Felsic volcanism taken place pre- and post-Kuroko mineralization in the vicinity of the Hokuroku district have been chronologically studied based upon dating data and microfossil data. The evolution of the volcanic activities was compared with those in northern part of Honshu. It is suggested that the volcanic sequence in the Hokuroku district is correlated to those of Northern part of Honshu in relation to the general evolution of the arc tectonics and Kuroko deposits were generated in the transitional stage from the volcanism under back-arc environment to that of island-arc environment. The evolution history of the Neogene volcanic activities in northern part of Honshu including the Hokuroku district is summarized as follows:
∑The preceding period to 17Ma is characterized by the terrestrial volcanism composed of the former basaltic activity (25-22Ma) and the later granitoid intrusions (22-17Ma).
∑The second period during 17-8 Ma is subdivided into intermediate to mafic lava and the pyroclastics in the pre- to initial rifting stage (17-16Ma), submarine dacitic lava and massive pyroclastics in the back-arc depression prior to the Kuroko generation (16-13Ma), and intermittent felsic pyroclastics under the up heaving environment of the Kuroko basin and contemporaneous shallow to terrestrial felsic volcanism in the surrounding area (13-8Ma).
∑The following period to 8Ma is characterized by a limited extended acidic volcanism under shallow water environment along the marginal part of Kuroko basin and extensive andesitic volcanism under terrestrial condition in the surrounding mountains.

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Phosphorites collected from three seamounts off Japan have been analyzed for their major oxide, rare earth element (REE) composition and biomarkers. Two phosphorites, from Daiichi-Kashima seamount and Uyeda Ridge, were originated from limestone and another one from seamount "N" was altered from basalt. The only phosphate mineral investigated in the samples was carbonate-fluorapatite with excellent crystallinity. The REE patterns of the three seamount phosborites were similar to the REE pattern of sea water, suggesting that the REE pattern of phoshorite is not dependent on the type of original rock.
The biomarker distribution of seamount phosphorites were characterized by low carbon number n-alkanes with C₁₄ and C₁₈ dominant, low CPI (Carbon Preference Index), and the presence of MMA (monomethylalkanes). SEM observations showed that these samples consisted mostly of phosphatized filaments and with a smaller amount fraction of hexagonal apatite crystals. The mineral associations, geochemistry, textures and geological settings indicate that microbes played an important role of in the seamount phosphorite formation.

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The Bajo gold area is located in the northeastern part of the late Cenozoic Hohi volcanic zone, central Kyushu. Andesite lava and pyroclastic rocks predominate in the Bajo gold area. The volcanic sequence is divided into lower and upper parts by a key bed. Hydrothermally altered volcanic rocks and the associated gold bearing quartz veins are observed commonly in the area.
Eight volcanic rocks from the lower (four samples) and the upper (four samples) parts and two dyke rocks were dated with K-Ar method. The lower part ages are; 6.45 Ma by groundmass, 5.82 Ma by biotite, and 6.06 and 5.77 Ma by hornblende. The upper part ages are; 5.83-5.17 Ma by hornblende. The dyke rocks are; 5.48 and 5.34 Ma by hornblende. Therefore, it is concluded that the volcanic activity in the Bajo gold area lasted for at least 1.3 million years from 6.45 Ma to 5.17 Ma. Sericite and potassium feldspar separated from hydrothermally altered volcanic rocks, quartz vein and clay vein in seven different ore deposits were also analized with K-Ar method in order to examine the timing of ore mineralization. Sericite gave 4.57 and 3.86 Ma in the Bajo deposit, 5.26 and 5.38 Ma in the Tsurunari deposit, 4.83 Ma in the Fujiyama deposit, 4.32 Ma in the Yamaura deposit, 4.33 Ma in the Hinode deposit, 4.76 Ma in the Ohtaka deposit, and 4.28 Ma in the Hinoji deposit. Potassium feldspar gave 3.89 Ma in the Yamaura deposit. Therefore, it is concluded that the gold mineralization in the Bajo gold area occurred continuously for about 1.5 million years from 5.38 Ma to 3.86 Ma. This suggests that the gold mineralization began in the end of the volcanic activity in the Bajo gold area.

