Supercritical geothermal reservoir revealed by a granite–porphyry system

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Supercritical geothermal reservoir revealed by a granite–porphyry system Noriyoshi Tsuchiya ∗ , Ryoichi Yamada, Masaoki Uno Graduate School of Environmental Studies, Tohoku University, Aramaki-aza-Aoba, 6-6-20, Aoba-ku, Sendai 980-8579, Japan

a r t i c l e

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Article history: Received 16 January 2015 Received in revised form 23 December 2015 Accepted 30 December 2015 Available online xxx Keywords: Supercritical geothermal reservoir Porphyry copper mineralization Granite–porphyry system Mineral filling veins Geofluids

a b s t r a c t To understand the geological properties of a supercritical geothermal reservoir, we investigated a granite–porphyry system as a natural analog. Quartz veins, hydrothermal breccia veins, and glassy veins are present in Neogene granitoids in NE Japan. The glassy veins formed at 500–550 ◦ C under lithostatic pressures, and then pressures dropped drastically. The solubility of silica also dropped, resulting in formation of quartz veins under a hydrostatic pressure regime. Connections between the lithostatic and hydrostatic pressure regimes were key to the formation of the hydrothermal breccia veins, and the granite–porphyry system provides useful information for creation of fracture clouds in supercritical geothermal reservoirs. © 2016 Published by Elsevier Ltd.

1. Introduction Following the Great East Japan Earthquake and the accident at the Fukushima Daiichi Nuclear power station on 3.11 (11th March) 2011, geothermal energy came to be considered one of the most promising sources of renewable energy for the future in Japan. However, there are several geological and geophysical issues to consider. First is that ∼80% of the potential geothermal energy in Japan lies inside National Parks, second is Onsen (hot springs) problem which is conflict between geothermal developers and Onsen owners due to some misunderstandings of geothermal and hot spring resources, and another is induced seismicity related to the development of geothermal energy. The temperatures of geothermal fields operating in Japan range from 200 to 300 ◦ C (average ∼250 ◦ C), and the depths range from 1000 to 2000 m (average ∼1500 m). In conventional geothermal reservoirs, the mechanical behavior of the rocks is presumed to be brittle, and convection of the hydrothermal fluid through existing network is the main method of circulation in the reservoir. In order to minimize induced seismicity, a rock mass that is “beyond brittle” is one possible candidate, because the rock mechanics of “beyond brittle” material is one of plastic deformation rather than brittle failure (Asanuma et al., 2012; Muraoka et al., 2014).

∗ Corresponding author. E-mail address: [email protected] (N. Tsuchiya).

At Kakkonda in NE Japan, the exploration well WD-1a encountered the partly solidified Kakkonda Granite and inferred reservoir temperatures in excess of 500 ◦ C (Doi et al., 1998; Ikeuchi et al., 1998; Kasai et al., 1998; Kato et al., 1998; Matsushima et al., 2003; Muraoka et al., 1998; Sasaki et al., 2003; Tosha et al., 1998). The project called DSGR (Deep-Seated Geothermal Reservoir) was conducted by NEDO (New Energy Development Organization, Japan); nevertheless, there were no strong emissions of steam from the bottom of the well. In an attempt to understand the findings of DSGR, we have studied an exposed Quaternary granitoid (the Takidani Granodiorite), since it is analogous to the type of granitoid rock mass that might host a deep-seated (artificial) geothermal reservoir (Bando et al., 2003; Kano and Tsuchiya, 2002). From an engineering point of view, the Takidani Granodiorite is a suitable candidate as a natural analog for a HDR/HWR (Hot Dry Rock/Hot Wet Rock) geothermal reservoir, particularly under supercritical geofluid conditions. The Takidani Granodiorite is located at the boundary of the Eurasian and North American Plates (Harayama, 1992), and extensive silicic magmatic activity (both volcanic and plutonic) occurred through the Pliocene and Pleistocene. In addition, we have investigated hydrothermal activity in order to understand the evolution of supercritical geothermal fluids in certain geological settings. Temperatures over 350 ◦ C are in the “beyond brittle” condition (a temperature of ∼350 ◦ C coincides with the brittle–ductile transition), and the ways in which fractures develop under these conditions are unclear.

http://dx.doi.org/10.1016/j.geothermics.2015.12.011 0375-6505/© 2016 Published by Elsevier Ltd.

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Fig. 1. Map of the study area and the granitoid complex. The regional geology is based mainly on Osawa et al. (1981) and Fujimoto (2006), and was revised by MITI (1986). Numbers in the figures indicate dating results shown in MITI (1986).

Porphyry copper deposits represent natural “beyond brittle” analogs where fluids from molten material (magma) infiltrate a ductile rock mass at ∼600 ◦ C, and where lithostatic pressures cause fractures in the rock mass, creating a stockwork fracture system (Batkhishig et al., 2014; Mercer and Reed, 2013; Davies et al., 2008; Ingebritsen, 2012; Rusk and Reed, 2002). The large strain rates during fluid injection released from the host rock render the rock mass brittle, allowing it to fracture in tensile and shear modes. In these porphyry deposits, we are able to observe several kinds of fractures represented by millimeter- to centimeter-scale quartz veins (Bons, 2001; Okamoto et al., 2008, 2010), where quartz filled and plugged the fractures; apparently the quartz was precipitated during adiabatic decompression and cooling as the fluids traversed from lithostatic to hydrostatic pressure regimes. A granite–porphyry system, associated with hydrothermal activity and mineralization, provides a suitable natural analog for studying a deep-seated geothermal reservoir where stockwork fracture systems are created in the presence of supercritical geothermal fluids. In this paper we describe fracture networks and their formation mechanisms using petrology and fluid inclusion studies in order to understand this “beyond brittle” supercritical geothermal reservoir.

