Pd-bearing oxides and hydrated oxides in mertieite-II crystals from alluvial sediments of the Darya river, Aldan Shield, Russia

摘要

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.