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Fig. 3. Kakanda-Nord: distribution of Cu and Co average concentrations on cross-sections from X: 0 to X: 1600 (LOB = lower orebody; UOB = upper orebody).
Some Cu–(few Co) primary sulphide mineralisations occur in the Mwashya Subgroup in Congo, and also are stratigraphically controlled in dolomites of the Lower Mwashya.
The Congo-type stratiform deposits stretch from Kolwezi up to Kimpe (Fig. 2) and are generally characterized by two major Cu–Co orebodies, the “lower” and “upper” orebodies, totalling 15–55 m cumulative thickness (average: 20–25 m). The mineralisation is hosted in a transgressive supratidal to subtidal sedimentary sequence deposited under quiet, shallow-water conditions (Bartholomé et al., 1972, Cailteux, 1978a, Cailteux, 1983 and Cailteux, 1994). The host rocks contain blebs, nodules and lenticular beds of dolomite–quartz pseudomorphs after anhydrite and gypsum, and high contents of Mg, Ba, Sr, Li, B, Br can be linked to the deposition of sediments under saline evaporitic conditions (Bartholomé et al., 1972, Katekesha, 1975, Cailteux, 1978a, Cailteux, 1983, Cailteux, 1994 and Moine et al., 1986).
The lower orebody host-rocks include (Table 2): (1) a massive chloritic–dolomitic siltite known as Grey R.A.T. (“Roches Argilo-Talqueuses”); (2) a fine-grained stratified dolostone (D.Strat. “Dolomie Stratifiée”); (3) silicified-stromatolitic-dolomites forming laminites alternating with thin chloritic–dolomitic silty beds (R.S.F. “Roches Siliceuses Feuilletées”). The Upper Orebody host-rocks include (Table 2): (1) the basal Dolomitic Shales (S.D.B., “Shales Dolomitiques de Base” also called S.D.1a); (2) an overlying coarse grained impure dolostone (B.O.M.Z., “Black Ore Mineralised Zone” also called S.D.1b) which is sometimes missing in the succession (e.g. in the Kambove area). A generally “barren” reef-type stromatolitic dolomite (R.S.C., “Roches Siliceuses Cellulaires”) occurs between the two orebodies. Ores are known on 0.4–1.0 m thickness along the contact between this reef dolomite and both lower and upper orebodies. The chloritic-silty-dolomitic lenses or layers locally interbedded within the R.S.C. are also mineralised (e.g. Kamoto). In some deposits (e.g. Kambove-Ouest), the primary stratiform mineralisation extends to the overlying carbonaceous dolomitic shales S.D.2a, up to the base of the S.D.2b. The organic matter content is variable, generally low, although local high contents have led to the development of black shales and dolomites in R.S.F.-R.S.C.-S.D.B units (Cailteux, 1983).
The Congo-type mineralised succession is very regular along strike (Fig. 2), showing the same lithological succession for >350 km, from Kolwezi (Demesmaeker et al., 1963, François, 1973 and Katekesha, 1975), to Tenke-Fungurume (Oosterbosch, 1950 and Oosterbosch, 1951), Kambove-Kakanda (Cailteux, 1978a and Cailteux, 1983), Kabolela (Lefebvre, 1976a and Lefebvre, 1976b), Etoile (Lefebvre and Cailteux, 1975) and Lubembe (Lefebvre and Tshiauka, 1986 and Tshiauka et al., 1995). However, there is a clear across-strike lithofacies variation marking a progressive evolution from more near-shore (north) to more reefal (south) environments (François, 1973, François, 1974, Lefebvre, 1979, Cailteux, 1978a, Cailteux, 1978b, Cailteux, 1983 and Cailteux, 1994). This palaeo-environmental variation seems to correlate with different copper–cobalt grades in the rocks (François, 1973 and François, 1974, and details below). The northern (present coordinates) near-shore sequences (“Long” and “Kilamusembu” facies) are characterized by the absence of stromatolites, the occurrence of dolomites and arenites in the Dolomitic Shales Formation and of arenites in the Kambove Formation. In these two sequences, the lithostratigraphic units usually hosting the orebodies are barren or poorly mineralised (e.g. Dipeta Syncline between Tenke and Fungurume), except in the Tenke deposit. The Kilamusembu facies occurs only in the Kolwezi area and represents a transitional facies between Long and Musonoï facies. The southern sequences (“Musonoï” and “Kalumbwe” facies) are marked by: (a) clasts of stromatolites; (b) stromatolites in R.S.C.; (c) lack of arenites in Dolomitic Shales and Kambove Formations. There are no dolomites in the Kalumbwe facies Dolomitic Shales Formation (e.g. Kakanda-Nord). This sequence hosts the most important copper–cobalt deposits (e.g. Kamoto, Fungurume), with only a few barren or poorly mineralised zones. The southernmost reef sequence (“Menda” and “Luishia” facies) is marked by algal bioherms in R.S.C. and in the Kambove Formation. The lithostratigraphic units usually hosting the orebodies are barren or poorly to well mineralised (e.g. Kambove-Ouest, Luishia, Luiswishi).
Sub-economic orebodies (generally <1 wt.% Cu) and small economic deposits (locally >2 wt.% Cu) occur in dark-grey to black carbonaceous metapelites forming the S.D.2d and 3b (Fig. 4 and Fig. 5; Table 2). However, the metals in these units are strictly bound to thin organic matter-rich horizons indicating deposition under strong reducing conditions.
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Fig. 4. Cross-section X: 90 E through the Kambove-Ouest deposit and Roan breccia below the oxidized zone (modified from Cailteux, 1983); KTO = Kamoto Formation, SD = Dolomitic Shale Formation, KVE = Kambove Formation; R.A.T. = Roches Argilo-Talqueuses.