Proceedings of an Advisory Group Meeting Vienna, 26—29 November 1979 jointly organized by )AEA and NEA (OECD) Uranium Exploration Case Histories
tNTERNATtONAL ATOMtC ENERGY AGENCY, V!ENNA, 1981 Cover picture:
Geology of the Key Lake area (Canada) showing the location of mineralized glaciai boutders transported by an esker. The mineralized boulders were the first indication of the presence of rich uranium accumulations in the region. Improvement of the glacial geology, together with lake sediment sampling, geophysical surveys and drilling, finally led to the discovery of two orebodies containing about 75 000 t U 3 O 8 with an average grade of 3% U 3OS (from Gatzweiler et al.).
LEGEND
[ j Athabasca Formationx*X Mieralized Boulders
Aphebian Metasediments жз=зУ Esker f +*+!*] Archean Granitoids Fault Zones
I Ore Zones Airborne EM Conductors URANIUM EXPLORATION CASE HISTORIES
PANEL PROCEEDINGS SERIES
URANIUM EXPLORATION CASE HISTORIES
PROCEEDINGS OF AN ADVISORY GROUP MEETING ON CASE HISTORIES OF URANIUM EXPLORATION JOINTLY ORGANIZED BY THE INTERNATIONAL ATOMIC ENERGY AGENCY AN D THE OECD NUCLEAR ENERGY AGENCY AND HELD IN VIENNA, 2 6 -2 9 NOVEM BER 1979
INTERNATIONAL ATOMIC ENERGY AGENCY VIENNA, 1981 URANIUM EXPLORATION CASE HISTORIES IAEA, VIENNA, 1981 STI/PUB/584 ISBN 92-0-141081-6
The Agency's Statute was approved on 23 October 1956 by the Conference on the Statute of the IAEA held at United Nations Headquarters, New York; it entered into force on 29 July 1957. The Headquarters oí the Agency are situated in Vienna. !ts principal objective is "to accelerate and enlarge the contribution of atomic energy to peace, health and prosperity throughout the world".
O IAEA, 1981
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Printed by the IA EA in Austria October 1981 FOREWORD
In 1976 the OECD Nuclear Energy Agency and the International Atomic Energy Agency established a Joint Group o f Experts on Research and Development in Uranium Exploration Techniques. The aim o f the Group's programme was to improve the rate o f discovery o f uranium deposits and to increase our knowledge o f the size o f the uranium resource base. Several areas were identified in which existing techniques could, with international co-operation, be significantly improved and new techniques developed to assist in the discovery o f uranium deposits, and in this connection 'Uranium Exploration Case Histories' was identified by the Group as a subject o f prime importance. The Joint Group considered that although there was an adequate diffusion o f information about the methodology and retrospective scope o f each available exploration technique, the effective and final performances of each technique in different geological, topographic and climatic conditions were not well known. The Group therefore proposed to collect and systematically analyse information about general exploration practice, to indicate the degree to which each had been successful and to publicize the various approaches adopted. It was considered that the most effective way to initiate this procedure would be to hold a meeting of the senior exploration personnel from the world's mining industry who have participated in the discovery o f major or new-type uranium deposits during the past three decades. The purpose o f the meeting was to describe the methods used and the history of the discovery of these deposits. From the various organizations and companies invited to participate in the meeting, nineteen favourable responses were received. Twenty successful programmes were described in the nineteen contributions, representing a wide spectrum o f exploration methods applied in different geographical and geological conditions of the more important or promising uraniferous areas of Argentina, Australia, Brazil, Canada, Czechoslovakia, Greenland, France, South Africa, Sweden, United States o f America and Zambia. The OECD Nuclear Energy Agency and the International Atomic Energy Agency wish to express their gratitude to all the organizations and companies that contributed, by means o f papers prepared by their respective highly qualified personnel, to the success o f the meeting on Uranium Exploration Case Histories. Both Agencies would, at the same time, like to recognize the valuable financial contributions o f the United States Department o f Energy and the Bundesanstalt für Geowissenschaften und Rohstoffe o f the Federal Republic o f Germany which make possible the publication o f the present proceedings. EDÏTORÏAL NOTE
77ie papers and с?мсм5л;'о?м /¡a^e &een edited йу t/¡e ediVori'a/ sta/f о/ t/ie /nternati'ona/ Htomi'c Energy Agency to t/¡e e ffe c t consideren/ necessary /or t/¡e reader's assistance. Г/ie fiews expressed aad M e genera/ sty/e adopfed remain, /¡owe^er, t/¡e responsi^i/i'ty о/t/ie named ant/iors or participants. /n addition, t/¡e riews are noy neí?essari/y t/iose o/ t/¡e governments o/ t/¡e nominating ТИет^ег States or o/ t/¡e nom¡nat;';ig organizations. Wierepapera /;aye óeen incorporated into t/iese A*oceedings wit/iOMt resetting /?y t/ie Agency, t/n's /ias óeen done wit/i t/ie ¿now/edge o/f/;eaMt/iors and M eirgovernrneHi a: The Espinharas uranium occurrence, Brazil (IAEA-ÁG-250/2) ...... 2 ÆD. FMc/и, У. da Fonfe, К ДмсАгам, К ГАа/смл Discussion ...... 11 The Southern Karoo uranium deposit, South Africa (IAEA-AG-25 0/15) ...... 15 Дои?/! И/h'can И /owí'c Energy Foard Discussion ...... 20 Discovery o f the Sierra Pintada uranium district, Mendoza Province, Argentina (IAEA-AG-250/6) ...... 23 F. /¿odn'go, A F . ßcF/мсо Discussion ...... 54 The Westmoreland uranium deposit, Queensland, Australia (IAEA-AG-250/3) ...... 59 //. D. FMc/M, H^.F. Дс/ü'nd^ayr Discussion ...... 71 Methodology and history o f discovery o f uranium mineralization in North-Western Province of Zambia (IAEA-AG-250/13) ...... 75 F. Afcncg/ze/ Discussion ...... 94 Poços de Caldas and Itataia: two case histories o f uranium exploration in Brazil (IAEA-AG-250/14) ...... 99 J.AÍ.A Forman, /l.C. /1 Discussion ...... 136 Exploration of the uranium reefs of Cooke Section, Radfontein Estates Gold Mining Company (Witwatersrand) Ltd, South Africa (IAEA-AG-250/16) ...... 141 B.D. 5*?ewarf Discussion ...... 169 Discovery o f the Jabiluka uranium deposits, East Alligator River Region, Northern Territory of Australia (IAÈA-AG-250/19) ...... 171 J. C. D. К Мэ.;/?ег Exploration o f the K ey Lake uranium deposits, Saskatchewan, Canada (IAEA-AG-250/5) ...... 195 Я. Ga?zwe;7er, Д. Д. Fan Discussion ...... 219 Midwest Lake uranium discovery, Saskatchewan, Canada (IAEA-AG-25 0/1) ...... 221 F. Rcoff Discussion ...... 240 From armchair geology to a deposit in a new uranium province (IAEA-AG-250/8) ...... ,...... 243 C. FMHJrocÆ Discussion ...... 275 Exploration o f Bernabe Montano complex o f uranium deposits, New Mexico, USA (IAEA-AG-250/9) ...... 279 D.Æ Porfer Discussion ...... 291 Discovery o f uranium deposits near Oshoto, East Central Powder Riverbasin, Wyoming, USA (IAEA-AG-250/10) ...... 293 ÆF. 5Yo;'c^, & Í . ßo^fz, Ai.О. ßMsweM The Bernardan deposit, Haute-Vienne, France (лйогГ (IAEA-AG-250/18) ...... 307 P. Cé/y Discussion ...... 308 Exploration o f the Panna Maria uranium mine, Karnes County, Texas, USA (IAEA-AG-250/12) ...... 31 1 E.C. BowwwM, N.Æ. Cygan, М Я . Discussion ...... 327 Discovery o f the Cluff deposits, Saskatchewan, Canada (,s/io;'?co?MMM4¡'ca^'oHj(ÍAEA-AG-250/17) ...... 329 7. D arc/e/ Discussion ...... 330 Uranium exploration in the N.Västerbotten—S.Norrbotten province, Northern Sweden (IAEA-AG-250/4) ...... 333 ß. Discussion ...... 350 Exploration history o f the Kvanefjeld uranium deposit, Ilimaussaq intrusion, South Greenland (IAEA-AG-250/11) ...... 353 ß .i. М'е/лен Uranium mineralization in the Bohemian Massif and its exploration (IAEA-AG-250/19) ...... 389 A/. О. Ж Discussion ...... 396 W O RKING GROUP REPORT ...... 401 Chairmen o f Sessions ...... 403 Secretariat ...... 403 List o f Participants ...... 405 CASE HISTORIES IAEA-AG-250/2 THE ESPINHARAS URANIUM OCCURRENCE, BRAZIL H.D. FUCHS Urangesellschaft mbH, Frankfurt/Main, Federal Republic o f Germany J. da FONTE, V. SUCKAU, V. THAKUR NUCLAM, Recife, Brazil Abstract THE ESPINHARAS URANIUM OCCURRENCE, BRAZIL. Nuclam has been exploring for uranium in Brazil since 1976. During this period one uranium ore body has been found in the vicinity of Espinharas, a village in Paraiba State, north east Brazil. According to present knowledge, the mineralized ore body is caused by metasomatic action. The history of discovery and the exploration work u n til the end of 1979 is given, showing the conceptual change with increasing knowledge of the mineralized zone. INTRODUCTION For a period o f about twenty years uranium prospecting and exploration in Brazil was conducted by the Comissâo Nacional de Energia Nuclear (CNEN), the Brazilian Atom ic Energy Commission, which was superseded by Nuclebras in 1975. Most o f the results o f the work done by CNEN is published or is readily available, and CNEN was always prepared to allow visits to the areas where uranium mineralizations were reported. This and the belief that the geology of Brazil is most favourable for uranium deposits caused Urangesellschaft to visit Brazil for the first time in 1973. In the course o f joint field trips by Brazilian and German geologists, and as a result o f the previous intensive desk research by CNEN, we were able to select specific areas which showed good potential for uranium deposits. In 1975 Urangesellschaft was invited to perform uranium exploration in selected areas jointly with Nuclebras. For this purpose a joint Brazilian/German Company with the name NUCLAM (Nuclebras Auxiliar de Mineraçâo) was founded in 1976. Its shareholders are Nuclebras (51%) and UrangeseHschaft (49%). 3 4 FUCHS et a). F/C. 7. Íccaíí'c/? o/ f/?e Ар:'л/:йгал ^га/!;мм pro/ecf. Nuclam's investigations were based on the previous work o f CNEN and Nudebras. They consisted mainly in regiona] radiometric airborne surveys, a regional carborne survey, and a ground check o f discovered anomalies. On the basis o f these data, Nuclam selected various working areas, one o f which iies in the north-east o f Brazil, where the uranium occurrence o f Espinharas was discovered (F ig .l). GEOLOGY The following simplified description is given for a better understanding of the geological situation o f this area: Between two highly metamorphosed blocks possibly o f Archean age, pelitic and psammitic sediments o f the Ceara Series have been deposited. The Ceara Series can be subdivided into the older Caico and the younger, possibly conformably overlying, Serido Group. The lower part of the Caico Group most likely belongs to the basement, where the contact is strongly migmatized. During later regional metamorphism the sediments were intruded by granites and partly migmatized. ÏAEA-AG-250/2 5 PROSPECTION r In 1976 Nuclam started prospection in the Serido area. The size o f the prospected area was approximately 38000 km^, and it represented a terrain o f highly metamorphosed Precambrian basement rocks. The initial work resulted in finding hundreds o f airborne and carborne radiometric (Th/U) anomalies. A new, very detailed, carborne survey (average 1 km road per 1 km^) and, for selected areas, a helicopter survey were performed. The studies included recheck and reclassification o f all previously found anomalies. Anomalies with relatively high U-values, with reasonable U/Th ratios, and anomalies o f reasonable size were studied in more detail. One o f the interesting anomalies was that o f Espinharas. It is located right inside and close to the small town o f Sâo Josë de Espinharas, which has a population o f 800 and is situated roughly 25 km north o f Patos, the third largest city o f Paraiba State. Espinharas can be reached by an all-weather dirt road. The mineralized zone is located about 200 m above sea level. The topography changes from flat plains to rugged hills; in general, the terrain is slightly undulated. The temperatures vary between 20° and 35°C. The average precipitation, confined to the period January to May, approaches 450 to 500 mm per year. The anomaly o f Espinharas was first discovered during a carborne survey in 1972, but it was not considered to be o f high priority. The strongly weathered and porous host rock was at that time assumed to be a meta-arkose. Re-investigations by Nuclam in 1976 revealed the 'intrusive' nature o f the radioactive rock, which was identified as a syenitic dyke. On resampling the anomaly it became obvious that it was over 2 0 0 0 m long and was related to a somewhat peculiar magmatic rock. Systematic and detailed ground prospecting of the anomaly began late in 1976. The first surface samples o f Espinharas contained less than 250 ppm U^Og but more than 1 0 0 0 ppm ThO ^. The U/Th mineralization occurs in two different albite-rich types o f rock within the Serido Group. One of them is possibly a completely metasomatically altered granite with apatite but hardly any primary quartz. The other is a meta- somatically altered amphibole-biotite-gneiss enriched in plagioclase and depleted in quartz. The first step in prospecting was a systematic radiometric ground survey: Baseline 2450 m long Striking 70°E Cross-sections every 50 m Reading stations every 20 m Additional readings on radioactive outcrops This survey indicated a radioactive zone more than 1 km long, with an average width o f over 40 m. Trenches were then dug across the anomalous zone, 200 m 6 FUCHS et ai. FVC.2. ^.митле^ o/ w¡'neraZi'zafi'07!. apart, some o f them.being more closely spaced. Deep pits were dug at points o f special interest to obtain fresh rock material and to study the tectonic setting. AH the trenches and pits, both shallow and deep, were radiometrically measured in detail by scintillometer Scintrex GtS 2 and spectrometer Scintrex GIS 3; they were sampled and mapped on a 1 : 1 0 0 scale. First leaching tests on surface rocks indicated that the uranium was not intergrown with ThO^ or P^Og. A substantial amount o f uranium was contained in limonite, formed during a period o f weathering (lateritization). This led to the assumption that, during weathering, uranium must have been available from leachable uranium minerals rather than from complex U-Th minerals or apatite, as those minerals hardly weather. On this assumption, higher primary uranium concentrations could be expected below the weathered surface. Reconnaissance mapping in the vicinity o f the main zone resulted in the discovery o f two additional mineralized areas, one o f them located 100 m SSW and the other about 800 m north of the main zone. Based on airphoto interpretation, the surrounding area was mapped on a 1:10000 scale. A stream sediment survey covering an area o f approximately 82 km^ commenced in December 1976 and was completed and evaluated in 1977. This survey did not lead to any new results. On the basis o f the mapping and trenching results o f 1976, it was decided to perform a limited drilling programme in 1977. Some additional surface work was conducted in order to better define the geometry o f the mineralized body and locate the initial drill sites. The work comprised'. Plane table mapping, 1:1000 scale; Digging deep pits at the margins o f the mineralized zone; Magnetic survey. IAEA-AG-250/2 7 FfC.J. Fi'ríf ¡Mferprííaíí'on o/ f/¡e dr¡'H secfi'oMí w¡f/¡ гЛе sfeep/y ¿¡'ppi'ng The plane table mapping resulted in a better delineation o f the mineralized zone at surface and in understanding the complex structure then expected. The latter had already been indicated by detailed radiometry. The magnetic survey did not produce any conclusive data which would have helped to define the mineralized body in depth at that time. The drilling programme was set up as soon as it became evident that further surface work could not deliver additional information. Its principal aims were to find: The geometrical shape o f the mineralized body; Any down-dip continuation of the mineralized body; and The possible positive change in grade and U/Th ratio towards depth. FIRST DIAMOND DRILLING PROGRAMME The outlined surface mineralization was drilled along 200-m sections (i.e. 1000 W, 800 W, 600 W). The borehole locations were based on the assumption that the mineralized body dips steeply and that the mineralization should be 8 FUCHS et a!. intersected between 50 and 100 m vertical depth. A total of thirteen coreholes were drilled in 1973, of which only two did not intersect mineralization. Actual drilling commenced in August 1977 with three rigs. Drilling progress was unexpectedly fast and the programme was completed on time, in spite o f some minor breakdowns. AH the holes were surveyed with a Tropari instrument. It turned out that the holes steepened and deviated more than expected, especially those with an inclination to the north. AM the radioactive cores were sampled at 50-cm inter vals and analysed (U chemically, Th radiometrically). The results can be summarized as follows: U/Th ratio is much more favourable in depth (0.3 to 1.0 at surface compared to 1 . 0 to 1 0 . 0 in core material); U values o f core material are higher than those o f surface samples; Minor amounts of calcite are always present (3% to 8 %); The body dips quite irregularly about 70° to the north; The mineralized zone extends over 1 km. The following statements could also be made: The U/Th ratio is more uniform and higher in samples o f mineralized gneiss than in samples o f pure 'dyke' material; The U/Th ratio varies widely in samples o f pure 'dyke', for no conceivable reason; The samples collected from contact gneiss/dyke show a behaviour similar to mineralized gneiss samples. This is consistent with the fact that the concentration o f uranium and its separation from thorium is greatly influenced by the geochemical environment. In the case o f the Espinharas anomaly, the favourable environment is furnished by biotite- gneiss and amphibole-biotite-gneiss. The metasomatic nature o f the mineralized rock was confirmed by these observations. The geological picture at the end of the first drilling programme looked as if we were dealing with an irregular vein-type ore body (Fig.2), which dips steeply north (Fig.3). As it was not possible to connect all mineralized sections, it was often necessary to introduce faults. The first drilling programme proved a potential source o f uranium at Espinharas but, still, little was known about the shape, the down-dip extension, and the average grade o f the mineralized body. A more systematic drilling IAEA-AG-250/2 9 programme during the second half o f 1978 was necessary in order to understand the mineralized body better. SECOND DIAMOND DRILLING PROGRAMME The objective o f the second drilling programme, which started in 1978 and was finished at the beginning o f 1979, was primarily to increase the level o f confidence o f the estimated reserves in the upper levels and to explore deeper levels for additional reserves. Detailed geological logs for each hole and the assay data for each core sample were prepared, as well as a summary o f the drill-hole data and ore zones at various cut-off grades. The overall drilling performance-was quite satisfactory, with a core recovery o f more than 90%. In general, the mineralized rocks were easy to drill. Up to 50 m were drilled by one rig in two working shifts, although a more typical performance was 20 m/day. The drill-holes had a tendency to deviate towards the east from their original direction and to increase their angle o f inclination. The most important results o f the second drilling programme are only now available and can be summarized as follows: (a) The originally assumed steeply dipping dyke-like shape o f the mineralized zone is no longer valid. There are indications now (Figs 4 and 5) that there are several mineralized zones more or less parallel to the general foliation (3 5 °) to the north-west and are positioned in a more imbricated fashion. The present drill pattern is therefore not optimal and interpretation is difficult. (b ) There are different high-grade lenses (average grade above 0.12%) which can be followed down-dip within the general mineralized zone. ю FUCHS et a!. F/C.3. ^4 ге-:яУегргеГаУ:оп о / ^ е dn'H secfi'ons ¿náícaíes í/:ar f^e wMera/;ze¿ AoA'es Aaye y/:e same <í:'p as ;Ae regona//b/i'af;on (c) The type o f mineralization means that occurrence o f uranium, thorium, phosphate and yttrium has not changed, except that the overaH grade may perhaps become somewhat higher towards depth. The drill-pattern is not yet close enough to give meaningful reserve figures, but the first estimates are as follows (drill indicated up to 1 0 0 m vertical depth): PlOOtUgOg, 0.108% U^Og cut-off: SOOppmUgOg or SlOOtUgOg, 0 . 1 2 2 % U^Og cut-off: VOOppmU^Og RESULTS On the basis o f the results so far, it can be stated that at Espinharas there is a good potential for a uranium deposit in respect o f reserves, but it is still to be proven that, with the present relatively low average grade, a feasible mining operation can be achieved. Only further work can give the answer. IAEA-AG-250/2 11 DISCUSSION D. TAYLOR: Did you do any open-hole drilling? H.D. FUCHS: It was all diamond core drilling. D. TA YLO R .' is there any reason fo r that? H.D. FUCHS: I think we had to do core drilling for this type o f work because, first, we had no rotary equipment and, second, we had to drill inclined because we first assumed that our ore body dipped at about 70°, and as there was thorium we had to have cores because logging would not work at all. We had a high thorium content, sometimes ten times more than uranium, and no logging method would have helped. D. TAYLOR: Do you think that you have a better control now than you could possibly have had with percussion drilling? H.D. FUCHS: We shall always have problems with the material obtained from percussion drilling as we have to assay only our cores or our material which we cannot get from percussion drilling. I don't think we can change because we have to know exactly what material we penetrate, and percussion would not help there even though it would be much cheaper. P. BARRETTO: Could you give us some idea of the magnitude of the drilling programme, the first and second phases, and when the uranium concen tration starts changing with depth? Is there any relationship to the water table? H.D. FUCHS: The first year we bored about 16 holes and in the second year about 30 holes with an average depth o f about 100 to 300 m. Water table is very difficult to assess there; it is a very dry area and I don't think we can really talk about a water table. However, we have a weathered zone which is about 10 to 15 m deep, and below this level more or less fresh rock is indicated by calcite. But we still do not know whether there is a cementation zone because, at an increase o f the grade to depth and between about 50 to 80 m, there seem to be higher uranium values. This 50 to 80 m would not correspond to the weathering surface, but we have a suspicion that we have a cementation zone there, which means that some uranium from the top was leached slightly down-dip (uranium was very mobile there). Again we have calcite, but there are no indications from the drill-core that we still have a porous material, so there are difficulties in trans porting the uranium from the weathered zone to deep levels. C. TEDESCO: I should like to know something about the type of climate and the weathering one might see, and about thorium variation, and whether or not it is constant from surface to depth, and then something about the type o f deposits, and how you interpret and classify these deposits. H.D. FUCHS: This part o f NE Brazil is rather dry. It has about 400 mm rainfall a year, but this is distributed over only a few months; lateritization is not too strong and it may be a type o f savannah. Then, the thorium variation is very difficult to assess. First, the thorium is, o f course, much higher at surface because ! 2 FUCHS et a!. it is far less teachable than the uranium, so we have a ratio at surface which is favourable to thorium. When we go down in depth it changes, but within the ore body there are chnges too. We find a completely altered granitic-vein-type material which contains a relatively large amount o f thorium. This brings us to another question. When the metasomatic solutions have gone into the host rock, the thorium content decreases and the U/Th ratio improves, this may indicate that uranium was far more mobile than thorium and was moving into the. host rock far better than the thorium. As to the type of deposit, we call it at the moment a metasomatic type of mineralization, possibly similar to some in the USSR but, frankly, we cannot yet answer this question. B.L. NIELSEN: You mentioned the intrusion of the dyke, syenitic or granitic, as the associated metasomatism including a disappearance o f the quartz. You also mentioned the association o f uranium with the limonitic material. What is the spatial correlation between the limonitized and the metasomatized zones? H.D. FUCHS: Most likely the feldspar-rich dyke was originally a granitic- type rock, which was completely changed by feldspathization (albitization, leading to a complete depletion of quartz). The limonitized zone at surface is purely a weathering feature and has nothing to do with ore-forming processes. B.L. NIELSEN: Was the uranium originally contained in the refractory minerals or within the upper schists? H.D. FUCHS: We can say we are sure that the uranium was not and is not connected with the thorium (at least not 80%). They are separate. Probably they were introduced in different phases because the uranium is fairly fairly leachable. We have a recovery, quite a high acid consumption in the order o f 85—90%, which is an indication that the uranium is not connected with thorium. B.L. NIELSEN: You might have had refractories containing both uranium and thorium, but the metasomatization might have broken these refractories down and caused a natural separation o f the elements. H.D. FUCHS: We still think the refractories came in with the metasomatic action and were not primarily in this granitic-type material. I think this is more likely than to claim that they were originally there but became separated through metasomatic action. R. G ATZW E ILE R : What is the structural setting o f the deposit? Is metaso matism generally related to regional and local faulting? H.D. FUCHS: Espinharas is not too far away from the east-to-west-running Patos Linement and within an area o f secondary fault zones, which may be lelated to the Patos Linement. The mineralized zone itself does not follow a distinct fault zone, but seems more to follow a wider tension zone, which is, however, not yet clearly established. IAEA-AG-250/2 13 D.A. PORTER: On the selected helicopter surveys, how many times back ground were the valid uranium anomalies? H.D. FUCHS: This is difficult to answer, as there were quite high uranium readings, indicating small uranium showings, and lower ones which showed more important targets. The Espinharas anomaly showed about five times background uranium values. The satellite deposit Arraras (900 m o ff the main zone) showed no values above background. D.A. PORTER: From what altitude were the helicopter surveys made? H.D. FUCHS: The helicopter surveys were flown at an average altitude o f 25 m. !AEA AG 250/15 THE SOUTHERN KAROO URANIUM DEPOSIT, SOUTH AFRICA SOUTH AFRICAN ATOMIC ENERGY BOARD* Pretoria, South Africa b ) ' f . D . 7 1 9 E A '1 S ' prgjgM/gJ ¿у F.D. Abstract THE SOUTHERN KAROO URANIUM DEPOSIT, SOUTH AFRICA. The first indication of the uranium mineralization was given by a carborne radiometric survey. The mineralization occurred in the Lower Beaufort Series (P-T2) south of the known 'dolerite line', on a flank of a shallow dipping anticline. Ore-grade mineralization was found in March 1976. The host rock is a fine-grained greywacke with interbeds of siltstone and mud- fragment conglomerate. The mineralization consists of pods of varying thickness, lateral extent and grade. It occurs mainly in organic-rich tabular pods which may be found in the mud-fragment conglomerate, sandstone lenses or in silty horizons. Organic debris probably acted as a reductant. The uranium minerals are coffinite and minor amounts of unidentified urano-organic compounds; no visible secondary minerals occur, not even in the outcrops. The associated minerals are pyrite, arsenopyrite and molybdenum (in unidentified form). Small amounts of copper and vanadium were identified. Two possible sources for uranium are postulated, (a) the contemporaneous tuffaceons material, and (b) the rock of fragments in the sediments. The genetic implications of the accessory minerals are being investigated. 1. INTRODUCTION The discoverers, a major mineral exploration company, began active explora tion in the southern Karoo in October 1974 with the commencement o f airborne radiometric surveying. No airborne radiometric anomalies were found over the discovery site, but the farm was optioned because a regional geological evaluation indicated that it was prospective for uranium. This was the first farm optioned by the company in the southern Karoo. The first surface occurrence o f uranium was discovered during carborne radiometric surveys along trails. A fter detailed geologic and radiometric mapping to evaluate the surface occurrences, drilling began in October 1975 and the first subsurface ore-grade mineralization was found in March 1976. * This case history is furnished by the mineral exploration company responsible for the discovery and is presented with the company's consent by the SAAEB. 15 16 SOUTH AFRICAN ATOMIC ENERGY BOARD 2. GENERAL GEOLOGY 2.1. Regional setting The uranium deposit, here informally called the 'Southern Karoo' (SK ) deposit, is situated within two farms (Fig. 1), south-east o f Beaufort West in the Cape Province o f South Africa. The deposit occurs in the 'SK Sand' which forms a small portion o f the Lower Beaufort Series o f Permo-Triassic age. It is estimated that the SK Sand occurs a few thousand metres above the Lower Beaufort/Ecca contact which is exposed on the southern edge o f the Karoo Basin near Rietbron. The SK deposit lies south o f the 'dolerite line', and no intrusive rocks were discovered during drilling. 2.2. Local setting Structurally, the SK area consists o f a series o f east-west trending anticlines and synclines with dips up to five degrees on a flank (Fig.2). Some monoclinal structures have dips o f up to thirty degrees. Two prominent faults have been inferred in the area from drilling, but there are probably many more smaller ones. The fault along the southern flank o f the SK anticline is a normal fault with the upthrown side on the north. In one place the displacement is 30 m. This fault has played an important role as the uranium mineralization on the downthrown side has been protected from erosion and oxidation, thus preserving a major portion o f the deposit. The fault in the south appears to be a reverse fault with about 45 m o f throw in places. There has been some oxidation o f the uranium mineralization along this fault. The SK Sand is exposed on the flanks o f the shallow-dipping SK anticline. The outcrop width varies from 25 to 125 m. 'K o ffie k lip " is developed irregularly along strike for several hundred metres. A t its widest, it covers 15 m o f outcrop. High radioactivity can be found in portions o f the Koffieklip, but much of it is barren o f uranium. The better surface mineralization occurs at Discovery Ridge on the south flank o f the SK anticline. A fter detailed geologic and radiometric mapping to evaluate the surface occurrences, drilling began in October 1975, immediately down-dip from Discovery Ridge. The first ore-grade mineralization was drilled in March 1976. * 'Koffieklip' is an Afrikaans term meaning 'coffee rock'. This is a surface weathering phenomenon which forms crusts, a few to two or three cm thick, of iron and manganese oxides and carbonates. ÍAEA-AG-250/15 17 OUTCROP-MíNERAUZED RY RtDGE F/C.2. о/ Ж m jMÓ-yMr/cce. 18 SOUTH AFRICAN ATOMIC ENERGY BOARD SOUTH WEST ^ ^ WORTH CAST А йэ?юп Crí7W (a) ^770/t/ /!-F WEST EAST №/^?0/7g (b) I ¿r/ммд ^arw/]/ с -z? Оле /7o F/C.J?. CroM-íecf¡'o?!í fAe 5Á* ¿ерою'г. 3. THE URANIUM DEPOSIT 3.1. Host rock The host rock for the uranium mineralization is a light to dark grey, very- ñne-grained to fine-grained greywacke with interbeds of siltstone and mud-fragment conglomerate. The general composition is about one-third quartz, one-third feldspar and one-third rock fragments. The calcite content varies considerably, but is as high as 30% in places. The SK Sand is up to 60 m thick and was deposited by a river that flowed north-easterly into the basin. The sand has all the characteristics o f a fluvial sand !AEA AG 250/15 19 system; point-bar complexes, abandoned channels, clay plugs, mud-fragment conglomerates, etc. The sandstone comprises at least two major cycles. The uranium mineralization generally occurs in the older cycle. Figure 2 shows the approximate limits o f the SK Sand in the subsurface as delineated by a considerable amount o f drilling. The maximum width o f the sand system is approximately 3000 m. Figure 3 shows cross-sections through the SK deposit. In both (a) and (b ) the upper cross-sections show some o f the struc tures in the area. The lower cross-sections, which have the top o f the SK Sand as the datum, show more dramatically the variations in thickness in the sand. The sporadic distribution o f uranium ore pods is graphically illustrated. Sizes and geometry o f pods shown are diagrammatic. 3.2. Uranium mineralization Detailed drilling in the SK deposit has outlined a large area o f anomalous uranium mineralization (Fig.2). Within this broad area there are ore-grade pods o f varying thickness, lateral extent and grade. The geometry and dimensions o f the pods are similar to those found in the Grants Mineral Belt in New Mexico, USA. Uranium mineralization occurs most commonly in organic-rich tabular pods which may be found in mud-fragment conglomerate, very-fine-grained sandstone lenses or in silty horizons. The organic debris probably acted as the reductant for the uranium. The principal ore minerals are uranium silicates chemically analogous to coffinite, with a wide range o f uranium content. Minor amounts o f unidentified urano-organic compounds are also present. Uraninite is extremely rare. Pyrite, arsenopyrite and molybdenum (in unidentified form) commonly occur with the uranium. The calcite content in the 'ore' pods is extremely variable. Some pods contain abundant calcite while other pods, equally rich in uranium, contain no calcite at all. Small amounts o f copper and vanadium have been detected in assays, but no appreciable amounts o f thorium, silver or gold have been found. The genetic implications o f the accessory minerals are still being investigated. No visible secondary uranium minerals have been found in the deposit, not even on outcrop. 3.3. Source o f uranium The most likely sources o f uranium are: (a) tuffaceous material deposited contemporaneously with the sediments, and (b ) from the matrix o f these sediments. Tuffaceous material in the Lower Beaufort sediments has only recently been recognized and its distribution is somewhat uncertain. In the SK area, there are several 'chert' (light green, highly siliceous rocks) horizons which could be o f 20 SOUTH AFRICAN ATOMIC ENERGY BOARD volcanic origin. Diagenesis o f indigenous volcanic material could release the contained uranium, and perhaps some o f the accessory metals, into solution. The SK Sand contains a high proportion o f rock fragments that could have contained uranium which was remobilized during diagenesis. 4. DETAILED EXPLORATION By the end o f 1977, exploration drilling had outlined a deposit that could be o f economic interest. However, deposits o f this kind have never been mined in the southern Karoo, so there is no established infrastructure and no history o f mining costs, best extraction methods, etc. It was decided that a pre-development programme consisting o f additional drilling, preliminary bench tests, and trial mining be commenced in 1978. A decline (or inclined shaft) was put down to the ore zone and various mining methods, including wide-raise stoping and face- scraping, were tried. I f the uranium deposits in the southern Karoo are to be exploited economically it will be necessary to find the cheapest method o f extracting the ore. This pre-development programme is now complete, and assessment o f the large volume o f data collected is under way. 5. CONCLUSIONS The SK uranium deposit was discovered in early 1976. Uranium mineraliza tion occurs in thin irregular pods with organic debris in very-fine-grained to fine grained sands o f fluvial origin. The principal primary ore mineral is coffinite, with pyrite, arsenopyrite and molybdenum in some form as important accessory minerals. A mining investment decision will be made at a later date. DISCUSSION D.A. PORTER: Is the mineralization related to the redox-front? P.D. TOENS^ : With the exception o f the surface outcrop and the first few metres in the subsurface, the host sandstone is entirely reduced. The concept o f redox fronts, as understood in the better known Colorado Plateau deposits, cannot be applied to this deposit or to other deposits in the Karoo. ^ P.D. Toens (S.A.A.E.B.) was not present at the meeting but answered the questions in a written communication. IAEA-AG-250/15 21 D.A. PO RTER: There are many alterations in the underlying mudstone. Is the ore consistently o f the same colour, and is there any study of trace element variation through the sandstone and enclosing sediments? P.D. TOENS: There is no visible evidence of alteration (associated with mineralization) in the underlying mudstone. However, no detailed geochemical study has been made of trace element variation (such as Li, В, V, etc.) through the sandstone and enclosing sediments. D.A. PO RTER: What is the average grade o f the deposit? P.D. TOENS: Both the cut-off grade used and average grade are company confidential information and, further, under terms o f the South African Atomic Energy Act, may not be released. C. TEDESCO: What is the nature o f the secondary minerals at surface? P.D. TOENS: There are no visible secondary uranium minerals at surface, unlike many other occurrences in the Karoo, where uranium phosphates and arsenates are the commonest found. AH the oxidized uranium occurs in the dark brown weathering crust (referred to in the paper as 'koffieklip') apparently as urano-organic complexes. In the subsurface, several uranium associations have been recognized, amongst which are coffinite (dominant) and unidentifiable urano-organic complexes. Oxidation appears to have preferentially removed the former and preserved the latter. D. TAYLOR: Was the exploration by drilling done by rotary-percussion rigs or also by rotary-coring? Could you give some information about drill-hole spacing, type o f gamma-logging, etc.? P.D. TOENS: Most of the exploration and pre-development drilling on the prospect was done by rotary-percussion rigs and comprised many hundreds o f holes. Hole diameters ranged from about 11.5 cm to 16.5 cm (4.5 to 6.5 inches). A limited amount o f coring was done through the host sandstone in holes pre collared with percussion rigs. Information on total drilling density or the total metres drilled is not available for release. Drill-hole spacing ranged from several hundred metres to 25 metres on a grid pattern. The progress o f outlining the ore pods followed the same pattern as that documented for many sandstone uranium deposits in the USA. A ll holes were radiometrically logged using Mt. Sopris equipment, and both gamma and electric logs were obtained for each hole. Records were taken in analog form and in about half o f the holes a printed digital tape also recorded depth and gamma signal (in cps) for expanded-scale re-runs in mineralized zones. IAEA-AG-250/6 DISCOVERY OF THE SIERRA PINTADA URANIUM DISTRICT, MENDOZA PROVINCE, ARGENTINA F. RODRIGO, A.E. BELLUCO National Atomic Energy Commission of Argentina, Buenos Aires, Argentina Presen fed Ay P. Abstract DISCOVERY OF THE SIERRA PINTADA URANIUM DISTRICT, MENDOZA PROVINCE, ARGENTINA. Since 1956, uranium-bearing minerals have been known to exist in Sierra Pintada, Mendoza Province, Argentina. Based on paragenetic considerations, a first radiometric prospection was carried out, leading to the discovery of two groups of anomalies (Puesto Agua del Toro and Cuesta de los Terneros), such as vein-type deposits, with uraninite and 'yellow minerals' and one sandstone-type deposit (Puesto La Josefa), related to sediments with carbon trash. Many anomalies of both types have been found prior to 1959, but aii of them turned out to be of small size and continuity. Some recent geological research and surveys in the area, and a reduced drilling programme carried out on selected anomalies, led to reinterpretation of the potential of the area. Furthermore, and as a result of an airborne radiometric prospection performed in mid-1968, numerous anomalies have been discovered. The main constellation of anomalies, along the Hanks of the El Tigre Brachyanticline, occurs in sandstones of Permian age. Explored by 80 000 m of drilling, they have shown the existence of several peneconcordant lens-shaped ore bodies of economic size, w ith uranophane on the surface and prevailing uraninite and some brannerite, coffinite and davidite below the water table. Reserves exceed 20 000 tonnes of UßOg. A new regional programme with a 4-km drill-grid initiated in [978 ]ed to the discovery of new ore bodies which are at present being evaluated. The alternatives and discontinuities during the development of the district, the prospecting and exploration techniques employed, and the results achieved in the different stages of the operation are discussed in detail. This case history attempts to illustrate the developing philosophy which was successfully applied in Sierra Pintada, with emphasis on the following points: (a) the need for adequate geological knowledge of the area; (b) the advantage of a massive survey (in this case, air survey); (c) the necessity for exploration (drilling) in order to define the anomalies and make their evaluation possible; and (d) the convenience of extending exploration when geology and control factors have been properly surveyed and recognized. I. HISTORY OF THE SIERRA PINTADA DISTRICT In 1956 the Comisión Nacional de Energía Atómica (C N E A ) discovered several small radiometric anomalies in sandstones and volcanic rocks in the Sierra 23 24 RODRIGO and BELLUCO Pintada District by surface reconnaissance prospecting. A ground foHow-up and some limited mining operations carried out in the 1950s did not show any significant uranium occurrences, especially in the volcanics. In 1960, CNEA sponsored an airborne survey training programme, using the San Rafael Airport, and some irregular flights were made over northern and central Sierra Pintada, using Royal 118-B equipment with one-head 3 in. X 2 in. crystal. No interest was shown in the discovery o f two small anomalies in Permian sediments and the verification o f two earlier recorded anomalies in travertine deposits in Las Peñas and Los Reyunos. However, the discovery o f uranium ore bodies in sandstones, made in 1967 during a small-scale drilling programme, together with up-dated regional geological knowledge, made a new interpretation of the uraniferous possible potential, and focused attention on the prospection of the Permian sandstones. A detailed radiometric airborne survey was therefore undertaken in 1968 at a line spacing o f 250 m over an area o f 3450 km \ and many new anomalies were located. Ground follow-up o f these anomalies led to the discovery o f uranium occurrences in the Cochicó Group sandstones on the western flank o f the El Tigre Brachyanticline. These cover an area o f 96 km \ o f whichi 60 knP pertain to the 'Dr. Baulies' and 'Los Reyunos' uranium mines. The occurrences were systematically prospected by surface trenching, sampling and drilling during the period 1968—75. More than 600 boreholes, totalling about 60 000 m, were drilled. A ll the deposits were located at the top o f the Atigradas Sandstone Member o f the Los Reyunos Formation, in a very conspicuous strati graphie position. On the basis o f a working hypothesis o f the favourability o f the Atigradas Sandstone Member and the existence o f sporadic outcrops in Sierra Pintada, further geological surveys o f reconnaissance drillings were initiated in 1978 on the eastern slope o f the range, north o f the El Tigre Brachyanticline. Tw o new deposits (Cerro Carrizalito and El Tigre Dam) and several positive holes were discovered. 2. LOCATION: GEOGRAPHICAL FEATURES The Sierra Pintada uranium district is situated in Mendoza Province, Argentina, 25 km west of the city of San Rafael (population 90 000) and 250 km south o f the city o f Mendoza (population 500 000), the capital o f the province (Fig. 1). The Sierra Pintada is a 100-km-long and 30 to 40-km-wide mountain range running north-to-south, which forms the northern part o f a very conspicuous morphostructural unit, the San Rafael Block, with a core of Paleozoic and Triassic rocks, whose borders are partially covered by Tertiary continental strata and, more gradually, by Quaternary and Recent sediments. The hilly environment, 800 to 2000 m.a.s.l., with a relief reflecting the faulted structure, is deeply cut by the perennial rivers Diamante and Atuel, fed !AEA AG 250/6 25 F /C .7. Locaron o/ fAe ^¡'erya P:'nfa¿a мгаитм A'yfncf. 26 RODRIGO and BELLUCO FVC.2. <&'егга Anfada, ге^'ояа/ geo/ogy. by melting snow from the Cordillera de ios Andes 200 km to the west. The region has a semi-arid climate, with an average summer temperature o f 22°C and an. average winter temperature of 7°C. Annual rainfall, supplemented by occasional snowfall, is 350 mm. A number o f local roads and tracks traverse the deposits, and a paved road links them with San Rafael and with other urban centres o f the province. A railway line from San Rafael passes about 5 km to the east from the main ore bodies. ÍAEA-AG-250/6 27 TABLE L SIERRA PINTADA URANIUM-BEARING DEPOSITS: SIMPLIFIED STRATIGRAPHIC TABLE AGES FORMATIONS LtTHOLOGY AND FACIES URANIUM SHOWS QUATERNARY VARIOUS FORMATIONS p ied m o n t , etc. Bosattic and ondesitic tovas \/^\/\ЛЛ/\Л/^\/\Л/^\ЛЛ/^и\ЛУ\ЛЛА/\ЛЛЛ/\Л PLIOCENE RIO SECO DEL ZAPALLO MIOCENE AtSOL Continento) sediments. \л/u^лллPENEPLДNAI)ON\лллл/ Continento) sediments, TRIASSIC PUESTO VIEJO tgnimbrites. Ca. CARRtZALtTO < Q- N 3 PERMIAN- 5 o A6UA DE tC CE LOS NOOUES \/\/\ллл/\лллллл/\л/^\л/^\ллл/\/\ллл/\л QUEBRADA TRIASSIC SosQHic focies. DEL PIMIENTO (?) PUNTA Los Enriques DEL AGUA deposit TOBA VíEJA GORDA tu ffs with c atco reo u s s a n d s to n e key MEMBER b e d s. wtNTRAFORMAHONAL UNCONFORMtTYW^ 23° ARENtSCAS PERMIAN AT)GRADAS MEMBER l g Red fangtomerotes and conglomerates, œ t* PSEPH)HC a lte rn a tin g with yeitow s o n d sto n e s an d MEMBER Co. COLORADO S (= BRECHA VERDE Pm) DEL tMPERtAL stones, siltstones and green and grey shales. DEVONIAN LA HORQUETA schiste, etc. CAMBRO- PONON TREHUE Marine limestones ORDOVICIAM PRECAMBRIAN C? LA VENTANA 28 RODRIGO and BELLUCO A jet plane service is in operation between San Rafael and Buenos Aires. San Rafael is essentialiy a farming community with hospitals, a university and other facilities, industria! development is rather limited. Near the main uranium deposits there is a small village, 25 de Mayo, located about 15 km from the site, which could be developed to provide housing during the exploitation o f the mines. The El Nihuil and Valle Grande dams on the Atuel River provide hydroelectric power (324 MW) and the Nihuil-Mendoza 132-kV power line crosses the centre o f the uranium district. Tw o other dams on the Diamante River, which are in an advanced stage o f construction (Agua del Toro and Los Reyunos),will double the present electricity output. A high-voltage line connects the Sierra Pintada main uranium deposit with the Los Coroneles power plant on the Diamante River. The El Tigre stream crosses the main ore body and will have to be diverted. It has a permanent flow o f clear, slightly salty water o f 0.2 to 3.0 trP - s "', which rises during the summer rains into short floods o f up to 1 0 0 0 m^ - s " '. The main system o f the Andes Mountains is seismically active, and nine earthquakes between 6.3 and 8.0 on the Richter Scale, with epicentres near Mendoza and San Juan, 250 and 400 km north of Sierra Pintada, respectively, have been recorded during the past 85 years. No earthquake has been recorded in - the San Rafael and Sierra Pintada areas, but ripple effects or aftershocks o f the large earthquakes further to the north are felt in the district. 3. REGIONAL GEOLOGY In recent years an adequate geological background knowledge has been obtained. The area was totally covered by 1:200 000 regular geological surveys, and three quadrangles were published in 1956, 1964 and 1972, respectively. Special thesis papers and specific geological and photogeological surveys carried out by CNEA have contributed to a progressively increased understanding o f the region (Fig. 2). Table I shows a simplified geological scheme, and a brief description o f the regional geology follows: .Рт-есятл&п'яи (.?/ Cerro Z-й HenfazM Form aron It is considered that some amphibolites and granites, strongly tectonized and intruded by quartz veinlets, aplites and pegmatites, might pertain to this period. Ролом Гге/змс Forwafio?? Reduced outcrops of whitish limestone overlying the above metamorphics and intrusives are assimilated to an old geosyncline in the Precordillera structural unit, 150 km northwards. IAEA-AG-250/6 29 DetwiMM.* ¿a Сгомр This is a complex sequence o f clastic sediments, grey wackes and pelites which crop out over large areas in a typical geosyncline flysch facies more than 1 0 0 0 m . thick. Metamorphism with sericitic and chloritic schists increases from south to north as a consequence o f intrusions o f granodiorites and diorites. С&г^ошУегомл.' Æ/ /wpen'a/ Forwaf;'OH Unconformably overlying the La Horqueta Group are conglomerates, sand stones and shales, with marine episodes, into a regressive facies which varies in thickness from 1 0 0 to several hundred metres. The Upper Carboniferous layer, which consists o f red and green conglomerates, crops out in the western and southern slopes o f Sierra Pintada. It is not present in the El Tigre Brachyanticline, where it has probably been removed by erosion from the Hercinian Orogeny, prior to the deposition o f the Permian sediments. FerwMn. Сос/м'со G'rotvp This is integrated by a very heterogeneous sequence o f clastic and pyroclastic sediments, with a wide distribution over the whole San Rafael Block. It is divided into two formations: Los Reyunos and Punta del Agua. The sequence starts with red fanglomerates and conglomerates alternating with yellow sandstones and pyroclastics, deposited on a very irregularly eroded surface. Fine to very coarse-grained arkosic sandstones follow, with parallel and cross-bedded stratification interbedded with tuffitic beds, presenting frequent lateral and vertical changes o f facies. A combined fluviatile-aeolian transport is assumed. These are the main uranium-bearing rocks. Los Reyunos Formation culminates in a compact, purplish-grey to violet, lithic crystalline tu ff with several calcareous sandstone key-beds. Punta del Agua Formation is a repeated sequence o f brick-red sandstones and conglomerates, with pyroclastic and tuffaceous materials, which predominate towards the top. СфрегРег^;'ям-7г;'ам;'с. Cerro (hrn'za/zYo Сгомр A very complex sedimentary-volcanic sequence overlies the Cochicó Group. It consists o f a complete effusive cycle o f basalts, followed by dacites and rhyolites. The volcanics cycle is interbedded and culminates with continental sediments o f the Lower to Middle Triassic period. 30 RODRIGO and BELLUCO rerfMry-QMafernary A fter a peneplanation stage, during the Jurassic, Cretaceous and Lower Tertiary periods, thick Miocene and Ptiocene continental clastic sediments were formed. Tertiary conglomerates, sandstones and tuffaceous sandstones are overlain by several Quaternary basaltic and andesitic flows and recent foothill deposits. Travertine deposits are also frequent. Structural scheme and geological history Sierra Pintada, as part o f the San Rafael Block, is a well defined geological and structural regional unit, dominated by intense and repeated faulting. The predominant structural feature o f the central uraniferous area is the 2 0 -km-long El Tigre Brachyanticline, which stretches from the Diamante River in the north to Los Chañares in the south. It is traversed by a large number o f faults, which vary in strike from NW-SE, through E-W to NE-SW. The historical and geotectonic evolution is marked by: (a) An Eopaleozoic geosyncline, flysch type, with intense folding and basic intrusives, reflecting variable metamorphism and deformation (Akadian-Bretonic Orogeny). (b) A regressive sequence with peripheral molassic deposits and an uplift, with faulted structures during Carboniferous times (Asturian Orogeny). Some acid intrusions are related. (c) Molassic deposits filling an active relief during the Permo-Triassic, with strong volcanism, a dominating folding and progressive block-faulting (Saalic Orogeny). A fter the deposition o f old Mid-Triassic sediments, a long period o f pene- planation occurred up to the Miocene and it was assumed that this had a direct relation to the mobilization and deposition o f uranium. Continental sediments coming from the western Andes covered the whole area during the Upper Tertiary, alternating with repeated episodes of \ulcamsm (mainly basahic). At the end of the period, a great-radio folding and a reactivation o f faulting during the uplifting o f the Andean Orogeny led to the complex and intricate fault-block structure o f the pre-Tertiary rocks, affecting the continuity and spatial position o f the uranium ore bodies. Tertiary sediments were totally eroded at the core o f Sierra Pintada, and some fluviatile terrace deposits, new basaltic volcanics and travertine deposits occurred during the Quaternary. ÏAEA-AG-250/6 31 4. FIRST-STAGE EXPLORATION (1956-60) 4.1. Ground prospection (1 9 5 6 -5 9 ) A t the beginning o f this period, geological information was either scarce or not available. Nevertheless, numerous small occurrences o f lead, silver, arsenic, fluorine, etc., and some paragenetic hypotheses prevailing at that time, led to the selection o f the crystalline formations o f this area for carrying out a reconnaissance ground prospection. A Precision Instrument 111-B, Ц in. X 1 in. N a l(T l) crystal scintillometer was utilized, with irregular grid itineraries, densified near the location o f the ore deposits mentioned. Puesto Agua del Toro was the first uranium discovery on the western flank o f Sierra Pintada, in the Diamante River Valley. It is located in a brecciated fault, in the contact o f an andesite with sandstones, tuffs and volcanic agglomerates, which were postulated as Triassic. The occurrence, consisting exclusively o f yellow minerals, is 15 to 20 m long and 0.20 to 0.50 m thick, the grades varying from 0.1% to 0.2% U 3 OS. It was explored by trenches and edged near the surface. Not far away, at Puesto Agua de la Josefa, a second model o f a punctual anomaly in sandstones with trash carbon was discovered (postulated at that time as pertaining to the Carboniferous period). In addition, high radioactive values in the acid effusives were proven. A t the same time, at La Cuesta de los Terneros, on the eastern slope o f Sierra Pintada, 10 km south o f El Tigre Brachyanticline, several anomalies o f both models were found. Priority was given to the more frequent occurrences, such as vein-type deposits, in relation to brecciated faults in Permo-Triassic effusive^rocks. An exploratory effort was made, by means o f a number o f minor mining operations over selected anomalies, resulting in punctual and discontinuous bodies a few metres to 30 m long and 0.5 to 2 m thick. Uraninite and yellow minerals were related to a hydro- thermal process. Despite the discovery o f one occurrence, Los Chañares, in fluviatile sandstones, in the contact with a rhyolite dyke, no effort was expended on the sedimentary model, and the real ore genesis was not properly interpreted at that time. 4.2. Airborne prospection survey (1960) An airborne radioactive prospecting programme was carried out, its main target being the training o f personnel on this kind o f survey, using some facilities offered by the San Rafael Airport with Cessnas 180 and 170, the property o f the local Air Club. A Royal 118-B scintillometer, with one non-collimated head of 3 in. X 2 in. N a l(T l) crystal was mounted. 32 RODRIGO and BELLUCO \ The area was selected mainly because o f its proximity to the La Cuesta de los Terneros anomalies. The totally irregular survey, as rim or с/н'ел de с/м.ме flight itineraries over some 1 2 0 0 krrP, in spite o f the inadequate equipment and the excessive terrain clearance, led to the detection o f two sandstone-type anomalies at Los Mesones and Los Caimanes, and two more anomalies in travertine deposits (Las Peñas and Los Reyunos). Unfortunately, the ground checking did not allow the potential mineralization o f the sedimentary formations to be defined, and the district was abandoned. 4.3. Comments on the results of the 1956—60 period The inadequate survey prospection, the scarcity o f geological information on a regional basis, and the erroneous priority given to the vein-type deposits, led to a ten-year delay in developing the Sierra Pintada uranium district. The negative results were based on: (a) Lack of flexibility in evaluating the metaHogenetic models; (b)The tendency to sacrifice general prospecting targets by premature develop ment of early findings; (c ) Inadequate equipment and surveying techniques in airborne prospection; (d)Hesitation (or impossibility, because o f budgetary problems) in exploring by drilling in some conspicuous occurrences (subsequently, at Los Mesones and Los Chañares, some interesting ore bodies were revealed); (e)The limitation of prospection to excessively restricted environments without prior regional analysis. 5. SECOND-STAGE EXPLORATION (1967-75) 5.1. Prior reconnaissance and geological reinterpretation During the 1960s some CNEA personnel received training,including visits to the principal uranium-developed countries. A t the same time, geological knowledge o f Sierra Pintada was advancing considerably. Three regular geological quadrangles at a 1 . 2 0 0 0 0 0 scale and several theses led to a better understanding o f this complex morphostructural unit. Late in 1967, a preliminary analysis o f existing background data on uranium within the framework o f the local geology and known world uranium deposit models led to the selection of representative anomalies and, profiting from CNEA's recently acquired capacity to improve drilling, 30 to 50-m-deep holes were bored in each o f them. IAEÁ-AG-250/6 33 F/C. J?. ^ДТЯТЯЯ мар. 34 RODRIGO and BELLUCO Results were entirely positive in Los Chañares, where mineralization was verified in fluviatile Permian sandstones with evident sedimentary control — thickness o f up to 4 m and grades varying from 0.1% to 0.2% UgOg. A fter a final reinterpretation o f all the available uranium data, and with the support o f new geological mapping, an airborne prospecting survey was programmed to cover the Permian Cochicó Group outcrops and especially the Atigradas Sandstones Member over an area o f 3000 km^. 5.2. Total gamma airborne survey (mid-1968) The main equipment, technical specifications and operating conditions were as follows: Conventional cartographic maps at a scale o f 1:100 000 were used for the flights, and enlarged copies at a scale o f 1:50 000 were used for data plotting. Aircraft: A Cessna 180-B. Scintillometer: MP-10 equipment made by CNEA was used, consisting of one-head 5 in. X 2 in. N a l(T l) crystal, with a lead collimator, with scales o f 250, 500, 1000, 3000 and 10 000 cps and time constants o f 0.5, 1 and 1.5 seconds. Radio-altimeter: AN/APN 1. The strip camera was a continuous French Cameflex, model S, with 35-mm film. Synchronized recorders, in tandem, with fiducial mark devices: two Ester- line Angus. Line night intervals were 250 m at a terrain clearance o f less than 100 m. Flight directions were in general normal to main structures, except over El Tigre Brachyanticline, where topographical conditions made it compulsory to adopt N-S strike directions. Data recovery from records was punctual, at every 250 m, and was plotted after manual clearance correction at the 1 0 0 -m base level. Contour maps were made at intervals o f BG X values o f 1.10, 1.40, 1.70 and over 1.70 (Fig. 3). Operative costs, including airborne photography at 1:20 000 scale over El Tigre Brachyanticline area was US $6.60/km^, at 1968 values. In these conditions, 270 hours were flown in 135 days and on 649 lines, covering a 3450 kirP surface. The results, notably effective, can be summarized as follows: (a) AH the later anomalies were detected; (b ) The constellation o f the El Tigre Brachyanticline anomalies was clearly defined; and (c) Some very valuable geological data were obtained. IAEA AG 250/6 35 5.3. Ground-checking (late 1968) The definition of the airborne radioactive anomalies of the El Tigre Group had been proven from the very beginning,so, before elaborating the isocontour maps and final interpretation, a classical geological survey mapping with selected detailed cross-sections at a 1:10 000 scale was therefore launched (Fig. 4). Simultaneously, another working group started the ground-checking o f some principal anomalies, using the French scintillometer SPP 2. As a result o f the clear stratigraphie control, and owing to topographic limitations, a regular grid prospection was exceptionally used, and the checking had to be done by means o f cross-sections 10 to 25 m apart between lines, with readings at every metre. In addition, for the follow-up o f the ore bodies in covered areas, such as La Terraza, by radon prospection, the French EPP 10 emanometer had to be used. The method proved suitable for extending the known ore bodies or main faults. However, no results were obtained over the virgin sectors. Geochemistry applied from the very beginning to water, soils and stream samples did not give positive results even in the neighbourhood o f the main out cropping ore body. Adequate knowledge of the El Tigre Brachyanticline mineralized area was obtained by means o f the ground-checking, the specific geological survey and the interpretation o f photographs taken during the airborne survey. Late in 1968, more than 15 proven occurrences were located and their stratigraphie positions and lithostratigraphic controls defined. All occurrences were located in the uppermost sequence o f the then redefined 'Areniscas Atigradas Member' of the Los Reyunos Formation in the Permian-Triassic Cochicó Group (see Table I). During the same period, six drill-holes were bored in the El Tigre I front and the continuity o f the mineralization was proven, with thicknesses varying between 4 and 18 m and with grades varying between 0.09 and 0.19 U^Og. Subsequently, a programme was initiated for implementing a topo-geological survey at a scale o f 1 : 1 0 0 0 which would cover the whole mineralized area (5000 X 6000 m) and serve as support o f development activities and drill-hole locations. 5.4. Exploratory operations A total o f 600 holes were drilled in the El Tigre area (about 60 000 m). Except for the main ore body (El Tigre 1-La Terraza), the grid was rather irregular (50 m to more than 100 m), starting at the discovery sites. The main ore body, which is divided by faults into three blocks, was recognized by the almost regular grid (50 m apart — this being established by Ch LOCAHON OF URANtFEROUSOCCURRENCES ¡ ORG ad BELLUCO andRODRIGO Г ¡AHuvial ^ 1 Bosattic fíows Y У[ Acid dyhes ГУТУ) Tuffs IAEA-AG-250/6 ^VG.4. Ceo/ogy о/ ^6 F/ 7%re ¿zr^c. bJ 38 RODRIGO and BELLUCO geostatistical methods). Taking into account the existing topographical problems, it was possible to bore 241 drill-holes totalling 23 628 m and distributed as follows: Block A: 131 holes 11 800 m Block B: 58 holes 5 634 m Block C: 52 holes 6 194 m O f these, 133 drill-holes were completely cored, 103 were cored only in the mineralized interval, in NX and BX diameters with excellent core recovery (average 90%), and five holes with cuttings recovery. Further, and in order to obtain suitable samples for hydrometallurgical assays, a 320-m gallery was excavated in Block A and a 35-m shaft and 206-m gallery in Block B. in addition to the core recovery, all the drill-holes were logged with GM Sensor Equipment. The radioactivity/grade correlation was positive. Total expenditure during the exploration stage amounted to approximately 5 million US $ at 1974 values. A problem that still persists is the high cost o f drilling, which is about four to five times higher than in the USA, and the contractor's explanation o f this cost is not acceptable to CNEA. 5.5. The constellation o f the El Tigre uranium deposits 3.3.7. I,oca?geo?ogy The Permian-Triassic Cochicó Group (consisting o f sediments with repeated intercalations o f tuffs and varied pyroclastics) lies unconformably over the above-mentioned Precambrian(?) to Carboniferous basement. The group is subdivided into the Los Reyunos Deposit Formation (Permian) and the Punta del Agua Formation (Uppermost Permian to Mid-Triassic). (a ) ¿05 Леумло.у Fo?T?Mf;'oM This consists o f a transgressive filling which overlies an irregular relief imposed by the Hercynian Orogeny over the former basement. It comprises. Í;) 77;e A*ep№z*c ТИйти&ег The Psephitic Member consists o f polymictic fanglomerates and conglomerates which are composed o f angular blocks and pebbles embedded in a distinctive red arenaceous matrix. It contains small intercalations o f cross-bedded yellowish sandstone and two pyroclastic beds, the lower one agglomeradic and the upper tuffaceous. IAEA AG-250/6 39 i;'z) i/ze yIrenMCÆS This member, which is 70 to 100 m thick, consists o f a cross-bedding o f fine to medium grained calcareous feldspathic argillaceous sandstone, with a thin bed o f tuffite (key-bed) commonly developed about 50 m above its base (Fig. 5). The fresh sandstone varies in colour from dark greenish-grey to greenish-gray and is occasionally reddish owing to staining by iron oxides. Thin beds o f coarser-grained, less argillaceous sandstone are common. The most abundant constituent is quartz, occurring as poorly sorted subangular to subrounded grains which frequently show bipyramidal crystal outlines characteristic o f phenocrysts in porphyritic rhyolites. Rounding o f these grains may be due to reabsorption by the groundmass o f the host rock, rather than to sedimentation processes. The feldspar grains are predominantly plagioclases, varying from albite to oligoclase in composition,and usually have a high degree o f alteration to kaolinite. The matrix, which constitutes up to 20% o f the rock, consists o f clay minerals, fine-grained sericite, chlorite and interstitially precipitated calcite. The latter also occurs as an alteration product o f feldspar, often completely replacing pre-existing grains. The Atigradas Sandstone includes two preferential stratigraphie levels to host the uraniferous bodies. The main, or upper, position occurs in the sandstone above the tuffite, but subsidiary ore bodies are developed in the sandstone sequence below the tuffite. The contact between the Atigradas Sandstone and the underlying Psephitic Member is characterized by interfingerings in the peripheral zones of the El Tigre structure. Small-scale planar cross-bedding, indicating deposition from the south-west, has been observed at the southern extremity o f Tigre I. A shallow water depositional environment is postulated, followed by minor scouring, together with an accumulation of aeolian sands, prior to the deposition of the overlying Toba (tuff) Vieja Gorda Member, both of them into a desertic climate. 77ze 7b&a He/a Gorda Ме?м&ег This member is estimated to be up to 200 m thick. It is a purplish-grey to violet lithic tu ff made up o f quartz, feldspar, biotite and other mafic minerals. It also contains lapilli and xenoliths o f the Devonian La Horqueta Formation. A reddish agglomerate a few metres thick, containing two to three thin beds o f white tuffite, occurs between 80 and 110 m above the base o f the tuff. An erosional surface marks its top. RODRIGO and BELLUCO IAEA-AG 250/6 4! 77!efMnfade/^4gMaForwaf;oM This.formation was deposited over the Toba Vieja Gorda Member on a rugged erosional surface. It occurs westwards from the western limits o f the El Tigre- La Terraza ore body and north o f it. The basal part consists o f brick-red sandy conglomerates with intercalations o f pale yellow and pink sandstones; the upper part is a sequence o f coarse-grained clastic sediments interbedded with pyroclastic rocks and lavas. Its thickness is estimated at several hundred metres. f c) Loca/ л?гмс?мга/ paffern The El Tigre Brachyanticline corresponds to a former closed anticline, superimposed by several diastrophic cycles leading to the present fault-block structure. The deformation process started with a disharmonie folding (anticlino rium), which affected the Psephitic Member, and a smooth anticline acting rather symmetrically over the subsequent formations. The uranium-bearing Areniscas Atigradas Member participates in this last deformation. This compressive tectonism was followed by several fracturing cycles which contributed to the complex faulting o f the periclinal system in relation to the anticlinorium structure, with a parallel east-west strike on the western slope o f the structure. Stratigraphie throws vary widely, reaching up to 70 m. 3.J.2. А/а/и ore &об?;'ел fa) 7¡¡'gre/-Za Terraza This ore body crops out in the central-western flank o f the El Tigre Branchyanticline. Tw o peneconcordant ore bodies were outlined by means o f diamond drilling surveys in the Areniscas Atigradas Member. The upper (or main) ore body covers an area o f about 1800 m from north to south and about 600 m from east to west. The lower ore body covers a surface o f 800 X 250 m. The upper ore body top is generally 10 to 20 m below the overlying Toba Vieja Gorda Member and the lower contact may extend right down to the tuffite key-bed. It varies in thickness from a cut-off o f 0.04% to a maximum o f 30 m and an average o f 10 m. The ore body crops out, or sub-outcrops, beneath a thin overburden cover in the east and pinches out abruptly to less than 0.35 m thickness in the south and west. In the north-west and north, its boundaries have not been defined and it may continue below the 800 m.a.s.l. altitude. The lower ore body is located below the tuffite key-bed, usually about 10 to 15 m below the upper ore body in Block С and about 20 m in Block A. Its average thickness in the delineated blocks is about 5 m, petering out to below 0.5 m along the edges. 42 RODRIGO and BELLUCO The vertical value distribution corresponds to a well-shaped, lenticular habit o f higher-grade zones which cannot be individually correlated over large distances. The average grade, at a 0.04% U^Og cut-off, is 0.125% for the main body and 0.09% U^Og for the lower one. Tw o east-west transverse normal faults divide the deposit into three main blocks, referred to as A , В and C, from south to north. The major fault o f the zone, which separates Blocks A and B, has a southward downthrow o f approxi mately 70 m. The faults between Blocks В and С have a total southward down displacement o f about 50 m. The 'loss o f ground' caused by these faults is up to 80 m wide (which accounts for the division into separate blocks). In addition, there are numerous normal faults with steeper dips and north west, south-west and west trends. The first two sets are generally more significant, with displacements o f up to 35 m. The majority o f these faults have downthrows to the south-west, south and south-east. The most notable exception is the north eastern fault (exposed in the open pit), which has a downthrow o f 30 m to the north-west. No reverse faults have yet been recognized. The main ore body dip varies between 12° and 35°. It has a general dip which varies between 20° and 30° to the north-west in Block A and towards the north-north-west in Blocks В and С The present interpretation o f the distribution and spatial position o f the mineralization has been attained through 23 east-west cross-sections and 160 correlation sections between drill holes. f&J Bgre íf-íí/ and Media Luna A/Í-HÍ This is the second most important uranium ore body in the District and occurs about 1.2 km south of Tigre I. It is about 1000 m long by 300 m wide, extending from Tigre II in the north to Media Luna III in the south. The mineralized beds, occurring 25 to 30 m below the Toba Vieja Gorda Member, crop out in the east and dip to the west at 18° to 20°. The total mineralized thickness varies from 2 to 20 m, averaging about 8 m. The grade, at a cut-off o f 0.04%, is 0.096% U 3 ()g. Í cj ¿од СаысАол /-// Gaucho I and II occur in the uppermost part o f the Areniscas Atigradas Member, dipping 20° to the west-north-west. They are a lens-shaped body, covered by an eroded relic o f tuffs, 15 to 2 0 m thick and located south-west from Tigre 1-La Terraza, extending about 200 X 250 m. They consist o f three pene- concordant beds, each o f them 1 to 2 m thick, separated by thin, low-grade ore intercalations. They have been explored by means o f 60 drill-holes (2500-m drillings), the average grade being 0 . 1 % U^Og. tAEA AG 250/6 43 Í J) ¿a Terraza N orfe This deposit is located close to the Tigre Í-La Terraza ore body, from which it is probably separated by a barren zone o f faulted discontinuity. Reconnaissance drilling initiated in 1978 proved the existence o f this 7-m-thick 'blind' body with a grade o f 0.09% U^Og. Its outline has not yet been deñned or delineated, but it may well be a natural extension o f the main ore body. Isolated holes made it possible to recognize mineralization 6 km northwards in a similar position. fe ) (№ A constellation o f small ore bodies: Gaucho IH-IV, La OHada, La Caverna and Los Chañares, crop out south-east o f Tigre I. The bodies are several tenths o f a metre wide, with thicknesses o f up to 2 m and ore grades o f less than 0 . 1 % UgOg. Exploration is now in progress to define their importance. 5.6. Deposits removed from the main area La Pintada Anticline deposits are located 6 km west-south-west from the El Tigre Brachyanticline and in the same stratigraphie position. They comprise several lenses over an area o f 500 X 200 m, with a similar type o f mineralization. N ot far from these deposits, the Los Enriques deposit occurs in similar sandstones. A group o f five radioactive anomalies with yellow uranium minerals has been found at Pantanito, 30 km west from El Tigre in the same Permian sediments. Work has not yet been initiated. On the western slope o f North Sierra Pintada, at Cariizalito, 20 km north o f the El Tigre deposit, uranium occurrences crop out along hundreds o f metres, with a content of up to 0.05% U^Og. An isolated hole (4 m thick and with a similar grade) cuts the mineralization 300 m to the east at a depth o f 100 m. A t Rincón del Atuel, on the eastern slope o f Sierra Pintada, a vein-type deposit occurs in Triassic effusives o f the Cerro Carrizalito Group. It crops out along 400 m and has been intersected by about ten drillings up to 50 m deep. The ore bodies appear discontinuous, up to 2 m thick and with up to 0.1% U^Og grades. A group o f occurrences o f a type which is not yet well known has been located in some Triassic effusives and sediments south-east o f the area (Las Abejas, La Rinconada, El Totem, Los Buitres and El Nihuil), some of them with a very marked radioactive disequilibrium. They have not yet been explored. A summary o f the results, ages o f the host rocks and types o f occurrences is given in Table II. 44 RODRIGO and BELLUCO TABLE H. AGE OF HOST ROCKS AND TYPES OF OCCURRENCES OF SIERRA PINTADA URANIUM DEPOSITS TYPE AGE DEPOSIT TYPE AGE DEPOSIT Many orebodies in the El Ф p Cerro C arrizal i to W Ф p La Pintada Ф Q Las Peñas Ф p P antanito Many orebodies at Cuesta de Ш "R los Terneros Ф p Punta del Agua Ш "R Arroyo El Alumbre Ф p Los Enriques В Las Abejas Ф P La Josefa Ш *R La Rinconada P-H Agua del Toro Ш *R El Totem "R Rincon de Abajo Ф С Puesto Aleguino Ш Ü P ircas del Mesón Ф c Central No. 1 Nihuil Oam Ф P Los Reyunos Ф p Nihuil Ф Q Don Manuel Ф p Cañadón Generoso Ф 0 Saladillo Grande В *6 Rincón del Atuel Sedimentary-type deposits С Carboniferous Vein-type deposits P Permian "R Triassic 0 Quaternary 5.7. Ore mineralogy The bodies are typical peneconcordant lenses ranging from a few to hundreds o f metres in size and up to 20 m thick. A cloud o f similarly shaped, variable stratiform bodies occurs in the main lenses, either isolated or joined in irregular chains, which expand along the north-south strike o f the structure. The host-rock lithological units are made up o f 'arkoses' or feldspathic sandstones, fine to medium grained, yellow on the surface and greenish-grey in the depth. The sandstones consist o f quartz, acid plagioclases and potassic feldspars, with minor lithic volcanic glass, paraectinites, etc. Apatite and zircon are rather scarce. Sericitization, calcification and kaolinization o f the feldspars are marked. The calcite content o f the ore is an important factor in uranium extraction and for this reason the weighed average CaCOg was sampled and analysed regularly in most boreholes. The main value o f the CaCO^ content is about 6.5% in the El Tigre 1-La Terraza ore body and 5% in the El Tigre III and Media Luna deposits. ÎAEA-AG-250/6 45 In the thin, highly mineralized and coarser-grained beds, black pitchblende is present interstitially in layers or patches; brannerite, coffinite, liebigite and davidite are also present, but have not been identified macroscopically. Yellow uranophane occurs near the surface as a thin film along the cleavage and joint planes in the sandstone. Pitchblende grains o f 1 to 12 jum in size, or in veinlets 0.2 to 1.5 mm thick, occur: (a) Interstitially and as fracture fillings in quartz and feldspar grains; (b) Intimately associated with y Fe^O^ and iron-rich chloritic material in the matrix; and (c) Associated with structures suggestive o f a cellular type o f organic matter. The pitchblende has a microcrystalline texture and presents an X-ray diffraction pattern similar to uraninite but contains no thorium. The presence o f coffinite, brannerite, davidite and liebigite was confirmed by X-ray diffraction analysis. Brannerite is frequently either completely or partially enclosed as a thin film over anastase and leucoxene. Microprobe scans have indicated that uranium is frequently found finely disseminated on the leucoxene grains. Brannerite grains may be pseudomorphous after anastase. There is an irregular vertical distribution o f values o f the uranium minerals. In general, however, higher contents tend to occur in the central part o f the ore bodies, with a relatively abrupt decrease towards the top, and a more gradual one towards the edges and the base o f the lenses. Deep below, some pyrite occurs, 2 to 5 mm in diameter, which rapidly evolves near the surface into hematite and limonite. Trash carbon has also been checked with grades o f about 0.1% C. To summarize, the ore in the Sierra Pintada district is practically monometallic. 