Alteration of Subsurface Granitic Rock in Okayama Area, Japan

Abstract

In order to understand the alteration process in subsurface granitic rocks, drilled core (100 meters deep) taken from Mesozoic granitic rock distributed in the Chugoku area of Japan has been investigated. The rock is characterized by the coarse size of rock-forming minerals such as quartz, plagioclase, orthoclase and biotite. The borehole logging and core observations show that there are several fractured zones, and almost all these structural features are associated with alteration zones that have been formed by water-rock interaction. The studied samples were collected from the altered zone at the location of 40 to 50 meters below the ground surface. Detailed analysis of the pore geometry, mineralogical observation and geochemical analysis were carried out to reveal the changes of textual and geochemical characteristics due to water-rock interaction. Alteration feature of rock-forming minerals such as plagioclase and biotite shows that the granitic body has two stages of thermal and water-rock interactions. One is the high-temperature hydrothermal alteration after the magma solidification, and the other is relatively low-temperature alteration probably due to groundwater circulation after the being exposed at the surface. This kind of methodology used for rock forming minerals of subsurface granitic rock can be applied to understand the alteration history due to the water-rock interactions during the formation of granitic body.

Major element distribution in Archean banded iron formation (BIF): influence of metamorphic differentiation

“…, 2007a,b), igneous rocks (Michibayashi et al. , 1999, 2002; Yoshida et al. , 2008a), microbial deposits (Yoshida et al.…”

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

“…The relatively shallow generation of Type-C alteration is indicated by the precipitation of Fe 3+ hydroxides under aerobic conditions (Cornell & Schwertmann, 1996;Ichikuni, 1966;Kamiya et al, 1971;Yokoyama & Tarutani, 1979), where precipitation commonly occurs due to the oxidation of Fe 2+ followed by hydrolysis at neutral pH and low-temperature (ambient to near-boiling) conditions. In the Fe 3+ hydroxides (limonite) are generally present at grain boundaries and in fractures at the outcrop-to-microscope scale from oxidized meteoric water, which descended to depths of approximately 200 m and leaches Fe 2+ from the iron-bearing minerals (e.g., biotite and amphibole) of the surrounding granite (Akagawa et al, 2004;Earle, 2019;Iwatsuki & Yoshida, 1999;Nishimoto et al, 2008;Yoshida et al, 2008). These mechanisms are consistent with limonite formation by weathering, as shown in Figure 6a,b.…”