Engineering Geology 183 (2014) 80–95

Contents lists available at ScienceDirect

Engineering Geology

j ourna l homepage: www.e lsev ie r .com/ locate /enggeo

Effects of hydrothermal alterations on physical and mechanicalproperties of rocks in the Kuril–Kamchatka island arc

Frolova Julia a,⁎, Ladygin Vladimir a, Rychagov Sergey b, Zukhubaya David a

a Faculty of Geology, Lomonosov Moscow State University, 1 Leninskie Gory, 119992 Moscow, Russiab Institute of Volcanology and Seismology, Far East Division RAS, 9 Piip Boulevard, 683006 Petropavlovsk-Kamchatsky, Russia

⁎ Corresponding author.

http://dx.doi.org/10.1016/j.enggeo.2014.10.0110013-7952/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 October 2013Received in revised form 26 May 2014Accepted 13 October 2014Available online 23 October 2014

Keywords:Hydrothermal systemsAlterationVolcanic rocksPhysical and mechanicalPropertiesKuril–Kamchatka arc

The hydrothermal systems of the Kuril–Kamchatka island arc are hosted in volcanic formations of Neogene–Quaternary age. Thermal water alters volcanic rocks and transforms them into hydrothermal rocks such assecondary quartzites, various propylites, zeolitic rocks, argillic rocks, clays, opalites, and quartz–feldsparmetasomatites. Mineralogical alteration of rocks contributes to changes of their physical and mechanicalproperties. The relationship between type of hydrothermal alteration and rock properties is quite variable anddepends on a number of factors including parent rocks, РТ-conditions, chemical composition and type of fluid,and duration of fluid–rock interaction. The study showed that high-temperature fluids cause consolidation andstrengthening of rock, decrease in their porosity and permeability, and remove hygroscopic moisture. Thistendency is observed independently of fluid chemical composition. Property variations caused by low-temperature subsurface fluids are more complicated and diverse but their “deterioration” prevails i.e. a decreasein density, strength and elastic modulus, formation of secondary porosity, high hygroscopy, softening andswelling in water saturated environment.Special attention is given to the near-surface alteration zones which affects the selection of sites for power plantconstruction and can impact the integrity of their foundation. The subsurface horizon of hydrothermal clays is themost problematic zone due to high porosity, plasticity, hygroscopy, compressibility and occasionally swelling.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Geothermal energy is a renewable, relatively inexpensive, environ-mentally friendly, and a domestic source of heat. It represents bothscientific and practical interests. The former concerns the origin, evolu-tion, and structure of hydrothermal systems as well as ore formationand entrapment. Of practical interest is the exploitation of thermalwater and energy as well as economical mineral and ore extraction.Geothermal energy is presently used for electricity production in 24countries with a total worldwide installed capacity from geothermalpower plants exceeding 10 GWe. More than 70 countries use geother-mal energy for heat supply with direct-used capacity of about 48 GWt

(Lund and Bertani, 2010). Traditionally a suite of different methodsis applied to investigate geothermal fields. Commonly geological,geochemical, hydrogeological and geophysical methods are used tounderstand the dynamic processes controlling the activity withingeothermal fields. This manuscript focuses on physical and mechanicalproperties of rocks and their application in determining the competencyof foundation materials for power plant construction.

Assessment of rock properties and their variation due to alterationprocesses play an important role in the study of hydrothermal systemsand exploitation of geothermal fields. It is most important to know thereservoir properties of host rocks as porosity controls reservoir capacity,and permeability characterizes water transport through the stratum.Density, strength and elastic properties are essential for characterizingthe state of stress in hydrothermal systems. Changes in the stress fieldmay cause either fracturing and formation of secondary permeabilityor a fracture collapse followed by a decrease in permeability. Physicalandmechanical properties provide data that can be used for geothermalreservoir numerical modeling, geophysical log interpretation, drillingoptimization in geothermal fields, and assessment of heat transport inthe reservoir. In addition, understanding of the properties of hydrother-mally altered rocks (mostly weak and clayey) is needed for designingengineering projects such as constructions of power plants, buildings,or pipelines in areas subjected to geothermal processes.

Hydrothermal rocks have beenwell studied in the pastwith respect tomineralogy (Naboko, 1963; Ellis, 1966; Rusinov, 1972; Kristmannsdóttirand Tómasson, 1978; Korzhinsky, 1982; Reyes, 1990; Zharikov andRusinov, 1998 and many others). More than 100 secondary mineralswere discovered in hydrothermal systems. The following hydrothermalfacies were distinguished in various hydrothermal systems: opalites(opal, tridymite, cristobalite, ferric oxides, kaolinite, alunite); argillic

81F. Julia et al. / Engineering Geology 183 (2014) 80–95

rocks (kaolinite, smectite, high-silica zeolites, opal); propylites — low-temperature (chlorite, calcite, quartz, laumontite, prehnite, sericite),medium-temperature (chlorite, quartz, wairakite, albite, epidote, sericite,prehnite), and high-temperature (chlorite, actinolite, quartz, wairakite,epidote, muscovite); and secondary quartzites (quartz, muscovite, gar-net). In spite of the well-studied mineralogy of hydrothermal rocks theirpetrophysical parameters are only relatively poorly known. Under theaction of thermal water, physical and mechanical properties of rockschange widely. Many publications reveal that alteration processes gener-ally result in mechanically weaker rock (Potro and Hürlimann, 2009;Rigopoulos et al., 2010; Coggan et al., 2013) while some other studiespropose more variable property changes (Sigurdsson et al., 2000;Bozkurtoğlu et al., 2006; Lutz et al., 2011; Siratovich et al., 2012;Wyering et al., 2012). Our previous studies have shown that these chang-es depend on a number of factors including parent rocks, РТ-conditions,composition and type of fluid, and duration of fluid–rock interaction(Frolova et al., 1999, 2010, 2011). Depending on factors mentionedabove, volcanic rock can transform to substantially altered hydrothermalrock. An example of significant alteration and “deterioration” of proper-ties is the transformation of hard igneous rocks with brittle failure intoplastic clays with ductile deformation, which is often observed in thenear-surface zone of geothermal fields. In contrast, weakly consolidatedporous tuffs can transform to hard and dense secondary quartzites orhigh-temperature propylites.

The changes of rock properties in geothermal areas, which can benatural or due to geothermal exploitation, may e.g. lead to gradualstructural transformation of the hydrothermal system hydrodynamicand temperature regimes and decrease borehole production that can af-fect geothermal field energy exploitation. In addition, changes in rocks'physical and mechanical properties can furthermore trigger hazardousgeological processes in the geothermal fields such as landslides and hy-drothermal explosions (Figure 2a,b). The most notable example is thecatastrophic landslide in the Geysers Valley on Kamchatka which oc-curred in 2007 (Sugrobov et al., 2009; Kiryukhin et al., 2010). The hostrocks of the hydrothermal system are highly porous pumice-rich tuffs.Thermal water partially transformed them to hydrothermal altered

Fig. 1. Location of the studied hydrothermal systems. 1 — Baransky, 2 — North-Paramush

clayey soils that result in increased slope instability which may finallylead to failure (Figure 2b).

2. Geological setting and geothermal conditions

Russia is rich in geothermal resources. Many regions have hot waterreservoirs with temperatures ranging from 50 to 300 °C at depths from200 to 3000 m (Povarov, 2000). The richest geothermal heat reservoirsoccur in the Kuril–Kamchatka island arc which is located in the north-western segment of theCircum-Pacific belt. These geothermal resourcesare closely associated with active volcanism and its tectonic positionabove a subduction zone. Tens of low- and high-temperature hydro-thermal systems are located in this region. Several types of geothermalmanifestations are present there such as fumaroles, steaming ground,mud pools, geysers, boiling or hot springs. A number of geothermal elec-tric power plants are operating in the region (Mutnovsky, Pauzhetskygeothermal fields, and some fields on the Kurils); but some other fieldsare used for space heating, recreation and green houses.

Most geothermal areas are associated with Pleistocene to Holocenecalc-alkaline volcanoes. The host rocks of hydrothermal systems arevolcanic or volcaniclastic types of Neogene–Quaternary age. They be-come intensely altered to e.g. propylites, zeolitic rocks, argillic rocks,secondary quartzites, opalites, and quartz–feldspar metasomatites.The location of the studied hydrothermal systems is shown in Fig. 1and a brief description of their salient features is given below.

The Baransky hydrothermal system is located on the slope of the vol-cano Baransky in themiddle part of Iturup Island (Southern Kurils). It isa high-temperature hydrothermal system in a progressive stage of evo-lution with temperatures above 300 °C at a depth of 1200m (Rychagovet al., 2005). Two geothermal power plants “Tuman” are in operationwith a total installed capacity of 3.6 MWe. The hydrothermal system ishosted in Pliocene–Pleistocene volcaniclastic rocks interbedded withlavas and occasionally crossed by dykes. A subvolcanic diorite intrusionis assumed to be the heat source of the hydrothermal system. The entirerock section is impacted and altered by thermal fluids. The following al-teration zones are recognized from the surface down to 1200 m depth,

ir, 3 — Koshelevsky, 4 — Pauzhetsky, 5 — Mutnovsky, 6 — Paratunsky, 7 — Essovsky.

