Geochemical and Isotopic Properties of Fluids from Gold-Bearing and Barren Quartz Veins of the Sovetskoye Gold Deposit (Siberia, Russia)

Abstract

This paper reports on the comparative study of fluids trapped in inclusions in gold-bearing and barren quartz veins at the Sovetskoye gold deposit, Yeniseisky ridge, Siberia, Russia. The host rocks are greenschist facies metasedimentary rocks of Upper Proterozoic age. Within the deposit, some veins contain 10 to 25 ppm of Au (gold-bearing veins); others contain less than 1 ppm of Au (barren veins). Fluid inclusions trapped in barren veins are predominantly low-salinity-H2O (4.0–8.0 mass % NaCl equiv.) with variable contents of dissolved CO2 (<12.5 mol %), CH4 (<1.3 mol %), and N2 (<1.7 mol %). Homogenization temperatures vary from 200° to 410°C, and the calculated pressures of entrapment are <1.5 kbars. Inclusion fluids trapped in the host schists are predominantly aqueous (97.6–99.3 mol % H2O), with CO2 contents below 2.4 mol percent. The fluids trapped in ore-bearing quartz inclusions are different. Inclusions show higher homogenization temperatures (up to 630°C) and salinity (up to 20–25 mass % NaCl equiv). Calculated pressures range from 0.7 to 2.0 kbars. Bulk gas chromatographic data of fluids extracted from native gold have contents of CO2 that range from 29.3 to 62.0 mol percent, N2 from 2.7 to 13.2 mol percent, and CH4 from 0.0 to 2.8 mol percent. In gold-bearing quartz the contents of CO2 range from 18.0 to 41.0 mol percent, N2 from 0.1 to 0.2 mol percent, and CH4 from 0.3 to 3.0 mol percent. Raman spectroscopic data of fluid from gold-bearing quartz revealed CO2 contents of individual inclusions as high as 98.2 mol percent, N2 up to 66.4 mol percent, and CH4 up to 88.7 mol percent. Fluids of the country-rock schists which host gold-bearing quartz veins are also enriched in CO2 (up to 49.5 mol %). The host rocks for both barren and gold-bearing quartz veins show consistent REE patterns characterized by a negative Eu anomaly. Fluids from barren quartz veins show similar REE patterns although with a less conspicuous negative Eu anomaly, whereas fluids from gold-bearing quartz show a marked positive Eu anomaly. Similar isotopic values of sulfur in pyrite, pyrrhotite, and arsenopyrite disseminated in country rocks ( δ 34S = 14.7–21.6‰), and in barren ( δ 34S = 12.9–19.9‰) and gold-bearing quartz veins ( δ 34S = 13.3–17.9‰) suggest that sulfur in sulfides was derived from the same source, most likely being the adjacent country rocks. CO2 trapped in fluid inclusions of barren and gold-bearing quartz has distinct isotopic compositions ( δ 13C = −8.1 to −10.2 and −4.9 to +5.2 ‰, respectively), and the δ 13C values are positively correlated with Au contents in veins. Helium isotope composition of both types of quartz veins (3He/4He = 0.04–0.05 Ra for gold-bearing veins and 3He/4He = 0.02 Ra for barren veins) indicates a crustal provenance of mineral-forming fluids. The 40Ar/39Ar dating of sericite demonstrates that the barren veins (890-850 Ma) are significantly older than the gold-bearing veins (830-730 Ma). On the basis of their ages, barren veins are interpreted as synmetamorphic. The age of the gold-bearing veins corresponds to the age of the local granitoids (850-720 Ma), suggesting that the origin of the Sovetskoye deposit is related to the tectonic activity syntetctonic magmatism.