High-temperature silicified zones as potential caprocks of supercritical geothermal reservoirs

•Granite-porphyry systems preserve similar features to those in geothermal systems.•A fossil supercritical geothermal system was observed in tazawako granite complex, NE Japan.•Several quartz filling veinlets were formed under sub- and super-critical states.•Supercritical geothermal system components: a heat source, reservoir, and caprock.•A high-temperature silicified zone is a potential caprock of a supercritical geothermal reservoir. We examined the silicification zone in a granite-porphyry system in order to investigate the caprocks in a supercritical geothermal system. At the top of the granite intrusion, a high-temperature silicification zone contains three generations of quartz. The earliest generation formed at higher temperatures and has bright luminescence and high Ti contents. This quartz is the phenocryst that formed in the granitic magma. The second and third generations formed at lower temperatures and have weak to dark luminescence and low Ti contents. This quartz is formed as replacement and in microfractures and cavities in the earlier quartz, suggesting it is a caprock for a supercritical geothermal reservoir.