Proterozoic Unconformity and Stratabound Uranium Deposits

Home , Barstow Formation, Rammelsbergite

IAEA-TECDOC-315

PROTEROZOIC UNCONFORMITY AND STRATABOUND URANIUM DEPOSITS

REPOR WORKINE TH F TO G GROU URANIUN PO M GEOLOGY ORGANIZED BY THE INTERNATIONAL ATOMIC ENERGY AGENCY EDITEY DB JOHN FERGUSON

A TECHNICAL DOCUMENT ISSUED BY THE INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1984 PROTEROZOIC UNCONFORMITY AND STRATABOUND URANIUM DEPOSITS IAEA, VIENNA, 1984 IAEA-TECDOC-315

Printe IAEe th AustriAn y i d b a October 1984 PLEASE BE AWARE THAT ALL OF THE MISSING PAGES IN THIS DOCUMENT WERE ORIGINALLY BLANK The IAEA does not normally maintain stocks of reports in this series. However, microfiche copies of these reports can be obtained from

IN IS Clearinghouse International Atomic Energy Agency Wagramerstrasse5 P.O. Box 100 A-1400 Vienna, Austria

Orders shoul accompaniee db prepaymeny db f Austriao t n Schillings 100, for e forcheque a th f m IAEth f mo n i o n i r Aeo microfiche service coupons which may be ordered separately from the INIS Clearinghouse. FOREWORD

e greaTh t surg interesf eo d activittan n exploratioi y uraniur nfo m deposits over the last decade has added significantly to our knowledge of uranium nature geologth uraniuf d eo an y m deposits informatioe . th Muc f o h n that has been developed by government and industry programmes has not been widel ybenefie availablth d systematif mann o ti ha yd t ean no case s csha gathering, organisatio publicationd nan . Witcurrene th h t cut-bacn i k uranium exploratio d researcnan h efforts therreaa s li e danger that much knowledge oth f e gained wil lose lb t and, witanticipatee th h d resurgence of activities, will again have to be developed, with a consequent loss of time, money and effort. In an effort to gather together the most important informatio e type th uraniuf o sn no m deposits ,seriea reportf o s beins i s g prepared, each covering a specific type of deposit. These reports are a producAgency'e th f o t s Working Grou Uraniun o p m Geology. This group, which has been active since 1970, has gathered and exchanged information on key questions of uranium geology and coordinated investigations on important geological questions. The Working Group has been carrying on several projects to prepare comprehensive report majon o s r type uraniuf so m deposits. These report plannee ar s completior fo d d releasnan 1984n ei . Finding wore th k f so will be presented at the International Geologic Congress in Moscow in August 1984.

The topics of the Working Group on Uranium Geology projects and the projece nameth f so t leaders are:

Sedimentary Basin Sandstond san e Type Deposits- Warren Finch

Uranium Deposit Proterozoin i s c Quartz-Pebble Conglomerates- Desmond Pretorius

Vein Type Uranium Deposits - Helmut Fuchs

Proterozoic Unconformity and Stratabound Uranium Deposits - John Ferguson

Surficial Deposits - Dennis Toens. e dedicatioth o e succest projecte Th th e d effort ndu an f o ss i sf o s the project leader d theian s r organisations e activth d e, an participatio n and contribution of world experts on the types of deposits involved. The Agency wishe exteno t s s l thankinvolve projecte it al d th o t sn r i d fo s their efforts e reportTh . s constitut n importanea t additioe th o t n literatur uraniun o e m geolog sucs e expectea har d an y havo t d warea m reception by the member states of the Agency and the uranium community, world-wide.

I.A.E.A., Vienna 198y ,Ma 4

EDITORIAL NOTE

The authors who have contributed to this project are all actively engaged in research wor n areai k s relatin theio t g r respective contributions. This advantag s alloweeha updater fo d d syntheses which frequently includes work that has a restricted distribution and additionally incorporates new data. It is hoped that this report wil exploratior lfo held ai p Proterozoir n serva fo n s ea c unconformity and stratabound uranium deposits.

m indebtea I Delo t d l Staffor typinr fo d g mos f thito s manuscripo t d an t DraftinR theBM g productio e Officth r fo e numberouf no s line drawing mapsd san , in particular woul,I d lik thano et k Ivon Chertok, Ingo Hartig, Natasha Kozin and Diana Pillinger.

John Ferguson CONTENTS

STRATIFOR STRATA-BOUND MAN D TYPES

Proterozoic strata-bound uranium deposit f Zambio s Zaird aan e ...... 7 . L. Meneghel Uranium in Early Proterozoic Aillik Group, Labrador ...... 35 S.S. Gandhi Olympie Th copper-uranium-golm cDa d deposit, Roxby Downs, South Australia ...... 9 6 . D.E. Roberts, G.R.T. Hudson Strata-bound uranium deposits in the Dripping Spring Quartzite, Gila County, Arizona, USA: A model for their formation ...... 95 C.J. Nutt Lianshanguae Th n uranium deposit, Northeast China: Some petrologicad an l geochemical constraints on genesis ...... 115 Zhong Jiarong, Guo Zhitian

UNCONFORMITY-RELATED TYPES

Pine Creek Geosyncline, N.T...... 5 13 . G.R. Ewers, Ferguson,J. R.S. Needham, T.H. Donnelly Uranium mineralizatio t Turena e Creek, Western Australia ...... 7 20 . G.N. Lustig, G.R. Ewers, C.R. Williams, FergusonJ. Unconformity-related uranium deposit Athabasce th n si a basin region, Saskatchewa7 n 21 ...... V. Ruzicka Uranium geology, Beaverlodge Area ...... 269 D.M. Ward Geology and discovery of Proterozoic uranium deposits, central district of Keewatin, Northwest Territories, Canada ...... 285 A.R. Miller, J.D. Blackwell, L. Curtis, W. Hilger, R.H. McMillan, E. Nutter The RioPreto uranium occurrences, Goiâs, Brazil ...... 313 P.M. Figueiredode Filho Proterozoic unconformity and strata-bound uranium deposits: Epilogue ...... 325 . Ferguson/ PROTEROZOIC STRATA-BOUND URANIUM DEPOSITS OF ZAMBI ZAIRD AAN E

. MENEGHEL L AGIP Nucleare, Milan, Italy

Abstract

e KatangTh a System, hos o uraniut t d coppean m r mineralization, is several thousands of metres thick ana rests unconformably on an older complex of crystalline rocks and meta-sediments and is locally covered by Karoo sandstones or Kalahari sands. e depositioTh e Katangth f o na System took place durin e Latth ge Proterozoic in a wide complex basin extending from Shaba provinc n Zairi e e throug a largh e par f Zambio t d intan a o eastern Angola. The sediments were affected by different grades of metamorphism, tectonic events, and by thermal events associated with post-tectonic metamorphism. e bas th f Katango e t A a syste m4 know8 ther e nar e copper deposit 2 uraniu4 d an s m occurrences s suggestei t I . d thal al t the known uranium and copper occurrences are of an essentially syngenetic sedimentary origin. The mineralisation is found in the Lower Roan Formatio ne Katang neath e bas th f ro e a System occurrin n rocki g s produce n similai d r environmental conditions and thus being stratigraphie controlled, however, their areal distributio s localisei n d producin a regionag l metal zonation. e uraniuth Man f o ym occurrences hav a typicae l vein aspect. These transgressive relationships are not inconsistent with a syngenetic origi s evidencea n e veith n y b morphologyd . Exploration activitie d genetian s c studie e ongoinar s g throughout this metallogenic province.

INTRODUCTION The Copperbelt areas of Zaire (Shaba Province) and Zambia which host the uranium deposits are located on the southern flank e Congoth f oe majo Cratoth rf o structurane whicon s i h l blocks e Africath f o n continent sout f Saharao h . 00 TABLE 1. Correlation of the Katanga System of the Domes, Zambian, and Shaban Copperbelt Areas

Domes area Zambian Copperbelt area Shaban Copperbelt area

McGregor, 1964; Edwards, 1974; Arthur, 1974; Mendelsohn, 1961; Bind d Mulgrewan a , 1974 Francois, 1974; Cahen, 1974; Cailteux, 1977 Klinck, in press; and Appleton, in press

Stratigraphie units Rock type Stratigraphie units Rock type Stratigraphie type Rock type

Shale Sandston d shalan e e Tillite Tillite West Lunga Kundelungu Shale Kundelungu Sandstone and shale formation Limestone and dolomite group Dolomite and shale supergroup Dolomite (Tillite) Tillite Tillite Grand Conglomerate and Mwashya Carbonaceous shale Carbonaceous shale supergroup Black shale Mwashya group Luigishi Schists Argillite (Roan unit 4.2) formation Dolomite and magnesite Dolomite Dolomite Wamikumbi formation Dolomitic schists (Roan unit 1) (Roan unit 4.1)

Talc-specularitic quartzite Roan group Argillite, shale dolotnitic schists Roan supergroup Dolomitic sandstone (Roan units 2,3, and 4) Dolomite Quartzite (marker) Quartzite (Roan uni) 5 t Shale (Roan units 3(?) and 2.3) Wushingwi Mica schists Argillite, micaceous quartzite, Siliceous dolomite formation Biotite schists impure dolomite (Roan uni) 6 t (Roan units 2.1 and 2.2)

Local conglomerate (Quartzite, conglomerate) Dolomitic sandstone (Roan unit 7) (Roan uni) 1 t

Basement complex: granites, schists, gneisses, etc. Uranium mineralisatio s firswa n t discovere e Shabth n ai d Province of Zaire in 1915 and sometime later in the Zambian Copperbelt e discoverieth , e North-Westerth n i s n Zambian Domes Area are fairly recent. For sometime these three districts (Fig. 1) have been regarde s indépendana d e anotheon f o t r forming three separate depositional basins s suggestea , e facth ty b dtha t there ar e marked differences betwee e threth n e areas A mor. e careful examination of the geological environments demonstrates however tha n eac i tf thes o h e area e lithologieth s s merge into each other indicating that they form a single depositional domain. The three areas are considered to be a part of a single geological e unitmineraliseTh . d area e thoughar s o havt t a e commo e same linkeb nth e o origi o t t importand d an n t metallogenetic epoch; later tectonic overprinting has masked their petrogenetic links. This modelling served as the basis for the recent discover f severao y l uranium showing d depositan e s th n i s Domes Are f Zambio a y Agib a p S.p.A. Whil a considerable e amoun f dato td informatio an a s i n currently available on the Copperbelt area of Zambia and Zaire, the studiee basin'e Dometh th n sn o o ss Are d generaan a l palaeo- geograph d evolutioan y e stilar n l scant d fragmentaryan y . More specific informatio e Dometh sn o nAre a geolog s includei y n i d e otheth thirs a areasr e concernepaperfa ar ss a ;e interth d - ested reade s referre i re availabl th o t d e literatur d partican e - ularly to Benham et al. (1976); Binda and Mulgrew (1974); Cahen (1974); Derrick d Oosterbosan s h (1958); Derrick d Vaean s s (1956); Demesmaeke . (1962)al t e r; Francois (1974); Garlick (1972); Oosterbosh (1962); Thorea u Tried e Terdonct d e u k (1933)

REGIONAL GEOLOGICAL SETTING e KatangTh a Systeme coppere bas th f th whico e l t a ,al ,h cobalt, uranium and nickel mineralisations are found (Table 2), unconformably rests on an older complex (gneisses, mica schists, quartzites, phyllite d granite an s localli d an )y covere y Karob d o sandstones or Kalahari sands. e reconstructioTh e histore Katangth th f f o no y a System basin is quite difficul t onlno t y becauss largit s ef it o e sizd an e 051 SAND

I '-'.^ KATANGA SYSTEM

BASAMENT COMPLE

Fi. Distributio1 g e Katangth f o na System (after Francois, 1962)

original complexit t alsbu yo becaus s domainit e s underwent different tectono-metamorphic evolutions. The lack of adequate regional mapping make r furthefo s r difficulties t followI . s that virtually unknown areas separate other areas for which a rich harves f dato ts availablei a . e depositioTh e mainlth f yo n marine Katanga System took e perioplacth 0 m.y n d 1,3084 i de .an 0 m.y. (Cahen, 1970a n )i wide complex basin extending from the Shaba province in Zaire throug a largh e par f Zambio t d easteran a n Angola e southTh . - western deposit e coverear s y morb d e recent formations (Fig, ) .1 The full sequence is several thousands of metres thick: at the base are found the copper deposits of the Copperbelt areas of Zambia and Shaba.

10 ê •5

11 In the Zambian Copperbelt area the sequence is subdivided, from bottom to top, into the Roan, Mwashya and Kundelungu Groups e RoaTh n Grou. s subdivide2) (Tablei p d an 1 s d int prea o - dominantly clastic, Lower Roan sub-Group, up to 1,000 m thick, an a predominantld y dolomitic Upper Roam n 0 sub-Group80 o t 0 50 , thick (Binda and Mulgrew, 1974). In Shaba the sequence is subdivided inte Roanth o , Mwashy d Kundelungan a u Supergroups, the Roan Supergroup being subdivided into four groups (Cahen, e Dometh 1974) n sI Aree .sequenc th a s subdividei e d from bottom p intto oo t Wushingui, Wamikumbi, Luigishi d Wesan , t Lunga Formations. There a generaappear e b o lt s increas n metamorphii e c grade in the Katanga System from east to west and from north to south. In Shaba ther s littli e e evidenc f metamorphismo e e th n i d an , Zambian Copperbel e metamorphith t c grade normally doet exceeno s d greenschist faciès (Mendelsohn, 1961). In the Solwezi area mineral assemblages indicate metamorphis e greenschisth f o m t faciès, with local developmen f amphibolito t e faciès (Arthurs, 1974). Lower Roan schists contain quartz, muscovite, and traces f phlogopito d iroan e n oxides, wit r withouo h t chlorite, biotite, or epidote. Kyanite occurs as individual crystals and needles, n coarsoi r e aggregate e unconformitth t a s y betwee e basementh n t and rocks of the Lower Roan Group. In the Kabompo Dome area, e metamorphith c grade reached amphibolite e basemenfacièth n i s t and in the Wushingwi and the Wamikumbi Formations (Appleton, in prep.; Edwards, 1974). Assemblages characteristie th f o c Wushingwi Formation (Lower Roan e kyanitar ) + quartze , quart+ z tal + phlogopitc + chloritee d kyanitan , + quarte + ztal + c phlogopite + chlorite. Chlorite is indicative of post-tectonic retrogressive metamorphism.

Shaban Copperbelt area e arcuatTh e Shaban Copperbelt cover n area sf approximatelo a y . km Uranium 0 30 , copper, cobal d nickean t e founar l d together, mainly alon e southerth g n e arcmargith ,f o particularln t a y Kalongwe, henda, Kasompi, Swamb d Shinkolobwan o e (Fig. 2) . Copper-cobalt mineralisation, sometimes accompanied by uranium, it Husonoisa e centra founth c n ar i de l, th parKamoto f o t , Mashumba, Kalumbwe, Mwinga, Mashitu, Kambove, Luishy d manan ay

12 Table 2. Distribution of Copper Deposits and Uranium Occurrences in the Katanga System

Shaban Copperbelt Zambian Copperbelt Domes area Total

Stratigraphie Number of Cu Number of U Stratigraphie Numbeu C Numbe U f o Stratigraphi rf o r e Numbeu C Numbe U f o Numberu f C o rNumbe U f o rf o r units deposits occurrences units deposits occurrences units deposits occurrences deposits occurrences

K.s.3 K.s.2.2 K.s.2.1 K.s.l2 . 3 K.s.l. 3.1 K.s.l. 2. 2 K.s.l1 .2. K.s.1.1 K.u. tillite K.i.1.4 K.I. shale K.I. shale K.i.1.3 2 K.i.1.2.2 K.I K.I K.i. 1.2.1 limestone limestone K.i.1.1 K.I. tillite K.I. tillite R.4.2 Mwashya Mwashya R.4.1 2 Upper Roan Upper Roan (R.u. 1) 1? 2 1? R.37.R.2.3 R.u.2 R.I. 4, R.I. 3 2 4 R.2.1.R.2. 4 8 2 5 61 4 5 Lowe3 22 r Roa7 1 n R.I R.I, .6 .5 R.I? 45 R.I. 7 ? Basement Basement 1? 1? Total 66 22 17 5 3 15 86 42

Roan subdivision from Binda et al. (1974) ^Correlation between Roan of Shaban and Zambian Copperbelt established by Cailteux (1977) One deposit of hydrothermal origin and one of supergene enrichment (Francois, 1974) The four deposits are of supergene enrichment (Francois, 1974) Abbreviations = Kundelungu K : « Roan R = ;superior s ; = inferior i ; « upper u ;= lowe 1 ; r other localities e northerth c onln ar I ye n. traceth par f o t s of copper and cobalt, not accompanied by uranium, have been found.

The mineralised dolomitic rocks of the Lower Roan Group, in Shab e foun ar as separat a d e masses enclose n brecciai d n I . central Shaba e rock e th onl, ar sy slightly deformed t farthebu , r south they are complexly folded and faulted. Because the contact between the Katanga System and the basement is not exposed, it is difficul o reconstruct t e paleoenvironmentth t . Francois (1974) suggested that transgressio d terrigenouan n s sedimentation initially took place on a continental platform in an oxidizing environment with the formation of a Roan 1 unit. The Roan 2, 3 and 4 units were then deposited under euxinic conditions, accompanie y locab d l volcanic activity, especiall e Roath 4 n t a y level, and with the concomitant development of copper and uranium mineralisation at the Roan 2 level. Finally, the basin was filled with Kundelungu sediment a variet n i s f environmentao y l conditions. Mineralisation occurs in the basal 30 to 40 m of the Roan 2 unit of the Lower Roan Group (Table 2). The host rock is generall a siliceouy s dolomite e maith , n minerals being dolomite, quartz, magnesite, sericite, and chlorite, with apatite, tourmaline, and monazite as accessory phases. Original clayey sediments were weakly metamorphosed, wite formatioth h f authigenio n c sericite ana chlorite (Francois, 1974). The usual ore mineral assemblages are chalcocite-bornite-carrollite or chalcopyrite- carrollite. The common uranium minerals are thorium-free uraninit d secondaran e y uranium minerals e highesTh . t uranium concentration e foun ar sn fractur i d e zone r neae o wates th r r table. All the disseminated black uranium minerals are localized at the base of the zone of copper-cobalt mineralisation (Cahen et al, 1971). Recent isotopic age determination indicate that Roan sedimentation began more than 1,000 m.y. ago (Cahen, 1974). Three subsequent orogenic phases were dated at 840 + 40 to 0 m.y2 + . 0 (Cahen 0 m.y.62 2 d 0 + m.y.an , 71 0 1970)67 ,e Ag . determinations (Cahe t al.e n , e uraniu1971th n o )m minerals from different localities give the following results: ^706 m.y., 670 + 20 m.y., 620 + 10 m.y., 582 + 15 m.y., 555 + 10 m.y. and 520 + 20 m.y. (Fig. 3). Modification of the protore has been

14 brought about, with local conversion into rich ore y tectonic,b , volcani d thermaan c l events d alteratioan , n phenomena. The uranium-copper-cobalt-nickel association is found in a different isopic zone from that of the main copper-cobalt association (Oosterbosch, 1962) mineralisatioe ,th a typica s ha n l vein aspect with epigenetic characteristics. The origin of the mineralisation, particularly its structural control, is a controversial one A metallogeneti. c epoch different froe th m copper-cobalt association has been postulated (Oosterbosch, 1962). A magmatic origi s beeha n n proposee depositth d t an Shinkoda s - lobwe have been classifie s hydrothermaa d l (Thorea t al.e u , 1933; Derrick Vaesd an s , 1956; Derrick Oosterbosch& s , 1958; Dahlkamp, 1978. Ngongo-Kashisha (1975o )tw suggeste th r fo s associations a unique metallogenetic process and Francois (1974) suggest a ssimila r origin. . (1971Caheal t e n) consider that the uranium deposits are due to the remobilisation of pretectonic mineralisation.

Zambian Copperbelt area e ZambiaIth n n Copperbelt, which cover m lonk n arega 2 s 11 a m widek 6 5 , uraniud an m mineralisatio s founi n t Nkanaa d , Mushoshi, Luanshya, and Chibuluma in the basal part of the Katanga system. Mineralisation occurs as disseminations or fracture fillings stratigraphically below the copper ores, as at Chibuluma n rocki f r shalloo so , w water origin adjacene th o t t cupriferous zonesNkant a s ,a (Mendelsohn, 1961) e hosTh t. rock e generallar s y argillite, fine-grained carbonaceous quartzite, micaceous quartzite, and shale; they represent the earliest sediments deposited under reducing conditions, as in the Shaban e areacommoTh .n accessory mineral e rutilear s , apatite, zircon, monazite and pyrite. The ore-bearing formation was affected by o uthret p e phase f deformatioo s n durin e Lufiliath g n Orogeny and by the same thermal and volcanic events and alteration processes as the sediments of the Shaban area. The uranium minerals are uraninite and its alteration products. Age determinations show that there are several generations of uranium mineralisation: 520 ± m.y., 468 +; 15 m.y., 0 m.y3 .+ 5 (Cahe 0 m.y.23 4 36 t al.d + e 5n , an , 1971)e firsTh .t generation included copper, cobalt, irod uraniuman n t subbu , -

15 sequentl e proportionth y e otheth rf o smetal s decreased until only uranium remained. No uranium mineralisation older than 520 m.y. has been found. The dissemination and the vein-type uranium mineralisation have clear epigenetic characteristics (Cahe t al.e n , 1961)e uraniuTh . m mineralisation contrasts with the copper mineralisation, for which a syndepositional origin can be regarded as established.

Domes Area e DomeTh s Aree northwesterth a n (Figi ) 2 . n provinc f Zambio e a extends for about 300 km from west to east and includes the Kabompo, Mwombezhi, Solwezi and Luswichi Domes. Rocks of pre- e areth a n e i basemenKarobelon th e o ag ot g t e complexth d an Katanga System. Gneisses, schists and granites of the basement complex occur in the cores of the Kabompo, Mwombezhi and Solwezi Domes. In e Kabompth o Dome e basementh , t rock e dividear s d int) gneisses(a o , including leucogneiss, scapolite gneiss, and biotite-hornblende gneiss and (b) psammitic and pelitic rocks affected by two phases of migmatisation (Klinck, in press). Appleton (in press) similarly describe n oldera s , partly migmatised gneiss complex and a younger formation of schists, both feldspathised and unfeldspathised. A gneiss group found in the Solwezi and Mwombezhi Domes comprises biotite gneiss, schistose gneiss, amphibolite, and hornblendite. A later granite group, separated from the gneiss group by a migmatite zone, is composed of marginally foliated granite (Arthurs, 1974). The high radioactive background over large areas suggests thae basementh t t complex could have beee originath n l e uraniusourcth f o em e founth n i d basal Katangan rocks. The lower units of the Katanga System crop out peripherally around the domes, whereas the higher stratigraphie units are found e surroundinth farthen i t ou rg country. e KabompIth n o Domd adjacenan e t areas e Wushingwth , i Form- ation unconformably overlies the basement complex. The formation is composed of a sequence of talc-muscovite-kyanite quartzites and schists, with intraformational breccia d conglomeratean s s (Appleton n prep.)i , A basa. l conglomerate occurs locallyt A .

16 some localities, notable Solwezth n i yi Dome area e basth f ,o e the formation is not clearly defined, the assumed contact being a zone of kyanite schist (Arthurs, 1974). The Wushingwi Form- ation gives ris o prominent e t quartzitic ridges. The overlying Wamikumbi Schist Formation comprises mainly biotite schists, variously with garnet, kyanite, actinolite, carbonate minerals, and scapolite. Quartzites, marbles, graphitic phyllites, banded ferruginous quartzite d schistosan s e tillites are also included. In the Kansanshi borehole NK6 in the Solwezi area e formatioth , s representei n y dolomitb d d an e muscovite-chlorite-talc schist, passing into micaceoud an s dolomitic quartzite (Arthurs, 1974). Unfoliated basic metavolcanic rock se Wamikumb occuth n i r i Formatio e soutth d an o ht n e Kabomp e nortth th f o ho o t Dome (Appleto n pressi n : Klinckn i , press) as lenticular masses up to 3 km in length. In western part of the Domes Area, the Luigishi Formation appears to overlie conformably the Wamikumbi Formation (Edwards, 1974), but in other areas, especially to the north of the Kabompo dome, it represents a more arenaceous faciès of the Wamikumbi formation (Appleton n prep.)i , t includeI . s calcareous psammitic schist d psammitesan s , variously containing biotite, scapolite, actinolit d garnet an e Wes Th t. Lunga Formation which follow a thic s i sk sequenc f foldeo e d pelitic metasediments with units of marble and tillites (Edwards, 1974). The tillites are considered to be predominantly glacial in origin, although in part they were probably modified by mass flow processes. The youngest Katangan unit, the Kanyidma Formation e northwest th s compose ,i n i cropt I t f calou s.o d - citic marbles (Appleton, in prep.). In the Wushingwi Formation the development of cross-bedding d intraformationaan l breccia a sedimentar n i s y sequence including carbonate (some probable bioherms) and coarse elastics, together with apatit d graphitean e y indicatma , a shalloe w water, near-shore, reducing marine environment of deposition. Near- shore, possibly lagoonal, condition e considerear s o havt d e prevailed at the beginning of the deposition of the Wamikumbi Formation, followe a transitio y b d o marint n e sedimentatione Th . intercalatio f carbonato n e rock y implma s y depositio n shalloi n w water. Edwards (1974), considere e Luigish e rockth th df o s i

17 Formation to have been originally sandy marls but was unable to find identifiable sedimentary structures. Shallow water deposition, possibly in a shelf environment, is suggested by Klinck (in press) for the West Lunga Formation. Resulting from the absence of age determinations, the correlation of the Katanga sediments of the Domes Area can only be based on litho-stratigraphic features and on tectonic and morphological considerations. The Wushingwi, Wamikumbi Schist, Luigishi, and West Lunga Formations are included in the Katanga System because they lie unconformably on the basement complex e lithologicallanar d y similae Katangth o t ra e rockth f o s Copperbelt. Althoug e Wushingw e rockth th hf o s i Formatioe ar n relatively highly deformed and metamorphosed, they can be correlated with similar arenaceous units aroun e Mwombezhth d i and Solwezi Domes d witan , h arenaceous Lower Roane rockth f o s Copperbelt. If the tillites are indeed of glacial origin, the West Lunga Formation may be correlated with Kindelungu of the Copperbelt (Edwards, 1974). The Katanga metasediments contain unfoliated gabbro, meta- gabbro, metadolerite d granitian , c rocks. Gabbroic rocks, including olivine gabbro, commonly form sills and plugs in the Luigishi and Wamikumbi Formations, and there are minor occurrences in the West Lunga Formation. Rock f granitio s c composition include adamellite, quartz monzonite, tonalité, and diorite. Hornblende- epidote adamellite n Katangai s n metasediments younger thae th n Wushingwi Formation in the northwest of the Domes Area may represen e post-Lufiliath t n Hook granite (Appleton n press)i , . The Domes Area lies within the southern portion of the Lufi- lian arc and is composed of Katangan rocks subjected to three phase f deformatioo s d metamorphisan n m durin e Lufiliath g n Orogeny (840-620 m.y.) e mod Th f formatioo e. e dometh s i sf o n uncertain, but it is generally believed that they were formed e Lufiliath lat n i e n Orogeny, possibl e interferencth y b y f o e folds of the second and third phases (Klinck, in press). Three major low-grade stratiform copper deposit e knowar s n in the Lower Roan of the Mwombezhi Dome. The ore-bearing horizon is a slightly carbonaceous shale. Deposition of the host rock ana distribution of the sulfides are closely related to the palaeotopography. Mineralisation seems to be of syn-

18 sedimentary origin related to reducing conditions (Benham et al., 1976). Uranium mineralisation occurs in the Lower Roan Group, m arounk e domes0 whicth d60 hr . fo crop t Uraniuou s m mineralisatio s confine i no horizons tw o e belot d on e abov on ,d a w an e quartzite marker horizon. At Kalaba and Katontu around the Kabompo Dome, uranium arsenates and vanadates, carnotite, gold, copper sulfides, traces of nickel, cobalt, lead, chromium and molybdenum occua brecciate n i r d talc schist just above th e quartzit e Wese th e eashorizont th f o Lungt o T a. Riverm k 5 1 , froe westerth m e Kabompn th edg f o eo Dome, uraniu d coppean m r mineralisation occur a carbonaceou in s sKatanga shalthe of e n System, and metatorbernite has been identified in siltstone. Two boreholes, locate a radioactiv n o d e anomaly, intersected leached uranium mineralisation at a 40 m depth (Edwards, 1974). Uran- inite, brannerite, metatorbernite, monazite, molybdenitea an , gold occu n quartz-carbonati r e e Kansanshveinth t a s i copper mine . Legg(C Arthursn i , , 1974). Below the quartzite marker horizon, uranium mineralisation occur s uraninitea s , disseminate n smali r ldo veinss yelloa d ,an w secondary minerals. Five of the 15 known occurrences have been investigate y drillinb d d encouraginan g g results have been obtained. In the Kawanga area (Kabompo Dome), uranium occurs mainly as traces of autunite coating mica schist cropping out in the o smaltw are f lo a radiometric anomalies. Drilling intersected uranium mineralisation in mica schist below the quartzite marker horizon. Discontinuous mineralisatio r moro n e occuron t a s levels the thickness of which can be several metres and has a U 0 j0 o0 percent1 conten o e t hol On p eu t. intersected mineralisation of such a grade over a thickness of about 25 m at a depth of more than 200 m. The mineralisation consists of thorium-free uraninite disseminated in schist or filling fractures several centimetres wide, and of secondary uranium minerals coating fracture surfaces. Kyanite schist froe mineraliseth m d zone carries 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 occur s individuaa s l crystaln i r o s

19 aggregates. Accessory minerals include iron oxides, apatite, monazite d rutile an ,e uraniu Th . m minera s autunitei l A . dark biotite schist n whici ,e schistositth h s markei y y flakeb d s f biotito d muscovitan e d equidimensionaan e l grain f quartzo s , occurs below the mineralised zone. Tourmaline, apatite, zircon and opaque minerals are also present. The rock overlying the mineralised zone is a talc schist, containing unorientated plates f talo d crystalan c f rhombio s c pyroxen d kyanitean e . Accessory minerals include monazite and scapolite. During the course of exploration for copper in the Mwombezhi Dome area, Mwinilunga Mines Limited found occurrences of uranium on the surface and subsurface in the Lumwana area (McGregor, 1964; Benha t al.e m , 1976) e uraniuTh . m mineralisatio s founi n d in mic e aKawang th schist n i e Solweza th s a ,arean I i .Dom e area, numerous small occurrences of uranium mineralisation have been found, and drilling has been carried out at Dumbwa, Kapijimpanga, Kimale d Mitukulukuan , e mineralisatioTh . s i n simila o that r t already described. Secondary uranium minerals are found coating flakes of mica, talc, or chlorite along fracture n veinleti d an s s containing quartz, talc, muscovite, apatite, and kyanite. The uranium minerals include autunite, meta-autunite, sabugalite,phosphoranite, vandendrlesscheite, and gummite. At depth, the mineralisation consists of uraninite, disseminated or forming scattered veinlets, and secondary uranium minerals t Kawangaa e s besa ,Th t. mineralisation intersecten i d drill holes occurs at the Mitukuluku showing, where 1.4 percent U 0 was found over a thickness of 9 m (C. Legg, in Arthurs, 1974) 30 0o The host rock is quartz-mica-kyanite-talc-epidote-apatite-chlorite schist, underlyin e marketh g r quartzite. The absolute ages of six samples from the Kawanga, Kimale, and Dumbwa areas, were determine e Institutth t a d f Geologicao e l Sciences, Geochemical Division, London e meaTh e n. th valu r fo e 2 m.y1 + 207/20. 6 53 ) age s (U 6i s

e uraniuTh m mineralisatio e Domee th th f sf o no Are s i a s youngei r o samre ag thae e nShaba th tha f o nt Copperbels i d an t of the same age or older than that of the Zambian Copperbelt e e DomeZambiath th n n si I nAre d Copperbel an a. 3) (Fig. e th t mineralisation lies close to the basement where reactivation of granite gave rise to thermal events after such influences had

20 ceased elsewhere. In the Domes Area and in the Zambian Copper- belt, any uranium mineralisation ages older than 520 m.y. could have been suppressed by these later thermal events, during which uranium and other minerals were remobilized. In the Shaban Copperbelt, older uranium mineralisation was isolated in up- faulted e Loweblockth rf o sRoa d onlan n a ver yw mineral fe y s particularly sensitiv o temperaturt e e were thermally affected.

S2OE * I I A Z 582 Kamoto . . . Kolwtzi 555 52O • Kambova 'A Kalongw« • Swambo . «hmliolobwo ; 706 V I /IwmilMRfla ;**\ fLUBUMBASHt I 7 _ ' Kiw.ng ..^-..x.> '" 536 520. Kamanihi S36.l«n,al. \^iiu \ . Dumbw» 536

Luanthya* 365 \ •"Li

SOm K O 2O O 16

Fi Isotopig3 c age f Uraniuso m occurrence theKatangn si a system (Modified after Cahenet al,l97) l

E DEPOSITSTH ' GEOLOGICAL SETTING Shaban Copperbelt area e ShabaIth n n Copperbelt area Francois (1974) classifies 22 uranium occurrences as 7 proven deposits, 2 indicated deposits, 3 inferred deposits 0 showings1 d e mosan , Th t. notabl f theso e e deposits is Shinkolobwe Mine (Derricks and Vaes, 1956; Derricks and Oosterbosh, 1958), the only one exploited. Although no official data have been published e amounth , f uraniuo t m exploited

could have been over 30,000 tonnes (U-.On averaga t aa ) e grade between 0.4 and 0.5%. The mine was opened in 1921 and closed in 1960 as a result of a fire. The uranium was found as uraninite or as secondary minerals (over 20 species) in the lower part of the R2 units at the base e coppeth f ro mineralisation. Uranium preferentially concentrated in the mylonite zones but originally could have had a

21 lensoid or stratigraphie dissemination similar to but more irregular and patchy than that of the copper mineralisation. Further- more since the two elements have different geochemical behaviour characteristics they tend to fractionate. Protore could have undergone a long series of later remobilizations, the most importan f whico t h coul e e faultinlinke b de mineth th o t df s o g alloctonous tectonic unit enclosed int n impermeabla o e talc-schist breccia. Within this unie uraniuth t m could have been freo t e move and concentrate in small fractures. Such remobilisation would have been responsibl e productioth r fo ef economio n c uranium grades. The host rock is a slab of Roan Group involved in a thrust zone and enclosed into a tectonic breccia; it rests over younger Kundelungu Strata. The mineralisation is found along open fractures within the dolomite. Mineralisation stops e boundarth t a y wite enclosinth h g breccia e largeth ; r faults are always barren. After development of mineralisation in e slath b only minor displacements followed. Uranium minerals are found botn veini h s w decimetre(frofe a metra mo t e p u s thick d disseminatean ) e hosth tn i rockd . Uraniu s accompanwa m - d raran eW eartd , tracean Mo hi f N o selements , Co , Cu . iey b d Some native gold occurred as encrustations on uraninite. Gold grades from 10 to 60 grams per tonne. Some platinum and pallad- ium is also present . e followinTh g parageneses have been recognise - firsd t phase: magnesite d phase2n ; : uraninite d phase3r ; : pyrite, selenium, molybdenum, monazite, chlorite; 4th phase: Co-Ni sulphides (and tectonization); 5th phase: chalcopyrite. Absolute age determination n uraniuo s m mineral t Shinkolobwa s e yieldee ag d 0 m.y1 + _ . 0 >70 62 0 m.yFlui2 6_ d + m.y.an .d 0 inclusion67 , s found in the Cu-Co and U-Ni-Co-Cu mineralisation within unit R2 show identical characteristics suggesting that they were formed durin e samth ge metallogenetic event.

Zambian Copperbelt area e ZambiaIth n n Copperbel t leasa t t five uranium occurrences are knowne NkanTh .a mineralisatio e onle thath s on y ha s ti n

been so far exploited with a total production of 110 tonnes U0,

0 o0 o Uranium ore occurred in the transition zone between a copper

22 _{ C• N0 t «IT «ot" » i a &»«oto

I A'r**i. iCAieI Fig. 4.- Diagram illustrating the topographic control of Lower Roan ore deposition froNkane th m a area.

23 orebody and barren dolomites, this barren gap reflecting a buried 'basement' hill (Fig. 4) . Uranium mineralisation was present in complex argillitic sediments (the Chert yn alternatin i Ore d an ) g sand d argillan y - aceous dolomites (the Banded Ore). The dominant primary uranium mineral is uraninite, occurring as fine disseminations and blebs, with a little brannerite and coffinite. Near surface oxidation produced uranophane, beta-uranotil d gummitean e . Numerous calcite, quartz, feldspar and anhydrite veins cut the deposit and were mineralised with brannerite and uraninite and traces of bornit d chalcopyritean e . Mineralisatio s beeha n n found bot s veina hd disseminatan s - ions. Whee veinth n s intersec a disseminatet d mineralised horizon the strata-bound ore grade decreases suggesting that the veins uranium came froe disseminationth m s themselves. Quartz- feldspar veins that do not intersect the mineralised beds are always barren. Field evidence demonstrates thae entirth t e mineralisation postdated the Lufilian Orogeny. Absolute age determinations at Nkana yielded ages of 520 +_ 20 m.y., 468 + 15 m.y. and 235 + 30 m.y. The first generation (520 m.y.) showed the following mineral association: uraninite, melanite, bornite, chalcopyrite, digenite, calcite, chalcocite, quartz, biotite, albite, chlorite. The 468 m.y. old one showed uraninite, carbonate, pyrite, finall e thir e th y(23 on d 5 m.y.) showed botryoidal uraninite only..

Domes Area Over 15 uranium occurrences are known around the basement domes of North-Western Zambia. So far none of them has been exploite d exploratioan d s stil i ne bes Th lt goin know. on f go n them, namely Kawanga has proved reserves of a few thousand tonnes n averaga U t 0a e grad f abouo e t 0.4%. J0 0o

URANIUM ORE GENESIS AND CONTROLS The protore and its relation to copper mineralisation Stratigraphie correlations have been established between the Zambian and Shaban Copperbelts (Cailteux, 1976), but correlatio f theso n e areas wite Dometh h s Are s uncertaini a e Th .

24 basin f depositioo s e mosnth t wer r parfo ef limiteo t d extent, especially for the lower parts of the Katanga System, so that there are many local faciès changes. The basal sediments in the Zambian Copperbelt and Domes Areas are predominantly arenaceous, whereas those in the Shaban area are carbonates, but e associatear l al d with copper, cobalt d uraniuan , m mineralis- atione sedimentTh . s deposited later were more uniforn i m compositio f greateo d an nr lateral exten d arean t , therefore more readily correlatable e sedimentTh e Katang. th f o sa System were deposite e perioth n i dd 1,300 m.y.62 0o t , e thosth f o e Roan Group probably before 1,000 m.y. (Cahen, 1970). There is evidence of stratigraphie control, since the uranium occurrences and main radioactive anomalies of the Shaban and Zambian Copperbelt e Dometh d s an sAre a appeae b o t r restricted to a particular stratigraphie interval at the base of or adjacen e copper-bearinth o t t g horizon o occut n r i ro s different isopic zones e knowTh .n copper, cobalt d uraniuan , m deposit n l intervaoccua al sn m i r 0 f sedimento 15 l o t 0 10 s thick (30-40 m in the Shaban area) at the base of an 8,000 m thick sequenc f sedimentso e e Shabath n I ne blac th area. kl al , uranium oxide d moran s e tha 0 percen7 e uraniun th f o tm occurrences are localised at the base of the zone of copper mineralisation. Secondary uranium minerals are found along fault zones and other fractures in decreasing amounts away from the mineralised horizon . 1971)(Caheal t e n. Environmental controls are similar in the three areas for both the copper and the uranium mineralisation, as indicated by the presence of bioherms in the vicinity of the mineralised zones. It seems likely that the bioherms provided the required condition o caust s e initial syngenetic precipitatio f coppero n , cobalt and uranium in shallow marginal marine waters. Subsequent enrichment could have taken place during diagenesis e zonTh e. of mineralisation therefore correspond o reducint s g conditions in the depositional environment as attested by the presence of carbonaceous matter in most host rocks. Where oxidized minerals, such as hematite, rutile and ilmenite, are prominent, uranium mineralisatio s wea i nr absent o k ; thern a e e seemb o t s irreconcilability between uranium mineralisation and the presence of some oxides. The host lithologies, therefore, can be said

25 to vary quite considerably but not the chemistry of the deposit- lonal environmen e vitat th whic ls factorwa h . A mineral zonin s evidene i Zambiag th n i tn Copperbelt (Garlick 1972) , at Lumwana in the Domes Area (McGregor, 1964) , e Shabath ann i nd area (Oosterbosch, 1962); froe footwalth m l upwar r froe o shorelind th m e seawar e sulfidth d e minerals occur in zones represented by chalcocite, bornite, chalcopyrite, and e Shabapyriteth n I n are. a particularly, uranium occurs below the copper mineralisation (Cahen, et al. 1971). In the Zambian Copperbel d Domean t s Area, lithologie consideration suggest several cycles of transgression and regression, with the result that the uranium mineralisation occurs at various horizons withi e zon th f coppeno e r mineralisatio t exclusivelno d an n t a y the base n explanatioA . f thio n s distribution coul e thab d t initially precipitated uranium was again taken into solution as conditions changed from reducing during transgression to oxidising during regression. Regional mineral e Shabazoninth n i ng ares beeha a n studied by Ngongo-Kashisha (1975) who shows that in the southern part of the 300 km arc, uranium, copper, cobalt, nickel, monazite, and magnetite are predominant. In the central part of the arc, there is an increase in the amount of copper and cobalt and a decreas e amounth n f uraniumi o te , nicke d magnesitean l , with monazite present only in traces. Only traces of copper and cobal e norther e founth ar tn i da region e areann th O par f .-o t e Domel scalth a n si eArea e higheth , r grade f coppeo s r occurrence, such as Kimale, are almost free from uranium and coppe s abseni r t from richese uraniuth f o tm occurrences including Kawanga and Mitukutulu; however both of these elements are found in deposits of intermediate grade, such as the Kapijimpanga occurrence A simila. r relationshi s appareni pe Zambiath n i t n Copperbelt, where uraniu f gooo m d grade occur a wea n i ks copper e Shabath zon t n Nkanaa i en Copperbeld an , t area (Oosterbosch, 1962). According to syngenetic concepts, the mineral zoning appears to be related to the changing chemical and physical conditions withi e zon th f nsedimentatioo e e deptth f wate o hs a n r increased or decreased with the advance or retreat of the shoreline (Garlick, 1972) .

26 Convincing evidenc s availabl i ee syngeneti th r fo e c sedimentary origin of copper mineralisation; it seems that the evident stratigraphie, palaeotopographi d environmentaan c l controls requir a similae r interpretatio e uraniuth r fo mn mineralisation. These conclusion e corroboratear s e identicath y b d l fluid-inclusion characteristic e copper-cobalth f o s t mineralisatio d uranium-copper-cobalt-nickean n l mineralisation (Ngongo- Kashisha, 1975), the fluid inclusions would be expected to be differen e uraniuth f i t m association belonge n independena o t d t magmatic epoch as previously stated by Derricks et al. (1956, 1958) postulates ,a y Oosterboscb d h s generall(1962a d an ) y accepted in the geological literature, at least as far as the Shinkolobwe, Swamb d Kalongwan o e deposit e concernedar s .

Protore-changing processes e uraniuth Man f o ym occurrences hav a typicae l vein aspect e isotopith l an al de mineralc th age f e post-sedimentaryo s ar s . Thes et inconsisten no fact e ar s t wite previouth h s conclusions e processeith f s which affecte e protorth d e examinedar e . These processes include diagenesis, metamorphism, tectonic events, supergene enrichment and thermal events associated with post- tectonic metamorphism; they modified the protore and converted it into ore with vein morphology. In the Shaban area, the effects of metamorphism were slight. In the Domes and Zambian Copperbelt areas, the rocks were re- crystallise a certai o t d n an dexten e economith t c minerals were segregated into veins whic e boundear h y barreb d n zones. Generall t seemi y s that, metamorphism only resulte n minoi d r uranium migration over short distances. A major role in the protore-changing processes was played by tectonic events e followinTh . g generation f uraniuo s m mineralisatio e knownar n : >706 m.y. (Shinkolobwe) 0 m.y2 + . 0 67 ; (Shinkolobwe and Swambo); 620 + 10 m.y. (Shinkolobwe, Kalongwe, and 0 m.yLuishya)1 .+ 5 (Kambov55 ; 2 (Kimale1 e + West) 6 53 ,; Kawang 0 m.y2 + d _ Dumbwa. an a0 (Nkana 52 ); , Kansanshi, Mushoshi 5 m.y1 _ 0 m.yd + 4 Kamoto. an + 8 (Nkana. 46 6 (Luanshya 35 ); ); ); 0 m.y3 _ + . 5 e Shaba(Nkana)23 th d n I nan .area e rangth , f o e mineralisation is from 706 to 520 m.y., whereas in the other e earlie5 m.yTh area23 . e rang o t r th s s fro 0 datei e 52 m s apply

27 Tabl - Parageneti 3 e f Mineralo c s Association (modified after Cahe t al.e n , 1971)

Main syngenetic Age of minerals Paragenesis metal U e F u C o C i N

0 m.y.(Swambo67 ) (a)Uranmite;pyrite,monazite,chlonte ;vaesite, siegenite;chalcopyrite m.y0 62 .d an 0 67 (a)Magnesite;uraninite; py- (Shinkolobwe) rite .molybdenite.rnona- zite;vaesite,cattierite, siegenite;chalcopynte.

m.y0 62 . (Kalongwe) (a)Uraninite; pyrite, chlorite;carrolite;bornite, chalcopyrite

582 m.y. (Kamot) P o (a)Uraninite;chlorite;carrolite;bornite.chalcopyrite

555 m.y.(Kambove) (a)Uraninite;chlorite;carrolite;bornite,chalcopyrite

536 m.y. (Dumbwa, (b)Uraninite,muscovite, apa- Kimale, Kawanga) t ite.molybdenite,chlorite

520 m.y. (Nkana) (b)Uraninite,me Ionite, bornite,chalcopyrite, digenite.caleite,chalcocite, quartz.biotite,albite, chlorite;molybdenite

520 m.y. (Kansanshi) (b)Brannerite,chalcopyrite, calcite,pyrite,rutile bornite.molybdenite

520 m.y. (Musoshi) (b)Albite.uraninite,pyrite, carbonate,quartz,molybdenite

520 m.y. (Kamot) oP (b)Uraninite.carbonate.pyrite

468 m.y. (Nkana) (b)Uraninite.carbonate,pyrite

365 m.y. (Luanshya) (b)Pitchblende,calcite, dolomite

235 m.y. (Nkana) (b)Pitchblende

(a) Paragenetic order; (b) paragenetic association; P = Principal

28 to the Shaban area where older generations were isolated in up-faulted blocks and preserved from the successive thermal effects followin e tectonism th e Zambiag th n I n .Copperbel d an t Domes Areas, wher e originath e l mineralised zones remained close e basementtth o e uraniuth , m minerals were exposee th o t d succeeding thermal effects and repeated mobilisation and redeposition took place; consequently, more uranium generations exist. According to Cahen et al. (1971) the uranium generations of 706, 670 + 20, and 620 +_ 10 m.y. are connected with the first, second and third phases, respectively, of the Lufilian Orogeny. All three generations of uranium mineralisation are found at Shinlolobwe. The 670 m.y. generation represents an epigenetic reworking of the 706 m.y. mineralisation, and the 620 m.y. generatio 0 m.y a reworkin67 s e i .n th mineralisation f o g e Th . 0 m.y2 + . 0 generatio52 s beeha nn relate a larg o t de thermal event associated with post-tectonic metamorphism (Belliere, 1961) which affected the three areas but had its greatest effect in the southeast. This major thermal event has been confirmed by potassium-argo e determinationag n d othean s r geological considerations. Other lesser thermal influences up to 235 m.y. o causeag d further redistributio f uraniuo n m mineralisation. Isotopic ages and their references to geologic events can be questionable, but metamorphic events, three main tectonic phases, e majoon r thermal event d supergenan , e alteratio e welar nl known and documented facts in the Katanga System geology. e paragenetiTh c associatio e mineralor f o n s confirme th s hypothesis thae numbeth t f elemento r s decreases after each remobilisation e maie metalsth or n f o ,l nickelAl . , cobalt, copper, iron and uranium are present at the 670 m.y. stage. At 620 m.y. nickel has disappeared and cobalt, copper and iron disappear successivel 0 m.y. 52 5 m.y8 m.y.e 36 46 , th d . ,t an a y stages leaving only the highly mobile uranium at the final 235 m.y stage (Table 3). In the paragenetic order uranium is normally the first meta o appeart l e graduaTh . l impoverishmene th f o t paragenetic associations after each remobilisation, without introduction of new metals, supports the hypothesis of one original mineralisation. Supergene alteration affected the mineralisation to a greater or lesser extent. In the Shaban area the uranium minerals

29 occurring in up-faulted blocks were leached out and reconcentrated near the water table. Francois (1974) states that intense weathering often cancels form, original composition, and origin e mineraliseth f o d bodies e ShinkolobwTh . e deposie on s i t such e Zambiaexampleth n I n .Copperbel d Domean t s Areas, uranium enrichment occur n structurei s s neae wateth r r tablr o e in fracture fillings. The newly described uranium occurrences of the Domes Area support the view of Cahen et al., (1971) that the present uranium concentration e resultth e f remobilisatioo ar s e originath f o n l mineralisation, reflecting well-documented geological events with no endogenetic processes being involved. e uraniuth s a m r origifa s s concernedA i n o specifin , c investigatio s beeha n n carried out. Neverthelese b n ca t i s s bee assumede ha copper nth t i donr s fo a ,e tha t cami t e from the weatherin e basementh f o g t althoug a possiblh e contribution from submarine volcanism cannot be completely ruled out. It is however believed that whatever the source of uranium could have been, the protore formation has been syngenetic for both metals, uraniu d copperan m .

Analogy with other mineralisation e uraniuTh m occurrence e Katangth f o sa System show certain similarities with occurrences elsewher e worldth n i .e Reference is here made to the protore and not to the subsequent remobilisation events A roug. h compariso e attempteb n ca n d with deposits e Northerth f o n Territory (Australia d Saskatchewaan ) n (Canada). In the three regions the uranium bearing sediments were deposited at the base of a marginal marine sequence in a basin bordere y granitesb d , gneisses, migmatites d othean , r metamorphic rocks. The presence of bioherms in Australian deposits (Needha & Stuart-Smithm e Zambian-Zaireath , n 1976i d an ) n occurrences indicates a euphotic zone, i.e. a shallow water or lagoonal environment e presencTh . f carbonaceouo e s matter, adjacent to or intercalated with the mineralisation, suggests a reducing marine environment. The Olympic Dam deposit on the Roxby Downs Pastoral Station, South Australia, appears to have similarities to the deposits associated with the Katanga System (see Robert & Hudsons , this volume) e originaTh . l mineralisation

30 subsequently underwent different geological histories, not only e differenith n t region t als bu n differensi o t e placesamth e f o s region, leadine presenth o t ge mineralisationt th typ f o e .

CONCLUSIONS Uranium and copper mineralisation is present at the base of Katanga System. The mineralisation is considered to be closely relate marina o t d e transgression ove n anciena r t crystalline basement. An originally stratiform mineralisation was changed by later tectonic, metamorphic and thermal events, giving rise to the relatively high-grade vein-type occurrences now seen which represent epigenetic enrichmen e syngenetith f o t c protore. e ShabaIth n n Copperbelt area e stratiforth , m e naturth f o e uranium mineralisation, closely associated wite southerth h n e Lowefacièth rf o sRoa n Group, provide a targes r exploratiofo t n along 300 km of the mineralised arc. The probability of finding sub-outcropping mineralisation is minimal, but new technology - suc s alpha-detectioa h n coul e e searctesteb d th r hidden fo i hd n concentration f oreo s . Although large deposit f high-grado s e uranium mineralisatio e unlikel ar ne found b o t y, low-grade occurrences could provide valuable by-product uranium for the copper industrye Zambiath n I n .Copperbel d Domean t s Areas, where th e uranium isopic e Lowezoneth rf o s Roa nt wel no Grou le ar p localised, it may be necessary to elucidate the palaeogeography and palaeoenvironmen e knowth nf o t uranium occurrences. Although the geology of the copper deposits has been thoroughly investigated, less attentio s beeha n ne giveth o t n peculiar conditions of uranium mineralisation. It has been show t Shinkolobwea n , Nkana, Kawang d Mitukutulan a u that rich concentrations of copper and uranium are mutually exclusive, so that exploration techniques for copper detection are different from those requiree locatioth r fo f uraniumdo n . It is difficult to evaluate the possible uranium reserves of the Katanga System; at the moment exploration is still going e threth l e al area n i sn o involving international companies.

Tentativel e totath y l amoun f Uo t0 00 concentration present a t J O the base of the Katanga System could be estimated between 50,000 and 100,000 tonnes.

31 REFERENCES APPLETON, J.D. in press: The geology of the Kabompo Gorge area. Exploratio f degreo n e sheet 122 W quarter5N : Zambia Geol. Survey, Kept. 40. ARTHURS, J.W., 1974 e geologe Solwez:Th th f o yi area. Explor- atio degreof n e sheets 122 quarter6NW 1126and , , parof t SW quarter: Zambia Geol. Survey, Rept. 36_, 48. BELLIERE , 1961,J. : Manifestations métamorphiques dan a regiol s n d'Elisabethville: Pub. Etat d'Elisabethville, Katanga, Jul , 175-1791 y .

BENHAM, D.G., GREIG, D.D. and NINK, B.W., 1976: Copper occurr- e Mombezhenceth f o s i area northwestern Zambia: Leon. Geol., J71, 433-442. BINDA, P.L. and MULGREW, T.R., 1974: Stratigraphy of copper occurrence e Zambiath n i sn Copperbelt n Bartholomei , , P. , éd., Gisements stratiformes et provinces cuprifères: Liège, Soc. Geol. Belgique, 180-201.

CAHEN 1970, ,L. : Etat actuaa Geochronologll e d l u Katangiend e : Mus. Royal Afrique Centre, Annales ,, SeiSer80 n .i . Geol. 6J> 7-14.

______1974: Geological background to the copper-bearing strata of southern Shaba (Zaire) in Bartholome, P., ed. Gisements stratiformes et provinces cuprifères: Liège, Soc. Geol. Belgique, 57-77.

CAHLN, L., PASTLELS, P., LEDENT, D.r BOURGUILLOT, R., VAN WAMBEKE, L. and EBLRHARDT, P., 1961: Recherches sur l'âge absolu des minéralisations uranifere u Katangd se Rodesid t e au Nordd e : Mus. Royal Afrique Centre Annales, 41, 1-54.

CAHEN, L., FRANÇOIS, A. and LEDENT, D., 1971: Sur l'âge des uraninites de Kambove ouest et de Kamoto principal et révision des connaissances relatives aux minéralisations uraniferes du Katang u Copperbela t e a e Zambiad t : Soc. Geol. Belgique, Annales, 94, 185-198. CAILTEUX , 1976S. , : Corrélation stratigraphiqu s sédimentde e s e Roa de âg u ZambieShabed d n t e a . Soc. Geol. Belgique, Annales, 99, 31-45.

32 DAHLKAMP, F.J., 1978: Classification of uranium deposits: Mineralium Deposita , 83-10413 , . DERRICKS, J.J d OOSTERBOSCHan . , 1958,R. e Swamb:Th d Kalongwan o e deposits compared to Shinkolobwe: Contribution to the study f Katango a uraniu n Unitei m d Nations Internat.e Confth n o . peaceful use f atomio s c energy, 2nd, Geneva, Proc.. 2 , Survey raw material resources: New York, United Nations, 663-695. DERRICKS, J.J. and VAES, J.F., 1956: The Shinkolobwe uranium deposit. Current status of our geologic and metallogenic knowledg n Internati e .e peacefuConfth n o . l use f atomio s c energy, 1st, Geneva, Proc., _6. Geology of uranium and thorium: New York, United Nations, 94-128.

DEMESMAEKER, G., FRANCOIS, A. and OOSTERBOSCH, R., 1962: La tectoniqu s gisementde e s cuprifères stratiforme u Katangad s n i , Lombardd Nicolinian , , J. eds. R. , , Gisements stratiformes de cuivre en Afrique, Paris, Assoc. des Services Gëbl., 47-115. EDWARDS, R.A., 1974 e geologe Wes:Th th t f Lungo y a River area. Explanation of degree sheet 1224, NW quarter: Zambia, Geol. Survey Kept. 43, 86. FRANCOIS , 1974,A. : Stratigraphie tectoniqu t minéralisatione e s dans l'arc cuprieferes du Shaba, in Bartholomé', P., éd. Gisements stratiformes et provinces cuprifères: Liège, Centenaire Soc. Gé"ol. Belgique, 79-101. GARLICK, W.G., 1972: Sedimentary environmen f Zambiao t n copper deposition: Geologie Mijnbouw , 277-29851 , . KLINCK, B.A. in press: The geology of the Kabompo Dome area. Exploration of degree sheet 1224, NE quarter: Zambia, Geol. Survey Kept. 44 . MCGREGOR, J.A., 1964: The Lumwana copper prospect in Zambia: Unpub disser., Grahamstown, South Africa, Rhodes Univ., 171. MENDELSOHN, F., 1961: The geology of the northern Rhodesian Copperbelt: London, Macdonald and Co., 523.

NEEDHAM, R.S. d STUART-SMITHan , , P.G., 1976 e Cahil:Th l Format- ion-host to uranium deposits in the Alligator River uranium field. Australia: BMRJ , 321-3331 , .

33 NGONGO-KASHISHA, 1975: Sur la similitude entre les gisements uranifères (type Shinkolobwe) et les gisements cuprifères (type Kamoto u Shaba ) a Zaïre: Soc. Gë*ol. Belgique, Annales, 98, 449-462.

OOSTLRBOSCII, R., 1962: Les minéralisations dans le système de Roan au Katanga, in Lombard, J. and Nicolini, R., eds., Gisements stratiformes de cuivre en Afrique: Paris, Assoc, des Services Géol., 71-136. u TRILd THOREAU E t TERDONCKD e U , J. ,, 1933R. ,e gîtL : e d'uranium de Shinkolobwe-Kasolo (Katanga): Extrait des Mem. publiés par l'Inst. Colonial Belge, J_, 18-64.

34 URANIU EARLMN I Y PROTEROZOIC AILLIK GROUP, LABRADOR

S.S. GANDHI Geological Survey of Canada, Ottawa, Ontario, Canada

Abstract

Uranium occurrence e widelar s y distribute n i tuffaceousd - argillaceous beds and rhyolitic volcanic rocks of the Aillik Group of Early Proterozoic age. They include partially developed Kitts and Michelin deposits, containing 156 d 7360an U respectivelyt 6 . Uranium occurs mainly as disseminated fine pitchblende grains in lenticular concordant zones, and rarely as coarse pitchblende aggregates and thin veinlets. e occurrenceTh e argillaceous-tuffaceouth n i s s beds, including the Kitts deposit, are within a steeply dipping stratigraphie zone less m thictham long k d 0 ove 5 n0 an k 2 .r Thee interpretear y s a d 1syngenetic', wit a sourch f uraniu o en activ a n i me felsic volcanic field in adjacent terrain. The occurrences in the rhyolitic rocks, including the Michelin deposit, are in elongate structural- stratigraphie belt e characterizer zoneso sar d an , a pronounce y b d d enrichmen f o d sodtdepletioan a f o potasn h associated with mineralization. They are interpreted as 'epigenetic', related closel e felsi th n tim i yo ct e volcanism that forme e hosth dt rocks 1767±4 Ma ago accoring to a Rb/Sr isochron date. e mineralizeTh d zones were affecte a varyin o t d g degrey b e Hudsonian deformatio d metamorphisman n .e pitchblendeth Som f o e s

yielded nearly concordant dates clos o 175 t e. 0OtherMa s froe th m south near the 'Grenville Front 1 indicate disturbance of isotopic equilibrium at the time of Grenville orogeny approximately 1100 Ma ago. This is corroborated by the isotopic composition of lead (Pb: 206/204: 532.45; 207/204:97.163; 208/204:36.726) in galena from

the Michelin deposit e overwhelminglTh . y dominant uranogenic

6 20 component of this lead has a Pb/Pb 207 ratio of 0.1585 consistent witextractios it h t 118a n a fro0M m d pitchblende176ol a 7M .

35 INTRODUCTION e uraniuTh m distric f centrao t l Labrador coas bees tha n explored intermittently since the first discovery of a pitchblende vein there in 1954. Exploration to date has located a number of occurrences of uraniu d molybdenuman m , includin w promisinfe a g g depositd an s prospects although none have been productiv s yeta e . e firsTh t comprehensiv e geologica th stud f o y l settind an g genetic aspects of the uranium occurrences in the district was published by the writer in 1978. Since then additional exploration work has been carried out, and several geological studies have been done, notabl Clary yb k (1979), Evans (1980), Kontak (1980), Whitd an e Martin (1980), Bailey (1979), . (1982 Gowed al d Ryaan t an e )rn Kay (1982). These have added valuabl w perspectivesene datd an a . Many problems however remain unresolve n thii d s geologically complex and economically promising area. Divergent opinions have been expresse n severao d l important aspect f e geologareo sth s a a f o y summarize . (ibid)y Goweb al dn attempA t e r . s madi t e hero t e emphasize those aspects that are relevant to the genesis of the uranium deposits.

REGIONAL GEOLOGY Early Proterozoic Aillik Group was deposited on an Archean banded gneiss complex (Figs. 1 and 2) , and was deformed into east- northeast to north trending folds, metamorphosed in the range of upper greenschist to lower amphibolite faciès, and intruded by granites, during Hudsonian cycle approximately o 175 ag o 155 t 0a M 0 (Gandh t al.e i , 1969, Gandhi ,e south 1978)th o ,T .thes e rocke ar s bounded by the 'Grenville Front', which is the northern limit of an extensive area affected strongly by deformation, metamorphism and intrusions approximately 100 o (Stevensono 0120ag t a 0M , 1970). Positio e d 'Grenvillnaturth an n f o e et welFrontno le knowar ' n i n this region becaus f scarcito e f outcropo y o extensivt e du s e glacial drift cover.

Aillik Group The Aillik Group is dominantly a bimodal sequence of basaltic and rhyolitic volcanic rocks interbedded with clastic sediments (Fig. 3). Rhyolitic volcanic rocks predominate in the upper part of the sequence. Faciès variations over short distance commone ar s d ,an stratigraphie relations are obscured by varying degree of deformation and intrusions. Thickness of the group is estimated as in the order

36 J | j Hadrynian l u supracrusi(a/s

i Grenvtlle T 1~ gnet. eiss gramfJ I C I e ^_- -j3 Neoheliktan '----"] supracrusfa/s

P* * Elsonian ^__ _ I intrusions

- ~] Paleohelikian E __ supracrustalsJ __ »pheb/an supracrustals gneiss granite

n Archean gneiss _L J granite

56"

Figure studyL the Location of areageneraland map geological featuressurroundingthe of regionthe of eastern Canadian Shield (from Gandhi, 1978). of 7600 m (Gandhi et al., 1969) or 8500 m (Clark, 1973J . It is informally divided inte lowe d th ouppe an r r Aillik groups. e loweTh r Aillik grou s exposei p e northwesterd th onl n i y n part of the study area along the Kitts-Post Hill belt near Kaipokok Bay (Figs. 2 and 3). On the southeast side of the bay, it is approximately 2700 m thick, generally dips steeply to the east- southeast, and is overlain by the upper Aillik group which is up to 900 m thick in this belt (Marten, 1977). Contacts of the sequence with the Archean basement to the northwest are faulted and sheared. On the southeastern side, the basement is thrust over the sequence. The thrust zone is obscured in part by intrusion of granodiorite gneiss and in part by extensive drift cover.

37 Figure 2. General geological map showing the Aillik Group and uranium occurrences in the group, Kaipokok Bay-Big River area, Labrador (from Gandhi, 1978, with minor modifications). Uranium occurrences: 1: Long Island, 10: Knife, 19: Burnt Lake, 2: Anderson Ridge, 11: Nash East, 20: Emben, 3: Kitts 'A' zone, 12: Nash Main, 21: Winter Lake, 4:zones,'C' Kittsand 'B' 13: 22: Nash West, Pitch Lake, 5: Kiwi, 14: Witch, 23: Round Pond-FaHs Lake, 6: South, 15: 'J' & 'B1, 24: John Micheiin, 7: Punch Lake, 16: Anna Lake North,25: Sum'!, 8: Gear, 17:26: Michelin, Cape Makkovik 9: Inda, 18: Rainbow, molybdenite deposit

The basal unit of the lower Aillik group is metasedimentary, and is comprised predominantl f micaceouo y s schist d gneissean s s derived from siltstone d sandstonesan s , with pyriti d graphitian c c horizons. It is approximately 800 m thick. A basaltic unit capping Post Hill, and extending to the southwest across the Kaipokok Bay, is underlain and overlai e metasedimentar th y somb nf o e o t p u y s i rocks d an , 1000 m thick (Fig. 3). The metasedimentary rocks from Kitts to Post

38 UPPER AILLIK GROUP

LOWER AILLIK GROUP

Quartz feldspar porphyry, extrusive intrusiveand KITTS-POST HILL BELT Rhyolite flows, tuffs, agglomerate, ignimbrite; interbedded volcaniclastic sediments

Conglomerate, agglomerate, sandstone, sittstone, banded quartzite; mostly felsic volcaniclastic sediments; minor shale limestoneand

Basaltic flows, massiveand WHITE BEAR MOUNTAIN-WALKER pillowed; mafic tuffs LAKE AREA Sittstone, sandstone, shale; some pyritic and graphitic beds

Figure . 3 Generalized lithostratigraphy e Aillikth f o Grou thren pi e parts of the study area and tentative correlations.

Hile overlaiar l y well-exposeb n d massiv d pillowean e d basalt flows and mafic tuffs, approximately 900 m in thickness. The basalts range from marginally subalkaline (tholeiitic o mildlt ) y alkaline, with soda predominant over potash (Evans, 1980; White and Martin, 1980). e thicTh k basaltic unis overlaii t a relativel y b n y thin stratigraphie zone or unit, less than 50 m thick, and referred to her s 'argillaceous-tuffaceoua e s unit1, n whicimportana s i h t hosf to uranium occurrences t I include. s cher d argillitan e t th o t e northeast, and laminated tuffs and tuffaceous siltstones to the southwest describes s showi ,a t Figurn I i n . d4 e further under 'Host Rocks f uraniuo ' m occurrences t gradeI . s into overlying felsic tuffaceous siltstones and banded quartzite, in the southwestern part e belt th d ofintan , o interbedded mafic tuffs, siliceoud an s calcareou e middls th bed s overlai n i e i beltes th d par an ,f y o tb n

39 NE

-^ ' :^^^^r^i^^^J~*~^ V"*"-*1 1- ' -^"v^ V v v V v V V V V V V V V V V V V V V V V V V \f V v v v v V V V vvvvvvvv V V V V V V V V V V V

Anderson Kilts A B and South Gear Inda Knife East Nash Nash zoneC s Ridgshowine g showing zones zone m NasMa h west Showing rone zone zone

Boundaryof Intertedded siltstones argillaceous- Cherty quartzite, local laminatedand tuffs magnetite layers tutfaceous unit

Felsic tuffs, siltstones, sandstone, Laminated tuffs, Basaltic flows, massive varicoloured and pillowed banded quartzite

Mafic tuffs, siliceous and Argillite, pyrrhotrte-nch Stratiform zoneof calcareous beds lenses disseminated uranium

Figure 4. Relationship of stratigraphy and uranium occurrences in the Kitts-Post Hill belt, Labrador (modified after Gandhi, 1978).

pillowed basalt to the north. The laminated tuffs and tuffaceous siltstones contain abundant felsic layers that are interpreted as derived from rhyolitic tuffs. Influx of this felsic material marks the beginnin f felsio g c volcanis e uppeth rf o mAilli k group, hence the argillaceous-tu faceouf s uni s interpretei t e basad th her s la e unit of the upper Aillik group. Stratigraphically equivalent rocks t ar exposee remaindee Aillino e th th e are n th f i dko a f o Grour o t p the e belteasd soutth an t . f o h e argillaceous-tuffaceouTh s uni overlais i t n assemblaga y nb f eo interbedded sandstone, siltstone, waterlain felsic tuffs, fine grained banded quartzit d calcareouan e s beds, wit n aggregata h e thickness of approximately 300 m. They dip steeply to the east- southeast. To the southeast, a quartz feldspar porphyry dyke striking paralle e widem bed d uptth 0 an so 30 o,t l separate bede th ss from a conglomerate-agglomerate unit which is approximately 260 m thick. East of this unit are a mafic volcanic lens, and rhyolitic flows and tuffs that are southern continuation of the thick and extensive rhyolitic unit near Makkovik Bay (Gandhi et al., 1969; Gandhi, 1978) . They may in part be interbedded with the sedimentary assemblage conglomerate-agglomeratth d an e e e quartunitTh . z feldspar porphyr s relatee felsii y th o t cd volcanic activite th f o y upper Aillik group.

40 Appearance of the sedimentary assemblage deposited in shallow water, with contributions from the felsic volcanic activity, in the sequence of Kitts-Post Hill belt indicates a significant change in the depositional environments and provenance of the basin characterize y extensivb d e pillow basalt d deepean s r water sediments e loweth rf o Ailli ke earlieth grou n i pr stages (Gandhi, 1978, p. 1497). Rocks of the upper Aillik group are extensive in the Aillik- Makkovi e Whitkth areed an Beaa r Mountain-Walker Lakee areath o t , northeast and south of the Kitts-Post Hill belt respectively. Generalized lithostratigraphic sequence in the two areas is shown in o n market Figurtherbu e ar r , e3 e bedo t maks e definitive correlation sothere froth e are d betweeon mo t an ,a n these aread an s the Kitts-Post Hill belt. A few tentative correlations however are suggested as shown in the figure. The oldest rocks in the two areas, exposed in the cores of major anticlines e interbedde(Figar , .2) d sandstones, silstones, banded quartzites with some magnetite-rich beds and intraformational conglomerates, totalling over 1000 m in thickness. At a few localities shales, calcareous a chert-magnetite-amphibolbeds d an , e iron formation, are present (Gandhi, 1978). Some of the beds in the unit show small scale cross-bedding and graded bedding. The conglomerates contain abundant rhyolitic clasts along with other lithologies t placea d san , grad ee interbeddear into r o , d with, rhyolitic agglomerate. Basaltic volcanic rocks occur at the upper boundary of the sedimentary unit d als an ,n rhyoliti i o c sequence abov . Thee it e ar y more abundant in the Aillik-Makkovik area than in the White Bear Mountain-Walker Lake area. They are relatively thin, with a maximum thickness of 150 m, compared to the basaltic units in the lower Aillik e stratigraphicallgroupar d an , y higher than them. Rhyolitic volcanic and volcaniclastic rocks, and genetically related thick sill-like bodie f quarto s z feldspar porphyry n pari , t extrusive and in part intrusive, extends over most of the Aillik- Makkovi d Whitan k e Bear Mountain-Walker Lake areas d attaian , n a n aggregate thickness of over 3500 m in both the areas (Figs. 2 and 3). The rhyolitic rocks are varied in character, and include feldspar porpnyritic, subporphyriti d nonporphyritian c c flow d tuffssan , fine grained massiv d flow-bandean e d rhyolites, fino t coarse e pyroclastics, air-fall tuffs and ash-flow tuffs or ignimbrites, and laharic flows. Henc e volcanith e c activit t s partlno wa y f i y largely, subaerial. Some of the tuffs are reworked by surface

41 waters. Some lense d bed an f so calcareous d siliciclastian s c sediment e alsar so interbedded wite rhyolitith h c rocks. The quartz feldspar porphyry contains coarse subhedral to euhedral crystal f feldspao s d blu an ra fine n quarti e grainet se z d quartzo-feldspathic matrix, and is deformed to a varying degree. In the White Bear Mountain-Walker Lake area, the porphyritic unit is approximately 1500 m thick, and is generally massive to weakly foliated, but strong foliation and lineation are developed at many places, and obscure contacts with the rhyolitic rocks. In some outcrops e porphyritith , c rock apparently grades inte rhyolitith o c rocks that include ignimbrites (Stevenson, 1970; Watson-White, 1976; Gandhi, 1978; Bailey, 1979) n additio I e majo. th ro t nunit , there are dykes and irregular small bodies of relatively little deformed quartz feldspar porphyr e area th d thes n an ,y represeni y ema t younger e intrusionsAillik-Makkovith n I . k area a quart, z feldspar porphyritic gneiss unin estimate a s extensiv i ts ha d d an e thickness of 3050 m (Gandhi et al., 1969). This may however be an over- estimate because the Aillik Group as a whole in this area is tightly folded, but the folding is not apparent in this essentially uniform s gneissiunitIt . c o t textur e s i du interpretee e b o t d recrystallization of quartz feldspar porphyry, with development of quart d hornblendan z e streak d growtan sd deformatioan h f o n porphyroblast f feldspao s r (Clark, 1971, 1973; Gandhi, 1978; White and Martin ,s shar 1980)ha d generall t an pI . y concordant boundaries with other n a a rhyolitisil r e o b l y ma ce area t th rockI .f o s extrusive unit. An assemblage of conglomerate, siltstones, banded quartzites and rhyolitic rocks e eas exposeg Bigh th e AilliktBi n th t o sidt a df o e- Makkovi e youngesth e kb e areAillity th parma a f o kte Grouth n i p study area. The rhyolitic rocks of the upper Aillik group are marginally subalkaline d shoan , w effect f alkalo s i metasomatis s discussea m d later (Watson-White, 1976; Gandhi, 1978; Evans, 1980; Whitd an e Martin, 1980; Gowe t al.e r , 1982). Rb/Sr isotopie ag c determination e rhyolitith n o s c rocks reporte y b Watsond - White (1976), Gandhi (1978 d Kontaan ) k (1980 e a M 167ar 6 )1 6 ± (4 point isochron, recalculated), 1659 Ma (errorchron, unreliable) and 1767 ± 4 Ma (5 point isochron) respectively. These are regarded as minimum ages.

42 Intrusions Rocks intrusive into the Aillik Group include pre-kinematic amphibolitic dykes, small gabbro bodies d quart,an z feldspar porphyry dyke d irregulaan s r bodies; large synkinematic intrusion gneissif so c granodiorite to monzonite, referred to as Long Island gneiss, and lenses of granite gneiss; late kinematic to post-kinematic granite plutons, the gabbro-diorite differentiated intrusion of Adlavik Bay; and younger diorite-syenite, diabase and lamprophyre dykes. The quartz feldspar porphyry intrusions and some of the granitic plutons are believe e epizonab o t d l intrusions e relaterhyolitith o t d c volcanic e uppeth rf o sAilli k group. The gneissic granodiorite of Long Island yielded a K-Ar date of 1832 ± 58 Ma (Gandhi et al., 1969) which is anomalous with respect to the Rb/Sr isotopi ce rhyolitedateth n o s s intrude . Thiit y ma sy b d o excest e bdu se argon inherited from basement rocks and/or froe th m numerous mafic inclusion f unknowo s n origin. Other K-Ar, Ar-Ard an , Rb-Sr dates on rocks of the Aillik Group, metamorphosed dykes, Adlavik gabbro and granitic intrusions range from 1750 to 1500 Ma (Gandhi et al., 1969, Gower et al., 1982). They suggest a long perio f Hudsoniao d n intrusive activity durin e latth ge Aphebiao t n early Paleohelikian time. A large gabbro sheet-like body trending east-northeast near the southern boundary of the study area (Fig. 2) is one of the Helikian Michael gabbro intrusions which are extensive in the Grenville province (Stevenson, 1970; Gower et al., 1982). Other younger dykes in the study area range in age from 1000 to 400 Ma (Gandhi et al., 1969; Gandhi, 1978; Gower et al., 1982).

Structure e Kitts-PosIth n t Hill belt e unconformitth , y betweee th n Archean e basemenAillith d k an tGrou s obliteratewa p y intensb d e tectonic sliding. Furthermore, along the east side of this belt, the basemen s thrusi t t ovee groupth r . Marten (1977) interpretex si d stages of Hudsonian deformation: two early stages of thrust-slicing and tectonic interleavin e beltth n ,i g followe y threb d e folding Aillie eventth n ki s Group o mino tw e majo r,d on an onesa rlat d e,an stag f faulting o ee wes th t n O sid .f Kaipoko o e k Bay,e rockth f o s lower Aillik group display similar polyphase deformation, according to Ryan and Kay (1982) who, in addition, suggested the possibility of post-Archean migmatization in the basement prior to the tectonic sliding.

