Zeolites in the Smrekovec volcaniclastic rocks , northern Slovenia

Volcaniclastics from the Upper Oligocene Smrekovec volcanic complex comprise autoclastic deposits, locally resedimented hyaloclastite deposits, pyroclastic deposits, volcaniclastic debris flow and turbidite ash flow deposits and reworked turbidite ash flow deposits. Particularly coarser-grained rocks underwent changes in mineralogy characterised by the development of zeolites and related new-formed silicate minerals: albite, quartz, chlorite, interlayered chlorite/smectite, prehnite, pumpellyite and sphene. Among zeolites, laumontite is the most widespread mineral; it primarily occurs in veins and as interstitial cement but may also replace volcanic glass, pj^ogenetic plagioclases and fine-grained matrix. Other zeolites heulandite, heulandite-clinoptilolite, analcime, stilbite, yugawaralite and thomsonite are less abundant , and are more localised in occurrence. The formation of zeolites and other new-formed silicate minerals is related to hydrothermal conditions generated by emplacement of high-level intrusive bodies into soft, water-saturated sediments.


Introduction
Volcaniclastic material is particularly susceptible to alteration processes. Being formed at much higher temperatures than that of the depositional medium, it is generally not in equilibrium with its low-temperature sedimentary environment. In response to essentially diffrent chemical and physical conditions on the Earth's surface, volcaniclastic constituents undergo the changes in mineralogy, characterised by reactions of hydration. These changes are particulary pronounced in aqueous environments.
Volcaniclastic sediments may be deposited in environments with high chemical gradient of reacting solutions, in areas of high-temperature gradients and/or hydrothermal activity, and in subsiding sedimentary basins. Herein, volcaniclastic material is subjected to physical and chemical conditions departing further from those that prevailed during deposition and early stage diagenesis. As a result, many of the initially stable minerals become unstable or metastable, whereas the stability conditions of many of other secondary minerals are still far from being attained. The mineral reactions taking place are a response to this instability and tend to establish or re-establish equilibrium between various phases and between the phases and the environment.
Zeolites form by weathering upon surface conditions in alkali soils of semiarid areas by interaction of volcaniclastic constituents with alkaline soil water. Hay (1970,1978) has described the alteration of trachytic glass being replaced by phillipsite, chabasite and analcime.
Another environment favourable for the zeolite formation is the so-called open hydrologie system. Meteoric water percolating through a tuff reacts with volcanic glass to increase in pH and alkalinity until zeolites precipitate in interstitial pores and voids of dissolved glass shards. Zeolites and other new-formed minerals are distributed in vertical zones consisting of surface soil, fresh tuff or slightly altered opal-and montmorillonite-cemented tuff, and zeolitic tuff. This type of zeolitization was described for the first time in volcaniclastics of alkali-basaltic composition from Hawaii by H a y and lijima (1968a, b) and lijima and H a r a d a (1968). The open system alteration was also recognised in the Pliocene alkali basaltic volcaniclastic rocks at Grad, NE Slovenia (Kralj, 1995). Common zeolites encountered in this type of environment are phillipsite, chabasite and analcime; gonnardite and natrolite may also occur In tuffs of rhyolitic composition clinoptilolite and abundant smectites form by interaction of silicic glass with percolating ground water (lijima, 1984). The present rather complicated situation in northern and north-eastem Slovenia is associated with global tectonic processes of Late Cretaceous to Tertiary subduction and collision of the continental African and oceanic European plates and their segmented parts, Apulia and the Pannonian fragment (Oberhauser, 1980;Roy den, 1988;Dercourt et al., 1986). In early Miocene, the Pannonian fragment separated from Apulia and began to escape eastward from the collision zone in Eastern Alps. Due to the mentioned eastward escapement, an extension of the Pannonian fragment began, being followed by subsidence, and consequently, the formation of a back-arc basinthe Pannonian basin.
