Petro-Geochemistry, Genesis and Economic Aspect of Syenitic and Mafic Rocks in Mindif Complex, Far North Cameroon, Central Africa

Syenitic and mafic rocks in Mindif Complex (Far North of Cameroon) were surveyed and characterized to classify them, understand their formation history, and assess their economic interest. Syenitic bodies (hololeucocratic microsyenites; mesocratic aplitic quartz-syenite; leucocratic porphyritic quartz-biotite syenite, and leucocratic porphyritic biotite-syenite) are silica-oversaturated to silica-saturated, alkaline, and metaluminous. Hololeucocratic microsyenites are structural oriented rocks, cooled in shallow depth from low trace and REE dry residual alkaline melts. Mesocratic aplitic quartz-syenite also crystallized in shallow depth from a much Ba-rich less dry residual melt. Leucocratic porphyritic quartz-biotite and biotite syenitic stocks represent two different rock types cooled from hydrous-rich melts in deep seated environments. The Mindif syenites probably crystallized in crustal source magmas (with important alkali feldspar accumulation) from partial melting of pre-existing igneous protoliths. Medium to coarse-grained-peraluminous granite found at the edge of pink microsyenitic dykes (in contact with granite host), is probably a crystallized product from magmatic mixture between the intrusive syenitic melt and a melt from partial fusion of the granite host rock. Mafic igneous rocks in Mindif are peridotgabbro and gabbro with different characteristics. Peridotgabbro, alkaline, holomelanocratic, medium-grained and ultrabasic, is an REE and incompatible elements depleted rock crystallized in shallow depth from a more evolved mantle source magma with plagioclase accumulation. Tholeiitic gabbro, melanocratic, and also medium-grained, was cooled in shallow depth, from a low-K and slightly LILE-enrichment magma probably originated from partial melting ultradepleted mantle source in sub-duction environment. Syenites and peridotgabbro with their high aluminum content (up to 18.5 wt%) greater than that of many parent rocks of lateritic bauxite in Cameroon, can be good parent rocks for bauxite-rich soils if found in a plateau topography and hot-humid tropical climate. The geochemical features of Mindif syenites are close to that of some syenites mined and used as raw materials in ceramic and glass industries. This similarity shows that these syenites are good for glass and ceramic manufacture.

Syenite's textures range from fine to coarse-grained, porphyritic and even pegmatitic; features highly related to their environment and depth of formation [1] [3] [8] [9]. The features presented above are useful for identification and differentiation of a syenitic rock, and, can also help to understand the condition under which these rocks were formed. Syenites present distinctive petrography and geochemical features; important in differentiating them and their characterization. They are also useful in constraining their crystallization history including petrogenesis and tectonic setting [2] [3] [4] [10] [11].
Syenitic rocks are very good in mining and processing industries as they can host gemstones and ore minerals [9] [12] [13] [14] [15] [16], or can enrich soil profiles with some minable metallic elements [17]. They can also be mined as industrial mineral or raw material for some industries [18] [19] [20]. Gem quality corundum was found in some syenites (e.g., sapphire-host syenite xenoliths in Cerova Mountains, Slovakia: [18]; corundums in syenite pegmatites from the be of great economic interests. Uncharacterized syenitic complex is found in Mindif at about 10 km SE of Maroua, Far North Region, Cameroon (Figure 1(c) adapted from [25] and [26]). Rocks in this complex are still not well characterized as limited information is available on them. Their type is still not differentiated, and the geological conditions under which these rocks were formed are still to be elucidated. Additionally, their economic interest is still to be assessed. In this paper, we used field geology, petrographic studies and element geochemical analyses to: 1) characterize and classify rocks found in the Mindif syenitic complex; 2) understand their formation history; and 3) assess their economic potential. Figure 1. Sketch geologic maps locating the study area within the local and regional settings (with data from [25] and [26]) (CLG: Chad Lake Group; BG: Benue Group); SGp (PG: Poli Group; AG: Adamawa Group; WCG: West Cameroon Group).

