Petrographic and mineralogical study of hydrothermal alteration of the Rokko Granite at Hakusui‐kyo and Horai‐kyo along the Arima‐Takatsuki Tectonic Line in western Japan

Abstract

AbstractField, hand specimen, and microscopic investigations alongside X‐ray diffraction analyses revealed four types of hydrothermal alteration (Type‐A, ‐B, ‐C, and ‐D) based on the mode of occurrence of altered rocks and alteration mineral assemblage at Hakusui‐kyo and Horai‐kyo along the Arima‐Takatsuki Tectonic Line (ATTL) in western Japan. Type‐A alteration locally occurred as gray alteration halos with sulfide minerals. Type‐B and ‐C alterations were confined to fault gouge veins and occurred as greenish‐gray veins and brown veins, respectively. Type‐C alteration crosscut Type‐B alteration. These alterations were associated with a number of granitic fragments including cohesive breccia and micrographic facies. Type‐D alteration occurred locally in brown sediments. Different mineralogical features in the four alterations are summarized as (Type‐A) illite; (Type‐B) chlorite; (Type‐C) limonite (Fe3+ hydroxides and goethite) and calcite; and (Type‐D) limonite. We propose that the alterations can be broadly divided into Paleocene hydrothermal alteration (Type‐A) and post‐Late Miocene hydrothermal alteration (Type‐B, ‐C, and ‐D): Type‐A alteration occurred at approximately 200 °C during hydrothermal activity after a granitic intrusion in Late Cretaceous; Type‐B, ‐C and ‐D alterations occurred under hydrothermal activity accompanying deep fluids with repeated ascents invoked by the seismicity of the ATTL after the Late Miocene. The fluids may have been the “Arima‐type thermal waters” (i.e., mixtures of convective groundwater and Na‐Ca‐Cl‐HCO3‐type fluids). Type‐B alteration occurred in fractures at depths where the temperature was ≥150 °C. Type‐C alteration overprinted Type‐B alteration as a result of mixing of new deep fluids and descending oxidized meteoric water near the surface. Fe3+ hydroxides and calcite precipitated from the fluids due to the oxidation of Fe2+ and the degassing of CO2, respectively, at ambient to near‐boiling temperatures. When the ascending fluids gushed out from the fractures, they generated Type‐D alteration at the surface under similar temperature conditions due to the oxidation of Fe2+.