Chlorite and epidote chemistry of the Yandong Cu deposit, NW China: Metallogenic and exploration implications for Paleozoic porphyry Cu systems in the Eastern Tianshan

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The Yandong porphyry Cu deposit in the Eastern Tianshan terrane is the largest in Xinjiang, NW China (metal reserve: 2.3 Mt Cu @ 0.67%). Three periods and six alteration and mineralization stages were identified in this study, i.e., Porphyry period (including Stage I: propylitic alteration; II: quartz-magnetite alteration; III: phyllic alteration and early mineralization); overprinting period (including Stage IV: overprinting mineralization and Stage V: late vein) and supergene period (Stage VI: supergene mineralization). Stage IV was the main Cu mineralization stage at the Yandong Cu deposit, which is characterized by the mineral assemblage of chalcopyrite+chlorite+anhydrite+calcite.Electron microprobe analysis (EMPA) data indicate that Stage IV chlorite contains lower SiO2 and MgO, but higher FeO, Al2O3 and Fe/(Fe+Mg) than Stage I chlorite. LA-ICP-MS analyses indicate that Stage IV chlorite contains lower Cr, Ni, Co, B, Ca and Sr, but higher Sc, Ga, Sn, Ti, Zn and Mn than Stage I chlorite. Stage IV epidote contains lower B, Zr, Ba and Ti, but higher Sn, Y, Ga, Ag, U, Y, Cu and Sr than Stage I epidote. Concentrations of Ti, Zn, As, V, Sc and Cu are highest near the orebody for both Stage I and IV chlorite, and Au and Sn in Stage IV chlorite are highest near the orebody. Titanium content of Stage I epidote is highest near the orebody, but V, Sc and Zr are lowest there. Stage I chlorite at Yandong is geochemically similar to the giant Batu Hijau porphyry Cu-Au deposit (Indonesia), but Stage IV chlorite contains higher Fe, Al and As, lower Mg and Sn than the latter. Stage I epidote at Yandong is geochemically similar to the porphyry and skarn deposits in the central Baguio district (Philippines), but Stage IV epidote has higher Sn, Ga and U, but lower Zr, Ti and Ba than the latter.Ore deposit geology and alteration mineral chemistry indicate that the Yandong Cu deposit is an atypical porphyry deposit. The adakitic tonalite porphyry and quartz albite porphyry (granitic) may have formed the early and overprinting Cu mineralization, respectively. Chlorite Ti and V, and epidote Sb show similar spatial variation patterns in Cenozoic and Paleozoic porphyry deposits, which may be used for exploration targeting for ancient porphyry deposits, such as the Yandong Cu deposit.