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GEOREF RECORD

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

David Selley, Robert Scott, Poul Emsbo, Lyudmyla Koziy, Murray W. Hitzman, Sturt W. Bull, Mark Duffett, Stanislas Sebagenzi, Jacqueline Halpin and David W. Broughton
Structural configuration of the Central African Copperbelt; roles of evaporites in structural evolution, basin hydrology, and ore location (in Metals, minerals, and society, Antonio M. Arribas (editor) and Jeffrey L. Mauk (editor))
Special Publication (Society of Economic Geologists (U. S.)) (January 2018) 21: 115-156

Abstract

The Central African Copperbelt is the world's premier sediment-hosted Cu province. It is contained in the Katangan basin, an intracratonic rift that records onset of growth at approximately 840 Ma and inversion at approximately 535 Ma. In the Copperbelt region, the basin has a crudely symmetrical form, with a central depocenter maximum containing approximately 11 km of strata positioned on the northern side of the border of the Democratic Republic of Congo and Zambia, and marginal condensed sequences <2 km in thickness. This fundamental extensional geometry was preserved through orogenesis, although complex configurations related to halokinesis are prevalent in central and northern parts of the basin, whereas to the south, relatively high-grade metamorphism occurred as a result of basement-involved thrusting and burial. The largest Cu+ or -Co ores, both stratiform and vein-controlled, are known from the periphery of the basin and transition to U-Ni-Co and Pb-Zn-Cu ores toward the depocenter maximum. Most ore types are positioned within a approximately 500-m halo to former near-basin-wide salt sheets or associated halokinetic structures, the exception being that located in extreme basin marginal positions, where primary salt was not deposited. Stratiform Cu+ or -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 with residual evaporitic brines. A likely signature of the ore fluids, the brines were generated during deposition of the basin-wide salt-sheets and occupied voluminous sub and intrasalt aquifers from approximately 800 Ma. Associated intense Mg+ or -K metasomatism was restricted to these levels, indicating that capping and enclosing salt remained impermeable for prolonged periods of the basin's history, isolating the deep-seated aquifers from the upper part of the basin fill. From approximately 765 to 740 Ma, the salt sheets in the Congolese part of the basin were halokinetically modified. Salt was withdrawn laterally to feed diapirs, ultimately leading to localized welding or breaching of the former hydrological seal. At these points, deeper-level residual brines were drawn into the intrasalt stratigraphy to interact with reducing elements and form the stratiform ores. It is probable that salt welding occurred diachronously across 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 variable timing of this fundamental change in hydrologic architecture is poorly constrained to the period of halokinetic onset to the earliest stages of orogenesis; however, the geometry of the ores and associated alteration patterns demands 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 development was more likely. In situ reducing elements that host Zambian-type stratiform Cu+ or -Co ores were in continuous hydrological communication with subsalt aquifers, such that ore formation could have commenced from the approximately 800 Ma brine introduction event. The nonhalokinetic character of the salt in this region allowed the intact seal to have maintained suprahydrostatic pore pressures, facilitating fluid circulation until late stages of basin growth and possibly early stage orogenesis. Leachate data from ores positioned in the depocenter maximum and southern parts of the basin that underwent relatively 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 increased permeability of halite. While it is certain that halite dissolution occurred during and after orogenesis, conditions favorable for salt dissolution may have existed locally during extension in the depocenter maximum. The permeability of salt increased to a point where it became the principal aquifer. The salt's properties as an aquiclude lost, originally deep-seated residual brine mixed with new phases of evaporite dissolution-related brine to produce ores at middle levels of the basin fill. During the final stages of ore formation, recorded by postorogenic Pb-Zn-Cu mineralization in the depocenter maximum, the salinity of fluids was dominantly derived from the dissolution of remnant bodies of salt.


ISSN: 1547-3112
Serial Title: Special Publication (Society of Economic Geologists (U. S.))
Serial Volume: 21
Title: Structural configuration of the Central African Copperbelt; roles of evaporites in structural evolution, basin hydrology, and ore location
Title: Metals, minerals, and society
Affiliation: University of Tasmania, ARC Centre for Excellence in Ore Deposits, Hobart, Tasmania, United States
Pages: 115-156
Published: 20180101
Text Language: English
Publisher: Society of Economic Geologists, Littleton, CO, United States
ISBN: 9781629496405
References: 97
Accession Number: 2020-016594
Categories: Economic geology, geology of ore depositsStructural geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., geol. sketch maps, 1 table, strat. col.
S23°00'00" - S10°00'00", E26°00'00" - E28°00'00"
Secondary Affiliation: U. S. Geological Survey, USA, United StatesUniversity College Dublin, IRL, IrelandMineral Resources Tasmania, AUS, AustraliaUniversity of Lubumbashi, COD, Congo, the Democratic Republic ofIvanhoe mines, CAN, Canada
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2020, American Geosciences Institute.
Update Code: 202011
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