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PO1CHEFS1ROOM

GEOKONGRES 84

20STE GEOLOGIESE KONGRES VAN DIE 20TH GEOLOGICAL CONGRESS OF THE

GEOLOGIESE VERENIGING GEOLOGICAL SOCIETY

VAN SUID-AFRIKA OF SOUTH AFRICA

POTCHEFSTROOM, 9 — 13 JULIE/JULY, 1984

OPSOMMINGS ABSTRACTS

DEEL 1 ALGEMEEN PART 1 GENERAL AFKORTINGS/ABBREVIATIONS

(GS) Geostatistiek Geostatistics

(M) Mineralogie en Petrologie Mineralogy and Petrology

(0) Oopsittings Open Sessions

(P) Plakkaat Poster

(S) Sedimentologie Sedimentology

(V - D) Ventersdorp- en Dominiumopeenvolgings Ventersdorp and Dominion Successions

* Presidensiele Rede Presidential Address INHOUD/CONTENTS

(P) BARNARDO, D.J. and G.F.J. HORN Bauxite in Natal 1

(M) BEUKES, G.J., A.E. SCHOCH, H. DE BRUIYN, W.A. VAN DER WESTHUIZEN en L.D.C. BOK "n Buitengewone mineraalparagenese in sillima= niethouđenđe gesteer.tes van die Pofadder-distrik. . 4

(O) BEUKES, N.J. and J.P. VAN WYK Thrust faulting along the edge of the Kaapvaal Craton in Griqualand West 8

(O) BRISTOW, J.W. Plinian air-fall activity and rhyolitic dome building in the Bumbeni Complex, Southern Lebombo . 11

(S) CAIRNCROSS, B. Depositional environments and coal seam charac= teristics in the north-eastern Witbank Coalfield.. 14

(P) CAMISANI-CALZOLARI, F.A.G.M., W.J. DE KLERK and P.J. VAN DER MERWE South.' African uranium resource estimates 17

(GS) CARTER, C.J. and R. SCHNEEWEISS Application of geostatistical techniques at the Otjihase Cu-Zn-Ag-S orebody, South West Africa/ Namibia . 19

(S) CHRISTIE, A.D.M. Depositional models for coal seam formation in the Klip River Coalfield, northern Natal 22

(S) COLLISTON, W.P. and J.C. LOOCK The determination of the stratigraphie polarity in Proterozoic metasedimentary rock using sedimentary cycles 26

(0) COLLISTON, W.P., H.E. PRAEKELT, D. STRYDOM, G. VAN ASWEGEN, H.J. BLIGNAULT and A.E. SCHOCH The recognition of low angle thrusts in central Bushmanland, Namaqua Mobile Belt 29 (M) CONRADIE, J.A. en A.E. SCHOCH Petrologiese en petrochemiese verwantskappe van die Koperbergsuite, Namakwaland 33

(M) DIXON, ROGER Sugilite and the associated mineral assemblage from Wessels Mine, Kalahari Manganese Field 36

(0) EBERLE, D. and E.H. STETTLER A quantitative geophysical stratigraphy in the Western Bushveld Complex 40

(M) GERINGER, G.J., H. DE BRUIYN, B.J.V. BOTHA and W.A. VAN DER WESTFUIZEN The geochemistry and petrogenetic relationships of some granites with mafic inclusions in the Keimoes Suite, the Namaqua Mobile Belt - A possible, magma mixing model 4 5

(S) GERRARD, I., K. STALLBOM and P. UNSTEAD Late Aptian to Cenomanian submarine fans, west coast, Republic of South Africa 49 (V - D) GROBLER, N.J. and N.A. BLEEKER A Makwassie-Rietgat complex SW of Warrenton, Northern Cape 52 / (M) HALLBAUER, D.K. Archaean granitic sources for the detrital mineral assemblage in Witwatersrand conglomerates 53

(M) HALLBAUER, D.K. and M. NAMI Gold distribution in Witwatersrand palaeo-placers 57

(S) KARPETA, W.P. The sedimentology and stratigraphie setting of the "A" reef placers, Aandenk .Formation, on President Steyn Gold Mine, Welkom, Orange Free State 61

(M) KLEYENSTÜBER, A.S.E. The mineralogy of the Voëlwater sedimentary rocks in the North-Western Cape, South Africa ... 63

(GS) KRIGE, D.G. The use of geostatistics in defining and re= ducing the uncertainty of grade estimates 66 * KYLE, D.L. Mineral discovery - The philosophy and strategy 6 7

(GS) LEMMER, I.C. Estimating recoverable reserves in a new way ... 72

(0) LOOCK, J.C. Zircons from the granites and gneisses of the Namaqua Mobile Belt and the interpretation of radiometric ages 77

(GS) MAGRI, E.J. and J.S. MOSTERT Some applications of geostatistics in mineral exploration 80

(M) MARSH, JULIAN Geochemistry of Karoo basalts and dolerites in the northeastern Orange Free State : Recognition and origin of two new Karoo basalt magma types . 87

(P) MARSH, JULIAN Karoo basalts of the Springbok Flats, Transvaal: Geochemistry and correlation 91

(S) MASON, T.R. Late Trias^ic Isopodichnus from the Natal Drakensberg 95

(S) MASON, T.R. and A.D.M. CHRISTIE Palaeoenvironmental significance of Ichnogenus Diplocraterion Torell from the Vryheid Forma= tion of Natal 99 (V - D) MCCARTHY, T.S. and A.B. CADLE The geology of the Bezuidenhout Valley graben .. 103

(O) MEYER, R., J.H. DE BEER and S.J. JOUBERT Geoelectrical study of the Bushveld Complex .... 105

(S) MILLER, R. McG., E.E. FREYER and I.W. HaLBICH A turbidite succession equivalent to the complete Swakop Group 108 (M) MIYANO, T. and N.J. BEUKES Mineralogy and petrology of the amphibole asbestos-bearing rocks of the Penge Iron For= mation, Transvaal Supergroup, Penge area 109

(M) MOORE, A.E. ' The post-Gondwanaland alkaline igneous province of Southern Africa 114

(GS) MORRISON, D.L., and S.L. MILLER What can be done with computerized gold data values 118

(P) OBERHOLSTER, R.E. Some examples of the application of geology • and mineralogy to building materials research 122

(P) SAGGERSON, E.P. and L.M. TURNER The metamorphic map of South Africa (1:1 000 000) 125

(M) SCHOCH, A.E., en E. KLATT Die verspreiding van pekels en kooldioksied in vloeistofinsluitsels van die gneise en gemeta= morfiseerde sedimentere gesteentes van die Noodwes-Kaap 128

(M) SMITS, G. The mineralogy on planes of unconformity within the Central Rand Group of the Witwaters= rand 133

_v (M) SOUTHWOOD, M.J. The influence of mineralogical parameters on the leachability of low-grade nickel deposits. 135

(V - D) STANISTREET, I.G. and T.S. MCCARTHY The tectono-sedimentary setting of the Ventersdorp outlier at Kromdraai, north-west of Johannesburg 138

(GS) STEFFENS, F.E. 1 Introduction to geostatistics. 140

(O) STRYDOM, D. Techniques employed during the structural- , stratigraphic mapping of the Proterozoic rocks of Western Bushmanland 142 (M) SWASH, P.M. The role of the mineralogist in the copper- mining industry 145

(P) TERBLANCHE, J.C. "n Fasieanalise van die basale Beaufortgroep in die Noordwes-Orar je-Vrystaat 148

(M) THERON, J.C. and A.E. 3CHOCH Uranium and thorium distribution in the Contreberg granite, Darling 150

(M) TORDIFFE, E.A.W. and A.E. SCHOCH Multi-element interpretation of regional stream sediment geochemical survey results from the Bushmanland region 154

(P) TWEMLOW, S.G. The Archaean gold-telluride-sulphide and gold-telluride mineralisation of a multiple stage hydrothermal vein deposit at the Com= moner Mine, Zimbabwe 156

(O) VAN ASWEGEN, G. The significance of the Gladkop Suite for the "Basement Problem" in the Namaqua Mobile Belt. 159

(M) VAN REENEN, D.D. Characterization of metamorphic fluid compo= sition associated with retrograde metamorphism in the Limpopo Belt in South Africa 163

(GS) VAN TONDER, G.J. Geostatistics applied in geohydrology 166

(S) VISSER, J.N.J. The problem of facies analysis in the Permo-Carboniferous Dwyka Formation 173

(M) VON RAHDEN, H.V.R. & M.J.E. Quantitative X-Ray diffraction: Fact or fantasy? 177

(GS) WOOD, I.D. An application of geostatistics in the Eastern Transvaal Coal Fields 178 ADDENDUM

* WL VAN WYK: Role of the South African Government in the Mining Industry. (S) N STAVRAKIS & N HILLER: A Triassic Arid-Zone Anastomosed Fluvial Deposit from the Karoo Basin. (S) BG ELS: Sedimentological Controls of Gold Distribution in the Middelvlei Reef, Doornfontein Gold Mine. (S) MP MULLINS: Gold Distribution as related to Sedimentary Facies in the VCR on Deelkraal Gold Mine, Carletonville Goldfield. (S) IZAK C'. RUST: Sedimentary Facies.

********

]- BAUXITE IN NATAL

D.3. BarnardD and G.F.J. Horn

Geological Survey, Private Hag X 112, Pretoria, 0001

Following the discovery of bauxite deposits in the Mooi River, Ldeza and Ngome areas in Natal by L private concern, the Geological Survey launched a reconnaissance txploretion program tD locate Dther similar deposits in this province.

Bauxite is an aluminous, laterit^c weathering product of a suitable parent rock. It contains ksolinite, goethite-limonite end qusrtz, with sufficient quantitie? of aluminium hydroxide in the form of gibbsite (AlgCb^HgD) to be Economically viable as a source of alumina (AI2O3) and aluminium metal. Bauxite deposits in Western Australian deposits operate at s cut-Dff Df around U8r,'j oibbsite, which is the lomest-arade material currently mined. ""

In South Afriud, the most suitable conditions for the formation Df bauxite are found in Natal. Potential targets UJBTE selected on the basis of three factors important in the process of bauxitisstiDn, i.e. precipitation, geomprphology and parent rack.

Selected targets were inspected in the field and surface samples of the parent rock and it's directly associated weathering products were collected. Any promising targets were then followed up by auger drilling and samples of potential bauxitic material mere collected.

Samples of the parent rock and the weathering products were analysed in the laboratory of the Geological Survey by mesns of XRF for major oxides while loss on ignition was determined gravimetrically. In addition, analyses of the quartz content of samples of the weathering products were carried out by XRD.

Available alumina is the amount of Al2U- extractable from bsuxitB under plant conditions. A technique was devised tD calculate the gibbsite content, end from this the percentage of available alumina, using the concentrations of qutrtz, Fe203jloss on ignition and re- active silica. The latt.^ ^.s. the difftrence ast-.S-n the totsl silica content end thL j.-njura j_f qu:.rtz in th= scruple. IncicLtec resources Df materiel cznzsir.Lrc mere then 2Q*-. avziliule clun-.in^ LEIE EubEsqusntly G^ti-rnl-in f^r biX;. bi.lt results by using the polygcnsl msthco. Tct^l incicii^L :C.LJJ:CEE EMU"! to 129 Mt cf bauxitic matsrial contsin-'-in b:t. ttr. Zl -_~.C 37;. r-i/iilrblE ^lumir.r.

D.J. BarnarcD ano G.F.J. The genesis of bauxite duperies on scv^rrl factors. Tlv_ Lnnu^l precipitation should be mare thtr, 1 ODD nm end thL weathering profile must bE UEII drained. Th = psrtnt rock must bE aluminous while elE- vated plateau-like landforms are idcsl for the formation of bauxite. A landform of this nature indicates that an aree is relatively stable and that the influence of erosion is nEgligable compared to the surrounding, lower-lying countryside. Karoo dolerite containing about 15% Al2 O3 and which occur as sheets on these plateaux, was found to bE the most suitable parent rock, fin average, annual ambient temperature of 2D°C is ideal.

The concurrent occurrence of other favourable factors with thE above-mentioned parameters, resulted in a broad zone of relatively small deposits paralleling the coast from Deza in the south to Ngome in Northern Natal. ThE single most important additional factor is drainage. It is while groundwater, which is continuously replen- ished by fresh rain-water, is draining through the soil and weathering profile that soluble elements such as Si, K, l\la, Mg, Ca and Mn are leached from the environment, leaving an aluminous residue. The influence Df leaching is confined to twD regions in the soil and rock profile i.e. the reaction zone and the main drainage environment. The reaction zone is situated between the doleritic parent rock and the bauxitic weathering material derived from the dolerite while the main drainage environment is contained in the joints and palaeo-JDints in the fresh rock and weathered matErial respectively. The reaction zone can be between 0,5 and 3D mm thick End consists of illite-montmorillonitE, kaolinite, gibbsite and semi-weathered minerals. Hydrolysis taking place in this zone is responsible for the breakdown of thE parent minerals, eg.

+ 5 AlSi3Oe + U H2D -•• KAl5Si702D(uH)u + U (K + OH") + 8 SiO2 Qrthoclase + water illitt + cations + anions + silica

2 KAl.Si^O^ClH). +7H,0*5 Al^SiJ^COH) . + 2 (K+ + DH~) + Illite + water kaolinite + cations + an^ans + silica

5 Al2Si205(0H)1+ + 5 H20 ^ 5 (Al^.3 H20) + 10 SiO Kaolinite + water gibbs_te + silica

During the formation of the "primary" clay minerals, cations ETE released from the parent minerals end have to be rEmoveo from the leached environment before any gibbsitE can form. ThE thicknEss of the reaction zone is dependent on the leaching potential of thE main drainage environment. If clay-formation has set in in the main drainage area, solutes frDtn thE reaction zone cannot be removed et a sufficient rete to prevent zhz thickE-r.ing cf thE clayey impErmE- eble reaction zone. If the nain criineoE ZTZZ LZ SEturetec by grounri-ustEJ EnrichED in solutss, the fcrmstiar: of "primary" clay minersls uill nat only bE Enhsncec b:jt the slrEEdy-formsc gibbsit-i bs transforn£c back tc "sEconnEry'1 ksoliniti;: cliy. Irisslly,

D.3. BarnErdn and G.F.J. Hern the main drainage environment will therefore consist of open joints and palaeo-joints in the frbBh rock end weathering profile.

The results from drilling optiLtiDns an various targets in Natal suggest that the distribution Df bcuxita an E target is controlled by a subsurface bedrock topography forming ridges and basins or valleys filled with in situ wpathLred material. Kaolin:te is the main mineral constituent of the soil profile in the central deeper parts of these basins uhere saturation by a high ground-water table is evident. On the flanks of the basins and on the ridges, thinner soil profiles pr.vail. Both lateral drainage of the ground-water towards the central deeper areas of the basins and vertical drainage via joints facilitate the formation of predominantly gibbsite-bearing profiles.

On a regional scale the presence or absence of bauxite, subsurfact bedrock features and well-developed soil profiles can be ascribed to a cyclic relationship between the rate of erosion and the rate of chemical weathering.

1 D.3. Barnardo and G.F.3. Horn ! h BUITENGEWONE MINERAALPARAGENESE IN SILLIMANIETHOUDENDE GESTEENTES VAN DIE POFADDER-DISTRIK

G.J. BEUKES, A.E. SCHOCH, H. DE BRUIYN, W.A. VAN DER WESTHUIZEN en L.D.C. BOK (Universiteit van die Oranje-Vrystaat)

Tydens die ondersoek van sillimaniethoudende gesteentes wes van Pofadder, op die plase Hotson (gedeelte van Wortel 42) en Koenabib 43, is n buitengewone paragenese aangetref. Dit bevat onder andere die seldsame minerale natroaluniet (Schoch et al., 1984), zaheriet (Beukes et al., 1984), en die nuwe raineraal hotsoniet (Beukes et al., 1983). Die alurainium-minerale kom voor in biotiet-sillimanietskis van die Wortelformasie (Praekelt et al., 1983).

Onreëlmatige liggame van sillimaniet wat langs strekking wissel van enkele meter tot hon derde meter in grootte, kom in die biotiet-sillimanietskis voor. Die grootste lense is tot onlangs ontgin in klein oopgroefmyne op die plaas Hotson. Die mineraalassosiasie onder bespreking is gevind in die groef wat bekend staan as Hotson 6, en wat eintlik op die aangrensende plaas Koenabib voorkom. Are van hoofsaaklik wit veranderingsprodukte, enkele millimeters tot tien sentimeter breed, kom in die sillimaniet voor.

Die veranderingsprodukte omvat silikate: kaoliniet, pirofilliet, topaas, muskowiet, biotiet, oksiedes: diaspoor, rutiel, ilmeniet en seldsame korund, gehidreerde oksiedes: volop goethiet, minerale afkomstig vanaf sul fie de s (chalkopiriet, piriet, covelliet): chslkantiet, atacamiet en seldsame brochantiet, sulfate ; natroaluniet, zaheriet, gips, en fosfate : hotsoniet en seldsame turkoois. Die afsetting verteenwoordig die tweede aangekondigde voorkoms van zaheriet (Beukes et al., 1984) en die eerste vindplek van die gehidreerde fosfaat-sulfaat hotsoniet (Beukes e_t al., 1983).

Hotsoniet se chemiese samestelling is Al,. (PO. )? (SO,), (OH),.. 16H->O. In handmonster is die mineraal wit en krytagtig, met h dowwe syagtige glans en aardagtige breuk. Die hardheid op die Mohsskaa] is 2,5, en

1 J G.J. BFlTvES et al. die digtheid is 2,06. Dit is opties anisotroop met *n vlekkerige uitdowing en het h baie lae dubbelbreking van minder as 0.003 met brekingsindekse van alfa = 1,519 en gamma = 1,521. Die mineraal is kriptokristal- lyn, en skandeei—elektronfotomikrografie toon latvormige mikrokristalle van tot 15 tnikron in lengte. Die dominante lyne in die X-straal diffrak- siespektrum is (D(A), I, hkl): 10,05 (100) 010; 8,45 (40) llï; 5,20 (70) 112; 5,01 (10) 020; 4,63 (20) 102; 4,43 (10) 220; 4,34 (7) 212; 3,91 (7) 012; en 3,67 (10) 211. Die mineraal is triklien, ruimtegroep PI, met a = 11,23 A, b = 11,66 A, c = 10,55 A en alfa = 112°31', beta = 107°32' en garnira - 64°27'. Die selvolume is 1135,37 A3. Die infrarooi- spektrum word gekenmerk deur absorpsiebande by 950, 1150, 1700 en 3600 cm , en differensiëldifferensiëlee termiese analise verskaf endoterraiese pieke by 110°C, 520°C, en 840°C.

Die zaheriet van hierdie voorkoms se chemiese samestelling stem ooreen met die data vir materiaal vanaf die eerste voorkotns in Pakistan

(Ruotsala en Babcock, 1977), nl. Al12(SO4)5(OH)96.20H2O. Dit kom in noue assosiasie met natroaluniet en hotsoniet voor in smal are, as wit tot lig-blougroen kriptokristallyne materiaal. Die mineraal is opties twee-assig positief, met h matige 2V en brekingsindekse van alfa = 1,498 en gamma = 1,499. Die hardheid op die Mohssk?.dl is 3,5 en die digtheid 2,01. Skandeer-elektronmikrofotografie toon h geöriën teerde golwende tekstuur en gerolde buisvormige veseis. Die diffraksiepatroon stem ooreen met "n trikliene simmetrie, ruimtegroep PI (selparameters: a = 18,47 A, b = 19,45 A, c = 3,77 A, alfa = 95°6', beta = 91°44\ gamma = 80°20') en word gedomineer deur een intense refleksie by d = 18,12 A met hkl = 100. Die infrarooi-spektrum vertoon absorpsiebande by 905, 960, 985, 1150, 1170 en 3600 cm , en differensiële termiese analise gee endotermiese pieke by 190°C, 470°C, 685°C en h doublet by 880°C en 910°C.

Petrografiese en veldgetuienis vir die assosiasie sillimaniet-natroaluniet- zaherite-hotsoniet, ondersteun h genetiese model met vier mineraal- reaksies. Hotsoniet word gevorm deur h reeks reaksies waarin hidrasie en sulfatisasie h rol speel: die opeenvolgende transformasie van sillimaniet na natroaluniet, natroaluniet na zaheriet en zaheriet na hotsoniet. In enkele gevalle kan zaheriet direk uit sillimaniet vorm. Die

G.J. BFIJKES et al. feit dat natroaluniet gevorm het en nooit aluniel nie, dui daarop dat Vi hoë Na/K verhouding teenwoordig was tyderis die eerste reaksie. Die voorgestelde reaksies kan as volg geskryf word:

H 3(Al2O,.SiO2) + Na2O + 4SO3 - w-2~ .,„2 2-3 Sillimaniet Natroaluniet

Natroaluniet Zaheriet

2_3 ,, + 5SO, + 33H2O = 6A12O..^~3 Sillimaniet Zaheriet

•

11(6A12O3.5SO3.33H2O) + 12P2O5 = 6(11A12O3.2P2_5 3 Zaheriet Hotsoniet > r + 19SO3 . 2

m Die sulfaat benodig vir die vorming van natroaluniet, zaheriet en hotsoniet, is verskaf deur die oksidasie van sulfiedes soos chalkopiriet * en piriet, wat volop is in die newegesteentes. Die bron van die fosfaat " wat nodig is vir die transformasie van zaheriet na hotsoniet en vir die • vorming van turkoois, is nog onbekend.

VERWYSING v

Beukes, G.J., Schoch, A.E., Van der Westhuizen, W.A., Bok, L.D.C, j and De Bruiyn, H. (1983). Hotsonite, a new hydrated aluminum phosphate-sulphate from Pof adder, South Africa. Aroer. Miner, (in pers).

Beukes, G.J., Schoch, A.E., De Bruiyn, H., Van der Westhuizen, W.A. and Bok, L.D.C. (1984). A new occurrence of the hydrated aluminium sulphate zaherite from Pofadder, South Africa. Miner. Mag, (in pers).

G.J. BEUKES c-'c al. Il Praekelt, H.E., Colliston, W.P. and Schoch, A.E. (1983). The stratigraphie interpretation of a highly deformed Proterozoic region in Central Bushmanland, South Africa, First Correlation of structurally separated metasediments of the Aggeneys subgroup, Precambrian Research (in pers).

Schoch, A.E,, Beukes, G.J. and Praekelt, H.E. (1984). A natroalunite-zaherite-hotsonite paragenesis from Pofadder, Bushman- land, South Africa. Can. Miner, (in pers).

Ruotsala, A.P. en Babcock, L.L. (1977). Zaherite, a new hydrated aluminum sulfate. Amer. Miner., 62, 1125 - 1128.

t 1 i • I fi .

I, G.J. BEUKES et al. THRUST FAULTING ALONG THE EDGE OF THE KAAPVAAL CRATON IN

GRIQUALAND WEST

N.J. BEUKES AND J.P. VAN WYK Department of Geology, Rand Afrikaans University JOHANNESBURG 2000

The presence of a low angle thrust fault system of post Olifantshoek-Gamagara age along the western margin of the Kaapvaal craton as described by Visser (1944) has been veri- fied by detailed stratigraphic mapping in the area. Thrusting caused rocks of the Transvaal Supergroup to overly the Gama- gara Formation which is a correlative of the basal Mapedi- Formation of the Olifantshoek Group.

The Gamagara and Mapedi Formations overly the Transvaal sequence with a marked angular uniformity. The amount of vertical stratigraphic duplication caused by the thrust fault system is symmetrically distributed north and southwards from the centre of the Maremare dome. On the dome itself the Ongeluk Lava directly overlies the Gamagara Formation. However, north- and southwards from there progressively lower units of the Transvaal sequence overlie the Gamagara Formation until near Rooinekke in the south and in the Kalahari manganese field in ' the north the total Asbesheuwels iron-formation sequence and -' part of the Campbellrand dolomite sequence follows on top of , it (Fig. 1) . j The thrust plane generally dips at an angle of 10°-11° to the j west but it has been folded slightly by post Olifaj.cshoek deformation. In outcrop the thrust plane is often obscured by i ferruginization and silicification (forming massive jasperoids) and the fact that much of the movement took place along shale 1

1 N.J. BEUKES and J.P. VAN WYK units in the sequence. Milonites and ultramilonites (chert) are, however, locally developed where brittle rock types such as quartzite are transgressed. Shales progressively become folded, sheared and massive towards the fault plane.

The significance of the recognition of the thrust faulting is that some of the stratiform and stratabound ore deposits in the area may be duplicated; the presence of the Asbesheuwels sequence above the fault plane in the west opens up new areas for possible crocidolite exploration; laterally equivalent sedimentary lithofacies deposited remote from each other are brought into close juxtaposition; the red bed sequence of the Gamagara Formation does not belong to the Transvaal Supergroup but developed at a later age; and the zone of oxidation at the base of the Mapedi- and Gamagara Formation with it's associated iron- and manganese deposits is of a single age corresponding to boundary between the Proterophytic and Paleophytic eras which is approximately 2 200 - 2 000 m.y. old. The thrust faults also necessitates a re-interpretation of recent geological maps and the formal stratigraphic subdivision of the area.

Reference :

Visser, D.J.L., 1944. Stratigraphic features and tectonics of portions of Bechuanaland and Griqualand West. Trans. Geol. Soc. S. Afr., 47, 197 - 254.

N.J. BEUKES and J.P. VAN WYK - 10 -

:. :\ > -

- I . •. . I > >

2. l> >

Thruff fouft MapadI, Ludmow and Gamogare Formation* Unconformity

Uakgonytnt Oicxnictit* Unconformity Koegas Subgroup Atbnhetnnlf Subgroup fc 61 CampbellrorxJ Subgroup

Fig. 1. Diagram illustrating the relationship of stratigraphic units to the thrust fault. 11

PLINIAN AIR-FALL ACTIVITY AND RHYOLITIC DOME BUILDING IN THE BUMBENI COMPLEX, SOUTHERN LEBOMBO

J.W. BRISTOU De Beers Geology Department, Box 47, Kimberley, 8300

Pyroclastic rocks may, broadly speaking be divided into 3 main groups with respect to their mode of emplacement viz. (i) pyroclastic ash-flows or ignimbrites (ii) air-fall or plinian deposits and (iii*1 surge deposits. Pyroclastic surges are high density, high velocity flows. Surge deposits are characterised by sedimentary bedforms and typically show reverse grading. They are represented by ground surges which usually form in the early stages of ash-flow eruptions and base surges which are the products of explosive maar-type eruptions. The latter include kimberlite and lamproite eruptions. Air-fall or plinian deposits typically cover vast areas. They tend to drape or mantle topography, are reasonably well sorted and in general consist of clast supported fragments. These deposits usually form as a result of fall-out from plinian column eruptions and may or may not be associated with ash-flow deposits. Pyroclastic ash-flow or ignimbrite deposits are generally extensive and flat lying. They are typically poorly sorted and are matrix supported. They may be exceptionally thick and range from welded to unwelded deposits. Recent work has shown that many of these deposits form by plinian column collapse processes. However, there are indications that some extensive and often extremely thick flows which typically show well developed flow textures may have been emplaced by processes other than simple column collapse. These deposits appear to have been characterised by high temperatures and low volatile content at emplacement. They were probably emplaced in an out-welling type processes in which there was only a very minor component of column collapse. The Lebombo rhyolites, viz. Jozini and Mbuluzi Formations, appear to represent excellent examples of high-temperature ash-flows emplaced by out-welling type processes. The Lebombo rhyolites are extensive and have been described in detail by Bristow (1976, 1983), Cleverly (1977) and Saggerson and Bristow (1983).

BRISTOW 1 12

Plinian air-fall deposits appear to be rare in the Lebombo. However, a localized, well developed air-fall deposit is found at Nxwala in the Bumbeni Complex of the southern Lebombo. This deposit and associated rocks, and mode of emplacement are described in some detail below. The Nxwala air-fall deposit is poorly bedded and has a minimum thickness of about 2m. It consists of copious amounts of pumice and much less abundant lichic fragments consisting of basaltic material and flow banded rhyolite. The tuff deposit can be divided into a thin lower horizon consisting of fine-grained ash and a thicker upper horizon consisting of coarser pumice fragments and a higher proportion of lithic fragments. The air-fall tuff is closely associated with obsidian flows (now represented by perlitic pitchstone), pumice-pitchstone breccias and flow-banded rhyolite. These rocks are collectively referred to as the Nxwala Suite. In thin section the above rock types show broad petrographic similarities with respect to phenocryst type. In addition, analytical data show that the rocks represent a chemically coherent group implying that the lavas and pyroclastics represent a comagmatlc suite. Reconstruction of the emplacement history of the Nxwala Suite indicates that the obsidian flows (perlitic pitchstone) were emplaced in the initial stage of volcanic activity. Due to their high viscosity these lavas are unlikely to have flowed far from their eruptive source resulting in doae-like structures above and adjacent to their source vent(s). Subsequently, the high viscosity and rapid cooling of the rhyolitic obsidian would have caused eventual capping of the vent(s) from which these rocks were emplaced. Rapid build-up of volatiles would have occurred below this cap or plug and ultimately excessive gas-pressure within the underlying magma chamber is considered to have caused ] collapse and fragmentation of the rhyolitic dome capping the vent(s). Emplacement of the coarse pumice-pitchstone breccia with an attendant plinian eruption column of fine ash would have followed. The coarse breccia could possibly be equated to a ground surge which typically occurs in the initial stages of pyroclastic eruptions (Fisher and Waters, 1970) or else could represent a vent breccia. The basal layers of fine ash (air-fall) which overlie the coarse breccia reflect rapid decompression of the underlying rhyolitic melt as the conduit plug was removed. This would have led to shock waves passing down the vent, disintergrating the highly viscous, vesiculated rhyolite

BRISTOW 2 13

into fine angular pumice fragments and shards (see Heiken, 1979) . Consequently deposition of a fine-grained basal air-fall deposit followed the emplacement of the coarse pumice pitchstone breccia, forming an evenly bedded air-fall deposit draped over the breccia. Following breccia formation and fine air-fall deposition, active plinian eruption continued, leading to the formation of the upper zone of coarse air-fall material. Concentration of lithic fragments (particularly basalt fragments) at the base of, or within, individual beds suggests that repeated vent constriction was the main cause of repetitive cycles noted in the upper air-fall zone. Plinian activity eventually ceased due to devolatilization of the underlying silicic magma body. Consequently the final stage of the Nswala eruptive event was marked by the emplacement of a relatively crystal rich rhyolite lava. Emplacement of this rhyolite led to the formation of a lava dome characterised by highly contorted flow banding, though subsequent erosion has removed much of the original topographic expression of this dome. The rock types and emplacement characteristics of the Nxwala suite are well documented in felsic volcanic provinces, particularly amongst the caldera complexes of the western U.S.A. eg. Medicine Lake Highlands, San Juan Mountains, Jemez Mountains and Mogollon-Datil volcanic province. A more recent analogy is also found at Mount St. Helens which has shown repeated plinian ^activity associated with collapse of the dome which formed in the crater left after the major eruption of May 18, 1980.

REFERENCES Bristow (1976), MSc, Univ. Natal Bristow (1983), Trans. Geol. Soc. S. Afr., 85 (3) Cleverly (1977), D. Phil., Univ. Oxford Fisher and Waters (1970), Am. J. Sci., 268. Heiken (1979), Am. J. Sci, 67. Saggerson and Bristow (1983), Bull. Volcanol., 46 (3).

1 BRISTOW 3 1 14

DEPOSITIONAL ENVIRONMENTS AND COM SEAM CHARACTERISTICS IN' THE NORTH-EASTERN WITBANK COAI FIELD. B. Cairncross Dept. of Geology, University of the Witwatersrand, Johannesburg.

A subsurface investigation of the depositional environments of the Vryheid Formation in the north-eastern Witbank Coalfield reveals a number of sedimentary environments which affect coal distribution and quality.

The Vryheid Formation attains a maximum thickness of 90m and overlies Pre-Karoo basement composed predominantly of Bushveld Complex felsite. Dwyka tillit.e is confined to isolated depressions in the basement. Stratigraphically, the sequence comprises gravels, sandstones and mudstones which separate a number of exploitable coal seams. These seams are numbered from No. ] at the base to No. 5 at the top. The overall sedimentary sequence is similar to other parts of the Witbank Coalfield; however, a number of significant differences exists. Sediments are coarser grained, in that very coarse-grained sandstones and granule grade gravels dominate the succession. These sediments predominate below the No. 1 seam, arc associated with the No. 2 seam, and comprise the bulk of the succession for those sedimentary intervals capped by the No. 3- 4 and 5 seams.

Sandstones between the basement and the No. 1 seam represent fluvial deposits derived from the reworking of underlying unconsolidated till and from a northerly granitic source terrain. These sediments are planar cross-bedded, formed by the- deposition of downstream migrating bed forms. Fining-upward sediments terminate the sequence indicating waning flow conditions. These deposits were gradually encroached over by vegetation that gave rise to extensive peat-forming swamps, later producing the No. 1 and 2 coal seams.

J B. Cairncross 1 15

The No. 1 and 2 soam parting is normally 5km) shallow, sandy rivers traversing the swamp in braided, bed- load systems. These originated in the north-west and flowed towards the south-east, resultingin the No. 2 seam being split, over wide areas, into the 2 lower (2L) and 2 upper (2U) sub-seams. Localized splitting occurs in the 21' seam in the west, forming the 2A seam above.

Band studies of the economically exploitable No. 1 and No. 2 coal seams show persistent lateral and vertical macroscopic trends. Bright, bright and dull, and dull coal bands are continuous over widespread areas. The basal zones of these two seams contain the brightest coal. No. 1 seam is the highest quality seam, except where downgrading has taken place through inorganic contamination. Numerous internal partings in the No. 2 seam cause quality variations.The 2L/2U fluvial parting has high ash and low C.V. coal adjacent to channel flanks, while lower ash and higher C.V. coal is found further away from the channel irargins.

Coarsening-upwards sedimentary cycles above the No. 2 seam typify prograding deltaic systems. Sequence thickness and sand body geometry indicate shallow-water lobate deltas. Overlying these deltaic sandstones is a fining-upward sedimentary unit which formed by fluvial deposition over the delta plain deposits.

The coarse- clastic sediments above the \o. ?. 4 and 5 seams were deposited on extensive braided alluvial plains with coarse sand and gravel transported by bed load processes. Coalescence of channels resulted in thick sand bodies inhibiting peat accumulation due- to: pinchout of coal adjacent to channel axes, oxidation of peat in topographically elevated areas and removal of underlying coal and associated sediments by channel scouring.

J B. Cairnrross. 1 16

The large proportion of coarse elastics, relative to other documented areas of the Witbank Coalfield, suggests that the study area was situated close to the source of sediment supply during the deposition of the Vryheid Formation. Coarse-grained fluvial deposits contained in the No. 2 seam most likely represent the upstream equivalents of the anastomosing channels in the No. 2 coal further to the south and south-west. Similarly, coarse-grained sandstones and gravels associated with the No. 3, 4 and 5 coal seams are the proximal fluvial sediments which fed more distal deltaic deposits situated further south in the basin.

Fluvial channel systems associated with peat accumulation have a deleterious effect on the ash and C.V. of adjacent coal seams. The ash and C.V. decrease and increase respectively with distance away from the channel systems.

B. Cairncross 17 2 3660114-

SOUTH AFRICAN URANIUM RESOURCE ESTIMATES

F.A.G.M Camisani-Calzolari

W.J. de Klerk P.J. van der Merwe

NUCLEAR DEVELOPMENT CORPORATION OF SOUTH AFRICA (PTY) LTD

ABSTRACT

This paper deals primarily with the methodologies used by the Nuclear Development Corporation of South Africa (Pty) Ltd (NUCOR) in arriving at the assessment of the South African uranium resources. The Resource Evaluation Group of NUCOR's Geology Department Is the body responsible for this task which is carried out on an on-going basis. A major reassessment exercise is undertaken biennially to coincide with the compilation of South Africa's contribution to the joint Nuclear Energy Agency (NEA)/ International Atomic Energy Agency (IAEA) publication "Uranium Resources Production and Demand". The latest South African assessment was completed during li)83 and consisted of the evaluation of 44 mines and in excess of 200 deposits and occurrences.

The evaluation is done on a property-by-property basis and relies upon data submitted to NUCOR by the various companies Involved in uranium mining and prospecting in South Africa. Resources are classified into Reasonably Assured, Estimated Additional and Speculative categories as defined by the NEA/IAEA Steering Group on Uranium Resources. Each category is divided into three cost categories, i.e. resources exploitable at less than $80 kg U, at $80-130/kg U and at $130-260/kg U. Resources are reported in quantities of uranium metal that could be recovered after mining and metallurgical losses have been taken Into consideration.

1 1 16

The evaluation of a uranium venture Is carried out in various steps, of which the most important in order of implementation, are : geological interpretation; assessment of in-situ resources using techniques varying from manual contouring of values to geoetatlsticE, depending on their suitability; feasiblity study and estimation of recoverable resources.

