ORE GEOIDGY REVIEWS ELSEVIER Ore Geology Reviews 10 (1995) 67-94 The Witwatersrand Basin, South Africa: Geological framework and mineralization processes Laurence J. Robb *, F. Michael Meyer i Department of Geology, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa Received 19 December 1994; accepted 19 June 1995 Abstract The Witwatersrand Basin formed over a period of 360 Ma between 3074 and 2714 Ma. Pulses of sedimentation within the sequence and its precursors were episodic, occurring between 3086-3074 Ma (Dominion Group), 2970-2914 Ma (West Rand Group) and 2894-2714 Ma (Central Rand Group). Detritus was derived from a mixed granite-greenstone source of two distinct ages; the first comprises Barberton-type greenstone belts and granitoids > 3100 Ma old, and the second consists of the greenstone belt-like Kraaipan Formation and associated granitoids < 3100 Ma old. Subsequent granitoid plutonism was episodic and coincided with hiatuses in sediment deposition, but continued throughout the evolution of the basin. Many of the provenance granitoids are characterized by hydrothermal alteration, are geochemically anomalous with respect to Au and U, and may represent viable source rocks for palaeoplacer mineralization. Tectonically, the basin evolved in response to processes occurring within a Wilson cycle, associated with the encroachment and ultimate collision of the Zimbabwe and Kaapvaal cratons. Metamorphism of the Witwatersrand Basin occurred at ca. 2500, 2300 and 2000 Ma. The first two events coincided with the progressive loading of the basin by Ventersdorp and Transvaal cover sequences, whereas the last reflects intrusion of the Bushveld Complex and/or the Vredefort catastrophism. Mineralization is concentrated in the conglomerates of the Central Rand Group and is represented by a complex paragenetic sequence initiated by early accumulation of detrital heavy minerals. This was followed by three stages of remobilization caused by metamorphic fluid circulation. An early event of authigenic pyrite formation at 2500 Ma was followed at 2300 Ma by maturation of organic material, fluxing of hydrocarbon bearing fluids through the basin and the radiolytic fixation of bitumen around detrital uraninite. This was followed at around 2000 Ma by peak metamorphism which resulted in the widespread redistribution of gold and the formation of a variety of secondary sulphides. Post-depositional fluid conditions were such that metal solubilities were low and precipitation mechanisms very effective, resulting in the superimposition of both primary and secondary mineralization. 1. Introduction not often repeated in the course of geological history. Crustal processes occasionally interact at a particular The major mineral deposits of the world form as a point on the earth's crust in such a way as to form metal result of a fortuitous combination of events which is concentrations that are virtually unique, examples of which might include Sudbury, the Zambian copper belt, * Corresponding author. the Viburnum trend, the Bushveld Complex, etc. The t Present address: lnstitut f'tir Mineralogie und Lagerstattenlehre, Witwatersrand Basin is a classic example of where a Rbeinisch-Westfalische Technische Hochschule, 52056 Aachen, variety of geological processes have coincided in time Germany. 0169-1368/95 / $09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10169-1368(95)00011-9 68 L.J. Robb, F.M. Meyer~Ore Geology Reviews 10 (1995) 67-94 and space to produce the world's greatest gold prov- minor rudaceous, iithologies deposited in a fluvio-del- ince. In just over 100 years of mining, about 45 000 taic environment in the centre of the Kaapvaal Craton tons of gold (more than 35% of all the gold mined in (Fig. 1 and Fig. 3). Until recently it was considered to the history of mankind; Handley, 1990) and over be early Proterozoic in age and, consequently, to have 150 000 tons of uranium have been produced from the developed subsequent to the formation of the craton. Witwatersrand Basin. As will become clear in this review the basin is now The nature of the geological processes which con- viewed as an integral part of the Kaapvaal Craton, hav- tributed to this prodigious accumulation of metals has ing formed during the latter stages in the development been much debated ever since the commencement of of what is now known to be one of the most extensively mining activities, in the area which was to become mineralized segments of the earth's crust• Johannesburg, in 1886. Controversies such as the tim- ing of mineralization and the tectonic framework for 2.2. Age constraints basin deposition, as well as uncertainties in the age of sedimentation, the limits of the basin and the source of Even as recently as 1986 the Witwatersrand Basin gold and uranium, have restricted geological under- was believed to have been deposited during the early standing in the region• Over the past decade, however, Proterozoic era, between about 2.6 and 2.3 Ga (Allsopp a number of advances have contributed to resolving and Welke, 1986)• Age constraints depended largely many of the problems of