A thesis submitted in fulfilment of the
requirements for the degree of
PhD in Geology
University of the Witwatersrand / Numerous small base-metal deposits occur in the acidic rocks of the Bushveld Complex, and
modern exploration programs are currently re-examining this metallotect in an attempt to refine
the current working hypothesis for mineralisation in these granites. The hypothesis proposed
for the origin of mineralisation is multifaceted, encompassing both spatial and temporal
relationships between at least three episodes of ore formation. The first episode of
mineralisation (typified by the Zaaiplaats tin deposit) occurred at relatively high temperatures
(>600'C to 4000' C), and resulted in the formation of orthomagmatic cassiterite, scheelite and
an early generation of fluorite. At lower temperatures (200°C<T<400°C), where processes
were essentially fluid dominated, a mesothermal Cu-Pb-Zn-As-Ag-Au assemblage was
deposited (exemplified by the Spoedwel, Boschhoek and Albert copper and silver deposits). A
third episode of mineralisation resulted in the formation of an Fe-U-F assemblage and is
recognised at several, but not necessanly all, of the deposits examined (for example, the Albert
silver deposit). The extended nature of this three-stage paragenetic sequence is considered to
reflect widespread mixing between an early fluid derived by H20-saturation of the granitic
magma and an external meteoric/connate fluid, circulation of which was stimulated by the long-lived
high heat-productive capacity of the Bushveld granites, as well as exhumation of the
metallotect;
The early high-temperature Sn/W assemblage was precipitated while magmatic fluids
dominated the system. With time, the pluton cooled and was subject to regional uplift.
Fractures developed, acting as conduits for external fluids of meteoric and/or connate origin.
The late magmatic fluids, enriched in incompatible metals (and volatiles), interacted with the
latter fluid, resulting in the localised precipitation of a secondary, lower-temperature mineral
assemblage (Cu-Pb-Zn) in the zone of fluid mixing. As the external fluid component became
progressively more dominant, the paragenesis changed, forming the :final Fe-U-F assemblage.
The formation of these three different, temporally separate assemblages is adequately explained
in terms of a fluid mixing model, wherein the concentration ofmetaIs and localisation of ore
deposits are controlled by lithology and structure. / Andrew Chakane 2018
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/24690 |
| Date | January 1998 |
| Creators | Freeman, Lauren Anne |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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