Age and petrogenesis of Li pegmatites in southern Newfoundland

Abstract

Canada’s efforts to develop low-carbon technologies has increased the demand for critical minerals, including lithium. Southern Newfoundland is a region displaying favourable geological conditions to host lithium pegmatites, particularly in the aureoles of voluminous, geochemically evolved plutonic rocks. The present research is focussed on the Killick pegmatite field, a swarm of lithium-cesium-tantalum (LCT) pegmatites discovered in 2021 in southern Newfoundland. Fieldwork focussed on mapping and sampling multiple spodumene-bearing pegmatite dikes. The dykes intruded metasiltstone, meta-quartz arenite, and metavolcanic rocks of the Dolman Cove formation. Due to the homogeneity of the >10 dykes in the Killick swarm, three dykes form the focus of petrographic and mineral chemistry data collection (Kraken, East and Hockey Stick dykes). The dykes are of variable thickness with irregular widths ranging from <1 up to <5m. The textures in the Kraken dyke are mostly equigranular and dykes show no internal zoning, but the East and Hockey Stick dykes have irregular zoning patterns including layered aplite zones in the contact with the host rock and coarser-grained cores with spodumene up to 15cm in length. Most plagioclase grains are albite, which likely formed due to the alteration of K-feldspar. Muscovite is present as both primary and secondary muscovite. Spodumene is acicular in shape, pale green, up to 15 cm in length, and it formed during the late magmatic stage. In some cases, the spodumene is altered to clay minerals. The garnet grains are Mn-rich spessartine and is either magmatic or xenocrystic. The tourmalines are acicular, schorl in composition, and mainly present in the contact zones of the pegmatites with the host rocks. The columbite – tantalite minerals are mostly manganocolumbite to manganotantalite that are rich in HFSE. Apatite, monazite, and beryl are also present in minor proportions. The high Li and Cs content in the dykes and the presence of spodumene, spessartine, and columbite-tantalite group minerals indicate that the pegmatites are highly fractionated. Taken together, these new data provide a better understanding of the magmatic-hydrothermal processes that resulted in the emplacement of LCT pegmatites in southern Newfoundland. The pegmatites may be related by a remelting or anatexis of nearby, voluminous, geochemically evolved two-mica granites (Peter Snout and Rose Blanche plutons) or originated by the remelting of the metasedimentary host rocks