Fig. 2. Landslides in hydrothermal clays. a — Mutnovsky geothermal field, b — the Geysers Valley.

82 F. Julia et al. / Engineering Geology 183 (2014) 80–95

i.e. acid sulfate leaching (opal, tridymite, cristobalite, ferric oxides, kao-linite, alunite); argillic rocks (smectite, high-silica zeolites, opal); argillicpropylites (corrensite, illite-smectite, calcite); zeolitic propylites;medium–high-temperature propylites (chlorite, quartz,wairakite, albite,epidote, sericite); and secondary quartzites (quartz,muscovite). The reg-ular vertical zoning is disturbed by “boiling” zones with quartz, adularia,and wairakite. The properties of 188 cores recovered from 11 boreholesinside the geothermal area and 65 cores from a borehole outside thegeothermal area were investigated (Frolova et al., 1999, 2011; Ladyginet al., 2000).

North-Paramushir hydrothermal system is confined to the VernadskyRidge, in the northern part of Paramushir Island (Northern Kurils). Thegeological section down to 2500 m depth consists of Miocene–Pleisto-cene lapilli tuffs and ash tuffites, interbedded with andesites and basal-tic andesites. The hydrothermal system has the following alterationzones: low temperature opalites, low-temperature propylites (chlorite

+ illite + quartz), quartz–adularia metasomatites with sericite,medium-temperature propylites (quartz + chlorite + epidote+ mus-covite), and secondary quartzites (quartz + muscovite + epidote).Temperature reaches 180–250 °C at depths of 1.5–2.5 km. It is assumedthat this is the progressive stage of evolution for the system (Rychagovet al., 2002). The properties of 35 cores from5 boreholes and 20 samplesfrom outcrops were investigated.

Koshelevsky hydrothermal system occupies the southernmost po-sition on the Kamchatka Peninsula and is located on the slope of avolcano of the same name. It is a high-temperature, steam-dominated system presently under investigation (Rychagov et al.,2012). Temperatures reach 260 °C at 1100 m depths (Belousovand Sugrobov, 1976). The host rocks consist of effusive andvolcaniclastic types of Neogene–Quaternary age. Several thermalfields are known within the Koshelevsky volcano; two of those arevery large i.e. — Low- and Upper-Koshevevsky — and differ in

83F. Julia et al. / Engineering Geology 183 (2014) 80–95

their geochemical and thermodynamic conditions. The cover of hy-drothermal clays is characteristic for Low-Koshelevsky field and asurface zone of acid sulfate leaching with opalites is developed inthe Upper-Koshelevsky field. Forty-two samples of andesites andtuffs were studied at near-surface alteration of the thermal fields.

The extensively studied Pauzhetsky hydrothermal system is situatedin South Kamchatka on the slope of Kambalny Ridge inside a volcano-tectonic depression. The first geothermal power station in the USSRhas been operating there since 1967. Presently the installed capacityof Pauzhetskaya geothermal power plant is 11MWe. The hydrothermalsystem is hosted in Neogene–Pleistocene tuffs (from ash to agglomerategrain size) alternating with lava flows and intersected by dykes. Therocks show intense thermal alteration (temperature is up to 180–200 °C), resulting in mineralogical and petrophysical zoning. It is sug-gested that the system is in a regressive stage of development(Rychagov et al., 2005) as seen by a low-temperature assemblagesuperimposing a high-temperature one. The following alteration zonesare distinguished from the surface down to 600 m depth: argillic alter-ation (smectite, high-silica zeolites, opal), zeolitites (laumontite andcorrensite), and propylites (chlorite, calcite, sericite, albite, occasionallyepidote). “Boiling” zones composed of quartz and adularia (occasionallywith wairakite and prehnite), are developed in some fault structures.The system is liquid-dominated where the main reservoir is found inhighly permeable but totally zeolitized lapilli tuffs. The caprock consistsof argillized ash vitric tuffs. A hydrothermal clay horizon is developednear the surface in the thermal fields. We investigated the propertiesof 202 cores recovered from six boreholes inside the geothermal fieldand 21 unaltered samples taken from outcrops located outside the geo-thermal anomalies (Frolova et al., 1999, 2006).

Mutnovsky hydrothermal system is one of the most prospective andthe best studied geothermal fields. It is located approximately70–80 km south of Petropavlovsk-Kamchatsky city. The region hasbeen the site of extensive drilling for geothermal development sincethe middle of the 1970s. Two geothermal power plants are presentlyunder operationwith installed capacities of 12 and 50MWe; they supplyelectricity into the power grid of the Kamchatka Peninsula. This is a high-enthalpy system fluidwith temperatures up to 280 °C (Rychagov, 2005).It is assumed that the system is fracture-dominatedwhere themain pro-duction zone coincideswith a fault (Kiryukhin et al., 2004). The dominantstratigraphy in the geothermal system consists of a complex volcanic se-quence of Oligocene–Miocene toHolocene age. It consists of tuffs, tuffites,breccias, andesites, basaltic andesites, dacites, and ignimbrites. The entiregeological section is intensely altered. The following alteration zonesoccur down to depths of 1000m: sulfate acid leaching (opal, chalcedony,cristobalite, tridymite, alunite, kaolinite, Fe hydroxides), argillic (smec-tite, high-silica zeolites), low-temperature propylites (illite, calcite, chlo-rite), and medium-temperature propylites subdivided into quartz–wairakite–prehnite and quartz–epidote–chlorite metasomatites. Theproperties of 75 samples from four boreholes and 45 samples from sur-face outcrops were investigated ranging from fresh to altered.

Paratunsky hydrothermal system is located 45 km south-west fromPetropavlovsk-Kamchatsky city. Thermal water is used for recreation,district heating, and green houses. The system is controlled by a faultzone and hosted in the Upper Oligocene–Low Miocene volcanic se-quence consisting of tuffs and lava units (basalts and basaltic andesites).Volcanic rocks are altered up to medium–high-temperature propyliteswith epidote, chlorite, albite, quartz, sericite, wairakite, and occasionalactinolite. The properties of 34 cores recovered from five boreholesdown to depth 2200 m were investigated.

Essovsky hydrothermal system is located on the Sredinny Ridge. Thegeological sequence consists of Neogene–Pleistocene tuffs and lavas.There are several hot springs in this region with fluid temperatures upto 95 °C and dissolved solids of 3 g/l (Pilipenko, 1998). The system hasso far only been studied in a preliminary way. Argillized tuffs withsmectites and zeolites were sampled in outcrops. The properties of 23samples were studied.

3. Database and methods of study

3.1. Database

Thedatabase of thehost rocks to the hydrothermal systemshas beenestablished and processed. It contains data obtained from a number ofhydrothermal systems of the Kuril–Kamchatka island arc includingPauzhetsky, Mutnovsky, Koshelevsky, Essovsky, Paratunsky, North-Paramushir, and Baransky. The database consists of three parts: geolog-ical description, petrological characteristics, and rock properties. Thegeological part contains data about rock location, age, and series.The petrological part includes mineral and chemical compositions,structure, secondary mineralogy, hydrothermal facies, and intensity ofalteration. The database comprises various rock types including basalts,andesites, dacites, tuffs, tuffites, and ignimbrites. The intensity ofhydrothermal alteration covers the entire spectrum from fresh to totallyaltered rocks.

About 750 samples from the different hydrothermal systems werecollected, studied andprocessed. Amajority of the sampleswere obtain-ed fromboreholes (down to 2500m)or natural outcrops; a few sampleswere also taken form open pits. In the laboratory each sample was sep-arated into several specimens (from 1 to 6) for physical andmechanicalmeasurements. Specimens had cylindrical or prismatic shape with alength-to-diameter ratio from 1:1 to 2:1. Several tests were carriedout for each property, and finally the mean values were calculated foreach sample.

3.2. Physical and mechanical measurements

Rock properties are subdivided into physical andmechanical param-eters. All measurements were performed in accordance with standardtesting procedures (Trophimov and Korolev, 1993) which is similar tothe standard of the International Society for Rock Mechanics (ISRM,2007). Physical properties include bulk density (ρ), grain density(ρs), open (effective) (no) and total (n) porosity, gas permeability,hygroscopic moisture (Wg), water absorption (W), velocity of ultrason-ic P- and S-waves (Vp, Vs). Total porosity was calculated through grainand bulk densities. Determination of water absorption and open (effec-tive) porosity was made using water saturation techniques at roomtemperature and atmospheric pressure. Water absorption capacity bymass (in percent) was calculated as the mass of the absorbed water inreference to the mass of dry rock. Subsequently, the open porositywas calculated through water absorption capacity, bulk density anddensity of water (1.0 g/ cm3). Open porosity characterizes the potionof open pores which are connected in the rocks and can be filled bywater. The gas permeability was measured by the steady state method.Ultrasonic wave velocity (both P- and S-waves) was measured with ul-trasonic pulse transmission technique (State Standard, 21153.7–75).The values of travel time (tp and ts) were calculated using the time cur-sor on the oscillogram. The velocities were calculated from the corelength and the travel time measurement using the formula: velocity =core length / travel time. The frequency of pulser was 1 MHz for densesamples and 250 kHz for porous samples. The measurements weredone in dry as well as in water-saturated states.

