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.
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The spark to put together this volume on banded iron formation (BIF)-related high-grade iron ore was born in 2005 during a steamy night in Carajás where the iron research group from the Universidade Federal Minas Gerais, Vale geologists, Carlos Rosière and Steffen Hagemann, were hotly debating the hypogene alteration genesis for the high-grade, jaspilite-hosted Serra Norte iron ore deposits. A couple of caipirinhas later we decided that the time was opportune to put together a volume that captured the new and innovative research that was being conducted on BIF-related high-grade iron ores throughout the world. We had little problem convincing our South African colleagues Jens Gutzmer and Nic Beukes to join the effort and decided that the 2008 biannual Society of Economic Geologists' (SEG) meeting in South Africa would be the perfect place to present this project through a combined field trip and workshop near Sishen.
The enthusiastic support that we received from the research community, SEG, and industry to put this volume together was generated by the significant increase in exploration activity, and with it the need for more detailed information on what exactly controls the location of high-grade iron orebodies, and renewed research interest around the world in models for the genesis of BIF-related high-grade iron ore, and particularly the relative importance of hypogene and supergene processes in formation of high-grade ore.
This volume concentrates on new research on the characteristics and metallogenesis of BIF-related high-grade iron ores. It contains a state of the art series of papers on established and new iron ore districts and deposits, the different components of the BIF iron mineral system, and how to best explore for this ore type. Although the emphasis of many of the contributions to this volume is on the hypogene aspect of high-grade iron ore formation, it is important to note that most BIF-related iron ore districts have a very pronounced supergene overprint due to deep lateritic weathering. The transformation of many hypogene iron orebodies of reasonable grade and size to the giant deposits exploited today can be related to this geologically recent supergene overprint; most of the past and still much of the present mining of high-grade iron ore relates to soft ore interpreted in most cases to be the direct result of supergene processes. Also mentioned here should be the recent resurgence of a syngenetic model that advocates the formation of chert-free BIF