Mineralogy, Petrography and Geochemistry of the Ina Syenite (Linté, Central Cameroon) as a Source of Industrial Feldspars ()
1. Introduction
Syenites are leucocratic to mesocratic felsic igneous rocks belonging to intermediate plutonic rocks with trachyte as the volcanic equivalent. These rocks are present in most tectonic contexts except in mid-ocean [1]. They are formed by the cooling of a crustal or mantellic magma saturated or undersaturated with silica and outcrop as allochthonous (batholith), dyke or vein within the granite massif that it intersects [2] [3] [4] [5]. Syenites display coarse grain texture, porphyritic texture or sometimes pegmatitic texture. They are essentially composed of alkaline feldspars (content ≥ 75%), in particular orthoclase and microcline and often sodium plagioclase (around 15%). Apart from feldspars, the rock contains quartz, mafic minerals such as biotite, pyroxene and amphiboles. Magnetite, ilmenite, apatite and titanite constitute the accessory phases [6].
The geological characteristics of syenites and their deposits make them an industrial mineral exploited for economic and strategic purposes worldwide. In the field of civil engineering, syenite has long been exploited as freestone [6] [7] [8]; in agriculture as a source of potash for soil enrichment [9] [10] [11]. Gold mineralization and rare earth elements are also exploited in some syenites [12] [13] [14] [15] [16]. They are also used as filler in the paint industries [17]. Syenite is exploited as an alternative source of feldspathic flux for the ceramic and glass industry [3] [6] [10] [17] - [24]. Feldspars are mainly mined in albitites, aplites and pegmatites [17] [22]. However, these deposits are being depleted at a tremendous rate while the global demand for feldspars remains strong, forcing manufacturers to look for new rock ores rich in feldspars and relatively poor in quartz and ferromagnesian minerals such as syenites.
In Cameroon, several feldspathic rocks were tested for ceramic purpose [20] [25] [26] [27]. Although having concluded that these rocks could be used as feldspar ore for industrial uses, these studies were limited to isolated outcrops on quartz-feldspar veins or pegmatites with a limited extension. It therefore appears necessary to find deposits whose characteristics would dictate the prospects for industrial development, such as alkaline syenites. The syenites of Linté (Centre Region-Cameroon) are an important consideration [28]. They are made of two batholiths: a Northern batholith at Ina and a Southern batholith at Linté. Little work has been carried out on these rocks whose enormous potentialities can be exploited although this valorisation, particularly as a source of industrial feldspathic fluxes, depends on knowledge of its physicochemical properties.
This study focuses on the Ina batholith in order to assess its potential as a source of feldspars for industrial uses. Petrographic analysis has been done to identify the different facies, geochemical analysis to understand the distribution of major elements and mineralogical analysis to characterize the rock mineralogy. These rocks were classified and the most probable facies for exploitation as a source of feldspars is identified.
2. Geographic and Geological Context
Located approximately 80 km north of Ngambétikar (Centre region Cameroon), the Ina syenites (Figure 1) cover ~ 1000 km2 and outcrop within the Precambrian syntectonic granites (Weecksteen, 1957). The study area extends for ~ 350 km2 between latitudes 05˚45'N and 05˚59'N and longitudes 011˚46'E and 011˚59'E located in a transitional equatorial to tropical climate zone, and covered a vast smooth peneplain whose altitudes dominantly range from 500 to 700 m. The monotonous topography is interrupted by some inselbergs (up to 600 m
![]()
Figure 1. Study area location ((a), (b)) and simplified geological setting (c) modified after [28].
in height) and Mount Wé (2335 m) is the highest hill in the area. We also note an alternation of hills, plains, and lowlands in the sector nestled within the vast depression of the Tikar plain to the south of the Adamawa plateau and the Banyo massifs [29].
The North Equatorial Pan-African Fold Belt is a major collisional belt that holds the region from the West African board to Est African [30] [31]. It is the southernmost branch of the Brazilian Pan-African domain and extends between Cameroon, Tchad, the Central African Republic, the North part of Congo craton and the western Nigeria shield [31] [32]. Among the three different litho-structural domains of the North Equatorial Pan-African Fold Belt defined in Cameroun by [31], the Linté syenite, on which the syenite from Ina batholite depends, belongs to the Centre Cameroon geodynamic domain of the Pan-African North Equatorial Fold Belt. This domain covers a large area stretching from the north of the town of Bafia to the south of the locality of Poli [33]. It is characterized by large rockslides including the Central African Shear Zone [34], the Foumban-Tibati-Banyo Fault [35] [36] and the Sanaga Fault located to the East of Bafia and marked on the 1/500.000 geological of Cameroon, Douala East sheet [28] [37].
