山形県大峠の浅熱水性Au-Ag系における熱水変質帯と雲母鉱物

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タイトル別名
  • Hydrothermal Alteration Zoning and Micaceous Minerals in the Otoge Epithermal Au-Ag System, Yamagata Prefecture, Japan
  • Hydrothermal Alteration Zoning and Mica

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The Otoge mine is located near the southern border of Yamagata Prefecture. Since the discovery of Au-Ag mineralization near the mine, the area has become an active target of exploration for gold. Pyroclastic rocks in the area, which are mostly dacitic to rhyolitic compositions, were subjected to intense hydrothermal alteration.<BR>Based on mineral assemblages, six alteration zones: the propylitic, smectite, interstratified I/S, sericite, kaolin and pyrophyllite zones, were defined. Micaceous minerals are the most common hydrothermal products except the smectite zone. Expandability in the micaceous minerals becomes higher with the elevation increase. Polytypes of micaceous minerals are 2M1, 1M, lMd and 2M2. The 1M type is the dominant polytype. 1M sericite (≤10%S) is close to muscovite and phengite in chemical composition, and shows close genetic relationship with Au-Ag mineralization.<BR>Chemical and mineralogical data suggest that the pyrophyllite and kaolin zones formed by acid hydrothermal fluids, and the sericite zone associated with Au-Ag bearing quartz veins resulted from a near neutral pH hydrothermal activity. The crosscutting relationship and age data indicate that the acid hydrothermal alteration was followed by the near neutral pH sericitization and Au-Ag mineralization. All these hydrothermal activities occurred during 4.7-4.0 Ma.

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  • 資源地質

    資源地質 46 (260), 355-368, 1996

    資源地質学会

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