Electrum-sulfide mineralization of the Jinan-Jeongeup Au-Ag area was deposited in three stages of structurally controlled quartz and calcite veins that fill N-S and NW-SE-trending shear zones. Radiometric dating indicates that mineralization is Late Cretaceous age (75-80 Ma). Fluid inclusion and sulfur isotope data show that ore mineralization was deposited between 330°C and 250°C from fluids with salinities of 1 to 8 wt.% equiv. NaCl and a δ³⁴S_Σ value of 3 to 6 per mil. Evidence of fluid boiling indicates a range of pressures from ?? 40 to 110 bars, corresponding to a depth of ?? 500m in a hydrothermal system which alternated from lithostatic toward hydrostatic conditions. Gold was carried as a thiosulfide complex in slightly acidic, chemically reducing fluids. Au-Ag deposition was likely a result of cooling coupled with boiling.
Measured and calculated hydrogen and oxygen isotope values of ore-forming fluids indicate a significant meteoric water component in the Jinan-Jeongeup ore fluids. Comparison reveals that all Korean Au-Ag deposits display various degrees of ¹⁸O-enrichment relative to meteoric water, produced by exchange with hot igneous rocks. However, individual mines and districts have relatively narrow ranges of hydrogen and oxygen isotope compositions which are directly related to their gold/silver ratios and which reflect their depths of formation. Silver-rich epithermal deposits display the smallest ¹⁸O shifts, indicating a lesser degree of water-rock interaction at shallow depths of formation (<750m). Korean-type gold-silver deposits display intermediate ¹⁸O shifts, indicating a moderate degree of water-rock interaction at intermediate depths of formation (750 to 1, 500m). Gold-rich mesothermal deposits display the largest ¹⁸O shifts, indicating the highest degree of water-rock interaction at relatively great depths of formation (>4.5km). This suggests a relationship between depth and degree of water-rock interaction in Korean deposits. All of these gold-silver-bearing deposits have fluids which are dominantly evolved meteoric waters, but only deeper systems with higher degrees of igneous rock interaction are exclusively gold-rich.

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Geological survey of the Kume-jima Island, 90 km west of the Okinawa-jima Island, has been carried out, and petrographical description on the middle to late Miocene igneous rocks of the Ara-dake Formation has been made to clarify the nature of the rocks. The analytical results of the major elements for 62 samples were reported as well.
The rocks are distributed in the eastern half of the island, and most of the activities were subareal ones. The rocks consist of lavas, pyroclastics, and a minor amount of hyaroclastites. Andesitic lavas are dominant and dacitic lavas are minor. The lavas are divided into 7 units: nearly aphyric andesitic rocks are found only in the units 1 and 5, while the others consist of porphyritic andesites and dacites with abundant plagioclase phenocrysts. Some dacitic rocks include hornblende with/without biotite as phenocrysts.
The rocks of the units 1 and 5 are rich in TiO₂ and P₂O₅, and plotted in the tholeiitic field in the FeO^*-FeO^*/MgO diagram. In contrast the others are calc-alkalic. The rocks of the units 2-7 are characterized by the highest K₂O contents among the other Cenozoic volcanic rocks from the central Ryukyu Islands and Okinawa Trough region.

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Middle Miocene basaltic rocks in the back-arc region of the NE Japan arc have distinct trace element compositions from those of the Quaternary basalt in the NE Japan arc.
The basaltic rocks are generally less differentiated (FeO^*/MgO=1.0-1.5) and rich in Cr content (140-550 ppm) than the Quaternary basalt. In the southern part of the region, La/Yb ratio in the basaltic rocks does not increase toward back-arc side. The basaltic rocks generally do not show intensive depletion of Ta in n-MORB normalized patterns. These patterns of the basaltic rocks are similar to those of transitional MORB and within-plate basalt.

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