Unnamed (near Kohlsaat Peak)

Occurrence, Undetermined

Commodities and mineralogy

Main commodities Au; Cu; Mo; Pb; Zn
Other commodities As
Ore minerals arsenopyrite; chalcopyrite; galena; pyrite
Gangue minerals quartz; tourmaline

Geographic location

Quadrangle map, 1:250,000-scale TL
Quadrangle map, 1:63,360-scale A-6
Latitude 62.238
Longitude -152.729
Nearby scientific data Find additional scientific data near this location
Location and accuracy Curtin, Karlson, O'Leary, Day, and McDougal (1978) locate this silt sample site approximately one and a quarter mile N20E of Kohlsaat Peak in the southwest quarter of Section 23, T. 25 N., R. 19 W., of the Seward Meridian.

Geologic setting

Geologic description

Curtin, Karlson, O'Leary, Day, and McDougal, (1978) recorded a silt anomaly with up to 0.75 ppm gold from a stream draining Kohlsaat Peak. In the late 1980s Cominco American Incorporated (unpublished report) collected rock samples of quartz-sericite-chlorite altered intrusive upstream from the published stream silt site that contained values ranging from 2.43 to 7.96 ppm gold from samples with minor arsenopyrite and chalcopyrite. Foley and others (1997) report six rock samples from the Kohlsaat pluton with maximum values of 14 ppb gold, 4 ppb palladium, and 5 ppb platinum.
Reiners and others (1996) describe the Kohlsaat pluton as a concentrically zoned body with a biotite-granite-porphyry core that intruded seriate and porphyritic intermediate composition rocks. The latter include olive-, pyroxene-, and biotite-bearing quartz syenite, quartz monzonite, and monzonite. Lamprophyric mafic and ultramafic rocks that range in composition from peridotite to alkali gabbro and monzonite are found as large xenoliths at the northeastern margin of the pluton. The granite-porphyry core is altered to sericite, quartz, carbonate and tourmaline assemblages, with minor enrichments of copper, gold, lead, molybdenum, and zinc. They have postulated that the composite pluton at Kohlsaat formed from depleted mantle melts that were contaminated during several stages of mafic magma generation by crustal components.
The Kohlsaat pluton is one of the larger composite plutons (Tcp) in the Yentna region described by Reed and Nelson (1980). These composite plutons form a 65-km-long curvilinear belt from Mt. Estelle in the south to Cascade Creek in the northeast that primarily intrude Mesozoic marine sedimentary rocks (KJs). Anomalous gold, platinum-group elements, copper, chrome, nickel and arsenic are reported from these plutons (Reed and others, 1978; Reed and Nelson, 1980; Nelson and others 1992; Foley and others, 1997). Gold and platinum group element placers have been worked at sites downstream from these bodies (Mertie, 1919; Cobb, 1973).
See TL020, TL023, TL052, and TL005.
Geologic map unit (-152.731248492754, 62.237422632989)
Age of mineralization Late Cretaceous/early Tertiary; mineralization is interpreted to be linked to the Kohlsaat pluton which has been determined to be 65.6 +/- 1.9 m.y. old by Reed and Lanphere, 1973.
Alteration of deposit Carbonate, chlorite, sericite, and quartz alteration.

Production and reserves

Workings or exploration Reconnaissance mapping, stream silt and rock sampling by the U.S. Geological Survey (Curtin, Karlson, O'Leary, Day, and McDougal, 1978; Reed and Nelson, 1980) and Cominco American Incorporated in the late 1980s (unpublished report) have been done in this vicinity.
Indication of production None

References

MRDS Number 10308443

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.
Reed, B.L., and Lanphere, M.A., 1973, Alaska-Aleutian Range batholith: Geochronology, chemistry, and relation to circum-Pacific plutonism: Geological Society of American Bulletin, v. 84, p.2583-2610.
Reiners, P.W., Nelson, B.K., and Nelson, S.W., 1996, Evidence for multiple mechanisms of crustal contamination of magma from compositionally zoned plutons of the Alaska Range: Journal of Petrology, v. 37, p. 261-292.
Reporters Madelyn A. Millholland (Millholland & Associates)
Last report date 8/10/1998