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TitreEvaluating crustal sulfur sources in magmatic Ni-sulfide deposits: Application of a new multiple S isotope method to the Voisey's Bay Ni deposit
AuteurHiebert, R; Bekker, A; Wing, B
SourceL'Association géologique du Canada-L'Association minéralogique du Canada, Réunion annuelle conjointe, Recueil des résumés vol. 35, (2012), 2012 p. 56-57
LiensOnline - En ligne
Séries alt.Secteur des sciences de la Terre, Contribution externe 20140526
ÉditeurAssociation géologique du Canada
RéunionGAC-MAC Joint annual meeting; St. Johns, NL; CA; mai 27-29, 2012
Mediapapier; en ligne; numérique
SNRC14D/01; 14D/08; 14C/05; 14C/04
Lat/Long OENS -62.0833 -61.5000 56.3333 56.1667
Sujetsisotopes; rapports des isotopes de soufre; géochimie du nickel; sulfures; gîtes sulfureux; gîtes magmatiques; modèles sédimentaires; Suite Plutonique de Nain ; Gneiss de Tasiuyak
ProgrammeÉtude des gîtes magmatiques de Ni-Cu-EPG, Initiative géoscientifique ciblée (IGC-4)
Résumé(disponible en anglais seulement)
It is generally accepted that crustal contamination is required for the formation of significant magmatic Ni-Cu-PGE sulfide deposits. Either the addition of external S or SiO 2 promote early sulfide saturation. The most direct indicator of the S source is S isotopes. However, the traditional use of d 34 S values is inadequate in deposits where sedimentary sulfides of Archean age in the footwall might not have significantly different d 34 S values from those of mantle S. Even where sediments have variable d 34 S values, d 34 S signatures can be reset to magmatic values by equilibrating large amounts of silicate magma with initial sulfide melt. We used new multiple S isotope methods to constrain the relationship between d 34 S and d 33 S values, which is helpful to differentiate between high-temperature 57 equilibrium fractionations found in magmatic S, and low-temperature kinetic fractionations found in sedimentary S. The Voisey’s Bay Ni-sulfide deposit, Labrador is hosted by atroctolitic conduit system connecting two subchambers of the Voisey’s Bay intrusion. The Voisey’s Bay intrusion is a part of the Nain plutonic suite and intruded at approximately 1.3 Ga along the boundary between the Proterozoic Tasiuyak Gneiss of the Churchill province and Archean gneisses of the Nain province. Several models for the formation of this deposit have been presented, but there is still significant controversy over the cause of S saturation in the magma to form the deposit. The general model suggests that assimilation of an unknown silica-rich rock, likely in a mid-crustal magma chamber, was followed by assimilation of a large amount of sulfidic Tasiuyak gneiss, leading to sulfur saturation prior to emplacement. However, the Tasiuyak gneiss does not have a high concentration of sulfur (typically <<1 wt%), and traditional d 34 S analysis cannot distinguish between mantle and crustal sources in this deposit. High-temperature equilibrium relationships are not preserved in our measured d 33 S and d 34 S values. Instead they indicate that a kinetic process is responsible for S isotope fractionations in the mineralization, troctolite, and Tasiuyak gneiss. The observed slope of the data on a d 33 S vs. d 34 S plot is consistent with bacterial sulfate reduction, suggesting a marine sedimentary protolith to the Tasiuyak gneiss. This signature has apparently been inherited by the troctolite and the ineralization during assimilation of the Tasiuyak gneiss, despite the equilibration of the sulfide melt with a very large amount of silicate magma, resetting the d 34 S values in the deposit to magmatic values.