Mercury Pollution in Gold Mining — The ASGM Crisis the Trade Cannot Ignore
Mercury Pollution in Gold Mining — The ASGM Crisis the Trade Cannot Ignore
The largest single source of anthropogenic mercury, and the international response under the Minamata Convention
Mercury pollution from artisanal and small-scale gold mining (ASGM) is, by current scientific assessment, the largest single source of anthropogenic mercury emissions to the global environment. The United Nations Environment Programme estimates that ASGM contributes approximately thirty-eight per cent of global anthropogenic mercury emissions — a figure that exceeds the contribution of coal-fired power generation. The pollution arises from the use of mercury to extract gold from ore through a process known as amalgamation, practised by an estimated ten to twenty million miners across more than seventy countries, predominantly in sub-Saharan Africa, South America, and Southeast Asia.
How amalgamation works and why it persists
Mercury amalgamation exploits the same chemical principle that underlies fire-gilding: gold dissolves readily in liquid mercury at room temperature to form a soft amalgam. ASGM miners crush gold-bearing ore, mix it with water, and add liquid mercury to the slurry, agitating until the mercury captures the fine gold particles into an amalgam mass. The amalgam is then squeezed to remove excess mercury and heated — typically over an open flame in a tin can or improvised retort — to vaporise the mercury and leave behind a sponge of impure gold ready for further refining.
The process persists despite its environmental and human-health costs because it is technically simple, requires minimal capital equipment, and produces gold rapidly enough to support cash-flow needs in subsistence economies. Alternative processes — cyanide leaching, gravity separation with shaking tables, direct smelting of concentrates — require either more capital, more processing time, or technical expertise that smallholder operations cannot easily access. Mercury can be obtained on informal markets in any region with active ASGM activity, often smuggled across borders despite formal trade restrictions.
The pollution pathway
Mercury enters the environment at multiple points in the amalgamation cycle. During amalgamation itself, mercury-rich tailings are typically discarded into watercourses, contaminating sediments and surface water. During the heating stage, mercury vapour is released directly to the atmosphere, where it can travel thousands of kilometres before deposition. The vapour is acutely toxic to the worker performing the heating and to anyone in the immediate vicinity; downwind communities and downstream water bodies receive the broader fallout.
Once in aquatic environments, inorganic mercury is methylated by sulphate-reducing bacteria into methylmercury, the bioaccumulative form that concentrates in fish and ascends through the food chain. Communities dependent on fish from contaminated waters experience methylmercury exposure orders of magnitude greater than ambient levels, with documented neurological, cardiovascular, and developmental effects. Children and pregnant women are at particular risk; methylmercury crosses the placenta and the blood-brain barrier and produces irreversible developmental injury at exposure levels that in adults might pass unnoticed.
The 1956 outbreak at Minamata Bay in Japan, where industrial methylmercury discharge from the Chisso Corporation chemical plant produced thousands of cases of severe neurological injury and birth defects, is the foundational case study in mercury epidemiology and the namesake of the international convention now governing mercury use globally.
The Minamata Convention
The Minamata Convention on Mercury was adopted in October 2013 in Kumamoto, Japan, and entered into force in August 2017 after the requisite number of national ratifications. The convention is an international treaty under the United Nations Environment Programme that commits signatory parties to phase down mercury supply, trade, and use across multiple sectors, with specific provisions on ASGM in Article 7 and Annex C.
For ASGM, the convention requires parties whose territory hosts significant artisanal mining activity to develop and implement national action plans to reduce and, where feasible, eliminate mercury use. The plans must address baseline measurement of mercury use, formalisation of the artisanal sector, public health responses, and gender and child protections. The convention also bans certain practices outright — whole-ore amalgamation, open burning of amalgam in residential areas, burning of amalgam in residential areas without a retort capturing the mercury vapour, and amalgamation of cyanide leach tailings.
Implementation has been uneven. By 2024, approximately one hundred and forty parties had ratified the convention, and a substantial number had submitted national action plans for ASGM. Actual reduction of mercury use on the ground has been slow, however, because the formalisation and alternative-technology investment required is substantially more difficult and expensive than the treaty drafting. Several major ASGM-producing countries — including Indonesia, Colombia, the Philippines, and Peru — have made measurable progress; others have national action plans on paper but limited operational change.
The trade response
Mercury pollution in ASGM is increasingly visible to the gold and jewellery trade through the supply-chain transparency requirements of the Responsible Jewellery Council, the OECD Due Diligence Guidance for Responsible Supply Chains of Minerals from Conflict-Affected and High-Risk Areas, and the London Bullion Market Association's Responsible Gold Guidance. These frameworks require gold refiners and downstream users to assess the origin of their gold and to avoid sourcing from operations using mercury in ways that violate the Minamata Convention's prohibitions or otherwise expose workers and communities to unacceptable harm.
Compliance is uneven and verification is difficult, particularly for the secondary refining sector and for gold passing through informal markets in countries with weak regulatory capacity. The Fairtrade and Fairmined certifications, applied to specific smallholder cooperatives that have invested in mercury-free or significantly reduced-mercury processing, provide one route by which jewellery manufacturers can source gold with documented improvement against the ASGM mercury baseline. The volume of certified gold remains a small fraction of global supply, but the segment is growing as retail and high-jewellery customers express increasing concern about supply-chain ethics.
Alternative technologies
Mercury-free ASGM is technically possible and is in operation in several certified cooperatives. The principal alternative processes are gravity separation using shaking tables, sluices, and centrifugal concentrators; direct smelting of concentrates with borax flux; and small-scale cyanide leaching with appropriate effluent management. Each has trade-offs in capital cost, processing time, recovery efficiency, and operator skill requirement. Mercury-free retorts that capture vapour during burning, while not eliminating mercury use, can substantially reduce occupational and community exposure during the most acute phase of the pollution pathway.
The challenge for the development sector is not the existence of alternatives but the capital, training, and market-access required to deploy them at scale across ten to twenty million miners working in geographically dispersed, often informally organised operations. Progress is being made — the UNDP-led planetGOLD programme is one of the most substantial international initiatives — but the pace of transition will measure in decades rather than years.