The Formation Conditions and Sources of Ore-Forming Fluids of the Nikolaevskoe Gold Deposit (South Urals)

We studied fluid inclusions, trace elements, and oxygen, carbon, and sulfur isotope ratios in minerals from stockwork sulfide-carbonate-quartz ores of the Nikolaevskoe gold deposit confined to volcanic island arc porphyry' intrusions. The study shows that fluid inclusions in quartz were homogenized at 260-200 degrees C and those in later formed calcite, at 227-205 degrees C. The fluids contain aqueous K-Mg-Na chloride solutions with salinity of 4.1-9.6 wt.% NaCl eq. Raman spectroscopy revealed CO2 (29-34 mol.%), CH4 (40-55 mol.%), and N-2 (8-30 mol.%) in the fluids. According to LA-ICP-MS data, quartz has low contents of Al (11.7-102 ppm) and Ti (0.05-0.64 ppm), which indicates its deposition from weakly acid low-alumina fluids at <350 degrees C. The REE patterns of calcite show accumulation of heavy lanthanides (La-N/Yb-N = 0.2-0.9), evident of high fluid alkalinity, and negative Ce (0.39-0.82) and positive Eu (1.99-5.25) anomalies. The negative Ce anomalies are due to meteoric water in the fluid and the fluid interaction with limestones. The positive Eu anomalies reflect the high-temperature (>250 degrees C) environment that existed before the calcite crystallization. The Y/Ho ratio in calcite (28.7-54.1) suggests that the fluid has magmatic components and components extracted from limestones and contains seaw'ater. The delta O-18(H2O) values (3.6-5.6 parts per thousand) of the fluid testify to the participation of magmatic and meteoric w'aters in the ore formation. The delta C-18(CO2) values (-4.1 to 1.4 parts per thousand) point to carbon of magmatic nature and carbon extracted from limestones. The heavy sulfur isotope composition of pyrite (6.75 to 9.87%o) and arsenopvrite (8.7 parts per thousand) might be due to sulfur supply from the host rocks or to the participation of |SO4](2-) of seawater in the ore-forming process. According to the results obtained, the Nikolaevskoe gold deposit is an island arc (Au +/- Cu)-quartz-sulfide deposit transitional between porphyry and epithermal types.