Fig. 4

a High magnification BSE image and b RGB maps of Ca–Kα (red), Fe-Kα (green), and S–Kα (Blue), which are obtained from the same area shown as (a) in Fig. 3a. The dotted lines and the color are the same as that shown in Fig. 3b. The upper region in this area (lithology 1) contains a lot of magnetite, pyrrhotite, and Ca–Mg carbonate (dolomite) of > 10 µm size. In the lower region, such large secondary minerals were less abundant (lithology 2). c High-magnification BSE image obtained from the area (c) shown in (a). d, e BSE image of dolomite grains, which corresponds to the same area shown as a square in (c), as (e) and Fig. 4a, respectively