Jana Just

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.

Development of magnetic fabrics during hydrothermal alteration in the Soultz-sous-Forêts granite from the EPS-1 borehole, Upper Rhine Graben

Abstract The Variscan, magnetite-bearing Soultz-sous-Forêts granite is found between 1420 and 2230 m of the EPS-1 borehole situated in the Upper Rhine Graben (France). Our study focuses on the changes of magnetic properties that occur during the progressive hydrothermal alteration and fracturing of the Soultz granite after emplacement. The magnetic susceptibility (κ) of the granite is between 10 and 80 × 10−3 SI, and suggests that ferrimagnetic minerals are the primary carrier. During cooling and later tectonic and hydrothermal overprints, including the formation of the Rhine Graben, the granite was deformed under brittle conditions and partially altered by hydrothermal fluids. Along with this fluid activity, oxidation of magnetite to hematite occurred and reduced κ (< 1 × 10−3 SI). AMS analysis on oriented samples documents the history of progressive transformation from primary magmatic fabric to tectonic fabric during hydrothermal alteration and faulting. The fresh granite with multidomain magnetite grains shows sub-horizontal magnetic foliations and randomly oriented magnetic lineations within the foliation plane. This fabric is similar to the magmatic fabric reflected by biotite. Transformation of the magnetic fabric started with localized magnetite oxidation along NW-SE oriented micro-cracks, which are probably associated with a late-magmatic alteration (stage I). Elongated and co-aligned magnetite relics within the newly formed hematite caused a well-defined NW-SE trending magnetic lineation and steeper magnetic foliation. Later alteration associated with intense brittle deformation (stage II) initially adopted this magnetic fabric, but intense cataclasis destroyed it. The geometry and orientation of magnetic fabric clearly indicate a hydrothermal alteration, which relates to the acting tectonic stresses in the post-emplacement history of the Soultz granite.