Why Is Gold Found in Quartz? The Chemistry Explained Simply
Gold and quartz travel together in the same hot underground water and drop out under the same conditions. Here is why, and how to tell promising quartz from the barren kind.
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Ask an old prospector where to look for gold and there is a good chance the answer will be "follow the quartz". It is solid advice, with one important catch: most quartz has no gold in it at all. Understanding why the two minerals often appear together also explains how to tell a promising vein from a worthless one.
Same water, same journey
Gold and quartz are carried by the same hot, mineral-rich water moving through cracks deep underground. Geologists call it a hydrothermal fluid. At depth, under high temperature and pressure, this water can hold a surprising amount of dissolved silica, the raw material of quartz. It can also hold tiny amounts of gold.
The gold does not float around as loose metal. It travels bonded to other chemicals, mainly sulfur in the form of bisulfide. Think of the sulfur as a taxi that keeps the gold dissolved. As long as conditions stay steady, gold and silica both stay in solution.
Same trigger, same moment
Things change when the fluid moves into a new environment, usually a crack that has just opened because a fault slipped. Three things tend to happen at once:
- Pressure drops suddenly. The fluid may even boil.
- Temperature falls as the fluid rises toward cooler rock.
- Chemistry shifts as the fluid reacts with the walls of the crack.
Silica becomes far less soluble as the fluid cools and loses pressure, so quartz starts to crystallise and fill the crack. At the same moment, boiling and chemical reactions strip away the sulfur that was holding the gold. With its taxi gone, the gold drops out and gets trapped inside the growing quartz.
So gold is not attracted to quartz. They simply share a ride and get off at the same stop. The full sequence is covered in our guide to how gold is formed.
Why most quartz is still barren
Quartz is one of the most common minerals in the Earth's crust. Silica-rich water moves through rocks almost everywhere, but gold-bearing fluids are rare. A quartz vein only tells you that hot water once passed through. Whether that water carried gold is a separate question.
Prospectors have a name for the barren stuff: bull quartz. It is milky white, massive and glassy, with clean surfaces and very little else in it. You can walk across kilometres of it without finding a single speck.
What gold-bearing quartz tends to look like
No single feature guarantees gold, but these signs make a vein worth a closer look:
- Rust staining and brown or orange patches. Sulfide minerals such as pyrite weather into iron oxides. Gold often travelled with those sulfides.
- Boxwork and empty cavities. Small holes with rusty, cell-like walls show where sulfide crystals have rotted away.
- Banding. Thin parallel layers, sometimes slightly different colours, show the vein was opened and refilled many times. Each pulse of fluid was a fresh chance to deliver gold.
- Darker or grey streaks. Fine-grained sulfides and arsenopyrite can give quartz a grey or bluish tone.
- Other minerals mixed in. Patches of rock from the vein walls, carbonate minerals or visible metallic grains all point to a more complex, more interesting fluid history.
- Context. A vein near old mine workings, or in an area known for gold, is a better bet than one in the middle of nowhere.
If you see something yellow in a quartz sample, test it before getting excited. Pyrite and chalcopyrite fool a lot of people. Our pyrite vs gold guide covers seven quick checks.
Where gold-quartz veins form
Gold-bearing quartz shows up in a few geological settings:
- Orogenic deposits, formed during mountain building. These are the classic gold-quartz reefs of the Mother Lode in California, Bendigo and Ballarat in Australia, and the greenstone belts of Ontario and Western Australia. See our page on orogenic gold deposits.
- Epithermal deposits, formed close to the surface above volcanic systems. Their quartz is often banded, finely layered and sometimes looks like agate. See epithermal gold deposits.
- Stockworks around porphyry intrusions, where networks of thin quartz veinlets carry gold and copper through a large body of rock.
From quartz veins to river gold
Most of the gold in creeks started out in quartz veins. When those veins reach the surface, weathering breaks them apart. Quartz is tough but lightweight, so it rolls and tumbles downstream. Gold is roughly seven times denser, so it settles quickly and collects in the low spots.
That is why quartz pebbles in a creek, especially rusty ones, are a useful clue. They show that veins exist somewhere upstream, and that the stream has been eroding them. Combine that with heavy black sand in your pan, and you are reading the same story the old-timers read.
Can you get the gold out of quartz yourself?
For a specimen, many collectors leave the gold in the quartz, because a good gold-in-quartz piece is often worth more than the gold alone. For ore, the gold has to be freed by crushing the rock to a fine powder and then separating the gold by gravity.
Avoid any method that uses mercury or cyanide. Both are toxic, both damage rivers and soil, and both are tightly regulated or banned for hobby use in most countries. Crushing and careful panning recovers most of the coarse gold a hobbyist will find.