Sleitat

Prospect, Active

Commodities and mineralogy

Main commodities Sn; W
Other commodities Ag; Bi; Cu; Ta; W
Ore minerals arsenopyrite; bismite; cassiterite; chalcopyrite; ferrotantalite; loellingite; pyrite; sphalerite; stannite; wolframite
Gangue minerals clay; muscovite; quartz; topaz; tourmaline; zinnwaldite

Geographic location

Quadrangle map, 1:250,000-scale TA
Quadrangle map, 1:63,360-scale A-3
Latitude 60.0439
Longitude -157.0882
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Sleitat prospect is centered on a saddle at an elevation of 1,725 feet, between the two high peaks of Sleitat Mountain. The prospect is near the center of section 31, T. 1 S, R. 45 W. The location is accurate.

Geologic setting

Geologic description

The occurrence of granite and peripheral gold-bearing quartz veins was reported on Sleitat Mountain by Mertie (1938), but subsequent exploration has shown that the principal mineralization is a tin-, tungsten-, and silver-bearing sheeted greisen system (Farnstrom, 1991; Burleigh, 1991; Hudson and Reed, 1997, Ellis, 2006). The greisen deposit was discovered during a Cominco Alaska regional exploration program in 1983, and subsequently evaluated in the mid-1980s by detailed surface mapping and sampling, and 9 drill holes (Farnstrom, 1991). 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 comprehensive NI 43-101 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. In 2006, Brett drilled 5 holes.
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). The cassiterite-bearing, greisen sheets trend east-west and are developed within the biotite-muscovite granite, zinnwaldite granite, and hornfels.
The greisen zones include 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 mainly concentrated in the north half of the stock; a second zone along the south border 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-grained quartz veins with concentrations of up to 50-60 percent cassiterite (Burleigh, 1991). Cassiterite is disseminated in the greisen, concentrated in cores of greisen veins, and is 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 loellingite with inclusions of bismite has been identified as disseminations in biotite-muscovite granite (Burleigh, 1991). 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). Individual greisen zones locally have high tin grades. For example, one 47.7-foot 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). Four of the five holes drilled by Brett Resources (2006) in 2006 intersected mineralization comparable to the earlier Cominco results. The 2006 drilling including 104 meters with 0.24 percent tin and 6.48 grams of silver per ton, and 66 meters with 0.29 percent tin and 14.10 percent silver.
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). Burleigh (1991) showed that much of the eroded tin-bearing material had migrated downslope and along the small streams that head in the Sleitat highland.
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.
Geologic map unit (-157.090419113799, 60.0431887722563)
Mineral deposit model Sn greisen deposits (Cox and Singer, 1986; model 15c).
Mineral deposit model number 15c
Age of mineralization 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).
Alteration of deposit Greisenization, late clay development, oxidation including iron- and scorodite-staining.

Production and reserves

Workings or exploration 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 panned-concentrate survey in nearby drainages (Burleigh, 1991). 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 comprehensive NI 43-101 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. Subsequently, Brett drilled 5 holes in 2006.
Indication of production None
Reserve estimates Burleigh (1991) estimated that the Sleitat deposit contains 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.

References

MRDS Number A013460

References

Brett Resources Inc., 2006, Brett's Alaska Tin Drilling Yields Encouraging Intercepts: http://www.brettresources.com/s/NewsReleases.asp?ReportID=152664&_Type=News-Releases&_Title=Bretts-Alaska-Tin-Drilling-Yields-Encouraging-Intercept (News Release, Oct 6, 2006)
Ellis, W.T., 2006, Technical report on the Sleitat Mountain tin-silver deposit, southwest Alaska, NI 43-101 report: Unpublished Technical Report for Brett Resources, Inc., 57 p. (posted on www.sedar.com, April 10, 2008).
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.
Reporters Travis L. Hudson (Applied Geology, Inc.); D.J. Grybeck (Port Ludlow, WA)
Last report date 6/5/2008