鄂尔多斯盆地流体动力学过程及其砂岩型铀矿化

鄂尔多斯盆地是我国重要能源基地,近年来砂岩型铀矿勘查进展明显。盆地形成演化中的流体动力学及其铀矿化是能源矿产勘查的基础依据,因而备受关注。运用Basin2TM软件数值模拟了鄂尔多斯盆地构造-沉积过程中流体的温度场、压力场、流动方向、流动速率和地热梯度等。伴随盆地的形成和演化,沉积体内流体温度和压力不断增高,流动逐步定向,速率逐渐增大,早白垩世盆地流体的温度达250℃,压力至(700~800)×101.33kPa,地热梯度38.3℃/km,并从盆地中西部沉降中心沿东部斜坡向盆地东部边缘大规模长距离渗出流动,它是砂岩型铀矿化主体在盆地边部形成的重要机制。盆地流体在砂岩中发生铀矿化时,伴随大量方解石蚀变矿物的形成,其δ13CV-PDB变化于-2.7‰~-14.0‰,δ18OV-SMOW介于18.4‰~20.0‰,反映铀矿化盆地流体中CO32-类或CO2主要是沉积有机质脱羟基产物,少部分为海相碳酸盐岩溶解产物,盆地流体中碳酸铀酰是铀元素的主要存在形式。铀矿化低温蚀变成因高岭石的δ18OV-SMOW介于12.6‰~13.7‰,流体包裹体的δDV-SMOW变化于-116‰~-133‰,H、O同位素组成指示盆地流体是经中生界沉积演化的大气降水,具有沉积建造水的性质。环盆地边部是砂岩型铀矿的重要找矿方向。

The Fluid Dynamic Processes and Its Uranium Mineralization of Sandstone-type in the Ordos Basin, China

The Ordos is an important energy-resource basin,and the sandstone-type uranium survey evidently develops now.The fluid dynamics and its uranium mineralization during the basin formation and evolution are essentials for the energy-resource survey and follow many interests.The fluid temperature-and pressure-field,the direction and speed of fluid flow,the geothermal gradient during the basin structure-sedimentary processes are all modeled by Basin2TM.As the formation and evolution of the basin,the basin fluid temperature and pressure increased,the fluid flow became directional and the speed rose.In the Early Cretaceous period,the basin fluid temperature was up to 250 ℃,the pressure to(700~800)×101.33 kPa,the geothermal gradient was 38.3 ℃/km,the large-scale and long-distance basin fluid flow from the subsidence center in the middle-western part of the basin to the eastern margin of the basin along the eastern slope,and this was the key mechanism of the sandstone-type uranium mineralization at the margin of the basin mainly.A lot of alteration calcite formed in sandstones as the uranium mineralization by the basin fluid,CO2-3 or CO2 in the uranium-bearing fluids were mainly from a de-hydroxyl group of organic matter and partly from the dissolution of the marine carbonate according to the C-and O-isotopic compositions(δ13CV-PDB =-2.7‰ to-14.0‰,δ18OV-SMOW=18.4‰ to 20.0‰),and a carbonic uranium-acyl may be the main uranium-bearing form in the basin fluid.The alteration kaolinite's δ18OV-SMOW is 12.6‰ to 13.7‰,and the fluid inclusion's δDV-SMOW is-116‰ to-133‰,and this indicates that the basin fluid is an evolutionary meteoric water in the Mesozoic system and of formation water character.The margin of Ordos basin is important for uranium ore survey.