砂岩型铀矿的“双阶段双模式”成矿作用

Two-stage and Two-mode Uranium Mineralization for Sandstone-type Uranium Deposits

长期以来,盆地中砂岩型铀矿的成矿与找矿仅关注表生低温氧化作用形成的铀矿化,忽视了复杂地质演化过程中多阶段、多模式的成矿作用。中国北方东部盆地自晚中生代以来经历了伸展-挤压-伸展的多阶段演化过程,铀成矿作用必将对这种多阶段构造演化密切响应。本研究通过中国北方东部巴音戈壁盆地塔木素矿床、二连盆地哈达图矿床、松辽盆地南部钱家店—白兴吐矿床等大型、巨型砂岩型铀矿床的精细解剖,分析矿床中矿体的变化特征和含矿目的层流体-岩石相互作用的标记,探索矿床形成的构造、沉积、铀源、还原剂等成矿控制因素,尤其是(古)太平洋板块在不同阶段、以不同的方式俯冲给研究区盆地带来的深刻影响。以区域构造演化为主线,确定早白垩世至晚白垩世早期,盆地在古太平洋板块高角度俯冲下发生伸展裂陷和拗陷作用,厘定砂岩型铀矿含矿目的层巴音戈壁组(K;b)辫状三角洲、赛汉组(K_(2)bs)辫状河、姚家组(K_(2)y)辫状河及曲流河等沉积环境。晚白垩世晚期,由于古太平洋板块的俯冲由高角度转变为低角度,盆地首次出现由伸展裂陷转变为挤压抬升的正反转演化,表生含铀含氧流体与砂岩相互作用,形成以赤铁矿、褐铁矿化等氧化作用为标志的早期氧化带型"卷状"铀矿化。始新世以来,太平洋板块的俯冲再次由低角度转变为高角度,盆地构造由挤压抬升转变为伸展张裂的负反转演化,导致正断层与基性岩浆活动,并伴随热流体与含矿目的层砂岩相互作用,出现大量的Fe、Mg碳酸盐、热液硫化物、绿泥石、绢云母等蚀变组合,铀矿体形态由原来的"卷状"变成了"透镜状"、"囊状"、"板状",并伴有高温钛铀矿的出现,形成晚期热液叠加铀矿化。两个成矿时期和两种不同方式的成矿作用被本文凝练为"双阶段双模式"铀成矿。结果表明,针对在中国北方东部提出的"双阶段双模式"铀成矿作用模型,下一步找矿上既要关注早期的氧化带型铀矿化,更要注重晚期的热流体叠加改造型铀矿化。

Uranium mineralization by low temperature epigenic oxidized fluid flow(EOF) in sandstone type-uranium deposits(SUD) has been emphasized by uranium geologists and explorers for long time, ignored of the complex geologic processes and multi stage mineralization. But SUDs in the basins of north China are difficultly genetically explained by one single mineralization stage and mode of EOF because the basinal geologic evolution of the eastern north China reveals extensional-compressional-extensional tectonic processes.Multi-stage uranium mineralization in the region responses this kind of tectonic setting changes. Taking the large/huge scaled SUDs like Tamusu in Bayingobi Basin, Hadatu in Erlian Basin and Qianjiadian–Baixingtu in south Songliao Basin for example, we generalize and comprehensively analyze the features of uranium orebodies,the interaction between uranium-bearing fluid flow, and the controlling factors like tectonics, depositional facies,uranium sources and reducing agents, especially the subduction ages and modes of the(paleo-) Pacific Plate for mineralization. Based upon the tectonic evolution, the depositional facies and spatial distribution of hosting sandbodies for Tamusu(K_(2)b), Hadatu(K_(2)bs) and Qianjiadian–Baixingtu(K_(2)y) deposits are interpreted as braided delta, braided river for both latter two, respectively, during the initial extensional stage for a basin under the high-angle subduction of the paleo Pacific Plate. During the Late Cretaceous, the basins firstly positively inverse evolution from extension into compression due to the subduction from high angle into low angle of Pacific Plate.Uranium-oxygen-bearing fluid flow(UOF) penetrates hosting sandstone from the basinal margin into basin center.The interaction between UOF and sandstone results in hematitization and limonitization in the oxidized zone and the early uranium mineralization in the redox zone. Since Eocene, the basins secondly negatively inverse evolution from compression into extension due to the subduction from low angle into high angle of Pacific Plate.The normal faults and basic magma activities cause the interaction between hydrothermal fluid flow(HFF) and mineralization hosting sandstones. A large amount of Fe and Mg carbonates, hydrothermal sulfides, chlorite and sericite come into the sandstone. Meanwhile, the uranium mineralization is reworked and uranium orebodies change from “rollfront” into “lenticular”, “cystid” and/or “tabular”, hydrothermal uranium mineral brannerite occurring in deposits. This process is taken as the late uranium mineralization in the study area. The early and late two uranium mineralization is here refined as “Two-stage and two-mode mineralization” for SUDs in north China.Hence, both UOF and HFF mineralization should be paid enough attention in the theoretic study and exploration in north China basins, even in northeast Asia.