An alternative model for the formation of hydrous Mg/Ni layer silicates (‘deweylite’/‘garnierite’) in faulted peridotites of New Caledonia: I. Texture and mineralogy of a paragenetic succession of silicate infillings

Detailed textural and mineralogical investigations carried out in mineralized veins of the New Caledonian peridotites reveal three major periods of infilling and alteration closely linked to post-obduction tectonic activity. The first two periods of infillings are related to the alteration of hydrothermal serpentines, mostly found in the thick serpentine network of the peridotites, into fine-grained serpentine-like residues and newly formed talc-like minerals of weak but variable swelling capacity. The alteration of serpentine into talc-like minerals is limited during the first period of infilling and almost completed during the second one. In some fault zones of the New Caledonian peridotites, talc-like minerals are replaced by sepiolite. Such alterations are reported in both the Ni-free and Ni-rich zones of the infillings (i.e. in the white `deweylite' and greenish `garnierite' of the fault zones, respectively). They led to the individualization of the hydrous Mg/Ni silicate ore, which is nowadays found in fractures of the saprock and saprolite units of thick lateritic profiles. The third and last period of infilling is assigned to the accumulation of silica and crystallization of quartz. This succession of clay minerals (serpentine-like and talc-like minerals, sepiolite) and quartz in the infillings is interpreted as the result of sequential exportation of Mg and redistribution of Ni along the reactivated faults, generating periods of increasing Si activity in solutions. In this paragenetic model, the meteoric water infiltrating repeatedly the permeable network created by post-obduction tectonic activity would have interacted with a low-temperature hydrothermal field following the exhumation and cooling of the ophiolite nappe. In the less permeable parts of the fault network, restricted leaching conditions would have generated greater alkalinity in solutions and therefore favored the crystallization of sepiolite instead of finely divided talc-like minerals.