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GRAINS OF Pt–Fe ALLOY AND INCLUSIONS IN A Pt–Fe ALLOY FROM FLORENCE CREEK, YUKON, CANADA: EVIDENCE FOR MOBILITY OF Os IN A Na–H₂O–Cl-RICH FLUID

Andrei Y. Barkov¹, Robert F. Martin^1,, William LeBarge² and Yana Fedortchouk^2,¶

¹ Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7, Canada
² Yukon Geological Survey, Ministry of Energy, Mines and Resources, PO Box 2703, Whitehorse, Yukon Y1A 2C6, Canada

E-mail address: bobm{at}eps.mcgill.ca

We report results of detailed electron-microprobe analyses (EMP) of fourteen grains of Pt–Fe alloy, ca. 0.4 to 1 mm, principally droplet-shaped or roundish, from Florence Creek, Yukon. We also describe new occurrences of Pt–(Pd)–Fe alloy from Arch Creek (up to 11 wt.% Pd) and Canadian Creek, Yukon. An extensive compositional series is observed at Florence, in which values of PGE/(Fe + Cu + Ni) vary from 2.2 to 5.4. Minor Ir and Os display a contrasting behavior in this series. The observed levels of Ir are generally greater than those of Os. The Ir correlates negatively with Pt (the correlation coefficient, R, is –0.81; 140 EMP point-analyses); in contrast, the correlation between Os and Pt is slightly positive (R = 0.64). Probably, at a higher temperature of crystallization, Ir readily replaces Pt in the structure of a Pt–Fe alloy. As a result, Ir did not attain its maximum, and the observed inclusions of exsolution-induced Ir-dominant alloy, hosted by the Pt–Fe alloy, are moderately enriched in Ir (52–54 at.%). An inclusion of erlichmanite (4.2 wt.% Rh) and a partial rim of Au–Ag alloy (Au_0.80Ag_0.18) were observed. Droplet-like inclusions of highly aluminous silicate minerals, hosted by grains of Pt–Fe alloy, are rich in Na, K and in Cl (up to 2 wt.%). Veinlets and micrograins of an Os-rich alloy (86–92 wt.% Os) are intimately associated with micrograins of nearly pure albite, Ab_97.8An_1.7Or_0.5, and with an intermediate member of the chamosite–clinochlore series. The observed association chamosite + albite + native osmium, infilling a cavity in a host grain of Pt–Fe alloy, was likely deposited from a late Na–H₂O–(Cl)-bearing fluid phase at a postmagmatic–hydrothermal stage, at a temperature less than 700°C. The Os and Na were possibly mobilized and transported as Na–(Os)-rich complexes related to the sodium hydroxy- or oxo-osmates. The precipitation of the metallic Os phase could be related to a change in physicochemical conditions, e.g., in fluid pressure or redox potential, leading to instability of these Na–(Os)-rich hydroxy- or oxo-osmate complexes. We suggest that a mineralized zone rich in chromite–magnesiochromite and hosted by an Alaskan–Uralian-type complex is the likely provenance for grains of Pt–Fe alloy recovered in Florence Creek.

Keywords: platinum-group elements, platinum-group minerals, Pt–Fe alloy, PGE alloys, native osmium, volatile transport, Na–H₂O–Cl-bearing fluid, ultramafic–mafic rocks, placer deposits, Florence Creek, Arch Creek, Canadian Creek, Yukon.