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International audienceIn New Caledonian Ni deposits, the richest Ni silicate ores occur in fractures within the bedrock and saprolite, generally several tens of meters to hundred meters below the present-day surface. Fracture-related Ni silicate ore accounts for high Ni grades, at least a few weight percent above the average exploited grade (2.5 %). These Ni-rich veins are affected by active dissolution-precipitation processes at the level of the water table. Ni in solution is precipitated as silicates in thin layer cementing joints. This mineralization is characterized by chemical and mineralogical concentric zoning with an outer green rim around an inner white zone composed, from the edge to the centre of the block, (i) a highly oxidized and altered zone, (ii) a green pure Ni-rich pimelite zone, (iii) a zone (limited to a few centimetres) with a mixture of Ni-poor kerolite and Ni-rich pimelite and intermediate colours and (iv) a large white Mg-kerolite mineralization zone. This study proposes that the concentric zonation results from evapo-precipitation process related to alternate periods of hydration and drying, induced by water table movements. This extensive dispersion of Ni in concentrically zoned ores can partly explain the rather monotonous Ni grade of the bulk exploitation at the base of the regolith with values between 2 and 3 wt%

The stable isotope compositions of veins provide information on the conditions of fluid-rock interaction and on the origin of fluids and temperatures. In New Caledonia, magnesite and silica veins occur throughout the Peridotite Nappe. In this work, we present stable isotope and clumped isotope data in order to constrain the conditions of fluid circulation and the relationship between fluid circulation and nickel ore-forming laterization focusing on the Koniambo Massif. For magnesite veins occurring at the base of the nappe, the high d18O values between 27.8‰ and 29.5‰ attest to a low temperature formation. Clumped isotope analyses on magnesite give temperatures between 26°C and 42°C that are consistent with amorphous silica -magnesite oxygen isotope equilibrium. The meteoric origin of the fluid is indicated by calculated d18Owater values between -3.4‰ to +1.5‰ Amorphous silica associated with magnesite or occurring in the coarse saprolite level displays a narrow range of d18O values between 29.7‰ and 35.3‰. For quartz veins occurring at the top of the bedrock and at the saprolite level, commonly in association with Ni-talc-like minerals, the d18O values are lower, between 21.8‰ and 29.0‰ and suggest low-temperature hydrothermal conditions (~40-95°C). Thermal equilibration of the fluid along the geothermic gradient before upward flow through the nappe and/or influence of exothermic reactions of serpentinization could be the source(s) of heat needed to form quartz veins under such conditions. Dear Editor, Please find attached the revised version of manuscript #GCA-D-15-00678, entitled "Paired stable isotope (O, C) and clumped isotope thermometry of magnesite and silica veins in the New Caledonia Peridotite Nappe", by B. Quesnel, P. Boulvais , P. Gautier, M. Cathelineau, C.M. John, M. Dierick, P. Agrinier and M. Drouillet, which required moderate revisions before final acceptance for publication in Geochimica et Cosmochimica Acta. You can find on this file our answers (in red below) to reviewer's comments. Accordingly to these comments, the text and some figures have been modified. For the text, we have let these changes visible on the manuscript to make easier your lecture. Note also that we have taken into account the minor editorial suggestions of reviewers (for example those noticed in yellow boxes in pdf files); these changes do not appear in the new version. We have also built new tables that could be useful for the geochemists community; these calculations have been suggested by reviewers 1 and reviewer A. Gilg (we now present a temperature range for quartz formation of 40-95°C, instead of 40-80°C in the previous version; conclusions remain unchanged). These tables are introduced as electronic supplements to preserve the ms as short and clear as it was.Best regards, Benoit Quesnel and co-authors *Cover LetterDear Editor, Please find attached the revised version of manuscript #GCA-D-15-00678, entitled "Paired stable isotope (O, C) and clumped isotope thermometry of magnesite and silica veins in the ...