Study On Indosinian Granite And Its Tungsten Mineralization In Central Hunan,South China

Granites with different ages are widespread in South China,especially Mesozoic granites are most important in this area;these granites constitute one of the largest granite provinces in the world.During the past several decades,a lot of researches mostly had focused on the late Mesozoic（Yanshanian）granites and their mineralization,but less attention had paid to the early Mesozoic（Indosinian）granite,and its mineralization has been ignored for a long time.Recent studies have revealed that the affected area and intensity of Indosinian magmatism in South China are seriously underestimated by the the traditional opinion,and there is an Indosinian tungsten mineralization event,but it is poorly constrained up to now.Indosinian granites are relatively developed in central Hunan,and some tungsten deposits related to these granites have been discovered in recent years and exhibit a good prospecting potential.Therefore,this doctoral dissertation takes Baimashan,Ziyunshan and Dashenshan in central Hunan as the major studying objective.Based on the isotopic chronology,elemental geochemistry（major and trace elements）and isotopic（Sr-Nd isotope,Hf isotope）geochemistry of these granitic plutons,in combination with the previous researches,with the help of research methods similar to big data,this dissertation places emphases on the granite types,its magma source regime,petrogenetic process,and the formation tectonic background and its physicochemical conditions.Based on the above,the spatiotemporal and genetic relationships between tungsten deposit and the relevant granite are further discussed.Some important achievements and new opinions have been obtained in the dissertation as follows:（1）The Baimashan pluton is a composite one,and the Shuiche superunit formed in the late Caledonian period（424.9±2.2 Ma）;the Longtan superunit,Xiaoshajiang superunit and Longzangwan superunit were all emplaced at the Indosinian period（211～228 Ma）.Based on the above dating,the chronological framework of the Baimashan granitic complex is reconstructed for the first time.（2）The time limit of Mesozoic granite in central Hunan is first restricted at 185～239 Ma.The granite in this region mainly belongs to the Indosinian pluton,which overturns the traditional opinion that there are many Yanshanian granites in central Hunan.（3）Indosinian granites in central Hunan exhibits moderately-fractionated signatures.It displays moderate Si O₂ content,high Ti O₂content,alkaline and low P₂O₅concentrations,and strong positive Rb,Th,U and Pb anomalies,moderate negative Nb,Sr,P and Ti anomalies,low Rb/Sr and high Zr/Hf ratios.The rare earth elements concentrations in these granites are generally low and there exists an obvious fractionation between LREE and HREE,with weak Eu negative anomaly.The high(⁸⁷Sr/⁸⁶Sr)_I,ε_Hf（t）value,andε_Nd（t）value are ascertained for these granites.（4）The Indosinian granites in central Hunan mainly belongs to the calc-alkaline I-type granite,and a few can be classified into the calc-alkaline S-type granite;The formation temperatures of these granites are relatively high,its zircon Zr saturation temperature can be reached at666～841℃,with an average of 767℃.Meso-and Neo-Proterozoic argillaceous and sandy clastic rocks in mid-and upper crust are responsible for the formation of the Indosinian granites in central Hunan,probably a small amount of mantle-derived materials are involved in the granite formation.The formation mechanism of Indosinian granites in this region can mainly be ascribed to the partial melting.（5）By mean of the Sm-Nd isotopic dating on scheelite,the accurate formation time of the Baojinshan gold-tungsten deposit was determined at 207.8±1.5 Ma.（6）In spite of distinct differences in mineralization types,ore-forming element assemblages and host strata of tungsten deposits in central Hunan,almost all tungsten mineralization took place at 196 Ma to227 Ma.There is an extensive Indosinian metallogenic event in central Hunan.（7）Tungsten deposits in central Hunan,spatially,temporally and genetically,are closely associated with Indosinian granites.Granites in this region exhibit an excellent metallogenic potential.The petrogenetic-metallogenic model of Indosinian granite in central Hunan is established for the first time.This thesis studies on Indosinian granite and its tungsten mineralization in central Hunan,not only improves the systematic study of Mesozoic granite and its tungsten mineralization in South China,but also provides a typical example of Indosinian granite and its tungsten mineralization all over the world.