Various origins have been proposed for the itabirite and associated hematite ores of Minas Gerais, Brazil. The U.S. Geological Survey, in cooperation with the Brazilian Departamento Nacional da Produqao Mineral, has undertaken a comprehensive program of mapping of these Precambrian deposits, which bear many similarities to other Precambrian iron formations. Itabirite is a finely laminated quartz-iron oxide rock with more or less dolomite, corresponding to James's oxide facies. It occurs principally in the middle group of the Minas series-a eugeosynclinal assemblage of quartzite, schist, chemical precipitates, graywacke, and volcanic rocks. Iron and silica were precipitated rhythmically in a shallow restricted basin under somewhat acid conditions that inhibited the precipitation of carbonates. Erosion of moderately deformed rocks of the Minas series furnished sediments for the overlying conglomeratic Itacolumi series. Severe post-Itacolumi deformation folded and thrust-faulted rocks of both series; the accompanying regional metamorphism recrystallized the chert and iron oxide to quartz, specular hematite, and minor amounts of magnetite. Ultramafic intrusions antedate this diastrophism; granodiorites and various basic dike rocks are younger. Hydrothermal replacement of breccia zones in itabirite and of dolomitic beds associated with the iron formation produced high-grade specular hematite ore nearly free of impurities. The areal distribution of the deposits indicates that the solutions followed fault zones. Heated meteoric water may have been responsible for the replacement, as minerals of obvious magmatic origin are absent. Since early Tertiary time the area has been uplifted several thousand feet. Leaching and cementation related to the present surface have modified both the iron formation and the hematite deposits, giving rise to various types of commercial ore. © 1953 Society of Economic Geologists, Inc.