Geochronology of the Voisey's Bay intrusion, Labrador, Canada, by precise U-Pb dating of coexisting baddeleyite, zircon, and apatite

The emplacement history of the Voisey's Bay troctolite intrusion, that host s the major Ni-Cu-Co sulfide deposit of the same name, has been studied usi ng precise U-Pb geochronology of baddeleyite, zircon and apatite. The badde leyite U-Pb ages of multiple drill core samples of troctolite and gabbro in dicate that all of the mafic rocks studied from different components of the Voisey's Bay intrusion: Eastern Deeps, Discovery Hill Zone and Reid Brook Zone, and from the adjacent Red Dog area, were emplaced at 1332.7 +/- 1.0 M a. On the basis of combined geological and geochronological evidence, it is suggested that the Voisey's Bay Ni-Cu-Co deposit was formed during the sam e period. The zircons coexisting with the ca. 1333 Ma baddeleyite show a di versity of ages. The zircons from normal troctolite and some of the olivine gabbro samples are coeval with the baddeleyite, while zircon from the vari ed textured troctolite and feeder olivine gabbro are much younger at 1305.0 +/- 0.8 Ma. The identical ages of the younger zircon population and the Vo isey's Bay syenite that cuts the mafic rocks indicate a link between zircon growth in the mafic rocks and contact metamorphism, related to the emplace ment of the syenite, Various mechanisms of zircon growth were probably invo lved, including reaction of the 1333 Ma baddeleyite with a silica-enriched fluid with formation of a secondary polycrystalline zircon, and zircon crys tallization from syenite micro-veins in the mafic rocks. The mean Pb-207/Pb -206 age of 1303.5 +/- 2.6 Ma of the Voisey's Bay apatites is similar to th e age of the younger zircon population. The apatite age may either be a res ult of resetting the U-Pb system in response to the syenite intrusion, or m ay reflect the closure of the system during regional cooling and cessation of fluid circulation. The presence of xenocrystic zircon in a Discovery Hil l Zone feeder olivine gabbro indicates that the Voisey's Bay magmas were co ntaminated with 1.90 Ga crustal rocks. (C) 1999 Elsevier Science B.V. All r ights reserved.