Because the choice of an evaluation method is dictated to some extent by statistical considerations, frequency distribution curves of the uranium accumulation variable in characteristic deposits are Illustrated and discussed.

As 90 X of South African uranium resources occur in association with other metals, primarily gold, the techniques used In assessing the potential of a deposit where uranium will be extracted as a by- or co-product are also elaborated upon. 19

APPLICATION OF GEOSTATISTICAL TECHNIQUES AT THE

OTJIHASE Cu-Zn-Ag-S OREBODY, SOUTH WEST AFRICA / NAMIBIA

C J CARTER R SCHNEEWEISS

Geologists

Tsumeb Corporation Limited, Tsumeb SWA/NAMIBIA

SYNOPSIS

Application of geostatistical techniques to drillhole data from the Otjihase orebody has produced a significant improvement in the accuracy of ore reserve estimates. A parallel analysis of orebody footwall data has produced contoured information to aid planning of contour-stoping, to predict 'bad ground1 and orebody faulting, and has been of considerable assistance in the deci- pherment of other locally significant structural features.

1. ORE RESERVE ESTIMATION

Production of semivariograms for all relevant variables in the portion of the orebody currently being mined revealed significant zonal anisotropies in the distributions of copper, sulphur and silver which correlate well with earlier interpretations.

As a first approximation, simple point kriging was used to pro-

C J CARTER / R SCHNEEWEISS 1 20

duce a twenty-five metre gridded data set from drillhole compo- site information and appropriate transitive semivariogram models for all variables - orebody thickness, metal and sulphur grades, and footwall elevation (following the removal of the regional trend).

Further processing, on a point by point basis, resulted in the production of a multicolour, X-Y plotted projection of orebody blocks (25 m x 25 m), each block containing operator-specified information such as ore tonnage, metal/sulphur grades or tonnages, thickness, and the associated estimation variances.

A reconciliation with 1982 mine production figures was carried out by using both the traditional, polygon-based ore reserves and the computerised 25 m ore block inventory. The results of the reconciliation (Table 1) show a dramatic improvement for both ore and contained metal tonnages.

TABLE 1 : 1982 PRODUCTION AND ORE RESERVE RECONCILIATION

VARIABLE PRODUCTION POLYGON METHOD BLOCK INVENTORY

Ore Tonnage 420 283 m tons 500 580 (+19,1%) 416 469 (-0,9%) Cont. Sulphur 97 817 " 117 708 (+20,3%) 96 204 (-1,6%) Cont. Copper 8 935 10 220 (+14,4%) 8 780 (-1,7%) % Sulphur 23,3% 23,50 (+ 0,9%) 23,10 (-1,0%) % Copper 2,13% 2,04 (- 4,2%) 2,11 (-0,9%)

The contour stoping method employed at Otjihase necessitates the mining of a 'waste wedge' below the orebody footwall. The dilution factor for any given ore block is estimated from stope width, orebody thickness and footwall dip. The calculation of

3/....

1 1 21

3. this footwall dilution on a block by block basis represents a significant advance over the old 'average dip and thickness' method.

The slight underestimation of all variables by the kriging method results from the assumption that footwall dilution is at zero grade.

2. OREBODY FOOTWALL ANALYSIS

Following removal of the regional trend by a least-squares plane fitting method, the footwall 'residuals' are treated as for any other variable. Kriging on a 25 m grid produces contour maps sufficiently accurate for stope planning purposes.

Contouring of the residuals themselves shows a pattern of gentle folding with two major trends. Geological follow-up work indicates that faulting occurs on the flanks of these "rolls" and that bad ground may be expected where peaks and troughs are seen. 22

DEPOSITIONAL MODELS FOR COAL SEAM FORMATION IN THE

KLIP RIVER COALFIELD, NORTHERN NATAL.

A.D.M. CHRISTIE

DEPARTMENT OF GEOLOGY, UNIVERSITY OF NATAL, DURBAN.

This paper describes the correlation and depositional environment of coal

seams within the Vryheid Formation of the Klip River Coalfield. The area

under investigation extends from Alcockspruit in the north to Helpmekaar in

the south (Fig. 1).

\o jo ^PAULPIETERS BURG \ KILOMETRES

—s. \ UTRECHT s \ h^ ^\ VPYHEID / '^NEWCASTLE \

i / > I DANNHAUSER \S

V/'j ..-' /[ou"0*i, \GLENCOEJ \\ Wow*** \ (w*m KAROO ftASIh Jy \\ / MELPMEKAAR^

Figure 1. Locality Map.

Three informal subdivisions of the Vryheid Formation are recognized :

1) DUNDEE MEMBER

This member is not more than 300m thick and extends from the base of the Vryheid

Formation to the base of the No 1 Seam. It comprises between 3 and 5 upward-

coarsening cycles which vary in thickness from 20 to 130m. Each of the cycles 23

relates to a major transgressive/regressive episode in which fluvio-deltaic

sediinentation dominated. Parts of the coastline were wave dominated and some

deposition was influenced by tidal processes.

2) GLENOOE MEMBER

The Glencoe Member is between 30 and 80m thick and extends from the base of

the No 1 Seam to the top of the No 4 Seam (Fig. 2). Owing to the confusion

in coal seam nomenclature within the coalfield, a numeric system has been

adopted :

TRADITIONAL REVISED

Leader Seam No 4 Seam

Top Seam No 3 Seam

Intermediate Seam No 2C Seam

Bottom Seam No 2B Seam

Extra Bottom Seam No 2A Seam

No 4 Seam No t Seam

Sedimentary associations indicate that the No 1 Seam peats accumulated in a

lower delta plain environment following delta abandonment at the end of a major regressive episode. Although this seam is thin, its areal extent suggests that conditions favourable for the accumulation of peat were widespread

for a short time.

This was followed by a further regressive episode during which prograding

deltas were dominant, and the formation of the No 2 Seam peats. The nature of the partings between the coals of the No 2 Seam reflect shallow embayment,

crevasse splay, minor distributary channel and fluvial floodsplain depositional 24

associated environments. An upward decrease in bioturbation^with coals of the No 2

Seam suggests that peat accumulated in environments that were progressively

less influenced by the brackish to marine basin waters.

The parting between the No 2 and No 3 Seams varies from a fine-grained sequence

less than 50cm thick to linear deposits of coarse-grained, pebbly sandstone up to 25m thick and 15km wide. This interval is thought to represent argillaceous overbank floodplain deposits flanked by laterally confined sandy braided rivers. Peat accumulation was most rapid within the interchannel areas, and continued after the abandonment of the river channels to form the

No 3 Seam. This seam is absent or poorly developed southwest of Wasbank and this is attributed to peat accumulation being affected earlier and to a greater extent here than the more proximal areas by the marine transgression which followed. The No k Seam caps a further regressive deltaic event, and this was the last notable phase of peat formation within the Klip River Coal- field during Vryheid Formation time.

To the west of the main road linking Ladysmith and Newcastle the coals become markedly thinner and more shaly and the parting between seams increases in thickness. This change, which is taken to indicate platform instability, relates to a steepening of the eastern side of a north-south trending pre-

Karoo basement valley.

3) DANHAUSER MEMBER

This member comprises 2 to 3 regressive deltaic deposits. It is between 80 and 140m thick and grades upward into the Volkrust Formation. 25

PALAEOCURRENT DATA

Palaeocurrent directions indicate a southwesterly prograding coastline between Alcockspruit and Dundee, and southerly to southeasterly to the south of Dundee throughout the deposition of the Vryheid Formation.

STEIN COALSPRUIT

TROUGH CROSS-STRA1 PL«NAR CROSS-STRAT FLA'' STRA1 RIPPLE CROSS-LAW SlPHONICHNUS SKOLITrtUS DIPLOCRATERlO" SPiBODESMOS

Figure 2. Section through the Glencoe Member on the

farm Stein Coal Spruit approximately 10km southwest

of Glencoe. 26

THE DETERMINATION OF THE STRATIGRAPHIC POLARITY IN PROTEROZOIC METASEDIMENTARY ROCK USING SEDIMENTARY CYCLES

W.P. Colliston and J.C. Loock

(University of the Orange Free State)

The correct determination of the stratigraphic facing direction is a prerequisite in stratigraphic analyses. In the metasedimentary rocks of the Aggeneys Subgroup, in central Bushmanland, primary sedimentary structures such as ripple marks and crossbedding, have almost totally been obliterated by metamorphic reconstitution. Sandstones have been transformed into coarsely crystalline metaquartzites and the shales into mica schists. However, various sedimentary cycles are still recognizable, and are compared with similar cycles from less deformed rocks of Devo- nian age, possibly deposited in similar environments.

The techniques used in this comparison show that it is possible to define a stratigraphic polarity for any sequence of metasedimentary rocks. Stratigraphic polarity, as determined from sedimentary cycles, can be successfully employed in unravelling structural and stratigraphic complexities.

First Order Cycles. A common type of first order cycle developed in offshore deposits along linear clastic shorelines, is represented by sandstone beds commonly less than two metres thick and bounded by sharp contacts, A fining upwards cycle is indicated by a decrease in grain size near the top of the bed and a corresponding increase in clay fraction. In Bushmanland the stratigraphic facing direction or polarity of a metaquartzite bed can be determined by observing the gradual increase in mica schist content towards the upper contact.

Second Order Cycles. Second order cycles composed of a stacking of first order cycles, are prominent in the field and can be recognised more easily than first order cycles. In sediments of linear clastic shorelines, regressive (upwards coarsening), transgressive (upwards fining) and

f 1 K.P. COLLISION AND J.O. 1.O0CK 1 27

regressive- transgressive (both symmetrical and asymmetrical) cycles can be found. Observations in Devonian terranes show that upwards coarsening cycles are the most common. Such a cycle is defined by:

(i) an increase in grain size upwards, (ii) an upwards thickening of the sandstone beds, and (iii) an increase in the percentage of quartz upwards in any chosen interval.

In Bushtnanland second order cycles are common and easily recognised. For example an upwards coarsening cycle is defined by an upwards thickening of the metaquartzite beds and by a corresponding increase in the percentage of quartz in a given interval. In the field, when a second order cycle is found, care must be taken in examining the metaquartzite beds in order to determine the correct polarity (stratigraphic facing or younging direction) of the first order cycles. Once this has "been done, it will be seen that the second order cycle consists of a stacking of first order cycles. The correct polarity can then be assigned to the second order cycle.

In metamorphic terranes it may sometimes be difficult or impossible to determine the correct polarity of the first order cycles. When this is the case the polarity of a second order cycle is arbitrarily defined as the coarsening direction. The stratigraphic facing direction is thus not stated or implied. Such a second order cycle with an incompletely defined polarity is still very useful in unravelling complex structures, provided that it is applied in a consistent fashion.

Stratigraphic Models. A limited number of models can be used to recognise and interpret stratigraphic columns resulting from sedimentation on deltas and linear clastic shorelines.

When first and second order cycles together with the coarsening or fining direction are graphically displayed, a model of exceptional usefulness is created. The nature of contacts or transitions should also be shown. Such models can be used for deducing polarity in metasedimentary rocks and for stratigraphic analysis in general.

W.P. COLLISION AND J.C. LOOCK Every model encompasses a unique polarity or correct stratigraphic facing direction which cannot be inverted.

From the above it is clear that polarity can be determined if first and second order cycles and the nature of the contacts are used in conjunction with conceptual models.

1 W.P. COLLISTOX AND J.C. LODCK 1 29

THE RECOGNITION OF LOW ANGLE THRUSTS IN CENTRAL BUSHMANLAND, NAMAQUA MOBILE BELT W.P. COLLISTON, H.E. PRAEKELT, D. STRYDOM, G. VAN ASWEGEN, H.J. BLIGNAULT AND A.E. SCHOCH

(University of the Orange Free State)

Following the success of the Western Namaqualand Geotraverse in the Proterozoic Namaqua mobile belt (Blignault et al., 1983), further investigations began in this region in 1981. The main emphasis was placed on examining the nature of the metasediments of the Aggeneys Subgroup within the central bushmanland portion of the Namaqua mobile belt in a 3300 km2 study area extending from Wortel in the north, situated on the contact zone with the Groothoek thrust (which separates the study area from the Haib igneous complex), to Aggeneys in the south, and from Achab in the east to Geselskapbank (Strydonr. in prep.) in the west.

This paper is mainly concerned with the structural development of the eastern part of the area, and specifically the recognition of low angle thrusts that are described for the first time for the economically important Aggeneys area, and for the Wortel-Haramoep area to the north. A model of overthrusting was independently developed from observations at three geographically separated study areas: the Aggeneysberge, Swartberg (Black Mountain) and Wortel-Haramoep. The research-approach used was that of detailed mapping combined with stratigraphic- structural analysis, which resulted in the recognition of classic thrust structures as reported from thin-skinned fold and thrust belts such as the Appalachians! U.S.A.

The stratigraphical succession of the Wortei-Haramoep mountain area (Praekelt et al., in press) consists of the Karadous (bottom), Hotson, Witberg and Wortel Formations (top). A tectonic contact has been identified towards the top of the stratigraphic sequence: a major thrust fault separates the juxtaposed stratigraphic units of the Witberg and Wortel Formations. This regional thrust plane (the Haramoep thrust fault) occurs virtually everywhere within 50 m to 600 ID of a distinctive marker ] horizon towards the base of the Witberg Formation. 1 W.P. COLLISTON. et al . 30

The Wortel Formation forms the hanging wall of the Haramoep thrust fault for a distance of at least 40 km in the transport direction and is also carried for approximately 150 km along the regional strike. This fault geometry is comparable with that of decollement-style thrust faulting. The decollement horizon is primarily restricted to a narrow stratigraphic interval within mechanically weak (incompetent) layers consisting of a sequence of metapelites with interbedded quartzite. The Hara- moep thrust plane is generally parallel to bedding and to the regional sillimanite-grade fabric, and has an irregular corrugated form, climbing up and down the stratigraphy. The transport direction is towards the south-west, as determined from imbricate thrusts, stretching lineations and overturning of the rocks of the Witberg Formation beneath the Haramoep thrust fault. A typical cross-section through the central Haramoep Mountains (Fig. 1) shows key features of the thrust, namely the ramp, imbricate thrusts, branch lines, leading and trailing thrust surfaces, synclinal fold beneath the thrust, the autochton and the allochton.

Fig. 1 Cross-section of the Haramoep thrust in the central Haramoep- Wortel Mountains showing the decollement nature of the fault and the simple ramp geometry.

In the Aggeneysberge a major tectonic discontinuity or thrust separates basal gneisses from overlying metasediments. The end members of these rock uiiits are duplicated along this structural zone, and it is argued that the duplication phenomena are not due to isoclinal folding or lit—

. r. 31

par-lit intrusion, but to imbricate thrusting. Similar structural contacts separating gneisses and metasediments have also been regionally recognised, namely in the basal regments of the Achab-Namiesberge, the Gamsberg and the western face of Swartberg.

Thrust geometries are recognized within the Aggeneys area, (in the Aggeneysberge and at Swartberg-Broken Hill) as well as elsewhere (at Achab-Namiesberge and Gamsberg). At Swartberg two major thrust, sheets with associated imbricates and recumbent macro isoclinal folds were identified. The two structural units also exhibit highly contrasting stratigraphic successions. The thrusts sheets are defined by stratigraphy and by localised shear effects, resulting in the excision of lithologies, especially at the inter-thrust contacts.

Structures in the Aggeneysberge are comprised of four macro flat-lying to gently inclined isoclinal folds, with limbs oriented easterly and which can be traced out along strike for some 6 to 17 km. These macro isoclines verge southwards, with fold axes plunging shallowly to the east-north-east, parallel to the mineral lineations. Each macro isocline has its own specific stratigraphy, which is lithologically distinct from all the others, and is bound by discrete zones of deformation generally parallel to bedding and the regional foliation. Ductile deformation features (e.g. highly deformed pebbles, ductile shears, stretching fabrics and an associated upper medium grade regional fabric) are characteristic of both the macro isoclines and bounding deformation zones. The thrust relationships between the basal gneisses and the overlying rocks is borne out by the discordant contact with the macro isoclinal folds. All the abovementioned geometric features are akin to those in classical thrust geometry, and the various macro recumbent to inclined isoclinal folds are interpreted as being representative of folded thrust sheets or fold nappes, with the bounding deformation zones representing thrust surfaces. These deformation zones predominantly occur within a sequence of layered schists.

The working hypothesis applied in many thrust belts is that thrusts propagate in the direction of tectonic transport. The implications for the

1 U.T. f"l]]J10\. t-t al. 32

study-area are that emplacement took place from north to south with the Klipbok fold nappe first, followed by the Dabbiepoort fold nappe, followed by the Quarry fold nappe and finally by the Soutkloof fold nappe. The upper portion of the basal gneiss contains the lowest retaining thrust surface or floor thrust for the fold nappe structures. The direction of thrusting is towards the southwest as obtained from the maximum extension direction and direction of overfolding. This transport direction is also supported by the inferred movement direction derived from imbricate thrusts in the Swartberg thrust sheets.

The proposed model of subhorizontal thrusting involving movement between rock types of different ductility contrast appears to be regionally applicable, and also appears to differ from the typical well documented type of rigid block thrust geometry; it is thus suggested that the deformation process of thrusting occurred within a ductile or relatively high grade plastic regime. This model also proposes profound changes to the conventional genetic/stratigraphic interpretation of the metasediments of the Aggeneys Subgroup in central Bushmanland.

REFERENCES

Blignault, H.J., Van Aswegen, G., Van der Merwe, S.W. and Colliston, W.P., 1983: The Namaqualand geotraverse and environs; part of the Proterozoic Namaqua mobile belt. In: B.J.V. Botha, (Editor), Namaqualand Metamorphic Complex, Spec. Publ. 10, Geol. S. Afr.

Praekelt, H.E., Colliston, W.P. and Schoch, A.E.: The stratigraphic interpretation of a highly deformed Proterozoic region in Central Bushmanland, South Africa: First correlation of structurally separated metasediments of the Aggeneys Subgroup. Precambrian Re- search (in press).

Strydom, D.: h Struktureel-stratigrafiese studie van die metasedimente en ander metamorfiete van noordwestelike Boesmanland (Ph.D. thesis, University of the Orange Free State, in preparation).

W.P. COLLISTON, et al. 33

PETROLOGIESE EN PETROCHEMIESE VERWANTSKAPPE VAN DIE KOPERBERGSUITE, NAMAKWALAND

J.A. CONRADIE EN A.E. SCHOCH

Die Koperdistrik van Namakwaland beslaan ongeveer 3000 km2 en word gekenmerk deur b suite van intermediêre tot mafiese intrusies waarvan sommige ekonomiese konsentrasies Cu-sulfiedes bevat. Die mafiese suite, bekend as die Koperbergsuite, het ongeveer 1100 miljoen jaar gelede ingedring in die gneisse en graniete van die Okiep-distrik, en word veral met laat-tektoniese "steilstrukture" geassosieer. Gangvormige voorkomste wat oos-noordoos strek is oorheersend (Exploration Depart- ment Staff of the O'okiep Copper Company Limited, in pers.).

Hoewel meer as t> duisend van hierdie mafiese intrusies bekend is in die gebied, is slegs sowat dertig ekonomies gemineraliseer. !n Petrografiese en petrochemiese ondersoek is uitgevoer om vas te stel of ekonoiniese voorkomste onderskei kan word van swak gemineraliseerde liggame (Conradie, 1983). Ongeveer h honderd monsters van sewe-en-dertig lokaliteite is petrografies ondersoek, terwyl sowat tagtig hoof- en spoorelement analises uitgevoer is.

Stollingsteksture is oorheersend in die mafiese gesteentes. Metamorfe teksture van sommige liggame dui op verskillende grade van vervorming, en word toegeskryf aan lokale steilstruktuur-vervorming tydens en nâ inplasing. Rotstipes wat algemeen voorkom sluit in andesien-anor- tosiet, leukodioriet, mika-dioriet, hipersteen-dioriet, glimmeriet en hipersteniet. Magnetiet-ryke meladioriet en horingblendiet is beperk tot enkele voorkomste. Andesiniet en leukodioriet is deurgaans swak gemineraliseer. Die ertspotensiaal van Vi liggaam blyk dus afhanklik te wees van die proporsie mafiese minerale hipersteen, biotiet/flogopiet en horingblende, en in Vi mindere mate van die oksiedminerale. *n Mafiese mineraal-inhoud wat 25 volume-persent oorskry kan beskou word as aanduidend van ekonomiese potensiaal.

J.A. CONRADIE EN A.E. SCHOCH 34

Rekenaar-modellering van petrochemiese data identifiseer andesiniet en leukodioriet as h chemiese familie, duidelik onderskeibaar van die meer mafiese ertsdraende tipes. Die petrografiese waarnemings word hierdeur gestaaf. h Ternere diagram, saamgestel uit die Niggli-waardes al, alk en fm, diBkrimineer ook bevredigend tussen nie-ertsdraende andesiniet en leukodioriet, en die meer mafiese voorkomste wat ertsliggame bevat. Enkele meladioriet-voorkomste stem nie ooreen met genoemde petrochemiese neigings nie.

Ongeveer sestig mineraalanalises is uitgevoer met behulp van !n elektron- mikrosonde, op hipersteen, bruin mika en plagioklaas van ertsdraende en nie-ertsdraende voorkomste. Die tnineraalsamestellings kan egter nie gekorreleer word met die ertspotensiaal van h liggaam nie. n Moontlike uitsondering is bruin mika wat effens verryk is in Mg in die perkniete. Analises van magnetiet-ilmeniet pare is aangewend vir geotermometrie en suurstof-geobarometrie. Die oksiedminerale vertoon feitlik endlid-same- stellings en verskaf geëkstrapoleerde temperature en fO^-waardes wat aansienlik laer is as die eksperimentele minima van Buddington en Lindsley (1964), Potensiele ertsdraers kan nie op grond van die verkreë fOp-waardes onderskei kan word nie, en die lae temperature van 300 - 400°C dui op in uitsonderlik stadige afkoelingsgeskiedenis.

In Ondersoek van vloeistofinsluitsels in sorgvuldig geselekteerde monsters van die mafiese gesteentes word tans onderneem. Dit kan reeds genoetn word dat h onekonomiese magnetiet-ryke mika-dioriet h hoogs komplekse multimodale vloeistofpopulasie bevat met In kooldioksied: water verhouding van ongeveer 80:20. Indien aangeneem word dat die hoofpieke vir kooldioksied- en water-insluitsels geneties verwant is, verteenwoordig die hoofpopulasie h vormingstoestand van ongeveer 200°C en 1 Kb. Daar is egter h spektrum van insluitsel-eienskappe tot by gevalle wat h hoë druk mag verteenwoordig, maar die data is nog onvoldoende vir sinvolle interpretering. Die resultate vir die hoofpopulasie stem egter ooreen met In model wat stadige afkoeling en drukver- ligting postuleer, en waarvolgens die finale afsluiting van die vloeistofinsluitsels plaasgevind het by ongeveer dieselfde temperatuur as waarby die oksiedes finaal in ewewig gebring is.

J.A. COKRADIE EK A.E. SCHOCH 35

VERWYSINGS

Buddington, A.F. and Lindsley, D.H. (1964). Iron-titanium oxide minerals and synthetic equivalents. J. Petrol., 5, 310 - 357.

Conradie, J.A. (1983), Petrological and petrochemical aspects of the Koperberg Suite, Namaqualand. M.Sc. tesis (ongepubl.), Univ. van die Oranje-Vrystaat.

Exploration Department Staff of the O'okiep Copper Company Ltd. (1981). The geology of the copper deposits of the Okiep District, (in pers).

J.A. CONRADIE EN A.E. SCHOCH 1 36

SUGILITE AND THE ASSOCIATED MINERAL ASSEMBLAGE FROM WESSELS MINE, KALAHARI MANGANESE FIELD

by: ROGER DIXON, GEOLOGICAL SURVEY OF SOUTH AFRICA

The sugilite occurrences at Wessels Mine have previously been described, but not the associated mineral assemblages. A petrographic, electron-microprobe and x-ray diffraction investigation has revealed a complex mineral assemblage which contains a number of minerals new to the Kalahari manganese field. The sugilite and its associated skarn-type assemblages are restricted to a few localized areas in the lower ore body at Wessels Mine, between the "Red Line", a ferruginous layer, and the "40cm Red Line" below it. This horizon is bounded above by siliceous banded ironstones and below by the lower ore body.

The assemblages shows a horizontal layering, with the more siliceous minerals at the top and less siliceous minerals at the bottom of he succession. This horizon is thought to have formed by a combination of thermal metamorphism and metasomatism by hot, alkali rich fluids, the alkalinity of which is confirmed by the presence of portlandite, Ca(OK)2, which is stable only under high PH conditions. The vertical succession of the assemblages which constitute the discrete bands is, from top to bottom, depicted in Figure 1.

Assemblage 10 is the most widespread association and occurs as an orange-brown, fine grained calc-silicate rock which is present as thin layers or lenses which are laterally fairly persistent, and are usually not more than 5cm thick. This glaucochroite-bearing calc-silicate rock shows parallel layers of ellipsoid blebs, a millimetre or two in diameter, which are coarser grained than the ground mass of the rock. These blebs contain diopside, pectolite and glaucochroite, compared with the ground mass which is composed of intimately mixed andradite, wcllastonite and pectolite. 37

FIG I. VERTICAL SUCCESSION OF THE SUGIUTE AND ASSOCIATED MINERAL ASSEMBLAGES LOWER ORE-BODY, WESSELS MINE.

BANDED IRON FORMATION "RED LINE" (1) QUARTZ (2) QUARTZ + SUGILITE + HEMATITE (3) SUGILITE + MN-PECTDLITE + 3 DSUMILITE PHASES (i») SUGILITE

(5) UESUUIANITE (6) DI0P5IDE + WDLLA5T0NITE + PECTOLITE + AEG-AUGITE (7) DIDPSIDE + UIDLLASTDNITE + PECTOLITE + ANDRADITE (8) (7) + QUARTZ (9) AIMDRADITE "kO cm RED LINE" (1D) GLAUCOCOCHROITE + WOLLASTOIMITE + PECTOLITE + ANDRADITE + DIOPSIDE MM - ORES HAU5MAIMNITE + BRAUNITE

PORTLANDITE

ROGER DIXON 38

The number of discrete assemblages developed in any one site is variable, but a distinct succession is observed. The order in which the different assemblages formed in the succession is 4, 9,8,7,6, 2 and 1 and lastly 5 and 3 . With the development of these bands there is a concomitant increase in the amount of alkalies in the bulk composition, reaching approximately 15 percent in assemblage 3,

Equilibrium textures, represented by a granoblastic equigranular polygonal texture, are present in layers containing assemblages 1,2 and 4. Minerals such as wollastonite, glaucochroite (CaMnSiO ) and vesuvianite indicate a fairly high temperature of formation for this assemblage. Considering the experimentally determined lower limits for the formation of these minerals a minimum temperature of approximately 400°C, in the presence of a high t-HjO, can be postulated. Decarbonation reactions in the underlying orebody, however, would lower the PHaO and hence increase the temperature of formation. In fact similar assemblages from elsewhere are usually considered to have formed at higher temperatures, beginning at approximately 500°C (Winkler 1976).

Sugilite, a member of the osumilite group, has the composition

KNa, (Fe Mn Sr) 2 (LiAlFe)3 Si^ 0 . In assemblage 3 sugilite of this composition occurs in association with three other members of the osumilite group: one of which has a composition similar to Roedderite (Mg-rich); one is similar to Eifelite (Na-rich) and the third is Ca-rich and has no known analogue. In addition manganoan pectolite (MnO=12 to 17 percent) is also present in this association. The Mg-rich phase crystallized in an acicular, hexagonal habit in small ovoids (1 to 2mm in diameter) which contain sugilite, and which indicate that this Mg-rich phase crystallized from the rim inwards, as in a vug.

Experimental work by Schreyer and Seiffert (1967), with a similar bulk composition, but simplified to the quaternary system KjO-MgO- AI3O3-S1OJ, yielded a phase of the osumilite group as the only crystalline phase, coexisting with a gas of impure H20, at 1 Kbar

PHaO in the range 650° to 700°C. This phase apparently formed in 39

the presence of liquid. Aside from this osumilite-group mineral phase, other minerals were also formed during the experiment at lower temperatures (minimum 500°C).

The overall assemblage, therefore, indicates a moderately high temperature of between 500°C and 600°C. The presence of andradite with diopside in assemblages 7 and 8 indicates an initial C02- rich fluid. This alkali-rich fluid phase was progressively enriched in Ha0 relative to C02 and after the decarbonation stage, thess fluids reacted with the wollastonite to yield pectolite.The fluid nevertheless maintained a relatively constant temperature and produced the assemblages described.

References:

Schreyer, W. and Seiffert, F. (1967) Contr. Mineral, and Petrol. 14,343-358.

Winkler, H.G.F. (1976) Petrogenesis of Metamorphic Rocks. 4th ed. Springer-Ver- lag Inc. 334p. 40

A Quantitative Geophysical Stratigraphy in the Western Bushveld Complex

D. Eberle !) S E.H. Stettler 2)

Geophysics Division, Bundesanstal t f Li r Geowi ssenschaf ten und Rohstoffe, P.O. Box 510153, 3000 Hannover 51, West Germany.

Geophysics Division, Geological Survey, Private Bag XI12, Pretoria 0001.

As part of a research project to improve interpretation of airborne geophysical data in terms of economic geology, the southwestern parts of the Bushveld Complex were studied using magnetic, electromagnetic, and radiometric profiling. The Rustenburg-Brits area was selected due to its known high potential for mineral occurrences.

Ground measurements were made along several geophysical traverses proceeding from the Magaliesberg area northwards to the Elands River and Sand River areas with a maximum measu- ring interval of no more than 25 m. From the data collected along these traverses, the lateral distribution of various geophysical parameters (magnetization, apparent resistivity, and gamma-radiation) was established within the different units of the Rustenburg Layered Suite (fig. 1: Line E).

The Basal and Critical Zones of the layered Suite are weakly magnetized; low magnetization values of 0.06 - 0.08 A/m are predominant. This assessment merits some attention, for high magnetization is frequently observed in basic and ultrabasic rocks due to weathering and serpentinization. Low apparent 4 1

resistivity (10 - 50 Ohm metres) and gamma-radiation values (mean total count rate 1250 cpm; crystal size 3" diameter * 3" thick) are characteristic for these two zones, which are important in terms of their mineral potential.

The Gabbro Main Zone can be magnetically subdivided into at least two sections. The lower part has an intermediate magnetization of about 0.25 A/m; the upper part is more strongly magnetized, revealing an average strength of 0.6 - 0.8 A/m. Elevated apparent resistivity of about 100 Ohm metres (especially within the lower magnetic subdivision) and very low gamma-radiation values (mean total count rate 1000 cpm) are characteristic of the Gabbro Main Zone.

Excessive magnetic anomaly amplitudes of more than 10 000 nT were often obtained in the magnetite gabbro or Upper Zone of the Rustenberg Layered Suite. Strong magnetization with maximum values of nearly 20 A/m is common. The apparent resistivity decreases, reaching values of only 10 - 50 Ohm metres. The gamma-radiation values considerably increase to total count rates of about 2500 cpm.

Finally, the Bushveld Granites are geophysically significant in virtue of their high apparent resistivity, which vary from 200 to 5000 Ohm metres, and their high gamma-radiation with an average total count rate of 7000 cpm. The magnetization decreases to low average values of 0.2 - 0.3 A/m.

It is obvious from the data that the amount of magnetite in the mafic units of the Rustenburg Layered Suite is a function of their stratigraphic distance from the Bushveld G r e n i t e s . 42

The transformation of the amplitude of the total magnetic intensity into magnetization values, which do not depend upon geomagnetic latitude, as well as the calculation of the mean values were done by computer. Another program was developed to automatically determine the boundaries of the different geological units along a profile. All the Bushveld data have been used with an automatic multivariate analysis program so as to be able to recognize still unknown, similar layered intrusion complexes on the basis of air- and ground- borne multi-method geophysical data.

The spectral analysis of an airborne magnetic survey made by the Republic of South Africa (sneet "Wolnuterskop" 2527 DA, sheet "Hekpoort" 2527 DC) yielded a simple model for the vertical distribution of the roc* magnetization. Two magnetic horizons, which have a magnetization of 0.5 to 1.0 A/m, have been located at relatively shallow mean depths of 400 and 1500 m below surface.

As the anomaly contours of near surface and outcropping magnetic sources are distorted by the flight characteristics (line spacing 1000 m/flight height 150 m), the upper horizon must be geologically correlated with prominent outcropping magnetic bodies like hortholinite dunite pipes, which pre- ferably occur in the Basal and Main Zones, or like the magnetite bands of the magnetite gabbro zone. The vertical distribution furthermore indicates that the Basal and Criti- cal Zones of the Layered Suite are nearly non-magnetic, which is in good agreement with the observed lateral magnetization distribution. Due to the absence of magnetic horizons within the basal members of the Layered Suite, the base of the Bushveld Complex cannot be outlined by any magnetic survey. 43 (A/m).

RHQA lOHU'U) 100QO.C

I SDO0.Q 2500.0

fig. O*HM A-RADIATION 9 9

• y~~-'—*-J~

910

2C 45

THE GEOCHEMISTRY AND PETROGENETIC RELATIONSHIPS OF SOME GRANITES WITH MAFIC INCLUSIONS IN THE KE1MOES SUITE, THE NAMAQUA MOBILE BELT - A POSSIBLE MAGMA MIXING MODEL

Geringer, G.J., De Bruiyn, H., Botha, B.J.V., and Van der Westhuizen, W.A. Dept. of Geology, University of the Orange Free State, Bloemfontein

Mafic inclusions are well documented features of granites in high grade metaraorphic terranes and are also present in the 1100 - 1250 Ma granite^ of the Keimoes Suite which is intimately associated with calc-alkaline volcanics of the Jannelsepan Group. It is interpreted as a calc-alkaline magmatic arc along the eastern margin of the Proterozoic Namaqua mobile belt where it intruded the high grade metamorphites of the Korannaland Sequence as late to post tectonic diapiric intrusions.

[ j Janneteepan Groip 1—~j undifierentiatea i , gratnotQ

Fig. i The Keimoes Suite and investigated plutons

GFRI ,GFR. G.1. ft al. 46

Four granites, namely the Colston, Straussburg, Kanoneiland and Ek- steenskuil plutons were selected for study of the genetic relationship between the granite and the inclusions.

Field relationships

In general the Colston, Kanoneiland and Eksteenskuil granites are grey, coarse-grained porphyritic biotite granites with subordinate hornblende. It is corundum normative and do not contain normative diopside. Mineralo- gically these granites can be classified as S-type granites. The Strauss- burg intrusive is a coarse-grained, porphyritic, biotite hornblende granite containing blue opaline quartz. It is diopside normative with no normative corundum and fits the mineralogical description for I-type granite.

The inclusions in the granites vary in shape and size and in abundance.

STRAUSSBURG O

Fig. 2 Modal composition of granite and associated inclusions.

GtRINGFR. G. J . e-1 a] . 47

In the central part of the plutons the inclusions are generally rounded and pillow-like with sharp contacts. Towards the margins the inclusions become elongated and often strongly foliated, which probably resulted from flow during diapiric intrusion. The inclusions in the Colston, Kanoneiland and Eksteenskuil granites are tonalitic to granodioritic (Fig. 2) Two sets of mafic inclusions are present in the Straussburg granite, the one porphyritic and the other non-porphyritic. The non-porphyritic inclusions are tonalitic to granodioritic while the porphyritic inclusions resemble charnockitic adamellite porphyry (Fig. 2). An interesting feature of these inclusions is the presence of blue opaline quartz, an aspect which is considered to be important in the genesis of these rocks. Chemical and genetic relationships. Chemically the granites and their inclusions reveal linear correlations which indicate strong genetic relationships between these rock types (Fig. 3). Biotite analyses indicate that the inclusions are of magmatic rather than metamorphic origin which suggests that the inclusions are not restites of metamorphic precursors (Fig. 4). COLSTON The biotite composition also shows disharmonic crystallisation of the #. tea , + at granites and the mafic inclusions. It is argued that the inclusions in the

rmAUSSBURG Colston, Kanoneiland and C owwn Eksteenskuil granites, as well as the non-porphyritic inclusions in the Straussburg granite, represent early crystalli- KANONELANO sation products which O dtMTl formed at higher temperature and lower water pressure conditions. Multi-stage crystallisation EKSTEENSK*. and emplacement is § o % oo o invoked for these granites.

Fig. 4 Biotite compositions show magmatic affinity of inclusions.

The porphyritic inclusions in the Straussburg granite which formed at higher temperature and slightly lower water pressure conditions than the non-porphyritic inclusions, crystallised independently from the non-porphyritic inclusions. It is concluded that these inclusions represent products of mixing of granitic and charnockitic magma. The porphyritic texture of the inclusions suggests rapid crystallisation of immiscible charnockitic liquid in granitic magma. The process of magma mixing changes the compositon of the granite magma to granodiorite. Mixing also explains the presence of blue opaline quartz in both the inclusions and the host granite.

