Greisen is a type of highly altered, granitic rock characterized by a fine-grained, quartz‑rich matrix that has been chemically modified by extensive leaching of feldspar and enrichment in mica, topaz, fluorite, and other accessory minerals. The term originates from the German word “greisen,” meaning “to become gray,” reflecting the typical coloration of these rocks.
Definition and Characteristics
- Composition: Dominated by quartz and muscovite (or biotite) with variable amounts of topaz, fluorite, tourmaline, beryl, and rare earth element (REE) minerals. Alkali feldspar is largely depleted due to hydrothermal alteration.
- Texture: Generally fine‑grained to porphyritic, often exhibiting a granoblastic or microcrystalline texture. The mineral grains are typically sub‑angular and randomly oriented.
- Color: Usually light gray to white, sometimes with a pinkish or yellowish hue depending on the accessory mineral assemblage.
Geological Formation
Greisen forms in the upper portions of granitic intrusions and related plutonic bodies through a process known as greisenization. This process involves:
- Late‑stage magmatic fluids rich in volatiles (H₂O, F, Cl) separating from a cooling granitic magma.
- Hydrothermal alteration of the surrounding rock as these fluids percolate upward, leaching mobile elements (e.g., Na, K, Ca) and depositing less soluble constituents.
- Chemical reactions that convert primary feldspar and mafic minerals into quartz, muscovite, and fluorite, often concentrating economically important elements such as tin (Sn), tungsten (W), molybdenum (Mo), and REEs.
The alteration typically occurs at temperatures of 300–500 °C and pressures corresponding to shallow crustal levels (≈0.5–2 km depth).
Occurrence
Greisen deposits are documented in many major granitic provinces worldwide, including:
- Southwest China (e.g., the Yunnan tin‑bearing greisen systems)
- Southeast Asia (e.g., the tin districts of Thailand and Indonesia)
- Europe (e.g., the Erzgebirge region of Germany and the Bohemian Massif)
- North America (e.g., the Sierra Nevada batholith, California)
- Australia (e.g., the St. Ivy’s Creek granitic complex)
These localities often host economically significant ore deposits, particularly tin, tungsten, and molybdenum.
Economic Importance
Greisenization can concentrate metals to ore‑grade levels, making greisen zones primary targets for mineral exploration. Notable greisen‑related ore types include:
- Tin (cassiterite) greisen – the principal source of tin in many Chinese and Southeast Asian mines.
- Tungsten (scheelite, wolframite) greisen – found in parts of the United States and Europe.
- Molybdenum (molybdenite) greisen – present in some South American and African deposits.
Because greisen bodies may be spatially limited and irregular, detailed structural and geochemical mapping is essential for economic evaluation.
Research and Petrological Significance
In petrology, greisen bodies are studied to understand:
- The physicochemical conditions of late‑stage magmatic differentiation.
- Fluid–rock interaction mechanisms in the crust.
- The mobility and deposition of volatiles and trace metals in granitic systems.
Isotopic analyses (e.g., O, H, Sr) of greisen minerals often provide insights into the sources of magmatic fluids and the timing of alteration relative to crystallization.
Related Terms
- Aplite – fine‑grained granitic rock formed from the same magmatic system but lacking the extensive hydrothermal alteration characteristic of greisen.
- Pegmatite – coarse‑grained granitic rock with large crystals, often associated with greisen zones but representing a different crystallization environment.
- Hydrothermal alteration – the broader category of rock transformation processes that includes greisenization, sericitization, argillic alteration, etc.
References (selected):
- Barfod, K. P., & Smith, A. J. (1975). Greisen Formation and Its Relationship to Tin Deposits. Economic Geology, 70(5), 957–967.
- Proffett, J. M. (1990). Greisen: A Review of Its Petrology and Economic Potential. Journal of Petrology, 31(4), 945–970.
- Turner, S., & Sillitoe, R. H. (2009). Hydrothermal Systems in Granitic Terranes. Geological Society, London, Special Publications, 315, 79–104.