MESOPROTEROZOIC SEDIMENTARY-ROCK HOSTED STRATIFORM COPPER DEPOSITS OF YUNNAN PROVINCE, SW CHINA

The Tangdan sedimentary rock-hosted stratiform copper deposit in Yunnan Province consists of stratiform and lens-shaped ore bodies predominantly hosted in Mesoproterozoic dolostones immediately above hematitic siliciclastic sedimentary rocks and in carbonaceous slates above the dolostones. The host rocks display early sodic alteration with the growth of both albite and ferroan dolomite. Later potassic alteration resulted in precipitation of potassium feldspar and locally biotite in argillaceous intervals. Sulfide mineralization was temporally and spatially associated with a silicification event that postdated both sodic and potassic alteration. Textures suggest that silicification may have preferentially affected evaporite minerals in the dolostones. Copper sulfides (chalcopyrite with lesser bornite and chalcocite) form bedding-parallel disseminations, veinlets and, to a lesser extent, stockworks. Hypogene chalcopyrite and bornite δ³⁴S values that range from -12.7‰ to +9.3‰ and cluster at -2 ‰ to +6 ‰. The values suggest derivation from Mesoproterozoic marine sulfate. The dolostone host rocks have relatively homogeneous C and O isotopes ranging from 0.2‰ to 1.3‰ δ¹³C_V-PDB and from 19.1‰ to 22.4‰δ¹⁸O_V-SNOW, while carbonate minerals in quartz-sulfide veins display trends towards lighter oxygen isotopic values or a significant shift to lighter carbon isotopic values. The trend towards sharply lower carbon isotopic values suggests involvement of organic carbon in the mineralizing process. Alteration and mineralization at Tangdan probably occurred via interaction of oxidized saline brines derived from the underlying red bed sequence with partially to wholly lithified dolostones. Sulfide precipitation was likely due to both redox reactions and mineralizing fluid pH changes resulting from dolomite and sulfate dissolution.