Phyllomanganate vein-infillings in faulted and Al-poor regoliths of the New Caledonian ophiolite: periodic and sequential crystallization of Ni–asbolane, Alk–birnessite and H–birnessite

Mineralogy, chemistry and spatial distribution of phyllomanganates, found in abundance at the bottom of thick Ni-laterite deposits, were established on dislocated vein-infillings showing banded and fibrous patterns (i.e. colloforms with rows of tiny boxworks) to decipher their conditions of formation in intensively faulted and Al-poor regoliths of the New Caledonian ophiolite (Koniambo klippe). Phyllomanganates of the vein-infillings belong to a Ni-rich, 9.5 Å type (Ni-asbolane) and two 7 Å types of contrasted alkali (K and Na) contents (Alk–birnessite and H–birnessite). Periodic development of radial cracks and differentiation of sequences of phyllomanganates enabled distinguishing up to five episodes of infillings. The first outer infilling of a botryoidal shape is devoid of Ni–asbolane and made of flakes of Alk–birnessite of different lengths and thicknesses, mixed and then replaced toward the inner section of the infilling by fine-grained H–birnessite. For the other infillings, the sequence of phyllomanganates is invariably the same from their outer to inner parts: (1) laths of Ni–asbolane in sealed cracks, (2) flakes of Alk–birnessite at the margin and extremity of the cracks or isolated in (3) a continuous groundmass of fine-grained H–birnessite. The relative proportions of these three phyllomanganate species can change drastically from one infilling to another. That of Ni–asbolane is closely related to the development of cracks, which are both important in the last two infillings. Thermal effects (e.g. cooling of a hot fluid on an older surface deposit) likely contributed to the development of radial cracks and, by self-organized diffusion and precipitation processes, to the sequential production of phyllomanganates (Ni–asbolane, Alk–birnessite and then H–birnessite) of decreasing particle sizes. As for the Mg/Ni phyllosilicate (“garnierite”) ore found at greater depth in the vein-infillings of the saprolite, the Ni/Co phyllomanganate (“chocolate”) ore observed abundantly as relics (dislocated veins) at the transition with the overlying laterite could also result from the early hydrothermal alteration of Al-poor peridotites following the emergence, dislocation and cooling of the ophiolite nappe onto the New Caledonian basement.