Hydrothermal evolution and source of metallogenic materials of Beiya Au polymetallic deposit,western Yunnan,China

The Beiya porphyry-skarn gold-polymetallic deposit is one of the largest gold deposits in China and it also contains significant amounts of silver and base metals. The ore-bearing monzonitic granite porphyry occurs as a stock, of which the skarn type gold-copper-iron ore bodies are controlled by the contact zone between alkali-rich monzonitic granite porphyry and the limestone, and the gold-silver polymetallic mineralization is controlled by interlayer structure. Alteration and mineralization occur around the intrusion and exterior of monzonitic granite porphyry. Ore mineral formation sequence is as follows: skarn minerals→magnetite→pyrite→chalcopyrite/bornite+pyrite+gold→pyrite+galena+gold(silver). Petrographic studies of fluid inclusions indicate that the following types of inclusions exist in the pre-mineralization quartz-pyrite stage: gas-liquid two-phase inclusions(L-type), three-phase inclusions with daughter minerals(D-type) and gas-rich inclusions(V-type). The colorless transparent quartz in the main gold-chalcopyrite-pyrite stage mainly consists of L-type and V-type inclusions, whereas the inclusions in the late gold-silver-galena stage are mainly L-type. The evolution of ore-forming fluids shows a trend from high temperature, high salinity to medium-low temperature and low salinity. Medium-low density fluids play a dominant role in mineral component migration and transportation. Fluid cooling and boiling are the main mechanisms of gold-copper precipitation, while the involvement of atmospheric water and pH reduction are the main mechanisms of gold-silver polymetallic precipitation. The fluids in the quartz-pyrite stage before mineralization and the main gold-chalcopyrite-pyrite stage are dominated by magmatic water, while in the gold-silver-galena stage the fluids are dominated by atmospheric water. Isotope tracers show that S and Pb are mainly derived from monzonitic granite porphyry, not from limestone of the Beiya Formation.