| Abstract |
An active geothermal system developed around a shallow intrusive body has several similarities with a fossil hydrothermal system of porphyry copper deposit, although they differ in hydrothermal alteration, salinity of circulating fluids and metal concentrations. This paper describes hydrothermal systems and related fractures around an intrusive body based on the comparison between porphyry copper and active geothermal systems. Intrusive-related active geothermal systems such as the Geysers in California, Tongonan in Philippines, Kakkonda in Japan, and Larderello in Italy have several common characteristics. That is: (1) They are present above and around an intrusive body. (2) Circulating fluids mostly originate from meteoric water, but some trapped in fluid inclusions in early stages are highly saline, suggesting involvement of magmatic fluids. (3) Contact metamorphism is dominant around an intrusive body with minor potassic alteration. (4) The deep fluid system is controlled by fractures around an intrusive body. However, a fracture system model is difficult to reconstruct geologically. By way of comparison, porphyry copper systems are summarized as follows: (1) The hydrothermal system is developed around a stock-like intrusive body. (2) Highly saline magmatic fluids are involved in an earlier stage, while meteoric fluid is present in later stages. (3) Potassic and phyllic alterations are developed around an intrusion. (4) Two types of fracture systems are recognized. They are radial and concentric structures in the early stage, and diagonal and high-angle ones in the late stage. The hydrothermal activities at the late stage of porphyry copper system are quite similar to those in present geothermal systems. If those activities are controlled by similar physico-chemical conditions, we could apply a fracture model of the late stage porphyry copper system to exploration of present geothermal systems. |