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New Caledonia holds one of the largest supergene nickel ore deposits worldwide. Two main ore types are recognized: oxide and hydrous Mg-Ni silicate types. Our study focuses on the latter type, usually located in elevated parts of the Peridotite Nappe, forming the so-called plateau and slope deposits. Understanding the controls of these nickel ore deposits is important in order to optimize the resource exploitation; however, to date there are only a few models to refer to, and structural aspects are rarely documented. From its formation at an oceanic ridge to Eocene obduction, the Peridotite Nappe of New Caledonia underwent several episodes of brittle fracture that mostly preceded weathering. We suggest that inherited fractures coated with serpentine polymorphs play a major role in the Ni enrichment process. Fracture analysis shows that thi s early fracture network shares the same orientations with lineaments of peridotite massifs and controlled erosion and plateau dissection during uplift events. The subsequent steep slopes together with circulating waters within serpentine-filled fractures parallel to underlying valleys led to slope failure. The collapse of plateau edges provoked plateau decompression and hence multidirectional extension. Gravity-driven faulting caused preferential opening of joints within the hanging walls of faults. The lateral permeability contrast favored weathering and eventually resulted in an increase of Ni content in the saprock. The controlling faults have a specific polyphase infill, which could be a metallotect and a potential guide for nickel exploration. [ABSTRACT FROM AUTHOR]

Dolines are closed geomorphological depressions which are surface manifestations of karstic systems. Usually developed on limestones, they also typify the morphology of the New Caledonian landscape, particularly on the southern massif of the main island (known as Massif du Sud). The specificity of dolines here lies in their development on ultramafic rocks. They are evidences of subsidence, suffosion and collapse phenomena resulting from dissolution weathering of peridotites. However, extensive underground drainage systems are still not yet recognized. Semi-automatic mapping of dolines is carried out on a 148 km2 area of the Massif du Sud from a high accuracy LiDAR digital elevation model. In this area 8601 dolines ranging from 1 m2 to 2 km2 are identified and morphologically characterized with precision. Most are small, shallow and round-shaped, yet more complex shapes are locally observed. Size distribution analysis allows the setting of a threshold of 20 000 m2 above which surface processes rather than chemical weathering control doline evolution. Doline density analysis reveals high concentrations on flat areas where ferricrete overlies the complete weathering profile, especially in the case of elevated rainy watersheds. Dolines are aligned and elongated along a north 135° ± 5° major fracture direction, which is inherited from the obduction of the Pacific Plate upper mantle in the Late Eocene. Finally, we propose a pioneering morphometric typology of dolines that provides important clues as to pseudokarstic activity. We define collapse, bowl-shaped and flat bottom dolines. Collapse and bowl-shaped dolines are assumed to denote active pseudokarst. They may widen and deepen, or eventually be filled by sediments. They are distinguished from flat bottom dolines that are partially to completely filled, which suggests that they are associated with paleo-pseudokarsts. However the groundwater flow paths associated with the genesis and evolution of dolines must be clarified, thus collapse and bowl-shaped dolines should be hydrologically monitored. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

To clarify the serpentinite correlation and kinematics in New Caledonia, we focus on the northwestern boundary of the Massif du Sud and the Mont Do Massif, in the Boulouparis area. On the basis of airborne electromagnetics data, structural analysis, and field data, we propose a significantly different model to the one commonly proposed for Peridotite Nappe emplacement. The interpretation is that serpentinite slivers in basement actually correlate with the Peridotite Nappe instead of older basement terrane serpentinites. Therefore the basal Peridotite Nappe fault is not as planar and simple as previously thought. Some portions of the Peridotite Nappe are involved into major shear zones with the basement by duplexing or internal thrust within the peridotite. Most of the kinematic indicators, in the serpentinite slivers, and in the substrate, show a N-S shortening accommodated with lateral (both dextral and sinistral) senses of shear. Serpentinite plays a major role in this process by accommodating most of the deformation and by partitioning strain between the substrate and the nappe, i.e. between the lower and the upper plate. This model is consistent with the dextral oblique convergence model recently proposed by several authors for the emplacement of the subduction-obduction complex of New Caledonia. [ABSTRACT FROM AUTHOR]

New data collected in the mining areas of the Tiébaghi Massif, a klippe of the Peridotite Nappe located in the north of New Caledonia, are presented. Data were collected in open cast nickel mine pits and chromite underground works, notably along a 1.2-km long tunnel which penetrates the deep geological structure of the massif. Surface and sub-surface geology, microtectonic observations and kinematic analysis of fracture planes, Electrical Resistivity Tomography, micron-scale Raman spectroscopy and Transmission Electron Microscopy were used to characterise the different tectonic regimes and associated mineralisations. Three stages of deformation and their kinematic axes are identified. Fractures coated with antigorite-tremolite minerals are associated to supra-subduction melt intrusions in a compressional regime characterised mainly by thrust and transcurrent faults. They are followed by fractures coated with polygonal serpentine formed in an extensional regime. The two former mineral phases are commonly superimposed on the same plane. Third-stage consist of fractures associated with supergene minerals deposited in an extensional regime, associated with gravity driven normal faulting. Fractures associated with this supergene stage have reactivated the former hypogene network. [ABSTRACT FROM AUTHOR]