OF THE ZEOLITE-RICH TUFFS IN THE PETROTA-PENTALOFOS AREA , EVROS COUNTY , GREECE

The zeolite-rich tuffs of the Petrota-Pentalofos area were deposited in the Orestias Basin during the Eocene. They are up to 100 m thick and extend more than 15 Km in a long axis. Most of the outcrops consist of frequent alternations of very fine grained tuff with pumice and lapilli tuffs, the latter containing detrital fragments of the Mesozoic substrate of the basin. X-ray Diffraction, Scanning Electron Microscopy and light microscopy analysis on quarry and borehole samples has shown that the tuffs are composed mainly of clinoptilolite and minor cristobalite, with a small proportion of detrital constituents (quartz, mica-schist) and pyrogenic crystals (feldspars, quartz, biotite). Minor amounts of mordenite randomly occur in some of the northern outcrops, closer to the occurrences of lava. ICP-AES chemical analysis of the tuffs gives evidence that the original magmas were of quartz-latite composition. The tuffs rest on pre-Cenozoic metamorphic basement and pass gradationally upwards into sandstone and limestone. Evidence is given for deposition of the tuffs in a supra to infra-littoral environment. The zeolitic tuffs originated as epiclastic volcanic sediments, transported by water from the source of the eruption. The transformation of the volcanic glass of the tuffs to zeolite and cristobalite has taken place by meteoric waters in an open hydrological system existed during the Tertiary. The zeolitic rocks are currently being exploited as an animal feed supplement.


Geology of the Zeolitic Thff Occurrences
The volcanic rocks, including the zeolitic tuffs, occur in a NW-SE belt extending through the villages of Petrota (site n.Q 5) and Pentalofos (site n.Q 1).In the hilly country close to the Bulgarian border the exposure is good, but further to the SE exposure is mainly confined to small quarries, and in a large part of this area the Eocene is mainly confined to small quarries, and in a large part of this area the g:i BULGARIA order to minimise preferred orientation.Selected samples were also subjected to repeated treatment with hot hydrochloric acid to remove the zeolite, enabling better identification of the nonzeolitic components.
External standards for X-ray diffraction were made up from mixtures of pure minerals.In the case of clinoptilolite, pure material of an appropriate composition was not available for making standards, so all measurements were made against a natural sample of clinoptilolite-rich, feldspar-poor rock from Pentalofos whose zeolite content had previously been determined by thermogravimetric analysis.This sample contained 9.6 % of zeolitic water, equivalent to 71 % of clinoptilolite by weight.Chemical analyses were made by ICP-AES.For SiO z ,

Introduction
The mineralogy of approximately 250 samples was determined by X-ray diffraction using a Philips X'Pert-APD computercontrolled diffractometer equipped with a monochromator and APD analytical software.The rock powders were prepared in cavity mounts, levelled off by knife edge without pressing, in

Analytical techniques
In Evros County, northern Greece, there are zeolite deposits of both sedimentary and hydrothermal type (Kosiaris et al., 1987;Skarpelis et al., 1987;Marandos et al., 1989;Tsolis-Katagas andKatagas, 1990, Koutles et al., 1995;Stamatakis et al., 1996).The hydrothermally formed zeolites are associated with mixed sulphide and gold mineralization and occur only in small amounts.In contrast, large sedimentary zeolite deposits occur in both the Alexandroupolis and Orestias Tertiary basins, Evros County and are characterised by the abundance of clinoptilolite and/or mordenite (Stamatakis et al., 1996).Those in the Orestias basin are derived from the alteration of Eocene pyroclastic rocks; the compositions of the magmas in this region range from basaltic andesite to rhyolite and trachyte (Andronopoulos, 1977;Kosiaris et al., 1987;Eleftheriadis et al., 1989).
The Pentalofos-Petrota area is situated in the north-western part of the Orestias basin and lies on the eastern edge of the Rhodope metamorphic massif.In the western part of the area (fig.1), Eocene sediments and volcanic rocks lie directly on the pre-Cenozoic metamorphic basement and dip in a generally north-eastward direction, although locally they show steep dips and faulting due to intrabasinal tectonics.Towards the east the Eocene is covered unconformably by clastic Neogene sediments in the central part of the basin (Andronopoulos, 1977;Noussinanos, 1991).
