Sedimentary-rock hosted gold deposits in Nevada

Twin Creeks mine Nevada Carlin-type gold ore

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, sand an ...

-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 met ...

deposits. These deposits are characterized by invisible (typically microscopic and/or dissolved) gold in arsenic rich pyrite and

arsenopyrite Arsenopyrite ( IMA symbol: Apy) is an iron arsenic sulfide (FeAsS). It is a hard ( Mohs 5.5-6) metallic, opaque, steel grey to silver white mineral with a relatively high specific gravity of 6.1. When dissolved in nitric acid, it releases eleme ...

. 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 Carlin may refer to: People * Carlin (name), including a list of people with the name Places * 4121 Carlin, a main belt asteroid * Carlin, Nevada, United States * Carlin How, a village in northern England, UK * Carlin Precinct, Calhoun County, I ...

, the first large deposit of this type discovered in the

Carlin Trend The Carlin Unconformity or Carlin Trend is a geologic feature in northeastern Nevada which represents a period of erosion or non-deposition likely associated with a collision between a tectonics, tectonic crustal block called a terrane and the N ...

Nevada Nevada ( ; ) is a U.S. state, state in the Western United States, 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. N ...

Geology

The Carlin type deposits show enrichment in the elements

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, but ...

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 time ...

mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...

thallium Thallium is a chemical element with the Symbol (chemistry), 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 W ...

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. Th ...

. 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 when ...

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 g ...

, although also

silicates In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is al ...

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 **Sider ...

s are either dissolved or converted to the silicates by silicate rich hydrothermal water. For example,

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

is transformed into

jasperoid Jasperoid is a rare, peculiar type of metasomatic alteration and occurs in two main forms; sulfidic jasperoids and hematitic jasperoids. True jasperoids are different from jaspillite, which is a form of metamorphosed chemical sedimentary rock, and f ...

. 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 minerals ...

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) feldsp ...

. 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 The Great Basin is the largest area of contiguous endorheic basin, endorheic watersheds, those with no outlets, in North America. It spans nearly all of Nevada, much of Utah, and portions of California, Idaho, Oregon, Wyoming, and Baja California ...

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.

References

External links

"The Rush to Uncover Gold’s Origins"

Geotimes The American Geosciences Institute (AGI) is a nonprofit federation of about 50 geoscientific and professional organizations that represents geologists, geophysicists, and other earth scientists. The organization was founded in 1948. The name of ...

, April 2006 {{DEFAULTSORT:Carlin-type gold deposit Economic geology Ore deposits Gold mining

Geology

Mining

See also

References

External links