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high heat flow, whereas the deposits in Samos were for- med in a .... 4- 6. General features of these tuffs are the presence of fine-grained zeolite partially re-.
Mineral. Deposita 3 1,473- 481 (1996)

MINERALIUM DEPOSITA ©Springer-Verlag 1996

The zeolite deposits of Greece M.G. Stamatakis 1, A. Hall 2, J.R. Hein 3 1

Depa rtment of G eology, Na tional University of Athens, Pa nepistimi opolis, A no llissia 157 85 Athens, Greece Depa rtme nt of G eology, Royal Holloway, University of L ondon, Egham, Surrey T W20 OEX, UK 3 US G eological Survey, 345 Middlefield Rd. MS 999, Menlo P ark, California 94025, USA 2

Received: 6 April 1995 / Accepted: 10 April 1996

Abstract. Zeolites are present in altered pyroclastic rocks at many localities in Greece, and large deposits o f potential eco nomic interest are present in t hree areas: (1) the Evros regio n of the province of Thrace in the no rtheas tern pa rt of the Greek mainland; (2) the islands of Kimolos and Po liegos in the western Aegean; and (3) the island of Samos in the eastern Aegean Sea. The deposits in Thrace are of Eocene-Oligocene age and are rich in heulandite and/o r clinoptilolite. Those of Kimol os and P oliegos a re mainly Quaternary a nd are rich in mo rd enite. Those of Samos are Miocene, and are rich in clinoptilo li te and/ or analcime. The d eposits in Thrace are believed to have formed in an open hydrological system by the actio n of meteoric wa ter, and those of the western Aegean islands in a similar way but under conditions of high heat fl ow, whereas the d eposits in Samos were fo rmed in a saline-alkaline lake.

Large zeolite deposits in sed imentary rocks a re mostly of Tertiary age, older zeolites generall y having bee n transfo rmed by diagenesis into other aluminosilicates. In most cases t he zeolites originate by the direct t ransformation o f volca nic ash, o r by diagenesis of sediments con taining a volcaniclastic component and/ or bioge nic silica (Hay 1977). The glassy component of volcanic ash is t he single most importa nt precursor for zeolites in sedimentary rocks (lijima 1988). Conditio ns in Greece durin g th e T ertiary were particularly favo urable fo r t he fo rmation of sedimentary zeolite d eposits because of: (1) repeated and ex tensive vo lca nism, with a high proportion o f p yroclastic products, (2) high hea t flow, a nd (3) an arid clima te during the Neogene, leading to the d evelo pmen t o f saline co ntinenta l basins. Zeolites occur in T ertia ry sedimentary rocks at many localities in Greece (Fig. 1), and large deposits o f pote ntial economic interest are presen t in th ree areas: (1)

Correspondence to: A. Hall

the Evros regio n of t he province o f Th race (Th raki) in the no rtheast of t he Greek mainlan d; (2) the islands of K imolos a nd P oliegos in the western Aegean; a nd (3) the island of Sam os in t he eastern Aegea n Sea. T he age, mineralogy, and paragenesis o f the depos its is different in each of th ese three areas.

