New data on PGE alloy minerals from a very old collection (probably 1890s), California

摘要: We report results of multiple electron-microprobe analyses nine grains alloy minerals, 2–5 mm in size, rich the platinum-group elements (PGE), from a unique, very old collection (~1890s) placer material Trinity Co., California. Osmium, iridium, ruthenium, and rutheniridosmine are principal species poor Fe (typically <0.5–1 wt%); they appear to be primary. An Fe-enrichment (up 4.6 wt%) is observed lamellae Ir-Ru-Os exsolved Fe-poor Os-Ir-Ru host, also rim-like alteration-induced phase developed along margin some alloy. Much greater levels 19.1 wt%), incorporated via substitution mechanisms: → Ru, (Os + Ru), Ir, were documented three Fe-Os-Ru-Ir alloys, which attain Fe-dominant compositions, i.e., hexaferrum. These alloys associated exotic phases enriched unconventional elements, such as W-(Mo)-bearing rutheniridosmine, (Os,Ir)5(W,Mo) newly recognized (Ir,Os)5(W,Mo), secondary, formed under conditions low fugacities O2 S2 result interaction primary with reducing fluid phase. No Pt-Fe found; these only occur Pt3Fe-type (isoferroplatinum or Fe-rich platinum) Pt2Fe-type inclusions (<50 μm), enclosed matrix Ir- Os-dominant Ru. The Pt2Fe appears compositional variant platinum, possibly reflecting lower limit Pt content possible mineral platinum. Au-Ag alloy, ranging up Au0.99, precipitated pseudomorphously by subsolidus reaction between residual Au-Ag-rich melt exsolution-induced phases. Micro-inclusions olivine, hosted ternary Os0.33Ru0.33Ir0.30, extremely Mg (Fo95.1–95.4), probably high-temperature involving chromite magnesiochromite. likely derived mineralized zone ultramafic rocks, chromitemagnesiochromite overall S, ophiolite complex northern