43 In the Aillik-Makkovik area, Gandhi et al. (1969) recognized two broad, open anticlinal structures with synkinematic granite gneiss domes at the core, and adjacent tightly folded zones with local shearing and mylonitization. More detailed structural studies by Clark (1971, 1973, 1979) have shown four stages of deformation, of e whicmos e th secont th h s importanwa d t one t resulteI e . th n i d northerly trending upright folds a subvertica, l schistositd an y subhorizontal mineral lineatio e planth f schistosityo n ei n . e WhitIth n e Bear Mountain-Walker Lake area, sedimentary units along the north side are tightly folded with axial planes nearly vertica r o overturne le northwesth e th o t n di t t (Figbu , .2) rhyolitic rocks to the south the folding is obscure because of lack of marker units. Foldin a smal n o gl scale, however s observei , t a d many places. The degree of development of foliation in the rhyolites varies considerably acros e regionasth l east-northeast trend (Gandhi, 1978) . Foliation commonly dips 50 to 60 degrees to the south- southeast, subparallel to recognizable lithologie boundaries in the rhyolites n echeloE . n shear zone d mylonitizean s d zone presente sar . Bailey (1979) has interpreted a number of faults subparallel to the regional tren t relativbu d e movements alon t knowngno thee .ar m

Tectonics e AilliTh k Grou s affectee i Hudsoniapth y b d n structural, metamorphi d intrusivan c e events that resembl n orogenia e c cyclet ,bu its lithological characters, in particular the abundant rhyolitic volcanics interpretee ,ar Watson-Whity b d e (1976), Gandhi (1978)d ,an White and Martin (1980) to indicate a nonorogenic, continental rift environmen f o depositiot n rather tha a classican l géosynclinal accumulation e rhyolitiTh . c rock e marginallar s y subalkalint a d an e least som f thee o subaerial ar m . They predominate ovee basaltth r s in a bimodal sequence of volcanics which lacks andesitic rocks. The absence of andésites is an important consideration in interpretation of the tectonic setting. Basalts of the lower Aillik group are considered a part of the bimodal volcanic sequence of the Aillik Group as a whole by White and Martin (1980), whereas Clark (1979) and Wardle and Bailey (1981) proposed a fundamentally different tectonic setting for the lower Aillik group from that of the upper Aillik group, and proposed a major angular unconformity separating the two groups. The lower Aillik group, according to these authors, was deposited in a shelf environment transitional southwards into deeper water environment,

44 similar to that of Labrador Trough. Clark (ibid) suggested that depositio lowee th rf no Ailli k grou s followepwa collisioy b d o tw f no cratons, erosion of the group in the Aillik-Makkovik area, developmen f o transcurrent t faults, major felsic volcanisd an m plutonis d continuean m d deformation. Wardl Bailed an e y e (ibidth n o ) other hand favoure a rifd t environmen e uppeth rr fo Aillit k group, but regarded the apparent synorogenic timing of its felsic volcanism as problematic. e possibilitTh a majo f o yr unconformity betwee e lowed th n an r upper Aillik groups was earlier considered by Gandhi (1976) but field evidence for it was not found. Wardle and Bailey (ibid) inferred an angular unconformity from what they considered as relatively more intense polyphase deformatio d highean n r grad f metamorphiso e e th n i m lower Aillik grou comparison i p o simplt n e structural styl d lowean e r grade of metamorphism in the upper Aillik group. The differences, however, are not as pronounced as they suggest, and can be attributed to the original lithological differences and to inhomogeneous deformatio d metamorphisan n m durin a singlg e 'orogenic' typ f cycleo e as proposed by Gandhi et al. (1969) and Marten (1977). Wardle and Bailey (ibid) also referre a polymic o t d t conglomerate e th uni n i t Kitts-Post Hill belt, describe y Marteb d e n th e bas(ibid f th o e s a ) upper Aillik group. Accordin o t Gandhg i (1970, 1978d an ) Evans (1980) however e conglomeratth , s i structuralle d an y stratigraphically above the east dipping siltstone-banded quartzite unit, which contains abundant waterlain felsic tuffs and conformably overlie e uranium-bearinth s g argillaceous-tuffaceous e unitTh . conglomerat s thui en intraformationaa s e uppel th uni rn i tAilli k group rather tha a basan l conglomerate markin majoa g r unconformity. Southern part of this 260 m thick and over 9 km long unit is describe s metamorphosea d d rhyolitic agglomerat y Burbidgb e e (1977). Evans (ibid) interprete a submarinunie s th da t e y caselaharan n ,I . similar agglomeratic and conglomeratic intraformational units also occur elsewher e uppeth n ri eAilli k group. Some structural readjustment of Hudsonian faults and shears in the proximity of the 'Grenville Front' is likely to have occurred durin e Grenvilliath g n events (Gandhi, 1978)r exampleFo . n a , Ar/A rn biotit o dat a sampla f 100M o n e i 5 e 4e± fro a sheam r zone obtaine y Archibalb d d Farraan d s reportea r . (1982n Goweal i d t e r, p. 46), indicates Grenvillian effects. Gower et al. (ibid) inferred one major east-west fault extending from the head of Kaipokok Bay along the Adlavik Brook to the mouth of the Big River, and a dextral displacement of 18 km along it, and regarded it as Grenvillian in

45 age. Bailey (1979 s interpreteha ) e broath d d mylonitized zoned an s nearly vertical L fabrics in the White Bear Mountain-Walker Lake area as Grenvillian developments along Hudsonian normal faults. Sheared and mylonitized zones howeve rr nort fa occus Cap a hs a re Makkovik where Grenvillian effects are likely to be negligible, and are therefore regarded here as Hudsonian structures. Presence of muscovit e rockth n somi sf e Adlavi o eth sout f o hk Brood an k alteration of the post-Hudsonian east-west trending gabbro sheet are attributed to the Grenvillian metamorphism by Gower et al. (1982).

URANIUM OCCURENCES Over 70 uranium occurrences* or groups of occurrences, containing pitchblende with negligible thorium, are known in rocks of the Aillik Group in the study area (Fig. 2), and some of these are economically promising prospects. Several pegmatitic uranium and thorium occurrences are found in the Archean basement complex and in granites intrusive into the Aillik Group. In addition, there are a numbe f molybdeniteo r , pyrit d fluoritan e e occurrence e Aillith n i ks f theo closelGroupe w ar m onlt fe ,bu a y associated wite uraniuth h m occurence e groupth n i .s These minerals howeve e e commoth ar r n i n pegmatites associated with the intrusive granites.

Distribution e uraniuth Mos f o tm occurrence Aillie th n ki s Grou e locatear p d in four elongate stratigraphie-structural belts or zones, namely the Kitts-Post Hill belt, Cape Makkovik-Monkey Hill belt, Falls Lake- Shoal Lake-Bernard Lak White beltth d e Bea,an r Mountain-Walker Lake bel s describeta (Fig) 2 . y Gandhb d i (1978). Kitts-Pose Thosth n i e t Hill belt are hosted by a stratigraphie zone of argillaceous and tuffaceous beds, less m thictha d 0 traceabl5 nan k m k r ove 0 fo e2 r along the strike. In the other three belts, the host rocks are rhyolitic flow d tuffsan s .

Shap d Sizan e e Most of the uranium occurrences are lenses or groups of echelon lense f o disseminates d uranium minerals, subparallee th o t l moderately to steeply dipping host beds or lithologie units, and to

* Uranium concentration of approximately 0.01 per cent over 10 m strike length r greatero , , detected radiometrically and/oy b r analyses of representative samples.

46 Tabl : 1 eApproximat e tonnag averagd ean e grad importanf eo t uraniud man molybdenum deposits in the Kaipokok Bay-Big River area, Labrador Deposit or Average grade Total U prospect (no.) Tonnes (per cent U) (tonnes) Kitts (3,4) 347,000 0.449 1560 Inda (9) 496,000 0.119 590 Nash Main (12) 216,000 0.187 404 Gea) (8 r 33,000 0.102 34 Michelin (17) 7,366,000 0.100 7366 Rainbow (18) 90,000 0.127 144 Burnt Lake (19) 146,000 0.067 98 Sunil (25) 344,000 0.024+ 82 0.102% MoS2

Cape Makkovik (26) 2,000,000 0.250% MoS2 +locallp u y to 0.004% U Data from Gowe al.t e rA.Ad ,an ., 1982 61 Burgoyne . ,p , Brinco Mining Limited, Vancouver, British Columbia, personal communications, 1984. Number bracketn i s : Location numbe Fign i r . .2

Uranium resources in the Kitts, Nash and Michelin deposits are regarded as 'Estimated Additional Resoruces 1 in high price category in terms of NBA/IAEA classiciation. Estimates for the Cape Makkovik molybdenum deposit include 30 per cent dilution by barren dykes.

the regional structural trend. The lenses are a few tens to a few hundreds of metres long in both the strike and dip directions, and up w metrefe ta os thick. Elsewhere uranium mineral e disseminatear s d irregularly and/o e confinear r o veint d f variablo s e orientation. High-grade veins and aggregates do occur, but they are rare. Exploration to-dat s outlineha e a smald l high-grade deposit, namely the Kitts deposit, and a larger, lower grade deposit, namely the Michelin deposit n additioi w prospectsd fe an , a n s listea , n i d Tabl . 1 ea numbe Ther e f othear o er r occurrence t thee bu sar y insufficiently explore o arrivt d t theia e r grad d tonnagean e d an , mos f theo t m appeae smalle b f lowe o o t r o r r grade thae oneth ns listed in the table.

Host Rocks A distinctive argillaceous-tuffaceous unit along the Kitts-Post Hill belt and the rhyolitic rocks east of this belt are the hosts of uranium occurrences in the Aillik Group (Figs. 2 and 4). Recognition

47 of the regional stratigraphie control of uranium mineralization in the Kitts-Post Hile mosth l t bels importanwa t te th guid n i e discovery and exploration of prospects along the belt (Gandhi, 1970, d shoulan 1977, d) havn a impace n o furthet r exploration. Stratigraphie control in the rhyolitic sequence is less evident, due e lacmainl f th markeo k o t y r units t threbu , e major stratigraphie- structural zone r belto s s have been recognize s favourabla d e ones, namely the White Bear Mountain-Walker Lake belt, the Cape Makkovik- Monkey Hill belt, and the Falls Lake-Shoal Lake-Bernard Lake belt (Gandhi, 1970, 1978). Exploration efforts in the past were concentrated along these belts.

Argillaceous-tuffaceous unit (Kitts-Post Hill Belt): This unit is les ss continuou i m thathic d 0 5 nan k sm k alon 0 e 2 strikth gr fo e from the Kitts deposit to the Nash West showing (Figs. 2 and 4). Lithological variations are common along and across the strike e northth o ,T argillit . (Fig4) . e C bed d san hose Kitt, B th t, A s zones, and other occurrences including the Kiwi, Anderson Ridge, South and Gear showings (Fig. 2). The beds are grey to dark grey and a few millimetres to several centimetres thick. They are tightly folded, contorted, intrude y gabbrob d , sheare d metamorphosean d t a d the Kitts deposit (Gandhi, 1978). They include some thin bedd an s lenses rich in graphite (1 to 2.5 per cent) and pyrrhotite (up to 10 per cent, with minor pyrite and chalcopyrite) that are the main host f pitchblendeo s . Magnetit s preseni e n varyini t g amountsd an , is locally abundant and forms distinct layers as at the Gear showing. Calcite occurs mainl s veinlets a yr cen t pe a t 7 d amount o an ,t p u s Kitts deposits. Pink variety of calcite is common at uranium occurrences. Pelitic beds commonly have porphyroblasts of garnet, and andalusite or chiastolite in a matrix of hornblende, sodic plagioclase, quartz, and some pyroxene and chlorite. Sillimanite tufts are developed in a buff white bed in contact with a gabbro intrusion near the Kitts deposit and are interpreted to have formed by contact metamorphism (Gandhi, 1977, 1978). Siltstones, metamorphosed to biotite-albite-quartz assemblages, and mafic tuffaceous beds and lenses are interbedded with the argillaceous beds. The siltstones (referred to as 'greywacke1 by Evans, 1980) contain little or no graphite, but contain cummingtonite- anthophyllite ± actinolite and minor magensium chlorite. Available chemical e analyseargilliteth f o 1 reportes (1 s y b d Gandhi, 1977, and 9 by Evans, 1980) and of the greywackes (14 reported by Evans, ibid) show close affinities to basaltic rocks.

48 The argillite beds overlie and are locally interbedded with chert which at places contains magnetite layers. Relatively more competent chert has been brecciated and disrupted into isolated lenses in incompetent argillite during deformation. It is regarded as inorganic, volcanogenic in origin (Gandhi, 1978) . The chert and argillite apparently pinch out to the south of the Inda showing (Fig . 4) .Faciè s change e mosar st marke t thia d s showing, and mafic tuffs, interbedded mafic and felsic tuffs, argillaceous and calcareous beds are present. Pitchblende occurs in maia n stratiform lend thresan e associated lense slightlt sa y higher stratigraphie levels e maiTh .n host rock e finar se grained, well- laminated tuffaceous beds, with green mafi d gred pinan can y k felsic laminae. Thee overlaiar y y calcareoub n s mafic tuffaceous beds, which also host some pitchblende. Pitchblend closels i e y associated with aegirine-augite in the laminated tuffs and with phlogopite in calcareous mafic tuffs (Evans, 1980). The latter also contain garnets which are vanadiferous. In the northern part of the Inda showing, some low-grade zones have disseminated pitchblend sodin i e c plagioclase-magnetite-sphene rich layers containing quartz, hornblend garnet± e , accordin o Evant g s (ibid). Spott d leaan yn mineralize de Knif zoneth d Nas an t ea hs East showings are in the rocks similar to that of the lower mineralized Inde zonth a t showingea Nase Th .h Mai Nasd nan h West depositn i e sar a distinctive laminated subunit that overlies pillow basalt and mafic tuff e subuniTh . fins i t e grained, tuffaceous with green, dark grey, bufd pinan fk laminae e mineralTh . s presen e pyroxenar t e (salite), garnet (grossular-andradite, vanadiferous), epidote, ferroan pargasite, albite, microcline, magnetite, calcit d minoan e r sphene (Evans, 1980). Red colouration due to hematite is developed in the rocks where uranium content is relatively high. The laminae are contorted with folds plunging commonle southth o .t y Uranium concentration is commonly greater at fold hinges than along the limbs. Hos te Punc th rock ht a sLak e m showingk 3 , 7) s . (FigNo , 2 . e Kittsoutth sf o hdeposit , where uranium occurrence e vein-typar s e and disseminated-type, include mafi d laminatean c d tuffs similao t r the Nash West zone, accordin Hardeo t g r (1981). The Witch showing, located approximately 5 km southwest of the Nash Wes n gre ti o pin t ys showin i k ) coloured14 g . (FigNo , 2 . felsic tuffaceous beds. These beds may be stratigraphically close to the laminated tuffaceous subunit of the Nash West and Nash Main deposits.

49 B prospec & J s locatee i t Th d approximatel m southwesk 2 1 y f o t the Witch showin ge wes. 15)(Figth No t n , o ,shor2 . f Kaipokoo e k Bay, and is in sheared rhyolitic tuffs and associated mafic flows or e uppesillsth e Aillirf o ,th par f ko t Group (Rya d Kayan n, 1982). The rocks may be stratigraphically close to those at the Witch showing. Uranium-bearing zones, according to Lever and Davidson (1978) e stratiformar , . In addition to the stratiform occurrences mentioned above, several uranium-bearing vein d fracture-fillingan s s occur alone th g Kitts-Post Hill bel e southwestth e.gt a . t corne f Lono r g Island an d one kilometre to the west of Punch Lake. These are hosted by quartzite-siltstone-argillite, basalt d paraschistsan s . Metapelitic hos te Ann rockth a t a Laks e North occurrencem k 8 , nort f Walkeo h r Lak granitn . ei 16) No e (Fig , ar ,2 e. gneiss-granite terrain (Willy, 1981). They appear to be similar to the argillaceous-tuffaceou so remotunitto d t isolatethee an bu e,ar y d froe uni o makth mt t a reliable e correlation.

Rhyoliti ce uppe rockth rf o sAilli k group: Rhyolitic flows, tuffs, and fragmental rocks are hosts to numerous uranium occurrences east and south of the Kitts-Post Hill belt. The rocks vary considerably in texture, but chemically they fall within a rather small compositional range. Some unitn abundanca e magneti ar o st e f du o ce accessory magnetite. Textural varieties distinguishe e fiel e th massiv ar n di d an e flow banded rhyolites, coarse feldspar porphyritic and subporphyritic rhyolite flow r o tuffss , laminate d thinlan d y bedded tuffs, fragmental rocks with rhyolitic lapilli, bomb d blocksan s d an , tuffaceous sedimentar e yMicheli th rocks t A n. deposit, several mineralized lenses are subparallel to a sequence of interbedded coarse feldspar porphyritic and subporphyritic rhyolitic tuffaceous beds and fine grained, thin felsic beds and lenses, although in detail the mineralized lenses transect the lithological boundaries at low angles (Gandhi, 1978). Extensive quartz feldspar porphyry and porphyritic gneiss, which may in part be intrusive and in part ignimbritic, are hosts to several occurrences, most of which are e contactclosth o t e s with other rhyolitic rocks e contactTh . e ar s sharp, and distinctive as the rhyolites are generally fine grained and lack the coarse quartz crystals that are abundant in the porphyry and prophyritic gneiss. Most of the occurrences along the Cape Makkovik-Monkey Hill belt are along such contacts either between the major units or of lenses of rhyolites in the porphyritic gneiss.

50 e rhyolitiTh c rock e deformear sa varyin o t d g degree, from relatively little deformed to schistose and highly streaked quartzo- feldspathic gneisses, and mylonitized or granulated varieties. They e bufar f white, pale pind lighan k t gre n colouri y d havan ,a e distinctive red colouration, due to finely disseminated hematite, at and around high-grade uranium concentrations. Some of the host tuffs are more mafic than rhyolitic tuffs e.g. at the Rainbow prospect, 2 km south of the Michelin deposit (Fig. 2, No. 18), the host bed is chloritic and calcareous. It overlie a mafis c tuffaceous s overlaii bed d ,an y b aphanitin c rhyolite. The host rock resembles some of the host rocks at the Inda and Gear showing Kitts-Pose th n i s t Hill belt. Furthermore, layers, lense d aggregatean s s ric n mafii h c silicate e preferentiallar s y mineralized in some of the occurrences e.g. at the Burnt Lake showings . 19)(FigNo ,, 2 .stratifor m mineralized zones occun i r relatively more mafic layers containing aegirine-augit d magnesioan e - riobeckite (Bailey, 1979; Kontak, 1980). A number of chemical analyses of unmineralized and mineralized rhyolitic rocks have been reporte y b Barud a (1969 ), Watson- White (1976), Minatidis (1976), Gandhi (1978), Bailey (1979), Evans (1980), Kontak (1980), White and Martin (1980) and Gower et al. (1982) . Gower et al. (ibid) listed 125 analyses of 0 group1 f unmineralizeo s f theo d l mrocks al excepa grou d r ,an fo pt of 4 dacitic rocks, are within a range of 70 to 78 per cent silica, 3 to 5 per cent soda and 4 to 6 per cent potash. No significant chemical difference e apparenar s t amon e groupth g f porphyritio s c rhyolite, nonporphyritic rhyolite, rhyolite tuff and quartz feldspar porphyry. The mineralized rocks are texturally identical to the unmineralized equivalent rocks ,e finel excepth r y fo t disseminated pitchblende and hematite amounting to a fraction of a per cent in the mineralized rock. Ther s howevei e a significanr t increas n sodi e a and corresponding depletio a lesse o n t potasi n r d extenan h f o t silica, reflecte n albitizatioi d f feldspao n r (Gandhi, 1978; White and Martin, 1980; Evans, 1980)t A othe.r localities, potash enrichment and corresponding soda depletion are observed in the unmineralized rocks (Watson-White, 1976; White and Martin, 1980). Alkali metasomatis s widespreai m e uppeth n ri dAilli k group. Hence some of the rhyolitic rocks have alkaline or mildly peralkaline affinités, although in general the rocks are high K., calc-alkaline in character.

51 Mineralogy Pitchblende with negligible thorium s virtualli , e onlth yy uranium-bearing phase in most of the occurrences. Coffinite and soddyit e associatear e d with pitchblend w occurrencefe a n i t e bu s these minerals are quantitatively insignificant compared to pitchblende. Yellow secondary uranium minerals are developed in the surface exposures, particularl t high-grada y e spots. e Kitts-Posth In t Hill belt, pitchblende occurs mostls a y disseminated grains less than 10 microns in diameter along thin laminae and beds and as small string-like aggregates along bedding e argillaceous-tuffaceouth plane n i s s host rocks (Hughson, ; Gandhic , b 195 , ,a 8 1977, 1978; Evans, 1980) t alsI .o occurn i s high-grade concentration foln i s d hinges, veine d shearsth an sn i s ,a Kitts deposit. Thes e apparentlar e o locat e ldu yremobilizatio d an n concentration, during deformation e originallth f o , y disseminated mineral grains. In addition, there are late stage calcite veins which at places carry pitchblende.

e mosTh t important silicate mineral associated with pitchblende is hornblende. It contains abundant inclusions of opaque minerals including pitchblende s normalli t I . y gree n thii n n sectionst bu , s brownisha d colouratiore h n highli n y radioactive spots. Textures observed at the Kitts, Gear, Inda and Nash deposits show that the hornblende is a post-mineralization metamorphic mineral (Marten, 1977; Gandhi, 1978; Evans, 1980). Other mafic silicates thae alsar to associated with pitchblend t placea d san e carry inclusions of it, are pyroxene, biotite, and chlorite. The latter is common along shear zones. Electron microprobe analyses done by Evans (1980) show iron-rich characte f theso r e minerals e alsH .o found clinozoisite, pumpellyite and prehnite which apparently represent retrograde metamorphism. Quartz, albit d sphenan ee ar e other silicates commonly present. Sulphides, graphite and magnetite are present in varying amounts in the occurrences along the Kitts-Post Hill belt, but there is no apparent systematic spatial relationship between these minerals and pitchblende. Pyrrhotite, pyrite, chalcopyrite d tracean , f o s molybdenit d arsenopyritean e e argillitee commoth ar , n i e n Th . sulphides occu s disseminatea r d grains, layers, lense d alsan s a o veinlets. Hematitt see no n argillitei ns i s commoei t n i bu n, tuffaceous beds ric n felsii h c components. Analyses of mineralized samples and bulk ore samples show only traces of precious metals, thorium, yttrium and other rare earths,

52 and generall w concentrationlo y f baso s e metals, molybdenud an m vanadium (Hughson, ; c 1958Gandhi , b , a , 1978; Evans, 1980). Fluorite is rare or absent in the uranium occurrences of the Kitts- Post Hill belt contrasn i , e occurrence th o somt tf o e n rhyolitii s c rocks elsewhere in the Aillik Group. In the rhyolitic host rocks, uranium occurs as disseminated pitchblende or uraninite grains less than 10 microns in diameter, commonl s a inclusiony n mafii s c silicate d alsan t sa grai o n boundaries and in streaky aggregates with hematite and mafic minerals. Uraniferous grains and aggregates are concentrated along foliatio d sheaan n r planes. At the Michelin deposit, the main host and closely associated mineral e metamicar s t sphen d aegirinan e e augite, which form aggregates with alkali amphibole, ilmeno-magnetite partially oxidize o hematitet d , zircon, calcite d apatitean , d scarcan , e phlogopite, andradite fluorit d pyritean e n quartz-albiti , e rock. Variatio n abundanci ne mafith cf o emineral s imparts distinctive banding to some of the coarse feldspar porphyritic units. The bands are parallel to the lithologie boundaries, and range in widths up to several centimetres. In the nonporphyritic hosts rocks, the bands are light grey, reddish brown and dark grey (Gandhi, 1976). Compositions of pitchblende or uraninite, shpene, aegirine, alkali amphibole and albite were determined by energy dispersive spectra and x-ray diffraction (Gandhi, 1978; Evans, 1980, White and Martin, 1980). Feldspa n boti r h phenocrysti d groundmasan c s phases of strongly mineralized rocks is nearly pure albite, with less than 1 per cent anorthite and orthoclase molecules (Gandhi, ibid, and Evans, ibid), whic s i indicativh f o sode a metasomatisme Th . phenomenon is also evident from the chemical data. Analyses of 57 mineralized rocks reported by Gandhi (ibid), Evans (ibid) and Whit Martid an e n (ibid) shoÛ K2 r wcen d pe Naa tan 2 1 O conteno t 7 f to les r centspe tha5 , 0. nwherea e unmineralizeth s d rhyolitic rockn i s comparison contain 3 to 5 per cent NaaO and 4 to 6 per cent K2O. Ther s alsi e a osmal l decreas n silici e a contene mineralizeth n i t d rocks compare e unmineralizeth o t d d rocks. Whit d Martian e n (ibid) suggest that relatively mildly metasomatized rocks evolvey b d Na-for-K exchange, and more intensely metasomatized rocks evolved by (Na ± Al)-metasomatism, involving silica depletion, rather than by simpl exchangen io e . Evans (ibid) recognized increasing intensitf o y metasomatism towarde dril e zoneth or e corln i th f ssectiono e e h s studied, and outlined inner, outer and transional metasomatic zones that envelope the ore zones. Some K20-enrichment is observed at the margins of the metasomatized zones.

53 Available data show little difference in contents of Th, rare earth elements, Mo and F between the mineralized and unmineralized

S sho d a wan l shou C C n increased a w, an Th 2 d C0 an ,, Pb rocks , Zr . decrease with desilicatio f o rockn s accordin o t Whitd g an e Martin (1980). Data presented by them, and by Gandhi (1978) and Evans (1980) , show that the strongly mineralized rocks commonly e th n i r Z m pp 0 contai70 o t n contras i n0 r 30 100Z o 150o t 0m t t pp 0

unmineralized rocks A slighlt. y highe 2 contenC0 r n mineralizei t d rocks (0.9 to 1.3 per cent) compared to unmineralized rocks o 0.6(0.6 t r cent6 0sample3 pe a s 14 suitnotei ) n f i o des from 6 dril4 l holes, analyse y Brineb d x Limite n searci d r guidefo h o t s mineralization. Mineralog e occurrence th f moso y f o t n rhyolitei s similas i s o t r that at the Michelin deposit, with some variations in relative proporiton f mineralo s d intensitan s f albitizationo y . Thue th s showings at Burnt Lake (Fig. 2, No. 19) are in coarse feldspar quartz porphyritic rhyolite, chemically simila e Micheli o th thost r t a e n deposite associatear d an , d with nonporphyritic rhyolite, tuffsd an , lenses of lapilli tuffs and agglomerate (Gandhi, 1978). Mineralogical studie y b Kontas k (1980), including electron microprobe analyses, show that uraninite or pitchblende is preferentially associated with aegirine-augite, acmite and magnesio- riebeckite, that occur as disseminated grains, aggregates and layers or bands up to 0.5 cm thick. The pyroxenes predominate over amphibole. Other minerals present are biotite, zircon, apatite, hematite, magnetite, sphalerite, chalcopyrite, coveJlite, bornite, chalcocite, fluorite and galena. The feldspar is nearly pure albite. Kontak (ibid) regarded the mineralization as stratiform. This is also seen in the Emben South showing, 5 km east of Burnt Lake . 20)No ,(Fig, 2 wher. e mineralizatioo t s restricte5 i n 0. a o t d 1.5 m thick unit containing up to 10 per cent aegirine and 80 per cent albite d notablan , e amount f riebeckiteo s , magnetite, sphene, apatite, pyrite and some quartz, calcite and fluorite. The unit is tightly folde m wito dantiform0 tw hwithi15 x d n area 0 nan s 15 a synforms plunging moderatel e southwesth o t y t (Minatidis, 1976; Gandhi, 1976; Willy, 1982). A few small occurrences of pyrite, molybdenite and fluorite are found in rhyolitic rocks close to the east shor f Walkeo e r Lake. Cape Ith ne Makkovik-Monkey Hill belt, disseminated molybdenite, pyrite and fluorite occur with disseminated pitchblende (uraninite) at the Sunil showing (Fig. 2, No. 25) and in the Cape Makkovik molybdenum deposit (Fig. 2, No. 26) which contains only traces of

54 pitchblend n i some e places e depositTh . e stratiformar s Sunie th ; l showing is 350 m long and up to 5 m thick, and the Cape Makkovik m thick 5 2 o t deposio . t 0 6 s p 230i Theu p t 0m di yd lon an g 70 degrees to the east and are drilled to a depth of 100 to 150 m. They contain approximately 5 per cent pyrite and subordinate amounts of molybdenite (Table 1). The sulphides in the deposits occur commonly as discrete streaks several millimetres long, parallel to the foliation or schistocity of host rocks, and locally as veins (King, 1963; Gandhi et al., 1969; Barua, 1969; Gandhi, 1978). Pyrite also occurs as euhedral crystals scattered through the host rock. Pitchblende occur e s a Sunistreakth s lt a sshowin g (Barua, 1969). It is commonly not associated intimately with molybdenite-pyrite streaks e hosTh .t rhyolitic roc t bota k h deposits is strongly enriched in soda and depleted in potash as at the Michelin s albiti deposite feldspa th t i ed n an i ,(Morser , 1961; Gill, 1966; Gandhi, 1967; Barua, ibid). Fluorite occur s purpla s e and colourless crystals closely associated with the sulphides. Other minerals present are quartz, hornblende, magnetite, zircon, apatite, actinolite, sphene, garnet, acmite or aegirine, epidote and calcite. Trace f chalcopyriteo s , sphalerite, galena, pyrrhotit f silveo d an re are presene Sunith n li t showing. In the vicinity of Makkovik, weak radioactivity occurs in skarn- like aggregates of pyroxene, garnet, magnetite and hematite, sphene (titanite) , epidote, pyrite and scheelite in lenses of limestone that occur in the predominantly rhyolitic sequence (Beavan, 1958) . Occurrence e Shoath ln i sLake-Fall s Lake-Round Pond zone also contain disseminations of fine grained pitchblende in rhyolitic flows, tuffs and relatively more mafic tuffaceous units. Host rocks are enriche n sodd depletei d an a n potasi d h (Gandhi, 1978). Traces f silveo d seleniu occurrencew an r fe notee a ar mn i d s (Gandhi, ibid). The main occurrence at Winter Lake (Fig. 2, No. 21), approximately 12 km southeast of Monkey Hill peak, is at the folded contact of rhyolitic tuffaceous rock and what appears to be a mafic dyke. Pitchblende disseminations and veins occur at the crests of minor folds, along the contact for 100 m strike length. An associated narrow sulphide-calcite vein at one end of the showing contains sphalerite, covellite and stromeyerite (Beavan, 1958). A number of veins carrying chalcopyrite, pyrite, pyrrhotite, molybdenitd an e fluorite occur near Round Pond (Fig. 2, No. 23) in conglomerate underlying rhyolitic volcanic rocks containing lean disseminated pitchblende mineralization (Gandhi, 1969). Vein f pitchblendo s e ar e more Roune commoth dn i nPon - dFall s Lake are tha) a23 n (Fig. No , .2

55 elsewhere in the Aillik Group. The vein at Pitch Lake (Fig. 2, No. 22), 5 km to the south-southeast of Monkey Hill peak, which was the first uranium occurrence discovered in the area in 1954, contains massive and botryoidal pitchblende, gummite, hematite and fergusonite (Beavan, 1958). It occurs in a large amphibolite (mafic lava) lens intrude y graniteb d .

Pitchblende and Galena Age Determinations U-Pb isotopic ratios of pitchblende samples are plotted on the concordia diagram (Fig. 5). Nine samples from the Kitts deposit yielded nearly concordant results. One of these, as reported by Gandhi (1978) , gave an essentially concordant date of 1730 Ma from the Pb207/Pb206 ratio. Eight other samples analyzed by Kontak (1980) yielded results indicating a very small range close to a concordant date of 1770 Ma. e sampleth f o s o representeTw d disseminated-type mineralization whereas others represented vein-type mineralization. The data indicate a single crystallization event approximately 1750 Ma ago, with little loss of radiogenic daughters since. Samples from the Inda and Gear showings, on the other hand, yielded discordant results representing uranium loss or lead gain. The precise mechanism that can cause this is not known.

064

s Q_

o Kins deposit A Gear showing • Inda showing 0 Michelin deposit 01 A John Michelin showing EmbeD n showing x Pitch Lake showing • Burnt Lake showing

Figure 5. Concordia plot of pitchblendes from uranium occurrences in the Aillik Group, Kaipokok Bay - Big River area, Labrador (after Gandhi, 1978, with additional data points from Kontak, 1980).

56 Evans (1980) estimated age f pitchblendeo s e Kittsth n i ,s Inda and Nash West deposits from elemental U/Pb ratios obtained by microprobe analyses of pitchblende grains. His calculated ages are: r r Kitt4 fo grainsIndfo (2 a s aM a M )5 , 9 18417811 20 ± 5± 4 (16 grains), and 1795 ± 159 Ma for Nash West (four sites in one large grain). Amon e uraniuth g m occurrence n rhyolitii s ce upperockth rf o s Aillik group, the John Michelin showing with disseminated pitchblende, located 2 km southwest of Makkovik (Fig. 2, No. 24), yielde a nearld y concordant dat f 174o e a fro5M m Pb /Pb ratio 207 206 (Gandhi, 1978). Thi s veri s ye dateclosth so t ee obtaineth r fo d Kitts deposit. Uranium mineralization predates at least the last stages of Hudsonian deformation and metamorphism (Gandhi, 1978) , hence these dates represent the time of these events. Together with a Rb/Sr isochro e nrhyoiti th dat f r 1767±o efo c a 4rockM m k 4 s northwes Michelie th f to n deposit (Kontak, 1980 )thes, e dates impla y rather short time mineral for formation of the host rocks, uranium mineralization and Hudsonian deformation and metamorphism. Samples from the Michelin deposit and the Emben showing gave relatively much younger Pb207/Pb206 dates, of 1244 and 1364 Ma respectively. Bote sampleth h s howeve e froar rm high-grade 'vein- type' concentrations exposed in trenches, and probably represent remobilizatio f uraniuo n m from originally disseminated-type, lower grade, more extensive mineralization (Gandhi, 1978). Two samples from the Burnt Lake showings gave discordant results; their Pb207/Pb206 dates are 1770 and 1680 Ma (Kontak, 1980). The older date is from a drill core sample and the other one is from a trench e concordisampleth n O . a diagram thes o samplese onetw eth s d an , from the Michelin deposit, and from the Emben and John Michelin showings, plot close to a line which intercepts the concordia at approximately 1800 Ma and 1100 Ma. This indicates that the disseminated-type uranium mineralization as represented by the John Michelin sample is close to 1800 Ma in age, and in case of the Burnt Lake, Michelin and Emben samples, it was affected by an episode that disturbed its isotopic equilibrium approximately 1100 Ma ago. Thi s supportei s a dat f y 101o eb d a 7obtaineM y Evanb d s (1980r fo ) pitchblende grains in a foliated mafic dyke in the Michelin deposit, usin e electroth g n microprobe n gootechniquei ds i agreemen t I . t with an Ar"*°/Ar39 date of 1004 ± 5 Ma mentioned earlier on biotite in a sample from a regional shear zone in the area (Gower et al., 1982, . 46)p .A K-A r dat n anotheo e r foliated, mafic dyke, approximately m e Micheli2thickc 5th n i , n deposit, reporte y Gandhb d i (1978)s i , somewha. Ma 6 2 t± younge9 91 t a r

57 The episode of disturbance of isotopic equilibrium and/or local redistribution of uranium leading to high-grade concentrations, approximately 1100 Ma ago coincides with the time of Grenville orogeni cs therefori cycle t I . e attribute e effectth o f t o ds Grenvillian structural and metamorphic events on the rocks in the souther e studn th par yf o tare a neae 'Grenvillth r e Front'. Mafic dyke e presenar s t neaMichelie th r d Embean n n sample localitiesd an , Gandhi (1978 . 1515-1516p , ) suggested that local redistributiof o n e intrusio th e dykes o th e uraniut dyke Th f e .o n s du me b ther y ma e however are more than 10 m from the localities and their contact metamorphic effects, as in case of other dykes in the study area, are restricted to within a few centimetres from the contacts, beyond which there is no visible effect on the country rocks including the mineralized rhyolitic rocks. Furthermore, ages of these dykes are not known. Their effects on the samples thus remain speculative. In case they were emplaced r durino prio o t rg Grenville time, their influence on isotopic equilibrium of the samples, if any, would be difficul o isolatt t e froe effectth m f Grenvilliao s n structurad an l metamorphic events. The pitchblende vein at Pitch Lake yielded a Pb207/Pb206 date of a (BeavanM 4 93 , 1958; Gandhi, 1978) e veiTh n. dips e gentlth o t y s exposei sout a north-facind n an ho d g cliff alon a faulg t trending east-northeast s possibli t I . e thae relativelth t y young a dat s i e result of loss of radiogenic lead due to movements along the fault or duo weatheringt e . A recent isotopic study of galena from the Michelin deposit strongly support e interpretatioth s n froe pitchblendth m e dataf o , disturbance of isotopic equilibrium and local redistribution of uranium in occurrences in the southern part of the study area, during Grenville timee samplTh .s froi e a rarem , lenticular aggregatf o e coarse grained calcite, amphibol d galenaan e , approximateln i m c 0 1 y diamete m thickc d upt 5 an r1. o, subparalle o regionat l l foliationn i , mineralized coarse feldspar porphyritic rhyolite. It is located m souta coars 5 f o 1. h e feldspar porphyritic dioritic dyke whics i h approximatel m thicc d trend0 an 2 ky s east-southeast s pari f o t I . the mineralized rhyolite sample SSG-157-'75 describey b d Gandhi (1978, p. 1508-1509; from south wall of drift, 241 m east of crosscut) e isotopiTh . c analysi s performewa s y b Geosped c Consultants Limited (Edmonton, Alberta, Canada), and yielded ratios as follows: Pb2°6/Pb2°": 532.45, Pb2°7/Pb2°":97.16d an 3 Pb2 ° 8/Pb20 "* : 36.726. Because of the overwhelming dominance of uranogenic component in this lead, choice of any reasonable

58 Proterozoic common lead will yield essentiall same th ye Pb207/Pb206 ratio (0.1585e radiogenith r )fo c component. e ratiTh o puts contraint e maximuth d minimun o san m m timf o e formation of galena and of the source pitchblende. The calculated maximum age of the source pitchblende, assuming a continuous accumulatio e radiogenith f no c lea galenn i d a unti e presenth l t time, is 2440 Ma. The calculated maximum age of galena, and hence minimum age of the source pitchblende, assuming that all the radiogenic component was generated and accumulated within a very short time interval, is 1490 Ma. Further constraints are imposed by the known geology, the pitchblende data and the Rb/Sr isochron date of e rhyolitith r 1767±fo ca 4M hos t rocks (Kontak, 1980). Calculations using 1767 Ma as the age of the host rocks as well as of the initial uranium mineralization, whic s supportei h y b theid r geological relations and the pitchblende data, yield 1180 Ma as the time of galena formation. Other calculated pair f dateo s r formatiofo s f o n the source pitchblende galenth f ao respectiveld an e y are: 175a 0M e youngean Th d 113d 180 an 120an . r8 0a Ma , M 1dateMa n thesi s e pairs are in approximate time range of 1200 to 1000 Ma generally accepte r Grenvilliafo d n orogeny. Thee thu generan ar yi s l agreement wite tim f th episodiho e c los f radiogenio s c lead from pitchblende indicate e concordith y b d a plot.

Genesis The main features of uranium mineralization in the Aillik Group are:

e pitchblend th Mos) (i f o t s fineli e y disseminated, (ii) High-grade veinlets and fracture fillings that occur at and close to the zones of disseminated pitchblende appear youngee b o t r than d mosan , t probably resulted from local remobilization and concentration of originally disseminated pitchblende. (iiie disseminateTh ) d mineralization predate least a dlase th tt phas f o Hudsoniae n deformatio d an metamorphisn m (Gandhi, 1978). f o pitchblend e e disseminateag th e n (ivi Th e) d zones i s approximately 1750 Ma, and corresponds closely with the oldest Rb/Sr isochron dat obtainea f 176e M eo th 4 7 ± r fo d upper Aillik group rhyolites (Kontak, 1980). (v) Disseminated pitchblend n i stratifor s i e m layerd an s lenses that occur within relatively thin stratigraphie

59 zonesn argillaceous-tuffaceoua n i ; é sKitts th uni n i -t Post Hil ldominantln i belt d an , y felsic tuffaceous units e rhyolitiith n c sequenc e uppeth n ri e Aillik group, (vi) The favourable stratigraphie zones are characterized by rapid lithofacies variations, (vii) Rhyolitic host rocks are strongly enriched in soda and depleted in potash, (viii) Uraniu s preferentialli m y associated with layerd an s lenses ric n graphitei h , pyrite, magnetit d hornblendan e e e argilliteith n d witan , h sodic pyroxene, sodic amphibole and e rhyolitisphenth n i e c rocks, (ix) Disseminated molybdenite, pyrite and fluorite occur in e disseminateth som f o e d pitchblend ee nortzoneth n hi s but are scarce or absent elsewhere, (x) Thorium and rare earths do not increase with uranium in the mineralized zones, (xi) Finely disseminated hematite is common in the mineralized zones, except for those in the argillite. (xii) Isotopic equilibrium in occurrences to the south near the 'Grenville Front1 was affected by Grenvillian events approximately 115 a 0M ago, which caused local redistribution of uranium and formation of rare galena rich in uranogenic component.

These features, in particular the close spatial and temporal relationships of the rhyolitic rocks and uranium mineralization, invite linking genetically the mineralization to the felsic volcanic activity tha s extensive uppewa t th e tim rth f o e Ailli t a e k group. A genetic model proposed by Gandhi (1978) and modified slightly here, invoke ) syngeneti(a s c depositio f o uraniun m derived from felsic volcanic y meteorib s c waters n euxinii , c basine son like th e where hos e Kitts-Post th bed f o s t Hill belt were depositedd an , (b) epigenetic mineralizatio e rhyolitith n i n c rocks elsewhery b e fluids generated at depth in the magma that gave rise to the rhyolites. The thick rhyolitic sequence is believed to be subareal in par t leasta t , froe presencth m f unito e s interprete s air-fala d l tuft d agglomeratesan s , ignimbrite d laharsan s . (a) Influx of felsic tuffaceous material at the time of depositio argillaceous-tuffaceoue th f no s hos Kitts-Pose t th uni n i t t Hill bel , tmark 4) e beginnine d th (Figsfelsisth an 3 .f c o g volcanism that produced a source of uranium-enriched sediments as well as of uranium in solution in meteoric waters. Strongly reducing

60 environment prevaile e basith n par i nf d o twher e argillits wa e deposite s indicatea d y sulphideb d d favoure d an graphit an s, it dn i e precipitatio f uraniuo n m froe meteorith m c waters that enteree th d basin during sedimentation and diagenesis. Reducing conditions adequate to precipitate uranium in the same fashion, extended most probably where interbedded mafic and felsic tuffaceous, and calcareous e sedimentth host f o unis t were deposited. Such conditions may also have prevailed in some lakes during the subaerial volcanism, and may have led to concentration of uranium in lacustrine sediments (Sherborne et al., 1979; Curtis, 1981). Mafic tuffaceous-calcareous host bed f restricteo s d e Rainboextenth t a tw zony represenema t suc n environmenta h . (b) Uranium mineralization in the rhyolitic rocks on the other hand was brought about by reaction of mineralizing magmatic fluids with the host rocks (Gandhi, 1978). The fluids were enriched in sodium, uranium d volatilean , n particulai s . SucCl r d COhan 2F , fluids were probably generated throughout the felsic volcanic cycle, but may have culminated during the waning stages of volcanism. They migrated through permeable avenuee sequenceth n i s , viz. non-welded tuffs and pyroclastic rocks, devitrified lavas, zones of frequent interlensin f variouo g s lithologica lf shearo unit d d faultan an s s due to incipient structural disturbances prior to major deformation. They reacted with the wall rocks, and brought about soda metasomatism and uranium mineralization in favourable zones. Uranium was precipitate y reactivb d e titanium d iron-bearinan - g minerals, namely sphene, pyroxene and hornblende. The precipitation was most probably aide y f boilinb volatileso d f of g . Original distributioe th f o n reactive minerals in the host rocks as disseminations and along certain layer d lensesan s s largeli , y responsibl e observeth r fo ed distributio f o pitchblenden . Molybdenite-pyrite-fluorite mineralization may be synchronous with the uranium mineralization or may represen a separatt e stag f evolvino e g magmatic fluidss It . increasing abundance to the north suggests a regional metallogenic zoning. Studies on Tertiary rhyolitic ash and lava flows in the western United States and experimental work show that heated, mildly alkaline oxygenated meteoric waters play an important role in leaching of uranium from devitrifying rhyolitic source rocks and precipitating it at favourable site o t fors m deposits (Roshol t al.e t , 1971; Zielinski, 1978, 1979, 1981; Zielinski et al., 1977). In case of the upper Aillik group, differing views have been expressed regarding relative importance of meteoric waters and juvenile fluids from the

61 volcano-plutonic complex, in bringing about the observed uranium mineralization. Gandhi (1978) regarde t unlikeli d y that mineraliza- e rhyolitith tion i n c rock s broughwa s t about entirele th y b y meteoric waters without any magmatic input, in view of the intense soda metasomatism which resembles fenitization, and the mineralogy, textur d geologicaan e l e mineralizesettinth f o g d zonesw Lo . temperature hydrous mineral e notablar s y t i seemabsent d san , unlikely that evidenc f theio e r presenc s completelwa e y obliterated by later metamorphism. Kontak (1980) favoured a combined role of meteoric waters that leached uranium from glass shard d felsian s c tuffs, and the volatile-rich fluids emanating from vents. White and Martin (1980) visualized convection cells of mildly alkaline aqueous fluid f meteorio s c origin near intrusive plug d arounan s d inferred subjacent plutons relate o volcanismt d . . Gowe(1982al t e r ) also postulated similar convection cells, but regarded the upper Aillik group as resting unconformably over deformed lower Aillik group at the tim f mineralizationo e . Evans e othe(1980th rn o ) hand invoked w temperaturelo onle th y , neutra o weaklt l y alkaline, oxidizing goundwaters of meteoric and/or of connate marine origin, for leaching f uraniuo s wela ms zirconiua l m froe felsith m c volcanic source rocks under diagenetic to zeolite faciès pressure-temperature conditions. The metals, according to him, were transported and also precipitated at low temperature (zeolite faciès), and redox changes and adsorption on Fe-Mn-Ti hydroxides played an important role in the precipitation. In contrast, Whit d Martian e n (ibid) emphasize a higd h temperaturf o e precipitation of uranium, above the upper limits of the greenschist faciès, implied by desilication of the Na-metasomatized host rocks (now albitites). Post-mineralization deformatio d metamorphisan n m makt i e difficult to evaluate the influence of structure, in particular shearing e transporth n i ,d depositioan t f uraniumo n . Marten (1977) and Evans (1980) postulated a premineralization deformation in the Kitts-Post Hill belt, that produced permeable shear zones in the host unit, and derivation of uranium from the overlying felsic tuffaceous bed y solutionb s s that eventually e sheadepositeth n ri zonest i d . It seems improbable, however, thapostulatethe t d favourable shear zones were restricte e incompetenth o t d t argillaceou d tuffaceouan s s host beds and were parallel to the bedding, and that such shear zones were not developed in the underlying and overlying units of the Aillik Group. Marten (ibid) furthermore visualize e mineralizinth d g f solutionmetamorphio e b o t s c origin, rather tha f magmatio n r o c meteoric origi mixtura e two r th o n. f o eTexture mineralizee th f o s d

62 zones however show that the growth of metamorphic minerals like hornblende effectively protecte pre-existine th d g uraniue hosth tn i m rock. This featur s i inconsistene t e withypothesith h f o s metamorphic remobilization of uranium from the source rocks.

CONCLUSIONS Uranium in the Aillik Group is genetically related to the felsic volcanic rocks thae thic d ar textensive an k d predominanan , t over e basaltuppeth e grouprn e i th bimodalsTh par f .o t volcanic sequence, with interbedded clastic sedimentary rocks, was deposited a icontinentan l rift environment. Circulatio f mineralizino n g fluid f eitheo s r magmati r meteorio c r mixeo c d origin, througe th h rhyolitic rocks, som f whico ee subaerialar h , either introducer o d redistributed uranium in permeable zones at the time of volcanism or soon after it, but prior to major Hudsonian deformation and metamorphism. Some uraniu s transportewa m y meteorib d c watern a o t s euxinic basin at the beginning of the felsic volcanism where it was deposited syngeneticall r durino y g diagenesi n argillaceoui s s beds n ani interbedded d mafid felsian c c tuffaceou d calcareouan s s sediments. Isotopic data support the genetic hypothesis, and indicate the tim f initiao e l uranium mineralizatio n 180i n 0o 175t 0a rangeM . They also revea a disturbancl f o isotopie c equilibriue th n i m occurrence e soutth ho t s nea e "Grenvillth r e Front1 approximately 1150 Ma ago.

Acknowledgements Brinco Mining Limited kindly permitted inclusio f unpublisheo n d information presented here. o R.Tt e .. Ruzicke ThankGeologicadu V th Bele d f ar an slo a l Survey of Canada, Ottawa, for critical review of the paper and valuable comments. Discussions with R.T. Bell, S.M. Roscoe and R.I . e GeologicaThorpth f o e l Surve f Canado y a resultea n i d considerable improvement of the manuscript. The opinions expressed here, however writer'e ar , s own.

REFERENCES Bailey, D.G., 1979: Geology of the Walker Lake — MacLean Lake area, 13K/9, 13J/12, Central Mineral Belt Labrador; Newfoundland Departmen f Mineo td Energy an s , Mineral Development Division, Report 78-3, 17 p.

63 Barua, M.C., 1969: Geolog f uranium-molybdenum-beao y g rockrin f o s the Aillik-Makkovi y Ba areak , Labrador; Unpublished M.Sc. thesis, Queen's University, Kingston, Ontario, 76 p. Beavan, A.P., 1958: The Labrador uranium area; Proceedings of the Geological Associatio . 137-145 p f Canada o n, 10 . v , Burbidge, G.H., 1977: A metamorphosed rhyolite breccia in the Aillik Group near Kaipokok Bay, Labrador; Unpublished B.Sc. (Rons.) thesis, Queen's University, Kingston, Ontario. p 4 1 , Clark, A.M.S., 1971: Structure and lithology of a part of the Aillik Series, Labrador; Proceeding e Geologicath f o s l Associatiof o n Canada, v. 24, p. 103-106. ______1973 A :reinterpretatio e stratigraphth f o n d an y deformatio e Aillith f ko n Group, Makkovik, Labrador; Unpublished Ph.D. thesis, Memorial University . John'sSt , , Newfoundland, 346 p. ______1979: Proterozoic deformation and igneous intrusions e iMakkovinth par f o t k Subprovince, Labrador; Precambrain . Research95-114p , 10 . . v , Curtis, L., 1981: Uranium in volcanic and volcaniclastic rocks — examples from Canada, Australia and Italy; American Association of Petroleum Geologists, Studies in Geology, No. 13, p. 55-62. Evans, D., 1980: Geology and petrochemistry of the Kitts and Michelin uranium deposits and related prospects, Central Mineral Belt, Labrador; Unpublished Ph.D. thesis, Queen's University, Kingston, Ontario, 312 p. Gandhi, S.S., 1967 :A pétrographi e stud f 196o y 6 drill core from Cape Makkovik area, Labrador; British Newfoundland Exploration Limited, Unpublished document G-67011. p 6 ,2 ______1968: Report on exploration in Aillik-Makkovik area, Labrador, during 1967: Brinex/Cominco joint venture; British Newfoundland Exploration Limited, Unpublished document G-68009. p 5 4 , ______1969: General features of mineralization in the Round Pond area, Aillik-Makkovik area, Labrador; British Newfoundland Exploration Limited, Unpublished document G-69054. p 2 ,1 ______1970: Geolog d uraniuan y m occurrencee th f o s g RiveKaipokoBi — r y areaBa k , Labrador; British Newfoundland Exploration Limited, Unpublished document G-70004. p 3 3 ,

64 Gandhi, S.S., 1976: Geology, isotopic aged origian s f uraniuo n m occurrences of Kaipokok Bay — Big River area, Labrador; British Newfoundland Exploration Limited . Unpublishe, d document G-76001, 25 p. ______1977: The relationship of stratigraphy and uranium mineralizatioe th Kitt n si n area, Labrador; British Newfoundland Exploration Limited, Unpublished document G-76013, 73 p. ______1978: Geological settin d genetian g c aspectf o s uranium occurrences in the Kaipokok Bay - Big River area, Labrador; Economic Geology, v. 73, p. 1492-1522. Gandhi, S.S., Grasty, R.L. d Grieve,an , R.A.F., 1969 e geologTh : y and geochronology of the Makkovik Bay area, Labrador; Canadian Journal of Earth Sciences, v. 6, p. 1019-1034. Gill, F.D., 1966: Petrograph f molybdenite-bearino y g gneisses, Makkovik area, Labrador (abstract); Canadian Mining Journal, v. 89, p. 100-101. Gower, C.F., Flanagan, M.J. ,d Bailey an Kerr , ,A. , D.G., 1982: Geology of the Kaipokok Bay - Big River area, Central Mineral Belt, Labrador; Newfoundland Department of Mines and Energy, Mineral Development Division, Report 82-7, 77 p. Harder, D.G., 1981: Detailed exploration at Punch Lake South Prospect, Are , LabradoraA ; Brinco Mining Limited, Unpublished document G-81009, 28 p. Hughson, M.R., 1958a: Mineralogical repor a uraniu n o te sampl or m e ('K-2') from British Newfoundland Exploration Limited; Kitts project, Makkovik area, Labrador, Newfoundland; Canada Dept. Mines Tech. Surveys, Mines Branch, Ottawa, Report IR-58-54, 9 p. ______1958b: Mineralogical repor uraniua n o te sampl or m e {'K-4') from British Newfoundland Exploration Limited; Kitts project, Makkovik area, Labrador, Newfoundland; Canada Dept. Mines Tech. Surveys, Mines Branch, Ottawa, Report IR-58-94, 6 p. ______1958c: Mineralogical report on a uranium ore sample ('K-5') from British Newfoundland Exploration Limited; Kitts project, Makkovik area, Labrador, Newfoundland: Canada Dept. Mines Tech. Surveys, Mines Branch, Ottawa, Report IR-58-162, 5 p.

65 King, A.F., 1963: Geologe Capth e f Makkovio y k peninsula, Aillik, Labrador. Unpublished M.Sc. thesis, Memorial University, St. John's, Newfoundland, 114 p. Kontak, D.J., 1980: Geology geochronology and uranium mineralization in Central Mineral Belt of Labrador, Canada; Unpublished M.Sc. thesis, Universit f Albertao y , Edmonton, Alberta. p 8 ,37 Lever, T. and Davidson, G., 1978: Reconnaissance geology and prospectin Albere th f to g Lake area, including detailed geology B prospect & J e oth f, Are Labrador, B a ; Brinco Mining Limited, Unpublished document G-78025. p 3 1 , Marten, B.E., 1977: The relationship between the Aillik Group and the Hopedale gneiss, Kaipokok Bay, Labrador; Unpublished Ph.D. thesis, Memorial University . John'sSt , , Newfoundland, 389 p. Minatidis, D.G., 1976 A comparativ: e stud f o tracy e element geochemistr d mineralogan y f o somy e uranium depositf o s Labrador, and evaluation of some uranium exploration techniques in glacial terrain; Unpublished M.Sc. thesis, Memorial University . John'sSt , , Newfoundland. p 6 21 , Morse, S.A., 1961: Pétrographi ee Ailli th note n ko s molybdenite showing, Labrador (NTS 130/3); British Newfoundland Exploration Limited, Unpublished document G-61003, 4 p. Rosholt, J.N., Prijana and Noble, D.C., 1971: Mobility of uranium and thorium in glassy and crystallized silicic volcanic rocks; Economic Geology, v. 66, p. 1061-1069. Ryan, A.B d Kay an ., 1982,A. : Basement-cover relationshipd an s plutoni e c Makkovith rock n i sk sub-province, nortf o h Postville, coastal Labrador (13J/13, 130/4) n i Curren; t Researc r 1981fo h , Edite C.Fy b d . O'Driscol R.Vd an l . Gibbons, Newfoundland Department of Mines and Energy, Mineral Development Division, Report 82-1, p. 109-121. Sherborne, J.E. Jr., Buckovic, W.A., Dewitt, D.B., Hellinger, T.S., and Pavlak, S.J., 1979: Major uranium discovery in volcaniclastic sediments, Basi d Rangan n e Province, Yavapai county, Arizona; American Associatio f Petroleuo n m Geologists Bulletin, v. 63, p. 621-646. Stevenson, I.M., 1970: Rigole d p Groswateareaan ma t y , Ba r Newfoundland (Labrador); Geological Survey of Canada, Paper 69-48, 24 p.

66 Wardle, R.J. Baileyd an , , D.G., 1981: Early Proterozoic sequencen si Labrador; in. Proterozoic Basins in Canada, P.H.A. Campbell, éditer; Geological Surve Canadaf o y , Paper 81-10 . 331-359,p . Watson-White, M.V., 1976 A petrologica: l stud f acio y d volcanic e Aillirockth n par i sf k o t Series, Labrador; Unpublished M.Sc. Thesis, McGill University, Montreal, Quebec. p 2 ,9 White, M.V.W., and Martin, R.F., 1980: The metasomatic changes that accompany uranium mineralization in the nonorogenic rhyolites e Uppeth rf o Aillik Group, Labrador; Canadian Mineralogist, v. 18, p. 459-479. Willy, A.J., 1981: Initial diamond drilling on the Anna Lake North Prospect, Summer 1981, Extended Licence No. 1524 BX, Area B, Labrador; Brinco Mining Limited, Unpublished document G-81008, 61 p. Willy, A.J., 1982: Detailed exploratio Maie Soutd th an n n hno Zones, Emben (Matthew Ben) Showings, Area A, Labrador; Brinco Mining Limited, Unpublished document G-82003. p 4 4 , Zielinski, R.A., 1978: Uranium abundanc d distributioan e n i n associated glassy and crystalline rhyolites of the western United States; Geological Society of America, Bulletin, v. 89, p. 409-414. ______1979: Uranium mobility during interactiof o n rhyolitic obsidian, perlite and felsite with alkaline carbonate 0 kg/cm21 = 2;P Chemica , C solutions0 12 l = Geology T : , v. 27, p. 47-63. ______1981: Experimental leaching of volcanic glass: implications for evaluation of glassy volcanic rocks as sources of uranium; American Association of Petroleum Geologists, Studie . 1-12p Geology n , i s .13 . ,No Zielinski R.A., Lipman, P.W d Millardan . , H.T. Jr., 1977: Minor element abundance n obsidiani s , perlit d felsitan e f calco e - alkaline rhyolites; American Mineralogist . 426-437p , 62 . v ,

67 THE OLYMPIC DAM COPPER-URANIUM-GOLD DEPOSIT, ROXBY DOWNS, SOUTH AUSTRALIA

D.E. ROBERTS, G.R.T. HUDSON Roxby Management Services Proprietary Ltd, Unley, SA, Australia

Abstract

The Olympi copper-uranium-golm Da c d deposi Soutn i t h Australi discoveres awa d n areaina s 1975l ha exten t I . t exceeding 20km^ with vertical thicknessef o s mineralization up to 350 metres. The deposit is estimated to contain in excess of 2,000 million tonnes of mineralized material with an average grade of 1.6 percent copper, 0.06 percent uranium oxides, and 0.6gm per tonne gold. The unmetamorphosed coarse clastic Proterozoic sediments hostine th g mineralization are probably no older than 1580 Ma and appear to fill a graben in granitic rocks beneath 350 f unmineralizedmo , flat lying Adelaidea Cambriao nt n sediments. Disseminated strata-bound bornite-chalcopyrite-pyrit e dominanth s i e t typf o e sulfide mineralization with lesser amount f o younges r transgressive chalcocite-bornite. Uraninite, with lesser coffinit branneritd an e e occurs wite hth sulphide mineralizatio a variet n i n f texturao y l form rard an se earths minerals bastnaesite and florencite occur in, and adjacent to sulphide mineralized zones. Gold and silver are a minor but significant component of the mineralization.

INTRODUCTION

The Olympi copper-uranium-golm Da c d deposit constitute a smajo r resourcf eo copper and uranium with an areal extent exceeding 20km^ and vertical thickness of mineralization measure tenn i d o hundredt s f metreo s s (Robert Hudsond an s , 1983). The deposit contains at least 2,000 million tonnes of mineralized material wit averagn ha e percen6 grad1. f eo t copper, 0.06 percent uranium oxide (U3Og), ang/tonn6 0. d e gold. Higher grade mineralizatio s beeha nn estimate0 45 t a d million tonnes of 2.5% Cu, 0.08% UgOg and 0.6gm/tonne Au. In addition significant concentration f raro s e earth silved san r occur deposie Th . locates i t d on Roxby Downs pastoral station, 650 kilometres north-northwest of Adelaide and 25 kilometres west of the opal mining town of Andamooka (Fig. 1).

69 Pi PEAKE AND C JOENISON RANGES

Lowe o Middll r e Proleroioi~ * \ c

Fig. 1. Geologic map of the southern part of South Australia showing the main lithologie <3c tectonic features and the location of the Olympic Dam deposit.

Exploratio r coppenfo Westery b r n Mining Corporation Limited which resulten di e discoverth e deposith s focusef o ywa t n Souto d h Australi n 197i a 2 following research durin e perioth g d 196 9o 197t 1 which relate e formatiodth f stratao n - bound copper deposit o solutiont s s thad aquireha t d copper during oxidative alteration of basalts (Haynes, 1972). In the Stuart Shelf region, regional gravity d magnetian c data resulting from e surveyCommonwealtth y b s h Bureaf o u Mineral Resource e Soutth d h an sAuscralia n Departmen f Mineo t d Energan s y suggeste e possibilitdth f basaltyo interpretatioe t th deptsa d han f gravitno d yan magnetic anomalies lineamend an , t mapping resulte e selectioth n di f specifino c target areas, including the Olympic Dam area (Fig. 2). In 1974 a commitment was mad drilo t e l stratigraphi e selecteth f o edo holetargettw e n o sth o tesr t s fo t presence of favourable host rocks in the Upper Proterozoic cover sequence, and source rocks in the basement. The first hole, RDI in the Olympic Dam area, was completed in July 1975 after intersectin metre8 g3 f 1.0so 5 percent coppebasemene th n 3 i rdepta 35 t a f t ho metres. This encouraging, although uneconomic, mineralizatio drilline th o t g d nle

70 Fig. 2. Positive gravity and magnetic anomalies associated with the Olympic Dam deposit. Magnetic contours in nanoteslas; gravity in milliGals; H = centre of anomaly; RDI = discovery hole.

oa patterf wer 4 e areaf RD holefirse n o th barren d Th n ts.i an 5 two3 RD , RD , intersecte8 anRD d d additional mineralization containin perceng1 t copper6 RD , gradewer7 w anlo eRD d9 .barren RD RD1 d an ,0 intersecte metre0 1 d17 2. f o s percent copper and over 0.05 percent uranium oxide and provided the first indication of significant uranium mineralization and the great potential of the area that has been confirmed by subsequent drilling. The gravity anomaly whic attributes hwa shalloa o dt w basemen f basaltio t c rock is now known to result from the dense hematite-rich breccias associated with the mineralization magnetie Th . c anomal stils yi l largely unexplaine modellind dan g indicate t i results s fro a mlarg e magnetic body beyon e currenth d t deptf o h drilling. Hos te typ rockth f strata-boun eo r fo s d copper deposit predictee th n di original concep e t areoccumineralizatiod th no a an n o i rd t confines ni rocko dt s beneat Uppee hth r Proterozoic sedimentary sequences. Detailed assessmen e prospecth f o t t commence n 197a i joind s 9a t venture between Roxby Mining Corporation wholla , y owned subsidiar f Westeryo n Mining Corporation Limited, BP Australia Limited and BP Petroleuni Development

71 TABLE 1

TYPICAL Cu-U MINERALIZED DRILL HOLE INTERSECTIONS

Drill Hole Interval Thickness Cu U,0n Au / i* " \ (m) (m) (%) (kg/tonne) (gm/tonne)

RDI 1 3539 3- 38 1.05 0.10 -

RD10 348 - 518 170 2.12 0.59 -

RD19 422 - 436 14 2.00 tr 15.4 684 - 758 74 3.04 0.45

RD20 0 3658 9- 211 2.10 0.80 -

RD24 9 3464 3- 306 1.00 0.20 811 - 1003 192 0.60 0.16 - 100 - 1123 8 125 1.10 0.33

RD39 4 3641 8- 146 3.25 0.68

Location of drill holes indicated in Figure 3.

o <0.5% Cu OR 20m% - 100m0 2 %• • 10 0- 200m % 200 - 500m% 500m%

WHENAN SHAFT

N 0 00 0 20 î

1- .k m \J W ». _.. —————j Fig . 3 Locatio. f diamonno d drill holes showing greater tha percen5 n0. t copper intersections Table intersectior fo Se . e1 numberen ni d drill holes.

72 Limited. Intensive vertical diamond drilling ove aren a r excesn squarai 5 2 f so e kilometre resultes sha succesa n di s rat ovef e o percen 0 8 r t mineralized drill holes Whenae (Fig , TablTh 3 . . ne1) Exploration Shaf bees ha tn complete 500o dt d man underground exploratio d bulan nk samplin n progressi s i l gobservations Al . , interpretation speculationd san s presente thin di sdat n papeo ae froar r m diamond drilling.

REGIONAL GEOLOGICAL SETTING

The Olympic Dam deposit occurs within the Stuart 'Shelf Region (Fig. 1), an area of flat-lying Adelaidea d Cambriaan n n sediments separated froe Adelaidmth e Geosyncline to the east by a major zone of north-south faulting termed the Torrens Hinge Zone by Thomson (1969), (Preiss et al., 1980). To the west and southwest sedimene th , t e Stuarcoveth f o rt Shelf laps onto exposed e rockth f so Gawler Craton (Rutland et al., 1980). The basement to the Stuart Shelf is considere consiso dt f Gawleo t r domai withis i n t i rock nd thian s s basement that e Olympith deposim Da c t occurs correlatioo N . n betwee e Olympith n m Da c succession and specific units of the tectonically composite Gawler domain can be made at this stage although regional stratigraphie data suggest that the Olympic Dam sequence is undeformed Middle Proterozoic sequence. The deposit straddles a west-northwest trending photolineament corrido intersectioe th t a r majoa f no r north-northwest trending gravity lineament (O'Driscol d Keenihanan l , 1980; O'Driscoll, 1982). Early Proterozoic metasediments and reworked Archaean gneisses of the Gawler Craton (Fig. 1) were strongly deformed and intruded by granite as a result of the Kimban Orogeny (1820 to about 1580 Ma; Webb et. al., 1982). Voluminous subaerial felsic volcanics (The Gawler Range Volcanics e relativelar ) y flat lying and form a large basin-like feature in the approximate centre of the Gawler domain, surrounded by post tectonic granites (Fig. 1). The volcanics and associated granites lack penetrative deformatio havd an ne ages between about 1525 and 1450 Ma (Webb et. al., 1982). The Adelaide Geosynclin e easth t o t econtain a s10k m thick sequenc f Uppeeo r Proterozoi d Cambriaan c n sediment d minoan s r basic volcanics e BedTh a. Volcanics which e soutoccuth n hi r easter ne Stuar e th par th f o tn i Sheld an f Geosyncline have been date t 107da (Weba 6 M Coatesd ban , 1980) rocke Th f .so e Geosynclinth e displa deformatioya markea M 0 d50 granitd nan e intrusion event, the Delamerian Orogeny but those of the Stuart Shelf are unaffected by the 500 Ma event.

73 In the Olympic Dam area about 350 metres of cover sediments are separated from e underlyinth g basement rock a majo y b s r unconformity numbeA . f prospecto r s containing sub-economic coppe r copper-uraniuo r m mineralization have been identified outside the Olympic Dam area by drilling of geophysical anomalies. The styl f mineralizatioo e n somi n f theso e s veri e y broadly simila o that r t under discussion.

GEOLOG OLYMPIE TH DEPOSIF M YO CDA T

Stratigraphy The host sequence for the Olympic Dam mineralization is un metamorphosed mediu o t coarse-grainedm , potassium feldspar-rich, alkali feldspar granite surrounding sedimentary breccias or rudites (Roberts and Hudson, 1983). Dolerite intrudes the granite and breccias, but not the cover sediments. Breccias have been classified intbroao otw d groupe basith f clasn o s so t typd ean matrix content: (1) Monomict: one clast type present. Matrix content usually less tha percentPolymict0 ) n1 (2 r moro d o an ;e tw :clas t type varioun si s proportions and matrix content 10 to 80 percent, usually greater than 30 percent. The polymict grou s i furthep r divided into three sub-groups e clason ) t (1 typ: e predominant (greater tha percen0 8 n clasf o clasts)o t tw t) type(2 , s predominant (about 40 percent each), and (3) three or more clast types with none predominant. The two clast and three or more clast groups contain most of the significant mineralization.

A large number of different clast types occur in the polymict breccias including altered basement granite; felsic, intermediat mafid an e c volcanics; various types of hematite-rich rocks including banded iron formation and hematitic siltstone; arenite; carbonate; fluorite; barite and sulfides (Fig. 7). Reworked clasts of breccias are common, particularly towards the top of the sequence. Clast sizes . cm 3 range o th t n e1 i t mos e bu ar t m 12 rango t p eu Eight different types of matrix are defined on the basis of composition and colour. Crystalline blac gred kan y hematite, granular brown hematit earthd ean y red hematit e maith ne matriar e x type t sericitebu s , chlorit siderite-ricd an e h type e alsar so observed. Arkosic matrice alse sar o commo somn ni e e partth f so deposit. The grain size of the matrix material is 0.05mm to 2mm and it is generall yfine-grainea d equivalen claste th f so t presen brecciae th n i t . Siltd yan clayey matrices are uncommon. The sediments have been divided o intmaitw on lithostratigraphic unitse th , Olympic Dam Formation and the Greenfield Formation. Both formations have

74 I A | Dolerite

GREENFIELD FORMATION Unconformity MEMBERS /vvvvvvvv V V V [w] Volcanic V

[Tj Hematite Breccia

[sj Lower Silicified

OLYMPIC DAM FORMATION MEMBERS Upper Granite D Breccia

[UT| Brooks

[D| Whenan

[ 7 Blac| k Hematite

Lower Granite D Breccia

] [T Alkali Granite

Fig. 4 Diagrammati. c composite stratigraphie sectio f pre-Adelaideano n units. Lithologies are summarized in Table 2. Approximate vertical extent is 1,500 to 2,000m.

Upp*r Grinlt« Br«cel« Member GREENFIELD FORMATION m OLYMPIC OAM FORMATION 1 Brooh«-Wh»nan-BI»c I k Hcmallt* M«mb«r>

ALKALI GRANITE LU Sldvrlt« rich zon« Fig . Generalize5 . d geologi cOlympie plath deposi f -450nm e o th cDa t ma t level.

75 TABLE 2

OLYMPIC DAM STRATIGRAPHY

Unit Thickness Lithology (max.)

Andamooka Limestone 60m Massive, locally bedded limestone.

Wilpena Group

Arcoona Quartzite 200m White & red massive & cross bedded sandstone.

Corraberra Sandstone 20m Red, massive & cross bedded sandstone.

Tregolana Shale 200m Thinly laminate gree. & d nd re shales.

- - UNCONFORMITY ------

Greenfield Formation

Volcanic Member 250m Interbedded altered felsic volcanics, volcanic breccias, conglomerates & banded iron formation.

Hematite Breccia Member 500m Hematite rich breccias with thin interbeds of polymict breccias & hematite siltstones. Veine silicy dbariteb d aan .

Lower Silicified Member 275m Strongly hematized & sihcified polymict breccias dominated by quartz, hematite & altered volcanics.

Olympic Dam Formation

Upper Granite Breccia 1000m Granite breccia & weakly polymict granite Member breccias, matrix-poor. Sencite & hematite altered. Chalcocite-bormte mineralized zones.

Brooks Member 300m Polymict breccias wit hvarieta f clasyo t types - sencitized felsic volcanic clasts diagnostic. Discrete felsic volcanic lenses. Variable matrix of brown and black hematite. Reworked clasts of underlying members. Variably mineralized with bornite (chalcocite), & chalcopynte. Sidente clasts locally abundant.

Whenan Member 350m Polymict breccias, red & black hematite clast matricess& . Cyclic zonef so matrix-rich & matrix-poor breccias. Bedded & graded units common. Strongly mineralized with bormte-chalcopynte (pyrite).

Black Hematite Member 150m Black hematite dominated breccias bots ha clas matrix& t . Specular hematite, chlonte & siderite in matrix. Porous, often vuggy. Pyrite (chalcopynte) mineralized.

Lower Granite Breccia 800m+ Granite breccia with thin polymict breccia Member lenses often containing distinctive chlonte altered volcanic clasts. Chlonte altered contact with above member & granophyre clasts.

76 WEST EAST

GREENFIELD FORMATION OLYMPI FORMATIOM CDA N MEMBERS MEMBERS

J Volcani[V COVERc O Upper Granite Breccia

DOLERIT ] Hematit[A E e Breccia ] Brook(V s \~] Lower Silicified E3 Whenan

•~~~ Unconformity 0 Black Hematite Fault FT] Lower Granite Breccia

Fig . East-wes6 . t cross sectio e Olympith deposif nm o cDa t 200,OOOa t N (A-An 1i Fig. 5) .

several members. Contacts between the formations and members within formations are often disconformable. Drilling has not penetrated the base of the Olympi Formationm cDa . A composite stratigraphie sectio basemene th f no t unit illustrates si Figurn di , e4 their broad distribution in plan at the -450m level, and in section at 200,OOON is shown in Figs. 5 and 6. Table 2 summarises their lithology.

Olympi Formatiom cDa n e OlympiTh Formatiom Da c n excesi s i nf 1000 o s m thic d comprisean k s five member s indicatesa Tabln di (Roberte2 Hudsond san , 1983). Lower Granite Breccia Member: This member consists of predominantly matrix-poor breccias containing basement granite clasts with rare, thin lenses of pyrite and hematite-rich polymict breccias. The breccias show varying degree f alteratioo s sericiteo t n , hematit chlorited an e . Chlorite alteration occur t bota se uppe th d loweh an r r contact e polymicth f o s t breccia lenses and is particularly well developed below the Black Hematite Member somn I . e areas microgranite dykes intrude thit s no membe o d t bu r extend intoverlyine oth g members.

77 c

Fig.7. Photographs (A-D) of cut sections of drill core and photomicrographs (E and F) of stratabound mineralization. A. Polymict breccia of granite (G), hematite (H), fluorite (black), and chalcopyrite (cp) with disseminated chalcopyrite and fluorite in the matrix. Cross-cutting fluorite vein. Note gross bedding; top = right. B. Thin graded-bedded units in Whenan Member. Note gradin f chalcopyritgo e (white), micro-faultin load gan d cast around granite clast (G), top = right. C. Clastic and disseminated chalcopyrite (white) in matrix-rich breccia. D. Disseminated and clastic bornite (bn) in red-brown hematite matrix- rich breccia with altered granite clast (G). E. Chalcopyrite (cp) with pyrite (py), intergrown with hematite laths. F. Bornite (bn) with fine-grained annular zones of hematite, and chalcopyrite (cp) in hematite with quartz clasts and a zoned intergrowt f uraninite-bornitho e (u).

78 Black Hematite Member : basae Thith s lsi membe e maith nf o rmineralize d zone t containI . s bed f massivso e unbrecciated hematit wels ea breccias a l s with black crystalline hematite dominant both as clasts and in matrices. Spéculante, fluorite, siderite and chlorite are present in the matrix and the breccias contain thin vugg poroud yan s zones. Pyrite-rich mineralizatios ni characteristi brecciae th f co thin si s member. Whenan Member: This member consistthickm serie15 a )f o st o f thisp o n(u beds of polymict matrix-rich breccias and matrix-poor granite-rich breccias aggregatin a tota o t gl thicknes n excesi s f 200o s mann mi y arease Th . breccia intercalatee sar d with thin bedded silty lense arenitid san c sediment bands hematite Th lowee . th n rei bed dominantls si y e blac thath d n kan i t beds towards the top of the member is reddish brown. Sedimentary features such as graded bedding (Fig. 7B) and upwards-fining beds up to 3 metres thick occur, and series of upwards-fining beds are not uncommon. Load casts, scours and, less commonly, cross-beddin e presentgar . This member hosts most of the chalcopyrite-bornite mineralization in the deposit. Most sulfidee oth f s occue matrix-ricth n i r h beds. Brooks Member: This member is not always developed and granite clasts are more abundan te underlyin thath n ni g Whenan Member e matriTh . x content is variabl s predominantli d an e y brown hematite. Characteristic clastn i s this membe e sericitizear r d felsic volcanic reworked san d Whenan Member and Black Hematite Member clasts. Siderite-rich clasts are also common in some areas. Mineralization is commonly restricted to the matrix-rich zones an s irregularli d y developed. Bornit chalcopyritd an e e dominanth e ar e t sulfides with minor chalcocite in some areas. Discrete lenses of pale green sericitized volcanics are common. Upper Granite Breccia Member: Although lithologically simila Lowee th o t rr Granite Breccia Member, this member is not chloritic, but is usually sericite, hematite and silica altered. Mineralization is restricted to thin (less than 10m thick) lenses, and veins composed of hematite, fluorite, chalcocite and bornite. Thin lenses and dikes of strongly sericitized felsic volcanics are common. Distribution of Members: The five members of the Olympic Dam Formation are not always intersected in drill holes and they display complex faciès relationship mann i s y e northwesterareasth n I . ne deposi th par f e o tth t three middle member e extensivelar s y interbedded wit e Uppeth h r Granite Breccia Member although features of the Black Hematite Member, Whenan Membe d Brookan r s Membe e observed ar e northeasr th n O . te flanth f o k

79 main mineralized zone (Fig. 5) siderite is a major matrix and clast component in the three members and their definition is less clear, particularl e Blacyth k Hematite Member. Elsewhere e Blacth , k Hematite Membe d Whenaan r n Membee separateb y ma rbarrey db n granite-breccia lenses.

