No abstract

The Ivindo Basement Complex (IBC) comprises the largest exposure of Meso‐ to Neoarchean rocks in the northwestern segment of the Congo Craton in the Republic of Congo. The age and petrogenetic setting of the IBC are not clear due to a lack of detailed studies. In the Souanké area, which is on the northeastern margin of the IBC, the most exposed rocks are charnockites, tonalite/granodiorite, and trondhjemite. Based on geology, geochemistry, and zircon U–Pb‐Lu–Hf isotopes, this paper constrains the petrogenesis and tectonic significance of these granitoids for the evolution of the IBC. Poor‐quality zircons did not allow for age determination and Lu–Hf isotopic characteristics for the charnockite. However, zircon U–Pb dating of the tonalite/granodiorite and trondhjemite yielded the weighted mean 207Pb/206Pb ages of 2,895 ± 9.4 Ma and 2,889 ± 9.2 to 2,897 ± 5.0 Ma, respectively. Zircon Hf isotopic (εHf[t]) values of +1.22 to +4.55 and Th/La >0.12 for the tonalite/granodiorite and trondhjemite indicate that these rocks were derived from partial melting of an older mafic reservoir (TDM2 ages range from 3,028 to 3,230 Ma), possibly triggered by mantle upwelling or plume activity. All the studied IBC granitoids are peraluminous, calcic‐alkalic, relatively sodium‐rich (Na2O/K2O > 1), and contain low Ni, Cr, and Yb contents, with strongly fractionated rare‐earth element (REE) patterns. On standard‐normalization diagrams, the IBC rocks exhibited similar distribution patterns in the trace element geochemistry as average Archean TTGs, except for high U, Nb, and Ta concentrations in the IBC trondhjemite. These high values may indicate amphibole dehydration melting (which sequesters Ta and Nb) at low‐pressure conditions at the source for the trondhjemite generation, while the other IBC rocks experienced high‐pressure melting. The 2,900–2,800 Ma tectono‐thermal event registered in the IBC from our study was also previously documented for the Ntem Complex TTGs of Cameroon (northwestern equivalent of the IBC) and elsewhere in other cratons such as in the Bação Complex (São Francisco Craton) of Brazil and, therefore, represents an important period for the Archean crustal evolution in geologic history.

The Nabeba high-grade iron deposit (Republic of the Congo) is hosted by banded iron formation (BIF) in the Ivindo Basement Complex, which lies in the northwestern part of the Congo Craton. The Nabeba BIF is intercalated with chlorite-sericite-quartz schist and comprises two facies (oxide and a carbonate-oxide). In this study, whole-rock and LA-ICP-MS magnetite geochemistry of the BIF was reported. Magnetite samples from both BIF facies had fairly similar trace element compositions except for the rare earth element plus yttrium (REE + Y) distribution patterns. The high V, Ni, Cr, and Mg contents of the magnetite in the Nabeba BIF could be ascribed to the involvement of external medium-high temperature hydrothermal fluids during their deposition in relatively reduced environment. The Post-Archean Australian Shale (PAAS)-normalized REY patterns of the Nabeba BIF magnetite were characterized by LREE depletion coupled with varying La and positive Eu anomalies. Processing of the information gathered from the geochemical signatures of magnetite and the whole-rock BIF suggested that the Nabeba BIF was formed by the mixing of predominantly anoxic seawater (99.9%) with 0.1% of high-temperature (>250 °C) hydrothermal vent fluids, similar to the formation mechanism of many Archean Algoma-type BIFs reported elsewhere in the world.

The continuous aquifers of the Congolese Cuvette basin constitute the seat of significant water tables reversibly supplying the Congo River basin as well as the water tables they drain. Consisting mainly of sandy-clayey-sandstone formations, these aquifers contain groundwater which is the main source of drinking water supply for communities in this sector of the departments Cuvette and Cuvette-Ouest. The need for water being more and more acute, these aquifers have been subjected to drilling operations in order to supply drinking water to these populations. The information obtained from these operations is data which made it possible to obtain information relating to the lithology, the hydrogeological parameters of these aquifers and information on the depth and on the lateral variations of the water table. The purpose of this present work is to contribute to the hydrogeological characterization of the aquifer of the Cuvette basin in the departments of Cuvette and Cuvette-Ouest via data from the analysis of the technical sheets of drillings carried out by the ASPERBRAS Company, collected at the Ministry of Major Works and territory planning. This characterization was made using Visual MODFLOW Flex 2015.1 software coupled with Surfer 10 software and Rock Works 17 software. The hydrogeological interpretation of the data shows that this aquifer, whose constitution is essentially made up of sands, clayey sands and clay, is continuous, porous and free. This composition gives the aquifer of Cuvette basin a heterogeneous character. In addition, a piezometric map was produced in order to indicate the direction of the water flows which turned out to be from west to east towards the lowest areas occupied by the valleys of the rivers and rivers. The conceptual model shows that the depth of the aquifer deceases considerably from the Cuvette-Ouest towards the Cuvette.