Revista Brasileira de Geociências 12(1-3): 522-530, Mer.iSet., 1982 - São Paulo

THE NATURE AND GENESIS OF ARCHAEAN GOLD MINERALlZATION lN SOUTHERN AFRICA

M.J. VILJOEN*

ABSTRACT Attention is drawn to the long history of gold mining in Southern Africa and the -very large number of mines known. The elose spatial association between these Archaean depo.. sits and greenstone belts is stressed. Ultramafic (komatiite] dominated successions occur ar the base of many greenstone belts and are followed by mafic (tholeiitic) arrd more felsic successions. Sediments generally terminate the. greenstone belt stratigraphy. Classiflcation schemes for Arehaean gold deposits in southern Africa are briefly reviewed. The possible prirnary source of gold from the greenstone stratigraphy, particularly from ultramafic and mafic rocks is briefly reviewed. The importance of sedirnentary breaks in the volcanic stra­ tigraphy in the early concentration of gold is discussed. The importance of the presence of reactive or available gold in rocks which is suitable for mobilization and further concentration is stressed. The Steynsdorp palagonitic tuff horizon is given as an example of a subeconomie syngenetic depoeit. The Consort mine in the sarne belt is considered to be a syngenetic banded sulphide iron forrnation which was enriched sufficiently to give an ore deposit. "The gold-quartz veins ofthe Steynsdorp field occur in competent metatholeiites and represem examples of typical gold-quartz Iode deposits. Of importance in the "formation of these deposits is the presence of an inirusive tonalitic granite. and lhe depcsition in a structural site in a lower greenschist metamorphic regime. Finally, a c1ass of gold, and gold antimony mineralization associated with highly earbonated komatiitic zones is described. Important examples .include the Sheba mine of the Barberton belt, Consolidated Murchison antimony mine of the Murchison belt and the Gaika and Globe ano Phoenix mines of the Midlands beIt of Zimbabwe. '

INTRODUCTION Evidence of aneienl gold mimng in Southern Africa as well as to Archaean greenstone belts activity in Southern Africa is widespread. Modern European elsewhere in lhe world (Viljoen and Viljoen, 1969a ; Anhaeus­ interesl in gold and gold mining however began in com par­ ser et ai., 1969). atively recenl times with the discovery by Carl Mauch in At lhe base ofmany greenstone belts is an extensive devel­ 1866 of the Tati goldfield in Botswana and the Umfuli opment of a variety of magnesiurn-rich mafic and ultra­ goldfield in Zimbabwe (Anhaeusser, 1976). A total of 4,000 mafie metavolcanic roeks now known world 'wide as gold mines are known frorn the Southern African region komatiites. Associated with the komatiites are generally and total gold production up to 1976 has been estimaled at subordinare layers offelsic tuffs and associaled banded iron approximately 80 million ounces of gold. (Anhaeusser, formations, This assemblage has collectively been referred 1976). to as the Lower Ultramafic Unit and in many areas has been There is no doubt about lhe close spatial (and genetic) extensively intruded, metamorphosed, dismembered and assoeiation between Archaean greenstone belts and gold partly granitized by the intrusion of largely tonalitic granitic deposits. This paper examines the nature of the S,nithern gneisses (Viljoen and Viljoen, I969a-c ; Anhaeusser et al., African greenstone belts and their contained gold deposits 1969) (Fig. I). and speculates on the primary source of gold as well as the Above the Lower Ultramafic Unit there is an abrupt factors that have lead to its anomalous concentration to change in the nature of lhe volcanicity in the successions form economic gold deposits. that are collectively grouped into the Mafic to Felsic Unit. Cyclically alternating ma fie and intermediate to acid volca­ GEOLOGY OF SOUTHERN AFRICAN GREENSTO­ nic rocks, as well as a wide variety or pyroclastic ando chem­ NE BELTS Gold in Southern Africa is largely confined ical sedimentary rocks predominate and include tholeiitic to two ancient granite greenstone bloeks, the Kaapvaal and basalts, andesites, dacites, rhyodacites, cherts, banded iron­ Rhodesian cratons which are separated by the Limpopo -formations and phyllites, as well as some stromalolitic Ii­ high-grade metamorphic mobile belt (Fig. I). Rocks within mestones (Viljoen and Viljoen, 1969d; Anhaeusser, 1976). the cratons have ages ranging from 2,700 to 3,500 Ma. Overlying the volcanic sequences in the greenstone belts Scattered within the granitic rocks are numerous volcano­ are rocks of essentially sedimentary character made up of sedimentary keels or greenstone belts covered mainly on the either an argillaceous assemblage, consisting ofgreywackes, westernsectors ofboth cratons, by younger eover formations shales, and siliceous chemical precipitates (banded ferru­ (Fig. I - Anhaeusser et ai., Í969; Anhaeusser, 1976). Studies ginous eherts and iron-forrnations), or an arenaceous as­ in the well developedand well preserved Barberton greens­ semblage, consisting of conglomerates, quartzítes, sub­ tone belt have led to the establishment of a stratigraphic greywackes, sandstones, anel shales, with subordinate jas­ rnodel that is broadly applicable to other greenstone belts pilitic cherts and banded iron-formations (Fig. I).

