A New Look at the Geology of the Zambian Copperbelt
Published:January 01, 2005
David Selley, David Broughton, Robert Scott, Murray Hitzman, Stuart Bull, Ross Large, Peter McGoldrick, Mawson Croaker, Nicky Pollington, Fernando Barra, 2005. "A New Look at the Geology of the Zambian Copperbelt", One Hundredth Anniversary Volume, Jeffrey W. Hedenquist, John F. H. Thompson, Richard J. Goldfarb, Jeremy P. Richards
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The Zambian Copperbelt accounts for approximately 46 percent of the production and reserves of the Cen tral African Copperbelt, the largest and highest grade sediment-hosted stratiform copper province known on Earth. Deposits in the Zambian Copperbelt are hosted by the Neoproterozoic Katangan Supergroup, a rela tively thin (~5 km) basinal succession of predominantly marginal marine and terrestrial metasedimentary rocks that lacks significant volumes of igneous rocks. The stratigraphic architecture of the Katangan Supergroup in the Zambian Copperbelt is comparable to that of Phanerozoic rift systems. The basal portion of the sequence (Lower Roan Group) contains continental sandstones and conglomerates deposited in a series of restricted sub-basins controlled by extensional normal faults. These largely terrestrial sediments are abruptly overlain by a re gionally extensive, variably organic rich marginal marine siltstone/shale (Copperbelt Orebody Member, or “Ore Shale”) that contains the majority of ore deposits. This horizon is overlain by laterally extensive marine car bonates and finer grained clastic rocks that evolved through time into a platformal sequence of mixed carbon ate and clastic (Upper Roan Group) rocks with abundant evaporitic textures, including widespread breccias thought to record the former presence of salt, now dissolved. Rocks of the overlying Mwashia and Kundelungu groups are dominantly shallow marine in origin.
Three significant tectonic events affected the basin. Extension associated with early rifting led to the devel opment of isolated fault-controlled basins and subsequent linkage of these basins along master faults at the time of Copperbelt Orebody Member deposition. A later period of extension occurred during late Mwashia to early Kundelungu time (~765–735 Ma) and is associated with limited mafic magmatism. Basin inversion and later compressive deformation (~595–490 Ma) culminated in upper greenschist-facies metamorphism (~530 Ma) in the Zambian Copperbelt.
The majority of ore deposits in the Zambian Copperbelt occur within a 200-m stratigraphic interval centered on the rocks of the Copperbelt Orebody Member. Deposits are broadly stratiform and are grouped into argillite- (70% of ore) and arenite-hosted (30% of ore) types. The distribution, geometry , and size of deposits are fundamentally controlled by early subbasin fault architecture and the availability of both in situ and mobile reductants, the distribution of which is linked to basin structures. Argillite-hosted deposits occur within rela tively dark and locally carbonaceous siltstones and shales, suggesting the former presence of an in situ organic reductant. These deposits are laterally extensive with strike lengths up to 17 km. Arenite-hosted deposits occur in both the footwall and hanging wall of the Ore Shale and have maximum strike lengths of 5 km. They occur at sites that were geometrically favorable for mobile hydrocarbon or sour gas accumulation. Both argillite- and arenite-hosted deposits contain so-called barren gaps of weakly to unmineralized strata that are typically asso ciated with the fault-bounded shoulders of early subbasins.
Two mineralization assemblages occur in the Zambian Copperbelt. The volumetrically dominant type con sists of prefolding disseminated and lesser vein-hosted Cu-Co sulfides. The most typical sulfide assemblage in the deposits is chalcopyrite-bornite with subsidiary chalcocite and pyrite. The Zambian Copperbelt is unusual among sediment-hosted stratiform copper districts in having abundant Co and low Ag, Zn, and Pb. The Cu-Co sulfide carrollite is widespread in the district, although cobalt is present in economic quantities in only some deposits on the western side of the district. The Zambian Copperbelt also contains ubiquitous, but volumetri cally minor, Cu-U-Mo-(Au) mineralization in postfolding veins.