Greenfield Formation The Greenfield Formatio o 700t p mu s thic i ncomprised an k e verth s y hematite- rich upper units of the stratigraphie sequence preserved in downfaulted blocks, it has both conformable and disconformable contacts with the Olympic Dam Formation (Fig. 5). The unit has been subdivided into the Silicified Member, the Hematite Breccia e MembeVolcanith d an rc Member e SilicifieTh . d Member consists of polymict breccias containing clasts of hematite, quartz and volcanics in hematite-rich matrices brecciae Th . strongle sar y Silicifie veined dan silic y db a and barite. The members of the Greenfield Formation are poorly mineralized.

Distribution of Members: The Greenfield Formation is restricted in its distributio e Silicified withith an n , it n d Membee Volcanith d an r c Member e alsar o restricted e SilicifieTh . d Membe s developei r d alon e easterth g n part, and the Volcanic Member appears to be restricted to the northern and eastern parts of the Greenfield Formation block (Fig. 5).

Structure

Locally the sequence hosting the Olympic Dam mineralization is confined by a fault-bounded trough informally called the Olympic Dam Graben. The graben has a probable northwest trend with a length in excess of 7km and a maximum width of e sedimentar5km th e bas f Th o e. y sequenc t beee grabeno th n s n i eha n intersecte e sequencn drillini th d d an g e therefor a minimu s ha e m thicknesf o s 1km. The main structural elements of the graben are shown in Fig. 8. The grabe s archei n d abou a northeast-trendint g axis. Limited evidence also suggests local arching within the graben about north-west axes with the development of small localised domes and basins in some areas (N. Oreskes and W. Brown, pers. comm., 1982). Dip-slip and strike-slip faults occur parallel to both the graben and main arch axis and fault displacement is concentrated on the northeast margie grabenth f o n . Faults roughly paralle e grabeth o t nl axie ar s displaced by those at a high angle to the axis. None of the faults dislocate the cover sediments. The arching, faulting and erosion prior to the deposition of the cover sediments exposed lower member e Olympith f Formatiom o s Da ce th n no

80 \ \ DOLERITE

GREENFIELD FORMATION

LIMIT BROOKS WHENAN BLACK HEMATITE MEMBERS

Fig . Pla8 . n showing main structural element Olympie th grabenf so m cDa . unconformit e vicinit e arche th cresth th n f i yf o t (easo y f Whenao t n Shaft). Upper members of the Olympic Dam Formation are thicker to the southeast and northwest. Remnants of the previously more extensive Greenfield Formation have been preserve downfaulten di d block arche crese th th f .n o tsi Thee th Olympi n i Formatiom ° Da c20 o t e nsouthwes th ° dip 15 o t s t a t southeastern half of the graben. Limited data suggest a flatter north or northeast dip in the northwest half. Local steepening and reversal of dips are observed near faults, synsedimentary slump breccia d locaan s l domes. Widespread slump structures are present in thinly bedded units. Intense t shearinbeeno ns observeha g t intervalbu d f closelo s y jointed an d fractured drillcore, and zones of very intense alteration, could be indicative of faults. Ther e markeear d variations over short lateral distance depte th o n ht si distinctive units within the stratigraphie sequence. Thick lithological units disappea d uppean r r stratigraphi ee adjacen b unit n ca so lowet t r stratigraphie units indicating vertical displacement. Extensive lateral (strike-slip) displacement also occur graben-file th n si l sequence with maximum displacement parallee th o lt graben long-axis mann I . y areas contemporaneous (growth) faults dislocate eth lower member e Olympith t f affecFormatiom o sno e BrookDa co th d t t sbu n

81 Membe Upped an r r Granite Breccia Member. Evidenc f widespreaeo d penetrative deformation and intense folding has not been observed.

Alteration

All rocks in the Olympic Dam Graben are altered to varying degrees. The dominant alteration minerals are hematite, sericite, chlorite, silica and carbonate type.o Ther alteratiof tw so e ear weaa n- k pervasive type affectinl gal distinctrocka d san , more intensive type relate mineralizationo dt .

. 1 Pervasive alteration. Hematite, sericit chloritd ean e alteration affectl al s rock types with dolerite least affected. Localized area f silico sd an a carbonate alteration occur. This styl f alteratioo e s mosi n t apparenn i t breccias with a high granite component where feldspars and ferromagnesian minerals are replaced. 2. Intense alteration. Intense hematite and chlorite alteration are associated with mineralization in the lower parts of the Olympic Dam Formation. Sericite and silica alteration are predominant in the upper members of this formation.

The broad vertical zonatio intensnof e alteratio usegeneraa be das ncan l guideto position in the stratigraphie sequence when distinctive units are lacking.

MINERALIZATION

Copper, uranium, rare earths, gold and silver mineralization occur in the Olympic Dam deposit. The bulk of the potential ore grade mineralization occurs in the Olympic Dam Formation. There is normally a direct relationship between the amount of mineralization and the amount of matrix in the host breccias.

Copper Mineralization Two distinct type f coppeo s r mineralizatio e definee basinar th f sulphido sn o d e assemblage, rock associatio d stratigraphian n e position (Robert d Hudsonan s , 1983)e oldeTh r. mineralizatio s i strata-bounn e youngeth d s largeli an dr y transgressive. Significant uranium, rare earths and gold mineralization is associated with bot typeo he normalltw ar stypese Th .y mutually exclusivt ebu some areas of overlap occur.

Bornite (chaleocite)-chalcopyrite-pyrite strata-bound mineralization: The strata-bound type occurs in the three hematite matrix-rich members of the

82 1000m

Greenfield Formation Block

Transgressive Chalcocite-Bornite

Strata-Bound Bornite-Chalcopyrite-Pyr ite

distributioe . FigPla9 th .f no f sulfidno e mineralization types.

Olympic Dam Formation throughout the graben (Fig. 9), but reaches its maximum thicknes o 350t p m(u s e thicknortth n hi ) east creste zonth f eo main structural arch. Most of the Sulfides occur in the matrix as uniform disseminations (0.1-2mm) that make up 5 to 20 percent of the rock but some e presenar s coarsa t e clasti n sizi ecm (Figfragment5c o . t . 7) p u s Replacemen micro-cavitd an t y fill texture e commo somar sn i d e nan finel y bedded thin layers stratiform sulfide e presentar s . Rock clasts containing disseminated sulfides sulfidd an , e rim clastn so abundane sar somn i t e areas.

The dominant gangue minerals in the strata-bound type are hematite, quartz, sericit fluoritd ean hign ei h grade copper zones. Vertical zoning of sulfides is broadly consiste.it, grading from a relatively sulfur- rich copper-poor assemblage (pyrite(chalcopyrite)) at the base of the Black Hematite Member, to a relatively copper-rich sulfur-poor assemblage (bornite- chalcopyrite) at the top of the Whenan Member, and in the Brooks Member. Supergene processes are not considered to be a major factor in sulphide zonation (Robert Hudsond san , 1983). Minor ore minerals in the strata-bound mineralization are carrollite, cobaltite, native silver, silver tellurid goldd ean .

83 . PhotograpFiglu . photomicrograpd an ) h(A f transgressivo ) h(B e mineralization. A. Disseminate d iensoian d d chalcocite-Dornite (white,) with fluorite (olackn i ) hematite matrix, ß. Complex vermiform intergrowth of chalcocite (white) and Dornite (gray).

Transgressive chalcocite-bornite mineralization: Cross cutting veins and irregular conformable lense f o uranium-bearins g chalcocite-bornite mineralization are found in the Brooks Member and Upper Granite Breccia Member of the Olympic Dam Formation (Fig. 10). This younger mineralization is confined to linear zones parallel to the long axis of the graben with the main zone being at least 6km long and up to 0.7km wide (Fig. 9). Smaller zones are developed to the northeast and southwest of the main trendthickese Th . t intersection f mineralizatioso f thino sn i typ e ear exces f e thicknes300mo s Th . f mineralizeo s d t necessarilzoneno s i s y dependent on the host unit thickness or lithology and there is no consistent

84 relationship between the concentration of sulfides and matrix content as in strata-boune th d mineralization.

The grain size of the sulfides varies from sub-micron inclusions in hematite to massive clasts up to 10 centimetres across, but most are less than 0.5mm. In all textural types the chalcocite and bornite are complexly intergrown (Fig. 10). The dominant gangue constituents are hematite, fluorite and quartz with lesser sericite and chlorite. Mino e mineraor r l constituent e nativar s e golsilverd an d , digenite, covellite, cobaltite copper-nickel-cobala , t arsenat nativd ean e copper.

Uranium Mineralization Three uranium mineral species have been identified pitchblende Th . e varietf yo uraninite (130%) is the most abundant. Coffinite (U(SiO4)1_x(OH)4x) is subordinate o uraninitt e while brannerit (U,Ca,Ce)(Ti,Fe){ e 2C>6) account r onlfo sa y mino r proportion of the uranium mineralization. The grain size of the uranium minerals varies from sub-micro severao nt l millimetre graine t mos th lese sbu f sar o ts than 50 microns. Uraninite and coffinite are intimately associated and occur throughout the mineralized zones with sulfides, sericite, hematite, fluorite and to a lesser extent, chlorite. Their form varies markedly and includes globular aggregates, micro- veinlets penetrating grain boundaries, fracture coatings, grai crystad an n l rims and oriented intergrowths with sulfides (T.R. Peachey pers. comm., 1981, Grey, 1978), (Fig. 11). A close association with bornite occurs in areas of uranium mineral veining. Brannerit restrictes ei s distributioit n di usualld nan y occur Brooke veinn i sth n si s Membe e Olympith Formationf m o r Da cs closel i t I y. associated with rutile (anatase), sericit d hematitan e d usuallan e y occur s patcha s y aggregatef o s irregular grains within sericite or as grain composites with hematite. The brannerite grain e alwayar s s surrounde 0 microns1 y smalo t b d 1 ( )l grainf o s titanium oxide (rutile, anatase). Zircon crystal graind salsan e ar so presene th n i t hal f oxidoo e grains.

Other Mineralization

Rare earths minerals bastnaesite (Ce,La)CO3(F,OH) and florencite (CeAl3(PO4)2(OH)g) and free gold are associated with both types of copper- uranium mineralization.

85 D

Fig. 11. Uranium mineralization, photomicrographs (A-D) and scanning electron micrographs (E, F) of typical mineral textures. A. Coarse-grained uraninite (dark gray) with coffinite (light gray) replacin d surroundinan g g bornite (white). B. Uraninite veinlets (gray) with bornite (white) in quartz. C. Concentrically zoned intergrowt f chalcopyritho e (white), bornite (gray uraninitd an ) e (dark gray) in quartz with hematite and chalcopyrite. D. Uraninite-coffinite (u) surrounding a silicate clast with hematite (white). E. Very fine-grained uraninite (white) rimming bornite. F. Uraninite (white) in bornite; note crystallographic control of veinlet afteF & r E s( Grey , 1978).

86 TABLE 4

IMPOSITIO MINERF NO A Strata-bound Transgress! ve bn-cp-py cc-bn 1 2 3 4

Cu % 1.15 1.72 2.55 2.80

U3°8 440 640 335 440 Au 0.4 0.5 2.4 0.15 Ag 1.0 2.0 7.0 4.0 La 1300 1700 1800 200 Ce 2000 2700 2650 2200 Pb 90 85 30 60 Zn 130 80 60 85 Ni 300 290 280 4 Co 135 120 10 70 Mo 50 48 110 36 As 85 65 130 60 Sb 8 6 4 4 Bi 4 8 4 4 Te 4 10 10 10 Hg 0.08 0.05 0.05 0.05 Sn 42 30 45 20 Cd 1 1 1 1 Pt 0.05 0.05 0.05 0.05 Pd 0.01 0.05 0.05 0.05 Y 110 135 35 90 F % 1.37 1.50 0.90 0.90 BaO 7250 3450 9300 3520 Fe % 37.70 30.40 17.40 21.50 44.50 SiO2 % 29.60 34.50 49.10

CO2 % 0.97 1.90 0.20 0.08 0.50 0.80 0.15 P2°5 % 0.20 MnO % 0.10 0.10 0.01 0.01 S% 2.03 2.00 0.66 1.13

Concentration unlesm pp sn s i stated .

bn-bornite, cc-chalcocite, cp-chalcopyrite, py-pyrite

1. and 2. - Composite bulk samples from 10 (5 in each) drill hole intersections near Whenae th n Shaft.

3. - Composite bulk sample from 6 drill hole intersections south of the Whenan Shaft.

4. - Composite bulk sample from 7 drill hole intersections in the northwest part of deposite th .

All analyses by Comlabs Pty. Ltd., Adelaide, South Australia.

87 Lower Granite* Breccia Member

Fig. 12. Down-hole plot from drill hole RD55 of Cu, U3O8, La, Ce, F and Fe in strata-bound mineralization showing relationshi f thespo e element lithologyo st .

LITHOLOGY La Ce Fe

wt% ppm Ppm ppm wt% wt% 2 4 6 810 CO o to o O o O o O O o O

Lower

Silicified

Member

Upper

Granite

Breccia

Member

Fig. Down-hol13 . e plot fron i me F dril d lan holF , e, UCe RD43 Cu , Og f La o 7, transgressive mineralization.

88 Geochemistry The general compositiomaio tw n e mineralizatioth f o n n type e indicatear s n i d e concentrationth d Tablan e mai3 e th eelements or nf o s copper, uranium, lanthanum, cerium, fluorin d irod theian an ne r relationship lithologo t s e th n yi strata-bound mineralizatio e illustratear n generaA n Figi d . .l12 positive correlation is evident for these elements in matrix-rich breccia units. The correlation is not as well defined on a detailed scale but positive correlations exist between uranium and the light rare earths lanthanum and cerium. A less defined positive correlation exists between these elements and fluorine. Ore element relation transgressive th n si e mineralizatio showe nar Fign . ni 13 . A numbe generaf o r l trend apparene sar t from simple element plots. These are:

Copper, uranium, iron, lanthanum, cerium and fluorine are concentrated in matrix-rich polymict breccia units. Iron increases towards the base of strata-bound mineralized zones. Copper distributio mors ni e uniform toward base sth f mineralize eo d zones. Uranium and lanthanum plus cerium can occur in relatively high concentrations without associated high copper concentrations but the reverse is rarely observed. Cobalt concentration is elevated in the lower, more pyrite-rich, parts of strata-bound mineralized zones. Barium content tends to be antipathetic to copper, uranium, lanthanum, cerium. Copper and uranium contents are low in siderite-rich zones which are usually lower in the sequence except in the northeast part of the main mineralized area. Lead and zinc contents are very low throughout the copper mineralized area. Silver content e highetendb o bornitn t si r e rich mineralized s zoneit t sbu distribution is erratic within these zones.

DISCUSSION

The Olympi deposim exampln a cDa s i tver a f eo y unusual typ f sediment-hosteeo d mineralization and no analagous deposits appear to have been documented. As the delineatio n earl a e deposi t th ya f stag s o ni datd te incomplete an ear a e th , following discussion is a preliminary attempt at synthesizing some of the features of the lithology and mineralization.

89 Formation of the Deposit The initial activity leading to the formation of the Olympic Dam deposit appears havo t e bee developmene nth thica f o tk pil f sedimentareo y breccias (the Olympic Dam Formation northwesa n i ) t trending troug responsn hi majoo et r faultinge Th . essential features of the pile are a series of matrix-rich polymict breccia units (Black Hematite Member, Whenan Member and Brooks Member) which contain strata-bound copper, uranium, rare earth d golan sd mineralizatioe ar d an n underlain and overlain by barren granite breccias. The breccias were probably deposited in an arid subaerial environment by debris flows, mudflows, lahar d rocan sk avalanches along active fault scarps. Depositional environment coarse-grained an s d sediments with similar lithological characteristic e Olympio thost sth deposin m i e Da c t have been describee th n di literature (see Roberts and Hudson, 1983) but none of these examples is mineralized.

The first mineralizing event occurred during sedimentation, with deposition of strata-bound hematite, sulfides, uraniu rard man e earths minerals, golsilverd dan , fluorite, siderite and barite. This event could have been related to geothermal activity resulting from volcanism but has some similarities with stratiform copper deposits (Robert Hudsond an s , 1983) latee Th r. epigenetic chalcocite-bornitd ean gold mineralization with associated uranium and rare earths was introduced in favourable structural and lithological zones during the waning stages of the earlier mineralizing event (Fig. 14). Fluorite, barite, siderit hematitd an e e veins were also forme t thida s time. e mineralizeTh e dsequenc th par f o t s overlaii e y distinctivb n e hematite-rich breccias, banded iron formation, volcanic breccia conglomeratesd san , felsic lavas and tuffs which indicate that volcanism occurred during depositio t leasa f no t part of the breccia sequence. The thinly bedded iron formations were probably deposite a play n di a lake environment with iron possibly being introduced froma fumarolic source. Rapid faciès change e basith f depositiono n i s , contemporaneous faultind an g later faulting produced a complicated geometry in the mineralized zones. Dolerite dykes intruded the sequence which was then eroded and later covered by stable shelf sediments. The absence of penetrative deformation and metamorphism in the deposit indicates an age younger than the 1580 Ma and older deformation eventGawlee th f so r domaisoute oldeth d ho an nt r tha dolerite nth e dikes which do not intrude the Adelaidean cover sequence.

90 Bornite Veins Chalcopyrite Pyrite

Chalcocite Bornite Sericite Quartz Alteration

Lense Matrin i s x Rich Breccias Chalcocite Chalcopyrite (Pyrite) Bornite

Bornite (Chalcocite) Bornite-Chalcopyrite

Chalcopyrite (Pyrite) Siderite Pyrite (Chalcopyrite) Chalcopyrite-Pyrite

Pyrite (Chalcopyrite)

Chlorlte Alteration

Fig. 14. Diagrammatic representation of the two types of mineralization in the Olympic Dam deposit.

The unusual associatio f copperno , iron, uranium, rare earth goldd overale san th , l oxidized host environment e sulfidth , e zonin d alteratioan g n relateo t d mineralization, pos a enumbe f interestino r g conceptual problems e higTh h . potassium, rare earths, barium and fluorine contents are suggestive of alkaline- igneous activity in the region but the alkaline igneous rocks have not been found at Olympic Dam. The copper, iron and gold probably originated from a mafic source. Some of the hematite may also have resulted from the in-situ oxidation of siderite, magnetite and chlorite.

The Hematite Association The strata-bound mineralization occurs within hematite-rich rocks. e Mucth f ho hematite appear o havt s e been deposited from solution textured an , s indicate simultaneous, rhythmic and sequential deposition of sulfides and hematite. The associatio f o hematitn e with sulfides e sulfide-hematitth , e texturese th , occurrenc baritof e e adjacen mineralizeto t d zones antipathetithe , c relation between barit e d relativsulfidesth an e d an e, absenc f sulfideo e n sideritei s - bearing rocks indicate that the sulfide precipitation could have been caused by the simultaneous oxidation of ferrous iron and reduction of sulfate. The ferrous iron could have been introduced by hot springs along the active faults into the sulfate- rich water f playso a Olympie laketh graben sm i cDa n (D.W. Haynes, pers. comm., 1981).

91 An analogous process of sulfate reduction and ferrous iron oxidation has been described by Mottl et al. (1979) and Seyfried and Dibble (1980). The well- developed chlorite alteration zone below the mineralization may be the result of initial percolatio f ferrouo n s iron-rich electrolyte inte underlyinth o g porous breccia. The higher level, transgressive, chalcocite-bornite-gold mineralization appears to e epigenetib d latean c r thae strata-bounth n d mineralization e commoTh . n associatio f sulfideo n d directlan s y precipitated e well-developequartth d an z d alteration halo around sulfide veins suggests thae temperaturth t f depositioeo n was probably highe rstrata-boune thath n ni d type.

Uranium and Rare Earths The uraniu rard man e earth mineral textures indicate that they were deposited both durin afted e maian gth r n sulfide mineralizing event e compositioTh . d nan close association of uranium and rare earth minerals with sulfides, fluorite and hematite indicate that fluorine, iron, carbonate, phosphate and sulfur were closely related to the transport and deposition of uranium and rare earths. The association of uranium-fluorine is a common one in many volcanic-related uranium deposit n i continentas l environments (Curtis, 1981 d supportan ) e th s mineralization-volcanism hypothesis. The very close association of uranium and directly precipitated hematite in what appears to be a low-temperature stratabound deposit is very unusual (Nash et al., 1981). However, the intimate associatio e tetravalenth f o n t uranium specie hematitd an s e could indicate that the ferrous iron oxidation and reduction of hexavalent uranium (present as a sulfate playth n ai e a possibllakes s wa ) e precipitating procese r somth fo sf o e uranium (D.W. Haynes, pers. comm., 1982).

Conclusions

The most unusual feature e Olympith deposif m o s cDa t are: e associatioth (1) f higo n h concentration f irono s , coppe d golan rd with high concentrations of uranium, fluorine and rare earths; (2) the evidence of simultaneous rhythmic and sequential deposition of sulphides and hematite; (3) the deposition during sedimentation in a very high energy environment; and (4) the size of the deposit.

The present uncertainty of interpretations will diminish with further drilling, underground developmen researchd an t untid an , l thi accomplisheds si , attempto t s explain the genesis of the deposit will continue to be speculative.

92 Acknowledge™ ent This pape s abridgei r d from that publishe Economin di c Geology, Vol, No.5.78 . , August 1983 and we thank the Editor for permission to reproduce parts of it here. e gratefuW e ar managemene th o t l f Roxbo t y Management Services Pty. Ltd., Western Mining Corporation Limited and British Petroleum Limited for permission to publish this paper. A number of the staff of RMS and WMC have contributed to date th a presente thane w d k dthean m all. Mrs. J. Hall typed the manuscript, Mr. J. Collins and Mr. C. Furstner prepared the figures.

REFERENCES

Curtis , 1981L. , : Uraniu volcanin mi volcaniclastid can c rock s- example s from Canada, Australia and Italy, in Goodaell, P.C., Waters, A.C., eds., Uranium in Volcanic and Volcaniclastic Rocks; Am. Assoc. Petroleum Geologists Studies in Geol. 13, p.37-54. Grey, I.E., 1978: An X.R.D. and S.E.M. study of uranium and rare-earth bearing mineral n W.M.C.'i s s Olympi coppem Da c r deposit. Unpubl. C.S.I.R.O. Mineral Chem. Communication, June 1978. Haynes, D.W., 1972: Geochemistr f altereo y d basalt d associatean s d copper deposits: Unpub. PhD. thesis, Aust. Nat. Univ., Canberra, A.C.T., 244 p. Mottl, M.J., Holland, H.D. d Corran , , R.F., 1979: Chemical exchange during hydrothermal alteration of basalt by seawater - ÏÏ. Experimental results for sulfud an , r speciesMn , Fe : Geochem t Cosmochime . . Acta , 869-88443 , . Nash, J.T., Granger, H.C.d Adamsan , , S.S., 1981, Geolog d conceptan y f o s genesi f importano s t type f uraniuo s m deposits. Econ. Geol. 75th Anniv. Vol., 63-116. O'DriscoU, E.S.T., and Keenihan, S.L., 1980: The Toowoomba-Charleville Lineament in southern Queensland: Aust. Petrol. Explor. Assoc. Jour., 20, 16-24. O'Driscoll, E.S.T., 1982: Pattern f discovero s e challength - y r innovativfo e e thinking: 1981 P.E.S.A. Distinguished Lecture. Petrol. Expl. Soc. Aust. Preiss, W.V., Rutland, R.W.R. Murrelld an , , 1980e PrecambriaB. , Th : f Soutno h Australia - the Stuart Shelf and Adelaide Geosyncline, in_ Hunter, D.R., ed., Developments in Precambrian geology 2, Precambrian of the Southern Hemisphere; Amsterdam, Elsevier Scientific Publishing Co., 327-360. Roberts, D.E. Hudsond an , , G.R.T., 1983Olympie Th copper-uranium-gol:m cDa d deposit, Roxby Downs, South Australia: Econ. Geol. , 799-82278 , . Rutland, R.W.R., Parker, A.J., and Pitt, G.M., 1980: The Precambrian of South Australi e Gawleth - a r Province Hunter_ in , , D.R., ed., Developmentn i s Precambrian Geology 2, Precambrian of the Southern Hemisphere; Amsterdam, Elsevier Scientific Publishing Co., 309-327.

93 Seyfried, W.E. and Dibble, W.E., 1980: Seawater-peridotite interaction at 300°C and 500 bars : implications for the origin of oceanic serpentinites: Geochem. et Cosmochim. Acta, 44, 309-321.

Thomson, B.P., 1969: Precambrian basement cover - the Adelaide System, in_ Parkin, L.W., ed., Handboo f Souto k h Australian geology: South Aust. Geol. Surv. Govt. Printer, Adelaide, 49-83. Webb, A.W., and Coats, R.P., 1980: A reassessment of the age of the Beda Volcanics on the Stuart Shelf, South Australia, South Aust. Dept. Mines and Energ . 80/6y KeptNo . . Bk .

Webb, A.W., Thomson, B.P., Blissett, A.H., Daly, S.J., Flint, R.B., and Parker A.J., 1982: Geochronology of the Gawler Craton, South Australia: South Aust. Dept. Mines and Energy Kept. Bk. No. 82/86.

94 STRATA-BOUND URANIUM DEPOSITS IN THE DRIPPING SPRING QUARTZITE, GILA COUNTY, ARIZONA, USA A model for their formation

C.J. NUTT, U.S. Geological Survey, Denver Federal Center, Denver, Colorado, United State Americf so a

Abstract

Uranium deposits in the Proterozoic Dripping Spring Quartzite e stratabounar n diagenetica1li d y altered, potassium-ricd an h carbonaceous volcanogenic siltstones e depositTh . e localizear s d near Proterozoic diabase intrusions and along monoclines that de- fore essentiallth m y flat-lying sequence. Uraninit d coffinitan e e are both in vertical veins and in sub-horizontal to horizontal veins along stylolite d beddinan s g e planesdisseminatear d an , n i d fine-grained, potassium-feldspar-rich layers. e uraniuTh m deposit e Drippinth n i s g Spring Quartzite ar e thought to be diagenet ic/sedimentary concentrations that were later remobilized during diabase emplacement. Uranium, which may have been released froe volcanogenith m c sediments during diagenesis, could have been transporte d concentratean d n carbonaceoui d s siltstone e tim th f diagenesio e t a s d stylolitan s e formation. Circulating fluids associated with diabase emplacement remobilized and concentrated uranium in carbonaceous siltstones close to diabase.

INTRODUCTION Uranium deposit e Proterozoith n i s c Dripping Spring Quartzite have been known sinc e 1950'sth e , when more tha0 occurrence10 n s were identifie n Gili d a County, Arizona, e Sierrmostlth n i ay Ancha region (Fig . .Renewe1) d interes n thesi t e deposits durine th g 1970's resulted in additional exploration and drilling. The deposit d prospectan sf particulao e ar s r interest because they e onlth y e knowar n example e Uniteth n i ds State f Proterozoio s c strata-bound uranium deposit n essentialli s y flat-lying, only locally metamorphosed, sedimentary rocks. Grange d Rauan r p

95 (1969a,b), who studied the deposits in detail during the period of e 1950'smininth n i g, proposed thae uraniuth t s derivewa m d from diabase intrusions, but Williams (1957) and Nut t (1981) suggested thae depositth t s were remobilized sedimentary concentrations. This paper, n extensioa e ideath f o f so n Nutt (1981), is in part based on the study of core drilled by Wyoming Mineral Corporation e Workmaith n n Creek ared an a on data supplied by Uranerz, U.S.A., Inc.

REGIONAL GEOLOGY The Dripping Spring Quartzite is part of the Proterozoic Apache Group in east-central and southeastern Figur stude . th 1 Locatioe yf o are p ama n Arizon ae Apache Th (Fig . ,2) .

in Gila County, Arizonai, horizontaU.S.A., l sequencf o e y near

showing uranium depositd an ) (• s a monoclines. Modified from Grange d an clastir c sedimentary, carbonate, d an volcanogeniRaucp (1969a,b)rocks, . unconformably overlies a basemen f foldeo t d locallan d y metamorphosed volcanid an c sedimentary rocks, the Alder Group and Mazatzal Quartzite, that were intruded by granitic rocks (Fig. 2). The zircon U-Pb ages of the basement rocks range from about 1705 m.y. for the Alder Group (Ludwig, 1974; oral comm., 1983 o 1430 t m.y)3 ± 0r graniti. fo c intrusions (Silver, 1968; Ludwi d Silveran g , 1977)e Th . Proterozoic Troy Quartzite lies unconformably above the Apache Group. Grange d Rauan r p (1964, 1969a), Shride (1967)d an , Bergquist and others (1981) have mapped and described the rocks in the Sierra Ancha region. Smith (1969, 1970), Smit d Silvean h r (1975) d Nehr an d ,Prin an u z (1970) studied diabasee sillth n i s Sierra Ancha. Wilso nd Wir deran n (1939A th n (1981so d an ) ) examined regional relation e basementh f o s th t r rocks i W d Anderd an , an n so (1981) have suggested regional tectonic settings for the Proterozoic rocks.

96 The uranium deposits in the Dripping Spring Quartzite, as well as outcrops of the Apache Group, are predominantly in Gila County (Fig. 1), and par ticularily in the Sierra Ancha region, although the Dripping Spring Quartzite is present elsewhere in southeastern Arizona e SierrTh . a Ancha

Troy I Arkos«. pebbly region is in the southern ••ndsion«. quartz!te QuartziteN part of the Colorado Plateau Unconformity Province a zon n i e, UNNAMED BASALT transitiona e Basith d o an nt l UPPER PART Argillite, dolomite Range Province to the south. Mescal s typicaA l throughoue th t Limestone1 UNNAMED BASALT LOWER PART Colorado Plateau e stratth , a Dolomite Unconformity e nearlar y horizontal except

UPPER PART where folde y monoclineb d s Siltstone u parn (I t volcano- (Fig. 1). Faults and joints ö genie), arkosic N sandstone cut the Apache Group. O Dripping Durin e 1950'th g s 21,36t 0 +-< < Spring oA \ at \ of ore with an average grade t_ Quartzite LOWER PART CL Arkosi feldd can - spathic sandstone, of 0.24 percent U^Og were quartzite T3 produced from 14 mines TJ BARNES e Drippinth n i g Spring -CONGLOMERATE MEMBER Quartzite, 80 percent of Tuffaceous siltstone, e productioth n coming from Pioneer arkose, feldspathic Formation' sandstone the Workmad n Re Cree d an k SCANLAN CONGLOMERATE Bluff properties (Ottod an n .MEMBER Unconformity others, 1981). An additional Alder Group, Mazatzal Granite, meta- 1,170 t averaged 0.07 Quartzite, sedimentard yan and 1430± meta-volcanic rocks 30m.y.-old percent. Recent drilling and older granites at Workman Creek has outlined Figure 2. Stratigraphie section of the n oreboda y wit n estimatea h d Apache Group. Modified from Granger 4,41 t U0 0g indicaten i d and Raup (1964). 3 place resources at a cut-off grad f 0.0o e 5 percent and an average grade of 0.11 percent.

Apache Group The Apache Group, consistin e Pioneeth f o g r Formation, Dripping Spring Quartzite d Mescaan , l Limestone (Fig , overlie2) . s 1430 ± 30 m.y.-old granitic plutons (Silver, 1968; Ludwig and Silver, 1977) and is intruded by 1100-1150 m.y.-old diabase (Silver, 1960; Smith and Silver, 1975). In the Sierra Ancha, the

97 1440 m.y.-old Ruin Granite (Ludwi d Silveran g , 1977) underliee th s Apache Group. The Pioneer, the oldest unit in the Apache Group, consist a hasa f o sl conglomerate e Scanlath , n Conglomerate Member, overlain by tuffaceous siltstone containing devitrified glass shards (Gastil, 1954), arkose, and feldspathic sandstone. Geologists from Uranerz, U.S.A., Inc., report that in the Sierra Anch11-devee w a a 1 oped regolith separate e underlyinth s g Ruin Granite froe Scanlath m n Conglomerate Member e basaTh . l conglomerate, the Barnes Conglomerate Member, of the Dripping Spring Quartzite unconformably overlies the Pioneer Formation and is overlai y fine-graineb n d sandston d siltstonan ee Drippinth f o e g Spring Quartzite. The Mescal Limestone unconformably overlies the Dripping Spring Quartzit d consistan e a lowe f o sr membe f cherto r y dolomite with halit en uppea mold d ran s membe f chertyo r , stromatolitic dolomite, argillite, and, locally, basalt. Basalt flowp ca s the Mescal. A s tratigraphica1ly continuous, 4.5- to 1 2.0-m-thick carbonate breccia underlying the carbonate sequence has been interprete y Shridb d e resule th (1967 f e dissolutioo b t o t ) f o n evaporites and subsequent collapse of higher portions of the section. The Troy Quartzite, although not part of the Apache Group, is Proterozoic in age and was probably deposited in the same basin as the Apache Group. Fine- to medium-grained arkose, pebbly sandstone d quartzitan , e unite th mak p ,u e which lies unconformably abov e Mescalth e . Diabase intrudes nearly all sections of the Apache Group and the Troy Quartzite. The diabase occurs predominantly as sill-like sheets 10-35 m 0thick t contactbu , e locallar s y discordante Th . large Sierra Ancha sill, forme y multiplb d e intrusion a high f o s- alumina magmaa centra s ha , l laye f feldspathio r c olivine-rich diabas d uppe an d elowe an r r layer f olivino s e diabase (Smith, 1970). The diabase is extensively deuterically altered along the contacts. Granophyre, coarsely recrystallized rocks, and hornfels occur alon e contacth g t separating e Drippindiabasth d an e g Spring Quartzite e granophyreTh . s forme y meltinb d d remobilizaan g f o n tio sedimentary rocks (Smith, 1969; Smith and Silver, 1975). Miaro- litic cavitie e e granophyrescommoth ar s n i n .

98 Dripping Spring Quartzite The Dripping Spring Quartzite - actually misnamed as it is largely compose f potassium-feldspar-rico d h rocks dividei - s d into lower and upper parts (Fig. 2) by Bergquist and others (1981). Granger and Raup (1964) defined additional units during their study of the Dripping Spring Quartzite and the uranium deposits, but the regional mapping unit e sufficienar s r thifo ts discussion. e loweTh r part, 85-11 m 0thick , consist f fineo s o mediumt - - grained arkosi d feldspathian c c sandstone, underlai e basath y lb n Barnes Conglomerate Member. Chemical analyses from Smith (1969) show thao arkositw t c sandstones collecte e Dragooth n i dn Mountains (southeas e Sierrth f ao t Ancha3 )8. havd 0 an ^ econtent 2 6. f o s percent, which are higher than the normal values for arkosic and feldspathic sandstones. Minor quantitie f fine-graineo s d potassium feldspa e matrith e seen ar n i xthiri n n sections from some samples e loweoth f r part. e uppeTh r part e ore-bearinth , g sequence s 75-11i , m 0thic k and consist f siltstono s d arkosian e c sandstone. Rare impure carbonate rock e preservear s s pyroxene-rica d h layer n contaci s t metamorphosed siltstone. Carbonaceous siltstones, locally interlayered with arkosic sandstone ,e loweoccuth n ri r sectiof o n the upper par d hose uraniuan tth t m deposits. Arkosic sandstone and siltstone (not carbonaceous) compose the upper section of the upper part. Stylolites are common in the upper part, where as many as 30 stylolite e centimeter-thicson occua n i r k layer. Stylolites commonly separate coarse d fine-grainean - de concenstratar d an -a trated within fine-grained layers. Residuum, which accumulates as the more soluble material is lost to intrastrata] fluids, is along the stylolite surfaces and consists predominantly of sphene, TiU2 , carbonaceous matter d pyritean , .

Mineralog d chemistran y f siltstoneo y e uppeth n ri s part The siltstones in the upper part are composed predominantly of fine-grained potassium feldspar that is generally <5um in diameter (Grange d Raupan r , 1964). Grange d Rauan rp (1964) identifiee th d feldspa s monoclinica r , excep n hornfelsii t c rocks where th e feldspa s microclinwa r e (triclinic) . DesborougA . G . h (oral comm., 1983) confirme y X-rab d y diffractio e presencth n f boto e h monoclinic feldspa d microclinean r t alsbu , o identified microclinn i e

99 rock t sappea no tha o d tr hornfelsi n hani c d specimen. Potassium feldspar composes from approximately 50 to more than 90 percent of e siltstonth e ore-bearinth n i e g sequenc e uppeth f ro e part (visual estimate from thin sections). The chemical composition of these rocks reflects this high feldspar content with correspondingly high o 13.O t value 8K^ 8 f perceno s t (Tabl . Medium-graine1) e d quartz and feldspar grains, commonly with irregular boundariese ar , scattered throug e potassium-feldspar-ricth h h layers. Siltstone tras s t igraphically e uppehigheth n ri r part have potassium feldspar content visually estimated from thin sectiono t s be commonly 50 to 60 percent, but X-ray diffraction work has also identified samples containing as much as a 95 percent potassium feldspar (G.A. Desborough, oral comm., 1983). Smith (1969) noted that these carbon-poo re upperockth n ri s sectioe uppeth f ro n part were also K„0-rich.

e significancTh e presencth f o ef fine-graineo e d potassium feldspa d higO valuesan rK^ h . HigO contenK^ h d largan t e amount f fine-graineo s d potassium feldspar in siltstones suggest that the rocks are authigenically altered volcanogenic rocks (Kastner and Siever, 1979). The presence of authigenic feldspar in altered tuffs has been well documente y Shepparb d d Gudan d e (1968, 1969, 1973) e feldspaTh . r that forms during diagenesis of marine strata is monoclinic (G. A. Desborough, oral comm., 1983). Although diagenesis has destroyed primary textures e higO e abundanccontenth ,l^ th h d an tf fine o e - grained potassium feldspar in the upper part of the Dripping Spring Quartzite indicate that this unit has a large volcanic component. The rocks composed of about 95 percent feldspar may be diagenet ically altered tuffs d thosan , e with lesser amountf o s feldspa e probablar r y reworked volcaniclastic rocks interlayered with detrital quartz and feldspar. w Na2lo Ud contentan e hig 0 Th 2 K hf theso s e rocks cannoe b t easily explaine y isochemicab d l alteratio f volcanio n c rocks- Sa . line pore water trapped or evolved during diagenesis of the Dripping Spring Quartzit r fluido e s froe overlyinth m g evaporite- bearing Mescal Limeston e possiblar e e source f potassiumo s . Sodium y havma e been los o solutiont t s during formatio f potassiuo n m feldspar from zeolites, which form as an intermediate product

100 Table 1—Chemical analyses of »auples fron ehe Dripping Spring Quantité |AS

1-1721 2-1192 4-383 5-95* 5-1265 7-71* 7-S87 DS78-58 DS78-20A29 SlOj 62.2 56.8 63.2 43.6 57.8 61.3 59.0 62.9 62.0

Al20j 15.2 14.1 14.8 10.8 14.8 16.2 14.7 16.4 15.2 Fe2°3 2.2 2.8 3.6 11.1 4.2 2.1 4.0 0.28 1.7 FeO 1.3 1 .7 * 2.8 1.6 1 .2 1.7 0.08 4.9 MgO 0.46 2.7 0.42 5.2 1.8 0.62 1.3 0.12 2.3 CaO 1.4 3.1 0.41 0.97 0.82 0.68 0.20 0.18 0. 18

Na20 0.21 0.2 0.68 0.07 0.11 0.10 0.11 0. 16 0.23

K20 12.5 11.9 11.5 8.0 12.1 13.8 12.3 13.3 8.8 + H20 0.47 1.2 0.89 2.6 1.3 0.62 1.1 0.82 2.7

H20" 0.10 0.79 0.41 2.5 0.86 0.24 0.82 0.40 0.25 TlOj 1.2 1.2 1.3 0.95 0.94 0.77 0.72 0.92 0.66 P2°5 0.17 0.09 0.09 0.15 0. 16 0.17 0.08 0.18 0. 13 MnO 0.04 0.08 <0.01 0. 10 0.03 0.01 0.04 0.02 0. 10 co2 0.01 0.32 0.01 0.11 0.07 0.12 0.02 0.02 0.02 Total Z 97.5 97.0 97.3 89.. 0 96.6 97.9 96.0 95.8 99.2 LOI * * * 2.7 0.6 0.31 0.68 3.1 * Organic C I 0.05 0. 14 0.63 1.15 0.27 0.13 0.25 2.92 0.03 U (ppm) 299 4210 6.73 11,200 2690 611 8420 16.4 5.18 Pb (ppm) 40 500 <20 1020 200 90 830 48 544 Cu (ppm) 1000 930 90 41 ,000 640 30 2400 7 145 Mo (ppm) 50 56 <10

. I Upper part» hornfelsic volcanogenlc siltstone o microscopicallK . y visible uranium minerals, but radloluxograph shows radioactivity concentrated In finely layered strata. Sample from Workman Creek core. . 2 Upper part, finely laminated hornfelsic volcanogenlc slltstone with visible uraninitd an e coffinite. Sample from Workman Creek core. . 3 Upper part, hornfelsic carbonaceous volcanogenl tones It i .s c Sample from Workman Creek core. . 4 Upper part, hornfelsic carbonaceous volcanogenic stIt stone with coffinite both disseminated and In veins. Sample f rom Workman Creek core * . 5 Upper part, hornfelsic voIcanogenlc siltstone with uranlferous chlorite veind an s coffinite. Sample from Workman Creek core* 6. Upper part, hornfelsic volcanogenlc siltstone* Uranium minerals are not visible microscopically, but radloluxographs show disseminated radioactivity. Sample from Workman Creek core. 7. Upper part, hornfelsic volcanogenlc slltstone with coffinite In veins and disseminated. Sample from Workman Creek ore. . 8 Upper part, altered(T) carbonaceous volcanogenlc slltstone. Collecte* OttonK . J ,y b frod m SE V* sec. 11, T5N, R14E, McFadden Peak Quadrangle, Arizona (1:62,500). . 9 Upper part, altered(7) slltstone with volcanogenlc componen d fine-grainean t d sandstone. Collected by J. K. Otton, from near Buckaroo Flats, T7N, R13E, Copper Mtn. Quadrange, Arizona (1:24,000).

between volcanic glas d feldsparan s : NaAlSi2Ûg'H2 SÎU- 4 0 + 2 + 4 (SurdatnNa KAlSiK> + - " 0 2 H ,3 01977)+ 8 . The carbonaceous siltstone in the lower section of the upper e highesth parO s valuesK^ ha tt e mosth , t authigenic potassium feldspar d presumablan , e greatesth y t volcanic component t abovbu , e

101 average K^O values and the presence of fine-grained potassium feldspar elsewher e Drippinth n i e g Spring Quartzite indicate that e sequencth muc f y o havh ma e e some volcanic component. Detailed stratigraphie sampling is needed to show the abundance and distribution of potassium-feldspar-rich rocks. Interestingly, the argil- e Mescath lit n i le Limestone alss higO contentha oK^ h , suggesting that thi e Mescas th par f o lta volcani alss ha o c component.

e effecTh f diagenesio t amorphis t e e m Drippin th d n an so m g Spring Quart zi t e Diagenesi s profoundlha s y altere e Drippinth d g Spring Quartzite. Volcanogenic siltstones now have unusually high K^O contents and are composed of fine-grained potassium feldspar, and volcanic textures and vo1 caniclas tic fragments have been largely destroyed. Detrital quart d feldspaan z r grain d overgrowthan s f o s the same irregulaar e n outlini r e becaus f extensivo e e dissolution, especially where the grains are in contact with authigenic potas- m feldsparsiu . Widespread stylolite occurrence suggests extensive dissolution and fluid circulation. Because dissolution that results in stylolite formation may cause thinning of the stratigraphie section - as much as 50 percent volume loss (Glover, 1969) - the presence of numerous stylolite e Drippinth n i s g Spring Quartzite suggests that n appreciabla e e sectioamounth f s beeo tha n n removed. Dissolved sediment y includma s e evaporites a possibl, e sourc f K^Oo ed an , carbonate rocks, which are now only locally observed as calc- silicate layer n hornfelsii s c rocks. Diabase intrusion e founar s d throughou e thinlth t y bedded ore- bearing upper part. Although macroscopically visible effects of the diabase Drippinth n o e g Spring Quartzit e restrictear e o t d granophyre and re crys t al1ized rocks within about 30 m of the diabase (Grange d Raupan r , 1964 d metasomatisan ) s confinewa m o t d rocks neae diabasth r e (Smith, 1969; Smit d Silveran h , 1975)e th , widespread presence of sphene and what Granger and Raup (1964) called spotted rocks (rocks with spherically shaped altered spots) suggests that diabase intrusions extensively altere e Drippinth d g Spring Quartzite. The occurrence of microcline, instead of monoclinic feldspar, outsid e immediatth e e vicinit e diabasth f o y e suggests that a large volume of rock was affected by low-grade thermal metamorphism, probably caused by extensive fluid movement

102 and heating of the Dripping Spring Quartzite during diabase emplacement; the extent of metamorphism probably depended on the permeabilit e rocksth f o .y Detailed samplin f stratigraphio g e sections and X-ray identification of the types of feldspar in the rock e needear s o determint d e actuath e l exten f metamorphiso t n i m the Dripping Spring Quartzite. Smit d Silvean h r (1975) suggested thae presencth t f o e miarolitic cavitie e granophyreth e n extensivi sth d an s e alteration of diabase near contacts indicate that water from the country rocks circulated throug e diabaseth h .

Depositional Environment of the Apache Group The Apache Group was predominantly deposited in a shallow water environment in a stable cratonic setting (Anderson and Wirth, 1981) e occurrencTh . f carbonaceouo e s siltstone e Drippinth n i s g Spring Quartzite is indicative of deposition of these sediments in a restricted environment. Carbonate rocks, evaporite molds, and stromatolites in the overlying Mescal Limestone suggest a supra- tida o intertidat l l setting e presencTh . f relico e t glass shards in the Pioneer Formation, high I^O contents and authigenic potassium feldspar in the Dripping Spring Quartzite and Mescal Limestone, and basalt in the Mescal Limestone is evidence of volcanic activity at the time of deposition of the Apache Group. The products of this intermittent volcanic activity were most voluminous during deposition of the Pioneer, the upper part of the Dripping Spring, and the upper part of the Mescal and following deposition of the Mescal.

GEOLOGIC SETTIN F URANIUO G M DEPOSITS The uranium deposits in the Sierra Ancha region are strata- e Drippinbounth n i d g Spring Quartzit e deposite Th (Fig e . ar 3) s. in the carbonaceous volcanogenic siltstone sequence in the upper part e neaar , r diabas ee predominantlsillsar d an , y restricteo t d strata abov d beloan e a wquartzit e stratigraphie marker bede .Th most productive areas in the Sierra Ancha were near Workman Creek d BlufanRe d f (Fig . .Grange3) d Rauan r p (1969a) noted uranium enrichmen a carbonaceou n i t se Mesca shalth n i le Limestone; this uranium occurrence is essentially the same as those in the Dripping Spring Quartzite where uraniu s localli m y enriche n carbonaceoui d s zones in siltstones. Uranium occurrences in the Sierra Ancha were

103 1 1 1 00' TERTIARY

MIDDLE PROTERO2OIC

Dripping Spring Quartzite

Troy Quartzitd an e Mescal Limestone d Pioneean r Formation of Apache Group

Apache Group, undifferentiated, including Dripping Spring Quartzite

- Monocline, showing trac d plungean e : dashed where approximately located

Fault

Uranium deposit

3345'—

2 mi j 1 2 km

Figure 3. Uranium deposit localities and geologic map of the Dripping Spring Quartzit e Sierrd diabas th e arean eth f o a a n i eAnch d Cherran a y Creek monoclines (Cherry Creek monocline is coincident with faults). Geology is from Bergquist and others (1981), Granger and Raup (1964), and Wilson and others (1959). Deposit location . Grangee C fro . ar s mH r (written comm., 1983). also found by geologists of Uranerz, U.S.A., Inc. in uraniferous quartz-hematite-apatite matrix filling fracture d sheaan s r zonen i s the granitic regolith at and near the unconformity separating the Ruin e GranitApachth d an ee Group. Althoug e depositth h e strata-boundar s , uranium concentration was apparently influenced by both structure and the presence of

104 diabase. Granger and Raup (1969a) were impressed by the proximity of diabase to uranium deposits, and an airborne magnetic survey conducte y Uranerzb d , U.S.A., Inc. confirm e associatioth s f o n diabase and uranium anomalies (oral comm., 1983). It should be noted, however, that in the Sierra Ancha diabase sills are so widespread in the Dripping Spring Quartzite that most outcrops of Dripping Spring Quartzite are near diabase or, possibly, near areas wher e diabasth e s beeha e n eroded .e deposit th Man f e o y ar s localized along the trend of the monoclines (Fig. 1), most commonly on the downdip side. Uranerz geologists have correlated the numerous deposit e Workmath f o sn Creek area wite intersectioth h n n anomalouoa f s magnetic zond complean e x faults associated with the Sierra Ancha monocline d BlufRe fe Th deposi. s situatei t d near diabase intruded along a fault and deposits are also clustered near the Cherry Creek monoclin d associatean e d faults . (Fig3) . Williams (1957) states that the ore is localized along north- northeast and north-northwest fractures that formed prior to ore formation. The uraniferous quartz-hematite-apatite fracture and breccia matrix in the regolith at the unconformity is also concentrated along the trend of the monoclines, near the uranium deposits in the Dripping Spring Quartzite. Uranium in the Dripping Spring Quartzite is both disseminated and in veins. The mines worked in the 1950's were developed primaril n high-gradi y e vertical veins. Core froe Workmath m n Creek area, however, shows that uraniu s alsi m o strata-bounn o d bote outcroth h d thin-sectioan p ns concentratei scal d an e n i d horizontal and subhorizontal veins, commonly along bedding planes and stylolites (Fig. 4). Even in coarse-grained recrystallized rocks uraniu s concentratei m d along remnant stylolites. Dissemi- nated uraniu n fine-grainedi s i m , potassiu feldspar-ricm— h layers; uraniu s conspicuousli m y absent from coarse-grained clastid an c pyroxene-rich layers (Fig. 4). Uraniu e occuror m s uraninita s d uraniuan e m silicate mineral(s). Uranium silicat s identifiewa e e presencth y b df o e uranium and silicon in qualitative analyses on the electron microprobe energy dispersive X-ray analysis system, and powder camera X-ray work confirmed the presence of coffinite mixed with other unidentified fine-grained minerals. Secondary uranium mineral e alsar s o present. Radioluxograph f fine-graineo s d samples that contai o visibln n e uranium minerals shoe presencth w f o e

105 s e t i I o I y st

\stylolite-rich ___, zone coarse-grained, lens

Figur . 4 eRadioluxograph f thin-sectioso n samples froDrippine th m g Spring Quartzite. Light areas have high radioactivity . Radioactivita . s i y disseminated and concentrated along stylolites. Radioluxograph exposure time, 46.5 hours. b. Radioactivity is noticably absent in coarse-grained lens within fine-grained, stylolite-rich section. Radioluxograph exposure time 5 hours7. , .

uranium, either in submicros copic grains, on grain boundaries, or incorporate n minerali d s suc s sphenea h . Sphene-fi lled stylolites are commonly very radioactive. Uraninite (called uraninite 1 in Granger and Raup, 1969a) is in veins and disseminated in layers that contain as much as 40 percent uraninite. Grains are euhedral and subhedral and < 20 v m in size. An unidentified uranium-silicate mineral commonly forms thin rims around uraninite, and coffinite is concentrated along the edges of some uraninite-rich layers. Uraninite is associated with sphene, chalcopyr , pyriteite , galena, molybdenite, pyrrhotite, graphite w places, fe and a n ,i ilmenite l thesAl . e minerals, except graphite y contaima , n inclusion f uraniniteo s ; uraninite inclusion e als ar n srecryi o s tallized feldspar matri d phlogopitan x e Coffinite (called uraninite 2 in Granger and Raup, 1969a) distribution is similar to that of uraninite. Disseminated coffinite is yellow brown in transmitted light, <25ym in size, and euhedral to subhedral. Coffinite in veins locally has a mottled gray-brown color in reflected light and differing ability to trans- t lightmi ; this variabilit n coloi yd reflectivitan r s probabli y y due to the presence of fine-grained minerals mixed with the coffinite. Fine-grained galena cubes are invariably scattered through coffinite and aid in identification of the coffinite. Coffinit s associatei e d with chlorite, chalcopyrit e, pyrrhotite , pyrite, molybdenite d galenaan , . Veins of coffinite contain inclusions of uraninite that make p majou o minot r re veins partth f o .s These uraninite inclusions,

106 the thin rims of uranium silicate on uraninite, and the coffinite concentrated alon e edgeth gf uraninite-rico s h layers suggest that coffinite formed after uraninite. Alteration, which occurs only locally in the Dripping Spring Quartzite deposits s primari , y compose f chloriteo d , iron sulfides, iron oxides d unidentifiedan , , fine-grained, clea o brownt r , highly biréfringent mineral s mosi d t an commos n coffinite-rici n h rocks. Coffinite, chlorite, pyrite, and chalcopyrite occur together in pods thae scatterear t d throughout uraniferous rocks. Neae or r mineral d veinsan s , feldspar grains hav a cloudye , brown appearance probably caused by partial alteration to clays. Radioactive jarosite veins fill cross-cutting fractures. e majoTh r element chemistr f uranium-bearino y d barrean g n siltstone sample s quiti s e similar (Tabl . Bot1) e h have anomalously higO valuesK^ h , indicating potassiu t introduceno s wa m d during ore-forming events. Magnesiu d Fe2Uan m ß value e highesar s t in chlori tically altered rockse th e latte o th ,t n pare i r du t presenc f fino e e iron sulfide d oxidean s s mixed wite chloriteth h . Other metals are also appreciably enriched in the uranium deposits 2 sample4 n I f hig.o 0 s siltston 1^ h e froe Workmath m n Creek prospect, molybdenum values ranged from 7 les29 so t tha 0 1 n ppm and averaged 59 ppm (values of less than 10 - the lower limit of detection - were assigned a value of 5 ppm); in these same samples copper values ranged from 27 to 41,000 ppm and averaged 1,998 ppm. Within the prospect, copper and uranium correlate positively; the correlation coefficient for the 42 samples is 0.7108 n contrastI . , withi e deposith n t molybdenu o corren s ha m- lation with either coppe r uraniumo r e correlatioth ; n coefficient for molybdenu d coppean m s -0.0857i e molybdenur th d an , - uranium m correlation coefficient is -0.0652. Anomalous silver (0.7-70 ppm), cobalt, nickel, zirconium, tin, and lanthanum were reporte n verticai d l vein d walan s l rocks (Uranerz, U.S.A., Inc., written comm., 1983),t althougno s i t i h clear that these elements were carried by the same fluids that carrie e uraniumth d . Silver value e commonlar s y greatee th n i r wall rock e veinss th tha n i .n e Vanadiu or t enriche e no th s i mn i d zones and zinc values are not high in the core from Workman Creek, although zinc is enriched in veins sampled by Uranerz. Lead values average 121 ppm in the Workman Creek core, with the range of values from les so 1,02t tha 0 02 n ppm.

107 The age of high-grade veinlets of uranium minerals were determined to be about 1050 m. y. (Granger and Raup, 1969a), approximately the same age as the diabase sills. A more extensive project e determinationoag f f uraniniteo s d coffinite-rican - h samples might clarify the relationship between diabase intrusions and uranium deposition.

SOURCE, TRANSPORT, AND DEPOSITION OF URANIUM There are at least two possible sources for uranium in the Sierra Ancha region. The volcanogenic component within the Dripping Spring Quartzite could have been a nearby source of uraniume underlyinth d an , g Ruin Granite constitute a possibls e distal source. The Ruin Granite typically has 2.5 to 8 ppm uranium (Anderson and Wirth, 1981) and the uraniferous quartz-hematite- apatite fracture and breccia matrix in the regolith has as much as m uranium135pp 0 . Uranium derived froe granitth m r regolito e h could have moved up structures into the Dripping Spring Quartzite and been reduced in carbonaceous zones. However, because of the extensive diagenesis, dissolution, and thermal metamorphism that have affected and apparently moved fluids through the Dripping Spring Quartzite, the simpler and favored model derives uranium from altered volcanogenic rocks. The diabase, thought by Granger and Raup (1969a) and Neuerburg and Granger (1960) to be the source of uranium, was probably most importan a hea s a tt sourc n movini e g fluid d uraniuan s m withie th n sedimentary rocks. Only local metasomatis e siltstonth f o m e occurred along the edges of the diabase, and there is no evidence for developmen e diabasth n f i eitheto e n earla r r lato y e stage volatile fluid carrying incompatible elements. On the contrary, the extensive deuteric alteration and the miarolitic cavities in e granophyreth s indicate that fluids moved froe countrth m y rock into the margins of the diabase. No large scale loss of uranium along the margins of the diabase has been documented. e uraniue quartz-hematite-apatitTh th n i m e fracturd an e breccia e granitimatrith n i x c regolit y havma h e been derived from either the Dripping Spring Quartzite or the granitic basement. Becaus e uraniuth e m occurrence e granitth n i se alon e unconth g - formity are in the vicinity of the Dripping Spring Quartzite deposits, these apatite-rich concentration e exploratiob y ma s n

108 guides to undiscovered uranium deposits in the Dripping Spring Quartz. ite Uranium derived from the Dripping Spring Quartzite could have been liberated from volcanogenic siltstones during the diagenetic alteration that formed the potassium feldspar. Analyses of the uranium conten f volcanio t c zircons froe Drippinth m g Spring Quartzite coul e use b do determin t d e volcanith f i e c rocks were enriched in uranium. Uranium, released to solutions during diagen- esid stylolitan s e formation, could have been deposited whert i e encountere a reducind g environment associated with carbonaceous matter. Grange d Rauan rp (1969a) describe small, low-grade stratabound concentrations of uranium in the Dripping Spring Quartzite that may be protore that developed during the extensive diagenesis. Inclusions of uraninite in metamorphic minerals suggest that sedimentary or diagenetic uranium concentrations were later metamorphosed . Regardless of the validity of the postulated development of a sedimentary protore, the spatial association of diabase and uranium deposite presencth d f an verticaso e l uranium veins suggest that the diabase was involved in remobilization of uranium and formation e depositoth f s thew a occurs no ye presenc Th . f fine-graineo e d microcline, indicativ f thermao e l metamorphism e altereth n i , d volcanogenic siltstones suggests extensive fluid circulation i n permeabl ee upperockth e n Drippinri sth par f o t g Spring Quartzite, perhaps during diabase intrusion. Fluid drawn toward the diabase y havma e traveled along bedding planes, stylolites d througan , h porous rock, carrying uranium originally liberated during diagenesis and depositing uranium in reducing environments associated with carbon-rich sedimentary rocks. Concentration of uranium along stylolites, bedding planes, vertical structures, and in fine- grained potassium-feldspar-rich layers was probably dependent on the permeability of these rocks and structures. Cemented clastic layer d raran s e carbonate strata, which contain little uranium now, were impermeable to the uranium-bearing fluids. The distribution e depositoth f s nea a quartzitr e stratigraphie marke d suggestbe r s that these strata wer n impermeabla e e barrie o eithet r r diagenetic or hydrothermal uranium-bearing fluids, or, alternatively, that uranium-bearing fluids preferentially traveled through this unit. Areas disrupted by monoclines and associated fracturing probably

109 were most susceptibl t circulatinho o t e g fluids, and, therefore, were the areas most likely to have concentration of uranium. Chloritic alteration remobilized uranium locally t apparbu , - ently had little effect on the geometry of ore bodies. The timing e chloritioth f c alteration even s unknowni t . The association of uranium, copper, and molybdenum in the deposits may be indicative of deposition of copper and molybdenum from the same circulating fluid that carried uranium or may be a result of a separate period(s) of fluid movement along the same structures.

DISCUSSION The elements proposed as critical to the formation of the deposit e Drippinth n i s g Spring Quartzite are: volcanogenic sediments, which may have been the source of uranium; extensive diagenetic alteration and dissolution during stylolite formation that could have liberated and transported uranium; carbonaceous matter and (or) sulfur that could have established a reducing environment for deposition of uranium; and diabase intrusions that remobilized and concentrated uranium. Alteration only locally affecte e uraniuth d m distribution. There are intriguing similarities between the deposits in the Sierra Anch ae largregioth d e an Australian d Canadiaan n n unconformity-type deposits: Proterozoic age; strata-bound character of the deposits; shallow water, carbonaceous host rocks; n unconformita y separatin e ore-bearinth g g sequence fro a granitim c or gneissic basement; development of a regolith at an unconformity; uranium enrichmen e regolitth n i t h neae unconformityth r ; apatite- rich concentrations near deposits d diabasan ; e intrusions. Striking d perhapan , s critical, differences betwee e Drippinth n g Spring Quartzite and unconformity-type deposits are the lack of metamorphis d intensan m e alteratio e Drippinth n i n g Spring Quartzite and the differences in ages: the Dripping Spring Quartzite and basement rocks are Middle Proterozoic; the larger unconformity-type deposits are in Lower and Middle Proterozoic rocks overlying Archean basement. Regardless of their relationship to unconformity-type deposits, the uranium deposits in the Dripping Spring Quartzite may be considered an intermediate stage between strata-bound sedimentar r diagenetio y c concentratione th d an s

110 remobilized, high-grade deposits in metamorphosed and highly altered rocks.

ACKNOWLEDGEMENTS . I GrangethanC . r H sharink fo r g information fros hi m detaile de Drippinstudth f o y g Spring Quartzite r valuablfo , e discussions, and for assistance with figure 3. I am grateful to Wyoming Mineral Corporation for allowing me to sampl d studan e o Uranerzyt cord an e , U.S.A., Inc r allowin.fo g access to its data.

REFERENCES CITED Anderson, Phillip d , Wirth1981an R. , . :K , Uranium potentian i l Precambrian conglomerates of the Central Arizona Arch. Rep. U.S. Depart. Energy GJBX-33(81). p 1 12 , Bergquist, J. R. , Shride, A. F., and Wrucke , C. T., 1981: Geologic e Sierrth maf o ap Ancha Wildernes d Saloman s e study area, Gila County, Arizona. Misc. Field Studie p U.SMa s . geol. SurvE M . 1162-A, 1 plate, scale: 1:48,000. Gastil, R.G. 1954: Late Precambrian volcanis n southeasteri m n Arizona. Amer. J. Sei., 252 , 436-440. Glover, J.E., 1969: Observations on stylolites in Western Australian rocks . royaJ . l Soc . .Aust.W , 12-1752 , . , Jr.d RaupB. an Granger , . , R ,1964 C. . :H , Stratigraphe th f o y Dripping Spring Quartzite, southeastern Arizona. Bull. U.S.

geol . Surv. JLJL§-8> 119 p. Granger, H. C., and Raup, R. B., Jr., 1969a: Geology of uranium deposit e Drippinth n i s g Spring Quartzite, Gila County, Arizona. Prof. Pap. U.S. geol. Surv. 595, 108 p. Granger, H. C., and Raup, R. B., Jr., 1969b: Detailed descriptions of uranium deposit e Drippinth n i s g Spring Quartzite, Gila County, Arizona. Open-File Rep. U.S. geol. . Surv.p 5 14 , Kastner, Miriam d Sieveran , , Raymond, 1979 w temperaturLo : e feldspar n sedimentari s y rocks. Amer . SeiJ . .279, , 435-479. Ludwig , 1974K. , : Precambrian geologe centrath f o yl Mazatzal Mountains, Arizona. Ph.D thesis, California Institutf o e Technology. 218 p. Ludwig, K., and Silver, L. T., 1977: Lead isotope inhomogeneity in Precambrian igneous feldspars. Geochim t cosmoche . . Acta, 41 , 1457-1471.

Ill Nehru, C. E., and Prinz, Martin, 1970: Petrologic study of the Sierra Ancha sill complex, Arizona. Bull, geol. Soc. Amer., 81 , 1733-1766. Neuerburg, G. J., and Granger, H. C., 1960: A geochemical test of é uraniue sourcth or diabasr n fo a me s deposita ee th f o s Dripping Spring district, Arizona. Neues Jb. Miner. Abh., 94, 759-797. Nutt, C. J., 1981: A model of uranium mineralization in the Dripping Spring Quartzite, Gila County, Arizona. Open-File Rep. U.S. geol. Surv. . 8J[-52p 9 4 4, Otton, J. K., Light, T. D., Shride, A. F., Bergquist, J. R., and Wrucke, TheobaldT. . C , , P.K., Duva l, J.S. d Wilsonan , , D.M., 1981: Map showing mineral resource potential of the Sierra Ancha Wilderness, Salome Study Area d vicinityan , , Gila County, Arizona. Misc. Field Studie p U.SMa s . geol. Surv. ME-1162-H1 , plate , scalep 6 , : 1:48,000. Shapiro, Leonard, 1975: Rapid analysis of silicate, carbonate, and phosphate rocks--revised edition. Bull. U.S. geol. Surv. 1401, 76 p. Sheppard, R. A. and Gude, A. J., 3d, 1968: Distribution and genesis of authigenic silicate minerals in tuffs of Pleistocene Lake Tecop a, Iny o County, California. Prof. Pap. U.S. geol. Surv. 597, 3 8 p. Sheppard, R. A. and Gude, A. J., 1969: Diagenesis of tuffs in the Barstow Formation, Mud Hills, San Bernardino County, California. Prof. Pap. U.S. geol. Surv. 634, 35 p. , 1973d GudeSheppardJ. an . . A :,A Zeolite. R , d associatean s d authigenic silicate mineral n tuffaceoui s g Bi se rockth f o s Sandy Formation, Mohave County, Arizona. Prof. Pap. U.S. geol. Sur v. 830. p 6 3 , Shride, 1967F. . A ,: Younger Precambrian geolog n southeri y n Arizona. Prof. Pap. U.S. geol. Surv. . 566p 9 8 , Silver, L. T., 1960: Age determinations on Precambrian diabase differentiates in the Sierra Ancha, Gila County, Arizona [abs.]. Bull. geol . Soc . Amer. , 71, 1973-1974. Silver, L. T., 1968: Precambrian batholiths of Arizona. Spec. Pap. geol. Soc. Amer., 121, 558-559. Smith, Douglas, 1969: Mineralogy and petrology of an olivine diabase sill complex and associated unusually potassic

112 granophyres, Sierra Ancha, central Arizona. Ph.D thesis, California Institut f Technologyo e . p 4 31 , Smith, Douglas 1970: Mineralog d petrologan ye diabasith f o y c rocka differentiate n i s d olivine diabase sill complex, Sierra Ancha, Arizona. Contrib. Mineral. Petrol. , 95-11327 , . Smith, Douglas, and Silver, L. T., 1975: Potassic granophyre associated with Precambrian diabase, Sierra Ancha, central Arizona. Bull, geol. Soc. Amer. 503-513, 6 8 , . Surdam, 1977C. . R :, Zeolite n closei s d hydrologie systemsn i : Mumpton , (Ed.)A. . F , Mineralog d geologan y f naturao y l zeolites. Mineral. Soc. Amer, short course notes, 4, 65-91. Williams , 1957J. . F :, Structural contro f uraniuo l m deposits, Sierra Ancha region, Gila County, Arizona. U.S. Atomic Energy Commission RME-3152, 121 p. Wilson, E. D., 1939: Pre-Cambrian Mazatzal revolution in central Arizona. Bull, geol. Soc. Amer. , 1113-116450 , . Wilson, E. D., Moore, R.T., and Peirce, H.W., 1959: Geologic map of Gila County Arizona. Arizona Bureau of Mines, scale: 1 :375,000.

113 THE LIANSHANGUAN URANIUM DEPOSIT, NORTHEAST CHINA Some petrological geochemicaland constraints on genesis

ZHONG JIARONG ZHITIAO GU , N Beijing Research Institut f Uraniueo m Geology, Beijing, China

Abstract

e LianshanguaTh n uranium deposit e locatee Lianoninar sth n i d g Provinc f northeastero e n China e depositTh . s occua transit n i r - ional zone lying betwee n Archaeaa n n crato d Lowean n r Proterozoic miogeosynclinal sequences. Three types of uranium mineralisation are present in the area; syngenetic mineralisation occurs in the basal psammiti ce Loweunitth rf o sProterozoi c rocks which, during metamorphis perioe th n di m 190s partl wa o 1800t , y0Ma mobilised d concentratean meta-sedimentse th n i d . Late-orogenic granite emplacement in the Lower Proterozoic saw the development of uranium- bearing alkali solutions that initially produced metasomatic white migmatit n fracturi e e zones margina e granitesth o t le grade Or . s are relatively rich in these alkali hypo-meso metasomatic uranium deposits. With precipitatio f potassiuo n d sodiuan m m froe th m metasomatising solutions uranium became further enriched in the evolved fluids. Precipitation of uranium from these fluids produced meso-epithermal mineralisatio e fissurth f o ne filling l typethreAl .e type f uraniuo s m mineralisation have been overprinted by tectonic movements associated with activation of the platform cover in the period 90 to 220 Ma.

INTRODUCTION Lianshanguan uranium deposi s locatem souti tk f 0 o h 12 d Shenyang, eastern Liaoning province t 40°59'a , d 123°30'an N E (Fig.l) e geologicaTh l settine regioth s similaf i no g o othet r r uranium-producing regions occurring in Precambrian platforms. Ground radiometr s revealeha y d that some uranium occurrences occur near the unconformity surface separating Lower Proterozoic sequences and Archaean granite. In 1978, a uranium deposit was found having surface exposur d extensionan e s verifie drillingy b d .

115 e preliminarTh y resulte earlieth f o sr wori s publisheFe wa kn Qi y b d & Hu Shaokang (1980) . n thiI s pape e presenw r e resultth t f x^oro s k recently carried out which includes regional setting, petrolog d geochemistryan y . This data has enabled further constraints to be placed on metallogenic modelling for the province.

REGIONAL GEOLOGICAL SETTING The Lianshanguan uranium deposit is located in the transitional zone betwee n Archaeaa n n e Northercratoth f o n n China Platfor a Lowe d ran m Proterozoic Miogeosyncline sequence (Liaohe Group) which occurs on the southern edge of the Lianshanguan Dome Anticline. Three tectono-stratigraphic unit e presene areaar sth n ,i t namel e Archaeath y n crystalline basement a ,Lowe r Proterozoic metasedimentary sequence and an Upper Proterozoic-Phanerozoic sedimentary platform cover (Fig. 1).

Fig 1 .Generalize d geologi f Lianshanguao p ma c n region shoxvin e locatio th e uraniug th f o nm deposit 1 . Upper Proterozoic-Phanerozoic sedimentary cover Loxver Proterozoic Units . Metavolcani2 c. Schis3 rockts 4. Quartzite Archaean 5. Metamorphic strata K-granit. 6 e 7. White migmatite (alkaline metasomatite) . Faul8 t 9. Uranium deposit

116 Crystalline basement Crystalline basemen s composei t f Archaeao d n K-granitd an e xenoliths of Anshan Group which are distributed in the core of anticline. The dominant rock is monzogranite with lesser granodiorite. U-Pb concordia on the granitic zircon indicates an age of abou e latTh te 253 Lowe. 0Ma r Proterozoic regional metamorphism locally produced gneissic textures in the granites and metasomatic effects were e Th responsibl . e introductioNa th d r an fo e K f o n older Anshan Group rocks now comprise amphibolite, chlorite-actinolite schist, granulite, banded magnetite quartzite and mica-quartz- schist. Originally these rocks are thought to have been basic calc-alkaline volcanic rocks and argillo-arenaceous iron-rich sediments. Resulting from the late Archaean intrusive event, the metamorphic units mostly occu s screena r d xenolithan s e granitith n i s c rocks. The crystalline rocks and inclusions have anomalously high U-contents thus making them a potential provenance source for the uranium deposits.

Metasediment series e LoweTh r Proterozoic Liaohe Group, forme miogeosynclinn i d e conditions s wideli , y develope n Lianshanguai d s ni areat I . distributed along the margin of the Archaean craton, and unconformably overlies it. In the Lianshanguan area, the Liaohe Group surround n arca s h anticline e loweTh e Liaoh.r th par f eo t Group comprises metamorphosed sandstone, conglomerate, graphite-bearing biotite schist d marblan , e intercalated with acid volcanics. Thick-bedded carbonates intercalated with schist comprise the middle portion e uppeTh .r Liaohe Group schis d slat an e tinter ar e - calated with quartzite e sedimentTh . s produc a whole e rock Pb-Pb isochron age of about 2185 Ma suggesting an age of deposition.

Platform type sedimentary cover e sedimentarTh y cove s mainli r y distribute e northerth n i d n par f Lianshanguao t n area (Fig . .1) Uppe r Proterozoic rocks comprise slightly metamorphic feldspathic-quartzite-sandstone, marl and shale; thes e unconformablar e y overlai e Cambrianth y b n - Ordovician sediments with no angular discordance. The Palaeozoic rocks consis f sandstoneo t , shal d limeston an e frequentl ar d an e y in faulted contact with Archaean-Lower Proterozoic sequences.

117 EVOLUTIO E ARETH AF O N In the Lianshanguan area the Archaean greenstone-gneiss terrane underwent multiple metamorphis d complean m x deformation, with fold faulan d t structures well developed. Followinn o g these tectonic event e Archaeath s n craton underwent erosiod an n peneplanation. Then crust rifted in Lower Proterozoic times and a new geosyncline was developed. Initial sedimentation saw the accumulation of terrigenous course elastics, in which the main uranium-bearing unit occurs. The Lianshanguan area is situated in the margina le geosyncline partth f o s . With increasing sedimentation marine incursion occurred in the deepening basin with pelitic d carbonatan e sediments being deposited. Sedimentation terminated wit a majoh r tectonic event datet a d o 180t 190. 0 0Ma Metamorphis m reached amphibolite facièd an s additiona W trenE- l d linear folds were developed. Reactivation e basementh f o t granites resulte n emplacemeni d t inte overlyinth o g formations. e differencOwinth o t g n physico-mechanii e c properties betwee e crystallinth n e rocd sedimentaran k y strata frequent intense compressions took place alon e interfacth g f theso eo tw e units. Mica-quartz schist with marked schistosity occupy the centra le compressepartth f o s d zones e mobilTh . e elements, such as K and Na, were activated and carried in solutions to bring about metasomatism within these structural zones, thereby producing the white migmatite zone rangin n widti g h from ten f metren o si o t s . m Thi0 exces10 s f alkalo s i metasomatis s controlleha m e th d location of economic uranium mineralisation in the area. The Lianshanguan area is flanked by the Hercynian orogenic belt to the north and by the western pacific plate in the east. Movements within these tectonic belts has resulted in the platform rocks of the Late Paleozoic-Mesozoic era, being faulted and intruded e platforTh . m activatio s resulteha r n movemeni d f o t radiogenic lead producing e environgalene th th n i af o s Lianshanguan uranium deposit.

GEOLOGY OF THE URANIUM DEPOSITS e LianshanguaTh n uranium deposit occur n quartziti s d schisan e t e bas af th Loweto e r Proterozoi d additionallan c y e occurth n i s white migmatit d alterean e d fracture zones develope e granitth n i de within the unconformable structural zone. The ore-bearing

118 horizons belon o Liaoht g e Group, Langzishan Formation e totath , l thicknes f whico s e representativs 300-63i On h . 0m e sectios a s i n follows :

Overlying beds : dolomitic marble, metamorphic volcanic rock Langzishan Formation : Upper member : 9) Graphite mica schist ) Tremolit8 e marble intercalated with mica-schist, granulit m thick 1 (2 e) Middle member ) Garne7 : t mica schist intercalated mica-quartzit m thick 0 (4 e) ) Feldspathi6 c quartzite intercalated with garnet mica schist (22 m thick) 5) Graphite-staurolite-garnet mica- schist (99 m thick) ) Garne4 t hornblende schist, garnet biotite schism thick 0 (1 t) Lower member ) Mic3 a quartzose schist, staurolite- garnet-mica-schist intercalated with mica-quartzite (ore-bearing bed) (20 m thick) 2) Quartzite, feldspathic-quartzite, metamorphosed conglomerate intercalated with mica-quartzite (ore-bearing bed) m thick 0 (3 ) 1) Metamorphosed granitic sand-conglomerate (Ancient weathering mantle) (1- 4m thick) . Basement Red granite

From the section it is apparent that the Langzishan Formation is composed of a suite of terrigenous detrital rocks, belonging to the littoral and neritic faciès. The dark grey quartzite and mica-quartz schist in the second and third beds are the primary uranium-bearing units.

The main economic uranium mineralization is distributed along the metasomatite-structural zone betwee e Granitth n e Compled an x Lower Proterozoic metamorphic rock e mineralizesTh (Fig . 2) . d zon s concordani e t wite beddingth h , trending MW-S d dippinEan o t g the S at angles varying from 30° to 60°. The uranium ore-bodies e locatee brecciatedar th n i d , jointe d deformean d d zone withie th n white migmatite, as well as in quartzite or mica-quartzite schist.

119 Fig. 2. Geologic map of the Lianshanguan uranium deposit 1 . Quartzite 2. Schist 3. Marble 4. Metavolcanic rocks 5. White migmatite 6. K-granite 7. Fault 8. Uranium ore bodies

Tabl . 1 Uraniue m mineralization characteristics

type f uraniuo s m metamorphosed hypo-mesothermal meso-epithermal mineralization mineralization sedimentary metasomatic filling characters host rocks dark grey quartz- white migmatite altered granite ite mica quartzite biotite granulite e or for f o m layer, layer-like lens, vein-like veinlet bodies texture & impregnated, cemented breccia,oolitic structure cemented spherulitic, concentric economic uranium uraninite meta-pitchblende pitchblende minerals associated pyrite, galena, alkali feldspar, calcite, minerals graphite, biotite, chlorite, flourite quartz garnet muscovite, galena, pyrite, galena, pyrite hematite v/all rock unaltered albitization, fluaritization, alteration chloritization carbonatization, sericitization silicification, hematitization temperaturf o e 450°-550°C 280°-250°C 170°-230°C mineralization U-Pb dating 2,100 m.y. 1,900 m.y. 1,830 m.y.

