Open pit mine

The Northern Belle pit in the Candelaria District, Mineral County, Nevada. The pit is large enough that I couldn't fit it all in the picture. This was one of the pits that fueled the 20th century heap-leach operations in the district. In a heap leach, low-grade crushed ore is stacked on top an impermeable sheet, usually thick plastic, and a leaching solution is sprinkled on top--often using the same sort of sprinklers that would be used for irrigation. The leaching solution trickles thru the heap, dissolving out the desired metal, and flows out the side on hitting the impermeable sheet. The leachate--the "pregnant solution" in the jargon--is then collected and the metal extracted. The solution contains reagents that dissolve the desired metal(s)--dilute cyanide, in the case of precious metals, because of the extraordinary affinity of the simple cyanide (CN-) ion for gold and silver. The technique allows extraction of metals from very low grade deposits, at the cost, of course, of excavating enormous quantities of rock. And cyanide contamination is a very real concern.

The left inset shows a different view of the pit, almost to the bottom. Altho it probably now contains some water after this season's winter storms, the lack of a pit lake shows how dry the area is--even by Nevada standards! The right inset shows a reclaimed heap leach--it's the smooth, almost trapezoidal ridge dominating the skyline. At least the sides have been bulldozed so they're now more shallow than the angle of repose, which lets vegetation (such as there is!) get re-established.