Roberts No. 2

Prospect, Inactive

Commodities and mineralogy

Main commodities Ag; Au; Cu; Pb; Zn
Ore minerals chalcopyrite; galena; marcasite; pyrite; pyrrhotite; sphalerite

Geographic location

Quadrangle map, 1:250,000-scale MH
Quadrangle map, 1:63,360-scale C-5
Latitude 63.5922
Longitude -146.2491
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Roberts No. 2 prospect is at an elevation of about 6,800 feet about about 3.7 miles north-northeast of McGinnis Peak. It is about 0.7 mile northwest of the center of section 29 T. 15 S., R. 8 E.

Geologic setting

Geologic description

The rocks in the area are derived from Devonian felsic to intermediate volcanic rocks, mainly andesite, dacite, and quartz keratophyre flows and tuffs, and from Devonian or older shale, marl, and marble (Nokleberg and others, 1992 [MF]; 1992 [Open-File]; Bittenbender and others, 2007). The rocks were intensely deformed into chlorite schist, limestone, and greenstone that were first metamorphosed to lower amphibolite facies in the Early Cretaceous and then retrograded to lower greenschist facies in the mid-Cretaceous.
The Roberts #2 prospect is one of several volcanogenic massive-sulfide deposits that lay along the north side of the Alaska Range (Lange and others, 1993, Newberry and others, 1997). The prospect was first identified by Nokleberg and others in the early 1980s (Nokleberg and Lange, 1985; Nokleberg and others, 1991; Lange and others, 1993). It was examined and sampled by the Bureau of Land Management in the early 2000s (Bittenbender and others, 2007). The prospect was examined and sampled by the American Copper and Nickel company in the 1980s but there are no workings on the prospect.
The mineralization at the Robert #2 prospect consists of massive to semi-massive lenses of sulfides in a layer up to 30 feet thick that can be traced for about 200 feet (Nokleberg and others, 1991; Lange and others, 1993; Bittenbender and others, 2007). The sulfides are mainly pyrrhotite, some oxidizing to marcasite, with lesser pyrite, chalcopyrite, sphalerite, and galena. The hanging wall of the layer is graphitic calcite-quartz schist. A 45-foot-thick layer of gray marble lies about 75 feet structurally below the sulfide layer; some metaconglomerate and metagraywacke is also present.
Samples collected by the U.S. Geological Survey contained up to 0.69 percent copper, 0.3 percent lead, 2.3 percent zinc, and 45.4 parts per million (ppm) silver (Nokleberg and others, 1991; Lange and others, 1993). Bittenbender and others (2007) collected 12 samples. The highest grade sample in place was a 18 feet that contained 44 parts per billion gold, 1.0 ppm silver, and 2,470 ppm copper. One float sample contained 1.17 percent zinc.
Geologic map unit (-146.251388472475, 63.591786803531)
Mineral deposit model Kuroko massive sulfide (Cox and Singer, 1986; model 28a).
Mineral deposit model number 28a
Age of mineralization Probably Devonian, the pre-metamorphic age of the host rocks.
Alteration of deposit Not specifically noted.

Production and reserves

Workings or exploration The Roberts #2 occurrence was first identified by Nokleberg and others in the early 1980s (Nokleberg and Lange, 1985; Nokleberg and others, 1991; Lange and others, 1993). It was examined and sampled by the Bureau of Land Management in the early 2000s (Bittenbender and others, 2007). The prospect was examined and sampled by the American Copper and Nickel company in the 1980s but there are no workings on the prospect.
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.
Lange, I. M., Nokleberg, W.J., Newkirk, S. R., Aleinikoff, J.N., Church, S.E., and Krouse, R.H., 1993, Devonian volcanogenic massive sulfide deposits and occurrences, southern Yukon-Tanana terrane, eastern Alaska Range, Alaska: Economic Geology, v. 88, p. 344-376.
Newberry, R.J., Crafford, T.C., Newkirk, S.R., Young, L.E., Nelson, S.W., and Duke, N.A., 1997, Volcanogenic massive sulfide deposits of Alaska, in Goldfarb, R.J. and Miller, L. D., eds., Mineral deposits of Alaska: Economic Geology Monograph 9, p. 120-150.
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