Summary:
The target mineralisation at the Project’s V1 and V2 deposits are characteristic of typical nickel laterite deposits formed in a seasonally wet tropical climate, on weathered and partially serpentinised ultramafic rocks.
The Vermelho nickel deposits consist of two hills named V1 and V2 (after Vermelho 1 and Vermelho 2), aligned on a northeast-southwest trend, overlying ultramafic bodies. The ultramafic bodies have had an extensive history of tropical weathering, which has produced a thick lateritic profile of nickel-enriched limonite and saprolite at V1 and V2.
The V1 and V2 deposits form flat lying topographical highs, where V1 body reaches heights of around 500 m, standing out from the adjacent terrain about 175 m whereas the V2 body has a maximum height of 450 m. The flattened terrain adjacent to the two bodies represents the gneiss ground that hosts the layered intrusion of Vermelho. The V2 hill is located approximately 2 km southwest of V1. The V1 hill has a deformed convex-concave shape (convex to northeast), and extends for approximately 2.4 km east-west, ranging from 700 m to 1.6 km north-south. The V2 hill has an east-west elongation, and extends for approximately 1.9 km east-west, ranging from 600 m to 900 m north-south. The V1 deposit has an average thickness of 53 m and a maximum thickness of 146 m, whereas the V2 deposit has an average thickness of 56 m and a maximum thickness of 115 m.
The ultramafic bodies are erosional relicts of the upper sheet of a three-layer intrusion represented, from bottom to top, by a mafic zone (gabbros, gabbro-norites and leuconorites), a pyroxenitic zone (orthopyroxenites and chromitites, with c. 50 m thickness) and a peridotitic zone (serpentinised dunites and harzburgites, with c. 150 m thickness). The northeast-southwest oriented, 10 km long and 2 km wide, layered intrusion has intruded a package of gneisses and migmatites belonging to the Xingu Complex. Late sub-vertical diabase dykes intersect the three layers in different directions. Various chromitite levels have been identified at the southern sides of both V1 and V2 within the pyroxenite zone.
At Vermelho, two main mineralisation types are observed: silicate at the base (saprolite) and oxide at the top (limonite) of the weathering profile.
• Limonite: the oxide material, with 1.2% nickel oxide (NiO) on average, is composed predominantly of goethite, and also contains chlorite, spinels and silica. In this case, the nickel is highly concentrated in chlorite (average 12% NiO), whereas in goethite NiO content ranges between 0.9% and 1.7%. As a result, the presence of chlorite, even in minor quantities, is important in elevating the grade of the oxide ore. Locally, higher grades of mineralisation can also be due to the presence of nickeliferous smectites.
• Saprolite: the mineralogical composition of the silicate zone, with 1.8% NiO in average, consists largely of serpentine, chlorite, and spinels, with quartz and goethite in minor amounts. Serpentine and chlorite are the main nickel-bearing minerals, nickel being about equally distributed between the two phases (2% to 3% NiO). There is no significant development of an enriched transition mineralisation type between the oxide and silicate horizons.
Barren zones within the lateritic profile are represented by serpentinite blocks, strongly silicified bands, mafic or pyroxenitic dykes, and ferruginous concretions (common at the top of the section). A transitional saprolitic zone occurs at the base of the mineralised zone, with nickel grades usually ranging from 0.25% and 0.5%.
The Vermelho laterites are mineralogically similar to the limonitic laterites of Western Australia and, like the Australian ores, are amenable to pressure acid leaching. A possible treatment route of the higher grade saprolite ores is the RKEF process as envisaged for HZM’s Araguaia deposits.