Roberts No. 1

Occurrence, Probably inactive

Commodities and mineralogy

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

Geographic location

Quadrangle map, 1:250,000-scale MH
Quadrangle map, 1:63,360-scale C-5
Latitude 63.5971
Longitude -146.2664
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Roberts No. 1 occurrence is about 3.8 miles north-northeast of McGinnis Peak at an elevation of about 7,500 feet. It is about 0.2 mile south-southeast of the center of section 19, 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]; 2002 [\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 #1 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). There are no workings on the occurrence and no published record of industry activity.
The Roberts #1 occurrence 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). As described by Bittenbender and others (2007), the mineralization at the prospect consists of semi-massive to massive sulfides that form several layers up to 6 feet thick that are conformable to the foliation of the calcareous quartz-chlorite schist and limestone host rock. The sulfides are mainly pyrrhotite and pyrite with some chalcopyrite, galena, and sphalerite The thickest layer extends for about 60 feet. Mineralization in float to the southeast suggests continuity to the Roberts #2 occurrence (MH010). A sample across 3.2 feet of mineralization in place contained 102 parts per billion (ppb) gold, 1.3 part per million (ppm) silver, 94 ppm lead, and 1.58 percent zinc. Float samples contained up to 97 ppb gold, 30.1 ppm silver, 1.63 percent copper, 2.56 percent lead, and 7.49 percent zinc.
Geologic map unit (-146.268688725198, 63.5966867575303)
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. The mineralization and host rocks have been subject to regional amphibolite- and greenschist-facies metamorphism in the Cretaceous.

Production and reserves

Workings or exploration The Roberts #1 occurrence was first identified by the U.S. Geological Survey 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). There are no workings on the occurrence and no published record of industry activity.
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