49 Airbome radiometric survey between 63° and 66°N, southem West Greenland Karsten Secher The airborne radiometric survey in West Greenland, initiated in 1975 (Secher, 1976), was completed in 1976 with the comp1etion of the region between Søndre Strømfjord and Frederikshåbs Isblink - an area of 42 000 km 2 • This made a total of 100 000 km 2 thus investigated in West Greenland (Steenfelt et ai., 1977). The programme was carried out as a collaboratioil between GGU and the Research Establishment, Risø. The aircraft, a Britten-Norman Islander, was chartered through Greeillandair Charter AIS. Field opera- tions were carried out from the GGU field station at Søndre Strømfjord Air Base. Equipment and methods used during the radiometric survey were as in 1975 (Secher, 1976). Follow-up field work on the ground will take place during the next few years. Remarks on the radioactivity The survey area is situated within the pre-Nagssugtoqidian block where very little was previously known about radioactivity. The present investigation hasshown some general trends concerning the radiation level ofthe rock units and the detected anomalies, which are briefly presented below. The rock units follow the divisions described by Escher & Watt (1976). The Archaean gneisses are mainly of enderbitic and granodioritic composition. Both types have a low radiation level but with our present knowledge it is impossible to distinguish between them radiometrically. The anorthosites within the gneiss complex also have a very low radiation level and are not possibie to distinguish from the surrounding rocks. Granites display variable, mostly higher, radiation leveis, distinguishing them from the gneisses. Isolated radioactive highs may be due to pegmatites. Known occurrences of allanite cause locally increased radiation, as around Fiskenæsset and Sukkertoppen. Supracrustal rocks within the survey area, e.g. Isukasia, Ivisfutoq and Ravns Storø, show low radiation levels and therefore cannot be distinguished from the gneisses. The suprac- rustal rocks of the Godthåbsfjord area are situated, together with granite units, in a NE-SW trending zone, 30 km wide (from Nunatarssuaq to the mouth ofBuksefjord), in which there is a concentration oflocalities with radiation 2 to 3 times higher than the rocks to the north and south. Some anomalies detected within this zone are obviously related to pegmatites. The Phanerozoic carbonatite complex at Qaqarssuk, south-east of Sukkertoppen (explored by Kryolitselskabet Øresund AIS), is marked by several anomalies, mainly along the western border of the complex. References Escher, A. & Watt, W. S. 1976: Summary of the geology of Greenland. In Escher, A. & Watt, S. W. (edit.) Geology o/ Greenland, 10-16. Copenhagen: Geol. Surv. Greenland. 4 Rapport Dr. 85