Main Controlling Factors Of Mesozoic Tungsten-Tin Polymetallic Mineralization In South Hunan

Located in the structural superposition between the Qin-Hang and Nanling metallogenic belts,the South Hunan area hosts abundant tungsten-tin polymetallic deposits associated with the Yanshanian biotite monzogranite,which provides a good window for studying tungsten-tin metallogenesis.The enrichment of tungsten and tin is generally considered to be related to the highly differentiated granites,but it remains unclear which factors control the large-scale tungsten-tin mineralization in South Hunan.In this thesis,typical tungsten-tin polymetallic deposits such as Furong,Xianghualing,Xintianling,Shuangyuanchong and Jiepailing in South Hunan were taken as the research objects to study the contribution of the source of ore materials,magmatic-hydrothermal processes and physicochemical conditions of mineralization,and then to demonstrate the main controlling factors for large-scale tungsten-tin mineralization.The following achievements and conclusions have been obtained:(1)The ore-forming fluids have similar source and composition.All of the W-Sn polymetallic deposits have complex but similar mineral assemblages in ores and altered rocks,with ore minerals including cassiterite,chalcopyrite,wolframite(or scheelite),sphalerite,galena and altered minerals including fluorite,topaz,mica,K-feldspar,chlorite and spinel.Thus,the ore-forming fluids required for the formation of W-dominated and Sn-dominated deposits likely have similar sources and compositions.The mineral assemblages suggest that the ore-forming fluids are rich in volatiles of B and F,and contain mineralization elements of W,Sn,Cu,Nb and Be.We thus consider that the ore-forming fluids responsible for the Li-Be-Nb-Ta-W-Sn-Bi mineralization of different ore deposits in South Hunan sourced from the same granitic magma chamber.(2)Compositional maturity of the source area provides the material base for Mesozoic large-scale tungsten-tin mineralization in South Hunan.Based on the study of magmatic zircon and detrital zircon in southern Hunan,the southern Hunan region experienced at least six magmatic tectonic events at about 2.5Ga,1.3-1.1Ga,950 Ma,800Ma,650 Ma and 430 Ma before Mesozoic,the zircon ?Eu,LREE/HREE,Y/Ho,and Nb/Ta values of the same tectonic-magmatic event in South Hunan vary greatly.Along with the ages getting younger,the LREE /HREE and Nb/Ta values display a tendency becoming larger,suggesting that the magma differentiation degree has not increased during geological historical evolution.The zircon ?REE,Y,Th,U,Nb and Ta contents as well as LREE/HREE and Nb/Ta values increase with ages becoming younger,suggesting that the compositional maturity of source area increases accordingly,and this tendency shows a positive correlation with the intensity of W-Sn mineralization.Our results indicate that the W-Sn mineralization of South China is mainly controlled by high compositional maturity of source area.The zircon trace elements can be used as an important index to trace the scale of tungsten-tin mineralization.(3)The volatile-rich hydrosilicate melts generated by magma liquid immiscibility represent the key factor for the super enrichment of tungsten and tin.Two types of volatile-rich hydrosilicate melts have been identified in the W-Sn polymetallic deposits of South Hunan.The first type is Na-and volatile-rich and represented by the albite-tourmaline-quartz pockets hosted in altered alkali-feldspar granite.The second type is K-and volatile-rich and represented by the lithian muscovite-fluorite pockets or blocks hosted in altered and mineralized granite porphyry.They have close spatiotemporal and genetic relationship and represent different products of magma liquid immiscibility.In South Hunan,the K-and volatile-rich melt is probably responsible for the greisen-type,the structure-altered zone-type and the K-feldpar-granite-type W-Sn polymetallic mineralization,while the Na-and volatile-rich melt is linked to the albite-granite-type Nb-Ta mineralization.(4)The sudden drop of pressure represents the key factor for the massive precipitation and mineralization of tungsten and tin.The phenomenon of sudden pressure drop is widespread in various ore fields of Mesozoic W-Sn polymetallic deposits in southern Hunan,forming cryptoexplosive breccia and various structures of sudden temperature drop caused by sudden pressure drop,such as solid solution separation structure,post syngenetic crystal structure,zircon high temperature metasomatic structure,etc.