Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.583981
Title: Roles of magmatism, contamination and hydrothermal processes in the development of Platreef mineralization, Bushveld Complex, South Africa
Author: Holwell, David
ISNI:       0000 0003 5238 4598
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2006
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Abstract:
The Platreef is a highly complex, pyroxenite-hosted Ni-Cu-PGE deposit. It is located at the base of the northern limb of the Bushveld Complex, South Africa, in direct contact with a variety of country rock sediments and Archaean basement. The interaction of the Platreef magma with these diverse country rock lithologies during emplacement had a profound effect on the style and distribution of the mineralization on both regional and local scales. Geometrically, the Platreef was emplaced as a thin, sill-like intrusion, with pre-formed PGE- rich sulfide droplets entrained within the magma. Sulfur saturation is likely to have occurred in a deep staging chamber or conduit prior to emplacement, and where immiscible sulfide droplets became enriched in PGE, base metals and semi metals. After emplacement, the PGE and semi-metal rich sulfide liquid cooled to form 'primary' assemblages of IPGE-rich pyrrhotite, IPGE-, Rh- and Pd-rich pentlandite, chalcopyrite, Pt and Pd tellurides and bismuthides and electrum within the feldspathic pyroxenites of the Platreef. Assimilation and metamorphism of some sedimentary footwall rocks, particularly the dolomites of the Malmani Subgroup, released large volumes of volatiles into the Platreef magma. This hydrothermal activity redistributed PGE and base metal sulfides (BMS) into the footwall rocks, and in places overprinted the 'primary' assemblages, and occasionally decoupled PGE from BMS, with the petrology of the reef and footwall, and the mineralogy of the platinum-group minerals, significantly affected. Each locality along the strike of the Platreef with a different footwall lithology has its own unique hydrothermal history directly related to the nature of the local floor rocks. For example, where the floor is composed of anhydrous basement gneiss, volatile activity was relatively insignificant, and partial melting of the floor allowed the percolation of PGE-rich sulfide liquid to penetrate the footwall. The nature of the floor rocks also controls the type and amount of contamination in the Platreef. Sulfides in country rock shales, for example, are assimilated into the Platreef magma and locally upgrade the S content at such localities, although this did not trigger S saturation. Footwall sulfates, such as anhydrite, cannot be assimilated, but can interact with the Platreef sulfides through hydrothermal leaching when sufficient fluids had been released during assimilation. After emplacement of the Platreef, a significant period of cooling occurred, such that the Platreef was almost completely crystallized, during which time some ductile deformation occurred. The gabbronoritic hangingwall magma was then emplaced, forming a magmatic unconformity over the Platreef, occasionally exploiting shear zones to intrude finger-like bodies down into the Platreef. Where the hangingwall magma assimilated mineralized Platreef thin zones of PGE mineralization developed at its base. The magmatic intrusion of the Platreef can be considered to be distinct from that of other magmatic units in the northern limb of the Complex. Its Ni-Cu-PGE sulfide mineralization is orthomagmatic in origin however, complex interaction with a variety of country rock lithologies has locally altered the style and distribution of the mineralization, to form unique mineralogical associations and assemblages along strike. The understanding of local country rock control on features such as the mineralogy of the ores and the extent of remobilization into the footwall is critical in optimizing exploration, mining and mineral processing techniques.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.583981  DOI: Not available
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