Zeolitized pyroclastic rocks have been located in the NW part of the Orestias basin, in at least six major outcrops.All six outcrops fall within the Pentalofon Sequence of Andronopoulos (l978a), and correspond approximately to his «Cohesive gritbreccia».They lie in a belt extending for 15 km from NW to SE (fig.1).The southernmost edge of the formation occurs at the village Ammovounon, where the tuffs gradually pass into sandstone (Andronopoulos, 1978b).However, similar tuffs are more widely developed in the Bulgarian territory to the west associated with acid and intermediate lava eruptions (Yanev & Bardintzeff, 1997).The subject of this paper is to describe the extent and composition of these zeolite tuff outcrops and to discuss their origin and genetic relationship."""' t::i::':l _ ~==:;-Eocene is concealed by overlapping QUfllernary deposits.Within the volcanic belt.actual lava arc seen at onJy a fcw places in the Petrota area; further to the south-east the volcanic succession consists cntirely of tuffs.
West of the volcanic belt.the Paleogene success ion consists of conglomerate and sandstone of lacustrine or bnlckish water fades with occasional thin intercalations of lignite.80th these continental deposits and the pyroclastic rocks were deposited direclly onto the metamorphic basement.As shown by surface outcrops and borehole data, the tuffaceous rocks of the Pelrota-Pentalofos area are conformably overlain by marlstone and siltstone.Small isohlled outliers of marine limestone of Upper Eocene age discordantly cover either the tuff!; or the marlstone or sometimes the metamorphic basement (fig.2) (Andronopoulos, 1978a&b).
The main outcrops of the tuff are as follows (fig.I): (I) Pentotlofos.Poorly to intensely zcolitized wffs occur in an area of about 0.5 km 2 , surrounded by grey-brownish claystone and marlstone.Rarely, the mClamorphic basement occur in the nearby stream gorges.covcred by green-whitish pumice tuff that hOSI large clasts of the basement rocks.The tuffaceous rocks of the area can be divided into three Iithological divisions.The uppermosl division is represented by a greyish~while fine-grained IUff, lhe middle division is represented by very fine-grained green IUFf, and the lowennost division is composed of green-whilish lapilli luff that hosts many heterogeneous c1asts of various sizes (figs. 2 & 3).
(2) Twin Hills.A small outcrop (0.05 km 2 ) of zeolilic tuff reSts on metamorphic basement (chlori-te~mica-schist).This outcrop is the most severely affected by tectonic activity of the five, and dips panly genlly to the east'.and partly with a high angle to the south easL Most of the marlstone layers that exhibitturbidilie character, arc fossil-rich, containing benthic and plangtonic Foraminifers, as well as small tube~like bivalves of shallow marine origin (Teredo Sp.).Rare gastropods also occur.Some of the Foraminifers have been transporled via small channels for short dislances.In addilion.scattered, recryslallized sponge and echinoid spicules are commonly present.Notably, sponge spicules along with other siliceous tests are also referred from the Oligocene zeolitic lurFs of the nearby castem Bulgaria (Djurova and Aleksiev 1990).The fauna of the bivalves is indicative for the existence of turbidity currents in the basin.which cause a suspension of the nulrienls that are requisite for their growth.llle shell of the bivalves is recryslallized containing silica replacing caldle.A local accumulation of microfossils commonly occurs in some mariSlone beds, due 10 the low sedimentation rates existed after the deposition of the tuffs.Additionally, all micro-fossils detected define an inner infra-liltonll environment of deposil ion (maximum depth of about 80 m).The zeolitized tuff grades into marlslone that encloses thin films of tuffaceous material, and linally to marlstone-siltstone that exhibits turbiditic character.
As it is resulted by the XRD analysis on repre-sClllative tuff samples, the zeolite tuff is very rich in c1inoptilolilC (!able I).