5.8. Ore genesis The following hypothesis for ore genesis is supported: The uranium is derived from Permian-Triassic acid effusives which poured out to the south or south-west o f the Sierra Pintada uranium district. During peneplanation, possibly during the Upper Mesozoic and Lower Tertiary periods, the uranium was leached and mobilized in aqueous solutions in which the presence o f CO 2 led to the formation o f stable, complex specimens, such as UDC or UTC, together with Ca, Fe, etc., which migrated to the Areniscas Atigradas Member. The precipitation o f uranyl ion was generated by the presence o f oxygen in the aquiferous and hydrated ferric oxides, such as the a and y varieties, controlled by the СОз pressure. С\ ORG ad BELLUCO andRODRIGO F O R M A T iO M S H T H 0 L 0 6 Y f TOBA ViEJA 60R0A Mb. Tuffs LOS REYUNOS 8 0 0 L Tuftitic key-bed 0 E P 0 S 4 Fm. ARENtSCAS AUGRAOAS Mb. ] ) Sandstones FÍC.6. 7-íc Геггдия зге IAEA AG 250/6 47 Intimate association with the у Fe 2 0 3 iron-rich chlorite in the matrix o f the sandstone suggests that its precipitation could also be influenced by the presence o f Fe and by variations in the Eh values. Higher mineralization in the upper part of the paleo-aquifer level, red, reddish-brown and red-violet in colour, corresponds to the location of the у РезОз variety, where bacteria find a more favourable environment for reducing uranium. Minor participation in the uranium fixation is attributed to the action o f organic matter due to the scarce content o f carbon trash which corresponds to the arid depositional environment o f the sediments. 5.9. Ore reserves The ore reserves estimate is based on the drilling exploration data. Although a regular 50-m grid was the aim, the rugged relief made this impossible. Spacing between boreholes is therefore less than 70 m, the borehole sheets serving as the basis for estimating the ore reserves. Co-ordinates, collar elevation, lithology, graphical representation o f total у -logs, sampling data and uranium values were obtained by this method. In addition, the СаСОд content is given for the whole or a part o f the mineralized zones in a number o f boreholes. The following sampling procedure was used: (a) A ll cores were radiometrically scanned to select the mineralized zone, which was then split into samples varying in length according to the radioactivity level, but generally about 1 m long. These samples were analysed radiometrically; in a large number o f boreholes, values o f over 0.05% U^Og were checked through chemical analysis. (b ) When chemical analysis was not carried out systematically, it was carried out at regular intervals to check calibration standards. The correspondence o f radiometrically and chemically determined values is excellent for all the boreholes, and there is no evidence o f disequilibrium. The thorium content o f the ore is relatively low, and consequently its effect on radiometric analysis is minimal. (c) Radiometric and chemical analyses o f drill cores made it possible to determine the mineral grade and it was necessary in a few cases — mainly at the beginning of the drilling - to take into account the radioactivity/grade correlation for the purpose o f the estimates. (d ) Estimates o f volumes were based on the structural interpretation made on topographical and geological profiles (Figs 6 and 7) and on plants and zoneo- graphical profiles. (e) Thickness has been corrected for each borehole intercept on the basis o f the average dip calculated for each section o f the deposit. In the same way, the surface o f influence o f each borehole intercept was corrected to calculate the blocks. 00 m.o.s.l. ^oo - 9 5 0 . ORG ad ELUCO C LLU BE and RODRIGO 9 0 0 - ORE ZONEOGRAPHY FORMATIONS aooL UTHOLOGY - 0 .0 4 % TOBA V!EJA GOROA Mb. ^ 1 ^ 7 ^ Tuffs ^ооб % ^ U30, 3 Tuffitic key-bid D E P O S i T ^ ^ ( ARENtSCAS AUGRADAS Mb. j ^-- 4 >o.to % Sondstones PSEPHtHC MEM8 ER Congtomerotes /^YC. 7. /-¿c íerrcza ore crojj-jecn'o/!. IAEA-AG-250/6 49 F7G. Д. ngye / ía Terraza, ore zoneography алс/ open-p;'f jeAeme. 50 RODRIGO and BELLUCO TABLE III. SIERRA PINTADA DISTRICT: URANIUM ORE RESERVES (t U 3OS)* OCCURRENCES REASONABLY ESTIMATED ASSURED ADDITIONAL TOTAL RESOURCES RESOURCES Tigre 1 - La Terraza 16 000 - 16 000 La Terraza North 800 500 1 300 T igre III 1 000 500 1 500 Media Luna 1-11-111 1 400 500 1 900 El Gaucho 1 -11-H I-IV 300 200 500 Los Chañares** 100 200 300 Minor ore bodies at El Tigre** 500 1 000 1 500 La Pintada** 100 200 300 Cerro Carrizalito** 100 500 600 Positive isolated holes** 300 3 000 3 300 TOTAL 20 600 6 600 27 200 * In the range of US $90-95/kg U3OS (equiv. US $ 40- 43/lb UgOs). ** With preliminary exploration. The calculations o f ore reserves are based on geostatistical concepts applied in: (a) Estimates o f thickness and ore grade in the sections o f borehole intercepts without drill cores; (b) Correction o f grade assigned to each block; (c) Calculation of accuracy for each block and of global accuracy; (d ) Classification o f mineral categories according to the French nomenclature. Sampling o f each borehole was performed on fractions o f different thicknesses, determined according to intervals o f radiometric values (normally 1 m or less). The calculation o f thickness and grade for each borehole intercept was made by selecting fractions according to a chosen cut-off grade. Where cores were missing, both parameters were determined by the statistical correlation o f radiometry and grade which made it possible to build up the corresponding curve based on borehole cores through chemical analysis. IAEA-AG-250/6 51 To define the average grade o f each block, the Matheron Corrector was used for correcting the grade o f the borehole centred in the block. This method made it possible to avoid over- or under-evaluation o f the blocks, the area o f influence being subject to the reconnaissance grid which was assigned, and the accuracy o f the estimate then calculated. Once the accuracy had been established, the accuracy o f the metal obtained by multiplying the product o f the thickness by the grade tonnage o f each block was classified into Reserves, Resources and Perspectives. Blocks with an accuracy level o f 6 8 % probability, which did not exceed a value o f 50%, were classified as Reserves; those exceeding this value were considered to be Resources and those which were devoid o f accuracy or whose values were higher than 100% were classified as Perspectives. The cut-off grades used were 0.04, 0.06 and 0.1% L^Og on first estimates. Recently, a cut-off o f 0.03% U^Og was added. Calculations o f ore reserves for the Tigre 1-La Terraza ore body were also made by Golden Associates by means o f computers and taking into account the open-pit preliminary design. Estimate differences are within the normal margin o f error for ore reserve calculations. A zoneography o f the economic ore body and a preliminary scheme for open-pit exploitation are shown in Fig. 8 . Estimated reserves at the end o f 1979, based on the N E A-IA E A classification, are shown in Table III. 6 . THIRD STAGE OF DEVELOPMENT (LATE 1977 UP TO THE PRESENT TIME) 6.1. Exploration In spite o f the positive results obtained during the second stage, the develop ment of the regional potential o f Sierra Pintada virtually stopped for a period of two years. A new programme was conceived in mid-1977 based on the geological aspects and the assumption that the Areniscas Atigradas Member is often present with no significant outcrops. The main targets include the follow-up o f its paleogeography, regional distribution and composition. Three potential uraniferous areas were postulated: North-east Sierra Pintada, from El Tigre north towards the Carrizalito and the Las Peñas occurrences; The central western slope o f Sierra Pintada, centred in the Agua de La Josefa and Pantanito group o f anomalies; and The south Sierra Pintada, south o f the El Nihuil Dam, with several types of occurrence. 52 RODRIGO and BELLUCO Two photogeological studies were carried out: the first over 3200 km^ at a scale o f 1 : 2 0 0 0 0 and the second over 6 6 0 0 0 krrP at a scale o f 1:500 0 0 0 , using Landsat imagery. Simultaneously, a geological ground survey was launched in north-east Sierra Pintada with checks o f selected locations and the performance o f detailed cross-sections. A t the end o f 1979, about 230 km had been completed at a scale o f 1:2500. Interest was focused on the Areniscas Atigradas Member, on the sandstony carboniferous El Imperial Formation and on some sedimentary sequences o f the Triassic which are considered to be excellent host rocks. The acid effusives have also been considered as possible uranium sources (see Table I). A preliminary scheme concerning these, mostly buried, formations was completed at the end o f 1977, taking into account the considerable doubts concerning their structural position conditioned by the complex block-faulting o f Sierra Pintada. Subsequently, a drilling programme was initiated on a 4-to-6-km grid, with an average depth o f 350 m, for the sole purpose o f obtaining sound geological knowledge. AGerhartOwenWidco Logger, Model 3500 PSL, was used for total 7 , resistivity and self-potential for cutting recovery and multilogging. Results were entirely satisfactory, the existence o f the Areniscas Atigradas Member at accessible depths having been proven in many localities. Four holes intersected uranium ore with grades o f up to 0.1% U^Og. Many others gave positive anomalies (0.02 to 0.03% UsOg). A second 10 000-m drilling programme was initiated late in 1979 over an area of 5 X 2 km north from the El Tigre 1-La Terraza up to the Diamante River, on a 400-m grid, chosen because o f the size o f the known economic ore bodies in the district. A few drill-holes were completed by the end o f 1979. One o f them inter sected an ore body at the El Tigre Dam in the Diamante River which was 1.60 m thick and had a grade o f 0.1% U^Og. The remainder were anomalous, with contents varying from 70 to 300 ppm U^Og. The programme is being implemented, and a new one involving some 2 0 0 0 0 m is to be initiated as soon as possible. The results obtained raise reasonable hopes for future development. The existence o f very favourable host rocks and the mobilization and fixation o f uranium have now been proven without any doubt and with an excellent continuity, within a triangle o f 45 km base (El Tigre-La Pintada-Pantanito-Agua de La Josefa) and 50 km height (up to the north o f Carrizalito). 6.2. Mining and milling programmes Following some minor mining exploitations, a programme was initiated early in 1978 at the El Tigre III ore body. A pit was opened which will produce 2 0 0 0 0 0 tonnes o f ore per year, the third part o f which, with agrade o f 0 . 1% U^Og, is being sent to the old Malargue Mill, and the remainder, with a grade o f 0.07% IAEA-AG-250/6 53 U^Og, is processed locally by heap-leaching methods in a plant which started operating in September 1979. The geological concept o f the deposit was satisfactorily proven during this operation. A major programme to exploit El Tigre 1-La Terraza, by subcontracting the engineering, erection and operation o f the mine and mill (on the basis o f an agree ment to purchase a fixed volume o f yellow cake o f up to 700 t U^Og per year) is now in the final stage o f negotiations. The complex is expected to start operating early in 1983. ACKNOWLEDGEMENTS The authors wish to express their thanks to all the colleagues at the Nuclear Supply Direction o f CNEA who contributed by sending papers and making suggestions and who assisted in the preparation o f this document. SELECTED BIBLIOGRAPHY ANTONIETTI, C., Yacimiento nuclear Dr. Baulíes, San Rafael, CNEA Internat Rep. (1969). BELLUCO, A.E., DIEZ, J.D., ANTONIETTI, C., ACHEN, H., VALERD I, C.J., "Los depósitos uraníferos en las Provincias de Mendoza y Neuquén", in Proc. 5th Congr. Geol. Arg., Buenos Aires, 1974, Vol. 2. BELLUCO, A.E., PARERA, C.A., Exploración costa sur del Río Diamante, Puesto La Josefa, CNEA Internal Rep. (1956). BELLUCO, A.E., RODRIGUEZ, E., Anteproyecto Pían de Expíoración, CNEA Internal Rep. (1975). BELLUCO, A.E., SUAREZ, G., Informe Geología Distrito Uranífero Sierra Pintada, en Bases para Licitación, CNEA Internai Rep. (1979). COCO, A.L., Relevamiento aeroradimétrico sobre áreas anómalas Los Reyunos-El Tigre-Los Chañares, CNEA Internai Rep. (1968). DAVIDS, N., Informes reservas yacimientos Estructura El Tigre, CNEA Internai Rep. (1978-79). DESSANTI, R., Descripción Hoja Geológica 27-C Río Diamante, Mendoza, Rep. Dir. Nac. Geol. Miner., Buenos Aires (1956). GONZALEZ DIAZ, E., Descripción Hoja Geológica 27-D San Rafael, Mendoza, Rep. Dir. Nac. Geol. Miner., Buenos Aires (1972). M AZZIERI, G., PEREZ, E., PRIETO, A.O., Informe sobre los resultados de la exploración geoiógico-mmera en e) braquianticlinal del Tigre y sus manifestaciones nucleares más importantes, CNEA Internal Rep. (1971). NICOLLI, H.B., "Consideraciones sobre la génesis de depósitos uraníferos en areniscas: Distrito Sierra Pintada, Depto. San Rafael, Prov. de Mendoza", in Proc. 5th Congr. Geol. Arg., Buenos Aires, 1974, Vol. 2. 54 RODRIGO and BELLUCO ORTEGA FURLOTTI, A., RODRIGUEZ, E.J., PRIETO, A.O., VALD IVIEZZO , A., "Et nuevo distrito uranífero de Sierra Pintada, Prov. de Mendoza", ibid. POLANSKY, J., Descripción Hoja Geológica 26-C La Tosca, Prov. de Mendoza, Rep. Dir.. Nac. Geoi. M in e rBuenos Aires (1964). RODRIGO, F., Descubrimiento de importantes yacimientos de uranio en ta Sierra Pintada, San Rafael, Mendoza, Mundo Geot. № 4, Buenos Aires (1971). RODRIGO, F., BELLUCO, A.E., "Programa nacional de desarrollo de ios recursos uraníferos de la Argentina", Uranium Deposits in Latin America: Geology and Exploration (Proc. Regional Advisory Group Mtg. Lima, 1978), IAEA, Vienna (1981) 395. RODRIGUEZ, E., VA LD IVIEZZO , A., Investigación geológica semiregional Sierra Pintada, CNEA Internal Rep. (1970). SANGUINETTI, J., Fotointerpretación geológica semidetallada, Distrito Sierra Pintada, CNEA Internal Rep. (1977). STIPANICIC, P.N., Conceptos geoestructurales generales sobre la distribución de los yacimientos uraníferos con control sedimentario en la Argentina y posible aplicación de los mismos en el resto de Sudamérica", Uranium Exploration Geology (Proc. Panel Vienna, 1970), IAEA, Vienna (1970) 205. STIPANICIC, P.N., RODRIGO, F., FRIZ, C.J.T., LINARES, E., "Provincias uraníferas argentinas", in Proc. 23rd Int. Geolog. Congress, Prague, 1968. DISCUSSION P. BARR ETTO : From 1969 to 1975 you carried out exploratory work in the Sierra Pintada area and used severa! techniques: airborne surveys, emanometric surveys and geochemistry. Which o f these really indicated the interesting items to follow up? What were your experience and results in other areas? P. STIPANICIC: The first good indication o f the area's favourability was based on geological considerations. Before 1968 some airborne surveys which were carried out with lines that were too wide and irregularly spaced did not give good results, owing to the highly radioactive background prevailing in the area. The 1968 detailed airborne survey, with a grid o f 250 m, gave an excellent result because the Permian sandstone bearing the uranium mineralization is covered by a thick (up to 200 m) piroclastic tuff with high radioactivity. When lines too widely spaced were used, the highly radioactive background o f the tu ff masked the anomalies corresponding to the outcrops o f some ore bodies, but with a 250-m regular grid, the major part o f the uranium outcrops, including those o f small volume, could be located by airborne survey, the remaining being discovered by radiometric prospecting on foot. The altitude o f flights was around 100 m. Classical emanometry gave good results in Argentina. Many years ago (1 9 5 3 -5 6 ) we used the old laboratory instruments, the Ambron ionization chamber, which gave us good results for defining and following the continuation o f the buried uranium bodies in Cosquin, Cordoba. A fter that, we used the French emanometers (by sniffers), also with excellent results, as in Paso de Indios (Chubut). In Sierra Pintada, the method proved useful for extending the known ore bodies or main ÏAEA-AG-250/6 55 results. Geochemical surveys were not very successful in Argentina (including the Sierra Pintada District). P. BAR R E TTO : What was the direction o f flights at Sierra Pintada? P. STIPANICIC: Normally, crossing the structure with few parallel lines, when topographic conditions obliged us to do so, or for control. F. SCOTT : Did the faults limit the mineralizing process? What is the thickness o f the ore bodies? P. STIPANICIC: They are of typical stratiform type and lenticular shape. Faulting is later than the mineralizing process and only produced the division o f the ore bodies in blocks. The maximum thickness o f the main ore body (Tigre I- La Terraza) is 30 m for a cut-off o f 0.04% U 3OS, and the average thickness is around 1 0 m. H. FUCHS: Is there any clear relation between the tu ff with abnormal uranium content and the mineralization at Sierra Pintada? P. STIPANICIC: I presented a paper on this kind o f problem during the IA E A Panel on Uranium Exploration Geology in Vienna in 1970. According to my idea, which was followed by other geologists, many o f the sandstone-type uranium deposits o f Argentina are closely related to the presence or availability o f large peneplanized areas sculptured in fertile uranium sources, such as granites, acidic tuffs, etc. Peneplanation and adequate climatic conditions have favoured uranium leaching from the igneous source rocks and the uranium was afterwards deposited in any kind o f host rocks offering favourable chemical and physical characteristics. The morphology o f the resulting ore body could therefore be variable, stratiform, vein-type, etc. In the Sierra Pintada geological environment, Devonian, Carboniferous, Permian and Triassic are very rich in granites, grano- diorites, andesites, rhyolites; tuffs, etc., and all this variety o f uranium source rocks was peneplanized in more than one stage. The first large peneplanation occurred between the Middle Permian and Lower Triassic; another great stage o f pene planation was recorded at the end o f the Upper Triassic as well as during several stages in Tertiary times. The uranium could have been leached from these peneplains at different geological times. In my opinion, the more important factors are the availability o f large igneous and peneplanized outcrops and the presence o f favourable host rocks. I believe that the uranium o f the Sierra Pintada ore bodies was mainly extracted from the Permian-Triassic tuffs. H. FUCHS: In north-eastern Australia (Queensland) we have a similar geological framework, with tuffs on top o f the Permian sandstone, but there we have indications that mineralization probably came with the younger volcanic acid tuffs and the uranium mineralization in the sandstone originated by apophysis o f the tuffs. In your example, it seems that it was just the erosional surface which allowed the uranium extraction from the source rocks. P. STIPANICIC: I would like to emphasize the factor just mentioned: large peneplanized crystalline surface, presence o f favourable host rocks and climatic 56 RODRIGO and BELLUCO conditions could meteorize the outcropping source rocks,allowing the uranium leaching af ялу Zwie. In Argentina, with a surface close to 3 million krrP, there are some good examples. In the central part of the country there is a nesocraton composed o f granites, granodiorites, syenites, etc., varying in age from Ordovician to Triassic, but the majority are o f Hercynian age. Large peneplanized surfaces were sculptured from Late Cretaceous times (Laramie diastrophism) but also immediately after important Tertiary diastrophic phases. Within this crystalline environment, which covered a surface o f more than 1 0 0 0 0 0 km \ one can find exogenic uranium deposits o f different types, formed during different times, but always related to some peneplanized igneous source area, from where the uranium was leached. Thus the Cosquin stratiform uranium deposits (Punilla Valley, Córdoba Province) are included in Eocene sediments and were formed using the products coming from the crystalline Laramie peneplain. But in the same geological environment, the uranium deposits o f Los Gigantes district were formed by uranium leached in recent times from the source rocks o f a young peneplain (Upper Tertiary) and precipitated as ore shots in the same tectonized granites, in the joint o f two main fault systems. Moreover, the same superficial and recent uranium solutions were able to form small accumulations o f calcrete type (San Luis). J. D ARD EL: What are the accessory minerals? P. STIPANICIC: The mineralogy is very simple: uraninite is the prevailing mineral with subordinate brannerite and coffinite. The relationship o f uranium mineralization with carbonaceous materials does not seem to be clear, owing to the scarcity o f the latter. In shallow levels there is a relation to the content o f some colloidal iron-oxides (y Fe^O^). D.A. PO RTER: Uraninite is below the water table; but what is the mineralization above the water table? P. STIPANICIC: Above the water table, uranophane. D.A. PO RTER: What is the control o f the mineralization? P. STIPANICIC: I don't know if Messrs Rodrigo and Belluco have a good answer to your question, because, to my knowledge, there is not a well proven explanation o f the control o f the mineralization. There is no clear paleo-channel control; there is no clear redox-front control, in spite o f the fact that some geologists are in favour o f this idea. There is no clear relation to lithological changes, because porosity and permeability seem to be similar elsewhere. The only objective indication is the apparent control by some iron-oxides. D.A. PORTER: What is the age of the mineralization? P. STIPANICÍC: I am not sure, but from geological considerations I believe it could be Tertiary. During Tertiary times there were large peneplanized surfaces o f acidic igneous rocks. Climatic conditions from Tertiary times until now did not change much (except during the Quaternary) and we have proved in some uranium districts that in very recent times a large amount o f uranium was leached from tuff during a very short period of time. In the Atuel Canyon, a few km IAEA-AG 250/6 57 south-west o f Sierra Pintada, a Permiart-Triassic tu ff shows very high radioactive value, which in some cases reached up to 3000 and 4000 cps (SPP2), but corresponding chemical analyses only indicated between 30 and 50 ppm U. Dr. Nicolli carried out a very interesting study on the subject (using the Mössbauer effect) and he proved that practically all the uranium contained in the tu ff was leached in the last million years, or, more possibly, in the past half million years. For all these reasons, I believe that the uranium mineralization in the Sierra Pintada district could be Tertiary. M. MATOL1N: What kind o f emanometric prospection was used? What type o f sampling and depth o f sampling? P. STIPANICIC: For the particular case o f Sierra Pintada, emanometry by sniffers was used, and the air samples were normally taken between 0.50 m and less than 1 m. According to my information, trace-tracks (or similar methods) were not very successful in some testing areas. IAEA AG 250/3 THE WESTMORELAND URANIUM DEPOSIT, QUEENSLAND, AUSTRALIA H.D. FUCHS Urangesellschaft mbH, Frankfurt/Main, Federal Republic of Germany W.E. SCH tND LM AYR Urangesellschaft Australia Pty. Ltd., Melbourne, Australia Abstract THE WESTMORELAND URANIUM DEPOSIT, QUEENSLAND, AUSTRALIA. Twenty-three years have passed since the initial discovery of uranium mineralization at the Westmoreland Locality in NW Queensland, Australia, in 1956, and the exploration history of the deposit, given in this paper, is a colourful one. Several exploration groups worked the areas at various intervals dictated by the initial discovery efforts and their revival in the late 1960s; financiai difficuities; resumption of work by farm-out; disappointment on reassessment of data; further farm-outs; and, finally, resumption of systematic work that located further mineralization and now takes the prospect towards potential feasibility. 1. GEOLOGICAL SUMMARY The Westmoreland Project is situated in NW Queensland, Australia (Fig. 1 ), and lies in an area o f sediments and volcanics at the south-western margin o f the McArthur Basin and the northern side o f the Murphy Tectonic Ridge, which separates the McArthur Basin and Nicholson Basin. Lithological units outcropping in the area are listed in Table I in order o f stratigraphy (and see Fig.2). Basic dykes follow main structural features and have been recognized to penetrate rocks of the 'Nicholson Granite Complex', Cliffdale Volcanics, Westmore land Conglomerate and the Seigal Volcanics. In most cases, dykes are highly altered and extremely variable, frequently showing evidence of brecciation and shearing. In the Westmoreland Conglomerates sills have been recognized. The rock units are folded over the Murphy Tectonic Ridge, the folding being gentle, rarely exceeding 25°, with open concentric folds plunging slightly east wards. There have been at least three periods o f faulting, leaving two prominent sets o f fault directions: an older NE trending fault system which is slightly offset to the NW by a younger NW trending system. 59 60 FUCHS and SCHINDLMAYR FYC.7. ¿ocaf;'cn о/ гЛе ¿Л-дя/мм Fro/ecf. TABLE I. STRATIGRAPHY Cainozoic Ailuvium Cretaceous Porcellerite, siltstone, sandstone, conglomerate Upper Peters Creek Volcanics Acid to intermediate lavas, basic dykes, Proterozoic? tuffs and minor sediments Lower Seigal V olcanics Amygdaloidal basalts and interbedded Proterozoic sediments Westmoreland Sandstone, conglomerate, siltstone Conglomerate and thin mudstone/shale Nicholson Medium to coarse grained muscovite- Granite biotite granite Cliffdale Acid and intermediate lavas and tuffs Volcanics Archean? Murphy Metamorphics Muscovite-biotite schists and migmatite ÏAEA-AG-250/3 61 FYG. 2. ^eo/ogi'caZ iecfi'on. 2. EXPLORATION HISTORY 2.1. Phase 1,1956—1959: Discovery and first investigation by Mt. Isa Mines (M IM ) In 1956 a group o f base-metal explorers, operated by MIM, selected a large area o f NW Queensland and the adjacent Northern Territory for exploration in search of: Volvanogenic Cu mineralization (Type Redbank); Stratabound Pb-Zn mineralization (McArthur River Type); Syngenetic U mineralization (Blind River Type); and Iron ore 62 FUCHS and SCHINDLMAYR F/C.J. A'^pZyK'cfgec/ogica/ map. 7 = Де^ Free FaM/? Zone // = CbyjfJa/e FaM/r Zof¡e 7 = /асДг, Caree a/!cf ¿a7¡g¡ /,елм^ 2 = /MMHagMnna J = Зме and ОмГса?ир. Field work by MIM commenced in August 1956 and consisted mainly o f regional geological and geochemical traverses. While MIM field work was continuing, the Bureau de recherches géologiques et minières (B M R ) in November 1956 con cluded a systematic airborne radiometric/magnetometric survey over parts o f the area. Flight data were passed to MIM field teams daily. The BMR survey located significant anomalies on outcrops o f Westmoreland Conglomerate, and two days later the anomalies were confirmed on the ground by MIM teams. The anomalies were caused by secondary U minerals in a sand stone o f the Westmoreland Conglomerate. Owing to the approaching rainy season, no further work was done that year. IAEA AG-250/3 63 Work resumed in May 1957, and mineralization in outcrop was found to occur at various levels within the sandstone as discontinuous lenses with generally low grades. In addition, mineralization was also discovered in an erosional gully, following the Red Tree Fault Zone, a regional fault structure (Fig.3). In 1958, eleven percussion holes were drilled to test subsurface extensions o f surface mineralization followed by two diamond drill holes in early 1959. With few exceptions, the holes located only thin and discontinuous lenses of low- grade mineralization at shallower depth well above the water table. At this stage, MIM concluded that the mineralization was of little economic significance and work ceased. However, three Mining Leases were pegged and applied for. In retrospect, MIM had located most o f the radiometric anomalies along the Red Tree Gully that were later to spark o ff a major drilling exercise and lead to the discovery o f high-grade vertical mineralization, but had failed to test any o f these in detail. 2.2. Phase 2, 1959-1967 From 1959 to 1967 the area lay vacant and untouched by any further systematic uranium exploration, following the worldwide lack o f interest in uranium. 2.3. Phase 3, 1967—1972: Queensland Mines In 1967, a company called Australian Oil Exploration, which was shortly afterwards renamed Queensland Mines Ltd (QM ), in its search for Blind-River-type uranium mineralization, applied for an area o f 700 km^ in far NW Queensland, on the basis o f BMR publications containing a number o f radiometric anomalies in a conglomerate unit that could be indicative o f uranium mineralization o f some significance. Field work commenced in September 1967 from fly camps and took the form o f geological and radiometric reconnaissance traverses on foot and horseback to check on the ground the various radiometric anomalies reported by the BMR. Early in 1968, a significant radiometric anomaly (M oogooma Prospect) was located on Westmoreland Conglomerate just north o f a major fault escarpment. This was subsequently investigated by radiometric gridding, mapping and, finally, core drilling, the light-weight core rig being transported on horseback. A small lens o f rich secondary mineralization was outlined in the vicinity o f an erosional gully (Red Tree Fault) striking north-east. In August 1968 QM contracted a private company to carry out an airborne radiometric and magnetometric survey. The survey, flown with more sophisticated equipment than BMR had available twelve years earlier, confirmed most o f the BMR anomalies and located others. Field check commenced in the last quarter o f 1968 and continued throughout the first half o f 1969. In general, this field work 64 FUCHS and SCHINDLMAYR TABLE H. RESERVES IN 1969 tUiOs Red Tree Zone 7830 at 0 .2 % UgOs Jack Lens 1350 at 0.15% U 3 OS Total 9180 o f 1968 and early 1969, together with the interpretation o f the airborne geo physics, resulted in the definition o f various prospective areas (Red Tree Fault Zone, Moogooma Prospect, Tjuambi Prospect, Vinemaree Prospect, Contact Prospect, Long Pocket Prospect). A first assessment o f these areas revealed that U minerali zation frequently occurred in or near joint zones or erosional gullies trending north east within the Westmoreland Conglomerate. On the strength o f the 1968 assessment o f the Red Tree Fault Zone and in view of the mineralization obvious from surface outcrop and drilling by MIM, QM decided to embark in 1969 on a major drilling programme, with the strata- bound sandstone mineralization as the main target. The programme o f percussion and diamond drilling began in May 1969. Initially and over most o f the period, three diamond core rigs and two percussion drill rigs were employed from contractors of Mt. Isa. Drilling apparently started on the sandstone to the east o f the MIM N o .l Lease, where it soon encountered substantial secondary mineralization at depth (the ore body was called Jack Lens), whilst only limited attention was given to the investigation o f the mineralization associated with the dyke. It was not until a drill-hole testing secondary mineralization near the edge of the Red Tree Gully unexpectedly penetrated dyke rock with high-grade mineralization associated with it that the emphasis o f the drilling programme swung away from testing low- gra TABLE IH. RESERVES AT END OF DRILLING PROGRAMME t U^Og Red Tree Fault Zone 8970 at 0.22% U^Og Garee Lens 2760 at 0.13%U3Ûg Jack Lens 2580 at 0.14% U^Og Langi Lens 190 at 0.12% U^Og Totai 14500 at 0.18% U3OS (cut-off 0.05% UgOg) Whilst the percussion drilling to outline the Jack Lens and the nearby Langi and Garee Lenses o f horizontal mineralization was based on a statistical drilling pattern of about 33 m, which is sufficient for a first estimate of contained resources, the drilling along the Red Tree Fault Zone over a strike length o f more than 6.5 km was somewhat less systematic. On the basis o f only a limited number o f multiple-hole sections and the prevailing single-hole sections at irregular spacing, the geometry o f the vertical lenses o f high-grade primary mineralization was, in the initial assessment, not recognized as being rather discontinuous. The reserves (all categories) calculated are shown in Table III. Concurrently with the drilling programme, some exploratory mining was carried out in the Red Tree Fault Zone, in the Jack Lens area and the Tjuambi Project, with a total length of openings of 280 m. These openings not only pro vided further information on the nature and distribution o f the mineralization and on the host and country rocks but also provided material for subsequent mineralogical and petrographical examinations as well as for first bench-scale metallurgical test work. These first treatment tests indicated that the ore was amenable to standard acid leaching with good recoveries. When, in early 1970, the exploration concept became heavily biased towards the discovery and outlining of primary U mineralization associated with NE-trending fault zones, an intensive programme was initiated for a very detailed radiometric gridding with detailed geological mapping along all such zones as the Red Tree and Long Pocket Areas. In mid-1971, the Nabarlek uranium deposit was discovered and, under standably, QM henceforward concentrated all its efforts on this more rewarding prospect. Field work at Westmoreland was therefore suspended in March 1972 owing to lack o f funds and doubts relating to stated ore reserves at that time. 66 FUCHS and SCHÍNDLMAYR 2.4. Phase 4, 1967—1971: Additional MIM investigations on their three mining leases In 1967, stimuiated by the fact that QM's activities revived interest in the Westmoreland area, MIM carried out a heiicopterborne scintillometer survey over the three Red Tree uranium leases. Following QM's successful drilling in mid-1969, MIM also embarked upon a diamond-drilling programme to investigate further the extent of the secondary mineralization within the N o .l Lease in the light o f QM's results on the Jack Lens. Their intention was also to test their No.2 and No.3 leases for vertical mineralization as encountered by QM between the two leases. Fifteen diamond holes were drilled, to a total o f 750 m, during September through December 1969, followed by 42 diamond holes between September and November 1970 for a total o f 1710 m. A review o f the resource calculation by MIM's partner C R A in 1973 stated that the maximum possible resource of secondary U mineralization, indicated by drilling today and applying no cut-off, was 1630 t U 3OS at 0.115% U^Og. C R A concluded that, in Leases No.2 and No.3, there was insufficient evidence o f continuity of mineralization to allow a calculation or even an estimate of resource tonnage. 2.5. Phase 5, 1972—1975: Queensland Mines Some reassessment o f data was made during 1973, especially with respect to the resources calculations. Reserves and resources were substantially down graded (see Table IV).QM stressed in a review report o f early 1974 that the resources o f the Red Tree Dyke are strictly potential only, as insufficient data was available for a reliable calculation o f reserves. The report concluded that, owing to insufficient proven reserves and the remoteness o f the prospect area at the time, the deposit would not be a viable operation, although a comprehensive prefeasibility study had not been undertaken. The review report recommended that QM retain all the current tenements in the prospect area and seek a joint venture partner to undertake further exploration on the tenements on behalf of QM. During the period 1972 to 1974, owing to the abandonment o f the field camp, the collapse o f the company, and change o f personnel, much o f the original field records were unfortunately lost. During 1974 and early 1975, QM approached a large number of exploration companies with an offer to participate in further exploration of the leases. However, conditions were prohibitive. It was only due to the financial misery o f QM in early 1975, to the danger o f having to forfeit the area for not meeting expenditure commitments and to the negotiating skills o f the Management o f the Australian Company Mines IAEA-AG-250/3 67 TABLE IV. RESERVES AND RESOURCES IN 1973 Ore body 1971 resources (all) 1973 resources (all) (t ЧзОз) (t ИзОз) Red Tree Joint Zone 8970 at 0.22% U^Og 2460 at 0.60% U3OS Jack Lens 2380 at0.14%U30g 1980 at 0.23% Garee Lens 2760 at 0.13%из0з 2520 at 0.25% U3OS Langi Lens 190 at 0.12% HOat 0.12% U3OS Total 14 500 7070 Administration Pty. Ltd., that a joint venture with more favourable conditions was finalized, enabling field work to be resumed after a three-year break. 2.6. Phase 6 , 1975: Mines Administration (Minad) Early in 1975, Minad had successfully negotiated a joint venture with QM, whereby they could, in successive steps, earn a 60% interest in the titles. In return, they had to pay QM a certain amount o f cash at signature and they were obliged to expend an additional amount on further exploration of the area within the next four years. Minad hoped, first, to confirm by diamond-drilling the Jack Lens the type and distribution o f ore for a re-assessment o f resources, and, second, to establish the continuity o f vertical-type mineralization as a means to upgrade the potential ore o f the Red Tree Dyke. In 1975 Minad drilled three diamond holes in the Jack Lens which confirmed the grades and thickness o f ore intersections o f QM's percussion holes in the upper levels but left some doubt concerning deeper levels o f mineralization. The reserves o f the Jack Lens were re-evaluated on the data o f that drilling, and similar adjustments were made to the reserves o f the Garee Lens and Langi Lens (see Table V ). Minad also drilled eight inclined diamond holes in the Huarabagoo and Huarabagoo East areas along the Red Tree Fault Zone in an effort to test the lateral and downdip extension, but it largely failed to do so. Minad therefore con sidered the potential for substantial ore reserves along the Red Tree Fault Zone to be severely downgraded. They concluded at the time that the prospect was not economically viable, that a substantial review o f all data would be required and that the possibility of other horizontal-type mineralization, as indicated at Huarabagoo and in untested areas under alluvial cover, should be explored if work was to continue. 68 FUCHS and SCHINDLMAYR TABLE V. SUCCESSIVE RE-EVALUATIONS ON BASIS OF DRILLING DATA Ore body 1971 1973 1975 (t U3OS) (t U30g) (t U3OS) Jack Lens 2580 at 0.14% UgOg 1980 at 0.23% U 3O8 1405 at 0.16% UgOg Garee Lens 2760 at 0.13% U3O8 2520 at 0.25% U3OS 1500 at 0.18% UsOg Langi Lens 190 at 0 . 1 2 % U3OS 1 10 at 0 . 1 2 % UgOs 1 1 0 at 0 .] 2 % U3OS Totat 5530 4610 3015 However, Minad considered the risk too high to proceed atone and approached severa! companies in !ate December 1975 to farm out haif o f their interest. Agreement in principie was reached with UrangeseMschaft Australia (U G A ) on 28 December 1975. 2.7. Phase 7, 1976 to present: QM, Minad, UGA Joint Venture Operated by Minad, a field programme for 1976. was designed and field work began in June o f that year. For the purpose o f the search for Jack-Lens-type ore bodies, it was assumed that the sandstone should be drilled 30 m away from the Red Tree Fault Zone. Initially 14 holes, totalling 1613 m, were drilled. One hole penetrated accumulated mineralized intersections o f 2 m at 0.10% UgOg at shallow depth in the Westmoreland Conglomerate. A second-phase drilling programme o f 35 1 additional metres investigated the newly found mineralization in four further coreholes. AH of them contained some mineralized intersections of potential economic interest, giving rise to hope o f horizontal-type mineralization of some size. For the first time, a down-hole logging o f the core holes was undertaken, but correlation o f assays and chemical assays showed considerable discrepancies due to radiometric disequilibrium conditions ranging from 0.5 to 1.9. Having earned a 15% interest in the Joint Venture by October 1976, Minad decided not to contribute to the expenditure o f the subsequent stage o f earning a further 30% of QM. UGA alone proceeded and took over operatorship of the project in November 1976. Whilst geochemistry and radiometric surveys added little o f significance to the potential o f the area, substantial and extensive radon anomalies became apparent in the Junnagunna area. Later drilling confirmed that the anomalies could be correlated to subsurface mineralization. !AEA-AG-250/3 69 FoHow-up drilling o f the mineralization at Junnagunna successfully proved the presence o f substantia! horizontal mineraiization at shallow depth with grades locally exceeding 1% L^Og. The mineralized area remained essentially open in most directions. The potential resources were tentatively estimated at about 540 t U^Og at 0.19% ^O g. The relationship of the mineralization to the Red Tree Fault Zone remained open, as the drilling failed to locate the dyke. In addition, some more holes were drilled at Garee Lens to improve understanding o f the mineralization. Several other holes were drilled to investigate continuation of Red Tree Zone; they were barren. In summary, exploration in 1977 proved the existence o f further areas o f significant horizontal-type mineralization at shallow depth and added substantially to the potential resources o f the whole prospect area. In 1978 the main aim was to increase the potential o f the Junnagunna Lens. A t Junnagunna, 15 o f the 29 holes intersected mineralization o f potentially economic thickness and grade, inferring a horizontal-type ore body o f significant size. The estimate o f contained potential resources lies in the order o f 3600 t U 3O8 at 0.15%. The zone o f mineralization now measures about 600 X 800 m and remains largely open to the ENE and to the north. Compared to the 1977 geological resource figure, a net gain o f 3600 t potential ore has been achieved, which, although probably inflated by the low cut-off grades and the lack o f consideration o f minimum thickness or overburden ratios, is rather encouraging. In 1977/78 the Junnagunna, Sue, Outcamp and General Long Pocket areas were investigated (Figs 4 and 5). Eighty-one drill-holes indicate the following potential resources (at a cut-off o f 300 ppm): Junnagunna: 3600 t UgOg at 0.15% U3Ûg Junnagunna South: 270 t U30g at 0.055% UsOg Sue: 675 t U30g at 0.11% U 3O8 Outcamp: 945 t U 3OS at 0.085% U 30g Together with the recalculated resources o f QM (Jack, Garee and Langi Lenses) at a cut-off o f 0.08% UgOg, the total resources are in the range o f 10 800 t ^ O g with an average grade o f 0.18% U^Og. First prefeasibility considerations during 1978 indicated that geological resources of approximately 13 500 t would be required to establish a viable mining operation. During 1978 the interest-earning period o f the Joint Venture expired and equities are now distributed as follows subject to adjustments on investment decision: Queensland Mines 40% Urangesellschaft Australia (operator) 37.5% Minad 22.5% 70 FUCHS and SCHîNDLMAYR ^7C.4. DriM-йо/е /осадой af Умялаумяпа. F7C.3. ^ecfiOT! af Уияла^мяна a^ /o?' F¡'g..?J. and, from September ¡978 onwards, all parties were obliged again to contribute. The 1979 programme was geared to finding additional potential resources by outlining and defining the Junnagunna ore body and by testing radon anomalies for further areas of mineralization. Some 3200 m o f diamond drilling were completed by late September 1979. Although results are not yet fully to hand, a further increase in the potential o f the Junnagunna area is apparent. U G A as operator is confident that the additional geological resources required to establish a viable mining operation will be located. IAEA-AG-250/3 71 DISCUSSION P.' BARRETTO: This is a very interesting example of a case history and of how the idea o f the mineralization changed and reserves were redefined. Was there any special reason for not using logging techniques at the early stage o f the exploration? H.D. FUCHS: It is not really up to me to say this because a different company did it, but I think it was just a mistake. We have seen now that the disequilibrium is tremendous; there was a variation o f between 0.6 and 1.9, and therefore just by sampling and by the relatively superficial work that was done, they came up with these absolutely wrong figures. Because we don't know the equilibrium situation we now even have difficulties in using logging. It changes from place to place and therefore we do diamond drilling to get cores which will produce exact chemical results. Naturally, in this area it is no problem to do core drilling as it is not deep. P. BARR ETTO : From your answer I presume that the secular radiometric situation was not known at the time. H.D. FUCHS: Not so far as I can recollect. P. BARRETTO: Was any other technique for detecting hidden mineralization used? H.D. FUCHS: Since we saw that radon survey worked quite well, we did not try any other method. C. TEDESCO: Did you try Track Etch over a Basalt Cover? H.D. FUCHS: No. The deposit is just on the rim o f the basalt cover, but it is not covered by volcanics. C. TEDESCO: Can you say that Track Etch worked where another radon detection method did not? H.D. FUCHS: No. We only tried Track Etch. D.A. PO RTER: In some sedimentary deposits where you have this dis equilibrium, the negative will be an oxidized ground and the positive will be a reduced ground. Are there any clues like that for the disequilibrium? H.D. FUCHS: We have not enough information yet. We notice that there is mostly oxidized mineralization in the oxidized area because we are so close to surface. We also have reduced pockets, but our drill pattern is still too far away for us to be able to really correlate this. We have not done it in detail. D.A. PORTER: If you are right, the positive disequilibrium might not occur down the hole. Is there no vertical correlation? H.D. FUCHS: We have not yet done this correlation. D. T A Y L O R : Is the radon anomaly associated directly with the shape o f the drill body or with the faults that occur close to the ore body? H.D. FUCHS: The shapes o f the ore bodies are schematic. In some cases high anomalies did not have any uranium underneath. The whole surrounding area 72 FUCHS and SCHINDLMAYR was anomalous, so we did not reaHy took for peaks in the radon anomaiy ; we just iooked for regionai anomalies which are quite high (3 to 4 times background areas) and we found mineralization below, but usually the higher-grade material was not below the peaks o f the radon anomalies. We were not able to find the continuation o f the fault; it is only schematic (actually we have several other cross-cutting faults) and the fault more or less disappeared. Only this year we drilled a few inclined holes to intersect the fault somewhere. So we could not follow the zone with radon. B. GUSTAFSSON: Were radon measurements limited to the dyke area? H.D. FUCHS: No, we measured the whole area with radon. B; GUSTAFSSON: What was the spacing o f the measuring grid? H.D. FUCHS: The grid was about 50 feet in the anomalous areas. M. MATOLIN: You mentioned that the coefficient of radioactive equilibrium varied very extensively. The figures you gave prove it. Do you think that in this situation the gamma log, or probably the core analysis, you used for deposit evaluation is the correct one, or do you intend to use in the future other techniques for deposit evaluation? H.D. FUCHS: We would prefer not to do core drilling any more and would like to find a more effective and cheaper method. There are several types o f equipment on the market but they are very difficult to get in this area, because we are not able to do down-hole logging to measure uranium only and not the decay products in order to avoid the disequilibrium problems. I don't think there is even yet a proper method in the USA. So we still have to do core drilling to get proper assays. If somebody knows a method o f doing this, it would be very helpful. M. M A TO LIN : There are investigations on these new methods in the USA and in other countries, but there are probably no available units at present. Some firms in the USA offer these units, but I am not sure about the performance o f this equipment. F. SCOTT. Proto-actinium had a good correlation, but it was cheaper to do core drilling than to use this method. H.D. FUCHS. I know there is a californium unit too, but this is too complex so I think we should be old-fashioned and still do core drilling. D.A. PO RTER: Where do surface radon anomalies stand in relation to the mineralization? H.D. FUCHS: With quite a few exceptions, the following general statement can be made: little lateral displacement o f the soil radon anomaly was observed in relation to the underlying mineralized sandstone, in spite o f approximately 15 m thickness o f cover rocks, including at least 10 m o f basic volcanics (Seigal volcanics). D.A. PO RTER: How many times background were the radon anomalies? H.D. FUCHS: The anomalies were about two to four times background with peaks of up to 100 times background. !AEA-AG-250/3 73 С. TEDESCO: Could you give us some details about (a) the characteristics o f the stratiform deposits in Proterozoic continental sediments, and (b ) the type o f geochemical prospecting that was effective and the environment in which it was applied. H.D. FUCHS: (a) We have not yet found any sedimentological or geochemical reason for the concentration o f uranium in the uppermost part o f the Westmoreland Conglomerate. There are still two opposite views about the genesis of the ore: (i) the uranium precipitation is closely connected with the basic dykes, sills and Seigal volcanics; and (ii) the uranium precipitation is a purely syngenetic or epigenetic process within the sediments, without any magmatic influence. (b ) The radon method (alpha-trace) was by far the most effective. IAEA-AG-2S0/I3 METHODOLOGY AND HISTORY OF DISCOVERY OF URANIUM MINERALIZATION IN NORTH-WESTERN PROVINCE OF ZAMBIA L. M ENEGHEL AG IP SpA Zambia Branch, Lusaka, Zambia Fre.s'CMfgi/ C. fapadi'a Abstract METHODOLOGY AND HISTORY OF DISCOVERY OF URANIUM MINERALIZATION IN NORTH-WESTERN PROVINCE OF ZAMBIA. Substantially negative results were obtained by an airborne spectrometer survey over an area of approximately 60 0 0 0 km^ where geological mapping had revealed the presence of probable Roan age metasediments, the host rock of the famous Shinkotobwe uranium deposit. In spite of the negative results Agip SpA decided to continue exploration on a purely theoretical geological basis. The geological history of the Katanga Basin was reviewed, corre lations established and evidence of a sedimentary origin of the known uranium occurrences, previously believed to be magmatic, collected. The Lower Roan Formation, which is the lowest part of the Katanga Sequence, was identified as the host rock of original mineralization. Photogeological interpretation revealed a band of Lower Roan some hundred metres thick dipping at approximately 30° and encompassing four large windows of basement. A long and hard campaign of detailed prospection over the formation on a 1 0 0 -m grid revealed fifteen uranium occurrences and several tens of anomalous areas. The subsequent drilling campaign intersected scattered mineralization, locally thick and of good grade, in vein-like type deposits. A uranium province has been brought to light but its economic significance is still to be established. A drilling campaign to evaluate the uranium reserves is planned. 1. FIRST PHASE 1.1. Preliminary selection o f area Geological mapping o f the North-Western Province of Zambia was carried out over a period o f years by various mining companies and by the Geological Survey Department o f Zambia. The presence in the region of metasediments of probable Roan age was established. This formation is well known in Shaba (previously the Katanga Province o f the Belgian Congo) and in Zambia to be the host rock for various mineralizations, such as copper and uranium (Shinkolobwe, Swambo, Kalongwe, Mindola and elsewhere). 75 76 MENEGHEL On the basis o f this information Agip in 1970 selected an area o f 60 000 km^ for detailed studies. Much o f the geological data concerning the region was not published, but it was available at the Geological Survey Department in Zambia on open files. I.2. Exploration methods The size o f the area, the difficulty o f access and the large distribution o f the outcrops suggested the use o f an airborne spectrometric survey for preliminary investigation. A total o f 100 666 line km were flown with lines spaced at 800 m, orientated perpendicular to the general geological strike. An Aero Commander and a Britten-Norman Islander equipped with magneto meters and spectrometers with Nal(Tl) detectors o f two 8 in. X 4 in. and one II.5 in. X 4 in. crystal volume were used. The airborne spectrometric survey was carried out by a contractor. A total o f about 1000 radiometric anomalous areas were selected, 650 o f which were classified as due to thorium, 250 to thorium and uranium, and 100 to uranium alone. O f the 1000 anomalies, 750 (o f which ten were uranium) were over magnetic rocks or schists and gneisses o f the Basement Complex (a term used to describe rocks of Pre-Katanga age) and 250 (including 90 due to uranium) were over sedimentary and metasedimentary rocks o f Katanga age. AM the uranium anomalies and most o f the others were checked by teams consisting o f a geologist, an assistant and a compass-man transported by a Bell 47/G 38—1 or a Bell 206/A Jet Ranger helicopter. A total o f 1300 helicopter hours were flown. The anomalous areas were classified in groups according to their probable cause. None of them was considered sufficiently interesting from a theoretical point o f view to justify deeper exploration. 2. SECOND PHASE 2.1. Research programme In 1972, a new exploration philosophy was adopted which eventually proved successful in the discovery o f some mineralization. A report o f this new philosophy and its application forms the main part o f this paper. New discoveries o f uranium in the Northern Territories o f Australia and re-evaluations o f the genesis o f Shinkolobwe and other uranium occurrences in the Roan Group played an important part in the development of the new approach to exploration. The new programme included a study, within the Katanga Basin, of: (a) The conditions o f deposition o f the Roan Group, and stratigraphie correlation between Shaba, the Copperbelt and North-Western Province; and IAEA-AG-250/Ï3 77 (b ) The conditions o f deposition o f uranium in marine environments and a study o f the origins o f uranium mineraiization in the Roan Group previousiy thought to be magmatic. It was not possible to establish firm stratigraphie correlation between the three areas, but it was possible to establish a satisfactory depositional sequence o f sediments o f the Katanga Basin, particularly the Roan Group, which represents the base o f the sequence. The deposition o f the Katanga Sequence, o f mainly marine origin, began between 840 and 1300 million years ago in a wide, complex basin extending from Shaba Province in Zaïre through a large part o f Zambia, notably the Copperbelt area and the North-Western Province area (the Domes area). The south-western boundaries o f the sequence are now masked by more recent formations (Fig. 1). The entire sequence is several thousands o f metres thick, and the basal parts contain the copper deposits o f the Copperbelt o f Zambia and of Shaba Province. The sequence is subdivided from bottom to top into the Roan Group, Mwashya Group and the Kundelungu Series (Table 1 ). The lower parts o f the sequence were deposited in basins o f limited extent, so that there are many local facies changes. The sediments deposited later were more uniform in composition and of greater lateral extent, and are therefore more readily correctable. The deposition o f the Roan Group took place in a shallow- water, near-shore, marine environment. The basal sediments o f the Group were deposited in an oxidizing environment during a major transgression, while the other sediments containing the mineralization were deposited in euxinic conditions, accompanied by local volcanic activity. Finally, the basin was filled with the rest o f the sequence under a number o f environmental conditions. The sediments were subsequently metamorphosed and tectonized several times; in Shaba the Roan Group was thrust over higher members o f the sequence and faulted in separate masses enclosed in breccia. The mineralization has been strongly influenced by tectonic, volcanic and thermal events and alteration phenomena. The second point o f the new programme was strongly influenced by findings in the Northern Territory o f Australia. Taking into account the deposition conditions o f uranium in marine environments, we oriented the prospecting towards fine-grained, shallow-water, near-shore, marine sediments o f the first transgression o f the Katanga sequence where organic life and reducing conditions existed. Evidence o f a sedimentary origin o f the known uranium occurrences in the Lower Roan was difficult to collect. It is, however, possible to prove that the known mineralization is a reworked mineralization of sedimentary origin. Evidence supporting such a conclusion was collected during the entire second phase of research, and is summarized below. TABLE I. CORRELATION OF KATANGA SYSTEM OF DOMES, COPPERBELT AND SHABA AREAS Stratigraphie Kabompo Dome Mwombezi Dome Solwezi Dome Copperbelt Area Shaba Area unit (McGregor [3]) (Mendelsohn [4] (François [6 ] and Binde [5]) and Cailteux [7]) Kundelungu Mainly argillaceous Limestone Dolomite Shale Sandstone and shale Series limestone (tillite) (tillite) Tillite Tillite Shale Sandstone and shale Dolomite and shale Dolomite Tillite Tillite Mwanshya Carbonaceous shale Carbonaceous shale Carbonaceous shale Carbonaceous shale Black shale (R. 42) Group Dolomite schists schists Argillite Upper Roan Magnesite Dolomite Dolomite Dolomite Dolomite (R. 41) Cu Subgroup Dolomitic limestone Dolomitic schists Cu Dolomitic schists Cu (R.u.l) Cu Lower Roan Soapstone and Cu T alc-specularitic Talc-specularitic Argillite, shale Dolomitic sandstone Subgroup breccia U quartzite quartzite Dolomitic schists Dolomite Specularitic quartzite Quartzite (marker) Quartzite (marker) (R.u.2)(R.1.3) Shale (R.3? ) (R..23) (R.1.4) Quartzite (marker) Mica schists Cu Mica schists Cu Siliceous dolomite Quartzite (R.1.5) Sericitic quartzite Cu Biotite schists U Biotite schists U (R.21)(R.22) Cu Biotite schists U Local conglomerate Local conglomerate Argillite Cu Dolomitic sandstone U Local conglomerate Micaceous quartzite (R.l) Impure dolomite ? (R.1.6) Quartzite Conglomerate (R.1.7) Basement Granites, schists, Granites, schists, Granites, schists, Granites, schists, ? Complex gneisses, etc. gneisses, etc. gneisses, etc. gneisses, etc. IAEA AG-250/13 79 f/G .A Di'yin'bMfi'on o/fAe Æafanga ^a/fer FrcnyoM [2p. TABLE H. DISTRIBUTION OF COPPER DEPOSITS AND URANIUM OCCURRENCES IN THE KATANGA SYSTEM Shaba ^ [6 j Copperbelt s-b D om es area T o ta l M etres N o . o f C u N o . o f U Stratigraphie N o. o f Cu N o. o f U StratisrapMc N o . o f Cu N o . o f U N o . o f Cu N o . o f U g ra p h ic Ks 3 K s 22 Ks 21 Ks 132 K.u. Tillite K s 13 ! Ks 122 Ks 121 Ks 11 K .i. 14 K.I. Shale K.I. Shale K .i. 13 - 8 0 0 0 K .i. 122 2' K.I. Lime- K .i. 12! K .I. 11 K.I. Tillite K.l. TiHite R. 42 M w anshya M w an sh ya 1? !? R . 4 ! 2 2 (R .u . 1) " . a n R . 3 ? ,R .2 3 R.u. 2 R.I. 4, R.I. 3 R. 21, R.22 64 22 R.I. 6, R.I. 5 17 5 3 15 84 42 4d 100 R . i ? R.Í. 7 *? Basem ent Basem ent 1? 1? T o ta l 66 22 17 5 3 15 86 42 IAEA-AG 250/13 81 It is considered that the mineralization took place during a single métallo génie phase which was associated with the marine transgression that occurred at the base o f the Katanga sequence. That phase began with the deposition o f the Roan sediments and ended with the start o f the Lufilian Orogeny, although minor mineralizations subsequently took place. The uraniferous occurrences and the main radiometric anomalies o f the Shaba, Copperbelt and Domes areas appear to be limited to the same geological formation in the footwall of copper-bearing horizons. The known deposits occur in a thickness o f sediments o f 100 to 150 m (30 to 40 m in the Shaba area) at the base o f a sequence o f sediments several thousand metres thick (Table II). In the Shaba area, all the black uranium oxides and more than 70% o f the uranium occurrences are localized at the base o f the zone o f copper mineraliza tion. The secondary uranium mineralization is found at a distance from the base, along fault zones and other fractures [8] . The presence o f bioherms in the vicinity o f the mineralized zones indicates that the conditions o f deposition in the Domes, Copperbelt and Shaba areas were similar. It seems likely that the bioherms provided the humic acid required for the precipitation o f copper, cobalt and uranium in shallow waters and in a zone where the position o f the shore line varied during periods o f marine transgression and regression. On the Copperbelt, at Lumwana in the Domes area [3] and in the Shaba area, it is shown that from the footwall upwards the sulphide minerals occur in zones represented by chalcocite, bornite, chalcopyrite and pyrite. In the Shaba area particularly, uranium and molybdenum tend to occur at the footwall o f the copper mineralization. The mineral zoning appears to be related tö the changing chemical and physical conditions within the zone o f deposition as the depth o f water increased or decreased with the advance or retreat o f the shore line. In the Copperbelt and Domes areas, there appear to have been several cycles o f transgression and regression, with the result that the uranium mineralization occurs at various horizons within the zone o f copper mineralization, and not exclusively at the base. An explanation o f this distribution is that the uranium was repeatedly precipitated and taken into solution as the conditions changed from reducing during transgression to oxidizing during regression. Regional mineral zoning in the Shaba area has been studied by Ngongo- Kashisha [9] , who shows that in the southern part o f the 300-km-long arc, uranium, copper, cobalt, nickel, monazite and magnesite are predominant. In the central part o f the arc, there is an increase in the amounts o f copper and cobalt and a decrease in the amounts o f uranium, nickel and magnesite, with monazite present only in traces. In the northern part, however, only copper and cobalt are found in traces. 82 MENEGHEL F7G.2. /sofopi'c о/мгая;'м?я оссм^еясм ;'я f/¡e А'аГая^а ^ ífe w ^o^¡/¡'e¿ a/fer Cc/¡en ef я/, [g ] / ÍAEA-AG-250/13 83 It is considered that the regional minerai zoning is reiated to the change in water depth, which caused a change in the deposition conditions during the period o f marine transgression at the base o f the Katanga sequence. In the Domes area, it is seen that the higher grades o f copper occurrence, such as Kimale, are almost free from uranium, and that copper is virtualiy absent where the uranium grade is high. A similar relationship is found in the Copperbelt, where good grades o f uranium occurrences are copper-free at Mindola. In the Shaba area, Ngongo-Kashisha [9] saw that the two mineral associations, uranium-cobalt-copper-nickel and copper-cobalt, have identical fluid inclusions, suggesting a single métallogénie phase. The pitchblende found in Shaba [10] and in the Domes area is free o f thorium; this is quite different from the thorium-uraninite assemblage found in rocks o f magmatic origin. Such characteristics point to a sedimentary origin and no other. The uranium mineralization of Shinkolobwe, Swambo and Kalongwe, however, have a vein form and have been considered by some as typical examples o f deposits of magmatic origin. A study o f the events and processes undergone by the mineralization during geological time explained the vein-like aspect and confirmed their sedimentary origin. Events and processes that affected the mineralization after its mainly syngene- tic original formation include metamorphism, tectonic events leading to the forma tion o f epigenetic deposits, supergene enrichment o f epigenetic deposits, and thermal events associated with post-tectonic metamorphism resulting in further redistribution o f uranium mineralization. In the Shaba area, the effects o f metamorphism were slight. In the Domes and Copperbelt areas, the rocks were recrystallized, and locally economic minerals were segregated into veins which are bordered by barren zones. The following ages o f different generations o f uranium mineralization were established: > 7 0 6 million years (m .y.) (Shinkotobwe); 670 ± 2 0 m.y. (Shinkolobwe, Swambo); 620 ± 10 m.y. (Shinkolobwe, Kalongwe, Luishya); 555 ± 10 m.y. (Kambove West); 520 ± 20 m.y. (Nkana, Kansanshi, Musoshi, Kamoto, Kawanga, Kimale); 468 ± 15 m.y. (Nkana); 365 ± 40 m.y. (Luanshya); and 235 ± 30 m.y. (Nkana) (Fig.2). The uranium generations o f > 706 m.y., 670 ± 20 m.y. and 620 ± 20 m.y. are connected with the first, second and third phases o f the Lufilian Orogeny affecting the Katanga sequence. The 520 ± 20 m.y. generation is related to a thermal event associated with post-tectonic metamorphism [ 11] which affected the three areas, and had its greatest effect in the south-east. Other lesser thermal influences up to 235 m.y. ago caused further redistribution o f uranium mineralization. Supergene alteration affected the remobilized occurrences to a greater or less extent. In places, secondary uranium mineralization occurs in structures 84 MENEGHEL TABLE HI. PARAGENESIS OF URANIUM MINERALIZATION* Loss of main Age of mineralization Paragenesis syngenetic elements Ni Co Cu Fe U 670 m.y. (Swambo) Uraninite, pyrite, monazite, chlorite, vaesite, siegenite, chalcopyrite 670 and 620 m.y. Magnesite, uraninite, pyrite, (Shinkolobwe) molybdenite, monazite, vaesite, cattierite, siegenite, chalcopyrite 620 m.y. (Kalongwe) Uraninite, pyrite, chlorite, carrolite, bornite, chalcopyrite 582 m.y. (Kamoto P.) Uraninite, chlorite, carrolite, bornite, chalcopyrite 555 m.y. (Kambove) Uraninite, carrolite, chalcopyrite 555 m.y. (Dumbwa) Uraninite, pyrite, chalcopyrite 543 m.y. (Kimale) Uraninite, quartz, muscovite, apatite, pyrite, chalcopyrite, albite, carbonate, molybdenite 535 m.y. (Kawanga) Uraninite, chlorite, kyanite, mica, pyrite 520 m.y. (Nkana) Uraninite, melonite, bornite, chalcopyrite, digenite, calcite, chalcocite, quartz, biotite, albite, chlorite, molybdenite 520 m.y. (Kansanshi) Brannerite, chalcopyrite, calcite, pyrite, rutile, bornite, molybdenite 520 m.y. (Musoshi) Albite, uraninite, pyrite, carbonate, quartz, molybdenite 520 m.y. (Kamoto P, Uraninite, carbonate, pyrite Kawanga, Kimale) 468 m.y. (Nkana) Uraninite, carbonate, pyrite 365 m.y. (Luanshya) Pitchblende, calcite, dolomite 235 m.y. (Nkana) Pitchblende * Modified after Ref. [ 8 ]. IAEA AG 250/13 85 near the ground-water level, as at Shinkoiobwe in the Shaba area and at the Kawanga and Mitukuluku occurrences in the Domes area. Geochronology and paragenesis relation confirm remobilization processes affecting a decreasing number o f elements. A ll the main metallic elements, nickel, cobalt, copper, iron and uranium, are present at the 670 m.y. stage. A t 620 m.y., nickel has disappeared, and cobalt, copper and iron disappear respectively at the 520, 468 and 365 m.y. stages, leaving only uranium at the final 235 m.y. stage (Table III). Remobilization processes destroyed the original sedimentary mineralization, and vein-like occurrences were formed. (The deposits in the Alligator River Province o f the Northern Territory o f Australia seem to have had the same origin as the uranium mineralization o f the Katanga sequence and to have been affected by similar events.) Before these deposits were discovered, occurrences o f comparable grade had been found only at Shinkolobwe in the Shaba area. High-grade occurrences were also found in the Domes area but, unfortunately, they are erratic and discontinuous. As at Alligator River, carbonates and organic materials are common in the mineralized rocks o f Shaba and in the Copperbelt: the content o f carbonates is up to 10% o f the rock and that o f carbon up to 1.5%. Bioherms occur along almost the whole length of the mineralized arc. Observations are incomplete in the Domes area, but carbonates and organic substances are common above the marker quartzite horizon. Graphite occurs in the Dome areas at the Lumwana copper deposits [12]. Recently, probable bioherms have been discovered by the Geological Survey o f Zambia in the Lower Roan o f the Solwezi Dome area. These considerations allowed us to conclude that the uranium mineralization o f the Katanga basin is o f sedimentary origin and to establish the more interesting areas where exploration should be concentrated. In the Shaba area, the stratiform nature o f the uranium mineralization, closely associated with the southern facies o f cupriferous mineralization, could provide a target for exploration along 300 km of the mineralized arc. Although enriched outcropping uranium mineralization is unlikely to be found, low-grade occurrences on the footwall o f the copper zone could provide valuable byproducts. In the Copperbelt and Domes areas, where the uranium mineralization is not confined to the copper-bearing formation, it would be appropriate to elucidate the paleogeography and paleo-environment at the times the known uranium occurrences are presumed to have been formed. Although the geology o f the copper deposits has been thoroughly investigated, less attention has been given to the peculiar conditions o f uranium mineralization. It has been shown at Shinkolobwe and Mindola that rich concentrations o f copper and uranium do not occur in association, so the guides to the discovery o f copper are different from those required for locating uranium. 00 Ch MENEGHEL J. ¿/гдл/м??? /я гЛе Сгомр. !AEA AG 250/Í3 87 2.2. Exp!oration methods On the basis o f the above resuits, appropriate methods o f exploration were used. First o f all it was decided to carry out a photogeological interpretation of the area in order to outline the boundaries o f the Roan Group, which is readily identifiable on the existing 1 : 30 000-scale aerial photographs. This work revealed a strip a few hundred metres wide, dipping at about 30°, contouring the four large windows o f the Basement. This strip forms an area o f approximately 60 k n P . Such a strip was prospected on foot with a 100-m grid (locally 50 m or 25 m) by teams consisting o f a geologist or a prospector, a compass-man and two or three local labourers. The base line only was picketed every 50 m. The scintillometers used were SRAT SPP2-NF of the Saphimo Stel, remarkable for their ruggedness and sensitivity, even if a little too heavy for routine work. This campaign o f prospecting operations revealed some fifteen uranium occurrences and several tens o f anomalous areas (Fig.3). 2.3. DriHing, logging and sampMng The drilling targets were established according to the size and intensity o f the anomalies and the presence o f visible uranium minerals within the anomalous areas, it was verified later, during drilling operations, that surfaced anomalous areas are not always related to mineralization, while deep mineralization need not have any surface expression. Drilling operations were carried out using drilling machines that were available in the country and were easily transportable in that region. Wagon drill-holes were executed with Ingersol Rand Crawler machines, which are mobile and reasonably cheap. The minimum guaranteed depth was 60 m. Core drill was carried out mainly with Hoy Sullivan model 22 with Longyear wire-line equipment and double tube core barrels. Depending on requirements, core was recovered either from the entire hole or only from the promising horizon. Since the uranium appeared to be in equilibrium, a number o f holes were non cored, with cheaper and faster results. Drilling operations were carried out by a contractor. Every hole was logged with the Mount Sopris gamma-logger, a useful instrument, easy to handle and sufficiently accurate if used carefully with suitable corrections. The mineralized core was sampled at 25-cm intervals within the anomalous peak established by the scintillometer S R A T SPP2 (the mineralization is very irregular). The grade resulting from chemical analyses was compared with that 88 MENEGHEL ^ Y C .4. ÆaAompo Dome.' р/яно/йс^м, íAow/^f/iewe^M/a/'diJM'AMfí'ono/Mran/Mw mweraHzafíon. The пмумЬегу a&ove c¡rcM¡ar уесМтч corre^ponJ Ю f/wse o/ m feríecfeJ /eyeZí, wí'fA fAe умг/асе area proporfi'owZ fo ?йе prodMcf ^/*ade ^!we^ f/че/сяея. !AEA-AG-250/!3 89 * x Wagon drill shallower than U-bearing horizon * Barren hole ° Low radioactive hole О Low mineralized level ê Mineralized level 50 - Wagon drill; R - Rotary; D - Diamond MENEGHEL F7C .J. Й аЬо?про D om e. р/ап yecfí'on о / ¿r¡7/ /¡o/es. /пУ еяее?;оя о / а g o o ^ wM era/izaf/on ¡n fAe Mica jc/¡ Hf recepf¡ve /ormafion. /Vo/e f/¡e fe ^ wegM/ал ¿¡уУг;^мУ;о/! о/мгдл;м/я. IAEA AG 2 5 0 /! 3 9] calculated by radiometric logs. Significant differences were sometimes found within a single hole, but the average o f such differences is balanced. The differences are due mainly to the irregularity o f the mineralization, consisting sometimes of isolated grains o f pitchblende. 2.4. Diagnostic feature o f the mineraHzation The uranium mineralization discovered occurs in the Lower Roan Group, which was the target o f the exploration. The Lower Roan extends for several hundreds o f kilometres along the margins o f the domes, with widths o f a few hundreds o f metres. The main occurrences o f uranium mineralization are confined to a distinct horizon, below a quartzite marker horizon. The minerals are thorium-free pitchblende, disseminated or in small veins, and yellow secondary minerals. Five areas have been investigated by drilling. In the Kawanga area o f the Kabompo Dome area, uranium occurs mainly as traces o f äutunite coating mica schist outcropping in the area o f two radio- metric anomalies. A t depth, drilling intersected uranium mineralization in mica schist underlying the quartzite marker horizon. Discontinuous mineralization occurs at one or more levels whose thickness can be several metres, with a U 3OS content o f up to 1%. One hole intersected mineralization o f similar grade along a thickness o f about 25 metres at a depth o f more than 200 m. The mineralization consists o f small grains o f pitchblende disseminated in the schist or along fractures several cm wide, and secondary uranium minerals coating fracture surfaces (Figs 4 and 5). A specimen o f clear-grey mineralized kyanite schist contains quartz, epidote, kyanite and muscovite, and has granoblastic and poikiloblastic textures. Quartz shows a mosaic texture, and epidote is present as irregularly shaped grains or as prismatic crystals. Kyanite occurs as individual crystals or in aggregates. Accessory minerals include iron oxides, apatite, monazite and rutile. The uranium mineral is autunite. Dark biotite schist, in which the schistocity is marked by flakes o f biotite and muscovite and equidimensional grains o f quartz, occurs below the mineralized zone. Tourmaline, apatite, zircon and opaque minerals are also present. The rock overlying the mineralized zone is a talc schist, containing unorientated plates o f talc and crystals o f rhombic pyroxene and kyanite. Accessory minerals include monazite and scapolite. During the course o f exploration for copper in the Mwambezhi Dome area, Mwinilunga Mines Ltd. found occurrences o f uranium on the surface and under ground in the area o f Lumwana [3,12]. The uranium mineralization is found in mica schist. In the Solwezi Dome area numerous small occurrences o f uranium mineraliza tion have been found, and drilling has been carried out at Dumbwa, Kapijimpanga, 9 2 MENEGHEL Kimale and Mitukuluku. The mineralization is similar to that previously described. Secondary uranium minerals are found coating flakes o f mica, talc or chlorite occurring along fractures and in veinlets containing quartz, talc, muscovite, apatite and kyanite. The uranium minerals include autunite, meta-autunite, sabugalite, phosphuranilite, vandendriesscheite and gummite. The heavy fraction o f the mineralized rock includes 40% to 60% kyanite, 25% to 40% micas, tourmaline, rutile, iron oxides, epidote and apatite. At depth, the mineralization consists o f pitchblende occurring disseminated in the host rock, or along fractures and secondary uranium minerals, as at Kawanga. The best mineralization intersected in drill-holes occurs at the Mitukuluku, where 1.4% t^Og was found over a thickness o f 9 m. The host rock is quartz-mica- kyanite-talc-epidote-apatite-chlorite schist, underlying the marker quartzite. At the end o f the second phase o f exploration it was possible to conclude that a uranium province had been discovered but that its real significance was still uncertain and the methods to be used to locate most economically significant uranium concentrations were still to be established. 3. THIRD PHASE 3.1. Critical review o f the results The second phase revealed that uranium enrichments are erratic and masked by barren, thick soil and rocks. The host horizon does not possess any useful physical characteristics which enable geophysical methods to be used. The paleoconditions were difficult to establish, mainly owing to lack of outcrops and the transformations suffered by the rocks. On the other hand, the localization o f uranium enrichments by drilling is too long and expensive. For these reasons, drilling operations were suspended and new techniques o f investigation examined. A test of the Track-Etch technique was made in two areas, but the results were rather discouraging: they emphasised only surfaced radiometric anomalies but none o f the buried mineralization. Since no other geophysical or paleogeological studies appeared applicable, it seems necessary to resume the drilling operations. 