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Friday, April 13, 2012

Gold Occurrences in the Guyanas

The Guyana Shield   CIA

The Guyana’s share the Guyana Shield with its larger neighbors Brazil, Colombia and Venezuela; this has been known to be a gold bearing region since its earliest days.  Throughout this area there have been many small scale miners that haven’t been known for their environmentally friendlyattitude.  At a high level conference between the governments of Brazil and the countries composing the Guyanas has brought forth a working agreement to rein in the small scale gold producers in the Amazon Basin.

During the 1990s a gold rush developed that engulfed the Guyanas that included Surinam, French Guinea and British Guinea as well as parts of Brazil, Colombia and Venezuela.  It has been estimated that over 100,000 people are earning their livelihood from the gold fields in these countries many of which are artisanal mining ventures that are damaging the rain forest.  These miners range from the national and Brazilian gold miners, operators and concession holders as well as heavy equipment owners. They're a rudimentary minds with small to medium scale with gold mining activities and often generate a legacy of extensive degradation as well as terrible social conditions both while gold was being mined and after the mining ceases.

The Guyana Shield is one of three cratons that are found on the South American plate. This craton is composed of 1.7 billion year old geological formation is found in northeast South America and forms part of the northern coast. The higher portions of this Shield are called the Guyana Highlands that is where the mysterious table-like mountains called
Tepuis are located.  This Shield is also the home of several very high waterfalls including the Angel Falls that is the highest in the world.

The rocks of the Guyana Shield correlate with another Shield found the northwest Africa with many of the same events traceable in both continents. Here are found rocks metasediments, metavolcanics (greenstones) and quasi-horizontal sediments causing horizontal layers of sandstone, Quartzsite, shale, and conglomerates. These rocks are heavily intruded by sills of younger mafic intrusives such as gabbro that were in place during the opening of the Atlantic Ocean during Jurassic times. This is very similar to the gold bearing rocks found in Portugal and Spain as well as North Africa.

The Precambrian rocks forming the shield are composed of three basic rock types including folded metasedimentary and metavolcanics rocks.  In some places there are coarse to fine grained sedimentary rocks that have been intruded volcanic rocks.  You can also find intrusive rocks of Mesozoic age in the folded rocks.

Much of the Guyana Shield consists of rocks like those found in the Canadian Shield with alternating belts of greenstone and granite with interleaving deposits of conglomerate.  A great deal of placer gold has been deposited in the rivers and streams of the region.  Most lode gold is found in fault zones of in the conglomerates like those found in the Abitibi Greenstone Belt of Canada.