URANIFEROUS NEPHELINE SYENITES AND RELATED ROCKS IN THE ILIMAUSSAQ AREA, JULIANEHAAB DISTRICT, SOUTH WEST GREENLAND

Concentrations of radioactive minerals are found in the eudialyte- bearing nepheline syenites of the Ilimaussaq complex in South West Greenland. The radioactive minerals may occur as accessories in the melanocratic nepheline syenite, called lujavrite. The lujavrite is considered to be the youngest rock of the complex, in which it occupies a considerable volume. The U content averages about 150 ppm. In restricted areas concentrations of 500 to 2000 ppm have been found. The Th content is 2 to r times that of U. Radioactive minerals may also occur as local concentrations in inclusions of older, recrystallized nepheline syenite in the lujavrite and as concentrations in recrystullized zones of deformation in some of the oldest nepheline syenites of the complex. Only the lujavrite may be considered as possible low-grade U and Th sources. The other occurrences are mainly of genetical interest. The most important radioactive mineral is steenstrupine, with about 0.3% U and 2.7% Th. Steenstrupine may alter to various poorly defined radioactive alteration products. Monazite, britholite, microlite, and thorianite have been found in small amounts. The eudialyte of the nepheline syenites is faintly radioacti ve. Steenstrupine is found in rocks, in which the eudialyte has been altered hydrothermally. (auth)