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Thursday, April 5, 2012

Global Metallogeny and Gold Deposits


Map of the Kolyma Basin in the far east of Russia.
by Kmusser


The discovery of gold in the Kolyma Basin of the far Eastern Russia by the Soviet geologist Yuri Bilibin during the 1920s gave rise to a new science called Metallogeny and Global Tectonics. This gold producing area was later the subject of a book about the Soviet system of forced labor by Aleksandr Solzhenitsyn titled the Gulag Archipelago

In order to work three criteria have to be met, but in essence they say that mineral deposits can be found worldwide when these conditions are met.

1)      Mineral deposits are formed whenever energy is released at plate boundaries whether they are converging, diverging or transform plates.
2)      Plate tectonics are the controlling factor for the formation of mineral deposits.
3)      By reconstructing fragments of plates provide a useful tool in exploration for new mineral deposits.

In any tectonic setting for the production and accumulation of mineral deposits several requirements have to be fulfilled there has to be either a spreading center, a mountain building episode at a plate convergence or collision boundary, in craton rift centers, or cratonic basins.

The mid-Atlantic Ridge outlined by dotted lines.  This is a typical spreading center.


Deposits at Spreading Centers

In active spreading centers such as the mid-Atlantic Ridge or in the Red Sea where Africa is separating from Arabia are metallaiferous sediments on the flanks of the ridges that in many places are also marked by black smokers that contribute to the metal deposits.  Although these sediments primarily contain as sulfides iron, zinc, copper, lead, silver and gold; some deposits such as in the Red Sea containing iron, copper and zinc.

At some ridges important deposits of manganese oxide deposits are lain down.  This is especially important along the mid-Atlantic Ridge.

Another set of minerals found in ultramafic rocks that are called ophiolites are asbestos, chromite and nickel.  These minerals are found in Phanarozoic mountain belts where they are transported by tectonic movement.  The chromite is often found in Podiform deposits in ultramafic rocks most notably serpentines.  A different type of deposit of the same origin is massive sulfide deposits of iron and copper sulfides also associated with these ophiolite deposits.

A typical convergent plate margin aka a subduction zone.  


Deposits at Convergent Plate Margins

There are actually two types of convergent plate margins, one of them is where two continents are converging and the other is where two island arcs are converging metal deposits are commonly found at either type of plate margin.  The largest of these margins is the Circum Pacific Belt that contains major metallic deposits including over half the world’s production of copper.  All the metals can be found at convergent margins.

The zoning of mineral deposits in one of these zones is quite apparent with different zones being encountered the further you are away from the margin.  These varying deposits are liberated from the plate the further it descends beneath the mantle wedge with tin coming out of the slab at a depth of about 300 kilometers.  These metals come up with magmatic fluids and are concentrated in hydrothermal and magmatic fluids.  Epithermal deposits are commonly found in this regime.  Oil and gas are found associated with this type of convergent margin.  In some places hydrothermal fields are also found.



Collision Boundary Deposits
  
Collision boundary deposits are a wild mélange of differing types of mineral from a wildly differing variety of environments ranging from deposits associated with spreading centers to deposits found at plate margins.  Typical of these deposits are those found on the ocean floor that are spotted across the width of the plate that are subducted beneath the continental plate with the metal deposits being brought up into the margin by tectonic movements.  These ocean floor deposits are the primary deposits from which others are derived.

Death Vallet, California the yellow dots show the location of mines. This is a typical example of a cratonic rift system.
USGS


Cratonic Rift Systems

 Hot spots in the mantle of the earth cause a blister to form at the surface of the earth that causes three cracks to form on the surface two of them form an ocean and the third is called a failed arm of the sea or an aulacogen. A good example of this type of feature is the Ottawa Aulocogene in Canada.  Usually any granites emplaced during the early stages of this kind of development are rich in tin and fluorite.  Later in their development aulacogenes collect large accumulations of evaporates and other metallaiferous deposits.  In their late stages they are apt to develop deposits of fluorite, barite and carbonatites.  These can be characterized by deposits of niobium, phosphorus, rare earth elements (REE), uranium, thorium and in places tin bearing granites.  Geothermal fields also occur along rifts because of the upwelling of magma along the rifts. 

The Dead Sea from space.   NASA


Cratonic Basins

Cratonic basins are where the inflow of water from the sea causes an accumulation of organic debris from which petroleum products are derived.  The basin is also a place where evaporate deposits are laid down especially salt hence the association of salt and petroleum products.  The heat required for the transformation of organic matter is supplied by the burial of the debris under layers of sediment.  An example of a cratonic basin is Death Valley in eastern California.  One of the products of the ongoing evaporation is borax.  With continued rifting the basin usually becomes filled with water so that the circulation in the system goes from restricted to unrestricted with the depositation of organic matter ceases so does the depositation of evaporites.

Petroleum products aren’t the only thing deposited in cratonic basins like all the landforms described above gold and other metals are also deposited.  By understanding these features you have a pretty good idea of how metals are deposited so you can design an effective exploration plan.


3 comments:

  1. Excellent and comprehensive article, John! Very well done, a LOT of information. ~R

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