Formation of the Dongmozhazhua Pb-Zn Deposit in the Thrust-Fold Setting of the Tibetan Plateau, China: Evidence from Fluid Inclusion and Stable Isotope Data

摘要

The Dongmozhazhua deposit, the largest Pb-Zn deposit in south Qinghai, China, is stratabound, carbonate-hosted and associated with epigenetic dolomitization and silicification of Lower-Middle Permian—Upper Triassic limestones in the hanging walls of a Cenozoic thrust fault system. The mineralization is localized in a Cenozoic thrust-folded belt along the northeastern edge of the Tibetan plateau, which was formed due to the India-Asia plate collision during the early Tertiary. The deposit comprises 16 orebodies with variable thick?nesses (1.5-26.3 m) and lengths (160-1820 m). The ores occur as dissemination, vein, and breccia cement. The main sulfide assemblage is sphalerite + galena + pyrite + marcasite ± chalcopyrite ± tetrahedrite, and gangue minerals consist mainly of calcite, dolomite, barite, and quartz. Samples of pre- to post-ore stages calcite yielded δ~(13)C and δ~(18)O values that are, respectively, similar to and lower than those yielded by the host limestones, suggesting that the calcite formed from fluids derived from carbonate dissolution. Fluid inclusions in calcite and sphalerite in the polymetallic sulfidization stage mostly comprise liquid and gas phases at room temperature, with moderate homogenization temperatures (100-140°C) and high salinities (21-28 wt% NaCl eq.). Micro-thermometric fluid inclusion data point to polysaline brines as ore-forming fluids. The δ and δ~(18)O values of ore fluids, cation compositions of fluid inclusions, and geological information suggest two main possible fluid sources, namely basinal brines and evaporated seawater. The fluid inclusion data and regional geology suggest that basinal brines derived from Tertiary basins located southeast of the Dongmozhazhua deposit migrated along deep detachment zones of the regional thrust system, leached substantial base metals from country rocks, and finally ascended along thrust faults at Dongmozhazhua. There, the base-metal-rich basinal brines mixed with bacterially-reduced H_2S-bearing fluids derived from evaporated seawater preserved in the Permo-Triassic carbonate strata. The mixing of the two fluids resulted in Pb-Zn mineralization. The Dongmozhazhua Pb-Zn deposit has many characteristics that are similar to MVT Pb-Zn deposits worldwide.