Résumé
Serpentinite-hosted massive magnetite ore bodies are reported for the first time in the Late Cretaceous Sabzevar ophiolitic belt, northeastern Iran. They show irregular and discontinuous shapes with variable sizes ranging from 30 to 60 cm. Chromian spinel grains are observed within both magnetite ores and host serpentinite. Magmatic chromian spinels, (Cr,Al)-spinel I, with compositions close to (Mg-0.6,Fe-0.4)(Cr-1.2,Al-0.75,Fe-0.05(3+))O-4 are preserved in the host serpentinite where they display a porous alteration rim composed of Cr-bearing chlorite and three different spinel-structure minerals: Cr-spinel (Fe-0.6,Mg-0.4)(Cr-1.4,Al-0.4,Fe-0.2(3+))O-4, named Cr-spinel II (second generation), magnetite and ferritchromite, nominally FeCr2O4. In the magnetite ore body, no (Cr,Al)-spinel I is found and Cr-spinel II occurs as relict cores surrounded by ferritchromite and magnetite. Detailed X-ray elemental mapping revealed that the 200 mu m-thick magnetite rim is composed of two magnetite types with different minor element compositions: the first rim found at the contact with ferritchromite is thin (20 mu m; magnetite-I); the thicker outer rim contains numerous Fe-poor and Mg- and Si-rich silicate inclusions (magnetite-II). Observations at the TEM scale allows to identify ferritchromite which occurs as a micrometer-sized rim between Cr-spinel II and magnetite I. Thermodynamic modelling of the phase relationships in the studied Sabzevar serpentinite suggests that Cr-spinel II is produced along with chlorite during a first alteration stage at temperatures between 725 and 575 degrees C in the course of peridotite-water interactions. A second hydrothermal alteration stage producing ferritchromite and magnetite is inferred from the thermochemical modelling at temperatures < 400 degrees C under high H-2 fugacity. This latter stage corresponds to the serpentinization of the Sabzevar oceanic peridotite and associated podiform chromitite deposit. The two alteration stages are interpreted as the result of the interaction between seawater and oceanic mantle at two different depth ranges in the course of its exhumation. Our thermodynamic calculations and textural relationships revealed that Cr is immobile and Fe is the main element to be transferred to the magnetite ore during alteration processes. Fe possibly originated from direct transport of the Fe2+ produced during olivine dissolution or from the dissolution of nano-sized magnetite grains initially formed in the host serpentinite during early serpentinization. Mass balance calculation reveals significant iron transport at a scale > 10 m during serpentinization.