The Stable Isotope Geochemistry Of The Liwu Copper Deposit In Jiulong, Sichuan

The Liwu copper deposit in Jiulong is one of the large-sized copper deposits in the west of Sichuan province and its orebody is a formation of medium to high grade proterozoic metamorphic rocks. The geotectonic position of this deposit is located in the Kangtien basement compolex belt of the southwest edge of Yangtze plate, the southeast edge of Songpan-Ganzi orogen and the northwest of Muli-Jinping arc thrust tectonic zone. The deposit is the part of proterozoic Cu-Zn polymetallic metallogenic belt of Jiulong-Muli metamorphic-thrust complex zone. Although the predecessors have already completed the geological exploration and some scientific research work, there are still widespread controversies about the genesis and metallogenic regularity of the deposit. Because of the lack of mineral resources and ore reserves, the mine has been classified as crisis mine by country. According to that, this essay discusses the source of metallogenic material, the nature of ore-forming fluid and the genesis of mineral deposit about the Liwu copper deposit through the study of stable isotopes to provide the theoretical basis for both surface and deep mineral exploration, which has great theoretical and practical significance.Based on the regional geological background and the study of the ore deposit geological characteristics, this essay combines with previous research results and verifies that Liwu rock group in the central portion of Jianglang dome is the ore-bearing layers of the copper deposit, which is considered as a formation of marine volcanic-turbidite clastic rocks through the systematic sampling and laboratory study on country rock of the deposit. The footwall of the ore-bearing layers clings to metamorphic volcanic rocks- subvolcanic rocks, indicating the relation of the space and the genesis between the mineralization and the magmatism.The orebody of Liwu copper deposit mainly appears in bedded, near-bedded and irregular lenticular shapes and roughly arranges in bedding and imbricate structures due to the orebody is obviously controlled by the formation, which is basically the same as the main stage of schistosity of the country rock and its the ductile shear zone. The main structures of ores include massive structure, brecciated structure, disseminated structure and banted structure. They have different proportions in different places, which is related to the degree of metamorphism reformation. The ores have the typical structure characteristics of hydrothermal filling and replacement, mainly including subhedral-allotriomorphic structure, metasomatic structure, exsolution structure and leaf structure, etc. The ore minerals mainly include copper pyrite, pyrrhotine, marmatite and iron pyrite, etc. The gangue minerals include quartz, biotite, chlorite, garnet, tourmaline and calcite, etc. The country rock in the ore district has suffered severe alteration and the alteration zone mostly appears in the middle of Liwu rock group, mainly including biotitization, silication, chloritization, garnetization and tourmalinization.This essay focuses on the study of stable isotopic composition such as H, O, S and Pb, and then discusses the source of metallogenic material, the nature of ore-forming fluid and the genesis of mineral deposit.The ?18O values of fluid are between-0.93‰ and 7.35‰. Apart from one point, other values are all positive, with 3.09‰ on average. This shows the significant characteristic of the wide variable range of the ?18O value for mixing the water. So water in ore-containing solution is the mixture of magmatic water and metamorphic water. The ?D values of fluid are between-76.3‰ and-61.4‰. All values are negative with-70.1‰ on average, which is largely different from the stable isotopic composition of standard mean ocean water and primary aqueous solution. The ?D values are close to magmatic water with-80‰~-40‰ and metamorphic water with-60‰~-20‰, performing the mixed characteristics of magmatic water and metamorphic water as well. This test result shows the ?34S values of different types of sulfides in the ore district have little difference and performs the consistency of sulfur sources of different metal sulfides. The ?34S values of different metal sulfides are all positive with small difference and relatively concentrated distribution. The variation range of S isotopic composition is small with 2.70‰~8.97‰, which is basically the same as the variation range of 