Abstract

Coarse-grained mertieite-II [Pd8(Sb,As)3] crystals (up to 2.5 mm across) with inclusions of sperrylite and intergrown with keithconnite [Pd3(Te,Bi)], cooperite-braggite, a Pt-Pd-Hg alloy, a (Pd,Pt)9Te phase and Au-Ag alloy occur in alluvial sediments in the Darya river, Aldan Shield, Russia. The primary PGM assemblage underwent alteration with formation of a very fine-grained mixture of Pd-bearing oxides and hydrated oxides. The Pd oxide/hydrated oxide aggregates (up to 100 μm wide and ~1 mm long) occur along crystallographic planes, on grain boundaries and fractures, occasionally intergrown with goethite. The Pd-O-bearing compounds formed after (1) mertieite-II (Sb content reaches 23 wt.%), (2) keithconnite (variably enriched in Te, up to 7 wt.% and Bi, up to 6 wt.%) and (3) potarite (with high Hg content, up to 35 wt.%). The variably oxidized and hydrated PGM assemblage can be attributed to supergene alteration and small-scale element redistribution under cool climatic conditions with a limited degree of leaching.

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