Stable and radiogenic isotope constraints on the magmatic and hydrothermal evolution of the Nechalacho Layered Suite, northwest Canada

The agpaitic nepheline syenites of the Nechalacho Layered Suite represent the youngest intrusive phase of the Blatchford Lake Igneous Complex, Canada, and host hydrothermally altered eudialyte-rich layers that constitute one of the world's largest rare earth element (REE) and niobium deposits. Three distinct peripheral rare metal pegmatites, the T-, R- and S-Zones, contain high-grade Li, Be, REE and Nb deposits. Based on stable and radiogenic isotope data, we constrain the age of the Nechalacho Layered Suite, identify its magmatic source and interpret the hydrothermal alteration that modified the ore and gangue mineralogy. In situ Laser ICP-MS U-Pb dating of igneous zircon yielded a concordia age of 2176.0 +/- 2.7 Ma for the Nechalacho Layered Suite. Initial epsilon Nd-2176 Ma between 1.1 and 3.0 and averaging 2.0 indicates a derivation from mantle magma that was isotopically similar to modern ocean island basalt. The delta O-18(melt) values calculated for magmatic aegirine, annite and zircon at 700 degrees C range from 5.1 to 7.8 parts per thousand; delta D-melt is -60%. For the earlier alkaline granite and quartz syenites of the Blatchford lake Igneous Complex (Grace Lake Granite, Thor Lake Syenite, Rim Syenite), delta O-18(melt) calculated for fayalite, riebeckite, richterite and quartz ranges from 6.7 to 8.1%. and delta D-melt from -99 to -93 parts per thousand. Together with the Nd isotope ratios, this indicates a common, deep-seated feeder system of mantle magma that evolved rapidly to generate granites, quartz syenites, and finally nepheline syenites, with little or no crustal contamination. The Nechalacho Layered Suite exsolved a late -magmatic aqueous-carbonic fluid that caused auto-metasomatism associated with ankerite-bastnasite-(Ce) mineralization, producing hydrothermal carbonates with delta C-13 from -6.3 to 2.7%. and delta O-18 from 11.3 to 15.0 parts per thousand. Similar ratios previously reported for ankerite from the T-Zone demonstrate a genetic link to the pegmatites at the magmatic-hydrothermal interface. Hydrothermal monazite-(Ce) from the Basal Zone of the Nechalacho deposit yielded a U-Pb concordia age of 1871 +/- 14 Ma and Pb-208/Th-232 ages of 1852 14, 1835 57 and 1854 27 Ma. This indicates monazite formation during a late regional hydrothermal event associated with faulting and emplacement of the Compton Intrusive Suite along the southern margin of the Slave Craton. The temperature determined for this late overprint using oxygen isotopes of quartz and magnetite is similar to 300 degrees C and the fluids had delta(OH2O)-O-18 of 3.1-93%. and delta D of -24%., indicating the presence of a regionally pervasive amagmatic crustal fluid. (C) 2016 Elsevier B.V. All rights reserved.