*J.C.I. Geological Dept., P.O. Bcx 590, Johannesburg 2000, Republic of South Afrtca Revista Brasileira de Geociências, Volume 12 (I~3), 1982 523

ln an attempt to establish the regional and stratigraphic controls of gold occurrences in Zimbabwe, Collender (1965), suggested that the deposits could be elassified in terms of the rock types in which the ore bodies are found. His six­ -fold scheme ofclassification is listed as follows: deposits in the granites, deposits in ultrabasic rocks, deposits in Bula­ wayan-jaspilites, deposits in Bulawayan lavas, deposits in Shamvaian sediments, and gold deposits in stocks and intrusives. Recently, Fripp (1976) introduced a four-fold classifica­ tion ofgold deposits in Zimbabwe based upon the geological ORffNaTQNf BUTa nature ofthe ore bodies, These were Iisted as: stratiform \ S~OIA,So>OllO deposits ofmineralized banded iron-formation, stratabound 2 !,lI OARW~ 3 5~MAYA'SAUS6URV "massive" sulphides, quartz Iodes, veins, slockworks and A !,l)IJI(O siliceous shear zones, and stratabound disseminated mine­ IlollOlAllOS S lolWANESI ralization in c1astic rocks. J WfOl~ li Mo:~1 A five fold mineralogical elassification was introduced by 9 OOll,UMIAll IG fEll';llURG Lightfoot (1934) based on the distinclive mineralogical III1IoWUMA characteristics of 110 of lhe larger Iode gold deposits in 11 SRUKWE,GW,lO I) MASHAaA Zimbabwe. The main groups are as follows: pyritic gold­ 11 fORl V~IOR1A 1\ llHl~GWl -quartz veins in 73 Iodes, pyritic impregnations along shat­ IS IlUClfWM,IWSOUTH AFRICA 13 MUAÇlI'S~N ..' li'., -o A simple twofold classification of Zimbabwian gold de­ 1~ aARII(R!o~ \ ~ Ii"". JI) MUltlERSllll* posits into stratabound and non-stratabound deposits, si­ 31 AMAlli\ '~ milar to that of Fripp (1976) has recently been proposed by Foster and Wilson (in press). The stratabound deposits are lfOfNP ",.".,.". further subdivided inro four main types: iron-forrnations, . •. (; •• ,t o,•• . f."'Cn'''o banded sulphides, volcanielastic-hosled and clastic hosted. ~ M.',"""". "o,"'.1"19nOOU'CO",PI.' ~ A«M'"~""".' QO...... ""9"'''''.' Non-stratabound deposits are divided into veins and mine­ OIlHtlSTONE BEl' uTl

