Petrography and Mineralogy of the Quartz and Quartz-Feldspar Sulphide Veins in the Pan-African Syenitic Massif of Guider (North Cameroon)

In the syenitic pluton of Guider (593 ± 4 Ma) in the North-West Cameroon domain of Central African Fold Belt, mineralized N-S to NE-SW vertical or sub-vertical quartz and quartz feldspar veins has been recently identified. In this contribution, we present petrography and mineralogy of these veins, in order to constrain their genesis and emplacement mechanisms based on detailed field work, petrographic studies and chemical characterization of minerals by using an electron probe microanalyser (EPMA). Field observations and vein microstructures show that the emplacement of the veins has been controlled by the dextral N-S trending strike-slip shear zones related to the regional D2 deformation phase. The results of mineralogical analysis reveal the co-presence of silicates and metallic minerals that include magnetite, ilmenite, pyrite, bismuthite, galena (very rare) and sulphide complexes (BiPbS, BiAgPbS, FeBiPbCuS, BiFe(TeS), FeBiPbS, BiPbCuS). The gangue is represented by quartz (quartz 1, quartz 2 and quartz 3), feldspars, sericite, chlorite, yellowish brown clay minerals, and hematite. The textural relationships between sulphides, quartz and alteration products show that the mineralization is essentially syn- to late-D2 and suggest that syenitic country rock and dextral shear zones have played an important role in the metallogenesis of these veins. This mineralization shows characteristics for copper-bearing calc-alkaline deposits, but differs from these by its more extensive alteration and its abundance in hematite. Substitutions of Al(IV) by Si(IV) in sericite associated with the sulphide mineralization and cataclastic deformation suggest that the temperature of trapping of the fluids is between 230°C and 275°C.


Introduction
Mineralization associated with more or less differentiated plutonic rocks and that associated with the concentration of hydrothermal fluids are the main primary sources of mineral resources on our planet [1] [2] [3]. Sulphides are rare and occur as ores of arsenic, tungsten, tin, bismuth, tellurium, gold, copper and antimony; quantitatively, pyrite and/or arsenopyrite are dominant. Gold is in the form of electrum or maldonite with bismuthinite and bismuth [4] [5].
Prospecting missions carried out on the Garoua East geological reconnaissance map in the North Cameroon region by the Bureau de Recherches Géologiques et Minières (BRGM) report gold, cassiterite, copper, galene-blende, rutile, ilmenite and monazite occurrences in sediments of several "mayo" or temporary watercourses [6] [7] [8]. The origin of these metals remains undetermined, especially for those located north of the Benue trough, where no metallogenic studies are carried out, compared to the Poli sector (south of the Benue trough), which has been the subject of several metallogenic studies [9] [10] [11].
Within the Pan-African syenitic massif of Guider, located north of the Benue trough, in the northern domain of the Pan-African Fold Belt in Cameroon ( Figure 1), [12] has reported the presence of quartz and quartz-feldspar veins with various sulphide and oxide mineralization indices.
However, apart from the petrological and structural studies carried out on this intrusion and its country rocks by [12] [13], no studies are carried out on these quartz and quartz-feldspar veins. The goal of this paper is to carry out a petrographic, mineralogical study of the quartz and quartz-feldspar veins of Guider on the one hand, and to constrain their genesis and emplacement mechanisms on the other hand.
For this purpose, in this paper, we present the results of field and microscope observations and result of EPMA. Based on these data, we: 1) constrain the geological context of the emplacement of these veins and its mineralization; 2) discuss the origin of fluids and mineralization processes; 3) specify quartz generations and mineralization; 4) define the metal paragenesis and species chronology.

