EBSD study of Quartz c-axis orientations in the silicified host rocks of the Kasuga gold deposit, Southwest Japan

摘要

The Kasuga High Sulfidation gold deposit is hosted in the silicified Late Miocene andesite of Nansatsu Volcanic Rocks in the southern part of the Satsuma Peninsula, Southwest Japan. Optical microscopic observations and X-ray fluorescence spectrometry (Horiba XGT-5000) analysis of two silicified andesite samples from the Kasuga ore body revealed that the rocks underwent different degrees of hydrothermal alteration and no tectonic deformation afterward. Crystallographic orientations of quartz composing the silicified andesite samples studied using Electron Backscatter Diffraction, EBSD (EDAX GE-1, TSL-OIM) displayed different trends in the c-ax-is orientations between the samples. The c-axes of quartz grains are mainly oriented sub-horizontal or plunging at low angles in the less-silicified andesite. The highly silicified sample has larger quartz grains with c-axes plunging at high angles. The difference in c-axis orientations is provably related to the silicification processes in the earlier and later stages of hydrothermal alteration. The results of this study suggest that quartz c-axis orientations of rocks deformed under low strain regimes are potentially inherited from initial undeformed state, for example crystallization from hydrothermal solutions.