The adakite and mineralization of the Shaxi porphyry copper-gold deposit, Central Anhui

Abstract：Adakites are intermediate to felsic igneous rocks, andesitic to rhyolitic in composition (basaltic members are lacking). They have trondhjemitic affinities (high-Na2O contents and K2O/Na2O 0.5) and their Mg no. (0.51), high contents of Ni (20×10-6-40×10-6) and Cr (30×10-6-50×10-6) contents are higher than in typical calc-alkaline magmas. Sr contents are high (>300 ×10-6, until 2000 ×10-6) and REE show strongly fractionated patterns with very low heavy REE (HREE) contents (Yb≤1.8×10-6, Y≤18 ×10-6). These rocks are depleted in Nb and Ta compared with other igneous rock. The tectonic formation of adakites are still disputed at present, the points of views are: 1) the melting of young oceanic crust during the subduction；2) delamination or foundering of dense mafic lower crust rocks (e.g., eclogite and garnet pyroxenite) in mafic lower crust to the mantle during continental orogenesis. Consequently, the Fe-Cu-Mo mineralization is closed related to the formation of adakites. On detailed review of geochemical data and geological environment in Shaxi porphyry Cu-Au deposit (central Anhui), we fund the close link between adakite and the regional Cu-Au mineralization. The mineralized model for Shaxi porphyry Cu deposit can be explained by the subduction of the western Pacific plate in the early Cretaceous, whose process released large ion lithophile element (LILE)-rich fluids rise up into the mantle wedge, inducing both its metasomatism and partial melting, trigger a large mount of Cu-Au and fluids materials to the crust environment and melting the subduction plate formed the adakitic assemblage and Shaxi porphyry Cu-Au deposit.