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Structural configuration of the Central African Copperbelt: roles of evaporites in structural evolution, basin hydrology, and ore location

Selley, D, Scott, R ORCID: 0000-0001-7885-3108, Emsbo, P, Koziy, L, Hitzman, MW, Bull, SW, Duffett, M, Sabagenzi, S, Halpin, J ORCID: 0000-0002-4992-8681 and Broughton, DW 2018 , 'Structural configuration of the Central African Copperbelt: roles of evaporites in structural evolution, basin hydrology, and ore location' , Economic Geology and The Bulletin of The Society of Economic Geologists, vol. 21 , pp. 115-156 , doi: 10.5382/sp.21.07.

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Abstract

The Central African Copperbelt is the world’s premier sediment-hosted Cu province. It is contained in theKatangan basin, an intracratonic rift that records onset of growth at ~840 Ma and inversion at ~535 Ma. In theCopperbelt region, the basin has a crudely symmetrical form, with a central depocenter maximum containing~11 km of strata positioned on the northern side of the border of the Democratic Republic of Congo andZambia, and marginal condensed sequences The largest Cu ± Co ores, both stratiform and vein-controlled, are known from the periphery of the basinand transition to U-Ni-Co and Pb-Zn-Cu ores toward the depocenter maximum. Most ore types are positionedwithin a ~500-m halo to former near-basin-wide salt sheets or associated halokinetic structures, the exceptionbeing that located in extreme basin marginal positions, where primary salt was not deposited. Stratiform Cu ±Co ores occur at intrasalt (Congolese-type), subsalt (Zambian-type), and salt-marginal (Kamoa-type) positions.Bulk crush-leach fluid inclusion data from the first two of these deposit types reveal a principal association withresidual evaporitic brines. A likely signature of the ore fluids, the brines were generated during deposition of thebasin-wide salt-sheets and occupied voluminous sub and intrasalt aquifers from ~800 Ma. Associated intenseMg ± K metasomatism was restricted to these levels, indicating that capping and enclosing salt remained impermeablefor prolonged periods of the basin’s history, isolating the deep-seated aquifers from the upper part ofthe basin fill. From ~765 to 740 Ma, the salt sheets in the Congolese part of the basin were halokinetically modified. Saltwas withdrawn laterally to feed diapirs, ultimately leading to localized welding or breaching of the formerhydrological seal. At these points, deeper-level residual brines were drawn into the intrasalt stratigraphy tointeract with reducing elements and form the stratiform ores. It is probable that salt welding occurred diachronouslyacross the northern and central parts of the basin, depending upon the interplay of original salt thickness,rates and volumes of sediment supply during accumulation of salt overburden, and tectonism. The variabletiming of this fundamental change in hydrologic architecture is poorly constrained to the period of halokineticonset to the earliest stages of orogenesis; however, the geometry of the ores and associated alteration patternsdemands that mineralization preceded the characteristically complex fragmentation of the host strata. Thus,while an early orogenic timing is permissible, mineralization during the later stages of extensional basin developmentwas more likely. In situ reducing elements that host Zambian-type stratiform Cu ± Co ores were in continuous hydrologicalcommunication with subsalt aquifers, such that ore formation could have commenced from the ~800 Mabrine introduction event. The nonhalokinetic character of the salt in this region allowed the intact seal to havemaintained suprahydrostatic pore pressures, facilitating fluid circulation until late stages of basin growth andpossibly early stage orogenesis. Leachate data from ores positioned in the depocenter maximum and southern parts of the basin that underwentrelatively high grade metamorphism record mixing of residual and halite dissolution-related brines. Salt dissolution was likely triggered by emergence of diapirs or thermally and/or mechanically induced increasedpermeability of halite. While it is certain that halite dissolution occurred during and after orogenesis, c onditionsfavorable for salt dissolution may have existed locally during extension in the depocenter maximum. The permeabilityof salt increased to a point where it became the principal aquifer. The salt’s properties as an aquicludelost, originally deep-seated residual brine mixed with new phases of evaporite dissolution-related brine to produceores at middle levels of the basin fill. During the final stages of ore formation, recorded by postorogenicPb-Zn-Cu mineralization in the depocenter maximum, the salinity of fluids was dominantly derived from thedissolution of remnant bodies of salt.

Item Type: Article
Authors/Creators:Selley, D and Scott, R and Emsbo, P and Koziy, L and Hitzman, MW and Bull, SW and Duffett, M and Sabagenzi, S and Halpin, J and Broughton, DW
Keywords: Central African Copperbelt, evaporites, sediment-hosted ore
Journal or Publication Title: Economic Geology and The Bulletin of The Society of Economic Geologists
Publisher: Society of Economic Geologists, Inc.
ISSN: 1547-3112
DOI / ID Number: 10.5382/sp.21.07
Copyright Information:

© 2018 Society of Economic Geologists, Inc.

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