It remains undefined whether the Smrekovec volcanism is related to an active continental margin or to one of the collision combinations: island arc -active continental margin -passive continental margin (Gill, 1981). However, chemical composition of the Smrekovec intermediate volcanics is not very characteristic of orogene andesites (Kralj, 1997). It indicates that tholeiitic magma very possibly underwent a differentiation due to crystal fractionation. Consequently, basalts, basaltic andesites, acid andesites, dacites and finally rhyodacites evolved in time, forming a volcanic suite. The Smrekovec volcanism may be related to local extension and leakage at the plate boundary, as it is the case in central California (Dickinson & Snyder, 1979a, b).
Smrekovec volcanic activity built a complex of submarine stratovolcano(es) with a significantly elevated relief composed of lavas, high-level intrusive bodies, autoclastic deposits, pyroclastic deposits and syn-eruptive resedimented volcaniclastic deposits (Kralj, 1997). The early stage of volcanic activity was dominantly non-explosive. Basalts and basaltic andesites were emplaced as submarine lavas or high-level intrusive bodies. The style of fragmentation was mainly autoclastic, related to chill and quench processes. The late-stage volcanic activity is characterised not only by non-explosive volcanism of acidic andesitic to rhyodacitic composition, but also by explosions, either combined hydrovolcanic and magmatic, or solely hydrovolcanic. Juvenile material, chiefly pumice and glass shards, became relatively abundant. Explosive volcanic activity was probably instrumental in generation of volcaniclastic debris flows and turbidite ash flows. Their deposits are recently the most widespread throughout the Smrekovec volcanic complex.

Zeolites and accompanying secondary minerals in the Smrekovec volcanics
Some of the volcaniclastic, autoclastic and coherent volcanic rocks have undergone the changes in mineralogy, characterised by the development of zeolites and other new-formed minerals: interlayered chlorite/smectite, albite, quartz, prehnite, pumpellyite, epidote, sphene, apophyllite, alkali feldspars and amphiboles (K o v i č & Krošl-Kuščer, 1986;K o v i č, 1988). They are abundantly developed on the contacts of high-level intrusive bodies with the enclosing sediments or in their vicinity. Particularly zeolitization was strongly controlled by porosity and permeability of sediments and is more pronounced in the coarser-grained volcaniclastics. This can easily be recognised in interbedded coarser-and finer-grained volcaniclastic rocks from the same profile: in coarser-grained varieties, laumontite, prehnite and pumpel-lyite developed, whereas interbedded, well-sealed fine-grained volcaniclastics do not contain either laumontite or prehnite and pumpellyite.
Rock composition controlled the kind of zeolite developed, although to some extent only. Laumontite occurs in the rocks of various composition, from basaltic to rhyodacitic. It replaces the primary constituents (volcanic glass, fine-grained matrix or plagioclases) or infills interstitial space, voids or fissure systems. On the other hand, clinoptilolite and heulandite occur mainly in hyaloclastites of acid andesitic to dacitic composition replacing volcanic glass and infilling vesicles and the rock pore space. Analcime and thomsonite developed in some complexly altered rocks of basaltic to basaltic andesitic composition. Herein, analcime replaces formerly developed laumontite, and albitised plagioclases. Yugawaralite and stilbite are characteristic vein minerals, and do not seem to be influenced by the host rock composition.
Studies of zeolites and accompanying new-formed minerals in the Smrekovec volcanics are based on X-ray diffraction (determination of mineral composition of 94 powdered samples, and determination of cell parameters for 3 analcimes), pétrographie investigation under the microscope (86 thin sections), elemental analysis by scanning electron microscope and energy dispersive X-ray spectrometry (30 zeolites and accompanying new-formed minerals), and combined chemical analysis -wet, atomic absorption spectrometry and emission spectrometry with inductively coupled plasma source (4 zeolite bearing rocks and 3 separated analcimes).

Laumontite -Ca^iAl^Si^ßO^g).I6H2O
Laumontite is the most widespread new-formed zeolite in the Smrekovec volcanics. Very commonly, it can be encountered in veinlet systems (plate 1, fig. 1). It also infills vesicles of volcanic lithic fragments (plate 2, fig. 1, 2) and interstitial pore space (plate 2, fig. 3). Replacements of the primary constituents -pyrogenetic plagioclases (plate 2, fig. 4) or volcanic glass (plate 3, fig. 1, 2) are somewhat less abundant. In general, the amount of laumontite rarely exceeds 20 wt.% of the whole rock, even in the most extensively altered volcanics. The average laumontite content, determined by X-ray diffraction method in 48 of the laumontite-bearing rock samples, ranged between 5 and 15 wt.%. The accompanying new-formed minerals determined are quartz, albite, chlorite and interlayered chlorite/smectite, written in the order of descending abundance. The amounts of prehnite, sphene, pumpellyite, epidote or apophyllite were beyond the X-ray detection limits; these minerals can only be recognised under the microscope.