The Centre Cameroon domain is marked by several generations of calc-alkaline granitoids with high potassium content, intruded in the high grade gneissic Paleoproterozoic basement [38] [39] [40] [41] [42]. They form a wide variety of rocks like granites, diorites, gabbro and syenites. Depending on the age, the granitoids are syntectonic (630 - 620 Ma), late tectonic (600 - 580 Ma) and post-tectonic (550 Ma) [31] [43]. The plutonic rocks of Linté batholith mainly consist of syenite and monzonite and were emplaced between 590 and 599 Ma, suggesting their classification as syn- to post- collisional granitoids related to the Pan-African Orogeny [44]. Most granitoids are syntectonic belonging to the potassic shoshonitic calc-alkaline series [2]; they are metallic to slightly peraluminous [38] [45] [46] and orthogneissified with a calc-alkaline tendency, high potassic to aluminous [32] [36] [38] [40]. These granitoids have orientations sub-parallel to the Centre Cameroonian Shear Zone.
3. Material and Method
3.1. Field Relationships and Sampling
On the Ina batholith, rock outcrops are widespread and heterogeneous. Identification and description of petrographic types and their sampling have been done based on colour, texture, structure and macroscopic characteristics. Observations were also made on special features such as veins, deposit mode and relationships between the different outcrops. Seventy-five samples were taken from 45 rock outcrops (see sample locations reported on Figure 2).
The outcrops of the Ina batholith are abundant and heterogeneous with many blocks, slabs and domes. The colour of the rocks observed on these outcrops varies from whitish (Figure 3(a)) to dark grey (Figure 3(d)). Numerous erratic, angular or rounded blocks and even monoliths (Figure 3(a)) characterize the
![]()
Figure 2. Topographic map of study area with sample locations from modified from Digital Terrain Model (USGS, 2021).
abundant block outcrops. The slab outcrops are large with more or less flattened surfaces (50 to 800 m2, Figure 3(b)), often marked by veins. The outcrops do not present any sign of alteration as is the case with blocks outcrops. The domes outcrops form hills of several meters high (Figure 3(d)). The base of these wide domes is boarded by angular blocks detached from the walls of the main hill.
![]()
Figure 3. Characteristics outcrops of the Ina batholith. (a) blocs outcrops; (b) slab outcrop; (c) vein on slab and dome outcrop; (d) hill outcrop.
Note that these different types of outcrops are not specifically related to the petrographic types recoded on the Ina batholith syenite and have no structural orientation. Granite, monzonite and granodiorite are sporadically observed as boulders while syenite forms a large slab of ~800 m2 in the centre of the study area (Figure 1 and Figure 4(b)).
3.2. Petrography
The rock samples were described macroscopically. Textural analyses were performed on thin sections by optical and electron microscope. Forty thin sections were prepared at the Geosciences Department of the University of Padova, Italy. The observations were made using an optical microscope at the University of Liege (Belgium). In order to complete the observations and to determine the semi-quantitative composition of some minerals like magnetite, ilmenite, hematite and perthites, thin sections were observed using a QEMSCAN FEI Quanta 650F electron microscope coupled with energy dispersion spectrometry at RWTH Aachen, Germany.
3.3. X-Ray Diffraction (XRD)
The mineralogical analysis was carried out on more than sixty rocks samples. Mineralogical phases were determined by X-ray diffraction (Bruker D8-Advance Eco 1 kW diffractometer) using monochromatic CuKα radiation of 0.020˚ operated at 40 kV and 25 mA using Cu-Ka1 radiation (I = 1.5406 Å) at the laboratory of “Argiles, Géochimie et Environnements sédimentaires AGEs”, University of Liege (Belgium). Identification of the mineral phases of bulk materials (random powders < 250 µm in size) was carried out by using EVA software for qualitative estimation and TOPAS software (™ Bruker) software.
3.4. Major and Trace Element Analysis
Major and selected trace elements were analysed using an ARL PERFORM-X 4200 (Rh X-ray tube) X-ray fluorescence spectrometer, Laboratory of Petrology, Geochemistry and Petrophysics of the University of Liège (Belgium). Major element analyses were performed on lithium tetra- and meta-borate fused glass discs prepared with 0.35 g of rock powder. Raw data were corrected following the Traill–Lachance algorithm and calibrated using 66 international standards (basalts, syenites, granites, ultramafic rocks, minerals, and soils). Trace elements (Co, Cu, Ga, Nb, Ni, Rb, Sr, Y, Zn, Zr, Ce, Ba, Cr, and V) were measured on pressed powder pellets. Except for Cr and V, data were corrected for matrix effects by Compton peak monitoring.
