Summary:
Beryllium is produced from Spor Mountain, a super-large volcanic-hosted bertrandite deposit in Utah.
The Spor Mountain district is situated-on the western margin of the Thomas caldera, one of at least three volcanic subsidence structures formed during Oligocene time (Shawe, 1972). These structures lie in an east-west trending belt of igneous rocks and mineral deposits, called the “berylium belt of western Utah" (Cohenour, 1963), or the "Deep Creek- Tintic belt" (Hilpert and Roberts, 1964), which also includes other metal deposits outside the Spor Mountain district.
The western margin of the caldera, marked by a narrow zone of faults and landslide breccias, is located at the east side of Spor Mountain. Beginning in early Miocene time, normal faulting cut both Paleozoic rocks and Tertiary volcanic rocks, producing the fault-block structure and topography typical of basin-and-range systems. All the faults were potential pathways for mineralizing fluids.
Tertiary volcanic rocks of the Spor Mountain Formation consist of two members, the vitric tuff and an overlying porphyritic rhyolite. The tuff formation is dated at 21 million years (Lower Miocene). The two members occur together in most places and are restricted to the vicinity of Spor Mountain. The porphyritic rhyolite member crops out as flows, domes, and small plugs. (Davis 1984).
Deposit Types
Beryllium was discovered in the area in 1959 when a rockhound collected opal nodules from the vitric tuff unit. These were tested by a nuclear berylometer at Beryllium Resources Inc. Beryllium exploration started in the area in 1960.
In the Spor Mountain mining district, beryllium is concentrated in the upper part of the beryllium wiff member of the Spor Mountain Formation. Beryllium ore bodies are from 5 to 10 feet (1.5-3 m) thick and extend as much as 2 miles (4 km) along strike. In detail, the ore bodies are complex and offset by small faults. Basin-and-range faults, having hundreds of feet of offset, tilt the ore bodies 10 to 30 degrees west.
The downdip minable extent of these ore bodies, which can be as much as 1,000 feet (300 m), is limited by an overburden of hard topaz rhyolite. The host tuff unconformablv overlies older rocks and fills northeast trending paleovalleys. (Griffitts, 1964; Davis, 1984). The continuous extent of the tuff indicates that it probably covered most topographic features.
The berylium bearing vitric tuff rests unconformably on older volcanic rocks of Tertiary age and sedimentary rocks of Paleozoic age.
Mining operations by Materion within the beryllium tuff member have encountered many variations in particle size and composition of the ore zone. The beryllium tuff deposits have been partially altered by hydrothermal (epithermal) fluids to a fine-grained mixture of montmorillonite-kaolinite clay, potassium feldspar, silica minerals, and fluorite. Distinctive zones of argillic and feldspathic alteration enclose the beryllium deposits.
The bertrandite ore mineral of beryllium is submicroscoric, disseminated in the tuff, and can be concentrated in fluorite nodules.