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Tuesday, May 3, 2011

Geosynclines and Gold Deposits

Geologic Provinces of the World.  USGS


A geosyncline is a long narrow trench that forms offshore from a continental margin that receives not only the sedimentation from the continental craton but other sources of sedimentation from deepsea sources.  Given enough time a geosyncline will develop into a subduction zone where the oceanic plate is subducted beneath the continental plate. In effect what this does is create a monstrous heat engine operating in the crust and mantle of the earth. In effect what a geosyncline is a giant heat engine several thousand kilometers long where oceanic crust is subducted beneath a continental plate. 

There are other geosynclines that can develop when two oceanic plats collide in this case it seems that the smaller plate is subducted beneath the larger plate.  The result however is almost the same as 20 to 30 kilometers away an arc of volcanic islands arises where many of the islands support a fine crop of andesitic volcanoes that erupts magma midway between basalt and rhyolite called andesite. Many of the volcanoes found in the Andes Mountains erupts this type of magma hence the word andesite the lava named after the Andes.

A late Cretaceous Subduction Zone on the west coast of the United States.  The geosyncline is the area depicted in white on the illustration.  Notice that some of this material is being subducted.  USGS


If this sounds like a geology lesson it is because this is all part of the Plate Tectonics theory that posits that there are about 20 tectonic plates of varying size that move across the face of the globe at speeds that measure in centimeters per year, or about as fast as a fingernail grows.  We live on and limit our mining to the crust of the earth that is roughly 10 kilometers thick.  Below the crust lies the mantle that proceeds down towards the center for a depth that is measured in hundreds of kilometers.  The mantle has about the consistency of tooth paste.  Below the mantle is the mesosphere that is more solid then the mantle due to far greater pressure.  The mesosphere is measured in thousands of kilometers.  At the bottom of the mesosphere is the core of the earth that is liquid descending ever downwards until you finally encounter the solid core of the earth.  Some geologists believe this is a single crystal of iron although there is really no way of proving this. 

Cross Section of the Earth   USGS


To introduce another concept is a measurement called the “Clarke” that is the average abundance of an element in the earth’s crust. The crustal abundance of gold averages 0.0000002 grams per ton from this number in order to achieve a commercial deposit, it is necessary to mobilize this gold where it is concentrated in some sort of trap where the gold makes a viable deposit; this is one of the many functions of a geosyncline.

As the material being subducted into the earth’s mantle it becomes superheated including the water that is riding on the plate that is being subducted into the lower crust and mantle.  This also includes any of the mineral deposits that dot the oceanic crust like polka-dots that are the from black smokers. It also includes the minerals that exist in solution in the salt brine that saturates the ocean floor.

Once all of this mixture reaches a sufficient depth the process of dewatering causes the water to boil, and at enough depth and pressure this water becomes supercritical, and also contains a witch's brew of chemicals that cause it to become highly corrosive and capable of dissolving just about anything that comes in contact with. The corrosive components of this water include chlorine, carbon dioxide and sulfur. Another function it serves is to lower the melting point of rock. By stripping the lighter elements from the rock it produces granite that works its way up through the crust of the earth in a manner that is very similar to what you observe in a “lava lamp.”

Gold is also mobilized by the same process and is carried through the surface of the earth where it is deposited as epithermal deposits in the roof rock over the granite intrusion, or in a contact metamorphic aura around the volcanic neck. This is the primary source of gold from which all other deposits of the metal are derived. It is also the primary source of practically all the other mineral deposits on earth.

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