Mesozoic shale, siltstone, sandstone and carbonate rocks and Eocene volcanic, volcaniclastic and pyroclastic rocks are intruded by late Eocene granitic batholith and related acidic dikes. As a result of hydrothermal activity associated with the granitoid, two distinct skarn zones of calcic skarn zone and magnesian skarn zone were formed in the carbonate rocks.
     Cl-bearing amphiboles of ferroactinolite to potassium rich ferrohastingsite composition are restricted to the calcic skarn zone and associating with salite, grandite, magnetite, calcite and sometimes with scapolite. The Cl content of amphiboles shows positive relationships with Fe²⁺/(Fe²⁺+Mg) and K/(K+Na) and a negative relation with Si/(Si+Al^IV). In contrast, F bearing phiogopite is a major mineral in the magnesian skarn. F content exhibits positive relationship with wt% of SiO₂ and MgO. The estimated log(f_H2O/f_HF) equilibrating with the phlogopite ranges from 4.6 to 5.5 at 400°C. Cl rich scapolite is widespread in both calcic and magnesian skarn zones. The scapolite has a compositional range of 23 to 40 mole% meionite in marialite-meionite solid solution series. X_C1 of the scapolite ranges from 0.44 to 0.98 (mostly more than 0.75). Scapolite of X_C1=0.98 from the hornfels is the most Cl-rich scapolite ever reported. Cl content of the scapolite positively correlate with Na/(Na+Ca+K). Calculated X_NaCl values for fluid in equilibrium with scapolite from the Sangan deposits reach up to more than 55 wt% equivalent NaCl.
     The Cl-rich hastingsite, Cl-rich scapolite, F-rich phiogopite, and fluid inclusions with high salinity (up to 70 wt% eq. NaCl) in granitic rocks and in skarn zones suggest that hydrothermal fluids were derived from the granitoid magma. The ore elements such as Fe and Cu could be transported by solutions as metal-chloride complexes. Large amounts of cations such as Al, Si, and Ca incorporated in the fluids should be derived from the country rocks and from the breakdown products of early skarn minerals.

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During retrograde metamorphism of carbonate-rich basic schists from the Asemi-gawa area, a part of host calcite formed near the peak of metamorphism has been locally replaced by Fe-Mg containing two different types of carbonate inclusions. One type is Fe-dolomite (FeCO₃=8.8 mol.%) accompanied by Cal₂(FeCO₃=1.5%, MgCO₃=2.7%) in the inner part of the replacement. The other one, Mg-ankerite (FeCO₃=16.6%) is accompanied by Cal₃(FeCO₃=3.2%, MgCO₃=7%) and Cal₄(FeCO₃=3.2%, MgCO₃=0.6%) in the outer part such of the replacement. An origin of carbonates can be explained by the reactions between a fluid phase containing Mg²⁺ and Fe²⁺ and a host calcite₁ during retrograde metamorphism.

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A unique, interlayered sill-sediment structure occurs along the lower contact of a Miocene basaltic andesite sheet intrusion on Rebun Island, northern Hokkaido, Japan. The intrusion is a sill or laccolith 1,200×800 m across and more than 150 m thick, which has intruded alternating diatomaceous mudstone and siltstone. The interlayered sill-sediment structure is 25 m high and 70 m wide in cross section and comprises alternating fluidally shaped basaltic andesite sills and sediment layers. Each basaltic andesite sill forms a thin wedge-like projection, 10-20 m long and 30-60 cm thick, which has quenched glassy margins. The sediment layers are 10-25 m long and 30-100 cm thick, and consist of massive diatomaceous mudstone. Contacts between the basaltic andesite sills and sediment layers are mostly sharp. In places, the margins of the basaltic andesite sills are brecciated, forming peperite zones 50×100 cm wide. The peperite consists of polyhedral clasts of basaltic andesite 5-20 cm across, separated by massive, indurated mudstone. Some large clasts show a jigsaw-fit texture.
     The interlayered sill sediment structure is inferred to have formed by parallel, sheet-like injections of basaltic andesite magma along bedding planes of wet and poorly consolidated sediment in an upper bathyal marine environment (several hundred metres deep). During parallel injections of the basaltic andesite magma into the wet sediment, a large amount of water vapour was generated in the wet sediment, and thin water vapour films were maintained continuously at the magma-sediment interface, insulating the magma from the host sediment. Following injections of the magma into the wet sediment, the temperature of the magma declined, so that the water-vapour films on the magma gradually disappeared. The breakdown of the insulating vapour films at the magma-sediment interface resulted in quench fragmentation of the magma, generating small zones of peperite around the basaltic andesite sills.

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