湘西花垣李梅铅锌矿区古热液卡斯特特征及其成因研究

发育于湘西花垣县李梅铅锌矿区内下寒武统清虚洞组藻灰岩中的一套角砾岩,具古卡斯特特征。溶蚀洞穴、溶蚀崩解、溶蚀变形构造以及特殊的洞穴充填角砾和洞穴矿物可作为鉴别和区分不同于一般沉积角砾岩的主要特征。X衍射、包体测温、包体成分及氢氧同位素组合分析表明:本区古卡斯特为一种低温(150℃±)、高盐度(14.89～39.24％)、弱酸性(pH=5.75.t=150℃)至弱碱性(pH=6.88、t=150℃)、还原型(Eh:-0.7721～-0.8699)的CaCl型深层热卤水的溶蚀作用所造成。

THE CHARACTERISTICS OF THE LI MEI PALEOKARST IN WESTERN HUNAN PROVINCE, CHINA, WITH PARTICULAR REFERENCE TO ITS GENESIS

In the lower Cambrian Qingxudong formation, a paleokarst was developed with a bioherm algal limestone over what is now Huayuan County, western Hunan Province. The paleokarst possesses the following characteristics. These are referred to as dissolution cavities, cave fillings and deformation structures. The nest-like dissolution cavities, 0.5-2m in diameter, are ball-shaped chambers and most of them are interconnected each other by one or more than one conduits. The solution collapse breccias are mainly fillings in the cavities and differ from sedimentary breccias in the following: 1) The karst breccias are incongruent with sedimentary structures of the host strata and limited in areal extent. 2)The solutional collapse breccias are purely algal limestone which was derived from the country rock. 3) It possesses soietional disaggregation structure, crystal-crust structure. solutional deformation structure and micro-dissolution texture. 4) The breccies have a very bad sorting and roundness. Both the cave formation and filting in L_1 Mei area occurred by hot brine dissolution and precipitation. This is clear from the following geochemical and geological evidence: 1) Mineralization in the caves includes formation of lead-zinc, dolomite, baryte calcite, silica, fluorite, etc. 2) The degree of ordering of the speleothemic dolomite is as high as 0.9537. and the Mg/Cavalue is almost I, indicating the dolomite formed by hydrothermal processes., in contrast to those associated with hydrothemal karst; the degree of ordering of the dolomite in the host rock is much lower. 3) The speleothemic calcite possesses hydrothermal-origin fluid inclusion and shows a homogenization temperature about 100-150℃. 4) The inclusion composition studies indicate the karst solution is highly salme (14.89％ to 39.24％), Weakly acidic, reducing and CaCl-type underground water. 5) Hydrogen and oxygen isotopic composition also shows that the kant solution fluid is underground brine.