Iron Cap

Prospects, Active

Commodities and mineralogy

Main commodities Cu; Fe
Other commodities Ag; Au
Ore minerals chalcopyrite; magnetite; pyrite; pyrrhotite
Gangue minerals calcite; chlorite; diopside; epidote; garnet; hornblende; quartz

Geographic location

Quadrangle map, 1:250,000-scale CR
Quadrangle map, 1:63,360-scale C-2
Latitude 55.64901
Longitude -132.39142
Nearby scientific data Find additional scientific data near this location
Location and accuracy The center of the area that contains the Iron Cap prospects is about 0.4 mile northwest of the top of Tolstoi Mountain and about 0.5 mile south of the center of section 4, T. 72 S., R. 85. The workings and geology of the Iron Cap and nearby prospects are shown on plate 17 of Warner and others (1961).

Geologic setting

Geologic description

The rocks in the vicinity of the Iron Cap prospects consist mainly of greenstone and clastic sedimentary rocks interlayered with limestone. The area is cut by a multitude of closely spaced, northeast-trending, nearly vertical dikes of diorite, granodiorite, diabase, basalt, and andesite. There is a major fault southwest of the prospects that can be traced for at least three miles. About 1,500 feet to the northeast, there is a contact with a large granodiorite body that postdates the dikes. The geology of the area and the location of the workings are shown in detail by Warner and others (1961, plate 17).
The deposit crops out in an area of about 1,000 by 2,000 feet, but most of the mineralization is confined to an area of about 150 by 200 feet delineated by numerous trenches. The mineralization consists of pods and lenses as much as 200 feet long and 20 to 30 feet thick of magnetite with subordinate pyrite and chalcopyrite. A few of the lenses consist largely of pyrite, pyrrhotite, and chalcopyrite, with subordinate magnetite. The ore bodies preferentially replace what was probably calcareous shale or sandstone (as distinct from the greenstone) and are roughly conformable to the layering. The ore bodies are associated with tactite composed of diopside, andradite garnet, hornblende, epidote, and blue-green chlorite, and are cut by late veinlets of quartz and calcite.
The Iron Cap prospect was explored around 1900 by an adit about 100 feet long, by numerous pits and trenches, and, in 1901, by several hundred feet of drilling (Brooks, 1902). During WW II, the U.S. Bureau of Mines drilled the deposit , collected numerous samples, and carried out a dip needle survey (Erickson, 1948). The dip-needle survey suggested that the ore bodies are shallow and do not extend outside the area of the known magnetite lenses. However, the American Copper and Nickel Company recently commissioned an aerial geophysical [aeromagnetic?] survey that located several strong anomalies in the area (Anzman, 1998; Hedderly-Smith, 1999 [Inventory]). The work by the Bureau of Mines defined about 90,000 metric tons of reserves with 40 percent iron, 0.25 percent copper, and trace gold and silver (Erickson, 1948; Warner and others, 1961; Maas and others, 1995).
The Iron Cap prospect is one of many copper-iron deposits on the Kasaan Peninsula having similar geology and origin (Warner and others, 1961; Eberlein and others, 1983; Brew, 1996). The rocks on the peninsula consist mainly of andesite ('greenstone' in much of the older literature) interbedded with about 25 percent sedimentary rocks comprising approximately equal amounts of limestone or marble, calcareous mudstone and sandstone, and graywacke and conglomerate. These units are part of the Luck Creek Breccia of Silurian and Devonian age, but many of the sedimentary units are similar to and probably grade into rocks of the Silurian and Ordovician, Descon Formation. The bedded rocks are intruded by a profusion of Silurian or Ordovician dikes, sills, and irregular masses of porphyritic gabbro, basalt, andesite, diorite, dacite, and granodiorite. Near some of the deposits, these intrusions may make up 20 percent or more of the outcrop and usually are associated with the development of tactite and alteration of the greenstone. The area subsequently was intruded by several large Silurian or Ordovician plutons; they are mainly granodiorite but locally are diorite and gabbro.
The ore deposits are typically small and of irregular shape; often the ore bodies form lenses or mantos. Some of the deposits conform to the layering in the greenstone and sedimentary rocks. The principal ore minerals are chalcopyrite, pyrite, and magnetite; hematite is often present and a little molybdenite occurs in some deposits. Most of the deposits are associated with tactite or skarn with varying amounts of actinolite, calcite, chlorite, garnet, diopside, epidote, and hornblende. There was significant by-product silver and gold in the ore that was mined in the past, and the gold values in some deposits are high enough to have encouraged exploration in recent years. Marble is more common in the deposits in the western part of the peninsula, where the gold values are generally higher as well (Wright and Wright, 1908; Wright, 1915; Warner and others, 1961; Myers, 1985; Bond, 1993; Maas and others, 1995).
Early interpretations of the ore deposits on the Kasaan Peninsula emphasize their contact metamorphic origin and their probable Mesozoic age (for example, Warner and others, 1961). However, recent radiometric dating and mapping indicate that the deposits formed in a Silurian or Ordovician, arc-related environment characterized by deposition of andesite and submarine sedimentary rocks that were intruded by swarms of dikes of varying composition, mineralized, and then intruded by large granodiorite plutons (Hedderly-Smith, 1999 [Inventory]).
The copper deposits of the Kasaan Peninsula were known to the Russians and the first claim was staked in 1867. Most of the production and development occurred from about 1900 to 1918, especially from 1905 to 1907, when copper prices soared and a smelter was built at Hadley on the north side of the Kasaan Peninsula. After World War I, copper supply exceeded demand, prices fell, and there has been no further copper production since 1918 (Wright, 1915; Warner and others, 1961; Roppel, 1991; Maas and others, 1995). However, because of the intense and widespread mineralization on the peninsula, the area has repeatedly been re-examined for copper, iron, and gold, notably during WW II (Warner and others, 1961) and in the last several decades.
Geologic map unit (-132.393103383813, 55.6486427438327)
Mineral deposit model Cu-Fe skarn (Cox and Singer, 1986; model 18d).
Mineral deposit model number 18d
Age of mineralization The deposit formed in a Silurian or Ordovician, submarine arc-related environment characterized by the deposition of volcanic and sedimentary rocks, the intrusion of swarms of dikes of diverse composition, and the emplacement of several large plutons.
Alteration of deposit Development of calc-silicate skarn.

