Canadian Mineralogist Vol. 28; pp. 363-368-(1990) THEAUTHIGENIC MINERALS FORMED FROM VOLCANIC EMANATIONS AT SOUSSAKI, WEST ATTICA PENINSULA,GREECE . KONSTANTINOSG. KYRIAKOPOULOS Departrnentof Geology,University of Athens,Panepistimiopolis, Ano llissia,157 84 Athens,Greece R.AY KANARIS-SOTIRIOU Departmentof Geologlt,Univ*sity of Sheffield,Mawin Street,Sheffield 51 3JD, UnitedKingdom MICHAEL G. STAMA-TAKIS Departmmt of Geology,University of Athens,Panepistimiopolis, Ano llissia, 15784 Athens, Greece ABsrRAcr en Ni, Co et Cr. Les formuleschimiques des mindraux authigeneset leursrelations sur le terrain font penserque The Soussakiarea, situated in the west Attica Peninsula, lessulfates de Fe, Mg et Ni, ainsique la magn6site,la dolo- Greece, represents the north\ryestern end of the South mite et, danscertains cas, l'opale-CT, sont li6s d l'alt6ra- Aegean Plio-Pleistocene volcanic arc. Postvolcanic tion hydrothermaledes serpentinites, tandis que alunite, phenomenacontinue to the present day, and include emis- opale-CT,cristobalite, et quartzsont associ6s d I'alt6ration sion of gasessuch as H2S, CO2, and water vapor. Reac- despetits massifs rhyodacitiques. La plupartdu gypseet tion betweenthese fluids and pre-existing rocks, consist- deI'anhydrite et unepartie de la calcite,l'aragonite et la ing mainly of serpentinite,chert, marlstone, limestone, and dolomiten6oform6es seraient dues i la d6compositiondes subordinate rhyodacitic lava, has resulted in the formation marneset descalcaires des roches hOtes. of glpsum, sulfur, silica polymorphs, and Fe and Mg sul- fates; kaolinite, anhydrite, carbonates, and Ni and K-Al Mots-clds:arc volcanique 6gden, sulfates, exhalations vol- sulfates also are present in some cases. The authigenic caniques,altdration hydrothermale, Grdce, minerals form a crust up to l0 m in thickness on top of partly altered basementrocks. The sulfates,carbonates and silica polymorphs are enrichedin Ni, Co and Cr. The chem- Ir.lrRoDUCTIoN ical formulas of the authigenicminerals and their field rela- tionships suggestthat the Fe, Mg, Ni sulfates, as well as In the Aegeanregion (Fig. 1), extensivevolcanic magnesite,dolomite and some of opal-CT, are related to activity has taken place from Tertiary through to hydrothermal alteration of serpentinites, whereasalunite, times (Innocenti et al. 1979, Fytikas et quartz Quaternary kaolinite, opal-CT, cristobalite and are associated is locatedabout 65 km rhyodacite.The al, 1984),The Soussakiarea with the alkration of small bodiesof major (Fig. part of the gypsum and anhydrite, and some of the west of Athens, near the Canal of Corinth 2) neoformed calcite, aragonite and dolomite, may be caused and representsthe northwesterly end of the South by the decompositionof the marlstonesand limestonesof Aegeanvolcanic arc. Here, volcanic activity com- the bedrock. mencedduring the Plioceneand continuedthrough the Pleistocene.Postvolcanic activity is stjll observed Keywords: Aegean volcanic arc, sulfate minerals, volcanic today, manifested as emanations of warm fluids in emanation, hydrothermal alteration, Greece. a localizedarea of roughly 200 m x 700 m called Psorochoma(Figs. 2, 3) (Papastamatiou193?, Pe SoMMAIRE 1972,Innocenti et al. 1984,Fyrikas e/ a/. 1988).This La r6gion de Soussaki, situde dans la partie occidentale activity is characterized by the emission of hydro- de la p6ninsule de l'Attique (Grbce),repr€sente I'extrdmit€ gen sulfide, carbon dioxide and water vapor nord-ouestde l'arc volcaniqueplio-pleistocbne. Les ph€- (Papastamatiou1937). nomdnespost-volcaniques continuent jusqu'i ce jour, Basementrock-types of the Soussakiarea include comme en temoignent des 6missionsde gaz (H2S, CO2, et pre-Neogeneserpentinite and chert, whereasmarine vapeur). La r6action de ces fluides avec les roches encais- and fresh-watermarlstone and limestoneof Pliocene principalement santes(constitudes de serpentinites, chert, age partly cover the older rocks @apastamatiou marnes,calcaires, et lavesrhyodacitiques) a abouti d la for- 1937). Locally, small bodies of dacite-rhyodacite g;ryse, polymorphes mation de soufre, de silice,et sulfates the former rocks. The de Fe et de Mg; kaolinite, anhydrite, carbonates, et sulfa- lavas have erupted through partly tes de Ni et de K-Al sont aussi prdsentsdans certains cas. volcanic glassofthese rocks is still unaffected Les min6raux authigbnesforment une cro0te 6paissede l0 by the postvolcanic phenomena. The serpentinite m sur les roches du soclepartiellement alterees.Les sulfa- basementoccurs as elongatebodies near the areaof tes, carbonates et les polymorphes de silice sont enrichis the major center of volcanic emanations. 