Characteristics of skarn deposits and related acid magmatism in Japan

Abstract

Most Japanese skarn deposits are genetically related to acid igneous activity of Cretaceous or Miocene age. The granitic rocks in Japan have been subdivided into a magnetite-bearing and a magnetite-free or ilmenite series by Ishihara (1977). This paper describes the characteristic features of skarn deposits associated with the two granitic rock series. 1. Epidote skarn is usually found in the deposits associated with magnetite-bearing granitoids, whereas plagioclase-clinopyroxene skarn is present instead of epidote skarn in those associated with ilmenite granitoids. 2. Ferrobustamite, ideally Ca 5 FeSi 6 O 18 , occurs in some deposits associated with ilmenite granitoids. 3. Some scheelite skarns, characterized by the assemblage grossular-hedenbergite, are also associated with ilmenite granitoids. 4. Auriferous skarn deposits tend to be associated with magnetite-bearing granitoids.These facts reveal a correlation between the oxidation states of granitic magma and those of related skarn-forming aqueous solutions. The correlation strongly suggests that the skarn-forming solution is essentially derived from the granitic magma. In relatively shallow environments, the correlation is not clear, suggesting an interaction of fluids with various oxidation states.