REE and other minerals in the West Moose River Pluton and associated rocks, Cobequid Highlands, Nova Scotia, and their association with fracturing

Abstract

The West Moose River pluton consists principally of an A-type granite emplaced immediately north of the Cobequid fault into Avalon terrane rocks in the latest Devonian-Early Carboniferous. Continued strike-slip fault movement in the shear zone caused the formation of pervasive fractures within the pluton, which have been shown to host REE-bearing minerals. The purpose of this study was to gain a better understanding of the source, timing, and distribution of REE-bearing minerals and their relation to regional events. Samples were collected from late mafic dykes, surrounding country rock, and the granite itself. Primary and secondary mineralogy were investigated using petrographic microscope, scanning electron microscope, and whole rock analysis. In total, 7 REE-bearing hydrothermal minerals were identified, with both the LREE- and HREE- type found in each lithology. The A-type granite seems the most likely source of hydrothermal REEs, and magmatic zircon for HREEs in particular; HREE-bearing zircon was the only primary REE-mineral identified, and occurred exclusively in the granite. Due to paragenetic association with carbonates, Fe-oxyhydroxides, and chalcopyrite, LREE remobilization likely occurred during the 323 Ma iron -oxide-copper-gold mineralization event. Hydrothermal HREE-minerals are either coeval with, or predate this event, and often contain Nb and Ti. LREE-minerals are most concentrated in late minette dykes, which show a positive correlation between REE concentrations and the amount of fracturing. The halogen rich content of minettes may provide favourable conditions for the precipitation of LREEs, and should be considered as future exploration targets and areas of research.