[HRE13-P05] Subseafloor mineralization in the Shimokawa Cu-Zn ore deposits in Hokkaido
Keywords:submarine hydrothermal activity, copper, chlorite, marine sediments
X-ray diffraction (XRD) results of the analyzed host rock samples show that chloritization and sericitization took place, not only in the footwall but also hanging wall samples, possibly by hydrothermal alteration. This is supported by increasing iron content in chlorite, which was indicated by its second to first intensity peak ratio in their XRD profiles. Furthermore, results of microscopic observation demonstrated that the hydrothermal activity responsible for the mineralization was divided into two stages: the early stage in which iron was mainly concentrated and the later stage in which copper and zinc was mainly concentrated. Distribution of the ore minerals formed by the later stage hydrothermal fluid, which flowed along diabase sill, indicates that the mineralization occurred epigenetically, after the formation of the sills. These results suggest that Shimokawa ore deposits were formed by hydrothermal activity beneath the seafloor and are, therefore, likely to represent sub-seafloor replacement ore deposits.