RARE NATURAL POLYCOMPONENT ALLOYS BASED ON GOLD AND COPPER FROM A PLATINUM PLACER IN THE KONDER ALKALINE-ULTRABASIC MASSIF, SOUTHEASTERN ALDAN SHIELD, RUSSIA

Abstract

Gold occurring in the platinum placer of the Konder alkaline-ultrabasic massif is represented by the following varieties: copper gold (6.9-25.0 wt % Cu; single grain with 44.3 wt % Cu)-Au3Cu (cuproauride), AuCu (tetraauricupride), and rare AuCu3 (auricupride); copper-platinum (up to 11.8 wt % Pt)-(Au, Pt)Cu; and copper-palladium (up to 10.3 wt % Pd)-(Au, Pd)Cu; as well as silver alloys (up to 62.7 wt % Ag and 4.8 wt % Cu)-from Au, Au3(Ag, Cu) to electrum and küstelite. Together with individual grains, gold also forms rims and microinclusions at the margins of cubic crystals of low-impurity high-Fe isoferroplatinum (9.5-11.5 wt % Fe) derived from phlogopite-magnetite clinopyroxenites (Nekrasov et al., 1994). This gold variety was not found in association with other types of the Konder nuggets genetically related to dunites, i.e., with those (a) of irregular shape with a significant admixture of chromite crystals and (b) rounded with rare chromite inclusions. The gold minerals and alloys are accompanied by various minerals of platinum-group elements (PGE), e.g., tetraferroplatinum, tulameenite, Pd-hongshiite, complex sulfides and arsenides of Pd and Pt, as well as antimonides, bismuthides, tellurides, stannides, plumbides, palladium germanide, etc. According to experimental and geochemical data (Nekrasov et al., 1994; this paper), the Ag-Cu-Ag alloys of the Konder massif were formed at 200-450°C and low fO2 (10-15 to 10-25) and fS2 (up to 10-25). According to X-ray diffraction data, the silver gold is disordered and has a face-centered cubic lattice. The copper gold has a tetragonal cell with the degree of ordering decreasing from CuAu to CuAu3 and Au.