Scientists have long known that veins of gold are formed by mineral deposition from hot fluids flowing through cracks deep in Earth’s crust. But a study published today in Nature Geoscience has found that the process can occur almost instantaneously — possibly within a few tenths of a second.

The process takes place along 'fault jogs' — sideways zigzag cracks that connect the main fault lines in rock, says first author Dion Weatherley, a seismologist at the University of Queensland in Brisbane, Australia.

When an earthquake hits, the sides of the main fault lines slip along the direction of the fault, rubbing against each other. But the fault jogs simply open up. Weatherley and his co-author, geochemist Richard Henley at the Australian National University in Canberra, wondered what happens to fluids circulating through these fault jogs at the time of the earthquake.

What their calculations revealed was stunning: a rapid depressurization that sees the normal high-pressure conditions deep within Earth drop to pressures close to those we experience at the surface.

For example, a magnitude-4 earthquake at a depth of 11 kilometers would cause the pressure in a suddenly opening fault jog to drop from 290 megapascals (MPa) to 0.2 MPa. (By comparison, air pressure at sea level is 0.1 MPa.) “So you’re looking at a 1,000-fold reduction in pressure,” Weatherley says.

Flash in the pan
When mineral-laden water at around 390 °C is subjected to that kind of pressure drop, Weatherley says, the liquid rapidly vaporizes and the minerals in the now-supersaturated water crystallize almost instantly  — a process that engineers call flash vaporization or flash deposition. The effect, he says, “is sufficiently large that quartz and any of its associated minerals and metals will fall out of solution”.

Eventually, more fluid percolates out of the surrounding rocks into the gap, restoring the initial pressure. But that doesn’t occur immediately, and so in the interim a single earthquake can produce an instant (albeit tiny) gold vein.

Big earthquakes will produce bigger pressure drops, but for gold-vein formation, that seems to be overkill. More interesting, Weatherley and Henley found, is that even small earthquakes produce surprisingly big pressure drops along fault jogs.

“We went all the way to magnitude –2,” Weatherley says — an earthquake so small, he adds, that it involves a slip of only about 130 micrometers along a mere 90 centimeters of the fault zone. “You still get a pressure drop of 50%,” he notes.

That, Weatherley adds, might be one of the reasons that the rocks in gold-bearing quartz deposits are often marbled with a spider web of tiny gold veins. “You [can] have thousands to hundreds of thousands of small earthquakes per year in a single fault system,” he says. “Over the course of hundreds of thousands of years, you have the potential to precipitate very large quantities of gold. Small bits add up.”

Weatherley says that prospectors might be able to use remote sensing techniques to find new gold deposits in deeply buried rocks in which fault jogs are common. “Fault systems with lots of jogs can be places where gold can be distributed,” he explains.

4 Comments

1. 1. RobinT 04:46 AM 3/20/13

   This idea is not new. I wrote a paper in Guide book to Gold Deposits of NW British Columbia in 1987, describing the Cinola Epithermal Gold Deposit (now the Harmony Deposit), on the Queen Charlotte Islands, B.C. and gave two public scientific presentations on this deposit.
   I recognized the banded vein nature occurring in a fault jog between two faults. I recognized boiling textures (quartz after calcite) and hypothesised that boiling occurred during abrupt seismic events creating tremendous pressure release and causing, deposition of gold from a saturated solution, and even used the term 'seismic veins' for this scenario. It was well known and described previously by several authors prior to this that boiling textures and therefore boiling is related to gold deposition. It was thought that cyclical boiling occurred as part of the hot spring system (eg Old Faithful, in Yellowstone Park).
   
   However, for me the eye opener was seeing open centres to the banded gold bearing quartz veins created after the hydrothermal activity ceased, created by ongoing seismic activity. It was clear to me at that time there was a clear relationship between supersaturated gold bearing fluids flash boiling and depositing gold and seismic activity.
   
   But still, nice paper confirming my observations.

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2. 2. Very Curious 03:06 AM 3/21/13

   This would explain how fractures through Carbonate materials which would ordinarily react with Silicon Dioxide to form the corresponding silicates are filled with Quartz or Silicon Dioxide with no reaction products at the contacts of the Silicon Dioxide to the Carbonate materials.
   
   In this way cracks or fractures can form which lead to a silica water source, fill with the fracture with silica saturated water and precipitate out the silica or quartz into the fracture or fractures quickly without any chemical reaction occurring at the Silicon Dioxide -Carbonate interface.

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3. 3. gmperkins 04:54 PM 3/22/13

   @RobinT its good to know that there is more background on this, this was a 'new idea' to me though and the article is well written. It is interesting and it isn't intuitive (to me at least) that we'd have such big pressure drops but after some thought it actually makes perfect sense.
   
   But, it still doesn't explain how to make gold from lead ;)

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4. 4. drmvandongen 11:58 PM 3/26/13

   In addition to above comments, the most common reference to this concept is from Sibson (1987) and has since been one of the key concepts quoted in introductory courses and textbooks on ore deposit formation. For example, see the commonly used 'Introduction to Ore Forming Processes' by L. Robb (2005), p. 142-145. Any undergrad geoscience student learns of this principle of the fault valve/suction pump in mesothermal gold deposit formation.
   
   Not withstanding that Dion and Dick did a good job, it puzzles me that the editors apparently saw novelty in this paper. To me, it suggests that what ore deposit researchers take for granted, may not be such common knowledge to the general public.
   
   
   
   
   

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