Genesis of the Pic de Fon Iron Oxide Deposit, Simandou Range, Republic of Guinea, West Africa
I. L. Cope, J. J. Wilkinson, A. J. Boyce, J. B. Chapman, R. J. Herrington, C. J. Harris, 2008. "Genesis of the Pic de Fon Iron Oxide Deposit, Simandou Range, Republic of Guinea, West Africa", Banded Iron Formation-Related High-Grade Iron Ore, Steffen Hagemann, Carlos Alberto Rosière, Jens Gutzmer, Nicolas J. Beukes
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The Pic de Fon iron oxide deposit is located at the southern end of the Simandou Range in the southeastern part of the Republic of Guinea, West Africa. The deposit has a strike length of 7.5 km, is approximately 0.5 km wide, and is open at depth and to the south. Stratigraphy consists of three banded iron formations (BIFs: Lower, Middle, Upper), of which the upper two may be selectively enriched to 65 percent iron over a thickness of at least 250 m.
Two episodes of magnetite growth were followed by oxidation to martite (syn-D2, proposed as Eburnean II, 2100–2000 Ma) and subsequent bladed microplaty hematite that replaced gangue (dominantly quartz) mesobands. Key iron mineral phases consist of recrystallized martite, hematite overgrowths, and bladed microplaty hematite. Immobile element and density data through selected enrichment transitions suggest that, although the process can involve locally up to a 36 percent net gain in iron, silica removal is the principal control of enrichment, with 33 to 38 percent compaction related to silica loss.
Oxygen isotope data for separated quartz (δ18O(V-SMOW) 14.0–16.4‰) and hematite (δ18O(V-SMOW) –0.7 to +1.3‰) from nonenriched BIF suggest closure of oxygen isotope exchange during retrograde metamorphism (Eburnean II?) at temperatures of 215° to 280°C. Hematite from enriched high-grade rocks exhibits generally lower δ18O(V-SMOW) values of –8.9 to +2.0 per mil. This 18O depletion supports ore-stage hematite equilibration with a moderate-temperature, isotopically light, evolved meteoric fluid within a shallow-crustal hydrothermal system. Iron isotope analyses indicate a general decrease in δ56Fe(IRMM-014) of 0.2 to 0.6 per mil during enrichment, confirming nonconservative behavior of iron.
It is proposed that hydrothermal activity initiated post-D2 and was driven by either post-Eburnean II orogenic collapse or a poorly constrained thermal event at approximately 1500 Ma. Needlelike microplaty hematite is possibly associated with structural reactivation during the Pan-African orogeny (750–550 Ma). Loss of silica and redistribution of iron continues to the present day as the result of strong subtropical weathering.
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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