Production and reserves

Workings or exploration The Iron Cap prospect was explored around 1900 by an adit about 100 feet long, by numerous pits and trenches, and, in 1901, by several hundred feet of drilling (Brooks, 1902). During WW II, the U.S. Bureau of Mines drilled the deposit , collected numerous samples, and carried out a dip needle survey (Erickson, 1948). The dip-needle survey suggested that the ore bodies are shallow and do not extend outside the area of the known magnetite lenses. However, the American Copper and Nickel Company recently commissioned an aerial geophysical [aeromagnetic?] survey that located several strong anomalies in the area (Anzman, 1998; Hedderly-Smith, 1999 [Inventory]).
Indication of production Undetermined
Reserve estimates The work by the Bureau of Mines defined about 90,000 metric tons of reserves with 40 percent iron, 0.25 percent copper, and trace gold and silver (Erickson, 1948; Warner and others, 1961; Maas and others, 1995).

References

MRDS Number A010084; A010105; A010141

References

Anzman, J.R., 1995, Airborne geophysical survey, Kasaan Peninsula, Alaska, 12 p. (Unpublished report held by Sealaska Corporation, Juneau, Alaska).
Bond, R.W., 1993; The mineralogy and geochemistry of the Kasaan Peninsula, iron-copper-silver-gold skarns, Prince of Wales Island, southeastern Alaska: Salt Lake City, University of Utah, M.Sc. thesis, 130 p.
Glavinovich, P.S., 1987, Sealaska Corporation Minerals Breifing Book, December 1987: Sealaska Corporation, 40 p. and plates. (Unpublished report held by Sealaska Corporation, Juneau, Alaska).
Hedderly-Smith, D.A., 1993, Report of the 1992 field season-Sealaska minerals reconnaissance project: Sealaska Corporation, 60 p. (Unpublished report held by the Sealaska Corporation, Juneau, Alaska.)
Hedderly-Smith, D.A., 1997, Report on the 1995 and 1996 work on the Kasaan Peninsula-Sealaska minerals project: Sealaska Corporation, 87 p. (Unpublished report held by the Sealaska Corporation, Juneau, Alaska.)
Hedderly-Smith, D.A., 1998, Report of the 1995-1997 work on the Kasaan Peninsula-Sealaska minerals project: Sealaska Corporation, 130 p. (Unpublished report held by the Sealaska Corporation, Juneau, Alaska.)
Hedderly-Smith, D.A., 1999, Inventory of metallic mineral prospects, showings and anomalies on Sealaska lands, 1988 through 1998: Sealaska Corporation, Juneau, Alaska, 217 p. (internal report held by Sealaska Corporation, Juneau, Alaska).
Myers, G.L., 1985, Geology and geochemistry of the iron-copper-gold skarns of Kasaan Peninsula, Alaska: Fairbanks, University of Alaska, M.Sc. thesis, 165 p.
Roppel, Patricia, 1991, Fortunes from the earth: Manhattan, Kansas, Sunflower University Press, 139 p.
Reporters D.J. Grybeck (Applied Geology)
Last report date 5/1/2004