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Natural abundance of gold and silver in the earth and sea

Occurrence in the earth's crust

Occurrence of gold and silver in the earth's crust

Precious metals can only be mined profitably where they occur relatively high up in the earth's crust and in clustered numbers. For a long time it was a great mystery why precious metals are so unevenly distributed on earth and why some regions have large deposits while in other areas no precious metals are found at all. Today we know that billions of years of geological changes due to plate tectonics, volcanism and ice ages led to the current distribution of the various precious metals in the earth's crust. 

Most of the accumulations of gold and silver found in the earth's crust are of hydrothermal origin. Of primary importance for the formation of ore deposits was the combination of divergent and convergent shifts of the boundaries of the continental plates and a corresponding volcanic activity, which resulted in the formation of magma and - together with the elements contained therein - the ascent of magmatic melts towards the surface. Thereby, the elements, previously inaccessibly enclosed in the earth's mantle, were transported into rock layers close to the surface, crystallized after the magmas cooled down and accumulated there. The gold and silver deposits that accumulate in the earth's crust in this way are called primary deposits. Since silver is less likely to form compounds with other metals than gold, it is found in enriched form closer to the surface in the earth's crust. In addition, silver dissolves more readily and has a lower density, which favors geothermal transport from deeper and enrichment in higher soil layers.

As a result of weathering processes, primary deposits as well as less enriched but still ore-bearing rock formations can erode, causing precious metals to enter water, for example.[3] Although the elemental precious metals are very stable, they can also go into solution under certain conditions, especially silver. Gold, on the other hand, tends to be transported by water in undissolved form. Due to the lower flow velocity at the inner edges of river bends, sediments enriched with gold may form there. The sediments weather, the heavy precious metals remain and re-sedimentation occurs. As a result of these permanent weathering and sedimentation processes, enrichments gradually occur, which are referred to as secondary deposits.

Some of the world's most important gold and silver deposits are listed below, differentiated according to their respective formation processes:[4]

  • Mesothermal deposits:
    They are linked to mountain-building processes and were formed by plate tectonic shifts of the ocean crust. The Greenstone Belt was formed three billion years ago. Deposits are found in Africa, Brazil, northern Canada, Siberia and Western Australia. The Circumpacific Fire Belt formed 150 million years ago by compressing and fanning out rock layers. Deposits were formed in the Andes, Central America, the Rocky Mountains, and Australia. The Mother Lode in California extends 270 km along the California coast with successive quartz veins.
  • Epithermal deposits:
    They were formed by magmatogenic hydrothermal fluids that transported the ores trapped within them into volcanic veins near the surface. The heavy ores sank and gold and silver accumulation occurred. One deposit of this type was the Comstock Lode in Nevada, one of the largest silver deposits ever found. It also includes silver deposits in Mexico, lead-silver-zinc deposits in Canada and the U.S., and tin-silver-bismuth deposits in Bolivia. Such deposits can form in short periods of time, sometimes in as little as 50 thousand years, such as in the Lihir Islands in Papua New Guinea. Carlin-type deposits formed in less than 50 mill years. The Carlin trend in Nevada is among the largest gold deposits. Because of the light rock, the gold here is very evenly distributed and not visible to the naked eye.
  • Fossil placers:
    These secondary deposits were formed about two billion years ago by the deposition of boulders and gravels in river channels. Through their weathering, the relatively weathering-resistant precious metals accumulated. In placer deposits, the proportion of silver is low because it is increasingly dissolved during transport. Fossil gold placers include South Africa's 52 thousand km2 Witwatersrand Basin, the largest gold deposit ever found and responsible for about a quarter of the world's gold production to date. Rivers from the hinterland flowed into the basin and paleosols formed in the form of debris fans. The sediments, several kilometers thick, were formed by ongoing erosion and sedimentation processes.
  • Recent placers:
    If gold enters rivers, gold particles can be deposited in places where the water does not have sufficiently high flow. The gold is gradually separated from other minerals by the movement of the gravel in the riverbed, in the form of thin flakes. They almost always occur near larger primary gold deposits. In rare cases, the formation of so-called nuggets may occur.

