Castle Island

Mine, Inactive

Commodities and mineralogy

Main commodities Barite
Other commodities Ag; Cu; Pb; Sn
Ore minerals barite; bornite; chalcopyrite; galena; pyrite; pyrrhotite; sphalerite; tetrahedrite

Geographic location

Quadrangle map, 1:250,000-scale PE
Quadrangle map, 1:63,360-scale C-4
Latitude 56.6521
Longitude -133.1667
Nearby scientific data Find additional scientific data near this location
Location and accuracy The location of the Castle Island mine is well known. However, the original outcrop of the deposit was little more than a large rock at the northeast end of 'Castle Island'; that original outcrop has now been completely mined out to below sea level. Note the mine which is almost universally called 'Castle Island' is not identified as such on the USGS 1:63,360-scale topographic map and none of the Castle Islands is specifically labeled as Castle Island on the current (2007) maps. The island that is the site of the mine and is described here is an islet about about 650 yards long, located about 1,500 feet south of Big Castle Island, which is shown on current topographic maps.

Geologic setting

Geologic description

The Castle Island barite deposit was known before World War I (Burchard, 1914; Buddington, 1923; Buddington, 1925; Buddington and Chapin, 1929) and was studied several times subsequently (Williams and Decker, 1932 (DGGS IR 117-1); Race, 1963 (DGGS PE 117-9). It was mined nearly continuously from 1966 to 1980 by a succession of companies: Alaska Barite Co from 1966 to 1969; Inlet Oil from from to 1975; and Chromalloy America from 1975 to 1980. The mine closed in 1980 and all the mining equipment and buildings were removed from the island. The original deposit was a small outcrop at the northeast end of the island that was entirely removed by mining. Much of the mining was done underwater from an offshore barge that used a dragline to recover ore fragmented by submarine blasting. The total production was about three-quarters of a million tons of barite, almost all of which was mined from 1968 to 1980 as direct shipping ore.
The ore body consisted of a lenticular, massive barite lens about 300 feet long and up to 200 feet thick that was mined to a depth of about 130 feet below sea level. [This description of the mineralization is synthesized from Burchard (1914); Buddington (1923); Buddington (1925); unpublished written and oral data from David Carnes, U. S. Bureau of Mines; unpublished field notes, analyses, and laboratory studies by the D.J. Grybeck; analyses summarized in Grybeck, Berg, and Karl (1984); and Still and others (2002).] The exact stratigraphic relations are unclear because most of the deposit was under salt water. However, examination of unpublished drilling data and cross sections maintained by the mine indicates that the barite lens probably occurred along the trough of a symmetrical syncline that trends about N30W with limbs that dip about 60NE. The hanging wall was limestone and gray schist; the footwall was graphitic calcareous schist. The drilling also indicated a considerable tonnage of lower grade barite interbedded with 'gray schist,' 'chert,' and 'graphitic schists,' and the possibility of at least one more high-grade barite lens offshore. Mine-run material was massive, white to light gray, almost pure barite that almost invariably contained a percent or so of sulfides as tiny disseminated grains. Assays of the massive barite indicate that it typically contained about 0.5 to 2 percent zinc, about 0.5 percent lead, a small amount of copper, and about 1 ounce of silver per ton. Under the reflecting microscope, the sulfides are sphalerite, galena, pyrite, pyrrhotite, bornite, tetrahedrite-tennantite, and chalcopyrite, together with minor amounts of other unidentified ore minerals, all as tiny, generally equant grains (D.J. Grybeck, personal observation, 1996). Examination of waste dumps provide many samples that show all transitions from massive barite to layered pyrite(-sphalerite-quartz)-barite rock with the other sulfides noted previously that are disseminated though the rock in minor amount. The association of barite, layered sulfide-barite rocks, schistose metafelsite(?), and black carbonaceous, calcareous phyllite at the mine indicate that it is part of the Triassic Duncan Canal-Zarembo Canal belt of dismembered, volcanogenic massive-sulfide deposits described by Berg and Grybeck (1980) and Berg (1981).
The Castle Islands largely, and the island on which the barite mine occurs specifically, consist largely of Upper Triassic Hyd Group rocks which are dominantly felsic and intermediate volcanic flows and breccia, limestone and argillite. However, some of the islands of the Castle Islands also consist of Devonian limestone and Quaternary basalt whose relationship to the Hyd Group rocks is probably structurally complicated and largely hidden under water (Brew, 1997; Karl and others, 1999).
Geologic map unit (-133.168430143618, 56.6517592155839)
Mineral deposit model Barite facies of a Kuroko massive-sulfide model (Cox and Singer, 1986; model 28a); alternatively a barite facies of a Sierran Kuroko model (Bliss, 1992; model 28a.1).
Mineral deposit model number 28a or 28a.1
Age of mineralization Late Triassic based on the age of the host rock.

Production and reserves

Workings or exploration The Castle Island deposit was known before World War I. It was mined nearly continuously from 1966 to 1980 by a succession of companies: Alaska Barite Co from 1966 to 1969; Inlet Oil from from to 1975; and by Chromalloy America from 1975 to 1980. The mine closed in 1980 and all the mining equipment and buildings were removed from the island. The original deposit was a small outcrop at the northeast end of the island that was entirely removed by mining. Much of the mining was then done underwater from an offshore barge that used a dragline to recover ore that was fragmented by submarine blasting. Essentially, the mine was operated as a submarine open pit from a camp on the island. An earlier phase of drilling and sampling on the original barite outcrop that ultimately resulted in mining the deposit was documented by Race (1963 [PE 117-9]) and Williams and Decker (1932 [IR 117-1]).
Indication of production Yes; medium
Reserve estimates In 1977, Carnes (1980) inferred that the deposit contained 390,000 tons of low-grade barite resources with a grade of 83 percent BaSO4 and 315,000 tons of higher grade barite resources. In 1980, Holdsworth (1980) estimated that the deposit contained 69,600 tons of ore-grade material. (From 1977 to 1980, the mine produced about 35,000 tons of barite.) When it closed, the mine had little if any reserves that could be economically mined with then-current technology (oral and written communication, 1996, from David Carnes, U. S. Bureau of Mines, who was the mining engineer in charge of the mine over most of its life).
Production notes The exact production was not systematically reported but total production was about three-quarters of a million tons of barite, most of which was mined from 1968 to 1980 as direct shipping ore. Swainbank and others (1995) indicate that the total production was 776,390 tonnes (865,000 tons) of raw and refined barite produced from 1963 to 1980. Still and others (2002) say that Alaska Barite Company produced 234,000 tons of barite by surface mining from 1966 to 1969 and that Inlet Oil and Chromalloy America produced another 552,888 tons of barite from 1970 to 1980 from the submarine portion of the barite lens.

References

MRDS Number A010345; A010666

References

Carnes, R.D., 1980, Update of the Bureau of Mines Mineral Availability System: U.S. Bureau of Mines unpublished report. (On file at the Bureau of Land Management, Minerals Information Center, Juneau, Alaska.)
Holdsworth, P.R., 1980, Unpublished letter to Martin Epner, President of Chromalloy America Corporation. (On file at the Bureau of Land Management, Minerals Information Center, Juneau, Alaska.)
Still, J.C., Bittenbender, P.E., Bean, K.W., and Gensler, E.G., 2002, Mineral assessment of the Stikine area, central Southeast Alaska: U.S. Bureau of Land Management Technical Report 51, 560 p.
Reporters H.C. Berg (Fullerton, California) and D.J. Grybeck (Port Ludlow, WA)
Last report date 4/8/2007