A granitic rock that has been altered by hot mineral-rich fluids, containing quartz and mica, commonly found in tin and tungsten mining regions.
From German 'Greisen,' derived from 'greis' meaning 'gray,' named for the typically grayish color of these altered granite rocks found in Saxony.
Geologists love greisen because it's like a geological treasure map—wherever you find this altered granite, valuable tin and tungsten minerals are usually hiding nearby, making it a prospector's best friend.
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