Horsfeld

Prospect, Inactive

Commodities and mineralogy

Main commodities Cu
Other commodities Ag; Au; Mo
Ore minerals chalcopyrite; magnetite; pyrite
Gangue minerals actinolite; potassium feldspar; quartz

Geographic location

Quadrangle map, 1:250,000-scale NB
Quadrangle map, 1:63,360-scale A-1
Latitude 62.0443
Longitude -141.216
Nearby scientific data Find additional scientific data near this location
Location and accuracy This prospect is in the headwaters of a small, unnamed north tributary to Horsfeld Creek; it is about 6,000 feet southwest of elevation 7253 (Joe) and 3,300 feet south-southwest of peak 6895. The site is near the center of the north boundary of the NW1/4 of section 16, T. 3 N., R. 23 E. of the Copper river Meridian. This is locality 7 of Richter and others (1973), locality 52 of Richter and others (1975), and National Park Service locality WRST-85 (unpublished data). Cobb and Richter (1980) included this prospect under the name 'Horsfeld'. It is located to within a few hundred feet.

Geologic setting

Geologic description

At the Horsfeld prospect, pyrite and chalcopyrite occur in quartz veinlets and as disseminations in monzonite and monzonite porphyry of the mid-Cretaceous, Klein Creek pluton (Richter and others, 1973; 1975). This porphyry copper and molybdenum deposit has a central 200- by 700-meter zone of potassium feldspar, actinolite, and magnetite alteration, and a 800- by 1,500-meter outer zone of unspecified hydrothermal alteration. Surface samples contain less than 0.5 percent copper, and average 0.10 percent copper and less than 0.01 percent molybdenum (Hollister and others, 1975; Richter and others, 1975). Gold and silver were detected in some surface samples (Matson and Richter, 1971 [OFR 71-202]). The deposit is estimated to contain 60 million tons with an average grade of 0.20 percent copper and very low amounts of molybdenum (Richter and others, 1975). The deposit is in the Klein Creek pluton near its contact with Jurassic or Cretaceous siliciclastic rocks; the associated hydrothermal alteration zone extends across this contact (Richter and others, 1973).
Geologic map unit (-141.218131691646, 62.0439759046027)
Mineral deposit model Porphyry Cu-Mo (Cox and Singer, 1986; model 21a)
Mineral deposit model number 21a
Age of mineralization Mid-Cretaceous. The diorite of the Klein Creek pluton that hosts the deposits has a K/Ar date of 111 +/- 3.6 Ma (Richter, Lanphere, and Matson, 1975).
Alteration of deposit This porphyry copper and molybdenum deposit has a central 200- by 700-meter zone of potassium feldspar-actinolite-magnetite alteration, and a 800- by 1,500-meter outer zone of unspecified hydrothermal alteration. Oxidation has resulted in abundant iron-staining.

Production and reserves

Workings or exploration Surface exploration and some diamond drilling have been used to evaluated this deposit.
Indication of production None
Reserve estimates The deposit is estimated to contain 60 million tons with an average grade of 0.20 percent copper and very low amounts of molybdenum (Richter and others, 1975).

Additional comments

The prospect is in the Wrangell-St. Elias National Preserve.

References

MRDS Number A011422

References

Hollister, V.F., Anzalone, S.A., and Richter, D.H., 1975, Porphyry copper belts of southern Alaska and contiguous Yukon Territory: Canadian Mining and Metallurgical Bulletin, v. 68, no. 756, p. 104-112.
Richter, D.H., Lanphere, M.A., and Matson, N.A., Jr., 1975, Granite plutonism and metamorphism, eeastern Alaska Range, Alaska: Geological Society of American Bulletin, v. 86, p. 819-820.
Reporters Travis L. Hudson (Applied Geology)
Last report date 11/24/2002