The Barro Alto mineral resource is primarily saprolite overlain by laterites and extends for 35 km in an arc from southwest to northeast. The deposit is in the Barro Alto mafic-ultramafic complex, which is part of the Pre-Cambrian shield in the state of Goiás. The complex is composed of a sequence of serpentinised dunites and pyroxenites, enveloped by gabbros.
The mineralisation corresponds to the surficial weathered portions of the serpentinites. There are three types of saprolitic ore. The West type ore tends to have higher nickel grades and silica/magnesia ratios than the East type and Plain type ores.
The Barro Alto complex is bounded by metamorphic volcanic-sedimentary sequences (Ferreirafilho et al., 1992) and is a major mafic-ultramafic layered intrusion that was subjected to granulite facies metamorphism. It is composed mainly of gabbronorite, with minor dunite, peridotite, norite, gabbro, anorthosite and banded clinopyroxene-garnet amphibolite (Moraes and Fuck, 2000). In the complexes of Niquelandia and Barro Alto, lateritic weathering has formed important reserves of extractable Ni ore, representing around 2 million tons of metallic Ni (Raous et al., 2010). Ultramafic areas are characterized by a natural enrichment in heavy metals. In the case of the Barro Alto complex, this enrichment is mainly in Ni, however the Cr content has been reported as high as 6030 mg kg-1 (Ratié et al., 2015) and 46800 mg kg-1 (Raous et al., 2010), which is not high ........
