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The formation process of myrmekites in granitic rocks can help us to understand the mass transfer between minerals and hydrothermal fluids during the deuteric stage. The Toki granite, central Japan, has three types of myrmekites. Type A myrmekite is defined as a single layer. Type B myrmekite shows a composite texture consisting of two layers, namely, a myrmekite layer and an albite (Ab)-rich layer that is free of vermicular quartz. Type C has a composite texture with the following three layers: two myrmekite layers separated by one Ab-rich layer. Micropores are found in these myrmekites in the undeformed granite, which enable quantitative determinations of the volume decrease during myrmekite formation by measurement of the area of micropores. The areal relationship between the micropores and vermicular quartz in the myrmekites exhibited a high correlation (R2 = 0.8352), thus indicating that the genesis of the micropores is evidently related to myrmekite formation. We derived the reaction equations for myrmekite formation based on the singular value decomposition method. The matrices for singular value decomposition involve the following volume factors: volume change during the reaction and volume ratios of the product minerals. The singular value decomposition indicates that the myrmekites are produced through the consumption of plagioclase and K-feldspar with an inflow of H4SiO4, Na+, and H+ from the hydrothermal fluid, accompanied by an outflow of Al3+, Ca2+, and K+ into the fluid, which constitute essential mass transfers during myrmekite formation. The difference of pH in the hydrothermal fluid and inflow amounts of H4SiO4 and Na+ can explain the reason why micropores occur in the myrmekites but not in the Ab-rich rim (layer); the smaller inflow of H4SiO4 and Na+ from the hydrothermal fluid with lower pH conditions yielded micropore production during myrmekitization, and the larger inflow of H4SiO4 and Na+ from the hydrothermal fluid with higher pH conditions yielded the formation of the Ab-rich rim (layer) with few micropores. The sequential variations in the chemical characteristics of the hydrothermal fluid during the sub-solidus conditions were characterized by a gradual decrease in H4SiO4 (Si4+), Fe2+, Mn2+, Mg2+, and Na+ and a gradual increase in Ca2+, K+, H+, and F–.

This study, along with our previous studies (Yuguchi et al. 2015, 2019a), reveals the hydrothermal alteration processes in a pluton, with a focus on the mass transfer between minerals and hydrothermal fluid. It also depicts the sequential variations in fluid chemistry as alteration progresses. Hydrothermal alteration of the Toki granite in Tono, Japan—the study area of this research—progressed through the successive processes of chloritization, plagioclase alteration, and precipitation of a carbonate. This paper describes the alteration processes of hornblende chloritization, K-feldspar chloritization, and the formation of fracture-filling chlorite through petrography and mineral chemistry. A set of singular value decomposition analyses was conducted to obtain reaction equations for the chloritization processes, which facilitates the quantitative assessment of mass transfer between the reactant and product minerals, and the inflow and outflow of components through the hydrothermal fluid. Several types of chloritization reactions (including biotite chloritization) can be characterized by their reaction with the inflow of Al3+, Fe2+, Mn2+, and Mg2+ and the outflow of H4SiO4, Ca2+, K+, and F–. The age and thermal conditions of hornblende chloritization (64–54 Ma and 330–190 °C), K-feldspar chloritization (68–53 Ma and 350–210 °C), and precipitation of fracture-filling chlorite (66 and 63 Ma, 340 and 320 °C) overlap with those of biotite chloritization (68–51 Ma and 350–180 °C). The chloritization reactions (this study and Yuguchi et al. 2015) and plagioclase alteration (Yuguchi et al. 2019a) represent sequential variations in fluid chemistry at temporal conditions from 68 to 51 Ma as the temperature decreased from 350 to 180 °C. As the alteration proceeds, the concentrations of aluminum, iron, manganese, and magnesium ions in the hydrothermal fluid decrease gradually, and those of calcium, hydrogen, and fluorine ions increase gradually. Hornblende chloritization is associated with formation of magnetite and ilmenite. The thermal conditions of the hydrothermal fluid yielding the formation of magnetite and ilmenite can be interpreted by the chemical characteristics of chlorite around their associated minerals. The formation temperature of magnetite was higher than that of ilmenite, implying a decrease in oxygen fugacity in the hydrothermal fluid with the decrease in temperature from 280–310 to 220–250 °C.

