Carlin–type gold deposits are sediment

Sediment

Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

-hosted disseminated 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...

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

 and arsenopyrite

Arsenopyrite

Arsenopyrite is an iron arsenic sulfide . It is a hard metallic, opaque, steel grey to silver white mineral with a relatively high specific gravity of 6.1. When dissolved in nitric acid, it releases elemental sulfur. When arsenopyrite is heated, it becomes magnetic and gives off toxic fumes...

. The deposit is named after the first large deposit of this composition found in the Carlin Unconformity

Carlin Unconformity

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 tectonic crustal block called a terrane and the North American Plate. The collision occurred during the...

, Nevada

Nevada

Nevada is a state in the western, mountain west, and southwestern regions of the United States. With an area of and a population of about 2.7 million, it is the 7th-largest and 35th-most populous state. Over two-thirds of Nevada's people live in the Las Vegas metropolitan area, which contains its...

.

Geology

The Carlin type deposits show enrichment in the elements 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...

, antimony

Antimony

Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...

, mercury

Mercury (element)

Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

, thallium

Thallium

Thallium is a chemical element with the symbol Tl and atomic number 81. This soft gray poor metal resembles tin but discolors when exposed to air. The two chemists William Crookes and Claude-Auguste Lamy discovered thallium independently in 1861 by the newly developed method of flame spectroscopy...

 and 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...

. This enrichment is created by hydrothermal circulation with a temperature of up to 300 °C. The underlying rocks out of which the minerals are dissolved are normally silt

Silt

Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...

y carbonates

Carbonate rock

Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite and dolostone, which is composed of the mineral dolomite .Calcite can be either dissolved by groundwater or precipitated by...

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

Dolomite

Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....

 is transformed into jasperoid

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 from jasper which is a chemical sediment.Sulfidic...

. Another alteration is the formation of clay minerals by interaction of water and feldspar

Feldspar

Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

. 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 those kind of deposits. 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. Carlin type deposits have also been found in China and Macedonia.

External links

• "The Rush to Uncover Gold’s Origins", Geotimes, April 2006