鄂尔多斯盆地南部店头铀矿床油气-铀成矿机理探讨

Discussion on the oil gas-uranium metallogenic mechanism in Diantou uranium deposit in southern Ordos basin

以鄂尔多斯盆地南部店头铀矿床为研究对象,总结研究区地层、构造、油气渗出和铀矿床特征,从油气渗出、氧化渗入、铀矿体与断缝体及铀-油气相互作用四个方面探讨了油气渗出作用与铀成矿关系。认为店头地区发育系列NEE向、近EW向断缝体,是深部延长组油气向上运聚的主要通道,伴有油浸、脉状石膏和黄铁矿等;油气渗出作用表现为油浸砂岩、褪色蚀变砂岩和地沥青三种样式,其中铀成矿期形成的地沥青或凝胶化有机质对铀沉淀富集起关键作用,成矿期后形成褪色蚀变砂岩或油浸砂岩只是对铀矿起到保矿作用;放射性元素对地沥青和凝胶化有机质催化热解作用,有机质活性及还原吸附能力增强;矿石中沥青铀矿或铀石围绕凝胶化有机质或黄铁矿赋存,具有多期环带状产出的特点,伴生方铅矿、闪锌矿和黄铜矿等中低温热液硫化物;渗出流体长期作用下的水-岩反应致使直罗组砂岩总体硬度大,密度较大,孔隙度和渗透率较低。认为店头地区铀成矿受渗出还原作用与渗入氧化作用动态作用成矿,形成高品位铀矿化,NEE向和近EW向断缝体是区域铀成矿关键控矿要素。旨在进一步阐明店头矿床铀成矿机理,识别并构建区域铀矿找矿标志,为选区和铀矿勘查部署提供依据。

The study focused on Diantou uranium deposit in the southern Ordos basin,and clarified the characteristics of the stratigraphy,structure,oil impregnation,and uranium deposit in the study area.The mechanism of oil and gas infiltration and uranium mineralization was explored from four aspects:oil and gas seepage,oxidation,the relationship between uranium ore bodies and fractured bodies,and the interaction between uranium and oil and gas.The development of a series of NEE and near-EW trending fault fracture bodies in Diantou area,which were produced by tectonic fracture zones and high-angle jointed dense zones,were the main channels for the upward migration and accumulation of oil and gas in the deep Yanchang formation,accompanied by oil leaching,veined gypsum,pyrite,etc.The oil and gas exudation can be divided into several periods,manifested in three styles:oil-leached sandstone,fade altered sandstone,and gelatinized organic,among which the gelatinized organic matter or ground asphalt formed during the uranium metallogenic period played a key role in the uranium precipitation and enrichment.The formation of gray-green paleooxide sandstone or oil-leached sandstone after the metallogenic period only played a role in preserving uranium mineralization.In addition,radioactive elements produced catalytic pyrolysis effect on organic matter and bitumen,and the activity and reductive adsorption capacity were enhanced,and uranium elements were accumulated and enriched to form high-grade ores during the oxidation transformation process during the metallogenic period.Pitchblend or uranite in the ore occurs around gelatinized organic matter or pyrite,associated with galena,sphalerite,chalcopyrite and other medium and low temperature hydrothermal sulfides,the sandstone of Zhiluo formation in this area are usually of high hardness,high density,low porosity and permeability,which were caused by medium and high diagenesis,and also the result of water-rock reaction under the long-term action of exudated fluid.The fracture body of Diantou deposit is closely related to the industrial orebody and is an important locating factor,which provides a basis and a train of thought for the selection area and uranium exploration.The fault and associated fracture system also improve the physical properties of the host sandstone.The oil and gas from Yanchang formation migrated up to Zhiluo formation and Lower Cretaceous through the fault,providing sufficient reducing materials for uranium mineralization.The gelatinized organic matter or bitumen formed in the metallogenic period played a key role in the enrichment and precipitation of uranium,and the fade altered sandstone and oil-leached sandstone formed after the metallogenic period played a role in ore preservation,the complex water-rock reaction was the main reason for the high diagenetic degree of Zhiluo formation in this area.The organic matter was activated by the heat effect of uranium element,which resulted in the accumulation and enrichment of uranium source and formation of high-grade uranium ore.