Cu-Co sulfides display complex textural relationships that are best explained by multistage ore formation. Diagenetic to late diagenetic mineralization is indicated by the typically nonfracture-controlled distribution of both sulfide and gangue phases, replacive textures of Cu-Co sulfides after diagenetic cements and pyrite, and an approximate 815 Ma Re-Os isochron age for sulfide precipitation at the Konkola deposit. Brines ca pable of mobilizing metals were most likely generated during development of evaporitic environments in units of the Upper Roan Group, and/or subsequent dissolution of these evaporites to form the Upper Roan Group breccias.
Late diagenetic to early orogenic mineralization is recorded by prefolding bedding-parallel veinlets and tex turally and compositionally comparable disseminated Cu-Co sulfides. An Re-Os isochron age on Cu-Co sul fides from two arenite- and one argillite-hosted deposits of 576 ± 41 Ma is consistent with early orogenic hy drocarbon or sour gas production. The minor Cu-U-Mo-(Au) mineralization event occurred following postpeak metamorphism, at approximately 500 Ma.
The Zambian Copperbelt ore province is characterized by stratigraphically and laterally widespread meta somatism that records a protracted history of basinal brine migration. Although the alteration history is com plex, it can be broadly categorized into an early Ca-Mg-SO 4, anhydrite- and dolomite-dominant stage involv ing brine reflux below the level of Upper Roan Group evaporites; a second, K-dominant stage characterized by widespread and commonly intense development of K-feldspar and locally sericite, best developed in rocks of the Lower Roan Group and associated with ore; and a third, Na-dominant stage characterized by development of albite, commonly at the expense of earlier-formed K-feldspar. Albite dominates in Upper Roan Group brec cias and Mwashia-Lower Kundelungu strata. It is also locally associated with a late Cu-U-Mo-(Au) vein event. Although none of these alteration types are direct guides to ore, they demonstrate widespread brine circula tion within the lower parts of the Katangan Supergroup.
Figures & Tables
One Hundredth Anniversary Volume
From the first issue in 1905 onward, Economic Geology has been the main publication for those who study mineral deposits; indeed, it is now difficult to imagine economic geology without Economic Geology. It is interesting to ask, therefore, Who were the farsighted people who founded the journal, and Why did they think a specialized publication devoted to mineral deposits was needed?
Let us first address the question, Who were the founders? They were the 12 men who collectivelydecided a new publication was needed, who then planned the financial structure to support the venture, and who served as the original editorial group. All were employed by, or associated with, the U.S. Geological Survey. Josiah Edward Spurr suggested the need for a journal sometime in November or December 1904. After informal discussions, nine of the founders met in the office of Waldemar Lindgren in the headquarters of the U.S. Geological Survey in Washington, D.C., on May 16, 1905, and founded the Economic Geology Publishing Company. The sole purpose of the company was the publication of a journal ‘...devoted primarily to the broad application of geologicprinciples to mineral deposits of economic value, and to the scientific description of such deposits, and particularly to the chemical, physical, and structural problems bearing on their genesis.’ Initial financing for the new company was raised by the sale of 80 shares at a cost of $25 per share.
Eight of the men at the founding meeting formed the first board of directors; Spurr was president, Frederick L. Ransome, secretary, and George O. Smith, treasurer. Other members were Arthur H. Brooks, Marius R. Campbell, Walter H. Weed, Waldemar Lindgren, and a young academic from Lehigh University in Pennsylvania, John D. Irving. Theninth man at the meeting was H. Foster Bain. Irving was appointed editor. Lindgren, Ransome, and Campbell from the U.S. Geological Survey, together with three academics, James F. Kemp of Columbia University, Heinrich Ries ofCornell University, and Charles K. Leith of the University of Wisconsin, were appointed associate editors. The initial board members, the editor, and associate editors are the people we now recognize as the founders of Economic Geology. Two others, Frank D. Adams, of McGill University in Canada, and John. W. Gregory, of Glasgow University in Scotland, were subsequently added as associate editors, and a third person, W. S. Bayley of the University of Illinois, was appointed as business editor, but