Grain size variation of sediments and sedimentary rocks generally complicates the assessment of the degree of chemical weathering as measured by the Chemical Index of Alteration (CIA) value. Mineralogical and geochemical research was carried out on Miocene and Pliocene fluvial sediments in southwest Japan in order to evaluate the effects of grain size on the degree of chemical weathering. A strong positive linear trend is usually exhibited between Al2O3/SiO2 ratios and quartz intensities obtained by X-ray powder diffraction, suggesting that the Al2O3/SiO2 ratio may be suitable as a grain size index for semi-consolidated and unconsolidated sediments. Most of the fine-grained sediments, for which Al2O3/SiO2 ratios are over 0.4, have higher CIA values of over 90. In contrast, coarse-grained sediments, for which Al2O3/SiO2 ratios are lower, especially below 0.2, display a wide range of CIA values. The CIA values for intermediate grain sized samples, for which Al2O3/SiO2 ratios range from 0.2 to 0.4, reflect distinctive differences in the degree of chemical weathering between Miocene and Pliocene sediments. Within this range of Al2O3/SiO2 ratios, the Miocene sediments have higher CIA values (86–98) than Pliocene sediments (71–92). To compare the degree of chemical weathering among some sediments or sedimentary rocks with variable grain sizes, it is desirable to evaluate with the Al2O3/SiO2 ratios limited to the intermediate grain size range.

This study presents position-by-position $t-T$ paths within a granitic pluton based on thermochronological data, and describes their constraints and their relationship with fracture frequency, as an example from the Toki granite, central Japan. The cooling paths have position-specific characteristics; a single $t-T$ path does not represent the cooling behavior of the entire pluton. Such position-specific $t-T$ paths enable us to evaluate three-dimensional thermal evolution within the granitic pluton, and thus can clarify the detailed formation history of the entire pluton after the incipient intrusion of the granitic magma into the shallow crust. This study reveals the relationship between position-specific $t-T$ paths and fracture frequency, and thus provides a criterion for evaluating the fracture population in terms of thermal stress., 本研究は、中部日本に位置する土岐花崗岩体を研究対象とし、熱年代学的な手法で冷却履歴(温度-時間履歴)を取得した。10本のボーリングコアから採取された15試料に対して、ジルコンU-Pb年代,黒雲母K-Ar年代,ジルコンフィッション・トラック(FT)年代,アパタイトFT年代、およびFTデータの逆解析からなる温度-時間履歴を提示した。得られた岩体内部の複数地点の冷却履歴の相違を比較し、その位置的な相違を生む原因について言及した。これは熱進化モデルの構築に資するデータとなる。また花崗岩内における物質移動は、水みちとして機能する割れ目により大きく支配されることから、割れ目の分布特性と得られた温度-時間履歴の関連について検討を行った。

Biotites occur with varying degrees of alteration within a granite. This study analyzes the relationships among alteration indicators, areal microvoid fractions in chloritized biotite, and macroscopic fracture frequencies in the Toki granite, central Japan, to establish the genesis and development processes of fractures in granite. Appropriate characterizations for the frequency distribution of macroscopic fractures in granite can assist in understanding potential hydrogeological applications, which contributes to safety evaluations for geological disposal and storage. Borehole 06MI03, drilled to a depth of 191 m, was used to obtain samples for the analysis. In total, 24 samples that depicted variations in the macroscopic fracture frequency were selected. Petrographic alteration indicators using biotite chloritization as innovative methods are proposed to evaluate the extent of hydrothermal alteration and fracture frequency within granites. The alteration indicators are defined as the ratio between the alteration product area and the original mineral area. Furthermore, the volume of microscopic fractures and micropores in the mineral was quantitatively characterized by the areal fraction of microvoids in minerals through image analysis. Samples with high macroscopic fracture frequencies correspond to a high number of areal microvoid fractions and large alteration indicators. Microvoids, which are the source of macroscopic fractures, occurred at temperatures between 350 and 780°C and can be evaluated by intrinsic factors, such as alteration indicators. Subsequent faulting and unloading (extrinsic factors) developed microvoids into macroscopic fractures. Intrinsic factors are used to evaluate the source of macroscopic fractures, and therefore contribute to the characterization of present and future distributions of macroscopic fracture frequencies.

