Fluid evolution of the Jiawula Ag-Pb-Zn deposit, Inner Mongolia: mineralogical,fluid inclusion, and stable isotopic evidence

摘要

The Jiawula Ag-Pb-Zn deposit lies in the renowned Ag-polymetallic metallogenic province of northern China. The origin of this structurally controlled ore body is linked to fluid migration and mineralization along cogenetic fault systems. Sulphur isotopic compositions suggest that the ore-forming aqueous solutions were derived mainly from deep magmatic fluids. Hydrogen and oxygen isotopic compositions indicate that these fluids were magmatic during early stages of ore formation and meteoric during late-stage mineralization. Lead isotopic compositions indicate that this metal was derived mainly from a mantle source, and to a lesser extent from a crustal source. Collectively, the isotopic data indicate that formation of the Jiawula Ag-Pb—Zn deposit was ultimately a reflection of late Yanshanian (140-120 Ma) volcanic-subvolcanic hydrothermal activity. The addition of meteoric water to these magmatic hydrothermal fluids created favourable conditions for mineralization. During ore formation, metallogenesis took place in a relatively open, non-equilibrium system under conditions of low (δ34)SΣs~ and an intermediate oxidation state.Microthermometric study of fluid inclusions indicates homogenization temperatures of 180-260°C. Salinities, densities, pressure, and depth of ore-forming fluids ranged from 0.18 to 12.62 wt.% NaCl eqv., 0.637 to 0.976 g/cm3, 3.44 to 162.05 bar, and 0.5 to 1.5 km, respectively. Laser Raman studies of single-phase fluid inclusions show that the ore-forming fluids belong to the H_2O-NaCl system. Analysis of bulk chemical compositions of fluid inclusions indicates that the ore-forming fluid can be classified as the Na~+-Ca~(2+)-SO_4~2-Cl fluid type. All obtained geochemical data demonstrate that the ore-forming fluids of the Jiawula Ag-Pb-Zn deposit are medium- to low-temperature, medium- to low-pressure, medium- to low-salinity, and low-density fluids. Based on their compositions, they can be classified into two end-members: magmatic hydrothermal fluid and meteoric water. The key factors allowing for metal transport and precipitation during ore formation include the sourcing of magmatic fluids with high contents of metallogenic elements and the mixing of these hydrothermal fluids with meteoric waters resulting in the formation of a large Ag—Pb-Zn deposit. In terms of genetic type, the Jiawula deposit can be regarded as a volcanic-subvolcanic hydrothermal vein Pb-Zn—Ag ore deposit.