内蒙古东胜铀矿区后生蚀变的稳定同位素特征及其地质意义
The epigenetic alteration stable isotope characteristics of the Dongsheng uranium ore district and their geological implications
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摘要: 近十几年来，在鄂尔多斯盆地北部伊盟隆起东胜及其以西杭锦旗一带，相继发现了东胜等系列大型-特大型砂岩铀矿，目前已成为中国砂岩铀矿发现潜力最大的地区。矿床的主要特征是矿体受直罗组砂岩绿色蚀变带控制；同时，在容矿层及其附近伴生有砂岩白色化、后生碳酸盐化及方解石脉、黄铁矿化等蚀变现象。通过后生蚀变的C、H、O、S 等稳定同位素方面研究、示踪和探讨了蚀变的形成过程和成因及其与铀矿化的关系。镜下特征显示，铀矿石中存在的大颗粒结核状黄铁矿与铀矿物共生，两者为同时期的产物；该类样品S 同位素表现为较大的负值，表明硫质来源的微生物作用特征明显；与盆地内可能提供硫源的直罗组煤屑、延安组煤层、延长组和延安组原油、上古生界煤系地层等有机S 同位素对比，发现铀矿与上古生界煤系烃源岩的有机硫源关系密切。延安组顶部大规模白色化砂岩的高岭石H、O 同位素表明，其成因与低温热液作用有关；容矿层后生方解石C、O 同位素表明，其形成主要是有机质作用提供了碳源，且其碳质来源主要为上古生界气源岩，而与奥陶系及延长组油气的碳源无关。后生作用流体包裹体的测温及其H、O 同位素组成表明，成矿流体以低温的大气降水为主，且与上古生界天然气一起形成低温热液的气-水混合流体；正是这一特点的流体作用形成了本区与铀矿化关系密切的上述各类后生蚀变现象。各类后生蚀变的稳定同位素特征表明，东胜铀矿床形成过程中存在后生低温热液和油气还原改造作用两大特点。本次研究结果为东胜铀矿区的成因认识提供了证据。Abstract: Some large to giant sandstone-type uranium deposits were discovered in Yimeng uplift area of the north Ordos basin, especially in Hanggin Banner-Dongsheng area, in recent decades, which suggests that this area is the most favorable potential area in search for sandstone-type uranium deposits. The main characteristics of these uranium deposits are under the control of green sandstone alteration zone in Zhiluo Formation, with some epigenetic alteration phenomena, such as the whitened sandstone, carbonation and calcite vein, and pyritization. In this paper, the authors studied and discussed systematically the formation process, genesis and the relationship with uranium mineralization by means of carbon, hydrogen, oxygen, sulfur stable isotope tracing. The microscopic observation shows that there are large particles of nodular iron pyrites in the uranium ore in association with uranium minerals, and both are products formed at the same time. The larger negative data of sulfur isotope in this type of pyritization shows that microbial effect on sulfur source quality is obvious. The sulfur isotope comparison with dispersed slack of Zhiluo Formation, coal seams of Yanan Formation, crude oil and coal-bearing Yanchang Formation and Yan'an Formation of the Upper Palaeozoic period shows that the sulfur is close to the Upper Palaeozoic coal organic sulfur. The hydrogen and oxygen isotopes of kaolinite from large white sandstone located at the top of Yan'an Formation show that the genesis was related to low temperature hydrothermal process. The carbon and oxygen isotope features of calcite from the ore-bearing bed indicate that the carbon sources were mainly provided by organic function, whereas the main carbon source was from gas source rocks of Upper Palaeozoic,with no relation to the Ordovician and Yanchang Formation oil and gas. The hydrogen and oxygen isotopic composition of the epigenetic fluid inclusions indicates that the ore-forming fluids were mainly composed of low temperature atmospheric precipitation, and formed the low temperature hydrothermal gaswater mixed solution together with gas of Upper Palaeozoic. This feature of fluid effect led to the formation of all kinds of epigenetic alteration in this area which was closely associated with uranium mineralization. In short, the kinds of epigenetic alteration of stable isotopes show that the Dongsheng uranium deposit is characterized by late epithermal solution and reformation of oil and gas. The results obtained by the authors provide evidence for the genesis of the Dongsheng uranium deposit.
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