A metamorphosed, potassic alteration zone associated with the Bleikvassli ZnPbCu orebody, Northern Norway

Abstract

The Bleikvassli ZnPbCu deposit is located in amphibolite facies schists and gneisses in the Rødingsfjellet Nappe Complex, North Norwegian Caledonides. A microcline gneiss occurs as a lens-shaped body, up to 250 m thick, in the footwall and as small, irregular bodies in the hanging wall. The microcline gneiss contains 40–75% microcline, and is characterized by a spotty texture. The footwall microcline gneiss is chemically zoned, with highest K 2O content (10–12 wt.%) and highest alkali-to-alumina ratio (0.76) in the central part. The mica and plagioclase contents increase towards the margins of the gneiss, which grades laterally into the surrounding kyanite-mica schist. The field relations and the chemical composition of the gneiss indicate that the potassium-rich protolith formed by addition of alkalies, SiO 2 and heat to a pelitic sediment. The microcline gneiss has thus been interpreted as the central part of a hydrothermal alteration zone associated with the Bleikvassli orebody.