Summary:
Deposit Types
The magmatic nickel-copper-PGE deposits occur as sulphide concentrations in a variety of magmatic mafic and ultramafic rocks.
Sulphide droplets often form within the ultramafic intrusion through contamination of the parental, mantle-derived magma with sulphur from adjacent rock units or by assimilation from the crust. As these sulphide droplets circulate through the magma by convection, they scavenge nickel, copper and the platinum-group elements from the magma, as these elements have a strong chemical affinity for sulphur. Since the sulphides are heavier than the magma, they sink through the magma and accumulate at the base of the intrusion as pockets or layers of sulphides that crystallize during the cooling of the magma to form mineral deposits.
According to classical classifications of nickel sulphide deposits, the Samapleu and Yepleu deposits are interpreted to be part of a differentiated, ultramafic and mafic feeder dykes system of the layered Yacouba complex (Gouedji et al., 2014).
The Samapleu licence is located adjacent to the large nickel-cobalt laterite deposits of Sipilou North, Foungouesso, Moyango, and Viala.
As is common in numerous documented intrusions, the emplacement of the Samapleu sequences is related to intense tectonic activity. However, the specific character of the Samapleu sequences is the fact that the magmatic intrusion originated from the lower continental crust at a depth of about 25 km.
The magmatic Ni, Cu and PGE deposits are subdivided into two main groups: 1) the deposits in association with ultramafic; and 2) the deposits with gabbroic sequences (Eckstrand, 1984).
The Samapleu mineralization is part of a typical ultramafic sequence. When compared, all the Ni-Cu sulphides share some characteristics (Naldrett, 1999):
- An ultramafic to picritic parent magma;
- Proximity to a major tectonic structure;
- Presence of rocks enriched in sulphides;
- Depletion in chalcophile elements in the intrusive rocks;
- Geochemical evidences of interaction between the magma and the host rocks and presence of, or proximity to, a dynamic magmatic conduit (feeder dykes).
The Sipilou Sud laterite deposit is characteristic of typical nickel laterite deposits formed in a seasonally wet tropical climate, on weathered and partially serpentinized ultramafic rocks. Features of nickel laterites include (Ellias, 2002):
- The nickel is derived from altered olivine, pyroxene and serpentine, which constitute the bulk of tectonically emplaced ultramafic oceanic crust and upper mantle rocks.
- Lateritization of serpentinized peridotite bodies occurred during the Tertiary period and the residual products have been preserved as laterite profiles over plateaus/amphitheatres, elevated terraces and ridges/spurs.
- The process of formation starts with hydration, oxidation, and hydrolysis.
- The warm/hot climate and the circulation of meteoric water (the pH being neutral to acid and the Eh being neutral to oxidant) are essential to this process. Silicates are in part dissolved, and the soluble substances are carried out of the system.
- This process results in the concentration of nickel in the regolith in hydrated silicate minerals and hydrated iron oxides; nickel and cobalt also concentrate in manganese oxides. The regolith hosting nickel laterite deposits is typically 10 m to 50 m thick, but can exceed 100 m.
- Concentration of the nickel by leaching from the limonite zone and enrichment in the underlying saprolite zones is also common. Leaching of magnesium +/- silicon causes nickel and iron to become relatively concentrated in the limonite zone. Nickel is released by recrystallization and dehydration of iron oxy-hydrides and is slowly leached downwards through the profile, both vertically and laterally, re-precipitating at the base with silicon and magnesium to form an absolute concentration within the saprolite.
- The degree of the nickel concentration and the detailed type of regolith profile developed is determined by several factors including climate, geomorphology, drainage, lithology composition, and structures in the parent rock, acting over time.
- A typical laterite profile contains three distinct horizons (limonite, transition and saprolite).
Mineralization
Mineralization at the Main, Extension, and Grata deposits consists predominantly of pyrrhotite, pentlandite and chalcopyrite, with subordinate amounts of pyrite, PGE and chromite. According to Gouedji (2014), and based on drill data, mineralization is preferably hosted in pyroxenite, although local zones rich in sulphides were identified within the peridotite units. In addition, strong sulphide mineralization also occurs at the gabbro-norite contact of the main zone of Samapleu.
Sulphide mineralization types at Main, Extension, and Grata deposits are matrix textures, nettextures, droplets, breccia, dragged sulphide sometimes with semi-massive sulphides, massive, veins, veinlets and are characteristics of magmatic mineralization types. Main, Extension, and Grata sulphides are formed by immiscibility due to the production of early sulphide liquid from mafic and ultramafic silicate melts.
The textural relationships reflect typical magmatic sulphide processes, whereby the parent melt reached sulphur saturation, leading to the development of an immiscible sulphide melt; this sulphide melt sequestered the chalcophile elements from the residual silicate magma during the emplacement of the mafic-ultramafic complex.
Information obtained from SNC’s geological mapping, geophysical survey data and detailed borehole observations suggests that the Main deposit is composed of upper and lower maficultramafic blocks. The upper block extends from the surface to a maximum depth of 150 m. The Lower block is separated from the upper block by a shallowly southwest dipping fault causing a displacement of approximately 75 m.
Platinum-group elements (“PGEs”) (palladium, platinum, and rhodium) are also present and are associated with the sulphide phases, either as a distinct mineral phase or included within the structure of the principal sulphides. The specific members of the platinum-group minerals ("PGMs”) identified are:
- Michenerite (PdBiTe);
- Mocheite (PtTe2);
- Rh-Cobaltite-Gersdorthite (NiAsS);
- Irarsite ((Ir, Ru, Rh, Pt)AsS);
- Hollingworthite ((Rh, Pt, Pd)AsS);
- Merenskyite (PdTe2).