High permeabilities of Quaternary granites in Japan and its implications for mass and heat transfer in a magmatic-hydrothermal system

The permeability of Quaternary unaltered granite from deep geothermal wells in Japan is higher than 100 mu d, which is one order of magnitude higher than the reported laboratory permeability of various granitic rocks. Mean crack-width, which ranges from 0.12 to 1.30 mu m, has a positive correlation with the measured permeability. The granite is considered to be permeable just after the solidification and subsequent alteration, and/or compaction reduces the permeability and crack-width. Grain boundaries may be important as a migration path for degassing and circulation of hydrothermal fluid in the magmatic-hydrothermal stage.