PLATINUM-GROUP MINERALS FROM A PLACER DEPOSIT IN BURWASH CREEK, KLUANE AREA, YUKON TERRITORY, CANADA

Abstract

Five placer grains of Pt–Fe alloy, ca. 0.5 to 2 mm, roundish in shape, and hitherto unconfirmed from this locality, were recovered from a placer deposit in Burwash Creek, Kluane area, Yukon. Our results of multiple electron-microprobe analyses (EMP) display the presence of elevated levels of Pd (0.49–5.84, mean 2.38 wt.%), Rh (1.05–1.94, mean 1.50 wt.%), Ir ( 45.7–47.5 Mg 37.5–48.0 Fe 4.9–16.8 , mg # = 100Mg/(Mg + Fe 2+ ) in the range 69.0–90.7], plagioclase (Ab 57.5–60 An 35.1–39.9 Or 2.6–4.8 ), and sodic–calcic amphiboles (0.6 and 1.1 Na apfu ), some of which are rich in Cl (0.72 and 1.17 wt.%, or 0.17 and 0.36 Cl apfu , respectively). The analyzed inclusions of PGM are members of the bowieite Rh 2 S 3 – kashinite Ir 2 S 3 solid-solution series, miassite (Rh,Pt,Pd) 17 S 15 , cooperite PtS, isoferroplatinum Pt 3 Fe (or Fe-rich platinum), vasilite (Pd,Cu,Pt) 16 S 7 , keithconnite Pd 20 Te 7 , an unusual Pt–Pd–Ni–Cu alloy ( i.e. , Pd-rich platinum: Pt 47.7–52.8 Pd 29.3–37.9 Ni 6.3–8.0 Cu 6.1–7.5 ), and a sulfotelluride of Pd, Pd 11 Te 2 S 2 (?), which may represent a new isomertieite-type compound. The trace-element composition of the silicate melt inclusions determined using laser-ablation ICP–MS shows a notable enrichment in large-ion lithophile elements. We infer that the reported association of PGM and the trace-element composition of silicate melt inclusions observed at Burwash Creek probably correspond to an Alaskan–Uralian-type mineralization; considered less likely is a relation with a Triassic ultramafic–mafic complex (sill-like) of so-called Kluane-type, also developed in the placer area.