鄂尔多斯盆地北缘砂岩型铀矿热液改造的铀成矿效应:来自黑云母绿泥石化过程的制约

Uranium Metallogenic Effect of Hydrothermal Fluid Transformation in Sandstone-Type Uranium Deposits in Northern Ordos Basin:Constraints from the Study of Biotite Chloritization Process

鄂尔多斯盆地北缘铀矿集区是中国北方最重要的砂岩型铀矿产地之一,为查明该区砂岩型铀矿中热液改造的铀成矿效应,综合应用薄片鉴定、扫描电镜与电子探针等手段对黑云母绿泥石化过程的矿物-地球化学特征开展研究.研究表明,鄂尔多斯盆地北缘砂岩型铀矿含矿层砂岩中黑云母多发生绿泥石化,常沿黑云母的解理和边缘进行蚀变交代,并伴随着Si,Fe,Ti,U等元素释放;与黑云母绿泥石化有关热液温度在137.3~208.3℃之间,为偏碱性-还原性质的中-低温含烃盆地热卤水.鄂尔多斯盆地深部广泛发育富铀烃源岩,在持续埋藏与热事件作用下排烃与迁出的铀可随盆地热卤水沿断裂等通道运移,理论上可为该区砂岩型铀矿提供深部铀源.当其运移至含矿层发生侧向迁移时,通过萃取砂岩中分散吸附铀和溶蚀早期形成铀矿物,造成铀的重新活化,并以有机络合物或有机胶体等形式迁移.伴随着黑云母绿泥石化,形成绿泥石化黑云母-铀石、绿泥石化黑云母-黄铁矿-铀石、绿泥石化黑云母-锐钛矿-铀石、锐钛矿-铀石等热液蚀变矿物组合,为砂岩型铀矿中热液参与铀成矿提供了直接矿物学证据;在还原性容量、温度、压力与酸碱度的变化部位,以铀石的形式沉淀于黄铁矿、有机质与亮晶方解石周边,造成铀的进一步富集成矿,为砂岩型铀矿中热液改造的铀成矿效应提供了依据.

Uranium(U)-concentrated areaof northern Ordos Basin is one of the most important sandstone-type U deposits in north China.In order to find out the Umetallogenic effect of hydrothermal fluid transformation in this area,the mineral-geochemical characteristics of biotite chloritization process were studied by thin section identification,scanning electron microscopy(SEM)and electron probe(EMPA).The results show that biotite in sandstone-type U deposits in north Ordos Basin mostly occurs chloritization,which is often altered and metasomatized along the cleavage and edge of biotite,accompanied by the release of Si,Fe,Ti,U and other elements.Hydrothermal fluid related to altered biotiteis basin brine with hydrocarbon and has the characteristics of alkaline-reducing and medium-low temperature(137.3~208.3℃).Moreover,U-rich source rocks were widely developed in the deep part of the Ordos Basin and the hydrocarbon expulsion and emigrated U can migrate along the faults and other channels with the geothermal brine in basin under the action of continuous burial and thermal events,which can theoretically provide deep U sources forsandstone-type U deposits in this area.When it migratedlaterally to ore-bearing layer,the U-rich hydrothermal fluid was formed by extracting U fromthe ore-bearing sandstones and dissolving the U minerals formed in the early stage.U is reactivated and migrated in the form of organic complexes or organic colloids in hydrothermal fluid and precipitated in the form of coffinite around pyrite,organic matter and calciteat the changing parts of reducing capacity,temperature,pressure and pH,resulting,which provides a basis for the Umetallogenic effect of hydrothermal fluid transformation in sandstone-type U deposits.In addition,the above fluid-rock reaction was easy to cause biotite chloritization,forminghydrothermal altered mineral assemblages,such aschloritized biotite-coffinite,chloritized biotite-pyrite-coffinite,chloritized biotite-anatase-coffinite,anatase coffinite,which also provides direct evidence for the participation of hydrothermal fluid in sandstone type U deposits.