The Chemical Composition of Chlorites from Hydrothermal Ore Deposits and its Applicability to Geothermometers

Compositional and structural properties of clay minerals from hydrothermal systems show highly diverse and systematic variations related to their localities. The chemical compositions of chlorite, one of the most common clay minerals, are thought to have a close relationship with its formational environments such as geological conditions, temperature, chemistry of hydrothermal solution, etc. In this study two chlorite geothermometers were considered by using the chemical composition of chlorites from some hydrothermal ore deposits in Japan. The two chlorite geothermometers are the Cathelineau's thermometer giving an empirical temperature scale of chlorite formation in an active geothermal field, and the Walshe's thermometer giving a thermodynamic temperature scale based on exchange reactions of chlorite solid solutions. On the basis of a comparison of fluid inclusion geothermometry, it is concluded that the two geothermometers can be applied to different compositional types of hydrothermal chlorite. The Cathelineau's thermometer fits well for Mg-chlorite, and the Walshe's thermometer fits for Fe-chlorite, indicating that both two chlorite geothermometers can be used as exploration indicators for geothermal reservoirs and hydrothermal ore deposits. In addition, it must be emphasized that the impurities in the chlorite should be examined in applying the chlorite geothermometers to chlorites.