South Pass Greenstone Belt - Economic Geology

Economic Geology

Gold was discovered in the Lewiston district of the South Pass granite-greenstone belt in 1842 along the eastern margin of the complex. Other discoveries followed including the discovery of high-grade gold mineralization at the Carissa Mine along the western flank of the belt in 1867. Some high-grade gold was recovered from the Lewiston and the South Pass-Atlantic City districts. One large 34-ounce nugget resides in the Los Angeles Museum of Natural History that was recovered from Rock Creek along the western flank of the greenstone belt.

In addition to gold, significant iron ore deposits were found along the northern edge of the South Pass greenstone belt. Some minor copper, silver, tungsten, asbestos, beryl (aquamarine), ruby including a diamond have also been reported within the region. Development of the large taconite (iron ore) deposits in the 1950s led to the opening of the Atlantic City iron mine, which produced more than 90 million tons of iron ore between 1962 and 1983 from banded iron formation in the Goldman Meadows Formation. Extensive gold development took place across the entire Precambrian greenstone belt in the late 1860s to early 1870s, and again in the 1930s until World War II. Lode deposits occur primarily in strike-trending shear zones within metagreywacke and hornblendic orthoamphibolites, and placer operations have developed at various times along most streams downstream from these shear structures.

Hausel reported cursory investigations of the historic Carissa Mine during the late 1980s indicated this mine hosted an identifiable reserve of more than 109,000 ounces averaging 0.343 to 0.850 oz/ton with a cutoff grade of 0.299 oz/ton. But due to scattered drilling and lack of deep drilling on this property, this high-grade zone is probably much more extensive along strike and depth. The high cutoff grade combined with the essentially untested large tonnage low-grade mineralized zone associated with the property (>1000 feet wide and >1000 feet long) suggests actual gold resources are much greater. In addition to the Carissa property, a distinct belt of prospects and historic mines lie between the Carissa mine and Miners Delight mine along a 4 to 5 mile northeasterly trend in the belt. This provides a structurally favorable zone for a major gold resource. Mapping by Hausel identified numerous shear structures along with steeply plunging ore shoots associated with isoclinal and open folding that may be favorable for saddle reef-type gold mineralization. There is little evidence that any of the historic operations recognized the steeply plunging structures as being significant.

The margins of the greenstone belt contain major gold anomalies within Tertiary sediments eroded from the greenstone belt. The Cathedral Bluffs Tongue of the Eocene Wasatch Formation, is estimated to host more than 28.5 million ounces of placer gold in the Dickie Springs-Oregon Gulch area.

Some aquamarine beryl is known in pegmatite in the South Pass Granite in the Anderson Ridge area in the northwestern corner of the belt. Non-gem beryl is reported from several coarse-grained tourmaline-beryl granite pegmatites in the area.