別府―島原地溝帯外部に分布する浅熱水性金鉱床のK-Ar年代

抄録

A large number of epithermal vein-type gold deposits are distributed in the northern-central Kyushu. K-Ar ages were determined for sericite and K-feldspar separated from hydrothermally-altered rocks and quartz-K-feldspar veins, and for quartz-alunite altered rock, which were collected from gold deposits located outside the Beppu-Shimabara Graben, to examine their age of gold mineralization. The results are 2.61±0.06 and 2.74±0.13 Ma in Ogata, 2.48±0.06 and 2.75±0.06 Ma in Kinsei-Hoshino, 3.55±0.12 Ma in Tamarai, 3.59±0.09 and 3.53±0.10 Ma in Asahi, 2.87±0.23 Ma in Magane, 3.54±0.08 Ma in Usa, 3.92±0.09 Ma in Yamaura and 8.05±0.24 Ma in Bajo.
Sericite age of Bajo deposit is 8.05 Ma, which is significantly older than those of other deposits. This is due to that relic muscovite of the host granitic rock mixed into the mineral separates to be dated. Newly and previously obtained K-Ar age data revealed that the gold deposits in outside of the Beppu-Shimabara Graben were formed in a period (1.5 million years) between 3.92 Ma and 2.48 Ma in Pliocene and that the gold mineralization occurred immediately after the end of major volcanic activity. The ages also show that the gold mineralization in Pliocene has close relationship with the volcanism.