Types of uranium mineralization Uranium mineralization occurs in three situations: sediment- y metamorphosedar ; hypo-meso hydrothermal metasomatic d mesoan ;- epithermal filling. They differ from one another in terms of their host rocks, mineral association, texture and structure of

120 uranium ore, wall rock alteration e bodie, or ford occurrf an o sm - enc e majo eTh d trac(Tablan r . e 1) eelement hose th t f o rocks n i s the U-deposits are shown in table 2 .

(i) Metamorphosed sedimentary mineralization The uranium mineralization of the metamorphosed sedimentary type is distributed within the structural-metasomatic zone

Table 2. Major & trace elements of host rocks in the Lianshanguan U-deposit host rock quartzite urano- granite altered white ore-bearing ferous granite migmatite white quartzite migmatite main element(%)

Si02 94.03 93.59 75.21 75.68 75.16 7 ] . 76

Ti02 0.06 0.06 0.11 0.08 0.13 0.14 A12°3 2.38 1.26 13.24 13.24 14.0 12.56 Fe2°3 0.18 0.29 0.81 0.81 0.69 0.56 FeO 0.59 0.72 0.91 0.52 0.85 0.90 MnO 0.01 0.01 0.03 0.03 0.03 0.04 MgO 0 0 0.19 0.17 0.17 0.28 CaO 0.20 0.24 0.48 0.60 0.52 1 .08

Na20 0.23 0.07 3.05 3.58 4.38 4.11 0.15 0.31 5.44 4.69 3.30 2.36 P2°5 0.02 0.04 0.20 0.13 0.17 0.09 S 0 0.66 C 0 0.02 - - - - Na20 1.53 0.23 0.56 0.76 1.33 1.74 trace elements (ppm) Cu 62 80 16 31 84 Zr 72 43 45 48 83 Ga 4 7 13 14 11 Cr 20 10 12 12 14 Ni 15 20 10 12 19 V 14 30 10 13 14 Pb 15 250 8.4 25 240 U 12 730 8.7 16 753 Th 7.4 34 45 37 38 Y 45 17 38 35 45

121 NE24° Fig . 3 Cros. s sectioe th f o n Liashanguan uranium deposit 1. Mica schist 2. Amphibole schist 3. Interbedded zone of quartzite and schist 4. Quartzite . Whit5 e migmatite . K-granit6 e 7. Metamorphosed sedimentary uranium ore body . Metasomati8 c hydrothermal uranium ore body . Shea9 r zone . Faul10 t

Fig. 4. Microuraninite (grey) with pyrite 'white) distribute in the groundmass

e hosTh t (Figrock . e dark-gre3) .ar s y quartzite, mica-quartz schist and biotite granulite. Ore bodies are stratified or strataform with only moderate thickness. Generally e grador e th , is less tha 0.1% U.

There is no clearly defined boundary between uranium mineralized zone d walan s l e rockcompositio s Th rathei .e ror f simpleo n , the main economic uranium minerals are microuraninite, occurring in the voids of detrital mineral grains or in form of disseminations include n biotitei d , garnet, feldspa d othean r r minerals (Fig. 4) .

The chemical composition of ore-bearing and barren rocks e mineralindicateth n i , -S a slighs, C , tFe enrichmen, K , AI f o t ised rocks. A positive correlation exists between U and the total . C Thi d contenan s S correlatio f o t n suggests that uranium owes its concentratio o reducint n d absorbenan g t conditions. Addition- ally ferrous iron and pelitic material may have aided the concen-

122 tration process. Trace elements of both ore and wall rock zones are almost identical except for Pb which was probably produced by radioactive . decaU f o y

(ii) Alkali hypo-meso metasomatic mineralization This type of uranium mineralization is strictly controlled by the structural-metasomati e bodieor c e zonese Th occuth . n i r structural breccia and border fracture zone of the white migmatite, s lensea r lenticleso s e gradOr s relativeli .e y rich, locally reaching 5% U. The main economic uranium mineral is meta- pitchblende occurrin n colloidai g l form. Pentagona d hexagonaan l l crystal-grain e seeb n e presen ca undear s d ran t high magnification e paragenetiTh . (Fig5) c. mineral sequenc s albitei e , K-feldspar, chlorite, muscovite, minor amoun f pyriteo t , galen d pyrrhotitean a .

e originaTh l rocks comprisin white th g e migmatite were mainly quartzite, feldspar-quartzite and granite. During the metasomatic process abundant Na and K was introduced to form sodic plagioclase and K-feldspar. Relative to the granite the white migmatite has an increased NaÔ/KÔ ratio (0.56 to 0.76) and with quartzite, the ratio has increased from 0.29 to 1.33 (Table 2). It shows that the alkalic metasomatising solutio s enrichewa n n sodiumi de Th . NaÔ/KÔ rati mucs i o h e ore-bearinhigheth n i r g metasomatic rock, averaging 1.74. This indicates that there is a close relationship between uranium mineralization and Na-metasomatism. Apart from the higher Pb content in the ore, the remaining trace elements show little variation within mineralized and non-mineralized zones.

(iii)Mesoepithermal filling type mineralization This type of uranium mineralization occurs mainly within fractures of altered granite. The ore bodies form mono-veins and stockwork or reticulated-veins occur on a small scale.

The main economic uranium minerals of this type are pitchblende having spheruliti r oolitio c c texture e paragenetiTh . c sequence of the opaque minerals are pyrite, galena, sphalerite and chalcopyrite e ganguth ; e mineral e mainlar s y compose f calciteo d , fluorite, quartz and sericite. The pitchblende is in close association with sulphides, dark-gre r ligho yd calcitre t d darkan e - violet fluorite.

123 Fig. 5. Meta-uraninite with pentagonal and hexagonal form

500-

2 100- 3895m 0pp

50- 2220 ppm (u)

O. a cn 310 ppm (Si) ) 41 x37m pp 0

10- X680 ppm

r E y D d SmEN d G u e C a L Yb

Fig. 6. Chondrite-normalized REE patterns of different genetic type uranium ore 1. Schist . Metamorphose2 d sedimentary uraniue or m . Hydrotherma3 l metasomatic uraniue or m 4. Hydrothermal filling uranium ore

GEOCHEMISTRY Rare-earth elements Rare-earth elements of ore and host rocks are shown in table 3 and figure 6. The chondrite-normalized patterns of rare-earth element e ore-bearinth f o s g quartzite, schis n Langzishai t n Formatio d Basemenan n t Granit e quitar e e similar e La/YTh .b ratio

124 Tabl . 3 eRare-eart h element e threth ef o stype f o s uranium mineralization (ppm)

mineralization metamorphosed hypo-mesothermal meso-epithermal types sedimentary metasomatic filling sample no. 292 177 242 317 121 245

La 60.4 19.4 24.1 51.6 190.4 136.5 Ce 109.1 23.4 56 88.7 407 236.1 Pr 14.6 3.1 10.1 17.1 68.7 33.3 Md 37.6 11.4 26.7 51.8 227.1 101.1 Sm 9.7 4.5 7.1 31.5 106 24 Eu 3.1 1.3 1.8 6.5 22.2 3.3 Gd 8.5 3.8 3.4 28.6 67.3 15 Dy 4.5 2.6 3.4 15.4 30.9 5.9 Er 4.1 1.9 3.1 11.4 41.1 4.2 Yb 3.2 1.6 3.0 8.7 19.8 3.4

EREE 254.8 68.5 138.7 311 1180.5 563 LREE ÏÏÏÏEE 11.4 5.8 9.6 3.8 6.3 18.5 La 7b" 18.9 9.3 8 5.9 9.6 40.1 Eu Eu* 1.13 0.94 1 0.66 0.76 0.5 U 370 740 680 2220 38950 310

is indicatin15 betwee and 8 n g enrichmen lighthe t in trare - earth elements. The reason for the Eu-anomaly is unclear. There are some differences between the REE patterns of ore-bearing rocks d normaan l quartzit e zone or d schist an se Th hav.e loweE RE r concentrations. Usually with uranium increase there is a reduction of REE content probably because uranium is concentrated in the fine grained detrital rocks with only minor amount f heavo s y minerals. The REE pattern within ores of the hydrothermal-metasomatic type are similar. The higher the uranium content, the more abundant are the REE. The fractionation of light rare-earth element s poor i sLa/Ye th ,d drop ban rati, s 10 betwees i od an 4 n e uraniuath s m content rises. Although there is a low uranium content in the ore of the hydrothermal filling type of mineralization the REE content is

125 relatively high witLa/Ye th h b ratio woult reachinI d . appea40 g r that the low temperatures associated with this type of mineralization results in light REE enrichment.

Sulfur Isotopes e sulfuTh r isotope d sundr an f pyriteo se yor e , th bot n i h wall rocks, indicate thae uraniuth t m mineralizatio s charactha n - eristic f boto s h sedimentary metamorphis d metamorphosean m d hydrothermal alteration (Fig. 7).

os34 fe mean T

0 -1 5 - 0 5 + 0 +1 5 1 + 0 +2 Fig. 7. Sulfur isotope composition of the pyrit n hosi e t rock d uraniuan s e or m 1. Pitchblende vein 2. White migmatite 3. Uranium-bearing quartzite and schist . Quartzit4 d schisan e t

f pyriteo S I 6 Q , presen n quartziti t r schisto e , varies from -8.4 o +16.8%t % e averagth ; e valu s +6.1% i e fac Th t. thae th t / o 6 S range is similar to sulfate present in palaeo-marine waters suggests a similar origin. The pyrite associated with uranium / o minerals in the ore-bearing quartzite has an average ô S of +6.3% Thi e wals aboui sth e saml th s ta roce k value se quartz founth n i d- ite host suggestin a sedimentarg y origi r thifo ns typ f uraniuo e m mineralization.

/ o The 6S of pyrite in the white migmatite and vein filling type uranium mineralization all have positive values. This indicates thae sulfuth t r doet comno s e fro a deem p sources i t bu , probably derived from the reconstituted country rocks; the associated uranium is also probably derived from the wall rocks having been mineralized after hydrothermal metasomatism.

U-Pb Radiometrie isotopes U-Pb isotope dating shows that the Lianshanguan uranium deposit is a product of multistage mineralization. Three diff-

126 erent metallo-genetic age f uraniuo s m mineralizatio e datedb n ca n. The age of metamorphosed sedimentary uranium mineralization is + 129 2114 -no? ^a ' ^e alkali-0 nietasomatic stage is dated at Q -r7 +'3'} e hydrothermath d an ; lMa fillin^ 7 _ 1891 g stag. t 182Ma a eQ 9_£ e laso ageTh tw ts approac e threth e modeef o th hstage le ag U-Psb white oth f e migmatite (191 Ma)7 7 7+ . Additionall e threth y e types of uranium mineralization have all been overprinted by tectonic movements associated with activatio e platforth f o n m during the period 90 to 220 Ma.

FLUID INCLUSIONS e formatioTh n temperatur e veith n f o pitchblende s beeha e n measured by the decrepitation method, and by the homogenization temperature of fluid inclusions in fluorite, calcite and quartz associated with the pitchblende. The decrepitation and homogenization temperature f theso s e minerals coul e divideb d d into tw o groups e firsTh s 280°-350°i .t Ce hypo-mesohydrotherma founth n i d l rninerogenesis, this represent e formatioth s n temperatur f alkalio e c metasomatic U-mineralization e mineraTh . l associatio f whico n s i h meta-pitchblende, alkali feldspar, quart d smalan z l amountf o s sulfides. The second is 160°-230°C occurring in the meso- epithermal minerogenesis, this represents the formation temperature of hydrothermal filling uranium mineralization. The main mineral associations here are pitchblende, calcite fluorite and microcrystalline quartz.

METALLOGENETIC CONTROLS As stated above, the formation of Lianshanguan uranium deposit underwent complex processes, it is controlled by sedimentation, regional metamorphism, structural movement and metasomatism. The major control factor e differen e variouar s th r sfo t mineralization types; these controls can be generalized as follows:

Uranium source Th abundancU e d U-extractinan e e maig th ratn f o hoste n i s this region are arranged in tables 4 and 5 and in figure 8. It show sU content thae th t f theso s el highe al rock e rar s than average crustal abundances U abundanc e Th .f quartzit o e d thaan e t of the white migmatite are especially high, they also have high U-extracting rates. This suggests that thera hig s i he degref o e

127 Quartzi te Samples

Fig . 8 histogra. e th f o m uranium content of the main host rocks.

gmai e m 1 1 t e t i Wh 70 Samples

10 20 30 40 0 10 20 30 40 ppm

Table 4. Uranium content of the main host rocks

li thology numbef o r average content mean square samples ot uranium^ ppm) deviation( 6X)

grani te 77 6.25 4.50 schist bl 6.65 5.60 quar tzi te 126 10 .20 8. 72 white 70 14 .36 8.84 migma ti te

Tabl . 5 eUraniu m extracting rat f maio e n host rocks

li thology uranium^ppm) extracting extracting rate(%) condi t ion

grane it 9.2-14 6.0 in normal tempera- schist 16 14.1 ture and 1% Na-CO- quartzi te 21.0-88 23.1 solution, the ratio white 19-130 2b.4 of sample to solut- migma ti te ion is 1:10 mobilized uranium in these rocks. As the basement granite and ore-bearing hosts in this region also have high U contents it is apparent that the area was anomalously eririchea in uranium. The basement granite, undergoing a long period of weathering and erosion, provided the uranium source for the formation of syngenetic U-bearing horizon e basath ln i s member e Loweth rf o s Proterozoic. Economic uranium grades were produced during metamorphis d metasomatisman m .

128 Sedimentation Most of the uranium occurrences in the region are localised in terrigenous detrital rocks of the Langzishan Formation which has stratabound characteristics. This uranium-bearing formation was deposited on the margins of the Archaean craton under miogeosyncline conditions during Lower Proterozoic timese LangzishaTh . n Formation unconf orinably overlie e basementh s t granite complex, belonging to the lower part of a transgressive cycle. This Formation, although widely distributed, only has uranium concentrated wher e lithologith e e lithofacie d palaeogeographian s c condit- ione favorablear s . e spatiaTh l distributio e originath f o n l horizons, ricn i h uranium s controllei , e basth e y b dtroug h near shoreline conditions. In the late Archaean, the basement granitic complex was exposed to the surface and underwent peneplanation. On the peneplain surface local rivers scoured troughs which controlled the thickness and lithofacies of the U-bearing horizons. In general, the Lianshanguan deposi S s strikinlocateN i t a n i d g basal trough with better uranium values mainly distribute e centr th d easter n an ei d n e trougslopth f ho e (Fig. .9)

120°

Fig. 9. Longitudinal section of the Lianshanguan uranium deposit ^symbol n figuri s ) a s3 e

The interbeds of sandstone and argillite in the base trough are the favourable lithofacies for formation of horizons rich in uranium. The meta-sedirnentary uranium mineralization in the Lianshanguan deposit is present mainly in black-grey quartzite and mica-quartz schist developed in the basal part of Langzishan Formation. Where the ratio of quartzite thickness to the total thicknes e ore-bearinth f o s g zon s lesi e s than 35%, uranium

129 grades improve. Where this ratio is higher than 50% mineralizatio s poorl thii nAl s. suggests thae sedimentarth t y lithofacies exerted a controlling role in the distribution of uranium mineralization. The change of sedimentary facies is strongly controlle e paleogeographith y b d c environment e uplifteth n I .d part e thick-beddeth s d macroclastic sediments n oxidizinoccua n i r g environment, whic s unfavorablwa h r uraniufo e m precipitatiod an n concentration e distath n I l e basafacie. th f lo s trough argillic and sandy sedimentary beds were produced. This zone lies in a transitional redox environment, that was favorable for uranium precipitatio d concentrationan n . Also these interbedded sandstone d argillitean s s produced permeabl d impermeablan e e layers, whic s advantageouwa h e circulatinth o t s f U-bearino g g ground water resultin n furthei g r uranium precipitation. Those lithologies rich in reducing mediums resulted in local uranium concentration. The U-bearing hosts of the Lianshanguan deposit are rich in clays, carbonaceous matter and pyrite and posses a dars k colour. These rocks hav a sulfue r conten f 0.05o t - 0.98%, 0.05-0.74 n organii % c carbon a positiv; e correlatios i n presen e totat. C th betwee Thi d ld an an samoun S U nsuggest f o t s that U-bearing horizons were formed under weak reducing conditions. Regional metamorphism The ore-bearing rocks in the region have undergone a low to medium grad f regionao e l metamorphism e metamorphiTh . c grads wa e favorabl r uraniufo e m concentration, wite loweth h r grade metamorphic rocks being a favorable place for the production of metasedimentary uranium mineralization. The main metamorphic mineral associations of the ore-bearing strat e staurolite-almandine-biotite-muscovite-quartar a d an z muscovite-biotite-plagioclase-quartz, which e greenbelonth o t -g schist facies or almandine-staurolite subfacies of the amphibolite facies e Mg/Mg+Fe+MTh . n distribution coefficiene tth (Figf o ) .10 biotite-garnet minera e rockl th pai n si r suggest a stemperatur f o e formation of about 500°-550°C at a pressure of 5-10 kb. Hydro- thermal solutions associated with this metamorphism reconcentrated the syngenetic uranium to form micro-grained uraninite, which is held in the metamorphic silicate minerals, or distributed along mineral grain boundarie d cracksa resulan s s A metamorphisf o t. m the meta-sediinentary uranium deposits were formed from original syngenetic concentrations.

130 Fig. 10. Relationship between distribution coefficient of Mg/Mg+Fe+Fn for the garnet- biotite pair 0-2 0-4 0-6 0-8 1-0

Biotite Mg/Mg + Fe *• Mn

Tectonic condition Tectonism was an important controlling factor of the distribution of uranium in the Lianshanguan uranium deposits. Tectonic movements associated with late Lower Proterozoic times resulten i d structural deformation characterize y plastib d e courscth flown eI . of plastic fold deformation, tectonic emplacemen e reactivateth f o t d basement granite took place alon e foldinth g g axi o fore t s th m Lianshanguan arch anticline a late-stag t A . f granito e e emplacement compressive fractures were formed along the contact zone between the granite and meta-sedirnentary rocks and also parallel to folding axes. These faults possess characteristic f multistago s e activation, which were welde r filleo d y whitb d e rnigmatite th n i e early stage and subjected to brittle faulting in the later stage that gave ris o fracturet e d crusan s h zones. These fractures provide the passageway for uranium-enriched fluid migration. These zones are the favorable sites for the hydrothermal rich ore bodies. The major economic orebodies of the deposit are located in strike faults and interstratified fracture zones (.Fig. 3). The main ore- zones are brecciated and developed in areas where the dip changes from gentl o steept ee crushe th ; d e elongatezoneth f o s d belf o t white migmatite formed along interstratified fracture zones. Other structure forms such as schistosity, gneissosity and rnicro- crack e alsar s o favorable site r uraniufo s m concentration.

Migma-metasomatism and alkalic hydrothermalism e hydrothermaTh l uranium mineralizatio n thii n s regios i n spatially closely related to the alkalic metasomatism that produc- e whiteth d e migmatite e maiTh n. expressions are:

131 ) a Hydrothermal uranium mineralization occurrine th n i g inner structural breccia zones or border fracture zones of the white migmatite. Pitchblende is especially in close association wite latth he fine-grained albite. The correlation between U and Na is positive. b) The paragenetic association of minerals and trace elements of ore-bearing and non ore-bearing white migmatite s essentialli s y similar e compositioTh . n of uranium ore is rather simple, apart from the higher o othen E rRE metallogeni d an b P , Th contenc , U f o t elements display enrichment. c) The uranium metallogenic age is similar to the age of the white migmatite. U-Pb dating of white migmatite f alkalio e ag ce hydrothermath i s d 1,91an , Ma 7 l metasomatic uranium mineralizatio s 1,89i n . 1Ma e formationaTh ) d l temperatur f pitchblendo e s betweei e n 250 -350°C, belonging to the range of hypo-mesotherrnal hydro thermal ism. The above information suggests that the hydrothermal uranium mineralization has characteristics of succeeding to and evolving from the migma-metasomatic solution in respects of time, space, composition and temperature. The metallogenic hydrothermal solut- s derivewa n io d froe alkalith m c migmatite solutio s postit n -i n migmatitic metasoiuatic stagee controllinTh . g rol f migmao e - metasomatism alkalic hydrothermalism in uranium mineralization can be summarized as follows:

e migmatizatioTh ) a n thii n s regio s generatewa n d during the regional metamorphis e latth e n i Lowem r Proterozoic era. In the late stages of regional metamorphism reactivatio e basementh f o n t granitic complex resulted in mobilisation of elements such as K, Na and U. This metasomatism occurs on both sides of the contact zone and forms the primary alkalic U-bearing fluid faciès (migmatitic solution).

The migration of this alkalic U-bearing fluid through the fracture zones resulte n wali d l rock metasomatism and extractio f uraniumo n e U-bearinTh . g primary alkalic solution became concentrate n locai d le whit partth ef o s

132 migmatite (e.g. uranium content and extracting rate of white migmatit n Lianshanguai e n l highedeposial e rar t than those of other rocks). These solutions became further enriched in uranium to form the late alkalic solutions, thus acting as a rich uranium source for the hydrothermal uranium mineralization. e Na-bearinTh ) b g alkalic hydrothermal solution formen i d the post-rnigma-metasomatic stage, uranium coule b d carried in the form of a carbonate uranyl complex. e solutionth f o e more H higheth p ,Th ee stablth r e this uranyl complex became A favorabl. e depositional site could have been produced by decreasing temperature d pressurean , changing redox conditions, reducinH p f o g (caused by separating alkalic metal from solution in the proces f alkalio s c metasomatism) e uranyTh . l complex could be uncoupled under these circumstances with reduction taking place producing pitchblende. ) Migmatitic r hydrothermao c l alkalic metasomatise th f o m area is mainly represented by the replacement of potassium by sodium. Because the ion radius of sodium is smaller than tha f potassiumo t e equal-ioth , n replacement of K by Na resulted in an increase in void space withi e mineralth n s resultin n highei g r porosity e rocksa permeabloth f d an , e spongy textures i t I . suggested that inhomogeneity resulting from metasomatism led to a reduction in the mechanical strength of the rocks allowing them to be easily fractured and thereby forming favorable structural traps for uranium mineralization.

E SUGGESTETH D GENETIC MODEL Petrogenetic modelling of the uranium mineralization at the Lianshanguan e summarisedeposib n ca t s followsa d : Afte e latth re Archaean orogeny e uraniuth , m enriched metaraorphic rocks and granite complex underwent weathering and erosion. Labile uraniu s leachewa m t froou dm these rockd an s carried by runoff into the offshore basal trough. The uranium was fixed by absorption on clays and reduced by carbonaceous matter, pyrite and changing redox conditions to form low-grade

133 syngenetic uranium concentrations in the basal part of the Lower Proterozoic psammitic sequence. Subsequently the rocks were metamorphosed during the late Lower Proterozoic to greenschist amphibolite faciès. e syngenetith Par f o t c uraniu s mobilisewa m d during metamorphism and concentrated to form the meta-sedimentary type of mineralization. Late-orogenic granite emplacement took place durin e regth g - ional and metamorphic event. This event produced a series of compressive fault d interstratifiean s d fracture zones paralleling the granite contact. During this tectonisra uranium-bearing alkali migmatitic solutions were produced. These metasomatising solutions migrated along fracture zone d replacemenan s e walth l f o t rocks took place to form an elongated belt of white migmatite. In this process, alkali solution extracted uranium froe surroundth m - ing rocks; with precipitation of some K and Na the uranium became enriched. These solutions further evolved into the uranium-rich post-migmatitic alkalic hydrothermal solutions n thesI . e hydrothermal solutions uranium was transported as the carbonated uranyl complex. Under suitable physiochemical conditions uranius wa m precipitate n fracturi d e systems producin e alkalith g c hydrothermal metasomatic type of mineralisation. After formation of the alkalic metasomatic uranium mineralization e residuath , l hydrothermal solution migrate o lowet d r pressure regimes. With falling temperature, and separating out of the alkalic metals, the solutions evolved became weakly acid and formed meso-epithermal mineralization of the fissure filling type. These deposit e characterisear s e minerath y b dl association sulfide-pitchblende or fluorite-pitchblende found in tension cracks far removed froe originath m l thermal source.

ACKNOWLEDGEMENTS The authors would like to thank the Liaoning Exploration Team of Uranium Geology for making information available to us.

REFERENCES QIN FEI & HU SHAOKANG., 1980: Present explortion status of the Lianshanguan uranium deposit, northeast China: in Ferguson, Johd A.Ban n . Goleby (Eds) "Uraniu e Pinth en i Creem k Geosyncline" (655-662) Proc. of IAEA 760p.

134 PINE CREEK GEOSYNCLINE, N.T.

G.R. EWERS . FERGUSON,J , R.S. NEEDHAM Burea Mineraf uo l Resources, Canberra CityT ,AC T.H. DONNELLY Baas Becking Geo-Biological Laboratory, Canberra City, ACT Australia

Abstract

e PinTh e Creek Geosyncline comprise f chronoo s m abouk 4 -1 t stratigraphic mainly peliti d psammitian c c Early Proterozoic sediments with interlayered tuff units, resting on granitic late Archaean complexes exposed as small domes. Sedimentation took place in one basin, and most stratigraphie units are represented throughout the basin. The sediments were regionally deformed and metamorphose t 180 a d. 0Ma Tightl y folded greenschist faciès strata in the centre grade into isoclinally deformed amphibolite faciès metamorphics in the west and northeast, granulites are presene extremth n i te northeast d post-orogenian e Pr . c continental tholeiites d post-orogenian , c granite diapirs intrude th e Early Proterozoic metasediments, and the granites are surrounded by hornfels zones up to 10 km wide in the greenschist faciès terrane. Cover rock f Carpentariao s n (Middle Proterozoicd an ) younger ages rest on all these rocks unconformably and conceal the original basin margins.

The Early Proterozoic metasediments are mainly pelites which are commonly carbonaceous, lesser psammite d carbonatesan s d an , minor rudites. Volcanic rocks mak p abouu e 0 percene 1 t th f o t total sequence. The environment of deposition ranges from shallow-marin o supratidat e d fluviatilan l e th r mos fo f e o t sequence, and to flysch in the topmost part.

The Early Proterozoic strata are overlain with marked angul- arit y mostlb y y subhorizontal fluviatile sandston d basalan e f o t Middle to Late Proterozoic age. A veneer of Mesozoic and Cainoz- oic sediments conceals much of the Early Proterozoic sequence, particularl n coastai y l areas.

135 The uranium deposits post-date the -1800 Ma regional metamorphic event; isotopic datin f uraninito ge or d e galenan th e n i a bodies indicates ages of mineralisation at -1600 Ma, -900 lia and -500 Ma. The ore bodies have a number of features in common; they are stratabound, located within breccia zones, are of a shallow depth, containe n rocki d s that underwent low-temperature retrogressive rnetamorphism and metasomatism, and occur immediately below the Early/Middle Proterozoic unconformity. It is suggested that these ore bodies were produced by downward percolating meteoric waters transporting uranyl complexe so reduct whic e du -h ing conditions in the breccia zones precipitated uranium oxicte.

As continental crustal development in post-2,200 lia times appears mainl o follot y w uniformitarlan lines e onlth ,y variable which could explain the concentration of vein-type uranium in the 2,200-1,70 perioa 0li d a steadilappear e b o t sy evolving atmosphere. s suggesteIi t d that during these time e hydrospherth s s wa e sufficiently oxidising for uranyl transport, but that rapidly reducing conditions were met short distances into the lithosphère. Reduction resulted in precipitation of uranium as DO- from meteoric water into suitable structural traps, which were largely developed during period f prolongeo s d erosion e structuraTh . l trapy ma s also have been active durin e earlth g y sedimentatio e Middlth f eo n Proterozoic cover rocks. Rapid developmen f impermeablo t e cover rocks preserved the uranium deposits.

INTRODUCTION e PinTh e Creek Geosynclin a roughl s i e y triangular aref o a 2 about 66 000 km south and east of Darwin, which contains Early Proterozoic metasediraents, dolente and granite interspersed with minor granitoid Archaean basement domes. It is surrounded by younger sedimentary basins from Middle Proterozoic to liesozoic in s mantlei aged n manan ,i d y part y Cainozoib s c depositse Th . economic uranium occurrences define 3 fields, the Rum Jungle and Alligator Rivers fields each centre n inliero d f Archaeao s n base- e Soutth ment d h an Alligato, r Valley n arefiela f extensivn o ai d e dip-slip faulting (Fig. 1).

Of the present known reserves of vein-type uranium in the western world, approximately 90% are confined to rocks of Proteroz-

136 GEOLOGY OF THE PINE CREEK GEOSYNCLINE CRETACEOUS } Sandstone stltslone

~~! Stltstone sandstone minor limestone PERMIAN j conglomerate CAMBRIAN | L/mestone sandstone stitstone basalt ORDOV'CIAN I conglomerate Sandstone snate siltstone dolomite FITZMAURICE GROUP | conglomerate Siltstone shale sandstone minor AUVERGNE GROUP dolomite

BULUTA GROUP Siltstone minor dolomite

TOLMER GROUP Sanc-alt-^ dolomite siltstone

MOUNT RIGG GROUP Sandstone conglomérats limestone

Sandstone minor limestone siltstoie KATHERINE RIVER and conglomerate CROUP Interbedded intermediate basicto volcanics

OENPELLI OMneDOLER'T| o fd dolente | E and differentiates

EDITH RIVER I Acid and mino basic volcanics VOLCANICS pyroclasticsI sandstoner Granite adametlite granodionte minor syenite

HERMIT CREEK SchistI gneiss amphiöolite migmatite METAMORPHICS MYRA FALLS METAMORPHICS i Pelitic schist lit par lit gneiss

LITCHFIELD COMPLEX '•$, Granite granodionte pegmatite 1 migmatile

NIMBUWAH COMPLEX I Migmatite granite granodionte

NOURLANGIE SCHIST QuartzI schist

Dolente1 differentiatesand ZAMU DOLERITE | amphlbollte in east FINNISS RIVER Siltstone greywacke sandstone acid to GROUP basic lavas pyroc/astics SOUTH ALLIGATOR Carbonaceous ferruginousand shale with GROUP chart bands carbonate tuff andésite bil MOUNT PARTRIDGE Sandstone siltstone shale quartzite anVose GROUP I conglomerate dolomite scnlsl and gneiss m east Sc sl 8 ss FORMATION Ül 9" ' Quartzite carbonaceous with j carnanate^ 3naoaic silicate gneiss lowern i part .,..,„„.,. „„„ , I ; 1 Calcareous and carbonaceous shale sandstone NAMOONA GROUP i Pn limestone, dolomite erkose conglomerate \ i schist ana marble inleas eastt MANTON GROUP Magnesite dolomite conglomerate

KAKADU GROUP Leucogneiss quartzite schist

NANAMBU COMPLEX Leucograntte migmattte gneiss granite schist

RUM JUNGLE Gneiss granite schist metssedtments COMPLEX —t—*• Syndine showing plunge Prevailing foliationof dip strike and WATERHOUSE COMPLEX Grange gn&iss amphtboliie migmalite —&— Overturned synclinp Significant uranium mine or deposit dolomite metasetftments —3—*" Anticline showing plungi sc Prevailing strataof dtp strike and Uranium prospect or minor mine f thie I ageoi s. skewnes s reali s f conditiona uniqu,o t se e s i s require . o account dit r fo t

The development of uranium in quartz-pebble conglomerates, confined as they are to the period 2,800-2,200 Ma (Robertson, 1974), has been ascribed to changing plutonic activity in the period 3,000-2,700 Ma (Moreau, 1974) as well as to the prevailing atmospheric conditions (Simpso d Bowlesan n press)n i , . Develop- ment of quartz-pebble conglomerate uranium deposits thus coincides with major tectonic changes correspondin e firsth to t gdevelopmen t of stabilised cratons and ensuing development of intracratonic sedimentary basins (Glikson, 1979; Sutton, 1979). This tectonism, coupled wite firsth h t developmen f algao t l colonies (Nagy et al., 1976) create a favourabld f conditiono e t th se e r fo s formation of quartz-pebble conglomerate uranium deposits (Pretor- ius, 1975).

If, however e post-2,20 th e loo w ,t a k a period0M , which also corresponds to the first extensive development of red beds (Cloud, 1976 ther, ) y unique an doe t appeae b eno s o geologicat r l and/or chemical conditions to account for the original concentration of vein-type uranium deposits within the 2,200-1,700 Ma time span.

Our studie f vein-typo s e uranium deposit e Proterozoith n i s c have been confined mostl o thost y e withi Lowee th n r Proterozoic sequenc e Pinth e f Creeo e k Geosyncline. Using thir s ou are s a a type-model, we have attempted to account for the extensive developmen f vein-typo t e uranium deposits originally formed withie th n time span 2,200-1,70. 0Ma

REGIONAL GEOLOGY f Earlo m k y 4 Proterozoi1 o t p U c metasediments with interbedded volcanics and mafic sills are preserved in a layercake sequence which generally thins westwards (FigA ; 1 Tabl. . 1) e regional metamorphic event between d 180187an 00 m.y. metamorphosed the strata to mainly greenschist faciès; however, amphibolite and minor granulite faciès dominates in the northeast, in the Alligator Rivers region. Proven Archaean rocks are restricted to mainly granite-gneis m JungleRu e th , f Waterhouso s d an e Nanambu Complexes, which form mantled gneiss domes near the presently exposed wester d easteran n n margine inlierth f o s.

138 TABLE 1. SUMMARY OF ARCHAEAN TO MIDDLE PROTEROZOIC STRATIGRAPHY OF THE PINE CREEK GEOSYNCLINE

Thick- Unit Lithology ness Age (m) (Ma) MINOR DOLERITE quartz dolerite dykes and small plug-like 1200 + 35 Sw a bodies öS MUDGINBERHI PKONOLITE phonolite dykes 1 1316 + 50 t* o MUNMARLARY PHONOLITE äW §U TOLMER GROUP sandstone, dolomite, siltstone 1000 0 KATHERINE RIVER GROUP sandstone, conglomerate, minor greywacke, 1200 1648 + 29 (basalt) s - siltstone. Interbedded basalt-andesite volcanics and pyroclastics

OENPELLI DOLERITE layered tholeiitic dolente lopoliths <250 1688 + 13

EDITH RIVER GROUP ignimbrite, microgranite, rhyolite, minor 1200 1808 + 8 (ignimbrite) (J X basalt and cherty sediments; basal sandstone, arkose 2« l g Z POST-OROGENIC biotite granite, adamellite, syenite, 173 180- 0 0 GRANITo < : E EMPLACEMENT granodiorite (numerous plutons) ) in ' T1 EL SHERANA GROUP rhyolite, greywacke, siltstone, sandstone, basalt

< 2 o -HMYRA FALLS METAMORPS HIC layered schist, gneiss (metamorphosed and 1800 s"! 't NOURLANGIE SCHIST partly migmatised Early Proterozoic sediments) a " NIMBUWAH COMPLEX granitoid nugraatite, granite, gneiss, schist 180 1873- 0 (anatexi Earlf so y Proterozoic granite)

ZAMU DOLERITE layered tholeiitic dolerite sills and minor <2500 0 17 191 + 4 dykes

FINNISS RIVER GROUP siltstone, slate, shale, greywacke, arkose 1500- (flysch) quartzite, schist, minor interbedded 5000 volcanics

SOUTH ALLIGATOR GROUP pyritic black shal d siltstoneean , chert- <5000 3 (dacite188 + 4 ) (shallow marine bande d nodulatean d d hematitic siltstond ean chemical, volcanic) black shale, algal carbonate, banded iron formation, jaapilite, tuff, greywacke near top MOUNT PARTRIDGE GROUP sandstone, siltstone, arkose, shale, conglom-

-18 7 (supratidal, fluviatile) aceous near base with lenses roagnesite

NAMOONA GROUP pyritic carbonaceous shale and siltstone <3500 (shallow marine, calcareous in places, calcareous sandstone, chemical, detrital, tuff, agglomerate; arkose, sandstone and supratidal) massive dolomite in west. EARL Y PROTER C

KAKADU GROUP sandstone, arkose, siltstone, conglomerate, -1000 (fluviatile) quartzite, schist, gneiss

NANAMBU COMPLEX granite, augen gneiss, leucogneiss, minor 1800 (gneiss) quartzit d schisan e t (includes accreted Early -2500 (granite) Proterozoic metamorphics)

Sj S RUM JUNGLE COMPLEX coarse, medium porphyritid ,an c adamellite, O in WATERHOUSE COMPLEX biotite-muscovite granite, migmatite, gneiss, 2500 S3 schist, pegmatite, meta-diorite, banded iron formation

The higher grade metasediments grade northeastwards into the migmatitic Myra Falls Metamorphics, where in places sedimentary 1 resistors' indicate the sedimentary parentage and allow correlation with the un-migmatised terrane. The intensity of folding also increases generally towards the northeast - tight to isoclinal folds with stee pe centra limbth n i sl parts progress through overturned tigh o isoclinat t l fold o polyphast s e isoclinal folds in the Alligator Rivers region. In the extreme northeast pre-

139 orogenic granit s partlwa e y migmatised along wite surroundinth h g metasediments. The metamorphics are unconformably overlain by valley-fill felsic volcanic d pyroclastican s d thes an e sintrude ar e y postb d - orogenic granites and dolerite. All these rocks are overlain with marked regional unconformity by Middle and Late Proterozoic plateau sandstone and volcanics.

Basement Complexes Archaean ages are established for the Rum Jungle and Water- house Complexe e westerth n i s ne Geosyncline th par f to r fo d ,an parts of the Nanambu Complex in the east (Page, 1976; Page et al., 1980; Richard t al.e s , 1977). Walpol t al.e , (1968) interpreted the Litchfield Comple d Hermian x t Creek Metamorphicse th n i , extreme west s wholla , r partlo y y Archaean; little isotopic work s beeha n don n theso e e rocks Berkmat bu , n (1980) reports muscovite- biotite schist in the Litchfield Complex as a probable correlative of the Burrell Creek Formation (topmost unit in the géosynclinal pile) d Needhaan , t al.e m , (1980) suggest that granitee th n i s Complex have some feature n commoi s n wite migmatitith h c granites. Page (1980) indicates that the Rb-Sr systematics of the Litchfield Complex are similar to those of the 1870-1800 Ma Nimbuwah Complex.

Rum Jungle and Waterhouse Complexes The Rum Jungle and Waterhouse Complexes are roughly oval 9 granite-gneiss domes up to 400 km in the west of the Geosyncline. e dominanth Thee ar yt m JunglrocRu ke eunitth districtn i s , with anomalous concentrations of uranium (12 and 13 ppm respectively, Ferguson & Winer, 1980) . Each Complex contains several granite and gneissic granite phases with veins and dykes of pegmatite and amphibolit d tourmalinan e e veins. Metasediments, banded iron formation, volcanic d metabasican s e Complexeth n i se though ar s t to pre-date the granites. On field relationships the leucocratic granite of the Rum Jungle Complex is considered to be the youngest phase; this rock typf abouo ee givetag 240n a sa (Richard 0M & s Rhodes, 1967; Pag t al.e e , 1980). Rhodes (1965) comparee th d Rum Jungle Complex to a mantled gneiss dome; Stephansson and Johnson (1976) suggested that both Complexes were domed by solid- state diapric intrusion of granite during the 1870-1800 Ma orogeny.

140 Nanambu Complex Rocks of this Complex are poorly exposed over an area of m betweek 0 e 8 Sout th d ny Eas b an h m tabouk Alligato 5 4 t r Rivern i s the northeas Geosynclinee th f o t . Needha Stuart-Smit& m h (1980) defined three groups of rocks in the Complex: Archaean two-mica granite; porphyroblastic foliated biotite granite and augen gneiss which is Archaean granite metamorphosed by the 1870-1800 Ma event; and pegmatoid leucocratic gneiss, migmatit d schisan e t whics i h Early Proterozoic arkosic material accrete e granitith o t d c Complex at 1870-180 . 0Ma Fiel d distinctio t phase3 alway no e s th i s f o n e relationshipag cleare Th phasee .th f so s were indicatey b d isotopic dating techniques (Page et al., 1980). On the southern and western margins of the Complex the leucocratic gneiss may contain resisters of Early Proterozoic quartzite, and the leucocratic gneiss grades out into meta-arkose of the Early Proterozoic metasedimentary sequence. The Nanambu Complex is a likely source of uranium for the Alligator Rivers area uranium deposits, as the two-mica Archaean granite gneiss is known to contain accessory uraninite (McAndrew & Finlay, 1980). Mean uranium content is 9 ppm U (Ferguson & Winer, 1980).

Géosynclinal sedimentation e stratigraphTh e Earlth yf o yProterozoi c sedimentd an s interbedded volcanic rocks is summarised in table 1, and diagrammatic relationship e show ar sn figur i n . 3 e f sedimento m t leask A 4 d volcanic1 t an s s were depositen i d parts of the basin. The margins of the basin are concealed by younger strata, so the size of the basin is unknown. The basin is probably continuous under cover with other Early Proterozoic sequences in northern Australia such as the Arnhem, Tennant Creek, Granites-Tanami and Arunta Blocks (Fig. 2). Sedimentation took place between 2400 Ma (the age of the youngest basement rocks) and 1870 Ma (the age of pre-metamorphic granodiorites) in the northeast (Pag t al.e , 1980). Volcanics interbedded wite th h sediment e Soutth n h (i Alligatos r Group f abouo )e tyielag n a d 1890 Ma (R.W. Page, pers. comm. May 1983). The sediments are mainly pelites, whic commonle ar h y carbonaceous and/or pyritic and/ or calcareous at depth. Other lithologies present are sandstone, crystalline carbonate, calcarenite, tufd conglomeratean f .

141 Money Shoa / lArafur a Basin

r^NANAMBU COMPLEX/

Bonapart eJUNGLM GulRU f E COMPLE Basin LITCHFIELD COMPLE COEN INLIER FITZMAURi££_MOBIlE ZONE/\

Carpentaria Basin Browse Basin BATTEN FAULT ZONE

\NICHOLSON £W/« •/'!•• LAWN HILL PLATFORM

• HALLS CREEK TENNANT CREEK INLIER . INLIER . . X Georgina THE GRANITES- Basin i • ' . TANAM• I INLIER l Broken River 21 «Off DAVENPORT Embayment , ARUNTA INLIER INLIER • I

NGALIA BASIN.

• AMADEUS BASIN

129°00- MUSGRAVE BLOCK

OROGENIC PROVINCES PLATFORM COVERS

/ f Tasman Trans-Australian / / m Central Australian Central Australian • * y/ North Australian * *. • • North Australian

West Australian

Fig. 2 Principal tectonic elements, northern Australia (Plum t al.e b , 1981).

The psammitic units are mainly fluviatile. The provenance e Namoonth d f Kakado an a u Group psammite s probablwa s e adjacenth y t basement complexes, whilst grainsize distribution suggests a northern provenanc Moune th tr Partridgfo e e Groupa wester d an , n source for the Finniss River Group (Stuart-Smith et al., 1980). Widespread algal carbonate and evaporite pseudomorphs indicate mainly very shallow to supratidal environments for the carbonates (Crick & Muir, 1980). The pelites were probably deposited in neritic conditions, and the greywackes may represent tectonic deepening of the basin heralding the 1870-1800 Ma event. Sedimentation took place in an intracratonic basin covering o at least 100 000 km . The earliest known Early Proterozoic sediments, the psammites of the Narnoona and Kadadu Groups, rest

142 unconformabl n Archaeao y n basemen d exposuran t s generalli e y confine o areat d s adjacen o exposet t d basement. These basal sediment y extenma s d ove e basir th muct depth a nf o h . e Thosth f eo Namoona Group surround the Rum Jungle and Waterhouse Complexes,and contain arkosic rudite, psammite, conglomerate, and minor shale. The Kakadu Group, which is best developed adjacent to the Nanambu Complex, comprises mainly meta-arkos d paragneissan e , whicn i h places grade into leucogneis Complexe th f o s . These psammite e overlaiar s y carbonaceousb n d calcareouan s pelite d psammitean s Massoe th f no s Formation, which extends from west of the Rum Jungle Complex almost to the South Alligator River; further east it is probably equivalent to the lower member of the Cahill Formation, a partly calcareous and carbonaceous sequence of micaceous quartzo-feldspathic schist, with lense f massivo s e carbonate near its base (Needham & Stuart-Smith, 1976). The Cahill Formation is host to uranium and other mineralisation in the Alligator Rivers area. Volcanic breccia and basalt of the Stag Creek Volcanic e conformablar s e Massoth n no e Formatioe th n i n centra e Geosynclinel th par f o t . Elsewhere, constitutine th g Mount Partridge Group, a wedge-shaped assemblage of sandstone, siltstone and conglomerate of the Mundogie Sandstone and Crater Formation, overlie Massoe th s n Formation t representI . n a s extensive fluvial fan which grew from the north, although coarser JunglRum clastethe arein s a indicate rejuvenatio basemenof n t inlierweste th n .i s Stromatolitic carbonat e Celith d an af o e Coomalie Dolomites indicate that Archaean basement formed islands in the Rum Jungle area until at least mid-Mount Partridge Group time. The alluvial fan deposits fine upwards into banded partly carbonaceous siltston Wildmae th f o en Siltstone s equivalenIt . t m JunglRu e eith n area Whitee th , s Formation, hosts U-Cu-Pb-Zn deposits at or near the contact with the Coomalie Dolomite below. Minor mafic volcanic units lie near the top of the Wildman m Jungle Ru m eask f o t0 Siltston .10 o t p u e A distinctive assemblag f iron-rico e h sediments, carbonate, an e Soutd th tuf hf o fAlligato r Group rests n placei , s unconformably, over the older groups. The lowest member of this group, the Koolpin Formation mostls i , y pyritic carbonaceous shale which commonly contains chert-band d nodulesan s , algal carbonated ,an

143 banded iron formation. These rock e generallar s y altered near- surfac o chert-bandet e d hematite siltston d massivan e e siliceous rocks. Chert-banded hematitic siltstone also s intera crop t ou s- beds in the overlying massive tuff and argillite of the Gerowie Tuff. Rocks similae ferruginouth o t r s siltston e Koolpith f o en Formation also crop out in the Alligator Rivers Uranium Field, e criticaanar d o correlatiot l e northeasterth n i n e n th par f o t Geosyncline. Conformit e ash-falth f o y le Gerowi rockth f o se Tuff verifies e chronostratigraphith c e sedimentarnaturth f o e y e pileTh . Gerowie Tuff is overlain by Kapalga Formation rocks, which are a transitional suite between Koolpin-type rocks (less iron-rich and less cher e Koolpit th tha n i nn Formation d greywackan ) e th f o e Finniss River Group, with interbed f Gerowio s e Tuff-type tufd an f argillite. Uraniu + golm d deposit e Soutth hf o sAlligato r Valley area are hosted by pyritic highly carbonaceous shale of the Koolpin Formation, and commonly lie on the faulted contact with pyroclastic sandston f post-orogenico e , late Early Proterozoic age. The uppermost part of the Early Proterozoic succession is represented largel monotonoua y b y s sequenc f siltstoneo e , slate, shale, greywacke, and minor arkose, quartzite, conglomerate, and schist, of the Burrell Creek Formation of the Finniss River Group. In the west, where greywacke is mostly volcanolithic, these rocks grade laterally and upwards to quartz sandstone and minor conglomerate of the Chilling Sandstone (Walpole et al., 1968). The Burrell Creek Formation commonly displays turbiditic sedimentary features, including graded beds, scour marks, flute castd an s flame structures.

Transitional igneous activity At or near the end of sedimentation, the Zamu Complex, a suite f continentao l tholeiites wit a composith e thicknes f abouo s t , intrudekm 5 e sediment1. th d s mainl s sillsa e ysill Th .s were folde d metamorphosean d d wite hosth h t e 1870-180rockth y b s a 0M regional event (Ferguso Needham& n e mediu, th 1978) o hign t I m h. grade metamorphic domain and in the contact aureoles of the post- orogenic granite e rock th e amphibolites ar s t elsewherbu , e th e original texture d mineralogan s e partlar y o whollt y y preserved; in places hornfelsic zones in the adjacent sediments are apparent.

144 e rockTh s range from quartz dolerite with felsic differentiates, to dacite, and are commonest in the South Alligator Group. The close of Early Proterozoic sedimentation is probably indicated by intrusion of granodiorite and tonalité of the Nimbuwah Complex e extremth n i , e northeas e Geosynclineth f o t t aboua , t 1870 Ma (Page et al., 198Q). This event may also have triggered off a long period of deformation and metamorphism which culminated at aboue whic ag s ubiquitousl ti e h 180th , 0Ma y recorde y K-Ab d r isotopic systems in the metamorphics and many of the basement rocks of the region. The tonalités and granodiorites of the Nimbuwah Complex underwent partial migmatisation in this period, as did the Early Proterozoic sediment e samth en i areas . Resisterf o s quartzite, carbonaceous schist, calc-silicate gneiss and marble, indicate that some of these rocks were originally of the Kakadu Grou r Cahilo p l Formation; those metamorphic rock o whict s o n h sedimentary parentag e objectivelb n ca e y determine e ascribear d o t d the Nourlangie Schiste partlth r yo , metamorphically differentiated Myra Falls Metamorphics (Needham, 1982) . The metamorphics were intruded by numerous post-orogenic granite plutons, whic e hrang ag havf abouo n ea e t 1800-174a 0M (Page et al., 1980). Several plutons coalesced to form the Cullen Granite batholit e Geosynclinee centrth th n f i o eh . e th Thi s i s 2 largest granitic body (3600 km ), and extends under cover rocks of

the Daly River Basin (Tucker et al., 1980). Diapiric intrusion of 'solid state 1 granit m Jungle Ru belod Waterhouse an eth w e Complexes s beeha n describe mechanisa s a d r rejuvenatiofo m f theso n e Complexe y uplifb s t (Stephansso Johnson& n , 1976). Felsic and minor mafic volcanics and volcaniclastics of the late Early Proterozoi l SheranE c a Grou d Editan p h River Volcanics (-176 ; LeggMa 0 Walpoln i o t al.e , 1968 e probablar ) y comagmatic with the diapiric granites. A significant period of erosion betwee e 1870-180th n a metamorphi0M d deformatioan c n eventd an , extrusion of volcanic rocks in these two groups, is indicated by their valley-fill nature.

The final igneous event prior to Middle Proterozoic platform sedimentation was intrusion at 1699 Ma (Page et al., 1980) of a series of continental tholeiitic lopoliths - the Oenpelli Dolerite - a tdeptm k abou 2 h 1- t(Stuart-Smit Ferguson& h , 1978 ). These intrusions consist of porphyritic olivine dolerite, quartz dolerite

145 and granophyre sheets, in places over 250 m thick, and minor dykes. The lopoliths form long, arcuate basement ridges to platform sedimentation, indicating another substantial perio f erosiono d . e patterTh f sedimentatioo n n followe y orogenesib d a d an s period of post-orogenic igneous activity seen in the Pine Creek Geo- synclin s consisteni e t wite modeth hf evolutioo l f Proterozoio n c mobile belts describe y Gliksob d y nb (1976)G PC d appliee an ,th o t d Stuart-Smit t al.e h , (1980).

Cover rocks Middle Proterozoic plateau-forming sandston d minoan e r interbedded, mainly intermediate, volcanic e Kombolgith f o s e Formation rest with marked unconformit n oldeo y r rocks, coverin e easterth g n extent of the Early Proterozoic metasediments and their gneissic equivalents whicw inlier e exposefe ar ha n si dwithi e sandstonth n e plateau. Minor dolerite intrusions of Late Proterozoic age cut the sandstone in places; some additional similar intrusions may be represente y lonb d g linear magnetic patterns coincident with major fractures in the sandstone. These intrusions, and Late Proterozoic phonolite dykes which intrude the Nanambu and Nimbuwah Complexes, represen e finath t l known igneous e regioeventth n i ns (Page et al., 1980) . Late Proterozoic sediments are exposed at the margins of extensive Cretaceous tableland e southwesth e Pinn th i se f o tCree k e sandstone-dominanth Geosynclinef o e ag e Th . t Tolmer Group which e samth en i cropare s obscuret i aou s o relationshipn s a , e ar s exposed betweed rockan f provet o si n n Late Proterozoic aget i ; appear se Lat basath e o t Proterozoil c sequence e northerTh .d an n southern margine Geosynclinth f o s e concealear e y Mesozoib d d an c Palaeozoic strata. A palaeo-weathering profil s commoi e n belo e Kombolgith w e Formation, and a rare regolith is developed in some places. A regolith is common below the Cretaceous, and lateritisation took place durin e Tertiaryth g . Hays (1965) identified three Cainoz- c lanoi d surfaces whic e commonlar h y superimposed d extensivan , e deep weatherin e developedar ) m g0 6 profile .o t p (u s The proximity of Kombolgie Formation sandstone to the uranium deposit e Alligatoth f o s r Rivers ared Soutan a h Alligator Valley, and of the Depot Creek Sandstone Member of the Tolmer Group to the uraniu m Jungl Ru deposit e eth area f s o focuses ha , d attention o n

146 the sandstone as a possible source of uranium or as a vital part of uranium ore genesis. The Kombolgie Formation is not however rich in uranium and airborne radiometric anomalies in the Formation are restricte o latéritet d s developed downslope from interbedded intermediate to mafic volcanic members (Needham et al., 1973). Heavier elements may be concentrated in purple beds which are commo n somi n e area se e unitlithologie neath Th e bas .th f r o e s are mainly coarse, clean, moderately well-sorted subangulao t r sub-rounded buf o whitt f e quartz sandstone, with pebbly horizons and conglomerate beds more common neae baseth r . Claste ar s mainly moderately to well-rounded quartzite and vein quartz; clasts of Early Proterozoic rocks are rare. There are very rare siltstone beds up to 5 cm thick. The sandstone is commonly devoid matria f o x except neare e basth re wher whita e e clay matris i x often present. The rock ranges from loose and friable, to well indurated o silicifiet , n stablo d e exposed surface d alonan s g joints. e unconformitTh y morphologa plan s ha e y simila o that rf o t the undulating sand plains which cove e rGeosynclin th muc f o h e today. The unconformity undulates gently with an amplitude of aboum althoug 0 2 t h ther e scatterear e d basement hill d ridgean s s up to 250 m. The Oenpelli Dolerite forms large arcuate basement ridges up to 100 m high and the post-orogenic granites in places form plateaux about 100 m above the general level of the unconform- e unconformitTh ity. y plan s displacei e y numeroub d s faults which mainly thro e sandstonth w ew ten onlfe f metres o sa y . Howevera , throw of at least 200 m has been substantiated for the Koongarra reverse fault (-Foy & Pedersen, 1975) . Extensive faulting of block f Kombolgio s e Formation sandston s recordei e r neae o th n ri d Ranger 1 and Jabiluka 1 ore bodies (Eupene et al., 1975); Hegge, 1977). Ther y broae an doe t appeae db no s relationshi o t r p between the elevation of the unconformity and the distribution of mineralisation. The contact at the base of the Depot Creek Sandstone Member is regionally anomalously radioactive, but the unconformit e Kombolgi th e bas f th o e f o ye Formatio o radion s ha -n metric expression. However, metamorphic d granitean s s beloe th w Kombolgie Formatio e commonlar n y altered placen i , o mort s e than 50m belo unconformitye th w .f o p Granitto gneisd e an th e t a s this palaeo-weathering-profile are commonly altered to a blotchy pale-gree d purplan n e quartz-clay rockn placeI . s this profile

147 may be more radioactive than its unweathered parent, but no analyses are available of the amount and distribution of uranium within this profile.

DISTRIBUTIO F URANIUO N M OCCURRENCES Sixty-seven uranium occurrences (Table 2) with visible uranium minerals, or significant uranium prospects, are scattered mainly in the east in a broadly Nt-trending belt which terminates abruptl a linea t a y r edge alon e Soutth g h Alligator Valley uranium field trend v alonan NV , da g running from JunglRu m o Katherinet e , (Fig. 4). The pattern ^artly reflects areas of most intense exploratio r uraniumfo n , i.e. aroun e deposit th e thred th ef o s major fields d alon an e ,Stuar th g t Highway (the latte y handb r - held geiger counters clutche y 'weekenb d d prospectorse th n i ' 1950's; Annabell, 1971) , but some alignments are indicated which seem to control the broad distribution of the deposits. This apparent contro s strongesi l e Soutth n hi t Alligator Valley area, where most of the deposits are close to a major NW strike-slip fault (Cric t al.e k , 1980).

In an analysis on the distribution of metallic mines and prospect e Pinth en i sCree k Geosyncline, Needha n press(i m ) indicates a strong relationship between the distribution of uranium occurrence e Cahillth d an ,s Whites d Koolpian , n Formations (Fig. 5a; 26, 13 and 22% of occurrences respectively. e dominancTh f favoureo e d stratigraphie unit n determinini s g uranium distributio s evei n n clearer when uraniu s expressei m s a d tonnes U.,0 (Fig« 5b). There is also a strong positive relation-

ship betweenR distribution of uranium occurrences and proximity to Archaean masses (Fig. 6), although no occurrences of significance are known within them. A low-order relationship is evident with lineament s bot(a sh lineament densit d lineamenan y t intersections), and alignments represented by the distribution of uranium deposits could indicat a fundamentae l fracture controe th o t l distribution. No relationship is evident with the distribution e post-orogenith f o c granites, even thoug e uraniuh th mos f o tm prospects in the south are hosted by them. The relationship between uranium distributio e present-dath d an n y positioe th f o n cover-rock unconformity may be more apparent than real, and more properly relatee thinneth o t dr Mesozoi d Cainozoian c c cover over

148 TABLE 2. URANIUM MUTES AND PROSPECTS OF THE PINE CREEK GEOSYNCLINE (aaended from Needhaa, 1981)

«* TT 'S

Black Rock U 1 prospect disseminated irregular, disseminated uraninite chlorit mice* a quartz Nimbuwah retrograded unconformity, îfault, metaMorphic hydrothermal offset by faulting schist Complex m.amphibolite chlonte alteration related* Tadpole V 2 prospect irregular to planar, thin uraninite in pegmatite quartz mica schist + M n.amphibolite pegmatite hydrothermal quart ze it Babarlek 3 U 12,000 1.98* steep tabular to shallow massive and disseminated chlonte mica quarts Myra Palls retrograded shear zone ?unconformity, metamorphic hydrothermal (stockpiled. cigar pitchblende, near-surface schist Metamorphic 3 m.amphibolite îchlorite alteration un e. relate strataboun? d , d being treated) oxidised halo (relict) " u 4 prospect tabula 0 6 r secondary near surface quartz mica schisd an t Cahill * graphite schist Formation Oumgarn J T 5 prospect tabula 0 6 r secondary near surface quartz mica schist and H m.amphibolite stratabound, quartz stratabound, metamorphic graphite schist stockwork breccia hydrothermal epigenetic+ , , uncf. related?

Oorrunghur * 0 6 prospect secondary near surface quartz mica schist It « Caramal 7 ö prospect arcuate ban ° d?0 sooty pitchblendn i e hematitic chlorite quarts Myra Palls retrograded unconformity, ?stratabound stratabound?, metamorphic breccia veins schist Metamorphics o.amphibolite (relict), chlorite alter- hydrothermal + epigenetic, ation? uncf. related?* Béatrice 0 8 prospect irregular o planat r steep minute disseminatiod nan sericite hematite mica Nimbuwah « shear zone breccias, metamorphic hydrothermaf - l vein-filling filmf o s chlorite quartz schist Complex unconformity epigenetic, uncf. related? sooty pitchblende, secondarie weatheren i s d zone Arrara U 9 prospect tabular shallow secondary, at base of chlorite quartz sericite Koolpin » pre-Kombolgie weathering stratabound, uncf. related, pre-Kombolgie weathering schist Formation? zone, ?stratabound, ?fault ?epigenetic profile Jabiluka 1 Ü 0 1 (3,484) 0.25e* folded layers and leases disseminated & Tein filling quartz sericite chlonte Cahill retrograded stratiform, unconformity, stratabound, metamorphic 0-70 , in 4 main strat- pitchblende, A selvages to + graphite schist Formation m.amphibolite chlonte alteration, hydrothermal & epigenetic, iform ore-bearing horizons chlorite A alteration zones. brecciation uncf. related? Minor coffinite and brannerite

Jabiluka 2 u A , U (203,8001 1 ) 0.39* » " it H H n it (8.1 Au) 15T A,W u Jabiluka 3 U 2 1 prospect simila o Jabilukt r 2 A a1 simila o Jabilukt r 2 & 1 a H N Ranger 5 13 U proapect n m.amphibolite stratabound? Ranger 66 a 14 testing b«st reported irregular breccia pipe soot collofory& m pitch- chlontised breccia& it retrograded chlorite alteration & stratabound & metamorphic incomplete drill inter- blende. Minor secondaries pegmatoid, quartz seric- m.amphibolite brecciation at Nanambu hydrotherma epigenetic+ l , sections 13 m • ite chlorite schist; CompleVCahilm F l uncf. related? 0.36*0 m 8 , nearby carbonate contact 0 0.76* m 8 2 , 0.38*

Ranger 4 U 5 1 prospect 'medium to low irregular to tabular chloritic pegmatoid m « ti H grade1 small body brecci n dolomiti a e

7J u A , U prospec6 1 t 'viable ore body disseminated pitchblende, chlorit graphite+ e schist, » retrograded stratabound, chlorite strataboun metamorphid+ o likely1 near-surface secondaries hematitic near surface m.amphibolite alteration, unconformity hydrothermal •»• epigenetic, uncf. related? 17 B (727) pitchblend n fracturei e & s chlorite schist, chloritic chlorite alt°ratiod nan breccia zones, minor thuc- breccia brecciatio t Nanambna u olite, near-surface second- CompleJt/Cahill Pm contact, aries unconformity Table 2 cont'd

00 t, aj °° w 1 i - * o o ~ « . t G "• 4i o '^ « .n e .e t 3 ~* § 3 >, 3 1 t- i-i 1 ï ^ | i ï î 1 ! n Es I û: X 3 S 1 1 2 J 1 s

»ustato» 1 18 U (confidential) best reported disseminated pitchblend n i e Lowe thilC r l schists Cahill retrograded chlorite alteratio d an n btrataboun metamorphi• d+ c drill intersect- 0 0.48S6 m ion 5 ,1. s CompleVCahil m F l uncf. related? 0 0.215 m 5 6 ^.ontact, unconfcroity Anomal4 Range U . 9 No 1 y1 r prospect 'thin, low grade n tt mi nera lsai toi n' No 3 OrebodU . 0 2 y (50,000) 0.36 shallow-dipping tabular n » o lenaoit d body with conformable based an , blind, down-dip lenses No. 1 Orebody 21 0 (50,000) 0.386 inverse cone with down-dip conformable filmy pitchblende mainly gneiss, silicified Nanamb& tarb-u chlo e nalterationt , sil- hydrotherma epigeieti+ l c lenses in chlorite rock and chlor- bonate, massive chlorite, Complex icifi.cation, unconformity - probably carbonate s Ijt- No. 2 Anooaly 22 U prospect ' significant ite schist, and carbonac- graphite schist C<»1 " ion. Uncf. i elated' primar second& y - ecus schist, particularly r catit. i ary mineralisat- ion1 No 9 tnoialU . 3 2 y prospect 'medium grade n H secondary mineralisationo (n 1 deeper drilling) No. 5 taooaly 2« V prospect n ti Koongarra 25 U (12,300) 0.34656 coalesced stratiform disseminated pitchblende, quartz chlorite * graph- " " stratabound to stratiform stritabouid + oetaoorphic lenses form elongate secondary tail near ite schists, garnet chloride alteration, brecc- hydrothermal + epigeneti , wedge, dip 55 surface quartz muscovite chlorite iation, ?faulting, imcf. related' schist 'unconformity 0 6 Tiv2 e Sisters prospect no visible uranium miner- carbonaceous phyllite Nou'-langie " carbon, unconformity als Schist Graveside 27 U prospect tabular steep lodes Hit«, nearby aeta- dolerite U 8 2 AnouQ J 2 y prospect 0.99* 'yellow-grepn micacpois weathered tuffac«>ous " Stag Creek faulting, ?stratdbound syngenetic/supergene uranyl phosphate' shal basal& e t Volcanics

Teagues 29 Ut Au linear near-surface patchy secondaries along carbonaceous siltstone, Koolpin Fm & l.greenschisL faulting, carbonaceous eoigenetic faulted sst/sltst contact sandstone Coronatio t Ss n shaK , nearby acid n fracturei & s volcanics, unconformity Bockhole 30 U,t i Au 152 n 1.156 t t several irregula rt t tabular patchy pitchblende H , near- Au included steep bodies along reverse surface secondaries under El fault Sherana O'Dwyers 31 U, Au - » u A , D 2 3 Sterrets " H H K " H H D 3 3 Airstrip prospect near-surfac4 x e no visible minéralisation " Koolpin n it it radioactivity Formation U 4 3 Sandstone prospect 300 ppm U 0 , " andésite, sandstone Plun Tree unmetamorphosed stratabound supergone , TK 485m 0pp lenses Volcanics t a 0 U m pp 4 1 ( depth) 3 8 El Sherana West 35 H,Au,Ag 185 0.8* irregular to carrot-shaped patches, vein dissemin4 s - cherty ferruginous silt- Koolpi 4 B nF l.greenschis t carbonaceous shale, near- epigenetic .007 Ag bodies aligned along ation f pitchblendeo s , stone , carbonaceous Coronation Sst by acid volcanics, Tfault- strike mainl n fracturesyi . Near siltstone, sandstone ing, unconformity D, Au El Sherana % 226 0.5* surface secondaries. .33 Au Minor Au, Pb Koolpin 37 U 3 0.12* sooty 4 massive pitch- carbonaceous shale greensch. 1 Koolpit is n ?fauiting, unconformity epigenetic blende in fractures Formation Scinto 6 36 U 3 0.15* secondary mineralisation sheared and jointed Coronation unmetaaorphosed n epigenetic supergeneT volcanics Sandstone Palette 39 D, Au 124 2.5* pipe incline, 5 4 d vein 4 massive nodular carbonaceous shale, Koolpin Fm & l.greenschist H epigenetic Au incl. under cuts across shale/ pitchblende, veins native sandstone Coronation Sst El Sheraufl sandstone contact gold, minor secondaries Skull 40 V 3 0.5* irregular pitchblende nodules in carbonaceous shale Koolpin l.greenschist Tfaulting, unconformity epigenetic shears Formation

n Saddle Ridge 41 0 78 0.2* steep tabular secondaries, mainly meta- bleached carbonaceous Koolpi& m F n carbonaceous shale, nearby * torbermte, minor sooty shale, rhyolite 4 tuff Coronatiot nSs acid volcanics, fault, pitchblende below orebody unconformity Scinto 5 42 U 22 0.4* irregular secondaries, minor pitch- bleached whitd re & e Koolpin M n supergene? blende ferruginous shale Formation

Coronation Hill 43 U, iu 75 0.3* several irregulae or r sooty pitchblende, dissem- volcanolithic and carbon- Coronation unmetamorphosed carbonaceous shale, brecc- " shoots Sandstone lation, unconformity gold Sleisbeck 44 B 3 0.46* irregular near-surface pitchblende, secondaries, chZoritic carbonaceous Kapalga l.greenschist ?breccia ?epi gene tic in breccia with phosphat d minoan e r schist, quar mudte, tzi - Formation Cu, Hi, Co, As atone

ABC 45 U prospect 0.4$ shallow irregular autunite, phosphuranylite interbedded tuff and Kombolgie unmetamorphosed host volcanics, ?nearby epi geneti c/ syngene ti ct amygdaloidal basalt Formation deep fracturing

Edith River 46 U prospect patchy bodies in N or disseminated torbermte & ailicified greisenised Cullen Granite " hydrothermal alteration N NW shears meta-autunit apatite+ e& granite zone quarts4 z veininn gi hematite in quartz veins granite

n 1 H H tt 47 V prospect " • Tennyson s 48 u prospect it it n H It H H

it it ff ff It It 49 u prospect 0.1-0.2$ n 50 0 prospect at surface " " " N ff N tt

n N H n 51 u prospect H " " H Hör 4 eO'Connor s 52 D prospect « " " H H » Yenbeme 53 prospect steep tabular reefs, greisenised quartz aplite aplite dykes in granite altered aplites near roof M paralle aplito t l e dykes. dykes containing wolfram of granite Also disaem. in joints mino4 r molybdenited an , Bi , WMo , 0.125 Mo & mica-rich seams torbermt quartn i e z veins. n 160 wo Fe, Cu, Bi sulphides at depth

54 f f K n -I Yenbeme 55 v prospect patchy bodies in N or disseminated torbermte & silicified greisenised . hydrothermal alteration II NW shears meta-autunite + apatite 4 granite zones 4 quartz veining in hematit n quarti e z veins granite Fergus son River 56 u prospect patchy bodies in N or disseminated torbermte& silicified grftisenised hydrothermal alteration zones II NW shears meta-autunite + apatite & granite 4 quartz veining in granite hematite in quartz veins Fleur de Lys 57 H, Cu 0,15 patche n conformabli s e uranini te+pyri te+chal- siltstone, slate Koolpin l.greenschist altered shear jointd an s s hydrothermal ; possible shear zone joint4 s s4 copyrite in shears. U+Cp Formation e xhal at i ve/epigeneti c along bedding planes George Creek 58 D 1 0.2» pods & stringers in weak patchy pitchblende, pyrite, it Burrel* l Creek • • steep N shear chalcopyrite in joints 4 Formation

at surface Tabl cont'2 e d Uï to

c*x S £ l ' °° S t- c c c ; 'i«! S I 1 c ET " £ 1 - •2 c - o S 1 1 •ê so 3 1 K X n o S tfc o 1 1

Adelaide River 59 D 45 0.5* steep N shear zone, pitchblende, pyrite, chal- - - irregular ore zone copyrit irregulan ei r pitches 345° quartz veins dissen& , n i . host rocks White's 60 U.Cu.Pb, 1069 0.16* roughly confomable steep uraninite , torbernite , sheared graphitic seric- Whites l.greenschlst Whites Fa/Cooaalie Dolomite syogenetic + apigenetic Co, Ni 1.8 4g, 861 Co, 25.7 g/T 4g 17-27 m wide NE shear chalcopyrite, bornite + itic, chlontic & pyritic Formation contact, stratabound, shear ^ supergene , Cu 0 00 5 1 2.2* Cu zone galena as disseminations shale zones on synclinal limb, 4100 Pb 0.6* Pb + veins nearby basenent granité, unconformity

Dyson1 s 61 U 534 0.34* conformable NE she^r 8 m vein * dissem. uraninite, sheared graphitic shale Whites Fœ & wide, steep saleeite hematitic dolomite Coomalie Dolomite Mount Fitch 62 D, Cu (1500) .042* 2 U bodies, E 110x30x20 m fine ?thucolit n clayei e y chlorite aericite griph- White& B F s (1885 Cu) 290,00« 0T deep, N l60x110x100 m deep, graphite schist bands, ite schist, magnesite Cooaalie 0.65* Cu 3 Cu lenses in -J tan con- veinlets in magnesite. Dolomite formable enriched zone Near surface Cu-enri chment in clays Mount Burton 63 ü, Cu 12.6 0.17* small conformable lens dissem. torbemite, pitch- pyritic black slate, Whites 100 Cu 1.3* Cu blende , malachite , chalco- minor quare t i tz Formation cite

Dolente Ridge 64 Ü.Pb.Cu, prospect N Zn Area 55 65 U,Pb,Zn, prospect irregular pods in shears pyritic black shale, Whites Fm & Cu & plunging fold axes dolomite Coomalie Del OBI te Rua Jungle Creek 66 U prospect chlontic A graphitic Whites shale Formation Rum Jungle Creek South 67 D 2612 0.4* horizontalx ciga» 0 6 r fine sooty pitchblende chlontic A graphitic « deptm 0 3 h , m 5 24 coatings, minor veins & pyritic shale stockworks SOUTH ALLIGATOR JUNGLM RU E AREA CENTRAL AREA HINGE ZONE ALLIGATOR RIVERS AREA

