Nunatak

Prospect, Active

Commodities and mineralogy

Main commodities Pb; Zn
Other commodities Ag; Au; Cu
Ore minerals arsenopyrite; chalcopyrite; galena; gold; magnetite; pyrite; pyrrhotite; sphalerite
Gangue minerals quartz; sericite

Geographic location

Quadrangle map, 1:250,000-scale MH
Quadrangle map, 1:63,360-scale B-1
Latitude 63.2503
Longitude -144.0919
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Nunatak prospect is approximately 4 miles south-southeast of the confluence of Robertson River and Rumble Creek. It is at an elevation of about 6,000 feet elevation, south of the toe of the remnant glacier in the SW1/4 section 16, T. 17 N., R. 7 E., Copper River Meridian. The location is accurate.

Geologic setting

Geologic description

The Nunatak deposit is in metarhyolite of the upper Lagoon unit of Devonian age. The deposit is a massive sulfide sheet-like body at least 9 feet thick. Its true thickness is undetermined because its lower portion is terminated by a fault against an underlying gabbroic sill. The Nunatak is a concealed deposit discovered by step-out drilling of a favorable stratigraphic layer (S.S. Dashevsky, oral communication, 2003). The prospect is one block of what was once part of a single continuous massive sulfide sheet that extends more than 2 miles along strike and is more than 40 feet thick. The sheet is now offset by high-angle faults into six deposits (Dashevsky and others, 2003). The metavolcanic rocks at the surface above the Nunatak prospect are part of the Tiger unit; however, the massive sulfide mineralization is in metavolcanic rocks of the upper Lagoon unit intersected by drilling into the underlying metavolcanic rocks of the upper Lagoon unit.
The Lagoon unit is a succession of dark gray, rusty, phyllitic metamorphosed mudstones interbedded with light-gray to white to pale-green siliceous quartz-sericite(-chlorite) schist. Locally the rocks contain coarse-grained blue quartz eyes and rare fragmental volcanic textures preserved as chloritized lithic fragments. The upper part is dominated by white to pale green, massive to laminated quartz-eye-bearing quartz-sericite(-chlorite-pyrite) schist; finely laminated schist with minor metamorphosed black mudstone; and thin intercalations of quartzite and fine-grained metamorphosed grit. The protoliths of the volcanic rocks of the upper Lagoon unit are mainly rhyodacite and dacite, but they also include rare rhyolite and minor andesite and basalt (Dashevsky and others, 2003).
Geologic map unit (-144.094150606374, 63.2499243840579)
Mineral deposit model Kuroko massive sulfide (Cox and Singer, 1986; model 28a).
Mineral deposit model number 28a
Age of mineralization The Drum unit, which is part of the metamorphic sequence that includes the rocks at this deposit, has been dated at the Devonian-Mississippian boundary on the basis of one SHRIMP U-Pb zircon age of 359 +/- 6 Ma at the nearby DD South prospect (MH325).
Alteration of deposit Development of sericite, quartz, pyrite, and carbonate in the hanging wall; extensive chlorite and magnesium enrichment and sodium depletion in the footwall.

Production and reserves

Workings or exploration The Nunatak deposit is a concealed deposit discovered when previously drilled core was re-examined and was found to have missed the target mineralized layer because of structural complications and insufficient depth. In 1997 American Copper and Nickel Company drilled one core hole totaling 479 feet and intersected 9 feet of massive sulfide (S.S. Dashevsky, oral communication, 2003). Stratigraphic projection was instrumental in recognizing this blind deposit.
Indication of production None

Additional comments

The unpublished data that is cited can be seen by contacting Grayd Resources Inc. in Vancouver, B.C., Canada (www.grayd.com), or Northern Associates Inc. in Fairbanks, Alaska.

References

References

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.
Reporters W.T. Ellis (Alaska Earth Sciences) and A.W. Wyatt and S.S. Dashevsky (Northern Associates Inc.)
Last report date 3/20/2003