Epithermal ore deposits are classified into low sulfidation system (LSS) and high sulfidation system. Epithermal vein type ore deposits of LSS are formed by chemical and physical changes of hot water in the fracture zone. This paper propose the formation model of epithermal vein type ore deposits of LSS, based on the characteristics of the hydrothermal alteration in Hosokura ore deposit, other epithermal ore deposits and active geothermal areas in Japan.
Initial hydrogeology of the hydrothermal system is composed of permeable near-surface aquifer of cold groundwater and permeable fracture zone in impermeable hydrogeologic basement. Neutral pH hot water rises along the fracture zone and flows into near-surface aquifer. This hot water mixes with cold groundwater at the near-surface aquifer leading to deposition of ore minerals within the aquifer and the fracture zone below it due to temperature drop (about 250°C→200°C). The hot water forms large scale neutral hydrothermal alteration zone also along aquifer.
Near-surface aquifer partly changes into horizontal and impermeable "alteration cap rock (ACR)" by hydrothermal alteration. Under this ACR, hot water comes to the boil in the fracture zone and ore minerals deposit in the fracture due to gas (H₂S, CO₂) escape and subsequent change of pH. Steam and gas are separated from hot water and rise through near-surface aquifer. Oxdation of H₂S forms acid hot water to form a small-scale acid hydrothermal alteration zone over a large scale neutral one.
The bonanza is formed where temperature of hot water falls and hot water boils, with the upper limit of it being controled by near-surface aquifer.