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Mineralium Deposita

, 43:575 | Cite as

Anhydrite pseudomorphs and the origin of stratiform Cu–Co ores in the Katangan Copperbelt (Democratic Republic of Congo)

  • Ph. Muchez
  • P. Vanderhaeghen
  • H. El Desouky
  • J. Schneider
  • A. Boyce
  • S. Dewaele
  • J. Cailteux
Letter

Abstract

The stratiform Cu–Co ore mineralisation in the Katangan Copperbelt consists of dispersed sulphides and sulphides in nodules and lenses, which are often pseudomorphs after evaporites. Two types of pseudomorphs can be distinguished in the nodules and lenses. In type 1 examples, dolomite precipitated first and was subsequently replaced by Cu–Co sulphides and authigenic quartz, whereas in type 2 examples, authigenic quartz and Cu–Co sulphides precipitated prior to dolomite and are coarse-grained. The sulphur isotopic composition of the copper–cobalt sulphides in the type 1 pseudomorphs is between −10.3 and 3.1‰ relative to the Vienna Canyon Diablo Troilite, indicating that the sulphide component was derived from bacterial sulphate reduction (BSR). The generation of \({\text{HCO}}_3^ - \) during this process caused the precipitation and replacement of anhydrite by dolomite. A second product of BSR is the generation of H2S, resulting in the precipitation of Cu–Co sulphides from the mineralising fluids. Initial sulphide precipitation occurred along the rim of the pseudomorphs and continued towards the core. Precipitation of authigenic quartz was most likely induced by a pH decrease during sulphide precipitation. Fluid inclusion data from quartz indicate the presence of a high-salinity (8–18 eq. wt.% NaCl) fluid, possibly derived from evaporated seawater which migrated through the deep subsurface. 87Sr/86Sr ratios of dolomite in type 1 nodules range between 0.71012 and 0.73576, significantly more radiogenic than the strontium isotopic composition of Neoproterozoic marine carbonates (87Sr/86Sr = 0.7056–0.7087). This suggests intense interaction with siliciclastic sedimentary rocks and/or the granitic basement. The low carbon isotopic composition of the dolomite in the pseudomorphs (−7.02 and −9.93‰ relative to the Vienna Pee Dee Belemnite, V-PDB) compared to the host rock dolomite (−4.90 and +1.31‰ V-PDB) resulted from the oxidation of organic matter during BSR.

Keywords

Anhydrite Copperbelt Cu–Co stratiform ore deposits Fluid chemistry Bacterial sulphate reduction Democratic Republic of Congo 

Notes

Acknowledgements

We are grateful to Steve Roberts and Robert Moritz for their constructive reviews and thoughtful suggestions and to Bernd Lehmann for editing this paper. We would like to thank E. Pirard (Université de Liège) for the permission to study borehole 120 from Kamoto. We are grateful to Dr. M. Joachimski of the University of Erlangen for the stable isotope analysis. We thank Herman Nijs for the careful preparation of the numerous thin and polished sections.

This research is financially supported by the research grants G.0585.06 and G.0414.08 of the FWO-Vlaanderen.

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Copyright information

© Springer-Verlag 2008

Authors and Affiliations

  • Ph. Muchez
    • 1
  • P. Vanderhaeghen
    • 1
  • H. El Desouky
    • 1
  • J. Schneider
    • 1
    • 2
  • A. Boyce
    • 3
  • S. Dewaele
    • 4
  • J. Cailteux
    • 5
  1. 1.Geodynamics & Geofluids Research Group, Afdeling GeologieK.U.LeuvenLeuvenBelgium
  2. 2.Centre for Archaeological SciencesK.U.LeuvenLeuvenBelgium
  3. 3.Isotope Geoscience UnitSUERCGlasgowUK
  4. 4.Department of Geology and MineralogyRoyal Museum for Central Africa (RMCA)TervurenBelgium
  5. 5.Département Recherche et Développement, EGMFGroupe G. Forrest InternationalLubumbashiDemocratic Republic of Congo

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