Abstract
The Airnbe Formation is a stratigraphic unit of the Archaean Guarinos Group (Guarinos greenstone belt, Goias, Central Brazil) consisting of (I) magnetite- and/or haematite-muscovite iron formations, quartz being absent; (ii) metahydrothermalites representing fossil exhalative centers with preserved vents surrounded by hydrothermal alteration haloes containing tourmaline, chloritoid, magnetite, chlorite, and muscovite, and (iii) minor metaconglomerates and metashales. Tourmaline, chloritoid, magnetite, chlorite, and muscovite were quantitatively analysed in a microprobe to determine compositional variations of mineral species common to different alteration haloes, to the two main facies of the iron formations, and to both, to suport the intepretation of the mineral composition of the protoliths of metahydrothermalites and iron formations. The composition of metamorphic minerals suggest that the metahydrothermalites protoliths were probably composed by variable proportions of kaolinite, diaspore, magnetite, pyrophilite, illite, Na-montmorillonite, while muscovite of the iron formations is interpreted as metamorphosed felsic volcanic ash.
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