Green Tuff Region Research Articles

Research history of the Green Tuff region in NE Japan in relation to the Sea of Japan formation

Research history of the Green Tuff region in NE Japan in relation to the Sea of Japan formation

グリーンタフ地域の硬質頁岩卓越層を基岩とする流れ盤型地すべりの地質的特徴

地すべり多発地帯のグリーンタフ地域における新第三系中部～上部中新統遠音別川層硬質頁岩卓越層を基岩とする古期大規模地すべり (オシンコシン地すべり) 地域の地すべりは, 基岩地質による地すべり分類では, 熱水変質帯地すべりである。この地域の地すべりは, 流れ盤型であり, 移動体構成物質による地すべり分類では, オシンコシン地すべりのような大規模熱水変質岩地すべり, 2006年町道オシンコシン線地すべりのような中規模岩屑・土砂地すべり, 1999年町道オシンコシン線地すべりのような小規模岩屑・土砂地すべりなどが見られる。この型の地すべりの発生は硬岩の硬質頁岩厚層中の多量のスメクタイトを含む軟岩の軟質凝灰岩薄層と密接な関係を持つ。

Unique Origin of Skarn at the Ohori Base Metal Deposit, Yamagata Prefecture, NE Japan: C, O and S Isotopic Study

AbstractThe Ohori ore deposit is one of the Cu–Pb–Zn deposits in the Green Tuff region, NE Japan, and consists of skarn‐type (Kaninomata) and vein‐type (Nakanomata) orebodies. The former has a unique origin because its original calcareous rocks were made by hydrothermal precipitation during Miocene submarine volcanism. Carbon and oxygen isotope ratios of skarn calcite and sulfur isotope ratios of sulfides were measured in and around the deposit. Carbon and oxygen isotope ratios of the skarn calcite are δ13C = −15.51 to −5.1‰, δ18O = +3.6 to +22.5‰. δ13C values are slightly lower than those of the Cretaceous skarn deposits in Japan. These isotope ratios of the Kaninomata skarn show that the original calcareous rocks resemble the present submarine hydrothermal carbonates at the CLAM Site, Okinawa Trough, than Cenozoic limestones, even though some isotopic shifts had occurred during later skarnization. δ34S ratios of the sulfide minerals from the Kaninomata and Nakanomata orebodies are mostly in a narrow range of +4.0 to +7.0‰ and they resemble each other, suggesting the same sulfur origin for the both deposits. The magnetite‐series Tertiary Kaninomatasawa granite is distributed just beneath the skarn layer and has δ34S ratios of +7.5 to 8.1‰. The heavy sulfur isotope ratio of the skarn sulfides may have been affected by the Kaninomatasawa granite.

Comparison of stable isotopes, ratios of 36Cl/Cl and 129I/127I in brine and deep groundwater from the Pacific coastal region and the eastern margin of the Japan Sea

Abstract Fifty-three samples, including brines associated with oil and natural gas reservoirs and groundwater samples from deep boreholes, were collected from the Pacific and Japan Sea coastal regions in Japan. The 129 I/ 127 I and 36 Cl/Cl ratios, and stable isotopes (δD and δ 18 O) are compared to investigate differences related to the geotectonic settings of the two regions. The δD and δ 18 O data indicate that brine and groundwater from the Pacific coastal region reflect mixing of meteoric water with connate seawater in the pores of sedimentary rocks. On the other hand, brine and groundwater from the Japan Sea coastal region have been hydrothermally altered. In particular, brines associated with petroleum accumulations at Niigata and Akita showed the same isotopic characteristics as fluids found in the Kuroko deposits of the Green Tuff region in northeastern Japan. There is little difference in the 36 Cl/Cl ratios in brine and groundwater from the Pacific and Japan Sea coasts. Most brine and some deep groundwater, except those from the Pleistocene Kazusa Group, have already reached the average secular equilibrium ratio of 9.9 ± 2.7 × 10 −15 for their mudstone and sandstone reservoirs. There was no correlation between the 36 Cl/Cl ratios and differences in geotectonic setting between the Pacific and the Japan Sea coast. The molar I/Br ratio suggests that the I in all of water samples was of biogenic origin. The average 129 I/ 127 I ratio was 290 ± 130 × 10 −15 to 294 ± 105 × 10 −15 in both regions, showing no relationship to the different geotectonic settings. The uncontaminated brine and groundwater samples are likely to have retained the original 129 I/ 127 I ratios of marine I released from the old organic matter stored in sedimentary rock.

