In-Depth Analysis of Complex Multiphase Oxidative Transformations in Iron Sulfides (Pyrrhotite and Pyrite) Within Migmatitic Gneiss
2025
Mateusz Dulski | Janusz Janeczek | Roman Włodyka
Raman imaging and K-means cluster analysis of individual mineral grains supplemented by scanning electron microscopy, electron probe microanalysis, and X-ray powder diffraction were applied to study fine-grained, multi-component products of the pyrrhotite three-stage oxidative alteration in migmatitic gneiss. During the first stage, related to the kaolinization of feldspars in gneisses, pyrrhotite was replaced by marcasite via intermediate amorphous iron sulfide. Increased oxygen fugacity caused the localized crystallization of either maghemite or ferric (oxyhydr)oxides. Even higher oxygen fugacity and an increase in solution pH during the second stage of alteration resulted in the partial replacement of marcasite by pyrite, followed by the replacement of both sulfides by Fe oxides (hematite, maghemite, magnetite) and ferric (oxyhydr)oxides (goethite, feroxyhyte). The final stage of sulfide oxidative alteration resulted in the predominance of sulfates of the alunite&ndash:jarosite series over ferric oxyhydroxides and relicts of Fe sulfides. Quartz&ndash:calcite&ndash:pyrite hydrothermal veins were affected by the most recent weathering, which resulted in the crystallization of the dominant alunite&ndash:jarosite-series minerals (alunite, jarosite, Al-jarosite) and ferric (oxyhydr)oxides (goethite, lepidocrocite).
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