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A Review of Some Recent Concepts of the Bushveld

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A Review of Some Recent Concepts of the Bushveld C awd.ian tulineralogist Vol. 17, w. 233-256 (1979) A REVIEWOF SOMERECENT CONCEPTS OF THE BUSHVELDGOMPLEX' WNN PARTICULARREFERENCE TO SULFIDEMINERALIZATION GERHARD VON GRUENEWALDT Lnstitute lor Geological Research on the Bushveld complex, university of Pretoria, 0083Pretoria, Republic of SouthAfrica ABsrnAcr de roches mafiques i ultramafiques qui constituenl lo complexeont 6t6 mises en place il y a 2O95(25\ m,a., pr€sque entidrement dans la couverture s6di- lff t of Ni metal, With resources of close ta 22 x m"ntaire peu d6form6e du craton du Kaapvaal' Bushveld Complex tle layered mafic rocks of the dont les $tructures gouvernent l'em'placementainsi repository of low can be considered the largest que la forme actuelle des diff6rentes infrusions. grade, Ni-sulfide deposits in the world. disseminated Deux -ug^a* ont prodrut la s6quencestratifi6e du are contained in several min- These vast resources complexe; Ie premier, i caracGre ultramaf,ique, a of which the Merenskv Reef. the eralized layers 6t6 suivi par plusieurs 6pisodes de maematisme UG2 chromitits layer and the Platreef are the most thol6itique. Ces magmas parentaux' pauwes en The four mafic to ultramafic compart- irportant. soufre, auraien! par incorporation de S, COz et HzO whicb constitute the layered sequence of the ments lib6r6s lors de rEactionsm6tamorphiques, donn6 les were emplaced. 2095!25 m.y. Bushveld Complex horizons min6ralis6s situ6s i la base du complexe. into a relatively undeformed sedi- ago, essedtially les hypothdsesvisant i. expliquer les con- cover of the Kaapvaal craton. The loci of On revoit mentary centrations de sulfures; la compaction de cumulats as well as the present-day configura' emplacement et la migration ascendantede liquides r6siduelsinter' various intrusions are controlled largely tion of the cumulus (qui constituaientplus de 2O/o d'un empi' structural features of the Kaapvaal craton. by the lement de cristaux cumulus) sont des processus Evidenco is presented in support of a bimodal importants de la formation des adcumulats et des the magmas that gave rise to the layered character of diff6rents types de peqmatitesmafiques. Ces mOmes the complex. The initial magma is con- sequen@ of liquides ont aussi influenc6 (1) la cristallisation des sidered to be ultramafic and to have been followed poches isol6es de magma prds de la base.de.. la by several heaves of magma of tholeiitic character. produisant ainsi le type de min6ralisa' parental of complex were ap- chambre. These magmas the tion du' banc Merensky et (2) la concentration parently poor that incorporation in the in sulfur so des 6l6mentsdu goupe du platine dans les niveaux magrna of S, COs and HzO liberated during meta- de chromitite de la zone critique sup6rieure. morphic reactions resulted in mineralized layers (Traduit Par la R6daction) near the floor of the intrusion. The hvpotheses to account for sulfide conoentration are reviewed. and ascending intercumulus liquids, Compaction INTRoDUcTIoN which constitute more than 20Vo of a pile of cu- mulus crystals,. are important processes in tle formation of adcumulates and the different types It is well known that the layered mafic rocks of mafic pegmatites. These liquids must also have of the Bushveld complex house the wor"ld's had an effect, firstly, on the crystalization of largest reserves of the platinum-grot'p tnetals, "isolated" batches of magma near the floor of the chromium and vanadiu,m (Willemse 1969, von give of chamber to rise to the Merensky Reef type Gruenewaldt 1977), estimated to be about 86, and, secondly. in tle concentration mineralization 83 and 64Vo, rcspe*tiv,e1y,by van Rensburg & of platinum-group elements in chromitite layers (L977). These authors consider the of the upper critical zone. Pretorius world's reserves of nickel to be approximately 50 x 100t and South Alricals share in these to be SoMlrerns 5 x 10' t, contained in the Merensky Reef, which is presently being min66 at several local- Vu ses r6serves de ?2 x 10ot de Ni, on peut con- ities. If, however, the estinated tonnages of sid6rer le complexe stratiforme du Bushveld comme nickel metal sontained in the UG2 chromitite gisements le plus vaste entrep6t de de sulfures de layer and tle Platreef (Table 1) are added to du monde. Ces ressources se nickel diss6min6s this figure, total nickel resources in the Bush- trouvent localisdes i plusieurs niveaux min6ralis6s, Complex amount to a substantial' 22 x LU t" dont les bancs Merensky et Platreef et la chromitite veld one of the largest UG2 sont les plus impor.tants. Ires quatre chambres making the Bushveld Complex ?33 234 TIIB CANADIAN MINERALOGIST repositories of low-grade disseminated nickel Tne SsrrrNc AND SHApE oF TrrE Cotupr,sx sulfide ores in the world. Nowhere in the corn- plex does nickel occur in sufficiently large Regional tectonic setting quantities to be exploited on its own; the