Mineralogical, geochronological, and geochemical characterization of Early Devonian aquamarine-bearing dykes of the Zealand Station beryl and molybdenite deposit, west central New Brunswick

Abstract

The Zealand Station beryl (aquamarine) and molybdenite deposit is located 25 km northwest of Fredericton, New Brunswick, along the northeastern cusp of the Hawkshaw Granite, previously dated at 411 ± 1 Ma (U–Pb titanite), of the multiphase Devonian Pokiok Batholith. A late-stage, southeast-trending, pegmatite–aplite dyke has abundant aquamarine associated with pegmatitic sections. An exposure of a pegmatitic dyke is predominantly quartz and K-feldspar that exhibits a border, intermediate, and core zone. The main pegmatite–aplite dyke has been dated at 400.5 ± 1.2 Ma using U–Pb thermal ionization mass spectrometry on magmatic zircon. This is consistent with the 404 ± 8 Ma age using the chemical U–Th – total Pb isochron method from the pegmatitic beryl-rich section. These ages link these pegmatitic to aplitic dykes to the Allandale Granite, which is the youngest (402 ± 1 Ma by U–Pb on monazite) and most evolved phase of the Pokiok Batholith. The granitic aplite and pegmatite dyke samples are predominantly magnesian with one pegmatite sample being ferroan (FeOt/(FeOt +MgO) = 0.64–0.94); the samples are slightly potassic and calc-alkaline with strong peraluminosity (A/CNK = 1.23–4.76). The various phases of dykes were derived from magma with crustal A-type source characteristics similar to the Allandale Granite. The Sm–Nd isotope values for the aplite dyke (εNd(400) = –2.15) and the Allandale Granite (εNd(400) = –1.6) reflect some assimilation of metasediments relative to other phases of the Pokiok Batholith. The pegmatite and aplite dykes are high-level, rare-earth element pegmatite phases (Nb–Y–F-type) with some Li–Cs–Ta-type characteristics.