Opaque minerals of common granitic rocks were studied microscopically. The granitoids were divided into (i) a magnetite-bearing magnetite-series and (ii) a magnetite-free ilmenite-series. Each series has the following characteristic assemblages of accessary minerals:
Magnetite-series: Magnetite (0.1-2 vol.%), ilmenite, hematite, pyrite, sphene, epidote, high ferric/ferrous (and high Mg/Fe) biotite;
Ilmenite-series: Ilmenite (less than 0.1 vol.%), pyrrhotite, graphite, muscovite, low ferric/ferrous (and low Mg/Fe) biotite.
The mineral assemblages imply a higher oxygen fugacity in the magnetite-series granitoids than in the ilmenite-series granitoids during solidification of the granitic magmas. The boundary separating the two series is probably near the Ni-NiO buffer.
The magnetite-series granitoids are considered to have been generated in a deep level (upper mantle and lowest crust) and not to have interacted with C-bearing materials; whereas the ilmenite-series granitoids were generated in the middle to lower continental crust and mixed with C-bearing metamorphic and sedimentary rocks at various stages in their igneous history. The former carries porphyry copper-molybdenum deposits and the latter accompanies greisen-type tin-wolframite deposits. Lack of porphyry copper deposits in the Mesozoic orogeny belts in East Asia is related to a general paucity of the magnetite-series granitoids in this terrane.

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Regional geological study was carried out in the famed gold-silver quartz vein mining area of the central part of western Izu Peninsula (ca. 195 km²), which belongs to the southern Fossa Magna geotectonic province of the late Cenozoic Green Tuff orogeny. The constituents, stratigraphy and tectonic development of the area are summarized as follows:
The results of this study, stratigraphic correlation in particular, were discussed in detail in comparison with many previous works. It is concluded that the constituents in this area consist of the lower unit of the Miocene Yugashima formation and the upper unit of the Pliocene Tanaba formation and Nekko dacite group.
The geologic history of the upper unit was initiated by basic volcanism associated with block subsidence (1, 000 m±). The second stage is acid magmatism that occurred in the subsided area and formed many dome structures in the pre-existing rocks. The final stage is seen as Quaternary volcanism and hot spring activity at present.
Most of gold-silver quartz veins tend to cluster around the dome structure. The ore veins were formed in tensional fractures originated in normal faulting related to the dome formation, as best exemplified at the Seigoshi mine. Acid intrusive bodies are inferred to exist under these domes. It is concluded that the gold-silver deposits were formed by the ore solution derived from the hidden subvolcanic intrusives during the second stage of dacitic activity.

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Minor elements in the ore minerals from the Mamut porphyry copper deposit, East Malaysia, have been analyzed by ion microprobe. Analyzed ore minerals are chalcopyrite, pyrite and pyrrhotite. These mineralsoccur in quartz veinlets in the biotite zone, tremolite-actinolite zone, hornblende zone of adamellite porphyry, and serpentinite. Besides major elements, Ti, Cr, Mn, Co, Ni, Cu and Zn were detected as minor elements in most samples, and Zr or Pb was also detected in a few samples.
The present result shows several valuable suggestions to the genesis of this deposit. (i) High concentration of Ni in chalcopyrite from serpentinite suggests that the source of nickel was the serpentinite, while the uniformity of zinc content in chalcopyrite regardless of the kind of country rock suggests that zinc in ore-forming fluids was not derived from the country rocks.(ii) The fact that the sulfides from the hornblende zone of adamellite porphyry where gaseous inclusions are dominant are generally poor in minor elements, indicates that minor elements were more concentrated in highly saline solutions than in sulfide minerals. (iii) The fluid inclusion data (NAGANO et al., 1977) and the distribution of minor elements between chalcopyrite and pyrite indicate that no remarkable temperature gradient existed in mineralization of the core and ore-shell. A wide range of the distribution coefficients, therefore, suggests that some pairs of chalcopyrite-pyrite were not formed in equilibrium.

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The partition of silver between solution and electrum was used to estimate H₂S concentration in the Kuroko forming solution. The calculation was made for a 1 molal NaCl solution at 250°C. The Ag/Au ratio in the ore solution was assumed to be identical with their crustal abundance ratio. Electrum containing as much as 35% silver can be deposited from the solution with a_HS-=10^-4.5, which corresponds to 100 ppm as H₂S at pH5. Gold is dissolved as thio-complex Au(HS)₂-in the ore solution.

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