| Abstract |
The heat source for a magmatic-related hydrothermal system is an intrusive body. Some clues as to the existence of a suitable heat source can be gained from the presence of young volcanoes in the area. However, active volcanoes usually either have no convective hydrothermal system, or are associated with systems which have too much magmatic fluid input to be exploitable. Many exploitable systems are not associated with active volcanoes that are young enough to be directly related to the heat source of the system. Hydrothermal systems go through a predictable life cycle, which must be understood to make sense out of the stage of evolution of any system encountered in exploration. In this section we will first describe typical changes that a hydrothermal system can go through in a single cycle, and then look at the possibilities of more complicated histories when hydrothermal systems become rejuvenated. These are not just theoretical considerations. They are based on observations in real systems. For example, in the Creede fossil hydrothermal system in Colorado, a great deal of fluid inclusion work has been done, and a “stratigraphy” has been erected based on colour banding in the abundant sphalerite in veins, which permits good time control (Figure 1). Twenty separate episodes of sphalerite deposition can be recognised. While these overall show a sequence of decreasing salinity and temperature with time, as would be expected with progressive dilution and cooling of a initial magmatic fluid, they also reveal two major cycles of rejuvenation plus some minor fluctuations. |