Unnamed (logging road, north side of Gypsum Creek)
Occurrences, Undetermined
calcite; diopside; epidote; garnet
This record consists of several occurrences of mineralization in rock pits and roadcuts along about 3 miles of a logging road on the northeast side of Gypsum Creek. For this record, the site is plotted on the road at about the midpoint of the occurrences at the southwest corner of sec. 34, T. 45 S., R. 64 E. It is location P-19 of Bittenbender and others (1999), and MAS no. 0021140307 (U.S. Bureau of Land Management, 2002). The location is accurate.
The rocks in the vicinity of these occurrences are part of the Mississippian, Iyoukeen Formation (Lathram and others, 1965). The upper half of the formation is cherty fossiliferous limestone; the lower half is fossiliferous shale with minor limestone. A Cretaceous hornblende-biotite quartz monzonite stock is just north of the prospect. and is probably the cause of the skarns at these occurrences.
This site includes two borrow pits about 2.5 miles apart along a logging road (Bittenbender and others, 1999). In the east pit, there is a zone about 60 feet wide of sheared hornfels and garnet-epidote-diopside-calcite endoskarn. There is locally abundant pyrite and pyrrhotite, and minor chalcopyrite in the hornfels and skarn. A sample of massive pyrrhotite contained 116 parts per million (ppm) copper and 310 ppm nickel, but no silver or gold. Similar skarn is exposed in the west pit where a 2-foot-wide zone of mineralized hornfels contains pyrite and pyrrhotite. A sample contained 2,210 ppm copper. A sample of rubble in the pit contained up to 1,950 ppm copper, 28 ppm molybdenum, 300 ppm cobalt, 0.4 ppm silver and 50 parts per billion gold.
Copper skarn (Cox and Singer, 1986, model 18b).
Probably related to a nearby Cretaceous intrusive.
Alteration of deposit
The mineralization is associated physically and probably genetically with hornfels and skarn.
Only surface sampling by government geologists.
Bittenbender, P., Still, J.C., Maas, K., and McDonald, M., Jr., 1999, Mineral resources of the Chichagof and Baranof Islands area, southeast Alaska: Bureau of Land Management, BLM-Alaska Technical Report 19, 222 p.
Lathram, E.H., Pomeroy, J.S., Berg, H.C., and Loney, R.A., 1965, Reconnaissance geology of Admiralty Island, Alaska: U.S. Geological Survey Bulletin 1181-R, p. R1-R48.
U.S. Bureau of Land Management, 2002, Alaska mineral locations database report (Sitka quadrangle), July 2, 2002, 205 p. [http://imcg.wr.usgs.gov/dem.html]
Donald Grybeck (U.S. Geological Survey)