The Influence of the Magmatic to Postmagmatic Evolution of the Parent Rock on the Co Deportment in Lateritic Systems: The Example of the Santa Fé Ni-Co Deposit (Brazil)

Abstract

The Santa Fé Ni-Co deposit is a major undeveloped lateritic deposit located in the Goiás State of Central Brazil. The deposit comprises two properties that together have indicated resources of 35.7 million tonnes (Mt), grading 1.14% Ni and 0.083% Co, and inferred resources of 104.3 Mt at 1.03% Ni and 0.054% Co. The laterite was derived from Late Cretaceous alkaline ultramafic lithologies that experienced an initial silicification from Eocene to Oligocene, followed by lateritization and partial reworking in Miocene-Pliocene. The deposit is characterized both by oxide- and phyllosilicate-dominated ore zones. In the former, Ni- and Co-bearing hematite and goethite dominate the supergene mineralogical assemblage, while ore-bearing Mn oxyhydroxides occur as minor components. In the phyllosilicate-dominated horizons the major Ni-carrying phase is chlorite. Multivariate statistical analyses (factor analysis and principal components analysis) conducted on the drill core assay database (bulk-rock chemical analyses) showed that significant differences exist between Ni and Co distributions. The Ni distribution is not controlled by any clear geochemical correlation. This is because the highest Ni concentrations have been measured in the ferruginous and in the ochre saprolite zones, where Ni-bearing minerals (chlorite and goethite) are mostly associated with reworked material and only in a limited way, with zones affected by in situ ferrugination. Cobalt has an atypical statistical distribution at Santa Fé if compared with other laterites, correlated not only with Mn but also with Cr in the majority of the laterite facies. From microchemical analyses on several potential Co-bearing minerals, it was found that the Co-Cr association is related to elevated Co contents in residual spinels, representing unweathered phases of the original parent rock now included in the laterite. This element distribution is atypical for Ni-Co laterite deposits, where Co is normally associated with Mn in supergene oxyhydroxides. In the case of the Santa Fé laterite, the Co concentration in spinels is likely related to magmatic and postmagmatic processes that affected the original parent rock before lateritization, specifically (1) orthomagmatic enrichment of Co in chromite, due to its high affinity to spinels in alkaline melts, and (2) trace elements (i.e., Co, Mn, Ni, and Zn) redistribution during the hydrothermal alteration of chromite into ferritchromite. The Santa Fé deposit represents a good example of how the prelateritic evolution of a parent rock strongly affects the efficiency of Co mobilization and enrichment during supergene alteration. Based on the interpretation of metallurgical test work, a fraction of total Co between 20 and 50% is locked in spinels.

The origin of supergene nickeliferous chlorite in the Santa Fé Ni-Laterite Deposit, GO, Brazil

“…Bedrock mineralogy was investigated by Barbour (1976) and Barbour and Hypolito (1983) and, more recently, by Putzolu et al (2021). They have described a magmatic mineral association composed mostly of olivine with minor pyroxene plus chromite, magnetite, ilmenite, perovskite, and apatite.…”

“…The lateritic profile developed over peripheral pyroxenites is generally thicker, but the Ni grade is at its minimum. Putzolu et al (2021) provided more detailed information about the lateritic profile and proposed a new classification of the regolith units, which is adopted in this publication: saprock; saprolite (divided into green, ferruginous, and ochreous); transition zone; and pisolitic soil layer (Fig. 2).…”

“…Whole-rock geochemical analysis, used for the mass balance study, has been conducted on part of the dataset previously studied by Putzolu et al (2021), which were provided by Teck Cominco Ltda. We selected 8,657 analyses from 535 drill holes performed on the dunite-derived regolith and on 90 drill holes over pyroxenite-derived regolith (Fig.…”

“…The Santa Fé deposit was discovered in the 1970s; it was first studied by Barbour (1976), Oliveira and Melfi (1979), Oliveira and Trescases (1980), Barbour and Hypolito (1983), and was later reviewed by Carvalho e Silva and Oliveira (1995). Exploration work was reinitiated in the early 2000s with intensive drilling, which produced a large amount of new data that were further studied by Golightly (2010) and Putzolu et al (2021). Putzolu et al (2021), by means of geochemical and mineralogical analyses, have identified that, rather unusually, Ni-rich chlorite occurs throughout the lateritic profile.…”

“…Exploration work was reinitiated in the early 2000s with intensive drilling, which produced a large amount of new data that were further studied by Golightly (2010) and Putzolu et al (2021). Putzolu et al (2021), by means of geochemical and mineralogical analyses, have identified that, rather unusually, Ni-rich chlorite occurs throughout the lateritic profile. To explain its occurrence, the authors above proposed that chlorite formation was triggered by the hydrothermal alteration from chromite to ferritchromite.…”