Mechanical parameters refer to the strength characteristics of rocksand their potential for deformation and include elastic modulus (E),Poisson's ratio (ν), uniaxial compressive (σc) and tensile (σt) strength.The uniaxial compressive strength test was performed by standard test-ing procedures in accordance with State Standards 21153.2-84 (1984)and ASTM D7012 (2013). The universal testing machine Controls1500 kNwas used for loading. Uniaxial compressive strengthwas deter-mined for samples in dry and water-saturated states. Then, the soften-ing coefficient (Csof) was calculated as the ratio between strengthvalues inwater-saturated and dry states. The tensile strengthwas deter-mined by splitting of cylindrical specimens based on State Standard21153.3-85 (1985) suggested methods. The elastic constants were

84 F. Julia et al. / Engineering Geology 183 (2014) 80–95

calculated from the measured wave velocities and the bulk densityfollowing the ASTM D2845 (2008).

3.3. Petrography

Property measurements were accomplished with geological andpetrological examinations. All samples were studied petrographically(optical microscope “Olympus” BX-41). Secondary minerals were alsoidentified using X-ray diffraction (DRON-3). Bulk analyses were carriedout, and then analysis of clay-sized separates (less than 2 μm size frac-tion) was performed on a portion of the samples to obtain precise claymineral composition. Microprobe analysis was conducted for a portionof the samples (electron microscopes Camebax SX-50 and LEO 1450VPwith microprobe apparatus INCA 300) to study the morphology ofpore space and chemical alteration that occurred during the hydrother-mal process. A combination of detailed petrological investigation withpetrophysical data leads to a more comprehensive understanding ofthe structure and evolution of the geothermal system.

4. Results

4.1. Unaltered volcanic rocks

As noted above the host rocks of the hydrothermal systems in theKuril–Kamchatka region are of volcanic origin — effusive lavas (basalts,basaltic andesites, andesites, dacites) and various tuffs and tuffites (fine

Fig. 3. Comparison of unaltered tuffs and effusive rocks. a — bulk density,

ash, coarse ash, lapilli, agglomerate). The properties of the fresh equiva-lent rocks were studied in detail because they control to a large extentthe rate, intensity and character of the alteration. It was concludedthat effusive lavas and tuffs differ significantly by their properties andintensity of alteration (Figure 3, Table 1).

4.1.1. Effusive lavasEffusive lavas are strongly bound by crystals, which have been

formed during lava solidification. These rocks are dense with lowporosity, water absorption and permeability (0.025 mD), and highcompressive strength, velocity of ultrasonic waves and elastic modulus(30–70 GPa). Tensile strength ranges in terms of 6–15% (10% inaverage) of the compressive strength that indicates a brittle type offailure. Fresh effusive rocks are predominantly resistant towater satura-tion (Table 1).

Lavas form commonly impermeable horizons in hydrothermalsystems. Gas vesicles which are often observed in lavas are basically iso-lated as they are covered by glassy film and don't transport fluids. Insome cases effusive lavas host a fracture-dominated reservoir. The frac-ture network can be of primary or secondary origin. Primary fracturesare formed due to the effect of thermal stress that is developed duringlava cooling and hardening. Secondary fractures form during tectonicmovements which are typical for the Kuril–Kamchatka region. Basicallythey contribute more to the reservoir permeability than primaryfractures.

b — porosity, c — P-wave velocity, d — uniaxial compressive strength.

Table 1Physical and mechanical properties of unaltered volcanic rocks in the Kuril-Kamchatkaregion.

Properties\rocks Tuffs Effusive lavas

Bulk density (g/m3) 1.47 2.690.86–2.13 2.45–2.87a

Porosity (%) 44.3 4.925.0–63.8 1.7–13.1

Open porosity (%) 35 2.210.5–54.9 0.1–4.4

Water absorption (%) 25.8 0.85.4–52.8 0.1–1.7

P-wave velocity (km/s) 1.83 4.760.7–3.1 2.9–5.8

Uniaxial compressive strength (MPa) 15 1631–42 82–250

Softening coefficient 0.42 0.760.10–0.84 0.60–1.0

Tensile strength/uniaxial compressive strength (%) 18 105–40 6–15

Number of samples 66 28

a Mean values are shown above the line, with the max–min range shown below.

Fig. 4. Bulk density (a) and porosity (b) of unaltered tuffs. 1. Pumice rich tuffs; 2— fine ashvitric tuffs; 3 — coarse ash crystal tuffs 4 — lapilli lithic tuffs.

85F. Julia et al. / Engineering Geology 183 (2014) 80–95

4.1.2. TuffsTuffs have a cementation type of contact between grains that have

formed during the consolidation and lithification of loose pyroclasticdeposits. In comparison with effusive lavas they are characterized byhigher porosity, water absorption and permeability and lower valuesof mechanical properties (Table 1; Figure 3). The compressive strengthof tuffs is one order of magnitude less than that of the lavas. Tensilestrength of tuffs ranges in terms of 5–40% (18% in average) fromcompressive strength. It indicates that tuffs are more inclined to sufferductile failure than effusive lavas. In contrast to the effusive rocks,tuffs basically soften at water saturation because cementation contactsbetween grains become weaker.

Properties of tuffs depend mainly on primary characteristics (sizeand composition of grains) as shown in Fig. 4. In a sequence of fineash–coarse ash–lapilli tuffs, average value of bulk density strongly in-creases as 1.28–1.59–1.79 g/cm3 (Figure 4a), grain density changes2.63–2.69–2.84 g/cm3; porosity decreases 50–41–37% (Figure 4b).Such correlation between grain size and densities can be related to thecomposition of tuff grains. Fine ash tuffs are predominantly composedof vitric shards with rare crystals of plagioclase or quartz; coarse ashtuffs are crystal-rich (plagioclase, and rare grains of ferromagnesianminerals), whereas lapilli tuffs consist mainly of lithoclasts which aremuch denser than glass or crystals. The most porous (56%) with thelowest density (1.11 g/cm3) are pumice-rich tuffs.

P-wave velocity increases slightly in a sequence of fine ash to coarseash and then lapilli tuffs as 1.7–1.95–2.05 km/s in accordance withdensity increase and porosity decrease. Fine and coarse ash tuffs exhibitalmost the same strength (average values are 11 and 13 MPa,respectively),whereas lapilli tuffs are approximately two times stronger(average value is 24 MPa). Saturation by water reduces strength forall tuffs but to different degrees. Fine ash tuffs are the easiest tosoften and are characterized by the lowest softening coefficient(Csof = 0.35), coarse ash tuffs have a softening coefficient equal to0.41; lapilli tuffs have higher coefficient equal to 0.52. The ability tosoften correlates with water absorption which strongly decreases in asequence: fine ash–coarse ash–lapilli tuffs as 33–21–14%. Also it wasnoted that Neogene tuffs are denser and characterized by higher valuesof mechanical properties in comparison with Pleistocene rocks becausethey are more consolidated and lithified. Tuffs are the most commonhost rocks of hydrothermal systems in the Kuril–Kamchatka arc. Typi-cally, they form porous or fracture-porous reservoirs, although highlyargillized vitric ash tuffs define caprock on the hydrothermal systems.

The rate and intensity of hydrothermal alteration depend to thesome extent on the type and stability of the initial rock. Factors thatincrease alteration are high porosity and permeability of parent rocks,fractures, grain structure with weak cementation, high content ofunstable volcanic glass which is very sensitive to alteration, and basalticcomposition. Primary rocks on the other hand with low porosity andpermeability, especially those with good crystallinity and rhyoliticcomposition are more resistant to hydrothermal alteration andpetrophysical changes.

The rate of alteration appears to be less in the effusive rocks andmuch more intense in tuffs due to their high permeability and weakcementation.

4.2. Hydrothermal rocks

Under the action of thermal water, parent rocks (effusive lavas, tuffs,and tuffites) change their composition, pore-space morphology andvolume, and properties, finally transforming into hydrothermal-metasomatic rocks. The original mineral assemblages of volcanic rocks(volcanic glass, plagioclases, and ferromagnesian minerals) are dis-solved and replaced by secondary minerals among which quartz, chlo-rite, zeolites, sericite, clays, siliceous minerals, epidote, calcite, albite,adularia, prehnite, alunite, and pyrite are the most abundant (Figure5). Volcanic glass and olivine are the most sensitive to alteration;orthopyroxene and plagioclase followed by clinopyroxenes are morestable, while magmatic albite and quartz are the most stable. The

Fig. 5. Hydrothermal rocks: images on the left are the specimens; images on the right arephotomicrographs of thin sections (crossed nicols). Ms — muscovite, Qz — quartz, Ep —

epidode, Ca— calcite,Wr—wairakite, Sr— sericite,ML—mixed— layer, Lm— laumontite,Z — zeolites.

86 F. Julia et al. / Engineering Geology 183 (2014) 80–95

intensity of alteration inside a hydrothermal system changeswidely from fresh or weakly altered volcanic rocks to 100% alteredhydrothermal-metasomatic rocks. The alterationhas basically a pseudo-morphic character when original texture is saved in spite of entiremineralogical transformation. In particular, zeolites, sericite and clayminerals often totally replace the crystals of plagioclase, or chlorite isdeveloped by pyroxene.