This study presents the K-Ar geochronology for hydrothermal K-feldspar in plagioclase alteration, including methodology and application to the Toki granite, in central Japan. Borehole samples from the Toki granite were collected and mechanically and chemically processed to separate plagioclase from the rock and remove bulk impurities. The sample fraction of cleaned plagioclase powder was further processed to a smaller size fraction, allowing separation of the altered K-feldspar from the plagioclase host. The resulting K-feldspar represented the hydrothermal alteration product and was characterized crystallographically as microcline, and its K-Ar ages were measured. The results of the K-Ar dating and petrographic characterization indicated that in this setting, plagioclase alteration occurred through a combination of solid-state replacement and dissolution–precipitation processes. The K-feldspathization age enables constraint of the temporal conditions of the solid-state replacement process to 62.2 ± 1.4 Ma. The time-temperature (t-T) path of the sampling site is an effective tool for determining both the timing and thermal conditions of the hydrothermal microcline formation in plagioclase alteration. The combination of the t-T path and the microcline K-Ar age provides formation temperatures of about 307–325 °C. The timing and thermal conditions of solid-state replacement (62.2 ± 1.4 Ma and 325–307 °C) indicate an older age and a higher temperature than those of dissolution–precipitation (59.2 ± 1.4 Ma and 305–290 °C: Yuguchi et al., 2019A). The plagioclase alteration consists of serial processes from solid-state replacement to dissolution–precipitation. Addition of the thermal conditions and timing into petrography have implications for the sequential phenomenal variation in granite., K-feldspar K-Ar age; Plagioclase alteration; Solid-state replacement; Dissolution and precipitation; K-feldspathization and illitization; Toki granitic pluton

Fracture zones around faults (damage zones) in crystalline rocks such as granite can act as significant transport pathways because the permeability of damage zones around active faults will likely be increased by the fracture network system. Understanding the characteristics and long-term behavior of damage zones along smaller scale faults is important for the safety assessment of deep underground facilities in the plutons of an orogenic belt; for example, facilities for the disposal of high-level radioactive waste. Therefore, this paper describes the features of a damage zone and the long-term behavior of its hydrogeological structures associated with faulting in an underground environment based on the results of geological and hydraulic investigations 300 and 500 m below ground level at the Mizunami Underground Research Laboratory, Central Japan. Detailed borehole and gallery wall investigations show the distributions of fractures, fracture fillings, predominant fracture orientations, groundwater inflow points, and hydraulic transmissivity in and around a damage zone. The results indicate that there are three stages in the development of hydrogeological structures. The first stage is the formation of background fractures that formed after the temperature of granitic plutons decreased through the ductile–brittle transition, forming fractures under brittle deformation conditions. The second stage is the formation of a damage zone and corresponding increase in hydraulic permeability as a result of the formation of relatively small fractures associated with faulting. The third stage is the formation of fracture fillings because of the infiltration of groundwater through the damage zone, decreasing the hydraulic permeability of the zone. In the late third stage, unconsolidated clayey fillings form in the large fractures associated with faulting resulting in decreased permeability of the fractures in the damage zone around the fault. These results underline the importance of understanding the development stages to evaluate the effect of faulting in orogenic belt plutons.