Laumontite crystals are very seldom transparent in a hand specimen (plate 1, fig. 1); most commonly they are earthy whitish. Crystal size ranges from some 10 pm up to 2 mm; the largest crystals formed in veins. Elemental analysis of ten laumontite samples by scanning electron microscope and energy dispersive X-ray spectrometry (SEM-EDX) revealed that laumontite may also contain small amounts (approx. up to 2 wt.%) of K2O ( fig. 2) besides calcium; sodium has not been detected in any of the examined samples.
Laumontite commonly replaces volcanic glass along with albite and quartz as accompanying new-formed minerals (plate 3, fig. 1, 2). Intergrowths of laumontite, albite and quartz are sometimes very fine-grained, detectable by X-ray diffraction only, although sometimes they may be recognised under the microscope, too. Replacements of volcanic glass by laumontite are often observed in hyaloclasts of autobrecci- Consequently, local hydrothermal conditions arised affecting predominantly marginal parts of the high-level intrusive body and the hyaloclasts of peperitic breccias. The laumontite-albite-quartz mineral assemblage commonly replaces spherical areas of hydrated volcanic glass produced by perlitic cracks (plate 3, fig. 1, 2). Laumontite from the Smrekovec volcanics can also replace pyrogenetic plagioclases and alkali feldspars, although the majority of plagioclases is albitised. According to Coombs et al. (1959), laumontite replaces the anorthite component in plagioclases whereas the albite component alters to fine-grained aggregates of albite. Some of alkali feldspars are altered to laumontite and secondary alkali feldspars. The two new-formed minerals are not intimately intergrown but replace crystal grains in the form of irregular patches, attaining a few tenths of mm in length.
Microfissures developed in the Smrekovec volcanics are often infilled solely by laumontite (table 1) although veinlets containing besides laumontite also one or two other zeolites -i.e. analcime (plate 3, fig. 3, 4), stilbite, yugawaralite, or yugawaralite and analcime, can also be encountered. On the other hand, the prehnite association with laumontite is rather common, not only in veins, but also in interstitial infillings of volcaniclastic and autoclastic rocks (plate 1, fig. 3, 4; plate 2, fig. 3). Laumontite postdates and also replaces prehnite. This is a very exceptional relationship between the two minerals, since in burial environments where prehnite replaces laumontite, the situation is opposite (Boles & Coombs, 1975;1977, Thompson, 1971. According to the activity diagram of phase relations for laumontite, heulandite and prehnite (Boles & Coombs, 1977), heulandite alters either to laumontite or prehnite when the activity of hydrous silica ajj4SiQ4(aq) decreases. Both reactions are strongly controlled by the activity ratio аса2+/(ан+)^-The reaction from laumontite to prehnite occurs unlikely in the presence of waters saturated with quartz, since silica is, along with water and the H"^ ions the reaction byproduct (Boles & Coombs, 1977. Instability of prehnite and its conversion to laumontite evidenced in the Smrekovec volcaniclastic rocks could therefore be related to the decreased ratio аса2+/(ан+)^ in reacting solutions; additional favourable conditions might be the increased activity of hydrous silica aH4Si04(aq) ^^^^ the decreased temperature of reacting solutions.

Heulandite (Na,K) Ca^(AlgSÌ2jOj2} • 24H2O and clinoptilolite (Na,K)e(AleSÌ3o072) • 2OH2O
Heulandite and clinoptilolite form a continuous solid solution series along the join between the stoichiometric fomulae given above (Mumpton, 1960;Gottardi & Galli, 1985). Heulandite, clinoptilolite and numerous members of the heulandite-clinoptilolite solid solution series altogether belong to the heulandite group; for this reason, the name heulandite may sometimes refer to the whole genus. For proper distinction of heulandite and clinoptilolite at least the thermal test of Mumpton (1960) must be applied.
