松辽盆地西南部含铀岩系沉积时的古气候演变及其与铀成矿关系

Paleoclimate Evolution and Uranium Mineralization during the Deposition of Uranium-Bearing Rocks in the Southwest of Songliao Basin

含铀岩系沉积时的古气候条件对铀成矿起着重要的控制作用,但是长期以来未引起人们足够重视.以松辽盆地西南部HLJ地区含矿目的层姚家组下段为研究对象,通过对代表性钻孔岩心的泥岩样品进行全岩主、微量元素和TOC测试,采用K/Na、aAlNa、CPA、CIX、PIA、CIW、CIA等多项化学风化指标来判断样品化学风化作用强度,重建其沉积时期古气候条件.同时,结合U和TOC含量及Fe^(2+)/Fe^(3+)比值来探讨含铀岩系沉积时的古气候条件对铀成矿的制约.结果表明,目的层碎屑岩经历了由弱-强-弱的化学风化作用,显示其沉积时的古气候由相对寒冷干旱-相对温暖潮湿-相对寒冷干旱的演变规律.这种古气候变化导致了铀储层砂体中TOC含量低(平均值0.05%),层间氧化带发育规模大,铀矿体主要形成于远离蚀源区的盆地腹部;形成于潮湿气候条件下的暗色泥岩具有更高TOC(平均值0.61%)和U含量(最高可达885×10^(-6)),其沉积时发生了强烈的预富集作用而成为后期成矿过程中重要铀来源之一,其成岩期排泄出还原性孔隙水可能是铀储层砂体中构建氧化-还原屏障的主要因素.沉积时的温暖潮湿气候增强了蚀源区化学风化作用强度,有利于母岩铀元素活化和淋漓及沉积区泥岩铀的预富集作用.

Although,the paleoclimate plays an important role in controlling uranium mineralization during the deposition of uranium-bearing rocks,it has not attracted enough attention of uranium geologists for a long time.The lower member of Yaojia formation is the ore bearing target layer in the southwest of Songliao basin.Taking the lower member of Yaojia formation as the research object,the contents of the main and trace elements and total organic carbon of mudstone samples from representative boreholes were tested.A number of chemical weathering indexes,such as K/Na ratio,aAlNa,chemical proxy of alteration(CPA),modified chemical index of alteration(CIX),plagioclase index of alteration(PIA),chemical index of weathering(CIW),chemical index of alteration(CIA),are used to judge the intensity of chemical weathering of the samples,and reconstruct the paleoclimate conditions during the depositional period.At the same time,combined with the contents of U and TOC and the ratio of Fe^(2+)/Fe^(3+),the constraints of paleoclimate conditions on uranium mineralization during the deposition of uranium-bearing rock series in the study area are discussed.The results show that the clastic rocks in the study area have experienced the change of chemical weathering intensity from weak to strong to weak,indicating that the evolution law of paleoclimate from relative cold and drought to relative warm and humidity to relative cold and drought.This paleoclimate change leads to low TOC content in uranium reservoir sand bodies(average value 0.05%)and large-scale development of interlayer oxidation zone,so uranium ore bodies are mainly located in the hinterland of the basin far away from the erosion source area.Dark mudstone formed in humid climate has higher TOC(average 0.61%)and U content(up to 885×10^(-6)).It has become one of the important sources of uranium due to its strong preconcentration during deposition.The discharge of reducing pore water during diagenesis may be the main factor for the construction of oxidation-reduction barrier in uranium reservoir sand bodies.During deposition,the warm and humid climate enhances the intensity of chemical weathering in the source area,which is conducive to the activation and leaching of uranium in the parent rock and the uranium preconcentration of mudstone in the sedimentary area.