EXAMPLES OF OI FFER EN T TYPES OF G OLO ..... :. :.:.:. O EPOSITS FROM SOUTHERN AFRICA No atte m pl '. .: : : : . ....: :. ', ' : ' .. will hc made in this papel' lo cla ssity go ld dcposits. and .: ), " ; , : .': no hroad gcner alizat ions co ncemi ng thc fcaturcs ofdiffcrent classes 0 1' de posi ts will bc givcn. Instcad a num her 01' ind ivid ual dc posits. ...vhic h disp lay ccnain charac tcristic fcat urcs that havc bcc n used in the class ilications ofprcvio us wo rkcrs will be descri bcd in dctail. So me 01' tbcse de pos its show diffcrcnt styles 01' m incralizati on that ca n he class ificd im o to tally separa re ea tego ries. Figurv 2 - Geo(oRJ' of IiI(' Stcvnsdorp Gotdfieki. Barberton belt. .~ ho w i"K distríbut ion oI x old mines ( (~/i('/" V íljaen elo al. . j/')(5 9) Syngenetic volcanic related deposits A num ber 0 1' go ld dcposits and occurrcnccs situa tcd within scdimc ntary a blaek opaq ue groundmass with flak cs 01' ch lorite, lesser breaks in volcanic dominat cd succc ssions and co nside rcd to q ua rtz and grai ns 0 1' carbo na tc. T he o paq ue gro und rnass is be 0 1' a syngcnctie type have rece ntly bccn describcd 0 1' re­ apparently la rgely ca rb onaccous and the rock is de scr ibcd fer red lo in So ut hcrn Africa . T hey vary from ano malous but as a ca rbonace o us tuffaceo us sha le (Viljoen et al .• 1969 ). sub-economic go ld co nccntrations in tuffaccous and palago­ Thc idea that this horizon in part co ntains or igina lly glassy n ilic b reaks (e.K. th e tull aceou s hori zo n 01' the StcYlldorp (palagon itie ) komat iitic ma terial is favo ured now (Viljoen. a rca 01' the Bal'berton greensto nc belt) to important well in press). bedded or layered aml appa ren lly largcly syngenelic slra­ It was fOUfl d that th e tuffaccous palagonitic ca rbonate tifo rm go ld ore oodies wilh in ko ma tiitic volcanic piles (e.R. ho rizon is d istinctly enriched in six 01' eig ht elements analy­ the Co nso rt go ld mine 01' the Barbcrton hclt ) (Viljoen . in sed; gold . arsenic . co pper. nick el tita nillrn and iron with prcss). Examp lcs 0 1' the abovc typcs 01' dcposits aml oceur­ rcnccs arc d iseu ssed bclow. an anomalo us go ld eonle nl of750 ppb (Viljoen el ai., 1969 ), It was co nclllde d th at this ho rizon represcn ts the ter m ina · I. S7 EYNSD ORI' TUFl,cBARBERTON GRITNSTONE tion 0 1' a peri od o f komatiitic vlllcan icity in whi ch go ld and BELT ln a d isellssio n o n metall ogen esis aml or e eontro l in other meta is were ano ma lo llsly enriched fro m the und er­ lhe Sleynsdo rp go ld lield 01' lh e Barbert on Belt , lhe presence Iying lavas by late hyd roth errnal . vo lcanie related processes o fa luffaceo lls horizon with anomalous go ld alld base meta l and perhaps wealhe ring . aide d possib ly by lhe presen ce 01' concentrations is dcscribcd by Viljocn el aI. (1969 ). T he primitivc lire fo rm s nolirish ing dur ing the pcriod of qui es. ho rizon oec urs at th e to p of the Komati Fo rma tio n irnrned i· cence in bctwecn the komat iitie ou tp ourings. al ely underl ying lh e ehe rly Middle Marker which defines lhe T hc process o f mo bilizin g th e low co nce ntra tions 01' go ld tight Stey nsdo rp an liclina l struc ture in this arca (Fig. 2 ). d i scu s~ed above and eo nee ntrating it int o Iode vein de po sits T he Ko ma li Fo rma lion in the Steynsdo rp anticline co m­ in ad jo ining mo re cornpetent rock types is d iscussed laler. pri ses a lower seq uencc ofoft en ca rbo na ted kom atiites ovcr­ lain hy high ly sheared ko ma tiitic basalt s. The tuffaceo us 2. CONSORT GO LD M INL'-B ARBERTON GRU:NS7'O­ hor izon takes the fo rm 0 1' a rcsistan l, hlack weathe ring . NE BU .T Th e go ld m ineralizal ion 01' lhe Co nso rl Gold br oad ly layered . da rk gree n to hlack rock with almost pure. M ine is co nside red to rep rese nt an exa m ple a I' a largely irregular carbonate bands. Th e mafi c co mpo nenl com pr ises prirnary cco no m ie go ld eonecntration o nce again c10sc ly Revista Brasileira de Geociêncías, Volume 12 (1-3), 1982 525

