Unnamed (Francis Island)

Mine, Inactive

Commodities and mineralogy

Main commodities Ag; Au; Cu; Zn
Other commodities Bi; Sb
Ore minerals bornite; chalcocite; chalcopyrite; magnetite; malachite; pyrite; pyrolusite; sphalerite; tetrahedrite
Gangue minerals calcite; tremolite

Geographic location

Quadrangle map, 1:250,000-scale MF
Quadrangle map, 1:63,360-scale C-1
Latitude 58.626
Longitude -136.177
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Francis Island mine site is on the southwest side of the island. The original site, described in several older reports (Buddington, 1926; Smith, 1933; Reed, 1938; Rossman, 1963) is now, apparently, largely covered by landslide debris. The occurrence is number 28 of MacKevett and others (1971), number 47 of Cobb (1972), and number 94 of Kimball and others (1978). It is accurate within 0.1 mile.

Geologic setting

Geologic description

The Francis Island occurrence is on the easternmost contact of a granitic intrusion of Cretaceous age. The intrusion is mostly underwater, but a substantial granitic mass, inferred to be part of the same pluton, is exposed on the south part of Marble Mountain across Whidbey Passage from Francis Island. The granitic rock is intruded into marble of Devonian and Silurian age (Brew and others, 1978; pl. IA).
Mineral deposits occur in sheared tremolitic marble and in pyroxene-garnet tactite about 5-feet thick at the granitic (quartz diorite) contact. Small rich pods of bornite and other minerals in tactite were described in older reports, including Buddington, (1926, p. 56), Buddington and Chapin (1929, p. 323), Smith, (1933, p. 323), Reed (1938, p. 69) and Rossman (1963-B 1121-K, p. K51). Some of the material was mined. This site has apparently been covered by landslide debris. A mineralized fault zone 10 feet thick can be followed for 50 feet before it is covered with surficial debris; MacKevett and others (1971, fig. 8) show the probable relation of the original site to the fault zone, which locally contains chalcopyrite, bornite, tetrahedrite (?), chalcocite (?), pyrite, and probably sphalerite. Malachite and pyrolusite occur as secondary minerals. MacKevett and others (1971) reported 7000 ppm copper, 1000 ppm zinc, 200 ppm antimony, 150 ppm bismuth, 20 ppm tin, and 1.46 ounce per ton silver in a selected sample of copper-stained metamorphic rock collected at or near the fault zone. Magnetite occurs locally in the tactite. A soil survey suggests the possibility of buried mineral deposits in the area (MacKevett and others, 1971, fig. 8).
The U.S. Bureau of Mines (Kimball and others, 1978) reported a trace of tungsten and 7 ppm silver in float samples from the area.
Geologic map unit (-136.178841783109, 58.6256321873054)
Mineral deposit model Copper skarn or tactite (Cox and Singer, 1986; model 18b).
Mineral deposit model number 18b
Age of mineralization Cretaceous (?).
Alteration of deposit Marble locally contains tremolite, and at intrusive contacts is altered to pyroxene - garnet skarn. Chlorite and tremolite also occur in calc-hornfels.

Production and reserves

Workings or exploration Surface cuts, now concealed by landslide debris, probably date back to 2 claims staked on the island in 1923, as reported by Buddington (1926) and repeated by Smith (1933) and Reed (1938). Buddington reported gold-silver-bearing bornite in a rich sulfide pocket in the tactite. MacKevett and others (1971) obtained maximum values of 7000 ppm copper, 1000 ppm zinc, 200 ppm antimony, 150 ppm bismuth, and 1.46 ounce/ton silver in a selected sample of copper-stained rock found near the inferred buried site.
Indication of production None
Production notes Possible small production, no records.

Additional comments

The deposit size is limited by the size of the island. A tactite or skarn affiliation is consisted by the high bismuth content found by MacKevett and others (1971).
The site is in Glacier Bay National Park and Preserve.

References

MRDS Number A013179

References

Reed, J.C., 1938, Some mineral deposits of Glacier Bay and vicinity, Alaska: Economic Geology, v. 33, p. 52-80.
Reporters C.C. Hawley (Hawley Resource Group)
Last report date 4/14/1999