GERINGtR. G.J. et al. "Late Aptian to Cenomanian submarine fans, west coast,Republic of South Africa" I Gerrard, K Stallbom and P Unstead The identification of ancient submarine fans off the South African west coast (figure 1) is based on the interpretation and correlation of geophysical well logs,including the dipmeter, drill cuttings and cores. Micro- paleontological data are used to define the chroiiostratigraphy and water depth at the time of deposition of the various seismic subdivisions. The tectonic setting of such seismic intervals is interpreted from numerous seismic profiles perpendicular to the coast. The generalized submarine fan model of Mutti and Ricci Lucchi (1972) is used to define the slope, inner, mid- and outer-fan and channel facies. The depositional lobes (suprafan lobes of some authors) generally build up the mid-fan but by definition car. occur anywhere on the submarine fan system where each lobe is a smaller scale and usually finer grained version of the much larger submarine fan. The geophysical log and dipmeter motifs described by Selley (1978) for the subsurface recognition of deep-sea sands are used as a guideline to interpret thinniug- and/or thickening-upward cycles. Detailed examination of cores resulted in the recognition of "classical" turbidites, different types of sediment gravity flows and mineralogical associations typical of deep-sea sands.

Detailed studies show that the microfauna present are divisible into in- situ and exotic populations on the basis of their state of preservation and the nature of the sediments with which they are associated. The in-situ microfauna, which include radiolaria, are found in the argillaceous sediments and indicate a deep-water (circa 200 m) depositional environment. The exotic microfauna and flora are found in the sandy sediments and comprise transitional to inner shelf bivalves, brackish water ostracods, terrestrial megaspores, seeds and large quantities of lignitic detritus.

Dark grey siltstone is the predominant lithology of the slope facies. The deformed bedding structures seen in the cores and interpreted from the dipmeter data are probably due to slope instability. The inner fan facies is generally sandy comprising debris and grain flow deposits. The lithology of channel deposits is typically a clean (very low clay content), massive, carbonaceous and glauconitic sandstone within which rare laminations show poorly defined grading, overturned beds and fluid escape structures. Mid-fan (depositional lobes or suprafan) deposits characteristically show very small scale, sharp and continuous parallel laminations within interbedded sandstone and shale cycles. These cycles get thinner further down the fan (or lobe) until they merge with the outer fan deposits. Interlami- nated fine-grained sandstones, siltstones and shales of the outer fan eventually grade to structureless black basinal shales. A characteristic of all west coast submarine fans is the high sand:shale ratio and very clean and well-sorted nature of the sandstones.

Present-day submarine fans can occur in a wide range of continental margin situations (subducting, trans-current, rifted/drifted). Anciert. analogues to some of these settings have been described from the geologic record (California, New Zealand, Agulhas Bank, North Sea etc.). However, it is difficult to recognize some of the modern features in the ancient analogues because of differences in scale and morphological structural configurations of the ancient analogues. 50

The accumulation of Late Albian to Cenomanian age submarine fans (about 1000-to-2800 DI in thickness) of the west coast was controlled by a steady rate of subsidence in the proto-Orange basin, following the onset of the main drift stage separating the South American and African continents, coupled with a large amount of terrestrial detrital supply. Our terms slope, fan and basin plain are not identical to the physiographic and structural elements, the continental slope, rise and abyssal plain, of modern continental margins. Our terms define physiographic and depositional elements of relatively deep basins but not necessarily abyssal plains. Optimum conditions for the formation of channels are prograding shelf sites where sediment is supplied more rapidly than it can be fully redis- tributed by alongshore processes. The sediment source for the submarine fans is from a coastal complex, and/or the shelf. The lithological characteristics, namely the proportion of mud to coarse sediments and the distribution of facies in a submarine fan depends essentially on the source of the sediments and the volume supplied in each event through time, the nature of the transport process, the configuration and dimensions of the depositional basin and the tectonic stability and balance between sedimentation and subsidence.

It is important then to use criteria from both ancient and modern submarine fans in subsurface exploration techniques. Submarine fans can contain potential hydrocarbon reservoirs because sandstones may occur in most of the facies of a submarine fan system. The sandstones are interbedded with potential source rocks. The channelized inner- and mid-fans form stratigraphic traps due to the sharp lateral variations in thickness of the sandstone channel fill. In a prograding submarine fan system with a high sand input, such as that on the west coast, the sheetlike deposits of the outer fan are also prospective. However, the occurrence of traps in these deposits is more subtle relying on permeability barriers or some form of secondary deformation.

To successfully map submarine fans for hydrocarbon reservoirs early formed stratigraphic and/or structural traps that were available at the time of early migration must be recognised. These sandstones have greater potential for production than those in late formed structural traps.

References 1. Mutti, E and Kicci Lucchi, F, 1972; Turbidites of the Northern Appenines : introduction to facies analysis, Internat. Geol. Rev., V. 20, no. 2, pp. 235-166.

2. Selly, R C, 1978; Ancient sedimentary environments, Chapman and Hall, Ltd., London. 1

SUBMARINE DEVELOPMENT

DOMINANT STRUCTURAL DIP

OUTUfC OF ORAMGE BASK ORIENTATION OF SUBMARINE FANS (NO RELATIVE LOCAUTY PLAN SCALE IMPLIED! omecTioN or FAN PROMIADATION INTERPRETED FROM BOREHOLE DIPMETER ZOO LOG

^i^,.nc- i • POSITION AND RELATIVE ORIENTATION OF LATE FIGURE 1 . ALBIAN TO CENOMANuN AGE SUBMARINE FANS OFF THE SOUTH AFRICAN WEST COAST.

- L 01731 52

A MAKWASSIE-RIETGAT COMPLEX SW OF WARRENTON, NORTHERN CAPE

N.J. GROBLER* and N.A. BLEEKER+

(University of the Orange Free State; +GENCOR) This positive topographic feature, 10 km long and 2 km wide, is doubly plunging and elongated north-south. The core consists of quartz porphyry (dacitic to rhyolitic) of the Makwassie Formation overlain by rocks of the Rietgat Formation of diverse lithology: highly amygdaloidal and altered tholeiitic basalt and volcaniclastics in the form of coarse volcanic breccia and water-borne lithic tuff which is chertified in some localities. Carbonates and thin-bedded tuff with carbonate interbeds appear in one locality.

The rocks of the Rietgat and Makwassie Formations ar«.- overlain unconformably in the west and east by feldspathic quartzite and sporadically- developed matrix-supported conglomerate of the Bothaville Formation. Shallow-water conditions of deposition are deduced. As the Bothaville J Formation pinches against the Makwassie-Rietgat high it has a variable thickness and is absent in some localities. It is conformably overlain by J tholeiitic basaltic to andesitic lava of the Allanridge Formation. Lenses of lava appear near the top of the Bothaville quartzite while lenses of quartzite are found in the lower Allanridge lavas, pointing to a gradation from sedimentary to volcanic conditions.

Some prominent N-S photo linears appear over and in the vicinity of the complex. One of these has caused minor displacement in the bedded tuff. They are interpreted as rejuvenated feeder fissures for the Makwassie Formation. The latter is not laterally continuous but confined to isolated localities, probably where there were the necessary deep-going fissures to tap a subterranean magma reservoir.

N.J. GROBLER and N.A. BLEEKER 53 TRM-' Z

ARCHAEAN GRANITIC S0URCE5 FOR THE DETRITAL MINERAL ASSEMBLAGE IN WITWATERSRAND CONGLOMERATES

]• D K HALLBAUER

] CHAMBER OF MINES OF S.A. RESEARCH ORGANIZATION, JOHANNESBURG

1 A granite-greenstone terrain, similar to that exposed in the Barberton Mountainland, has been proposed in recent yesrs as "1 a model source area for the Witwatersrand detritus. —' Although there are abundant granite exposures known along the margin of the Witwatersrand Basin, comparatively few J greenstone occurrences have been observed.

i Recent investigations into the heavy mineral content of some Archaean granites along the north-western margin of the ! Witwatersrand Basin revealed the presence of small amounts of gold particles in a number of granite exposures. They ~i are occasionally accompanied by a conspicuous sulphide ~" mineralization comprising pyrite, chalcopyrite, sphalerite, - molybdenite and, infrequently, arsenopyrite, gersdorffite - and galena. 5ome granites were found to contain up to 0,23S _. pyrite.

A genetic relationship between the gold particles in granites and those in Witwatersrand conglomerates is indicated by the same range in fineness from about 800 to 960 (Fig. 1), but perhaps more significantly by a similar ~ mercury content ranging from 0,5 to 2,0 per cent. A number of gold particles from granites were found to be intergrown - with pyrite, chalcopyrite or gersdorffite as has been often reported for detrital gold in the conglomerates.

Trace element analysis end inclusion studies of the granitic 54

pyrite have equally shown a number of features reminiscent of detrital pyrite. Noteworthy is the conspicuously high tellurium content, its high correlation to the silver content and numerous inclusions of albite and micaceous minerals.

Larger amounts of pyrite in granites are usually accompanied by nodules of carbonaceous matter similar to those known as "fly-speck carbon" from Witwatersrand conglomerates. These nodules are up to 1 mm in diameter, often intergrown with sulphide minerals and contain small inclusions of thorianite " " or uraninite and REE-minerals. The presence of almost identical counterparts in conglomerates (Fig. 2 and 3) ^ ' requires a revision of earlier postulations and a primary, granitic origin for "fly-speck carbon" is proposed.

The presence of sericitized plagioclase, large unaltered microcline and chloritized biotite in most mineralized ! granites points to a K-Silicate alteration similar to that associated with porphyry deposits.

Other mineralized granites are characterized by their " content of blue opalescent quartz, similar to that often - observed in Witwatersrand conglomerates, accompanied by an . albitization of the original feldspars.

Although detailed petrographic and geochemical studies are still in progress, an intimate relationship between the components of some Archaean granites end detrital minerals in Witwatersrand conglomerates is already evident and a granitic origin for the majority of conglomerate components * is proposed.

D K HALLBAUER 55

20 Ventersdorp Contact Reef

•ioJ

til 750 800 850 900 950 1000

20 Carbon Leader Reef n=1200 ,t

750 800 850 900 950 1000

201 ArchGean Granites n=190

800 .50 900 950 1000 750 ineness -> Pig-are 1 Histograms showing the range of gold particle fineness for the Ventersdorp Contact Reef (Carletonville area), the Carbon Leader Reef and for particles fron some localities of Archaear. granites NW of Carletonville.

D K HALLBAUEH 56

JD:tVK 7 INCLUSION IN CAPIOM ESIG OPTEC ELfflEHTS fOUNt: PBESETILT' n»et nut Tine:2 COUNTS/SECOND: ? 15? FUL••L SCALE:• l« LINEAR l-2t• KEU • ie EU/CH • •• ••i ^M • HHH ^M ••• •|a••• ••• •1 HHI •It* SB m """•m —i 15 U

Il:lK4, INCLUSION IN CHP1OX ELEnENTS FOUNi: HEAD TinE: 21X COUKTS.'SECONS: I'SSB FULL SCdLE: Jif e-je KEU i-» E'I-CH

u L —*—, y ;

Fignre 2 Energy dispersiye spectra of uraninite inclusions in carbon nodules from an Archaean granite { top ) end the Yentersdorp Contact Reef ( Deelkreel mine ) showing a striking similarity in cocpositioE.

D K HALLBAUE?. 57

GOLD DISTRIBUTION IN WITWATERSRAND PALAEO-PLACERS

D.K. HALLBAUER AND M. NAMI

CHAMBER OF MINES OF 5.A. RESEARCH ORGANIZATION, JOHANNESBURG

ABSTRACT For many years the concept of a single, prograding fluvial fan with one entry point has been the dominating model of goldfields within the Witwatersrand Basin'l). By accepting such a model one also has to accept the presence of well mixed detrital material. In this case the composition of individual mineral groups such as gold, detrital pyrite, quartz and others might cover a broad range but it should nevertheless be almost constant throughout the fan. Based on this concept and by extrapolating mineral compositions from smaller areas to the whole basin even collective terms such as 'the Witwatersrand pyrite1 have been used(2) jn tne past. Detailed mineralogical and geochemical investigations have, however, shown that significant differences not only exist in the composition of gold, detrital pyrite and other minerals from different goldfields within the basin but thBt significant differences exist within reef horizons. Fineness measurements on several thousand single gold particles and their statistical analysis as well as measurements of the silver-mercury distribution in single grains^' clearly indicate that: (a) a homogenisation of gold particle composition during a solution stage'*' did not take place, (b) the silver content of gold particles still reflects or is the primary silver content at the time of deposition, 58

(c) the sediments of a particular goldfield were not dispersed through one entry point into the depositional environment. Two major groups or facies could be distinguished for the Carbon Leader. Their regional distribution is well correlated to known palaeo-current directions. This result seen in conjunction with sedimentological observations points to a two or multi-stage depositionel history of the Carbon Leader. Within the VCR several units could be discerned on the basis of gold particle composition end the contents for 24 trace elements in detrital pyrites (Fig. 1). Although local reworked fan deposits have been distinguished^) the results nevertheless indicate that reworking of elder reefs played e minor role in supplying detrital material to the VCR placer. Different primary desposits in close proximity of the Witwatersrand Basin have thus to be assumed to account for the lack of mixed gold particle populations. The distribution of gold in Witwatersrand conglomerates is of course not only influenced by the number, nature and location of primary sources but was also controlled by fluvial transport and the conditions of the despesitional environment. The depositional environment of the Carbon Leader placer may be interpreted from a multitude of observations as a deposition from successive sheet floods. The detrital material was discharged onto an extensive, braided alluvial plain from two directions, northwest and northeast. Each flood event was charaterized by different hydrodynamic conditions. The distribution of gold and final concentration was probably greatly influenced by e transport of fine-grained gold in suspension as indicated by mathematical modelling (Fig. 2). Thus could the Carbon Leader placer on the basis of

O.K. HALLBAUER AND M. OJAMI 59

lithology, texture and internal sedimentary structures be divided into seven different types which in conjunction with the different source deposits influence the distribution pattern of the gold.

REFERENCES 1. Pretorius, D.A. (1981). Economic Geology, 75th Anniversary volume, pp 117-138. 2. Saeger, R. (1976). Ecom. Geol. Res. Unit, Inf. Circular No. 105. 3. von Gehlen, K. (in press). Mineralium Deposita.

I 4. Button, A. (1961). U.S. Dept. of Energy, Report No. J GDBX-3(81). 5. Krapez, B. (1979). M.Sc.Diss., University of the Witwatersrand.

-751—'—•—L '-9 -B -7 -t -5 -<. -3 -7 -1 0 1 2 3 t 5 6 7 t 9 Conanical Variable 1 igure 1 Grouping cf de-.rit&l pyrites froc the \2r. ar.a some Elsburg reefs fror t:.e leelkraal r.ir.e :y Etepvrise discriciri-ir.t ar-tlysir c'1 tr.e^r trace eiecer.t ccr.ter.zt. The results shov: the existence cf tvc V?r. facies at leelkrs&l but dc sot indicate revcrkir.r cf Zlsiurg reefs ( I.'c£.I>34, "rl <_• *J32 ';.

1 D.K. HALLBAUER AND H. NAMI 1 60

I.s- .1 1.4- 1.3-

1.2-

1.1-

1.0-

O.B-

0.7- H 0.6-

0.5-

0.4-

0.3-

0.2-

0.1-

0 1 1 1 1 1 1 1 1 I I 0.001 0.002 O.0O3 0.004 0,005 COOS 0.007 0.008 0.009 0.010 HYDRAULIC GRADIENT Figure 2 Water depth, gradient and velocity requirements for the transportation of quartz pebbles, and the flow conditions for which gold particles will be transported in suspension.

J j

D.K.. HALLBAUER AND K. NAKI 61

THE SEDIMENTOLOGY AND STRATIGRAPHIC SETTING OF THE "A" REEF

PLACERS, AANDENK FORMATION, ON PRESIDENT STEYN GOLD MINE.

WELKOM, ORANGE FREE STATE

W.P. KARPETA

A detailed sedinentological study of the "A" Reef placers

on President Steyn Gold Mine was undertaken to establish

the environments of deposition and stratigraphic setting of

the placers within the Aandenk Formation of the Central

Rand Group.

Underground investigations have shown that two placers are present on President Steyn Gold Mine, the lower being called the Witpan Placer and the upper, the Uitsig Placer. Both have gold mineralization.

The Witpan Placer is up to 2 metres thick and consists of oligomictic, large pebble conglomerates and mature, siliceous quartzites. Three facies associations have been identified the Main Channel Facies Association, the Subsidiary Channel

Facies Association and the Terrace Facies Association, each characterized by differing lithofacies combinations and gold mineralization patterns. The Witpan Placer is thought to have been deposited in a gravelly braided stream system resembling the modern Donjek River. Extensive within-channe 1 compound lateral and medial braid bars were deposited during flood events followed by Quartzites during lower flood stages.

On most of number 2 Shaft, President Steyn, the Uitsig

Placer is represented by the reworked top of the Witpan

Placer, indicating fairly continuous deposition in this area.

On number 1 Shaft and the northern part of number 2

W.P. KARPETA 62

Shaft the Witpan Placer is overlain by up to 1 metre of flat

bedded argillaceous quartzites laid down ephemeral flood events in a partially abandoned part of the stream system.

These deposits resemble those described from the moderii

Bijou Creek.

Overlying these argillaceous quartzites is a 10 cm. thick, oligomictic, medium pebble conglomerate, the Uitsig Placer, which is, in turn, overlain by up to 5 metres of trough and planar cross bedded, siliceous and subordinate argillaceous quartzites. These were probably deposited by a sandy braided stream similar to that of the modern South Saskatchewan River, with the Placer representing the basal lag gravels.

Palaeocurrent directions were NNW to SSE for both Placers.

Eight detailed sections through the Aandenk Formation on

President Steyn have revealed up to five fining upward sequences, each showing an upward decrease in clast size and conglomerate percentage, and having a characteristic pebble assemblage. The presence of these sequences indicates tectonic control of sedimentation.

The "A" Reef placers occur approximately 2 metres above the base of the second fining upward sequence in the Aandenk

Formation. This sequence begins with up to 3 metres of poorly sorted, polymictic, small cobble conglomerates and very coarse grained argillaceous quartzites which were probably rapidly deposited by a pulse of sediment laden flood water asso= ciated with tectonic uplift in the source area. Partial incision with extensive reworking of the underlying sediments led to the formation of the "A" Reef placers.

W.P. KARPETA 63

THE MINERALOGY OF THE VOELWATER SEDIMENTARY ROCKS IN THE NORTH-WESTERN CAPE, SOUTH AFRICA

A.S.E. Kleyensttiber

Council for Mineral Technology RAU Applied Mineralogy Research Group

ABSTRACT ] More commonly known as the Kalahari manganese field, the VoSl- water Jasper Formation in the Hotazel area on the eastern side J of the Dimoten syncline is a relatively undisturbed unit of mixed volcanogenic-chemical sedimentary rocks. The angle of j dip of these rocks varies from 5° to 20° to the west.

This finely banded carbonate-silicate-manganese lutite sequence J of rocks must be unique in regard to the tremendous accumulation of manganese repositories of manganese in the World, J Three depositional cycles for manganese can be observed within J these sediments. Fifteen drill cores, sampled at intervals of approximately 1 m through the thick (110 in) Voe'lwater sediments, were studied by microscope, X-ray diffraction, and electron-microprobe methods. J The mineralogy was studied on a regional scale, with the emphasis on the manganese mineralogy.

Mineralogically, the Kalahari manganese field can be subdivided I into two specific types of areas, where there are differences in mineral distribution and composition but no obvious sedi- 1 mentological differences through the entire rock sequence. Two- -" thirds of the basin is comprised of the so-called Mamatwan-type , area, in which minerals are found that show virtually no alte- i' ration (other than diagenetic influences) from the time of their formation. The manganese ore in thir area has a manga- r nese metal content of between 35 and 38 per cent by mass.

A.S.E. Kleyenstiiber 64

The remaining one-third of the northern portion of the basin has been called the Wessels-type area. Here the mineralogy has changed appreciably, and the manganese ore exhibits a higher manganese metal content in the region of 44 to 48 per cent by mass. The minerals show metamorphic recrystallization and have a much larger grain size than those in the Mamatwan-type area.

Within the Mamatwan-type area the following mineralogy is evident. Minerals in the lowermost banded ironstones and lutites " are : quartz (recrystallized chert), the silicates, talc, stilpnomelane, and minnesotaite. Carbonate compositions are repre- «• sented by members of the dolomite-kutnahorite series and calcite. Kutnahorite is the most common carbonate species and occurs as well-crystallized oSides and fine banding in the lu- _ tite. Hematite is representative of the oxide phase. No primary magnetite is present.

Grading upwards towards the first braunite-lutite, primary 1 rhodochrosite replaces the kutnahorite, quartz and the silicates being virtually absent. With the rhodochrosite, bemen- •1 tite, which is the manganese equivalent of greenalite, starts -1 making its appearance. Hematite is still the only oxide pre- . -. sent.

• Within the braunite-lutite (manganese ore), braunite is the 'j primary manganese mineral, with jacobsite and hausmannite pre- | sent to a lesser extent. Bementite, friedelite, magnesium- I rich tephroite, feitknechtite, pyrochroite, and todorokite are " minor mineral constituents.

«r The gangue minerals associated with the ore are calcite and 1 kutnahorite as carbonates, with hematite as the oxide. Grading out of the braunite lutite, the mineralogy described .] above is reversed, and the rocks grade back into banded ironstones. This cyclic pattern in sedimentation and mineralogy * •1 is repeated three times. 1 1 A.S.E. Kleyenstiiber 1• 1 65

The mineralogy of Wessels-type area is varied and different, but not its sedimentology. Primary minerals in the banded ironstones are quartz (recrystallized chert), andradite, and acmite. Calcite is the only carbonate species and hematite, mostly in the form of specularite, is the only iron oxide present. Minor amounts of piedmontite, banalsite, hornblende, sugilite, serandite, pectolite, and sturmannite are encountered.

Hausmannite, braunite II, bixbyite, and manganite are the chief constituent minerals of the manganese ore in this Wessels-type area. Ths minor manganese minerals include marokite, gaude- froyite, n>agnesium-rich tephroite, and a new braunite species.

Gangue minerals associated with this ore are baryte, andradite, calcite, gypsum, and portlandite.

Comparison of these two areas shows mineralogical differences that can be attributed only to metamorphic and metasomatic effects. This is possible since, within the Wessels-type area, the sediments were subjected to more structural deformation than those in the Mamatwan-type area.

Supergane manganese minerals in the form of peroxides are encountered only in the present-day outcrop areas. These are at the mines of Black Rock, Hotazel, Langdon, and Mamatwan. The peroxides are represented by pyrolusite and ramsdellite, while cryptomelane and manjiroite are supergene minerals found at the Mamatwan Mine.

Comparison of the mineralogy with the sedimentology bears out the cyclic nature of the deposition of the Voelwater sediments, involving transgression and regression relative to a volcanic source.

A.S.E. Kleyenstuber 66

THE USE OF GEOSTAT1STICS IN DEFINING AND REDUCING THE UNCERTAINTY OF GRADE ESTIMATES

The underlying statistical philosophy of confidence limits for estimates of the mean based on limited data is explained. It is shown that this approach, where the data used is confined to the population area, provides a definition of uncertainty which cannot be expanded to a probability distribution of the unknown mean. This has particular significance in the cases, almost invariably so for ore grades, where the frequency distributions of the basic data are skew.

The alternative approach, Bayesian in principle, is examined as the more logical alternative. In this approach information outside the population area concerned is integrated with the 'inside' data to yield not only an estimate with narrower limits of error, but also a probability distribution for the unknown grade. This is essential where feasibility studies for new mining properties ere done and where the overall risk of failure due to the uncertainties of grades, costs, prices and othar estimates, dictate the use of a risk analysis model. This approach also provides a quantification of the 'experience factor' introduced by geologists and mining engineers where a straight statistical estimate of the 'inside' data is incompatible with outside geological and other information.

It is common sense that if an ore grade estimate fells well outside the range of the known grades for similar deposits either adjacent to and in the same continuous host formation (e.g. the Witwatersrand basin) or in geologically similar conditions elsewhere in the world, this 'outside' information will inevitably feature in the final anlysis. This could at least be by way of a subjective adjustment to the grade estimate.

An interesting local example is analysed in some detail to demonstrate the two approaches covered above, i.e. the expected gold grade from the high grade section of the Free State Geduld mine. This mine section was developed and mined first and could be valued originally on only 5 boreholes. The resultant statistical estimate is shown to be unrealistic and the alternative geostatistical approach is used of introducing all the available 'outside' data from boreholes then available for the whole main sector of the Orange Free State Goldfield. The result is closely comparable with the actual ]• underground development for this section of the Free State Geduld mine.

It is also shown that this alternative approach is in principle the same ] technique as that of 'regression' and 'kriging' effectively and successfully in use for some 2 decades for ore reserve estimates on some large South African gold mines.

i D.G. KRIGE 1 DGK/DOH y 20.2.19B4 67

PRESIDENTIAL ADDRESS 1984

D.L. Kyle Anglovaal Limited 56 Main Street Johannesburg, 2001

MINERAL DISCOVERY - THE PHILOSOPHY AND STRATEGY

ABSTRACT

Mineral exploration leading to the discovery of ore has been

recorded in both modern history and as far back as many thousands of

years B.C. The ancient peoples of the Mediterranean region explored

for metals within land based travel distances of those areas, largely

in the Middle East and Africa. Historical evidence indicates

navigational ability to reach and regularly travel to the American

continent. High grade easily workable mineral deposits were their

targets and in the discovery of these It is indicated that they turned

over most occurrences In many of these regions. The ease of

extraction of the metal from these deposits must have been a paramount

target for these early prospectors. War and other catastrophes in the

Mediterranean region from time to time lost the people the abilty to

travel and/or navigate and thus prospecting and metal winning took

place In cycles associated with these events while specific targets

varied by consumer demand for such metals as gold, copper, zinc, (in

the manufacture of bronze) and iron. There must have been a cost

element in these exploratory ventures, however they may have been less

committed to domestic and commercial demands than we are todav.

-2- 68

More recent history has shown that there has been no real change

in the minerals policies and the economics of production and consumer

demand. National minerals policies are set variously to secure the

supply of metals worldwide, to encourage local production and to

ensure that the metal supplies are obtained at the cheapest price or

most appropriate method. These national policies vary considerably

_ between the developed and the underdeveloped countries and depend on

. ' such aspects as source of supply, energy costs, labour costs and

"" availability, and the economic aspects of manufacture varying from

simple to heavy industry and then to high technology. Since the 2nd

World War there has been an increased demand for and consumption of

_ metals; we have seen a change from Britain to America and currently to

the Far East in the monopoly of heavy machinery manufacture. The

change in the oil price in 1973 and its effect on energy costs and

j transport has again affected the national mineral strategies. The

replacement of the heavy machinery industry by high technology t i.

J manufacture in Britain and America is forcing the conventional supply

-I of ferrous and non-ferrous base metals to the Far East, and

production, transport and markets are now having to take account of I this swing j This paper discusses these national strategies in a brief manner

and then concentrates on the philosophy of modern mineral exploration

and those strategies which relate to mineral discovery (finding mines)

and the economics of such exploration and discovery.

-3- 69

Discovery strategies are discussed in an attempt to assess the

[ most effective application of modern methodology and to relate this to

the strategies involved in sound exploration management* The

.1 corporate strategy and the approach by the Exploration Management

1 Team is identified as being paramount to the success ratio and rate in i

the finding of mines. The structure of the management team and the

1 necessary higher standards of technical professionalism are viewed as

being critical to successful mineral discovery, especially at present

. I and more so in the future. This corporate strategy must take

1 account <.i the technical aspects of the probability for mineral possible distribution of ore bodies. The prerequisites for successful discovery in a selected area by relating geological concepts to the exploration staff are discussed, as some are better at finding mines J than others. J

Exploration risk is related to the political, financial and

professional aspects. Corporate survivial in mining is identified as

being dependent on these risks as they are compared to the recognition

J of the absolute necessity to replace depleting corporate ore reserves. J Survival in the present climate of the minerals industry has been

J threatened by a decade of some of the lowest metal prices in real

terms, in tha recorded history of mining. The threatened and actual

-1 take-over of major mining comp&nies by oil conglomerates has had to be

I weathered, in some cases successfully and in others unsuccessfully. -4- 70

The more recent disenchantment of oil companies for metal mining

has now added a new perspective in this survival of the established

mining companies and the possible withdrawal, either partial or

complete, from the minerals scene-

The principles of exploration procedure are considered to enable

the early discussion in this paper to be placed Into context; some case histories of the corporate management of particular exploration

successes are discussed. The value of mineral discovery as related to the cost and lead time of exploration and the statistics of exploration expenditure is weighed up.

In the opinion of the author, optimism must prevail in the face of the gloom and doom theory of the exhaustion of minerals and the lack of ability to find these in economically mineable quanltities.

We draw strength from the firm knowledge that the elements so important for the metals Industry and the survival of mankind as we know it occur in the earth's crust in varying quantities. Scientific progress in the earth sciences together with the sophistication of physical probe methods are considered to be adequate tools to continue to discover economically viable deposits. Even If these discoveries become, in good time, few and far between, the economics of the world and the price of metals will through our technological advances be such as to allow the exploitation of perhaps lower grade ore; particularly when taking into account the differing conditions of cost and energy, transport, human skills and production and financial objectives in different parts of the world. These conditions will

-5- 71 allow for a lengthy continued exploration of mineral resources before the economics of the time precipitate other sources of the elements required. It would therefore appear that the Incipient plans for ocean mining may appear to be a little before their time, although one must take note of changing economics via the advance in ocean mining technology. The supporters of the early acquirement of metals from space must surely be considered dreamers compared to the rest of the ordinary explorers.

Although the aspects of historic and natural policies and the future of the mining industry is discussed, this address concentrates largely on mineral exploration statistics read against the cost of discoveries and their ultimate value in money terms and to corporate survival; the important structure and attitude of exploration management teams; the very necessary professsionallsm and dedication of exploration staff and the application of the science of geology and its satellite fields to the discovery process. 72

ESTIMATING RECOVERABLE RESERVES IN A NEW WAY

I.C. Leramer

Gold Fields of South Africa

Feasibility studies of mining propositions often have to be based on very few boreholes, because of the depth of the deposit and the cost of drilling. It is therefore essential to extract as much information as possible from the available data. At the same time, it would be very interesting to know how close the resultant predictions come to reality.

The proposal is to employ generalized indicator kriging in this J context, and to illustrate to what extent global recoverable reserves can be forecast at such an early stage. For this purpose we use a computer-simulated tabular deposit of nickel laterite origin, called Stanford II \ The 'lease area* is 550 metres long and 110 metres wide (see Fig. 1). It is 'perfectly known* through 60500 'core' samples on a square metre grid. For simplicity sake, constant thickness of mineralization will be assumed. This rich set of data will only be used as background information whenever we need to construct a picture of the 'true reality*.

The lease area can be seen as five contiguous squares of 110 x 110 metres each. To simulate a typical exploration situation, five boreholes are 'drilled*, one each in the centres of these squares. Furthermore, as is usually the case with deep boreholes, four deflections are 'drilled* off each sveh borehole, to the North, South, East and West, to intersect the mineralization at a deviation distance of 10 metres from the original hole. In practice it is usually not ] possible to pinpoint these positions so accurately, but, as we shall see further on, it is only necessary to estimate the average of a set of such deviation distances as accurately as possible.

1 Figure 1. Lease area of Stanford II indicating the given boreholes with crosses. The contour lines are based on the exhaustive 'true* information available : dotted line - 0,0 X Cu; broken line - 0,5% Cu; solid line -1,0% Cu; hatched areas - above 1,5% Cu. 1 1 73

We thus start out wi'h five clusters of samples, one around each borehole, and can make no assumption that the grades follow a standard distribution like a normal or lognorraal one. The mean grade of the 25 samples is 0,58%, let us say, copper (Cu). The variance (which is too low due to clustering) is 0,28 (%).

If we briefly check the 'true* data at this point, we ilnd that the mineralization is very variable and about a third of all the grades is waste. The five clusters missed the higher grade patches, but it also hit a smaller percentage total waste.

The fact that the true average grade is 0,58% Cu, is thus pure chance. However, the highest sample grade is 1,68% Cu, whereas the highest true grade is 3,45% Cu and the true variance of the deposit is 0,38% .

As far as the sparse borehole data is concerned, Figure 2a shows a histogram of the 5 average cluster grades and Figure 2b a histogram of the 25 sample grades, which, ^ocause of the clustering, could not be considered representative of the deposit. Compare these with the solid line curve in Figure A, the true histogram for the deposit.

,2. it

b* i 1 ! J j 1 . Grade (% C) ''* Grade

Figure 2a. Histogram of Figure 2b. Histogram of 25 averaged cluster grades. sample grades.

The clustering of the data, on the other hand, affords us an unusual opportunity to estimate the behavioir of the grade variogram close to the origin reasonably well and this is important information. With the level of data available, we cannot hope to establish a directional 1 anisotropy and the decision is made at the outset to calculate an >mni- directional grade variogram (if the thickness were not constant, a grade x thickness variogram). Also, small nuances in variogram model rould not be picked up by such sparse data, so we will settle for the best single-structure spherical tnodel^ that can be flttad to the experimental variogram.

Figure 3a shows the calculated experimental variogram (solid line) and the best spherical model (broken line) that was fitted to it, given by

v(h) = 0,12 + 0,335[l,5(h/105) - 0,5(h/105) 3J , h •'- 105

= 0,455 , h > 105 1 74

*(h) 1fa

°A-

03.-• V 0,1- ' /I1 \

1 • ko K irv ^M ]• Figure 3a. Omni-direetional ex- Figure 3b. Omni-directional experi- perimental variogram based on 25 mental variogram based on 60500 samples (solid line). The broken samples plotted for lags of 5 line shows model fit. The number metres. Each point represents in of pairs involved In ralrulating excess of 40500 pairs and some each point is given In brackets. points more than double that.

i The experimental /ariogram shows a hole effect (and so does the 'true variogram' in fact), but the spherical model gives an adequate fit of the experimental variogram in Figure 3a up to distances of 90 metres, which determines our next decision not to krige over distances larger than that.

! Figure 3b shows the 'true omni-directional experimental variogram*, based on the 60500 samples, for comparison and illustrates the fact that if one succeeds in estimating the behaviour of the variogram close to the origin, as well as the sill value, rather well, then one stands a good chance of ending up with a fairly representative variogram for all further calculations. Each point on the experimental variogram plot represents an average distance for the set of pairs used to calculate that point, and, as remarked before, it is thus only necessary to estimate this average deviation distance for the first few points reasonably well. '1 Following tii^ logic of a generalized indicator kriging calculation, each 110 x 110 square metre block is digitized into 16 evenly spread discretization points, and ordinary point grade kriging is performed for each such discretization point separately using a sample search radius of not more than 90 metres in order to get the average point kriging variance over the deposit. Taking the square root, the average kriging standard deviation is used to set up generalized indicators at each sample point of the form (see 1)

(Z;z) - z)/o] 1 1 75

where Z is the sample grade, z is the cutoff grade and o is the kriging standard deviation. Clearly, if one rhooses a set of, say, 15 cutoff grades: 0.0001, 0.2, 0.4, 0.6, .... 2.8, this will result in a

set of 15 corresponding values iG(Z;z) at each sample point. (The first cutoff grade is chosen so that it will Isolate the waste). For e&ch 110 x 110 square metre block, discretized into 16 points, we now krige 15 times, going down the set of cutoff grades, using the grade variogram model and the appropriate 1Q -value at each sample point:

16 (z) 1/16 (z) block "discr 5

and n* ( "discr

for each cutoff grade z, and using the 5 samples in each square. The z s e ect ve t ie X's are the ordinary kriging weights. g, ^ ck^ ^ * ff i ly ^ predicted grade cumulative histogram value at grade z for a 110 x 110 square metre block and the average of tiie 'J'GKI V ~ values gives the estimated point grade cumulative histogram for tne lease :

* (z) 1/5 A * (2) — i lease i=l block i from which we derive the histogram values by means of successive subtraction for illustration purposes.

V 0,3 "True His+ocfa 3 H.»to^r.«

"7rm

Variance-. o,3S (%y J j Figure 4 'True* grade histogram for the lease based on 60500 samples (solid line), compared with the estimated histogram for the lease based J. on 5 clusters of sample data (broken line). The waste values in both rases are indicated by a 'spike* of zero grades to the left of the origin.

J 76

The estimator as illustrated here is slightly biased, and if one h

Figure 4 shows the true histogram of the 60500 values (solid line) and the predicted histogram (broken line).

Note that this estimated histogram predicts the existence of grades higher than the highest sample grade rather well and also the existence of a certain amount of waste. Also, note the degree of refinement when compared with the 'raw* histograms in Figures 2a and 2b.