2. Geological setting The study area is located in central Akita Prefecture, Tohoku District, NE Japan. In the vicinity of the area, volcano-sedimentary rock sequences of Paleogene to Neogene age were deposited around a basement of Cretaceous granitoids. The tectonic setting was one of an intra-rift rise formed during the period of back-arc spreading of the Sea of Japan that started at 28 Ma and continued until 13 Ma. Paleogene sequences since the Eocene are mainly made up of terrestrial andesite lavas with subordinate pyroclastic rocks, and they represent continental margin volcanism prior to back-arc opening. These sequences were followed by volcanoclastics with several basaltic lava flows in the periphery of the study area, as back-arc volcanism continued during the period 20–13 Ma. After 13 Ma, the peripheral area gradually changed to a bathyal environment, but the study area itself remained as a small continental rise, the result of differential uplift and corresponding intrusions of granitoids. Granitic intrusive activity occurred intermittently in the area. In the eastern margins of the area, diorites and dioritic porphyries were intruded during the period 24–19 Ma, and in the western margins of the area similar rocks were emplaced at 7.2–6.0 Ma. Numerous quartz–porphyry or dacite dikes were also emplaced at 11–8 Ma

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Fig. 2. Detailed geological map of the study area. Sample names beside filled circles are discussed in the following figures and tables. Mineralization from A to D is as follows: (A) glassy veins with copper mineralization, (B) Cu–Pb–Zn quartz veins, (C) disseminated Cu–Mo mineralization, and (D) gold mineralization.

around the granitic complex (Osawa et al., 1981). The details of the distribution of granitic rocks, with K–Ar dating results from MITI (1986), are shown in Fig. 1. 3. Petrology and mineralization of the granitoids 3.1. Petrology of the granitic complex According to the strict classification of felsic plutonic rocks, most of the Cretaceous, Paleogene, and Neogene granitoids in NE Japan are granodiorites in terms of their compositions, and according to microscopic and XRD analysis. The details of our methods of XRF analysis follow Kimura and Yamada (1996). Many dark enclaves of quartz syenite to adamerite composition are found in the marginal parts of individual granodioritic intrusions, and these are thought to have been derived from the parental magma of the granodiorite. The Paleogene and Neogene granodioritic rocks can be divided into three different types on the basis of pet-

rography. The first is a holocrystalline granodiorite characterized by large hornblende crystals, and it is mainly distributed in the upstream area of Koaizawa (KIZ) and the downstream area of Ohmizuhata (OMH) catchments, as shown in Fig. 2. The second is a granodiorite porphyry, which is distributed along the northern and southern margins of the holocrystalline granodiorite. The boundaries between the holocrystalline and porphyritic rocks are transitional. The third type is quartz porphyry, and it can be found in the marginal part of the Ohmizuhata granodiorite porphyry as dikes that were extruded from the granodiorite porphyry, and some of them appear to be pegmatitic dikes because of the presence of graphic intergrowths and perthite. Almost all the modal compositions of these rocks plot in the granodiorite field in the IUGS classification (after Streckeisen, 1974), as shown in Fig. 3a. The rocks are made up of quartz, plagioclase, hornblende, and some K-feldspar, in descending order of volumetric importance. Biotite was found in only one specimen of the Ohmizuhata granodiorite porphyry. Hornblende is com-

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Fig. 3. Modal and normative compositions of granitoids and enclave (a) Modal compositions based on the Q–A–P diagram (IUGS classification after Streckeisen, 1974). (b) Normative compositions using CIPW norms (Blundy and Cashman, 2001). Modal compositions are estimated under the microscope, assisted by XRD data. Enclave data are excluded from the normative diagram because of their mafic compositions. Numerals on the normative diagram refer to isobars in MPa. Abbreviations: KIZ = Koaizawa area; OMH = Ohmizuhata area.

monly altered to chlorite, and some of the plagioclase is affected by sericitic alteration. The three different rock types of holocrystalline granodiorite, granodiorite porphyry, and quartz porphyry are identified based on variations in the proportions of their minerals and their textures. A plot of norms on the granite system of Blundy and Cashman (2001) suggest that the KIZ granodiorite-adamerite was emplaced at somewhat deeper levels than the OMH granodiorite porphyry. The granodioritic rocks (KIZ granodiorite-adamerite in Fig. 3b) lie on the 100–200 MPa cotectic line, whereas the granodiorite porphyries (OMH (granodiorite porphyry)) lie along the 50–100 MPa cotectic line. The quartz porphyries (OMH (quartz porphyry) in Fig. 3b) also plot in a similar area, between the 100 and 200 MPa cotectic lines. The SiO2 versus FeO(t)/MgO diagram, after Miyashiro (1974), shows that all the granitic rocks, including their mafic enclaves, are calk-alkaline (Fig. 4). The fact that the quartz porphyry plots within the tholeiite field may simply be the result of enrichment in iron sulfides due to mineralization. Harker diagrams (Fig. 5), where the mafic enclaves and the host granitic rocks are connected by tie lines, indicate that all granitoid members form a linear trend due to some genetic relationship. The decreases in major oxides, except Na2 O and K2 O, with increasing SiO2 , would be the result of fractional crystallization. However, the relationships between the enclaves and host granites for Na2 O and K2 O show a retrograde tendency against increasing SiO2 , which could be explained by Na2 O easily escaping from the host rocks during plagioclase alteration, and K2 O being an incompatible element in shallow-level granitic rocks.

