The genesis of thorium-rich monazite placer deposits in Sri Lanka

Abstract

The monazite placers of Sri Lanka are among the world's most thorium rich sediments. This study of the stream sediments and rocks from an area in the southwest of Sri Lanka shows that the highly metamorphosed aluminous schists and gneisses and also granitoid rocks of the Highland and Southwest Groups ofthe Precambrian of Sri Lankaare the ~robable source rocks for the thorium-rich monazite. Themanmatic fluids known to have-pervaded the aluminous sediments during the intense folding and metamorphism under granulite facies conditions are considered to have been thorium rich. The P-T conditions of-metamorphism proved to be ideal for the formation of a variety of gem minerals including gem monazite and other associated heavy minerals.