Geography and Geological Settings
The Far North Region of Cameroon is made of high and low lands. The high lands are mountainous or form isolated inselbergs. Mountainous topographies are found in the West and SW of the region, precisely, in Kapsiki Plateau (altitude: 1000 m), and part of the Mandara Mountains [27]. Spots and isolated hills (forming inselbergs) are found in the south and southeast, with some of them located in Mindif, Moutourwa, Lara, and Boboyo. The North and a great portion in the East of the region are low.
The Far north of Cameroon is composed of three main geological units ( Figure 1(b)): 1) Quaternary deposits found in Lake Chad Basin and many other parts of the region [28] [29] [30] [31]; 2) volcanic to sub-volcanic complexes related to the Cameroon Volcanic Line [27] [31] [32]; and 3) igneous and metamorphic rocks of the Pan-African Cameroon Mobile Belt [25] [33]. Quaternary materials, mainly found in low lands, are visible in the north and in the east. Some were deposited in and along many rivers (e.g., Mayo Diamaré, Mayo Kani, Mayo Danay, Mayo Tsanaga, Mayo Boula, Logone and Chari). These materials are mostly made up of alluvial and lake-type clay deposits [25] [28] [29] [34], and alluvial and ancient sands [25]. Supergenes deposits were developed on gabboic rocks [30], soapstones, gneisses, migmatites, syenites, and granitoids [25]. Volcanic and sub-volcanic rocks are dominantly found at the Kapsiki Plateau, although, spots are found at hilly reliefs around the town of Maroua. Kapsiki Plateau, the northernmost volcanic structure of the Cameroon Volcanic Line [35], is an asymmetrical horst which covers an area of 150 km 2 and lies along the Cameroon-Nigeria border [25]. The volcanic rocks in this plateau are alkaline trachytes and basalts, and perakaline rhyolites [36] [37]. Ankaramites, phonolites, and hawaiite are also found [27]. The magmatic and metamorphic rocks forming the basement in the Far North region of Cameroon vary from one locality to another. Around Kapsiki Plateau, they are assumed Precambrian age migmatites and anatectites; intruded by granites, and cut or overlain by trachytic, rhyolitic, and phonolitic plugs, basaltic necks or flows [27]. The basements rocks in other areas in this region are gneisses, amphibolites, schistes, soapstones, diorites and/or granitoids locally overlain or cut by basaltic flows, dykes or granitic and syenitic stocks [25] [33]. Mindif in the South East of Maroua is dominantly covered by superficial materials (laterites, ancient sands, recent alluviums) partly overlying, gneisses, amphibolites, and soapstones [25]. Part of these rocks is intruded by undifferentiated and undated syenitic inselbergs (with height exceeding 550 m) and various domes.

Field Work and Analytical Methods
Geological survey and characterization of syenitic bodies and mafic rocks were carried out in Mindif. Located outcrops were described based on the following parameters: color, texture and structure, mode of occurrence, mineralogical composition, the existence of specific features (dykes, veins, veinlets, lens, pock-

Results
Field and petrographic data for each rock type are presented separately after its nomenclature and classification. The major, minor, trace and rare earth element composition of those rocks are also presented.

Nomenclature and Rock Classification
Plotted data in SiO 2 versus Na 2 O + K 2 O diagram of [39] (Figure 2) distinguish: syenites, gabbro, peridotgabbro and granite. Syenites are all plotted in alkaline field, and contact granite, on alkaline-sub-alkaline dividing line. Gabbro and peridotgabbro are plotted in sub-alkaline and alkaline field, respectively.  Light-grey syenite also forms domes and sub-rounded to rounded very large blocks in the south and eastern part of the locality. Some blocks enclose mafic xenoliths (similar to those found in whitish syenite) (Figure 3(b)). This leucocratic rock is coarse-grained to porphyritic. It is mainly composed of microcline and few plagioclases. Very few biotite (Figure 4(b)), clinopyroxene, zircon and opaque minerals are found. Clinopyroxene, opaque minerals and biotite are dominantly found together. Zircon crystals prismatic to pyramidal in shape, occurs as inclusion in biotite. The proportion of microcline and biotite is less than those found in whitish syenite.
Dark-grey syenite cropping in the west of the study area, is a mesocratic and quartz-bearing; partly underlying a porphyritic very coarse-grained, sheet-like and highly altered granitic cover rock. It is medium-grained and mainly consists of orthoclase (with few sericite or perthitic texture) and plagioclase. Few biotite and clinopyroxene are associated to very few quartz, titanite and opaque minerals. Orthoclase laths are generally larger than that of plagioclase. Sericite, visible in some orthoclase, locally forms micro-veinlets. International Journal of Geosciences Pink syenites (Figure 3(c)) (found in the west), are hololeucocratic (no visible ferromagnesian minerals), highly fractured, and form dykes. They cut across highly weathered granitic country rock (similar to the one underlying dark-grey and whitish syenite). The main directions of those syenitic dykes are N60E90, N120E90, and N40E60SE. The composition and grain size change towards the contact with the wall-rock. They are fine-grained (≤1 mm in size) in the core, but, medium-grained towards a granitic transitional rock linking the pink syenite with the host rock. Pink syenite is composed of plagioclase, and microcline, and orthoclase (with sericitization), and few muscovite. The contact granite is hololeucocratic, medium to coarse-grained rock with textural zoning. It is coarse-grained at the contact with the country rock and medium-grained close to pink syenite. It is mainly made of quartz and microcline with few plagioclases (Figure 4(c)). Mineral's size increases towards the granitic host rock.