Quartz from a granitic pluton is found to have formed through sequential growth events under different mechanisms and crystallization temperatures, which can provide new insights into magmatic processes of granitic magmas that were eventually consolidified into plutons. The events were identified using (1) the description of crystal shape and occurrence, (2) the study of the internal structure with cathodoluminescence (CL), and (3) derivation of the crystallization temperatures based on TitaniQ thermometry. The magmatic quartz crystals from the Toki granite, central Japan, are characterized as having the following internal structures: oscillatory zonation, no-oscillatory zonation with luminescence gradation (gradational zonation), and heterogeneous CL. The quartz crystals with oscillatory zonation were formed in the temperature range of about 800 °C to below 700 °C, which is referred to as oscillatory zoning temperature (OZT) conditions. The CL zonation pattern was controlled by the temperature conditions and titanium diffusivity in the melt (magma). The crystallization process of quartz within the Toki granite reveals the cooling processes of the granitic pluton; the lithofacies with a high frequency of oscillatory-zoned quartz underwent slower cooling under the OZT conditions than those in other lithofacies.

Bulk chemical composition and mineral composition of the Pliocene sediments in southwest Japan reveal the effects of source rock compositions and grain sizes, as well as the relationship between sedimentary environments and the degree of chemical weathering in the Pliocene warm period. A wide variation in the degree of chemical weathering, source rock compositions and grain sizes was observed with vertical and spatial changes in the sedimentary environment. Coarse-grained sediments, deposited in sandy channels and their floodplains, indicate a lower but wide range of CIA values. This result suggests that the degree of chemical weathering can be modified by changes in grain size due to hydrologic processes. In contrast, stagnant water sediments, composed of homogeneous fine-grained sediments, display a wide variety of REE compositions and Al2O3/SiO2 ratios mainly caused by variation in source rock compositions and grain sizes. However, the CIA values of the stagnant water sediments are over 90, such that these sediments display a tight cluster near the Al2O3 apex on the Al2O3-(CaO + Na2O)-K2O ternary diagram. This study demonstrates CIA values are generally above 90, which is due to the increased leaching of Ca, Na, and K with exposure to under intense chemical weathering conditions even if the source rocks and grain sizes are variable. Regardless of the wide variety of source rock compositions and grain sizes, REE and kaolinite-rich clay mineral concentrations in these sediments are indicative of the intense weathering conditions that prevailed in the Pliocene period (3–4 Ma) in southwest Japan.

Rock matrix permeability is mainly controlled by microcracks and is important for long-term solute transport, but may be highly heterogeneous. This study aims to identify the factors influencing the permeability of granite matrix by analyzing: physical properties, crack distribution, mineral composition, and the concentration of main elements in 48 core samples taken from three deep drillholes in the Mizunami area, central Japan. All drillholes intersect the Tsukiyoshi fault at depth. Permeability of core samples, measured by a gas permeameter, largely increases in the fault and fracture zones, which may be caused by the development of cracks as the total length of cracks extracted from thin-sections is correlated with measured permeability. Although a significant correlation is identified between permeability and P-wave velocity, a representative elastic property of rock, this correlation is enhanced by classifying the samples into two groups by the Mn/Fe concentration ratio. This ratio is identified as a key factor to separate lithofacies differentiated by the source magma. Thus, lithofacies is another control factor for permeability due to the difference in mineral composition. Moreover, permeability shows significant negative and positive correlations with Si and Ca concentrations, respectively. These concentrations are probably affected by dissolution of silicate minerals and calcite generation in the hydrothermal alteration process, which can develop both pore space and crack network and consequently, increases rock permeability in addition to crack generation and development. Therefore, a combination of hydrothermal alteration before the emplacement of granite and strong faulting are the predominant processes for controlling permeability.

