Orange Hill

Prospect, Inactive

Commodities and mineralogy

Main commodities Ag; Au; Cu; Mo
Ore minerals chalcopyrite; molybdenite; pyrite
Gangue minerals anhydrite; biotite; calcite; chlorite; epidote; gypsum; potassium feldspar; quartz; sericite

Geographic location

Quadrangle map, 1:250,000-scale NB
Quadrangle map, 1:63,360-scale A-4
Latitude 62.204
Longitude -142.8449
Nearby scientific data Find additional scientific data near this location
Location and accuracy This prospect is on the northeast flank of Orange Hill. It is in an unnamed drainage that flows north-northwest to near the Orange Hill airstrip, which is 0.5 mile northwest of the prospect. The site is near the northeast corner of section 20, T. 5 N., R. 14 E. of the Copper River Meridian. This is locality 2 of Richter (1973), locality 22 of Richter and others (1975), and included in National Park Service locality WRST-31 (unpublished data). Cobb and Richter (1980) included this prospect under the name 'Orange Hill'. It is located to within a few hundred feet.

Geologic setting

Geologic description

The Orange Hill porphyry copper prospect was first discovered and staked as early as 1902 for its possible gold content (Mendenhall and Schrader, 1903). By 1940, 18 mining claims had been patented, including contiguous claims that extend southeast across contact metamorphic deposits (Moffit, 1943, 1954; also see NB044). Over the years, the prospect has been extensively explored by open cuts, short adits, and much diamond drilling.
Pyrite, chalcopyrite, and molybdenite occur in quartz veinlets and as disseminations in quartz diorite and granodiorite intruded by quartz plagioclase porphyry (Richter and others, 1975). Hydrothermal alteration is well developed in the Orange Hill deposit (Linn, 1973; Hollister and others, 1975; Richter and others, 1975). A central 400 by 2,000 meter altered zone contains abundant biotite, quartz veinlets, minor K-feldspar, chlorite, and sericite. It is cut by late anhydrite veins. An outer 1,000 by 3,000 meter altered zone contains chlorite, minor sericite, and anhydrite. The deposit has been extensively oxidized and iron-oxide staining is abundant. Surface copper grades are less than 0.4 percent and average about 0.25 percent. Surface molybdenum grades are as high as 0.8 percent but average about 0.01 percent. The average grade of the non-oxidized mineralization is estimated to be 0.35 percent copper and 0.02 percent molybdenum (Richter and others, 1975). Gold values range up to 0.04 ounce per ton and probably average about 0.005 ounce per ton (Van Alstine and Black, 1944; Richter and others, 1975 ). The average silver grade is estimated to be 0.01 ounce per ton (Van Alstine and Black, 1944). Three estimates of the tonnage and grade of the Orange Hill deposit are: 1) 210 million tons grading 0.04 per cent copper and 0.02 percent molybdenum (Van Alstine and Black, 1944); 2) 86 million tons grading 0.3 or more percent copper and 0.015 percent molybdenite (Linn, 1973); and 3) 320 million tons grading 0.35 percent copper and 0.02 percent molybdenum (Richter and others, 1975). Richter and others (1975) estimate the deposit contains 1.6 million ounces of gold and 3.2 million ounces of silver.
The Orange Hill deposit is within quartz diorite and granodiorite of the southwestern border zone of the mid-Cretaceous Nabesna pluton. The granitic rocks in turn are intruded by quartz-plagioclase porphyry that is also inferred to be Cretaceous in age (Richter, 1973). Many varieties of hornblende-plagioclase porphyry form irregular intrusive bodies in the Nabesna pluton and other regional bedrock units in the area; these are considered to be Tertiary in age (Richter, 1973). K/Ar ages for the Nabesna pluton range from 105 +/- 4.0 to 114 +/- 3.4 Ma (Richter and others, 1975; Richter, Lanphere, and Matson, 1975). Country rocks to the Nabesna pluton at Orange Hill include upper Paleozoic metavolcanic rocks, Permian limestone, Permian or Triassic argillite, and Triassic, Nikolai Greenstone (Richter, 1973).
Geologic map unit (-142.847027068724, 62.2036105074013)
Mineral deposit model Porphyry Cu-Mo (Cox and Singer, 1986; model 21a)
Mineral deposit model number 21a
Age of mineralization Mid-Cretaceous. The intrusive host rocks to the Orange Hill deposit are considered to be part of the Nabesna pluton. K/Ar ages for the Nabesna pluton range from 105 +/- 4.0 to 114 +/- 3.4 Ma (Richter and others, 1975; Richter, Lanphere, and Matson, 1975). Many varieties of hornblende-plagioclase porphyry form irregular intrusive bodies into the Nabesna pluton and other regional bedrock units in the area; these are considered to be Tertiary in age (Richter, 1973).
Alteration of deposit A central 400 by 2,000 meter altered zone contains abundant biotite, quartz veinlets, minor K-feldspar, chlorite, and sericite. It is cut by late anhydrite veins. An outer 1,000 by 3,000 meter altered zone contains chlorite, minor sericite, and anhydrite. The deposit has been extensively oxidized and iron-oxide staining is abundant.

Production and reserves

Workings or exploration The Orange Hill porphyry copper prospect was first discovered and staked as early as 1902 for its possible gold content (Mendenhall and Schrader, 1903). By 1940, 18 mining claims, a millsite, and a homestead had been patented. The mining claims include several contiguous claims that extend southeast across contact metamorphic deposits (Moffit, 1943; 1954; also see NB044). Over the years, the prospect has been extensively explored by open cuts, shafts, short adits, and much diamond drilling. The underground workings total only 250 to 300 feet and most were reported be caved by 1944 (Van Alstine and Black, 1944).
Indication of production None
Reserve estimates The average grade of the unoxidized mineralization is estimated to be 0.35 percent copper and 0.02 percent molybdenum (Richter and others, 1975). Gold values range up to 0.04 ounce per ton and probably average about 0.005 ounce per ton (Van Alstine and Black, 1944; Richter and others, 1975). The average silver grade is estimated to be 0.01 ounce per ton (Van Alstine and Black, 1944). Three estimates of the tonnage and grade of the Orange Hill deposit are: 1) 210 million tons grading 0.04 per cent copper and 0.02 percent molybdenum (Van Alstine and Black, 1944); 2) 86 million tons grading 0.3 or more percent copper and 0.015 percent molybdenite (Linn, 1973); and 3) 320 million tons grading 0.35 percent copper and 0.02 percent molybdenum (Richter and others, 1975). Richter and others (1975) estimate the deposit contains 1.6 million ounces of gold and 3.2 million ounces of silver.

Additional comments

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

References

MRDS Number A011395; D000306

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.
Linn, G.W., 1973, Geology of Orange Hill, Alaska: Berkeley, University of California, M.A. thesis, 119 p.
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