中国北方晚中生代陆相盆地红-黑岩系耦合产出对砂岩型铀矿成矿环境的制约

摘要

为研究中国北方陆相盆地红层-黑色岩系对砂岩型铀成矿的制约,文章研究了国内外红-黑岩系与砂岩型铀矿赋存岩层的时空关系,筛选了10万余m岩心钻探资料,选择准噶尔、鄂尔多斯和松辽盆地这3个典型产铀盆地,通过编制盆地钻孔柱状图、典型地区连井剖面图及关键岩层的地球化学测试等方法,对红-黑岩系和砂岩型铀矿的赋存岩层进行了垂向、横向上综合分析与对比.研究发现:北方陆相盆地自西向东铀矿赋存地层的时代由中侏罗世过渡到晚白垩世;晚中生代至少存在6次大规模的富氧红层沉积事件:Ⅰ中侏罗世—晚侏罗世早期(Bathonian-Oxfordian),Ⅱ早白垩世早中期(Berriasian-Barremian),Ⅲ早白垩世中期(Barremian),Ⅳ晚白垩世早期(Cenomanian),Ⅴ晚白垩世中期(Coniacian)和VI晚白垩世晚期(Campanian).其中第Ⅰ、Ⅴ和Ⅵ期红层之下沉积了时代相近的黑色层,与之构成"红-黑岩系"的沉积结构,是北方砂岩型铀成矿的3个重要层位.典型盆地内地球化学表明,红层与黑色层的B、Sr和Cu元素含量及Fe2+/Fe3+、B/Ga、Sr/Cu和FeO/MnO比值具有明显的差异,结合黑色岩层中草莓状黄铁矿、碳屑、油斑和红层中碳酸盐岩的发育,认为红层为相对较强氧化环境,黑色层为相对较还原环境.连井剖面资料显示红层、黑色层与砂岩型铀矿空间关系密切,铀矿多产于红层与黑色层之间过渡带上,呈板状矿体赋存于灰色、绿灰色砂岩和细砂岩中.一般红层与黑色层垂向距离超过500 m不利于成矿.晚中生代陆相盆地内耦合产出的黑色岩系和红色岩系是古沉积环境由还原向氧化转变形成的垂向分带,前者为铀矿物质沉淀提供了"障",后者为表生流体溶解铀矿提供了"场".文章初步提出了红-黑岩系垂向环境变化制约着北方陆相盆地砂岩型铀大规模成矿作用的新认识.这些认识不仅对砂岩型铀矿成矿环境、成矿规律及成矿模式研究具有重要意义,更对目前正在开展的砂岩型铀矿勘查工作具有实践指导意义.%Based on the 973 program and the northern sandstone type uranium survey program, this study tried to explain the"red and black"sandstone constraint on uranium mineralization. On the basis of summarizing temporal and spatial relationship between red layers and black layers of global sandstone uranium deposits and the sieving of more than 100,000 meters core drilling data of the northern continental basin, the authors selected Junggar, Ordos and Songliao basin as typical research objects. Through compilation of drilling chart of the basin, investigating well-connecting section of typical mining area and geochemical testing of key strata, the authors made a vertical and horizontal comprehensive analysis and comparison of"red and black"sandstone and uranium-bearing strata, and the results show that the formation ages of uranium-bearing strata gradually change from Middle Jurassic to Upper Cretaceous from west to east in China. There were at least 6 large-scale oxygen-rich red sedimentary events in Late Mesozoic period, which are respectively Ⅰ. Middle Jurassic-late Jurassic early stage (Bathonian-Oxfordian), Ⅱ. Early-middle stage of the Early Cretaceous (Berriasian-Barremian), Ⅲ. Middle stage of Early Cretaceous (Barremian), IV. Late stage of middle Cretaceous (Cenomanian), V. Middle stage of late Cretaceous (Coniacian), and VI. Late stage of late Cretaceous (Campanian). The red layer and the black layer below constituting"red- black color structure" in Ⅰ, Ⅴ and Ⅵ stage are 3 important uranium-bearing strata in China. Geochemical and fossil data of the red and black layers in the typical basins show that the content of B, Sr and Cu and the ratios of Fe2+/Fe3+, B/Ga, Sr/Cu and FeO/MnO are obviously different between them. The content of Fe2O3 in the red layer is obviously higher, and the ratio of U and U/Th in the transition zone is obviously higher than other sides. Combined with development of strawberry pyrite in black layer and carbonate rocks in red layer, the authors hold that the red layer is a relatively strong oxidizing environment and the black layer is a relatively reducing environment, which respectively provide oxidation-reduction conditions for the mineralization of sandstone uranium ore. The spectrum of uranium deposits is related genetically with red and black layer, industrial orebodies occur in tabular form in gray and grayish green sandstone. The red and black layer with the thickness of more than 500m is unfavorable for mineralization. The black and red layer formed in late Mesozoic continental basin has vertical zoning to represent the depositional environment change from oxidizing to reducing environment which provides"obstacle"and"field"for mineralization respectively. In conclusion, the authors put forward the "double color structure"metallogenic prospecting model for sandstone uranium deposit, in which the upper layer serves as the red oxide barrier, whereas the lower layer serves as black reducing barrier, and the sandstone uranium deposit (ore spot) transition zone is characterized by grayish green, gray sand enrichment and mineralization. The understanding obtained by the authors is significant not only for improving the metallogenic environment knowledge, metallogenic regularity and metallogenic model of sandstone uranium deposit but also for guiding the investigation of sandstone uranium deposits.