con- The assemblageof well-exposed Precambrian tribution of the associated platinum-group rocks on the Kaapvaal craton displays a fairly metals, Au and Cu make these large dissemi- continuous record of the evolution of the crust nated strlfide deposits economically viable. from the early Archean, some 3600 m.y. ago, This review is confined essentially to tle to late Waterberg times, t1250 m.y. (Hunter latest thoughts and developments concerning the 1J74a, b). Detailed sttrdies on the many rock environmsal and mode of emplacement of the units that comprise the Kaapvaal sraton have complex, the composition of the magmas that shown that the evolution of the crust during gave rise to the layered mafic rocks of the this 2300 m.y. period was dominated by east- complex and the magrnatic sul,fide concentra- northeast and north-northwest structural trends tions contained within them. A basic knowledee and that these structures are also important of the various lithologies that comprise tie controls on the present configuration of the Bushveld Complex is assumed. For this infor- lithologies preserved on the craton. The east- mation, the reader is referred to the many northeast direction is especially well-developed review-type papers (Wager & Brown 1968, in the eastern part of the Kaapvaal craton and Willemse 1969, Hunter 1976, Vermaak & Lee is manifest in the prominent Murchison linea- 1978, Hunter & Hamilton l97S). Although in ment (Fig. 1), a site of repeated tectonic re- moet of these reviews an attempt was made to activation from Archean to late WaErberg cover all aspects and components of the Bush- times, as well as in several other linear east- veld Complex, the approach differs from review northeasterly-trending structures @utton 1973, to review; this is understandable when the large Hunter 1975). The north-northwesterly trend amount of literature on the Bushveld Complex is represented by tle so-called Amsterdam is considered. Well over 800 articles, tleses linea,ment and the Kraaipan trendo as well as and research reports are listed in two recent a broad north-northwesterly-trending arch that bibliographies (Molyneux et aJ. 1976, Knowles is the location of a number of domical base- t978). ment windows. Termed "Vryburg arch" by (1975), TABLEI. PNOMI]IOilSAI'ID RESOURCES OF P@ + AU, NI ANDCU Hunter this consists of a number of III iIINERAI.IZEOLATERS OF THEBTJSHIELO COWLTIT positive, dome-like features, the magnitude of lilEREl{'XY ,,r.r!ffr' which varied at different times during the depo- REET LAYER PLATRETF TOTAT sition of the sedimentary successionsthat over- lie the basement. At other times, however, PGE1 Au Proportlonsxx these positive areas also coincided with the 59 41 42 axes of maximum deposition of sedimentary Pd 25 34 46 basins (Brock & Pretorius t964). Rr t2 4 Rh 3 9 3 According to Hunter (L975), the develop- Ir I t.9 0.8 ment of the Kaapvaal craton, rigid enough to 0s 0.8 0.6 sustain and preserve ensialic basins, took place tu 3.2 0.4 3.4 about 3000 m.y. ago. On this crust a series of Redery grade 5.5 6.9 3.0 9n/t distinct depositional basins developed, the basin Res@rces. t8.15 32.50 12.24 62.89 kg r l0o axes and northern flanks of which migrated across the craton .from southeast to northwest l{l (Anhaeusser 1.973, Hunter 1974b). In these ConEnt , 0.18 varlable variable kr@ery grade 0.7 3.6 basins the sedimentary and volcanic rocks of, Kg/l in ascending order, the Pongola, Dominitrm Res@rces- 4.3 3.8 14,7 tx 10o Reef-Witwatersrand, Ventersdorp, Transvaal and Waterberg Supergroups accumulated. Sedi- Content z 0.ll l0 varlable mentological studies suggest that, in general, ns@ery grode 0.8 1.8 the southern kg/t margins of these basins reflect t"'fl"i3u 7.30 9.95 gradual subsidense in contras,t to the more aotive northern flanks, and that transport di- xrvalues \fter von Gruen*aldt (1977); bNen fM Co6ins I Vemak fi976). rections werg except for local deviations, mostly from the northern and northeastern sides of the basins (Brock & Pretorius 1964, de Villiers 1967, Visser 1969, Button 1973, 1975). SULFIDES IN TTIB BUSTTVBLD COMPLBX 235 THE STRUCTURALSETTING OF THE BUSHVELDCOMPLEX o l@ eoo LEGEND ...-..-.htdddof d€d 2tt@ !ffittcd i) a*rm O o*tty nqi a @tur .F-- b tbg'dft bts -.-.- k@*y Rd BASEI{ENTDOTES ! Chsi66t 8 V@d€fod 2 lrolope 9 Klsl€dop 3 Mdblo Holl lO filolrl&q 4 Domllt@ ll irobpo 5 \k@ 12 Gabqong 6 Joh@6hrg 13Mdo@o 7 Dq6 14 C@@dil3Ris Fro. 1. The structural settlng of the Bushveld Complex. An explanation for the development and the inflation caused by subcrustal thermal expanqion. migration of tle sedimentary basins with the During such times the lithosphere would have associated volcanic activity was put forward by been in a state of tension and cond,itions would Pretorius (1965), who considers the basins to have prevailed favoring the formation of dykes' have developed under tensional conditions in In soitrast, the Bushveld Complex was intrtrded an environment where vertical tectonics pre- at the termination of Transvaal sedimentation, vailed.
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