Calculations of the Cross, Iddings, Pirson and Washington standards (CIPW) were made on the basis of the major element contents to determine the normative composition.
4. Results and Discussion
4.1. Petrography
The petrographic types of the Ina batholith consist of alkaline syenite, quartz syenite, monzonite, granite and granodiorite. The area is dominated by Neoproterozoic high-K calc-alkaline syenite and monzonite intruding the Paleoproterozoic basement [28].
4.1.1. Syenite
On macroscopic scale, syenites are the most common petrographic type in the study area. The rocks are massive leucocratic to melanocratic with porphyritic texture (Figure 4(a)) or phaneritic texture (Figure 4(b)). The porphyritic facies are the most abundant, often with a pegmatitic character. These are alkaline syenites made by prismatic phenocrysts of potassium feldspars with malgachite facies (Figure 4(d)). These feldspars are grey to dark-grey in colour (Figure 4(b) and Figure 4(d)), often pinkish (Figure 4(a) and Figure 4(c)), and show bright and shimmering faces with the Carlsbad twining (Figure 4(a)). These sub-automorphic feldspar crystals are joined by an interstitial microlithic ferromagnesian matrix. By shallow alteration, the feldspars remain in relief and the rock shows a rough characteristic surface. Ferromagnesian minerals are dominated by millimetric lamella of biotite. The feldspars from the porphyritic facies are disseminated in a finely crystallized background with biotite and some quartz (Figure 4(b)).
Microscopic observations reveal two syenite facies: alkaline syenite and quartz syenite. The alkaline syenite has a porphyritic texture with a pegmatitic character. It almost exclusively shows large phenocrysts of alkali feldspar with plagioclase in some samples. These phenocrysts are linked to each other by a ferromagnesian
![]()
Figure 4. Macroscopic photograph of porphyritic alkaline syenite (a) and quartz syenite (b) identified on the Ina batholith with pinkish feldspars phenocryst (c) and dark-grey feldspars phenocryst with malgachite facies.
matrix consisting of biotite, amphibole and pyroxene entangled and moulded along the crystals of the feldspars or in inclusion. The accessory phases constitute of apatite, titanite and some zircon. In quartz syenite, the rock is grainy in texture and exhibits the same mineralogical characteristics as alkaline syenite except for quartz and higher mesostasis.
Orthoclase is the most abundant alkali feldspar in the rock (80% to 90% modal). It occurs as large (3 to 5 cm), clear automorphic to sub-automorphic with well-marked cleavages, exhibiting the Carlsbad twin (Figure 5(a)). Only a few spots show, in polarized light, the distinctive grid pattern of microcline. These feldspars phenocrysts are marked by numerous inclusions of opaque minerals characteristic of the malgachitic type-facies illustrated by the whitish spots in Figure 5(c) and Figure 5(d). They are numerous occurrences of perthitic micro textures with exsolutions of sodium feldspars in the form of sparks. These sodium feldspars are close to the albite identified by ESEM-EDX (Figure 5(d)). On the one hand, a marginal substitution is observed, followed by a chessboard substitution where albite forming pericline twinning are oriented in both direction of alkali feldspar (Figure 5(c)).
Plagioclases are rare (10% to 20% modal) with automorphic to sub-automorphic crystals. They are in the form of large areas with a oscillatory concentric zonation (Figure 5(b)) or in the form of elongated rods and marked by a polysynthetic twin of different composition and optical orientation. These plagioclases may contain inclusions of ferromagnesian and opaque minerals (Figure 5(b)).
![]()
Figure 5. Micro-photographs of Ina syenites. (a) Orthoclase phenocrysts with Carlsbad twin; (b) plagioclase phenocryst with zoned twin; (c) microscopic ferromagnesian in alkali feldspars phenocryst observed by ESEM-EDX; (d) perthites in alkali feldspars phenocryst observed by ESEM-EDX; (e) ouralitization in feldspar phenocryst; (f) magnetite and hematite observed by reflected light in the matrix of feldspars phenocryst; (j) apatite crystal and (k) titanite crystal in the matrix of feldspars phenocrysts. Or: orthoclase, Plg: plagioclase, Bt: Biotite, Cpx: Clinopyroxene, Amp: Amphibole, OM: Opaque minerals, Apt: Apatite.
The cloudy aspect of some plagioclases and potassium feldspars indicates some alteration.
