As said earlier, the syenitic rock around Ayetoro occurs as a ‘boss’ 1.5 km west of the town, that intrudes mainly quart-biotite schist. The rock is massive, being devoid of structures as folds and fractures; light grey in colour and dull in appearance ( Plate 1(A) ), which gave the impression that it either lacks quartz or contains it sparingly. It is medium-grained in texture and looks syenitic being thought to compose mainly feldspars and minor biotite. Under the microscope, Ayetoro granitoid comprises 70% felsic minerals and 30% ferromagnesian minerals as observed by their low and high relief, respectively, with plane polarized light ( Plate 2(A) ). With the polars crossed, the felsic minerals were found to be feldspars and quartz; while the ferromagnesian minerals constitutes predominantly biotite ( Plate 2(B) ). The feldspars are alkali with microcline, identified by grid twinning, being the most abundant ( Plate 2(B) ). Quartz was identified by undulose extinction and lack of cleavage ( Plate 2(A) & Plate 2(B) ), and seems to occur in the same proportion as microcline. Biotite was recognized by brown to green colour, and strong pleochroism ( Plate 2(B) ). This mineralogy of mainly microcline, quartz and biotite as well as the texture of the rock indicate that the syenite-looking granitoid of Ayetoro area is actually microgranite.

Sasaro granitoid occurs as ‘stock’ near the town where it intrudes marble and quart-biotite schist in the Igarra Schist Belt [4] [5] . The rock is dark grey in colour and highly dim in appearance ( Plate 1(B) ) indicating that it lacks in quartz. The rock shows an extensive brecciation and quartz-veining, which indicate that it is highly sheared. In hand specimen, the rock is medium-grained and contains feldspar, biotite and pyroxene. In thin section, Sasaro granitoid consists of almost 65% felsic and 35% ferromagnesian minerals as observed under the plane polarized light ( Plate 3(A) ). Under cross nicols, the felsic minerals are, essentially, feldspars, which are Na-rich plagioclase (i.e. albite—identified by pericline twinning) and alkali feldspar, mostly orthoclase—identified by Carlsbad twinning ( Plate 3(B )). The ferromagnesian minerals are biotite; hornblende, which is pleochroic in shades of yellow-green to green to blue-green; and pyroxene, identified by prismatic habit and presence of at least two orthogonal set of perfect prismatic cleavage planes ( Plate 3(B) ). The texture and mineralogy of the syenitic rock in Sasaro area suggest that it is micromonzonite.

Chemical bulk rock analysis has been used to further classify the syenitic rocks at Ayetoro and Sasaro as well as to uravel their tectonic settings. Concentrations of main metal oxides (i.e. SiO₂, Al₂O₃, Fe₂O₃, MgO, CaO, Na₂O, K₂O, TiO₂, P₂O₅, MnO and Cr₂O₃), in % and some trace elemants (i.e., Ba, Ni, Sr, Zr, Y, Nb, and Sc), in ppm, obtained from the bulk rock analysis are given on Table 1 for the syenitic rock at Ayetoro. The SiO₂ content of Ayetoro granitoid ranges from 69.73% to 70.19% with an average of 69.89%, which classifies as acidic rock ( Table 1 ). Also, a comparison of (Na₂O + K₂O) content, which ranges from 9.11% to 9.55% with an average of 9.33% and Al₂O₃ content that ranges from 14.31 to 14.77% with an average of 14.54% shows that for Ayetoro syenitic rock the (Na₂O + K₂O) content is generally less than the Al₂O₃ content ( Table 1 ). This has further classified it to subalkaline rock with (Na₂O + K₂O) < Al₂O₃. This

Plate 1. Outcrops of (A) Ayetoro and (B) Sasaro Granitoids within Igarra Schist Belt.

Plate 2. Photomicrograph of Ayetoro Granitoid Comprising Microcline (mc), Quartz (qt) and Biotite (bt). Bar Scale: 2 µm (A ppl, B xpl).

Plate 3. Photomicrograph of Sasaro Granitoid Comprising Orthoclase (or), Albite (al), Biotite (bt), hornblende (hbl), and Pyroxene (prx). Bar Scale: 2 µm (A ppl, B xpl).

subalkaline magma series can be further subdivided to K-rich (high K-type) calc-alkaline magma series considering the fact that K₂O content, which ranges from 5.13% to 5.64% with an average of 5.37% is higher than Na₂O content, which ranges from 3.91% to 4.03% with an average of 3.96% ( Table 1 ). The TAS (total alkali versus silica) diagram of plutonic rocks after Middlemost [13] is a diagram routinely used to classify such plutonic rocks and has been employed here ( Figure 5 ). The diagram reveals that all the five samples from Ayetoro granitoids plot in the field of granite. This has confirmed the classification from petrography, which indicated that Ayetoro granitoid is medium-grained granite (microgranite).

