Hydrothermal Systems
Abstract
Disequilibrium is a temporary disturbance of a system that usually exists in a more or less steady state. Overprinting is the replacement of one mineral assemblage by another due to changes in physical and/or chemical conditions (temperature, pressure and/or chemistry). Recognising disequilibrium and overprinting in fossil hydrothermal systems can be important in mineral exploration, because some of the mechanisms that cause overprinting also cause mineralisation. It is also relevant when assessing the potential of geothermal fields for energy development, and in identifying geothermal hazards.
There is a close association between prograde overprinting and mineralisation where overprinting is due to renewed magmatism in a stable or waning hydrothermal system. Retrograde overprinting is associated with mineralisation where it is due to a sudden pressure drop following catastrophic unroofing of a hydrothermal system.