Metasediments in the Alahina Sector and Associated Mineralization (North-Eastern Guinea)

The Alahina sector is located in the North-East of Guinea, precisely in the Siguiri volcano-sedimentary basin. It consists mainly of meta-sediments traversed by Paleoproterozoic pyroclastite, granite, monzogranite and granodi-orite veins as well as Mesozoic dolerite and gabbros veins. This article presents new data, on the one hand, on the geochemical petrographic features and the origin of the metasediments of this zone, and on the other hand, on the genetic model of the gold mineralization that they contain. Field observations, as well as petrographic and geochemical studies reveal that the me-ta-sediments consist of sericite and chlorite schists. They belong to the groups of shales and grauwackes. Their protholites are moderately altered (60 < CIA < 80) andesitic, basaltic and granodioritic rocks emplaced in a geotectonic oceanic island arc and/or active continental margin. Their chondrite norma-lized REE patterns show a moderate LREE enrichment (La N /Yb N = 6.31 -13.24) and a flat heavy rare earth patterns (HREE). This spectrum is almost identical to those of the “Post-Archean average Australian Shale” and Early Proterozoic Greywackes. Two types of polyphase gold mineralization occur in the Alahina sector: disseminated and veined. They consist of particular of grains and nanoparticles pyrite associated with gold, magnetite, hematite, ilmenite. The hydrothermal alteration accompanying this mineralization consists of silica,


Introduction
The Alahina sector is located in northeastern Guinea, in the Siguiri volcano-sedimentary basin, also belonging to the Paleoproterozoic domain of the Man Ridge (Figure 1(a)). Sediments in the Siguiri Basin (Figure 1(b)) rest on an Archean metamorphic basement and include pelititic formations to which the schistose, argilitic to sandstone-feldspathic rocks of the Alahina sector are attached ( Figure 2). The maximum age of these rocks, calculated on zircons (U-Pb), is 2124 ± 7 Ma [1] [2]. These meta-sediments coexist in the Alahina sector with felsic, plutonic and volcanic magmatic units consisting of granodiorites, monzogranites and pyroclastites whose age dates back to 2092 ± 5 Ma (U-Pb on zircons, [2]- [6]). The similar geological contexts, marked by magmatic, tectonic and metamorphic activity at the green shale gradient, have resulted in West Africa, the establishment of a large gold stock in the meta-sediments [1] [7]- [14]. For this reason, we have carried out the present study which aims to characterize the meta-sediments and mineralization of the Alahina sector from petrographic and geochemical data based on major and  [16] and modified). Open Journal of Geology and Loulo, Mali) [1], atypical orogenic gold (e.g. Massawa deposit in Senegal) [22], residuum gold (Yatela deposit in Mali) [23], placer type deposits (e.g. Tarkwa deposit in Ghana, Pigois et al., 2003), 4D orogenic gold (e.g. Siguiri gold deposit, Guinea) [3].
From a stratigraphic view point, the Paleoproterozoic lands of the Man's Ridge is composed of two lithological assemblages [16] [24], that are also outcropping near our study area (Figure 3): the lower Bririmian (B1), tholeiitic (2166 ± 66 Ma) [25], consisting mainly of detrital sediments and carbonate rocks, and the upper Birimian (B2), more heterogeneous, composed of sedimentary rocks (volcanoclastites, turbidites, shales and carbonates) and calc-alkaline magmatic rocks dated between 2100 and 2150 Ma [26]. It should be noted that the upper Birimian is in some places covered with Tarkwaian fluvio-deltaic sediments [10].

Material and Methods
We followed two methodological steps: 1) field work: • geological mapping of the Alahina sector with selection of sample collection sites (artisanal gold mining sites, former mechanized gold mining quarries); • structural measurements and sampling. Samples collected: meta-sediment and hydrothermal quartz vein; 2) laboratory work • making thin sections and polished sections; • petrographic study of the samples under a microscope; • geochemical analysis of the samples; • treatment, analysis and interpretation of the results.
Sixteen (16)  Our samples are treated according to the same analytical protocol. A representative sample of about ten centimeters in diameter is selected. Half of this sample is archived, the other half is divided into two quarters. The first quarter was used for the chemical analysis of major and trace elements by X-ray fluorescence and ICP-MS/OES on powders, the second quarter is used for the manufacture of polished sections and thin sections.
The metallographic microscope used, type Olympus BX60, is equipped with 4 magnification objectives, ×5, ×20, ×50, ×100. It is possible to add objectives. This microscope makes observations in natural light and in polarized light from lighting positioned above the sample, through the lens. The main characteristics of minerals determined in natural light are mineral color, pleochroism, mineral form, zonation, reflectivity, hardness and cleavage. In polarized light, the determining characteristics were polarization color, intensity of anisotropy, twins, scattered light, and corrosion to light.
The analysis of the major elements is made from powders with a particle size less than 40 µm, obtained after grinding and sieving in an agate mortar in order to avoid any contamination. These powders are then subjected to a lithium borate (LiBO 2 ) fusion and then to aqua regia digestion prior to X-ray fluorescence spectrometry. The solution obtained, subjected to highly energetic X-ray radiation, reacts by emitting radiation characteristic of the elements that make up the To do this, we used internal standards, Indium and Rhenium, previously incorporated in each sample, a machine blank, the international geostandard, and a 2% HNO 3 solution that passes under the argon flux every half -hours to decrease the memory effect on each sample. A standard Barium solution is also applied to the rate of bi-charged ions.
Optical emission spectrometry consisted of applying electrical energy in the form of an arc or spark (under argon) between the sample and an electrode to vaporize the atoms. These atoms or ions excited in the plasma create an emission spectrum specific to each element of the sample.