Peridotgabbro, Gabbro and Amphibolite
Peridotgabbro found in the east of the area, is very dark in color and holomelanocratic ( Figure 4(d)). This rock locally encloses few felsic to greenish veinlets probably product of fluid's circulation. It is dominantly composed of long plagioclase laths associated with olivine crystals (euhedral to well-rounded) ( Figure   4(c)). Few orthopyroxene and clinopyroxene with sub-hedral to euhedral shape are found. Part of the olivine crystals encloses regular to irregular cracks, or fine-grained crystals of the same mineral. Few anhedral to euhedral opaque minerals occurs at the contact with some olivine crystals. Gabbro found in the south-east of the study area is dark-grey, melanocratic, and have no clearly defined shape. It is medium-grained and composed of clinopyroxene, orthopyroxene, plagioclase, and few opaque minerals.
Amphibolite is a melanocratic and pneumato-granoblastic rock outcropping in the east of the area. This rock is weakly-schistose "amphibolite schist" with its features similar to those of xenoliths found in light-grey and whitish syenites.
This rock is medium-grained and mainly consists of brownish amphibole crystals. These crystals (sub-hedral to euhedral) are mainly aligned (parallel to SW-NE direction). Few amphibole crystals are associated with sub-rounded to anhedral quartz in quartz-rich alignments. Sub-hedral clinopyroxene and very few plagioclase laths are spotted in quartz-rich alignments. Opaque minerals are found in both amphibole-rich and quartz-rich parts.

Geochemistry
Major and minor elements (in oxide) abundance and CIPW norms (

3) Spider diagram and REE patterns
The primitive mantle-normalized multi-element patterns (N-MORB) (normalization values after [40] spider diagram (Figure 7    Ouo-Léré Basin (North Region of Cameroon: [41]). The distinction between the study peridotgrabbro and gabbro is clearly shown in Figure 2; as their plots fall in different positions. Peridotgabbro is alkaline whereas, gabbro is sub-alkaline in TAS diagram ( Figure 2). The CIPW norms show the presence of nepheline in peridotgabbro, which is absent in gabbro (with normative hypersthene).
2) Trace and rare earth elements Trace element contents in gabbro and peridotgabbro (Table 2), relatively distinguish: 1) high elements suite (101 -345 ppm: Sr, Ni, V, Cr, Cu, Zn, and Zr ); 2) low element suite (15 -80 ppm: Ba, Co, Sc, Y, and Ga); and 3) very low (0.03 -9.20 ppm: Nb, Li, Rb, Be, Hf, Sn, Ta, Th, Pb, Cs, and U). Incompatible element classification shows that within (LILE) and (HFSE), Sr (345 ppm in peridotgabbro and up to 252 in gabbro) and Zr (23 ppm in peridotgabbro and up to 101 in gabbro), are the highest. Other distinctive features are the Sc (≤186 ppm), V (≤294 ppm), Ni (≤258 ppm), Cu (102 ppm) contents in gabbro, which are higher than those found in peridotgabbro (with highest Zn: 117 ppm). These trace element compositions are different to those in basalts at Kapsiki plateau studied by [25]. Trace element abundances in gabbro are much close to those in microgabbro and basalt cropping the Mayo Ouo-Léré Basin [41]. Calculated trace elements ratios in Table 2 Figure   8(c) is almost flat for gabbro (with no Eu anomaly); but weakly increases from La to Eu and decreases from Eu to Lu for peridotgabbro.

