Compositional evolution of tourmaline in the petalite-subtype Nyköpingsgruvan pegmatites, Utö, Stockholm Archipelago, Sweden

Abstract

The classic petalite-subtype Nyköpingsgruvan pegmatites are located on the northern part of Utö Island, Stockholm archipelago, Sweden. They consist of two genetically related pegmatite bodies, the southern (minor) and northern (major), transecting the Nyköpingsgruvan iron formation. The pegmatite zones are named according to their most characteristic mineral: (1) spodumene, (2) pink K-feldspar + albite intergrowth, (3) lepidolite, (4) coarse sacchardoidal albite (>1 mm), (5) petalite, and (6) fine saccharoidal albite (<1–2 mm). The internal tourmaline composition evolves through the following crystallization sequence: Al-rich schorl → phases intermediate between schorl and elbaite → “fluor-elbaite” with variable Fe contents → elbaite → phases intermediate between elbaite and rossmanite → Ca-bearing elbaite-rossmanite → Ca-bearing elbaite. The dominant substitutions are Na ⇄ □ (□ = vacancy) with minor Ca variation at the X site, and 2Fe2+ ⇄ Al + Li at the Y site. The negative correlation between Fe and (Al + Li) and between Fe and Mn in tourmaline is due to fractionation of the pegmatite melt. The negative correlation between □ at the X site and F at the O(1) site is caused by crystal-chemical constraints and controlled by f(F2). The presence of Ca-bearing elbaite-rossmanite, Ca-bearing elbaite, apatite and microlite in the fractionated pegmatite zones indicates that late-stage Ca-enrichment is probably due to conservation of Ca during consolidation of the pegmatite by Ca-F complexes in the melt. Most tourmaline throughout the pegmatites is veined by cookeite, indicating decreasing salinity and low F-activity in the late low-temperature hydrothermal fluids. In the BIF, the exocontact tourmaline is dominantly schorl-dravite with variable Ca contents and in the aplitic veinlets within the iron formation, the exocontact tourmaline is dominantly (Ca, Mg)-bearing schorl, whereas “fluor-elbaite”-schorl dominates mica schist along the contacts with the pegmatites. In the contaminated, dolomitic-marble-hosted Grundberg outcrop, the endomorphic tourmaline is Li-rich (mainly liddicoatite and elbaite) with (Mg, Fe)-rich fracture-infillings (mainly dravite and schorl).