De Putter, Th. & Mees, F. 2016. ‘Re-appraisal of the world-class Paleoproterozoic manganese deposit of Kisenge (Katanga, DRC)’. 5th International Geologica Belgica Meeting. Book of abstracts, book of abstracts. 85. (PR)

Résumé de colloque

The Kisenge area, in the southwestern part of the Katanga province (Democratic Republic of the Congo), hosts a poorly documented world-class Paleoproterozoic manganese deposit, occurring in outcrops along an East-West trending 6 km-long series of small hills. The area belongs to the southern part of the Congo Craton, represented mainly by schists, gneiss, migmatites and amphibolites. The Mn ore deposits of the region have an assumed Late Paleoproterozoic age, based on geological context (youngest age of the underlying basement is ~2 Ga: pers. comm. A. Boven), on similarities with equivalent deposits in western and southern Africa, and on a single radiogenic age for a cross-cutting pegmatite body (1,853±89 Ma; Ledent et al., 1962). The primary Mn ores are rhodochrosite-dominated limestones or marbles, with varying abundance of Mn-rich garnet (spessartine), grading to non-calcareous garnetite. Within limestones/marbles, garnets often contain abundant rhodochrosite micrite inclusions, hence suggesting a relatively early formation, in a non-lithified sediment. Sterile intercalations consists of graphitic shales and mudstones. The deposits are Mn-rich (c. 50%) and contain significant quantities of accessory metals (up to 0.1% Co, Ni, Zn). The deposits are intensely folded and dip steeply to the south (70°) in the outcrop area. They are crossed by veins representing a complex hydrothermal paragenesis, with predominantly pyroxmangite (MnSiO3), tephroite (Mn2SiO4), and secondary rhodochrosite. The rhodochrosite marble displays relatively flat (Lan/Ybn~1) or hump-shaped (LaN/YbN~0.4) REY patterns, with low light-to-heavy REE ratios (PrN/YbN~0.8) and positive Ce anomalies (1.3 < Ce/Ce* < 2.3). This is compatible with sediment deposition in a stratified basin with a redoxcline, and hence of late Paleoproterozoic age (typically from c. 1.9 Ga onward). Along the surface, the deposits have undergone intense weathering, transforming the carbonate ore into supergene Mn oxide deposits, composed of cryptomelane and relatively minor lithiophorite, manganite and pyrolusite. Ar-Ar age determination of K-bearing Mn oxides (cryptomelane) has identified several stages of supergene ore development, including major episodes around 10.5 Ma, 3.6 Ma and 2.6 Ma, providing a record of Cenozoic geodynamic evolution of the region. The c. 2 Ga Kisenge Mn deposit is comparable to roughly contemporaneous (c. 1.9 to 2.2 Ga) deposits located elsewhere in Africa (Burkina Faso, Ghana, Gabon, South Africa), as well as in Brazil. Some of these deposits, including Kisenge, are located along the margins of the proto-Congo Craton, which included the Gabon and São Francisco blocks.