东胜砂岩型铀矿后生蚀变地球化学性质及其成矿意义

Geochemical Characteristics of Epigenetic Alteration in Dongsheng Sandstone-type Uranium Deposit and its Metallogenic Signification

东胜砂岩型铀矿具有大规模控矿绿色蚀变和漂白蚀变带、其氧化蚀变仅为蚀变残留等特征而与国内吐哈、伊犁盆地南缘等铀矿明显不同,从而引起人们的关注。本文通过对研究区该主要后生蚀变相关的地球化学研究,探讨了东胜矿床形成的特殊性:早期形成铀矿化的古层间氧化带在强还原性环境下,经二次还原作用改造形成了控矿的绿色蚀变带,二者皆具有低有机碳,低ΣS,高Th/U值的特征。铀矿化蚀变有机碳、δD、δ18O含量在各蚀变阶段中最高,Th/U值最低。分布于延安组顶部的漂白蚀变是表生氧化岩石在还原性环境下经还原改造形成,其Th/U值高,TFe含量低。总之,东胜矿床各种蚀变地球化学指标指示的矿床成因信息是,绿色蚀变和漂白蚀变及铀矿化的形成处于一种相对较强的还原性环境。在成岩作用,古氧化蚀变、漂白蚀变、绿色蚀变以及铀矿化的序列演化中,流体作用的物理化学性质或环境的过渡性转变出现在铀矿化阶段。对黄铁矿硫同位素以及砂岩碳酸盐胶结物碳、氧及其包裹体氢、氧同位素的分析,认为流体主要由大气降水、来自下部的煤成气以及与含矿目的层有机质脱羧基作用有关的有机酸混合组成。因而,东胜矿床的形成是无机—有机流体相互作用的结果。

The characteristic of DongSheng sandstone type uranium deposit is different from that of Turpan-Hami and Yili basin for its large-scale ore-controlled green-color alteration zone, bleach alteration zone and paleooxidation alteration just as the residual alteration, which cause investigators＇ attention .This paper analyse the particularity of DongSheng sandstone type uranium deposit＇s formation from the aspects of alteration geochemistry.. According to the analysis, the authors considered that the paleo-interlayer oxidation zone of uranium mineralization in early period came into ore-controlled green-color alteration zone by the alteration of second deoxidize under the strong reduction environment. The two is characterized by low organic carbon and total sulfur and high Th/U ratio. In the period of uranium mineralization alteration is characterized by the most high organic carbon,δD, δ18Oand the lowest Th/U ratio. The formation of bleach alteration which distribute on top of Yan＇an formation is the supergene rock caused by deoxidize and alteration under reduction environment, as a result it has high Th/U ratio and low TFe. In a word ,the most important common ground in kinds of indexes of alteration geochemistry indicating the formation of green-color alteration zone and bleach alteration zone is placed in a relative hard deoxidize environment. In the sequence of diagenetic action, paleo-oxidation alteration, bleach alteration,green-color alteration as well as uranium mineralization alteration, thephysical-chemical characteristics of fluid or environment come into being changed with rules, but the transition appear at the period of uranium mineralization alteration. Meanwhile, according to the analysis of sulfur isotope in iron pyrites and carbon, oxygen isotope in the cementation of calcite ＆ hydrogen-oxygen isotope in inclusions of carbonation roke, the authors thought that fluid had been made of meteoric water, coal bed gas which come from deep source and the organic acid after removing-carboxyl of organic matters in the ore-bearing Dongsheng uranium deposit is the reciprocity of organic and target stratum . As a result, the formation of inorganic fluid.