Formation process of the vein in the Nakamura formation of the Mizunami Group

Abstract

This paper discusses the formation of veins less than 20 cm in width which are abundant in the Nakamura Formation of the Miocene Mizunami Group. These veins consist of a central clastic and adjacent dense portions. Field observations and chemical and X-ray diffraction analyses show that the clastic part was formed by hydraulic fracturing under hydrothermal conditions, and that light (Lm) and dark colored parts (Dm) of the veins were formed by precipitation of opal-CT from hydrothermal fluids. The formation of Lm and Dm are probably related to fluids that dissolved pumice fragments and acidic fluids formed in a reducing environment, respectively. Fluid penetration of a host rock with intergranular porosity adjacent to the clastic part of a vein and subsequent precipitation of Lm formed the dense part of the vein. Based on microscopic observation and XRD analysis, hydrothermal alteration did not affect the dense parts of the veins, unlike the host rocks. Progression of water-rock interaction occurred after the veins solidified.