Geochemistry of biotite and chilled margins in the Voisey's Bay troctolite

Glasgow, Jennifer (2014) Geochemistry of biotite and chilled margins in the Voisey's Bay troctolite. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Voisey's Bay troctolite is one of the oldest members of the Nain Plutonic Suite (NPS), and hosts the Voisey's Bay Ni-Cu-Co deposits. This study involves detailed mineralogy, geochemistry and oxygen isotope chemistry of the Voisey's Bay Complex (VBC) chamber, feeder and marginal rocks to determine: (1) the geochemical-isotopic fingerprint of biotite in troctolite, and (2) the composition of chilled margins on troctolite as indicators of parental magma composition and of the role of contaminants within the VBI. There are variations in the Ni and F contents of biotite in chamber versus feeder rocks as determined via electron microprobe analysis. Oxygen isotope analyses of biotite separates indicate that δ¹⁸O ratios of biotite in the Western Deeps and Mushuau intrusions are enriched relative to the mantle water value of 5.7 ‰, whereas ratios of biotite in the Eastern Deeps, Discovery Hill and Ovoid are depleted. Gabbronorite chilled margins are present in three settings within the VBI, and formed when hot magma quenched against country rock. Several chilled margins show evidence of country rock contamination, however, three chilled margin samples represent uncontaminted mafic magma parental to the VBI. The geochemical variations seen in biotite and chilled margins can be explained by a multiple-stage emplacement history that included at least two distinct mega-pulses of magma.

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