Abstract: The Triassic Taiyangshan porphyry copper-molybdenum deposit in Tianshui City, Gansu Province is tectonically located in the West Qinling Orogenic Belt. The bulk of Cu-Mo mineralization is associated with monzonite porphyry and quartz monzonite porphyry, and occurs as stockwork veinlets and disseminated ores in the porphyries and the surrounding Devonian host rocks. Oxygen fugacity is an important physical-chemical parameter in petrology and an obvious controlling factor in magmatic-hydrothermal mineralization. We conducted LA-ICP-MS trace elemental analyses on zircons from the ore-hosting porphyries at Taiyangshan, aiming at calculating the magma oxygen fugacity and constraining its ore formation implications. The average oxygen fugacities were ΔFMQ+3.04 and ΔFMQ+3.15 for ore-bearing monzonite porphyry and quartz monzonite porphyry, respectively, characterized by high f(O2) values. The pre-mineralization quartz diorite porphyry, however, yielded an average oxygen fugacity of ΔFMQ-1.83, a much lower f(O2) value. Combined with our previous petrogenesis results, we therefore proposed that the oxidized magma is associated with enriched mantle, whose addition brought sulfur and copper while increased oxygen fugacity.

Key words: Taiyangshan deposit, oxygen fugacity, porphyry deposit, West Qinling