3.2. Mineralization Four distinctive types of mineralization are recognized in and around the granodioritic complex (Fig. 2), as follows. (A) Weak copper mineralization accompanied by glassy veins (mentioned later) is found in the holocrystalline granodiorite. Tiny chalcopyrite grains occur along the grain boundaries of minerals in the holocrystalline rocks, and along the margins of glassy veins. The bulk copper assays are up to 100 ppm, slightly higher than background levels. (B) Small but very high-grade Cu–Pb–Zn quartz veins occur where the granodiorite porphyry is in contact with the holocrys-

Fig. 4. Magmatic trends shown on the SiO2 vs. FeO(t) /MgO diagram. Arrowed lines show a couple of holocrystalline rocks and their enclaves. The dotted line between the calc-alkaline and tholeiite fields is from Miyashiro (1974). Abbreviations: Enc = enclave; GD/DP = granodiorite and diorite porphyry; QP = quartz porphyry. FeO(t) is FeO total as Fe2 O3 . Abbreviations: KIZ: Koaizawa area; OMH: Ohmizuhata area; GD: granodiorite; DP: granodiorite porphyry; QP: Quartz porphyry.

talline granodiorite. More than 10 quartz veins, some up to 10 cm wide, contain 3–5 wt% Cu and 9–27 wt% Zn. It is noteworthy that several characteristics of these veins, such as orientation, density, and formation temperature, are common to both the quartz and glassy veins mentioned above. (C) Cu–Mo mineralization in the OMH (Ohmizuhata) granodiorite porphyry is economically important. Disseminated zones of mineralization with 0.3–0.6 wt% Mo and 0.1–0.2 wt% Cu were intersected sporadically over the 800-m length of a previous drill hole (MITI, 1986). Chalcopyrite and molybdenite are concentrated in the fracture zones of the granodiorite porphyry. (D) Gold mineralization is also important in the quartz porphyry of the Ohmizuhata area, and assays show 0.8–2.7 g/ton

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Fig. 5. Harker diagrams for the major elements. The arrowed lines and abbreviations are as in Figs. 3 and 4.

Au in zones ranging from 1 to 5.4 m in width in the strongly silicified parts of the quartz porphyry. Evidence of hydrothermal activity associated with this mineralization can be observed, and it is considered to represent a fossil of geothermal activity in and around the Paleogene to Neogene granite–porphyry system.

to have taken place under boiling conditions, because precipitation of quartz occurred simultaneously with very weak chemical reactions during extremely short periods of activity, such as a phreatic episode or earthquake. 4.3. Glassy veins

4. Veinlets The granodiorites, granodiorite porphyries, and quartz porphyries all contain several types of veins. Fig. 6 shows representatives of these veins, and among these we were able to recognize three types: quartz veins, hydrothermal breccia veins, and glassy veins, as described below. 4.1. Quartz veins The quartz veins are generally planar, and they are filled mostly with quartz (Fig. 6a). Their widths range from 5 to 20 mm, and the veins cut the foliation of the granitoids. The simple tabular shapes of the quartz veins indicate typical brittle behavior which was involved in their formation. The quartz veins can be observed in all the granitoid types. 4.2. Hydrothermal breccia veins Hydrothermal breccia veins are mainly found in the porphyritic rocks as discordant bodies with widths of 50–100 mm. Brecciated material includes fragments of the host rock, and the fragments are angular (Fig. 6b). Fragment sizes vary widely, ranging from several centimeters to a few millimeters. The observations indicate that brecciation (in other words, brittle failure) of the host rock occurred during hydrothermal activity. The fact that the corners of the fragments retain their angularity indicates that the associated hydrothermal solutions had only a weak chemical reactivity, because evidence of dissolution was not observed. Additionally, angularity of fragments suggests minimal transport, which means ‘in situ’ brecciation. Apart from the angular fragments, the hydrothermal breccia veins are filled mostly with quartz. Davies et al. (2008) classified several types of hydrothermal breccias and veins as tectonic breccias, fault breccias, hydraulic breccias, hydraulic implosion breccias, phreatic breccias, and hybrid breccias. The mechanism of formation of the hydrothermal brecciation in our study area is not clear, but we consider it

Dark gray to black glassy veins are found mainly in the granodiorites. Some tiny examples range in width from 1 to 10 mm, but most are 50–100 mm in width, with a preferred orientation. Some of the main glassy veins also appear to have been injected into the host rock, and these field observations suggest that they could be viewed as pseudotachylites (Toyoshima, 1990); nevertheless, their mechanism of formation remains unclear. We continue, therefore, simply to describe this type of vein as a “glassy vein”. Glassy veins cut across mafic and fine-grained enclaves, as shown in Fig. 6c, and the glassy veins are cut by quartz veins. The timing of the formation of the hydrothermal breccia veins remains unclear, except that these veins cut the granodiorite. Altogether, these observations indicate the following order of formation and depths for the veins. The glassy veins were the first to be formed at relatively deep levels; the hydrothermal brecciation veins were then formed at moderate depths; and the quartz veins were the last to form during hydrothermal activity. 5. Fluid inclusion microthermometry We prepared doubly polished, 100 ␮m thick plates of the glassy and quartz veins that cut the granodiorites, granodiorite porphyries, and quartz porphyries. All of fluid inclusions show two phase and primary properties. The sizes of the inclusions are always less than 10 ␮m so that the salinity of the fluids could not be measured. Homogenization temperatures, Th , were measured using a Linkam heating stage. Fig. 7 shows histograms of Th for two-phase fluid inclusions in various kinds of veinlets. The median Th of the glassy veins was 343 ◦ C, and the Th in quartz veins in granodiorites with Cu–Pb–Zn polymetallic mineralization was 330 ◦ C. In contrast, Th values in the porphyritic rocks were relatively low, so that the Th in quartz veins in granodiorite porphyries with disseminated Cu–Mo mineralization was 246 ◦ C (median value), and the Th in quartz veins in quartz porphyries with gold mineralization was 245 ◦ C (median value). Higher values of Th were obtained in glassy and quartz veins that cut granodiorites (A & B in Fig. 7) and

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Fig. 6. Occurrence of veinlets. (a) Quartz vein (indicated by pencil) in quartz porphyry. (b) Hydrothermal breccia vein in granodiorite porphyry. (c) Glassy vein and fine-grained mafic enclave in granodiorite. (d) Glassy vein.

