How gold veins form: a pressure-and-cracks story

Part of our educational gold series: a simplified look at how gold-bearing veins can form in structurally controlled systems such as Clogau.

Gold veins do not form at random. They develop when hot, mineral-rich fluids move through faults and fractures in the rock, then deposit gold when pressure, temperature or fluid chemistry changes.

One of the best-known settings for this is orogenic gold, where deposits are linked to major fault systems. A useful way to picture it is the fault-valve idea: faults seal, pressure builds, then the system cracks open again. Fluids rush through, quartz is deposited, and over repeated cycles a vein system begins to grow. In simple terms: crack, fill, seal, repeat.

That is why many gold deposits are not made of one simple vein, but of a whole network of quartz veins and fractures built in pulses over time. A single vein can also record several stages of mineralisation, from early quartz, to sulphides that may host microscopic gold, to later coarser native gold forming in open spaces.

At Clogau, the geology points to this kind of structurally controlled mesothermal, or broadly orogenic-style, gold system. The veins are associated with Cambrian black shales, local greenstones or microdiorite, and quartz-sulphide-carbonate veining within shear zones. Importantly, gold is not spread evenly through all the quartz. Historically, miners chased localised shoots and pockets, where the right structures met the right host rocks.

That is the key idea: the pathways for fluid may be broad, but the best gold concentrations are often very local, forming where structure, pressure change and favourable rock types come together.

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