四川盆地西南缘二叠系页岩铜银矿床地质地球化学特征和流体对流成矿机制

GEOLOGICAL AND GEOCHEMICAL CHARACTERISTICSAND GENESIS BY FLUID CONVECTION FOR PERMIANKUPFERSCHIEFER CU-AG TYPE DEPOSITS AT THE SOUTHWESTERN EDGE OF THE SICHUAN BASIN

四川盆地西南缘乐山—沐川地区的页岩铜银矿床产于上二叠统宣威组上部。地质和常量元素、稀土元素、热释光等地球化学证据表明， 页岩铜银矿床形成于成岩期，成矿流体和成矿物质主要来源于下伏的峨嵋山玄武岩。流体动力学分析表明成矿流体是以单环对流方式进行迁移，流体在玄武岩中迁移并淋滤Ｃｕ 、Ａｇ 金属形成含矿流体，沿西侧基底隆起上升，然后沿宣威组向盆地中心横向迁移，在盆地中部下渗返回玄武岩中形成一个长约１５ ｋｍ 、高约３００ ｍ 的近水平的对流环。进一步的分析与计算表明，该区具有良好的形成中型铜、大型银页岩矿床的地质、构造和流体动力学条件，但难以形成超大型的页岩铜银矿床。

Kupferschiefer type Cu-Ag deposits occur in the Upper Permian Xuanwei Formation in the Leshan-Muchuan region at the southwestern edge of the Sichuan basin.Evidences from geology,major element,REE and thermoluminescence analyses suggest that these Kupferschiefer type Cu-Ag deposits were formed during diagenesis and the ore-forming fluids were derived from the underlying basalt.Fluid dynamic analyses show that the ore-forming fluids migrated in a unicellular convection,fluids migrated through the basalt,leaching Cu and Ag from basalt and forming ore-bearing solusions,up the western basement highs,moved laterally along the Xuanwei Formation toward the basin centers,presumably to sink back down into the basalt,completing a convection cell which was about 15 km long,300 m high and subhorizontal.Further analysis and calculation suggests that there are good geological,tectonic and fluid-dynamic conditions to form middle-size Cu and large-size Ag Kupferschiefer type deposits,but superlarge doposits are unlikely to be formed in this region.