Magnetic Facies and Polymetallic Sulphides Deposit in the Mafic and Ultramafic Intrusions of Samapleu (Western Côte d’Ivoire) ()
1. Introduction
Copper-nickel sulphide deposits are generally associated with ultrabasic formations [1] [2]. Other types of deposits have developed in a sedimentary environment where bacteriological activities on biogenic sulphur concentrate metal-bearing ions from supergene alteration of ancient magmatic complexes [3]. In Côte d’Ivoire, mafic and ultramafic intrusions, including Sama Main (SM) and Extension 1 (E1), have been observed within Archean formations in the Sipilou region where cupper-nickel occurrences have been described at Samapleu [2] [4] [5] [6] [7]. This work aims to study the relationships between different magnetic facies, geological formations and associated polymetallic sulphides in order to contribute to a better characterization of the metal concentrations at Samapleu.
2. Study Area and Geological Context
The study area is located approximately 7 km south of Samapleu, a village in the department of Sipilou in western Côte d'Ivoire (Figure 1). It lies between 856,400 to 859,315 North latitudes and 616,850 to 636,280 West longitudes. Its geology is associated with the Liberian orogeny [8] and consists of grey granulitic gneiss and pink granulite with local migmatization (Figure 1). Within this granitic basement, mafic units composed of gabbro and gabbro-norite, and ultramafic units represented by peridotite, pyroxenite and chromitite are found in intrusions [7] [9].
3. Analytical Methods
The methodological approach focuses on mapping, petrographic and geochemical characterisation of core samples from drillings. Located in a tropical zone, the Samapleu site is covered by a large lateritic layer through which few crystalline rocks outcrop. The mapping is mainly based on ground magnetic data and hammer prospecting. Application of Fast Fourier Transform (FFT) filters to the magnetic data produced maps of apparent magnetic susceptibility (
) and Analytical Signal (AS) according to the following equations:
and
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Figure 1. Study area and geological map [9].
With
;
et
where, H(r): downward continuation;
: correction of the geometric effect;
Reduction to the pole; F: Total geomagnetic field; h: depth in ground units; I: geomagnetic inclination; Ia: pole reduction amplitude inclination; D: geomagnetic declination; θ: latitude; r: wavenumber.
The first (
) provides information on the distribution of (acidic, basic and ultrabasic) units and thus facilitates geological interpretation. The second (AS) presents maximum amplitudes on the fault and/or contact and highlights discontinuities to discriminate magnetic facies linked to the ferromagnesian oxide’s distribution in the rock matrix [10].
The petrographic and geochemical characterization of the formations is based on the sampling method and analytical techniques. To do this, samples of the SM and E1 intrusions as well as the surrounding rocks were taken from the drill holes for the production of polished thin sections. The silicate minerals in these rocks were studied under transmitted light with Zeiss AXIOSKOP 40 and Leica microscopes. Sulphides and oxides were also observed with a Leica DMLM microscope in reflected light. The geochemical study involved a total of 18 samples from the boreholes whose 10 came from the intrusions (SM and E1) and 8 from the surrounding rocks. They were conditioned according to the methods of [11], then melted with lithium metaborate (LiBO2) and dissolved in nitric acid (HNO3). Major elements were analysed by optical emission spectrometry (ICPOES Icap 6500) with radial flare. And lastly, minor and rare earth elements were analysed by mass spectrometry (ICP-MS X7 from Thermo).
4. Results
4.1. Mapping
The magnetic susceptibility (k) map shows three (3) domains that provide information on the acidity of the rocks and help in the mapping (Figure 2(a)). The first domain is characterised by low k values (k ≤ 0.152) and reflects, at low latitudes, the magnetic response of formations rich in ferromagnesian minerals. It is superimposed with the outcrops of mafic and ultramafic rocks that constitute the E1 and SM intrusions. The second domain is marked by high k values (k ≥ 0.162) and corresponds, in our tropics, to the signature of quartzo-feldspathic rocks relatively poor in ferromagnetic minerals. This domain covers a large part of the prospect and is superimposed on the granulite corps. Between these two entities, there is a domain characterised by intermediate values of magnetic susceptibility which are marked by the yellow-orange colour. This domain corresponds to the magnetic response of formations relatively rich in magnetite (Figure 2(a) and Figure 2(c)). The analytical signal map discriminates the domains into four (4) magnetic facies that are related to the geological structures (Figure 2(b)).
