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碳酸岩是一类具有低密度、低粘度、富挥发分, 易富集Nb、U、Th、REEs、P、Sr、Ba等不相容元素的较为特殊的岩浆岩,是极为重要的资源宝库。以往主要对这类岩石中的稀土矿化研究较多，但对其中的铀成矿作用的研究相对较少。我国小秦岭地区分布有碱性岩-碳酸岩带。近年来，位于该带上与碳酸岩脉有关的华阳川铀多金属矿床被重新确定为一个的大型铀-铌-铅-稀土多金属矿床，在其邻区还分布有大石沟钼-铀-铅-稀土多金属矿床。本文以华阳川与大石沟多金属矿床为研究对象，在总结前人研究资料的基础上，基于详细的矿区地质工作，利用场发射扫描电镜(SEM-EDS)、电子探针(EMPA)、离子探针(SIMS)等技术手段，对华阳川与大石沟矿床中的主要铀矿物开展了系统的矿物学、矿物化学与年代学研究，着重探讨了矿床中主要铀矿物的矿物学特征、矿物化学特征及成矿年代学特征。结合等离子体质谱(ICP-MS)手段，对华阳川与大石沟矿床中不同类型方解石碳酸岩及华阳川不同期次的碳酸(盐)岩中的方解石开展了稀土元素分析，初步探讨了不同类型碳酸岩(脉)与不同期次方解石之间在稀土组成上的差异，并结合矿石、围岩中有用元素的线性分析，总结了有用矿质元素U与∑REE之间的联系。主要取得以下成果及认识：(1) 华阳川铀铌多金属矿床中的主要铀矿物为铌钛铀矿，其次为晶质铀矿，并含少量长白矿、铀钍石、钍石、铌钇矿/黑稀金矿等含铀矿物。大石沟Mo-U-Pb-REE多金属矿床中的主要铀矿物为铌钛铀矿、钛铀矿、晶质铀矿。(2) 基于铌钛铀矿电子探针与能谱分析，发现了华阳川与大石沟矿床中的早期铌钛铀矿均发生了不同程度的后期热液蚀变。热液蚀变会导致铌钛铀矿中的Si含量升高，Ca、U等活动元素丢失，一般而言，弱蚀变会导致Ca含量显著降低，而U仅发生少量丢失，但较强蚀变甚至可以完全交代铌钛铀矿。(3) 年代学特征显示，华阳川矿床与大石沟矿床中均存在两期晶质铀矿年龄，其中华阳川矿床的两期年龄分别为~200 Ma与~129 Ma；大石沟矿床的两期年龄分别为203~206 Ma与~127 Ma，两个矿床的两期年龄在误差范围内较为一致。结合矿物学与野外地质特征，我们认为两个矿床的铀成矿作用均以印支-燕山期之交(200 ~ 206 Ma)的铀成矿作用为主，与碱性岩-碳酸岩有密切的成因联系，而燕山期(127 ~ 129 Ma)的铀成矿作用仅占次要地位。(4) 晶质铀矿矿物化学特征显示，华阳川早期(~200 Ma)铀成矿形成于岩浆-高温岩浆热液体系，晚期(~129 Ma)的铀成矿则是高温热液体系的产物；大石沟早期铀成矿(203~206 Ma)形成于高温岩浆体系，晚期铀成矿(~127 Ma)形成于高温岩浆热液体系。(5) 华阳川矿床中的铅矿化与晚期热液成因的方解石-沸石脉有密切的成因联系，与U-Nb-REE之间应是空间上的伴生关系，其形成时限应晚于~200 Ma；大石沟多金属矿床中的年代学结果显示，钼成矿(221 Ma)略早于铀成矿(200 ~ 206 Ma)，暗示Mo与U可能并不是同时代的产物，而是一种空间上的伴生关系。(6) 基于弱矿化样品及矿石样品全岩微量元素分析，发现矿床中U与∑REE之间存在较高的线性相关性，表明铀矿化较好的区段也常伴随较高的稀土矿化，说明矿床中也具有较高潜力的稀土矿化，这一认识需要引起生产部门的重视。因此，在研究U成矿或开展铀矿选冶的过程中，关注铀成矿的同时，也应加强对其中稀土成矿的关注。(7) 尽管两个矿床在铀矿物组合特征上存在一些差异，然而结合矿体产状特征及矿床所处区域地质背景，综合铀矿物的矿物学、矿物化学与年代学特征表明，华阳川与大石沟矿床可能具有相似的成因，即铀铌(稀土)成矿可能都与碳酸岩有成因联系，且都经历了后期较为相似的热液蚀变过程。两矿床中的燕山期铀矿物的成因可能是大量早期铌钛铀矿蚀变后，少量铀被带出后再次沉淀的结果，但不排除燕山期岩浆活动提供了部分铀源的可能。综合上述研究成果，我们初步认为华阳川与大石沟矿床可能是一个主要形成于印支-燕山期之交(200 ~206 Ma)的，并被燕山期(127 ~ 129 Ma)岩浆活动所(叠加)改造过的主要与碱性岩-碳酸(伟晶)岩脉有成因联系的U-Nb-REE多金属矿床。

Carbonatite is a magmatic rock characterized by the low density, low viscosity, rich volatile and enrich in Nb, U, Th, REEs, P, Sr, Ba and other incompatible elements In the past, Lots of studies have been made on rare earth mineralization in carbonatite. However, few of them focused on uranium mineralization.There is an alkaline igneous rock-carbonatite belt distributed in the Lesser Qinling area of China. Recently, the Huayangchuan uranium polymetallic deposit located in the belt associated with carbonatite veins has been redefined as a large uranium- niobium-lead-rare earth polymetallic deposit. And also, there is a molybdenum- uranium-lead rare earth polymetallic deposit named Dashigou distributed in adjacent area. Based on the previous research and the detailed geological work of the mining area.The systematic mineralogy, mineral chemistry and chronology of the main uranium minerals in the Huayangchuan and Dashigou deposits have been made , using field emission scanning electron microscopy, electron probe and ion probe analysis, plasma mass spectrometry. The mineralogical characteristics, mineral chemistry and mineralization age of the main uranium minerals, and rare earth elements and trace elements of single minerals in the deposits are revealed. The difference in rare earth composition between different carbonate rocks and calcite are discussed based on the classification of carbonate rocks in the mining area. The relationship between U and ∑REE are revealed as well. Main conclusions are achieved as follows:(1) The results show that primary uranium mineral in the Huayang-chuan uranium-niobium polymetallic deposit is betafite,followed by uraninite，with a small amount of coffinite, uranothorite, changbaiite and samarskite/euxenite, while betafite, brannerite and uraninite are the main uranium minerals in the Dashigou Mo-U-REE polymetallic deposit.