6. Formation temperatures and pressures of the veinlets 6.1. Methodology

Fig. 7. Homogenization temperatures for the fluid inclusions. The half-tone bars are data excluded from further consideration because of their isolated positions. A–D indicate the veins types described in Figs. 2 and 11.

lower values were found in quartz veins that cut porphyritic rocks (C & D in Fig. 7). Those bimodal populations (higher values in A and B, lower values in C and D) indicate that two different processes occurred. Higher temperature fluids were captured within the glassy veins and lower temperature fluids were trapped in the quartz veins. Homogenization temperatures should be converted to adequate pressure conditions. It was not possible to calculate pressure corrections using the salinity of the fluid inclusions in this case, but the highest median value (343 ◦ C) for the glassy veins is considered to represent >500 ◦ C under pressures of 200 MPa (∼7–8 km in depth, which is explained later) (Bodnar and Viryk, 1994; Bodnar et al., 1985).

The chemical compositions of the minerals in the granitoids and the veins were analyzed using an electron probe micro analyzer (EPMA; JEOL JXA-8200) in the Graduate School of Environmental Studies, Tohoku University, Japan. For plagioclase and amphibole, the accelerating voltage, beam current, and beam diameter were set at 15 kV, 12 nA, and 1–5 ␮m, respectively. For analyzing Ti concentrations in quartz, an accelerating voltage of 20 kV, a beam current of 100 nA, and a beam diameter of 5 ␮m were used (cf. Huang and Audétat, 2012). Peak titanium was measured at 300 s and background Ti at 150 s, using a PETH crystal, whereas Al and Si were measured at 10 s peak and 5 s background using TAP crystals. Rutile, K-feldspar, and wollastonite were used as standard materials for Ti, Al, and Si, respectively (Table 1). The temperatures were estimated by applying the geothermometer of Holland and Blundy (1994) for hornblende–plagioclase pairs in the host rocks and veins, or by using plagioclase inclusions in hornblende. The pressures were determined using the Al-in-hornblende geobarometer (Schmidt, 1992), with the pressure-dependent calibration from Anderson and Smith (1995). The temperatures of quartz growth were estimated by using the Ti-in-quartz geothermometer (Wark and Watson, 2006). The pressure-dependent calibration of Huang and Audétat (2012) is suitable for the range of pressures relevant to our study.

6.2. Hornblende–plagioclase pairs The granodiorites are made up of quartz, plagioclase, hornblende, and small amounts of K-feldspar. Black glassy veins are composed of hornblende, quartz, plagioclase, and K-feldspar, together with magnetite, rutile, and titanite as accessories. We applied the hornblende–plagioclase geothermometer to pairs of hornblende and plagioclase in the host rocks. Unfortunately, the grain sizes of those pairs in the glassy veins were too fine for determining the chemical compositions. Table 2 lists the chemical

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Table 1 Representative analyses of whole rock chemical composition by XRF. Ser. No.

Sample name.

1 KIZ 2 KIZ 3 KIZ 4 KIZ 5 KIZ 6 KIZ 7 KIZ 8 KIZ 9 OMH 10 OMH 11 OMH 12 OMH 13 OMH 14 OMH 15 OMH 16 OMH 17 OMH

807-02 807-02 822-FL 822-FL 822-02 906-FL 822-01 807-04 807-06 807-FL 807-FL 906-06 906-06 807-02 906-03 727-01 906-02

Remarks Enc Hlc Enc Hlc DC GD GD DP DP Enc Hlc Enc Hlc DP QP QP QP

dykewith glassy vein Hb rich with quartz vein

altered aplite mineralized pegmatite

SiO2

TiO2

Al2 O3

Fe2 O3

55.30 61.87 52.42 63.16 78.25 62.46 64.14 74.45 75.90 65.50 66.36 66.81 71.26 70.43 74.96 74.47 76.66

0.74 0.66 0.75 0.47 0.18 0.56 0.53 0.37 0.09 0.54 0.45 0.50 0.38 0.36 0.09 0.33 0.10

17.14 15.13 17.24 16.35 11.50 16.23 16.77 11.14 12.39 15.52 15.6 16.36 15.33 14.62 12.28 13.90 12.05

9.37 7.63 10.24 6.10 1.72 7.04 6.74 4.36 2.28 6.03 5.19 4.58 3.33 3.94 1.37 3.86 1.92

MnO 0.23 0.18 0.21 0.10 0.03 0.14 0.10 0.09 0.03 0.15 0.08 0.10 0.07 0.05 0.02 0.11 0.02

MgO 3.95 3.21 4.28 2.21 0.25 2.59 0.10 0.09 0.03 1.86 2.01 2.16 1.49 0.92 0.11 0.1 0.02

CaO 6.01 5.31 7.66 5.39 0.14 4.92 5.58 0.03 0.19 2.47 4.26 1.47 1.06 2.12 0.16 1.28 0.21

Na2 O

K2 O

P2 O5

Total

3.10 2.72 3.77 2.92 2.08 3 3.48 0.01 3.88 5.29 3.17 4.03 3.99 3.65 4.00 4.09 3.98

2.89 1.62 2.29 2.09 4.06 1.72 1.68 6.31 4.54 2.80 1.99 2.61 3.03 2.89 4.63 1.77 4.71

0.20 0.14 0.15 0.10 0.08 0.17 0.12 0.08 0.08 0.17 0.10 0.29 0.10 0.10 0.08 0.08 0.08

