| Nanling tectonic belt is an important granitic uranium deposit which developed on severe deformation folding basement.. This belt developed large-volume granite and rift basins in south china during earlier Mesozoic. The Guidong pluton, locating in the south central Cathaysian and east central of Dadongshan-Guidong-Wuliting granite belt, belongs to Nanling tectonic belt. This pluton regionally located at the cross of post Caledonian uplift of Fujian and Jiangxi province, and hollow zone of Hunan, Guizhou and north Guangdong province. The Xiazhuang uranium ore field is right in the southeast of Guidong pluton.Field geological survey and research show that the Xiazhuang ore filed lies in the Guidong granite body which emplaced in wall rock except west direction. The major granitic plutons include Luxi, Xiazhuang, Maofeng and fenshui’ao, all of which emplaced in earlier Mesozoic. According to the latest LA-ICP-MS zircon U-Pb dating method, they emplaced respectively in246.2±4Ma,245.4±6Ma,227.1±5Ma. The main elements and trace elements indicate that the granitic plutons in the ore field are all the products of post collision tectonic background. The northern parts, some southern parts and east parts of wall rocks are Cambrian stratum-a suit of sandstone and slate with flysch building, most of these rocks undergone mylonitized deformation. The rest of the southern parts of wall rocks are mainly fine grained greywacke, quartz sandstone, siltstone and argillaceous shale.There are mainly NNE, NEE and the near EW-trending structures in Xiazhuang ore field, which pattern as chess-board. The EW direction structures are the five group mafic dikes, distributing equidistantly in spatial, from the north to the south they are Shuikou-Zhuanshanxia, Huangpi-Zhangguangying, Mingzhuhu-Zhaixia, Luxi-Xianrenzhang and Zhongxinduan respectively, of which the mainly rock type is uralited diabase. According to the SHRIMP and LA-ICP-MS zircon U-Pb dating method, the EW direction diabase emplaced in the initial granite in193±4Ma; the NEE structures include northern Huangpi fault and the southern Mashishan fault, between which the Xiazhuang uranium ore field lies; the NNE structures are the Mingzhuhu fault, Xinqiao-Xiazhuang Silicated fault,102-shijiaowei fault, Xianrenzhang fault, Taiping’an fault,6009fault and NNE diorite porphyrite veins. Structural deformation and micro-structural research show that, the NEE fault belts mainly consist of large-scale quartz veins with later keatite innerpenetrated; while the NNE faults undergo silication in granite body with brittle deformation and ductile deformation, and fault gouge, cataclastic granite in brittle fault zones while protomylonite in ductile fault zones especially in6009fault. Besides the field outcrop and quartz C axis fabric of the6009fault indicate that it went ductile-brittle two structural deformations up to now. Because of frequent tectonic movements and intensive alteration since Mesozoic, especially in Cenozoic the tectonic activities play an important part to uranium mineralization. The results of ESR dating tell us that, the tectonic evolution in Cenozoic can be divided in to three stages:I (65.5-55.0Ma) differential uplifting of land mass and colossal faults movements, Ⅱ (20.0-40.3Ma) intermittent faulting and displaying upward trend,Ⅲ(2.2-20.0Ma)frequently hydrothermal activities with small-scale.Experiment of uranium element of the granite shows that the granite formed in early Mesozoic is high in uranium, uranium content in Maofeng, Xiazhuang and Luxi is15.9ppm^5.46ppm、5.84ppm respectively and up to24ppm. Isotope research shows that rocks bearing uranium ore and the granite are homogeneous in geochemical characteristic, it can be inferred the granites are source area of later uranium mineralization. The emplacement of mafic dikes in early Yanshanian supplies amount of reducing agent such as Fe2+, S2-, which change U6+toU4+; above all the tectonic movements in Yanshannian are the key factors to uranium mineralization. The tectonic movements in Yanshannian produced NEE and NNE-trend structure systems in ore-field The NEE-trend structures control the basic framework of the ore-field and applying channel for the deep fluid while the NNE-trend and its secondary structures directly control the spatial distribution of ore-deposits and ore-bodies. Controlled by the fault structure, the ore bodies are usually in a side line, or in lenticular/varicose shape. In the south of the ore-field, the "intersection" uranium ore-deposits are influenced by the NWW and the NNE-trend structure. Ore-bodies intersect with basic dikes or silicified fault zone in the form of reverse joint, oblique joint and nearly parallel joint. So what mentioned above greatly contribute to constructing the uranium ore-forming mechanism and metallogenic model in Xiazhuang uranium ore-field in north Guangdong province.Intracontinental subduction and orogenesis took place in South China in late Paleozoic, acid magmatism after orogeny made amount of uranium element concentrate in the granite (220-245Ma), this in fact finished pre-enrichment of uranium mineralization. Extending period, mainly meaning to the EW five group mafic dikes, emplaced in193±4Ma according to the latest chronology research. Large amount of liquid rich in alkali and volatile matter such as CO2CH4、 F、H2O, went upward along continent crust fault, reacting with the granite and making U element out in the form of UO2F42-or UO2(CO3)22-. Mineralization period, former structural liquid went upward along fracture reacting with the granite and producing uranium ore, which are characterized in low fugacity of O2, low temperature and pressure and weak acidity in the form of pyrite-grayish black quartz uranium mineralization type. This type formed by filling is mainly in130-100Ma in northern part of uranium ore field. Late mineralization activities behave that the structural liquid continues going upward and reacts with liquid of earth’s surface, getting further metasomatism with wall rocks. So this alteration of cataclastic rocks lead small veins or dissemination uranium mineralization. From tectonic environment standpoint, there are two magma systems of different tectonic environment, squeezing orogeny finished enrichment of U element and forming granite rich in alkali, and the extending of mafic dikes in early Jurassic finished the emigration of U element to shallow earth surface, finally the basic conditions of uranium mineralization. From dynamic mechanism of the earth, the orogeny processing in Indo-Chinese and Yanshannian completed the leaching and emigration of U element, thus large-scale uranium mineralization took place in Yanshannian. The severe tectonic movements in Cenozoic have some negative factors to uranium mineralization enrichment. |
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