A METAMORPHOSED, POTASSIC ALTERATION ONE ASSOCIATED WITH THE BLEIKVASSLI ZN-PB-CU OREBODY, NORTHERN NORWAY

The Bleikvassli Zn-Pb-Cu deposit is located in amphibolite facies schi sts and gneisses in the Rodingsfjellet Nappe Complex, North Norwegian Caledonides. A microcline gneiss occurs as a lens-shaped body, up to 2 50 m thick, in the footwall and as small, irregular bodies in the hang ing wall. The microcline gneiss contains 40-75% microcline, and is cha racterized by a spotty texture. The footwall microcline gneiss is chem ically zoned, with highest K2O content (10-12 wt.%) and highest alkali -to-alumina ratio (0.76) ih the central part. The mica and plagioclase contents increase towards the margins of the gneiss, which grades lat erally into the surrounding kyanite-mica schist. The field relations a nd the chemical composition of the gneiss indicate that the potassium- rich protolith formed by addition of alkalies, SiO2 and heat to a peli tic sediment. The microcline gneiss has thus been interpreted as the c entral part of a hydrothermal alteration zone associated with the Blei kvassli orebody.