(3) Paleslra.An inlier (> 0.5 km::!) of zeolitic tuffs is surrounded by Quatemary sands and clays.The tuffs here dip generally to the east al approx-imately IO-IY'.At Paleslra, massively bedded coarser units (beds> 30 cm thick) are interbedded with thinly bedded finer units (beds < 10 cm thick), and in some beds there is a gradation from fine into coarse upwards (fig.4).Channel-fill structu.res and lenses up to I m of sand and gravel are visible in a quarry here, and similar mUlerial is present in lhe borehole Pal-G I.The lenses of loose dark brown to black sand and gravel are exclusively composed of fragments of metamorphic rocks such as gneiss, schist and quartzite.They have a north-south long axis, laterally thinning in the tuffaceous host rock.Northeast-Southwest oriented faults with throws of some 10 m are visible in the front of the existing quarry.As a result, the northernmost Palestra zeolite deposit contains only the lowermost part of the succession, whereas the southernmost deposit is more complete.
(4) Petrota. 1 km south-east of Petrota village there exist several small abandoned quarries in an area of about 0.5 km 2 , developed on the zeolitic tuffs.It represents the northern extension of Palestra tuff, having the same sedimentological features and mineralogical composition.
(5) Petrota Paleochorafa.Two outcrops of about 0.3 km 2 of pumice and lapilli tuff occur NE of Petrota village in close proximity to basement rocks.The are associated with claystone, volcanic conglomerates and lava flows.
(6) Petrota Guardhouse.There are small outcrops of less than 0.3 km 2 of tuffs surrounded by volcanic breccio-conglomerate and volcanic domes.In the Petrota Guardhouse and Petrota-Paleochorafa areas, pink or pale green lava is irregularly present adjacent to the main tuff outcrops.The groundmass of these lavas is not zeolitized but is highly silicified and rich in cristobalite.
At all these outcrops the tuffaceous material varies in grain size; coarse volcanic conglomerates and lapilli tuff predominate in the north (Petrota), and fine-grained ash tuff predominates in the south (Pentalofos).Pumice tuff intervals occur frequently in all outcrops but are more abundant at the lowermost stratigraphic levels.In general, the colour of all the tuffs is white (coarser tuffs) or pale green (finer tuffs), but the greenish tuffs usually weather to a creamy colour.
The age of these tuffs and marlstone has been determined as Upper Eocene (Andronopoulos, 1977(Andronopoulos, & 1978a)).From field observations, channel sampling and boreholes measurements, the maximum thickness of the tuff is some 60 m in the southernmost outcrop, increasing to some 90 m in the north, near Petrota.
At Pentalofos, Palestra and Petrota, the tuff is partially exploited by the local authorities to use as road covering during wet periods.-Turbiditic l:iyers from P"J.lcstra quarry.NOle the finegrained zeolite tuff UI lhe head of 'he hamOlcr and the coarser tuff at its handlc.A small congloffierme horizon with channel fill .struclUre is shown at the lOp.

Petrography
The parellfOllm'a Lav:'ls which are believed to represent the parental material of the zeolitic rocks occur at Petrota. as small outcrops and as fmgmenls in volcanic breccias.They are Imites, and consist of plagioclase and K-feldspar phenocrysrs with minor biotile in a cryptocrystalline groundmass.identified by X-ray diffraction as a mixture of crislobalite and K-feldspar.The lavas vmy in colour from pale green 1'0 pink.

The zeolitic '1uffs
There are wide variations in grain size.but the main petrographic features of the tuffs are the same at all the localities.Typically.a coarse-grained turf contains from 10-40 % of crystals, 60-90 % of (devitrified) glassy matrix, and 0-20 % of lithic clasts.Pumice claSIS are present in some turfs but not others.
The crystals are mostly feldspars (plagioclase and sallidine), quartz und biotite, and less commonly muscovite.In a coarse tuff the size of the crystals nonnally ranges up to about 0.5 mm.Cryslals are often broken, and in Lhe case of micas severely bent.However, euhedral crystals are not uncommon, and rounded cryslals also occur.The feldspars and biolile ,Ire normally fresh, excepl in outcrop specimens that show obvious weathering.A minority of biolite crystals are chlorilized.The mineralogical composition of the volcunic (i.e.pyrogenic) material of the tuffs does not vary appreciably across Ihe bell.
