Canadian Mineralogist Yol. 27, pp. Ul-245 (1989) FORMATIONOF SULFATESAT THE THIAPHESAREA OF MILOS ISLAND: POSSIBLEPRECURSORS OF KAOLINITEMINERALIZATION AKTSE. KELEPERTSIS Department of Geolog:t, Notional Univenity olAthens, Panepistimiopolis, Ano ltisia, GR-I5784Athens, Greece ABSTRACT the South Aegean active volcanic arc (Fig. l). Only a small exposrueof metarirorphic rocks occursat the southernpart of the island; theserocks belongto a The physicochemicalconditions at the Thiaphes area of flysch formation (phyllites) associated with Milos Island are favorable for tJte recent formation of ophiottes and allochthonousblocks of lirnestones g;psum, sul- alunogen, alunite, natroalunite, melanterite, (Fytikas 1977).Most of the island consistsof vol- fur, sylvite, quartz,'cristobalite and kaolinite. The local rocks (Fig. 2), which are p4rt of the southern environment is characterizedby gas emi3sionsrich in H2S canic which formed during and CO2, thermal waters enrichedin SOaandNaby mixing Aegean island. arc, and lhe with seawater,porous and permeablealluvial aluminosili- Pliocene as a consequenceof thenorthward'subduc- cate-rich soils, and the presenceof atmospheric 02. Altera- tion of the African plate beneath the Aegean plate tions started in the Quaternary in an acid environment and (Fytikaset al.1984). Thevolcanicrocks belongto a continue up to the present day in areasof H2S-rich fluids. calc-alkaline sedesof andesitesand rhyolites aicod- The hydrated sulfates represent precursors of the very paniedby tuffs, ignimbritesand pyroclasticrocks. common kaolinite mineralization on the island. Kaolinite there is a high-enthalpy geothermal was also formed by the hydrolysis of feldspars. On Milos, field (Fytikas & Marinelli 196). The heat flow was responsiblefor intensehydrothermal actii{ty, which Keywords: hydrothermal alteration, sulfates, kaolinite, ' causedwidespread bentonization.and alunitization, acid environment, Milos, Greece. kaolinization and the formation of various hydrothermal mineral deposits(kaolinite, bentonite, sulfur, barite, galena, alunite and mangauese So[,ttr4erns oxides).Kaolinite depositsare especially large; most of them are being exploited. Kaolinite is usedas raw Le milieu physico-chimique dans la r6gion de Thiaphes, material in the paper and cementindustries. sur I'lle de Milos, en Grbce, est propice d la formation Fumaroles(up to l02oc), submarinegas leakage natroalu- r€centede I'associationde alunogbne,alunite, (>50'C), thermal springs (up to 75"C) and hot glpse, qirartz, nite, m€lanterite, soufre, sylvite, cristobalite grounds (ui.to 100"C) are some surface thermal et kaolinite. Des €missionsgzLzeuses riches en H2S et CO2 sitesof altdredvolcanic rocks, caracterisentce milieu. Sont aussiimportants les eauxthe(- manifestations.At the males,enrichies en SOaet Na suite au mdlangeavec I'eau active mud volcanos, lignitic deposits, and volcanic de mer, un sol alluvionriaireporeux, perm€able,riche en emarrations (Velinov et al. 1970, Ballatds et al. aluminosilicates, et la pr€sencede I'oxygdne atmosph€ri- 1980, hydrous sulfate (e.g., rozerite, melanterite, que. Les ph€nombnesd'alt6ratron ont debute au Quater. rdmerite, halotrichite, epsomite and alunite) are naire dans un milieu acide, et continuent aujourd'hui dans formed. en H2S. Les sirlfates hydratds seraient les 6ndroits enrichis paper the mineralogyand the con- les pr€curseurs de la kaolinite, qui est r6pandue sur l'fle. This reportson L'hydrolyse des feldspaths a aussi produit de la kaolinite. ditions of formatlon of sulfate minerals and other secondriry products, espeiially kaolinite; in the Thiaphes area near the village Adamanda (Ftg. 2). (Traduit par la R6daction) GBOIOCYOT..|HE THIAPMS AREA Mots-cl4s: alt6ration hydrothermale, sulfates, kaolinite' milieu acide, Milos, Grdce. The areain which humid-hot soils and gasexhala- tions occur is approximately I km northeast of Adaqanda; it is known as Thiaphes, meaning "sul- INTRODUC'IION fur concentrations".The soils are unconsolidated alluvia[depositsconsistirig ofpebbles, sands, Silt and clay derived by weathering of the surrounding The island of Miltis is located in the central part of geological formations, such as tuffs, rhyolitic and 241 242 THE CANADIAN MINERALOGIST Frc. l. Map of Greeceshowing Milos Island and the activevolcanic arc. rhyodacitic lavas, and lahars, which are younger RESULTS than Upper Pliocene(Fyrikas 1977). Where gas emanationstake place, the soils are Mineralogy altered and coveredwith thin layers of secondary minerals that make the soils yellowish, greenish, The mineralsidentified by X-ray diffractometry white and brownish black. The soils are damp, gypsum, probably as a result of the effect of the underlying include alunogen, melanterite, natro- (all quartz, cristobalite, geothermal