摘要： 内蒙古巴音戈壁盆地因格井坳陷下白垩统巴音戈壁组上段是中国高放废物黏土岩处置库重点预选区段。通过对该段沉积岩开展岩石学、矿物学和地球化学等研究,发现该热水沉积岩岩相呈深灰色、灰色,具有纹层/条带状、网脉状、同生变形、“斑状”、块状共5种典型构造,主要矿物组分为白云石、铁白云石、方沸石、伊利石、钠长石和黄铁矿。依据主要矿物组分,将热水沉积岩分为混积岩、泥质白云岩、泥质沸石岩及含白云石沸石泥岩4类。地球化学测试结果表明: 热水沉积岩中主量元素铁、锰富集;深源气液型微量元素Mo、Sb、Zn、As、Sr、Ba丰度相对较高;稀土元素总量变化范围大,平均值为119.85μg/g,轻稀土明显富集,重稀土亏损,δCe未见明显亏损(平均值0.99),δEu中等偏强的负异常(平均值0.62);δ¹⁸O值以负偏为主。根据氧同位素值计算出的热水沉积物形成温度介于40.41~64.87℃之间,Fe-Mn-(Cu+Co+Ni)三角图解中热水沉积物均落入喷流沉积区。结合区域构造背景,认为该套热水沉积岩是在干旱的气候条件下,偏还原的半咸湖—咸湖湖水与深部岩浆热液相互作用形成的低温“白烟型”热水沉积岩,其中阿尔金断裂带在100—120Ma的活动是热水沉积岩形成的主控因素,其为白云化流体提供了运移通道。对巴音戈壁盆地下白垩统巴音戈壁组热水沉积岩的研究,有助于确定高放废物黏土岩地质处置库重点目标层位。

关键词: 热水沉积岩, 岩石学特征, 地球化学特征, 因格井坳陷

Abstract: The Upper Member of Lower Cretaceous Bayingebi Formation in Yingejing sag,Bayan Gebi Basin,Inner Mongolia is the key pre-selected section of clay rock repository for China’s high-level radioactive waste(HLW). The studies of petrological,mineralogical and geochemical for the sedimentary rocks in this section show that the cores of hydrothermal-sedimentary rocks are dark-gray and gray,and have five typical structures,i.e. laminated and banded,stockwork,syngenetic deformation bedding,porphyritic massive structures. The hydrothermal-sedimentary rock is predominantly composed of dolomite,ankerite,analcime,illite,albite,and pyrite,which are typical minerals of exhalative sedimentary rocks. There are 4 types of hydrothermal-sedimentary rock based on the difference among major minerals. Geochemical characteristics confirm that the main elements are iron and manganese. The typical deep-source gas-liquid trace elements such as Mo,Sb,Zn,As,Sr and Ba are relatively abundant. The average value of $\sum$REE is 119.85 μg/g and is characterized by enrichment of light REE and depletion of heavy REE. There is no significant enrichment or loss of δCe(average value 0.99),and δEu shows a medium strong negative anomaly(average value 0.62). The values of δ¹⁸O are mainly negative bias,indicating the formation temperatures of lake-facies hydrothermal-sediments ranging from 40.41℃ to 64.87℃. The samples of Fe-Mn-(Cu+Ni+Co)-ternary diagram all fall into hydrothermal sediment area. The comprehensive analyses show that this is a set of low-temperature “white smoke type” hydrothermal sedimentary rock associated with magmatism and deep hydrothermal fluid under the mainly arid depositional environment within the brackish-saline lake. The 100-120Ma tectonic activity in the Altun fault zone was the main controlling factor for the formation of exhalative sedimentary rocks,which provided the migration pathway for dolomitized fluids. The study on hydrothermal-sedimentary rock with unique mineral composition provides scientific basis and new ideas for claystone geological disposal repository of China’ HLW.

Key words: hydrothermal-sedimentary rock, petrological characteristics, geochemical characteristics, Yingejing sag

中图分类号: 

• P588.2