Ann Fork

Prospect, Active?

Commodities and mineralogy

Main commodities Au; Cu; Ni; Pd; Pt
Ore minerals chalcopyrite; ilmenite; magnetite; pentlandite; pyrite; pyrrhotite

Geographic location

Quadrangle map, 1:250,000-scale MH
Quadrangle map, 1:63,360-scale B-4
Latitude 63.3278
Longitude -145.8641
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Ann Fork prospect is at an elevation of about 4,280 feet, about 2.8 mile north-northeast of the junction of the North Fork and West Forks of Rainy Creek. It is about 0.5 mile east-northeast of the center of section 29, T. 18 S., R. 10 E.

Geologic setting

Geologic description

The Ann Fork prospect is near the north margin of the Late Triassic, Rainy mafic-ultramafic complex that intrudes volcanic and sedimentary rocks of the Tetelna and Slana Spur Formations of Pennsylvanian and Permian age (Stout, 1976, Nokleberg and others, 1992; Nokleberg and others, 1992; Bittenbender and others, 2007). The 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 . The bas of the complex is a layered gabbro as much as 1,500 feet thick.
The prospect was first sampled by Barker (1988); it was examined and sampled by industry in the early 1990s, and it was examined and sampled by the Bureau of Land Management in 2002 (Bittenbender and others, 2007).
The mineralization at the Ann Fork prospect consists of semi-massive, net-textured, and disseminated sulfides, mainly pyrrhotite, with pentlandite, chalcopyrite, magnetite, and ilmenite, in gabbro dikes(?) that cut serpentinized peridotite and pyroxenite (Bittenbender and others, 2007). The mineralization is locally brecciated and oxidized to gossan; the brecciated mineralization is probably remobilized from primary magmatic mineralization in the Rainy complex. Some of the semi-massive mineralization has up to 80 percent sulfides across 1 to 2 feet. Garnet-pyroxene skarn has replaced marble beds near the contact of the Rainy ultramafic-mafic complex. Dunite in the complex is strongly serpentinized near a diorite intrusion at the contact.
Bittenbender and others (2007) collected 4 samples up to 6.5 feet long in the gabbro. The highest grade was a sample of semi-massive sulfides with 39 parts per billion (ppb) gold, 2.15 percent copper, 1,820 parts per million nickel, 72 ppb platinum, and 375 ppb palladium. Barker (1988) had previously analyzed samples that showed similar metal values.
Geologic map unit (-145.866357837469, 63.3273844219676)
Mineral deposit model Primary and remobilized nickel-copper-PGE mineralization in and adjacent to a differentiated mafic-ultramafic complex.
Age of mineralization Genetically related to the emplacement of a Late Triassic mafic-ultramafic complex.
Alteration of deposit Garnet-pyroxene skarn has replaced marble beds near the contact of the Rainy ultramafic-mafic complex. Dunite in the complex is strongly serpentinized near a diorite intrusion at the contact.

Production and reserves

Workings or exploration The prospect was first sampled by Barker (1988); it was examined and sampled by industry in the early 1990s, and it was examined and sampled by the Bureau of Land Management in 2002 (Bittenbender and others, 2007).
Indication of production None
Reserve estimates None.
Production notes None.

References

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.
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