Admiralty mining district

The Admiralty mining district is a mining area in the U.S. state of Alaska which consists of Admiralty Island. Silver and base metals are mined, with gold recovered as a by-product.

Admiralty mining district
Alaska Mining District
Admiralty mining district
Coordinates: 58°05′N 134°38′W
CountryUnited States
StateAlaska

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 tons of 0.25 ounce-per-ton gold ore occurred in the mid-1930s. No significant placer mining was done on Admiralty Island. About 500,000 ounces of gold, almost all from Greens Creek, have been recovered from the Admiralty district.

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 (120,000 m2) 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 20 miles (32 km) south of Juneau, Alaska, on Admiralty Island within the Admiralty Island National Monument.[2] The mine produces silver, gold, zinc and lead from a structurally and mineralogically complex VMS deposit. Geologists discovered zinc anomalous stream sediments in the Greens Creek basin in 1973. Follow-up in 1974 led to discovery of a hillside secondary iron oxide gossan containing highly anomalous lead, zinc and silver. Core drilling to test the unoxidized bedrock under the gossan was initiated in 1975 and penetrated a 82 feet (25 m) thick lens of massive sulfide mineralisation. An extensive exploration drilling program began after the discovery hole and positive results led to full-scale development beginning in 1987 and production of metal concentrates in 1989. Production was halted due to low metal prices, for a few years in the mid-'90s. Concentrates are trucked 8 miles (13 km) from the mine/mill complex to a port on Hawk Inlet; from there they are shipped worldwide for smelting and refining.[3][4]

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.[5] 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.[6] 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, which departs Juneau at 5:00AM and 5:00PM daily.[7]

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.[8][9]

See also

Gold mining in Alaska

References
  1. HECLA MINING CO/DE/ - HL Annual Report (Regulation S-K, item 405) (10-K405) ITEM 1. BUSINESS.(1)
  2. Greens Creek Geology Page
  3. Taylor, Cliff D.; West, Andrew W.; Lear, Kerry G.; Hall, Tim E.; Proffett, John M. (2010). "Geology and Metal Zoning of the Greens Creek Massive Sulfide Deposit, Southeastern Alaska". Geology, Geochemistry, and Genesis of the Greens Creek Massive Sulfide Deposit, Admiralty Island, Southeastern Alaska. U.S. Geological Survey. doi:10.3133/pp1763.
  4. Gold Pour Slide Show Archived February 16, 2008, at the Wayback Machine
  5. Hecla Mining Company - News Release
  6. Hecla Mining Company Archived May 9, 2008, at the Wayback Machine
  7. Greens Creek Mine
  8. Economic Geology Monograph 9, Mineral Deposits of Alaska, VMS Deposits, Alaska, Newberry et al., 1997, pp137-140
  9. "Abstract Listing - Text Format". Archived from the original on 2011-07-06. Retrieved 2008-03-26.
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