How Is Gold Formed? From Exploding Stars to River Gravels
Gold is made in stellar collisions, carried by hot underground water and finally concentrated by rivers. Here is the whole journey, step by step.
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Every flake of gold in a prospector's pan has been through three separate journeys. It was made in space, it was gathered underground by hot water, and in many cases it was moved again by a river. Understanding those three steps is the single most useful thing you can learn if you want to know where gold might be found.
Step one: gold is made in space
Gold cannot be made inside an ordinary star. Stars fuse light elements into heavier ones, but that process stops at iron. Anything heavier, gold included, needs a sudden flood of neutrons that only the most violent events in the universe can supply.
The strongest evidence points to collisions between neutron stars. In 2017, astronomers detected gravitational waves from one of these collisions and then watched the glow of freshly made heavy elements spread out from the same spot. Some rare types of supernova may also contribute. Either way, the gold on Earth is older than the Sun and was already present in the cloud of dust and gas that formed our solar system.
Step two: most of Earth's gold disappeared
When the young Earth was mostly molten, heavy iron sank to the centre to form the core. Gold dissolves easily in molten iron, so the great majority of the planet's gold went down with it. That is why gold is so rare at the surface. The average rock in the continental crust contains only about 4 parts per billion of gold, or roughly 0.004 grams per tonne.
Many geologists think some of the gold we mine today arrived slightly later, delivered by meteorite impacts after the core had already formed. Whatever its exact source, the gold left in the crust is spread very thinly. To make an ore deposit, nature has to collect it and concentrate it hundreds or thousands of times.
Step three: hot water does the collecting
The main collecting agent is hot, salty water moving through cracks deep underground. Geologists call it a hydrothermal fluid. This water can come from cooling magma, from rocks being heated and squeezed during mountain building, or from rain and seawater that soaked down and warmed up.
At high temperature and pressure, these fluids can dissolve tiny amounts of gold, usually bonded with sulfur or chlorine. A fluid moving through kilometres of rock can quietly pick up gold from a huge volume of it. The important part is what happens next: something makes the fluid drop its load in one small place.
Gold falls out of solution when:
- the fluid cools as it rises toward the surface,
- the pressure drops suddenly and the fluid boils, which happens when a fault slips and a crack opens,
- the fluid reacts with iron-rich rock, which strips out the sulfur that was holding the gold,
- two different fluids mix and the chemistry changes.
The same fluids also carry dissolved silica. When conditions change, silica comes out as quartz, often at the same time as the gold. That is why gold so often sits inside white quartz veins, and why old prospectors followed quartz across the hills.
The main types of gold deposit
Geologists group gold deposits by the setting in which the fluids did their work. You do not need to memorise all of them, but knowing the big four helps you read geological maps and reports.
| Deposit type | Where and how it forms | Typical depth | Well-known examples |
|---|---|---|---|
| Orogenic (lode) | Fluids released during mountain building travel up large faults; gold sits in quartz veins | Several kilometres | Mother Lode (California), Golden Mile (Kalgoorlie, Australia), Bendigo (Australia) |
| Epithermal | Hot springs systems above volcanoes; gold drops out as the fluid boils near the surface | Usually under 1.5 km | Hishikari (Japan), Comstock Lode (Nevada) |
| Porphyry | Large, low-grade bodies of copper and gold around a cooling intrusion | 1 to 4 km | Grasberg (Indonesia), Bingham Canyon (Utah) |
| Carlin-type | Microscopic gold inside tiny pyrite grains in sedimentary rock | 1 to 3 km | Carlin Trend (Nevada) |
Each type has its own guide: orogenic, epithermal and porphyry. Orogenic and epithermal deposits are the ones that produce visible gold in quartz, and they are the main source of the nuggets and flakes that hobby prospectors find. Porphyry and Carlin-type deposits hold enormous amounts of gold, but it is usually invisible to the naked eye.
Step four: rivers concentrate it again
Once a gold-bearing vein reaches the surface, weather starts to break it down. Rain, frost and plant roots crack the rock. Quartz and the other minerals wash away, but the gold stays as grains, flakes and occasionally nuggets.
Gold is about 19 times heavier than water and roughly seven times heavier than quartz. When streams carry it downhill, it is the first thing to settle wherever the water slows down. Over thousands of years, this natural sorting builds up rich layers called placer deposits (see lode vs placer gold). The best spots are usually:
- the inside of river bends, where the current is slower,
- cracks and low points in the bedrock under the gravel,
- behind large boulders and below waterfalls,
- old river channels that are now dry and sometimes sit high above the modern stream.
Placers can also be ancient. The Witwatersrand Basin in South Africa is a fossil placer around three billion years old, and it has produced a very large share of all the gold ever mined, by many estimates around a third or more.
What this means if you want to find gold
The geology above boils down to a few practical rules:
- Gold comes from somewhere. Placer gold in a creek means there is, or was, a source upstream. Areas with known lode mines are the best places to start.
- Look for the clues of hot fluids. Quartz veins, rusty stained rock and old mine workings show that hydrothermal fluids passed through.
- Think like water. In streams, gold collects wherever the current loses energy. Bedrock is where the richest material usually sits.
- Black sand is a good sign. Heavy minerals such as magnetite travel with gold, so a pan that ends with plenty of black sand is worth a closer look.
Before you head out, check who owns the land and what the local rules allow. In many places gold prospecting needs permission, and some areas are closed completely.
Frequently asked questions
Is gold still forming today?
Yes. Active hot spring systems in places such as New Zealand and Japan are depositing small amounts of gold right now. The process is far too slow to notice in a human lifetime, though.
Why is gold found with quartz?
The same hot fluids that carry gold also carry dissolved silica. When the fluid cools or boils, both drop out together, so gold ends up locked inside the quartz.
How old is the gold we find?
The gold atoms themselves are older than the solar system. The deposits are much younger: they formed anywhere from a few million to more than three billion years ago, depending on the region.