The town of Okondja is the capital of the Sébé-Bricolo department, located north of Franceville, in the Haut-Ogooué province. It is accessible by road from Libreville over a distance of approximately 700 km, as well as from Franceville over a distance of 150 km and from Lastourville over a distance of 130 km.

The population density is low and mainly concentrated in the departmental capital. The rest of the population lives in villages located along the four main roads that 3 connect Okondja to Franceville via Akiéni and Andjoho, on the Okondja-Lastourville road and on the Okondja-Makokou road [1][2]. Figure 1 shows the localisation of Okondja area in the Franceville Basin.

Figure 1.Location of the Okondja area in the Francevillian basin.

In the Okondja region, as in the rest of the Haut-Ogooué province, the climate is equatorial, with two rainy seasons alternating with two dry seasons of similar intensity. Average annual rainfall varies from 1600 to 1800 mm. The average daily temperature is 24.6˚C year-round, reaching a maximum of 29˚C and a minimum of 21˚C [1][2]. The lowest monthly rainfall is recorded from June to September. The longdry season (June to September) is well-defined and accompanied by relatively low temperatures, averaging 23˚C [1][2]. This is the most favorable time for fieldwork.

In the mining province of Haut-Ogooué, the Franceville Basin is composed of four sub-basins: the Booué Basin, the Lastoursville Basin, the Franceville Basin, and the Okondja Basin [3]. Stratigraphically, the sedimentary thickness of the Booué Basinvaries between 200 m and 300 m, while that of the Lastoursville Basin is on the order of 400 m to 600 m. The sedimentary thicknesses of the Franceville Basin vary from 2500 m to 3000 m, and the Okondja Basin has sedimentary thicknesses of nearly 4000m [1][4]. Lithostratigraphically, the Franceville Basin is composed of five successive units (or groups) [3][5][6]:

1) Francevillian A is essentially composed of coarse detrital rocks (quartz-arenites, more or less conglomeratic);

2) Francevillian B reflects the transition to finer sedimentation, predominantly pelitic to ampelitic, and locally sandstone to conglomeratic and/or dolomitic. This sedimentation is accompanied by volcanism of varying extent and expression depending on the area;

3) Francevillian C is characterized by the presence of jasper layers several meters thick, covering a very wide geographical area. Its lithological characteristics and ubiquity make it a key lithostratigraphic marker;

4) Francevillian D is quite similar to Francevillian B; essentially pelitic to ampelitic, but with a strong volcanic imprint (tuffs, lavas, volcanosedimentary facies);

5) Francevillian E marks a return to predominantly sandstone sedimentation, but with recurrences of fine-grained facies (pelites).

Francevillian B is very well developed in the Franceville and Okondja basins, but is almost absent from less subsiding areas, such as the Lastoursville shoal or the Booué Basin [3].

The FB Formation exhibits two sedimentary evolutions from base to top, leading to its division into two subformations, FB1 and FB2 [3][5][7]:

1) The FB1 Formation, whose base is primarily composed of fine sediments such as black claystones rich in organic matter and pelites, alternating with brecciated, carbonated sandstone layers.

2) The FB2 Formation, approximately 100 m thick, is characterized by beds of fine and massive Poubara sandstone, reaching several tens of meters in thickness, highly silicified, sometimes graded with trough bedding, and the presence of hummocky cross-stratification (HCS).

The exploration permit area consists of steep hills and plateaus dissected by a hydrographic network of streams that drain into the Sébé River, a right-bank tributary of the Ogooué and the main collector of the permit area, along with its tributary, the Lebaye River, which gave its name to one of the six mineralized plateaus within the permit. All the manganese deposits are located in the Sébé basin. The Lebaye, flowing from north to south, crosses the area perpendicularly and divides the plateau that bears its name into two almost equal parts. In terms of relief, the topography shows variations in altitude from 300 to 470 m, with the NW and SW hills being the highest points on the plateau. The Lebaye Plateau, in particular, consists of plateaus and steephills traversed by small streams [2]. Figure 2 shows the stratigraphic formations of the Okondja geology.

Figure 2.Stratigraphic formations of the Okondja geology.

