Constraints of magmatic oxidation state on mineralization in the Beiya alkali-rich porphyry gold deposit, western Yunnan, China

摘要

The Beiya gold deposit is the biggest Cenozoic gold deposit in the Jinshajiang-Ailaoshan alkali-rich porphyry metallogenic belt within the Sanjiang region. Porphyry intrusions are widely distributed at the Beiya deposit. In this study, we investigate the lithological and geochemical characteristics of the Beiya alkali-rich porphyries and compare zircon Ce4+/Ce3+ ratio and magmatic oxygen fugacity (fO2) of the ore-bearing and ore-barren porphyries. The research shows that the ore-bearing intrusion is monzogranite porphyry in the area of Wandongshan, Hongnitang and their surrounding ore blocks. The intrusions have similar U-Pb zircon ages of ～36?Ma, and all the porphyries display patterns of enriched LREEs and depleted HREEs. The ore-bearing monzogranite porphyry is characterized with high zircon Ce4+/Ce3+ ratio (average of 80) and high fO2 value (average of △FMQ?=?+3.7), whereas the ore-barren monzogranite porphyry and granite porphyry are characterized by much lower zircon Ce4+/Ce3+ ratios (average of 24 and 20, respectively) and low fO2 values (average of △FMQ?=?+1.2 and △FMQ?=??0.5, respectively). This indicates that the ore-bearing porphyry intrusions had much higher fO2 of magma than the ore-barren porphyry intrusions. We suggest that oxidized magmas are more favorable to porphyry Cu-Au mineralization under the intra-continental and collisional settings, which is similar to that in arc environments. Such a conclusion may potentially be used in regional exploration for porphyry Cu-Au deposits.