Platinum enrichment at low temperatures and related microstructures, with examples of hongshiite (PtCu) and empirical 'Pt2HgSe3' from Itabira, Minas Gerais, Brazil

摘要

Low-temperature Pt enrichment is illustrated with reference to two hypogene platiniferous minerals: (i) a Pd-Cu-bearing 'Pt2HgSe3', an unnamed phase; and (ii) hongshiite, PtCu, with up to 0.7 wt% Hg. The former has patches oxidized to a Pt-O species with curvilinear microcracks originated by desiccation. The latter displays parallel sets of Cu-depleted, Pt-Hg-enriched patches, along which microfractures occur. Such microstructures are interpreted to arise from two processes: (i) oxidation and formation of a hydrated Pt-O compound, which tends to succumb to a Pt-rich alloy by deoxygenation and dehydration, leaving behind curvilinear desiccation cracks; (ii) metal leaching conducive to native Pt by removal of Cu from hongshiite via micropores that coalesce along crystallographic planes to result in reticulated microcracks. Platinum enrichment either by oxidation or metal leaching ideally produces characteristic microcracks, which may be indicative of the origin, i.e. detrital vs. supergene, of alluvial and eluvial Pt-rich alloys.