Borborema

BORBOREMA (PT: Borborema; RU: Борборема) is the region with skarnous deposits of the ores of tungsten and rare-metallic pegmatites at the north-east of Brazil. Most of these deposits have been discovered and are being developed since the start of the 40-ies of the 20th century.

The region is formed of the metamorphosed depositions of the Brazilian shield of the Proterozoic age of the Ceará series (quartzites, silicified micaceous schists, greywackes, arkoses, conglomerates, biotite-garnet schists, lenses of limestones, marbles, amphibolite, and tactites). The rocks form anticlinal fold, and are breached by the intrusions of granites, aplites, diorites, monzonites, pegmatites, are broken by numerous tectonic disturbances (which are often filled with veinous quartz). The deposits of the ores of tungsten (approximately 300) stretch themselves as two stripes in the direction of the north-eastern strike, are associated with the fine-grained skarn-alike rocks, namely, tactites, which are formed of garnet, pyroxene, epidote, calcite, and quartz, and also of hornblende, vesuvianite, wollastonite, fluorite, and tourmaline. Ore bodies are of the lenticular, less often irregular shape; the length is several kilometers, the thickness is up to 2 metres. Average content of WO3 within the ore is 0.8%. The total reserves within 85 major deposits and ore manifestations are 9.2 million tonnes of ore. The largest deposits are Kishaba, Malhada-da-Angico, Malhada-Limpa, Breju, Bodo (Bodu), Cafuca, Bonito.

The development of these deposits is performed by the opencast (Kishaba, Bonitu) and underground methods. The Bodo, Kafuka, and Malhada-Limpa deposits have been unsealed using inclined adits (the depth is more than 40 metres), the Malhada-da-Angico deposit has been unsealed by vertical shafts. The ore is processed mostly at the mining enterprises of the "Mineração Tomaz Salustino" Brazilian firm. There is produced mainly 75% scheelite concentrate (2160 tonnes per year), the largest part of which is exported into the USA. Pegmatites are situated in the form of strip, which is stretched from the north to the south for 75 kilometres, with the width of approximately 50 kilometres. Pegmatitic veins (the total quantity is several thousands, the length is 200-500 metres, the thickness is 25-75 metres) have mainly north-eastern or northern strike and steep dip. Most open pit mines are concentrated to the south of the Pedra Lavrada city, along the border between the Paraíba and Rio Grande do Norte states. The largest of them are Cerradoziñho, Piedras Pretas, Serra Branca, Piaba, Tankiñhus, Alto Feio, and others. Pegmatites contain tantalite, beryl, spodumene, lithiophilite, cassiterite, quartz, feldspar, and so on. At the Cerradoziñho deposit, there also exist rock crystal, banded aquamarine, smoky quartz, and tourmaline. The content of tantalite within the ore is 0.025%, and of beryl is 0.1-0.2%. The extraction of the rare-metallic minerals from pegmatites is conducted manually by prospectors. The sites of prospectors have the shape of trenches (the depth is 10-15 metres), which are traced at the both sides of the central axial quartzous part of the vein. The ore mass is raised onto surface, where it is crushed and screened, the wastes are thrown away into dumps. The beneficiation is performed using the manual sorting of the ore to 50-65%. Tantalite is extracted mainly from eluvium near the outcrops of pegmatites onto the surface of the Earth. During the 1980, there has been extracted 130 tonnes of Ta-concentrate (in terms of Ta2O5). The marketable tantalite concentrate with the content of Ta2O5 of 53%, of Nb2O3 of 3%, and of SnO2 of 6%, is mostly exported.