Zeolite deposits of Thrace The province of T h race contains zeoli te deposits of bot h sedimenta ry a nd hyd rothermal origin. T he hydrothermal occu rrences are associated with mixed sulphide a nd gold mineralization and are not p resent in sufficient q uantity to be of economic in terest. I n contrast, the sedimentary zeolite deposits that occu r in the areas o f MetaxadesPetrota a nd Lefkimi-Ferrai are presen t in large quantity a nd have high zeolite con tents, i.e. zeolites are the main rock-forming mine rals. T he tuffaceo us depos its of Metaxades-Petrota and Lefkimi-Ferrai were deposited in adjace nt shallow marine basins, separated by a barrier of Mesozoic rocks. T he L ejkimi-Ferrai area is sit uated in the Alexandroupolis basin. The Te rtia ry succession in this area is partly marine and partly non-mar ine, a nd com prises Eocene-Miocene clastic sedimentary rocks, limestones, lavas and tuffs, and several lignite seams (Andronopoulos 1977). T he lavas and p yroclastic rocks are O ligocene in age, and have compositions ranging from a ndesite to rh yoli te (Papadopoulos J980; In noce nti et a!. 1984; Kosiaris et al. 1987; Skarpel is et a l. 1987). Pyroclastic rocks are represen ted by ash -fall and lapilli tuffs. Some tu ffs show cross-bedding whilst in ot her places they are unstratified. I n some places the tuffs overlie U pper Eocene clastic sedimenta ry rocks and elsewhere they we re directly deposited on p re-Oligocene metamo rphic basement. The type o f depos itio nal enviro nmen t th at existed during p yroclastic volcanism has not been established. Terrestria l, lacustrine, and shallow mari ne features have all been described, b ut sedimentary structures in the tuffs suggest subaqueous deposition. T he

474

' Petrota

N

i

• Lefkimi • Ferrai 0

~ ~~

Samos

\::"-,

~ (Na + K) and in clinoptilolite (Na + K) > Ca. Electro n microprobe analyses of the Metaxades example by Tsolis-Katagas and Katagas (1990) show that Ca is the principa l cation, i.e. Ca > (Na + K) and also K ~ Na, although the SijAl ratio is greater than 4. The Metaxades mineral also breaks down on heating to 500 C, which is typical of heul andite but not clinoptilolite. Nevertheless Tsolis-Katagas and Katagas (1990) found that minor amounts of K-rich clinoptilo li te and mordenite coexist with the predominant heulandite in the Metaxades deposit.

475 The tu fTs fro m M etaxades all co ntai n heula nd ite and opal-CT , an d mos t also contain K-feldspa r, wh ich is partly of vo lca nic orig in and partly second a ry. T he presence of seco ndary K-felds pa r in these rocks was revealed by the high conte nt of fi xed a mmo nium (H all et a l. 1994) and by SE M observatio ns (th is study). The p rese nce of seco nda ry K-feldspa r (ad u la ria) at P etrota has p revio usly been descri bed by K irov e t a l. ( 1990). Some petrogra phic rela tio nships o f t he tu ffs fro m Thrace a re shown in Figs. 4- 6. G ene ral fea tures o f these tu ffs a re the presence of fine-grained zeo li te pa rti ally replacing glass a nd o f well-crystallized zeolites filling cavities, the prese nce of rhomboid (adularia -like) second a ry K-fcldspa r, a nd the overgrowth of prisma tic o r ta bula r zeo li tes (he ul a ndite o r clinoptilo lite) by fi b ro us zeolites (mainl y mo rd enite).

T he conditions t hat led to the zeolitization of the deposits a t Lelki mi and Metaxades are no t yet clea r. The Lelkimi zeol itic rocks were derived from a dacitic-an desitic pa rent material and the Metaxades zeolitic rocks from a more felsic pa ren t material. The occurrence of a uth igenic K- feldspa r at Metaxades and P etrota suggests simila ri ties with saline-alkaline lake deposi ts, but this was not t he environment of de position beca use t he zeolitic tuffs a re followed by shallow marine to nea rshore limesto nes a nd there are no evaporites in t he succession. The tuffs of the M etaxades a rea a re qu ite fi ne-grained, so we ass ume tha t they were de posited from vo lca nic cent res fa r from the d eposi tio nal a rea, and a ny geot hermal co ntrib uti o n can be excl uded. The Miocene an d/o r P liocene rocks which overlie t he Oligoce ne tu ffs were deposi ted o nly in t he cen tral part of the basi n, a nd therefo re b urial

