Greisen is the glittering, altered remains of granite that hides valuable metal deposits underground.

Nature · Published 3 October 2026 Swipe more like this →

In short: When granite cools deep within the Earth, it releases gas-rich fluids that transform the rock into something entirely new. This process, known as greisenization, strips away feldspar and biotite, leaving behind a shimmering, quartz-rich stone that often serves as a geological treasure map for tin, tungsten, and other rare metals.

Greisen is a specialized modification of granite, an intrusive igneous rock typically composed of quartz and feldspar. While granite is the most common plutonic rock in the Earth's crust, greisen represents a distinct, altered state. It is defined by the essential presence of quartz and white mica, specifically muscovite, while notably lacking the feldspar and biotite that characterize standard granite. This mineral shift gives the rock a silvery, glittering appearance, as the layered muscovite crystals reflect light.

The formation of greisen is an endogenous process occurring during the final cooling stages of granite emplacement. As magma solidifies, it releases gas- and water-rich fluids that are forced through the interstitial spaces of the cooling granite. These fluids migrate toward the upper margins of the intrusion, where they become trapped beneath a sealed roof of overlying rock. This environment, typically found at depths between 0.5 and 5 kilometres, allows for the intense chemical alteration of the original granite into greisen.

This transformation is not merely a change in appearance; it is a vital indicator for mineral exploration. Because the residual fluids of granite crystallization concentrate incompatible metals, greisen zones are often rich in minerals such as cassiterite, wolframite, and molybdenite. Depending on the intensity of the alteration, the rock may retain its original granitic texture or lose it entirely, becoming a massive, quartz-muscovite rock. These deposits are frequently associated with S-type granites, which originate from the melting of sedimentary protoliths within thick, tectonically active fold belts.

The economic significance of these formations has led to mining operations worldwide, from the Ore Mountains in the Czech Republic to the Anchor Mine in Tasmania. By studying these zones, geologists can trace the path of hydrothermal fluids that once surged through the crust. Whether appearing as small veins or expansive zones in the roof of a batholith, greisen remains a testament to the volatile, late-stage chemistry that defines the life cycle of granitic intrusions.

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