TOLMEft GROUP

MIDDL E LAT •>jXTLr\/\/v\/Xjxr\/>j^/VVV\/^^ \j^y\A/\/v\/v\/V'\/\/v\/\/\/>j^/%y^

'/vV.V ELSHERANA AA „ andA « «A , -EDIT,. * , , H RIVER GROUPS,, î*,î. A

\/\A/VVVV\/\/\/\/V\/Vlj\rv\j 'WW/VWX/WWWWV j ' j / ? / ? ; ; • ? i

' / ' ! ' / i 1 1 l 1 l — " — t Bonni- —~ M f — e— Formatio— — — n— -=-,% — —! ^ { Myr1 , a j i Falls ^ i ' :»*** + **»,.» A, T <->,!> « -« *,T*-(i,.< T A _"<./<*•-• ^v» i* 1*vA .*î — Formation — - — e 7C— )')))> ' i ~*~^ — i— * % m ' Metamorpnic i 1 s ———————————————————

~~~~~~~~~~~ "Wildman Siltstonenj^nr^UTUTJ ^ iîiîif l j z j J ; / ' i ' ' j ' ' / ; ' l 8 I'l 1 1 EARL Y PROTEROZOI C PRO T ! !'!',')')')';') 1 1 1' ' I l 1 ! 1 l Crater Formation

Cahill Formation F S P GROU N A MOUN T PARTRIDG E SOUT H ALLIGATO R mvER J ! i O ) ) — — -^-' 7 .^^J' — —1_ -'-*— >/ — -— S E 1« Öeestons formatio - -^j-Masson^qrmation_j^j^^ n _ KAKADU GROUP ~~\L^- \ \

RUM JUNGLE and W ATE R HOUSE NANAMBU COMPLEX COMPLEXES

Carbonate Psammite dolomite magnesite

V V V V V V Basic volcanics ) Schist\1 gneiss V V V

' Greywacke-. ' v

Fig 3. Diagrammatic stratigraphy of the Pine Creek Geosyncline

Early Proterozoic bedrock neae cliff-likth r e Middlee th edg f o e- Late Proterozoic plateau sandstones e thinneTh . r coves ha r allowed more effectiv f conventionao e us e l uranium exploration technology in these areas in contrast to the poorly and ineffect- ually explored areas marked by thicker cover, particularly near the ocast.

153 Fi. 4 Distributioy f significano n t uraniuw occurrences in the tine Creek Geosyncline. (Numbers cross-refer to Table 2)

South Alligatoj valley Uianium Field

50 km

Middle Proterozoic to Mesozoic cover units Tolmer Group Kombolgie Formation

Oenpelli Dolente Edith River Volcanics Post otogenic granite Nimbuwah Complex Litchfield Complex

Hermit Creek Metamorphics Nourlangie Schist Myra Falls Metamorphics

Zamu Dolerite Meeway Volcanics Berinka Volcanics Dorothy Volcanics

Chilling Sandstone Burrell Creek Formation Kapalga and Mount Bonnie Fm. Gerowie Tuff Koolpin Formation Wildman Siltstone Mundogie Sandstone Stag Creek Volcanics j Cahill Formation

Masson Formation 322300 Kakadu Group Coomalie Dolomite Crater Formation Celia Dolomite Beestons Formation

Nanambu Complex Rum Jungle Complex A - B Waterhouse Complex

0 10 20 30 0 2000 4000 6000 8000 10000

f mineo prospect. d san No s Tonnes U308 Fig 5. Distribution of uranium ) numbeA f occurrenceso r d an ,

B) tonnes U.,0 production and reserves in Pine Creek rock units Q 90 -

70-

60 -

50-

40-

30-

20-

10-

20-

10-

in grämte 0-5 5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 =»50 Distance (km) IS/NT/,is

Distance from Archaean granite Distance from post-orogenic granite

Fi. 6 Proximitg f Pino y e Creek uranium occurrences from Archaean and Proterozoic granite

GEOLOGICAL SETTIN F DEPOSITO G S The clustering of uranium occurrences naturally defines 3 major uranium fields a scatte d an ,f prospect o r s aligneW N d between Darwin and Katherine (Fig. 4). Within each field the uranium deposits share many common features and lie in the same host formation e hosth tt formatiobu , s differeni n n eaci t h field,

Descriptions of individual uranium depsoits and some pros- e Pinpectth en i Crees k Geosyncline have been given elsewhern i e literature (e.g. Ferguson and Goleby, 1980). Table 2 summarises the main features of these occurrences in terms of their grade, tonnage, geometry, host formatio d lithologiesan n e or d an , characteristics, and also outlines possible controls on mineralisation n thiI s. sectio e threth n e main uranium fielde ar s

156 132°30' 133°00' 12°00'

12°30'

(u) Uranium deposit • Uranium prospect

5 UJ h- O œ CRETACEOUS SOUTH ALLIGATOR GROUP —1 N OS uj gr-O- t> Koolpin Formation QOCO 5<-<°> Kombolgie Formation z • s£^ 0. n o MOUNT PARTRIDGE GROUP O < 1- Wildman Siltstone r-"-- > Oenpelli Dolent^ ^^ e O K + + UJ Mundogie Sandstone > O 5 1- Nabarlek and Tin Camp Granité M o O LU LU O < + _-_- M c/j DEFORMATIOd Nan O Cahill Formation 0 O rr o METAMORPHISM 0. < rr Z KAKADU GROUP LU LU > 2 Nouilangie Schist —i I- O f !, 0 Kud|umarndi Quartzite 0 rr 2 ?' l '*i rr < O_ < Myia Falls Metammphics LU o UJ iV V; O ce H Mount Howship Gneiss c-^ W NIMBUWAH COMPLEX ||||||| < 2 LU Mount Basedow Gneiss o<: z. m i- Zamu Dolente Illl a , Z ' s. NANAMBU COMPLEX î UJ 0 < rr 00 Fi. 7 GeologgAlligatoe th f o y r Rivers uranium Field discussed, with the emphasis on similarities and differences between deposits rather than detailed deposit descriptions. Deposit e Alligatoth n i s r Rivers Uranium Fielde (ARUFth e ar ) most recent (post-1970 A.D.) and spectacular U discoveries in the Pine Creek Geosyncline (PCG), and are either in the early stages of mining or yet to be mined. Some of these deposits have been the subjec wida f eo t variet f detaileo y d geochemica d petroan l -

157 logical studies developmena d an s t proceeds further studien ca s be expecte o enhanct d r understandinou e f themo y gcontrastB . , the deposits in the Rum Jungle Uranium Field (RJUF) and South Alligator Valley Uranium Field (SAVUF) were discoveree th n i d 1950's, and mine and milling operations ceased in the 1960's. They did not receive the same attention and consequently cannot be documented as thoroughly as the Alligator Rivers deposit.

ALLIGATOR RIVERS URANIUM FIELD (ARUF) 2 The Alligator Rivers Uranium Field covers about 22 500 km in the northeast of the PCG and contains the Jabiluka, Ranger 1, Koongarra and Nabarlek deposits (Fig. 7). The area is distinct from other parts of the geosyncline in that metamorphic grade (mediu o high-grade)t m d degrean , f deformatioo e n (four periods f deformatioo d isoclinaan n l folding) e markedlar , y greater (Ferguson, 1980; Needham & Stuart-Smith, 1980). The uranium discoveries were mad n 197i ed 19710an . Subsequent formatiof o n a National Park in the area has precluded effective exploration of the region since the discovery years. e areTh a betwee e Sout d th Easn an h t Alligator Rivers i s dominate e Nanambth y b du Complex, which contains granitoids that recor dd -180Archaeaan o 250a t ) erogeni0M p Ma 0 (u n c eventsd an , accreted Early Proterozoic feldspathic and quartzitic gneiss and schist n placee gneisi Th , .is ss (mainl ye complex)soutth f o h , gradational outwards into Mount Basedow Gneiss, the lowest recognisable part of the Early Proterozoic metasedimentary sequence e MunmarlarTh . y Quartzit a quartz-ric s i e h equivalent of the gneiss on the western flank of the complex. The overlying Cahill Formation onlaps in places to rest directly on Archaean granite Nanambth f o eu Complex. This formation containe th l al s uranium deposits and most of the uranium prospects of the area, and is the prime guide to uranium exploration. Needham and Stuart-Smith (1976) have divide e Cahilth d l Formation into tw o members a lowe: r member, whice uraniu th s hosi ho t tm mineralis- s characterisei atio d an n e presencth y b df carbonaceouo e s units and carbonate rocks d includean , n interlayerea s d sequenc f mico e a schist, chloritised feldspathic quartzite, quartz schist, para- amphibolit d calc-silicatan e e rock; n uppea r more psammitic member which contains quartz schist, feldspathic schist, feldspathic quartzite d conglomeratean , e CahilTh . l Formatio s overlaini n ,

158 in places unconformably Nourlangie th y b , e Schist monotonoua , s sequence of mainly quartz mica schist. Ferruginous siltstone with finely crystalline quartz bands exposed in the north may be metamorphosed equivalents of the Koolpin Formation. e Easth East f Alligatoo t r Rive e Earlth r y Proterozoic metasediments grade into metamorphically differentiated schist and

gneiss of the Myra Falls Metamorphics. Some distinctive lithologie s e mappe'resistersa b t n ou dca s e correl1 b whic n ca h- ated with the Kakadu Group and Cahill Formation. 'In the extreme east, pre-orogenic granite intrude s metamorphoset -187a dwa a 0M d and partly differentiated along witmetasedimente th h o fort s m the migmatitic Nimbuwah Complex s compositioit ; n ranges from graniti o granodioritit c e northth n o tonalitii t c, n placei c s south of Nabarlek. The orogenic event probably began at the time of intrusion of the pre-deformation granite d culminatean , t aboua d t 180. 0Ma Pre-orogenic tholeiitic dolerite sills (Zamu Dolerite) are preserved throughou e are th s tstrongla a y foliated amphibolite. Post-orogenic (1730-178 ) granit0Ma - trondhjemite e forms small stocks east and south of Nabarlek, and post-orogenic (-1690 Ma) 'tholeiitic dolerite intruded as extensive lopoliths throughout the regio e Nanambn th eas f o tu Complex. The Middle Proterozoic Kombolgie Formation forms a subhorizontal sheet of sandstone and interbedded mainly andesitic volcanics about 200-100 0m thick , generally marke a shee y b dr cliff rising abruptly above the lower undulating terrain formed e oldebth y r rocks A majo. r erosional perio s representei d y b d the unconformable contact, as at least 1-2 km of supracrustals were removed to expose the Oenpelli Dolerite lopoliths. Small pocket f tuffaceouo s s sediments alon e contacth g probable ar t e distal equivalent mainle th f yo s felsic late Early Proterozoic volcanism centree Soutth n hi dAlligato r Valley Uranium Field. Minor phonolite and dolerite dykes were emplaced between about 1370-120 a ago0M , concentrate Nanambe th d n Nimbuwai dan u h Complexes. Mesozoi d Cainozoian c c marin o terrestriat e l deposits form a northward-thickening veneer (up to 80 m thick) in the northwest, and colluvial sand and latérite masks much of the Early Proterozoic rocks throughou e regionth t .

159 Jabiluka, Ranger, Koongarr e othe th d mos an rf a o t prospects are stratabound within the Early Proterozoic Cahill Formation. Nabarlek is hosted by the Myra Falls Metamorphics, but this unit is thought to be a correlative of the Cahill Formation (Needham, 1982)e carbonatTh . e rocks neaJabilukae th r , Ranged an 1 r Koongarra deposits were derived from evaporites (Crick & Muir, 1980), indicatin a supratidag l depositional environment. Cahill Formation carbonate west of the Nanambu Complex, where no significant uranium occurrences are known, was deposited under open marine conditions (Needham, 1982); this and the stratabound e depositnaturth f o e s suggest that supratidal depositioa s wa n possible prerequisite for uranium concentration in the lower Cahill Formation.

The major deposits (geological plans and cross sections are given in Figures 8-14) are in close proximity to a sharply defined unconformity with overlying Middle Proterozoic Kombolgie Formation sandstone n unconformita ; y which approximates closely wite th h present day land surface. The Kombolgie Formation generally forms a prominent escarpment in close proximity to the deposits; although the Jabiluka 2 deposit is concealed by up to 220 m of Kombolgie Formation sandstone e depositTh . s hav a strone g spatial association with granitoid/metasedimentary contactse Th . Archaean Nanambu Complex is known to exist 1.5 km south of Jabiluka; it forms the footwall to the Ranger orebodies; and at Koongarra, it crops out 6.5 km NNW of the deposit. At Nabarlek, the late Early Proterozoic Nabarlek Granite crops out 16 km to the s e deposibeeha th eas n d f o intersectetan t a vertica t a d l depth m belo e 0 orebodyth 47 w f o .

Hos te majo rockth ro t sdeposit predominantle ar s y quartz muscovite chlorite schists (Figs 8-14) whic e commonlar h y characterised by segregation banding (in places accompanied by crenulation e threth ef o mineral e )e dominan b on wher y y ma san e t in individual bands e sequencTh . t Nabarlea e k differ n thai s t quartz e minotendb r o absento t rs , except towar e southerth d d en n of the orebody and in metasediments below the Oenpelli Dolerite. l depositsIal n , quartz e tabulartendb o t s , have wavy extinction, and to show horse-tail fractures passing continuously across adjacent grains and containing minute inclusions. In strongly deformed schists, the quartz has developed brittle fracture and pressure

160 Kombolgie Formation sandstone overlying Cahill Formation

Topographic contours (metres above sea fevef I Fi. 8 Solig d geolog f Jabilukuranium-gol2 o y d an 1 a d deposits (adapted from revised unpublished data by courtesy of Pancontinental Mining Ltd)

SOUTH NORTH JABILUKA 1 Kombolgie Formation sandstone SOUTH NORTH JABILUKA 2 Cahill Formation schists

UGS Upper Graphit» Sarlas HWS Hinging Wall Sanas MMS Mam Mm« Sanas FWS Footwall Sanas LMS1 Lowar Mma Sanas 1 LSS1 Lowar Schist Sanas1 IMS 2 Lowar Mma Sana* 2 LSS2 Lowar Schist Sanas 2

Uraniumore

WEST EAST

Fig 9. Generalised long-section and cross-sections of Jabiluka 1 and 2 uranium-gold deposits (adapted from revised, unpublished data by courtesy of Pancontinental Mining Ltd)

161 Surface expression of orebody

Fig 10. Generalised geological plaf o n the Ranger 1 orebodies and prospects (after Eupene & others 1975; Hegge & others 1980)

* N ANOMALo9 Y

solution features, which has resulted in partial recrystallisation. Usually muscovite and less commonly biotite and chlorite show two periods of growth. Early distorted flakes define the schistosity, ana later undeformed flakes are superposed on the foliation. Fine sericitic mica is generally present, and may occur as an alteration produc f minoo t r feldspar t NabarlekA . , sericite rich patchet se s in a chlorite matrix give rise to minor spotted schists, and elongated grains of leucoxene +_ sphene, which are parallel to the schistosity, give some rock a fleckes d appearance. Porphyroblasts f almandino e garne e locallar t y abundan l depositsal n i t . Thin quartz veins, which are both discordant ana concordant to the schistosity and appear to predate and postdate the regional meta-

162 -200m

Zon f surfaceo e oxidation

Chlonte rock

Footwall shear zone

Dolente

Pegmatite, pegmatoid

Hanging wall schist

Upper mine schist

Lower mine chert

Chlontic carbonate Cahill Formation

Lenticle schist

Recrystallised carbonate

Footwall schistand gneiss

—A— Base f weatheredo zone

&%% Approximate ore outline

Faults B. most N and breccia zones shownnot Cross-sectio orebod1 Rangee . th No yf 1 ro n (after Eupene & others, 1975; Hegge & others, 1980)

163 o\

CARRENT ARIAN

Kombolgie Formation

50m immun Escarpment CARRENTARIAN Kombolgie Formation r Koongarrav— reverse fault Sandstone conglomerate — — Cross fault

Main primary zoneore EARLY PROTEROZOIC Cahill Formation Quartz ch/onte schist Outline— — secondaryof zoneore f^HJ Graphitic quartz chlome schist Fig 12. Plan of the Koongarra deposit (after Foy & :edersen, 1975; Hegg others& e , 1980) Siliceous bands

"| fault breccia

Outline primaryof secondaryand zonesore

—n — Base of weathered zone

Fig 13. Cross-section of the Koongarra No. 1 orebody alon Figurn gi linA 7 e(afteA- & e y Fo r Pedersen, 1975; Hegge & others, 1980) SW NE

16/D53-1/7

50ir _) Quartz-m/ca schist, siliceous chlorite schist

Ch/onte-mica schist, micaceous chlorite schist —A— Base of weathered zone

Chlorite schist •••••••• Pit outline

Chlorite rock ± hematite ± senate ^^^ Ore zone

. FiCross-section14 g e th e souther th f o t a d en n Nabarlek orebody. Modified from mine section 9553; publishe y courtesb d f o y Queensland Mines Ltd

morphism, are common at Jabiluka, Ranger and Koongarra, and rarer at Nabarlek. In the vicinity of all of the deposits, a pervasive chlorite dominated alteration halo surrounds the mineralisation. At Jabiluka, this halo extends into the overlying Kombolgie Formation sandstone cover sequence (Gustafson and Curtis, 1983) and is known to extend laterally more than 200 m from ore (Binns et al., 1980). The chlorite ranges from cryptocrystalline to flaky, and generally occurs as a partial or complete replacement of biotite and muscovite t JabilukaA . completelt i , y pseudomorphs garned an t amphibole, and in tourmaline-bearing granitoid pegmatite dykes, alkali feldspar has also been extensively chloritised. At Ranger chloritisatioe th , 1 mors i n e restricte d lesan ds intensee th ; Hangingwall Sequence contains fresh garnet and K-feldspar in pelitic schist, and unaltered hornblende and plagioclase occur in interbedded amphibolite. Minor K-feldspar, plagioclase, and hornblende persise Uppeth rn i tMin e Sequence. Chloritisatios i n

165 most intense Range th e zonesn or i re , especiall e Loweth y r Mine Sequence, where feldspars in granitic pegmatites have altered to chlorite and dolente has been completely chloritised. At Koongarra, garne s eithei t r completely chloritise r surroundeo d y b d an outer layer of chlorite elongated parallel to the schistosity. The garnet ma^ contain helical trails of opaque inclusions. Foy & Pedersen (1975) have reported feldspar in several drillholes and have also noted the presence of partially or completely chloritised hornblende. Chloritisation is pervasive at Nabarlek in metasediments abov d belo e Oenpellan e th w i Dolente t alsi ;o affects the margins of the dolente and the underlying Nabarle* Granite, which has been intersected below the orebody at a aepth of 470 m. Chlorite forms pseudomorphs after biotite, actinolite, muscovite, d appearan o havt s e replaced garnet.

Wite exceptioth h f Nabarleko n e uraniuth , m depositl al e ar s associated with massive bedded dolomite and/or magnesite. This associatio s characterisei n e disappearanceth y b d r markeo , d thinnin f carbonato g e sequence se zonesa correswithior d e an ,th n- ponding increase in the degree of disruption and brecciation in the orebodies. Chert replacemen e carbonateth f o ta commo s i s n feature (Lwers and Ferguson, 1980), and Eupene (1979) has postulated that this disruption in the ore zones has been caused by the silicification of carbonate resulting in volume reduction and subsequent collapse. Ferguson et al. (1980) have suggested that solution of massive carbonate led to the formation of collapse dolines, and that these acted as structural traps and sites for ore- formation. At Nabarlek, the absence of massive carbonate in the vicinit e orebodth f o yy indicates that collapse dolinet no e ar s relevan e structurath o t t l settin f thio g s deposit (Ewer t al.e s , 1983) .

Accessory minerals found in the wall rocks of some or all of the deposits include tourmaline, apatite, sphene, leucoxene, zircon, epidote-group minerals, rutile, gypsum and opaques. Euhedral prisms of pleochroic green tourmaline, sphene, apatite and epidote- group mineral e commo e concentratear sb d ten o an nt d n muscovitei d - rich and chlorite-rich zones. Rutile and gypsum are generally rare, though sagenitic growth f rutilso e commoar e n flaki n y chlorite in wall rocks at Nabarlek. The opaques include graphite, hematit d minoan e r sulphide (mainly pyrite). Graphite occurs

166 throughout the sequence at Jabiluka, whereas it is confined to the base of the Upper Mine Schists at Ranger, and is restricted to a schist overlying the primary mineralisation at Koongarra. Graphite was reported in handspecimen only from schist adjacent to the Nabarlek orebody (Lee d Tammekandan s , 1979); however presencs it , e t beeno ns confirmeha mora n ei d recent study (Ewer t al.e s , 1983). Hematit a commo s i e n accessory mineral, particualrl n wali y l rocks at Nabarlek, wher t occuri e s fina s e dissemination r elongato s e grains which paralle e schistosityth l . The two most striking features in all of the deposits are the tendenc r mineralisatiofo y o occut n r within zone f brecciationso , and for it to be intimately associated with chlorite. Reasons for this brecciatio e associatioth d e witan or n hf o chloritn e have been considere a variete genesi n or i d f o y s model d wile an sb l dealt with later in this chapter. However, it is worth noting that, unlik e otheth e r deposits e Nabarleth , k orebody occurn i s what is interpreted as a shear zone (Anthony, 1975) and is grossly discordant with the enclosing metasediments. The breccia fragment e typicallar s y chlorite- and/or sericite-bearing meta-arenites, intensely chloritised material, and chert which is commonly a replacemen f carbonateo t t JabilukaA . , Range d Koongarraan r , fragments may contain graphite or hematite. At Nabarlek, the original composition of the fragments (and also the matrix) is difficult to ascertain because of the intensity of chloritisation and a later sericitisation in the ore zone (Ewers et al., 1983); however, it is reasonable to assume that the fragments are from the enclosing host rocks. In all deposits, the fragments are usually angula d rotatedan r . Texture d lithologan s y generally indicate thae brecciatioth t n developed afte e regionath r l metamorphism (ca. 1800 Ma) and that there was a high percentage of voids between the fragments. The breccias are cemented by chlorite _+ quartz +_ hematite + uraninite, and at Jabiluka, Ranger and Koongarra, graphite and minor sulphides can occur in the matrix. Graphite has not been identified in the Nabarlek ore-zone rocks; however, recent isotopic work on minor carbonate associated with the mineralisation suggests that organic materia s originallwa l y present (Ewers et al., 1983). At Ranger, vugs and fractures which remaine e timde breccia th opeth et a n s were initially cemented, have subsequently been infilled with euhedral quartz and chlorite, and can also contain hematite, sulphides, and carbonates. There

167 is also evidence that later deformatio e Rangeth n i rn deposis ha t caused some breccia e rebrecciateb o t s d before being cementea d second time with chlorite.

Massive chlorit sericit_ + e + ehematit e rock d intenselan s y altered schist e Nabarlee commoth ar s n e i zonnd highligh or kan e a t significant difference between ore zone rocks at Nabarlek and those e otheith n r deposits t NabarlekA . , hematit d sericitan e e ar e major constituents and are spatially related to mineralisation even though their developmen s mino i td seemingl an r y unrelate o minert d - alisatio e otheth n ri ndeposits . Recent pétrographie wors ha k indicated that the ore zone at Nabarlek has undergone progressive and pervasive sericitisation (probably about 920 Ma), resulting in the widespread replacement and breakdown of chlorite, the formation f hematiteo e solutioth d d redepositioan ,an n f uraninito n e (Ewers et al., 1983). To date, these events have not been recognised in the other deposits. Multiple generation f chlorito s e have been observel al n i d deposits; the grainsize, colour, pleochroism, and interference colours of these different generations are wide ranging, and their relationship e anotheon o t s r vere zonesor y e complex,th thern I e. appears to be no correlation between mineralisation and the composition or texture of a particular variety of chlorite. Chlorite compositions in both the host rocks and ore zone overlap and are highl a lessey o variablt rd extenan e, Mg witt , hFe respec, Al o t t Si; although, in general, Mg-rich and Al-rich varieties predominate . The primary ore mineral assemblage in each deposit is dominated y uraniniteb , though minor coffinit d branneritan e ee occuth n i r Jabiluk d Nabarlean a k deposits. . Eupen(1975al t e )e have also reported minor brannerite at Ranger. Thucolite is known from Jabiluka (Binn t al.e s , s bee1980ha nd reportean ) d froe Rangeth m r 1 No. 3 orebody (G.H. Taylor in Eupene, 1979). The close proximity of all deposits to the present land surface and the absenc a sufficien f o e t thicknes f coveo s r rock n mosi s t instances s exposeha e primarth d y mineralisatio e effectth o t f weatheringno s . Uraninit s beeha e n oxidise d replacean dn arra a f y secondaro b yd y uranium minerals, to form a wide low grade envelope, which is particularly well developed ovee Koongarrth r d Nabarlean a k deposits.

168 e uraninitTh s invariabli e y associated with chlorite, particularly where it fills interstices between breccia fragments. In many instances, subsequent sericitisatio e Nabarleth n e i n or k zone has resulted in the wholesale replacement of chlorite and the preservation of residual massive uraninite; however, these residuals commonly contain uraninite-chlorite intergrowths.

The uraninite exists in a variety of forms - disseminated uraninite occurs as cubes 10-100 urn across, but these may coalesce to form clusters, strings d massivan , e uraninite. Colloform textures with associated concentric banding d radiaan , l fractures are common in massive varieties, and can give way to reticulated and lacework patterns where silicate inclusion e abundantar s . Uraninite also forms thin veins coating foliation planes and these may become braided where the uraninite becomes interstitial to flaky micas. Fine stringers of uraninite sometimes fill fractures which are discordant to the foliation and appear to postdate the earliest mineralisation. Complex intergrowths in which chlorite exert s platit s y habit against uraninit e alsar eo common. Rare massive spherules of uraninite up to 0.7 mm in diameter are found at Jabiluka. At Ranger, the disseminated form is particularly common, and "atoll-type" textures occur where only the rims of uraninite cubes tak a polishe ; Ewer d Fergusoan s n (1980) have reporte e progressivth d e replacemen f theso t e cubes froe corth me outward y chloriteb s , thereby establishing that t differena , t times uraniu s beeha m n remobilised n polisheI . d sections t higa , h magnification and with the aid of oil immersion lenses, the alteratio f uraninito n o darket e r varietie s visibls ascribei si d an e d to the partial oxidation of U to U . This alteration occurs along grain boundaries n fracturesi , s isolatea d an , d patchen i s massive uraninite. The extent of the alteration is very pronounced in residual uraninite found in sericitised ore-zone rocks at Nabarlek.

Sulphide occurs in minor amounts in all deposits, though it should be added that there is no correlation between the presence of sulphur and the concentration of uranium (Ferguson and Winer 1980; Binn t al.e s , 1980). Galen moss i a t abundants it ; relationship to the uraninite and its Pb isotopic composition indicate a radiogenic origin (Hills and Richards, 1976). It usually occur s disseminatea s d euhedral grains (generallm y 5 < y

169 across), fracture filling n uraninitei s s stringera d an , d an s irregular grain n massivi s e chlorite. Pyrit d chalcopyritan e e ar e present and can be replaced by galena. Traces of chalcocite and covellite have been found in all deposits, and bornite, digenite, arsenopyrite, cobaltite d marcasitan , e have been identifiet a d Nabarlek. Hegge (1977 s reporteha ) e rarth de occurrencf o e tellurides (altaite, tellurbismuth, and melonite) at Jabiluka. Weathering at Nabarlek has led to the breakdown of copper sulphides e minoth rd an supergene enrichmen f nativo t e coppe d cupritean r . Gols beeha d nl deposit noteal n i d s except Nabarlek d occuran , n i s economic quantities in the Jabiluka 2 orebody. Graphite is found in the ore zones at Jabiluka, Ranger, and Koongarra; however, there appears to be no correlation between the presence of carbon and concentration or uranium, and there is no clear evidence to suggest that the removal of carbon (as say methan d carboan e n dioxide) produce a widespread d reducing environmen n whici t h uranium deposition took place g .carbon vu Veid an n- ate sl deposits occual n i a r rar t thee bu ,ear y featurt a e Nabarlek. While the field relationships and age dating have provided a reasonably coherent picture of the sequence of events on a regional scale (Pag t al.e e , 1980 e relationshith ), e d maitiminth an p nf o g mineralising event in each deposit and subsequent events in the ore zones to the regional framework is less certain. It seems to be generally accepted basie th f fielo sn d o laborator, an d y work, that mineralisation took place later than the 1800 Ma regional metamorphism, althoug e evidencth h e indicates that remobilisation has occurred in the ore zones, probably many times. Uraninite and galena ages indicat o maitw e n period f possiblo s e mineralisatior o n mobilisation, namely -1600 Ma and -900 Ma (Hills & Richards, 1976; J.H. Hills, R.W. Page, B.L. Gulson & G.H. Riley, pers. comm.). Rb-Sr ages on rocks from the alteration zones associated with uranium mineralisatio t Nabarlea n d Jabilukan k a giv n averaga e e f abouo mode e tag l 160 , additionall0Ma a youngey r event <92, Ma 0 is recorde e sericitise t Nabarleth a d d an n bote i k or dh ore-zone rocks (Page et al., 1980). K-Ar muscovite ages from altered and unaltered Cahill Formation rocks at Jabiluka are consistent at about 1800 Ma - an indication that the alteration associated with thie zonor s e took place belo e thresholth w d retention temperature at about 35Û°C (Page et al., 1980).

170 Stable isotopes Stable isotope studies of bedded sulfides (dominantly pyrite) in graphitic rocks froe Cahilth m l Formation, adjacen e majoth o rt t uranium ore deposits (Jabiluka 1 & 2. Koongarra and Ranger 1), hâve 6 34S values in the same range as mantle sulfur (Table 3). This indicates that mantl ee sulfu sulfuth s re wa rsourc th r fo e formatio f theso n e sulfide d thaan s t Archaean-type conditions (i.e. anaerobic world conditions) prevailed during sedimentation (cf. Skyrin d Donnellyan g , 1982) e sulfideTh . s formed from hydrothermal solutions probably derived by volcanogenic processes. S value g sulfideô f vu vei o s e r o Th n s froe uraniu34th m m deposits mentioned above have, relative to the bedded sulphides, a wide rang eo +7% t e presencfro »Th 6 - m(Tabl . f o e3) e S-enriche d 6 34 S values, outside the standard deviation limits of mantle sulfur, indicates sulfate was now present (Rye and Ohmoto, 1974) at the tim f uraniuo e e formationor m e variablTh .S value 6 e bese ar st 34 explaine s metaa d l sulfide precipitatio n organic-rici n h zones from

H2S which was produced by anaerobic sulfate reducing bacteria. Bedded carbonates (mainly dolomite) from samples adjaceno t t e thzoneor e s C havvalue 6 e s around 0%« n accori , d with normal marine carbonate <5-1 0 C values (Keith and Weber, 1964). Their 610 Q values, however, rang o +19%»anet fro3 e significantl+1 m ar d y below the range reporte r earlfo d y Proterozoic marine carbonates (Veizer and Hoefs 25%J,+ 1976o ,t indicatin5 ;+1 g some groundwate- re r crystallisation. The 6 13 C and ô 18O values of vein or vug carbonate from the ore zone e variablar s e rangin 20%»+ o 0%»ang,o t t fro 0 7 + d-2 m respectivel C value 5 < se yTh indicat(Tabl . 3) ee carbonate formation, at least in part, from organically derived CO, while

the 61 80 values show to a greater degree the effect of groundwate9 r recrystallisation. g carbonatevu Vei r o n s shoo reasonabletw w T O "1 O correlation lines between 6 C and 6 O values for the two most extensively sampled deposit d Koongarra)s an (Jabiluk 2 d an 1 .a These correlations suppor e argumen th tn sit i uf o treaction n i s which oxidising groundwaters have recrystallised carbonatd an e oxidised organic matter. Unde e anaerobith r c conditions within the breccia zones sulfate reducers were active A greate. r groundwater reactio s indicatei n t Koongarra d d thi an as supporte i s y b d the significantlsignif y altere e zonor d e organic matte C value ô r s (Tabl. 3) e

171 Tabl : 3 eIsotopi c composition mineralf o s s associated withd an , from strata adjacent to, major uranium deposits from the Alligator Rivers Area (Ayre d Eadingtonan s , 1975; Donnelly and Ferguson, 1980; Ewers et al., 1983)

Locatio d mineraan n l form Isot opic Composi tions % 6 S(CDT)* 613C(PDB)* 6180(SMOW)*

1. Alligator Rivers Area Bedded sulfides in barren Cahill Fm adjacent to: Jabiluka 1 & 2) Koongarra ) +2 +_ 1 Ranger 1 )

g sulfidevu Vei r o nbreccin i s e zoneor a s Cahill Fm Jabiluka 1 & 2) Koongarra ) -6 to +14 Ranger 1 )

Bedded carbonates barren Cahill Fm av. 0 +13 to +19

g carbonatvu Vei r o n n breccii e e or a zones Cahill Fm Jabiluk) 2 & 1 a Koongarra ) 0 o t 0 -2 0 +2 o t 7 + ) Range 1 r

Sedimentary organic matter in breccia ore zones Jabiluk2 & 1 a 4 -2 o t 0 -3 Koongarra 5 -1 o t 2 -2

Ore zone carbonate Nabarlek -20 9 t+2 o o -1t 5 5 +2

. Sout2 h Alligator Rivers Area Bedded sulfide in barren Koolpin Fm -1.6 Sulfides associated with uraniue or m 2 +1 o t 6 -

Mantle Sulfur 2 _ + 2 +

Early Proterozoic marine carbonates 0 +_ 4 +15 to +25

Sedimentary organic carbon 5 + _ 5 -2

'•International reference standards: CDT - Canyon Diablo Troilite PDB - Peedee Belemite SHOW - Standard Mean Ocean Water

172 Table 4: Isotopic compositions of minerals from (1) three base metal deposits in the Rum Jungle area, and (2) barren organic-rich sedimentar yPine rockth e f o Crees k Geosyncline (Donnell d Robertsan y , 1976)

Location and mineral form Isotopic Composition. % s

1. Woodcutters Pb/Zn sulfide deposit - 3.5 +1 o +14.t 0 7 in Whitem sF graphite av.-15.7

. Brown2 s Pb/Zn sulfide deposin i t -5.4 to +9.4 +5. -6.o t 59 +9.7 to +13.4 Whites Fm graphite -19 to -30

3. Mount Bonnie Pb/Zn/Cu sulfide +0.2 to +6.0 av.-11.5 cc. av.+ 1 1.6 cc deposiMoune th tn i tBonni m F e . +2.av 0 av.- 8.8 do. o d 8 av.8. + graphite -22.3

Massive carbonate

Coomalie Dolomite (av. of 2 samples) +0.9 + 15.8

Celia Dolomite (av. of 2 samples) +5.2 + 15.0

Disseminated and vein sulfides from carbonaceous-rich sedimentary rocks from various strata

Kapalga Fm . +12.av 4

Koolpim nF av. + 2.6

Wildman Siltstone av. +14.8

Coomalie Dolomite av. + 5.0

Donnelly and Ferguson (198Q) showed, using sulfur isotope geothermometry, that bedded or ore zone coexisting sulfides record the same temperature (T ~270°C). A similar temperature was obtained froe sulfidon m e s pai. (1980)y Binnwa al b r t t I e s . suggested in both studies that the temperature event was post-ore, s suggestewa ant i d d thae even th y havt ma t e beee -168th na M 8 (Page t al.e , , 1980) emplacemen Oenpelle th f o t i Dolerite.

In contras e stablth o et t isotope studieJabilukae th f o s , Ranger and Koongarra deposits, which indicate low temperature ore zone reactions, Ewers et al. (1983) have noted that isotopic composition e Nabarleth f e o zons or ke carbonate (Tabl ) indicat3 e e high temperature reactions w fluid/roclo A . k ratio produced 180-enriched carbonates. Low fluid/rock ratios were also found in 0 stud f 6 fluiao y 1d8 reactions involvin Oenpelle th g i Dolerite

173 at Nabarlek (Ypma and Fuzikawa, 1980). The Nabarlek 13C-depleted

carbonates (to -20%0) indicate incorporation of organically derived CO.- n theii , r formation. A wide variety of geochemical studies on deposits of the ARUF have been undertaken; however, they usually relate to a particular t permino orebodo d t d comparisonan y s betwee e depositsth n r To . the Jabiluka orebodies, Ferguson and Winer (1980) noted that where wholeroc U valuek s exceede 0 ppm4 d ,U correlate , As , dW wit, Cu h d wheran o e C whole-roc d an c S , U valuek Pb , NbsMo , were belo0 3 w o correlationn m pp s existed. . (1980Binnal t e )s have concluded that altered schiste broath n d i salteratio n halo surroundine th g Jabiluka orebodies have undergon e followinth e g changes compareo t d their unaltered amphibolite faciès equivalents: enrichmeni L n i t

and Mg, and to a lesser extent Scr V, Ga, and Nb; and depletion in Na, Ca, Sr and Ba, and to a lesser extent Cr, Mn, Fe, P, Zn and Pb. b werR d e an e alsimmediat Ith K no , m) 1 e (< vicinit e or f o y depleted. They also concluded that uraninite at Jabiluka was significantl , radiogeniY , y Se enriche , , rarPb Ça c en i eartd h . Sn G.A d an . element, CowaAg , n Mo s (reporte , (RLE)As , n Cu ,i d Ferguson et al., 1980) established that uraninite from Jabiluka also concentrates Li, B, Bi, Sb, Ga, Co and Ti. The concentration n particularE (heavi RE E f RE o y s alsha ) o been reporte r orefo d - zone samples from Jabiluka, Ranger and Koongarra (McLennan and Taylor, 1980). Mercury enrichment has been observed in primary ores from Jabiluka, Nabarle d Rangeran ke highesth ; t concentrations being in gold-rich samples from the Jabiluka 2 orebody (Ryall, 1981; Ryall and Binns, 1980). Marked increases have been noted in the U/Th ratios in the ore zones of all deposits when compared to U/Th ratios in intrusives and metasediments throughout the geosyncline (Ferguso d Wineran n , 1980; Ewer t al.e s , 1983); this has been interpreted as indicating that U rather than Th was transported under oxidising conditions in the hexavalent state to sites of deposition (Ferguson et al., 1980). Ewers and Ferguson (1980) reported that the + F3 e 2+/F e ratio of chlorite appeared to decrease with increasing uranium contene whole-rocth n i t k sample, and suggested that redox reactions involving Fe in the chlorite and uraniu n solutioi m n (transporte s uranya d l complexes) could lead to uranium deposition. A study of the groundwater geochemistry near the Jabiluka deposi s indicateha t d that uranium concentration n thesi s e waters

174 are low and not useful as a prospecting tool (Deutscher et al., 1980). At Koongarra, the distribution of secondary uranium minerals, which for e weatherea dispersiom th n i n d fa n zone above the primary mineralisation s beeha , n interprete n termi d f o s lateral groundwater movement (Snelling, 1980). Radiometrie dis- equilibrium between uranium and its daughter isotopes has also been show o result n t from weatherin e circulatioth d an g f groundwatero n s (Dickson and Snelling, 1980). Shirvington (1980) has suggested that the measuremen8 t23 o f4 23 U/ U activity ratios in soil samples derived from weathered schists associated with uranium mineral- a usefu isatioe b ly exploratioma n n tool.

Fluid inclusion studies have been confined to the Jabiluka and Nabarlek deposits (Ypma and Fuzikawa, 1980) and are restricted to fluid inclusions occurring in quartz and carbonates, mainly in late stage veins. Although uraninite can be associated with such veins and related to the fluids from which they formed, the significance of these fluids in terms of the earlier main mineralising events must be considered to be suspect. It has already been noted thae mineralisatioth t l deposital n i ns primaril i s y associated with chlorite a minera, t amenablno l o fluit e d inclusion studies. At Nabarlek, quartz is a minor constituent in the ore zone and is unrelated to the primary mineralising s abseneventi t i t; from most rocks, although minor amounts occur in breccia fragments, some schists, and in late veins. Ypma and Fuzikawa (1980) have observed two types of fluid inclusions in quartz and calcite associated with mineralisation in the Nabarlek and Jabiluka deposits: type 1 inclusions contain saline brine dominate y CaCl,,b d , MgCl~ d NaCl an ,d hav an , e homogenisation temperatures between 100° and 160°C; type 2 inclusions are characterise w salinitie lo e vapou y th b d n d CO,i r ,an s phase .

In summary, there are major features common to all deposits e ARU io whicth nt F h various authors (Eupene, 1979; Hegg t al.e , 1980; Ewers and Ferguson, 1980; Needham and Stuart-Smith, 1980) have drawn attention. These are:

a spatia ) 1 l relationshi e deposit th Earlye th f o po t /s Middle Proterozoic unconformity which approximates to the present day land surface,

175 2) the proximity of deposits to the Kombolgie Formation escarpment, and to the contact between Early Proterozoic metasediment d granitoian s d complexes, e stratabounth 3) d natur f deposito e s withi e Earlth n y Proterozoic Cahill Formation,

) 4 wite exceptioth h f Nabarleko n e presencth , f massivo e e bedded carbonates outsid e zone or d ethei an s r dis- appearanc ee zoneswithior e ,th n 5) brecciation e zoneswithior e ,th n 6) an intimate association between primary uranium mineralisation (predominantly uraninite) and chlorite,

7) an intense and pervasive chlorite dominated alteration halo in host rocks surrounding the mineralisation.

RUM JUNGLE URANIUM FIELD (RJUF) Uraniu s firswa m t discovere n northeri d n Australim Ru t a a Jungle in 1949, and mining and milling continued until 1970.

The Rum Jungle and Waterhouse Complexes are basement highs to the Early Proterozoic sediments (Fig. 15). Both contain schist, gneiss, granite gneiss d severaan , l varietie f graniteo s . Rhodes (1965) m JungldivideRu e eth dComple x intx unitssi o , which in orde f decreasino r g relative schisar d gneisse an tag e ; granite gneiss; metadiorite; coarse granite; large feldspar granite; and leucocratic granite. Veins and dykes of pegmatite and amphibolite d quartz-tourmalinan , e vein e alsar so present. Johnson (1974) recognised the same units in the Waterhouse Complex, and also note e presencth d f bandeo e d iron formatio n boti n h complexes. Richards et al. (1966) determined a Pb-U age measured on zircons m JunglRu e eth Complexr f fo 255o d Paga M 0an , e (1976) reportea d 2400 Ma Rb-Sr isochron for the Waterhouse Complex. Richards and Rhodes (1967) obtaine a Rb-Sd r whol f aboueo roce r tag kfo 240 a M 0 the leucocratic granite of the Rum Jungle Complex, which proved e comple thal unitth al te Archaeanf o sar x . Rhodes (1965) compar- m JunglRu e eeth dComple a mantle o t x d gneiss dome; Stephansson d Johnsoan n (1976) sugges m Junglt Ru d thaWaterhouse an eth t e Complexes were domed by intrusion of late Early Proterozoic granite at depth.

176 e NamoonTh a Group represent a basas l rudite, arenite carbonate assemblage (the Beestons Formatio d Celian n a Dolomite) which make up a conformable sequence encircling both granitic complexes, except north of the Rum Jungle Complex, and west of the Waterhouse Complex, where they pinch out.

Coarse pebbly sandstone and conglomerate of the Crater Formation represents the unconformable base of the Mount Partridge Group, which oversteps onte complexeth o n places.i s The basement complexes formed islands at least until Crater Formation t commonltimei s a , y contains clast f bandeo s d iron formation derived from the Rum Jungle Complex. The Coomalie Dolomite above the Crater Formation is similar lithologically to the Celia Dolomite; both contain common clasts after gypsum and halite indicatin a supratidag l environmen f depositiono t n i d an , places extensive algal mats of Conophyton sp. are developed in the Celia Dolomite. Both carbonate unit pervasivele ar s y altereo t d magnesite a late d ran , alteration even s evidencei t y abundanb d t development of talc.

m JunglOutsidRu e eth eare e Mouna th mos tf o tPartridg e Group is represente y interbandeb d d grey siltston d blacan e k carbonaceous siltstone of the Wildman Siltstone, but at Rum Jungle this is largely represente y shallow-wateb d r faciès variants e Coomalith ; e Dolomit s overlaii e y calcareoub n s carbonaceouse shalth f o e White r neao t sra Formationthis i st i contac d an , t thal al t uraniu d base-metaan m l mineralisatio s found i ne Acaci th p : Ga a Quartzite Wildmae Membeth f o rn Siltstone represent a highers - energ ye shallophasth n i ew marin o intertidat e l conditions which persisted in the area almost to the end of Mount Partridge Group time. The South Alligator Group rocks thin in the Rum Jungle area and are not known further west, where Burrell Creek Formation greywacke and siltstone form an extensive monotonous terrain. Except for some uranium prospects of no economic importance (mainly in the Koolpin Formation east of the Waterhouse Complex) these unit e essentiallar s y unmineralise m JunglRu e eth arean i d .

Durin e -180 th ga orogen M 0 e depositionath y l dips aroune th d Archaean basement highs were accentuated e Earlth yd an Proter, - ozoic sequence folded along axes trending mostly nort o northt h -

177 east. Dips are commonly about 60°, and the sediments regionally metamorphose w grade e Giantlo Th o t .d s Reef Fault a dextra, l wrench fault, trends northeasterl d displace an m yJungl Ru e eth s Comple d Earlan x y Proterozoic unit y aboub s t five kilometres, west block north. The subsequently produced re-entrant of Early

Proterozoic metasediments into the area of Rum Jungle Complex is known as 'The Embayment,1 where most uranium and base metal deposit e aree locatedth ar a f o s .

The Early Proterozoic sequence is overlain unconformably by the Middle or Upper Proterozoic Depot Creek Sandstone Member. The uranium deposits at Rum Jungle, Adelaide River and George l neae presenal Creeth re ar kt eastern limi f outcroo e t th f o p r immediatelo , at sandston e ar yd belowan e e unconformitth , y surface. Like the Kombolgie Formation, the Depot Creek Sandstone Member overlie e oldeth s rs mostl i rock d yan s quartz sandstone with minor pebble conglomerate beds, but there are no interbedded volcanics.

The RJUF contains numerous prospects and significant uranium deposit t Dyson'sa s , White's m JunglRu , e Creek South (RJCS)d an ,

Mt Fitch (Table 2; Figs 15 & 16). Over 4300 tonnes U000 have -J O been mine d treatean d d froe firsth m t three deposits t FitcM d han , has reserves of 1500 tonnes U 0 . Unlike deposits in the ARUF, j0 o0 these deposits contain considerably smaller tonnages of U_0 , may 3 o0 be associated with significant base-metal mineralisation (particularly Cu), and occur in a low-grade regional metamorphic terrain. Geological cross sections for the Dyson's, White's, and RJCS deposit e givear sn Figur i ne referre b 6 (ann 1 e ca do t d in relation to the following text).

The deposits all show a strong stratigraphie and lithological control; they occur within carbonaceous, pyritic d chloritean , - rich shale and schist of the Early Proterozoic Whites Formation, whic n turi h n n faulteoverliei s i dr o scontac t with massive carbonate belonging to the Coomalie Dolomite (Berkman, 1968; Fraser, 1975; Berkman and Fraser, 198Û). Unlike the other deposits e RJCth ,S orebodt localiseno s a carbonati y t a d e contact, although dolomit s beeha e n intersecte n drillholei d s withim 0 7 n e orebody th m belo e orebodief 0 o Th 10 w. d e betweean ar sm 0 3 n e presenth t land surface a surfac; e which coincides closely with

178 CAINOZOIC

TOLMER GROUP I-O

Zamu Dolente i

FINNISS RIVER GROUP Burrell Creek Formation

SOUTH ALLIGATOR GROUP Mount Bonnie Formation

O Gerowie Tuff N O CC Koolptn Formation UJ O MOUNT PARTRIDGE GROUP IT Q- Wildman Siltstone Whites Formation t3°00' - Coomalie Dolomite

Crater Formation

ANTOM N GROUP Celia Dolomite

Beestons Formation

r er RUM JUNGLE and 1 si WATERHOUSE COMPLEXES

Uranium© deposit

• Uranium prospect 10 km

uroo'

Fig 15. Geology of the Rum Jungle Uranium Field

an unconformity separatin e Earlth g y Proterozoic rocks from overlying sandston d hematite-quartz-breccian e f probablo a e Late Proterozoic age e orebodieTh . e typicallar s y associated with zones of shearing or brecciation, and as a consequence of their proximit e surfacth o t ye have been weathere o givt d a variete f o y secondary uranium minerals whic e predominantlar h y phosphatic. e uraniuTh m mineralisatio m JunglRu e eth depositn i n s varies s mineralogiit n d associatioan y n with other metals t RJCSA . , the mineralisatio s dominantli n y uraninite (base-metal sulphides are virtually absent) occurring as fine sooty coatings on shears and joints in pyritic and dolomitic shale ana schist; some veins and stockworks are developed on a minor scale at the contact with underlying graphitic shale (Berkman, 1968). Several near surface pods of saleeite in the weathered zone were also mined. At Dyson's, saleeit dominane th s i e minera or t d occuran l s throughout the orebody, particularly on cleavage planes, joints, and fractures in graphitic shales. Sklodowskite and autunite are

179 LATE PROTEROZOIC Depot Creek Sandstone Member

Whites Formation

Coomalie Dolomite

Uranium mineralisation

Fault

Present ground surface

Fig 16. Diagrammatic sections of U deposits at Dyson's (A) m Jungl,Ru e Creek Sout) (B h and White's (C). (Modified after Berkman, 1968 and Fraser, 1980). minor, and uraninite occurs in graphitic and strongly pyritic shales below the base of oxidation at 25 m. Base metals are again t BurtoM absent e Th n .deposi t occur graphitin i s c shales interbedded with grey pyritic quartzit d consistan e f torbernito s e at the surface grading to uraninite at depth. Malachite, chalcocite and native copper are present in the oxidised zone. Mineralisatio e White'th n i ns orebode mosth ts i s compley ha a an x been describe y Spratb d t (1965 s consistina ) o orebodiestw f o g a , copper-uranium orebody overlain by a base-metal orebody in sheared graphitic, sericitic, chloritic and pyritic shales. The copper- uranium orebody mainly contains uraninite, secondary uranium minerals, chalcopyrite, and bornite. The base metal orebody has been subdivided into three zones: a copper-cobalt zone (mainly

180 chalcopyrite, bornite, chalcocite, linnaeit d carollite)an e a , cobalt-nickel zone (mainly fine grained linnaeite, carollite, bravoit d gersdorffite)an e a cobalt-lea a an , d zone (mainly galena with minor carollit d sphalerite)an e t FitcM e hTh copper. - uranium deposits are hosted by chloritic, sericitic and graphitic schists, wite maith hn uranium orebody containing fine grained uraninite (Craven, 1983) and confined to a major breccia zone in magnesite. The copper mineralisation is mostly secondary (malachite, native copper, and chalcocite) and confined to residual clays overlying the magnesite (Berkman and Fraser, 1980).

Studies of the petrology, geochemistry and geochronology of the deposits in the RJUF are very minor. Roberts (1960) deduced from mineragraphic studies of ore samples from White's deposit, that uraninite and pyrite mineralisation preceded a period of shearing whic s latewa h r followe e introductio, th Co y b d, Cu f o n anb sulphidesP d . Richards (1963) obtaine 207a d Pb/f o 206 e Pbag 1015 Ma on a uraninite sample from White's, but concluded from Roberts' work thae uraninitth t s invariablwa e y altered an d therefore probably older. No fluid inclusion studies have been undertaken on any of the deposits.

Stable isotopes To date only limited stable isotopic studies have been carried out in the Rum Jungle area. The following discussion is a summary of a preliminary study (Donnelly and Roberts, 1976) and more recent studie y Donnellb s d Cric n prep.an y (i kd an ) n prep.)(i Donnell . .al t e y

Sulfides in Barren Organic Rich Sedimentary Rocks: Sulfides from barren sedimentary rocks (pyrite and pyrrhotite) from various stratigraphi m JunglRu ee uniteth aref o s a hav a range f o e 34 positiv S value ô e s (Tabl , whic4) e h shoo distinctw w t distributions. These date interpretear a s indicatina d o generationtw g s of sulfide: one formed during sedimentation, the other was introduced from hydrothermal fluids after consolidatioe th f o n e firsS valuesedimentsth 6 tf o se generatio Th . n sulfid34 e ar e suggested to be similar to that found for bedded sulfides in the Cahill Formation, indicating formation from sulfu f mantlo r e origin e morTh e. positiv e S introducevalueth ô ef o s d sulfides34 indicates their formation from high temperature (>250°C) sulfate

181 (u) Uranium deposit

• Uranium prospect or smafl deposit

(< WO tonnes U308)

(_) CAINOZOIC 0 ce r Ol- S EOCENE - $ OL- Z LU LU 5 ou ce. CRETACEOUS oc ce 1 LL ïïg § Kombolgie Formation o a. FINNISS RIVER GROUP___ Q ? Oenpelh Dolente in Burrell Creek Formation EDITH RIVER GROUP SOUTH ALLIGATOR GROUP >- K Plum Tree Creek Volcanic: "•'-"'•"'. Mount Bonnie and Kapalga Formations K Kurrundie Sandstone Shovel Billabong Andésite O U Gerowie Tuff O

Fig 17. Geology of the South Alligator Valley Uranium Field

182 reduction. Although there is only limited isotopic data on massive carbonate e aree facth th a t f o stha t thei 0 value6 r o d s l fi t exceeno d ~+15 % simila, e valueth o e bast sr th foune r metafo d l vein carbonate s significant(Tabli , 4) e . Withi e Alligatoth n r Rivers area bedded carbonates have 6 l R 0 values which range up to ~+20%, the average value for early Proterozoic marine carbonate. These data suggest that in the Rum Jungle Area significant amounts f carbonateo s have been recrystallised r formedo , , froe hydroth m - thermal fluids; a conclusion in agreement with the extent of introduction of the second generation pyrite or pyrrhotite. No stable isotopes have been carried out on minerals associated with uranium mineralisatio e RJUFth .n o n Three base metal deposits have however been investigate e stablth d e an disotop e values presented in Table 4. The significantly positive ô3 4 S value e sulfides or foun r fo ds indicate e presencth s f sulfato e e and it is suggested that these deposits are the result of the pervasive hydrothermal fluids moving through the ore zones as well as the country rocks.

SOUTH ALLIGATOR VALLEY URANIUM FIELD (SAVUF) e uppeTh r reachee Soutth hf o sAlligato r River flow alona g northwest-trending bel f tightlo t y folded Early Proterozoic géo- synclinal metasediment m widek s0 1 .m lonabou k y b g0 Thes 6 t e strata are deeply eroded and form strike ridges up to 200 m high in places, the tops of which are commonly capped by late Early Proterozoi o Middlt c e Proterozoic felsic volcanic d sandstonean s . This area is known as the South Alligator Valley. Basins of the felsic volcanic d sandstonan s e flank this northwest-trending belt, which in its narrowest part contains twelve of the thirteen uranium deposits of the South Alligator Valley Uranium Field. Sleisbeck, the thirteenth deposit, lies 32 km to the southeast (Fig. 17)e depositTh . s were discovered between 195 d 19542an , and mine and milling operations ceased in 1965. The Early Proterozoic géosynclinal sequence generally youngs eastwards, but forms a complex syncline in the central part where e westerth muc f o he nstructur th lim f o b s concealei e y youngeb d r rockse MassoTh . n Formatio e oldesth s ti n unid containan t s mainly carbonaceous shale, siltstone, carbonate, calcarenite and sandstone e pelitiTh . c rocks, particularl e carbonaceouth y s

183 ones e stronglar , y iron-staine o brice surfacet dd shal th re kt a e , and form low undulating ridges. The calcarenite (porous sandstone at surface) and sandstone form continuous ridges which rarely n heighti exceee Stam Th g0 .6 d Cree k Volcanic a sequenc s i s f o e poorly exposed altered basalt breccia, flows, tuff, and dark green tuffaceous shale conformably above the Masson Formation, and is about 100 s 0expose i m thick t I d. alon e southerth g n flana f o k prominen d continuouan t s ridge commonl m hig 0 h 10 ye forme th y b d unconformably overlying Mundogie Sandstone, which contains feldspathic quartzit d conglomeratean e .

Rocks of the South Alligator Group dominate the géosynclinal sequence in the valley and rest unconformably on older rocks, although owing to the strong folding clear unconformable contacts are seen only in fold hinges north of the area. The basal unit e Koolpiith s n Formation, interbedded dolomite, siltstond an e carbonaceous shale. Its base is marked by a massive chert-banded ferruginous siltstone (carbonaceous shale with carbonate bands at depth) y massivb r o , e dolomite with algal structures. Strongly ferruginised or silicified beds form continuous ridges up to 40 m high but other strata rarely crop out. Tuff and argillite of the Gerowie Tuff are interbeaded with the upper part of the Koolpin Formation, and thicken upwards as interbeds of Koolpin Formation rocks become progressively thinner. They are probably related genetically to the Shovel Billabong Andésite, a flow of variolitic andésit d microdioritan e e 100-30 m 0thic ke neath e bas th f ro e Gerowie Tuff. The Mount Bonnie and Kapalga Formations, separated by the South Alligator Fault, form the uppermost part of the group e similaar d ran assemblage f chert-bandeo s d ferruginous siltstone and shale, with greywacke, but only the Mount Bonnie Formation contains e tufffaul Th y hav.ma t e been active during deposition, o thas t upward movemen e easth t o removan t sido ty hav d ma ele el by erosioy tuffaceouan f o n s sediments deposited there during South Alligator Group time.

The Fisher Creek Siltstone is the youngest unit in the géo- synclinal sequence, and contains a monotonous sequence of siltstone, feldspathic sandstone, phyllite, greywacke and arkose.

The Zamu Dolente forms extensive sills mainly in the Koolpin Formation. The sills are folded with the géosynclinal sediments

184 and comprise a tholeiitic differentiated suite of mostly quartz dolerite.

Durin e 180 th ga orogeni 0M c even e géosynclinath t l sequence s metamorphosewa o low-gradt d e (most sedimentary texturee ar s preserved; pelitic rocks have a well developed slaty cleavage and contain sericite, chlorit d raran e e epidote d psammitian , c rocks are commonly fractured and veined by quartz) and deformed into overturned tigh o isoclinat t l folds with subhorizontal axes.

Following orogenesis, two suites of dominantly felsic volcanics and related volcanoclastics accumulated in a graben-like structure roughly coextensive with the South Alligator Valley. Each is valley-fill in character and strongly unconformable at the base, and they are separated in time by intrusion of the Malone Creek Granite. The older El Sherana Group contains basal coarse sandstone of the Coronation Sandstone, massive rhyolite, ignimbrit l RhyolitePu l d mino Pu an d einterbedde an e r, th tuf f o f d greywack d siltston an ee Toili th f so e Formation whic e hornar h - felsed by the granite. These rocks are tightly folded in contrast to the generally gently folded rocks of the younger Edith River Group, which includes basal polymictic conglomerate and sandston e Kurrundith f o e e Sandstone overlai n extensiva y b n e sheet of ignimbrite with minor basalt of the Plum Tree Creek Volcanics. These rocks unconformably overlie granite of similar age to the Malone Creek Granite outside this area.

Earlier workers (Walpole et al., 1968) described volcanic vents in the Pul Pul Rhyolite, including one which hosts a uranium deposit, but more detailed examination indicates that exposure of intrusive centres in the area are confined to small syenite plugs along the South Alligator Fault near the Malone Creek Granite.

e Edit th e relativ Th f ho Rivee ag e r Group volcanisf o d an m intrusio Oenpelle th f o n i Dolerite lopolithst ye s a t -169 a s i 0a M not known, but the dolerite intrusion probably represents the last e perioeventh f n 'Transitionao i dt l Igneousd Activityen e th t a ' of the Early Proterozoic. The dolerite and both volcanic suites form a very rugged terrain over which sandstone of the Middle Proterozoic Kombolgie Formatio s depositewa n d with marked unconformity .

185 MIDDLE PROTEROZOIC KATHERINE RIVER GROUP "J Kombolgie Formation -. .~\ EARLY PROTEROZOIC EL SHERANA GROUP Pul Pul Rhyolite

Coronation Sandstone

SOUTH ALLIGATOR GROUP Koolpin Formation

Uranium mineralisation

Fault

Fig 18. South Alligator uranium deposits. A. Generalised cross section of Palette (after Shepherd, 1967; and Cric t al.e k , 1980) . CompositB . e cross section of Rockhole (after Hare, 1961; Ayres and Eadington, 1975; and Crick et al., 1980). . LongitudinaC l projectio l SheranE l L f d o n an a Sherana West (after Foy, 1975 and Crick et al., 1980)

Faulting too e episodek th e are placn th i an f géosyncli o se - inal sedimentation, igneous activity, and platform sedimentation, formed a graben as a depository for the volcanic and volcaniclastic rocks, and probably provided foci for extrusive pipes. Géosynclinal sequence rocks have been thrown againste rockth f o s volcanic suites and platform cover. Thickened sequences of Mesozoic and Tertiary rocks indicate that faults in the area have remained active until relatively recent geological time.

Deposits in the SAVUF field differ from those in the ARUF and RJU y theib F r association with Early Proterozoic sedimentary rocks

186 higher in the sequence, and in some deposits, an association with acid volcanic rocks; however they are similar in that they occur near an Early Proterozoic/Middle Proterozoic unconformity, and like the Rum Jungle deposits, occur in a low-grade metamorphic terrain. Generalised sections throug e morh th some f signifo e - icant deposit e SAVU e th illustrate ar n Fi s n Figuri d . 18 e

An idealised orebody in the South Alligator Valley has the following main features:

occurt i ) s1 belo Earle th w y Proterozoic/Middle Proterozoic unconformity, usually in fractured cherty ferruginous and, at times carbonaceous, siltstones of the Early Proterozoic Koolpin Formation and adjacent sandstone lenses of the Coronation Sandstone, juxtaposed by faulting,

e mineralisatioth 2) n doe t occuno s r mor em belo tha0 w10 n the unconformity, and does not extend more than a few metres abov t (foi e r example refe o cross-sectiont r f o s the Palettl SheranE d an e a deposits). Abou 0 percen7 t t of the production from the South Alligator Valley deposits has come from the Koolpin Formation and the remainder from the Coronation Sandstone,

3} mineralisation is localised by major to minor faults, shears d fracturesan , ,

4) uraninite is either massive or occurs as veins and small lenses, and is associated with minor galena, chalcopyrite, pyrite, and native gold. Rutherfordine, niccolite, gersdorffite, clausthalite and coloradite have also been recorded,

n oxidisei 5) e zonesor d , phosphate-rich secondary uranium minerals predominate and include gummite, metatorbernite, autunite, phosphuranylite, uranophane d soddyitean , . more th e f significano o Tw t deposits exhibit some divergence from this idealised orebody; the Coronation Hill deposit occurs in a breccia of unknown origin containing fragments of altered El Sherana Group bnd blocks of carbonaceous shale; and the Saddle

187 Ridge deposit consists almost entirely of secondary uranium mineral s associatei d an s d with l tufE d rhyolit an e f th f o e Sherana Group.

Hills and Richards (1972) and Cooper (1973) have reinterpreted b isotopP d an Ue measurements obtaine y Greenhalgb d d Jeffrean h y (1959 n uraniuo ) e specimenor m s froe SAVU d th havm an F e indicated an age of 815 to 710 Ma, with a possible remobilisation or further phas f mineralisatioo e t approximatela n . n i ThiMa s 0 i s 50 y agreement with the two generations of uranium mineralisation reporte y Threadgolb d d (1960 r orefo ) s from Rockhole l SheranaE , , and Palette.

Apart from a geochemical and pétrographie study by Ayres and Eadington (1975 f carbonaceouo ) s shale d acian s d volcanic rocks associated with the uranium mineralisation, there are no further substantive studies of this type reported for deposits in the SAVUF. Ayres and Eadington (1975) concluded that U correlates with Cu, V, and Ga, but not with carbon, even though there is a close association between uraninit a carbonaceouan e s shales.

Stable isotopes Although onl a limitey S stud e zon6 f barredor o y e d an n 34 sulfides was carried out there are features in common with similar studiee majoth rf o s uranium deposits. Tha, sulfidis t n i e barren carbonaceou S valu 6 se a shalcompatibl s ha e e wit34h derivation from a mantle sulfur source, while introduced ore zone sulfides have a 5 34S range which the authors interpret as indicating sulfide formatio e e actionresulth th f s o f a tnsulfato s e reducing bacteria. The origin of these deposits, the authors suggestw e resultemperaturlo th f s o twa , e fluid transporf o t oxidised uraniu d reductioan m d precipitatioan n n whee fluith n d reached reduced zones.

CONTROLS OF ORE FORMATION In the development of any genetic models applying to the uranium deposits of the Pine Creek Geosyncline (PCG) cognisance a numbe f o f featureo r s commo o thest n e deposits would havo t e be taken into account. These features include:

188 Strataboun e zonesor d e natur.th f o e

Association of ore bodies with zones of brecciation or shearing.

The regional association of most of the mineralisation with carbonate rocks and an antipathetic relationship with carbonate in the ore zones.

Shallo e depositsor we deptth f .o h

The intense and pervasive chloritisation associated with the ore zones, wall rocks and cover rocks.

Absence of mineralisation in the Middle Proterozoic cover rocks despite the sometimes abundant presence of chloritised zones.

The spatial relationship of the deposits to the Early/ Middle Proterozoic unconformity which approximates to the present-day land surface.

High U/Th ratios in mineralised zones.

e widTh e sprea f appareno d t age f mineralisationo s .

Proximity of the deposits to the overlying Middle Proterozoic cover rocks.

e proposeTh d supergene genetic mode r uraniulfo m mineralisation whic e develow h t puniversall no her s i e y accepted an d the reade s referrei r o Binnt d t al.e s , (1980 d Gustafsoan ) & n Curtis (1983) for alternative models. The proposed model is based primarily on the deposits in the ARUF which are the most extensively studied a numbes therA e .f ar featureeo r s common to the ore zones within the three uranium fields of the PCG it would seem likely that they shar a commoe n origin. Development of breccia zones took place after regional metamorphism. The brecci e zoneor a s associated with carbonate-rich sequences were probably produced during peneplanation of the Early Proterozoic strat d beforan a e depositio Middlf o n e Proterozoic cover rocks took place. Solutio f theso n e carbonate rocks appear o havt s e produced collapse wite subsequenth h t developmenn a f o t

Î89 insoluble residue of breccia fragments. At Nabarlek and in other deposits from the RJUF and SAVUF, brecciation appears to be tectonically controlled by shear and fracture systems. These breccia zones probably pre-date e developmenth d e Middlth f eo t Proterozoic cover rocks. Besides the development of breccia fragments in the solution cavities, other insoluble residues probably included micas, chlorite, clays, quart d graphitean z . Texture d lithologiean s s clearly indicate thae brecciath t s developed afte e regionath r l metamorphism (Ewer & Fergusons , 1980) and that a higther s h wa epercentag f voido e s betwee e fragth n - ment t timea s s reachin 0 percen5 g y volumeb t e deptTh f .o h solution cavity developmen t Jabilukaa t , whic s thoughi h o t t m belo Middle 0 extenth w60 o et d Proterozoic cover s probablwa , y facilitate e presenc a th palaeotopographie are th f y o b adn i e c high of at least 250 m (G.A. McArthur, pers. comm.). The low grade alteration within the ore zones suggests that the main mineralisin w gtemperatur lo even s wa t d tooan ek place after the development of the brecciation and fracturing. The palaeoclimate durin e peneplanatioth g e Earlth yf o nProterozoi c surface wits abundanit h t developmen f breccio t a zones wa s probably arid and oxidising as indicated by the red-bed character- Middle isticth f eo s Proterozoic cover rocks (Gustafso & Curtisn , 1983). Lateritic soils were probably developed and clays from these soils could also have contribute e insolublth o t d e material presene breccith n i ta s alsi zones ot I possibl. e that decaying matter, now represented and identified as thucolite in some deposit s washewa s d into these trapse suggesteTh . d rolf o e bacterial sulphate reduction and the presence of biogenic carbonates withi e breccith n a zones further support e presencth s f o e organic matter (Donnell Ferguson& y , 1980)e introduceTh . d material could have been enriched in uranium, and have been capabl f furtheo e r enrichment through absorption.

e Archaeath Mos f d Earlo t an n y Proterozoic rocks withie th n PCG are enriched in uranium relative to world abundances for equivalent rock types (Ferguso Winer& n , 1980)e ArchaeaTh . n granitoid e though e ar representativb s G o withit tPC e e th n th f o e provenanc Earle eth areyr Proterozoifo a c sediments. These granitoids and the later Early Proterozoic high level granitoids and their extrusive e enrichear s 6 time o n uraniut i ds 2 y b m

190 world average abundances for granitoids (Ferguson et al., 1980). e uraniue granitoidTh th n i m s presene i accessors th n i t y minerals uraninite, monazite, zircon, uranothorite, xenotine and apatite (McAndrew & Finlay, 1980). The accessory uraninite in these granitoids ensured the presence of labile uranium in the provenance rocks. The Early Proterozoic metasedimentary sequence is als a potentiao l sourc f uraniumo e ; however, uranium contents in unmineralised areas of the PCG average no more than 6 ppm and rarely exceed 14 ppm for any given formation or rock type (Ewers, 1982) e syngenetiTh . c concentratio f uraniun o na s wa m insufficient concentrating mechanism to produce ore deposits as all of the uranium deposits post-date the regional metamorphic event. There is no apparent concentration of uranium during the main regional metamorphic event (Ferguson & Winer, 1980). The abundant late-tectonic I-type granitoid d extrusivan s e equivalents which developed durin e -180th g a 0regionaM l metamorphic event have slightly higher uranium contents than the Archaean granitoids. e bodieor e e natursth Th suggesf o e t that a ther s wa e downward percolatio ore-formine th f o n g solutions e absencTh . e f mineralisatioo e coveth n ri n rocks suggests thae maith t n mineralisation event took place during peneplanation of the Early Proterozoic rocks. Evidenc n suppori e a downwar f o t d percolat- e mineralisinth f o n io g solutions includes ponding effects with uranium enrichmen e uppeth rn o t sid f steeplo e y dipping, impermeabl e zones e or barrier or e ,e th bottominth n i sf o t ou g zones against impermeable mafic intrusives as at Jabiluka One, and Nabarle d limitean k d verticae bodiesor e th .l l extenal f o t In that photosynthesis probably took place fro t leasa m t 250a 0M (Cloud, 1976) it is likely that younger meteoric waters were oxidising to varying degree as suggested by the red bed character- e Kombolgiisticth f o s e sandstones. Under these situationt i s is likely that U was transported as the U complex. As Th is diadochic wit U h (Naumov4+ , 1959) U/Te ,th h ratia critica s i o l parameter in deciding the valence state in which U is being transported. That the uranium was probably transported in the hexavalent state is supported by the marked increase in U/Th ratios within the ore bodies (>500) as compared to the granitoids (<1.1) granitic pegmatites (-12.5 d blacan ) k shales (-0.3) (Ferguso Winer& n , 1980; Ewers, 1982) e presencTh . f o e

191 carbonates and micas in the metasediments would buffer groundwater pH to neutral or slightly alkaline. Under these conditions phosphate, carbonate and hydroxy uranyl complexes would be the most important (Langmuir, 1978). With increasing temperature, thee minoar y r speciel pH'al st t temperaturea a s C 0 10 f o s (Langmuir, 1978). Since P is not an important element in any of the rocks associated with the ore zones, whereas the REE's sho e we transporteknowb ar e enrichmenzoneo t or nd e an s th n di t by carbonate complexes (Kosterin, 1959) uranyl carbonate complexe e preferredar s e zone or .s presene swa th Tha „ n i C0 t is supported by the presence of chert after carbonate - a condition resulting from increased P„ou„ Q (von Engelhandt, 1977). Experimental studies indicate that organic materia d clayan l s readily adsorb large quantities of uranium in the pH range 5 to 8.5 (Rozhkova et al., 1959; Muto et al., 1965; Doi et al., 1975). Mut t al.e o , (1965) found that uranium, n presumei e b o t d a metastable state when adsorbe n clayso d s releasewa , d an d forme n timi d a stable e uranium minera e processd thatth an l n i , , the clays recovered their sorptive capacity. Mut t al.e o , (1965) quot n enrichmena e t facto f 10,000o r e experimentaTh . l worf o k Giblin (1980) shows that the adsorption capacity of kaolin, known to have the least effective sorption properties of all clay minerals, reached a maximum Kd of 35,000 at 25°C and pH 6.5. In order for desorption to take place the surface charge of the adsorbed uraniu n mus io e madmb t e incompatible wite negativth h e sorbent surface cla r th clay-sizeo y f o e d chlorite. This incompatibility can be brought about by increasing the C0„ content at.constant pH, raising the pH (Langmuir, 1978), or varying the temperature (Giblin, 1980). The accumulation of organic material within the breccia zones would create an anaerobic environment in which CH. and CO- might be present at a pH between 5 and 7. It is postulated, that whil e clay th d organie an s c material were accumulating within the breccia zones, the plumbing system of these structures was such that low-temperature CO^-rich groundwater, carrying uranium as uranyl carbonate complexes, passed throug s likelhi themt I y . that these meteoric solutions were capable of transporting the other elements that are characteristically enriched in the ore deposits, namely, W, As, Nb, Mo, Li, d REE'an B s; althoug, e zoneMg or , e s Au th hcoul , Cu d , Co , Sc have partly inherited this geochemistry froe carbonaceous-ricth m h

192 residue produced during development of the solution breccias. e maiTh s nprobabl wa sourc U f o ye derived from dissolutiof o n uraninite withi e Archaeath n d Lowean n r Proterozoic granitoids and their extrusives wit a furtheh r contribution froe metath m - sedimentar e solutionth y f roco kH s p e pile initiad Th an . h E l could have evolved significantly before they reached the sites of uranium deposition in breccia zones. On entering the breccia zones reductio H woulp n i dn enhanc e adsorptioth e U ont f o n particulate matter e reductioTh . f uranyo n l complexe y havma s e been facilitated by the oxidation of Fe 2 + in clay, chlorite or other Fe-bearing minerals (Ewer Ferguson& s , 1980).

e brecciTh a zones developed during peneplanation, would s depositionaa continu t ac o t e l site r uraniufo s m afte e onseth r t of Middle Proterozoic sedimentation, provided the following conditions were met: uranium continued to be leached from provenance rocks by groundwater;

acces f theso s e uranium-bearing waters inte th o structures remained possible.

Presumabl e structureth y s would initially remaie n th ope o t n introductio f surficiao n le oxidation-reauctiowaterth d an s n boundary would remain substantiall e samth en i positioy n within the structure. uranium deposition would only stop once sufficient sedimentation had taken place to result in the following changes :

Compaction of the regolith, particularly over the topographic lows which must have overlai e brecciasth n , thereby sealing the structure and preventing further access f uranium-bearino g waters.

Loading and dewatering of clays within the trap which would reduce permeability and develop chlorites from clays (Rowsell and De Swart, 1976) .

Uranium could have been adde o that d t already presene th n i t open structures within the Early Proterozoic rocks during the earlier part of the Middle Proterozoic sedimentation. Other authors have suggested that mineralisation coul e formeb d d during

193 the early depositio e coveth rf o nrock s (Kalliokosk t al.e i , 1978) ant wouli d a logica de b see o lt m extensio e overalth f o nl model presented here.

It is suggested that once the deposits were effectively sealed, all subsequent events recorded within the deposits, includin e rangth gf isotopio e c date d higan s h temperature features, represent reworking in situ. In the ARUF high heat- flow conditions were probably present in the area due to igneous activity associated with the pre-Kombolgie Oenpelli Dolerite at 1,688 Ma and intercalated lava flows in the Kombolgie Formation at 1,650 Ma (Page et al., 1980). It is probable that the heat associated with the latter igneous event resulted in the -27C temperatur0 e recorde e ore-zonth n i d e from stable isotope data; additionally, amorphous uranium oxides could have crystallized to a disseminated euhedral form about 1,600 Ma. This euhedral uraninite is particularly well developed at Ranger and Ranger also records the -1,600 Ma event very clearly.

Russian studie f hydrothermao s l uranium deposits (Tugarinov and Naumov, 1969) indicate that the formation of uraninite occurs C whereaabov 0 25 e s pitchblende e massivth , e microcrystalline and commonly colloform variety of uraninite, forms at temperatures t exceedinno g 220° d probablan C y less than 200°C e -1,60Th .a 0M dates recorded withi e ore-zoneth n s would belon o thit g s period. It is suggested that the Type II fluid inclusions found at Nabarlek and Jabiluka (Ypma and Fuzikawa, 1980) may represent the original mineralising groundwater.