F ig. 4a - d. SEM photographs of a zeolitic tuff from Avdella, T hrace: a void containing tabular heulandite, with minor overgrowth of fibrous mordcnitc, b heulandite overgrown by fibrous m ordenite, c rhomboid K-feldspar crystals occur ring in a void along with tabular heulandite, d a ltered vesicular glass completely a ltered to matted zeoli te, and showing a K-feldspar rhomb and tabu lar crystals of heulandite resting on the surface

476

Fig. 6a, b. SEM photo of a zcolitic tuff from Mak rylofos. near Ferrai, Thrace: a glass shards showing the walls of microscopic pipe vesicles with interstitial clay minera ls, b fibrous and bladed zeolites lini ng a cavity in the same specimen

Zeolitic rocks of Samos

Fig. Sa- c. SEM p hotographs of a zeolitic tuff from Mctaxades, Thrace: a rosenes of tabular he ulandite: b a large crystal of secondary K-feldspar of adularia ha bit, c void lined with silica fibres, possibly replacing bacteria or fungus

diagenesis is no t a plausible explanation for zeoli te formati o n. For these rea sons, the fo rma tion of zeolite deposits in this a rea is attributed to the action of meteoric wate r in a n o pen hydrological system.

The island of Samos co nta ins two small Neogene sedimentary basins sepa ra ted by metamorph ic basement rocks. Volcanic tuffs occur in the Neogene succession in both basins, and have been ex tensively zeol itized in the western Karlovassi Basin, but are zeolitized ha rdly a t all in the eastern Mi tilini Basin. The succession in the Karlovassi Basin is composed of late Miocene lacustrine a nd terrestrial rocks, followed by Pliocene freshwater marls to ne. M iocene basaltic to silicic la vas o ccur alo ng t he eastern ma rgin o f the basin. Lacustrine sediments are represented by as hes a nd tuffs, claystone, marls tone, limestone, and lenses or t hin seams of eva po rite minerals such as gypsum, coleman ite, and celestine (Stamatakis and Economou 1991). The t uffaceous rocks have undergone diagenetic tra nsfo rmation to

477

zeolites (clinoptilolite, analcime, chabazite, mordenite), silica (o pal C-T), and boron-bearing K-feldspar (Stamatakis L989a, b). C li noptilolite and analcime are the most abundant zeolites. There is an irregular but ro ughly concentric zonation of the secondary minerals in the altered tuffs: (1) a marginal zone of detrital (non-authigenic) silicates an d carbonates; (2) zeolitic zo nes characterized by abundant clinoptilolite and/or analcime; and (3) a central zone containing boronbearing K-feldspar and sporadic occurrences of borate minerals (Stamatakis 1986; 1989a,b; Helvaci et al. 1993). This pattern of zonation, together with the B-bearing nature of the feldspar a nd the presence of evaporite minerals, is characteristic of a saline-alkaline lake environment. The deposits are similar to others of the same age in neighbouring regions (western Turkey and so uthern former Yugoslavia), as well as to those in the western United States. The zeolitic rocks in the Karlovassi basin are exceptionally fine-grained and porous, but are well indurated. They are texturally homogeneou s and often display a conchoidal fracture. They are usually very pale green in colour.

Zeolites in the western Aegean Zeoli tized tuffs occur on the islands ofSantorini, Kimolos, and Poliegos in the western Aegean. They differ from those of Thrace not only in their age a nd depositional environment but also in their post-depositional evolution and secondary mineral assemblages. Santorini is a complex stratovolcano that has been active since the Pliocene. Pyroclastic deposits predominate over lavas, and magma types range in composition from basalt to rhyodacite (Pichler et al. 1980; Huijsmans et a l. 1988). Zeolite-bearing tuffs have been described from the pre-caldera pyroclastic succession at Akrotiri, at the southern end of Thira Island. There, the dacitic pumice tuffs of the Lumaravi-Archangelos unit have been altered to zeolites, mainl y mordenite and/o r clinoptilolite (Kanaris 1981; Stamatak is 1986; Tsolis-Katagas and Katagas 1989). Subsequently the tuffs were infiltrated by seawater, and they now contain trace amounts of marine evapo rite minerals, mainly halite. The soluble evapo rite minerals tachyhydrite, sylvite, and halite are commonly present in pore spaces and as a coating on the o uter surfaces of samples. All the authigenic minerals are randomly distributed, whe reas the evaporite minerals are enriched near the coastline. The tuffs were originally deposited in a shallow-wa ter nearshore environment, as indicated by the bivalve and foraminiferal fauna and by cross-bedding in the finer tuffs. They have never been buried to great depths and zeolitization ca n be attributed to two facto rs, namely the high heat flow of the area since the Pliocene and the subaerial exposure of the material during the Quaternary. Generally the material is coa rser grained than in west Thrace, a nd it co ntains blocks of acid and basic volcanic rock up to 1m across. Therefore the tuffs were deposited much closer to the e ruptive centre and the high heat flow was probably decisive in the zeolitization. H owever, the zeolite

