Rom. J. Mineralogv. vol. 80. p. 11-20, BucureEti.2000

ZEOLITE FORMATION IN THE LOWER MIOCENE TUFFS'NORTH-WESTERN TRAN SYLVANIA, ROMANIA

I. SEGHEDI. A SZAKÁCS. I. V.A'NciHELI}). ( ' . c]oS.rEA

lnstitutrrt Geologic a| Romárriei. str. (iaransebeq nr' l. R()-79678 Bur:ureqti 32

Key worcls: Rh.yolites. (jlirr<lPtilolite. Opal-(i.t.. N'íorrlenite. l)iagenesis .]i.ansylva-

nian Ba.sirt.

Abstract: Zeolites ar.e co6mo1 alt,eration product,s of silicic, nrostly vitric, Badenian

(Lower lV{iocene) tufts in the northwestern part, of thr: T'ransylvanian Basirr. T'his studv

of lí0 samples from sections at'Iá4>iu, Vale, Bobálna, AluriE and $oimeni showed t'hat

reworked fallout tuíTs arrd aslr.florv trrfls hacl been ex[ensively altered to a miner.al a.s-

senrltlage dorninatecl lrv zeolitr:s. The relative abundance of zeolites and associated

rninerals were rletermined b.v X-ray lrowder cliffi'action. Selected saurples wele also

stuclie4 by SEIVT and optical methods to ascert'ain the genesis and paragenetic rela-

tionships ol the minerals. Zeolit'ic tuí{s consist mostlv of clinoptilolite (4G90 ,vo| and

' opal-CT. Tr.ace atttottnts oí fibrous m<lrc|ertite and phillipsite were detected by SEM.'Ihe comrnolr par.agerretic seqlrence is volcanic' glass-clinoptilolite-mordenite. Opal-(-:T

crvslallized at the same tilre ars clinoptilolit,e or later. '.lhe origin of the zeolites and

associatecl nrinerals in these cleposits is shorvu to be diagenetic. Hydrolysis of the

silicic glass brolglrt al>out, ll, saline alkalinc pore wat,er', trappetl cluring marine sedi-

ruentation of t[e;;yroclastic nraterial a.ncl subseqttent burial accon-rpanied by increase

of t,enrperalure (u1r t,o RooC'). rvas followerl by formation of zeolites at the ex1>ence of

glass and zeolite an<l opal-C"l pre'<'ipitaliott itt pore spaces'

Introductiorr

This iuvestiga,t ion cleals wit lt the cl ia,genet, ic hist,ory of an irnportant beclclecl tuft ' sequence depositecl

urai l ly ip the Baclenian (Lower Miocene) t iure over a large area, lnostly in t 'he Tl 'ansylvanian Basin' The

stratigraphic posit, iol of t i le tuffsequeltce rvithin the Nl ioceue secl itur:ut,ary rocks provides an excel let lt trtarker

horizo-1. Dri l i ipg ha.s shown that this tuff conrplex occurs throughout' the whole Transylvauian Ba'sin with

outcrops along the $outheastern, western aucl rrorthertr llrargilrtr. It, is also lvidespread in other parts of the

region, as intramount,ain basins of the Apuseni Mountains! or extertral border of the (larPathian berrd (Fig'

f yl 1.ne preseut investigation is restrict,ed to its classical occurretrce atea., where the tuffsequence crops out'

between the towns of i-rluj-Napoca a,ncl Dej. In this a,rea it, is knou'n in the literat'ure a.s the "Dej Tuff'

(posepny, 1867). Eviclencoof zeol ites. ma,inly cl inopti lol i te, in t 'he De.i Tufts wa's reportecl by Popescu et al ' ,

i f gZl l *'d later confirnred by Popescu ancl Asvaclurov (1978). I\ ' lordenite was re,:orclecl by Istrate (1980) and

later by Berlelea' ancl Avra1n (l-ggl). Other invest, igat, ious rer,orcl ing the 1>resence of zeol it 'es in the strrcly

area i t r c luc le t ' l rose by Bede lean ( ]982) . ( i l re rgar i e t a l ' . (1991) . Rácá tá ianr r e t a l . , ( l99 l ) anc l Bár :bat ' e t a I . .

