Fluid inclusions from the Jinchang Cu–Au deposit, Heilongjiang Province, NE China: Genetic style and magmatic-hydrothermal evolution

• Two distinct categories of fluid inclusions were recognized in terms of temperature and salinity.
• Unmixing of saline fluids and vapor-rich fluids directly exsolved from magmas as immiscible fluids.
• Cu–Au metallogenic temperatures vary in range of 480–200 °C.
• It is an oxidized intrusion-related Cu–Au deposit.

The Jinchang Cu–Au deposit is located in the easternmost part of the Central Asian Orogenic Belt (CAOB). The mineralization here is associated with Cretaceous porphyritic intrusions of ca. 120–110 Ma and is mainly hosted within intensely altered Mesozoic granites, breccia pipes, ringed and radial faults. The gold ores are disseminated in the wall rock, and also occur as massive auriferous quartz-sulfide veins in fractures and breccias. Fluid inclusion data from breccias, chalcopyrite–quartz vein, and pyrite–quartz vein display homogenization temperatures in the range of 200 to >550 °C and the fluids show variable salinity in the range of 0–58 wt.% (NaCl equiv.). The halite absent fluid inclusions shows a slight increase in salinity with a sharp decrease in temperature. We interpret that the high-salinity brine and low salinity vapor- or aqueous-rich fluids from ores represent fluid unmixing from magma. The fluid inclusion data from Jinchang demonstrate that the main mineralization took place at temperatures from 480 °C to 200 °C, and evolved from high to low salinities. Geology and fluid inclusion data suggest that the Jinchang deposit is an oxidized intrusion-related Cu–Au deposit. The voluminous hydrothermal magnetite formed during high temperature potassic alteration of wall rocks in this ore deposit can be used as a potential prospecting tool.