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Figure 2. Geophysical and geological map. (Extension 1 = E1; Samapleu Main = SM). (a) Apparent susceptibility of magnetic field [6]. (b) Analytical signal of magnetic field [6]. (c) Samapleu schematic geological map [12].
Thus, the very high magnetic facies (VHMF) reflect a strong basic signature and has a NE-SW orientation. Its axis shows some distortions and is interrupted by NW-SE oriented F2 fault.
The high magnetic facies (HMF) have two (2) main directions. It borders the VHMF laterally in a NE-SW direction and, to the south of the prospect, has a NW-SE orientation. To the northwest and southeast, the low magnetic facies (LMF) does not seem to define a preferred orientation but rather corresponds to the magnetic response of the pre-existing basement. In the north-western part of this complex, moderate magnetic facies (MMF) are observed in places, with a reduced lateral extension, but mainly oriented NE-SW (Figure 2(b)).
The superposition of magnetic facies and geological hammer observations allow to connect the peridotites/pyroxenites response to the VHMF and the gabbro-norites to the high magnetic facies (HMF). These two facies define the mafic-ultramafic sequence of the E1 intrusion. It is elongated in a NE-SW direction over a length of about 2 km and a width of 50 to 200 meters, dipping 70˚ to 80˚ to the South East, parallel to the major gneissic foliation. The E1 intrusion shows some distortions due to the effects of fractures whose main one being F2 and trending NW-SE (Figure 2(b)).
To the south of the prospect, the intrusion of Sama Main (SM) is characterised only by the high magnetic facies (HMF), which reflects the magnetic response of the gabbro-norite. It is less elongated, subrounded and oriented NW-SE with a subvertical dip trending NE.
However, the apparent magnetic susceptibility map indicates at the SM intrusion level, in addition to the basic units, the magnetic signature of the acidic rocks (Figure 2(a)).
Similarly, the geological map also notes the existence of peridotites and/or pyroxenites on its northern margins (Figure 2(c)). These controversies between the high magnetic facies (HMF) and the geological units within the SM intrusion will be highlighted with the results of petrographic and geochemical studies. The moderate magnetic facies (MMF) is associated with quartzo-feldspathic formations and jotunite/enderbite assemblages that locally concentrate ferromagnesian minerals. They occur to the northwest and south of the prospect and are mainly NE-SW trending.
The low magnetic facies (LMF) is the signature of the Archean granulite assemblages. Although having a homogeneous structure in places, it’s characterised by a gneissic structure frequently marked by early foliations of variable direction with a strong dip of 70˚ to 90˚ towards the South East or North West [12].
4.2. Petrography
4.2.1. Rock Formations
Two sets of rocks are recorded on the prospect, namely country rocks and the Samapleu’s mafic-ultramafic intrusions (SM and E1).
The host rocks include more or less gneissic granulites, jotunite/enderbite assemblage and quartzo-feldspathic formations.
Gneissic granulites may have a foliated (Figure 3(a)) or equant structure. They are massive, generally with a granoblastic to grano-nematoblastic texture.
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Figure 3. Macrophotography and microphotography of surrounding rocks. (a) Macroscopic aspect of a gneissic granulite; (b) Granulite seen in unanalysed polarised light (LPNA); (c) Macroscopic aspect of a sulphide jotunite; (d) Jotunite seen in LPNA; (e) Macroscopic aspect of a quartzo-feldspathic formation; (f) Quartz-feldspathic formation seen in polarised light analysis (LPA). Qtz = quartz; Pl = plagioclase; Opx = orthopyroxene; Amp = amphibole; Bt = biotite; Gr = graphite; Sulf = sulphide.
From the mineralogical viewpoint, they are mainly composed of orthopyroxene (Opx), oligoclase, quartz and biotite in variable proportions. Quartz (40% to 50%) is very variable in size and has an undulating extinction. Oligoclase (25% to 35%) often contains exsolutions of potassium feldspar. Opx (bronzite), which represents 20% to 30% of the rock, is of medium-sized (0.1 to 0.3 mm) with some porphyroblastic crystals (4 mm to 1 cm). It has a biotite or amphibole reaction crown (Figure 3(b)) in places. Ilmenite, as well as sulphides (pyrite and pyrrhotite), appear in small proportions (≤2%) and zircon is incidental.
