Re–Os sulfide (chalcopyrite, pyrite and molybdenite) systematics and fluid inclusion study of the Duobaoshan porphyry Cu (Mo) deposit, Heilongjiang Province, China

Abstract

The Duobaoshan porphyry deposit, located in the northwestern part of the Lesser Hinggan Range, is one of the biggest porphyry Cu (Mo) deposits in the Central Asian orogenic belt in China. The Duobaoshan porphyry deposit occurs in granodiorite and volcanic rocks of the Middle Ordovician Duobaoshan Formation. Six types of veins have been identified in three ore-forming stages as follows: a quartz–potassic feldspar vein in the early ore-forming stage, an early stage quartz–molybdenite vein, late stage quartz–molybdenite and quartz–chalcopyrite–pyrite veins in the middle ore-forming stage, and quartz–pyrite and calcite–quartz veins in the late ore-forming stage. The following four types of fluid inclusions are distinguished from various quartz veins: two-phase aqueous, pure gas phase, CO2-bearing and daughter mineral-bearing inclusions.The ore-forming fluid for the early ore-forming stage belongs to the H2O–CO2–NaCl system, which is characterized by high temperatures (>550°C), intermediate salinities (16.2–18.1wt% NaCl eqv.) and high CO2 content. The ore-forming fluid from the middle ore-forming stage evolved to the H2O–CO2–NaCl system, which is characterized by intermediate to high temperatures (230–450°C) and high/low salinities (0.8 to >65.3wt% NaCl eqv.) and is also rich in CO2 and metals. The ore-forming fluid finally reached cool temperatures (110–200°C), low salinities (3.9–8.4wt% NaCl eqv.) and was CO2-poor. Intensive fluid immiscibility or boiling occurred when the ore-forming fluid with temperatures of 230–450°C and pressures of 10–41MPa ascended to 4.1km, inducing the escape of CO2, depressing the solubility of fluid, and depositing abundant metal sulfides.The total Re and Os concentrations of chalcopyrite and pyrite range from 0.15 to 2.95μg/g and 0.74 to 15.01ng/g, respectively. Analyses of seven chalcopyrite and pyrite samples yielded isochron ages of 482–486Ma, and the model age of one molybdenite sample is 485.6±3.7Ma. The mineralization age of the Duobaoshan deposit is between 482 and 486Ma, and mantle fluid and materials were involved in the mineralization process. The Duobaoshan deposit is a classic porphyry Cu (Mo) deposit that formed in an early Palaeozoic island arc tectonic setting.