Journal of Geosciences, 60 (2015), 163–170 DOI: 10.3190/jgeosci.194 Original paper Geochemistry of Mn-bearing rocks hosted in rhyolites from Hadjer Bigli (SE Lake Chad, Cameroon Line): preliminary results Gbambié Isaac Bertrand MBOwOu1*, Nilson Francisquini BOTELHO2, Ismaïla NGOuNOuNO1 1 School of Geology and Mining Engineering, University of Ngaoundere, P.O. Box: 115 Meiganga, Cameroon; [email protected] 2 Geosciences Institute, University of Brasilia (UnB), Asa Norte, Brasilia, DF, CEP 70910-900 Brazil * Corresponding author Manganese-bearing rocks consisting of anhedral Mn- and Fe-oxyhydroxides (cryptomelane, goethite), kaolinite, tiny cerianite and zircon grains (< 10 µm), and euhedral to subeuhedral quartz crystals, are hosted within rhyolitic rocks of Hadjer Bigli volcano (SE Lake Chad). Their Ce (1328 ppm) and Zn (1563 ppm) contents are increased, with high ∑REE values (1573 ppm).The Mn-rich rocks have been shown to be one of the most important environments for adsorption of some trace elements. The high Zn contents are considered to be the result of water–rock chemical reactions at in- termediate to high temperatures. The sedimentary processes led likely to the concentration of manganese in the Lake Chad water and beneath, well before the volcanic activity. According to mineral chemistry and the whole-rock data, rhyolitic magma interacted with the near-surface manganese formations already concentrated by sedimentary processes. Keywords: manganese, trace elements, Hadjer Bigli, Lake Chad, Cameroon Line Received: March 30, 2015; accepted: August 5, 2015; handling editor: M. Štemprok 1. Introduction Cretaceous rhyolitic rocks. In many places, these bodies display tectonic control and rock-association that permit Manganese deposits can be formed by hydrothermal, their correlation to deposits generated in continental sedimentary, and supergene processes in various envi- basin. ronments (e.g. lakes, internal seas, archipelagos, shallow Lake Chad is located in central Africa, to the North marine, deep marine, as well as terrestrial). Deposits of of the Congo Craton and to the East of the West African various size, grade and origin, host oxidic manganese Craton. The rhyolites from Hadjer Bigli and those from ores (El-Hasan et al. 2008). However, more than 95 % of Hadjer el Hamis and Hadjer al Hider (formerly named all deposits exploited today are of sedimentary genesis, Hadjer Hider) occur at the northernmost termination of having originated from ancient ocean water. Manganese the Cameroon Line (Mbowou et al. 2012), which is an is generally intimately associated with iron. Both are active intraplate tectonomagmatic zone, with associated redox-sensitive elements with weakly oxidized ionic alkaline volcanism, developed on both oceanic and con- species (Fe2+, Mn2+) that are fairly soluble in water in tinental lithosphere. Oriented N30°E, it crosses the Gulf surface environments under oxygen-deficient (anaerobic) of Guinea and Cameroon for more than 2000 km, from conditions. In contrast, they are both oxidized to poorly Pagalu Island to Lake Chad (Vicat et al. 2002; Déruelle et soluble Fe3+ and Mn4+ in aerobic environments. al. 2007). According to Vicat et al. (2002), these rhyolites Various trace elements can be associated with iron can be related to a tectonomagmatic stage of the western and manganese deposits. Thus, the distribution of trace central Africa rift system. elements can be used for monitoring of the geochemical The economically most valuable manganese deposits processes that contributed to the genesis of the manga- are associated with ancient and recent weathering crusts nese deposits. The REE mobility for example, is con- (retrograde diagenesis zone), primarily in the near equa- trolled mainly by the redox potential of the environment. torial areas of South America, Africa, Australia, and Therefore the REE patterns can be used in tracing the India. Therefore, these regions are most promising for post-depositional processes (i.e. diagenesis) (De Carlo the discovery of new high-grade manganese ore deposits 1991). Polgári and Gutzmer (2012 and references therein) (Kuleshov 2011 and references therein). demonstrated the particular difficulties associated with The aim of this study is to present the first mineral identifying the exact genetic history of stratabound man- chemistry of the Mn-bearing rocks from the SE Lake ganese deposits. Chad and their geochemical data, in order to identify the Manganese-bearing rocks discovered in the Hadjer genetic processes which led to the high concentration of Bigli volcano (SE of the Lake Chad) are hosted by Late some trace elements. www.jgeosci.org Gbambié Isaac Bertrand Mbowou, Nilson Francisquini Botelho, Ismaïla Ngounouno 2. Geological setting sedimentary formations hide the Precambrian basement and the extensions of the tectonic evolution. In Chad, the The Lake Chad