A comparison of trace element geochemical characteristics between the Platreef deposit of Bushveld Complex and the Jinchuan Cu-Ni-PGE sulfide deposit and its significance

Abstract：Major and trace element geochemistry of rocks and ores from the Platreef deposit of the Bushveld Complex was systemically studied in this paper by using new ICP-MS analytic method for the purpose of making a comparison with geochemistry of the Jinchuan Cu-Ni-PGE sulfide deposit and probing into the genesis of this type of deposits so as to guide their prospecting work. It is concluded that ∑REE values of the Platreef rocks are close to values of lherzolite from the Jinchuan Cu-Ni-PGE sulfide deposit, with very similar REE patterns. Chondrite-normalized REE patterns of the ore-bearing rocks from both deposits assuming right-inclined fractionation, with enrichment of LREE and smoothness of HREE; nevertheless, Platreef rocks have a positive Eu anomaly，whereas Eu negative anomalies are evident for most lherzolite and ores from the Jinchuan deposit. This might be a reflection of insignificantly varying plagioclase/orthopyroxene ratios during the crystallisation of the rocks. it is evident that, relative to the Jinchuan deposit, the Platreef deposit has higher contents of strongly fractionated incompatible elements but shows a trough-shaped profile with pronounced depletion of Nb and Ta，suggesting a model of crustal contamination during mantle plume magma emplacement, evolution and ore formation that resulted in the geochemical features of trace elements similar to arc picrite. Jinchuan ore-bearing magma is enriched in Fe, Ti and highly incompatible lithophile elements, exhibiting geochemical features of Ferropicrites trace elements; and hence it is thought to have been derived from conrrespondingly enriched mantle ( EMI) sources even if there probably also existed some crustal substance contamination during the rock-forming and ore-forming process. The primitive mantle-normalized PGE patterns of platreef rocks are arch-shaped with the enrichment of Pt, Pd and Ru relative to Os, Ir and Rh, and incompatible elements Cs, Rb, Th, U and LREE as well as chalcophile elements Cu and Ni, suggesting that the residual mantle peridotite from which the Bushveld Complex derived has undergone infiltration and contamination-mantle metasomatism by external mineral-enriched fluids containing highly incompatible elements, which may be responsible for the fractionation of PGE from mantale-derived magma.