Trace element geochemistry of iron-(oxy)-hydroxides in Ni(Co)-laterites: Review, new data and implications for ore forming processes

Iron-(oxy)-hydroxide (FeO/OH) phases are abundant in all supergene ore deposits. The most common FeO/OH phase in supergene environments is goethite, although hematite, lepidocrocite, ferrihydrite, and maghemite can also occur. Natural FeO/OHs are rarely chemically pure, as a range of metal cations can be readily incorporated into their mineral structure. Although an extensive body of literature exists on the scavenging action of synthetic FeO/OHs, there is a general lack of studies of natural systems and, more specifically, of studies dealing with the geochemistry of trace elements in FeO/OHs associated with supergene ores. Furthermore, although it is known that FeO/OHs in supergene ore systems typically contain elevated levels of useful metals like REE, Sc, V, Co, Mn, Cr, and Ni, in most cases, these phases are considered as gangue and hence, the metals are not recovered. Only in the case of Ni(Co)-laterite deposits the FeO/OHs are often exploited for Ni and Co, and sometimes for Sc. Most previous works on Ni(Co)-laterite deposits have focused on the lateritization process of the parent rocks and the mineralogy of the resulting Ni(Co)-bearing minerals. Only rarely have published studies focused on REE, V, Sc, and PGE deportment within FeO/OHs. In this study, we describe new mineralogical and chemical data (XRPD, SEM-EDS, EPMA, ICP-AES, LA-ICP-MS, and TEM-HRTEM) obtained from a range of natural FeO/OH samples collected from four important Ni(Co)-laterite deposits, namely Wingellina (Western Australia), Piauí (Brazil), Karaçam and Çaldağ (Turkey). In the course of this study, we investigated the geochemistry of goethite and hematite within the oxidation zone of the respective laterite profiles, evaluating the deportment of minor metals such as Ti, Sc, Cr, Ni, Co, V, Zn, and Mn. Although derived from different parent rocks located in different geographic areas, the FeO/OH samples collected share a number of common features. In particular, there are commonalities in ore textures, mineralogy, and metal deportment. Based on multivariate statistical analysis, the chemistry of the studied FeO/OHs define three major elemental associations: i) Mn–Al–Ti–Sc–V as evident in goethite samples from Wingellina; ii) Mg–Ni–Si–Zn as exemplified by samples from Karaçam and Piauí, and iii) Cr–V as illustrated by the Çaldağ samples. These contrasting geochemical footprints can be explained in terms of first- and second-order controls with the chemical composition of the parent rock representing the first-order, and favorable pH conditions for the fixation of trace elements within FeO/OHs representing the second-order control. Seasonality and maturation may be additional factors influencing FeO/OHs mineralogy, as periods of arid climate may have favored the dehydration of some FeO/OHs to form more stable species (such as goethite to hematite) over time. In summary, our observations have helped to better understand the ore deposition model relating to surficial weathering systems and have also established the parameters that control the distribution of economically relevant by-product metals in FeO/OHs in diverse conditions during the formation of Ni(Co)-laterites.