The matrix Iypically shows the outlines of cuspate glass sh<lrds, with concave margins representing the fOnller walls of gas bubbles (Plate I; A, B).
Some intact bubbles are preserved.On the whole the glass shards are not flattened and intact bubbles are circular rather than oval in cross section.The lerminations of the cusps are broken rather than streaked out.TIle maximum size of the recognisable glass shards is generally similar to that of the Jelict crystals (plate I; D, E).
Recognisable pumice fragments include some that are naltened and some th:.1t are nol.The flallened ones arc not all oriented in the plane of the bed• ding and they are sometimes mixed with delrital (non-volcanic) material.Non-nanened pumice fragments include some which conlain microscopic pipe vesicles.
The microcrystalline material which constitutes the devitrification product of the gluss shards is anisotropic, but in most cases individual crystals can not be distinguished by optical microscopy.However, in some of the larger glass shards, the secondary mineral (i.e.zeolile) is seen to form radiating crystal aggregates which huve nucleated from the walls of the shard (figs.5 & 6).X-ray diffJaction shows that the microcrystal.linematerial is a mixture of secondary minerals.mainly clinoptilolile but also cristob,llite and in a few cases mordenite or smeCtile.A small proportion of the K~feldspar muy also be secondary.since il occurs in microscopic veinlels CUlling the tuffs at Pcmalofos, and secon• dary K-feldspar of adularia habit has been detected at Petrola by SEM examination (Kirov et al., 1990); however the majority of the K~feldspar is primary, as seen from the sanidine phenocrysts present in the rocks.
Mordenile is much less abundant th:.n c1inoptilolite, occurring randomly und almost exclusively in The lithic dosts include micn-schist.quartzite, gneiss and lava.Lithic c1asts are present at all the localities, bUl generally the biggest blocks occur near to the volcanic centre of Petrota.The fuffs at Penralofos confain fewer derrital constituents and the dusts are only of small size.The metamorphic c1asts are mostly made up of varying proportions of quartz and muscovite.The larger ones could be described as mica-schist.but some of the smaller ones are nearly all muscovite or nearly all quartz (Plme I; E, F).A few larger clast's are made up only of quartz with a granobl;lstic texture and could be described ns quartzite.A minority of the schist clusts comain biolite, indicating that the biotite in the rock may include bolh pyrogenic and detrital contributions, Many of the dispersed crystals in the IUff may be relicts from broken up lithic c1asts: this is particularly true of muscovite (which never occurs <IS a prim ary mineral in volcanic rocks) and epidote, which is present in very small amount).
The fine~grained tuffs show the same features as the coarser grained fUerS.Typically a fine-grained lUll' contains crystals up lO 0.1 mm and (devilrilied) glass shards of the same size.Lilhic c1asts are not nonllillly present in the fine tuffs but muscovite is often present.which could represent dispersed detrital material.is seen as a moderalely uniform particle size of glass particles where the rock was mainly vitreous (Plme I; C), and as roughly equal particle size or vitric and non~vitric grains where detrital or pyrogenic cryslals are present (plme I; D). h can be seen from Plate I; C lhal-the extended cusps lhat would be present in bubble-wall shards of a newly fmgmended pumice-OllS glass have been rounded off, giving the rock the lcxture of an immature glass sand.
Zeolilization posl-d:ues the sedimentary rewor• king because the clinoptilolilC I:ulls which have grown in Ihe glass shards are perpendicular lO the margins of the shards, and do 110t cross the margins al all.Sedimentary re-working of the tuffs is indicaled by lwO pelrographic features of lhe dcvhirfied glassy particles (in addition to their admixlure with non~vol• canic detrital grains and lithic clasts).These are rounding and sorling.The cuspale terminations of lhe bubble•wall shards are nOl sharp or strclched out, bUl have been rounded, i.e. abraded, in a way lhitl could nol be achieved simply by exlreme vesicullltion (see Plale I; C).On Ihe hand specimen or outcrop senle, sorting is shown by the repealed inlcrbedding of fine and coarse-grained tuff (turvidiles) Wilh sharply defined bedding planes that can not be matched in recent ash-fall or as.h•now deposits.Quantitative measuremellls of the mineralogical variation were carried out by X~ray diffraction on a tOlalof 122 samples from lWO of the boreholes (Pen G2 and Pel GI).The Pelrola GI borehole core was divided into 66 sections each 1-2 m long for XRD measurement, and fable I shows the resuhs averaged over 3 main illlervals.The Penlalofos G2 borehole core was divided into 56 sections e.lch appro• ximalely I m long, :md table 2 shows the resulls averaged over 2 main intervals.

