Tungsten-tin Decoupling Mechanism At Deposit Scale

The decoupling of tungsten and tin is a very common phenomenon in the mineralization process,but the mechanism of the decoupling is remain unclear.The Nanling region is the largest tungsten-tin metallogenic belt in China and even in the world.And,it is an ideal place to carry out decoupling mechanism between granitic magmatism and tungsten-tin metallogenesis.Although tungsten and tin metallogenesis are closely related to the high evolved granites,the mineralizations of tungsten and tin show significant decoupling phenomenon in the ore-forming fluid evolution process,different metallogenic events in the mining area and the spatial zoning of the same metallogenic event.Now,a preliminary study has been carried out on the decoupling of tungsten and tin from the same metallogenic event at the scale of metallogenic belt.However,there is still a lack of systematic research on the decoupling mechanism of different scale metallogenic events to tungsten and tin deposits in this area.The Wangxianling deposit is a large-sized tungsten and tin deposit in southern Hunan Province,which is closely related to highly evolved granite,but the W and Sn mineralizations are spatially related to tourmaline granite and biotite granite respectively.In this study,various types of granites and representative tungsten and tin deposits in the area are taken as anatomical objects.We have systematically carried out studied on geochronology,petrogeochemistry and in situ element geochemistry on minerals.Furthermore,the decoupling mechanism of tungsten and tin mineralization in the mining area is discussed.The main achievements are summarized as follows:(1)The LA-ICP-MS zircon U-Pb ages of medium-coarse tourmaline dimica granite in central facies and fine tourmaline dimica granite in marginal facies are 229±2.5 Ma and 217.8±3.9 Ma,respectively.The LA-ICP-MS zircon U-Pb ages of biotite granite concealed beneath tin ore bodies are 154.0±1.3 Ma,and the quartz vein type tungsten ore in Shuishan area are obtained.The Re-Os age of Molybdenite associated with wolframite is 219.5±3.4 Ma.Based on previous studies,it is considered that tungsten mineralization in this area is closely related to the medium-fine tourmaline-mica granite in the margin facies of Wangxianling intrusion,while tin mineralization should be related to the concealed biotite granite of Late Jurassic.It is clear that the tungsten and tin mineralization in this area should be the results of the Late Triassic and Late Jurassic metallogenic events,respectively..(2)It is clear that both the tourmaline granite and biotite granite in the area are highly differentiated and reduced granites.Although biotite granite shows more mantle-derived components,the two types of granite related to mineralization are mainly derived from the metamorphic basement in the area.By comparing the zirconium saturation temperatures of the two types of granite,it is found that the zirconium saturation temperatures of the tourmaline-mica granite related to tungsten mineralization are significant higher than those of the biotite granite.Combined with previous studies,it is proposed that magma evolution and redox state alone cannot account for the W-Sn decoupling in this area.Instead,Low temperature melting dominated by Muscovite dehydration and high temperature melting dominated by biotite dehydration may be the fundamental reasons for W-Sn decoupling at deposit scale.(3)Our data on the ratios of Zn/Nb,Co/La and Sn of the tourmaline in tourmaline granite show that the tourmaline in tourmaline granite is obviously different from tourmaline in typical tin-forming granite,providing mineralogical evidence that the tourmaline granite in this area is tin-barren granite.In addition,in situ element geochemical data on hydrothermal tourmaline show that the in situ element geochemical analyses on hydrothermal tourmaline in quartz veins and greisen can effectively indicate the enrichment and precipitation of various ore-forming materials in hydrothermal fluids.