Discussions
Field, petrographic and geochemical data are used to characterize each rock type cropping in the Mindif complex. The same data are used for pretrogenesis, tectonic evaluation and to assess their economic interest.

Characterization and Classification
Five rock types (syenitic bodies, contact granites, peridotgabbro, gabbro and amphibolite) present specific features that can be used to characterize and classify them.

Syenitic Bodies and Granitic Contact Rock
Syenitic rocks in Mindif complex are four types in color: pink, dark-grey, light-grey, and whitish. This difference is also texturally and compositionally visible; with pink syenite being fine to medium-grained, dyke-like and with no ferromagnesian mineral; dark-gray syenite (medium-grained, mesocratic and with few quartz); light-grey syenite (leucocratic, coarse grained to porphyritic with few ferromagnesian minerals) and whitish syenite (leucocratic coarse-grained to porphyritic, and with relatively higher proportion of ferromagnesian minerals). These variations are probably due to different crystallization environment and cooling history. Fine and medium-grained syenites are probably products of rapid to slow cooling of their magmas at shallow depth [9] [42]. Coarse-grained to porphyritic syenites crystallized from slow cooling of melts at deep seated environments [9] [42]. These differences are also geochemical (see plots in Figures 5-8) with some elements being highly concentrated in some syenites than in others, typifying high enrichments of specific elements in their source magmas. In TAS diagram (Figure 2), and with their CIPW norms, two groups are distinguished: (1) alkaline quartz-bearing syenites and (2) alkaline quartz-barren syenites (showing that they were cooled from alkaline magmas). The alkaline nature of the syenites is clear shown, as their plots fall in shoshonite series [43] (Figure 9(a)) and alkali field [44] (Figure 9(b)), and the Na 2 O + K 2 O > 11 wt% (compatible with value presented in [45] and alkaline plutons in [46]). Quartz alkaline syenites enclose the highest REE contents which coupled with their relatively low Zr value, differentiate them from other syenites. Excepting the contact granite which is peraluminous, all the syenites  [47]; and (d) molecular Al 2 O 3 /CaO + Na 2 O + K 2 O versus SiO 2 plots in [48]. International Journal of Geosciences are metaluminous, as show in Figure 9(c) [47]. Syenites and contact granite found in Mindif are I-type granitoids [48] (see Figure 9(d)) showing that their source magmas are from igneous origin. Petrographic differences coupled with alkalinity distinguish for four main groups of syenitic rocks: 1) hololeucocratic alkaline microsyenite; 2) mesocratic alkaline aplitic quartz-syenit; 3) leucocratic alkaline porphyritic quartz-biotite syenite; and 4) leucocratic alkaline porphyritic biotite-syenite.