Heulandite is very common in hydrothermally altered basic volcanics as vesicle and fissure filling. Sedimentary occurrences of heulandite with proper evidence are rare. On the contrary, clinoptilolite is a veiy rare hydrothermal mineral and has been shown to be the main constituent of many sediments, and is hence much more abundant in the Earth's crust than heulandite (Gottardi & Galli, 1985).
In the Smrekovec volcanics heulandite replaces volcanic glass of acid andesitic composition, being accompanied by cristobalite/quartz and montmorillonite; it also infills pore space in the same rock. Heulandite has not been encountered as vesicle and fissure filling in the rocks of more basic composition; therein, laumontite is the predominant zeolite.
Common hostrocks of heulandite are resedimented hyaloclastites (plate 4, figs. 1, 3, 4) which also contain pumice lapilli and glassy, fine-grained matrix of similar composition. Plagioclases are fresh. The new-formed minerals are very fine-grained and can not be recognised under the microscope. Heulandite crystals sometimes attain up to some hundred pim (plate 3, fig. 1); smectite montmorillonite occurs in globular aggregates having a few hundred pm in diameter (plate 3, fig. 1). Heulandite and cristobalite or microcrystalline quartz are intimately intergrown when replacing volcanic glass. Besides heulandite, very small amounts of analcime may locally occur According to Mumpton (1960) clinoptilolite remains stable after being heated for twelve hours at 600°C, whereas the heulandite lattice collapses. X-ray diffraction patterns of four thermally treated samples have confirmed the presence of heulandite; in one of the samples solid solution heulandite-clinoptilolite with predominating heulandite component has been determined (figs. 3a, 3b).
Resedimented hyaloclastites with pumice lapilli occur in the form of scarce, small and isolated erosional remnants on the top of the mountain range from Komen to Smrekovec. They are also to be found along the southern and northern slopes of Komen, Krnes and Smrekovec, dipping outward from the top of Komen towards the southeast and northwest, respectively. In general, the hyaloclastites contain heulandite, heulandite-clinoptilolite, smectite and quartz. Locally, they can be found altered to laumontite, albite, quartz, interlayered chlorite/smectite and traces of analcime. The laumontite-albite-quartz-chlorite/smectite-(analcime) mineral assemblage occurring in the same rock layer as the heulandite-cristobalite/quartz-smectite could indicate the presence of the progressive zeolite reaction pattern: silicic andesitic/dacitic glass -> clinoptilolite-cristobalite/quartz-smectite (mordenite)-heulandite-analcime-cristobalite/quartz-smectite laumontite-albite-quartz-interlayered chlorite/smectite. However, the relationship between laumontite and heulandite seems to be more complicated. In heulandite-bearing hyaloclastites, laumontite locally occurs in very small amounts being developed as the replacement of volcanic glass in larger hyaloclasts or as interstitial cement. Herein, laumontite was found to be partially replaced by clinoptilolite-heulandite (plate 4, fig. 2). Microscopic observation and X-ray analysis indicate the transformation can be either direct or related to prior alteration of laumontite to kaolinite or montmorillonite. The occurrence indicates that a post-hydrothermal process, diagenesis or halmyrolysis, must be superimposed on the earlier alteration.

Analcime
Naiß(AlißSi320gß).16H20 Besides some subordinate occurrences of analcime developed during the progressive alteration of silicic andesitic or dacitic glass to heulandite, smectite and cristoba-lite, analcime may also show very exceptional style of formation. In the northern slopes of Smrekovec, extensively altered autoclastic and volcaniclastic rocks occur containing up to 60% of analcime (plate 1, fig. 2). Herein, analcime replaces formerly developed laumontite and albitised plagioclases, and is accompanied by interlayered smectite/chlorite.
