Greisen

Greisen is a hydrothermally metamorphosed granitic rock. It is composed mostly of light-colored mica (muscovite, lepidolite, zinnwaldite) and quartz.

Greisen is a type of endoskarn. It means that greisen in most cases formed inside the granitic pluton itself that provided heat and hydrothermal fluids to initiate the greisenization. Therefore, granite seems to have metamorphosed itself to greisen by late hot magmatic fluids that contained many chemical elements incompatible in common minerals forming granitic intrusions. Hence, greisen is usually a natural concentrate of somewhat unusual minerals. Some of them may be economically interesting. These minerals are cassiterite (tin ore), fluorite (fluorine), rutile (titanium), lepidolite (lithium), tourmaline (contains boron but used mostly as a semi-precious gemstone), wolframite (tungsten), topaz, etc.

It should not be a surprise that the term ‘greisen’ was originally a mining term. It originates from Germany (the Ore Mountains) and meant a rock that contained tin ore (cassiterite) with quartz, mica, and little or no feldspar.

Greisen in most cases is a metamorphosed S-type granite (‘S’ refers to sedimentary protolith). Such granites typically have a relatively high content of incompatible elements and therefore often give rise to pegmatites too. Greisen is usually coarse-grained and gray in color with a glittering appearance because of high mica content. The fluids that cause greisenization usually follow cracks and fissures in the granitic pluton (and sometimes adjacent country rocks also) but sometimes these cracks are so closely spaced that almost the whole of the granite is turned into greisen.