Sedimentary rock-hosted strata-bound copper deposits are widespread in the Kangdian region of the Central Yunnan and southern Sichuan provinces, southwest China. The deposits occur within weakly metamorphosed rocks of the late Paleoproterozoic to early Mesoproterozoic Dongchuan Group and are spatially associated with discordant breccia bodies that are interpreted as having formed from salt diapirs. The Tangdan deposit, the largest in the region, consists of stratiform and discordant orebodies predominantly hosted in dolostones immediately above hematitic sandstones and siltstones and in black shales above the dolostones.
Host rocks for the deposit display a complex paragenetic sequence of alteration and mineralization. Early sodic alteration resulted in the growth of both albite and ferroan dolomite. Later potassic alteration resulted in the precipitation of potassium feldspar and locally biotite in argillaceous layers. Sulfide mineralization was temporally and spatially associated with silicification that postdated both sodic and potassic alteration. Textures suggest that silicification may have preferentially affected evaporite minerals in the dolostones.
Copper sulfides form bedding-parallel disseminations, veinlets and, to a lesser extent, stockworks. Copper sulfides are dominated by chalcopyrite with lesser bornite and chalcocite. Hypogene chalcopyrite and bornite from the Tangdan deposit have δ34S values that range from −12.7 to +9.3‰ and cluster between −3 to +5‰. The values suggest derivation from Mesoproterozoic marine sulfates. The dolostone host rocks have relatively homogeneous C and O isotope values ranging from 0.2 to 1.3‰ δ13CV-PDB and from 19.1 to 22.4‰ δ18OV-SNOW. Carbonate minerals in quartz sulfide veins display both a trend toward lighter oxygen isotope values and a trend to significantly lighter carbon isotope values. The light carbon isotope values suggest 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. The styles of alteration and mineralization at the Tangdan deposit are similar to those observed in the Central African Copperbelt, particularly the dolostone-hosted orebodies in the Mines Series of the Democratic Republic of Congo.