January 2024 LIP of the Month

Mafic Magmatic Record of the Greater Congo craton

C. Djeutchou ^a,b,*, M. de Kock ^b, R.E. Ernst ^c,d, F.G. Ossa Ossa ^a,b, A. Bekker ^e,b

^a Earth Sciences Department, Khalifa University of Science and Technology, P.O. Box: 127788, Abu Dhabi, United Arab Emirates

^b Department of Geology, University of Johannesburg, Auckland Park 2006, South Africa

^c Department of Earth Sciences, Carleton University, Ottawa, ON, Canada

^d Faculty of Geology and Geography, Tomsk State University, Tomsk, Russia

^e Department of Earth & Planetary Sciences, University of California, Riverside, CA 92501, USA

Extracted and modified from: Djeutchou, C., de Kock, M., Ernst, R. E., Ossa, F. O., & Bekker, A. (2024). A review of the Intraplate Mafic Magmatic Record of the Greater Congo craton. Earth-Science Reviews, Volume 249, 104649.

For full details see the published paper

An overview of the Greater Congo Craton

The central and eastern African geology is dominated by the Greater Congo craton (GCC), which extends across nineteen countries. It consists of several distinct Archaean blocks that have been amalgamated over billions of years through tectonic processes (Fig. 1). Some geoscientists defined it as a circular continent-scale Congo River basin (now more typically referred to as the Congo basin) of central Africa with a surrounding rim of spatially discontinuous Proterozoic and Archaean exposures (Pasyanos and Nyblade, 2007; Crosby et al., 2010; Kadima et al., 2011). Other authors referred to it as the proto-Congo craton and define it as an assemblage of Archaean nuclei welded together at ca. 2.1–1.8 Ga as a result of the Eburnean collisional orogeny during the Columbia (Nuna) amalgamation and before the ca. 550 Ma assembly of Gondwana (Pinna et al., 1996; Waele et al., 2006a, 2006b, 2008; Begg et al., 2009; Fernandez-Alonso et al., 2012) that defined the GCC. The Archaean components of the GCC are subcircular in shape and variable in size (Fig. 1). They are commonly referred to as blocks, shields, nucleus, or cratons, and are generally exposed at the margins of the GCC (Begg et al., 2009; Fernandez-Alonso et al., 2011; Dirks et al., 2015; Jelsma et al., 2018). The Archaean crust generally is composed of older granulite–gneiss belts and granite–greenstone association with younger granites and late Archaean sedimentary basins, mobile belts, dykes, and layered intrusions (Milesi et al., 2006; Begg et al., 2009; Westerhof et al., 2014; Dirks et al., 2015). A large portion of the GCC was subsequently buried under the Phanerozoic sedimentary cover.

Intraplate Magmatic Events on the Greater Congo Craton

Numerous generations of mafic dyke swarms and sills cross-cut the Precambrian terranes of the GCC, but hardly any of these intrusions are yet linked to volcanic units preserved within the supracrustal volcanosedimentary successions (Djeutchou et al., 2023). Some of these magmatic events form part of previously recognized LIPs. Generally, ultramafic and mafic bodies as well as alkaline intrusions have been shown to be associated with ore deposits including nickel sulfide deposits, which are locally enriched in copper, cobalt, PGE (Platinum Group Elements), gold, and oxide- and silicate-phase nickel–cobalt laterite deposits formed by weathering of ultramafic bodies, and massive and stratiform accumulations of chromite, ilmenite, and vanadium-bearing magnetite (e.g., Peck and Huminicki, 2016). This metallogenic potential is one of the main reasons for the recently increasing interest in LIPs (Ernst et al., 2013a, 2013b; Ernst and Jowitt, 2013). Furthermore, LIPs are also associated with the supercontinent breakup, regional uplift, and hydrocarbon maturation (e.g., Ernst, 2014). A review by Djeutchou et al. (2024) highlight the gaps in current knowledge and the necessity for further studies on each magmatic event in the GCC to evaluate their metallogenic potential (Fig. 2). Additionally, this study enhances our comprehension of the geological evolution of this cratonic block, contributes to our understanding of the early formation and assembly of the African continent, and improves our understanding of global tectonic processes.

