Lost River-Cassiterite dike exogreisen Mine, Port Clarence District, Nome Borough, Alaska, USA
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Geology: The pre-mineral Cassiterite dike crosscuts Ordovician limestone and dolomite above a buried and mineralized granite cupola. The dike strikes northwest, dips moderately south, and extends at depth into parts of the Lost River skarn deposit. It ranges in thickness between 3 and 21 feet but in areas of previous stoping, widths of 5 to 10 feet are common (Sainsbury, 1964, plate 10). It is extensively altered over 2,200 feet of strike in the mine area. The dike was probably emplaced along a fault and some post mineralization displacement on this structure has occurred (Sainsbury, 1964, p. 10). Originally a leucocratic and porphyritic felsic rock, the dike is extensively replaced by quartz-topaz-fluorite greisen with disseminated cassiterite and sulfide minerals such as stannite, arsenopyrite, pyrite, galena, chalcopyrite, and sphalerite. Sulfide-rich veinlets containing cassiterite crosscut the greisen in many places. Wolframite is present in greater amounts in deeper parts of the mineralized dike where its mode of occurrence is similar to that of cassiterite, including its presence in crosscutting sulfide-bearing veins. Overprinting clay (kaolinite) alteration is common throughout the deposit. This alteration can completely obliterate preexisting textures and mineralogy, leaving only cassiterite grains in a clay matrix. Sainsbury (1964, p. 36) emphasizes that the clay alteration was superimposed on previously mineralized rock and that it did not affect tin distribution. Mining operations in the 1950's produced 309 tons of tin from 51,000 tons of ore averaging 1.13% tin (Lorain and others, 1958). Tungsten was not recovered during these operations. Some parts of the deposit were of higher grade; distinct ore shoots with greater than 2% tin were present (Hudson and Reed, 1997, p. 458). The higher grade tungsten zones contained 0.8% WO3. Sainsbury (1964, p. 50) has calculated reserves for two types of ore; 200,500 tons grading 1.3% tin and 0.125% WO3 and 105,000 tons grading 0.76% tin and 0.6% WO3. Sainsbury (1964, p. 51) suggests that the known and inferred ore with greater than 1% combined tin and WO3 could be about 430,000 tons. The deposit is open to the southeast and potential exists to the west on the other side of Cassiterite Creek.
Workings: In addition to many surface trenches, significant underground workings exist at the Lost River Mine. Most of these are on the Cassiterite dike but some deeper exploratory drifts encountered the buried Lost River granite cupola. The underground workings include adits, drifts, declines, raises, and shafts that total several thousand feet in length (Sainsbury, 1964, plate 10). These workings are developed on five levels with over 500 feet of vertical extent and with individual drifts being up to 1,100 feet long. Many diamond drill holes have been completed from both the surface and underground.
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). Fine-grained, leucocratic granite collected from a Lost River Mine dump has been dated at 80.2 +/- 2.9 my (Hudson and Arth, 1983, p.769).
Alteration: Greisen has extensively replaced the felsic Cassiterite dike over 2,200 feet of strike and several hundred feet of dip. Later kaolinite replacement has overprinted much of the greisen.
Production: Lode production from the Lost River Mine is all from the Cassiterite dike exogreisen. Production includes 5.6 tons of concentrate containing 3.5 tons of tin and 0.6 tons of tungsten in 1913 and 309 tons of tin in concentrate produced between 1952 and 1955 (Lorain and others, 1958, p. 7).
Reserves: Sainsbury (1964, p. 50) has calculated reserves for two types of ore; 200,500 tons grading 1.3% tin and 0.125% WO3 and 105,000 tons grading 0.76% tin and 0.6% WO3. Sainsbury (1964, p. 51) suggests that the known and inferred ore with greater than 1% combined tin and WO3 could be about 430,000 tons. The deposit is open to the southeast and potential exists to the west on the other side of Cassiterite Creek.
Commodities (Major) - Sn, W
Development Status: Yes
Deposit Model: Exogriesen. This deposit has characteristics of both tin vein model (15b) and
11 entries listed. 8 valid minerals.
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Cobb, E.H., 1975, Summary of references to mineral occurrences (other than mineral fuels and construction materials) in the Teller quadrangle, Alaska: U.S. Geological Survey Open-File Report 75-587, 130 p. Cobb, E.H., and Sainsbury, C.L., 1972, Metallic mineral resources map of the Teller quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-426, 1 sheet, scale 1:250,000. Hudson, T.L., and Arth, J. G., 1983, Tin granites of Seward Peninsula, Alaska: Geological Society of America Bulletin, v. 94, p. 768-790. Hudson, T.L., and Reed, B.L., 1997, Tin deposits of Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 450-465. Lorain, S.H., Wells, R.R., Mihelich, Miro, Mulligan, J.J., Thorne, R.L., and Herdlick, J.A., 1958, Lode-tin mining at Lost River, Seward Peninsula, Alaska: U.S. Bureau of Mines Information Circular 7871, 76 p. Sainsbury, C.L., 1964, Geology of the Lost River mine area, Alaska: U.S. Geological Survey Bulletin 1129, 80 p.