363 364 THE CANADIAN MINERALOGIST ahydrite NiSoo.6HrO; native element:sulfur; car- bonates:magnesite, dolomite, calcite,aragonite; K- Al hydroxysulfate: alunite; silica polymorphs: opal- CT, low cristobalite, q\ara; clay minerals: kaolinite, smectite; sulfide: marcasite; oxides-hydroxides: scarceFe-oxides, hydroxides. The sulfate aggregatesrich in roemerite and nickel hexahydriteusually havea greencolor, whereasthose containing rozenite, epsomite, hexahydrite, melan- terite, anhydrite, or alunite are white. Melanterite- and rozenite-rich samples are easily identified becausethey exhibit curved, elongatecrystals and silky, cotton-like aggregates,respectively. Opal-CT, low cristobalite and quartz form hard, massivebod- ies, black-greyto olive greenin color. The presence of native sulfur givesayellowish color to aggegates. Transparent gypsum crystals are widespread in all the sulfate aggregates, but may form individual massesup to 5 m thick, especiallyclose to marlstone and limestone (Fig. 3). In contrast, near serpentinite Ftc. l. The locationof the Soussakiarea in the south masses,magnesite and opal-CT occur in abundance, Aegeanarc. in addition to the Ni, Mg and Fe sulfates(Frg. 3). Most of the ferrous, ferric, nickel and magnesium sulfates occur as efflorescenceson the inner walls of cavesexcavated before and during the SecondWorld In the area, the reaction of warm, acidic fluids War for the extractionof native sulfur. It is notice- with the pre-existingrock-types created a seriesof able that the three Fe-sulfateshave not beenfound authigenicminerals, in particular sulfates,carbonates together (Table l). In some caves, different and silica polymorphs. Thick, unconsolidatedmasses pilageneseshave beenobserved (Table l), consist- of thesesecondary minerals up to l0 m in thickness ing of either alunite with opal-CT, or kaolinite with commonly occru near the sitesof fluid emissionas qvartz, or low cristobalite with opal-CT. In the last a result of decompositionof silicateminerals in the two cases,unaltered volcanic glassis present. pre-existingrocks. Ni, Co, Cu, Zn and Cr contentsof the aggregates This study discussesthe assemblagesof the secon- are reported in Table 2. Lead concentrations were dary minerals, the distribution of Ni, Co, Cl, Zn found to be below the detectionlimit (5 ppm). Nickel and Cr, and relationshipswith the type of parent concentrationsshow an enorrnousrange, from 15 material. Sampleswere collectedfrom the sitesof ppm in samples enriched in native sulfur and car- major postvolcanicactivity at Psorochoma(Fig. 3). bonate, to 9800ppm in samplescontaining nickel hexahydrite (Tables l, 2). Cobalt concentrations ANALYTICALMBTHons generallyfollow those of nickel, with valuesbetween 5 and 355 ppm. Although Znand Cu valuesare rela- Mineralogical determinations were carried out tively low in comparisonwith the Ni and Co con- usingX-ray-diffraction analysis.Concentrations of centrations, the former elements do occur in Ni, Co, Cu and Zn were determined by atomic increased concentrations wherever the Ni and Co absorption analysis,whereas concentrations of Cr valuesare high (Table 2, Fig. 4). and Pb were determinedby colorimetry. All the ana- A statistical treatment of trace-elementconcentra- lyseswere done at the University of Athens. The ana- tions showsthat thereis a good correlationbetween lytical results are given in Tables I and 2. Ni and Co (correlationcoefficient R = 0.89),moder- ate correlation betweenNi andZn (R = 0.64), Co and MlNnnerocy AND GEoCHEMISTRY Zn, and poor correlation between Ni and Cu (R:0.28), Cr and Zn (Fie. 4). The identified authigenic minerals (Table l) can be subdivided into chemically related groups as fol- lows: hydrous ferric and ferrous sulfates:rozenite DISCUSSIoN FeSO..4H2O,melanterite FeSOa.TllrQ, roernerite * Fd"Fe3 (SO)o.l4HrO; hydrous Mg-bearing sul- In the field of volcanic emanationsat Soussaki, fates: hexahydrite, epsomite; Ca-bearingsulfates: the predominanceof Fe and Mg hydrous sulfates, gypsum, anhydrite; hydrous Ni sulfate: nickel hex- the presenceofNi sulfate and a correspondinglyhigh AUTHICENIC MINERALS FROM VOLCANIC EMANATIONS 365 gkqrti 3;7"57' \-\;: SAR ONIKOS GULF 3fs4' q-lk ffi4 tT-Tl1 Fls )r" 2 -6 ./ I3 @z t 23"42' 23"g6 Frc. 2. Geologicalmap of the Soussakiarea (from Fytikas el a/. 1988,slightly modified). Legend: I alluvial deposits, 2 marly limestones,marls, conglomeratesof Plio-Pleistoceneage, 3 conglomerates,sandstones and marly limestones with lignitic intercalations(Pliocene), 4 ophiolite complex,5 carbonaceoussequence (Middle Jurassic- Upper Tri- assic),6 dacitic lava flows and domes,7 tuffs, 8 fault (a observed,b inferred), 9 area studied. !e1\\;x \:>;i) F uorty tirnestones D^-Tr] CoSOaU..tlrg elfiaescenee llTll A otd Ug higtEr h!/drorcdqrtemtio'' iorms ot lv-v.l Serpentinite my:i*gEff*** t Kf iffi*" Frc. 3. Schematiccross-section at the southernend of the study area at