Magnetite Cliff

Prospects, Inactive

Commodities and mineralogy

Main commodities Cu; Fe
Other commodities Ag; Au
Ore minerals chalcopyrite; magnetite
Gangue minerals diopside; garnet; quartz

Geographic location

Quadrangle map, 1:250,000-scale CR
Quadrangle map, 1:63,360-scale A-2
Latitude 55.24903
Longitude -132.62844
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Magnetite Cliff prospects are at an elevation of about 1,600 feet, about 1.2 miles northwest of Copper Mountain and about 0.6 mile northwest of the southeast corner of section 27, T. 76 S., R. 84 E. This record also includes several copper-bearing magnetite occurrences in skarn about 2,800 feet east of the Magnetite Cliff prospects; these have been called the 'Upper Magnetite' occurrences.

Geologic setting

Geologic description

The Magnetite Cliff prospect is one of several similar iron skarn deposits in Jumbo Basin (Kennedy, 1953; Herreid and others, 1978; Maasand others, 1995). The rocks in Jumbo Basin consist of intensely folded marble, calcareous schist, and quartz-mica schist, overlain, possibly unconformably, by a thick greenstone unit; all are part of the Wales Group of Late Proterozoic or Cambrian age (Herreid and others, 1978; Eberlein and others, 1983; Brew, 1996). The metamorphic rocks are intruded by a large Cretaceous stock that is mainly granodiorite but locally varies to gabbro. Altered andesite dikes and sills are common. The deposits at the Magnetite Cliff and nearby prospects are part of a band of skarn up to 1,000 feet wide at the marble-granodiorite contact.
It is uncertain when the Magnetite Cliff prospect was discovered, but it was identified by symbol before 1908 as part of the Jumbo Group (Wright and Wright, 1908). The prospect was developed by 4 short adits but has not produced any ore. Several similar occurrences nearby that are included in this record are often called the 'Upper Magnetite' deposits (Kennedy, 1953; Maas and others, 1995).
The Magnetite Cliff prospect consists of 3 tabular magnetite lenses that contain 2 to 3 percent chalcopyrite (Kennedy, 1953). In contrast to the Jumbo Mine (CR150), there is no molybdenite. The largest of these lenses is about 300 feet wide, up to 50 feet thick, and can be traced down dip for several hundred feet. The lenses strike about N25W and dip 30-60NE; they are in the band of skarn at the marble-granodiorite contact. The skarn consists almost entirely of diopside and garnet; the marble-skarn contact is sharp and little marble remains in the skarn. The granodiorite near the contact is intensely altered and locally is totally replaced by diopside and garnet.
The Upper Magnetite deposits are 5 small, irregular-shaped masses of magnetite with a few percent of disseminated chalcopyrite, scattered over an area about 1,200 feet long. They occur in roof pendants of marble and skarn next to the granodiorite or in skarn in the granodiorite. As at the Magnetite Cliff prospect, the skarn consists mostly of diopside and garnet, but it also contains some quartz. The contact between the skarn and marble is gradational.
Before WW II, the Magnetite Cliff prospect was usually described as part of the Jumbo Mine. In his detailed study, Kennedy (1953) did extensive geological and geophysical work on the Magnetite Cliff prospect, as well as on the similar magnetite bodies nearby. The deposits have since been examined several times and drilled, usually as part of more extensive exploration that included the Jumbo Mine and other copper properties nearby. The Anaconda Company worked in the area in the 1950s and conducted several geophysical surveys in the 1960s (Gonnason Exploration, 1963; Hings, 1964; Klobusicky, 1965). Hanna Mining examined the property (Hogg , 1965), and Cominco Alaska Exploration sampled and mapped the area in 1989 and 1990.
Kennedy (1953) estimates that the Magnetite Cliff prospect has approximately 370,000 tons of indicated and inferred ore with a grade of about 45 percent iron and 0.73 percent copper. None of the other magnetite occurrences has more than a few thousand tons of ore and collectively they probably contain less than 50,000 tons. The current annual minerals report of the State of Alaska (Swainbank and others, 2002) estimates the 'Jumbo' reserves as 650,000 tons of ore with an average grade of 45.2 percent iron, 0.75 percent copper, 0.01 ounce of gold per ton, and 0.08 ounce of silver per ton. Those reserves probably include the reserves defined by Kennedy at the Magnetite Cliff and nearby deposits.
Geologic map unit (-132.630094755764, 55.2486788070805)
Mineral deposit model Iron-copper skarn (Cox and Singer, 1986; model 18d).
Mineral deposit model number 18d
Age of mineralization Iron-copper skarn probably related to Cretaceous granodiorite exposed at the surface nearby.
Alteration of deposit Magnetite lenses associated with diopside-garnet skarn. Andesite dikes and sills are altered.

