Study On Ore-forming Environment And Process Of Gejiu Skarn Sn-Cu

Gejiu ore concentration area is located in Youjiang Basin,the western margin of South China Plate.Typical Skarn Sn and Cu deposits are developed in the area,but the distribution characteristics are more tin in the north and more copper in the south.The reasons for the distribution difference are not clear.To solve this problem,this paper selected the tin deposit of Gaofengshan deposit in Gaosong ore field and Zhuyeshan copper deposit in Laochang ore field as the research object.Based on detailed field geological work and indoor petrographic observation,the geochemical characteristics of ore-forming rock mass（containing biotite）and representative minerals at different stages（garnet,pyrite,sphalerite）were studied.Combined with the U-Pb dating of garnet and vesuvianite,the Ore-forming environment and process of tin and copper deposits are identified,and the reasons for the differential enrichment of tin and copper deposits are preliminarily analyzed.The study indicates that the REEs characteristics of the ore-forming granites（Masong rock mass and Laoka rock mass）are similar,and they are all crust-mantle synmelting granites with high silicon,rich alkali and high volatile content.The degree of differentiation of the Laoka rock mass is higher than that of the Masong rock mass,and it has experienced strong crystallization differentiation and melt-fluid interaction to a certain extent.The formation temperature of the Masong rock mass is obviously higher than that of the Laoka rock mass,and the formation pressure and emplacement depth of the Laoka rock mass are both higher than that of the Masong rock mass.The formation environment of the two rock masses is the transition environment from low oxygen fugacity to high oxygen fugacity,and halogen（F,Cl）is generally enriched in the Laoka rock mass,and the degree of halogen enrichment is higher than that in the Masong rock mass.Two generations of garnet（Grt-GS1 and Grt-GS2）from Gaofengshan deposit and Grt-ZS from Zhuyeshan deposit are characterized by low∑REE content,LREE deficit,HREE enrichment and negative Eu anomaly.The garnet（Grt-ZX）in Zhuyeshan deposit is characterized by LREE enrichment,HREE depletion,strong positive Eu anomaly,and relatively high contents of∑REE,trace elements（Cu,V,Cr,Co,Ni,etc.）and halogen（F,Cl）.All types of garnet grow slowly and form in a closed or nearly closed,nearly equilibrium and low W/R ratio environment（diffusion metasomatism）.Grt-GS1,Grt-GS2 and Grt-ZS are all formed in a weakly acidic fluid environment,while Grt-ZX is formed in a near-neutral fluid environment rich in F and Cl.The f O₂ of garnet in Gaofengshan deposit is lower than that in Zhuyeshan deposit,and the oxygen fugacity from Grt-GS1 to Grt-GS2 is obviously increased.The pyrite（ZX-Py）in skarn near the basalt of Zhuyeshan deposit has high Co,Ni and Cu contents,which is consistent with the element characteristics of basalt,indicating that the basalt may provide a certain amount of Cu.The structure and trace element characteristics of sphalerite indicate that the sphalerite in Gaofengshan and Zhuyeshan deposits was formed in a medium-high temperature and low sulfur fugacity environment,and the formation temperature of GS-SP in Gaofengshan deposit is higher than that in Zhuyeshan deposit（ZYS-SP）,and the log f S₂ of fluid in Zhuyeshan deposit is higher than that in Gaofengshan deposit.U-Pb dating of garnets show that the skarn is 88.2±1.3 Ma（MSWD=0.72,n=26）and 82.3±3.7 Ma（MSWD=0.9,n=38）in the vicinity of Zhuyeshan basalt.U-Pb dating of vesuvianite shows that the skarn near Zhuyeshan granite is 84.1±0.5 Ma（MSWD=1.3,n=34）.The formation age of skarn in the two deposits is similar to that of granite.The skarn near the basalt is formed by metasomatic granitic magma and surrounding rock,and the basalt provides a large amount of copper and halogen.The differences of tin and copper ore-forming environment include:the temperature of initial magma of ore-forming rock mass,the degree of differentiation,the content of halogen and the difference of oxygen fugity of ore-forming fluid.With the increase of oxygen fugacity of the ore-forming fluid in the late skarn-oxide stage,a large amount of tin is enriched and precipitated in the tin-rich ore-forming fluid from the granite in Gaofengshan deposit.In Zhuyeshan deposit,a large amount of copper and halogen（F,Cl）were extracted from the early basalt by large-scale melting of the granitic magma.The halogen further enhanced the evolutionary differentiation of the granitic magma and further enriched copper.Subsequently,copper was precipitated and ore-formed in the late sulfide stage at 286℃～333℃and log f S₂ of-12.2～-9.5.