associated with komatiitic volcanics. Mineralization is rich in chert, arsenopyrite, pyrrhotite, and ankerite (Fripp, associated with a fine grained siliceous .zone known as the 1976), Consort "Contact" or Consort "Bar" (Hearn, 1943; Vil­ Other deposits with certain syngenetic characteristics joen, 1964; Voges, in press). include the Giant and the New Found out, as well as other The Consort Contact is situated on the contact between smaller deposits in the Gadzema greenstone belt north of underlying Onverwacht Group rocks and overlying Fig Hartley, where gold mineralization is associated with ban­ Tree Group rocks at the western extremity of a linear fold ded ironstones interlayered with komatiites and felsic schists. structure. the Lily syncline. Komatiitic rocks dominate the (Anhaeusser, 1976; Wiles, 1957). The Sherwood Starr mine Onverwacht Group in the arca, and t~ke.the form ofa range in the Que Que area also represents an auriferous sulphidic of arnphibolites, serpentinites and allied rocks equated banded jaspilite of possible syngenetic origin. with kornatiitic basalts and komatiites respectively. The Many more examples of the above type could be cited, overlying Fig Tree Group rocks consist of hornfels derived particularly in the lower ultramafic, komatiitic sector of from greywackes and shales (Viljoen. in press). the greenstone stratigraphies, suggesting a possible genetic Economic mineralization is confined to the western ex­ connection between these deposits and kornatiites (Viljoen, tremity of the synclinal structure where the contact is stron­ in press), gly folded. ln close proximity to the Consort Contact, the lt should be pointed out that many of the above deposits, footwall komatiitic rocks are massive, greenish grey, mag­ although showing certain apparently syngenetic charac­ nesium-rich arnphibolites cornprising tremolite with locally teristics such as stratabound, bedded or banded mineraliza­ developed hornblende. (Viljoen, 1964; Voges, in press). tion, also show epigenetic characteristics. These include Some antigorite and chlorite can also be present and these the structural control of ore shoots, fracturing and younger rocks are equated with the Geluk type of komatiitic basalt veining in better mineralized areas, as well as other features as defined by Viljoen and Viljoen (1969b), which were probably responsible in many cases for the final The Cansort Bar averages 4 m in thickness and is a stron­ enrichment and localization of payable ore bodies. gly laminated and highly siliceous cherty rock varying in colour from black to a well layered green and brown rock, Epigenetic deposits lnto this category are classed var­ the layers being 1-5 mm wide, and far more pronounced ious vein or shear zone gold deposits of the type that are so in fold structures. Interlayered sulphide-rich bands with widespread and important in the Archaean, They invariably varying proportions of arsenopyrite and pyrrhotite are pre­ occur some distance from a granite contact in a lower greens­ sent in the Bar and such arcas constitute the ore zones being chist metarnorphic regime and are sitedwithin fractures, mined for gold. Sulphide impregnations with associated breaks and shears and other structural disturbances within gold are also found within amphibolites underlying the more cornpetent rock types. These often take the form of Consort Bar. Apart from the sulphides arsenopyrite and mafic (tholeiitic) lavas, banded iron formations as well as pyrrhotite, vein quartz. tourrnaline, topaz and calcite veins certain felsic rock types and other scdimcnts, The old are also present in the ore. Minor chalcopyrite and pen­ Steynsdorp gold field of the Barberton belt represents a tlandite is also found and free gold is often present, typical example of this type and is described below. The Consort Bar is considercd to have had an origin similar to the mineralized tuff horizon overlying the Kornati 1. STEYNSDORP GOLD F1ELD-BARBERTON BELT Formation in the Steynsdorp area described above, Late The Steynsdorp Gold Field was of minor importance and phase hydrothermal activity related to the waning phase of only operated for about ten years towards the end of last the komatiitic vulcanicity is considered to have been an century. Being a poorly mineralized area it has not been important mineralizing agent. Mineralizing hydrothermal subjected to complicating processes such a metarnorphism fluids could also have been derived from circulating hydro­ and wall rock alteration on a scale that has obscured the thermal systems leaching metais from the pile of recently particularly clear relationships that exist between gold erupted komatiites during a quiescent period before the rnineralizauon, stratigraphy and structure (Viljoen et at., deposition of the overlying Fig Tree Group rocks. The 1969; Viljoen, in press). hydrothermal solutions are considered to have been rela­ The gold rich palagonitic and tuffaceous accumulation tively high temperature, rich in silica, iron, sulphur, arsenic at the top of the Komati Formation in the tight Steynsdorp and gold and to have becn responsible for the formation anticlinal structure has been referred to earlier. This pala­ of the vulcanosedimentary Consort Contact. (Viljoen, in gonitic tuff occurs imrnediately below a persistent, narrow, prcss). The deposit could thus be classed into a category, banded cherl horizon known as the Middle Marker. Vir­ noted earlier, of stratiform mineralized banded iron for- tually ali the auriferous veins mined in this region occur in " mation, The significance of younger structure and younger the volcanics of the overlying Hooggenoeg Formation, concentrating mechanisms in the possible enrichment of generally in close proximity to the Middle Marker, the only the deposit are not fully understood. exceptions being mineralized veins in porphyry bodies in­ trusive into the underlying Komati Formation (Fig. 2) ], OTHER POSSIBLE SYNGENETlC DEPOSITS A num­ (Viljoen el ai" 1976). These vein deposits were only respon­ ber of other gold deposits are situated wilhin well layered sible for some of the gold produced from the Steynsdorp banded iron formations often in komatiitic volcanic accu~ field, most having come from an alluvial digging known as mulations in Southern African greenstone belts" ln the Fullerton Creek (Fig, 2). Vubachikwe mine area of the Gwanda belt, Fripp (1976) A number of different types of gold occurrences were describes gold ore bodies in several thin beds of banded iron described in the area by Viljoen el ai. (1969). Gold-quartz formation which are interlayered with mafíc and komatiitic veins form the main type and are considered to have been volcanics, 'The ore bodies are stratiform and are confined emplaced into dilatent zones associated with faulting and lo beds of sulphide and mixed sulphide carbonate facies of jointing in the more competent rock types; mostly the banded iron-formalion which consist of layers alternately tholeiitic volcanics of the lower portiol1 of the Hooggenoeg 526 Revista Brasileira de Geocíêncías, Volume 12 (1.3), 1982

