When not used responsibly, mercury and cyanide are highly toxic.
Cyanide leaching is used by large mining operations to separate gold from ore. This is done by leaching the rock on engineered, lined pads or by crushing the rock to the consistency of sand, then adding water to form a slurry that is mixed with a cyanide solution. Gold particles bond with the cyanide in solution and are extracted from the slurry. Cyanide pollution is a major concern. A rice-grain sized dose of cyanide can be fatal to humans and concentrations of 1 microgram (one-millionth of a gram) per litre of water can be fatal to fish.
While the wide sale use of cyanide is a relatively recent development, mercury has been used for centuries as a cheap and easy method to extract gold. Like cyanide, it is a deadly toxin. Once mercury enters the environment, its vapors become trapped in the atmosphere, precipitate onto the ground, and run into the water supply. When exposed to organic matter, methyl mercury is formed. This compound is stored in animal fat and accumulates over time, reaching levels in fish that can be thousands or millions of times higher than in the river.
Mercury is still used by small scale artisanal miners because it can extract as much as 60% of the gold. It is mixed with gold bearing mud and gravel into an “amalgam,” which is then burned out with a torch or even over an open fire, releasing mercury vapor into the atmosphere and exposing miners and bystanders to toxic fumes.