Pétrographie work shows that solution of the disseminated uraninite and its redeposition as stringers and colloform uranin- s occurreit ARUe ha e th F n i depositd s (Ewer d Fergusonan s , 1980; Binns et al., 1980) and it would seem that the conditions required for this to take place may be very mild. The -900 Ma event can be correlated with epeirogenesi d developmenan s f Adelaideao t n (Upper Proterozoic) sedimentation to the south of the Pine Creek Geosyncline. Similarl e a -50dateM th y0 s associated wite th h SAVUF deposits could be attributed to epeirogenesis associated witdevelopmenthe h earlthe y of tPalaeozoi c Daly River Basito n the south of the PCG. Hydrostatic imbalance at high levels in the crust would be well-developed during periods of thermal activ-

194 ity associated with epeirogenesis which could subsequently remobilis e uraniumth e .

To preserve these ore-zones it was necessary that impermeable cover rocks develop rapidly. The dominantly coarse-elastics with lesser intercalated volcani ce overlyinunitth f o s g Middle Proterozoic Formations appear to be excellent cover-rock protect- ion once the depth of sedimentation was sufficient to seal the uranium deposits.

DISCUSSION The events described for the period 2,200-1,700 Ma in the PCG do not appear to be unique to that period and could equally apply to low-temperature vein-type deposits in younger rocks. During the post 2,200 Ma period, the essential requirements of uranium source, transport, depositio d preservatioan n n that accounr fo t vein-type uranium deposit Earln i s y Proterozoic rocks would appear to apply up to the present. The only apparent post-1,700 a variablM e compare e 2,200-1,70th o t d e hydroa perio0M th s i -d sphere, whic s progressivelwa h y evolvin n oxygenatea g d atmosphere (Boichenko et al., 1975). The transitional oxidising atmosphere perioe ith n d 2,200-1,70 a whils0M t sufficiently oxidisino t g transpor e hydrosphereth n e uranyi th tn io lt rapidl ,me y reducing conditions short distances into the lithosphère. This allowed

precipitatio e samUO and th f e o n t , a tim e reducing conditions 2 encountere e lithosphèrth n i d e stabilise e uraniniteth d . With rapid development of cover rocks the uranium ore-bodies were preserved.

s suggestei t I e dpos th tha tn i t1,70 a perio0M e atmosphth d - ere was sufficiently oxidising so that groundwater conditions were generally inadequat e reductioth r d fo precipitatioe an n f o n uranium on any large scale. The ultimate sink for uranium under these circumstances would have been the oceans, where uranium is added to sea-water, altered floor basalts, black shales and other pelagic sediments (Fyfe, 1979). This broad dissemination of uranium in the marine environment did not favour the formation of extensive vein-type uranium deposit e posth t n i s1,70 a period0M .

o uraniuN m deposits have been discoveree th o dat t dn i e Middle Proterozoic cover rocks in the PCG and workers in

195 Australia have not had the opportunity to study this type of mineralisation e readeTh s referre. i re companio th o t d n papers in this volume concerning uranium mineralisation in the cover rock n somi s e uranium deposit n northeri s n Canada.

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BERKMAN, D.A., 1968: The geology of the Rum Jungle uranium deposits. Australas. Inst. Min. Metall. Symposium, Uranium in Australia, 12-31.

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205 URANIUM MINERALIZATION TURET A E CREEK, WESTERN AUSTRALIA

G.N. LUSTIG*, G.R. EWERS**, C.R. WILLIAMS*1", J. FERGUSON** * Pancontinental Mining Ltd, Sydney, NSW ** Bureau of Mineral Resources, Canberra City, ACT Australia

Abstract

Uranium mineralisation which appear e unconformityb o t s - related has been discovered at a contact between Middle Proter- ozoic sandstone and Early Proterozoic shale, greywacke, and dolomite in the Turee Creek area, W.A. The nature of this contac s controversiali t y represenma t i ; n a faulta r o t unconformity along which ther s beeha en minor movement. Although the mineralisation is still being evaluated and the ore controls e definedb o t hav t a smal, ye e l uranium deposit consistinf o g approximatel 0 tonne00 3 s64 ygradin g 0.124% U.,u s alreadgha y been delineated e mineralisatioTh . n consist f uraniniteo s , carnotite, phosphuranylite, and metatorbernite, and is hosted by clay zones, hematitic and/or carbonaceous shale and.their brecciated equivalents, and chert breccias which form a sequence of uncertain e contacth n i t e zoneag . Relationship e compleo rapit ar s e ddu x faciès changes withi d betweean n n drill sections a tendenc d ,an y for unit o pinct st alonou h g strik d dowan en dip.

INTRODUCTION The Turee Creek area is in the Pilbara region of Western Australia, approximately 1200 km north-northeast of Perth (Fig. 1). Exploration for uranium in this area began in approximately 1973, with the initial target being possible "roll-front" deposits within the Middle Proterozoic sandstones. Although sub-economic occurrence f sandstone-hosteso d mineralisation were locatede ,th

Present address: Hunter Resources Ltd, 10 Spring Street, Sydney NSW 2000, Australia.

207 LEGEND

Middl. | Bangemal I l Group P~"| Bresnahan Group

frV| Manganesp Group

E«rly | Wylop o Group Protarozoic | j Hamvrsley ft Tür« Ck Grpa

jv;j Fort«8cu« Group

Archaa! • Granlta+ 1 n , matasadimants, b_d rnalavotcanic«

Pig1 e GeologicaPilbar.th f o ap Regioma l n (modified after Goode, 1981)

emphasis switche e Earlth o yt d Proterozoic sediments becausf o e some similarities with the Alligator River region of the Northern Territory. Initial prospecting involved ground follow-u f airborno p e radiometric anomalies, with sub-economic uranium mineralisation being located within shale d greywackean s e Earlth yf o sProterozoi c Mount HcGrath Formatio e Angelth n i on River area (Fig ) wes2 . t Turee th ef o Creek homestead marke e mos Th Figurn i tü . 1 e significant mineralisatio s discoverewa n n 1980-8i d 1 arid, although currently uneconomic, investigations are continuing. These uranium deposits are associated with the contact between the Middle Proterozoic sandstones and the Early Proterozoic shales, greywackes, and dolomites. The BMR's involvement in a petrological, geochemical, and stable isotope study of the Angelo River mineralisation began in late 1982, and the results obtained thus far are of a preliminary nature. e uraniuTh m deposit t Angela s o River represen a typt f o e unconformity-related mineralisation, distinctly different from both the Alligator River and Athabasca types of mineralisation. e mineralisatioTh t clearlno s i ny located withi e overlyinth n g sandstone nor the underlying flysche faciès sediments, but within a contact unit of disputed sedimentary or tectonic origin. The relationship between this unit and either sequence is uncertain.

208 \

LEGEND Ed Dolarite SYMBOLS

| Ebi I Kundarong Sandstona —————— Gaological contact MIDDLE PROTEROZOIC Brasnahan Group Pbc | Cherrybooka Congtomarata —•*••!?••• Fault with ralatlva movemant

Ewo I Aghburlon Formation —— Fault/unconformity!?] Wyloo Group EARLY PROTEROZOIC E«d I Duck Creak Dolomit« ——|—— Anticline

e.m| Mount McGrath Formation ———|—— Syncllna

-J- Strika t dip of badding

^—————• Badding trand Unas Pig. 2 Regional Geology, Angelo River Area

REGIONAL GEOLOGICAL SETTING e TureTh e Creek are s locatee southeri a th t a d n e margith f o n Hamersley Basi e nare s underlaiTh (Figi a . .y Earl1) b n y Proterozoic sediments and minor volcanics of the Wyloo Group, whic e unconformablar h y overlai y arenitesb n , felclspathic arenites, and conglomerates of the Middle Proterozoic Bresnahan Group (Daniels, 1968). The Wyloo Group was deposited in an elongated trough along the south-western e Hamerslemargith f o n y Basis ni (Figd an ) .1 separate dn active basi a fro th e resy b nth f me o t zon f basemeno e t high sParaburdoe referreth s a o t do Hinge Zone (Goode, 1981). Foldin s thoughi g o havt t e controlle e configuratioth d e th f o n s activbasinwa d ean , bot hd durin prioan o t gr deposition.

Wyloo Group sedimentation commenced after a brief period of subaerial exposure, followin e dominantlth g y chemical sedimentation of the Hamersley Group. The lowermost clastic units are thought to be terrestrial in origin, with later transgressive

209 marine deposition. Shallow marine conditions existed during carbonate deposition. The Early Proterozoic sedimentary sequence was subsequently folded and metamorphosed to greenschist facies during the Ophthalmian Orogeny (1700-1800 Ma). The Middle Proterozoic Bresnahan Group, possibly represents a molasse sequence, deposite n adjoinina n i d g basin developea s a d result of the Ophthalmian Orogeny.

GEOLOGICAL SETTING OF THE DEPOSITS Stratigraphy and lithologies e oldesTh t recognise e Wylod th uni of e Angel o tGrou th n i op River area has been correlated with the Mt McGrath Formation (Table 1). It consists of a succession of greywacke and shale with minor dolomite, which represents a turbidite sequence, overlain by a sequence of sandstone, dolomite and dolomitic shale, which passes int a othicke r sequenc f shaleo e s with minor dolomite. e sequencth a volcanif o s i ep to Neace sedimentarth r y breccia. e overlyinTh g Duck Creek Dolomite occur t severaa s l locations withi e area th nd consist an , f dolomiteo s , chert brecci d minoan a r carbonaceous shale. The dolomites are frequently silicified at surface, and ironstones have been developed on many outcrops. The Ashburton Formation forms the uppermost unit of the Wyloo Group and consist f interbeddeo s d shale, mudstone, siltston d greyan e - wackee greywackeTh . e micaceousar s a ,featur e which appearo t s be particularly diagnostic of this formation. The clastic sediments in the Wyloo Group are characterised by fine grained assemblages of quartz, chlorite, K-mica, possibly clay minerals, and minor opaques. Variations in the relative abundance of any of these minerals can give rise to laminations. The opaques may contain carbonaceous material but usually consist of hematite with some limonite staining along fractures. Very fine grained pyrit d chalcopyritan e e accessore rarear ar e s a , y minerals such as tourmaline and zircon. Brecciation of Wyloo Group lithologie e Angelth n i os River are s commoni a . The Bresnahan Group sediments, which unconformably overlie the Wyloo Group, consist of a basal conglomerate belonging to the Cherrybooka Conglomerate passing upwards into a unit known as the Kunderong Sandstone e CherrybookTh . a Conglomerate occurt a s e Angelth e westerf th o d Riveen n r area "and contains cobbles

210 Table 1 - Stratigraphie units of the Turee Creak Area.

AGE STRATIGRAPHIC LITHOLOGY UNIT QUATERNARY Colluvium, alluvium CAINOZOIC —————————— -UNCONFORMITY ——————— — TERTIARY Colluvium, calcrete, ironstone, silcrete, sandstone. ~ ——————— -~ UNCONFORMITY ———————— " KUNDERONG MIDDLE SANDSTONE Quartz arenite, sub-arkose, arkose PROTERO - ZOIC CHERRY - BOOKA Conglomerate Carpentarian] GROU P CONGLOM. BRESNAHA N ——————————— UNCONFORMITY ————————— ASHBURTON FORMATION Greywacke, siltstone, shale EARLY DUCK CRK. PROTERO- Dolomite, dolomitic shale, chert ZOIC DOLOMITE

MT.McGRATH Greywacke, siltstone, shale, sandstone,

WYLO O GROU P FORMATION carbonaceous shale, dolomite.

and coarse pebbles of quartz, quartzite, hematitic chert, jaspilite and banded iron formation occurring within a coarse sand matrix. The Kunderong Sandstone is the dominant Middle Proterozoic unit within the Turee Creek area. It consists of a sequence of medium to coarse grained, and sometimes pebbly, arenite, feldspathic arenite, and arkose. Coarse cross-bedding is common e feldspaTh . s mainli r y microclin s eithei d ran e pale pink, due to fine grained hematite, or buff white where sericitised. The matrix usually contains fine K-mica, chlorite and minor opaques d detrita,an l tourmalin s commoni e . Hematitic, chloritic, and kaolinitic alteration are widespread suggesting a complex redox history and resulting in some rocks developing a mottled appearance. At the Angelo B-zone (Figs 2, 4), the contact between the Kunderong Sandstone and Mt McGrath Formation is marked by a thick silicified breccia which has been alternately described as a silicified fault breccia, a Middle Proterozoic silcrete, or silicified Kunderong Sandstone. The interpretation of this unit is controversial and is largely dependent on whether the Bresnahan-Wyloo boundary is viewed as a faulted contact or an unconformity. The silicified zone is underlain by a clay zone. e Kunderon th e bas Af th to e g Sandston Angele th n i oe A-zone a sequenc ) (Fig3 , 2 f sshale/breccio e a units consistinf o g hematitic and/or carbonaceous shale, their brecciated equivalents, and mixed breccias containing chert, shale, silicified dolomite d sandstonan ) (? e fragment a silt n i sy matri e developedar x .

211 NW LEGEND

!•:'.'•] Kunderong Sandsion«

BH Clly

|J2| Carbonacaous shala braccla

l~3 Hamatltlc chart braccla & carbonacaous shala braccia H Dolomit MtF . « IcGrath __ Fm. |_Ü3 Graywacka

Uranium minaralisation

Fig. 3 Generalised cross-section of Angelo River A-Zone

NW SE

LEGEND

|:'.-!!l Sandstona j Dolomltip^ c shala KUNDERONG Mt. McGrath SANDSTONE P'tj Bracclatad Bandston* Formation p=rj Dolomlta

£[j Slllclflad braccla ^ Graywacke/slltstone rrn fcj::] Clay Icarbonacaous) YM Uranium mlnarallzatlon

EB1 Clay Pig. 4 Generalised cross-section of Angelo River B-Zone

Relationships are complex with rapid faciès changes within and between drill sections a tendenc d an ,r unit fo yo pinct st ou h along strik d dowan e n dip. These units have been considereo t d be basae th par lf o tBresnaha n Group r alternativelo , s Wyloa y o Group sediments within a fault zone. Some of the brecciation appears to be tectonic, but other breccia units are clearly of sedimentary origin.

212 Structural Geology The dominant structure within the Angelo River area is the contact between the Middle Proterozoic Bresnahan Group, and the Early Proterozoic Wyloo Group (Fig. 2). Initial mapping in the Turee Creek area (Daniels, 1968} has shown this contact as a fault. Subsequent work by Pancontinental geologists has led to varied opinions. Some have considere n unconformity a e contac e th db o t t , with minor movement alon e unconformitth g y surface. Curtis (1982) concurred with this interpretation d felan , t thae th t carbonaceous unit associated with mineralisation at the contact represents restricted marine embaywents existin e timth f o et a g initial alluvial sedimentation in the Bresnahan Basin. Hirono and Suzuki (1983) have interpreted silicifiea sandstone breccias that occur at several locations along the contact as evidence of post-Kunderong faulting. The contact is a focus of intense shearing and brecciation withi Wyloe th n o Group sediments d angulaan , r relationships exist between the unconformity/fault surface at several locations. The regional strike of the Kunderong Sandstone is approximately at right e Angelangleth o t os River contact e strikth t ebu , commonly swings to a sub-parallel orientation within 100 to 300 m of the contact (Fig. 2) . A prominent east-west oriented fault, locally knows a n "Frank's Fault" intersects the main Early Proterozoic-Middle Proterozoic contact west of the mineralised "A-Zone". The fault dip t approximatela s s downthrow0 degreeha 7 e ysout th a an o ht s n the southern block relativ e northerth o t e n block. The Early Proterozoic sequence has been folded into a broad, northwesterly plunging anticline, that is cored by the Mt McGrath Formation. Foldine Bresnahath n i g n Group occurs neae th r contact which has resulted in synclinal anticlinal structures parallel to the contact. A large northeasterly plunging syncline within the Kunderong Sandstone is present at the eastern end of . the Angelo River area.

Uranium Mineralisation Although ther s mineralisatioi e n withi e Wyloth n o Group sedimentary rocks away froe contactth m e ,know th mos nf to uranium mineralisatio s withii n e contacth n t zone s stateA . d previously the nature of this contact is somewhat controversial, and there-

213 fore some uncertainties about the stratigraphie position of the mineralisation exist. Along the contact there are two main mineralised zones e Angelth , oe Angel A-Zonth a a an eB-Zon e (Fig. 2). e AngelTh o A-Zon ea smal s (Figi l ) .3 uraniu m deposit consisting of approximately 643 000 tonnes grading 0.124% U,0,,. The deposit is approximately 400 m long, with a maximum thickness f approximatelo . m Uraniu0 3 y m mineralisatio s hostei n y b d hematitic and/or carbonaceous shale, their brecciated equivalents, and chert breccias that form a sequence of uncertain age within the contact zone. Uranium mineralisation is irregular in distribution and commonly neither the mineralisation nor the host rocks can be correlated from hole to hole. e AngelTh o B-zon s discovere wa eo sever t e du en 1980 i d an , drilling problem s beeha s n incompletely delineated. Mineralis-

a maximuatio s ha nm wit m 5 a gradhwidt 8. f 0.047f o eo h % U,00 J O and is associated with clay (which is in part carbonaceous) and brecciated Kunderong Sandstone. The lower horizon has a maximum intersectio f 10.o n m 5averagin g 0.438% U^Od mineralisatioan o s i n associated with cross-cutting iron oxide veins within a clay zone which underlie a ssilicifie d brecci f disputeo a d origin. U-Pb isotope data indicate that the age of U mineralisation from this lower zon s approximateli e a (PigeonM y 0 1013 _ + 5 , 1981). Uraninite, carnotite, phosphuranylite and metatorbernite have been identified froe Angelth m o A-zon d B-zonean e . Auto- radiographs of thin sections prepared from core containing U indicat m >10pp e followin0 th e g features n brecciateI . d Wyloo Group sediments U occur, s finely disseminated throughout shale fragments and matrix material. The breccia fragments are not always mineralised e facth t t thabu , t some enrichear e s cleai d r evidence to suggest a syngenetic concentration in the Early Proterozoic sequence n mineraliseI . d zones s usualli ,U y distribute s secondara d y minerals eithe n lati r e fractures a r o s isolated grains. Uranium also occur n fracturei s d an s disseminated form associated with opaques. In many instances, these opaques are secondary iron oxides which have either scavenged U or have associated secondary U minerals, though some y represenma t unconfirmeye s a t d uraninite concentrations. Although sulphides are not generally related to the U distribution, the concentration of U around the margins of some

214 Table 2 - Correlations on trace element data at 99 percent confidence limits for samples containing >50 ppm U (*denotes sample population too small, <10 samples). The most significant correlations (i.e. where I r I >0.70) are underlined. ELEMENT CORRELATES WITH Ba Sr a G U r Z b R n Z F i L h T b N n Z e S U F a G i L _ Zr B b R a B Sr Pb As U Ce Nd La Th Nb Zr Ga B Rb U As Rb Co Pb Li Zr Mb B F Th Rb Ga Li V b R B a G F h T r Z Nb Y La Ce Nd F Pb Ce La Nd F Pb B Nd Ce La F Pb Sc B Ga Rb Cu Ni V Ga B Nb Co Ni As U Ni Co Se Cu Se b R i L Zn Sn Mo * Ga B _Z_r Nb Se Th Rb F V Li b P o C U As S F a G b R a L d N e C b N :Z_. rLJ e C V h T b N b R r Z a G e S B

sulphide grains indicates that localised reducing conditions may have exerted some influence. e tim th f writingo e t A , only trace element date availar a - able and the interpretation of this data is at an early stage. e result Th a correlatio f o s n matrix calculate r thosfo d e samples wher U econcentratione summarise ar m pp n excesn 0 i i 5 d e f o sar s Tabl . 2 eCorrelation 9 percen9 e th tt a sconfidenc e limit (for

215 48 samples, r -0.37) have been ranked in descending order according to the absolute value of the correlation coefficient. The most significant correlations (i.e. where I r I >0.70) are underlined. Uranium correlates, though not strongly, with elements having different geochemical affinities (i.e. some are chalcophile, others are lithophile). The absence of strong correlations involving U, particularly with respect to Pb, probably reflect the mobility of U in a deeply weathered environmen s indicatea t e widespreath y b d d developmen f secondaro t y U mineral d botan sa primar h d radiogenian y . Pb ce sourcth r fo e Surprisingly o elemenn , t correlate f thesO s 8 wit4 e . S h sample 9 sample3 s , containinU e eithe ar sm pp r0 breccia>5 g s or have been brecciated, and they encompass the full range of lithologies represente t Turea d e Creek, though shale breccias predominate. Twenty-five sample e eithear s r knowr o n suspecte e carbonaceoub e hematitic o ar t d 4 2 d an .s

CONTROL N ORE-FORMATIOO S N Fo a varietr f reasonso y e genesior n a ,e s th mode r fo l mineralisation at Turee Creek cannot be advanced at this stage. e truTh e e Bresnahan-Wylonaturth f o e o contact (i.e. whether it is a fault or unconformity) has not been resolved, and the mineralisatio s associatei n d with lithologies whic e complear h x f uncertaio e anar d n age. Petrological, geochemicald an , stable isotope studies are also at an early stage.

REFERENCES CURTIS, L.W., 1982: Initial finding a stud f f drilo so y l core samples from the Angelo Creek uranium prospect, Turee Creek Area, Western Australia, unpublished consulting reporr fo t Pancontinental Mining Ltd. DANIELS, J.L., 1968: Explanatory notes on the Turee Creek 1:250 000 Geological Sheet, West. Aust. Geol. Surv. ______1975: Bresnaha Mound an n t Minnie Basins, West. Aust. Geol. Surv. Mem, 2, p!43-147. GOODE, A.D.T., 1981: Proterozoic geolog Westerf o y n Australin i a D.R. Hunter (Ed.) Precambrian of the southern hemisphere: Ams ter d am, Els eyi er, pl05-203.

216 HIRONO SUZUKId an . , 1983,S , S. e 198:Th 2 repor mineralogicaf o t l studies on rock samples from Turee Creek Western Australia, unpublished PNC Exploration (Australia) Pty Ltd report. PIGEON, R.T., 1981: Report on lead isotopic composition of rock powders submitted by Pancontinental Mining Ltd, unpublished consulting report for Pancontinental Mining Ltd.

217 UNCONFORMITY-RELATED URANIUM DEPOSITS IN THE ATHABASCA BASIN REGION, SASKATCHEWAN

. RUZICKV A Geological Surve Canadaf yo , Ottawa, Ontario, Canada

Abstract

Uranium deposits e sub-Athabascrelateth o t d a unconformity contain large tonnage f higo s h grade uranium resources. Their distributio s structuralli n d lithologicallan y y controllee th y b d unconformity, steep faults, pelitic rocks in the crystalline basement and clastic sedimentary rocks in the cover. The deposits contain monominerallic or polymetallic mineralizations, which are surrounded by alteration haloes. The deposits formed 1050 to 1300 Ma ago under hydrothermal conditions.

INTRODUCTION Discoveries of uranium deposits related to the sub-Athabasca unconformit late th en i 1960y s triggered intense exploration activity and extensive geological studie t onl Saskatchewan no si y Canadd an n a but also aroun worlde th d .

Economic importanc f thio e s deposit type attracte e latth en i d 1970s more tha companie0 n6 s which operated more tha0 exploratio15 n n projects with a total annual budget exceeding 60 million Canadian dollars.

As of the end of 1982 uranium resources of the Athabasca region constituted almost a half of Canada's uranium endowment in the Reasonably Assured and Estimated Additional low price resource categories.

GEOLOG ATHABASCE TH F YO A REGION The Athabasca region is a part of the Churchill Geological Province of the Canadian Shield. It is built up of Archean and Aphebian crystalline basemen d youngeran t , Helikian, supracrustal rocks.

219 The crystalline basement Lewr d Sibbalan y d (1979) subdivide e crystallinth d e basement complex into lithostructural domains and these domains were grouped in major crustal units designated from northwest to southeast as Western Craton, Crée Lake Mobile d ZonRottenstonan e e Complex (Fig. 1).

The Western Crato s interpretewa n a relativel s a d y stable foreland borderin e northwesth n i e gHudsonia th t n erogenic belt. This crato f mainls builo i n p u ty Archean katazonally metamorphosed rocks with some remnants of Aphebian supracrustal rocks. It is bounded nort d northwes an he Athabasc th f o t a Basin e regioth y b n Macdonald Fault and the Thelon Front and in the southeast by the Virgin River Zone. During the Hudsonian Orogeny the craton was affected by intense brittle to ductile deformation and by retrograde metamorphism.

The Crée Lake Mobile Zone, e Hudsoniawhicth s par i f ho t n orogenic belt, comprises a core of Archean granitic gneisses and Aphebian shelf to miogeosynclinal metasedimentary rocks including graphiti d pyritian c c metapelites t consistI . f o Wollastons , Mudjati d Virgian c n River domains. The Wollaston domain, which is the southeastern marginal part of the Crée Lake Mobile Zone, contains elongated Archean granitic domes flanke y b Aphebiad n metasedimentary rocks including quartz conglomerates, quartzites, aluminous, graphitic, pyriti d nonan c - graphitic pelite d semipelitesan s , calc-silicate rocks, banded iron formation, volcanic arkose greywackesd san e lowermosTh . t horizon i n the metasedimentary sequence, the Wollaston Group, are graphitic metapelites. Thee fine ar yo medium-grainet - d rocks consisting chiefly of quartz, feldspars, biotite and graphite (Sibbald, 1979). Garnet, tourmalin d pyritan ee presen ar e n lessei t r amounts. This horizon underlie 1 r exampluraniu'B fo s d an em 1 Collin'A y Ba s deposits. The Wollaston Group contains several such pelitic horizons, which usually contain graphite. These horizon e intersar - e vicinitcalateth n i f o severady l uranium deposits commonly with calc-silicate rocks containing locally scapolite and albite. These scapolite/albite rocks in the Wollaston domain are interpreted by some geologists (e.g. Ray, 1978; Chandler, 1978 s )havina g been forme y evaporitib d c processes. The most common rocks of the Mudjatic domain are Archean granitoid gneisses locally containing pegmatitic intercalations.

220 104 60

U DEPOSITS HELIKIAN E ^ Athabasca Group

1 Eagle Point APHEBIAN-HELIKIAN

2 CollmA y sBa I____I Martin Formation APHËBIAN 3 CollmB y sBa |p^r^:| Wollaston Lake-Virgin River Belt 4 Rabbit Lake ARCHEAN 5 Midwest Lake I; .'"T;!) LATE Tazin Belt, LaRonge Bait, Granite 6 Cigar Lake | I MIDDLE Fontaine Belt, Rottenstone Domain 7 Key Lake ] " EARLf Y Black Lake Wedg e, Clearwater Block 8 "D"

9 Peter km Major Shear Zone 10 Ace-Fay-Verna 11 Mauricy eBa 12 Fond-du-Lac Figure 1. Major crustal units lithostructuraland domains Athabascathe of region (modified from Lewry Sibbaldd an , 1979), Circled numbers: (1) Rabbit Lake ~ Collins Bay area (4) Car swell Structure (2) McClean Lak e- Midwes t Lake- (5) Maurice areaBay Waterbury Lake area (6) Fond-du-Lac area (3) Key Lake area

221 e VirgiTh n River domai a mobil s i n e bel f Archeao t n granitoid core Aphebiad san n supracrustal rocks boundes e northwesi th t i ;n i d t by the Virgin River — Black Lake Shear Zone. It represents a major Hudsonian mobile front, analogous to the Grenville Front at the southeast margin of the Superior Structural Province (Lewry and Sibbald, 1979) .

Rottenstone Th e Complex, which border Crée th se Lake Mobile Zone e southeasttth o , comprise broaa s d bel f plutonio t c rock f mainlo s y Hudsonian age (Lewry and Sibbald, 1979) and older intrusive rocks. The Wathaman Batholith, which is a major intrusive body in this complex, originated durin a tectonig c event, which took place about 1880 Ma (Bell and Macdonald, 1982). The Peter Lake domain, separated from the Wollaston domain by the Needle Falls Shear Zone, comprises a variet f o granitiy o t basic c plutonic rocks e basith ; c rocks represent the oldest intrusive suite in the complex (2177 ± 110 Ma; Ray and Wanless, 1980).

Geochronology of the crystalline basement Bell and Macdonald (1982) summarized and interpreted results of geochronological studies on the Precambrian in Saskatchewan and concluded e majo,th thatr) (a plutoni: c events took plac n foui e r periods: at about 2510 Ma (e.g. intrusion of 'Wollaston Inliers1); 2,180 Ma (e.g. formation of Donaldson Lake gneiss); 1,880 Ma (e.g. intrusion of Wathaman batholith); and 1740 Ma (the "main" Hudsonian e felsievent)th ) c(b ;hypabyssa l rocks were emplacet a d about 1580 Ma; (c) the three main metamorphic episodes culminated at 1875 ± 20 Ma; 1762 Ma; and especially 1680 - 1655 Ma; the latter episode represent n importansa d distinctan t anatectice eventth ) (d ; supracrustal rocks of the Wollaston Group were deposited 1875 - 2500 Ma ago; and (e) the intrusion of lamprophyre dykes took place at about 1740 Ma ago.

The Helikian supracrustal rocks Prio o depositiot r e Helikiath f o n n sedimentarye rockth n i s Athabasca Basi e crystallinth n e basement complex underwent chemical weathering in a warm, moist climate (Macdonald, 1980) . This weathering produce a regolitd h simila o modert r n lateritic soilp u s to several tens of metres thick (Ruzicka, 1975) . The Helikian supracrustal sequence (Athabasca Group s depositewa ) n o thid s regolit r locallyo h , wher e regolitth e s erodede unalterewa h th n o , d basement. The sedimentation took place in three structural sub-

222 Table 1: Lithostratigraphic units of the Athabasca Group (After Ramaekers, 1979 and 1980) . Maximum Thickness §< Formation metres Lithology o 0 50 £, CarswelDolomitel , some chert •§ Douglas 200 ) 0(? Psammites, pelites en 0 8 u TumaPebbl Lakye sandstone £ Otherside 350 Sandstone, siltstone •0 £ 12 Locker LakPebble y sandstone, conglomerate {g Wolverin 0 70 e Point Psammites, pelites, ••H phosphorites IH Lazenby Lake 118 Pebbly sandstone, "rg sandstone Manitou Falls 1400 Sandstone, basal conglomerate 0 30 Fair PoinConglomeratit c quartz- rich sandstone with clay matrix

basins (Ramaekers, d 1980reflectean ) a generad l e uplifth n i t Wollaston Fold Belt and perhaps along some of the faults in the eastern portion of the Athabasca Basin (Ramaekers, 1979) . Therefore the easter e basinn th par f o ,t i.e e .vicinite th thath n i f to y uplifted area, contains mainly fluviatile deposit f detrituso s , wherea distae sth l (i.e. western) part contains lacustrin r shalloeo w marine sediments e AthabascTh . a sedimentary sequence resemblea s typical molasse assemblage which, after deposition underwent, severe diagenetic changes (ibidem). The Athabasca Group has been subdivided (Ramaekers, 1979 and 1980) into nine formation e easterth sn I (Tabln . par e ) 1 eth t basal unit, the Manitou Falls Formation, is made up of quartz sandstones with variable amounts (up to 20 percent) of interstitial clay. The source of the detritus was apparently to the northeast or to easte ManitoTh . u Falls Formatio maximua s ha n m thicknes f abouo s t 1400 m. e basaTh le Athabasc rockth f o s ae westerGrouth n i pn parf o t the Athabasca Basin were designate e Faith r s a dPoin t Formation. This formation, m thickabou 0 builconglomeratif s t30 ,o i p tu c sandstone with clay-rich matrix. The detritus was derived from a nearby sourc d depositean e , apparentlyin d , lacustrin r shalloo e w marine environment. e ManitoTh u Falls Formatio s overlai i e nLazenb th y b ny Lake Formatio Wolverine th d nan e Point Formation, which consis pebblf to y quartz sandstone and of pelitic sedimentary rocks respectively with a maximum thickness abou 0 metres70 t .

223 e LockeTh r Lake Formation which overlie e Wolverinth s e Point Formation in the western portion of the Athabasca Basin consists of pebbly poorly sorted sandstone, whic m thicks abou i h0 12 t. e uppermosTh t e memberAthabascth f o s a Group, namele th y Otherside, the Tuma Lake, the Douglas and the Carswell Formations were deposited apparently in a paralic environment. Most of them consist of sandstone with abundant interstitial clay; in the Carswell Formation carbonate prevails. The combined maximum thickness of these members exceeds 600 metres. e sedimentatioTh n withi Athabasce th n a Basi s intermittentlwa n y accompanied by explosive volcanism; ash beds occur, for instance, in the Wolverine Point Formation (Ramaekers, 1980)! Locall e clastith y c sedimentary rocks contain substantial amount f phosphateo s s e uppe(e.gth Wolverine rn i .th par f o t e Point Formation; Ramaekers, 1980), organic substance, sucs kerogea h n (e.g. in the basal part of the succession; Ruzicka, 1975; Ruzicka and LeCheminant, 1984) or layers of sulphides (e.g. in the Beartooth Island area). The deposition of the Athabasca Group rocks took place probably during the 1340 to 1450 Ma period (Bell and Macdonald, 1982) as indicated by the Rb-Sr isochron on tuffaceous shales from the Wolverine Point Formation and diabase dykes cutting the Athabasca Group sedimentary rocks.

Structural features of the Athabasca region e structuraTh l pattercrystalline th f o n e basemen s dominatei t d by northeasterly trending geological units, most of which are bounded by major faults or shear zones. The Virgin River Shear and Black Lake Fault Zones separat e Westerth e n Craton froe Créth me Lake Mobile Zone. The Needle Falls Shear Zone separates the Crée Lake Mobile Zone from the Rottenstone Complex. The northeasterly striking fault syste s complementei m y northwesterlb d d northerlan y y striking fault sets. Basic intrusive rocks, which were emplaced after the deposition e Athabascoth f a Group sedimentary rocks, hea e northl th som f -o e westerly and northerly trending faults. A multi-ring structure, the Carswell Structure (Currie, 1969), occurs in the western part of the Athabasce Basin. The origin of this structure is not unanimously explained: it is interpreted either as having been formea meteoriti y b d c impact (Innés, a 1964 s a )r o

224 cryptovolcanic feature (Currie, 1969); the latter interpretation appears to be more reasonable.

URANIUM METALLOGENY OF THE ATHABASCA REGION Two major important episodes dominate the uranium metallogeny of the Athabasca region) Mineralizatio(1 : n associated e witth h Hudsonian orogeny about 1740 Ma; and (2) mineralization associated with the tectonic events about 1240 Ma. Processes associated with the first episode were active mainly withi crystalline th n e basement rocks wherea e mineralizatioth s f o n the second episod s speciallwa e y e sub-Athabascrelateth o t d a unconformity. Uranium deposit e firsth tf o s episodee occuth n i r Beaverlodge area e ,Athabasc th nort f o h a Basin (Tremblay, 1972). Mineralizatio depositn ni secone th f dso period occuralteree th n si d crystalline basement rocks below, withi e altereth n d sedimentary rocks above the unconformity or, most commonly, within the altered interface between the crystalline basement rocks and overlying clastic sediments, i.e. alon sub-Athabasce th g a unconformity.

e firsTh t episod uraniun i e m metallogeny Tremblay (1972) recognized four stage n formatioi se th f o n Beaverlodge uranium deposits, which are hosted by crystalline basement rocks of the Western Craton: (i) Deposition of uranium- bearing sediments; (ii) mobilizatio concentratios f uraniuno it d an m n during granitization; (iii) remobilizatios f it o uraniun d an m concentration during mylonitization (about 175 a 0M ago d an ) (iv) remobilization and concentration during late fracturing (about d later)an 124 a e regarde0M H . granitie th d d argillaceouan c s rocks of the Archean Tazin Group as the ultimate source of uranium in the Beaverlodge deposits. Bell and Macdonald (1982) recognized two periods of prominent mineralizatio e crystallinth n i n e basement rock n Saskatchewani s : (1) Syngenetic mineralization in pegmatites with absolute age of uraninite about 1860 Ma (which would correspond with Tremblay's secon d) epigenetistage(2 d )an c mineralizatio pitchblendn i n e veins with absolut aboue eag t 174 , (whic0Ma indenticas i h l with Tremblay's third stage of mineralization in the Beaverlodge area) . These two periods coincide with igneou d metamorphian s c e regioeventth n i ns namely syngenetie th : c mineralization wite "Wathamath h n Plutonism" d "Wathamaan . 188 ) (c 0Ma n Peak f metamorphiso " n metasedimenti m s (187Ma)0 2 epigenetie ± ;5th c mineralization wit emplacemene hth f to

225 "Younger Granites" along with lamprophyre dykes (1740 Ma) and with the second ("Main") peak of me t amorph ism (c. 1762 Ma). The second period, e epigenetii.eth . c mineralization, represent a majos r metallogenic event associated with magmatic and post-magmatic processes of the Hudsonian orogeny. These processes included extensive mylonitization (Tremblay, 1972), feldspathic (commonly sodic) metasomatism (Robinson, 1955), carbonatization and hematitizatio e rockth sf o nhostin g uranium and/or polymetallic deposits (Ruzicka, 1971 . Simila) r processe thoss sa e associated with the "Younger Granites e "Mainth r "o " pea f (Hudsoniano k ) metamorphism are recognizable in other regions of the Canadian Shield. Langford (1977), however, speculated thae Beaverlodgth t e pitchblende deposits originated from uranium-bearing meteoric and near- surface ground water. Uranium mineralizatio firse th tf n o perio Belf do Macdonald lan d is widespread in Churchill Structural Province, but the mineralized pegmatites and mineralization in them are very irregularly distribute w gradea ruledlo s anda f ,o ., Uranium mineralizatio e seconth f do n perio f o Beld d an l Macdonald occurs in fractures in certain lithostratigraphic units y (e.gCompleFa . f o Tremblayx , 1972) which were affectey b d metasomatic and hydrothermal processes. The main mineral assemblages are either pitchblende and U-Ti phases ('brannerite1) or (rarely) uranium-polymetallic associations. The ultimate source of uranium in these deposit s i unknowns t mighi e b :deep-seatet d fluids, granitized and mylonitized uranium-bearing sediments (Tremblay, 1972), residual evaporitic brines, connate fluidr o s crystalline basement rocks. Unaltered crystalline basement rocks underlyin e Athabascth g a Group locally contain elevated amounts of uranium and associated elements, such as nickel, cobalt and molybdenum (Table 2). These rocks might be sources not only of the Hudsonian epigenetic mineralization, but also of the mineralization associated with the sub-Athabasca unconformity.

The second episode of uranium metallogeny e maiTh n ore-forming processe secone th f dso episod f uraniueo m mineralization took place after deposition of the Athabasca Group rocks, i.e. during their diagnesis, under elevated thermal conditions. They were accompanie y retrogradb d e metamorphise th f o m host rocks (e.g. argillization, depletion of graphite), magnesian metasomatism and redistribution of iron, silica and aluminum oxides.

226 Tabl: 2 e Some elemental constituents from crystalline basement rocks, Athabasca Basin. Sample analysem pp n i s Element 123 Ça 730 17000 340 Na 329 >10000 720 Mg >20000 >20000 >20000 Ti 9000 10000 6900 Mn 110 2600 510 Ag <5 <5 <5 As <2000 <2000 <2000 B 98 53 62 Ba 130 140 680 Be 6 5 3 Ce 500 <200 200 Co 20 52 57 Cr 170 140 120 Cu 14 18 180 La 240 <100 <100 Mo <50 <50 <50 Ni 190 110 220 Pb <700 <700 <700 Sb <500 <500 <500 Sn <200 <200 <200 Sr 300 150 89 U 48 14 17 V 330 210 320 Y 41 50 <40 Yb 7 <4 <4 Zn <200 <200 <200 Zr 380 200 210

Sampl : Graphiti1 e c pelite froe footwale th mth f o l Deilman bodye or n . Sample 2: Unaltered garnetiferous gneiss from the footwal Midwese th f lo t Lake deposit. Sample 3: unaltered graphitic pelite from the footwall of the Midwest Lake deposit. Analytical method: Optical emission spectroscopy using 12B-DR spectroscope (samples 1-3)

a rul s eA illite and/or chlorite form haloes aroune th d mineralization, whereas kaolinite occur a greate n i s r distance from the mineralization. The host rocks in the vicinity of the mineralizatio e hematitizear ne sandstonth d n i highean d p eu r limonitized. Graphite in the pelite immediately below the mineralizatio usualls i n y depleted e sandstonTh . e abov e zonth f eeo limonitizatio oftes i n n silicified (Fig. 2) . Distribution of the ore bodies is, as a rule, structurally controlle y intersectionb d f striko s e faults wite unconformitth h y

227 KAOLINIT• :• E I IN THE MATRIX

^^i^é'Ê^^^H^MA'\tZED SEDIMENTARY 'aOCK^^^^^(^^fp-&^ ^Äl^ftiÄMÄiÄEA•^?^'-']^<>£$^:i^?-b^^ Ä fmmwmwmmmwmïfffm i L L i T E •f^mm-^mmmmm^m-:muèiwïïm ï^miïm^mmï^ ENVELOPE mmïïï^mMMm^Mïmïmm^ wï$fmm mmmmm

QUATERNARY APPROXIMAT ° . METRE3 ° , 2 S ° . E1 SCAL ? I E

MINERALIZATION HELIKIAN p;-:;v-.;-:.-;j SED. ROCKS OF THE fob&a ATHABASCA GROUP FAULT APHEBIA) N(? ^^^1 REQOLITH

K^JyJ METASEDIMENTARY APPROXIMATE LIMITS OF f^^Sj ROCKS ALTERATION HALOES ARCHEAN (?) 1+ + + +1 GRANITOIDS

FigurgeneralizedA . e2 cross section throug hmineralizea d body associated wit sub-Athabasce hth a unconformity.

228 and lithologicall y graphitib y c pelit r semipelito e e horizone th n i s crystalline basement. Age of mineralization, as determined on seven pitchblende samples (Tremblay, 1982) , spans an interval from 1075 to 1320 Ma. The mineralization is either monomineralic (pitchblende) or polymetallic (U, Ni, Co, Bi, Cu, Pb, As, Y, Zn, Mo, Ag, Au in various proportions). Mineralogical geothermometric analyses of samples from the Key Lake deposit indicate (Dahlkamp and Tan, 1977; Pechmann and Voultsidis, 1981; Ruzicka and Littlejohn, 1982; Ruzicka, 1982) that the uranium minerals crystallize temperaturet da s from 135 o 150°C°t , Pagel (1975 . othe(1980e Paged al th ) an t rn le ) o hand determinedn ,o the basi f fluio s d inclusion studie n sampleo s s froe Clufth m f Lake area, that temperatur f mineralizatioo e s probabl e wa rangn th n ei y from 60° to 260°C.

URANIUM DEPOSITS e uraniuTh m deposit se Aphebian-Helikiath relate o t d n unconformit Athabasce th base f th eo t a ya Basin occur beneat covea h r e Helikiaf rockth o f o s n Athabasca Grou , wheror p e these rocks were eroded, under unconsolidated Quaternary deposits e coveTh .r rocks abov knowe th e n deposits var thicknesn i y sRabbit a frol ni mt Lako t e y Lakemetre0 1 Ke 5 metre 16 d ,t Collin a san t McClea a s y Ba s n Lake, 0 metre21 Midwest a s 0 metret45 Lakd Cigat a san e r Lake. Individual deposits contain resources comprising tonnages of uraniu mw hundre frofe a mo severa t d n thousandte l s tonne n orei s s grading from 0.2% to more than 10% U (Table 3). Areal distributio e depositth f o ns controlle i s y structurab d l and lithological featuree crystallinth f o s e e basementh d an t sedimentary cover. e AthabascTh a regio s beeha nn subdivide n thii d s paper into area s a follows s) *Rabbi(1 (Fig : t) .1 Lak e— Collin s Bay; (2) McClean Lake — Midwest Lake — Waterbury Lake; (3) Key Lake; ) Carswel(4 l Structure Mauric) (5 ; ) Fone (6 Lacu Bayd d .;an

RABBIT LAKE - COLLINS BAY AREA

The most important deposits in this area are: Rabbit Lake, Collin d Eagly "Aan Ba s , 1 ' e Collin 'B Point y n additioI Ba s. o t n these deposit e Raventh s , Horsesho d Wesan et Bear deposits contain identified uranium resources.

*Number n bracketi s s refe o circlet r d number Figurn i s . 1 e

229 K) U> O Table 3: Published uranium resources in selected uranium deposits associated with the sub-Athabasca unconformity. Ore U U Area/Deposit Tonnes % Tonnes Reference Rabbit Lak- e Collins Bay area Rabbit Lake 5 655 000 0.34 19 230 1) Clark et al., 1982 Collins Bay B N.A. 3 N.A29 1 1 Denne. r an<3 Reeves, 1981 West Bear N.A. N.A 5 .42 Tremblay, 1982 McClean Lake— Midwest Lake— Waterbury Lake area McClean Lake 353 800 1.53 5 384 Saracoglu et al., 1983 Midwest Lake 1 200 000 1.60 0 125 9 Canada Wide Mines (1980) Key Lake area Key Lake 0 2.180 7 27 32 8 22 9 6 Saskatchewan Mining Development Corporation (1983) Carswell Structure D 123 000 4.2 1 521 5 Bayda (1978) Mauric y areBa e a Maurice Bay N.A. <0.42 7 57 Lehnert-Thied an l Kretchmar, 1979 Fond-du-Lac area Fond-du-Lac N.A. <0.2 5 138 Tremblay, 1982

N.A. Informatio t availablno n e ) Productio1 t beeno ns excludedha n . c*=S% OLE PQ9NT CO IKE ISLAND COLLINS BAY A — ZOND . E /ACOLLINS BAY B ZONE a - 58 15

FIRST GRAYLING

_ «HOOK/LAKE -rfABEMT LAKE JOHN) 'BEAVER LAKE * WOLLASTON LAKE • AHORS

Lampin Lak«x^>*~i

SOUTH LAMPI' N

•STAN REASOO T P DU N

— 58 L.'."3 ATHABASCA GROUP (HELIKIAN)

A U DEPOSIT • U OCCURRENCE

Milchlell Lake

0 10 ELDORADO* WEST BEAJ MILES i_

57 45 45' 30 104 103 15

Figure . 3 Uranium occurrence Rabbitn si Lake Collins~ area.y Ba

231 NW

QUATERNARY

F>l OVERBURDEN

HELIKIAN ll ATHABASCA GROUP

APHEBIAN

L'\»j UPPER GNEISSES

/ FAULT HHIJ MASSIVE META-ARKOSES

| | PLAGIOCLASITE

MASSIVE DOLOMITE

mH REGOLITH

INTRUSIVE l.'tyy BIOTITE MICROORANITE

REGOLITH

LOCAL CHLORITIZATION / FAULT

I PERVASIVI E CHLORITIZATION

| I GRAPHITIC CHLORITIC ALTERATION

DOLOMITIZATIO CHLORITIZATIOD NAN N

PERVASIVE DOLOMITIZATION met res

Figure 4. Geological section through the Rabbit Lake deposit; (a) lithology; (b) alteration. After Sibbald Heinm e (1981).

The Rabbit Lake deposit This deposit is located about 5 km west of the west shore of Wollasto north-northeasm k 4 n 35 Lak d ean Ronga L f eto (Fig. .3) Uranium mineralizatio Rabbie th n ti n Lake deposi hostes i ta y db suit f o Aphebiae e nWollasto th rock f o s n Group (Hoevd an e Sibbald, 1978). Sibbald (1978) recognized in this suite three rocks

232 Figure . 5 Radiating crystals f o sklodowskite n i quartz; Rabbit Lake deposit, Saskatchewan. Transmitted light.

units: (1) soda-rich feldspathic quartzite at the base; (2) an assemblage of calc-silicate rocks, feldspathic quartzite, marble and graphite-bearing rocks ) layere(3 ; d gneis sf o quartzomad p u e - feldspathic rocks intercalated with amphibolite. This suite is intrude y fine-graineb d d granites (microgranite), (Fig. 4) . e deposi th e are e IAphebiaf th no th a t n rocks trend northeasterly (065°) and dip southeasterly (65°); the host rocks represent a northwestern limb of a major syncline. They are dislocated by the 065°-trendin ° SE-dippin30 d an g g Rabbit Lake Fault. The host rocks are strongly altered. Chloritic alteration took e pre-mineralizatioth plac n i e n period durin e latth ge stagef o s Hudsonian Orogeny, kaolinization and illitization during the mineralization period and argillization mainly due to weathering during the post-mineralization period until present time (Tremblay, 1982). e bod s or s cigar-shapei longesy it e Th n i t d dimensioan d n metre0 55 horizontase th long n I . l plane projectio e zons or i ee th n 5 metre 12 metre5 o 36 t s o 0 t wides9 5 lont extendd i 21 ;an a g o t s 0 metresdept15 f o h .

233 The mineralization consists mainly of pitchblende and coffinite, but sklodowskit g (U0(M e2 )2(Si03)2(OH)2*5H a commo 2s Oi ) n secondary mineral (Fig. 5) along with minor amounts of kasolite, woelsendorfite, uranophane and boltwoodite. Minor amounts of other metallic minerals are associated with uranium mineralization: galena, pyrite, rutile, nickelind an e marcasite. Calcite and phyllosilicates are most common gangue minerals. Elemental assemblages associated with uranium in the Rabbit Lake deposit include vanadium, molybdenum, nickel, copper, lead, barium, boron, magnesium and phosphorus in amounts exceeding the clarkes of these elements. e minimuTh e pitchblendmth absolutr fo e ag e mineralizatios ha n been determined by U-Pb method as 1281 ± 11 Ma (Gumming and Rimsaite, 1979); however uraniu s beeha mn later rejuvenated several times. Decrepitation tests on dolomite, which accompanied uranium mineralization as a gangue in the Rabbit Lake deposit, indicated temperatur f crystallizatioo e n fluio t 245°a nd dan C inclusionn i s euhedral quart 135t a z o 160°°t d allowinCan effectr fo g pressurf so e at 180 225°o °t C (Little, 1974). A similar typf o mineralizatioe s noticewa n d m k abou 1 2 t northeas f Rabbito t Lak n Spurjaco e k Island (Chandler, 1978).

The Collins Bay 'A1 Zone The Collins Bay 'A' Zone is located about 10.5 km north of the Rabbit Lake deposit near the eastern edge of the Athabasca Basin (Jones, 1980; Tremblay, 1982) uranium-polymetallis It . c mineralization occurs in the hanging wall of the Collins Bay Fault under over burden and a small remnant of a sandstone layer of the Athabasca Grou e bodp s or enclosei rocksy e Th n clay.i d , which gradually changes with dept o clay-alteret h d Aphebian rocks (regolith) fresh graphitic and/or biotitic paragneis Archead san ) granitoi(? n d gneiss (Fig. 6). The Aphebian rocks are folded into a northeasterly trending anticline. Distribution of the mineralization is structurally controlle y b intersectiond e northeasterlth f o s y trending and southeasterly dipping Collins Bay Fault, the altered zone along the sub-Athabasca unconformity, and by northwesterly trending cross faults. The high grade core of the Collins Bay 'A 'a lensoi Zons ha e d s shapeaboui metre0 t 5 i t; s o longt p u , 30 metres wide and up to 22 metres thick. Some drillhole inter-

234 W

Water

QUATERNARY

[~ '• <] OVERBURDEN HIGH GRADE CORE HELIKIAN

[ ) ATHABASCA GROUP CLAY ALTERATION

APHEBIAN

jj|^j PARAGNEISS (GRAPHITIC) COLLINS BAY FAULT

L-ÏH3 PARAGNEISS

ARCHEAN(?)

GRANITOID GNEISS

Figure 6. Generalized cross section through the Collms Bay 'A' Zone. After Jones (1980).

Table 4: Analyses of selected drill core samples from the ' ZoneCollin'A !y sBa

Constituents ppm

Sample No U Ni Pb Ag Au

1 2.33 0.56 0.20 3.77 0.03 2 59.78 1.07 9.18 29.14 10.63 3 64.87 0.96 9.11 30.51 24.31 4 61.22 1.20 8.89 25.37 33.46 5 13.40 5.09 1.18 23.66 1.92 6 53.17 1.69 6.16 43.54 15.94 1 35.19 3.66 7.08 1,349.8 10.35

Samplewere5 o t : s1 - randomly collected from the central portion of the 'A1 zone; Sample 6 and 7 were: randomly collected from the northeastern portion of 1 thzone'A e ;

235 sections contained uraniu n exces0 perceni m5 f o s U tove r more than 10 metres. The ore consists of pitchblende associated with rammelsbergite, pararammelsbergite, some galena, sphalerit d traceean f silveo s d an r gold. (Table 4).

Collin1 Zon'B ey sBa The Collins Bay 'B1 Zone is located about 8 km north of Rabbit Lake deposit (Jones, 1980; Tremblay, 1982). Uranium mineralization occurs mainly in clay altered sandstone of the Athabasca Group, which unconformably overlies highly altered Archean granitic gneiss. The Aphebian paragneiss, which is locally graphitic, occurs mainly to the east of the mineralization and only e deposi th e norther e n contact f th i s wito or i t t t e d i a h Th en n. bodies of the 'B1 Zone follow the trend of the Collins Bay Fault. e bod s or aboui y e tTh 1200 metre 0 metre15 s o longt s widp u , e and in average 17 metres thick. It contains high grade pods in the northern, central and southern part of the deposit with ore grades exceeding 20% U and 20% Ni over 2 metres. In additio o pitchblendt n d coffinitan e e mineralizatioth e n includes gersdorffite, skutterudite, nickeline, millerite, galena, chalcopyrit d pyritan e e (Fig . .7) Elementa l assemblagee th n i s Collins Bay 'B1 zone include Ag, As, Co, Cr, Cu, Ni, Pb, Ti, V and Zr (Tabl. 5) e Several smale bodieor l s occur betwee e Collinth ny Ba s 1 Zones'B d 'A.an 1 Their distributio controlles i n y intersectionb d s of the Collins Bay Fault with the sub-Athabasca unconformity. These e bodieor e callear s d 1 ZonesCollin'F d y 'D.an Ba s 1 1 ,Thei'E r mineralizatio1 Zone'B d s an (Fig ' . 'A 8) s simila.e i n th o t r

Eagle Point deposit The Eagle Point deposit occurs aboum northeask e 2 tth f o t ' Zons 'A Collihos It ey tBa s(Fig rock. e Aphebia3) .ar s n metasediments suc s feldspar-cordierite-biotita h e gneiss, pyritic garnetiferous gneiss, feldspathic quartzite and graphitic paragneiss, which are intercalated with granitic pegmatoids. The host rocks exhibit monoclinal layering and faulting and strong argillizatio d hematitizatioan n r o limonitization n which follow faults, bedding-plane r foliationso .

236 Galena m.» —?- Pitchblende „

Figur . e7 Photomicrograp f o pitchblendeh associated Figur . e8 Photomicrograp f pitchblende,o h partly coatedy b with gersdorffite d nickeline;an Zone.1 'B Collins y Ba galena, associated with gersdorffite and nickeline. Collins Bay Reflected light. 'D1 Zone. Reflected light. to <0 01 0) •H 01 x: ü —t 4J 0) U C •H 01 x: x: § n r j ^ i [ vu •H —H O) • • • • • lit h wit pitchblend e re d sandston e in rH rH l-l lit h wit pitchblend e fl3 (0 ro rH ro •o m >O o O 0) O O 0) o 0) 0 0 • • • rH O CM « r- 0 • CMvo o « o D> DÏ4J rH Ul l-i ro n. CM *° O o 0 0 o o m o r- o o u l-i ro rH rH 0) g^. rH A A V a <"oo e CQ m CQ PQ CQ CQ ro ^* j *^ -^m ° moo o o o o o roO rH 00 0 rHo ro l CM in nl Ul A -rH rH CQ 1 3 CM 4-1 4-> O o o o 0 O O 0 O O0 O 0o o o o a C rH in • C V V V V TD Jii c o Dl CT>4-> 43 0 u C C >J in CO Co r Mn i CM Q) •H H •r H CI ' O ro oo o vo ooo vo VO *3* ^* x: Ul Ul <0 w *-' O Cn i Mo o o o- r ino H r C~M C - O 0 CM rH 4-1 3 3 -0 in o o o o o o o roO O ro 0 0 0 rH 0) ro C -CM 0 0 r^ * Z «r**V«l Wf H o o o o o o o o o o o o o 0 0 O O o t-X l-i u n) 0) •H e o 0 U c V v v V V rH 0) ra 4-) 4-1 XI XI 01 «5 rö >i rO r-o ro i-l l-i CT> 4J in t • • • • • in rH • >t O O u c , r-l (Q fQ ro rH ro "O in CM TJJ o •H 0) C XI X! (U m o o • 0 • H r « ro • r~ 0 • rH rrj 4J TJ -rH O H U- U CQ V A A V 0 0) rH •D H .r £ • T3 c 4J a> cn en o 4J 0) Ul ü C 4-> • 4-i ro j_l f^ 01 U "H •H 4-1 -H (U • rH 14J iH >1•O ü <1J -rHü ! •HX rH rH f£ 0) O rH (U •a W U e ra -P 4-1 t> •o -o IXS c O (P 4-> 0) 0) 0> C ro c-c O N NNO

• I C r*1 r*1 £>1 i-l in 4-1 1 4- H i H H r C 1 1 W ra o o r o d) 0) (0 C C C V H e • • 3 CO N Z 2 EH rH

238 .83 Table 6: Elemental assemblage in a weakly mineralized part of the Eagle Point deposit. Sample 1 2

Si02 74.0 84.0

A1203 12.0 8.5

Fe203 0.4 0.4 CaO 0.25 0.15

K2O 3.4 1.7

% Na2O 0.2 0.2 MgO 2.5 1.9

Ti02 0.38 0.16

P20S 0.05 0.04 MnO 0.06 NF

As NF NF Ba 340 100 Co 10 NF Cr 90 50 Cu 10 40 Nb 10 10 Ni 20 20 m pp b P 20 410 Sr NF NF U 10 12 Y 20 10 Zn 80 10 Zr 180 210 Rb 200 80 F 900 500 Cl 200 400

N.B.: N.Ft founno d.— 2 collecteSample d an 1 s d from drill cord an e analyzed using 12B-DR

Uranium mineralization occur lensen si s arrange narron i d w zones and consists mainl f pitchblendeo y , which occur n smali s l cubes, veins, botryoid r associateso d with carbon, coffinite, uranophand ean boltwoodite (Figs. 9 & 10). Its distribution is controlled by the alteration zones. Gersdorffite, millerite, chalcocit d galenan e a occur locall n smali y l amounts. Elemental assemblag a weakl n i e y mineralized part of the deposit includes Cr, Pb, Ni, Zr, Rb, F and Cl (Table 6) .

240 Unlike the Collins Bay zones the Eagle Point deposit occurs entirely in the altered Aphebian basement rocks below the sub- Athabasca unconformity, is stratabound and its mineralization is confined to at least fifteen sheet-like zones. The general strike of the rocks and mineralized zones is east-northeast and dips are mineralizatioe Th . SE identifier ° fa bee70 s o ha no s n t ° d40 alona g m dept 0 45 h distanco dow t m widt nd 0 an 20 hf abou o eo t t p u 200 , 0m the dip.

Other deposit Rabbie th f tso Lak eCollin— arey sBa a In addition to the Collins Bay zones and Rabbit Lake and Eagle Point deposits, the Rabbit Lake — Collins Bay area includes the West Bear, Raved Horseshoan n e deposit d severaan s l small uranium occurrences (Fig. .3) The West Bear deposit occurs about 43 km southwest of the Rabbit Lake mine. Distributio e mineralizatioth f o n e deposith n s i ni t controlled by the sub-Athabasca unconformity, which is beneath overburden and 15 to 20 metre thick sandstone of the Athabasca Group. e mineralizatioth Mos f e altereo tth n i d s i pyritin d graphitian c c semipelite e Wollastoth f o s n domain just belo e unconformityth w . Some mineralization occurs in the sandstone immediately above the unconformit withid yan clae nth y alteratio nunconformitye zonth t ea . In additio o uraniut n mineralizatioe th m n contains higher amountf o s arsenic, lead, nickel, vanadium, zirconiu d silvean m r (Tabl. 7) e The Raven and Horseshoe deposits occur about 5.5 km southeast of the Rabbit Lake min e hose Th (Figt . rock.3) f boto s h deposite ar s mainly altered graphitic guartzites and biotite gneisses with minor amphibolites and calc-silicate rocks of the Wollaston Group. The pitchblend d uranophanan e e mineralizatio s irregularli n y distributed in fractures and disseminated in the altered host rocks.

MCCLEAN LAK MIDWESE- T LAK WATERBURE- Y LAKE AREA Deposits with identified uranium resources in this area are: McClean Lake, Dawn Lake, Midwest Lake and Cigar Lake. In addition several occurrences d Mallean B ,n JE suc Laks a he neae McCleath r n Lake J neaB deposir d an McArthut r River, occu n i thir s area (Fig. 11).

The McClean Lake deposit occurs about 11 km northwest of the Rabbit Lake mine (Brummer et al., 1981; Saracoglu et al., 1983; Tremblay, 1982; and Fig. 11). The deposits contains three east-

241 -Ê(O» to Tabl : 7 eSpectroscopi c analyse f selecteo s d samples froWese th mt Bear deposit, Sample Constituent Si >2.0 >20.0 15.0 Al 1.5 2.0 2.0 Fe >2.0 >3.0 >3.0 Ca % 0.033 0.3 0.5 Na 0.0054 N.F. N.F. Mg 0.67 5 0. 1- 0.2 - 0.5 Ti 0.11 1.0 1.0 P N.A. N.F. N.F. Mn 0.031 0.2 0.1 Ag <5 100 70 As 0 100 9 10 000 15 000 Bi N.A. 2 000 700 Ce <200 N.F. N.F. Co 360 1 000 700 Cr 91 200 150 Cu 570 1 500 500 La <100 N.F. N.F. Nb <50 N.F. N.F. Nd Ppm N.A. N.F. N.F. Ni 1 700 2000 - 5000 1000 - 3000 Pb 0 >1000 0 20 000 15 000 Sb <500 N.A. N.A. Sc N.A. 100 100 Su <200 N.A. N.A. Sr 24 1 000 1 000 U 70 85 700 0 1200 5 V <20 2 000 5 000 Y 67 100 200 Yb <4 N.A. N.A. Zn 0 100 5 N.F. N.F. Zr 100 3 000 1 500 t availablno N.A - . e Sample 1: analyzed using 12B-DR N.F. - Not found Sample 2 & 3: analyzed using SQ23 1 04 30 10335 IS I 5830 • -58°30

MALLE N LAKE

JEB

tf«

TOR WA L T i McCLEAN .NORTH

McCLEAN SOUTH

à. U DEPOSIT

U OCCURRENC • E

0 1 5 0 KM

58 . 58 Figure . Uranium11 occurrences n i McClean Lake - Midwes t Lake OJ Waterbury Lake area. north-east trending mineralized zones, eac f whico h h consistf o s several ore lenses. These zones occur at the sub-Athabasca unconformity which is about 165 metres below the surface. The footwal e zoneth sf o lconsist f Aphebiao s n rocks including graphitic and pyritic metapelites, feldspathic quartzit d amphibolitean e . e hanginTh g wall consist f coarso s r pebblo e y sandstone th f o e Manitou Falls Formation. Uranium mineralization, pitchblendd an e coffinite commonls i , y associated with illite, chlorit d metallian e c minerals sucs a pararammelsbergiteh , nickeline, gersdorffite, safflorite, chalcopyrite, siderite, hematite, goethite, bravoitd an e pyrite (Brummer et al., 1981).

The Dawn Lake deposit is located about 20 km west of the Rabbit Lake mine (Clarke and Fogwill, 1981; Tremblay, 1982; and Fig. 11) . Its four mineralized zones are designated as 14, 11, 11A and 11B (Fig .e mineralization Th 12). , consisting mainl f pitchblendo y e (massive, botryoidal and sooty), some uranoan carbon and secondary uranium minerals, is accompanied in one of these zones with cobalt and nickel arsenides and copper sulphide, whereas in the other zones the uranium mineralization is monomineralic. The mineralization e altereth occur n i ds basement rocks (Zone 11B), withie th n argillized interface along the unconformity (Zones 11A and 14) and within the sedimentary rocks of the Manitou Falls Formation (Zon ee basemen 11)Th . t host rock e chloritizear s d pelites with variable amount f o disseminates d graphite, biotite, pyritd an e locally cordierite; calc-silicate rocks and granitic pegmatites locally contain tourmaline e argillizeTh . d interface alon e subth g- Athabasca unconformity includes weathere de basemen th zon f o e t rocks (saprolite or regolith) and products of diagenetic alteration. The e Manitoth rock f o us Falls Formation include basad an l intraformational conglomerate, sandston d claan ey intercalations. Distribution of the mineralization is structurally controlled by intersections of the sub-Athabasca unconformity with strike- and cross-faults (Fig. 12).

The Midwest Lake deposit is situated 25 km northwest of the Rabbit Lake mine (Fig; Scott11 . , 1981; Tremblay, 1982. ) The deposit occurs along the sub-Athabasca unconformity, which is abou 5 metre19 t s belo e surface deposite centrth wth th f t o ea e , and extends alon n intersectioa g f thio n s unconformity wita h northeasterly trending fault zone (Figs. 13 and 14). It is about 900 metres long, up to about 110 metres wide and 36 metres thick.

244 LONGITUDINAL SECTION

11AZONE SCALE i———£——Too

QUATERNARY MINERALIZATIOU N | OVERBURDE ' ' [ N

HELIKIAN / | ] ATHABASCA GROUP / FAULT ARCHEAN AND APHEBIAN METAMORPHIC BASEMENT (UNDIVIDED) DAWN LAKE METAPELITE

$ CALC-SILICATE ROCKS FOLD AXIS

+\ GRANITE PEGMATITE

Figure 12. Dawn Lake deposit. Modified after Clarke and Fogwill (1981).

245 SECTION A

MINK ARM OF MIDWEST LAKE

SCALE

0 10 0 8 0 6 0 4 0 2 0 [ j SANDSTONE ORE METRES MASSIVE ORE

FAULT PLAN VIE F OREBODWO Y

Figure 13. Plan view of Midwest Lake deposit. (After 'Canada Wide Mines Midwest- Lake Uranium Project'; Canada Wide Mines, 1980).

The uranium mineralization occurs (i) in a massive or spherulitic form (Fig. 15); (ii) in a mixture of pitchblende with coffinite; and (iii) in a sooty form (Ruzicka and Littlejohn, 1981). Associated nonradioactive mineral e nickelinear s , millerite, maucherite, rammelsbergite, gersdorffite, nickelhexahydrite, retgersite, calcite, chlorite, halloysite, hematite and goethite. Mineralization in the sandstone consists mainly of massive and sooty pitchblende occurring interstitially or as fracture filling in quartz grains (Fig. 16). A generalized conceptual mineral succession postulates five stage f o mineralizatios n (ibidem; Fig. 17)) massiv(1 :r o e spherulitic pitchblende ) firs(2 ; t generatio f nickeno l arsenided san sulphides ) secon(3 ; d generatio f nickeo n l arsenide d sulphidean s s and pitchblende-coffinite assemblage ) sulpharsenide(4 ; d sootan s y pitchblende; (5) gangue and post-ore minerals. The main elemental constituent Midwese th n i st Lake deposit are n additio,i o uraniumt n , nickel, cobalt, arsenic, molybdenum, titanium, vanadiu d zirconiuan m m (Tabl. 8) e In addition to concentrations in the main ore bodies at the unconformity some mineralizatio s depositewa n n i fractured s connectin e bodieor e sth g wit e surfacth h d latean e r incorporaten i d

246 •• rV? =fff.P-e\jyytfYfiD'O- -TW

SCALE 0 5 0 4 0 3 O i 0 1 5 0

QUATERNARY

PPUl OVERBURDEN SANDSTONE ORE

HELIKIAN MASSIVE ORE pill ATHABASCA GROUP-SANDSTONE f-v-r-r'v-r-r-l / [5g|g| ATHABASCA GROUP-CONGLOMERATE / FAULT ARCHEAN

fS^fjj GNEISS (BIOTITE.PELDSPAR.CHLORITE.GRAPHITE^ '- r »

Figure 14. Cross section through Midwest Lake deposit. (After 'Canada Wide Mines - Midwest Lake Uranium Project', Canada Wide Mines, 1980).

247 248 MINERAL ASSEMBLAGE

Rammelsbergite

Pitchblende and Coffinlte

Pitchblende (sooty)

Calclte.Hematite. Chlorlte, Goethlte, Halloyslte .Retgerslte Nfckel-hexahydrlte

Figure 17. Generalized mineral succession in the Midwest Lake deposit. Dashed line (----) indicates replacement process; dotted line (.....) indicates that the younger minera bees ha l n derived from another mineral(s).

glacial e driftsurficiath n thi I y .s wa l dispersion haloes indicated presence of concealed ore bodies and thus exploration targets. K-Ar analyses of sericitic and chloritic clay collected from the mineralized zone indicated f o a M absolut121 3 3 e 2 ag ± e (Stevens et al., 1982), which can be considered as the main age of the Midwest Lake deposit.

The Cigar Lake deposit occurs near the southwestern shore of Waterbury Lake (Fig. 11). Its mineralization is structurally controlle mylonita y b d e basemen th zon n i e t rocksn alterea y b , d sub-Athabasce th zon t a e a unconformit y graphitib d an y c metapelites Wollastoe oth f n Group (Ruzick LeCheminantd an a , 1984) mylonite Th . e zone has been subjected to faulting and cross-faulting. Retrograde metamorphism resulte n extensivi d e clay (kaolinite, illitd an e chlorite) alteratio e hosth t f o nrockso phaseTw f .uraniuo s m mineralizatio ne deposit a polymetallicth occu) n (i i r : , which

249 to

Table 8: Selected elemental constituents in selected drill core samples from the Midwest Lake deposit. (From: Ruzick d Littlejohnan a , 198 Canadd 1an a Wide Mines Limited, 1980).

Constituent Percenn i s t

Sample 1) 1) 2) 2) 2) 1) 2) 2) 1) 4) 4) Element 1 2 3 4 5 6 7 8 9 10 11 Ag 0.0068 <0.0005 0.015 n.d. 0.003 <0.0005 0.0015 n.d. <0.0005 tr. 0.0068 As 7.2 <0.20 >10.0 n.d. 3.0 0.23 5.0 n.d. 0.35 1.68 9.62 B 0.0068 0.024 n.a. n.a. n.a. 0.068 n.a. n.a. 0.0085 n.a. n.a. Ba <0.0005 0.0082 n.a. n.a. n.a. 0.0078 n.a. n.a. 0.0033 n.a. n.a. Be <0.0003 0.0004 n.a. n.a. n.a. 0.0016 n.a. n.a. <0.0003 n.a. n.a. Bi n.a. n.a. n.a. n.d. 0.1 n.a. 0.15 n.d. n.a. 0.017 0.057 Co 0.22 0.013 1.0 0.3 0.5 0.003 0.2 < 0.05 0.29 0.19 0.11 Cr <0.0005 0.019 0.015 0.003 0.015 0.033 0.01 0.02 <0.0005 0.019 0.023 Cu 0.033 0.0024 0.15 0.007 0.007 0.0014 0.15 0.02 0.00073 0.08 0.42 Mo 0.49 <0.005 n.a. n.a. n.a. <0.005 n.a. n.a. <0.005 0.023 0.136 Ni >2.0 0.02 > 0.7 0.2 >1.5 0.35 > 1.5 < 0.005 0.12 0.94 4.80 Pb 0.082 <0.07 3.0 n.d. n.d. <0.07 n.d. n.d. <0.07 0.04 1.19 Sr 0.0028 0.0052 n.d. n.d. 0.007 0.0029 0.003 n.d. <0.001 n.a. n.a. Ti 0.24 0.93 0.2 0.05 2.0 0.66 1.0 0.7 0.066 0.79 0.73 U3) 0.02 0.007 27.2 4.8 2.2 0.14 14.4 17.3 0.009 0.25 11.79 V 0.014 0.062 0.3 0.015 0.3 0.49 0.3 0.1 0.0055 0.11 0.34 Zn <0.02 <0.02 n.d. n.d. n.d. <0.02 n.d. n.d. <0.02 0.11 0.52 Zr 0.025 0.032 < 0.005 0.03 0.3 0.02 0.5 0.15 0.014 n.a. n.a.

tr- trac. e Locatio Samplesf no : n.a- analysi. availablt sno e n.d. - not detected (i.e. below the detection limit) 1 DDH 78-40: sandstone 2 58 18'N, 104 06'W: chloritized . 1 Analyzed using 12B-DR 3 DDH 78-42: regolith . Analyze2 d using 5Q-23 4 DDH 78-40: sandstone 3. Analyzed by Neutron Activation 5 DDH 78-18: regolith 4. Analyzed by a commercial laboratory 6 DDH 78-18: regolith 7 DDH 78-18: sandstone 8 DDH 78-70: sandstone 9 DDH 78-15: sandstone 10 Typical sandstone ore 11 Typical massive ore ; Zn d an r Z , As includes , Mo , addition C i , , Pb o uranium t n, Co , Ni , this phase prevails at the base of the ore bodies; (ii) a monomineralic, which consist f pitchblendso botryoidaln i e , massive, granular, brecciated or finely disseminated forms; this phase usually occurs in the upper portions of the mineralized bodies, i.e. in the sandstone. The main mineralized zone occurs abou 0 metre45 t s beloe th w surface, but lesser amounts of mineralization were encountered about 100 metres below the present surface.

KEY LAKE AREA y LakKe ee areTh s locatei a da m RongnortL k abou d f 0 o an eh24 t m south-southwesk abou 0 18 t e Rabbie th Th f to . Lak1) e. min(Fig e uranium mineralizatio e bodiesor n o occurtw : n i sGärtne d an r Deilman . 18)Fig . n; d Tremblayan (Gatzweile1981 ; , al. 1982 , t e r The ore bodies are closely associated with the sub-Athabasca unconformity, basement tectonic features, such as several layers of graphitic metasedimentary pelitic a majorock d ran s reverse fault zonee faulTh . t zon paralles i e l wite foliatioth he basemen th f no t rocks, i.e. 060° to 070°, dips 40 to 60 degrees to the north-west and dislocates both the basement and cover rocks vertically up to 30 m. The graphitic pelites are part of the Aphebian Wollaston Group, comprising biotite-plagioclase-quartz-cordierite gneiss, garnet- quartz-feldspar-cordierite gneiss, amphibolite, calc-silicate rocks, migmatit d granitan e e pegmatite e WollastoTh . n Group rocks unconformably overlie Archean granitic rocks, which occu s northa r - easterly elongated domal structures e AphebiaTh . n sedimentt a e ar s the sub-Athabasca unconformity, are strongly altered and contain chlorit d illitean e .e alteratioth Som f o e, apparently is n , regolithic t som s bu diageneti,i e c hydrotherma r o relatel o t d tectonic movements along the fault zone. The sedimentary rocks of the Athabasca Group rest unconformabl basemene th n yo te ar rock d an s 0 metre6 e depositso th ut se pare th f thico an i .k Thee ar y conglomeratic sandstone at the base and quartz sandstone in the rest e sequenceoth f e sedimentarTh . y rock e alterear s d with hematite, kaolinit d chloritean e . The uranium mineralization occurs in at least four main generations (Ruzicka and Littlejohn, 1981): (i) as thorium-free euhedral

crystals of a alpha U30? compound; (ii) as massive, commonly botryoidal, pitchblende; (iii) as pitchblende-coffinite aggregates; s soota ) yan (iv pitchblended e mosTh t. common mineral assemblages e depositoth f s consis f pitchblende-coffinito t e aggregates with

251 105 45

1OOO 2OOO Metres

Athabasca Approximate Location of Fault Group Athabasca Unconformity Aphebian — — — Geological Contact Ore Zone Lake Archean Camp

Figure 18. Geological setting of the Key Lake deposit. (After Tremblay, 1982).

Figure 19. Snowflakes of gersdorffite with dark cores of bravoite from Deilmann body,Lakeore Key deposit. Reflected light; chaZkopyrtte= p c .

252 gersdorffite and sooty pitchblende. In addition to clay, calcite and/or siderite are the main gangue minerals. Associated with the uranium mineralizatio e alsar no millerite, nickeline, bravoite (Fig. 19), rammelsbergite, chalcopyrite and galena. According to analyses of selected samples the elemental assemblages of the mineralization consis f arsenico t , cobalt, copper, nickel, lead, titanium and vanadium (Table 9). U-Pb isotope analyses on 34 samples of pitchblende from the Key Lake deposit indicated three main age f mineralizationso : ~1,22; 8Ma ~960 Ma; and ~89 Ma (Gatzweiler et al., 1979); Wendt et al. (1978) reported an age of 1270 Ma also on pitchblende. The ~1,228 Ma age fits well wite absolutth h mineralizatione e th age f o s s obtainen o d other sub-Athabasca unconformity related deposits. The associations of pitchblende/bravoite and pitchblende/alpha- triuranium heptaoxide, found in the Key Lake deposits, indicate a temperatur f o crystallizatioe n between 135d 150°°an C (Dahlkam t al.e p , 1977; Pechman d Voultsidisan n , 1981; Ruzickd an a Littlejohn, 1982). The Gärtne Deilmand an r n deposit e 145d 120ar san 0 0 metres long respectively, their horizontal width is up to 100 metres and their vertica 0 metresl 15 dimensio o t .p u s i n

CARSWELL STRUCTURE The Carswell Structure is located in the western part of the Athabasca Basin, about 140 km southwest of Uranium City (Fig. 1). e centraTh e structurl th cor f o e e comprises Aphebian gneissed an s granitoids, classifie pressn (i )y Alons. b d intal o groups t tw ooe : an older Earl River Comple xa younge d (200 an o 196 rt 5) Pete0Ma r River Gneiss. This cor s surroundei e y Helikiadb n sedimentary rocks of the Athabasca Group: William River Subgroup, consisting of conglomerate, sandston peli-ted ean ; Douglas Formation with sandstone, siltstone and mudstone; and Carswell Formation consisting mainly of dolomite (ibidem). These rocks are intruded by diabase dykes (dated by Wanless et al., 1979, as 949 ± 33 Ma). The youngest, volcanic- like rocks, are Cluff Breccia, dated on two samples by a K-Ar method a (CurrieM 8 2 ± ,9 1969)46 d an . a M 5 5 ± 6 48 s a e knowTh n deposit se south-centra th occu n i r e l th par f o t Carswell Structure e vicinitth n i ,f Cluf o y f Lake (Fig. 20)o .T date, uranium resources have been identified in the following deposits: D, N, R, OP, Claude and Peter River (Fig. 21).

253 to

Table 9: Selected elemental constituents in selected drill core samples from the Key Lake deposit. (From: Ruzicka and Littlejohn, 1981) .

Constituents in Percent Sample 1) 1) 2) 2) 2) 2) 2) 2) Element 1 2 3 4 5 6 7 8 Ag < 0.0005 < 0.0005 n .d. < 0.001 0.03 0.03 0.03 0.02 As < 0.20 < 0.2 2.0 0.7 3.0 > 10.0 > 10.0 10.0 Ca 0.073 0.017 10.0 3.0 10 .0 1.5 1.5 10.0 Co 0.002 < 0.001 0.1 0.2 0.07 0.3 0.3 0.1 Cr 0.017 < 0.0005 < 0. 001 < 0.001 < 0 .001 < 0.001 0.03 0.001 Cu 0.0014 < 0.0007 0.05 0.02 0.07 0.2 0.07 0.03 Mn 0.011 0.002 0.5 0.3 0.3 0.05 0.02 0.2 Mo < 0.005 < 0.005 n . a. n.a. n.a. n.a. n.a. n.a. Ni 0.019 0.0022 > 0.7 > 0.7 > 0.7 > 0.7 > 0.7 0.7 Pb < 0.07 < 0.07 2.0 1.0 3.0 3.0 1.5 5.0 Sr 0.03 0.005 0.1 0.03 0.07 0.02 < 0.001 0.15 Ti3 0.90 0.029 0.2 0.15 0.2 0.2 0.15 0.17 u ) 0.0048 0.0155 23.0 20 .4 31 .6 ' 30.8 32.1 26.9 V 0.033 < 0.002 0.1 0.07 0.1 0.07 0.15 0.07 Zn < 0.02 < 0.02 < 0. 1 < 0.1 < 0.1 < 0.1 < 0.1 0.1 Zr 0.058 0.0055 < 0.005 < 0.005 < 0.005 < 0.005 < 0.005 0.005

n.a . - analysis not available Location of Samples: n.d . - not detected (i.e. beloe th w detection limit) 1. DDH 1962 (Deilman) graphitic schist t detecteno y T an hs n wa i d sample 2. DDH 1967 (Deilman) sandstone 3. DDH 5052 (Gärtner) regolith with pitchblende 1. Analyzed using 12B-DR 4. DDH 5052 (Gärtner) regolith with pitchblende 2. Analyzed using 5Q-23 5. DDH 5057 (Gärtner) regolith with pitchblende 3. Analyzed by Atomic Energy Canada y Ltdb . 6. DDH 5062 (Gärtner) regolith with pitchblende Neutron Activation 7. DDH 5050 (Gärtner) regolith with pitchblende 8. DDH 5057 (Gärtner) regolith with pitchblende 5830- -5830

i o 5815 -5815

HELIKIAN CARSWELL FORMATION U DEPOSIA T

DOUGLAS FORMATION

I WILLIA I M RIVER SUBGROUP MAIN FAULT APHEBIAN F7%] METAMORPHIC BASEMENT

Figure . Geology20 f Cars-wello Structure. (Simplified afte rpress)n i Alonso , aL . t e

deposiD e Th t occurs alone sub-Athabascth g a unconformity (Ruzicka, 1975) e wholTh . e sequenc overturnes i e o tharocke s d th t s of the Peter River Gneiss Formation rest upon the sedimentary rocks of the William River Subgroup. The paleosurface of the basement rocks is deeply weathered and has a thick regolith. The highest uranium concentration occurs in the basal sequence of the Athabasca s confineGroupi t I . d almost entirel o bituminout y s pelite, which

255 10930

5825—i

HELIKIAN

| WILLIA[ M RIVER SUBGROUP APHEBIAN tr^a PETER RIVER GNEISS DEPOSIU A T

Ë----3 EARL RIVER COMPLEX • U OCCURENCE

Figure . 21 Geology d distributionan f uraniuo m occurrence a par f n o Cars-welli st Structure vicinitye th m f Cluff o Lake. (After Alonso t al. e press) n i , .

grades transitionall o sandstonet y e maiTh .n uraniu e mineraor m s i l pitchblende; coffinit d uranoaan e n carbon occu n lessei r r amounts. The associated polymetallic mineralization includes pararammelsbergite, native selenium and, selenide f leado s , bismuth, nickel and cobalt, native gold, altaite, galena, chalcopyrite, pyrrhotite and pyrite. U-Pb isotope analyses indicat 1 deposi'D e e th tabsolut f o e ag e as 1100 Ma (Tapaninen, 1975) , but Gancarz (1979) using U-Pb and Pb-Pb methods suggested two main phases of mineralization (1050 Ma and 800 Ma) and a remobilization phase (234 Ma); the remobilization phase coincides e Hercynia wite rangth th hf o e n uranium metallogenic epoch in Europe (Ruzicka, 1971; Ball et al., 1982). deposiD e s beeTh ha tn a higdepleted s h wa grad t e I bod.or e y 0 metre(Tabl14 , s3) e long metre2 ,1 metre2 s1 wid d an se thick.

256 MAIN ZONE B ZONE SW

MAURICE CREEK ZONE

QUATERNARY

I" ,~^'j Overburden Faults HELIKIAN

[""""I ATHABASCA GROUP ,Sandstone U Mineralisation

APHEBIA) N(?

Gneiss.locally graphitic

ARCHEAN (?)

F^I Qranltoids

Figure 22. A sketch (not to scale) showing distribution of mineralized zones in Maurice Bay area. Modified from Lehnert-ThieZ et al., 1981 and Uranerz Exploration and Mining Limited, personal communication.

e remaininTh g e depositCarswelth n i sl Structure ar e monometalli consisd can t mainl f pitchblendo y coffinited ean , N e Th . , ClaudOP d Pete, R an e r River deposits occu quartzo-feldspathin i r c gneiss, metapelit d quartzitean e Peteth rf o eRive r Gneiss Formation (Fig. 21). Their mineralizatio s structuralli n y controlley b d steeply dipping faults and subhorizontal mylonite zones.

MAURIC AREY EBA A The Maurice Bay area is located about 80 km west-southwest of Uranium City on the north shore of Lake Athabasca and northwest rim of the Athabasca Basin (Fig. 1). Uranium mineralization occurs in the Main Zone, the A Zone, the B Zone, and in a few additional zones. All the mineralized zones are spatially relatee sub-Athabascth o t d a unconformity (Lehnert-Thiel and Kretschmar, 1979; Lehnert-Thiel et al., 1981; Tremblay, 1982). The basement rocks in the Maurice Bay area are granitized metasediments, apparentl Tazie th para y nf o tBel t (Tremblay, 1982), which are the main host rocks of the Beaverlodge uranium deposits. The basement rocks were affected by Hudsonian Orogeny and are locally albitic and some are graphitic. The tops of the basement rocks at the unconformity (regolith) are strongly hematitized and chloritized

257 oo

