QUARTZ C-AXIS FABRIC OF TECTONICALLY UNDEFORMED ROCKS FROM KASUGA GOLD MINE, SOUTHWEST JAPAN

Application of Electron Backscatter Diffraction (EBSD) to geology for studying rock textures explaining natural deformation mechanisms, etc. is a growing practice. Quartz c-axis crystallographic preferred orientations (CPO’s) are used since long to understand the concluding stages of deformation in a geological setting (e.g., Lister & Hobbs, 1980, Journal of Structural Geology 2, 355 – 370). However, a study of rocks structurally untransformed since their formation seems to be overlooked. C-axis CPO's in the oriented rock samples of volcanic rock from Kasuga High Sulfidation gold deposit, Makurazaki, Japan were studied using EBSD and Orientation Imaging Microscopy (OIM) to discover the microscopic fabric that may exist in rocks in their primary structures. The deposit is hosted in silicified andesite which is a product of the hydrothermal activity that followed Cenozoic volcanism in the area. The andesite has been silicified to various extents due to hydrothermal alteration. We have chosen two representative samples for this study, highly-silicified andesite and less-silicified andesite. In hand specimen the samples differ only in colour and, porous cavities are comparatively abundant in the less-silicified rock. Petrographic study of rock thin sections using microscope confirmed the absence of tectonic deformation features and the presence of euhedral and anhedral quartz in both samples. EBSD coupled with OIM Analysis served to define the influence of degree of silicification on the c-axis fabric of these undeformed rocks. Though in the (0001) pole figures c-axis maxima are randomly distributed for both types, the less-silicified rock has concentration of maxima at the rims of the figures meaning c-axis orientation of quartz grains is nearly parallel to the horizontal; however, the highly-silicified sample has these maxima saturated towards the centre of the pole figures suggesting that quartz crystallized nearly vertically or with its c-axis at high angle to the horizon in this rock. We may conclude that degree of silicification also affects the orientation of constituent grains that appears as varying c-axis fabric in undeformed rocks making the apparently simple textures more complex. Also, EBSD provides powerful tools for observing texture in rocks that are not shown by more traditional methods.