The Niki-Niki massif ( Figure 2 ) is mainly composed of granulites, pyroxenites and talcschists.

They are generally garnet-free. They break into milky-white to grayish balls ( Figure 3(a) ). Microscopically ( Figure 3(b) ), they show a granoblastic texture composed of plagioclase, clinopyroxene, and amphibole (green hornblende). Plagioclase is abundant and occurs as phenocrysts with polysynthetic twinning. Clinopyroxene (diopside) are sometimes affected by slightly microfractured or intensely cracked subautomorphic blasts. Amphibole consists of large, elongated, subautomorphic crystals containing oxide inclusions. They are generally very little represented and constitute early minerals in the form of large patches associated with pyroxene. Quartz is fairly rare and occurs in xenomorphic form as inclusion in plagioclase and amphibole.

They outcrop in lenses within the granulites. These rocks are often amphibolitized (amphibolo-pyroxenites) and associated with talcschists. These are often cumulating, containing either coarse or fine-grained pyroxene ( Figure 3(c) ). They are dark gray to black, with a massive structure and a dense spheroidal splitting. Microscopically, they show a granoblastic texture made of clinopyroxene, orthopyroxene, amphibole (green hornblende) and plagioclase ( Figure 3(d) ). Clinopyroxene (diopside) are lightly colored in green, with numerous cracks and undulatory extinction. Orthopyroxene (hypersthene) is pinkish with undulatory extinction. They are highly cleaved and contain inclusions of opaque minerals (ilmenite, rutile, magnetite). Plagioclase displays phenocrysts with polysynthetic twins and undulatory extinction. Amphibole (green hornblende) appears as elongated phenocrysts joined to pyroxene and plagioclase.

They outcrop in balls, laminates or lenses. They are milky white or greenish, with a schistose or laminated structure ( Figure 3(e) ). Microscopically, they display a lepidoblastic texture with serpentine, talc and chlorite ( Figure 3(f) and Figure 3(g) ). Serpentine is fibrous and associated with talc, chlorite and opaque minerals (ilmenite, magnetite). Talc appears elongated and spread out. It is often associated with chlorite and derived from retromorphosis of amphibole.

The Niki-Niki massif rocks are characterized by silica contents ranging between 42.45 and 51.86 wt% ( Table 1 ). They are mainly granulites, pyroxenites and talcschists. The analyzed samples show high Al₂O₃ (up to 17.06 wt%) and Sr (up to 580.91 ppm) content, which implies their richness in plagioclase. MgO contents reach 28.72 wt% and Fe₂O₃ up to 14.60 wt% indicating pyroxene cumulate.

According to Harker diagrams ( Figure 4 ), Niki-Niki massif rocks show a positive trend between MgO and FeOt, CaO and a negative trend with SiO₂, TiO₂, Na₂O, K₂O and P₂O₅.

In the trace elements (Rb, Y, La, Ba, Ni, Ce, Sr, Cr) versus SiO₂ diagrams ( Harker, 1909 ) ( Figure 5 ), Niki-Niki massif rocks show a positive trend between SiO₂ vs Ba, Sr, Y and a negative trend with Ni, Sr, La and Ce.

Chondrite-normalized REE ( Nakamura, 197 4) ( Figure 6(a) ) and positive mantle normalized spider diagrams ( McDonough & Sun, 1995 ) ( Figure 6(b) ) show that Niki-Niki massif rocks are moderately fractionated (2.09 ≤ (La/Yb)N ≤ 4.84). They are slightly enriched in LREE (1 ≤ (La/Sm)N ≤ 1.82) and show generally flat heavy rare earth element spectra (1 ≤ (Gd/Yb)N ≤ 1.67). They show slight negative anomalies in Ce, Gd, Dy, Er and positive in Pr, Sm, Ho and Tm.

Primitive mantle normalized spider diagrams ( McDonough & Sun, 1995 ) ( Figure 6(c) ) show that the Niki-Niki massif rocks exhibit negative deplation in Rb, Nb, Ce, Pb, Eu, Ta, Pr, P, Zr, Ti and positive enrichment in Ba, U, La and Pb. The negative Eu anomaly indicates the role of plagioclase during the fractional crystallization processes. The negative anomaly in Nb is characteristic of continental tholeiites and crustal contamination. It suggests that the magmas of these rocks are linked to a plate convergence magmatic environment (subduction zone) where Nb is trapped. The negative anomaly in Ti reflects ilmenite fractionation in these rocks.

