The Magmatism And Genesis Of The Bengge Gold Deposit In Northwestern Yunnan

The Bengge gold deposit is located in the southern Yushu-Yidun arc,eastern Tibet Plateau.Shoshonitic and ultrapotassic syenite intrusions at Bengge region are different from the spatially and temporally associated calc-alkaline arc-related intrusions.As one of the few known Au deposit,the Bengge gold deposit,differs from other porphyry Cu deposits,which are well-developed in the Yidun arc-related magma belt.This article aims to study the origin and evolution of the syenitic magmas and constrain their geodynamic setting.At the same time,after describing the geological features of the gold deposit,combined with research on the ore-forming fluid and material source and the evolution process,it is suggested that the Bengge gold deposit belongs to the Intrusion-related Gold System.The trachyte(220.1±2.7Ma),biotite pyroxene syenite(218.4±4.1Ma)and pegmatite syenite(210.4±5.2Ma)in the Bengge region are the products of the same magma event,which is due to the subduction of the Gangzi-Litang Ocean during the late Triassic.Geochemical and Sr-Nd isotope data show that the syenitic magma is originated from the low degree partial melting of the enriched mantle which is metasomated by the subducted crust and/or fluid and mixed with a small amount of asthenospheric magma.The miarolitic cavities occurring in highly fractional crystallization pegmatite syenite recorded the evidence of CO2-rich volatile phase exsolved from the highly evolved magmatic systems.So we propose that extension regime in the Bengge area lead to the upwelling of asthenospheric mantle and the decompression melting of enriched mantle.The ore-forming process of gold is clearly associated with the syenite intrusion via research on the fluid inclusion within quartz-carbonate veins,combining with insitu sulfur isotope and trace elements analyzed by SHRIMP and LA-ICP-MS,respectively.The gold is extracted from the magma in the form of Au(HS)2-,while the ore-forming fluid,containing CO2-H2O–NaCl,exsolved from magmatic systems.The migration of aurifeours fluid gave rise to the fluid "boiling" in low pressure environment,resulting in the ore-forming fluid immiscibility and separating to H2 Ophase and CO2 phases.Escaping of CO2 and H2 S vapor in the phase separation reduced total sulfur content in the ore-forming fluid and decreased the solubility of Au ion,and led to the gold precipitation.Recrystallization caused by deformation and brittle fracturing/brecciation of early pyrite,wetted the inclusion of Au-Te-Pb-Sb-Bi.Gold and all other elements are reactive and “digestive ripening" in the late oreforming fluid and homogeneous occurred as attice gold in the late "foam texture" pyrite.