Tectonic Movement and its Relation to the Mineral Resources in the Green Tuff Region

Tectonic Movement and its Relation to the Mineral Resources in the Green Tuff Region

Fluid Inclusions in Some Neogene Ore Deposits in the Green Tuff Region

Fluid Inclusions in Some Neogene Ore Deposits in the Green Tuff Region

Geology of the Green Tuff Region in Japan

Geology of the Green Tuff Region in Japan

Some Characteristics of the Neogene Mineralization in the Sanin Green Tuff Region, Southwest Japan.

Some Characteristics of the Neogene Mineralization in the Sanin Green Tuff Region, Southwest Japan.

Vitrinite reflectivity and authigenic minerals in the Neogene sediments of the Green Tuff region, Japan.

Vitrinite reflectivity and authigenic minerals in the Neogene sediments of the Green Tuff region, Japan.

Origin of hydrocarbons in the Green Tuff region of Japan: 129I results from oil field brines and hot springs in the Akita and Niigata Basins

The Akita and Niigata Basins are part of the Green Tuff region of Honshu, Japan, which hosts major occurrences of oil and gas and of hydrothermal sulfide-sulfate Kuroko Deposits. The Green Tuff formation is associated with the opening of the Japan Sea, which occurred in the Early Miocene and was accompanied by large-scale volcanic activities. We measured halogen concentrations and 129I/I ratios in oil and gas field brines and hot springs in order to determine the origin of iodine and, indirectly, of hydrocarbons in this area and their association with geological processes during the formation of the Japanese Island Arc. Based on the concentration of iodine, these samples fall into three categories: Type A (I > 190 μM), Type B (150 μM > I > 90 μM), and Type C (I < 50 μM). Samples in Types A and B come predominantly from oil/gas fields and have 129I/I ratios below 300 × 10 − 15 , corresponding to ages essentially in the Eocene epoch. In contrast, samples in Type C are generally from hot springs and have 129I/I ratios above 400 × 10 −15, corresponding to Oligocene to Miocene ages. Source formations for Type C were accumulated during the active opening of the Japan Sea, however, those for Types A and B probably are organic-rich Eocene sediments or older basement rocks which predate the volcanic formations associated with the backarc spreading of the Japanese Island. The observation that oil and gas is predominantly found in reservoir formations of middle to late Miocene age suggests mobilization of these brines in association with the tectonic and volcanic/hydrothermal activities during the opening of the Japan Sea which also led to the formation of the Kuroko Deposits in the same area. The hot spring samples of Type C reflect these processes most directly. Because of the close association of iodine with methane and other hydrocarbons, oil and gas in the Green Tuff area probably is also derived from sources considerably older than the current reservoir formations.

Formation processes of thermal waters in Green Tuff: A geochemical study in the Hokuriku district, central Japan

