Greisen is a hydrothermally metamorphosed granitic rock. It is composed mostly of light-colored mica (muscovite, lepidolite, zinnwaldite) and quartz.
Greisen with cassiterite from Namibia. Width of sample 10 cm.
It is a type of endoskarn. It means that these rocks 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. This is the reason why 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.
In most cases this rock 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. It 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 in adjacent country rocks also), but sometimes these cracks are so closely spaced that almost the whole granite body is turned into greisen.
I found greisen where Irish Creek debouches into South River in Rockbridge County, VA. The upper reaches of Irish Creek is where cassiterite is found–the only place in the U.S. It was once mined there.
That is also the place where old Joe Clark, of the old-time song, and his brother, Nelson, received a land grant after serving in the War of 1812. A record of the land grant is in the Rockbridge County Court House. When the brothers arrived to claim their grant they found that it was “straight up.” Nevertheless, they stayed and produced many children with their respective Native American wives.
Correction: It was cassiterite that was mined in the upper reaches of Irish Creek. It was mined for the tin.