Rapid River Prospect, Lost River Valley, Port Clarence Mining District, Nome Census Area, Alaska, USAi

Regional Level Types
Rapid River Prospect	Prospect
Lost River Valley	Basin
Port Clarence Mining District	Mining District
Nome Census Area	Census Area
Alaska	State
USA	Country

Location: Rapid River is the major west tributary of Lost River. Its mouth is located one mile upstream from the mouth of Lost River on the Bering Sea. This prospect is located in the valley of Rapid River, about 5.5 miles upstream from its confluence with Lost River. The area of mineralization extends across Rapid River valley but it is best developed on a small knoll on the north side of the drainage at 471 feet elevation. This is locality 3 of Cobb and Sainsbury (1972). Cobb (1975) summarized relevant references under the name 'Rapid R.'.
Geology: Banded fluorite-chrysoberyl veinlets, veins, pipes, and irregular replacements are present in Lower Ordovician limestone and dolomite in the footwall of the Rapid River fault. Sainsbury (1969; 1972) maps the Rapid River fault as a 12-mile long east-west trending thrust fault in the southern part of the York Mountains although stratigraphic relations across the fault suggest normal displacement. The fluorite-chyrsoberyl mineralization is locally present over an area 4,400 feet long (east-west) and about 1,000 feet wide. Lamprophyre dikes are also common in this area and some dike borders localize fluorite-chyrsoberyl replacements. Individual veins are generally 1 to 3 inches wide and spaced 6 to 18 inches apart but some irregular replacements are up to 25 feet across. Structures with a general east-west orientation, such as joints, small faults, and dike borders are a dominant control on mineralization. The mineralization includes minor euclase, bertrandite, and phenakite (?). Gangue minerals are diaspore, white mica, tourmaline, and hematite; fine-grained silica is locally present along the mineralized zones. BeO contents of 12 samples range from 0.15 to 1.05%; fluorite content of two samples is 57% (Sainsbury, 1963, p. 11). Diamond-drilling by Newmont Mining Company in 1963 encountered mineralization in the footwall of the Rapid River fault a few hundred feet below the surface. This drilling encountered sulfide mineralization in the cores of some veins. The sulfide minerals include galena and sphalerite; minor cassiterite is associated with the sulfides (Sainsbury, 1969, p. 77). Newmont attempted to find this early drill data in 1998 but was unsuccessful. However, the old project files did contain a report on metallugical testing of samples obtained by the 1963 drilling (Porter, 1964). One sample, a composite from drill holes 9 through 14, weighed 100 pounds and containted 12.0% CaF2, 0.086% BeO, and 56.4% CaCO3. A second sample, a composite from drill holes 2, 8, 21, 22, and a 10-foot channel in a surface trench, weighed 50 pounds and contained 27.4% CaF2, 0.22% BeO, and 34.5% CaCO3. Bench-scale sink-float tests showed that this material could be upgraded to about 50% CaF2 and 0.35% BeO. The metallugical test results indicated that the beryllium was associated with silicate minerals and did not directly follow fluorite through the procedures.
Workings: A few dozer trenches and several diamond drill holes have been completed on the prospect.
Age: Assumed to be related to the development of tin systems in the Lost River area and therefore Late Cretaceous, the age of the tin-mineralizing granites there (Hudson and Arth, 1983).
Alteration: Some lamprophere dikes have tactite borders but these may not be related to mineralization. The origin of dolomite and dolomite breccia is also not clear here. Mass balance calculations show significant SiO2, Al2O3, alkali, and fluorine enrichment with mineralization (Sainsbury, 1968, p. 1567).
Reserves: Not defined

Commodities (Major) - Be, fluorite; (Minor) - Ag(?), Cu, Pb, Sn, Zn
Development Status: None
Deposit Model: Fluorite- and beryllium-bearing veins and replacements in Ordovician limestone

Other Databases

Link to USGS - Alaska:	TE036

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References