Rare Earth Element Geochemistry and Abundances in Syenites and Charnockitic Rocks of Selected Locations within Southwestern Nigeria

Syenites: The syenites studied were found in the following areas: Awo, Iwo, Okeho, Ayetoro-Oke, and Iwere-Oke. The syenite of Iwo is found as a low-lying and very extensive outcrop; quartzo-feldspathic and pegmatitic veins were observed in the outcrop. It is grey in colour with fine- to medium-grained texture. The Okeho, Ayetoro Oke, and Iwere Oke syenite outcrops were found as low-lying and huge continuous boulders. They are dark green to dark grey in colour with medium- to coarse-grained texture. The petrographic analysis for syenites reveals mineral assemblages of quartz, plagioclase, biotite, olivine, pyroxenes, hornblende, microcline, augites, and opaque minerals (Figure 2a–c).

Figure 2:

Charnockites: The charnockite samples were collected from: Oye, Otun, Ido Osi, Ado, Ikere Ekiti, Akure, and Osuntedo. The charnockitic rocks of Oye occur as low-lying outcrops, boulders, and as road cut in some places. The charnockites in Ado-Ekiti, Ikere, and Akure occur mostly as hills and as large, elongated, and rounded boulders in close association with the porphyritic Older Granite. They are coarse grained with a colour variety ranging from dark green to greenish grey and whitish grey as observed in the sample from Ado-Ekiti. The petrographic analysis revealed mineral assemblages of quartz, plagioclase, orthoclase, biotite, pyroxenes, hornblende, microcline, and opaque minerals (Figure 3a–c).

Figure 3:

REE-bearing accessory minerals were studied using their chemical compositions as observed via Scanning Electron Microscopy. The observed REE in the charnockite occurred along the rims of zoned apatite (Ca₅ (PO₄)₃ (F, Cl, OH) grain (Figure 4a and b). The zoning might be due to a change in temperature or composition of the fluid from which the crystal crystalised.

Figure 4:

Apatite was observed to be associated with a mineral suspected to be monazite, which is an REE-concentrating mineral more enriched in LREE. REE minerals in syenites are small and scarce (Figure 4 c and d). Their composition is also very different to the REE minerals in the other samples. The REE-bearing mineral identified in the BSE image of the syenite is suspected to be monazite, on the basis based of its chemical compositions (Table 1).

The total REE (∑REE) ranges from 342.54 to 675.15 ppm. The syenites are more enriched in LREE, with values ranging from 325.66 to 648.24 ppm, and Ce, being the most prevalent REE, HREE ranged 19.03-26.91 ppm. The Iwo quartz potassic syenite is characterized by higher ∑REE than the syenites from other areas, with a value of 675.15 ppm. The Kajola and Iwere–Oke syenites have comparable values of 469.17 ppm and 462.69 ppm, respectively, for their ∑REE, while the syenites from Ayetoro–Oke have the lowest ∑REE value of 325.66 ppm (Table 2).

The ∑REE of the charnockitic rocks varies with location, ranging from 224.26 ppm in the charnockite of Otun to 801.03 ppm in Akure charnockite. The charnockites are more enriched in LREE with values ranging from 211.28 ppm to 754.03 ppm and relatively depleted in HREE with values ranging from 8.64 ppm to 47.7 ppm. The Akure charnockite is characterised by anomalously high REE contents of 801.03 ppm compared to other charnockites. The syenites and charnockites are characterised by high concentration of LREE and relatively low contents of HREEs with significant LREE/HREE fractionation.

The rocks having more LREE than HREE may be a result of newly formed P-bearing minerals such as apatite or monazite being important carriers of LREE [6, 7]. That the syenites and charnockitic rocks are characterised by high REE concentrations, which is due to the presence of high values of LREE, suggests REE-concentrating minerals (especially monazite, which contains LREE), which indicates that their original magma had crustal input [8].

The (La/Yb)_N values ranged from 29.36 to 48.75 for the syenites and from 24.43 to 48.29 for the charnockites; the (La/Sm)_N values in syenites is 4.35 : 5.82; and 4.54 : 9.18 in the charnockites. The (Ce/Yb)_N ranges from 21.19 to 38.49 for the syenites, and from 17.76 to 28.93 for the charnockites. The high values of the normalised ratios of La to Yb, La to Sm, and Ce to Yb in the different rock types are evidence of a high degree of fractionation, which shows that the REE patterns are LREE enriched.

According to Ukaegbu and Ekwueme [9], the La/Yb, Ce/Yb, and La/Sm values are normally higher in residual products than in primary melts. Hence, (La/Yb)_N values higher than 5 are an indication of magmatic differentiation [10]. This implies that even though the magma might have originated from a mixed source, differentiation was important in the formation of these rocks [8].

The syenites showed negative Eu anomalies of 0.65–0.82, suggesting that a large amount of plagioclase was removed from felsic magma during fractional crystallisation. It could also be due to the presence of pyroxenes in them, as pyroxene-rich cumulates acquire a negative Eu anomaly [11]. The charnockitic rocks showed both a negative and a positive Eu anomaly of 0.63–1.87, indicating accumulation of plagioclase during the fractionation of magma for the later (Figure 5). According to Weill and Drake [12], magma crystallising stable plagioclase has most of the Eu being incorporated into the plagioclase mineral leading to a higher concentration of Eu in the mineral relative to other rare earth elements in the magma.

Figure 5:

The Ce anomaly ranges from 0.92 to 0.98 in the syenites, and from 0.90 to 0.93 in the charnockitic rocks. Ce anomalies reflect the oxygen fugacity in the environment of formation, [13]. According to Boynton [13], when there is no large negative Ce anomaly, it indicates formation under reducing conditions; a large negative Ce anomaly indicates formation under oxidising conditions.

In order to determine the mineralisation potential of the syenites and charnockites, the REE-bearing minerals in the rocks were searched for, the mineral apatite and monazite were observed, and a comparative study was carried out comparing between the syenites of this present study with related rocks, which includes the following: the nepheline syenites of the Gboko area, with ∑REE = 130.26 ppm; quartz syenites of the Prakasam alkaline province of India ∑REE = 1685.3 ppm; the quartz syenites from the Nigerian younger granites province ∑REE =1084.6 ppm; and the syenites of Capituva Massif, Brazil ∑REE = 453.28 ppm in order to determine the abundance of ∑REE in the study area syenites relative to those of other areas, (Table 3).

Charnockitic rocks of the study area were compared with related rock types to determine their ∑REE abundance relative to those of the related rock types. The comparison was done with Charnockites/enderbites of the Obudu Plateau; Garnet-charnockitic gneiss of Congo; charnockites from Kogi, Nigeria; charnockite gneiss from Norway; and charnockites from Pallavaram, Madras city, India (Table 4). The abundance of REE in the syenites and charnockites, when compared to concentrations from regions where REE has been studied and shown to be substantially enriched, suggests that the syenites are fairly enriched, while the charnockites are moderately enriched.

Further comparison was done for both the syenites and the charnockitic rocks with carbonatites from Wu dyke, Bayan Obo, North China, an area where REE is mined. The REE concentration in the rocks of the study areas had very low ∑REE relative to the very high REE contents of this rock and are therefore not be economically viable.