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

- Seven pyrite separates from the Zijinshan Cu-Au deposit yield an Re-Os isochron age of 103 ± 4 Ma, coeval with the Luoboling Cu-Mo deposit.
- The initial Os isotope composition of 0.45 suggests that the Os in the hydrothermal fluids may be derived from the crust with input from the mantle.
- Analysis of one pyrite from the deposit shows a disturbance of the Re-Os system, and thus it should be cautious in the interpretation of such data.

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 ± 4 Ma 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 2 km 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.