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Chapter 6: Kiruna-Type Iron Oxide-Apatite (IOA) and Iron Oxide Copper-Gold (IOCG) Deposits Form by a Combination of Igneous and Magmatic-Hydrothermal Processes: Evidence from the Chilean Iron Belt

By
Adam C. Simon
Adam C. Simon
1
Department of Earth and Environmental Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, Michigan 48109-1005
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Jaayke Knipping
Jaayke Knipping
2
Institut für Mineralogie, Leibniz Universität Hannover, Callinstraße 3, 30167, Hannover, Germany
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Martin Reich
Martin Reich
3
Department of Geology and Andean Geothermal Center of Excellence (CEGA), FCFM, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile
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Fernando Barra
Fernando Barra
3
Department of Geology and Andean Geothermal Center of Excellence (CEGA), FCFM, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile
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Artur P. Deditius
Artur P. Deditius
4
School of Engineering and Information Technology, Murdoch University, 90 South Street, Murdoch, Western Australia 6150, Australia
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Laura Bilenker
Laura Bilenker
5
Department of Earth, Ocean, and Atmospheric Sciences, 2020–2207 Main Mall, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4
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Tristan Childress
Tristan Childress
1
Department of Earth and Environmental Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, Michigan 48109-1005
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Published:
January 01, 2018

Abstract

Iron oxide copper-gold (IOCG) and Kiruna-type iron oxide-apatite (IOA) deposits are commonly spatially and temporally associated with one another, and with coeval magmatism. Here, we use trace element concentrations in magnetite and pyrite, Fe and O stable isotope abundances of magnetite and hematite, H isotopes of magnetite and actinolite, and Re-Os systematics of magnetite from the Los Colorados Kiruna-type IOA deposit in the Chilean iron belt to develop a new genetic model that explains IOCG and IOA deposits as a continuum produced by a combination of igneous and magmatic-hydrothermal processes. The concentrations of [Al + Mn] and [Ti + V] are highest in magnetite cores and decrease systematically from core to rim, consistent with growth of magnetite cores from a silicate melt, and rims from a cooling magmatic-hydrothermal fluid. Almost all bulk δ18O values in magnetite are within the range of 0 to 5‰, and bulk δ56Fe for magnetite are within the range 0 to 0.8‰ of Fe isotopes, both of which indicate a magmatic source for O and Fe. The values of δ18O and δD for actinolite, which is paragenetically equivalent to magnetite, are, respectively, 6.46 ± 0.56 and −59.3 ± 1.7‰, indicative of a mantle source. Pyrite grains consistently yield Co/Ni ratios that exceed unity, and imply precipitation of pyrite from an ore fluid evolved from an intermediate to mafic magma. The calculated initial 187Os/188Os ratio (Osi) for magnetite from Los Colorados is 1.2, overlapping Osi values for Chilean porphyry-Cu deposits, and consistent with an origin from juvenile magma. Together, the data are consistent with a geologic model wherein (1) magnetite microlites crystallize as a near-liquidus phase from an intermediate to mafic silicate melt; (2) magnetite microlites serve as nucleation sites for fluid bubbles and promote volatile saturation of the melt; (3) the volatile phase coalesces and encapsulates magnetite microlites to form a magnetite-fluid suspension; (4) the suspension scavenges Fe, Cu, Au, S, Cl, P, and rare earth elements (REE) from the melt; (5) the suspension ascends from the host magma during regional extension; (6) as the suspension ascends, originally igneous magnetite microlites grow larger by sourcing Fe from the cooling magmatic-hydrothermal fluid; (7) in deep-seated crustal faults, magnetite crystals are deposited to form a Kiruna-type IOA deposit due to decompression of the magnetite-fluid suspension; and (8) the further ascending fluid transports Fe, Cu, Au, and S to shallower levels or lateral distal zones of the system where hematite, magnetite, and sulfides precipitate to form IOCG deposits. The model explains the globally observed temporal and spatial relationship between magmatism and IOA and IOCG deposits, and provides a valuable conceptual framework to define exploration strategies.

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Metals, Minerals, and Society

Society of Economic Geologists (SEG)
Volume
21
ISBN electronic:
9781629496405
Publication date:
January 01, 2018

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