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NONREDOX TRANSFORMATIONS OF MAGNETITE-HEMATITE IN HYDROTHERMAL SYSTEMS

Hiroshi Ohmoto

Astrobiology Research Center and Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802

Corresponding author: email,ohmoto{at}geosc.psu.edu

The transformation of magnetite to hematite, or hematite to magnetite, in nature has generally been considered a redox reaction and linked to a specific redox state of fluid; however, a nonredox reaction, Fe₂O_{3 (hm)} + Fe²⁺ + H₂O = Fe₃O_{4 (mt)} + 2H⁺, may have been the principal mechanism for the transformations of iron oxides in nature, especially in hydrothermal environments. For example, the transformation of goethite and/or hematite (primary precipitates) to magnetite in banded iron-formations (BIFs) probably occurred through nonredox reactions with Fe²⁺-bearing hydrothermal fluids during the accumulation of a BIF sequence, rather than through redox reactions involving organic matter during and/or after the BIF deposition. The proposed mechanisms for the transformation of magnetite to hematite provides new exploration strategies for hematite-rich secondary ores, extending the target for orebodies to much deeper zones below the paleosurface. Another important implication of the proposed mechanism is that the presence or absence of magnetite and/or hematite in geologic formations may or may not provide meaningful information on the redox state of fluid.

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