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

• Published in: Contributions to mineralogy and petrology 2021-06, Vol.176 (6), Article 43
• Main Authors: Eslami, Alireza, Malvoisin, Benjamin, Brunet, Fabrice, Kananian, Ali, Bach, Wolfgang, Grieco, Giovanni, Cavallo, Alessandro, Gatta, G. Diego
• Format: Article
• Language: English
• Subjects: Chlorite; Chromite; Composition; Cretaceous; Dissolution; Dissolving; Earth and Environmental Science; Earth Sciences; Fugacity; Geology; Grains; Hydrothermal alteration; Inclusions; Iron; Magnetite; Mass balance; Mineral Resources; Mineralogy; Modelling; Olivine; Ophiolites; Ores; Original Paper; Peridotite; Petrology; Phase relationships; Sciences of the Universe; Seawater; Serpentinite; Serpentinization; Silicates; Silicon; Spinel; Thermodynamic models; Transport
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-021-01799-0

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 3+ 0.05 )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 3+ 0.2 )O 4 , named Cr-spinel II (second generation), magnetite and ferritchromite, nominally FeCr 2 O 4 . 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