Hydrothermal alteration in the Iwami-ginzan (silver mine) district, Shimane Prefecture, Japan

摘要

Hydrothermal alteration associated with silver mineralization in the Iwami silver mine district is characterized by intense potassium introduction. Formation of white mica minerals (sericitization) was found in dacite which hosts the Eikyu silver vein system at depth. Abundant adularia occurs surrounding the area of the Fukuishi silver deposit. Small amounts of sericite and sericite/smectite (I/S) mixed layer minerals are also associated with adularia; sericite and sericite/smectite mixed layer minerals have replaced adularia or occur in matrix. Smectite alteration with slight kaolinite alteration is present around the adularia and sericite zones. Smectite occurs in volcanic glass and small fragments, but kaolinite replaced plagioclase phenocrysts along fractures. The altered rocks with higher potassium content (>8wt%) may indicate the vents of mineralized fluids in the area.