Mercury and CO2 emissions from artisanal gold mining in Brazilian Amazon rainforest – Nature

Gold is a commodity that has captivated the human imagination for thousands of years. Demand remains undiminished, with about 4,000 to 5,000 metric tons being produced every year worldwide. About one-third of this production is recovered from recycling (2021: 1,136 t); the rest is newly mined (2021: 3,581 t) (ref. 1). But the mining process is associated with a high environmental impact. Due to gold’s low content in rock, large amounts of energy are required. Major concerns arise due to environmental damage by chemicals, social issues and regulatory and governance issues. Recycling gold could substantially reduce environmental impact2, especially because large quantities of gold are in circulation. It is estimated that the anthropogenic gold stock is about 200,000 tons1. But the search for gold goes on because for many impoverished people, digging for gold is synonymous with digging for money. Artisanal and small-scale gold mining (ASGM) supplies about 700 tons every year1 under particularly problematic conditions. Impacts such as deforestation, the release of toxic chemicals such as mercury and the high impact on climate change associated with fossil energy consumption cannot be ignored. In ASGM, problems are exacerbated by sometimes adverse working conditions and regulatory and governance problems caused by unregulated and sometimes illegal mines.

In numerous field surveys and visits to a high number of ASGM sites in Brazil, mainly in the period of 2018–2022, we investigated mercury use and impact on climate change, providing valuable empirical data for further scientific discussion. Several points make Brazil an interesting object of study. The distribution of ASGM based on the population involved in mining3,4 shows that with the exception of China, ASGM hotspots are located in tropical rainforests. Brazil is the country with the highest ASGM population in the Amazon rainforest. The world’s largest ASGM district, the Tapajós River basin, which is where we conducted our fieldwork, is also in Brazil5. Especially in this climate zone, economic activities such as ASGM, the protection of nature and biodiversity and the rights of indigenous peoples clash, as in hardly any other place on earth. ASGM thus represents a prime example of conflicting sustainability goals.

The use of mercury still plays a decisive role in ASGM, although its harmful effects on health and the environment are well known. ASGM in Brazil, which is often in remote areas where the government is unable to enforce the law, is an illustrative example of this. In 2017, the United Nations Minamata Convention on Mercury came into force with ratification by around 140 countries (including Brazil). ASGM is one important issue in the convention, with article 7 requiring countries to draft National Action Plans (NAPs) for handling and reducing mercury usage in ASGM6,7,8. But implementation has been slow as gold from ASGM is an economic factor and mercury still plays an important role here. Since the 1990s, attempts have been made to recover mercury through simple distillation devices, called retorts, and thus reduce its release into the environment9. But it is unknown how effective this method is. Additionally, in Brazil, ASGM is being mechanized through the use of excavators, which raises the question how this affects the carbon footprint of gold10.

Artisanal gold mining is prohibited in certain areas of the Brazilian rainforest, for example, in Reservas Indígenas and Parque National11. In these areas, ASGM is illegal and punishable by law. Outside these areas, mining is allowed with a permit. This involves registration at the national mining office (Agência Nacional de Mineração) and filing an environmental impact assessment (licenciamento ambiental) issued by the federal states12. This is a highly bureaucratic process, and as a result, few miners (garimpeiros) have the complete permit. However, they have the possibility to obtain a permit for mineral exploration with prospect mining (pesquisa mineral com lavra experimental) for a limited time, which is then used to mine gold. Although this practice is not illegal, it is in an irregular grey area. There is also illegal gold mining, in which the gold is laundered through legal mining operations that produce no or only small quantities of gold13. In addition to deficiencies in the law, there are virtually no official controls in the vast rainforest, apart from a few spectacular actions by the environmental authorities. As a result, most gold mines are informal operations, making it difficult to visit these mines, to gain trust and to conduct surveys. For this study, we visited a large number of both illegal and informal mines and exemplary licensed mines, which is a key feature of this study. A complete ban on gold mining in the Brazilian rainforest would fail as it would be impossible for the state to enforce and would only push gold miners further into remote areas where the potential for conflict with indigenous people would increase. On the other hand, greater formalization—that is, simpler and faster authorization of gold mining accompanied by stronger monitoring—would be a first step towards improving mining conditions.

