Brief Study On Fluid Inclusion Characteristics And Ore Genesis Of Liwu Copper Deposit In Jiulong County Of Sichuan Province

Liwu copper deposit which lie in Jiulong County of southeastern Ganzi Tibetan Autonomous Prefecture in Sichuan is one of the most large-scale copper deposit in Yunnan, Guizhou and Sichuan province, and formed in moderate- deep metamorphic rocks. Its tectonic location lies in the western margin of the Yangtze plate and the southeast of Songpan- Garze orogenic belt, Muli- Jinpingshan arcuate nappe structures which is southwest- northeast direction formed a dome chain composed by a series of metamorphic core and miscellaneous rock bodys. All these rock bodys located in the north-west and trailing edge of the nappe structures. These dome structures include Chias, Wachang, Changqiang, Jianglang, Taka, Sanya, and they are a series minor dome anticline which are in NNW-SN eaxial direction. As important regional ore-controlling structures, these domes played an important role in copper and gold polymetallic mineralization in the area. Liwu copper deposit and its peripheral deposits(mine sites) include Heiniudong, Dalaoxionggou, Shanghai bottom, Boxianglin, Baiyanzi, Sunyelin etc. precisely located in Jianglang dome structure.As the main body of the regional structure, the dome structure contains a variety kind of faults and folds, which are the most important ore-controlling structure in mine area. Among them, Faults mainly are cyclic stripping faults which include a series of detachment fault, detachment fault and ductile shear zone, and they all surrounded the metamorphic core(Miscellaneous Rock). While fold structures are represented by a variety kind of bedding shears, slippage and folds in different lithological formation which are caused by positive slip motion of the detachment fault. The formation of the core part in Jianglang dome sturcture is mesoproterozoic stratum called Liwu rock group(Pt2l), the Ordovician, Silurian, Permian and Triassic stratum are distributed from inside out. The volcanics and subvolcanic rocks generally have deeply deteriorated in the area. And these rocks are mainly composed of metamorphic mafic volcanic rocks, metamorphic mafic subvolcanic rocks and metamorphic neutral- alkaline subvolcanic rocks. Rocks in mining area have experienced strong tectonic deformation and metamorphism as well as hydrothermal filling metasomatism. Non-directional regional foliation and hydrothermal alteration minerals in the area show that strong hydrothermal alteration has a close contact to copper mineralization.Predecessors have carried out some geological exploration and scientific research on Liwu copper deposit. But there are still some differences and disputes in the genesis of type, ore-forming fluids source and migration mechanism, mineralization stage and so on. For this reason, we selected the topic of fluid inclusion study of Liwu coppe deposit on the theis, based on macro understanding of the geological characteristics of the deposit, through intensive observation and comprehensive sampling, mineral microscopic petrography, petrogeochemical, ore deposit geochemistry and focused on testing homogenization temperature, freezing point temperature of near-wall rock and ore to calculate inclusion salinity, density, pressure, mineralization depth, ultimately to get the recognition of the ore-forming fluid source, the fluid nature of the mineralization, evolution and migration mechanisms.There are a variety types of of fluid inclusions in Liwu copper deposits, such as: gas-liquid two phase liquid-type inclusions, pure liquid phase inclusions, three-phase liquid CO2-containing liquid-type inclusions, pure gas type inclusions and daughter mineral-containing inclusions. Among all the inclusions the gas-liquid two-phase liquid type inclusions accounted for the largest proportion. And Inclusions forms mainly include lens-shaped, irregular-shaped and elongated. Their particle size are between 8-25?m. The homogenization temperature and freezing point temperature have been determined through a large number of gas-liquid two-phase liquid type inclusions.Then the salinity, density and pressure of the inclusions can be calculated as well as the depth of mineralization can be roughly estimated.Homogenization temperature measurement results of inclusions in transparent minerals of ore and host rocks concentrated in 118.5 ~ 369.5 °C, and with an average of 244.0 °C. Among them the homogenization temperature measurement results of inclusions in ore samples range 148.8 ~ 373.9 °C, with the average of 261.35 °C, also the range can be properly divided into two groups: 186.3°C ~ 286.5 °C, with average of 236.4 °C; 300.6 °C ~ 373.9 °C, with the average of 337.25 °C. Apart from that, an extremely small amount of inclusions' temperature exceed 400 °C. It can be indicated that the main mineralization has at least experienced two mineralization stage, with the process of high temperature to low temperature. In