Figure 23. Photomicrograph of pitchblende (P) associated Figure . 24 Photomicrograp f o mineralizatioh n from with chlorite (Chl) and pitchblende-chlorite mixture Fond-du-Lac deposit. Q = quartz, Fe = h'monite, (P ± Chl) in sandstone, Maurice Bay deposit (Main Zone) P = pitchblende. Reflected light. (Q) = quartz grains. Reflected light. (Fig. e 22)sedimentarTh . ye Athabascth rock f o s a Group which overlie unconformably the basement rocks, are at the unconformity chloritized and limonitized, brecciated and, further upward, silicified. e mineralizatioTh e Maith nn i nZon s controllei e a steepl y b d y dipping fault and occurs in both the altered basement rocks and in the altered sandstone. In the A Zone the mineralization occurs along fracture d disseminatean s e basementh n i d e sandstonet th rock d an s . In the B Zone the mineralization occurs entirely in the altered sandstone. In one of the additional small zone mineralization occurs as subhorizontal lens in the sandstone near the surface. The largest resources (Table 3) are in the Main Zone, which is 1500 metres long, up to 200 metres wide and up to 10 metres thick. Pitchblende and coffinit e maie mineralth or n e ar e s (Fig. 23); thee commonlar y y associated with chlorite and/or limonite (Table 10). Uranerz Exploration and Mining Limited reported (verbal communication) absolute age of most of the uranium mineralization (determinte U-Py b d b metho. GermanyW n i d somt s 1,20f a )o bu e , 0Ma the pitchblende from the Main Zone reportedly yielded 2500 Ma. Absolut mineralizatiof o smale e th eag lf o zone e reportes on swa n i n d . byMa Uraner0 25 s a z

Tabl : Selecte10 e d elemental constituent selecten i s d drill core samples from the Main Zone, Maurice Bay deposit. (From: Ruzicka and Littlejohn, 1981). Constituent Percenn i s t

Sample 1) 1) 2) 1) 1) Element 1 2 3 4 5 Ag n.d. n.d. < 0.005 n.d. n.d. As n.d. n.d. < 0.2 n.d. n.d. Co n.d. n.d. < 0.001 n.d. n.d. Cr 0.01 0.03 < 0.0005 n.d. n.d. Cu 0.001 0.05 < 0.007 0.005 0.007 Mo n.a. n.a. < 0.005 n.a. n.a. Ni 0.02 < 0.0015 < 0.001 0.02 < 0.0015 Pb 0.2 1.5 < 0.07 n.d. n.d. Sc n.d. < 0.0007 n.a. n.d. n.d. Ti 0.3 0.3 0.48 0.1 0.1 5.84 32.5 0.14 7.70 11.1 V 0.01 0.07 < 0.002 0.005 n.d. Zn n.d. n.d. 0.11 n.d. n.d. Zr 0.015 < 0.005 0.0096 n.d. n.d.

availablt no n.a .- e Location of Samples: detectet no - n.dd. (i.e. belo detectioe th w n limit) MB-21K 1.DD : sandstone MB-87K DD . :2 regolith t detecte no y sampl an s Tn wa hi de MB-24K 3.DD : altered sandstone MB-46K 4.DD : regolith 1. Analyzed using 5Q-23 MB-98H 5.DD : regolith Analyze. 2 d using 12B-DR 3. Analyzed by Neutron Activation

259 FOND-DU-LAC AREA The Fond-du-Lac area is located at the south shore of the e Athabasc th e norther Athabascf th o t m a a ri nd Basi an a n (Fig. ) .1 The Fond-du-Lac deposit, the only known deposit in this area, occurs about 90 km east-southeast of Uranium City (Homeniuk et al., 1980; Tremblay, 1982). Most of the uranium mineralization of the Fond du Lac deposit occurs in the sandstone of the Athabasca Group within 35 metres above the sub-Athabasca unconformity (Homeniuk et al., 1980). The host rocks, which contain carbonate cement, are hematitized, limonitized and chloritized e maie mineralTh or n. , pitchblende, occurs a s coatin n quarto g e sandstonz th grain n i r so e matri mixtura s a x e with limonite (Fig. 24). Coffinit s lesi e s abundant. Accordino t g Homeniuk et al., (1980) the main mineralization took place . Ma 0 8 d rejuvenatios an it a M d 1100-1205 an 21 o t ag a n a 0M

A CONCEPTUAL GENETIC MODEL FOR THE SUB-ATHABASCA UNCONFORMITY-RELATED DEPOSITS Conceptual genetic models for the sub-Athabasca unconformity- related deposits have been propose y b severad l authors (e.g. Knipping, 1974; Little, 1974; Morton, 1977; Langford, 1977; Hoevd Sibbaldan e , 1976, 1978; McMillan, 1978; Dahlkamp, 1978; Munday, 1979; Ruzicka, 1979, 1982; Kirchner et al., 1979; Page t al.e l , 1980; Ramaekers, 1980 Carlee d ; , 1981; Clar t al.e k , 1982; Tremblay, 1982) . Some of the authors consider that the concentration of uranium took place in several stages and that several processes were involve formation i d e depositsth f o n . Taking into account similaritie dissimilaritied san s among these deposits, fiel d laboratoran d y observations d studiean , n similao s r deposits outside Canada (e.g. Ferguson t al.,e , 1980; Cricd an k Muir, 1980; Binn t al.e s , 1980; Brookins, 1980 e followinth ) g factors for establishing a conceptual model are recognized: (1) The main episode of uranium mineralization in all the sub- Athabasca unconformity-related deposits took place from 105 o about 0 t 130 a ago0M ; (2) most of the pitchblende, the main ore-forming mineral, was deposited under temperatures between 130° and 260°C; (3) the mineralization was associated with retrograde metamorphism, which commonly resulte n clai d y alteration; tourmalinization of the host rock and magnesian metasomatism were common companion f thesso e processes;

260 (4) deposition of the mineralization was structurally controlle e sub-Athabascth y b d a unconformit d steean y p faults intersecting this unconformity e hosth ;t rocks were, as a rule, dislocated along these faults; (5) at least one additional period of uranium mineralization (remobilization ?) took place about 200 to 300 Ma ago, i.e. durin e tim th f gHercyniao e n Orogeny; (6) uranium might have been derived from several sources. Some unaltered basement rocks, such as granitoids and metapelites, contain above normal amounts of uranium and associated elements which constitut e bodiesor e th ;e these elements could have been incorporated into mineralizing solution y leachingb s . Altered sedimentarye th rock f o s Athabasca Group are depleted of some elements, such as U, Ni, As and Co; these elements might have been incorporated in connate water r residuao s l brine d thuan s s might become another possible source for the mineralization; (7) hydrothermal solutions associated with diagenetic, tectonic or deep seated processes acted as an important factor in the formation of the ore; ) remobilizatio(8 d redistributioan n mineralizatioe th f o n e du n to hydrodynamic conditions are continuing processes, which have caused modificatio e depositth f o nd developmen an s f o t dispersion haloes in their vicinity. Therefore formatioe uranium/polymetallith f o n c deposits associated with the sub-Athabasca unconformity should be classified as polygenetic.

ACKNOWLEDGEMENT The author acknowledges co-operation of the following companies in supplying information on their properties: Amok Ltée, Asamera Inc., Canadian Occidental Petroleum Ltd., Eldor Resources Ltd., former Gulf Minerals Canada Ltd., Key Lake Mining Corporation, S.E.R.U. Nucléaire (Canada) Ltée, Saskatchewan Mining Development Corporation and Uranerz Exploration and Mining Ltd. Minerals show n photomicrographno s were identifie laboratorien i d f so the Geological Surve f o Canady y b A.La . Littlejohd an n G.M. LeCheminant.

261 REFERENCES Alonso , ,ChipleyD. , Körung,D. , ,E. Law , McAleenanA. , , ,C. Preciado, J., Tona, F. and Wheatly, K., in press: Geology of the Carswell Structure p 1:50,00ma e publisheA b i o t 0jj ; y b d Amok Ltd/Ltée. Ball, T.K., Basham, I.R. and Michie, U. McL., 1982: Uraniferous vein occurrence f south-weso s t Englan — dParagenesi d genesisan s ; in Vein-Type and Similar Uranium Deposits in Rocks Younger than Proterozoic, International Atomic Energy Agency TC 295/9, p. 113-158. Bayda, E.D. Justice, 1978: Cluff Lake Board of Enquiry, Government of Saskatchewan, Regina, Saskatchewan. Bell, K. and Macdonald, R. , 1982: Geochronological Calibration of e Precambriath n Shiel n i Saskatchewand \ Summarir ;f o y Investigations 1982; Saskatchewan Geological Survey, Miscellaneous Report 82-4, p. 17-22. Binns, R.A., McAndrew, J. and Sun, S.S., 1980: Origin of uranium mineralizatio t a Jabilukan n i "Uraniu; n i Pinm e Creek Geosyncline 1. FergusoJ d edite an A.By b n d. Goleby; International Atomic Energy Agency, Vienna . 543-562p ; . Brookins, D.G., 1980: Syngenetic model for some early Proterozoic uranium deposits; evidence from Oklo n 'Uraniu i ;e Pin th en i m Creek Geosyncline1 . FergusoJ edite y d b A.Bdan n . Goleby; International Atomic Energy Agency, Vienna . 709-720p ; . Brummer, J.J., Saracoglu, N., Wallis, R.H. and Golightly, J.P., 1981: McClean Uranium Deposits Saskatchewan, Part 2, Geology; in Canadian Institut f o Minin ed Metallurgan g & Geology y Division Uranium Field Excursion Guidebook, Sept. 8-13. Canada Wide Mines Ltd., 1980: Preliminary Environmental Impact Statement, 1980 A ;repor •>t t Atomic Energy Control Board, Canada. Unpublished. Chandler, F.W., 1978: Geolog e Wollasto th f par o yf o t n Lake Fold Belt, North Wollaston Lake, Saskatchewan; Geological Survey of Canada, Bulletin . 277p 5 2 , Clark, R.J. McH., Homeniuk, L.A. and Bonnar, R. , 1982: Uranium geologe Athabascth a comparison i yd an a n with other Canadian Proterozoic Basins; in Canadian Mining and Metallurgical Bulletin, Volume 75, Number 840, April, 1982, p. 91-98.

262 Clarke, P.J. and Fogwill, W.D., 1981: Geology of the Asamera Dawn Lake Uranium Deposits Athabasca Basin, Northern Saskatchewan; in Canadian Institute of Mining and Metallurgy Geology Division Uranium Field Excursion Guidebook, Sept. 8-13. Crick ,d I.HMuiran . , M.D., 1980: Evaporite d uraniuan s m

mineralization in the Pine Creek Geosyncline; in "Uranium in the Pine Creek Geosyncline 1 edited by J. Ferguson and A.B. Goleby; International Atomic Energy Agency, Vienna; p. 531-542. Gumming, G.L. and Rimsaite, J., 1979: Isotopic studies of lead- depleted pitchblende, secondary radioactive minerals and sulphides from the Rabbit Lake uranium deposit, Saskatchewan; Canadian Journal of Earth Sciences, v. 16, p. 1702-1715. Currie, K.L., 1969: Geological Notes on the Carswell Circular Structure, Saskatchewan (74K); Geological Survey of Canada, Paper 67-32. p 0 ,6 Dahlkamp, F.J., 1978: Geological Appraisal of the Key Lake U-Ni Deposits, Northern Saskatchewan; Economic Geology, v. 73, p. 1430-1449. Dahlkamp, F.JTan d , 1977an .,B. : Geolog mineralogy d Ke an y e th f o y Lake U-Ni deposits, northern Saskatchewan, Canada; in Geology, Mining d Extractioan , n Processin f Uraniumo g M.J. ed ,. Jones; London, Institute of Mining and Metallurgy, p. 145-158. de Carle, A.L., 1981 y LakKe : e e Geologth Deposit f n o a y s- overview n Canadiai ; n Institut f o Minine d Metallurgan g y Geology Division Uranium Field Excursion Guidebook, Sept. 8-13 109-130. p , . Denner, M.J. and Reeves, P.L., 1981: Uranium Reserves and Potential of Northern Saskatchewan; Saskatchewan Mineral Resources, Miscellaneous Report 81-5, 22 p. Ferguson , Ewers,J. , G.Rd Donnellyan . , T.H., 1980: e th Mode r fo l development of economic uranium mineralization in the Alligator Rivers Uranium Field; rn 'Uranium in the Pine Creek Geosyncline '. Ferguso J d edite an A.By b n. d Goleby; International Atomic Energy Agency, Vienna; p. 563-574. Gancarz, A.J., 1979: Chronology of the Cluff Lake Uranium Deposit, Saskatchewan, Canada; International Uranium Symposium in the Pine Creek Geosyncline, N.W. Australia, June; Extended Abstracts.

263 Gatzweiler, R., Lehnert-Thiel, K., Glasen, D., Tan, B., Voultsidis, V., Strnad, J.G., and Rieh, J. , 1979: The Key Lake Uranium-nickel deposits; Canadian Minin d Metallurgicaan g l Bulletin. 73-79p , 72 . . v , Gatzweiler Schmeling, . R , Tand , 1981an B. ,. B , : Exploratioe th f o n Key Lake uranium deposits, Saskatchewan, Canada; in Uranium exploration case histories; International Atomic Energy Agency, Vienna, 1981, AG-250/5, p. 195-220. Heine, T., 1981: The Rabbit Lake deposit and the Collins Bay depositsM GeologCI n ij y; Division Uranium Field Excursion Guidebook, September 8-13, 1981, Saskatchewan, p. 87-102. Hoeve, J. and Sibbald, T.I.I., 1976: The Rabbit Lake Uranium Mine; in Proceedings of a symposium on Uranium in Saskatchewan, ed. C.E. Dünn, Saskatchewan Geological Society, Special Publication no. 3, p. 331-354. ______e 1978genesith n f :RabbiO o s t Lakd othean e r unconformity type uranium deposit n northeri s n Saskatchewan, Canada; Economic Geology, v. 73, no. 8, p. 1450-1473. Homeniuk, L.A., BonnarClarkd an . ,R R.J. McH., 1980 e Fond-du:Th - Lac Uranium Deposit, Saskatchewan; Canadian Mining and Metallurgy, Toronto Meeting, Ontario, April, v. 73, Abstract. Innés, M.J.S., 1964: Recent advances in meteorite crater research at the Dominion Observatory, Ottawa, Canada; Meteorics, v. 2, p. 219-249. Jones , 1980,B. e Geologe Collin:Th th y Uraniuf o yBa s m Deposit, Saskatchewan; Canadian Minin d Metallurgicaan g l Bulletin, v. 73, no. 818, p. 84-90. Kirchner, G., Lehnert-Thiel, K., Rich, J. and Strnad, J.G., 1979: y Lakd JabilukaKe an e : Model r Lowefo s r Proterozoic Uranium Deposition; International Uranium Symposiu e Pinth e f Creeo m k geosyncline, N.W. Australia, June; Extended Abstracts. Knipping, H.D., 1974 e concept:Th f supergeno s e versus hypogene emplacement of uranium at Rabbit Lake, Saskatchewan, Canada; iri Formation of Uranium Deposits, International Atomic Energy Agency, Vienna, p. 531-548. Langford, F.F., 1977: Surficial origi Nortf no h American pitchblende and related uranium deposits; American Association of Petroleum Geologists Bulletin, v. 61, no. 1, p. 28-42.

264 Lehnert-Thiel, K. and Kretschmar, W., 1979: The Geology of the Maurice Bay Deposit; Canadian Mining and Metallurgy, Winnipeg Meeting, Manitoba, September, Abstract. Lehnert-Thiel, K., Kretschmar, W. and Petura, J., 1981: The Geology of the Maurice Bay Uranium Deposit; iji Canadian Institute of Mining and Metallurgy Geology Division Uranium Field Excursion Guidebook, Sept. 8-13. Lewry, J.F d Sibbald.an , T.I.I., 1979 :A revie f pre-Athabasco w a basement geolog n i Northery n Saskatchewan n r Uraniu; m Exploration Techniques, Proceeding a Symposiu f o s m heln i d Regina, 1978, November 16-17, edited by G.R. Parslow, Saskatchewan Geological Society, Special Publication Number 4, p. 19-58. Little, H.W., 1974: Uraniu n Canadai m n i Repor; f Activitieso t , Par Geologica, A t l Surve f Canadao y , Paper 74-1A . 137-139,p . Macdonald, C.C., 1980: Mineralogy and geochemistry of a Precambrian regolite Athabascth n i h a Basin; Unpublished M.Sc. thesis, University of Saskatchewan. McMillan, R.H., 1978: Genetic Aspect & Classification of Important Canadian Uranium Deposits; Canadian Minin d Metallurgicaan g l Bulletin, v. 71, p. 61-67. Morton, R.D., 1977: The western and northern Australian uranium deposit s— exploratio n guide r exploratioo s n deterrentr fo s Saskatchewan? n Proceedingi ; a Symposiu f o s n Uraniuo m n i m Saskatchewan C.E. ed , . Dünn, Saskatchewan Geological Society, Special Publication no. 3, p. 211-255. Munday, R.J., 1979: Uranium mineralizatio northern i n n Saskatchewan; Canadian Minin d Metallurgicaan g l Bulletin , April72 . v ,, p. 102-111. Pagel , 1975,M. : Cadre géologiqu e gisementd e s d'uranium dana l s structure Carswell (Saskatchewan-Canada): Etude des phases fluides; Thèse de Docteur de Spécialité (3e cycle), Université de Na,ncy. Pagel Sheppardd Poty, an .M. . ,B , S.M.F., 1980: Contributio somo nt e Saskatchewan uranium deposits mainly from fluid inclusiod an n isotopic data; in Uranium in the Pine Creek Geosyncline, Proceedings of the International Uranium Symposium on the Pine Creek Geosyncline, Sydney 8 Jun4- ,e 1979, jointly sponsored by BMR and CSIRO in co-operation with IAEA; International Atomic Energy Agency, Vienna . 639-654,p .

265 Voultsidisd Pechmannan n vo , . 1981,E ,V. : Further resulte th n o s petrograph y Lakd Ke mineralogan eye th uranium-nicke f o y l orebodies, Saskatchewan, Canada n Abstracti ; M GeologsCI y Division Uranium Symposium and Field Tours, Saskatchewan, September 8-13, 1981, p. 14-15. Ramaekers , 1979P. , : Stratigraph Athabasce th f o y a Basin Summar. in ; y of Investigations 1979, Saskatchewan Geological Survey, Miscellaneous Report 79-10, p. 154-160. ______1980: Stratigraph d Tectonian y c e Historth f o y Athabasca Group (Helikian Northerf )o n Saskatchewan Summarn ;r y of Investigations, 1980; Saskatchewan Geological Survey, Miscellaneous Report 80-4 . 99-106p , . Ray, G.E., 1978: Reconnaissance Geology: Wollaston Lake (West) Area (Part of NTS Area 64L) ; in Summary of Investigations 1978, Saskatchewan Geological Survey, Miscellaneous Report 78-10, p. 25-34. Ray, G.E Wanlessd .an , R.K. geologicad an , e 1980ag e :lTh historf o y the Wollaston, Peter Lake, and Rottenstone domains in northern Saskatchewan; Canadian Journa f Earto l h Sciences, Volum7 1 e Number 3, p. 333-347. Robinson, S.C. , 1955: Mineralogy of Uranium Deposits, Goldfields, Saskatchewan; Geological Survey f o Canada , Bulletin, 31 , . p 8 21 Ruzicka, V., 1971: Geological comparison between East European and Canadian uranium deposits; Geological Surve f o Canaday , Paper 70-48, 195 pp. ______1975: Some metallogenic features of the "D" uranium deposit at Cluff Lake, Saskatchewan; in Report of Activities, Geological Survey of Canada, Paper 75-1C, p. 279-283. ______1979: Uraniu d thoriuan m n Canadai m , 1978;n i Current Research, Part A, Geological Survey of Canada, Paper 79-1A . 139-155,p . ______1982: Studie n uraniuo s m metallogenic provincen i s Canada Proceedingn i ; e Symposiuth f o s n Uraniuo m m Exploration Methods Paris lst-4th June 1982; Nuclear Energy Agency, Organization for Economic Co-operation and Development; p. 143-155. Ruzicka, V. and LeCheminant, G.M., 1984: Uranium deposit research, 1983 Currenn ;r t Research, Par Geologica, A t l Surve Canadaf o y , Paper 84-1A . 39-51,p .

266 Ruzicka, V. and Littlejohn, A.L., 1981: Studies on uranium in Canada, 1980; in Current Research, Part A, Geological Survey of Canada, Paper 81-1A . 133-144,p . ______1982: Notes on mineralogy of various types of uranium deposit genetid san c implications Currenn i ; t Research, Par Geologica, A t l Surve Canadaf o y , Paper 82-1A . 341-349,p . Saracoglu, N., Wallis, R.H., Brummer, J.J. and Golightly, J.P., 1983: The McClean uranium deposits, northern Saskatchewan — discovery; The Canadian Mining and Metallurgical Bulletin, April, 1983 1-17. ,p . Saskatchewan Mining Development Corporation, 1983: Annual Report, 1982; Regina, 1983 Scott, F., 1981: Midwest Lake uranium discovery; in Uranium Exploration Case Histories; International Atomic Energy Agency, Vienna, 1981, AG-250/1, p. 221-242. Sibbald, T.I.I., 1978: Uranium Metallogenetic Studies, Rabbit Lake: Geology n Summari ; f o Investigationy s 1978, Saskatchewan Geological Survey, Miscellaneous Report 78-10 . 56-60,p . ______1979: NEA/IAEA Test-Area: Basement Geologyt ir ; Summary of Investigations 1979; Saskatchewan Geological Survey, Miscellaneous Report 79-10, P. 77-85. Stevens, R.D., Delabio, R.N., and Lachance, G.N., 1982: Age determination d geologicaan s l studies, K-Ar isotopic ages, Repor Geologica; 15 t l Surve Canadaf o y , Paper 81-2 . 34-35,p . Tapaninen, K., 1975: Geology and metallogenesis of the Carswell area uranium deposits; Canadian Institut f Minino e d Metallurgyan g , Edmonton Meeting, Alberta. Tremblay, L.P., 1972: e BeaverlodgGeologth f o y e mining area, Saskatchewan, Canada Geological Survey of Canada, Memoir 367, 26. p 5 ______1982: Geologuraniue th f yo m deposits relatee th o t d sub-Athabasca unconformity, Saskatchewan; Geological Survef o y Canada, Paper 81-20, 56 p. Wanless, R.D., Stevens, R.D., Lachance, G.R. d Delabioan , , R.N., 1979: Age determinations and geological studies, K-Ar isotopic ages, Repor Geologica; 14 t l Surve Canadaf o y , Paper 79-2. Wendt , HohndorfI. , d Voultsidisan , Lenz,A. , ,H. , 1978,V. : Radiometri e determinatioag e n sampley o nLak Ke ef o sUraniu m Deposit; United States Geological Survey, Open-File Report 78-701.

267 URANIUM GEOLOGY BEAVERLODGE AREA

D.M. WARD Eldor Resources Ltd, Saskatoon, Saskatchewan, Canada

Abstract

A brief summary is presented of the geology of the Beaver- lodge Area with particular emphasis e uraniuplaceth n o dm mining propertie f Eldorado s o Resources Limited e deformeTh . d metasedimentar d volcanian y c rocks withif Aphebiao e aree th nar a n and Helikian age. These rocks have undergone at least three periods of orogeny, namely, the Kenorian (-2560 m.y.), Hudsonian (-1750 m.y.) and Grenvillian (-1000 m.y.). The final e Hudsoniaphasth f o e n Orogeny cause e activatioth d f majoo n r faults with initial emplacemen f epigenetio t c pitchblende deposits taking place and with probable widespread remobilisation during the Grenville Orogeny. Dates as recent as 200 m.y. have also been recordee uraniuth r fo md mineralisation. Genesis of the numerous uranium deposits in the Beaverlodge Area remains uncertain s generalli t I . y considered thae th t deposits are related to metamorphic events and are the result of remobilisation withi d froan nm donor metasediments.

INTRODUCTION The purpose of this paper is to provide a brief summary of the geologe Beaverlodgth f o y e are f northero a n Saskatchewan with particular emphasis on the uranium mining properties of Eldorado Resources Limited (formerly Eldorado Nuclear Limited). An attemp s beeha t n mad o accentuatt e e more th some f o e controversial aspecte geologicath f so l o summarizrecort d an d e as briefly as possible the concepts of various proposed genetic models. Onlmose th yt recen d majoan t r publication Beavere th n o s- lodge area have been include e selecteth n i d d referencee th d an s reader is referred to those publications for a more complete record l geologicaAl . l map d recordEldorade an sth f so o mining

269 operations are maintained in storage in the National Archives, in Ottawa. Total production of uranium from the Beaverlodge district ove a lifetimr 2 years3 f o e, commencin d terminatin an n 195i g0 g

in June 1982, has exceeded 30,500 tonnes of U000, °f which in J O exces f 21,00o s 0 tonnes were produced froe mineth mf Eldorado s o Resources. Although this volume of production is comparable to the reserves of many of the more recent discoveries associated with the Athabasca Basin, the long production life coupled with the large number of geologists who have been associated with the Beaverlodge area has resulted in the accumulation of a considerably greater technical data base. As a consequence there may be a tendency to assume that the geology of uranium deposits of the Beaverlodge are mors i a e complex than that associated with the Athabasca Basin.

GENERAL GEOLOGE ARLTH AF O Y The Beaverlodge area is located within the Canadian Shield, e northwesnortth f o h e Athabasct th edg f o e a Basi d nortan n f o h Lake Athabasca in the province of Saskatchewan. The area is situated in the western Craton subdivision of the Churchill Structural Province e rockTh .s withi e areth na have been subdivided into three groups (Tremblay 1972) which include the folded and metamorphosed Aphebian meta-sedimentary rocks of the Tazin Group, Aphebian meta-sedimentary rocks of the Murmac Bay Formatio d Helikiaan n n sedimentar d volcanian y ce rockth f o s Martin Formation (Fig. 1). e areRockth a f o havs e undergon t leasa e t three periodf o s orogeny. The earliest was the Kenoran (about 2560 m.y. ago), which was followed by the most prominent the Hudsonian (about 1750 m.y. ago) and finally by the Grenville (about 1000 m.y. ago). e KenoraTh n orogen s considerei y o havt d e resulte n metai d - morphis o product m e Archean gneiss dome d granulitean s s plus syngenetic uraniu n pegmatitesi m e HudsoniaTh . n orogeny resulted in widespread metamorphism and mylonitization of the Tazin Group and Murma y FormatioBa c n wite lateth h r stages having markea d perio f activo d e mountain building e depositioth , e Martith f o n Formation sediments, and eruption of the Martin volcanic rocks. The final phase of the Hudsonian also caused the activation of the ma3or faults includin t LouisC S faults AB e d th ,g an .Blac y Ba k

270 HP—T~l mtttieniitie granitt I •*• *l fildipir roch

Pig 1 *Generalize d geologBeaverlodge th f yo e Area

271 Initial emplacemen f epigenetio t c pitchblende deposits occurred at this time while, the Grenville orogeny is credited for further widespread remobilisation.

HISTORICAL REVIEW Durin e gol th ge 1930'sd th rus n i h, pitchblends wa e discovere e propertth t a df Nicholso o y n Mine e norts th Ltd hn o , shore of Lake Athabasca. The occurrence remained a mineralogical curiosity until Eldorado Minin d Refininan g g (now Eldorado Resources) staked mineral claims in the Fish Hook Bay region immediately east and north of the Nicholson property. Eldorado then commenced prospectin r uraniufo g m throughou e ared th tan a succeeded in identifying hundreds of uranium showings, which led e drivin n exploratioth a o f t o g n adit near Martin Lak n 194i e 8 e sinkinano prospecth dtw f o g t shaft e n 1949i Ac e Eagls th d , an e shafts (Fig. 1). The greatest success from underground exploration and developmen s achievewa t d froe inclineth m e shafAc d t situated parallel to, and in the footwall of the St Louis Fault. After a decision was made to go into production the Ace shaft was deepened to a vertical depth of approximately 240 m and a vertical production shaft (Fay s collarewa ) n 1951i d , approximately 120e th 0m wes f o t Ace shaft. Further underground shaft development included the Verna shaft approximatel e e shafth Ac n e i t th m eas f 0 o t 90 y hangin t LouiS e go Winzsth waltw faul f d o el an tshaft o t s d Vernan ay develo Fa orebodie e y th pWinz Fa t deptha e se Th . shaft achieved a vertical depth of approximately 1.8 km below surface. Lateral underground development from these shafts totalled in excess of 136 km by the time the mine closed. Several satellite mines were developed on the Eldorado holding d providean s d supplementar t varioua e or ys times throughout the life of the property. These included the underground Hab, Dubyn d Martian a n Lake mine d smalan s l opee n th pit n i s Verna, Eagle, West Fay d Dubynan , a areas e largesth , t beinpi t g the Böige t locatepi r e Vernd th eas af o t shaft. Increasing production costs coupled with declining market prices and ore reserve grade necessitated the termination of production from the area in June 1982. Production for the Eldorado properties is summarised in table 1. If production is included from other operations the

272 TABLE 1 ALL TIME MINE PRODUCTION ELDORADO RESOURCES LIMITED BEAVERLODGE OPERATION Mine Average grade Tonnes

Fay 0.28 12,305 Verna 0.19 6,934 Hab 0.43 934 Open Pits 0.20 657 Dubyna 0.22 195 Martin Lake 0.30 7 Eagle 0.11 0 Total 0.25 . 21,033

districe th tota r fo lt becomes 33,713 tonnes U000 with mosf o t 3 O the additional production coming from the Gunnar Mine at the Crackingstone th f o sout d en h e peninsul d froan a m small underground orebodies located west of the Black Bay Fault.

REGIONAL GEOLOGY AND STRATIGRAPHY a stratigraphiFigur s i 2 e e colum f geologo n d stratigran y - aphy of the area containing uranium deposits of Eldorado's mining operations. Most of the rock nomenclatures used throughout the histore operationth f o y s have been include r crosfo d s reference purposes. The Tazin Group comprises a large variety of metamorphic granulite, sedimentar d volcanian y c rockse majoTh .r uranium host rock e containear s d within thi a broa sn i groud d zonan p e of mylonitization and faulting. Although significant in surface exposure, the Murmac Bay Formation did not contribute in a significant way to the uranium production for the area. The Martin Formation is made up of the remnants of a Helikian supracrustal basin lying unconformably on the Tazin Group. The rocks consist of a succession commencing with a basal angular conglomerate through a rounded upper conglomerate grading into an arkosic sandstone which in turn grades into a sequence of red-bed siltstones e FormatioTh . n also contains andésite d basaltian s c flows. A small amount of uranium production was achieved from

273 ELDORADO RESOURCES LIMITED BEAVERLODGE OPERATION

GEOLOGY LEGEND z X llj § e o; CC 0 UJ ce UJ UJ o o. o MARTIN FORMATION t 1 -1400 1490 • Diabase 13 z"- Stltstone and Shale <2 Z z° 2 1799 • > 1 630 •« Andésite, Porphyry, Basalt** ?ï S _l °" UJ 1 I3 ' X Arkos Sandstond ean e 1 1 1 U Conglomerate — i8oo TAZIN GROUP O METASEDIMENTS N 1975 • % Pegmatite, Pegmatitic Alaskite O tx Orange Mylonite, Feldspar rock. Mylomtic Alaskite UJ Mylomtic Mica Schist/Para Gneiss, z Argillite, Paraschist r- * Meta Argillite. Migmatite

O Epidotic Argillite, Greenschist, „. . ,. ... ., , , , _ Siliceous Argillite, Ultra Mylonite K CD Phyllomtic Amphibolite Ul 0. X Quartzite Silica, Siliceous Mylonite a < Diopside, Cale-Silicate rocks Dolomite -2100 n/MUAi rtcnu i kvc r'ucicc 2100 • Alaskite,* Leucocratic Gneiss ... . „ Mylonite «4O fn % Silica) (Mafics I5%) Kakinte

Pig 2 .Stratigraphi e column the basaltic-flows and from within the basal conglomerates, at topographic lows in the unconformity. Structural features of the area include the St Louis Fault (Fig) whic1 . h strikes e southeasnortheasth o t d d dip° an tan t 50 s along which the Tazin host rocks for the ma3or uranium producing deposit e distributedar s t Loui.S e sNorte th faul th f o hs i t Donaldson Lake anticline a broa, d fold structure which plunges e southwestgentlth o t y wits southeasit h t limb lying paralleo t l tht LouiS e s northwess it faul d an t t limb being e offseth y b t Black Bay fault (Section A-A1, Fig. 1). The Black Bay fault also strikes northeast, is situated 6 km west of the St Louis fault, and is slightly offset by the St Louis fault. These two fault

274 structure C faule connecteAB ar se t th whic y b d h dips approximately e southwestth o t 45 , abut e Blac e southwesy th Faulsth Ba k t a t t enf Fredetto d t LouieS Lake d ssplayth e an e faul th f n sof i t d Eldoradol vicinit e th of o ytownsite e otheTh .r major structural feature of the area is the Martin Lake syncline which ia synclinas l trough wit hd whic maximuan h° 75 m o t dip p u s plunges to the north where it is offset by the ABC fault. Although uranium mineralisation occurs in nearly all Tazin and Martin rocks, the major concentrations lie within metasediments of the Tazin Group distributed along the St Louis fault. Lesser amount f uraniuo s e productioor m s beeha n n achieved from basal Martin rocks west of the Fay Mine shaft and very minor amounts from volcani cMartie rockth f nso Group.

ST LOUIS FAULT AREA The majority of rocks seen underground and on surface in the immediat et Loui S vicinit e sth e TaziFaul f th o e pary ar ntf o t Group. Figur show3 e s generalised section f geologo s d an y uranium mineralisatio n boto n he fault sideth n generaI f .so l terms the section commences with the Foot Bay Gneiss. This unit, which e Donaldsoform th core f th so e n Lake Anticlines i , represented in the lower levels of the Fay Mine in the extreme footwal e faultn augeth a f s o a l,n gneiss, whose texturs i e primarily a result of cataclastic deformation. It varies in grain size with plagioclase augen ranging up to 3-4 cm in diameter set in a fine quartzo-feldspathic matrix. The unit does not host any significant uranium mineralisation and due to s distancit e t oftefroe faulno th ms ni t encountere e minth en i d workings. The second member of the Tazin Group is known as the Donaldson Lake Gneiss. This unis alsha to undergone severe cataclastic deformatio d withi an nmine s representeth ni e a y b d fine grained highly laminated mylonit f granito e e composition. e laminationTh se relativel b ten o t d y flat lying where thee ar y encountere e extremth n i de footwal e faul th t ten o bu tf t d lo steepen as they approach the fault. Only occasionally do these mylonites host uranium mineralisatio d thean nn usualle th t a y contact with overlying units. Near surfac d awaan ey froe th m St Louis fault however, this unit is not so highly mylonitised and

275 HANGING WALL DEPOSITS VERNA MINE

*c ——~S ^-^ MW

HANGING WALL AND FOOTWALL DEPOSITS ACE-FAY MINE

Martin Formation

metasomatic granite feldspar rock orange mylonite

I I mica seh i s t,argi l l i te

P-I^Ï-IJ epidotic amphibolite

s 11 ica ,si liceous mylonite

IDL I Oonaldson Lake gneiss

_X pitchblende orebody

Pig. 3 Sections showing generalized geology in hanging wall and footwall of St. Louis Fault host e Dubyne th sth b Mine Ha ad e an deposit, th 8 zon3 f o e th , deposit f Nationao s l Exploration. e nexTh t serie f rocko a comples s i s x sequence which hosts the majorit e uraniuth f o ym deposits t occura considerI r . fo s - able distanc ) alont LouiS e km footwal th e 5 gs th (> e faul f o l t where it hosts the deposits of the Fay Mine ana in a wedge located e hangwal ith e nfault th f o l, t LouiswhicS e s d formei hth an , y b d Larum faults, wher t hosti ee Vern th s a mind relatean e d deposits. e sequencTh e consists mainl f whao ys generall i t y referres a o t d a mylonitic mica s composeschisti t I .d primaril f albito y e feldspar with accessory chlorite and muscovite and has been intensely e greatesmylonitizedth r fa t y B quantitie. f uraniuo s m occu n thii r s roc s branchina k g network f veino s s usually display- ing cymoidal structural patterns. Near the base of the sequence there usually occurs an amphibolitic unit which is thought to be

276 metavolcanic rock. It is represented by dark green to black phyllonitic and epidotic amphibolite or chlorite-epidote rock. e uni Th s characteristicalli t y laced wit a networh f fino k e epidote veinlets, is associated with relatively large bands of silic r siliceouo a se seconmyloniteth s di mosd an ,t productive rocr uraniufo k m mineralisation. Althoug t providei h n a s excellent marke e uraniur th e bashorizo th f o e m r fo producinn g sequence it does occasionally occur well up within the mylonitic i mica schist especially in the hanging wall of the fault. e uppermosTh e Tazit th uni nf o e tvicinitGrou e th th n i pf o y t LouiS s faula controversia s i t l rock indeed t occurI a .s a s mylonite which is granitic in composition and frequently difficult to distinguish from the mylonites of the Donaldson Lake Gneiss. It contains megascopic porphyroclasts of plagioclase lying in a fine grained quartzo-feldspathic matrix n whic i ,e feldspath h r contains fine inclusion f hematitso e givin e roc th ga characterk - istic orange colour. Due to this characteristic colour and texture, the rock has been referred to as feldspar rock or orange porphyroclastic mylonit y minb e e geologist metasomatia s a d an s c quartz feldspar granit y othersb e e uni Th mucs i t. h more brittle than the underlying mylonitic mica schist and therefore tends to host uranium deposits in lens-shaped stockworks consisting of tiny cross cutting fractures or breccia zones. Controversy existo whethet n alteratios a e unia s s th ri t f o n underlying units resulting from metasomatis r albitizatioo m f o n the underlying units adjacent to the fault, or is perhaps an ancient paleoweathering of the underlying units. Uranium deposits occurrin n thii g s w gradeunilo te b ,ten o irregulat d r in shap d extremelan e y errati mineran i c l content. e th Mos f o t production froe verth my deepest e minelevelth f ,o s alone th g footwall of the fault was derived from this unit. Martin Formation rocks also e occuth n surfacn o ri d an e underground workings in the St Louis Fault area. The most common occurrence are the coarse angular basal conglomerates whic he hangin occuth t LouiS n i re g sth wal Faultf o l , wesf o t the Larum Fault. Uraniu s minewa m d froe conglomeratth m n i e topographic lows generally forme y dowb d n faulted blocks, which, within the limits of the underground workings, extended to a depth of 150 m. At one location uranium was mined adjacent to a Martin amygdaloidal basal e footwalt th e fauldyka th n i t ea tf o l

277 depth of approximately 240 m. In access crosscuts driven south y ShafFa froe t th mtowar e maith dn footwal e zoneor l , porphyritic andésite dykes of the Martin Formation are exposed. This intrusive unit appears to occur as a single narrow dyke extending f surfaco o u t withip m d branche 0 an e 84 n t deptha s s into several dyke t approximatela s y 106 0m fro m surface ,e limi whicth f o s ti h the exposuree immediatth n I .e footwal d adjacenan e l th o t t fault plane, Martin rocks have been see o occut n o deptt r n i h . km Considerabl 8 exces1. f o s e controversy existo whethet s a s r these rocks actuall Martie a tectoniar y r o n c breccia. However large blocks of polymict conglomerate and bedded and cross bedded arkosic sandstone containing lense f siltstoneo s s have been mapped from surface to the deepest levels of the mine. Uranium has been know o occut n n closi r e proximit e Martith o t nye rockth n i s footwal e faul t th rarel bu tf o l y withi e rockth n s themselves. Uraninite is the most abundant and widespread uranium bearing minera e hos th n mose mylonitit i l f th o rockst n I .c mica schist and associated amphibolites, and especially in the lower levels of the Fay Mine minor uranium depleted brannerite occurs as the extremities of the ore bearing structures. In the uppermost e Tazith uni nf o tGrou p (orange mylonite-feldspar rock) bran- e mosneritth ts i abundane e bearinor t g mineral wher t occuri e s in network f Mg-chlorito s e bearinor e eth e corg veinth f o et a s structures. At the ends of the structures the brannerite gives way to a uranium depleted variety of brannerite and uranium becomes enriched in a siliceous pitchblende rock, while the extremities of the structure are relatively brannerite free.

ORE GEOLOGY With the exception of surface production from the Eagle area, from Hab Mine and from Dubyna Mine all uranium production for the Eldorado Beaverlodge operation has been achieved from mylonitised metasediment d granitian s c rocks located withim 0 40 n of the St Louis Fault. Figure 4 is a simplified stratigraphie section indicating the proportion of total production plus remaining drill indicated resource r majofo s r rock units. Upon terminatio f productioo n n froe operatioth m n estimata n e was made of remaining reserves and resources. Reserves are classified as all uranium bearing rock from which production was

278 MARTIN FORMATION z RELATIVE U CONTENT CONGLOMERATE u ARKOSE X ANDESITE / BASALT

TAZIN GROUP (MYLONITIZED)

METASOMATIC GRANITE FELDSPAR ROCK 2O% ORANGE MYLONITE

MICA SCHIST ARGILLITE 48«

m tu x o.

EPIDOTIC AMPHIBOLITE 26% SILICA , SILICEOUS MYLONITE

DONALDSON LAKE GNEISS 5%

Pig. 4 Proportion of total Uranium conten stratigraphin ti e column FOOT BAY GNEISS Ul O c being achieved and for which mining development had been completed at the time of closure. Resources, on the other hand includes reserves plus all drill indicated uranium, whether under developmen r havino t g been mothballe r possiblfo d e future development, should economics warrant it. Therefore, in order to provide an estimate of the quantities of uranium within each unit of the stratigraphie section, production plus resource s beeha sn used. By far the greatest amount of uranium was contained within the top 3 units of the Tazin Group. The largest producers were the vein type deposits occurrin e mice th th an i t gschisa d an t base of the mica schist within the metavolcanics (epidotes argillites) and associated silica units. The veins generally displayed branching cymoidal structural patterns both longitudinalld an y vertically, with those located in the footwall of the St Louis Fault being the most elongated, continuous and of highest grade. Individua e veinor l s ranged from leso mort sem thatha1 0. nn 1.5 m in width.

279 A considerable amount of uranium was produced from the upper e Tazith uni nf o tGroup e mylonitizeth , d metasomatic granitr o e orang s moreit e mylonite o t brittl e Du e. nature this unit tended to provide a stockwork of tiny cross cuttings fractures hosting uranium mineralisation rather than the more continuous vein of the mica schist units. This unit generally lies stratigraphically belo e Martin-Tazith w n unconformitt S e footwal th e th n i yf o l Louis Fault bot n surfaco t hdeptha d an .e Production froe th m uppe e Tazir th uni nf o t therefore also includee s th som f o e materia le Marti froth e bas th f mno e y ShaftrockFa e s ,th wes f o t in the hanging wall of the fault, where the stockworks or breccias occurre n downfaultei d d topographi e Martinc th low n i s - Tazin unconformity. Most orebodies distributed along the St Louis fault plunged e wesd tdisplayeth oan t a remarkabld e continuity with deptht A . the time of closure the deepest production was being achieved from a stockwork breccia deposit in the footwall of the St Louis Fault at a vertical depth in excess of 1800 m.

FACTORS CONTROLLING MINERALISATION Many factors have been reported to have a major controlling influence on uranium mineralisation in the Beaverlodge area. The most dominan f thoso t e factor e listear s s followsa d : l TaziAl n. A Group host rock e highlar s y mylonitizet bu d retai a recognisabln e stratigraphie order. B. All uranium deposits are spacially related to major faults and occur in secondary structures. . C Uranium bearing structure e confinear s o elongatet d d zones that are both stratiform and crosscutting and form cymoidal fracture patterns. D. Wall rocks of uranium bearing structures are hematitised. l uraniuAl . E m deposit e spaciallar s y relatee th o t d Martin-Tazin contact. . e vicinitF Rockth n i sf uraniuo y m deposits have been subjecte o somt d e late stage soda-metasomatise th n i m for f albitizationo m . Although graphit n importana s i e t association with uranium deposit e Athabascth n i s a basin s presenc it e Beaverlodg, th n i e e deposit ss significanta doe t appeae b no s o t r . s Firsi t i t

280 characteristically associated with ore bearing structures at the base of the mylonitic mica schsits, within and adjacent to the epidotic amphibolite. Secondly, at one known location, it occurs within an ore-bearing shear zone immediately below the Martin Formation west of the Fay Shaft. The third notable occurrence is in ore bearing fractures in the uppermost unit (mylonite) of the Tazin Eagle Grouth n ei pMin e area, situate e cresth f o tn o d the Donaldson Lake anticline. In all cases the graphite occurs on slickenside faces but is not considered to be sufficiently ubiquitou majoa e b r o e controt factos th n i f ruraniulo m mineralisation in the Beaverlodge area.

In addition to the above, geochronology of uranium deposits in the Beaverlodge area has been the subject of considerable work by a number of geologists. To address this subject in detai wels i l l beyon e scopth df thieo s paper n summarI . y therefore, age dates of epigenetic pitchblende mineralisation are highly discordant e earliesTh . t dates that have been recorded ordee th f 170aro rn i eo 180 0t 0 m.y d coincidenta.an l wite th h Hudsonian orogeny. Age determinations have also suggested o 110t clustering 0 m.y90 e s 0 recenm.y.a th 20 aren d .i s an a t

Statistics, group summaries with corresponding histograms, fo a totar f approximatelo l e determinationag 0 12 y s were mady b e Robinson (1955), Koeppel 1968 and Slater 1982 using Pb206/U238, Pb207/U23 Pb207/Pb20d an 5 6 isotopic compositions. Histograms illustrate the suggestion of the above-mentioned clusterings, while the statistical summaries and plots suggest that the two methods using the uranium-lead isotopes are comparabl lead-leae th t bu e d isotope t correlatno o sd wels a e l with the other two and on the average indicate considerably older agese effectTh . f episodiso c lead loss consequentld ,an y the discordanc e uraniuth f o ym ages e therefor,ar e illustrated.

Fluid inclusion studie n attempa n i s o determint e temperaturf formatioo e n have also been quite extensiv d widan e e ranging in reported results. Temperatures have ranged from greater than 400°C to as low as 60-70°C. Sassano (1972) suggested that depositio y havma ne occurred ove a lonr g perio f timo d an e perhaps that temperature f formatioo s n have gradually decreased over that time. It was not uncommon in the deeper levels of the Fay Mine in particular to encounter pockets of highly saline

281 (CaCl) water under extremely high pressure s generallwa t I . y felt that these waters represented remnants of original formational waters or hydrothermal fluids.

CONCEPTS OF GENETIC MODELS Genetic Models proposed for uranium deposits in the Beaverlodge area have ranged from exclusively hypogene to exclusively supergene, depending largely what factors controlling mineralisatio e considerear n e significantb o t d . Neither geochronological nor fluid inclusion studies have been able to provid a definitive e guida specifi o t e c genetic modele Ag . dates of uranium mineralisation have, however, defined possible period f remobilisatioo s f uraniumo n e oldesth , t coinciding roughly with the late stages of the Hudsonian orogeny, and the most recent being withi e las0 m.yth n20 t . Fluid inclusion studies also have provide a widd e rang f resultso e y , an whic r fo h single period coul e interpreteb d a resul e b f eitheo to t d r deep- seated hydrothermal fluids or of relatively cooler surface water heated purely by diagenesis. Wha moss i t t commo o nearlt n l modelsal y s thai t uranium probably originated from weatherin e pre-existinth f o g g Archean terrane which contained uraniu n granitii m d pegmatitian c c rocks. The uranium was concentrated syngenetically in Apinebian pelites s anthewa d n mobilised during subsequent metamorphic events. Genetic models var t thia y s point wit d durinhan respecw gho o t t what metamorphic event the ma3or period of mobilisation occurred to bring the uranium to its present site of deposition. Proposals range from uranium being moved durin d immediatelan g y following metasomatis a deevi mp thrust faults produce y widb d deed an e p zones of mylonitisation, to during the mylonitisation. If the uranium was deposited syngenetically in Tazin sediments, the present vein systems might reflect old stream channels or shor ee Tazi lineth n i s sedimentary sequence. This would hel o explait p e remarkablth n e continuity down e plungth f o e stratiform vein system st Loui S alon e sth gFaul s showa t n i n figure 3. e uraniuIth f a dees movevi wa mpp u dthrust s associated with mylonitisation, the fault association and specific alteration halos woul e explainedb d .

282 A supergene model would have uranium released through weathering of the Aphebian metasediments and older Archean rocks probably during Hudsonian times. Uranium solutions would migrate downward opea vi sn fracture Tazin i s n meta-sediments immediately below elongated fracture controlled basins or valleys on the Martin-Tazin unconformity. Late tectonic activity durin Hudsoniae th g n orogeny and related to faulting, folding and subduction of Martin rocks along the northeast trending St Louis and related faults, resulted in the remobilisation of uranium downwards in tensional fractur e presens zoneit o t st depositional site. e greatesTh t amoun f remobilisatioto n woulde occuth n i r immediate vicinit e faulth tf o yresultin e greatesth n i g t concentration and greatest preservation of the uranium mineralisation at this location. Areas away from the fault such as across the west of the Donaldson Lake anticline would contain erratic mineralisation i n fractures which ten o accumulatt d elongaten i e d zones parallel e pre-Marti witth e axi f th ho s n valleys. Most mineralisation seen at the present surface in these areas would therefore represent root f erodeso d deposits, explainin e lac f structth go k - ural continuity. In summary the complete genesis of the numerous uranium deposits in the Beaverlodge area remains uncertain. Most workers agree however, that the deposits are related to metamorphic events and are the result of remobilisation within and from donor metasediments.

CONCLUSIONS e BeaverlodgTh e mining distric s playeha t a majod r roln i e the histor f Canadiao y n uraniu e mriche th mining d r Ha deposit. s associated with the Athabasca Basin been discovered first, the Beaverlodge story might never have occurred. However, greater bedrock exposures of mineralisation has naturally attracted attention to the Beaverlodge area and it was this attention that lead to discoveries in the Athabasca Basin. It appears that through minin d continuinan g g discover w depositne e th f o yn i s Athabasca Basin that the interpretation of the geology of the associated deposit s becomini s g increasingly more complex. There- fore, what might have initially been interprete s substantiaa d l

283 geological differences between the Beaverlodge deposits and the Athabasca deposits will very likely be interpreted as being similar and indeed merely as variations within a common geological environment.

ACKNOWLEDGEMENTS e authoTh r wishe o than t se managemen th k f Eldorado t o Resources Limited for granting permission to publish this paper. The contribution of our 85 geologists who were involved in the exploration, development and production of the Eldoraao Beaver- lodge operatio e gratefullar n y acknowledged.

SELECTED REFERENCES KOEPPEL d Histore Uraniuan th , 1968 e f V. ,o Ag ym : Mineralization of the Beaverlodge Area, Saskatchewan. Geol. Surv. Canada, Paper 67-31. SASSANO , 1972e NaturG. ,Th e Uraniud :Origi th an e f o nm Mineralizatio y MineFa e , th Eldorado t a n , Saskatchewan. Ph.D. Thesis, University of Alberta. SLATER, J.R., 1982: Some Relationships Between Deformatiod an n Ore Genesis at the Fay Mine, Eldorado, Saskatchewan. Unpublished M.Sc. Thesis, Universit f Albertao y . SMITH, E.E.N. (in press): Geology of the Beaverlodge Operations Eldorado Nuclear Limited. C.I.M. Spec. Publ., Geology of Canadian Uranium Deposits.

TREMBLAY, L.P., 1972: GeologBeaverlodge th f o y e Mining Area, Saskatchewan. Geol. Surv. Canada, Memoir 367.

284 GEOLOG DISCOVERD YAN Y OF PROTEROZOIC URANIUM DEPOSITS, CENTRAL DISTRIC KEEWATINF TO , NORTHWEST TERRITORIES, CANADA

A.R. MILLER Geological Surve f Canadayo , Ottawa, Ontario J.D. BLACKWELL Cominco Ltd, Vancouver, British Columbia L. CURTIS L. Curti Associated san s Inc., Toronto, Ontario W. HILGER Urangesellschaft Canada Ltd, Toronto, Ontario R.H. McMILLAN . NUTTEE , R Westmin Resources Ltd, Toronto, Ontario Canada

Abstract

Proterozoic supracrustal successions, Central Districf o t Keewatin e similae Proterozoiar ,th o t r c basin f northero s n Saskatchewan which contain economic uranium deposits. Because e geologicath f o l similarities, this district experiencea d dramatic increase in uranium exploration during the 1970's. Deposit f varyinso g type were discovere n thii w uraniud ne s m province indicating a potential comparable to the Northern Saskatchewan uranium province. In this paper, examples of three e morth e f -importano t uranium deposit type e describedar s . Early Aphebian shallow marine pelitic to psammitic metasediment e uppeth rf o sAme r group host subeconomic uranium concentrations. Mineralisation interprete s syngenetia d o earlt c y diagenetic is restricted to drab to red fine clastic sediments that displa a moderaty e magnetic susceptibility. Late Aphebian structurally controlled depressions, Baker Lake Basin, Yathkyed Lakd Angikunan e i Lake subbasinse ar ,

285 filled with continental sediment d volcanican se Dubawn th f o st Group. Structurally controlled uranium mineralisatio s mosi n t common adjacent to the margins of these basins. The Lac Cinquante deposit located in the northeastern Angikuni Lake subbasi s hostei n n Archeai d n metasedimentar d metavolcanian y c rocks immediately adjacent to the Dubawnt unconformity. Mineral- isatio s localisen closi ni r eo n proximiti d a highl o t y y fractured brecciatean d d unit consistin f interbeddeo g d chloritic tuffaceous and sulphide + graphite metasediments. In the eastern Baker Lake Basin uranium mineralisatio s localisei n n i d thermal altered Kazan Formation arkose near alkaline dyke complexes.

Conglomerat d sandstonan ee Helikia th f o e n Thelon Basie li n unconformabl a highl n o y y weathered basement complex e LonTh e. Gull deposi s hostei t n retrogradei d d alterean d d psammitio t c pelitic metasediments and undeformed granite near the erosional edge of the basin. Mineralisation is structurally controlled and associated with zone f intenso d sMg-metasomatism an - K e . These features paralle le unconformity-relate thosth f o e d deposits of N. Saskatchewan,

INTRODUCTION e Baker-TheloTh n uranium district liee th s f withio e on n most remote area f Canada o se District th n i , f Keewatio s d an n Mackenzie, Northwest Territories. a subpolaThi s i s r region, beyond the northern limit of forest cover, marked by subdued topography and extensive lake and river systems which can be ice covere r ninfo de e yearmonthth .f o s Existing transport- ation infrastructure is minimal, comprising only float aeroplane access, a single land air strip at the village of Baker Lake, and barge m inlanaccesk 0 d28 s from Hudso y througBa n h Chester- field Inlet and Baker Lake. The village of Baker Lake is situated 0 kilometre64 se por d railwanortth an t f o h y terminal Churchill, Manitoba. In spite of the remote location and high costs of explorat- e ionregio th ,s bee ha nn subjecte o severat d l period f intenso s e uranium exploration by both private and government-owned mining companies e mosTh .t recent exploration period dating from 1974 e tim th f writingo eo t s largeli , e e recognitioresulth th y f o t n

286 of marked regional geological similarities to the productive Athabasca Basin (700 kilometres to the southwest) and the Pine Creek Geosyncline region in the Northern Territory of Australia (Curtis and Miller, 1980), and the fierce competition for land Athabasce ith n a Basin, which forced companies wit a commitmenh t to uranium exploration but a restricted land position northward. Earlier discoveries in the Baker-Thelon area, made during the period 196 o 1970demonstrated 8t ha , d that uranium mineralisation was present, though not as economically viable deposits, and together wit e rapidlhth y accumulating data from discoverien i s Saskatchewan and Australia, set the stage for a major effort in the region and the establishment of a new camp (Bundrock, 1981) . This paper outlines the igneous and sedimentary record of Proterozoic succession n thii s s distric d usean ts this recors a d the framework to outline the exploration history and geology of the most important uranium prospects and deposits. The geology and uranium metallogeny are presented in three sections: ) EarlA y Aphebian metasedimentary belts Ame) i :r group consisting of shallow marine to continental deposits and ii) undifferentiated metasedimentary belts ) LatB ; e Aphebian Baker Lake Basi d subbasinsan n : Dubawnt Group continental sedimentary and volcanic rocks ) HelikiaC : n Thelon Basin: Dubawnt Group continenta d shalloan l w marine sedimentary rocks.

) EarlA y Aphebian Metasedimentary belts Amer group Early Aphebian metasedimentary belts are principally found in the region bounded by the Baker, Aberdeen and Amer lakes (Figur . Thes1) e e belt e largelar s y confine Armie th to t dLak e Block (Heywood and Schau, 1978) which is bounded to the north by the Amer-Meadowban e Chesterfiel e soutth th y kb ho t Faul d an td Fault Zone (Schau et al., 1982). The Amer group is preserved in a west-southwest trending southwest plunging synclinorium that is covered in the southwest e Helikiabth y n Thelon Formation e bel Th s expose.i t r morfo de than 120 km with an average width of about 50 km. The Amer group is interprete s earla d y Aphebia ns correlatio baseit n o d n with the Hurwitz Grou f southero p n Keewatin (Bell, 1969, 1971). However the age of the Amer group is poorly bracketed between

287 (O oo 66 66°

cSg-rff'ï" V* t *-V? . J gV'^f^' *

106 104 102° 100 ORDOVICIAN ARCHEAN Ultramafics: komatiites, serpentenites Fossiliferous limestone Basic metavolcanics, massive and pillowed flows, pyroclastics, HELIKIAN tuff, and agglomerate; metagreywacke, mixed basic and intermediate metavolcanics, amphibolite. carbonate iron formation, calc-sllicate rocks; v l Basalt quartzite, quartz pebble conglomerate, black slate, includes some gabbro

Dolostone, siliceous dolostone s^ Fault

T" I Thelon Formation: sandstone, pebbly sandstone, conglomerate (B) Baker Lake Basin

APHEBIAN (Y) Yathkyed Lake sub basin

Porphyritic diorite-gabbro (includes McRae Lake dyke) (A) Angikuni Lake sub basin

Granite parn I : t fluorite bearin rapakivd gan l textured TF Tulennalu Fault

Syenite: quartz syenite; includes Martell intrusive suite F Amer-MeadowbanM A k Fault

F C Chesterfield Fault Zone Pitz Formation if Uranium deposit, prospect Dubawnt Christopher Island and KunwaX formations Group Locatio Theloe th Baked f no n an r Lake Basins, Districts of Keewatm and Mackenzie, South Channel and Kazan formations Northwest Territories, Canada **drpi-%C V ~ Amer group and equivalents

APHEBIAN and/or ARCHEAK

Orthoquartzite, quartz pebble conglomerate, muscovite schist and phyllite,

Basic metavolcanics, metapelites, calc-silicate rocks, graphitic schists, metagreywacke, iron formation

Migmatite, irregularly layered granitoid gneiss, paragneiss; amphibolite, cataclastlc augen gneiss

Fig 1. Geology of Central District of Keewatin, N.W.T. (from Miller ana to oo LeCheminant, in press) K) VO O

Tabl . 1 eStratigraphi e e Amecolumth r f o groupn , Central Distric f Keewatio t n

Lithology Remark quartz arenite, arkose, maroon preserved only at the western end of the sandstone, shal d siltstonan e e with belt rare stromatolitic dolostone. fine grained pink feldspathic cyclic with overall upward fining trend; upper clastic arenite, interlaminated arenitd an e main uraniferous unit at interface sequence siltstone, calcareous trough-cross between arenite and siltstone bedded arenite, local conglomeratic breccia; cappe y massivb d o t e laminated siltstone grey black, green and maroon shales upward coarsening trend and siltstone, interbedded fine grained feldspathic and quartz arenite, thin stromatolitic dolostone laminate d locallan d y stromatolitic dolostone, with minor intercalated transitional shale, fine-grained quartz arenite, sequence cher d conglomeratean t ; very local vesicular mafic flow d sillsan s . pyritic grey to black carbonaceous siltston d shalean e , interbedded chert and fine grained quartz arenite. basal thicK quartz arenite and quartz- fluvial to near shore marine lower clastic pebble orthoconglomerate with minor sequence shale and iron formation. 2801+24/-21 Ma (U/Pb, zircon) from the basement complex adjacent e Ameth r o t group (Mille d LeCheminentan r n pressi , d an ) 1849+18 Ma (U/Pb zircon) from an undeformed quartz syenite that intrudes Amer metasediments (Teli d Heywoodan a , 1978). Stratigraphy and sedimentology of the Amer group have been described by Heywood (1977) , Tippett and Heywood (1978) , Knox (1980), Patterson (1981 . (1983)Telid al Table.1 e an )t se e a; . The group consists predominantly of two clastic sequences, a lower white orthoquartzit n uppea d r an e interbedded sequencf o e feldspathic sandstone, siltstone, mudstone and arkose. These two sequences are separated by a transitional sequence that includes thin discontinuous dolomitic limestone at the base and intercalated mudstone-siltston e topth .t a eMino r mafic igneous units occur alon e folth gd bel t approximatela t e samth ye stratigraphie level as thin locally conformable flows (Patterson, 1981) and as gabbroic sills (Telia et al., 1983). Metamorphic assemblages in the northeast segment of the fold belt indicate upper greenschist-lower amphibolite faciès regional metamorphism (Patterson, 1981) wit grade hth e decreasin o subt g - greenschis e south-wester th e bel n th i tt f o (Knox d en n, 1980, Teli t al.e a , 1983). Deformational events affectin metae th g - sedimentary sequence are: i) west trending folds and northerly verging thrust faults, ii) southwest trending folds, and iii) northwest trending normal faults (Tippet d Heywoodan t , 1978; Knox, 1980; Patterson, 1981; Teli al.t e a , 1983).

Exploration for and geology of uranium mineralisation in the Amer group The discovery of economic uranium deposits in Wollaston Group metasedimentary rocks adjacent to the Athabasca Basin, . SaskatchewaN 1968n i n , prompted Aquitaine Compan f Canado y o t a conduct reconnaissance airborne surveys in other geologically similar regions. Reconnaissance airborne radiometric and magnetic surveys were conducted ovee northeasterth r n portioAmee th r f o n belt because abundanth f o e t outcrop compare e southwesterth o t d n portion. Radiometric anomalies were identified which initiated a progra f grouno m d radiometric surveys, prospecting, soil geochemistr d diamonan y d drilling (Aquitaine Compan f Canadao y , 1970; Laporte, 1974). Re-evaluation of the northeast Amer bel y Comincb t n 197i o 7 involving detailed mapping, radiometric

291 S(r«tibouA4 uriniu n grouAn mpm clMlIC W

Bisemenl complex

Eirly Aphchiu Rtgolith

O C_^ Uranium occurrence Amgr group tnd melasedirnonttry bt/ti

Fig 2. Schematic cross section, Schultz and Amer Lakes, Northeast Thelon Basin

survey d diamonan s d drilling indicated subeconomic uranium concentrations e discoverTh . f uraniuo y m mineralisatioy b n Westmin Resources Ltd in the southwestern portion of the Amer group has influenced the exploration for unconformity-related mineralisatio n thai n t region. Stratiform and stratabound uranium with minor base metals is hoste y severab d l lithologies withi d finnre e dra d clastian b c units situated neae boundar e th transitionar th f o y d uppean l r e Ameunitth r f o grous p (Figur . e uraniu2) th eMuc f o hm mineralisation is restricted to this stratigraphie position along the length of the synclinorium (Miller and LeCheminant, in press). Mineralised zones, lenticula n plad i sectionr an n e comprisear , d f stackeo d discontinuous concordant psammiti d pelitian c c beds. These zones may have a composite strike length to 1000 m and thickness of 3-5 m. Mineralised zones show variable grades of o 0.18t 0.05 % LUO om (Laporte ove2 1- r , 1974; Lor t al.e d , 1981), J o Mineralisatio s containei n a wid n i ed variet f lithologieo y s of the upper Amer group: siltstone, sandy siltstone, feldspathic sandston d calcareouan e s equivalents (Knox, 1980; Taylor, 1978; Curtis and Miller, 1980). Rapid lateral and vertical lithological variation e interstratifleth f o s d mudstone-siltstone-feldspathic sandstone are interpreted to have been deposited in a tidal flat-restricted bay environment. Mineralisation localised within siltstone, along silt-sand interface r withio s n sandy beds exhibit a consistencs f boto y h macro d microscopian - c features,

292 alteration assemblage d elementaan s l associations alone th g entire e Amelengtth r f o group he mineraliseTh . d lenses havn a e elevated magnetic susceptibility and a red coloration which contrasts with drab hues commo n adjaceni n t strata. Uraninite, coffinite and subordinate U-Ti-Si compounds occur as ultra fine (1-10 microns) disseminated grains replacing framework feldspar and interstitia o quartz-feldspat l r framework grain n psammitii s c s disseminatea bedd an s d spheroidaan d l aggregate n pelitii s c beds (Curtis & Miller, 1980). Disseminated pyrite, galena, chalcopyrite, bornite, molybdenite,trace arsenopyrite and cobaltit e associatear e d with uranium mineralisation. Radio- active strat e characterisear a y goethitb d d celadonitan e e interstitia d replacinan o t l g feldspar framework grains, hematite, disseminated authigenic subhedral to euhedral magnetite and interstitial iron-chlorite and quartz. Complex metal assemblages and early diagenetic alteration support the interpretation that the stratiform mineralisation is comparable to sandstone- type mineralisation whic s controllei h y initiab d l porosity, permeabilit d redoan y x barriers. Mineralised zones explore t constitut no dato t do d e e economic ore bodies because of i) remote location, ii) size, e lensesford gradth an mf d iiio ean , ) geometr e stratith f o y- form lenses due to multiple deformation. However potentially economic zones of uranium mineralisation may occur in fold closures where mining thickness coul e createo b dt e du d recumbent folding.

Exploration concepts e followinTh g exploration concepts have been usefun i l prospecting and delineating the stratiform mineralisation in the Amer group ) recognitioa : e mineralisatioth f o n e restricb o t n - ted to a specific lithostratigraphic position and environment, b) recognition of a primary sedimentologic control, and ) coincidencc f mineraliseo e d strata with moderate magnetic susceptibility. Undifferentiated metasedimentary belts Several discrete sedimentary and volcanic-bearing sequences containing orthoquartzite strata are recognised south and south- Amee th easr f grouo t p (Ashton 1981, 1982; Scha t al.e u , 1982) e termeanar d d Archean and/or Aphebian (Figur . 1) ePolyphas e

293 to VD -P- Table 2. Stratigraphy of the Dubawnt Group, Baker Lake Basin and subbasins

Unit Lithologies Remarks

Mackenzie Diabase mediu o coarst m e grained gabbr d quartan o z NW trending: 120 a (Rb/Sr4M ) gabbro

Thelon Formation conglomerate, sandstone, pebbly sandstone

Basic Intrusions porphyritic leucogabbro; medium grained NE trending dyke-like intrusions: (McRae Lake dyke) gabbr d quartan o z monzonite 1735 Ma (K/Ar)

Granite alkali-feldspar granites; porphyritic circular to multilobate; fluorite- biotite-hornblende, biotite and biotite- bearing; granophyric, miarolitic to muscovite granites rapakivi textures

Basalt black amygdaloid basalt intercalated with laminated green siltstone

Pitz Formation rhyolite, minor rhyodacite and dacite; fluorite and topaz-bearing; interflow rhyolite breccia and conglomerate, red quartz-rich sediments represent debris lithic sandston d siltstonan e e flod streaan w m channel deposits

Kunwak Formation conglomerate, lithic sandstone, siltstone, alluvial fan, braided rive d lacustrinean r and mudstone; minor amygdaloidal dacite deposits

Christopher Island Formation trachybasalt, trachyandesit d trachytean e ; subaerial potassic alkaline lavas and d relatean d alkaline intrusive volcaniclastic conglomerates, wacke, pyroclastics with interflow fluvial complexes (includes Kartell sandstone and siltstone deposits: 178 a 6(Rb/SrM o 183t ) 0 (K/Ar) Syenites) mediu o coarst m e grained syenite-quartz small stocks, sills and dykes: 1832 Ma syenite; minor biotite pyroxenite; (K/Ar) biotite lamprophyre

Basal clastic successions polymictic conglomerate, red to grey alluviad braidean n fa dl fluvial deposits, (Kazan Formation, South arkose and arkosic wacke, siltstone minor eolian deposits; clastic wedges Channel Formation, Angikuni and mudstone derived from easd southeastan t . Formation) folding, northerly directed thrustin d lithologicaan g l differences have made stratigraphie correlations with the Amer group tentative. Some of these sequences may be equivalent to the Archean Prince Albert Group, Melville Peninsula (Schau et al., 1982). Scattered uranium occurrence e knowar s n throughout these sequences however their positio e stratigraphyth n i n , alteration and associated minerals is poorly known (Curtis & Miller, 1980).

) LatB e Aphebian Baker Lake Basi d subbasinan n s Sedimentary and volcanic units of the Dubawnt Group were divided intx formationsi o y Donaldsob s n o format(1965tw d an )- ion f moro s e local extent were established (Blake, 1980; LeCheminant et al., 1979a; see Table 2). The most complete record of the Dubawnt Group is preserved in the Baker Lake Basin. This complex graben, 50 km in average width, can be traced more m frok tham0 30 easternf Bake o d ren n Lak o Tulemalt e u Lake (Figur . 1) eIsolate d Dubawnt Group successions wesf o t Tulemalu Lakd southeasan e f Dubawno t e tfaulteb Lak y ma ed remnant a southwester f o s n extensio e basinth f .o n Wesf o t Forde Lake, the Baker Lake Basin is bounded by Tulemalu Fault. To the northeast the basin extends across the projection of this fault. East of the Tulemalu fault the boundaries of the Baker Lake Basin are generally parallel to earlier structural trends in the basement and are defined by unconformities or steep normal faults. Hergeneralla e basi s th e ha n y asymmetric cross-section with northwest dipping basa d bedre ls sequences overlaia y b n thin veneer of alkaline volcanic rocks (Donaldson, 1965, 1967; Blake, 1980). o smalleTw r intracratonic basins Yathkyee th , d Lake subbasin (Curti d Milleran s Angikune , th 1980 d an )i Lake subbasin (Miller and LeCheminant n pressi , e southeasar )e Tulemal th f o t u fault. Structurally and stratigraphically these subbasins are similar to easter ne Bake partth rf o sLak e Basin (Bad d Blakean e , 1977; Blake, 1980). Initial developmen e Baketh rf o t Lake Basi d subbasinan n s s stronglwa y influence pre-existine th y b d g tectonic framework particularly major faults. Basal redbeds of the South Channel and Kazan formations were deposited in response to faulting and shedding of debris from adjacent highland areas. These format- ione interpretear s s alluviaa d l fan-braided river deposits

295 having a source area to the south and southeast (Donaldson, 1965, 1967) . Redbed sedimentation was succeeded by widespread voluminous subaerial volcanic eruptions that produced potassic alkaline lavas, pyroclastics and comagmatic stocks, sills and plutons of the Christopher Island Formation. Alkaline flows are interbedded with debris flows and fluvial deposits which display the complex interaction between volcanism, faulting and fluvial deposition e ChristopheTh . r Island Formatio t diachronouno s i n s (Curtis and Miller, 1980) as it is probably the only valid time- stratigraphic event in the Baker Lake Basin, instead its relation- a widshi o et p variet f underlyino y g units reflecte th s relatively independent histories of deposition in the various subbasins of the Baker Lake Basin (Blackwell, in prep.). Volcanic rocks of the Christopher Island Formation are alkaline, with shoshonitic affinities (Blake, e characterise1980ar d )an d

bi contents yT Na+ hig , 20 0,2 Fe K h K w d ,0:Nalo an hig , a 2B >1 0h P contents and erratic but high U and Sr contents. Covering e Christopheth , m k r0 1 Islanx 5 n area df o aconstitute e th s largest alkaline provinc e worldth n i .e Whole rock Rb/Sd an r mineral K/Ar radiometric dating indicate a minimum age for alkaline volcanis f abouo m t 183 0a (LeCheminanM t al.e t , 1979b). Continued block faulting following the waning of alkaline volcanism created local subbasins that were infilled with Kunwak Formation red clastic sediments. Depositional environments range from alluvia o braidplait n fa l o lacustrint n e (LeCheminant et al., 1981).

Large scale magmatism resumed with rhyolitic eruptions and emplacemen f epizonao t l granite plutons. Pitz Formatios i n comprised of high-K, silica-rich, topaz-bearing rhyolite lavas, lithic-crystal tuffs with intercalated quartz-rich volcaniclastic rocks. This sequence record a histors f contemporaneouo y s silicic volcanism, mass wasting and stream-channel deposition. Pitz volcanic rocks are comparable to topaz rhyolites as defined by Burd Sheridaan t n (1980 d Bur an )t al.e t , (1982). Elongate multilobate plutons of rapakivi granite intrude Dubawnt Group rock west of Tebes3uak Lake. Northeasterly trending bodies of porphyritic leucogabbro-quartz monzonite, including the McRae Lake dyke, intrude the latter rocks and have yielded a whole

296 roc e youngesk Th K/A . rt dat Ma intrusionf 1732 o e5 e 5± th n i s aree regionallar a y extensive MacKenzie diabase dykes (whole rock Rb/S Patchet, r Ma 120 9 3 t al.4e + t , 1978) which intrude isolated capping f Theloo s n Formation wes f Tulemalo t u Fault (Figure 1). In summary the Baker Lake Basin and subbasins develope a continenta n i d l extensional regim t aboua e t 1.9-1.8 Ga. Uranium mineralisation has been recognised in each formation of the Dubawnt Group and basement complex to the Baker Lake Basin and subbasins (Mille d LeCheminantan r press)n i , . Zonef o s brittle failure are potentially the best exploration targets and have yielded the largest number of occurrences (Miller, 1980) wite highesth h t uranium concentrations. Structurally controlled mineralisatio s spatialli n y ) faulterelatei : dto d margins e basinsth ) f faul o ii d ,cataclasti an t c zones cuttinr o g adjacent to the basal unconformities, iii) specific lithologies in the Archean-Aphebian basement and Dubawnt Group, and iv) structures that have suffered repeated movement e mosTh t. significant fracture controlled uranium deposit discovered to date, Lac Cinquante deposit, is situated near the northeastern rim of the Angikuni Lake subbasin. Exploratio c CinquantLa d Geologe an nth f eo y deposit, Angikuni Lake subbasin c CinquantLa e Th e deposit contains drill indicated reserves 3 millioo5. f tUOg K n g (Northern Miner, 1982) e initiaTh . l discovery of mineralisation on the property was made by Pan Ocean l LtdOi n 197.i 4 durin a reconnaissancg e airborne radiometric prospecting program. At the time of discovery, only 1:1,000 000 scale maps were availabl e regioth n f Yathkyed-Angikuni o ne i lakes; however the rims of these subbasins were targeted based upo e succesn Ocean'th nPa f so s exploration alon e easterth g n margin of Baker Lake Basin in the late 1960's. Uranophane- bearing conglomerat s detectewa e d durin e airbornth g e surved an y provided the impetus for ground acquisition. Follow-up work included regional airborne magneti d electromagnetian c c surveys and detailed ground scintillometer, magnetic and VLF surveys. On c thCinquantLa e e property, detailed ground prospecting revealed numerous fracture controlled uraninite-hematite-carbonate veins within Archean metavolcanic rocks which are basement to the Dubawnt Group on the property. The key to ongoing exploration

297 was the recognition by Pan Ocean personnel of the association betwee F conductorVL n d uraninite-bearinan s g vein systems. e northeasTh t terminu e Angikunth f o s i Lake subbasis i n underlain by an Archean greenstone belt which on a regional scale is subordinate in volume to granite gneiss and granodiorite. This greenstone belt is correlated with the Henik Group of southern Keewatin (Bade, 1980). Fault and mylonite zones trendin W transecN gd E-Wan tE N ,thi s basement compled an x contro distributioe th l f Dubawno n t Group rocks (Bade, 1980). The deposit is hosted in a sequence predominantly of propylitized pillowed and massive basic metavolcanics, meta- gabbro d subordinatan s e massiv d fragmentaan e l felsic metavolcanics with related metasediments. Few lithological markers are apparen n thii t s sequence e exceptioon e th , o thit n s general- isation is a thin, 1-2 metre, tuffaceous chert and chert- graphite-sulphide unit which occurs discontinuously through the principal zone of mineralisation. This zone is termed the Main Zone. Similar cherty carbonate and graphitic metasediments of possible exhalative affinity have been note n outcron i di d an p drill core elsewhere in the district. Greenschist grade basalti o andesitit c c volcanics consist of chlorite, epidote, relict plagioclase (An3 5) with minor quartz, clino-amphibole, albite, carbonate, pyrite, sphene, anatase and sericite. Interbedded tuffaceous metasediments contain quartz, chlorite, sericit d carbonatan e e with minor pyrit d Fe-Tan e i oxides e chert-sulphidTh . e marker unit, sometimes associated with U-Mo mineralisation, consists of chlorite, quartz a carbonaceou, s chlorite mix, sericite, pyrit d raran e e chalcopyrite and sphalerite. e contacTh t betwee e greenstonth n d overlyinan e g Dubawnt Group is marked by an increase in hematite and to a lesser extent carbonate. This hematite-carbonate paleo-weathering profile is up to 5 metres thick with pronounced variations that reflect pre-Dubawnt paleo-topography. Coarse grained conglomerat e latth e f Aphebiao e n Dubawnt Group overlie e basementh s t complex. Boulde o pebblt r e sized granitic to granodioritic gneiss framework clasts characterise the conglomerate although lithologies of the greenstone terrain

298 are present. Basal conglomerat overlais i e n conformably and gradationally by pink to red arkosic grits, siltstone, mud- ston d conglomeratean e . Minor clast f Dubawnso t mafic volcanic e basarockth n li s red bed succession indicate alkaline volcanism accompanied alluvial fan-braided river sedimentation. Because of this minor volcanic component e basa,th l redbeds have been mappes a d Christopher Island Formation (Bade, 1980 thet )bu y occupe th y same stratigraphie positio d displaan n y essentiall e samth ye lithological relationships as South Channel and Kazan formations in the Baker Lake Basin (LeCheminant et al., 1976, 1977, 1979a,b; Donaldson, 1965). Christopher Island Formation phlogopite- phyric lavas, lapilli tufd intercalatean f d volcanic-rich sediments overlie the redbeds (Blake, 1980). The alkaline volcanic r contentsS d an s, .displaBa , Cr , y Ni elevate , Th , U d Lamprophyre d sill-likan s e intrusions comagmatic with volcanic rocks occur throughout the stratigraphy. In the vicinity of the deposit, major penetrative faults and shear e subparallear s W foliationWN o t le principa Th . l conductive zones, one of which coincides with the ore deposit, also strik n thii e s direction. Extensive shearing sub-parallel o foliatiot s indicatei n y transpositiob d f pillowo n d an s locally extreme stretching of pillows. Although the Main Zone directio s indicatei n F conductoVL y b d r axi t 290°-300°a s , e surfacth mos f o te mineralisation spatially associated wite th h Main Zonf cros o s controlle i et sse fracturesa y b d , termed gash veins, trending 040 -060 . These vein systems composed of carbonat hematit+ e uraninit+ e e with subordinate chalcopyrite, pyrite and rare arsenopyrite are discontinuous and of variable width, 1-1. 5cm The Main Zone structure, approximately 1100 m long, coincides wit bana h f volcanogenio d d volcaniclastian c c metasediments ranging from 1 to 8 metres and averaging 2.5 metres in thickness. Mineralisation occurs within this stratigraphie unit or in very close proximity to it and has the form of plunging pods of uraninite veins (Miller et al, in press). The ore zone is 300-400 m long with an average width of 1 metre and has been drill tested to 270 m depth. Extensive fracturing and localised cataclastic textures characteris Maie th en Zond brecciatioan e n is typical of the cross-cutting 'gash veins'.