Mica is mainly biotite (<7% modal). It is interstitial and appears as small and sometimes distorted flakes. Some greenish brown flakes underline the transformation of biotite into chlorite during weathering (Figure 5(a)). They line up along the edges of the large feldspar crystals with the other ferromagnesian minerals. Pyroxenes (~5% modal) are mainly clinopyroxenes with the characteristic twins of augite. Rare orthopyroxene (i.e., hypersthene) surrounds amphibole (Figure 5(e) and Figure 5(g)). The amphiboles (green hornblende) are xenomorphic and interstitial in the rock, generally associated with biotite and pyroxenes.
Rounded, sub-angular or angular grains of various sizes and shapes of opaque minerals are scattered in the rock, often in inclusions in other minerals such as K-feldspars. These minerals are identified on reflected light as hematite, ilmenite, hemo-ilmenite (Figure 5(f)) and magnetite (Figure 5(f)). They represent < 2% modal of the total rock volume.
The accessory minerals are mainly represented by apatite and titanite. Apatite appears as small elongated prisms embedded in other minerals (Figure 5(g)). Titanite (Figure 5(h)) is observed in small crystals with irregular sub-automorphic contours. Its inclusion in biotite indicates a primary biotite like that of the Monguélé syenites in Southern Cameroon [47].
Quartz is abundant in the grainy facies of syenites (7% modal). It occurs as small ≤ 1 mm sub-automorphic crystals, in interstitial position and seems to be crumpled between the large crystals of feldspars giving it an appearance of a filament (Figure 5(a)). Most often, it presents a wavy extinction and is associated with pyroxenes and amphiboles.
4.1.2. Others Granitoids
1) Granite
Granite samples correspond to leucocratic alkaline granites with a porphyritic texture. They contain pink alkali feldspars in phenocrysts and grains of plagioclase feldspars often mixed with quartz (Figure 6(a)). The significant presence of slightly oriented biotite flakes between feldspars phenocryst makes these rocks biotite granites (Figure 6(b)). Quartz is present as small grains disseminated in a mesostasis formed by ferromagnesian minerals and plagioclases.
At a microscopic scale, the rock is massive, leucocratic with grainy texture. It mainly consists of quartz (~30%) and orthoclase associated with plagioclase, biotite and pyroxene. Quartz is observed as sub-automorphic crystals of millimetre size. Most microcrystal are subcircular with rolling extinction. Some quartz microcrystal is also present as inclusions in feldspar phenocryst.
Orthoclase (Figure 7(a)) occurs as millimetric to centimetric crystals with automorphic to sub-automorphic borders. The weathering gives a cloudy appearance to the feldspars. This cloudy appearance represents an association of potassium feldspars and sodium feldspars leading to a perthitic microtexture. Plagioclase is observed in elongated sub-automorphic rods of millimetre to centimetre
![]()
Figure 6. Macroscopic photograph of granite with phaneritic (a) and porphyritic texture (b) samples identified on the Ina batholith.
![]()
Figure 7. Micro-photographs of alkaline granite; with perthitic feldspars and sub-automorphic quartz (a) and myrmekite microstructure (b) observed in the granite samples of the Ina batholith.
size with common polysynthetic twins. The presence of quartz intergrowths in the alkali feldspars leads to an observed myrmekite microtexture (Figure 7(b)).
Biotite occurs as lightly coloured flakes in light to dark brown. These flakes are deformed, frayed and eventually chloritized or even associated with titanite. Pyroxenes are rare (<1% of rock volume), occurring as sub-automorphic augite, interstitial and generally associated with biotite. Opaque minerals (<1% of rock volume) occur in grains of varying sizes in association with ferromagnesian minerals. Accessory minerals are represented by rare titanite observed in the interstices and in association with biotite.
2) Monzonite
Both monzonite and quartz monzonite are found at the site of Ina batholith. The rocks are massive, leucocratic to melanocratic with quartz and without quartz respectively. They have a granular structure where the size of the feldspar crystals varies from millimetric to centimetric. Small crystals of ferromagnesian minerals (biotite, pyroxene, amphibole, and opaque mineral) are more abundant in the quartz monzonite facies (Figure 8(a))
The most abundant minerals in thin section are large feldspars notably orthoclase characterized by wide zones (about 2 cm) all marked by the Carlsbad twinning. Plagioclases stand out for their polysynthetic twins. They are elongated rods about the size of the orthoclase (Figure 8(b)). The feldspars in monzonite are characterized by a pinkish colour like in granite. The plagioclases and K-feldspars have a modal composition of ~70%.
