Geochemical Characteristics And Ore Genesis Of Shimensi Tungsten Deposit In Dahutang

Shimensi tungsten deposit of Dahutang area, located in the west part of the east metallogenic belt of Jiangnan uplift in the Lower Yangtze metallogenic province and the north side of Qin-Hang Belt, is a large tungsten deposit discovered newly in recent years. Depending on the basis of previous studies, the paper deeply discusses the source of ore-forming material, ore-forming fluid and the ore deposit evolution by systematic research including geological sampling, indoor slice identification, fluid inclusions test and stable isotope research, and mainly achieves the following cognitions:(1) The cryptoexplosive breccias in Shimensi deposit of Dahutang area present tubular and radish shape, which have typical brecciated texture. The breccias have different sizes, which mainly present lacerate, serration, sharp prismatic and subprismatic; breccia can be cohered and its ingredients are the same as its surrounding rocks; cataclastic quartz phenocryst can be cohered and there are the same composition between matrix and surrounding rocks. In addition, there are many cracks in quartz phenocryst almost without wavy extinction. Therefore, the breccias are indentified as cryptoexplosive breccias.(2) The homogenization temperature of tungsten ore fluid inclusions of cryptoexplosive breccias in Shimensi deposit of Dahutang area is between 152.1℃ and 387.1℃ and mainly concentrates on 150℃ to 250℃; the salinity value is between 4.8% and 17.3%, average value is 11.2% and mainly concentrates on 8% to 14%. The density of ore-forming fluid is between 0.64 g/cm3 and 1.02 g/cm3 and its average value is 0.92 g/cm3. The pressure of ore-forming fluid is between 42.1 MPa and 117.4 MPa and its average value is 61.4 MPa. The compositions of gas phase is mainly H2 O and little CO2. So, the ore-forming fluid belongs to the system of middle-high temperature, middle-lower salinity and Na Cl-H2O±CO2. The ore-forming fluid has experienced mixed effection of high temperature and salinity magmatic hydrothermal migmatizate and low temperature, low salinity meteoric water during its evolution. The effection of mixing fluids and boiling is the main factor of complex decomposition and deposits in the ore bearing fluid.(3) Veinlet disseminated type deposit can be divided into two phase including early scheelite stage and later scheelite-sulfide stage. The homogenization temperature of early scheelite stage is between 162.9℃ and 367.8℃ with average value of 236.9℃ and concentrates on 170℃ to 250℃ and 270℃ to 350℃ individually. The salinity is manily between 6.7% and 13.1% with average value of 10.2% and concentrates on 8% to 12%. The density of ore-forming fluid is between 0.72 g/cm3 and 0.98 g/cm3 and its average value is 0.89 g/cm3. The pressure of ore-forming fluid is between 44.9 MPa and 102.0 MPa and its average value is 66.0 MPa. The compositions of gas phase is mainly H2 O and little CO2. The ore-forming fluid belongs to the system of middle-high temperature, middle-lower salinity and Na Cl-H2O±CO2. The ore-forming fluid is mixed with magmatic hydrothermal migmatizate and meteoric water. The effection of mixing fluids is the main factor of deposits in the ore bearing fluid.(4) The homogenization temperature of later scheelite-sulfide between 153.8 ℃ and 375.6 ℃, concentrating on 170℃ to 270℃ with unimodal property. The salinity is manily between 7.2% and 13.2% with average value of 10.8% and concentrates on 8% to 13%. The density of ore-forming fluid is between 0.67 g/cm3 and 1.00 g/cm3 and its average value is 0.88 g/cm3. The pressure of ore-forming fluid is between 42.7 MPa and 103.1 MPa and its average value is 69.6 MPa. The compositions of gas phase is mainly CH4, N2 and CO2 by Laser Raman microprobe. The ore-forming fluid occurs fluid unmixing with CO2 vanishment during its evolution. The effection of unmixing fluids is the main factor of deposits in the ore bearing fluid.(5) The results of sulfur isotope test show that the δ34S of veinlet disseminated deposit is between-2.6‰ and-0.3‰ and δ34S of cryptoexplosive breccia deposit is between-1.7‰ and 0.2‰, which both have obvious tower distribution characteristics. The δ34S of veinlet disseminated deposit is lower than cryptoexplosive breccia deposit. Sulfur of cryptoexplosive breccia deposit comes from deep magma or upper mantle, but sulfur of veinlet disseminated deposit mainly comes from deep magma or upper mantle and may have been contaminated by the rock. The δ13CV- PDB of calcite is between-11.42‰ and-5.76‰ with average value of-7.06‰ and its δ18OV- SMOW is between 7.13‰ and 16.34‰ with average value of 11.79‰ which shows carbon is mainly derived from the deep magma or the upper mantle, but do not rule out the possibility of mixed organic carbon. Combined with the researching results of Pb isotope, it can be inferred that the ore-forming material within the mining area mainly come from S granitic magma of Dahutang during yanshanian, but does not rule out the possibility of extraction part of metallogenic material surrounding rock by the ore-forming fluid during it rises.