The area of mining exploration permit No. G2-569, known as “Okondja Manganese”, is delimited in the form of a polygon. This exploration permit covers an area of 747.2km2 and contains six plateaus mineralized in Mn (see Figure 3).

Figure 3.Position of the 6 manganese prospects of the GMF permit including the Lebaye plateau in the center and in blue.

Observation of the profiles at the Lebaye site, from top to bottom, revealed that: (1) the manganese-bearing pisolite horizon (PIS), covered by altered clay formations (argillites) and vegetation (COV), with a thickness varying between 1 and 7 m, is considered barren; (2) the transition horizon (ZTR), corresponding to a mixture of pisolitic and platy materials with a ferruginous clay matrix, constitutes the top of the FB1 formation; (3) the platy ore (PLA), including the BLO, MAS, and ZTR types, is composed of black pelites, intercalated sandstones, banded pelites, or bedded dolomites, with alternating black shales and black sandstones, and characterized by massive sandstones or Poubara sandstones. All these elements constitute the main phase of the run-of-mine materials of the exploitable mineral resource. (4) The massive zone (MAS) and the clayey zone (AMN), or Mn protore, will constitute the base of the FB1 formation.

These observations suggest marine sedimentation subjected to the action of storm waves typical of the upper offshore [7].

According to [1] and [5], all these facies mentioned above were emplaced as debris flows that passed through canyons feeding submarine fans during the dismantling of the basin margin after periods of base level collapse. After the deposition of these breccias, an alternation appears between light-colored layers corresponding to dolomitic sandstones and dark layers, which are black dolomitic claystones rich in organic matter. This facies is named “banded pelites” in the works of [6] and [8][9].

On the other hand, and despite the observation of numerous similarities with other deposits of the same Francevillian Supergroup, particularly from a geological point of view (lithology, metallogeny, mineralization typology, etc.), the Lebaye deposit appears as a distinct, complex, and specific deposit in many respects:

the heterogeneous nature and discontinuity of its Mn mineralization.the extreme variability in the quality and thickness of the mineralized horizons.to the west of the plateau, on the right bank of the Lebaye River, the Mn mineralization is ferruginous; this is not the case in the eastern half of the plateau.the deposit exhibits a very thick overburden layer at its summit, exceeding 6 meters in some places, while this layer is less thick on the slopes.The mineralization there is of a soft, friable and relatively hardened nature in the platelet zone and much less consistent in the transition zone.

A distinctive feature of the FB Formation is the sudden appearance of carbonates. These occur primarily as dolomite (CaMg [CO₃]₂), but are also visible as ankerite (Ca [Fe²⁺, Mn²⁺, Mg] [CO₃]₂), siderite (FeCO₃), and later as the manganese species rhodochrosite (MnCO₃). Volcanic and hydrothermal activity in the Okondja Basin likely contributed to and supplied manganese and iron in the seawater [3].

The manganese content of the seawater was probably very high, which also resulted in a high manganese/iron ratio. Indeed, at the end of the Archean and the beginning of the Proterozoic, numerous iron deposits occurred, but the conditions for manganese precipitation were not met. A model has been proposed by [6] that suggests the precipitation of manganese and iron as oxides on a platform where the dissolved oxygen concentration was high at the surface due to intense biological activity and the rapid burial of organic matter [10].

Periods of transgression favored the transport of Fe²⁺ and Mn²⁺ to the surface via upwelling currents. During regressive periods, these elements precipitated as oxides. Since Mn²⁺ precipitation is inhibited by the presence of Fe²⁺ in the environment [7][11], the Fe²⁺/Fe³⁺ chemocline was reached before that of Mn²⁺/Mn⁴⁺, thus favoring the formation of iron-bearing deposits below the manganese layer.

During diagenesis, these elements were incorporated into carbonates, sulfides, and silicates. Mn²⁺ is thought to have reacted with CO₂ or HCO₃⁻ resulting from the degradation of organic matter below the reducing seawater/sediment interface to form Mn carbonates [6][10].