J If selection is contemplated, further adjustments based on the anticipated size of the mining selection unit, l*]fe an affine J . correction which can be calculated from the variogram , will have f ,\ ]• to be made to the histogram. This will then allow one to forecast | * tonnage, quantity of metal and average grade of the recovered ore for different pay limits. With our hindsight we realize that both the variogram and the point grade histogram have been estimated well enough to allow such forecasts of recovery functions with good result.

Which is quite a refined level of information to obtain from five J boreholes with their deflections.

Acknowledgments

J The method illustrated here was developed as part of PhD studies at Stanford University with Prof. A.G. Journel. The study opportunity ' "T| was afforded by Gold Fields of Soutli Africa. • J I Use of the Stanford II computer-simulated deposit is acknowledged.

f References J l. Lemraer, I.e. Estimating Local Recoverable Reserves via Indicator £ Kriging, Geostatistics for Natural Resources Characterization, 2nd 7 NATO Advanced Study Institute, 1984, D. Reidel Publishing Company, * "I Dordrecht, Holland. 1 2. The computer-simulated deposit, Stanford II, was developed at i -» Stanford University by members of that school of geostatistics.

7 3. See, for instance, Journel, A.G. and Huijbregts, Ch.J., Mining ^ Geostatistics, 1978, Academic Press, London, Reprint 198], p. 163.

4, Reference 3, p. 471.

1 I 77 T&*J:

1 ZIRCONS FROM THE GRANITES AND GNEISSES OF THE NAMAQUA

' MOBILE BELT AND THE INTERPRETATION OF RADIOMETRIC AGES

I J.C. LOOCK

I (University of the Orange Free State) A study of zircon concentrates from selected granites and gneisses of Bushmanland and !"amaqualand revealed that radiometric ages can be interpreted by taking the physical and crystallographic properties of 1 crystals and grains into consideration. Within limits, an indication of the geological history of the rocks can also be obtained.

Selected samples collected by various workers were submitted to the CSIR for a determination of the isotopic ratios of U and Pb. As the calculated radiometric ages appeared anomalous in some cases, and could not be related to known geological events, a microscopic and X-ray diffraction study of zircon concentrates was undertaken. For examination by means of a petrographic microscope, a small sample of a concentrate was mounted on a standard glass slide by using a resin of high refractive index. The aims of the microscopic studie was (i) to J determine the number of zircon populations present in each sample, and (ii) to see whether the population represented granitic or gneissic zircons and (iii) to detect the presence of annealing.

The habit (Pupin, 1980), shape, and presence or absence of inclusions, cores, zoning, and overgrowths of zircon suites were studied. Physical properties such as colour, roundness, surface texture, lustre and degree of transparency was also noted. It was found that the zircon suites are either unimodal or bimodal. Round brown or pink grains, usually with concentric zoning, are interpreted as derived or inherited "gneissic" zircons, indicating assimilation of older rocks. Clear and colourless to light pink prismatic crystals with dipyramidal terminations aiid zoning parallel to the crystal faces, are named "granitic" zircons and are interpreted as representing the young, newly-formed generation.

J.C. LOOCK 78

Zircons annealed through a prolonged metamorphic event and thus subjected to resetting of the isotopic system, can be recognised by their glistening irregularly rounded surface, their transparency and the absence of zoning. Inclusions, if present, occur in clusters and are not scattered throughout the crystals as in the case of granitic zircons.

In general it was found that the X-ray powder diffraction patterns of the granitic zircons (non annealed) are more diffused than those of the annealed zircons- an observation which corresponds with that of Koppel and Grunenfelder (1971).

The results of the investigation of three zircon suites are briefly discussed.

Wyepoort Granite, Spektakel Suite. Radiornetric age: 1132 Ma

Acicular granitic zircons indicate that the calculated age is most probably the true age of emplacement. A few round brown zircons indicate some contamination by older material.

Konkyp Gneiss, Little Namaqualand Suite. Radiometric age; 1098 Ma

The zircon population is bimodal, consisting of gneissic and granitic types. Pronounced annealing of the crystals indicate that the radiometric age represents the age of metamorphism or transformation of a much older rock into a gneiss .

Brandewynsbank Gneiss, Gladkopsuite. Radiometric age: 1456 Ma

Both granitic and gneissic zircons a~ e present. Necked crystals indicate a gneiss-forming event. Partial annealing and the presence of small cores, leads to the conclusion that the radiometric age may be a mixed age. The rock should be older than the radiometric age and the time of gneiss formation much younger.

J.C. 10OCK 79

I Lower intercept ages

These ages, obtained from the concordia plots, range from the Late J Triassic to the Holocene, but may be geologically meaningless. However, according to the dilatancy model, these dates could be related to the Gondwana rifting and subsequent uplift and erosion.

J REFERENCE

j Koppel, V. and Griinenfelder, M., 1971. A study of inherited and newly formed zircons from paragneisses and granitised sediments of "j the Strona-Ceneri Zone (Southern Alps). Schweiz. Mineralog. Petrog. Mitteilungen, 51, 385-409. 1 -1 Pupin, J.P., 1980. Zircon and granite petrology. Contr. Mineral. Petrel. . 73, 207-220. i

1 1

J.C. LOOCK .1 8 0 J SOME APPLICATIONS OF GEOSTATISTICS IN MINERAL EXPLORATION By E.3. MAGRI and J.5. MOSTERT

ABSTRACT

The use of geostatistical methods in the estimation of ore reserves for feasibility studies and later for selective mining and grade control are well known. Applications of geostatistics in mineral exploration such as the J design of sampling campaigns i.e. sample size, spacing, etc, the design of sample preparation methods, the selection of analytical techniques, etc are often neglected. This paper highlights the importance of the use of geostatistical concepts in the various exploration stages of a mining venture through the analysis of two case studies. J 1. OPTIMISATION OF A BASE METAL GEOCHEMICAL GRID A known occurrence of apparently stratiform synsedimentary copper mineralisation in a sequence of calcareous snd arenaceous metasediments J' served as the conceptual model for a regional exploration programme. Orientation geochemical sampling indicated that conventional traverse sampling of the B soil horizon and analysis of the minus 80 mesh fraction for base metal content by atomic absorption spectrometry was J' effective in identifying and delineating anomalous zones.

In the course of the regional geochemical survey, a number of anomalous zones were identified which required follow-up detailed soil sampling to provide sufficiently accurate definition of the anomalies in order to enable the siting of boreholes for further investigation. The large number of soil samples envisaged in the detailed geochemical surveys J prompted a geostatistical study to optimise future grid designs in similar environments so as to take the minimum number of samples, yet still achi3V> the required degree of confidence in the anomalies and J their spatial disposition. I A 2000 metre long, east-west striking zone of anomalous copper values -J was indicated by the regional survey in soil covered, gently northward sloping terrain. The complete absence of outcrop and the broad nature ~j of the regional anomaly led to a detailed sampling programme at the _j somewhat conservative sample spacing of 10 metres along north-south traverses speced at 100 metre intervals along strike. ] Consider, for example the distribution of copper values shown in Fig. 1. While this sampling grid accurately delineated the significant copper anomalies, a geostatistical analysis of the data showed that ] considerable savings could be achieved in reducing che north-south density of sampling without sacrificing confidence and accuracy. ] 1

E.3. MAGRI .l.S. MOSTERT _ The logarithmic semivariograms shown in Tig. 2 show clearly the difference in ranges for the N-S and E-W directions, as would be --. expected. New rectangular geochemical grids can then be designed so that the ratio of the sampling intervals in the E-W and N-S directions ~ is similar to that of the semivariogram ranges. The sampling interval on strike is determined by the size of the anomaly being sought. For example, if the appropriate distance between the traverses of a new — regional grid in a similar geological environment was 400m, then an adequate dip sampling interval can be obtained from the following relation:

dip distJ"ce _ dip range strike distance strike range

"" in this case 1 dip distance = 222. * *Du = 155m -i 850 "I _ Naturally geological criteria are of prime importance and often J constitute the overiding factor in determining grid dimensions, but this method supplies an additional tool for the design of geochemical grids.

2. THE DESIGN OF SAMPLE PREPARATION METHODS

A promising tin prospect in the advanced stages of an exploration ~! programme presented problems related to the optimum and most cost —i effective sampling preparation procedures to be adopted.

-1 Cassiterite mineralisation occurs as disseminations in fracture bound j steeply dipping lodes crosscutting both the strike and dip of moderately to steeply dipping, east-west striking felsites. The apparently erratic -. distribution of tin values in the lodes and the generally fine grain J size of the cassiterite dictated the necessity of taking samples of relatively large volume. Accordingly, in order to study the distribution of tin values along the strike of the lodes, a detailed j trench sampling campaign was carried out. As a result of this exercise, —. over 6000 samples, each weighing between 10kg and 15kg, were collected.

"? Due to the fairly lsrge mass of each sample, sample preparation became a jt problem. In order to gain some understanding of the magnitude of the errors involved, the following procedures were tried nut using 30 -, randomly selected samples:

Samples were crushed on site yielding a product containing approximately 50% to 60% under 10 mesh material.

— 2.2 Ea'.n sample w-s thoroughly mixed and riffled to obtain two duplicate So (es of approximately one kg each (stream A and B).

E.J. MAGRI J.5. M05TERT

1 82 2.3 At this stage the samples were transported to an analytical laboratory in 3ohannesburg where three different sampJe preparation methods were tried out on all the test samples, as shown in Fig. 3. In all cases, two duplicate analytes were obtained from each stream.

2.4 Some representative results obtained using method (a) are shown in Tig. 4. They clearly show that the method yields a total coefficient of variation (including field and laboratory errors) of 26,AS, which is considered to be inadequate.

2.5 At this stage a screening test was carried out in order to gain understanding into the size-distribution of the mineralised particles as well as to arrive at the necesssary degree of sample fineness required before further splitting of the samples in order to yield acceptable levels of error. The results obtained for one arbitrary sample are given in Table 1 and show that material below 12 mesh yields acceptable coefficients of variation, these however correspond to laboratory errors only.

2.6 From the above results it was clear that samples need to have a higher degree of fineness before further splitting. Therefore a jaw crusher, that can handle high volumes of throughput, was adjusted to yield a product containing more than 90% under 10 mesh material. This corresponds to method (b) shown in Fig. 3 and produced the results shown in Fig. 5. Judging by the total coefficient of variation of 15,2% this method yielded an unacceptable total error.

2.7 Acceptable results were obtained by pulverising the entire samples obtained from the field splitting (± 1kg each). The pulverising, however, was done with a disk pulveriser that can handle fairly large volumes rather than using the Ziebtechnik swing mill, which yields a much better product but would not be able to cope with the necessary throughput. The results are given in Fig. 6 which shows a coefficient of variation due to field and laboratory srrors of 5,2%.

2.8 It is a well known fact that disk pulverisers can produce serious sample contamination, especially in samples containing high amounts of gold. Therefore, it was decided to pulverise samples in the order shown in Fig. 7. In all cases five analytes were assayed for each sample.

E.3. MAGRI 3.S. MOSTERT -' 6 3

1 Test 1000X000X000X000

Test 2AAABAAABAAABAAA

J 0: homogeneous samples derived from a single low sample containing ± 0,2% Sn.

J X: high contaminants containing ± 19* Sn derived from a single high sample.

j A: homogeneous samples derived from a single high sample

-* containinq ± 4,5% Sn.

I B: Contaminants containing no tin.

Fig. 7 Disk pulveriser contamination test.

The results of the sample contamination tests are given in Table 2 and indicate that for tin samples the contamination due to the I disk pulveriser is minimal. —i < Table 2 Disk pulveriser contamination test.

J Test Samples No. of Samples Mean Standard Derivation

J 1 low before high 20 0,0193 0,0021 contamination

1 1 low after high 15 0,0248 0,0033 contamination

"j 2 High before low 20 4,59 0,256 contamination

High after low 15 4,502 0,0668 J- contamination CONCLUSIONS

" Statistical analysis in the form of histograms and variograms can help in the design of soil geochemistry sampling grids.

J Sample preparation methods should be specially designed for the size and type of sample being analysed. When sampling a new ore occurrence, a ] pilot sample preparation study should be made in order to ensure adequate levels of error. This paper clearly shows the multidisciplinary nature of geological exploration. Therefore, ideally, the different disciplines should work J together right from the start of the project. 1 ACKNOWLEDGEMENT The authors are grateful to Anglovaal for permission to publish this paper.

E.3. MAGRI 3.S. MOSTERT k, t t__J U—J I—J L_e-J l_-J I J J i I I I. _J I 4 I I I J I i

TABLE 1 SCREENING ANALYSIS OF AN ARBITRARY SAMPLE

STATISTICAL ANALYSIS MESH SIZE FRACTION WEIGHT % % OF TOTAL MM N HIGH LOW RANGE MEAN S.D. C.O.V. Sn

+ 3,35 42,3 16 3,35 1,17 2,18 1,77 0,68 38,4 39 o - 3,35 + 1,7 14,2 4 2,52 1,94 0,58 2,22 0,24 10,3 16 in - 1,7 + 1,4 4,5 6 2,30 2,14 0,16 2,22 0,09 4,1 5 12 -1,4 +0,85 9,9 8 2,44 2,33 0,11 2,40 0,04 1.7 12 20 - 0,85 29,1 8 1,87 1,78 0,09 1,81 0,03 1,7 28

AVERAGE SAMPLE VALUE = 1,93% Sn

EJM/D.1H 20.2,1984 85

o143 O

143 O 143O

fig. I DETAIL GEOCHEMICAL GRID Sample spacing N-S = 10m Sample spacing E -W = 100m

0 30 Sill

ID ]

fig.2 SEMIVARIOGRAM OF In (Cu% >* N-S n = I48S i + E—W meon=98ppm S =0,269 J REF NO 63/79 1 E J MA6RI and JS MOSTERT L. - » I I _ J

15kg - 20kg SAMPLE.

Fig 3. Somple preparofion 3- me f hods. CRUSHING* 50V. - 10" m ON SITE j RIFFLING. I < |'l»g STREAM A) |» Ug STREAMBJ £2" J / 1 1 //. Fig. 4. Coarse crushii, i n) Further riffling. C + C.O.V. = 26,4% / b) Furfher crushing ro 90% - 10* and riffling. JHB Laboratory in * 1- + ±/ c) Disk pulverizing and riffling. + ly*

I I | «230gAg|

by XRF I 2 3 All four valust roportad. %Sn STREAM AI

Ixl 2-- Fig. 5. Fine crushing

C.O.V. = 15,2'/, Fig. 6. Pulverizing c V) C.O.V = 5,2%

I ••

I 2 3 12 3 4 % Sn STREAM Al % Sn STREAM AI B3/P0 87

GEOCHEMISTRY OF KAROO BASALTS AND DOLERITES IN THE NORTHEASTERN ORANGE FREE STATE : RECOGNITION AND ORIGIN OF TWO NEW KAROO BASALT MAGMA TYPES

Julian Marsh Department of Geology, Rhodes University, Grahamstown, 6140

One of the most significant results emerging from the Karoo Volcanics Project of the NGP is the recognition of a number of geochemically distinct basalt magma types occurring within the lower part of the Karoo volcanic pile in the northeastern Cape and southern Lesotho. Moreover, these different types are invariably confined to the stratigraphically mappable units defined by Lock et al. (1974) and those described and mapped by various research students at Rhodes University. An outstanding problem of the volcanic geology of the Central Karoo area is to determine how extensive these stratigraphic units and their distinctive basalt types are, and to determine whether hitherto unrecognized types are present elsewhere in the Central area. In order to answer some of these questions a reconnaissance study was made of the Karoo basalt outliers in the northeastern Orange Free State between Clocolan (27.55°E;28.19°S) and Golden Gate (28.62°E;28.58°S). Samples collected in this study were supplemented by a suite collected on the Monontsha Pass (28.78°E;28.58°S) during the NGP.

Features which can facilitate stratigraphic subdivision such as laterally persistent sandstone horizons and extensive tephra layers do not appear to be present within the basalt lava seguence. Nevertheless evaluation the major and trace element chemistry of samples from the lowermost part of the volcanic succession indicates that at least three different basalt magma types occur in the area and that flows of these types occupy consistent stratigraphic positions. Therefore it is possible to recognize a chemical stratigraphy within the basalts similar to that known to occur in the northeastern Cape, although at present the lack of definable field or petrographic characteristics may make mapping of these basalt types difficult. Fig. 1 illustrates the distribution of basalt outcrops m the northeastern OF? and a schematic section showing '..he distribution of the three magma types along the section line- delineated on the rr.ap. Average composition of the 3 basalt types are presented m Table 1 and they are easily discriminated on a plot of Zr/Nb vs Zr/Y (Fig. 2) where they

Marsh 88

"* are also compared to some of the main basalt types from the northeastern Cape. It is clear that one of the basalt types recognized in the OFS is equivalent to the voluminous Lesotho type and it is significant that it occurs in the upper part of the succession in both areas. The other two types have no counterpart in the northeastern Cape. The basalts with Zr/Nb- 10 and Zr/Y =4.0-4.6 form the lowermost lavas in the west and are designated the Wonderkop type. In the east the lowermost lavas have Zr/Nb> 34 and Zr/Y > 4.7 and are designated the Golden Gate type. Both types are present in some areas (Mount Morkel and the Monontsha Pass - Fig. Kb)) and " in such cases the Golden Gate type underlies the Wonderkop type. Fig. 3 shows various interelement relationships for the different types. , , Included on these plots are the data for a number of dolerites which intrude the lavas and underlying sedimentary strata. Also shown are low-pressure (gabbro) fractional crystallization trends from a typical primitive (high ! low Zr) Lesotho-type parent magma. It is clear that the dolerites are probably differentiates of Lesotho type magmas but the Wonderkop, Golden Gate and Lesotho types are not related by low pressure fractionation. Relationships between Zr, Ti, Nb, Y, Cr, and Mg^ can also be used to reject 1 derivation by variable degrees of partial melting of a chemically homogeneous source and a relationship through high-pressure fractionation. Another possibility is that the various basalt types are related through J crustal contamination. Full examination of this possibility must await consideration of isotopic data. Marsh and Eales (1984), Hawkesworth et al. J. (1984) and Bristow et al. (1984) have evaluated in detail the role of crustal contamination in the petrogenesis of Karoo iavas and have concluded L that it is unlikely that it has been a significant process in the generation of the Central area basalt types. It is believed that this conclusion "I applies equally well to the Wonderkop and Golden Gate types. The distinctive chemical features of these types ultimately reflects those of m, their source rocks and this supports proposals that an enriched chemically heterogeneous mantle, probably located in the lithosphere, is the source • material for the Central area basaltic magmas.

REFERENCES , Bristow, J.W., Allsopp, H.L., Erlank, A.J., Marsh, J.S. and Armstrong, R.A. (1984) Spec. Publn. geol. Soc. S. Afr., 12_ (in press).

Marsh 89

Hawkesworth, C.J., Marsh, J.S., Duncan, A.R., Erlank, A.J. and Norry, M.J. (1984) Spec. Publn. geol. Soc. S. Afr., 13_, (in press). Lock, B.E., Paverd, A.L., and Broderick, T.J. (1974) Trans, geol. Soc. S. Afrc. 77, 117-129. Marsh, J.S. and Eales, H.V. (1984) Spec. Publn. geol. Soc. S. Afr. 13 (in press).

- 26* 3C'

UESOTHO TVfE GB WoNfcEtcOr TyPE £|KTE T/Yt

MotCEL Fig. 1: Basalt outcrops in the northeastern OFS with a schematic-section showing distribution of the 1 main basalt types. J Table 1

Average catpositicn of three basalt types

Lesotho Wonderkop Golden Gate SiO, 51.50 52.26 52.41 TiO, .97 1.04 1.03 Al.q, 15.19 15.82 15.16 F^O, 11.30 10.37 11.01 HlO .37 .15 .17 MjO 7.42 6.62 6.33 CaO 10.70 10.66 10.48 Na,0 2.12 2.24 2.42 K.0 .58 .72 .60 P,O5 .13 .17 .14

Fb 11.7 12.5 14.7 ST 154 236 172 Y 24.7 25.6 27.2 2r 91 115 135 tto 3.6 11.3 3.1 2n 80 76 64 Qi 94 75 97 Ni 97 9€ 97 Co 51 4? 48 Cr 355 391 300 V 249 24E 261 Nc. of samples 12 4 9

Marsh 90

f^-M0SHtSH'i FORt> ^

\1 yhiiKifa curve Moskesk's 5.O- iP ** B*M RIVER. *

* 4.0-

• Sf • ''" "^ LESOTHO 30 i i i i i 30 40 Zr/Nb

Fig. 2: Zr/Y vs_ Zr/Nb. Dots - Lesotho type; triangles - Honderkop type; stars - Golden Gate type; Squares - dolerltes.

Foes 14 14-0 A - * 12 • n u A • 10 no & .QAICCO -^v>. . Zr B CtltkS KNEE) loo D / * • ""•-•'

•

EU • 2 0s- • **n t t i o - 11 /& S LESOTMo-^^ "~\ *£' 1 1.0 •••* r J i# i • O.JV * & D u 1 03 ,*••**•;' , 0 * vtACTlOMWTOH ^ • * J2 s 200- O.BVO ^7 c •3 • too ( 1 y '^ (?) 35 4.O *.S 24

&'•/' • MOWEtH'i Fig. 3: Interelement relationships for basalts. ffO IJO ISO Symbols as in Fig. 2. 91

KAROO BASALTS OF THE SPRINGBOK FLATS, TRANSVAAL: GEOCHEMISTRY AND CORRELATION

Julian Marsh Department of Geology, Rhodes University, Grahamstown, 6140

2 Karoo basalts underlie an area of about 3750 km of the Springbok Flats. The Basalts are poorly exposed and nearly all the available information on these rocks has been obtained from borehole intersections. The basalts are confined to two synclinal basins elongated north-east and are downfaulted against older rocks on their northern margins. Borehole

data indicate that in the northeastern basint the basalts have a maximun thickness of about 460 m (Visser and Van der Merwe, 1959). In the southwestern basin the preserved basalt sequence is thicker, being in excess of 750 m. Individual flows vary from 1 to 30 m in thickness and are characterized by amygdaloidal bases and tops. Flow-top breccias are common. Four borehole cores, two from each basin, were logged and sampled for detailed petorgraphic and chemical analysis. Most of the flows are of fine- grained aphyric or sparsely plagioclase-phyric basalt. In one borehole (RTL1) from north of Immerpan siding in the northeastern basin, 5 flows, totalling about 25 p\ of a plagioclase macroporphyritic basalt, occur interbedded as a unit with the aphyric lavas about 300 m above the base of the volcanic succession. Table 1 presents and compares the average chemical composition of the Springbok Flats basalt with those of the Sabie River Formation of the Lebombo and the widespread Lesotho type of the Central Karoo area. The macroporphyritic basalts in borehole RTL1 are similar to the Sabie River basalts of north-central Lebombo although in detail the former have lower an MgO, total iron, Ni, Cr and Sc and higher A12°3 °' incompatible elements

(Y, Zr, Nb, K2O, P^, Ba, Sr). Bristow (1982) has described flows in the Sabie River Formation near the Lataba River which are petrographically indistinguishalbe to the macroporphyritic basalts in RTL1. The aphyric basalts exhibit strong compositional affinities to both the Lesotho and Sabie River basalts from the southern Lebombo but are distinctive in having higher Cr and Co (Fig. 1). Preliminary Sr- and Nd- isotopic data support these conclusions (Fig. 2). The macroporphyritic

Marsh 1 92

samples have £„ and cK1J which are identical to incompatible element- Sr Nd enriched lavas of northern Lebombo. The aphyric lavas exhibit a wide range in positive e similar to that of the Sabie River basalts of southern 1 Lebombo, yet the former have much less negative e^, being similar to those of the Lesotho and other Central area basalt types. J Vertical compositional variations encountered in single boreholes in the northeastern (RTL1) and southwestern (LB1) basins are illustrated in Fig. 3. The distinctive composition of the macroporhyritic lavas in RTL1 is evident. Their five-fold enrichment in Ba, Sr, Zr and P, yet similar Ni , SiCL, V and lower Cr and Sc when compared with the aphyric lavas is inconsistent with these types being related by fractional crystallization. Amongst the aphyric lavas more evolved basalts in terms of Mg^ and Cr occur towards the top of the sequence but this evolved character is not apparent in the data for other element (e.g. Zr, and in RTL1, Ni). In LB1 the P-CWZr ratios are suggestive of the presence of two types of basalt and this distinction is apparent when other ratios, e.g. Zr/Y, are considered. Either one or both of these types are present in the other boreholes and they may be interbedded. More work is necessary to characterize the two types more fully and to determine their extent and stratigraphic significance. SACS (1980) currently correlates the Springbok Flats basalts with the Letaba Formation, the name applied by SACS to all mafic lavas below the rhyolites in the Lebombo. However, work in the Karoo volcanics project of the NGP has resulted in considerable refinement of the volcanic stratigraphy in the Lebombo (Cleverly and Bristow, 1979). The term Letaba Formation is 1 now reserved for olivine-rich lavas which are chemically and petrographically quite unlike the Springbok Flats types and their correlation with the Letaba Formation should be abandoned. On the basis of data presented here, correlation of the Springbok Flats basalts with either the Sabie River Formation or the Lesotho Formation (Lock et al., 1974) is possible. Until more data are available it is proposed that the basalts discussed here be grouped into a Springbok Flats Formation with the presence 1 of macroporphyritic lavas strongly supporting a correlation with the Sabie River Formation. 1 This study also allows the geographical extent of the incompatible element-enriched clan of Karoo Lavas to be refined. Their southern limit 1 Marsh 93

extends from between Komatipoort and the Sabie River in the Lebombo through the northeastern basin of the Springbok Flats to Serowe in eastern Botswana.

REFERENCES Bristow, J.W. (1982) Trans, geol. Soc. S. Afr., 85, 167-178. Cleverly, R.W. and Bristow, J.W. (1979) Trans, geol. Soc. S. Afr., 82, 227-230. Duncan, A.R., Erlank, A.J., and Marsh, J.S. (1984) Spec. Publn. Geol. Soc. S. Afr., 13_, in press. Hawkesworth, C.J., Marsh, J.S., Duncan, A.R., Erlank, A.J. and Norry, M.J. (1984) Spec. Publn. geol. Soc. S. Afr., 13, in press. Lock, B.E., Paverd, A.L., and Broderick, T.J. (1974) Trans, geol. Soc. S. J Afr.. 21, 117-129. Visser, H.N. and Van der Merwe, S.W. (1959) Geol. Surv. S. Afr. Bull. 31, 97pp. Male 1 T Average compositions of various Karoo basalts Springbok Flats Letaba SRBF SRBF aphyric nacrophyric N.L. N.L. S.L. Lesotho

SiO2 51.44 51.74 49.97 51.92 52.84 51.50

TiO2 .99 3.26 3.07 3.23 1.44 .95

A12O3 , 15.37 15.30 8.22 13.13 14.87 15.69 Fe2°3 11.01 11.86 12.02 13.28 12.32 10.96 MnO .17 .15 .16 .17 .17 .16 Hgo 7.5B 3.93 15.52 5.34 5.51 7.01 CaO 9.90 8.43 7.07 7.98 9.27 10.69

J Na2O 2.34 2.35 1.43 2.34 2.55 2.17 ¥> .77 2.26 2.10 2.08 .78 .70 T P2°5 .14 .66 .45 .53 .25 .16 Ea 293 1024 917 853 295 174 Sr 203 1068 1002 856 316 192 Rb 19.4 45.6 55.4 45.1 19.1 12.0 Y 26.4 45.0 27.8 36.0 28.6 24.4 Zr 105 506 402 349 134 94 MJ 4.0 29.1 19.0 19.5 5.3 4.9 Zn 79 113 110 120 96 86 Cu 77 124 83 113 130 87 Ni 89 64 827 84 76 94 Co 56 49 61 49 48 48 Cr 352 79 804 85 136 283 V 249 237 204 24t 261 240 Sc 32.5 17.7 21.1 24.0 29.9 33.0 No. of samples 39 6 19 9 37 49

• From txmcan et al. (1984) 1 Marsh 94

" 1 O / • /• Crr n / f • t sot / O \

o • 400 to . o ip.. * f 1"•*-. ^LESOTHO j \ Fig. 1 300 i *% 35 Zr vs Cr and Hg Ho.. Dots-borehole \) ° V WM; stars-borehole LB1; circles- o SO borehole RTLI excluding the 200 1 / V UCSOTHO \ macrophyrlc samples. \ o"" 0 \ o 100 \ ive «. p. N OK«o

1 1 1 • I 1 1 . • it\ 60 100 (20 (40 ICC 120 140 Zr pp<*\

-1O 0 *2O '"XL—SAM Ktvtlt F. \^ Fig. 2: Sr- and Nd- isotoplc compositions -2 at 190 my. Dots -4 aphyric basalts; circles macrophyrlc -t basalts. Field for other basalts from -e Hawkesworth et al. -10 (1984) -u

•708 •710

"J J

Tf to /So ID io 1 —T—i—i—i—i—r

Fig. 3: Vertical composition v variations in RTLlCabove,and LBI (left). i1 A 300-

J 95

LATE TRIASSIC ISOPODICHNUS FROM THE NATAL DRAKENSBERG

-. T.R. Mason Department of Geology & Applied Geology, -" University of Natal, Durban. 4001

_ Invertebrate trace fossils were collected from the Stormberg Group in

the rugged terrain of the Natal-Lesotho border on the Drakensberg Escarpment

"" (Fig. 1). The sedimentary succession at the collecting locality spans

the Clarens Formation - Elliot Formation contact zone and is of late

Triassic age {Figs. 2, 3). A cross-bedded silt-sandstone (Beukes, 1970)

_,. of probable aeolian origin was taken to be the local base of the Clarens

•" Formation. The underlying Elliot Formation comprises sandstone, siltstone

^ and mudstone, characteristically finely laminated and with numerous

desiccation cracks. These sediments were probably lacustrine, and

vertebrate trackways produced by bipedal dinosaurs have been recorded

_ from other parts of the Drakensberg (Van rijk et al_., 1975) from similar

" sediments of Beaufort age (Permo-Triassic). The Elliot Formation sediments

~] most striking characteristic is their bright red colour. The colour m

banded mudstones and fine-grained sandstones are extensively bioturbated

but it was not possible to identify these traces.

:. The ichnofauna was collected from thin flaggy sandstones and the most

readily identified traces were assigned to the ichnogenus Isopodichnus a Bornemann which is usually considered to be indicative of fresh water.

-> Both Rusophycus- and Cruziana-type Isopodichnus were found and these

variants are responsible for the lively controversy over the nareing

j of these ichnofossils. Triassic ichnofaunas from East Greenland contain

Isopodichnus but Bromley and Asgaard (1979) claim Isopodichnus to be j. a junior synonym of Cruziana. Other workers (Trewir, 1975; Pollard

et a_l-, 1982; Pollard, 1981; Pollard, pers. comr,., 19B3) continue

T.R. MASON 96

to regard Isopodichnus as a separate freshwater ichnogenus. The confusion arises from the fact that had these trace fossils occurred in Palaeozoic rocks they could have been considered as traces produced by trilobites.

The South African Triassic traces were probably made by branchiopods in shallow ephemeral lakes or lacustrine margins.

ORANGE / ^ FREE NATAL N STATE / ''<;>iv

PIETERMARITZBURG LESOTHO 2* t 'DURBAN

'/TRANSKEI x-

10 20 30 10

LESOTHO

TRACE FOSSIL LOCALITYV

Nata! Lesotho *

WATERSHED KOP 27J9fr

' ATHULE 27 20km

Fig. 1 Locality maps showing the posit :CT. cf the Isopodichnus collectm- s:te. The trvrr. cf Underberg is shown or. both ir,apE as 6 rtfere:.:-: poi nt.

T.R. MASON" 97

] Lithotype Inferred Environment

Dune cross-bedded Aeolian ] silt-sandslone with cross-beds up to 1.5m thick Greenish-buff, thin bedded flaggy sandstone Lacustnne Fme-qrained sandstone and red-brown interbedded mud laminae Finely laminated reddish-brown mudstone and siltstone

Sandstone Lake margins

Red muddy sandstone and siltstone

• TRACE FOSSILS AT THESE HORIZONS J

Fig. 2 Sedimentary succession and inferred environments of deposition for the Stormberg sediments at the collecting site.

DRAKENSBERG BASALT GROUP Jurassic

[STORMBERG CLARENS FORMATION ELLIOT FORMATION GROUP] i MOLTENO FORMATION Triassic BEAUFORT UPPER BEAUFORT GROUP Triassic GROUP LOWER BEAUFORT GROUP Permian

ECCA VOLKSRUST SHALE FORMATION VRYHEID FORMATION Permian GROUP PIETERMARITZBURG SHALE FORMATION KARO O SUPERGROU P DWYKA FORMATION Permian

Fig. 3 Stratigraphic nomenclature used by the author.

T.R. MASON 98

J REFERENCES Beukes, N.J., 1970. Stratigraphy and sedimentology of the Cave Sandstone stage, Karroo System. Second Gondwana Symposium, S. Africa. J Proceedings and Papers, C.S.I.R., Pretoria, 321-341. Bromley, R. and Asgaard, U., 1979. Triassic freshwater ichnocoenoses from Carlsberg Fjord, East Greenland. Palaeogecgraphy, Palaeo- J climatology & Palaeoecology 28, 39-80.

Pollard, J.E., 1981. A comparison between the Triassic trace fossils J of Cheshire and south Germany. Palaeontology 24, 555-588.

Pollard, J.E., Steel, R.J. and Undersrud, E., 1982. Facies sequences J and trace fossils in lacustrine/fan delta deposits, Hornelen Easin (M. Devonian), western Norway. Sedimentary Geology 32, 63-87. 1 Trewin, N.H., 1975. Isopodichnus in a trace fossil assemblage from the Old Red Sandstone. Lethaia 9, 29-37.

Van Dijk, D.E., Hobday, D.K. and Tankard, A.J., 1978. Permo-Triassic lacustrine deposits in the eastern Karoo basin, Natal, South 1 Africa, 225-239. I_n Matter, A. and Tucker, M.E., Eds., Modern and Ancient Lake Sediments. Spec. Publ. Int. Ass. Sediment. 2. Blackwell Scientific Publications, Oxford, 290 pp.

1 "I.

T.R. MASON 99

— PALAEOENVIRONMENTAL SIGNIFICANCE OF ICHNOGENUS DIPLOCRATERION TORELL FROM

"~ THE VRYHEID FORMATION OF NATAL.

T.R.Mason & A.D.M.Christie

Department of Geology and Applied Geology, University of Natal, Durban 4001

— Field work by members of the coal research group at the University of

~ Natal, Durban, has produced much new information about the sedimentary

environments of coal deposition in the Vryheid Formation of northern Natal.

Many exposures suitable for detailed observation and measurement of the

_ ichnofauna (trace fossils) have been found and one of them is described in

— • this paper. »

"" The section is located in Lusthof Stream, about 25 km southeast of

Dundee, Natal (Fig.1). The section log and palaeoenvironmental interpret-

ations based on the sedimentary structures are presented in Figure 2. For

_] descriptive convenience three divisions of the succession are recognised,

—, and the ichnofauna of the middle division is dominated by abundant.

vertical, spreiten-bearing, U-shaped burrows identified as ichnogenus

I Diplocraterion Tore 11 1870. Other elements of the ichnofauna include

Skolithos and Siphonichnus•

_J' On the bedding surfaces of the Diplocraterion-bearing cross-bedded

—f sandstones the vertical U-burrows form attenuated dumb-bell shapes which

often intersect one another and are equated with the 'Hiihnertreppen'

] (chicken tracks) of the German Triassic (Blanckenhorn 1910). The dumb-bells

appear to be weakly oriented with respect to the trough cross-bed directions I _J measured in the same rock unit.

— The nomenclature of trace fossils is chaotic because of the large

""". number of poorly-defined ichnofenera and ichnospecies accumulated over the

! past 100 years and the lack of modern monographic revisions of trace fossil

f T.R. MASON' and A.D.M. CHRISTIE 1 groups. One of the few recent monographic papers concerns U-burrows and the

synonymy of Corophioides, Diplocraterion and Polyupsilon (Fiirsich 1974).

Fiirsich defines Diplocraterion using two significant features, the vertical

U-burrow and the form of the spreite. These criteria place the Natal speci-

mens in ichnogenus Diplocraterion polyupsilon (Smith 1893). D. polyupsilon

is also known from the Carboniferous of Scotland (Knox 1973). Figuie 3

illustrates our interpretation of events leading to the formation of the

Natal specimens. We conclude: Diplocraterion was a suspension-feeding

organism; as it grew it enlarged its burrow in a bidirectional sense, both vertically and laterally; preservation of these trace fo6sils provides evidence for alternating periods of erosion and deposition when the Vryheid

Formation was being laid down; the Karoo basin was fully marine at this time.

[ VRTHEID*/

DUNDEE y/j^ >, VBUFFALO B1VER

N\ t\

Area of ^ \ ^_^ |LUSTHOF STREAM] /f. /; V Mam Map ^^^^ ^ \ \ \J r\ ^*\ ) \SPITSKOPA ^^ /LUSTHOF^^ /MAIN KAROC f>'•:'•'• . / B*S(ft DURBAN. / v^sy s (\ 9 KnOMETREt I." X ^\ Mj\

Figure 1. Location of the farm Lusthcf and the strea-T. section.