Calcium and magnesium-bearing sabugalite occurs as aggregations of yellowish platy crystals in veinlets or druses in conglomerate from the oxidized parts of the Tono uranium deposit, Central Japan. X-ray powder diffractometry of this mineral has reflections consistent with previous powder diffraction data of sabugalite. It is included in the monoclinic system with space group C2/m and calculated cell parameters of a = 19.68A, b = 9.89A, c = 9.82A α = γ = 90°, β-96.93° and V = 1897.83A3. Chemical analysis yields a formula of (Ca0.10 Mg0.09)Σ0.19Al0.53(UO2)2.04((PO4)1.99(AsO4)0.01)Σ2.00·11.22H2O. EMPA mapping shows that the mineral is compositionally uniform with no micron-scale layering. Charge of cations including Ca and Mg in the cation-H2O layer is 1.98 being identical to that of autunite group minerals. This suggests that the charge balance in the cation-H2O layer of the mineral could be made by the alkaline earth or alkaline elements rather than by hydrogen ions.

A petrographic study of sandstones from the Miocene Mizunami Group in Central Japan has been performed on core samples from a single borehole, in order to evaluate the provenance of the sedimentary rock. Evaluation of the provenance is based on bulk mineral, heavy mineral and plagioclase contents and on whole rock chemical compositions. The sandstones studied are divisible into three types; the first type is characterized by the occurrence of biotite and plagioclase ranging from albite to oligoclase, the second type is characterized by the dominance of amphibole and labradorite with pyroxene (clinopyroxene > orthopyroxene), and the third type is characterized by the dominance of pyroxene (orthopyroxene > clinopyroxene) and andesine with lesser labradorite, bytownite and anorthite. The first type is interpreted to be derived from the basement granite, whereas the others were derived mostly from volcanic ash, judging from their mineral compositions. The volcanic activity that supplied the volcanic ash to the Mizunami Basin occurred in two phases, distinguishable by variations in their mineralogical and geochemical compositions, an indication of change in character of the volcanic activity. This petrographic study of the sandstones in the Mizunami Group suggests that unrecognized volcanic activity occurred around the Mizunami Basin, even though potential provenance of the volcanic ash has not yet been identified.

Holocene environmental changes were reconstructed by analyzing diatom assemblages from sediment cores YM, KZN, NK, and MC from the Nobi Plain, central Japan. Five diatom assemblage zones were identified: (1) at the beginning of the Holocene, freshwater species were dominant; (2) then, marine and brackish-marine species increased, indicating transgression; (3) in the middle Holocene, proportions of marine and brackish-marine species became almost constant, with marine species dominant; (4) marine species began to be replaced by freshwater species, indicating marine regression as a result of delta progradation; and (5) freshwater species again became dominant. These diatom assemblages correlate with previously defined lithological units: zones 1 and 2 with unit B (fluvial to coastal plain), zone 3 with unit C (inner bay or prodelta), zone 4 mainly with unit D1 (delta front slope), and zone 5 with units D2 (delta front platform) and E (delta plain and flood plain). Although the shoreline migrated landward (transgression) faster than it migrated seaward (regression), transgressive diatom assemblage changes (decrease in marine-brackish water species) took up to 1000 years, whereas regressive changes required only a few hundred years. Diatom analysis is useful for reconstructing not only Holocene sea-level changes and sedimentary environments but also local geographic effects.

The usefulness of electrical conductivity (EC) of stirred Holocene deltaic sediments from three cores from the Nobi Plain, Japan, was assessed as a proxy for marine transgression and regression. Lack of correlation of EC with mud content for the sediments with >20% mud suggested that permeability was a negligible factor. Marine deposits showed high EC (>0.9 mS/cm), terrestrial deposits low EC (

It is important to understand how long-term geological phenomena such as uplift and erosion influence deep hydrogeological and hydrochemical environments, and to predict the influence of such long-term geological phenomena in the future for the geological disposal of nuclear wastes. From the viewpoint of groundwater flow, it is necessary to estimate long-term topographic changes, and to evaluate their influence on deep groundwater flow conditions. In this study, the influence of long-term topographic change on deep groundwater flow conditions was numerically assessed. The general paleotopographic features of a wide area were estimated and groundwater flow simulations were carried out around the Tono area. As a result, the effects of long-term topographic changes and hydraulic features of faults on groundwater flow conditions, such as hydraulic gradient, velocity distribution, flow paths, and lengths, were confirmed. In general, if topographic characteristics such as locations of major mountains and valleys around the site have not changed, the groundwater flow paths will not significantly change. The methodology, which is proposed in this study, used to understand groundwater flow evolutions due to long-term topographic changes is efficient for identifying detailed assessment areas and is recommended based on the results of this study.

