Stateline Mining District, Iron County, Utah, USAi

Regional Level Types
Stateline Mining District	Mining District
Iron County	County
Utah	State
USA	Country

The Stateline mining district lies along the Nevada border in extreme northwestern Iron County, 57 mi west-northwest of Cedar City and 7 mi north of the Gold Springs district. The Stateline district veins were discovered in the early 1890s and by 1896 most of the known mines had been discovered. Some on-site mills were erected but were not very profitable. The district was a modest Au-Ag producer (about 2540 ounces Au), nearly all from Utah, with sporadic production between about 1897 and 1948 (Thomson and Perry, 1975). Total district metal production at modern metal prices is estimated at $4.3 million. The principal operations were the Ofer, Johnny, and Gold Dome underground mines.
The Stateline district lies in the Paradise Mountains of the Basin and Range Province. The rocks exposed at Stateline consist of a thick sequence of Miocene volcanic rocks. From older to younger the sequence consists of andesitic flows, ash-flow tuffs, and rhyolitic flows. These volcanic rocks are cut by pale olive rhyolite dikes (~16.5 Ma), which are locally cut by small quartz veins near the Ofer mine. Precious metal production is derived from a series of conjugate, north-northeast-trending (N. 5°–20° E.) and more productive west-northwest-trending (N. 70°–80° W.), epithermal, low-sulfidation, brecciated, quartz-adularia- carbonate-pyrite ±fluorite veins (USGS Model 25c), and host the district’s principal mines, the Ofer, Johnny, Gold Dome, and Margarette. The veins are typically about 3 to 20 ft wide and can be traced on the surface for several hundred to a few thousand feet. The north-northeast-trending veins dip to the west at about 60° and the west-northwest striking veins dip an average of 65° north (Smith, 1902). The Au:Ag ratio of the ores increases from west to east across the district (Smith, 1902). The Ofer mine to the west produced high-grade Ag ore (700 ppm Ag) and the Johnny mine to the east averaged just 110 ppm Ag, but had better Au values (Thomson and Perry, 1975).
The ore is in localized shoots within the veins. The ore shoots on the east-west vein set rake about 45° NW approximately paralleling the intersection of the two vein sets. Wall rocks of the veins are generally argillized. The principal Ag mineral in the district is chlorargyrite associated with other unidentified non-sulfide Ag-bearing minerals in the Fe, Mn, and Mo oxides. The Ofer and Johnny mines reached primary argentite, sylvanite, and pyrite ore (Thomson and Perry, 1975). A 0.2-sq-mi area of pervasive phyllic alteration, disseminated pyrite, quartz veining, and anomalous Mo (to 7600 ppm Mo) occurs at and south of the Ofer mine.