The behaviour of chromium and its isotopes in nickel laterites

Abstract

Ni laterites are important metal deposits that form by oxidative weathering of exposed ultramafic rocks in the Critical Zone; a key process for element cycling and supergene metal enrichment. Most continental chromium is hosted in ultramafic rocks and redox–dependent Cr release during laterite formation largely contributes to the Cr budget and the isotope signal that is transported via rivers into the oceans. However, the fractionation behaviour of Cr isotopes during Cr oxidation, in particular during the passage from rock to soil, is poorly understood. The aim of this study is to identify the major driving factors that govern Cr isotope fractionation during ultramafic rock weathering and subsequent Ni laterite formation. Three Ni laterites (Çaldağ, Karaçam, São João do Piauí) were investigated to evaluate the impact of the geochemical composition, the mineralogy, and the conditions of formation on the Cr isotopic composition. This study also presents the first comprehensive characterisation of the São João do Piauí Ni deposit and reveals that weathering under semi–arid climates can lead to the formation of a valuable Ni laterite given a distinct chemical composition of the parent rock. The results show that the total range of Cr isotopic compositions in modern (Cenozoic to Mesozoic) Ni laterites can be highly variable (Δ53/52Cr = 0.26 to 1.58 ‰) and that the isotopic evolution is primarily controlled by the distribution of the Cr between silicates and oxides. The early stages of weathering are characterised by a significant loss of Cr from silicates, which is accompanied by a large Cr isotope fractionation, whereas loss of Cr from oxides only occurs in the later stages. Based on these findings, a Cr isotope fractionation model was formulated that includes the chemical composition of Cr–bearing minerals of the parent rock and uses a Cr isotope fractionation factor that was estimated from the Cr isotopic composition of analysed pore water (δ53Cr = 4.31 ‰). Secondary controls that influence the Cr isotopic compositions of Ni laterites include the Mn content of the rock and the external conditions of laterite formation, such as the height of the water table and the evolution of the local relief.

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ORCID for Rachael James: orcid.org/0000-0001-7402-2315

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