Laurence J. Robb

The Witwatersrand Basin, South Africa: Geological framework and mineralization processes

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.

A review of the geochronological constraints on the evolution of the Kaapvaal Craton, South Africa

Abstract All the published U–Pb geochronological data from zircon, titanite, sphene and monazite, and Pb–Pb evaporation data applicable to the Palaeoarchaean-to-Neoarchaean geology of the Kaapvaal Craton are employed to reconstruct the history and crustal architecture of the ca. 3600–2500xa0Ma cratonic basement in South Africa. Only data interpreted as representative of the age of crystallisation, or the 207 Pb / 206 Pb ages from zircon xenocrysts, have been considered in constructing graphs of cumulative probability versus age of rock formation. In this manner, it has been possible to identify the main magmatic events that have contributed to the assembly of the craton. For convenience of data handling, the Kaapvaal Craton has been subdivided into eastern, central, northern and western geographic domains. These domains are not intended as discrete geological terranes although each one has its own geological and chronological character, although consanguinity of events is also evident. The oldest rocks so far recognised are located in the Swaziland–Barberton areas where ages >3600xa0Ma have been recorded. The early stages of shield development are best exposed in the Barberton Mountain Land where it is now apparent that continent formation took place by magmatic accretion and tectonic amalgamation of small protocontinental blocks. At Barberton, several diachronous blocks, formed between 3600 and 3200xa0Ma, have been identified, each of which represents a cycle of arc-related magmatism and sedimentation. This phase of crustal development was followed by a period of Mesoarchaean cratonic magmatism, particularly prevalent between approximately 3100–3000xa0Ma, and marked by the development of a major, crescent-shaped, juvenile arc that was accreted onto the northern and western margins of the evolving Kaapvaal Shield. Cratonisation was accomplished by the emplacement of major granitoid batholiths, which thickened and stabilised the continental crust during the early stages of this cycle. Subsequent evolution of the craton, between 3000 and 2700xa0Ma, was associated with continent–arc collision, during which time the Witwatersrand Basin and its correlatives were deposited as foreland sequences, followed by episodic extension and rifting, when the Gaborone–Kanye and Ventersdorp sequences were developed.

U-Pb ages on single detrital zircon grains from the Witwatersrand Basin, South Africa: Constraints on the age of sedimentation and on the evolution of granites adjacent to the basin

U-Pb ages of single detrital zircon grains from various stratigraphic horizons in the Dominion and Witwatersrand sequences provide constraints on the maximum age of sedimentation as well as indicating the pattern of age distribution in the (granitoid) source area providing detritus into the basin. Zircon ages in the Dominion sediments range from 3191-3105 Ma with a geometric mean (documentclass{aastex}nusepackage{amsbsy}nusepackage{amsfonts}nusepackage{amssymb}nusepackage{bm}nusepackage{mathrsfs}nusepackage{pifont}nusepackage{stmaryrd}nusepackage{textcomp}nusepackage{portland,xspace}nusepackage{amsmath,amsxtra}nusepackage{wasysym}npagestyle{empty}nDeclareMathSizes{10}{9}{7}{6}nbegin{document}

Geology, geochemistry and isotopic characteristics of the Archaean Kaap Valley pluton, Barberton Mountain Land, South Africa

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U–Pb and Pb–Pb isotopic studies relating to the origin of gold mineralization in the Evander Goldfield, Witwatersrand Basin, South Africa

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Three examples illustrating the analysis of organic matter associated with uranium ores

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