Rhyolite Prospect, Aniak Mining District, Bethel Census Area, Alaska, USAi

Regional Level Types
Rhyolite Prospect	Prospect
Aniak Mining District	Mining District
Bethel Census Area	Census Area
Alaska	State
USA	Country

Location: The Rhyolite prospect is on the south slope of Juninggulra Mountain on the divide between the Iditarod and Kuskokwim Rivers. It is marked by prospect pits and trenches over an area about a square mile in the S1/2 sec. 35; and SW1/4 sec. 36, T. 22N., R. 50 W., of the Seward Meridian. Some of the mine dumps are shown on the 1:63,360-scale USGS topographic map. The coordinates are at the center of the area. The prospect is locality 4 of Miller and others (1989). The location is accurate. (The location of this prospect in Cobb (1972, [MF 368]) is incorrect.)
Geology: The rocks in the vicinity of Juninggulra Mountain are mainly sandstone and shale of the Upper Cretaceous, Kuskokwim Group. The sediments are intruded by a multiphase, granite porphyry, intrusive complex at least 3.5 miles long and 1.2 mile wide (Cady and others, 1955; Bundtzen and others, 1998). Sainsbury and MacKevett (1965) also show several subordinate types of igneous rocks on Juninggulra Mountain including small diabase or lamprophyre dikes containing ubiquitous specks of pyrite, and amygdaloidal intermediate dikes. A large sill of the granite porphyry cuts these older, more mafic dikes. The Juninggulra Mountain igneous complex is along the trend of the Donlin Creek dike and sill swarm in the southwest Iditarod quadrangle ARDF ID171 to ARDF ID179 (Bundtzen, Miller, and Hawley, 2004). Gray, Gent, and others (1997) reported that sericite from vein mineralization at the Rhyolite prospect has a 40Ar/39Ar age of 70.9 Ma, or about the same age of vein mineralization radiometrically dated in the Donlin dike and sill swarm (Bundtzen and Miller, 1997; Szumigala and others, 2000). At the Rhyolite prospect, cinnabar-stibnite mineralization is confined to steeply dipping, lenses and veins in silica-carbonate altered zones within a large granite porphyry sill (Bundtzen and others, 1998). The cinnabar occurs in three, widely-spaced locations in fractured dike rock and graywacke, and is disseminated in altered dike rocks. The cinnabar occurs as masses from 0.4 to 4 inches thick; the largest masses are at the intersections of the veins (Sainsbury and MacKevett, 1965). Most of the mineralized veinlets are associated with N50W steeply dipping faults. The primary mercury mineral is metacinnabar (Bundtzen and others, 1998). Samples assayed by the U.S. Geological Survey and the U.S. Bureau of Mines contained from less than 0.02 to 54.0 percent mercury and from less than 0.05 to 66.3 percent antimony (Maloney, 1962, 1968; Meyers, 1985). Metacinnabar, quartz, and carbonate veins sampled by Bundtzen and others (1998) also contained up to 574 parts per million (ppm) zinc, 136 ppm arsenic, 279 ppm antimony, 0.8 ppm silver, and 21 ppm tin.
Workings: A few stringers of cinnabar were discovered by prospectors in 1953. R.F. Lyman and Joe Stuver did bulldozer trenching in 1956 and 1957 (Meyers, 1985). In 1958 and 1959, the U.S. Bureau of Mines explored the Rhyolite prospect with about 3,000 feet of trenching and sampled many exposures of mineralization (Maloney, 1962, 1968). C.L. Sainsbury geologically mapped the Rhyolite prospect by plane table in 1959 (Sainsbury and McKevett, 1965). The prospect was restaked by Henry Waterford in 1971. In 1974, Resource Associates of Alaska, Inc. conducted surface sampling of the prospect on behalf of Calista Corporation and outlined two zones with elevated mercury values (Muntzert and others, 1975). Bundtzen and others (1998) produced a 1:63,360-scale geologic map of the Juninggulra Mountain area and sampled selected trenches at the Rhyolite prospect.
Age: Gray and others (1997) reported that sericite from vein mineralization at the Rhyolite prospect has a 40Ar/39Ar age of 70.9 Ma.
Alteration: Sericitic alteration of granite porphyry; silica carbonate alteration of mafic dikes and the largest granite porphyry sill.

Commodities (Major) - Hg, Sb; (Minor) - Ag, As, Sn, Zn
Development Status: None
Deposit Model: Silica-carbonate mercury (Cox and Singer, 1986; model 27c).

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Link to USGS - Alaska:	SM008

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References