Mica/smectite mixed-layer minerals in alteration zones surrounding the Yokota Kuroko deposit, Aizu district, Northeast Japan

Abstract

Core samples from 14 boreholes were investigated to determine the smectite % and composition of exchangeable cations of mica/smectite in alteration area around the Yokota Kuroko deposit, which consists of upper and lower orebodies. The upper orebodies occur in the horizon between the Oshio and Takisawagawa Formations, and the lower orebodies in the strata 150 to 200m lower than the former.
Near the upper orebodies, the smectite % of mica/smectite decreases with depth from about 70% to 20% within 10m below the boundary between the Takisawagawa and Oshio Formations. On the otherhand, at a distance away from orebodies, smectite is converted to mica/smectite containing 40% to 30% smectite at 30 meters or more lower than Oshio-Takisawagawa boundary and mica/smectite zone increases in thickness. Stratiform ore bodies and veinlets are restricted within the zone of 10% smectite in the isopleth map at the bottom of the rhyolite of the Takisawagawa Formation.
Major exchangable interlayer cation in clay fractions in the Takisawagawa to the lowermost part of the Oshio Formations are generally Na+, while those in the lower to middle parts of the Oshio Formation are Ca²⁺.
The changes in smectite % and composition of exchangeable interlayer cation near the boundary between the Takisawagawa and Oshio Formations indicate that a decrease in hydrothermal activity and change in chemical composition occurred at the final stage of Kuroko mineralization.