Formation conditions, composition and ore-forming sources of the Bolshoy Karan gold-porphyry deposit (the Southern Urals)

Research subject. We studied PT conditions, composition and sources of ore-forming fluids of the Bolshoy Karan goldporphyry deposit (South Urals) confined to the Main Urals Fault zone at Southern Urals.

Methods. Temperatures of ore mineral formation, salt composition and fluid salinity were estimated using a Linkam TMS-600 cryostage equipped with an Olympus BX 51 optical microscope. The gas composition of fluid inclusions was determined using a Horiba LabRam HR800 Evolution Raman spectrometer and an Olympus BX-FM optical microscope. Trace elements were detected by means of LA-ICP-MS analysis. С and О isotopic composition was identified using a MAT 253 (Thermo Fisher Scientific) mass-spectrometer. S in the minerals of sulphide-carbonate-quartz ores was identified using a DeltaPLUS Advantage mass-spectrometer.

Results. It is shown that the fluid inclusions in quartz are homogenized at 370–310ºC, while those of the latest calcite – at 234–200ºС. The fluids contain Mg-Na chloride solutions with a salinity of 3.0–11.9 wt % NaClэкв. According to LA-ICP-MS data, quartz contains high Al contents (916–1556 ppm) confirming its formation from a high aluminous acid fluid. The REE distribution spectra in calcite are characterized by the accumulation of light lanthanides (LaN/YbN = 3.4–9.11) pointing to the acid fluid composition, as well as by negative Ce (0.58–0.88) and positive Eu (1.51–3.61) anomalies. The negative Ce anomaly may have been caused by interactions between the fluid and host limestones. The positive Eu anomaly reflects the existence of a middle-temperature environment (>250ºC) prior to  calcite crystallization. Y/Ho values in calcite (29.3–35.6) suggest the presence of magmatic components and those extracted from limestones. The values of δ18О in calcite vary from 14.7 to 19.8‰, while those of δ13С – from –4.1 to 0.7‰. The values of δ18ОH2O for the ore-forming fluid, which were  calculated based on average homogenization temperatures of fluid inclusions in calcite (230ºС), vary from 6.5 to 11.5‰, while δ13ССO2 – from –3.21 to 1.6‰. δ34S values in pyrite ranged from –0.60 to – 1.50‰.

Conclusions. Our data confirm the formation of the gold-porphyry mineralization of the Bolshoy Karan deposit under mesothermal conditions. Magmatic fluids played the key ore-formation role. The  geochemical peculiarities of interactions between the fluid and host rocks were revealed.

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