内蒙古东胜地区砂岩型铀矿赋矿地层方解石胶结物C、O同位素特征和成因模型

Carbon and oxygen isotopic characteristics and genetic model of calcite cements in sandstone-type uranium deposits in Dongsheng area of Inner Mongolia

砂岩型铀矿赋矿地层方解石胶结物C、O同位素组成对研究成矿机制具有重要的理论和实践意义。前人在成矿流体来源定性判断方面做了大量的研究工作,但缺乏对成矿混合流体的组分特征及相关参数的定量认识。根据内蒙古东胜地区东南部皂火壕和西北部纳岭沟铀矿床赋矿地层直罗组方解石胶结物C、O同位素组成特征,建立了渗出热卤水和渗入地层水两种不同流体混合生成方解石胶结物C、O同位素组成定量成因模型,并研究了两种流体来源、溶解碳的浓度比、流体比例以及热液温度等综合因素。除少数样品方解石胶结物为地层沉积时形成外,大部分方解石胶结物是热卤水与地层水两种流体混合作用的结果,古生界有机酸脱羧作用导致热卤水中富含CO_(2)。东胜地区砂岩型铀矿成矿流体地层水与热卤水比例为0.5~0.9,地层水所占的比例较大;热卤水与地层水溶解碳浓度比主要范围为1.5~5.0,部分大于10.0,热卤水中溶解碳浓度较高,是富含CO_(2)的流体。混合流体温度分为两个主要范围55~80℃、90~140℃,结合古盐度和盐度指数反演,表明成矿流体为有机与无机混合成因的低温热液流体。用砂岩型铀矿赋矿地层方解石胶结物C、O同位素组成,可定量模拟两种流体溶解碳浓度比、流体比例和方解石形成时温度等流体成矿条件,解释碳酸盐胶结物成因,以便从成矿流体角度更好地理解成矿作用过程。

Objectives:The carbon and oxygen isotopic compositions of calcite cements in the host strata of sandstone type uranium deposit have important theoretical and practical significance for the study of metallogenic mechanism.The predecessors have done a lot of research work on the qualitative judgment of the source of ore-forming fluid,but the quantitative understanding of the component characteristics and related parameters of ore-forming mixed fluid is still lacking.Methods:The carbon and oxygen isotopic compositions of calcite cements in different sandstones inZhiluo Formation in Zaohuohao uranium deposit in southeastern Dongsheng area were analyzed,and the carbon and oxygen isotopic compositions of calcite cements in Zhiluo Formation in Zaohuohao uranium deposit and Nalinggou uranium deposit in northwestern Dongsheng area were studied.Results:A quantitative genetic model of carbon and oxygen isotopic compositions of calcite cements produced by the mixing of two different fluids between the seeping hot brine and the seeping formation water was established,and the comprehensive factors such as the source of the two fluids,the concentration ratio of dissolved carbon,the proportion of the fluids and the temperature of the hydrothermal solution were studied.Conclusions:Except for a few calcite cements formed during stratum deposition,most calcite cements precipitated as a result of mixing of hot brine and formation water.Decarboxylation of Paleozoic organic acids resulted in CO_(2) enrichment in hot brine.The ratio of formation water of ore-forming fluid to hot brine in sandstone type uranium deposit in Dongsheng area is 0.5 to 0.9,and the ratio of formation water is more than two-thirds;the ratio of dissolved carbon in hot brine to formation water is mainly in the range of 1.5 to 5.0,and some of them are more than 10.0;the concentration of dissolved carbon in hot brine is high which the fluid is enrichment in CO_(2).The mixed fluid temperature can be divided into two main ranges of 55℃ to 80℃ and 90℃ to 140℃.Combined with the inversion of paleosalinity and salinity index,it shows that the ore-forming fluid is a low-temperature hydrothermal mixed fluid of organic and inorganic origin.The carbon and oxygen isotopic compositions of calcite cements in sandstone type uranium deposits can be used to quantitatively simulate the ore-forming conditions of the two fluids,such as the ratio of dissolved carbon concentration,the ratio of the two fluids and the temperature of calcite formation,so as to explain the genesis of carbonate cements and better understand the ore-forming process from the perspective of ore-forming fluids.