Cobol; Mine Mountain; Cobol North Mine, Chichagof District, Skagway-Hoonah-Angoon Borough, Alaska, USA
|Latitude & Longitude (WGS84):||57° 51' 15'' North , 136° 12' 43'' West|
|Latitude & Longitude (decimal):||57.85417,-136.21194|
The mine is in West Chichagof-Yakobi Wilderness.
Location: The Cobol Mine is identified as an abandoned mine and marked by an adit symbol on the USGS D-7 topographic map (1997 ed.). It is at an elevation of about 750 feet at the northwest foot of Mine Mountain on northwest Chichagof Island. The mine is 0.2 mile north of the center of sec. 29, T. 46 S., R. 57 E. It is location P-34 of Bittenbender and others (1999), who call it 'Mine Mountain', or 'Cobol North'; location 11 of Cobb (1972, 1978); and MAS no. 0021140025 (U.S. Bureau of Land Management, 2002). The location is accurate.
Geology: Johnson and Karl (1985) describe the rocks in the area of the Cobol Mine as Mesozoic or Paleozoic siliceous metasedimentary and metavolcanic rocks that are intruded by an elongate stock of Cretaceous or Tertiary quartz diorite and tonalite. The mine is at the contact between the metamorphic and intrusive rocks. The rocks are cut by high-angle faults of diverse, but mainly northwest, strike. The most prominent is the Border Ranges Fault, a regional-scale, northwest-striking, steeply-dipping fault whose trace is about 3 miles southwest of the mine. Cobb (1972, 1978), citing Buddington (1925) and Reed and Coats (1941) reports that the Cobol Mine claims were located and development begun in 1922. The deposit consists of auriferous, sulfide-bearing, quartz fissure veins in albite-quartz diorite and greenstone that are intruded by an aplite dike. The wallrock is sericitized near the veins. The vein that was mined was about 2 feet thick and contained generally sparse sulfides, including arsenopyrite, sphalerite, and galena, with minor pyrite, chalcopyrite, and pyrrhotite. Free gold occurred in the vein and in gouge along the hanging wall. Mining and milling in 1933-35 produced about 1,000 ounces of gold from 135 tons of ore from a stope about 70 feet long and 40 feet high. The mining equipment was removed in 1936. Bittenbender and others (1999), citing Kimball (1982), call this property the Mine Mountain Mine and note that it was developed by an adit and 250 feet of crosscuts and drifts. Samples across a 1.2-foot-wide vein in a 70-foot stope assayed up to 2.45 ounces of gold per ton. Samples from 120 feet of other workings contained up to 0.15 ounce of gold per ton across veins 0.2 to 0.7 foot thick. Isotopic studies indicate that the gold-quartz veins in coastal southern and southeastern Alaska are Eocene, about 50 Ma in age (Haeussler, 1995; Goldfarb, 1997; Goldfarb and others, 1997).
Workings: Bittenbender and others call this property the Mine Mountain Mine and note that it was developed by an adit and 250 feet of crosscuts and drifts.
Age: Isotopic studies indicate that the gold-quartz veins in coastal southern and southeastern Alaska are Eocene, about 50 Ma in age (Haeussler and others, 1995; Goldfarb, 1997; Goldfarb and others, 1997).
Alteration: The wallrock is sericitized near the veins.
Production: Mining and milling in 1933-35 produced about 1,000 ounces of gold from 135 tons of ore that was mined from a stope about 70 feet long and 40 feet high. The mining equipment was removed in 1936.
Commodities (Major) - Au, Cu, Pb, Zn
Development Status: Yes; small
Deposit Model: Polymetallic vein (Cox and Singer, 1986; model 22c).
7 valid minerals.
This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.
Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org
201.3 - 252.17 Ma
|Metamorphic rocks of Baranof Island|
Age: Triassic (201.3 - 252.17 Ma)
Description: A heterogeneous unit of metasedimentary and metavolcanic rock of slightly higher metamorphic grade than the overlying Goon Dip Greenstone. Because the Goon Dip Greenstone has been correlated with the Nikolai Greenstone (Jones and others, 1977), which overlies the Skolai Group of Southern Alaska, the rocks of this unit may be equivalent to the Skolai rocks. However, there is very little lithologic correspondence between these two units. Thus, this unit is lithologically distinct from the Skolai Group, but apparently occupies a similar stratigraphic position. NOTE: More extensive description in pamphlet.
Reference: Wilson, F.H., Hults, C.P., Mull, C.G, and Karl, S.M. (compilers). Geologic map of Alaska. doi: 10.3133/sim3340. U.S. Geological Survey Scientific Investigations Map 3340, pamphlet 196.