Forrnation in lhe proximity of lhe Middle Marker. Mosl 2. OTHER EPlGENETlC DEPOSITS The important of the gold occurrences associated with faults immediately basic features of lhe very wide range of Archaean Iode gold above lhe Middle Marker, in lhe nose and along lhe eastern deposits is exemplified by lhe Steynsdorp gold deposits, limb of lhe Steynsdorp anticline, are of this typc, Several An almosl identical control to that described for Steyns­ deposits on the western limb of the anticlinal structure, dorp, has also been recognised in the Shangani belt of Zirn­ including the relatively large Comstock mine also occur in babwe (Viljoen, in press), Here most of the gold-quartz the faults immediately above the Middle Marker. A num­ veins occur in faults and shears within competent tholeiitic ber of deposits in this region occur about I km west of lhe greenstones immediately overlying a large accumulation or Middle Marker and appear to be relaled to a major high less competent, komatiite dominated successions (Viljoen, angled wrench fault, the Kromberg fault. in press). Once again a heat source was created by the in­ The trace of the fault is marked by a number of gold­ lrusion of a tonalitic granite along lhe eastern flank of lhe -quartz veins, the larger ones showing a distinct parallelism Shangani belt. This mobilized gold which moved lo a lower ' with lhe fault trace. Veins of this type were probably lar­ temperature regime, there to be deposited in suitable struc­ gely responsible for lhe forrnation of lhe alluvial deposits tures in the first competent rock type to be encountercd. of Fullerton Creek. Numerous other gold deposits in Zimbabwe appear lo The veins, lenses and stringers of white quartz vary in have a basically similar control and include for example width from a few millimeters to three mctres, and carbonate, many deposíts in lhe Bulawayo, Filabusi and Gwanda belts. commonly in lhe form of ankerite and sidcrite generally Many larger deposits in Zimbabwe also appear lo be related accompanies lhe quartz as gangue. Sulphides in the form to granite stocks and are controlled by suitable structural of pyrite, arsenopyrite and some chalcopyrite are rare and sites. ln the Golden Valley area lo lhe northwest ofGalooma mosl of lhe gold was apparently free milling. The adjacent in lhe Midlands belt of Zimbabwe, gold/quartz veins occur metabasalts are propylítically altered and have narrow apparently in broadly the sarne stratigraphic horizon in an bleached aureoles containing chlorite, quartz and carbonate are approximately 3-4 km from lhe contact of a small tona­ (Viljoen et ai" 1969). litic body known as the Lion Hill stock (Anhaeusser, 1976; A feldspar porphyry body intrusive into lhe Komati For­ Fripp, 1976). The granitic intrusive appears to have been rnation of lhe Steynsdorp aruicline. hosts complex sulphidic responsible for the development of a concentric tensional quartz veins whích were mined at the Gypsy Queen mine. system around the area of intrusion. The most favourable These complex ores conrain arsenopyrite, pyrite and lhe tensional sites were created along the contacts of felsite base melai sulphides sphalerite and galena. The galena porphyry layers within lhe volcanic stratigraphy and it generally contains a large number of inclusions of tetrahe­ is at these localities that the gold-bearing quartz veins of drite. bournonite, argentite and pyrargyrile. The mineralized lhe Golden Valley and Patchway mines. and also many veins of the Gypsy Queen mine are considered to be centro­ other smaller mines, was deposited. Scheelite is a common lIed bythe fracluring ofthe cornpetent porphyry body which accessory mineral associatcd with lhe epigenetic veins of hosts them. this area. That many of lhe gold/quartz veins are controlled by The irnportant Dalny mine about 15 km north of Golden the cornpetent fractured metatholciitcs of the Hooggenoeg Valley represents a good example of a mineralized shear Formation is supported by lhe distribution ofdia base dykes zone within voleanic rocks of lhe Bulawayan Group. The in the area. Tbcse are well developed in the metatholeiitcs orehody is a heavily propylitized sheared zonc, lhe sheared and often stop abruptly at lhe Middle Marker. Where dykes rocks being impregnated with arsenopyrite and pyritc. The persisl into the sheared, incornpetent komatiites of lhe deposit would fali into lhe calegories of what have been Steynsdorp anticlinal core, they are invariably disconti­ termed siliceous shear zones 01' pyritic impregnations along nuous and form isolated trains. A robusl dyke is developed shatter belts. in lhe porphyry body lhal hosts the Gypsy Queen rninerali­ Numerous other important epigcnetic deposits exist in zation and like the latter is also controlled by lhe relative southern Africa (Anhaeusser, 1976) but their main features compelency of this body (Fig, 2) (Viljoen et ai" 1969). are largely similar lo those of the deposits described above. The question of the origin of the gold in the veins descri­ bed above was considered by Viljoen et ai. (1969). II was argued that relatively high proportions of gold are presenl Carbonated komatiite related deposits As noted in the primitive kornatiitic volcanics and particularly in the . earlier an important range of Archaean gold or gold/anti­ palagonitic tuff horizon underlying these volcanics as noted mony deposits have recently been shown to be associated earlier. This gold together with other volatile elcments is with highly altercd (carbonated) and often strongly sheared, considercd to have been mobilized as a result of the heat linear zones of komaliite. The komaliiles are often highly generated by lhe inlrusion of lhe tonalilic Sleynsdorp gra­ altered to a range or tale, talc-carbonate and quartz-car­ nite. The Occurrence of gold within primary sulphides in bonate rocks. The lalter rock lype frequently shows signs of the komatiitic accumulation allowed for its ease of mobili­ gold and related mineralization and in suitable structural zation. Gold and associated elements are considered to have sites is lhe host to importanl Archaean gold and slihnile/gold moved down a temperature gradient away from the granite Iodes (Viljoen, 1979; Pearlon, 1979; Viljoen. in press; contact and to have becn deposited in suitable structural Pearlon, in press). traps within a lower teinperature greenschist metamorphic sequence. Ideal siles would have been lhe fraclured and I. SHEBA GOLD MINE·BARBERTON GREENSTONE faulted tholeiiles immedialely above the Middle Marker as BELT A number ofdifferenl slyles of mineralizalion occur well as lhe compelenl porphyry body of lhe Gypsy Queen in the Sheba 'and Fairview mine areas and according to the mine (Viljoen eI ai" 1969; Viljoen, in press). The vein depo· clª~&ification schemes outlined earlier, could be c1assed into sits can be considcred as good cxamples or what have been l!11'~~ dislinclive categories. An importanl lype of minerali· classifled as quartz Iodes 01' pyritic gold-quartz veins. zation has recenlly been ascribed lo lhe calegory of carbo· Revista Brasileira de Geocíências, Volume 12 (H), 1982 527