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Wakamatsu chromite mine has a largest chromitite-pod called "nanago-ore body (7th ore body)" which is 40×210×25m (2.1×10⁵m³) in size in Japan. The other chromitite-pods which are intermediate size (4.5×10⁴-1.2×10³m³) are also distributed in Wakamatsu mine. We have clarified that the relationships between chromian spinel in chemistry and size of chromitite for exploration for podiform chromitite. Major results of this work are as follows.
(1) Nanago ore body is larger than the other chromitite bodies by one order of magnitude.
(2) Chromitite bodies are divided into two groups by Cr# of chromian spinel from chromitite that are high-Cr# group (Cr#:0.52-0.57, nanago and 10th ore bodies) and low-Cr# group (Cr#:0.41-0.48, 51st, 52nd, 53rd, 54th, 55th, and 56th ore bodies).
(3) Cr# of chromian spinel in dunite envelops varies from 0.45 to 0.62 in "nanago and 10th ore bodies group" and 0.48 to 0.57 in "the other chromitite ore bodies group."
(4) Chromian spinels from dunite envelops in both Nanago and the other chrmitite bodies are plotted in relatively high-Cr# and low-V₂O₃ field on Cr#-V₂O₃ diagram of Matsumoto and Arai (1997).
The results of above characteristics clearly show that spinel precipitation and concentration are more intense at nanago ore body than at the other chromitite bodies of the Wakamatsu mine. And that to explore the podiform chromitite, Cr#-V₂O₃ relation is useful petrological exploration tool not only for relatively large chromitite but also for intermediate one.

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Major, trace, and rare-earth element geochemistry of Early Archean (3.5 Ga) hydrothermally altered greenstone are reported in order to clarify geochemical features of seafloor hydrothermal alteration including carbonatization and silicification and to estimate CO₂ flux sunk into the altered oceanic crust by the hydrothermal carbonatization. The greenstone collected from the Marble Bar area, eastern Pilbara Craton, can be divided into five types on the basis of microscopic characteristics; dolerite, basalt, unidentified rock, highly silicified rock, and interpillow material. The dolerite is subdivided into relatively fresh and altered ones based on the presence or absence of primary clinopyroxene. The basalt and unidentified rock are subdivisible into carbonatized and non-carbonatized ones.
The whole-rock chemical composition of the relatively fresh dolerite is essentially similar to that of modern midocean ridge basalt (MORB) except for enrichments of large-ion-lithophile elements such as K₂O, Rb, and Ba. Compared to the relatively fresh dolerite, other altered rocks are enriched in K₂O, Rb, Ba and is depleted in Na₂O, reflecting the presence of K-mica that replaces the primary plagioclase. The CO₂ enrichment in the carbonatized rocks is attributed to the formation of carbonate minerals, indicating that significant amounts of CO₂ in the circulating seawater was added to the oceanic crust during the hydrothermal alteration. Moreover, relationships between CO₂ and CaO, MgO, Fe₂O₃* (total iron as Fe₂O₃), and MnO in the altered greenstone imply that there was essentially neither gain nor loss of Ca, Mg, Fe, and Mn during the carbonatization. This suggests that the oceanic crust trapped CO₂ in the hydrothermal solution by using Ca, Mg, Fe, and Mn in the oceanic crust.
Based on our results, a net sink of CO₂ in the oceanic crust by the Early Archean seafloor hydrothermal alteration is estimated to be (0.27-2.7)×10¹³mol/yr. This carbon flux can be comparable to the present total carbon flux by carbonate precipitation, carbon burial, and seafloor hydrothermal alteration, suggesting that the carbonatization of oceanic crust by the seafloor hydrothermal activity played an important role as a sink of CO₂ in the Early Archean atmosphere and ocean.

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