Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XXIII, No.1


Petrological Characteristics of the Granitoids Around the Yaguki Mine, Fukushima Prefecture, Japan : With Special Reference to the Relationships between the Yaguki-type Granodiorite and Ore Mineralization

Fujikawa, Osamu;Matsueda, Hiroharu

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Granodiorite bodies of the Yaguki-, the Ohbisagawa-, and the Yokokawa-types are distributed around the Yaguki contact metasomatic copper-iron and tungsten ore deposits. The tungsten mineralization might have occurred later than the copper-iron ore mineralization in the Yaguki ore deposit on the basis of field evidence. The Yaguki-type granodiorite (GD), lying beneath the ore bodies, is considered to be closely related to both of the ore mineralizations at the deposit. Based on the magnetic susceptibility and the mode of occurrences of Fe-Ti oxide minerals, the Ohbisagawa- and the Yokokawa-type granodiorites belong to the magnetite-series granitoids, while the Yaguki-type GD belongs to the ilmenite-series granitoids. The upper intrusion surface of the Yaguki-type GD body dips gently to the northeast and shows local dome structures. The ore bodies are mostly distributed around these domes. Based on bulk chemical compositions, the distribution of alkali contents of the Yaguki-type GD is in accord with that of the ore bodies such as No.1, No.2, and the Nambu-Hinokiyama ore bodies. MnO content in ilmenite within the Yaguki-type GD body tends to increase toward the west as a whole and is not correlated to the D. I. of the host rocks. Although fluorine contents of apatite in the Yaguki-type GD do not greatly change with the increase in D. I., chlorine contents slightly increase with increasing D. I. in the early stage of magmatic differentiation (D. I.=64-68) and then decrease in the later stage (D. I. >68). Three-phase fluid inclusions, consisting of liquid, vapor, and halite, are observed in the Yaguki-type GD. On the basis of several assumptions, magma of the Yaguki-type GD might be initially unsaturated with water, but it became saturated with water during magma crystallization. The differentiated magma of the Yaguki-type GD might be coexisted with an aqueous fluid containing chlorine and base metals. The aqueous fluids moved up to the sedimentary rocks above the granodiorite along the fissure zone. Mineralization occurred at the boundary between slate and limestone beds. Namely, not only the geologic structure but also the nature of magma might control the distribution of ore bodies in the Yaguki ore deposit.