Podiform magnetite ore(s) in the Sabzevar ophiolite (NE Iran): oceanic hydrothermal alteration of a chromite deposit

Serpentinite-hosted massive magnetite ore bodies are reported for the first time in 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 observed within both ores host serpentinite. Magmatic chromian spinels, (Cr,Al)-spinel I, compositions close (Mg0.6,Fe0.4)(Cr1.2,Al0.75,Fe3+0.05)O4 preserved serpentinite where they display a porous alteration rim composed of Cr-bearing chlorite three different spinel-structure minerals: Cr-spinel (Fe0.6,Mg0.4)(Cr1.4,Al0.4,Fe3+0.2)O4, named II (second generation), ferritchromite, nominally FeCr2O4. In body, no I is found occurs as relict cores surrounded by ferritchromite magnetite. Detailed X-ray elemental mapping revealed that 200 μm-thick two types minor element compositions: at contact thin (20 μm; magnetite-I); thicker outer contains numerous Fe-poor Mg- Si-rich silicate inclusions (magnetite-II). Observations TEM scale allows identify which micrometer-sized between I. Thermodynamic modelling phase relationships studied suggests produced along during stage temperatures 725 575 °C course peridotite-water interactions. A second hydrothermal producing inferred thermochemical < 400 under high H2 fugacity. This latter corresponds serpentinization oceanic peridotite associated podiform chromitite deposit. The stages interpreted result interaction seawater mantle depth ranges its exhumation. Our thermodynamic calculations textural Cr immobile Fe main be transferred processes. possibly originated direct transport Fe2+ olivine dissolution or nano-sized initially formed early serpentinization. Mass balance calculation reveals significant iron > 10 m