Relationship between solidification depth of granitic rocks and formation of hydrothermal ore deposits

Chemical analysis of biotite in representative granitic rocks in Japan shows that the total Al (Al-T) content changes with the metal type of the accompanying hydrothermal ore deposits and increases in the following order: Pb-Zn and Mo deposits < Cu-Fe and Sn deposits < W deposits < non-mineralized granitic rocks. The Al-T content of biotite in granitic rocks may be a useful indicator for distinguishing between mineralized and non-mineralized granitic rocks. A good positive correlation is seen between the Al-T content of biotite and the solidification pressure of the granitic rocks estimated by sphalerite and hornblende geobarometers and the mineral assemblages of the surrounding rocks. These facts suggest that the Al-T content of biotite can be used to estimate the solidification pressure (P) of the granitic rocks. The following empirical equation was obtained: P (kb) = 3.03 x Al-T - 6.53 (+/- 0.33) where Al-T designates the total Al content in biotite on the basis O = 22. According to the obtained biotite geobarometer, it is estimated that Pb-Zn and Mo deposits were formed at pressures below 1 kb, Cu-Fe and Sn deposits at 1-2 kb, W deposits at 2-3 kb and non-mineralized granitic rocks were solidified at pressures above 3 kb.