A complex alteration history leading to analcime development can be observed in a 50 metres thick profile in the northern slopes of Smrekovec. The early stage of alteration is characterised by an intrusion of basaltic andesite into volcaniclastic sediments. Andesite marginal parts were autobrecciated and autoclasts partially admixted to the enclosing sediments. A plagioclase-rich dyke of similar composition cuts the andesite. By contact metamorphism and hydrothermal activity related to the andesite emplacement laumontite extensively developed in the layer of autoclastic andesite along with albite, quarz, interlayered chlorite/smectite and traces of sphene. Small amounts of prehnite and pumpellyite also occur in this autoclastic layer that was situated immediately above the source of heat. Herein, pumpellyite may replace plagioclases along with albite and prehnite (plate 5, fig. 1, 2) or infills vesicles in autoclasts (plate 5, fig. 4). The laumontite-albite-quartz-chlorite/smectite mineral assemblage is developed above the andesite intrusive body for over 120 metres, up to the top of Smrekovec. However, the laumontite content in the section is fairly variable, but is generally much lower in volcaniclastic rocks (5-20 wt.%) than in the autoclastic layer where it may attain up to 50 wt.%. Interstratified fine-grained volcaniclastic rocks, even if situated in close vicinity of the intrusive andesite, contain only traces of zeolites, whereas plagioclases are completely albitised and the matrix replaced by interlayered chlorite/smectite. This high-level intrusive body of basaltic andesitic composition is interrupted by another andesite body -probably a feeder dyke -which is of acidic andesitic composition. Analcime is closely related to this late-stage intrusion and predominantly follows previous alteration replacing laumontite. It is very localised in occurrence; at a distance of some 10 metres laterally from the intrusion, analcime becomes very scarce -often below the X-ray detection limit. Herein, incomplete replacements of laumontite by analcime are commonly encountered (plate 3, fig. 3, 4). Closer to the intrusion, analcime becomes more pronounced, replacing not only laumontite but also albitised plagioclases (plate 1, fig. 2). Analcime is particularly abundand in autoclastic rocks that previously underwent extensive laumontite alteration. The replacement of laumontite by analcime is accompanied by crystallisation of alkali feldspars (plate 5, fig. 3). The presence of alkali feldspars was confirmed by elemental analysis of seven analcime-rich samples by scanning electron microscope and energy dispersive X-ray spectrometry ( fig. 4). As already mentioned, laumontite may also contain, besides calcium, small amounts (approx. up to 2 wt.%) of K2O ( fig. 2); during the reaction from laumontite to analcime, potassium might have been fixed by crystalisation of alkali feldspars. Together with alkali feldspars, up to 200 mp sized exsolutions of thomsonite sometimes occur Chemical analyses of four analcime-or laumontite-bearing rocks important for interpretation of analcime occurrence in the Smrekovec volcaniclastics is shown in table 2. The rock samples no. 3 (Sm 34/51) and no. 4 (Sm 34/11) are texturally alike and also, similarly extensively altered. The only conspicuous difference is in the type of zeolite developed: the rock sample no. 3 contains analcime, and the rock sample no. 4, laumontite. It is very interesting that no obvious distinction between the abundances of major elements can be observed, although at least the difference in the sodium and calcium contents would be expected. The rocks could have undergone some ion exchange processes in interlayered smectite/chlorite clay minerals after crystallisation of zeolites. Analcime has been separated almost completely from the bulk samples of extensively altered rocks by the use of heavy liquids. Three relatively pure analcime samples containing no other minerals detectable by X-ray diffraction were obtained. The analcime samples were investigated by the means of X-ray diffraction method (tables 3, 4, 5) and combined wet chemical analysis, atomic absorption spectrometry and optical emission spectrometry with inductively coupled plasma source (table 6). The results have shown that analcimes are cubic, low-silica and calcian varieties. No solid solution with wairakite (A oki & Minato, 1980;Harada & Sudo, 1976) can be assumed.
