Melt recharge, f O2-T conditions, and metal fertility of felsic magmas: zircon trace element chemistry of Cu-Au porphyries in the Sanjiang orogenic belt, southwest China

摘要

The magmatic hydrothermal Pulang Cu deposit (Triassic) and the Beiya Au-Cu deposits (Eocene) are located in the Sanjiang copper porphyry belt, southwest China. Zircon chemistry was used to constrain the magmatic evolution and oxidation state of the porphyries. The results show that porphyries of the Beiya district formed from an early oxidized melt and a later relatively reduced and more evolved magma, whereas Pulang experienced a normal Cu porphyry evolutionary trend. The Pulang porphyries crystallized from more oxidized magma (?FMQ +?2.9–4.6, average?=?4.0?±?1.0, n ?=?3) with an average temperature of 709?±?6?°C compared to the Beiya porphyries (?FMQ +?0.6–3.5, average?=?1.9?±?1.3, n ?=?5) with a mean magmatic temperature of 780?±?22?°C. These data, combined with data from other Cu- and Au-rich porphyries in the Sanjiang belt (i.e., Machangjing Cu, Yao’an Au), are consistent with previous experimental work showing that elevated Cu and Au solubilities in magma require oxidizing conditions. A compilation of existing geochemical data for magmatic zircons from fertile and barren porphyry systems worldwide establishes an optimal diagnostic interval on Ce_(IV)/Ce_(III)-T~(Ti-in-zircon)and (Eu/Eu*)~(N)plots for generating magmatic hydrothermal Cu-Au deposits.