北山花岗岩裂隙围岩蚀变矿物学及地球化学特征

Mineralogy and Geochemical Characteristics of Rock Alteration Surrounding Fractures in Beishan Granite

在地质流体的作用下,花岗岩裂隙两侧常发生不同程度的围岩蚀变,蚀变可能会改变围岩的矿物组成、化学组分乃至对放射性核素的阻滞性能。因此,查明深部岩体裂隙围岩的蚀变特征是处置库场址评价的重要内容之一。以北山新场和沙枣园花岗岩体深钻孔岩心为调查对象,借助显微分析、X射线荧光法等分析测试手段,研究分析了裂隙两侧岩石蚀变的矿物学及地球化学特征,初步探讨了蚀变作用对于围岩还原能力的影响。研究结果表明:北山花岗岩裂隙蚀变岩仅在少数裂隙两侧分布,主要类型为因矿物交代作用引起的钠长石化、黏土化、赤铁矿化和绿泥石化等,每种蚀变作用过程使得围岩的矿物组分与化学组成发生特定变化,除Na_(2)O、K_(2)O、CaO、Fe和LOI等元素以外,其他主量元素含量在蚀变作用中变化微弱或无变化。仅从Fe(Ⅱ)含量变化上分析,北山花岗岩裂隙蚀变作用对围岩还原能力的影响很小。

As a result of the influence of hydrothermal fluid,the granite adjacent to the fracture generally occurs alteration at different degrees which may vary its mineralogical,chemical composition and reducing capacity.Thus,revealing the alteration characteristics surrounding the fractures in the deep rock is one of the important tasks of site characterization for the geological disposal repository.The mineralogy,geochemical characteristics,and reducing capacity of rock alteration were investigated and analyzed,using microscopic analysis and X-ray fluorescence method.The rock samples surrounding the fracture were taken from Xinchang and Shazaoyuan sites in Beishan area.The results show that the altered rocks rarely exist around the fractures,and the main types of rock alteration surrounding the fractures are albitization,clavization,hematitization,and chloritization caused by mineral metasomatism.The altered rock takes some specific changes in mineralogy and chemical composition compared with the unaltered rock.Except for elements,such as Na_(2)O、K_(2)O,CaO,Fe and LOI,etc.,the major elements show no changes or very slight changes in altered rocks.The variation degree of reducing capacity,i.e.Fe(Ⅱ)between the altered rock and fresh rock,is generally very small.