Thermal waters from Green Tuff region in the Hokuriku district, central Japan were investigated to reveal formation process of water, based on the chemical and isotopic compositions of water and dissolved gases in water. The thermal waters of local meteoric origin are typical Green Tuff-type water, Na–Ca–Cl–SO 4-type or Na–Cl–SO 4-type, which well reacted by water–rock interaction and is found to be saturated with calcite and undersaturated with gypsum. The Green Tuff-type waters have relatively high 3He/ 4He ratio indicating high contribution of mantle-derived helium, and have extremely low total carbon/ 3He (Ct/ 3He) ratio compared with the ratio of MORB due to removal of carbon species from waters caused by calcite deposition. In contrast, a mineral spring water close to an active fault is found to be an immature water with less water–rock interaction containing a large amount of gaseous CO 2 and is undersaturated with both calcite and gypsum. This water has high 3He/ 4He ratio and Ct/ 3He ratio indicating the high contribution of mantle-derived helium and deep-seated CO 2, and likely provided through the fault. We proposed a chemical evolution model considering existence of both chemically immature CO 2 gas-rich water and mature carbon-poor water which well reacted with rocks. Deep-seated CO 2 and mantle-derived helium are upwelling via faults and are supplied to the groundwater of local meteoric origin, and an immature water containing high content of CO 2 and mantle-derived helium is formed. During the circulation of this immature water in the Green Tuff layers, water–rock interaction such as ion exchanges with minerals, dissolution of gypsum and deposition of calcite occurred. At the interaction with rocks, deposition of calcite occurs and results in a loss of Ca 2+, and decrease of Ca 2+ brings dissolution of gypsum and more calcite deposition. Consequently, almost all the supplied CO 2 is eliminated from the water. Furthermore, we made a quantitative evaluation of the chemical evolution of Green Tuff-type water using geochemical modeling code PHREEQC taking account of addition of gas components, and found the good agreement with observational results. This study reveals that the supply of large amounts of CO 2 and subsequent water–rock interaction are necessary for the formation of “Green Tuff-type” water with high 3He/ 4He ratio and extremely low Ct/ 3He ratio.

Kuroko and Hydrocarbon Deposits from Northern Honshu, Japan: A Possible Common Hydrothermal/Magmatic Origin?

Abstract. Northern Honshu is the most important area for mineral and oil resources in Japan. Many kuroko deposits and oil and gas fields are distributed in two belts along the northeast Japan arc, the kuroko metal‐belt on the Pacific side and the oil‐belt on the Sea of Japan side. The kuroko deposits are located mainly in the Green Tuff strata which formed as a result of submarine vol‐canism during the late Miocene and Pliocene. Most of the source rocks of the oil and gas deposits formed at the same time as the kuroko deposits and some of them are located in reservoirs of hydrothermally‐altered volcanic rocks in the Green Tuff region. There is general agreement that the kuroko deposits formed as a result of submarine hydrothermal and magmatic activity whereas almost all petroleum geologists and geochemists consider that hydrocarbon deposits were generated independently of such activity. Since the discovery of hydrothermally‐generated petroleum in the Guaymas Basin, Gulf of California, however, it is clear that petroleum can be formed almost instantaneously in terrestrial and submarine hydrothermal areas. The paleo‐northeastern Sea of Japan is therefore considered to be a potential area for hydrothermal petroleum generation because thick organic‐rich sediments overlie an active submarine volcanic area. Several lines of geological and geochemical evidence suggest the possibility of hydrothermally‐enhanced maturation of organic matter and the contribution of magmatic activity to the formation of these deposits. Although most of the oil and gas in northern Honshu has been generated conventionally as a consequence of the high geothermal gradients there, it appears that some of the oil and gas fields may have formed as a result of extensive hydrothermal and magmatic activity during the late Miocene to Pliocene. Because of the much steeper angle of the faults in the vicinity of the Hokuroku basin than in the Akita basin, the magmatic contribution to the kuroko mineralization would have been far greater than to the oil and gas deposits of the Niigata and Akita basins. We therefore propose a strong relationship between metal and oil and gas generation in northern Honshu based on the structure and tectonics of the northern Honshu arc‐backarc system.

Heavy Sulfur‐isotope Enrichment in Middle Miocene Onnagawa Mudstones in the Green Tuff Region, Northeastern Japan

Abstract. Sulfur isotope ratios are analyzed for a series of mudstones in a successive section in the Taiheizan area in the central Green Tuff region, northeast Japan. The δ34S values for the Uyashinai Mudstone Member (Nishikurosawa stage), the Onnagawa Formation and the Funakawa Formation range from ‐45 to ‐14%, ‐20 to +4% and ‐30 to ‐10%, respectively. There is a marked positive shift in δ34S values at the Nishikurosawa/Onnagawa boundary and an upward δ34S‐decreasing trend in the Onnagawa and Funakawa Formations. The present data provides evidence for environmental change in the form of stratification of the seawater, initiated at the Nishikurosawa/Onnagawa boundary and persisting during deposition of the Onnagawa Formation, followed by gradual oxygenation during deposition of the Funakawa Formation.