Observation of samples obtained in the field shows a predominance of brownish clay facies, pisolites, and massive blocks of Mn-rich sandstone, sometimes altered by the presence of iron and ferruginous clay, Mn-rich clay platelets, and variegated ampelites ranging in color from reddish to brown and black, with intercalated Mn layers. All these facies originate primarily from detrital sedimentary rocks, thus suggesting supergene alteration of the FB1 horizon.

The Lebaye deposit was therefore formed by supergene alteration of the black and/or variegated ampelites, initially rich in manganese carbonate (max 13%), to produce ores rich in manganese oxide [2][4][5]. The supergene profile of the mineralized bodies is almost identical from one plateau to another. These ores are confined to the upper parts of the plateaus. Lateral continuity and the total extent of the ores are linked to the size and morphology of the plateaus.

The average manganese content in the platelet-bearing manganese lithofacies, which is the main ore, is 35%. The average manganese content in the clayey-sandy horizon with pisolites is 15.5%.

From a mineralogical perspective, manganese is found primarily in the form of:

manganese oxides, with or without silica;pyrolusite (MnO₂);psilomelane (mMnO•MnO₂•nH₂O);manganese silicates such as braunite (MnII (MnIII)₆SiO₁₂);iron oxides and hydroxides: magnetite-hematite and goethite;manganese carbonate: rhodochrosite (MnCO₃) to a lesser extent;silica, for its part, is found largely in the form of quartz.

NGM, taking into account the specific geological and geochemical characteristics of its ore, decided to adopt dense medium separation (DMS) technology to improve the efficiency of the recovery process and ensure better control over the final product.

The different types of materials identified as feeds to the dense medium separation circuit exhibit chemical similarities and several types of minerals in common [9]. Particle densities range from 2.43 for pisolites to 3.36 for block goethite.

The ore in flakes has a density of 3.29; the densities of the laterites vary according to their iron and manganese content. Pyrolusite, cryptomelane, and todorokite or lithiophorite (these two phases are indistinguishable by XRD) are identified manganese minerals. kaolinite is the major clay minerals. Iron is found in hematite and goethite. Silica and quartz are also associated with clay minerals. The run-of-mine (ROM) feed to the DMS unit would a priori consist of ores predominantly of pisolite type, secondarily, and especially platelets, blocks, massive ore and mixed transition facies [9].

These materials can be easily separated based on their density to obtain ore concentrates with few or no associated pisolites. Blocky goethite, which is iron-rich and denser than the flake ores, could be found among the concentrates. The implication of the results showing flake ore as the main phase of the run-of-mine materials is that, during ore processing, pisolites could be considered gangue when determining resources and reserves. Consequently, it can be concluded that PLA-type ores, including BLO, MAS, and ZTR types, would constitute the phases forming the exploitable mineral resources.

According to international geological prospecting standards for the elements Fe, Mn, and P, some elements are considered valuable because they can be recovered, even in small quantities, without additional costs during mining; others are considered detrimental when these elements negatively impact the quality of the ore. In the case of manganese, the substance of interest in this study, the potential detrimental element is phosphorus.

A practical classification of manganese and iron ores is established according to their Mn and/or Fe content, and their detrimental elements [9].

Based on their manganese (Mn) and iron (Fe) content, we distinguish:

Rich ores: more than 50% Mn + Fe (Moanda, Bélinga);Medium ores: 30 to 50% Mn + Fe;Poor ores: less than 30% Mn + Fe.

Table 1 summarizes the average geochimical results for Mn%, P% and Fe% by zone.

Table 1. Average geochemical results for Mn, P and Fe by zone.

In relation to this classification and based on the analytical data from NGM, the Lebaye manganese ore, with an average content of 47% manganese and iron, undoubtedly falls into the category of medium-grade ores.

Regarding the presence of phosphorus, the analytical data from NGM and its predecessors show that phosphorus levels are relatively low, with an average of 0.071% P with a P/ore ratio of 0.0018 (for an average deposit content of 37% manganese). Large mineralized zones exist where phosphorus is present in almost trace amounts.

From a hydrogeological point of view, it would be wise to carry out an in-depth hydrogeological study in this area of the future quarry in order to estimate the potential interactions between infiltration waters and the hydrogeological environment and to better understand their impact [9].