Mineralogy, ore-forming fluids and geochronology of the Shangmachang and Beidagou gold deposits, Heilongjiang province, NE China

Abstract

The newly discovered Shangmachang epithermal gold (>11t Au; Heilongjiang province, NE China) and Beidagou epithermal tellurium-gold (>5t Au) deposits are located at the northeastern margin of the Central Asian Orogenic Belt and are hosted in Early Cretaceous volcanic rocks. At the Shangmachang deposit, gold occurs largely as electrum microparticles (5–120μm) with a fineness of approximately 401–697 in quartz, calcite, and pyrite. Silver-bearing minerals include electrum, polybasite, and minor argentite. The main mineral paragenetic assemblages consist of pyrite-electrum and galena-polybasite, while the hydrothermal alteration minerals include quartz, calcite, sericite, chlorite, and pyrite. Trace elements in the gangue minerals show that the Al contents of gold-bearing quartz veins are lower than those of pure quartz veins (early stage). Fluid inclusions in quartz and calcite mainly comprise liquid-vapor, and the components contain a liquid phase consisting of Na+, Cl−, and SO42− and a gas phase consisting of N2, CO2, and CH4. The Shangmachang fluid inclusions homogenize at 142.8°–303.0°C and are of low salinity (0.2–8.8wt% NaCl equiv.), while the Au mineralization formed at 150°–225°C. The calcite δ18Ofluid values range from −17.8‰ to −13.1‰, and the carbon isotope compositions range from −8.2‰ to −4.5‰. Zircon U-Pb dating of the Longjiang Formation andesite yields 119.4±1.9Ma, and the granite yields 118.6±1.2Ma, suggesting that local magmatism occurred during the Early Cretaceous. Calcite Sm-Nd dating suggests that the Au mineralization has an age of approximately 113.6±4.0Ma.At the Beidagou deposit, the main mineral paragenetic assemblages consist of sphalerite-galena-hessite and chalcopyrite-altaite-petzite-hessite, while the hydrothermal alteration minerals include quartz, calcite, anhydrite, chlorite, and pyrite. The components of the fluid inclusions in quartz contain a fluid phase consisting of Na+, K+, Cl−, and SO42− and a gas phase consisting of N2, CO2, CH4, and C2H6. The Beidagou fluid inclusions homogenize at 133.8°–370.1°C and are of low salinity (0.2–8.7wt% NaCl equiv.), while the Au mineralization formed at 175°–250°C. The δ18Ofluid and δDSMOW values of the quartz samples from Beidagou range from −8.2‰ to −5.0‰ and −101.6‰ to −92.8‰, respectively. The Al contents of gold-bearing quartz veins are higher than those of pure quartz veins. Rb-Sr dating of ore-bearing quartz veins suggests that the Te-Au-Ag mineralization has an age of about 115.5±4.4Ma.In summary, the interpretation of new data from the geological setting, fluid inclusions, O-H-C isotopes, and geochronology, suggests that the ore-forming fluids predominantly comprised meteoric fluids, while the metal content was derived from a mantle source at both the Shangmachang and Beidagou deposits. A geochemical model suggests that the fluids responsible for quartz deposition had a neutral pH and that adiabatic boiling and mixing triggered the gold, silver and tellurium precipitation. Both Shangmachang and Beidagou are interpreted as low-sulfidation epithermal gold deposits.