Step Mountain; WK Prospects, Black District, Southeast Fairbanks Borough, Alaska, USA
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Location: The Step Mountain prospects cover about 5 square miles and are located in sections 7-9, 17, 18, and 20, T. 8 N., R. 31 E., of the Fairbanks Meridian, just to the north of Step Mountain. The coordinates are for the approximate center of the area. The location is accurate. These prospects are located within Doyon, Limited selected or conveyed land.
Geology: The Step Mountain Pb-Zn prospects are in a doubly plunging antiform that forms a dome-shaped mountain. The Permian Step Conglomerate and the Nation River Formation are exposed at its core. A northwest- or west-trending fault cuts the center of Step Mountain and may host additional mineralization (Northstar Exploration, Inc., 1999). The zinc minerals consist of smithsonite, a light gray to white oxide mineral that is not believed to be an alteration product of sulfides, and hydrozincite. The lead mineral is galena (Doyon, Limited, 1986). The lead and zinc minerals are in quartz veins(?) in Permian limestone breccia zones and lenses, in fossiliferous calcareous siltstone, and in layers overlying fossiliferous calcarenite, where it locally replaces coquina in an upper packstone unit (Mosher, 1990; Schmidt, 1997). Along the crest of the anticline, the conglomerate, sandstone, and cross-cutting quartz veins contain only slightly anomalous base-metal values (Northstar Exploration, Inc., 1999). Vitrinite reflectance data from the Devonian Nation River Formation and the Mississippian Ford Lake Shale in the Step Mountain antiform suggest that these rocks were exposed to temperatures of approximately 240 to 285 degrees Celsius (Underwood and others, 1992). There are an estimated 35,000 to 850,000 tons of economic grade Zn, which includes 4.9 meters of 2.7 percent Zn, 3.6 meters of 18.8 percent Zn, and 40 cm of 19.7 percent Zn (Northstar Exploration, Inc., 1999). This prospect was discovered in the late 1970s and has been explored by short trenches and diamond drilling (Northstar Exploration, Inc., 1999).
Workings: The prospects have been examined since the late 1970s, and several short trenches and some diamond drilling have been completed (Northstar Exploration, Inc., 1999).
Age: Permian or younger.
Reserves: There are an estimated 35,000 to 850,000 tons of economic grade Zn, which includes 4.9 meters of 2.7 percent Zn, 3.6 meters of 18.8 percent Zn, and 40 cm of 19.7 percent Zn (Mosher, 1990).
Commodities (Major) - Pb, Zn; (Minor) - Ag, Cd
Development Status: None
Deposit Model: Carbonate-hosted Zn-Pb (Schmidt, 1997; Cox and Singer, 1986; model 32b)
5 entries listed. 5 valid minerals.
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Doyon, Limited, 1986, Mines, prospects, and geochemical anomalies on Doyon, Limited, regional overselection lands, Alaska: Fairbanks, Alaska, Doyon, Limited Report 86-01A, v. 1, 150 p. (Report held by Doyon, Limited, Fairbanks, Alaska.) Mosher, Greg, 1990, Step Mountain project, Geological exploration and diamond drilling, 1990: Pasminco Exploration Ltd. for Doyon, Limited, unpublished report 92-77, 19 p. (Report held by Doyon, Limited, Fairbanks, Alaska.) North Star Exploration, Inc., 1999, 1998 Annual Report Kandik, Block 7: Fairbanks, Alaska. (Report held by Doyon, Limited, Fairbanks, Alaska.) Schmidt, J.M., 1997, Shale-hosted Zn-Pb-Ag and barite deposits of Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral deposits of Alaska, 1997: Economic Geology Monograph 9, p. 35-65. Underwood, M.B., Brocculeri, T., Bergfeld, D., Howell, D.G., and Pawlewicz, M., 1992, Statistical comparison between illite crystallinity and vitrinite reflectance, Kandik region of east-central Alaska, in Bradley, D.C., and Dusel-Bacon, Cynthia, eds., Geologic studes in Alaska bu the U.S. Geological Survey, 1991: U.S. Geological Survey Bulletin 2041, p. 222-237.