3.2. Costs The costs incurred during these phases are o f little general significance because they are strictly dependant on the local situation. We believe it is more useful to indicate the amount o f work involved in this type o f exploration in a country with such difficult access and such great supply problems. IAEA-AG-250/13 93 The anomalies must be screened mainly by helicopter: the tack o f tracks and bridges makes it impossible to reach the anomalous areas by car. The heli copter must be fast, capable o f carrying four or five passengers, and powerful enough to be used even during the hottest hours o f the day. A standard prospecting team (geologist, prospector, compass-man, three local labourers) using a four-wheel-drive car can carry out an average o f 10 line km per day, taking radiometric readings every 50 m and doing geological and topographical observation. A wagon drill, with local operators, can drill an average of 50 m a day. A core drill can drill approximately 15 m a day. 4. CONCLUSIONS The prospecting activity in North-Western Province started late in 1970, was suspended in 1975, and was resumed for a short period in 1977. The first phase ended in 1972 with negative results. Was the decision to carry out the long and expensive second phase fair and wise? It depends on where the answer comes from and when. We can conclude as follows: a large area with a good potential uranium reserve has been revealed and it is open to future development; the exploration costs per pound o f the proven reserve are still relatively low. The third phase is now under consideration. What will be the wisest and fairest decision? REFERENCES [1] CAHEN, L., Etat actuel de la géochronologie du Katanga, Ann. Mus. Roy. Afr. Centr., Sei. Géol. 65 (1970). [2] FRANÇOIS, A., "La tectonique des gisements cuprifères stratiformes du Katanga", Proc. Symp. Gisements Stratiformes de Cuivre en Afrique, Paris, 1963, Part 2. [3] McGREGOR, J.A ., The Lumwana Copper Prospect in Zambia, Dissertation, Rhodes Univ. (1964), unpublished. [4] MENDELSOHN, F., The Geology of the Northern Rhodesia Copperbelt, MacDonald, London (1961). [5] BINDA, P.L., MULGREW, T.R., "Stratigraphy of copper occurrences in the Zambian copperbelt", Proc. Symp. Gisements Stratiformes et Provinces Cuprifères, Liège, 1974. [6] FRANÇOIS, A., "Stratigraphie, tectonique et minéralisations dans l'arc cuprifère du Shaba", ibid. [7] CAILTEUX, S., Corrélation stratigraphique des sédiments d'âge Roan du Shaba et de Zambie, Ann. Soc. Géol. Belg. 99 (1977). [8] CAHEN, L., FRANÇOIS, A., LEDENT, D., Sur l'âge des uraninites de Kambove ouest et de Kamoto principal et révision des connaissances relatives aux minéralisations uranifères du Katanga et du Copperbelt de Zambia, Ann. Soc. Géol. Belg. 94 (1971). [9] NGONGO-KASHISHA, Sur la similitude entre les gisements uranifères (type Shinkolobwe) et les gisements cuprifères (type Kamoto) au Shaba, Zaïre, Ann. Soc. Géol. Belg. 98 (1975). 94 MENEGHEL [10] BUTTGENBACK, H., Les minéraux de Belgique et du Congo belge, Paris (1947). [11] BBLLIERE, J., Manifestations métamorphiques dans la région d'Elisabethville, Publ. Etat d'Elisabethville (1 July 1961). [12] BENHAM, D.G., GREIG, D.D., NtNK, B.W., Copper occurrences of the Mombezhi area Northwestern Zambia, Econ. Geol. 71 2 (1976). SELECTED BIBLIOGRAPHY CAHEN, L., "Geological background to the copper-bearing strata of Southern Shaba (Zaire)", Proc. Symp. Gisements Stratiformes et Provinces Cuprifères, Liège, 1974. CAHEN, L., SNELLING, N.J., Données radiométriques nouvelles par la méthode potassium- argon: Existence d'une importante élévation post-tectonique de la température dans les couches katangiennes du Sud du Katanga et du Copperbelt de la Zambia, Ann. Soc. Géol. Belg. 94 (1971). MENEGHEL, L., "Uranium occurrence in the Katanga System of north-westem Zambia", Uranium Deposits in Africa: Geology and Exploration (Proc. Regional Advisory Group Mtg. Lusalca, 1977), IAEA, Vienna (1979) 97. OOSTERBOSCH, R., Les minéralisations dans le système de Roan au Katanga (LOMBARD, J., NIC0L1N1, P., Eds), Proc. Symp. Gisements Stratiformes de Cuivre en Afrique, Paris, 1962 (Part 2). DISCUSSION B.L. NIELSEN: A tot o f money has been spent on an extensive drilling programme. Could you tell us how many targets were drilled and what were the criteria for selecting these targets? L. M ENEGHEL' : We drilled several holes on various anomalies but only in four cases was a detailed drilling campaign carried out and encouraging mineralization intersected. The drilting targets were established on the basis o f the size and intensity o f the anomalies and the presence o f visible uranium minerals at the surface. The Lower Roan near the contact with the Basement has been confirmed as the main host rock but this contact is several hundred km long and no valid criteria could be determined to select the most favourable zones. We verified that surface anomalies are not necessarily related to mineraliza tion, while deep mineralization intersected does not necessarily have surface expression. H.D. FUCHS: What is the metamorphic grade of the Lower Roan Group in the working area? ' Mr. Meneghel answered some questions in writing after the meeting. IAEA-AG-250/13 95 L. MENEGHEL: The average grade o f regional metamorphism is o f the green schist facies, although the Lower Roan has also been affected by dynamic metamorphism. The mineral assemblage in the Lower Roan mica schists, in approximate order of mineral abundance is quartz and muscovite with some chlorite, kyanite, biotite or epidote. Kyanite is often abundant and in large crystals. H.D. FUCHS: Have you yet been able to find any features like structures, younger granitic intrusions, etc., which could lead to a secondary enrichment process o f the already discovered proto-ores? L. MENEGHEL: In most cases uranium has been remobilized. Secondary enrichment has been found along small fractures or coating thin layers o f chlorite or biotite. Metamorphic segregation veins are also found. I believe that in the areas studied the uranium migration was only minor and covered only short distances. The causes o f migration were multiple: metamorphism, tectonic events, thermal events and alteration. The isotopic ages o f uranium generations show a clear relation to the ages o f these events. H.D. FUCHS: It looks as if you are dealing with what we call proto-ores, i.e. really with the syngenetic mineralization in these marine black shales, and you have some weak concentration with your weak metamorphism. Probably you ought to look for traps, which are the main problem in unconformity situations. C. PAP ADIA: That is our problem. In fact, the geological potential of the formation is extremely high. The problem is to transform the geological potential into an exploitable ore body, owing to the particular condition of the mineraliza tion resolved as small pockets, small lenses, scattered without any apparent rule. We tried to establish a correlation o f the uranium lenses, but this was not possible, even though we used a close-grid for the boreholes: 20—25 m. The mineralized bodies are very rich but they are too small to allow possible correlations. H.D. FUCHS: We have a similar situation in many other places, for example in the Wollaston Belt where we don't have unconformity but just the metamorphic belt, where the mineralization is more or less concordant to the old foliation, as well as in Pine Creek in the Schultz Lake area, etc. So I agree that the Zambian area is highly favourable. But the final step is still missing, and it is only present in Shinkolobwe where there is a higher metamorphic grade with an isoclined folding and axial plane foliation which were filled with the solutions, probably meta- ^ morphic and not hydrothermal solutions from a genetic point o f view. So I think you will have difficulty in Zambia if you don't come to areas with higher grade o f metamorphism or if you use some granitic intrusions which lead to a remobiliza tion, or if there is unconformity which we don't have in Zambia, where we have a reworking o f the solutions along unconformity. C. PA PA D IA : We have not yet found Lower Roan formation with high-grade metamorphism; generally it is low, and maybe too low. 96 MENEGHEL P. STIPANICIC: I would like to congratulate Mr. Meneghel for clarifying the problem related to the genesis o f the parallel mineral belts o f Zambia (Cu with subordinated U) and Zaïre (U with subordinated Cu). The mineral deposits o f both belts were considered for many years to be typical vein-endogenous type, in spite o f the lack o f conclusive arguments,and serious evidence to the contrary, but especially ignoring the opinion o f a leading metallogenetist like Schneiderhohln who, about fifty years ago, stated that the Zambia Copper Belt's mineralization (which included Nkana, Nchanga, etc., uranium ore bodies) was not related to igneous-hydrothermal processes but to sedimentary-exogenous ones which were later masked by regional metamorphism^. During private discussions at international meetings held during the last six years, and also in published papers^, I suggested the 'sedimentary-non-endogenous' origin for the Shinkolobwe, Swambo and Kolongwe uranium deposits o f Zaire, but I did not receive a very warm reaction, except only from Mr. Cameron (IA E A ) and Mr. Rodrigo (CNEA, Argentina). In this regard, I believe that the confusion about the interpretation o f the metallogenesis o f the Zaïre uranium deposits was due to a narrow focussing o f the problem and the discarding o f valuable regional geological and metallogenetic concepts. Thus, the Shinkolobwe, Swambo and Kolongwe uranium deposits (Zaire Belt), as well as those small deposits o f Nkana, Nchanga, etc. (Zambia Copper Belt), are not only ахс/мл/уе/у contained in sedimentary rocks o f the Lower Roan Group o f the Mine Series, but they also show a dominant stratiform pattern, to which pseudo-vein sectors are sometimes connected. They are not, however localized in the main faults but appear as small veinlets into secondary fractures. In spite o f this, Derriks and Vaes^ made a tremendous effort in 1955 to demonstrate the 'endogenous-igneous' origin o f the Shinkolobwe deposit which then appeared to be an exception within the whole Upper Katanga mineral district. The main arguments used by Derriks and Vaes to support their idea were: (a) the uranium abundance as opposed to scarcity o f copper; (b ) the possible genetic independence o f uranium and copper; and (c ) the connection o f the Shinkolobwe mineralization to some igneous bodies 'proved by the presence o f several 'pegmatites' or 'igneous apophyses' inside the uranium mine, according to Thoreau and du Trieu^. 2 SCHNEIDERHÖHLN, H., Min. Mag. London 46 (1932). ^ STIPANICIC, P.N., Curso Latinam. Prosp. y Explor. Uranium Depos., Buenos Aires, CIEN/CNEA (1978). ^ DERRIKS, J.J., VAES, J.F., "The Shinkolobwe uranium deposit", Peaceful Uses of Atomic Energy (Proc. Int. Conf. Geneva, 1955) Vol. 6, UN, New York (1956) 94. s THOREAU, J., du TRIEU, R., Mem. Inst. Colon. Belge I 8 (1933). IAEA AG-250/13 97 Two years later, Derriks and Oosterbosch^ applied the same genetic model o f Shinkolobwe to the new Swambo and Kolongwe uranium deposits, although copper was extremely abundant in the latter! The most surprising fact, however, is that the 'igneous-hydrothermal' origin o f the Shinkolobwe-Swambo and Kolongwe deposits was also supported — or at least accepted until recently, by many uranium geologists, in spite o f some arguments to the contrary expressed by different authors on the following points: (a) The Shinkoiobwe and Kolongwe mineralizations (700—550 m.y.), hosted in sediments o f the Mine Series (840—620 m.y.),are much younger and obviously cannot be related to the granitic plutons o f the Katanga Geosyncline aged 1100—1300 m.y. (see Drysdall et a l7 ). (b ) The clear evidence presented by Garlick^ on two aspects related to the Zambia Copper Belt's mineralization: (i) The total inconsistency of the zonal mineralization; and (ii) The total inconsistency o f the relationship between the mineralizing process and 'some igneous rocks', apparently proved by the presence o f abundant 'pegmatites', due not only to the difference in age mentioned above, but also because these 'pegmatites' turned out to be merely cross-bedded recrystallized arkoses! According to the descriptions by Thoreau and du Trieu, the Shinkolobwe 'pegmatites' also seem to be residual arkoses. (c) The Shinkolobwe uranium mineralization decreases downwards and shows neither endogenic roots nor vertical mineral zoning. This work of Dr. Meneghel is an outstanding contribution to our knowledge o f the Zambian-Zairian uranium mineralization, which apparently could be best explained through models related to Precambrian unconformities. ^ DERRIKS, J.J , OOSTERBOSCH, R., "The Swambo and Kalongwe deposits compared to Shinkolobwe: contribution to the study of Katanga uranium", Peaceful Uses of Atomic Energy (Proc. 2nd Int. Conf. Geneva, 1958) Vol. 2, UN, Geneva (1958) 663. ^ DRYSDALL, A.R., et al., J.R. Geol. Min. Soc. (Netherlands) I 3 (1972). s GARLICK, W.G., J R. Geoi. Min. Soc. (Netherlands) 1 3 (1972). IAEA-AG 250/14 POÇOS DE CALDAS AND ITATAIA: TWO CASE HISTORIES OF URANIUM EXPLORATION IN BRAZIL J.M.A. FORMAN, A.G. ANGEIRAS Nudebrás, Rio de Janeiro, Brazii Abstract POÇOS DE CALDAS AND ITATAIA. TWO CASE HISTORIES OF URANIUM EXPLORATION IN BRAZIL. Zirconium-bearing minerals have been known in Poços de Caldas Plateau since 1887, although associated radioactive material was suspected only in 1948, following fogging on photographic plates. High U^Og contents were then discovered in caldasites (zircon + baddeleyite) which occur in the alkaline complex. Exploration commenced in 1952 with the object of evaluating uranium reserves associated with Zr ores. In 1965 amorphous uranium minerals and jordisite were found in the Agostinho deposit. This type of occurrence then became the main exploration objective. Later in 1972, pitchblende, coffinite and jordisite were found in the present Campo do Cercado deposit. Reserves exceed 26 000 t UßOg in the Plateau. The Itataia deposits were discovered following a carborne radiometric survey in 1975 over the Precambrian basement rocks of NE Brazil, during which more than a dozen anomalies were found. The importance of the anomaly at Itataia was only appreciated in mid-1976, and a very large uranium-phosphate ore body was later suspected in 900-m.y.-old marbles and gneisses. Drilling began in mid-1977 and the deposit is at present in an advanced stage of evaluation. Mineralization is known to a depth of 300 m. The technology to be employed in the extraction of uranium has been developed by Nuclebrás. Reserves of uranium exceed 122 500 t U^Og. The grades of U^Og vary from 500 to 9000 ppm and the grades of P2O3 from 12% to 37%. These two contrasting case histories illustrate the exploration philosophy employed on the Poços de Caldas and Itataia deposits. Part I BRAZIL'S URANIUM EXPLORATION PROGRAMME General Information Uranium exploration has been carried out in Brazil since 1972 but with particular intensity during the last five years. An initial objective was evaluation o f the uranium reserves associated with zirconium ores o f the Poços de Caldas Plateau. 99 100 FORMAN and ANGEÏRAS URANIUM DEPOSITS SEOtMEMTARY BASINS )-ITATAIA 2 - POÇOS DE C A L D A S BRAZILIANNE FOLDED BELTS 3-RIO PRETO 4- LAGO A R E A L PRE-BRAZILIANNE CRATONIC AREAS 5 ^MORINOPOLIS 6 - QUADRILATERO FERRIFERO 7- FISU EIR A F/C.7. yeo/oyica/ А??яя:л.! о/мгал/мм <п ßrcz;7. Until 1970, uranium exploration was mainly confined to the Paleozoic and Mesozoic sedimentary basins. Some reconnaissance was, however, also carried out in the Precambrian domains, and this led to the selection o f large areas for aerogeophysical surveys. From 1974 to 1978 there was a significant increase in airborne gamma-ray surveys over the Precambrian basement,amounting to 3 000 000 kirP o f a total area o f 4 300 000 km^ o f Precambrian rock units (Fig. 1). ÏAEA-AG-250/14 101 The promising results o f those surveys led to a shift in emphasis from the sedimentary basins to the Precambrian domains. Areas in a variety o f geological environments were delimited for detailed studies, including the deposits at Itataia in the north-eastern part o f the country. Here uranium is associated with phosphates in an unusual type o f mineralization. Important, too, are the Lagoa Real deposits located in the south o f the State o f Bahia. Mineralization occurs in diatexites and metatexites with the development o f uraninite in shear zones associated with strong albitization. The mineralization has been attributed to fluids originating during a phase o f reactivation o f an ancient platform. Uraninite age is 820 m.y., while the country rocks are 2500 m.y. old. These deposits are in an advanced stage o f evaluation. Although still at an initial stage o f drilling, the region o f Rio Preto in the Central Brazilian State o f Goiás is also a promising area. Nearly a hundred occurrences of secondary mineralization are known in Lower Precambrian schists overlain by quartz-sandstone o f Middle Precambrian age. The geological setting favours an unconformity-type model and uraninite has been found in some drill holes at depths o f 60 metres. In the same State o f Goiás, small uranium deposits occur in Devonian arkoses at the northern border o f the Gondwana Paraná Basin. Mineralization is Cretaceous in age and related to roll-front processes. Although the known deposits have been evaluated, there is continuing regional exploration for additional similar occurrences. The rapid progress made after 1975 (when known reserves amounted to 11 040 t) in identification and exploration o f uraniferous districts can be attributed to a rational sequence o f stages, including: fa) The selection and verification of potentially favourable areas following integrated studies based on all available information. Geological thinking is fundamental at this stage. anaf This follows as a result o f the preliminary studies. Specific programmes are established in favourable areas based on traditional prospecting methods. During this stage, preliminary viability studies, including the calculation o f reserves, are also carried out. For the philosophy o f evaluation, all prospects are evaluated irrespective of the degree of mineralization but taking into account their potential, so that expenditure does not exceed what may be found. The estimation o f reserves is based not only on conventional techniques (blocks, isolines, parallel sections, etc.), but also on statistical and geostatistical methods such as krigeage and co-krigeage, using available programmes developed by Nuclebrás personnel. The use o f these various techniques, both consecutively and simultaneously, permits a high degree o f precision in the evaluation. The concept o f гедеггел and ;'йй?;'ся?еб? гелегуед (reasonably assured), and щ/еггеб? гелегуел (estimated additional) is based on a series o f criteria. These include the estimation methods used, the regularity o f the sampling grid, the grid 1 0 2 FORMAN and ANGEIRAS TABLE I. NUCLEBRAS AND SUBSIDIARY: URANIUM RESERVES AT APRIL 1979 (t UgOs) REASONABLY # ESTIMATED TOTAL NUCLEBRÁS KNOWN "u" DISTRICTS ASSURED ADDITIONAL 1 - POÇOS DE CALDAS PLANALTO 20 ООО 6 800 26 800 / 7 F C C 4 есс F / F C C F F C C F F C C Д C C C 2 -FIGU EIR A - PR 7 ООО 1 ООО 8 ООО 3 - QUADRILÁTERO FER RÍFER O -MG 5 ООО 10 000 15 ООО 4 -AMORINÓPOLIS-GO 2 ООО 3 ООО 5 ООО 5 - CAMPOS BELOS - GO 500 500 1 ООО 6 - ITATAIA - CE 4 8 ООО 7 4 5 0 0 ¡2 2 5 0 0 7 - LAGOA REAL - BA - 5 500 5 500 SUB-TOTAL NUCLEBRAS 8 2 5 0 0 )0) 3 0 0 )8 3 8 0 0 NUCLAM - KNOWN " U " DISTRICTS ESPINHARAS-PB 5 ООО 5 ООО to ООО SUB-TOTAL NUCLAM 5 ООО 5 0 0 0 Ю ООО TOTAL NUCLEBRAS NUCLAM 8 7 5 0 0 )0 6 3 0 0 )9 3 8 0 0 * NEA-IAEA classification. spacing, the percentage o f core recovery, the relationship between the number o f chemical and radiometric data from the mineralized horizons, the degree o f correlation o f the regression line between radioactivity and chemical grade, and, finally, the relative error with respect to the most probable value o f U^Og tonnage at the 95% confidence level. It should be emphasized that the maximum value o f the errors considered is compatible with the class o f reserves reported and that, for measured reserves, one o f the techniques always used is geostatistics. Also for measured reserves, the relationship used between data from chemical and radiometric analysis is greater than that internationally adopted. The calculations are greatly facilitated by delayed neutron analysis for U^Og which permits a large number o f samples to be processed quickly. Reported reserves are defined as those economically viable on the basis o f an international value o f US $40 per lb ^ O g . The work done so far has led to the evaluation o f reserves as shown in Table I. Poços de Caldas and Itataia have been chosen to illustrate two distinct and contrasting case histories o f uranium exploration in Brazil. The uraniferous deposits at Poços de Caldas were the first to be investigated in Brazil and have been explored almost continuously for 27 years. The initial exploration at Poços de Caldas was directed towards the uranium-bearing caldasite (1952—1965) as it was thought that uranium could be extracted economically ÏAEA AG 250/14 103 from refractory ores. However, just as interest was beginning to decline after persevering and careful investigation o f more than 150 anomalies throughout the alkaline complex, the Agostinho mineralization was discovered in 1965, and this greatly enhanced the economic prospects of the area. This important event caused a shift in emphasis from attempts to extract uranium from zirconium ores to prospects with the following characteristics: High U/Zr ratios; Presence o f hydrothermal potassic alteration; Contact between foyaite and tinguaite; and Regions of tectonic reactivation The Cercado Mine was discovered partly because these factors were taken into account, but principally owing to extensive drilling on a closed grid which was facilitated by the availability o f funds for this purpose from 1970 onwards. Drilling was o f particular significance owing to the paucity o f outcrops and the presence o f a thick weathered cover. Next to drilling, ground radiometry and rock chemistry contributed most to the discovery o f the Cercado Mine. Electroresistivity and seismic refraction were found most useful for the definition and delimitation o f the structural framework o f these deposits. in contrast to Poços de Caldas, the itataia deposit has only been discovered and explored since mid-1976. All known uranium occurrences in NE Brazil were identified by geological and radiometric surveys following an integrated study o f all available information. The ground surveys led to the discovery o f ore-grade deposits at Itataia, where exploration and evaluation followed a logical and usual sequence of investigation. This was facilitated by good outcrops and an absence o f significant chemical weathering. The mineralized bodies appear on the surface and their unusual characteristics contrast with the country rocks. Consequently, geological and radiometric maps at once delimited the surface extension o f the ore bodies (0.25 km^). Part II POÇOS DE CALDAS CASE HISTORY 1. LOCATION AND GENERAL GEOLOGY The alkaline complex of Poços de Caldas is located about 400 km west of Rio de Janeiro in the southern part o f the State o f Minas Gerais, adjacent to the boundary with the State o f Sao Paulo. It has a favoured location near the most important industrial centres o f the country. The climate consists o f a rainy season from October to March followed by a dry season from March to September. 104 FORMAN and ANGEÍRAS SCALE PHONOLITE POTASSIC ROCK FOYAITE TINGUAITE TUFFS TIN GUAITE RING DYKE LUJAURITE / CHIBINITE CONTACTS FENITE / GNEISS FAULTS F/C.2. Ceo/o^i'ca/ мар Je CaMaí a^aKne comp/ex. Annual rainfall averages 1685.5 mm, 83% being concentrated in the rainy period. A thick weathered zone covers the central part o f the complex and fresh rock exposures are scarce. The complex was formed by a plume-generated alkaline intrusion [ 1 ] emplaced in Precambrian gneissic rocks of the Brazilian shield. The complex is roughly circular in shape, having an area o f approximately 1000 km^. A t least 14 circular structures can be identified on satellite photographs o f the plateau. These were initially interpreted as representing volcanic necks, but it now seems more reasonable to consider that they were caused by plug-like intrusions. ÏAEA-AG-250/14 105 The Poços de Caldas complex is very interesting on account o f the wide range o f rocks present, which include coarse-grained igneous rocks, volcanics and sedi ments. The most common rock types belong to the nepheline-syenite family and, in order o f decreasing abundance, these are tinguaite, phonolite, nepheline syenite, foyaite, lujaurite and chibinite [2—4]. Some of the extrusive rocks found include tuffs, ash, volcanic breccias and welded tuffs (Fig.2). In the central part o f the structure occur very altered rocks in which potassium has been introduced by 'hydrothermal' activity. The original rocks were tinguaite and phonolite. The best outcrops o f lujaurite and chibinite are found in the north, while some Mesozoic sediments occur in the north-east. To the south-east the granitic and gneissic rocks o f the basement show intense fenitization. Outcrops on the Planalto o f Poços de Caldas are scarce, and weathering has produced a deep and extensive soil cover up to 20 m thick. The details o f the geological relationships are therefore best observed in underground workings, the Cercado Mine and the Agostinho deposit. 1.1. Tectonics The regional and local tectonics of the complex has greatly influenced the development o f the structure as well as the mineralization. The complex is cut by two roughly perpendicular fault systems: N60W and N40E. These faults follow the main regional trends and were reactivated at the time o f the initial intrusion. The regional fault trend can be observed in the mine in addition to a localized fault system trending N10E to N25E [5]. 1.2. Evolution o f the complex The geological evolution o f the complex was described in 1959 [6]. Recently, a different model in six simplified stages was proposed [ 1], here reduced to five and somewhat modified (Fig.3). The first stage involved the injection o f an alkaline plug into Precambrian gneisses o f the basement complex forming a roughly circular structure. The domed rocks were fractured upon uplift and as the intrusion cooled it shrank and collapsed, forming a caldera. The second stage involved the renewal o f magmatic activity which may in part have been related to the shrinking and collapse o f the phonolite/tinguaite plug at the end o f the first stage. Tinguaite was injected between the host rock and the plug. During the third stage o f the development o f the complex there was further collapse o f the phonolite/tinguaite plug and erosion to form a crater-like depression with tinguaite ring dykes surrounding the plug. The fourth stage was characterized by a second phase o f alkaline intrusion forming shallow emplacements o f lujaurite, chibinite and foyaite within the phonolite/tinguaite plug. The Cercado area was 106 FORMAN and ANGEIRAS STAGE 1 STAGE ) FORMATtON OF DOME CONTRACTtON OF STAGE 2 STAGE 3 )NTRUS)ON OF TtNGUAtTE EROStON OF DOME R!N6 DYKES-UPUFT OF DOME MINOR SUBAERtAL PRIMARY MINERALIZATION VOLCANISM TUFFICITIC BRECCIAS PRIMARY ORES -TINGUAITE BASEMENT BASEMENT BASEMENT LUJAURITE CmatNITE STAGE 4 STAGE 5 UPLtFT CERCADO AREA PRESENT StTUATION TUFFtCmC BRECCtATION F/C. J. o/Popoí cfe СаМм cowp^x. ÍAEA-AG-250/14 107 uplifted and tabular bodies of tufficitic breccia were formed by the movement of gases along penecontemporaneous fractures. Primary mineralization accompanied this thermal event and concentrated in the breccias. Some small-scale subaerial volcanism (ankaratritic lavas, tuffs and pyroclastics) occurred in the north-western part o f the Planalto. In the final stage o f development, the Planalto was eroded and there was a redistribution o f the primary uranium mineralization and the formation o f secondary ores. Economic deposits o f bauxite formed as the result o f weathering o f the alkaline rocks. It is suggested that the chemical evolution o f the syenitic rocks had evolved from miaskitic into strongly agpaitic characters [7]. Whole-rock К /Ar determinations carried out on the alkaline rocks have shown ages between 80 m.y. and 62 m.y. [8]. On the other hand, XU-Pb determinations performed in zircon from 11 samples o f caldasite veins have indicated an average age o f 98 m.y. [9]. 2. EXPLORATION AND DEVELOPMENT 2.1. Historical background Economic interest in the Planalto de Poços de Caldas was engendered by the works o f O.A. Derby in 1887 [10], von Sachsen Coburg in 1889, and Hussak in 1899 [11], who referred to the presence o f zirconium-bearing minerals. In September 1948, the presence of radioactive elements in zirconium-bearing minerals was determined from fogging on photographic plates [12]. Samples studied in the Laboratorio da Produçâo Mineral showed that the radioactivity was due to uranium ( < 1% U^Og). In 1952 in Washington, DC, White and Tolbert o f the USGS carried out radiometric determination on samples o f rocks from various sites, verifying that the samples containing zirconium-bearing minerals from Poços de Caldas were radioactive. Some o f these contained as much as 3%UgOg [13]. The radioactive material occurs in rocks known as сяИя,м7е which are a mixture o f zirconium oxide (baddeleyite) and zirconium silicate (zircon). Caldasite, first named in 1887 [ 10], was originally found in veins; it is a massive, grey-coloured and very heavy rock. In August 1952, White informed the Brazilian Conselho Nacional de Pesquisas o f the presence o f uranium in caldasite. Later, in October 1952, uranium exploration at Poços de Caldas began. The history o f uranium exploration in Brazil with specific reference to Poços de Caldas can be divided into five clearly defined periods. FORMAN and ANGEIRAS A t this time very little was known about the complex, although it has been established that uranium was associated with the caldasites. So, one o f the objectives was to estimate the reserves o f caldasite, and one o f the first steps was to make a survey o f all the prospects o f this rock in an area o f 440 km^ centred on Poços de Caldas. However, no metallogenetic hypothesis or model had been considered at this early stage o f exploration. Effective exploration commenced in 1953 when the Departamento Nacional de Produçâo Mineral (DNPM) contracted Levantamento Aerofotogramétricos SA (L A S A ) to make an airborne survey o f an area o f 1330 km^. A scintillometer type EA189 made by Electronic Associates and a Fluxgate magnetometer (Model II) made by Gulf Research and Development Corp. were mounted in a Douglas DC-3 aircraft. About 5300 line km were flown in a north-south direction at an altitude o f 150 m (line spacing was 250 m). This work revealed the presence of 44 major radioactive anomalies and many other smaller ones. O f the 44 anomalies, 38 were considered to be important, most o f which coincided with the caldasite prospects. During this period, the exploration programme included regional mapping at a scale o f 1:50 000 over an area o f 1000 km^, some drilling (1600 m ) and the opening o f galleries. Deposits o f thorium and rare earths had been discovered at Morro do Ferro. These were described in 1957 [14], and details o f the work carried out around the 38 prospects o f uraniferous caldasite were reported in 1966 [15]. The aspects o f the zirconium mineralization were studied in 1967 [7]. The prospects at Campo do Cercado and at Taquari were considered to be the most promising. By the end o f this period, 50 000 to 250 000 tonnes o f caldasite concentrates had been estimated, containing approximately 60% to 85% Zr02 and an average o f 0.5% U^Og. The reserves ranged from 250 to 1250 tonnes o f U^Og. The 'French period' o f technical co-operation marked the start o f systematic exploration at Poços de Caldas. This had as its objectives (a) the study o f the economic possibilities o f mining caldasite and extraction o f the uranium and (b) exploration for other types of uranium mineralization with specific reference to reserves and ease o f extraction. To achieve this, a permanent technical staff was installed to execute various types o f detailed studies. The exploration during this period was based very largely on a reinterpretation o f the geophysical survey o f 1953. Complementary field studies included the following four programmes.' (1 ) A survey o f the old workings and prospects o f caldasite which were deter mined to contain uranium on the basis o f the airborne survey. Prospects with high Th/U ratios were excluded from this study. As a result, over 30 o f the old caldasite !AEA-AG-250/!4 109 workings and prospects were examined systematically and mapped on various scales (1:10 000; 1:1000; 1:50 or 1:20). The purpose of the programme was to study their occurrences in detail with emphasis on the topography, radiometry, petrology and mineralogy. (2) 5улГеуиа^'с рголресГ/оп.' For exploration purposes the Planalto de Poços de Caldas was divided into a north, a central and a southern zone; these zones were then subdivided into sectors, each with an area o f about 20 km^. The sectors were numbered according to their chronological order o f study and were designated by the initial o f the zone where they were situated. For example, C-09 was the ninth sector to be examined in the central zone o f the Planalto. A survey was carried out in a selected area o f about 400 krrp in the central zone, the most mineralized o f the three. The geology, with particular attention to structure, was mapped on a scale o f 1:10 000 over an area o f 363 krrP, and a radiometric map o f an area o f 389 krrP was prepared on the same scale and on a grid o f 75 X 200 m. Enlarged aerial photographs were used as base maps. One hundred and sixty-six anomalies were found, some of which were grouped into constellations. These were later investigated. (3 ) Defa;7ec? a d o rn e raû?;'ofne?ry.' By way o f concluding the general study o f the alkaline intrusive o f Poços de Caldas, a radiometric survey was flown over the northern and southern zones. A SPA-2 scintillometer was mounted in a Bell-300 helicopter and 682 km o f radiometric profiles were made on a 500 X 250 m grid covering about 340 km. More than 40 anomalies in 13 constellations were found. Two of them were found in the northern zone and 11 in the south, but all o f them were situated adjacent to the central zone. Most o f these anomalies were found in the alteration zones o f gneiss and fenite adjacent to the contact with syenitic rocks. One o f the main objectives o f these surveys was a general understanding of the geology and radiogeology of the complex. Considerable difficulties were caused by the variety o f instruments used and the types o f measurements made. Pollution from the old caldasite workings complicated matters further. As a result of combined geological and radiometric investigation, the following conclusions were established (Fig.4): (a) The rocks o f the Planalto o f Poços de Caldas have regional background values o f about 40 mR, and, in function o f this, areas having radiometric values greater than three times the.regional background were considered to be anomalous and were selected for further study. (b) The anomalies having the most uniform radiometric values were found to occur in areas underlain by fine-grained tinguaite near the contact with foyaite bodies. (c) Most o f the anomalies with values exceeding five times the regional background could be attributed to veins o f caldasite at the site o f old workings 110 FORMAN and ANGEÍRAS 20 29hm — L О TIN5UAITE RING DYKE RADIOMETRIC SCALE t.8 2.3 3.3 BG FAULT . F/C.4. ^adiowefric щар o//*OfOí de CaMaí aZAraZme co^pZex. and prospects and to concentrations in alluvial deposits. The strong anomalies at Morro do Ferro differ from the others o f this type inasmuch as they are due to concentrations o f thorium and rare earths present in magnetite dykes. (d ) Another group o f anomalies with values 3 to 4 times the regional back ground are most common in areas underlain by very altered potassium-rich rocks which are often overlain by argillaceous soils up to 20 m thick. Good examples o f this are the anomalies which later developed into the Cercado Mine and Agostinho deposit. However, in many cases some o f these anomalies were attributed to super ficial pollution, including tailings from the old workings. 1AEA-AG 250/14 HI (e) In addition to the above types, other anomalies of different radiometric values and geological contexts were found. Such anomalies were characteristic o f areas underlain by volcanic tuffs with surface mineralization and occurrences of lujaurite and chibinite-containing eudialyte. In the extreme south-east o f the Planalto, some anomalies were attributed to radioactive feldspathic veins. (4 ) Defa;7ed eva/ыяП'оя a/reaJy ле/ecfed рголресй ял ßngidas, /1^ол?;'и/:ол, /ndfc;'o 70, Га^мап and 7м/о&' These prospects were chosen for their different geological and radiometric characteristics. The object was to look for mineralization types other than the caldasite. The conclusions were as follows [16]: (a) The classical deposits o f caldasite, such as those occurring at Brigidas, were unlikely to be o f economic value, from considerations both o f reserves and o f the physical and chemical properties o f the ore. (b) The deposits at Agostinho appeared to be more interesting on account o f the high (5 times higher than usual) U/Zr ratios as exposed on the surface. Tw o linear structures were identified, extending to the north and south o f the main propsect for a distance o f 300 to 400 m [17]. Amorphous uranium, associated with pyrite, fluorite, tiny veins of molybdenite and accessory microcrystalline caldasite, was found after drilling on the northern structure where oxidized rocks disappeared into a highly fractured zone. The occurrence was quite unexpected [16]. (c) At Tufos another type of mineralization was found, composed of micro- crystalline caldasite associated with colloidal uranium dispersed in clays formed by weathering o f the tuffs. When surface studies were carried out at Taquari and Indicio 70, nothing o f interest was found. A t the same time, importance was being given to the Agostinho deposit. A geological map o f the entire complex constituted part o f a final synthesis o f the four programmes carried out during the period. This included a survey o f the old caldasite workings, detailed geology, rock chemistry and radiometry. It was then possible for the first time to have a preliminary understanding o f the general relationships between the tectonic elements and the rock types with the minerali zation. This allowed criteria to be established which led eventually to the discovery of the economic deposits. These criteria included the U/Zr ratios, the distribution of 'potassic' rocks and their confinement to a central reactivated zone. An anomaly, designated Mancha A —C-0 9 , was discovered in 1963 during a ground radiometric survey. This anomaly was considered to be of minor importance compared to the others at the time of discovery. However, it was investigated in detail in 1971 and became part of the Cercado Mine. 112 FORMAN and ANGEÏRAS 7966-7970 During this period, preliminary evaluation o f the reserves was carried out at the Agostinho prospect, including the sinking o f shaft (42 m ) and the opening o f galleries and cross-cuts (350 m), which followed the highly fractured breccias of tinguaite filled with pyrite, fluorite, jordisite, zirconium-bearing minerals and iron and manganese oxides. Uranium occurs as amorphous oxides and is also associated with the zirconium-bearing minerals. This constituted the first detailed evaluation o f the reserves o f soluble uranium-molybdenum ore at Poços de Caldas [18]. The 166 anomalies found during the systematic prospecting carried out in the course o f the 'French period' in the central zone were investigated for their economic potential. Although a number o f prospects similar to Agostinho were studied and in some cases even drilled, only the Agostinho deposit was considered to be o f economic interest at the completion o f these studies and was until 1970 the only uranium working in which galleries had been opened. 7P77-7974 During this period exploration became more specific; the underground development at Agostinho, where beneficiation tests were performed on ore samples, was accelerated. Reserve calculations amounted to 3200 tonnes o f U3O8 o f which 1100 tonnes were measured while the remaining 2100 tonnes were estimated. However, technical and economic viability studies concluded that the deposit was uneconomic in terms o f the market value o f uranium at the time o f this evaluation. In 1971 the Mancha A —C—09 anomaly was drilled for the first time. A drill hole penetrating a tinguaite breccia 52 m below the surface at Campo do Cercado was found to contain coffinite and pitchblende in very small quantities. However, this was the first time distinct uranium minerals had been found in the rocks o f this alkaline complex [19]. Following the initial discovery, a number o f uranium- associated minerals were found, e.g. uranium associated with molybdenum; pitchblende and coffinite associated with a carbonaceous pyritic substance. Zirconium-bearing minerals were also found. In addition, the rocks from this area were determined to have higher U/Zr ratios than any others from the Planalto. The geometry and characteristics o f this body were not recognized immediately owing to difficulties in determining its shape and extension. Although the geometry and trends o f the body were not known, it was thought at the time to be similar to the subvertical and faulted tinguaite breccia at Agostinho. The higher U/Zr ratio suggested that the uranium was not related to the zirconium 1AEA-AG 250/14 113 minerals and that the mineralization continued below the weathered zone. It was concluded that the ore was soluble and quite different to the other types found at Poços de Caldas. About 15 000 m o f diamond drilling were carried out between 1970 and 1974. By the end o f 1974 it was realized that at least three mineralized bodies existed at Campo do Cercado and that each o f them had distinct characteristics o f geometry and mineralization. In 1974 a pilot plant was built adjacent to the Campo do Cercado deposits since by this time the physical and chemical qualities of the ore had been recognized as superior to those o f Agostinho. 7 9 7 3 -7 9 7 9 With the establishment o f Nuclebrás in 1974, it was decided that the Campo do Cercado deposit showed the most promise o f fulfilling the short-term necessities o f the Brazilian Nuclear Programme. A development programme was therefore established on the basis o f the available information in order to provide answers to the outstanding problems, such as: (a) Geological knowledge of the three ore bodies: their geometry, genetic model, and spatial distribution o f the grades o f molybdenum and uranium. (b ) Definition o f the overall reserves, using conventional, statistical and geostatistica! methods to add a further degree o f confidence to the values obtained. (c) Attainment o f large representative samples for processing studies. (d) Attainment of geological parameters for the future development of the deposit. Between 1975 and 1979, the area was exhaustively drilled. Some 105 000 m o f evaluation drilling, mainly diamond drilling, were carried out and led to the identification of four distinct types o f mineralization in three ore bodies ('A ', 'B' and'E'): Primary vein mineralization in tinguaite breccia ('A ' ore body); Secondary mineralization of the oxidation-reduction type (the lower part of 'E' ore body and the upper part of 'B' ore body); Diffuse mineralization, 'amas' in pyroclastic rocks (lower part o f B' ore body); Pockets of reduced ground preserved in oxidized rock (upper part of 'E' ore body). In addition, about 2500 m o f galleries were opened on three levels. It should be stressed that none o f the above characteristics can be recognized on the surface owing to the very high degree o f weathering and alteration. The rapid development and understanding o f the Campo do Cercado deposit was made possible by the availability of funds which permitted extensive drilling. 114 FORMAN and ANGEIRAS TABLE II. CAMPO DO CERCADO MINE: CORE ASSAY VERSUS RADIOMETRY INTERVAL (m) COFE ASSAY (изОа%) RADIOMETRY (cps) 0 1.00 0.455 175 1.00 - 2.00 0.455 236 5.00 - 6.00 0.091 . 515 6.00 - 7.00 0.103 487 7.00 - 8.00 0.107 387 42.00 - 43.00 0.122 1 860 43.00 - 44.00 0.150 1 162 44.00 - 45.00 0.195 2 284 45.00 - 46.00 0.210 2 538 3. SAMPLING AND ANALYTICAL PROCEDURES Only the mineralized horizons o f the drill cores were sampled and the selection was made on the basis o f the radioactivity in excess o f 1500 cps (SRAT/SPP-2). The radioactivity was initially used as a mineralization indicator, in the primary ores the correlation between the radiometry and the chemical values was found to be fairly consistent. However, in the secondary ore zones o f the upper part o f 'E' ore body, the correlation was found to be poor owing to positive disequilibrium conditions. Here only chemical values were found to be representative. This condition was not recognized at first, and after 1975 it was necessary to resample and analyse according to statistical procedures [20] (Table II). 4. ADDITIONAL METHODS 4.1. Geochemistry A total o f 4335 geochemical samples o f water, soil and stream from different parts o f the Planalto were collected for geochemical analysis. However, owing to the pollution caused by the tailings from old workings and prospects, this technique was abandoned. Radon emanometry (Track-Etch) was also used but with little success. !AEA-AG-250/14 115 4.2. Ground geophysics Some 10 390 resistivity measurements were made, principally in the areas o f Brígidas and Agostinho. This technique proved useful in identifying veins and fault zones. Resistivity and seismic refraction survey contributed considerably to determining the structural characteristics o f the Cercado Mine. 5. GEOLOGY OF THE CERCADO MINE The Cercado Mine is divided into three ore bodies which lie on the border o f a secondary crater inside the great cauldron [5]. The ore bodies were described initially by A.G. de Oliveira, M.O. Fraenkel o f Nuclebrás.and later by J.E. Tilsley [1] and J.C. Biondi [21] (see Fig.5 and Table III). 5.1. 'A 'o r e body This body (0.4 km^) consists o f tinguaite and phonolite cut by vertical or steeply inclined dykes o f tufficitic breccias. These rocks are dark in colour and composed o f fragments o f tinguaite surrounding by a fine-grained matrix o f tiny rock particles. Primary mineralization associated with these breccias includes uranium, molybdenum and pyrite. In the north-eastern part o f 'A ' ore body, the mineralization is secondary, being associated with a redox front from leaching o f the breccias. The characteristics o f this mineralization are identical to those of 'E' ore body. The country rocks are sometimes mineralized near the contact with highly mineralized breccias. This process is often facilitated by intense fracturing, causing a local increase in the porosity. 'A ' ore body contains about 20% o f the total reserves o f the Cercado Mine. Average grades are 0.21% U 3 O 3 and 0.95% M 0O 3. 5.2. E' ore body This body (0.4 km^) occurs in highly altered and fractured tinguaite and phonolite. It is thought to be faulted against 'B' ore body which lies to the south-east. A ll o f the mineralization in this ore body is secondary and is very variable in character within the same rock type. Three types o f mineralization have been recognized. The first occurs in the higher levels to a depth of approximately 120 m but is concentrated in the upper 10 m. The uranium is found in limonite, by which it was probably adsorbed. The second and third types are more important and occur in an interval subtended between the oxidation front above and by the weathered zone below. 116 FORMAN and ANGEtRAS CIRCULAR STRUCTURE E Ä S 3 BRECCIAS ^ E%%%%¡AMAS [ ^ - ^ j pYROCLASTIC ROCKS EI^HpHONOLITE^ — — FAULTS NOOULES F7C.J. ЗсйетаГм со?ярояГе с/-ом-^есУ:'о/! o/ Cercado depojí'f í/rofn [5]/ TABLE IH. POÇOS DE CALDAS PLANALTO RESERVES (tonnes) REASONABLY ESTIMATED DEPOSITS TOTAL ASSURED ADDITIONAL CERCADO MINE 17 200 4 600 21 800 ' ЧзОа ADDITIONAL AREAS 2 800 2 200 5 000 TOTAL POÇOS CE CALDAS 20 000 6 800 26 800 MEASURED AND DEPOSITS INFERRED TOTAL INDICATED МоОз CERCADO MINE NA * 25 000 25 000 ^ ZrOz CERCADO MINE NA 172 400 172 400 3 * Not announced. ' Grade 0.0847%. 3 Grade 0.П %. ^ Grade 0.81%. The intense fracturing striking north-east is a factor which contributed greatly to the concentration o f the uranium mineralization as black nodules. The re mobilization resulted in the formation o f secondary mineralization at the inter section o f perpendicular fault sets. . The ore bodies associated with the redox front are variable in shape (beds, apophyses and prisms), and their distribution was controlled either by the migration IAEA-AG-250/14 117 o f the oxidation front and/or by structural features. The galleries cut mineralized bodies o f all shapes, including narrow and subvertical zones as well as irregular pockets o f 'amas'. The development o f the amas was controlled by the rock weathering. This alteration is thought to have been caused by hydrothermal solutions and fumaroles. In general, the 'E' body mineralization is subhorizontal and amounts to about 15% o f the reserves o f the Cercado Mine. Average ore grades are 0.117% U^Og, 0.93% ZrO^ and 0.09% M0O 3. 5.3. 'B' ore body This body (0.54 km^) is located in a volcanic assemblage o f tuffs, lavas, ashes and breccias cut by intrusives o f variable shape and size. This pyroclastic material was deposited in a depression on the syenite mass. Some o f the mineralization can be considered to be primary, in contrast to another type which is clearly secondary. The former occurs in a zone above the nepheline-syenites which lie at some depth below the surface, and accompanies the contours o f this rock body. The average values o f uranium increase with decreasing distancé to the nepheline- syenites, while those o f zirconium do not change with increasing depth. It has also been noted that the thickness o f mineralization increases with depth, which supports the view that the mineralization is controlled by the pyroclastic/nepheline syenite interface. High porosity is thought to have greatly influenced the distribution o f the mineralization and its concentration in amas. Minor secondary mineralization occurs at the oxidation surface and is considered to be o f little economic importance. Both types of mineralization amount to about 65% of the reserves known at Cercado Mine. Average grades are 0.074% U^Og, 0.072% ZrO^ and 0.11% M 0O 3. 6. ORIGIN OF PRIMARY MINERALIZATION The primary ores occur in injection breccias which penetrated the alkaline intrusives. These breccias are generally tabular in shape, steeply inclined and vary abruptly in thickness both along strike and down dip, depending upon the degree of tectonic control. To varying degrees they follow faults and fractures which originated during the intrusion o f the younger nepheline-syenites and upon the shrinking o f the magma. Gases escaping along such fractures and conduits caused a relatively high degree o f abrasion, and, depending on the duration and distance o f transport, produced a breccia composed o f subangular to rounded fragments in a fine-grained matrix. This process is known as tufficitic brecciation. J.E. Tilsley [ 1 ] was the first to suggest that this process might be operative at Cercado, and this view was subsequently adopted and developed by J.C. Biondi [21, 22]. The gases involved in this process may have been released as the result o f retrograde boiling o f the magma or, alternatively, by the release o f pressure upon 118 FORMAN and ANGEIRAS TUFFtCmC DYKE MtNERAHZED, ENERGY TO SYEN !TE DYKE PENETRATE I FOLLOWING HOST ROCK FOR TUFftCmC BRECOA GREAT DtSTANCE ШСН ENERGY CHANNEL a tNCORPOR- AUNC BRECOA FLUtOtZAHON CHANNEL (TUFF)C!HC BRECC'A) ELEMENTS MtNERALtZED Bx ELEM EN TS INCLUDE ALL ROCKS TRAVERSED ( NO SCALE ) .F/C.d. №'негяй2я?;'ол о/ ге/яи'<9н.уй;р o/ 77/л/еу [1]/ failure o f the enclosing rocks. However, it is likely that both these processes were operative. The breccias may be mineralized along their entire length or along part o f it only, depending on the availability o f metal-rich gases and volatiles. Both high- and low-temperature suites have been described and these may occur separately or may be superimposed following the general rules o f degassing phenomena. However, a vertical zonation has been observed whereby the high-temperature metals such as zirconium tend to be concentrated in the lower parts o f the intrusion, while in the upper levels the mineralogy suggests a low-temperature environment. ÏAEA-AG-250/M 119 The superimposed mineral suites may represent multiple degassing events. The minerals found in these tufficitic breccias include zircon, baddeleyite, pitchblende, coffinite, pyrite, sphalerite, galena and fluorite. Thorium is also present but its mineralogy has not been defined. 7. FORMATION OF SECONDARY ORE MINERALS The secondary uranium ores o f the Planalto de Poços de Caldas are directly related to the weathering process. During weathering,the primary mineralization contained in the tufficitic breccias is released during oxidation and carried down wards by ground-water. When this uranium-rich water enters a reducing environ ment, the contained metal is precipitated as black nodules, pitchblende and pyrite. The sequence o f formation o f secondary ores has been summarized [ 1 ] with reference to Fig.6 as follows: 1. Hydrolization (complete or partial) o f the tinguaite; 2. Development o f an oxidation/reduction front at a level below surface where the chemical oxygen demand removes all free oxygen from descending ground-water; 3. Dissolution o f uranium at the oxidation front as the front advances; 4. Re-precipitation o f uranium in reducing condition ahead of, and usually below, the oxidation front. The oxidation/reduction front is generally tabular; horizontal irregularities are common and these generally have an amplitude o f 1 to 3 m. 'Tongues' o f oxidation may follow fractures or faults tens of metres below the general position o f the front. In addition to the secondary uranium minerals, secondary minerals of molybdenum are formed, including jordisite and ilsemanite. Part III ITATAIA CASE HISTORY 1. LOCATION AND REGIONAL GEOGRAPHY The Itataia region is located about 170 km west o f Fortaleza, in the central part o f the State o f Ceará (NE Brazil). The climate there is semi-arid all year round. Irregular rainfall may occur from January to April and average rainfall is less than 500 m. Consequently, good and extensive fresh rock exposures are a common feature. 120 FORMAN and ANGEtRAS SCALE О 25 50 km ' < . ¡ — L 39°30 4° о + + t\+ ^ + + \ + , ,+ -r -fstû.QUITER + 0 . + + -^ + + + + + + -)^'/ 4 °Э 0 '- + + + + V + + + + + + yi- + + + *h/ "t* + + + + + + + ^/ / + + + -f^^+ + + + + + + + + + + + + +'\/ + + + + \ + + + +*^+^-^+ -ь + + + <+ + JAGUAR!BEANA FOLDED BELT + + + SANTA QU!TER!A MEOtAN MASStF P-U ANOMAHES: FAULTS CARBORNE FRACTURES o AiRBORNE STRUCTURAL TRENDS ^ ITATAIA DEPOSIT F/C. 7. Ceo/og:'ca/ /яар o / Cenfra/ Ceará ^fafe.' мгая/мм/рАоурАоял jMfn'cf. ÏAEA-AG-250/Î4 12! ln 1965, Kegel divided the north-east o f Brazil into orogenic blocks [23] and thereby established a tectonic base which is followed to this day. Kegel's tectonic model was adapted and placed within a geochronological framework based on age determinations o f rocks from this part o f the country [24, 25]. The two main tectonic elements pertinent to the Itataia deposit were described. These are the Santa Quitéria Massif to the north-west and the Jaguaribeana Fold Belt(Fig.7). 1.1. The Santa Quitéria Massif This consists o f a highly deformed granitic crust in which two or more tectonic and magmatic cycles have been superimposed. These rocks have been highly metamorphosed.. Sedimentary and volcanic rocks, and possibly basic rocks, have been subjected to a high degree o f migmatization in which the migma could be either allocthonous or autochthonous [26]. The evolution of the protocontinental stage occurred 2.5 billion* years ago [27]. The primitive rocks were transformed almost completely, reactivated and homogenized by granitization o f the crust. This event may have occurred about 2.0 billion years ago, as calculated from relicts o f amphibolites in granites and migmatites, and was followed by isotopic reorganization 1.3 b.y. as well as 540 m.y. ago. This latest tectonic event occurred at the end o f the Brazilianne Cycle. 1.2. The Jaguaribeana Fold Belt The rocks of this tectonic domain were formed from sediments which were composed mainly o f carbonates, arkoses and sometimes marls. These sediments were deposited on a granitic basement. The rocks were metamorphosed to the grade o f the amphibolite facies. The carbonate rocks and the marls were metamorphosed to marbles and various types of calcosilicate rocks whose varied mineralogy reflects the nature o f the original sediments. The Jaguaribeana Fold Belt is delimited by two transcurrent faults known as the Rio Groairas and Itatira faults. The rock-units have been highly folded and faulted owing to the differential movements o f the stable blocks to the west and east. Antithetic faults with curved surfaces and with their concave faces directed southwards are clearly visible on ERTS images. These indicate a release o f tectonic compression and confirm the presence o f stable blocks to the west and east delimiting a central mobile zone. The style of folding has not yet been defined. It is very likely that two or more generations o f folds exist, but this will be confirmed only after a regional structural analysis. The principal phase o f folding and metamorphism occurred ' Here one billion = 10^. 1 2 2 FORMAN and ANGEIRAS about 650 m.y. ago and was accompanied by localized crustal anatexis which gave rise to the formation o f granites at the time o f this tectonic cycle. 1.3. Granitic rocks In regional terms, it was concluded [27, 28] that there were various phases o f magmatism and that these were related in time and space to the tectonic evolution as a whole. It appears that a high degree o f sialization occurred in the pre-orogenic phase since only a few relicts could be identified with certainty in the interior o f the older blocks (e.g. the Santa Quitéria Massif). Katazonal granites were formed during the orogenic phase (650 m.y.). These are autochthonous and are located principally in the older stable blocks. Their pétrographie, structural and textural trends suggest an origin by anatexis related to the general mobilization o f the pre-Brazilianne Basement. The late orogenic granites (550 m.y.) are poorly developed in Ceará [28]. However, post-orogenic granites (460 -510 m .y.) are know to occur in the central north-western part o f the state. The preferential emplacement o f plutons in the Santa Quitéria Massif is noteworthy since bodies o f reduced size occur in the folded areas. The emplacement o f the post-orogenic granites was controlled by extensive regional faults, especially those formed at the time o f compressional release, in addition to those in the border zones between the different tectonic elements such as the Santa Quitéria Massif and the Jaguaribeana Fold Belt [27, 29]. These zones may have functioned as 'thermal channels' [30]. The post-orogenic magmatism originated by fusion of the lower crust or by fractional crystallization o f a basic primary magma [28]. On the basis o f their chemical composition, these granites can be considered as leucocratic types with strong alkaline tendencies [31]. The post-orogenic granites are particularly interesting since they contain high values o f UgOg (an average o f 30 ppm). Similarly, the values o f P^Og (0.65% to 1.3%) are high compared to those normally found in granitic rocks [27]. 2. HISTORICAL BACKGROUND During the 1960s, systematic exploration and prospecting for radioactive minerals in NE Brazil were carried out almost exclusively in the Paleozoic sedimentary basins (Jatoba, Piaui-Maranhâo, etc.). In the early 1970s, some sporadic reconnaissance was undertaken in the Precambrian terrains o f the north east. However, a systematic programme including drilling was carried out in the granitic province o f Seridó. The results were not particularly encouraging, either in the sediments or in the basement rocks. Systematic investigation o f the Precambrian basement in the State o f Ceará began with a carborne radiometric survey in 1975. This survey covered an area IAEA AG-250/14 123 o f 35 000 km^ in the north-central part o f that state. Although basic geological information was lacking, certain mineral occurrences were already known, including scheelite, columbite-tantalite, beryl, spodumene and ambligonite. Uranium minerals are found sporadically in the pegmatites o f the region. The results o f this survey included the discovery o f 273 anomalies during 11 167 km o f radio- geological traverses along roads and tracks. The equipment used included a portable scintillometer SRAT-SPP-2 mounted on a four-wheel-drive vehicle. Twelve anomalous areas (Fig.7) were selected for further study on the basis o f common geological factors, geometry and chemical values. A ll twelve areas had high values o f U^Og (1000 to 3000 ppm UgOg and 12% to 30% P ^ s ), and were related either directly or indirectly to vuggy, quartz-poor, feldspathic rocks. The areal distribution o f this unusual rock type drew attention to the probable regional character o f the mineralizing event. No metallogenetic model had then been considered. The unusual rocks were called albite-syenites on the basis o f their mineral content, specifically the absence o f quartz. The mineralization was consequently related to alkaline rocks. This approach was later shown to be incorrect [27, 32]. One o f the anomalies discovered during this car-mounted survey was no more than a single boulder lying on a dry stream bed. The boulder was about 50 cm in diameter, reddish in colour, dense, massive, cryptocrystalline,and assaying 1500 ppm UjOg and 30% P ^ s . This was considered to be o f low priority in the schedule o f follow-up studies on account o f the form occurrence. However, during a geological reconnaissance survey in July 1976, Nuclebrás geologists discovered an extensive outcrop o f such uranium- and phosphate- rich rock in an area about 1.5 km to the north o f this anomaly. These rocks had radiometric values o f 7000 to 10 000 cps (SRAT-SPP-2) and, on chemical analysis, revealed exceptionally high values for this type o f association (2500 ppm U^Og and 30% РзОз). Pink feldspathic, vuggy and radioactive rocks were also found in the same place. This occurrence was referred to as Anomaly 62 and later became known as the itataia deposit. 3. PRIMARY EXPLORATION The importance o f Anomaly 62 (Itataia) was only recognized in July 1976 owing to the outcropping radioactive rocks. As the result of a preliminary reconnaissance, a 2-km^ area was delimited to be investigated in detail. Exploration crews commenced primary exploration o f the Itataia anomaly during the second half of 1976. The usual ground techniques of detailed radiometric and geological surveys were adopted in view o f the local features o f the area. 124 FORMAN and ANGEtRAS SCALE О 200 400 600 аоо lOOOm ' ' ' ' . . . < RADIOMETRIC SCALE < 300 ¡200 4800 >7)00 6g ISORAD CONTOURS-CPS (SRAT/SPP-2) !} GALLERIES, ! PROJECTED WHERE TRACED " О DRILL HOLES F7C.& .RaA'omefric map o/7fafa¡'a Jepo^Y. 3.1. Radiometry Topographic and radiometric surveys were carried out on a 1:1000 scale along north-south traverses with lines spaced at 50-m intervals and with readings every 25 m over an area o f 2 km^. The exploration team consisted o f one geologist and two field technicians equipped with SRAT-SPP-2 scintillometers. The radiometric background o f the country rocks (marbles and gneisses) was less than 100 cps. Maximum values attained 12 000cps. The mineralized bodies were perfectly contoured by the isorads (Fig.8). 3.2. Geological mapping A detailed geological map was made in early 1977 on a 1.2500 scale in the 2-km^ area o f the radiometric survey. The geological mapping was carried out along the traverses used for the radiometry. Information was transferred to a IAEA-AG-250/14 125 base map, and a distinction was made between rock outcrops and blocks. Enlarged 1:10 000 air photographs were used for structural studies and gross contact demarcations. The mapping team consisted o f two senior geologists. Shallow test pits and trenches were opened concomitantly with the mapping. These permitted determination o f the strike o f lithological succession in areas o f overburden. This geological information, when considered together with detailed surface radiometric character, often permitted favourable lithologies to be traced between areas where bedrock information was lacking. The general relationships shown on the geological map confirmed the isorad contours. The surface expression o f the ore body suggested a thick dyke-like structure (Fig.9), but this interpretation was later shown to be mistaken. Numerous samples collected during this phase were sent for various types o f analysis (analysis o f oxides, trace elements, petrography and mineralogy) in order to make a first approximation o f the mineralization model and the lithological and structural controls. The results obtained surpassed what was hoped for, since the existence was determined o f structures and mineralized bodies with thicknesses o f tens o f metres and average grades o f 2500 ppm o f UgOg and 33%РзО;. 3.3. Ground geophysics V L F and electroresistivity were carried out in late 1977 in an attempt to trace the lateral extension o f buried structures and mineralized bodies identified during the geological mapping. These methods gave satisfactory results; they confirmed the existence and the preferred orientation of some buried mineralized bodies following a strike N70°N. Some faults suggested by the geological mapping were also confirmed. 4. DRILLING PROGRAMMES The surface exposure o f the Itataia ore bodies was fully delineated by the geological/radiometric surveys. Decision to drill was quickly taken. Exploration services performed at Itataia before the drilling programme, including the carborne survey, amounted to US $670 000 (at July 1979 prices). The drilling programme began in July 1977. Twenty-nine holes were drilled to a total extent of 5000 m. Cores were taken throughout the entire length of the holes and the recovery rate was about 90% in each core withdrawal: The drill holes were sited on a wide-spaced grid that could be closed if required. During this programme , the average depth o f the holes was 150 m, following which they were all logged with a Mount Sopris 2000 for natural gamma, resistivity and self-potential. In 1979, a Mount Sopris 3000 logger was used for this purpose, 0 too 200 300 400 50 0 6 0 0 7 0 0 8 0 0 m ^ i — i --i ______,______¡------! SCALE OMN n ANGEIRAS andFORMAN FELDSPATWtC A M D /O R CONTACTS S0)L COVER f ^ + ^ + 3 SRAWITIC ROCKS MASStVE C0LL0PHAM)TE БЭ MAR8LES & y* FAULTS MARBLES WtTM ANTtFORMAL STRUCTURES COLLOPWAM'TE GNEiSSES F/C.P. map o^poy/f. 1AEA-AG 250/14 127 F/G. 70. /fafaía сго^-мсУ;ол. allowing all three logs to be recorded simultaneously during a single run. Mineralized rocks were intercepted in all 29 drill holes,and the dyke pattern, as suggested by the geological mapping, was shown to be incorrect. Thick veins and lens-like massive bodies were defined up to 150 m depth (Fig. 10). The results obtained during this first drilling programme were highly significant. Allowing for an even distribution o f the mineralization, it was possible to infer the existence o f at least 18 000 tonnes o f UgOg at itataia, which justified another drilling programme of a further 10 000 m. in view o f the promising results, drilling was reinitiated in mid-1978 and terminated in January 1979. Forty-two holes were drilled to a maximum depth o f 300 m each and to a total o f 10 129 m during the programme. About 70% o f the drilling had as its objective the blocking out o f reserves. The remaining 30% was carried out for exploration purposes with specific reference to the lateral extent o f the mineralization. The holes were sited on a 160-m-square grid, including those o f the first drilling programme. 128 FORMAN and ANGEÏRAS The results stressed the size o f the ore body and showed that massive mineralization occurred from the surface to depths o f 150 m and that stockwork- type mineralization continued to depths greater than 300 m. Some holes also revealed extensive stockwork mineralization some 10 m to 20 m below the out cropping marbles. This had already been suspected but was not clear from surface studies (Fig. 10). At the end 1978, computer calculations based on the conventional method o f blocks estimation, indicated reserves o f 71 000 tonnes o f U^Og o f which 34 000 tonnes were considered to be in the category o f measured and indicated reserves (reasonably assured) and 37 000 tonnes as inferred reserves (additional estimates). By this time the great potential o f the Itataia deposits had been recognized and thus economic evaluation programmes attained a high priority and were accelerated. The drilling grid was closed to 80 m with a contract for a further 10 000 m o f diamond drilling. It had as sole objective the blocking out o f reserves. The above programme began in January 1979 and was scheduled to end in November 1979. Core /ogg/ng was conventional and included records of rock types, composition, texture, colour index, structure, foliation, weathering, mineralization features (massive, veinlet, stockwork, impregnation, etc.) and additional drill-hole information. All these data were correlated with the radiometry, self-potential and resistivity logs, which resulted in a final composite hole log prepared on a 1:100 scale. This procedure proved very useful during the core sampling and facilitated geological correlations. The ra (a) It permitted good approximation for the grades calculated from the logs, as the equilibrium conditions approximate 100%; (b ) It provided assay information in horizons o f lost core and assisted in the statistical procedures o f sampling; (c) Radiometric backgrounds in barren holes and/or barren zones assisted considerably in differentiation and correlation o f rock types; (d ) It provided reasonable in-situ estimates o f grade values. The downhole SP and resistivity logs were also useful in defining lithologie boundaries. Rock types such as gneisses, marbles and mineralized horizons gave good characteristic responses which assisted in the correlation o f the lithological units. 5. SAMPLING AND ANALYTICAL PROCEDURES The cores from the mineralized horizons were first sampled according to geostatistical requirements [20] using a support o f 0.25 m for homogeneous lithologie segments. Irregularities caused by lithologie breaks and core recovery IAEA-AG-250/Ï4 129 were adjusted by using statistical techniques for error distribution. This was achieved by the correlation between the drill cores and the composite hole log. By this procedure all the lost core could be evaluated as if the recovery rate had been 100%. The necessity to determine grades and reserves o f phosphates o f economic importance required samples to be obtained o f all the mineralized horizons. In addition to UgOg, ThO^ and P 2O5 systematic analysis (delayed neutron and X-ray fluorescence), hundreds o f analyses for trace elements by emission spectrography were carried out to determine the geochemical background and to look for other elements of potential economic interest (Pb, Zn, Mo, Ag). Concomitantly, radioactive disequilibrium studies were performed in core samples. These studies indicated equilibrium between the chemical and gamma- ray spectrometer values (correlation coefficient 0.96). In 1979, support o f the sampling was increased to 0.50 m, in view o f the regularity o f the mineralization, bearing in mind the aspects o f lithological breaks and/or core recovery. To accelerate the analytical determination, analysis by delayed neutron activation was replaced by a RIGAKU 'programmable' X-ray analysis system. Routine analytical procedures were established for the Itataia ores. Each sample was automatically analysed for U 3O8, P2O3, SÍO2, А^Оз, CaO and Fe2Û3 to obtain basic information for milling and dressing tests. 6. UNDERGROUND WORKINGS A 400-m-long gallery was opened in 1979 to intercept the central part o f the main ore body and to obtain large volumes o f different ore types for milling and dressing studies in Nuclebrás laboratories. Underground observations con firmed the drill-hole interpretation, including the geological correlation and the ore-body geometry and dimensions. By the beginning o f 1980 a further 1000 m o f galleries would be contracted. 7. GEOLOGY OF THE ITATAIA DEPOSIT The Itataia deposit is situated in the Jaguaribeana Fold Belt, in rocks attributed to the Caico Group. The principal country rocks are paragneisses (sillimanite- granite-biotite gneisses) with a well developed planar fabric. Intercalated with these gneisses is a large lens o f carbonate rock at least 10 km long. Lateral gradations are common and include graphite-marbles and calcosilicate rocks rich in phlogopite, diopside, tremolite, scapolite and quartz. 130 FORMAN and ANGEIRAS Both the marbles and the gneisses are cut by several granite and granite- pegmatite apophyses in addition to small granitic cupola. These intrusives have been affected intensively by deuteric processes, and drilling has confirmed this at depth. The marbles and gneisses are very highly folded. The latest axial planes are subvertical and the fold axes dip gently to the east (5° to 10°). The Itataia ore bodies are located on one o f the regional compressional release faults and, before the mineralization episodes, the marbles were uplifted on to the gneisses. This tectonic event accounts for the local geomorphology whereby the marbles have their topographic expression as a small hillock some 110 m high, while the surrounding lowlands are underlain by gneisses. The marbles constitute an 'upper block', while the gneisses are considered as a 'lower block', both o f which are mineralized,and have been considered as an upper and lower ore body respectively. The fault zone formed an important channel along which mineralized fluids were able to pass (Figs 9 and 10). Most o f the exploration and evaluation to date has been carried out on the upper block, and the reserves and grades quoted elsewhere only refer to this body. The ore bodies are contained in massive lens-shaped and veinlike structures of variable sizes and shapes which are concordant with the surrounding marbles. Numerous apophyses and veinlets cut not only the gneisses and marbles but also the 'episyenitic' rocks forming stockworks (Table IV). Two types o f ore bodies have been recognized. The first consists of uniform massive collophanite and the second of veinlet and stockwork ores in marbles and 'episyenitic' rocks as well as in impregnations in gneisses. In fact, impregnation is a general feature which affects all the lithologies in varying degrees. . The uranium occurs in cryptocrystalline hydroxy-apatite, which is also of commercial interest as a phosphate source. The rock in which this mineral occurs has been referred to as сойорйаи^е because 90% of it is composed of crypto- crystalline hydroxy-apatite and the remaining 1 0 % o f calcite, ankerite, iron oxides and graphite. Anomalous values of thorium and rare earths o f the yttrium series are found (Th - 162 ppm; Y = 300 ppm; La = 5 ! ppm; Sr = 3800 ppm). Mo, Ag, Zn and Pb may attain high values in the marbles, but this is a local phenomenon. There are also large quantities o f albite, chlorite and hematite in the feldspathic 'episyenitic' rocks, while anatase has been identified in small amounts. The present reserves of Itataia are shown in Table V. Uranium is evenly distributed in the collophanite, as shown by fission track and microprobe analysis. Two views have been proposed on the nature of the uranium in the collophane. The first is that calcium has been isomorphously replaced by uranium in the collophane structure. The second holds that the uranium is associated with finely dispersed oxides or that it has been adsorbed by the collophane. However, it is significant that no discrete uranium species has yet been found. IAEA-AG-250/14 13! TABLE IV. ITATAIA DEPOSIT: UsOgANDP^O; CONTENTSOF DIFFERENT ORES ORES (%) " 3° s P2O5 U/Th Range Range Hassive Collophanite 0.15 - 0.9 30.0 - 38.0 > 9.7 Feldspathic Rocks 0.07 - 0.15 7.5 - 12.5 > 6.5 Marble (stockwork) 0.04 - 0.10 5.0 - 15.0 > 4.5 TABLE V. ITATAIA DEPOSIT RESERVES (tonnes) Reasonably Estimated TOTAL Assured Additional ЫзОа 48 000 74 500 122 500 Measured and Inferred TOTAL Indicated P2O5 Not Announced > 13 400 000 > 13 400 000 8 . ORIGIN OF THE MINERALIZATION The mineralization evolved during four distinct episodes or stages [27]: ( 1 ) The initial stage was related to the intrusion of a number o f post-orogenic granitic bodies (400—500 m.y.) along zones o f compressional release associated with the large regional faults. Some intense hydraulic fracturing affecting country rocks might have been caused by the intrusions themselves. These granites have an anomalous uranium content of 30 to 40 ppm. However, the P 2O 5 values are above normal for these rock types (0.65% to 1.3%). (2) Intense late magmatic activity o f a deuteric nature accompanied the emplacement o f granites. Dequartzification, albitization, chloritization and apatization transformed the granites into feldspathic rocks ('episyenites' ленды ZafMÀ while albitization, analcitization and chloritization caused the gneisses to retrogress to green-schist facies while the marbles have been partially scapolitized. Euhedral fluor-apatite formed in vuggs and cavities developed during this 132 FORMAN and ANGEIRAS epimagmatic stage. With reference to the French feldspar-episyenites [33], it is probable that this process occurred at temperatures approaching 350°C. Uranium was enriched to an average of 750 ppm and РзОз to an average of 12%. (3) The third stage in the evolution o f the mineralization is characterized by local deformation of the country rock as well as the 'episyenites'. This caused a brecciation of the rocks and greatly increased their porosity. (4) During the fourth and final stage, cryptocrystalline hydroxy-apatite was extensively deposited from saline heated fluids (^-130°C, and approximately 22% equivalent CaC^ [34]), which impregnated the pore-spaces and cavities formed during the previous stage. In addition, there was widespread replacement of calcite and feldspar by phosphate in the marbles and 'episyenites', respectively. Uranium and phosphorus were enriched considerably (average values are in excess of 2500 ppm UgOg and 34% P ^ ) . Mineralization proceeded intermittently owing to pulsatory conditions (similarly with a polyascendant model). Periods of charge (low fluid velocity) alternated with discharge (high fluid velocity), activated by a fluid convection system related to the cooling of 450—500-m.y.