addition In the process of ore fluids' migration to the surrounding rock, the temperature decreased significantly, which showed the main mineralization of deposit is dominated by hydrothermal filling and metasomatism.Finally it can be mesured that the freezing temperature range among-16.55 °C to-6.58 °C. According to inclusions freezing point-salinity conversion formula, it can be acquired that the salinity is in range of 4.43 ~ 33.82wt%, among them, the salinity of inclusions mainly concentrate in the range of 11.69 ~ 20.80wt% in ore samples, and with average of 16.25wt%. The results show that ore fluids have a obvious character of high salinity, indicating that ore-forming fluids in Liwu Copper deposit have a significantly character of high salinity.According to the relationship equations of inclusions homogenization temperature, salinity density, the density range which can be calculated are 1.20~1.54g/cm3 and 1.20~1.54g/cm3, with the average of 1.3728g/cm3, moreover, it can be confirmed that ore-forming fluid density is about 0.901g/cm3 ~1.026g/cm3 through calculation diagram pattern in density fluid system, and main metallogenic fluid density is approximately 1.015g/cm3. These two kinds of density analysis models showed ore-forming fluid density of Liwu copper deposit varied deeply at wide range, and multi –sourced character of ore-forming fluid. The metallogenic pressure of Liwu copper deposit is within 441.66~911.40×105Pa through the pressure calculation formula. If only considering the lithostatic pressure, according to the homogenization temperature and salinity ranges, it can be roughly calculated that depth is 435-450 m during the main ore-forming period, which shows it's a shallow deposit for the metallogenic depth below 3km, and the fluid p H value varies from 6.5~6.8, which shows alkalescence evironment.According to the analysis of inclusions H-O isotopes and inclusion component, it can be declared in this paper that the source of ore-forming fluid have significantly characters of multi –sourced and high salinity. The water in fluid included magmatic water?meteoric water?sea water and metamorphic water varied widely in density and salinity, which directly resulted that the different sources of water mixed in different proportions, so that caused large change in salinity. These features and characteristics match with temperature and pressure' broad range when the deposit were formed.Through the arrangement and analysis of the major elements, trace elements, rare earth elements and other field, it has be determined that the original rock in the schist and quartzite of ore-bearing rock series in Liwu copper deposits is a typical sedimentary rock, plagioclase amphibolite protolith's original rock is basic volcanic rocks. In addition quartzite, schist protolith originated from the same sediment source area, and metabasite have a certain genetic relationship with oceanic tholeiite. It can be defined that there was an initial enrichment of copper at the initial phase of the metamorphic volcanic-sedimentary rocks' forming process, which formed a possible original ore source beds of copper ore.In this paper, with microscopic mineralography research, there are obviously filling and replacement structure, dissolution and Residual structure, also some gangue minerals debried obvious crumb structure which present sedimentary rocks. These phenomena have further confirmed the formation of the deposits have significant relationship with the early marine volcanic eruption- clastic sedimention. In addition, through the research on ore mining under the microscope, it can be confirmed that there are at least two generations chalcopyrite: the chalcopyrite and sphalerite which are typical solid solution separation structure implied that they are from same generation, also there are another period chalcopyrite, and chalcopyrite replaced galena, galena replaced sphalerite, sphalerite replaced and dissoluted pyrrhotite. Among them, pyrrhotite formed earlist, chalcopyrite formed latest.It also can be seen occasionally that galena replaced chalcopyrite, reflected that there are several superimposed mineralization in the mining area, and mineralization has a great relation with the late hydrothermal metamorphism, all these studies revealed that hydrothermal have a typical character of multi-stage.To sum up, this paper suggest characteristics of fluid inclusions in Liwu copper deposit which revealed that the ore-forming fluid has the characteristics of multiphase and multi-stage forming evolution, And pressure generated in late Indo- Yanshanian tectonic orogeny has pushed Ore-forming fluid to transport. It provided the main mineralization kinetic and thermal energy in the formation of deposits. We can basically determine that the Liwu Copper Deposits genesis of type are metamorphogenic deposit, which mainly formed from multi-stage hydrothermal filling metasomatism, have great relationship with early marine volcanic eruption-clastic deposits.