塔木素铀矿床下白垩统巴音戈壁组含铀砂岩成岩特征及其与铀矿化关系研究

Study of the Diagenetic Characteristics of Uraniferous Sandstone and Its Relationship with Uranium Mineralization of the Bayin Gobi Formation,Lower Cretaceous Tamusu Deposit

位于巴音戈壁盆地南部的塔木素铀矿床为典型的硬砂岩型铀矿床,这种特殊硬砂岩型铀矿与我国北方其他典型砂岩铀矿床最主要的不同就是表现在成岩特征上。运用岩石学、岩相学、岩石地球化学、阴极发光、扫描电子显微镜等对塔木素铀矿床下白垩统巴音戈壁组上段(K_(1)b^(2))含铀砂岩成岩特征及其与铀矿化的关系进行了研究。结果表明含铀砂岩碎屑物主要有石英(平均含量12.38%)、长石(平均含量50.33%),胶结物主要有石膏(平均含量12.57%)、铁白云石(平均含量3.20%)、含铁白云石(平均含量9.33%)、白云石(平均含量9.76%)及少量杂基(平均含量2.62%),以孔隙式和基底式胶结为主。含铀砂岩具有盐湖盆地早成岩阶段B期和晚成岩阶段A期的成岩特征,成岩的水介质条件为高矿化度(35.4 g/L)的碱性水(pH=7.52)。在早成岩阶段B期形成了以化学胶结作用、水岩作用、溶蚀作用及交代作用为主的成岩特征。胶结物具有由铁白云石—含铁白云石—白云石—石膏的生成顺序,碳酸盐胶结物由中心到边缘依次为自形铁白云石、半自形—自形的含铁白云石、最外面为他形—半自形的白云石。石膏形成最晚,并可见石膏交代石英、长石及白云石现象。在此过程中,高矿化度水中的Na^(+)替换了斜长石中的Ca^(2+),使斜长石全部转化为钠长石,并在斜长石表面形成了溶蚀微孔洞并沿解理形成微裂隙。受碱性地下水影响,基本无自生胶结作用及黏土矿物生成。晚成岩阶段A期以溶解作用为特征,酸性的地表水溶解了砂岩中的碳酸盐胶结物并形成了溶洞。含铀砂岩整体具有孔隙度低及渗透率低等特征,不利于层间氧化作用的形成。根据含铀砂岩成岩特征及其与铀矿化关系的研究发现,塔木素地区铀矿化具有沉积成岩及层间氧化双重成因特征。

The Tamusu uranium deposit in the southern part of the Bayin Gobi Basin is a typical hard-sandstone uranium deposit,but with very different diagenetic characteristics from other sandstone-type uranium deposits in northern China.The characteristics of uraniferous sandstone(K_(1)b^(2-2))and its relationship to uranium mineralization in the Tamusu deposit have been studied In recent years by petrology,petrography,geochemical,scanning electron microscope(SEM)and so on.The detritus is composed of quartz(average content 2.38%)and feldspars(average content 50.33%).The cements comprise gypsum(average content 12.57%),ankerite(average content 3.20%),ferruginous dolomite(average content 9.33%),dolomite(average content 9.76%)and a small amount of complex matrix material(average content 2.62%),causing both basal cementation and porous cementation.The diagenetic process is characterized by early stage B diagenesis and later stage A diagenesis,with alkaline water(pH 7.52).Early stage B diagenesis is characterized by chemical cementation,water-rock interaction,dissolution and metasomatism.The formation sequence of the cements is ankerite-ferruginous dolomite-dolomite-gypsum;the carbonate cements,from center to edge,are automorphic ankerite,automorphic-hypidiomorphic ferruginous dolomite,and hypidiomorphic-xenomorphic dolomite.Gypsum forms last,with metasomatism of quartz,feldspar and dolomite.In this process,plagioclase is altered to albite due to the presence of Na^(+)in the highly saline water replacing Ca^(2+),with dissolution microcavities formed on the surface of the plagioclase and microcracks along cleavage planes.During early stage B diagenesis,no auto-cementation takes place,and clay minerals are affected by alkaline groundwater.During late stage A diagenesis,acidic surface water containing SO^(2)_(4)-dissolves the carbonate cement,forming cavities.The uraniferous sandstone does not favor interlayer oxidation due to its low porosity and permeability.It is inferred that the genesis of uranium mineralization in the Tamusu deposit occurs by sedimentary diagenesis and interlayer oxidation.