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

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

Location: The Idaho prospect is located on the east bank of Lost River just below the mouth of Tin Creek. This is about 4 miles upstream of the mouth of Lost River on the Bering Sea and 1.9 miles downstream from the Lost River Mine (TE048-TE051). This is locality 7 of Cobb and Sainsbury (1972). Cobb (1975) summarized relevant references under the name 'Idaho'.
Geology: The Idaho prospect is on a low bench (about 200 feet elevation ) on the east bank of Lost River. Here Ordovician limestone in the hanging wall of the Rapid River fault is locally exposed along cut banks of Lost River and its east tributary Tin Creek. Extensive fluorite veining in the limestone merges northward across Tin Creek to that exposed in the Lost River Valley prospect. The original Idaho prospect was a small area of chalcopyrite-pyrrhotite-fluorite mineralization exposed on the east bank of Lost River (Knopf, 1908). Sainsbury noted the presence of tin, sulfide minerals, and fluorite (1969, plate 1) and of beryllium rock and sulfides in this general area (1969, plate 5). Three types of mineralization are present here (Hudson, 1990): (1) fluorite and beryllium-bearing veins and replacements in dolomite; (2) magnetite-fluorite-idocrase (?) replacement veins (laminar skarn or wrigglite) along fractures in the carbonate rocks; and (3) sulfide-rich veins (and replacements?). Dolomite outcrops along the south side of lower Tin Creek contain irregularly sheeted fluorite veins. Individual veins are up to 3 inches wide and zones up to 1 foot wide can have up to 20 thin veinlets. Channel and grab samples of this mineralization contain up to 19% fluorine and 0.023 to .063 % beryllium. The presence of 0.023% beryllium in 4-foot wide zones containing sparse fluorite veins suggests that some of the mineralization is disseminated. Laminar magnetite-fluorite-idocrase (?) replacement veins (wrigglite) are common in outcrop along the east bank of Lost River. These veins, which vary from hairline seams to 3 feet wide, contain elevated tin, tungsten, beryllium, fluorine, base-metal, and silver. Five samples (Hudson 1983; 1990) contained 0.14 to 0.36% tin, 0.06 to 0.26% tungsten, 0.012 to 0.029% beryllium, 13 to 15% fluorine, and 0.3 to 4.7 ppm silver. Zinc, determined in only one of these samples, was 1.05%. Highly oxidized sulfide-rich mineralization is present in a vein exposed in a dozer trench on the east bank of Lost River. The trench exposes a 4-foot wide oxidized zone that strikes northeast and dips south. Sulfide remnants in this zone are massive pyrrhotite-chalcopyrite-sphalerite rock with minor constituents such as fluorite. Four sulfide-rich samples contained up to 0.19% tungsten, 0.64 to 3.72% copper, 0.14 to 14.3% zinc, 5 to 35 ppm silver, and weakly anomalous tin and lead (Hudson, 1983; 1990). Airborne and ground magnetometer surveys show that a large, 600+ gamma positive magnetic anomaly is centered on the Idaho prospect area. This feature combined with the widespread surface mineralization and alteration, the results of a soil geochemistry survey, and structural setting (interpreted to be in a downdropped hanging wall block of the Rapid River fault) suggests the presence of significant skarn and/or replacement mineralization at depth (Hudson, 1983; 1990). A vertical diamond drill hole was drilled in 1995 to test this interpretation. This drill hole was collared slightly east of the sulfide vein exposed in the dozer trench. It reached a total depth of 983 feet (Drechsler, 1995). It encountered sulfide-bearing veins, veinlets, and disseminations in the upper 100 feet including one interval between 40.5 and 45 feet that contained up to 50% massive pyrrhotite and chalcopyrite. Steep dipping magnetite-calcsilicate veins and veinlets, less than 1 inch wide and commonly less than 0.25 inch wide, are common below 100 feet. These appear to increase in intensity below 500 feet including two intervals (640 to 678 feet and 938 to the bottom of the hole) that are stockworks. Fluorite-bearing veins are scattered through the hole but appear to be more abundant at depth. Two thin (0.25 and 0.75 inch wide) felsic dikes are present between 910 and 930 feet. The sulfide-rich interval between 40.5 and 45 feet assayed 1.74% copper, 2.2% zinc, 16 ppm silver, 3.79% CaF2, 250 ppm tin, and 55 ppm tungsten. Other selected intervals in the hole were anomalous in base metals, tin, and tungsten but significant grades were not intercepted. Two five-foot samples representing the interval between 432 and 442 feet contained 340 and 650 ppm tin and 540 and 320 tungsten; a sample representing the 715 to 720 foot interval contained 11% fluorite, 160 ppm tin, and 375 ppm tungsten; and two five-foot samples representing the interval between 818 and 828 feet contained 10 and 11% fluorite, 520 and 640 ppm tin, and 480 and 510 ppm tungsten.
Workings: One dozer trench, a surface soil geochemical survey, outcrop geologic mapping and sampling, an airborne magnetic survey, a ground magnetic survey, and one vertical 983-foot diamond drill hole have been completed on the prospect.
Age: The age of the mineralization is 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: All of the veining in the area can be considered a type of alteration in the outer fringes of a significant hydrothermal system. This includes fluorite, laminar magnetite-fluorite-idocrase, and the sulfide-rich veins. Dolomitization is also locally devloped but it is not known how this is related to mineralization.
Reserves: Not defined

Commodities (Major) - Ag, Cu, Pb, Sn, Zn; (Minor) - Fluorite, W
Development Status: None
Deposit Model: Skarn, vein, and replacement in limestone. Tin skarn (model 14b) and/or replac

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Link to USGS - Alaska:	TE040

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References