DW

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 63.2534
Longitude -144.1038
Nearby scientific data Find additional scientific data near this location
Location and accuracy The DW prospect is approximately 3.5 miles south-southeast of the confluence of Robertson River and Rumble Creek. The prospect is at an elevation of 5,700 feet and 1.1 mile south-southeast of peak 6220, about one-third mile east-southeast of the center of section 17, T. 17 N., R. 7 E., Copper River Meridian. The location is is accurate. The deposit is partly covered by a glacier.

Geologic setting

Geologic description

The DW massive sulfide deposit is in felsic metavolcanic rocks of the upper Lagoon unit of Devonian age. The deposit is a sheet-like body as much as 19 feet thick. The prospect is one block of what was once part of a single continuous massive sulfide sheet that extended more than 2 miles along strike and is more than 40 feet thick. The deposit is now offset by high-angle faults into six deposits (Dashevsky and others, 2003). The DW mineralized horizon projects up-dip to a thin distal edge exposed at the DW East (MH335) massive sulfide prospect.
The rocks surrounding the DW prospect are part of the Tiger unit; however, the massive sulfide mineralization was intersected by drilling into the underlying metavolcanic stocks of the upper Lagoon unit. Drilling has defined an inferred resource of 0.4 million tons that average 0.4 percent copper, 1.7 percent lead, 4.8 percent zinc, 58 parts per million (ppm) silver and 1.4 ppm gold, accompanied by high arsenic and antimony values.
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 (-144.106050654258, 63.253024279511)
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 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 DW prospect was discovered by finding massive sulfide float boulders during stream-sediment reconnaissance in 1976 (Rodney A. Blakestad and others, unpublished Resource Associates of Alaska Inc. report, 1976). Extensive geological, geochemical, and geophysical surveys were conducted from 1976 to 1981 by Resource Associates of Alaska and from 1994 to 1998 and by American Copper and Nickel Company. Eleven core holes totaling 6,804 feet have been drilled. Electromagnetic geophysical surveys were instrumental in recognizing this blind deposit.
Indication of production None
Reserve estimates The DW deposit has an inferred resource of 0.4 million tons that average 0.4 percent copper, 1.7 percent lead, 4.8 percent zinc, 58 parts per million (ppm) silver and 1.4 ppm gold. The resource calculation includes only those contiguous bodies with a true thickness greater than 8 feet and a gross metal value of greater than $80.00 per ton at 1998 prices (Dashevsky and others, 2003).

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