Comparison of Smectite–Corrensite–Chlorite Series Minerals in the Todoroki and Hishikari Au–Ag Deposits: Applicability of Mineralogical Properties as Exploration Index for Epithermal Systems

The tri-smectite(S)–corrensite(Co)–chlorite(C) series minerals from two epithermal deposits show a discontinuous stepwise sequence of different mixed-layering of chlorite and smectite layers, and there are differences in the mode of occurrence of the two deposits. The Al/Si ratios and Fe/(Fe + Mg) ratios of the S–Co–C minerals vary closely related to mixed-layering and mode of occurrence. The S–Co–C minerals as a product of direct precipitation from ascending hydrothermal solutions may reflect fluid chemistry that originated in water–rock interaction at deeper strata. The differences in mixed-layering of the S–Co–C series minerals may be related to different thermal and redox conditions affected by fluid mixing and boiling, and to kinetic factor such as time length of hydrothermal activity that affected vein formation. The corrensite and Co–C minerals as a product of hydrothermal alteration involving dissolution, re-precipitation, and crystallization, may undergo smectite-to-chlorite transformation in epithermal systems. The transformation and distribution of corrensite and Co–C minerals in the host rocks around vein areas may have been controlled by thermal conditions related to fluid mixing and water/rock ratios. In addition, the corrensite and Co–C minerals with high Fe/(Fe + Mg) ratios may be affected by the host rock. The factors influencing the conversion of the S–Co–C series minerals may be similar in both ore veins and host rocks. However, it is emphasized that fluid/rock ratios may be a major factor influencing the conversion of the S–Co–S series minerals in host rocks. Additionally, the time length of hydrothermal activity that affected vein formation may be an important factor influencing the conversion of the S–Co–C series minerals in ore veins.