东胜地区直罗组含铀砂岩中方解石的碳氧同位素组成及铀成矿意义  

Carbon and Oxygen Isotopic Composition and Uranium Mineralization Significance of Calcite of Zhiluo Formation Uranium-bearing Sandstones in Dongsheng Area

作　　者：汤超[1] 司马献章[1] 朱强[1] 冯晓曦[1] 陈印[1] 陈路路[1] 刘晓雪[1] 

机构地区：[1]中国地质调查局天津地质调查中心(天津地质矿产研究所),天津300061

出　　处：《高校地质学报》2016年第4期698-706,共9页Geological Journal of China Universities

基　　金：中国地质调查局项目(1212011307800DD20160128);国家重点基础计划项目(2015CB53000)联合资助

摘　　要：方解石是东胜地区直罗组含铀砂岩中重要的胶结物类型,同时碳酸盐化与铀成矿作用关系密切。通过方解石胶结物岩石学、矿物学、碳氧同位素分析,研究了含铀砂岩中方解石碳氧同位素组成、沉淀机制及铀成矿意义。研究表明,东胜地区砂岩类型为长石岩屑砂岩、岩屑砂岩和石英砂岩,粘土矿物主要由蒙皂石、伊利石、高岭石和绿泥石组成,方解石类型以含铁矿方解石为主,其次为铁方解石。方解石δ^(13)C_(PDB)为-15.7‰^-1.6‰,平均-9.08‰,δ^(18)O_(PDB)为-15.6‰^-10‰,平均-12.4‰,显示其形成与有机酸脱羧作用有关,碳来源为有机碳。碳氧同位素分析数据计算表明,与方解石平衡的水相氧同位素组成变化范围较宽,为-7.66%-9.71‰,推测较轻的同位素组成具有封存大气降水的特征,而较重同位素组成则反映成岩成矿过程中深部富含油气低温热流体的加入。综合分析认为,东胜地区直罗组砂岩中方解石是地表水和深部油气共同作用的结果:早期有机酸促使长石类骨架颗粒溶蚀,形成石英颗粒次生加大边,并伴随着自生高岭石沉淀;后期随着大量烃类注入砂岩中,成岩成矿环境由酸性向碱性转变,还原性增强,介质水中的CO_2与Ca^(2+)圾Fe^(2+)结合形成含铁为特征的方解石,沉淀在原生粒间孔和各类次生溶蚀孔隙中。整个过程都伴随有铀元素运移和沉淀,暗示东胜铀矿床是地表水和深部油气混合作用形成的。Calcite is an important cement type of Zhiluo Formation uranium-bearing sandstones in Dongsheng area,and the carbonation is closely related to the uranium mineralization.Based on petrographic,mineralogical,carbon and oxygen isotope analysis of calcite cements,the composition,precipitation mechanism,and uranium mineralization significance of carbon and oxygen isotope of calcite were researched in the study area.The results show that sandstones include feldspathic lithic sandstone,lithic sandstone and quartz sandstone,and clay minerals are mainly composed of smectite,illite,kaolinite and chlorite and iron-containing calcite,followed by iron calcite,as the main types of carbonate cements.The δ^(13)C_(v-pdb) is- 15.7‰^-1.6‰ with an average of-9.08‰.The δ^(18)O_(v-pdb)is-15.6‰^-10‰ with an average of-12.4‰.The results indicate that the formation of calcite is related to the organic acid decarboxylation and the carbon source is organic carbon.Analyses of carbon and oxygen isotopic results reveal that the composition of aqueous phase oxygen isotope,balance to the carlcite,has a wide range of values from-7.66‰ to 9.71‰.We presumed that the lighter isotope composition has characteristics of sealed atmospheric precipitation and the heavier isotope composition reflects the participation of deep low-temperature thermal fluid with oil and rich-gas during the process of diagenetic mineralization.The comprehensive analysis indicated that calcite in Zhiluo Formation sandstones of Dongsheng area was a result of interaction of surface water and deep oil and gas.Early organic prompted feldspathic matrix particle dissolution and the quartz grains overgrowth edge was formed,along with kaolinite precipitation.Later,with a large number of hydrocarbons injected into sandstones,diagenetic metallogenic environment changed from acidic to basic and the reducibility is enhanced.The combination of CO_2,Ca^(2+),and Fe^(2+) in medium water formed the iron calcite and precipitated in all kinds of primary and secondary pores.The

关 键 词：方解石胶结物 碳氧同位素 成岩成矿环境 古流体 东胜地区 

分 类 号：P619.14[天文地球—矿床学] P597.2[天文地球—地质学]