T.R. hlASOS and A.D.M. CHRISTIE 101 ] I

] PALAEO - LITHOLOGY ENVIRONMENTAL I INTERPRETATION ] 55- COARSENING-UPWARD SEQUENCE GRADING DISTRIBUTARY I FROM MASSIVE S1LT5TONE THROUGH CHANNEL ALTERNATING SANDSTONE AND SILTSTONE 50- '] TO EBOSIVELY-BASED. TROUGH CROSS- BEDDED COARSE-GRAINED SANDSTONE I PRODELTA. ] 45- EMBAYMENT 1 MEGARIPPLED PEBBLY SANDSTONE ] 40- ALTERNATING SANDSTONE AND SILTSTONE I SVNAEPESIS STRUCTURES SHOAL WATER DELTA COMPLEX ] 35- PLANAR CROSS-BEDDED (OCCASIONAL I TROUGH CROSS-BEDS! AND THIN C77////// SILTSTONE INTERCALATIONS ] 30-

I 77//V/71 BASAL PEBBLE CONGLOMERATE ] 25- MASSIVE SILTSTONE AND SUBORDINATE PRODELTA/ I RIPPLE CROSS-LAMINATED SANDSTONE EMBAYMENT

] 20-

I TROUGH PLANAR AND HERRINGBONE CREVASSE SPLAY ] CROSS-BEDDED SANDSTONE WITH 15- SUBORDINATE SILTSTONE TIDAL I D DEPOSITS DISTRIBUTARY .] CHANNEL 10- I GIANT CROSS BEDS 3ILBERTIAN/YUKON .] TYPE DE..TA I 5- ] I ] 1 ] 1 Figure 2. The successicn measured in Lusthof Stream. The two trace fossil 3 zones are defined on the basis of the dominant ichnofossil. 1 ] 1 I T.R. MASON' and A.D.M. CHRISTIE 102 J

i I I \ I ] RFCOlONISATION ]

EROSION OF FINE 5EDIMENT L»YER AND UNDERLYING CROSS-BEDDED SANDSTONE BY SAME PROCESS AS AT STAGE I .

ICHNOFAUNA KILLED BY FRESH WATER AND SEDIMENT INFLUX

i i II % i COLONISATION BY DIPLOCRATERION DURING SEDIMENTATION BY SUSPENSION SETTLING J ••NORMAL'1 MARINE CONDITIONS

DEPOSITION OF PLANAR AND TROUGH CROSS-BEDDED SANDS1ONE DURING FLOOD AND FRESH WATER INFLUX

Figure 3. Reconstruction of the sequence of events leading to the preservation

of Diplocraterion in the Lusthof Stream section. J REFERENCES

Blanckenhorn.M. 1916 Organische Reste im Mitteleren Buntsandstein. Sitz.

Ber• Ges• £. Beforderg. ges. Naturwiss. Marburg. 21-43.

Fiirsich, F. 1974 On Diplocraterion Torell 1870 and the significance of

morphological features in vertical, spreiten-bearing,

U-shaped trace fossils. Jour. Palaeo. 48, 952-962.

Knox, R.W.O'B. 1973 Ichnogenus Corophioides. Lethaia, £, 133-146.

Smith, J. 1893 Peculiar U-tubes in sandstone near Crawfurdlar.d Castle and

Gowkha Quarry, near Kilwinniug. Geol.Soc.Glasgow. Trans.

9_, 289-292.

Torell, 0. 1870 Petrifacata Suecana Formationis Cambricae. Lunds. Univ.

Arsskr. 6. Avdel. 2, No VIII, 1-14.

T.R. MASON and A.D.M. CHRISTIE "1 103

THE GEOLOGY OF THE BEZUIDENHOUT VALLEY GRABEN

T.S. McCarthy & A.B. Cadle

; The Bezuidenhout Velley graben structure extends eastward frotr a point west of central Johannesburg to Kempton Park, where it disappears beneath younger cover, a distance of some 35km. The maximum width ~] of the graben is 3km. The graben is filled by rocks correlated with J the Ventersdorp Supergroup.

-\. The graben is bounded along much of its southern margin by the Riet- J fontein fault. This northerly dipping fault is characterized by the occurrence of overturned West Rand Group strata on its southern side, , where dips as low as 55° north have been recorded. At two localities along the southern margin, Central Rand Group rocks lie adjacent to " the graben, and separate it from the Rietfontein fault. In the suburb of Kensington, Central Rand Group rocks lie adjacent to the graben, ~"j while to the west, West Rand Group interposes. Diamond drilling and J surface exposures in this area indicates that brecciated West Rand Group rocks actually overlie Central Rand Group rocks along a northerly -) dipping fault. South of Edenvale, at the defunct Rietfontein Mine, J a thin but normal succession of Central Rand Group strata lie juxtaposed against the southern flank of the graben, but to the north of the Rietfontein fault.

J The northern contact of the graben transgresses the Hospital Hill Sub- group stratigraphy, and in the east lies against basement. The exact ~| nature of this contact is in dispute: Mellor regarded it as uncom- J fortnable, while investigations in the city area by de Beer suggest the presence of a fault. The contact is not normally exposed, but -i at one locality, the occurrence of a diamictite has been cited as J, evidence for an unconformity. At this locality, West Rand Group strata, which dip at 34°S, are overlain by a tuffaceous diamictite containing J sandy lenses, grading upwards into a coarse, sandy diamictite and finally into a plane bedded sandstone with scattered clasts. The clasts, which are sub-rounded to angular, consist predominantly of quartzite, with some shale and an occasional lava clast. Dips in this "1 sequence average about 5P°S, but tight folding is evident, which is not J evident in the adjacent Witwatersrand Supergroup strata. Disparity of dips between the axial plane of these folds and the ubiquitous cleavage, ] Outcropsuggest stha withit thn ethe latte- graber ins arsuperimposede few, but .a generalized stratigraphy can be constructed. Apart from the sediments cited above, the most of the northern margin of the graben is characterized by lavas: con- ] sisting mainly of homogenous typos, but with a prominent porphvrii ic horizen ocruring near the base in tlir suburb of Dcwctsliof. lava tops containing abundant amygdales are locally present. The lavas are

] 1.5=. McCarthy & A.B. Cadle 104

overlain by a thick sequence of volcaniclastic sediments which make up most of the succession. The sediments consist of tuffaceous units, lithic sandstones, and a variety of conglomeratic rocks ranging in coarseness up to boulder beds. In the far eastern portion of the graben, the volcaniclastic rocks abut directly on basement, and no lava is present. The material constituting these sediments is almost entirely volcanic in origin. Scour channels and graded beds are frequently observed in the sandstone units. Particles consist of fragments of lava, and individual feldspar crystals have been observed. In the conglomeratic rocks, clasts consist of sub-rounded to well-rounded pebbles, cobbles and boulders of assorted basic lava types, Quartzite clasts are very rare. The matrix also consists of sand-sized volcanic rock fragments and crystals, and fragmented amygdales. Clasts are moderately well sorted, and both clast and matrix, supported types occur. In the eastern portion of the graben, clasts consist predominantly of acid lavas, often containing kaolin pseudomorphs of large phenocysts of alkali feldspar. ]• North of the old Rietfontein Mine on the southern flank of the graben coarse vein quartz and quartzite conglomerates, interbedded with quartz- itic sandstones apparently interfinger with the volcaniclastic sedi- ] ments.

i Provenance area for the sediments in the graben was undoubtedly the -J Klipriviersberg lavas. The nature of the sediments suggest rapid erosion in the provenance area and rapid deposition in a fluvial -j system. The paucity of lavas intercalated in the volcaniclastic sedi- j tnents implies that volcanic activity had largely ceased by the time sediment accumulation started. It is suggested that the area in ques- -. tion was originally covered by Klipriviersberg lavas. Graben develop- I ment comnenced in late Ventersdorp times, and eroded lavas were rapidly deposited in the subsiding graben. The source of the acid volcanic clasts is enigmatic. At a late stage, Witwatersrand Supergroup strata ~| may have been exposed in local host blocks, contributing to the graben •J" fill. Two periods of post-graben compression are evident in folding and steeping of dips to near vertical in the central portion of the —| graben, and in the later cleavage.

I J

1.=. McCarthy & A.B. Cadlc 105

GEOELECTRICAL STUDY OF THE BUSHVELD COMPLEX by R. Meyer, J.H. de Beer and S.J. Joubert Geophysics Division, NPRL, CSIR, P.O. Box 395, Pretoria

During 1981 a research project was started to study the structure of the Bushveld Complex by means of geoelectrical deep soundings as part of the National Geoscience Programme. To date a total of 50 soundings have been completed with maximum current electrode spacings ranging between 5 and 40 km. Of these soundings 13 are in the eastern compartment whereas the rest are in the western compartment of the Bushveld Complex.

Initially the soundings were sited in such a way as to determine the geoelectric stratification of all the major geological subdivisions of the Bushveld Complex as well as the directly underlying upper formations of the Pretoria Group.

The graphitic shale unit at the base of the Silverton Formation, being extremely conductive (resistivity <0,1 Tim), acts as a geoelectric marker horizon and is clearly recognized on the sounding curves. This conductive layer ] is overlain by the more resistive upper sections of the Silverton Formation, the resistivity of which varies between 1000 and 3500 fim depending on the j amount of diabase present in the succession. Further up in the succession this is followed by the even more resistive Magaliesberg and Rayton Forma- { tions having resistivities of between 6000 and 10 000 fim. Initially it was thought that the Rustenburg Layered Suite is only represented by a single "I geoelectric unit with resistivity between 250 and 600 flm. Subsequent soundings done during 1983 however revealed a much more complex and not yet ful- 1 T ly explained geoelectric layering within the different zones of the layered . -i sequence. The Lower and Transitional Zones are almost entirely composed of . _, high resistivity rocks such as norites and pyroxenites. The same remark is J valid for the Critical Zone and Lower Subzone of the Main Zone, although •. soundings done on outcrops of the Lower Subzone of the Main Zone indicate a | conductive (^250 Jim) geoelectric layer for the upper 200 m followed by a re- | sistive layer of at least one order of magnitude higher resistivity. This j conductive layer can not yet be explained in terms of bulk rock types. It

L 106

is therefore expected that this conductive geoelectric layer is in some way J related to the Upper Subzone of the Critical Zone. The upper gabbro-norites T of the Main Zone are again highly resistive. Lithologically upward in the -* succession the Main Zone is followed by the Bierkraal Magnetite Gabbro or «. Upper Zone. The numerous magnetite layers in the lower parts of this Zone, J result in a resistivity as low as about 100 Rm. From the above it is thus clear that the Rustenburg Layered Sequence consists of a very complex geo- J electric succession.

"1 Finally the Bushveld acid phase, the Lebowa Granite Suite, comprising gra- -' nites and granophyres, is the most resistive geoelectric unit of the Bush- «j* veld Complex with resistivities varying between 3000 and 20 000 Sim. The J granites in the western Bushveld and to the west of the Crocodile River * Fragment have resistivities of only about 3000-4000 Qm whereas resistivi- J ties determined in the Eastern Bushveld on similar rocks yielded values of as high as 20 000 fim.

-1 Some of the main conclusions that can be drawn at this stage of the survey -i are the following:

1) The conductive graphitic shale horizon in the Silverton Formation occurs j below the mafic as well as the acid rocks of the Bushveld Complex in the Eastern and the Western Transvaal and could easily be traced geoelectrical-

2) The Lebowa Granite Suite thickens towards the centre of the Complex but "!' is not thicker than 3000 m. 3) Indications are that the mafic sequence in the eastern compartment, be- I low the Nebo granite becomes thinner towards the centre of the Complex. This is in agreement with interpretations of the Bouguer gravity anomalies. ! 4) In the western Bushveld, however, the mafic sequence seems to be present below the granite except for an area to the north and east of the Crocodile River Fragment. 5) Soundings on the Crocodile River Fragment did not reveal the presence of the graphitic shale of the Silverton Formation. No Bushveld type rocks would seem to underlie this Fragment.

R. Meyer, J.H. de Beer and S.J. Joubert p2 107

6) The Nebo granite in the Eastern part of the Complex has a transverse resistance of almost one order of magnitude larger compared with the granites north of Rustenburg. This is attributed to a difference in resistivity and not thickness of the granite. The difference in resistivity could be related to differences in the post-emplacement tectonic histories of the two granite units. 7) The electrical resistivity method has proved to be well suited to study the structure in depth of the Bushveld Complex but more soundings are J required to draw representative depth sections of the Complex. J J' January 1984

1 J

-a' n J

R. Meyer, J.H. de Beer and S.J. Joubert p3 108 1 1 A TURB1D1TE SUCCESSION EQUIVALENT TO THE COMPLETE SWAKOP GROUP

R.McG.MILLER, E.E. FREYER and l.W. HaLBlCH

ABSTRACT The Damara Sequence along the lower Ugab River is entirely turbiditic and consists of 3 schist units separated by 2 lime- stones, all of which are correlated with the Swakop Group. The ,\ basal unit is correlated with horizons further east that directly overlie Nosib rocks. The lowermost turbidites therefore imply that the coastal arm of the Damara Orogen had developed into a deep-water ocean by the end of Nosib times, the Pan-African South Atlantic Ocean. Indications are that 1 all sediments were derived from the east. Ice rafting of glacial debris is suggested by the occurrence of a 1,5 m granite boulder in an area where the turbidites are considered to be most distal.

1 t

MILLER, FREYER and HaLBlCH J 3 109 MINERALOGY AND PETROLOGY OF THE AMPHIBOLE ASBESTOS-BEARING J ROCKS OF THE PENGE IRON FORMATION, TRANSVAAL J SUPERGROUP, PENGE AREA T. Miyano and N.J. Beukes j Department of Geology, Rand Afrikaans University Johannesburg 2000 j ABSTRACT

J The Penge Iron Formation of the Transvaal Supergroup, Penge and Malipsdrif areas, is characterized by abundant amphibole ]• asbestos (amosite and crocidolite). The mineralogy and petrology of the iron-formation were described using core samples selected from diamond-drill hole BH 13 at Penge and some rock specimens collected in the Malipsdrift area.

Both crocidolite and amosite occur in the Malipsdrift area, but crocidolite (or riebeckite) is of a very small amount at Penge. Because both areas have been subjected to contact metamorphism associated with the Bushveld Complex, low-grade metamorphic minerals like greenalite, minnesotaite, stilpnomelane, and siderite are absent or very scarce. The minerals in the Penge rocks are grunerite, micas (ferri-annite, biotite), quartz, carbonates (calcite, ankerite), chlorite (Fe- ]• rich)*, stilpnomelane (needle)*, pyrite, magnetite, riebeckite, garnet***, hornblende**, tourmaline**, pyrrhotite, and chalco- I pyrite, in the approximate order of abundance. Furthermore carbonaceous matter occur abundantly. Chemical compositions | and mineral assemblages are listed in Table 1.

""? Primary stilpnomelane is replaced mainly by ferri-annite, bio- •^ tite, and/or grunerite. Siderite, ankerite, and/or iron- -r oxides (hematite, magnetite) may react with quartz to form -i, grunerite. Some grunerite may replace minnesotaite. As a __ result, calcite is more abundant and iron-oxides and quartz J are relatively less than in the low-grade metamorphic iron-

T. Miyano and N.J. Beukes J 1 10

""] formation (e.g. Beukes, 1980). On these reactions, carbonaceous matters seem to have played an effective role as a redu- -> cing agent of ferric iron during metamorphism.

Retrograde stilpnomelane and chlorite replace ferri-annite and J biotite with decreasing temperature. Diabase sills which predate the intrusion of the Bushveld Complex may be responsible ! for formation of tourmaline, hornblende, and biotite, the occurrence of which are restricted to the contact aureole of the ] sills. It is worthwhile noting that some amosite and grunerite replace crocidolite (or riebeckite) which may be of very low- to low- grade metamorphic origin. The grunerite may grow to longer fibre and prisms after replacement. However, prismatic riebeckite in the Malipsdrift area, cut amosite fibre in many places. Riebeckite formation requires a more effective barrier against the escape of Na+ at higher temperatures (Miyano J & Klein, 1983). Considering that folding in the Malipsdrift area is stronger than at Penge, there may exist a structural "| barrier against the escape of Na+ derived from pre-existing crocidolite (or riebeckite). Accordingly, riebeckite can be -j reprecipitated because of sufficient soda availability (or - high A.Na+/AH+) provided that temperature decreases only sUght- _, ly. Such replacement and reprecipitation are well explained _ by figure 1. The diagram was constructed at 350°C, because __ the peak metamorphic temperature is estimated to be inbetween J 250 and 450°C based on the mineral assemblages described above. Fig. 1 shows that riebeckite can be replaced by grunerite with increasing temperature (arrow A) and reprecipitates with decreasing temperature (arrow B) at constant ANa+/AH+•

Effect of additional Mg in riebeckite and grunerite can be considered using an ideal solution model between Fe- and Mg- end members. With decreasing X^e (or X^|u), the boundary of _ riebeckite-grunerite association shifts to a lower log

T. Miyano and N.J. Beukes 11 J at constant ANa+/AH+- Accordingly the effect is similar to that of increasing temperature. Because there is little chemical difference between prismatic and fibrous forms of rie- J beckite or grunerite, the two possible reactions (arrows A and B) of riebeckite -> grunerite and grunerite—>• riebeckite may + + take place at similar T-f(O2) - A(Na )/A(H ) conditions. Probably the former occurs progressively and the latter retrogressively.

35DT I

-20

HtmlMr

\ MagnHit* J 30

A* \\

Grunfrrite \ «0

i i i 2 4 e IDG ato./

+ + Fig. 1 Isobaric log f(02) - log ANa /AH diagram at 350°C.

The diagram was constructed on the basis of thermodynamic data from Halgeson et al (1974, 1976, 1981) and Miyano and Klein (1983 a, b). REFERENCES: Miyano, T. and Klein, C. (1983) Evaluation of the Stability Relations of amphibole asbestos in metamorphosed iron- formations. Mining Geology, Japan., 33, 213-222. Beukes, N.J. (1980) Lithofacies and stratigraphy of the Kuru- man and Griguatown Iron-formations, northern Cape Province, South Africa. Trans, geol. Soc. S. Afr., 83, 69-86.

T. Miyano and N.J. Beukes L,_J I 1 » I I I L-J L._J I J I i L.__j I J

Table 1. Representative electron microprobe analyses of iron-bearing minerals from the Penge Iron Formation

@ Si0 A1 O TiO FeO MnO MgO CaO Na K 0 Total 2 2 3 2 2° 2 Gru(1) 50.37 0.00 0.00 39.69 0.00 6.20 0.15 0.00 0. 00 96.41 Gru(2) 50.37 0.11 0.00 37.88 0.78 7.63 0.14 0.04 0. 10 97.05 Gru(3) 50.09 0.20 0.08 39.96 0.41 5.78 0.28 0.09 0. 00 96.89 Gru(4) 50.72 0.63 0.00 36.55 0.16 8.29 0.24 0.19 0. 02 96.80 Rk(1) 52.32 0.22 0.00 33.32 0.00 3.76 0.37 6.70 0. 23 96.92 Rk(2) 5 3.26 0.23 0.00 32.61 0.00 3.87 0.24 6.91 0. 00 97.12 Rk(3) 52.60 0.13 0.00 32.41 0.00 4.09 0.95 6.42 0. 26 96.86 Micadi 38.65 8.55 0.47 32.20 0.14 6.51 0.02 0.05 8. 09 94.68 Mica(2) 36.72 10.06 0.90 30.57 0.08 7.37 0.07 0.01 9. 51 95.29 Mica(3) 33.94 13.49 1.58 31.25 0.04 4.48 0.05 0.08 9. 28 94.19 Mica(4) 33.83 15.62 1 .26 26.62 0.37 6.35 0.17 0.04 9. 65 93.91 Cald) 0.04 0.01 0.04 2.49 1.74 0.00 55.70 0.02 0..01 60.05 Cal(2) 0.05 0.02 0.00 2.72 1.50 0.00 55.90 0.02 0..05 60.26 And) 0.01 0.00 0.02 19.83 2.59 4.83 27.33 0.02 0..00 54.63 An (2) 0.01 0.00 0.00 16.70 1 .16 8.90 27.97 0.00 0.,01 54.75 Sd(1) 0.01 0.01 0.01 50.44 1 .97 2.53 3.61 0.00 0..01 53.59 Sd(2) 0.01 0.00 0.06 47.82 3.86 3.72 2.92 0.03 0..02 58.44 St.i.1* 45.15 6.37 0.00 32.86 0.13 2.82 0.01 0.14 2..39 89.87 Chi* 24.28 18.39 0.00 36.93 0.15 7.21 0.01 0.00 0..02 86.99 Ho** 40.79 13.06 0,19 25.42 0.28 3.61 10.56 2.07 0..71 96.69 To** 37.51 25.60 0.60 13.08 0.05 4.75 1 .26 2.17 0,.05 85.07 Gad)*** 36.99 19.75 0.16 15.27 20.07 0.21 5.72 0.00 0..00 98,17 Ga(2)*** 37.11 19.71 0.11 18.08 15.78 0.29 6.37 0.02 0..00 97.47 @ All Fe as FeO. * retrograde or secondary. ** raetamorphic mineral due to diabase sill (?). *** detrital (?) . Gru(1): amosite (MA314); Gru-Mt+Qtz+Rk, Gru(2): fibrous grunerite rosette (1/33); Gru-Qtz-Fan -An+Py+Mt, Gru(3): needle-like grunerite (1/101); Gru-Bio-Cal-(Chi)+Qtz, Gru(4): prismatic grunerite (1/146); Gru-Qtz-Bio-An-Mt+Py+(Chi), Rk(1): crocidolite (MA321); Rk-Mt+Gru+Qtz, Rk(2): prismatic riebeckite (MA297); Rk-Gru+Qtz+Mt+Fan, Rk(3): prismatic riebeckite in ainosite (MA304); Gru-Rk+Mt, Mica(1): platy ferri-annite (1/33); Gru-Fan+Qtz+An+Py+Mt, Mica(2): platy ferri-annite in carbonaceous band (1/51); Qtz-Gru-Fan-(Chi)+Py+Mt+Rk+Chal, Mica(3): biotite in calcite band (1/99); Cal-Bio-Gru+(Stil)+Py+Qtz, Mica(4): biotite in carbonaceous-rich band (1/133); Bio-Gar-Cal+Py+Mt+An, Cal(1): calcite (1/8F); Qtz-Gru-Fan-Cal, Cal(2): calcite in biotite band (1/113); Bio-Gru-Qtz-Cal+Rk+Py+(Chi), An(1): ankerite (1/33), Gru-An-Fan+Mt, An(2): ankerite (1/142); Qtz-Gru-An-Fan, Sd(1): Qtz-An-Sd-Gru+Py+Chal, Sd(2): calcite-siderite vein (1/101); Cal-Sd+Qtz+Bio+Gru, Stil: needle-like stilpnomelane (1/98); Fan-(Stil)-Gru-Cal +Alb, Chi: chlorite (1/107); Fan-Gru-(Stil)-(Chi)+Py, Ho: hornblende (1/112); Qtz-Bio-Gru-Ho -To+Py+Mt+Cal, To: tourmaline (1/112); Qtz-Bio-Gru-Ho-To+Py+Mt+Cal, Ga(1): spessartine in carbonaceous-rich band (core,1/133); Bio-Gar-Cal+Py+Mt, Ga(2): same as Gall),(rim).

Mineral abbreviations:

Alb=albite, An=ankerite, Bio=biotite, Cal=calcite, Chal=chalcopyrite, Chl=chlorite, Fan=ferri-annite, Ga or Gar=garnet, Gru=grunerite, Ho=hornblende, Mt=magnetite, Py=pyrite, Qtz=quartz, Rk=riebeckite, Sd=siderite, Stil=stilpnomelane, To=tourmaline

i—i »—i i—i i—i i—i i—•} r—i I—1 r—i I i 1 1 14 1 THE iOJT-GONDV>;.;:Ai:.?:i> /.LK/.n:ra IGWSOUS PROVINCE O? SOUTHERN AFRICA A.E. hoore, Box 10072, Gaborone, Botswana. The disruption of Gondwanaland was accompanied by a widespread episode of basaltic volcanism in southern Africa, spanning the 1 period from the Jurassic to the early Cretaceous. Large sub- volcanic alkaline complexes with a similiar age spread, which define major igneous lineaments to the west of the subcontinent, represent a distinctly different expression of igneous activity apparently also related to the breakup of Gondwanaland (Marsh, 1983). The post-Gondwanaland history of southern Africa was characterized by a markedly different style of volcanism, represented by widespread clusters of relatively small alkaline volcanic .1 pipes, with ages falling mainly in the time range from mid- Cretaceous to mid-Tertiary. ] The younger alkaline volcanic pipes show a systematic distribution of rock types across the subcontinent. Kimberlites are T concentrated in the centre of the country, with diamondiferous J pipes occurring in very old cratonic areas. Olivine melilitites are found in the vicinity of Gamoep and Garies in ^ Namaqualand, near Sutherland, Robertson and Riversdale to the I south of the country, and in Natal to the east, thus forming a discontinuous ring about the central kimberlite clusters. Closer to the margins of the margins of the subcontinent are 1 the phonolites of the Klinghardt mountains (south western J Namibia), an alkali olivine basalt diatreme near Lambert's Bay on the west coast of South Africa, and the trachytes and trachybasalts of the Alphard Bank on the southern continental shelf. In chemical terms, this zonation in rock types is J represented by a remarkably systematic trend of decreasing _ average bulk rock MgO concentration for the different alkaline T pipe clusters from the centre towards the margins of the J continent.

"1. Euhedral olivines in kimberlites have compositions approaching -i those of typical mantle olivines, indicating crystallization in equilibrium with very Kg-rich magmas. Euhedral olivines "t in the Namaqualand olivine melilitites are characteristically J Fe-rich relative to those in kimberlites, indicating crystallization from more evolved liquids. The trend of decreasing _ MgO content of alkaline pipes from the centre towards the 1 margin of the subcontinent may therefore be related to the J degree to which the different magmas evolved. 1 It is proposed that primary alkaline magmas beneath much of J the subcontinent corresponded closely to an average Kimberley- type kimterlite in "ccripatible" element concentration. Kajor 7 element n.ixinr modelling, trace element constraints and exper- ]. irr.entnl considerations sue.-est that dee]' seated fractionation of the hif'r. ;resrure :->.nser olivine, orthoryroxene, garnet, m clinoryroxer.f :.nd il-nenite from a liquid equivalent to an j aver=re kir. berlitc could yeild an evolved liquid similiar in J

A.S. Koore 1 15

* "compatible" element concentration to an averoce G^moep-type olivine melilitite (Table 1). Further high pressure fractionation of the latter liquid involving olivine, clinopyroxene and ilmenite would produce a liquid similiar in "compatible" ; element concentration to an average Garies-type olivine mel- - . ilitite (Table 2). Simple crystal fraction models alone do not account for "incompatible" element variations however, "* which appear to reflect the imprint of a variety of other processes, including variable degrees of partial melting and possibly mantle heterogeneity and magma mixing. The spread in bulk rock compositions within the Garies group of pipes appears to be related essentially to intermediate pressure fractionation involving the dominant phenocryst phases (olivine, melilite, perovskite and titanomagnetite) (Table 3). J It is thus possible to rationalize chemical variations of kimberlites and olivine melilitites in terms of a unified magma T ' evolution model. However, the wide geographic distribution of J these different rock types and differences in age and initial Sr isotopic ratios show that they cannot form a comagmatic - ' series related to a single parent magma. Rather, it would appear that the rational chemical relationships reflect the operation of common igneous processes in broadly similiar tectonic environments (i.e. a stable craton). 1 J The proposed crystal fractionation model linking kimberlites and olivine melilitites implies that magmas generated beneath -] areas closer to the continental margin are more likely to j evolve by crystal fractionation than those generated beneath the central parts of the subcontinent. This could be related _ to a steepening of the geotherm from the central part of the I craton towards the continental margins, coupled with the effects of temperature, pressure and liquid composition on the solubil- ity of C02 in basic and ultrabasic magmas. These factors also "| determine the depth at which CO2 begins to separate from the J magma. In olivine melilitites, this component appears to be lost at relatively high pressures as an immiscible carbonate liquid. Retention of C02 to shallow depths in kimberlites results in subsequent loss as a CC^-rich vapour phase, leading to marked adiabatic cooling and an unusual F-T path for these magmas. This in turn results in marked mineralogical differences in the groundmass assemblages of kimberlites and olivine melilitites. Ages for the post-Gondwanaland pipes suggest episodes of ir.ore intense volcanic activity separated by intervening periods of less intense or no volcanic activity. The periods of more intense vclcanisis correspond to rrsjcr breaks in the offshore sedimentary record, possibly related to continental ur.lift. The low initial Sr ratios characteristic of the poet- C-ondwanaland alkaline volcanics surrestf that their source area may have teen metascrr.r.tically enriched in "incompatible" elements shortly prior to igneour activity. It follows that such a metasoriiatie event may have beer; rerponsifcle for initiating both the continental uplift and the associated volcanism.

Koore 1 16

There are inadequpte data to establish the relationship between kimberlites and olivine melilitites and other alkaline volcanics or) the subcontinent in any detail. Nevertheless, the systematic chemical trends shown by the alkaline pipe . clusters, together with a common association with episodes of continental uplift, suggest that they collectively constitute a major alkaline province, which it is proposed should be called the post-Gondwanaland alkaline igneous province of southern Africa. TABLES Tables 1 - 3 show the results of least squares linear mixing calculations (Bryan et al, 1969). In Table 1, the average South African (Kimberley-type) kimberlite has been recalculated on an anhydrous, calcite-free basis using unpublished data for fresh kimberlite samples kindly made available by J.J. Gurney and C.R. Clement.

TABLE 1

"Mixing" calculation for a hypothetical parental liquid equivalent to an average South African kimberlite, and hypothetical derivative liquid equivalent to an average Gamoep-type olivine melilitite.

Derivative Parent (Av. Gamoep) (Av. S •A. Kimberlite) Obs Obs Calc %Diff

SiO2 35,52 42,85 42,85 0,00 TiOo 5,74 ''1,50 1,50 -0,18 AI2O3 5,07 4,88 4,88 -0,03 Fe2O-j 16,46 9,64 9,65 0,06 MgO 19,41 31,76 31,76 0,00 OaO 11,97 4,90 4,90 -0,03 Na20 1,55 0,88 0,51 -41,72 K2O 1,44 2,17 0,33 -84,74 P2O5 1,01 1,03 0,23 -77,35 Sum of squares of differences: 4,15 Phase proportions used in Mix

Vector Coefficient

Average Gamoep 0,2235 Orthopyroxene 0,1729 Clinopyroxene 0,0957 Olivine (F091 ) 0,3178 Garnet 0,1623 Ilicenite 0,0006 Total

A.E. Koore 1 17

TABLE 2 Derivative (Av. Garies) Parent, (Av. Gamoep.) Obs Obs. C-lc % I>ifi

SiO2 32,74 35,52 55,63 0,31 T1O2 6,38 5,74 5,69 0,84 AI2O3 6,99 5,07 5,09 0,33 Fe?O3 19,75 16,46 16,59 0,81 MgD 11,58 19,41 19,30 -0,59 CaO H,75 11,97 11.65 -2,66 Na20 1,51 1,55 1,21 -22,03 K2O 1,91 1,44 1,31 -8,74 1,81 1,01 -^ p2o5 1,25 23,31 Sum of squares of differences: 0,39 Phase proportions used in Mix Vector Coefficient Average Garies 0,6864 Olivine (Foq1) 0,1757 Clinopyroxene 0,1077 Ilmenite 0,0251 J Total U,9950 Table 3 Mix relating the most Mg-rich (parental) and Mg-poor (derivative) bulk rock compositions in the Garies group of olivine melilitites

Derivative (AMJ281) Parent (ART 478) 1 Obs' Obs. Calc. % Diff

SiO2 34,46 34,40 34,62 0,63 j TiO2 6,97 4,92 5,03 2,14 J - A12O3 8,68 6,24 5,67 -9,18 Pe2O-5 19,37 16,78 16,80 0,13 "{ KgO 8,11 15,77 15,58 -1,25 J- CaO 15,80 14,39 14,24 -1,02 Na2O 2,44 1,91 1,86 -2,71 -i K20 1,30 2,03 0,75 -63,23 J P205 1,42 1,76 0,78 -55,50 Sum of squares of differences: 3,05 i Phase proportions used in Kix -1 Vector Coefficient AKJ 26T 0,5486 1 Olivine (F085) 0,2064 j Kelilite 0,1713 Perovskite 0,0140 Titanoroarnetite 0,0273 j Total 0,9676 References. - Bryan W.3., Finder L.V.. and Chayes F. (1969) Science 163, 926-7 j Karsh J.S. (1975) Earth rlanet. Sci. Lett. 18, 317 - 25.

A.3. Koore 1 18

WHAT CAN BE DONE WITH COMPUTERIZED GOLD DATA VALUES

By D.L. Morrison, Survey Department S.L. Miller , Management Services Division Gold Fields of South Africa Limited

An investigation into the feasibility of introducing geostatistical techniques for the estimation of ore reserves on the gold mines of the Gold Fields of South Africa group was commenced in October 1979. A pilot system was developed on a PRIME 550 minicomputer and in j parallel a program of data capture began for selected mines and reefs. The pilot system consists of a suite of some sixty programs and utilities. It has a simple data base design and is totally j interactive. This greatly facilitated the extensive testing, developments and refinements that were necessary to evaluate and _ prove the system.

The project to evaluate the geostatistical techniques was carried out jointly by the Survey Department and the Operations Research Unit of "1 the Management Services Division. This project lasted close on two J years. A report was compiled and a presentation made to the Executive in August 1983 after which permission was granted to n proceed with the System Analysis for a routine production system _] based on these techniques for all the Group gold mines.

~ In all some half a million sample values were collected over a J variety of mines and reefs. Data capture was done at the mines using graphic workstations (Tektronix 4054 and digitizer). For each data point the following information was captured:- Lo coordinates, J. cm.g/t value, channel width, stope width, reef width, data type J (stope, development or borehole), and working place. The data was sent to Head Office where it was vetted before being loaded to the "7 , appropriate data base. J ' The original idea was to develop a system ostensibly just for ore -. reserve determination. It soon became apparent however that having ( a large mass of data available in a centialized and convenient form ~~ opened the doors to a wide range of potentially useful applications. The following are some areas in which the pilot system has been put i to use. J

T 1. Ore Reserve Calculation

_ Sercivariograms (based on block averages) are calculated (for i both cm.g/t and channel width) and modelled by a single structure spherical model. Simple kriging is performed and every block in a grid is estimated. ! 119 i Overlaying the ore reserve block coordinates on the kriged t grids, and by making use of a stope width/rhanne1 width , . relation, the following Is kept for each ore reserve block : block headers, area, tonnage, em.g/t value, stope width and , grade. From this Information It Is easy to construct any of the I . required ore reserve reports (e.g. value segregation).

] 2. Contour Plots j A kriging run results in grids of values for ctn.g/t and channel I width. These two grids may then be combined to produce a channel value grid (kriged cm.g/t/kriged channel width), or by feeding In a stope width/channel width relationship, a stope value grid (kriged cm.g/t/stope width). Contours based on these J grids are then produced using the Surface II package. These contours have been found to be extremely useful for purposes of understanding, interpretation, planning, forecasting and 1 geological modelling.

Contours of the kriging error may be used to guide the layout of ~j ore reserve blocks.

The production system will also retain the elevation for every -, sample point. It will then be possible to produce contours of ! elevation for a particular reef or difference In elevation between overlying reefs.

3. Off-Reef Samples

"I Off-reef samples are Inherently identifiable In that their _i em.g/t value and channel width are zero. This can be due to the unavoidable stoping of intrusions (e.g. dykes or sills) or the displacement of the reef into the hanging or footwall. Plots of these samples provide management with a useful monitoring J tool and reinforce sampling department reports. it is envisaged that in the production system recording of off- reef sampling will distinguish between Intrusions, reef in J hanging or footwall, or partially exposed reef. It will then be possible to produce plots of the different forms of off-reef ~| sampling, which could be updated on a regular basis.

A. External Waste

i Contour plots of the external waste status in any specified area of a data base can be produced in a very short time. The external waste (the difference between stoping width and channel ! width) at a sampling point is compared to any channel -* width /stoping width standard which gives an external waste which is either greater than or less than the standard for the "? channel width at that point. J' J J 120 -» The contours are suitably coloured, resulting in a highly I illuminating picture and a very useful monitoring device.

] 5. Variogram Estimators

It was found that variograms based on block averages highlighted "1 the underlying structure more clearly than did those based on _J points. They are also less susceptible to outliers and the calculation time is much faster (typically over a hundred times •^ as fast). Making use of block averages also speeds up the J kriging process and means that many more data points are Implicitly taken into account when kriging a block.

