Study On Metallogenic Mechanism Of The Giant Dahutang Tungsten Polymetallic Orefield In Jiangxi Province,China

Dahutang giant tungsten polymetallic ore field in Jiangxi provience is one of the largest tungsten mines in the world that discovered in recent years.There are multistage ore-related Yanshanian granites in the ore field.The main mineralization type is vein-disseminated scheelite.And it also has a variety of mineralization types,such as coarse vein type wolframite and cryptoexplosive breccia type tungsten?copper,molybdenum?,forming a"multi-integration"tungsten?copper,molybdenum?deposit.Systematic research and in-depth analysis of the diagenesis and ore-forming background,ore-controlling factors,ore-forming fluid characteristics,and mineralization processes of the Dahutang tungsten deposit,understanding the genesis and metallogenic mechanism of the Dahutang tungsten deposit,and establishing the Dahutang-type mineralization model has important theoretical and practical significance for the theoretical research and prospecting exploration for the tungsten polymetallic ore in the Jiangnan orogenic belt and even the entire South China region.LA-ICP-MS monazite U–Pb dating of the coarse-grained porphyritic biotite granite?CPBG?,fine-grained porphyritic biotite granite?FPBG?,fine-grained biotite granite?FBG?and biotite granite porphyry?BGP?yield emplacement ages of 147.9±1.1 Ma,146.4±1.1 Ma,138.6±0.98Ma and 142.8±1.7 Ma,respectively.Whole-rock geochemical results indicate that the four granites should be classified as S-type granites,but BGP has distinct features transitional between S-and I-type granites.They were possibly generated by partial melting of upper crustal pelites and basic volcanic rocks with different proportion from the Neoproterozoic Shuangqiaoshan Group in the source.Geochemical characteristics suggest that they were derived from two magma chambers?the CPBG,FPBG and FBG vs.the BGP?and experienced different evolutionary processes and experienced different degree of magmatic differentiation during magmatic evolution.FeO contents show only minor variation with Anorthite contents and there is a significant positive correlation between Sr and Ba content in plagioclase crystals,indicating that both magmas experienced chemically closed evolution affected by only thermal mixing and/or decompression,but without chemical mixing with mafic magma.The four granites contributed tungsten rather than calcium to the scheelite mineralization owing to their small volume and low calcium contents,but the Neoproterozoic biotitic granodiorite most probably provided large amounts of calcium for the scheelite mineralization because of its large size and high calcium contents.The formation time of cryptoexplosive breccia and corase quartz veins in the Shimensi and Shiweidong deposits are 142.0±0.6 Ma and 136.1±0.5 Ma,respectively,which are both formed in the Early Jurassic,representing the formation time of cryptoexplosive breccia type mineralization and quartz large vein type mineralization.The cryptoexplosive breccia in the Shimensi deposit should be caused by the biotite granite porphyry.The quartz vein type mineralization in the Shiweidong deposit may be formed by the late hydrothermal action of the fine-grained porphyritic biotite granite.After the fluid dissolved from the strongly reducing parent magma,the oxygen fugacity gradually increases during the migration from the parent rock to the surrounding rock,the CO₂content gradually increases and the fluid gradually becomes weak from strong reducibility.The fluids in the ore-forming stages such as ore-bearing quartz veins,cryptoexplosive breccia cements and ore-bearing quartz veins are weakly oxidizing-oxidizing fluids,with a large amount of CO₂,CH₄ and N₂ gases are generally present in the inclusions.The?³⁴S values of sulfides from crypto-explosive breccia,quartz vein-type and fine vein-disseminated type in the Dahutang ore field vary from-2.05 to-0.33‰,-36.34^-0.02‰and-10.07^-1.53‰,respectively.The large-scale variation of?³⁴S and the large negative value away from 0 indicate that the sulfur in the Dahutang ore field cannot be pure magmatic sulfur,and there should be sulfur from sedimentary rocks.Sulfur in late quartz veins mineralization stage should have a large amount of sulfur from sedimentary rocks.The partial melting of the tungsten-rich metamorphic sediments of the Shuangqiaoshan Group in the Neoproterozoic and the highly differential crystallization of the magma led to the initial and further enrichment of tungsten in the magma.The tungsten mineralization in the Shimensi deposit is greatly controlled by the redox state of the ore-forming parent magma.Oxygen fugacities of the CPBG and FPBG declined from early?most above the NNO buffers?to late stages of fractional crystallization?between the NNO and QFM buffers?because of the higher degree of magmatic differentiation in the late stages.The two porphyritic granites?the CPBG and FPBG?are most likely the main contributors of tungsten,while the FBG and BGP are mainly responsible for copper and molybdenum in the Shimensi deposit.Prolonged multiphase magmatism and prolonged W-Cu-Mo mineralization play important roles in the formation of Shimensi large tungsten polymetallic deposit.