Metallogeny of supergene cobalt mineralization, New Caledonia

Abstract

In New Caledonia, weathering profiles developed on ultrabasic rocks host economic Cobalt (Co) mineralization of supergene origin. Cobalt gradually concentrates from the fresh ultrabasic rock to the interface between the saprolitic and residual ferruginous layers, where it accumulates, because Co minerals are unstable in the upper part of the profile. In places where the depth of weathering varies markedly as a result of heterogeneities in the parent rock, stronger accumulations build up in the depressions, as Co is partially leached from the upper profiles. The Co mineralization observable today is essentially recent and is still being deposited. However, some lithiophorite, which does not dissolve with continued weathering, survives as a remnant from previous weathering periods. Cobalt mineralization occurs as several bluish black‐grey minerals, including heterogenite, asbolan (from Ni‐rich to Co‐rich), lithiophorite and minerals with intermediate composition between asbolan and lithiophorite. Each of these minerals has its own Eh‐pH stability field and is, therefore, limited to certain horizons, depths and macroscopic habits. Some replace silicate minerals along the joints in the saprolite, others replace plant roots at the base of the ferruginous layer. Heterogenite and the most cobaltiferous varieties of the other minerals occur only in places where Co accumulation is particularly strong. Guidelines are proposed for prospecting potential Co‐rich areas and for the visual assessment of Co grades.