Relation of Mercury, Uranium, and Lithium Deposits to the McDermitt Caldera Complex, Nevada-Oregon

Abstract

The McDermitt caldera complex, located along the Nevada-Oregon border, is a Miocene collapse structure 45 kilometer in diameter. Large-volume rhyolitic and peralkaline ash-flow tuffs were erupted from 17.9 to 15.8 m.y. ago, leading to the formation of overlapping and nested calderas. Emplacement of rhyolitic ring domes, located primarily along the western margin of the calderas, represents the last phase of volcanic activity. The complex is the site of large deposits of mercury, an are deposit and several occurrences of uranium, and widespread occurrences of lithium. Mercury deposits at Cordero, McDermitt, Bretz, Ruja, and Opalite occur along ring fractures in sedimentary rocks that °fill the calderas. Near the deposits the rocks are altered to zeolites and within the ore zones to potassium feldspar and silicate minerals. Although the mercury deposits contain anomalous concentrations of uranium, the most important uranium occurrences are restricted to rhyolitic ring domes emplaced along the western margin of the calderas. 1 Lithium occurrences are located in tuffaceous rocks that are altered to zeolites and potassium feldspar. Concentrations of lithium ranging from 0.1 to 0.68 percent are associated with the clay mineral hectorite. The rhyolitic rocks erupted from the McDermitt caldera complex are enriched in mercury, uranium, and lithium and are likely source rocks for these elements in the ore deposits. Tuffaceous caldera fill-sediment averages 0.29 ppm mercury, 22 ppm uranium, and 236 ppm lithium; large-volume ash-flow tuffs contain up to 0.26 ppm mercury, 20 ppm uranium, and 300 ppm lithium.

Evolution of volcanic and tectonic features in caldera settings and their importance in localization of ore deposits

“…This environment has not generally been a site for exploration, so that examples of deposits are not common. In the McDermitt caldera complex (Rytuba and McKee, 1984), the ring fracture fault of the Long Ridge caldera provides the structural control for the Cordero mercury deposit (Rytuba and Glanzman, 1979 The Atlanta district, Nevada, is •tnother district in which precious metal deposits outside a caldera may be related to early tumescent structures. Deposits outside the structural boundary of the Indian Peak caldera are localized along inward-dipping normal faults (Miller et al, 1989).…”

“…Water in the moat may become important as a recharge reservoir for hydrothermal systems focused along the ring fracture. In the McDermitt caldera complex, the ring fracture faults of the Long Ridge and Jordan Meadow calderas localized hydrothermal mercury and uranium deposits (Rytuba and Glanzman, 1979), as well as molybdenum and precious metal deposits. Where the hydrothermal fluids reached the moat sedimentary sequence, they spread laterally along permeable horizons to form stratiform mercury, antimony, and uranium deposits (Rytuba, 1981).…”

Eruptive history of the rhyolitic Kane Springs Wash Volcanic Center, Nevada

“…The most abundant outcrops of these basalts are along the edges of Kane Springs Wash, suggesting they erupted during its formation. [Rytuba and Glanzman, 1979], and some granitic ring complexes of Nigeria [Turner, 1976]. In particular, many parallels can be drawn between the Spearhead member of the Thirsty Canyon Tuff and succeeding lavas at the Black Mountain center, and the lavas and tuffs of the Kane Springs Wash center.…”

“…Cognate trachyte inclusions are commonly found at the tops of voluminous peralkaline tuff sheets at these two centers, suggesting the magma erupted as ash-flow tuff may be a relatively thin, volatile-charged zone capping a trachytic dominant volume. [Smith, 1979;Hildreth et al, 1980]. Emplacement of the central trachyte-syenite complex within the Kane Springs Wash caldera is time-equivalent to resurgent doming found in many continental calderas [Smith and Bailey, 1968;Bailey et al, 1976;Steven and Lipman, 1976].…”

“…Cook [1965] named and described The Kane Springs Wash center, located in the eastern Basin and Range province, is one of a number of volcanic centers ringing the Basin and Range that erupted peralkaline tuffs 16-14 Ma ago. These include the Silent Canyon center [Orkild et al, 1968], parts of the McDermitt complex [Rytuba and Glanzman, 1979], the Caliente cauldron complex [Noble and McKee, 1972], and several centers in northeastern Nevada [Noble et al, 1968[Noble et al, , 1970Korringa, 1973]. The Kane Springs Wash center lies along an east-west belt of predominantly silicic volcanic rocks ~15 to ~6 Ma in age which is the southern terminus of a southward sweep of volcanism through the Basin and Range during the Tertiary [Stewart et al, 1977].…”