Ann Creek

Prospect, Active?

Commodities and mineralogy

Main commodities Ag; Au; Cu; Ni
Other commodities Co; Cr; Pb; Pd; Pt
Ore minerals chalcopyrite; galena; pentlandite; pyrite; pyrrhotite; sperrylite

Geographic location

Quadrangle map, 1:250,000-scale MH
Quadrangle map, 1:63,360-scale B-4
Latitude 63.3392
Longitude -145.7794
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Ann Creek prospect trends northwest for about a mile and parallels a short tributary to lower Ann Creek. The center of the prospect is about is 1.2 miles west of the mouth of Ann Creek and near the center of section 23, T. 18 S., R. 10 E. Figure 20 of Bittenbender and others (2007) has an aerial photography of the prospect.

Geologic setting

Geologic description

The Ann Creek prospect is in a tabular ultramafic-mafic sill 80 to 120 feet thick that trends approximately east for about a mile on the north side of a small tributary to lower Ann Creek (Rose, 1965). The rocks north of the sill are dark, siliceous sedimentary rocks and light-colored tuff. The rocks south of the sill are andesite and dacite. The rocks on both sides are part of the Pennsylvanian and Permian, Slana Spur Formation; the different lithologies rocks suggests it was intruded along a fault (Rose, 1965). Airborne magnetic data indicate that this body may connect with the Late Triassic, Rainy ultramafic-mafic complex three miles to the west (W.T. Ellis, oral communication, 2001). The Rainy complex is up to 5,000 feet thick, dips shallowly north, and can be traced for more than 15 miles. It is mainly serpentinized dunite with subordinate peridotite, pyroxenite, and gabbro (Stout, 1976; Nokleberg and others, 1992 [MF]; 1992 [Open-File]; Bittenbender and others, 2007). The base of the sill is a layered gabbro as much as 1,500 feet thick.
The Ann Creek prospect was discovered in the 1950s (Barker and others, 1985) and subsequently examined and sampled by Saunders (1962), Hanson (1963), Rose (1965), and the Bureau of Mines from 1981 to 1984 (Barker and other, 1985; Barker, 1988). Falconbridge Exploration U.S., Inc. examined the property in 1997 and had airborne, total-field-magnetics and resistivity surveys flown over it (Wells, 1998; Pritchard, 1997). The Bureau of Land Management examined and sampled the prospect several times in the early 2000s (Bittenbender and others, 2007).
The rocks in the sill at the Ann Creek prospect vary from fresh dunite, peridotite, and mafic gabbronorite to serpentinite. A dike of serpentinized peridotite extends southeast from the gabbronorite; it appears to be cut off by a fault. Near the east end of the sill, shattered and serpentinized diorite or quartz diorite is in contact with the dunite. Rose (1965) believed that the diorite may be a differentiate of the ultramafic-mafic Rainy complex.
Small lenses of massive sulfide occur in an approximately 50-foot-thick layer of gabbro that also contains disseminated pyrrhotite, pyrite, pentlandite, chalcopyrite, and a trace of galena (Rose, 1965). Serpentinite at this location also carries small amounts of sulfides. Disseminated and massive sulfide samples contained 0.17 to 2.01 percent nickel and 0.1 to 0.61 percent copper (Saunders, 1962). Samples of sulfides from a 1-foot vein assayed 0.20 ounce of gold per ton and 0.32 ounce of silver per ton (Rose, 1965). Foley (1992) collected a sample from a massive-sulfide lens that contained 1.9 percent nickel, 3.5 percent copper, 0.02 percent cobalt, 540 parts per billion (ppb) palladium, and 340 ppb platinum. Sperrylite was identified in a gabbronorite sample that contained 0.44 percent nickel, 0.3 percent copper, 0.02 percent cobalt, and 340 ppb palladium.
The Bureau of Land Management examined the Ann Creek prospect on several occasions in the early 2000s and collected 35 samples (Bittenbender and others, 2007). Their best sample was from a massive sulfide lens about 1 foot thick in serpentinized peridotite. It contained 35 ppb gold, 483 ppb platinum, 825 ppb palladium, 7,890 parts per million (ppm) copper, 6.74 percent nickel, and 1,765 ppm cobalt. Such massive sulfide lens were rare. They also collected several measured samples to determine the average grade of the deposit. A sample 40 feet long averaged 30 ppb gold, 1 ppm silver, 100 ppm cobalt, 1,000 ppm copper, 1,800 ppm nickel, 105 ppb platinum, and 145 ppb palladium.
Geologic map unit (-145.781657012695, 63.3387861803581)
Mineral deposit model Nickel-copper-PGE mineralization in a differentiated mafic-ultramafic sill.
Age of mineralization Genetically related to the emplacement of a Late Triassic mafic-ultramafic complex.
Alteration of deposit Local serpentinization.

Production and reserves

Workings or exploration The Ann Creek prospect was discovered in the 1950s (Barker and others, 1985) and subsequently examined and sampled by Saunders (1962), Hanson (1963), Rose (1965) and the Bureau of Mines from 1981 to 1984 (Barker and others, 1985; Barker, 1988). Falconbridge Exploration U.S., Inc. examined the property in 1997 and had airborne, total-field-magnetics and resistivity surveys flown over it (Wells, 1998; Pritchard, 1997). The Bureau of Land Management examined and sampled the prospect several times in the early 2000s (Bittenbender and others, 2007).
Indication of production None
Reserve estimates None.
Production notes None.

References

MRDS Number A011792

References

Barker, J.C., 1988, Distribution of platinum-group elements in an ultramafic complex near Rainbow Mountain, east-central Alaska Range, IN Vassilou, A.H., Hausen, D.M., and Carson, D.J.T, eds, Process Mineralogy VII, Applications to mineral beneficiation: Proceedings of the Metal Society SME/AIME Joint [Annual] Meeting, Denver, Colo., p. 197-220.
Bittenbender, P.E., Bean, K.W., Kurtak, J.M., and Deininger, James Jr., 2007, Mineral assessment of the Delta River Mining District area, east-central, Alaska: U.S. Bureau of Land Management, Alaska Technical Report 57, 675 p.
Pritchard, R.A., Dighem V survey for Falconbridge Exploration U.S. Inc., Mt. Hayes area, Alaska: Unpublished report, prepared by Geoterrex-Dighem, Report #649, 104 p., 1 CD.
Wells, K., 1998, Report on the 1997 work program for Falconbridge Exploration U.S., Inc. on the Forbes Nickel Project (PN 5-998)-Mt. Hayes quadrangle, Alaska: Unpublished report, Falconbridge Exploration U.S., Inc., 41 p.
Reporters W.T. Ellis (Alaska Earth Sciences), C.C. Hawley (Hawley Resource Group), and W.J. Nokleberg (USGS); D.J. Grybeck (Contractor, USGS)
Last report date 5/13/2012