Borehole samples
The Petrola-Paleochorafa borehole samples show high zeolite conlenl, comparing with the other boreholes samples.rising to over 65 % throughoul the top 40 111 of the borehole.Approximmely 15•20 % of the rock in lhis borehole consisls of primary minerals.including both detrilal and pyrogenic components.Most of the feldspar and a proportion of llle quartz and the mica (biotilc) are pyrogenic, i.e. they represent phenocrysts preselll in the volcañ ic glass at the lime of eruption.The remainder of lhe quartz and anolher part of the mica (muscovite) are delrital, since it was observed lhal quartz•museovite schist fragments are the mOSl common type of non•volcanic malerial included in the tuffs.At Pelrota the metamorphic componenl makes up a relatively small proportion of the tuffs.The princi• pal secondary minerals in the turf aI Pelrota are zeolile and cristobalite.The amount of cristobalile is between 10 and 16 %, and there is a weak negalive correlaljon between the amounts of zeolite and cristobalite.The crystalline constituents of the luff   amount to approximately 85-95 % of the total rock, the remainder being non-crystalline, i.e. an amorphous residue from the alteration of the original volcanic glass.No unaltered glass was observed.
The Pentalofos borehole samples are broadly similar to those from Petrota, but show slightly different proportions of the main constituents.There is a higher proportion of detrital minerals, indicated by higher quartz and mica, which is mainly muscovite.There is a slightly lower proportion of pyrogenic minerals, indicated by lower feldspar.Unlike at Petrota, the tuffs from Pentalofos show a clear positive correlation between the two main secondary minerals, zeolite and cristobalite.There is a much higher proportion of amorphous material in the Pentalofos tuffs than at Petrota.

Quarry samples
Channel sampling was performed in the Pentalofos, Twin Hills, Palestra and Petrota quarries (table 1).In Pentalofos sampling was performed in an active quarry, where 43 samples were obtained, representing the uppermost 43 m of the tuff.In these samples, the uppermost tuff layers of nine metres thick have a greyish colour, containing more detrital components, such as micas, feldspars and guartz.Smectite is present in some intervals, whereas non-zeolitized volcanic glass is present at that level (fig.3).The next 23 m are very rich in clinoptilolite that is only accompanied by minor cristobalite and volcanic glass (table 3).This succession is more fine grained than the former, representing an ash flow deposit.The lowermost 11 m of the tuff are mainly represented by porous, pumice-like material rich in detrital fragments, especially close to the basement.
The fine grained tuffs that constitute the middle of the section, totalling 23 m seem to be the richest in clinoptilolite of any zeolitic tuff that has been reported from Greece.Twin Hills and Palestra channel samples have similar X-ray mineralogy to those from Pentalofos quarry.They are mainly composed of clinoptilolite with minor cristobalite, Geochemical data potassium and socium-feldspars.The coarse-grained layers contain more pyrogenic and detrital Table 2 shows the chemical composition of the components, such as feldspars, quartz and mafic freshest available lava from Petrota, and zeolitic minerals.
tuffs from the same locality.The lava (analysis 2) Interestingly, Pentalofos quarry that is 200 m lies outside the normal range of magma composisouth of the 3 boreholes show higher percentage of tions, having Si0 2 too high for a rock with no prizeolite comparing with the borehole samples.mary quartz, and a K20/Na20 ratio higher than any In general, field and laboratory measurements on common lava type.The phenocrysts are completely surface and boreholes samples in all outcrops of fresh, e.g. the plagioclase is very clear and has wellthe area, show an increase of the non-zeolitic matepreserved normal and oscillatory zoning and the rial in the tuff samples, from south-east (Pentalofos biotite is free of chloritization or any other obvious quarry) towards to the north-west (Petrota Guardsecondary alteration.However, the former glassy house), owed to the frequent presence of detrital groundmass is clearly enriched in K 2 0 and Si0 2 , horizons or constituents in each particular tuff represented by K-feldspar and cristobalite.Presulayer.
