Geology of the Tenke-Fungurume Sediment-Hosted Strata-Bound Copper-Cobalt District, Katanga, Democratic Republic of Congo*
Wolfram Schuh, Richard A. Leveille, Isabel Fay, Robert North, 2012. "Geology of the Tenke-Fungurume Sediment-Hosted Strata-Bound Copper-Cobalt District, Katanga, Democratic Republic of Congo", 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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Tenke-Fungurume is one of the world's premier copper deposits with almost 17 billion pounds (7.7 million metric tons, Mt) of contained copper as reserves and mineralized material at a combined average grade exceeding 3 wt % Cu identified by Freeport McMoRan Copper and Gold, Inc. (Freeport). It is also the world's largest known resource of potentially mineable cobalt. Tenke-Fungurume lies within the Central African Copperbelt, which straddles the border between the Democratic Republic of Congo (DRC) and Zambia. The Central African Copperbelt is the largest copper- and cobalt-producing belt in Africa and one of the three most important copper regions in the world. The belt also contains half of the world's cobalt reserves.
The Tenke-Fungurume district consists of over 120 separate mineralized tectonic blocks, 30 of which have been drilled by Freeport to resource definition level with mineable reserves defined within 14 of those. Since 2006, Freeport has drilled approximately 350,000 m in over 2,000 diamond drill holes defining recoverable proven and probable reserves of 141 Mt at 3.00% Cu + 0.32% Co (8.4 billion lbs Cu and 860 million lbs Co) with an additional 105 Mt at 3.31% Cu and 0.307% Co as mineralized material adding a further 7.6 billion lbs of contained copper. Production is by open-pit mining and agitated leach processing at a current rate of 281 million lbs copper and 25 million lbs cobalt per year. Tenke-Fungurume is the third largest copper mine in Africa and the largest cobalt mine in the world.
All current production and reserves are from oxide ores with oxidation typically extending to depths of 80 to 150 m. Main oxide ore minerals include up to 91% malachite and up to 9% pseudomalachite and libethenite; oxide cobalt is mainly heterogenite. The oxide zone is underlain by a 50- to 200-m-thick zone of mixed ore characterized by pink cobaltoan dolomite, chrysocolla, and chalcocite. Sulfide mineralization below the mixed ore has been drilled to a 1,900-m depth. Sulfide minerals are chalcocite, bornite, carrollite, chalcopyrite, and scant pyrite.
The Central African Copperbelt coincides with the Neoproterozoic Lufilian arc, a platform sedimentary sequence of the Katanga Supergroup deposited between 880 and 570 Ma. Extension was followed by the 560 to 500 Ma compressional Lufilian orogeny that folded and thrust the ore-hosting sediments to the northeast.
The large majority of the copper-cobalt ores in Tenke-Fungurume are strata bound, hosted in the Mines Series of the Roan Group. The bulk of the ore occurs directly above a regional stratigraphic contact between red, oxidized hematitic beds below, and gray, reduced carbonate beds above. The former are reddish Roches Argileuses Talqueuses (RAT) lilas, the latter are gray algal dolomites of the Mines Series. This major redox boundary underlies copper-cobalt mineralization everywhere in the Central African Copperbelt.
Field and petrographic observations indicate that hypogene copper-cobalt mineralization took place in two main phases; an initial diagenetic event formed strata-bound, disseminated to fine laminated copper sulfides, and a second overprinting sulfide event generated crosscutting epigenetic veins related to the Lufilian orogeny. Deep supergene oxidation during the Tertiary formed the oxide orebodies.
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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.