Petrography and uplift history of the Quaternary Takidani Granodiorite: Could it have hosted a supercritical (HDR) geothermal reservoir?

The Quaternary Takidani Granodiorite (Japan Alps) is analogous to the type of deep-seated (3-5 km deep) intrusive-hosted fracture network system that might support (supercritical) hot dry/wet rock (HDR/HWR) energy extraction. The I-type Takidani Granodiorite comprises: Porphyritic granodiorite, porphyritic granite, biotitehornblende granodiorite, hornblende-biotite granodiorite, biotite-hornblende granite and biotite granite facies; the intrusion has a reverse chemical zonation, characterized by > 70 wt% SiO₂ at its inferred margin and < 67 wt% SiO₂ at the core. Fluid inclusion evidence indicates that fractured Takidani Granodiorite at one time hosted a liquiddominated, convective hydrothermal system, with <380°C, low-salinity reservoir fluids at hydrostatic (mesothermal) pressure conditions. 'Healed' microfractures also trapped > 600°C, hypersaline (∼35 wt% NaCl_eq) fluids of magmatic origin, with inferred minimum pressures of formation being ∼600-750 bar, which corresponds to fluid entrapment at ∼2.4-3.0 km depth. Al-in-hornblende geobarometry indicates that hornblende crystallization occurred at about 1.45 Ma (7.7-9.4 km depth) in the (marginal) eastern Takidani Granodiorite, but later (at ∼1.25 Ma) and shallower (∼6.5-7.0 km) near the core of the intrusion. The average rate of uplift across the Takidani Granodiorite from the time of hornblende crystallization has been 5.1-5.9 mm/yr (although uplift was about 7.5 mm/yr prior to ∼1.2 Ma), which is faster than average uplift rates in the Japan Alps (∼3 mm/yr during the last 2 million years). A temperature-depth-time window, when the Takidani Granodiorite had potential to host an HDR system, would have been when the internal temperature of the intrusive was cooling from 500°C to ∼400°C. Taking into account the initial (7.5 mm/yr) rate of uplift and effects of erosion, an optimal temperature-time-depth window is proposed: For 500°C at 1.54-1.57 Ma and ∼5.2±0.9 km (drilling) depth; and 400°C at 1.36-1.38 Ma and V3.3E0.8 km (drilling) depth, which is within the capabilities of modern drilling technologies, and similar to measured temperature-depth profiles in other active hydrothermal systems (e.g. at Kakkonda, Japan).