mably this type of alteration was a precursor to the Besides Pentalofos quarry, smectite is present in zeolitization which has affected the vitric tuffs.small amounts (up to 7 %) and in near-surface sam- The compositions of zeolitic tuffs from the three pIes from Petrota borehole, but it is absent in the main outcrops are shown in tables 2a, b & c, and 3. rest of the borehole sections, and is presumed to be In general, all the compositions are similar to one a surface weathering product unrelated to zeolitizaanother, and show the high Si0 2 (even higher on a tion. H 2 0-free basis) and low Fe, Ti, Mg expected of fel-

TRACHYANDESITE
and CaO and lower in Na20 and K 2 0 than the other deposits.These differences suggest cation exchange by Ca-Mg-bearing groundwater, possibly related to the presence of limestone in the Paleogene succession overlying the zeolitic beds.Unpublished electron microprobe analyses of the zeolite in these rocks show that although it is clinoptilolite (i.e. with high Si/AI ratios), Ca is the predominant exchangeable cation (Prof.A. Kassoli-Fournaraki, personal comm.).The Na20 content of all the zeolitized rocks is very low, i.e. much lower than that of any felsic lava type, and is not correlated with either CaO or K 2 0. It appears that Na20 has been strongly depleted by the zeolitization process, which has two implications: (1) that zeolitization took place in an open hydrological system; and (2) the water in which the original ashes were deposited was not seawater.
The trace elements fall into two groups.The high field strenght elements are relatively immobile during hydrothermal alteration and have concentrations appropriate to felsic volcanic rocks as has been mentioned above, whereas the univalent and divalent elements show large variations attributable to cation exchange.This is particularly apparent in the distribution of Sr, which shows a similar pattern of distribution to Ca but far more extreme.In Pentalofos, Ca is higher than average and Sr is extremely high, whereas in Petrota Ca is lower than average and Sr is extremely low.This variation could possibly be related to cation exchange with groundwater that has or has not previously interacted with limestone.In view of the relative selectivity of clinoptilolite for various cations, i.e.K> Ba > Na > Sr > Ca (Tsitsishvili et al., 1992) it is surprising that strontium is so strongly enriched in the Pentalofos tuffs, when potassium and barium are within the normal range for igneous rocks.This suggests that the tuffs have interacted with strontium-rich waters at some time in their history.
There are two possible sources of strontium-rich water that may be considered: either the tuffs were deposited in hypersaline water of marine origin (strontium is the most abundant cation in seawater after sodium, magnesium, calcium and potassium); or they have interacted with groundwater derived from limestone undergoing aragonite to calcite transformation (strontium can substitute for calcium in aragonite but not in calcite or dolomite).A hypersaline environment of deposition is very unlikely because of the very low sodium content of the zeolitic rocks, and reaction with strontium-rich groundwater is much more probable.This is confirmed by the distribution of Sr and other exchangeable cations in the borehole samples (table 7).There Figure 7 shows the relations between the immobile element ratios Zr/Ti and Nb/Y, which are widely used for magma characterisation.On this plot the compositions of the tuffs from the three outcrops plot close together, and all lie at the silicic end of the trachyandesite field, i.e. corresponding to an original magma of latite or quartz-latite composition.This is consistent with what can be seen of the relict pyrogenic minerals, i.e. plagioclase, alkali feldspar and biotite are abundant, but quartz occurs either in small amounts or is absent.
The greatest variation in major element concentrations is in the exchangeable cations Mg2+, Ca 2 +, Na+, and K+ (table 4).Compared with the other two deposits, the rocks from Petrota are lower in MgO and CaO and higher in Na20 and K 2 0. In this respect they are closest in composition to the probable original composition of a felsic magma, although even so the Na20 content is too low for a fresh rock.The rocks from Pentalofos are higher in MgO Origin of the zeolites is a general trend for calcium, magnesium and strontium to increase towards the top of the succession and for potassium and sodium to increase towards the bottom.In effect, the entire zeolitic tuff deposit has acted as a gigantic ion exchange column.
