Origin of the giant Allard Lake ilmenite ore deposit (Canada) by fractional crystallization,multiple magma pulses and mixing |
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| Authors: | Bernard Charlier Olivier Namur Simon Malpas Cédric de Marneffe Jean-Clair Duchesne Jacqueline Vander Auwera Olivier Bolle |
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| Institution: | 1. Department of Earth Sciences, The University of Hong Kong, Hong Kong;2. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China;3. Geological Survey of Finland, 02150 Espoo, Finland;4. ProjecTerrae, Room 33, 23/F, On Hong Commercial Building, 145 Hennessy Rd, Wan Chai, Hong Kong;1. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China;2. John de Laeter Center for Isotope Research, Department of Applied Geology, Curtin University, Perth, WA 6845, Australia;3. University of Chinese Academy of Sciences, Beijing 100049, China |
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| Abstract: | The late-Proterozoic Allard Lake ilmenite deposit is located in the Havre-Saint-Pierre anorthosite complex, part of the allochtonous polycyclic belt of the Grenville Province. Presently the world's largest Fe–Ti oxide deposit, it had a pre-mining amount in excess of 200 Mt at grades over 60 wt.% hemo-ilmenite. The main ore body is a funnel-shaped intrusion, measuring 1.03 × 1.10 km and 100–300 m-thick. Two smaller bodies are separated by faults and anorthosite. The ore is an ilmenite-rich norite (or ilmenitite) made up of hemo-ilmenite (Hem22.6–29.4, 66.2 wt.% on average), andesine plagioclase (An45–50), aluminous spinel and locally orthopyroxene. Whole-rock chemical compositions are controlled by the proportions of ilmenite and plagioclase ± orthopyroxene which supports the cumulate origin of the deposit. Ore-forming processes are further constrained by normal and reverse fractionation trends of Cr concentration in cumulus ilmenite that reveal multiple magma emplacements and alternating periods of fractional crystallization and magma mixing. Mixing of magmas produced hybrids located in the stability field of ilmenite resulted in periodic crystallization of ilmenite alone. The unsystematic differentiation trends in the Allard Lake deposit, arising from a succession of magma pulses, hybridisation, and the fractionation of hemo-ilmenite alone or together with plagioclase suggest that the deposit formed within a magma conduit. This dynamic emplacement mechanism associated with continuous gravity driven accumulation of Fe–Ti oxides and possibly plagioclase buoyancy in a fractionating ferrobasalt explains the huge concentration of hemo-ilmenite. The occurrence of sapphirine associated with aluminous spinel and high-alumina orthopyroxene (7.6–9.1 wt.% Al2O3) lacking exsolved plagioclase supports the involvement of a metamorphic overprint during the synchronous Ottawan orogeny, which is also responsible for strong textural equilibration and external granule of exsolved aluminous spinel due to slow cooling. |
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