Gold, Earthquakes, and Time

How long does it take to form veins of gold in the rock? (a) millions of years; (b) less than a second.

If you guessed (b), you overestimated.  It might just be a few tenths of a second.  So reported Richard A. Lovett in Nature News about a new study in Nature Geoscience on the relationship between heat, pressure, precipitation, and earthquakes.

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.

The idea is that hot fluid flashes into vapor in the fault jogs during earthquakes, causing nearly instant precipitation of the fluids within them under rapid depressurization.  This is called “flash vaporization.”  Small earthquakes appear to be more efficient at gold vein formation than large ones.  A slip of merely 130 micrometers in a 90-centimeter fault zone can result in a 50% pressure drop, allowing the rapid precipitation of gold and quartz.

The caption in a photo of a gold vein accompanying the article says, “Veins of gold, such as this one trapped in quartz and granite, may deposit when the high-pressure water in which they were dissolved suddenly vaporises during an earthquake.”

Lovett still promoted the idea that large veins of gold might take hundreds of thousands of years to form out of the “tiny” veins hypothesized in the study.

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.”

Nevertheless, it’s still a “flash in the pan” to use Lovett’s clever double metaphor.  Given that so many small earthquakes can occur in a few years, it would seem hundreds of thousands of years are not necessary to find gold in them thar hills.  The authors of the original paper said, “Multiple earthquakes progressively build economic-grade gold deposits.”

See also the 10/15/2006 entry, “Gold Can Form in a Geological Instant.”

It takes a lot of findings like this to overcome the programming many students receive in school that everything on earth takes millions of years.  Darwin was enamored with the idea of slow and gradual processes accumulating large changes over time.  That, however, is a worldview preference, not a fact of nature.