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 angstrom type (Ni-asbolane) and two 7 angstrom 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.