Pseudo-Karst Silicification Related to Late Ni Reworking in New Caledonia

Silicification in New Caledonian pseudo-karsts developed on peridotite was assessed using delta O-18 and delta Si-30 pairs on quartz cements. The objective was to document the chronology of pseudo-karst development and cementation relative to geomorphic evolution. The latter began at the end of the Eocene with the supergene alteration of peridotites and the subsequent formation of extended lateritic weathering profiles. Neogene uplift favoured the dismantling of these early lateritic profiles and valley deepening. The river incision resulted in (i) the stepping of a series of lateritic paleo-landforms and (ii) the development of a pseudo-karst system with subvertical dissolution pipes preferentially along pre-existing serpentine faults. The local collapse of the pipes formed breccias, which were then cemented by white quartz and Ni-rich talc-like (pimelite). The delta Si-30 of quartz, ranging between -5 parts per thousand and -7 parts per thousand, are typical of silcretes and close to the minimum values recorded worldwide. The estimated delta O-18 of -6 to -12 parts per thousand for the fluids are lower than those of tropical rainfall typical of present-day and Eocene-Oligocene climates. Evaporation during drier climatic episodes is the main driving force for quartz and pimelite precipitation. The silicification presents similarities with silcretes from Australia, which are considered predominantly middle Miocene in age.