Table 2 gives the bulk contents of main oxides in % and some trace elemants in ppm, for Sasaro syenitic rock. In its own case, SiO₂ content, which ranges from 53.95% to 55.71% with an average of 54.76 ( Table 2 ), classifies the rock as intermediate. Also, when compared the (Na₂O + K₂O) content, which ranges from 8.37% to 9.34% with an average of 8.93% with Al₂O₃ content that ranges from 11.89% to 12.53% with an average of 12.31%, the (Na₂O + K₂O) content of Sasaro granitoid is also less thani its Al₂O₃ content ( Table 2 ). This further classifies the rock as subalkaline magmatic rock, which can be further subdivided to K-rich (high K-type) calc-alkaline magma series. This being that the K₂O content of the rock, which ranges from 6.16% to 6.95% with an average of 6.66% is higher than Na₂O content, which ranges from 2.20% to 2.39% with an average of 2.27% ( Table 2 ). The TAS diagram of Middlemost [13] shows all the five samples from Sasaro rock plot in the field of Monzonite ( Figure 6 ). This result from geochemical classification of the Sasaro plutonic rock has further buttressed that of the petrographic classification, which showed it to be monzonite (micromonzonite).

Major- and trace-element discrimination plots have been used to infer the tectonic conditions present at the locations of Ayetero granite and Sasaro monzonite. That is, whether the places are near a tectonic plate boundary or in a more interior position on the plate.

The R1, i.e. [4Si – 11(Na + K) – 2(Fe + Ti)], versus R2, i.e. [6Ca + 2Mg + Al], plot of Batchelor and Bowden [14] for Ayetoro granite ( Figure 7(A) ) shows it to be orogenic granite as all its samples plot in the field of late orogenic granites. The trace element geotectonic discrimination Y versus Nb diagram ( Figure 7(B) )

after Pearce et al. [15] revealed the Ayetoro granite to be volcanic arc or syncollisional. The major elements granite tectonic discrimination plots of Maniar and Piccoli [16] are given in Figure 8 for Ayetoro granite. The SiO₂ versus FeOt /(FeOt + MgO) plot ( Figure 8(A) ), M/AFM, i.e. [MgO/(Al₂O₃ + Fe₂O₃ + MgO)], versus F/AFM, i.e. [Fe₂O₃/(Al₂O₃ + Fe₂O₃ + MgO)], plot ( Figure 8(B) ) and C/ACF, i.e. [CaO/(Al₂O₃ + CaO + Fe₂O₃)], versus F/ACF, i.e. [Fe₂O₃/(Al₂O₃ + CaO + Fe₂O₃)], plot ( Figure 8(C) ) show that Ayetoro granite formed from post orogenic granitoid magma, which intruded the sedimentary succession of Igarra Schist Belt in an island arc, continental arc, or continental collision environment. The A/NK, i.e. [Al₂O₃/(Na₂O + K₂O)], versus A/CNK, i.e. [Al₂O₃/(CaO + Na₂O + K₂O)], plot ( Figure 8(D) ) shows that Ayetoro granite is a metaluminous igneous rock.

The F/ACF, i.e. [Fe₂O₃/(Al₂O₃ + CaO + Fe₂O₃)], versus C/ACF, i.e. [CaO/ (Al₂O₃ + CaO + Fe₂O₃)], plot of Maniar and Piccoli [16] for Sasaro monzonite ( Figure 9(A) ) shows it to be orogenic as they mostly plot in post orogenic magma zone. The M/AFM, i.e. [MgO/(Al₂O₃ + Fe₂O₃ + MgO)], versus C/AFM, i.e. [CaO/(Al₂O₃ + Fe₂O₃ + MgO)], plot, also of Maniar and Piccoli [16] shows that Sasaro monzonite formed in either an island arc, continental arc or continental collision environment ( Figure 9(B) ). Also, the A/NK, i.e. [Al₂O₃/(Na₂O + K₂O)], versus A/CNK, i.e. [Al₂O₃/(CaO + Na₂O + K₂O)], plot of Maniar and Piccoli [16] for Sasaro monzonite ( Figure 9(C) ) shows it to be metaluminous igneous rock like Ayetoto granite. The FeO_t-MgO-Al₂O₃ ternary plot of Pearce et al. [15] restricts Sasaro monzonite to an island arc environment as all its samples plot in the field of oceanic ridge or floor ( Figure 9(D) ), which is in agreement with the presence of marble among the metasedimentary successions intruded by Sasaro monzonite.

