Yandong large-size porphyry copper deposit is located in the Late Paleozoic Dananhu island arc in Eastern Tianshan,Xinjiang.Lower Carboniferous Qi'eshan Group occurred in the Yandong region.New results recognize a volcanic apparatus at Yandong that includes two cycles comprising five lithofacies.The first cycle consists of the effusive(basalt and andesite),explosive(auto-brecciated lava)and pyroclastic phases(tuff).The second cycle is characterized by the intermediate and felsic intrusions.The intrusions,including the diorite porphyry,aplite diorite and plagiogranite porphyry,are ore-bearing intrusions.Both diorite porphyry(including aplite diorite)and plagiogranite porphyry host the bulk of the copper mineralization at Yandong and have been overprinted by the two distinct alteration systems.Diorite porphyry has been overprinted by three alteration assemblages,including the central sericite-chlorite assemblage and chlorite-sericite assemblage and distal propylitic assemblage.Cu-Fe sulfides are closely associated with the sericite-chlorite assemblage.Plagiogranite porphyry,cutting the early-stage diorite porphyry,has undergone the central phyllic and surrounding strong and weak sericite alteration.Intense phyllic alteration produced the high-grade copper ore at Yandong.The two alteration systems at Yandong were characterized by rare potassic alteration and widespread sericite alteration.