( 1 9 9 1 )

General geology

In the T'ans"vlvanian Basiu the Lorver Miocene seclimeut,aly strat,a, n'hich followecl the e1>icontirlelrtal-

continental l,)or.ene-Oligu.",r" ro.i inrent.ation. f<>t'trt a cliscotrt' ittuous post,-tect'ouic cover sequeltce over the

[lpper.(.retaceorts-Pul"ág.u" deposits (Sá'rrcltr|esr:rr. l!)84). 'Ilre Dej Trrff Conrplex, sorrre 20-l00 rrr tlrick.

occllrs lear to t,5e t,op oithe Lonver l\,Iiocene succession (Popescu et al., 1995). During the Eally Miocene

t,he Tra.sylr,a.nia,u Basin underwent progressive tectonic subsideuce. T'he whole complex comprises 4 to 5

'rajor volcanicla^stic cycles, each inclucling reworkecl ash-flolv ancl fallout tuffs at the ba.se, fallout' tuffs ancl

.eworked tuffs i1 the miclclle part,, and tufEtes interbeclcled with psarnitic to siltic sediments in the upper

o)-,á'*l" f

s,í.:.:";, /ooo ooofltr./ .o ooar - o o o

f o o

-r.-., Ífqj

o

r 7 g o '! a , o o o

!",-1ös",,,i,Wr.

i;:::i!,:Wo o o o o o oo o o o o o

o o o o o 0 0 ,

iBl,r",,i'"1

s E N ! I YNTAINS

T,2 l. SEGHEDI et al.

W.7. o!'*s'

..u KRA INA\.\

{"?.. o bo o o

q - \

?í:o

o oo

o o- o o

I .BUCHAREST

F i g . l - I o ca t i o l r and r l i s t l i b r r t i r ) no f Ne ( )gc r r e i gneous rocks i l r l t o r r r an i a l : l n r r e r ( ' a r ; : a t l r i ; r 1 [ l r r i t s : 2 : ( )u t c r ( ' a rpa th i a '( ln i ts l i l : Ac id caic-a lkal inevok:anism: a) cr .oppingorrt . b) bul ier l ;4: Inter.rncc l iatecair_alkal iner lagmat isnr: a) intrus ive.b) stratovolcanic, The fi'arne slrows the study alea.

palt'. As a rvltole, the courplex has a rhyolitic character (Seghecli ancl Sza,ka,cs. l991). St,rat,igraphica,l ly, theDej ,t.uÍtCotrr|l|ex

is cottstraitred t'o Langlriau (Lower Badeniarr) (Popescu, l$70). but a I(-Ár age cleternti.ttatiotr. obt'ainecl on biotite Íiour tlre basa| level oÍ.t'|re t:otrr1>lex, gave l8.0t0.l-l N,1a (Berggrelt alrcl Popescu,unprrblishecl dat,a). T'he volcanic lrorlrce area is sti l l rrlrknown.

