Geology,geochemistry And Genesis Of The Yuanlinzi Magmatic Hydrothermal Sn-Cu Deposit In Inner Mongolia

Yuanlinzi deposit is a new copper-tin symbiosis deposit discovered in the southern Great Xing’an Range（SGXR）in recent years.It is a typical copper-rich tin deposit.A total of 15 industrial ore bodies were found in the mining area,including 4tin copper bodies and 11 copper bodies（associated tin）.The total ore content of the deposit is 1,548,900 tons,with an average grade of 1.24%copper and 19,255.35 tons of metal,and an average grade of 0.27%tin and 4,228.48 tons of metal,indicating that the deposit size is close to medium.The ore minerals in the ore include cassiterite,arsenopyrite,pyrite,chalcopyrite,sphalerite and molybdenite,gangue minerals are mainly quartz,sericite,chlorite,calcite,fluorite.The surrounding rock alteration of the deposit mainly includes silicification,sericitization and chlorite.The ore-forming stage of the deposit can be divided into quartz-chlorite-cassiterite stage,quartz-sulfide stage and quartz-fluorite-carbonate stage.LA-ICP-MS U-Pb dating of the cassiterite in the deposit is 137.96±0.55 Ma,which indicates that the tin mineralization of the deposit was formed in the Early Cretaceous.The trace element data show that cassite is deficient in light rare earths,relatively enriched in heavy rare earths（（La/Lu）_N=0.008-0.041）,and has high content of Sc,Zr,Nb and Ta high field strength elements,indicating that the ore-forming fluid has high temperature,high F and high field strength elements.In situ S-Pb isotope data show thatδ³⁴S values of chalcopyrite and pyrite range from 2.53 to 4.92‰and5.16 to 6.20‰respectively.The ²⁰⁶Pb/²⁰⁴Pb ratio of pyrite is 17.328-18.071,the²⁰⁷Pb/²⁰⁴Pb ratio is 15.16-16.23,and the ²⁰⁸Pb/²⁰⁴Pb ratio is 37.756-38.733,indicating that the source of sulfur in the deposit is a single source of deep magmatic sulfur,and the lead isotope results of pyrite show that the lead in the deposit is mixed source of crust and mantle.Mesozoic intrusive rocks in the mining area are divided into two stages,Early Triassic and Early Cretaceous.The Early Triassic intrusive rock is granodiorite,and the results of LA-ICP-MS U-Pb dating are 247.6-246.3 Ma.The granodiorite is quasi-aluminous and belongs to the calc-alkaline series.The total rare earth elements of the granodiorite rocks range from 68 to 136 ppm,with an average value of 98 ppm.The samples are enriched in light rare earth elements and deficient in heavy rare earth elements.Granodiorite is rich in Rb,Th,U,Ta,La and Ce,but deficient in Ba and Nb.The ⁸⁷Sr/⁸⁶Sr ratios of granodiorite samples are 0.7066-0.7068(I_Sr is 0.7034-0.7041),ε_Nd（t）values are 2.15-3.53,and the two-stage Nd model ages(T_DM2)are 732-845 Ma.The Early Cretaceous intrusive rocks in the mining area are granite,and their lithology includes granite porphyry（GP）,amazonite-bearing alkaline granite（AAG）,K-feldspar granite（KFG）,amazonite-bearing porphyritic granite（APG）,and fine-grained granite（FGG）.LA-ICP-MS U-Pb dating of the granite is 145.4-141.9 Ma,The granite are quasi-aluminous-weak peraluminous and belong to the high potassium calc-alkaline series.The total rare earth elements range from 91-241 ppm with an average content of 168 ppm,showing enrichment of light rare earth elements and deficit of heavy rare earth elements,with strong negative Eu anomaly（δEu=0.12-0.45）.The granites are rich in Rb,Th,U,Ta,La,Ce and Hf,and deficient in Ba,Nb and Sr.The granite samples have ⁸⁷Sr/⁸⁶Sr values of 0.7107-0.7497(I_Sr is0.7020-0.7065),ε_Nd（t）values of 1.89-3.00,and two-stage Nd model ages(T_DM2)of690-781 Ma.Geochemical data and Sr-Nd isotope data indicate that the Early Triassic granodiorite is I-type granite,which is the product of partial melting of the new lower crust;The Early Cretaceous granite is A-type granite,which was formed by partial melting of potassium-rich felsic rocks in the middle and upper crust,and the separation and crystallization of potassium-feldspar,plagioclase,apatite and other minerals occurred during the formation process.Combined with the regional tectonic setting,we can see that the Early Triassic granodiorite was formed in the post-collision extensional environment after the closure of the ancient Asian Ocean,and the Early Cretaceous granite was formed in the low pressure and high temperature environment after the closure of the Mongolic-Okhotsk Ocean and under the oblique subduction of the ancient Pacific plate.Yuanlinzi tin-copper deposit is a part of the Early Cretaceous Sn polymetallic mineralization in the southern Great Xing’an Range（SGXR）,and has a close genetic relationship with the Early Cretaceous magmatism.The Ce⁴⁺/Ce³⁺ratio of zircon in the granite gradually decreased with the granite formation age from morning to night（145-143 Ma）,but suddenly increased in the late granite（142 Ma）(mean Ce⁴⁺/Ce³⁺=178.1),which is higher than typical metallogenic magmas of tin deposits and similar to porphyry copper deposits.Neither the Early Triassic granodiorite nor the surrounding rock strata in the mining area have the potential to provide Cu in the deposit.Therefore,the Cu in the deposit is most likely to be derived from injection of copper-bearing melt/fluid differentiated from relatively oxidized mafic magma.