-old granite bodies. Transport of large quantities o f calcium phosphate would have been possible only in acid medium (low-pH fluids). Movement o f these fluids in the host marbles provoked an increase in pH, the alkalinity favouring rapid deposition of uraniferous coHophane. Such conditions of rapid deposition are apparently con firmed by the cryptocrystalline fabric o f the collophane and the absence of uranium minerals. The uranium would have been adsorbed by the collophane with, possibly, some replacement of Ca in the apatite. Fluid inclusions in the apatite indicate formation temperatures o f the order of 13 6 ° С and high salinity (^ 19.5 % to 22% equivalent CaCl^ ), while quartz fillings in cavities,in the collophane indicate temperatures of 50°C and low salinity (^"5% equivalent CaCl^), evidence of a distinct decrease in salinity during deposition of the late minerals [34]. The low CO2 content suggests low pressure deposition. Daughter minerals in the fluid inclusions are quartz apatite, Mg, Ca and Ba sulphates, and Mg and Al chlorides. It is not known in what form the uranium was transported in the fluids. However, in view of the predominance of the Cl " anion, it is possible that uranium was transported as chloride. The role of СГ in uranium mineralization is pointed out by several authors [35, 36]. 9. REGIONAL INVESTIGATIONS In view of the economic importance o f the special type of uranium mineralization discovered by the carborne survey, especially the large Itataia ore body, Nuclebrás decided to execute additional regional investigations: ÏAEA-AG-250/Ï4 133 9.1. Airborne geophysics In late 1977, Levantamentos Aerofotogramétricos SA (LA S A ) was contracted to carry out an airborne gamma-ray magnetometer survey in view o f the wide geographical distribution of the uranium mineralization. The main factor in the choice of airborne geophysics as a method of regional prospecting was the excellent exposure, thin soil cover, intermittent stream network, difficulties o f access and, principally, awareness that the mineralization was of a regional nature. Two Norman-Britten Islander and one Douglas DC-3 aircraft were used in the survey in three distinct areas, totalling 38 000 knP. Operational characteristics were uniform and included altitude of 150 m, speed of 220 km -h"' and north- south flight lines spaced 500 m apart. The geophysical equipment used included a gamma-spectrometer 'Exploranium' DIGRS, Model 3001, containing nine 6 in. X 4 in. crystals as well as a 'Geometries' Model 803 magnetometer. The data from 78 000 km o f flight lines were recorded simultaneously in analog form and digitally on magnetic tape. In the processing, 441 anomalous records were identified which, on the basis of essentially physical parameters, correspond to 42 anomalous zones. The data obtained during the airborne gamma-ray survey were analysed in 1978 by Nuclebras personnel. Thirty new anomalies were selected for verification. As a result, fifteen showings o f phosphate-uranium mineralization were recognized, among which four were coincident with the Itataia deposit and two have been correlated with anomalies already discovered by the carborne survey carried out in 1975. The remaining anomalies represented new P-U areas (Fig.7). These anomalies are at present being investigated by field crews. 9.2. Geological mapping An area of 260 km^ around Itataia was mapped on a 1:25 000 scale to investigate the distribution of anomalies which had been detected by the airborne geophysics. These are distributed following marble and granitic intrusives. Geological mapping was carried out using 1:25 000 aerial photographs,and the information was transferred to photomosaic base maps at the field camp. A detailed lithologie and structural map was made which emphasized the economic importance o f the additional uranium-phosphate showings surrounding Itataia. 9.3. ERTS images Multispectral analysis of LANDSAT images was carried out with an Image 100 analyser in order to define characteristic standards of the Itataia mineralization that could be used in the identification o f similar areas. This was not found very satisfactory owing to the relatively small size o f the target. 134 FORMAN and ANGEÏRAS Satellite images did, however, prove to be very useful for the regional tectonic studies that are being undertaken. Part IV FINAL REMARKS The discovery and development o f the Itataia deposit cannot be easily compared with Poços de Caldas on account o f the relative scales of the problems involved. At Itataia the fact that the exploration target was identified from the time o f discovery permitted prospecting and exploration to be carried out in a logical sequence with minimal waste of resources in terms of manpower and funds. However, at Poços de Caldas the initial exploration target was abandoned following the detailed evaluation o f the caldasite deposits and the recognition of a second and more promising target. The good outcrop around Itataia allowed reliable geological maps to be made quickly and cheaply. At Poços de Caldas the outcrop is poor and the weathered zone very thick. Consequently, surface prospecting and mapping were slow and comparatively expensive. At Itataia, the ore bodies appear on the surface while those at Poços de Caldas (Cercado) were found by drilling. The mineralization chemistry of the Itataia deposit is simple although the type of deposit is unusual. The mineral chemistry of the Poços de Caldas deposit is more complex and, in addition, the mineralization is unconventional. At Poços de Caldas, the anomalies were initially identified by aerogeophysics and later confirmed by ground radiometry. Rock chemistry was used to separate the caldasite zones (Zr > U) from the others (U > Zr). The ore deposits and reserves were determined by drilling and underground workings. At Itataia, the anomalies were identified during a carbome survey while geology and ground radiometrics defined the surface expression of the under lying deposits. The subsurface characteristics of the ore body and associated reserves were determined by drilling. ACKNOWLEDGEMENTS The authors wish to express their thanks to M.Ö. Fraenkel, E.B. Bastian and M.H. Waring, who assisted in the preparation of this paper. The work o f all Nuclebras geological staff is acknowledged. IAEA-AG-250/14 135 REFERENCES TILSLEY, J.E., "Poços de Caldas", in Uranium Exploration in Brazil, David S. Robertson & Associates, Toronto (1974/1976); Int. Rep. Nuclebrás, Rio de Janeiro (1976). ULBRICH, H.H.G.H., PUCCI, F.G., "DiferenciàçSo mineralógica nos tinguaitos da pedreira Bortolan, Poços de Caldas, MG", in Proc. 30th Congr. Soc. Brasil, de Geología, Recife, Bol.No.l (1978) 93 (abstract). ULBRICH, H.H.G.J., ULBRICH, M.N., BAGNOLI, E., "Estrutura e petrografia do tujaurito de Poços de Caldas, MG", ibid., p.92 (abstract). ULBRICH, H.H.G .J., VLACH, S.R.F., "O nefelina sienito hibrido do Maciço Alcalino de Poços de Caldas, MG", pp.93—94 (abstract). SANTOS, R., "Geology and mining development of the C-09 uranium deposit", Uranium Deposits in Latin America: Geology and Exploration (Proc. Regional Advisory Group Mtg. Lima, 197S),IAEA, Vienna (¡9 8 1 ) 533. ELERT, R., Contribuiçâo a geología do Maciço Alcalino de Poços de Caldas, Bol. Fac. Fil. Ciênc. e Letr., Univ. Sâo Paulo, No.237 (Geol. 18) (1952). FORMAN, J.M.A., The geology and zirconium ore deposits of the Poços de Caldas Plateau, Brazil, MSc Thesis, Dept, of Geology, Stanford University (1966). AMARAL, G., et al., Potassium-argon ages of alkaline rocks from Southern Brazil, Geochim. Cosmochim. Acta 31 (1967) 117. DUTRA, C.V., O método Pb/a e idades de zircôes do Maciço Alcalino de Poços de Caldas, MG, Bol. Inst. Geología, Ouro Preto, 1 (1 9 6 6 ) 125 —135. DERBY, O.A., On nepheline rocks in Brazil, with special reference to the association of phonolite and foyaite, Part I, Q.J. Geol. Soc. London (1887). HUSSAK, E., Mineralogische Notizen aus Brasilien, III. Theil, Zur Kenntniss der sog. "Favas der brasilianischen Diamantsande und über die mineralischen Begleiter des Diamants von Brasilien", Tschermaks Mineral. Petrog. Mitt. 4 (1889). FRAYA, R:, Zircónio, historia, apiicaçôes e ocorrência, Rev. Min. Met. (Rio de Janeiro) 13(1948). WHITE, M.G., PIERSON, C.T., Sumario da prospecçâo para minerais radioativos no Brasil: Período de 1952 a 1960, Com. Nac. Energia Nuclear, Rio de Janeiro, Bol. No.l (1974) 1 4 -1 6 . WEDOW, H., The Morro do Ferro thorium and rare-earth ore deposit, Poços de Caldas District, Brazil, US Geol. Surv. Bull. 1185, Washington (1957) D-l/D-34. TOLBERT, G., The uraniferous zirconium deposit of the Poços de Caldas Plateau, Brazil, US Geol. Surv., Bull. 1 185-C, Washington (1966) C-l/C-28. OLIVEIRA, A.G. de, Aspectos geológicos da mineralizaçao de uranio em Poços de Caldas, in Proc. 20th Congr. Soc. Brasil, de Geología,Rio de Janeiro, Publ. No.l (1966) 35 — 36 (abstract). GERSTNER, A., Missao Brasil (1961/1966): Relatório Geral de Sintese, Com.Nac. Energia Nuclear, Rio de Janeiro, Bol. No.2 (1974). OLIVEIRA, A.G. de, Uränio no Planalto de Poços de Caldas, in Proc. 22nd Congr. Soc. Bras, de Geología, Belo Horizonte, Resumo das Com. (1968) 30 (abstract). GORSKY, V.A., GORSKY, E., Contribuiçâo à mineralogía e petrografia do Planalto de Poços de Caldas, Com. Nac. Energia Nuclear, Rio de Janeiro, Bol. N o.13 (1974). JOURNEL, A.G., Giostatistique minière, V oi.i, Centre de morpho¡ogie mathématique, Fontainebleau (1977). BIONDI, J.C., Relatório de cálculo de reservas, C-09, Internal Rep. Nuclebrás, Rio de Janeiro (1976). 136 FORMAN and ANGEIRAS [22] BIONDI, J.C., A origem dos pipes e diques de brecha — modelo de implosao/fluidizaçào, in Proc. 30th Congr. Soc. Brasil, de Geología, Recife, Vol.3 (1978) 1202—1212. [23] KEGEL, W., Lineament-Tektonik in Nord-est Brasilien, Geol. Rundsch. 54 (1965) 124 0 -1 2 6 0 . [24] BRITO NEVES, B.B., Regionalizaçào geotectónica do Pré-Cambriano Nordestino, PhDThesis, Univ. Sâo Paulo (1975). [25] BRITO NEVES, B.B., et al., Reavaliaçao dos dados geocronológicos do Pré-Cambriano do nordeste brasileiro, in Proc. 30th Congr. Soc. Brasil, de Geologia 6 (1973) 261—271. [26] COSTA, L.A.M. da, PIRES, F.R ., ANGEIRAS, A.G., A preliminary suggestion for a migmatogenic map, Acad. Bras. Ciênc. (Rio de Janeiro) 42 (1970) 5 17—519. [27] WERNICK, E„ HASUY, Y., BRITO NEVES, B.B. de, As regiôes de dobramentos nordeste sudeste, in Proc. 30th Congr. Sociedade Brasil de Geologia, Recife, Vol. 6 (1979) 2 9 4 3 -2 5 0 6 . [28] SANTOS, E., MELLO, C.B.M., Diversidade do plutonismo granítico do Nordeste, ibid., pp. 2 6 2 4 -2 6 3 4 . [29] ANGEIRAS, A.G., NETTO, A.M., CAMPOS, M. de, Mineralizaçâo fósforo uranífera associada a epissienitos sódicos no Pré-Cambriano cearense, ibid., Bol. N o.l, p.341 (abstract). [30] RAMBERG, H., "Model studies in relation to intrusion of plutonio bodies", Mechanism of Igneous (NEWALL, G., RAST, N., Eds), Gallery Press, Liverpool (1970) 27 1 —286. [31 ] FARINA, M., "Perspectivas metalogenéticas de alguns granitos pós-orogénicos do nordeste brasileiro", 8 ° Simposio de Geologia do Nordeste, Atas, Campiña Grande (1977) 121 — 129. [32] ANGEIRAS, A.G., Aspectos preliminares de mineralizaçâo uranífera na área de Itataia, Internal Rep. Nuclebrás, Rio de Janeiro (1977). [33] LEROY, I., Les épisyenites non-minéralisés dans le massif de granite a deux-micas de Saint Sylvestre (Limousin, France), Thèse: Docteur de spécialité, Université de Nancy (1971). [34] FUZIKAWA, K., Estudo preliminar de inclusoes fluidas em rochas do Projeto Itatira, Internai Rep. Nuclebrás, Rio de Janeiro (1978). [35] KOSTOV, I., "Crystallochemical differentiation and localization of uranium ore deposits in the earth's crust", Recognition and Evaluation of Uraniferous Areas (Proc. Techn. Committee Mtg. Vienna, 1975), IAEA, Vienna (1977) 15. [36] RICH, R.A., HOLLAND, H.D., PETERSEN, V., Hydrothermal Uranium Deposits, Developments in Economic Geology, Vol.6 , Elsevier, Amsterdam (1977). DISCUSSION B.L. NIELSEN: The importance of the location of the Cercado and Agostinho deposits at the margin of the secondary craters was pointed out. What is the relation between the craters and the formation o f the tuffaceous breccias of stage 5 o f the intrusive history? J.M.A. FORMAN: Image interpretation indicates that both Agostinho and Cercado deposits are associated with the secondary craters formed during stage 4 at the same time as the tuffaceous breccias. Nevertheless, intense weathering has not allowed a more direct correlation which could only be done with further drilling. As the main targets, at the moment, are elsewhere (Itataia, Lagoa, Real, etc.), drilling will only be done in the future in order to confirm the image interpretation. IAEA-AG-250/14 137 B.L. NIELSEN: Have you any information on whether uranium has been introduced from the intrusion into the surrounding gneiss terrain; for instance, into the regional fault structures? J.M.A. FORMAN: No uranium has been found in the basement rocks surrounding the plateau. Some uranium values are found in the basement but associated with an alkaline structure, to the south-east of the plateau, which does not.outcrop but can be easily seen in the satellite images. P. BARRETTO: You indicated that the post-orogenic granites have 30—40 ppm uranium, sometimes up to 100 ppm. Where is this uranium located? A.G. ANGEIRAS: We don't know at the moment. However, when rock samples are treated with nitric acid 2jN and 30 minutes o f slow heating, 70% to 80% of total U^Og is leached. The granites are also anomalous for РзО$ (0.65% to 1.32%) so maybe the uranium is associated with apatite crystals. P. BARRETTO: The age o f these granites is approximately 500 m.y. I suppose these are К /Ar age determinations and refer to remobilization o f the old shield rocks. Is this correct or are they of the intrusive type? A.G. ANGEIRAS: The ages have been determined by К /Ar and Rb/Sr methods (see Ref. [27]). The ^Sr/^Sr isotopic ratio indicates that such granites are formed at the lower crust by fusion. They are in fact stock and plug-like intrusions of a post-orogenic magmatism, at the closure o f the Brazilianne Cycle in South America. It should also be mentioned that anomalous U 3O3 values have been found in 450-500 m.y. granites from different places in Brazil. H.D. FUCHS: I understand you use episyenitization just as a descriptive term, describing a depletion of quartz and introduction of feldspar (albite). A.G. ANGEIRAS: Besides the dequartzification and strong albitization (Na^O up to 10%) there was widespread chloritization of the biotites, dis appearance o f the К feldspars and haematitization. The sequence of alteration conforms to this described by Leroy [32] for the French feldspar episyenites, including the anomalous РдО$ values (these are by far higher in the Leará uraniferous district). As the mode o f occurrence, source o f the vuggy, quartz-poor rocks of the district, there is some similarity with typical French episyenites. For the Itataia deposits, since the type of mineralization is unusual, the term is being used as episyenites sensu latu as feldspathic rock is a rather descriptive expression. M. MATOLÍN: Are the results of gamma surveys in Brazil (airborne, carborne, ground) reported in relative values (cps) or are the instruments calibrated? J.M.A. FORMAN: Ground surveys are reported in cps and all the calibration of instruments in specially prepared calibration pads. Aerial surveys are reported in U, Th, К and total count channels; readings are corrected for cosmic and aircraft background. Calibration pads are being built at a selected airport. P. BARRETTO: One and a half years passed between the discovery of the uraniferous block and the discovery o f the outcropping collophanite. What was the reason for this delay, and what led to the decision to return to the area? 138 FORMAN and ANGEtRAS A.G. ANGEIRAS: The boulder anomaly was considered to have priority in mid-1976 on account of the occurrence. The decision to return to the area one year later was due to Nuclebrás' policy o f field investigation of all radiometric anomalies proven, after chemical analysis, to be uraniferous. Besides, the value of L^Og for the boulder was found to be the highest of the twelve anomalies detected by the car-mounted survey. J.M.A. FORMAN: Remember I said at the beginning that it is very important in this business o f uranium exploration to keep an open mind. We had a few problems with our geologists when a boulder was found with a type of mineralization not known elsewhere and no-one wanted to pay any attention to it. But since the boulder contained about 2 0 0 0 ppm of uranium, the decision was made to see what it was in spite o f the fact that it was not known elsewhere, and I think this is very important. If you try to tie every deposit to a genetic type that is well known and well described you might overlook something like Itataia. D. TAYLO R: How do you get the uranium out o f the ground and out o f the ore? J.M.A. FORMAN: The mineralization phenomenon is regional. In Itataia, which is the smaller area (2 km^), as Mr. Angeiras mentioned, there is a lot of phosphate; for the whole o f the deposit we have something like 1 1 % average P2O5 . Physically it is possible to concentrate that to bring it up to about 30% P2O 3 . The process is to produce the phosphoric acid and get uranium out of that if you have a production o f phosphate and uranium. This is a common technique on sedimentary phosphates ranging up to 2 0 0 ppm uranium; it is feasible and economical. But in the case o f much higher grades in Itataia, this is easier than the present plants in France and Florida that produce uranium from phosphate. D. TAYLO R: Could you describe the uranium extraction process to be used at Itataia? Details of anticipated costs and recovery would be most interesting. J.M.A. FORMAN: At present, ore-dressing studies are being carried out and the fluxogramme for the process is: Mechanical preparation of the ore Physical concentration of phosphate and uranium by flotation Acid leaching Filtration Clarification of the phosphoric liquor Extraction and re-extraction by solvents Uranium precipitation Calcination o f the precipitate to UO 3 Concentration of the phosphoric acid (free from uranium) by evaporation Fertilizer production ÍAEA-AG-250/14 139 The costs will depend on the scale to be chosen for the mill. 500 tonnes per year of uranium concentrates will be produced at under US $40 per lb, with phosphoric acid at competitive costs, at the prevailing market prices. Recovery for both uranium and phosphate is above 94%. IAEA AG 250/16 EXPLORATION OF THE URANIUM REEFS OF COOKE SECTION, RADFONTEIN ESTATES GOLD MINING COMPANY (WITWATERSRAND) LTD, SOUTH AFRICA B.D. STEWART Johannesburg Consolidated Investment Co. Ltd, Johannesburg, South Africa Abstract EXPLORATION OF THE URANIUM REEFS OF COOKE SECTION, RANDFONTEIN ESTATES GOLD MINING COMPANY (WITWATERSRAND) LTD, SOUTH AFRICA. The Cooke Section ore bodies he in the South West Witwatersrand, South Africa, adjacent to the outcropping Centra) Rand deposits which have been exploited on a large scale since 1886. Since the early 1900s several programmes have been carried out to locate the south-westerly, subsurface extensions of the Central Rand deposits. The programme which ultimately led to the delineation of the Cooke Section deposit was initiated in the early 1950s. This programme covered an area roughly 150 km^ and holes were spaced out at intervals of 3 to 5 km. Some positive results were obtained, and this led to a follow-up programme in which 8 8 holes were drilled, resulting in the Cooke Section mining operation. Payability was found to occur in a group of conglomerates known collectively as the Middle Elsburg reefs, but not in the well-known reefs of the Central and West Rand. The Middle Elsburgs, though auriferous and uraniferous, have not been found payable along the Central Rand, and were not the primary target of the initial programme. It is estimated that the total resources amount to 72 million t of ore, of which about 36 million t are considered to be viable under current economic conditions. This case study clearly demonstrates that an exploration programme must always be flexible enough to cater for the unexpected developments that are usually encountered. The exploration emphasises the need for patient, systematic analysis of all data, particularly when the ore body being prospected is deeply buried. 1. INTRODUCTION The South African gold and uranium deposits are contained within an elliptical sedimentary basin o f early Proterozoic age, known as the Witwatersrand basin, which is situated in the central interior o f the country (F ig.l). These deposits are unique in many respects, and therefore much o f what will be described in this paper may be applicable only to other Witwatersrand-type deposits. The purpose o f the paper, however, is to describe how the complicated stratigraphy, sedimentology and structure o f the ore bodies was unravelled, and to 141 4 STEWART 142 F/C.A íocaíK?H o/ íAe ¿ял'л. 144 STEWART IAEA AG-250/t6 145 stress the importance o f careful analysis and re-analysis of all geological information when carrying out an exploration programme. At the outset it must be stressed that uranium, important as it may be, is only a by-product o f the South African gold-mining industry and therefore, historically, the motivation for prospecting the Witwatersrand deposits was on the basis o f its gold potential. It is only in the past few years that uranium has in certain instances been elevated to that o f a co-product. Cooke Section is a recent extension to a, very large, and old, gold mine called Randfontein Estates Gold Mining Co. Witwatersrand Ltd (REGM), a subsidiary of Johannesburg Consolidated Investment Co. Ltd (JCI) which is situated in the West Rand gold field (Fig.2). The new extension was found in an area referred to as the South West Witwatersrand, which lies adjacent to a very large field known as the Central Rand. There are a number o f reasons why it remained undetected for about seven decades after the discovery o f gold near Johannesburg. The most significant are that: it is concealed beneath younger formations; it occurs in a structurally complicated area; the deposits are relatively small and highly variable; and, perhaps most important, it occurs in a part o f the stratigraphy that had never before yielded any economic values. A further contributing factor is that the conglomerates that produced enormous quantities o f gold in the Central Rand were considered to be non-economic in the South West Witwatersrand and the area was therefore by-passed in favour of more lucrative targets. 2. LOCALITY Cooke Section lies some 40 km west o f Johannesburg, and a few km south west of the most westerly of the Central Rand mines. It is situated on the farms, Luipaardsvlei 243 IQ, Gemsbokfontein 290 IQ, Panvlakte 291 IQ, and Gemspost 288 IQ (Fig.3). The area was settled more than a century ago, and REGM started mining operations in 1890. Over the years it acquired surface and mineral rights covering large tracts o f ground, including Panviakte, which it purchased in mid-1890. Prospecting on this farm, however, only started some 55 years later. 3. HISTORICAL BACKGROUND Although the Cooke Section deposits were only located in 1962/3 (i.e. about 76 years after the discovery of gold in the Central Rand), much was known about the Witwatersrand geology in general and about the South West Witwatersrand in particular. Prior to its discovery, the area had been comprehensively mapped, assorted papers had been written on a variety of geological aspects, and the activities of the mining companies operating in the area and in the surrounding fields were well known from newspaper articles and published company reports. 146 STEWART While no formal documents reviewing all this background are to hand (Pretorius [ 1 ] has reviewed the development o f geological thought for the basin as a whole), a great deal o f what had been learnt over the years was built into the prospecting philosophy which resulted in the discovery o f Cooke Section. This understanding is reflected in some o f the early structural plans. To appreciate the problems encountered during the exploration, a brief description of the geology and the prospecting episodes is necessary. 3.1. Regional geological setting The surface geology was mapped between 1910 and 1915 by E.T. Mellor [2] and published in 1917. This map has been found extremely reliable and has served as a base for much of the subsequent exploratory work. From a summarized version of the map (Fig.2) it is noted that the Central Rand gold mines lie along the Upper Witwatersrand System outcrops, with the quartzites and interbedded conglomerates striking east-west and dipping southwards. These are almost conformably overlain by lavas o f the Ventersdorp System. The Witwatersrand rocks extend relatively uniformly to the west as far as Durban Roodepoort deep gold mine, at which point they start to curve to the south-west before being terminated against an upthrown block, known as the Witpoortje horst (or gap). At this position the Witwatersrand rocks are buried by younger rocks of the Transvaal System which form an extensive blanket persisting for tens o f km. Thus all the extensions o f the Witwatersrand ore bodies are buried at depths ranging from zero to more than 3000 m. ïn the Cooke Section area, the Transvaal rocks are 300 to 1000 m thick. A more detailed version o f the surface geology of.the South West Witwaters rand is shown in Fig.4 (based mainly on Mellor's map, but extended southwards by Cousins [3], and the stratigraphie column' is shown in Fig.5. Payability has been found in each of the seven zones o f conglomerate in the Upper Witwatersrand System, and each o f these groups may contain several payable conglomerate bands. In most cases, however, only one or two reefs have been found to be payable on any particular mine, while on some mines three or four conglomerates from perhaps two reef zones have proved mineable over large areas. The major exceptions to this are the mines of the West Rand basin where at least 23 different reefs from five reef zones have been mined. Some o f those, for example the White Reef on parts of REGM, have been mined as primary uranium ore bodies. ' The South African Geological Survey is in the process of introducing lithostratigraphic nomenclature for the entire South African stratigraphie column. Since this is not available in its final form, the original (and familiar) nomenclature is used in this paper. IAEA-AG-250/Í6 147 -WG.4. №eíf H^i'fwaíer^a^d iMr^acegeoZogy ['a/fer AfeZ^ora^d CoMMí [2,3]. 4^ oo g s UJ SYSTEM 1 >- (Л : Г) Upper Government z < ART STEW о: Shales 0) rr û: UJ О ûi Leer ceno,,*- Queras Ш < z CL < CL ^ ! z Shdes or,d 0^.л,= в F/C. J. Сенегя/zzeí? (7б)/м^я о / H^.yf [2 ]/ IAEA AG-250/16 149 3.2. Nature o f ore bodies The ore bodies of the Witwatersrand are congiomerates which vary from a few cm in thickness to as much as 6 m and occur as extensive tabular sheets covering areas from a few to many tens o f krrP. Although the conglomerates are extremely extensive along strike, they degenerate rather rapidly down the paleoslope. Consequently the zone of payability is found to be a narrow strip extending several tens o f km along strike, but only 4 to 6 km down the paleoslope. Much work has been done on the sedimentology o f these deposits, yet geologists are still at variance as to the exact nature o f the sedimentary environments in which they were deposited. The general consensus is that they are either fluvial or shallow marine in origin, with the gold and uranium concentrated syngenetically. As already mentioned, the ore bodies are primary gold deposits but virtually all o f them contain some uranium. Although there is usually a good correlation between gold and uranium content, the proportion o f uranium to gold varies from reef to reef. Furthermore, the ratio for an individual reef tends to vary gradually, but in a systematic way, across the ore body, the higher ratios usually being found in the more distal parts o f the depository. In low-grade uranium reefs the ratio may be as low as 5 to 1, while in ore bodies where uranium is the major product it may be in excess o f 1 0 0 0 to 1 . 3.3. Neighbouring mines Before the preliminary drilling started in the vicinity of Cooke Section, mines were operating to the west, north and north-east, and after completion of the preliminary drilling, but before the detailed prospecting started, an additional mine came into production to the south. Nevertheless, certain aspects o f the structure were not resolved, and the detailed stratigraphy in the upper part o f the system had not been described. It was also recognized that the area being examined was rather distant from the edge o f the basin, and there was therefore much speculation as to which conglomerates could still be payable so far down the paleoslope. 3.4. Previous drilling in the area Attempts had been made to locate extensions to the Central Rand and West Rand ore bodies from about the beginning o f the century, and various programmes were conducted in the first decade. Further programmes were carried out in the early 1920s and again in the mid to late 1930s. The positions o f these holes are shown in Figs 6 and 7 [4]. Most work was carried out in the northern part of the area and served to elucidate some o f the subsurface geology; specifically, it demonstrated that the Witpoortje horst swung southwards to the east o f Venterspost and Libanon gold 150 STEWART F/G. 6. proipecf^nHmjpri-iP/^. IAEA AG 250/16 151 F7C. 7. Г/:е ScMf?! M'fwafewancí.' pro^pecf 7 P7 J - ^PJO. ÏAEA-AG-250/16 153 -WG.9. ГАе H^'fwaferírand.' proípecídr¡Hi'n^/P30-7P60. 154 STEWART mines. Thus it could be shown that these mines were not part o f the Central Rand structural block. A simplified interpretation of the sub-Transvaal structure is shown in Fig.8 . The drilling also provided information about the stratigraphy and the approxi mate elevation of the major reef zones. However, while some payable intersections were reported from the Main Reef zone of conglomerates, none of the driHing gave more than a vague hint that the Middle Elsburg reefs were developed, let alone highly payable, even though one borehole actually traversed the economic zone some 700 m to the north-east o f Cooke No. 1 shaft system. This borehole, called LG (drilled between 1938 and 1940), actually penetrated a part o f the mine where recovery grades are about 14 gt'* of gold and roughly 140 ppm U^Og, but failed to intersect the reef because it was cut out by a dyke. The results o f this work are not compiled (some data is to hand and some results were verbally communicated), but a proportion o f what was learnt was built into the prospecting programmes of the early 1950s when a series of holes were drilled on a wide grid across Gemsbokfontein, Panvlakte, and the area to the south (Fig.9). During this phase, a programme o f ten rather widely spaced holes were drilled in the vicinity of Cooke Section, the actual positions being chosen to meet geological objectives. Eight o f these were intended to test the potential of the Main-Bird Series conglomerates on the horst, while the remaining two were to check the structure and to test the potential of the reefs in the downthrown block to the east o f the horst. As part of this programme, a further series of holes were drilled on Waterpan 292 IQ and Modderfontein 345 IQ. This drilling met with almost immediate success, with good values intersected in the Upper Elsburg reefs, and thus the initial exploration programme was expanded to an evaluation programme, and some ten years later, in 1960, Western Areas Gold Mining Co. Ltd (WAGM) began. North of WAGM, one o f the holes intersected an 'interesting' value, but owing to the WAGM evaluation programme, follow-up driHing was deferred until 1961. 3.5. Geophysics Gravity and magnetic surveys were carried out in the area to assist in the interpretation of the structure. Once it had been realized that the horst block swung southwards through the area, it seemed reasonable to use a gravimetric technique to delineate the position o f the lava mass, which was believed and subsequently demonstrated to be present to the east of the fault. This proved unsuccessful since the anticipated anomaly resulting from the density difference between the lavas and quartzites was masked by the thick cover o f dolomites. However, a survey carried out in the western part of Luipaardsvlei suggested a significant fault paralleling the main IAEA-AG-250/16 155 structure with a downthrow to the west. To date it has not been possible to confirm this displacement. Airborne and ground magnetic surveys were also carried out, the objective being to locate and trace the magnetic horizons of the Lower Witwatersrand System as was done by Krahmann [5] along the West Witwatersrand Line. Although Krahmann was very successful in tracing these magnetic beds below the Transvaal cover, no anomalies were detected, and thus it was deduced that none o f them sub-outcropped immediately below the dolomites and must in fact be deeply buried below younger Witwatersrand rocks as well as the overlying systems. 3.6. The uranium boom The presence o f radioactivity in the Witwatersrand conglomerate was first suspected by Pirow [ 6 ] as early as 1920; in 1924 Cooper [7] identified uraninite in concentrates gathered from Central Rand mines, it was not, however, until the uranium boom started that prospect boreholes in the South West Witwatersrand were assayed for uranium (actually L^Os). Prior to the 1950s only gold assays were carried out and thus an intersection was considered to be payable on its gold content alone. Once the importance o f uranium in the conglomerates had been appreciated and systematic sampling results became available, it was found that many conglomerates previously considered subeconomic had now become viable. This provided an added stimulus to prospecting and made the mining houses look very closely at the marginal gold deposits. 3.7. Conclusion This then is the background against which the programme to prospect the Cooke Section areas was initiated. The general setting was understood but there were many gaps in the detail. The prime target was gold, although a contribution from uranium was recognised as a distinct possibility. The major reef zones had been intersected in places, with disappointing results, and where payability was found it was very deep (below 2000 m). One anomalous value occurred in borehole GB 2, and this was considered worthy o f follow-up work. 4. COOKE SECTION EXPLORATION Borehole GB 2 was one o f the holes sited in the downthrown block, and although the values were not spectacular, it was decided to drill one follow-up hole in the vicinity. This hole, GB 3, proved successful, intersecting a 1.32-m-thick Middle Elsburg conglomerate at a depth o f 968 m below surface and having an average value o f 20.0 ppm o f gold and 690 ppm UgOg. 156 STEWART 4.1. Geology of the prospect area The general geology o f the area to be prospected was reasonably well under stood when drilling started in 1962. A contour plan o f the base o f the Transvaal System, showing a fairly flat but slightly channelled surface, was available, and early structural interpretations of the pre-Transvaal geology proved reasonably accurate. From drilling it was found that the Transvaal rocks ranged from 300 to 700 m thick. The dominant feature of the structure is the horst block which runs approximately from north to south (Fig.8 ) and is bounded on the east by the Panvlakte fault. This fault was believed to have a downthrow to the east o f at least 1 0 0 0 m. To the west o f the fault, only rocks belonging to the Main-Bird Series had been intersected, and it was assumed that the remainder of the succession, as well as all the overlying Ventersdorp rocks, had been eroded during the pre-Transvaal peneplanation. To the east o f the fault it was believed that almost the entire Witwatersrand succession was preserved, as well as some o f the Ventersdorp lavas. Indications o f a gentle anticline (or monocline) had been found, and it was there fore anticipated that across this fold axis the uppermost beds, including the Upper Elsburg conglomerates, would be absent. The regional strike was believed to be roughly north-south, the dips on the horst being in excess o f 45° to the east. Dips on the downthrown side were believed to be between zero and 1 0 ° to the east. The stratigraphie column as described by Mellor [2] had been found to apply in the area, although the description was too generalized to apply in detail. It should be noted that although the target was the Middle Elsburg reefs, they were generally found to be very poorly developed along the outcropping Central Rand, so much that Mellor made no reference to them in his original description. This is why little attention had been paid to these conglomerates. Although they had been intersected in boreholes drilled elsewhere in the area, descriptions o f the reef zone itself were inadequate and those of the hanging wall quartzite succession were extremely generalized. The quartzites lying between the Upper Elsburg and Kimberley reefs were usually described as being 'monotonous'. It was clear from the geology o f neighbouring mines and from the holes drilled in the area, that the Ventersdorp lavas transgressed across the Upper Witwatersrand rocks. It was to be expected therefore that in those parts of the prospect area where lava separated the Witwatersrand from the Transvaal rocks, the lava could rest on any part of the Kimberley/Elsburg series. Although the regional structure was considered to be relatively simple, it was anticipated, in keeping with typical Witwatersrand ore bodies, that faults ranging in size from 1 to 2 0 0 m would be intersected in the boreholes. The target as it was in 1962 was a zone o f conglomerates, generally poorly developed in the Central and West Rand, but shown to contain payable values in ÏAEA-AG-250/16 157 ÆiC.70. pT-oípee^WZZi'n^iPóO —7970. 158 STEWART the South West Witwatersrand. The ore bodies were believed to occur at approxi mately 1 0 0 0 m below surface, and unconformably overlain by the younger systems o f rocks. No clearly defined stratigraphie marker beds had been described in the hanging wall o f the reef zone and there was therefore a distinct chance that the ore zone could be missed, particularly since the amount o f conglomerate in the zone represented less than 10% o f the section, the remainder being quartzite. The presence o f faults and dykes made correlation even more problematic. 4.2. Drilling programme At the start o f the programme the area held was about 150 krn^ and included most o f Gemsbokfontein and Panvlakte, and part o f Luipaardsvlei and Gemspost. Part o f the area was held by REGM, while the remainder was owned by JCÎ. The prospecting was planned and supervized by JCI's own staff, and most of the drilling was done by the JCI Drilling Division, though drilling contractors were used at times. The initial plan was to drill holes on a 2500-ft (800-m) grid, but as results became available this was modified. Ultimately, the spacing of the holes was roughly 800 m along strike (i.e. the north-south direction) but at times holes were drilled as close as 300 m in the dip direction. A total of 8 8 holes were drilled in and around the Cooke Section lease (Fig. 10). In the so-called Cooke 3 area, which is the most uraniferous part o f the property, a total o f 30 holes and 49 000 metres were drilled, which cost US $1.04 million. A similar programme would cost at least US $3.0 million in 1979. 4.3. Drilling procedure The standard procedure for gold prospecting in South Africa is to prospect by means o f diamond core drilling, using narrow-diameter holes — either Nx or Bx which are, respectively, 7.6 or 6.0 cm in diameter. This was the method used to prospect the South West Witwatersrand and most o f the holes were drilled Bx diameter. Sullivan-50 drills were used and operated continuously for 136 hours each week, closing down on Sundays. The crew per rig was three drillers and fourteen labourers working three shifts per day. The prospect area is covered by a thick succession o f Transvaal rocks, consisting almost entirely of dolomitic limestones in which are narrow chert bands. These rocks decompose at surface to form a thick clayey residuum in which are fragments of chert. This unconsolidated layer varies from 20 m to 80 m thick and, because of this, the upper part of the hole was piloted using a percussion-type drill. Once core drilling started, the entire hole would be cored and thus an almost complete section would be available for examination. On completion o f the hole, wedges would be placed above the reef zone in order to make additional inter IAEA-AG-250/16 159 sections o f ore horizon. The standard procedure was to obtain four intersections, but in exceptional cases (poor core recovery, or large variation in reef development) additional deflections would be made. On average, each hole took from four to five months to complete. The major part o f the drilling programme was completed in five years, although the latest drilling in the area was as recent as 1976. 4.4. Core handling All the core was transported to the JCI Drilling Division's base camp (which coincidentally is situated very close to the prospect), where it was logged. Most of the dolomite and lava cores were discarded after logging, but selected complete sections were preserved. Withwatersrand System cores, on the other hand, were boxed for permanent storage in core trays holding 30 ft (9.1 m) o f core. These cores would then be available for further study. After boxing and logging, the entire succession, at times more than 1 0 0 0 m, was sampled for gold and uranium. The sample widths were 60 cm in rock where no values were anticipated, and approximately 30 cm where values were possible. Gold was determined by the standard fire assay technique, and uranium (reported as UgOg) by a radiometric method. Both uranium and gold assays were checked periodically at different laboratories. 5. PROSPECTING RESULTS Soon after the positive result had been obtained in borehole GB 3, six holes were started simultaneously, three to the north and three to the south of the positive intersections (GB 2 and GB 3). The results o f this drilling produced a very confusing picture since the sequences intersected were all quite different. Although it was subsequently shown that four o f the six holes intersected the Middle Elsburgs, the sequences intersected in the holes were surprisingly dissimilar. Not only was it difficult to correlate them stratigraphically, but these initial driHing results confused the structure rather than elucidated it. Typical o f the kind o f results obtained are those of boreholes PV 5,16 and 31 in the Cooke 3 area. Though all three have actually intersected the major gold and uranium reef in the zone, they are all very dissimilar. Figure 11 shows the major lithologies and systems intersected as well as the positions o f the best values. However, when the quartzites were examined in detail (depicted diagrammatically in Fig. 12) it was very difficult to correlate distinctive bands from one hole to the other. The collar positions o f the three boreholes are shown in Fig. 13. It was evident early in the programme that the development o f a detailed stratigraphie column was necessary, and that only once this was available could 160 STEWART PV5 PV 16 PV3! -ЬгЕ PE SE ywvv TRANSVAAL SYSTEM VENTERSDORP SYSTEM 1WITWATERSRAND Dyke 1- J SYSTEM 4 2 0 0 m HIGH-GRADE REEF MODERATE-GRADE REEF LOW-GRADE REEF F/GJ A CcoAe ^ íecfíon.' geoíogicaí low ing ¡¡'fhoiogiM. :AEA-AG-250/!6 PV 5 P V 16 PV3) F7C..Z2. Coo^e^jecfMM.' ¿efa;7e¿bore/¡oíecoZMmn& 162 STEWART ÏAEA-AG-250/16 163 each borehole be subdivided into stratigraphie units and the shape, size and attitude of the ore body be pieced together. 5.1. Development of stratigraphie column The succession in which the payable conglomerates occur are mainly medium- grained, grey or greenish-grey quartzites. There are coarse and finer phases, but the quartzites are in general rather uniform and, after a cursory inspection,.may be described as monotonous. However, after very careful examination, distinctive variations were recognised and certain of these were found persistent enough to be used to subdivide the stratigraphy. . Unfortunately, few boreholes were described in sufficient detail to allow the logs to be used for developing a stratigraphie column. The cores therefore had to be re-examined. The method used was to select two holes which, from the logs, appeared to . have similar successions. The core trays for the entire sequence of quartzites and conglomerates were laid out in long rows (at times as many as 80 trays in a single row), with the holes to be compared laid out side by side so that the anticipated correlatives were adjacent to each other. In this way, it was simple to compare the details and thus recognise such similarities as existed. Once a part of the sequence was found to be common to the two holes, the entire rows o f core were moved so that these comparable parts of the sequence were alongside each other.. The rest of the succession was then compared in detail. This process was repeated with additional holes. It was soon found that the finer-grained, denser quartzites were the most persistent and were therefore used as marker bands to split up the succession. Once this was done it was possible to compare the lithologies between the marker bands. Special attention was paid not only to the dominant rock type but also to the nature of any variations that might be present and to the type of contacts between different quartzite types. In this way a crude stratigraphy was developed. This was then tested against other holes, using the same technique of direct comparison. As the result of this detailed examination, significant variations were recorded, not only in the actual lithologies comprising a particular unit, but also in the thicknesses of individual units. The presence of faults and dykes was an additional complicating factor, and it was therefore several months and at least 2 0 holes later before a reasonably reliable column was generated. The stratigraphie units were named by a set of symbols in which letters and numbers were used. The reef zone lies within the major group of rocks designated by the letter E. This group was further sub divided into nine units and these were named, from bottom to top, El to E9. In those cases where further subdivision was possible, letters were again used, and thus the E9 unit was divided into E9A to E9G subunits, once again from bottom to top. 164 STEWART F/CJ4. Соойе ^ леся'оя.' FPF/c Mrani'MfT! ^муп'ймй'ои. ÍAEA-AG-250/16 165 Соо^е^^еся'ои.' yfopeí/ü'c^neíío/FPF/cree/. 166 STEWART fVG .id. CcoÆe J лес?;'оп.' FPF/c Ю МГ7а морссА. IAEA-AG-250/! 6 167 Overlying the E group of units is the UE group, within which are seven units, although only the very basal part of this sequence (the UE1 unit) is considered part o f the Middle Elsburg reef zone. This study was carried out concurrently with the drilling. Once the stratigraphie column was available, all the holes in the prospect were zoned in detail and only then could the continuity o f individual conglomerates be demonstrated and the structure of the area elucidated. 5.2. Middle Elsburg reef zone From these detailed studies three important conglomerates were found to be present in the Middle Elsburg reef zone. These were traced from-hole to hole and it was thus possible to indicate not only their lateral extent, but also the distribution of their gold and uranium content. Figures 13 and 14 depict such distributions for the E9E/c reef. Similar plans have been prepared for all the potentially economic conglomerates. It has been found that simple contouring o f borehole values can be misleading. However, since these deposits are clearly detrital in origin, it would be anti cipated that sedimentological and stratigraphie maps o f the area would show similar trends to those o f the gold and uranium distribution. Two such plans indicating the thickness of the E9E/c reef and an isopach map for the stratigraphie interval from the E9E/c to the UE1A are shown in Figs 15 and 16 respectively. Distinct similarities are noted. Similar correlations have been demonstrated for parameters such as pebble size, conglomerate to quartzite ratio, and uranium to gold ratio for the three major reef horizons, as well as for isopachs of a number of stratigraphie units. Much of this work required further reference to the cores, and now some 17 years after prospecting started, the cores are frequently consulted in order to 'fine-tune' the correlation, value distribution and structure. 5.3. Valuation Once reliable grade and reef thickness (or stope thickness) maps are available, it is relatively simple to compute the ore reserves. In the case o f the Middle Elsburgs the presence o f gold and uranium renders the calculation more difficult since each block o f ground must be assessed on the basis of its combined metal content. The problem is magnified these days by the widely fluctuating gold price. In the Cooke 3 area the total uranium resource has been found to be some 72 million tonnes of ore. Of this about 36 million tonnes are considered payable at present. 168 STEWART 6. CONCLUSION The ore bodies in the Middle Elsburg reef zone were found to be payable in the South West Witwatersrand many years after gold was first mined in the vicinity. That they were missed for so long seems surprising, but this was due mainly to the complexity of the Witwatersrand geology. Additional contributing factors were: (a) The reefs are buried beneath several hundreds of metres of younger formations; (b) Payability was not anticipated in the Middle Elsburgs; and (c) The early prospecting targets were the Main Reef conglomerates and, once those were considered unpayable, interest in the area waned. Even when payability was found in the initial boreholes, it still required a tremendous amount of closely spaced prospect drilling and many hours of detailed examination of the cores before it could be shown that sufficient reserves existed to sustain a mining operation. This case history demonstrates the need to have as detailed an understanding of the structure, stratigraphy (with special reference to the conglomeratic zones) and, if possible, the nature of distribution of the mineralization in the area before detailed prospecting starts. Detailed compilation o f all data at the outset is essential. Much research is being carried out by the South African mining houses and the Transvaal and Orange Free State Chamber o f Mines to develop short-cut methods (such as trace-element chemistry to facilitate correlation) to aid the prospecting geologist. However, in the final analysis, the tried and tested method of drilling fully cored boreholes is by far the most successful. Magnetometric and gravimetric surveys have also been found useful in many, but not all, areas. All programmes must be sensitive enough to recognise all potential ore bodies irrespective o f the circumstances in which they occur, without becoming excessively cumbersome and costly. It is also essentia! that careful studies o f all cores be carried out, particularly those where indications o f payability are observed, however slight they may be. ACKNOWLEDGEMENTS The author would like to record his thanks to all his colleagues with whom he worked during the prospecting of the Cooke Section. It was as a result of many informal discussions that a large amount of unpublished information was communicated and many o f the problems relating to the geology o f the area resolved. IAEA-AG-250/16 169 A special word of thanks is due to Mr. E.H. Jones (Consulting Geologist of JCI), who supervised the prospecting and subsequent interpretation of the result, for his invaluable guidance and objective criticism. Thanks are also extended to the staff o f JCI Geological Department for their assistance in preparing the text and diagrams and, finally, to the management of Johannesburg Consolidated Investment Co. Ltd for permission to publish this paper. REFERENCES [1] FRETORIUS, D.A., The depositional environment of the Witwatersrand goldfields: A chronoiogicai review of speculations and observations, Miner. Soc. Eng. 7 1 ( i 975). [2] MELLOR, E.T., The geology of the Witwatersrand — An explanation of the geological map of the Witwatersrand Goldfield, Geol. Surv. S. Afr. (1917). [3] COUSINS, C.A., The stratigraphy, structure and igneous rocks of the Transvaal System at Western Areas Gold Mine, Trans. Geol. Soc. S. Afr. 65 2 (1962). [4] TECHNICAL MAP SERVICES, JOHANNESBURG, Borehole Data, Witwatersrand System (Orange Free State and Transvaal) (1950). [5] KRAHMANN, R., The geophysical magnetometric investigations on West Witwatersrand areas: between Randfontein and Potchefstroom, Transvaal, Trans. Geol. Soc. S. Afr. 39 (1937). [6] PIROW, H., Distribution of the pebbles in the Rand banket and other features of the rock, Trans. Geol. Soc. S. Afr. 23 (1920). [7] COOPER, R.A., Mineral constituents of Rand conglomerates, J. Chem. Metall. Min. Soc. S. Afr. 24 (1924). DISCUSSION J.M.A. FORMAN: Do the proximal/distal facies concepts apply to this deposit? What are the ratios o f uranium to gold? B.D. STEWART: Distinct proximal and some distal facies exist, although they differ in certain aspects from those found elsewhere in the Witwatersrand. U/Au ratios vary across the deposit, being lowest in the proximal facies and highest in the distal facies. J.M.A. FORMAN: Do you find differences in the colour of the quartz pebbles associated with uranium? B.D. STEWART: No correlation between the colour of the quartz pebbles and the uranium content has been recognized. We have noticed that in the more uraniferous conglomerates some of the pebbles display a characteristic fracturation. J.M.A. FORMAN: Does organic material (thucholite) play an important role in the Au/U deposition? B.D. STEWART: Organic material is present in reefs and is associated with increased gold values, and in certain instances with enhanced uranium content. 170 STEWART D. TAYLO R: What downhole logging techniques do you use and do they not assist in the correlation between holes? B.D. STEWART : Downhole logging is not carried out by JCI, since all the Witwatersrand cores are sampled for uranium. Some companies are known to have used downhole radiometrics and have had a certain degree o f success — I don't know the exact degree. R. GATZWEILER: How do you determine the payability of an ore block? Do you treat one element as the major cost bearing and the other as by-product or are they treated equally? B.D. STEWART : A grid is superimposed over the area, and the mean grade for gold and uranium estimated for each block. From this it is possible to establish the value of the resource or reserve in monetary terms, and they are thus treated equally. This, of course, only applies when both gold and uranium are recovered. D.A. PORTER: Smoky quartz is associated with some sandstone uranium deposits. Is there much smoky quartz in the conglomerates? If so, does it appear to be associated with the uranium? B.D. STEWART : Smoky quartz pebbles are present; their percentage in the Middle Elsburg reefs is not available, but it is probably less than 20%. No association between smoky quartz pebbles and uranium has been recognized. It should be noted that some conglomerates in the Upper Witwatersrand System have a mudi higher percentage of smoky quartz pebbles than the Middle Elsburgs, but in these cases they contain low uranium values (less than 1 0 0 ppm). ÍAEA-AG-250/19 DISCOVERY OF THE JABILUKA URANIUM DEPOSITS, EAST ALLIGATOR RIVER REGION, NORTHERN TERRITORY OF AUSTRALIA J.C. ROWNTREE, D.V. MOSHER Pancontinental Mining Ltd, Sydney, Australia Abstract DISCOVERY OF THE JABILUKA URANIUM DEPOSITS, EAST ALLIGATOR RIVER . REGION, NORTHERN TERRITORY OF AUSTRALIA. The Jabiluka One and Two uranium deposits occur in Lower Proterozoic metasediments of the Cahill Formation. The Jabiluka One deposit coincides with a 'window' through unconformabiy overlying Middle Proterozoic sandstone of the Kombolgie Formation; Jabiluka Two is completely covered by that sandstone. The observed part of the Cahill Formation exhibits four horizons which are favourable hosts for uranium mineralization. These have been folded into an east-southeast-striking, gently plunging, asymmetric flexure. The host rocks are mainly chlorite and/or graphite schists and their brecciated equivalents which have undergone initial regional prograde metamorphism to amphibolite facies, then retrograde metamorphism to greenschist facies. Mineralization consists of uraninite, mainly filling open spaces and to a lesser extent in disseminated form. Chlorite alteration is intimately associated with the uranium mineralization. A portion of the Jabiluka Two deposit contains economic concentrations of gold. Although the deposits are generally stratabound, structural preparation appears to be the most significant ore control on a local scale. The Jabiluka case history illustrates an effective philosophy which was successful in exploration for stratabound uranium deposits in the East Alligator River Region. This philosophy encompassed the following points: (a) The value of regional appraisals in selection of a property; (b) The recognition of the detection limits of airborne radiometric surveys; (c) The importance of ground prospecting for low-order point-source radiometric anomalies which cannot be detected by airborne survey; (d) The importance of evaluating all anomalies and the flexibility to change priorities as further exploration results are obtained; (e) The necessity of establishing the controls on the mineralization before proceeding with further exploration; (f) The necessity of exploring extensions of favourable lithologies to test for periodicity of mineralization even where cover precludes surface expression; and (g) The desirability for modification of exploration techniques on different types of anomalies. 171 172 ROWNTREE and MOSHER 1- INTRODUCTION During th e p e r io d 1960 to 1967 v i r t u a l l y no uranium e x p lo r a t io n was carried out in Australia. Total Australian production of U-Од up to 1975 was 10,000 short tons U^Og of which 7,800 short tOn§ were exported and approximately 2,200 short tons were stockpiled (1). After the revival, of uranium exploration in 1967 a substantial increase in U^Og reserves has resulted primarily from the discovery since 1969 of four major uranium deposits in the East Alligator River Region, approximately 240 kilometres east of Darwin, Northern Territory, Australia (Fig. 1). The four deposits in this Region contain a total of approximately 350,000 short tons of contained U^Og (Table I). Although this paper is concerned with the case history of the discovery of the Jabiluka uranium deposits, it is useful to put the discovery of these deposits in the context of exploration in the East Alligator River Region. Table II outlines the airborne and/or ground radiometric surveys which resulted in the discovery of the four deposits. 2. HISTORY OF THE JABILUKA DEPOSITS Pancontinental Mining Limited acquired exploration rights over an area of approximately 18 0 square kilometres in the East Alligator River Region in 1970 after a regional assessment of the Pine Creek Geosyncline resulted in the selection of this area for uranium exploration. i It was necessary to carry out a fixed-wing spectrometer survey in late 1970. However, this survey was of limited use,primarily due to ground conditions at the time. The first ground exploration program,which was carried out in 1971, included a helicopter spectrometer survey and ground radiometric surveys. One large anomaly complex and several point-source anomalies including the Jabiluka anomaly were discovered during this work. Getty Oil Development Company Limited acquired a substantial minority equity in the property and provided the funds which permitted further exploration. Drilling towards the end of 1971 resulted in the discovery of economic uranium mineralisation at Jabiluka One. Based on a working hypothesis concerning the periodicity of uranium deposits along strike, further exploratory drilling was carried out in 1973 to the east of Jabiluka One. Jabiluka Two was discovered approximately 500 metres east of Jabiluka One in mid-1973 as a result of this drilling (Fig. 2). 3. REGIONAL GEOLOGY The oldest rocks in the Pine Creek Geosyncline of the East Alligator River Region are a sequence of Lower Proterozoic IAEA-AG-250/19 173 TABLE I. REASONABLY ASSUMED RESOURCES AND ESTIMATED ADDITIONAL RESOURCES OF FOUR URANIUM DEPOSITS REASONABLY ASSUMED RESOURCES AND URANIUM DEPOSIT ESTIMATED ADDITIONAL RESOURCES Í Short tons of contained U3 O3 ) JABILUKA 2 0 4 4 0 0 RANGER 100 3 5 0 Peko-Wallsend Limited/ Electrolytic Zinc Co. of Australasia KOONGARRA 32 5 0 0 NABARLEK 9 5 0 0 Queensland Mines Limited NOTE: RAR and EAR recoverable at prices less than $US 10/lb 4^ TABLE II. SURVEYS WHICH RESULTED IN DISCOVERY OF THE FOUR URANIUM DEPOSITS (/rom Яе/ [2]) . F!XEDW)NG RADtOMETR)C SURVEY GROUND RAD!OMETR)C SURVEY URANtUM ANOMALY ANOMALY ANOMALY ANOMALY DEPOStT DATE EXTENT EXTENT CENTRE TYPE )NTENS)TY TYPE !NTENS)TY Severa! 7000 m 15 6000 m 250 MOSHER andROWNTREE RANGER 1969 f tight Comptex 1500 m 500 m High One 100 m 10 Point 90 m KOONGARRA 1969 ttight Source tine 90 m High Three 1800 m 150 m 50 NABARLEK 1970 ftight 500 m Comple, 1500 m 2 Point JABtLUKA 1970 N ОТ DETEC TED 105m ж 45m 80m X 40m 1AEA-AG 250/19 175 F7C.2. Ceoío^ racfi'omgfri'c.:. phyllites, schists and quartzites of the Kakadu Group and cherts, quartz schists, quartzites and carbonates assigned to th e C a h ill Form ation o f th e Namoona Group (3 ). The C a h ill Formation is the host rock for the stratabound uranium mineralisation in the Region. Migmatisation of the Lower Proterozoic metasediments resulted in the development of gneiss, lit par lit injection gneiss and syntectonic granites and pegmatites referred to as the Nanambu Complex. Unconformably overlying the Lower Proterozoic rocks is a thick succession of Middle Proterozoic sandstones and conglomerates of the Kombolgie Formation. The erosional remnant of this Formation forms the Arnhem Land Plateau which rises to 200 metres in the eastern portion of the East Alligator River Region. Partial peneplanation during the Mesozoic resulted in the development of relatively thin Cretaceous Mullamen beds over the low-lying portion of the northern part of the Region. Further partial peneplanation during the Tertiary, generally to a level a few metres below the Middle Proterozoic unconformity, resulted in the development of extensive laterites. Recent eluvial and alluvial sandy deposits form a thin but persistent cover over the low-lying areas of the Region. 176 ROWNTREE and MOSHER F/C.J. 7%e УдМыАд алеа. IAEA-AG-250/19 177 4. EXPLORATION ENVIRONMENT The climate in the East Alligator River Region consists of a monsoonal season from December to April followed by dry season from May to November. Annual r a i n f a l l i s a p p ro x im a te ly 1400 m illim etres and the average daily maximum temperature ranges from 31° to 36° Celsius. The low-lying areas of the Region are covered with a moderate growth of eucalyptus and paperbark trees; however, the Arnhem Land Plateau supports a light growth of paperbarks and scrub (Fig.3). The extensive lowland in the western portion of the Region is approximately 10-50 metres above sea level. The lowland, which is flooded in the monsoonal season, contains numerous permanent water channels and seasonal drainage channels. The majority of the exploration work is carried out during the dry season from June to December when working and access conditions are easiest and the support costs are lowest. The Jabiluka property in the central portion of the Region lies approximately 240 kilometres due east of Darwin. There is a paved main road to the Region from Darwin; however, local access routes consist of gravel roads which provide reasonable access during the dry season. 5. SELECTION OF THE PROPERTY The property was selected because it was within the Lower Proterozoic Pine Creek Geosyncline which contains known uranium mineralisation in the South Alligator River Region and Rum Jungle Region (Fig. 1). The Bureau of Mineral Resources' preliminary geological maps of the East Alligator River Region were based primarily on air photographic interpretation, together with wide spaced ground traverses and were of limited reliab ility. In 1970 Pancontinental acquired the property (Fig. 4) and initiated a preliminary investigation which included geological reconnaissance and radiometric traverses as a basis for the fixed- wing radiometric survey which it was necessary to carry out due to work commitments on the property. The fixed-wing survey was of limited use,primarily due to ground conditions at the time, and did not provide a suitable basis for further ground exploration. Prior to commencement of ground exploration in 19 71, preliminary base maps were prepared from government aerial photos of the area. A structural interpretation of the property was also carried out utilising the preliminary geological information acquired during the property reconnaissance in 1970. 178 ROWNTREE and MOSHER F/C. 4. УяМмАя.Ргсрег?у. 6. PRIMARY EXPLORATION Exploration crews commenced primary exploration on the Jabiluka property in 1971 using selected ground and airborne techniques. 6.1, Helicopter Radiometric Survey A helicopter mounted radiometric survey was carried out during the 19 71 primary exploration program after the basic nature of the radiometric anomalies on the property had been determined by ground orientation surveys (4). The survey was flown late in the 1971 dry season,which was the optimum period to obtain the best definition of radiometric anomalies. The survey was carried out over a 75 square kilometre area using an AV4 McPhar Unit 4 channel spectrometer with a crystal volume of 5000 cm^ (Fig. 5). Air speed was 90-95 kilometres per hour and the mean terrain clearance was 90 metres. Flight line spacing was 18 0 metres with lines flown both north-south and east-west. Flight lines were recovered photographically. Uranium/thorium ratios from the helicopter radiometric survey were utilised as a result of the ground radio metric orientation work which showed that this provided the best anomaly definition. Anomaly priorities were established for subsequent ground radiometric surveying. IAEA-AG-250/19 i 79 AREA SURVEYED HEUCOPTER ADDtHONAL ANOMAHES GROUND ANOMALtES FÍC.J. J4aá:o?nefr¡cano?naHM. The helicopter survey was more cost effective than a ground radiometric survey which was carried out over a 20 square kilometre portion of the same survey area. Although the ground radiometric survey provided a much more detailed coverage of the test area, it was felt that a substantial increase in the rate of coverage was needed in order to complete the surveying of all accessible areas of the property during the limited exploration season. It cost a total of $105 per square kilometre (Dec. 197 5 Australian dollars) for helicopter radiometric coverage at 180 metre line spacings in both flight directions. It cost a total of $300 per square kilometre at 30 metre line centres for the equivalent ground radiometric survey coverage. There was an order of magnitude increase in the speed of coverage comparing the helicopter survey to the ground survey. The helicopter survey therefore provided a reasonably rapid and inexpensive method of delineating the larger anomaly complexes and allowed the ground exploration program to be focused on areas of interest which required further detailed ground radiometric coverage and prospecting. 6.2. Reconnaissance Prospecting Reconnaissance prospecting was carried out to locate airborne radiometric anomalies on the ground and to cover 180 ROWNTREE and MOSHER favourable accessible areas in more detail, and particularly to ensure detection of point-source radio- metric anomalies not usually picked up in aerial radio- metric surveys. Exploration teams consisting of one geologist and one or more field technicians were equipped with spectrometers with the sound level set at 1^ times background broadband (B.B.). The instruments were the same make and were calibrated several times a day using a standard thorium disc. Anomalies from different teams were therefore relative and easy to compare and stack according to p r i o r i t y . Ground traverses were made on rough grid pattern at approximately 30 metre line spacings with continuous instrument operation. Locations of anomalies were flagged and the highest counts on the uranium and broadband channels recorded. Ground conditions, terrain and surficial geology were also recorded in anomaly areas. Large areas were covered quickly and effectively and as a result t-he entire property excluding the Arnhem Land Plateau was prospected. Several new anomalies, including Jabiluka, were discovered that had not been indicated in the airborne surveys (Fig. 5). 6.3. Geological Mapping Geological mapping was carried out using aerial photos, and the information was transferred to photo mosaic base maps at the field camp. In the earlier programs the geological mapping was concentrated along the margin of the Arnhem Land Plateau where outcrops of Lower Proterozoic metamorphic rocks were exposed. The lowland areas did not appear to afford much scope for useful geological mapping because of the extensive overburden cover. Familiarisation with the geomorphology of the Region during later programs resulted in the recognition that the harder weathering lithological units such as cherts and silicified schists were reflected by eluvial material which, when mapped, permitted the determination of the strike of the lithological succession in overburden covered areas. This geological information when considered together with the detailed surface radiometric character often permitted favourable lithologies to be traced between areas where bedrock information was obtained using other techniques. 6.4. Auger Programs A u ger d r i l l i n g was c a r r ie d o u t u sin g a Gemco 210B au ger d rill mounted on a Bombardier Muskeg tractor equipped with a hydraulic blade for site preparation. IAEA-AG-250/19 181 Auger programs employed were of two types : 1. Grid line drilling will be considered under Section 7.2. 2. Grid pattern auger drilling was carried out to evaluate large areas, on a grid pattern with h o le s i n i t i a l l y a t 150 m etre c e n t r e s . Some in fill drilling at 50 metre centres was necessary. Grid pattern drilling aided geological mapping, outlined bedrock radiometric anomalous areas and provided bedrock samples for analysis in areas of overburden cover. 7^______SECONDARY GROUND EXPLORATION 7.1. General The primary exploration radiometric surveying, geological mapping and grid auger drilling provided the basis for secondary exploration in areas which were geologically and/or radiometrically favourable. Secondary exploration commenced in some areas as soon as enough information had been collected from the primary exploration phase to establish priorities. The first listing of priorities tended to place too high a priority on the anomaly complexes. In the final analysis the third priority anomaly at Jabiluka proved to be the best illustrating the importance of continued flexib ility throughout the exploration program. A rigid priority classification based on the airborne survey or the primary phase of exploration would have concentrated most of the exploration effort in the wrong areas and, as a result, decreased the chances of discovering Jabiluka. During secondary ground exploration,different techniques were often used to obtain information under different ground conditions. For example, sampling of bedrock geology was carried out using four techniques as illustrated below: Technique Reason Used Channel sampling Bedrock exposed. T ren ch in g Overburden less than 3m (practical lim it for the backhoe was 4m) . Auger drilling Overburden over 3m. Shallow percussion Thin sandstone cover d r i l l i n g . overlying metamorphic ro c k s . 182 ROWNTREE and MOSHER 7.2. The Jabiluka One Anomaly The Jabiluka. One anomaly occurs along the western margin of an outlier of Kombolgie Formation sandstone (Fig. 5). There is no radiometric anomaly over the J a b ilu k a Two a re a . At Jabiluka One the unconformity between the sandstone and underlying metamorphic rocks is present along the scree slope but the contact is completely obscured by the sandstone scree. There are no metamorphic rock outcrops. The anomaly was first detected during the reconnaissance prospecting program as a small area of surface anomalism showing approximately 2 x background B.B. (Fig. 2). A more detailed examination of the area uncovered an anomalous fragment of metamorphic schist 8 x background B.B..and an anomalous anthill located on the scree slope which gave broadband readings of 50 x background B.B. The Jabiluka One anomaly was progressively given greater priority and several secondary exploration techniques were employed to test the area further. A detailed radiometric survey was carried out over an established grid at Jabiluka. The survey outlined two anomalous overburden covered areas (Fig. 2 ). One area (105m x 45m) exten d ed n o rth w e s t-s o u th e a s t and a vera g ed 2 x background B.B. The second area (80m x 40m) extended east-west and averaged 1^ x background B.B. In addition to the grid, readings between grid lines were recorded. The highest single point reading was 6^ x background B.B. Pits were hand-excavated below the two point-source anomalies at Jabiluka and were successful in discovering uranium mineralised float. One metre below the surface of the stronger anomaly a cobble of quartz schist was uncovered which assayed 8% U^Og. Geological mapping was completed to provide a controlled base map for plotting results of the secondary exploration program . In thin overburden areas adjacent to the scree slope a backhoe was used to dig three trenches across the anomaly axis. These trenches cut the near surface extensions of the Jabiluka One ore zones and provided the first quantitative information on grades of mineralisation, geological association and structure (Fig. 6 ). Grid line auger drilling was used to evaluate individual anomalies in more detail. Holes were drilled at closely spaced intervals (3-10m apart) to trace in bedrock anomalous zones first detected by other techniques. Holes were generally drilled to greater depths (15-25m) than on the grid pattern auger drilling which was carried out IAEA-AG-250/19 183 -100 ? O-J. AiRBORNE <л 1 0 0 — 1 5 J - GROUND F7 C. 6. One croM-^ecfion. during the primary exploration phase on the property in order to provide a better geological section. The auger drilling supplemented trench information in areas of thick overburden,thereby assisting in the positioning of the in itial exploratory diamond and percussion d rill holes at Jabiluka One. A sixty by sixty metre drilling pattern was adopted for the in itial drilling program because surface results indicated a target at least that large and the spacing seemed close enough to carry out preliminary geological correlations. D rill Hole 1 was positioned to intersect the down-dip extension of the best trench results below the zone of weathering where primary uranium mineralisation could be expected. D.D.H. 1 intersected 22 metres of primary uranium mineralisation averaging 0.73% U^Og. The near-surface uranium zone at Jabiluka One is thinner and of much lower grade than the down-dip extension inter sected in D.D.H. 1 (Fig. 6). In the East Alligator River Region extrapolations made from surface indications would . be misleading if leaching or enrichment of uranium and the lithological or compositional controls on the mineralisation were not considered in evaluating the potential for extensions. 184 ROWNTREE and MOSHER One very important conclusion drawn from an assessment of other stratabound deposits in the Region was that periodicit; of uranium deposition both along strike and down dip was a possibility within the Jabiluka host rocks. To evaluate this hypothesis during the 1973 program,north-south lines of three holes each at 60m spacing were drilled at 120m intervals to the east. The 120m by 60m d rill pattern was the maximum grid spacing which could be used to intersect a target half the size of Jabiluka One. On the fourth fence to the east, Jabiluka Two was intersected. 8. DELINEATION OF THE JABILUKA DEPOSITS The Jabiluka One deposit was delineated after the in itial drilling by core drilling on a thirty by thirty metre grid pattern. Cores showing anomalous radioactivity were split, sampled and assayed for U^Og using X.R.F. analysis. The Jabiluka Two deposit was delineated on a sixty by sixty metre grid pattern using a combination of core drillin g and open hole drilling techniques. To the south and east the depth to ore at Jabiluka Two was greater than the practical lim it of the percussion d rill rigs,and drilling in this area was carried out by core drilling. Where possible,drill holes were precollared above the ore zone by percussion drillin g or other open-hole techniques. This improved productivity and lowered overall meterage costs without lowering the quality of the data obtained in the ore zones.. Percussion holes were sampled at one metre intervals and samples laid out book-form on a cleared site beside the hole. Two representative samples of each metre sample were collected and retained as a permanent geological record,and composite samples were taken of each anomalous radiometric zone. Radiometric, self-potential, single-point resistance and drift surveys were conducted down all d rill holes. For these surveys a contractor supplied an air-conditioned truck-mounted unit capable of producing analogue results for radiometric single point resistance and S.P. surveys. All three of these surveys were, carried out simultaneously on a single run and were recorded on the same analogue printout. Digital radiometric readings were taken at 0.2-metre intervals on the in itial probe run and anomalous zones were re-run on a 0.1-metre interval. The data was recorded on a digital tape as total broadband counts. Loggin probes were calibrated in a drill-hole which was used as a test pit. К factors were determined by equating core assays with down- hole radiometric results. Few exploration programs warrant settin up an artifical test pit at the.onset and in most cases a drill hole is used for this purpose. However, studies should be under taken to ensure the radiometric response down the hole is in equilibrium and that other factors such as casing, fluid, hole diameter.and radiometric intensity etc. do not adversely influence the results. IAEA-AG-250/19 185 TABLE III JABILUKA DEPOSITS: CORE ASSAY VERSUS RADIOMETRIC ASSAY (DH 176) CORE ASSAY RADIOMETRIC ASSAY INTERVAL (metres) RESULT %из0в INTERVAL (metres) RESULT %изОв 27 0 0 41 25 6 0 49 3 9 0 06 4 0 0 06 110 0 85 10 3 0 76 13 0 0 93 14 4 0 74 5 0 0 08 6 7 0 09 5 0 0 06 4 6 0 08 The downhole radiometric surveys were of value for the following rea so n s: Radiometric results permitted accurate assay results to be calculated from percussion holes. .