Formation of platinum-group minerals from an evolving sulfide liquid at Sudbury, Canada

Chalcophile and platinum-group elements (PGE) are collected by a magmatic sulfide liquid and form PGE (± NiCu) deposits. Early-crystallizing monosulfide solid solution (MSS) concentrates Os, Ir, Ru and Rh (IPGE) and the residual liquid concentrates Cu, Pt, Pd, Ag, As, Bi, Te, and Sn. It is important to determine the host phases of the PGE, which are sulfide minerals and/or platinum-group minerals (PGM), in order to understand the petrogenesis of the deposit and to improve PGE extraction. Previous work shows that PGM form by: \ud \ud 1) exsolution from base metal sulfides, 2) crystallization from the residual Cu-rich liquid and/or a late-stage immiscible melt and 3) remobilization during metamorphism or by hydrothermal fluids. \ud \ud We have investigated the origin of PGM from Sudbury Ni-Cu-PGE deposits by combining a PGM study with wholerock data and laser ablation-ICP-MS analysis of the sulfides. We found that a large proportion of the PGE are hosted in As-, Bi-, Te- and Sn-rich PGM, which formed over a wide range of temperatures during the evolution of the sulfide deposit. The amount of As in the initial sulfide melt, which varied according to the As content of the assimilated country rocks, was critical in determining whether As-PGM (IrAsS, RhAsS ± PtAs2) crystallized early (1200-900°C) from the sulfide liquid togther with MSS. An As-rich sulfide melt crystallizes sulfarsenides so that the co-existing MSS is depleted in Ir and Rh whereas an As-poor sulfide melt does not crystallize sulfarsenides and the IPGE remain in MSS. The amount of Bi, Te and Sn increases during sulfide fractionation so that the following Pt-Pd-minerals crystallized from the residual liquid. Solitary grains of Pd2Sn and PtSn crystallized early (1300-800°C) from the Cu-rich sulfide liquid whereas an unnamed Pt(Sn,Bi)Te phase together with numerous Pt-Pd-BiTe-Ag minerals crystallized as composite grains from microdroplets of an immiscible late-stage melt (1000-600°C). The small amount of Pt, Pd, Bi and Te, which partitioned into MSS, later exsolved (< 600°C) as laths of (PtPd)(BiTe)2 and PdBiTe and were remobilized during greenschist metamorphism