Sarnplirrg arrd nrethods of study

Saurples were collected along the westetn out,crops of the tansylvania,p Basi1, bet,weeu Cliceu-Giurgesjtiancl (l luj-Napoca (Fig. 2). Generalized lithological colunrns of the Bobalna Hil l alcl AlpniE o." rh.r*jr.,l,tFigute 3 with the santltled iutetvals. Material was collect,ed s,herever ap obvious clralge iri l it,hnl<rgo'ruasobscrvecl. Str.at' igr.a'phir: s<.t.tiotrs Wel.e lneasu|.etl at all tlre lor:atÍotls ((.iceLr-(i iurgr:l it i. 1.ir 'uluI IJi l|-Dej. Ra1laDt 'at 'u l r r i , A l r rn ig, Pagl iqa, $oinre l r i auc l C 'oru;) u, i th a t 'orrr ;>lex sarupl ing{trr pct , r .6gra,p l r i r .a l oSseryat io.s . 50s i t t t tp les wcte e 'xatt t ine<l f rott t a l l t ,ho st 'uc l ier l prof i les ' l ' l re i l r r r iuera l c .onr1:os i t ic l r r ( ' l 'ab le l ) was c[eter l r i 'ec lby \-ra.v powclel'diffraction t,echtriqur: (XllD) aft,er hea,t treatnrent at 5500('. usirrg a l)r.o'-lj l iustrurrreutand Ni-fi ltered ('uha radiation ("' electron nrir:loscopy (SEM) wa.s carriecl nut *itl a R,DMMA 202tllStrUment.

Petrography and rniner.alogy

ful cu; xt s r oy.t ir ut d n t. i. t, r o s r, o p t r f t aturt s.|rvo trrailt volcanir.:la'stic ror.'k types lrar,e lleen foutrd: teworke<| pvro<'last'ics a'lrc| ttrÍliÍes. ,llre firs| ones

are grou1>ed in secotrdary as|r-florv t'uffs arld secoltd fall-out trrffs. The color oí.these tocks is wlrite. l iglrt8l'een or green. The last color cltaracterizes zeolite-riclr vit,roclastic tuffs. T[e rocks a,re indura,te, 1a'ingcotrchoidal fractures. The tuffs generally are fiue-grained. well-graclecl, showing parallel beclcling strat,if ica-tiorr. ' lhe rervorked ash-flow tuffs are massive ancl richer in crysta,ls.

Cíceu:iuroesi

i.tat+t ' f { +t . + . íI + tÍ t l

ERLA

CLUJ

sp)+ '+ + + r, . F + { f . tt + r ++ + +

(ili[N.p'\ ll"EF

ZL:)LITE h.oRI|ÍATloN lN LOWEIt i|[lo(,E!tE ?'t,IFF.s

F ig .2- ( ie r re r 'a lgeo log ica l sketc l - r . : ,Í thet t r l r . t ' l lwes teLt tbot . r le to f t l te l . t . i rnsy lvaI r ia t rFJas i t r showingt I t r ' a t .eaof i t rves t iga t io r r .

l : Qt ra t r : rnary de1 los i t s : 2 : Sat .nra t ian s t ra ta : j ] : Bac len ia t t : a ) De j s t r .a ta . l l ) M i r .eq s t r 'a ta ; 4 : Lower .M i l l cene s t la ta : í :

Pa leogene s t ra ta ; 6 : Locat ion o f the sec t ions .

t . )

L1 I. SEGHEDI et al.

Bobó l no

nt

T-

I

f

I

II

Atun ig V .

I

I

2

I

I

I

!

' ,,-'

2,3.r lll

_ - _ l

Fig. 3 - (leneralisecl |itho|rlgic co|umns of the Dej .Iitff

se<1uen.-.es fi.onr B.lbá|rra Hil| and AluniE;sections show the sampling intervals. I, II, III and IV indicate the major volcaniclastic sequences: 1:ash-flow tuffs; 2: fallout tuffs; 3: interbedded fallout tuffs, epiclastic rocks and sedimentiu'y strata.

In thin section, the main petrographic colnponents include vitroclasts, crystaloclasts and lithoclasts.Based on the relative amount of these components, the rocks can be defiued es vitrocla.stic and/or crystalo-clastic tuffs, a,ncl crystal-vitric/vitric-crystal tuffs The lithoclastir: component is less abundant, particularlyfound at the base of the tttaiu rewolked ash-flow t,ufls. The vitric component consist,s of glass, shards, pumiceand tna.ssive glass fraglttettts. The crystalocla.sts are most,l.v' (luartz, plagioclase, biot,itr', greeu arnphibole,alkali 'feldspat alld accessory pyroxeue, allanite and opaque nriuerals (tita,no-rnagnetite, i lmenite).

