Fluid Inclusions and Stable Isotope Geochemistry of Gold Mineralization Associated with Fine-Grained Granite: A Case Study of the Xiawolong Gold Deposit, Jiaodong Peninsula, China

The Xiawolong gold deposit, located in the Muping-Rushan gold metallogenic belt (eastern Jiaodong Peninsula), is a newly discovered deposit that developed in the late Early Cretaceous as fine-grained granite. Gold mineralization, which mainly occurs in the middle of fresh fine-grained granite dikes, consists of stockwork-style and disseminated ores. They are characterized by middle-high-temperature mineral assemblages, such as molybdenite and magnetite, associated with gold-bearing pyrite. Four types of primary fluid inclusions, contained within the quartz grains from the gold-bearing disseminated and stockwork-style fine-grained granitic ores, were identified based on microthermometry and Raman spectroscopy. The types identified were type 1 aqueous inclusions with middle-high temperature (201 to 480 degrees C) and middle-low salinity of 0.18 to 17.00 wt.% NaCl equiv.; type 2 H2O-CO2 inclusions, which show middle-high temperatures (218 to 385 degrees C), middle-low salinities (1.23 to 13.26 wt.% equiv. NaCl), and variable XCO2 (0.031 to 0.044); type 3 daughter mineral-bearing inclusions with high temperature (416 to 446 degrees C) and relatively constant and high salinity (28.59 to 32.87 wt.% NaCl equiv.); and type 4 CO2 fluid inclusions, which possess a bulk density of 0.405 to 0.758 g/cm(3) and a constant XCO2 (0.952 to 0.990) (according to the decreasing abundance of fluid inclusions). The delta O-18(water) range is between 3.4 and 5.9 parts per thousand, and the range of the delta D is from -97.1 to -77.4 parts per thousand, which indicates that the ore-forming process is of a magmatic water origin. The delta S-34 values possess a narrow range between 4.5 and 9.3 parts per thousand, indicating the source of the Mesozoic Kunyushan granitoids. The Pb isotopic compositions of pyrite show that the Mesozoic Kunyushan granitoids are the main lead source for pyrites. Types 1, 2, and 3 fluid inclusions coexist in the same view field of the quartz grain, which are suggested to occur as the result of fluid immiscibility because of the boiling of a single homogeneous NaCl-CaCl2-KCl-CO2-H2O system. The fluid immiscibility, rather the fluid mixing and wall-rock sulfidation, is the mechanism of gold precipitation in the Xiawolong deposit. Compared with both the "Linglong-type" and "Jiaojia-type" gold deposits in the Jiaodong Peninsula in terms of geological-petrographic evidence and all of the available geochemical data, it can be concluded the Xiawolong gold deposit is of magmatic hydrothermal origin, having a genetic relation to the fine-grained granite.