Abstract

Recent revisionist criticism of the modified placer theory of the origin of the Witwatersrand ores is singularly unconvincing. In response to that criticism it is shown that the so-called enigma of the sulfidation of black sands is a myth; sulfide assemblages viewed as unusual are quite plausibly attributed to a source effect involving detrital transport; prolonged weathering and recycling under oxygen-deficient atmospheric conditions have favored preservation of detrital sulfides, native elements, and uraninite, although many of these minerals have been reconstituted to varying degrees as a result of diagenesis and metamorphism; a supposed example of structural control of mineralization is invalid.Evolution of the mineralogy of the Witwatersrand ores is intimately linked to extensive reworking and recycling of constituents derived from deep weathering of source rocks. These rocks are composed of about 60 percent arenaceous sequences, 15 to 20 percent greenstone belt rocks, and 20 to 25 percent metamorphic and felsic igneous rocks of the sialic crust. Revisionist arguments for a widespread hydrothermally-induced metasomatic origin of reported Precambrian paleosols associated with the ore-bearing conglomerates are shown to be ill-founded and wrong. In summary, revisionist criticism of the modified placer hypothesis proves to be, at best, premature. The placer hypothesis, as presently modified, is alive and well, enjoying a formidable predictive capacity as the favored working hypothesis.

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