Secondary nt.ineralsThin-section petrography, XRD and SEM data show that the vitric matrix of the tuffs was partially

or entirely alterecl to zeolites and cristobalite (opal-CT) (Table l). Other minerals, such as quartz andplagioclase, represeut prilnary coustituents. Rhyolitic glass iu the samples from the locality of Coruq isrnostly unaltered, although interstitia,l calcite is locally abundant Iu the altered sanrples the growth ofeuhedral secondary tniuerals was observed within iuterstitial voicls bet,ween t,he slrards.

ClinoptiloliteThis rnineral is the maiu secondary product in all the studied sections occurring in a wide range of

euhedral fortns, including blades, laths, plates with the characteristic coffin shape, and blocky crystals. Thecrystals are about 1-30 microns long (Fig. 4). The shards are pseudomorphosed and the larger crystals havegrown perpendicular t'o the sha,rds, fi l l iug the interstitial cavities (Fig. a). The XRD pattern of clinoptiloliteis very simila,r to that of heula,ndit,e (Mumptou, lg60), but after thermal treatmeut at 550 0C heulanditebreaks down, wlrereas clinoptilolit 'e is unaffect'ed. T|re 8.95 Á ancl 3'95 Á 1leaks are uttec1uivocal (Fig. 5).

Opal-CT lepispheresDiagenetic sil ica,. kuown previously as low-cristobalit,e, is frequently preseut, besides eliuoptilolite in the

studiecl sectious. Silica lepispheres are microspheroida,l boclies of blaclecl crystals, consisting of more or lessdisordered cristobalit,e and t,rydimite, randornly intergrowu, being defined as opal-CT'by Oehler (1975) (.Figs.6,7). Opal-CT crystall ized aI the same time as clinoptilolite, or later. In Figure 5 the sharp peak at 4.05Á i. d.'" to opal-C ancl trot opaI-CT (Jorres arrd Segrrit, 1971). As a result of the lreat treat'nrent the verybroad reflection of Opal-CT arouud 4.3, 4.1 and 3..9 A has been couverted to the low-cristobalite-l ike peakin this difractograrn.

MordeniteThis nrineral, found only in vitric fallout tuffs at Tárpiu, Aluniq and Vale, was detected by SEM. It

occurs as randontly distributed fibers on cliuoptilolite and opal-CT, but never as a direct alterat,ion productof the volcanic glass (Fig. 6).

ZEOLITE ITORVIATION lN LOWER IvIIOCENE TLII'FS

TabIeMineral associations and relative frequerrcy of

'rrrineral species

of srrlr.cteid Dr'.i T\rff sarrrplcs tlctcrntirx'<l hy XII'D aualyses.

