Geology, Geochemistry And Genesis Of The Baoshan Copper Polymetallic Deposit, Hunan Province

The Baoshan deposit is the largest Cu deposit in the central section of the QinHang ore belt, which is located at the intersection of Qin-Hang NE-trending ore belt and Nanling EW-trending polymetallic metallogenic belt. Based on detailed field investigation and microscope observation of the Baoshan copper depsosit, this paper takes the representative Baoshan copper-polymetallic deposit as an example. We systematically conduct the studies of mineralogy, elemental geochemistry, fluid geochemistry and stable isotope geochemistry and obtain the following preliminary conclusions:(1) The mineralizations of the Baoshan copper polymetallic deposit can be divided into 4 stages, i.e., skarn stage, retrograde stage, metal sulfides stage and fluorite-calcite vein stage.(2) Electron microprobe analyses of the skarn minerals show that the garnets in the Baoshan deposit are mainly andradite-grossularite and varied from andradite to grossular during the evolution process. The pyroxene is diopside which appears with minor hedenbergite. The amphibole is dominated by actinolite, followed by cannilloite. The end member of the epidote group is dominated by epidote, and contains a little brunsvigite and ripidolite. Based on the above, it is suggested that the skarn in the Baoshan copper-pollymetallic deposit is calcareous skarn, with minor magnesium skarn.(3) We futher studied the microscale in-situ of the garnet by LA-ICP-MS analysis. Most of the garnets are depleted in LILEs and enriched in HFSEs. Rare earth element patterns of the andradite and grossularite are similar to those of the granodiorite porphyry in the Baoshan deposit, namely, LREE enrichment and HREE depletion with significant differentiation between HREE and LREE.(4) The study of fluid inclusions indicates that the main types of fluid inclusions for Baoshan deposit are liquid-rich inclusions. The ore-forming fluid of Baoshan deposit belongs to system of H2O-NaCl. There is a little CO2 in the gas phase and a few NaCl daughter minerals in the fluid inclusions; The homogenization temperatures of the fluid inclusions range from 430°C to >550°C(mainly above 550°C) and the salinities are between 13.4% and 21.98% NaCleqv in the skarn stage. The homogenization temperatures of the fluid inclusions range from 211.8°C to 395°C(mainly 260~320°C and 360~395°C) and the salinities range from 3.71% to 19.53% NaCleqv in the retrograde stage. The pressure of this stage changed from lithostatic pressure to hydrostatic pressure. In metal sulfides stage, the homogenization temperatures of this stage range from 156.7°C to 323.1°C(mainly 190~240°C) and the salinities range from 3.71% to 19.84% NaCleqv. In fluoritecalcite vein stage, the homogenization temperatures are between 100°C and 266.5°C(concentrated in 145~180°C) and the salinities are 3.71%~19.84% NaCleqv. The density of the fluid keeps increasing which is centered in 0.65-1.05g/cm3. The trapping pressure can be estimated to be about 23.8~169.9Mpa which changed from lithostatic pressure to hydrostatic pressure, equivalent to a depth of 2.25~6.29 km which is centered in 4~6km. The copper polymetallic mineralization in Baoshan deposit is related to the fluids from early high-temperature, high salinity to late lowtemperature, low salinity.(5) The H and O isotope study of different minerals indicates that the hydrogen and oxygen data fall mainly in the magmatic water and the transition belt of the magmatic water and meteoric water of the δD-δ18O diagram. Thus the ore-forming fluid was mainly magmatic water in the early stage, with progressive mixing with proportionally more cooler meteoric water during the process of fluid evolution. The mixing of high and low temperature fluids maybe the main mechanism for copper polymetallic precipitation.(6) According to the above studies, we preliminarily discussed the physicochemical conditions of the formation of the ores and the mechanism for copper polymetallic precipitation. The fluid has a high oxygen fugacity in the skarn stage and a low oxygen fugacity in the last three stages. During the process of the fluid evolution, the changes of the temperature, pressure, salinity, pH, oxygen fugacity and fluid mixing processes are responsible for metal minerals precipitation. Compared Baoshan deposit with the typical skarn deposit associated with porphyry deposit, the lack of the boiling inclusions may indicate that the ore-forming pressure is relatively high and the porphyry deposit is lacking in this deposit.