Gold is very chemically non-reactive, as a general rule, which can make it difficult to separate out from the other metals that it naturally occurs with. However, gold readily forms an amalgam with mercury, and this amalgam can be easily separated back into its two component parts. Although there are other methods for separating gold out from the ore it is found in, amalgamation with mercury is by far the cheapest. Mercury has a similar affinity for silver and copper, and is also used in the mining of those metals.
However, mercury is a highly potent neurotoxin, made even more dangerous by its tendency to accumulate in the tissues of animals, including humans. Mercury pollution that finds its way into the water is the ultimate cause behind widespread mercury contamination of seafood. In addition to causing serious developmental problems in infants and children, mercury poisoning can cause sensory impairments, lack of coordination and other neurological symptoms in adults.
Other significant sources of mercury pollution include cement production, waste disposal (from crematoria, sewage sludge and hazardous and municipal waste), caustic soda production and, finally, production of mercury itself.
Historically, mercury was used in large quantities in all types of gold mining. Since the 1960s, large gold mines have mostly shifted away from the use of mercury, but it is still very popular with small-scale miners. In addition, much of the mercury expelled into the environment from large mines in earlier decades remains there to this day; an estimated 45,000 metric tons used in California gold mines, for example, have yet to be recovered.
Because gold also dissolves well in a solution of potassium cyanide, many modern mines treat their ores with cyanide instead of mercury; therefore, gold mining is also a major source of cyanide pollution worldwide.