98.94 98.47 99.00 98.88 98.30 98.83 99.24 96.93 99.40 100.33 99.21 98.91 100.03 99.08 97.70 99.99 99.74

FL: float, Enc: enclave, Hlc: holocrystalline rock, GD: granodiorite, DP: diorite porphyry, Dc, dacite, QP: quartz porphyry

Fig. 8. Equilibrium calculations using the hornblende–plagioclase geothermometer (Holland and Blundy, 1994) and the Al-in-hornblende geobarometer (Anderson and Smith, 1995; Schmidt, 1992). Plagioclases have Ab > 0.7. Hrn: hornblende, Pl: Plagioclase, ed: edenite, tr: tremolite, ri: richterite, H&B94: Holland and Blundy, 1994; AD&SM: Anderson and Smith, 1995; Schmidt92: Schmidt, 1992. Each line shows calculated result by using individual geothermometry and geobarometry.

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Table 2 Representative microprobe analyses of hornblende and plagioclase for sample 807-07. Temperatures for the hornblende and plagioclase pair were calculated on the basis of the geothermometer of Holland and Blundy (1994). Pressures were estimated using the Al-in-hornblende geobarometer (Anderson and Smith, 1995; Schmidt, 1992); cf. Fig. 8. KIZ 807-07 Amp2 p33

KIZ 807-07 Amp2 p35

SiO2 46.06 0.44 TiO2 7.61 Al2O3 18.29 FeO 0.64 MnO 11.05 MgO 11.88 CaO Na2 O 0.82 K2 O 0.71 0.04 Cl2 0.00 F2 97.54 total Cations normalized to 13 for Si + Ti + Al + Fe + Mn + Mg 6.83 Si4+ 0.05 Ti4+ 1.33 Al3+ 0.77 Fe3+ 2.27 Fe2+ Mn2+ 0.08 2.44 Mg2+ 2+ 1.89 Ca 0.23 Na+ 0.13 K+ total 16.02 Anions 1.99 OH Cl 0.01 F 0.00 KIZ 807-07 Pl p31 SiO2 TiO2 Al2 O3 FeO MnO MgO CaO Na2 O K2 O total Cations per 8 oxygens Si4+ Ti4+ Al3+ Fe2+ Mn2+ Mg2+ Ca2+ Na+ K+ total Xab Xan Xor P-T estimates

KIZ 807-07 Amp2 p37

KIZ 807-07 Amp3 p43

46.42 0.57 7.61 17.99 0.67 11.35 11.97 0.97 0.75 0.08 0.00 98.39

46.86 0.60 7.24 17.32 0.69 11.74 11.94 0.76 0.65 0.12 0.00 97.91

47.08 1.45 7.75 16.98 0.50 11.63 11.50 1.11 0.65 0.11 0.00 98.77

6.83 0.06 1.32 0.70 2.21 0.08 2.49 1.89 0.28 0.14 16.01

6.89 0.07 1.25 0.73 2.13 0.09 2.57 1.88 0.22 0.12 15.95

6.86 0.16 1.33 0.62 2.07 0.06 2.53 1.79 0.31 0.12 15.84

1.98 0.02 0.00

1.97 0.03 0.00

1.97 0.03 0.00

KIZ 807-07 Pl p36

KIZ 807-07 Pl p38

KIZ 807-07 Pl p45

62.14 0.00 23.62 0.20 0.04 0.00 4.95 8.83 0.45 100.24

64.48 0.00 23.09 0.24 0.02 0.00 4.04 9.35 0.31 101.53

61.61 0.00 24.39 0.50 0.00 0.01 5.87 8.56 0.32 101.27

61.71 0.00 24.20 0.23 0.09 0.00 5.46 8.71 0.27 100.67

2.76 0.00 1.23 0.01 0.00 0.00 0.24 0.76 0.03 5.02 0.74 0.23 0.02

2.81 0.00 1.19 0.01 0.00 0.00 0.19 0.79 0.02 5.00 0.79 0.19 0.02

2.71 0.00 1.27 0.02 0.00 0.00 0.28 0.73 0.02 5.03 0.71 0.27 0.02

2.73 0.00 1.26 0.01 0.00 0.00 0.26 0.75 0.02 5.02 0.73 0.25 0.01

Amp Pl

Amp2 p33 Pl p31

Amp2 p35 Pl p36

Amp2 p37 Pl p38

Amp3 p43 Pl p45

T [◦ C] P [kbar]

686 3.29

685 3.23

685 2.94

691 3.27

compositions of the hornblende–plagioclase pairs in the granodiorites, and Fig. 8 shows the calculation of equilibrium temperatures and pressures based on the hornblende–plagioclase geothermometer (Holland and Blundy, 1994) and the Al-in-hornblende barometer (Anderson and Smith, 1995; Schmidt, 1992). The possible ranges of temperature and pressure of the host granodiorites lie within 650–700 ◦ C and 3–3.5 kbar (∼300 MPa), respectively. Based on the assemblages of normative minerals described above, the granodiorites (KIZ granodiorite–adamerite on Fig. 3b) plot on the 100–200 MPa cotectic line. The hornblende–plagioclase pair indicated relatively higher pressure

condition due to early crystallization in granitic magma. Pressure estimates can be related to emplacement depths for the granitoids (Bando et al., 2003), so that the depth of emplacement in our case might be ∼200 MPa (7–8 km depth at ␳ = 2.6). 6.3. Ti in quartz Table 3 shows the titanium contents of the quartz and estimated temperatures. Fig. 9a shows a glassy vein in granodiorite. It is difficult to identify direct evidence for shearing inside granodiorite, however, brittle deformation (shearing) should have occurred

