Unnamed

Occurrence, Inactive

Commodities and mineralogy

Main commodities Ag; Au; Cu
Ore minerals bornite; chalcopyrite; gold; unknown platinoid; unknown silver mineral
Gangue minerals calcite; diopside; garnet; wollastonite

Geographic location

Quadrangle map, 1:250,000-scale TL
Quadrangle map, 1:63,360-scale C-5
Latitude 62.554
Longitude -152.197
Nearby scientific data Find additional scientific data near this location
Location and accuracy C.C. Hawley and Associates, Inc. (1978, locality C, Fig. 4.1-C(3)) show this occurrence 3.4 miles from the peak of Dall Mt. on a bearing of S61E in the southwest quarter of Section 36, T. 29 N., R. 16 W., of the Seward Meridian at about 3600 feet within Denali National Park and Preserve.

Geologic setting

Geologic description

At this occurrence massive Middle or Upper Devonian limestone (Dl), Paleozoic siliceous black argillite, and minor siltstone (Pzus) are intruded by the Cascade pluton, a 64.6 m.y. old composite pluton (Tcp) which consists of granite-porphyry, intermediate phase rocks and lamprophyre (Reed and Nelson, 1980). The mineral occurrence is separated from the intrusion by a few feet of pyritic, silicified black argillite. Massive bornite and chalcopyrite occur in a replacement band 0.3 to 3 feet wide, with free gold in thin calcite-diopside seams within banded tactite. One sample contained 25% copper, 41 oz/ton silver and 0.7 oz/ton gold over a three foot width (C.C. Hawley and Associates, Inc., 1978). Foley and others (1997) report locally abundant bornite within the pluton and anomalous gold in the lamprophyre (31 ppb) and in granite-porphyry (88 ppb). They also note anomalous platinum (300 ppb) in pan concentrate samples at the eastern margin of the pluton. The pluton has converted the country rocks to a tactite assemblage which includes diopside and wollastonite.
This occurrence is part of 10-mile-long tactite zone related to Cascade pluton; another anomalous area lies about three quarters of a mile southwest of this site (TL066) where bleached tactite zone in pebble conglomerate and extends for hundreds of yards between Cascade pluton and a small outlying stock.
The Cascade pluton has a composition similar to the Kohlsaat pluton (Reed and Nelson, 1980; Nelson and others, 1992). Foley and others (1997) report mean values of 20 ppb gold, 1 ppb palladium, 42 ppb platinum, 139 ppm nickel, and 26 ppm copper from six rock samples of the Cascade pluton. Anomalous gold, platinum-group elements, copper, chrome, nickel and arsenic are reported from the composite plutons mapped by Reed and Nelson (1980) in the southern Alaska Range (Reed and others, 1978; Nelson and others 1992). Gold and platinum group element placers have been worked at sites downstream from these bodies (Mertie, 1919; Cobb, 1973). See also TL020, TL023, TL052, and TL053.
Geologic map unit (-152.199227782993, 62.5534411198623)
Mineral deposit model Cu skarn deposits (Cox and Singer, 1986; model 18b).
Mineral deposit model number 18b
Age of mineralization Late Cretaceous/early Tertiary; mineralization is interpreted to be linked to the Cascade pluton dated as 64.6 +/- 1.8 m.y. by K/Ar methods (Reed and Lanphere, 1972).
Alteration of deposit Development of calc-silicate mineral assemblages in tactitized limestone, and silicification of argillite and siltstone (C.C. Hawley and Associates, Inc., 1978).

Production and reserves

Workings or exploration Reconnaissance mapping, prospecting, stream silt, pan concentrate and rock sampling are all that have been done here. Foley and others (1997) report mean values of 20 ppb gold, 1 ppb palladium, 42 ppb platinum, 139 ppm nickel, and 26 ppm copper from six rock samples of the Cascade pluton. Anomalous gold, platinum-group elements, copper, chrome, nickel and arsenic are reported from the composite plutons mapped by Reed and Nelson (1980) in the southern Alaska Range (Reed and others, 1978; Nelson and others 1992). Gold and platinum group element placers have been worked at sites downstream from these bodies (Mertie, 1919; Cobb, 1973).
Indication of production None

Additional comments

The Cascade pluton has also been called the Dall Stock (C.C. Hawley and Associates, 1978). This occurrence is part of a 10-mile-long tactite zone related to Cascade pluton; another anomalous area lies 1 mile southwest of this site: bleached tactite zone in Dall Limestone pebble conglomerate extends for hundreds of yards between Cascade pluton and small outlying stock. This site is located within Denali National Park and Preserve.

References

MRDS Number A011566

References

Foley, J.Y., Light, T.D., Nelson, S.W., and Harris, R.A., 1997, Mineral occurrences associated with mafic-ultramafic and related alkaline complexes in Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 396-449.
Nelson, S.W., Foley, J.Y., and Nelson, B.K., 1992, Metal-rich lamprophyric and associated alkaline mafic, ultramafic and intermediate to felsic rocks from composite plutons, central Alaska Range, Alaska [abs.]: International Conference on Arctic Margins, Anchorage, Alaska, September 2-4, 1992, Abstracts with Program, p. 43.
Reporters Madelyn A. Millholland (Millholland & Associates)
Last report date 8/10/1998