It Is desirable from the point of view of a production system J that the variogram estimators show Resistance (non-sensitivity to outliers) and Robustness (not susceptible to changes in underlying assumptions) properties. An exercise to compare J different variogram estimators showed that the Cressle-Hawkins estimator is consistently stable and that it should definitely be considered a candidate for inclusion in the production 1 svstem.

' *>. Comparison of Different Kriging Methods

It was necessary to determine which kriging method should be incorporated in the production system for routine use in ore J reserve calculations. Testing was carried out for both cm.g/t and channel width estimation on three mines. _j The methods considered were ordinary, simple and multigaussion kriging. Evaluation criteria were laid down against which _ testing was done. These included block factors, the components I of the estimation error and conditional unbiasedness. It was concluded that for the time being the relatively easily understood and implemented simple kriging procedure was 1 adequate.

"1 '* Sampling Spacing

•^ The current method of estimating the grade of a stope for the | next month is based on the average of the skin samples on the -* stope. If the varlograra is known then it is possible to quantify the precision of the estimate. An exercise on ^ different stope face sampling patterns was undertaken. Using J. graphs of precision versus sample spacing the following observations were made :-; 121

- There Is a clear tradeoff between gain in precision and the cost of reducing sample spacing.

- Curves may differ significantly between different areas of a mine.

~"| - It is important to take any anlsotropy into account when —i laying out ore reserve blocks and this should be done whenever possible.

At times it would pay to take historic data into account (e.g. the average of the last two months sampling) while at other times there would be a loss of precision if ] this were done. J CONCLUSION Capturing gold sample values and storing them in a form from which it is easy and fast to extract relevant Information has resulted in an 3" ever increasing demand from users for a variety of applications. There are considerable benefits to be gained from progressing to a ] production system via a pilot one.

J 122 "1 SOME EXAMPLES OF THE APPLICATION OF GEOLOGY AND MINERALOGY ] TO BUILDING MATERIALS RESEARCH ] R E Oberhoister ] National Building Research Institute, CSIR, Pretoria. ] INTRODUCTION Geology and mineralogy find wide application in the building materials industry and are essential for the study and characterisation of raw ] materials such as clay minerals, sand, rocks and natural pigments, which are used for the manufacture of building materials; this training is also important for research into manufactured products such as burnt clay bricks, tiles, cement and concrete. A special training in techniques for ]' the identification of minerals, in chemistry and in crystal chemistry is essential to understand and predict reactions that may occur between ] different minerals, or under certain environmental conditions. The application of geology and mineralogy to building materials research is illustrated in the following description of reactions with harmful consequences that can occur in building materials.

I UNSTABLE QUARTZ Although reaction between cement and aggregate is essential for the development of bond in concrete, undesirable reactions occur under certain ] conditions between the alkalis in cement and particular minerals in the aggregate. The following examples involve unstable forms of quartz. J' Alkali-silica reaction. This reaction occurs when the alkalis in cement react with the minerals opal, tridymite and cristobalite if they are -| present in the aggregate and this reaction results in deleterious [• expansion of concrete. It was first identified more than 40 years ago in the USA. I Alkali-quartz rock reaction. It has become evident in countries such as -• the USA, Canada and South Africa that quartz-bearing rocks such as greywacke, hornfels, quartzite and granite-gneiss are also subject to 1 deleterious alkali-aggregate reaction in concrete. The opinion is that J chert, chalcedony and microcrystalline-, cryptocrystalline- or strained quartz react with the alkalis of the cement. m j The reaction between the alkalis and the unstable forms of quartz results in the formation of an alkali-silica gel. The gel expands when it absorbs m water; if a pressure of about 10 MPa develops, the tensile strength of the '• cement paste is exceeded, which results in cracking of the concrete. This reaction can be diagnosed in concrete by the identification of a • white product both on aggregate fracture planes and in voids in the

DR RE OBERHOLSTER 123

mortar. This reaction product is difficult to characterise because it is not easy to separate in pure form, and also because it probably contains J both crystalline and amorphous products. However, the following properties are fairly universal. It is light brown to colourless under a petrographic microscope and mostly anisotropic, with refractive indices between 1,494 and 1,510. Under a scanning electron microscope it has a characteristic appearance and EOXA data indicate a Ca-K-Na silicate with the following molar ratios CaO/SiO2, 0,16 - 0,28; Na20/Si02, 0,01 - 0,18 and K20/Si02, 0,06 - 0,12. The most prominent XRD peak is at about 12,2 A which shifts to about 11,1 A under dehydration. The greatest incidence of cracking of concrete struc- J tures by alkali-aggregate reaction has been recorded in the South Western Cape, with hornfels and greywacke aggregate of the Malmesbury Group; deterioration of concrete as a result of this reaction has also been recorded in the Eastern Cape with orthoquartzite of the Table Mountain Group and on the Reef with quartzite of the Witwatersrand Supergroup.

SULPHATES AND SULPHIDES During the past sixty years in Norway, deterioration of concrete and upheaval of foundations have been experienced as a result of the presence of slightly metamorphosed pyrrhotite-containing shales of the middle Cambrian/lower Ordovician age. Similar problems have also been encountered in other countries such as the USA, Canada and South Africa. In many cases the problems encountered result from the presence of minerals such as marcasite (FeS2), pyrite (FeS2) and pyrrhotite (Fe7S8-FeS) associated with carbonaceous shales. Oxidation reactions of the sulphide minerals. It is believed that the oxidation of the sulphide minerals is chemico-biological. Iron sulphide is first oxidised to sulphate:

2FeS2 + 2HzO + 702 -»• 2FeS0* + 2H2S0., The next reaction cannot proceed chemically in an acid environment and is ]• thought to be completely attributable to bacterial oxidation:

02 + 2H2S0-, + 2Fe2(S0,,)3 The ferric sulphate can react with unreacted sulphide as follows:

3 + FeS2 + 8H2O + 15FeS0.. + 8H2S0., Oxidation of the ferrous iron to ferric iron by bacteria of the •^ Ferrobacillus-Thiobacillus group re-occurs. The reaction takes place in a j pH range of between 2 and 4,5. ^ Acid attack. Coarse aggregate or sand from gold-mine dumps is often used j as fill under concrete floors and pavements, or between concrete columns -1 • where soil which contains expansive clay minerals has been removed. The sulphide minerals in the fill are oxidised; the resulting sulphuric acid "! attacks the cementitious component of the concrete. In Welkom a pH of 2,8 J has been recorded in water-saturated fill. 1 DF, R E OBERHOLSTER 1 1 124 Soil and foundation heave. If the sulphide-bearing rock contains calcite, the sulphuric acid will react with it to form gypsum which has a molar volume double that of caicite. The growth of gypsum crystals between rock layers can force the rock apart. The formation of jarosite KFe3(50 02(0H)e and ferric sulphate also results in a volume increase of, respectively, 1. 115 and 170 per cent. The volume increase resulting from the formation of these minerals causes heave of the strata, which can lead to the cracking of structures built on 1 it. ] Sulphate attack of concrete. If Portland cement products, such as concrete, are in contact with sulphuric acid, gypsum and other sulphate 1 salts, reaction with the cement hydration products can occur. Sulphuric acid reacts with Ca(0H)2 to form gypsum which has a molar volume more than double that of Ca(0H)2:

Ca(OH)2 + H^O* •+• CaS0,,.2H20. Gypsum from the above reaction or from other sources, can react with hydrated tricalcium aluminate of the cement paste, to form the mineral ettringite, as follows:

3Ca0.Al203.6H20 + 3CaS0,,.2H20 + 24H20 + 3CaO.Al 203.3CaS01(.32H20. Ettringite has a molar volume more than 41/2 times that of tricalcium aluminate hydrate, and formation of it in concrete can lead to serious cracking. Ettringite occurs naturally at locations such as Crestmore in California. Although deterioration of concrete as a result of sulphate attack is ] fairly common, it is seldom encountered because of the use of sulphide-bearing aggregates. However, it was found that serious cracking of the walls and plinth walls of houses at the mining community of Penge, ]• was attributable to the use of tailings containing a carbonaceous sulphide-bearing cummingtonite slate, in the concrete floors and cement ] bricks. The main sulphide constituent of the slate was pyrrhotite with a molar Fe/S ratio of 0,96, as determined by EDXA. Selective dissolution methods ] were employed to determine the mineral sulphur, organic sulphur and pyrrhotite contents which were found to be respectively between 0,84 and 1,05 per cent, 3,20 and 2,30 per cent, and 2,25 and 2,81 per cent in different parts of the rock. The corresponding total carbon content was, respectively, 6,01 per cent and 4,06 per cent. The result of the reaction between the oxidation products of the slate aggregate and the hydration products of the cement, was the mineral thaumasite, CaSiO 3.CaSO,,.CaCO 3-15H20, which is sometimes found in deteriorated Portland cement products. This rcineral occurs in nature, at places such as Bingham in Utah and in the Fairfax Quarry, Centreville, Virginia.

DK F E OEERKOLETER 125

"f THE METAMORPHIC MAP OF SOOTH AFRICA (1:1 000 000) j by

! E.P. Saggerson and L.M. Turner

i _l The initiative to publish maps showing the distribution of metamorphic

•» rocks and facies was taken during a meeting of the International Commission

on Petrology and Volcanism of the Upper Mantle Project in Copenhagen

j in 1965. In June 1966 the General Assembly of the Commission for the

Geological Map of the World (C.G.M.W.) approved the plan and eventually

_l a Subcommission, under the chairmanship of Professor Zwart of the

^ • University of Leiden, was created. South Africa made its contribution

to the Metamorphic Map of Africa through the coordinator for Africa. i —' At the inaugural meeting of the current C.S.P. programme, the Geological

^ Survey of South Africa indicated that as part of its contribution,

a metamorphic map would be published as one of the new series of maps

I on a scale of 1-. 1 000 000 which also include new geological, mineral

and tectonic maps. i The purpose of this metamorphic map is to provide a graphic representation 1 _j- of the distribution of the main types of metamorphism and associated

-> rock types through geological time. This map together with its explanation

is essentially a record of thermal processes on a regional scale and

j differs from the geological map in that few lithostratigraphic units are delineated, for metamorphic events overlap rocks of different i j composition and age. m The presentation mainly follows the scheme accepted by the C.G.K.K.

— and published in I.U.G.S. Newsletter No. 2 of 1967. The roetamorphic

rocks have been classified according to the definitions of pressure i i E.P. SAGGERSON and L.M. TURNER 126 I

facies series which include low pressure/temperature (contact metamoiphism),

' low pressure/temperature, intermediate pressure/temperature and inter-

"» mediate high pressure/temperature (dynamothermal metamorphium), facies

series based on the stability fields of the Al2Si05 polymorphs and

I other diagnostic minerals, mineral reactions and mineral assemblages.

Because of the unique geological record in South Africa different regimes

I of regional metamorphism can be distinguished. These include regional

^ dynamothermal effects, geothermal and contact effects, burial and shock

effects. Burial effects in the Witwatersrand and other Proterozoic

basins are preliminarily assessed whilst increasing coal ranks and

changes in spore colours are shown for parts of the Karoo Sequence.

The different facies series, coal rank, shock metamorphism etc. are

; represented by their own sets of related colours with increasing intensity

_ of metamorphic grade, whilst diagnostic minerals are also shown (where

—' essential to interpretation). Localised effects have been omitted

T for the sake of clarity. Granitoid rocks and migmatites are a common

rock association in metamorphic belts and are shown by suitable symbols

j with distinctive colouring to depict their metamorphic and tectonic

^ associations.

Many terranes display evidence of polymetainorphic events, vertical

_! stripes of appropriate colour being overprinted by the younger event

•" represented by horizontal stripes. The age of metamorphism is shown

by italicised symbols according to its age within an erathem.

— In presenting a pre-publication hand-coloured copy of the map as a

poster display, contributions and constructive criticism are invited

so that this first edition of the metarorph^c map when published will

be generally acceptable. In the written synthesis, time and structural

~" E.P. SAGGERSON and L.M. TURNER 1 127 1

associations are additional dimensions that have been taken into acco

-' so developing a framework for understanding the crustal processes involve*

_) The authors wish to thank all those contributors to date who have si v willingly supplied data, some of it as yet unpublished, and pay a tribute

to the South African geological community who have so readily responded

I to the spirit of this project that can have far-reaching effects

particularly in the economic field. All contributions will be suitably

I acknowledged. That it will provide a framework for and stimulate further

- research and investigation only time will tell.

E.P. EAGGERSON and L.M. TURNER 128

DIE VERSPREIDING VAN PEKELS EN KOOLDIOKSIED IN VLOEISTOFINSLUITSELS VAN DIE GNEISE EN GEMFTAMORFISEERDE SEDIMENTêRE GESTEENTES VAN DIE NOORDWES-KAAP A.E. SCHOCH EN E. KLATT (Universiteit van die Oranje-Vrystaat) Die studie van die vlugtige fases wat geassosieër is met die stratge- bonde ertsvoorkomste van die Noordwes-Kaap, vereis b kennis van die aard en verspreiding van vloeistof-insluitsels in die omgewingsgesteen- tes. Daar is nog nie voorheen data versamel oor hierdie aspek nie. In hierdie verslag word die resultate van h ondersoek aangekondig vir drie gekose monsterlokaliteite in die ooste, weste en noorde van die studie- gebied. Die tydrowende aard van volledige mikrotermometriese studie is in ag geneetn, en deeglike petrografiese ondersoek van sowat 250 monsters het die fondament gevorm vir die seleksie van 60 preparate vir bepaling van die eienskappe van die vloeistof-insluitsels. Dit was die voorneme om met h baie breë regionale benadering te begin, en geleidelik die resolusie te verbeter, tot by die uiteindelike karakterisering van elke belangrike forroasie sowel as spesifieke ertsdistrikte. Hierdie proses het nou sowat halfpad gevorder, maar dit bly steeds verassend hoedat uiteenlopende gesteentetipes van h spesifieke gebied soortgelyke vloeistofinsluitsel-populasies bevat. Tot op datum is reeds sowat 4 000 mikrotermometriese bepalings gedoen (smelttemperatuur °C=Tm, homoge- niseringstemperatuur °C=Th, ekwivalent-persentasie = e, digtheid volgens gemiddelde isocnor = d):

1. AUGRABIES-GEBIED (Fig. 1). Hoewel die eienskappe van die waterinsluitsels h baie breë spreiding vertoon vanaf suiwer water tot by versadigde pekel, kan dit nie gewyt word aan afnekkingsverskynsels nie, en is daar essensieël !n trimodale patroon. Die grootste populasie het min opgeloste stowwe (Tm -2) en h minimum insluitingstemperatuur van 130°C (Th). Daar is ook twee belangrike pekelpopulasies (Tm -7, eNaCl = 10 en Tm -18 tot -22, eNaCl = 20 tot 22). h Aaneenlopende spektrum van Tm tot so laag as -50 dui egter op die teenwoordigheid van kalsium- chloried-pekels, selfs versadigde pekels (eCaCl.,=50). Die homogenise- ringsspektrum is unimodaal tot bimodaal, sodat die hoofpiek ooreen moet stem met beide die suiwer water en die belangrikste pekels. Daar is !n waarneembare neiging vir die Th om te styg van wes na oos (A: Augra- biespark 140, BI: Renosterkop 150, C: oos van Augrabies (dorp) 160). Daar is egter h aaneenlopende spektrum van waardes tot so hoog as Th 320.

Die meeste monsters bevat amper geen kooldioksied nie, behalwe in die omgewing van Renosterkop. Afgesien van die teenwoordigheid van klein hoeveelhede kooldicksied in die waterinsluitsels, soos geïllustreer deur die smelting van klatraat-yse (BI: Tm +10 tot +12). is daar h komplekse kooldioksied-populasie wat gewoonlik suiwer is (Tm -56,6), maar soms h klein hoeveelheid koolwaterstof bevat (lm -57,8, eCH, = 3). Dit- homogeni- seringsspektrum is op die minste trimoriaaï (BJ: Th b, 14 en 25 tot 29), aanduidend van 'n komplekse barometrif-se geskiedenis. Kombinering van die prominentste ptkel-piek (d=0,93) en kooldioksied-piek met die hoogste digtheid (d=0,91), lei tot h Dl-rekoristrukfaie van 1,4 Kb en 210°C.

A.l . SCHOCH en I . KI Al 1 1 29

A (588) (418)

Bi (348)

200 300 -40 -20 0 20 [*C]

B2 (450) j -20 -10 30 CC]

Fig. 1 Histogramme vir graniet, gneis, kwartsare en kwartsiet (Augrabies-graniet, Renosterkop-kwartsiet, Brabees-graniet- ] gneiss, (Praekelt, in voorb.)) vanaf Augrabies-park (A), Renosterkop (B) en oos van Augrabies (dorp) (C). Die histogramme aan die linkerkant verteenwoordig stnelttempera- 1 ture (Tm) van water, en dié aan die regterkant verskaf passende homogeniseringstemperature (Th). Die onderste diagram (oningekleur, B-,) verskaf data oor homogenisering van kooldioksied vir monsters van Renosterkop. Die getalle gee in elke geval die aantal lesings.

2. GARIES-OMGEW1NG (Fig. Z). Oor die algemeen verteenwoordig die water-insluitsels h. swak pekel (Tm -2, eNaCI=4 tot Tm -5, eNaCJ=7) saaiTi met suiwer water, mar.r die waterpopulasie is meer kompleks as die indruk wat geskep word deur die smeltingiiemperature. Die trimodale tot bimodale homogenisering^'-pekirun-. het 'r, hoofpiek waarvan die waar- de geleidelik hoer word vanaf tiit gebicri oos van Garies (C: Tii 140), deur die westelike area (B: Th J90) tot by Vi maksimum in die noorde (A : Th 270). Kooldioksied is seldsaam behalwe in kwartsare in die

A.I . SCH"CH en [ . KI Ai! 130

noorde. Laasgenoemde bevat pekel (Tm -4 tot -11) van lae graad (Th 110 tot 130) saam met onsuiwer kooldioksied (Tm -57,7, eCH =3) van middel-graad (Th +20 tot +24), om h moontlike DT-rekonstruRsie van 200°C en 1,3 Kb te ondersteun. Met die uitsondering van h bietjie kooldioksied in een van die gneise, verteen woordig hierdie vlugtige populasie klaarblyklik *n laat episode met betrekking tot die pekel- episode.

A (91) (91)

B (152) (114)

C (226) (172)

200 300 PC] -20 o ro

Fig. 2 Mikroterrnometriese resultate vir graniet, gneis, kwartsiet en kwartsare uit die Garies-omgewing. Die gebied noord van Garies word ver teen woordig deur die histogram gemerk A, die gebied na die weste deur B, en die ooste deur C. Die linkerhandse histogramme verteenwoordig smelttemperature van water (Tm) en die ander gee die data vir die passende homogeniseringstemperature (Th). Die getalle verteenwoordig aantal lesings.

3. GESELSKAPBANK (Fig. 3). Die vloeistof-insluitsels in h "miloniet"- sone op die rand van die Naab-oorskuifblad (Strydom, in voorb.), verteenwoordig waarskynlik beide h spreiding van primêre eienskappe en In globale monstering van vlugtige bestanddele oor h groot gebied ween s vloeistof-be weeglikheid op die skuifskeurvlakke . h Breë Spektrum word vertoon van swak pekels (A: Tm ~3) , sterk pekels (Tm -18, eNaCl= 21) en kalsiurrchloried-pekels (Tm -24, Tm -34 tot -36), met passende homogenisering (Th 100, 130, 160, 180). Üeg van die vervorming- sone word egter h verbasende ooreen stemming gevind tussen monsters geneem in die T'hammaberg-formasie (Strydom, in voorb.), wat bekend is vir die voorkoms van sulfiedes, en die Beenbreek-formasie. Daar is

A.i . samen ei, i. Ki.vn 131 ]

oorwegend swak pekels (B en D: Tm -2, eNaC]=4), met h komplekse ] barometriese geskiedenis (Th bimodaal 140, 190). Die Beenbreek-formasie bevat egter nie net swak pekel nie (B: Tm -2, Th unimodaal 180), maar ook sterk pekel (Tm -20) en h klein hoeveelheid kooldioksied. Hierdie oorkoepelende eienskappe verskil van die eienskappe van gesteen- ]. tes in die Naab-oorskuifblad en die Geselskapbankformasie (Strydom in voorb.), wat swak pekel het (C: Tm -2) maar h hoogs komplekse J barometriese geskiedenis (Th kwadrimodaal 150, 220, 260, 320-350).

J (90) J

J (132)

(173) C (161) "| D (92)

— 100 200 300 [*C] ] -40 -20 0[*C]

1 Fig. 3 Resultate vir die gebied om Geselskapbank, A: miloniet in die Naab-oorskuifblad, B: kwartsiet uit die Beenbreek-formasie, C: granuliete en gneise uit die Naab-oorskuifblad en Gesel- skapbank-formasie en D: kwartsiet uit die T'hammaberg-formasie. Die linkerhandse histogramme gee smelttemperature (Tm) van water, en die ander diagramme verskaf passende homogeni- serings-temperature (Tin). Die getalle verteenwoordig aantal lesings. SLOTSOM. Die genoemde "agtergrond"-gegewens word tans gebruik as fondament vir die uitbreiding van die waarnemings tot die studie van

A.F. SCHfCH en F. KL ATI 132

mineralogies identiese monsters uit verskillende gekarteerde formasies van die gemetamorfoseerde sedimentêre gesteentes (Van Aswegen et al. (1984), Praekelt et al. (1983), Colliston et al. Q984)) en die ekonomies-belangrike eenhede. Die mees verassende regionale eienskap tot dusver gevind, is die swak verteen woordiging van kooldioksied in granulitiese gesteentes, in kontras met dïz Skandinawiese granuliete (Klatt en Schoch (1984), Klatt U979), Touret (1981)).

VERWYSINGS Strydom D.: h Struktureel-stratigrafiese studie van die metasedimente en ander metamorfiete van noordwestelike Boesmanland. (Ph.D. proefskrif, Universiteit van die Oranje- Vrystaat, in voorbereiding).

Van Aswegen, G., Strydom, D., Colliston, W.P., Praekelt, H.P., Blignault H.J., Schoch, A.E., Botha, B.J.V. (1984): A contribution to the tectonic and stratigraphie correlation of Proterozoic rocks in Namaqualand and Bushmanland, South Africa, Simposium WG3, Internas. Geol. Kongres.

Praekelt, H.E., Colliston, VI.P. en Schoch, A.E. (1983): The stratigraphie interpretation of a highly deformed Proterozoic region in Central Bushmanland, South Africa: First correlation of structurally separated metasediments of the Aggeneys Subgroup. Precam- brian Research (in pers). Colliston, W.P., Praekelt, H.E., Strydom, D. , Van Aswegen, G., Blignault, H.J., Schoch, A Z. (1984): The recognition of low angle thrusts in Central iiushmanland, Namaqua mobile belt. e J Abstr., 20 Geol. Ver. S. Afr, Potchefstroom. Praekelt, H.E. (1984): Die geologie van die gebied rondom Augrabies (2820 C). (M.Sc. proefskrif in voorbereiding, Universiteit van die Oranje-Vrystaat).

— Klatt, E., Schoch, A.E. (1984): Comparison of fluid inclusion • characteristics in high grade metamorphic rocks from Finnish • — Lapland with medium to high grade rocks of the Northern Cape Province, South Africa. Internas. Geol. Kongress. ' 1 , _ Klatt, E. (1979): Die Flüssigkeitseinschlüsse in den Granuliten Nord- lapplands und ihren Rahmengesteinen. Fortschr. Mineralogie, 57, - -. 62-63.

— Touret, J.L.R. (1981): Fluid inclusions in high grade metamorphic rocks. Hfs. 8 in Short Course in Fluid Inclusions, (Red. L.S. -• Hollister, M.L. Crawford), Min. Ass. Canada, Short course Hand- book 6, 182-208.

i — i: 133

THE MINERALOGY ON PLANES OF UNCONFORMITY WITHIN THE CENTRAL RAND GROUP OF THE WITWATERSRAND

G. SMITS

^ Council for Mineral Technology

~~ EXTENDED ABSTRACT

~~\ The ores of the Witwatersrand occur in placers that formed on fan- ™ shaped accumulations of debris filling depressions between granite domes —i on the western and northern edges of the depository. Recurrent periods of •«< tectonic instability in the provenance resulted in rejuvenation of the head-tributary river system and were responsible for the cyclic 1 sedimentation on the fans. The uplifts also led to repeated reworking of earlier sediment, and the recycling process, particularly that of ore- bearing conglomerates, was an important factor in the progressive concentration of heavy minerals.

—i J The fining-upward cycles are separated by unconformities that mark breaks ir> the stratigraphie sequence. During these breaks, deposition J ceased for considerable periods, sometimes for millions of years, or erosion caused the loss of earlier sediment. The major ore-bearing reefs —i !* rest on such planes.

—i Observations by microscope disclose that the concentration of heavy J minerals frequently started before deposition of the basal conglomerate, forming a thin veneer on the palaeosurface. At certain sites on these j ancient erosion planes, conditions were favourable for the development of algae and bacteria. The algal mats played an important role in the j concentration of gold and uraninite before the onset of a new cycle, since the quantities of these ore minerals occurring in association with kerogen far exceed those occurring in the overlying reef. However, the mechanisms "" responsible for the concentration of the two minerals differ, urar.inite — being entirely enveloped by kerogen in its original detrital state, whereas -»" gold precipitated in voids available within the carbonaceous matter and at _ the contacts with the over- and underlying sediments.

G. SMITS 134 J In addition, the common association of gold, sulphides, sulpharsenides, J and (U,Ti)-bearing minerals of authigenic origin with chlorite suggests that sorption by clay minerals was a process that contributed significantly to the concentration of metal ions. After burial, diagenetic and metamorphic processes gradually led to the release and redistribution of these metal ions and resulted in the precipitation of these neoformed J components. J The role of the unconformities in providing optimum conditions for the preconcentration and chemical modification of the detritus in general, J and the ore minerals in specific, towards the end of a break in sedimentation of extreme duration, appears to have been of considerable significance. J

i j

G. SUITS 135

THE INFLUENCE OF MINERALOGICAL PARAMETERS ON THE LEACHABILITY OF 7OW-GRADE NICKEL DEPOSITS J M.J. SOUTHWOOD Council for Mineral Technology Private Bag X3O15 Randburg 2125

J EXTENDED ABSTRACT The projected expansion of the South African stainless-steel industry provides an incentive for the re-evaluation of low-grade nickel deposits in the Republic. Current nickel production is dependent upon the demand J for the platinum-group metals (PGMs), since nickel is a co-product of PGM production from the Merensky Reef of the Bushveld Complex. In 1980, South ^ Africa was responsible for 5,8 per cent (30 000 t) of the Western World's primary nickel production. This status is, however, liable to decline with ^ increasing PGM production from the relatively nickel-poor UG-2 Layer of the J Complex.

j Several alternative nickel resources are known in South Africa, but they are generally low-grade occurrences that are sub-economic at present. mi Acid-bacterial heap leaching provides a potential low-cost benef iciation process, but its usefulness depends in the first instance upon the •i mineralogical characteristics of the ore. i- i Firstly, the mode of occurrence of nickel (and other economic metals) I must be established. The metals must be in acid-soluble form (i.e. in sulphides rather than in silicates) and associated with an abundant supply ^ of ferrous iron, which acts as a substrate for bacteria (commonly i J Thiobaaillus fevrooxidans). Highly oxidized or residual nickel deposits «• are therefore unlikely to be suitable, i Secondly, the ore must be sufficiently permeable to allow lixiviant to ; penetrate. The permeability of a fresh, igneous rock is essentially negligible. However, alteration iiay considerably enhance its permeability — as a result of the development of a metamorphic fabric, or of microfractures

Mi qmTTHunnn 136

due to deuteric or hypogene processes such as serpentinization.

Gangue mineralogy is a third important parameter, since the presence of acid-consuming minerals will greatly elevate the quantity of acid consumed by the ore. Carbonate minerals are most deleterious in this respect; every 1 per cent by mass of calcite in an ore will consume J approximately 10 kg of 98 per cent sulphuric acid per tonne of ore. Magnesite or dolomite may form from the breaking down of serpentine J minerals, and are theiefore common constituents of nickel sulphide ores. Certain chlorite minerals also consume acid, but their consumption is less easily quantified because of their variable composition and J incongruent dissolution. Acid consumption may also increase as a result of adsorption by colloidal silicate particles. Desliming is therefore desirable before the lixiviant is applied.

J Sulphide mineralogy, sulphide chemistry, and ore intergrowths are important factors influencing the dissolution rate of metals. The J dissolution rate of sulphide is controlled to a large extent by galvanic interaction between different phases in mutual contact. A mineral with a J low rest potential will undergo enhanced anodic dissolution when in contact with a phase of higher rest potential. The relative rest potentials in aqueous acid solution of major sulphide phases pertinent to J this study are given below: J pyrite > chalcopyrite > pyrrhotite « pentlandite > violarite (?)

Intergrowths between these phases are therefore of considerable importance. Preliminary studies indicate that image analysis can be used in the quantification of parameters, such as grain size and areas of mutual contact between sulphide phases, so that an assessment can be made of the importance of galvanic processes during leaching.

The primary magmatic assemblage consists typically of pyrrhotite, pentlandite, and chalcopyrite, with only minor quantities of pyrite. During hypogene or supergene alteration, however, pyrite may form at the expense of pyrrhotite, and secondary nickel sulphides such as violarite and millerite may form froir. pentlandite. Preliminary studies indicate that violarite has a higher dissolution rate than pentlandite; indeed, the solution behaviour of the nickel sulphides might be expected to mirror

1 J M.J. SOUTHWOOD 137

that of the copper sulphide series, where activation energy decreases with increasing oxidation.

In summary, the mineralogical suitability of a nickel ore for acid leaching is greatly influenced by the extent of hypogene, supergene, and (to a lesser degree) metamorphic alteration. These processes may facilitate the development of permeability, the production of acid- consuming gangue minerals, and the modification or, in extreme cases, the elimination of sulphide mineral phases.

J 1

1 M.J. SOUTHWOOD J • 58

THF TECTONO-SEDIMLN TAR^ ^ETI INC, Of Till ViMI K -DORP J . OUTLIER AT KROMDRAM. NORTH-WI?! 01 .IQHVWI HirRG_

-i I.G. Stanistreet and T.~. McCarthy

The Kromdraai exposure of the Ventersdorp Supergroup is situated ""I north-west of Johannesburg. It offers a unique opportunit \ tu vttidy a J sedimentary component of the supergroup at the most north-east 01 ly extent of its outcrop.

j Three main sedimentary facies may be distinguished:

(1) A diamictite facies ~j (2) A planar stratified sandstone facies -I (3) A trough cross-bedded sandstone facies

Fine diamictite units are interbedded with planar strai ified and trough J cross-bedded sandstones. This association interfingers with thick, coarse diamictite units, containing boulders in excess of 2 metres -. diameter. The finer grained units make up a greater proportion of the j sequence towards the north-west of the outlier, whereas the diamictite facies comprises the entire sequence in the south. The vast majority of clasts in the diamictite facies consist of orthoquartzite, probably derived from the West Rand Group, but a significant number of clasts -J1 in the diamictite facies in the south comprise quarts pebble conglomerates, some of which are auriferous. The latter were almost certainly ~"| derived from a Central Rand Group source. Rare amygdalnKl.il lava clasts _j have been found. These were presumably derived from oldc;- lavas of the Ventersdorp Supergroup (Klipriviersberg Group?). On the basis of the _ lithologies present, the section is tentatively correlated with the Platberg Group.

The diamictite facies represents deposition by mud-flows. The pne- "*| dominance of this facies suggests deposition on semi-arid fans. -J The planar stratified sandstone facies would then represent sheet-flood deposits on the fan surface. The trough cross-bedded sandstone facies -1 probably represents rare fluvial reworking of the fan accimulation. I The facies distribution would suggest proximal fan deposit s in the southeast and more distal deposition towards the northwest .

The existence of semi-arid fan systems implies deposition adjacent to "" active fault scarps. This is probably related to the ir.idc !•• Vent ersdor-p phase of horst-graben tectonics, documented from other pait> <>i tin- I "* basin. Clast assemblages in the 'IJamictites suggest that source rocks on adjacent horsts included Centril Rand Group strata, and there-Tore it is possible that Central Rand Group sequences have been preserved in I the associated grabens. I I I I.G. ^t ar.i st t f et ari(i 1.-. M<( art :i\ 139

The Ventersdorp Supergroup outlier has been tectonic-ally emplaced into ~ its present position on a major thrust. It has overridden a complex _J basement including Archaean greenstones, granites and rocks of the ; Witwatersrand Supergroup. The thrust, plane is locally represented by — tectonic melanges containing large blocks of Witwatersrand Supergroup and Ventersdorp Supergroup lithologies. The matrix to the melange ~" includes mylonite and pseudotachylite. The Ventersdorp Supergroup _. outlier shows no obvious internal dislocation. This contrasts with the underlying Witwatersrand Supergroup rocks at Zwartkops, where -. several thrust slices have recently been documented. It is our con- I tention that the structural complexities in the Witwatersrand Super- group outlier were caused by proximity to the major thrust, and as such these rocks lie in a schuppen zone below the major thrust. The "1 - fact that younger rocks are thrust onto older rocks is a consequence -J of the earlier, syn-sedimentary horst-graben phase.

~t - The tectonic melange along the major thrust zone is overprinted by the _• regional, east-west cleavage. It is therefore later than the thrusting and represents the penultimate phase of deformation. This cleavage is _, regional in extent, but has experienced minor deformation during post- ' Transvaal upright folding.

The inference that the Ventersdorp Supergroup strata formed in a "1 graben containing Central Rand Group strata has considerable economic •j implications. The upper portion of this graben has been laterally displaced during the thrusting. The determination of tectonic transport -[ direction is therefore important, as it may lead to the location of _] Central Rand Group remnants preserved in this and possibly other similar grabens. We favour an easterly transport direction, away from the -. Ventersdorp basin axis. This would imply that Central Rand Group j sedimentation occurred north of the Rand anticline.

I.G. >tanistreet and J.S. Mc-farthv HO

INTRODUCTION TO GEOSTATISTICS

Certain statistical techniques, such as least squares regression, analysis of variance, discriminant analysis and principal components analysis are well known to geologists, and are applied almost routinely in research wort- in the earth sciences. There is one very fundamental assumption that is basic to these techniques, namely that of independence of the observations, which may not be valid for certain types of data in the earth sciences. For this kind of situation, alternative analyses have to be used, especially analyses which utilize the dependence among observations. "Geostatisties" is a term used for an approach which was developed for this type of situation, especially with a view to mining applications, but it has been used in several other fields, notably soil science, hydrogeology, air pollution control and meteorology.

To illustrate the effect of the interdependence among data, consider the data from the boreholes of an exploratory study of an ore deposit. The geologist will normally not choose the drilling sites at random, but he would rather concentrate on areas where high values were found. This would mean, however, that high values will be over-represented, and therefore a simple arithmetic mean will be an overestimate of the amount of ore present in the deposit. It is clear that a weighted average should be used, and Geoscatisties offers techniques for computing the weights.

A geostatistical study usually involves one or more regionalized variables (such as ore values, atmospheric pressure, pH of soil). A regionalized variable is assumed to vary gradually in space (or, in some applications, over time), which implies that there is a spatial dependence between the values of the variable, with points clvse together more likely to have similar values than points which are far apart. The first step in a geostatistical study will usually be to model this dependence, and the mathematical tool used for this is the variogram, which is related to the correlogram. At this stage there must be an interaction between the geologist and the geostatistician, since the variogram must reflect the available knowledge about the geology of the phenomenon, while on the other hand the variogram may suggest something about the geology which might be of interest to the geologist.

The next step in the geostutistical study is usually estimation, for

F.E. Steffens 141

example estimation of the amount of ore in an ore body. The estimation technique is called kriging. There are several variations of kriging available, depending on the assumptions that one is willing to make, such as ordinary kriging, lognormal kriging, universal kriging and multigaussian kriging. The variogram may also be used to study the need for additional data, for example for planning a drilling campaign. In addition, it en- ables one to simulate the phenomenon that is being studied. A conditional simulation will reproduce the data in the points where data are available, and it will reproduce the dependence properties embodied in the variogram, but it will provide randomly generated values for all other points. Such a simulated deposit may be used, for example, to help in planning the mining operations.

Geostatistics is a highly specialised field, not usually covered in ordinary Statistics courses. It is potentially very useful in the earth sciences, but specialized training is needed in order to utilize its full potential. To the statistician certain aspects of the subject do not seem quite rigorous, and there is still scope for development work to be done by statisticians.

F.E. Steffens 142

_____ PJ MAPPING OF THE PROTEROZOIC ROCKS OF WESTERN RUSHMANLAND

J D. STRYDOM

I (University of the Orange Free State) I ~"| The area under discussion is situated north-east of Springbok (Fig. 1). i •" Iwselberg outcrops of plutonic, metasedimentary and metavolcanic rocks • ~-% represent a highly deformed terrane. In general the area is characte- J rised by the presence of large open synforms, antifi us and subvertical I shear zones (Fig. 1). The wave length of the open folds varies from 5 J to 20 km, of which the Geselskapbank structure is the most prominent. I The subvertical and easterly orientated shear zones are best developed I in the central part of the study area with an anomalous influence on the I geometry of the open folds.

