白音查干凹陷下白垩统腾格尔组白云岩成因机理研究

In the recent years, a set of zeolitic dolostones and zeolite stones, with single layer thickness of 0.2-1.5m, has been discovered within the deep lacustrine mudstone of the Lower Cretaceous Tenggeer Formation in the Baiyinchagan Sag. Data from thin sections, electronic probe microscopy, energy spectrum and whole rocks X-ray diffraction show that the mineralogy is composed of ankerite (Fe 0.5%-5%), natrolite, analcite, sjogrenite, barite and pyrite. Ankerite crystals, with gain size vary from 10μm to 200μm, are characterized by abundant crack, wavy extinction and saddle-shaped. Shaly and micrite texture， and intraclastic texture with intraclast composed by dolomite and zeolite are well developed and several typical structures are also identified, including contemporaneous deformation structure, stockwork structure and breccia structure. The dolostone with high content of U (40~60ppm) and Th (60~100ppm) shows extreme anomalous response in gamma ray logs (700~1100API), which is 2~4 time higher than that of normal mudstone. The constituent and content of trace element such as Cu、Ni、Co 、Fe、Mn are consistent with that of modern submarine hot water deposition in the Red Sea. Furthermore this set of deposition is mainly distributed along the basement rift. All the above characters provide evidences that these dolostones are originated from the hydrothermal deposition. A few cases like this hydrothermal deposition have been reported all over the world in the past years. The process and mechanism of hydrothermal sedimentation is not well understood and there is no some mature depositional model so far. Thus the study on the hydrothermal deposition will further enrich the modern sedimentary theory,find out the more hydrothermal ore deoposits and provide new oil and gas reservoir targets suggestion for future petroleum exploration.

近年来在白音查干凹陷下白垩统腾格尔组深~半深湖相泥岩中发现一套单层厚0.2~1.5m的沸石质白云岩和沸石岩。岩石薄片、电子探针、能谱及X衍射分析表明：白云岩层系由铁白云石（含Fe0.5%~5%）、钠沸石、方沸石、水镁铁石、重晶石、黄铁矿组成。白云石晶体10~200μ,具裂纹，波状消光，鞍状特征。岩石具泥粉晶结构和内碎屑结构，内碎屑为白云岩和沸石岩。发育同生变形构造、网脉状构造、角砾构造。自然伽马700~1100API，为正常泥岩的2~4倍，铀和钍含量分别高达40~60ppm和60~100ppm。Cu、Ni、Co 、Fe、Mn等微量元素组成和分布与现代红海海底热水沉积一致。平面上沿基底断裂分布。多种资料表明这是一套热水沉积岩。目前国内外对这类沉积报道较少，研究程度低，对其沉积机理与沉积过程缺乏深入了解，没有统一的沉积模式。通过此研究，有助于丰富和完善现有的沉积学理论，寻找更多的热水沉积矿床。

内蒙古二连盆地白音查干凹陷下白垩统腾格尔组发育一套湖相白云岩，本次研究对该套白云岩的矿物组成、结构与构造特征、矿物组合关系、地球化学特征等进行了系统深入仔细研究，在此基础上对岩石进行识别，讨论其成因。研究表明：研究区白云岩其矿物组分复杂多样，除白云石外，还发育大量的沸石（钠沸石和方沸石）、黄铁矿、菱镁矿及重晶石等高温热液矿物，并混有粘土矿物、钾长石和斜长石等泥质陆源碎屑；白云岩结构构造复杂多样，结构可以划分为热水沉积内碎屑结构、泥晶结构和团块结构3种；构造可以划分为纹层状构造、条带状构造、网脉状构造、同生变形构造、角砾状构造、蝌蚪状构造、星散状构造等9种类型。白云岩微量元素富集Pb、Sb、Tl、W、Mo、Bi，亏损V、Cr、Co、Ni，暗示存在中高温深源碱性热液流体加入。不同类型白云岩的稀土元素在LREE与HREE的富集特征、Tb，Yb和Dy等异常方面存在明显不同，说明形成白云岩的热液不仅有深部热液流体也有湖水。白云岩具有相似的δ18O（-20.5~-11.0‰VPDB）和δ13C（+1.4~+4.5‰VPDB），但部分泥晶白云岩表现出不同的同位素特征（δ18O，-8.5~-2.3‰VPDB；δ13C，+1.4‰~+8.6‰VPDB）。白云岩的87Sr/86Sr值从0.71774到0.72080不等，明显地高于早白垩世海水，而略低于地壳锶同位素值。其中粉-细晶白云岩形成的均一温度为167-283℃，其盐度为11.8-23.2 wt％NaCl，说明形成白云岩的热液流体为高温、高盐度流体。综上认为在裂谷盆地地质背景下，来自深部的富含CO32-、SO42-和Na+、Al3+、Si4+、Ca2+、Mg2+、Fe2+、Ba2+等离子的热液流体沿各级断裂上涌，喷出或溢流至湖底，与冷的湖水混合，引起周围环境变化，促进白云石及其共生矿物的沉淀。热液流体为形成白云岩提供基本组分，而湖水为白云岩的形成提供沉积环境。腾格尔组白云岩属于热水沉积成因，称之为热水沉积白云岩。本区热水沉积岩岩石学和地球化学特征研究可为其它地区的热水沉积岩的识别提供参考，该成果有利于丰富和完善现有的沉积学理论，寻找更多的热水沉积矿床。