Basically the distribution of hydrothermal facies has a zonal charac-ter according to fluid composition, pH, temperature, and pressure. Thefollowing facies are distinguished in geological section of “generalized”hydrothermal system (toward the surface): secondary quartzites,propylites (high–medium-temperature and low-temperature — tran-sylvanian, zeolitic, and argillic), argillized rocks often with high-silicazeolites, hydrothermal clays and opalites. Quartz–feldspar rocks aretypical in faults where they are formed due to thermal water boiling.Secondary mineralization and average values of properties are shownin Table 2.

4.2.1. Secondary quartzitesAccording to Bogatikov and Popov (2001) secondary quartzites are

formed under the action of high-temperature acid fluids (Т =300–550 °С,рН=1–4), and basically occur in deep horizons of a hydro-thermal system. Commonly they are developed around subvolcanicbodies inside a hydrothermal system. Secondary quartzites were

studied in Baransky and North-Paramushir systems. They are massiveand fine- to medium-grained rocks, composed of quartz and sericite/muscovite; epidote, pyrite, titanium oxide, and garnet are present inlesser amounts. There are two generations of quartz: the first one occursas a fine-grained matrix whereas later quartz fills veins forming thinbranching network. Sericite is developed as fine flakes withinplagioclase phenocrysts and muscovite as vein fillings. The intensity ofalteration is basically so high that the initial differences in propertiesbetween tuffs and effusive rocks disappear.

As shown in Table 2, secondary quartzites are dense (2.45–2.75 g/cm3), exhibit relatively low porosity (9.4–15%) and areimpermeable (0.04 mD) hydrothermal rocks, with water absorptionunder 4%. Dense structure and microcrystalline quartz cement appar-ently provide high mechanical strength of the rock σc = 77–158 MPaand elastic modulus E = 30–46 GPa. Strength is almost the same indry as well as in water-saturated state, thus secondary quartzites arerelatively resistant to water saturation.

It should be noted that secondary quartzites are by nature imperme-able rock, but when highly fractured due to an intense hydrodynamicregime they host high temperature fracture-dominated reservoir.

4.2.2. PropylitesPropylite alteration is formed under the action of nearly neutral

(from slightly acid to slightly alkaline) sulfate–hydrocarbonate-chloridethermal water with temperature varied in a range of 150–350 °С(Zharikov and Rusinov, 1998). The primary rock composition stays rath-er stable during alteration with the exception of СО2, H2S, Н2О, andother volatiles. Typical minerals in propylites are chlorite, epidote, al-bite, quartz, prehnite, sericite, zeolites, calcite, adularia, pyrite, hematite,and sphene. Several types of propylites are distinguished depending ontemperature conditions and mineral assemblages. In particular,propylites are classified in three temperature facies (Korzhinsky, 1982):1) high-temperature — quartz + epidote + actinolite; 2) medium-temperature — quartz + epidote + chlorite; and 3) low-temperature— quartz + calcite + chlorite.

4.2.2.1. Medium–high-temperature propylites. Medium and high-temperature propylites occur in Baransky, North-Paramushir,Mutnovsky, and Paratunsky hydrothermal systems. The alteration as-semblage includes chlorite, quartz, epidote, sericite, albite, wairakite,prehnite, ore minerals, and actinolite (the last one is typical for high-temperature propylites). They replace primary minerals, and fill poresand veins. Medium and high-temperature propylites are considered inone group because they are rather similar inmineralogy and are charac-terized by high strength and elastic properties. By their properties thepropylites are identical to secondary quartzites — they are dense, havelow porosity and low permeability (0.01–0.5 mD), with high values ofcompressive strength and elastic modulus (E = 22–48 GPa formedium-temperature propylites and 40–74 GPa for high-temperaturepropylites). Saturation by water doesn't influence strength (Table 2).Grain density varies in a range 2.70–2.93 g/cm3; and water absorptiondoesn't exceed 8%. High values of mechanical properties are the resultof the development of hard secondary minerals, which fill pores andfractures and substitute matrix and phenocrysts. The largest contribu-tion to increased mechanical properties includes the filling of intercrys-talline and intergranular micropores. The contacts between grainsbecome stronger and denser, which reinforces the cementation of therock.

4.2.2.2. Low-temperature propylites. Low-temperature propylites are themost abundant facies in the hydrothermal systems. They are subdividedinto chlorite–calcite–illite (Transylvanian), zeolitic and argillicpropylites.

4.2.2.3. Transylvanian propylites. Transylvanian propylites consist of cal-cite, chlorite and sericite/illite with quartz, adularia, albite, and ore

Table 2Physical and mechanical properties of hydrothermal rocksc.

Hydrothermal facieses Bulkdensityg/m3

Porosity%

Waterabsorption,%

Vp, km/s σc,MPa

Csoft Secondary minerals

Secondary quartzites (10)b 2.52a

2.45–2.75129.4–15

3.22.0–4.0

4.303.85–5.15

11377–158

0.760.65–1.0

Quartz, muscovite(epidote, garnet, titanium oxide)

Quartz–feldspar metasomatites (“boiling”) (46) 2.201.70–2.51

209.7–36

6.32.0–16

4.103.20–5.0

8325–171

0.820.70–1.0

Quartz, adularia, wairakite

Propylites High temperature (74) 2.572.32–2.78

5.60.4–11

1.20.1–4.5

4.653.20–5.85

13050–328

0.750.50–1.0

Quartz, chlorite, muscovite,wairakite,epidote, albite, actinolite

Medium temperature (17) 2.422.18–2.71

146.2–24

4.30.8–8

3.802.90–4.55

7344–120

0.840.65–1.0

Quartz, chlorite, sericite/muscovite,wairakite, epidote, prehnite, albite

Lowtemperature

Transylvanian (15) 2.141.89–2.41

2418–37

7.64.2–16

3.252.20–4.30

4325–73

0.680.48–0.89

Chlorite, illite, calcite

Zeolitic (40) 1.591.07–2.0

4026–59

207.5–29

1.00.60–2.60

5.61.4–22

0.490.18–0.85

Laumontite, chlorite

Argillic (29) 1.901.6–2.27

3321–43

124.8–21

2.301.60–3.45

207–39

0.460.27–0.67

Mixed-layer clays (chlorite–smectite,smectite–illite), calcite, silicaminerals

Argillic rocks Clays (11) 1.161.0–1.52

5746–64

– – Soak andswell

Smectite, kaolinite, mixed-layer clays,silica minerals

Argillites with zeolites(7)

1.691.50–1.75

4035–45

167.5–18

1.71.0–2.1

206–40

0.2–0.5 Clinoptilolite, mordenite, heulandite,smectites, opal

Opalites Opalites (29) 1.661.42–1.89

3210–38

142.4–23

2.942.30–3.35

3223–53

0.830.69–1.0

Opal, chalcedony, cristobalite,tridymite, quartz, Fe hydroxides

Opal–kaolinite (5) 1.721.33–2.01

3625–51

125–23

1,821.60–2.0

84–14

0.500.42–0.58

+kaolinite

Fe–opalites (7) 2.222.01–2.5

2927–32

8.88.4–9.3

3.763.0–4.6

3819–70

0.550.31–0.80

+ hydroxide Fe

Quartz–alunite (5) 2.352.29–2.46

115,7–17

3.32–5

4.583.75–5.0

6527–122

0.950.9–1.0

Quartz, alunite

a Mean values are shown above the line, with the max–min range shown below.b Quantity of samples is specified in brackets.c Only properties of intense to totally altered rocks are shown.

87F. Julia et al. / Engineering Geology 183 (2014) 80–95

minerals. The matrix of volcanic rock is replaced by chlorite, illite andsome calcite. Plagioclase is partially altered to albite, adularia, calciteand illite. Calcite, chlorite and quartz replace pyroxenes and fill poresand fractures. Ore minerals such as pyrite, hematite, magnetite, andsphene are also common. Alteration has a pseudomorphic characterwhen primary rock textures are observed in spite of intense mineralog-ical alteration. The intensity of propylitization varies from weak to in-tense, being about 50% in average.

Transylvanian propylites are rather dense rocks (bulk densityexceeds 2.0 g/cm3). Variation in compressive strength is from 25 MPa(occasionally 15 MPa) to 73 MPa as seen in Table 2. High values arecharacteristic when quartz and calcite are abundant in a matrix, strong-ly cementing the rock. On the other hand, the dominance of illite andchlorite (occasionally with some interlayers of smectite) results in me-chanically weaker rock.

4.2.2.4. Argillic propylites. Themain criteria for defining argillic propylitesis the mineralogical change of illite and chlorite to mixed-layer clayssuch us illite–smectite and chlorite–smectite. The other secondarymin-erals are calcite and low-temperature silica minerals. The cement iscomposed of clays with silica minerals and grains or clusters of calcite.The crystals of plagioclase are almost totally leached and partiallysubstituted by chlorite (or chlorite–smectite) and calcite surroundedby rims of illite–smectite. Pores are filled with chlorite or mixed-layers, however some pores are empty.

