Admiralty mining district
Encyclopedia
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.
, on Admiralty Island
within the Admiralty Island National Monument
. The mine produces ores 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 (14 km) from the mine/mill complex to a port site on Hawk Inlet; from there they are shipped anywhere worldwide for smelting and refining.
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. 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. 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.
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.
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...
which consists of Admiralty Island
Admiralty Island
Admiralty Island is an island in the Alexander Archipelago in Southeast Alaska, at . It is 145 km long and 56 km wide with an area of 4,264.1 km² , making it the seventh largest island in the United States and the 132nd largest island in the world. It is one of the...
. Most of the gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
recovered from the Admiralty district is a by product of silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
and base metal mining.
The Alaska Empire underground lode mine recovered gold from quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of 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
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
-nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
-cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
ore, without gold, from a Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
gabbro
Gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
-norite
Norite
Norite is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite and hypersthene with olivine. Norite is essentially indistinguishable from gabbro without thin section study under the petrographic microscope...
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.
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 acres (117,358.9 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. It is located approximately 25 km south of Juneau, AlaskaJuneau, Alaska
The City and Borough of Juneau is a unified municipality located on the Gastineau Channel in the panhandle of the U.S. state of Alaska. It has been the capital of Alaska since 1906, when the government of the then-District of Alaska was moved from Sitka as dictated by the U.S. Congress in 1900...
, on Admiralty Island
Admiralty Island
Admiralty Island is an island in the Alexander Archipelago in Southeast Alaska, at . It is 145 km long and 56 km wide with an area of 4,264.1 km² , making it the seventh largest island in the United States and the 132nd largest island in the world. It is one of the...
within the Admiralty Island National Monument
Admiralty Island National Monument
Admiralty Island National Monument is located on Admiralty Island in Southeast Alaska. It was created December 1, 1978, and covers 955,747 acres of Tongass National Forest in the Panhandle of southeast Alaska...
. The mine produces ores of silver and gold, and concentrates of zinc and lead, from a structurally and mineralogically complex VMS
Volcanogenic massive sulfide ore deposit
Volcanogenic massive sulfide ore deposits are a type of metal sulfide ore deposit, mainly Cu-Zn-Pb which are associated with and created by volcanic-associated hydrothermal events in submarine environments....
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 (14 km) from the mine/mill complex to a port site on Hawk Inlet; from there they are shipped anywhere worldwide for smelting and refining.
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. 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. 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.
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
Sedimentary exhalative deposits
Sedimentary exhalative deposits are ore deposits which are interpreted to have been formed by release of ore-bearing hydrothermal fluids into a water reservoir , resulting in the precipitation of stratiform ore....
models. Most of the ore body is zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
-lead-silver-gold-barium
Barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...
rich; some zones have elevated copper values. The deposit is hosted in metamorphosed, multiply deformed, and hydrothermally-altered mainly marine Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
sediments and mafic volcanic rocks within the exotic Alexander terrane
Terrane
A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...
. 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
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
, sphalerite
Sphalerite
Sphalerite is a mineral that is the chief ore of zinc. It consists largely of zinc sulfide in crystalline form but almost always contains variable iron. When iron content is high it is an opaque black variety, marmatite. It is usually found in association with galena, pyrite, and other sulfides...
, galena
Galena
Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...
, and tetrahedrite
Tetrahedrite
Tetrahedrite is a copper antimony sulfosalt mineral with formula: 12Sb4S13. It is the antimony endmember of the continuous solid solution series with arsenic bearing tennantite. Pure endmembers of the series are seldom if ever seen in nature. Of the two, the antimony rich phase is more common...
/tennantite
Tennantite
Tennantite is a copper arsenic sulfosalt mineral. Its chemical formula is Cu12As4S13. It is grey-black, steel-gray, iron-gray or black in color. A closely related mineral, tetrahedrite has antimony substituting for arsenic and the two form a solid solution series. The two have very similar...
are the major sulfide minerals.