Barkov, AY; Fleet, ME; Nixon, GT; Levson, VM
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 Sigma-PGE:(Fe + Cu + Ni) ratio of 3.6-5.6, "(Pt,Ir)(2)Fe"-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 ocommon Ir-dominant lr-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)(lrRh,Pt)(2)S-4, unusual sperrylite-type [(Pt,Rh,Fe)(AS,S)(2-x)] and platarsite-type [PtAs1-xS1+x or unnamed Pt(S,As)(2-x)] phases, and unnamed (IrRh,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 starmides from the Noril'sk complex, indicate the following sequence of crystallization: (Pt,lr,Rh)(3)Fe - (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 degrees 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.