Firesand River is an approximately circular carbonatite complex some 2.4 km in diameter giving a strong aeromagnetic anomaly. It is intruded into Precambrian granites and syenites (Algoman) and greenstones (Keewatin) which are variably fenitized with the development of green pyroxene and biotite. The core of the complex is a dolomitic carbonatite which forms a hill. It is ferruginous and contains some quartz and baryte. It is referred to as rauhaugite by Heinrich and Vian (1967, p. 252) who describe a baryte-smoky quartz-carbonate rock within it. Surrounding this is sovite which is strongly foliated concentrically with steep inward dips. There are possibly several sovitic cone sheets cutting the greenstones. The sovite is a coarse rock of calcite with variable biotite, apatite, magnetite, pyrrhotine, aegirine, pyrochlore and locally actinolite. Mixed carbonate-silicate rocks occur as screens or possibly dykes in the southern part of the sovite and cutting the greenstones to the northwest. They comprise carbonate and varying proportions of biotite, pyroxene, nepheline, Ti-garnet, apatite and a range of other unidentified minerals. Lamprophyric dyke rocks of olivine, biotite, augite and accessories occur in the vicinity of the intrusion. There are a few radiating carbonatite dykes and veins.