Abstract
The Fe Mn ore deposits of Urucum are located close to the Brasilian-Bolivian border. They appear to be considerably younger (Late Proterozoic) than most other banded iron ore formations. The Mn ores occur in three or four well-defined beds. Oxygen isotope determinations have been carried out in Fe-poor and Fe-rich layers and on a few Mn ores. Due to the extremely small grain size, a conventional mineral separation was impossible. However, since the banded iron ores consist of two components only (SiO 2 and Fe 2O 3), a determination of the Fe content allows a calculation of the oxygen isotope composition of the pure end-members. The so calculated δ 18O-values vary between +21.3 and +26.4‰ for quartz and between +0.8 and +6.5‰ for hematite. Isotope equilibrium may have been reached in some but not all samples. Calculated temperatures vary between 250° and 280°C, depending upon which calibration curve is used. Such temperatures are higher than indicated by maximum burial depths. The δ 19O-values appear to reflect diagenetic and burial metamorphic conditions rather than primary conditions.
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