High salinity fluid inclusions in the Yinshan polymetallic deposit from the Le–De metallogenic belt in Jiangxi Province, China: Their origin and implications for ore genesis

A fluid inclusion investigation of the polymetallic mineralization at Yinshan from the Le–De metallogenic belt in Jiangxi Province of China has been carried out using petrographic and microthermometric techniques. The data obtained here indicate that three major types of fluids were involved during the formation of the deposit. They are type I vapor-rich, type II liquid-rich and type III halite-bearing inclusions within the H₂O–NaCl system. The high salinity fluids represented by type III inclusions, being unusual to the distal part of an intrusion-centered ore-forming system such as Yinshan, have been interpreted as the product of direct exsolution of a crystallizing magma, rather than a result of fluid immiscibility from a low salinity fluid. Evidence used to support such an interpretation includes the mode of homogenization of type III inclusions exclusively via halite dissolution, spatial separation of type I and type III inclusions on microscopic scale, the consistent phase ratios within the inclusions concerned, and considerable deviation in homogenization temperature for both type I and type III inclusions. Trapping conditions for type I inclusions were estimated to be around 440 °C and 260 bars, while type III inclusions were constrained to be trapped at least above 900 bars and > 500 °C. The formation temperatures for type II inclusions range from 270 to 390 °C if a lithostatic pressure of 260 bars is assumed. Pressure fluctuation determined by this fluid inclusion study coupled with decreases in salinity and temperature as result of the potential fluid mixing are supposed to have played an important role in triggering the precipitation of ore minerals from the hydrothermal solution.