Fluid inclusion study on Wulaga gold deposit in Heilongjiang Province and implications for ore genesis

Abstract：Wulaga gold deposit is controlled by both fracture zone and intermediate-acidic porphyry. In the mining district, Heilongjiang Group metamorphic rocks are considered as the main source bed. The Yanshanian plagioclase-granitic porphyry is the wall rock and provided the heat source for the gold mineralization process. The locations of the ore bodies are the places of fault intersection or the fault extensional sections due to attitude change. And the ore bodies are distributed in raking and stepwise pattern. The ore minerals are mainly composed of native gold, pyrite, marcasite, stibnite and native silver while the gangue minerals are mainly chalcedony quartz, colloid opal, carbonate, adularia, sericite and kaolinite. Gold ores are of colloidal, fine grain and cataclastic textures and veinlike, stockwork and brecciated structures. Thus, the gold ores show typical low temperature characteristics in texture, structure and mineral assemblages. Based on microscopic study, it is found that aqueous two-phase and small amount of pure aqueous-phase fluid inclusions dominate in quartz. It is suggested that the ore-forming fluid belongs to H2O-NaCl system and the fluid is characterized by low salinity (from 1.22% to 5.4% NaCl.eqv) and low density (from 0.88 to 0.96 g/ cm3). The forming temperature ranges from 140℃ to 220℃. The average value of pressure is 23.24 MPa and the mineralizing depth is about 2.32 km. With respects to stable isotopic features of the fluid inclusions, the values of δDSMOW are between -78.25‰ and -132.64‰ while those of δ18OH2O are from -3.7‰ to 6.23‰. It is concluded that the ore forming fluids are mainly meteoric water. Finally, it is suggested that the deposit belongs to a low sulfadation epithermal gold deposit that was formed from neutral, reduced and low temperature hydrothermal system.