Geologic Characteristic And Genesis Mechanism Of The Shuikoushan Pb-Zn Deposit In Changning City, Hunan Province

The Shuikoushan is a large polymetallic ore-field, located in the middle of Gan-Xiang-Gui-Yue fold belt of South China fold system and in the northern margin of NS-Leiyang-Linwu fold belt, comprising of Shuikoushan lead-zinc deposit, Kangjiawan lead-zinc deposit, Shi’aoling lead-zinc deposit, Longwangshan gold deposit and Xianrenyan gold deposit etc. The Shuikoushan lead-zinc deposit lies in the center of the orefield, adjacent to the Kangjiawan lead-zinc deposit in east and next to the Longwangshan gold deposit in south. Based on field geological investigation, we systematically studied the geochemical properties, forming age and material source of granodiorite, the physicochemical condition and source of ore-forming material through the methods as petrochemistry, isotope geochemistry, fluid geochemistry, zircon LA-ICP-MS U-Pb dating and stable isotopes for ore etc. With reference to previous research results, we discussed the metallogenic mechanism and genesis of the Shuikoushan lead-zinc deposit.Major element analysis of the Shuikoushan granodiorite shows that SiO2ranges from60.65%to60.99%, Al2O3ranges from15.05%to15.67%, MgO ranges from4.62%to6.59%, K20+Na20ranges from6.81%to7.06%, K2O/Na2O ranges from1.22to1.89, A/CNK ranges from1.13to1.34.(La/Yb)N ranges from9.98to11.26with Eu negative anomalies (8Eu=0.77～0.83).The granodiorite is enriched in LILES(Rb, Th, U, La) and Pb while Ba、Sr and HFSES(Nb, Ta, Ti) are depleted. Zircon LA-ICP-MS U-Pb dating age yields of156.03±1.0Ma (MSWD=0.33,2σ, N=20). Hf isotope analysis indicates that the granodiorite has low εHf(t)=-10.78---8.71, with mean value of-9.71; TDMC ranges from1.75Ga to1.88Ga, with mean value of1.82Ga, suggesting granodiorite magma source mainly came from the lower crust in the Paleoproterozoic.There are mainly two types of primary fluid inclusions in ore-bearing calcites, including aqueous-rich and vapor-rich two-phase inclusions. Microthermometry shows that the homogenization temperatures range from92℃to345℃with two peak values of120℃～160℃and280℃～320℃. The salinities range from0.35%to19.84%NaCl eq., with two peak values of9%～12%NaCl eq. and15%～18%NaCl eq.. The density of fluid inclusions ranges from0.668g/cm3to1.083g/cm3, with an average value of0.929g/cm3. Metallogenic pressure is estimated about18.3MPa and metallogenic depth is approximately1.8km. The research results indicate that the deposit formed in a low temperature epithermal environment.Ore δ34S range from-2.42‰to-0.23‰, with an average value of-1.10‰, indicating a deep source for magma and maybe partly contaminated by the crustal substance. Ore206Pb/204Pb values range from18.444to18.538, with a mean value of18.150;207Pb/204Pb values range from15.654to15.783, with a mean value of15.726;208pb/204pb values range from38627to39.066, with a mean value of38.872, indicating a crust source. Values of δ13C VPDB of limestone in stratum range from-0.5‰to0.3‰, with an average value of0‰,δ18O SMOW values range from14.1‰to16.8‰, with an average value of15.5‰;δ13C VPDB of ore-bearing calcites values range from-1.8‰to-0.7‰, with an average value of-1.3‰,δ18O SMOW values range from15.0%o to16.5%o, with an average value of16.0‰.The C-O isotopic compositions of ore-bearing calcites are similar to that of the limestone, inferring the carbon of ore mainly derived from limestone of Early Paleozoic stratum.During150to165Ma, the lithosphere in southern Hunan was tensional and thinning, causing mantle upwelling and magmatic underplating. With a large-scale of crustal substances melting, the acidic magma derived from the lower crust and mantle intruded along the fault, and ultimately emplaced in the near-surface, formed the Shuikoushan granodiorite. The mineralized ore-forming fluids containing Pb, Zn, Cu, Au elements migrated into the near-surface, setting and enriching at proper tectonic position under the action of mixing effect of tectonic decompression and atmospheric precipitation.