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A Ni-Cu-Co-PGE massive sulfide prospect in a gabbronorite dike at Lac Volant, eastern Grenville Province, Québec

By
Hassan Nabil
Hassan Nabil
Sciences de la Terre, Université du Québec à Chicoutimi, 555 Bv. de l'Université, Chicoutimi, Québec G7H 2B1, Canada
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Thomas Clark
Thomas Clark
Géologie Québec, Ministère des Ressources naturelles, 5700 4e Avenue Ouest, Bureau A-210, Charlesbourg, Québec G1H 6R1, Canada
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Sarah-Jane Barnes
Sarah-Jane Barnes
Sciences de la Terre, Université du Québec à Chicoutimi, 555 Bv. de l'Université, Chicoutimi, Québec G7H 2B1, Canada
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Published:
January 2004

The Lac Volant Ni-Cu-Co-PGE prospect, located 75 km northeast of Sept-Iles in the Grenville Province of Québec, is an example of magmatic sulfide mineralization associated with mafic magmas in a high-grade metamorphic terrain. Disseminated and massive sulfides occur within a 20- to 25-m-thick zone in a gabbronorite dike intruding the gabbronorite of the host Matamec Complex. The average composition (n = 29) of the sulfides is estimated to be 2.0% Cu, 1.5% Ni, 0.12% Co, 67 ppb Pt, and 256 ppb Pd. The age of the dike is 1351 ± 6 Ma (U-Pb zircon age), which is identical to the age of the Rivière-Pentecôte anorthosite (1354 ± 3 Ma) located 130 km to the southwest. The dike originated by means of multiple injections of sulfide-bearing, tholeiitic magmas derived from a depleted, N-type mid-ocean ridge basalt (N-MORB)-type source. The dike is chemically similar to the gabbronoritic host, suggesting similar parental magmas. Geochemical variation within the dike was caused by fractional crystallization of silicates (orthopyroxene, clinopyroxene, and plagioclase) and by ingestion of crustal rocks. Assimilation is suggested by the presence of xenoliths of granite and metasediment within the dike or narrow branching dikes; enrichment of the dike in Rb, Th, Ba, and light rare earth elements; and a negative Ta anomaly with respect to Th. The observed composition of the dike can be explained by 15% crustal contamination, which would have occurred at depth before the magma began to crystallize mafic minerals. Massive, matrix, and disseminated sulfides, interpreted to be of magmatic origin, are composed of pyrrhotite (the principal species, 75%), pentlandite (altered to bravoite and violarite), chalcopyrite, and pyrite. Magmatic breccia structures are common in gabbronorite containing disseminated sulfides. The presence of metasedimentary xenoliths and a high S/Se ratio (9,000–16,000) suggest that sulfide saturation was caused by contamination of the magma. The sulfide liquid interacted with a small volume of magma (R = silicate liquid/sulfide liquid = 200). Impoverishment of the sulfides in platinum-group elements (PGE) relative to Ni and Cu may have been caused by the loss of PGE during an earlier and deeper episode of sulfide separation. Limited sulfide fractionation is suggested by the chemical similarity of the three types of sulfide. Metamorphic recrystallization produced xenomorphic pyrrhotite and idiomorphic pyrite crystals up to 25 cm and 4 cm in size, respectively. Alteration by meteoric water caused the transformation of pentlandite into bravoite and violarite. The Lac Volant prospect is similar in terms of composition, style of emplacement, magma type, and age to certain Norwegian deposits (the Ertelian and Flåt deposits in the Sveconorwegian Province) and to the Voisey's Bay deposit in Labrador.

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Contents

GSA Memoirs

Proterozoic Tectonic Evolution of the Grenville Orogen in North America

Edited by
Richard P. Tollo
Richard P. Tollo
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James McLelland
James McLelland
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Louise Corriveau
Louise Corriveau
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Mervin J. Bartholomew
Mervin J. Bartholomew
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Geological Society of America
Volume
197
ISBN print:
9780813711973
Publication date:
2004

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