Unnamed (near Broxson Gulch)

Occurrence, Undetermined

Commodities and mineralogy

Main commodities Cu
Other commodities Ag; Au; Co; Ni; Pd; Pt; Zn
Ore minerals chalcopyrite; covellite; digenite; pentlandite; pyrite; pyrrhotite

Geographic location

Quadrangle map, 1:250,000-scale MH
Quadrangle map, 1:63,360-scale B-5
Latitude 63.3458
Longitude -146.0829
Nearby scientific data Find additional scientific data near this location
Location and accuracy This occurrence is at an elevation of about 4,900 feet on the easternmost narrow ridge between the two forks of of Broxson Gulch that meet just above its mouth on Eureka Creek. It is about 1.8 mile northeast of elevation 4080 and about 0.6 mile northeast of the center of section 19, T. 18 S., R. 9 E.

Geologic setting

Geologic description

As originally described by Rose (1965), this occurrence is in amphibolitized 'serpentinite' inferred to be in the hanging wall of a major thrust fault. Rose (1966) later described the rock as basalt, part of the Mississippi to Pennsylvanian, Rainy Creek basalt unit which is at least 1,500 feet thick. Boulders containing massive sulfides were found along a 20-foot-wide zone on the north side of the ridge. Rose's samples contained 0.9 percent copper, a trace of zinc, and less than 0.1 percent nickel. Fifty feet south, a layer of pyrrhotite a few inches thick crops out for several feet. Nokleberg and others (1991) collected a sample with pyrite, pyrrhotite, pentlandite, and chalcopyrite that contained 500 parts per million (ppm) cobalt. Samples collected by Foley (1992) of massive-sulfide-bearing boulders in the area contained as much as 2.4 percent copper, 0.66 percent nickel, 0.1 percent cobalt, 13 ppm silver, 416 ppm gold, and 137 parts per billion (ppb) platinum, and 137 ppb palladium.
Bittenbender and others (2007) reexamined and sampled the site. They identified the rocks as strongly altered volcaniclastic rocks made up mainly of fine-grained amphibole and epidote with relict plagioclase clots. They sampled boulders up to 1.5 feet in size which originally identified by Rose. The volcaniclastic rocks contain small pockets of disseminated pyrrhotite and chalcopyrite with minor covellite and digenite and semi-massive sulfides. Samples contained up to 4,800 ppm copper, 633 ppm cobalt and up to 23 parts per billion (ppb) gold and 23 ppb platinum-group elements.
A 1982 unpublished report by the Bureau of Mines (cited in Bittenbender and others, 2007) describe a '...persistently mineralized sulfide zone adjacent to the contact of hornblende gabbro with thermally metamorphosed limestone. The mineralization has a width of 15 to 20 feet and was traced intermittently along strike for approximately 600 feet...'. Samples were said to contain from 0.1 to 1 percent copper and 0.2 to 0.4 percent cobalt. However, the 1982 Bureau of Mines report notes the presence of limestone, a significant difference from the descriptions of Rose (1965, 1966) and Bittenbender and others (2007).
Geologic map unit (-146.085165765626, 63.3453809106977)
Mineral deposit model Massive sulfides with chalcopyrite and pyrrhotite but notably different descriptions of the geology.
Age of mineralization Ambiguous in view of the different descriptions of the rocks in the area.
Alteration of deposit Host rocks are highly altered but may be due to metamorphism.

Production and reserves

Workings or exploration Sampled by state and federal agencies several times between 1966 and the early 2000s. Claims staked over it in the 2000s.
Indication of production None
Reserve estimates None.
Production notes None.

References

References

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