Mixed layer mica/smectite in the Miocene sediments of the area southeast of the Lake Towada, Northeast Japan

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.