mineralogy and distribution in space is similar to that of western Thrace. Ash of the same age and c hemical composition as that of Santorini occurs in Crete (S tamatakis et al. 1991 ) and has not been transformed to zeolites, suggesting the importance of heat flow in the formation of the Santorin i zeolites. However, the tuffs and la vas of Aegina island do not show any transformation to zeolites, even t hough the Aegina is one of the Quaternary volcanic centres of the sout h Aegean vo lcan ic a rc, and wo uld have had high heat flow along with Santorini. Electron microprobe analyses show that the main exchangeable cations of the Santorini zeolites are: clinoptilolite K > Ca > Na; mordenite Na > K > Ca (TsolisKatagas and Katagas 1989), or clinoptilolite K > Na > Ca to Na > K > Ca (Kitsopoulos and Dunham, this volume). The clinoptil olite occurs in the matrix of the tuffs, filling cavities as a precipitate, and pseudomorphing glass shards. The mord enite overgrows clinoptilolite and evidently formed later. The paragenetic seq ue nce of clinoptilolite followed by mordenite may reflect the formation of clinoptilolite during the initial stage of zeolitization followed by the development of mordenite as a result of seawater infiltration. The islands of Kimolos and Poliegos are situated in the central-western part of the sou th Aegean volcanic arc (Fig. 1). Both islands are composed of Pliocene to Recent volca nic rocks (Fytikas and Vougioukalakis 1992; Kanaris 1989, 1993). Rhyolitic to andesitic lavas occur throughout the islands. The tuffs of Kimolos Island were deposited along with thin marl and diatomaceous h orizons in a nearshore environment. Pyroclastic rocks include pumice ignimbrite, volcanic breccia, lapilli tuffs and ash fall tephra. Most of the volcanic rocks have been altered, yielding perlite, bentonite, K-feldspar, zeoli tes (clinoptilolite and mordenite), kaolinite, al unite, and crystalline and amorphous silica. Small bentonite bodies occur above some of t he zeolite deposits in the NE part of Kimolos, but in the SW part of the island the zeoli tized rhyolitic-trachytic lavas do not have any overlying succession. ln con trast, in Poliegos Island the zeolitized tuffs are partially covered by rhyolitic lava flo ws, were deposited on land, and are unbedded o r thicklybedded. The predominant zeolites are clinoptilolite and/ or mordenite on Kimo los, and mordenite alone on Poliegos. The zeol itic rocks in Kimolos and Poliegos have a pale green colour, and most probably formed in open hydrological systems. Zeolite formation is not directly related to the hydrothermal activi ty wh ic h has occurred on the islands. The small barite-galena veins that cut the zeolitized tuffs in both the islands contain silica polymorphs but no clinoptilolite or mordenite. The most promising occurrences from an eco nomic point of view are those of mordenite in the SW part of K imolos island a nd in the weste rn part of Poliegos island, where mordenite is locally the main rock-forming mineral. Figure 7 shows SEM photographs of a completel y zeolitized rock from Kimo los wh ich also contains seco ndary gypsum. The zeolite-rich rocks from Kimolos and Poliegos are currently being evaluated for use in soilconditioning a nd as a cement additive.