Regional Geology
The Neoproterozoic basement of Cameroon which covers a great part of the country is part of the Central African Fold Belt. This orogenic belt formed during Brasiliano/Pan-African orogeny by collision between the converging West African craton, Congo craton and Saharan Metacraton, is mainly subdivided  (Figure 1(b)). The domain consists of the Neoproterozoic Poli Group and several generation of granitoid [14]. The Poli group is dominantly constituted of Neoproterozoic medium-to high-grade schists of volcanic to volcano-sedimentary origin, emplaced between 700 -665 Ma upon, or in the DOI: 10.4236/ojg.2020. 103013 238 Open Journal of Geology vicinity, of young magmatic arcs [17]. The granitoids, into formation of the Poli Group, are grouped into pre-, syn-to late and post-tectonic calc-alkaline granitoids dated between 660 -540 Ma.
At the scale of the study area, geological works carried out by [13] [14] shows that the Neoproterozoic basement is primarily constitutes of composite orthogneisses, syenitic pluton and numerous dykes of varying composition including mafic dolerites dykes, felsic dykes and some quartzo-feldspathic dykes intruding the previous rocks. Orthogneissic rocks (627 -632 Ma) are dominantly made up of metadiorite, metatonalite and metagranodiorite, describe as basic to intermediate metaplutonic rocks by [18], and minor amphibolite occurring commonly as lenticular layers within orthogneiss. Because of their macroscopic features of strong deformation at solid state, they have been interpreted as a pre-tectonic granitoids. These formations suffered at least two phases of ductile deformation (D 1 and D 2 ). D 1 fabrics are commonly overprinted by D 2 and poorly exposed. However can be seen locally at the hinge of some isoclinal folds, where the foliation S 1 is sometimes oblique to the foliation S 2 of D 2 . D 2 phase of deformation, widely-developed in orthogneissic rocks, is responsible for the development of an S 2 foliation, L 2 lineation, folds F 2 and shear markers C 2 . S 2 foliation is the most widely developed planar fabric. It is defined by felsic (quartz-plagioclase) layers alternate with biotite-hornblende layers and preferential orientation of minerals (amphibole, biotite, K-feldspar, and quartz) that are mainly oriented N-S to NNE-SSW with steep dip angles. Lineations are commonly mineral lineation defined by alignment of amphibole and plunges mainly to the SSW with a subhorizontal angle. Shear sense indicators associated with the D 2 event (C 2 ) include asymmetric boudins, asymmetric and sigmoidal quartz aggregates or feldspar porphyroclasts and asymmetric Z folds revealing dextral strike-slip tectonic.
The syenitic pluton is a small elongated massif in the N-S direction of around 70 km 2 in the city of Guider ( Figure 2). It is dominated by pink and grey syenite with subordinated quartz-diorite. Pink and grey syenite exhibit a granular textures with abundant alkali feldspar and variable proportion of Fe-Mg minerals (amphibole and biotite). Available isotope chronology data attribute an age of 593 ± 4 Ma to the Guider pluton (U/Pb zircon; [13]).
The same authors, on the basis of analysis on microstructures and magnetic fabrics of this pluton and its country rocks, conclude that the emplacement this pluton is late tectonic.

Sampling and Analytical Methods
Sixteen quartz vein and twenty five quartz-feldspar vein samples were collected from outcrops; but only seven are considered in this study. There are MG3B, MG1B and MG2 for quartz veins and MG3D, MG3C, MG1C and MG1D for quartz-feldspar veins. Samples were first examined and described on a macroscopic and mesoscopic scale. Each sample was carefully shoot using a high resolution digital camera. Areas of interest were marked for thin sections making. Textures of sulphide minerals and associated mineral assemblage were observed on polished thin sections, using the backscattered electron mode (BSE) of Hitachi S4800 scanning electron microscope (SEM), equipped with an EDS spectrometer at GeoRessources laboratory (Nancy, France) to characterize the paragenetic assemblages of the samples. The mineral chemical compositions were obtained using an electron probe microanalyser (EPMA) CAMECA SX-100 at GeoRessources laboratory (Nancy, France). The electron microprobe, type CAMECA SX100, is equipped with an electron gun (0.2 to 50 kV), with a tungsten filament coupled to five wavelength dispersion spectrometers, allowing the analysis of the elements from Boron to Uranium under the following analysis conditions (Primary beam: 10 nA, Acceleration voltage: 15 kV, Beam diameter: 2 μm).
The result obtained is compared with a file containing the number of X photons of the same nature emitted under the same conditions by a compound of known composition. After correction of the matrix effects, the elementary composition of the analyzed area, generally in the range of 1 to 5 μm 3 , is obtained.
The composition is given in percentage of elements (% oxides, % weight or atomic %). The raw results made it possible to calculate the structural formula of the minerals.

