[Daumic (OP)]

In few years hydraulic fracturing has revolutionized the world of energy by the production of shale gas and shale oil.

It is perhaps possible that fracking can reach another resource in the depth of the Earth: gold.

A new theory established by geochemists (1) describes a transport of gold by trisulphide ion in hydrothermal deposit. Trisulphide ion chelates gold and facilitates its transport towards the ground surface by water. But the stability of trisulphide ion depends of temperature and pressure. Trisulphide ion decays at a depth of some kilometres and leaves a first deposit of gold. According to this theory, a second transport by chloride and sulphide ions explains the gold deposits near the surface.

We can imagine a deep gold deposit under each hydrothermal gold deposit. The deep gold deposits are probably more massive than the upper deposits because the transport by trisulphide ion is more efficient than the transport by chloride and sulphide ions. These deep gold deposits are not accessible by classical process of mining.

These deep deposits are perhaps accessible by hydraulic fracturing. A depth of some kilometres is not a problem. The shale oil deposits of Permian Basin exploited in Texas by fracking have an equivalent depth of some kilometres. How can we extract gold? Perhaps by the following process:
-   two vertical wells to reach the deep layer of deposit,
-   horizontal drill between the vertical wells with a hydraulic fracturing,
-   circulation of water with gold chelatant in the fractured zone, for example pyridinethiol (2).

If this process works, gold extraction by fracking can be the beginning of a new chapter of fracking industry: the deep mining.

(1) Sulfur radical species form gold deposits on Earth (https://www.pnas.org/content/112/44/13484 [nofollow])

(2) Pyridinethiol‐Assisted Dissolution of Elemental Gold in Organic Solutions
 https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201810447 [nofollow]

[evan_au]

Pyridinethiol

This seems to be the chemical in question: https://pubchem.ncbi.nlm.nih.gov/compound/4-Mercaptopyridine

It irritates the skin and eyes, and is lethal (LD50) if ingested at an amount of 0.75g/kg - so don't drink it!
- But using hydrogen peroxide as a solvent is also dangerous - but not for long; I expect hydrogen peroxide would break down to water and oxygen as soon as it hit dirt.
- People opposed to fracking are concerned about various chemicals used to fracture the rocks - well, these ones would provide real cause for concern!
- Plus the problem of disposing of salty groundwater, which can pollute freshwater streams and lakes.

Another issue is that even after you  have a reaction with gold, it isn't really soluble in water, so you would have real problems pumping it to the surface. Gold is very dense, so any gold you extracted would probably stay at the bottom of the well.

...and what's so good about gold, anyway? It's value is mostly propped up by artificial scarcity, and fears of a world recession following COVID-19.

[Daumic (OP)]

The classical gold mining is also a very pollutant process, using cyanide or mercury (1). Furthermore, the small gold content of deposits makes necessary the grinding and treatment of a big amount of rocks.
If gold mining by fracking is possible, the gold value can amortize quickly the cost of drilling and fracturing. After the gold extraction, the drills and fractured zone remain for another use, like geothermal energy. The great value of gold can facilitate the development of geothermal energy.

(1) https://en.wikipedia.org/wiki/Gold_mining [nofollow]