The depositional process of the Latest Pleistocene to Holocene shallow-marine and fluvial successions is investigated using five 14C-dated borehole cores from the Kiso River delta, central Japan. The delta succession provides high-quality records of the transgression and regression of an enclosed delta system and reveals the development of fluvial-dominated coastal lowlands. Based on a facies analysis, the cored sediments are classified into five sedimentary units: A) basal gravel, B) fluvial and intertidal sand and silt, C) transgressive lag deposit, prodelta mud, and sandy silt, D) delta-front-slope sandy silt and sand, and delta-front-platform sand, and, E) delta-plain and fluvial sand and silt. Detailed age-depth curves of these cores are reconstructed based on 108 AMS 14C ages including 83 new 14C ages. These curves show patterns similar to those of temporal variations in accumulation rates, and are divided into four sections (I to IV) from the base to the top: I mainly consists of unit B with rapid accumulation (7.3-21.4mm/yr); II mainly consists of unit C with moderate accumulation (2.4-2.8mm/yr); III mainly consists of unit D with rapid accumulation (6.7-17.8mm/yr); and, IV consists of unit D and E with moderate accumulation (1.3-3.9mm/yr). The section boundaries of I/II and II/III, which coincide with the unit boundaries of B/C and C/D, reflect the inundation of a flood plain by seawater, developing a bay environment, and the subsequent progradation of the delta–front slope, respectively. The ages of these boundaries constrain the estimation of the expanding rate of the bay and progradation rates of the delta. The expanding rate of the bay is estimated at 10m/yr during the period 10200-7900 cal yrs BP, and the progradation rates of the delta are 3-4m/yr (6500-4100 cal yrs BP), 5m/yr (4100-1300 cal yrs BP), and 10m/yr (1300 cal yrs BP to the present). These age data indicate that the transition from transgression to regression occurred between 7800 and 7300 cal yrs BP. The geological cross-section along the Kiso River along with the isochrones constructed by over a hundred 14C dates indicates the following depositional history of the Kiso River delta: (1) from 10000 to 7280 cal yrs BP (K-Ah horizon) —During the expansion of the bay, onlap of unit C on unit B is visible, and unit C overlapped all the core sites just before the K-Ah fall, and (2) from 7280 cal yrs BP to present—Isochrones younger than 6500 cal yrs BP are similar to each other and cross the unit boundaries of C/D and D/E. This demonstrates that units C–E are contemporaneous heterotopic facies formed in response to delta progradation after the K-Ah fall.

飛騨山脈南部，梓川上流部セバ谷沿いに分布する礫層中から産する材化石の14C年代を測定し，約4.8万年前の年代値を得た．従来，この礫層はMIS 4に相当する波田面構成礫層とされていたが，今回の測定により，それより新しい時代の礫層であることが明らかとなった．

The basic concept of deep geological disposal of high-level radioactive waste is to isolate the waste from the human environment for the long term. Because the Japanese islands are located in a geologically active area, geological phenomena such as exhumation and fault activity must be considered by any safety assessment connected with deep geological disposal. The Tono Uranium Deposit, central Japan, has been affected by such geological phenomena during the interval since its formation, and so it is a suitable analogue for evaluating how this might be done. The present natural analogue study of the Tono Uranium Deposit (Tono Natural Analogue Project) was started in 2001 with the main aim of studying a so-called ‘worst-case scenario’ for performance assessment (PA). The project involved characterizing the geology, hydrogeology, geochemistry and microbiology of the deposit and obtaining quantitative information about specific times in the past, as a means for developing, and building confidence in, conceptual and numerical models. This project applied systems analysis, which has been widely undertaken in PAs of deep geological isolation. Systems analysis involves a systematic identification, classification and screening of features, events and process (FEPs) that occur or have occurred in and around the deposit. Based on the site data, important FEPs were identified.

