Mineralogy and age of the pollucite bearing hydra pegmatite in southern newfoundland, Canada

The Hydra pegmatite in southern Newfoundland is a recently discovered Cs-rich pegmatite, and it is the first documented occurrence of pollucite in the Canadian Appalachian Orogen. Hydra is a zoned pegmatite dyke with six well-defined zones that have sharp contacts with the host mylonitic metavolcanic rocks. The pegmatite zones include an upper border zone, intermediate zone 1, core zone, aplite zone, intermediate zone 2, and lower border zone. The dyke has silicate minerals (schorl, elbaite, polylithionite-trilithionite, muscovite-Li-phengite, siderophyllite, beryl, pollucite, garnet, zircon, quartz, albite and microcline), phosphate minerals (fluorapatite, monazite, and xenotime), and oxide minerals (columbite-(Mn), magnetite, and pyrochlore). Based on the minerals present, Hydra is classified as a lepidolite sub-type pegmatite of the LCT pegmatite family. The paragenetic sequence shows an evolution from a less evolved magma in the border zones, to an extremely geochemically evolved core zone. The fractionation is tracked by changes in the mineral assemblages. For example, siderophyllite is only present in border zones, whereas lepidolite is present only in intermediate and core zones. Pollucite is Pol 82 Anl 18 and shows a diffuse Na alteration on the rims and along fractures that was likely a product of minor analcimization. The presence of Cs in the form of pollucite indicates a high degree of magmatic fractionation. A new tourmaline 40 Ar/ 39 Ar age of 374 ± 4 Ma indicates that the pegmatite dykes were emplaced during the waning stages of orogenic events in the northern Appalachians.