Abstract
Object of study. The paper presents the mineralogical and petrographic study results of metamorphosed and metasomatized komatiites and komatiitic peridotites from the Ozerki soapstone deposit and Pentinsuo prospect, located in the Kostomuksha greenstone structure of the Karelian Craton, Fennoscandian Shield. Material and methods. Surface and drill core samples of various mineral and structural-textural varieties of altered komatiites were studied by optical microscopy, electron microscopy with an attachment for microanalysis, XRD phase, thermogravimetric and chemical analysis. Results. Soapstone formation in the investigated localities displays a multistage alteration and associates with the superimposed alteration of Mg-rich metakomatiite flows and olivine cumulates due to influx of carbon dioxide bearing fluids enriched in calcium and potassium. The chemical composition of initial komatiite (MgO content of the rock) and the degree of its fracture intensity are the main factors controlling soapstone formation. In differentiated lava flows soapstone is formed mainly in high-magnesium cumulate zones characterized by a high content of serpentine. In the flow top and spinifex zones an early amphibole-chlorite-magnetite mineral association is preserved in varying degrees. Talc and carbonate are formed by the decomposition of serpentine and amphibole. The chlorite content in soapstone is controlled by the Al2O3 concentration in the respective flow zones. Conclusion. The carbonate-chlorite-talc is the general natural type of soapstone associated with thin komatiite lava flows of distal volcanic facies. In more thick proximal lava flows the formation of soapstone of chloritecarbonate-talc and talc-carbonate composition is possible. The most prefered prospecting areas for soapstone are fields proximal to the eruptive vent, main lava conduits and subvolcanic analogues of komatiites.
Highlights
Surface and drill core samples of various mineral and structural-textural varieties of altered komatiites were studied by optical microscopy, electron microscopy with an attachment for microanalysis, XRD phase, thermogravimetric and chemical analysis
Soapstone formation in the investigated localities displays a multistage alteration and associates with the superimposed alteration of Mg-rich metakomatiite flows and olivine cumulates due to influx of carbon dioxide bearing fluids enriched in calcium and potassium
Talc and carbonate are formed by the decomposition of serpentine and amphibole
Summary
Месторождение талькового камня Озерки и проявление Пентинсуо расположены в юго-западной части Костомукшской зеленокаменной структуры, локализованной в западной части КостомукшскоГимольского зеленокаменного пояса Карельского кратона (рис. 1, 2). Месторождение талькового камня Озерки и проявление Пентинсуо расположены в юго-западной части Костомукшской зеленокаменной структуры, локализованной в западной части КостомукшскоГимольского зеленокаменного пояса Карельского кратона 1. Схема геологического строения Костомукшской зеленокаменной структуры, по [Кулешевич, 2004] с упрощением. 1 – метакоматииты; 2 – метабазальты; 3 – метагаббро; 4 – диориты; 5 – гранит-порфиры; 6 – тектонические нарушения; 7 – направление кровли лавового потока; 8 – элементы залегания сланцеватости; 9 – контуры участков: 1 – Озерки-1, 2 – Зеленая горка, 3 – Пентинсуо; 10 – дорога. Метакоматииты Костомукшской зеленокаменной структуры характеризуются содержаниями MgO (в пересчете без летучих) от 18 до 29 мас. На проявлении Пентинсуо метаультрамафиты формируют крутопадающее пластообразное тело северо-восточного простирания протяженностью 500 м, мощностью 200 м, сложенное в центральной части метаперидотитами, к периферии сменяющимися массивными, подушечными лавами метакоматиитов, потоками коматиитовых и толеи товых метабазальтов 1 – chlorite-amphibole and (carbonate)-amphibole-talcchlorite rocks; 2 – carbonate-chlorite-talc rock (soapstone); 3 – metabasalts; 4 – metagabbro; 5 – faults; 6 – drill holes; 7 – iron content of minerals: in the numerator – dolomite/ talc, in the denominator – chlorite; 8 – spinifex texture zones; 9 – direction of lava flowtop; 10 – road
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