Gilpatrick Dike

Mine, Inactive

Alternative names

Sprague and Byers

Commodities and mineralogy

Main commodities Ag; Au
Other commodities Cu; Pb; Zn
Ore minerals arsenopyrite; chalcopyrite; galena; gold; molybdenite; pyrite; pyrrhotite; sphalerite
Gangue minerals calcite; chlorite; quartz; sericite

Geographic location

Quadrangle map, 1:250,000-scale SR
Quadrangle map, 1:63,360-scale C-7
Latitude 60.6138
Longitude -149.575
Nearby scientific data Find additional scientific data near this location
Location and accuracy The mine is located in the NE1/4 section 14, T. 6 N., R. 2 W., of the Seward Meridian, on the south side of the divide between Summit and Slate Creeks. There are three adits on the Slate Creek side of the divide at elevations of 2,850, 3,300, and 3,400 feet, along with numerous prospect pits and open cuts. On the Summit Creek side of the divide, there are trenches and prospect pits above 2,400 feet elevation. This is location 29 of Cobb and Richter (1972), location 36 of MacKevett and Holloway (1977), location 54 of Cobb and Tysdal (1980), and location S-253 of Jansons and others (1984). This location is accurate to within a quarter of a mile.

Geologic setting

Geologic description

The deposit at the Gilpatrick Dike mine consists of a mineralized Eocene felsic dike (the so-called Gilpatrick dike) and of quartz veins in graywacke and slate of the Valdez Group of Late Cretaceous age adjacent to the dike (Mitchell and others, 1981). The dike contains quartz-filled fractures that carry native metals and sulfides, including gold, arsenopyrite, galena, and sphalerite and some chalcopyrite, molybdenite, and pyrrhotite. These veins and veinlets range in width from a fraction of an inch to 8 or 10 inches (Tuck, 1933).
The slate and graywacke 2 to 10 feet from the dike contain quartz veins that are parallel to the dike. The veins are 2 to 12 inches wide and contain the same ore minerals as those in the dike (Tuck, 1933).
The dike and adjacent veins are offset by several dextral faults. The presence of gouge on one wall of the dike indicates post-mineralization movement along another fault parallel to the dike. The dike continues for several miles (see Fresno Creek, SR145), but development at this site is restricted to a few thousand feet of its length.
The wall rocks near the veins locally display extensive chlorite-sericite alteration. The highest grade of ore coincides with the zones of greatest alteration.
Geologic map unit (-149.577154575242, 60.6132295134656)
Mineral deposit model Low-sulfide Au-quartz veins (Cox and Singer, 1986; model 36a)
Mineral deposit model number 36a
Age of mineralization Eocene or younger; the mineralization is in and adjacent to Eocene dike rock.
Alteration of deposit The dike and the country rocks near the veins display locally intense chlorite-sericite alteration.

Production and reserves

Workings or exploration There are three levels of adits, at 2,850, 3,300, and 3,400 feet elevation. The lowest adit is 265 feet long, the middle adit has caved in, and the upper adit is 50 feet long. There are also numerous trenches and prospect pits.
Indication of production Yes; medium
Reserve estimates Reserves are 2,000 tons of ore at 0.89 ounce of gold per ton and 0.65 ounces of silver per ton (Hoekzema and Sherman, 1983).
Production notes Most production occurred from about 1937 to 1948. Recorded production was 3,545 ounces of gold and 1,099 ounces of silver from an estimated 3,664 tons of ore (Hoekzema and Sherman, 1983).

Additional comments

The location labeled Oracle Mine on the Seward C-7 quadrangle map is the a mill site and is not the location of the working.

References

MRDS Number A010390

References

Hoekzema, R.P., and Sherman, G.E., 1983, Mineral investigations in the Chugach National Forest, Alaska (Peninsula study area): U.S. Bureau of Mines in-house report; held at U.S. Bureau of Land Management Alaska State Office, Anchorage, 524 p.
Smith, S.S., 1917, The mining industry in the Territory of Alaska during the calendar year 1915: U.S. Bureau of Mines Bulletin 142, 65 p.
Smith, S.S., 1917, The mining industry in the Territory of Alaska during the calendar year 1916: U.S. Bureau of Mines Bulletin 153, 89 p.
Reporters Jeff A. Huber (Anchorage)
Last report date 3/7/2000