Greisen

Greisen is a hydrothermally altered granitic rock that forms in the late stages of granitic intrusions when fluids rich in silica, fluorine, and metals infiltrate the surrounding rock. This alteration leads to the replacement of primary feldspar minerals by quartz and mica, producing a quartz-rich, often granular to mosaic texture. Greisen is most commonly associated with tin and tungsten mineralization, and it frequently contains cassiterite as an ore mineral, with tungsten minerals such as wolframite or scheelite occurring in tungsten-rich varieties.

Formation and mineralogy are the defining features of greisen. The alteration is driven by high fluorine activity and moderate to elevated temperatures, typically in the range of a few hundred degrees Celsius, in near-surface settings. The result is a rock dominated by quartz and muscovite (mica), commonly with accessory minerals such as tourmaline, topaz, and, occasionally, fluorite. The textures can vary from fine-grained to coarser aggregates, and greisen zones often form discontinuous caps or envelopes around tin-tungsten ore bodies within granitic terranes.

Geographic occurrence is worldwide where granitic intrusions have given rise to tin- and tungsten-bearing systems. Greisen zones are economically important because they host cassiterite-rich ore and, in tungsten districts, associated tungsten minerals. Although greisen itself is a product of alteration rather than a primary lithology, it serves as a key indicator of near-ore hydrothermal activity and helps delineate zones of mineralization within granitic districts. The term derives from German, reflecting its long-recognized role in European tin-tungsten deposits.