The Asahi Mountains, located along the border between Yamagata and Niigata Prefectures, lies to the northwest of the Tanagura Shear Zone, which represents the boundary of pre-Neogene basement rocks. The Asahi Mountains are comprised of Cretaceous plutonic rocks of various lithologies, of which the Nishiasahi basic intrusive complex composed of quartz dioritic rocks and gabbroic rocks represents the first phase of intrusive activity. Geochemical modeling suggests that the differentiation of the quartz dioritic rocks resulted from the fractional crystallization of 25.4% amphibole, 23.0% plagioclase, 8.2% biotite, and 1.1% magnetite, whereas the gabbroic rocks were formed by the accumulation of these fractionated minerals. The gabbroic rocks have slight geochemical variations, which can be explained by the selective accumulation of the minerals. The initial Sr and Nd isotopic ratios of the Nishiasahi basic intrusive complex, corrected using an age of 120 Ma, are in the range of 0.70697-0.70797 and 0.51223-0.51229, respectively. This suggests the existence of an enriched source material, resembling the case of the Ryoke basic intrusive rocks in the Southwest Japan arc. The primitive magma of the quartz dioritic rocks geochemically corresponds to the experimental melt composition obtained by Rapp and Watson (1995) for amphibolite under the conditions of 1075 ℃ and 0.8 GPa. Therefore, an enriched mafic lower crust (i.e., enriched amphibolite) would be appropriate for the source material of the Nishiasahi basic intrusive complex.
Bi minerals have been discovered from the Fukueison mine, situated in the Uetsu region, Tsuruoka City, Yamagata Prefecture, Japan. Bi minerals occur in quartz veins which consist of chalcopyrite, pyrite and chlorite, accompanied by minor molybdenite, sphalerite and arsenopyrite, hosted in the Nishitagawa granodiorite. Bi minerals are composed of bismuthinite as a major constituent and native Bi, Bi-sulfosalts (krupkaite, cosalite and galenobismutite) and Bi-Te-S minerals (tetradymite, joséite-A, joséite-B and ingodite) as minor constituents. From the paragenesis and texture of Bi minerals, these minerals might have been formed by a change of fS2 and fTe2 conditions. Comparing to other Bi-bearing Mo deposits, Inner Zone of Southwest Japan, it is considered that Bi is closely related with Cu mineralization.