Mercur Mining District, Tooele County, Utah, USAi

Regional Level Types
Mercur Mining District	Mining District
Tooele County	County
Utah	State
USA	Country

The Mercur (Lewiston, Camp Floyd) mining district lies in the southwestern Oquirrh Mountains about 35 mi southwest of Salt Lake City. The district was initially organized in 1870, was intermittently productive from 1871 to 1998, and the largest production was from 1890 to 1913 and 1983 to 1998. Mercur is the largest primary Au producer in Utah at 2,605,037 ounces Au along with 1,183,724 ounces of Ag and 3469 flasks of Hg, recovered. Mercur is the sixth most productive district in Utah in terms of total metal value and the largest Hg producer. Total district metal production at modern metal prices is estimated at $2.9 billion. The Mercur Hill, Marion Hill, and Sacramento are the largest open pit mines in the district (Mako, 1999).

The Oquirrh Mountains are among the easternmost in the Basin and Range Province. Most of the mineralization in the Mercur district is concentrated between the crest of the Ophir thrust-cored anticline and its hinge line to the east. The Mercur Au deposits are localized near an east-northeast-trending set of normal faults including the Lulu graben, Eagle Hill tear fault, and Carrie Steele fault. Portions of the district also show a pronounced east-northeast-trending, subvertical joint set. The ores are largely confined to the Mercur member in the lower limestone section of the Mississippian Great Blue Limestone. The Mercur member is a slope-forming, heterogeneous sequence of black, thin- to medium-bedded, carbonaceous, fossiliferous, and Fe-rich (over 1% Fe) limestone, calcareous sandstone, calcareous siltstone, and shale. The best ore host is the thin-bedded, calcareous, sandstone and siltstone of the Mercur beds (Mako, 1999).

The most readily recognized hydrothermal alteration in the Mercur district is the extensive silicification at the contact between the Magazine sandstone and underlying Topliff member limestone. Later argillic alteration and decalcification are more immediately associated with Au ores than this earlier silicification event. In argillic alteration and decalcification, carbonate is removed and detrital phyllosilicate minerals are altered to kaolinite and sericite. Mineralization is localized primarily by the intersection of east- northeast-trending faults and favorable Mercur member host horizons, principally the Magazine sandstone and Mercur beds. Gold mineralization is spatially associated with argillic alteration and decalcification (Mako, 1999). Gold occurs with late arsenian pyrite overgrowths, orpiment, and thallium minerals (USGS Model 26a). Geochemically, the Au ores typically are enriched in As, Ba, Hg, Sb, Si, and Tl and depleted in Ca, Mg, and Sr.
There are two primary Eocene-Oligocene intrusive phases in the district: Porphyry Hill biotite monzonite porphyry and Eagle Hill Rhyolite. The likely age for the Mercur mineralization is between 39 and 31.6 Ma; a late Eocene age is preferred based on the preponderance of mineralization of this age in the Oquirrh Mountains (Krahulec, 2011).

The initial discovery of the Mercur mining district was the “Silver Ledge” in 1869. The Camp Floyd district was organized and the town of Lewiston (known later as Mercur) was established the following year. For a period of about 11 years the oxidized silver ore was mined, but by 1881, the rich silver ore was exhausted and the district was temporarily abandoned. The district was later reorganized in 1894 after the discovery of gold ore.

During the silver mining period in 1879, a Bavarian immigrant/prospector named Arie Pinedo found a vein of cinnabar, which is a sulfide of mercury. He named his claim Mercur after the German word for mercury. This name stuck and eventually the mining district and former town site of Lewiston adopted the name of Mercur. In 1883, gold was found in an area called the Gold Ledge using assay methods. The miners were confused by the results because the gold was not visible to the naked eye and could not be recovered by panning or other gravitational methods. In 1890, the newly developed cyanide gold recovery method enabled the recovery of microscopic gold. In 1898, the Golden Gate mill in Mercur, the largest mill in North America at the time, was constructed to process 1,000 tons of ore per day. In 1902, the town of Mercur was destroyed by a fire and soon afterward be came a ghost town. The area rejuvenated in 1933 when gold prices increased from $20.67 to $35 per ounce. Gold mining ceased in the district during World War II after the passage of the Federal Mine Closing Act, which banned all gold mining in the United States during the war. At this time, siliceous ores were mined instead, and used as a flux agent by smelters. Gold production resumed in the district in 1983 and estimated ore reserves were 15 million tons at 0.09 ounces gold per ton of ore. As mining progressed, additional reserves were discovered, but by 1997 all gold deposits were exhausted and efforts to find additional economic reserves were unsuccessful. The mine closed after the last of the stockpiled ore was processed for its gold content and heap leaching was completed.