Constraints Of Magmatic Oxidation State On Mineralization In The Beiya Alkali-rich Porphyry Gold Deposit,Western Yunnan,China

Many porphyry deposits are closely related to the oxidized magmas,which are usually associated with the arc environment.In recent years,many studies have demonstrated that there are abundant porphyry deposits being formed in intra-continental environments.However,there are still lack of systematic studies in the aspects of magma oxygen fugacity and constrains of mineralization.The Beiya gold deposit is the biggest gold deposit in the Jinshajiang-Ailaoshan alkali-rich porphyry metallogenic belt within the Sanjiang region under the intra-continental environment.Porphyry intrusions are widely distributed at the Beiya gold deposit.The study for magma oxygen fugacity and constrains of mineralization is unclear.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 porphyries.1.Systematic LA-ICP-MS U-Pb zircon dating results indicate that ore-bearing porphyries and ore-barren porphyriesare coeval,at about 36 Ma.2.The ore-bearing porphyries have higher zircon Ce⁴⁺/Ce³⁺ ratio and magmatic oxygen fugacity than the ore-barren porphyries.3.The geochemical characteristics of the ore-bearing porphyries and ore-barren porphyries are similar to adakites geochemically.The source of porphyries in Beiya gold deposit is thickened lower crust.The source of the ore-bearing porphyries may be influenced by the EMII enriched mantle.The EMII enriched mantle is characterized by high S and high oxygen fugacity,owing to the oceanic crust subduction.Under high fo2 conditions,the majority of sulfur exists as SO₂^4-in the magma,causing the concentration of Cu and Au in magmatic-hydrothermal ore-forming fluids.