Mineral. Deposita 22, 227-235 (1987) MINERALIUM DEPOSITA © Springer-Verlag 1987 Hydrothermally altered peraluminous Archaean granites as a provenance model for Witwatersrand sediments R. Klemd 1 and D. K. Hallbauer 2 1 Geology Dept., P.O. Box 524, Rand Afrikaans University, Johannesburg, 2000, Republic of South Africa 2 Chamber of Mines of South Africa Research Organization, P.O. Box 91230, Auckland Park 2006, Republic of South Africa Abstract. Hydrothermal alteration of peraluminous gra- nitic rocks, occurring at Varkenskraal, northwest of the Witwatersrand Basin, consists of two major styles. The first style, pervasive in nature, is represented by two stages: i) A propylitic alteration and ii) A disseminated sericitization. Both alteration stages were accompanied by a deposition of disseminated sulphides. The second style is represented by a vein related alteration in four stages including potassium feldspar, minor carbonate and minor sulphide mineralization. Nodules of uraniferous carbonaceous matter and small amounts of particulate gold could possibly be associated with either one or both styles. Hydrothermally altered rocks of similar nature have been observed in a number of other localities close to the Witwatersrand Basin. Their abundance, large volumes and low-grade, disseminated pyrite/gold mineralization indi- cate that rocks of this type could have been the provenance of the Witwatersrand gold-bearing sediments. One of the striking features of the Witwatersrand Basin, which contains the world's largest concentrations of gold and uranium, is the predominantly arenaceous nature of its several thousand metres of accumulated sediments. Al- though argillaceous layers, such as the Khaki Shale in the Welkom Goldfield of the Basin, occasionally form marker horizons, they nevertheless represent a small part of the overall composition of the Central Rand Group succession of sediments in which quartz accompanied by minor sericite and chlorite abound. The interbedded gold and uranium bearing conglom- erate beds are dominated by milky or greyish coloured quartz pebbles. The heavy mineral assemblages contained in these layers are comprised of over ninety per cent pyrite with varying amounts of arsenopyrite, cobaltite, molyb- denite, occasionally sphalerite, uraninite, chromite, zircon. rutile, tourmaline, gold, and small amounts of platinum group minerals as the most common detrital mineral species. In the absence of any substantial and conspicuous pre- Witwatersrand gold deposits in the immediate vicinity of the Witwatersrand Basin, the Archaean Barberton gold deposits have been suggested as a provenance model for the Witwatersrand deposits (Viljoen etal. 1970). The detrital components in Witwatersrand sediments, however, exhibit many features which not only point to proximal sources (Hallbauer 1982) but are also difficult to reconcile with characteristics of minerals from the Barberton area. Considerations arising from research on the heavy mineral assemblage in Witwatersrand conglomerates therefore led to the proposal that granitic rocks, or perhaps low-grade porphyry-type deposits, were the major primary sources for these sediments (Hallbauer 1982). Recently a number of mineralogical and geochemical similarities have been found between gold, sulphides, and other heavy minerals occurring in granitic rocks along the perimeter of the Witwatersrand Basin and those in Wit- watersrand conglomerates (Hallbauer etal. 1986). The association of gold, copper, tungsten and other metals with granitic rocks has long been known, and a large number of data concerning the origin of the mineralization in plutonic rocks is available. The mineralization is often found to be directly related to hydrothermal alteration. Thus, hydrothermally altered Archaean granites (sensu lato) of the Western Transvaal were investigated as a possible source of at least some of the mineralization found in the Witwatersrand gold fields (Hallbauer 1984; Robb and Meyer 1985). In this paper a detailed petrographical and mineral/ chemical study of fresh drill core of hydrothermally altered granites of the Varkenskraal area (Fig. 1) is presented as an example of several known, similar plutons. Prior to this work, a broad petrographical and geochemical study of granitic rocks of the Western Transvaal was conducted by Breitkreuz (1985). The granitic rocks which are the subject of this study, are exposed over an area of some 10kin 2 south of the Klerkskraaldam west of Johannesburg (Fig. 1). They appear to be part of a far larger pluton (Klemd et al. 1986). The strongly weathered and locally mylonitised rocks are partly overlain, unconformably, by post-Witwa- tersrand Black Reef conglomerate and Ventersdorp lavas, in some areas by pre-Witwatersrand rocks of the Dominion Group. The age of the Ventersdorp lavas is not yet precisely known, and ages ranging from approximately 2,300Ma (Van Niekerk and Burger 1964) to about 2,700 Ma (Crockett 1971; Van Niekerk and Burger 1978) have been reported. For the granitic rocks at Varkenskraal no age dating has yet been undertaken so far. However, sericite in the 'palaeosol' which overlies the Archaean granites displays a Rb/Sr model age of about 2,370 Ma (Klemd et al. 1986), presumably dating the time ofsericite formation.