How Gold Forms: The Geology Behind NZ's Goldfields

Understanding where gold comes from can help you find more of it. Gold follows rules, even if it doesn't always feel that way when you're panning an empty pan. Here's the science behind the shine.

Where Does Gold Come From?

All the gold on Earth arrived from space — literally. Gold is formed in supernova explosions and neutron star collisions, then scattered through the universe. When Earth coalesced 4.5 billion years ago, gold was mixed into the molten rock. Most sank to the core, but some remained in the crust — and that's what we're mining today.

In New Zealand, gold was concentrated millions of years ago through a process called hydrothermal deposition. Hot, mineral-rich fluids circulated through fractures in the rock, and when conditions changed (temperature dropped, pressure released, chemistry shifted), gold precipitated out and formed veins.

The Otago Schist

Most of Otago's gold is hosted in a rock type called schist — a metamorphic rock formed when sediments are heated and compressed deep in the Earth over millions of years. The Otago Schist is a vast formation covering much of Central Otago, and it contains gold throughout.

The gold isn't evenly distributed. It tends to concentrate in quartz veins that cut through the schist. These veins formed when fluids flowed through fractures in the rock, depositing silica (quartz) and, sometimes, gold.

The Macraes Mine — New Zealand's largest — is extracting gold from exactly this kind of deposit: massive quartz veins running through the Otago Schist.

From Reef to River

The gold that prospectors find in rivers didn't start there. It eroded out of hard-rock source deposits over millions of years. Glaciers ground the mountains. Rivers cut through the land. And gradually, gold particles were liberated from their quartz hosts and washed downstream.

This is alluvial gold — gold that's been transported and deposited by water. It's typically fine and flat, having been battered against rocks during its journey. The further gold travels from its source, the smaller and smoother it becomes.

Why Gold Concentrates

Gold is extraordinarily dense — about 19 times heavier than water. When water flows over a mixed bed of gravel, sand, and gold, the gold sinks quickly while lighter materials wash away. Over time, this creates natural concentrations.

Gold accumulates in predictable locations:

• Inside bends of rivers: Where water slows and drops its load.
• Behind large boulders: Eddies that trap heavy particles.
• In bedrock crevices: Once gold falls into a crack, it rarely escapes.
• On clay or false bedrock layers: Where downward migration stops.
• At the head of gravel bars: Where fast water meets slow water.

Understanding these trap points is the key to productive prospecting. You're not looking for gold everywhere — you're looking for the places where water has done the concentrating work for you.

Different Goldfields, Different Geology

Practical Implications

What does this mean for recreational prospectors?

1. Follow the geology. Look for schist country with known quartz veins upstream. That's where the gold came from.
2. Read the water. Gold follows hydraulic rules. Learn where fast water becomes slow water.
3. Go to bedrock. The best gold is usually on or near bedrock. If you're only working surface gravel, you're missing the good stuff.
4. Think about source. How far has the gold travelled? Coarser gold means you're closer to the source. Fine flour gold means long transport.

Geology won't tell you exactly where to dig, but it will tell you where NOT to waste your time. And that's half the battle.