How Is Gold Ore Formed? A Guide to Gold Deposit Types

Gold does not sit in the ground waiting to be picked up. It is dispersed through the Earth’s crust at a few parts per billion, and a deposit only exists where geology has concentrated it hundreds or thousands of times. Understanding how gold ore forms is not academic — the mechanism that concentrated the gold also decides its grain size, its host mineral and how hard it will be to recover.

This guide covers the main gold deposit types, what each one looks like in the ground, and how that translates into the processing route a plant will need.

Where gold comes from

Gold is a siderophile element: it prefers to bond with iron and, early in Earth’s history, most of it sank into the core. The gold that remains in the crust arrived through later magmatic and hydrothermal activity, carried in hot, mineral-rich fluids that rose along faults and fractures.

As those fluids cool, change pressure or react with surrounding rock, the gold comes out of solution. Where it precipitates inside a fracture system, you get a lode deposit. Where rock containing that gold is later weathered and eroded, and the dense gold particles are concentrated by flowing water, you get a placer deposit. Almost every gold deposit is a variation on these two stories.

The main types of gold deposit

Orogenic and lode gold

Orogenic deposits form where hydrothermal fluids move through fault systems during mountain-building. The gold precipitates in quartz veins and in altered wall rock, commonly alongside sulphides such as pyrite and arsenopyrite. These are the classic “lode” deposits, and they are the mainstay of hard-rock gold mining.

Their behaviour in the plant depends on grain size. Coarse, free gold responds well to gravity concentration early in the circuit; fine gold locked inside sulphide grains will report to a flotation concentrate instead.

Placer and alluvial gold

Placer deposits form when a primary deposit erodes and rivers carry the liberated gold downstream. Because gold is dense and chemically inert, it settles out in riffles, behind boulders and on bedrock, while lighter material is flushed away. The result is free gold in sand and gravel — coarse near the source, finer far downstream.

Placer gold is the easiest to recover in principle, because it is already liberated. In practice, clay, cemented gravel and very fine gold all cause trouble, and the alluvial gold recovery route lives or dies by how well the circuit washes and size-classifies the feed before gravity concentration.

Oxidized versus sulphide gold

Near the surface, weathering oxidizes sulphide minerals and liberates much of the gold. This oxidized zone is free-milling: the gold is accessible to cyanide, and recovery is usually straightforward. Deeper down, the same deposit becomes sulphide ore, where gold is locked inside pyrite or arsenopyrite and often invisible even under a microscope.

That transition is one of the most consequential facts in gold processing. Sulphide or refractory gold resists cyanidation and may need roasting, pressure oxidation or bio-oxidation before any leaching will work. We cover this in detail in our guide to oxidized gold ore processing.

Why deposit type decides the flow sheet

The geology sets the rules. A placer deposit needs no comminution at all — the gold is already free — while a hard-rock lode deposit may need three crushing stages and fine grinding to reach the liberation degree the ore demands.

Notice that the same metal produces completely different plants. A 3 g/t orogenic ore with coarse free gold might use a simple gravity circuit; a 3 g/t Carlin-type ore of the same grade might need a pressure-oxidation autoclave. Grade alone tells you almost nothing about the plant.

From deposit to recovery

In a hard-rock plant, the path from run of mine ore to gold bullion usually runs through four stages. Comminution crushes and grinds the ore to the fineness of grinding needed for mineral liberation. Gravity concentration or flotation removes coarse or sulphide-hosted gold early. Leaching dissolves the remaining gold into a cyanide solution, and electrowinning or zinc precipitation recovers it from solution before smelting.

Each stage has an efficiency that can be measured and improved, and each has a cost. That is why the geological work matters so much up front: knowing whether your gold is free or locked tells you which of those stages you actually need.

Frequently asked questions