开鲁盆地钱家店凹陷含铀岩系姚家组沉积环境及其富铀意义

Sedimentary environments and uranium enrichment in the Yaojia Formation, Qianjiadian depression,Kailu Basin,Nei Mongol

关键词:  下白垩统  姚家组  沉积环境  钱家店凹陷  开鲁盆地  内蒙古

Key words:  Lower Cretaceous  Yaojia Formation  sedimentary environment  Qianjiadian depression  Kailu Basin  Nei Mongol

摘要:

利用10多口钻孔岩心、电测曲线和分析化验资料,对钱家店凹陷含铀岩系姚家组沉积环境及古气候进行了分析。姚家组为一套以中细粒为主的砂质辫状河沉积产物。含矿层主岩为岩屑长石砂岩和长石岩屑砂岩,粒度概率累积曲线以悬浮总体含量较高为特征的两段式。一个完整的垂向序列由含砾砂岩或中细砂岩向上变细为粉砂岩和少量泥岩组成的正韵律。碎屑组分含量变化和重矿物分析表明,沉积物源主要来自盆地西北部古隆起的酸性岩浆岩和变质岩。泥岩元素分析、孢粉分析及泥岩颜色等指示,姚家组沉积期古气候主要为亚热带干旱气候,在这一大的气候背景下有过程度较弱的短暂潮湿化事件发生。干旱—半干旱气候为层间氧化带的形成提供了有利的气候条件,周边古隆起或更远山地的酸性火山岩、古老结晶岩和变质岩为该区提供了丰富的铀源,广泛发育、物性较好的辫状河道砂体为铀矿化提供了良好的储渗空间。

Abstract:

The sedimentary environments and palaeoclimates in the uranium-bearing Yaojia Formation in the Qianjiadian depression,Kailu Basin are discussed on the basis of well logs,electric logs and laboratory data.The Yaojia Formation is heaped up by the medium-to fine-grained sandy braided stream deposits.The host rocks of ore beds consist of lithic arkose and arkosic arenite.The grain size probability cumulative curves for the sandstones display the features of saltation and suspension populations,i.e.,those of typical fluvial deposits. Sedimentary structures include scour filling structure,scour-faceted structure, large-scale trough cross-bedding,wedged cross-bedding,parallel bedding,massive bedding,graded bedding,and wavy cross-bedding.The vertical sequences exhibit a fining-upward normal rhythmic sequence from gravelly sandstones or medium-to fine-grained sandstone passed upwards into siltstone and mudstone.The analysis of clastics and heavy minerals shows that the sediments are derived from the acidic magmatic rocks and metamorphic rocks on the palaeouplift in northwestern basin.The chemical analysis and colour index of mudstone and sporopollen analysis indicate that the palaeoclimates are dominated by the arid climates of the subtropical zone in which there occurred weak and ephemeral moistening events here and there.The arid-semiarid climates facilitate the formation of interstratal oxidized zones.The acidic volcanic rocks,old crystalline rocks and metamorphic rocks from the above-mentioned palaeouplift or distal mountain areas may serve as the uranium sources,and the braided stream sandstones as good reservoir rocks for uranium mineralization.