Sulfide Ore Smelting at the Naganobori Copper Mine Recorded on Speleothems from the Ogiri No. 4 Pit on the Akiyoshi-dai Plateau, Yamaguchi, Japan

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At the Naganobori Copper-Mine site, Akiyoshi-dai, Yamaguchi, southwestern Japan, the mining of copper ores dates back from the end of the 7th century to the beginning of the 8th century. The copper ore smelting was considered to be performed near the mine. Although copper ores in the oxidizing zone should have been used in the early stages, but later the sulfide ores in the enriched and primary zones, it has not yet been clarified as to when the smelting of the sulfide ores was started. The environmental change information for the past two thousand years was extracted from two growing stalagmites in the Ogiri No. 4 pit. Their fluorescent annual microbanding information was used for dating. The sulfate concentration was almost constant from 100 to 1400 A.D., and then clearly increased, suggesting that the extensive smelting of the sulfide ores had started. The concentration gradually increased, reached a maximum in the early stages of the 1700s, and then increased again from 1900. The magnesium concentration changed, almost synchronizing with the sulfate concentration. The possible reduction of the biomass by acid rain or leaching of the magnesium ions from the soil was suggested. The records extracted were in good agreement with those partially extracted from ancient documents, ancient picture maps and topographical maps. The copper used for the Great Buddha of Nara casting has the high possibility of being smelted using copper ores from the oxidizing zone.

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