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Nature and longevity of hydrothermal fluid flow and mineralisation in granites of the Bushveld Complex, South Africa

  • L. J. Robb (a1), L. A. Freeman (a1) and R. A. Armstrong (a2)

Abstract

The Lebowa Granite Suite of the Bushveld Complex is a large, 2054 Ma old, A-type batholith, characterised by numerous relatively small magmato-hydrothermal, polymetallic ore deposits. The mineralisation is represented by a three-stage paragenetic sequence: early magmatic Sn-W-Mo-F ores (600°C > T > 400°C), followed by a Cu-Pb-Zn-As-Ag-Au paragenesis (400°C > T > 200°C) and then late-stage Fe-F-U mineralisation (< 200°C). The first stage of mineralisation (typified by the endogranitic Zaaiplaats tin deposit) is related to incompatible trace element concentration during crystal fractionation and subsequent fluid saturation of the magma. Evolution of the late magmatic fluids as they were channelled along fractures, as well as mingling with externally derived connate or meteoric fluids, resulted in the deposition of the second stage of mineralisation (typified by the fracture-related, endogranitic Spoedwel and Albert deposits and the exogranitic, sediment-hosted Rooiberg mine) which is dominated by polymetallic sulphide ores. As the externally derived fluid component became progressively more dominant, oxidation of the polymetallic sulphide assemblage and precipitation of hematite, pitchblende and fluorite occurred generally along the same fracture systems that hosted the earlier sulphide paragenesis.

Small hydrothermal zircons trapped along quartz growth zones from the Spoedwel deposit yield a U-Pb concordia age of 1957 ± 15 Ma. Whole-rock Rb-Sr age determinations from the Lebowa Granite Suite fall in the range 1790 ± 114 Ma to 1604 ± 70 Ma and are interpreted to reflect alkali element mobility and isotopic resetting during exhumation of the Bushveld granite. In contrast to thermal modelling which indicates that hydrothermal activity should have ceased within 4 my of emplacement, isotopic evidence suggests that mineralisation was long-lived, but episodic, and that fluid flow events were linked to major periods of Palaeo- and Mesoproterozoic orogenic activity along the margins of the Kaapvaal Craton. During these orogenic episodes, fluid flow was enhanced by tectonically induced fluid over-pressuring and/or exhumation of the Bushveld Complex.

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