The Gerfed gold deposit: fluids and PT-conditions for quartz vein formation (Yenisei Ridge, Russia)

At the Gerfed gold deposit, fluid inclusions were studied by thermobarometry, gas chromatography, Raman spectroscopy, and ICP MS in quartz samples of three types: quartzites, Au-poor (<1–2 ppm) feathering veins, and Au-rich (>2.8–10 ppm) feathering veins. It has been found that these three types were produced from fluids differing in composition and thermobarogeochemical parameters. The quartzites formed from low-salinity (<7.0 wt.% NaCl equiv) homogeneous fluids of essentially aqueous–chloride composition at 120–230 °C and 0.1–0.5 kbar. The gas phase in these fluids comprises H2O, CO2, CH4, and N2, with CO2/(CO2 + H2O) = 0.04–0.15 and CO2/CH4 = 2.2–3.8. The Au-poor feathering veins formed from homogeneous and heterogeneous fluids at 150–300 °C and 0.5–2.0 kbar. The salinity of the fluids increased to 10 wt.% NaCl equiv. The gas phase in them comprises H2O, CO2, N2, and CH4. Here, CO2/(CO2 + H2O) = 0.09–0.17 and CO2/CH4 = 2.2–2.3. The Au-rich feathering veins formed from heterogeneous and more saline (6.0–23.3 wt.% NaCl equiv) CO2–H2O fluids at higher temperatures (150–400 °C) and pressures (1.1–2.5 kbar). In this fluid CO2/(CO2 + H2O) = 0.18–0.27 and CO2/CH4 = 4.1–20.8. All three quartz types show negative Eu anomalies and a distinct predominance of LREE over HREE. Differently directed trends of REE and Eu/Sm in the quartzites and feathering veins suggest that the fluids were produced from different sources. The fluids of the gold-bearing quartz veins are enriched in K, Li, and Rb, and those of the Au-poor feathering veins, in Sr and Na. The quartzites have low Rb and Sr and similar Na and K contents. Areas with a high and bonanza gold content in feathering-vein stockworks formed when high-temperature saline H2O–CO2 fluids were superposed on the Au-poor quartzites and feathering veins.