Geological reconnaissance of some Uruguayan iron and manganese deposits in 1962

Abstract

The Valentines iron deposit, Florida, Uruguay This report is the result of a request by the Uruguayan Government for technical assistance in the field of geology by the U.S. Geological Survey under the auspices of the U.S. Agency for International Development and the Corps of Engineers, United States Army. Geological reconnaissance was made in the Departamentos (counties) of Florida, Tacuarembo, Rivera, and Cerro Largo from January 5-15, 1962. The Valentines iron deposit is located about 261 km north of Montevideo, Uruguay. Exploration of the deposit was started about three years ago by Ingeniero Juan H. Caorsi and Q.I. Don Juan C. Goni of the Instituto Geologico del Uruguay. The deposit contains an estimated 23,500,000 tons of hematite-martite-magnetite iron ore that averages about 40 percent Fe. The ore occurs in highly metamorphosed itabirite of Precambrian age that is surrounded by coarse-grained metasomatized granite. The itabirite formation is about 50 meters thick and has been encountered down dip at a depth of 240 meters below its surface exposure. A few granite and amphibolite sills, commonly less than 1 m wide, lie within the itabirite. The deposit has been separated into three distinct parts along strike by small right-lateral slip faults or normal faults; the northern Apretado body, the central Aurora body, and the southern Esabella body. The orebody lies on the western flank of a gently plunging NNW-trending anticline. Mining of the ore and its transportation by railroad to Montevideo would present no special difficulties, but shipping by small barges should be investigated. Geological reconnaissance in the Tacuarembo and Rivera Departamentos Three mineralized areas lie in an area near the town of Minas de Corrales in the Departamento de Rivera; they are the Cerro Amelia, the Cerro de Papagayo, and the Cerro Iman. The Cerro Amelia is composed of small bands of iron-rich rock separated by an amphibolitic or mafic rock. Selective mining would be necessary to extract the 31,000 tons per meter of depth of iron-rich rock that ranges from 15 to 40 percent metallic iron. The Cerro de Papagayo district contains many small, rich deposits of ferruginous manganese ore. The ratio of Mn to Fe varies widely within each small deposit as well as from deposit to deposit. Some ferruginous manganese ore contains 50-55 percent manganese dioxide. Although there are many thousands of tons of ore in the district, small-scale mining operations are imperative. One deposit, the Cerro Avestuz manganese mine, was visited. The manganese ore body lies within contorted highly metamorphosed itabirite that contains both hard low grade and soft high grade ferruginous manganese ores estimated to average 40 percent Mn. About 38,000 tons of manganese ore is present in this deposit. The Cerro Iman is a large block of itabirite that contains about 40 percent Fe. The grade is variable and probably runs from less than 35 percent Fe to more than 50 percent Fe. No exploration has been done on this deposit. It is recommended that the Cerro de Iman area be geologically mapped in detail, and that a geological reconnaissance be made of the area that is between the Cuchilla de Corrales and the Cuchilla de Areycua/Cuchilla del Cerro Pelado area.