Limestone in the Sako-nishi area of the Kamioka Zn-Pb mine, central Japan, is depleted in ¹⁸O and ¹³C around the mineralized zone owing to hydrothermal activity in the Cretaceous to Paleogene age. Based on δ¹⁸O values, the Sakonishi limestone was divided into four groups, A (>15‰), B (10-15‰), C (5-10‰), and D (<5‰), and the relationships among the groups were analyzed in terms of their chemical compositions. The concentrations of S, Mg, and Sr in the limestone are independent of its δ¹⁸O value, suggesting that these elements, which mostly substitute for the structural sites in carbonate, did not exchange significantly with the hydrothermal fluid. In contrast, the concentrations of Al, Fe, Mn, P, and Zn tend to increase with decreasing δ¹⁸O value; they are 5 to 15 times more enriched in the highly altered group-D limestone than in the least altered group-A limestone. This enrichment is largely a consequence of the formation of Al-Fe bearing hydrothermal minerals (i.e., chlorite and epidote) and suggests that the isotopically altered limestone was formed by the exchange of C and O with the pervasive hydrothermal fluid, accompanied with the crystallization of Al-Fe minerals through a possible reaction with the host limestone. The elemental composition of limestone may serve as a potential tool for the exploration of skarn deposits. In the Kamioka mine, the Al content (ca. 0.1%), Mg/(Mg+Fe+Mn) (ca. 0.35 by weight), Al/Mg (ca. 0.5), and Al/Sr (ca. 2.0) can be used as indicators for determining the altered limestone and for focusing mineralized zones.

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The timing of igneous activity in the Amakusa area, the most southern part of the Inner Zone of Kyushu, was determined on two samples by K-Ar age dating (quartz porphyry: 16.7±0.3 Ma, granodiorite: 14.6±0.3 Ma). These and previous ages indicate that Miocene felsic igneous activity and continuous mafic igneous activity, composed of high Mg andesite, occurred between 19 and 14 Ma. Basalt activity took place at 10 and 7 Ma. The felsic and mafic igneous activity occurred intermittently at 1-3 m.y. intervals in the Amakusa area.
Miocene igneous activity was protracted more than 5 m.y. in the Amakusaarea and also elsewhere in the Inner Zone of Kyushu, for example, Tsushima (19-14 Ma), Goto (20-7 Ma) and Koshikijima (15-7 Ma). In contrast, the ages of Miocene igneous activity in the Outer Zone of Kyushu extend for only about 2 m.y., around 15-13 Ma, which indicates that the range of Miocene magmatic activity in the Inner Zone of Kyushu was longer than that in the Outer Zone. It is possible that differrent styles of magmatic activity occurred during the Miocene in the Inner and Outer Zone of Kyushu, i.e. back-arc and fore-arc tectonic settings, respectively. In the back-arc area the upper lithosphere expanded, became thin and fractured as a result of being pulled by the subducted lithosphere. After that, magma from the asthenosphere ascended intermittently along the cracks in the fractured upper lithosphere, which caused extensive and intermittent magmatic activity. This is one possible explanation why Miocene magmatic activity is protracted in the Inner Zone which was a back-arc area.
The Miocene magmatic activity in the Tsushima and Goto area started at 20-18 Ma, which is almost the same time when the Sea of Japan opened. It maybe that a large amount of magmatic activity was associated with the opening of the Sea of Japan at about 20 Ma in the region of the Sea of Japan, Oki, Tsushima and Goto.
Magmatic activity in the Goto and Koshikijima region occurred up to 8-7 Ma, the latest Miocene, which is the same time when magmatic activity in the older Okinawa Trough started. If the Goto region was located in the extensional area from the Okinawa Trough, it is possible that magmatic activity related to the older Okinawa Trough also occurred in Goto and Koshikijima.
It has previously been proposed that the Amakusa area belongs to the Inner zone of Kyushu, but the granitoid rock series and the existence of high Mg andesite in the Amakusa area is similar to that found in the northern Outer zone or Setouchi volcanic zone. The Amakusa area clearly needs a more detailed study.

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Major and minor elements and mineral chemistry are presented for 19 green rock samples from the El Roble mining area. These rocks are compositionally bimodal ranging from 47.4 to 52.5% SiO₂ and from 56.2 to 62.7% SiO₂. The basic rocks are divided into three types according to TiO₂ Content. The type 1 rocks are richest in TiO₂ (2.4-2.8%), and their multi-element spider diagrams normalized against N-MORB and other geochemical discrimination diagrams indicate that they are similar to T-MORB or OIB. The type 2 rocks are lower in TiO₂ (1.3-1.6%) than those of the type 1, and their multi-element spider diagrams resemble N-MORB, but merge the island arc characteristics together in rather high content of LIL elements. The type 3 rocks are lowest in TiO₂ (1.1-1.3%) among the basic rocks and are transitional between MORB and island arc basalt in their multi-element spider diagrams and minor element (Ti, Zr, V and Sr) chemistry. The intermediate rocks, the type 4, are lower in TiO₂ (0.2-0.8%) than the type 3 rocks and include boninite. They have the typical characteristics of island arc and/or fore arc in their multi-element spider diagrams, with distinct minus anomaly of Nb, and in Ti, Zr, V and Sr contents. The El Roble ore deposit occurs closely related with the type 2 and 3 rocks which have suffered the ocean-floor alteration ranging from greenschist facies to a transition state from greenschist to amphibolite facies. In contrast, the mineral assemblage of alteration minarals in the type 1 and 4 rocks indicate the prehnite-pumpellyite facies. Comparing with the geochemical data for the Recent volcanic rocks at variable tectonic settings, it is estimated that the type 1, 2 and 3 rocks were formed at the back-arc rift and the type 4 rocks at the island arc and/or fore arc. In the process of spreading and closing of the Colombian back-arc basin of Cretaceous age, the El Roble ore deposit probably formed at the spreading axis when it approached to the subduction zone and the back-arc magma was generated by mixing of T-MORB and island arc magma components.

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