Data from the Tono Uranium Deposit of central Japan were used to develop an improved approach for simulating uranium migration and retardation, while taking into account both long-term environmental changes and uncertainties in data. Based upon extensive field and laboratory investigations, conceptual and numerical models for environmental perturbations, including uplift, subsidence and faulting, were constructed. Model development was based on a novel adaptation of a safety assessment methodology that previously has been applied to radioactive waste repositories. A ‘reference scenario’ was developed using a systems analysis approach. This scenario is a best estimate of how the geological system and the uranium deposit reached their present states and includes descriptions of all major environmental perturbations. Uranium is mobilized from the uppermost Toki granite under relatively oxidizing conditions, and is then transported by groundwater into overlying sedimentary rocks. There, reducing conditions promote uranium deposition. A specially designed numerical model simulated the main features of this scenario. Many simulations were performed to identify key uncertainties to which the timing of ore deposition and uranium distribution are sensitive. A key finding is that retardation of U by processes other than precipitation of discrete U minerals, most probably sorption on solid phases, contributes significantly to the stability of the ore deposit. Sorption could potentially be important for confining the U within the sedimentary rocks in spite of environmental changes such as exhumation and seismic pumping. The approach could be used elsewhere, to assess the safety of deep geological high-level radioactive waste (HLW) disposal. A related application would be at potential future waste disposal sites, to prioritize site characterization so that the most safety-relevant uncertainties are reduced. There are also possible applications in other fields, most notably to assess the implications of alternative ore genetic models.

Data characterizing the mineralogy, hydrochemistry and geomicrobiology of the Tono region of central Japan were used to interpret geochemical constraints on the origin and stability of the Tono Uranium Deposit. The derived constraints are compatible with models of deposit formation, which call for leaching of uranium from the upper weathered zone of the Toki Granite by relatively oxidizing groundwaters that are near-neutral to moderately alkaline and carboniferous. The oxidizing groundwaters then migrate into mudstones and sandstones of the overlying Toki Formation, where the uranyl species is reduced by water–rock–microbe interactions to uranous species, sorbed by various detrital and authigenic phases and eventually precipitated as uraninite, coffinite and the metastable, amorphous hydrous oxide, UO 2 ( am ). Formation of the Tono deposit may have been more or less continuous up to the present time. The modern hydrochemical system, upon which the genetic model is based, began to evolve about 15 Ma when seawater was flushed out of the sedimentary cover and basement granite by fresh, meteoric waters during a period of uplift and erosion preceding Pliocene to Pleistocene sedimentation. Recharge with meteoric water continued to the present, which suggests that palaeohydrochemical conditions were probably similar to those observed in the region today when the Tono deposit began to form about 10 Ma. Redox environments in the Tono region inferred from in-situ Eh measurements in deep boreholes and calculated potentials for the SO 4 2− /HS − redox couple appear to be controlled by heterogeneous reactions involving Fe(III)-oxyhydroxides. Metastable equilibria and particle-size effects associated with these reactions produce a range of possible redox environments that are equally compatible with both the relatively oxidizing and reducing groundwaters of the Toki Granite. This compatibility extends to sedimentary porewaters, where the redox environment is also controlled by microbially mediated sulphate reduction, oxidation of organic matter and precipitation of sulphide minerals. Redox conditions have been stable during at least the past several tens of thousands of years based on palaeoredox indicators interpreted from the trace element contents of fracture calcites. The pH and carbonate contents of palaeogroundwaters and modern groundwaters of the Tono region were, and are, controlled mainly by calcite equilibrium.