478

Whilst th e only authigenic mineral in the chert is chalcedony, the porcellanite contains sm ectite, opal-CT, clinoptilolite, and barite as au thigenic minerals (Stamatakis and Hein 1993). The silica polymo rphs, silicates, and barite formed at the expense of siliceous, organic and calcareous material. C linoptilolite occurs by itself, o r along with th e other silicates and barite, usually replacing siliceous tests, but much less commonly forming prismatic crystals. In some rocks, clinoptilo lite is the major constituent after the removal of carbonates. Boles and Wise (1978) considered that volcanic glass is not a pre-requisite for the formation of clinoptilolite in deep-sea sedimen ts, a nd volcanic detri tus has no t been recognized in the clinoptilolite-bearing sediments of the Ionia n I slands. Furthermore, these sediments have not undergone either hydrothermal alteration o r metamo rphism. The fo rmation of clinoptilo lite in these rocks was therefore attributed to burial diagenesis of sediments containing biogenic silica (Stamatakis et al. 1989).

Chemical composition of the zeolitic rocks in Greece

Fig. 7a, b. SEM photographs of a mordenite-rich tuff from K imolos Jsland: a fibrous morden ite growing into a cavity from a massive mordenite ma trix. Small spherical bodies of silica are also visible: b A vug in massive morden ite. The crystals partially fi lling the vug are gypsum (including the 20 ~tm wide cluster). A vein of mixed mordenite and gypsum is present along the top edge of the photograph

Minor occurrences It is like ly that zeolites will eventually be found in man y of the Tertiary pyroclastic rocks of Greece, in similar depositional and d iagenetic environments to those of the known deposits. A rather distinctiv·e, although minor, occurrence is in some deep-sea non-volcanic sediments of the Ionian Islands, off the west coast of Greece. In three of these island, namely Zakynthos, Kefa llinia, a nd Lefkas, there exist siliceous sed iments (cherts and porcelanites) of Aqu itanian to Langian age (early to middle Miocene). In their least-altered fo rm, these rocks are represented by the diatomaceous rocks in Zakynthos (S tamatakis et al. 1988, 1989). These contain relics of d iatom s, radiolaria and sponge spicules, but have been altered to cherty and porcellanous rocks. It has been established that the original sediment was deposited in a deep-sea environmen t (Dermitzakis 1977; Mirkou 1979).

Table 1 shows a selection of chemical analyses of zeoli tic rocks from Greek localities. These zeolitic rocks were derived from a variety of parent materials, but have been considerably modified by the zeolitization process. Tt is not possible for any of the localities to make a quantitative calculation of the material gains and losses involved in zeolitization because of the difficulty of identifying equivalent unzeolitized mate ri a l with wh ich to make a compan son. Some indication of the composition of the vo lcanic precursor materi als may be obtained from their contents of immobile trace elements. F ig ure 8 shows a plot of Zr/Ti0 2 against Nb/Y which effectively differentiates between magmas of different composition and is relatively unsusceptible to the effects of post-magmatic alteratio n. The zeoli tic tuffs of Santori ni and Kimo los lie in the dacite-rhyodacite field, which corresponds to what is known of their actual parent material. The samples from Lefkimi (Thrace) also fall in this field. The sam ples from the other localities in Th race lie in th e trachyandesite field, although the original diagram of Winchester and F loyd (1977) is poorly constrained at the bou ndary between rh yolite and trachyandesite, so a rhyolitic protolith may be possible for the Metaxades tuffs. The zeoli tic tuffs from Samos are clearly much different from those of the other areas and the immobile element diagram suggests a trachytic protoli th. The major element com positions were very severely modified by the process of zeolitization. For example, the a lkali contents of the zeolitic tuffs from Samos shown in Fig. 9 are far removed from those of normal igneous rocks. F or those elements that are subject to cation exchange, the present concentrat io ns probably represent neither the original rock composition nor the end-product of the zeolitization process, but are the result of subsequent interaction with local gro undwa ters. Very high cesium contents are a notable featu re of the zeolitic rocks from Samos. In the case of the analcimebearing rocks (e.g. specimen DD10c in Table 1) this is

