Mineralogical and Geochemical Characteristics of Hydrothermally Altered Basalt in the Kuroko Mining Area, Japan: Comparison with Ridge Basalt Alteration

Authors: Shikazono, N., Shimizu, M., Utada, M.,
Conference: Japan International Geothermal Symposium Session: Geology and Geochemistry
Year: 1988 Language: English
Geo Location:
Abstract: The hydrothermally altered basalt in the Kuroko mining area can be mineralogically divided into two types; chlorite-rich and epidote-rich one. The chlorite-rich type is characterized by chlorite~ albite, calcite, pyrite, quartz and small amounts of iron-poor epidote and actinolite, and they occur as replacement of groundmass and phenocrysts, and vesicle is filled with chlorite. The epidote-rich type is characterized by iron-rich epidote, quartz, calcite and small amounts of hematite and magnetite, occurring as vesicle filling. Generally, brecciated rock suffered the chlorite-rich type alteration, while massive rock is changed to the epidote-rich one. Fluid inclusion studies show that alteration of basalt to the chlorite-rish and epidote-rich assemblages occurred at the temperature range of 230-250 C and 250-280oC, respectively, implying that generally temperature increased from chlorite stage to epidote stage. The above data indicate that permeability and temperature are important factors causing different type of alteration (epidote and chlorite alterations). Texture of alteration minerals suggests that chlorite formed earlier than iron-rich epidote. Wi th proceeding of hydrothermal al teration, iron content of epidote, increases and CaO content of feldspar decreases. MgO/FeO ratio of chlorite depends on the coexisting minerals; that coexisting with pyrite is low (1.1), while that coexisting with hematite and epidote is high (1.4).
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