Quartz is quite abundant in quartz monzonite but its content varies between the outcrops. They are wavy extinction, automorphic to sub-automorphic crystals. The relative concentration is less than 15%. Biotite is also marked by the presence of flakes which, like in other rocks, are gradually transformed into chlorite. Biotite is also observed in clusters of minerals whose relative abundance is about 10%. Among the other ferromagnesian minerals, we note the presence of amphiboles (i.e. green hornblende) and clinopyroxene (e.g. augite). Opaque minerals are present in inclusion in alkali feldspar or plagioclase. A myrmekitic microtexture is also observed.
3) Granodiorite
Granodiorites are massive, brown in colour with an aphanitic structure. These rocks present large centimetric phenocrysts (up to 5 cm) of potassic feldspars sporadically dispersed in a finely grainy matrix (Figure 9(a)) with fine flakes of biotite and small translucent quartz crystals.
On the thin section, the granodiorite is marked by the presence of large centimetric phenocrysts of feldspars dispersed in the finely grained matrix (Figure 9(b)) with fine biotite flakes and small quartz crystals. Feldspars crystals present a malgachitic facies, also visible at a macroscopic scale as syenite.
Granodiorite is made up of alkaline feldspar and plagioclases with quartz, biotite and amphibole as secondary minerals. Opaque minerals constitute the accessory phases. Orthoclase is sub-automorphic and is observed either in large crystals up to 5 cm, or in the form of finely grained crystals all devoid of perthite. The phenocrysts present the Carlsbad twin. Plagioclases appear as small, xenomorphic, whitish crystals abundant in the rock. The polysynthetic twin scratching is rare in crystals. Orthoclase and plagioclase contain inclusions of opaque
![]()
Figure 8. Macroscopic photograph of quartz monzonite sample (a) of Ina batholith and his micro-photographs showing alkali feldspars and plagioclase phenocryst with sub-automorphic quartz and ferromagnesian minerals (biotite, pyroxene and opaque minerals).
![]()
Figure 9. Macroscopic photograph of granodiorite sample (a) of Ina batholith and his micro-photographs showing alkali feldspars phenocryst with ferromagnesian minerals (biotite, pyroxene and opaque minerals).
minerals. Pale yellow to greenish biotite appears as fine flakes or in clusters wide of crystals of orthoclase and plagioclase. Amphibole is disseminated throughout the rock with a relatively high abundance compared to biotite. They are observed in small crystals of olive-green colour and seem to stretch in the length. Rare opaque minerals (magnetite) are included in feldspars.
4.2. Quantitative Mineralogy
Diffractograms and indexing of characteristic hkl lines allowed the identification of the mineral phases and their proportions in each. The indexing diffractograms were selected and grouped according to facies (Figure 10). The mineralogical composition of the syenite samples is similar (Figure 10(a)) with orthoclase, microcline, albite, anorthite, biotite, augite, green hornblende and some quartz. Except the presence of muscovite in granite and monzonite, the other rock samples of Ina batholith reveal the same mineralogical phases as those identified in the syenites (Figure 10(b)) but with different proportions.
The average quantitative estimation of the different mineralogical phases is given in Table 1. It is noted that the amount of plagioclase (albite and andesine) is about ~40%. This abundance of plagioclase underlines the presence of cryptoperthites invisible under an optical microscope but detectable by XRD [48].
4.3. Geochemistry
4.3.1. Classification and Nomenclature
The typology of the plutonic rocks suited is based on the classification of [49] which compares the wt% composition of Na2O + K2O (total alkali) and SiO2 oxides. The major element composition of the rocks samples of Ina batholith is consistent with syenite, monzonite, quartz monzonite, granite, granodiorite and monzodiorite (Figure 11). However most of the samples plot in the field of alkaline syenites with a high modal content in orthoclase (~85 wt%).
4.3.2. Major elements
The abundance in major and trace elements and the calculated CIPW (i.e., Cross, Iddings, Persson and Washington) standard for each rock type is presented in Table 2. The major element data of Ina batholith rocks samples shows
![]()
Figure 10. XRD pattern of alkaline syenite samples (A) and some granitoids samples (B) of Ina batholith.
![]()
![]()
Table 1. Quantitative estimation of the mineralogical phases in the Ina alkaline syenite (a) and Monzonite (b) derived from Topas software (™ Bruker). The abundance is calculated on an average of 25 samples for (a) and 15 for (b).
