K-Ar ages of volcanic rocks and gold deposits in the Bajo gold area, central Kyushu
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The Bajo gold area is located in the northeastern part of the late Cenozoic Hohi volcanic zone, central Kyushu. Andesite lava and pyroclastic rocks predominate in the Bajo gold area. The volcanic sequence is divided into lower and upper parts by a key bed. Hydrothermally altered volcanic rocks and the associated gold bearing quartz veins are observed commonly in the area.<BR>Eight volcanic rocks from the lower (four samples) and the upper (four samples) parts and two dyke rocks were dated with K-Ar method. The lower part ages are; 6.45 Ma by groundmass, 5.82 Ma by biotite, and 6.06 and 5.77 Ma by hornblende. The upper part ages are; 5.83-5.17 Ma by hornblende. The dyke rocks are; 5.48 and 5.34 Ma by hornblende. Therefore, it is concluded that the volcanic activity in the Bajo gold area lasted for at least 1.3 million years from 6.45 Ma to 5.17 Ma. Sericite and potassium feldspar separated from hydrothermally altered volcanic rocks, quartz vein and clay vein in seven different ore deposits were also analized with K-Ar method in order to examine the timing of ore mineralization. Sericite gave 4.57 and 3.86 Ma in the Bajo deposit, 5.26 and 5.38 Ma in the Tsurunari deposit, 4.83 Ma in the Fujiyama deposit, 4.32 Ma in the Yamaura deposit, 4.33 Ma in the Hinode deposit, 4.76 Ma in the Ohtaka deposit, and 4.28 Ma in the Hinoji deposit. Potassium feldspar gave 3.89 Ma in the Yamaura deposit. Therefore, it is concluded that the gold mineralization in the Bajo gold area occurred continuously for about 1.5 million years from 5.38 Ma to 3.86 Ma. This suggests that the gold mineralization began in the end of the volcanic activity in the Bajo gold area.
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