All the quarry exposures are small, and in most of the intervening areas the tuffs are covered by younger deposits.Additionally, the continuity of the deposits is interrupted by faulting.Subject to these limitations, our interpretation on the nature of the deposits is as follows.
The tuffs are not air-fall deposits because beds do not have a uniform thickness over a distance of more than a few metres, and many of them are much too coarse.They are not pyroclastic surge deposits because they do not show the dune-like structures characteristic of surges and they are too well-bedded with many sharply defined bedding planes.They are not pyroclastic flow deposits, also because they are too well-bedded, and in addition there is no welding and only occasional flattening of glass particles (although these features are not present in all ash flows).
The deposits are interpreted as ash that has been re-deposited by water.Bedding and turbiditic structures represented by fine-grained to coarser, pumice tuff alternations are well developed, and low-angle cross-bedding is present in all deposits studied.Channel filling structures are present in Palestra quarry.Non-volcanic detritus is common, especially at Pentalofos and especially in the lower part of the tuff succession.The predominant and most easily recognisable non-volcanic clasts in all five outcrops are those of metamorphic origin, which include minerals characteristic of both low and high metamorphic grades, e.g.chlorite and garnet, suggesting derivation from a large or mixed source area.A characteristic non-volcanic mineral is muscovite, which occurs dispersed in the tuffs as well as being a constituent of metamorphic clasts.There are also abundant large fragments of lava in all the tuffs of the Petrota outcrop, whereas they are absent from the southernmost tuff outcrops of Palestra, Twin Hill and Pentalofos.
The zeolitic tuffs of this area originated as epiclastic volcanic sediments, i.e. they were transported by water from the source of the eruption.They are partially mixed with non-volcanic detritus, but the volcanic component of the tuffs consisted predominantly (c.90 %) of glassy particles at the time of deposition.This glass has been altered to zeolites, whilst the associated pyrogenic crystals remain almost unaltered.
The tuff laminae in the marlstone overlying the main tuff formation and the grey uppermost tuff itself have only partially been transformed to zeolite and cristobalite.Going stratigraphically downwards, the fresh glass has been gradually transformed to clinoptilolite plus minor cristobalite.
This gradual and stratigraphy-related alteration of volcanic glass is characteristic of the action of meteoric waters in an open hydrologic system.It may be possible to determine the time of original zeolitization.The tuffaceous formation was uplifted during the late Eocene, as shown by the unconformity between it and the overlying Upper Eocene reefal limestone that discordantly covers all the older formations.Acid to intermediate tuffs that were erupted in a later stage (Oligocene) near Neon Chimonion (30 km SE of Pentalofos) are not zeolitized.Their original glassy material is partially transformed to smectite and opal but not zeolites.This is evidence that the zeolitization process took place between the late Eocene and Oligocene.At that time the older tuffs were uplifted and affected by circulating meteoric waters in an open hydrologic system.The solutions were rich in silica, as it is resulted by the presence of silicified woods in the clastic series of the Eocene-Oligocene rocks (Andronopoulos, 1977, Prof. E. Velitzelos, 1996 person. comm.).Subsequent modification of the zeolitized tuffs by cation exchange with circulating groundwater could have taken place at any time up to the present day.
Groundwater is still active now, as all the pyroclastic rocks studied here are intensely fractured and have relatively high porosity.The lower part of the tuff succession is currently water-saturated, as shown by several water tables that were met during the drilling project in the area.The pH measured in the water table at 13.5 m depth in the PenG2 borehole of the Pentalofos area is between 7.5 to 8.0.Within the fractures of the tuff, groundwater has deposited brownish/whitish films of secondary minerals, such as smectite and calcite.Smectite could be formed in expense of clinoptilolite, as has been described from other clinoptilolite deposits (Stamatakis, 1989).The potential formation of smectite before and after clinoptilolite (Hay, 1977;Stamatakis, 1989) is evidence that zeolites can easily be transformed to other authigenic minerals at the same diagenetic and post-diagenetic conditions.