The jotunite-enderbite assemblage indicates a decimetric to metric inter-stratification with paragneiss and quartzo-feldspathic formations. It is green in colour with brownish-purple flecks and is massive (Figure 3(c)). The constituent minerals are medium to coarse in size (0.2 - 8 mm) and have a granoblastic texture under microscopic observation.
Opx (hypersthene, 50% - 70%) is locally transformed into amphibole (cummingtonite). Garnet has a proportion of 5% - 10% in the rock. Amphibole and biotite are locally coronitic (Figure 3(d)). In these rocks, magnetite is relatively present in places. The jotunite-enderbite set contains more than 10% sulphides (pyrite, pyrrhotite and chalcopyrite) generally disseminated or in the form of veins, with locally tapered graphite (size ≤ 1 mm) (Figure 3(c) and Figure 3(d)).
Quartzo-feldspathic formations are massive and whitish with brown or green flecks (Figure 3(e)). Microscopically, it is enriched in quartz, plagioclase and secondarily in hypersthene, garnet, biotite and amphibole (Figure 3(f)). In places, quartz (0.1 mm to 1 cm) reaches up to 40% to 70% of the rock and feldspar (plagioclase) is between 10% and 30%. Like garnet, hypersthene is locally around 10%. Biotite and amphibole are less than 7%. This rock contains less than 5% of sulphides (pyrite, pyrrhotite) generally disseminated, with locally high proportion of graphite and magnetite in places.
The mafic and ultramafic intrusions concern the SM and E1 zones. They are quite similar and all have cumulative textures. They are grouped into two units, one of which, ultramafic, is made up of peridotites and pyroxenites, and the other mafic is made up of norite, gabbro-norite and anorthosite.Within the ultramafic unit, the peridotite consists mainly of weakly serpentinised lherzolite and secondarily of harzburgite and dunite. It is massive, very magnetic and blackish-green in colour (Figure 4(a)). In places it is banded with serpentine and weakly mineralised with sulphide. Microscopically, it is composed of olivine, serpentine, Opx, clinopyroxene (Cpx), amphibole, spinel and sulphide (Figure 4(b)). The olivine (70% - 90% of the rock) is cumulus-shaped, sub-rounded and has minerals of 0.1 - 3 mm in diameter. Serpentine (3% to 4%) borrows the olivine's fissure networks and is associated with magnetite. Opx and Cpx represent less than 15% of the rock. Green amphibole (5% to 20%) is interstitial between
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Figure 4. Macrophotography and microphotography of the rocks of the Samapleu intrusions. (a) Macroscopic aspect of a peridotite; (b) Peridotite seen in LPNA; (c) Macroscopic aspect of a pyroxenite; (d) Pyroxenite seen in LPA; (e) Macroscopic aspect of a gabbro-norite; (f) Gabbro-norite seen in polarised light analysis (LPA). Ol =olivine; Spl = spinel; Opx = orthopyroxene; Cpx = clinopyroxene; Amp = amphibole; Pl = plagioclase; Sulf = sulphide.
the olivine crystals. Sulphides and spinels represent less than 5% of the rock.
Pyroxenites are composed of pyroxenite in the strict sense (ss), olivine pyroxenite and plagioclase pyroxenite. They are massive, green speckled with brown, locally magnetic and strongly mineralised in sulphide (Figure 4(c)). Microscopically, they consist of Opx and Cpx, amphibole and plagioclase, olivine, spinel and sulphides in variable proportions (Figure 4(d)). Opx is more abundant (more than 60% of the rock) than Cpx, which makes less than 15% of the rock. Amphibole and plagioclase are 10% - 15% of the rock and are xenomorphic to sub-automorphic. Olivine is less abundant and spinels are interstitial in the pyroxenes. Sulphides are abundant and approach 50% in the pyroxenites (Figure 4(c)).