Field Observations
The mineralized quartz-feldspar and quartz veins occur mostly at the central part of the Guider pluton. At the surface, they are characterised by 0.3 to 0.6 m DOI: 10.4236/ojg.2020.103013 240 Open Journal of Geology thick and length of ~100 mostly directed 10˚N -15˚E steeply dip to W ( Figure   2(a)) consisting essentially of quartz and more or less altered feldspars. These veins often present tight shear planes oriented 20˚N -30˚E. In the immediate surrounding rock these tight shear planes are also present in association with quartz veinlets that give to the rock a fibrous appearance (Figure 2(b)). At the scale of the sample, the rock has a reddish or yellowish colour due to the alteration of the feldspars into clay products and the presence of iron oxides. Quartz is also covered with oxides ( Figure 2(c)), suggesting the hematization phenomenon that generally accompanies mineralized quartz veins.  Open Journal of Geology quartz-feldspar veins and rare in quartz veins. In quartz-feldspar veins, they occur as perthitic, xenomorphic, deformed and more or less pseudomorphosed by sericite ( Figure 3(b)). In mircoshear planes, quartz, sulphides and oxides fill the microcrack within feldspars or are associated with alteration products ( Figure   3(c)). These very tight planes give a crumbled appearance to the feldspar stretched in the microshear planes of the rock (Figure 3(d)).

Petrography and
Secondary muscovite (sericite) occurs in quartz fissures in textural equilibrium with sulphides and oxides or associated with alteration products of feldspar crystals. Chlorites occur as oriented lamellae or in quartz fractures and pressure shadows around opaque minerals (sulphides and oxides).     [20]. Micas are characterized by a correlative substitution between Si IV and Al IV (Figure 5(b)) and between Fe 2+ and Al VI (Figure 5(b)).

Oxides
Oxides are analysed in quartz veins and quartz-feldspar samples. In

Sulphides
Electronic microprobe analyses show that sulphides are represented by pyrite (FeS 2 ) in quartz and quartz-feldspar veins, bismuthinite (Bi 2 S 3 ), galena (PbS) and complex sulphides. SEM images ( Figure 6) highlight the textural relationship between the different types of analysed sulphides.   (Table 4).

Bismuthinite
Bismuthinite is the second most abundant type of sulphide after pyrite in the analyzed rocks. The mineral has a higher reflectivity than pyrites, which makes it possible to distinguish them from the latter. In quartz veins (MG1B, MG2),   (Table 5). In addition to Bismuth and sulphur, the analyzed crystals still contain traces of lead (1.07 -4.62 wt%), copper (0.99 -5.45 wt%) and rarely gold (≤0.15 wt%) and selenium (≤0.06 wt%). Traces of Se suggest a partial substitution of S 2− by Se 2− .

Galena
Galena is detected in the sample of the MG2 quartz vein. It crystallizes in the cracks of the pyrite. Due to the very small size of these crystals, only one analysis is performed. The crystal has a content of 77.66 wt% Pb and 16.38 wt% sulphur (Table 6). Iron and bismuth are in trace form with values of 2.51 wt% and 2.85 wt% respectively.

