Minerals of the Au-Cu-Ag System in Grains from the Placers of the Olkhovaya-1 River (Eastern Kamchatka, Russia)

Abstract

Heterogeneous grains in the heavy fractions of the placers of the Olkhovaya-1 river (Kamchatka Cape Peninsula, Eastern Kamchatka, Russia) containing Au-Ag, Au-Cu, and Au-Ag-Cu particles were analyzed using scanning electron microscopy, electron-probe microanalysis, and X-ray powder diffractometry. The analyses showed that auricupride dominates in some grains, whereas in others, the main phases are tetra-auricupride or Ag-rich and (or) Au-rich alloys. It was revealed that in the central parts of some grains of exsolution structures, auricupride Cu3.04–2.74Au0.96–1.26 (Ag < 1 wt.%) occurs in intergrowths with low-fineness Ag0.86–0.67Au0.09–0.33Cu<0.05 solid solutions (160‰–480‰), and tetra-auricupride Cu1.12–0.87Au0.88–1.11Ag<0.02 (Ag < 2 wt.%) occurs with higher-fineness Au0.73–0.62Ag0.2–0.38Cu<0.07 solid solutions (520‰–850‰). We also observed, mainly in the peripheral parts of the grains, insignificant amounts of secondary phases that were compositionally similar to Cu2Au, Cu3Au2, Au3Cu2, Au2Cu, Au3Cu, high-fineness gold with impurities of Cu and occasionally Ag (>850‰), and pure gold. In intergrowths with auricupride and tetra-auricupride, we also observed earlier-formed silicates (garnet, pyroxene, chlorite, epidote, titanite), syngenetic sulfides (pyrrhotite, bornite, galena), and later minerals (chalcocite, covellite, anilite, cuprite, goethite, etc.). The XRD analysis of the peripheral parts of some grains showed the presence of auricupride (Cu3Au), tetra-auricupride CuAu (I) and Cu(Au0.92Ag0.08) (II), and gold. The profiles show the absence of peaks of the Au3Cu phase and other Au-Ag-Cu phases identified by the EMPA, which is likely due to their low concentrations in the samples or their structural similarity to gold. It is assumed that the probable source of Au-Ag-Cu mineralization in the Olkhovaya-1 river placers is located in the upper reaches of watercourses that erode the ultrabasic massif of Mounts Soldatskaya and Golaya (Kamchatka Cape Peninsula).