479 Table 1. Chemical analyses or zeol ite-bearing tufTs fro m various loca lities in Greece AK4 Si0 2 Ti0 2 Al 2 0 3 Fe2 0 3 MgO MnO CaO a2 0 K2 0 P20 s L.O. l. Ba (ppm) Ce Co Cr Cs Cu La Li Nb Nd Rb Sn Sr

v y

Zn Zr

59.66 0.41 13.62 2.71 1.34 0.1 1 3.47 3.60 1.71 0. 11 11.36 559 51 8 I

I 4 31 8 9 18 47 1 308 45 23 41 153

AK7 62.32 0.38 13.28 2.78 1.30 0.11 2.63 3.47 2.27 0.09 10.47 364 47 7 13 I 5 28 14 10 16 78 I 389 45 22 41 190

SM96b 61.33 0.17 15.28 2.56 1.65 0.06 2.40 0.52 4.85 0.05 11.04 69 130 5 18 66 7 78 120 1.10 39 444 16 11 86 16 69 78 539

DDI Oc

SM36o

57.20 0.29 16.30 2.97 3. 11 0.03 1.52 3.00 7.25 0.08 7.24

60.25 0.30 16.35 3.29 2.21 0.10 1.15 1.49 10.05 0.08 4.40

48 198 5 15 Ill 7 11 5 283 72

56 424 7 Ill 15 52 87 705

75 178 7 44 23 9 104 219 66 49 544 5 122 22 53 90 578

MTXI 71.09 0.08 10.22 0.52 0.59 0.02 3.42 0.62 1.89 0.06 11. 12 282 39 10 6 7 I 19 6 23 18 140 3 1927 6 57 18 96

MTX2 71.79 0.07 11.44 0.44 0.45 0.05 1.43 0.49 5.39 0.03 6.68 91 44 5 4 3 I 21 7 30 18 149 2 951 4 23 18 69

AK4: San torini (mordenite-rich) AK7: Santorini (clinoptilolite-rich) SM96b: Samos (clinoptilolitc-rich) DD10c: Samos (analcime-rich) SM36o: Samos (K-fcldspar-rich) MTX I: Metaxades (heulanditc-rich) MTX2: Metaxadcs (K-feldspar-rich)

perhaps not surpn s111g because the high selectivity of analcime for Cs is well-known. Howeve r, there is a lso high Cs in all samples from Samos, even th ose which do not co ntain analcime (e.g. SM96b and SM36o in T a ble l). so the high Cs may be partl y due to the orig inal magma composition. The high rare earth contents (Ce, La and N d) of the Samos sa mples are a lso charac te ristic of an a lkaline parent magma. All the pyroclastic rocks st udied here have relat ive ly high porosity, and fo r two areas it is possible to estimate the co mpositio ns of the m ost recent a mbient wa ters to which the zeol ites have been exposed. At Metaxades, the presence of limestone relicts in the succession immediately ove rlying the zeo lite- rich tuffs suggests that the a mbie nt wa ters wou ld have been calcium-rich, a nd this is co nsiste nt with a relatively high Ca/(Ca + Na + K) ratio for the Metaxades hcul a ndite-ric h rocks (e.g. MTX I in Table 1) a nd high Ca/(Ca + Na) in the K-feldspar-heulanditc rocks (e.g. MTX 2). T his is also pres umabl y the reason why heul andite is the main zeo lite in the M e taxades a rea, rather tha n clinop tilolite, whi ch is the commonest zeolite in m ost o ther zeolitic deposits derived from a rh yo litic pro to lith. I n cont rast, o n Santorini the infi ltration of seawater into the lowe r pa rt o f t he volca nic

pile res ulted in Na-rich bulk rock compostttons and to the la te formatio n o f Na-rich mo rde nite overgrowing clino ptilo lite.

