The concentration of PGE in the Earth s crust with special reference to the Bushveld Complex.

The Earth's mantle is the principal reservoir from which platinum-group element(PGE)concentrations in the crust are derived.The transfer of the PGE is accomplished by two main methods,first the development of mantle partial melts and their intrusion into the crust,and second the emplacement of mantle slAbs in the subduction/collision zones.The first mechanism is far more important than the second.Once in the crust,a number of mechanisms serve to concentrate the PGE sufficiently and they can be exploited economically as the principal product,rather than as a by-product.These include(i)the development of an Ni-Cu sulfide liquid in a mafic intrusion,the concentration of this liquid,followed by cooling and fractional crystallization that results in a residual sulfide liquid highly enriched in Cu,Pt,and Pd;(ii)the formation of layers of very high-PGE tenor sulfides at specific horizons within a layered intrusion,either with or without associated chromitite;(iii)the emplacement of magma carrying PGE-rich sulfide along the margins of layered intrusions;(iv)the delayed separation of immiscible sulfides until the late stages of the differentiation of a layered intrusion;(v)chromite crystallization without the development of sulfide immiscibility;(vi)hydrothermal redistribution and concentration of PGE from zones of low grade disseminated sulfide;(vii)secondary concentration of PGE along with chromite during recrystallization of Ural-Alaskan intrusions and the subsequent development of placer deposits during the weathering of these bodies;and(viii)the concentration of Pt during the formation of black shale deposits.The Bushveld Igneous Complex of South Africa hosts 75% of the world's resources of Pt,54% of Pd resources,and 82% of Rh resources,and contains examples of mineralization formed by processes(ii),(iii),(iv),(v),and(vi)listed a little earlier in the article.Of these,process(ii)accounts for 90% of the current economic reserves and resources,and type(iii)for 9%.The Merensky Reef(32% of total resources)is a PGE-enriched horizon that contains 1-3 thin seams of chromite,and an average of 1-3 wt% sulfide,across the mining width.The sulfides are thought to have been the principal collectors for PGE.The Reef results from two or more influxes of hot,sulfide-bearing,mafic magma that give rise to the horizon.The thickness of the ultramafic cumulates(mainly orthopyroxenite,although includes peridotite in some areas)as a result of these influxes and varies from 50 cm to several meters,although mining is usually focused on a zone that is rarely greater than 1 m in thickness.The genesis of the Reef is still debated,some arguing that the PGE have been concentrated from below by ascending hydrothermal fluids,and others arguing that they have been carried from above by sulfides settling from the magma,giving rise to the Merensky pyroxenites.What is clear is that the pyroxenite,norite,and anorthosite overlying the Reef are composed of minerals derived from two magma types,one rich in MgO(-12 wt%)and Cr and poor in Al2O3(-12 wt%)and the other with the composition of a typical tholeiite.The UG-2 chromitite accounts for 58% of the economic resources,and comprises of a chromitite seam 60 cm-1 m thick(sometimes divided by an internal parting of pyroxenite)and 1-3 overlying thinner seams of chromite.The sulfide content of UG-2 is significantly lower that that in the Merensky Reef,ranging from 0.5 to 1.5 wt%,although the sulfides are thought to have played a role in the concentration of at least some of the PGE.There are up to 13 chromitite horizons below that of the UG-2,and all contain PGE,although the total PGE contents and the(Pt+Pd)/(Ru+Ir+Os)ratios are much lower than those of UG-2.High 87Sr/86Sr ratios found within the pyroxenite "partings" within UG-2 suggest that mixing with melted roof rocks may have played a role in causing both chromitite and sulfide to form.The Platreef is the main example of type(iii)mineralization and currently accounts for 9% of the total resources,although active exploration is occurring along this zone and this proportion will probably rise in the future.The Reef is much thicker than the Merensky Reef and UG-2,and is currently mined open and cast over a thickness of about 50 m.The Platreef is zoned,ranging from an upper,orthopyroxene cumulate to a lower zone of pyroxenite,feldspathic pyroxenite,and norite that is interacted strongly with shale,iron formation,and dolomite sediments forming the immediate footwall.In this article,it is suggested that the Platreef is the consequence of several surges of magma that were responsible for different units,including the UG-2 and Merensky Reef,within the main chamber of the Bushveld.These magmas were displaced and exited up the walls of the chamber in response to new influxes of magma entering the main chamber.Cylindrical,zoned pipes of ultramafic rock containing very high Pt grades cut cumulates in the lower part of the Bushveld complex,and were thought to be the consequence of hydrothermal remobilization.None are currently in production,and they constitute a historic PGE resource that never contributed significantly to the overall resources of the complex.