- Radiometric results provide assay information in zones of lost core. Radiometric assays in core-holes served as a check against the X.R.F. results from split core samples (Table III). Anomaly character observed in the analogue printout assisted in the correlation of ore zones. Radiometric backgrounds in non-anomalous holes assisted considerably in differentiation and correlation of lithological units. In core-holes the analogues provided a fast method for picking the anomalous intervals to be split for assay. Radiometric results provided immediate assay results in the field. The downhole S.P. and resistance surveys were useful in defining lithology boundaries,especially in open holes. Rock types such as graphite schists and dolomites gave good character responses 186 ROWNTRBE and MOSHER which assisted in the correlation of lithological units. In the field office, checks against the downhole electrical and radiometric survey results and a thorough examination of the percussion chips with the use of a binocular microscope, made it possible to produce a detailed geological log of percussion holes. D rift surveys were taken separately in each hole (generally at 25-metre intervals) using a multi-shot Eastman instrument. Core logging was conventional and included the recording of the rock types, composition, colour index, texture, structure, foliation, weathering, mineralisation and drill-hole information. Core and percussion logging was carried out using a geological logging form designed for the project. The form standardised the information record and facilitated geological correlations even where holes on the same section were logged by different geologists in different years. 9. GOLD MINERALISATION Early in the exploration program on the property,several surface samples gave significant gold values which confirmed that a gold association existed in the uranium deposits of the East Alligator Region. Assays of composite samples of the Jabiluka uranium deposit indicated the presence of gold in possibly economic quantities. A subsequent sampling and drillin g program within the Jabiluka Two uranium deposit delineated a gold orebody containing, 410,000 tonnes of indicated reserves having a grade of 16.1 g gold per tonne and 120,000 tonnes of inferred ore at an average grade of 12.5 g gold per tonne. 10. COMPARISON OF COSTS AND PRODUCTIVITY Although cost figures for exploration drilling are quickly dated,it is useful to show the cost trends and the cost effectiveness of exploration drilling at Jabiluka over the p e r io d 1971 to 1975. D uring t h i s p e r io d a t o t a l o f s ix exploration programs were carried out, some of which were carried out during the dry season and others during the wet season. The cost figures in absolute terms would only be applicable to areas of similar physiography and to programs involving a similar level and type of drilling. Figures quoted are in December 1975 dollars. During the period December 1971 to December 1975 inflation in Australia was approximately 66% on a cu m u la tive b a s is . Figure 7 shows the basic comparison of costs for drilling and downhole radiometric logging, together with drilling product ivities during the six programs carried out from 1971 and 1975. Core drilling costs have remained relatively constant in direct contractor terms; however, there has been a substantial decrease IAEA-AG-250/19 Í87 Indirect costs ¡ПЗ Direct costs ¿-й! Learning curve— Contractor teaming curve—— F/G. 7. Drt'/Zmg соям and prodMcí¡'v:í^. in indirect core drilling costs as a result of a fairly rapid learning curve for onsite technical personnel. Overall core drilling costs have therefore dropped substantially, despite a fivefold increase in the average hole depth from 60 to 300 metres from 1971 to 1975. Percussion drillin g costs show that contractor charges have doubled since 1971 as a result of original contractor under estimation of difficu lt drilling conditions mostly related to ground water problems. Since 1971/73 contractor costs for percussion drilling have remained relatively constant. Radiometric logging costs increased substantially from 1971 to 1975 due to a virtual monopoly on contractor logging services in Australia and a relatively low cumulative drilling rate. High costs were overcome in part in the 1974/75 wet season program by logging all of the 1974/75 program holes during the 188 ROWNTREE and MOSHER 1975 dry season program. This results in a substantial saving in contract radiometric logging costs per metre drilled for both p rogram s. Figure 7 shows that drilling productivities increased from 1^ to 5 metres per man day from 1971 to 1975. This reflects a substantial increase in output by both the contractor and onsite company personnel. Levels of 3 to 4 metres per man-day (equivalent to 15 to 20 metres per geologist-day) are acceptable. 11 OTHER EXPLORATION TECHNIQUES , 11.1. General Although the exploration techniques as discussed for Jabiluka proved to be the most effective in that area, the application of these techniques had to be modified in exploration over other areas of the property. The small point source radiometric anomaly at Jabiluka One limited the area for detailed secondary exploration; however, the eight-kilom etre-long anomaly complex on the southern portion of the property required a modified approach. Anomaly complexes in the East Alligator River Region often reflect uneconomic uranium mineralisation within generally unfavourable lithologies. Therefore it is necessary to delineate the favourable lithologies,which are often unrelated to surface anomalism,before effective secondary exploration can proceed. During the secondary exploration on the southern portion of the property favourable lithologies were delineated by surficial geological mapping, trenching, grid line auger drilling and deep lithological core drilling. This work represented an additional step in the exploration program over this portion of the property compared to that at Jabiluka where the point source anomaly occurred directly over the favourable lithologies. Further drilling along strike and down dip within the favourable lithologies outlined resulted in the discovery of economic uranium mineralisation on the southern portion of the property. 11.2. Techniques Preliminary testing of several other exploration techniques was carried out over selected areas of the Jabiluka property. The brief description of techniques in Section 11.2 is primarily for the sake of completeness; most techniques were of limited technical success or were not cost effective and/or were bypassed as results were obtained by more direct methods. ÏAEA-AG-2S0/19 189 Geochemical Sampling Limited geochemical sampling was carried out on auger samples at Jabiluka One. The only consistently anomalous element was copper; however, it did not provide as good a definition of the uranium anomaly as geochemical U ^s. Geobotany In 1971 a g e o b o ta n ic a l o r ie n t a t io n program was carried out to determine plants in the area that could be used to indicate uranium (5). Results showed a wide variation in uranium content of the species sampled, even within known anomalous areas; however the eucalypts including gums, mallee and stringybark,showed the most encouraging results and could be useful indicators in evaluating overburden areas prior to employing more direct methods. The geobotanical program was discontinued because the use of direct bedrock sampling techniques such as auger drillin g was preferred. Alpha-Particle Survey Alpha-particle surveys were carried out over portions of Jabiluka One and Two as well as over several other areas of the property. Results of the surveys indicate: That in areas of known surface radiometric anomalism the alpha-particle survey detected th e same anomalism (F ig . 8 ) . That the alpha-particle survey did not detect J a b ilu k a Two w hich i s o v e r la in by a minimum of 20 metres of Kombolgie Formation Sandstone (F ig . 8 ) . That alpha-particle surveys would appear to be more effective to evaluate areas which are overlain by dry loosely consolidated overburden. That alpha-particle anomalies can be detected along faults and fracture zones in the Kombolgie Formation sandstones; however, no anomalies outlined in this way have been evaluated by drilling to date. It is interesting to note that an auger hole can be drilled to a depth of 20 metres for a total cost of approximately $200 including radiometric 190 ROWNTREE and MOSHER оE -.150 RADON CUP SURVEY —t 0 ÛQ n150c¿ GROUND RADiOMETRtCS ^ ^ Oa. V) 6 JABtLUKA ONE JAB!LUKA TWO and geological logging. An alpha-particle sample point costs a total of approximately $40. The auger drilling was more effective than the equivalent alpha-particle sampling as an exploration technique on the property because of the invaluable bedrock geological information obtained and because of the rapid turnaround on auger information compared to the nine week turnaround for alpha-particle results. Radon Emanometry A radon emanometer was tested during the 1971 exploration program to determine if this technique could be utilised in overburden covered areas of the property (6). Although positive results were obtained,moisture and dust contamination in the ionisation chamber prevented reproducibility of readings. Mercury Sniffer Survey A "mercury sniffer" survey was conducted over anomalous areas of the property using a truck- mounted instrument. The survey failed to outline any significant anomalies,apparently because of low concentration of mercury in the host rocks and because clay-rich overburden limited the migration of mercury vapour. This IAEA-AG-250/19 191 technique does not appear to have application for uranium exploration in the East Alligator River Region. Airborne Magnetic Survey An aeromagnetic survey was carried out in 197 0 with the fixed-wing airborne radiometric survey. The magnetics were useful to delineate.areas u n d e rla in by th e Nanambu Complex and t o d e t e c t late-stage basic intrusives along major structural features under the Kombolgie Formation. It was not possible from the survey to detect magnetic contrast among the Lower Proterozoic lithologies. The aerial magnetics were not flown at optimum altitudes because they were flown with the radiometrics. Ground Magnetic Survey Results obtained from ground magnetic surveys were erratic and were of little use in exploration. The laterite cover is believed to be partly responsible for spurious readings which prevented a reliable interpretation. VLF - Electromagnetic Survey . VLF - EM surveys were carried out in the Jabiluka area in 1971 in an attempt to trace graphitic zones located during the trenching program (6). Several anomalies were outlined but it was not possible to reliably distinguish between a true graphitic conductor and a simple geological change in conductivity. The technique was not employed in subsequent programs because it was bypassed as results were obtained by more direct methods. 12. CONCLUSIONS Case histories of exploration programs are generally regarded as useful only if specific new techniques are developed or if an exploration philosophy is shown to be effective in the discovery of an economic deposit. The exploration at Jabiluka is not a classical case of the development of new techniques because of the rapid progression to known direct exploration techniques such as trenching, augering and deep drilling and the bypassing of indirect techniques in which there is the greatest scope for new developments. Direct exploration techniques were employed extensively because it was a high-priority property in a known uranium region. 192 ROWNTREE and MOSHER The Jabiluka case history illustrates an effective philosophy which was successful in exploration for stratabound uranium deposits in the East Alligator River Region. This philosophy encompassed the following points: The value of regional appraisals in selection of a property. The recognition of the detection limits of airborne radiometric surveys. The importance of ground prospecting for low-order point-source radiometric anomalies which cannot be detected by airborne surveys. The importance of evaluating all anomalies and the flexibility to change priorities as further exploration results are obtained. The necessity of establishing the controls on the mineralisation (for example the lithological controls on the Jabiluka property) before proceeding with further exploration. The necessity of exploring extensions of favourable lithologies to test for periodicity of mineralisation even where cover precludes surface expression. The flexibility to modify exploration techniques employed on point-source anomalies in order to properly evaluate anomaly complexes. Some aspects of this philosophy may be useful in exploration for similar uranium deposits in other areas. REFERENCES (1) DEPARTMENT OF MINERALS AND ENERGY, BUREAU OF MINERAL RESOURCES, GEOLOGY AND GEOPHYSICS, Mineral Resources Report No. 6 Uranium Deposits, Canberra (1974). (2) PEDERSON, C., NORANDA AUSTRALIA LTD. - EUPENE, G., GEOPEKO LTD. - ANTHONY, P., QUEENSLAND MINES LTD., Personal communication, Sydney (1976). (3) THE AUSTRALASIAN INSTITUTE OF MINING AND METALLURGY, Eighth.Commonwealth Mining and Metallurgical Congress, Geology of Australian Ore Deposits, Australia and New Zealand, Melbourne (1965). IAEA-AG-250/19 193 HOWE, A.C.A., AUSTRALIA PTY. LIMITED, Proprietary Report, Report on the Helicopter Radiometric Survey, Report No. 52, Sydney (1972). HOWE, A.C.A., AUSTRALIA PTY. LIMITED, Proprietary Report, Report on Geobotanical Sampling Program, Report No. 53, Sydney (1972). HOWE, A.C.A., AUSTRALIA PTY. LIMITED, Proprietary Report, Interim Report on the Exploration Work conducted over Authority to Prospect 2013, Report No. 48, Sydney (1971). IAEA-AG-250/5 EXPLORATION OF THE KEY LAKE URANIUM DEPOSITS, SASKATCHEWAN, CANADA R. GATZWEILER, В. SCHMELING Uranerzbergbau GmbH, Bonn, Federal Republic o f Germany B. TAN Uranerz Exploration and Mining Ltd, Saskatoon, Saskatchewan, Canada Abstract EXPLORATION OF THE KEY LAKE URANIUM DEPOSITS, SASKATCHEWAN, CANADA. In 1969, one year after the discovery of the Rabbit Lake uranium deposit, exploration started in the Key Lake area as part of a major uranium rush into Northern Saskatchewan, and within the frame of a joint venture. The area was not chosen on the basis of a particular metallogenetic concept. The lack of exploratory success in 1969 and 1970, together with the introduction in March 1970 of foreign ownership restrictions for uranium mining in Canada, discouraged six of the nine companies forming the original joint venture. In 1971 the three remaining companies decided to continue under a redefined concept, based on the knowledge obtained from the Rabbit Lake deposit (Uranerz had acquired a 49% share in 1970) and from the newly discovered uranium deposits in the Pine Creek Geosynctine, Australia. In the same year, exploration work resulted in the discovery of two high-grade mineralized boulders and significant radioactive and geochemical anomalies 5 km SW of the Key Lake deposits. Sub sequent exploration, aimed at finding the source of the mineralized boulders, comprised geological, glacial geological and ground radiometric surveys, boulder tracing, air-photo interpretation, lake sediment and muskeg sampling surveys, radon surveys, ground magnetic, gravity, electromagnetic and IP surveys, and driHing. The systematic exploration efforts resulted in the discovery of the Gaertner ore body in July 1975 and the Deilmann ore body in June 1976, where glacial geology, lake sediment sampling, magnetic and electromagnetic surveys were the key methods in defining the drilling targets. Three further years and a total of about 2400 drillholes were needed to fully delineate the two ore bodies. 1. INTRODUCTION The Key Lake uranium deposits are located in Northern Saskatchewan, Canada, 700 km road distance north of Saskatoon, the base o f Key Lake Mining Corporation (KLMC), which is the operating company for the Saskatchewan Uranium Joint Venture (Fig. 1). This comprises the Saskatchewan Mining 195 196 GATZWEILER et at. F7C. 7. íocaf;'on wcp o/Æey ¿ероя7. ÏAEA-AG-250/5 197 Development Corporation (SMDC), Eldor Resources Ltd, and Uranerz Exploration and Mining Ltd, a wholly owned subsidiary company o f Uranerzbergbau GmbH of Bonn, Federal Republic of Germany. A project feasibility study is close to completion. Start of production is anticipated for 1984. The deposits and their probable genesis have been discussed by various authors (Dahlkamp and Tan, 1977;Dahlkamp, 1978; Kirchner et a!., 1979). Case histories have been compiled by Kirchner and Tan (1977) and Tan (1980). The exploration history o f Key Lake covers a period o f ten years, and started with regional work in 1969. The general Key Lake area was selected for more detailed work under a new exploration concept in 1971. The discovery of. high-grade mineralized boulders and major radioactive and geochemical anomalies 5 km south-west of the deposits during the same year provided strong encourage ment for continued exploration which led eventually to the discovery of the Gaertner ore body in July 1975 and the Deilmann ore body in June 1976. Three further years were necessary to fully define and quantify the deposits. 2. GEOLOGICAL SETTING AND SPECIAL FEATURES The deposits occur within the Wollaston Fold Belt o f Hudsonian age (1 750 m.y.) at the south-east rim o f the Athabasca Basin. They belong to the Proterozoic unconformity related type. Most o f the area is covered by surficial deposits, in particular lakes, muskegs and glacial debris. There is a clear inter relationship between surficial geology and bedrock geology (Fig. 2): (a) The tectonically weakened ore-bearing structural zone has been scoured by glaciation and is now occupied by a glacial trough filled with outwash sand and basal till up to 90 m thick. (b) An esker has eroded part o f the Gaertner deposit and transported ore material up to 6 km to the south-west, causing chemical dispersion in lake sediments and muskegs. Outcrops of bedrock are very scarce. The bedrock geology is therefore largely based on the interpretation o f geophysical and drillhole data. During the pre-discovery period o f exploration very little was known about the basement geology of the area; only broad hypothetical analogies were drawn from other parts o f the Fold Belt. In 1976 the first geological map was compiled by Thiel and Rich based on an interpretation o f a combined airborne electromagnetic and magnetic survey (Questor) and on mapping to the south o f the area by Ray. Elongated, NE' striking periclinal structures are interpreted as Archean basement domes. These are overlain by Aphebian rocks which commonly contain graphitic horizons in their basal portion. These are indicated as major electromagnetic (EM) conductors (Fig.3). 00 AZELR t aï. et GATZWEILER TILL AND SLOPE WASH OUTWASH SAND. GRAVEL. КАМЕ MORAINE I I MUSKEGS ESKER OREBODY X* ORE BOULDERS LAKES WC.2 . area.' умгДем/ yeo/o^. IAEA-AG-250/5 199 TABLE I.. KEY LAKE URANIUM DEPOSITS: DIMENSIONS, TONNAGES AND GRADES GAERTNER DEILMANN OREBODY OREBODY LENGTH ( m ) 1 400 1 200 MAX. WIDTH ( m) 80 200 VERT. EXTENT ( m ) 70 190 DEPTH BELOW ( m ) 30-100 20-210 SURFACE AV. LENGTH TO 35:1 15:1 WIDTH ORE TONNAGE ( t ) 900 000 2 100 000 AV. GRADE t'/.UsOa) [CUT-OFF: 0.05 '/.) - 3 - 2.5 CONT. METAL OXIDE H UsOa) 27 000 52 500 (ич[)[0к lb s UsOg) 59.4 115.5 The Key Lake ore bodies are closely associated with such graphitic basal metasediments and lie on the north-west flank o f a major pericline close to the edge o f the erosional margin of the unconformably overlying Athabasca sandstone o f Middle Proterozoic age. They are also controlled by a major fault zone which is parallel to the foliation o f the Hudsonian gneisses and is developed at the base of one of the graphitic gneiss units. This fault zone dips 40 to 60 degrees to the north-west. It extends vertically through the plane o f unconformity and cuts the overlying sandstones. It is a reverse fault which has caused a downthrow o f the south-east block of up to 30 m. North and NNW-trending cross-faults are of a younger age and are o f minor importance. The deposits occur partly within a basement window due to glacial erosion over the tectonically weakened fault zone. Another important feature is the post-Hudsonian and pre-Athabasca paleo- weathering o f the basement rocks which is commonly observed near the deposits ('regolith'). It diminishes with increasing depth below the sub-Athabasca 200 N GATZW EJLER EJLER GATZW t a).et FÍG. Geology o / Xey La^e а;*еа, bosed од geoiogicai mapping, G.ß. Ray, J976, a^d w?er- рт-еГай'ол о/рАуд'са/ ¿ай, Æ. íeA/í^^y-r/ü'e/ a/¡¿ У. Я;'с/!, 7^7á. ÏAEA-AG-250/5 201 unconformity. In fault zones the paleoweathering extends to greater depth and is not easily distinguishable from hydrothermal-type alteration. The deposits are overlain by up to 90 m of glacial gravel and lakes with a maximum depth o f 26 m. The ore bodies trend parallel to and are centred on the trace of the intersection between the north-east striking fault and the sub- Athabasca unconformity. However, some apophyses o f ore extend along the unconformity and downwards along graphitic horizons. Table I summarizes dimensions, tonnages and grades o f the ore bodies. Attention is drawn to the fact that the bodies are largely vein type in shape and represent a relatively small and difficult target for drilling due to their modest max. width (80—200 m) and vertical extension (70—190 m). The ore grades are extremely variable and reach 35% ЧзО§ and 20% Ni over 2-m sections. The contained metal decreases by only about 1 0 % if the cut-off grade is increased from <0.1 to 1% UgOg. Geostatistical investigations showed that the metal distribution is less anisotropic along strike and in the vertical dimension than across strike. The ore minerals are uranium oxides plus some uranium silicates and nickel sulphides and sulpho-arsenides. They are closely intergrown. Significant trace- element concentrations o f Co, Mo and Pb exist, but they were not important in the exploration history o f the deposits. 3. HISTORY OF EXPLORATION 3.1. Prior to 1971 The discovery of the Rabbit Lake uranium deposit in late 1968 was the beginning o f a major uranium rush into Northern Saskatchewan (Fig. 1). Uranerz, which was founded the same year, acquired an equity in two joint ventures which held exploration permits close to the margins of the Athabasca Basin. This was then considered a more prospective location than the interior o f the basin: however, the areas were not chosen on the basis o f a particular metallogenetic concept. The Northern Saskatchewan Uranium Joint Venture (NSUJV) was headed by Inexco with 30% equity. Uranerz held 10%, and the remaining equity was distributed amongst seven other companies. The main exploration methods applied at that time were airborne radiometry and spectrometry carried out by a contractor on a quarter-mile grid pattern with ground follow-up including footborne radiometry, reconnaissance-type mapping, geochemical and radon sampling and ground geophysics (magnetic and electromagnetic surveys). No significant exploratory success was obtained in 1969 and 1970. In March 1970 the Canadian Government announced the introduction of foreign ownership 2 02 GATZWEILER et al. restrictions for uranium mining. This, together with the lack o f success, caused an exodus o f uranium explorers from Canada and from Saskatchewan in particular. Early in 1971 only two other companies besides Uranerz remained in the NSUJV, each then holding an equity of 33.3%. The three companies agreed to continue exploration under a redefined concept in more favourable areas. Uranerz formulated this new concept by applying its knowledge of the Rabbit Lake deposit — the company had bought a 49% share in this deposit in 1970 — and newly discovered uranium deposits in the East Alligator River Region o f the Northern Territory o f Australia. The main criteria were: 1. Prospective areas had to cover the unconformity between Aphebian metasediments o f the Wollaston Belt and the Athabasca Formation. 2. The Aphebian metasediments had to be o f shallow water deposition, including calcareous rocks. 3. The deposits searched for were fault-controlled and - more important - such faults could intersect the overlying unmetamorphosed Athabasca Sandstone. This could consequently have an effect on the erosional pattern o f the sandstone margin. The new target area stretched along the south-east margin o f the Athabasca Basin, and the joint venture could now work without major active competition, as the area of main interest was vacant. Additional information became available through study o f exploration reports submitted to the Saskatchewan Government by earlier involved companies. From 1968 to 1970 the general Zimmer-Key Lake area was occupied by a small Canadian company which had carried out some airborne spectrometry and some limited prospecting, including radon surveys of lake waters and soils. The results o f these investigations, however, did not encourage further work by that company although the reports mentioned anomalous radon content in the waters o f Zimmer Lake. 3.2. Summer 1971 Held season The 1971 summer programme consisted o f regional reconnaissance-type work with the object o f delineating smaller target areas. The following work was carried out; Structural airphoto interpretation; Geological and radiometric traversing to map the margins of the sandstone; and A regional radon survey o f all lakes accessible by light aircraft on floats. The anomalous radon concentration in Zimmer Lake was confirmed. Follow-up work located two highly radioactive swamps about 1 km east of Zimmer Lake. During a further check o f these swamps two boulders o f massive IAEA-AG-250/5 203 uranium-nickel ore were found about 500 m north-east o f Zimmer Lake and 5 km south o f the Key Lake deposits. Late in 1971 one claimblock (CBS 1878) was staked, covering an area of about 6 km^. 3.3. Summer 1972 field season During the 1972 summer field season this claimblock was investigated in detail. Systematic radiometric, magnetic and electromagnetic (EM 16) surveys combined with geological mapping, lake sediment and muskeg sampling were carried out. The surrounding areas were further investigated by reconnaissance surveys. Eighty-five mineralized boulders were discovered in three areas within the claimblock. At that time the technique o f systematic boulder-tracing was not known to the operators and was therefore not employed. The purpose of the magnetic and electromagnetic surveys was to locate tectonic structures. Results were not satisfactory. Only towards the end of 1979 was the necessity for further land protection recognized; consequently one further claim block and two mineral prospecting permits were applied for. 3.4. Winter 1972/1973 field season The interpretation o f the results obtained from the summer work led to the wrong assumption that a structurally controlled uranium-nickel deposit might exist in the vicinity o f the radioactive swamps and boulder fields north of Zimmer Lake. To outline drilling locations a vertical-loop EM survey was contracted. This EM survey located several zones o f higher conductivity which were then tested by a 19-hole core-drilling programme. No ore-bearing inter sections were obtained. Meanwhile, an air-photo study was contracted to an experienced glacial geologist. An esker was recognized as being responsible for the transport of the ore boulders, which were located at the termination point of the esker. The conclusion then was that the ore boulders originated from an area around Sea horse Lake, 5 km north o f the drilled area (Fig.4). 3.5. Summer 1973 and winter 1973/! 974 field seasons Early in 1973, B. Tan became responsible for the project and remained in charge until 1977. This is emphasized because continuity o f personnel is con sidered a very important factor for successful exploration. Tan implemented geochemical lake-bottom sediment sampling as a further tool. The organic-rich lake sediments produced the most effective results both at the reconnaissance and the detailed level (Tan, 1976). Very high uranium and nickel values ranging ÍAEA-AG-250/5 205 from 199 ppm to over 700 ppm were found, and a very distinct cut-off o f the anomalous zone was also indicated. The uranium content o f over 100 ppm in Kinikinik Lake decreases drastically to only 5 ppm in Key Lake (Fig. 5). The results confirmed the new target area as outlined by the glacial studies. An additional radiometric anomaly was discovered close to the north shoreline o f Seahorse Lake. Soil samples o f the anomalous area assayed up to 1% UgOg. A subsequent radon survey in the same area revealed a broad anomalous zone. The continued glacial studies could trace the esker beyond Seahorse Lake by recognizing a linear-shaped depression which was interpreted as a 'tunnel valley' or ice-walled channel. In this part o f the glacial stream no sediments were deposited. Now it became clear that somewhere along the tunnel valley a rich uranium-nickel ore body had been eroded; its location, however, was only broadly defined by the geochemical anomaly about 1 0 0 0 m wide. The next logical step was a multiple geophysical approach to narrow down the target zone. The programme consisted o f a ground magnetic—IP/resistivity and a vertical-loop EM survey. The latter was not successfully completed because of instrument failures. All geophysical work at this time was done by contractors. The magnetic contours showed a low intensity trend parallel to the strike o f base ment rocks (Fig.6 ). It indicated the presence of an alteration zone likely to be associated with a fracture zone. The trend direction of this magnetic 'low' coincided also with a resistivity 'low' and partly with an EM conductor (Key Lake). The IP survey produced several isolated anomalies which were interpreted at that time as bodies o f disseminated sulphidic mineralization which possibly provided the source material o f many o f the ore boulders. 3.6. Summer 1974 field season Though several drill targets had been outlined by the foregoing geophysical surveys, the summer season of 1974 started with a radon Track Etch survey covering the area of the geophysical anomalies. The background scattered quite erratically ranging from 1 to 10 tracks/mm^. One cup location had a reading o f 45 tracks/mm^. This area was subsequently tested with one drill-hole, with negative results. Another low-order anomaly with values between 20 and 30 tracks/mm^ occurs about 50 m north-west o f the Deilmann ore body. At that time it did not arouse any attention. The subsequent drill programme consisted o f ten holes with a total of 1068 metres o f BQ-size diamond core. The first hole tested the crosspoint of a VL-EM conductor with an IP anomaly in the Key Lake area. Thick graphitic gneiss was intersected but with no apparent mineralization. The graphitic horizon explained sufficiently the EM and IP anomalies. Its importance with regard to the lithostratigraphical ore control was not recognized at that time. FVC.J. ÆeyiaÆecrea.' conferí IAEA-AG-250/5 207 Assay results were only received after completion o f the programme. Two samples taken from an intersection with a weak increase in downhole radiometry contained 50 and 680 ppm U^Og and surprisingly high nickel values o f 0 .6 % and 1.1% Ni, respectively. More than one year later it was realized that DDH 1 had in fact intersected the 'roots' o f the Deilmann ore body. Six additional holes were drilled to test the other IP anomalies. AH results were negative. Holes 3 and 4, however, revealed different types o f paleosoil (regolith); hole No.4 showed strong hematitization, whereas green alteration predominated in hole No.3, indicating a reducing environment. Also, a weak radiometric anomaly was encountered in hole No.3 but, unfortunately, no core was recovered from this section. 3.7. The 1975 field season In January 1975 Uranerz personnel continued the vertical-loop EM survey which had been started the year before by a contractor. A strong conductor was indicated within the 200-m-wide zone o f low magnetic response. To locate the centre o f this conductive zone more precisely, an additional horizontal-loop EM survey with a coil separation o f 100 m was carried out. The resulting conductor was especially well developed at the north-east end o f Karl Ernst Lake (location o f DDH 15). On the basis o f these results, holes DDH 11 to DDH 15 were drilled along the conductor. While holes 11 to 14 are (ironically) located almost exactly between the two ore bodies and showed only a few weak radioactive zones, hole No. 15 inter sected 7 m o f highly radioactive material. Again, no core could be recovered. Since this hole was located within the tunnel valley, the radioactivity was interpreted as being caused by a high concentration o f ore boulders. On the basis of this wrong interpretation, drilling along the conductor was discontinued and additional holes were drilled around the location of DDH 15 to find the source o f these boulders, but no further significant intersections were obtained. Moreover, since the ice had started to melt, drilling on the lake was no longer possible. Since this hectic winter field season ended with such a confusing array of results, a deliberate programme o f systematic evaluation of all data obtained was collectively carried out to isolate some meaningful parameters. The following conclusions were reached: (1) Only holes drilled close to the main EM conductor had intersected radio active zones, i.e. DDH 1, 3, 12, 13 and 15. (2) Therefore, the combined vertical-and horizontal-loop EM surveys were confirmed as the best tools to locate any ore-bearing structure. (3) Holes with radioactive intersections mostly showed chloritic alteration within the paleoweathering zone. 0 GTWIE e al. et GATZWEILER 208 ELECTROMAGNETS SURVEYS - — VERHCAL LOOP CONDUCTORS - HORIZONTAL LOOP CONDUCTORS MAGNETIC LOW t S!MPUF)ED TRENO ONLY ) ). P. SURVEY (CAUSATIVE BODIES ONLY ) j o LOCATION , OiRECTfON AND NUMBER OF DtAMOND DRtLJ HOLES SECTION COMPARE FlG.7 FÍC.6. сотрйе^#еосйет;ся?ян(У#еорЬум'са?гедм??л. IAEA-AG-250/5 209 TABLE IL KEY LAKE EXPLORATION DRILLING STATISTICS NUMBER YEAR OF METERS REMARKS HOLE 1969 / 71 - - L im ite d .d rillin g outside Zinmer-Key Lake area E o f Zimmer Lake - 1 st Quarter 1973 1973 25 1 098 target: EM-conductors underlying radioactive swamp 1974 10 1 068 1975 11 1 127 tarset^ см-conductor and bou)der field ^ ^ ^ 1975 4 387 Key Lake area - sunmer hole 25 discovery: 7 m core at 7.4 1 U^Og/4.2 Ï Mi 1973/75 50 3 681 TOTAL UNTIL DISCOVERY 1975 41 3 727 Gaertner orebody - post-discovery 1976 409 45 344 ^Îpr^lenL^!^lf*De°LnnteLd^ 1977 891 97 783 D elineation of Gaertner and Deilmann orebodies 1978 803 88 010 Delineation drilling, some HQ fill-in drilling 1977/78 233 28 255 HQ and HQ metallurgical-, geotechnica!-, fill-in drilling 1973 / 78 2427 266 800 GRAND TOTAL, assayed core = 12.000 m (4) It was considered imperative to improve the core recovery. (5) Further drilling should be on profiles across the interpreted ore structures. After break-up of the ice in June 1975 a further detailed horizontal-loop EM survey was carried out on the land bridges between Seahorse Lake, Karl Ernst Lake and Dieter Lake. This time a coil separation o f 200 m was employed to increase the depth penetration to about 100 m. The interpretation o f this survey showed a distinct conductive body between Karl Ernst Lake and Seahorse Lake at a depth o f about 30 m dipping 70° to 80° to the north. A t the same time, a gravity survey was carried out by a contractor. The results, however, could not be correlated with the EM results. 2 1 0 GATZWEILER et a). F7C. 7. Centra/ parf o /f /м Саел^пелс^е ^eo/o^/ca/ cro^-íecí/on. The subsequent drilling programme was centred on the land bridge between Seahorse und Karl Ernst Lakes using 10-m-spaced holes on 25-m profiles perpen dicular to the conductive zone. The first hole (DDH 22) intersected about 20 m of highly radioactive material, but again no core could be recovered from the mineralized section. The same applies to the two subsequent holes, DDH 23 and 24. Finally, the drilling method was changed. So far the contractor had used BW pipe and tricone bits to penetrate the basal till. Then the BW pipe was pulled out and the hole was continued with BQ core drilling. Quite often, as was the case in hole DDH 15, the drillers continued with the tricone bit far into the brecciated basement. Now, after penetration o f the basal till, the hole was reamed with BW pipe to the interface o f basal till and bedrock. Then BQ core drilling was continued through the BW casing. Furthermore, the use o f special drilling muds and a double tube core barrel increased the core recovery substantially. On 29 July, about 7 m o f high-grade uranium-nicke! ore was recovered from hole No. 25. The assay of this core resulted in an average o f 7.4% L^Og and 4.2% Ni. By following the main conductive zone as mapped by the vertical-and horizontal-loop EM survey and by applying the same drill pattern, the Deilmann ore body was discovered in June 1976. tAEA-AG-250/5 211 NW SE FÏG .& paff o/í^e De¡7mann ore &oáy, íi'mp/:/¡'ed ^ео/о^'са/ cm^-^eefion. 3.8. Post-discovery exploration Another two years and a total of about 2400 drill-holes were needed to fully delineate the two ore bodies. Table It summarizes the drilling programmes. Pre-discovery drilling comprises four programmes with a total of 50 holes and 3681 m. The post-discovery drilling includes some larger-diameter holes for recovery .of material for metallurgical and geotechnical tests and for checking continuity of high-grade mineralization and the general validity o f sampling achieved by the 25 m X 10 m drilling pattern. The ore boulder field south o f the Gaertner ore body which contains several thousand tonnes o f U3OS was delineated largely by a new drilling method called sonic drilling, which allows core recovery of the unconsolidated glacial sediments. 212 GATZWEILER et a!. TABLE m. KEY LAKE: APPLIED EXPLORATION METHODS MODE METHOD STRATIGRAPHICAL MAPPING AERIAL PHOTO INTERPRETATION and SURFIOAL GEOLOGtCAL GLACIAL GEOLOGtCAL STUDY AND LAKE and STREAM MATER SAMPLING GEOCHEMtCAL )NVE5TtGAH0N5 LAKE, STREAM and SWAMP SEDIMENT SAMPLING SOIL and ROCK SAMPLING; BOULDER TRACING GE0B0TANICAL and BIOCHEMICAL SAMPLING GAMMA RAY SPECTROMETRY AtRBORNE AEROMAGNETICS ELECTROMAGNETICS TOTAL COUNT RADIOMETRY and GAMMA RAY SPECTROMETRY RADON EMAN0METRY; TRACK ETCH and ALPHANUCLEAR MAGNETICS GRAVITY GEOPHYStCALj гопнмп SEISMIC SURVEYtNG ) INDUCED POLARIZATION and D.C.-RESISTIVITY SELF POTENTIAL and MISE-A-LA-MASSE ELECTROMAGNETIC METHODS: VERTICAL LOOP, HORIZONTAL LOOP, TURAM and VLF RADIOMETRIC LOGGING: GROSS GAMMA, GAMMA RAY SPECTROMETRY and DOWN HOLE NEUTRON ACTIVATION RESISTANCE and SELF POTENTIAL LOGGING DENSITY DETERMINATIONS DRtLL CORE POROSITY and PERMEABILITY TESTS AND RADIOMETRIC EQUILIBRIUM DETERMINATIONS SAMPLE RADIOMETRIC AGE DETERMINATIONS Percussion drilling was also tested but without significant success. Despite the initial difficulties, the overall core recovery in mineralized sections reached 85%. A total o f about 36000 core split samples, mostly 30 cm long, were assayed routinely for uranium and nickel and less frequently for arsenic. About 1 0 0 0 0 determinations o f wet and dry bulk density were performed, including in particular high-grade samples. 7exf continued on p.2i 7. TABLE IV. KEY LAKE: GEOLOGICAL AND GEOCHEMICAL METHODS APPLIED UNTIL END OF 1978 SURFiCtAL GEOLOGtCAL AND GEOCHEMICAL tNVESDGAHONS METHOD OBJECTIVE DONE BY TIME STRATIGRAPHICAL MAPPING regional and detailed mapping Joint Venture Operator 1971 - '75 AERIAL PHOTO INTERPRETATION support for mapping J.V. Operator; Contractor 1971 '73 GLACIAL GEOLOGICAL STUOY support for detailed mapping J.V. Operator; Contractor 197) - '74 LAKE and STREAM MATER SAMPLING reconnaissance prospecting J.V. Operator 1971 - '76 SAMPLING of ORGANIC and INORGANIC MATTER in reconnaissance prospecting J.V. Operator; Contractor 1972 - '76 LAKES, STREAMS and SMAMPS and detailed mapping SOIL and ROCK SAMPLING; mapping and prospecting J.V. Operator 1971' - '75 BOULDER TRACING GEOBOTANICAL and orientative investigation J.V. Operator, DMRS 1974 '76-'77 BIOCHEMICAL SAMPLING TABLE V. KEY LAKE: GEOPHYSICAL METHODS APPLIED UNTIL END OF 1978 GEOPHYStCAL INVESTIGATIONS MODE METHOD OBJECTIVE DONE BY HME GAMMA RAY SPECTROMETRY regional гесоппШм.се on behalf of GSC; 1968; '75 Joint Venture Operator 1973 AtRBORNE AEROMAGNETIC on behalf o f GSC; 1968; '75 " ^ r ^ r a tapping div. Contractors 1976; '78 ELECTROMAGNETIC 1976; '73 "^t^ctÜra/map^ng QUESTOR-.^GEOTERRÉX; SANDER TOTAL COUNT RAOIOMETRY and GAMMA RAY SPECTROMETRY 'lnSle"ned'^eI^№ions Joint Venture Operator up to 1977 RADON EMANOMETRY Including . TRACK ETCH and ALPHANUCLEAR Joint Venture Operator 1972-'76 MAGNETIC detailed structural mapping. J.V .Oper.; Contr.: RENTING 1973; '75-'78 GRAVITY local orien ta tiv e survey Contractor: KENTING 1975 GROUND SEISMIC Contractor: KENTING; 1973 SURVEYS ¡.e^nai're^rch рг^гаш on behalf of OMR 1976 INOUCEO POLARIZATION and O.C.-RESISTIVITY d etailed structural mapping J.V. Oper.; Contr.: KENHMC 1973; '76; '78 SELF POTENTIAL and MISE-A-LA-MASSE local orlentative surveys J.V. Operator 1976; '77 ELECTROMAGNETIC: VERTICAL LOOP J.V. Oper.; Contr.: KENTING 1973-'78 HORIZONTAL LOOP J.V. Oper.; Contr.: KENTING 1975-'78 TURAM Contractor-. GEOSEARCH 1977 VLF and VLF-Radiohm 1973; '76; '78 GAMA SPECTROMETRIC LOGGING discrim ination of U. Th, К J.V. Operator 1974-'76 GROSS GAMMA LOGGING system, drill hole investigations J.V. Oper.; Contr.: ROKE 1976 - cont. DOWN HOLE NEUTRON ACTIVATION LOGGING H thol./stratlgr. Investigations J.V . Oper.-, C ontr.: ROKE 1976 - cont. RESISTANCE and S-P LOGGING support for H tM . detent. J.V. Operator 1976 - cont. TABLE Vi. KEY LAKE: METHODS FOR DRILL CORE AND SAMPLE EXAMINATIONS APPLIED UNTIL END OF 1978 DRtLL CORE AND SAMPLE ¡NVESHGAHONS METHOD OBJECTIVE DONE BY TIME DENSITY DETERMINATIONS orientative tests; at div. COMMERCIAL LABs 1976 support for ore reserve calcul. Joint Venture Operator 1976 - cont. POROSITY and PERMEABILITY TESTS to investigate rockmech. parameters at div. COMMERCIAL .LABs 1976 - '77 RADIOMETRIC EQUILIBRIUM DETERM. support for eU^Og calculations Contractor: LORING LAB. 1976 - cont. RADIOMETRIC AGE DETERMINATIONS to investigate ore genesis Contractor: BGR 1976 - '77 MINERALOGICAL INVESTIGATIONS to investigate ore genesis at UEB, Bonn 1976 - cont. SYSTEM. CHEMICAL ASSAYING to investigate mineralization at div. COMMERCIAL LABs continuously and ore grade concentration and UEM/UEB L)NE 9+650 E UNE 11+700 E GATZWEILER et at. 216 A-A' B-B' EM-SURVEY HORiZONTAL LOOP (UEM 1975 !976) % - 1 000 Wi ¿ 200 m sep. - * - П m RES)ST)V)TY SURVEY ( UEM 1976 ) Я m m g at GRAV4Y SURVEY S.P SURVEY (KENUNC 1975 1 (UEM 1976 1 < MAGNETtC TOTAL )NTENS)TY ï ( KENTtNG SURVEY 1973 ) 6 M ATHABASCA FORMATEN BASEMENT ! i t FAULT ZONE z z z Z Z z z GRAPHIC ROCK 8 8 8 8 8 8 8 о m <0 ^ ORE ZONE ООО) O о O) e t F7G.P. Хеу ¿аЛсе. coMpí'ZgdgeopAyí/ec/ pw/i'Fei ftvo я'уярй/ïei? ÍAEA-AG-2S0/5 2]7 F/C. 7 0. DeveZopwenf o/Á^ey ía/ce expZorafion coíf& With a few exceptions, alt drill-holes could be radiometrically logged. The large amount of radiometric and chemical data allowed a close correlation, so the final ore reserve calculation is therefore based on radiometric data. Reserve calcu lations employ a three-dimensional geostatistical method (see Figs 7 and 8 ). 3.9. Summary o f exploration methods Tables III- V I and Fig.9 give an overview o f the methods which have been applied during exploration in the Key Lake area. The most important with respect to the final success were: (1) Glacial geological studies; (2) Lake sediment sampling, which led to definition o f the correct target area o f approximately 5 km^ ; and (3) Magnetic and electromagnetic surveys which finally defined the ore- bearing structure as a drill target. Since the discovery, airborne magnetic and electromagnetic surveys have become an important tool for regional and semidetailed structural mapping. Although knowledge o f the mineralization and its setting has increased substan tially since the discovery, it has not so far been possible to develop any essentially new methods to improve the effectiveness o f exploration. Meanwhile more emphasis has been placed on rock geochemistry. Refined gravity, helium and geobotanical sampling are still at the testing stage. 218 GATZWEILER et a). 3.10. Cost of exploration The development of exploration expenditure incurred by the Joint Venture between 1969 and 1979 is shown in Fig. 10. Pre-discovery expenditure amounts only to about 20% o f total expenditure. While drilling costs amounted to less than 2 0 % until the discovery, they were naturally the major cost item during the post-discovery period. With an anticipated start of production in 1984 the 'lead time' for Key Lake is 15 years! 4. SUMMARY AND CONCLUSIONS Some factors which in the authors'* opinion greatly influenced the final success should be mentioned: ( 1 ) Sufficient financing of the project despite a prevailing adverse political climate (mainly foreign ownership restrictions); (2) Constructive co-operation between the Joint Venture partners,which allowed Uranerz to realize its exploration concept; (3) A young, enthusiastic and persistent team o f explorationists keen to apply a wide variety o f methods and always prepared to learn from mistakes; and, last but not least, (4) A good portion of 1 u с к ! ACKNOWLEDGEMENTS The authors wish to thank the management of Uranerzbergbau GmbH, Bonn, and Key Lake Mining Corporation of Saskatoon for permission to publish this paper. They also acknowledge the co-operative effort o f all geologists o f the Uranerz group involved in the discovery and evaluation of the Key Lake deposits. F. Bianconi reviewed and discussed the manuscript. Drawings were prepared by G. Moeller. BIBLIOGRAPHY Dahlkamp, F.J., and Tan, B., 1977: Geology and mineralogy of the Key Lake U-Ni deposits, Northern Saskatchewan, Canada, in Jones, M.J., sd., Geology, mining and extractive processing of uranium: London, Inst. Mining and Metallurgy, p. 145-158. ÍAEA-AG-250/5 219 Dahlkamp, F., 1978: Geologie appraisal of the Key Lake U-Ni Deposits, Northern Saskatchewan, Econ. Geol., vol. 73, 1430-1449. Gatzweiler, R., 1978: Case History Key Lake. Unpublished presentation Alberta Section CIM. Gatzweiler, R., Lehnert-Thiel, K., et al, 1979: The Key Lake uranium-nickel deposits. The Canadian Mining and Metallurgical Bulletin, July, 1979. Kirchner, G. et al, 1979; Key Lake and Jabiluka models for lower proterozoic uranium deposition. In: Intern. Uranium Symposium on the Pine Creek Geosyncline, N.T. A u s t r a l i a . Kirchner, G. and Tan, B., 1977: Prospektion, Exploration und Entwicklung der Uranlagerstätte Key Lake, Kanada. Erzmetall Band 30, Heft 12. Lagally, U., 1976: Abschlußbericht über die Uran prospektion im Northern Saskatchewan Joint Venture/ Saskatchewan Uranium Joint Venture. Unpublished Uranerz internal report. Tan, B., 1976: Geochemical case history in the Key Lake area. Saskatchewan Geological Society. Special Publication No. 3. Tan, B., 1980: Prospektion, Exploration und Lagerstätten erkundung der Uranvorkommen am Key Lake in Saskatchewan, Kanada. Dissertation am Institut für Lagerstättenforschung und Rohstoffkunde an der Technischen -Universität Berlin. DISCUSSION В. GUSTAFSSON: Have you any idea o f the ice movement and have you established some kind o f boulder train? R. GATZWEILER: The general glaciation direction is from the north-east, almost parallel to the strike of the Wollaston Belt. The initially discovered boulders occur at an esker delta. The esker contains boulders throughout with the exception o f the section where it has not deposited sediments, i.e. the 'tunnel valley'. B. GUSTAFSSON: If I remember, you have some till in the area. R. GATZWEILER: Yes, we have this broad magnetic low structure which mainly indicates the strong alteration halo o f the ore-bearing structure. However, it partly coincides with a glacial trough. This is largely filled with outwash sand, but at the base we find several metres o f basal till. J. DARDEL: Is the uranium content o f the boulders representative o f the average content o f the ore bodies? 2 2 0 GATZWEILER et ai. R. GATZWEILER: The first two boulders found are representative for high-grade uranium-nickei basement mineraiization. Many of the boulders found in 1972 in the southern area represent sandstone ore, generally with lower contents in uranium and nickel. H.D. FUCHS: What is the exact metamorphic grade o f the host rock? R. GATZWEILER: The metamorphic grade o f the host rocks is upper amphibolite facies. The mineral assemblage o f biotite, cordierite and garnet points to a low-pressure metamorphic environment. H.D. FUCHS: Could you explain a little more about the type o f alteration within the mineralized zone? R. GATZWEILER: The alteration comprises kaolinization, chloritization and some hematitization. The alteration within the fault zone is partly due to paleoweathering and partly due to hydrothermal alteration. Research work is in progress to distinguish these two phenomena. G. BUNDROCK: I presume that in the order of 50 million lb UgOg have been eroded between the two ore bodies by glacial action. Do you have any idea whether some of that can be recovered or mined from the boulder field and the till? R. GATZWEILER: I am very surprised at your assumption. But you could be roughly right; we have delineated about 5000 t of boulder ore immediately adjacent to the ore body. This will be recovered. D.A. PORTER: Would you discuss the model used for the source and deposition o f uranium at Key Lake? R. GATZWEILER: Initially very little was known about the bedrock geology o f the working area. Therefore, considerations on the source o f uranium had very limited inñuence on the exploration concept. The general Kenoran and Hudsonian granitic terrain was known to be anomalously high in uranium. As regards deposition, a major tectonic structure and possibly calc-silicate lithologies were considered important. Later a model of multicycle deposition and enrichment evolved, starting with the formation of a sedimentary-type primary deposit within the basal sequence of Aphebian sediments close to an Archean basement dome. ÍAEA-AG-250/1 MIDWEST LAKE URANIUM DISCOVERY, SASKATCHEWAN, CANADA F. SCOTT Esso Minerais Canada, Toronto, Ontario, Canada Abstract MIDWEST LAKE URANIUM DISCOVERY, SASKATCHEWAN, CANADA. The discovery of the Midwest Lake uranium deposit in Saskatchewan came some ten years after the start of exploration. The original mining rights were acquired on the basis of regional published, geology and proximity to the earlier discovery. Aerial radiometric surveys led to the location of a train of radioactive, glacially transported sandstone boulders and cobbles. The source of these mineralized erratics did not outcrop, and an extensive series of magnetic, electromagnetic, seismic and gravity surveys was carried out in an unsuccessful attempt to identify the source location. These surveys were followed by several programmes of diamond drilling, geochemical surveys and Pleistocene geological studies. None of these programmes or surveys encountered bedrock mineralization. When information about ore controls in the Athabasca Basin became avaiiabie, a limited programme of three 300-m wildcat diamond-drill holes was proposed. The second of these holes cut weak radioactivity in a poorly cored intersection. This intersection was at an unconformity at a depth of 200 m. The programme terminated prematurely with early melting of lake ice. The first hole in the subsequent winter's follow-up drilling intersected uranium values in excess of 8 %. INTRODUCTION Ten years elapsed between the acquisition of mining rights at Midwest Lake, Saskatchewan (see Fig.l), in 1968 and the first intersections o f ore-grade material in place in January 1978. Subject to limitations of weather and geological theory, exploration was carried out each year. In this study the state o f knowledge at each phase of exploration is described, together with the advances in knowledge derived from testing the various geophysical indications and geological models. LAND ACQUISITION In 1966, Numac Oils Ltd, a small independent Canadian Oil company, acquired a spread o f uranium prospecting lands in the Beaverlodge area of 221 222 SCOTT §. northern Saskatchewan. This acquisition was prompted by a perceived growth in demand for uranium, consequent on the growth in construction of nuclear power facilities throughout the world. Imperial Oil Ltd, and later Bow Valley Industries and others, agreed to join Numac in exploration of these lands. For the next two years, extensive field work was completed without defining an economic ore deposit. The joint exploration venture was operated by Numac and directed by Murray Trigg and George Woollett, two independent and knowledgeable uranium geologists. At this time the economic uranium occurrences in Saskatchewan were limited to the Beaverlodge district. Low-grade uraniferous pegmatites were known to occur at Black Lake, Charlebois Lake, Foster Lake, Cup Lake and Lac La Ronge (Fig.2). In 1968, Gulf Minerals Canada, an affiliate o f Gulf Oil Ltd, announced a uranium discovery in the Rabbit Lake area, south-west of Wollaston Lake. This announcement led to a 'rush' for adjacent land, and the Numac joint venture obtained four permit areas near the Gulf discovery in December of that year (Fig.3). Because o f the snow cover, an exploration programme o f these permits, on the recommendation of geologists Trigg and Woollett, was deferred until the summer o f 1969, when radiometric surveys would be feasible. The initial ÏAEA-AG-250/1 223 ! #^'^^BLACKL^KE BEAVERLODGE CHARLEBOtS LK^ WOLLASTON LK.) ALTA. MAN. FOSTER LAKE * CUP LAKE % LAC LA RONGE # LA RONGE I PRtNCE ALBERT SASKATOON t ) * ' t— IOOMILES4 1 ¡ i i s F/C.2 . мгди/Mw оссмггенсм, /я/fer С^СЖар №. /043^4). stage o f exploration on the four permits was a reconnaissance geochemical and geophysical programme. The geochemical work involved the analyses o f a one-mile (1.6 -km) grid o f lake water samples for radon and uranium. These analyses did not identify any areas for more detailed examination. The aerial radiometric survey was carried out with a Cessna 180 fixed-wing aircraft, carrying a scintillometer with a 13-cm crystal. Line spacing was 400 m at an altitude of 100 m. The results o f this survey, as shown in Fig.4, indicated several weakly radioactive trends striking south-west across Permit 8 , one of the four permits covered. Ground prospecting along these trends showed the majority o f the radio- metric indications to be related to weakly radioactive granite boulders and cobbles, which came from a source area in the basement rocks several tens o f km to the north-east. ^ 7ex? ccnfmMea on p .2 ji. SCOTT ЯС.4. ^4erMZ гс^;'о?яеМс я/rvey аж? АомМе;- Угс;'п, /PáP. IAEA-AG-2S0/1 F/C. J. СеорА^я'сй/ ямр, / Р 70. F/С .б. Зеймн'стМгргеУаЯ'ой, /Р70. 226 SCOTT F/C .7. Afa^HefomefeT'.yMrye.)', 7970. F/G. & FLFe?ecfrofMa^Hef!'c.!Mn'gy, 7P70. ÏAEA-AG-250/1 227 /УС. P. Cran'fy^T-ye^,/P70. F/C./ 0. y4Fí,FC ^м?'^'ey conducfor ахм, /P 70. 228 SCOTT F/G.Í2. Reároc&áríMAoíe!, 797Л IAEA-AG-250/1 229 F7G.74. Aadon ;'я M'cfwesf LaA:e waferi, 7974 сомни per MnnMfeJ. 230 SCOTT F/C. 73. /Мен m 7974. TRENCH I О 500 0 500 О 500 О RADIOACTIVE SANDSTONE CLASTS RADtOMETFnC- TRENCH 2 COUNT PER -о - SECOND TRENCH 3 -о- F7C.7d. OvírAurden УгепсА pw/i/ei, /coding 797.?. IAEA-AG-250/1 231 Surface prospecting to check one aerial radiometric occurrence, south west of Midwest (sometime called McMahon) Lake, disclosed a narrow train of boulders and cobbles showing secondary uranium minerals in fractures in Athabasca sandstone (Fig.4). This train o f radioactive boulders could be followed for a distance of 3.2 km. The richest of these erratics contained 5% U^Og. Since careful prospecting to the north-east o f Midwest Lake did not locate any similar boulders, it was assumed that they originated from a source beneath the lake. Underwater radiometric surveys below the surface of the lake did not show any radioactivity and it was concluded that the source area was covered by post-glacial lake bottom sediments. Examination o f the fractured nature of the radioactive cobbles suggested that their source would be related to some structure in the Athabasca sandstone and an attempt was made to define this postulated structure by geophysics. Since the target area was assumed to lie beneath Midwest Lake, it was necessary to wait until the formation of sufficient ice in the winter o f 1969-1970. In the spring o f 1970, a series o f geophysical tests was carried out. A reflection seismic survey, over lines 720 m apart (Fig.5) was completed in an attempt, first, to determine the thickness of sandstone over basement, 232 SCOTT LEGEND CENOZOIC QUATERNARY RECENT Я Н PALUDAL- PEAT ^HfREEZE-THAW-BOULDER FIELDS PLEISTOCENE STRATIFIED DRIFT Я Н OUTWASH -GRAVEL-SAND ЗЖИ KAMES.ESKERS, CREVASSE FILLINGS - GRAVEL, SAND NONSTRATIFIED DRIFT П П TILL-SILTY, MEDIUM SAND (CONTAINS '---- URANIFEROUS SANDSTONE CLASTS) улюж "BOULDER" TRAIN * ESKER CREVASSE FILLING DRUMLIN ( NOT ALL SHOWN ) — STRIATIONS SCALE О . . IOOO .____ i____ ZO) O O FEET FYG.7& 797$. and second, to seek interruptions in such thickness which could identify fault traces. The seismic records were noisy and subject to various interpretations, one o f which suggested an average basement depth of 200 to 230 m, interrupted by a low point o f 300 to 330 m beneath the topographic low expressed by the lake (Fig.6 ). A magnetometer survey over lines at 130-m spacing was designed to seek interruptions in trends or changes in magnetic patterns which could be related to faulting or changes in depth o f basement features. The plotted results, (Fig.7) showed a constant gradient over the survey area with no features which could be attributed to fault interruptions. A VLF electromagnetic survey (Fig.8 ) at a frequency o f Hz, was designed to detect any linear, shallow, conductive features in the Athabasca sandstone which may have localized mineralization. The survey demonstrated a current ÏAEA-AG-250/1 233 concentration coincident with the outline o f Midwest Lake, due to lower resistivity lake bottom sediments. A gravity survey was run to see if any sharp gradient changes existed which could be related to faulting. The results o f this survey (Fig.9), when corrected for water and overburden depths, did not show any anomalous features. A long wire AFMAG (designated by the contractor as AFLEC) (Fig. 10), survey was completed, in an attempt to detect a deeper linear conductive feature than might be detected in a VLF survey. The results were interpreted to indicate a linear feature with an imprecise axis which roughly corresponded to the axis o f the lake. This geophysical response trended on land to the north and south o f the survey grid, and could be interpreted to be a separate deeper bedrock feature than the shallow VLF response. On compilation o f the geophysical response, eleven inclined diamond drill holes (Fig. 11) were put down from the ice of Midwest Lake to test the various geophysical responses. These holes failed to detect either radioactivity or permissive structures. 234 SCOTT W 9! 278 93 184 95 238 97 185 99 277 E ILAKE O'BURDEN URANtUM CONTENT ! o.i% - 1% S 1% -5 % S 5 % + SANDSTONE BASEMENT 30 METRES — ) F7G.20. Dr[7ísícfíon, Í97& After the failure o f the geophysical programme and consequent drill testing, the accumulated data was reviewed in 1971 and a different exploration technique devised. It was assumed that mineralization should subcrop beneath overburden. Ninety-one short vertical holes (Fig. 12), for a total o f 1700 m of drilling, were put down. None of these holes detected uranium. The failure o f this second drill programme coupled with a failure in the forecast growth of uranium demand, led to a reassessment o f the exploration programme. Investigations in 1972 and 1973 were largely geochemical studies, designed to focus on areas with anomalous uranium content. Analyses were made o f uranium and radon in soils, uranium in lake sediments, and radon in lake waters. The uranium content o f lake sediments increases slightly with water depth but did not identify any notable areas o f concentration (Fig. 13). Radon values (Fig. 14) were slightly near the shore, in the vicinity o f granite boulders along IAEA-AG-250/1 235 the east side o f the lake and at the south-east end where drainage from the boulder train enters the iake. When no definite driii targets were developed from the geochemical studies, further work was carried out on the boulder train. A radon gas in soil survey gave additional definition to the mineralized glacial material (Fig. 15). Eleven overburden trenches were put down to bedrock and showed mineralized clasts erratically distributed, as in Fig. 16. Most o f the mineralized erratics range in diameter from 5 cm to 15 cm. Overall radioactivity increased in the southern trenches. This fact was interpreted as due to rapid comminution o f the radioactive material down-ice. This suggested that the source o f the boulders might be near the north end o f the boulder train. Twenty-five short inclined holes (Fig. 17) to a total o f 800 m were drilled to test this hypothesis, but again, no mineralization was encountered. 236 SCOTT О 150 METRES ! " ' h ' 0 4 0 0 FEET 80 + 00 N F/C.2 2 . P'grfiù'aMoop7P7& Biac^c curves.' 4F№4G pro/iiei; y/:a^e¿ cMr^ei.' fe^ù'ca/-/oop p/*o/¡7eí. In 1976, a careful study was made of the Pleistocene geology o f the property in another attempt to identify a probable specific source area for the boulder train. The net result o f these studies indicated a most likely source immediately up-ice from the train, a second less likely source area slightly further (Fig. 18). During this period, geological information was becoming generally available about the Saskatchewan uranium discoveries from Mokta at Cluff Lake and Uranerz at Key Lake. Although specific details were not available, one factor these new discoveries had in common (and this was probably applicable to the producers at Rabbit Lake and Beaverlodge) was the proximity o f the ore deposits to the unconformity. This relationship gave impetus to a new approach to exploration at Midwest Lake. The rationale was that "since extensive exploration had failed to locate the subcrop course o f the uraniferous float, this source area probably represented a target too small for development at this remote location". A variation to this scenario, developed by Esso geologist Leo Kirwan, was that the intersection o f the presumed uraniferous structure with the unconformity between basement rocks and overlying Athabasca sandstones could be the location o f an important uranium deposit. ÎAEA AG-250/1 237 TABLE I. EXPLORATION COSTS, MIDWEST LAKE, TO 1979 Geophysical Geochemical Geological Drilling Other Total 1969 (108)" 229 (158) 335 564 1970 (52) 105 (72) 146 (86) 174 (16) 32 457 1971 (4) 8 (24) 47 (58) 114 169 1972 (6) 11 (22) 41 (4) 7 59 1973 (16) 27 27 1974 (2) 4 (4) 5 (6) 9 18 1975 (26) 35 (120) 161 196 1976 (22) 27 27 1977 (10) 11 (70) 80 (10) 11 102 1978 (62) 67 (2570) 2655 (12) 13 2735 1979 70 2246 14 2330 Totals 518 351 299 5430 86 6684 " Parentheses denote current dollars. Based on all the geophysical and geological data available, three holes were spotted from the ice at Midwest Lake to test the unconformity at a presumed depth of 300 m. Two o f these holes were barren, but the third intersected 2 m with radioactivity immediately above the unconformity, with poor core recovery. At this time, drilling had to be suspended owing to poor ice conditions. The depth to the unconformity proved to be 200 m rather than the 300 m interpreted from the seismic surveys. When Midwest Lake froze over in 1978, drilling resumed. The first drill hole found two mineralized intervals, 9.5 m o f 0.13% U 3O 3 and 1.2 m of 8.73% UgOg. Drilling was continued with four rigs and,up to September 1979, 336 exploration holes had been put down. The results o f this drilling showed a discontinuous zone of uranium mineralization, extending for 4 km, still open to the north (Fig. 19). The drilling pattern is based on a series of cross-sections 39 m and 60 m apart, with hole spacing at 30 m. Three types of uranium mineralization were noted (Fig.20): 238 SCOTT fa) -Sandsfcwe fype Erratic intersections, generally lower grade, occur in the sandstone, extending upward from the unconformity for a distance o f 170 m. On the drill spacing used it is difficult to correlate mineralization between drill holes. Massive and disseminated uraninite, with cobalt and nickel arsenides, occur at and above the unconformity. The mineralization appears localized near a 2 0 -m change in elevation which may represent a fault displacement. This type o f ore shows good continuity along and between the drilled sections. fc) .балетнем? fype Uraninite with nickel and cobalt arsenides occurs in steeply(? ) dipping fractures extending down from the unconformity for a distance of at least 100 m. Wallrock alteration is pervasive in the vicinity of these 'veins'. Because o f the drill pattern we do not yet know the attitude of these fractures and have not been able to correlate them between drill holes. As with the sandstone type, the contribution o f this type o f mineralization to the presently recognized uranium reserves is not important. The exploration stages we have identified are as follows: f;') .йесомиаммисе — / 969 ' Aerial surveys, lake-water analyses and ground prospecting, which resulted in the discovery o f a mineralized boulder train. fn) Geop/?y.s;'ca/ ,sfag — 7977 —79 7J.' Geochemical studies, and further unsuccessful drilling. ffyj Geo/og;'ca/ - 7976. Pleistocene geological studies. ( у7 -C f м'7 DTscorery аи(7 — 797&-7979. Subsequent to the discovery o f the ore deposit, additional geophysical surveys were carried out on the property. ÍAEA-AG-250/1 F7C.2J. уомгсе, 7979. 240 SCOTT A Turam survey (Fig.2I) over the north end o f the structure showed coincidence of a shallow conductive feature with the earlier VLF anomaly apparently related to lake bottom sediments. Vertical-loop and AFMAG electromagnetic surveys (Fig.22) over the south end o f the structure showed a weak, shallow, linear feature which agrees roughly with the trend of the drilling. This is interpreted as weak conductivity associated with a fracture zone in the overlying sandstone. Table I gives a rough breakdown o f our exploration costs distributed between activities and expressed in 1979 Canadian dollars. As an example of hindsight, Fig.23 shows the boulder train in relation to the point where the 'sandstone' uranium mineralization comes closest to the present surface. Based on our interpretation o f the drill results, this source area has a maximum strike length of 100 m. This is another example of a useful feature of prospecting in terrain which has been subject to continental glaciation. Mineralized glacial erratics are usually detectable over a much larger area than the mineral deposit source. The highly competitive nature o f the mining exploration industry in Canada made a large contribution to the original selection o f the mineral lands at Midwest Lake. If the Numac Group had not made an early decision, some other group would have acquired the holdings. The early discovery o f the high-grade boulder train led to a continuing effort, in spite o f the lack o f success, for the ensuing eight years. Finally, the development o f new theories based on the dissemination o f information among Canadian geologists led to the eventual discovery o f the deposit. DÎSCUSSION H.D. FUCHS: From the picture you have shown, the mineralized body lies more or less horizontal, which seems to be quite different from the other mineralized bodies in the area. Is the mineralization confined mostly to the regolith? How is the distribution o f the mineralization to the sandstone, to the regolith, to the basement? F. SCOTT: Away from the ore, the development o f regolith is only some tens of a cm thick. Because o f the strong alteration associated with mineralization, it is difficult to locate the unconformity exactly. I would estimate that 75% of the ore is in the sandstone. G. BUNDROCK: What is the composition o f those volcanic rocks intercalated with the sandstones? F. SCOTT: I think that the igneous rocks that cut the Athabasca sandstone are typical diabase dykes but in some cases they have been altered to muds and we assumed that some of them may be sill-like and conformable as well as dyke-like IAEA-AG-250/1 241 and unconformabie. The drill pattern we had o f vertical holes has not been an adequate test, and in a couple of sections we have igneous rocks which are earlier than mineralization and earlier than the alteration. R. GATZWEILER: Do you see a zonation o f the mineralization and alteration? F. SCOTT: The mineralization in the sandstone seems to differ from the unconformity and basement ores, and seems to have minor chalcopyrite and sphalerite as accessory minerals, rather than the nickel and cobalt arsenides. D.A. PORTER: Is the long narrow train related to a fracture system or to the unconformity? F. SCOTT: A 10- to 15-m change in the elevation o f the ore is associated with the long train (not directly), and we feel that that may represent some kind of fracture. D.A. PORTER: May I ask about the alteration of the halo — is it a radioactive halo? F. SCOTT: We almost always have some alteration with mineralization. D.A. PORTER: Do you see alteration before you see mineralization? F. SCOTT: Generally, yes. On some of these fractures we have up in the sandstones, it is just a matter of a few cm of altered rocks before we get to the mineralization. I would say the alteration is always larger than the visible radioactivity. J. DARDEL: Do you have reducate facies in the sandstones and have you identiñed some redox-front in the mineralized sandstones? F. SCOTT: We have not identified any obvious reductants in the sandstone. In drill cores, we have seen many examples o f miniature 'roll-fronts' on a cm scale. J. DARDEL: Are the ages o f the mineralizations in the sandstones and in the unconformity comparable? F. SCOTT: The ages of mineralization from Midwest Lake are comparable to other deposits in Saskatchewan. C. TEDESCO: Did you find graphite in the mineralized gneisses? F. SCOTT: Yes, but not necessarily in the same location as mineralization. Graphite is associated with the ore 70% o f the time, but 30% of the time it is not. C. TEDESCO: Do you believe the uranium halo in the sandstones to be contemporaneous or more recent than the unconformity related to the main ore body? F. SCOTT: I believe that the halo in the sandstone is earlier than the mineralization and that the mineralization is of one age, but some o f our geologists think that the higher level sandstone ore is more recent than the unconformity deposit. D.A. PORTER: Some publications mention Red Bed sequences in the Athabasca sandstone. Do you describe the Athabasca as such in the Midwest area? 242 SCOTT F. SCOTT: I have seen what we would call 'Red Beds' higher up in the Athabasca sequence. The sandstones at Midwest Lake are white, grey or buff coloured. D.A. PORTER: Would you describe the alteration associated with uranium in the sandstone host and the metasedimentary host? F. SCOTT: The alteration in the sandstone can be described as 'sericitization', with minor chlorite, principally due to potash solutions. With the unconformity ore, alteration minerals include sericite, magnesian chlorite, serpentine (septachlorite?) diagnostic o f both magnesian and potassic solutions. In the basement rock the dominant alteration is serpentinization, chloritization and the production of clay minerals. Some secondary quartz may be present. P. BARRETTO: From the figures showing the lake, the drilled holes and the ore zone, it seems that you have a wide range of high-grade ore 1—5% and 25% but also above the unconformity. Is there any stratigraphie or structural control for this high-grade ore? F. SCOTT: We feel that there are specific structural and/or lithological controls for the higher grade mineralization, but we have not yet identified such controls. D.M. TAYLO R: Were all the drill holes cored? Were the holes down-hole logged? Is there any disequilibrium? F. SCOTT: All holes were cored and, wherever possible, logged. We have no evidence o f disequilibrium and between 0.1% and 9.0% U^Og we have excellent agreement between the chemical assays o f core and the radiometric logs. tAEA-AG-250/8 FROM ARMCHAIR GEOLOGY TO A DEPOSIT IN A NEW URANIUM PROVINCE G. BUNDROCK UrangeseHschaft Canada Ltd, Toronto, Ontario, Canada Abstract FROM ARMCHAIR GEOLOGY TO A DEPOSIT IN A NEW URANIUM PROVINCE. Armchair geological considerations based on scarce data available in 1973 regarding supergene uranium mineralization (unconformity-related uranium mineralization) led to a five- year exploration effort in the Baker Lake area, Northwest Territories,Canada. Exploration procedure, techniques applied, and the results of this work are reviewed. Area selection had to be based on a 1:1 000 000 geological map in the absence of more detailed information. The location of the rim of the basin is extremely poorly defined owing to extensive glacial cover in the area. Uranium had been discovered in the general area in previous exploration efforts (1969—1971), but in settings apparently different from the mineralization sought at the time. The initial field work entailed regional geological mapping and prospecting surveys, airborne geophysics and geochemical surveys in an area of 4000 km^. Over a period of five years this area was enlarged, and ground totalling 10 000 km^ was surveyed. Owing to the short summers, work in this remote area is generally restricted to a 2^ to 3 months field season. In 1977 the project proceeded to exploratory drilling of a target detected in 1974 close to Sissons Lake. Mineralization was encountered, and best intersection exceeded 1% U^Og over a width of 33 m. The geological setting and certain characteristics of the mineralization suggest that the find represents mineralization of the unconformity type. Increased exploration in the Baker Lake area gave rise to local concern that the activities interfered with hunting and that caribou migration routes would be adversely affected. A land freeze and a court injunction were imposed on all explorers,which severely restricted activities during 1977—1979. In large areas, exploration could proceed during only one month per year. All work at Sissons Lake was affected and only restricted follow-up of the 1977 discovery was possible during 1978 and 1979. On the basis of present information, it is assumed that the discovery can be developed into a viable deposit. Judging from several additional anomalies detected during 1978 and 1979 by all companies, it seems likely that the area might become the second important province with unconformity-related mineralization in Canada. 243 244 BUNDROCK INTRODUCTION Urangesellschaft mbH, Frankfurt/Main, F R G , has been active in exploration for uranium in Canada since 1968. During an initial period it was the company's policy to join forces with experienced and knowledgable Canadian exploration and mining companies as only limited expertise existed within the company.- In 1973 Urangesellschaft substantially increased its activity in Canada and for this purpose an exploration entity vas established which was incorporated in 1974 as Urangesellschaft Canada Limited (in the following abbreviated UG Canada). During the start-up period of UG Canada a variety of genetic models was researched. Efforts concentrated on exploration for uranium.miner alization of the following three types: 1. Elliot Lake type quartz-pebble conglomerates, 2. Roessing-Bancroft type magmatic mineralization, 3. Rabbit Lake-East Alligator type - "supergene mineralization" (old nomenclature) - "unconformity related vein-type mineralization" (new nomenclature) This presentation is entirely restricted to UG Canada's exploration efforts for the third type of deposit in one particular region of Canada, the Baker Lake area. Northwest Territories (Figure 1). Over a period of six years,exploration work has progressed from arm chair geological evaluation to very detailed ground investigations. During the course of this work UG Canada has encountered significant mineralization at the Lone Gull. Showing, Sissons Lake, which we expect to be able to develop in due course into a viable deposit. Following this first discovery, a large area with favourable lithological and tectonic setting is likely to develop into a new uranium province with unconformity-related vein-type mineralization. This province would be second in Canada to the Athabasca Basin in northern Saskatchewan and Alberta. EXPLORATION ENVIRONMENT The project area lies 300 km north of the tree line. The vegetation consists of tundra grasses, bushes and lichen. Despite a prevalence ÏAEA AG 250/8 245 /. 0/,Й7%^7- íZ^f¿Z. of lakes and swamps,the area is essentially a semi-desert. The ground is permanently frozen to a yet undetermined depth. During the field season a top layer of a few inches to some feet thaws. The climate is arctic. Extremely severe winter conditions last from December to March with temperatures often around -40° to -60° C,which is aggravated by high winds. During the summer,which is essentially a 2 to 2.5 month period between the middle of June and early September, average daily températures reach 2° to 10° C. Due to the northern location (200 km south of the Arctic circle) day light is decreased to a few hours during the winter months. In the middle of the field season extended daylight allows up to 18 hours of operation. The area lies 50 to 100 km north and west of Baker Lake. Baker Lake is a settlement of approximately 1200 people of mostly native origin (Eskimo). 246 BUNDROCK The settlement has port facilities and access to salt water. Vessels of up to 500 tons find passage. An airstrip of limited length which is serviced by regular commercial scheduled flights is in existence. Baker Lake is situated some 600 km north of Churchill, Manitoba, which has railway, port and airstrip facilities and 700 km north of Lynn Lake, Manitoba, which has railroad and airstrip and is connected by road to southern Canada. All mobilization for the project is done from Baker Lake or from Lynn Lake. Some supplies are brought in by boat to Baker Lake and are then redistributed over the land during the winter or by small float-equipped aircraft. Additional freight is often flown in from Churchill to Baker Lake or to its field destination during the winter or spring months utilizing frozen lakes as natural airstrips. All transportation of personnel during the field season is carried out by helicopters or on occasion by small fixed-wing aircraft. Piston-type helicopters were used originally but are now superseded by jet helicopters of the type Hughes 500 D and others. REGIONAL GEOLOGY Geologists familiar with the latest discoveries in the Athabasca area, Canada, and the deposits and discoveries in the East Alligator area, Australia, will consider the geology of the central parts of the Canadian Northwest Territories a tantalizing situation. In northern Saskatchewan, the 400 x 300 km large Athabasca basin of Proterozoic continental sandstone overlies mainly Archean basement. A multitude of highgrade uranium deposits have been discovered,particularly along the eastern margin of the basin where it overlies lower-Proterozoic metasediments. In the central part of the Northwest Territories a similar basin of continental sandstones - the Thelon basin - with an extent of 500 km north-south and 200 km east-west overlies mostly Archean basement, but again rests locally on lower-Proterozoic metasediments particularly in the northeastern parts (Fig. 2). ÏAEA-AG-250/8 247 F7G.2. DMfr¡'&Mf:on o/M'íMíe Proferozo¡'c w Cenada. The central part of the basin is occupied by a game sanctuary and is not open to exploration. The southern area is very remote and the least known, and has dicouraged exploration for many years. The northeastern part, which here is referred to as the Baker Lake area, seems most attractive due to its possible equivalent setting to the Athabasca situation. The central area of the Keewatin District belongs to the Churchill structural province. Prevailing rocks are Archean gneisses and greenstones hJ 100* 00 BUNDROCK 64 6 4° ÏAEA-AG-250/8 !00° 94° F/G. J.. GeoJogy о/ Аойе о?