Production and reserves

Workings or exploration Prospecting in the area began by at least 1879, although copper was reported in the area earlier by Russians and Indians (Wright, 1915; Kennedy, 1953; Herreid and others, 1978; Roppel, 1991). By 1905, considerable reserves had been defined at the nearby Jumbo Mine (CR150), which produced copper ore from 1907 to 1923. The Magnetite Cliff deposit was found early in the history of the area, but in early reports it was usually described as part of the Jumbo property (Brooks, 1902; Wright and Wright, 1908; Wright, 1915; Wright and Fosse, 1946). The most detailed study of the Magnetite Cliff deposit is by Kennedy (1953), who described the rocks in detail and mapped the surface and underground workings. The deposit has since been examined several times. The Anaconda Company examined the area in the 1950s and conducted several geophysical surveys in the 1960s (Gonnason Exploration, 1963; Hings, 1964; Klobusicky, 1965). Hanna Mining examined the property (Hogg , 1965), and Cominco Alaska Exploration mapped and sampled the deposits in 1989 and 1990.
Indication of production None
Reserve estimates Kennedy (1953) estimates that the Magnetite Cliff prospect has approximately 370,000 tons of indicated and inferred ore with a grade of about 45 percent iron and 0.73 percent copper. None of the other magnetite occurrences has more than a few thousand tons of ore and collectively they probably contain less than 50,000 tons. The current annual minerals report of the State of Alaska (Swainbank and others, 2002)--as have their annual reports for many years--estimates the 'Jumbo' reserves as 650,000 tons of ore with an average grade of 45.2 percent iron, 0.75 percent copper, 0.01 ounce of gold per ton, and 0.08 ounce of silver per ton. Those reserves probably include the reserves defined by Kennedy at the Magnetite Cliff and nearby deposits.

Additional comments

The Magnetite Cliff prospect is on a large block of patented claims. The land around it has been conveyed to the Sealaska Corporation or is under application for transfer to them.

References

MRDS Number A010683

References

Burton, W.D., 1924, Report on the Jumbo mine, Sulzer, Alaska, 6 p. (Unpublished report held by the Bureau of Land Management, Mineral Information Center, Juneau, Alaska).
Hogg, N., 1965, Eskil Anderson properties-Jumbo Basin, Prince of Wales Island, Alaska: Unpublished report to Hanna Mining Company, 8 p. (Unpublished report held by the Bureau of Land Management, Mineral Information Center, Juneau, Alaska.)
Klobusicky, T., 1965, Examination of the Jumbo Mine, southeastern Alaska: Duval Mining Company, 10 p. (Unpublished report held by the Bureau of Land Management, Mineral Information Center, Juneau, Alaska).
Leavens, P.B., and Thomssen, R.W., 1977, Famous mineral localities: Prince of Wales Island, Alaska: Mineralogical Record, v. 8, no. 1, p. 4-12.
Roppel, Patricia, 1991, Fortunes from the earth: Manhattan, Kansas, Sunflower University Press, 139 p.
Smith, S.S., 1917, The mining industry in the Territory of Alaska during the calendar year 1915: U.S. Bureau of Mines Bulletin 142, 65 p.
Smith, S.S., 1917, The mining industry in the Territory of Alaska during the calendar year 1916: U.S. Bureau of Mines Bulletin 153, 89 p.
Reporters D.J. Grybeck (Applied Geology)
Last report date 5/1/2004