I A structural-stratigraphic method was used to map the supracrustal "I rocks, and by this means important discordant and concordant contact I ~» relationships between various units were discovered. Units with fairly -. uniform and laterally continuous characteristics were grouped together _ J and mapped as part of the Khurisberg, Nab and Haib Subgroups. An | allochtonous unit, mapped as the Geselskapbank formation, structurally I overlies the Heiorigas formation and the Nab subgroup. Attention can I be drawn to the fact that the Heiorigas formation is lithologically very n similar to the Haib Subgroup of the Richtersveld Province. It is further I pointed out that formal names for rocks used here have been published •* "J by SACS, 1980. • -i Crosscutting features are due to low angle thrusting and sheared J recumbent folding of the early isoclinal fold phases (F- and F, fold pe- I riods). In the Geselskapbank area several thrust faults were traced _J which involve the plutonic and supracrustal rocks on a regional scale. fl This caused the juxtaposition of rocks of the Vioolsdrif Suite/Haib Sub- I group (Heiorigas formation) against the Naab/suite/Geselskapbank B ' formation as well as against Gladkop Suite, the Oonab gneiss/Nab Sub- "I group and, to the south, the Klein -Namaqualand Suite /Khurisberg .3

J 143 Subgroup. These thrust units are represented by the Groothoek, J Skelmfontein (Blignault et ah, 1983), Geselskapbank and Naab nappe sheets (Van Aswegen et ah, 1984) (Fig. 1). 3 In the southern domain the Oranjefontein thrust fault separates rocks of the Copper District from the Bushmanland augen gneisses. This defor- 3 med thrust plane was identified by aid of small scale refoliation features and the linear distribution of the ancient Brandewynsbank Gneiss 3 along this plane. The fairly abrupt eastern termination of the Spektakel Suite, Nababeep Gneiss and the associated Koperberg Suite, has to date 3 presented considerable interpretative problems, but the recognition of the Oranjefontein structural discontinity is an attractive solution. 3 The interpretation and understanding of these thrust structures can be 3 demonstrated by a series of balanced longitudinal sections and cross sections drawn on different scales. From these sections the sense of movement is interpreted to have been towards the south-west. Large 3 distances of displacement is suggested by the presence of granulite facies rocks in one of the major nappes, seen against the medium grade 3 rocks of structurally underlying nappes.

3 REFERENCES 1 J Blignault, H.J., Van Aswegen, G., Van der Merwe, S.W. and Colliston, [ W.P., 1983: The Namaqualand geotraverse and environs; part of J the Proterozoic Namaqua mobile belt. In: B.J.V. Botha, (Editor), I Namaqualand Metamorphic Complex, Spec. Pubh 10, Geol. S. Afr. 3 | South African Committee for Stratigraphy (SACS), 1980. Stratigraphy of "J South Africa. Part 1 (Comp. L.E. Kent). Lithostratigraphy of the I Republic of South Africa, South West Africa/Namibia, and the -i Republics of Bophuthatswana, Traroskei and Venda: Handb. geoh J Surv. S. Afr., 8. J Van Aswegen, G. , Slrydom, D., Collistor., V, .P., Praekclt, H.E., Blignault, H.J., Schoch, A.E., and Botha, B.J.V. A Contribution to the Tectonic and Stratigraphic Correlation of Proterozoic Rocks in Namaqualand and Bushmanland, South Africa (in preparation).

1 Y

Fig. ] Major structures of the Geselskapbank-Areb area north-east of Springbok. 145

THE ROLE OF THE MINERALOGIST IN THE COPPER-MINING INDUSTRY

P.M. SWASH

Council for Mineral Technology Private Bag X3015 Randburg 2125

EXTENDED ABSTRACT

The mineralogist plays an important role in every phase of development ~~i , of any copper-mining venture. This paper shows how the mineralogist is —' involved in all aspects of copper production from the initial exploratory -, , phase through to mining, milling, smelting, and refining.

Copper minerals occur in more than one form, and respond differently ~j to different metallurgical techniques, e.g. sulphides float selectively when the normal commercial reagents are used, yet 'oxides' (malachite, "•j pseudomalachite, etc.) do not. In exploration, the initial mineralogical —' study is used as an aid in selection of the recovery technique most likely _, to be effective, and in the evaluation of reserves and the economic _J feasibility of a deposit. Examination of the minerals yielded information on the genesis of the ore-body, variations within the deposit, zonation, J alteration, etc. Such information may be useful in mining operations and as a guide to further exploration for nearby deposits of a similar nature. I Comparison of the mineralogy and geology of the ore deposit with that of known mines aids in the selection of mining and milling practices.

_J As economics and technologies change and cut-off grades become lower, marginal deposits, complex ores, and even dump material may be considered I worthy of exploitation. Mineralogical examination can therefore be expected to become even more important in future prospecting.

-* During mining operations, advance mine cores are often taken ahead .- of the area to be mined, and drill-core composites, representing mineralized J intersections of proposed mining blocks, are examined mineralogically. Recoveries and grades are then predicted for the length of time that a J certain mining block is expected to be exploited. A mineralogist may

P.M. SWASH 146

observe that the ore is present in the form of finely disseminated — sulphides and that beneficiation would be uneconomic, or that the ore -, may even be considered refractory with a very poor response to flotation. ^, He may therefore recommend that the mineralized block should not be mined.

^ The most important work of the applied mineralogist obviously concerns production, his main aim being to help in improving the overall efficiency ~~ of metallurgical operations. For example, the additional recoveries of — copper that can be achieved by very slight improvements in flotation or -, leaching techniques, represent large tonnages on plants with a high _ throughput. Thus, a decrease in the copper content of the tailings of a mere 0,1 per cent means a considerably improved recovery of copper and, hence, of increased revenue that soon repays the cost of any successful research.

••• An important tool in the solution of problems related to milling and concentration is a data bank of mineralogical parameters. This is accumulated by the monthly examination of composited mill products. Continuous monitoring of the mill products builds up a store of information for comparison with abnormal samples. Such a data bank supplies a frame of reference that can be used in the establishment of any trends or any changes in the response of the ore to flotation. It can also be used in the evaluation of any mineralogical changes that may arise as the result of alteration in the flotation circuit. The metallurgist can use the ; " mineralogical information as an aid in the adjustment of process parameters > - for optimum recovery of the metal. i '« j The following problems are commonly encountered on plants for the i concentration of copper. i I _ (i) High copper levels in the tailings obviously mean that recoveries of the total amount of copper will be smaller. In this instance, it is ' of importance for the metallurgist to determine whether the additional copper occurs as a sulphide, oxide, or refractory material ' (vermiculite or wad). High oxide levels occur in tailings from some mills because these mills were not designed for the recovery of oxides. j! "" This problem is not easily alleviated. If the additional copper in ~ the tailings is in the form of copper sulphides, the problem may be

P.M. SWASH due to operational factors such as poor grinding, overgrinding, or poor use of reagents, etc. The problem may also arise because there are changes in the mineralogical characteristics of the ore, in the sulphide minerals, or in the distribution of copper minerals within the rock, which may lead to reduced liberation of the sulphide grains.

(ii) The concentrate may have a high gangue content because of a change in the liberation characteristics of the grains. A slightly finer grind would then be recommended. Dilution of the concentrate may also result from poor operation or high concentrations of talc jr carbonaceous shale in the ore.

Mineralogical work applied to extractive metallurgy in the copper- mining industry involves assistance in the solution of pyrometallurgical and hydrometallurgical problems. These problems are normally of a non- routine 'trouble-shooting1 nature, and the mineralogical aspects of problems arising on the plants must be dealt with immediately and expertly.

The following are a few examples of samples that are commonly examined by a mineralogist in the copper industry:

leaching plants feeds, calcines, dusts, sinters smelters ' slags, mattes, refractory bricks, fluxes refineries wire bars, mould washes precious-metals plants dore slags, deselenized slimes plants for treatment of calcines. refractory copper ore (TORCO) 1A8

•N FASIE - ANALISE VAN DIE BASALE BEAUFORTGROEP J IN DIE NOORDWES - ORANJE - VRYSTAAT j Alhoewel daar reeds verskeie studies op die basale Beaufort elders in die Karookom uitgevoer is, is daar nog 'n groot tekort aan gedetailleerde studies J in die deel van die kom. ' -, Die gebied ter sprake is in die omgewing van Theunissen, Welkom en Venters- I burg (figuur 1). Die doel van hierdie ondersoek is om deur middel van fasies- ana lises die afsettingsomgewing vas te stel. Om dit moontlik te maak is ver- I skeie dagsome en kernboorgate in detail bestudeer en beskryf. Daar moet Jin gedagte gehou word dat die dagsome baie swak is en alhoewel kernboorgate ( 'n baie groot aanvulling in die verband was, bestaan daar egter nog gapings. T TierberSiegs tweg vae nstratigrafies die Eccagroee eenhedp en diee isFormasi hier teenwoordige Normandie, naamlin van kdi die eBeaufort Formasi- e groep. Die Formasie Tierberg bestaan uit 'n basale horisontaalgelaagde donker skalie- eenheid en 'n meer sanderiger opwaartsgrowwer sone. Die Beaufort bestaan hoofsaaklik uit growwe tot fyn sandsteen en enkele konglomeraatlae of -lense, en toon 'n oorgang na die onderliggende Tierberg- formasie.

Die volgende sewe dominante sedimetêre fasies is teenwoordig in die fluviale Beaufortgesteentes-

1. Growwe Sandsteen- Konglomeraatfasies 2. Kruisgelaagde Sandsteenfasies 3. Horisontaalgelaagde Sandsteenfasies 4. Riffelkruisgelaagde Sandsteenfasies 5. Afwisselende Moddersteen en Sliksteenfasies 6. Moddersteenfasies

7. Koolstofryke Skaliefasies

In die Oorgangsone aan die bokant van die Eccagroep is die volgende fasies geidentifiseer -

1- Laehoekkruisgelaagde en Horisontaalgelaagde Sandsteenfasies 2. Riffelkruisgelaagde Sandsteenfasies 3. Riffelkruisgelaagde Skalie / Sliksteenfasies 't* Moddersteenfasies

C~ TPDftl 149

ft" SO D WELKOM tf

OVENTCR8BURG

^ I

VERKLARIWG

P-T»« FORMASIE NORMANDIEN D THEUNISSEN j ft FOWMASIE TIERBERG

Figuur I Lokalitaitskoart van die g«bi«d j

1 J O J URANIUM AND THORIUM DISTRIBUTION IN THE J CONTREBERG GRANITE, DARLING J J.C. THERON AND A.E. SCHOCH « (University of the Orange Free State) J Two plutons of young granite, intrusive into other members of the Cape J Granite suite, occur to the northwest and southeast of Darling, Western Cape Province. In spite of their comparable temporal and structural J positions in the granite series, the two granites differ considerably in textural and compositional properties, making a comparative study desirable.

1 It has been demonstrated that the Klipberg granite (KB) has unique properties such as a preponderance of carbon dioxidic fluid inclusions over brine (Schoch, Winkler and Touret, 1980), the presence of small episyenite bodies (Schoch and Scheepers, 1982), and the occurrence of molybdenum mineralization (Scheepers, 1982). The episyenites are J characterised by replacement textures as a result of successive potassium-, sbdium-, fluorine- and calcium-metasomatism (Scheepers, J 1982). The radioelements are mostly present in primary minerals such as zircon, thorite, gummite and monazite, but to a lesser extent also in 3 secondary materials such as iron oxides, collectively providing whole rock values of approximately 30 ppm U«Og and 70 ppm ThO,. In general J« the process of episyenitisation lowered the radioelement content. The radioelement distribution pattern is characterised by small positive and I negative anomalies of a few m2, with a contrast of as much as 100 ppm

U_OR. A control area was also studied on the well-exposed coastal ~~J outcrops near Paternoster, yielding similar results for the Cape Colum- bine granite (CCG) (Scheepers, 1982). -The similarities between the —i Klipberg alkali granite and the various uranium bearing French granites —. necessitated the detailed investigation of the companion pluton at Contre- berg.

Petrographically the Contreberg granite (CB) differs considerably from j KB (Schoch and Theron, 1983). While the latter is a medium-evengrai- j 151

T ned rock, the latter is a mediumgrained porphyritic rock. CB has a | -* high percentage of subhedral potash feldspar phenocrysts which are up ' -. to 3 cm long, and which are highly directed, more or less parallel to , .J the north-northeasterly trending long axis of the dyke-like granite I body. The average modal composition (volume percent) shows that CB is J a two-mica granite in which potash feldspar predominates in spite of I fair amounts of dark minerals:

Quartz K-spar. Plag. Biot. Chi. Muse. MAIN GRANITE 37 42 7 8 2 3 ji ._FINEGRAINED TYP.E 38 46 8 5 1 1 I ' sericite and in the finegrained variety also allanite. The full alteration J sequence of brown biotite to green biotite to chlorite can often be I observed, and some feldspars are heavily altered to sericite and saus- I surite along microscopic cracks. The average major element analyses ( + I standard error of the mean, 2s/ /n) for CB and KB illustrates that the ~~| former is less felsic:

CB KB CB KB

SiO2 71,69(0,21); 72,86(0,83) CaO 1,70(0,04); 0,68(0,08) 13,89(0,09); 13,62(0,09) K 4,93(0,07); 4,86(0,61) A12°3 2° 3,07(0,09); 0,87(0,11) Na2O 2,34(0,06); 4,35(0,54) ji MgO 0,93(0,06); 0,15(0,02) TiO2 0,39(0,01); 0,10(0,01) • An outcrop map of CB was prepared on a scale of 1:1000, and a gamma- | spectrometric survey was done on a 10 m grid wherever outcrops per- • " mitted. As control area the coastal outcrops of the Slippers Bay granite * "1 (SLB) in St. Helena Bay was selected. The average radioelement con- ••-' tent proved to be considerably lower in CB and SLB than in KB and • (hybrid granodiorite, fingrained biotite and Darling granite or Vreden- 1..•m burCCGg , granite)although. iInitiallt is stily la nhighe instrumenr than t inwit thh e a envelopinthallium-activateg countrdy Narocl k 1 crystal of 0,16 1 was employed, but the desired sensitivity could not be obtained owing to the high background-to-anomaly ratio in CB and SLB.

J.C. THERON AND A.E. SCHOCH I 152

The necessary improvement was obtained when the crystal volume was increased to 0,35 1. For CB the average values are 6,0 ppm U,O and 3 8 28 ppm ThO2» but the same distribution pattern of small positive and r;J negative anomalies is present as was found in KB and CCG. The highest values for CB is 23 ppm UjOg and 43 ppm ThO2. W Albitisation is a prominent feature of the episyenitisation in KB and ~\ CCG (Scheepers, 1982), but similar processes did not take place in CB W and SLB. The correspondence between the properties of KB and CCG may be due to their fortuitous proximity to the regional Colenso fault, _ J along which the volatile substances responsible for the metasomatic alterations could have been introduced. The difference between the two i groups of young granites is clearly displayed on a diagram specifically | •• designed to reveal such metasomatic trends (De la Roche, 1977) (See J Fig.).

~| REFERENCES

T A.E. Schoch, P. WINKLER, J. TOURET (1980): A CO2-granite from the I ™ Saldanha-Darling batholith, South Africa. Abstr. 26th internat. -j geol. congress, Paris, I, 87.

| R. SCHEEPERS (1982): Die uraan, thorium-, en molibdeen-verspreiding *J in geselekteerde leukograniete van die Kaapgraniet-suite. Unpubl. • M.Sc. thesis, Univ. Stellenbosch, 256 p.

I A.E. SCHOCH, R. SCHEEPERS (1982): Uranium and carbon dioxide in ~J the Klipberg granite, Darling, Cape Province. Abstr., Symposium • for researchers in Uranium Geology, Pelindaba. 3 p. 1 • J A.E. SCHOCH, J.C. THERON (1983): Uranium and thorium distribution • _ in young granites of the Cape Granite suite near Darling. Abstr., _J Symposium for researchers in Nuclear Geology, Pelindaba, 12-14.

J.C. THERON AND A.F. SCHOCH 153

where AjB = (Al-2Na + 3 Mg + Fe + 2 Ti)555/(555-Q)

A2-K = (A1-2K + Fe -Mg -4Ca)555/(555-Q) and Q / = Si/3 -(K + Na + 2 Ca/3) (all cation proportions XI000)

Ab = Albite; An = Anorthosite; Or = Orthoclase; Mu = Muscovite; S = Siderophyllite; A = Annite; •]• E = Eastonite; P = Phlogopite

Klipberg alkali granite and episyenites (Scheepers, 1982)

Cape Columbine granite suite (Scheepers, 1982)

1 Fine-grained Contreberg granite (Theron, 1983)

Medium-grained por^hyritic Contreberg granite (Theron, 1983).

J.C. THERON AKD A.E. SCHOCH J 154 ] MULTI-ELEMENT INTERPRETATION OF REGIONAL STREAM SEDIMENT J GEOCHEMICAL SURVEY RESULTS FROM THE BUSHMANLAND REGION

]. E.A.W. TORDIFFE AND A.E. SCHOCH (University of the Orange Free State)

A comprehensive program has been initiated in Bushmanland for the • interpretation of the regional stream sediment geochemical maps compiled by the Geological Survey of South Africa. This investigation is intended to link the trace element distribution patterns to unpublished detailed ] geological maps of the area. The first results have been obtained for the region of Geselskapbank and Areb in north-western Bushmanland ] (Strydom, 1984, 1982). These preliminary results are reported in this presentation. Because most rock-forming and ore minerals consist of a ] multi-element assemblage, it can be expected that a multi-element approach would be more viable than single element interpretations. Careful ] grouping of elements was therefore necessary and for this purpose correlation coefficients were determined (Table 1). ] Table 1 Correlation coefficients exceeding 0,5 of trace elements in ] streai/j sediment samples from the Geselskapbank area (map 2918 AA). Mn Nb Ti Th u Y Zr Zn Co ] Mn 1 Nb 1 ] Ti 0,.64 1 1 Th 0.,73 0.,-9 1 U 0.,74 0 .63 0..89 1

Y 0..68 0 .66 1

Zr 0. 63 0 .69 0. 79 0 .85 0.68 1

Zn 0.57 - .52 -.60 -.67 1

Co 0.59 0.59

1 E.A.U. TORDIFFE and A.E. SCHOCH I 155 1 Contour maps of combinations of correlated elements reveal that in certain cases good relationships exist between specific statistical ranges and the regional geology. The gap statistic (Miesch, 1981) was used to determine the specific statistical ranges to be considered.

.1 Because some of the high anomalous areas on the contour maps are represented by only one sample, it was decided to omit all sample points exceeding one standard deviation from the selected specific set of data during the compilation of the contour maps. This resulted in smoother contour maps which could easily be interpreted and the definite correlation between the geochemical data set and the various mapped geological units is demonstrated on a series of overlay maps.

REFERENCES ] MIESCH, A.T., 1981. Estimation of the geochemical threshold and [ its statistical significance. J. Geochem. Explor., 16: 49-76.

T STRYDOM, D., 1982: Geological map of the Geselskapbank-Areb area. Bushmanland Research Project, Dept. Geology, Universi- ^ ty of the Orange Free State, Bloemfontein.

1984: "In Struktureel-stratigrafiese studie van die J tnetasedimente en ander metamorfiete van noordwestelike Boesmanland". Ph.D.-thesis in preparation. University of the i OFS, Bloemfontein.

E.A.W. TORDIFFE and A.E. SCHOCH .1 156

J The Archaean gold-telluride-sulphide and gold-telluride mineralisaticn of a multiple stage hydrothermal vein deposit at the Commoner nine, Zimbabwe. S C Twemlow Dalny Dine Chekari Zimbabwe The Commoner Mine is situated on the western edge of the Midlands greenstone belt, 50 km west-southwest of Kadoma. Although a small mine by J international standards, milling an average of 1500 tons of ore per month, it has nevertheless produced over 4000 kg of gold in over B0 years of almost continuous operation. The mine workings extend over a strike length J of 750m and shaft sinking is extending the development from 26 level to 31(-310m) level at a distance of 1050m on incline. Current geological interest in this deposit was initiated by the presence of coarse grained telluride minerals in ore exposed on 21 level J in 1978. In contrast to the major gold-producing mines in Zimbabwe which occur predominantly in the Bimodal Suite of the Upper Bulawayan (2*9-2*7Ca) the Commoner orebody is located in an outlier of folded sediments and minor J' volcanics which overlie the largely unproductive andesitic volcanics of the 1 Haliyami formation. To the west of the mine this greenstone succession has been intruded by a large tonalite-granodiorita batholith, and three smaller stocks of similar composition. These intrusions were eraplaced at the and of the Bulawayan period of volcanicity and have been deted at 2*65Ca. The deposit is a hydrothermal quartz-calcite vain which strikes NNE J and dips at 20-30 towards the west. The wallrock* exposed in the mine uorkings are predominantly pale green water-lain tuffs, with rippla marks and load casts, which are overlain by carbonaceous siltstones and black J pyritic phyllites. Small scale structures and changes in the nature of chemical precipitates indicate that these sediments were probably deposited in a shallow water marine environment. Pale grey-brown porphyritic felsites occur as dykes which predate the mineralisation, and later doleritic dykes J cut through the vein. On the upper levels of the mine the vein was concordant with the strata with minor cymoidal transgressions creating parallel ~| ore channels. On the lower levels, however, the vein maintains a dip of -* 20-30° while the wallrocks dip steeply eastwards, demonstrating that the strata were folded prior to the development of the host fracture. ~] The orebody ranges in appearance from a thin clay seam to a comp- J" osite vein of quartz, calcite, and included wallrock which attains 5m in thickness. Two major ages of gangue mineralisation comprise mn early mass- -] ive quartz, and a later laminated quartz-calcite. These phases can be I further divided into a number of sub-phases on the basis of differing mineral composition, and internal croascutting relationships. The early mineralisation is composed of quartz which contains variable amounts of wall- "1 rock fragments and has the appearance of a breccia cemented by quartz. As most of the wallrock blocks are angular and appear to have undergone little abresion it is thought that this breccia results from the propagation and *~T infilling of fractures created by hydraulic fracturing. This mechanism also j explains the repetition of partings of the same style of mineralisation in fractured quartz in numerous positions throughout the deposit. All known —, economic mineralisation is associated with one of the sub-phases of the J early quartz and ore shoots have been demarcated where this quartz ia continuous along the strike and dip of the vein. Other smaller lenses of ore- bearing quartz occur between the ore shoots and these are generally aligned 1 with the pitch of the ore shoots.

1 S G Twemlow 157

Ui.thi.ri the ora shoots the gold occurs in intimate association with macroacopic sulphide or tellurids mineralisation. An early sulphide assemblage occurs within a white acicular quartz as aggregates of fine grained minerals up to 10mm in diameter. In these aggregates tetrahedrite is generally dominant and encloses chalcopyrite, sphalerite, galena, ersenopyrite, hessita (Ag.Te), and gold. The gold is usually associated uith hessite and occurs as rounded blaba anclosad by either hessite or tetrahedrite. In a feu of the aggregates sphalerite or chalcopyrite are the major phases present end these enclose the same assemblage of fine grained accessory j minerals. Uithin one particular host mineral tha proportions of the other minerals present are remarkably consistent, even though the aggregates are separated from eachother by distances equivalent to two to three times J their diameter. It is thought that nucleation and co-precipitation are responsible for the deposition of fine grained mineral assemblages as dis- crate aggregates uithin tha quartz. In the Bulauayo ore shoot this mineral- J isation is enclosed in massive white quartz which protected it from modification and replacement by later fluids. In tha Salisbury, Gokwe, and C18 shoots the acicular quartz which encloses tha sulphide mineralisation was fractured by later movement both in the plane of the orebody and along J bedding planes which lie sub-perpendicular to the vein. Fluids carrying gold and tellurium were Introduced into these fractures and telluride minerals ware deposited, with gold and hematite, as cavity and breccia fill- J ings. In areas where the sulphide assemblage was exposed to these fluids this resulted in partial to total replacement of the sulphides by telluride minerals. Altaite (PbTe), coloradoite (HgTe), and hematite are the major phases with minor tellurides of Au, Ag, Au-Ag, Cu, Ni, Au-Cu, and Ag-Cu. J The presence of hypogene hematite in the breccia filling indicates that conditions of high oxygen fugacity obtained during the deposition of this J mineralisation. Coarse grained gold-silver tellurides ( petzite AuAg.Te-t sylvanite AuAgTe., and hessite Ag-Te) fill fractures which transect tha telluride braccia filling of the C18 ore shoot. The occurrence of granular inter- J growths of hessita and aylvanite in these veinlets indicates a maximum temperature of deposition of 170°C for thie mineralisation. The restriction of the tel^uride mineralisation to areas of pre existing sulphide ore J implies that the tellurides were deposited as the final cavity filling during the same phase of mineralisation as that which deposited the sulph- ~] ides. J Later fluids deposited further sub-phases of the early quartz and modified the pre existing vein by fracturing and recamenting with younger quartz, and by extensive pyritisation and chloritisation of uallrock fragments. Up to eight separate sub-phases of early quartz have been noted in certain areas of thick vein development and due to the variations in vein character produced by shearing correlation of these sub-phases across the deposit is exceptionally difficult. Physical and chemical evidence of the incorporation of ore bearing mineralisation in later sub-economic quartz is provided by the presence of included fragments of mineralised quartz, and by geochamical anomalies in the younger gangue mineralisation. The later laminated quartz-celcite occurs as laminated sheared zones which follow either the contacts of the vein or partings within the main body of the vein. Duplication of this style of mineralisation along several shears is common and it often constitutes a substantial proportion of the total vein material in certain areas. A gradual increase in the ratio of calcite to quartz during the deposition of the laminated mineralisation is indicated by the proportions of these minerals in successive

S G Tuemlow 158

partings within the vein. Early sub-phases of the quartz-calcite consist of vuggy quartz in which calcite occurs as the cavity filling whereas in later eub-phaaes euhedral quartz crystals coat the walls of fractures filled by coarsely crystalline calcite. The spatial relationship between the known gold deposits in the area and the granitic intrusions implies that there is a genetic connection of the Mineralisation to the emplaceaent of the granitic magma. Comparison of the physical and aineralogical characteristics of the Commoner orebody with those of the Tertiary gold-telluride deposits of the Circum Pacific Belt and the Archaean deposits of Canada and Australia indicates that this mineralisation was probably deposited in a near-surface environment. As other gold-telluride deposits of various ages are associated with intermediate igneous activity this reinforces the link with the granodiorite- tonalite batholith. Examination of the granitic bodies indicates that the present erosion level is close to the roof zone of the batholith. Several J greenstone roof pendants are located on the eastern edge of the intrusion and outcrops of granitic rocks, north and south of the Commoner Mine appear to be apophyses of an underlying eastern extension of the batholith. The fracturing and hydrothermal alteration of a wide zone within the margins of the batholith indicates that there was extensive fluid movement through this fracture envelope. Circulation, or expulsion of fluids from the roof zone of the batholith, and its thermal aureole, probably provided both the heat source of the hydrothermal system, and much of the chemical content of the mineralisation. It is possible that adjustments, in the overlying greenstones and sedimentary strata, during the intrusion of the batholith were responsible for the continual reopening cf the Commoner fracture to later fluids which deposited younger sub-phases of the early Mineralis- ation, The later quartz-calcite is dominantly shear controlled and possibly reflects a weakening of the hydrothermal system and a change in the nature of the mineralising fluid. This may have occurred during the tran- sition from active emplacement of the granitic magma, with the expulsion of large quantities of fluid, to the cooling and contraction of the batholith, with the generation of recirculating hydrothsrmal convection cells. Two tonalite stocks, outcropping in the greenstones along the I eastern margin of the batholith, display concentric zonation and vertical -^' mineral lineations implying rapid upward migration of the magma; features indicative of sub-volcanic intrusions. The association of intermediate ~~ intrusive and extrusive igneous activity with gold mineralisation in the _, • area around the Commoner Mine shows that this region bears a striking resemblance to the present island arc environment. This indicates that — there is a probability of considerable potentially economic gold mineral- j isation in a zone trending northwest along the margin of the tonalite batholith. _ The gold-telluride ores of the Commoner nine display features characteristic of both plutonic-hydrothernal and volcanic-hydrothermal - styles of telluride mineralisation. This hybrid nature is probably the result of the deposition of mineralisation derived from a plutonic source ~" in a near-surface sub-volcanic environment.

5 G Twealou 159

THE SIGNIFICANCE OF THE GLADKOP SUITE FOR THE "BASEMENT PROBLEM" IN THE NAMAQUA MOBILE BEL1

G. VAN ASWEGEN (University of the Orange Free State)

The positive identification of the basement for the metasedimentary and metavolcanic rocks of the western Namaqua Mobile Belt, has eluded geologists for many years. As a result of the National Geodynamics Project of the late seventies, major lithological units were stratigraphically described and Classified for the first time. One of these, the Gladkop Suite, could B^ considered likely "basement" in terms of its general appearance. It is the aim of this paper , however, to show that the Gladkop Suite is in fact not the basement for those metasedimentary units with which it i6 geographically associated and to discuss the significance of this to the "basement problem."

Historical background

Brief reference to some previous ideas about the stratigraphy in the western Namaqua Mobile Belt is pertinent to the theme of this paper.

Initially Joubert (1971, p. 16) considered the Nababeep type augen gneisses as basement to the metasedimentary pile (a view vigorously opposed by geologists of the O'okiep Copper Company) but the ample evidence of primary intrusive relations lead to the classification of those gneisses as the older part of the Klein Namaqualand Suite of syntectonic granitoids (Marais and Joubert, 1980).

Geologists of the O.C.C. tentatively suggested that the "grey granu- lites", collectively renamed "Brandewynsbank formation", are older than the metasediments (O'okiep Copper Company, 1975, p. 23). Bertrand identified banded grey gneiss with pre-Vioolsdrift Suite tectonic fabric in the Henkries area, and since he did not recognise similar structures in the Haib Subgroup metavolcanites, he concluded that the grey gneiss is the pre-Vioolsdrif basement. Moore (1977) interpreted banded grey gneisses on the farms Achab and Namies Suid as the basement for the Bushmanland metasedimentary and metavolcanic succession (Aggeneys Subgroup), but subsequent work has shown that major structural discontinuities between the gneisses and the metasedimentary units obscure the primary contact relations and structural patterns to such an extent that the above interpretation seems questionable (W.P. Collis- ton, pers. comm.) A combined B.P.I. (Univ. of the Witwatersrand) and University of Cape Town research project, sponsored by the C.S.P., is at present in progress to determine, amongst other things, the isotopic age relations between the Achab grey gneisses and the Aggeneys Sub- group .

The Gladkop Suite

This unit is fully described elsewhere (Van Aswegen, 1983) and only those aspects relevant to the basement problem is discussed here.

Three major gneiss units are distinguished namely the Steinkopf Gneiss (granodioritic to granitic grey gneiss with prominent leucosome veins in

G. VAN ASWEGEN 160 J places), Brandewynsbank Gneiss (granitic grey gneiss) and Noenoemaas- berg Gneiss (leuco-granitic gneiss, similar to some of the "pink J gneisses"). These gneisses are intimately associated on regional and outcrop scale and together form a "banded gneiss" complex comparable to the basement complexes of other mobile belts. The type locality for J the Suite is in the Steinkopf area. In general, by distinguishing primary from secondary features, it is shown that the precursors to the gneisses were homogeneous in terms J of composition, texture and structure and that the banding, especially well developed in the Steinkopf Gneiss, is of secondary origin. Intru- sive relations are established on grounds of the recognition of xenoliths J (on tnesoscopic and macroscopic scale) and observations of contamination and assimilation at the contacts. Such contact relations show that the J Steinkopf Gneiss is the oldest unit of the three. J J

N J J J Km. Scale Legeni Granite-, « [Klein Namaqualand • granite gneisses] „ , , and . 6 J Spektakel Suites Strike and Leucogneiss | direction JGladkop Suite of dip of Grey gneiss foliation/ 3 banding Metasediments locality described in text

Fig. 1 Generalized geological map of the eastern nose area of the i Ratelpoort synform. (adapted from Van Aswegen, 1981).

G. VAN ASWEGEV 161

One of the most important exposures revealing the age relations between the Gladkop Suite and metasediments is found in the eastern nose area of the Ratelpoort synform. Fig. 1 is a generalized geological map of the area showing map-scale xenolithic rafts of the Khurisberg Subgroup (Marais et al., 1980, p.275) within granitoid gneisses of the Gladkop, J. Klein Namaqualand and Spektakel Suites. At the locality marked with an arrow the metasediments consist of gar. et.-sillimanite-cordierite- quartz-feldspar rock, quartz-sillimanite rock.. s -^inated metaquartzite and banded quarts!-garnet and quartz-limonite rocks. Although massive white metaquartzites are absent at the outcrop, the lithologic types listed above are typical of the metapelitic association of the Khurisberg Subgroup in the immediate vicinity. The grey gneiss at the outcrop re- J sembles the Steinkopf Gneiss elsewhere in the Copper District in every respect including some spectacular ptygmatically folded leucosome veins. It was mapped as "grey granulite" by geologists of the O'okiep Copper Company.

The grey gneiss normally does not contain garnet, but within one metre of the contact with the garnetiferous metapelite garnet is present in the gneiss together with at least one discrete xenoiith and numerous schlieric remnants of the metapelite.

Similar contamination effects in the otherwise homogeneous Steinkopf Gneiss, where in contact with the Besondermeid Formation of the Een- riet Subgroup, demonstrate the time-equivalence of the Gladkop Suite with ths Vioolsdrif Suite. The latter is also clearly intrusive into the Eenriet Subgroup. A Vioolsdrif type isotopic age for the Steinkopf J Gneiss found by Barton (1983) confirms the time-equivalence. Discussion

If the Khurisberg Subgroup could be positively correlated with other successions currently grouped together as the Bushmanland Group, then a minimum age for these tnetasedimentary units appears to be fixed and time correlations with the 2000 Ma Haib Subgroup is corraborated. The basement would still be undiscovered. If, ou the other hand the Aggeneys Sequence is found to have been deposited at a later time on a Vioolsdrif-age basement, the stratigraphy of the metasedimentary rocks of the western Namaqua Mobile Belt is even more complex than currently realized. The Khurisberg Subgroup would then be part of the basement 1 onto which the Aggeneys Subgroup was deposited.

G. VAN ASWEGEN 162 I REFERENCES j Barton, E.S. Reconnaisance isotopic investigations in the Namaqua Mobile Belt and implications for Proterozoic crustal evolution - Namaqualand Geotraverse. In: Botha, B.J.V. (Ed.), Namaqualand J Metamorphic Complex. GeoI7~ Soc. S. Afr. Spec. Publ., _1?_, in press. J Bertrand, J.M. (1975). Granitoids and deformation sequence in the Goodhouse-Henkries area. A new interpretation of the relationship between rocks in the Vioolsdrif-Goodhouse area and the Namaqua- 3 land Bushmanland gneisses. In: Thirteenth anual report, Precam- brian Res. Unit., Univ. Cape Town, 145 pp.

J Joubert, P. (1971), The regional tectonism of the gneisses of part of Namaqualand. Bull. Precambrian Res. Unit., Univ. Cape Town, Vd, J 220 pp. Marais, J.A.H., Van Aswegen, G. and Van der Merwe, S.W. (1980). Okiep Group, 275-281. In: SACS. Stratigraphy of South Africa, Part 1 (Comp. L.E. Kent). Handb. geol. Surv. S. 3 Afr. 8, 690 pp.

I Marais, J.A.H. and Joubert, P. (1980). The Klein Naraaqualand Suite, J 294-299. In: SACS. Stratigraphy of South Africa, Part 1 (Comp. L.E. Kent). Handb. geol. Surv. S. Afr., ji, 690 pp.

J Moore, J.M. (1977). The geology of the Namiesberg, Northern Cape. Bull. Precambrian Res. Unit., Univ. Cape Town, ^0, 73 pp.

J O'okiep Copper Company Limited (1975). In_: Precongress excursion guide book No. 1, 16th Geocongress, geol. Soc. S. Afr., Stellen- _. bosch, 73 pp. t Van Aswegen, G. (1981). The intrusive nature of the Steinkopf Gneiss. In: Namaqualand Excursion, S.Afr. Geodynamics Project (Comp. "| H.J. Blignault) Geol. Soc. S. Afr. 50 pp. (1983). The Gladkop Suite - the grey and pink gneisses of Steinkopf. In: Botha, B.J.V. (Ed), Namaqualand Metamorphic Complex. Geol. Soc. S. Afr. Spec. Publ., W, in press.

