Study on the geochemical characteristics of ore-forming fluids and genesis of Xiaoxinancha gold-copper deposit, Jilin Province

Xiaoxinancha gold-copper deposit occurred in granodiorite-quartz diorite intrusions of Hercynian epoch. The mineralization of it mainly composed of large quartz vein type and fine quartz veinlet type, with molybdenite-quartz veins superimposed only locally. Fluid inclusion study shows that quartz in both of gold-copper ore veins and in molybdenite-quartz veins contain the same kinds of fluid inclusions, which include NaCl daughter mineral-bearing, gaseous and gas-rich as well as aqueous two-phase fluid inclusions. The similarity of homogenization temperature and salinity of the same kind of fluid inclusions reveals that the geochemical nature of ore-forming solutions of gold-copper and molybdenum mineralization have much in common. Hydrogen-oxygen isotopes of fluid inclusions imply they both came from magmatic fluids. Re-Os isotopic dating of molybdenite shows that molybdenum mineralization happened in ca. 109Ma, whereas (40)Ar-(39) Ar isotopic dating of quartz in gold-copper veins reveals that gold-copper mineralization happened in 123.35 +/- 0.8Ma. Combined with the results of chronological study of magmatic intrusions in the mining area, the conclusion were made that the origin of gold-copper mineralization mainly related with fine grained granodiorite intrusion of late phase of Yanshan, while the origin of molybdenum mineralization mainly related with granite porphyry of late phase of Yanshan, which successively emplaced after fine grained granodiorite intrusion in the mining area.