299 A comple + (Zn u C x ) + meta b P + l g assemblageA + o M + U , characterises the Lac Cinquante Main Zone ore body. This type f mineralisatioo s similai n o structurallt r y controlled occurr- e Bakeenceth n r i sLak e Basin (Millerg A d an , o M 1980} , U . are potentially economically recoverabl e depositth n i e . Hematite, chlorite, Fe-carbonate and albite are the commonest gangue minerals forming diffuse envelopes around veins and filling veins. Uraninite occurs as vein and fracture fillings in brecciated metavolcanic d metasedimentsan s s selvagea , o t s hematitised volcanic fragment n brecciai s d disseminatean s n i d pervasively altered wall rock. Very fine grained felty molybdenite accompanies uraninite and occurs with minor quantities of chalcopyrite e mineraTh . l containin gt beesilveno ns ha r identified.

Summarc CinquantLa d Genesie an yth f eo s Deposit The deposit is a structurally controlled vein system containing complex U + Mo + Ag + Pb + Cu + (Zn) mineralisation. e chertyTh , sulphidi d graphitian c c interflow sediment y welma s l have influenced the localisation of the host structures and created a local reducing environment for the precipitation of mineralisation. These earlier sediments may have contained traces of primary pyrite + sphalerite + chalcopyrite mineralisation . e MaiTh n Zone mineralisatio s beeha n n W emplaceWN a n i d striking structure which intersecte e latth de Aphebian unconformity. Cross-cutting NE and ENE veins were also mineralised durin e maith gn evente mineralisinTh . g fluids were oxidising and characteristically CO„-ric , d werAg an h e, enricheMo , U n i d a lesse . o t Na r d d an extenan b , P As t, Cu Explorationist n thii s s region have observe e preferentth d - ial associatio f uraninito n e vein n greenstonei s s neae basath r l Dubawnt Group unconformit s comparea y o granitt d d granitan e e gneiss and this has been used as an empirical exploration guide e genetith f e o regionc ith e n implicationOn . f thio s s obser- vation could be that the ultimate source of the uranium is the overlying Dubawnt Group volcanics and clastic sediments, which as previously noted are high in uranium and also coincidentally contain U-Mo mineralisation. Basic metavolcanic d interfloan s w sediments appear to be more favourable hosts than granites or

300 granite gneisses and this may signify a redox effect which is not t understoodye e mineralisatioTh . s structurait d an n l setting s inviteha d comparisons c CinquantbetweeLa e th n e deposid an t the Beaverlodge vein-type deposits in Northern Saskatchewan (Tremblay 1972, 1978; Miller, 1980; Mille d LeCheminantan r n i , press).

Exploration Concepts The following exploration concepts have proved useful in prospecting and outlining the deposit: (a) recognition that a late Aphebian unconformity truncates a sequence of Archean metavolcani d metasedimentaran c y rocks ) evidenc(b ; f enrichmeno e t in uranium in the overlying late Aphebian sequence; (c) recognition of dominant structural control; (d) coincidence of VLF conductors with mineralised structures; and (e) some lithological (redox?) control of mineralisation by cherty-sulphide and graphitic interflow sediments. These same units may also have exerte n influenca d e localisatioth n o e f structureso n .

Exploratio d geologan n f epigenetio y c stratabound mineralisation, Baker Lake Basin In 1974-75 Cominc d entereLt o a joind t venture witn Pa h o explort d e easterLt Oceath e l Oi nn Baker Lake Basin (Laporte et al., 1978). Radiometrie anomalies detecte y airbornb d e surveys were followe p witu d h regiona d detailean l d geological mapping, prospecting, geochemical surveys, a variety of geophysical survey d diamonan s d drilling. Uranium mineralisatio n Kazai n n Formation arkos d Soutan e h Channel conglomerat s beeha en describe n detaii d y Stantob l n (1979) and Miller (1980). Mineralisation comprising disseminated uraninite, coffinite, U-Ti-Si compounds, digenite, covellite, chalcopyrite, bornite, native coppe d nativan r e silver occurs within lamina d cross-beddean r d arkose, arkosic siltstone and conglomerate peripheral to crosscutting xenolithic lamprophyre dykes. The outer portion of the thermally altered sediments commonly grey, mottled pink to cherry red contain the highest grades of uranium-copper-silver mineralisation: uranium, to 0.31%, copper to 3.5%, silver to 15 ppm and anomalous concentrations of ,V, Mo, and Ba (Miller, 1980; Stanton, 1979). However the epigenetic cross-cutting mineralised zones are small, irregular, discontinuous along strike and confined to arkose and

301 conglomerate beds having higher porosity and permeability. Alteration assemblages include albite, hematite, anatase, carbonate mineral d quartzan s . Miller (1980) consideree th d mineralisation process to be a two-stage event. Metallic, sulphide influx and precipitation occurred due to convective flow f groundwateo r accompanying dyke emplacemen d thermah an E t d an l gradients in the outer portion of the thermal aureole. Later, uraniu s precipitatewa m d from oxygenated groundwate n zonei r f o s pre-existing sulphides. Stanton (1979) considered mineralisation o occut r entirely during dyke emplacemen d resultean t d from redox reactions between the lamprophyre dyke and heated metal-bearing groundwater.

Exploration concepts The following concepts proved useful in prospecting for epigenetic mineralisation within redbeds: a) distinctive colour changes; b) localisation of mineralisation to specific porous horizons in the arkosic sequence; and c) proximity of alkaline intrusive bodies.

C) Helikian Thelon Basin The Thelon Formation as outlined by Wright (1967) and Donaldson (1969), is largely preserved in an arcuate northeast trending basin which lies principally to the west and northwest e earlth yf o Aphebian belt d latan s e Aphebian basins (Figur. 1) e Thelon sediments lie unconformably upon a diverse basement comple f Archeao x n tonaliti o granitit c c gneisses, Aphebian metasediments of the Amer group and interpreted equivalents and late Aphebian continental sediment d volcanic an se Dubawn th f o st Group. Basement lithologies at and adjacent to the perimeter of the Thelon Basin display intense hematization and argillization and record a period of lateritic weathering prior to and during initial Thelon sedimentation (LeCheminant et al., 1983). Depth of weathering is variable based on lithology but feldspathic metasediments and granitoid gneiss can be weathered to depths greater than 50 m. Sedimentolog e Theloth f no y Basi s beeha n n studiey b d Donaldson (1969) and Cécile (1973) . Cécile (1973) subdivided the basin on the basis of various physical and sedimentologic parameters into four 'faciès' that broadly mirror the preserved

302 form of the basin. The basal 'faciès1 of the Thelon Formation is commonly conglomerati d interstratifiean c d with pebbly arkose and siltstone. Basal coarse to medium grained siliciclastics can be cemented by authigenic clay + quartz or syngenetic to early diagenetic uraniferous fluorapatite, goyazite and illite. Uraniferous phosphates have been found throughou n extensiva t e stratigraphie interval in the Thelon Basin, from the sub-Thelon saprolit o mort e e tha0 metre30 n s abov e basath e l unconformity (Westmin Resources private reports). A minimum age for the Thelon Formation of 1660 Ma (Pb/Pb, fluorapatite) was determined from a phosphate-cemented pebbly arkose overlying the weathered basement complex (Miller, 1983). Ninety percene basith s i nf o t characterised by mature sandstone, quartz arenite and lithic subarenite which are interpreted as continental to nearshore marine in origin (Donaldson, 1969). This thick sandstone sequence correspond e uppeth o r t s three "faciès f Cécilo 1 e (1973). Multicoloured siltstone-sandstone intercalate with and increase in e sandstonabundancth f o p ee to sequence neae th r . These fine elastic e overlaiar s y shallob n w marine stromatolitid an c oolitic dolostone-siliceous dolostone. Basalt is spatially associated wite uppeth h r most lithologies, howevers it , stratigraphie relationship is uncertain.

Exploratio d geologLone an nth e f Gulo y l deposit In 1973, Urangesellschaft Canada Ltd began a grassroots uranium exploration prograe Centrath n i ml Distric f Keewatio t n for unconformity-related mineralisation based on the available 1:1,000,000 map of Wright (1967) and on then current concepts pertainin o thit g s deposit type. Geochemical lake wated an r lake sediment surveys and a helicopter-mounted airborne system were undertaken on permits obtained in 1974 (Bundrock, 1981). A significant airborne uranium anomal s locatewa y n highli d y glaciate d drifan d t covered terrain neae erosionath r l edgf o e the southeastern Thelon Basin, south of Schultz Lake. Ground followup detected a uraniferous mudboil. In addition the drainage geochemical survey disclosed anomalous radon in a lake, e deposit m soutth k f 3 o hn intensifie a . o t Thid le s d exploration progra n 197 i md 197 5an 6 that incorporated prospecting, detailed mapping, geomorphology surveys, soil geochemistry, scintillometer, magnetometer P surveysI d an .F VL , Exploration diamond drilling, initiate n 1977i d , intersecte e grador d e

303 mineralisation of 1.0% U 0 over 33 m (Northern Miner, 1978). 30 0o Since then the deposit, called Lone Gull, has undergone extensive exploration drilling to 1983. Exploration has been focused on targets that display multiple coincidence f VLFo s , gravity, resistivity, radiometric and EM-16 anomalies in altered metasediments. The Lone Gull deposits contain 17 million kg U^Og e presenath t t stag f exploratioo e n (Hilge t al.e r , 1982). e LonTh e Gull deposi s hostei t y clastib d c metasedimentary e rocksinformallth f o , y called Judge Sissions Lake beltd an , unmetamorphosed fluorite-bearing granite which is intrusive into this metasedimentary sequence (Mille d LeCheminantan r n press)i , . In the immediate area, shallowly north dipping metasedimentary e subdivideb rock n ca s d int a loweo r group characterisey b d immature clastic sediments and an overlying group comprising supermature orthoquartzite that resembles orthoquartzite th f o e Amer group. The lower immature metasedimentary sequence is dominated by drab green to green-black sulphide-bearing biotite + muscovite feldspathic wacke, lithic feldspathic wacke and epidote-bearing equivalents. Pelite and lean oxide faciès iron formation intercalate with the psammitic units and increase proportionally eastward through the deposit area. Metamorphic assemblages indicate upper greenschist-lower amphibolite regional metamorphism followed by retrogression to lower greenschist faciès. Phlogopite-phyric lamprophyres, syenitic dykes, and non-foliated, medium grained, sulphide- and fluorite-bearing subporphyritic to porphyritic granites intrude this metasedimentary sequence. Difference e structurath n i se th ln i styl d an e lithological assemblag e immaturth f o e e metasedimentary sequence compared to the Amer group to the north suggest that these metasediment e oldear s r thae Ameth nr group d coul an ,e Archean b d . Basal silicified conglomerat d sandston an ee Helikia th f o e n Thelon Formation rest unconformably on weathered orthoquartzite, approximately 2 km north of the mineralised zone. In this area hematized orthoquartzite reflects pre-Thelon paleo-weathering. Northwest trending Mackenzie diabase dykes intrude all of the above lithologies. Potential uranium ore, occurrin a serie s a gf thre o s e pods having a combined strike length of 1900 m (Fuchs et al., in press) s controllei , a weln pari dy lb t defined east-northeast

304 j* , "l Fluorite granite p^<:l Aphebian metasediments muscovite-biotite teldspathic wacke chlontoid-garnet metapehte lean oxide iron formation | | Uranium ore zone [fftj3y| Overburden 100 ..••'* Interpretative alteration boundary

Fault

250m 100 Fi. Interpretativ3 g e drill section througe th h Lone Gull uranium deposit (from Miller and LeCheminant, in press) trending steeply north-dipping normal faul d psammitian t c lithologies that are highly susceptible to alteration. Alteration, present as an envelope about the major structure, is characterise a pervasiv y b d e d illiteFe-chloritean - ,Mg , minor smectite group clays, hematit d limonitan e e (Milled an r LeCheminant, in press; Figure 3). Mineralisation is most commonly present as fractures cross-cutting altered metasedimentary layering and granite and disseminations through intensely clay altered rocks. Multi-generation uraninit d coffinitan e e ar e associated with minor and variable galena, molybdenite, pyrite, chalcopyrite, clausthalit d nickelan e , bismut d bismuth-silvean h r tellurides. Anomalous contents of boron and vanadium are associated with mineralisation.

Summary of Exploration concepts The Lone Gull deposit exhibits several features in common with unconformity-related deposit Northern i s n Saskatchewan (Tremblay, 1978; Hoeve, 1978) e absencexcepth r f o fo et graphitic metasedimentary host rocks (Miller and LeCheminant, in press). Thus the following concepts have proved useful in delineating and exploring the Lone Gull deposit: i) position neae erosionath r l Helikiaa edg f o e n sandstone basin) ii ; spatial associatio f mineralisatioo n n with reactivated major structures; iii) a comprehensive geophysical program to

305 delineate the response of each system and coincidence of various methods;

Southwestern Amer group The southwestern portion of the Amer group and overlying Thelon Formation sediments have been the focus of continued uranium exploration utilizing unconformity-related models because this region possesses many features similae southeasterth o t r n Athabasca Basin and East Alligator River uranium field, Australia, This e Theloregioth f no n Basi s beeha nn extensively faultey b d NE trending structures as the Amer mylonite zone and parallel faults (Geol. Surv. Can. Maps 1566A, 1567A) and NW trending faults. Kirchne . (1980al t e )r suggested that lower Proterozoic pelites syngenetically enriche n uraniun i importanda e ar m t componen e generatioth n i t f unconformity-relateo n d depositn i s the Athabasca region. Stratiform mineralisation in upper Amer group pelites-psammites may represent a similar favourable situation. Carbonaceous/graphitic mudstone and siltstone in this same sequence may represent reduced strata that could react with oxidised uraniferous solutions. Numerous fracture- controlled occurrences wit a uraninite-illith e signature have been found in this region and these suggest similarities to the unconformity-related environment present in Northern Saskatchewan.

CONCLUSION Numerous factors interact to determine area selection by e exploratioth n geologist, howeve e Baker-Theloth r n region represents a target selection based almost exclusively on geological criteria. The region is remote, did not have the benefi f earlo t y prospecting discoveries d havdi a ereliablt bu , e geological bas f map o ed report an s y officerb se th f o s Geological Survey of Canada (Wright, 1967; Donaldson, 1965, 1966) e importancTh . f havino e n establishea g d geological framework cannot be overstated, and coupled with interpretative papers such as those by Donaldson (1967) and Fraser et al (1970), provided the analogical link to the Athabasca Basin in Saskatchewan and the East Alligator River area in northwestern Australia e discover Th e Rabbi.th f to y Lake, Cluff Lakd an e y LakKe e deposit n Saskatchewani s e Jabilukath d an , , Koongarra and Ranger deposits in Australia allowed geologists to focus in n areao s where higher metamorphic rank Lower Proterozoic supra-

306 crustal rocks are overlain with profound unconformity by undisturbed Middle Proterozoic fluvial quartz arenite units, with that unconformity surface marke y intensb d e regolithic alteration. That relationship e Baker-Theloexistth n i s n region, witnessed by the relative distributions of the Thelon Formation overlying the Amer Group and the informally named Judge Sissons Lake belt supracrusta le Shultrockth n i zs Lake area. The discover unconformity-relatede th f o y Lone Gull deposits confir e validitth m f exploratioo y y geologicab n l analogy within this geological setting. Notwithstanding the fact that an unconformity-type deposit occurs where it was predicted to be (and deposits of this type, with their higher grades and large tonnages represent t.he prime exploration target )othe, r aspectf o s the Baker-Thelon regio d strengtad n o thit h s area being selected as a prime exploration target. 1. The presence of moderate to high metamorphic rank "basement rocks f Archeao " d Lowean n r Proterozoic ages, with preserved belt f Loweo s r Proterozoic supracrustal sequences containing stratabound uraniferous horizons provide a "protore" situation for subsequent upgradin e Lower-Middlth t a g e Proterozoic unconformity. 2. The presence of numerous vein-type occurrences in adjacent basement terranes and regional relationships to those in Saskatchewan wite uraniuth h m e Beaverlodgveinth n i s e area immediately adjacen e lucrativth o t t e Athabasca Basin camp (Miller, 1980) suggest widespread uranium mobility. e presencTh . 3 f uranium-enricheo e d subaerial volcanic rocks such as the Christopher Island and Pitz formations, or the uraniferous fluorite-bearing granite e Nueltin-typeth f o s , which during lithification and subsequent weathering provided a source f labilo e uraniu r subsequenfo m t remobilizatio- re d an n deposition. 4. The multiple episodes of weathering and saprolite development recorde e Dubawnth n i dt Group stratigraphie recordd an , the flat-lying nature of this rock package, provides larger, areally extensive tract f prospectivso e ground.

307 REFERENCES

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309 HILGER , STUART,W. , PROSSER FUCHSd ,M. an , 1982. H. ,E , : Exploration of the Lone Gull property, N.W.T.; CIM Bull., 7J3, #839, p!30. HOEVE , 1978,J. : Classificatio f uraniuo n m deposit northern i s n Saskatchewan; in MAC Short Course in Uranium Deposits: Their Mineralogy and Origin, edited by M.M. Kimberley, Min. Assoc. Can. _3' 397-402.

KIRCHNER , LEHNART-THIEL,G. STRANDd RICK, an K. , . ,J , J.G., 1980: The Key Lake Ni-U deposits: A model for Lower Proterozoic uranium deposition. Uranium in the Pine Creek Geosyncline, Proceeding e Internationath f o s l Uranium Symposiue th n o m Pine Creek Geosyncline; International Atomic Energy Agency, Vienna, 1980, 617-630. KNOX, A.W., 1980 e uraniu:Th m mineralisatio e Ameth r f Grouo n p (Aphebian); Regional Geology, Geochemistr d Genesisan y , 66G/1, 66G/2, GGH/5, 66H/6. District of Keewatin, N.W.T.; unpub. M.Sc. Thesis, University of Calgary, 159 pages.

LAPORTE, P.J., GIBBINS, W.A., HURDLE, E.J., LORD , PADGHAM,C. , W.A. d SEATOÏMan , , J.B., 1978: Northwest Territories, Mineral Industry Report, 1975 Canada Indian Affaird an s Northern Development S 1978-5EG , . LAPORTE, P.J., 1974: Mineral Industry Report 196 d 1979an 0 Northwes, 3 f volo 2 . t Territories, eas f 104°o t ; Indian and Northern Affairs EGS 1974-1.

LeCHEMINANT, A.N., ASHTON, K.E., CHIARENZELLI, J., DONALDSON, J.A., BEST, M.A., THOMPSONTELLAd an . ,S , D.L., 1983: Geology of Aberdeen Lake map area, District of Keewatin: Preliminary report Currenn i_ ; t Research, Par , GeologicaA t l Survey of Canada, Paper 83-1A, 437-448.

LeCHEMINANT, A.N., IANNELLI, T.R., ZAITLIN, B., and MILLER, A.R., 1981: GeologTebesjuae th f o y k Lakp areama e , Districf o t Keewatin A progres: s report n Curreni : t Research, Par, B t Geological Survey of Canada, Paper 81-1B, 113-128.

LeCHEMINANT, A.N., LAMBERT, M.B., MILLER, A.R., and BOOTH, G.W., 1979a: Geological studies: Tebesjuak Lakp areama e , District of Keewatin; jin Current Research, Part A, Geolog- ical Surve f Canadao y , Paper 79-1A, 179-186.

310 LeCHEMINANT, A.N., LEATHERBARROW, R.W., and MILLER, A.R., 19795: Thirty Mile Lake (65P ) map-areaW^ , , Distric f Keewatino t , in Current Research, Par , GeologicaB t l Surve f Canadao y , Paper 79-1B, 319-327.

LeCHEMINANT, A.N., BLAKE, D.H., LEATHERBARROW, R.W.d ,an deBIE , 1977,L. : Geological studies: Thirty Mile Lakd an e MacQuoid Lake map-areas, District of Keewatin; Geological Survey of Canada, Report of Activities, Part A, Paper 77-1A, 205-208. LeCHEMINANT, A.N., HEWS, P.C., LANE, L.S. and WOLFF, J.M., 1976: Macquoid Lake (55M West Half) and Thirty Mile Lake (65P East Half) map-areas, Distric f Keewatino t ; Geological Surve f Canadao y , Repor f Activitieso t , Par , PapeA t r 76-1A, 383-386. MILLER, A.R., 1980: Uranium Geologe Easterth f o yn Baker Lake Basin, District of Keewatin, Northwest Territories; Geological Survey of Canada, Bulletin 330, 63 pages. MILLER, A.R. d LeCHEMINANT,an , A.N. ,pressn i : Geologd an y uranium metallogeny of Proterozoic supercrustal successions, Central Distric f Keewatino t , N.W.T. with comparisono t s Northern Saskatchewa CIMn i n M Special Volum n Uraniuo e m geology of Northern Saskatchewan.

MILLER, A.R., STANTON, R.A., CLUFF, G.R. and MALE, M.J., in press: Uranium deposit d prospectan s e Baketh rf o s Lake Basid an n subbasins, Central District of Keewatin, Northwest Territories CIMn i , M Special Volum n Uraniuo e m Depositf o s Canada. NORTHERN MINER, 1978: Urangesellschaft "U" find in Baker Lake area, N.W.T. Northern Miner, Sept 21, 1978, 1. NORTHERN MINER, 1982: Aber for n rapii d d expansion with acquisit- iof Marathono n , Northern Miner. 1 ,, Vol32 , .68 PATCHETT, P.J., BYLUND, G., and UPTON, B.G.J., 1978: Paleomag- e Grenvillnetisth d an m e Orogeny w Rb-SNe : r ages from dolerite n Canadi s d Greenlandan a . Eart d Planetaran h y Science Letters, Vol. 40, 349-364.

PATTERSON, J.G., 1981: Amer Lake n AphebiaA : n fold thrusan d t complex; unpub. M.Sc. Thesis, University of Calgary, 106 pages.

311 SCHAU, MIKKEL, TREMBLAY CHRISTOPHERd an , F. , , 1982,A. : Geology 'of Baker Lake map area, District of Keewatin: a progress report: in Current Research, Part A, Geological Survey of Canada, Paper 82-1A, 143-150.

STANTON, R.A., 1979: Genesis of Uranium-Copper Occurrence 74-1E, Baker Lake, Northwest Territories; unpublished M.Sc. Thesis, University of Western Ontario, 167 pages. TAYLOR, M.A., 1978 n :investigatioA e uraniuth f o nm mineralisation at the Sandhills Showing, District of Keewatin, North- west Territories; unpubl. B.Sc. Thesis, Queen's University, 51 pages.

TELLA, S., ASHTON, K.E., THOMPSON, O.K., and MILLER, A.R., 1983: Geologe Deeth p f Roso y e map-area, Distric f Keewatino t , N.W.T n Curreni . t Research, Par , GeologicaA t l Survef o y Canada, Paper 83-1A, 403-409. d HEYWOODTELLAan , S. , , W.W., 1978 e structura:Th l historf o y e Ameth r Mylonite Zone, Churchill Structural Province, District of Keewatin; Current Research, Part C, Geological Survey of Canada, Paper 78-1C, 79-88.

TIPPETT, C.R ., and HEYWOOD, W.W., 1978: Stratigraphy and structure of the northern Amer group (Aphebian), Churchill Structural Province, District of Keewatin; Geological Survey of Canada, Current Research, Part B, Paper 78-1B, 7-11. TREMBLAY, L.P., 1972: GeologBeaverlodge th f o y e Mining area, Saskatchewan; Geological Surve f Canadao y , Memoir 367, 265 pages.

TREMBLAY, L.P., 1978: Geologic setting of the Beaverlodge-type f vein-uraniuo m s comparisodeposiit d e aa t th o that nf o t unconformity-type; in MAC Short Course in uranium Deposits; Their Mineralog d Origian y n editey b d M.M. Kimberley . AssocMm , . Can 431-456, 3 . . WRIGHT, G.M., 1967: Geologe southeasterth f o y n barren grounds, e Districtpartth f o s f Mackenzio s d Keewatian e n (Operat- ions Keewatin, Baker, Thelon); Geological Survef o y Canada, Memoir 350, 91 pages.

312 PRETO RI E O URANIUTH M OCCURRENCES, GOIAS, BRAZIL

FIGUEIREDE D P.M. O FILHO NUCLEBRÄ S- SUPPM , Janeiroe d o Ri , Brazil

Abstract

The pre-Arai ( 1 1,200 Ma ) discordance is interpreted as a surface from which uraniu s leachewa m d from older (3,200 -2,50) 0Ma granites and gneisses, and transported by surface and ground waters. Prio o theit r r metamorphism e graphitith , c schiste Ticunzath f o s l Formation possessed the oxi-reduction conditions necessary to fix the uranium available at that time. The Basal Complex consists of Archaean and lower Proterozoic rp_ cks ( 3,200 - 2,500 Ma ) including gneissic granites and orth£ gneisses. The Ticunzal Formation ( 2,000 Ma ) which overlies the Basal Complex consists of paragneisses and biotite-muscovite graph_i te schists. The Basal complex and the Ticunzal Formation are in contact with granitic bodies, and together these underlie discordaii e metasedimenttlth y e Ara d Bambuth an if o s i groups whic e likear h_ wise separate a discordance y b d e Arad BambuTh an .i i f groupo e ar s Upper Proterozoic age and are sterile with respect to uranium.

INTRODUCTION Aerogammaspectrometric surveys carried out in 1983 over appr£ ximately 46,000 km2 of the northeastern part of the State of Goias revealed hundreds of radiometric anomalies in basement rocks. The morpholog e regioth s f characterizei o ny a dissecte y b d d plateau and ranges of Proterozoic quartzites and metaconglomerates attribu^ e Arath iteo t Groupd . These anomalies were evaluated during geolo^ gical reconnaissance, regional and local geological mapping, geophy_ sical survey d geologicaan s l drilling A numbe. f o ruraniu m occur_ rences in Proterozoic metamorphic and metasomatic rocks were found, the ages of which vary from 1,700 to 2,200 Ma. During the Uruassuan (1,400 - 1,000 Ma) and Brazilian (1,000 - 500 Ma ) cycles the rocks were rejuvenated isotopically. Various type f uraniuo s m mineralizatio veinn i n s have been ideii tified. These include the classical types of occurrence such as th£ se associated with pegmatites, with rocks attribute hydrothermao t d l

313 origin and with tectonic structures, along with mineralization types associated with unconformities suc s thosa h e definee classth n i d^ fication of Me Millan (1978) for Canadian deposits and by Ferguson and Rowntree (1980) for the Alligator River uranium occurrences. This paper summarizes the regional geological panorama of the Campos Belos-Cavalcante-Rio Pret oe Statareath f o f eo sGoia o c s vering some stateth e d e f comment10,00an o ,th smal 2 n km 0lo s uranium deposits around Rio Preto which are interpreted as being of the "unconformity" type.

GEOLOGICAL SETTING Geomorphology The regional uplift which initiated thepresent erosional cycle of the basins of the rivers Parana and Preto in the northeastern e Statth parf Goiâo f eo t s began afte e Loweth r r Cretaceous. Accor_ din o Brunt gd Schobbenhauan i s Filho (1976), thie sb surfac n ca e equatee Gondwanth o t d a Surfac f Kino e g (1956) e remnantth f ,o s which include the Chapada dos Veadeiros and the Serra Gérai do Para^ na. This regional uplift has a NE-SW trending axis with outcrops of the Archaea d Lowean n r Proterozoic basement preserve a n numbei d r of inliers at elevations of between 400 and 500 m, surrounded by the Upper Proterozoic quartzite uplands of the Serra do Santana which attain altitudes of 1,000 m to the north and 1,700 m to the south. e mosTh t significan f theso t e inlier e thosar s Campof eo s Belo_s_ Cavalcante and Rio Preto (Figure 1). The former is drained by the Rio Parana while the latter through the Rio Preto system drains iti to the Rio Maranhäo to the northwest of the area before it receives the waters of the Rio Parana. The orientation of the inliers shows a distinct northeasterly trend, the eastern border being controlled by large scale thrust faults and the northwestern one by broad synclines and anticlines with axes trendin e morth g e n N-SI erode. d westerlm yCa area f o s s Belospo e Paranth , a River flows over older rocks, leaving isola_ ted relicts of quartzitic rocks. Withi e erodeth n d part f boto e sCampo th h s Belos-Cavalcant_ in e lier as well as that of Rio Preto, there occur four types of geomor_ phological structures: (1) alignments of crests of the older faults, whether fault scarps or fault-line scarps with elevations between 500 and 600 m; (2) elevated and rounded forms composed of granjL^ toids attaining heights of 800 m; (3) hogbacks of quartzites which protect the relict hills attaining altitudes of about 700m; (4) the

314 LOCATION I N SOUTH AMERICA

I3°20'

HW 47«00'

- REGIONA 1 Fig. L GEOLOGE TH F O Y RIO PRETO-CAVALCANTE-CAMPOS BELOS AREAS STATE OF GOIAS, BRAZIL

315 Tertiary surface composed of detritus and latérite mapped by Sousa . (1980)eal t .

Regional Geology e principaTh l element e Precambriath f o s n geolog d strat_ian y _ graphy of Goiâs were defined by Barbosa et al. (1969). The regional mapping of the Cavalcante-Rio Preto area of Goias by Sousa et al. (1980) was based on this work and the stratigraphy was accordingly adapted. The mapping of Sousa et al. (1980) has served as a base for more recent studie d specificallan s y those relate o t uraniud m prospectio d explorationan n . The oldest rock unit in the area is the Basal Complex which contains rocks of Archaean and Lower Proterozoic age. Sousa et al. (1980) included within thi coarse th s e grained granite gneisses which are gre n colouri y , with K-feldspar porphyroblast s wela ss orthoa l _ gneis f granodiorite-tonalito s e composition, (typically coarse gra^ d neleucocractian d c wit a hslightl y conspicuous foliation). Ages of this complex very from 3,20 o 2,50t 0 a (Rei0M s Neto : AndradIn , e et al. 1981). Subordinately, there occur nuclei of migmatites and vein f pegmatiteo s . Accordin o Figueiredt g d Oesterlean o n (1981, ) the uranium mineralization which e wesoccur f th Campoo to t s s Belos is associated wit he Basa rockth f l o s Complex. Of the principal groups of rocks comprising the Basal Complex Marini et al. (1978) and Sousa et al. (1980) drew attention to anew uni f metasedimentaro t y rocks whic s name e wa hTicunza th d l Format^ , divisablon e into memberstw o e loweTh . r membe s composei r finf o d e grained biotite paragneiss which is light grey in colour, garnetif£ rous, finely foliated, generally cataclasti d intercalatean c d with biotite-tnuscovite schist ( which can be graphitic ). Amphibolites also occu s wela rs pegmatita l e vein d basian s c dykes. Minerals such as chlorite, garnet and tourmaline usually occur. The most common assessory minerals are zircon, rutile, apatite, goethite and opaques. The upper member, separated from the underlying unit by a gradational contact, is composed of muscovite-biotite schist, gen_e rally graphitic; schist containing garnet and tourmaline and usually quartzose, passin o quartz-schist g t vertically. Secondary minerals include epidote, chlorit d sericitan e e whil e morth e e common asses^ series include zircon, apatite and opaques. Also present in this member are veins of pegmatite and quartz. This Ticunzal Formation e uraniu e th hos th if so t m mineralizatio o PretRi e o th region n i n . This formatio s datei n t aboua d t 2,00 a (Rei0M s Neto n Andradi , t e e al., 1981).

316 Normally, two schistositles are clearly visible. S, is marked mainl y lenseb y f quartzo s , sericite/muscovit d biotitan e e derived, probably, by metamorphic segregation. This surface may coincide with the stratification plane of the original rock (So), an hypo thesis which is reinforced by the tectonics of isoclinal folds which gave rise to this S,. ~ surfacS e Th e indicate e mosth s t recent foliation planen O . many occasions, various stages of crenulation development can be observed with reorientation of the micaceous minerals and graphite e extentth o t that n somi , e places, ther s occurreha e totaa d l tran_s positio e oldeth f ro n shistosity (S,) n manI . y s casei ~ defiS s ^ nen thii d n sectio y narrob n w zone f opaquo s e minerals. The rocks of the Ticunzal Formation have been metamorphosed to biotite faciès. Indicator f partiao s l retrograde metamorphise ar m common. These include: chloritization of the biotite, alteration of the garnet to biotite and chlorite and sericitization of the plagioclase. This retrograde metamorphism is attributed to generalized cataclji sis which affected the whole area. The rocks of the Basal Complex and the TicunzalFormation are cut by bodies of granitic composition which Sousa etal. (1980) referred s "acita o d intrusives". These include bodies which have been intru_ ded only into the Basal Complex as well as younger bodies which cut the Ticunzal Formation e oldeTh . r intrusive e separateb n ca s d into those which contain tin mineralization and those which do not. e rockTh s associate n mineralizatiodti wite th h e restriar n £ teo certait d n bodies suc s thosa h f Sucurio e , Mangabeira, Fedra Branc d Riachan a s Cavalosdo o . Thes e lighar e t gre n colouri y ^ fi , o mediut e n m graine d composean d f quartzo d , feldspa d biotitean r . The rocks are of porphyroblastic texture and contain porphyroblast s f bluo e quart d euhedraan z s wela ls subhedraa l l f crystalo s feldspar. Hydrothermal pneumatolytic alteration phenomene ar a observed e rockTh . se for occuth f greiseno mn i r s mineralized with cassiterite and to a lesser extent with tantalite. According to Urdininea (1977), there is likely to exist a relationship between d tantalitan n ti e e th mineralizatio e golth d d whican n h occurs throughout the area. Attention was drawn to geochemical samples with anomalous uranium values suggesting a probable association of this element and its remobilization with the same mineralizing phenomenon. The granitic bodies which do not contain tin mineralization include the Teresina Granophyre, the Caldas Granite, the Morro da Mangabeira, Serre Serrs Mende th o Mocambodo a d a d an s . Accordino t g

317 Reis Neto ( In: Andrade et al., 1981 ), some radiometric datings suggest an age of 2,500 Ma for the Teresina Granophyre. Urdininea (1977), found anomalous uranium value n streai s m sediments derived from these areas. e youngeTh r "acid intrusives e thosar " e Ticunzae th whic t cu hl Formatio d havan n a egranitic-granodioriti c composition. According to Andrad. (1981)al t e emplacemene e ,th f theso t e bodie s relwa s £ ted to diapiric processes. They have been attributed an age of Reiy 1,60b sa Andrad: M 0Net In ( oal.t e e , 198. ) 1 e Basae rockth Th lf o sComplex e Ticunzath , l Formationd an , e acith d intrusive e separatear s d froe metasedimentth m e th f o s Arai Group by an unconformity representing an erosional phase. The Arai Grou s beeha p n divided into formationstw o e ArraiaTh : s Formji tion and the Trairas Formation. The Arraias Formation consists of fine-grained quartzites, well stratifie d laminatedan d . Fino t e medium-grained quartzite d mediuan s o coarse-grainet m d quartzites with cross bedding and ripple marks and a level of calcareous quartz schist also occur n severaI . l place a basas l conglomerate havina g schistose quartz matrix with pebbles of quartz and quartzite has also been observed. Volcanic rocks of acid to intermediate compos^ tion and represented by rhyolites, dacites and andésites are inter calatee sequenceth n i d , especiall e regioth f n Cavalcanteo i ny . The Trairas Formation consists of quartz-schists with lenses of friable quartzite and beds of metamorphosed siltstone. The Arai Grou s aboui p t 2,24 e younges th 0m thic d an kt date determined from the volcanic rocks is 1,200 Ma ( Sousa et al., 1980 ). The Arai Group constitutes a sedimentary cover overlying the unconformity under which the uranium occurrences are found in the areas of Rio Preto as well as Campos Belos. These sediments are reponsible for the topography of the Chapada dos Veadeiros, Chapada de Ägua Doce e serrath , f Caldaso s , Forquilha, Ticunza d othersan l , as well as for the spurs which occur at the margins of the inliers under consideration. Metasediment e basath f lo s formatio e Bambuth f o ni Group over_ lie unconformabl e Ara e rockth th i y f o sGroup . However e occurth , _ rence of the former is restricted throughout the region under study. Deposits of detritus and latérite of probable Tertiary age occur on the eroded parts of the Basal Complex forming terraces and plateaus of small extension as well as on the tablelands composed of quartzit e Arraiath f o e s Formation. e recenTh t sediment e composear s f beddeo d d o finmediumt - e - grained loose sands which are white in colour. These form deposits

318 SSE

LEGEND

t A OUARTZ - 8IOTITE SCHIST (TICUNZAL FORMATION)

GNEISS

LOG

SO- N70E/80NI S / W 350 J

URANIUM MINERALIZATION

Fig. 2-SCHEMATIC SECTION OF THE MODE OF OCCURRENCE OF URANIUM ORE IN INOO2, RIO PRETO PROJECT, GOIAS, NUCLEBRAS-SUPPM

on the bed and margins of some stretches of the Preto, Claro, das Pedras, das Almas and Parana rivers ( Sousa et al., 1980 ).

GENERAL CHARACTERISTIC E URANIUTH F O S M MINERALIZATION Features of the uranium mineralization in the Rio Preto area have been summarize y Figueiredb d o. (1982Filhal t s followe )oa : s

Lithological Controls e uraniuTh m mineralizatio no Pret occurinRi e o th aren s i gi a restricte e Ticunzath o t d l Formation. This mineralizatio s founi n d most commonl n quartz-muscoviti y ^ biotite schist which may be graphitic. The levels of schists coil tain little biotit e sterilar e e with respec o uraniumt t . Howeve, r this does not mean that the biotite-rich schists necessarily contain uranium. Quartz veins, composed generally of smoky quartz, are found frequently adjacen e mineralizationth o t t . Another typ f uraniuo e m mineralizatio s thani t which occurn i s the contact between a biotite schist of metasomatic origin and a gneiss belonging to the lower member of the Ticunzal Formation.

319 Structural Controls The principal control over the mineralization verified in sur_ face studies is the schistosity plane So/S, (Figure 2) since nearly e radioactivth l al e bands follow this trend. Besides this forf o m occurrence, subsurface studies have revealed that mineralization is present frequentl n fracturesi y . Another important structural e controcontacth s i lt zone between the metasomatic rocks and the gneisses of the Ticunzal For_ mat ion.

Mineralogical Association Biotite, pyrite, chalcopyrite, chlorit d hematitan e e nearly always accompany the uranium mineralization. However, it can not be stated that these mineral e alwayar s s present along with signif^ cant uranium grades.

Grades, Thicknes d Radioactivitan s y The best occurrences in the area of the Rio Preto Project are occurrences 001 and 002. These are small deposits less than 1,000 tonnes of U«0„ each. On the basis of drilling results, the grades

vary from 600 to 2,000 ppm U_0g and less than 100 ppm Th02. The mineralized thicknesse e abou ar se radiometri tTh 0,8 . m 5 c valuef so gamma logs of these two occurrences are generally high with maxima of t 20,00occurrenca d s 40,00t occurrenca an cp 0 s 1 cp 000 e e 002.

MINERALIZATION MODEL e eleveTh n most promising uranium occurrenceo PretRi e o th n i s area are associated with two distinct lithological contexts which fall into the same metamorphic rock domain. These consist of gra^ phitic schists and paragneisses of the Ticunzal Format ion»described by Sousa et al. (1980). In the first case the uranium concentrations are conditioned by the presence of schistose rocks, grey to dark grey in colour in the form of a dyke. These rocks possess sharp contacts and are enclosed in the paragneisses referred to above. In many cases, both rock-types have been affecte y cataclasisb d t depthA . e uraniuth , m mineralizatio n thii n s lithological contex s veri t y irregulad an r discontinous. It occurs exclusively in the form of uraninite as small fracture filling e contacth n i st zone wite gneisth h s country rock. This rock is the product of metasomatic alteration of the country rocks. Recent pétrographi d chemicaan e l studies carriet ou d n surfaco e sample d drilan s l cores have show a biotit ne b thi o t se

320 schist, sometimes containing abundant hornblend d apatitean e . Biotj^ te, presen a majo s a tr constituen s neoformedi t . The second and more usual lithological context is invariably a biotite-muscovite schist, generally graphitic and cataclastic. This schist encases pegmatite injections and veins of quartz which is nearly always of the smoky variety. In this rock-type, the uranium mineralization occurs as uraninite, sometimes in fractures and at other times, distributed alon e schistositth g y planes n botI . h coi\ texts (metasomatic rocks and graphitic schists), autunite and more rarely renardite, torbernite, rutherfordin d sabugalitan e th e ar e secondary uranium minerals which occur at the surface. According to Marini (1978), the controls over the uranium mineralizatio e essentiallar n y stratigraphi e graphitith n i e c schists. This contrast with earlier views developed during the initial stages of evaluation which considered that the uranium minjs ralizatio s controlewa n d structurall y faultsb y . However, this t commene e uraniuorigiauthono th th d f n di o o rnt m althougs it h concentratio s attributewa n o tectonit d c mechanism d foldinan s n i g which graphite played the role of a chemical barrier for mineralized solutions. Rodrigues (1978) proposed an epigenetic origin for the uranium whic y havma h e migrated through intensely cataclased zones in the graphitic schists. As a result of the integration of the various phases of the Rio Preto Project, it is reasonable to consider the hypothesis that the pre-Arai unconformity (1,200 Ma) was a surface over which ura_ niurn leached from the older gneisses and granites (2,500 Ma) was transporte y surfacb d d grounan e d waters e anomalouTh . s geochem^ l uraniuca m values adjacen e oldeth o rt t granite d gneissean s s des_ cribe y Urdinineb d a (1977) suppor e hypothesith t s that these rocks were sourcth e ee uranium rockth f o s . The graphitic schists possessed the physical and chemical cha^ racteristics which permited redox reaction and the fixation of any free uranium available at that time. The Ticunzal Formation is the hos e uraniut th roc f o km mineralization whil e paragneisseth e d an s quartzose schist e mineralizear s d only slightly. Another possibility worth considering involve e contribuitth s ^ on of fluids having hydrothermal characteristics in the phase of uranium concentration. This view takes into account the frequent occurrence of hematite and the argilization of the mineralized sec_ tions having significant uranium grades along with sulphide( s tassociate) e d with uraninite.

321 REFERENCES ANDRADE, S.M., SOUSA, E.L., OLIVEIRA, A.G d SANTOSan . , A.R., 1981: Projeto Rio Preto - Mapeamento Geologico 1:25.000 Relatorio Fina Fasee d l . NUCLEBRÂS-EGOI.P - CompanM y Repor - Restrictedt . BARBOSA, 0., BAPTISTA, M.B., BRAUN, O.P.G., DYER, R.C. andCOTTA.J. , 196C. 9 : Projeto BrasiliaGoias, Restricted Report. PROSPE o Departamentt C o Naciona a Produçad l o Minéral Rio de Janeiro, 225 pp. BRUNI, M.A.L. and SCHOBBENHAUS FILHO, C., 1976 : Carta Geologica do Brasi o Milionésimo-Folha l a , Brasiliexpl23 . SD i - a . MME/DNPMpp 3 16 cativ .- p ema texd an t FERGUSON d ROWNTREEan . J , , J.C., 1980 : Vein-type Uranium Deposits in Proterozoic Rocks - Renue de L'Institut Français da Pétrole. XXXV, (3), 485-496. FIGUEIREDO, A.M. and OESTERLEN, P.M., 1981 : Prospecçâo de urânio no Estad e Goiasd o . Rev. Bras. Geociências, 1 1 . (3), 147-152. FIGUEIREDO FILHO, P.M OLIVEIRA, .de , A.G. LIBERAL, G.S. andABRAHAO, J.R.S., 198 : 2Projet Preto Ri o - Relatorio o Final, NUCLEBRÂS-EGOI.PM. Company Report, Restricted. KING, L.G., 1956 : A Geomorfologia do Brasil Oriental - Revis ta Bras. Geogr. e Janeirod o ,Ri XVIII- . ) (2 , MARINI, O.J., 1978 : Projeto Rio Preto - Relatorio Mensal - Novem bro 1978. NUCLEBRÂS-DRM-SPPM-ERGOI -Company Report, Res trie t ed . MARINI, O.J., LIBERAL, G. da S., REIS, L.T. dos, TRINDADE, C.A.H. , and SOUZA, S.L 197, de . : 8Nov a unidade litoes trat_i_ grafic o Pré-Cambriand a o Estad: d o n I e Goiad o - s "XXX Congresso Brasileir e Geologia"d o , Recife, Resu s Comunicaçoesda o m . 126p , . Me MILLAN, R.H., 1978 : Genetic Aspects & Classification of Impor_ tant Canadian Uranium Deposits. Mineral. Assoca Ç . nada. Shorth Course Handbook, _3_: 187-204. RODRIGUES, C.S., 1978 : Projeto Rio Preto - Relatorio Final de Eta pa I - Poços e Trincheiras - NUCLEBRÂ'S - DRM - SPPM- ERGOI. Company Report, Restricted. SANTOS, A.R d VASQUESan . , R.C., 198 : 0Projet o Cavalcante, SÏntese Geologica -Relatorio Final de Fase - NUCLEBRÂS-EGOI .PM. Company Report, Restricted. SOUSA, E.L., ANDRADE, S.M d OLIVEIRAan . , A.G., 1980 : Projetm Ca o pos Belos/Cavalcante - Mapeamento Geologico 1:100.000

322 Relatorio Final de Fase - NUCLEBRÂS-EGOI.PM - Company Report, Restricted. URDININEA, J.À., 197 7: Projet e Geoquïmicd é a Regional Campo_ Be s los Cavalcant - Goiäse . Relatorio Fina a GEOMITEd l C Geologia e Mineraçâo Trabalhos Tecnicos Ltda. pajra EMPRESAS NUCLEARES BRASILEIRA NUCLEBR&S- A S/ S . 170p, maps and anexes.

323 PROTEROZOIC UNCONFORMITY STRATA-BOUND AN D URANIUM DEPOSITS Epilogue

J. FERGUSON Bureau of Mineral Resources, Canberra City, ACT, Australia

No detailed attempt is made to synthesise the diverse styles of mineralisation found in the Proterozoic rocks which are presented in this collection of papers. It appears to me unlikely tha a commot n thread would linl thesal k e deposits int a singlo e genetic framework. Although a number of these deposits appear to have a similar history of development when viewed in the gross aspect. The question could be asked as to why the Proterozoic was suc a favourablh e tim ee developmen perioth r fo d f uraniuto e or m bodies n particulai , e high-gradth r e ones a numbes wel,a s a lf o r other mineral commodities e sceptiTh .y repl ma c y thae th t Proterozoic covers a large time span amounting to a third of earth's histor d nearlan y geologicae y th hal f o f l recorde Th . Earl Middlo t y e Proterozoic period appear o havt s e been essential to the development of the high-grade unconformity related uranium deposits. Further exploratio s needei n e b d n beforca t i e established whethe e Zambia-Zaireth r , Labrado d Olympian r m Da c stratiform type f uraniuo s m deposits shoul e restricteb d a o t d particular era of the earth's history. The uranium deposits covered by this collection of papers group themselves into two main types, being:

) dominantla y stratiform types b) strataboun d unconformity-relatean d d types

I have chosen to look at these two groupings separately. The notable absenc Franceville papea th f n o eo r e Region, Gabonn i , this treatise is unfortunate and stems from the difficulty in communicating with authors in remote parts of Africa. Fortunately reference can be made to a range of published material on these deposits and this is included in the discussion presented here.

325 STRATIFORM TYPES Regional Setting A number of regional contrasts are offered by the three stratiform provinces, namely, Zambia-Zaire, Aillik Group- Labrador, and Olympic Dam Deposit-South Australia. The Labrador deposits are hosted in Lower Proterozoic rocks whereas the other two occurrences are hosted in Middle Proterozoic rocks. The rocks range from undisturbed and unmetamorphosed as in the Olympic Dam Deposit to highly deformed and metamorphosed as is the case for the Labrador Province. The Zambia-Zaire Province offers an interesting contrast in that the individual deposits range fro n unmetamorphosea m d condition grading througo t h deposits occurrin n lowei g r amphibolite grade. Contrasting rock types host the uranium mineralisation in the three provinces,they span the compositions from felsic volcaniclastics, argillites, arenites, rudite d calcareouan s s rocks. In all three provinces the uranium mineralisation occurs in the basal sections which unconformably overlie high uranium granitoids of the basement complexes. In these provinces there o irelationshin s a majo o t pr unconformity wite overlyinth h g rocks. Dolerite dykes cut the mineralised zones in two of the deposits and additionally lamprophyres are found in the alkaline igneous province of the Aillik Group, Labrador. Pre-Adelaidean lamprophyric rocke Cattl e founth ar st ea d Grid, Cu-deposit, Mt Gunson area some 85 km from the Olympic Dam deposit (J. Knutson pers. comm.). The rocks hosting the uranium mineralisation in the three provinces occu n anorogenii r c environments relate o extensiont d - al tectonics with rifting having been identified in two cases.

Geological setting of deposits In all three situations the initial uranium concentration, tha n somi t e places produced protor d elsewheran e e ore-grade, appears to have been stratiform and syngenetic with some development of epigenetic mineralisation. The epigenetic mineralisatio s usuallwa n y accompanie y upgradinb d f uraniuo g m values. This transgressive epigenetic uranium appeare postth n -i s lithification situation in the Labrador and Zambia-Zaire Province d appearan s o havt s e been developed shortly aftee th r syngenetic mineralisation. The transgressive mineralisation is

326 related to fractures, faults, breccias, joints, shears and mylonites. In all situations the main primary mineralisation w thoriuilo s m uraninite; minor coffinite, branneritd an e soddyit e presenar e n somi t e situations e Zambia-Zairth n I . e and Olympic Dam Deposit multi-element mineralisation is present, in particula , rarCu re earth elements, gol + silved - thesr e deposit l characteriseal e ar s y higb d h U/Th ratios. e rol Th f metamorphiso e n concentratini m g uranium appears to have been insignificant as evidenced by the Zambia-Zaire Province where no marked change is discernable within the differ- t metamorphien c grades encountered s suggestei t I . d thaf i t uranium mobilisation occurred it could only be over short distances and may account for the resetting of uraninite ages.

Source, Transpor d Depositioan t n In all three situations the deposits have a close spatial relationship wite basementh h d initiaan t l mineralisatios i n thought to have been syngenetic. It would appear likely that the syngenetic mineralisation, developed in the Lower Katanga e Zambia-ZairSysteth f o m e Province s derive,wa d from uranium enriched basement. A likely source for the uranium is the granitoids whereas the copper probably owes its origin to mafic rocks within the basement complex. The mineralisation associated with both the Labrador and Olympic Dam Deposit appear to be relate o alkalint d e igneous activity whic s identifiei h n i d the Aillik e mineralisatioGrouth y b p n being container o n i d adjacen o alkalint t e felsic volcanogeni e casc f th rockso e n I . the Olympi m DepositDa c , these alkaline igneous rocks havt no e been identified but the high uranium granitoid basement rocks e nearbath t y Cattle Grid copper deposit t Gunso,M n area, contain lamproitic rocks (pers. comm . Knutson)J . . Alteratios ha n playe a significand e mobilisatiot th rol n i e f uraniue o n th t a m Olympic Dam and Aillik Group Deposits being characterised by alkaline metasomatism; K being dominant in the former and Na e Ailliith n k Group Deposits n botI h. situations this alkaline metasomatism is probably related to the alkaline igneous activity wite possibilitth h meteoritia f o y c interaction. e resettinTh f uraniuo g m isotopic value a featur s i s f moso e t uranium deposits, particularly those that have undergone metamorphism, orogenesis, epeirogenesi d alteratioan s n producey b d

327 groundwaters. This hold r Labradofo s e rth e cas andth f o en i , Zambia-Zaire Deposit s pertineni s o thost t e that have undergone metamorphism. s suggesteIi t d thae copperth t ,e iroth d gol an t na d Olympi m DeposiDa c t probably originated fro mafia m c source. The uranium was probably transported in the oxidised state as uranyl complexes as the ore deposits are characterised by low thorium contents. In the Labrador and Zambia-Zaire situation carbonaceous matte d sulphidesan r , whic y havma h e acte s reductantsa d e ar , presenhose th t n i rockst . Evans (1980) also suggests absorption of uranyl complexes onto Fe , Mn and Ti hydroxides with redox reactions causing precipitatio f uraniniteo n e absencTh . e f reductanto e Olympith m t Deposia sDa c t suggests thae th t mechanis f precipitatioo m e oxidiseth f o n d uraniu y havma m e been brought abou y oxidatiob t f ferrouo n o ferrit s c iron. Later circulation by mineralising fluids in these deposits also resulte n transgressivi d e uranium being deposited. Discussion e rol Th f alkalino e e igneous activity seem a likels y mechanism for the development of uranium mineralisation in the d OlympiLabradoan m Da cr t obviouDepositse no th s i n i st bu , Zambia-Zaire Province. Althoug e structurath h l settin f theso g e deposits is very different, they share the aspect of having been develope a continenta n i d l regime during anorogenic periods. e developmenth r Fo f uraniuo t m deposit f thio s s type followth e - ing appear e essentialb o t s : ) 1 Establishmen a uraniu f o t m province eithen enrichea y b r d basement complex or association with U-enriched alkaline igneous activity. 2) Transpor s uranya t l complexes. A suitabl3) e environmen r syngenetifo t c concentration. 4) Mobilisation of syngenetic uranium mineralisation producing some transgressive zones of epigenetic uranium enrichment. 5) Sealing of deposit so that present-day oxidising ground and surface waters do not disperse the uranium by solution and transport.

328 STRATABOUN UNCONFORMITY-RELATED AN D D TYPES Regional Setting In most situation e unconformityth s , when present, separates the Lower Proterozoic froMiddle th m e Proterozoi e casth e s i s a c for the extensive Alligator Rivers Uranium Field and those of the Athabasca Basin Region. Others that share this time break e Beaverlodgarth e e Deposits, Brazilian Deposits, Turee Creed an k Thelon Basin Deposits. Elsewher e unconformitth e y separates the Archaea d Lowean n r Proterozoic r examplefo ; e Latth ,e Aphebian Basin mineralised zones and the Lianshanguan Deposit. Stratabound deposits that appear to have no spatial relationship to unconformities include those of the Dripping Spring Quartzite, Amer Group, and the Franceville Deposits (Diouly-Osso & Chauvet, 1979). In fact the Amer Group Deposits more correctly belong to the stratiform type but because the other two types of deposits in the Central District of Keewatin fall within this category they have been included here. Mineralisation occurs in diverse lithologies which display varying degree f metamorphiso s d alterationan m l situational n I . s the host rocks are either Lower or Middle Proterozoic in age, except for the Late Aphebian Basins, here the mineralisation is presen e Lowe th t onl rn no t i yProterozoi c rock t alsn bu si o metastrat Archaeae th f o a n basement. In the unconformity-related situations, the mineralisation is either restricted to the lower sequences of rocks or, alternatively, presen n rocki t s both abov d belo an e unconform th w - ity. Examples of the first type include the deposits of the Pine Creek Geosyncline, the Brazilian Deposits and the Thelon Basin. Example covee th rf so rock s containing mineralisation include Late Aphebian Basins, Beaverlodge area, Athabasca Basin Regio d Turean n e Creek. Wite exceptio th he stratifor th f o n d stratabounan m d Amer Group mineralisation, the economic grades of uranium occupy prominent dislocation features r examplefo , , fractures, faults, schistosity planes, breccias, shears and mylonitised zones. Metamorphic grade in the hosting rocks is generally greenschist to amphibolite faciès, although notable exceptions include zeolite e Francevillfacièth n i s d Drippinan e g Spring Quartzite Deposits. Additionally, the Amer Group uranium mineralisation

329 is contained in rocks ranging from zeolite through greenschist o lowet r amphibolite grade e uraniuTh . s usualli m y stratabound and in some local situations also has a stratiform disposition. In most of these deposits the mineralisation post-dates the metamorphism and deformation. However, a notable exception is the Lianshanguan Deposit wher e earlth e y syngenetic uranium mineralisatio s beeha n n migmatised. e hosTh t rock o thest s e uranium deposit e dominantlar s y meta-terrigenous rocks that are usually carbonaceous. Other rock types include metavolcanics, granitoids, volcaniclastic rocks, metaregolith and carbonates. Basement granitoids have a close spatial relationship with most of these uranium deposits and furthermore, where present, display anomalously high uranium content a numbe n I f o r. deposits thera spatia s i e l associatio f uraniuo n m mineralisation with dolerite intrusives and alkaline igneous rocks. In all situations the uranium deposits occur in tectonic regimes developed during anorogenic periods produced by extensional tectonics where continental and/or marine sequences dominate. Geological Setting of Deposits In som f theso e e deposits ther s initiawa e l syngenetic concentration of uranium in terrigenous sediments (Franceville, Amer Group, Beaverlodge, Lianshanguan) or volcaniclastic rocks e Drippinth n i gs a Spring Quartzite Deposits. This syngenetic uranium doet producno s e economic grades. However, diagenetic processes have upgraded some of the syngenetic concentrations. Epigenetic processes superposed on post-lithification syngenetic mineralisation is the main factor responsible for upgrading these uranium deposits to economically viable grades. e depositth Mos f e characteriseo t ar s y multiplb d e resettinf o g e uraniuth m mineralisation ages. Generally these mineral resetting ages correspond to periods of metamorphism, igneous events and epeirogenesis. Uraninite is the main primary uranium mineral in all situations, with some deposits containing minor coffinite and brannerite. A feature of these uranium deposits is that uranium can either occur with other economically viablee elementth n i r so mono-element situation. Even withi a singln e provincs i t i e possibl o fint e d both situations. Common associated economic

330 combinatioa f o e followingy th , an element Ni f e o nb , n Cu :ca s . Li d an i B , Sn , U , Zn , Nb , Au , Ag , Co , As , Mo , Pb Within the ore zones metasomatism is usually present and is usually accompanied by the introduction of one or more of the following major elements: Na, K, Mg and Fe. e ganguTh e mineralog n thesi y e uranium deposit widels i s y diverse,chlorit d iroan e n oxide d hydroxidean s s (mainly hematite and limonite) are common to most of these deposits but can also be mutually exclusive within individual deposits. Other relatively common minerals include clays, graphite (and carbonaceous matter), alkali feldspars, chert, quartz, carbonates and tourmaline. Common opaques include galena, pyrite, chalcopyrite, covellite, and sundry nickel sulphides. In the unconformity-related uranium deposits the depths to which mineralisation is found below the unconformity is usually shallow, being less than 300 m. The Beaverlodge Deposits are an exceptio s hera n e minin s takeha g n plac o deptht e f 180o s . 0m Source, Transpor d Depositioan t n Mos f theso t e deposit e juxtaposear s d with crystalline basement rock d wheran s e informatio s availablei n , these rocks are shown to be enriched in uranium and appear in most circum- e primarth stance e b yo t ssourc f uraniuo e me deposits founth n i d . In all occurrences the high U/Th ratios present within the mineralised zones suggest that uranium was transported in the oxidised state. The labile uranium extracted from the granit- d basemenoi t rocks usually result miln i s d syngenetic enrichment e sedimentth f o d regolithan s s derived from these sources. Amee th e casr I th f nGroueo p deposits e syngenetith , c enrichment approaches economic grades e exceptio th t thi s bu ,i s n rather thae ruleth n . Elsewher s beeha nt i enecessar o upgradt y e th e initial syngenetic mineralisation in order to achieve economic grades. In most syngenetic situations uranium appears to have been precipitate y reductiob d e presencth n i nf carbonaceouo e s matter. Diagenetic upgrading of some deposits has also taken place e gradOr .e concentratio f uraniugeneran o ni s ha ml been brought about by epigenetic remobilisation with deposition having taken plac n zonei e f disruptionso w temperaturlo e Th . e epigenetic movemen f uraniuo t m generally appear o havt s e been generated by surface and groundwater flow initiated by epeiro-

331 genesis e higheTh . r temperature epigenetic solutions appear to have been mobilise y metamorphismb d , granite intrusion, migmatisation and other igneous events. In some situations stable isotope data suggest the reductant appears to have been biogenic carbonaceous matter and/or bacteria lw occurr sulphatefe a -n I . s beeenceha nt i ssuggeste d thae absorptioth t f uraniuo n m onto e clayF -Mn-T r o s i hydroxide2+ s couplewa s d with later redox reactions involving these minerals resultin e precipitatioth n i g n f uraniumo . Fluid inclusion and stable isotope data suggest a wide variatio f temperatureo n s recorde e mineralisinth y b d g solution e generath n i sl range 60-30 . 0C Temperature t bota s h these extremes have been recorded within individual deposits. There is a wide variation in uraninite ages within individual deposits, within a province ages can range from Middle Proteroz- oic to Mesozoic.

Discussion It would appear that once a province has been enriched in uranium that all the succeeding sedimentary rocks derived from the basement rock e alsar so uranium enriched, although sometimes only mildl . so yThi s feature also generally appliee th o t s later developmen f igneouo t s rocks withi e provinceth n e Th . labile nature of uranium in the post-2200 Ma period is very evident judging from syngeneti d epigenetian c c upgrading found in most of these Proterozoic uranium deposits. In the syngenetic situation psammitic, pelitic and volcanogenic rocks host the mineralisation. In these situations there is a close spatial relationship with carbonaceous matter e largTh .e compositional variety of rock types that host the epigenetic uranium deposits at first suggest a wide spectrum of depositional controls n thesI .e situations structur e mai th appearn e b o t s control of uranium mineralisation rather than the primary host rock lithologye epigenetith l Al .c mineralisation zones share the featur f beino e g associated witw temperaturlo h e parageneses apparently brought abou y walb t l rock-solution reactione Or . zone e usuallar s y characterise e presencth y b df chloriteso e , clays, iron oxides, graphite and carbonaceous matter. It could be argued that in situations where graphite/carbonate matter is absent that this material was initially present and has sub-

332 sequently been consumed e alternativTh . e suggestio o uraniut n m being precipitated by organic reduction is one of absorption of uranium from solution onto particulate matter with redox reaction involving Fe causing deposition of uraninite. e generallTh y shallow depth o whict s h mineralisatios i n encountered in the unconformity-related deposits has led to the suggestion that these deposits were produce y surface-relateb d d supergene processes. The Beaverlodge Deposits are the exception as mining has proven ore to depths of 1800 m. The fact that the Beaverlodge Deposit e spatiallar s y relate o majot d r faults may account for the great depths of mineralisation encountered. o adopt s a i supergent e Ion f e mode t couli l e argueb d d that this mineralisation could be brought about by downward percolation of uranium solutions via deep tensional fractures related to major faults n alternativA . e interpretation could accoun r thifo t s mineralisatio e produceb o a result n s a df remobilisatio o t f o n syngenetic uranium during metamorphic events. The grade of metamorphism does not always play a significant e locatioth rol n i ef theso n e deposit s hostina s g terranes range from zeolite to amphibolite faciès. Within single geological provinces uranium deposits occu n rocki r s that have undergone metamorphic grades ranging from greenschis o uppet t r amphibolite e Pin th e case th f Creeo e n I k. Geosyncline uranium movement doe t appeae relateb no s o t ro thi t d s metamorphic gradient. It is to be remembered that in a number of deposits the uranium mineralisation post-dates the regional metamorphism being containe n zonei d f dislocationo s n somI e. terranes metamorphism does appear to have played a role in the generation and activatio f solutiono n s thay havma t e been responsiblr fo e the scavengin f uraniuo g m with precipitation taking placn i e traps under epigenetic conditions. Metasomatis n alsca m o result from the solutions associated with those metamorphic processes. l situationIal n e higth s h U/Th ratioe s or recorde e th n i d zones strongly suggest thae uraniuth t s transportewa m e th n i d oxidised state. The multiple ages recorded in the ore mineralisation of these deposits indicates high mobilit f uraniuo y m unde a number r f geologicao l conditions. This feature north ms i erathe r than the exceptio d evaluatioan n e timinth f ore-forminf o go n g events

333 should be treated with great caution. Uraninite dissolution d re-precipitatioan n unde w temperaturlo r e groundwater situations possibly related to epeirogenesis is a common feature. The large range of temperatures deduced from fluid inclusion and stable isotope dat e treateab equallo t ds ha wity h great caution as most of these deposits occupy open systems. It is imperative to identify which gangue minerals are related to uranium mineralisatio o identift d e geologicaan n th y l events e rangTh f .o e temperatures deduced from fluid inclusio d stablan n e isotope data is simply reflecting different periods of activity only some of which might be related to uranium mineralisation. Unfortun- ately stable isotope data is presently limited; where sulphur isotope e availablar s ee suggestiotherth s i e f bacteriao n l sulphate reduction as well as sedimentary source input. In the Alligator Rivers Uranium Field biogenic carbonaceous matter has also been identified. y accoune behaviouAn th f o t f uraniuo r m transporting solutions would also hav o takt e e accoune frequenth f o t t associatio f otheo n r economic elements foun n zonei d f uraniuo s m mineralisation. Where experimental wor s availabli k t wouli e d appear that most, if not all, of these elements can be transport- n oxidisinei d g solutions. Where determined e leas th ,i d radiogenic which explains the correlation of this element with uranium. The role of the unconformity in the development of these deposits is a tantalising one. This enigma is caused mainly by e facth t that uranium mineralisatio s eithei n r confineo t d rocks belo e unconformitth w s founi r do y both abov d beloan e w the unconformity. A few workers have suggested that the cover rocks are the main source of the uranium mineralisation. Other interpretations see the unconformity surface serving as an aquifer along which uranium-rich solutions were transported to produce the ore grades encountered. Yet other workers regard the rocks abov e unconformitth e s havina y g actea passiv s a d e imperviou p shieldinca s g uranium deposits from subsequent weatherin d transportan g . e protectivThos th o argu wh r e fo e e e coveth rol rf o e rocks explai e situatioth n n where thers i e mineralisation within these rock s havina s g resulted from remobilisation from below the unconformity. In this model it

334 is necessary for faulting and brecciation to have taken place in the cover rocks, these zones of dislocation then acted as avenues for solutions which derived their uranium content from the ore zones below the unconformity. It is also necessary e uraniue coveth th x rn i fi mrocks o t e absenc th ,a suitabl f o e e reductant in the Kombolgie cover rocks of the Alligator Rivers Uranium Field may explain why there is chloritisation of the Kombolgie but no associated uranium.

CONCLUDING REMARKS e higTh h solubilit f uraniuo y n oxidisini m g environments appears to be responsible for the diverse styles of mineralisation outline n thii d s collectio f paperso n e b . o t Wha s ha t answere s whethei d r thera rea s i le time dependence th n i y distributio f theso n e Proterozoic uranium deposits n ordeI .r to make this assessmen e uranius wela th o loo t s lt i a k m t i t cycle. Durin e 3800-300th g a periodM 0 e continentath , l crust develope y generatiob d f sodino c plutonism characterisey b d tonaliti d frondhjemitian c c magmas derived fro a peridotitim c upper mantle source (Glikson, 1979; Button, 1979). These Na- ricd uranium-depletean h d sialic rocks occu n closi r e associat- n wite earlio th h y greenstone belts n southerI . n Africae th , potassic-rich volcani d plutonian c c rocks develope t 300, a dMa 0 whereas in most other areas this development took place at 2700 Ma, corresponding to a major global thermal peak. With the advent of the development of K-rich granitoids in the period 3000-270 , uraniumMa 0 ,s incompatibl owinit o t g e behaviours wa , partitioned into these magmas by partial melting and fractionation of an undepleted upper mantle source. Some K-rich granitoids are richer in uranium than others, so it would appear thae uppeth t r mantle source peridotites were probably inhomogeneous leas,at theiin t r uranium content. Wherthe e uranium content of these granitoids exceeded 6 ppm, they represent potential provenance areas for the quartz-pebble conglomerate deposits. In that K-rich granitoids were develop- n earliea t ea d r stag n southeri e n Africa (i.e t 300a . 0 Ma),t i is not surprising that the uraniferous quartz-pebble conglomer- Dominioe th ate f o s n Reed Witwatersranan f d Systems fall into the older time slo f abouto t 2800-260 a (PretoriusM 0 , 1975)

335 whereas the Blind River deposits developed between 2500-2200 Ma (Robertson, 1974). The low oxidation state of the atmosphere t thia s time would have allowed detrital transpor f uraninito t e e reducinith n g environ e bedloath f f o streamso d o product s e the quartz-pebble conglomerate uranium deposits. e developmen Th e post-220 th d beda (Cloudn re er i s f a o t0M , 1976) indicates uranium, readily leached from the enriched provenance rocks, coul e transportee b d th n e uranyi th n s io a ld near-surface oxidizing atmosphere A feature Proterozoi.th f o e c unconformity and stratabound uranium deposits are the large U/Th ratios, whic e generallar h e highy th greate n -i d ran tha0 10 n grade ore zones can exceed 1000. These high ratios support the uranyl transpor ts diadochi i concep h T s a tc witU h The high-grade unconformity-related uranium deposits were develope e post-220th n i d a perioM 0 d being either hosten i d Lower or Middle Proterozoic rocks. It would appear from known discoveries that this time constraint is valid in the production of these deposits. In order to account for this time constraint it could be argued that earlier detrital uraninite was concentrated in pre- 2200 Ma sediments, as is the case for the quartz-pebble conglomerates, and that this syngenetic mineralisation was later mobilised and concentrated by oxidising solutions related to tectonic processes. An alternative hypogene argument would be to suggest that igneous activity in the Lower and Middle Proterozoic was U-enriched. However, some of these U-deposits lack a time and spatial association with igneous eventsa supergen f I . e mode s adoptei l e followinth d g argument could be advanced to account for the skewed time distribution of unconformity-related deposits (Ferguson & Rowntree, 1980; Robertson et al, 1978) . s continentaA l crustal developmen n post-220i t a timeM 0 s appears mainl o follot y w uniformitarian lines e onlth ,y variable which could explain the concentration of vein-type uranium by supergene processes in the 2200-1400 Ma period appears to be a steadily evolving atmosphere s suggestei t I . d that during these times the hydrosphere was sufficiently oxidising for uranyl transport, but that rapidly reducing conditions were met short distances inte lithosphèreth o . Reduction resulte n precipi d -

336 itation of uranium as U02 from meteoric water into suitable structural traps, which were largely developed during periodf o s prolonged erosion. The structural traps may also have been active durin e earlth g y sedimentatio e Middlth f eo n Proterozoic cover rocks. Rapid development of impermeable cover rocks preserved the uranium deposits from subsequent weathering and transport. In the post-1400 Ma period the atmosphere was sufficiently oxidizin o thas g t reductio n groundwatei n r situations was generally inadequate for the precipitation of uranium on any large scale other than in restricted continental sandstone situations. The ultimate "sink" for uranium under these circumstances would have beee oceanth n s s founwheri o t t di e concentrate in sea-water, altered floor basalts, black shales and other pelagic sediments. This broad dissemination of uranium within the marine environment did not allow any extensive uranium deposits to form in the post-1400 Ma period. A similar supergene genetic argument could be advanced for the dominantly stratiform U-deposit o explait s n their distribut- ioLowen i nd Middl an r e Proterozoic rocks A hypogen. e process n als ca e advanceAillie b o th d Olympi r an k fo dm Deposits Da c ; if this genetic mode s correci l ts difficul i the t i n o undert t - stand why uranium associated with alkaline igneous rocks should not in fact occur throughout the geological time scale. The apparent time skewness show r thesfo n e deposit a y onl ma se b y reflectio t undiscovereye f o n d younger occurrences. s cleaIi t r that onc provinca e s beeha e n enrichen i d uranium - whether these are basement rocks or alkaline igneous rocks thal rockal t s post-dating thi se provinceventh n i e t ar e usually anomalously enriche n uraniumi d . This later enrichment results from the highly labile behaviour of U under oxidising conditions whether they be hypogene or supergene. What is required in exploration to locate ore bodies is identifying the structural trap se eithe b whic n rca h syngeneti r epigenetico c .

ACKNOWLEDGEMENT G.R. Ewers kindly reviewed this Chapter.

337 REFERENCES CLOUD , 1976,P. : Major feature f crustao s l evolution. Du Toit Memorial Lecture 14, Geol. Soc. Sth Africa, Annexur o Volumt e . 79 e DIOULY-OSSO CHAUVET& . ,P , R.J., 1979 s gisementLe : s d'uranium de la région de Franceville (Gabon) 123-148. In: Uranium deposit African i s : Geolog & Explorationy . IAEA Panel Proc. Ser. Vienna 1979. EVANS , 1980D. , : Geolog d petrochemistran y e Kittth d an sf o y Michelin uranium deposit d relatean s d prospects, Central Mineral Belt, Labrador. Unpubl. Ph.D. thesis, Queen's Univ, Kingston, Ontario 312p. FERGUSON, J. & ROWNTREE, J.C., 1980: Vein-type uranium deposits in Proterozoic rocks. Revue de'Institut Francais du Pétrole, XXXV, (3), 485-496

GLIKSON, A.Y., 1979: Early Precambrian tonalite-trondhjemite sialic nuclei. Earth Sei. Rev. 15, 1-73.

PRETORIUS, D.A., 1975: The depositional environment of the Witwatersrand goldfields a chronologica: l revief o w speculation d observationsan s . Minerals Sei. Engng, 1_ , 18-47. ROBERTSON, D.S., 1974: Uraniu n quartz-pebbli m e conglomerates: Repor f workino t g groups, IAEA, Vienna, Proc. Series No. 571/PUB/374, 707-711.

ROBERTSON, D.S., TILSLEY, J.E HOGG& . , G.M., 1978e timeTh :- bound characte f uraniuo r m deposits. Econ. Geol., 73 , 1409-1419. SUTTON, J., 1979: Evolution of the continental crust. Phil. Trans . Soc.R . , A.291, 257-266.

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