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Fig. 9. (a) Photomicrograph of granodiorite including a glassy vein, and the area of an SEM image is indicated. (b) SEM image of quartz–K-feldspar aggregate in the glassy vein. Dark spots indicate the analytical point for Ti-in-quartz. Numbers with underlines show the titanium contents of the quartz, which all have zero values for this figure. (c) SEM image of the black part of the glassy vein. Black spots indicate analytical points for Ti-in-quartz, underlined numbers indicate titanium contents in quartz, and numbers in parenthesis are temperatures estimated using the method of Huang and Audétat (2012). (d) SEM image of primary quartz in granodiorite. Holes show analytical points for Ti-in-quartz, numbers with underlines indicate titanium contents in quartz, and numbers in parenthesis are temperatures estimated using the method of Huang and Audétat (2012). (e) SEM-CL image of the same area as in (d). Microfractures filled with quartz can be seen.

under differential pressure conditions (see Fig. 6d). Fig. 9b shows an SEM image of quartz (relatively dark) and K-feldspar (relatively bright) between tiny black veins. The irregularly shaped quartz is surrounded by K-feldspar with a mosaic texture. Dark spots indicate the analytical points for Ti in the quartz; however, this particular quartz contains no Ti. Fig. 9c shows an SEM image of a tiny black vein, and the relatively dark grains are quartz. The estimated temperatures for this quartz, as deduced from its Ti content, range from 646 to 787 ◦ C. Taking into account the equilibrium temperature of the hornblende–plagioclase pairs in the host rock (∼700 ◦ C), the formation temperature of the glassy vein is considered to be 650–700 ◦ C. In contrast, the Ti content of the quartz in the quartz–K-feldspar shown in Fig. 9b was zero, and we can put forward two reasons for this. One is that the fluid precipitating the quartz was depleted in titanium, and the other is that the temperature was less than 600 ◦ C, which is out of the range of the

Ti-in-quartz geothermometer (Wark and Watson, 2006). According to the observed textures (irregularly shaped quartz surrounded by a mosaic of K-feldspar), the quartz–K-feldspar aggregates in the glassy veins might have been precipitated from solutions at temperatures less than 600 ◦ C. Fig. 9d shows an SEM image and analytical points for Ti in quartz in a host rock. Some points showed more than 100 ppm Ti, and no Ti could be observed in the adjacent zone. Fig. 9e is an SEMCL image that shows quartz with brittle failure. Comparing Fig. 9d (SEM image) and e (SEM-CL image), the high-Ti points are in the bright CL zone, and the zero-Ti points are weakly luminescent zones of interstitial quartz filling microfractures. Oscillatory zoning in the CL image has been interpreted as reflecting quartz dissolution and precipitation due to oscillations in pressure (Rusk and Reed, 2002). Batkhishig et al. (2005) reported heterogeneous SEM-CL images of epithermal quartz that reflected complex hydrothermal events.

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Table 3 Representative microprobe analyses of titanium in quartz. Temperatures were calculated on the basis of Huang and Audétat (2012). Sample name KIZ KIZ KIZ KIZ KIZ KIZ KIZ KIZ KIZ KIZ KIZ KIZ KIZ KIZ KIZ KIZ KIZ KIZ KIZ KIZ KIZ KIZ a b

807-07 807-07 807-07 807-07 807-07 807-07 807-07 807-07 807-07 807-07 807-07 807-07 807-07 807-07 807-07 807-07 807-07 807-07 807-07 807-07 807-07 807-07

SiO2 qtz e1 p2 99.66 qtz e1 p19-1 98.88 qtz e1 p19-2 99.41 qtz e2 p3 99.33 qtz e2 p7 98.14 qtz e2 p10 100.05 qtz e2 p11 99.57 qtz2 line1 94.84 qtz2 line2 98.29 qtz2 line3 99.15 qtz2 line4 99.86 qtz2 line5 99.39 qtz2 line6 99.85 qtz2 line7 102.25 qtz2 line8 99.51 qtz2 line9 99.89 qtz2 line10 99.73 qtz2 line11 99.51 qtz2 line12 99.17 qtz2 line13 99.99 qtz2 line14 99.99 qtz2 line15 99.50

Al2O3 Total

Ti [ppm] SDa b

0.00 99.66 N.D. 0.01 98.89 N.D. 0.01 99.41 N.D. 0.05 99.40 117 1.39 99.53 35 0.01 100.07 93 0.01 99.59 125 0.01 94.86 58 0.04 98.33 N.D. 0.00 99.15 23 0.00 99.86 32 0.00 99.39 N.D. 0.00 99.85 N.D. 0.01 102.27 95 0.00 99.51 34 0.00 99.90 36 0.00 99.74 26 0.00 99.52 64 0.01 99.19 N.D. 0.00 99.99 N.D. 0.02 100.01 N.D. 0.05 99.56 N.D.

– – – 5 5 5 5 5 – 5 5 – – 5 5 5 5 5 – – – –

T [◦ C] SD – – – 779 646 751 788 696 – 606 636 – – 753 642 646 616 707 – – – –

– – – 5 13 6 5 9 – 18 14 – – 6 13 13 17 8 – – – –

Standard deviation. Not determined.