As seen in Table 2, argillic propylites are highly porous (21–43%)with the average density equaling to 1.90 g/cm3. In spite of high poros-ity, permeability is low ranging in terms of 0.04–0.91 mD. Variable con-tent and different clay composition cause a wide range of hygroscopicmoisture from 1.0 to 5.5%. High hygroscopy indicates a high contentof clays with smectite layers. P-wave velocity varies from 1.60 to2.5 km/s but reaches up to 3.45 km/s where calcite is dominant in ma-trix. Compressive strength changes from 7 to 39 MPa and is controlled

by the amount of clay and calcite. Saturation bywater causes a decreaseof compressive strength as a result of wedging out under the load, re-duction of friction between particles, and weakening of structural con-nections in clay minerals. P-wave velocity also changes in relation tosaturated state. It is well known that P-waves travel faster in waterthan in air so saturated rocks basically have higher values (Trophimovand Korolev, 1993). Meanwhile argillic propylites with high content ofclay minerals are characterized by up to 20–30% lower Vp in saturatedsamples in comparison with dry samples. This phenomenon is verycommon for all hydrothermal rocks containing clay minerals (Frolovaet al., 2006).

4.2.2.5. Zeolitic propylites. Zeolitization is developed under the action ofsubneutral to alkalineNa–Ca sulfate–chloridewith carbondioxidefluids(Senderov and Hitarov, 1970). The alteration assemblage includesabundant laumontite and chlorite; wairakite, prehnite, analcime, andcorrensite are also present. The intensity of alteration varies fromweak to 100%, while the intense and totally altered rocks are prevalent.

In weakly altered tuffs the matrix is occasionally substituted by fine-grained laumontite. Plagioclases are slightly zeolitized bymicrofractures.Ferromagnesian minerals (pyroxene and hornblende) stay unaltered orslightly chloritized. In moderately altered rocks, the matrix is totallyrecrystallized to fine zeolitic aggregate in association with chlorite (orcorrensite). Zeolites replace plagioclases and fill pores in rock. Horn-blende and clinopyroxenes stay unaltered while orthopyroxenes aresubstituted by chlorite and prehnite. Totally altered rocks consist ofcoarse laumontite aggregates composed of prismatic crystals cementedby later needle crystals and fine-grained chlorite–laumontite matrix.Chlorite is present at the edges and within laumontite aggregates, alsoon the contacts between grains. The casts of plagioclase are totally re-placed by zeolites. Ferromagnesian minerals are intensely altered byintracrystalline microfractures. Relicts of clinopyroxenes are foundeven in almost totally altered zeolitic propylites. Veins with zeolite and

88 F. Julia et al. / Engineering Geology 183 (2014) 80–95

chlorite intersect the rocks. In a process of zeolitization volcanic rocks to-tally change their mineralogy and structure, and turn to the weak andheterogeneous chlorite–zeolitic rocks with secondary granoblastic–micropoikilitic structure.

Zeolitic propylites are weak (σc = 5.6 MPa), highly porous (26–59%) and permeable (3.6–4.3 mD) rocks with water absorption varyingin the range of 10–29%. Zeolitic propylites are distinguished by extreme-ly low values of P-wave velocity in the order of 0.8–1.6 km/s, andaccordingly low elastic modulus. The reason for low velocities is thehighly microporous matrix as shown in Fig. 6. The matrix consists offine-size crystals of laumontite (3–5 μm) and leaves of chlorite(or corrensite) with weak contacts between crystals that attenuate thepropagation of ultrasonic waves through the medium.

Commonly zeolitic propylite stratum hosts porous-type reservoirin hydrothermal systems. The well studied example is Pauzhetskyhydrothermal system where the main production zone is correlatedwith intensely zeolitizied lapilli tuffs.

Thus, various facies of propylites differ greatly according to theirproperties. Fig. 7 shows a relationship between P-wave velocity againstdensity (Figure 7a,b) and compressive strength versus porosity (Figure7c,d) for different types of propylites. Medium-temperature rocks arethe densest among propylites with the highest mechanical strength.Calcite–illite–chlorite (Transylvanian) propylites although denseenough are more porous with lower elastic properties and strength.Argillization further reduces the strength and contributes to rockdecompaction. Zeolitic propylites are the weakest among propylitesand characterized by the highest porosity.

Despite the mineralogical variability of propylites, their mechanicalproperties are well correlated with their physical properties. Some em-pirical relations are shown in Fig. 7. As expected, the density of rock in-creases ultrasonic velocity and strength, in contrast, porosity decreasesmechanical properties (Figure 7). Linear regression is established be-tween P-wave velocity and bulk density (Figure 7b). The correlation ishigh with a correlation coefficient equal to 0.95. P-wave velocity ishigher in a saturated than in a dry state for high–medium-temperatureand Transylvanian propylites,whereas the opposite tendency in P-wavevelocity is observed for argillic and zeolitic propylites. Relationship be-tween uniaxial compressive strength and porosity is described by expo-nential equation (Figure 7d). Regression obtained for dry and water-saturated samples shows a similar trend although the curve for drysamples is located above the curve related to saturated conditions.

4.2.3. Argillic rocks“Argillization” is a metasomatic replacement of parent rock by clay

minerals (Zharikov and Rusinov, 1998). It is suggested that argillization

Fig. 6. SEM images of zeolitic propylites (a–b). Relict crystals of plagioclase inmicroporous laumrite, Qz— quartz, Pl — plagioclase.

is characteristic of thefinal stages of the hydrothermal process. Basicallyargillic rocks are formed at shallow depths due to interaction with low-temperature fluids (T= 50–150 °C). Argillization zones occur in all hy-drothermal systems although they have some peculiarities. Several fa-cies of argillic rocks are distinguished: zeolite–smectite, smectite,smectite–kaolinite, and kaolinite.

The most abundant minerals of argillic rocks are clay minerals of asmectite group (montmorillonite, beidelite, nontronite, saponite).They grow under the action of slightly acid or subneutral fluids with atemperature below 150 °С. A unique characteristic is the group ofhigh-silica zeolites including mordenite, clinoptilolite, and heulandite.They are formed under the action of alkaline fluids in a temperaturebelow 130–150 °С and often occur in association with smectites. Theminerals of the kaolinite group are also widely present. They aredeveloped in near-surface horizons under the influence of acid low-temperature fluids. Silica minerals such as opal, tridymite, andcristobalite often accompany clay minerals. The characteristic featureof near-surface argillization zone is a wide development of unstable as-semblages and metastable phases as a result of frequently changingconditions (temperature, chemistry of fluids, presence of steam, etc.)in this zone.

4.2.3.1. Smectite–zeolite rocks. Smectite–zeolite rocks were investigatedin detail in the Pauzhetsky and Essovsky hydrothermal systems.Secondary mineral assemblage consists of smectites, clinoptilolite,mordenite, heulandite, and silica minerals (opal, tridymite, andcristobalite). Glassy matrix of tuffs is totally recrystallized to fine-grained zeolitic aggregate. These zeolites are invisible in thin sectionsbut clearly observed in scanning electron microscope under high mag-nification. It is seen that matrix is composed of rectangular–tabularand lamellar microcrystalls of clinoptilolite (Figure 8a) in associationwith fiber-needle or sheaf-like crystals of mordenite (Figure 8a, b).Smectites are also abundant in association with zeolites (Figure 8c).The ratio of smectites and zeolites varies widely. Zeolites are often pres-ent as films around the grains forming filmy cement. The intensity ofprimary rock alteration is usually weak to moderate. Vitric material isintensely to totally altered, whereas crystals and lithic material arefresh or weakly altered.

Smectite–zeolite rocks are distinguished by low density (1.51–1.77 g/cm3) and high values of porosity (35–46%), water absorption(9–19%) and hygroscopic moisture (3–6%). The presence of smectitessignificantly reduces rock permeability in spite of high porosity. Fig. 8cillustrates the microstructure of smectites under an electron micro-scope. It is clearly seen that smectites are characterized by ultrasmallpores and have cellular microstructure with the “cell” size of about

ontite–chlorite matrix (Pauzhetsky hydrothermal system). Lm— laumontite; Chl— chlo-

Fig. 7. Properties of propylites. Scatterplots: a,b — P-wave velocity versus bulk density; c,d — uniaxial compressive strength versus porosity.

89F. Julia et al. / Engineering Geology 183 (2014) 80–95

5–7 μm.Ultrasmall pores don't transportwater: they are filled by boundwater, which can react with the rock but doesn't filter through the stra-tum. Also smectites swell in saturated state mudding intergranularpores and microfractures in a rock, additionally reducing permeability.That is why argillized horizons usually provide an impermeable caprockin hydrothermal system (Frolova et al., 2006).

Fig. 8. SEM images of argillized tuffs (Pauzhetsky hydrothermal system). a — fiber crystals of mmordenite; c — cellular microstructure of smectite. Sm — smectites, Clpt— clinoptilolite, Mrd —

Compressive strength of zeolite–argillic rocks varies from 6 to16 MPa. The strongest rocks are mainly cemented with fine-grained ze-olites and opal. The development of smectite “leaves” between zeolitescrystals leads to strength reduction. The rock composed of smectiteswith small amounts of zeolites and opal, is the weakest. Argillic rockssoften in saturated state; compressive strength decreases by 50–75%

ordenite and tabular crystals of clinoptilolite; b — fiber-needle and sheaf-like crystals ofmordenite.

