The Zealand Station beryl (aquamarine) deposit, West-Central NB: mineralogic, geochronologic, and petrogenetic constraints

Abstract

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.