- Scritto da Matteo Oberto
The link between gold and earthquakes
Is it a coincidence that gold is often associated with areas that in the past have been sites of earthquakes? Over 80% of the world's primary commercial gold deposits could be formed in a "flash".
Using a series of models, the researchers showed that earthquakes could act as a triggering agent for the deposition of large quantities of gold, which would pass in a few moments from being present in the aqueous fluids circulating in the fractures, in the solid state. There are several mechanisms that could be achieved in the genesis of important enrichments when an earthquake releases its accumulated energy:
Gold precipitates from aqueous fluids following a radical and sudden change in pressure conditions (depressurization);
Gold precipitates from aqueous fluids following an important change from a chemical point of view as a result of the mobilization of fluids external to its circuit, introduced as a result of seismic activity (mixing);
Gold precipitates as a result of the change in the status of aqueous gaseous fluid (boiling).
In all these cases, it is hypothesized that some types of primary gold deposits may be the result of the sum of gold accumulated by a series of innumerable earthquakes in geological time, even of minimum intensity.
Figure 1: In the main image we notice an important geological structure, which strongly conditions with its activity both the morphology and the genesis of important earthquakes over time. It should be noted, in fact, that earthquakes are the consequence of the liberation of the energy accumulated over time by faults, which reached a certain threshold can not do anything else but unleash it in the surrounding environment.
How exactly gold accumulates in the cracks occupied by the fluids is not yet fully clarified but from the data it is clear that typically these circuits are from 5 to 30 km deep and that the fluids that circulate there contain gold in the form of complexes mainly (gold typically moves in aqueous solutions bound to sulfur or other types of complexes).
The studies of Henley and Weatherley
Richard Henley of the Australian National University of Canberra and Dion Weatherley of the University of Queensland of Brisbane studied the dynamics of earthquakes and found that the pressure changes triggered by the earthquake are much more important than previously thought. Their model suggests that earthquakes can within a few milliseconds greatly increase the extent and volume of cracks in the deep rocks involved in the seismic energy in propagation at the speed of sound! The fluids that were previously found in those small cracks, therefore, have to deal with an impressive pressure change and sometimes even change the state of aggregation becoming gaseous; metals, on the other hand, precipitate.
Henley says: "The fluid can not get from the surrounding encasing rock to the center of the fracture fast enough to fill the void so the pressure drops from 3000 times the atmospheric pressure to pressures almost identical to those on the Earth's surface in an instant. The liquid vaporizes as a result and all the minerals it contains are deposited ".
The process, however, must be viewed at the scale of thousands if not hundreds of thousands of earthquakes and microsisms involving the same hydrothermal circuit in a period of time, in fact, Weatherley points out that studies have emerged to confirm that the large gold deposits should be the the result of the sum of many minimal episodes examined individually, but with a remarkable result given by their summation over time.
Figure 2, 3: Note how the gold settles in the free space left by the quartz, which crystallizing in a previous phase leaves little room for gold for its deposition.
A good chunk of world gold comes from the Witwatersrand Basin in South Africa, where it was deposited and concentrated over 2 billion years ago when erosion consumed gold-rich quartz veins outcropping in nearby mountain ranges. A doubt arises: "Even in ancient times the mechanisms were the same as those of today?".
We quantify the pressure difference that aqueous fluids undergo during an earthquake
A study published in Nature Geoscience has shown that the process can occur almost instantaneously, probably in the order of a few tenths of a second.
When an earthquake occurs, the rock blocks that delimit the main fault surface (roof and bed) slide along the fault line (fault mirror), rubbing against each other. Weatherley and his co-author, the geochemist Richard Henley at the Australian National University in Canberra, have collaborated together to solve some problems on the mechanisms of depositing gold from the much discussed fluids in those fateful tenths of a second.
The results that their calculations have revealed are surprising, in fact there is a rapid depressurization that sees the normal conditions of high pressure deep within the crust fall suddenly to pressures close to those we experience on the surface.
It is therefore useful to make an example: a magnitude 4 earthquake at a depth of 11 km, according to the calculations, would cause a sudden pressure drop from 290 megapascals (MPa) to 0.2 MPa. (In comparison, atmospheric pressure at sea level is 0.1 MPa).
In summary, in a few tenths of a second the local pressure could vary by about 1450 times! A notable variable in deposition of gold.
When water loaded with mineral salts at about 390 ° C is subject to that kind of pressure drop, Weatherley says, the liquid rapidly evaporates and the salts or complexes in the now over-saturated water crystallize almost instantaneously. This is a process that engineers call "flash" or "flash deposition" vaporization. The effect, he says, "is large enough to precipitate quartz and all its associated minerals and metals from the solution".
Subsequently the liquid percolates from the surrounding rocks in the vacuum, restoring the initial pressure. But this does not happen immediately, and so in the meantime a single earthquake can produce an instantaneous gold vein (even if tiny). It is then the summation over time of countless episodes to produce the primary auriferous deposit.