*еа. bJ 4^. 250 BUNDROCK which were metamorphosed and intruded during the Kenoran Orogeny at approxi- mateiy 2 500 million years. Following an erosional period, lower- Proterozoic shelf sediments were deposited in an area 1 500 x 3 000 km. These metasediments can be found,e.g., in the Amer Belt and equivalents in the Baker Lake area, the Hurwitz area south of the Baker Lake area, and in the Wollaston Belt, Saskatchewan. The lower-Proterozoic sediments were deformed and metamorphosed during the Hudsonian Orogeny which yields dates for the Baker Lake area between 1750 and 1850 m.y. The lower-Proterozoic rocks in the Baker Lake area show greenschist to amphibolite metamorphic grades. A number of intrusives were emplaced. Following the Hudsonian Orogeny the area was subject to rifting and a sequence of continental middle-Proterozoic sediments and volcanics of the Dubawnt group were deposited. Rocks of the lower Dubawnt group, dated around 1800 m.y., consist of redbed sediments, volcanics and interbedded volcano-clastic sediments. Large areas were affected by the extrusion of calc-alkaline volcanics. These extrusives are contemporaneous with late to post-orogenetic granites which were emplaced in other parts of the area. Following a long period of erosion,an extensive sequence of continental sandstones (Thelon Formation) was deposited. Maximum sandstone thickness is in excess of 2 000 m. At the later stage of deposition a marine ingression occurred,similar to the Athabasca Basin. Youngest rocks observed in the area are isolated outcrops of limestone of possible Ordivician age. SELECTION OF PROPE*RTIES At the arm-chair geological stage of evaluation very little information was available for the selection process. The data essentially consisted of a geological map at a 1:1 000 000 scale covering most of the Keewatin District, the Eastern part of the Northwest Territories, and a map at a 1:250 000 scale covering a small part of this. According to the model of supergene uranium mineralization, ground with highest favourability would be located in the immediate vicinity of the unconformity, preferentially where a variety of lithologies was present in the basement complex. On a priority basis,ground was chosen in the infrastructurally better situated northeastern part of the Thelon basin (Fig. 3). IAEA AG-250/8 25! Ground was selected which covers more than 150 km of unconformity. Of this total,40 km overlie lower-Proterozoic metasediments, another 40 km lie on lower-Proterozoic metasediments and what was then thought of as a greenstone sequence of questionable Archean or Aphebian age. The remaining 70 km sand stones were presumed tooverlie undifferentiated gneissic Archean basement. Prospecting permits were obtained and detailed preparatory work for the field season revealed that most permits selected had actually been pre viously held by other companies. In retrospect it seems likely that, had this fact been recognized prior to the permit application, the company might have refrained from selecting these areas. Of particular interest in this context is the fact that the prospecting permit on which the Sissons Lake discovery is located had been surveyed and worked for two subsequent field seasons in 1969 and 1970 by Central Del Rio Oils Ltd.. and COMINCO Canada. Work performed included airborne radiometric and EM survey by a leading Canadian contractor followed by a field season of ground evaluation. Five other permits and adjacent ground had been in vestigated by Aquitaine Canada Ltd. This.company had undertaken a large areal assessment in the Baker Lake region similar to efforts by other French groups, e.g. in northern Saskatchewan, which led to the discovery of Cluff Lake..Their work entailed regional assessment,including airborne work and geochemistry,which actually led to the first discovery of uranium in the lower-Proterozoic Amer Belt in the Baker Lake area. Many showings with outcropping radioactivity now held by various companies in the Baker Lake area were actually located during Aquitaine's effort. The possible significance of these showings, though, was obviously not recognized at a time when little data were available on unconformity-related mineralization. HISTORY OF LAND ACQUISITION To put the outset of UG Canada's work in proper perspective , mention should be made of the state of knowledge of unconformity-related uranium mineralization in 1973. At that time, very little information on situations in Australia and at the Rabbit Lake deposit was publicly available, data only starting to be forthcoming in 1974. Thus the Baker Lake project was of a very speculative grass-roots recon naissance nature at the time of initiation in 1973 when UG Canada applied 2 for 11 prospecting permits comprising 6 500 km (Fig. 4). BUNDROCK -fïG.4. ¡УС Canada рголресЯ'я^ ;и rAe йа^ет- ¿aA;e area 7Р74-7Р7Р. ю Ln bJ 254 BUNDROCK Following the results of the first field season, the company acquired 2 additional prospecting permits covering 3 500 km in 1975 and in 1976 a 2 further 1 000 km were added. During the 1977 field season one of the targets which had been discovered in 1974 was drilled and significant mineralization, e.g. H U ^ g over 33 m, was encountered in the first drill-hole. The exploration model of unconformity-related mineralization occurring in the Baker Lake area was thus successfully confirmed. As parts of permits had been released, the discovery necessitated a dramatic enlargement of UG Canada's ground not only around the discovery site, but also where low-priority anomalies from various surveys were now significantly upgraded by the discovery. Claim staking was not possible at the time of the discovery due to a land freeze and this undesirable situation left favourable ground unprotected until May 1978. Over a period of more than 6 months it was uncertain to what extent the company had managed to keep the discovery confidential. After termination of the freeze the most important ground could be acquired in competition with various companies within a few days. Landholding within the original 1974 ground, which had decreased to less than 5% prior to staking,were increased to cover approximately 20% of the original prospecting permits. Parts of this ground were actually re acquired to the extent of 80% and more. 2 Further permits were acquired in 1978 and 1979 totalling 1 500 km . During the period 1974 to 1979 the company has in total performed recon- naissance-type work m an area close to 12 500 km 2 on ground held by the 2 company. In addition, 2 500 km of open ground was surveyed. At present the ground held by the company in form of claims and permits amounts to 2 approximately 5 000 km . Interest in the area has recently come close to land booms in the Athabasca Basin area. DISCOVERY HISTORY OF THE LONE GULL DEPOSIT The Lone Gull deposit is located in the southern part of the 1974 permits. This part was surveyed in the second part of the 1974 field season. Geochemical sampling was undertaken at a.density of one sample per 6 km^ using lake waters and lake sediments. A helicopter-mounted airborne system tAEAAG-250/8 255 - total count 350 cm^ - flew selected areas along the erosional rim of the Thelon sandstones at a quarter-mile spacing. Earlier in the field season the system had had two successes in the northern area. The strip of ground surveyed by the airborne system had a width up to 5 km extending generally 1.5 km on to the sandstones and 2 to 3 km into the basement. A distinct uranium anomaly, which belonged to the top ten of the survey, was encountered. The anomaly was immediately followed up by aban doning the systematic flying. An area with a number of distinct frost boils was identified, which proved to be the source of the high radioactivity. Contemporaneously, drainage geochemical sampling had been performed and samples from the watershed of the Lone Gull deposit had already been collected. Due to a backlog in the fieldlab,samples from the area had not yet been analysed. Witin a day these results became available and a sample from a lake 3 km south of the deposit yielded an anomalous radon value. We speculate that this in itself might have caused semi-detailed follow-up which presumably should have led to the discovery, even without the prior success of the airborne survey. Immediately implemented prospecting in the area led to the discovery of a second radioactive showing approximately 1 km distant (Fig. 5). Trenching attempts in the frost boil area were unsuccessful due to the perma frost. Analyses of material from the frost boils yielded uranium values in excess of 1% U^Og. Due to the proximity of the showings to granites it was not obvious at the time of discovery that true supergene mineralization had been located. The possibility was considered that the mineralization might be of hydrothermal nature and associated with molybdenite-bearing fluorite granites, which show above background radioactivity throughout the area. Very detailed surveys were performed over the next two field seasons prior to drilling,which comprised: Geology Airphoto interpretation, Aeromag interpretation. Detailed prospecting of outcrops and boulders. Detailed mapping of outcrops and boulders, Geomorphology survey. Ъ 1 3 ^ < T.56 ÏAEA AG-250/3 257 ^ ^ other EM indications fltlIHH anomoious geochemistry .FYC.ó. Lone См/? й^а. Geochemistry Soil sampling on grids with various densities using: a) organic-rich top soils,and b) organic-free till material In addition Rn-gas in soils was determined. Geophysics Scintillometer surveys, Magnetometer surveys, VLF surveys,and IP surveys. The extent of various geochemical and geophysical anomalies detected around the discovery site to allow a comparison with recently published data on other unconformity-related uranium mineralization is summarized in Fig. 6. <У) 00 TABLE L LONE GULL ANOMALY SIGNATURE Ground Radiometrics Airborne Radiometrics Area Extent Area Extent Anomalous Response 400 m Spacing 200 m 2 X b g 10 00 2000 m lines 10 lines U channel 3 X bg ( 200 cps) 300 X 300 m 3 X bg .. rad. 250 250 m 1 line 10 X bg ( 500 cps) 100 X 200 m 10 X bg., rad . 100 300 m 1 line M local spot high cj ( 10 000 cps) Z О P3 О Drainage Geochemistry (U group combined) Grid Geochemistry (U only) О PS Area Area top 2.5% 300 X 200 m 0.6 km^ 5 X bg 250 X 7 50 m 10% 1250 X 2500 m 3.1 km ( 5 ppm) 20 X bg 150 X 200 m ( 20 ppm) 50 X bg 75 X 75 m ( 50 ppm) Local maximum (frost boil) 1% ÏAEA-AG-250/8 259 г О F/C. 7. CeneraZized сгомчесУюп, ¿one GtvZZ ore Aody. An attempt has been made to describe the 'signature' of the Lone Gull mineralization (Table I ). This can be used to derive the spacing of surveys to be selected for geochemical and geophysical work to achieve an assured discovery of the Lone Gull deposit. For further comparison of the Lone Gull mineralization with other uncon formity-related uranium mineralization, a typical cross-section which con tains the discovery hole is shown in Fig. 7 . 260 ARMCHAIR GEOLOGY PHASE RECONNAISSANCE PHASE BUNDROCK SEMI-REGIONAL PHASE tAEA AG-250/8 262 BUNDROCK Í7G.9. Æxp?oraf!on procedure, <íeya:7e¿ pAaíe. 264 BUNDROCK The discovery exhibits many of the features characteristic of other unconformity-related vein type mineralization, e.g. alteration halo, dist ribution of high- and low-grade mineralization etc. EXPLORATION TECHNIQUES The exploration techniques applied by UG Canada progressively from regional assessment to detailed evaluation are discussed below (see Figs 8 and 9). Airborne Geophysics An essential component of UG Canada's regional and semi-regional areal assessment consists of airborne radiometric surveys and multi-system air borne work. A tenfold enlargement of crystal volume from 350 cm^ and the addition of a magnetometer and VLF-EM system and accessories of altitude monitoring and flight pass control, and finally a digital data acquisition system, reflect the significant role the system has played over the years. Emphasis has changed from direct to indirect methods. Thousands of line km have recently been flown with the major objective of obtaining detailed litho logical and structural data from magnetometer and VLF-EM. The reason for this development lies in the fact that after the Lone Gull discovery the importance of this information was fully recognized. To indicate the significance of the company's use of the system, it should be mentioned that during the last three years the system flew, on a multiproject basis, in excess of 15 000 line km per year. It is estimated that internal costs amount to 50 to 60% of contractor costs. The major advantage besides cost benefits lies in the extreme flexibility of employing the system on short notice regardless of contract volume, in priority areas. Geochemistry Application of geochemical surveys is a further significant component of UG Canada's exploration efforts. This is particularly due to two facts: 1) In the project area, glacial deposits of sometimes considerable thickness impede collection of geological information and diminish the effectiveness of radiometric surveys. ÏAEA-AG-250/8 265 2) On a regional scale it permits coverage of large areas in relatively short periods with the provision to adjust sampling density to specific requirements. Since 1974 UG Canada has tested and applied various geochemical sampling techniques, used a variety of sample media, field or laboratory-assayed samples for up to 1 dozen elements, and applied various statistical evaluation techniques. The following outlines in a brief form our experience On a regional scale UG Canada has experimented in drainage geochemistry with different types of lake sediment samples, e.g. lake centre sediment samples; near-shore lake sediment samples. Collection of lake-centre sediment samples has become standard practice. 2 2 Sample density varies from one sample per 6 km to one sample per 2 km . Based on our present experience the latter sample density seems a minimum in the physical environment in which we are working, especially considering the type of mineralization searched for. Anomalous areas defined by the above regional geochemical surveys or areas considered favourable, based on geological, prospecting and airborne work, are further evaluated in a semi-regional/semi-detailed phase. During this 2 2 phase,sample density varies from two samples per km to four samples per km To obtain these densities in drainage geochemistry the sampling includes ponds, streams and swamps. Detailed geochemistry entails surveys as follows: 1) Soil sampling on prospecting traverses: One sample every ^ km (on traverses with b to 1 km spacing); 2) Low-density sampling of organic-rich soils ongrids; 3) Medium to high density sampling of surface till material on grids; 4) Sampling of deeper till material by portable drill. Our experiences with organic-rich samples show that in many cases anomalies can be sufficiently recognized using this technique on low-density sampling (75 X 75 m) and the amount of tedious detailed work (Case 3) can be signi ficantly decreased, which is of essence in view of costs and the brief exploration period. - 2 6 6 BUNDROCK A standard regional sample location yields one water and one sediment sample. These samples are analysed for the following 'uranium group elements' : Uranium in water Radon in water Uranium in sediment Radium in sediment Under special circumstances waters have been assayed in addition for Cu, Pb, Zn, and lake sediments have been assayed for Cu, Pb, Zn, Co, Ni, Ag, Mn, Fe. Lengthy shipping periods and poor turn-around at commercial labs during the peak of the field season meant that assay results were only obtained after termination of the field season. Partly in expectation of this, assaying of waters for radon and sediments for radium was performed in the field. This procedure was continued in subsequent seasons as additional information was obtained. Availability of radon and radium results within days after sampling allowed immediate resampling on a more detailed basis. The effectiveness is shown by the following: In the first year, field assaying of radon and radium (single or combined) yielded 57 anomalies which were either resampled or directly followed up by prospecting. After the field season, when uranium assays for water and sediment were received,no anomalous areas additional to those 57 indicated by radium and radon were found, although the significance of some was en hanced. Over the years data from close to 10 000 locations have become available. In retrospect it can be said that the apparent duplication of assays has paid off well. In many cases high single anomalies of any of the four uranium group elements, not backed up by another anomalous determination, could be traced to mineralization. In other cases, the combined presence of 2 or 3 elements, though only moderately anomalous, enhanced a situation and thus established a distinct anomaly group worth prospecting. Very often high single values, particularly uranium in water and uranium in sediment, which might have been considered erratic, were surrounded by lower though anomalous values in other elements and thus could be followed up without prior reconfirmation. Soil samples are generally only assayed for uranium, although in some cases they have been assayed for uranium and thorium. In specific situations they have been assayed for a multitude of elements such as uranium/thorium, cobalt, ÏAEA-AG-250/8 267 nickel, copper, lead, zinc, chromium, molybdenum, vanadium. Significant element association recognized at this stage are: uranium and lead uranium, copper, lead, zinc uranium, copper, lead, chromium The poor turnaround of commercial laboratories eventually led to the estab lishment of a company laboratory, operating during the field season and capable of analysing uranium in waters and uranium in sediments and soils. The lab uses the newly developed uranium laser technique which requires semi-skilled chemical technicians and achieves reasonable output. Results, particularly of field-dried and field-sieved samples, are available on priority samples within 12 hours after arrival in the lab. In addition, the lab allows the flexibility of using different extraction techniques at very short notice on small batches of samples. Ground Geophysics Ground geophysical methods, with the exception of radiometric prospecting, are only applied at the semi-detailed and detailed stage. Over the years, indirect methods have gained precedence over direct radiometric surveys. Our standard procedure of evaluation of areas with showings detected by prospecting or areas considered favourable on the basis of other data entails extensive grid-controlled radiometric surveys with readings taken at inter vals of 10 to 75 metres and recording of interstation highs. Due to extensive glacial overburder^ very often no lithology and structural information is available as outcrops are often kilometres away. To assist the geological interpretation and possibly directly locate the traps con taining mineralization indirect methods such as magnetics, VLF-EM, Horizontal Loop EM and IP surveys are applied. In this regard, UG Canada geophysicists, who had extensive base metal background, had to go through an educational process,as targets in uranium exploration are very different from base metal targets. A compilation of what type of response can be expected on certain types of targets is presented in TableII. After the discovery of Lone Gull,UG Canada has in the meantime tested the application of some 'second generation geophysical methods'. One method entails gravimeter work. 268 BUNDROCK TABLE H. APPLICATION OF GROUND GEOPHYSICAL TECHNIQUES TARGET SEARCH INSTRUMENTATION VLF Mag IP Max-Min Grav Shear Zone X X Shear Zone Altered X X X - X Shear Zone Mineralized X X X X X F ault X (X) - - (X) Fault Altered X X X - X Fault Mineralized X X X X X Fracture (X) (x) - - - Fracture Altered X (X) (x) - - Fracture Mineralized X X X (x) - Dyke - X (x) - (x) Disseminated Sulphides - (X) X - X Massive Sulphides X X X X X Lithological Horizon (x) (X) (x) - (x) Lithological Horizon X X X X (x) Mineralized x: Positive response expected (x): Possible positive response No response expected Normal VLF work in the Northwest Territories depends on two US trans mitting stations. These transmitting stations are remote and often not at an optimal direction. Problems with signal strength and this directionality can be overcome with VLF systems using a local transmitter. It seems that problems encountered during magnetometer work due to magnetic storms can be partly overcome by gradiometer surveys. UG Canada's ground geophysical field data accumulates at an ever increasing rate and is now computer-processed. This is cost effective and produces reports and maps in much shorter time. IAEA AG 250/8 269 TableIllattempts to show all major instrumentation presently offered the Canadian and North American market from a variety of manufacturers. The instruments utilized by UG Canada in its exploration efforts are in heavy outlines. D r i l l i n q Diamond drilling has been performed over a period of 3 years. Major diffi culties arise from the inclement climatic conditions, in particular from the fact that drilling can only proceed after sufficient melt water becomes available or outside temperatures have increased to the extent that heated drill water pumped over long distances does not freeae in water lines. Except for the top few feet, the ground is permanently frozen, and drilling and particularly servicing and surveying of the holes can only be achieved with the addition of salt to prevent freeze-up of the drill holes. While drilling on a set-up is in progress, the permafost conditions require that drilling continues around the clock in two or three shifts. In 1977 unexpected problems with land use permits made it impossible to utilize conventional drill equipment. Light-weight equipment (1000 lbs) of the type 'Hydra Mink' with a coring capacity of approximately 500 ft (BQ wireline) had to be utilized and six holes totalling 2 500 ft were drilled at a multiple of normal drilling costs. Since 1978 drilling pro ceeded with conventional equipment of the following type: BBS - 17 with a coring capacity of 2 000 ft (BQ wireline). Drilling is performed by a contractor with arctic experience and an average of 80 ft per shift which includes standby and moving times is generally a c h i e v e d . STAGES OF PHASED EXPLORATION Due to the fact that UG Canada acquired large blocks of land over consecu tive years, exploration work has been quite complex,with surveys at diffe rent levels of exploration occurring at the same time. An attempt has been made to diagrammatically present UG Canada's exploration procedure as applied in the Baker Lake area. Field work has to be performed 270 TABLE III. INSTRUMENTATION AVAILABLE GEOPHYSICS GROUND ECU IPMEN1 ELE с т а о м я : ч с т : с 5 gS Ж ö"o 5 } § COMPANY o á Üis § ! 1sa ЯЫ Й o!Ü АШ* ttEbEAR Alphmeter МЮЯЧ E lí„ t í H T í t СЖРЮЕ Unitized o o m cK W K tcs C EM mom Í í ü "-snMCMrs E r " BUNDROCK S i S * ° :* 2 ^ att? L ш тк no) кмкдс UDLiM Model 2000 E i E * ftWT SOPRIS 1000 - с C L W СИ. - 2 кг - 1 SHCERCEOFWSICS urn s BGS-1SL. 2 HA - 2 SE - 77 - й - 3 MBS - 2 SMT SPP-2 MF URHt i " VARI^ V 100-102 IAEA-AG-25 0/8 27! Yeor during a 2^ to 3 months field season due to climatic constraints. This essen tially forces us to concentrate mainly on data collection. The remainder of the year is available for extensive data processing and assessment. During a three to four year period, evaluation of any new area generally proceeds in four phases which are called: Reconnaissance phase. Semi-regional phase. Semi-detailed phase, Detailed phase. BUNDROCK TABLE IV. BREAKDOWN OF EXPLORATION EXPENDITURE, 1979 Major cost centres Transport 25% Drilling 2 2 % Personnel and travel 15% Fuel 1 0% Accommodation and support 8 % Assaying 5% Instruments ' 5% 90% Environmental study 5% 95% Miscellaneous 5% 1 0 0 % TABLE V. SUMMARY OF EXPLORATION COSTS PER km\ BAKER LAKE AREA: AIR PHOTOGRAPHY, GEOLOGICAL MAPPING AND PROSPECTING Type of survey Cdn. $ Air photography: black and white 17 colour 2 0 Geological mapping and prospecting: Regional 20 Semiregional 120 Detailed 350 ÏAEA AG-250/8 273 TABLE VI. SUMMARY OF EXPLORATION COSTS, BAKER LAKE AREA: GEOCHEMISTRY TYPE 0 F SURVEY UNIT С 0 S T ^ S e d im e n t W a t e r f o r S e d . DRAINAGE GEOCHEMISTRY f o r U u, Rn f o r U, Rn d e n s i t y . 1 p e r 5 km km^ $ 10 $ 9 $ 11 d e n s i t y . 1 p e r 1 km^ kn^ $ 40 $ 30 $ 45 d e n s i t y . . . 2 p e r 1 km^ km^ $ 60 $ 50 $ 70 S O IL f o r U s p a c i n g . 75 X 150 m $ 500 s p a c i n g . 50 X 50 m km^ $ 3500 T I L L f o r U s p a c i n g . . 250 X 250 m s i t e $ 1 0 s p a c i n g . 30 X 30 m s i t e $ 35 SOIL for GASES Rn ...... Track-Etch s i t e $ 22 . Alpha-Nuclear s i t e $ 22 He s i t e $ 55 SOILS for Pb -ISOTOPES s i t e $ 25 3 Canadian dotlars. Diagrams attempting to summarize relevant data such as Size of area in various phases, Exploration methods applied. Objectives of surveys. Status of anomaly, targets and showings,and Action required are presented in Figs 8 and 9. DETAILS OF COST UG Canada's efforts in the Baker Lake area amounted to a multi-million dollar exploration campaign during the period 1974 - 1979. .The development of expen diture volume over the years is presented in Fig. 10. Expenditures are not quoted in actual dollars for obvious reasons and units indicated are arbitrary. 274 BUNDROCK TABLE VH. SUMMARY OF EXPLORATION COSTS, BAKER LAKE AREA: AIRBORNE AND GROUND GEOPHYSICS, DRILLING TYPE OF SURVEY COST PER L IN E k m ^ AIRBORNE GEOPHYSICS (excl. fuel) Total Count 375 cm^ helicopter .... S 4 Spectrometer + Mag 1700 " $ T ' " " t u ^ p t e r Spectrometer + Mag 3400 " S10 " * t ! ^ p ° e r Large System 3400 " helicopter .... $13 (incl. Mag, EM-VLF, A n a l o g u e + D i g i t a l ) COST PER L IN E km GROUND G EOPHYSICS V L F ...... $ 45 M A X -M IN ...... $ 55 V e r t i c a l L o o p ...... $ 65 P u l s e EM ...... $ 55 Tu ram ...... $155 I P ...... $155 $160 DRILLING (DIAMOND DRILLING) COST PER METRE Indirect All Incl. 1 000 m $ 75 $ 30 $ 210 2 000 m $ 65 $ 27 $ 165 ' 4 000 m $ 60 $ 27 $ 150 10 000 m $ 55 $ 25 $ 135 ^ Canadian dollars. Expenditures shown are on a yearly basis and are separated only into explo ration and drilling costs. A break-down on a percentage basis of 1979 expenditures is presented in Table I . The cost distribution between various cost centres reflects the particular situation of the remoteness and the arctic climate of the Baker Lake area. t AE A-AG-25 0/8 275 An attempt has been made to summarize specific exploration expenditures for all relevant surveys and drilling operations in Tables V, VI andvil. These costs are expressed in 1979 Cdn.Dollars. Many of the detailed costs are essentially based on the fact that expenditures for a man per day, based on a 100-day field season, amount to $350.oo per day all-inclusive. ACKNOWLEDGEMENTS The success of UG Canada's exploration in the Baker Lake area is based to a large extent on the hard work and enthusiasm of many individuals participating over the years. To name a few individuals who are no longer with the company but who were instrumental in the selection o f the area and the first discoveries, acknowledgement should be given to Dr. U. Klinge, Robert H. Morse and Eric P. Onasick. DISCUSSION D. TAYLO R: Could you indicate the known extent of the mineralizations? G. BUNDROCK: The Lone Gull mineralization is now under investigation in its third year. Due to severe land-use restrictions and a court injunction, very little work could be performed during that period and the overall picture is piecemeal. Mineralization is now known and has been confirmed by drilling over approximately 2 km strike length. Mineralization seems to be discontinuous. C. TEDESCO r Please give some details about your experience in the use of the portable laser analyser. The manufacturer declares important interferences from Fe and Mn - do you also use it for sediments? G. BUNDROCK: Urangesellschaft has been involved with others besides the manufacturer in R&D of the laser instrumentation and techniques. We have now been successfully using the technique in geochemical exploration for two years; Our laboratory is located at our expediting base but I see no reason why it should not work under field-lab conditions providing a 1000 to 1500 W generator with reasonable voltage stability is available. We are using the laser for determinations of uranium in water, sediment and soils (except very organic-rich material). We have encountered problems with high organic contents in the solutions. Fe and Mn have not posed problems, maybe owing to relatively low concentrations in our samples. It should be possible to overcome problems of this sort by using a spiking method. At the present stage we do not think that laser assay results with our laboratory set-up are 'precision-assays'. This, in our opinion, is not necessary at the exploration geochemistry stage where often only top 2.5%, 5%, 1 0 % and/or top 2 0 % o f the assay data is taken into consideration. 276 BUNDROCK C. TEDESCO: Could you also use this for analysing sediments? G. BUNDROCK: Yes, that's what we are doing. You use a normal digestion process and then you go on to subdilution, and you can process that material. Actually the 30 000 samples I quoted are probably in excess of 25 000 samples o f lake sediments and soils and the remainder are water samples. P. BARRETTO: Concerning Mr. Tedesco's question, I should like to add that the only problem observed so far with the UA-3 analyser was with the laser beam. Frequent changes of the laser tube (3—4 months) are needed depending on the number of samples analysed. Concerning the technique itself, care must be taken in the case of murky water or high content of aluminium (causes precipitation) and iron or manganese (lower results). In analysing stream sediment samples, attention must be given to the solution pH as acidity will interfere with the fluorescence, lowering the results. It is the same for high salinity, but then the manufacturer suggests the use of FLURAN-2. G. BUNDROCK. We have one additional problem when we are dealing with hard water. Then we would prefer to send it to our own laboratory, otherwise we do not get any explainable results. B. GUSTAFSSON: What kind of till samples do you have? What do they look like? G. BUNDROCK: Glacial till - we attempt to get samples from the basal till which very often we do not manage. Rather than not take anything, we attempt to take the lower parts of the normal till, which partly gets us round possible leaching problems. Large areas have been beaches some thousand years ago, and the wave action has certainly removed some of the uranium. So we think by getting a deeper horizon we might get a more representative sample. And again, very often we want to avoid volcanic-rich polluted surface material. And that's when we are satisfied with normal till sample. But it would really be optimal to get down to basal till. B. GUSTAFSSON: Are you interested in a certain grain size in the till? Do you preconcentrate? G. BUNDROCK: No. We have done some tests on grain size but they were of no use to us. F. SCOTT: Is the frost-heaved material spread out down-ice? G. BUNDROCK: The frost-heaved material seems to be developed more or less directly over the sub-outcrop of the mineralization. The frost boils are partly located on a hill slope, and some down-hill movement can be observed. B. TAN: How far is the ore body from the unconformity (Dubawnt Sandstone)? G. BUNDROCK: The Lone Gull deposit is approximately 2 km south of the rim of the Thelon Sandstones. The vertical distance from the unconformity is uncertain. No remnants o f sandstone or regolith have been found in the drilling to date. From horst and graben features observable in the area we think it might be 50 to 150 m below the unconformity. IAEA-AG-250/8 277 D.A. PORTER: Was your first hole, which was a discovery, located on an airborne anomaly? G. BUNDROCK: The airborne anomaly is centred on a group o f frost boils. The first drill-hole was set up to intersect ground approximately 50 m below them. D.A. PORTER: What was the thickness of the overburden over the ore? G. BUNDROCK: Overburden thickness varied from 5 m to more than 30 m. B.L. NIELSEN: When you foresee the discovery of a new uranium province in the Baker Lake area, is this forecast mainly based on UG's discoveries in the area or on the geological similarities with the Athabaska region? G. BUNDROCK: The assumption of a new uranium province is in the first place based on favourability considerations in the context of similarities with the Athabasca area. In addition, exploration and mining companies, including UG, have come up with a number of interesting showings and anomalies during the last two years which do indicate that direction. ÏAEA-AG-250/9 EXPLORATION OF BERNABE MONTANO COMPLEX OF URANIUM DEPOSITS, NEW MEXICO, USA D.A. PORTER Conoco Inc., Denver, Colorado, United States of America Abstract EXPLORATION OF BERNABE MONTANO COMPLEX OF URANIUM DEPOSITS, NEW MEXICO, USA.. The Bemabe Montano discovery is a significant eastern extension of the Grants Minerai Belt, consisting of two nearly parallel mineralized trends with a combined strike length of about 14.5 km. One deposit with approximately 10^ lb of uranium oxide has been blocked out and several km of mineralized trend require additional delineation driHing. The mineraliza tion exhibits many similarities to Westwater Canyon Member ore deposits in other parts of the Grants Mineral Belt; one of the most significant is the continuation of the south-easterly trend that has persisted, with some breaks, for a length of over 175 km. As with other Grants Mineral Belt deposits, the mineralization is associated with multilevel húmate masses that are roughly parallel to the bedding of the Westwater Canyon Member host sandstone beds. These húmate masses and the associated uranium deposits show a marked preference for the margins of the thicker, more laterally continuous, channelways. The discovery of the Bernabe Montano complex of deposits is significant for several reasons. First, it opened up exploration in the distal fan facies where many geologists thought the uranium potential was relatively low. The discovery is potentially more significant in that it demonstrates the ability of detailed subsurface geologic mapping to suggest the location of high potential geologic trends in partially explored but favourable regions where the more traditional surface geologic and radiometric techniques are no longer effective in finding new deposits. 1. INTRODUCTION The Grants Mineral Belt in north-western New Mexico (Fig. 1) contains some o f the largest sandstone-type uranium deposits known and more than one third o f the known uranium reserves o f the United States o f America. The deposits occur in a belt 16-48 km wide extending for about 175 km in a north-west to south-east direction (Fig. 2). The well-known Ambrosia Lake District is situated midway along the Mineral Belt to the north of the town of Grants. This paper describes the exploration history leading to the discovery o f the Bemabe Montano complex o f deposits, located at the extreme south-eastern end of the Grants Mineral Belt. Special emphasis is given to the exploration philosophy and events that led to this major discovery. 279 280 PORTER F/C. i. Сеяега/ АэсаПон o / № w Л/exico, C ran íí №'яега^ Де/; ан üp< BERNABE MONTANO . F ш F # о 0 50 100 km or 1______:______I______! 1 fYC.2. СгаяУу М'негя/ ße?f /^сс^'оя о/Жоягсяо Сгял?. ÏAEA-AG-250/9 28! F7G. J. Generalized ео/мтпаг лесП'оп о / fAe Aíorr^on Formafi'on ¡'л f7¡e GranM Minera/ Де/f. Уас^р;7е and ^*¡?M¡?n Салуол are ргелен? о л ^ ¡n ¿oca/ area,!. 2. GENERAL GEOLOGIC SETTING The Grants Mineral Belt extends across most o f the southern margin of the San Juan Basin and is bounded on the east by the Rio Grande Trough and on the west by the Defiance Uplift (Fig. 2). The sedimentary rock units which host the uranium deposits are exposed in outcrops along the flanks of the Zuni and Lucero Uplifts just to the south o f the San Juan Basin. The regional dip of strata comprising the Grants Mineral Belt is approximately 2°—3° NNE into the central San Juan Basin. The dip is disrupted locally by folding and faulting (Hilpert (1963) and Kelly (1963)). The Bernabe Montano complex o f deposits is located about 60 km east of the Ambrosia Lake District. It is structurally situated on the western flank of the Rio Grande Trough and the south-eastern flank o f the San Juan Basin, where normal faults striking N25—ЗОЕ form a series o f narrow horsts and grabens that keep the host rock at 460—610 m depth until large faults on the east end of the trend drop it into the Rio Grande Trough beyond economic depths. This faulting is responsible for a reversal of dip in the Bernabe Montano area on the order of 3° to the south-east. 3. STRATIGRAPHY Sandstone beds of the late Jurassic Morrison Formation are the most important host rocks in the Bemabe Montano area and the rest of the Grants 282 PORTER Mineral Belt, and this brief stratigraphie review will therefore be limited to the Morrison Formation. The unit ranges in thickness from a thin wedge, partly due to pre-Dakota erosion, to over 180 m across this region. The Morrison Formation is subdivided into three main members which are, in ascending order, the Recapture, Westwater Canyon, and Brushy Basin Members (Fig. 3). The Recapture Member is the basal unit o f the Morrison Formation in the Grants Mineral Belt. It is variable in thickness and consists o f 15 m to more than 75 m of reddish-brown to grayish mudstones and siltstones with lenticular inter beds of gray limestones and gray to white, fine- to medium-grained,sandstones. The Recapture Member has a transitional conformable contact with the under lying aeolian Bluff sandstone and intertongues with the basal part of the overlying Westwater Canyon Member. The Westwater Canyon Member sandstone beds are the host for the uranium deposits of the Bernabe Montano Complex and most o f the rest of the Grants Mineral Belt. This unit consists of a series of feldspathic, medium- to fine-grained, sandstone beds that often contain lenses o f coarse to very coarse sands. The sandstones are generally gray to light reddish-brown in colour, poorly sorted, cross-bedded and moderately well cemented. The sandstone beds are separated by gray-green and locally red-coloured lenticular mudstone beds which are highly variable in thickness. The thickness of the Westwater Canyon varies from less than 30 m to more than 105 m and often has an arbitrarily defined intertonguing- type contact with the overlying Brushy Basin Member. The Brushy Basin Member ranges from 0 to approximately 90 m in thickness. It is a variable series of grayish-green bentonitic mudstones with subordinate amounts of lenticular gray siltstone and fine- to medium-grained sandstone beds that become more numerous in its basal part. Local sandstone channels in the Brushy Basin Member have been mapped and named for economic purposes, such as the Jackpile Sandstone at the top o f the Member in the Laguna District and the Poison Canyon Sandstone which occurs at the base of the Member in the southern Ambrosia Lake District. The Brushy Basin Member is separated from the overlying Cretacious-age Dakota Sandstone by a regional unconformity. Most regional geologic studies of the Morrison Formation have concluded that it is a complex sequence o f continental braided stream and flood plain-type sediments. These strata were deposited on a very large, wet, alluvial fan whose proximal part is believed to have been located in west-central New Mexico. The regional depositional patterns and source area in south-central Arizona have been well documented by Craig et al. (1955), Saucier (1976) and Galloway (1979). 4. EXPLORATION HISTORY AND GUIDES When Conoco started work in the area in late 1967, nearly two decades of successful exploration by industry had taken place along the southern margin ÏAEA-AG-250/9 283 of the Grants Mineral Beit. In areas such as the Ambrosia Lake District no significant amount of prospective exploration lands could be acquired at reasonable costs near existing deposits or at depths of less than 450 m. Conoco management, when faced with these economic barriers, decided to base its exploration strategy on the search for entirely new mineralized trends rather than buy relatively small parcels o f expensive lands that might contain extensions of well-known mineralized trends. To implement this decision Conoco had to: (1) explore outside the Grants Mineral Belt in New Mexico, or (2) develop an exploration programme significantly different from the outcrop and mine-extension techniques in use by companies established in the Grants Mineral Belt. Conoco attempted the first approach for approximately two years — with no success. The company then reassessed the situation and decided to try the second alternative by developing a different exploration programme for the Grants Mineral Belt. This programme was regional in scope and began with a review of the extensive geologic data o f the Morrison Formation produced by government agencies and industry. Field studies o f the prospective host rocks and visits to active mines were conducted concurrently with the review o f the geologic literature in order to develop a foundation for the development o f a realistic exploration technique for the region. This preliminary work provided a number of general characteristics that are common to most o f the uranium deposits in the Westwater Canyon Member host sandstones: (1) The host sandstones were deposited in a series of main channelways whose axes were generally parallel to the prominent, pre-Laramide north-west to south-east tectonic grain as displayed by the Zuni Uplift. (2) Most o f the uranium deposits lie roughly parallel to the north-west to south-east tectonic grain and the axes o f the host sandstone's channelway s. (3) Mapping in the Ambrosia Lake District by Santos (1970) suggests that ore deposits tend to be located in the more permeable zones of the host sandstones. (4) The ore bodies often occur as separate multilevel ore bodies enclosed in long 'trends' of low-grade uranium shows. These trends tend to be parallel to the local sedimentary and structural trends of a given area. (5) Two general types o f uranium ore bodies are known. These have been termed (a) 'primary', 'trend', or 'prefault'; and (b) 'secondary', 'stack', 'redistri buted', or 'postfault' ore. The first category, or 'primary' deposits, are found in . long, roughly tabular envelopes that parallel the main depositional trends. The second category, or 'secondary' deposits, are found in fracture and fault zones that generally cut obliquely across the sedimentary channelways and 'primary ore' trends. ( 6 ) Primary ore bodies are roughly tabular or blanket-shaped, but they generally have one long dimension which tends to be parallel to the trend of sedimentary channelways. 284 PORTER (7) Primary ore is often associated with dark húmate masses that are found roughly paraHei to the bedding o f the Westwater Canyon Member sandstones. ( 8 ) Coffinite, a uranium silicate, is identified as the main unoxidized ore mineral. (9) Uranium content in primary ores can exceed 1%, but an average grade approaching 0.20% U^Og is more common throughout the Grants Mineral Belt. (10) Abnormally high concentrations o f vanadium, molybdenum, selenium and arsenic tend to fringe the ore bodies (Squyres, 1969). (11) The host sandstone beds are often oxidized to a reddish-brown colour and are essentially barren on outcrops, while the uranium ore tends to be found down-dip in gray, reduced host rock. (12) Oxidation appears to have encroached down-dip and to the north until disrupted by primary mineralization or the ore deposits. 5. EXPLORATION TECHNIQUES Early exploration in the Grants Mineral Belt utilized airborne and surface radiometric surveys, geologic mapping, and exploration drilling down-dip from surface shows. These techniques had yielded several important discoveries, but after initial successes they were marginally effective in this region. Conoco's late entry into exploration made it necessary to employ a different exploration philosophy to provide guides towards the discovery of new mineralized trends with no surface expression. It was determined fairly early that in this region remote sensing, geophysical, geochemical and elemental analysis techniques were apparently largely insensitive to the primary factors associated with uranium, and, on a cost-result basis, could not be justified. A review o f the geologic literature suggested that the main favourable factors associated with uranium deposition in sandstones were related to: ( 1 ) depositional trends and facies in the channelway systems o f the host sandstone beds; ( 2 ) structural elements that disrupt these channelway systems; and (3) geochemical factors that influenced deposition and later movement of the uranium. The search for subsurface areas with these favourable characteristics required a synthesis of all available surface and drill-hole information that could be obtained. The techniques selected for this synthesis were adapted from subsurface geologic mapping techniques, such as isopach, lithofacies, sand-shale radio and structure contour maps, as routinely practised in exploration for hydrocarbons. Available information on the geochemical state (oxidized-reduced) of the host sandstone beds was superimposed on these maps. To develop the regional subsurface control required to make this mapping realistic, it was necessary to obtain a broad network of subsurface borehole logs. This was accomplished through many trades o f logs with other companies active IAEA-AG-250/9 285 FAN FACIES F/G. 4. Generated /ae¡'eí о/ гЛе IVeífwaíer Canyon /an /ron; Æe//y f7P77^ and GaHotvay^P79^. in the region. At that time, log trading was normal procedure in hydrocarbon exploration but not with the established uranium companies. In trading information, Conoco in many cases used data from holes outside the Grants Mineral Belt area. The logs traded were radiometric, electrical and lithologie. The locations of boreholes were plotted on series of regional subsurface geologic maps, and the stratigraphie and structure data derived from the borehole logs were integrated into an evolving interpretation of the Westwater alluvial fan. These maps helped identify the different facies and structural elements of the alluvial fan. They also demonstrated the geologic habitats of uranium mineraliza tion known at the time and helped to focus land acquisition efforts on areas with similar geology. Regional isopach maps o f the total Westwater Canyon Member proved to be the most useful of the subsurface maps constructed by Conoco during the early exploration years. These maps showed that three major deposition environments could be recognized across this large alluvial fan (Fig. 4). These were (1) a proximal facies near the head o f the fan in the south-western Grants Mineral Belt; (2) a medial facies in the central part of the Grants Mineral Belt; and (3) a distal part in the northern and south-eastern parts of the Grants Mineral Belt. Each o f these major regional fan facies is highly variable and can contain ore deposits. 286 PORTER The proximal fan facies is interpreted to have been a high-energy fluvia! regime. This facies can be recognized by its very broad Westwater Canyon channelways that are almost entirely massive sandstones. The medial fan facies is characterized by a more variable Westwater Canyon section characterized by large channelways that show considerable lateral variation between the massive sandstones along their axes and the sandstone beds with increased shale interbeds along the channelway margins. The distal fan facies has comparatively narrow channelways between broad interchannel areas of silty sandstones and shales. When known ore bodies and mineralized boreholes were superimposed on these regional isopach maps, a geologic picture o f the habitats o f uranium in the Grants Mineral Belt began to emerge. The maps showed the locations o f the principal channelways across the region and highlighted the affinity o f uranium deposits for channelway margins. Sections made across the channelways showed that ore bodies are less apt to occur: (1 ) where the Westwater Canyon Member section is almost entirely massive sandstone; ( 2 ) in areas o f excessively thick shale interbeds; and (3) in highly oxidized areas. These isopach maps also pointed out that ore deposits are often found where highly transmissive channelways are disrupted by structural deformation or stratigraphie variations. In areas of sparse borehole control these isopach maps were based on projections from known channelways and mineralization. Study o f these regional maps showed that one o f the most significant similarities across the Grants Mineral Belt is the south-easterly trend, with some breaks, o f both the main channelways and accompanying ore bodies. Significant deviations from this south-eastern pattern generally appeared to be associated with faults or fractures. 6 . BERNABE MONTANO DISCOVERY: ACQUISITION AND EXPLORATION Early regional isopach and mineralization mapping done by Conoco high lighted the potential o f the undrilled Bernabe Montano Grant, which at that time was located about 32 km north-east of the nearest uranium production. This mapping suggested that the south-easterly Westwater Canyon trends that were so prominent in the rest of the Grants Mineral Belt also seemed to be present in this area o f distal fan facies. Regional projections o f the stratigraphi- cally higher Jackpile sandstone's channelway and mineralized trends also seemed likely to continue their known north-easterly courses across the Grant. The Bernabe Montano Grant was offered by the Laguna Indian Pueblo at a sealed-bid lease sale in late 1970. Conoco was the successful bidder on the northern three-quarters o f the Grant with a sum o f nearly US $ 1 000 000 bonus on approximately 37 500 acres in a block 9—10 km wide by 16 km long (Fig.5). ÏAEA-AG-250/9 287 F/C. J. Жар о/йегляЬе M oníano СгаяУ í/!owing елр/огсУюя permif алеа o&fai'nej 6^ Conoco. Drilling was begun on the property in January 1971 and the channelway and mineralization projections were largely verified. Mineralization was found in both the Jackpile sandstone and the Westwater Canyon Member, but only the Westwater Canyon gave enough encouragement for follow-up drilling. Rapid evaluation of the Grant at a minimum cost was the main objective of the initial drilling programme. To meet this objective, a reconnaissance borehole pattern was devised to take maximum advantage of the long and short axial directions exhibited by known Grants Mineral Belt deposits (Griffiths and Singer, 1969). The Phase-1 drilling programme consisted o f staggered fences o f boreholes spaced 1220 m apart in a north-to-south direction and 1830 m apart in an east- to-west direction (Fig. 6 ). On alternate fences, boreholes were staggered about 600 m north to south to form a diamond pattern with a density o f five holes in about 10.5 km^. Seventy-two holes totalling approximately 31 000 m were drilled in the Phase-1 programme, at a cost o f approximately US $250 000. The Phase-11 drilling programme filled in the diamond pattern so that hole spacing was approximately 600 m in a north to south direction and approximately 900 m in an east-to-west direction, for a density o f four holes in about 5 km\ 288 PORTER )HOLE LOCATION The Phase-11 programme brought the total drilling to 198 holes and about 91 000 m, with direct costs at nearly US $500 000. The Phase-1 and Phase-11 drilling programmes covered the entire acreage block controlled by Conoco. This drilling yielded two holes with ore intercepts o f thickness greater than 1.8 m and grade higher than 0.1% U^Og. The drilling also defined two broad areas with anomalous radioactivity. The initial geologic evaluation, from the drilling, was conducted with downhole electric and radiometric logs and geologic study o f bit cuttings collected over 1.5-m intervals. No cores were cut during Phases 1 and 11. Additional drilling was conducted in 1971 to test other favourable geologic and anomalous areas. By the end o f 1971 two mineralized trends with a combined length o f almost 14.5 km were established. It had taken 300 holes with depths of 300-750 m to find these mineralized trends in the Westwater Canyon Member. Evaluation of this drilling also allowed the non-prospective northern third of the original block to be released at the end of 1971. ÏAEA-AG-250/9 289 О 5 km L------< F/C. 7. freienf acreage OMiù'ne yeneya/ ¿Mfn'&Míi'cn o/ iwo wain mmeraZizan'on írenífs. Subsequent drilling was limited primarily to delineation-type exploration of the two main mineralized trends (Fig. 7). The first delineation phase decreased hole spacing to about 300 m and then to about 150 m on north-to-south fences spaced approximately 900 m apart. The next delineation phase decreased the 900-m fence spacing to about 300-m spaced fences in selected areas across the north-east to south-west trending mineralization. Eventually the most favourable areas were drilled on a selective spacing of approximately 1 2 0 m with the mineable areas blocked out on about a 60-m grid. Over 2000 boreholes have been drilled to date on this project. 7. BERNABE MONTANO MINERALIZATION TRENDS Ore mineralization is found at every stratigraphie level of the Westwater Canyon Member in the Bernabe Montano complex of deposits. This gives the ore trends an initial appearance o f being highly discontinuous and poddy. However, in a restricted area within the mineralized trend the ore is usually confined to the upper, middle or lower parts o f the Westwater Canyon Member. 290 PORTER This variation is interpreted as being the resuit o f the rapid sedimentologic and geochemical changes that are characteristic of the distal fan facies. The rapid changes in sedimentologic and geochemical conditions may also be an important factor in creating the large continuous halo around this complex o f ore deposits. More detailed studies o f the nature of these ore bodies will have to await underground mine development. The grades of uranium mineralization vary considerably from several times background to high-grade intercepts in excess o f 1.0% eU^Og. Average, in-place, radiometric grades for individual ore pods vary between 0 . 1 2 % and 0.17% eU^Og using a 1.8 m/0.10% cut-off. Ore zones average up to 60 m or less in width. Engineered ore reserves of over 10 000 000 lb of uranium oxide have been blocked out to date. Considerable potential for additional ore reserves remains within the ore trends. The reader is referred to the recent paper by Kozusko and Saucier (1980) for a more comprehensive description o f the Bernabe Montano Grant's geology and ore deposits. 8 . GENERAL OBSERVATIONS ON THE BERNABE MONTANO DEPOSIT The Bernabe Montano complex of uranium deposits is in most respects typical o f those found in the Grants Mineral Belt, but this area has some interesting geologic variations. Published data on other Grants Mineral Belt deposits indicate that they have relatively narrow mineralized halos. The halo enclosing the two mineralized trends in the Bernabe Montano area is unusually large. Other unique geologic features of this area which do not appear to be genetically related to the mineralization are ( 1 ) the position of the oxidation interface on the north side of the mineralized area; ( 2 ) structural deformation by late Tertiary-age faulting that offsets ore in a series of horsts and grabens; and (3) the south-easterly dip of the strata. Oxidation along the northern edge o f the mineralized area may be responsible for the less extensive area and lower grades of the northern trend compared to the southern trend. The later Tertiary-age faulting is younger in age than that known in other parts o f the Grants Mineral Belt but is no more extensive than that present in some other areas. ACKNOWLEDGEMENTS The author would like to thank Conoco Inc. for permission to publish this paper. Special acknowledgements are due Mr. L.W. Koch and Mr. L.W. Heiny, who provided important management support for this exploration venture. . Acknowledgements are also due Conoco's Albuquerque District geological staff, who worked on the development o f the Bernabe Montano exploration programme and gave data assistance for this report. IAEA-AG-250/9 29! BIBLIOGRAPHY Craig, L. C., and others, 1955, Stratigraphy of the Morrison and related formations, Colorado Plateau region, á preliminary report: U. S. Geol. Survey Bull. 1009E, p. 125-168. Galloway, M. E., 1979, Morrison Formation of the Colorado Plateau, in Depositional and groundwater flow systems in the exploration for uranium: Bureau of Economic Geology, Univ. of Texas at Austin, Austin, Texas, p. 214-228. Griffiths, J. D., and Singer, D . A . , 1969, Size, shape, and arrangement of some uranium ore bodies, pt. 2 of U. S. Atomic Energy Comm. Rept. GJO-918-2: The College of Earth and Mineral Sciences Experiment Station, The Pennsylvania State University, 38 p. Hilpert, L. S.,1963, Geology and technology of the Grants uranium region: New Mexico Bur. Mines and Min. Res., Memoir 15, 277 p. Kelly, T. E., 1977, Geohydrology of the Westwater Canyon Member, Morrison Formation, of the southern San Juan Basin, New Mexico, in New Mexico Geological Society Guidebook, Twenty-eighth Field Conference, San Juan Basin III, p. 285-290. Kelly, v. C., 1963, Geology and technology of the Grants uranium region: New Mexico Bur. Mines and Min. Res., Memoir 15, 277 p. Kozusko, R. G., and Saucier, A. E., 1980, The Bernabe Montano Uranium Deposit, Sandoval County, New Mexico, in Symposium on Grants Uranium Region: New Mexico Bur. Mines and Min. Resources, Memoir 38 /In Press/. Santos, E. S., 1970, Stratigraphy of the Morrison Formation and structure of the Ambrosia Lake District, New Mexico, U. S. Geol. Survey Bull. 1272-E, 30 p. Saucier, A. E., 1976, Tectonic influence on uraniferous trends in the late Jurassic Morrison Formation, in Tectonics and mineral resources of southwestern North America: New Mexico Geol. Soc., Special Publication no. 6, p. 151-157. Squyres, J. B., 1969, Origin and depositional environment of uranium deposits of the Grants Region, New Mexico: Unpublished PhD dissertation, Stanford University, 228 p. DISCUSSION H.D. FUCHS: It must be quite difficult to follow the depocentres in areas where no drilling information exists. Is it possible to locate the depocentres/ channels by a detailed interpretation o f structure (by means of air-photo interpretation, etc.)? D.A. PORTER: The north-west to south-east trend as reflected by the Zuni Uplift is predominant and is the primary projection, especially in the 292 PORTER Westwater depocentres closest to the uplift. Mapping the regional structure by both surface and subsurface methods is very important as Jurassic-age structures influenced depocentre trends. Some of these older structures are mappabie from the surface as they have been active through Laramide time. P. BARRETTO: In what ways wouid the radiometric and/or geochemical halo of Bernabe Montano mineralization differ from other Grant Mineral Belt deposits? D.A. PORTER: From data that Conoco has been able to obtain from the literature and other companies, it appears that the low-grade mineralization or radiometric halo at Bernabe Montano is of greater areal extent than is normally associated with deposits in the Grants Mineral Belt. One explanation for this would relate to the position o f the Bernabe Montano complex in the distal facies o f the alluvial fan. The geochemical interface between oxidized and reduced sediments hosting the ore encroaches from the north at Bernabe, whereas in the Grant Mineral Belt it is common for the redox interface to encroach from the south. This reversal of the redox interface, however, is on the updip side in both instances, so the difference is strictly in response to structure. D. TAYLO R: What is the depth of the main mineralization? D.A. PORTER: The depth varies. In the northern trend it ranges from 300 m to 375 m. In the southern trend the range is on the order o f 475 m to 550 m. D. TAYLO R: What is your drill-hole spacing for blocking out the ore? D.A. PORTER: The ore was blocked out on approximately a 60-m grid. IAEA-AG 250/10 DISCOVERY OF URANIUM DEPOSITS NEAR OSHOTO, EAST CENTRAL POWDER RIVER BASIN, WYOMING, USA A.F. STOICK, K.L. BOLTZ, M.D. BUSWELL Nuclear Dynamics, Inc., Phoenix, Arizona, United States of America PresenfeJ Ж. К //аллеи Abstract DISCOVERY OF URANIUM DEPOSITS NEAR OSHOTO, EAST CENTRAL POWDER RIVER BASIN,WYOMING, USA. A uranium deposit on the east-central flank of the Powder River basin was discovered in 1952 by airborne radiometrics. Subsequent exploration, including driHing, paleochannel mapping, and geochemistry, between 1971 and 1977, discovered 6 million lb (2.7 million kg) of indicated and inferred uranium reserves and 34 million lb (15.4 million kg) of potential resources through an expenditure of US $4.7 million. Characteristics of the Oshoto ore body indicate that roll-front deposition is strongly influenced by facies changes within the fluvial Lance Formation. 1. INTRODUCTION The Sundance Project area is located near Oshoto, Wyoming, approximately 25 miles (40 km) north of Moorcroft, Wyoming. An area 10X35 miles (16 X 56 km) is included in this project. Figure 1 shows the Sundance Project in relation to the Powder River basin and surrounding structures. 2. INITIAL DISCOVERY The possibility of uranium occurrence in this area was first brought to the attention of A.F. Stoick in 1952. A local drilling contractor flew over the area with a scintillometer and noted anomalous count. Preliminary checks of all available surface outcrops revealed mineral occurrences in a light-pink oxidized- zone middle-Lance sands. Owing to unfavourable economics, additional investi gation was delayed until 1970. 293 294 STOICK et ai. Locotion of Sundance Project Cretaceous rocks f^ecambrian rocks Mojor synciino! axis Major anticiinat axis Out!ine of Powder River basin ^YC.7. Zocato/? Pro/ccf дяс? wc/or TABLE I. NUBETH JOINT VENTURE; SUNDANCE PROJECT COST HISTORY 27 ООО !20 220 98 HO [40 ООО 3 3 8 0 ,0 0 0 Totol Explor. 4 700 ООО !n Situ L€oc^. 2 600 ООО i 500.00 0 Phose 4 Production !970 ' 1 72 ' 1 74 1 76 t 78 } 80 ' ) 82 ) 84 ' YEAR ÏAEA-AG-250/10 295 Between 1952 and 1970, uranium mining in Lower Cretaceous sandstones in the Black Hills and Tertiary sandstones in the Powder River basin established evidence of uranium mineralization on a regional scale. Literature studies [1 ] indicated favourable host conditions in continental sandstones and shales on the UpperCretaceous Lance Formation. Development o f oil fields in the immediate area provided subsurface information indicating radioactive anomalies in the Lance Formation. A joint venture to explore the Sundance Project area was created between Nuclear Dynamics, Inc., and Bethlehem Steel Corporation. An investment of US $27 000 was made, with exploration efforts focusing on airborne radiometric surveys and outcrop sampling. Table I shows the cost history. 3. RESEARCH AND RECONNAISSANCE Airborne radiometric surveys were used to delineate favourable targets in Lance and Lance equivalents for land acquisition and subsequent exploration. Equipment used for airborne surveys included a Cessna 185 to 300 hp aircraft mounted with a Mount Sopris Model SC160-A Scintillator. The scintillator crystal was 7 in. (18 cm) in diameter and 2 in. (5 cm) thick. Flight lines were established and flown over a 15 X 35-mile area (24 X 56 km) on 1/3-mile (0.54 km) grids. Coverage was later expanded north to the Canadian border and south into Colorado. Airborne surveys located large low-intensity radiometric anomalies which were ground-checked to determine whether radioactivity was due to uranium decay. Some anomalous radioactivity was associated with sandstone outcrops exhibiting light-pink oxidation colours. An additional study o f oil-well logs in the vicinity was completed during Research and Reconnaissance efforts to aid in selection of land acquisition and exploration targets. Radioactive anomalies were noted and mapped according to stratigraphie position and location. Airborne and subsurface anomalies encompassed an area 10 X 35 miles (16 X 56 km) parallelling the western flank of the Black Hills uplift (Fig. 1 ). ' 4. LAND ACQUISITION On the basis of Research and Reconnaissance exploration results, a major land acquisition programme was organized. During 1971, 70000 acres (283 km^) incorporating State, Federal and private mineral rights, were procured for Nuclear Dynamics, Inc., at a cost o f US $120 220. An additional 44 000 acres (178 km^) were acquired, bringing the total cost o f land acquisition to US $218 330. 2 9 6 STOÏCK et al. TABLE II. EXPLORATION COSTS (IN US $) DRILL ' EXPLORATION STATISTICS DIRECT TOTAL HOLES. OFFSET TOTAL DEPTH DRILLING EXPLORATION PHASE DRILLED SPACING FOOTAGE RANGE COST/FOOT COST 1 A 42 1-2 mile 53 507 feet 800- $.80/ft (1.6-3.2 km) (16 309 m) 1800 feet (244-549 m) ($2.62/m) mile (6.4-8 km) В 641 25-1000 219 220 feet 200- . $.80/ft (66-818 m) 800 feet (7.6-305 m) (122-244 m) ($2.62/4i) $140 000 11 1432 25-1000 955 521 feet 200- $.66/ft (291 242 m) 1800 feet (7.6-305%i) (122-549 m) ($2.16/m) $950 000 III 2921 100-1000 1 763 296 feet 400- $.73/ft (537 453 m) 1000 feet (30.5-305 m) (122-305 m) (î2.39/m) $3 380 000 TABLE III. EXPLORATION OBJECTIVES AND RESULTS CRtTICAL EVIDENCE PHASE ' OBJECTIVES FIRST ENCOURAGING RESULTS for CONTINUATION e U 30g and 10.2ft (3.1 m) of depths of 180ft (55 m) and 1321ft (402 m ) . 15ft (4.6 m) of .31% eU^Og, along The 345th Phase III drill hole was the discovery of a 4.7 million pound.(2 132 000 kg) 6 million pounds (2 721 000 kg) UgOg and 34 million pounds (15 422 000 kg) IAEA-AG-250/Ï0 297 5. EXPLORATION DRILLING Exploration drilling was conducted in three phases. Phase I consisted of wide-spaced drilling on 1 to 2-mile ( 1 .6 to 3.2-km) centres. Exploration drilling in Phase II tested roll-fronts for grade and lateral extent of mineralization. Phase III included wide-spaced drilling for expansion o f mineralized areas and close-spaced drilling for precise delineation of potential ore bodies. Tables II and III summarize drilling costs, objectives and results for all phases. Phase I began with wide-spaced drilling to gain information on lithologie, stratigraphie, structural and radiometric characteristics o f the Sundance Project area. Forty-two holes provided the foundation for an expanded exploration effort. Subsequent drilling of 642 additional sites provided data for subsurface verification o f airborne radiometric anomalies, delineation of generalized oxidation-reduction boundaries and correlation o f gross lithologie units. Phase II exploration was accomplished on two scales. Closely spaced sites, 25 to 100 ft (7.6 to 30.5 m) offset, were drilled to test width, grade and mineral continuity along delinated oxidation-reduction boundaries. Wide-spaced drilling, 400 to 1000 ft (122 to 305 m), was employed to explore and extend additional roll-fronts. Phase III drilling included an extension o f Phase 11 wide-spaced drilling and concentration of close-spaced development drilling to deHne in detail areas known to contain mineralization. Roll-front boundaries were mapped (Fig.2) on the basis of gamma-curve configuration and the oxidized or reduced conditions of the host sands (Fig.3). Daily update o f these data provided a basis for selection of future drill targets. Additional exploration methods employed were: (a) Geochemical analysis o f core taken in series across a roll indicated that vanadium could be used as a potential pathfinder element for uranium in the barren interior zone; (b) Radon sampling of water wells located two anomalies in areas previously unexplored; and (c) Subsurface mapping of sand channels to locate areas o f high ground water transmission in relation to mapped oxidation-reduction boundaries. 6 . DRILLING AND SAMPLING Tungsten carbide insert bits were used in drilling 4532 exploration holes. Sampling was performed at 5-ft (1.5-m) intervals. Cuttings were washed, boxed, described in detail with a binocular microscope, and stored for future reference. Texf confi'nMed on page STOîCK et al. 500 tOOO 2000 FEET EOLOGY 8Y: M. D. BUSWELL IAEA-AG-250/10 299 О О О n 500 cps gg MINERALIZATION Э SAND 4 / I Vertical by Rubin , ¡970 gg ALTERATION Й SWALE Geotogy by M.D. Busweii, )980 IAEA-AG-250/10 301 A Active fron! F Passive front S Staqnonl fron n°er,.r ^ b o ^ d o ^ to GEOLOGY BY' 1 9 80 M. D. Buswell F7G.4. ¿owe/- sand MHiY 302 STOICK et al. F/G .J. t/ppef мне? ми:'? M!'nera?;'zaf:'oM. ÍAEA-AG-250/10 ßQß SAND ISOPACH 70 - tOO Feet thick Г?! 40 - 69 Feet thick f ] 0-39 feet thick О 500 !00Q 2000 GEOLOGY BY: f ,эао M.O. Buwe!) F / C . 6. lo w e r м н < 7 un¡Y морасЛ. 304 STOICK et d. SAND tSOPACH 70 - 100 Feet thick [Ж ] 40 - 69 Feet thick ( { 0-39 Feet thick GEOLOGY BY; M.D. 8uswe4 t9 80 F/C. 7. t/pper морасА. IAEA-AG-250/Ю 305 A 3-in. (7.6-cm) ID face-discharged diamond bit with a Ю-ft (3-m) core barrel was used to core 50 holes through mineralized horizons. Core recovery was consistently near 100% when the drilling fluid consisted of WOLF and Lowloss mixture, two Baroid, Inc., products. Uranium chemical disequilibrium was checked periodically throughout the Sundance exploration programme. Results indicated, for values above 0.02% U 3OS, that chemical uranium was enriched with respect to equivalent radiometric uranium oxide. Chemical disequilibrium below this value was found to be erratic and favoured neither radiometric nor chemical uranium consistently. 7. OSHOTO ORE DEPOSIT During Phase III, 1800 exploration sites were drilled on 100 to 500 ft (30.5 to 152 m) centres to delineate ore. Indicated reserves of 4.7 million lb (2 132 000 kg) have been assigned to the Oshoto deposit. Roll-fronts occurring in the Oshoto area form long, narrow, finger-like projections (Figs 4 and 5) oriented normal to the basin ward dip o f the sediments. Roll-fronts trending along strike were contrary to the working hypothesis of downdip migration o f oxidized groundwater. Buswell [3] shows paleochannels, trending north-south (Figs 6 and 7) to be an important factor in ore deposition in the Oshoto deposit. Oxidation reduction boundaries in the Oshoto area are divided into active, passive and stagnant fronts as defined by Galloway et al. [5] (Figs 4 and 5). Active fronts form well-developed rolls in areas where the oxidation reduction boundaries are normal to ground-water migration. Areas of maximum ground water transmission occur primarily in thick channel sand sequences. Passive fronts mark boundaries between oxidized and reduced sediments parallel to front trends, forming narrow and often discontinuous mineralization. Ore deposition along passive fronts occurs near channel margins or other permeability changes. Stagnant fronts occur as stranded islands or as enlongated embayments of reduced sediments where front migration is minimal. Stagnant front formation is attributed to permeability changes due to either facies changes or faulting. 8 . FUTURE EXPLORATION ON THE SUNDANCE PROJECT To locate ore bodies similar to the Oshoto deposit, emphasis placed on mapping regional and local paleochannel trends with respect to oxidation reduction boundaries would be an effective exploration method. Analysis of existing data would provide the basis for obtaining preliminary information on major paleochannels. The principal problem in conducting exploration in a complex fluvial system such as the Lance Formation would be the correlation and mapping of channel sand sequences. 3 0 6 STOÍCK et al. REFERENCES [1 ] ROBINSON, L .S., MAPEL, W.J., BERGENDAHL, M.H., Stratigraphy and structure of the northern and western flanks of the Black Hills uplift, Wyoming, Montana, and South Dakota, USGS Prof. Paper No. 404 (1964) 23—98 and 1 2 1 -1 2 3 . [2] GRÜNER, J.W., Concentration of uranium in sediments by multiple migration-accretion, Econ. Geol. 51 (1956) 4 9 5 -5 2 0 . [3] BUSWELL, M.D., Subsurface Geology of the Oshoto Uranium Deposit, Crook County, Wyoming, Master's thesis, South Dakota School of Mines and Technology (1980). [4] RUBIN, B., Uranium roll-front zonation in the southern Powder River basin, Wyoming, Earth Science Bull. 3 4 (1970) 5—12. [5] GALLOWAY, W.E., KRE1TLER, C.W., McGOWEN, J.H., Depositional and groundwater flow systems in the exploration for uranium, Bureau of Econ. Geol., Univ. of Texas at Austin, Research Colloquium (1979) 177—180. IAEA-AG-250/18 THE BERNADAN DEPOSIT, HAUTE-VIENNE, FRANCE* P. GELY Compagnie minière Dong Trieu, Paris, France Д/юг? со?и?иим;ся?юм Like many other deposits, the Bernardan was discovered during the 1960s, at the north-western corner o f the French Central Massif. The mineralized bodies consist o f vesicular episyenites carrying autunite, coffinite and pitchblende mineralizations, coffinite being the main uranium mineral. The bulk o f the assured reserves are contained in a main body of irregular columnar shape which has been explored to a depth o f 120 m. The maximum horizontal dimensions are 1 0 0 m in the north-to-south direction and 60 m in the east-to-west direction. There are other known bodies of smaller size in the immediate vicinity and to a depth o f 300 m. The proven reserves at present amount to more than 3000 t of uranium with an average grade higher than 0.5% U. Initial exploration was focused on the two-mica granite massifs o f the region. In the absence o f previous indications, the targets were chosen on the basis of similarity to French granites o f the same type serving as supports for deposits already known. The first regional shows were detected in the course o f carborne prospecting. Helicopter exploration of a 200-km^ area with close-grid spacing did not yield any information about the Bernardan site itself. The discovery of the deposit was a methodological success. The sequence adopted in the first exploration was essentially as follows: prospecting on the ground, resistivity measurements and numerous short drillings (average 7 m) deep enough to pierce the bedrock beneath the thin Eocene detritic cover. After a preliminary campaign o f oblique drilling, systematic exploration to an average depth of 1 2 0 m was carried out by means of percussion vertical drilling and core drilling according to a square-grid pattern ( 8 -m spacing). The evaluation of the deposit as well as the mining project was based entirely on the results o f these drillings. Open-cast mining operations were undertaken. The observations based on drilling have been confirmed, and the prospecting work is being continued both in extension and in depth. * The complete text was not submitted for publication. 