Djeutchou et al. (2024) identified 97 intraplate events in their compilation (Fig. 2), 10 of which are confirmed to be of LIP scale. Most documented events lack precise dating, and those that are precisely dated are often based on only one U-Pb date. Of these events, only 15 are Archaean in age, whereas 54 are Proterozoic, 14 are Phanerozoic, and 14 are undated. Only five of these events have associated palaeomagnetic data, but of variable quality.

Magmatic Barcode of the GCC and Possible Matchs with other Cratons

The GCC's magmatic barcode serves as a temporal fingerprint that can be compared with other terranes worldwide. If at least two magmatic events are shared, it may indicate paleogeographic proximity between the terranes. Without palaeomagnetic data combined with precise geochronology, Precambrian crustal blocks can only be considered as potential nearest neighbours to the GCC based on magmatic barcode matches. The magmatic barcode record for the GCC has been updated in Djeutchou et al. (2024), as shown in Figure 3.

The available age matches with other cratons, mostly presented in Ernst et al. (2013a, 2013b) and Ernst et al. (2021), were re-examined to identify potential neighbours of the GCC (Figs. 4 and 5). Only eleven magmatic events have been precisely dated using the U-Pb method on the GCC. However, the available geochronology on these events is insufficient to fully characterise each event in terms of area extent and age range (Djeutchou et al., 2024). The Kunene-Kibaran event is the only event with multiple dated units. The magmatic events that are less precisely dated or undated require further investigation. Many LIPs have been precisely dated and exhibit a single pulse lasting only a few million years, while others represent multiple short pulses spanning up to a few tens of millions of years (Bryan and Ernst, 2008; Ernst, 2014; Kasbohm et al., 2021; Ernst et al., 2021). For a 1770 Ma event, the age interval considered for the provisional barcode match among cratons could extend from 1745 Ma to 1795 Ma.

Metallogenic Potential

It has been shown that LIPs serve as the primary host for or are associated with the significant number of types of mineral deposits (Ernst and Jowitt, 2013). This is illustrated by the orthomagmatic Ni-Cu-PGE sulfide, Fe-Ti-V oxide, and Cr deposits associated with LIP events, where mineral deposits are a direct result of mafic-ultramafic magmatism (Ernst and Jowitt, 2013). Due to the genetic connection between LIPs and some kimberlites and carbonatites, rare earth elements (REEs), Nb, Ta, and diamonds, which are associated with these rocks, can also be directly linked with LIP emplacement. Ernst and Jowitt (2013) also highlighted that LIPs can have an important influence on ore genesis in hydrothermal systems. In addition, tropical weathering of mafic-ultramafic LIP units forms economically important Ni–Co laterites and Al bauxites, and weathering of associated carbonatites yields Nb, Ta, and REE laterites. Although the GCC has enormous economic potential (Goossens, 2007, 2009, 2013), it remains understudied and poorly understood, unlike the Kalahari craton in southern Africa. This lack of knowledge on the GCC geology is a major impediment to its metallogenic understanding and prospecting. Consequently, almost all magmatic events recorded on the GCC require future work combining geochronology, palaeomagnetism, and igneous geochemistry to reveal their unrecognized economic potential.

Conclusion

Many gaps in the knowledge of the GCC mafic record remain. Further multidisciplinary studies combining the latest developments in U–Pb geochronology and either paleomagnetism or igneous geochemistry, with ongoing mapping and sampling of relevant igneous rocks, will greatly broaden our understanding of this research. Consequently, the filling in of the LIP barcode record of the GCC will be more accurate. This would provide further constraints on palaeocontinent reconstructions, which could potentially lead to the discovery of new ore deposits.

References