The accumulation mechanism of giant ores in the Xikuangshan antimony deposit, central Hunan: Constraints from fluid inclusions hosted in calcite

Although a lot of efforts have been made on the Xikuangshan deposit, the key scientific problem why such giant ores were accumulated in the narrow Xikuangshan area still remains unclear.The detailed investigation on the evolution of the ore-forming fluid and on the mechanism of its ore precipitation will be helpful for solving the above problem, since the giant ore accumulation in the hydrothermal deposit resulted from the convergence and effective precipitation of the large-scale hydrothermal fluid.Calcite is common in the Xikuanghan deposit and is closely related to antimony mineralization, thus it is an ideal mineral to study the mineralization process in the Xikuangshan ore district.Based on the researches on the petrography and microthermometry of fluid inclusions hosted in calcite, the fluid evolution process and ore precipitation mechanism are preliminarily depicted, and then the accumulation mechanism of giant ores in the Xikuangshan deposit is discussed.It is shown that, various calcites share similar fluid inclusion types, but their fluid inclusions display the distinct differences in view of petrography, the fluid inclusions hosted in those calcites of the early mineralization are rare in amount and small in size, the inclusions hosted in calcite of the late mineralization are relatively developed and relatively large in size, and the inclusions in the post-ore calcites are the most in amount and the largest in size.It was revealed that there are twice independent mineralization events happened in the Xikuangshan ore district.The early mineralization is characterized by the hydrothermal fluid with relatively high temperature and moderate salinity, which is beneficial to the large-scale migration of antimony, and the fluid mixing is responsible for the ore precipitation of early mineralization; the late ore-forming fluid is a kind of moderate-temperature and low-salinity hydrothermal solution, the antimony concentration in this kind of hydrothermal solution is significantly lower than that in the early mineralizing fluid, and the ore precipitation resulted from the fluid cooling.Moreover, there is another hydrothermal event after antimony mineralization with low-temperature and low-salinity solution.The giant accumulation of ore in the Xikuangshan ore district is mainly ascribed to the early mineralization, with high-concentration antimony in the ore-forming fluid and the giant flux of fluid, and the ore precipitation mechanism of fluid mixing in the early mineralization stage are favorable for the formation of the giant antimony deposits.