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Geology

More gold under the forest

Two volcanoes in the Amazon may shelter vast reserves of precious minerals

In the south of Pará, between the Tapajós and Jamanxim rivers, two discrete hills hide two of the oldest volcanoes on Earth, formed almost 1.9 billion years ago, when the Earth was less than half the age it has today. Underneath them, at a depth that varies from 100 meters to one kilometer, there may exist amid the rocks vast deposits of gold, silver, zinc, copper and molybdenum, as recent studies published by geologists from the University of São Paulo (USP) suggest.

If the prospecting confirms what is indicated in the São Paulo team’s geological models, the Tapajós Gold-bearing Province, as the region is known, may lodge reserves of gold ten times larger than used to be estimated – the previous calculations suggested the existence of deposits of up to one hundred tons of gold, enough to justify starting exploration. “Our data indicates that there may be deposits of as much as a thousand tons there”, says Caetano Juliani, a researcher from the Institute of Geosciences (IGc) at USP and the coordinator of the studies. “In Peru”, adds Robert Rye, a researcher with the United States Geological Survey and a partner of the Brazilian team, “a team with similar geological characteristics has roughly 250 tons of gold to be exploited”.

The consequences of the research have not been restricted to the economic aspects. Two studies – one of them accepted for publication in Precambrian Research , an important point of reference in the area – propose a new model for the formation of these mineral deposits in the Amazon. It was previously believed that these minerals existed only in geological faults, but the discoveries of the group from USP indicate that these reserves also occur in volcanic rock, which extends far beyond the Tapajós, to reach the Xingu River to the east, and up to the border with the Guyanas in the north.

But the geologists have another reason for celebration: the volcanoes have suffered practically no erosion, nor the action of the movement of the Earth’s crust and are in a good state of preservation. Nowadays, one of them is 200 meters high and 1.7 kilometers in diameter, and the other 300 meters high and of a diameter as yet unknown. According to Juliani, they must have lost only400 meters in height – very little compared with the 20 kilometers that the rain, the wind and glaciers have taken away from the Serra do Mar range, formed only 600 million years ago.

For this reason, the researchers believe that they can get information on the Earth’s atmosphere of two billion years ago, through an analysis of the isotopes (atoms of one and the same chemical element, with different masses), stored in tiny pockets of liquids and gases found in minerals from the volcano, besides understanding better how the portion of the crust that makes up the Amazon was formed.

Frequent in the Tapajós region, the volcanoes used to spread rivers of glutinous and incandescent rock, the lava, and clouds of burning ashes that covered an area of thousands of square kilometers, from the Tocantins River, to the east, to the Serra do Cachimbo mountains to the west, and as far as the extreme north of Pará. An analysis of the satellite and radar images, added to the study of the composition of the rocks and the minerals, has shown that the two volcanoes are part of an area that showed intense volcanic activity in that period and remained active for up to as much as 40 million years.

It is the result of the development of a series of kettles – depressed circular regions grouped together, which can be associated with economically important mineral deposits. As the eruptions ceased, the magma from the inside of the Earth’s crust and from the surface cooled down, and then solidified, generating, respectively, the deep magmatic (plutonic) rock and the volcanic rock. It was in this process that the magma released liquids and vapors – the hydrothermal solution – that precipitated minerals made up of chemical elements like oxygen, sulfur and hydrogen, which are studied today for being able to reveal details of the temperature and composition of the fluids of that period.

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) (nº 98/02567-6); Modality Regular research benefit line; Coordinator Caetano Juliani – Institute of Geosciences at USP; Investment R$ 41,412.11 and US$ 14,997.00

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