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Wednesday, March 10, 2010

How Gold Is Formed

Thought this was interesting.

Our entire planet is composed of materials that originated from stardust.  However fascinating this fact may be, this does not help us much in understanding how gold occurs on Earth:

1) Gold is widely distributed in the Earth's crust with an average concentration of 0.005 parts per million.

2) Various geological processes serve to concentrate gold and other metals in certain places to produce matalliferous ore bodies that are economically viable to mine. 

3) Gold must be concentrated to about 5ppm, or 1000 times the crustal average concentration, and in sufficient volume to be able to recover at least 200,000 ounces (6 metric tons) to make an ore body economically viable.

4) Gold is concentrated in the bedrock mainly by volcanic activity and hydrothermal activity associated with it. It occurs most typically in mineralised quartz veins around and above granite plutons, but some of the richest deposits are found associated with massive sulphide ore bodies in Precambrian (usually Archaean) mobile belts.

5) Gold usually occurs as a native metal, often alloyed with small amounts of Silver, Lead, Copper, iron, and tiny amounts of rare Platinum Group Minerals. 

6) Gold is a relatively inert substance, and survives weathering and erosion very well. Many of the richest accumulations of gold are concentrated by alluvial action depositing gold from large catchment areas into river and stream channels.

7) Two of the largest and most famous gold ores found in Africa, the Witwatersrand formation (South Africa) and the Tarkwa formation (Ghana) are quartz pebble conglomerates that were formed by both processes of concentration (4 and 6). First it formed in quartz reefs through an externsive region, then it was eroded and concentrated more by alluvial action. The gold is found in veins within the quartz pebbles and as individual grains in the finer sedimentary matrix between them.

8) Gold is often found on or near the surface in alluvial deposits, where panning and dredging may be economically viable at concentrations as low as 1ppm.
In some of these areas where alluvial gold is found, there is no sign of the original ore bodies, because they have long since been eroded away, leaving only the concentration of gold surviving in stream sediments.