Geochemical Characteristics and Sources of Ore-forming Fluids of the Mayuan Pb-Zn Deposit, Nanzheng, Shaanxi, China

The Mayuan stratabound Pb-Zn deposit in Nanzheng, Shaanxi Province, is located in the northern margin of the Yangtze Plate, in the southern margin of the Beiba Arch. The orebodies are stratiform and hosted in breciated dolostone of the Sinian Dengying Formation. The ore minerals are primarily sphalerite and galena, and the gangue minerals comprise of dolomite, quartz, barite, calcite and solid bitumen. Fluid inclusions from ore-stage quartz and calcite have homogenization tempreatures from 98 to 337°C and salinities from 7.7 wt% to 22.2 wt% (NaCl equiv.). The vapor phase of the inclusions is mainly composed of CH4 with minor CO2 and H2S. The δDfluid values of fluid inclusions in quartz and calcite display a range from –68‰ to –113‰ (SMOW), and the δ18Ofluid values calculated from δ18Oquartz and δ18Ocalcite values range from 4.5‰ to 16.7‰ (SMOW). These data suggest that the ore-forming fluids may have been derived from evaporitic sea water that had reacted with organic matter. The δ13CCH4 values of CH4 in fluid inclusions range from –37.2‰ to –21.0‰ (PDB), suggesting that the CH4 in the ore-forming fluids was mainly derived from organic matter. This, together with the abundance of solid bitumen in the ores, suggest that organic matter played an important role in mineralization, and that the thermochemical sulfate reduction (TSR) was the main mechanism of sulfide precipitation. The Mayuan Pb-Zn deposit is a carbonate-hosted epigenetic deposit that may be classified as a Mississippi Valley type (MVT) deposit.