Complex Sulphides
Complex sulphides correspond to sulphides in which we have the combination of several elements. Referring to the literature, it has been difficult for us to classify them either as sulfosalts or sulfides. We group them here according to the content of the elements. Thus, two groups are highlighted: complex bismuth and lead sulphides (BiPbS), complex bismuth and iron sulphides (BiFe(S)) or (BiFe (S, Te)).      Complex sulphides are analyzed in the quartz-feldspar veins and quartz veins represented by the MG1C and MG1D and MG2 respectively ( Table 6). The crystals are elongated according to foliation S 2 or in pyrite cracks. In addition to the two cations Pb 2+ (2.21 -12.66 wt%) and Bi 3+ (6.39 -35.67 wt%), we note the presence of Cu (≤11.8 wt%), which suggests the presence of aïkinite (PbCuBiS 3 ). Iron is abundant in sample MG1D (9.79 -26.88 wt%) but rare in MG1B (0.14 -0.36 wt%). Gold is present in several crystals, but as a trace (≤0.26 wt%) in the MG1D sample where it is included in the pyrite. The transfer of Bi and Pb contents in a diagram shows a positive correlation between these two elements, suggesting substitution between them ( Figure 7). As the charges are not identical, it is probably necessary to consider another substitution for the mineral to be neutral. Otherwise, some sites will not be saturated, which characterizes many sulphides and sulphosalts (Cox, 1987).

Geological Context
The study area, in the locality of Guider, is located in the northern domain of the CPAC in Cameroon. This area consists of orthogneissic basement dated at 627 -632 [12], intruded by syenitic intrusion dated at 593 ± 4 Ma [13]. All of these metamorphic and plutonic rocks are cross cutting by quartz and quartz-feldspar veins. At least three deformation phases are highlighted in this sector: D 1 phase, in orthogneiss and amphibolites where it is transposed by D 2 phase. The D 2 phase is materialized by steep foliation with N-S to NNE-SSW direction with mineral lineations (L 2 ) shallowly plunging and dextral markers attesting a transpressive and dextral regime of this D2 in the Guider area [12]. According to [13] the NE-trending and dextral transpressive shear zone that acted in the Guider area ended after the emplacement of the Guider syenitic pluton.
Petrographic and mineralogical studies distinguish two types of quartz and quartz-feldspar veins: non-mineralized veins and mineralized veins. The 10˚N -15˚E orientation of the mineralized veins, the 20˚N -30˚E orientation of the veinlets and the microstructure of the veins suggest that the emplacement of mineralized veins took place before the end of transpressive deformation in its syenitic country rocks. We suggest that the mineralization of sulphides is associated to the late stage of D 2 phase of deformation.

Origin of Fluids and Mineralization Processes
Syenite, which is the intrusive massif in orthogneissic basement, is an under-saturated rock, therefore low in silica. It is necessary to question the origin of the quartz and quartz-feldspar veins more or less mineralized in this massif.
The search for the sources of the elements is one of the most difficult problems of metallogeny. It is now understood that the source of the vector that transports the elements can be independent of the source of the elements [21]. In addition to geochemical and isotopic data in particular, petrographic studies of the veins The syenite pluton which is a probable thermal source and the presence of many fracture planes blocked or not by veins suggest that the mode of circulation of mineralizing fluids south of Guider is forced convection (Figure 8). According to [23], forced convection, carried out around a warmer point (a pluton), corresponds to the repeated circulation (multiple pass) of the same fluids. Heavy cold fluids sink, heat up and become less heavy and rise. The density of the fluids is partially corrected due to the acquisition of dissolved elements at high temperature, which increases their density and therefore promotes their deposition.
In this way, around the Guider syenite pluton there has been a forced convection whose size depends on the size of the pluton, its shape, its level of placement, its permeability and the distribution of permeability in the surrounding, in particular the existence of horizontal discontinuities.