The rocks of the Niki-Niki massif appear in the TAS diagram of Middlemost (1994) ( Figure 7(a) ) in the gabbros field. The diagram of Irvine & Baragar (1971) ( Figure 7(b) ) indicates that these rocks show tholeiitic affinity. The diagram of Foley et al., (1987) and Shand (1943) ( Figure 7(c) ) shows that these rocks are meta- to peraluminous. The diagram of Meschede (1986) ( Figure 7(d) ) shows that the rocks of the Niki-Niki massif are distributed in the N-MORB (D) and intraplate tholeiitic and volcanic arc basalts (C) fields.

Geochemical results on whole rock ( Table 1 ) and field observations reveal the presence of significant anomalies for some of the base metals; notably chromium, nickel, vanadium, copper and cobalt ( Figure 8 ). Chromium anomalies range from 123 ppm to 2638 ppm; nickel anomalies range from 64 ppm to 1961 ppm ( Figure 8 ).

Among the seven prospecting wells, the NP5 located on the western slope of the Niki-Niki massif (X = 301,579 E, Y = 923,101 N, H = 296 m) showed significant gold showings along the toposequence ( Figure 9 ). It has a depth of 220 cm. Its profile consists of four horizons, from top to bottom: gravelly soil, ferruginous crust, gravel horizon, silty horizon ( Figure 10(a) ).

This is a thin horizon of soil made up of dark-red ferruginous gravel, exhibiting a lumpy structure and a sandy-clay loam texture. The fragments are sub-rounded and the entire layer is powdery, interspersed with numerous rootlets.

This horizon has a compact structure and a silty-clay texture with numerous ferruginous gravels ( Figure 10(b) ). These ferruginous nodules form an indurated horizon. The strong cohesion between the ferruginous nodules indicates extensive leaching. The nodules have a black core and an ochre-red periphery. In addition to these nodules, there are a few angular quartz fragments.

It consists of angular quartz pebbles loosely bound by a brick-red clay matrix. The pebbles range from centimetric to decimetric in size and are believed to have originated from the disintegration of a quartz vein located 10 meters east of shaft NP5. A panning test carried out on an excavated sample of angular gravel revealed the presence of gold (0.1 to 2 mm in size) ( Figure 10(c) ).

This horizon is clearly distinct from the gravelly horizon. It consists essentially of brick-red clay with the same macroscopic characteristics as the clay matrix of the overlying horizon. The few ferruginous gravels observed are softer and can be broken easily with the fingers. This horizon has a lumpy structure and a clayey texture.

The petrographic study of the Niki-Niki massif reveals a diversity of facies: granulites, pyroxenites and talcschists. Granulites are generally garnet-free with a mineralogical assemblage of Pl + Cpx + Opx + Hbl. Amphibolitization has led to granulites transformation into amphibolites characterized by Hbl + Pl + Qtz paragenesis, indicating a transition from granulite to amphibolite facies. Pyroxenites

have an Opx + Cpx + Pl + Hbl paragenesis and underwent an initial retrograde metamorphism in the amphibolite facies, where pyroxenes were replaced by green hornblende. Talcschists show a paragenesis of Tlc + Srp + Chl, with the appearance of talc and chlorite marking the transition to greenschist facies. These paragenesis and metamorphic conditions are similar to those in the Kabye, Agou and Akuse massifs ( Agbossoumonde, 1998 ; Agbossoumonde et al., 2001 ; Attoh & Morgan, 2004 ; Sabi, 2007 ).

The correlations observed in the Harker variation diagrams for major and trace elements, and the parallelism of the rare earth and multi-element spectra, suggest that the different facies evolved from a single magmatic source.

The slight negative anomalies in Nb-Ta reflect the crustal recycling influence (crustal contamination). The positive anomaly in Sr suggests plagioclase accumulation. The good linear correlation in the Harker diagrams reflects the role of fractional crystallization in the differentiation process during the evolution of the parent magmas of these massif rocks ( Agbossoumonde, 1998 ; Agbossoumonde et al., 2001 ; Sabi, 2007 ).

