### The Formation of Gold Deposits: A Hypothesis based on Earthquakes and Quartz Crystals
Gold has been revered for its allure, scarcity, and resistance to oxidation. Its worth arises not merely from human appreciation but also from its distinct chemical characteristics. Gold is extremely unreactive, meaning it does not readily bond with other elements, and it has minimal solubility in most solvents. These traits complicate its extraction from other substances, which is why a significant portion of the gold we extract originates from deposits where it exists in sizable, unblemished chunks. This leads to an intriguing question: If gold is so challenging to extract, how do natural processes manage to produce such extensive reserves of it?
A team of Australian scientists has recently put forth an exciting hypothesis that might answer this query. They propose that the emergence of substantial gold deposits could be associated with the piezoelectric effect in quartz crystals, instigated by the pressure from tectonic movements, such as earthquakes. This theory not only provides a possible explanation for gold deposit formation but also correlates several apparently unrelated geological phenomena.
#### The Influence of Orogeny on Gold Formation
Around 75% of gold extracted by humanity arises from orogenic gold deposits. Orogeny pertains to the tectonic activities that form mountains, and these gold deposits materialize in the fractures where two rock masses are sliding against each other. These zones are often abundant in heated hydrothermal fluids, which can dissolve gold, albeit in very low concentrations—typically under a milligram per liter of water.
Notably, these gold deposits are nearly always linked to quartz, a mineral that develops crystalline structures of silicon dioxide. This correlation between quartz and gold is not mere coincidence and forms the foundation of the new hypothesis.
#### The Piezoelectric Effect in Quartz
Quartz stands out among common minerals due to its piezoelectric nature, meaning it produces an electric charge when subjected to mechanical stress. The piezoelectric effect occurs because quartz crystals possess a lack of symmetry (they are non-centrosymmetric). When these crystals are deformed under pressure, a disruption in their internal electric structure results in an electrical potential, or voltage, across the crystal. This voltage is directly proportional to the applied mechanical force.
Quartz is also an insulator, which means the electric charge it generates does not dissipate easily. However, this charge can be neutralized by the transfer of electrons to or from materials that interact with the quartz, including fluids. This electron transfer can instigate redox (reduction/oxidation) reactions in nearby fluids, potentially causing dissolved ions to precipitate and deposit on the quartz.
#### A Self-Reinforcing Mechanism
When a tiny metal deposit, such as gold, develops on the surface of quartz, it can enhance additional electron exchange with the adjoining fluid, resulting in further metal accumulation at the same site. This mechanism could be self-reinforcing, as the initial deposit reduces the concentration of metal ions in the adjacent fluid, promoting additional ions to migrate to the deposition site. This implies that the fluid itself doesn’t need to continuously pass over the same area for the deposit to enlarge.
The hypothesis also necessitates a source of strain to initiate the piezoelectric effect. As these processes occur in active fault zones, where tectonic strain is prevalent, this condition is readily satisfied.
#### Experimental Validation
Investigating this hypothesis in active fault zones would be notably difficult due to their inaccessibility and the gradual nature of these processes. Nevertheless, the researchers executed a more straightforward experiment to explore the concept. They immersed quartz crystals in a solution containing gold, either as gold chloride ions or as suspended gold nanoparticles. The crystals experienced strain at a frequency comparable to that generated by minor earthquakes, and the experiment lasted for an hour.
The findings were revealing. Small gold deposits appeared on the pure quartz crystals, irrespective of whether the gold was in dissolved form or as nanoparticles. In the case of naturally occurring quartz, which already bore minute gold deposits, the new gold was deposited on the pre-existing sites instead of forming new ones.
The researchers also observed that much of the quartz in natural deposits is disordered, comprising numerous small crystals aligned randomly. In such disordered material, the piezoelectric effect of individual crystals tends to negate, indicating that gold will preferentially aggregate on single crystals. This insight aids in explaining why gold is frequently discovered in substantial chunks within these deposits.
#### Conclusion and Future Prospects
This hypothesis is intriguing as it accounts for a previously enigmatic phenomenon through established scientific principles. The experimental evidence supports the notion that the piezoelectric effect in quartz could significantly influence the creation of extensive gold deposits. However, additional research is required to measure the rate of gold deposition through this method and to compare these experimental findings with natural gold deposits.
As we advance our comprehension of these processes, we may reveal new insights into the geological conditions that foster the formation of gold deposits, potentially
