Large-scale albitisation of siltstones on Qeqertakavsak island, northeast Disko Bugt, West Greenland

Abstract

Massive, light-coloured, albite-rich rocks (65–75% albite, 10–20% quartz and 10–15% dolomite) occur in zones up to 1.5 km wide within a sequence of dark siltstones on the island Qeqertakavsak in the Disko Bugt area of West Greenland. Field observations and geochemical data show that these rocks were formed by pervasive metasomatic alteration of the siltstones during which large amounts of Na, Ca and CO 2 were added to the rock, and other elements, particularly Fe, K, Rb, Ba, Ni and H, were removed. The sediments underwent an increase in volume of 5–6% during alteration, and net mass movements during alteration are estimated as addition of ∼ 100 kg Na 2O, ∼ 70 kg CaO and ∼ 90 kg of volatiles (mainly CO 2) per cubic metre of the original sediment, whereas ∼ 75 kg FeO tot and ∼ 80 kg K 2O per cubic metre were removed. Part of the alteration is strata-bound, and perhaps contemporaneous with diagenesis. Elsewhere it is related to fracture zones in the sediments and syn- or post-tectonic. It is thought that the alteration is caused by percolating brines, perhaps related to evaporites which may have been present in the sedimentary sequence, and that these brines were set into convection by the emplacement of basic intrusions.