Quartz Generations and Mineralization
The siliceous phase is the most important gangue of the ore bodies. Microscopic study of the mineralized veins revealed several generations of quartz associated with them ( Figure 9). Quartz 1, with an S/C structure, was probably affected by a shear event that preceded its recrystallization into quartz 2. The arrangement of the crystals constituting the latter in a chevron fold, whose axis is parallel to the S 2 , is in favour of its crystallization during compression perpendicular to this planar structure compatible with the boudinage observed in the wall of some  Pyrite, xenomorph to subautomorphic, large cracked beaches are always associated with quartz 1 or disseminated in surrounding rocks. Euhedral to subautomorphic pyrite seems to be mainly associated with recrystallization quartz (quartz 2 and shear quartz), with sometimes ambiguous relationships with quartz 1. Bismuthinite sometimes present in quartz 2 contains other complex sulphides, oxides and pyrite inclusions. It therefore seems that the presence of sulphides ranges from the early phases of magmatism to the late phases of recrystallization and alteration of the veins and their hosts (syenite, gneiss and amphibolites). The magmatic source of the sulphides observed in these veins is therefore highly plausible. The more or less deformed magmatic rocks could have been subjected to repeated circulation of hydrothermal fluid, which may have remobilized the sulphides of the surrounding rocks to gradually and continuously enrich the veins. This mechanism can be considered for sulphide enrichment.
Oxides are represented by hematite with exsolutions of fibrous ilmenite and automorphic pyrite. This assembly indicates a reducing state of the hydrothermal fluid during crystallization or recrystallization of these oxides and sulphides [28]. However, these oxides may be of secondary supergenic origin [3]. It should also be noted that the relationships between sulphides, oxides, quartz types and host rock are not unambiguous and may be subject to discussion [29]. The textures of these sulphides and oxides appear to reflect remobilizations rather than crystallizations, resulting in the intersection of quartz grain boundaries, magmatic and tectonic structures. Sulphides have textures that can be interpreted in terms of their late emplacement relative to vein structure. Indeed, sulphides are essentially associated with quartz 2 or located at the C2 shear planes. Metamorphism coupled with tectonics played an important role in the mineralization of the Guider veins. However, further studies are needed to refine the genetic interpretation of mineralization.
From the petrographic and mineralogical studies, it appears that sulphides are associated with different generations of quartz and particularly with quartz 2 and quartz in shear zones. This can be interpreted in terms of the continuous and repeated remobilization of sulphides from its source (magmatic or hydrothermal) to feed the veins throughout their establishment and evolution. This explanation has also been proposed by [30] for gold from the Phanerozoic fold belt of Victoria in Australia and the late quartz veinlets, which intersect the latter, appear sterile.

Metal Paragenesis and Species Chronology
The mineralographic observations allow us to propose a metallic paragenesis, based mainly on the observed textural relationships. Paragenesis consists in interpreting the mineralogical evolution of the deposit [31]. In the veins studied, we note the existence of two morphological forms of quartz: 1) quartz, prior to early mineralization, is in the form of coarse grains and/or highly fractured crystals. Some fractures are filled with pyrite and/or hematite These sulphides are therefore being filled (as is the case for those in fractures) following a precipitation slightly later than this quartz or following a redistribution caused by a deformation; 2) recrystallized quartz may be found in fractures or borders of the previous quartz. The sulphides present with this type of quartz are cataclased in places. The pyrite is formed, more locally, by a precipitation contemporary to this recrystallized quartz. Pyrite crystals are observed in contact with recrystallized quartz grains, suggesting that this sulphide is not later than recrystallized quartz, but rather contemporary to it. Gangue minerals, such as chlorite, hematite and sericite, are found in these deformed zones of mineralization, where recrystallized quartz is more abundant. It is likely that the cooling fractures of Guider's syenite [13] were initially filled with quartz with pyrites, ilmenite and magnetite. This primary mineralization by filling was resumed by a subsequent deformation that produced the cataclase of quartz and primary sulphides (pyrite). The subsequent intense cataclase suffered by these veins and veinlets would have caused neomineralization and formation of small quartz grains and created openings allowing the placement and/or redistribution of sulphides, certain gangue minerals, and possibly locally, a new injection of sterile or mineralized quartz in late pyrite, bismuthinite and other sulphides with Bi, Ag, Cu, Pb. The sulphides present in the veins, veins and veintles were probably formed largely by a reaction between the fluids and the fragments of the host or the host itself on contact. Sulphides are very frequently directly attached to the altered syenite.
According to petrographic observations, most of the metal phases began to precipitate simultaneously. However, since pyrite contains inclusions of almost all other metal phases, it seems to have continued to develop after the formation of the other metal phases it has incorporated as well as some gangue crystals. Since most mineralized veins are close to the syenite pluton, there appears to be a spatial and genetic link between the presence of the host syenite and the presence of sulphides. The order of crystallization of the phases is shown in Micas are characterized by a correlative substitution between Si IV and Al IV and between Fe 2+ and Al (VI) (Figures not shown). However, there is no substitution between Fe 2+ and Si 4+ . According to [32] the substitution of Al IV with Si IV is temperature-dependent and varies between 230˚C and 275˚C, in the silicified and sericitized zone. They determined a temperature ranging from 250˚C to 310˚C in the chlorite zone at the Home Mine (Noranda, Quebec). These same types of substitutions and alterations are also found at the Tiouit Mine in anti-atlas oriental (south of Morocco) [33]). So, the same range of alteration temperatures and associated mineralization can be considered for Guider mineralized veins.

