Idiomorphic crystals (up to 3.5 mm) of ferroan platinum, cooperite and mertieite-II were found in a heavy-mineral concentrate from stream sediments of the Darya river in the Aldan Shield, Russia. Pt-Fe crystals display cubic and thin platy habits; occasionally they are twinned. The chemical composition ranges from Pt2.64Fe1.00 to Pt2.88Fe1.00 with Os, Ru, Ir, Rh and Pd below the analytical detection limit of the electron microprobe. X-ray diffractometry of Pt-Fe crystals suggests a F-centred cubic lattice, characteristic of ferroan platinum. Some of the ferroan platinum crystals have large (about 100 μm wide) cooperite overgrowth rims or are covered by a Au-Ag alloy. Cooperite also occurs as large euhedral crystals (up to 3 mm across, partly twinned). Crystals of mertieite-II are speckled with μm-sized (2–5 μm) inclusions of sperrylite and intergrown with minerals of cooperite-braggite solid solution, Pt-Pd-Hg alloy, keithconnite and Au-Ag alloy. Fractures along crystallographic planes of the mertieite-II crystals are filled with Pd-Pt-Fe-Sb-As-Hg-Te-Bi-bearing oxides. The coarse-grained PGM from the Darya have a geological setting similar to the Kondyor PGE placer 75 km to the northeast and are probably related to clinopyroxenite-hornblende-magnetite units of Alaskan-/Uralian-type intrusions.

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.