The mafic unit is massive and whitish-grey with brown flecks (Figure 4(e)). It is magnetic and weakly mineralised in sulphide. Microscopically, this unit is composed of Opx and Cpx, plagioclase and amphibole, biotite and sulphide in variable proportions (Figure 4(f)). Opx (5% - 40% of the rock) is subrounded and variable in size (3 mm to 1 cm). Cpx (≤15%) has a very heterogeneous distribution and locally favours norite formation. Plagioclase is quite abundant (30% - 40%) and variable in size (0.1 - 5 mm). Of variable shape, it is interstitial to the pyroxenes. Its proportion reaches locally 80% to 90% of the rock and forms the anorthosite. The association of plagioclase with pyroxenes gives in places a biotite reaction crown. Amphibole (5% - 10%) is xenomorphic and interstitial between pyroxenes and plagioclase. Oxides and sulphides are small and less abundant in the rock (0% - 5%).
4.2.2. Iron Oxide and Sulphide Mineralization
In the host rocks, iron oxides consisting of magnetite are contained in the jotunite/enderbite assemblage as well as the quartzo-feldspathic formations. In the jotunites/enderbites, magnetite (Fe3O4) is relatively present in places in sub-rounded and disseminated form. In the quartzo-feldspathic formations, it forms a banded alternation with a foliated structure (Figure 5(a)). Sulphides are generally less abundant within these rocks. They are disseminated and consist mainly of pyrrhotite, pyrite and secondarily of chalcopyrite (Figure 5(b)).
On the other hand, in the mafic and ultramafic intrusions (SM and E1), the oxides observed are mainly spinels (MgAl2O4) and magnetite in relatively small quantities. On the other hand, sulphides are abundant in the ultramafics, particularly in the pyroxenites, where they are globally disseminated, interstitial and in places form massive and brecciated sulphides (Figure 5(c)). They are mainly composed of pyrrhotite, pentlandite, chalcopyrite and secondarily of pyrite (Figure 5(d)).
4.3. Geochemistry
4.3.1. Host Rocks
Major element contents in the gneissic granulite are characterised by high contents of non-magnetic oxides [SiO2 (65.85% to 70.21%) & Al2O3 (14.49% to
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Figure 5. Macrophotography and microphotography of iron oxide and sulphide mineralisation of the Samapleu lithologies. (a) Macroscopic aspect of a magnetite-banded quartzo-feldspathic formation; (b) Sulphide paragenesis (pyrrhotite, pyrite) of a jotunite; (c) Macroscopic aspect of a brecciated sulphide pyroxenite; (d) Sulphide paragenesis (pyrrhotite, chalcopyrite, pentlandite) of a pyroxenite. Sil = silicate; Oxi = oxide; Po = pyrrhotite; Py = pyrite; Cp = chalcopyrite; Pn = pentlandite.
16.99%)] and low contents of magnetic oxides [Fe2O3 (2.98% to 4.94%) & MgO (1.42% to 2.85%)]. In the quartzo-feldspathic magnetite formations, grades are relatively high for both non-magnetic oxides [SiO2 (53% - 59%) & Al2O3 (13%)] and magnetic oxides [Fe2O3 (19 and 25%) & MgO (3% and 4%)].
The chemical analysis of the enderbite and jotunite shows similar contents of magnetic and non-magnetic oxides with a slight increase in alumina, for the enderbite, and magnesia for the jotunite. The contents of CaO, Na2O and K2O, MnO and TiO2 as well as some minor elements (Ni, Cr and Co) are generally low (Table 1).
The rare earth spectra (REE) of the host rocks are mainly enriched with the (La/Yb)n ratio varying between 4.57 and 217.13 except for the jotunite where the (La/Yb)n ratio is 0.73 (Figure 6).
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Figure 6. Rare earth contents (REE) of host rocks normalised to chondrite [14].
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Table 1. Major element (%) and rare earth element (ppm) composition of host rocks.
bd = belon detection.
The light rare earths (LREE) show an enrichment of 10 to 100 times higher than that of chondrite, with a (La/Sm)n ratio between 1.76 and 13.57, indicating depressed spectra. The heavy rare earths (HREE) spectra are relatively flat around 1 to 10 times the chondrite contents with a (Dy/Yb)n ratio between 0.69 and 1.64. Both a positive and negative Eu anomaly due to feldspar enrichment is observable [13]. The spectra suggest a calc-alkaline affinity.