The higher contents of titanium indicated igneous temperatures and primary quartz, and the absence of titanium in the interstitial quartz indicated a hydrothermal event associated with brittle failure under relatively low temperatures (<550 ◦ C) in Fig. 9d and e. Fig. 10 shows the distribution of Si, Ti, Al, Fe, Mg, Ca, Na and K in the granodiorite, including the glassy vein shown in Fig. 9a. Higher X-ray intensities for Si, Al, Fe, Mg, and Ca were recognized in the glassy vein. In particular, the Ca intensity was higher than either the Na or K intensity. These observations indicate that the glassy vein contains a Ca amphibole and epidote. The timing of formation and the relationships between the quartz–K-feldspar zone in the glassy vein and the interstitial quartz in the host grain (quartz) are not totally clear. However, they were not formed at temperatures lower than the quartz veins. Taking into account the analysis of fluid inclusions, the formation temperature of the quartz–K-feldspar zone in the glassy vein is considered to be 500–550 ◦ C, and the interstitial quartz in the host granodiorite might have formed at the same temperatures (Fig. 11). 7. Discussion 7.1. Supercritical geothermal reservoir In the classical geological sense, a “Deep-Seated Geothermal Reservoir” (DSGR) cannot exist above the plastic temperature of the reservoir rock (>400 ◦ C, depending on the rock type) because connecting fractures are absent in plastically deformable rocks, and the convective transport of fluids under ductile conditions is therefore expected to be weak. The permeability of the Earth’s crust largely governs important processes such as the advective transport of heat and fluid (Cox, 2010; Ingebritsen, 2012; Ingebritsen and Manning, 2010), and the permeability of the Earth’s crust is extremely heterogeneous, ranging from 10−23 m2 for intact crystalline rocks to 10−7 m2 for well-sorted gravels (Ingebritsen and Manning, 2010). Weis et al. (2012) described permeability changes that reflected temperatures and pressures of rocks on and inside the magmatic fluid plume of a porphyry copper system. Crustal-scale permeability shows a dynamic behavior, and Bando et al. (2003) noted that

volcano–plutonic complexes have the potential to satisfy the criteria necessary for the development of an artificial DSGR. Our study of fluid inclusion microthermometry and the petrological analysis of several kinds of veinlets in a granite–porphyry system have provided us with the following scenario for the development of a supercritical geothermal reservoir. Magmatic fluids moved through a hot granitoid intrusive body, which heated the host rock mass via conduction. Magmatic fluids associated with volatile material ascended under a lithostatic pressure regime. The granodiorite was emplaced at around 7–8 km depth, and the glassy veins formed around the top of the granodiorite (∼5 km depth). The glassy veins contained a great deal of water at a temperature of 500–550 ◦ C, and formed hornblende rather than biotite. The pressure regime was still lithostatic. However, when a magmatic fluid (or a supercritical geofluid) crosses the transition zone between magmatic and meteoric fluid regions, the pressure drops from lithostatic to hydrostatic conditions, and the temperatures drop from 550 ◦ C to less than 350 ◦ C within a very narrow range of depths. This is an episodic event like an earthquake, and Weatherley and Henley (2013) noted that flash vaporization, consequent on seismic slip, may underpin the formation of most gold deposits, including porphyry copper–gold deposits. Flash vaporization is a consequence of extreme pressure changes due to slip events in fault arrays. Rapid decompression produced an adiabatic expansion of overpressured fluids in the porphyritic rock mass. The vapor phase fluid has very weak chemical reactivity in terms of dissolution, but precipitation could easily occur by flash vaporization. The hydrothermal breccia veins formed as a result of adiabatic expansion due to slip sliding to form the jogs, and the lithostatic pressure regime can be connected with the hydrostatic pressure regime by flash vaporization. As fluid temperatures drop rapidly, the solubility of silica is drastically reduced (Fournier, 1991, 1999; Lowell et al., 1993; Manning, 1994; Rimstidt and Barnes, 1980; Akinfiev and Diamond, 2009; Saishu et al., 2012, 2014). Therefore, we propose that the hydrothermal breccia veins and the quartz veins in the porphyritic rocks were formed after such a connection of the lithostatic and hydrostatic pressure regimes. The glassy veins were formed under lithostatic pressure regime and were potential reservoirs of supercritical geothermal fluids, and then isolated supercritical fluids were exposed to hydrostatic pressure condition by flash vaporization, forming hydrothermal breccia veins. Mercer and Reed (2013) described a thermal profile for the Butte porphyry copper deposit (Montana, USA) that mimics an irregular pattern following active fractures at any given time and evolves by discrete cycles of dynamic, transitory, hightemperature hydrofracturing, fluid release, and vein formation that overprints cooler host-rock temperatures. They found that a magmatic-hydrothermal continuum represented in hydrothermal veins, ranging from ∼710 ◦ C to <440 ◦ C. Our study indicated that upper limit of formation temperature of the glassy veins ranges from 650 ◦ C to 700 ◦ C, and then supercritical fluids were trapped around 500 ◦ C to 550 ◦ C. Activities of supercritical fluids within almost same temperature range were recorded in both cases. No economically viable porphyry copper deposits have been found in Japan. Here, in the Kowaizawa–Ohmizuhara area, there is evidence of Cu–Pb–Zn mineralization, including gold mineralization, but the area is still very much in the exploration stage in terms of finding an economic deposit. However, centers of natural porphyry copper associated with mesothermal and epithermal deposits either evolved from a supercritical geothermal system to a conventional and subcritical geothermal system as they cooled, or they maintained a conventional geothermal system above the heat center throughout their lives at shallow levels where temperatures were ≤350 ◦ C.

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Fig. 10. X-ray intensity maps for the elements Si, Ti, Al, Fe, Mg, Ca, Na, and K for the boxed area in Fig. 9a with ROYGBIV scale. R is high and Vis low. The glassy vein shows relatively high intensities of Si, Al, Fe, Mg, and Ca.