307 308 GELY DISCUSSION P. BARRETTO: Did you try a geochemical survey in this small area and, if so, what was the result? P. GELY: No, we have not used geochemistry systematically because we felt that the contamination produced during the first reconnaissance work made the method not very effective. Secondly, the zone which we think has more interest is covered by Eocene sediments which turn out to be in discontinuity with the bedrock geochemistry. In this regard, we have a very good example in Bernardan, where a very rich mineralization (several % U^Og) is covered by only 0.50 m of sediments, which have only 2 - 3 ppm U. However, we use geochemistry quite systematically in other areas of France (for a large number o f elements, including uranium), and have obtained good results. P. BARRETTO: Even if the cover is completely unrelated to the bedrock geochemistry but the water table is very close to the surface, could some kind o f transport be produced into the cover forming some sort o f uranium halo? P. GELY : We have so far found no anomaly in the cover itself. It would be possible to have some good indications in the western part o f the area, where the cover is thinner. P. BARRETTO: When you carried out the radon survey by sniffers, how did you avoid contamination by atmospheric air? P. GELY: We put a plug on the top o f the hole and pumped the air for some twenty minutes. Usually we waited a few hours after the plug was taken out of the anomaly before making the radon measurements. M. MATOLIN: The Central Massif o f France is a part o f the European Variscan chain and you have certain evidence o f the radioactivity of this area. Does the area you describe belong to the granites with richer or with normal radioactivity? What is the normal radioactivity of the Central Massif granites and what is the value for the areas you describe? P. GELY. The two-mica granites represent the uranium-richer zones in the , French Central Massif in particular as well as in all the European Hercinian granites. The accumulation o f reserves in those areas represents the more important uranium potential of France with around 20 000 to 30 000 t U. But that does not mean that the two-mica granites are necessarily the more radioactive in surface; there are some biotite granites with high radioactivity in which the uranium seems to be fixed in a very stable form and cannot be mobilized easily. The basic uranium concentration o f the two-mica granites in the two areas I described is around 15 to 20 ppm U in altered zones. It has been shown that in the fresh two-mica granites the uranium is present as very small uraninite crystals and that the uraninite was apparently oxidized in the shallow levels, passing into solution and constituting, therefore, an important source which could be mobilized and redeposited in favourable structures. ÎAEA-AG-250/18 309 P. BARRETTO: What is the evidence to explain the genesis o f these vein deposits? P. GELY : First o f ail, the Pegy deposit is a vein deposit. The vein structure at Pegy goes in the direction of La Marche dislocation, nearest to a fauit. The Pegy vein structure is essentially a breccia which takes up materials of the wall rocks. The gneiss at the north is also in conformity with this location, and the gneiss is very close to the two-mica granites. We have been thinking mainly in terms o f an exogenic genesis for these uranium deposits, although we are not quite certain. But we consider it more important for a mine researcher to know about the favourable structures which are able to bear the uranium mineralization than where the uranium is coming from. P. BARRETTO: In the case o f the Bernardan deposit you have information down to 250 m from surface. Do you have some information on the other deposits? P. GELY: Not yet. Our efforts at present consist in preparing the open pit. P. STIPANICIC: Are the Fe^/Fe"^ ratios the same in the mineralized zones and in the nearest normal episyenites and is this total iron content (Fe*^ +Fe*^ ) the same as the total Fe+^ content in the reduzate facies? P. GELY: We do not yet have this information. Some time ago a systematic sampling of the mineralized zones and o f the country rocks was carried out for mineralogical studies and to analyse the disequilibrium problems. A.G. ANGEIRAS: In regard to exogenous versus endogenous model, I can mention some features which we found in the Itataia region. In the Lower Proterozoic we have small plugs o f granites which are intensely faulted and jointed and this type of vug-rock is developed from fractures inside the granite body. The climate is dry, and there is no chemical weathering at all. Some holes drilled down to 200 m show granites with 2 0 -3 0 ppm U. This could be a geo chemical process, because we have no evidence about weathering and oxidation in this area. J. DARDEL: What is the age of the mineralization? Is it Tertiary? P. GELY: We did not carry out any study on the subject, but we believe that it could possibly be Eocene. Mr. Dardel knows more about this problem because he knows the work done by CEA-COGEMA. It is very difficult to say anything about the mineralizations occurring some 50 km eastwards, 200 to 300 m.y. old, and located in large faults, but there is some residual mineralization of about 30 m.y. IAEA-AG 250/12 EXPLORATION OF THE PANNA MARIA URANIUM MINE, KARNES COUNTY, TEXAS, USA E C. BOWMAN Chevron Resources Co., San Francisco, California N E. CYGAN, M.H. ALIEF Chevron Resources Co., Denver, Colorado, United States o f America Abstract EXPLORATION OF THE PANNA MARIA URANIUM MINE, KARNES COUNTY, TEXAS, USA. The Panna Maria Mine is located in Karnes County on the coastal plain of south-eastern Texas, about 55 miles ( 8 8 km) south-east of San Antonio. Host rock for the uranium is the Tordilla sandstone member of the Upper Eocene Jackson Group that strikes north-east and dips one to two degrees toward the coast. Chevron became interested in uranium exploration in south-east Texas in 1971 as a result of reports of increased industrial activity in the area, some of which was apparently successful. Additional attractions were the inexpensive drilling and the fact that many of the deposits were less than 200 feet deep. Also, some petroleum leases held by Chevron contained provisions relating to other minerals that might permit driHing for uranium. Regional stratigraphie studies of the Upper Eocene, Miocene and Pliocene were completed in 1972 and an area of interest was selected in western Kames County with the Jackson group sands as the objective. Further studies narrowed the selection to the area between Hobson and Panna Maria, and the objective to the Tordilla sand Member of the Upper Jackson. The exploration model was a roll-front uranium deposit occurring along a redox front trending generally north-east. DriHing began in 1972, using rotary drills and contract drillers and loggers, and extended with interruptions until the end of 1974. Ore-grade mineralization was discovered in the 26th hole drilled in 1972. A total of 987 holes were drilled for a contract cost of US $ 146 000. Uranium resources at that time were estimated to be approximately 3.4 million lb (1.5 million kg) U 3O3 . Evaluation and development driHing during 1975 and 1976 increased the proven and probable reserves to 6 to 8 million lb (2.7 to 3.6 million kg) having a grade less than 0.1% U3ÓS. The mine and mill went on stream in February 1979. The Panna Maria area is located in Karnes County, Texas, approximately 55 miles (88 km) southeast of San Antonio and about 6 miles (9.6 km) north of Karnes City (Figure 1). The terrain is flat to gently sloping farm and grazing land, about 350 feet (105 m) in elevation and is easily accessible by hard-surfaced highways and farm roads. A few widely spaced oil wells and associated pro duction f a c ilitie s of the Panna Maria Oil Field are present in the area; how ever, production is low and there is little visible activity by the petroleum 311 312 BOWMAN et a!. F/G. i. íocafion map o/fanna Mana M'ne. F ÍG .2 . C hevron pgí?*oZeMW ^ а ^ м 7P 7 2 ('ba^e/row FargFe gíaZ. [2 ]/ !AEA AG 250/12 313 F/G.J?. RegionalgeoZogy and мгап:'и?н JMfn'cfy, Гела^ coa^^aZ pZaZn, 7972 ^aie^om FargV e efa¿ [2 ]/ industry. Petroleum production dates from 1961 and several productive leases are owned by Chevron U.S.A., a wholly owned subsidiary of Standard Oil Company of California. , Chevron Resources, then the Minerals Staff of Standard Oil Company of Califor nia, became interested in uranium exploration in the Gulf Coast of Texas in 1970 largely because of the marked increase in uranium industry activity in the region and the reported success of some of the companies in finding minable deposits. This interest was enhanced by studies of published material, by visits to mines, and by discussions with geologists familiar with the geology and the uranium deposits. An important factor contributing to our practical, rather than academic, in te re st was the fact that Chevron held a number of oil and gas leases distributed as single leases or small groups of leases along the Tertiary outcrop of the Gulf Coast for a distance of nearly 300 miles. Figure 2 shows the approximate location of the leases. Many of these leases were for the customary "oil, gas and other minerals," a few also contained the phrase "fissionable materials," and a very few mentioned uranium. The importance of this to the Minerals Staff was that if some of the leases were located in an area where the geology appeared to be favorable for the occurrence of uranium, such leases possibly could be used as "drilling islands" to test exploration ideas. 314 BOWMAN et al. Studies by Eargle and Weeks(l)(2) and others indicated that most uranium deposits in the Texas coastal plain were in fluvial or littoral marine sandstones of the Jackson Group (Upper Eocene), Oakville (Miocene) and Goliad (Pliocene) formations. Figure 3 is a generalized geologic map on which are shown the two major Texas uranium d is tr ic ts , the Karnes County and Live Oak County districts, and areas of uranium mineralization or high radioactivity. Eargle and Weeks (1,p.12,13) also suggested that the "roll-front" model described by Harshman(3) was applicable to some of the Texas uranium deposits. Other deposits appeared to be associated with faults which provided avenues of migration of petroleum-derived hydrogen sulphide gases that acted as the reductant and precipitant of the uranium(4). Regardless of the mechanism of uranium precip itatio n , i t was apparent th a t permeable continental or marginal marine sandstone host rocks were required. Moreover, if Chevron leases were to be used as d rillin g islands, a knowledge of the distribution, thickness and environment of deposition of the sandstones with respect to the location of the various leases was also a primary require ment. In view of these requirements, Mr. Cygan in 1971 carried out regional stratigraphie studies which resulted in a series of sand isolith maps extending from the United States-Mexico border northeastward to Louisiana. Information for these maps was obtained from published sources and other publicly available data, examination of outcrops and a review of proprietary information from Chevron petroleum exploration files. Although the petroleum industry had IAEA AG-250/12 315 Age Group Formation Member Ф с Ф о о Catahouta Tuft 2 Fashing Ctay Torditta Ss. Whitsett Dubose Ctay Jackson Deweesevitte Ss. Conquista Ctay Eocene Diiworth Ss. Manning Ciay F7G.J. OMfcroppi'n^i^afi'gyap/ü'cMTM'fî, АлглмСоыл?у, Гехм. drilled many hundreds of wells into and through the Tertiary section of the Gulf Coast, the majority of these holes were cased in the upper 500 to 1000 feet before running logs, with the result that little information was available in the cased holes concerning the near surface stratigraphy. An example of one of the maps is the paleoenvironmental map and isolith of sands in the Upper Eocene Jackson group shown in Figure 4. It will be noted on this map that two major areas of sand accumulation are present, one in Karnes County in the vicinity of the Uranium District and another about 60 miles (100 km) to the southwest. In both of these areas the total sand thickness within the Jackson Group exceeds 400 feet (120 m). Much of the sand shown on the map is near-shore marine, but there are several areas from Karnes County southwestward that have a thickness, greater than 100 feet (30 m) of nonmarine, fluvial sand. However, the greatest thickness of nonmarine sand appears to be in northwestern Karnes County where 300 to 400 feet (90-120 m) is found. An examination of the completed maps, together with known occurrences of radioactivity,enabled a preliminary selection of the optimum combination of stratigraphie unit and lease holdings. The Upper Jackson group, Whitsett formation in northwestern Karnes County was chosen as the prospective unit. Figure 5 shows the subdivisions of the Jackson group in Karnes County. Following this selection, a more detailed study of the geology of western Karnes County was made. The area studied is shown in Figure 6. Outcrops of sandstone were mapped, measured, described and sampled for uranium, and for paleoenvironmental determination by Cheyron's palynology laboratory in New Orleans. Color air photographs were used to assist in the surface mapping and topographic maps on a scale of 1:24 000 were used for base maps and elevation control. Geologic maps by Eargle and others(5) were available as were air radiometric maps by Moxham and Eargle(6). Results of the study were presented on 1 inch to 1 000 foot base maps. The outcropping Deweeseville and Tordilla sandstones of the Whitsett formation were found to be continental, deltaic distributary channel sands. The Deweeseville is light gray, fine to very fine grained, sub-angular quartz in a weak siliceous cement. The unit has very faint limonitic surface staining, is cross-bedded, and is made up of 6-foot beds of friable sandstone interbedded 316 BOWMAN et al. with sandy clays up Co 4 feet thick. The Tordilla sandstone is light gray, clay-rich, cross-bedded, and very fine grained, the grains consisting of quartz and shards of volcanic glass loosely cemented with silica. Oxidized uranium minerals were present in the Deweeseville sand of the Whitsett formation in the inactive Brysch mine 2.4 miles (4 km) northwest of Hobson, but no other uranium mineralization was seen in the rather sparse outcrops of the Deweeseville or Tordilla sands. Traverses across the outcropping or thinly covered sandstones with a hand-held scintillometer showed radioactivity only slightly higher than background. As noted by Eargle and others(4), air- \radiometric anomalies of + 700 and + 490 cps are present along the trend of the Tordilla and Deweeseville sandstones but no outcrop occurs coincident with the anomalies. (These two sandstones are included in the Stones Switch member of the Whitsett formation on the referenced map.) Chevron petroleum exploration well files provided many electric logs that furnished information essential to stratigraphie and structural mapping. The only gamma-ray logs in the near-surface Tertiary section in northwestern Karnes County were run by the U.S. Atomic Energy Commission in existing oil wells as part of a regional radiometric survey. Three holes were logged; two of these were updip from the Tordilla outcrop and exhibited small scale anomalies in the Deweeseville. The third log was run in June 1967 in the Chevron (Sotex) well, Felix Poll ok No. 1, about one mile (1.6 km) southeast of Hobson. An anomalous radioactive interval five feet thick was recorded between the depths of 188 and IAEA AG 250/12 317 193 feet (57.3 and 58.8 m) which, by rough calculation, allowing for attenua tion by two strings of casing and cement, showed a content of 0.012 percent eUgOg, apparently in the Deweeseville sandstone. As a result of this study, the group of Chevron oil and gas leases in the vicinity of the small community center of Panna Maria was selected as the initial prospecting area. See Figure 6. The Panna Maria project was initiated to evaluate the uranium potential of these leased areas. Reasons for identify ing the Panna Maria area for more intensive prospecting are summarized in the following lis t: 1. Previous uranium production from Eocene, Jackson Group sandstones of the Gulf Coast. 2. Continuity of Jackson sandstone units from producing areas through the area selected. 3. Air-radiometric anomalies over the Tordilla and Deweeseville sandstones. 4. Gamma-ray anomaly in cased well southeast of Hobson. 5. Probable coincidence in location of sands in Jackson Group with petroleum leases held by Chevron. As previously mentioned, Eargle (2, p. 13) suggested that the roll-front model was applicable to many of the Texas uranium deposits. This suggestion was reinforced by observation by the writers of well-developed rolls in some of the open-pit mines south and southwest of Panna Maria; these were very similar to the crescentic rolls seen in Wyoming. These factors, together with the results of our regional geologic studies,led us to select a roll-front uranium deposit in the Jackson sandstones as our exploration model. According to the model, the deposit would be bordered on the updip side by light-colored, oxidized sandstone and downdip by brown to medium-gray reduced (unaltered) sandstone. The most probable trend of the deposit would be north east, roughly parallel to the strike of the bedding. Dimensions of the deposit including the upper and lower limbs, or trailing edges of the crescentic roll probably would not exceed 300 feet (90 m) in the down dip direction and 1000 feet (300 m) parallel to strike. Its thickness would be about 20 feet (6 m) and the ore grade would average about 0.20 percent U3O0 . These dimensions and grade correspond to Eargle's typical open-pit deposit (2, p. 29) in the vicinity of the Weddington mine, approximately 8 miles southwest of Hobson. Under our plan, exploration would be confined to the area between the outcrop of the sands and the 500-foot depth contour. The depth limitation was based on the probable economic depth limit of open pit mining in this region. Underground mining of the virtually unconsolidated sandstones was considered to be economically unfeasible in view of the low price of uranium, $6 to $7 per pound, at that time. Accepting this model, four major questions must be answered by the exploration program: 1. Are the Jackson sandstones as well developed as the study of western Karnes County suggests? 2. Is an oxidation--reduction interface, or "redox front," present? 318 BOWMAN et at. 3. If a redox front is present, is uranium mineralization developed along the front? 4. If uranium mineralization is present, what is the grade and tonnage? Answers to these questions could be obtained only by drilling a number of holes. A reconnaissance program of a relatively small number of widely spaced holes would be needed to learn the thickness of the sandstones and whether an oxidation--reduction (redox) front was present. Progressively more holes at closer spacing would be needed to determine the distribution of mineralization and the grade and tonnage of possible ore bodies. In the following discussion, "Phase I drilling" and "reconnaissance drilling" are considered synonymous terms; Phase II, "outline drilling" and "infill drilling" are also considered to be synonymous terms to describe drilling used to determine the limits of mineralized areas and their grade and approximate tonnage. Early in 1972, a reconnaissance drilling program was designed to provide the needed logs and samples. Drilling locations were rather severely restricted in that drilling was to be done only on mineral leases or options obtained on lands already held by Chevron under petroleum leases. A 400 x 800 foot (122 x 244 m) grid was selected with the long axis of the grid being northeast, approximately parallel to the general strike of the bedding. Statistically, this grid would be almost certain to find the typical roll-front uranium deposit as defined in the exploration model, if the mineralization or an appreciable part of it was present under the leases scheduled for drilling. It was anticipated that about 65 holes would be drilled in this initial Phase I program. Drilling commenced on August 14, 1972,using two contractor-owned, truck-mounted rotary rigs of the type commonly used by petroleum companies for drilling seismic shot holes. Four and three-quarter inch tricone bits were used with IAEA-AG 250/12 3)9 <М я С з 350±20ft Surface а) о о л О Д " <с S o I—*— __——— : Shale и —rí os е >. л д Upper Jackson ЙО Mineralization^ L i g n i t e ____ Mineraiization -П V) Upper Limb <ы2Е 5" с О"- Yо 5 о м^ О ^ Ф Ш -g^ О ю -*2 0 0 ^ 5 о F/G.á'. Panna Магм crea, ícAemafíc croM-íeefíon an¿ geo/oy¡'c сс?м?ия. natural mud as the drilling fluid. At the start of the program, samples of cuttings were taken representing ten-foot (3 m) intervals, but it was soon apparent that the thin bedding and rapid changes in lithology were not ade quately represented by the ten-foot composite sample and the interval was reduced to 5 feet (1.5 m) for the remainder of this program and a ll subsequent programs. Samples were examined and logged on site by a geologist using a hand lens. Use of a binocular microscope proved to be impractical at the d rill site because the microscopist could not process the samples nearly as rapidly as they were recovered. A ll holes were logged by a contract logging truck immediately after drilling was completed and before moving the d rill rig to a new location. Gamna-ray self-potential and resistivity logs were recorded by analog; gamma-ray intensity in counts-pet-second was also digitally recorded during re-runs of mineralized zones. Cost of drilling was approximately $0.38 per foot ($1.25 per m). Cost of logging was approximately $0.14 per foot ($0.46 per m). No coring was done during the 1972 drilling program. Drilling and logging were completed on August 23, 1972,with 64 holes drilled for a total contract cost in 1972 dollars of $7 600. Surface damage payments to landowners of $20 per hole,and salaries, administrative costs and expenses of Chevron geologists and a permit man-party manager,are not included in these costs. The results of the 1972 Phase I drilling program are summarized in Figure 7. Sample examination established the presence of a northeast trending redox front in the Tordilla sandstone in the western part of the leased area. Northwest of the front, the Tordilla is oxidized and has a characteristic tan color attributed to the presence of limonite. Southeast of the front and downdip from it,the color of the sand changes to brown-gray and then to dark gray typical of the reducing environment. Carbonaceous material, although not abundant in the Tordilla, increases noticeably southeast from the redox front. A diagrammatic cross-section of the deposit as interpreted from the 1972 drilling is shown in Figure 8. 320 BOWMAN et ai. A A Northwest Southeast CM9 CM8 CM7 HM8 HM7 10 50 Meters -j- 200 Feet F/G. 9. Fanna Жаг:'а area, 79 72 (?ri'ZZ¡'H¿r pro^ww, c/-oyj-íeefi'oy! ^ Ore-grade mineralization, defined as 1500 counts-per-second or greater over a one-foot interval on the gamma-ray logs, was found in 13 holes in the western part of the drilled area, and near ore grade, between 1000 and 1500 cps, was found in 7 holes. These count rates are equivalent to 0.03 and 0.05 percent eU^Og, respectively. It is of interest to note that at this time cut-off grade in tne open pit uranium mines in this region was one foot of 0.05 percent U^Og and the average grade was 0.2 percent 4^0^. The first ore-grade hole drilled at Panna Maria was No. CM-llj which had 2 reet of greater than 0.05 percent eU^Og; it was the 26th hole drilled in the Phase I program. A series of cross-sections were made to assist in interpretation of the drilling results, particularly as to the validity of the assumed roll-front model but also to assist in calculation of reserves that might be present. The location of one of these, cross section A-A', is shown on the map in F ig ure 7.Figures 8 and 9 show our interpretation of this cross-section. The section is hung stratigraphically with the thin, usually mineralized, lignite zone at the top of the Tordilla as the datum, or reference plane. Dip is about 1-1/2 degrees to the southeast. The change in color of the Tordilla sandstone from yellow-brown in the updip oxidized zone to gray in the downdip reduced zone was readily apparent in the d rill cuttings and is shown diagramatically on the cross-section. Note that the d rill holes shown are about 400-500 feet (122-152 m) apart and that the vertical scale is 5 X the horizontal. Calculation from the gamma-ray log indicated that the mineralized zone in CM-7 contained 15 feet (4.5 m) of greater than 0.05 percent eU^Og and approximately 30 feet (9.1 m) greater than 0.03 percent eU^O., a ll in gray-brown sandstone. The lignite zone contained one foot of 0.05 to 0.1 percent. CM-8 showed 3 feet (1 m) of mineralization in the lignite, ranging from 0.03 to 0.20 percent, and a thin zone of less than 0.03 percent near the base of Che oxidized Tordilla sand. HoleHM-8 had a 5-foot (1.5 m) siltstone interval containing 0.05 to 0.15 percent eU^Og about 3 (1 m) feet above the lignite zone. The lignite zone itself had less than 0.03 ÍAEA-AG-250/12 32] percent but there was 3 feet (1 tn) of 0.05-0.10 percent two feet below the lignite in the upper part of the Tordilla sand. The cross-section shows the very tentative interpretation of a crescentic roll at hole CM-7; much more closely spaced drilling would be required to accurately determine its shape. Mineralization in the lignite zone and in the two intervals above and below the lignite in HM-8 did not appear to be directly related to the interpreted roll at CM-7. An estimate of the probable and possible uranium reserves was made using the cross-sections to interpret the possible extent of the mineralized area having a grade of at least 0.05 percent eU 0- through a thickness of 1 foot: probable 108 000 lbs (49 000 kg); possible I Z50 000 lbs (567 000 kg); total 1 358 000 lbs (616 000 kg) eU.O.. J о The encouraging results of the 1972 Phase-I reconnaissance drilling program prompted the acquisition of additional mining leases adjoining existing leases where it appeared that mineralization extended into the unleased lands. During 1973, the newly acquired leases were investigated by a 60-hole reconnaissance drilling program with the same approximately 400 X 800-foot grid used in 1972. Also during 1973, a follow-up Phase-11 drilling program of 124 holes was drilled on a 200 X 200-foot grid (60 X 60 m) to obtain a better definition of the distribution and grade of the mineralization found in 1972. Cores were taken through the mineralized interval in 9 holes for chemical assays and lithologie samples. The average thickness of the cored interval was approxi mately 30 feet (9 m), with about 90% recovery, and the cost was about $1.3.64 322 BOWMAN et al. A A' Northwest Southeast 4 0 - ] 5 0 1 0 0 F e e t -J_ *r __ < 8 25 Meters F7C.77. Ралла Afana area, 7Р7.?^гййл,у, сгсю-лесУмэл^-у!' per foot ($44.75 per m). Unit drilling and logging costs in 1973 were $0.31 and $0.11 per foot ($1.02 and $0.36 per m) respectively. Drilling started on March 15 and was completed on April 22. A total of 184 holes were drilled for a total of 46 044 feet (14 020 m) and an average depth of 250 feet (76 m). The coring costs are considered to have been exceptionally high and not representa tive of industry experience in the coring of the Jackson sandstones. As in 1972, the results of the drilling in 1973 were encouraging in that the roll-front uranium model continued to be valid as a guide to mineralization,and additional uranium was found in both the Phase-1 and Phase-11 programs. Fig ure 10 shows the area drilled in 1973. It w ill also be noted that the area held by Chevron under mining leases or options increased from that held in 1972. Two lines of close-spaced drilling and coring were established across the mineralized area to determine the geometry of the potential ore body. Drift surveys of the core holes were made to ensure correct location of mineral intersections. The location of one of these two "fences," A-A', is shown on the map in Figure 10, and the cross-section along the line A-A' is shown in Figure 11. Our interpretation is that the mineralization occurs in this particular locality in a nearly typical crescentic roll-front configuration with light-colored oxidized sandstone on the concave side of the "ore" development and brownish-gray to dark-gray sandstone on the convex side. Groundwater movement is assumed to have been downdip, from northwest to south east. Uranium content of the potential ore zone ranges from 20.5 ft. (6.3 m) averaging 0.13 percent UgOg the high-grade central part of the roll to 0.03 percent or less on the limbs ortrailing edges. The maximum thickness of the deposit is 25 feet (7.6 m), its width with a cut-off value(s) of one foot of 0.05 percent UcOg is 260 feet (79 m), and the depth to the top of the deposit is 180 feet (55 m). IAEA AG 250/12 323 r. / ! Outiine and in(ii) Driüing 1974 Recon. DriHing 1974 ^ O u tiin e and in(it) Driüing 1973 Approx. Location7 of Redox Front Recon. Dritting 1972 No DriHing-! 0 2000 4000 Feet 1------г*----- г-* 500 1000 Meters F/G . 7 2. Panna M ana area, /9 7 4 tZrZZZZng program . Comparisons of chemical (fluorimetric) analyses with radiometric calculations indicated that in the areas of oxidized host rock, mineralization was typically low grade and was out of equilibrium in favor of radiometrics, but in the areas of reduced (unaltered) rock the disequilibrium was dominantly in favor of chemical assays regardless of grade, i.e . the percent UßOg calculated from the gamma-ray logs was lower than was actually present. This comparison indicated that calculations of grade derived from the gamma-ray logging were sufficiently accurate for guidance of the exploration program and for preliminary resource estimates without applying a correction factor for disequilibrium. Estimates of uranium reserves based on the 1972 and 1973 programs were 1.2 million pounds (0.54 million kg) indicated reserves and 0.5 million pounds (0.23 million kg) potential reserves for a total of 1.7 million pounds (0.77 million kg) eU^O^. As a result of the 1973 drilling, additional.mining leases were acquired along the projected southwest and northeast trends of the mineralization. Drilling was resumed in March, 1974, and completed in December with approximately 140 Phase-1 reconnaissance holes being drilled on the new properties, and 599 Phase- 11 holes on a 100 x 100 foot (30 x 30 m) grid to further define the extent of mineral ization found by the 1973 drilling (Fig.l2). The pattern and the spacing of the reconnaissance drilling ranged from a 400 x 1200 foot (122 x 366 m) diamond in the northeast to a system of "fences," or lines, of drill holes in the southwest. The fences were oriented northwest-southeast approximately perpendicular to the regional strike; they were spaced 400 to 600 feet apart, and the holes within them were 100 feet apart. Drilling and logging equipment was similar to that used in 1972 and 1973. Hole depths for the 1974 program ranged from 210 to 230 feet (65 to 71 m), averaging 215 feet (66 m); total drilled footage was 159 200 feet (48 520 m). Sixteen holes were cored through the mineralized zone. A total of 508 feet (155 m) of 324 BOWMAN et al. A A Northwest Southeast FJG. 7 J. Panne №zr:'a area, 79 74 dn'A'ng program, cro^-yecf;on ^4 -vt'. core was cut, but insufficient information is presently available to calculate footage costs. Costs of drilling and logging were $0.55 and $0.14 per foot ($1.80 and $0.46 per ml respectively - a substantial increase over costs in 1973. The location and results of the 1974 drilling program are shown in Figure 12. The mineralized roll-front was extended about 1400 feet (430 m) southwest from its limit at the end of the 197 3 drilling. Another mineralized redox front was found in the newly acquired northeastern-most mining lease, with the strong probability that this was an extension of the mineralized area found in the 1972 program. Six additional holes drilled along the line of cross-section A-A' previously shown in Figure 11 permitted a more accurate definition of the geometry and uranium distribution,as shown in Figure 13. Although the shape of the area containing greater than 0.03 percent U^Og shown in Figure 13 is more e llip t i cal, or pod-like than crescentic, the similarity of the two cross-sections is apparent. The major difference is that part of the long upper limb of the crescent shown in the earlier interpretation appears to be a separate lens of mineralization. It seems likely, however, that at an earlier time the lens was a continuous part.of the upper limb, having later become separated because of solution. The width of the contiguous mineralized area in Figure 13 is 2Ù0 feet (61 m) compared to 260 feet (79 m) in the earlier interpretation. Drilling was completed in December 1974. Estimates made early in 1975 indicated that between 1.5 and 2.0 million pounds (0.7 and 0.9 million kg) were found by the 1974 program, bringing the total resource estimate to about 3.4 million pounds U^Og (1.5 million kg). IAEA AG 250/12 325 TABLE I PANNA MARIA AREA. TEXAS Summary of Exptoration Driüing NO. YEAR TYPE OF WORK HOLES DRiLLtNG LOGGtNG Cost Per Cost Per Tota! Feet/ Foot/ Feet/ Foot/ Cost Meters Meter Meters Meter Drilling Drifted Logged $ & Logging 1972 Phase I, 64 13 900 0.38 15 900 0.14 7 600 1.25 4 850 0.46 1973 Phase I & II, 184 46 000 0.31 76 200 0.11 22 800 Recon. & Outline 14 020 1.02 23 230 0.36 1974 Phase I & II, 739 159 200 0.55 201 200 0.14 115 500 Recon. & Outline 48 520 1.88 61 320 0.46 1972 1974 Totals 987 219 100 0.49 293 300 0.13 145 900 66 780 1.61 89 400 0.43 NOTES. A summary of the 1972 to 1974 drilling program is shown in Table I. It should be noted that the costs represent only those amounts paid to drilling and logging contractors for their equipment and services. Salary and administrative costs for Chevron's geologists, party manager, land and legal personnel, and management are not included, nor are costs of property acquisi tion. Costs are in dollars of the years shown. Note, also, that in each year the footage logged considerably exceeded the footage drilled because of repeated logging in the same hole at different scales of sensitivity. The cost per foot of logging is calculated using total feet logged, including repeat runs, rather than using the drilled footage. The program of Chevron's Exploration Department at Panna Maria was terminated at year-end, 1974. It was considered that sufficient uranium resources had been outlined to be of apparent economic interest and that the work remaining to be done was mostly of the nature of detailed evaluation of what had been found. Such exploration as remained to be done could be carried out during the course of the evaluation work. Evaluation and development drilling during 1975 and 1976 proved that the north eastern segment of mineralization was, indeed, connected to the central and southwestern part, bringing the total length of known mineralized redox front to 3.7 miles (5.9 km). Early in 1977, calculations showed a proven reserve of 6-8 million pounds (2.7-3.6 million kg). The Panna Maria mine and mill went on stream in February, 1979,with a throughput of 2 500 short tons (2 300 mt) per day with an average grade of less than 0.1 percent UßOg. This grade is less than half that anticipated in the exploration model (0.Z percent), but the marked increase in the market price of since the model was firs t applied in 1972 has made feasible both lowering tne average grade and increasing the size of the minable reserves. 326 BOWMAN et al. CONCLUSIONS 1. The roll-front model proved to be a valid premise on which to design an exploration program. Oxidized and reduced (unaltered) sandstones were readily identifiable in d rill cuttings, making location of the redox front a relatively simple matter in most of the area. In cross-section the mineralized roll-front is not a perfect crescent and its dimensions are different than were assumed in the model; it is longer, narrower, thicker and of lower grade: Length Width Thickness Grade Model: 1000 ft 300 ft 20 ft 0.2% Cross-section: Several 200 ft 28 ft < 0.1% 1000's ft 2. In areas such as Panna Maria, full use should be made of both subsurface and surface information relating to the stratigraphy of potential uranium host rocks. When exploring on a small lease holding, it is not enough to know that a particular sandstone is productive a few miles away; one should know that there is a high probability that a good host rock is present in the specific area of interest. 3. The spacing of d rill holes was satisfactory in that the redox front was readily located and most of its length was found to be mineralized. Had we been certain that a ll potentially economic mineralization would occur as visualized in the model, a number of holes updip and downdip from the front could have been omitted. We considered the mode.l to be a guide only, and therefore designed the drilling program to test a ll the acre age held. As a result some small, thin areas of low-grade mineralization were found that were not directly connected to the main ore zone, but are presently part oí the ore reserves. 4. It is apparent from the cross-sections that the thickest and highest grade part of the mineralized ro ll front is approximately 100 feet (30 m) wide; for this reason its geometry cannot be closely defined by a 100 X 100 foot (30 X 30 m) outline drilling pattern. We believe that in future explora tion of deposits of this type a series of "fences" of closely spaced holes should be drilled across the trend of the roll during the latter stages of the outline drilling program; the fences being 400 to 600 feet (120 to 240 m) apart and the holes within the fences being approximately 25 feet (7.6 m) apart. Also, several of the holes in each fence should be twinned by core holes through the ore zone. The additional knowledge gained by this closely spaced drilling would greatly assist the exploration group in deciding whether or not the much more detailed and expensive development drilling program should be recommended. Moreover, i f the results were encouraging, it very likely would be most helpful in persuading management to approve the necessary funds for development drilling. 5. Although the Panna Maria oil field, which produces from the Edwards limestone (Lower Cretaceous) at a depth of about 10 000 feet, lies beneath much of the uranium deposit, it is believed that the leakage of H2S or hydrocarbon gases from the petroleum reservoir did not play a major role in precipitation of the uranium in the vicinity of the Panna Maria mine. IAEA AG 250/12 327 REFERENCES 1. EARGLE, D. H. and WEEKS, A.M.D., Factors in the.formation of uranium deposits, Coastal Plain of Texas, Texas Univ. Bur. Economic Geol. Guidebook, No. 3 (1968). 2. EARGLE, D. H., HINDS, G. W., and WEEKS, A.M.D., Uranium geology and mines, South Texas, Houston Geol. Society, Houston, Texas, Guidebook, field trip of the Houston Geol. Soc. for the American Assoc, of Petroleum Geologists (1971). 3. HARSHMAN,.E. N., Uranium ore rolls in the United States, in Uranium Exploration Geology (Proc. Panel Vienna, 1970) IAEA Vienna (1970) ' 219. 4. KLOHN, M.L., and PICKENS, W.R., Geology of the Felder uranium deposit, Live Oak County, Texas, Society Mining Engineers, preprint No. 70-1-78, New York, N.Y. (1970). 5. EARGLE, D. H., TRUMBULL, J ., and МОХНАМ, R.M., Preliminary aeroradio- activity and geologic Map of Karnes City, NW quadrangle, Karnes County, Texas, U.S. Geol Survey, Geophysical Investigation Map GP 251 (1961). 6. МОХНАМ, R.M., and EARGLE, D.H., Airborne radioactivity and geologic map of the Coastal Plain area, Southeast Texas, U.S. Geol. Survey, Geophysical Investigations map GP-198 (1961). DISCUSSION D.A. PO RTER: What development spacing did you use for the proven reserve estimation? E.C. BOWMAN: Development spacing was 33 m on a rectangular grid. In addition, lines o f core holes were drilled across the deposit at intervals o f approximately 200 m with holes in these lines 17 to 33 m apart. D.A. PO RTER: A t what cut-off was the reserve calculation based? E.C. BOWMAN: Reserve calculation was based on a cut-off grade o f 0.61 m of0.03%U30g. H.D. FUCHS: You have shown that the thickening o f the sands o f the Jackson Group is more or less parallel to the indicated shore line and does not seem to follow the river system. What is the reason for this? E.C. BOWMAN: The thickest development o f the Jackson sands does appear, in total, to be approximately parallel to the shore line. In detail, however, the isoiiths o f the individual sands show axes o f thickness that approach perpendicularity to the coast in response to having been deposited in aggrading fluvial distributory channels. H.D. FUCHS: Can you recognize any faulting parallel to the shore line which may have influenced the sedimentation pattern? E.C. BOWMAN: We have not recognized the influence o f faulting on the distribution o f the Jackson sands in the Panna Maria area. IAEA-AG-250/17 DISCOVERY OF THE CLUFF DEPOSITS, SASKATCHEWAN, CANADA* J. D A R D E L Délégation aux matières nucléaires, Commissariat à l'énergie atomique, Paris, France 5%orf СОМТЙМТНСЯЙОЯ An airborne scintiHometric survey in 1967 over a part o f the Athabasca sandstone basin in the north-west corner o f the province o f Saskatchewan provided some contrast-free data at the centre o f the Carswell crypto-explosion structure. As a result o f ground reconnaissance o f one o f the anomalies to the north-east o f Lake C luff in July 1968, uraninite-pitchblende pebbles were found in the vicinity o f an abnormal contact between sandstones and a gneissic-granite basement, followed by the discovery o f in-situ mineralizations in pre-Athabasca metamorphites (C lu ff D), in Athabasca sandstones (CiuffsB andE)and in 'volcanic' breccias associated with the emplacement o f the Carswell structure (C lu ff F). The uranium content o f the erratic pebbles as nodules in the breccia zone o f C luff D can be as much as 70%. During the 1969 field season, exploration o f the contact between the gneissic- granite core and the sandstone aureole in the Carswell structure was continued together with systematic prospecting around the C luff uranium occurrences. The techniques used included scintillometry (ground and helicopter-borne), emanometry, ground magnetometry, geological survey, trenching and shallow drilling (518 m of coring with the Winkie minidrill). The main findings o f this season were: (a) The pattern o f glacial dispersion o f uranium-containing pebbles in the cover. (b ) The diversity o f rocks with uranium mineralization. (c ) The complexity o f the geological framework o f the in-situ mineralization. (d) The uranium-organic matter association in gneisses and sandstones, and the frequent presence o f gold associated with high uranium content. (e ) The important mineralization in crushed gneisses in the first borehole at occurrence N. (f) The important mineralization in a chlorite-rich rock in the first borehole at occurrence D o f vein type in gneisses; the mineralization would be in an overturned imbricated structure within the Athabasca sandstone near the unconformity under the metamorphic basement. * The complete text was not submitted for publication. 329 330 DARDEL During the subsequent seasons, from !970 to 1975, checking o f mining properties and a systematic study o f the mineralizations continued and new targets were set. Deposit N was developed by driHing in 1970 to 1971, deposit D in 1971 to 1972, and deposit Claude or ZA in 1972 to 1973. The study o f mineralizations О-P to the north o f D, R and F, to the east and to the south o f N, o f the source o f the glacial trains S and o f occurrences A A (Dominique) began between 1969 (F ) and 1974 (A A ). Exploration o f this region was rendered difficult by the over lapping o f a glacial and periglacial cover and a circular cryptoexplosion structure, the tectonic features o f which resemble those o f a diapiric plug; outcrops are rare, and always highiy dislocated. The techniques used were therefore numerous at the scale o f the Carswell structure (black-and-white and false-colour infra-red aerial photography, photo geology, helicopter-borne scintiHometry and spectrometry, airborne magneto- metry, regional geochemistry, study o f the cover and mapping) and at the detailed scale o f occurrences and targets (systematic scintillometry, emanometry, geochemistry, trenching, geological mapping, resistivity, magnetometry, VLF, gravity, seismic reflection and seismic refraction). Deposit D is located in an oblique and folded imbricated and overturned structure between sandstones and basement; the mineralization occurs as a thin layer in the sandstones a few metres away from the unconformity. The mineralization in the form of uraninite-pitchblende is in a phytlitic material with rpilcd detritic minerals. The ore is rich in organic carbon with appreciable concentrations o f boron, selenium and gold. Its average uranium content is 6 %. The N and Claude deposits are located in crush zones with a radial direction in relation to the Carswell structure. Deposit N extends into a brecciated corridor between granitoid gneisses and a variated gneissic sequence with more silica and alumina, while the Claude deposit is located in a fractured quartz-feldspathic gneiss complex. The main mineralization is in the form o f uraninite-pitchblende and coffinite accompanied by phyllites and carbonaceous matter. The average uranium content varies from 0.25% to 0.4%. The present reserves at C luff are 13 700 t o f uranium in an ore with an average uranium content o f 0.9%; in mineralizations, mainly in extension o f the deposits described above, the estimated additional resources are in the order o f 3800 t o f uranium in ores with an average uranium content o f 0.5%. DÎSCUSSÎON C. TEDESCO: My question pertains partly to your present paper but is more general also. I would like to know whether, in the episyenitization process IAEA-AG-250/17 that appears to be wide-spread in the area and connected with several types o f uranium deposits, the overall balance is a significant loss o f potash and if this loss might be represented by using the anomaly in the radiometric K-channei. J. D ARD EL: First, I am not sure that the final balance o f the episyenitization leads to the loss o f potassium. Secondly, in this case you have a granitic contribution which is quite potassic and which enriches the rock, which migmatizes in potassium. You have potassium in there that is a bit too strong and too high. I don't think the loss o f potassium is a very clear balance; what is clear, however, is that if the quartz leaves you have a loss o f quartz, and here I disagree with you a little. I f the quartz leaves, it means the rock is fractured. There is a tectonic history behind it: it crushes the rock; the rock becomes porous and permeable, so there is a loss o f quartz, and anything available becomes transported. So 1 don't think the loss of potassium is that clear so far as the balance is concerned. Don't you think, in general, that there could be a loss o f potassium in episyenitization which is associated with that? Let us say that, as far as the alteration o f the biotite into muscovite goes, it is different. H.D. FUCHS: I don't like the term 'episyenitization' very much because it implies a genetic term. A syenite is something which is intrusive. I think it is a good working term, especially when used in France, and Mr. Gélÿ has also mentioned it a lot in the literature, but I think we should not use the term episyenitization just for depletion of rock or quartz or whatever when you are talking about alteration or decomposition o f rocks. It could perhaps lead to some misunderstanding because I don't believe there is any classical hydrothermal action in this process. I am just suggesting that we ought to be a bit careful with this term because it is not familiar to English- or German-speaking people, and I would, therefore, like to have this term replaced by 'alteration in general' or 'decomposition'. P. BARRETTO.: What is the distance between the aerial survey anomaly and the first ore body? J. DARDEL: The first boulder is right under the anomaly; 5 m according to the photopoint. The ore body I mentioned is about 100 m. P. BARRETTO: Now that you know exactly where the ore bodies are, if you look at the aerial survey records can you identify any amplitude changes or other indications that could be related to the ore body mineralization? J. DARDEL: The airborne radiometric profile does not pass exactly over the deposit D (in any case, the deposit D crops out in the valley). No anomalous radio activity has been recorded in the surface over the deposit. IAEA AG 250/4 URANIUM EXPLORATION IN THE N.VÄSTERBOTTEN-S.NORRBOTTEN PROVINCE, NORTHERN SWEDEN B. GUSTAFSSON Geological Survey of Sweden, Luleâ, Sweden Abstract URANIUM EXPLORATION IN THE N.VÄSTERBOTTEN-S.NORRBOTTEN PROVINCE, NORTHERN SWEDEN. Over the past decade, the Geological Survey of Sweden (SGU) has discovered a uranium province N.Västerbotten-S.Norrbotten, in northern Sweden. The province contains some thirty prospects. SGU has an annual budget for uranium prospecting of US $6.5 million, of which half is government-financed and half on contract from the Swedish Nuclear Fuel Supply (SKBF). Uranium exploration is integrated with other Survey activities, particularly prospecting for other metals. The state-financed programme is concentrated essentially within the N.Västerbotten- S.Norrbotten uranium province. The uranium occurrences here are spatially related to terrestrial acid volcanics of Middle Proterozoic age. Some occurrences have been radiometrically dated, yielding an age of 1750 m.y., at which time the Svecokarelian Orogeny is considered to have reached its climax. Restricted sodium metasomatism occurs in connection with these occurrences. Glacial drift 3 —10 m thick obscures much of the bedrock and provides an excellent medium for exploration. The discovery of any new prospect has been followed by blanket coverage of the surrounding area using regional airborne radiometric surveying and geochemical and geological ground surveying. As drilling costs are high (US $70-130 per metre), much effort is con centrated on the detailed predrilling work. As an aid to geological bedrock mapping, detailed magnetic and electromagnetic methods are used. When (in nearly all cases) the mineralization is not exposed, a glaciogeological interpretation is made with the help of detailed boulder tracing within state systems, till spectrometry and seismic refraction. The uranium potential for the whole province is estimated to be 20 000 t U. The most important deposit so far is Pleutajokk. 1. INTRODUCTION Since 1967, uranium exploration in Sweden has been carried out exclusively by the Geological Survey o f Sweden (SGU). Before this, all state-financed uranium exploration was undertaken by AB Atomenergi. The transfer o f responsibilities was an attempt to integrate uranium exploration with other state-financed ore exploration programmes, in order to make maximum use of existing capacity and 333 334 GUSTAFSSON FVG.A í/гаятт м Sweden. IAEA-AG-250/4 КА KG КА KG 1 9 7 7 1978 1979 BB GEOLOGY KG GEOCHEMISTRY FF GEOPHYSIC AIRBORNE KA CHEMICAL ANALYSES FM " GROUND ВТ DRILLING F/C.2. D^fri&Mfícn o /expenditure ¡'л t/¡e ЖД^рго/ect. КА КА 1978/1979 1979/ 1980 BB GEOLOGY KG GEOCHEMISTRY FF GEOPHYSIC AIRBORNE KA CHEMICAL ANALYSES FM " GROUND ВТ DRILLING .ÜMfn'&MííOH о /expenditure ¡'л f^e gfafe-/[nanced ыгапшт exp/orafíon. 336 GUSTAFSSON know-how within various specialities of exploration and geological expertise throughout the country [ 1 ]. Initially the budget was rather modest but it increased rapidly and is now at about US $6.5 million o f which half is state-financed and half on contract from the Swedish Nuclear Fuel Supply (SKBF). The SKBF contract is restricted to a 35 000-km^ area in the central part o f Sweden and to a five-year period o f which two years remain (see Fig. 1). The invested expenditure is evenly distributed over this period. During the initial stages o f the exploration programme, regional surveys concentrating on airborne geophysics are emphasized. During the latter stages, detailed investigations involving extensive drilling consume the bulk o f the expenditure (Fig.2). The state-financed programme relates to the remaining part o f the country, and the detailed prospecting work is for the present essentially concentrated within the N.Västerbotten—S.Norrbotten uranium province, a 20 000-km^ area in northern Sweden (Fig. 1). A financial grant is awarded on a one-year basis, and expenditure between regional and detailed surveys, as well as between different specialities, is kept in balance from one year to another (Fig.3). 2. REGIONAL METHODS Airborne spectrometry and geochemical sampling o f organic stream material are frequently used in a regional exploration programme. For the former, 100 000 line km are flown annually, covering some 20 000 km^ or 4% o f the country. The methods used are V LF, magnetometry and gamma-ray spectrometry. Profiles 200 m apart are flown at a height o f 30 m and at a speed o f 70 m - s"* ; readings are taken during 30 m in every 40-m length. The results are plotted by computer on a three-component (three-colour) radiometric map [ 2 ]. Organic samples from stream edges are collected with a density o f 0.5 samples per km^. In anomalous areas samples are taken every 300 m along streams,which give a mean density o f about 3 to 4 samples per km^ [3]. Some 20 000 km^ are covered every year. The main object o f most o f these regional geophysical and geochemical surveys is not only uranium exploration, but exploration for other metals and minerals closely integrated with geological mapping. It is estimated that, at the present rate o f progress, Sweden should be adequately investigated by the above methods in twenty years. 3. DETAILED METHODS Uranium exploration in Sweden has during the last twelve years been con fined to the Precambrian rocks. As exposure is generally poor, geophysical, geo IAEA AG 250/4 337 chemical and glaciogeological studies have been an essentia! supplement to bedrock geological mapping. Fortunately, large areas have been built up by a till cover which has proved an excellent medium for exploration in northern Sweden [4]. Work on a local scale is therefore to a large extent concerned with studies involving different types o f till and moraine which characterize exploration in this area. Ore-bearing structures have been located using ground geophysics, magnetic and EM methods. Soil emanometry and snow emanometry have been used but have not yielded satisfactory results. Snow radiometry has given good results but is often overlooked in favour of the slower but more effective boulder-tracing [5]. Seismic refraction is used to determine the soil depth as an aid to glaciogeological interpretation and drilling. Geochemical sampling used in uranium exploration is nowadays restricted to organic material along streams and to peat sampling along swamp margins. Soil-mapping with glaciogeological interpretation is fundamental. Boulder-tracing and till spectrometry on the fine-grain material are two important applications to glaciogeology. During the last decades, glacial geology has played an increasing part in complex ore exploration. Studies of morphology and till material provide information about direction and distance o f glacial transport. Owing to the physical properties o f the glacier, the fine-grained fraction tends to occur more proximally to its source than the boulders [4], and this is utilized for geochemical till-sampling [ 6 ]. In uranium exploration, 'till spectrometry' is preferred,and for this method a 60-cm-deep pit is dug and boulders and pebbles removed. A gamma- ray spectrometer is placed at the bottom o f the pit and covered with fine-grained material from the pit walls before recording. The results are plotted as eU/eTh values which eliminate the background uranium variation, and this can be positively correlated with the thorium content. The uranium mineralizations are mainly monometallic. A ll rock samples collected by SGU are measured radiometrically and all drill holes are gamma-logged. Carborne and snowscooter-borne methods were used earlier with some good results [ 1 ] but were discontinued because they were uneconomic. In northern Sweden, local people are encouraged to participate in private prospecting during their free time, and SGU supports this by lending out, for example, scintillometers. Promising discoveries are recognized by an award- giving board. 4. THE N.VÄSTERBOTTEN-S.NORRBOTTEN URANIUM PROVINCE 4.1. Genera! features The province has a gentle topography varying between 400 and 800 metres above sea-level and is bounded to the west by the Swedish Caledonide mountain 338 GUSTAFSSON L E G E N D 0 20 40km 1 ^ * I CALEDONIAN FRONT ' '* ¡:;:;:;:g:№ ¡ GABBRO.DIORITE j GRANITE K?:-:- ACID VOLCANIC 1 I ! ^ KARELIAN CONTINENT I;',I OTHER suPRACRusTALsj ^ BOTHNIAN BASIN [¡¡¡lj!!l ARCHAEAN BASEMENT(4 . ' FAULT . . . o URANIUM OCCURRENCE F/G. 4. Geo/ogy and tiraMiMW оссмг/-йлсе^ о/?йе TV. мгапшт prOMHCf. IAEA-AG 250/4 339 range. The cümate is subarctic with a mean temperature in January o f — 12°C and in July o f + 13°C. Precipitation is 500 mm per year and is evenly distributed. The dominant soit profile is podzoiic in character and nearly half o f the area is covered by lakes. The rest comprises coniferous forests, swamps and barren mountains. Less than 1% is outcropped [ 1 ]. 4.2. Geological setting The province is located at the southern margin o f the Karelian Continent close to the Bothnian Basin marine sediments to the south. Along the contact between the terrestrial and marine sediments is located the Skellefte sulphide province which possesses many characteristics o f an island arc environment. It has been postulated that the contact represents a subduction zone in terms o f a plate-tectonic model. The supracrustal rocks have been folded, faulted, re crystallized and intruded by plutonites during different stages of the Svecokarelian Orogeny which culminated approximately 1750 m.y. ago. Volcanites are also preserved from a late stage o f the Orogeny. In Late Precambrian times an extensive erosion during 1000 m.y. took place. In Upper Precambrian times the Caledonian geosyncline was formed with deposition o f arenites and pelites on the Precambrian Peneplane [7] (Fig.4). 4.3. Uranium occurrences The uranium occurrences are associated with areas o f both pre-Orogenic and Late Orogenic terrestrial acid volcanics. They are deposited partly in metarhyolites and synkinematic granitoids of Svecokarelian age and partly at the contact between Upper Precambrian sediments and underlying regolith. The character of the former type is mostly hydrothermal, monometallic or with titanium, preceded by sodium metasomatism. The mineralizationshave been dated to 1750 m.y., which coincides with the culmination o f the Svecokarelian Orogeny. The uranium is high grade (0.1% U ) and spatially related to basic dykes. The sediment-regolith type is o f supergene character and occurs within the black kerogenic parts o f arenites, or in arenites with a clayish matrix, in the vicinity o f the uranium concentrations in the Precambrian basement [7] (Fig.4). 4.4. Case histories 4.4.7. DAs'coyerM The first three uranium finds were reported in 1969 and consisted o f (a) an archive sample o f Caledonian Front radioactive material; (b ) a radioactive drill- core from a manganese occurrence further to the north; and (c ) some uranium 340 GUSTAFSSON F/C.J. ProípeetMf /¡Mfoo' o/ fAe Л^. ^ä'jferAoyfen-&7Vorr&offen мгап/мл? province and ¡'Я М7*ал!'мл! potential. IAEA AG-250/4 341 secondary yellow minerals found by a group of SGU prospectors on boulders near the Pleutajokk stream. At the same time and in the same area, an extensive exploration programme for base metals took place which included geochemical [8] and airborne surveys. The uranium anomalies from these surveys singled out certain bedrock groups, especially recry stallized acid volcanics and the basal part o f the Caledonian Front sediments. During the next two to three years these . anomalous areas were followed up by geological mapping and boulder-tracing, resulting in the discovery of additional prospects [1] (Fig.5). Later experience has shown that every prospect o f economic interest gives anomalies resulting from both airborne radiometric surveys and geochemical sampling of organic material along streams. Thus the area covered by geochemical sampling and airborne geophysics has successively expanded and has resulted in new prospects. For example, in 1972 the NNE-SSW-trending uranium anomalous zone at Arvidsjaur was discovered and, in 1974, as a result of airborne surveys, the Sorsele uranium district was discovered the following summer (Fig.5). The area covered by geochemical sampling and airborne geophysics has continued to increase but there has been a limit to the number o f new mineralizations found. To the south and west the province is limited by geological boundaries, although further north and east the same rock types apparently exist but without uranium concentrations. What geological, geochemical or mineralogical factors serve to delineate this uranium province? A research programme has been initiated to answer these questions, and the answers may also identify further uranium provinces in Sweden. Improved methods o f exploration have supplied the motivation to re-examine the whole area, and with some success. An important step in this respect was made in 1974 when the airborne results were presented for the first time as three- component coloured maps. 4.4.2. Р/ем fa/o area As mentioned above, the first finding at Pleutajokk was made in 1969 by SGU prospectors. In the previous year a base-metal programme had been con ducted over the area using airborne magnetic and radiometric surveys, geochemical sampling and analysis of organic material along streams, stream-water and stream sediments. In addition to base-metal statistics, the resulting data indicated uranium anomalies on both the geochemical and the radiometric maps but, before follow- up groups could investigate the Pleutajokk area, the finding had already been made by the prospectors. In 1970 the surrounding area was thoroughly investigated by boulder-tracing, which indicated a target area of 15 km\ A ground magnetic survey using a 20 m X 40 m grid was conducted during the winter of 1970-71 over a 15.2-km^ area. A 1.3-knP area was also selected for radiometric measurements on a 10 m X 20 m grid on the snow cover. The magnetic map shows many excellent 342 GUSTAFSSON KfC.6. BouMer traim in areas ,4 ая^ B, Píeufa/ó^! ÎAEA-AG-250/4 343 bedrock structures hidden by a 4—10 m-thick till cover. The radiometric survey also located boulder trains and an area comprising granite boulders o f anomalously high background radioactivity. During the summer o f ]971 a soil emanometry survey was carried out in the same area as the snow radiometry, employing a similar measuring density. A boulder-train study was initiated which entailed detailed boulder-tr'acing carried out on a 40 m X 40 m grid in a 5.3-km^ area with the aim o f detecting every boulder of anomalous radioactivity for a boulder-train study (Fig.6 ). A t the same time, topographic features that may have affected the shapes o f the boulder trains were mapped (e.g. lakes, swamps, steep slopes, erosional channels in the moraine). As a result o f comparison between the boulder map, the snow radiometric map and the soil radon map, the radiometric highs could be perfectly correlated with the boulder trains, and there was an anomaly proximal to the boulder trains. The emanometry resulted only in spot anomalies, and the radon distribution in general was confusing. The boulders in areas A and В were interpreted as being locally derived. Diamond drilling commenced in 1972 within the proximal part o f the A - boulder train (Fig.6 ), and the mineralization was cut with the first drill-hole. The boulder train in area В can be divided into three or four clusters. They were originally interpreted as different trains from more than one source, distributed along the south-west side o f the main fault between areas A and В (Fig.7). 190 short percussion drill-holes, totalling 1060 m, in reconnaissance profiles across those clusters gave positive results only in the proximal part o f the large boulder train. In 1973 a diamond-drilling programme began on the indicated mineralization B, and by 1975 the principal mineralizations in areas A and В were relatively well known down to 150 m below surface, after 50 holes had been drilled yielding a total of 11 000 m. Later drillings have given more detailed information o f the near surface geology and provided extended knowledge o f the ore to 500 m depth. The maximum depth of the ore is still unknown, and the figures for reasonably assured resources are given in Table I. Exploratory drilling started in area С during 1973 but was discontinued in 1976 when intensive drilling began in areas A and B. Although some mineralizations had been indicated, they have not yet been followed up. In 1972 another airborne radiometric survey was carried out and in 1975 the results were displayed on three-component maps. Distinct uranium anomalies became apparent just north o f the mineralized area previously known. During the ground follow-up in 1975, three new boulder trains and a radio-, active outcrop were found. The area of detailed boulder-tracing was subsequently extended for another 5.4 km^ (Fig.8 ) and the area o f magnetic survey for 9.8 km^. An EM survey with 60-m coil spacing and 18 kHz was conducted in a 20 m X 40 m grid over 18.4 km^ to locate water-bearing crush zones, which have caused con siderable drilling problems. 344 LEGEND YOUNG GRANITE (1600 m.y.) PEGMATITE DOLERITE METAVOLCANIE , RHYODACIHC, OAEITiC TUFF GUSTAFSSON , ANDESITIC LAVA RHYOLITE WITH INTERLAIN , ANOESITIC LAVA AND TUFF FAULT, ERUSHZONE URANIUH OCEURRENCE If750 m.y.) PLEUTAJOKK GEOLOGY 0 1 2km PVG. 7. Gic/ogy a^d мгаТ^'мтя оссмггепсе^ о/ ?Ле Р/емУа/о^^ area. IAEA-AG-250/4 345 2km BOULDERS FROM SEMIREGIONAL __t BOULDER TRACING AREA OF DETAILED F BOULDER TRACING F/G.& ВсиИегУга;нл:л f/¡ePZeMfa/o^^area. ГАе frawsi'nareas^-Gwerewyesf^aye^w 7P77, areas F-G :'n /$7J-7d and areaD ¡n 797d. 346 GUSTAFSSON F /C . P. -S'frMcfMraZ pafferns F/G./0. DMinAMfion о/ fAe KraMi'MW w Д^, P^Mfg/pMr /мг^м/р.'-о/е^Я'ои^ 348 GUSTAFSSON TABLE I. REASONABLY ASSURED RESOURCES IN PLEUTAJOKK Calculation based on . Year t u %U No. of Total Information holes drilling from metres 1973 380 0.10 22 5 540 SGU 1974 940 0.17 40 6 630 SGU 1975 2200 0.14 51 11 110 SGU 1976 4000 0.10 113 24 010 SGU 1980 4000 LKAB NOTE: Mean driHing cost between 1977 and !978 was US $97/m but varied from US $64 to US $ 136/m for different prospects. A t swamp margins, 410 peat samples were collected 50—100 m apart, yielding more or less spot anomalies. These were followed up by careful boulder-tracing but could not be explained. However, this work led to the discovery o f the D boulder train. Till spectrometry was conducted in the E area o f Pleutajokk in a 50 m X 50 m grid. A strong eU/eTh anomaly was indicated within the boulder train and at 50—100 m proximal. Additional glaciogeological studies suggested a short transport. The magnetic map also shows a similar anomaly pattern about 70 m proximal to the grain as in areas A and В o f the main ore bodies. In 1976 all rights in the Pleutajokk area were transferred to the LK A B mining company. Exploration in the area was suspended and all effort was concentrated on a detailed drilling programme in areas A and B. In area B, which is the most promising, uranium-bearing veins form part o f a tabular body dipping 70° to 75° (Figs 9 and 10). In 1976 an ore estimation based on a minimum horizontal width o f 2.5 m, and a cut-off grade o f 0.05% U calculated down to a depth o f 450 m, yielded 3000 t U with 0.10% U and a mean width o f 5.6 m. The cores were analysed for uranium by X R F and delayed neutron activation; the drill-holes were gamma-logged and eU values calculated. There is good agreement between the chemical analyses and the gamma-logging results. The host rock is a metarhyolite with thin interbeds o f andesitic lavas . The main uranium concentrations seem to occur alongside doleritic dykes, probably acting as physical and/or chemical traps for the uranium-bearing solutions (Fig.7). IAEA AG 250/4 349 The anomalous metarhyolite comprises an area o f 35 krrP and is bounded in the north by volcanites of different facies with iow uranium content, and to the west and east by a younger granite. To the south there is a deep trough, occupied by Lake Hornavan. It was initially considered that the mineralizations were generated from the nearby granite, but geochronological studies involving a Rb-Sr whole- rock isochron study o f the granite gave an age o f 1591 ± 35 m.y., while the uranium mineralization is significantly older, 17381'° m.y., according to a U-Pb deter mination [9]. The uranium mineralizations are now believed to be epigenetic, having been remobilized from the rhyolite and redeposited during (a) devitrification and (b) the regional metamorphism which took place at that time. 5. URANIUM POTENTIAL OF THE N.VÂSTERBOTTEN-S.NORRBOTTEN PROVINCE The Pleutajokk deposit has now become a mining target with a reasonably assured potential in areas A and В o f 4000 t U (0.1% U). Estimated additional resources amount to 1000 t U and the potential resources o f the other parts o f Pleutajokk are estimated to be 6000 t U. The uranium potential o f the whole Pleutajokk area is considered to be in the region o f 10 000 t U. Several interesting prospects o f high quality, and estimated 1000 t U, have also been found in the immediate vicinity o f the Pleutajokk area. One o f the most important peripheral deposits is the Dobblon mineralization, which is a stratabound uranium mineralization in rhyolitic ignimbrites o f a late orogenic stage [10]. Reasonably assured resources are 4400 t U (0.027% U ) with richer horizons giving 1000 t U o f 0.04% U. A number o f occurrences around Dobblon are being examined but this work has so far yielded poor results, although some promising prospects remain to be investigated. The Kvarnân deposit is situated some 150 km eastwards. The uranium mineralization occurs as parallel lenses some 700 m long in a metasupracrustal rock-type. Extensive drillings have successively increased the ore volume so that, at the moment, reasonably assured resources amount to 1000 t U (0.05% U ) or 500 t U o f 0.1% U. This will probably improve during the next six months o f drilling. Within the Pleutajokk-Dobblon-Kvarnân triangle there are still a number o f promising prospects, mostly consisting o f boulder trains which, statistically, should contribute significantly to the uranium reserves in the province. Drillings have been conducted at eighteen such prospects, of which five were investigated thoroughly enough to be considered for exploitation. For the remaining thirteen prospects, drilling is continuing as planned. Drillings are also planned for twelve more new prospects. 350 GUSTAFSSON The uranium potential o f the whole province is estimated to be 20 000 t U o f which two-thirds consists o f 0.1 % U or more. in 1976, SGU relinquished exploration rights o f the Pleutajokk area to the LKAB mining company, who have estimated that if the reasonably assured resources increase from 3000 t U (0.1 % U) to 4000 t U, this should be adequate to start mining with a back-up dressing plant. This prerequisite o f an extra 1000 t U should be obtained from several small occurrences o f high-grade ore in the province (Fig.5). ' . < . REFERENCES [ I ] LUNDBERG, B., "Exploration for uranium through giacial d rift in the Arjeplog district, northern Sweden", Prospecting in Areas of Giacial Terrain (Proc. Symp. Trondheim, 1973) ÏMM London (1973) 31-43. [2] LINDEN, A.H., ÂKERBLOM, G., "Method of detecting small or indistinct radioactive sources by airborne gamma-ray spectrometry", Geology, Mining and Extractive Processing of Uranium (Proc. Mtg. London, 1977), IMM, London (1976) 113—120. [3] LARSSON, J.O., Organic stream sediments in regional geochemical prospecting, Precambrian Pajala district, Sweden, J. Geochem. Explor. 6 (1976) 233—249. [4] MINELL, H., Glaciologicat interpretations of boulder trains for the purpose of prospecting in till, Geol. Surv. Sweden, Mem. and Not., Ser.C, 743 (1978). [5] GUSTAFSSON, B., MINELL, H., "Case history of discovery and exploration of Pleutajokk uranium deposit, northern Sweden", Prospecting in Areas of Glaciated Terrain (Proc. Symp. Helsinki, 1977), IMM, London (1977) 72—79. [6] BRUNDIN, N.H., BERGSTRÖM, J., Regional prospecting for ores based on heavy minerals in glacial till, J. Geochem. Explor. 7 (1977) 1 — 19. [7] ADAMEK, P.M., WILSON, M.R., The evolution of a uianium province in N. Sweden, Proc. R. Soc. (London) 291 (1979) 355-368. [8] BRUNDIN, N.H., NAIRIS, B., Alternative sample types in regional geochemical prospecting, J. Geochem. Explor. 1 (1972) 7—46. [9] WILSON, M.R., SUNDIN, N.O., Isotopic age determinations on rocks and minerals from Sweden 1960—1978, Geol. Surv. Sweden, Rep. and Comm. 16 (1979). [10] LINDROOS, H., SMELLIE, J., A stratabound uranium occurrence within Middle Precambrian ignimbrites at Dobblon, northernSweden, Econ. Geol. 74 (1979) 1118—1130. DISCUSSION M. M A TO LÍN : Did you experience any difficulties with the detector in the low temperature o f snow-scooter technique, and how low was the temperature? B. GUSTAFSSON: We did not experience any difficulties due to the low temperature, which for weeks could be between —20°C and —40°C. We used the same crystals during the five winters when we carried out snow-scooter measure ments. The crystals were thermo-insulated to guarantee a change in temperature iess than ]°C per hour. The same crystals were used during the summers in ordinary ground radiometry work. ÍAEA-AG-250/4 35] C.'TEDESCO: Did you have any difficulties in using the snow-scooter? B. GUSTAFSSON: The probiem was that work had to be done in areas far ' from roads,.and that is why.snow-scopters were used. But the scooters needed repair occasionally, and this wasted a lot o f time. So they became too expensive to use. . . . D. TAYLOR: I know you have used both core drilling and deep percussion drilling in the Pleutajokk area. Can you comment on the use o f the two techniques? B. GUSTAFSSON: Deep percussion drilling was used in the Pleutajokk. where a lot o f core drillings had been made before and after percussion drilling. Percussion drilling could not go deeper than 100 m owing to the water pressure. Percussion drilling costs were about two-thirds those o f core drilling, although large differences in hole-diameter gamma-logging could be obtained with the same sound and with comparable results in both core-holes and percussion holes. There are good agreements between results from chemical analyses o f cores and gamma-logging in drill holes. Percussion drilling was performed between old core- holes in a surface near parts where the mineralization was already reasonably well known. A fter the percussion drilling, a core-drilling programme was set up to penetrate the mineralization at deeper levels and where more geological information was required. A.G. A N G E IR AS: In Brazil we have two important episodes o f uranium mineralization related to albitization. An early episode occurred at about 850 m.y. following shear zones; the country rocks are 2500 m.y. The later episode is related to the Itataia deposit, where mineralization is younger than 500 m.y. I would like to know some details about the early albitization such as: (a) What are the additional minerals to the albitization; (b) How are the uranium bearing albite-rich rocks graded into the country rocks? (c) What is the Na/К ratio in the country rocks? B. GUSTAFSSON: (a) Two general types have been distinguished: (i) albite- chiorite quartz; (ii) aibite-aegerine-riebeckite-chlorite (furthermore hematite, zircon, sericite and calcite can occur). (b) I do not know about this transition in detail, but for certain mineralization sudden changes can occur across one metre; for other mineralizations there is a gradual transition through the alteration zone into the country rocks. (c) The Na/К ratios in the country rocks, acid volcanics and granitoids vary between 3:5 and 1.1. The same ratios in the mineralizations vary between 3:1 and 100:1. Each mineralization has its specific ratio. The alkali content is 8 - 10%. B.L. NIELSEN: What kind o f control do you think the dolerite types in the Lilljuthatten area have on the distribution o f the uranium in the granite? It seemed obvious that a substantial part o f the anomalies was parallel to the dykes. 352 GUSTAFSSON В. GUSTAFSSON: Uranium occurrences in the vicinity of doterite dykes have been observed in several localities in the Precambrian basement in Sweden, but the reason has not been satisfactorily explained. There are several hypotheses for various areas depending on differences in occurrence. In Lilljuthatten, NW SKBF area, we believe that the same deep-seated structure controls both the dolerite dykes and the uranium mineralizations. A ll dykes are older than the mineralizations; immediately adjacent to the dykes there may have been chemical and mechanical traps for minor concentrations within the mineralizations themselves. B.L. NIELSEN: Did you investigate the magnitude of the absorption of the gamma rays in various thicknesses o f the snow layers in connection with your snow-scooter surveys, and did you make any correction for such absorption? B. GUSTAFSSON: We did some simple tests before we started. I have not seen the results, but the absorption due to the snow was very low. We have detected radioactive boulders below 2 m of snow cover; with a normal depth o f 1 m to 1.5 m there is no difficulty in detecting them. No corrections have been made. IAEA-AG-250/11 EXPLORATION HISTORY OF THE KVANEFJELD URANIUM DEPOSIT, ILÍMAUSSAQ INTRUSION, SOUTH GREENLAND B.L. NIELSEN The Geological Survey of Greenland, Copenhagen, Denmark Abstract EXPLORATION HISTORY OF THE KVANEFJELD URANIUM DEPOSIT, ILÍMAUSSAQ INTRUSION, SOUTH GREENLAND. The uranium deposit at Kvanefjeld is situated in the alkaline complex of Ilimaussaq in South Greenland. The deposit belongs to the category of disseminated magmatic low-grade uranium resources. The resources known at present are 27 000 t U in the class of reasonably assured resources and 16 000 t U in the class of estimated additional resources. The ore body is still open in two directions and the possibility of considerably increasing the resource figures is high. The average grade of the ore is 340 ppm U. Exploration work has reached a detailed development stage and material is at present being mined for extraction tests on the refractory ore. The exploration history dates back to the mid-1950s, and during the subsequent 25 years the deposit has been investigated with varying intensity. Yet its economic feasibility has not been proven and the decision for or against development awaits the results of the metallurgical pilot-plant tests and the solution of a number of environmental and legal questions. A lapse of three to five years is expected before a final decision can be made. The character of the exploration in the area is determined by the fact that exploration licences for radioactive materials are traditionally not given to private mining companies in Greenland. The Kvanefjeld deposit has accordingly been explored by Danish State institutions without direct commercial interest in the development; no centrally co-ordinated exploration, metallurgical and environ mental study has been carried out. Progress in the exploration has therefore been largely governed by the initiatives of single institutions. I. INTRODUCTION Before the exploration h istory on the K vanefjeld uranium deposit is presented, the present-day situation regarding the classification and size of the deposit will be briefly review ed. Kvanefjeld is situated in the alkaline complex of Ilimaussaq in South Greenland (Fig. 1). The deposit belongs to the category of disseminated magmatic low-grade uranium resources and the presently known resources are 27 000 tons 353 354 NÍELSEN F/C. 7. ¿осайол wap о/ //¡'^амяд^ comp/еж and уйе ^ffane/)eZd муал;'мл: ¿ерол'г fa/fer U in the class of reasonably assured resources and 16 000 tons U in the class of estimated additional resources. The ore body is still open in two directions and the possibility of increasing the resource figures considerably is high. The average grade of the ore is 340 ppm U with a cut-off of 250 ppm U. The exploration work has reached a detailed development stage and an a d it opened to provide a bulk sample fo r extraction tests on the refractory ore. A case h istory on the uranium exploration in the Ilimaussaq alkaline complex is presented in the following with the main objective of providing guidance of exploration elsewhere within similar geological environments. Viewed with hindsight, several mistakes were made during the 25 IAEA AG-250/11 355 years of exploration, although all decisions were thought to be rig h t or the best possible ones at the time they were taken. It is hoped that the description below of the logic or need of logic in the long sequence of events may help to adjust some present and future uranium exploration programme in the optimal direction. Some of the specific field operations are listed in Table I together with cost fig u re s where a v a ila b le . An appreciation of the uranium exp loration in the Ilimaussaq complex up to the present can only be made with an understanding of a number of conditions and criteria, some of which are specific for this particular project. 1) The exploration history has been very long, almost 25 years, and a decision to go into production has s till not been taken. The background fo r th is is mainly the marginal economy of the low-grade uranium ore. The deposit has nevertheless become gradually more attractive in view of the general price development on the uranium market. The long exploration h istory also means that the exploration techniques used range from rather p rim itive methods to recent advanced techniques. 2) The exploration has, from the very beginning, been carried out by Danish State institutions, each of them without any direct interest or dependence on the eco nomic feasibility of the project. 3) The uranium deposit at K van efjeld is situated within a subarctic area with an unusually high percentage of geological exposure. This implies that geological methods have played a p rin c ip a l ro le in the exp loration rather than geophysical or geochemical methods. 