Native gold in the Poldnevskoye demantoid deposit

Abstract

Research subject. Vein carbonate-magnetite-serpentine rocks with nests of gem garnet (demantoid) among antigorite serpentinites of the Korkodinsky massif in the Middle Urals. Aim. To characterize the composition of native gold and physicochemical conditions of its formation in a previously undescribed type of gold mineralization in ultrabasic rocks. Materials and methods. Grains of native gold from carbonate (calcite)-serpentine (clinochrysotile) veins with demantoid and host serpentinites were examined by scanning electron microscopy (JSM-6390LV by Jeol) and X-ray microanalysis (Cameca SX 100 with five-wave spectrometer). Results. Significant differences in the morphology and chemical composition of native gold grains from serpentinite and carbonate-serpentine veins were revealed. In serpentinite, these differences are represented by Au–Cu intermetallides, i.e., AuCu3 auricupride and AuCu tetraauricupride. The intermetallide compositions are close to stoichiometric with a Cu excess of up to 0.1 p.u. Rare inclusions of electrum (37–44 wt % Ag, sample grade 555–617‰) are present in tetraauricupride; copper and nickel sulfides are present in auricupride. Sulfide inclusions are represented by accretions of digenite (Cu/S = 1.88) with pentlandite, as well as digenite with flame-like phenocrysts of bornite-digenite solid solution. In the peripheral parts of auricupride grains, hypergene rims of up to 100 µm in thickness were detected. Hypergene gold in reflected light is brown in color, having a porous, collomorphic or monolithic-cracked structure. This gold is characterized by low Cu contents (1.4–10.6 wt %), relative to the primary auricupride composition (50.7–52.3 wt % Cu), a deficit of total components ranging from 1.5 to 20.0 wt %, and the presence of an oxygen line on EDX spectra. The grains of native gold from the vein carbonate-serpentine mass are composed of Au–Ag solid solution particles, belonging to high-grade gold (913–961‰, 4–10 wt % Ag), less frequently to lower-grade gold (808–866‰, 13.0–19.4 wt % Ag). Mineral inclusions in silver gold were not found. Conclusions. Differences in the composition of native gold from vein mass and serpentinite reflect the variability of physical and chemical conditions of its deposition. The deposition of Au–Cu intermetallides occured from fluids with low oxygen and sulfur fugitivities: log fS2 = –8…–20, log fO2 = –26…–39, and Au-Ag solid solutions under higher values of sulfur fugitivities and more oxidized conditions. It is assumed that cuprous gold is associated with antigorite serpentinization, when the hyperbasite body was in the depth. During its rise to the surface and decompression, reducing conditions were replaced by oxidizing conditions. As a result, the fluid became carbonic acidic. In such conditions, carbonate-serpentine veins with demantoid and native silver gold were formed. The presence of auricupride hypergene replacement products of collomorphic, porous and fractured structure, as well as the presence of oxygen line in EDX spectra testify both to the processes of redistribution of micropores formed during copper removal and a possible formation of oxide or hydroxide forms of gold and copper.