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Genesis of the Pic de Fon Iron Oxide Deposit, Simandou Range, Republic of Guinea, West Africa

By
I. L. Cope
I. L. Cope
Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, Exhibition Road, London SW7 2AZ, United Kingdom
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J. J. Wilkinson
J. J. Wilkinson
Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, Exhibition Road, London SW7 2AZ, United Kingdom
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A. J. Boyce
A. J. Boyce
Scottish Universities Environmental Research Centre, Rankine Avenue, East Kilbride G75 0QF, United Kingdom
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J. B. Chapman
J. B. Chapman
Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, Exhibition Road, London SW7 2AZ, United Kingdom
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R. J. Herrington
R. J. Herrington
Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom
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C. J. Harris
C. J. Harris
Rio Tinto Iron Ore, 2 Eastbourne Terrace, London W2 6LG, United Kingdom
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Published:
January 01, 2008

Abstract

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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Contents

Reviews in Economic Geology

Banded Iron Formation-Related High-Grade Iron Ore

Steffen Hagemann
Steffen Hagemann
Centre for Exploration Targeting, School of Earth and Geographical Sciences, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
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Carlos Alberto Rosière
Carlos Alberto Rosière
Centro de Pesquisas Prof. Manoel Teixeira da Costa, Instituto de Geociências, Universidade Federal de Minas Gerais, Av. Antônio Carlos 6627, Campus Pampulha, Belo Horizonte, MG 31270.90, Brazil
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Jens Gutzmer
Jens Gutzmer
Paleoproterozoic Mineralization Research Group, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa
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Nicolas J. Beukes
Nicolas J. Beukes
Paleoproterozoic Mineralization Research Group, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa
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Society of Economic Geologists
Volume
15
ISBN electronic:
9781629490229
Publication date:
January 01, 2008

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