Genesis And Metallogenic Model For The Kunshan W-Mo-Cu Deposit In The Dahutang Area,Northern Jiangxi Province

The Kunshan W-Mo-Cu deposit in Northern Jiangxi which is located in the central area of Jiuling tungsten-polymetallic ore concentration region, is a medium scale tungsten-polymetallic deposit discovered in Dahutang area recently. In this paper, based on the previous research results and field geological survey, detailed studies on geochronology, petrology and geochemistry of the Yanshanian intrusions, the microthermometry and composition of the fluid inclusions, the isotope geochemistry are conducted. On the basis of above achievements, we found out the ages and origin of magmatism and mineralization activities in the Kunshan deposit, the origin and evolution of the ore-forming fluids, and discussed genesis of the Kunshan deposit, also set up an ore-forming model of the Dahutang area. The innovative achievements and views are as follows.Unlike the ores of the Dahutang deposit that consist of veinlet-dissemination type, hydrothermal crypto-explosion breccia type, and thick-vein type which occur in the upper part of the Yanshanian granite and the contact zone with the Neoproterozoic granodiorite, the ores of the Kunshan deposit are tungsten, copper and molybdenum bearing quartz veins occurring in the top of the Yanshanian plutons and the contact zone of the Neoproterozoic Shuangqiaoshan Group low grade metamorphic rocks, and interestingly, the surface to deep ore zonation of W-Mo-Cu is obvious.The Yanshanian porphyritic granite in the Kunshan deposit is closely related to the tungsten-polymetallic mineralization, which also shows in the accordance between the diagenesis age（151.7±1.3 Ma, LA-ICP-MS U-Pb dating） and the molybdenum mineralization age（151.0±1.3 Ma, Re-Os dating）. The Yanshanian granite porphyry（136.6±2.5 Ma, LA-ICP-MS U-Pb dating） yielding after mineralization as veins or dikes in the Kunshan deposit, has a clear contact zone with surrounding rock. Petrological and geochemical study shows that both porphyritic granite and granite porphyry are characterized as high silicon, high potassium, calc-alkaline series of high fractional S-type granites. The εHf（t） values of porphyritic granite vary from-8.80 to 4.48 with the two-stage model ages t DM2 of 917 Ma to 1757 Ma. It is indicated that the source region of the granite were mostly from argillaceous sedimentary rocks, likely from the Neoproterozoic Shuangqiaoshan Group.The C-O isotopes of quartz and calcite veins suggest that the carbon（-6.3‰, on average） is mainly derived from the magma that affected by low-temperature alteration, while the carbon in late calcite also mixed with sedimentary organic material in the Shuangqiaoshan Group. Meanwhile, the δ³⁴S isotopes（-1.5‰³.0‰） of Chalcopyrite, pyrite and molybdenite samples indicate that sulfur mainly caomes from deep magma or upper mantle.The results of the microthermometry and composition analysis of the fluid inclusions show that the ore-forming fluid is characterized by medium-high temperature, low salinity, medium density. The main gas phase composition of fluid inclusions is H₂O, while the content of CH₄ and CO₂ is very low. The evolution of ore-forming fluid in the Kunshan deposit has gone through two stage at least which are boiling and fluid mixing. The H-O isotopes of quartz veins（dD=-92.0‰ ^-56.0‰、d18OH₂O=3.81‰ 9.07‰） show that the ore-forming fluids in the Kunshan deposit are mainly magmatic fluid.According to the geological characteristics, the origin and evolution of oreforming fluid, It’ s concluded that the zonation of W-Mo-Cu orebodies is closly related to the boiling of ore-forming fluid in early stage and sediment of high density fluid. Combined with geological background of the Dahutang area and main deposit, ametallogenic model of the Dahutang area have been set up. The Yanshanian magmatic activity in the Dahutang area, which originated from the same deep magma chamber and then intruded discontinuously after differentiation process, is related to the tectonic conversion of the Lower Yangtze region in Late Jurassic. The granite sequence is : porphyritic granite（150¹45 Ma）�'fine grained granite（¹45Ma）�' granite porphyry（ 143 Ma, 135Ma）. Magma activity took some deep ore-forming fluid and materials to the crust, and caused a series of mineralization.According to the summary and analysis of geological characteristics for W-polymetallic deposits in Jiangxi province, It is concluded that the tungsten deposits in Northern Jiangxi Province have an element assemblage of W-Mo-Cu and scheelite orebodies occur as veinlet-dissemination type or skarn type, while those in Southern Jiangxi Province is W-Sn and wolframite orebodies occur as quartz vein type. The Kunshan deposit is worth further investigation.