Mineralogy and Geochemistry of Eteké Eburnean Gold Deposit (Gabon)

The Gabon geology offered favorable and natural environments for the formation of various types of mineralization. The Etéké gold district, aim of this study, is located in the Ngounié province (southern Gabon) on the western edge of the Chaillu massif. Geologically, the gold mineralization is associated with the Eburnean orogeny and hosted in the Archean greenstone belts. Also, this deposit is covered by a significant vegetation cover and a very extensive lateritic weathering profile, which hinders the most accurate study. Through this paper, we aim to propose a genesis pattern of this mineralization via a multidisciplinary approach. To do this, a petrographic, metallogenic, and geochemical characterization has been established in the different sectors of the Etéké deposit. The studied deposits display varied facies which are encased in granitoid. They are essentially formed of abundant granitoid, and amphibolite compared to the volcano-sedimentary formations. These rocks display magmatic textures, affected by metamorphism, and not sufficiently preserved. Based on our multidisciplinary approach, the studied samples collected from the core’s boreholes allowed us to decipher a volcanogenic and metamorphosed origin of the gold genesis.


Geology of Gabon
stratigraphic sets [1] designated in a regional context. From the bottom to the top, we found the Archean basement, the Paleoproterozoic formations, the Neoproterozoic of Nyanga, and the Phanerozoic groups ( Figure 1): • The Archean domain, part of the Congo craton, extends from Congo to Cameroon crossing Gabon and represent the Precambrian basement of the central Africa [2] [3]. It consists of Archean cratons welded together by the Mesoproterozoic and Paleoproterozoic belts [4] [5] and represented mainly, in the Gabon territory, by the Chaillu massif and the North Gabon massif. Its establishment is the result of significant magmatic activities that led to the formation of diverse ultrabasic and granitoid rocks that are the subject of several datations in the Chaillu massif [1] [6] [7] [8].  [9] [10] [11]. In Cameroon, Central Africa, Congo and more particularly Gabon, the Eburnean orogeny has been recognized by a folded belt similar to the Phanerozoic belt dominated by modern plate tectonics [1] [12] [13]. In the central part of Gabon, this belt is characterized by the Ogooué mobile zone and particularly by the establishment of intracratonic basins in the Francevillian such as the Booué, Lastourville, Okouja and Franceville basins ( Figure  1) [10] [12] [14] [15] [16] [17]. These basins lay in discordance on the Archean base of the Chaillu massif. The well-known basin is the Franceville basin, since it contains manganese deposits [18] [19] [20] [21] and uranium [15] [16] [22]. • The Neoproterozoic domain includes geological formations represented by the Mayombe-African belt which dominates in the south of the country. This belt is formed by massive Paleoprotezoic granite covering the Neoprotezoic deposits of the Pan-African foreland basin [23].
• The Phanerozoic domain outcrops in the West at the location of the coastal basin linked to the opening event of the South Atlantic Ocean during the Lower Cretaceous [24] and in the East at the location of the Paleogene to Quaternary sandstones and the aeolian sands of the Batéké plateau [8] [25].

Geological Situation, Location and Description of the Eteké Gold Showings
From 1937 to 1959, the Eteké gold district, located in southern Gabon, was the primary source of gold production in the country. Several tons of alluvial gold were extracted [11] mostly in the Dando-Mobi sectors (ore of 2.3 -7.2 t grading 15 g/t), Ovala (ore 2 t grading 11 g/t) and Dango (5.8 t grading 8.4 g/t and ore 8.4 grading 2.4 -9.8 g/t). This corresponds to gold veins set up at the time of the Paleoproterozoic Eburnean orogeny between 2.0 and 2.2 Ga [26] and hosted in the greenstone belts of Archean ages of the Chaillu massif (Congo Craton) [1] [27]. These green rocks are essentially formed from amphibolites traversed by granitoid intrusions [1] [28].
In the Eteké region, the gold mineralization is linked to the Eburnean orogeny and it is the most interesting in this deposit category. Five primary sites have been recognized in the Etéké region ( Figure 2) and will be the subject of our study: • Dondo-Mobi: Prospect essentially hosted in ultrabasic rocks of the Etéké group, located between the thrust plane and the Staurolite mica schists of the Ogooué supergroup [28].
• Dango: The supporting bedrock of the Dango deposit is mainly composed of sedimentary rocks from the Massima supergroup [28]. The lithological terms are represented by black muscovite-chlorite schists and jasper recrystallized into massive or banded quartzites.  domes of remobilized Archean migmatites [28]. This synclinal filling is filled with sediments and lavas from the Etéké group, the Massima and Ogooué supergroups.

Material and Methods
The Etéké gold district is characterized by significant vegetation cover and a well-extended lateritic alteration profile. Core drillings data were therefore of paramount input to our study, as they enabled us to define the relationships be-

Dango Zone
The Dango zone corresponds to a zone with significant, massive and whitish si- It overlaps with an episode of SIM breccia expressed by fine cement hematitization ( Figure 2(h)).

Dondo-Mobi Zone
The Dondo-Mobi deposit has been known for its mineralization associated with hydrothermal alteration by the biotite and quartz veins. The metamorphosed volcano-sedimentary host is relatively homogeneous with a microgranular texture with andesitic compositions (Figure 3

Ovala Zone
The Ovala deposit has been characterized for its dominant clastic sedimentary rocks, highly developed muscovite schists, extensive silicification, and kyanite

Massima Zone
The studied cores in the Massima district expose a volcano-sedimentary sequence ( Figure 5(a)) injected into mafic sills. These rocks are weakly deformed, except in the graphitic shales zones they present a very strong deformation ( Figure 5(b)). These boreholes contain several gold zones closely related to disseminated pyrite in the silicified volcano-sedimentary zones ( Figure 5(c)).

