Fluids are bound to be involved in the formation of ophiolitic chromite deposits

Abstract

Chromite orebodies within the harzburgitic upper mantle have been investigated in several ophiolitic complexes, with a focus here on New Caledonia. The relationship between harzburgite and a dunite envelope around pods of chromitite points to a passive metasomatic transformation. The replacement of orthopyroxene by olivine leads to the necessary mobilization of Cr 3+ , not accepted by the neoformed olivine. A symplectitic intergrowth of enstatite + chromite at the interface between harzburgite and dunite may result from a silica-enriched fluid phase. Chromite in the orebodies contains micro-inclusions of olivine, clinopyroxene, orthopyroxene (rare), pargasite, aspidolite and nepheline. The olivine is hypermagnesian (up to 97 mol% Fo) and Ni-rich (0.25–1.05 wt%) and crystallized from a reducing fluid phase, not a magma. The pyroxenes and pargasite also are highly magnesian. The presence of Na in the environment accounts for pargasite, aspidolite and anomalously sodic nepheline. A Ni–Cu alloy is present. Chromite in the orebodies contains fluid inclusions, whereas chromite disseminated in harzburgite does not. The mixed fluid contains H 2 O (5 wt% equivalent of NaCl), with CO 2 and CH 4 in a 10 to 1 molar ratio. Values of δ 13 C VPDB (−22.1‰ for CO 2 and −28.1‰ for CH 4 , respectively) are typical of shallow marine hydrothermal systems. Experiments reveal that the precipitation of chromite from a fluid phase requires a reducing fluid and a temperature of ≤1050 °C. The appearance of olivine in the envelope results from the incongruent dissolution of enstatite in the fluid. The reaction is proven experimentally to be very efficient at a modest temperature (750 °C) and a low confining pressure (1.5 kbar). It leads to a shrinkage of the solid fraction, making the reaction self-propagating. Large volumes of harzburgite can be efficiently processed in an open, fluid-dominant reducing system. Relevant experiments show that the dissolution will increase in efficiency at higher temperatures and pressures. We do not negate the widely accepted explanation of chromitite formation by localized melt–harzburgite or melt–lherzolite interaction, but do point out that on the grounds of efficiency, such a process should be of secondary importance.