Economic potential At present, the on ly economic use o f zeolitic rocks in Greece is in the M etaxades-Petrota area, whe re zeolitic tuffs a re quarried fo r build ing sto ne for local use. Many of the buildings in Metaxades, Avdella and su rroundi ng villages are constructed o f sto ne blocks from nearby qua rries, giving these villages a unique appea rance in a coun try whe re concrete is t he normal construct ion material for do mestic buildings. The uses for wh ic h Gree k natural zeoli tes are cu rre ntly being conside red fal l in to three categories: (a) in animal rea ring, (b) fo r pollutio n control, a nd (c) as a cement additive. The first category incl udes a va riety of applications, as a n anima l feed supplement, as pet litter, a nd as a cleaning/filtra tio n agen t in fish fa rmin g. The seco nd category includes re moval of a mm o ni a a nd toxic metals from waste water, a nd possible re-use o f a m monia ted zeolite as a soi l conditioner.

480

PHONOLIT E

K-feldspar -rich

12

COMENDITE PANTELLERITE •SA

10

RHYOLITE

8

~

-------~~fT_

0.10

RHYODACITE DACITE

-

. LF .

PAo L PET

0

6

'

PEN

AK

N

~

KM

N

0 i=

0N

0

p

2 ANDESIT E

Chabazite - rich

+

RHYOLITE

TRACHYTE

+

Clinoptilolite -rich

4

TRACHYANDESITE

.. .



+

+ ANDESITE BASALT

ALKALI-BASALT

0.01

0

1----- SUB-ALKALINE BASALT

6

Fig. 9. Plot of Na 2 0 against K 2 0 for zeolitic tuffs from Sa mos. The average compositions of common volcanic rock types (from Le Maitre 1976) are shown fo r comparison 1.0

0. 1

5

10.0

Nb/Y

F ig. 8. Plot of ZrjTi0 2 against Nb/Y fo r zeolitic tufTs from eight areas in G reece, for comparison with the ranges for fresh volcanic rocks given by Wincheste r a nd Floyd (1977). The data a rc fro m unpublished work by the authors (Ha ll a nd Stamatakis). Abbreviations: AK, Santorini (Akrotiri, mean of 8); KM , Kim olos (mean of 21); LF, Lefkimi (mean of 6): /liT X. Metaxades (mean o f 28); PAL, Palestra (mean of 5); PEN, Pentalofos (mean of 7); PET. Petro ta (mean of 10): SA, Samos (mean of 12)

There a re ma ny fac tors that affect the economics of these applicatio ns, the most important being the size a nd grade of the deposi ts, their minera logy, transportatio n a nd processing costs, a nd e nvironmen ta l considerations. The last of t hese is pa rticularly important in Greece beca use of the environ men tally sensitive na ture o f some of the a reas in wh ich the deposits occur. The deposits on Santorini a re in a n isla nd with a la rge and growing to urist indus try tha t has stopped the ex tractio n of pozzola nic ash, whilst Samos is also highly dependent o n t he tourist ind ustry a nd qua rryi ng is subj ect to seve re restrictio ns. Exploitation o f the deposits o n Kimolos and P o liegos would incur high t ra nsport costs, and would be hampered by the lack of wa ter on these islands. T he deposits most likely to be exploited in the fo reseeable fut ure a re those in Evros (Thrace), because o f their purity, ease of access and avai labi lity in la rge qua ntity. Ackllow/edgemellls. We are grateful to D r. Andreas Magganas for valuable discussio ns, a nd to Sarah James, Claire Grate r, and Robert Oscarson fo r analytical assistance. Our work has been supported by travel grants from the Britis h Council a nd the Natio nal University of Athens.

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Edi torial handling: D.J . Morgan