The Geochemistry of Banded Iron Formation-Hosted High-Grade Hematite-Martite Iron Ores
J. Gutzmer, Benny C. Chisonga, Nicolas J. Beukes, Joydip Mukhopadhyay, 2008. "The Geochemistry of Banded Iron Formation-Hosted High-Grade Hematite-Martite Iron Ores", Banded Iron Formation-Related High-Grade Iron Ore, Steffen Hagemann, Carlos Alberto Rosière, Jens Gutzmer, Nicolas J. Beukes
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The whole-rock geochemistry of banded iron formation-hosted high-grade iron ores has long been ignored as a possible source of constraints on the physicochemical conditions of ore formation. In this contribution, available geochemical data, including major, trace, and rare earth element concentrations, from a selected number of high-grade hematite-martite deposits that represent supergene and hypogene ore-forming environments are collated. Geochemical data for high-grade iron ores are evaluated against the average composition of the BIF protolith, to gauge important trends of enrichment and depletion. Results reveal a generally very similar distribution of major and minor elements, irrespective of deposit type. The marked enrichment of iron is in all cases attributable to the effective removal of SiO2, MgO, CaO, as well as CO2. The often invoked immobility or even introduction of iron during high-grade iron ore formation is called into question by the observation that the increase in concentration of Al2O3 exceeds that of iron in almost all deposits. Furthermore, the distribution of redox-sensitive elements, such as Mn and V, suggests that during the transformation from BIF to high-grade hematite-martite ore fo2 remained effectively buffered by the oxidation of magnetite to hematite. Distinct enrichment of certain trace elements holds the promise to establish geochemical fingerprints to distinguish high-grade iron ore deposit types of different origin. This applies in particular to supergene high-grade hematite-martite ores, which are characterized by distinctly elevated concentrations of Sr and Ba and the efficient fractionation of LREE from HREE. Hydrothermal, magmatic-hydrothermal and supergene-modified hydrothermal deposits, on the other hand, appear not to have unique geochemical fingerprints. Enrichment of trace metals is usually restricted to single deposits but nevertheless provides an indication that more thorough studies may yield meaningful geochemical signatures to also distinguish different types of hypogene hematite-martite deposits.