Lithium-caesium-tantalum pegmatites in the Varuträsk area, Sweden: geological setting, timing, and source-transport-trap characteristics

This study investigates lithium-caesium-tantalum (LCT) pegmatites and associated pegmatites and granites in the Varuträsk area, northern Sweden, using a mineral systems approach (i.e. source-transport-trap). The research incorporates geological mapping both regionally and underground, analysis of geological structures and host rock competency, 3D modelling, lithogeochemistry, and zircon-monazite U–Pb SIMS geochronology to assess the genesis and controls on the Varuträsk LCT pegmatite system. The findings reveal four phases of intrusive magmatism within a supracrustal package predominantly composed of metagreywacke, metabasalt (amphibolite), and black shale, subjected to two fabric-forming deformation phases and at least two folding phases, pre- and syn-plutonism in association with the Svecokarelian orogeny. Pegmatites are controlled by brittle to brittle-plastic structures that intersect the tectonic fabric of the host rock, with host rock competency, characterized by uniaxial compressive strength, being a key factor determining the intrusion angle of the pegmatite. The earliest magmatic phase in the area is represented by a granodiorite pluton with a zircon ²⁰⁷Pb/²⁰⁶Pb age of 1885 ± 3.2 Ma, linked to the main Svecokarelian orogenic cycle. A granodiorite-tonalite intrusion related to the Transscandinavian Igneous Belt has a zircon ²⁰⁷Pb/²⁰⁶Pb age of 1801 ± 1.7 Ma. Three peraluminous S-type granites (plutonic Skellefte suite) yielded zircon ²⁰⁷Pb/²⁰⁶Pb ages of 1795 ± 1.7 Ma and 1792 ± 1.6 Ma, and a monazite ²⁰⁷Pb/²⁰⁶Pb age of 1798 ± 4.6 Ma. Small and irregular to elongated bodies of hypabyssal pegmatitic leucogranite with peraluminous characteristics (hypabyssal Skellefte suite), although not dated in this study, form a geochemical continuum with the Varuträsk LCT pegmatite and less evolved regional pegmatites, and are chemically distinct from the larger S-type plutons. A ‘simple’ muscovite pegmatite dyke, representative of the less evolved regional pegmatites in the area, produced a monazite U–Pb crystallisation age of 1780 ± 6.9 Ma. These results suggest that the hypabyssal pegmatitic leucogranites, regional less evolved pegmatites, and the Varuträsk LCT pegmatite represent the final stage (c. 1.78 Ga) of crustal maturation in the orogenic cycle, marginally postdating the regional migmatization and S-type granite plutonism event. However, further research is encouraged in this study to validate the time gap between plutonic and pegmatitic granites in the Varuträsk area. We propose a mineral system model encompassing a c. 1.80 – 1.78 Ga timeframe that involved initial anatexis of metasedimentary rocks to form S-type granite plutons during the late Svecokarelian orogeny. Subsequently, a resolvable younger magmatic event generated granitic pegmatite melts that utilized pre-existing structures as transport pathways. These melts were trapped in structurally favourable, brittle to brittle-plastic sites and the host rock competency influenced the emplacement angle of the pegmatites.