![]()
Figure 11. Igneous rocks classification of Ina batholith samples rocks plotted in total Alkali Silica (SiO2 versus Na2O + K2O) diagram according to [49].
a compositional range between 58.68 wt% and 63.63 wt% silica from intermediate rocks (syenite) and up to 77.02 wt% for saturated rocks (granite). The Al2O3 content range from 13.91 wt% to 19.81 wt% while the percentage of alkaline oxides (Na2O + K2O) range from 6.05 wt% to 13.69 wt% with the highest percentage in the intermediate rocks. The content of the other oxides is variable: 0.95 wt% to 8.09 wt% Fe2O3, 0.32 wt% to 1.45 wt% TiO2, 0.35 wt% to 3.52 wt% CaO, (0.13 wt% to 3.98 wt% MgO. The contents in MnO and P2O5 are low (<0.8 wt%, Table 2).
[50] defined a geochemical boundary in the TAS diagram used to differentiate plutonic suites where all syenite project within the limits of alkaline series (Figure 12(a) and Figure 12(b)). In the same diagram, some monzonite and granites project into the sub-alkaline series. Frost et al., (2001) [51] proposed a new classification considering the CaO content of samples and allows to define 4 domains between alkaline and calcic series (Figure 12(c)). This previous diagram shows that the majority of samples from Ina batholith project into the alkaline suite domain with some samples in the alkaline-calcic and calcic range. It should be noted that all syenite are plotted in the calc-alkaline series. Despite these small variations, the Ina batholith samples are characterized by a marked alkaline composition.
The relative abundance of silica, alumina and alkaline oxides [(SiO2 ˃ Al2O3 ˃ (Na2O + K2O)] and the ratio Al/(Na+K) > 1 indicate a peraluminous composition [52] [53] of most studied rocks (Figure 12(d) and Figure 12(e)). This peraluminous classification is also justified by the characteristic mineralogy of the Ina batholith rocks samples with the remarkable presence of biotite but also apatite and ilmenite as accessory mineral [52] [54] [55]. However, the K2O/Na2O ratio ranging from 1.69 wt% to 3.39 wt% underlines hyperpotassic composition, in agreement with the distribution in the ternary diagram of Marshall (Figure 12(e)).
When it comes to major element versus SiO2, the analysis of the evolution of the major elements refers to the silica saturation diagrams of [56] the Ina batholith rocks samples follow similar differentiation trends. Although the intermediate rocks, notably syenite shows more similarity. The evolution of Na2O and Al2O3 according to the silica content in syenite show positive correlations as Na2O and Al2O3 content increase when the silica decrease (Figure 13(a), Figure 13(c)). In the others granitoids, the plotted points are scattered and result in a less distinctive correlation. The evolution of CaO, TiO2, MgO, Fe2O3 and P2O5 reflects a similar negative trend (negative correlation) as shown in the Figure 13(d), Figure 13(e), Figure 13(g), Figure 13(i), The absence of strong iron enrichment in rocks as in this case (0.89 ≤ Fe2O3 wt% < 4.69)., indicates an alkaline series in magmatic differentiation [50].
There is a very weak correlation between K2O and SiO2 in the syenite and no correlation in the other granitoids rocks (Figure 13(b)). Instead, the latter allows for an abundant concentration of K2O in the silica content range of intermediate
![]()
Table 2. Chemical composition of the different rock samples of Ina batholith measured by XRF on bulk rock powder (wt%).
![]()
Figure 12. Magmatic classification series of rock samples of the Ina batholith. (a) Na2O + K2O vs SiO2 [50], (b) Al2O3 vs normative plagioclase (An wt%) [50], (c) Na2O + K2O - CaO wt % vs SiO2 [51], (d) A/NK vs A/CNK [53], and (e) Albite, anorthite and orthoclase ternary diagram of [57].
rock with very slight variations in their content (7.52 ≤ wt% K2O < 9.78). This can be explained by the abundance of alkaline syenite among the studied samples and also by the fact that these syenite are hyperpotassic [57] (Figure 12(e)).
The calculation of the CIPW standard allows to identify the theoretical composition of felsic and accessory minerals For Ina syenites, two facies are distinguished by their normative quartz composition: alkaline syenites with low quartz content (At most 4.77 wt%) and quartz syenites with quartz content ≥ 5 wt% The normative orthoclase is abundant, ranging between 38.39 wt% and 57.02 wt%. Plagioclase ranges from 24.42 wt% to 41.35 wt%. Ferromagnesian and accessory minerals are the same as observed in the thin sections. The anorthite
![]()
Figure 13. Hacker variation diagram of some major elements (in wt% oxide) versus SiO2 of the Ina batholith rocks samples [63].
proportion of plagioclases range from An2 to An26.30 proof that plagioclases are sodic and therefore, very close to albite.
