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Platinum-Group Element Mineralization of the Main Sulfide Zone, Great Dyke, Zimbabwe

By
Thomas Oberthür
Thomas Oberthür
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Published:
January 01, 2011

Abstract

The Great Dyke of Zimbabwe hosts the world's second largest reserve of platinum-group elements (PGE). Economic PGE mineralization is restricted to sulfide disseminations mainly in pyroxenites of the P1 layer, the Main sulfide zone, which is currently under extensive exploration. The producing Ngezi and Mimosa mines are the lowest cash-cost platinum mines in the world.

The Main sulfide zone has a fine structure made up by a number of successive, geochemically distinct layers and typical vertical element distribution patterns characterized by a general upward zoning sequence in the order → Pd Pt → base metal sulfides. With some overlap, a number of sublayers can be distinguished in the PGE subzone and in the base metal sulfides subzone of the Main sulfide zone. These layers and the element decoupling patterns are regarded to represent first-order, primary magmatic features of sulfide accumulation and concomitant scavenging of PGE in relationship to their different (and probably variable) sulfide/silicate partition coefficients.

The PGE are bimodally distributed in the Main sulfide zone: Large proportions of Pd and Rh are hosted in pentlandite, whereas Pt is dominantly present in the form of discrete platinum-group minerals (PGM). The distribution patterns of the various PGM within the Main sulfide zone suggest that a large fraction of the PGE, primarily concentrated in sulfide at magmatic conditions, was redistributed following the crystallization of sulfides in the subsolidus stage. PGE expelled from the annealing sulfides formed PGM with reactant partners like As, Te, and Bi, whose proportions and availabilities differ regionally. It is assumed that these reactions took place under largely isochemical conditions; however, chemical gradients within the Main sulfide zone and magmatic-hydrothermal fluids may have supported the small-scale redistribution of the PGE and the reactions that formed the PGM.

The current work on the Great Dyke emphasizes the role of sulfide generation and accumulation on PGE concentration which take place in the course of the magmatic evolution of layered intrusions. Sulfur saturation leading to sulfide segregation appears to be the most important factor in the primary magmatic concentration of the PGE. The enrichment of the economically most important Pd group PGE (PPGE) i.e., Pt, Pd, and Rh, is sulfide controlled. The geochemical offset patterns are regarded to reflect a first-order, dominantely magmatic control of the mineralization as these patterns are observed persistently over wide areas of the Great Dyke. In contrast, the different sulfide and PGM assemblages are viewed to represent second-order reaction products that came into existence downtemperature during annealing of the mineral assemblages. The variability of PGM assemblages appears to be controlled mostly by the presence of semimetals.

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Contents

Reviews in Economic Geology

Magmatic Ni-Cu and PGE Deposits: Geology, Geochemistry, and Genesis

Chusi Li
Chusi Li
Indiana University
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Edward M. Ripley
Edward M. Ripley
Indiana University
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Society of Economic Geologists
Volume
17
ISBN electronic:
9781629490243
Publication date:
January 01, 2011

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