nated komatiite related deposits (Viljocn. in press) and this Zwartkoppie Formation. The Zwartkoppie Formation 1Il is used here as an example of this type, The two othcr types the area consists of a grey talcose carbonate rock (grey of mineralization occurring in lhe arca are also described schist) green, fuchsitic carbonate and quartz-sericite schist and serve as examples of what could be terrned epigenetic (green schist ) as well as banded, black, white and pale quartz Iode dcposits and sulphide replaccment bodies Dr green banded chert (chert). The grey schist frequently occu­ impregnation Iodes. pies the coreofthe fold structure and is followed by green car­ The regional geological seuing of the Sheba and Fairview bonate which tends to occur immediately below the chert. mines has been outlined by Anhaeusser (1974), and more The grey and green carbonate rocks generally persist around recently the geology of the Sheba mine has been described the folds while the chert horizon is often missing along the by wagcncr and Wiegand (in press) while thal of the Fair­ northern limbs of the isoclinal struetures (Fig. 3). Most of view mine has been described by Wiggett et aí, (ln press ), the folds plunge lo the east at angles varymg between 10" Two major, juxtaposed. tight , synclinal fold structures. lhe and 40" (Wagencr and Wiegand, in press). Eureka syncline to the north and Ulundi syncline to the The green schist of the Zwartkoppie Formation hosts south, occur in lhe arca and are separated by lhe Sheba most ofthe ore in this Formation, while fewer, but very rich, fault (Fig. 3). The Eureka syneline is composed of arena­ ore bodies are loeated within chert. The grey schist of the ceous sediments of lhe Moodics Group while lhe Ulundi Zwartkoppie Formation is almost invariably barren (He­ syncline is cornposed of argillaceous sediments of lhe Fig ame, 1943; Grocneveld, 1973l. ln the chert bars the ore body Tree Group. Along the northern limb ofthe Ulundi structure is often a zone of irregular linked fractures ("crackle bre­ occur a number of tight anticlinal folds of lhe Zwartkoppie ceia") whilc in lhe grccnschist it is more in the form of a Fortnarion of lhe Onverwacht Group. These inelude lhe zone 01' impregnation (Wagener and Wiegand. in press). Hospital. Zwartkoppie and Birthday anticlines (Fig. 3). A number 01' important gold-bearing fractures branch Another tightly folded sliver of Zwartkoppie Forrnarion 01'1' the Zwartkoppie anticlinal structure to the northwest also occurs along lhe contact between lhe Eureka and and southwest to forrn a crude herringbone pattern (Fig. Ulundi synclines. Ali of the above rock assernblages have 3 - Groeneveld, 1973; Wagener and Wiegand, in press). been folded about a northwest trending axis to give lhe Where the transgressive fractures meet the competem ho­ broad arcuate structurc now sccn (Fig. J) (Viljoen, in press ). rizons 01' the Zwartkoppie Formation, they often change ln the Ulundi syncline. gold rnineralization is closely as~ direction. branch out and disappear. Improved mineraliza­ sociated with the Zwartkoppie anticlines comprised of the tion is oftcn found at such localities.

s ...... -. . ~ .. . j)' ....

fAULT -, HOSPITAL ANTICLlNE ,, - "">,------, ZWARTKOPPIE

... OUTHERN CRO;S- .... ANTICUNE I BAR t / / /~ SHEBA G.M. AREA

'.' -----j'.'Km

LEGEND

MOOOIES c=J QUARTZITE a CONGLOMERATE :J­ GROUP AREA "'" c=J SHALE 8 GREYWACKE SHEBA FORMATION FIG TREE .;, } GROUP

~ 8ANDED CHERT }}

-, ONVERWACHT GREEN FUCHsmc CAR80NATE 80 QUARTl _ i~::~~~:'E GflOUP ....\', SERICITE SCHIST {MAINLY CARBONATED KOMATllTESl . " mnnnn GREY CARBONATE €I TALC-CAR80NATE SCHIST

mIIl SILlCIFlEO MYLONITE ,\ \ ____ GOLO- 8EARING FflACTURE \ Q OPEN STOPES

Figure 3 - Geology ofthe area around the Shebu and Fairview mines, Barberton bett (from Vítjoen, in press) 528 Revista Brasileira de Geocíências, Volume 12 (1-3), 1982

