Madouzi-type (nodular) sedimentary copper deposit associated with the Emeishan basalt

Abstract

Ore minerals in the sedimentary-type Cu deposits in the Xuanwei Formation overlying the Emeishan basalt are dominated by copper sulfides and native copper. As the ores mostly exhibit concretionary structure, previous researchers named them the “Madouzi-type Copper Deposit”. Here the authors carried out mineralogical and isotopic studies on copper nodules in this ore deposit. The mineralogical study shows that copper nodules are composed of copper sulfides that have been cemented by ferruginous amorphous minerals, clay, and carbonaceous fragments in the modes of metasomatism and sedimentation. The nodules are preliminarily present as aggregates of gelatinous material. The isotopic analysis shows that the δ ¹³C_PDB values of anthraxolite are within the range of −24.8‰–−23.9‰, indicating that the anthraxolite is the product of sedimentary metamorphism of in-situ plants. The δ ³⁴S_V-CDT values of chalcocite are within the range of 7.6‰–13.1‰, close to those (about 11‰) of Permian seawater. The δ ³⁴S_V-CDT values of bornite and chalcopyrite are 21.6‰–22.2‰, similar to the sulfur isotopic composition (20‰) of marine sulfate, indicative of different sources of sulfur. The above characteristics indicate that the copper nodules were formed in such a process that Cu-bearing basalt underwent weathering-leaching and copper-bearing material was transported into waters (e.g., rivers, lakes, and swamps) and then adsorbed on clay and ferruginous amorphous mineral fragments. Then, the copper-bearing material was suspended and transported in the form of gelinite. In lake or swamp environment, it was co-deposited with sediments to form copper nodules. At later stages there occurred metasomatism and hydrothermal superimposition, followed by the replacement of chalcocite by bornite and the superimposition of chalcopyrite over bornite, finally resulting in the formation of the “Madouzi-type” nodular copper deposit.