Supergene manganese ore records 75 Myr-long Campanian to Pleistocene geodynamic evolution and weathering history of the Central African Great Lakes Region – Tectonics drives, climate assists
Graphical abstract
Introduction
The African continent hosts several large manganese deposits, many of them capped with a supergene cover (e.g. Beukes et al., 2016). Primary ores are mostly of Paleoproterozoic age, whereas the supergene cover formed much more recently, predominantly during the Cenozoic. Available 40Ar/39Ar ages for supergene manganese ores vary strongly between different parts of Africa, with Paleocene to Pliocene ores in West Africa (Hénocque et al., 1998; Colin et al., 2005; Beauvais et al., 2008) and younger, Eocene to Pliocene, ores in southern Africa (Gutzmer et al., 2012). Recent investigation on the ‘Thrust Manganese’ ore in the Kalahari Manganese Fields, which is located at the contact with the Kalahari unconformity, documents the precipitation of supergene apatite formed in the ore at 77 ± 7.5 Ma (Vafeas et al., 2018a, Vafeas et al., 2018b).
In Central Africa, the major Paleoproterozoic Kisenge deposit, in the western part of the Lualaba Province (formerly Katanga) of the Democratic Republic of the Congo (DRC), has yielded Mio-Pliocene ages for the supergene ores (Decrée et al., 2010; De Putter et al., 2015). Some other deposits occur in other parts of the former Katanga Province, as well as in the former Kasai Province. The geochronological study of their associated supergene ores is the subject of the present study. It considerably broadens the range of ages first obtained for the Kisenge Mn ore. They allow constraining the local geodynamics which is marked, from the Oligocene onwards, by the opening of the western branch of the East African Rift System (EARS) (Pik et al., 2008; Roberts et al., 2012; De Putter et al., 2015). At a continental scale, ages of supergene ores provide relevant constraints to understand the dynamics of cratonic denudation, in a continent where cratonic terranes cover huge areas (Beauvais and Chardon, 2013). As such process relates to 1st order factors, the correlation of ages of supergene ores at continental scale may therefore open a window on the dynamics of Africa under tectonic and/or climatic forcing (a.o. Sahagian, 1988; Burke and Gunnell, 2008; Begg et al., 2009; Beauvais and Chardon, 2013; Venancio da Silva et al., 2019). It is the purpose of the present study to bring additional age constraints on the manganese deposits of Mwene-Ditu and Kasekelesa.
Section snippets
Geological and geodynamic context
The substrate of the western part of the former Katanga Province and the southern part of the neighbouring former Kasai province, south-eastern DRC, consists of a poorly documented granitic-gneissic basement known as the Kasai Block, of Archean and Paleoproterozoic age (Cahen and Lepersonne, 1967; Lepersonne, 1974; Boven et al., 2011). Paleoproterozoic sediments, which unconformably overlie this granito-gneissic basement, locally include manganese deposits, most notably as primary carbonate ore
40Ar/39Ar method
Two batches of Mn oxide samples from Kasekelesa (RGM series) and Mwene-Ditu, Kasai (RG series), were analyzed by the 40Ar/39Ar method in step-heating using a CO2 laser probe coupled with a MAP 215 mass spectrometer. The procedure was described by Ruffet et al., 1991, Ruffet et al., 1995, Ruffet et al., 1997.
Irradiation of samples was performed at Mac Master Nuclear Reactor (Hamilton, Ontario, Canada) in the 8C facility. It lasted 50.967 h with a global efficiency (J/h) of 4.540 × 10-5 h-1. The
40Ar/39Ar data processing
All analyses were duplicated and sample RGM 7041 was analyzed three times due to the complexity of the observed results. This complexity of 40Ar/39Ar results is rather common, both because of the complex nature of Mn oxides and the often long and frequently polyphase history they record (e.g. Vasconcelos et al., 1992, Vasconcelos et al., 1994; Ruffet et al., 1996; Beauvais et al., 2008).
As recalled in detail by De Putter et al. (2015), even though cryptomelane (K(Mn4+7Mn3+)O16), which is the
Results
The results for Kasekelesa and Mwene-Ditu will be presented sequentially because their records are quite different. Validated ages for both sites are reported in probability density plots in Fig. 2 (to the left, Kasekelesa; to the right, Mwene-Ditu). In this study, sample RGM 7041 is undoubtedly emblematic of the difficulties encountered in the processing of the 40Ar/39Ar experiments (Fig. 3). Three grains were analyzed because of the complexity of the obtained age spectra and the importance of
Background 40Ar/39Ar ages in the study area
Supergene manganese oxide deposits that developed at the expense of the primary Mn-carbonate ores at Kisenge-Kamata have previously been dated through 40Ar/39Ar analysis (De Putter et al., 2015). Based on those results, a sequence of uplift events has been inferred to have occurred during the Late Miocene (at 10.5 Ma; 3.6 Ma; 2.6 Ma), with evidence for some earlier Miocene events (at c. ≥19.2 Ma, 15.7 Ma, 14.2 Ma and 13.6 Ma) (De Putter et al., 2015). Pre-Miocene events were not formally
Conclusion
The timing of supergene manganese ore formation in the south-east of the D.R. Congo, as recorded by 40Ar/39Ar ages, has implications for understanding the tectono-climatic evolution of a vast part of Africa, at a local to continent-wide scale.
At the local scale, the ages document the formation of supergene Mn ores, most likely synchronous with the development of other supergene ores (Cu, Co) and residual ores (Fe, Al), on a stable uplifted substrate which was exposed to the atmosphere (
CRediT authorship contribution statement
Thierry De Putter: Writing - original draft, Writing - review & editing, Methodology, Investigation. Gilles Ruffet: Writing - original draft, Writing - review & editing, Investigation, Formal analysis.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study is a contribution to the Paleurafrica research project (BR/121/A3/Paleurafrica) of the Belgian Science Policy Office (BELSPO). We thank Pr. Dr O. Friedrich for providing us with his δ18O dataset. Finally, we are grateful to P.M. Vasconcelos and an anonymous reviewer for their constructive comments. We would like to thank Prof. Dr. Oliver Friedrich for providing us with his δ18
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