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

Abstract

Serpentinite-hosted massive magnetite ore bodies are reported for the first time in the Late Cretaceous Sabzevar ophiolitic belt, northeastern of 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 (Mg0.6,Fe0.4)(Cr1.2,Al0.75,Fe 3+ 0.05)O4 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 (Fe0.6,Mg0.4)(Cr1.4,Al0.4,Fe 3+ 0.2)O4, 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 μm-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 μm; 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 °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 °C under high H2 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 Fe 2+ 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.

Hydrothermal alteration of chromitite-dunite pairs from the Sabzevar ophiolite (NE Iran): Chemical and nano-textural evolution of Cr-spinel

“…However, hydrothermal alteration and/or metamorphism may lead to significant changes in Cr-spinel texture and chemical composition (e.g. Colás et al, 2014;Barra et al, 2014;Eslami et al, 2021;Roy et al, 2022). With respect to major elements, Cr-spinel composition evolves towards an increase in iron (Fe 2+ and/or Fe 3+ ) and a decrease in magnesium and aluminum contents.…”

“…Spinel alteration has often been characterized in serpentinized ultramafic rocks which were submitted to subsequent metamorphism (e.g., Evans and Frost, 1975;Pinsent and Hirst, 1977;Burkhard, 1993). However, there is now a large number of studies which show that Cr-spinel alteration can already occur at the serpentinization stage (Spangenberg, 1943, Ulmer, 1974Ashley, 1975;Burkhard, 1993;Khalil & El-Makky, 2009;Mukherjee et al, 2010;Barra et al, 2014;Eslami et al, 2021). At the P-T conditions of serpentinization, a miscibility gap occurs in Cr-spinels (T < 600 °C, Sack and Ghiorso, 1991) which are thus expected to exsolve magnetite and ferrian Cr-spinel.…”

“…Over the years, a general scheme which is supported by thermochemical modelling, has emerged that considers the alteration of primary spinel under retrograde conditions to be a twostage process (Gervilla et al, 2012;Barra et al, 2014;Colás et al, 2014;Eslami et al, 2021). A first step produces Fe 3+ -poor Cr-spinel, sometimes termed ferrian Cr-spinel, through chlorite formation, possibly under reducing conditions and high silica activity (Colás et al, 2017).…”

“…In order to gain insight into hydrothermal Cr-spinel alteration under retrograde conditions (lithospheric mantle exhumation and cooling), the Sabzevar ophiolite (NE Iran) is remarkable since it also hosts magnetite ores that formed at the expense of Cr-spinel (Eslami et al, 2021).…”

“…Consequently, large amounts of iron must have been transferred from the surrounding dunite to form the magnetite ores. Eslami et al (2021) proposed that iron transfer over distances of more than 10 m occurred during serpentinization. Finally, Eslami et al (2021) showed, on the same samples, the added-value of sub-micrometer observation to characterize the Cr-spinel -magnetite replacement processes.…”