Metallogenic Model of Dongchuan-type Sedimen-tary Copper Deposits In China

摘要

Dongchuan—type copper deposit are a major type of sedimentary copper deposits in China. They occur in a transgressive copper—bearing rock series of the Middle Proterozoic Kunyang Group. The rock series is marked by variegated sandshales of the Yinmin Formation in the lower part, dolomites of the Luoxue Formation in the middle part and carbonaceous shales and marls of the Heishan Formation in the upper part. The formation of this type of deposits occurred in four stages. (1) The stage of formation of copper— and iron—bearing weathering crust: In the early Middle Proterozoic the crust was uplifted and the copper—bearing rock se-ries in the uplifted area underwent weathering and erosion and then copper—and iron—bear-ing weathering crust was formed at a dry-hot climate and in an oxiding and alkaline envi-ronment. (2) The stage of formation of copper-bearing source beds: In this stage the uplifted crust sank, and at adminantly dry—hot transitional climate, the copper- and iron—bearing weathering crust on old land was denuded, transported rapidly as fragments and in suspen-sion and deposited in hollows on sea basin margins. Meanwhile, erosion of bottom currents resulted in leading of copper and iron from copper- and iron-bearing weathered materials on the bottom of the sea basin, thus producing copper—bearing source beds (copper—bearing argillaceous—arenaceous dolomite) in the Yinmin Formation and the Luoxue Formation. (3) The stage of formation of stratiform copper orebodies: During the process of diagenesis, copper—bearing minerals having originally occurred in sediments were dissolved and entered intergranular fluids of the sediments. As a result of filter pressing, evaporation and crystallization dehydration, the copper—bearing intergranular fluids migrated upward and laterally toward sediments with a high porosity and good pore permeability and portions with structural fractures and then were entrapped and concentrated below impermeable bar-riers, thus forming stratiform copper orebodies in sediments with appreciable amounts of re-ducing agents. (4) The stage of formation of metamorphosed ore veins and pockets: The stratiform orebodies occurring in the Kunyang Group were deformed and metamorphosed during the Jinning movement, and under the effects of metamorphic hydrothermal processes, thus forming copper-bearing metamorphic ore veins and pockets.