field, whose presencehas been con- alunite, alunite sulfates), halite, and sylvite.of the two forms of silica, firmed by drilling of the surrounding alluvial sulfur; deposits. cristobalite is the newly formed mineral present, whereaspart of the quaramaybe of detrital origin. The aluminosilicatesare representedby kaolinite; ANALYTICALMETHoDS halite and sylvitealso are present. Powder X-ray diffractometry was usedto identify the hydrated sulfate aggregatesand the a$sociated Chemistry material. A Philips PWlOlO diffractometer was operated with Ni-filtered CuKo radiation at 36 kV The chemical composition of the sulfate-coated and 24 mA, at the Departmentof Geology,National aggregatesanalyzed is quite simple and is asexpected University of Athens. The samples(10) were also from the mineral determinations;concentrations of analyzedby atomic absorption$pectrometry for K, the major elements(Al, Ca, K, Na, Fe)correlate well Na, Al, Fe, Ca, Co, Ni, Pb,Zn, Cu and Sr. X-ray- with the mineral assemblages.For example, the diffraction data and chemicalresults are shown in samplewith high gypsum content (M5) is charac- TablesI and 2, respectively. terized by a high Ca content; the presenceof alu- KAOLINITE MINERALIZATION, MILOS, GREECE 243 Ftc,2.GeologicalsketchmapofMiloslsland(fromFytikasela/.1986)'withlocation of the studied area. Symbols: I metamorphic basement,2 Neogenesediments, 3 basal pyroclastic series (Middle-Upper Pliocene), 4 complex of domes and lava flows (Upper Pliocene),5 pyroclasticrocks, 6lava domes,7 Halepa and Plakes domes, 8 rhyolitic complexes(Upper Pleistocene),9 products of phreatic activity' l0 Quaternary sediments, I I study area. nogen Alz(SOo)tcl7H2O, alunite KA13(SO;2(OH)6, DISCUSSION: natroalunite NaAl(SOJz(oH)u and kaolinite FoRMATIoNoF ALTERATIoNMrsrners AI2Si2O5(OH)4is reflected by the high Al contents in the samples.Sodium and potassiumare presentin Hydrous sulfate minerals typically form in the alunite, natroalunite, halite and sylvite. The high oxidation zone of sulfide ores (Velinov et al. 1970' Fe-contentsin the samplesreflect the presenceof Good 1971,Zodrow et ql. 1979),but also havebeen melanterite(FeSO*r7H2O), the only iron sulfatede- found in areasof volcanic activity (Stamatakisel a/. tected.Ni, Pb, Zn concentrationsare neveranoma- 1987).From field observationsand publisheddata lous, suggestingthat other sulfate minerals are ab- (Fytikas 1977), the chemical environment at the sent.Cu, Co and Sr werenot detectedin the samples, Thiaphes area is favorable for the formation of exceptfor a Sr value of 230 ppm in a gypsum-rich hydrated sulfate minerals. This environment is char- specimen. All these results suggestthat the altered acterized by H2O, H2S and CO2emanations and by parent soils, which were affected by the hydrother- high humidity from the geothermal field. The SOa mal solutions and HrS gases,were low in trace ele- contents ofthermal watersthat apparently penetrate ments(Cu, Pb,Zn, Ni), and this is reflectedby their the permeablealluvial soilsare high and hencepH is mineralogicalcomposition. The alluvial soilsconsist low (4.0). of particles derived from the alteration of the sur- Thehydroussulfates attheThiaphes area of Milos rounding geologicalformations, which are mainly must be the alteration products formed by the reac- white tuffs and hydrothermally altered (mainly tion of H2Swith alluvial soils in a humid microen- kaolinized)volcanic rocks. In general,the soils are vironment of low pH. The abundancesof Al-sulfate composedof Si-, Al-, Fe-, K- and Na- bearing sili- minerals (alunogen, natroalunite, alunite) reflect the cates.Part of the Na is attributed to contamination existenceof Al-rich minerals in the soils. The occur- of the hydrothermalsolutions by seawater. rence of sulfur in thin layers in the surface around 24 THE CANADIAN MINERALOGIST TABTEI. ASSEIELACESOF MINERALS.AS DEDUCEDFROI,I XRD AI{ALYSES The formation oflypsum occursby the transfor. Sample . Mlneraloqy mation of the anorthite coriponent of plagioclase !n sulfur, alunogen,cristoballte accordingto the reaction: IQ al unogen, melanterite, hal I te, kaol inite, natrc- al uni te CaAl2Si2Os+ H2SO4 + 3H2O - M3 sulfur, hallte, quartz, sylvite, alunlte anorthite M4 aI unogen, nel anterl te, hat ite, kaol lnlte, hatro- alunlte, sylvite component M5 cristoballte, quartz, gypsum CaSOo.![1rO.* A-l2Si2O5.(OH)n t'|6 alunogen, crlstoballtd, quartz, gypsln gyp$um kaolinite n7 alunogen, cristobalite, quartz M]-I sulfur, crlstobalite As mentioned before, on Milos there are M4-t al unogen, cri stobal ite, melanterl te, natrcal unite; sylvite, quartz widespreadgaseous .emanations, thermal springs, M5-l sulfur, crlstobalite, quartz barite and sulfur deposits, etc,, as well as many kaolinite deposits.Most of thesedeposits