EBSD: Towards a better understanding of the role of deformation in the formation of schistose hypogene high-grade iron ore

Abstract

The Cuité iron ore deposit is located in the Guanhães tectonic block at the eastern margin of the São Francisco craton, and consists of schistose to massive lenses of high-grade ore (>50 wt% Fe) hosted in a tectonic sliver of iron formation and quartz mica schist of the Serra da Serpentina Group. The orebody is encased in a sheared domain of the Statherian Açucena granite of the Borrachudos Suite in association with quartz and pegmatite veins. The structure and mineralogy of the iron oxides distinguish four main textural types: a) the granular-textured hematite ore (GO), b) the schistose lamellar-granular or specularitic hematite ore (SO), c) the coarse-grained granular hematite ore (CGO) and (d) the massive magnetite ore (MO). The orebodies exhibit a ubiquitous continuous to domanial anastomosed schistosity defined by millimeter-sized elongated hematite plates (specularite), minor quartz, and shreds of kaolinite wrapping around granoblastic domains or laterally grading into coarse granular textured bodies. Magnetite occurs as single idioblastic grains or aggregates, usually oxidized to kenomagnetite/maghemite and martite, overgrowing the foliated fabric to form a massive textured ore. The sheared kaolinite is associated with the occurrence of anatectic pegmatite bodies and extensive hydrothermal alteration in the country rocks during the late collisional stages of the Brasiliano Orogeny which spanned from the Ediacaran to the early Cambrian periods. Textural EBSD (Electron Backscatter Diffraction) studies reveal a tectonic evolution during iron mineralization under ductile conditions, characterized by progressive (and overlapping) hematite crystal-plastic deformation, dynamic recrystallization, grain growth and the development of a typical CPO (crystallographic preferred orientations) pattern. The hematite [c]-axes form a maximum that coincides with the foliation pole extending along a girdle perpendicular to the stretching orientation. The textural modifications of the orebody developed during shearing and alteration by pegmatite fluids. The fingerprints of hydrothermal activity can be identified through the alteration of the country rocks, crystallization of minerals, and the mobilization of quartz by hot fluids, which is a major process of iron enrichment in shear zones.

The internal deformation of hematite contrasts with the observations of oriented strain-free fibrous specularite which is typically found precipitated in interstitial open spaces and strain shadow domains although in very small proportions in the Cuité deposit. This highlights the diversity of mechanisms involved in the formation of shear zone-hosted, schistose high-grade iron ore and brings new insights into its genetic interpretation.