Sleitat Prospect, Bristol Bay District, Dillingham Borough, Alaska, USA
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Geology: The occurrence of granite and peripheral gold-bearing quartz gash veins was reported on Sleitat Mountain by Mertie (1938, p. 91), but subsequent exploration has shown that the principal mineral deposit is a tin-, tungsten-, and silver-bearing sheeted greisen system (Farnstrom, 1991; Burleigh, 1991; Hudson and Reed, 1997). The greisen deposit was discovered by a Cominco Alaska regional exploration program in 1983, and subsequently evaluated in the mid-1980s by detailed surface mapping, sampling, and diamond drilling (Farnstrom, 1991). A composite granite stock that hosts most of the greisen sharply crosscuts and thermally metamorphoses mid-Cretaceous clastic sedimentary rocks of the Kuskokwim Group. The stock has a discontinuous border zone of medium-grained biotite granite and biotite-muscovite granite, and a core of fine-grained zinnwaldite granite. Felsic porphyry dikes crosscut hornfels peripheral to the stock. A K/Ar age of 56.8 +/- 2.8 Ma on muscovite from a late veinlet was reported by Burleigh (1991, p. 6). Greisen sheets trend east-west and are developed within the biotite-muscovite granite, zinnwaldite granite, and hornfels. The east-west trending cassiterite-bearing greisen zones are nearly vertical quartz-topaz-tourmaline +/- white mica veins and tabular bodies that vary from inches to 20 feet in thickness and coalesce to greater thicknesses in places. They are concentrated in the north half of the stock and in a second zone along the south border that includes some greisen sheets in peripheral hornfels. The individual greisen sheets are granular, massive, separated by less-altered granite, have disseminated clay-lined voids, and have cores that locally contain a few inches of coarse quartz veins carrying high concentrations (50 to 60 percent) of cassiterite (Burleigh, 1991, p. 14). Cassiterite is disseminated in the greisen, concentrated in cores of greisen veins, and in quartz-topaz veins that fill fractures in hornfels. Cassiterite-bearing veins in hornfels are up to 1.5 feet wide and a few hundred feet long. Small amounts of wolframite are disseminated in the greisen but it also occurs with arsenopyrite in quartz veins, especially in hornfels peripheral to greisen zones. Arsenopyrite is common in the greisen and veins. Up to 5 percent lollingite with inclusions of bismite has been identified as disseminations in biotite-muscovite granite (Burleigh, 1991, p. 16). Sphalerite is a minor but common constituent of the greisen and some stannite and chalcopyrite are associated with the sphalerite. One small grain of ferrotantalite was identified during SEM analysis of the greisen (Burleigh, 1991, p. 16). Individual greisen zones locally have high tin grades. For example, one 47.7-foot (true) drill intercept averaged 1.56 percent tin, and included a 5-foot-thick section grading 12.6 percent tin and 5.7 ounces of silver per ton (Farnstrom, 1991; Burleigh, 1991, p. 18). The Sleitat prospect is a deeply eroded tin-bearing system. The sheeted greisens, particularly those on the north side of the stock, are expected to diminish in size and in intensity of cassiterite mineralization at depth. However, mineralization in the relatively wide hornfels zone on the south side of the stock may indicate that the upper contact of the granite body is not steeply dipping there, or that a mineralizing zinnwaldite granite cupola could be present at depth (Hudson and Reed, 1997, p. 461). Burleigh (1991) showed that much of the eroded tin-bearing material had migrated downslope and along the small streams that head against the lode deposit. Solomon Resources Ltd. staked 3,520 acres on the Sleitat prospect in the summer of 2005 (Brett Resources Inc., 2006). They subsequently worked out a joint venture agreement with Brett Resources, Inc., and geologic and geochemical field work began in Sept. 2005. In 2005, Brett commissioned a extensive report on the deposit by William T. Ellis (2006) who analyzed and compiled the previous data on the deposit and did some limited verification sampling. Burleigh (1991) estimated that the Sleitat deposit contained a total of 28.6 million tons of ore with an average grade of 0.37 percent tin, 0.04 percent tungsten and 17 ppm silver. However, Ellis (2006) cautioned that these figures while not necessarily wrong do not meet current industry standards for determining mineral resources.
Workings: The Sleitat prospect was discovered and explored by Cominco Alaska in the 1980s. This work included detailed surface mapping, sampling, and some diamond drilling (Farnstrom, 1991). In 1989, the U. S. Bureau of Mines conducted additional surface examinations, geochemical sampling, surface magnetometer and radiometric surveys, and a pan concentrate survey in nearby drainages (Burleigh, 1991). Solomon Resources Ltd. staked 3,520 acres on the Sleitat prospect in the summer of 2005. They subsequently worked out a joint venture agreement with Brett Resources, Inc., and geologic and geochemical field work began in Sept. 2005. In 2005, Brett commissioned a extensive report on the deposit by William T. Ellis who analyzed and compiled the previous data on the deposit and did some limited verification sampling.
Age: Early Tertiary. A composite granite stock that hosts most of the greisen sharply crosscuts and thermally metamorphoses mid-Cretaceous clastic sedimentary rocks of the Kuskokwim Group. A K/Ar age of 56.8 +/- 2.8 Ma on muscovite from a late veinlet was reported by Burleigh (1991, p. 6).
Alteration: Greisenization, late clay development, oxidation including iron- and scorodite-staining.
Reserves: Burleigh (1991) estimated that the Sleitat deposit contained a total of 28.6 million tons of ore with an average grade of 0.37 percent tin, 0.04 percent tungsten, and 17 ppm silver. However, Ellis (2006) cautioned that these figures, while not necessarily wrong, do not meet current industry standards for determining mineral resources.
Commodities (Major) - Sn, W; (Minor) - Ag, Bi, Cu, Ta, W
Development Status: None
Deposit Model: Sn greisen deposits (Cox and Singer, 1986; model 15c)
16 entries listed. 12 valid minerals.
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Brett Resources, Inc., 2006: www.brettresources.com/s/Alaska.asp?ReportID=130030 (as of April, 2006). Burleigh, R.E., 1991, Evaluation of the tin-tungsten greisen mineralization and associated granite at Sleitat Mountain, southwestern Alaska: U.S. Bureau of Mines Open-File Report 35-91, 41 p. Cobb, E.H., 1972, Metallic mineral resources map of the Taylor Mountains quadrangle: U.S. Geological Survey Miscellaneous Field Studies Map MF-384, 1 sheet, scale 1:250,000. Cobb, E.H., 1976, Summary of references to mineral occurrences (other than mineral fuels and construction materials) in the Dillingham, Sleetmute, and Taylor Mountains quadrangles, Alaska: U.S. Geological Survey Open-File Report 76-606, 92 p. Ellis, W.T., 2006: www.brettresources.com/i/pdf/2006-02-03_Sleitat_43101.pdf (as of February, 2006). Farnstrom, H.E., 1991, Sleitat: A new tin-silver prospect in southwestern Alaska: Alaska Miner, v. 19, p. 12-14. 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. Menzie, W.D., and Reed, B.L., 1986, Grade and tonnage model of Sn greisen deposits: U.S. Geological Survey Bulletin 1693, p. 71-72. Mertie, J.B., Jr., 1938, The Nushagak district, Alaska: U.S. Geological Survey Bulletin 903, 96 p.