Stable isotope and fluid inclusion evidence of multistage fluidal activity in Baltic paleobasin: Silurian carbonate sequence in Kalana, Estonia
Date
2017-07-24
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Abstract
Eesti ja laiemalt Balti paleobasseini alal pole viimase umbes poole miljardi aasta jooksul toimunud selliseid vulkaanilisi või tektoonilisi sündmusi, mis võiksid kivimeid oluliselt kuumutada. Sellest hoolimata esineb maapinnalähedastes settekivimites mitmeid nähtusi, mis viitavad mõjutustele kõrgema temperatuuri poolt. Nähtusi nagu dolomiidistumine, polümetalse maagistumise ilmingud, kivimite diageneetilised gradiendid ja sekundaarne magnetiseeritus saab seletada kuumade lahuste ehk fluidide liikumise mõjuga.
Käesoleva doktoritöö eesmärgiks oli uurida hüdrotermaalsete fluidide iseloomu ja nende põhjustatud mineralisatsiooni Balti paleobasseinis Kalana karjääris avatud Siluri karbonaatsete kivimite põhjal. Vaatluse alla jäi kaks võimalikku hüdrotermaalsete fluididega seotud sündmust: 1) kaltsiidi-sfaleriidi soonte teke ning 2) barüüti ja erilise isotoopkoostisega kaltsiiti sisaldavate koopalaadsete moodustiste areng.
Sfaleriit (ZnS) on soontesse kristalliseerunud kahes etapis. Fluidisuletiste mikrotermomeetria annab esimese põlvkonna tekketemperatuuriks umbes 200°C, teisel 70-100°C. Tegemist oli kõrge soolsusega (u. 26%) CaCl2-se lahusega. Vaadeldud mineralisatsioon seostub tänapäeva Norras-Rootsis asuvate Kaledoniidide mäestiku tekkimisega u. 400 miljonit aastat tagasi ning on oma olemuselt nn Mississippi Valley tüüpi.
Barüüti (BaSO4) esineb vaheldumisi kaltsiidi (CaCO3) ja püriidiga (FeS2) lõhede ja koopalaadsete struktuuride seintel. Selle kaltsiidi teeb eriliseks tema isotoopkoostis, mis viitab süsiniku päritolule bio- või termogeensest metaanist, mida on bakterite poolt anaeroobselt oksüdeeritud. Sellise protsessi toimumist koobastes pole teadaolevalt varem kirjeldatud.
Barüüt on tekkinud kahe fluidi segunemisel, millest baariumi sisaldav tõusis sügavamalt Maa sisemusest. Sulfaatioon pärineb kas mereveest või mattunud orgaanika lagunemisel tekkinud H2S-i oksüdeerumisest mikroobide poolt.
Barüüt ja kaltsiit on arvatavasti tekkinud madalatemperatuurilise (u. 70°C) fluidi liikumisest põhjustatud protsesside tagajärjel
During the last half billion years Estonia and more generally Baltic paleobasin haven’t experienced tectonic or volcanic activity that could heat up rocks significantly. Nevertheless, some geological evidences like dolomitization, sphalerite/galena mineralization, diagenetic gradients and secondary magnetization of rocks indicate influence of hydrothermal fluid movement. The aim of this thesis is to study the fluid-driven mineralization, mineral assemblage and paragenesis, and fluid characteristics in fracture-controlled cave and vein systems in the Silurian carbonate succession in Kalana, central Estonia. Formation of calcite-sphalerite veins and speleothemes formed of barite and isotopically very light calcite were under special focus. Sphalerite (ZnS) occurs in two generations. Microthermometry of fluid inclusions suggest temperature of formation not less than 200°C and 70-100°C respectively. It was a CaCl2-type fluid with high salinity (ca 26%). The mineralization could be connected to formation of Caledonides around 400 Ma, and is similar to Mississippi Valley type mineralization. Barite (BaSO4) appears together with calcite (CaCO3) and pyrite (FeS2) on the walls of fractures and cave like structures. Calcite in those speleothemes has exceptional isotopic composition which suggest that its carbon source is methane which is anaerobically oxidized by microbes. Such process going on in caves hasn’t been described before. Barite is formed due mixing of two fluids: deeper and warmer Ba-bearing and shallower sulfate rich fluid. Sulfate originates from seawater or from oxidation of sulfide formed due mineralization of organic matter. Speleothem calcite and barite are formed due to activity of relatively low temperature (ca 70°C) hydrothermal fluids
During the last half billion years Estonia and more generally Baltic paleobasin haven’t experienced tectonic or volcanic activity that could heat up rocks significantly. Nevertheless, some geological evidences like dolomitization, sphalerite/galena mineralization, diagenetic gradients and secondary magnetization of rocks indicate influence of hydrothermal fluid movement. The aim of this thesis is to study the fluid-driven mineralization, mineral assemblage and paragenesis, and fluid characteristics in fracture-controlled cave and vein systems in the Silurian carbonate succession in Kalana, central Estonia. Formation of calcite-sphalerite veins and speleothemes formed of barite and isotopically very light calcite were under special focus. Sphalerite (ZnS) occurs in two generations. Microthermometry of fluid inclusions suggest temperature of formation not less than 200°C and 70-100°C respectively. It was a CaCl2-type fluid with high salinity (ca 26%). The mineralization could be connected to formation of Caledonides around 400 Ma, and is similar to Mississippi Valley type mineralization. Barite (BaSO4) appears together with calcite (CaCO3) and pyrite (FeS2) on the walls of fractures and cave like structures. Calcite in those speleothemes has exceptional isotopic composition which suggest that its carbon source is methane which is anaerobically oxidized by microbes. Such process going on in caves hasn’t been described before. Barite is formed due mixing of two fluids: deeper and warmer Ba-bearing and shallower sulfate rich fluid. Sulfate originates from seawater or from oxidation of sulfide formed due mineralization of organic matter. Speleothem calcite and barite are formed due to activity of relatively low temperature (ca 70°C) hydrothermal fluids
Description
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Keywords
voolised, mineraliseerumine, stabiilsed isotoobid, karbonaatsed kivimid, silur, Eesti, Baltoskandia, fluids (physics), mineralization, stable isotopes, carbonate rocks, Silurian, Estonia, Baltoscandia