| 鄂尔多斯盆地砂岩型铀成矿中两种流体系统相互作用——地球化学证据和流体动力学模拟 |
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| 引用本文：池国祥,薛伟.2010.Interaction of two fluid systems in formation of sandstone-type uranium deposits in Ordos Basin: Geochemical evidence and hydrodynamic modeling[J].Mineral Deposits,29(1):134~142 |
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| 基金项目:国家自然科学基金（40772061、40472054、40272050）、国家自然科学重点基金（40930423）、高等学校学科创新引智计划（B07011）、长江学者和创新团队发展计划（IRT0755）、国家重点基础研究发展计划（2003CB214606）、加拿大发明基金 |
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| 中文摘要:鄂尔多斯盆地内的铀矿床多产在盆地的边部，并主要以侏罗系直罗组 砂岩为主岩，铀矿体与 从红色到绿-绿灰色、从氧化向还原过渡带中的热液蚀变密切相关。与铀矿化有关的后生矿 物内流体包裹体研究曾得出60～180℃指示埋藏或热液环境的均一温度。铀矿化共生方解石 和高岭石的C、H、O同位素组成研究表明，热液方解石的δ13CV-PDB为-14.0‰～2.7‰、铀成矿流体的δDV-SMOW和δ18OV-SMOW分别是-130‰～-94‰和-9.1‰～4.8‰，C和H同位素组成指示出大量C和H来自烃类的氧化。盆地内流体流动的数值模拟表 明，在侏罗纪 白垩纪盆地边缘相对抬升和盆地内地层轻微变倾过程中，发育出2种流体流动系统，即渗出流和渗入流。渗出流系统受盆地上覆沉积压实作 用产生的超压驱动，起源于盆地下部，从盆地中心下部流向盆地边缘浅部；渗入流系统受 重力驱动，从盆地边缘向下流向盆地中心。笔者认为铀矿化的定位与这2种流体系统的混合过程密切相关，从侏罗系—白垩系渗滤出来的U6+被渗入的雨水流体搬运，当与渗出 的富烃盆地流体相遇时，烃类将U6+还原为U4+，并在2种流体系统的过渡带铀矿 物沉淀。2种流体系统间过渡带的位置受地形起伏大小控制。在白垩纪末期（65 Ma），假设从盆地中央到边缘的起伏为350 m，则2种流体系统的过渡带定位于上侏罗统（铀矿化地层）；在晚期因超压 逐渐消失引起的地形小起伏情况下，过渡带也能发育在这个层位。2种流体流动系统的过渡带能够较长时期的保持在同一个层位，因此，白垩纪之后地形起伏的变小和超压的消失相结合，是铀成矿十分有利的条件。 |
| 中文关键词:地球化学 砂岩型铀矿 C、H、O同位素 流体流动模拟 流体系统 鄂尔多斯盆地 |
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| Interaction of two fluid systems in formation of sandstone-type uranium deposits in Ordos Basin: Geochemical evidence and hydrodynamic modeling |
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| Abstract:The Ordos Basin is one of the top oil, gas and coal producers in China and is in creasingly recognized as an important uranium mineralization province. Uranium deposits occur near the margin of the basin and are mainly hosted in sandstones of the Jurassic Zhiluo Formation. The ore bodies are associated with alterations that indicate a transition from oxidizing to reducing conditions, accompanied by a change of color from red to green and grey. Previous studies of fluid inclusions in epigenetic minerals associated with uranium mineralization yielded homogenization temperatures from 60 to 180℃, indicating either deep-burial or hydroth ermal rather than early diagenetic environments. The δ13CV-PDB,δDV-SMOW, and δ18OV-SMOWof the fluids related to mineralization, inferred from the C-H-O isotopes of calcite and kaolinite associated with mineralization, range from -14.0‰ to -2.7‰, -130‰ to -94‰ and -9.1‰ to 4.8‰, respectively. The C and H isotope compositions indicate that significant proportions of the carbon and hydrogen in the mineralizing fluids were derived from oxidation of hydrocarbons. Numerical modeling of fluid flow in the basin indicates that two fluid flow systems were developed in the Jurassic and Cretaceous time when the margins of the basin were relatively uplifted and the strata of the basin were gently inclined. The upper flow system was driven by gravity in relation to topographic relief and the flow direction was downward and from basin margin towards basin center. The lower flow system was driven by overpressures related to sedimentcompaction and the flow was upward and from basin center towards basin margins. The authors hold that the location of uranium mineralization is associated with the mixing of the two fluid flow systems: Uranium was leached from the Cretaceous and Jurassic strata and carried by the descending meteoric fluids, and was reduced from U6+ to U4+by hydrocarbons carried by the ascending basinal fluids and precipitated at the interface of the two fluid systems. The location of the interface between the two flow systems is dependent on the magnitude of the topographic relief. At the end of the Cretaceous sedimentation (65 Ma), the interface was located in the Upper Jurassic (the main stratigraphic interval of uranium mineralization) if the topographic relief was 350 m from the basin center to margin. Such an interface location can also be reached for a lower topographic relief at a later time due to gradua l dissipation of the overpressures. Thus, a combination of decreasing topographic relief and dissipation of overpressures after the Cretaceous sedimentation was favorable for mineralization because the interface of the two fluid flow systems could be maintained in the same stratigraphic interval for a prolonged period of time. |
| keywords:geochemistry,sandstone-hosted uranium deposits, C, H and O isotopes, numerical modeling of fluid flow, fluid system, Ordos Basin |
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