Weathering processes and crystal chemistry of Ni-bearing minerals in saprock horizons of New Caledonia ophiolite

This paper proposes a detailed study of the formation of the main Ni ore that is today mined in the saprock horizon of New Caledonia ophiolite. More specifically, this study aims at, i) characterizing the nature and the crystal chemistry of the main Ni-bearing phases, and ii) better understanding the weathering processes that lead to such nickel enrichment in these horizons. The study focuses on a boulder (similar to 20 cm in diameter) sampled in the saprock facies of the Koniambo massif, reflecting the early stages of peridotite bedrock weathering. This boulder is explored with a large panel of techniques, from the centimeter to the nanometer scale, including microimaging and X-ray absorption spectroscopy at the Ni and Fe K-edges. A characterization along the cross section reveals important heterogeneities in textures, and a large concentration of nickel in the outer part through an intense and connected Ni-phyllosilicate network that develop at the expense of the primary lizardite network. The inner part is preserved from the weathering by supergene fluids. Primary lizardite initially contains about 0.4 wt% NiO, similarly to surrounding olivine and pyroxene. In the outer part, the main Ni-bearing phases are identified as polygonal serpentine (up to similar to 5 wt% NiO) and two successive generations of talc-like minerals that contain similar to 20 to 36 wt% NiO. In these two minerals, nickel is very likely to form nano-clusters in the octahedral layers. The Fe3+/Fe-total ratios in serpentine minerals increases regularly from similar to 0.5 in primary lizardite, to similar to 0.95 in polygonal serpentine, traducing increasing local oxygen fugacity with increasing weathering degree. From molecular modeling, we propose that the large excess of water systematically observed for these Ni-rich minerals can be explained by nanometer sized crystals, which potentially require many additional hydrogen bonds (i.e., mainly silanol groups) to passivate crystal edges and preserve electro-neutrality. On the other hand, the main driving force leading to important nickel accumulation in the saprock horizon is likely to be the strong pH gradient present at all scales in the regolith profile, including at the boulder scale, which have major effects on the hydroxide solubility. As the pH decreases in the phyllosilicate network, we observe, i) the weathering and oxidation of the primary lizardite network, ii) the formation of a newly formed lizardite, iii) the formation of Nirich polygonal serpentine, iv) the formation of successive generations of Ni-rich talc-like minerals.