Tin-Midway; Tin Creek #1 Prospect, McGrath District, Yukon-Koyukuk Borough, Alaska, USA
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Location: The Tin Creek-Midway deposit is exposed in a canyon of a major tributary of Tin Creek, about 2.4 kilometers northwest of the north end of Veleska Lake at an elevation of 2,250 feet (685 m) in the NE1/4 sec. 24, T. 27 N., R. 24 W., of the Seward Meridian. The reporter visited the site in 1981 and 1983.
Geology: The Tin Creek-Midway deposit (southern extension is known as Tin Creek #1) is perhaps the best-studied silver-base metal skarn deposit in the Farewell Mineral belt. It consists of a series of sulfide-skarn deposits that occur along contacts between dikes and carbonate sedimentary rocks, along faults, and as bedding replacements of calcareous units in siliciclastic rocks (Lu and others, 1992). The mineralization occurs where a composite, northwest trending dike and sill swarm (averaging granodiorite in composition) cuts highly deformed siliciclastic and carbonate sedimentary rocks of the mid-Silurian Terra Cotta Mountains Sandstone, a unit of the Dillinger subterrane (Bundtzen, Harris, and Gilbert, 1997). The sedimentary rocks have been transformed into marble, garnet-banded marble, banded hornfels, and sulfide-rich, calc-silicate skarn. Volcanic units of the Latest Cretaceous (65 Ma) Veleska Lake Volcanic Field (Bundtzen, Harris, and Gilbert, 1997) cap the section immediately south of the Tin Creek-Midway Skarn deposit. Three sets of faults are present: (1) N30E high angle faults; (2) N50E-trending low angle reverse or thrust faults; and (3) younger west to northwest trending high angle faults that cut the earlier, northeast trending faults (Szumigala, 1986, 1987; Lu and others, 1992). These latter faults apparently controlled the northwest-trending granodiorite dike and sill swarm. Practically all of these dikes contain endoskarns; i.e., the dikes were apparently intruded prior to skarn mineralization. Several granodiorite dikes in the prospect area have been radiometrically dated at 25-30 Ma (Szumigala, 1987; Solie and others, 1991). Mineralized areas explored by Anaconda Minerals Company (Reed, 1982) were designated the MW (Tin Creek- Midway) and TCI (Tin Creek #1) zones. Although both endoskarn and exoskarn occur at the Tin Creek-Midway deposit, only exoskarn contains significant silver-base metal mineralization (Lu, 1988). The Tin Creek-Midway skarn system is zoned from a predominantly garnet skarn with abundant chalcopyrite and stockwork quartz veining in the northern MW zone to a pyroxene (johannsenite) skarn dominated by iron-rich sphalerite (marmatite) and minor galena in the southern TCI zone (Lu and others, 1992). Microthermometric, isotopic, and microprobe data collected by Szumigala (1985,1987), Lu (1988, 1989), and Lu and others (1992) all indicate the Tin Creek-Midway skarn can be described as a low temperature (<250 degrees C), lead-zinc skarn in the southern TCI zone that zones into a higher temperature (>280 degrees C), copper-base metal skarn in the northern MW zone. A plutonic heat source is indicated near the northern MW mineral zone. Using the FeS mole fraction in sphalerite, Lu (1988) calculated that skarn formation took place at about 300 bars, or in a very shallow, subvolcanic environment Six drill holes penetrated the Tin Creek-Midway deposit. Channel sampling from the various sulfide horizons average 0.7 percent copper, 4.7 percent zinc, and 40.4 grams/tonne silver over sample widths ranging from 1.5 meters to 23.0 meters (Brewer and others, 1992). Surface grab samples summarized by Bundtzen and others (1997) contain 0.35 percent copper, 0.17 percent lead, 5.55 percent zinc, 0.05 percent cadmium, and 13.0 grams/tonne silver. Based on subsurface exploration work completed by Anaconda Minerals Company (Reed, 1982; Rob Kell, written communication, 1984), at least 353,000 tonnes of semi- to massive-sulfide mineralization is inferred in the Tin Creek-Midway skarn deposit.
