Podiform magnetite ore(s) in the Sabzevar ophiolite (NE Iran): oceanic hydrothermal alteration of a chromite deposit
Articolo
Data di Pubblicazione:
2021
Citazione:
Podiform magnetite ore(s) in the Sabzevar ophiolite (NE Iran): oceanic hydrothermal alteration of a chromite deposit / A. Eslami, B. Malvoisin, F. Brunet, A. Kananian, W. Bach, G. Grieco, A. Cavallo, G.D. Gatta. - In: CONTRIBUTIONS TO MINERALOGY AND PETROLOGY. - ISSN 0010-7999. - 176:6(2021), pp. 43.1-43.23. [10.1007/s00410-021-01799-0]
Abstract:
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 (Mg0.6,Fe0.4)(Cr1.2,Al0.75,Fe3+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,Fe3+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 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.
Tipologia IRIS:
01 - Articolo su periodico
Keywords:
Serpentinization; Epitaxial growth; Ferritchromite; Iron segregation; ASTAR
Elenco autori:
A. Eslami, B. Malvoisin, F. Brunet, A. Kananian, W. Bach, G. Grieco, A. Cavallo, G.D. Gatta
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