东胜铀矿床和钱家店铀矿床后生蚀变特征及其与铀成矿关系

【作者】 庞雅庆；

【导师】 向伟东；

【作者基本信息】 核工业北京地质研究院 ， 矿产普查与勘探， 2007， 硕士

【摘要】 东胜铀矿床和钱家店铀矿床是我国近年来发现的砂岩型铀矿床,这两个砂岩型铀矿床都具有独特的后生蚀变分带特征。本文主要从砂岩型铀矿床后生蚀变分带入手,并着重以砂岩型铀矿床后生蚀变的岩石学、矿物学、地球化学特征为主线,对东胜铀矿床和钱家店铀矿床进行解剖,并与典型层间氧化带型铀矿床进行对比研究,分析后生蚀变作用与铀成矿的关系,为成矿预测提供依据。本文以显微镜下观察为主,并辅以电子探针、扫描电镜等先进技术,认为东胜铀矿床含矿的直罗组下段砂岩为岩屑长石砂岩,主要蚀变矿物为蒙脱石、绿泥石、绿帘石、方解石、褐铁矿、黄铁矿、铀石等;钱家店铀矿床含矿的姚家组砂岩为长石岩屑砂岩或含长石岩屑砂岩,主要蚀变矿物为高岭石、方解石、褐铁矿、黄铁矿、沥青铀矿等。通过对东胜铀矿床和钱家店铀矿床后生蚀变分带宏观的观察和微观的研究,认为这两个矿床后生蚀变分带与典型层间氧化带分带略有不同。东胜铀矿床特殊的灰绿色蚀变带主要由灰绿色、绿色砂岩组成,蚀变矿物主要为绿泥石,认为灰绿色蚀变带是后期还原性流体改造的产物,可能是古层间氧化带的一部分。钱家店铀矿床特殊的漂白砂岩带主要由矿区范围内与铀矿化关系密切的灰白色砂岩组成,蚀变矿物主要为高岭石,认为漂白砂岩带是酸性弱氧化流体改造的产物,是氧化带的一部分。对漂白砂岩带岩石呈白色调的原因进行探讨,认为主要是由岩石的高岭石化,铁离子的迁移和有机质的氧化造成的。本文基本查明了东胜铀矿床和钱家店铀矿床不同地球化学类型砂岩中常量元素、微量元素和稀土元素的分布富集特征。东胜铀矿床灰绿色砂岩中有机质和全硫含量偏低、Al₂O₃含量偏高、CaO含量降低、U及其伴生元素含量略有增高、∑REE含量增高、LREE/HREE比值偏高、ΔFe₂O₃/FeO比值偏低等;钱家店铀矿床漂白砂岩中有机质和全硫含量偏低、SiO₂和Al₂O₃含量增高、CaO含量降低、U及其伴生元素含量略有增高、∑REE含量降低、LREE/HREE比值增高及ΔFe₂O₃/FeO比值略高等。通过对研究区野外调研,室内大量的蚀变岩石学矿物学镜下观察,并配合一定的分析测试手段等工作后,认为东胜砂岩型铀矿后生蚀变作用主要可分三类:层间氧化蚀变,还原硫化物蚀变,油气还原蚀变;钱家店砂岩型铀矿后生蚀变作用主要可分为两类:层间氧化蚀变、还原硫化物蚀变。后生氧化作用是使铀迁移、富集的重要条件,而后生还原作用与原生还原作用一起形成的还原地球化学障,则是铀沉淀成矿的关键因素,同时也是铀矿床能够保存下来的重要条件。更多还原

【Abstract】 Dongsheng and Qianjiadian uranium deposits are two sandstone-type uranium deposits discovered in China in recent years and have unique characteristics of the epigenetic alteration zones. The two deposits have been studied in detail mainly focusing on petrologic, mineralogical and geochemical characteristics of their epigenetic alteration zones. Compared with typical interlayer-oxidation zone sandstone uranium deposits, the relation of their epigenetic alteration to the uranium mineralization has been discussed providing for uranium metallogenic prognosis.Observing under the microscope and electron probe microanalysis and SEM reveal that the uranium-hosting sandstone of the lower member of Zhiluo Formation in Dongsheng uranium deposit is lithic-feldspar sandstone and main alteration minerals are smectite, chlorite, epidote, calcite, limonite, pyrite, coffinite, etc; the uranium-hosting sandstone of Yaojia Formation in Qianjiadian uranium deposit is feldspathic litharenite and main alteration minerals are kaolinite, calcite, limonite, pyrite, pitchblende, ect.Macroscopical observations and microcosmic studies of the epigenetic alteration zones of Dongsheng and Qianjiadian uranium deposits reveal that their epigenetic alteration zones are slightly different from those of the typical interlayer-oxidation zones. The unique gray-green epigenetic alteration zone of Dongsheng uranium deposit is mostly composed of gray-green and green sandstones, and alteration minerals are dominantly chlorite. It is considered that the gray-green epigenetic alteration zone is the product due to the alteration by the introduced fluids in the late stage, and maybe a part of the paleo-interlayer oxidation zone. The unique bleached zone of Qianjiadian uranium deposit is mostly composed of gray-white sandstones closely related to the uranium mineralization, and alteration minerals are dominantly kaolinite. It is proposed that the bleached zone is the product due to the alteration by the acidic weak-oxidizing fluids, and is a part of the interlayer oxidation zone. Main reasons for the bleached sandstone in the color of white are transfer of Fe²⁺, oxidation of TOC and kaolinization of sandstone.The distribution characteristics of major elements, minor elements and REE in sandstone of different geochemical types in Dongsheng and Qianjiadian uranium deposits are basically found. Compared with the gray sandstone in Dongsheng uranium deposit, the gray-green sandstone is characterized by lower contents of TOC, S, and CaO, and slightly higher content of Al₂O₃,ΣREE and U, and higher ratio of LREE/HREE, and lower ratio ofΔFe₂O₃/FeO, ect. Compared with the gray sandstone in Qianjiadian uranium deposit, the bleached sandstone is characterized by lower contents of TOC, S, CaO andΣREE, and higher contents of SiO₂, Al₂O₃ and U, and higher ratios of LREE/HREE andΔFe₂O₃/FeO, ect.Field investigations in the study area, and petrologic and mineralogical observations under the microscope and some analyses reveal that the epigenetic alteration of Dongsheng uranium deposit can mainly be divided into three types: interlayer oxidation, reducing sulfide alteration, reduction of oil-gas. Hower ,that of Qianjiadian uranium deposit can mainly be divided into two types: interlayer oxidation and reducing sulfide alteration. The epigenetic oxidation is the important condition for the migration and enrichment of uranium, while the reducing geochemical barrier composed of the epigenetic and original reduction is the key factor that uranium for the precipitation of uranium to form a deposit, and it is also the important condition for the preservation of uranium deposits.更多还原