Peridotgabbro, Gabbro and Amphibolite
Mafic igneous rocks in the Mindif complex include peridotgabbro (holomelamocratic and medium-grained), and gabbro (melanocratic and medium-grained). The medium-grained texture of peridotgabbro and gabbro shows that their source magmas probably cooled in shallow depth. These two rocks clearly show petrographic and geochemical differences (high olivine proportion in peridotgabbro and its relatively low SiO 2 content < 43.0 wt%; and very low olivine proportion and SiO 2 > 49.0 wt% in gabbro). This difference indicates that they are not from the same magma (peridotgabbro crystallized from silica depleted magma and gabbro, product of silica slightly enriched melt). Peridotgabbro could be originated from silica-undersaturated magma, as normative nepheline is determined. Gabbro with the absent of normative nepheline and quartz could be originated from a silica-saturated melt. The different between peridotgabbro and gabbro is shown in TAS diagram (Figure 2), as the first rock is plotted in alkaline field and the second, in sub-alkaline or tholeiitic. The tholeiitic nature of gabbro typifying that higher degrees of partial mantle melting were involved in the source magmas, as shown in [49] Y/Nb versus Zr/Nb of, Zr/Nb versus Zr/Y, and Zr versus Nb diagrams ( Figure 10). The other difference is the presence of normative hypersthene in gabbro and it absence in peridotgabbro. The relative high content alumina, iron oxide, MgO and CaO in peridotgabbro than in gabbro also differentiate this tow rocks. The high Al 2 O 3 content in peridotgabbro is probably due to important plagioclase crystallization in this rock and could classify it as high-Al basaltic rock [50]; and, noted high Fe 2 O 3 and MgO contents in this same rock can be justified by the presence of high olivine. The other difference is that of specific elements (e.g., Cr, Ni, Cu, Sr, Zr, Ba and REE) higher in gabbro than in peridotgabbro (V, Sr, and Zn-enriched). For [51], continental alkaline basaltic rocks are often enriched in trace elements relative to oceanic counterparts. The trace elements abundances in peridotgabbro (the only studied alkaline basaltic rock) are globally less than those in continental basaltic rocks presented in [50] and [51]. They can therefore be classified as oceanic alkaline peridotgabbro. The Th (<1.0 ppm) in gabbro is within the range limit in tholeiitic microgabbro and basalt found in Mayo Oulo-Léré Basin (North of Cameroon) which were classified as continental tholeiitic basaltic rocks [41]. The mafic rocks found within the Mindif complex can be named: 1) alkaline-nepheline peridotgabbro and 2) sub-alkaline olivine-hypersthene gabbro. Amphibolite-schist, melanocratic, fine-grained,  sheet-like, with pneumato-granoblastic texture relatively, is one of the oldest country rock in the study area as some fragments are reworked xenoliths in biotite porphyritic syenites.

Syenitic Bodies and Contact Granites
Alkaline syenites found in Mindif complex with their variations, may have different crystallization history. Fracture filling materials represent discordant bo-International Journal of Geosciences dies probably cooled in shallow depth (less than 1 km) whereas, stocks (mainly found in the locality), were cooled much deeper (probably at more than 1 km) in the Earth Crust. The alkaline syenites were crystallized from silica-saturated melts (those without quartz) and silica-over-saturated melts (those with quartz). The two rocks identified to have cooled from silica-oversaturated melts (dark-grey and whitish syenite: quartz syenites) have color, textural and compositional differences proving that they never crystallized at the same depth and therefore, may not be from the same magma or represent different stage of a crystallizing magma. What is contrasting, is that these two rocks were located very far each other. In addition, dark-grey syenite has medium-grained texture, highest Ba, Nb, Ta and Y contents and lowest Zr; which probably shows that it was crystallized at a much shallow depth (less than that of whitish syenite) from a Ba-Nb-Y-Ta-enriched magma. Dark-grey syenite locally underlay a highly weathered granitic cover, showing that its magma intruded the same granitic cover rock but at different depths and positions. The granitic cover is locally cut by numerous and highly fractured pink microsyenitic dykes. The presence of a dyke in the cover rock, shows that it was affected by extension or the force of a rising magma diaper [42]. These dykes show that the granitic cover rock was locally affected by post emplacement cataclastic tectonics with composite magmatic injections into the opened gabs. The dykes are vertical and inclined with different strike and dip, which suggests a different extension direction during magma injections along the opened fractures. These dykes can be classified as multiple as they have closely the same composition, if [42] dyke classification is considered. From the obtained dip values, it is suggested that opening of the fractures involved lateral and ENE-WSW incline displacements with the maximum development of the dykes occurring where the fracture planes were mostly vertical. The fracture filled rocks are fine to medium-grained, alkali feldspar-rich, and have no hydrous minerals; suggesting crystallizations from dry residual alkaline melts during late stages syenitic magmatic differentiations in shallow depth. The presence of normative olivine and absence of quartz in the microsyenites shows cooling from silica-saturated magma (s). The presence elevated TiO 2 , in this rock; show an enrichment of titanium in their source magma, and the possible crystallization of ilmenite [51]. Ilmenite crystallization in the source magma of all the Mindif syenites can be confirmed as they all have slight high TiO 2 content and normative ilmenite. The source magma of the syenitic dykes was also relatively Al-Zr-V-enriched but, silica and much alkali depleted; if compared with data of others syenites (Table 1 and Table 2). Contact granite at the edges of microsyenitic dykes and in contact with host rock, can be a product of magmatic mixture between the intrusive melt and magma from partial melting of the host rock. With SiO 2 = 74 wt%, plotted in peraluminous field (Figure 9(c)) and lowest elementary contents, the contact granite cooled from silica-oversaturated and peraluminous melt with this melt being generally chemically depleted.
The presence of hydrous mineral (biotite) in light-grey and whitish syenite and their coarse-grained and porphyritic texture may show crystallization from less dry to high hydrous melts during magmatic differentiation in deep seated environments. The presence of sericite in alkali feldspar in those two rocks can reflect volatile increase magmatic phase [52]. These two syenites could be crystallized products from different magma phases, as these rocks have different color and CIPW mineralogy, plotted separately in TAS diagram and others discriminating diagrams (e.g., Figure 5 and Figure 6). The light-grey and whitish syenites locally encloses amphibolite xenoliths which are probably reworked materials sorted from the country rock forming the magmatic chamber by the cooling syenitic magmas when ascending. The structural, textural and compositional similarities between these mafic xenoliths and amphibolite schist outcropping in the east of the study area, show that they were sorted from this rock.  Figure 9(d)).

