Discovery of the copper deposits with features of the Keweenawan type in the border area of Yunnan and Guizhou provinces

There existed intense Cu anomaly on the northeastern side of the geochemical boundary with NW strike in the border area between the Yunnan and Guizhou provinces. Through field observation, ore bodies of high-grade native coppers have been found. The copper mineralization was constrained by the ancient volcanic vents of Permian basalt eruption and the overlain strata of carbonaceous argillites. Native coppers with flaky, net veined and impregnated occurrences, fine-grained tenorites and massive chalcocites widely occur in volcanic breccias, tuffs, carbonaceous-siliceous argillites and siliceous bitumen rocks with bed thickness of about 15–80 m. Cu contents vary from 0.5% to 20%. The copper mineralization was tightly related to actino-lite-tremolitization, zeolitization and bituminization and involved in extensive reduction environments. Continental flood basalts erupted in mantle plume environments usually have high Cu concentrations (∼170 ×10−6 in the Emeishan basalts), which provided a copper source of mineralization. Thus, metallogenesis of the native copper deposits in the Yunnan-Guizhou border area is tightly associated with intensive crust-mantle and organic-inorganic interactions. The tremolitization and chalcocitization indicate that the metallogenic temperatures are in a range of 400–100°C. The geologic background and characteristics of ore and alteration for the native copper deposits in this area are somewhat similar to those of the Keweenawan native copper deposit in Michigan, USA.