High Oxygen Fugacity and Slab Melting Linked to Cu Mineralization: Evidence from Dexing Porphyry Copper Deposits, Southeastern China

Abstract

The Dexing porphyry Cu deposit is the largest Cu deposit in eastern China, with total reserves of 8.4 Mt Cu. The Dexing porphyries have geochemical characteristics typical of adakites: they are similar to examples in the Circum-Pacific Belt and in the Lower Yangtze River Belt but different from adakites from the Dabie Mountains and the Tibetan Plateau. Ce4+/Ce3+ and values calculated from zircon trace-element compositions vary from 495 to 1922 and from 0.51 to 0.82, respectively, and reflect high oxygen fugacity similar to that measured in or inferred for porphyry Cu-Au deposits in the South America. The high oxygen fugacity is consistent with abundant anhydrite and magnetite-hematite intergrowths in the porphyry, which indicate that the highest oxygen fugacity of Dexing porphyry reached the hematite-magnetite buffer. Based on the geochemical characteristics and the drifting history of the Pacific Plate, we propose that the Dexing adakitic porphyries formed through slab melting, most likely during subduction of an aseismic ridge in the Pacific Plate in the Mid-Jurassic.