Critical View on the Typology of Mineralization
The association of mineralized quartz veins with shear zones suggests that the Guider mineralization is of the mesothermal type [34]. Near-surface intrusions provide favourable sites for the formation and development of mineralization with characteristics ranging from epithermal to sulphides and copper porphyry [35].
The majority of porphyry deposits have the same overall evolutionary history, although they may differ in detail. Guider mineralized veins display some of the known characteristics of copper-bearing porphyry calc-alkali veins, but they differ from them by their considerably widespread and highly impregnated alteration and overall hematite richness. This abundance of hematite is a particularity of Guider's mineralized veins. Hematite is the most distinctive alteration product in these veins. This probably intermittent hematization from different sources distinguishes this deposit from the usual copper porphyries [36]. According to some authors [37] [38], considerable quantities of Fe can be extracted from an acidic to intermediate magma. Rocks that are in contact with a retrograde boiling of brine-laden fluids are capable of releasing appreciable amounts of iron [39]. Also, under conditions of low pH, an oxidizing medium and temperatures < 360˚C, magnetite transforms into hematite, with a sericite-clay-chlorite alteration assemblage. These conditions also favour the concentration of Au and sulphides [39] [40] [41]. The appreciable quantity of hematite in Guider's mineralized veins is probably produced, among other things, by the martitization of hypogenous hydrothermal magnetite or, incidentally, magmatic magnetite. The latter mineral and part of the hematite are locally transformed into pyrite. By way of comparison, the abundance of iron in the Marte mine (northern Chile) [36], is mainly represented by magnetite that has not been martitized as in the case of Guider.
Our results show that some aspects of alteration (serialization, chloritization, pyritization, silicification, and oxidation) that characterize porphyry-Cu deposits [42] do exist around the calc-alkali Guider pluton. However, their distribution is less regular and relatively more complex than that corresponding to the classical zonation described by the authors who have worked on Cu-porphyres [35] [41] [43]. The majority of the studies of these authors concern recent deposits where, zonation of alterations and associated mineralization are not affected by subsequent episodes (metamorphism, deformation, erosion). On the other hand, older deposits (such as the Pan-African Guider pluton) are probably affected, changed or displaced, so their recognition and classification become relatively problematic. A more detailed isotope-based study is needed to refine the nature, origin of the fluids and the classification of Guider's sulphide mineralization. The results obtained by this study show that some alteration aspects (chloritization, hematization, pyritization, sericitization) that characterize porphyry-Cu deposits do exist around the Guider pluton. However, their distribution is less regular and relatively more complex than that corresponding to the classical zonation described by the authors who worked on the Cu porphyres.