Exploration for magmatic Ni-Cu-PGE sulphide deposits: A review of recent advances in the use of geochemical tools, and their application to some South African ores

Abstract

Most major magmatic Cu-Ni-PGE sulphide deposits are thought to have formed by segregation of an immiscible sulphide melt from a silicate host magma, in response to processes such as magma mixing, rapid cooling, differentiation, and contamination. The metal content of the sulphides is governed by the concentration of the metals in the silicate host magma, the sulphide melt/silicate melt partition coefficients (D values) of the metals, and the R-factor during sulphide segregation. Fractionation between the metals occurs during partial melting of the upper mantle source, crystallization of oxides, platinum-group minerals (PGM), and silicates (mainly olivine and less so orthopyroxene), segregation of sulphide melt, and crystallization of the sulphide melt. The latter process may yield zoned ore bodies consisting of Os, Ir, Ru, Rh, Fe, (Ni)-rich monosulphide solid solution (mss) cumulate ore and fractionated sulphide ore rich in Cu, Pt, and Pd. It is possible to model these processes and thereby to estimate the potential of a magmatic body to host economic Cu-Ni-PGE sulphide deposits. The location of Cu-Ni-PGE sulphide ores may be facilitated by applying a number of geochemical tools. PGE-rich horizons within layered intrusions are particularly difficult to locate because the ore zones are generally thin compared to the thickness of the intrusions. Variation in Cu/Pd ratios of the silicate rocks may delineate the position of some of these horizons, since the strongly chalcophile Pd is preferentially depleted during sulphide segregation, resulting in an increase in Cu/Pd of the subsequently crystallizing overlying cumulates. Cu/Pd ratios may also be applied in sill-like bodies such as the Uitkomst intrusion, Mpumalanga, to estimate the potential for conduit-type deposits, and in lavas where they may help to locate possible Noril'sk-type mineralized feeder zones to basalt flows. Other methods of determining whether igneous bodies have experienced magma mixing and sulphide segregation are based on Ni contents of olivines, Ti contents, and Cr/Fe ratios of spinels, and petrographic features such as the presence of plagioclase inclusions within ferromagnesian phases. Se/S ratios may help to determine whether magmatic sulphide ores underwent post-magmatic (metamorphic or hydrothermal) sulphur loss. They may also discriminate between a magmatic or sedimentary source of the S, and between a magmatic or hydrothermal origin of the metals. Hydrothermal sulphide ores may further by distinguished from magmatic ores by means of significantly higher Cu/Ni and Pd/lr ratios of the former. Such distinction is important because it would be futile to investigate hydrothermal deposits for magmatic ore zonation.