Research On The Skarns Formation Mechanism Of The Shizhuyuan W-Sn Polymetallic Deposit,Hunan Province

The Shizhuyuan W-Sn polymetallic deposit is located in the central of the Nanling Range, and it is also a world class deposit with the characteristics of variety minerals, large scale, abundant mineral association and the complex metallogenic conditions.Take the Shizhuyuan deposit as the research object, based on the detailed field study, using some experimental methods such as EMPA, the rock geochemistry analysis and the fluid inclusion homogenization temperature. This paper mainly studies the properties of the Qianlishan granite rocks, the characteristics of the Alkali Metasomatic Veins in skarn and the ore-forming fluid geochemical. The main conclusions we got are as follows:(1) The results of the mineral assemblage analysis have showed that the skarn evolution always went through three stages:Skarns Stage, a stage forming the skarn minerals like Garnet, Diopside, VesuVianite, Wollastonite; Degradation Metamorphic Stage, a stage of forming some Oxidate like Scheelite, Magnetite; Sulfide Stage, a stage of setting Pyrite Chalcopyrite, Pyrrhotite, etc.(2) The results of the Albite’An content in the first two period Qianlishan granite have showed that its average value is always less than2, so it should belong to the pure Albite. The Mica in the first period of the granite body is Ferrimuscovite with a range4-5%, While in the second period its average value is less than5%. Therefore, according to the QAP illustration of the intrusive rocks, we renamed the first period granite as the Coarse Grained Porphyritic Alkali-Feldspar Granite, the second period granite as the Fine Grained Alkali-Feldspar Granite, and the last period granite as the Granite Porphyry unchanged.(3) The major mineral in Alkali Metasomatic Veins contain K-feldspar, Plagioclase, Quartz, Fluorite. The white Mica content is few with the content less than3%. Furthermore, it contains hardly Topaz and Tourmaline. So it is not the classic "Greisen Veins" like the previous described.According to the differences of the minerals association, we divided the Alkali Metasomatic Veins into two kinds which related to the two stages of the skarn evolution:the Early Alkali Metasomatic Veins and the Late Alkali Metasomatic Veins. The Early Alkali Metasomatic Veins contain more Plagioclase, Wolframite, and the process is related to the Skarns Stage and it is also the main ore-forming period of Wolframite. In contrast, the Late Alkali Metasomatic Veins contain more K-feldspar, Scheelite and Magnetite, so it is related to the Degradation Metamorphic Stage and it is also the main ore-forming period of Scheelite.(4) The types of the Fluid inclusions in Shizhuyuan are rich liquid fluid inclusions, two-phase fluid inclusions, rich CO2three-phase fluid inclusions, containing the son crystal multiphase. The fluid inclusions and the host minerals are mainly Fluorite, Quartz, Garnet, Scheelite, Diopside and Calcite.Skarns Stage:The ore-forming fluid has the characteristics of high homogenization temperature (600-350℃), high salinity (>15wt%), rich CO2and the fluid source is mainly magmatic water.Degradation Metamorphic Stage:the homogenization temperature (350-250℃) and salinity (2-10wt%) are falling rapidly, and the meteoric water began to mix magmatic water. In addition, the Calcite appears in Fluorite, and the Scheelite start to precipitate.Sulfide Stage:The ore-forming fluid tend to be stable with the characteristics of low homogenization temperature (250-120℃) and low salinity (1-4wt%).On the basis of the homogenization temperature test of rich CO2three-phase fluid inclusions, we worked out the range of metallogenic pressure is mainly focus on1800-3200bar. Finally we figured out the metallogenic depth is5.4-11.9km.In conclusion, the skarn evolution in Shizhuyuan went through Skarns Stage, Degradation Metamorphic Stage and Sulfide stage. The Skarns Stage is the main ore-forming period of Wolframite, and it is related the Early Alkali Metasomatic Veins. The ore-forming fluid has the characteristics of high homogenization temperature, high salinity and rich CO2; The Degradation Metamorphic Stage is the main ore-forming period of Scheelite, and it is related the Late Alkali Metasomatic Veins. In this stage, the homogenization temperature and salinity are falling rapidly, and the meteoric water mixed; Sulfide Stage:The ore-forming fluid tended to be stable with the characteristics of low homogenization temperature and low salinity.