Geological Characteristics And Genesis Of The Native Copper Deposit In Caizhushan,Eastern Tianshan,Xinjiang

The Caizhushan native copper deposit is located in the southeast of Shanshan County,Xinjiang.Its tectonic location belongs to the Aqishan-Yamansu Island Arc where the Siberian Plate and Tarim Plate assembled.The Caizhushan,Shilipo,Changchengshan native copper deposits and the Hongyuntan,Tieling,Shuanglongshan,Heijianshan,Yamansu Fe-Cu(Au)deposits constitute the south sub-belt of the Late Paleozoic metallogenic belt of Jueluotage in eastern Tianshan.A large number of geological studies have been carried out on native copper deposits such as Shilipo and Changchengshan,as well as Fe-Cu(Au)deposits such as Heijianshan and Yamansu in the metallogenic belt.However,few studies have been done on the Caizhushan native copper deposit since its discovery in 2015.Based on the investigation of the geological characteristics of the Caizhushan native copper deposit,this paper discusses the genesis of the Caizhushan native copper deposit by means of scanning electron microscopy(SEM),electron microprobe(EPMA)and copper isotope analysis.The following understandings are obtained:The basalt section in the lower part of Tugutubulake Formation of Upper Carboniferous is an important ore-bearing position of the Caizhushan native copper deposit.The formation is a set of basic to acid volcanic lava and intermediate acid volcanic tuff assemblages.The geochemical characteristics of volcanic rocks range from calc-alkaline series to high-potassium calc-alkaline series,reflecting the characteristics of continental arc volcanic rocks during the transition from compression to extension.The Caizhushan native copper body is lens-like and radially vein-like,with a length of 120?150 m on the surface and an apparent thickness of 2.1?25.6 m.The ore contains 0.1%?2.3%Cu,and concomitance 1.16g/t Au and 70.3g/t Ag.Spatially,the native copper deposit is associated with Fe-Mn mineralization,Tongcheng zoisite type lead deposit and silver deposit.The ore minerals are native copper,cuprite,chalcocite,digenite,covellite,malachite,atacamite,magnetite,ilmenite and psilomelane.Gangue minerals include quartz,epidote,chlorite,pumpellyite,actinolite,uralite,sericite,prehnite,calcite,anhydrite and kaolin.The ore structure includes irregular granular structure,inclusion structure,porphyritic structure,metasomatic relict structure and intersertal structure.The ore structure includes massive structure,vein structure,breccia structure,scattered structure,disseminated structure and crust structure.The main alteration types are epidotization,chloritization,tremolitization,silicification,pumpellyitization,prehnitization,carbonation,kaolinization.Under the microscope,the fracture shape garnet,diopside,tremolite and calcite bands are alternated,and the metallic minerals are distributed between the bands,indicating that garnet,diopside and some tremolite and carbonate minerals were formed before the native copper mineralization,representing the products of the primary sulfide deposit.The epidotization,silicification and carbonation,which are closely related to the native copper mineralization,partially cut through the stratum of the Hongliugou Formation in the Permian System,which indicates that the deposit should have formed in Permian.The observation under microscope shows that there are two kinds of native copper production states,one is bean granule produced in silicate minerals with chalcocite,the other occurs in the vein or block form between silicate minerals and is associated with cuprite.SEM analysis showed that native copper,ilmenite and magnetite were associated with silicate minerals.The interior of native copper is composed of more fine globular native copper,and the formation of native copper is related to chalcocite.The spherical native copper edge forms an S-rich band.The block native copper encased chalcocite and silicate minerals,and S is concentrated in native copper edges or silicate minerals.All these characteristics indicate that the native copper and chalcocite in the Caizhushan native copper deposit come from copper sulfide,and S in copper sulfide gradually precipitates outward to form native copper.Microscopic observation shows that the cuprite is usually distributed around the veins or massive natural copper.The results of SEM microscope show that Si O₂,Ca SO₄,Ca Cl₂ and K elements are involved in the formation of the cuprite,accompanied by silicification,potassification and anhydrite alteration.This indicates that with the addition of quartz,potassium feldspar and halide,the native copper formed cuprite,the plagioclase and potassium feldspar decomposed to form anhydrite and potassification,and the local cuprite was transformed into atacamite.The results of EPMA test show that the Cu content of the Caizhushan native copper ranges from92 to 99%,and Ag,Au,Mn,Fe,Co,Ni,Pb,Hg,Bi,Sb and Cr.The local Ag content is as high as69.47%,forming Cu-Ag-Hg alloy.When local original copper-bearing sulphides form native copper,the inherited sulphides have blue-gray tones due to the presence of Ag,Fe,Mn,Co and other elements.The w(Cu)of chalcocite ranges from 74.51%?1.15%and w(S)ranges from 16.42%?20.86%,which also contain Mn,Fe,Co,Ni,Zn,Pb,Ag and Cr elements.The consistency of trace elements in native copper and chalcocite indicates the genetic relationship between native copper and sulfide such as chalcocite is explained,which is consistent with the phenomenon observed under the microscope.The w(Cu)and w(O)of cuprite are range of 84.21%?89.15%and 11.02%?22.34%,and Fe,Mn,Au,Ag,Co,Ni,Pb,Ca,Cl,S.The atacamite metasomatizes the cuprite along fissures or interstitial minerals,and the ratio of Cl/S of the cuprite was 0.15?81.75,indicating that the formation of the copper deposit was related to the Cl-rich brine.Ilmenite and magnetite are distributed in silicate minerals,containing Mn O,V₂O₅,Mg O,Al₂O₃,Cr₂O₃,Co O,Ni O.The copper isotopic composition of the Caizhushan native copper deposit ranges from 0.55‰to0.61‰,which is close to that of the VMS type deposit.Combined with geochemical analysis,the non-mineralized bodies in the Caizhushan native copper deposit are more rich in S than the mineralized bodies.Therefore,in terms of the source of ore-forming materials,the copper isotopic composition of Caizhushan reflects that the Caizhushan native copper mineralization is related to the VMS deposit in the belt.Combined with metallogenic age,alteration characteristics and tectonic setting of occurrence,the formation process of cuprite and atacamite in the Caizhushan native copper deposit is similar to that of the IOCG type iron copper deposit in the Aqishan-Yamandsu arc belt,and may have the same genesis with that of the Fe-Cu(Au)deposits in the ore belt.Therefore,the eastern Tianshan plate subducted to the middle Tianshan block to the south at the end of Late Carboniferous,forming the Aqishan-Yamansu island arc belt,in which the Yamansu Formation VMS deposit is produced.Permian intrusion as the heat source and quartz as the flux.The fluid migrated upward along the fault and passed through the VMS orebody in Marine volcanic rocks.Melting caused a large amount of S in the copper sulfide to precipitate out to form chalcocite and native copper.The fluid carried ore-forming materials to the REDOX interface of hematite-magnetite in the IOCG type deposit,and native copper was oxidized to cuprite.The addition of silicate materials and brine makes the silicate minerals such as feldspar form anhydrite,silicification and kaolinization,and makes the cuprite form atacamite.Fe and Mn form the Fe-Mn mineralized alteration zone in the mining area.Au and Ag formed gold occurrence and silver occurrence.