Gold chemistry: why gold moves before it becomes visible

This post is part of our educational gold series for investors, looking at how gold forms, moves and becomes concentrated in mineral systems.

Gold does not usually move through the crust as visible metal. Long before it appears in a quartz vein, it may have travelled as dissolved gold in hot, mineral-bearing fluids moving through fractures in the rock.

In many hydrothermal gold systems, gold is carried as chemical complexes. Gold-sulphur complexes, especially hydrosulphide or bisulphide species, are particularly important. In more saline fluids, gold-chloride complexes can also play a role, allowing very small amounts of gold to stay dissolved and move through the system.

Gold is deposited when those fluids change. A drop in pressure, a fall in temperature, a shift in redox conditions, or a change in available sulphur can make the fluid less able to hold gold. At that point, gold may come out of solution and become trapped in quartz veins, fractures or associated sulphide minerals.

In the wider Dolgellau Gold Belt, published geology records show gold occurring alongside sulphide minerals such as pyrite and arsenopyrite. Arkadian, formerly Alba Mineral Resources, has also reported sulphides in quartz veining at Llechfraith, reinforcing the link between gold, sulphide minerals and fluid chemistry.

At Clogau, the deposition mechanism is believed to involve interaction between auriferous, or gold-bearing, fluids and highly reduced shales. That chemical setting helps explain how gold may have moved, where it was trapped, and why it can be so unevenly distributed underground.

Gold exploration is therefore also about understanding the pathways, the fluid chemistry, and the conditions that made invisible gold become visible.

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