Pyrite Re-Os isotope systematics at the Zijinshan deposit of SW Fujian, China: Constraints on the timing and source of Cu-Au mineralization

Abstract

Zijinshan is the largest high-sulfidation epithermal Cu–Au deposit on mainland China. The primary mineralization at the deposit is characterized by pyrite, digenite and covellite. Although some RbSr isochron ages of the fluid inclusions in quartz and the apparent K-Ar age of the Cu-bearing alunite alteration zone have been previously interpreted as the ore-forming age, the Rb-Sr and K-Ar dating systems are usually readily reset due to their closure temperature, and thus document the latest thermal event. In order to precisely determine the age of the Cu mineralization, eight-pyrite separates were Re-Os dated, and seven yielded an isochron age of 103±4Ma with an initial 187Os/188Os ratio of 0.45±0.14. This date is interpreted as the age of Cu mineralization, which is in agreement with geochronological data from molybdenite and the porphyritic granodiorite that hosts Cu-Mo mineralization at Luoboling (located 2km NE of Zijinshan), suggesting that these two deposits were formed during the same metallogenic event. The relatively low initial 187Os/188Os ratio indicates that the source of the Cu at Zijinshan is predominantly the crust with input from the mantle.