The team from USP began to study volcanoes in 1998, when a mining company from the region, Rio Tinto Desenvolvimentos Minerais, found minerals that in principle should not have been there. Flying to the Tapajós in single-engined planes that would land on precarious runways in the forest, the researchers arrived at the first volcano, located 120 kilometers to the southeast of Itaituba, the nearest town in Pará. They discovered the volcano one year later, and dating confirmed that it had arisen some 1,9 billion years ago, in a geological era with the name of paleoproterozoic. Last year, one of Juliani's students, Carmen Maria Dantas Nunes, proved that this volcano holds minerals generated in systems known as high sulfidation.

Formed close to the surface and associated with rocks that filled in the volcano's crater, these minerals were deposited by hydrothermal fluids of the primitive magma, at a relatively high state of oxidation. For this reason, from the surface of this volcano down to an estimated depth of 150 meters, alunite is found, a rare mineral in old terrain, rich in potassium and sodium, of a white and pink hue, much used as an ornamental stone and also as a source of sulfate. This was one of the important clues than called the attention for the economic potential of the region, in that a series of other studies had already associated the occurrence of alunite to that of economically more important minerals, like gold, copper zinc and molybdenum.

Until the discovery of this volcano, the oldest gold deposit formed by a high sulfidation system, located in Newfoundland, in Canada, dated back 570 million years. Its rocks, however, underwent alterations caused by variations in temperature and pressure. The Brazilian volcano, three times older, has preserved its original characteristics, so that the forecasts for the possible occurrence of mineral deposits, born of geological analysis, are expanding and may extend to beyond Brazil. "There are probably mineral deposits of this kind in Africa as well", points out Rye. Until 130 million years ago, when they started to separate, South America and Africa formed a single continent, which is why they show terrain of the same age and with similar geological structures.

In April, another of Juliani's pupils, Rafael Hernandes Corrêa Silva, described the second volcano, discovered in 2001. 50 kilometers away, to the north, from the first one, and a hundred meters higher, it was formed in the same era, but in a region where the magma showed a relatively low state of oxidation, resulting in a hydrothermal system known as low sulfidation. As a result of this origin, the rock there contained adularia, a mineral used in the manufacture of glass, chinaware and porcelain. Like alunite, abundant in the first volcano, adularia is associated with the formation of minerals like gold, molybdenum and copper.

The two volcanoes are located on the edges of the kettles, depressed circular areas with a diameter of up to 20 kilometers each. On the Tapajós of two billion years ago, the kettles grouped together and formed complexes over 50 kilometers long. It is known today that not only the volcanoes, but also the granitic rocks themselves associated with the evolution of the kettles, may house deposits of gold, silver, copper, zinc and molybdenum, as the researchers from USP have shown. The model that they have created also makes it possible to understand the distribution of the deposits of these minerals in the kettles. The more superficial ones, called epithermal, show minerals with a high concentration of gold, silver or copper, and constitute veins with a relatively small volume. But with the deeper deposits, the porphyries, the situation is reversed: the precious metals are found in relatively low concentrations and spread in large volumes.

This organization of the mineral deposits determines the way that the region can be exploited, not by sole prospectors, who are not able to remove the gold from the rock, but by large mining companies, which enjoy the equipment and the necessary capital. Even so, Juliani is afraid that the increase in the activity of prospectors may cause environmental damage similar to what took place in Serra Pelada, today a lake 70 meters deep. Roberto Dall'Agnol, from UFPA, who worked with the team from USP, is also concerned. According to him, the confirmation of the presence of the reserves will only benefit the economy in Pará, if it is not followed by the predatory extractivist model that is a mark of the history of the region.



The Project
Study of an Area typical of Gold Mined Zones of Hydrothermal Alteration from the Tapajós Region (PA)
Modality
Regular research benefit line
Coordinator
Caetano Juliani - Institute of Geosciences at USP
Investment
R$ 41,412.11 and US$ 14,997.00