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Carlin–type gold deposits are
"The Rush to Uncover Gold’s Origins"
Geotimes, April 2006 {{DEFAULTSORT:Carlin-type gold deposit Economic geology Ore deposits Gold mining
Carlin–type gold deposits are sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
-hosted disseminated gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile ...
deposits. These deposits are characterized by invisible (typically microscopic and/or dissolved) gold in arsenic rich pyrite and arsenopyrite
Arsenopyrite (International Mineralogical Association, IMA List of mineral symbols, symbol: Apy) is an iron arsenic sulfide (FeAsS). It is a hard (Mohs scale of mineral hardness, Mohs 5.5-6) metallic, opaque, steel grey to silver white mineral wi ...
. This dissolved kind of gold is called "Invisible Gold", as it can only be found through chemical analysis. The deposit is named after the Carlin mine, the first large deposit of this type discovered in the Carlin Trend, Nevada
Nevada ( ; ) is a state in the Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the 7th-most extensive, ...
.
Geology
The Carlin type deposits show enrichment in the elementsgold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile ...
, arsenic
Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, bu ...
, antimony
Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient ti ...
, mercury, thallium
Thallium is a chemical element with the symbol Tl and atomic number 81. It is a gray post-transition metal that is not found free in nature. When isolated, thallium resembles tin, but discolors when exposed to air. Chemists William Crookes a ...
and barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element.
...
. This enrichment is created by hydrothermal
Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...
circulation with a temperature of up to 300 °C. The underlying rocks out of which the minerals are dissolved are normally silt
Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel wh ...
y carbonates
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...
, although also silicates and other sediments are possible. The source of the heating for the water in the hydrothermal circulation is still under discussion. The material in the deposit is altered in a way that the carbonate mineral
Carbonate minerals are those minerals containing the carbonate ion, .
Carbonate divisions Anhydrous carbonates
*Calcite group: trigonal
** Calcite CaCO3
** Gaspéite (Ni,Mg,Fe2+)CO3
**Magnesite MgCO3
** Otavite CdCO3
** Rhodochrosite MnCO3
* ...
s are either dissolved or converted to the silicates by silicate rich hydrothermal water. For example, dolomite is transformed into jasperoid. Another alteration is the formation of clay mineral
Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces.
Clay mineral ...
s by interaction of water and feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feld ...
. The absence of base metal sulfides and the even distribution of the pyrite and arsenopyrite in the host rock are the most obvious difference to other sulfide deposits.
Mining
The Carlin–type deposits represent some of the largest hydrothermal gold deposits in the world. The invisible nature of the gold in the deposit makes it difficult to find deposits of that kind. The class of deposit was defined after the Carlin mine became a mass producer of gold in the 1960s and it was recognized that other deposits of that type should exist. Most of the mines in the Great Basin in the United States belong to the Carlin type. Similar "Invisible Gold" deposits have also been found in northern Canada, China, Iran, and Macedonia; but the relationship between these deposits and those in Nevada are debated.See also
* Gold mining in NevadaReferences
External links
"The Rush to Uncover Gold’s Origins"
Geotimes, April 2006 {{DEFAULTSORT:Carlin-type gold deposit Economic geology Ore deposits Gold mining