Paleoceanography and Heavy Metal Enrichment of Mudstones in the Green Tuff Region, Northeastern Japan

Abstract. A series of mudstones corresponding to N.8 to N.17 of Blow from the Uyashinai Mudstone Member, the Onnagawa Formation and the Funakawa Formation in the Taiheizan area of the central Green Tuff region, northeast Japan, was examined by chemical analysis. According to conventional chemical systematics, the mudstones are analyzed in terms of detrital, biogenic‐A (siliceous), biogenic‐B (calcareous) and hydrogenous components. The relative contribution of the biogenic‐A (siliceous) component increases in upward succession in the Uyashinai and Onnagawa mudstones, whereas the contribution of the biogenic‐B (calcareous) component is restricted to the lower Uyashinai mudstones. The contribution of hydrogenous components Zn, Pb, Cu and Ba tends to increase during the Nishikurosawa stage, and decreases near the Nishikurosawa/Onnagawa boundary (Pb and Ba) or in the Onnagawa stage (Zn and Cu). The observed enrichment of heavy metals in the Uyashinai mudstones is attributed to the onset of vigorous upwelling of deep water with higher nutrient and metal content associated with a contemporaneous change in global deep‐water circulation.

Benthic Foraminifera in Siliceous Black Ore from the Ezuri Kuroko Deposit, Hokuroku District, Japan

Abstract: Abundant benthic foraminifera have been identified in thin sections of the siliceous black ore in the Ezuri Kuroko deposit, Hokuroku, Japan. By treating samples with conventional hydrofluoric acid digestion techniques, sponge spicules and radiolaria have also been recognized in the residue. Under microscopic observations, 94 individual foraminiferal specimens have been detected. However, as it is difficult to identify species or genera by means of microscope observations alone, only a small number of genera have been identified based on morphology. The foraminiferal assemblage is composed predominantly of agglutinated species (83%) with subordinate calcareous species (17%), and is assigned to the Cyclammina Assemblage based on the preponderance of Cyclammina (57%). The foraminifera are generally well preserved within micro‐crystalline to cryptocrystalline quartz, and exhibit no obvious features related to compaction or secondary deformation.Textural observations suggest that the siliceous component of this rock was not derived from an allochthonous block but instead constitutes autochthonous proto‐Kuroko sediment. The Cyclammina Assemblage in the ore is different from recently described foraminiferal assemblages in the vicinity of present deep‐sea hydrothermal vents, but is identical to those found in black shales of the Onnagawa to Funakawa stages in the Green Tuff region. The proto‐Kuroko sediment is assumed to have been deposited in an oxygen‐deficient environment within a closed, deep‐seated basin.The existence of siliceous microfossils suggests that the silica in the siliceous ore did not originate from silica sinter deposits produced by submarine hydrothermal activities, but from a biogenic siliceous ooze, probably composed of diatoms.Sulfide mineralization in the interstices of some of the microfossils is inferred to relate primarily to bacterial sulfate reduction associated with the decomposition of organic matter. The later sulfide mineralization associated with larger crystals (which contain fluid inclusions with homogenization temperatures of approximately 250C) cuts across the siliceous masses and foraminiferal septa, and may have been formed after consolidation of the siliceous ooze, accompanying the formation of acidic intrusive rocks during the late Onnagawa stage.

