Skarn And Ore Genesis Of The Darongxi Tungsten Deposit, Western Hunan

Xuefeng uplift located in Western Hunan is an important Au-Sb-W metallogenic region in South China. In this region, tungsten deposits primarily occur as stratabound quartz vein type or skarn type. Lots of comprehensive researches on the quartz vein type tungsten deposits including Woxi and Zhazixi tungsten deposits have been undertaken. However, scarce attention is paid on the skarn type tungsten deposit such as Darongxi deposit in this area. Based on the previous researches, this dissertation applies the mineralogy, economic geology and geochemistry methods on the Darongxi scheelite deposit. In order to reveal the skarn formation, mineralization mechanism and ore genesis, the geological characteristics, skarn minerals’geochemistry, fluid inclusions and geochronology are emphatically studied. The main achievements are obtained in this study as follows:(1) Orebodies in the Darongxi deposit are comprised of tungsten-bearing skarn type and quartz stockworks type, which are stratabound and stratiform controlled by fractures and/or pores of different lithologic boundaries. In addition, several molybdenite-bearing quartz veins cutting the tungsten orebodies in this deposit are first discovered in this study.(2) Paragenetic mineral assemble characteristics show that the mineralization can be divided into two metallogenic periods. The former is skarn period including prograde, retrograde and oxide stages, and the latter is quartz-sulfide period containing early and late stages. Scheelite mainly occurred in the oxide stage.(3) Geochemical characteristics reveal that skarnoid in the Darongxi deposit should be classified as a reduced and stratabound skarn generated by hydrothermal diffusive metasomatism. Skarn is mainly made up of clinopyroxene and garnet, but most of clinopyroxenes are metasomated by garnets. Both belong to manganic calc-silicate minerals. Major element and rare earth element provide an evidence for the evolution processes of different type clinopyroxenes and garnets. Meanwhile, REE patterns of clinopyroxenes or garnets collected from different ore veins are similar, and tungsten contents display positive correlation with Eu anomaly in different geological bodies. These indicate that the fluids responsible for formation of skarn orebody, stockwork orebody and molybdenite-bearing quartz veins may be derived from the same source.(4) Fluid inclusion studies demonstrate that ore-forming fluid of the Darongxi deposit is characterized by low salinity and medium-high temperature. Homogenization temperatures of fluid inclusions in garnets range from324℃to400℃, and its salinities vary from2.24to10.98wt percent NaCl equiv, and mean density is0.65gram per cubic centimeter. By contrast, fluid inclusions of scheelite and quartz in the oxide stage have lower salinities from0.88to6.16wt percent NaCl equiv, whose homogenization temperatures range from182℃to356℃with an average density of0.80gram per cubic centimeter. From skarn period to quartz-sulfide period, the temperature tends to decrease, while salinity discrepancy changes may be affected by isolation environment. Laser Raman spectrum analysis data reveal that gas in fluid inclusions of skarn minerals is methane whereas that of mineral in quartz-sulfide period contains methane as well as carbon dioxide. Methane may result from the reaction between hydrothermal fluid and graphite in host rocks, but carbon dioxide could be derived from the primitive ore-forming fluid.(5) Re-Os accurate dating of molybdenite collected from the Darongxi deposit is223.3±3.9Ma, which coincides with the emplacement time of Dashenshan granite (224.3±1.0Ma). Thus, the formation of granite and tungsten mineralization in the Darongxi deposit should take place at the Late Indosinian (T3).(6) The Darongxi tungsten deposit belongs to stratabound metasomatism skarn deposit related to the Indosinian granite, which is generated by the diffusion metasomatism between calc wall rock and a medium-high temperature, low salinity and W-Mo-Cu hydrothermal fluid from the differentiation and evolution of magma during the Dashenshan granite emplacment.(7) The achievements obtained in this study imply that there exists multi-period tungsten mineralization in Western Hunan and Indosinian granitic mineralization may be widespread throughout South China.54figures,16tables and183references are incorporated in this paper.