Moukanda Zone
In this study area, the treated boreholes display gabbroic sequences with particle size varying from microgranular to granular. The invasion of the structural fabric seems to have a link with the grain size: the less grainy parts are the least affected by the deformation. Probably, it could be due to the metamorphic recrystallization.

Petrographic and Metallographic Characterizations of the Study Area
The Etéké Gold deposit represents a wide facies variety, it is encased in granitoids. These rocks have been located in the base of the Chaillu massif, and characterized with a magmatic texture that are not sufficiently preserved and affected by metamorphism. The samples prepared from the core's boreholes allowed us to determine the different facies below:

Sandy Chloritized Metapelite
It is a very thin facies composed of micaceous and sandy levels. These levels are composed of very fine crystals of quartz, intensely recrystallized and flattened according to the regional schistosity ( Figure 6(a)). Some white micas also appear  in these levels.
The micaceous levels are dominated by sericite and muscovite. These micas are generally oriented according to the regional schistosity ( Figure 6

Rubefied Quartzite
This facies have gathered mainly intense recrystallized quartz. The mineralogy observed indicates that the fragments of the studied rock would correspond to a quartzite. Nevertheless, some fragments with the same mineralogy but their particle size is coarser would correspond to quartz veins or faults that have been deformed in the same way as quartzite (Figure 7(a)). The rock cement is siliceous, more or less reddened (Figure 7(b), Figure 7(c)).
Indeed, it is strongly impregnated with iron oxides which has been invaded in  the fragments by the fractures and cracks that affect them. This facies are characterized by a strong impregnation of iron oxides in the form of micrometric to millimeter ranges covering the entire rock (Figure 7(d)).

Meta-Andesite
This rock shows a microlithic porphyry texture (Figure 8(a), Figure 8(b)) whose mineralogy is formed by: • Quartz in the form of crystals, recrystallized and forms the cryptocrystalline matrix of the rock. • The Feldspar, which is more or less abundant in these facies, is corroded at the edge. Its peripheries are frequently confused with the quartzo-feldspar matrix. Its size varies from 200 to 500 μm. It is usually very altered with a characteristic dirty appearance (Figure 8(c)).

Amphibolite
It is a deformed, recrystallized facies slightly oriented according to the direction of foliation. It is recognized by the following mineralogy: amphibole, biotite, chlorite, calcite and quartz. The Amphiboles are very common (Figure 9(a)), they are in the form of elongated crystals according to the direction of foliation.
Some crystals have a Poikilitic texture. Chlorites exhibit a radio-fibrous form ( Figure 9(b)) and biotites present in a fibrous aspect are also visible in the rock (Figure 9(b)).
In these facies, the Quartz, slightly present, are recrystallized forming triple points with rolling extinction and corroded by the matrix of the rock. However, Calcite is abundant and permeates the entire rock (Figure 9(c)). Under a metallographic microscope, this structure shows the presence of very fine iron hydroxide boundaries in the late cracks and fractures and very fine grains (<5 μm) of Gold and/or Silver have been also observed in quartz (Figure 9(d)).

Volcanics Rocks
The composition of the analyzed samples follows that of mafic rocks characterized by relatively low SiO 2 contents (43.35 wt% to 55.5 wt%). MgO content varies   [29], the rock samples have predominantly basaltic and basaltic-andesite rock compositions ( Figure 10).
On the Zr-MgO diagram, all the samples are discriminated to be ortho-amphibolite ( Figure 11(a)). The studied rocks have a calc-alkaline affinity ( Figure   11(b)) and they exhibited arc-basalt setting (Figure 11(c)). Trace element contents of the studied samples were also used to determine the possible tectonic setting of the rocks. The distribution of our samples on the triangular diagram of [30], shows that our amphibolites are predominantly grouped in the volcanic-arc basalts setting of the orogenic domain ( Figure 11(d)).
The granitoids samples, characterized by a diorite to granodiorite geochemical composition, are predominantly calc-alkaline and moderately to highly potassic rocks (Figure 13(a)). The nature of these rocks is also found to be metalliferous to peraluminous (Figure 13

Conclusions
The geological history of Gabon provides information on the formation of several strategic natural resources with valuable economic interest (gold, manganese, iron, etc.). Gabonese gold, aim of this study, is produced intermittently and    [40] and ((c), (d)) Tectonic discrimination diagrams [41] for the Etéké granitoïd samples. generated mainly from the Etéké region.
In the Eteké region, the gold mineralization is linked to the Eburnean orogeny. The Etéké gold districts display considerably significant alteration. The most common alterations are carboniferous and silicified impregnations. Otherwise, the ferriferous oxidations prove to be the major characteristic in certain deposits.
Five primary sites have been characterized in the Etéké region: • Dango zone: it is well-known by a SIM whose origin is synchronous with the deformation, on the basis of the tectonic fabric which is permeated in the silicification. this deposit corresponds to an orogenic mineralization system with significant silicification during shearing. • Dondo-Mobi zone: it represents a characteristic orogenic system formed by weak tectonic shortening and a usual geometry of sub-horizontal stress veins and more inclined shear veins. Random biotite is the hydrothermal alteration mineral as the mineralization was emplaced at slightly elevated temperatures.

Conflicts of Interest
The authors declare no conflicts of interest regarding the publication of this paper.