90 F. Julia et al. / Engineering Geology 183 (2014) 80–95

after saturation with water due to theweakness of smectites. Compres-sional wave velocity also decreases under saturation by 10–30% incomparison with dry samples.

4.2.3.2. Hydrothermal clays. Themost problematic rock from engineeringgeological point of view is hydrothermal clays which compose the near-surface horizons in the hydrothermal systems. Near-surface horizon ofclays is very common on active volcanoes and hydrothermal systems,occurring as a cover of several meters of thickness. It is a very heteroge-neous layer characterized by high porosity, plasticity, hygroscopy, andcompressibility.

Hydrothermal clays are well-studied mineralogically but limitedwith respect to engineering geology data. Potro and Hürlimann(2009) studied the alteration of phonolitic lava to weak, porous, silty,medium-plasticity clay-rich soil due to kaolinization. They showedthat the presence of significant amounts of kaolinite in the partially al-tered soil was the reason for a lower strength than in the fully alteredalunitic one. They concluded that the type of alteration, which was de-termined by the temperature and pH, was more important in control-ling the strength of the altered soil than the degree of alteration.Coggan et al. (2013) showed that the degree of kaolinization, and asso-ciated changes in the mineralogy of a granite is directly related to a re-duction in uniaxial compressive strength and bulk density.

Hydrothermal clays were studied in detail in the Koshelevsky,Pauzhetsky and Baransky geothermal fields. In particular, in the Low-Koshelevsly thermal field andesites are gradually transformed intoclays (рН = 3.5–5.5; Т up to 95 °C, Rychagov et al., 2008). Alterationstarts in fracture networks exposed to thermal water and steam. Thewalls of fractures are rapidly replaced by clay minerals. Graduallythe fractures propagate and expand, and new blocks of andesite are al-tered through argillization. In the end cores of andesites remain insurrounded by clayey mass (Figure 9). Relicts of andesites are intenselyfractured, ferruginizated, and argillized. It can be assumed that the pro-gressive fracturing occurs under the pressure of swelling in surrounding

Fig. 9. Transformation of andesite massif in clays (Low-Koshelevsky geothermal field). 1— andandesite.

claymass. Thus, the claymass is very heterogeneous, and contains hardrelicts of andesites. The composition of clays is mainly smectites ormixed-layer minerals. It has pseudomorphic texture inherited fromandesite.

Microscopic study has shown that the leaching process leads to theformation of secondary pores in the rock matrix and the initiation ofmicro-fractures simultaneously within phenocrysts. Then clay mineralsdevelop in fractures, and cover the walls of pores gradually filling theempty space. Volcanic glass is the most sensitive material and is thefirst replaced. Subsequently the microcrysts in the matrix are partiallydissolved dependent on their morphology. Alteration of plagioclasephenocrysts basically begins from microfractures and the inner part ofthe crystal, which is more anorthic or andesine in composition,whereasthe outer part of the crystal (basically albite composition) is more stableto alteration and succumbs to alteration at later stages (Figure 10).Development of microfractures, fillingwith smectites (content of smec-tites is 21–28%), decreased density of andesite from 2.57 to 2.16–2.32 g/cm3, and increased porosity from 1.4 to 10–26% all leads to theweakening of andesite from 61 to 23–26 MPa. The velocity of the P-wave changes from 5.7 to 3.6 km/s and elastic modulus from 54 to24GPa. The high content of smectite causes rock softening in a saturatedstate. Strength is almost the same in dry and saturated states for unal-tered andesite whereas it decreases by 40% due to saturation for alteredandesite.

The replacement of andesites by clays is accompanied by a strongreduction in density (andesites 2.5–2.6 g/cm3, clays 1.0–1.1 g/cm3),increase in porosity (from 5–8% to 50–60%) and hygroscopy. Water im-pregnation softens and swells the clay and increase plasticity (plasticityindex equal to 30). They form several meters thick cover in geothermalareas. Slope geological processes (landslides,mud-flows) are frequentlydeveloped in connection with hydrothermal clays.

Argillic rocks are basically developed in the uppermost subsurfacehorizon which often composes the foundation for geothermal electricpower stations. In particular, the Pauzhetskaya power plant is

esites massif, 2— fractures with clays, 3— hydrothermal clay mass; 4— relict fragments of

Fig. 10. Photomicrographs of thin sections (Low-Koshelevsky geothermal field) (parallel nicols). Alteration of plagioclase phenocrysts under argillization. a, b— initial stage of alteration.Smectites fill intra-crystalline microfractures; c—middle stage of alteration. Smectites substitute inner part of phenocryst; d— final stage of alteration. Pseudomorphosis of smectites byplagioclase.

91F. Julia et al. / Engineering Geology 183 (2014) 80–95

constructed on fine ash vitrous tuffs which have been altered byargillization and zeolitization. The tuffs are highly porous (42–63%)andweak (σc= 2–10MPa, occasionally 25MPa) with high hygroscopy(up to 5%), which correlates with the high content of smectites, replac-ing volcanic glass. Saturation by water decreases compressive strengthby 50–80% and also velocity of the P-wave by 20–30% due to the soften-ing of smectites. The pipeline which is transporting fluid from the wellto the power station crosses the hydrothermal altered clayey soils. Thefoundation of Upper-Mutnovskaya power plant consists of tuffs inter-bedded with andesitic lavas. The rocks are altered by argillization andlow-temperature propilitization, and the tuffs are altered in a higher de-gree. The rocks are rather dense (andesite 2.54–2.58 g/cm3; tuffs 2.37–2.41 g/cm3), with the strength σc = 68–72MPa for andesite, and σc =21–46 MPa for tuffs. The main problem is the reduction in strength in awater saturated environment by up to 50% for andesites and 80–85% fortuffs. The reduction in strength appears to be associated with a highcontent of mixed-layer illite–smectite clays, which have substitutedthe primary minerals.

4.2.4. OpalitesOpalites are formed in near-surface condition due to acid leaching

caused by sulfate waters with рН equal to 2–3 and temperatures nearto 100 °C. The lower boundary of the opalite horizon basically coincideswith groundwater level. Opalites are white colored rocks composed ofsiliceous minerals such as opal, chalcedony, quartz, tridymite, andcristobalite. These minerals provide stiff “framework” to the rocks asseen by the relatively high mechanical strength (σc = 23–53 MPa)and P-wave velocity (2.30–3.35 km/s), despite high porosity (25–38%)and low density (1.42–1.89 g/cm3). Siliceous minerals totally replaceprimary crystals although original texture is saved. Occasionally hema-tite and Fe-hydroxides are developed in association with opalites. Thisiron enrichment leads to an increase in grain density from 2.3–2.6 to2.8–2.9 g/cm3 and bulk density — up to 2.01–2.50 g/cm3.

The entire alteration sequence from fresh andesite to 100% alteredopalite was studied in the Upper-Koshelevsky geothermal field(Figure 11a). The fresh andesites are massive and porphyritic. Plagio-clase phenocrysts are abundant (size up to 2–3 mm) with rare pyrox-enes. Groundmass is glass-rich with microcrysts of plagioclase andpyroxenes (up to 0.1 mm). The glassy matrix becomes replaced byopal and cryptocrystalline cristobalite. Primary microcrystals graduallydissolve and also convert to siliceous minerals. Phenocrysts are leachedstarting from microfractures and inner parts of crystals and become fi-nally totally transformed to cristobalite. Some kaolinite is occasionallypresent. Opalite is the final product of total alteration into siliceousmin-erals although the primary structure of the andesite is still visible. Bulkchemical analysis shows the loss of major elements with the exceptionof Si and Ti during the opalization. Amount of SiO2 increases from 54.4%in the primary andesites to 91.1% in opalites.

Box plots (Figure 11) show the progressive change in density, poros-ity, P-wave velocity and elasticmodulus in relation to alteration intensi-ty. The loss of major elements and leaching of rocks leads to theformation of secondary porosity as seen by 8% primary porosity of an-desite and 37% in the opalite. Bulk density decreases from 2.62 to1.46 g/cm3 (Figure 11b) according to porosity growth (Figure 11c).Grain density decreases from 2.85 to 2.31 g/cm3 as a result of the devel-opment of siliceous minerals with low density in comparison with pri-mary minerals. Mechanical properties correlate with density andporosity. Elastic modulus deceases from 42 to 8 GPa (Figure 11d).Poisson's ratio ranges in terms of 0.28–0.31 for andesites and decreasesto 0.10–0.17 for opalites that indicates the high stiffness rigidity ofopalites supported by siliceous “framework”. Compressive strength isreduced four times from 125 MPa in andesites to 37 MPa in opalites(Figure 11e).

An increase in рН to 4–5 in the fluid usually leads to the formation ofkaolinite, alongwith siliceousminerals which results in rockweakening(σc ~ 10 MPa; Vp ~ 1.6–2.0 km/s), formation of hygroscopic moisture(1–5%), and reduction of permeability. This result conforms to other

Fig. 11. Influence of alteration intensity on andesite properties in the zone of acid sulfate leaching (Upper-Koshelevsky geothermal field). a— samples; b— bulk density; c— porosity; d—

P-wave velocity; e — elastic modulus; f — uniaxial compressive strength. 1. Unaltered andesite; 2 — weakly altered andesite; 3 — intensely altered andesite; 4, 5 — opalites.