岐阜県南東部に分布する中新統瑞浪層群土岐夾炭層について，本層中部の凝灰質砂岩のフィッション・トラック年代測定と記載岩石学的性質に基づいて，その堆積年代を検討した．凝灰質砂岩および土岐花崗岩中の斜長石の屈折率分布から，凝灰質砂岩は安山岩質の火山灰と花崗岩起源の砕屑物の混在物であることが明らかになった．また，凝灰質砂岩中のジルコンの粒子年代分布を統計的に解析し，同一年代と見なされる粒子集団を分類した結果，17～21 Maおよび56～67 MaのFT年代が識別された．古い方の年代は土岐花崗岩のFT年代（59～61 Ma）と一致しており，基盤から供給されたジルコンの年代を示し，若い方の年代は安山岩質火山灰の形成年代，すなわち瑞浪層群土岐夾炭層中部の堆積年代を示す．これまでに報告された本層上部のFT年代（約18 Ma）を考慮すると，基底礫岩を除く土岐夾炭層は約18～20 Maに堆積したと推定される．

The Cretaceous Toki granitic pluton of the Tono district, central Japan was emplaced in the East Asian continental margin at about 70 Ma. The Toki granite has apatite fission-track (AFT) ages ranging from 52.1 ±2.8 Ma to 37.1 ±3.6 Ma (number of measurements, n = 33); this indicates the three-dimensional thermal evolution during the pluton's low-temperature history (temperature in the AFT partial annealing zone: 60–120 °C). The majority of the Toki granite has a spatial distribution of older ages in the shallower parts and younger ages in the deeper parts, representing that the shallower regions arrived (were exhumed) at the AFT closure depth earlier than the deeper regions. Such a cooling pattern was predominantly constrained by the exhumation of the Toki granitic pluton and was related to the regional denudation of the Tono district. The age–elevation relationships (AERs) of the Toki granite indicate a fast exhumation rate of about 0.16 ±0.04 mm/year between 50 Ma and 40 Ma. The AFT inverse calculation using HeFTy program gives time-temperature paths (t–T paths), suggesting that the pluton experienced continuous slow cooling without massive reheating since about 40 Ma until the present day. A combination of the AERs and AFT inverse calculations represents the following exhumation history of the Toki granite: (i) the fast exhumation at a rate of 0.16 ±0.04 mm/year between 50 Ma and 40 Ma; (ii) slow exhumation at less than 0.16 ±0.04 mm/year after 40 Ma; and (iii) exposure at the surface prior to 30–20 Ma. The Tono district, which contains the Toki granite, underwent slow denudation at a rate of less than 0.16 ±0.04 mm/year within the East Asian continental margin before the Japan Sea opening at 25–15 Ma and then within the Southwest Japan Arc after the Japan Sea opening, which is in good agreement with representative denudation rates obtained in low-relief hill and plain fields.

Long-term stability of the geological environment is one of the important keys for deep geological disposal of high-level radioactive waste in the Japanese Islands due to their location in a tectonically active island-arc. Uranium occurrences in Japan have been subjected to many geological processes inherent to the island-arc setting. Geological environments associated with uranium mineralization are considered favorable for HLW disposal, because uranium mineralization is considered a natural analogue of the radionuclides in HLW. Studies on the long-term stability of the uranium mineralization in Japan can be instructive as these could provide useful information on the long-term stability of the geological environment. Information on host rock and mode of occurrence of uranium mineralization was compiled from published data. The mineralization occurs in these types of deposits, i.e., sedimentary formations, association with metallic ore mineralization of magmatic origin and stratiform manganese mineralization, pegmatite, and alluvial placer deposit. The mineralization occurs in various geological settings in Japan. This fact suggests that geological environments suitable for geological isolation are widely distributed in the Japanese Islands, despite their location in a geologically active area. This study will support building confidence in HLW disposal in the Japanese Islands.Copyright © 2010 by ASME