Two Cu-Co sulfide phases and contrasting fluid systems in the Katanga Copperbelt, Democratic Republic of Congo

摘要

The Katanga Copperbelt is the Congolese part of the well-known Central African Copperbelt, the largest sediment-hosted stratiform Cu-Co province on Earth. Petrographic examination of borehole samples from the Kamoto and Luiswishi mines in the Katanga Copperbelt recognized two generations of hypogene Cu-Co sulfides and associated gangue minerals (dolomite and quartz). The first generation is characterized by fine-grained Cu-Co sulfides and quartz replacing dolomite. The second generation is paragenetically later and characterized by coarse-grained Cu-Co sulfides and quartz overgrown and partly replaced by dolomite. Fluid inclusion microthermometric data were collected from two different types of fluid inclusions: type-I fluid inclusions (liquid + vapor) in the quartz of the first generation and type-II fluid inclusions (liquid + vapor + halite) in the quartz of the second generation. The microthermometric analyses indicate that the fluids represented by type-I and type-II fluid inclusions had very different temperatures and salinities and were not in thermal equilibrium with the host rock. Petrographic and microthermometric data indicate the presence of at least two main hypogene Cu-Co sulfide phases in the Katanga Copperbelt. The first is an early diagenetic typical stratiform phase, which produced finegrained sulfides that are disseminated in the host rock and frequently concentrated in nodules and lenticular layers. This phase is related to a hydrothermal fluid with a moderate temperature (115 to 220 deg C, or less if reequilibration of inclusions has occurred) and salinity (11.3 to 20.9 wt. percent NaCl equiv.). The second hypogene Cu-Co phase produced syn-orogenic coarse-grained sulfides, which also occur disseminated in the host rock but mainly concentrated in a distinct type of stratiform nodules and layers and in stratabound veins and tectonic breccia cement. This second phase is related to a hydrothermal fluid with high temperature (270 to 385 deg C) and salinity (35 to 45.5 wt. percent NaCl equiv.). A review of available microthermometric and ore geochronological data of the Copperbelt in both the Democratic Republic of Congo and Zambia supports the regional presence of the two Cu-Co phases proposed in our study. Future geochemical analyses in the Copperbelt should take into account the presence of, at least, these two Cu-Co phases, their contrasting fluid systems and the possible overprint of the first phase by the second one.