Porosity/Permeability of the tuff was a significant factor in zeolitization, because the massive volcanic bombs that are present in a zeolite-rich ground mass in the Petrota outcrop, do not show any degree of zeolitization.
Heat flow does not seem to have played an important role, because the volcanic glass of the lavas adjacent to the Petrota deposit is not zeolitized (+ low porosity/permeability).In addition, the only zeolite found in the tuffs apart from clinoptilolite is mordenite, which occurs in trace amounts in Petrota areas, which are the outcrops closest to the occurrence of lavas.There is no «zoning» of high-temperature zeolites near the contact of lava with the tuff.
However as mordenite is usually formed at higher temperatures than clinoptilolite (Sheppard, Gude & Fitzpatrick, 1988;Kirov et aI., 1990), it is suggested that in the Petrota area the nearby lava extrusions are partially responsible for the local formation of mordenite, which is absent from the other deposits that are further away from the volcanic centres.
The original tuffs were probably deposited in isolated depressions formed by severe tectonic activity that has created an intensely faulted basement (Noussinanos, 1991).On the other hand, late tectonic activity that has affected not only the basement but also the tuffs, and has cut any long zeolitic tuff deposit to small ones.

Commercial importance
The most promising zeolite deposits for mine development in the Petrota-Pentalofos area those of Pentalofos, Palestra and Petrota-Paleochorafa with total proven and probable reserves amounting to 42,700,000 tonnes of clinoptilolite-bearing tuffs.
Based on the results of exploration and specific chemical, physical and mineralogical analyses and tests, the Pentalofos fine-grained zeolitic tuff is the most important among all and can supply material that could be used in a wide range of application, including uses for animal feeding waste water cleaning, fish farming and as additive in cement.
Recently, mining by open-pit-method started in the southern part of the Pentalofos deposit.The fine-grained zeolitic tuff material is crushed, screened in various sizes and bagged in a treatment plant owned by Silver & Baryte Ores Mining Company S.A., Athens, Greece.Most of the extracted material is for swine nutrition and fish-farming applications so far.
Fig. I.-Map showing the studied areas and the sites of the zeolite deposits.

Fig
Fig.4.-Turbiditic l:iyers from P"J.lcstra quarry.NOle the finegrained zeolite tuff UI lhe head of 'he hamOlcr and the coarser tuff at its handlc.A small congloffierme horizon with channel fill .struclUre is shown at the lOp.
PI:lle 1.-Ph01omicrographs of lhill secliollS of zeohtic luffs (all the glass in Ihcse photographs is pseudomorphed by c1inoptilolilc): (A) A lfpieal view showing abraded cuspate glass shards and detrila or pyrogcnic grnins of quartz :lnd muscovite in a matrix of fine-gramcd glass particles.(8) Glass particles with concave margins (bubble waJls) and rounded tcnninations in a matrix of fine-grained glass.(C) Rounded glass p.1nicles in l\ matrix of finer-$rained glass; an elongated flake of detrital muscovite is visible In upper cenlre.(D) Detrlrlll and pyrogenic quartz.feldspar and muscovite alongside glas.'ish:lrds of similar panicle size.(E) A large c1ust of quartz-mica-schisl (centre) in 11 matrix of glass fragmenls.detrital grains and fine-.i\rainedground mass.Small crystals of gamet (high relief) are vIsible in l\ metamorphic clasl at upper right, (F) Same view liS E bUl belween crossed polars, Ihc low firsl order grey and fine grain size of the zeolitized groulldmllss conlraSl with the coarser grllin size ofthc melamorphic c1asts.Localities arc Paleslr:l (A, C) and Penllllofos (8.D. E. F).Photographs taken in planepolllrized light.except F.
[n lhin seclion, sorting Boreholes were drilled by the Silver and Barylcs mining company at Ihree sites in Penlalofos in an area 150 x lOO m, one sile in Paleslra.and one site in Petrola•Pelaochorafa (fig.I), All five drill holes, Pelt Cl.C2 & C3, Pall and Petl, at these sites CUl almOSl alllhe luffaeeous succession and finished by reaching the metamorphic basement.

Table 3 .
-Average compositions of the zeolitic tufTs.

Table 4 .
-Exchangeable cations in borehole samples from different depths.