论湘南地区“硅化带”的成因及其找矿意义

THE GENESIS OF "S1UCIFIED ZONES" IN SOUTHERN HUNAN AND THEIR ORE-PROSPECTING SIGNIFICANCE

湘南地区沉积盖层中存在着大量的所谓“硅化带”。实际上,它是由多个互不相连的角砾岩体所构成,有的聚集成群,或者呈完全孤立的山堡,个别处为明显的椭圆锥状中心型火山机构。角砾岩体多为椭圆状或等轴状,与围岩多呈刺穿状接触。经研究认为,该区“硅化带”是一种由岩浆气体及部分岩浆物质,在封闭还原条件下产生急剧爆发所形成的爆发角砾岩体群。它是一种极好的容矿控矿地质体,具有重要的找矿意义。

Lots of'silicified zones' exist in sedimentary cover of southern Hunan area. In fact, they are steep ridges and peaks composed of separate breccia bodies, which occur either in groups or as isolated hills with a few places assuming central type volcanic apparatus in the form of elliptic cones. The breccia bodies making up'silicified zones' mostly take elliptical or isometrical forms in plane and conical or cylindrical forms branching upward in profile, with the branches showing stock, dyke or irregular forms. They are exposed or concealed in various Middle Devonian-Cretaceous strata and mostly in piercing contact with wall rocks. It is inferred that their downward extension can reach more than one kilometers.The gravels and cements in the breccia body are mainly fractured products of the wall rocks and, next, magmatic materials and rock fragments derived from deeper strata. The brecciated gravels are sharply angular to pebble-like in form, and the cements are mainly siliceous, displaying mainly microcrystalline and cryptocrystalline textures and partly porphyritic texture.The breccia body exhibits apparent horizontal and vertical zoning, with well-developed porous and flow structures as well as some corrosion and reaction rim textures.The breccia body is obviously different from the wall rocks in such aspects as mineral constituents, chemical composition and trace elements, and displays apparent dehydration and intense post-magmatic hydrothermal alteration, commonly forming a secondary quartzitized'cap' in its upper part. These'silicified zones' are explosive breccia groups formed as a result of drastic explosion of magmatic gas and some magmatic materials under the sealed and reductive condition, and the explosion occurred respectively at 106 Ma and 63-61 Ma.As the explosive breccia body might yield large-sized uranium(molybdenum) deposits, gold deposits and polymetallic deposits, it serves as an excellent ore-hosting and ore-controlling geologic body and has important ore-prospecting significance.