4.3.2. Mafic and Ultramafic Intrusions (SM and E1)
The major elements of the mafic and ultramafic rocks (SM and E1) are characterised by highly variable contents, characteristic of rocks composed of cumulates. Indeed, they have low non-magnetic oxide contents [SiO2 (38.94% to 51.91%) & Al2O3 (2.65% to 19.20%)] and high magnetic oxide contents for the magnesian pole [MgO (10.59% to 32.52%)], the iron pole being relatively low [Fe2O3 (7.21% to 16.50%)].
Oxide contents (magnetic and non-magnetic) in the mafic rocks are slightly lower than in the ultramafic rocks. However, there is an enrichment of calcium [CaO (1.98% to 13.86%)] within the mafic rocks.
In addition, it is revealed that overall; the contents of Na2O, K2O, MnO and TiO2 are extremely low, below 2%.
Some minor elements such as Ni (238 to 2374 ppm), Cr (695 to 9428 ppm), Cu (46 to 1004 ppm) and Co (42 to 168 ppm) have moderately high contents (Table 2).
The rare earth spectra are globally flat for all the SM and E1 intrusive rocks, with a (La/Yb)n ratio that varies from 0.35 to 3.67. These rocks show a low enrichment, 1 to 10 times, compared to the chondrite content (Figure 7). They are slightly enriched in light rare earths (LREE) with a (La/Sm)n ratio varying from 0.43 to 3.32, and relatively constant in heavy rare earths (HREE) with the (Dy/Yb)n ratio ranging between 0.73 and 1.14. The positive Eu anomaly in gabbro-norite, plagioclase pyroxenite is related to the abundance of plagioclase within these rocks (Table 2).
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Figure 7. Rare earth contents (REE) of the SM and E1 intrusions normalised to chondrite [14].
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Table 2. Major element (%) and rare earth element (ppm) composition of the SM and E1 intrusions.
Peridot = peridotite, Ol = Olivine, Pyr = Pyroxenite, Pl = Plagioclase, Lherz = Lherzolite, Web = Webstérite, Gab-nor = Gabbronorite.
5. Discussion
5.1. Host Rocks and Magnetic Facies
The host rocks consist of gneissic granulites interspersed with quartzo-feldspathic formations and the jotunite-enderbite assemblage, which are locally magnetic and sulphurous. The gneissic granulites are essentially composed of cardinal minerals (oxides: Si/Al/Ca) and secondarily of ferromagnesian minerals (oxides: Fe/Mg) with respect to their geochemical compositions (see Table 1). This composition, which gives them weak magnetic facies (LMF), is a characteristic of acid rocks [12] [15] (see Figure 2(a), Figure 2(b) and Figure 2(c)).
Within these granulites, moderate magnetic facies units appear in places, superimposed on quartzo-feldspathic and magnetite jotunite-enderbite formations (see Figure 2(b) and Figure 2(c)). The latter concentrate, locally compared to the granulites, an enrichment in magnetic oxides (oxide: Fe/Mg) coupled with a relative depletion in non-magnetic oxides (oxide: Si/Al) (see Table 1). This inverse evolution of oxide contents amplifies their magnetic response compared to granulites. This explains the presence of moderate magnetic facies (MMF) patches within the low magnetic facies (LMF) that characterises the gneissic granulites of Samapleu (see Figure 2(b) and Figure 2(c)). The granoblastic and granonematoblastic textures observed within these rocks suggest that they are affected by a high-grade metamorphism (granulite facies), dated to the Liberian (2.8 Ga) that characterises the Archean domain in Côte d'Ivoire [8] [12] [16] [17] [18] [19] [20].
The high SiO2 and Al2O3, medium CaO and Na2O contents on the one hand, and the overall enrichment in light rare earths (LREE) on the other hand, show that these rocks come from the continental crust [13] [21]. With regard to the structures (textures, foliation, etc.) and age (2.8 Ga), the quartzo-feldspathic and jotunite-enderbite formations, with locally iron sulphide-rich magnetite (pyrite, pyrrhotite), are thought to have formed by anatexis of the Archean gneissic granulitic crust during the Liberian orogeny [12] [22] [23] [24] [25].
