The Loma de Hierro Ni-laterite deposit (Venezuela): Mineralogical and chemical composition

Nickel laterite deposits developed on ultramafic rocks have traditionally been a significant source of Ni and Co and recently of Sc. Although the Loma de Hierro deposit (Venezuela) has been in operation for more than 50 years, it lacks detailed studies on the mineralogical and geochemical composition of the lateritic profile. In this study, we present a geochemical and mineral description of the main carrier phases of Ni and Co in a complete profile of the deposit. The selected weathering profile has been developed from a partially serpentinized harzburgitic protolith and has a well-developed saprolitic horizon covered by a thin limonitic horizon. The geochemical signature of the profile is characterized by a significant Mg and Si decrease towards the top of the saprolite, with a clearly visible Mg discontinuity. The main Ni-bearing minerals are secondary serpentine (1-4 wt.% NiO) and kerolite-pimelite-dominated garnierite mixtures with serpentine (18-22 wt.% NiO). Limonite is rich in goethite (0-1.85 wt. % NiO), gibbsite, and Mn-oxy-hydroxides. The latter have intermediate compositions between lithiophorite and asbolane (2-13 wt.% CoO). The highest Sc grades (40-68 ppm) were observed in the limonite with amounts positively correlated with Fe content. Rare earth elements are mainly concentrated in the upper part of the saprolite horizon (60-80 ppm), while they have a lower content in the limonite (7-45 ppm). In this horizon, rare earth elements are clearly associated with Fe, indicating adsorption and/ or coprecipitation. This association was not observed in the saprolite, suggesting that other minerals (e.g., clay minerals) are controlling their concentration; more information is needed to identify the rare earth element-bearing minerals. The lateritic profile of Loma de Hierro can be classified as representative of hydrated Mg silicate deposits, and was formed in a context of continuous tectonic uplift and a low water table conditions favoring the development of a thick saprolitic horizon and the precipitation of kerolite-pimelite-dominated garnierites

Geology, geochemistry and mineralogy of the Loma de Hierro Ni-laterite deposit, Venezuela

Ni-laterite deposits host over 60% of the word land-based Ni resources (McRae, 2018). They formed from the weathering of Mg-rich ultramafic rocks exposed to the surface under favorable topography and climatic conditions. The typical Ni-laterite profile is characterized by a partially serpentinized parent rock (protolith) at the bottom followed by a saprolite horizon (dominated by secondary Si- and Mg-bearing minerals), and a limonitic horizon, dominated by goethite that evolves to hematite with time (e.g. Golightly, 2010). Ni originally liberated from olivine is concentrated in different secondary minerals such as goethite in the limonite and garnierites, secondary serpentines and/or smectite in the saprolite zone (Pelletier 1996; Villanova-de-Benavent et al. 2014). Ni-laterites are classified according to their dominant Ni-bearing ore in (a) hydrous-Mg-silicate, (b) clay and (c) oxide type deposits (Brand et al. 1998