The analcime occurrence bears evidence of a very complex alteration history of the Smrekovec volcaniclastic rocks. Analcime is superimposed on the earlier, laumontite yielding alteration, and is related to the late-stage emplacement of an acid andesite body -probably a feeder dyke. Experimental work on hydrothermal alteration of the Smrekovec volcanics (table 1, sample no. 1) performed by Barth-Wirsching (pers. comm.) indicates laumontite alters to analcime in closed or open system at the temperatures of above 150°C by action of sodium-bearing reacting solutions. Hydrothermal fluids responsible for the laumontite to analcime transformation could have been magmatic in origin but it is also possible that marine water from the sea-bottom became superheated when penetrating along the fissures opening the pathway of the ascending magma. Stilbite and jaigawaralite are typical hydrothermal zeolites (Gottardi & Galli, 1985). In the Smrekovec volcanics both stilbite ( fig. 5a) and yugawaralite (fig. 5b) occur only as vein minerals, being always accompanied by laumontite. Stilbite commonly crystallises at lower temperatures than laumontite (lijima, 1984;Boles & Coombs, 1975;L i o u, 1971a). Yugawaralite develops at higher temperatures than laumontite and in comparison with wairakite at lower pressures (L i o u, 1971b).
In veins, yugawaralite and laumontite may also be accompanied by analcime. One of the veinlets containing yugawaralite, laumontite and analcime occurs in a finegrained tuff which does not contain zeolites but is located in the vicinity of analcimerich rocks. Immediately above the contact with tuff, a few mm thick layer of finegrained laumontite and yugawaralite occurs. Above this layer, cubic crystals of anal- Table 3. X-ray diffraction pattern of analcime (sample N 34 1/4 L) Tabela 3. Zapis rentgenske difrakcije vprašenega vzorca analcima, izdvojenega iz kamenine v težki tekočini (vzorec N 34 1/4 L) Sample N 34 1/4 L, separated from altered volcaniclastic rock from northern slopes of Smrekovec; Philips diffractometer, Ni filtered CuK" radiation (X = 1.54051), slits 1°, 0.1 mm, 1°, scanning speed l°/min; cuMc cell parameter a = 13.1950 cime developed; the analcime crystals are of approx. equal size of 2-3 mm. On the analcime crystals, fine-grained laumontite occurs. The described succession of vein zeolites indicates that laumontite crystallised before and after the analcime. Analcime, occurring between the layers of calcic zeolites seems to crystallise during short episode of sodium-yielding hydrothermal activity.
Laumontite is a common zeolite in different environments. Upon burial and contact metamorphism, it forms from a zeolite precursor -most frequently heulandite, but also mordenite or clinoptilolite (Coombs et al., 1959;Boles & Coombs, 1975;1977;lijima & Utada, 1966;Utada, 1973). On the other hand, hydrothermal genesis of laumontite, attributed to those crystals filling veins and fractures with no obvious reaction of the mineralising fluid with the wallrock, is also rather common (Gottardi & Galli, 1985). For comparison of the laumontite occurrence in the Smrekovec volcaniclastics, the alteration upon contact metamorphism, encountered in Neogene sediments of Japan is particularly interesting. The following text is a very breef summary of the comprehensive work of Utada (1973). Neogene sediments surrounding volcano-plutonic masses underwent complex changes in mineralogy related to contact metamorphic, diagenetic and hydrothermal alteration. According to the assemblages of new-formed minerals eight alteration zones were recognised. Higher-grade zones are completely metamorphic and comprise: the homblende-plagioclase zone, the actinolite-plagioclase-chlorite zone, the prehnite-epidote-plagioclase-chlorite zone, and the chlorite-epidote-plagioclase-quartz zone. Lower-grade alteration zones comprising abundant zeolites are the following: the laumontite-chlorite-plagioclase-quartz zone, the analcime-heulandite-chlorite-montmorillonite-quartz zone, the mordenite-montmorillonite-opal/quartz or the clinoptilolite-mordenite-montmorillonite-opal zone, and the zone of altered volcanic glass, montmorillonite and opal. The laumontite-bearing zone commonly spreads in the outer areas apart from the intrusive mass but sometimes it also immediately surrounds intrusive bodies of small sizes. Laumontite replaces plagioclase phenocrysts, finegrained matrix and groundmass of various rocks, and is interspersed with other newformed minerals. It also occurs in druses and as a vein mineral. The original rock texture is relatively well preserved. In the Smrekovec volcaniclastics laumontite is the most widespread zeolite, developed as interstitial filling, a vein mineral or replacement of volcanic glass and plagioclases. The average laumontite content in altered volcaniclastic rocks rarely exceeds 20 wt.% of the bulk composition. The replacements of volcanic glass and pyrogenetic plagioclases are more localised in occurrence and related to the proximity of high-level intrusive bodies. The degree of zeolitisation is also strongly dependent on porosity and permeability of the host-rock; this relationship is the most obvious in the sections, composed of interbedded coarse-grained rocks containing abundant zeolites, and fine-grained tuffs which lack of zeolites, except for fissure fillings.
