Study on Ore-forming Fluid Inclusions and Geological Characteristics of the Gongpoquan Porphyry Copper Deposit in Beishan, Gansu Province

Gongpoquan copper deposit is a typical porphyry copper deposit located in the Gongpoquan island arc zone of Gansu-Mongolia-Beishan porphyry copper metallogenic belt. The host rocks are characterized by dacite porphyry, granodiorite porphyry and quartz diorite porphyrite. The ore minerals mainly consist of chalcopyrites and bornites, with few pyrites, sphalerites, galenas and digenites etc. The study of fluid inclusions in the ore-bearing vein-quartz concludes four types of fluid inclusions namely pure vapor inclusions (I-type), pure aqueous inclusions (II-type), nature aqueous-rich gas-liquid two phase inclusions (IIIa-type) and secondary aqueous-rich gas-liquid two phase inclusions (IIIb-type). The mineralization is closely related to nature aqueous-rich gas-liquid two phase inclusions, i.e. IIIa-type. This kind of fluid inclusions have homogenization temperature of 103-250℃, salinity of 0.18%-9.98%(NaCl_eq), and density of 0.75-1.0g/cm³, which shows that the ore-forming fluid is mesothermal, with low salinity and low density. Laser-Raman microspectroscopy analyses reveal that the liquid phase is mainly composed of H₂O, and the vapor phase of H₂, N₂ and CO₂. The rising and emplacement of porphyry provided ore-forming materials such as Cu and S for the formation of the copper deposit. Due to late tectonic activity, the porphyry became mobilized and generated ore-forming fluid through differentiation, the decompressing boiling of which resulted in separation of phases whilemigrating upward, and then Cu precipitated. Therefore, the decompressing boiling might be mainly responsible for the precipitation of metal in fluid.