Formation of Silica and Magnesite Veins in the Massif of Peridotite of Koniambo: Geometric and Stable Isotopes Data

The New Caledonian Peridotite Nappe hosts one of the largest nickel ore deposit in the world, in association with laterites developed at the expense of the peridotites. According to a per descensum model of fluid circulation, it is proposed that a genetic link exists between this supergene alteration and the numerous silica and magnesite veins found deeper in the nappe. This work, based on geometrical, mineralogical and stable isotope analyses, provides contstraints on the origin of these veins. For the magnesite veins, the homogeneous and high 6180 values attest for a low temperature formation from meteoric waters; consistently, clumped isotope thermometry provided temperatures between 26 degrees C and 35 degrees C. Silica veins display a large range in delta O-18 values, which suggests that they formed between surface conditions and low-temperature hydrothermalism. These results question the genetic link of some silica veins with supergene alteration and more generally the per descensum model of Ni-laterite ore formation.