Petrographic and mineralogical study of the sediment-hosted Cu-Co ore deposit at Kambove West in the central part of the Katanga Copperbelt (DRC)
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Abstract:
Kambove West is a sediment-hosted Cu-Co deposit in the Neoproterozoic Mines Subgroup, which is mainly composed of dolomitic shales and dolostones. Key factors and processes responsible for high-grade mineralization were identified by studying multiple cores. The stratigraphic position of the mineralized zones adjacent to the Roches Siliceuses Cellulaires and brecciated zones are the most important factor controlling the formation of a high-grade mineralization as they most likely acted as conduits for the mineralizing fluids. The presence of organic matter, stromatolite fragments and anhydrite pseudomorphs promoted mineralization. The organic matter caused a reducing environment required for the precipitation of the sulfides. Sulfate-reducing microorganisms, which were main components of Precambrian stromatolite communities, could also have caused the necessary reduction reactions. In addition to sulfate from the pores of the sediment, anhydrite formed a sulfate source.
A paragenesis with six stages was established based on microscopic observations and cold cathodoluminescence petrography. Two major hypogene mineralization phases, which consist of pyrite, chalcopyrite, bornite, chalcocite and carrollite, formed during diagenesis and low-grade metamorphism, and were succeeded by supergene remobilization, of which chalcocite forms the dominant Cu-phase in the cementation zone. The distinction between hypo- and supergene chalcocite is based on the identification of multiple microtextures, chalcocite polymorphism, and the association with iron and other (hydro)oxides or late generations of dolomite. However, none of the features are conclusive. The combination of them is highly suggestive for a supergene remobilization.Keywords:
Chalcocite
Bornite
Hypogene
Supergene (geology)
Covellite
Anhydrite
Stromatolite
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The banded iron formation (BIF)-hosted iron ore deposits in the lower greenstone succession of the Koolyanobbing greenstone belt, 50 km north of Southern Cross in Western Australia, are a ∼200 Mt high-grade Fe (>58%) pre-mining resource and represents one of the most important iron ore districts in the Yilgarn craton. Four hypogene alteration (ore-forming) stages and one supergene upgrading event took place: (1) During ore stage 1, LREE-depleted, transition metal-enriched, Mg-Fe (±Ca) carbonates replaced quartz in BIFs. The deposit-scale alteration was most likely induced by devolatilization of sea-floor-altered, Ca-Si-depleted mafic rocks in the vicinity of the BIF during early regional (syn-D 1), very low to low-grade metamorphism and was most strongly developed on reactivated BIF-basalt contacts. (2) Ore stage 2 involved the formation of patchy magnetite ore by a syn-D 2 to-D 4 dissolution of early carbonate. Enrichment of Fe 2O 3total in magnetite iron ore was by a factor of 2 to 2.4, and compatible trace elements in magnetite, such as Ga, V, and Al, were immobile. A subdeposit-scale ferroan talc-footprint proximal to magnetite iron ore in the largest deposit (K deposit) was associated with ore stage 2 and resulted from dissolution of magnesite due to reaction with silica in the BIF under greenschist facies conditions and potentially high fluid/rock ratio. (3) Magnetite growth, during ore stage 3, forming granular magnetite-martite ore is related to a subsequent hydrothermal event, occurring locally throughout the belt, especially in D 2b faults. (4) Ore stage 4 was associated with Fe-Ca-P-(L)REE-Y-enriched hydrothermal fluids, possibly from a magmatic source such as the postmetamorphic Lake Seabrook granite that crops out about 10 km west of the Koolyanobbing deposits and at the southern margin of the greenstone belt. These Ca-enriched fluids interacted with distal metamorphosed mafic rock and influenced the BIF-ore system in a small number of deposits. They were channelled through regional D 4 faults and caused specularitedolomite- quartz alteration, resulting in Fe grades of up to 68%. (5) Supergene upgrade (ore stage 5) by (further) gangue leaching in the weathering zone was most effective in carbonate-altered BIFs and magnetite ore. This process, together with supergene martitization and goethite replacement of magnetite, led to the formation of high-grade, locally (at K deposit) high P goethite-martite ore. At Koolyanobbing, the two geochemically distinct stages of Archean carbonate alteration clearly controlled the formation of hypogene magnetite-specularite- martite-rich ore and recent supergene modification, including the further upgrade of Fe ore. © 2012 by ECONOMIC GEOLOGY.
Banded iron formation
Greenstone belt
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Yilgarn Craton
Gangue
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The ore deposits of the Central African Copperbelt formed during a multiphase mineralisation process. The basement underlying the Neoproterozoic Katanga Supergroup that hosts the ore, demonstrates the largest potential as metal source. Various ore deposits that formed during different mineralisation phases are taken as case studies, i.e. Kamoto, Luiswishi, Kambove West, Dikulushi and Kipushi (Democratic Republic of Congo, DRC). The Sr and Nd isotopic compositions of gangue carbonates associated with these deposits is determined and compared with those of rocks from several basement units, bordering or underlying the Copperbelt, to infer the metal sources. The mineralising fluid of diagenetic stratiform Cu-Co mineralisation interacted with felsic basement rocks underlying the region. The Co from these deposits is most likely derived from mafic rocks, but this is not observed in the isotopic signatures. Syn-orogenic, stratabound Cu-Co mineralisation resulted mainly from remobilisation of diagenetic sulphides. A limited, renewed contribution of metals from felsic basement rocks might be indicated by the isotope ratios in the western part of the Copperbelt, where the metamorphic grade is the lowest. The mineralising fluid of syn- and post-orogenic, vein-type mineralisations interacted with local mafic rocks, and with felsic basement or siliciclastic host rocks.
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Hypogene
Sericite
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Batholith
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1. Introduction and geological setting Sediment-hosted stratiform copper deposits are characterized by including several types of sulfide and oxide minerals that exhibit different zoning and paragenesis. Stratiform copper mineralization in the Katanga Copperbelt (Democratic Republic of Congo; Fig. 1) took place in the Neoproterozoic Katanga Supergroup, mainly in the Roan Group, but also in the Nguba and Kundelungu Groups. The Nguba Group overlies the Roan Group and consists at its base of the Grand Conglomérat, a glacial deposit. The Nguba Group is overlain by the Kundelungu Group, with the Petit Conglomérat at its base. The latter is also a diamictite deposit (Cailteux et al., 2005). Two main ore bodies are present in the Mines Subgroup in the lower part of the Roan Group, i.e. the Lower and Upper Orebody. Between both ore bodies, a barren or weakly mineralized member, the Roches Siliceuses Cellulaire (RSC), occurs. The host rock of the two ore bodies is formed by dolomitic shales, siltstones and stromatolitic dolomites (Cailteux et al., 2005). The latter shales and siltstones contain lenticular layers and nodules of pseudomorphs after anhydrite and very early diagenetic pyrite (Cailteux et al., 2005). The RSC is interpreted to originally represent massive stromatolitic dolomites with some interbedded dolomitic siltstone, and thus biogenic carbonates. Figure 1. Location of the Central African Copperbelt and of the most important ore deposits (after Cailteux et al. 2005). N
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