Frequency symbols:++++ - very abundant

]il :3l5'oíl'L,..+ - sporadic

l 5

Samplenumber

Location Original rocktvpe

Mineralogy Frequency

rD-143-1 Iárpiu (5) TuÍÍ :linoptiloliteluartz

+++{.l

ÍD-144-2a 3obá|na (7) lgnimbrite :linoptilolitequartz

++++

TD-l44-2a 3obálna (7) lgnimbrite clinoptilolitequartzcristobalite

+++++++

TD-144-6 Bobálna (7) lgnimbrite clinoptilolitequarlzooal-CT

++{{{

rD-145-14 Bobálna (7) lgnimbrite :linoptilolitequartz

++++++

TD-154-4A Bobálna (7) lgnimbrite clinoptilolitequartzplaqioclase

+++++++++

TD-141-68 Vale (8) Coarse tuÍí clinoptilolite ++++

TD-139 Alunig (9) Medium tuÍí :linoptiloliteclagioclasecpal-CT

++++++

+

TD-139-14 Alunig (9) Fine tuÍí :linoptilolitecuartz

++++++

TD-139-18 Alunig (9) Fine tuÍÍ clinoptilolitequartzooal-CT

++++

+++

rD-139-4 Alunig (9) Fine tuÍÍ clinoptilolitequartz

+++++

rD-139-sC Alunig (9) Fine tuÍÍ cl inopti lo l i teouartz

+++++

rD-139-34 Alunig (9) Fine tuÍÍ ;linoptilolitequartz

++++++

TD-140-3 Alunig (9) Fine tuíf ol inopti lo l i te ++++

TD-140-6 Alunis (9) Fine tufÍ clinootilolite ++++

TD-134-4-2 Páo|isa (10) Fine tuÍÍ clinootilolite ++++

TD-145-4-2 $oimeni(11)Fine tuíÍ :linoptiloliteruartz

+++++

TD-137-1 Corug (12) Tuífite amorfous materialquartzcalciteplagioclasekaolinite

++++

+++

TD-137-0 Oorug (12) TuÍÍite calciteamoríous materialquartzolaqioclase

++++++++

| ő l. SEGHEDI et al.

XRDE L ] T{ O PT II ILO ILUTIE

OPAL- C

. {<!&0 0

2e{Rod Cu Kx)

Fig. 4 - X.ray powder diffraction diagrams for. clinoptilolite (sarup|e t4o-l3) arrd c|irropti|olite associaüed with opa|-C(sample f 39-18) from Aluniq, aftel heat treatment at 5bOoC.

EbillipsitePhillipsite occurs only at Tárpiu as a secondary alteration product of clirroptilolite associated with opal.

cr. It was detected by sEM and appears as laths of prismatic crystals (pig. z).

Carbonate mineralsCalcite is the tuost abundant secondary nrineral in the t,uffs a,t Corug. Its fonnation seelns to have

inhibited the developtnent ofzeolites so the glass shards are practically unalterecl.

fuSe""gg_of s"Sg11g!g;y n'rineralsThe paragenetic relatiorts are closely counected with the a,urount, of volcanic glass iu the cleposit. No evi-

dence wasfound that zeolites replaced other ruiuerals. The mainobserved parageneses are: (l)ctinoptilolite;(2) clinoptilolite - opal-ClT; (3) clinoptilolite - opal-CT - mordenite.

lnq\'

(\cll

ci

LNo)c.;

gE- ' * '

-< ' *

lq 5 ;.Í.{É.Éa \ a í \ | \ , m a n ( n ( Y r r ;

ocrrP;

o o l@ c o.O 3..

ZEOLrTE FORMATION IN LOWER MIOC:ENE T(TFFS

Fig. 5 - Sca.'ing electron micr.ograph showing relationships betwccn bla,l,'- ancl lath-shaped clinoptikrlite filling voids-

b;tween diageniz"d gl ."" shards; Págl iga occt l l . |c| l (]c: sr i t le bar- is 7 1rrrr l r ;ng.

Fig. 6 - geanni.g electlon micrograph of opal-CT lepispherep, fibmus nrordenite and clinoptilolite blades fi'om AluniQ:

scale bar is 0.5 Pm long.

L7

I 8 l. SECHEDI et a!.

Fig. 7 - Scanning electron micrograph of lath-shapecl phi l l ips i te crvsLals associatec l with opal-C 'T lepis l ; l rcres fr .1m Tarpiu; scale bar is 3 pnr long.

Genesis of zeolites and associated minerals

All the evidence provided by these investigations support the view that the described alteratio' productsare authigenic in origin. Hay (1993) has sumnrarized the major factors controll ing the forrnation and clis-tributiorr of zeolites aud other a'uthigeníc nrinera]s. Tlrese inclrrcle |rrineralogica'l conrpositiorr. pernreabil ityand age of the host rock, temperature variation, pl'essure ancl chernistry of t,he pore waters.

