Finding Gold in Fault Zones
Gold is usually associated with fault zones, but it is hard for a layman to recognize a fault some when he sees one. The same can be said for several geologists too. A fault zone on a bedrock map it often depicted as a pencil line on a map. That isn’t what a fault zone is in the field.
A fault zone is an area of shattered rock caused by movements in the crust of the Earth. This is the reason why earthquakes are associated with faults that happen when the strain on the rocks becomes so great the rocks break causing the earth to lurch forward rapidly causing an earthquake. The intensity of the earthquake is strictly a function of how much energy is released by the lurch.
One of the consequences of a fault zone and any associated earthquakes is the rock shattered forms an excellent conduit for hydrothermal fluids to come up from the depths of the earth bringing with it many minerals that are dissolved in the hot mineral charged waters. Finding one of these fault zones is a challenge that mineral seekers over the ages have solved.
The shattered rock found in a fault zone is called a “brecchia” that is often filled in with other minerals cementing the small angular fragments of rock back together. One of the best recognized of all mineral deposits containing gold is those found in a brechiated fault. Sometimes the rock found is soft enough to be ground into a clay like mass called “gouge” that also may contain gold and other minerals.
Faults cause a weakness in the surface of the earth that water with its power of erosion is always ready to take advantage of practically all rivers and streams follow fault zones because of this. The faults associated with these are often larger then the smaller faults that can be found almost anywhere.
Whenever you see a vein of another color in the surrounding rock it is there you are seeing a fault zone. Another indicator of a fault zone is seeing several veins of quartz or other minerals close together in a darker rock matrix.
A common feature of faults is a special structure called “slickensides. These are areas of rock that look like the rock has been “combed.” By feeling these you can determine which way the fault moved because the pointed features of the slickensides always points towards the “upstream” side of the way the fault moved.
Faults also show up on topographic maps as linear structures called liniments that can be seen in a large scale map. Other features indicating faults are off-set streams of any other linear feature.
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Oxford Dictionary of Earth Sciences, Michael Alleby, Oxford University Press, 2003