Notes on the Almaden Mercury Mine

Nutmeg Mountain, viewed from the southwest

Nutmeg Mountain, viewed from the southwest, April 2017

Between 1932 and 1972, Nutmeg Mountain, eleven miles east of Weiser, Idaho, was a mercury mine. The mining was episodic, not continuous. Usually, it took a war to drive the price of mercury, which is used in explosives, to levels that made mining and extraction profitable. Changes in mining technology also affected the level of activity.

Altogether, some 22,600 flasks of mercury were extracted from the Almaden Mine on Nutmeg Mountain. The process generated some 800,000 tons of waste rock – called “spoil” by the industry. The mercury deposits, in the form of cinnabar, mercury sulphide are in arkosic sandstones and siltstones of the Miocene Payette Formation. “Arkosic sandstones” are sands that are at least 25% feldspar from a river delta or lower channels. The sands have been lithified, turned into rock.

Hot, mineralized fluids rose along faults in the rock and formed a mineralized zone approximately 18 feet thick with the finely disseminated cinnabar as the primary ore mineral. The deposit is extensively silicified – mixed with high silicon rocks that also precipitated out –  and also contains opal and chalcedony with traces of pyrite. The ore as processed by grinding it to fine material, and then “roasting” the ore, cooking it at 600° F in the presence of oxygen, which causes the mercury to vaporize. The mercury vapors were captured and condensed out.

The ore body was not rich; over the life of the mine, it’s estimated there were about 3.6 pounds of mercury per ton of ore. As a consequence, the “spoil” is enormous.

Almaden Mercury Mine spoil, April 2017

Almaden Mercury Mine spoil, April 2017

The mine and works were at the summit. the spoil is the heap of reddish rock extending from the summit down to the toe in the lower right of the photograph. By modern super-mine standards, it’s not especially large but it is nonetheless immense.

Mercury is an acute neurotoxin. The amounts of various forms of mercury allowed in the workplace and in foods are extremely low.

Country Regulating agency Regulated activity Medium Type of mercury compound Type of limit Limit
US Occupational Safety and Health Administration occupational exposure air elemental mercury Ceiling (not to exceed) 0.1 mg/m³
US Occupational Safety and Health Administration occupational exposure air organic mercury Ceiling (not to exceed) 0.05 mg/m³
US Food and Drug Administration eating sea food methylmercury[^1] Maximum allowable concentration 1 ppm(1 mg/L)
US Environmental Protection Agency drinking water inorganic mercury Maximum contaminant level 2 ppb (0.002 mg/L)

The Idaho Geological Survey conducts surveys of abandoned mines. A report in 2007 discussed the Almaden Mine. There are stunningly high levels of mercury present in the rocks at the top of the mountain and in the spoil. Soil at ground squirrel burrows near the road, for example, tested at 27.6 parts per million mercury; that’s hundreds of times the EPA limits. The IGS concluded that the semi-arid conditions minimized the risk of transport. WC wonders if the IGS considered that the Great-horned Owls and hawks nesting in the area, feeding on the ground squirrels, might disagree.

The IGS noted that someone seemed to be harvesting the pretty, red-colored rocks for use in landscaping. On Nutmeg Mountain, the redder the rock, the more mercury it contains.

Sometimes WC suspects that human folly has no limits.

There’s quite a thorough history of the Almaden Mercury Mine published by the U.S. Geological Society if you are curious.