Minerals of the Brumado magnesite deposits, Serra das Eguas, Bahia, Brazil

The magnesite occurrence at Brumado in the Serra das Eguas district is one of the largest deposits of magnesite in the world and is host to a surprisingly species-rich mineral assemblage. The mineralized areas at Brumado are located in a syncline composed of a lower unit of dolomite marble and an upper unit of quartzite. These rocks are underlain by Precambrian gneisses, schists and amphibolites. The magnesite and associated minerals appear to be related to the intrusion of nearby igneous dikes. The igneous activity produced hydrothermal fluids that interacted with the preexisting dolomite to form magnesite. Fracture fillings of coarsegrained magnesite, quartz, alumino-silicates and other late-stage minerals also appear to be related to the same episode of hydrothermal mineralization (Bodenlos, 1954; Cassedanne and Cassedanne, 1978).

In addition to superb crystals of magnesite, dolomite and quartz, many well-crystallized accessory minerals occur at Brumado. Uvite and dravite are found as sharp crystals up to 5 cm in length with colors that range from black to green to red. Topaz occurs as fine yellow, orange, pink or purple crystals up to several cm in length. Sellaite, considered to be the world's finest, is found as crystals that exceed 5 cm in size. Hematite has been found in large, spectacular crystals in excess of 10 cm. The finest zeunerite/ metazeunerite are found in crystals up to 3.5 cm on an edge. Associated with these are druses of sklodowskite that equal any previously known examples. Beryl (varieties aquamarine and emerald) has been found in crystals to several centimeters in length and was one of the minerals mined from the area before the discovery of the magnesite lode. Other minerals encountered at Brumado include: tale, goyazite, svanbergite, chemotive-(Y), wakefieldite-(Y), florencite-(Ce), anhydrite, celestite, barite and numerous other species. Brumado certainly ranks among one of the most fascinating mineral-producing areas in the world and continues to supply collectors with superb specimens.

References

BODENLOS, A. J. (1954) Magnesite deposits in the Serra das Eguas, Brumado, Bahia, Brazil. U.S. Geological Survey Bulletin 975-C, 167 p.

CASSEDANNE, J. P., and CASSEDANNE, J. 0. (1978) Famous mineral localities: The Brumado district, Bahia, Brazil. Mineralogical Record, 9, 196-205.

Alexander U. Falster, William B. Simmons, Karen L. Webber, and James W. Nizamoff

Department of Geology and Geophysics

University of New Orleans

New Orleans, Louisiana 70148

Carlos P. Barbosa

Rua Cel. Roberto Soares Ferreira, 586

CEP 35030, Governador Valadares, Minas Gerais, Brazil

Richard V. Gaines*

*Deceased

Copyright Mineralogical Record Mar/Apr 2000
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