Rb-Sr Dating of Gold-Bearing Pyrite in the Jinchang Porphyry Cu-Au Deposit, Heilongjiang Province, NE China

The Jinchang Cu-Au deposit in Heilongjiang Province, NE China, is located in the easternmost part of the Central Asian Orogenic Belt. Rb-Sr analyses of auriferous pyrite from the deposit yielded an isochron age of 113.7 +/- 2.5Ma, consistent with previously reported Re-Os ages. Both sets of ages represent the timing of Cu-Au mineralization because (i) the pyrite was separated from quartz-sulfide veins of the mineralization stage in granite porphyry; (ii) fluid inclusions have relatively high Rb, Sr, and Os content, allowing precise measurement; (iii) there are no other mineral inclusions or secondary fluids in pyrite to disturb the Rb-Sr or Re-Os decay systems; and (iv) the closure temperatures of the two decay systems are 500 degrees C (compared with the homogenization temperatures of fluid inclusions of 230-510 degrees C). It is proposed that ore-forming components were derived from mantle-crust mixing, with ore-forming fluids being mainly exsolved from magmas with minor amounts of meteoric water. The age of mineralization at Jinchang and in the adjacent regions, combined with the tectonic evolution of the northeast China epicontinental region, indicates that the formation of the Jinchang porphyry Cu-Au deposit was associated with Early Cretaceous subduction of the paleo-Pacific Plate.