鄂尔多斯盆地东胜铀矿床成矿特征与成矿模式

Metallogenic characteristics and model of Dongsheng uranium deposit in Ordos basin,North China

东胜铀矿床产于中侏罗统直罗组下段的辫状河道沉积砂体中，铀矿体受灰色砂岩与灰绿色砂岩的接触带控制。灰绿色砂岩的化学成分中硅酸盐矿物二价铁含量高是该类型砂岩呈现绿色的重要原因。在矿物组成上，灰绿色砂岩主要表现为粘土矿物总量高，特别是绿泥石含量高。岩石学、矿物学和地球化学的证据表明，东胜铀矿区存在后生还原作用。灰绿色砂岩是控矿的古氧化岩石遭受还原性流体改造的产物，后生还原作用掩盖了古氧化蚀变带，并在一定程度上起到了保矿作用。东胜地区后生还原作用在时间上和成因上与河套断陷盆地的产生和形成具有密切联系。铀在矿石中主要呈铀矿物和吸附形式存在。通过电子探针和x一射线粉晶分析鉴定，确定东胜铀矿床矿石中的主要铀矿物为铀石，并含有少量钛铀矿和沥青铀矿。富矿石样品中铀石单矿物的铀铅同位素年龄集中分布于20～10Ma之间。通过研究，建立了鄂尔多斯盆地东胜铀矿床的“多阶段铀成矿模式”，即“成岩期预富集-层间渗入成矿-再改造富集-油气还原保护”的多阶段、多成因流体长期富集改造的铀成矿模式。

The Dongsheng uranium deposit in the Ordos basin, Northern China occurs in fluvial sandstones in the lower member of the Zhiluo Formation, Middle Jurassic. Uranium ore bodies are mostly located at or near the contact between gray sandstones and gray-greenish sandstones. The high contents of Fe^2＋ in gray-green host sandstones are considered as the most important reason why those sandstones are in gray-greenish or green color. In mineralogical aspects, there are more clay minerals, especially more chlorite in gray-green sandstones. Research data of petrology, mineralogy and geochemistry show that gray-green sandstones were altered by the epigenetic reducing alteration in the Dongsheng region. It is proposed that the gray-greenish host sandstones were previously oxidized sandstones and subsequently reduced by introduced fluids. The reduction process covered up the previously oxidized sandstones and protected uranium accumulations from the destruction by infiltrating oxidizing water. It is stated that the reduction process in the Dongsheng area was simultaneously and genetically related with the formation and development of the Hetao down-faulted basin located to the north of the Ordos basin. Uranium mainly occurs in uranium minerals and adsorption form. Data of the electron microprobe and X-ray powdered crystal analysis minor brannerite and pitchblend. Uranium-lead isotopic show that uranium minerals are mainly coffinite, ages of the uranium mineral-coffinite in high grade ores of the uranium deposit are centered at Miocene time between 20 Ma and 10 Ma. A ＂Multistage Uranium Mineralization＂ model is proposed by authors for uranium ore-formation of the deposit. The mineralization process could be divided into four main stages, namely the pre concentraion during diagenesis, the interlayer infiltration mineralization during Late Jurassic to Paleocene time, the reconcentration in Miocene, and the last stage of ore preservation exerted by reducing fluids after Pliocene time.