4.3.3. Trace Elements
The trace elements data of Ina batholith samples (Table 3) indicate high values in Ba which varies from 2159 to 4953 ppm in the syenite and from 2470 to 6664 ppm in the monzonite. In granite and granodiorite, Ba ranges from 892 to 2159 ppm and to 1905 to 2159 ppm, respectively. Sr is also characterized by high abundance from 848 to 1477 ppm in syenite and 356 to 2274 ppm in monzonite. On the contrary, Nb, Pb, Co, Cu, Y and Ga display the lowest values, ranging between 10 and 300 ppm at most.
The parallel evolution of trace and major elements in the Harker diagrams (Figure 14) shows similar behaviours with silica enrichment for Ba, Sr, Rb and Zr. In contrast Cr and Ni present a negative correlation with silica in all studied petrography types. The classification diagram of Whalen et al. (1987) [58] shows that most petrographic types of the Ina batholith samples belong to the others granites, notably designated by the letters “S” and “I” nomenclature according to [59] and MPG nomenclature of [60]. Recently these nomenclatures (S and I) have
![]()
Table 3. Trace element (ppm) of granite, granodiorite and monzodiorite of the Ina batholith. Note nd indicates values not detected by the XRF.
![]()
Figure 14. Hacker variation diagram of some trace elements of Ina batholith syenite [63].
been associated to granitoids and not only to the granites [54] (Figure 15(a)). The diagram of [61] confirms that most samples belong to the anorogenic domain of syn-collisional granitic magma (Figure 15(b)), with Y/Nb values < 1.2
![]()
Figure 15. Diagram showing the setting of Ina batholite rocks; (a) diagram from [58] and (b) diagram from [61].
ppm characteristics of most alkaline suites [62].
5. Potential of Ina Syenite as a Source of Feldspathic Flux
In general, several igneous rocks whose petrographic and geochemical classification obeys a certain standard are exploited as alternative telepathic flux raw material for ceramics and glass industries [18]. These include alkaline syenite and quartz syenite [3] [6] [19] [21] [22] [64] [65], monzonite and quartz monzonite [19], granite [17] [19] and even gneiss [66]. The rock samples of Ina batholith, according the geochemical characterization and nomenclature from the TAS diagram [49], belong to the range of these igneous rocks used as feldspathic fluxes.
Syenite, due to their specify, are among the most requested rocks as alternative source for feldspathic flux worldwide [4] [17] [22] [67] [68]. For this purpose, several characteristics criteria based on petrographic features, mineralogical and geochemical composition are analysed. These syenites are the most common petrographic type in the Ina batholith and therefore require special attention others petrographic types (monzonite and granite) that are also potentially valuable but are not enough present on the study site.
Petrographic analysis of rocks samples of Ina shows that they are mainly composed of bulky K-feldspars phenocryst, sometimes plagioclases phenocryst (at least 85 wt% of feldspar) and ferromagnesian minerals such as those in the syenite study by [19] [69]. Although feldspars phenocrysts is the main mineral, its purity is affected by the presence in the form of inclusion of ferromagnesian, accessory minerals but especially ferrous opaque minerals leading to the dark grey color of these feldspars with the so-called malgachite facies [70]. The recurrent observation of perthites in K-feldspars compensates for the low presence of plagioclases, particularly sodium plagioclase and thus the increasing of sodium content of Ina syenite as feldspathic flux raw material [17]. Quartz identified in some petrographic type also plays a key role in ceramic production at it increase stiffness, prevent deformation and reduce capillary cracking and thermal expansion of the product [71]. The fineness of the ferromagnesian matrix of these rocks has an impact on its structural nature which is also essential in that crushing and grinding could be done with reduced energy consumption [72].
On the geochemical level, the contents specially sought in commercial feldspathic flux raw materials are those of alumina, potassium, sodium, iron and titanium. The minimum required alumina content is 16 wt% [73]; yet the Ina syenite have high contents (15 ≥ wt% Al2O3 ≥ 20) for an average of 17.03 wt% (Table 2). The relation [(SiO2 ˃ Al2O3 ˃ (Na2O + K2O)] and the ratio Al/(Ca + Na + K) > 1 which is Alumina Saturation Index (ASI) justifies the peraluminous nature of all Ina batholith syenite samples. This in accordance to Shand diagram (1943) (Figure 16(a)) as the most part of the granitoids ores exploited worldwide [19]. This alumina improves the hardness, flexibility and mechanical strength of the ceramic products and chemical resistance of the glass [21] [65] [74].
