Study On The Geological Characteristics And Ore Genesis Of Tongkuangyu Copper Deposit In The Zhongtiaoshan Mountains, Shanxi Province

The Tongkuangyu Copper deposit is located in Yuanqu County, Shanxi Province, tectonically in the intersection area of Zhongtiao trigeminal Rift, which is an important Cu ore province in China extends along southern margin of the North China Craton.This paper presents detailed studies on geological features, isotope geochemistry, physical and chemical environment and porphyry geochemical characteristics of Tongkuangyu copper deposit, accompanied with a comparative analysis with other typical porphyry copper deposits home and abroad, summaries temporal and spatial feature, alteration characteristics and tectonic setting, discusses the origin of ore-forming metals and metallogenic mechanism.Strata exposed in the region consist of metamorphic albite granite porphyry and metamorphic quartz crystal tuff from Luotuo Feng group, upper Archean Jiangxian formation and metaso -ultrapotassic volcanic rocks of Xijinggou group and Shujinggou group. Up to now, it has been delineated 339 ore bodies (112 ore bodies in year 1992) with five main ore bodies (1,2,3,4,5 # ore body), mainly presents as lenticular, pod-like shape.Structure in the region is complex due to the multistage of deformation and metamorphism during Jiangxian-Zhongtiao movements. The copper deposit is hosted within the overturned fold or semi- open compound syncline on the south flank of Tongkuangyu compound syncline. There are two major faults in the mining area, one is the Tongkuangyu post-mineralization fault, nearly SN-trending, cut off 5 # ore body into two part, led to the stratum distortion on both sides; the other one is the thrust fault, nearly EW, a little far from the ore body and therefore little effect on it.Copper mineralization occurs as disseminated veinlets or veins, anhedral- subhedral granular or metasomatism dissolution textures, disseminated and veinlets filling structures. Ore mineral assemblage is relatively simple, mostly pyrite and chalcopyrite with minor amount of bornite, molybdenite and magnetite; gangue assemblage is quartz, sericite, plagioclase, calcite, chlorite, biotite and so on. Alteration consists of sericitization, silicification, biotitization, chloritization, carbonation and so on, among which silicification, sericitization and chloritization are closely associated with mineralization.Results from isotope geochemistry (including H, O, S, Pb and Sr isotope) indicate the involvement of hydrothermal fluids generally from magmatic compositions, with the participation of meteoric water. Metallogenic materials show a character of multi-sources (mantle, crust and many other sources) and process of deposition is controlled by the temperature and metamorphism.A fluid inclusion study was undertaken to constrain physical and chemical conditions of ore-forming fluids. Fluid inclusions are divided into two populations on the basis of petrography and microthermometry: typically liquid-rich two phase inclusions and daughter mineral-containing three phase inclusions, both of them are dominated by gas-liquid ratio ranging from 10% to 35 %. Homogenization temperatures of two-phase fluid inclusions are between 138.2℃and 253.6℃while three-phase type at 420 ~ 616℃, salinities of two-phase type are between 16.53 wt% NaCl.eqv and 31.04 wt% NaCl.eqv while halite daughter-bearing type at 47.6 ~ 70.92wt% NaCl.eqv. Pressure mainly concentrates in the 22 ~ 26MPa, an average of 24.1MPa, the depth of the deposit is about 2.2 ~ 2.6km. Hydrothermal fluids origin from magmatic source mixed meteoric water. All above features come to the conclusion that Tongkuangyu copper deposit is a typical porphyry deposit.The major elements analysis indicates the rock should be peraluminous calc-alkaline series; REE chondrite-normalized distribution curve is right tilting, implying the strong fracturation of LEE and extremely depleted in HEE. In addition to one sample with obviousδEu negative anomaly, the other samples show noδEu anomalies. Trace element analysis shows the enrichment of K, Ba, Rb and other large ion lithophile elements (LILE) and LREE, depleted in Y, Zr, Ti high field strength elements ( HFSE), inferring the presence of plagioclase and the absence of phlogopite in porphyritic magma source, enrichment of garnet and hornblende in residual magma phase.In the tectonic discriminal diagram for granite, all of the samples are plotted in volcanic arc granite (VGA) field and have the transition to syn-collision granites (Syn-COLG) field. In tectonic model diagram for Pb isotope, most of the samples are plotted between crust field and orogenic belt field. Petrochemically,the characteristics above suggest the Tongkuangyu copper deposit occurred in a syn-orogenic rift in the ocean-continent collision stage.According to the dating results, both mineralization and intrusion of the porphyry formed round 2.1 Ga, concurrent with the opening of Zhongtiao rift, after strong compression and uplift, accompanied by violent sodium-volcanic activity, leading to the formation of the metamorphic albitic granite porphyries and sodium medium-basic rock and volcanic sedimentary rocks of Bizigou and Songjiagou groups. Tongkuangyu copper deposit had been suffered from regional metamorphism and deformation activities during 1.9-1.8Ga.