Fluid evolution and geobarometry on the Ohtani and Kaneuchi tungsten-quartz vein deposits,Japan: oxygen and carbon isotopic evidence

The Ohtani and Kaneuchi deposits are tungsten-bearing quartz veins of hypothermal type which are hosted by Cretaceous granodiorite and sedimentary rocks respectively. Oxygen and carbon isotopic compositions were measured on minerals associated with the deposits to determine whether the ore-forming fluids were derived from related igneous bodies. The oxygen isotopic equilibrium temperatures of quartz-muscovite pairs within veins from the Ohtani and Kaneuchi deposits range from 530° to 190°C and from 400° to 300°C, respectively. The equilibrium temperatures of the granodiorite (600°C), greisen (590° to 530°C) and veins in the Ohtani deposit decrease in this order. The calculated oxygen isotopic composition of the Ohtani ore-forming fluid is around 12 (SMOW) above 500°C and lowers successively toward 3 as temperature decreases. The fluid at the earliest stage of mineralization could be in equilibrium with the granodiorite at about 600°C. These lines of evidence suggest that the granodiorite magma played a crucial role in the mineralization. The evolution trend of the Ohtani ore-forming fluid can be explained by a combined process of cooling due to heat conduction, adiabatic expansion and mixing of magmatic water with ground water. Calcites from the deposits have carbon isotopic ratios ranging from -11 to + 2 with the oxygen isotopic ratios in a narrow range. Since the CO₂/CH₄ fugacity ratio of the fluid is estimated to have been near unity, a slight fluctuation in fO₂ caused the large variation in the carbon isotopic composition of calcite. The formation pressures for the Ohtani and Kaneuchi deposits are calculated to be 1.5 kb and 1.4 kb, respectively, on the basis of the difference between the pressure-dependent homogenization temperatures of fluid inclusions and the pressure-independent isotopic equilibrium temperatures. These values are reasonable for the depths in which hypothermal ore deposits have formed.