Evidence for metre-scale variations in hematite composition within the Palaeoproterozoic Itabira Iron Formation, Minas Gerais, Brazil

摘要

Hematite is a mineral the chemical composition of which rarely differs significantly from stoichiometric Fe_2O_3. As such, little attention has been paid to the mineral chemistry of hematite in Precambrian iron formations, where hematite forms monomineralic high-grade orebodies. Electron microprobe analysis of hematite from two iron-ore deposits, Caue (Itabira district) and Gongo Soco, in the Palaeoproterozoic Itabira Iron Formation, Quadrilatero Ferrifero of Minas Gerais, Brazil, has revealed distinct variations in chemical composition with respect to Ti and Cr. Hematite containing Ti and/or Cr is of very local occurrence in the itabirite unit and shows a spatial relationship to hematitic, palladiferous gold-bearing veins (known as 'jacutinga'), occurring either within the veins (adjacent to, or included in, palladiferous gold grains) or in their vicinity. Where present, titaniferous hematite (to approx 1.3 wt. percent TiO_2) is lepidoblastic and defines a pervasive tectonic foliation (S_1). In contrast, Ti-free, chromiferous hematite (to approx 6.4 wt. percent Cr_2O_3) characteristically occurs as inclusions in palladiferous gold within Si-truncating 'jacutinga'. Replacement of granoblastic, Ti-free, chromiferous martite with relicts of magnetite by lepidoblastic, Cr-depleted, titaniferous hematite proves that Cr and Ti were mobile during metamorphism. Chromium was ultimately fractionated into the hematite found in auriferous aggregates within cross-cutting 'jacutinga'. A positive correlation between Cr and Pt in bulk-rock samples from the Itabira district suggests that Cr is a potential prospective guide for Au-Pd-Pt-bearing hematitic veins ('jacutinga').