Formation and evolution of the Høgtuva beryllium deposit, Norway

Abstract

Metagranite in the Hogtuva tectonic window, Nordland, Norway, hosts several spatially restricted Be mineralisations with phenakite as the main ore mineral. Geochronology of magmatic zircon (zircon-I) indicates crystallisation of phenakite-bearing and beryl-free metaluminous granite at 1787 ± 57 Ma and of phenakite- and beryl (beryl-I)-bearing peraluminous aplites and pegmatites at 1710 ± 59 Ma. Crystallisation of texturally distinct zircon (zircon-II) cogenetic with fluorite in peraluminous aplites and pegmatites occurred at 434 ± 14 Ma during Caledonian metamorphism in Be-mineralised and barren metagranite. Breakdown of phenakite post-dates growth of zircon-II and resulted in the crystallisation of metamorphic hogtuvaite (Ca2 $${\text{Fe}}_{3}^{2 + } {\text{Fe}}_{3}^{3 + }$$ Si4BeAlO20) in metaluminous metagranite and metamorphic beryl (beryl-II) in peraluminous metamorphosed aplites and pegmatites. Qualitative and quantitative assessment of mineral reactions implies that Be has not been mobilised to significant degrees during metamorphism, in spite of the presence of a fluorine-rich fluid, and that phenakite underwent in situ metamorphic reactions. Hogtuvaite crystallised as a metamorphic phase at the expense of phenakite, the anorthite component in albitic plagioclase, magnetite and possibly clinopyroxene. In peraluminous environments, the breakdown of phenakite to metamorphic beryl-II is accompanied by the formation of albite. A decreasing activity ratio of a(K+/H+) in the metamorphic fluid results in the reaction of K-feldspar and phenakite to beryl-II. Further decrease in a(K+/H+) finally results in complete transformation of the remaining K-feldspar into albite. The metamorphic breakdown of phenakite in the distinct Be-mineralisation types at Hogtuva is probably controlled by changes in the activities of Na+, K+ and H+ in the coexisting metamorphic fluid.