The Genesis Of Mica Minerals And Its Indication In Dahutang Deposit

The Dahutang W-Cu-Mo deposit is located in the Eastern Jiangnan uplifted belt with proved WO₃ resource reserves of 1.1 million tons,which is one of the largest tungsten polymetallic deposits in the world.The ore deposit is composed of by Shimensi and Shiweidong sections,and the ore types contains?1?veinlets disseminated,?2?cryptoexplosive breccia and?3?quartz large vein types.Based on the detailed geology investigation on Shimensi and Shiweidong section of Dahutang deposit,this study focused on the widespread mica in this deposit,analyzed minerals compositions,Li isotope geochemistry and ore-forming fluid,and further discussed its genesis.After that,this study further compared the W mineralization between Jiangnan uplift belt and the Middle-Lower Yantze river valley belt.The main research results are listed as follows:?1?There developed several types biotite in Dahutang deposit,including biotite in Jiuling granodiorite?type 1?,biotite in Yanshanian porphyritic granite?type 2?and biotite in Yanshanian fine-grained granite?type 3?.The type 2 biotite has low X_Mg value and Fe³⁺/Fe²⁺ratio,high Cl,concentrations and LREE/HREE value,with low oxygen fugacity,which can be considered as the met allogenic specificity indicator of W-forming granitoids.?2?There developed disseminated biotite?type 1?and agglomerated biotite?type 2?in Dahutang deposit,which were formed in the hydrothermal stage.And the disseminated biotite?type 1?formed within the W-mineralization stage,with high Mg,Rb,K,Nb,Ta,Sr and F concentrations,with low X_Mg and Fe³⁺/Fe²⁺values.?3?There developed 3 types hydrothermal muscovite in Dahutang deposit,which are pre-ore muscovite?type 1?,coeval muscovite with tungsten?type 2?and post-ore muscovite?type 3?.Compared with type 1 and type 3,the type 2 muscovite has distinctly lower K/Rb ratios,which can be considered as the metallogenic specificity indicator of W-forming fluid.?4?The mica Li isotopes results in in Dahutang deposit shows that,⁶Li in mica particles is easy to migrate along the direction of altered hydrothermal fluid,which recorded the migration and precipitation of tungsten.?5?The hydrothermal stage of the Shimensi and Shiweidong sections in Dahutang can be divided into three main assemblages:wolframite-scheelite-quartz assemblage,scheelite-wolframite-sulfide-quartz assemblage and veinlets scheelite-quartz assemblage,the results of the quartz fluid inclusions exhibit the ore-forming fluid was low-middle temperature and low-middle salinity fluid.And,from veinlets disseminated scheelite?quartz vein type scheelite,the ore-forming fluid evolved into decreasing magmatic component and REE concentration,increasing Na concentration,with decreasing the oxygen fugacity gradually.?6?After comparing of tungsten deposits in the Middle-Lower Yantze river valley met allogenic belt and the Eastern Jiangnan uplifted belt,it is found that the metallogenic granitoid intrusions in the two belts are metaluminous to peraluminous and high-potassium-calcium-alkaine rocks.All tungsten deposits in the Middle-Lower Yantze river valley belt are bearing in Paleozoic carbonate,while most tungsten deposits in the Eastern Jiangnan uplifted belt are bearing into Proterozoic metamorphic strata.The magmatic source and bearing strata are probably two important conditions to control the scales and element assemblages of the W-polymetal deposits in between the Eastern Jiangnan uplifted belt and the Middle-Lower Yantze river valley belt.