(2) It can be inferred the early betafite in the deposits experienced different degrees of late hydrothermal alteration, based on the electron probe and energy spectrum analysis of the betafite. The hydrothermal alteration will lead to an increase in the Si content in the betafite, and the leaching of active elements such as Ca and U will be lost. Generally, the weak alteration will result in a significant decrease in the Ca content but few in the U. However, the betafite can be completely replaced if the alternation is strong.(3) EPMA U-Th-Pb chemical age and SIMS U-Pb geochronology of the uraninite reveal that there are two phases of uranium precipitate in the two deposit. Two episodes of uranium mineraliztion in the Huayangchuan deposit are ~200 Ma and ~129 Ma respectively and two ages are 203~206 Ma and ~127 Ma for the Dashigou deposit. The ages of the deposits are consistent within the error range. Combining with mineralogical characteristics, uranium mineralization of the two deposits is dominated by the Indosinian-Yanshanian period (200 ~ 206 Ma), while the uranium mineralization of the Yanshanian period (127 ~ 129 Ma) is only secondary mineralization. It is demonstrated that the early uranium mineralization related to the alkaline reock and carbonatite. (4) The chemical characteristics of uraninite show that the early uranium mineralization (~200 Ma) is associated with the magmatic-high temperature hydrothermal system and the late uranium mineralization (~129 Ma) is associated with the high temperature hydrothermal systems. (5) The lead mineralization in the Huayangchuan deposit has a close genetic relationship with the calcite-zeolite vein of late hydrothermal genesis, and there should be a spatial correlation with U-Nb-REE and its formation time should be later than ~ 200 Ma; Chronological results in the Dashigou polymetallic deposit show that molybdenum mineralization (221 Ma) is slightly earlier than uranium mineralization (200 ~ 206 Ma), suggesting that Mo and U are not contemporaries, but a spatially associated relationship.(6) A high linear correlation between U and ∑REE content in the deposits are obtained, based on the analysis of trace minerals in weak mineralized samples and ore samples. It is indicated that the the rare earth mineralization is often accompanied with uranium mineralization Therefore, more attention should also be paid to rare earth mineralization in the process of studying U mineralization or carrying out uranium ore smelting.(7) Although there are some differences in the uranium mineral combination characteristics between the two deposits (such as brannerite only found in the Dashigou deposit), the deposits may have similar genesis and show genetic relationships with the carbonaite. The uranium mineralization in the late period (~127-129Ma) in the two deposits may be precipitated from the fluid whose uranium content was from the alternation of the early betafite. It can’t be excluded that the Yanshanian magmatism may provide some uranium source. In summary, the Huayangchuan and Dashigou U-Nb-REE polymetallic deposits show the genetic relationship with the Indosinian-Yanshanian alkaline igneous rock-carbonatite veins but superimposed by the Yanshanian magmatism activity.