Outcrops of nepheline syenite gneiss occur at three localities in the vicinity of Boca Nova, but airborne gamma-spectrometric surveys indicate the probable presence of an extensive body. Other areas to the north and southeast of Boca Nova, although with no outcrop, are also thought to conceal bodies of nepheline syenite (Lowell and Villas, 1983, Fig. 1_1). The nepheline syenite gneisses occur within folded pelitic metasediments of the Gurupi Group. The gneisses are banded with thin biotite-rich and thicker biotite-poor laminae and abundant concordant and discordant veins and lenticles which may grade into concordant pegmatites. The gneisses consist of albite-oligoclase, microcline, nepheline (30%), biotite and accessory carbonate, zircon, pyrochlore, sodalite, cancrinite and opaques. The pegmatites are mineralogically very similar except that biotite is less abundant. Lowell and Villas (1983) distinguish two generations of the dominant sialic phases, which together with the chemical data, are used to suggest a petrogenesis involving anatexis of previously metamorphosed igneous nepheline syenites.