Witwatersrand metallogene- on the limited availability of whole rock Rb-Sr and sis. The purpose of this review is to highlight some of Pb-Pb isotope determinations carried out on ante- and the pertinent features relating to the geological envi- supercedent rock formations, which have since proved, ronment and mineralization processes in the Witwa- in most cases, to reflect a resetting event that has been tersrand Basin, drawing especially on research carried superimposed on to the rock system and not the age of out in recent years. formation. Recent work has utilized precise, single zir- con U-Pb dating techniques in order to accurately con- strain the true age of Witwatersrand deposition; these 2. Geological framework results now show that the basin is Archaean in age and was deposited over an extended period of 360 Ma. 2.1. The Kaapvaal Craton A summary of recently acquired U-Pb zircon age data (after Barton et al., 1989; Robb et al., 1990a, b, The Kaapvaal Craton is the name given to the ancient 1992; Armstrong et al., 1991) is presented in Fig. 1 segment of continental crust which formed in southern and summarized below. Witwatersrand sediments Africa between about 3.7 to 2.7 Ga. Much of this con- overlie, either the Archaean granite-greenstone base- tinental nucleus actually formed prior to 3.1 Ga by the ment, or the volcano-sedimentary Dominion Group. formation of an extensive granitoid basement and amal- The maximum age of deposition is, therefore, con- gamation with arc-like oceanic terranes represented by trolled by the ages of these units. Several basement mafic/ultramafic volcanics and associated sediments granitoids which stratigraphically underlie Witwaters- (De Wit et al., 1992). Subsequent growth of the craton rand strata have now been dated and range in age was accompanied by further continental magmatic between 3174+9 and 3086+3 Ma. Volcanics in the activity, possible Cordilleran-style accretion of com- upper part of the Dominion Group yield an age of posite terranes along the margins of the proto-conti- 374 + 6 Ma and this figure represents the maximum nent, and the deposition of appropriate sedimentary available constraint for onset of Witwatersrand depo- basins. Although the question of when the Kaapvaal sition. Dominion sediments were deposited over a rel- continent attained its cratonic rigidity is a semantic one, atively short interval, between 3086 5:3 Ma (the age it is clear that by 2.7 Ga, or shortly thereafter, this of the Westerdam granite, Fig. 2, on top of which segment of crust was not subjected to further major Dominion sediments have been deposited) and orogenesis. The Witwatersrand Basin is an approxi- 3074 + 6 Ma, the age of overlying lavas. Detrital zircon mately 7000 m thick terrigenous sequence comprising and monazite grains extracted from Dominion sedi- mainly arenaceous and argillaceous, together with ments range in age between 3191 and 3105 Ma. The L.J. Robb, F.M. Meyer~Ore Geology Reviews 10 (1995) 67-94 69 ,'T! I i t • .:.::.'/...:/:.-~-.'/-::::" ".'.'...'.:.:.':":.:.::..":.:: ~ ':?~'..","...~i::--."i:-: - ~'I-:..-..'?."?:T-:..':;::,.:'.::.::.:.'.:::: I ';. "'. "'. ".'. ":. ".'0~ o • "."';" "', ";. "" r~ ;'.;':.'.':::.'":.':.': oo .:. • ". ":. • ", ,;.. • r~ • ...'.;,°-'.-'.,'.-'.,%,°. .-:..;.:;.-;,, v .::.::.:.',::.::.::. :',.".:.'.::..::..:::: > ..= ~u ,~::...:.::.:-...:.-. = I1-~ ~.~-.'.-..':::.::." Z ~_ ~.~.~-.:.: .o,~ i (~ -,.,..%,-,,-..-.,-,.,..,,,,.,-,..,,- L~ ,~ ~ ~ o~ ,~ ":-'"""'":'-'"::":::"':':""":':::':': -- I ~ ~ 0 e- L.jt.a ~ l < • I--L)- I z e- o 0o. m_ - '. • ~ ~ ~ 0 ~ zl.,j 8 ILl Z z ~ • o • ~,o ~ ~ ,, 8 ~3 .= • • • ~ | * • ~e il " " II fl II I 70 L.J. Robb, F.M. Meyer/Ore Geology Reviews 10 (1995) 67-94 of the 30 detrital grains independently analysed from the West Rand Group, none is younger than 2914 Ma, a feature which provides credibility to the age of the Crown lava. Detrital zircon grains from conglomerate reefs in the 72 q auriferous Central Rand Group become progressively z ~ g younger upwards in the stratigraphy and the youngest grains occur in the Elsburg and Ventersdorp Contact B ~ (VCR) reefs (2894+ 10 and 2780+5 Ma, respec- tively). The VCR must, therefore, have been laid down sometime after 2780 Ma ago, but before or at 2714 Ma which is the age of the Ventersdorp lavas (Armstrong et al., 1991). Central Rand Group deposition is more ~5oo'~,oo '~3'oo'32oo 31oo ~ooo 29o0 2800 ~oo difficult to constrain because the youngest detrital AO[ (Ma) Fig. 2. Frequency histogram showing distribution of single, detrital grains occur in rocks high up in the succession. If it is zircon and monazite ages from the Dominion and Witwatersrand assumed that the igneous (granitic?) rock within which sequences (data from Barton et al., 1989, Robb et al., 1990a, Robb the zircon formed was emplaced prior to the pulse of et al., 1990b). erosion and sedimentation which followed exhumation of that rock, then the Central Rand Group may have latter date provides an independent assessment of the been deposited subsequent to 2894 Ma (the youngest maximum age of deposition for the Dominion Group.