92 F. Julia et al. / Engineering Geology 183 (2014) 80–95

Fig. 12. Comparison of properties of high- and low-temperature hydrothermal rocks. a —

P-wave velocity versus bulk density; b — uniaxial compressive strength versus porosity.

93F. Julia et al. / Engineering Geology 183 (2014) 80–95

studies of the effect of kaolinization on mechanical properties (Potroand Hürlimann, 2009; Coggan et al., 2013).

4.2.5. Quartz–feldspar metasomatitesThese rocks are formed under particular thermodynamic and geo-

chemical conditions, which are characterized by a phase transitionfrom liquid to steam. These conditions are associated with faults andfracture zones, where intense boiling occurs due to a sudden pressurerelease. The boiling causes a temperature decrease, heat loss, gas re-lease, and increase of pH. These conditions are favorable for quartz–feld-spar metasomatites. Rockmatrix is replaced by cryptocrystalline quartzand adularia. Plagioclase is leached to “skeleton” crystals, partiallysubstituted by wairakite. Epidote and prehnite occasionally occur.Quartz–adularia rocks are distinguished by high density (2.0–2.51 g/cm3 in average), compressive strength (80–171 MPa), P-wavevelocity (4.0–5.85 km/s) and elastic modulus (30–50 GPa).Water satu-ration doesn't influence the mechanical properties; however, it shouldbe noted that large secondary pores are characteristic for this zone.Formation of this secondary porosity (n = 30–36%) causes reductionin density (ρ = 1.7–1.9 g/cm3) and mechanical properties of rocks(Vp = 3.2–3.5 km/s; σc = 30–40 MPa; E = 16–20 GPa).

5. Discussion

As considered above, thermal fluids strongly change the mineralogyand properties of host rocks. The temperature and pressure in hydro-thermal systems primarily control the changes in rock properties. Thedifference between low- and medium–high-temperature facies is illus-trated in Fig. 12.

The density of high-temperature facies exceeds 2.2 g/cm3 and poros-ity is below20%. P-wave velocity ranges from3.5 to 6.0 km/s in dry stateand remains the same or slightly increases up to 10–20% in saturatedstate. An increase of Vp indicates open pores and fractures which filledwithwater due to saturation. The effect of saturation on P-wave velocitycorrelates with water absorption. Samples in whichwater absorption islow (less than 1%) have almost the same Vp values in dry and saturatedstates whereas P-wave velocity increases 10–20% under saturation insamples in which water absorption equals 4–6%. The high-temperature facies have high compressive strength, exceeding50 MPa, in dry as well as in water-saturated states.

For low-temperature facies, density varies in a range 1.3–2.3 g/cm3,and porosity is above 20%. Compressive strength doesn't exceed50 MPa, and predominantly decreases in a saturated state. P-wave ve-locity is less than 4.0 km/s in dry state and influenced by saturation indifferent ways although commonly reduced. Thus, hydrothermal rockswith clay components are characterized by Vp reduction under satura-tion by 20–30%. Also, low-temperature rocks are basically hygroscopic,with bound water content of up to 5–6% whereas high-temperature fa-cies don't contain hygroscopic moisture in appreciable amounts.

“Boiling zone” composed of quartz and feldspar is placed on inter-mediate position between high- and low-temperature facies althoughthey are closer to the first one.

It has been concluded that deep, high-temperature fluids(T N 200 °C) cause consolidation and strengthening of rock, decreasetheir porosity and permeability, and remove hygroscopic moisture.This tendency is observed independently of fluid chemical composition.This is the result of increased hardness developed by the secondaryminerals (quartz, epidote, zeolites, albite, adularia, prehnite, and cal-cite), which fill pores and veins, and substitute groundmass and pheno-crysts. Minerals which fill the intercrystalline and intergranularmicropores make the largest contribution to rock strengthening. Thecontacts between grains become stronger and denser, reinforcing thecementation of the rock. Since the pores are completely filled by sec-ondary minerals, water absorption and permeability are low.

The changes described above of the properties are observed for botheffusive lavas and tuffs; however, in tuffs the alteration shows higher

contrast. It should be noted that the intensity of alteration can be sohigh that the rocks lose their primary features. In that case, the initialdifference in properties between tuffs and effusive lavas disappears.

Property variations caused by low-temperature subsurface fluids(Т b 150 °C) aremore complicated anddiverse. Depending onwhat pro-cess prevails (rocks leaching, precipitation of secondary minerals inpores and fractures, or replacement of primary minerals by secondaryminerals), both an increase or a decrease in physical and mechanicalproperties can occur. The leaching of rocks by acid fluids raises porosityand lowers density, ultrasonic velocity, and strength. Precipitation inpores leads to opposite property changes. The dynamic of property al-terations depends also on secondary mineral assemblage including themost extended clays (kaolinite or smectite depending on pH), alunite,opal, cristobalite, and high-silica zeolites. In this way chemical composi-tion and acidity–alkalinity of low-temperature thermal fluids controlnot only mineralogical composition but also physical and mechanicalproperties of rocks.

6. Conclusion

1. Hydrothermal systems in the Kuril–Kamchatka region are hostedwithin volcanic formation of Neogene–Quaternary age amongwhich tuffs and effusive lavas are the most common types. Compar-ison between unaltered effusive lavas and tuffs has revealed thatthey differ significantly by their properties and rate of alteration.The former are characterized by higher density, strength, and elastic

94 F. Julia et al. / Engineering Geology 183 (2014) 80–95

properties and lower porosity and permeability. The rate of alter-ation appears to be less in the effusive rocks and muchmore intensein tuffs due to their high permeability and weak cementation.

2. Under the action of thermal fluids the parent rocks are intensely al-tered and finally transformed to hydrothermal rocks such as second-ary quartzites, propylites (high–medium temperature and low-temperature — transylvanian, zeolitic, and argillic), argillized rocksoften with high-silica zeolites, hydrothermal clays, opalites, andquartz–feldspar metasomatites. The tendency of properties tochange can be variable depending on particularities of protolith, tem-perature, pressure, and chemical composition of the percolatingfluid, and duration of fluid–rock alteration. In some cases, rock “im-provement” is observed, i.e. consolidation, strengthening, a decreaseof porosity and permeability, and a removal of hygroscopic moisturein the rock. In other cases, rock “deterioration” occurs, i.e. formationof secondary porosity and permeability; a decrease of density andelastic modulus, weakening, and formation of hygroscopic moisture.

3. Hydrothermal facies differ greatly in their properties. High–medium-temperature facies (secondary quartzite, high–medium-temperaturepropylite) are the densest, with low porosity, high strength and elas-tic modulus, and are resistant to water saturation. Zeolitic propylitesare weak, highly porous and permeable rocks with extremely lowvalues of ultrasonic wave velocity. Argillic rocks are distinguishedby high porosity but low permeability. They are characterized byhigh hygroscopy and soften in a water saturated environment.Opalites have stiff siliceous framework which provides relativelyhigh strength and elastic properties in spite of highly porous struc-ture and low density.

4. Most problematic rocks are hydrothermal clays which compose thenear-surface horizons in the hydrothermal systems. They are highlyheterogeneous, contain hard rock debris, and are characterized byhigh porosity, plasticity, hygroscopy and compressibility. They softenor soak in saturated state, and sometimes swell increasing their vol-ume and forming additional internal pressure.

5. Alteration of physical and mechanical rock properties in geothermalareas leads to a variety of consequences: changes in engineering geo-logical features of the area; transformation of the hydrothermal sys-tem structure and changes in its hydrodynamic and temperatureregime; decreases in borehole production during field exploitation;and promotes hazardous geological processes in the thermal fieldsthat can result in landslides and hydrothermal explosions.

Acknowledgments

This research was supported by Russian Foundation for Basic Re-search (Grants No 13-05-00530 and No 13-05-00262). We are gratefulto the staff of Geothermic Laboratory, Institute of Volcanology and Seis-mology, FED RAS for scientific cooperation, discussions and field workduring many years and the staff of the Faculty of Geology, LomonosovMoscow State University, for analytical measurements.We would liketo thank Dr. R. Gordon Bloomquist and four anonymous reviewers fortheir constructive comments and suggestions and also David Zur forediting the manuscript and improving the English language.

References

American Society for Testing Materials. ASTM D7012-13. Standard test methods for com-pressive strength and elastic moduli of intact rock core specimens under varyingstates of stress and temperatures. American Society for Testing Materials, Pennsylva-nia, USA.

American Society for Testing Materials. ASTM D2845-08. Standard test method for labo-ratory determination of pulse velocities and ultrasonic elastic constants of rock.American Society for Testing Materials, Pennsylvania, USA.

Belousov, V.I., Sugrobov, V.M., 1976. Geological and hydrogeothermal conditions of geo-thermal regions and hydrothermal systems of Kamchatka. Hydrothermal Systemsand Thermal Fields of Kamchatka (pp. 5–23 (in Russian)).

Bogatikov, O.A., Popov, V.S., 2001. Petrography and petrology of magmatic, metamorphicand metasomatic rocks. Logos, Moscow (768 pp.).

Bozkurtoğlu, E., Vardar, M., Suner, F., Zambak, C., 2006. A new numerical approach toweathering and alteration in rock using a pilot area in the Tuzla geothermal area,Turkey. J. Eng. Geol. 87 (1–2), 33–47.

