Diagenetic Ore-forming Processes at Kamoto, Katanga, Republic of the Congo

Abstract

At Kamoto and in other Katangan deposits, copper and cobalt occur as sulfides (mostly digenite, chalcocite, bornite and carrollite) in two stratiform orebodies. The host rock is dolostone, chert and shale. There is no metamorphism. Mineralization was emplaced before major tectonic deformation.The two stratiform orebodies are separated by a barren interval. The lower one rests a few feet above an erosional surface which developed over consolidated red dolostone beds completely devoid of sulfides. Above the upper orebody, a sulfide fraction is present, consisting mostly of pyrite with some chalcopyrite.The mineralized sedimentary rocks present some of the features commonly observed in tidal-flat sediments. However, they have undergone a complex sequence of diagenetic transformations. Several authigenic (or partly authigenic) minerals have crystallized: dolomite, perhaps in part from hydromagnesite + CaC03; magnesite, from the remaining hydromagnesite; quartz; chlorite and other phyllosilicates; colorless tourmaline; pyrite; other sulfides: chalcopyrite, bornite, digenite, chalcocite, carrollite, etc ... New evidence is presented to show that this diagenesis took place at first in an environment devoid of copper and cobalt and then proceeded while the metals were brought in from an outside source.The features described may have resulted from a reaction between a hypersaline brine (with high pH and high Eh) flowing through the lower part of the Kamoto Dolostone, and a modified connate water (reducing and less alkaline because of abundant organic matter) present in the upper part.

Reactive transport modelling of ore mineral zoning and the paragenesis of copper sulfides in sediment-hosted stratiform ore deposits, the Katanga Copperbelt (DRC)

“…The model simulates the often observed replacement of pyrite by chalcopyrite as a first process during mineralization (cf. Bartholomé et al, 1972). This replacement of pyrite by chalcopyrite is followed by the replacement of chalcopyrite by bornite, bornite by digenite and finally digenite by chalcocite (El Desouky et al, 2010).…”

“…The model nicely simulates the results of Van Langendonck et al (2013), who described a vertical zoning of chalcocite to bornite, chalcopyrite and pyrite in the ore at Kambove West. The replacement of anhydrite by the sulfides, which is a characteristic feature of the stratiform ores both in the DRC and Zambia (Bartholomé et al, 1972;Annels, 1974;Cailteux, 1978), is reflected by the complete disappearance of anhydrite in the sulfide-mineralized parts.…”

“…This dolomite is nonhttp://dx.doi.org/10.20341/gb.2016.005 luminescent under cathodoluminescence petrography (Dewaele et al, 2006). Authigenic quartz is intergrown with the sulfides and precipitated in rocks of the Lower and Upper Orebody and in the low-grade intermediate zone (Oosterbosch & Schuiling, 1951;Bartholomé et al, 1972;Cailteux, 1994). This mineralization phase occurred around 800 Ma at shallow to intermediate burial depth (Muchez et al, 2015).…”

“…Paragenetic studies indicate that pyrite, often present as framboids, was the first sulfide phase to precipitate with a lateral and vertical distribution well beyond the ore deposits (Bartholomé, 1963). This early pyrite was replaced by chalcopyrite (Bartholomé et al, 1972). At Kamoto and Luiswishi (Katanga Copperbelt), this replacement is followed by a general succession of chalcopyrite, bornitecarrollite and chalcocite (El Desouky et al, 2010).…”

Alteration and ore distribution in the Proterozoic Mines Series, Tenke-Fungurume Cu–Co district, Democratic Republic of Congo

“…These strata host most of the DRC's (Democratic Republic of Congo) large, high-grade copper and cobalt deposits, but determining their timing and their lithological relationships to ore and to later events has challenged researchers since the 1930s. The still-ongoing, 80-year debate over the origin of the ores (e.g., Bateman 1930;Darnley 1960;Mendelsohn 1961;Bartholomé et al 1972;Annels 1974;Selley et al 2005) is one example of the capacity of the Mines Series to perplex and intrigue geologists who study its deposits.…”