West Dip Mining District, Tooele County, Utah, USAi

Regional Level Types
West Dip Mining District	Mining District
Tooele County	County
Utah	State
USA	Country

The West Dip (West Mercur) mining district is located on the southwest flank of the Oquirrh Mountains 15 mi south of Tooele in Tooele County. The district is a significant Au producer with a poorly documented historical production of about 36,900 ounces of Au between 1897 and 1941. Early Au production was severely hampered by the difficult metallurgy of the ores (Gilluly, 1932). Total district metal production at modern metal prices is estimated at $41 million. The district was heavily explored in the 1980s and 1990s for Au without ensuing development. The largest producers are the La Cigale and Daisy Au mines.
West Dip lies on the south flank of the Bingham–Park City mineral belt in the Basin and Range Province. West Dip is elongated on the southwest limb of the Ophir anticline. The Ophir anticline is a closed, asymmetrical thrust-cored anticline with a steep northeast limb and a gentler limb on the southwest, where Paleozoic sedimentary rocks dip about 55° west-southwest (Kroko and Bruhn, 1992). Mineralization occurs in association with the West Mercur fault along the range front and, to a much lesser extent, the subparallel Lakes of Killarney fault about a mile to the east. The West Mercur fault is approximately bedding- parallel (dipping 40°–60° southwest) and appears to have developed near the base of the Upper Mississippian Manning Canyon Shale. The southwest-dipping (55°–80°) Lakes of Killarney fault appears to be stratigraphically controlled near the contact between the Long Trail Shale and the Mercur member of the Middle Mississippian lower Great Blue Limestone in the footwall (Gilluly, 1932).
The old underground workings along the West Mercur fault, including the La Cigale, Omaha, and Daisy Au mines, generally consist of shafts inclined to the west-southwest. In many of these workings, the footwall is upper Great Blue Limestone and the hanging wall may be Manning Canyon Shale or lithified Quaternary alluvium (Gilluly, 1932). Better grade Ag mineralization occurs in jasperoids to the north and Au grades increase southward in argillized, bioclastic limestone having remobilized carbon (USGS Model 26a). The carbonate rocks adjacent to a narrow dike, east and uphill of the main West Dip workings, are also reportedly anomalous in Au, As, Hg, Sb, and Tl. The main West Dip ore deposits are associated with bleached, decalcified, argillized, weakly silicified host rocks and contain microscopic Au. As at the Mercur deposits, directly across the Ophir anticline to the east, geochemical work found the highest Au grades were associated with argillized and Fe-stained carbonate rocks. Some spotty, refractory ore grade pods of sedimentary rock-hosted Au mineralization having high carbon and pyrite were drilled near the Daisy mine. The mineralization at West Dip is very similar to the ores in the Mercur district, but the ores are higher in the stratigraphic section, underlying the Manning Canyon Shale rather than the Long Trail Shale at Mercur.