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Geology

Hitching a ride in steam

Interaction of water with rocks facilitated the accumulation of copper and the formation of amethysts

Eduardo CesarAmethyst (left) and native copper: perhaps in depositsEduardo Cesar

In the depths of the Guarani aquifer, the underground reservoir that supplies the cities of the southeast and south of Brazil, the water temperature is unlikely to exceed 60o Celsius. However, this water was once far hotter, to the point of changing the composition of the rocks that cover the aquifer  forming copper spheres and amethyst deposits as it rose to the surface.

A team from the Federal University of Rio Grande do Sul (UFRGS) concluded that the water of the Guarani aquifer must have reached 130oC around 135 million years ago, when carnivorous dinosaurs ran after herbivorous dinosaurs in the probably barren plains of the south and southeast of Brazil. Analyses of rocks have indicated that the water must have boiled and remained as steam for 1 or 2 million years, while a part of the liquid magma came out of the Tristão da Cunha plume, which heated the entire south and southeast region of South America, and settled in the middle of the layer of already crystallized basalt.

The water steam must have crossed the basalt, released the copper atoms from the minerals and carried them to the spherical cavities and fractures in which the copper accumulated. Likewise, the water vapor, in releasing, transporting and accumulating minerals while it moved away from the center of the earth, may have aided the formation of deposits of amethyst, a variety of quartz that is violet in color because of impurities, like manganese or iron, in the south of the country.

With this hypothesis, older concepts about the formation of these minerals are scuppered. “The copper and amethyst deposits of this region, the Paraná volcanic province, must have been formed at 150oC at the most, as a result of the interaction of the water and steam with the basalts, and not at 1200oC, as a consequence of the cooling of the basaltic lava, as was thought,” says the geologist Léo Afraneo Hartmann, a professor at UFRGS and coordinator of the team that for five years has been examining variations in the composition of the rocks that cover the aquifer. This layer, known geologically as the Serra Geral Group, comes to the surface in the south of  Minas Gerais, then spreads out at depths that reach 1,800 meters in the states of São Paulo and Paraná and rises to 800 meters below the surface in Rio Grande do Sul. “All the tests are confirming this new hypothesis.”

For two years, while doing his PhD with Hartmann as his tutor, Víter Magalhães Pinto collected samples of copper as much as 3 meters below the surface in 85 locations in the Vista Alegre district, on the border between Rio Grande do Sul and Santa Catarina. His purpose was to understand why the copper there, instead of being in deposits, as in other regions in Brazil, forms spheres with low impurity, whose dimensions vary from half the size of a closed adult fist to 200 kg, weighing between 500 grams and 200 kg. Farmers find them when they plough the land. Although their volume is insufficient for commercial exploitation, they can be melted and molded relatively easily into pans. The locals from the southern region used this copper to make spear heads and arrows.

At UFRGS and in Australia, where he did part of his PhD, Víter analyzed the succession of minerals accumulated in these cavities and cracks. “The copper was the last deposit phase of minerals in the rock cavities,” concluded Víter, hired in January as a professor at the Federal University of Roraima. Therefore, he thought, the copper must be more recent than the other minerals and must have been removed from the minerals, pyroxene and magnetite, which make up the basalts, by steam.

Detailed in an article about to be published in the International Geology Review, these findings converged with the PhD research of Lauren Duarte, conducted under Hartmann’s supervision as well. Lauren examined the amethysts from the Rio Grande do Sul town of Ametista do Sul and of Artigas, in Uruguay, inside elongated geodes up to four meters tall. She and Hartmann concluded that these precious stones must have been formed as a result of the action of steam on the basalt layer, as described in an article published in 2009 in the Journal of Volcanology and Geothermal Research.

“Two theoretical physicists, professors Marcos Vanconcelos and Joacir Medeiros, who were here from UFRGS, helped us a lot with the mathematical modeling of the temperature and water pressure, which explained what we saw in the field,” reported Lauren, hired last year as a professor at UFRGS. Since 2008, she has been part of the team from a laboratory that develops technologies that could lead to making the most, in economic terms, of waste from minerals and gems, like agate and amethyst.

 Water from gold
“In the Serra Geral there is still hot water today, although not as hot as before, and it’s still passing through the rocks that cover the aquifer,”  says Hartmann. The waters that reach the thermal spa of Iraí, in the north of Rio Grande do Sul, are warm and in some towns in São Paulo have dissolved silica in them. For two decades, Hartmann and his team examined how the water, combined with sulfur and chlorine at temperatures higher than 150oC, facilitated the formation of gold deposits in the Amazon Region, in Uruguay and in the Andes.

Supported by funding from CNPq (the National Scientific and Technological Development Council] ) and Fapergs (the Rio Grande do Sul State Research Funding Agency), Hartmann continues going into the field with his team. In August he is planning on going to Quaraí, in the west of Rio do Grande do Sul, with researchers, post-graduate students and members of the Brazilian Geology Society and the Brazilian Geological Service. In his opinion, there may be as many amethysts in the grassy plains of the pampas as there are in the Ametista do Sul hills.

“The gemological district of Los Catalanes, in Uruguay, on the other side of the border, has huge deposits of amethysts and on this side we’ve still not found any, but they must exist,”  he says, based on two articles that are about to be published – one in the Geological Magazine and the other in the International Geology Review. Hartmann also believes that copper may have formed deposits along the Serra Geral, and not just small and few and far between deposits. “In China they’ve already found and are mining copper with similar origins,” he says. “The indications we’ve seen so far in the south of Brazil are signs that there may be a deposit, but it’s only by systematically looking for it that we’ll know for sure.”

Scientific Article
DUARTE, L. C. et al. Epigenetic formation of amethyst-bearing geodes from Los Catalanes gemological district, Artigas, Uruguay, southern Paraná Magmatic ProvinceJournal of Volcanology and Geothermal Research. v. 184, p. 427-36, 2009.

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