Observation of Pore Spaces and Microcracks Using a Fluorescent Technique in Some Reservoir Rocks of Oil, Gas and Geothermal Fields in the Green Tuff Region, Japan

: Pore spaces and microcracks in representative oil, gas and geothermal reservoir rocks from the Green Tuff region, Japan, were examined using a fluorescent technique. This technique was developed to visualize microscopically pore spaces and microcracks filled with synthetic resin mixed with fluorescent paint under ultraviolet light. Various morphology of pore spaces and microcracks was clearly identified. Spaces in studied reservoir rocks are classified into following three types: pore spaces in matrix, pore spaces in particles, and microcracks. It is observed that valuable oil and gas reservoir rocks relatively include many pore spaces, while microcracks are important for geothermal rocks. Correlation between textural characteristics and porosity or permeability was found in the oil reservoir rocks. Effective permeability depends upon pore spaces in matrix more than upon other components such as pore spaces in particles and microcracks. Looseness in matrix caused by larger grain size of particles is strongly correlated with permeability. Pore spaces play an important role as a reservoir in oil and gas fields, but are less important in geothermal field. Instead, microcracks are important for geothermal reservoir system.

Basin evolution within the Kitakami Massif, northeast Japan: relationship between sedimentation, tectonics and volcanism in an incipient Neogene continental back-arc basin

Abstract The Ichinohe basin in the western margin of the Kitakami Massif, northeast Japan, consists of the Early to Middle Miocene Yotsuyaku and Keiseitoge formations and represents the incipient stage of Neogene back-arc sedimentation that extends toward the north and northwest. Megaclast breccias derived from Mesozoic basement rocks of the Kitakami Massif were transported from southern areas as debris flows. The debris-flow deposits of the Yotsuyaku Formation grade northwards along the basin axis into clast-supported, braided-stream deposits, where floodplains developed. In the center of the basin voluminous volcanic eruptives of the Keiseitoge Formation were derived from eastern and southeastern volcanic ranges and formed transversal distributaries. The deposition of both clastic and pyroclastic sediments suggests that volcanism and tectonic uplift took place simultaneously during basin evolution. Parallel-laminated, fine-grained sediments intercalated with lignite beds suggest lacustrine deposition, and large cross-beds of volcanic breccias indicate Gilbert-type progradation into the lakes. A number of small lakes in the center of the basin was probably formed as the result of damming tributary mouths by the main drainage from eastern and southeastern volcanoes or the reciprocal blocking by lahars and non-volcanic debris flows derived from the southern uplift. Damming episodes produced by thick lava flows were not recognized in this study. The evolution of the N–S trending, longitudinal Ichinohe Basin indicates that E–W extensional tectonism related to the opening of the Sea of Japan affected not only the Green Tuff Region, to the west of the studied basin, but also even the Mesozoic basement during the Miocene. Although the mingling of volcanic and basement-derived deposits indicates the complexities of sedimentary facies and provenances, our view of the stratigraphic architecture has relevance to the interpretation of the evolution of other extensional basin systems.

天然ガスの起源と生成環境 Ｎ２／Ａｒ比とＨｅ／Ａｒ比からのアプローチ

N2/Ar and He/Ar ratios and their isotopic ratios of gas samples from exploration and production wells of oils and natural gases in the Tohoku district, Japan, were measured for discussing the origins of natural gases. Their plots of N2/Ar and He/Ar ratios from the Green Tuff region and non-volcanic area show the common distribution with positive correlation, being lower than those of geothermal and volcanic gases which contain the magmatic gas with higher N2/Ar and He/Ar ratios in the Northeast Japan. In their distribution zone, the gases collected from formations with high maturity level of organic matter show high N2/Ar and He/Ar ratios, while the gases from the formations with its low maturity level give their low ratios. The natural gases of their high ratios contain thermogenic CH4 and N2 accompanied by Ar having high 40Ar/36Ar ratios deep-seated in the crust. On the other hand, the gases with low N2/Ar and He/Ar ratios are characterized mainly by biogenic CH4 and atmospheric N2 and Ar with low 40Ar/36Ar ratios. These results indicate that natural gases from the Green Tuff formations rarely contain magmatic N2 and CH4. Furthermore, the correlation between N2/Ar and He/Ar ratios of natural gases seems to be useful in estimating their origins and the environment of their generations.

