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 (^TAl) 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 ^TAl 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 T Al 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 ^TAl 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 × ^TAl - 6.53 (± 0.33) where T_Al 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.