The Central African Copperbelt: Diverse Stratigraphic, Structural, and Temporal Settings in the World's Largest Sedimentary Copper District
Murray W. Hrtzman, David Broughton, David Selley, Jon Woodhead, David Wood, Stuart Bull, 2012. "The Central African Copperbelt: Diverse Stratigraphic, Structural, and Temporal Settings in the World's Largest Sedimentary Copper District", Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe, Jeffrey W. Hedenquist, Michael Harris, Francisco Camus
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The Central African Copperbelt, including the Zambian Copperbelt, Congolese Copperbelt, and deposits in the North West Province of Zambia, is the world's largest and highest-grade sedimentary copper province, with approximately 200 Mt of contained copper and the world's largest cobalt reserves. It is hosted in Neoproterozoic metasedimentary rocks of the Katangan Supergroup (∼880 and ∼600 Ma) deposited in a series of intra-continental rift basins with abundant evaporite deposits. Early rift-stage continental rocks were overlain by a sequence of mixed evaporitic carbonate and clastic rocks, followed by a second period of renewed rift-stage clastic and mafic rocks. Widespread glacial and postglacial deposits covered this lower part of the basinal sequence, and mark an uppermost limit to the distribution of major copper deposits. Subsequent depositon of relatively monotonous, nonevaporitic basin fill clastic and lesser carbonate rocks preceded basin inversion during the Pan-African (∼590–500 Ma) Lufilian orogeny.
The Copperbelt contains copper deposits in a range of rock units at a number of different stratigraphic levels. These deposits display differing styles and textures of mineralization and alteration types. Deposits may contain either or both disseminated, generally fine-grained sulfides and vein-hosted, generally coarse-grained sulfides. Nevertheless, there are shared characteristics among most deposits. Deposits are hosted at stratigraphic or structural redox boundaries. Where deposits occur in the stratigraphically lowermost reduced rocks, overlying reduced or favorable rocks generally were not mineralized. Although redox was a fundamental control for mineralization, the most carbonaceous rocks within an ore horizon are commonly not economically mineralized. Ore sulfide zonation within deposits occurs on multiple scales, with complexity of zoning broadly related to the complexity of the host-rock sequence. Macrostructural controls on deposit position suggest that extensional faults were important in controlling fluid flow, either directly or indirectly through influence on sedimentary and probably diagenetic facies variation. The stratigraphic section within which the deposits are located was affected by regional potassic, magnesian, silicic, and/or sodic alteration controlled partly by lithol-ogy and indicative of the passage of basinal brines.
Mineralization in the Copperbelt appears to have occurred over a protracted period that spanned diagenesis, basin inversion, and metamorphism. This attests to the longevity of ore-forming brines resident within the Katangan basin and at least the upper part of its basement. The near-surface portions of deposits throughout the Central African Copperbelt have undergone oxidation and supergene enrichment and such enrichment has been important in upgrading the copper tenor of many deposits.
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Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe
It has been recognized for the past century that copper deposits, in common with those of many other metals, are heterogeneously concentrated in Earth’s upper crust, resulting in areally restricted copper provinces that were generated during several discrete metallogenic epochs over time intervals of up to several hundred million years. Various segments of circum-Pacific magmatic arcs, for example, have total contained copper contents that differ by two orders of magnitude. Each metallogenic epoch introduced its own deposit type(s), of which porphyry copper (and related skarn), followed by sediment-hosted stratiform copper and then iron oxide copper-gold (IOCG), are globally preeminent. Nonetheless, genesis of the copper provinces remains somewhat enigmatic and a topic of ongoing debate.
A variety of deposit-scale geometric and geologic features and factors strongly influence the size and/or grade of porphyry copper, sediment-hosted stratiform copper, and/or IOCG deposits. For example, development of major porphyry copper deposits/districts is favored by the presence of clustered alteration-mineralization centers, mafic or massive carbonate host rocks, voluminous magmatic-hydrothermal breccias, low sulfidation-state core zones conducive to copper deposition as bornite ± digenite, hypogene and supergene sulfide enrichment, and mineralized skarn formation, coupled with lack of serious dilution by late, low-grade porphyry intrusions and breccias. Furthermore, the copper endowment of all deposit types undoubtedly benefits from optimization of the ore-forming processes involved.
Tectonic setting also plays a fundamental role in copper metallogeny. Contractional tectonomagmatic belts, created by flat-slab subduction or, less commonly, arc-continent collision and characterized by crustal thickening and high rates of uplift and exhumation, appear to host most large, high-grade hypogene porphyry copper deposits. Such mature arc crust also undergoes mafic magma input during porphyry copper formation. The premier sediment-hosted stratiform copper provinces were formed in cratonic or hinterland extensional sedimentary basins that subsequently underwent tectonic inversion. The IOCG deposits were generated in association with extension/transtension and felsic intrusions, the latter apparently triggered by deep-seated mafic magmas in either intracratonic or subduction settings. The radically different exhumation rates characteristic of these various tectonic settings account well for the secular distribution of copper deposit types, in particular the youthfulness of most porphyry relative to sediment-hosted stratiform and IOCG deposits. Notwithstanding the importance of these deposit-scale geologic, regional tectonic, and erosion-rate criteria for effective copper deposit formation and preservation, they seem inadequate to explain the localization of premier copper provinces, such as the central Andes, southwestern North America, and Central African Copperbelt, in which different deposit types were generated during several discrete epochs. By the same token, the paucity of copper mineralization in some apparently similar geologic settings elsewhere also remains unexplained.
It is proposed here that major copper provinces occur where restricted segments of the lithosphere were predisposed to upper-crustal copper concentration throughout long intervals of Earth history. This predisposition was most likely gained during oxidation and copper introduction by subduction-derived fluids, containing metals and volatiles extracted from hydrated basalts and sediments in downgoing slabs. As a result, superjacent lithospheric mantle and lowermost crust were metasomatized as well as gaining cupriferous sulfide-bearing cumulates during magmatic differentiation—processes that rendered them fertile for tapping during subsequent subduction-or, uncommonly, intraplate extension-related magmatic events to generate porphyry copper and IOCG districts or belts. The fertile lithosphere beneath some accretionary orogens became incorporated during earlier collisional events, commonly during Precambrian times. Relatively oxidized crustal profiles—as opposed to those dominated by reduced, sedimentary material—are also required for effective formation of all major copper deposits. Large sedimentary basins underlain by or adjoining oxidized and potentially copper-anomalous crust and filled initially by immature redbed strata containing magmatic arc-derived detritus provide optimal sites for large-scale, sediment-hosted stratiform copper mineralization. Translithospheric fault zones, acting as giant plumbing systems, commonly played a key role in localizing all types of major copper deposits, districts, and belts. These proposals address the long-debated concept of metal inheritance in terms of the fundamental role played by subduction-metasomatized mantle lithosphere and lowermost crust in global copper metallogeny.