• journal_title：The Canadian Mineralogist
• Contributor：Andrei Y. Barkov ; Michael E. Fleet ; Graham T. Nixon ; Victor M. Levson
• Publisher：Mineralogical Association of Canada
• Date：2005-
• Format：text/html
• Language：en
• Identifier：10.2113/gscanmin.43.5.1687
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：43
• issue：5
• firstpage：1687
• section：Phase equilibria, platinum-group minerals, and ore deposits

Platinum-group minerals (PGM) from a selected suite of gold- and platinum-group-element (PGE) placer deposits in British Columbia have been analyzed using the electron-microprobe (EMP). The PGM placer grains (n = 70, 0.1–1.5 mm in size) principally comprise various Pt–Fe–(Cu) alloys: “Pt₃Fe”-type alloys [Fe-rich platinum (formerly, ferroan platinum: Bayliss et al. 2005) or isoferroplatinum], Fe-rich Pt with an atomic ∑PGE:(Fe + Cu + Ni) ratio of 3.6–5.6, “(Pt,Ir)₂Fe”-type alloy, members of the tulameenite–tetraferroplatinum solid-solution series extending from Pt₂Fe(Cu,Ni) toward PGE_1+x(Fe,Cu,Ni)_1−x, less common Ir-dominant Ir–Os–(Ru–Pt) alloys, subordinate Os-dominant alloys, and minor Ru-rich alloys and rutheniridosmine, the latter with an atomic Ir:Os:Ru proportion close to 1:1:1. Trace amounts of PGE sulfides and sulfarsenides: cooperite PtS, Ni-rich cuproiridsite (Cu,Ni,Fe)(Ir,Rh,Pt)₂S₄, unusual sperrylite-type [(Pt,Rh,Fe)(As,S)_2−x] and platarsite-type [PtAs_1−xS_1+x, or unnamed Pt(S,As)_2−x] phases, and unnamed (Ir,Rh,Pt)S (?) crystallized at a late stage in low-S environments. Some PGM grains contain micro-inclusions of diopside, augite, ferro-edenite, a potassian sodic-calcic amphibole (richterite?), talc, clinochlore and euhedral quartz. High values of mg# [100Mg/(Mg + Fe)] of the ferromagnesian minerals in these inclusions suggest highly magnesian source-rocks. Textural and compositional data, in particular the zoned intergrowths of Pt–Fe–(Cu) alloy grains, which broadly resemble the zoned Pt–Pd–Cu stannides from the Noril’sk complex, indicate the following sequence of crystallization: (Pt,Ir,Rh)₃Fe → (Pt,Ir,Rh)_1+x(Fe,Cu)_1−x or Pt₂Fe(Cu,Ni). The zoned Pt–Fe–Cu alloys likely formed by fractional crystallization of primary solid-solutions under closed-system conditions as a result of increase in the activity of Cu in the residual liquid after an early-stage (magmatic) crystallization of the Cu-poor core. The Cu-rich Pt–Fe alloys formed around these core zones and at their peripheries during a significant drop in temperature at a late stage of crystallization of the composite alloy grains. The compositions of micro-inclusions and exsolution lamellae of Os- and Ir-dominant alloys in Pt–Fe alloys imply uniform temperatures of equilibration within the range 750–800°C. The compositions of Cu-rich Pt–Fe alloys, PGE sulfides and sulfarsenides, and exsolution lamellae of Os- and Ir-dominant alloys, observed in intimate association with Pt–Fe alloys, are likely related to the crystallization of the coexisting Pt–Fe alloys. Narrow zones of Pt–Fe alloys richer in Pt and poor in Fe, observed at the boundary of placer Pt–Fe alloy grains, provide evidence for a removal of Fe and corresponding addition of Pt as a result of interaction with a low-temperature fluid. The terrane affinities, compositions and associations of the placer PGM examined appear consistent with two types of potential source-rocks, associated with Alaskan-type intrusions and the Atlin ophiolite complex. The preservation of faceted morphologies on many of the placer PGM grains implies a relatively short distance of transport from their source.