The Ni(Co) laterite of Santa Fé (Brazil)

The Santa Fé Ni(Co) laterite deposit is located in the Goiás State, in Central Brazil. Its resources amount to 40Mt @ 1.37% NI and 0.12% Co in the oxide zone. The laterite profile was developed over the Santa Fé alkaline complex. This is a Late Cretaceous ellipsoidal body, consisting of mafic and ultramafic rocks; it has a rough concentric zonation with dunites in the centre and peridotites, pyroxenites, alkaline gabbros and syenites at the border [1]. The laterite profile can be subdivided in seven horizons lying on the fresh serpentinized dunite rocks. From the top to the bottom: i) Pisolitic Soil (R1); ii) Massive lateritic crusts (R2); iii) Transition Zone (R3); v) Ochreous Saprolite (R5); vi) Ferruginous Saprolite (R6); vii) Green Saprolite (R7); viii) Saprock (R8). Between R3 and R5 some silcrete levels can occur. In order to characterize the Santa Fé ores, petrographic, mineralogical and geochemical studies were carried out on selected samples from each horizon, as well as from several drill-cores. The samples were examined using a range of techniques: wet chemical analyses by Inductive Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), X-Ray Diffraction (XRD), Scanning Electron Microscopy by Energy Dispersive Spectroscopy (SEM-EDS) and Electron Micro-Probe Analyses (EPMA). From the chemical analyses, iron is the most abundant element: the majority of Fe is concentrated in the upper horizons of the profile (from R1 to R6 and in minor part in R7). The other major elements detected are Cr and Ti, which seem to be concentrated in the upper part up to R6. Cobalt and Mn are well correlated and are abundant from the top of the profile to R5. Nickel is generally poor in the upper part, with its concentration elevated in R5 to R7. Magnetite is the main Fe-oxide occurring in all the horizons; goethite and hematite also occur in samples R1 to R6, while maghemite was found only in R5-R6 and R7. Clays are also ubiquitous: smectite occurs from R1 to R5; minor kaolinite occurs in horizons R1 and R5 and clinochlore is commonly found in all the horizons from R1 to R7. Quartz is also common in all the profile. Accessory minerals are: ilmenite, occurring in R1 and R3, lithiophorite, present in R3 and R5 and asbolane in R5. Saponite can be present in horizon R7 and R8, together with antigorite, lizardite, forsterite and enstatite. The R1-R3 horizons are pisolitic and the pisoliths generally grow around a magnetite core. Less frequently, hematite cores are surrounded by mixed Fe-oxy-hydroxides (goethite) in a fine grain Fe-oxy-hydroxide matrix. The R5 and R6 horizons contain detrital magnetite, and hematite in a pelitomorphic matrix consisting of Fe-oxy-hydroxides intermixed with clays (mainly chloritesmectite). R7 and R8 show interlocking texture, mainly consisting of phyllosilicates (i.e. chrysotile, antigorite or lizardite). Magnetite and Fe-oxy-hydroxides are seen to pseudomorph olivine grains. The magnetite is highly enriched in Cr (up to ~20wt.%) and may contain also minor impurities (i.e. Al, Mg and Ti <~3wt%). Hematite, hosts traces of Cr (up to ~2 wt%), Al (up to ~3 wt %) and minor Mn and V (<~0.5wt%). Goethite can host Ni (up to ~1.5 wt%), Si (up to ~2wt%), Mg (up to ~1wt %). Al, Mn, Cr and Ti can occur in even minor amounts (<1wt %). From preliminary analyses, the clay minerals seem to be poorly enriched in Co but are high in Ni (from ~2.5 up to ~11wt% in R5). Minor Mn-oxy-hydroxides were also detected in the samples from the R5 horizon; they are commonly enriched in Ni (up to ~11 wt%) and Co (up to ~2 wt%). Preliminary data thus suggests that the Ni is largely hosted in clay and goethite whilst the bulk of the Co is found associated with Mn-oxides.