This study is focused on element behaviors, mineral compositions, microstructures and distributional characters of remaining grains in the fault rock developed in Yongdang-ri, Yangbuk-myeon, Gyeongju City, Korea, using XRF, ICP, XRD, EPMA-BSE, optical microscope, laser grain size analysis and fractal dimension analysis methods in order to better understand the chemical variations in fault rocks during the fault activity, with emphasis on dependence of chemical mobility on mineralogy across the fault zone. Microstructures related to the fault gouge zone were characterized using grain size distribution and porosity analysis through BSE images, with emphasis on distribution characteristics of remaining grains and porosity.
The exposed fault core zone is about 1.5 meter thick. On the average, the breccia zone is 1.2 meter and the gouge zone is 20 cm thick, respectively. XRD results show that the breccia zone consists predominantly of rock-forming minerals including quartz and feldspar, but the gouge zone consists of abundant clay minerals such as chlorite, illite and kaolinite. Mineral vein, pyrite and altered minerals commonly observed in the fault rock support evidence of fault activity associated with hydrothermal alteration. Fractal dimensions based on box counting, image analysis and laser particle analysis suggest that mineral grains in the fault rock underwent fracturing process as well as abrasion that gave rise to diminution of grains during the fault activity. Fractal dimensions(D-values) calculated by three methods gradually increase from the breccia zone to the gouge zone which has commonly high D-values. There are no noticeable changes in D-values in the gouge zone with trend being constant. It means that the bulk-crushing process of mineral grains in the breccia zone was predominant, whereas abrasion of mineral grains in the gouge zone took place by continuous fault activity. It means that the bulk-crushing process of mineral grains in the breccia zone was predominant, whereas abrasion of mineral grains in the gouge zone took place by continuous fault activity. Mineral compositions in the fault zone and peculiar trends in grain distribution indicate that multiple fault activity had a considerable influence on the evolution of fault zones, together with hydrothermal alteration. Meanwhile, fractal dimension values(D) in the fault rock should be used with caution because there is possibility that different values are unexpectedly obtained depending on the measurement methods available even in the same sample.
As one of the main components of the fault rocks, SiO2 shows the highest content which ranges from 61.6 to 71.0%, and Al2O3 is also high as having the 10.8-15.8% range. Alkali elements such as Na2O and K2O are in the range of 0.22-4.63% and 2.02-4.89%, respectively, and Fe2O3 is 3.80-12.5%, indicating that there are significant variations within the fault rock. Based on the chemical characteristics in the fault rocks, it is evident that the fault gouge zone is depleted in Na2O, Al2O3, K2O, SiO2, CaO, Ba and Sr, whereas enriched in Fe2O3, MgO, MnO, Zr, Hf and Rb relative to the fault breccia zone. Such chemical behaviors are closely related to the difference in the mineral compositions between breccia and gouge zones because the breccia zone consists of the rock-forming minerals including quartz and feldspar, whereas the gouge zone consists of abundant clay minerals such as illite and chlorite. The alteration of the primary minerals leading to the formation of the clay minerals in the fault zone was affected by the hydrothermal fluids involved in fault activity. Taking into account the fact that major, trace and rare earth elements were leached out from the precursor minerals, it is assumed that the element mobility was high during the first stage of the fault activity because the fracture zone is interpreted to have acted as a path of hydrothermal fluids. Moving toward the later stage of fault activity, the center of the fracture zone was transformed into the gouge zone during which the permeability in the fault zone gradually decreased with the formation of clay minerals. Consequently, elements were effectively constrained in the gouge zone mostly filled with authigenic minerals including clay minerals, characterized by the low element mobility.
The changes in microstructures were accompanied by precipitation of the secondary minerals during multiple fault activities in connection with fracturing of main rock-forming minerals such as quartz and feldspar derived from the host rock. As the D-value gradually increases, the pore space in the fault gouge becomes smaller, showing the difference in microstructures. When fractured quartz grains are dominant, the D-values are 2.38, 2.60 whereas they are 2.69, 2.78 in association with clay minerals derived from the alteration of feldspar. The D-values of quartz grains in clay-rich matrix are 2.99, 3.03. Quartz and feldspar exerted a different influence on the formation process of microstructures in the fault gouge zone. Although large quartz grains highly fractured with abrasions caused the grain size reduction in the gouge zone during early fault activities, fine-grained quartz in the clay-rich matrix easily remained due to the reduction in friction resulting from clay minerals. Feldspar grains underwent more abrasion and dissolution, related to an increase in the amount of clay minerals newly formed. The reason why the minerals underwent different changes during fault activities is due to the essential reactivity inherent between the two minerals in granite. The mechanisms and environment by which the fault rocks form in the granite area are significantly complex processes which require multifaceted approaches based on the real microstructures.