GENESIS OF METASOMATIC ROCKS AND MINERALIZED VEINS AT THE BEREZOVSKOE DEPOSIT, CENTRAL URALS: EVIDENCE FROM FLUID INCLUSIONS AND STABLE ISOTOPES

Abstract

The Berezovskoe gold deposit near the town of Yekaterinburg in the Central Urals is controlled by an array of granitoid porphyry and lamprophyre dikes. Individual fluid inclusions in quartz from veins accompanying metasomatic rocks of the propylite, gumbeiite, and beresite-listwanite associations were examined by microthermometric techniques, and aqueous extracts of these inclusions were analyzed by ICP-MS techniques. The isotopic compositions of quartz, tourmaline, scheelite, and carbonates were analyzed, and it was established that mineralized quartz veins associated with the metasomatic rocks of the beresite-listwanite association at the Berezovskoe deposit were produced by at least two pulses of hydrothermal activity. The first of them was responsible for the origin of the early (scheelite + quartz I) assemblage, and the second formed the productive ore mineralization. The early assemblages crystallized at temperatures of 365-270 degreesC under pressures of 3460-1470 bar from solutions with salt concentrations of 14.9-2.4 wt % equiv. NaCl and a CO2 concentration of 6.3-3.1 mol/kg of solution. The density of the solution was 1.10-1.00 g/cm(3), and the density of the carbon dioxide phase was 1.04-0.78 g/cm(3). The quartz of the productive association crystallized at temperatures of 335-255 degreesC and pressures of 2860-810 bar from solutions with salt concentrations of 9.0-2.0 wt % equiv. NaCl and a CO2 concentration of 5.3-3.1 mol/kg of solution. The density of the solution was 1.08-0.96 g/cm(3), and the density of the carbon dioxide phase was 0.99-0.62 g/cm(3). The ore mineralization was deposited as a consequence of fluid boiling and devolatilization, which were caused by local tectonic motions. Gold-bearing mineralized veins developed at weak vertical variations in the physicochemical parameters, which was favorable for a great depth interval of ore deposition and a high ore potential at the lower levels of the deposit. The calculated oxygen isotopic composition of the aqueous phase in the fluid that produced quartz I (delta O-18(H2O) from +3.6 to +6.5 parts per thousand) and scheelite (delta O-18(H2O) from +2.8 to +5.4 parts per thousand) testify that the fluid was of magmatic and/or metamorphic provenance.