Mine waste tailings are the residual material left after ore is crushed, ground, and processed to extract valuable minerals. The fraction returned to the surface as waste is typically a slurry of fine rock particles, water, and the chemical reagents used in beneficiation. Tailings are stored in engineered impoundments — most commonly tailings dams — and are among the most consequential long-term liabilities of the extractive sector. Their failure has caused some of the worst industrial disasters of the modern era.

What tailings are

When ore is processed, the valuable mineral fraction is separated from the gangue by crushing, grinding, flotation, leaching, or other beneficiation steps. The valuable concentrate proceeds to smelting or refining; the gangue, now finely ground and chemically altered, becomes tailings. Particle sizes are typically in the silt and clay range, water content is high, and the mass of tailings produced per tonne of marketable product can be enormous — for many base-metal operations, more than ninety-nine per cent of mined rock ends up as waste.

The chemistry of tailings depends on the ore body and the processing route. Sulphide ores generate tailings that can produce acid mine drainage when sulphides oxidise on contact with air and water, releasing sulphuric acid that mobilises heavy metals into groundwater and surface streams. Cyanide-leach gold tailings carry residual cyanide. Uranium-mill tailings contain low-level radioactive daughter products. Each chemistry demands a tailored containment strategy.

Storage and engineering

Tailings are most commonly stored in surface impoundments retained by embankment dams. Three principal dam construction methods are used: upstream, centreline, and downstream. Upstream construction, in which successive embankment lifts are built atop previously deposited tailings, is the cheapest but the most prone to liquefaction failure under seismic loading or rapid loading. Downstream and centreline methods are more robust and are now mandated by most responsible operators and regulators for new construction.

Alternative storage methods include dry-stack tailings, in which the slurry is dewatered before deposition and stacked as a filter cake; paste tailings, in which a high-solids paste is deposited in thickened form; and underground backfill, in which tailings are returned to mined-out voids. Dry-stack and underground methods reduce dam-failure risk but cost more and are not feasible at all sites.

Failures and the modern reckoning

The 25 January 2019 collapse of Vale's Brumadinho B-I tailings dam in Minas Gerais, Brazil, killed 270 people and released about ten million cubic metres of tailings into the Paraopeba River. The disaster came less than four years after the November 2015 Mariana (Fundão) dam failure, also operated by a Vale joint venture, which killed nineteen people and remains the worst environmental disaster in Brazilian history. Brumadinho was an upstream-construction dam; investigations attributed the failure to static liquefaction of the saturated tailings.

The two failures triggered a global reassessment. In August 2020, the International Council on Mining and Metals (ICMM), the United Nations Environment Programme, and the Principles for Responsible Investment jointly issued the Global Industry Standard on Tailings Management (GISTM), the first internationally agreed standard for tailings facilities. The GISTM sets requirements across the full life cycle, from design and construction through operation, closure, and post-closure stewardship, with explicit zero-tolerance language for catastrophic failure with loss of life.

The gemstone trade's exposure

Gemstone mining generates far smaller tailings volumes than copper, gold, iron, or coal mining, but the same principles apply. Coloured-stone operations in Tanzania, Mozambique, Madagascar, and elsewhere produce tailings from washing and crushing operations that, while modest in volume, can still degrade local water quality if not contained. Diamond mining at industrial scale — the kimberlite operations of De Beers, ALROSA, Rio Tinto, and others — produces substantial tailings volumes from the dense-media separation circuits used to recover diamonds from kimberlite ore.

For artisanal and small-scale gem mining, the tailings issue is less about engineered impoundments and more about the cumulative impact of unrestricted alluvial workings on river systems. Rehabilitation of disturbed alluvial ground after mining is closely related and is treated separately under mine reclamation.

In the trade

Provenance-conscious buyers and major retailers increasingly want to know how the operations behind a stone manage tailings and other waste streams. The Responsible Jewellery Council's Code of Practices, the Kimberley Process for diamonds, and emerging coloured-stone traceability initiatives all touch on tailings management as part of broader environmental and social standards. The trade's exposure to tailings risk is reputational as much as operational: a high-profile dam failure at any mine in a producing region affects perception of stones from that region for years.

Closure and reclamation

Closed tailings facilities require active stewardship potentially for centuries. Cover systems, water management, and long-term monitoring are necessary to prevent acid drainage, dam degradation, and seepage. The cost of perpetual care is increasingly internalised at the design stage rather than deferred to post-closure, and financial assurance instruments are required by most modern mining jurisdictions to ensure funds are available for reclamation regardless of operator solvency.

Further reading

• ICMM — Tailings management and the Global Industry Standard
• Global Tailings Review — GISTM standard documents
• UNEP — Tailings management overview
• Responsible Jewellery Council — Code of Practices