Peridotgabbro and Gabbro
Peridotgabbro and gabbro are basaltic rocks crystallized from mantle source magma(s) in continental and oceanic settings.  [51]) and in the Mamfe Basin (qualified as high-Ba basaltic rocks: [50]). Slightly Ti-enrichment (a less mobile element) in peridotgabbro could be due to ilmenite crystallization in its source magma. Zirconium, Nb, Ta, and Hf whose ratios (Zr/Hf and Nb/Ta) are less changeable during crustal contamination and crystal differentiation processes show some similarities compare to that of primitive mantle are Zr/Hf (36.27) and Nb/Ta (17 ± 2.0), respectively [62]. The Nb/Ta ratios for Mindif gabbroic rocks are close to that of the primitive mantle which is not the case with Zr/Hf ratios (either less or greater than that of the primitive mantle). It can be suggested that mantle source magmas with crustal influences crystallized to give the Mindif gabbro and peridotgabbro. The mantle origin of their source magmas is shown in Nb/Th versus Zr/Nb and Zr/Y versus Nb/Y binary diagrams of [63] and [64] (Figure 11) in which gabbro are both influenced by deep depleted mantle (DPM) and oceanic plateau basalt (OPB) end-members (Figure 11(a)); and peridotgabbro influence only by DPM end-member ( Figure 11(b)).
The use of MgO and Mg # values of mafic rocks to interpret mantle event has been for rocks found along the Cameroon volcanic line [65]; an active N308E tectono-magmatic alignment, extending from Pagalu Island to Lake Chad [32].  basaltic magma crystallization (proven the existence of residual garnet in the source magma) [68]. Very high fractionation index corresponds to the depletion in HREE relative to LREE (otherwise, LREE enrichment) and indicate the presence of garnet as a residual phase in the sources of these rocks [51] [57] [69]. For [69] high (Tb/Yb) N in mafic volcanic rock also characterizes the presence of residual garnet in the mantle source. The very low (Tb/Yb) N ratios:1.3, coupled with the very low REE in peridotgabbro could just be due to limited presence of these elements in its source magma; as it would be difficult to certify the presence of residual garnet in the melt. This hypothesis is supported by their plots, very far to the garnet peridotite line and much close to transitional tholeiitic basalts (in La/Yb versus Dy/Yb discriminating diagram of [70] and [71]) ( Figure  12(a)). In Sm/Yb versus La/Sm binary diagram (Figure 12(b)), peridotgabbro is plotted a bit at the beginning of the Mid Ocean Ridge Basalt (MORB) line whereas, gabbro was plotted very far. This study tholeiitic gabbro is low-K, slightly LILE enriched and show pronounced Nd depletion. The features are close to those of Dabbah gabbros in Nubian Shield of Egypt [60] whose source magmas were suggested to be generated in subduction-related environments. The Nb/Th ratios have been used in differentiating arc gabbros (Nb/Th < 7.5) from non-arc gabbros (Nb/Th > 8.5) [72].

