Washington Dome Mining District, Washington County, Utah, USAi

Regional Level Types
Washington Dome Mining District	Mining District
Washington County	County
Utah	State
USA	Country

The Washington Dome mining district is located about 5 mi east of St. George in south-central Washington County. The district has no recorded production but includes a series of small prospects for Au-As-Hg. One prospect on the north flank of Washington Dome attempted Hg development (metacinnabar?) in the 1950s and a prospect on the east flank of the dome exposed some gypsum beds. The Au potential of the area was not initially recognized until prospectors in the 1980s received Au values from some jasperoid samples. The Washington Dome property was then extensively drilled as a grassroots Au project in the 1990s (Hayden, 2005).
The Washington Dome district is geologically situated in the transition zone from the Basin and Range Province to the Colorado Plateau. Washington Dome itself is a 2-mi-long, doubly plunging, anticlinal segment of the Sevier-age, northeast-trending Virgin anticline. The Virgin anticline is a major structural element of the transition zone, extending for 25 mi from the Arizona-Utah border on the southwest to Zion National Park on the northeast. The anticline is typically a broad to open, symmetrical fold with an amplitude of over 2000 ft and a wavelength of 4 to 6 mi. The oldest rocks exposed in the core of the dome are Early Permian Harrisburg Member of the Kaibab Formation. Just west of Washington Dome the Basin and Range-age Washington normal fault drops the Paleozoic strata under Quaternary alluvium (Hayden, 2005).
The principal Au prospects are found on the southeast flank of Washington Dome. Mineralization occurs in a 50- to 100-ft-thick silty limestone in the upper part of the Harrisburg Member of the Kaibab Formation. The mineralized zone is elongated parallel to the anticline and along reverse and normal faults over an area about 6000 ft long and 500 ft wide. The mineralization is localized at the intersection of north-south or northeast trending faults and favorable porous, permeable, and reactive silty and sandy limestone. Silicification is the dominant alteration accompanying Au mineralization, but decalcification, dissolution of gypsum, and the introduction of pyrite are also common. Jasperoid, quartz veinlets, calcite veins, and jarosite, hematite, and green As-staining (scorodite?) are all recognized locally. The area has widespread weak Au, >0.1 ppm, with strong As to 10,000 ppm, Sb to 220 ppm, Hg to 200 ppm, and Tl to 10 ppm. Three zones of weak mineralization were intersected by drilling, all on the southeast flank of the dome (Krahulec, 2011). The best hole intersected 200 ft of approximately 0.9 ppm Au from 105 to 305 ft depth. Washington Dome has the classic alteration and geochemical trace element patterns of a sediment-hosted Au system (USGS Model 19c).
Washington Dome is partially inside the Washington City limits and the private lands are being heavily developed for residential use.