McDermitt Mine, Opalite Mining District, Humboldt County, Nevada, USAi

Regional Level Types
McDermitt Mine	Group of Mines (Inactive)
Opalite Mining District	Mining District
Humboldt County	County
Nevada	State
USA	Country

A former Hg mine with Li occurrence located in secs. 27, 28, 33 & 34, T47N, R37E, MDM. The McDermitt mine itself is a large open-cut mercury mine (active 1975-1990), but the name is often used to more broadly include the adjacent older Cordero mine complex.

The Cordero mines and the McDermitt mine are located in the Opalite mercury mining district in the United States of America. The Opalite mercury mining district encompasses locations that are both in the state of northern Nevada and in the state of southern Oregon. The deposits are all related to a large volcanic center (caldera complex) called the McDermitt caldera complex. Mines in the Opalite district include those listed below^[1][2][3].

Bretz Mine, Malheur Co., OR
Cordero Mine, Humboldt Co., NV
Corderito Mine, Humboldt Co., NV
Crofoot Project, Humboldt Co., NV
Disaster Peak Property, Humboldt Co., NV
Lenway Mine, Humboldt Co., NV
McDermitt Mine, Humboldt Co., NV
Opalite Mine, Malheur Co., OR
Ruja Mine, Humboldt Co., NV

The Cordero mine was the first discovery of mercury in the district, quickly followed by the Bretz and Opalite mines to the north. These were both underground and open cut operations. In the 1970s, mercury was discovered in tuffaceous lake sediments adjacent to and just north of the Cordero mine. This deposit was exploited exclusively by open cut mining. It was named the McDermitt mine, and operated by Placer Amex until acquired by Barrick Gold Corporation. The McDermitt mine was the last operating primary mercury production mine in the USA when it ceased operations in 1990. The patented mining claims of the McDermitt mine were still owned by Barrick Gold as of March 2016, but the mine is closed and there is no mining activity in the district.

Mineralization is a Hg deposit with ancillary Li hosted in the Miocene Cordero Rhyolite. Cinnabar represents 50% of the ore and corderoite is 50%. U- and Li-enriched rhyolite was observed.

The following summary prepared by Michael Cox is drawn from the extensive body of references and more than five weeks of detailed field examination of the pit during the period of late 2014 to early 2017.

The McDermitt open pit mine was excavated in lacustrine and/or fluvial tuff under a thin mantle of recent (Pleistocene) alluvial gravel. According to Giraud^[4] (1986) and McCormack^[5] (1986) the tuff was only locally reworked by water prior to consolidation. Fragments are nearly always angular, suggesting the source material was unconsolidated pyroclastic debris rather than eroded consolidated rock layers. The parent rock is pyroclastic, with angular fragments of peralkaline rhyolite that contains alkali feldspar phenocrysts, sometimes several centimeters long or larger. During violent gaseous eruptions, the parent magma was fragmented at the vent, blown upward, and deposited on the surface, including into and over lakes and streams. The resulting pyroclastic ash and debris was laid down on the surface of the land existing at the time and later consolidated into rock. The volcanic rocks vary with depth and age, but the main ash deposit is broadly called McDermitt Tuff by Henry et al.^[6] (2016), and according to them, the main eruption of tuff occurred at 16.35±0.03 Ma. After the main tuff eruption, the volcanic source chamber collapsed to form an oval surface depression called a caldera. The McDermitt caldera measures 25 miles (ca. 40 km) north-south by 14 miles (ca. 23 km) to 19 miles (ca. 31 km) east-west. Hydrothermal fluids then deposited the antimony and mercury mineralization and silicified the parent tuff. This occurred at about 15.7±0.4 Ma, based on Noble et al.^[7] (1988) dating adularia from the open pit ore. Sometimes the silica and ore replacement is so thorough the parent rock textures and grains can no longer be recognized, even in thin sections under the microscope.

Other Databases

Link to USGS MRDS:	10197229

Other Regions, Features and Areas containing this locality

References