On the formation conditions of porphyry Cu-Mo polymetallic deposits in the Xiaosigou district

The host mass consists of quartz diorite-porphyry (marginal facies), granodiorite porphry (intermediate facies) and granite porphry (central facies). The petrographical facies are asymmetrically spread. The formation temperature of the rock mass ranges from 890 to 800°C, the pressure from 330 to 380 bars and the depth from about 1.5 to 2 km. Mineralization is controlled by the N-E contact zone and adjacent fault structures. Mo-mineralization occurs in the granite porphyry with strong potash metasomatism. Cu-Mo-mineralization is distributed in the inner contact zone of the rock mass, and the wallrock is K-silicified granite porphyry. Cu-pyrite-mineralization is recognized in skarn and serpentinite. Pb-Zn ore veins occur in marbleized limestones. Rock and mineral analyses, fluid inclusion studies and highT-P experiments indicate that extensive precipitation of Cu-bearing pyrite took place atT = 290–250°C, andP = 330–380 bars and that of copper atT = 400–310°C andP = 330–380 bars. Precipitation of Pb and Zn was followed by the transformation of hydrothermal solutions from alkaline to intermediate with decreasing temperature.