Origin and Economic Potential of Lithium-mineralized Granitic Pegmatites in Helgeland, North Norway

Abstract

A comprehensive set of analytical techniques, encompassing geological, petrographic, mineralogical, and zircon U-Pb-Hf isotopic compositions, was employed to investigate a relatively poorly known pegmatite field located in Helgeland, North Norway. The Paleoproterozoic basement gneisses in the area have U-Pb zircon crystallization ages between 1870 ± 12 and 1811 ± 6 Ma, meaning that they belong to the Transscandinavian Igneous Belt (TIB) of the Fennoscandian Shield. The Paleoproterozoic basement in the study area has been overprinted by metamorphism during late stages of the Caledonian orogenic event. The U-Pb zircon age determinations for granitic pegmatite magma emplacement define two discrete pegmatite generations during the late stage of the Caledonian collisional event. An older generation of pegmatites, with ages between 422 ± 5 and 412 ± 5 Ma, that formed by partial melting related to collision and thrusting during the Caledonian orogenic event. The younger generation of pegmatites has ages between 403 ± 6 and 387 ± 6 Ma, and these magmas formed during decompression in the latest stages of the Caledonian orogeny. Using fractionation indicators such as K/Rb and Nb/Ta in micas, the study identified a correlation between the U-Pb ages of the Caledonian pegmatites and their degree of magmatic differentiation; the younger pegmatites are more evolved. The Helgeland pegmatites form a cluster of highly evolved and more primitive granitic magmas with some minor rare metal enrichment such as Li-Cs-Ta in the younger and more evolved pegmatites. Hafnium isotopic data for zircons reveal slightly subchondritic to firmly juvenile εHfi values between -3.3 and +5 for the Paleoproterozoic gneisses, whereas the Caledonian pegmatites show a spread between -32 and -8. For the pegmatites, significant within-sample variations of up to 24 epsilon-units are recorded, which indicates magma derivation from an isotopically heterogeneous crustal source region that may have contained metasedimentary rock packages. The zircon Hf isotopic compositions reveal that the pegmatites formed by partial melting of the Paleoproterozoic basement during the latest stages of the Caledonian orogeny. The study demonstrates that partial melting of a ca. 1.8 Ga old metasedimentary rock from the Neverdalen locality can yield similar Hf isotopic compositions to the ca. 0.4 Ga old Helgeland pegmatites after its isotopic evolution for more than one billion years.