Gold Mineralizing Potential of the Late Triassic Calcareous, Carbonaceous and Sulfide‐Rich Sedimentary Rocks of the Isatomae Formation in the South Kitakami Belt, Japan

The South Kitakami Belt of Japan yields a number of mesothermal auriferous vein deposits, hosted in Paleozoic‐Mesozoic sedimentary rocks in the vicinity of Cretaceous intrusive rocks. We investigated lithology, mineralogy, and chemistry of the host sedimentary rocks (Late Triassic Isatomae Formation) in the Ishinomaki district to evaluate gold mineralizing potential in the belt. The formation consists of shallow‐marine continental shelf sedimentary rocks that underwent prehnite‐pumpellyite facies of contact metamorphism. The analyzed three representative slate samples of the formation contain diagenetic‐metamorphic minerals such as calcite, K‐feldspar, albite, pumpellyite, titanite, and apatite, in addition to micro‐nodules and seams consisting of carbonaceous matter (CM) and/or sulfides. The carbonaceous matter in the nodules and seams was partly or mostly substituted by sulfides; pyrite with minor cobaltite and pyrrhotite, which occur as framboids, compact spheres, and compact anhedral grains. Cobaltite contains rarely electrum grains. These samples are characterized by high Ca contents, and are depleted in Mg, Fe, Na, and K, compared to the underlying Paleozoic and Mesozoic shale and overlying Jurassic sedimentary rocks of the belt. A sulfide nodule‐bearing sample is enriched in metals such as As, Cr, Co, Ni, Cu, and Sb. Depletion of sulfur mole ratio of the pyrites, in particular, framboidal/spherical pyrite, compared to the stoichiometric ratio, suggests that the pyrites formed from monosulfides. Most of the trace elements such as As, Bi, Pb, Tl, Sb, and Mo are more enriched in pyrite than pyrrhotite. These analytical results of the Isatomae Formation indicate that this slate was accumulated in an anoxic seawater environment and enriched in metallic elements, which may have been adsorbed to Fe‐oxyhydroxide particles originally. The successive diagenetic reaction with pore water sulfurized the Fe‐oxyhydroxides to monosulfides, which finally transformed into pyrite. The narrow and enriched range (+0.3‰ to +0.5‰) δ 34 S values of sulfides in the Isatomae slate suggest that this diagenetic reaction occurred in a closed system, where seawater sulfate hardly diffused into the sediments. Subsequent prehnite‐pumpellyite metamorphism transformed pyrite to pyrrhotite, during which metamorphism, the trace elements including Au may have exsolved into metamorphic fluids. These results show a gold mineralizing potential for the Isatomae Formation.