The Zealand Station beryl (aquamarine) deposit, West-Central NB: mineralogic, geochronologic, and petrogenetic constraints
University of New Brunswick
The Zealand Station deposit is located 30 km northwest of Fredericton, NB, along the north eastern cusp of the Hawkshaw Granite, previously dated at 411 ± 1 Ma by U-Pb titanite. A late-stage, SE-trending, sparsely porphyritic aplite dyke with coarse-grained perthitic feldspar has locally abundant beryl (aquamarine) in the aplitic groundmass. A pegmatitic phase of the dyke crops out 50 m east of the map area; it predominately consists of quartz and K-feldspar, with granophyric texture, and minor wolframite. Beryl is also found near altered greisen pockets and along quartz-rich veins in the granitic host rock; the veins show two predominant orientations: 135°/90° and 010°/75°W. The aplitic dyke has been dated at 400.5 ± 1.2 Ma using U-Pb TIMS on magmatic zircon. This isconsistent with the 404 ± 8 Ma using the U-Th-Pb CHIME method on magmatic monazite using the EPMA technique: these ages link the aplitic dyke to the Allandale Granite that was previously dated at 402 ± 1 Ma by U-Pb (monazite); it is the youngest and most evolved phase of the Devonian Pokiok Batholith. In addition, two monazite analyses yielded an age of 510 ± 20 Ma using EPMA, indicating a supracrustal xenocrystic origin for some of the monazite. The veins and greisen pockets have up to 5.61 wt. % BeO, and up to 20 vol. % beryl in a part of the aplitic phase of the dyke. Beryl samples (n=15) were analysed by EPMA. The average H20 content of the Zealand Station beryl is 1.53 wt. % calculated by (Na20 + 1.4829)/1 .1771 and includes ferric iron (up to 1.44 wt. %), which is the dominant chromophore. The dyke samples have a granitic composition and are predominantly magnesian, slightly potassic, and calc-alkaline with strong peraluminousity (1 .23-4.76). The apliteII pegmatite dykes are highly fractionated, based on their characteristic high incompatible elemental abundances, such as Th (1 l-33ppm), Rb (218-327 ppm), Cs (6-22 ppm), and Ta (3-14 ppm) and elevated HREE (Y, Yb, and Lu); they also have very low Nb/Ta (2.6-7.1) and very high Rb/Sr (4.1-15.8) ratios. The dykes were derived from a probably igneous in origin and had characteristics typical of an A-type source, the age and geological setting of this granite indicate that it is syn- to post- collisional with the Acadian Orogen. As this magma rises there seems to have been increased contamination by metasediments of the recently thickened crust, possibly explaining some of the S-type petrogeochemical characteristics. The dykes are related to the Allandale Granite phase of the Pokiok Batholith by their age, petrology, geochemistry and Nd/Sm isotope ratios (ε-Nd of -2.15 for the dykes whereas the Allandale has a value of -1.6). The pegmatite and aplite are a high-level rare-earth element pegmatite with both NYF and LCT-type characteristics. The beryl geothermobarometer indicated that the dyke crystallized between 600° to 660°C and atapproximately 2.5 kbar.