area is mainly characterized by the oc- Precambrian basement outcrops in five distinct massifs: currence of sedimentary units and volcanic outcrops. the Mayo Kebbi Massif in the southwest, the Tibesti The geology of the South-east (SE) of the Lake Chad is Massif in the north, the Ouaddaï Massif in the east, the composed of Quaternary-to-recent sandy-clay and sand Guera Massif in the center and the Yade Massif in the units, of aeolian and fluvio–lacustrine origin. These units south (Isseini et al. 2012). cover the oldest sedimentary formations and surround The discovery of the Mn-rich rocks in the volcanic the rhyolitic domes and necks. Three volcanoes (Fig. 1): outcrops of Hadjer Bigli is of special interest regarding the Hadjer el Hamis (418 m a.s.l.); Hadjer Bigli (308 m a.s.l.) mineral exploration and the geodynamic significance of the and Hadjer Hidjer (283 m a.s.l.), with particular rhyolitic SE Lake Chad domain. It may provide some insights into compositions (pantellerite and comendite) are located to the petrological processes which led to their genesis. The the SE of Lake Chad (Mbowou et al. 2012). Mn-rich rocks are hosted in the rhyolites from the Hadjer The eruption of the rhyolitic rocks in this domain Bigli volcano (Fig. 2), where rhyolites are mined actually has likely triggered the uplifting of the Precambrian in an open quarry. Manganese mineralization had been basement as evidenced by the positive isostatic gravity previously discovered in the Goz Beida area located to the anomalies (from 0 to +20 mGal), which are explained by south of the Ouaddaï Massif (East of Chad). It consists of the local presence of near surface heavy material over gneiss, migmatite, granitoids, quartzites, micaschists and the asthenosphere (Louis 1970). Cenozoic to Quaternary amphibolites. A manganese hydroxide layer 0.2–0.3 m thick has been described in the Goz Beida area, with a strike length of 50 m (Kasser 1995). Moreover, Vicat et al (2000) had analyzed at the University of Nancy (CRPG, France) two Mn- and Zn-rich clays, sampled around of N’Djamena in the Lake Chad Basin. The geological setting of the Mn-bearing rocks from the SE Lake Chad is character- ized by the dominance of the sedimentary rocks which were intruded by volcanic rocks. The main body of Hadjer Bigli plug (308 m a.s.l) is made up of weathered reddish brown rhyolite, which is cut by green- ish dark rhyolitic dykes with a thickness of 1 to 3 m, and by dark Mn-rich rocks. This volcanic plug of c. 15–20 m height is slightly flattened on its top and bordered by mod- erated to steep slope flanks. The blocks of rhyolites of all sizes are mainly accumulated on the flanks and in the open Fig. 1 Location of the Lake Chad (Cen- tral Africa), the Atlantic Ocean and African Cratons (a). The sector of the Kapsiki Plateau (Northern Cameroon) is indicated (b) and the position of the Mn-rich rock occurrence (c) SE of the Lake Chad (Mbowou et al. 2012). 164 Geochemistry of Mn-bearing rocks from Hadjer Bigli, SE Lake Chad 14°40’14°40' to the ZAF program by deduction from the calibrated and calculated element proportions. Whole-rock samples were crushed to fine powder in a mill equipped with two agate jars at the Instituto de Geociências, Universidade de Brasilia. Representative whole-rock major- and trace-element analyses were carried out using ICP-ES and ICP-MS methods, respec- 70°E tively, at the Acme Analytical Laboratories. Total loss on ignition (L.O.I), total carbon and sulfur were determined 12°49’12°49' gravimetrically after heating or sintering at 1000 °C. Lithium borate fusion and dilute acid digestion of a 60°E 0.2 g sample powder were employed to ensure the correct analyses of the rare earths and refractory elements. Detec- tion limits (DL) were less than 0.05 wt. % for the major 250 m elements, 5 ppm for Ba and less than 0.6 ppm for the REE and the other trace elements. The analyses of total carbon and sulfur were performed by gravimetric meth- Sandy clay and sand units Mn-rich rocks ods at the low analytical detection limits (0.02 wt. %). Host rhyolite Dykes of rhyolite 4. Results Fig. 2 Geological sketch of Hadjer Bigli volcano. quarry. The greenish dark rhyolitic dykes (1−1.8 m 4.1. Petrography and mineral chemistry thick) are subvertical (dipping 60–70°E) and oriented N–S (Ganwa et al. 2009). Their distribution (trend, 4.1.1. Host rhyolites dip, thickness and number) may have been controlled by the regional extensional stress field (Déruelle et The host rhyolites are reddish brown (Fig. 3) with partly al. 2007). The Mn-rich rocks form sparse (5−8 vol. welded tuff appearance, characterized by the occurrence %), elongated, fusiform and amoebic bodies within of scarce small vesicles (≤ 0.5 cm across). The rock the rhyolite of the Hadjer Bigli volcano, with variable consists of quartz, alkali feldspar (nearly pure sanidine thicknesses up to 2 m. Or92−95; Tab.