The increase in FeOt versus MgO reflects the crystallization of pyroxene and amphibole. The decrease in TiO₂ and P₂O₅ versus MgO suggests the fractional crystallization of ilmenite and apatite. The decrease in Na₂O and K₂O reflects the enrichment of these rocks in acidic plagioclase ( Tairou et al., 2022 ; Guillot et al., 2019 ; Kpanzou, 2023 ).

The increase in Ba content suggests the high fractionation of plagioclase in these rocks. The decrease in Ni and Cr contents versus SiO₂ indicates the fractionation of ferromagnesian (pyroxene and amphibole) of these rocks. These elements could also be associated with MgO and FeOt in pyroxenes according to some authors ( Hamlaoui et al., 2020 ; Kpanzou, 2023 ).

The rocks of the Niki-Niki massif have a basic to ultrabasic composition. Their chemical characteristics allow us to classify them as gabbros according to Middlemost (1994) terminology. Petrographic characteristics reflect the chemical composition of these rocks with abundant hornblende and the presence of orthopyroxene and clinopyroxene. Negative anomalies in elements such as niobium, phosphorus and titanium, observed in multi-element spectra, are signature features typically associated with subduction zone magmas. The negative anomaly in Nb is characteristic of continental tholeiites and suggests crustal contamination or metasomatism from a mantle source ( Kpanzou et al., 2022 ; Kpanzou, 2023 ; Kpanzou et al., 2023a ; 2023b ).

The genesis of these rocks is therefore compatible with emplacement in a magmatic arc context with simultaneous crustal assimilation and fractional crystallization of magma probably derived from continental crust.

Based on the petrogeochemical data of the Niki-Niki massif rocks, such as their tholeiitic affinity and meta- to peraluminous signature, it can be proposed that they were emplaced in a probably extensive tectonic context like most of basic to ultrabasic massifs of the Pan-African suture zone ( Duclaux, 2003 ; Guillot et al., 2019 ; Kpanzou et al., 2022 ).

Geotectonic discrimination diagrams indicate that these rocks are emplaced in various settings: N-MORB, intraplate tholeiites and volcanic arc basalts. The negative anomaly in Nb is characteristic of continental tholeiites and crustal contamination. It suggests a context of subduction zone in which Nb is trapped ( Guillot et al., 2019 ; Kpanzou et al., 2022 ; Kpanzou, 2023 ).

The geochemical characteristics of the Niki-Niki massif rocks are similar to those of the Kabye, Agou and Djabatoure massifs rocks, such as high Sr and Ba values, a negative anomaly in Nb, and enrichment in light rare earth elements (LREE) compared to heavy rare earth elements (HREE) ( Affaton, 1990 ; Agbossoumonde, 1998 ; Duclaux, 2003 ; Sabi, 2007 ; Kpanzou, 2023 , Kpanzou et al., 2023a ). These rocks have also been subsequently contaminated by continental crust.

Despite the lack of isotopic data, petrogeochemical studies of the Niki-Niki massif rocks show similarities with those of the Kabye, Agou, Akuse and Djabatoure massifs, allowing to classify them in the same geodynamic context.

The highest content of Ni (359 to 1961 ppm) and Cr (2269 to 2638 ppm) are well above the average for common basaltic fluids, which range from 200 to 300 ppm for Ni and 400 to 500 ppm for Cr ( Bougault et al., 1980 ).

The concentrations of Ni (1961 ppm), Cr (2638 ppm), Cu (123 ppm), V (304 ppm), Co (120 ppm) and Mo (2 ppm) are also higher than the average values for the Earth’s crust, which are Ni (105 ppm), Cr (185 ppm), Cu (75 ppm), V (230 ppm), Co (29 ppm), and Mo (1 ppm) ( Taylor & McLennan, 1985, 1995 ). Similarly, the strategic prospecting work carried out by BRGM ( Dempster, 1967 ) also revealed concentrations of Au (1000 ppb), Cu (341 ppm), Cr (3053 ppm), Ni (2104 ppm), V (136 ppm) and Mo (20 ppm). The results of the current research confirm the existence of these mineral occurrences in the study area.

Gold showings in one of the shafts dug along the toposequence ( Figure 9 ) further corroborate BRGM’s findings, which indicated that gold panning occurs near outcrops of galena quartz veins ( Picot et al., 1989 ). The presence of these crushed quartz veins indicates a shear zone which is often associated with mineralization, represented here by the gold powder collected.

The gold dust and anomalous base metal values found in the study area represent an important target for exploration of these mineral occurrences.