It has recently become apparent that the grey and greens­ nite are a number of massive quartz stockwork ore bodies chists ofthe Zwartkoppie Formation in the area are derived with carbonate alteration and sulphide impregnation, which from ultramafic, probably largely kornatiitic rocks. Rem­ were mined as open stopes in the early history of the mine: nants of serpentinites have been found in the grey schists The most famous of these is the Golden Quarry which (Wiggett et ai., in press) and the ultramafic chemistryof contained phenomenal gold values (Fig. 3). These bodies both the grey and greenschist has also been recently esta­ are also considered to have been emplaced into the cornpe­ blishcd (Pearton, in press). Spinifex textured material has tent Moodies quartzite by a hydrothermal processo What also been widely reported from green and grey schists along caused the formation of such vertical (350 m in the case

the Zwartkoppie I anticline in particular, both on surface of Golden Quarry) very rich pipe-like stockwork Iodes in and in underground workings of the Sheba mine. The green this area, in contrast to the linear-vertical vein deposits and grey schists are therefore regarded as having been deri­ elsewhere in the Moodies quartzites, is unknown (Viljoen, ved from the carbonate alteration of spinifex textured in press). Another important type of sulphide impregnation peridotitic komatiite. The micaceous greenschists with mineralization occurs within argillaceous sediments of the which gold mineralization is associated has developed as a Fig Tree Group in close proximity to Zwartkoppie horizons. resu1t of the introduction of potassium which appears to ln the Sheba mine arca, an important new ore body, the be closely related to the introduction of the ore fluido The Main Reef Complex, has recently been discovered, associa­ origin of the mineralization is therefore closely analogous ted with the Birthday antieline at depth. The ore zone occurs to the mineralization at Consolidated Murchison, discussed within Fig Tree shale aiong the southern contact of the later. Birthday chert. The' mineralization takes lhe form of finely II is suggested that the chert bars of the Zwartkoppie disseminated arsenopyrite and minor pyrite. The variation Formation acted as important competent horizons during in the concentration of arsenopyrite often gives the shale a deforrnation and inlroduction ofthe Ore fluido Shearing look banded appearance. Thus, a1though the ore shoot is often place within the incompetent komatiites forming the cores banded and parallel to strike it diverges vertically from the of the Zwartkoppie antielines and it was here that major chert horizon and transgresses into the shale (Wagener and carbonate alteration occurred. It is considered to be of sig­ Wiegand, in prcss]. nificance that the most intense alteration resulting in the The major sulphidic ore bodies of the Fairview mine are formation of the greenschist with gold mineralization took very similar to the Main ReefComplex described above and place along the contacts of lhe chert bars where maximum form a complex intermeshed structure comprising three differential movement between competent and incompetent main ore shoots approximately 200-300 m in the hanging­ rock types occurred. wall of lhe Sheba fault and in close proximity lo a Zwart­ The kornatiites of the Zwartkoppie Formation in tlie koppie anticlinal structure. These ore bodies are steeply area thcrcforc represent major linear zones of carbonate plunging, tabular features averaging 60 m in strike extent alteration and of more localized gold mineralization. It is and 1.5 m thickness (Steyn, 1965; Wiggett et al., in press). irnpossible to speculate as to how much gold was present The origin of gold at the Fairview and Sheba mines is as a primary constituem in the komatiites of the Zwartko­ sornewhat speculative but has generally been ascribed to ppie Formation and how much rnight have been introduced hydrothermal processes (Steyn, 1965; Wiggett et ai, in press) from elsewhere. There could well have been a significant According to Wiggetl et ai. (in press), gold was derived from build up of gold however at the termination of komatiitic Onverwacht volcanics and Fig Tree sediments and was activity which formed the Zwartkoppie Formation as is mobilized by the intrusion of the Kaap Valley tonalite. The evidenced in the Steynsdorp area and the Consort Mine main mineralization is considered to have taken place du­ arca and discussed earlier. There is little doubt however that ring the younger F3 deformational phase which caused the youngcr hydrothermal alteration was an important mobilizer arcuation of the Eureka and Ulundi Synclines and resulted and concentrator of gold and that the komatiitic volcanics in the development of a variety of tension fractures which were the focus for this important phase of gold mineraliza­ became mineralized, Important features include the con­ tion (Viljoen, in press). finement of gold mineralization to the Zwartkoppie an­ Fractures branching from the mineralized Zwartkoppie tielinal strúctures or fractures branching from these (Vil-: horizons also contain gold mineralization which could bc joen, in press). Gold rnineralization is seldom found more considered as an epigenetic type, ln the Moodies rocks of than a few 100 m from the Zwartkoppie zones, suggesting the Eureka syncline, a distinctive type of mineraJization a close genetic and/or structural relationship between the within gold/quartz veins is developed. The veins, situated two. within structually competent quartzitc occur within tension Gold/quartz veins