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THE NATURE and GENESIS of ARCHAEAN GOLD Minerallzation Ln SOUTHERN AFRICA

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THE NATURE and GENESIS of ARCHAEAN GOLD Minerallzation Ln SOUTHERN AFRICA 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,IW<ZA ter belts in 26 Iodes, antimonial impregnations, along shat­ " O~OKO '8 S~.~GA~' ter belts in 6 Iodes, antimonial quartz veins in four Iodes. '9 aUlAWA'O,(lU(l' 1GfR.PUS' and cupriferous gold-quartz veins in one Iode. ·IIGWA@A II lOW,'R GWA~~A This oversimplification of the mineralogy was designed 1JIA11 to emphasize the dominant ore mineral in each category, l' IOIOMf 1; MMsnAMA there being numerous others besides gold and the particular lo sUlHEf<l.A~~ 11 fl[llIISlilJl1G ~~ 0~: mineral singled out for special mention. '. SOUTH 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<OLOGICAl U""S ~ftOOEStA BO'SWA~A SOU.ft AfRtCA SWAZllA~D ralized shear zones. Soam".. " f'ot.o.,"'''''~'.' ,M'w,vo" Oo,"wa'"t Viljoen (1979) and Pearton (1979) while not attempting S.b"w,,'" a comprehensive elassification of Archaean gold deposits, drew attention to a distinctive type ofcarbonated ultramafic Figure 1 - Greenstone belts of the Southern African Archaean ShOM gold and gold/antirnony deposit. These deposits occur in wing major firhological Units tfrom Anhaeusser, /976) what appear to represent linear zones of kornatiitic rocks which have been sheared and extensively altered to tale, tale-carbonato and siliceous carbonate assemblages. A va­ riety of komatiite related deposits in Southern Africa are described by Viljoen (in press) and the role ofkomatiites as CLASSIFICATION OF ARCHAEAN GOLO OCCUR­ possible source rocks from which gold was liberated during RENCES lN SOUTHERN AFRICA Gold. although metamorphism to form nearby Iode gold systems, is discus­ rare in quantity, is nevertheless a widely distributed element sed in the sarne paper. and occurs in ali of lhe main geological settings and rock types outlined above. As a consequence of its widespread development and modes of occurrence, there have been PRIMARV SOURCE OF GOLO lN THE ARCHAEAN many attempts lo elassify known gold occurrences. A fea­ The elose spatial association between gold deposits and ture of Archaean gold deposits however, is the marked dif­ greenstone belts has led a number of workers to the conelu­ ferences that exist between different
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