Admiralty mining district

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Coordinates: 58°20′N 134°40′W / 58.333, -134.667
Admiralty mining district
— Alaska Mining District —
Admiralty mining district

The Admiralty mining district is a mining area in the U.S. state of Alaska which consists of Admiralty Island. Most of the gold recovered from the Admiralty district is a by product of silver and base metal mining.

The Alaska Empire underground lode mine recovered gold from quartz veins in metamorphic rocks. Discovered and staked in the 1920s, production of about 20,000 tonnes of 0.25 ounce-per-ton gold ore occurred in the mid-1930s. The Funter Bay underground lode mine produced about 500 thousand tonnes of copper-nickel-cobalt ore, without gold, from a Mesozoic gabbro-norite pipe. No significant placer mining was done on Admiralty. About 500,000 ounces of gold, almost all from Greens Creek, have been recovered from the Admiralty district.

[edit] Greens Creek mine (operating)

The Greens Creek mine in the Admiralty mining district is the 5th largest silver producer in the world; gold is a byproduct. The mine is an underground operation, with surface disposal of tailings onto two 29-acre sites. The mine and mill site lie on 18 patented claims; the mine has mineral rights to 12 square miles (7500 acres) of surrounding land owned by the Federal government.^[1] It is located approximately 25 km south of Juneau, Alaska, on Admiralty Island within the Admiralty Island National Monument.^[2] The mine produces dores of silver and gold, and concentrates of zinc and lead, from a structurally and mineralogically complex VMS deposit. Geologists discovered mineralized outcrops in 1975, exploration drilling began in 1978; full-scale development was initiated in 1987 and production of metal concentrate began in 1989. Production was halted, due to low metal prices, for a few years in the mid-'90s. Concentrates are trucked nine miles from the mine/mill complex to a port site on Hawk Inlet; from there they are shipped anywhere worldwide for smelting and refining.^[3]

In 2007 Greens Creek produced 8.6 million ounces of silver, 68,000 ounces of gold, 63,000 tons of zinc and 21,000 tons of lead.^[4] Proven and probable reserves at Greens Creek (in 2006) are 33 million ounces of silver, 257,000 ounces of gold, and 237,000 tons of zinc contained in 2.3 million tons of ore grading approximately 14 opt silver, 0.11 opt gold, 10% zinc, and 4% lead. These numbers reflect the mill recovery rates of approximately 70% for all metals.^[5] New mine reserves have kept pace with production; the mine is expected to operate into the 2020s. Exploration continues on surrounding lands. The mine is the largest taxpayer in the Juneau Borough. Two hundred and eighty people are employed by the mine, three-quarters of whom are residents of Southeast Alaska. Most mine workers commute from Juneau on a high-speed ferry operated by the company.^[6]

Metal sulfides were deposited by hydrothermal fluids on or in the sea floor of a tectonically-active basin. The deposit exhibits features of both VMS and SEDEX models. Most of the ore body is zinc-lead-silver-gold-barium rich; some zones have elevated copper values. The deposit is hosted in metamorphosed, multiply deformed, and hydrothermally-altered mainly marine Triassic sediments and mafic volcanic rocks within the exotic Alexander terrane. Mineralization occurs as discontinuous vein or blanket-like bodies of laminated or replacement-textured massive sulfides; roughly along a structural lithology contact in the sediments. Pyrite, sphalerite, galena, and tetrahedrite/tennantite are the major sulfide minerals.^[7]^[8]