Metallogenic epochs in the Inner Zone of southwest Japan

The principal metallogenic epochs in the Inner Zone of southwestern Japan are classified by considering mainly radiometric and microfossil age data, as follows—(1) Early Cambrian Epoch: Characterized by podiform-type chromite deposits hosted in ophiolite complexes; these are the earliest-formed mineral deposits in the Japanese Islands, and probably formed in the shallow mantle of island-arc settings; (2) Carboniferous to Permian Epoch: Characterized by Besshi-type base metal sulfide deposits mainly associated with submarine lavas and tuffs of basaltic composition including their metamorphosed equivalents, and by bedded cupriferous pyrite deposits associated with submarine acidic tuffs. Both types of deposits would have been formed in a back-arc basin environment, geochemically influenced by an island-arc; (3) Middle Triassic to Jurassic Epoch: Characterized by most of the bedded manganese deposits associated with bedded cherts, including their metamorphosed equivalents, which would have formed by hot spring activity in an ocean basin environment with a limited current circulation. The manganese deposits and their host rocks are the result of seafloor accretion; (4) Late Cretaceous to Paleogene Epoch: Characterized by extensive continental-arc magmatism (volcanic and plutonic), resulting in formation of various mineral deposit types, related mainly to ilmenite series (Late Cretaceous) or magnetite series (Paleogene) granitic rocks. Related to the former are polymetallic skarns and veins containing Cu, Sn and W; also related to this metallogenic activity are stratabound hydrothermal clay deposits of kaolin, pyrophyllite, and sericite associated with pre-ilmenite series granitoids subaerial felsic pyroclastic rocks. Molybdenite-bearing veins and replacement-type sericite deposits are characteristics of the magnetite series terranes, with local skarns and vein occurrences. As with ilmenite — series granitoids occurring in southwestern Japan, eastward younging is also recognized in these mineral deposits, regardless of ore type; (5) Neogene Epoch: Characterized by intense submarine volcanism of acidic composition and associated Kuroko deposits in the Green Tuff region, a major tectonic division and a major metallogenic province of the Neogene Tertiary of Japan. These mineral deposits would be related to Japan Sea opening. In the `Non-Green Tuff region,' important mineralization events are represented by sedimentary uranium deposits, and veins characterized by Au, Sb or Hg occurrences; (6) Plio-Pleistocene Epoch: Characterized by epithermal Au–Ag and Au–Ag–Cu veins associated with subaerial andesitic to dacitic volcanic rocks of Plio-Pleistocene age. Volcanism of this and following epochs is characteristic of an island-arc setting; and (7) Quaternary Epoch: Characterized by volcanic sulfur deposits and minor iron sand deposits.

深部地層中のＨｅの移動現象と天然ガスの起源

He isotopic ratios of gas samples from exploration and production wells of oils and natural gases in the northeast Japan were measured for discussing the migration of He gas between the formations in the Green Tuff region. In Niigata and Akita Prefectures, He gases from the Green Tuff formations give the highest 3He/4He ratios, indicating magmatic origins. It is noted that both radiogenic and magmatic He gases were found from the different depths of the same formation in Niigata Prefecture. This feature strongly indicates that the magmatic He is not originated from volcanic rock, although volcanic rocks homogeneously distributes in the formation. In Akita Prefecture, He gases in the formation containing little volcanic rocks have the large proportion of magmatic He. Therefore, it can be inferred that magmatic He migrates from the Green Tuff formation to upper formations through faults or cracks.δ13C value of CH4 gas coexisting with radiogenic He found in the Niigata Green Tuff region was -28.1‰, indicating the thermogenic origin. The same feature was observed in the natural gases from the Iwaki-oki production wells located in the Pacific ocean. In contrast, in the Akita area, He and CH4 gases from the formation containing little volcanic rocks give magmatic 3He/4He ratios and biogenic δ13C values. These gases deviate from the positive correlation between the 3He/4He ratios and δ13C values of CH4 which have been reported for natural gases from Japanese island arcs. Therefore, the relation demonstrated previously has no meaning for discussing the origin of CH4 in natural gases. These results seem to support that the origins of CH4 having high δ13C values from the Green Tuff region in Japan are mainly not magmatic but thermogenic.