Le massif du Koniambo - Nouvelle Calédonie : Formation et obduction d'un complexe ophiolitique de type SSZ. Enrichissement en nickel, cobalt et scandium dans les profils résiduels.

Abstract : The geology of the island of New Caledonia consists of a patchwork of thick marine and terrigenous terraces. During late Cretaceous and Paleocene, the continuing break-up of the Gondwana margin resulted in the drifting of oceanic crusts in marginal basins, which finally reached the Eocene subduction zone of the newly formed Loyalty basin. During the Upper Eocene, the New Caledonian ophiolite was emplaced, following compression from the northeast, by thrusting over Late Cretaceous to Paleocene basalts and Mesozoic sediments. Ultramafic members are exposed over a total area of about 7 000 km2 in New Caledonia, with the Massif du Sud alone covering an area of 5 700 km2. A belt of isolated ultramafic massifs that belong to the New Caledonian ophiolite is present along the west coast of the main island of New Caledonia and the Koniambo massif is one of them. The Koniambo is located 270 km north-west of Nouméa. The massif is made up of a number of distinct and intricately overlapping fragments of an ultramafic ophiolitic nappe. There are three main geological assemblages: a) the Vavouto Sequence composed of serpentinised peridotites; b) the Dunitic Sequence; and c) the Harzburgitic Sequence. These ultramafic assemblages lie above slabs of oceanic crust, the Poya Unit, which are themselves laid out in complex overlapping formations. This thesis tries to link petrologic and mineralogical characteristics of these various assemblages to recent developments in geodynamic modeling related to ophiolitic complexes. Our study suggests that not only the volcanic sequence of a given ophiolitic complex can present affinities connected to a supra-subductive environment but also the associated mantle sequences. The intermingling of volcanic and volcano-sedimentary units at the base and the overlying of the Temala gabbro and the mantle units at the top, suggest a structurally inverted structural assemblage of an ophiolite suite. The ophiolitic sequence might have originated from a back-arc basin but evolved and migrated, eventually reaching a subduction zone. Several ultramafic members of the original sequence were strongly affected by their passage through this supra-subductive environment (SSZ) before being obducted onto the New Caledonia basement. The structurally inverted sequence contrasts with the less dismembered ultramafic sequences of the Massif du Sud. The laterite includes typical limonite and garnierite deposits in a highly complex geological succession of altered or weathered facies. Traditional methods were adapted to estimate and classify the Koniambo nickel-cobalt laterite resources in Nouvelle-Calédonie, for use in a feasibility study. As the resultant three-dimensional (3D) model is the basis for establishing the mining reserve and production schedule for a number of processing options. The model integrates the concept of geological horizons and geomorphic domaining to create a parent 3D block model. Estimation was conducted in unwrinkled space using Ordinary Kriging. The non-linear method of Uniform Conditioning was used to adjust the parent 3D model to reflect the proposed mining selectivity. Conditional Simulation was used to assess the risk associated with ore continuity and formed the basis for resource classification. Based on standards described by the Canadian Institute of Mining, Metallurgy and Petroleum (CIM), a combined Measured and Indicated mineral resource was established at 75.6 million tonnes grading 2.47% nickel and 0.059% cobalt at a 2.0% nickel cut-off. An additional Inferred mineral resource of 83 million tonnes grading 2.5% nickel and 0.07% cobalt was also estimated. Scandium enrichments are also described in the text.
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Marc-Antoine Audet. Le massif du Koniambo - Nouvelle Calédonie : Formation et obduction d'un complexe ophiolitique de type SSZ. Enrichissement en nickel, cobalt et scandium dans les profils résiduels.. Sciences de la Terre. Université de Nouvelle Calédonie, 2008. Français. ⟨tel-00432694⟩

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