Theories on Gold Nugget Formation (Part 3)
(Australian nugget with bits of white quartz still attached.)
After a recent detour (my series of posts about meteorites), I'm back to finish up my series on gold nugget formation. As I mentioned in my microbial theory post, there are three theories being bandied about these days that tend to refute the long-held view that gold nuggets are simply eroded out of vein or reef material and then reduced, smoothed, and worn by erosional or hydraulic factors.
The second modern theory on gold nugget formation is precipitation:
As best I understand it, precipitation involves the saturation of hydrothermal fluids with soluble gold chemical complexes that can create a type of silica gel known as a colloid instead of quartz (the typical gold-bearing crystalline structure). These colloids have an uncanny ability to stabilize gold in solution and allow tiny particles of the yellow metal to be transported in a type of suspension fluid. Toss in some geologic shear zones and significant fluid pressure drops and you have a recipe for the growth of microscopic gold grains.
As these colloidal grains are transported in solution, they accumulate along something called a "grain boundary" and begin to "grow." At certain boundaries these gold grains will actually accumulate in enough numbers to create gold nuggets. These precipitation nuggets can become very large over time and their distribution very widespread if the geological and mineralization factors are right.
Aussies Have Led the Way
Once again, it's been the Australians who've led the way in researching and developing new theories about gold nugget formation, including precipitation. Initially, the Aussies also subscribed to the standard theory that gold nuggets had been broken off some rich reef or vein and transported by water to present-day locations.
(Another Aussie beauty.)
What Australian researchers found out, however, was that many large drifts ("patches" here in the U.S.) contained gold nuggets whose size, composition, and fineness was, once again, totally unlike the gold eroding out from nearby veins or reefs.
"Favorable for Carrying Gold"
Aussie researcher A.C. Selwyn backed the precipitation theory about how these Outback nuggets were formed and arrived at their current locations:
"Nuggets may be formed and particles of gold may increase in size through the deposition of gold from meteoric waters percolating up or through the drifts. During the time of our extensive basaltic eruptions, these liquids must have been thermal and probably of a highly saline character, which made them favorable for carrying gold in solution.”
That's precipitation theory in a nutshell. In my final post in this series I'll bring your attention to the third theory of gold nugget formation, crystallization.
Until then, best of luck to you.
If you liked this post, you may want to read: "Theories on Gold Nugget Formation (Part 2)"
(c) Jim Rocha (J.R.) 2012
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