Earthquakes Deposit Gold in Fault Zones

[Stand Watie]

Interesting article from a science site.

Earthquakes Deposit Gold in Fault Zones



Gold deposits may be created in a flash—literally. Along fault zones deep within Earth's crust, small cavities filled with fluids rich in dissolved substances such as gold and silicate minerals can expand suddenly to as much as 130,000 times their former size during a major earthquake, a new analysis suggests. In such circumstances, pressure drops accordingly, driving a process the scientists call flash evaporation. And when the pressure in the cavity suddenly drops, so does the solubility of minerals in the water there. Along with substantial quantities of quartz, large earthquakes could deposit as much as 0.1 milligrams of gold along each square meter of a fault zone's surface in just a fraction of a second (gold-flecked chunk of quartz from the Red Lake Mine of Ontario, inset). Because mineral-rich waters continually infiltrate cavities along the fault, ore deposits accumulate with each passing quake. Typical rates of seismicity along a fault, such as the San Andreas fault zone shown in the main image, could generate a 100-metric-ton deposit of gold in less than 100,000 years, the team reports online today in Nature Geoscience. More than 80% of the world's gold deposits may have been formed via flash evaporation along fault zones, the researchers estimate. The new model may even lead to a new style of mineral exploration, they say: Just look for the ancient fault zones.

 

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[Stand Watie]

More here:
http://www.nature.com/ngeo/journal/v6/n4/full/ngeo1759.html

Flash vaporization during earthquakes evidenced by gold deposits

Much of the world’s known gold has been derived from arrays of quartz veins. The veins formed during periods of mountain building that occurred as long as 3 billion years ago and were deposited by very large volumes of water that flowed along deep, seismically active faults. The veins formed under fluctuating pressures, during earthquakes, but the magnitude of the pressure fluctuations and their influence on mineral deposition is not known. Here we use a simple thermo-mechanical piston model to calculate the drop in fluid pressure experienced by a fluid-filled fault cavity during an earthquake. The geometry of the model is constrained using measurements of typical fault jogs, such as those preserved in the Revenge gold deposit in Western Australia, and other gold deposits around the world. We find that cavity expansion generates extreme reductions in pressure that cause the fluid that is trapped in the jog to expand to a very low-density vapour. Such flash vaporization of the fluid results in the rapid co-deposition of silica with a range of trace elements to form gold-enriched quartz veins. Flash vaporization continues as more fluid flows towards the newly expanded cavity, until the pressure in the cavity eventually recovers to ambient conditions. Multiple earthquakes progressively build economic-grade gold deposits.