Fabric, Texture and Anisotropy of Magnetic Susceptibility in High-Grade Iron Ores from the Quadrilatero Ferrifero, Minas Gerais, Brazil

摘要

The iron protores of the Quadrilatero Ferrifero in Minas Gerais, Brazil, are quartz- or carbonate-banded laminated rocks (itabirite) which still display primary features similar to those encountered in non-metamorphic jaspilites, such as sedimentary layering, slump structures and porous granoblastic martite fabric with relictic kenomagnetite. Synmetamorphic deformation initiated recrystallization of the platy hematite variety (specularite) and by this gave rise to a variety of secondary grain fabrics and textures portrayed by the intensity of magnetic (low-field) susceptibility and by the geometry of corresponding anisotropy ellipsoids.rnFolded ores usually have a foliation roughly parallel to the compositional banding, an axial plane schistosity, lepidogranoblastic fabric and textures dominantly resulting from flexural slip. The stereogram of the corresponding (11.0) texture shows a single maximum parallel to the intersection line of banding and schistosity. The AMS is generally low; its ellipsoid is triaxial. Increasing strain and shearing obliterates the primary layering and induced the formation of ore mylonites with lepidoblastic (S-tectonites) or nematoblastic (L-tectonites) grain fabrics which results in varying shapes of the AMS ellipsoids. Schistose ores, finally, have girdle arrangements of hematite-(ll.O) poles, high susceptibilities and oblate shapes of the AMS ellipsoids, whereas strongly lineated ores display a single maximum parallel to the stretching direction, low to moderate susceptibilities and AMS ellipsoids with prolate shape. Secondary recrystallization and grain growth does not affect the lattice-preferred orientation.