The White Reef
view southwest from Silver Reef
Silver Reef, Utah, Geology Highlights
Geological Formations as viewed from the Museum at Silver Reef, Utah
By Robert F. Biek
Senior Scientist
Utah Geological Survey
Summary -
The Silver Reef mining district is unusual because of its ore-grade silver chloride that occurs in sandstone.
The silver chloride, also known as horn silver or chlorargyrite, is an inconspicuous mineral normally associated with the weathered parts of silver sulfide deposits such as Nevada’s famous Comstock Lode.
The fact that it took 10 years between the discovery of silver in sandstone in 1866 and the great Pioche Stampede of 1876, which ushered in the silver boom at Silver Reef, gives an idea of the difficulty early prospectors had in believing economic silver could be found in sandstone.
The silver is found in a white sandstone called the Springdale Sandstone, which is exposed as tilted layers on the flanks of the Virgin anticline, a large fold or upwarp. The sandstone was deposited in braided-stream channels and contains petrified wood and carbonized plant fragments. It was the porosity and permeability of the sandstone, in combination with carbonaceous material and the structural traps created by the anticline, that led to favorable conditions for the precipitation of silver.
Detail -
The Silver Reef mining district consists of four "reefs" on the northeast-plunging nose of the Virgin anticline: White, Buckeye, and Butte Reefs are on the anticline's northwest flank, whereas East Reef is on the anticline's east flank. A "reef" is a mining term for a lode or vein, and was also used by early pioneers to describe prominent ridges that were a barrier to travel. Each reef is held up by resistant white sandstone that geologists call the Springdale Sandstone Member of the Kayenta Formation. This is the same ledge-forming sandstone exposed above the town of Springdale at the entrance to Zion Canyon, from which the rock formation gets its name.
Early miners thought that each reef represented a distinct sandstone bed and it wasn’t until the careful geologic mapping of the late Brigham Young University geologist Paul Proctor in the late 1940s and early 1950s that the geology of the district was finally understood. Paul showed that the district is simply the northern part of the 20-mile-long Virgin anticline, an upwarp with thrust faults on its northwest flank that caused repetition of the ore-bearing sandstone layer.
The Springdale Sandstone was deposited in braided stream channels and contains petrified wood and carbonized plant fragments. It was the porosity and permeability of the sandstone, in combination with carbonaceous material and the structural traps created by the anticline and faults, that led to favorable conditions for the precipitation of silver, copper, and uranium in the district.
The original source of the metals is thought to be rock layers rich in altered volcanic ash in the underlying Petrified Forest Member of the Chinle Formation. Thus, the silver, copper, and uranium are not related to nearby igneous rocks of the Pine Valley Mountains or to the much younger basaltic lava flows, but instead are ultimately derived from volcanic ash that drifted east from 200-million-year-old volcanoes at the western margin of North America. Sulfide-poor, chlorine-rich brines leached the metals from the volcanic ash. As the brines migrated through the Virgin anticline, they encountered porous and permeable, locally carbon-rich sandstone and the metals dropped out of solution.
Loose rocks of high-grade silver chloride were first discovered near Harrisburg in 1866, and in-situ mineralization was found in 1868, but it was not until 1876 that the silver rush was underway in earnest. That it took 10 years between the discovery of silver in sandstone and the great Pioche Stampede, which ushered in the silver boom at Silver Reef (and the final collapse of Pioche, Nevada), gives an idea of the difficulty early prospectors had in believing economic silver could be found in sandstone.
The silver chloride is contained in chlorargyrite, an inconspicuous mineral normally associated with the weathered portions of silver-bearing sulfide deposits such as Nevada’s famous Comstock Lode. The principal mining activity in the district lasted just 12 years, through 1888, with lessee operations through 1909, after which major mining essentially ceased.
Prior to 1910, the district produced over 7 million ounces of silver from ore that averaged 20 to 50 ounces silver per ton, nearly 70 percent of which came from the prolific Buckeye Reef. Sporadic production between 1949 and 1968 yielded about 10 ounces of gold, 165,000 ounces of silver, 34 short tons of copper, and at least 2500 pounds of uranium oxide.
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