鄂尔多斯盆地双龙地区砂岩型铀矿富集、迁移和成矿机制

Enrichment,Transportation and Ore Forming Mechanism of Sandstone-type Uranium Deposits in Shuanglong Area,Ordos Basin

铀是一个国家经济发展和国防建设重要的矿产资源,鄂尔多斯盆地分布着数十个重要的砂岩型铀矿,是我国重要的产铀基地之一,然而关于砂岩型铀矿的富集、迁移和成矿机制仍然存在很大争议。本文以鄂尔多斯盆地研究程度相对较低的双龙地区砂岩型铀矿为研究对象,从构造背景、矿物蚀变特征和地球化学等方面入手,探讨其成矿机制。矿物蚀变特征表明,岩心样品自上至下在氧化带、过渡带(矿化位置)和还原带分别发育不同的蚀变类型。微量元素地球化学特征显示U与Th、Pb、Mo、Zr、Hf和REE等元素正相关性明显。对黄铁矿的研究表明,热液成因黄铁矿与铀的富集关系密切。过渡带样品中石英次生加大边的流体包裹体中主要成分为H2O、CO3^2-、HCO3^-和CH4等,表明成矿与碳酸盐热液有关。过渡带样品方解石C、O同位素特征(δ13CV-PDB=-12.4‰^-8.3‰,δ18OV-SMOW=13.0‰~15.1‰)和含CH4流体包裹体指示该地区受到有机质的还原作用。综合研究表明,中生代中期开始,鄂尔多斯盆地周围铀背景值较高的古老岩体为盆地提供了铀源;中生代晚期燕山运动造成深部碳酸盐低温热液沿断裂侵入到地层中,并携带六价铀进行迁移;白垩系烃类等有机质沿构造裂隙自下而上的运移导致了铀的富集;地表水等的流体动力导致含铀热液与有机质不断混合反应,将六价铀还原成四价铀,并在直罗组中沉淀富集成矿。

Uranium is an important mineral resource for a country’s economy and national defense.In recent years,a series of sandstone-type uranium deposits have been discovered in the Ordos Basin,thus this type of ores has become one of the most important uranium resources in China.However,so far,there are still great controversies about the enrichment,transportation and ore forming mechanism of the sandstone-type uranium deposits in the Ordos Basin.In this paper,we conducted an integrated research of tectonic background,mineral alteration and geochemical characteristics on the Shuanglong sandstone-type uranium deposit in the Ordos Basin to decipher its ore forming mechanism.The drill core samples of oxidation,transition(mineralization zone)and reduction zones from top to bottom show contrast characteristics of mineral alteration.The samples have U contents positively correlated with Th,Pb,Mo and REE.Pyrite is closely related to the uranium enrichment,indicative of a hydrothermal origin.The fluid inclusions of the samples in transition zone mainly contain H2O,CO2−3,HCO3−and CH4,which indicates that the ore-forming hydrothermal fluid is carbonate-type.The carbon and oxygen isotopic compositions(from−12.4‰to 8.3‰and from 13.0‰to 15.1‰,respectively)and CH4 bearing fluid inclusion indicate that this region was reductive due to the high contents of organic materials.The uranium rich rocks around the Ordos Basin acted as the uranium source since the middle Mesozoic.The deep carbonate-type hydrothermal fluid driven by the Yanshanian movement in the late Mesozoic percolated the strata along the faults and leached uranium in the rocks.The organic materials migrated upwards along the structural fractures during the Cretaceous and led to the uranium enrichment.Hydrodynamics of surface water led to continuous mixing of uranium hydrothermal solution and organic materials,which resulted in uranium precipitation,nrichment and mineralization in the strata of the Zhiluo Formation.