Sandra de Brito Barreto

The role of magmatic and hydrothermal processes in the evolution of Be-bearing pegmatites: Evidence from beryl and its breakdown products

Abstract Beryl and euclase crystals from the Mina do Santino and the Jacú pegmatites in the Borborema Pegmatite Province in northeastern Brazil contain several generations of melt and fluid inclusions, which allow interpretation of P-T-X conditions responsible for beryl crystallization and for alteration of a primary pegmatitic mineral assemblage to a mixture of hydrothermal minerals (euclase, bertrandite, kaolinite, and quartz). Primary melt and fluid inclusions hosted by beryl were trapped simultaneously. However, their homogenization temperatures are significantly higher (870-900 °C) than the values previously reported for pegmatitic systems (<712 °C) and should be treated with caution. An isobaric drop of temperature resulted in the exsolution of a fluid. A low-salinity CO2-enriched phase and a saline water-rich phase were trapped in pseudosecondary inclusions in beryl at a pressure of 2.1-2.7 kbar and temperature of 390-480 °C. Cooling of the country rocks below 400 °C caused a ductile-to-brittle transition and allowed infiltration of cold groundwater, which further decreased the temperature in the system to 190-240 °C. At the same time, the pressure dropped from a lithostatic (2.1-2.7 kbar) to a hydrostatic value (0.57-0.73 kbar). Consequently, minerals deposited under magmatic conditions (feldspars and beryl) became unstable and a newly formed hydrothermal mineral paragenesis (euclase, bertrandite, kaolinite, and quartz) overprinted the earlier one. The hydrothermal fluids responsible for the alteration differ from the earlier-exsolved fluids in having a lower salinity, lower homogenization temperature, the absence of CO2, and the presence of CH4.

Brazilian Quartz Deposits with Special Emphasis on Gemstone Quartz and its Color Treatment

The exploration of Brazilian quartz deposits started in the beginning of the twentieth Century, with intensification of production during the Second World War. Four geological environments are the sources for the different types of quartz in Brazil: (1) Neoproterozoic granitic pegmatites—gemstones and minor industrial quartz (2) Neoproterozoic hydrothermal veins—industrial quartz and minor gemstones, (3) Mesozoic basaltic sheets with amethyst and agate—gemstones and (4) Cenozoic secondary deposits—industrial sands. Industrial quartz occurs in Brazil as lascas and sands and the most important sources are the sedimentary deposits of Botucatu and Piramboia Formations and the hydrothermal veins of the Espinhaco Range. The production is mainly used in the metallurgical industry, in the process of production of ferrosilicon alloys and in the glass industry. Between 1996 and 2005, official data suggest a total production of 1,143,497 tons of lascas and up to 40 Mt of industrial sands. The measured resources of about 2,400 Mt, indicate a potential for growth in the industrial quartz market. The main production is located in the states of Sao Paulo, Santa Catarina and Minas Gerais. Transparent single crystals and lascas of quartz from pegmatites, hydrothermal veins and geodes in basalts are the most common gemological material in Brazil. Part of this production is used for color treatment via irradiation and heating, to produce more attractive gemstones, sometimes with colors that will not be found in nature such as the green-gold type.