Abstract
Core samples from 18 boreholes in the area southeast of the Lake Towada were investigated to determine expandability of mica/smectite in the Miocene sediments. The expandability decreases with depth from about 80 to 0% over intervals of 100 to 400m in the western part of the area, while intervals of more than 600m in the eastern part. The percentage of expandable layers in mica/smectite decrease from east to west. The expandable layer isopleth map at the top of the Yotsuzawa Formation shows a decrease in expandable layers from 80 to 10% at a distance of about 500m. The conversion of smectite to mica/smectite mixed layers begins at 0.6-0.8% vitrinite reflectivity, and the conversion to the 10% of expandable layers occurs in 1.2-2.8% vitrinite reflectivity.
Stratiform ore bodies of Kuroko and copper-lead-zinc veins are restricted to the zone less than 20% of expandable layers at the top of the Yotsuzawa Formation. The expandability of mica/smectite mixed layer in sediments is useful as a guide to mineral exploration.