The examined rocks are maittly rhyolitic reworked fallout and ash-flow tuffs, rich in volcanic glass. ThePrimary volcaniclastic deposits were redeposited in a marine environment In a,ll obser.ved sections thedeposits have a similar ruinera,logical cornposition and structure, aucl are r.nore or less equallv a,ftectecl byzeolitization. The roughly'constaut rnineralogical contposition of the rocks, in t,he w5ole cleposit, suggesrsthat no significant mass transfer was involved in the formation of diagelet,ic rrrinerals. The distributionof anv a,uthigerlir: urinerals in the deposits shoulcl reflect t,he chemistrv of the pore fluicls present duringdiagenesis. Experimeutal and theoretical approaches (e g lJess, 1966) suggest that the praip charact,eristicsof the pore rvatet'controll ing the dissolution and precipitation of either argil l ic pha-ses or zeolites are closelyrelated to the lelative activity of alkalis and hydrogen. High alkali ions/hyclrogel ions ratio and a r.elativehigh activity of si l ica favor the prevail ing generation of zeolit,es as opposed to clay nrinerals. This situatiopwould characterize generatiou of the zeolites described here. Such situations are typical of alkali 'e sali 'e lakeor tnarine environment (Hay, 1978, Iij ima, 1978). The presence of alkaline saliue water favorizes zeolitization,but' additional factors tnust also be involved. The Dej 'fuff

Clornplex was buriecl duripg the Miocene unclera significant cover of later sediments up to 3.5t0.5 km Sander.s, 1998). It seems likely that the clepositssuffered euough burial to give rise to siguifir:ant changes in temperature a1d pl'essure, which favored thediagenetic processes. The burial temperature^ of 80 0+10 0C (Sanders, 1998) is iu the esti 'rated ra'ge forclinoptilolite formation and stabil ity (49-83 0C1 in the present-day diagenesis i1 various areas worldwide( l i j ima, 1986, Ogihara, lg96).

Paleoenvironmental conditions during the Early Miocene times suggest t,ha,t, the seconclary rninerals inthe Dej tuffs were generated through hydrolysis ancl dissolutiou of sil icic glass b1, i lterst,it ial saline watersin a marine environment. The subsequent buria,l favolecl the diagenet,ic processes cllri lg t,he rise of t,errrper-ature and pressure The ahnost complete zeolit, ization of thc volcanic glass-r.iclr cleposits suggest,s t,hat the1;ore water, w]riclr caused the diagerresis, was of cognate tvpe. trappecl in t,he seclitrrerlts cluríng or shortly

ZEOLITE FORMATION IN LOWER MIOCENE TLIFFS

after the underwater cleposition of the volcaniclastics. Some tniuera,logical variation in the different studied

profiles could be related to different compositions of t,he interstitial pore fluids, but also to differences in the

colnposition of the protoliths. The crystallizatiorr sequence during tuff diagenesis was probably controlled

by grorvth kinetics allowing the rretastable precipitation of clinoptilolite and opal-CT as a result of kinetic

bariiers (Murata a1d Larson, 1975). followed in some cases by rnordenite or phil l ipsite generation. The lack

of t,ransfornatiou of volcanic glass in the case of [he occurtelrce suggests that' there were not favorable con-

diti<-,ns for zeolitization. The abundant, presence of authigenic calcite in the Coruq tuffs suggests a slightly

acidic elvholnlent, inherited by pore waters. that precluded the formation of zeolites at the expense of

volcanic glass.lVe can assultle that the amount, of zeolites in t,he dia,genetically transfortnecl tuffs closely reflects the

arnoult of primary volcanic glass initially present' in the volcaniclastic deposits.

AcknowledgernentsThe Geological Institute of Romania supported this research. We are gtateful to Hilary Downes and

Authony Hall for reviewing attd correcting the early versiotl of the manuscript.

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Reccioed: Octobr 20OOAccepted: December 20(M