Thermally induced alterations of minerals during measurements of the temperature dependence of magnetic susceptibility: a case study from the hydrothermally altered Soultz-sous-Forêts granite, France

Identification of magnetic minerals using the temperature dependence of magnetic susceptibility in low field (κ–T) combined with optical microscopy, microprobe, X-ray diffraction, and chemical analysis provided new constraints on the alteration of Fe-bearing minerals of the magnetite-bearing Soultz-sous-Forêts granite from the EPS-1 borehole (upper Rhine Graben, France). While relatively fresh granite shows largely reversible κ–T curves typical of magnetite, the altered granite revealed a wide variety of irreversible heating and cooling curves, which allowed an assignment to different alteration stages under specific geochemical conditions. Though paramagnetic minerals like Fe-bearing carbonates, pyrite, or antiferromagnetic hematite could not be detected according to their Curie or Neél temperature, they were identified due to reactions to new ferrimagnetic phases during the heating/cooling experiments at specific temperatures. Mineral reactions were proved by measurements of the single mineral phases hematite, Fe-carbonates, and illite. Our mineralogical results combined with the thermomagnetic measurements imply that first faulting of the granite occurred already during cooling of the magma, which caused a first magnetite oxidation event. During uplift of the granitic body and exposure to a paleo-erosion surface, strongly acidic fluids, emerged from pyrite oxidation, caused a decomposition of Fe-bearing minerals like martite (hematite derived from magnetite oxidation) and Fe-carbonates and an ongoing transformation of magnetite to martite. Subsequently, precipitation of fine-grained hematite was restricted only to the upper part of the pluton. In the deeper part of the borehole, pyrite was preserved from oxidation. In an active fault, zone martite was reduced back to magnetite, which can be explained with the occurrence of organic matter transported by fluids.