Abstract

A structural classification of podiform chromite orebodies from southern New Caledonia results in a division of deposits into three major types: discordant, subconcordant, and concordant, with penetrative structures (foliation and lineation) in the enclosing peridotite. Discordant deposits are very irregular in shape and clearly crosscut banding and foliation. Subconcordant deposits are generally tabular in shape and lie within 10 degrees to 25 degrees in strike and/or dip to the foliation. Concordant deposits are also tabular and lie parallel to foliation in peridotite; pyroxene lineation in host peridotite always indicates the elongation direction of these deposits. Within subconcordant and discordant deposits, chromite lineations are often oblique to those in the surrounding rocks; they follow local variations in the orebody shape and indicate possible deposit extensions at depth. The classification of chromite bodies corresponds to three stages of increasing deformation, as evidenced by chromite ore textures. Discordant deposits which are the least deformed are characterized by primary textures such as the nodular one, the foam texture, the chromite net, and the occluded silicate texture. Within subconcordant and mainly concordant deposits, massive, disseminated, and antinodular ores show evidence of strong deformation. The lack of geochemical distinctions between the different deposit types supports such an hypothesis.These chromite deposits in ophiolitic harzburgites are thought to have been formed beneath an oceanic spreading ridge. If the discordant pods are considered as representative of the original situation, it is proposed that the chromite has crystallized and has been dynamically concentrated along steep conduits traversing the enclosing harzburgite and feeding a magma chamber. Next the chromite-enriched pipes are caught up by plastic deformation in the mantle and tectonically reoriented toward the foliation.The chromite deposits in the Massif du Sud are located within a domain about 1.5 km thick in the harzburgites and dunite zones. This domain is limited upward by the first cumulates and gabbros and downward by the transition to a different plastic flow regime in the harzburgites.Finally, some guides to chromite prospecting and exploration are cited, applicable at different geologic scales and based on lithologic and structural criteria.

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