Mine tailings are the ore waste of mines, and are typically a mud-like material.
Worldwide, the storage and handling of tailings is a major environmental issue. Many tailings are toxic and must be kept perpetually isolated from the environment. Scale of tailings production is immense, since metal extraction is usually only ounces or pounds, for every ton of ore. Tailings containment facilities are regarded as the world’s largest man-made objects.
Mine tailings’ size and composition depends on the mining method. For hardrock metal mines, tailings are usually a very fine mud or powder, which is left over after ore is crushed and valuable minerals are extracted from it. Tailings may also contain chemicals used for mineral extraction.
Read more: http://www.groundtruthtrekking.org/Issues/MetalsMining/MineTailings.html#ixzz6UhiawlMP
Disposal of mine tailings is usually the single biggest environmental concern facing a hardrock metal mine, and creates very long-term environmental liabilities which future generations must manage.
Read more: http://www.groundtruthtrekking.org/Issues/MetalsMining/MineTailings.html#ixzz6UhiYClHJ
Toxic chemicals used to extract the valuable materials from the ore, such as the cyanide used in gold mining, remain in the tailings at the end of the process, and may leach out into ground water.
Read more: http://www.groundtruthtrekking.org/Issues/MetalsMining/MineTailings.html#ixzz6Uhidy88q
The above introduction and image are from Mining Tailings article 2014. Take 5 minutes to read in full.