The minimum content required for total alkali (K2O + Na2O) is 6 wt%. They are considered as the main melting oxides whose role is to control fusibility [17] [73]. However, the total alkali content of Ina syenite is very high (10.10 ≥ wt% K2O + Na2O ≥ 13.69) and the average is about 11.48 wt%. The Figure 16(b) [57] shows the potassic character of the Ina syenite. In addition, the result show
![]()
Figure 16. Geochemical classification of Ina syenite according to feldspar ores worldwide, (a) A/CNK vs A/KN Shand diagram (1943) and (b) Orthoclase-Albite-Anorthite ternary diagram of [57] and (c) wt% Fe2O3 + wt% TiO2 content of Ina alkaline syenite compared to [73] norm.
higher total alkali content of Ina syenite compared to the feldspar ore of the world’s specialized feldspars ores [Germany (9 wt%), Sweden (8.83 wt%), Norway (9.20 wt%), Czech (7.89 wt%) and Japan (7.58 wt%)] [17].
In the Ina syenite, the iron (1 ≥ wt% Fe2O3 ≥ 5) and titanium (0.4 ≥ wt% TiO2 ≥ 1) content are very high (Figure 16(c)) compared to the minimum required which varies from 0.01 to 0.5 depending on their use [73]. this amount of colored element reduces the purity of the feldspars and has a significant impact on its whiteness.
Indeed, according to data of Ina syenite, they can be suitable in ceramic and glass industries like some of other feldspars ores studied around the world (Figure 17) despite a high content of iron, titanium and other colored minerals which are considered harmful to ceramics and glass applications. Thus, several scientific works have shown that it’s possible to carry out purification and enrichment treatments of feldspar ores that increase the content of alumina and melting elements in the one hand and reduce the impurity contents. These treatments consist of magnetic separation that removes iron bearing and titanium bearing minerals [15] [75] [76] [77] [78]. Magnetic separation is most often followed by flotation adapted for colored minerals [75] [79] [80] [81]. Flotation also allows to isolate K-feldpar from Na-feldspar [82] [83]. The high ppm content of trace elements such as Ba and Sr in the Ina batholith syenite samples has no impact on the potential of these rocks as a feldspathic flux for the ceramic and glass industries.
At the end, ceramics tests are still needed to approve the use as raw material in these industries. The high traces elements content in Ba and Sr of Ina batholith samples has no impact on the potential of these rocks as a feldspathic flux for ceramics and glasses industries but also as a source of potassium for agricultural amendment [9]. In addition, the granite, monzonite and granodiorite of the Ina
![]()
Figure 17. Comparative curves of chemical composition of alkaline syenite of Ina batholith and some other feldspars ores suited around the world with SiO2 wt%/3.
site can also be used as a source of feldspathic flux. These rocks, including syenite, can be exploited as dimension stones [4] [7] [8].
6. Conclusion
The aim of this study was to characterise the rocks of Ina batholith with a particular focus on syenites in order to assess its industrial mineral potential. Based on petrography, mineralogy and geochemistry, the Ina batholith consists of syenites (alkaline syenite and quartz syenite) and other granitoids including granitic bedrock, monzonite and granodiorite intrusion. Syenite ranging in composition from predominantly alkaline syenite to minority quartz syenite is far the most promising petrographic type. Petrographic examinations of the samples show that the rocks mainly consist of alkali feldspar phenocryst and plagioclases well as minor amounts of various ferromagnesian mineral (chloritized biotite, clinopyroxenes, and amphiboles). These feldspar phenocrysts are characterised by their dark grey hue due to the presence of several opaque minerals’ inclusions (ilmenite, hematite, magnetite) and some accessory minerals (titanite). There are also numerous perthites and cryptoperthites established by exsolutions of sodium feldspars. In addition, quartz is observed as interstitial microcrystals or as micrographic intergrowth with alkali feldspar.
Geochemical analysis of the major elements reveals proportions of silica that range from intermediated rocks (58.68 wt%) to saturated rocks (7702 wt%). The results also indicate alumina contents range between 15.08 wt% and 19.80 wt% with an average of 17.03 wt%, total alkali range between 10.30 wt% and 12.30 wt% with an average of 11.48 wt%; K2O/Na2O ratio varies between 1.65 wt% and 5.51 wt% with an average of 2.58. In contrast, the iron and titanium contents are quite low (<5 wt%). These geochemical data justified the fact that all syenites are metaluminous and ultrapotassic. These data underline that alkaline syenites of Ina are more suitable for the ceramic and glass industry than some feldspathic fluxes ores of the same category mined throughout the world.
Acknowledgments
The data are part of the doctoral thesis of the first author at the Department of Geology, of the University of Liège (Belgium) in co-diplomation with the Department of Earth Sciences of the University of Yaoundé I, Cameroon. The first author would like to thank ARES-CCD for the PhD grant and technical support.