This occurrence comprises three separate nepheline syenite bodies covering some 200 km2 which are overlain by upper Amazonian Tertiary sedimentary rocks. The more northerly body intrudes the Araracuara Formation. The syenites comprise microcline (49-61%), albite (7-14%), nepheline (23-34%), cancrinite, a little biotite and opaques with accessory sphene, zircon and carbonate; possible monazite has been observed. The carbonate is of a brownish colour, possibly siderite, and is confined mainly to fractures. Pegmatitic facies of the syenite occur locally. Chemical analyses indicate K2O>Na2O.