Lost River-endogreisen Mine, Port Clarence District, Nome Borough, Alaska, USA
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Geology: The Lost River endogreisen deposit is developed at the roof of a highly differentiated, fine-grained granite cupola and in sheeted zones at depth within the cupola. The Lost River skarn deposit is developed in Ordovician limestone country rocks adjacent to and above the endogreisen deposit. Late hydrothermal breccia, with mica- and clay-rich matrices, have been superimposed on both endogreisen and skarn (Dobson, 1982, figure 4). 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). As described by Dobson (1982), the roof greisen is mica-rich, commonly 30 to 60% muscovite and probably zinnwaldite. The mica-rich greisen merges upward with hydrothermal breccias having a mica-rich matrix. Downward the mica-rich roof greisen gives way to quartz-rich greisen that characteristically contains tourmaline and sulfide minerals including pyrite, arsenopyrite, galena, sphalerite, and chalcopyrite. There are gradations from unaltered to completely greisenized granite but in places only thin, quartz-arsenopyrite, quartz-tourmaline, and greisen veins are present in granite. The roof greisen as mapped by Dobson (1982, figures 4 and 5), has dimensions of about 120 x 400 x 1,000 feet. Sainsbury's (1964, plate 9) compilation of grade data for this part of the deposit indicates that the general grade is a few tenths of a percent tin and a few hundreths of a percent WO3. The deeper sheeted greisen was defined by Dobson (1982, figure 4) but it is primarily known from intercepts in two diamond drill holes completed by Teasgulf Inc. in 1979. Hole TG 2 (collared at 275 feet elevation, azimuth of N 19 E, inclined at 67.5 degrees, and a total depth of 1,012 feet) encountered: (1) 0.19% tin, 0.74% copper, 0.95% lead, 4.32% zinc, 2.73 opt silver, and 0.01% WO3 from 633 to 638 feet; (2) 1.21% tin, 0.05% copper, 0.06% lead, 0.03% zinc, 0.18 opt silver, and 0.06% WO3 from 638 to 647 feet; (3) 0.84% tin, less than 0.03% copper, 0.18% lead, 0.43% zinc, 0.27 opt silver, and 0.01% WO3 from 800.1 to 814.5 feet; and (4) 1.33% tin, less than 0.03% copper, less than 0.04% lead and zinc, 0.04 opt silver, and 0.05% WO3 from 814.5 to 823.6 feet. Hole TG 3 (collared at 321 feet elevation, azimuth of N 19 E, inclined at 64.5 degrees, and a total depth of 1,037 feet) encountered: (1) 0.28% tin, 0.08% copper, 0.78% lead, 0.80% zinc, and 1.75 opt silver (tungsten was not determined) from 778.5 to 789.8 feet; and (2) 0.88% tin, 0.08% copper, 0.20% lead, 2.13% zinc, and greater than 3.4 opt silver (tungsten was not determined) from 796.3 to 801.4 feet. Other zones of weaker tin metallization and weaker to stronger base metal and silver metallization are present in these holes. The metallization is in highly silicified zones with sulfide minerals, tourmaline, and fluorite in granite.
Workings: This prospect has been explored by 750 feet of underground workings of the Lost River mine (32 and 195 crosscuts, Calcite drift, and 190 raise; Sainsbury, 1964, plate 9). The upper roof greisen has been encountered by many USBM and Lost River Mining Corporation drill holes and the deeper sheeted greisen zones hve been encountered by two Texasgulf Inc diamond drill holes.
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: There is a well-developed quartz-tourmaline-fluorite greisen at depth, a mica-rich greisen in the roof zone, and both mica- and clay-matix hydrothermal breccias above the roof zone. The greisen types appear to merge with one another and the breccias are late, overprinting assemblages.
Production: Production from the Lost River Mine has been from the Cassiterite dike exogreisen deposit (TE048).
Reserves: Resource estimates have not been separately determined for the endogreisen deposits; grades are commonly a few tenths percent tin and a few hundreths percent WO3 although some greisen sheets at depth have higher tin grades, base metals, and silver in places.
Commodities (Major) - Sn; (Minor) - Ag, Cu, Pb, Zn, W
Development Status: None
Deposit Model: Endogreisen including roof and sheeted greisen. Tin greisen model (15c) of Cox
12 entries listed. 9 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. Dobson, D.C., 1982, Geology and alteration of the Lost River tin-tungsten-fluorine deposit, Alaska: Economic Geology, v. 77, p. 1033-1052. 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. Texasgulf, Inc., 1979, Logs for diamond drill holes TG 1, TG 2, and TG 3, Lost River Mine area: Unpublished data submitted to Lost River Mining Corporation, Toronto, Canada.