emanating from the Zwartkoppie ho­ fractures radiating about the arcuate fold structure of the rizons were emplaced into competent fractured rocks of Eureka syneline (Fig. 3). the Moodies Group in the Eureka syncline. ln other areas They are well developed in the Fairview mine area and differcntial movement elose to the contact between Fig in the northern part ofthe Sheba mine area (Fig. 3) (Anhacu­ Tree Group shale and Zwartkoppie chert, resulted in lhe sser, 1974; Wiggett et ai., in press; Viljoen, in press). Robust developmcnt of sulphide bodies in argillaceous sedirncnts (10 mm-1.5 m) pale grey toblue black quartz veins occur adjoining the chert bars. in the Fairview mine arca while in the Sheba area, narrow fracture zones have been impregnated with small veins of 2. OTHER CARBONATED ULTRAMAFIC DEPOSITS dark quartz and disserninated arsenopyrite and minor pyrite. A number of other carbonated uitramafic deposits in Sou­ The veins are well developed in quartzites adjoining the thern. Africa are described by Viljoen (1979) and Viljoen (in Sheba fau1t but pinch out away from this feature in less com­ press). One ofthe most important ofthese is the Consolidated petent shale. ln the northern part of the Sheba mine area lhe Murchison antirnony/gold deposit of the Murchison range Sheba fault zone takes the form of a silicified mylonite. within the northern part of the Kaapvaal craton (Fig. I). Immediately north of this well developed and barren mylo- A number of importam stibnite/gold bodies occur within Revista Brasileira de Geoeiéncias, Volume 12 (1-3), 1982 529 a range of siliceous and fuchsitic carbonate rocks which suggesting that younger concentrating mechanisms are ne­ form a sedes of nodes within a regional tale and talc-carbo­ cessary to give an economic deposit. na te zone known as the "Antimony Line" (Pearton, 1979; Epigenetic dcposits of the well known gold/quartz vein Viljoen, 1979; Viljoen , in press). Extensive introduction of Iode type are extremely abundant but do not often form very hydrothermal carbonato followed by the introduction of large deposits. They oecur in a wide range of roek types but antimony. arsenic and iron sulphides together with other are perhaps best deveJoped in mafic tholeiitic greenstones volatile elements, is considered to have occurred and to have and associated more acid rock types and banded iron forma­ replaced and altered an original komatiitic horizon. A feature tions. Thc greater competency of such rock types is oftcn of the carbonate centres is their general zonal structure with an importanr controlling factor in the location of these vein a tale and ralc-carbonate periphery passing into a central deposits. Of imporrancc too is their oecurrenee in a lower core of more siliceous and often fuchsitic carbonate rocks. greenschist metarnorphic regime and the general proximity Mineralization occurs as irregular veins in extrernely siIi­ ofan intrusive, often tonalitic granite. Sueh granitoid bodies ceous phases or as irregular siliceous veining in rnagnesite are considered to have been importam rnobilizers of small breccias. amounts of gold from the volcanics and sedimcnls of lhe T9.e kornatiite zone is considcred to have acted as a major greenstonc belts. incornpetcnt horizon along whieh extensivo shearing took A number 01' important carbonate-rich gold and gold.an­ place, allowing for the introduetion of the hydrothermal timony deposits occur in southern Africa anel have recently fluids (Pearton, 1979 and in press ; Viljoen. 1979; Viljoen becn shown lo represent aitered u1tramafic (komatiitie) ro­ in press). cks. Thcy generally takc lhe form of linear zones of tale, Important deposits of this type in Zimbabwe inciude the talc-carbonate and siliceous (often fuchsitic} carbonate, Gaika and Globe and Phoenix mines near Que Que in the with mineralization being associatcd with the las: type. Midlands Belts. Here major shear zones oceur in an ultra­ The generation of carbonate hydrothermal fluids anel car­ mafic body which has been altered to a range 01' tale, talc­ bonate alteration of kornatiitic rocks thus appears to be -carbonate und siliceous carbonate rocks. Thc lauer occur importam In thc mobilization and concentration 01' a widcs­ in lhe centre ofalleration zones and host quartz vcins carry­ prcad type 01' gold deposit in the Archaean: ing gold anel lesse r stibnite mineralization.Iviljocn. 1979; The cxamples of the different styles of Archacan minera­ Viljoen, in press). Iization discussed in this paper are considcred to covcr the general featurcs of the majority of deposits in Southern CONCLUSIONS The vast range 01' diffcrent types 01' Africa. A numbcr of smaller occurrcnces such as those in gold dcposits in Southern Africa makes a detailed classifi­ granitic rocks and those in greenstone xenoliths havc no! cation exlremely difficult although in broad terms a general bccn discussed although in general the majority 01' thcse classification is possible. Most of the fairly clear syngenetic represem quartz-Iode iypes. deposits occur in scdimcntary breaks closcJy associated An important nCW mine in Zimbabwe, the Rcnco mine is with volcanie rocks and more particularly komatiitie vol­ anomalous in that it is one 01' lhe vcry fcw gold occurrcnces canics. suggesting a genetic connection. Very often the best within the high grade metamorphic assemblage 01' the mineralization occurs in fold structures, areas·of more in~ Limpopo mobile belt. Jt has fcatures suggestive 01' both tense fraeturing, or in other structurally disturbed sites, a syngenetic and an epigenetic origino

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