G. VAN ASWEGEK 163 ]

CHARACTERIZATION OF METAMORPHIC FLUID COMPOSITION ASSOCIATED ] WITH RETROGRADE METAMORPHISM IN THE LIMPOPO BELT IN SOUTH AFRICA ] by D.D. van Reenen Department of Geology, Rand Afrikaans University, P.O. Box 524, Johannesburg 2000

The presence of a well-established retrograde Opx isograd in the Southern Marginal Zone of the Limpopo belt in South Africa provided an excellent opportunity for a detailed study of the ] composition and evolution of the fluid phase towards the end of the thermotectonic history of this belt. This investigation was largely based on a detailed study of fluid inclusions in three carefully selected samples of meta-pelite accross this isograd. These samples satisfied the following two conditions:

(i) they are representative of a large number of samples which 1 have already been studied in detail from both a mineralogical and phase chemical point of view (Van Reenen, ] (1983) , and (ii) they have similar bulk chemical compositions but are characterized by three stages in the hydration of cordierite ] and hypersthene, namely : DR 54: granulite from the orthopyroxene zone north of the isograd. This sample shows virtually no sign of ] hydration and consists of the following mineral assemblage: garn. + opx. + cord. + biot. + Qz + ] pl'ag. DR 19: similar to the previous sample but with obvious ]. signs of the incipient hydration of cordierite. DV 43: this sample is from the ortho-amphibole zone south of the isograd and shows complete hydration and recrystallization to the following amphibolite ]. facies mineral assemblage: garn. + anth. + gedr. + kyan. + biot. + Qz + plag. ] The following two reactions controlled the hydration of cordierite and hypersthene accross this isograd: ] hypersthene + water = anthophyllite cordierite + water = gedrite + kyanite + quartz The first reaction is a true isograd (discontinuous) reaction ] and proceeds at a temperature of ± 780CC and a total pressure of ± 7 kbar if the water pressure equals the total pressure (Greenwood, 1963 and 1967) . These conditions place the hydration of cordierite and hypersthene in the present investigation within the stability field of sillimanite - an observation which does not agree with the fact that the hydration of

D.D. VAN REENEN cordierite actually proceeded within the stability field of kyanite and not of sillimanite. The only way in which the results of this investigation can be reconciled with the experimental data on the stability of hypersthene is to suggest that the hydrating fluids must have been extremely poor in H2O, being mostly composed of CO2. The influence of CO2 on the stability of hypersthene is as follows (Greenwood 1967, Fig.4 and 8) :

: pH20 = 0.05 ptotal : dehydration of anthophyllite occurs at t 660°C if ptotal = 1 7 kbar.

: pH20 = 0.025 ptotal : dehydration of anthophyllite occurs at t 625°C if ptotal = ± 7 kbar. Both these conditions will shift the dehydration curve from the stability field of sillimanite at high temperature to that of kyanite at much lower temperature. This suggestion fits the results of the fluid inclusion study which can be summa- rized as follows :

(i) All three samples are characterized by CO2- and H2O-rich fluid inclusions with exactly the same composition and range of densities.

(ii) the early high density C02-rich inclusions (with f= 1.0 gm/cm3) were composed almost entirely of CO2 with no recognizable H2O-rich phase. (iii)the high-salinity and high density H2O-rich inclusions were trapped at a much later stage in the history of these rocks. Isochores for these early CO2~rich inclusions project very near the suggested P-T conditions for the hydration of hypersthene under conditions where the fluid phase contained very little water. It is also interesting to note that the calculated P-T conditions along the orthopyroxene isograd based on the garnet-biotite geothermometer and the plag-garn-kyan-Qz geobarometer gave results (of T = 600°C-660cC and P = 6-7 kbar) which fit the data for the hydration of hypersthene under water-poor conditions almost exactly.

It is therefore suggested that the hydration of cordierite and hypersthene along the opx isograd of this study was controlled by a combination of decreasing temperatures accompanied by extremely CO2~rich hydrating fluids. The suggestion is furthermore made that this CO2~rich fluid, which evolved between 1 2.45 and 2.6 Ma ago, probably played an important role in the occurrence of gold (and antimony) mineralization in the greenstone belts on the adjacent Kaapvaal Craton. These belts show evidence of widespread carbonation and the available information is an indication that the associated mineralization occurred between i 2.4 and 2.5 Ma ago.

D.D. VAN REENEN 165

Greenwood, H.J. (1963). The synthesis and stability of anthophyllite. J.Petrology, 4, 317-351. Greenwood, H.J. (1967). Mineral equilibria in the system MgO-SiO2-H2O-CO2. In P.H. Abelson, Ed. Researches in Geochemistry, II. John Wiley and Sons, New York, 542-549. Van Reenen, D.D. (1983). Phase relations in cordierite-garnet- hypersthene-biotite bearing assemblages as a function of changing pressure and temperature in the Southern Marginal Zone of the Limpopo metamorphic complex, South Africa. Limpopo Volume, Geol. Soc. S. Afr. Spec. Publ. 8, 143-167.

D.D. VAN REENEN 166 3 GEOSTATISTICS APPLIED IN GEOHYDROLOGY

3 1. INTRODUCTION

3 Host of the interpolation methods used in geohydrology fail to quantify the accuracy of the estimates. The theory of regionalized variables 3 overcome this by pointing out the relationship between the spatial correlation of the variables and the precision of interpolation.

3 This paper is concerned with the useful contribution that kriging can make towards the interpolation of the following geohydrologic 3 parameters:

3 (i) Transmissivities (ii) Water-levels 3 (iii) Bouguer-anomalies 3 2. TRANSHISSIVITY 3 The semi-variogram of the transmissivities of the Cape Flats aquifer shows a hole-effect which can be fitted best with least square splines. T Application of these least square splines in the point kriging of transmissivities, yields more consistent test statistics then the -» commonly used semi-variogram models.

3. WATER-LEVELS

Because water-levels usually show some kind of first or second order drift over a region, universal kriging must be applied. For this study the following universal kriging models were applied to the [ Toppenish-Creek data:

G.3. van Tonder I

1 167 (i) Iterative calculation of the residual semi-variogram together with 1 (a) low-order polynomials (b) trigonometric base functions

(ii) Fletcher's penalty funciton applied to an intrinsic function of 1 order 1 (iii) Division of the universal kriging matrix in an upper and lower part, using only the lower matrix for interpolation. 1 3. BOUGUER-ANOHALIES

To see what usefull contribution kriging can make towards the reduction of Ve number of observed gravity-measurements in the dolomitic areas of South Africa, it is applied to the Bouguer-anomalies obtained from a 3 study on a part of the Zuurbekomcompartment. A reduction of ninety percent of the number of measurements gave rise to only a 1,1 percent loss in accuracy of the calculated volume of the weathered dolomite.

1 1 1 1 1

G.3. van Tonder 168

GEOSTATISTIEK IN GRONDWATERSTUD1ES

1 1. INLEIDING Geohidrologiese data is hoofsaakJik beperk tot die inligting wat uit boorgate verkry word. Omrede boorgate wyd verspreid in 'n studie- gebied voorkom, is dit dus noodsaaklik dat daar van een of ander interpolasiemetode gebruik gemaak word om inligting by punte te kry waar geen boorgate bestaan nie. Weens die gebrek van die konvensionele interpolasiemetodes aan doeltreffende fovtskatting by nie-eksperimentele punte, is dit raadsaam om liewer kriging vir J die skatting van parameters te gebruik. In hierdie seminaar sal aangetoon word hoe kriging toegepas is vir die skatting van die volgende geohidrologiese parameters:

(i) Transmissiwiteite (Ü) Watervlakke r (iii) Geofisiese groottes (spesifiek gravimetriese data)

2. KRIGING VAN TRANSMISSIWITEITE

As gevallestudie sal die Kaapse Vlakte-akwifeer naby Valsbaai beskou word. Die eksperimentele semi-variogram ( kyk figuur 1) toon 'n holte-effek. Gewone punt kriging is met verskillende semi-variogram passingsmod^Ue uitgevoer waarvoigens kleinste kwad/aat latfunksies ] die beste toetsstatistieke gelewer het. Hierdie model is opgeweeç teenoor tendensanalise en verskillende afstandsgeweegde metodes en het in al die gevalle die kleinste gemiddelde fout statistiek gelewer. Die transmissiwiteitskontoere er. die ooreenstemmende kriginf i standaardafwykingskontoere bereken volgens die kleinste kwadraat latfunksie krigingmodel word in figure 2 en 3 gttoon.

3. KRIGING VAN WATERVLAKKE

? Omrede watervlakke tot 'n baie groot mate van die topografie afhang word verwag dat die waargenome watervlakke altyd in 'n mindere of meerdere mate 'n drywing sal toon, wat impliseer dat universele kriging toegepas moet word. Watervlakke van die Toppenish Creek- j akwifeer wat 'n definitiewe drywing toon ( kyk figuur 4 vir die semi- variogram ) is vir hierdie gevallestudie gebruik. Die volgende verskillende universele krigingmodelle is op hierdie stel van watervlakke toegepas: 169

] (i) Iteratiewe bepaling van die residu semi-variogram tesame met (a) lineêre polinoom basisfunksies (b) trigonometriese basisfunksies

(ii) Fletcher se boete-funksie-metode toegepas op 'n intrinsieke funksie van orde 1

(iii) Opbreek van die universele krigingmatriks wat aanleiding tot *n onsydige interpolasiemetode gee.

] 3. KRIGING VAN GRAVIMETRIESE DATA Weens die hoë koste verbonde aan 'n gravimetriese opname is besluit om ondersoek in te stel na die moontlikheid om kriging te gebruik om die aantal waamemingspunte in so 'n opname te verminder sonder 'n noemenswaardige verlies van akkuraatheid in die berekening van die volume verweerde materiaal in dolomitiese gebiede. Vir hierdie doel is 'n deel van die Zuurbekomkomparte- ment beskou waar 1600 gravimetriese punt waard es uitgevoer is. Hierdie 1600 punte is gebruik om die diepte van verwering en dus ook die volume verweerde materiaal te bereken. Hierna is 145 punte uit die oorspronklike 1600 punte op 'n gelykverspreide rooster geselekteer en kriging is gebruik om die diepte van verwering ( kyk ïïguur 5 ) by 1600 punte te voorspel. Hierdie kontoere toon groot ooreenstemming met dié bereken vanaf die 1600 punte (kyk figuur 6) 'n ^ersentasiefout van 1,1 persent vir die volume verweerde dolomiet bereken met die 145 punte relatief tot dié soos bereken met die 1600 punte, toon dat geen noemenswaardige verlies in akkuraatheid geskied het nie.

1 1 1 170

• eksperimcntelc punte

— Utfu.iUie beiuderini

6 7 8 10

h (mi 1O3) ftguur 1 Kletnsle kwadmat latfitnksle passintsmodel van die eksperimentele semt-varioeram van die 1 trammlalwttelle In die Kmpte Vlakte. ] 1 1 1 1 1 1 fituur 2 TnnsmtssMultzkontotri van die Kaapse Vlakte soos nut kleivste kwadmat latfunksies nrkry.

1 171 J 4 .3

Figuur 3 Standaardafwykingikontoere ran die feskatte imnsmissiwltelle In die Kaapse Vlakle. Waardes van 100 en poter kan as duldende op onvoldoende tnligting bakou word. 3

• » 5 i: 14 it it k (U)

Fipiur 4 Eksperimentele semhiariotram ten die watervlakke in die Toppenbh Cnek-akwlfeer.

-1 172 J J J J J ]

Fifuur 5 Ditptt mn terv/trinf rm die dolomlet tn die Zunrbekomkompartement tootmet145datapunle tn krigint ftrby,

1 1

1 6 Dlepte tan rerwerint m die dolomlet In die Zuurbekomkompartemenl toot met die 1 600 datapunte rerkry. I 173 ' J THE PROBLEM OF FACIES ANALYSIS IN THE PERMO-CARBONIFEROUS I * DWYKA FORMATION

1 J.N.J. VISSER I Dept. of Geology, University of the O.F.S., Bloemfontein

[ Both unit scale as well as formational scale facies changes are present "1 in the Dwyka Formation in the main Karoo, Warmbad and Kalahari ( "• Basins. Unit scale lithofacies changes (Fig. 1) involve vertical and -. lateral textural grading (diamictite —*• shale), colour grading (reddish p J —»• greenish diamictite), and structural characteristics (massive —•• | bedded diamictite) which are imposed on the rocks by the depositional J environment (Fig. 2) as well as the sediment source. Whereas the latter factor is not generally recognised in non-glacial depositional environ- 1 ments, the often limited sediment transport distance during glaciation -J I resulted in depositional facies, consisting entirely of locally derived ~1 material, whic1- grade vertically into facies comprised of distantly f derived debris. Furthermore the recognition of lithofacies in the Dwyka ' -l is often hampered by the indiscriminate use of the genetic term "tillite" i -* for a wide variety of glacial as well as resedimented rocks, giving false ] impressions of a homogeneous sequence. Changes on formation al scale can best be described in terms of depo- 1 facies whereby the totality of local geographical conditions determines the lithologic nature of the sequence. In total a valley/inner platform and an outer platform facies were recognised in the Dwyka Formation ] (Fig. 3). The valley/inner platf-vrr iacies is characterised t/ a varied lithology in which tillite is often subordinate and bedded diamictites and ] rhythmites abound. The outer platform facies consists overwhelmingly of glacially deposited beds with only a few shaly units. Lateral relationships between the two facies are difficult to establish as they are most probably diachronous. It is suggested that over the largest part of the basin deposition of the outer platform facies was contemporaneous with extensive erosion of the plateau and inner platform areas. Deposition in the latter areas probably represents a late-stage glacial event.

! " J.N.J. VISSER "1 J 174 WEST NORTH SOUTH

1 ] 1

>f"ATi

A A A_A_< A/\ A~A A

AftftflAfl)

o--o — Massive diamictite -o O' Laminated diamictite — © — Faintly bedded diamictite j-«-_~_-| Shale/mudstone with ice- tv--^-1 rafted debris MAM Bedded diamictite Rhythmite shale AAAAA O O O o o o o o o Conglomerate Sandstone Mudstone

FIG. 1 STRATIGRAPHIC SEQUENCE OF THE BASAL PART OF THE DWYKA FORMATION IN THE HOTAZEL VALLEY, NEAR SISHEN. THREE BOREHOLES SPACED OVER H KM,

J.N.J. VISSER pi r- I twil .

Grounded Embayment iceberg zone

Sea-level

Lenses of massive diamicton

CO t/i Rhythmi te mud Laminated Massive diamicton Mud with ice- diairicton rafted debris Debris flow

Debris flow fan (diamicton Ice marginal fan(faintly bedded & gravel & sand & mud) diamicton & reworked diamicton)

FIG. 2 GLACIER RETREAT MODEL DURING DEPOSITION OF THE BASAL PART OF THE DWYKA FORMATION IN THE HOTAZEL VALLEY 176

Massive diamictite (subglacial tills)

75%/ / \25% Outer platform facies association

50%

Inner platform and valley facies association 25%

Grasdrif 1 (Warmbad Basin)

75% 50% 25%

Heterolithic bedded laminated diamictite J diamictite (sediment & rhythmite shale & gravity flow deposits) mudstone & sandstone (debris rain, suspension settling, trac- tional deposits)

FIG. 3 TRIANGULMR PLOT OF DEPOFACIES FOR THE DWYKA FORMATION IN THE KAROO, KALAHARI AND WARMBAD BASINS

J.N.J. VISSER 177

QUANTITATIVE X-RAY DIFFRACTION: FACT OR FANTASY?

<«. All quantitative X-ray diffraction analysis methods are

i based on the premise made by Klug and Alexander in 1954 that the

'" intensity of an X-ray diffraction pattern is directly proportional

I to the concentration of the component producing it, provided that

due allowance is made for absorption effects.

«J In this paper the authors provide an overview of other

-, factors which also materially affect the relationship between

X-ray intensity and the concentration of the component responsible

I ' for the X-ray diffraction pattern.

In recent years, much of the tedium of data acquisition and

— subsequent data processing has been minimized by the advent of

1 computer-control.'ed X-ray diffraction systems, but practical

aspects of sample preparation and presentation have enjoyed less

I attention, although these factors materially affect the quality of

the final analytical results.

J Used in conjunction with petrological studies and

—I conventional X-ray fluorescence. X-ray diffraction becomes a I — ' powerful tool for the rapid non-destructive determination of the

j gross compostions of rocks and the identification and

quantification of alteration Kir.?

J areas, and is thu? extremely useful in exploration. It is at this

—i stage still, however, at best a semi-quantitative analytical

"* technique.

VON RAHDEN, K.V.I?. & M.J.E. 178 J AN APPLICATION OF GEOSTATISTICS J IN THE EASTERN TRANSVAAL COAL FIELDS A major problem in the coal industry is the large number of J variable involved. For instance, in the Eastern Transvaal: l. There are 5 seams. 2. A seam may be divided into bands (that is sub-seams) and J with each band are associated physical variables (eleva- tions, widths, etc.,) as well as qualities of raw coal "I (calorific value, ash, volatiles, swell, moisture, Roga J index, phosphorus, sulphur, etc.,) 3) Furthermore the coal is processed - at Rand Mines it is •^ usual to wash at 8 different relative densities, with the J yield and qualities determined for each R.D.

Rand Mines have developed an extensive data base Management system to handle this data. A typical master file can have J over 500 labels, which in Geostatistical terms means that 500 2-values are associated with each X - Y co-ordinate. 3 To maintain a Kriging program under such circumstances is • formidable but not impossible. J The points to be discussed are the Kriging procedure that f has been adopted - the physical variables, raw coal qualities I _ and washed coal qualities. r -* 1. KRIGING PROCEDURE ~\ David's Krigiftg subroutine (ref. 1) which is reliable, has I J been chosen. To use his SIM16 matrix inversion (ref. 2) it has been necessary to deal with co-variances rather than the "l variogram. SIM16 has proved to be fast. I The variogram models catered for are the Spherical and _ Gaussian, with geometric and zonal Anisotropy.. I —^ An option has been incorporated for Kriging directly into arbitrary mine panels as well as into grid blocks - this has "*1 been necessary in order to cater for daily up-dates. | 2. THE PHYSICAL VARIABLES 1 -I _J It has been found essential to thoroughly screen ths data, first of all to correct errors, but more important to gain 1 _ insight into the geological structure. This screening has ! been considerafy facilitated by the use of computer inter- -* active colour graphics. Techniques used include the Histo- ! gram, a colour posting of the data, the variogram and general i":* computer checks. L: I: I.D.WOOD 179 J

Features looked for include outliers, co-ordination errors -1 (double points), abnormal drilling, geological discontinuities (faults, etc.,), mixed distributions and distortions in the distribution (no transformations have been made at J this stage). By dividing properties into geologically stable areas, good J variograms have been obtained, with zero nugget effect and a large range. J These variograms have been used for: 1. Optimising drilling patterns - the theoretical ideas are by Royle (ref. 3), but in practice, the weights have been J found using the Kriging subroutine with arbitrary Z-values 2. Kriging into mining panels - here a full trace giving name, co-ordinates, values and weights of each borehole J used in the Kriging has proved very useful in sorting out problems which have arisen. 3 The results have been reconciled with actual production. 3. THE RAW COAL QUALITIES Certain qualities (such as swell, Roga etc)show no regionali- sation and are not suitable for geostatistical analysis,.

Others (such as calorific value, ash, volatiles, etc.,) are regionalised.• It was again essential to thoroughly screen the data before finding the variograms. These variograms are well defined but show a nugget effect, which has been confirmed by experimental sampling. It is well known that there is correlation between these qualities. It is interesting that the variograros are very similar (fig.1) .

1.0

0.8

0.6 ' FIG. I NORMALISED 0.4 VARIOGRAM

0.2

500 1000 1500 2000 LAG

I.D. WOOD 180 j

J To simplify the procedure, a common normalised variogram has been assumed for all relevant raw qualities and has been used for evaluatiing drilling programs and for Kriging. J The estimation variance has been corrected individually for each quality by multiplying by it's variance. J 4. THE WASHED COAL QUALITIES The coal samples at Rand Mines are generally washed at 8 J different relative densities (that is 1.35 to 1.8 ). The variograms are similar and the approach , using a common normalised variogram, could be applied. However, as it is J the quality of panels and not boreholes which is of interest, mathematical functions have been fitted to the washing J curves (fig.2). 100 J ASH cv 40 32 J 80 60 / 30 28 J FIG.2 / 40 20 24 ] J 10 20

1.3 1,4 L5 16 17 T8 RD > J The yield curve will be used as an example to illustrate the procedure - polynomials are not appropriate and an arc-tan transformation has been made and a function, which must be linear in it's parameters, fitted. The type of function depends on the type of coal quality. The parameters of the mathematical curves were analysed and showed up distinct quality areas. These areas have been isolated and the variograms for the parameters in each area found to be similar. Using the variograms, the parameters were Kriged into panels giving an over-all washing curve for each panel (fig. 3). i

I.D. WOOD 181

FIG. 3

1.30 140 1.50 560 1.70 1.80 R.D. ] SUMMARY AND CONCLUSIONS

1. Careful vetting of all borehole data is essential - it J eliminates errors, shows the geological structure and highlights distinct quality areas. ] 2. A normalised common variogram has been used for raw qualities - it considerably simplifies maintenance of the Kriging program.

] 3. Mathematical functions have been fitted to the washing curves - this gives an over-all average washing curve for each panel , which is interactively available to the ] engineers for their mine planning. ] REFERENCES 1. David M.- Geostatistical ore reserve estimation Elsevier Scientific Publishing Co.-page 261. 2. Davis K., David M., Belisle J. - journal of the International Society of Mathematical Geology - Vol. 10 Ho.it (1978) - PG 369.

3. Royle A.G. ,Newton M.J.,Sarin V.K. - Geostatistical Factors in the Design of Mine Sampling Programs - Inst. Mining & Metal. , Vol 81, Sec A 1972.

ACKNOWLEDGEMENTS Acknowledgement is made to Rand Mines for the use of their data and for permission to publish this paper. Any errors are the responsibility of the author.

I.D. WOOD y

J ADDENDUM

1 1 1

ROLE OF THE SOUTH AFRICAN GOVERNMENT IN THE MINING INDUSTRY j BY W.L. VAN WYK

ABSTRACT The mining industry throughout the chain from exploration through development, processing and marketing to trade in derived products is a high risk business with many different and unique characteristics compared with other industries.

Although South Africa is committed to a system of private enterprise Government plays an important role in the mining industry because it supplies some vital ingredients necessary to stimulate and encourage investments and successful development.

] This paper describes and philosophizes on the role of government in supplying basic geologic information; mining laws; surety of title for prospecting and mining rights? tax structure; infrastructure of rail and harbour and leading facilities; providing sophisticated research and technical services in the fields of geology, metallurgy and international mineral commodity intellegence. J J A TRIASSIC ARID-ZONE ANASTOMOSED FLUVIAL

J DEPOSIT FROM THE KAROO BASIN

- INTRODUCTION

1 The early to middle Triassic Burgersdorp Formation at the top of the J Beaufort Group in the north eastern Cape consists of greyish-red and subordinate greenish-grey mudstones interbedded with light grey fine-grained sandstones. The sedimentary units are generally arranged into fining-upward cycles varying in thickness from a few metres to a few tens of metres.

! The mudstones dominate the succession, making up 70-80% of the forma- w tion. In most places they are massive although many fine examples of mudcracked surfaces with associated sand-wedge polygons occur. The "* . sandstones mostly show horizontal lamination, but trough cross-bsd- . ding, ripple cross-lamination and planar cross-bedding are locally abundant. Calcareous duricrust layers make up a minor part of the - t sequence (Stavrakis, 1979; 1980; Hiller and Stavrakis, in press).

*" In the past the sequence has been interpreted as being the product of _ a high sinuosity meandering river system (Johnson, 1976; Stavrakis, 1979; 1980; Hiller and Stavrakis, in press). However, in the last •"• mentioned work allusion was made to the possibility that the Burgers- drop Formation represents sedimentation in an anastomosing fluvial system in an area of reduced slopes at the northern (distal) margin of _ the Katberg Sandstone wedge.

ANASTOMOSED RIVER FACIES MODELS

In recent years, study of modern anastomosed fluvial systems has indicated that the morphology, processes, and deposits of these rivers differ markedly from the better-known braided and meandering systems and that a new facies model is warranted (Smith, 1983).

Papers by Smith & Smith (1980), Smith & Putnam (1980) and Smith (1983) describe the characteristics of modern and ancient anastomosed rivers and their associated deposits in wet climates from Canada. They describe anastomosed rivers as rapidly aggrading low energy channel and wetland complexes in which they recognise six subenvironments with associated deposits : multiple, interconnected, low-gradient, low 6inuosity sand or gravel bed channels, sand or fine gravel crevasse splay deposits, levee deposits of sand silt, lacustrine muds, marsh deposits of mud and organic material and peat bogs. Each of the above papers emphasise the importance of a wet climate in sustaining the abundant vegetation which stabilises the channel banks, however work by Rust (1981) and Rust & Legun (1983) in arid central Australia suggest that a wet climate need not be of fundamental importance. In the Lake Eyre Basin ephemeral stream flow is building a low channel- density anastomosed system on top of a relict braided sand sheet.

N. STAVRAKIS AND N. HILLER Study of the Australian deposits led Fust (19P)) to propose a preliminary facies model for arid zone anastomosing-fluvial systems. ThiB comprises a mud dominated succession vith minor isolated channel sands, deep dessication cracks, minor carbonaceous horizons, duri- crusts and evaporite horizons.

To achieve the rapid aggradation associated with anastomosed river systems, the sedimentary basin must subside continuously or the local base level controlling the basin must rise continuously (Smith 1983), so that the river is constantly trying to attain an equilibrium graded profile by rapid sedimentation (continuously building up its channel). J Thus anastomosed rivers are characterised by very low gradients and little excess energy producing channel sedimentation and "lacustrine" deposition in the interchanne^. areas. Smith & Putnam (1980) point out that the optimum structural sett ings for anastomosis would be J tectonically active intennontane basins and molasse foreland plains where subsidence is induced by mountain building. On the other hand the Lake Eyre Basin of Australia has been tectonically stable for a long time (Rust, 1981) thus allowing the very low gradients that seem to be necessary for anastomosis to develop.

J BURGERSDORP FORMATION AS AN ANASTOMOSED FLUVIAL SYSTEM The Burgersdorp Formation was deposited in a tectonically active molasse basin under semi-arid to arid conditions. It has been interpreted as the distal equivalent of the Katberg Sandstone, a braided J river deposit, and to have been deposited in an area of low slopes (Hillar & Stavrakis, in press).

J The succession displays many of the features of Rust's arid zone model in that ths sequence is mudstone dominated, contains deep dessication cracks and uuricrust horizons. However, there is one major difference 3 - the Burgersdorp Formation contains numerous crevasse splay sandstones whereas the Cooper's Creek deposits described by Rust (1981) contain very few, if any crevasse deposits. Crevasses are common in the humid regime Canadian deposits but the role of climate in crevasse J development is not clear. Splays characteristically occur where banks are stable, slopes low, and aggradation rates high and these conditions appear to have been met on numerous occasions during Burgersdorp J depositional history.

REFERENCES

Hiller, N and Stavrakis, N (In press) A Triassic fluvial system from the Karoo basin, South Africa. Palaeogeog; Palaeo- clitnat; Palaeoecol.

Rust, B.R. (1981) Sedimentation in ar, arid-zone anastomosing fluvial system : Cooper's Creek, Central Australia. J. sedim. Petrol. 51, 745-755.

N. STAVRAKIS AKD K. HILLER J .-_ I I in arid Central Australia evd a carhoriferous analogue in New Brunswick, Canada. I In : Modern and Ancient Fluvial Systems. Ed : I ~" Collinson, J.D. and Lewin, J. Spec. Publs. (6) int. Ass. Sediment; 385-392. § J Smith, D.G. (1983) Anastomosed fluvial deposits : modern examples from Western Canada. 2lL • Modern and Ancient Fluvial J Systems. Ed : Collinson, J.D. and Lewin, J. Spec. Publs. (6) int. Ass. Sediment., 155-168. , Smith, D.G. and Putnum, P.E. (1980) Anastomosed river deposits : I Modern and ancient examples in Alberta, Canada. Can. I J J. Earth Sci. (10) 1396-1406. J Smith, D.G. and Smith, N.D. (1980) Sedimentation in anastomosed river systems : examples from alluvial valleys near Banff, ( Alberta. J. sedim. Petrol. (50) 157-164. I' I J Stavrakis, N. (1979) Sedimentology of the Katberg Sandstone in the Eastern Cape Province. M. Sc. thesis (unpubl.) Rhodes i Univ., Grahamstown, 113 pp. I Stavrakis, N. (1980) Sedimentation of the Katberg Sandstone and adjacent formations iri the South-Eastern Karoo, basin. 1 Trans, geol. Soc. S. Afr., (83) 361-374.

ADDRESSES

N. STAVRAKIS N. MIXER ii BP COAL SOUTHERN AFRICA DEPT. OF GEOLOGY P 0 BOX 1554 RHODES UNIVERSITY JOHANNESBURG GRAHAMSTOWH 2000 6140 SEDIMENTOLOGICAL CONTROLS OF GOLD DISTRIBUTION IN THE MIDDELVLEI REEF, DOORNFONTEIN GOLD MINE

INTRODUCTION Doornfontein Gold Mine is the westernmost member of a group of mines in a gold-field known as the West Wits Line, situated between Westonaria and Carletonville. Gold ore is derived from two placer ore bodies, termed reefs, which occur near the base of the Johannesburg Subgroup of the Witwatersrand Supergroup. The older of these, the Carbon Leader, is the principal gold-producing reef, contributing about two-thirds towards the total ore production of 120 000 tonnes pet" month. The Middelvlei Reef, which occurs stratigraphically about 80 metres above the Carbon Leader, has a lower mean gold grade and exhibits a more erratic lateral gold distribution pattern than the latter. ]• LITHOLOGY AND SEDIMENTOLOGY 1 • The Middelvlei Reef consists of one or more oligomictic conglomerate beds, overlain by quartzite, often pebbly, and thin siltstone beds or laminae. The footwall of the reef comprises very coarse quart2ites, usually fairly argillaceous, while the hangingwall quartzites are finer- grained. Four conglomerate-, two quartzite- and one siltstone lithofacies were recognised in the reef. These ideally occur in fining-upward sedimentary cycles, commencing with conglomerate at the base, followed by quartzite and terminating in a siltstone lamina at the top. Two facies assemblage types were formulated; a single cycle type and a multiple cycle type. The cycles often exhibit scours in the footwall or in older cycles. Reef thickness varies from a few centimetres to more than two metres. An isopach map of the eastern part of the mine shows a north-south trend of thicker reef zones and an area of small uniform thickness in the north-east.

Palaeocurrent directions, determined from cross-bedding and the orientation of basal scours, indicate a unidirectional south-south-easterly palaeoflow in the eastern part of the mine and a unidirectional easterly palaeoflow in the western part.

Mean pebble size, expressed in terms of the apparent long axes, varies from -2,850 to -3,67$ and the standard deviation between 0,59$ and 0,80$. Quartz is the dominant clast type, illustrating a remarkable maturity of the pebble assemblage. It was found that the assemblage in the western part of the mine is somewhat B G Els

B G ELS 1 different from that in the east. Carbon, occuring mostly as small nodules {"fly specks"), is sometimes present in the thinner reef types. Several examples of syndepositional faulting, which demonstrably affected the sedimentology, were encountered.

INTERPRETATION AND SEDIMENTOLOGICAL MODEL The types and relative proportions of the various lithofacies, the cyclic fining upward sedimentary sequences, the unimodal palaeocurrent directions and basal scours are criteria that led to the conclusion that the Middelvlei Reef is fluvial in origin. It is suggested that the depositional ] environment was an alluvial plain (braid plain) into which sediment was fed from an area of high relief, via alluvial fans and dispersed on the plain by braided stream processes. The lateral variation in reef thickness is attributed to predepositional footwall topography as well as syndepositional scouring of the footwall. The different mean palaeocurrent directions and different pebble assemblages found in the eastern and western parts of ] the mine suggest two geographically different source areas. It is suggested that the carbon represents the remains of primitive algae which grew in quiet parts of the depositional ] system. The occurrence is interpreted as 'in situ' and marks periods of non-deposition.

] The occurrence of syndepositional faulting implies a relatively long period of deposition.

GOLD DISTRIBUTION "] The gold distribution in vertical profile and its J relationship to lithofacies were investigated by statistical comparison of pebble sizes and pebble packing densities in samples, with gold grade. No proportional relationship was I obtained, but the gold was found to be concentrated predominantly in the conglomerate facies.

The relationship between gold grade and sedimentary parameters was analyzed by Automatic Interaction Detection (A.I.D.) which revealed antithetic relationships between gold grade and (a) the proportion of interbedded quartzite (b) the number of conglomerate beds and (c) reef thickness.

The latter relationship is confirmed by comparison of isocon and isopach maps of the same area. Certain trends on the B G Els

B G ELS J isocon map were found to correspond to the general direction of a set of faults and dykes. This direction is roughly J perpendicular to the palaeocurrent direction. J- MODEL FOR GOLD CONCENTRATION The antithetic relationship between gold grade and reef thickness is attributed partly to winnowing (washing) of previously deposited reef material, during which some of the J barren lower mass fraction was removed.

The coincidence of certain isocon trends with structural features is explained by postulating that the latter represent minor syndepositional faults which created areas J of higher palaeotopography, upon which winnowing took place. The antithetic relationship between gold grade and the number of conglomerate beds is suggested to be due to reworking of sequences of interbedded gold-bearing gravel and barren sand beds, which resulted in the elimination of the latter and concentration of the gold in a single gravel layer.

A favourable factor for effective winnowing and reworking was depositional conditions approaching equilibrium, which resulted in a low net rate of sedimentation.

B G Els

B G ELS GOLD DISTRIBUTION AS RELATED TO SEDIMENTARY FACIES IN THE VCR ON DEELKRAAL GOLD MINE, CALETONVILLE GOLDFIELD

INTRODUCTION Deelkraal Gold Mine is situated on the south-western extremity of the Carletonville Goldfield. The principal reef mined is the Ventersdorp Contact Reef (VCR) which dips at 25° to 30° in a direction of 160° and suboutcrops against the Black Reef at an approximate depth of 1 600 metres below the surface. The VCR is unconformably underlain by conglomerates and quartzites of the Mondeor Conglomerate Formation, of which successively older horizons are truncated by the VCR in an easterly direction. A significant secondary tonnage is 1 derived by selective mining of the underlying Deej.kraal Reef of the Mondeor Conglomerate Formation.

SEDIMENTARY FACIES

A number of fluvial facies types, as well as one possible sieve deposit, are recognized on Deelkraal Gold Mine. These are shown in Figure 1 and are described below.

FACIES A This facies consists of a uniformly one-metre thick fining upward sequence of oligomictic pebble supported conglomerates i and minor quartzites, deposited on an extremely even palaeotopography. This facies is interpreted to have been deposited relatively quickly, during flood, conditions,with little reworking having taken place. i FACIES B "1 This "Black VCR" is a lobate-shaped, oligomictic, pebble- supported conglomerate interpreted to be a sieve deposit associated with facies Cl. Reef thicknesses range from 7 metres in the north to a contact in the south, and i palaeocurrents have a north-south direction. FACIES Cl This facies is situated in the central part of the mine and comprises a five to six metre sequence of conglomerates and quartzites. It is interpreted to be of fluvial channel origin. Reef thickness isopachs exhibit an approximate north-south direction.

M P Mullins FACIES C2 Facies C2 consists mainly of pebble supported oligomictic conglomerates and is interpreted to be a result of caolescing longitudinal bars associated with high energy conditions.

FACIES C3 This facies consists of coalesced longitudinal bars, minor sand filled channels, pebble lags and bare bedrock surfaces. Palaeotopographic differences become more marked towards the east. Intermittent flood conditions formed and reworked this facies.

GOLD CONTENT Optimum conditions for gold concentration in the VCR conglomerates existed during the deposition of facies C3. Highest values are recorded in thin reef (about 40cm) of this facies. Facies C2 and facies A exhibit local concentration within pay shoots, while both facies B and facies Cl have overall low gold contents because of the lack of any concentrating mechanism during deposition.

M P MULLINS J J J J J

]• ]•

LEOENP

Pokitocurnnt Dirtctton • Shaft

8 VCR FociM Typtt

Figure 1 DEELKRAAL GOLD MINE VCR FACIES DISTRIBUTION PLAN I _ 1 by | IZAK C. RUST 1 Department of Geology University of Port Elizabeth -; ._„, ,„,.,„. I together with the modern application of Walther's Law, and fortified "I1 by the current concept of actualistic or methodological uniformitaria- I nism, it serves as the mainstay of geological and sedimsntoiogical T hindcasting. The technique is highly successful in assisting us to | -* visualise the geological past. 1 . J The systematic study of sedimentological processes and modern [ environments has greatly enhanced our capability to recognise and J reconstruct the palaeoenvironment. The juxtaposed Keurbooms estuarine I area and the nearby exposures of the Jurassic Robberg Formation 1 illustrate how a knowledge of the modern sedimentological situation f * and relationships can help us to recognise palaeoenvironments as T represented in the rock record.

Izak C RUST