Geological And Geochemical Characteristics And Genesis Of The Skarn-type Scheelite Deposit In South Yangtian, Southeastern Yunnan

Nanyangtian scheelite polymetallic ore field,the largest tungsten polymetallic ore field at the western margin of Nanling tungsten-tin polymetallic metallogenic belt,lies at the southwest of Malipo County,Wenshan autonomous Prefecture,Yunnan Province,and geotectoniccally located at the intersection of Binhai Pacific tectonic field,southern China folds,Yangtze quasi-platform and Sanjiang folds.The ore field is composed of a series of deposits,such as Nanyangtian,Fava,Chayeshan and Wazha,and so on.The Nanyangtian deposit is a typical super-large skarn-hydrothermal vein type scheelite deposit in the ore field,discovered in the past ten years.It is an important window to better understanding the mineralization mechanism of tungsten in this erea.So,based on the previous studies,this paper focused on the geologic characteristics,fluid geochemistry,element and stable isotope geochemistry of the Nanyangtian scheelite deposit.By comprehensive comparison,the source and evolution of ore forming fluids and ore-forming materials,and metallogenic mechanism are discussed.The main advance achievements and conclusions from this study are listed as follows:1.There are two types of scheelite mineralization in the Nanyangtian mining area.One kind is the milkiness porphyritic mass texture and disseminated scheelite in the "layered" skarn,and the occurrence is more stable and the grade is higher.The other is produced in the feldspar quartz vein in the beige lumps,vein-like scheelite,Occurrence and grade change is bigger.The former is the main industrial ore type.By the paragenetic association of minerals,the mineralization can be divided into skarn stage,degradation of alteration stage,quartz-scheelite stage and calcite-sulfide stage.2.Skarn is the most important ore-hosting rock in the Nanyangtian mining area.The calcic skarn are the main skarn types.The main mineral assemblage of the skarns is diopside-hedenbergite-grossularite-hornblende-epidote,the main end member composition of the pyroxene is Di55-82Hd18-41Jo0.3-6,which of the garnet is Gro83-89 Alm7-9And4-5,indicates that the early skarns are mainly formed in the reducing environment.Mineral assemblage and distribution characteristics show that with the increase of garnet and diopside,scheelite is enhanced and the grade of ore is increased.It shows that the mineralization has a close relationship with the skarn,the scheelite mineralization is the product of skarnization.3.Nanyangtian ore-bearing skarn shows low Si,Ca-rich,relatively rich Fe,Mg,poor alkali,indicates that the original rock may be a high silicate content of silicic acid(35%-65%),which provids the foundation for the replacement of later ore-forming hydrothermal fluids.The total rare earth elements contents of the skarns are between the non-alteration surrounding rocks and Laojunshan granites(the Laojunshan granite pluton(66.74 ?g/g)<Nanyangtian skarn(138.997?g/g)<schist(200.225?g/g)),and they have similar distribution pattern by LREE enrichment,Ce anomaly is not obvious,moderate negative Eu anomaly,indicate that they have close formation relationship.Compared with different types of skarn,the skarn of Nanyangtian mining area is similar to the magmatic hydrothermal metasomatism skarn.So,the skarns in the Nanyangtian area are mainly formed by the hydrothermal metasomatism of the post magmatic hydrothermal from the Yanshanian period Laojunshan granite and the surrounding rocks.4.Fluid inclusion studies in the scheelite and skarn minerals show that the ore-forming fluid is characterized by medium-low temperature,middle-low salinity.The homogenization temperature of the fluid inclusions range from 221?423 ? in garnet and epidote skarn,177?260? in quartz and scheelite,and 173?227? in calcites.And the corresponding salinity of the fluid inclusions range from 0.18%?16.34%NaCleqv,0.35%?7.17%NaCleqv,and 0.35%?2.24%NaCleqv,respectively.The temperature and salinity of the ore-forming fluid are decreased from the early to late stage,and the scheelite mineralization is mainly related to medium temperature,medium-low salinity stage.The Raman spectrum microprobe analysis show that there are two types of fluid in the quartz and scheelite,one type fluid contains CO2,N2 and H2O,while another contains only N2 and H2O.It is infered that the migration of the ore-forming element is not related to the Cl and F riched fluids.The low salinity of the fluid inclusion of ore mineral and gangue mineral in Nanyangtian tungsten deposit indicate that the physi-chemical condition changes of the ore-forming fluids lead to the precipitation of ore,this changes are the comprehensive result of the metasomatic reaction between the tungstenic fluid and carbonatite,the cooling and mixing of fluids in the ore-forming process.The oxygen isotope ?18OsMOW of the ore-bearing skarn in Nanyangtian is between 5.8 ‰ and 9.1‰,which is basically consistent with the magmatic water(about 5.5 ‰ to 8.5 ‰).Fluid inclusions and stable isotope studies show that the ore-forming fluids of the Nanyangtian scheelite deposit are mainly magmatic sources and contain some stratigraphic and atmospheric water.5.Comprehensive research show that the ore-forming elements are enriched in the surrounding rocks by the regional metamorphism in Indosinian period,and hydrothermal metasomatism between the surrounding rocks and the ore-bearing magmatic-hydrothermal fluid from the Yanshan period Laojunshan granite result in the formation of Nanyangtian super large skarn type scheelite deposit.