REGIONAL ENVIRONMENTAL IMPACTS OF LODE GOLD MINING IN THE SIERRA NEVADA, CALIFORNIA

The Sierra Nevada region is a world-class gold-mining province that has produced about 2800t of gold since 1848, including 1000t from lode mines and the remainder from placers. There are more than 4000 lode gold mines with recorded production; about 70 percent of this production came from 25 mines. The deposits are classified as low-sulfide gold-quartz veins, also called orogenic gold deposits. The ore bodies occur in quartz-carbonate veins that contain a few percent sulfides, mainly pyrite, cutting hydrothermally altered metamorphic and plutonic host rocks. Arsenopyrite and arsenian pyrite are common; ores often contain more than 1000 mg/kg arsenic. Thus the main environmental problem associated with these mines is high levels of arsenic in mill tailings.

Another concern is mercury in tailings. Most gold mines in the Sierra Nevada used stamp mills to crush the ore, and mercury amalgamation to recover free gold. Total loss of mercury to the environment from lode mills is estimated at about 1100t. Before 1890, mercury losses to tailings ranged from 6 g/t to more than 20 g/t.

Tailings produced before 1912, when California required mill tailings to be impounded, are now entrained in stream sediments. Improvements in milling technology beginning in the 1890’s, the most important being cyanidation, greatly reduced mercury usage and losses by 1910, and modestly reduced the amounts of arsenic passed to tailings. Therefore, accumulations of mill tailings that now remain (about 40-60 MMt), should generally have average mercury concentrations <3 mg/kg, and arsenic concentrations <1000 mg/kg.

Although drainage from mine openings and tailings piles may show elevated levels of mercury (>100 ng/L) and arsenic (0.01-1.0 mg/L), discharges from individual sources of these types in the region tend to be small (<500 L/min), so notable affects on surface waters are local. Similarly, although arsenic contamination of groundwater exists in local areas, no widespread contamination clearly attributable to gold mining has been recognized.