Genesis of sediment-hosted stratiform copper–cobalt mineralization at Luiswishi and Kamoto, Katanga Copperbelt (Democratic Republic of Congo)

摘要

The sediment-hosted stratiform Cu–Co mineralization of the Luiswishi and Kamoto deposits in the Katangan Copperbelt is hosted by the Neoproterozoic Mines Subgroup. Two main hypogene Cu–Co sulfide mineralization stages and associated gangue minerals (dolomite and quartz) are distinguished. The first is an early diagenetic, typical stratiform mineralization with finegrained minerals, whereas the second is a multistage synorogenic stratiform to stratabound mineralization with coarse-grained minerals. For both stages, the main hypogene Cu–Co sulfide minerals are chalcopyrite, bornite, carrollite, and chalcocite. These minerals are in many places replaced by supergene sulfides (e.g., digenite and covellite), especially near the surface, and are completely oxidized in the weathered superficial zone and in surface outcrops, with malachite, heterogenite, chrysocolla, and azurite as the main oxidation products. The hypogene sulfides of the first Cu–Co stage display δ~(34)S values (-10.3‰ to +3.1‰ Vienna Canyon Diablo Troilite (VCDT)), which partly overlap with the δ34S signature of framboidal pyrites (-28.7‰ to 4.2‰ V-CDT) and have Δ~(34)S_(SO4)-Sulfides in the range of 14.4‰ to 27.8‰. This fractionation is consistent with bacterial sulfate reduction (BSR). The hypogene sulfides of the second Cu–Co stage display δ~(34)S signatures that are either similar (-13.1‰ to +5.2‰ V-CDT) to the δ~(34)S values of the sulfides of the first Cu–Co stage or comparable (+18.6‰ to +21.0‰ VCDT) to the δ~(34)S of Neoproterozoic seawater. This indicates that the sulfides of the second stage obtained their sulfur by both remobilization from early diagenetic sulfides and from thermochemical sulfate reduction (TSR). The carbon (-9.9‰to -1.4‰Vienna Pee Dee Belemnite (VPDB)) and oxygen (-14.3‰ to -7.7‰ V-PDB) isotope signatures of dolomites associated with the first Cu–Co stage are in agreement with the interpretation that these dolomites are by-products of BSR. The carbon (-8.6‰ to +0.3‰ VPDB) and oxygen (-24.0‰ to -10.3‰ V-PDB) isotope signatures of dolomites associated with the second Cu–Co stage are mostly similar to the δ~(13)C (-7.1‰ to +1.3‰ VPDB) and δ~(18)O (-14.5‰ to -7.2‰ V-PDB) of the host rock and of the dolomites of the first Cu–Co stage. This indicates that the dolomites of the second Cu–Co stage precipitated from a high-temperature, host rock-buffered fluid, possibly under the influence of TSR. The dolomites associated with the first Cu–Co stage are characterized by significantly radiogenic Sr isotope signatures (0.70987 to 0.73576) that show a good correspondence with the Sr isotope signatures of the granitic basement rocks at an age of ca. 816 Ma. This indicates that the mineralizing fluid of the first Cu–Co stage has most likely leached radiogenic Sr and Cu–Co metals by interaction with the underlying basement rocks and/orwith arenitic sedimentary rocks derived from such a basement. In contrast, the Sr isotope signatures (0.70883 to 0.71215) of the dolomites associated with the second stage show a good correspondence with the ~(87)Sr/~(86)Sr ratios (0.70723 to 0.70927) of poorly mineralized/barren host rocks at ca. 590 Ma. This indicates that the fluid of the second Cu–Co stage was likely a remobilizing fluid that significantly interacted with the country rocks and possibly did not mobilize additional metals from the basement rocks.