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Contrasting petrological and geochemical relationships in the Voisey's Bay and Mushuau intrusions, Labrador, Canada; implications for ore genesis

Chusi Li, Peter C. Lightfoot, Yuri V. Amelin and Anthony J. Naldrett
Contrasting petrological and geochemical relationships in the Voisey's Bay and Mushuau intrusions, Labrador, Canada; implications for ore genesis (in A special issue on Voisey's Bay Ni-Cu-Co deposit, Anthony J. Naldrett (editor) and Chusi Li (editor))
Economic Geology and the Bulletin of the Society of Economic Geologists (July 2000) 95 (4): 771-799

Abstract

New data on the Voisey's Bay and Mushuau intrusions indicate that the two bodies, which were formerly grouped as the Reid Brook Complex, differ markedly in age, petrology, geochemistry, and the scale of known mineralization. U-Pb dating of baddeleyite and zircon has revealed that the Mushuau intrusion has an age of 1.313 Ga, which is approximately 20 m.y. younger than the 1.333 Ga Voisey's Bay intrusion. The Voisey's Bay intrusion is host to the 124 million tons Ni-Cu-Co sulfide deposit (reserves+resources). Mineralization occurs at the base of an upper subchamber (the Eastern Deeps subchamber) and in a conduit that connects the Eastern Deeps to a lower subchamber (the Reid Brook subchamber). The mineralization is associated with a basal breccia that contains abundant reacted inclusions of paragneiss. Within the Eastern Deeps, the basal mineralization is overlain by a varied-textured troctolite that contains variable amounts of disseminated sulfide and gneiss inclusions. By comparison, only minor amounts of sulfide mineralization have been observed within the Mushuau intrusion. The Mushuau intrusion comprises a layered unit that has been intruded by a stellate-textured troctolite along its contact with footwall orthogneiss. The layered unit consists dominantly of melatroctolite at the margin with leucotroctolite toward the center and olivine gabbro at the core. The cumulates of the Mushuau intrusion are richer in olivine and thus have higher MgO contents than those of the Voisey's Bay intrusion. Notwithstanding this, olivines in the melatroctolite inclusions of the Voisey's Bay intrusion (which are thought to represent disrupted cumulus layers) contain up to 81 mole percent forsterite, whereas those in the melatroctolite of the Mushuau intrusion contain no more than 70 mole percent forsterite. Nickel depletion in olivine is more pronounced in the Voisey's Bay intrusion than in the Mushuau intrusion. Our data indicate that the parental magmas of both intrusions were broadly basaltic (MgO<8.1 wt %) and that the magma of the Mushuau intrusion was more evolved than that of the Voisey's Bay intrusion, yet still capable of forming olivine-rich cumulates. The troctolites of the Voisey's Bay intrusion have Ce/Yb = 22 to 27, whereas most troctolites of the Mushuau intrusion have Ce/Yb = 12 to 17. The rocks of the Mushuau intrusion are simple mixtures of mantle-derived magma and local orthogneiss, whereas the rocks of the Voisey's Bay intrusion are formed by multiple stages of crustal contamination including an early stage of contamination at lower midcrustal level and a late stage of contamination by Tasiuyak paragneiss at upper crustal level. The assimilation of the sulfide-bearing Tasiuyak paragneiss by magma at Voisey's Bay may have promoted initial sulfide saturation and/or increased sulfide oversaturation, thus contributing to the segregation of large amounts of sulfide liquid from magma. Continuing flow of magma through a dynamic conduit system at Voisey's Bay has caused the local accumulation of sulfide of sufficient size and grade to constitute mineable reserves.


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 95
Serial Issue: 4
Title: Contrasting petrological and geochemical relationships in the Voisey's Bay and Mushuau intrusions, Labrador, Canada; implications for ore genesis
Title: A special issue on Voisey's Bay Ni-Cu-Co deposit
Author(s): Li, ChusiLightfoot, Peter C.Amelin, Yuri V.Naldrett, Anthony J.
Author(s): Naldrett, Anthony J.editor
Author(s): Li, Chusieditor
Affiliation: University of Pretoria, Department of Earth Sciences, Pretoria, South Africa
Affiliation: University of Toronto, Department of Geology, Toronto, ON, Canada
Pages: 771-799
Published: 200007
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 32
Accession Number: 2000-054363
Categories: Economic geology, geology of ore depositsIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: Includes appendix
Illustration Description: illus. incl. sects., 3 tables, sketch maps
N56°19'60" - N56°19'60", W62°04'60" - W62°04'60"
N46°30'00" - N51°40'00", W59°19'60" - W52°30'00"
Secondary Affiliation: University of Pretoria, ZAF, South AfricaINCO Technical Services, CAN, CanadaRoyal Ontario Museum, CAN, CanadaUniversity of Toronto, CAN, Canada
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200018
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