Mineralogy And Fluid Inclusion Geochemistry Of The Chating Cu-au Deposit,Xuancheng City,Anhui Province

The Chating Cu-Au deposit in the Xuancheng City of Anhui Province is a newly discovered large porphyry Cu-Au deposit in the Xuancheng-Nanling Basin of the Middle and Lower Yangtze River Valley Metallogenic Belt in recent years.The exploration of the deposit has not yet been completed,and there are many geological problems waiting to be thrashed out,such as the origin and evolution of the ore-forming fluids,the genesis and metallogenic mechanism of the deposit.Based on the comprehensive collection and summary of the achievements of previous works,detailed field geological surveys and sample collections are conducted.Through petrographic?mineralogical?observation,electron microprobe analysis,geochemical study on the characteristics of major and trace elements of minerals and fluid inclusions,the geological characteristics of the Chating copper-gold deposit,the occurrence mode of gold,the genetic connection between gold and copper in the ores,the origin and evolution of the ore-forming fluids were further discussed.Geological characteristics of the Chating copper-gold deposit show that the deposit is a typical porphyry deposit.The intrusive rocks in the ore district are mainly quartz-dioritic porphyrite,dioritic porphyrite,spessartite.The ore bodies are mainly occurred in the quartz-dioritic porphyrite with lenticular forms.The ore-bearing intrusive rocks are characterized by zonal hydrothermal alterations,such as K-feldspar alteration and beresitization.The structures of ores are manly veinlets-disseminated,vein,disseminated.The main ore textures are cuhedral to subhedral-crystals,metasometic and solid solution.Ore minerals are mainly chalcopyrite,pyrite,sphalerite,magnetite,tetrahedrite,bornite,molybdenite,galena and gold-bearing minerals.Large amount of magnetite and anhydrite developed in the ores of the Chating copper and gold deposit,which shows that the metallogenic process may undergo a more oxidized environment and the ore-forming hydrothermal fluids have high oxygen fugacity.The Ce⁴⁺/Ce³⁺ratios of zircons in quartz-dioritic porphyrite,dioritic porphyrite and spessartite are respectively 479.24,272.56 and 282.53,which indicates that the ore host intrusive rocks have higher oxygen fugacity.High oxidizing magma is more conducive to mineralization of porphyry deposits.Chalcopyrite and pyrite are the main gold bearing minerals.The pyrites and the chalcopyrites at the second and third mineralization stages are rich in gold,whereas the quartzs at all stages and anhydrites at the third stage are almost gold-free.The occurrence mode of gold can be divided into two forms:"visible gold"and"invisible gold".The major minerals of the"visible gold"are electrum,followed by native gold."Visible gold"is found in the ores and they are mainly embedded as inclusion gold and interparticle gold,and a small amount of fissure gold included.The occurrence of"invisible gold"is nanoparticle native gold in pyrite.A part of gold exsolved from Cu-Fe sulfides with the temperature decreased.Pyrites from the Chating copper gold deposit enriched with LREE and depleted in HREE.The ratios Hf/Sm,Th/La and Nb/La of the pyrites indicate that Cl is more than F in ore-forming hydrothermal fluids.The REEs of anhydrite are characterized by enrichment of LREE and depletion of HREE.The decoupling mechanism between Y and HREE proved that the main migration form is mainly Cl complex.Fluid inclusion types are mainly liquid rich inclusions in the Chating copper gold deposit.They are often in the form of long strips,ellipsoid,irregular.The homogeneous temperature data of fluid inclusions show that the ore-forming fluid is medium-high temperature to medium-low temperature,medium-high salinity to low salinity magmatic hydrothermal fluid.From the early mineralization stage to the middle mineralization stage,the salinity and pressure of the ore-forming hydrothermal fluid decrease with the decrease of the temperature.The composition of fluid inclusions,trace elements of pyrite and S isotopic composition of anhydrite indicate that the ore-forming material is mainly derived from magmatic hydrothermal solution,and there may be atmospheric precipitation mixing in the later stage of mineralization.