The Pederneira mine, located a few kilometers north of the famous Cruzeiro mine in the Sao José de Safira area of Minas Gerais, Brazil, produced an extraordinary quantity of gem tourmaline crystals from 1980 to 2006. In 2010, U.S.-based Fine Minerals International began a geologic prospecting and underground mining project to evaluate the deposit's remaining potential. The lead author has been involved in the geologic study of Pederneira and collected new data and representative samples of pegmatitic rocks, minerals in pockets, and tourmaline gem material. A complete gemological characterization of tourmaline from this locality was conducted in collaboration with the University of Milan and the National Council of Research in Milan. The Pederneira tourmaline deposit is characterized by two major pegmatitic bodies: a lower one miners call Dilo, and an upper one known as Dada. These two masses are roughly prismatic in shape and dip to the southwest at a steep angle. They are interconnected, and bounded at the top and bottom by subhorizontal barren pegmatitic dikes. The host rock is composed of biotite- and muscovite-bearing paragneiss, with quartz and plagioclase, and accessory garnet and tourmaline (dravite). Both pegmatites exceed 30 m in width. The Dilo pegmatite contains large enclaves of host rock. Large, spectacular gem-bearing pockets are distributed along the core zone of both the Dilo and Dada pegmatites. Pederneira's gem tourmaline is characterized by a wide variety of red-to-purple and green colors. Bicolored and "watermelon" crystals are common. Blue-green to grayish blue and blue colors are rarer, as is cat's-eye material ( red or green). The high-quality gem crystals in the pockets largely consist of second-generation "pencils" that grow into the cavity in the crystallographic direction of the analogous pole (mostly pedial terminations) or the antilogous pole (mostly pyramidal terminations). Microprobe analyses of the gem material revealed compositions in the elbaite field, with variable contents of fluorine and a high rossmanite component in some pink samples. Foitite was confined to dark green to black portions of the crystals. The only significant minor elements were iron and manganese. Present information on potential future production is uncertain and significant investments with the mining of new exploration tunnels are in progress.
The Pederneira Pegmatite, Minas Gerais, Brazil: Geology and Gem Tourmaline
Valeria Diella;
2011
Abstract
The Pederneira mine, located a few kilometers north of the famous Cruzeiro mine in the Sao José de Safira area of Minas Gerais, Brazil, produced an extraordinary quantity of gem tourmaline crystals from 1980 to 2006. In 2010, U.S.-based Fine Minerals International began a geologic prospecting and underground mining project to evaluate the deposit's remaining potential. The lead author has been involved in the geologic study of Pederneira and collected new data and representative samples of pegmatitic rocks, minerals in pockets, and tourmaline gem material. A complete gemological characterization of tourmaline from this locality was conducted in collaboration with the University of Milan and the National Council of Research in Milan. The Pederneira tourmaline deposit is characterized by two major pegmatitic bodies: a lower one miners call Dilo, and an upper one known as Dada. These two masses are roughly prismatic in shape and dip to the southwest at a steep angle. They are interconnected, and bounded at the top and bottom by subhorizontal barren pegmatitic dikes. The host rock is composed of biotite- and muscovite-bearing paragneiss, with quartz and plagioclase, and accessory garnet and tourmaline (dravite). Both pegmatites exceed 30 m in width. The Dilo pegmatite contains large enclaves of host rock. Large, spectacular gem-bearing pockets are distributed along the core zone of both the Dilo and Dada pegmatites. Pederneira's gem tourmaline is characterized by a wide variety of red-to-purple and green colors. Bicolored and "watermelon" crystals are common. Blue-green to grayish blue and blue colors are rarer, as is cat's-eye material ( red or green). The high-quality gem crystals in the pockets largely consist of second-generation "pencils" that grow into the cavity in the crystallographic direction of the analogous pole (mostly pedial terminations) or the antilogous pole (mostly pyramidal terminations). Microprobe analyses of the gem material revealed compositions in the elbaite field, with variable contents of fluorine and a high rossmanite component in some pink samples. Foitite was confined to dark green to black portions of the crystals. The only significant minor elements were iron and manganese. Present information on potential future production is uncertain and significant investments with the mining of new exploration tunnels are in progress.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