Fig. 11. (a) Schematic model of the granite–porphyry system associated with mineralization in the study area. (b) P-T evolution of a supercritical geothermal reservoir. A–D in (a) correspond to mineralization and vein types shown in Fig. 2. The solid line in (b) indicates the transition of pressure regimes in the case of a geothermal gradient of 10 ◦ C/100 m. The study area, described in the main text, showed almost the same geothermal gradient. The emplacement depth of the granodiorite was ∼7 km, at 200 MPa, and fluids ascended to the top of the porphyritic rocks. The glassy veins were formed at temperatures of 500–550 ◦ C at ∼5 km depth (), 1 and then episodic flash vaporization was accompanied by earthquakes and related phenomena. Hydrothermal breccia veins were formed during the flash vaporization (), 2 and the pressure regime shifts from lithostatic conditions to the hydrothermal regime. The pressure of the geothermal fluid changed along the hydrostatic line, and then quartz veinlets were formed at 200–250 ◦ C () 1 ’, 2 and ’ 3 on the figure. 3 and ∼2.5 km depth. In the case of a geotherm of 15 ◦ C/100 m, each event shifts to a relatively shallow depth, indicated by ’,

7.2. EGS technology for supercritical geothermal reservoirs The critical point for pure water is at 374 ◦ C and 22 MPa (Sekiguchi et al., 2013), and the critical point shifts to higher temperatures and pressures in the case of brine (Bodnar et al., 1985), and moves to lower temperatures and higher pressures in an H2 O–CO2 system (Diamond, 2001). Tsuchiya and Hirano (2007) noted that the bottom of the WD-1a well in the Kakkonda geothermal field was in a supercritical state. Temperature and

depth (pressure) conditions in a supercritical geothermal reservoir strongly depend on the geothermal gradient. In the case of a gradient of 10 ◦ C/100 m, the possible depth for a supercritical condition is ∼4000 m, and it is <3000 m in the case of a gradient of 15 ◦ C/100 m. Important EGS technological problems that need to be solved in the development of a supercritical geothermal reservoir are as follows. How can one drill to touch and penetrate the supercritical region in the subsurface? How can one create fractures under “beyond brittle” conditions? And how can one control the induced seismicity in

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a supercritical reservoir? Technological advances in these areas are essential even for EGS in a conventional geothermal reservoir. The most important issue is the creation of fractures in “beyond brittle” rock masses under supercritical conditions. The stockwork of veins in granite–porphyry systems provides some hints for creation of fracture clouds in supercritical geothermal reservoirs. 8. Conclusions The systematic distribution of veins in a granite-porphyry system was investigated in order to understand geological properties of a supercritical geothermal reservoir. Coupled with petrological and mineralogical investigations (SEM, EPMA, SEM-CL and fluid inclusion), we demonstrate the evolution of natural hydrothermal fracturing to form several kinds of veinlets (quartz veins, hydrothermal breccia veins and glassy veins) in the rock mass under super- and sub-critical conditions. The glassy veins are interpreted to have formed in the supercritical fluid reservoir at 500–550 ◦ C under lithostatic pressures, and then pressures dropped drastically. The solubility of silica also dropped, and the quartz veins formed under hydrostatic pressures. Connections between the lithostatic and hydrostatic pressure regimes were key to the formation of the hydrothermal breccia veins. A supercritical geothermal reservoir has great advantages compared with conventional geothermal systems, including highentropy fluids and weak chemical reactivity (Tsuchiya and Hirano, 2007). Granite–porphyry systems can provide important lessons regarding the nature and development processes of supercritical geothermal activities, and they represent possible candidates for natural analogs of supercritical geothermal reservoirs. Vein stockworks and their evolution illustrate the integrated history of fracture networks and fluid connectivity in these system. Acknowledgments This work was supported by a Grant-in-Aid for Specially Promoted Research, Grant Number 25000009. Our study benefited from discussions with Prof. Mark Reed (University of Oregon, USA), and research members of JBBP (Japan Beyond Brittle Project). The authors would like to say thanks to anonymous reviewers and guest editor (Dr. Patrick Dobson) for variable comments to revise the manuscript. References Akinfiev, N.N., Diamond, L.W., 2009. A simple predictive model of quartz solubility in water-salt-CO2 systems at temperatures up to 1000 ◦ C and pressures up to 1000 MPa. Geochim. Cosmochim. Acta 73, 1597–1608. Anderson, J.L., Smith, D.R., 1995. The effects of temperature and fO2 on the Al-in-hornblende barometer. Am. Mineral. 80, 549–559. Asanuma, H., Muraoka, H., Tsuchiya, N., Ito, H., 2012. The concept of the Japan Beyond-Brittle Project (JBBP) to develop EGS reservoirs in ductile zones. GRC Trans. 36, 359–364. Bando, M., Bignall, G., Sekine, K., Tsuchiya, N., 2003. Petrography and uplift history of the Quaternary Takidani Granodiorite: could it have hosted a supercritical (HDR) geothermal reservoir? J. Volcanol. Geotherm. Res. 120, 215–234. Batkhishig, B., Bignall, G., Tsuchiya, N., 2005. Hydrothermal quartz vein formation, revealed by coupled SEM-CL imaging and fluid inclusion microthermometry: Shuteen complex, South Gobi, Mongolia. Resour. Geol. 55, 1–8. Batkhishig, B., Tsuchiya, N., Bignall, G., 2014. Magmatic-hydrothermal activity in the Shuteen area, South Mongolia. Econ. Geol. 109, 1929–1942. Bodnar, R.J., Burnham, C.W., Sterner, S.M., 1985. Synthetic fluid inclusions in natural quartz: III. Determination of phase equilibrium properties in the system H2 O-NaCl to 1000 ◦ C and 1500 bars. Geochim. Cosmochim. Acta 49, 1861–1873. Bodnar, R.J., Viryk, M.O., 1994. Interpretation of microthermometric data for H2 O-NaCI fluid inclusions. In: De Vivo, B., Frezzotti, M.L. (Eds.), Fluid Inclusions in Minerals: Method and Applications, 1. Short course of the working group (IMA), pp. 17–130. Bons, P.D., 2001. The formation of large quartz veins by rapid ascent of fluids in mobile hydrofractures. Tectonophysics 336, 1–17.

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