4) Traditionally, private mining companies have not obtained permission for the exploration of radioactive minerals; and because of this the uranium exp loration has been carried out by the Danish State and directed by the TABLE I. COST OF SOME OF THE SPECIFIC OPERATIONS ON THE KVANEFJELD PROJECT 356 Year Subject Remarks Cost (1000 US$) 1955 Geox 55 field expedition In total 13 persons in 93 days 1*7 1956 Geox 56 field expedition In total 12 persons in 73 days 19 1958 Drilling programme, field work In total 39 persons *320 1962 Drilling and mining of 200 t ore Approx. 30 persons in 100 days 230 1964-67 Detailed mappi'ng programme 2-5 field teams every year NIELSEN 1969 Drilling programme, field work In total 10 persons in 100 days 285 1974-76 Environmental study, "Narssaq 3-6 field teams every year project" Compilation not yet completed 345 1976 Small pilot plant experiment at Risz Additional 2 mill. D.kr. in "in kind" 380 contributions 1977 Drilling programme, field radiometry Approx. 20 persons in 90 days 1 180 1978-82 Uranium extraction programme incl. Programme not completed test mining and pilot plant 6 915 NOTE: Most work in Copenhagen, including all laboratory work and some field work, is "in kind" contributions which cannot be specified. Costs have been recalculated in US $ according to an average 1979 exchange rate of 1 US $ *- 5.25 D.kr. IAEA-AG 250/11 357 Geological Survey of Greenland. According to the Act on Mineral Resources in Greenland of 1965, the Danish State (the Minister for Greenland) possessed full authority regarding exploration and exploitation of mineral resources in Greenland. The new Mineral Resources (Greenland) Act 1978, which was issued in association with the introduction of Home Rule in Greenland, established a joint decision-making power to be vested in the Danish national authorities and the Home Rule authorities alike when it comes to essential disposition in respect of mineral resources. This means that exploration and exploitation of mineral resources in Greenland can only be initiated subject to agreement between the Danish National Authorities (the Minister for Greenland) and the Home Rule authorities. Licences and concessions under this new system are still to be issued formally by the Minister for Greenland. Greenland is a part of the Kingdom of Denmark and, together with Denmark, a member of the European Economic Community. The exploration comprises the following sequence of events: 1955-56: Regional radiom etric exp loration within the Ilimaussaq intrusion. 1958: First core-drilling programme at Kvanefjeld totalling 3728 m. 1958-61: Regional mapping of the Ilimaussaq in tru sion in the scale of 1:20 000. Laboratory tests on uranium , extraction. 1962: Core drilling in U mineralised areas outside Kvanefjeld. Test mining of 180 tons of ore and deepening of drill holes at Kvanefjeld. 1964-67: D etailed g e o lo g ic a l mapping of K van efjeld in the scale of 1:2 000. Continuous metallurgical tests. 358 NIELSEN 1969: Third core-drilling programme at Kvanefjeld, totalling 1621 m. 1970-76: Limited f i e l d work. M eta llu rg ica l work and feasibility studies. Environmental study on the geo chemistry in the Ilimaussaq area initiated. 1977: Fourth c o r e - d r illin g programme, t o t a llin g 5ЮЗ m. Extensive field gamma-spectrometer survey. Resources increased, from 5 800 tons U to 27 000 tons U. Small pilot extraction plant. 1978-80: Major change in applied extra ction technique. Improvement in recovery rate from 60% to 85%. Adit opened fo r the mining of 5 000 tons of ore fo r new pilot plant. Further environmental investigations. 2 THE EARLIEST HISTORY The onset of the uranium exploration in the Ilimaussaq intrusion is linked with the establishment of the Danish Atomic Energy Commission (AEK) in March 1955- Three months later the late President of AEK, Niels Bohr, in a letter to the Department of Greenland (Cranlandsdepartementet) stressed the importance of initiating exploration for radioactive raw materials on Danish territory. The AEK pointed at the potential of three alkaline intrusive complexes in South Greenland, Ilimaussaq, Grannedal-Ika and Igaliko, on the basis of consultion with the Geological Survey of Greenland (GGU). The AEK also pointed at the advantage of applying a number of prospectors in a field programme supervised by g e o lo g is ts as opposed to a technically more difficult aerial survey. The field work was recommended to begin immediately in order to gain the greatest possible experience, and the 1955 season should be regarded as a "test year". GGU selected three minor areas within Ilimaussaq and Crannedal-Ika for the first field season. The general geology of the Ilimaussaq intrusion was well known and described by Ussing (1912), and from the m in eralogical work by Lorenzen (1881) the steenstrupine found in some of the Ilimaussaq rocks' was known to contain IAEA-AG-250/11 359 thorium. The radioactivity of steenstrupine was also men tioned by Boggild ( 1905). This information was probably the main reason for the initial high priority of the Ilimaussaq intrusion as a favourable exploration target. The Grannedal-Ika complex was being mapped by GGU geologists during the field work in the Ivigtut area further to the north (Emeleus, 1964). Because of the late start of the field season no geologists were available for the project and it was decided to staff the project with Danish military personnel under the technical guidance of one engineer of geophysics and a part-time geological consultant from GGU's field team in the neighbouring areas. ' No detailed planning of the field work was carried out that year and the i n i t i a l phase of the exploration was not the best possible. The conclusions from the first years of field work were that fu rth er work should take .place in Ilimaussaq and that a regional aerial survey should be strongly considered. The g e ig e r measurements taken along lin es proved to be too slow a method for a regional exploration programme. Very l i t t l e evaluation was done on the data of the 1955 expedition, and because of this neither GGU nor the involved military authorities expected field work to be continued the following year. Nevertheless the AEK in the late spring of 1956 made arrangements for a second field season, and once again the survey was planned without adequate geological support. One geologist, however, took part in the field work which in 1956 was supplemented with an airborne scintillometer survey. During the 1956 opera tion the only aerial survey ever carried out within the Ilimaussaq area was completed. The systematic field work with the geiger counters in 1955 and 1956 resu lted in an almost complete coverage of the intrusion. Although the collaboration between the military personnel in charge of the "technical" prospecting and the geologist attending the field work was very limited after the return to Denmark it was concluded that the most important radioactive mineralisation 360 NIELSEN was found in the nepheline syenite, lujavrite, and particularly in the lujavrite of the Kvanefjeld area. Not only was the radioactivity generally higher here, but the lujavrite was also present in considerable quantity and a substantial part is exposed at the .surface. The aerial survey was extended the same year to the Grannedal-Ika and Ivigtut area to the north. However, the results of this work did not justify intensified exploration within that region. With the evaluation of the results from the 1955 and 1956 programmes, the uranium deposit at Kvanefjeld had been located. Although some regional work was carried out in the following year, the main effort was thereafter devoted to an elucidation of the discovery at Kvanefjeld. The following chapters will accordingly describe the deposit in a more systematic manner, its geographical and geological setting as well as the development of the various stages in the exploration of the deposit. The general history of the geological exploration of the Ilimaussaq complex up to 1966 was described by H. Szrensen (1967); this paper deals solely with the uranium exploration and with the Kvanefjeld area in particular. 3. GEOGRAPHICAL SETTING The Kvanefjeld area forms a 2.5-km 2 hilly plateau at an elevation of 500-700 metres. It is situated in South Greenland at a geographical position of 60°58' north and 46°00' west (Fig. 1). The distance to Narssaq, the local administrative centre and port with 2 500 inhabitants, is about 8 km. The climate is arctic with average temperatures at Narssaq of -3-3°C and +7-5°C for January and July respectively. Climatic conditions are not likely to put any serious limitation on a possible open-pit mining operation. In general,access to the area is easy and the nearby deep fjords are navigable for most of the year. 4. GEOLOGY Kvanefjeld is situated at the northern border of the Ilimaussaq alkaline complex belonging to the Gardar igneous ÏAEA-AG 250/П 361 province of South Greenland (Emeleus and Upton, 1976) (Fig. 1). The complex was emplaced at 1 168 Í 21 m.y. (Blaxland et al. 1976) into Ketilidian basement granites and an over- lying series of sandstones and lavas. The Ilimaussaq complex described by Ussing (1912), H. S0rensen (1958, 1970) and Fergusson (1964, 1970) comprises a series of alkaline to peralkaline syenites and granite, emplaced in four principal successive phases (Nielsen and Steenfelt, 1979): 1) marginal augite syenite, 2 ) alkaline acid magma, 3 ) agpaitic syenites including pulaskite, foyaite, sodalite foyaite, naujaite (sodalite-nepheline syenite) and kakortokite (eudialyte- nepheline syenite) and 4) lujavrite (mafic nepheline- analcime syenite). The lujavrite intruded the overlying consolidated syenites and country rocks probably in several pulses and it encloses abundant xenoliths of these rock types. In the Kvanefjeld area, which may be described as a huge intrusive breccia with the lujavrite forming the matrix, the average ratio between lujavrite and xenoliths is estimated at 2:3- Nielsen and Steenfelt (1979) describing the intrusive events at Kvanefjeld, proposed an emplacement by permissive intrusion with subsidence of the older blocks into the lujavrite magma. A simplified geological map of the Kvanefjeld area is shown in Fig. 2, and the character of the intrusive breccia is seen from Fig. 3- The uranium deposit is associated mainly with the lujavrite and partly with metasomatically altered lavas and hydrothermal analcime veins and pegmatites. The most compre hensive publication on the mineralisation at Kvanefjeld is given by Serensen et al. 1974- In principle the Kvanefjeld mineralisation is not different from lujavrites enriched in radioactive elements elsewhere within the Ilimaussaq intrusion, except for the size and the grade. Most of the uranium is contained in steenstrupine (Na^, Ce(Mn,Fe)H^((Si,P)0^)^ and a small amount in uranothorite. The uranium content in the steenstrupine varies between 0.2% and 1 .4% and the thorium content from less than 1% to 5% (Makovicky et al. 1980). The steenstrupine occurs as a disseminated accessory mineral in the lujavrite and the FYG.2. wap о/^Гуяне/уеМ. ГАе enhance fo Уйе аЛ7 w/н'сЛ и а/ ргелея? &e¡7!g ¿п'уея mfo Уйе JepoM'f м warA;e¿ wi'f/¡ а Мас^ dof. ^/fer№'e^ewa?!d^feey!/eZ^7P75J IAEA-AG-250/H 363 F/C.J. ¿м/avrifes m f/¡e nor;/¡ern parf о/ í/ie J?rane//eM p/afeaM, w/ü'c/] confai'ns c considerare parf o/f/¡e MraniMW resoi/rce. TVaM/aiYe xenoA/As i'n ^e /м/afri'íe are seen as di'sfi'nc; wAi'fe areas. 77:e s/¡arp coníací fo í/ie Zafas o/ f^e Fri'A/)'ord ForMMfion is cZear/y seen ¡n ;йе bacA:groMnd o/ f/¡e pAofoyrap/i. ГАе Za^e i'n ^e /oregroMnd corresponds ?o /a^e /í o/F^.2. F/G.4.. /ÍMíoradi'ograpA and corresponding driVZ-core samp/e s/iow/ng ?^pi'ca/ /icnogeneoMs di'sfri'&Mrion o/radioacr/fe e/emen^s /n /ine-grai'ned /м/afri'fe. ¿engr/i o/core approxi'maZe/y 73 cm. Dri'Z/-/!oZeJ#.¿?Ofn. ^/íerAfa^ovi'c^yeíaZ., 7P50./ 364 NtELSEN i______0.5 mm______] F/G..5. № cropAofo о/мсЯ'оя соггеурояЛ'я^Дм^и Мей ;'?Hgge o/zoned ^ре о/ .:?еел,уУгыр;ие. íengf?; о/*Aar approxwaye(y 0.J wïw. ^ /f e r А^а^ом;с^м efa/., 7Р<У6^ ÏAEA-AG 250/11 365 overall radioactivity of this rock corresponds typically at Kvanefjeld to uranium contents between 300 and 400 ppm U. The autoradiograph in Fig. 4 shows the distribution of radioactive steenstrupine crystals in fine-grained lujavrite. The position of the uranium within the ateenstrupine is shown on the fission track image in Fig. 5. The uranium is thought to have been carried by a volatile phase in the lujavrite magma possibly in a chemical complex with fluorine. During the final crystallisation the uranium, together with thorium and rare-earth elements, entered the steenstrupine. The larger part of the resource may thus be classified as a syngenetic "porphyry" or disseminated deposit. Some mobilisation of the uranium has nevertheless taken place and a secondary enrichment is found in shear zones or along the border zones of plastically deformed lava xenoliths. This mobilisation of the uranium is strongly associated with a deuteric alteration of the lujavrite (Makovicky et al.,1980) a phenomenon which also has a major influence on the extractability of the uranium (see chapter 7). A tentative genetic model for this type of deposit explains that the uranium is contained primarily in accessory minerals derived from the volatile phase in the latest intrusive stages of an alkaline igneous complex. Redistribu tion of the uranium may or may not take place. The content of a number of rare elements in the initial magma in several alkaline intrusions has been exceptionally high, and the uranium concentration of the earlier nepheline syenites is high, (in Ilimaussaq 2$ to 50 ppm mainly contained within eudialyte (Steenfelt and Bohse, 1975)). The general association between U, Th, Zr, Nb and REE in alkaline igneous rocks is described by Szrensen (1970, 1977) and Semenov (1974). Apart from Ilimaussaq, Pogos de Caldas (Brasil), Pilanesberg (South Africa) and Bokan Mountain (Alaska) are examples of alkaline plutons of high radioactivity. The mineral wealth of some alkaline plutons implies that production of uranium may be accompanied by a number of by-products. At Kvanefjeld the potential by-products include Th, REE, Nb, Li, Zn and F. 366 NIELSEN 5. EXPLORATION 5.1. Geotogica) methods Although the general geology of the Ilimaussaq intrusion was known from the work of Ussing (l912),no detailed geological information was available in 1957 when the Kvanefjeld area ' was selected for further uranium exploration. Detailed geological mapping at a scale of 1:2 000 of the geologically very complicated area was not initiated until 1964. This mapping, which was carried out by personnel and students from the Institute for Petrology at the University of Copenhagen, was completed in 1967 and the results were published by H. Szrensen et al. (1969). The field work and the evaluation of the geology was made in collaboration with the Geological Survey of Greenland. The' all-important geological mapping was more than 7 years behind the detailed geophysical exploration, which was initiated as early as 1Э57. This was clearly a disadvantage for the general planning of the first stages of the exploration,and several years were lost as the important proposals for drilling programmes could only be put forward with a detailed understanding of the geology available at the time. After the completion of the geological map in 1967, only limited geological field work was carried out except for the geological guidance of the drilling projects in.1969 and 19 77. For.a number of years a major part of the geological work was thus carried out as an academic university project. However, extensive mineralogical and geochemical laboratory research was made, particularly on drill core material. The research was made in connection with the drilling programmes and the metallurgical work on the ore (Makovicky et al. 1980, Kunzendorf et al., in prep.). The work by Nielsen and Steenfelt (1979) is the most recent contribution to the intrusive history of the Kvanefjeld area. It is believed that for further development of the deposit there is no immediate need for additional field geology. Detailed geological investigations are,however, essential for the evaluation of all forthcoming geophysical and geochemical work as well as for new drilling programmes. IAEA AG 250/П 367 From the chapter below on the organisational structure it will be understood that the expertise on the various aspects of the uranium deposit at Kvanefjeld is widely spread among a large group of people from a number of institutions. This is also true for the geological knowledge, and the difficulty in maintaining an "in-house" geological expertise, for instance at the Geological Survey of Greenland, is realised and is of major concern for the future development of the project. 5.2. Geophysical and geochemical methods As the geiger counter was the most important prospecting tool in the very early exploration phase, radiometric surveys remained the principal exploration methods at Kvanefjeld. After the discovery of this area a detailed survey with geiger readings in a 50 m and a 5 m grid over the most radioactive area was carried out in 1957. Rock samples were taken at many survey points and assayed in a field laboratory established nearby. The field work, which also included topographical mapping and geological reconnaissance, was a joint operation between AEK, GGU and Kryolitselskabet 0resund A/S, a private Danish mining company. However no systematic survey of the entire plateau was carried out and the survey focused on a minor area, the "mine area", where the radioactivity in a pegmatitic lujavrite and in metasomatised lavas was considerably higher than elsewhere on the plateau. It was, however, already realised that year that the area of comparatively high radioactivity only constituted a "shell-like" structure in what was considered an intrusive dome (Bondam, 1957, 1960). On the basis of the field radiometric survey in 1957) the first drilling programme was proposed and carried out in 1958 (see next chapter). Since 1957 several field programmes based on gamma-ray spectrometry have been performed (Levborg et al., 1968, 1971; Nyegaard et al.,1977). Apart from contributing to the understanding of the distribution of radioactive elements in lujavrites, these surveys have contributed to the 368 NíELSEN TABLE II. SPECIFICATIONS ON FIELD INSTRUMENTATION USED DURING THE URANIUM EXPLORATION AT KVANEFJELD Instrumentation Specification Period GM-counter Radiac A/N-PDR-27E, Admirai Corporation, USA During Geox 55 and 56 Früngel 2, Hamburg Geox 56 Superscaler, type HP Geox 56 Berkeley scaler, type 2080A, with IB-85 GM-tube 1957 Atomat, WR 57A 1958-70 Phillips, type PW 4010 and PW 4014 1958-70 Scintillation counter Harwell, type 14I3A Geox 56 Saphymo Stei, SPP2-NF 1977-79 Field gamma-ray spec. Ris0 design, collimated 3 in. X3 in. Nal detector 1966-70 Geometrics/Exploranium Disa GR 410, 1976 3 in. X 3 in. Na! detector Airborne gamma-ray Harwell, type 1444A, recorder 1442A Geox 56 spectrometer Logging equipment Mount Sopris, type 1000 gamma-logger 1958-69 Scintrics spectrometric logging system on GAD 6 1977 spectrometer, GS-3S logging probe ionisation chamber Reyter and Stokes Environmental Monitoring 1977 System, RS 111 Thermoluminescence LiF, Harshaw TLD700 1977 dosemeters scientific understanding and practical application of field gamma-ray spectrometry. The subject has been investigated intensively by the Rise National Laboratory, the research establishment of the former AEK. The radiometric surveys have also provided important information for a study of the natural radiation environment and the monitoring of the gamma exposure levels (Lavborg et al., 1980; Bztter- Jensen et al., 1980). IAEA-AG-250/H 369 F/C. 6. ZMZi'ng operafí'on ¡'я íAe /^vane//eZd area ¿n /97P. Together with the improved geological understanding, field radiometric surveys of the area have been the basis for the location of drilling sites in 1969 and 1976. All spectrometric surveys have been.followed by analytical programmes on surface samples. The instrumentation,which has always been carefully calibrated,is listed in Table II. 5.3. DriMing Four individual drilling programmes have been carried out at Kvanefjeld in 1958, 1962, 1969 and 1977 (Fig. 6 ). In 370 TABLE HI. SPECIFICATIONS ON CORE DRILLING OPERATIONS IN THE KVANEFJELD AND ILÍMAUSSAQ AREAS Year Area No. of holes Total length drilled (m) Equipment 1958 "Mine area" 30 Svenska Diamant bergborrnings, 3 728 type XC-42 1958 Kvanefjeld 6 1962 "Mine area" (deepening) 2 270 Svenska Diamant bergborrnings, type XC-42 NIELSEN 1962 Ilimaussaq 7 1 400 1969 Kvanef j eId 6 1 621 Svenska Diamantberg borrnings, type XC-42 and XF-90 1976 "Northern area" 24 4 256 Atlas Copco Diamec 250 and D-750 .1976 Steenstrup area 4 855 Total 77 Total 10 730 IAEA AG 250/11 371 total 10 730 metres have been drilled and altogether 70 holes have been drilled and cored with core recovery close to 100%. Details on the drilling equipment are presented in Table III. The first drilling programme in 1958 was planned on the basis of geological reconnaissance and the radiometric field work in 1957. A total of 3 728 m was drilled with the purpose of delineating the ore in the "highly active, domed mine area" including the extension of the ore beneath a major gabbro xenolith to the east (Fig. 2). Six of the 36 holes were drilled in other areas of increased radioactivity with the possibility of striking "ore". In the "mine area" 30 holes were drilled in an approximate 50-m grid. The average depth was about 100 m and the maximum depth 180 m. The main criteria for terminating each hole was drilling a section of minimum 6 m in lujavrite with less than 200 ppm U. The main result of the programme was the discovery of heterogeneously distributed uranium in the upper 30-m layer consisting of a mixed rock of lujavrite, pegmatitic lujavrite and metasomatised lava and gabbro. It was not possible to correlate either radioactivity or lithology between the holes. The radio activity of the lujavrite in the 6 holes outside the "mine area" was generally less than that found inside the "mine area". At this early stage of the project one was "looking" for ore with an average grade of about 500 ppm U, and with this assumption the ore body was thought to be closed in all directions and the total resource could therefore be calcu lated. The estimate was 4000 tons U. The exact grade figures a.re not available but they are supposed to be 400 ppm U and 200 ppm U for average grade and cut-off grade respectively.' The correlation between tonnage and grade was probably not fully realised in 1958, because of the small representation of fine-grained lujavrite ore in the "mine area". The sub sequent drilling programmes showed that a substantial part of the lujavrites outside the "mine area" contain close to 350 ppm U. This discovery was the main reason for the considerable increase in the resource figures after the large 1977 drilling programme. The principal difference between 372 NIELSEN Grade (ppm) (tons U) F/G. 7. Fn'nci'paZ /grade смгрел /or f^e мга/ч'мт-йеапи^ roe^ af ^уале/7'еМ. the tonnage/grade relationship in the pegmatitic lujavrite of the "mine area" and the fine-grained lujavrites of the entire Kvanefjeld is shown in Fig. 7. In 1962, 7 exploration holes, each 200 m deep,were drilled in lujavrite areas of comparatively high radioactivity at 7 different sites in the Ilimaussaq intrusion. None of these holes gave rise to more detailed studies and Kvanefjeld was still the main area of interest. In the same year two of the holes in the "mine area" were deepened to 200 m, and from the surface an adit of .20 m was driven into the upper part IAEA-AG 2S0/11 F/C.& af J^vane/jfeZd on f/:e iZope overZoo^i'ng f/:e TVarwat? FZpdaZ. №Aen comp/efed fAe JepfA o/?Ae аЛ'У w/ZJ ¿e aíoMí J000 m. 77¡e 20-m-deep ad¡Y/rom 7962 ¡л кем :'n fAe ыррег r¡g/!í-/¡and íMe o/ íZ¡e рйоЬ%гярй cZoíe fo fZ¡e horizon &eZow fZ¡e arrow. of the ore body (Fig. 8). The 180 tons of ore from this "mine constituted a bulk sample for the extraction tests in Denmark (see chapter 7). The bulk sample consisted of pegmatitic lujavrite and altered lava with an average grade of 630 ppmU. Since 1962 this sample has become less representative of what is believed to be the average ore, and particularly the extractability of the uranium from the pegmatitic lujavrite is different from the'extractability of the uranium from the fine-grained lujavrites. The drilling programme in 1969 amounted to 1621 m and comprised exploration drill holes in shear zones of possible uranium enrichment as well as a'450-m-deep drill-hole in the "mine area". Two holes were drilled in the "northern area", providing the first indications of deep subsurface extension of the "highly active" lujavrite situated in this area. A geological description of the deposit was made and thé 374 NIELSEN resource figures were re-estimated taking the results of the 1969 drilling into consideration (H. Sarensen et al.,1974)- The result of the block estimate was now 5 800 tons U with, an average grade of ЗЮ ppm and a cut-off value of 300 ppm. No tonnage-grade curve was produced, and the 300 ppm cut-off level was definitely chosen too high thus giving no information on the considerable amount of lujavrite with uranium concen trations between 250 and 300 ppm. Not until 1977 was a systematic drilling programme carried out in the "northern area" (Fig. 2). A total of 4 256 m was drilled in 200-m-deep holes with a grid spacing of 145 m. Additionally 4 exploration holes amounting to 855 m in total were drilled along the eastern continuation of the "northern area". The zones at the contact of the intrusion had a radioactivity above average at the surface; a feature which was not maintained at depth. With easier access to computer facilities the resource figures for the entire Kvanefjeld deposit were re-estimated for the third time and a tonnage- grade curve and corresponding average grade values were produced for the northern area. The figures are given in the Introduction, chapter 1. A strict budget prevented a desirable flexibility of the 1977 drilling programme and several holes were stopped in "ore", and the mineralised body is still open to the south-west. The "northern area" and the "mine area" were tied together with this drilling programme (Nyegaard et al., 1977). 6. LOGGING, ASSAYING, RESOURCE ESTIMATES Every drilling programme in the Kvanefjeld area has been accompanied by radiometric logging of the holes. The specifications of the logging equipment are outlined in Table II. The logging has been exclusively radiometric (scintillometric and spectrometric) with analogue recording in 1958, 1962 and 1969. The logging in 1977 was spectrometric with digital recording of the results (Lavborg et al., 1980). The main reason for the introduction of the more complicated spectrometric logging was that the radioactive rocks have ÎAEA-AG-250/H 375 varying Th/U ratios, typically from 5 to 1. They appear in^ complete radioactive equilibrium. In 1958 and 1977, when the large-scale systematic drilling programmes were carried out, the results of the logging were used for the resource calculations. Calibration of the instrumentation was in both cases performed by means of radiometric and chemical analyses on material from the drill cores. The T h /и ratio increases with increasing radioactivity of the ore and, as the calibration curve for the 1958 logging was constructed with too much weight on the "hot" samples, the uranium resources of that time were over estimated by an unknown percentage. The spectrometric logging results of 1977 were converted into element concentrations on the basis of calibration with an assay of U and Th every second metre of the drill core. Resource estimates were made both from the logging results and from the core assays. They show differences at various cut-off grades (Levborg et al., 1980). With a 250 ppm U cut-off level the estimate made from the logging data is 11 per cent greater than the estimate from the core analyses. This difference may be explained, but it can be concluded that as the Kvanefjeld project progresses to a higher development stage much more reliable control is needed on assaying and on resource estimates. The necessity for this is particularly important at Kvanefjeld, where one is concerned with low-grade deposits of marginal feasibility. At present a geostatistical resource estimate is being carried out on the basis of existing analy tical data, and it is foreseen that geostatistics will play an increasing role in the future evaluation of the deposit. The revised resource estimates following the drilling programme in 1969 were based on gamma-spectrometric scanning of all new and earlier drill-core material. The time-consuming scanning job was carried out in the laboratory and provided the basis for a reliable estimate (L0vborg et al., 1971). One of the reasons for choosing the scanning procedure was that relogging of the holes from 1958 was no longer possible. Most of the analytical work for U and Th and a number of other elements on rock samples from Ilimaussaq has been 376 NtELSEN TABLE IV. ANALYTICAL METHODS IN LABORATORY Method Elements Laboratory Gamma-ray U, Th, К Risa National Laboratory spectrometry Delayed neutron U - Rise National Laboratory counting Energy dispersive Th, Zr, Ca, Ti, V, Nb, Riso National Laboratory x-ray fluorescence Cr, No, Mn, Fe, Ni, Y, Cu, Rb, Zn, Ga, Sr, РЪ Spectrophotometry U, Th Rise National Laboratory Fluoride specific F Rise National Laboratory ion-electrode Optical spectro- Li , Be University of Copenhagen metry Instrumental N a , К, Sc, Cr, Man, Rise National-Laboratory neutron Fe, Co, Zn, Rb, Zr, activation Sn, Sb, Cs, Ba, La, Ce, Nd, Sm, Eu, Tb, Yb, Lu, Hf, Ta, Th X-ray fluorescence 'SiC^, TiO^ AlgO^, Geological Survey of Greenland FeO^O^, FeO, MnO, MgO, CaO, Ma^O, K^O, P2°5 made by Rise National Laboratory. Table IV shows the various methods applied and the elements analysed. The more recent resource estimates on laboratory analyses are all based on gamma-spèctrometry on 250 g crushed samples. This analytical facility is calibrated with international standards and an intercalibration has been carried out with the delayed neutron counting analyses and spectrophotometric analytical facilities situated at Rise. In spite of the availability of high-quality analytical facilities, it is realised that IAEA-ÁG-250/11 377 extensive analytical control work by other laboratories is now needed if a detailed economic feasibility of the project is to be evaluated. Most electronic data processing is carried out at the computer facility at Risa. 7. ORE PROCESSING The uranium minerals in the ore from the Kvanefjeld deposit are refractory, and the effort spent on the extraction of uranium from lujavrite during the 25 years is comparable in cost, manpower and time to all the remaining exploration work. The Chemistry Department at Risa has been in charge of this work during the entire period. It.is not the purpose here to go into details of the problems faced over the years and only a review of the research work will be presented. After the start in 1957/58 it soon became clear that neither of the classical techniques, acid leaching or carbonate leaching was successfully applicable in the case of lujavrite, and for a long period of time (almost 15 years) a sulphatising roasting of the ore was the most effective extraction technique. The ore was treated with SO^ at about 700° C and subsequent leaching with water yielded from 40 to 70% of the uranium as dissolved sulphate depending on the type of lujavrite and the alteration of the steenstrupine (Asmund, 1971, Hansen, 1977). Owing to the low grade of the ore,much work has been devoted to possible pre-concentration of the steenstrupine. However, none of the applied methods have proved effective. Before, the research on the. sulphatising roasting technique was definitively stopped, a minorpilot-plant experiment was made on the remaining 150 tons of ore recovered from the adit of 1962 (E. Szrensen et al., 1978). It was already realised at the time of this pilot-plant study that the available ore was almost twice as rich in uranium andlitho- logically very different from the rock type representing the major part of the presently known resource. The many years of generally meagre results on the ore-processing side 378 NIELSEN is without doubt one of the major reasons for the rather slow progress of the Kvanefjeld project during those years. After 1978 a new carbonate pressure leaching process was developed and the uranium yields of the autoclave experiments are at present in the order of 70-90%. The principle is that a mixture of ground ore and carbonate solution is treated for 15 minutes in an autoclave at 260°C and 75 atmospheres. By this process the steenstrupine is broken down and the uranium dissolved. After filtering, the uranium is precipitated from the carbonate solution by reduction with hydrogen. Mineralogical studies (Makovicky et al., 1980), including fission track methods and microprobe investigations, show that the pre-treatment state of hydration or deuteric alteration of the steenstrupine is the main controlling factor of the extraction with the highest yields from the least hydrated lujavrites. A new adit, about 1000 m long is at present being driven into the centre of the ore body (Fig. 8). The adit will provide material for the selection of a 5000-tons sample for a new pilot plant on the carbonate pressure leaching process, scheduled for 1981-1983. Although the ultimate average ore is not yet known, it is believed that the ore from the tunnel can be selected so as to cover most parameters that will influence the extraction process. 8. CONSULTANTS An appreciable number of external consultants have been attached to the project during its 25 years history. In the late fifties and early sixties most consultation was applied to various problems on preconcentration and leaching. According to the Chemistry Department of Rise, most of the reports from the consultants did not contribute much to the solution of the processing problem. In view of the specific nature of the ore and the metallurgical problems, the limited funds ascribed to consultation did not enable the consultants to come much further than understanding the problems. Only rarely were useful contributions to a solution obtained, and a general !AEA AG-250/П 379 TABLE V. CONSULTANTS AND CONTRACTORS OF THE KVANEFJELD PROJECT Major consultants Atlas Copco AB, Stockholm P.G. Kihlstedt, Kungliga Tekniska Högskola, Stockholm Prof. R.N. Pryor, Royal School of Mines, London David S. Robertson & Associates Ltd., Toronto F.L. Smith and Co. A/s, Copenhagen Major contractors Danish Arctic Contractors A/s, Copenhagen Geoteknisk Institut, Copenhagen Lurgi Chemie GmbH, Frankfurt am Main Norsk Anleggstransport AS, Oslo Ontario Research Foundation, Ontario Pihl & San A/S, Copenhagen Terratest AB (Svenska Diamantbergborrnings AB), Stockholm scepticism towards consultants was created. This may not be fair - but the result has been that the chemists working on the extraction process have worked as a partly isolated group mainly because no groups in the uranium industry deal with similar problems on low-grade uranium ores. At present, however, collaboration has been established to international metallurgical companies; and consulting engineers played an important role in the fundamental change from the sulphatising roasting to the carbonate pressure leaching process. A list of the consultants employed is presented in alphabetic order in Table V . Two major studies of the project including feasibility studies have been carried out, the first one directed by the late Prof. R.N. Pryor, Royal School of Mines, London in 1974 and the second one by D.S. Robertson and Ass. Ltd., Toronto, in 1977 with a revision in 1979- Both reports have underlined the high sensitivity of the rate of recovery to the economics of the project. In the latest report, based on a number of 380 NIELSEN assumptions, an estimate of an operating cost of US$ 17 per ton of ore is given. With an estimated price of US$ 44 per pound U^.0 , a revenue of $ 30 per ton of ore milled would be 3 о obtained. This estimate is considered sufficient to justify continuation of the study of the Kvanefjeld deposit as well as preparation of a feasibility study for commercial production of uranium. The immediate investigations to be carried out are: a) detailed ore reserve study and a mining plan, b) pilot-plant study on a bulk sample in connection with the development and testing of a complete flow sheet for the production of yellow cake and by-products, c) studies of the infrastructure and natural environment of the Kvanefjeld area, and d) elucidation of the legal and political implications in order to determine their effect on the economics of the project. With the limited mining traditions in Denmark and Greenland it is felt that external consultants must be increasingly integrated in subsequent stages of the Kvanefjeld project. It is thus seen that technically and economically a continuation of the project can be argued. The intensity and organisation of the future programme and an ultimate decision whether or not to go into production depends on development of the policy of the Home Rule Authorities concerning mineral extraction and in particular this project. It will also depend on Danish energy policy towards the introduction of nuclear power in Denmark. 9. ORGANIZATIONAL STRUCTURE In the introduction it was mentioned that the Kvanefjeld project is not conducted by a single private or State-owned company with a direct economic interest towards exploitation of the deposit. Apart from the short cooperation with the partly State-owned mining company Kryolitselskabet 0resund A/S in 1958,all activities have been planned and conducted by Danish State institutions of which the most important and involved over a long period of time are: IAEA AG 250/H 38! 1) The Riso National Laboratory, a research establishment of the Ministry of Energy. Risz was the original research establishment of the former Atomic Energy Commission (AEK). The expertise necessary for the Kvanefjeld project (processing of minerals, analytical facilities, field radiometric methods) have been developed internally in the course of the project. 2) The Institute for Petrology of the University of Copenhagen. The staff of the institute has a wide interest in the mineralogy and magmatic petrology of the alkaline Ilimaussaq intrusion. As a natural consequence of this, research on the geology of Kvanefjeld sensu lato has been studied by the institute on behalf of and in collaboration with the Geological Survey of Greenland. 3) The Geological Survey of Greenland is a directorate associated with the Ministry for Greenland and now also the Mineral Resource Administration for Greenland. The Geological Survey of Greenland (GGU) officially conducts all geological research on the Kvanefjeld deposit. It is advisor to the administrative bodies above and almost all uraniurn exploration carried out in Greenland is performed by GGU. A small number of other State organisations have been involved in various parts of the project. Most engineering field activities have been performed by contracting foreign and Danish companies. It will be seen that no structured organisation exists. The "Kvanefjeld Project" is at present not more than a collective name for a number of individual initiatives and research studies run by individual institutions. The lack of a firm organiäation is not necessarily a disadvantage especially during the early stages of an exploration programme. It is nevertheless felt now that the logistic complications of an intensified future programme will need an organisational structure "like that of a mining company" with a higher degree of economic autonomy. This is necessary in order to bring the present and future studies into balance. 382 NIELSEN A substantial part of the ongoing research on the deposit is financed by energy research funds from the Danish Ministry of Commerce and by the European Economic Communities. A steering group with a broad representation has been established in connection with this research and the improvement of the overall future organisation of the Kvanefjeld project is being under taken by the steering group. Ю. ENVIRONMENTAL RESEARCH Increasing concern over the past decade has been given to protection of the natural environment. This is also true for the Narssaq area around the Kvanefjeld deposit. Since it is a thinly populated area with negligible industrial activity, the area is at present regarded as without chemical pollution. Nevertheless it was known from an early stage that the Ilimaussaq intrusion, constituting a remarkable geochemical anomaly, might reflect some of its characteristic element concentration on the surrounding environment and ecological system. In order to describe and quantify this secondary geochemical dispersion a three-year research project (1973- 76), financed by the Danish National Research Council, was carried out by graduate students of geology and biology at the University of Copenhagen. The results of this "Narssaq Project" -will provide a fundamental basis for the regulation and control of the inevitable impact of the environment in connection with future mining operations. The staff of the "Narssaq Project" (described by Rose-Hansen and S 0rensen, 1979), has collected more than 3000 samples of soils and water, plants and animals from the marine, terrestrial and limnic environment which have been analysed by neutron activation analysis for about 40 elements. The results are not yet fully reported and the difficulty in having a research programme disseminated too widely as an educational study is once again emphasized. During the International Hydrological Decade 1963-73 the Narssaq area was studied as one of the Danish representative areas, and extensive information on the climatology and hydrology was collected. Additional environmental studies are associated with the IAEA AG-2S0/H 383 research on processing of the ore and mining of the large bulk sample. These studies, including collecting of meteorolo gical data, radioecology, chemistry of stream water, sea water, and a number of animals, are concentrated in the immediate vicinity of Kvanefjeld. They can at best only be regarded as an orientation study before a more thorough environmental programme can be initiated. The lack of organisational structure has presumably been a delaying factor in the establishment of a large-scale study, not only of the natural environment, but also of the influence of a mining project on a small, local community with an economy based on fishing and sheep farming. It is fully realised, and accepted, that the environmental concern will be a very important factor in the future development of the project. It. CONCLUSIONS The discovery of the Kvanefjeld uranium deposit in 1956 was made only one year after regional exploration was initiated within the Ilimaussaq complex. There is accordingly not a sophisticated sequence of steps to be described ending with the discovery of the deposit. In this respect the discovery was a very easy one. On the basis of the existing knowledge of the presence of radioactive minerals within the intrusion, a straightforward radiometric field and aerial survey was carried out, and the deposit was found, exposed at the surface, during the second field season. With this result in mind, one could say that the exploration approach was correct. If one should start exploration within a physically and geologically similar environment today, some kind of geo chemical exploration would probably also be considered. In the case of Kvanefjeld, however, geochemical exploration research seems to be of minor importance, partly because of the ex ceptional exposure of the rocks, and partly because of the refractory nature of the uranium-bearing minerals. If the time from the start of the exploration to the initial discovery was short, the elapsed time during 384 NtELSEN succeeding stages of development has been extraordinarily long, and the background and explanation for this is the real point of interest in the present exploration case history. The main reason is the low grade of the ore and the marginal economic feasibility combined with an-untraditional project management, no security in a long-term budget, and no traditional background in mining and milling. Part of the investigation in the nineteen-sixties was therefore not exploration in its classical sense but scientific geological research. It is believed that no private mining company would have maintained exploration for 25 years on a resource which to most mining people looks marginal or subeconomic. It is the author's opinion that if, theoretically, the Kvanefjeld project had been administered from the start on strict commercial principles, the area would have been abandoned and the project shelved after the first drilling programme in 1958. Renewed interest would probably have arisen in the middle seventies, and the project brought to the level it has reached today by intensive work. Although fictive, such a development could have been an important short cut which, however, would only have been possible under a different organisational structure. Overall, the total cost up to the present stage of development would then also have been less. An advantage of the actual small-scale work over many years is that there has been sufficient time for organisations involved to report and publish their investigations as illustrated in the long list of references. At present a regional uranium exploration programme is being carried out in the entire South Greenland (Armour-Brown et al., 1980), and a detailed gamma-spectrometric survey of the Ilimaussaq intrusion and the surrounding country rocks will be initiated in 1980. Thus, as a consequence of the development of the exploration at Kvanefjeld, the wish for a systematic regional exploration programme was triggered although the main aim would not be to find a "new Kvanefjeld". Contrary to the usual development of an exploration programme, the regional IAEA-AG-250/H 385 exploration in South Greenland was carried out after the more detailed survey in a small area. Such reverse exploration approach is not desirable, but it is a result of the rule that the presentation of some local exploration result is the justification for funding the regional surveys. ACKNOWLEDGEMENTS The present paper is published with the permission of the Director of the Geological Survey of Greenland. I acknowledge the help received from E. Sorensen, T. Lundgaard, J. Bondam and H. Sorensen regarding the early history of the Kvanefjeld project. A. Steenfelt, H. Sorensen, K. Secher, J. Jensen, S. Watt and G. Vigh kindly read the manuscript and proposed improvements to the text. BIBLIOGRAPHY Armour-Brown, A., Tukiainen, T. & Wallin, B. 1980: The South Greenland uranium exploration programme. Rapp. Grönlands geol. Unders. in press. Asmund, G. 1971: Chemistry and kinetics of the sulphating roasting of uranium-bearing silicates. Risa Report 253, 125 PP- Blaxland, A.B., van Breemen, 0. & Steenfelt, A. 1976: Age and origin of the agpaitic magmatism at Ilimaussaq, south Greenland: Rb-Sr study. Lithos 9 31-38. Boggild, O.B. 1905: Mineralogía Groenlandica. Meddr Grönland 32 625 pp. Bondam, J. 1957: Rapport over undersogelser ved uranforekomsten pa Kvanefjeldet (Narssaq) i sommeren 1957- Internal GGU report. Bondam, J. 1960: Uranundersogelser pa Kvanefjeld ved Narssaq. Med. dansk geol. Foren. 1_i 271-272. Bondam, J. & Sorensen, H. 1959:"Uraniferous nepheline syenites and related rocks in the Ilimaussaq area, Julianehaab district, Southwest Greenland'^ Peaceful Uses of Atomic Energy.(Proc. 2nd Int. Conf. Geneva, Sept. 1958) Vol. 2, UN, Geneva, 555. 386 NIELSEN Batter-Jensen, L., Christensen, P. & Nielsen, B.L. 1978: External radiation exposure associated with uranium- thorium mineralization on the Kvanefjeld Plateau, Greenland. Risa-M-1989, 15 PP, Risa National Laboratory. Batter-Jensen, L., Lavborg, L. & Nielsen, B.L. 1980: Gamma exposure levels from natural radioactivity in Greenland. Seminar on the Radiological burden on Man from Natural Radioactivity in the Countries of the European Community, Le Vesinet, France,4th-6th Dec. 1979- Emeleus, C.H. 1964: The Grannedal-fka alkaline complex, South Greenland. Bull. Granlands geol. Unders. 45 75 pp (also Meddr Granland 172 3). Emeleus, C.H. & Upton, B.G.J. 1976: The Gardar period in southern Greenland. In Escher, A. and Watt, W.S. (eds), Geology of Greenland, 153-181. Geological Survey of Greenland, Copenhagen. Ferguson, J. 1964: Geology of the Ilimaussaq alkaline intrusion, South Greenland. Bull. Grönlands geol. Unders. ¿9 81 pp. Ferguson, J. 1970: The significance of the kakortokite in the evolution of the Ilimaussaq intrusion, South Greenland. Bull. Grönlands geol. Unders. 89 193 pp (also Meddr Grönland 1_90 1 ) . Hansen, J.K. Gamborg 1977: Sulphatising roasting of a Greenlandic uranium ore, reactivity of minerals and recovery. Ris0 Report 355, 129 pp. Riso National Laboratory. Kunzendorf, H., Nyegaard, P.L., & Nielsen, B.L. in prep: The distribution of characteristic elements in the radioactive rocks of the northern part of Kvanefjeld, South Greenland. Lorenzen, J. 1881: Expeditionen til Julianehaabs Distrikt i 1876, III Undersagelser af Mineralerne i Sodalith- Syeniten. Meddr Grönland 2^ 43-81. Lavborg, L., Kunzendorf, H. & Hansen, J. 1968: Use of field gamma-spectrometry in the exploration of uranium and thorium deposits in South Greenland, Nuclear Techniques and Mineral Resources,IAEA, Vienna, 197-211. IAEA-AG-250/11 387 Lzvborg, L., Wollenberg, H., Sorensen, P. & Hansen, J. 1971: Field determination of uranium and thorium by gamma-ray spectrometry exemplified by measurements in the Ilimaussaq alkaline intrusion, South Greenland'. Econ. Geol. ^6 368-384. Levborg, L., Wollenberg, H., Rose-Hansen, J. & Nielsen, B.L. 1972: Drill-core scanning for radioelements by gamma- ray spectrometry. Geophysics 37 675-693. Lovborg, L., Botter-Jensen, L., Christiansen, E.M. & Nielsen, B.L. 1980: Gamma-ray measurements in an area of high natural radioactivity. 3rd International Symp. on the Natural Radiation Environment, Houston, Texas 23-28 April, 1978. Lovborg, L., Nyegaard, P., Christiansen,E.M. & Nielsen, B.L. 1980: Borehole logging for uranium by gamma-ray spectrometry. Geophysics. June 1980 Issue. Makovicky, M., Nielsen, B.L., Karup-Moller, S., Makovicky, E. & Sorensen, E. 1980: Mineralogical, radiographic and uranium leaching studies on the uranium ore from Kvane fjeld, Ilimaussaq complex, South Greenland. Rise Report, in press, Riso National Laboratory. Nielsen, B.L. & Steenfelt, A. 1979: Intrusive events at Kvanefjeld in the Ilimaussaq igneous complex. Bull. geol. Soc. Denmark 2_7 143-155- Nyegaard, P., Nielsen, B.L., Lovborg, L. & Sorensen, P. 1977: Kvanefjeld Uranium Project, Drilling Programme 1977. Internal GGU report. Rose-Hansen, J. & Sorensen, H. (eds) 1979: Geological, geo chemical and ecological studies in the Ilimaussaq area, South Greenland. Rapp. Grönlands geol. Unders. 95 79-81. Semenov, E.I. 1974: Economic mineralogy of alkaline rocks. In Sorensen, H. (ed) The alkaline rocks 543-552. John Wiley and Sons, London, New York. Steenfelt, A. & Bohse, H. 1975: Variations in the content of uranium in eudialyte from the differentiated alkaline Ilimaussaq intrusion, South Greenland. Lithos 8 39-45. 388 NtELSEN Sorensen, E., Lundgaard, T., Rasmussen, I. & Dibbern, H.C. 1978: Pilot plant' testing on uranium extraction from the Kvanefjeld ore deposit. RÍS0-M-21O2, Risa National Laboratory, 63 pp. Sorensen, H. 1958: The Ilimaussaq batholith, a review and discussion. Bull. Grönlands geol. Unders. 19 48 pp (also Meddr Grönland 162 3). Sorensen, H. 1967: On the history of exploration of the Ilimaussaq alkaline intrusion, South Greenland. Bull. Grönlands geol. Unders. 6_8 33 pp (also Meddr Grönland 181 3). Sorensen, H. 1970: Occurrence of uranium in alkaline igneous rocks. Uranium Exploration Geology (Proc. Panel, Vienna 1970) IAEA, Vienna, 161-168. Sorensen, H. 1970: Internal structures and geological setting of the three agpaitic intrusions - Khibina and Lovozero of the Kola Peninsula and Ilimaussaq, South Greenland. Can. Miner. 1_0 299-334. Sorensen, H. 1977: Features of the distribution of uranium in igneous rocks - uranium deposits associated with igneous rocks. Recognition and Evaluation of Uraniferous Areas (Proc. Technical Committee Meeting, Vienna, 1975) IAEA, V i e n n a , 4.7 -52 . Sorensen, H., Hansen, J. & Bondesen, E. 1969: Preliminary account of the geology of the Kvanefjeld area of the Ilimaussaq intrusion, South Greenland. Rapp. Grönlands 'geol. Unders. 1_8 40 pp. Sorensen, H., Rose-Hansen, J., Nielsen, B.L.,. Lovborg, L., Sorensen, E. & Lundgaard, T., 1974: The uranium deposit at Kvanefjeld, Ilimaussaq intrusion, South Greenland. Rapp. Grönlands geol. Unders. 60 54 pp. Sorensen, E., Lundgaard, T., Rasmussen, I. & Dibbern, H.C.: Pilot plant testing on uranium extraction from the Kvanefjeld ore deposit. Riso-M-2102, Riso National Laboratory, 63 pp. Ussing, N.V. 1912: Geology of the country around Julianehaab, Greenland. Meddr Grönland 3_8 426 pp. IAEA-AG-250/19 URANIUM MINERALIZATION IN THE BOHEMIAN MASSIF AND ITS EXPLORATION M. MATOLÍN, O. PLUSKAL, M. RENÉ Faculty of Science, Charles University, Prague, Czechoslovak Socialist Republic Abstract URANIUM MINERALIZATION IN THE BOHEMIAN MASSIF AND ITS EXPLORATION. Long-term systematic and planned uranium survey including airborne, carbome, ground, logging and laboratory radiometric measurements as well as geological and geochemical investigations have shown a difference in radioactivity of two regional geological units in Czechoslovakia. The higher regional radioactivity of the Variscan granitoid rocks of the Bohemian Massif differs from that of the West Carpathians and is associated with more frequent uranium mineralization. Endogenous vein-type uranium mineralization has a spatial association with high-radioactivity granitoids in the Bohemian Massif. Airborne prospection defined rock radioactivity features on a regional scale while surface and subsurface radiometric and geological investigations using various techniques localized important uranium deposits. Complex statistical evaluation of numerous geophysical and geological data was studied in order to delineate uranium-favourable areas. 1. INTRODUCTION Different radioactivity values were recorded in the rocks of the Bohemian Massif (also called the Czech Massif) and in those of the West Carpathians, Czechoslovak Socialist Republic. The preponderance of uranium deposits in the Bohemian Massif is linked with a substantially higher radioactivity of granitoid rocks. Investigation of the vein-type uranium mineralization revealed the spatial relation of this mineralization with Variscan granitoids. The uranium minerali zation was found to be associated with the development of disjunctive tectonics on a regional and a local scale, which is reflected in the morphology of the ore bodies. Two basic morphological types have been distinguished in the Bohemian Massif: the vein mineralization летми and the mineralization in tectonic zones. Pre-Variscan uranium mineralization has not been known in the Bohemian Massif. Various geological and geophysical exploration methods led to discoveries of uranium mineralization in crystalline and sedimentary rocks. 389 390 MATOLÍN et al. 2. GEOLOGICAL CHARACTERISTICS OF URANIUM DEPOSITS There are endogenous and exogenous uranium deposits in the Bohemian Massif. Following a world-wide trend, a detailed investigation of uranium deposits in young sedimentary formations of the Bohemian Massif is under way. Increasing attention is also being paid to the distribution principles and to the geochemical and structural conditions o f deposits in the crystalline series o f the Bohemian Massif. The vein-type uranium mineralization occupies a significant position in the métallogénie province of the Bohemian Massif both economically and scientifically; for example, for unravelling the important problems of distribution and geological position of ore deposits. Published ten years ago, a report on the general charac teristics o f the uranium deposits of the Bohemian Massif described the geological position and mineralogical conditions of vein-type uranium deposits and gave a general outline of individual genetic types [1 ]. From the point of view o f geological position, the basic classification of vein-type uranium deposits o f the Bohemian Massif into three métallogénie zones remains valid (Fig.l): (а) МэМяяиАми zone (deposits: Dylen, Chotëbor, Jasenice, Licomerice, Predborice, Pribram, Okrouhlá Radouft, Rozná-Olsí, Slavkovice, Vitkov, Zadni Chodov); F7G.A Map o/ ?Ae CzechoyiovaA: Sociait'st RepubHc showing mefaHogeni'c zones in fAe ДоАемп'ал Afassi/. (-^RoAemianMassif; W^esfCarpafAi'ans. (^МуМалмМсытл; ^^axoni'an-rAMri'ngi'an zone; ^ 5мб?е?;'с-.Могаи'ал zone; ^ Омгйле о /occurrences о /yein-fype ыгап;ыю nn'neraHzafton in fAe ßoAemi'an Jtiassi/! IAEA AG 250/Í9 391 (b) &2XOHMM-7 %Mn'Hg;an zone (deposits: Horni Siavkov, Jâchymov, Mëdënec); (c) Зиб?е?;'с-Л?огау;'аи zo^e (deposits: Harrachov, Javornik, Medvëdin, Prehrada, Prichovice). Two basic types o f uranium mineralization have currently been distinguished on the basis of structural, lithological and geochemical study of the conditions of vein-type uranium mineralization (J. Kominek in Ref. [2]): (a) Deposits in mineralized tectonic zones; (b) Deposits o f vein type леиди The deposits o f type (a) (Dylert, Okrouhlá Radouft, Rozná-ОШ, Zadni Chodov) are associated with the systems of thick deep-seated dislocation zones and pertinent feathered dislocations and with joints of lower orders with NNW—SSE and N —S strike. These deposits are distinguished by metasomatic alteration of rocks surrounding mineralized structures, frequent presence of finely dispersed graphite and organic substance in dislocation filling, and by unsharp mineralization outlines of ore bodies. The vertical range of the mineralization is mostly large. Study of the isotopic composition of S, С and О revealed relations of some deposits to the rocks of wider surroundings, which could indicate that sulphur in the sulphides or ore minerals could derive from these rocks, and that the isotopic composition of С and О of carbonates betrays the meteoric origin of water o f mineralized solutions. The deposits o f type (b) (Jâchymov, Pribram, Slavkovice) are characterized by the vein filling of dislocations and joints whose uranium minerals are mostly associated with carbonates. Investigations carried out in the Pribram district revealed that the localization o f mineralization is primarily controlled by structural factors and that the thickness of mineralized sectors is closely related to the mechanical properties o f host rocks. Also significant for the localization of mineralization is the oxidation reduction potential of individual series which build up the ore deposit district. In ore-bearing series it is two to three times higher than in the surrounding series. The deposits lying in the Upper Proterozoic series of the Barrandian Area, and in the íelezné hory zone situated to the east o f it, were found typically to contain uranium-organic complexes replacing the original, often massive, uranite mineralization. Their distribution increases either depth- wards (Pribram) or completely replaces primary mineralization (2elezné hory), preserving only unimportant relics. Judging by the latest investigation, the organic matter of uranium-bearing antraxolites is probably of biogene origin, as evidenced by the presence o f isoprenoid hydrocarbons of phytan and pristan along with the results of isotopic С analysis. The sediments whose hydrocarbons were mobilized by the operation o f magmatic sources are regarded as a source o f hydrocarbons. 392 MATOLÍN et at. FYC.2. Гурел o/^ranifoids о/ fAe RoAewian М ая;/¡n reiafion io pein-fype uranium mineraiizafion. Craniioids wifA more /requenf occurrence о /yein-^pe иган;'мп! mineraiizaiion; ^2^ Cranfíoi'dí wifA rare occurrence о/ ^ein-fypeuranium mineraiizafion, Zoned íype о/ uranium mineraiizafion; ^4^ Hein-fype uranium minera/izafion sensu strictu; í/ranium w;'nerai;'zañon in sedimeniary roc/:s. In the majority of deposits, attention is paid to endogenic aureoles of uranium and associated elements as weil as to the development of rock alteration in the surroundings of ore bodies, and isotopic and geochemical study is under way. Some hydrothermally altered granites in the vicinity of ore bodies were found to have increased uranium content and lower potassium content in the parts where mineralization is in the endocontact of granitoids. The processes leading to loss o f quartz and to the origin of rocks similar to episyenites occurred on a limited scale. The generally accepted spatial association of vein^type mineralization with Variscan granitoids is corroborated by later discovery of uranium occurrences in the endocontacts as well as in the exocontacts of these rocks. The largest proportion of vein-type uranium occurrences and deposits in the Bohemian Massif (Fig.2) is associated with granitoid bodies of: The Bor Massif, The Karlovy Vary Massif and the granites of the Saxonian side, The Krusné hory mountains, The Central Bohemian pluton, The Trebic Massif involving the apophyses lying at a distance several km from the main body. IAEA-AG-250/19 393 The Bohemian Massif Variscan granitoids with relatively small representation of vein-type uranium mineralization are as follows: The Central Moldanubian Massif, The Krkonose-Jizerské hory pluton and other bodies, The Variscan granitoids in the area o f the West Sudeten, The Smrciny Massif, Granite bodies at the boundary of the West and East Sudeten, The Zelezné hory pluton. The investigation failed to confirm the originally postulated sterility o f the small granite bodies of the Barrandian area, yet the occurrences of uranium mineralization belong rather to the second granitoid group. Exogenous uranium deposits occur in Permo-Carboniferous cover (Kladno- Rakovnik Basin, Lower-Silesian Basin), in Upper Cretaceous and Tertiary (Cheb Basin, Sokolov Basin). Uranium in the Hamr deposit was encountered in fresh-water sediments and shallow marine sediments of Cenomanian-Senonian age lying on Permo-Carboniferous phyllites and deeply weathered granites. 3- RADIOACTIVITY OF THE BOHEMIAN MASSIF GRANITOIDS Investigation of the granitoids is based on systematic airborne, ground and laboratory measurements of gamma activity of rocks carried out during the past twenty years, which yielded data on the radioactivity o f regional geological bodies encountered throughout the entire territory of the Czechoslovak Socialist Republic [3,4]. These data were also instrumental in drawing some conclusions on uranium deposits. The radioactivity map o f the rocks o f the CSR (Fig. j ) clearly shows the difference between the Bohemian Massif area, characterized by more frequent occurrence of extensive areas made up of rocks with increased gamma activity, and the West Carpathians area, displaying regionally low concentration of radio active elements. Tests indicating different radioactivity o f the rocks of the Bohemian Massif and those of the West Carpathians disclosed that: (a) There is no significant difference between sedimentary and metamorphic rocks; (b) There is no significant difference between magmatic rocks as a whole; (c) There is a significant difference between granitoid rocks (granites, granodiorites, diorites); and (d) 33% and 6.3% of the area of magmatic rocks in the Bohemian Massif and in the West Carpathians, respectively, show above-average radioactivity. 394 MATO LIN et ai. TABLE L UPPER LIMITS OF К, U and Th CONCENTRATIONS IN ROCKS frange о/90% о/о&легум? va/мм) К U Th (%) (ppm eU) (ppm eTh) Bohemian Massif 4.8 12 41 West Carpathians 4.5 6.7 22 Ranges of К, U and Th concentrations were determined by laboratory gamma-ray spectrometry in rock samples of both the geological units (Table I). The prevalence of Variscan plutons in the Bohemian Massif makes them an important factor in the formation o f deposits of Post-Proterozoic age. In the latter the vein-type deposits occupy a dominant position. The radioactivity of the granitoids of the Bohemian Massif regarded as the potential source of uranium mineralization was measured as total gamma-ray activity and expressed by the values in Table II. 4. SPATIAL ASSOCIATION OF MINERALIZATION WITH GRANITOIDS Study of spatial association has shown that the mineralization distributed in the exocontact of granitoid bodies or larger massifs is confined to a certain part o f their periphery. Such situation can be observed at the western margin o f the Bor Massif, at the eastern margin o f the Trèbic Massif, and in the area of the Krusné hory granitoids. Uranium mineralization is generally widespread in close proximity to, or within, granitoid bodies with higher clarke o f uranium, which do not as a rule belong to the varieties richest in uranium within the scope o f larger massifs. Vein-type uranium mineralization o f the Bohemian Massif is distinguished by remobilization of mineralization observed in individual deposits. In some cases the relatively younger mineralization (o f Kimmerian age) occurs on a larger scale in deeper parts o f deposits. Systematic study of the position of vein-type uranium mineralization o f the Bohemian Massif in the general structure o f the European Variscicum corroborated a number of common features o f this mineralization throughout the whole tectogene. One o f the features requiring further attention is the striking sterility o f earlier Pre-Variscan vein-type uranium mineralization which, judging by available data, is of general rather than only local validity. IAEA-AG-250/19 395 TABLE II. RADIOACTIVITY OF GRANITOID ROCKS OF THE BOHEMIAN MASSIF Radioactivity Rock (ppm eU) Granite and granodiorite 8.3-17.7 Pegmatite 4.8 - 9.8 Aplite 6.8 - 15.0 Melanocratic granite-melanocratic syenite 9.2 - 35.6 Phonolite 18.8 - 20.0 Diorite 1.0- 8.8 Note: Radioactivity is characterized by the intervai involving 64.2% of measured values. 5. URANIUM EXPLORATION Geological and geophysical methods have been used for uranium exploration during the last thirty years. Regional radiometric measurements located only sporadic new occurrences of uranium mineralization although they outlined promising areas where subsequent detailed work sometimes produced positive results. In evaluating the results of regional radiometric measurements, attention should be paid to the high degree of geological exploration of the area, as well as to the unfavourable physical conditions for detecting anomalous radioactivity of local small bodies representing vein-type uranium mineralization. The study areas were selected on the basis of evaluation of the geological situation and common knowledge o f the distribution o f radioactive elements in the rocks of promising areas. Positive results from one or more methods applied in the Bohemian Massif led to the discovery of 25 selected localities with uranium mineralization: Methods Positive results Geological revision of abandoned mining works and rocks 6 Ground gamma-ray survey 7 Carbome gamma-ray survey * 5 Gamma-logging 1 Emanometry 9 396 MATOLIN et a!. Structures of promising localities were investigated by means of, in particular, geo-electric methods, magnetometry, gravimetry and, less often, seismic methods. Studies were undertaken o f uranium mineralization prognoses and selection of promising areas on the basis of complex evaluation of geological features and geophysical fields using statistical methods [5]. REFERENCES [1] PLUSKAL, O., "Uranium mineralization in the Bohemian Massif", Uranium Exploration Geology (Proc. Panel Vienna, 1970),IAEA, Vienna (1970) 107. [2] HALBRSTÁT, J., KOMÍNEK, J., PLUSKAL, O., World Deposits of Radioactive Raw Materials, Rep. Faculty of Science, Charles Univ., Prague (1979) 76 p. (in Czech). [3] MATOLÍN, М., Radioactivity of Rocks of the Bohemian Massif, Academia, Prague (1970) (in Czech), 99 p. [4] MATOLÍN, М., Radioactivity of RocksofWest Carpathians, Rep. Charles University, Prague (1976) (in Czech), 127 p. [5] MATOLÍN, М., STEHLÍK, E., DEDÁCEK, K., "Determination of prospect of raw materials occurrences by statistical evaluation of geological and geophysical data", Proc. Symp. Hornická Pribram ve vëdë a technice, geoph. section, Pribram (1976) (in Czech) p.17—37. ADDITIONAL LITERATURE BOITSOV, V.E., LEG1ERSKI, Ya., Sequence and time of mineralization on certain hydrothermal uranium deposits, Geologiya rudnykh mestorozhdenij, Vol. X IX , No.l, (1977), p. 39—50 (in Russian). PLUSKAL, O., RENE, M., Fundamental problems of uranium metallogeny in European Variscan belt, Internal Rep. Faculty of Science, Charles Univ., Prague (1976) (in Czech). 108 p. DISCUSSION F. SCÖTT: You described one prospect where emanometry from a depth of 1 m failed to find an anomaly, while 3-m measurements located a uranium deposit. What is the reason for this? M. MATOLÍN: This is a very important question for uranium exploration in the second stage. In the first stage we covered the territory by emanometry, sampling from a depth of 1 m. As you know, this is a primitive technique. You try to get the air sample into your detection chamber, and you never know the proportion of soil air to normal air going through the hole. The results at the first exploration stage showed that if we can make these holes deeper (which is of course more expensive and has lower daily performances) we usually get better results. The practical problem is how to get the holes to a depth of 2 to 3 m. !AEA-AG-250/19 397 When the Czechoslovak Uranium Industry groups worked with holes up to 1 m, the productivity o f three-personnel groups was about 300 to 500 holes per day. The productivity with 3-m holes is much lower and only a limited area can be covered with this technique. However, when we study the penetration o f gases in the soil it is quite clear from the mathematical formulae and from experience too that the first 50 cm of the soil do not correspond well to the field o f radio activity which is underneath, and the deeper you go the more accurate results, corresponding to the radioactivity of the bedrocks, you will get. If there is an ore body at 3 to 4 or 6 m which cannot be detected by emanometry at 1 m, it will certainly be detected from a deeper hole. C. TEDESCO: Emanometry is used much more systematically in your country than elsewhere. Could you explain why emanometry was used and what results you achieved? M .M ATOLÍN: Emanometry gave us very positive results. We used quite simple techniques and the personnel who carried out the work were unskilled but numerous. This was from 1958 to 1964, when emanometry was used to cover the greater part of the Bohemian Massif. The external contacts as well as the granites themselves were used as territory for emanometric measuring. The instruments we used were primarily ionization chambers from the USSR, reported as SG-11. The radioactivity of the soil gases was read by a moving thread in the spectroscope. These were quite simple instruments, without any electronics, where you read the ionization current directly. Then later on we used the EM- 6 emanometers from the USSR where we used as a detection chamber a scintillation chamber covered by ZnS activated by silver, a well-known scintillation material for detecting alpha radiation. The results were read on the count rate meter. Finally, we are using either the Canadian ETR-1 scintillometer or our own RP-25 emanometer, which is similar. Both systems can determine whether uranium anomalies are of uranium or thorium origin. The reason we use emanometry is that, owing to the type of cover of our rocks, the direct techniques o f gamma-ray measuring is often unsuccessful. In these areas, however, we obtained positive results with emano metry. Measuring was done in situ; the radioactivity of soil gases was measured directly. There were many errors because the personnel were not well trained at that time, but from a statistical point of view, when the technique was applied on a large scale we obtained some results. Now the technique is much improved and we have much more accurate results; it is, however, used on a smaller scale. J. CAMERON: I would like to take this opportunity to congratulate Professor Matolin on his paper. We in the IAEA have been hoping for about twelve years to have a description o f the uranium deposits of our neighbouring country, Czechoslovakia, and I am delighted to be here to hear this first presentation, not only because it is an excellent presentation in itself, but also because it is wonderful to have this new information at the IAEA. When will the compilation which Professor Matolin said might appear next year be available? Will it be in Czech and/or English? 398 MATOLÍN et at. M. MATOLÍN: Thank you for your kind words. I think the book will probably be prepared in Czech with a summary in Russian and English. If there is sufficient interest, it should not be difficult to have it translated into any other language. The main point is that it wiH be issued. P. BARRETTO: Was the gamma-iogging at the first discovery applied to uranium-exploration holes only or to oil or anything else? M. MATOLÍN: Many holes were made, for example, for water, prospection for other metallic elements, and for many other geological reasons. There is a regulation that every borehole should be gamma-logged but, unfortunately, this was not always applied at the beginning o f the first stage of exploration. P. BARRETTO: The amount of detailed information collected and processed for the uranium resources and favourability study is very impressive. Is this a model or system developed and used by the industry or is it a research conducted by a university, where there would be much more time for such detailed analysis? M. MATOLÍN: The entire uranium exploration activity rests with the Czechoslovak Uranium Industry, who co-operate with certain external institutions for research work. The Charles University is one o f these and that is where I collaborate. We have some data on uranium exploration and the results of exploration in Czechoslovakia. This type of complex statistical evaluation of uranium-favourable areas was one o f the external projects which was carried out under my guidance at the University three years.ago. It has not yet been applied on a wide scale because there are certain obstacles, and therefore only minor parts of the territory have been evaluated from this point of view; the major part has been evaluated on the basis of individual geological and geophysical results and geological assumptions. WORKING GROUP REPORT WORKING GROUP REPORT* Forty-five cases o f exploration programmes which led to the discovery of important uranium deposits in the last three decades were selected by the NEA/IAEA Joint Group o f Experts on Research and Development in Uranium Exploration Techniques as possible good examples to be presented for the implementation of the R&D Proposal No. 1 : 'Uranium Exploration Case Histories' (see Foreword). The forty-five organizations or companies which conducted these exploration programmes were invited to participate in the meeting, and nineteen favourable responses were received covering the exploration case histories o f twenty uranium deposits. Some private companies were reluctant to provide information on their exploration work, considering it to be too 'sensitive'. The twenty examples submitted to the meeting represent about 45% of the total of the selected cases, and although they constitute only a very small fraction o f the discoveries made over the past three decades, they can be considered as very satisfactory and representative samples. All the participants voluntarily agreed to present information for the benefit o f the nuclear energy industry and it is hoped that these descriptions will assist others in planning and operating uranium exploration programmes round the world. In contrast to deposits o f other economic minerals, uranium is commonly not directly detectable by techniques known at present for more than a short distance from mineralization, and, in this regard, the cases presented show a wide spectrum o f exploration methods in a wide range o f geographical, geological and cultural conditions. The various exploration methods discussed demonstrate successful applications in some cases and negative results in others. One important fact that emerged from the discussions was that there was no specific technique with universal application. In several cases, discovery resulted from the application o f more than one technique. Success or failure depended on the selection of the correct technique to fit a particular type of deposit in a particular area. Although there are many similar features o f a particular deposit-type, each deposit is unique. Frequently the geological type o f a deposit was not recognized until after discovery and an appreciable number o f discoveries resulted from a re-interpretation of previously available data. The case histories presented show the basic technical and economic choices that were made in the selection o f favourable areas and in the selection of the exploration methods applied. In most cases it was important to determine favourable regions before selecting the detailed exploration techniques which Combined report of two Sub-Working Groups. 401 402 WORKING GROUP REPORT located the actual deposits. The size and grade o f the expected deposits are important, since a particutar quantity o f mineralization may permit extraction in one region but not in another. In addition to the technical factors o f geology, geophysics and geochemistry, there are a large number o f other local features that must be considered, such as: Amount o f previous activity Expected cost o f the programme Available skills Logistics Climate Proximity to infrastructure Environmental concerns Political concerns Available techniques Expected requirements for uranium. AM these factors must be examined and evaluated before an exploration programme is decided upon. It is recommended that the NEA/IAEA consider convening a second meeting on case histories o f uranium exploration in two to five years, the timing of such a meeting depending on the response received by the NEA/IAEA to the publication o f the papers presented at this meeting. CHAÏRMEN OF SESSIONS J. DARDEL France C. PAPADIA Italy G. BUNDROCK Canada M. MATOLÍN CSSR SECRETARIAT Scientific P. STIPANICIC Division of Nuclear Power and Reactors, Secretaries IAEA, Wagramerstrasse 5, P.O. Box Ю0, A-1400 Vienna, Austria D. TAYLO R OECD Nuclear Energy Agency, 38 boulevard Suchet, F-75016 Paris, France Editor M. LEWIS Division of Publications, IAEA, Wagramerstrasse 5, P.O. Box 100, A-1400 Vienna, Austria 403 LIST OF PARTICIPANTS BRAZIL Angeiras, A.G. Nuclebrás Engenharia SA, Forman, J.M.A. Av. Presidente Wilson 231-10, Rio de Janeiro CANADA Bundrock, G. Urangesellschaft Canada Ltd, Suite 3100, 2 Bloor Street East, Toronto, Ontario M4W 1A8 Scott, F. Esso Minerais Canada, 2300 Yonge Street, P.O. Box 4029, Station A, Toronto, Ontario M5W 1K3 Tan, B. Uranerz Exploration and Mining Ltd, 3633 Sources Boulevard, Doilard des Ormeaux, Quebec CZECHOSLOVAK SOCIALIST REPUBLIC Matolín, M. Department of Applied Geophysics, Faculty of Science, Charles University, Albertov 6, 12843 Prague DENMARK Nielsen, B.L. Geological Survey of Greenland, 0sterVoldgade 10, DK-1350 Copenhagen К 406 LIST OF PARTICIPANTS FRANCE Darde], J. 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