The role of magmatic and hydrothermal processes in the formation of miarolitic gem beryl from the Luumäki pegmatite, SE Finland

Miarolitic pockets from the 1628 Ma Luumäki pegmatite in SE Finland contain gem-quality beryl (heliodor) within a matrix of reddish hematite, biotite, muscovite, chlorite, quartz, kaolinite, and bertrandite. Two different types of gem-beryl-bearing miarolitic pockets were observed, one in the intermediate zone of the pegmatite close to the contact with the quartz core, and the other at the border of the pegmatite body, at the contact with the host-rock rapakivi granite. Although similar in appearance, the pockets were formed by two distinct processes, as demonstrated by their structural position and the trace element chemistry of beryl. The stage-I gem beryls formed in miarolitic pockets in the intermediate zone, when the last batches of highly fractionated melt strongly enriched in incompatible and fluxing elements and volatiles crystallized. This is demonstrated by the trace element composition of gem beryls (Sc, Fe, Ga and Cs contents), which are aligned on a trend with magmatic common beryls, which had crystallized earlier in the inner intermediate zone. Subsequently, the later, stage-II gem beryls formed in miarolitic pockets at the border of the pegmatite from a hydrothermal fluid that escaped from the inner part of the pegmatite. This is demonstrated by the distinct trace element composition of these gem beryls, notably the high Li and Na concentrations, the abundance of fluid inclusions, and the textural association with a particular type of K-feldspar, which is chemically distinct from the magmatic fractionation trend connecting all other magmatic K-feldspars of the Luumäki pegmatite. The hydrothermal fluid caused local alteration of some of the magmatic common beryl crystals, resulting in changes in their trace element composition, as well as alteration of the host-rock rapakivi granites. The two distinct stages of gem-beryl formation in the Luumäki pegmatite, therefore, record the transition from a silicate melt-dominated to a fluid-dominated system.