Laumontite and other zeolites show no obvious zonal arrangement. Away from extensively altered rocks encountered in close vicinity of high-level intrusive bodies, laumontite-cemented volcaniclastics grade into the rocks in which zeolites do not occur any more, not even as vein minerals. The only occurrence which could indicate the presence of two possible zones with defined progressive reaction pattern, is related to resedimented hyaloclastites spreading from the top of Komen towards the south-east and north-west. The hyaloclastites are generally altered to heulandite, heulandite-clinoptilolite, quartz and montmorillonite. Locally, laumontite, albite, quartz and interlayered chlorite/montmorillonite are encountered in the same type of rocks; due to extensive erosion of hyaloclastites it is uncertain whether the two alteration patterns occur in exactly the same layer. In heulandite-bearing rocks, scarce remains of laumontite occur being extensively replaced by clinoptilolite-heulandite. This relationship between the two minerals would hardly justify the progressive reaction pattern and the existence of zonal arrangement of zeolites. It strongly suggests that other mechanisms -diagenesis or halmyrolysis -must have operated after the hydrothermal stage of alteration. Yugawaralite is a vein mineral genetically related to crystallisation from hydrothermal fluids. Stilbite is very common hydrothermal zeolite although it can also be encountered in burial environments as is the case in Taringatura Hills, New Zealand (Boles & Coombs, 1975;1977). Analcime in the Smrekovec volcaniclastics is of hydrothermal origin formed during late-stage emplacement of a high-level intrusive body -most probably a feeder dyke.
If the zeolite occurrence in the Smrekovec volcaniclastics is compared with the previously described contact metamorphic alteration, it can be concluded that higher-grade metamorphic zones are missing. Zeolites do not show any obvious zonal arrangement although an enhanced rock alteration in close vicinity of the outcrops of high-level intrusive bodies suggests their emplacement must have been instrumental in the development of laumontite and other zeolites but was probably too small to produce zonation recognisable on larger scale. On the other hand, laumontite and heulandite locally replace volcanic glass indicating the precipitation from hydrothermal fluids could not have been the only mechanism responsible for the zeolite development.

Conclusions
The Smrekovec volcaniclastic rocks underwent alteration characterised by the development of zeolites and related silicate minerals: albite, quartz, chlorite and interlayered chlorite/smectite. Laumontite is the most widespread in occurrence; heulandite, heulandite-clinoptilolite and analcime may locally be abundant whereas stilbite and yugawaralite can be encountered only as vein minerals. Laumontite developed as replacement of the primary constituents -volcanic glass, pyrogenetic plagioclases and a fine-grained matrix, and as abundant interstitial filling and a vein mineral. Heulandite and heulandite-clinoptilolite occur abundantly in resedimented hyaloclastites of acid andesitic to dacitic composition. Herein, they replace volcanic glass and infill vesicles in glassy hyaloclasts or pumice lapilli. Analcimerich rocks are very localised in occurrence. Herein, analcime replaces previously developed laumontite, and rarely also albitised plagioclases. It formed during the late-stage emplacement of a dyke into already lithified and alterd volcaniclastic rocks.
Zeolites developed in the Smrekovec volcaniclastics, their occurrence and association with prehnite and pumpellyite indicate their formation to be closely related to local hydrothermal conditions generated in water-saturated sediments by emplacement of high-level intrusive bodies. This intrusives were obviously too small sources of heat to produce zonation on kilometre scale as encountered in contact metamorphic settings.
Quartz, interlayered chlorite/smectite and albite are widely developed throughout the Smrekovec volcanic complex, irrespective to finer-or coarser-grained texture of volcaniclastic rocks, their position or zeolite content. For this reason, they could have also developed upon shallow burial diagenesis.