The main take from this geotectonic study is that Ayetoro boss and Sasaro stock are orogenic quite like Igarra batholith, which is believed to be equally pre-, syn- and post-tectonic rocks [11] [17] ; and could have been formed by the same magma. The difference in composition among the varieties of Older Granite suites on one hand, and between Ayetoro microgranite and Sasaro monzonite, on the other, could be in the type of crustal rock digested. While Ayetoro microgranite intruded mainly biotite schist, Sasaro monzonite intruded marble mainly and, probably, could have been altered by it.

The mineralization potential of the plutonic rocks at Ayetoro and Sasaro have been discussed on the bases of their structural disposition and trace element contents, in ppm, from the partial rock analysis. The structural disposition enables the knowledge of whether features and structures that enhance the permeability of the rocks for hydrothermal-fluid flow are present or not. Whereas, the trace-element contents of the rocks give an indication of whether there has been some initial metal mineralization in them.

Ayetoro granite is massive and seems devoid of features and structures that can serve as passageways for hydrothermal fluids and loci to precipitate metallic minerals from them. Concentrations of partially extracted trace elements using aqua regia digestion in Ayetoro microgranite ( Table 3 ) shows that Au, Bi, Cd

Figure 8. Major element tectonic discrimation plots for Ayetoro granite after Maniar and Picolli [16] . A. SiO₂ versus FeOt/(FeOt + MgO), (B) M/AFM versus F/AFM, (C) C/ACF versus F/ACF, (D) A/CNK versus ANK. (Symbols: IAG—island arc granites, CAG—continental arc granites, CCG—continental collision granites, POG—post-orogenic granites, CEUG—continent-epeirogenic uplift granite, RRG—rift related granites).

and W are sparsely distributed, being mostly present below their detection limits in analytical equipment employed. Whereas, others like Ag, As, Co, Cu, Mo, Pb, Ba, Sb, Sr, Th, Tl, U, V, and Zn have low distribution because their average concentrations in Ayetoro microgranite (i.e. 20, 0.7, 3.5, 10.69, 1.17, 23.89, 49.6, 0.10, 19.8, 11, 0.24, 2.1, 17 and 22.2, respectively) are less than or comparable to their average abundance in felsic rock ( Table 3 ) after Taylor [18] . These sparse and low distribution of trace elements do not amount to anything in terms of mineralization of Ayetoro granite. The economic aspect of this massive microgranite body, with no sign of mineralization of any metal, is that it can be employed as an industrial rock. There is therefore a need to determine its strength for application in construction industry.

On the other hand, Sasaro monzonite is highly sheared, heavily brecciated, and contains a few quartz and quartzo-feldspathic veins. This plutonic body,

Figure 9. (A, B, C) Major element tectonic discrimation plots after Manir and Picolli [16] , [(A) C/ACF versus F/ACF, (B) M/AFM versus F/AFM, (C) A/CNK versus ANK. (Symbols: IAG—island arc granites, CAG—continental arc granites, CCG—continental collision granites, POG—post-orogenic granites, CEUG—continent-epeirogenic uplift granite, RRG—rift related granites)]; and (D) Ternary FeOt-MgO-Al₂O₃ plot after Pearce et al., [15] for Sasaro monzonite.

therefore, does not only contain required spaces suitable to serve as passageways and loci for hydrothermal fluids to traverse and deposit its metal load, but also evidences that hydrothermal fluids have traversed it. All the 18 partially extracted trace elements, i.e. Ag, As, Au, Bi, Cd, Co, Cu, Mo, Pb, Ba, Sb, Sr, Th, Tl, U, V, W and Zn, considered for this study are widely distributed in Sasaro monzonite because they are present at detectable levels, above the minimum detection limits of the ICP-MS ( Table 4 ). The average concentrations of Mo, Cu, Ag, Co, Au, Bi and Tl are much higher at 1.62, 87.17, 63, 16.5, 1.5, 0.26 and 0.6 ppm, respectively in the rock as compared to their average abundances in intermediate rocks, which are respectively 0.9, 35, 7, 10, 0.5, 80, and 0.5 [18] . This indicates possible mineralization of Ag, Au, Cu and Mo in Sasaro Syenite. The

Element Concentration in ppm. Ag & Au in ppb.

Element Concentration in ppm. Ag & Au in ppb.

average contents of other trace elements as As, Cd, Pb, Ba, Sb, Sr, Th, U, V, W and Zn in Sasaro monzonite is far less than their average abundance in intermediate rocks after Taylor [18] . Consequently, the economic aspects of Sasaro monzonite is possible presence of Au, Ag, Cu, etc. minerals in it, which warrants more detail exploration study.