Composition and sources of mineral-forming fluids of the Orlovka orogenic gold deposit (Southern Urals)

Fluid inclusions, REE and Y patterns, and carbon and oxygen isotope ratios in the minerals of sulfide-carbonate-quartz mineralization of the Orlovka orogenic gold deposit were studied. We have established that fluid inclusions in calcite and quartz homogenize in the same temperature range 217-170 °C and contain aqueous Mg-K-Na solutions with salinity of 3.0-6.4 wt.% NaCl equiv. According to the results of gas chromatography of inclusions in quartz, the gas phase is a mixture H2O (79-977 ppm) + CO2 (2.64-5.35 ppm) + CH4 (0.0020.018 ppm) ± N2 (0-1.22 ppm). The REE pattern of calcite shows accumulation of LREE ((La/Yb)N = 1.28-7.18), (La/Lu)N = 1.10-6.58 (indicating a predominance of REE sorption in the fluid), and weak negative Ce and positive Eu anomalies. Negative Ce anomalies in calcite might be due to the interaction of the fluid with limestones and to the presence of a small amount of meteoric water in it. The positive Eu anomalies reflect the high-temperature environment (> 200-250 °C) that existed in the fluid system before the crystallization of calcite. The 513CCO2 (-2.0 to + 0.9%c) values of the fluid, close to the carbon isotope composition of carbonates of the host rocks (-2.3 to + 1.9%c), testify to the metamorphogenic source of carbon. The 518OH2O values of the fluid depositing quartz (3.1 to 4.5%c) and calcite (4.0 to 4.6%c; one sample has 6.6%c) suggest that metamorphic water with an impurity of meteoric water prevailed in the fluid system. We propose a model for the gold deposit formation, which takes into account the generation of a mineral-forming fluid at the progressive stage of greenschist dynamometamorphism of the host rocks and the formation of gold mineralization at the regressive stage.