The BIF-hosted iron ore system represents the world's largest and highest grade iron ore districts and deposits. BIF, the precursor to medium- and high-grade (45-58% and 58-68% Fe, respectively) BIF hosted iron ore, consists of Archean and Paleoproterozoic Algoma type BIF (e.g., Serra Norte iron ore district in the Carajas Mineral Province), Proterozoic Lake Superior-type BIF (e.g., deposits in the Hamersley Province and Kaapvaal craton), and Neoproterozoic Rapitan-type banded iron formations (e.g., the Urucum iron ore district).
The BIF-hosted iron ore system is structurally controlled via km-scale normal and strike slip fault systems, which allow large volumes of ascending and descending hydrothermal fluids to circulate during Archean or Proterozoic orogenic or early extensional events. Structures are also (passively) accessed via downward flowing supergene fluids during Cenozoic times.
At the depositional site the transformation of BIF to medium- and high-grade iron ore is controlled by: (1) structural permeability, (2) hypogene alteration caused by ascending deep fluids (largely magmatic or basinal brines), and descending "warm" meteoric water, and (3) supergene enrichment via weathering processes. Hematite and magnetite based iron ores include a combination of microplaty hematite with little or no goethite, martite-goethite, specular hematite and magnetite, magnetite-martite, magnetite-specular hematite and magnetite-amphibole, respectively. Goethite ores with variably amounts of hematite and magnetite are mainly encountered in the weathering zone.
In most large deposits, three major hypogene and one supergene ore stages are observed: (1) silica leaching and formation of magnetite and locally carbonate; (2) oxidation of magnetite to hematite (martitisation), further dissolution of quartz and formation of carbonate, (3) further martitisation, replacement of Fe silicates by hematite, new microplaty hematite and specular hematite formation and dissolution of carbonates; and (4) replacement of magnetite and any remaining carbonate by goethite, hematite and formation of fibrous quartz and clay minerals.
Hypogene alteration of BIF and surrounding country rocks is characterized by: (1) changes in the oxide mineralogy and textures, (2) development of distinct vertical and lateral distal, intermediate and proximal alteration zones defined by distinct oxide-silicate carbonate assemblages, and (3) mass negative reactions such as de-silicification and de-carbonatisation, which significantly increase the porosity of high-grade iron ore, or lead to volume reduction by textural collapse or layer-compaction. Supergene alteration, up to depths of 200m, is characterized by leaching of hypogene silica and carbonates, and dissolution precipitation of the iron oxyhydroxides.
|Title of host publication||MINERAL RESOURCES IN A SUSTAINABLE WORLD, VOLS 1-5|
|Editors||AS Andre-Mayer, M Cathelineau, P Muchez, E Pirard, S Sindern|
|Publisher||ASGA-ASSOC SCIENTIFIQUE GEOLOGIE & APPLICATIONS|
|Number of pages||4|
|Publication status||Published - 2015|
|Event||13th SGA Biennial Meeting on Mineral Resources in a Sustainable World - Nancy Centre Prouvé, Nancy, France|
Duration: 24 Aug 2015 → 27 Aug 2015
|Conference||13th SGA Biennial Meeting on Mineral Resources in a Sustainable World|
|Period||24/08/15 → 27/08/15|