Bull Valley Mining District (Bull Valley), Washington County, Utah, USAi

Regional Level Types
Bull Valley Mining District (Bull Valley)	Mining District
Washington County	County
Utah	State
USA	Country

The Bull Valley mining district is in the rugged Bull Valley Mountains of northwestern Washington County, about 26 mi north- northwest of St. George. The district was organized in 1903 but no production has been recorded. However, some exploration was carried out by the U.S. Bureau of Mines in the 1940s with some additional drilling by U.S. Steel in the 1950s. Little work has been reported in the last half century. The Pilot Fe deposit has had the bulk of the exploration and has the largest known Fe resource in the district (Bullock, 1970).
The Bull Valley district lies on the Miocene-age, east-northeast-trending Iron axis of the Basin and Range Province. The Iron axis is a Sevier-age anticline intruded by Miocene quartz monzonites. In broad terms, the geology of the district is Jurassic and Cretaceous sedimentary strata overlain by a series of Miocene volcanic rocks. The sedimentary rocks in the district are passively intruded by an east-northeast-trending series of three small, Miocene (22 Ma) quartz monzonite porphyry stocks. This whole package is cut by a series of east-west-trending strike-slip faults, mostly having right-lateral displacement. The units exposed in the district are predominantly the Jurassic Carmel Formation carbonate and sandstone including the Homestake Limestone Member, Cretaceous Iron Springs Formation sandstone and shale, and Miocene Rencher Formation quartz latitic, welded, crystal-rich, ash-flow tuff and tuff breccia. The Homestake Limestone Member is the main host in the large Iron Springs Fe district to the northeast. The Bull Valley quartz monzonite porphyry stock, the largest of the three stocks, is a fine-grained quartz monzonite having phenocrysts of plagioclase, hornblende, and augite in a groundmass of quartz, K-spar, magnetite, apatite, zircon, and titanite. The stock domed the sedimentary rocks until it ultimately erupted violently producing a finer grained vent phase and the coeval Rencher Formation volcanics (Bullock, 1970; Biek and others, 2009).
The Bull Valley Fe orebodies are principally contact metasomatic deposits, breccia fillings and replacements, and veins associated with the porphyry stocks. The most important deposit in the district, the Pilot orebody, is a contact metasomatic- hydrothermal replacement of volcanics with thin intercalated limestones adjacent to the Bull Valley stock. The ores occur in volcanic xenoliths and limestone and volcanic replacements along the northeast margin of the stock. The ore zone dips north at about 20°. The Pilot orebody is an irregularly shaped zone, 400 ft thick and 1000 ft wide, and has been followed 900 ft or more down dip. Drill intersections range from 10% to 60% Fe. The main reported ore mineral is hematite with lessor magnetite and martite in a gangue of quartz, calcite, and apatite. A magnetic high covers the down dip extension of this orebody, despite the predominance of hematite in the orebody. The Pilot deposit has a subeconomic inferred resource of 15 to 20 million tons at 30.7% Fe (Bullock, 1970).