GEOLOGICAL AND GEOCHEMICAL FEATURES AND GENESIS BY FLUID CONVECTION FOR PERMIAN Cu-BEARING Cu-Ag TYPE DEPOSITS AT THE SOUTHWESTERN EDGE OF THE SICHUAN BASIN

摘要

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 thermolurninescence analyses suggest that these Kupferschiefer type CuAg deposits were formed during diagenesis and the ore-forming fluids were derived frorn 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 frorn basalt and forming orebearing solutions, 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 cycle which was about 15 km long, 300 m high and subhorizontal. Further analysis and calculation suggest that there are good geological, tectonic and fluid-dynamic conditions to form middle-scale Cu and giant Ag Kupferschiefer type deposits, but superlarge deposits are unlikely to be formed in this region.