Potential Uses of the Mindif Syenitic and Mafic Rocks
Syenites are often used in ceramic and glass industries [73]. One of the key factor uses to identify good syenite in manufacturing glass and ceramic is its geochemical composition [ [24] with SiO 2 (50 -60 wt%), Al 2 O 3 (15 -17 wt%) and Fe 2 O 3 (4.9 -5.7 wt%) were tested suitable for glass and ceramic industries. The alumina content required in glass ranges from 1.5% -2.0% in flat glass and up to 15% in some fiberglass [1]. Excepting the SiO 2 slightly greater than that of Bull's Run syenites, the content of the other oxides in Mindif syenites is much closer to that of Bull's Run syenite. This geochemical similarity shows that the Mindif syenites can be used in glass and ceramic industries. Nevertheless, manufacturing tests are still needed to approve their use as raw material in these industries. For better uses in glass and ceramic industries, the iron content can be reduced to an average of 0.81 wt%, and, that of alumina upgraded at above 20 wt% as was done red syenite in South Africa [24]. Syenites are mined and used as raw material for aluminum extraction in Russia [20], can host substantial amount of minable metals and/or gemstones [14] [22], or can weather to enrichment lateritic soil with aluminum in hot and humid tropical climate and plateau topography [17]. Mindif syenites with their significant alumina (up to 18.43 wt%) can enrich a lateritic soil with aluminum if found on a plateau relief and in a humid tropical climatic zone. The climate in Mindif is hot tropical sahelian, and the relief is a plain cut by syenite's stocks forming inselbergs (not suitable for the concentration of lateritic bauxite). Transparent, colorless and very angular uncharacterized gemstones were sampled during this study in eluvial materials close to the contact between pink syenites and weathered granitic cover rock in the west of the study area. Although their origin not certain, their very angular texture and pegmatitic grain size (generally above 1 cm) shows that they are proximal source(s) (probably from the underlying rock(s)). International Journal of Geosciences Gabbro and peridotgabbro found in Mindif Syenitic Complex are generally depleted in minable substance such as Ni, Co, Zn, V, and Cr. The contents of these elements never exceed 500 ppm, lower than the contents in mineralized gabbroic rocks and Ni-Co ultrabasic rocks (example of Lomié Ni-Co ultrabasic rocks in the East region of Cameroon studied by [74]). The Al 2 O 3 (15.82 wt%) in peridotgabbro is significant for weathering and formation lateritic bauxite, as this value greater than those found in the parent rock of (Al 2 O 3 :15.5 wt% in basalt) Bangam and (Al 2 O 3 :15.6 wt% in basalt) Fongo-Tongo bauxites in the West Region of Cameroon [75].

Conclusions
Mindif Complex is composed of syenites (pink, dark-grey, light-grey, and whitish), contact-granite, gabbro, peridotgabbro and amphibolite schist with different characteristics and crystallization history.
Pink syenites are dyke-like, fine to medium-grained cooled in shallow depth from much Al-rich and dry residual alkaline-silica-saturated melts during fractured tectonic-magmatic evens. Medium to coarse-grained-peraluminous granite found at the contact between pink syenites and granitic host, is probably a product of magmatic mixing between the intrusive syenitic melt and melt from partial fusion of the granite host rock.
Dark-grey syenite is a quartz-bearing medium-grained rock cooled in shallow depth from more Al-Ba rich-hydrous residual alkaline silica-oversaturated melt during fractional magmatism. Light-grey and whitish syenites are coarse to porphyritic stocks cooled deep-seated from alkaline silica-saturated and silica-oversaturated hydrous rich melts with important feldspar accumulations.
Peridotgabbro is a medium-grained, ultrabasic and alkaline rock, crystallized in shallow depth from more evolved mantle source silica-undersaturated magma with plagioclase accumulation. Gabbro is a medium-grained, crystallized in shallow depth from silica-saturated magma originated from partial melting ultradepleted mantle source in subduction environment. Amphibolite schist is a medium-grained, sheet-like, and pneumato-granoblastic metamorphic basement rock cut and reworked by biotite syenitic stocks.
Mindif syenites and peridotgabbro represent high-Al 2 O 3 rocks capable to enrich lateritic soil with aluminum under a humid tropical climate on a plateau topography. These syenites can be used in some cases in glassy and ceramic industries after treatment.