Petrographic Characteristics of Metasediments
From a macroscopic view point, the Alahina metasediments have a schistose facies, ranging from greenish-gray to yellowish-gray or brown (Figure 4

Nature and Provenance of Metasediments
The study of major and trace elements provides information about rocks and minerals formation conditions. Authors [17] [34] [35] [36] have studied rocks similar to those of this study and none of them reveal anomalies likely to modify the geochemical characters. Thus, we use the major and trace elements to highlight the petro-geochemical characteristics of the Alahina meta-sediments. Table 1 and Table 2 below mention respectively the composition in major and trace elements of the studied meta-sediments.   [29]. [Triangles in blue represent metasediment data from the work of [38]]. to the transformation of feldspars into clay minerals [39]. Rocks with CIA values between 60 and 80 with atmospheric alteration and CIA < 60 show no or little alteration [29]. The meta-sediments analyzed have CIA values between 62.5 and 77.51 and indicate an atmospheric alteration of the protholite of these rocks.
The intensity of the alteration as well as the nature of the original sediments was indicated by the ternary diagram A-CN-K and CIA [29] (Figure 6(c)).
Thus, our samples plotted in this diagram (Figure 6

Geotectonic Environment of Metasediments
The geodynamic deposition environment of the detrital sediments that led to the formation of the metasediments object of the present study is appreciated from the Open Journal of Geology discriminating diagrams of [32] and [33]. Thus, in the Roser and Korsch diagram, the analyzed meta-sediments are positioned in two domains: the oceanic island arcs margins domain and the active continental margins domain (Figure 9(a)).
However, the discriminant diagram from Bhatia shows a membership of metasediments in oceanic island arcs domain (Figure 9(b)).

Mineralizations Associated with Alahina Metasediments
The petrographic study showed mineralization within the meta-sediments already affected by tectonic deformations.    ) generating open folds whose axes have a direction varying between N80˚ and N100˚ (measured in situ). Veins V 2 are parallel to S 2 schistosity or NE shear zones and intersect veins V 1 (Figure 10(a)). They are arranged "in step" and affected by ductile-brittle D2 deformation characterized by driving folds and fractures of direction NE. Their thickness often varies from 2 cm to 30 cm. Veins V 3 intersect V 2 veins associated with S 2 schistosity following NE dextral shear (Figure 10(a)).
These different veins are generally recrystallized in silica, carbonates, micas, sulphides, oxides and precious metals thanks to the circulation of hydrothermal fluids responsible for neoformations.
• Phases of mineralization and their location Three hydrothermal phases have been distinguished: 1) a phase with silico-sericite-chloritized and sulphurized parageneses, often diffuse or stratiform, consisting of Qz I -Ser-Chl ± Ep ± Tr ± Act ± Cb ± Ap-Py I ± Apy I ± Mg I ± Leu I ± Hem I for veins V 1 which is localized in meta-sediments ( Figure 10(b) & Figure   10(c)); 2) two phases with sulphurised, locally serititized and/or chloritized sul-  found in Archean meta-sediments by [30] and [31]. The existence of Archean

Genetic Model of Mineralization Associated with Alahina Metasediments
The Alahina metasediments are affected by deformations and traversed by hydrothermal fluids with deposition of silica and sulphide [46] and/or oxidized Birimian sediments near the contact with the volcanites. Like these deposits, gold mineralization in Alahina sector is associated with shear zone-type deformations. The genetic mineralization model commonly accepted in such a geological context is that of a continuum of fluid flows from ductile stages to brittle stages in relation to the triggering of earthquakes, according to the "fault-valve" model proposed by [49]. Indeed, each time the fluids pressure exceeds the lithostatic pressure, it causes rocks fracturing in localized zones which will thus constitute drains facilitating hydrothermal fluids circulation, creating the mineralized zones [50]. With the decrease of pressure, these fluids precipitate their mineral charge which will be found thus in filling of veins. Two types of mineralization can be mentioned in the Alahina sector: disseminated and veined. The existence of disseminated mineralization that would be syngeneic is based on the presence of nanoparticles of pyrite, magnetite, hematite, ilmenite within the metasediments independently of the veins or veins.

Conclusions
Alahina metasediments in the northeastern Guinea area consist of sericite and