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In order to form heavier elements, atomic nuclei must capture free neutrons faster than they can escape due to radioactive decay. Metals heavier than iron (symbol FE, atomic number 26) need at least an exploding star to form rapid enough neutron capture. An element as heavy as gold (symbol AU, atomic number 79), theoretically could only form in the collision of two neutron stars (1).
Estimates of required creation events illustrated in this periodic chart:
How and why then did we get our gold on Earth? Simple!
See the following theory as illustrated.
A super massive star, or white dwarf, reaches its end of life, and supernovas, sending out heavy elements into space.
Not quite large enough to collapse into a black hole, the star’s core instead collapses into a Neutron star, composed almost entirely of neutrons, The extreme pressure causes the electrons and protons present in normal matter to combine into additional neutrons, partially supported against further collapse by neutron degeneracy pressure.
Somewhere in the vicinity, about a billion years ago by one new estimate (2), some of the heavy neutron material ejected from the Supernova hits the newly forming Earth (3).
The heavy material joined with the heavy iron in sinking to the center of our planet. As the core cooled, the center core temperature dropped below the melting point of iron causing the molten iron to crystallize and grow the inner core in size, a process that continues today. This spinning metal core of our planet creates the magnetic field that protects us from radiation. Less heavy metals cooled into the outer crust. The gold however, also sank to the center of the earth (4).
Somewhere in our galaxy, some 200 million years later than the supernova, two neutron stars collided sending out mass ejecta.
Earth is hit by about 20 billion, billion tons of heavy neutron asteroidal material, possibly again liquifying it. Enough gold, uranium and other heavy elements about equal in mass to all of Earth's oceans likely came to the solar system from just one collision of two neutron stars (5).
The impacting meteorites are stirred into Earth's mantle by gigantic convection processes (6,7). The neutrons began combing with other elements allowing heavier metals to form.
Pushing up into the mantle, these hot, mineral-rich fluids are circulated through cracks and fractures in the Earth’s crust, dissolving and transporting minerals, including gold along the way. These aqueous gold solutions were transported by hydrogen sulfide and chloride ligands, as well as by sulphur ions (8).
As these fluids encounter cooler conditions or suitable host rocks, they deposit their mineral content, including gold, in fractures, veins, or other rock formations. Over time, geological processes, such as erosion and tectonic movements, can bring these gold-bearing rocks to the Earth’s surface.
Types of Veins
Hydrothermal veins are formed when hot mineral rich fluids flow through rocks, and the minerals within the fluids crystallize in the fractures and cavities. Hydrothermal veins usually contain quartz along with ore minerals because fluids usually contain lots of dissolved silica. Gold is often found in veins with quartz. (9,10).
Pegmatites are formed when molten material expelled from igneous intrusions crystallizes very slowly. Extremely coarse-grained minerals and are formed from highly fluid, water-rich magmas.
Contact metamorphic veins occurs when magma aureoles enter into preexisting rock, metamorphizing surrounding rock. The aureole may release highly mineralized water that crystalizes in cracks in surrounding rock.
Shear zone veins occur during the movement of the rocks when intense pressure and friction can cause fluids to flow along the zone. Low angle detachment faults can also contain gold and silver.
Quartz
Similar to gold, quartz is a ‘mineral fluid’ which travels upward and solidifies into its mined form. Due to this, their chemical compositions are quite complementary and are, therefore, often found together. When quartz first begins to solidify, substances like pyrite or other sulfide minerals (which reside closer to the Earth’s crust) tend to form within the mineral. However, these substances do not stay in the mineral for long and usually rush out due to bad weather or other natural events. When this happens, it leaves behind voids and tiny porous cracks in the quartz. Later, when the fluid containing gold passes through the mineral, gold particles start to form in its crevices. Over time, this material reaches its more metallic state (11,12).
Rivers can breakup these rocks and separate the heavy gold which, being the heaviest of minerals, comes to rest in dips in the bedrock. Great place for us to find it!
(Picture from gold mining hike). src: Yes Im A Goldminer Don's mining Office
Asteroid Psyche appears to be made of solid metal, possibly including gold, nickel, and iron. It could be the core of an unformed or destroyed planet.
Launched in 2023, our Psyche spacecraft will arrive in Aug 2029.
Data collected by Evan Robinson
References
1) Watson, D., Hansen, C.J., Selsing, J. et al. Identification of strontium in the merger of two neutron stars. Nature 574, 497–500 (2019). https://doi.org/10.1038/s41586-019-1676-3
2) https://www.space.com/earth-core-billion-years-old.html
4) https://www.rdworldonline.com/where-does-all-the-gold-come-from/
6) https://geologyscience.com/minerals/gold/
7) Ampex https://learn.apmex.com/learning-guide/science/how-is-gold-formed/
8) Pokrovski et al. (2015). Sulfur radical species form gold deposits on Earth. Proceedings of the National Academy of Sciences. 112. 10.1073/pnas.1506378112.
9) https://geologyscience.com/geology-branches/mining-geology/vein-deposits/
10) https://www.sandatlas.org/what-do-you-mean-hydrothermal/
11) https://www.pnas.org/doi/full/10.1073/pnas.1506378112
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