5.2. Mafic to Ultramafic Intrusions and Magnetic Facies
The mafic and ultramafic rocks are intrusive within the Achaean crustal rocks. In Samapleu, they form two units, the Sama Main (SM), which is subrounded and oriented NW-SE and the Extension 1 (E1), which is 50 to 200 m wide and about 2 km long in a NE-SW direction (see Figure 2(b)). They comprise continuous mafic and ultramafic horizons composed of gabbronorite-norite and peridotite-pyroxenite respectively, which are made up of olivine, pyroxene and amphibole cumulates.
The mapping reveals a clear correlation, at the level of the E1 intrusion, between the magnetic facies and these lithologies. The gabbro-norites are superimposed on the high magnetic facies (HMF) and occupy the edges of the peridotite-pyroxenites, which are associated with the very high magnetic facies (VHMF) (see Figure 2(a), Figure 2(b) and Figure 2(c)). However, at the level of the SM intrusion, two controversies develop, on the one hand, by high magnetic facies which, on the other hand, is associated with gabbro-norites and peridotite-pyroxenites; and on the other hand, by the magnetic signature of acid rocks. The first is linked to a slight decrease in the magnetic oxide (MgO) content of the pyroxenes and olivines of the SM intrusion compared to E1 (see Table 2). This causes the magnetic spectrum of the peridotites-pyroxenites to drop to that of the gabbronorites-norites in the SM. The second is related to a slight increase in oxide contents, magnetic (MgO) and non-magnetic [SiO2 and CaO], in the gabbro-norites of the SM intrusion compared to E1 (see Table 2). Indeed, the increase in [SiO2 and CaO] attenuates the global magnetic response of the gabbro-norites, which, in the vicinity of the peridotite-pyroxenites, appear with a low magnetic response that characterises acidic rocks (see Figure 2(a)). Also, the increase in magnesia (MgO) content in the gabbro-norites contributes, at the level of the SM intrusion, to the conservation of the magnetic spectrum of these mafic rocks in the high magnetic facies (HMF).
The mineralogy and crystallochemistry of the SM and E1 intrusions indicate that they are derived from a magma of mantle origin by fractional crystallisation [26].
The parallel spectra observed on the rare earths indicate a common source of the SM and E1 formations (see Figure 7). They indicate moderate LREE enrichment and flat HREE spectra that would mark a mantle and crustal plume related origin [12] [27] [28] [29] [30].
Furthermore, the sulphide copper-nickel mineralization associated with these intrusions is magmatic in nature, formed by immiscibility of an early sulphide fluid from one or more silicate mafic and ultramafic magmas [12] [31] [32] [33]. These Samapleu intrusions are dated at 2.09 Ga (U-Pb age on rutile; Eburnian) and are, according to [5] contemporary with certain Birimian geodynamic events. They are thought to have been emplaced through fractures in a granulitic Achaean basement [2] [6] [12] [30].
6. Conclusion
The present study allows to link the high magnetic facies to the mafic and ultramafic intrusions, SM and E1, which are made up of gabbronorite-norite-anorthosite and peridotite-pyroxenite respectively. These rocks are rich in copper-nickel sulphides (pyrrhotite, pentlandite and chalcopyrite), particularly in the pyroxenites where they are globally disseminated, interstitial and in places form massive and brecciated sulphides.
The SM and E1 intrusions are of mantle origin, Eburnian in age (2.09 Ga; U-Pb age on rutile) and thought to have been emplaced through fractures within granulitic Achaean basement, which is linked to weak magnetic facies. The magnetite formations, composed of quartzo-feldspathic and jotunite-enderbite units, are associated with moderate magnetic facies. They are thought to be subordinated to anatexic mineral remobilization and have a low concentration of iron sulphides (pyrite and pyrrhotite). They are nickel-free and have a crustal origin with a Liberian age (2.8 Ga; U-Pb age on Zircon).
This study linked cupper-nickel sulphides to strong magnetic facies created by magnetite, spinel, olivine and pyroxene. On the other hand, iron sulphides present locally in quartzo-feldspathic rocks are rather associated with moderate magnetic facies caused by magnetite. This approach, which allows the characterization of the polymetallic sulphides deposit environment, could be exported to other regions with similar geological formations.
Funding
No funding was provided for this study.
Acknowledgements
The authors are grateful to the General Management of Sama Nickel CI for agreeing to make the field study data available to us for this academic project.