Coggan, J.S., Stead, D., Howe, J.H., Faulks, C.I., 2013. Mineralogical controls on the engi-neering behavior of hydrothermally altered granites under uniaxial compression. J.Eng. Geol. 160, 89–102.

Ellis, A., 1966. Volcanic hydrothermal areas and the interpretation of thermal waters com-positions. Bull. Volcanol. 29, 575–584.

Frolova, Yu.V., Golodkovskaya, G.A., Ladygin, V.M., Rychagov, S.N., 1999. Nature of engi-neering geological properties of hydrothermal metasomatic rocks of the Kuril–Kam-chatka region. J. Univ. Geol. Bull. Ser. 4: Geol. (3), 36–42 (in Russian).

Frolova, J., Ladygin, V., Rychagov, S., 2006. Petrophysical properties of argillitization zonein geothermal fields. Geotherm. Resour. Counc. Trans. 30, 909–912.

Frolova, J.V., Ladygin, V.M., Rychagov, S.N., 2010. Petrophysical alteration of volcanic rocksin hydrothermal systems of the Kuril–Kamchatka island arc. Proceedings of theWGC2010, April 25–30, Bali, Indonesia (Web. https://www.geothermal-library.org/index.php?mode=pubs&action=view&record=8006099).

Frolova, Yu.V., Ladygin, V.M., Rychagov, S.N., 2011. Patterns in the transformation of thecomposition and properties of volcanogenic rocks in hydrothermal–magmatic sys-tems of the Kuril–Kamchatka island arc. Univ. Geol. Bull. 66 (6), 430–438.

International Society on Rock Mechanics, 2007. In: Ulsay, R., Hudson, J. (Eds.), The Com-plete ISRM Suggested Methods for Rock Characterization, Testing and Monitoring.ISRM, pp. 1974–2006 (628 pp.).

Kiryukhin, A., Xu, T., Pruess, K., Apps, J., Slovtsov, I., 2004. Thermal–hydrodynamic chem-ical (THC) modeling based on geothermal field data. J. Geothermics 33, 349–381.

Kiryukhin, A.V., Rychkova, T.V., Droznin, V.A., Chernykh, E.V., Puzankov, M.Y., Vergasova,L.P., 2010. Geysers valley hydrothermal system (Kamchatka): recent changes relatedto landslide of June 3, 2007. Proceedings World Geothermal Congress 2010, Bali,Indonesia (25–29 April).

Korzhinsky, D.S., 1982. The Theory of Metasomatic Zoning. Science, Moscow (104 pp.).Kristmannsdóttir, H., Tómasson, J., 1978. Zeolite zones in geothermal areas in Iceland. In:

Sand, L.B., Mumpton, F.A. (Eds.), Natural Zeolites: Occurrence, Properties, Use.Pergamon Press, Elmsford, New York, pp. 277–284.

Ladygin, V.M., Frolova, J.V., Rychagov, S.N., 2000. Formation of composition andpetrophysical properties of hydrothermally altered rocks in geothermal reservoir.Proceeding, World Geothermal Congress, pp. 2695–2699.

Lund, J.W., Bertani, R., 2010. Worldwide geothermal utilization 2010. Geotherm. Resour.Counc. Trans. 34, 195–198.

Lutz, S.J., Zutshi, A., Robertson-Tait, A., Drakos, P., Zemach, E., 2011. Lithologies, hydrother-mal alteration, and rockmechanical properties in wells 15-12 and BCH-3, Brady's HotSprings geothermal field, Nevada. Geotherm. Resour. Counc. Trans. 35, 469–476.

Naboko, S.I., 1963. Hydrothermal Metamorphism of Rocks in Volcanic Areas. Publisher ASUSSR, Moscow (172 pp.).

Pilipenko, G.F., 1998. Present hydrothermal systems and thermal springs of Kamchatka.“Present Hydrothermal Systems and Epithermal Gold–Silver Deposits of Kamchatka”Russian–Japanese Field Seminar. Petropavlovsk–Kamchatsky, pp. 5–12.

Potro, R., Hürlimann, M., 2009. The decrease in the shear strength of volcanic materialswith argillic hydrothermal alteration, insights from the summit region of Teide stra-tovolcano, Tenerife. J. Eng. Geol. 104 (1–2), 135–143.

Povarov, O., 2000. Geothermal power engineering in Russia — today. Proceedings WorldGeothermal Congress 2000, Kyushu–Tohoku, Japan, May 28–June 10, pp. 1587–1592.

Reyes, A.G., 1990. Petrology of Philippines geothermal systems and the application of al-teration mineralogy to their assessment. J. Volcanol. Geotherm. Res. 43, 279–309.

Rigopoulos, I., Tsikouras, B., Pomonis, P., Hatzipanagiotou, K., 2010. The influence oflateration on the engineering properties of dolerites: the examples from the Pindosand Vourinos ophiolites (Northern Greece). J. Rock Mech. Min. Sci. 47 (1), 69–80.

Rusinov, V.L., 1972. Geological and Physical–Chemical Patterns of Propylitization. Nauka,Moscow, p. 204.

Rychagov, S.N., 2005. Hydrothermal–magmatic systems of island arcs: the structure andstages development. Proceedings of International Kuril–Kamchatka Field Workshop7 “Geothermal and Mineral Resources of Modern Volcanism Areas”, Petropavlovsk-Kamchatsky, pp. 117–140 (in Russian).

Rychagov, S.N., Belousov, V.I., Glavatskikh, S.F., Ladygin, V.M., Sandimirova, E.I., 2002.North-Paramushir hydrothermal–magmatic system: the characteristic of deep geo-logical structure and model of present mineral formation. J. Volcanol. Seismol. (4),3–21.

Rychagov, S.N., Korobov, A.D., Glavatskikh, S.F., et al., 2005. Evolution of metasomatic pro-cesses in structure of hydrothermal–magmatic systems in island arcs. Proc. Int. FieldKuril–Kamchatka Seminar “Geothermal and Mineral Resources in the Regions of Re-cent Volcanism” Petropavlovsk-Kamchatskii, pp. 207–216.

Rychagov, S., Davletbaev, R., Kovina, O., Koroleva, G., 2008. The characteristic of subsur-face horizons of hydrothermal clays in Low-Koshelevsky and Pauzhetsky geothermaldeposits. J. Bulletin of Kamchatka Regional Association “Educational-Scientific Cen-ter”. Earth Sci. 12, 116–134 (in Russian).

Rychagov, S.N., Sokolov, V.N., Chernov, M.S., 2012. Hydrothermal clays in geothermalfields on the South Kamchatka: the new approach and results of the studies. J.Geochem. (4), 378–392.

Senderov, E.E., Hitarov, N.I., 1970. Zeolites, Their Synthesis and Conditions of Formation inNature. Science, Moscow (283 pp.).

Sigurdsson, O., Gudmundsson, A., Fridleifsson, G.O., Franzson, H., Gudlaugsson, S.Th.,Stefansson, V., 2000. Database on igneous rock properties in Icelandic geothermalsystems. Status and unexpected results. Proc. World Geothermal Congress, pp.2881–2886.

Siratovich, P.A., Davidson, J., Villeneuve, M., Gravley, D., Kennedy, B., Cole, J., Wyering, L.,Price, L., 2012. Physical and mechanical properties of the Rotokawa andesite fromproduction wells RK 27_L2, RK 28 and RK 30. New Zealand Geothermal Workshop

95F. Julia et al. / Engineering Geology 183 (2014) 80–95

— 2012 Proceedings 19–21 November 2012, Auckland, New Zealand (https://www.geothermal-library.org/index.php?mode=pubs&action=view&record=8019178).

State Standard 21153.2-84, 1984 bb. Rocks. Methods for determination of uniaxial com-pressive strength. Publisher of Standards, Moscow (12 pp.).

State Standard 21153.3-85, 1985. Rocks. Methods for Determination of Tensile Strength.Publisher of Standards, Moscow, pp. 4–7.

State Standard 21153.7-75, 1984. Rocks. Methods for Determination of Compressionaland Shear Wave Velocities. Publisher of Standards, Moscow, pp. 29–35.

Sugrobov, V.M., Sugrobova, N.G., Droznin, V.A., Karpov, G.A., Leonov, V.L., 2009. GeysersValley — Pearl of Kamchatka212 pp. Scientific Guidebook. Moscow 978-5-9610-0099-3, (in Russian).

Trophimov, V.T., Korolev, V.A., 1993. Practicum by Soil Geology, (390 pp. (in Russian)).Wyering, L., Villeneuve, M., Wallis, I., 2012. The effects of hydrothermal alteration on me-

chanical rock properties of the Andesite Breccia and Tahorakuri Formation from theNgatamariki geothermal field, New Zealand and empirical relations between rockstrength and physical properties. New Zealand Geothermal Workshop — 2012 Pro-ceedings 19–21 November 2012, Auckland, New Zealand (https://www.geothermal-library.org/index.php?mode=pubs&action=view&record=8019243).

Zharikov, V.A., Rusinov, V.L., 1998. Metasomatism and Metasomatic Rocks. Publisher“Science World”, Moscow (492 pp. (in Russian)).