Workings: The Tin Creek-Midway skarn was investigated by the Alaska Division of Geological and Geophysical Surveys in 1980 (Bundtzen, Kline, and Clough, 1982). During 1981 and 1982, the Anaconda Minerals Company explored the deposit with surface sampling and 6 diamond drill holes (Reed, 1982; Brewer and others, 1992). Channel sampling of the various sulfide horizons average 0.7 percent copper, 4.7 percent zinc, and 40.4 grams/tonne silver over sample widths ranging from 1.5 meters to 23.0 meters (Brewer and others, 1992). Grab samples collected by Bundtzen, Harris, and Gilbert (1997) contained 0.35 percent copper, 0.17 percent lead, 5.55 percent zinc, 500 ppm cadmium, and 13.0 grams/tonne silver. Based on the Anaconda Minerals Company exploration work, about 353,000 tonnes of semi- to massive-sulfide mineralization (of unstated grade) is inferred at the Tin Creek-Midway deposit (Nokleberg and others, 1987). However, no official grade and tonnage data has been formally released (Reed, 1982; Brewer and others, 1992; Rob Kell, written communication, 1984).
Age: Not dated; inferred to be 25-to-30 Ma, based on ages of granodiorite dike swarm (Solie and others, 1991).
Alteration: Late epidotization of prograde skarns.
Reserves: Based on Anaconda Minerals Company exploration work, which included some limited diamond drilling, about 353,000 tonnes of semi- to massive-sulfide mineralization (of unstated grade) can be inferred at the Tin Creek-Midway deposit (Noklebeg and others, 1987).
Commodities (Major) - Ag, Cu, Pb, Zn; (Minor) - Cd, Co
Development Status: No
Deposit Model: Low temperature lead-zinc skarn deposit (Cox and Singer, 1986; model 18c).
Mineral List
12 entries listed. 10 valid minerals.
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References
Brewer, N.H., Puchner, C.C., and Gemuts, I., 1992, Farewell district, southwest Alaska Range: North Pacific Mining Company prospectus report, 21 p. Bundtzen, T.K., Harris, E.E., and Gilbert, W.G., 1997, Geologic Map of the eastern McGrath quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigations 97-14, 34 p., 1 sheet, scale 1:125,000. Bundtzen, T.K., Kline, J.T., and Clough, J.G., 1982, Preliminary geologiy of the McGrath B-2 quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Open-File report 149, 22 p., 1 sheet, scale 1:40,000. Lu, Chansheng, 1988, Skarn formation at Tin Creek, Alaska: Eugene, University of Oregon, M.Sc. thesis, 119 p. Lu, Changsheng, 1989, Mineralogy and geochemistry of skarn formation at Tin Creek, Alaska [abs.]: 28th International Geological Congress, Washington D.C., p. 2-33. Lu, Changsheng, Reed, Mark, and Misra, K.C., 1992, Zinc-lead skarn mineralization at Tin Creek, Alaska--Fluid inclusions and skarn-forming reactions: Geochimica et Cosmochimica, v. 56, p. 109-119. Newberry, R.J., Allegro, G.L., Cutler, S.E., Hagen-Levelle, D.D., Adams, D.D., Nicholson, L.C., Weglarz, T.B., Bakke, A.A., Clautice, K.H., Coulter, G.A., Ford, M.J., Myers, G.L., and Szumigala, D.J., 1997, Skarn deposits of Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 355-395. Reed, Mark, 1982, The Tin Creek sector of the Farewell district, Alaska: Anaconda Minerals Company internal report, 44 p. Solie, D.N., Bundtzen, T.K., and Gilbert, W.G., 1991, K/Ar ages of igneous rocks in the McGrath quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Public Data File 91-23, 7 p., 1 sheet, 1:250,000 scale. Szumigala, D.J., 1986, Geology and geochemistry of the Tin Creek skarn deposits, Farewell mining district, southwest Alaska: University of Alaska, Fairbanks, M.Sc. thesis, 144 p Szumigala, D.J., 1987, Geology of the lead-zinc skarn deposits at Tin Creek area, McGrath B-2 quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigations 87-5, 21 p., 1 sheet, 1:10,000 scale.
