Abstract

The Ashbrook mining district is located in the Goose Creek Mountains of extreme northwestern Box Elder County, Utah. The initial prospecting in the district was reportedly done in 1864, the first claim staked in 1873, and the district was organized in 1874. The Ashbrook district saw limited, sporadic production until 1920, when the Vipont mine became a strong silver-gold producer for four years. Erratic production continued from 1924 until the federal precious metal mine closure act of 1941 halted production for the duration of WW II. The district remained basically idle from 1942 until the old mill tailings were reprocessed by heap leaching from 1978 to 1984. Past district production amounts to about 180,000 tons of ore which yielded 3,440,000 ounces of silver (19 oz/t). The Vipont property still hosts an open-pitable mineral resource of approximately 430,000 tons at an average grade of 5.06 oz/t Ag and 0.01 oz/t Au (over 2 million ounces of silver contained) and several suggested exploration targets remain untested.

Geologically, the Ashbrook district had a long and complex history. The strata in the area of the Vipont mine consist of a sequence of Paleozoic quartzites, carbonates, shales, and sandstones. These units have been subjected to Late Jurassic to Early Cretaceous thrusting, folding, and low-grade metamorphism followed by severe Eocene to Miocene extension and intrusion by Miocene (±13 Ma) granite and rhyolite porphyry plugs, dikes, and sills. Mineralization is related to the rhyolitic intrusive bodies and the ore deposits have not undergone significant deformation.

Mineralization at the Vipont mine primarily consists of silver-rich veins, mantos, and stratiform disseminated ores hosted primarily by the Mississippian (?) Vipont limestone (Tripon Pass Limestone?) in a gently southwest-plunging syncline. The ore bodies are formed along favorable northeast-trending crenulations and faults within the syncline. A typical individual ore body may be 500 feet long, by 100 feet wide, by 8 feet thick. Mineralization generally occurs either near the hanging wall or footwall contacts of the Vipont limestone. Persistent mineralization is known to extend down plunge for about a half mile and has been accessed by stopes on numerous mine levels.

The Vipont ore bodies are northeast-elongate, lensoidal, stratiform zones of void filling, disseminated, replacement, and veinlet-style sulfide mineralization. The total sulfide content of the ore probably averages less than 10 weight percent. The primary hypogene minerals associated with Vipont ore, in addition to quartz and rhodochrosite, are (in general decreasing order of abundance) pyrite, arsenopyrite, galena, sphalerite, chalcopyrite, pyrargyrite, tennantite, pearceite, and argyrodite. Argentite, apparently supergene, is the most common ore mineral exploited in the oxidized zone. The Vipont ores are best classified a distal disseminated deposits (USGS model 19c), but may represent a transitional stage between the distal disseminated ores and polymetallic vein and replacement deposits (USGS model 19a).