Occurrence in water

Water also contains traces of gold and silver in dissolved form. The silver contents are significantly higher in fresh water than in salt water, but due to the vanishingly small amounts of fresh water compared to salt water, this is of no consequence.[5] The precious metals dissolved in the oceans are distributed relatively evenly over a very large volume of water: On average, one cubic kilometer of seawater contains 10 kg of gold and 1.2 kg of silver.[6] With a volume of about 1.5 billion km3 of seawater worldwide, this results in about 15 million tons of gold and 1.8 million tons of silver. Accordingly, gold is 8 times more abundant than silver in the world's oceans. The higher proportion of gold in seawater compared to the proportion of silver is apparently related to the fact that the silver content decreases more and more from spring water to river water to seawater, which is due to the fact that acidic water is more capable of transporting silver than neutral or alkaline water.[7]

Frequency of gold and silver in seawater

Fritz HaberIn the 1920s, Fritz Haber, a German Nobel Prize winner in chemistry, attempted to extract gold from seawater as a contribution to the reparation payments to be made by the German Reich. The yield of the water samples was an average of 0.004 mg of gold perm3 or ton of seawater,[8] which was 40% of the average gold content in salt water. Theoretically, 6 million tons of gold (60 times the currently known potentially mineable underground gold resources) and - if the quota of 40% for gold is also applied to silver - 0.7 million tons of silver (1.3 times the mineable silver resources in the earth's crust) could be extracted from seawater.

 

Comparison of gold and silver deposits

However, economically viable extraction of gold and silver from seawater is hardly conceivable given the extremely low concentrations and the huge volume of water.

More realistic is the use of another precious metal source in the oceans: On the East Pacific Ridge, so-called hydrothermal vents were discovered several thousand meters below sea level in the late 1970s. In the meantime, more of these "hot springs" have been found - mostly in places where continental plates collide. They function according to the following principle:[9] Cold seawater penetrates deep into the earth's interior through fissures in the seafloor and is heated there as a result of volcanic activity, causing minerals to dissolve out of the magma rock. The water, heated to several hundred degrees, then rises back to the seafloor together with the minerals and cools there. As a result, the minerals precipitate and are deposited on the sea floor. As the water circulates continuously in this way, deposits are formed over time that contain gold, silver or other metals, depending on the composition of the magma rock.

Companies specializing in deep-sea expeditions would like to recover these treasures, but are currently still faced with technical problems that are difficult to solve, as the metals would have to be recovered from ships. In view of the high costs, the prices of the metals would first have to rise considerably further for such a salvage operation several thousand meters below the surface of the ocean to be profitable.

Origin of gold nuggets

Until now, it was assumed that gold formation was a purely abiotic process. The recently demonstrated bio-mineralization of gold by microbes was therefore an extraordinarily remarkable discovery. According to this discovery, the bacterium Cupriavidus metallidurans is capable of converting gold ions dissolved in liquid into nanoparticles of pure metallic gold.[10] However, the role these microorganisms play in the context of gold mineralization is still largely unclear. It is possible that they are involved in the formation of the famous gold nuggets. Under certain circumstances, they could also become of practical importance, as it may be possible to increase the exploitation of deposits with the help of the bacteria.

Artificially producing gold and silver

Izzy Friedman,the Godfather of Silver, once opined:

"None of you can intellectually grasp the future price of silver. Any price you think of will fall short of the real price. What I say is true as long as silver cannot be artificially produced."[11]

Alchemists have always dreamed of artificially producing gold or silver by converting base metals into precious metals. Despite all their efforts, they have never found the Philosopher's Stone (gold) or the White Lion (silver). In the meantime, however, in some cases they have succeeded in transforming an already existing element into another one. One example is the fusion of hydrogen nuclei to helium. Since the helium nucleus has a lower mass than the sum of the masses of the hydrogen nuclei, the nuclear fusion releases energy. This process occurs every day in the sun and is what makes life on earth possible in the first place. This mechanism can be used for energy production (hydrogen power plants) or for military purposes (hydrogen bomb).

An anthropogenic production of new elements is also possible in the field of precious metals: With the help of a particle accelerator or a nuclear reactor, the mercury isotope Hg 196, which makes up 0.15% of natural mercury, can be converted into the gold isotope Au 197.[12] Since only this stable gold isotope exists, artificially produced gold cannot be distinguished from natural gold. However, due to the extremely high costs involved and the relatively low yield, artificial gold synthesis will probably never achieve economic significance for the extraction of precious metals, nor will the capture of metal-bearing meteorites or mining on other planets or on asteroids.

 

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