Abstract
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.
Highlights
Nickel laterite deposits are the product of weathering of ultramafic parent rocks at tropical latitudes (Gleeson et al, 2003; Freyssinet et al, 2005; Butt and Cluzel, 2013; Herrington et al, 2016)
The identification of major minerals was carried out using X-ray powder diffraction (XRPD) analyses, performed with a Panalytical X’Pert PRO MPD diffractometer equipped with an X’celerator PSD 2.1° detector at the Natural History Museum (NHM) in London (UK)
Ilmenite and Ti/Al oxide are found as exsolution lamellae in magnetite I and perovskite, respectively, their formation from alkaline melts is generally related to very different stages of the evolution of the orthomagmatic system
Summary
Nickel laterite deposits are the product of weathering of ultramafic parent rocks at tropical latitudes (Gleeson et al, 2003; Freyssinet et al, 2005; Butt and Cluzel, 2013; Herrington et al, 2016). The general features of a laterite profile, such as the mineralogy and chemistry, are products of the interplay between many factors, such as climate, relief, drainage conditions, tectonic lineaments, parent rock mineralogy, and degree of serpentinization (Gleeson et al, 2003; Freyssinet et al, 2005; Golightly, 2010; Porto, 2016; Putzolu et al, 2019). Nickel laterites can be subdivided in three main types: (1) oxide-type deposits, charac-.
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