钱家店凹陷含铀岩系姚家组高岭土化特征及高岭石氢氧同位素的流体指示意义

Characteristics and H-O Isotopes of Kaolinite in the Uranium-bearing Rocks from the Yaojia Formation,Qianjiadian Depression and its Implication

钱家店凹陷姚家组高岭石的形成与铀成矿存在流体上的密切联系。通过岩心观察、孔渗测试、光谱扫描及电镜扫描等方法分析了姚家组砂岩高岭土化蚀变空间特征,并对高岭石进行H-O同位素测试,以查明形成高岭石的相关流体及其特征。研究发现:①高岭土化蚀变往往与砂岩的漂白和铀矿化关系密切,灰‒灰白色砂岩渗透率明显高于红‒红棕色砂岩,联井剖面反映姚家组砂岩的高岭土化与漂白程度总体呈现出沿NW-SE向逐渐减弱的趋势;②姚家组高岭石发育,含铀矿段及围岩砂岩的高岭石含量占黏土矿物的65%~90%,高岭石的δ18OV-SMOW值为–1.03‰~8.82‰,δDV-SMOW值为–115.87‰~–105.51‰,显示高岭石与水具有明显的O同位素分馏现象。以H-O同位素反演流体温度为切入点,从流体温度、流体性质与流向以及流体控制因素的角度探讨形成高岭石的相关流体特征,得到以下认识:①δD-δ18O特征表明形成高岭石的流体温度在40~120℃,且研究区西北部的姚家组砂岩高岭石形成的流体温度明显高于其他地区;②与高岭石形成相关的流体可能主要是NW-SE向的较高温深层还原性含烃流体与SW和NW向的低温含氧地下水交汇的酸性流体耦合;③受控于掀斜与反转构造活动的构造天窗和西北部断裂是耦合流体形成的根本原因,而岩相非均质性和渗透性差异分别对地下水和还原性含烃流体有明显影响。

To find out the genesis and characteristics of the kaolinite in the sandstone of the Yaojia Formation in the Qianjiadian Depression,core observation,pore infiltration test,spectral scanning and electron microscopy were carried out on the sandstone based on integrated understandings of the tectonic-sedimentary system since Early Cretaceous in the Qianjiadian Depression.The results of XRD and hydrogen-oxygen isotope analyses of the kaolinite show that:(1)kaolinite alteration is closely correlated with sandstone bleaching and uranium mineralization.The permeability of grey-greyish-white sandstone is obviously higher than that of the red-reddish brown sandstone.The joint well profile shows that the degree of kaolinization and bleaching of sandstone in the Yaojia Formation tend to decrease along NW-SE.(2)Kaolinite is widely developed in the uranium-bearing sandstone and its surrounding sandstone of the Yaojia Formation,with contents as high as 65%to 90%.Theδ18OV-SMOW andδDV-SMOW values of these kaolinites are ranging from–1.03‰to 8.82‰and–115.87‰to–105.51‰respectively,which indicate distinct oxygen isotope fractionation between kaolinite and water.A few factors may contribute to the formation of kaolinite:(1)the temperature,dominantly 40℃to 120℃,and the highest temperature found in the northwestern part of the study area;(2)the related fluids include NW-SE oriented acidic deep-reducing hydrocarbon-containing fluids and acidic coupling fluids at the intersection of SW,NW and SE-oriented low-temperature oxygenated groundwater,with partially small-scaled mixed magmatic hydrothermal fluids;(3)the structural inlier and northwestern faults controlled by skew and reversal tectonic activities controlled the formation of the coupled fluids,while the lithology and permeability heterogeneity significantly affected the flow of the groundwater and reducing hydrocarbon-bearing fluids.