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USGS1953

Iron deposits of the congonhas district, minas Gerais, Brazil

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.

Economic Geology↗
USGS1951

Notes on the manganese ores of Brazil

Brazil has been a small but consistent source of manganese ore for world markets during the last five decades. Production both for export and for domestic use is likely to increase. Deposits can be classified into syngenetic (the primary manganese sediments in iron formation at Urucum) and epigenetic. The epigenetic deposits can be further subdivided into four classes: (a) The Queluz or Morro da Mina type, in which the manganese oxide ore has been derived by weathering processes from a manganese silicate-carbonate-sulfide protore. (b) The Lagoa Grande type, enclosed in itabirite, in which lean man-ganiferous horizons in the metamorphosed iron formation have been leached and the manganese concentrated by weathering processes. (c) Deposits in slightly manganiferous rocks of various types in which the manganese has been concentrated by weathering processes. (d) Deposits in barren rocks into which manganese oxides have been introduced from unknown sources. It is probable that the Lagoa Grande type is closely related in ultimate origin to the Urucum type, although the regional metamorphism suffered by the former obscures relations. The Urucum deposits and two deposits of the Morro da Mina type have large reserves of export-grade ore. Many deposits of the Lagoa Grande type are known and the aggregate reserves are large, although their grade makes them more suitable for domestic than for export use. Deposits of the other two types are numerous but small, and their grade seems to consign most ore from these deposits to the domestic market. © 1951 Society of Economic Geologists, Inc.

Economic Geology↗
USGS1946

Quartz crystal in Brazil

The peak production of Brazilian quartz crystal was reached in 1943 when 2411 metric tons were exported. Minas Gerais, Goiaz, and Bahia are the principal producing states. Minor quantities are obtained from Espirito Santo and Northeastern Brazil. Primary veins and pegmatites yield secondary deposits of eluvial, colluvial, and alluvial origin. The veins range in form from tabular or lenticular bodies, pipes, and pockets to composite lodes and bedded masses. The mineralogy is simple; accessory minerals are few and rare. A small proportion of clear crystals, occurring in vein masses of milky quartz, furnishes the commercial product. Filling, rather than replacement, is believed to be the dominant emplacement process. Pegmatites yield a little commercial quartz. Both residual (eluvial) and water-laid (alluvial) quartz deposits are economically important. Crystals range up to 40 metric tons in weight; those of 1 to 5 tons occur in many deposits. Quartz from every locality studied is twinned on both the Brazil and Dauphiné laws. Phantoms are abundant. Field studies reveal little difference between the final quartz from some pegmatites and that from veins. It is inferred that the veins may grade into pegmatites at depth and that both are products of the same mineralogenic epoch. Present evidence does not justify a closer dating for the formation of the veins and pegmatites than the early Paleozoic. There are few large quartz mines in Brazil. The impressive quartz production during the war years came from many small, scattered workings. Mining methods are primitive, and mechanization was feasible in relatively few areas. The ratio of mine-clean crystal to the total quantity of excavated material varies between 1:1000 and 1:100,000, with an over-all average for the best mines of the order of 1:10,000. Cost of mining ranges from $2 to $10 a kilogram. The average export value during 1944 was $12.50 per kilogram. The potential reserves of quartz crystal in Brazil are large, but actual or visual reserves in known districts are small. Future production capacity will be determined by economic factors.

GSA Bulletin↗