32 S /34 S values. The absolute change value of ?34S is 6.27‰ with significant tower effect and the peak concentrates around 6‰~7‰ with the obvious enrichment of 34 S, belonging to heavy sulfur type. This S isotopic composition is obviously lower than that of seawater(around 20‰) and marine evaporite(?34S=10‰~27‰), and higher than organic sulfur(low negative values), but within the range of metamorphic rock(?34S=-20‰~+20‰), sedimentary rock(?34S=-40‰~+50‰) and granite(small positive value near +10‰). Considering there are no organics and evaporite in the ore district and the ?34S value is similar with granite, showing total sulfur isotopic composition of ore-forming fluid is small positive value near zero, this explains the main sulfur source of this copper ore is mantle sulfur or homogeneous crustal sulfur. Also, it probably indicates that the Yanshan granite intrusion near the Liwu copper deposit is composed of the differentiation of mantle material. This essay tests the Pb isotopic composition of copper pyrite, pyrrhotine, marmatite and iron pyrite: 206Pb/204 Pb is 18.180~18.537,207Pb/204 Pb is 15.747~15.937 and 208Pb/204 Pb is 38.894~39.276. This indicates that the variation range of Pb isotopic composition of sulfide ore of Liwu copper deposit is large, with 9.76~10.17 of ? value and 9.97 on average. Combined with previous research results, this essay considers that the ore lead is the mixed lead of crustal lead and mantle lead and the lead source is largely supplied by the ore-bearing metamorphic rocks.The source of metallogenic material of the Liwu copper deposit is complex, with amulti-source characteristics.The marine volcanic-turbidite clastic rocks is the original ore-bearing layer of the deposit, which is formed in the Mesoproterozoic.Meanwhile, a sulfur of the metallogenic substancemainly from the upper mantle and mixed with a sulfur from seawater,the lead from the mixture of crust and mantle and the water in ore-forming fluid which mainly compose of the mixed magmatic water and metamorphic water.Due to the influence from metallogenic environment,tectonic movement and other factors, some of the water mixed with seawater and atmospheric precipitation.In general, the Liwu copper deposit is featured with sea facies volcanic clastic-metamorphic genesis.Combining with regional geological evolutionary history, the mineralization of Liwu copper deposit with a multi-period feature.This paper will divide the mineralization into four phases.In the first phase,a large transgressive activity accompanied by undersea volcanic eruptions which occur in the Mesoproterozoic Kangdian area basins formed a set of pyroclastic sedimentary rock. And some studies indicate that the initial enrichment of copper has formed the original ore source layer of the copper mine in that time.In the next phase Songpan-Garze orogenic belty tectonic activity caused the set of pyroclastic sedimentary rock suffered strong deformation and metamorphism in Indosinian-Yanshan.In the third phase,Jianglang metamorphic core complexes shaped into a dome and the crust remelting form a large number of intermediate-acidic granite in the deep.And the shaped activity also formed the bedding slip tape and fold in the metamorphic core complexes where provides routeways and ore hosting sites.Finally,the regional dynamic metamorphic fluid promotes the original ore source layer to form a rich copper ore.In summary, this paper on the basis of stable isotope studies, to analyze the minerals and ore-forming fluid source.From the metallogenic geological conditions, the spatial distribution of ore and the feature of mineral assemblages, the mineralization of Liwu copper deposit is limited by four constraints which is stratigraphy, tectonics, magmatism and metamorphism.The regional dome structure controls the spread of the deposit and the ore structure controls the amount and spatial distribution of the deposit. The tectonism, magmatism and metamorphism provides the main metallogenic dynamic energy and heat energy.And the regional dynamic metamorphism of copper promotes the next aggregation to form rich ore body. This paper based on the summarizes of the main ore-controlling factors of the deposit, to discuss the prospecting indicators, presented the main ore-searching directions of the periphera and deep prospecting, determine the direction of extension of hydrothermal alteration zone and predict that in Zhongzui,Heiniu cave and Haidigou have good prospecting potential where near the dome structure.