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.