This is particularly true for the handling of mercury in the mines. Contrary to many opinions, the purchase and use of mercury is allowed in Brazil under certain conditions, and mercury use in ASGM is still standard practice. There is a lack of clear specifications and government monitoring14. Although there was a strict government regulation in 1989 that required ASGM to register mercury recovery facilities15, this was withdrawn in 201516. Here, too, greater formalization of gold mining could substantially improve mining conditions. This would also help Brazil to comply with the spirit and the letter of the Minamata Convention, that is, to substantially reduce the use of mercury. But Brazil has not even fulfilled the formal requirements of the Minamata Convention, which it signed in 2017, and is now more than two years behind in submitting its NAP to the United Nations17,18.

A unique feature in the Tapajós region is that ASGM uses several different mining methods, which were thus able to be investigated in this study: mining of secondary deposits on land (baixão) and by dredges (dragas) and mining of primary deposits underground and in open pits (filão) (Fig. 1). In so-called primary deposits, gold is embedded in rock veins. When primary gold deposits are eroded and flushed out of the rock as fine gold dust or as small grains, they are called secondary deposits. Baixão is the most common method in the region. This process uses water jets to elutriate gold-bearing soil, pumping it over sluice boxes and amalgamating the gold with mercury, a process well described in many studies19,20. The gold production process of the dragas is very similar to the process on land but without the need for water jetting, as the sediments from the riverbeds can be pumped over the sluice box directly21. A common technique for mining the filão in ASGM uses amalgamated copper plates to trap the gold in the crushed ore22. Sometimes the crushed ore is also leached with cyanide instead of or in combination with mercury use before cyanidation6 on the amalgamated copper plates to extract the gold. The latter technique should be eliminated as part of the NAP of the United Nations Minamata Convention on mercury.

Fig. 1: The four main mining types of ASGM in the Tapajós.

Photographs show (clockwise from top left) underground mining of primary deposits (filão), open pit mining of primary deposits (filão), open pit mining (baixão) of secondary deposits and dredge mining (draga) of secondary deposits.

Social conditions in ASGM mines, deforestation rates and the effects on health of mercury in rivers and soils have been studied many times6,23,24,25. Our team published a separate article on the social aspects of the ASGM sector in the Tapajós region, for example, the absence of the state, the illegality and informality of mining operations, poor working conditions, difficult living conditions, unstable payment of wages and autonomy26. Much research on mercury in soils, plants, waters, fish and humans has also been done in the Brazilian Amazon rainforest since the end of the 1980s5,19,26,27,28. Studies of the specific amounts of mercury used and recovered in ASGM are, however, rare. They have small sample sizes of very few different types of mine, and there are problems in comparability21. Even less research has been done on energy intensity and the climate impact of ASGM. Studies on energy intensity, all in countries other than Brazil, mostly refer to individual mines and specific regions, and there are methodological problems in comparing the results29,30,31. The international discussion about climate change and the contribution of mining to greenhouse gas emissions is gaining importance, which was the motivation for us not only to look at mercury in ASGM but also to determine energy use.

There is still a gap in knowledge about the current extent of mercury use, retort efficiency and the carbon footprint of ASGM. The Tapajós region, which is known to produce at least 15 tons of gold per year13, provided the opportunity to collect a larger amount of empirical data for different mining methods within a reasonable amount of time. This included mines that were inaccessible to outsiders and about which little in situ data are typically published. In our study, we were able to collect and analyse data on energy consumption, mercury use, loss and recovery from over 100 data samplings at different mines. The Tapajós region is also well suited to this research because here ASGM shows a development in mechanization that might sooner or later take place in other regions of the world. In addition, since the 1990s, there have been several programmes to train miners in the use of retorts, but there is no record of how many of ASGM sites (garimpos) in the region use retorts

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