Re-Os dating of sulfides from the volcanogenic massive sulfide deposit at Gacun, southwestem China

The Re-Os isotopic compositions of sulfide ores were analyzed for the Gacun, a volcanogenic massive sulfide deposit in southwestern China, to constrain the timing of mineralization. Sulfide ores from the deposit have a wide range of Re and Os concentrations, varying from 80.2 to 1543.2 ppb and from 0.307 to 8.83 ppb, respectively, and yielded a limited field of high Re-187/Os-188 and high (OS)-O-187/Os-188 ratios, ranging from 1452 to 3309 and from 5.77 to 13.24, respectively. All sulfide samples yielded an isochron with an age of 217+/-28 Ma and an initial Os-187/Os-188 ratio of around 0.52+/-0.73. The Re-Os isochron age agrees with ages previously constrained by the other isotopic dating of the host rocks and fossil strata for the deposit. The rhythmic variation in (OS)-O-187/Os-188 and Re-187/Os-188 ratios within massive sulfide zone records a complicated process for ore-forming fluids episodically vented into the brine pool on the Mesozoic seafloor.