When you picture gold, you imagine visible flecks or veins. Carlin-type gold is the opposite: the gold is so finely scattered, locked invisibly inside other minerals, that you can't spot it even under a microscope. Yet these deposits are enormous.

How it forms

Hot fluids move through particular silty, carbonate-bearing sedimentary rocks and react with them — dissolving parts of the rock and depositing microscopic gold disseminated through a "favourable" host bed. The host rock and its chemistry are the key; there are no obvious veins.

This makes Carlin-type its own model — not the quartz veins of orogenic gold, and not the shallow volcanic setting of epithermal gold. The gold is spread invisibly through altered sedimentary rock.

Because the gold is invisible and the deposits are big but lower-grade, they're mined in large open pits with chemical processing. The Carlin Trend of Nevada — named after the town of Carlin — is one of the richest gold provinces on Earth.

Refractory gold — locked away

The invisible gold comes with a catch. Normally gold is recovered by crushing the ore and soaking it in a weak cyanide solution that dissolves the gold so it can be collected — a step called leaching. But in much Carlin-type ore the gold is sealed inside sulfide minerals (like pyrite and arsenic-bearing grains), so the cyanide simply can't reach it. Gold like this is called refractory — it resists the normal recovery method. (Ore whose gold leaches out easily is the opposite: free-milling.)

To win refractory gold you must first unlock it by breaking down the host sulfides before leaching. Three common ways: roasting (burning the sulfides off — but this can release sulfur and arsenic fumes, the same hazard we met with arsenides in Unit 2), pressure oxidation (cooking the ore with oxygen under high heat and pressure in a sealed vessel called an autoclave), and bio-oxidation (letting special bacteria eat the sulfides away). Each adds cost, energy, and complexity — so whether the ore is refractory can make or break a deposit's economics.