Of Heilongjiang Wulaga gold subvolcanic magma - Hydrothermal mineralization: Melt - Fluid inclusions evidence

The Wulaga gold deposit, located in the northeastern edge of the joint of Paleo-Asiatic and Pacific tectonic domains, is one of the most important gold deposits in China. The ore bodies are mainly hosted in secondary faults of the Wulaga fault, and cryptoexplosive breccia zone within the Tuanjiegou plagioclase granite-porphyry, and in the layer fractures of metamorphic Heilongjiang Group. There are three types of fluid inclusions in quartz phenocryst of plagioclase granite-porphyry, that is, melt inclusions, primary L-V and L-V-S inclusions, as well as secondary L-V inclusions. Glassy melt inclusions are characterized by acid magma (Si0 2 = 69. 5% ∼73. 8%), with the trapping of temperatures higher than 800°C. Secondary L-V inclusions in quartz phenocryst have 210 ∼ 350°C of homegenization temperatures (Th), which represents the features of sub-volcanic hydrothermal fluids and are coincide with those of pyrite-white chalcedony quartz stage (Ql), while salinities (5% ∼7%NaCleqv) are slightly higher than those of Ql. Gold mineralization of the Wulaga can be divided into three stages. Fluid inclusions in pyrite-early white chalcedony quartz stage (stage I) are characterized by aqueous solution with homogenization temperatures of 154 ∼ 355°C, mainly in 230 ∼21Q°C. Salinities of fluid inclusions are 1.3% ∼8. 2% NaCleqv, with densities of 0. 53 ∼0. 88g/cm 3. Homogenization temperatures of fluid inclusions in smoky gray chalcedony quartz-polymetallic sulfide stage (stage II) are 159 ∼ 196°C, mainly in 170 ∼ 190°C, with salinities of 2. 2% ∼ 3. 2% NaCleqv and densities of 0. 79 ∼ 0. 92g/cm 3. Those in calcite of carbonate-quartz stage (stage III) are mainly in 170 ∼ 270°C, with salinities of 0. 5% ∼ 2. 9% NaCleqv and densities of 0. 80 ∼ 0. 88g/cm. Ore-forming fluids in the main mineralization stage are characterized by mid to low temperatures, low salinities, and lack of CO 2, which is similar with epithermal deposit related with continental volcanic-subvolcanic rocks in the world. Melt and fluid inclusion study show that gold mineralization has a close relationship with plagioclase granite-porphyry, and it is possible for silicate magma to produce salt-aqueous solution through immiscibility in magmatic differentiation. The Wulaga gold deposit should belong to epithermal gold deposit related to Cretaceous plagioclase granite-porphyry.