Geology And Fluid Geochemistry Of The Hucunnan Cu-Mo Deposit, Tongling, Anhui Province

The Hucunnan deposit is a skarn-porphyry multiple Cu-Mo deposit, which is first found in the Tongling Cu(Au) ore-concentrated district of Anhui province and appears special and typical in the Middle-Lower Yangtze metallogenic belt. A comprehensive study has been carried out on the ore deposit geology and fluid geochemistry of this deposit in order to ascertain its fluid mineralization process.The fluid mineralization process is divided into three periods, i.e., high temperature pneumatolytic, middle to high temperature hydrothermal, and low temperature hydrothermal periods. Potash feldspathization and skarnization occurred in the first period, chloritization, epidotization and sericitization in the second period, and carbonization in the third period. Cu-Mo mineralization mainly took place in the second period, forming sulfide ore veins of molybdenite and chalcopyrite.Liquid-rich aqueous inclusions with daughter mineral-bearing multiphase inclusions are observed in minerals of the first and second periods, with minor vapor-rich aqueous inclusions in minerals of the second periods. In contrast, only liquid-rich aqueous inclusions occur in minerals of the third period. Homogenization temperature and salinity of fluid inclusions decrease from over 435℃ and 14.0~64.9% Na Cleq in the first period through 203~458℃ and 4.6~47.5% Na Cleq in the second period to 156~276℃ and 1.0~15.5% Na Cleq in the third period, respectively. From the early to the late mineralization period, the ore-forming fluids evolved from high temperature, high pressure and high salinity to moderate-low temperature, low pressure and low salinity fluids. Cu-Mo mineralization mainly took place in the quartz- molybdenite stage and the quartz- chalcopyrite stage of the second period. Depth of fluid mineralization ranges from 0.6 to 1.5 km.The hydrogen and oxygen isotopic analyses for the ore-forming fluids of the deposit indicate major derivation of the ore-forming fluids from magmatic water in the second period, but from mixed fluid of magmatic water with meteoric water in the third period. The carbon and oxygen isotopic data are consistent with derivation of carbon mainly from a mixed magma of mantle with crust source and dissolve marine carbonates.The geological and geochemical analyses indicate occurrence of fluid immisciblization, boiling and mixing during evolution of ore-forming fluids. The fluid immisciblization occurred in the first period, leading to high enrichment of ore-forming elements in the ore-forming fluids, while the fluid boiling existed in the second period, resulting in unloading of ore-forming elements in the ore-forming fluids responsible for the precipitation of many sulfides. The fluid mixing existed in the third period, resulting in further reducing temperature and salinity of the ore-forming fluids.