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Pro Gradu AJMA FACULTY OF TECHNOLOGY GEOLOGICAL CHARACTERIZATION OF ANORTHOSITIC ROCKS IN THE OTANMÄKI INTRUSION, CENTRAL FINLAND: CONSTRAINTS ON MAGMA EVOLUTION AND Fe-Ti-V OXIDE ORE GENESIS Anssi Johannes Mäkisalo MASTER’S PROGRAMME IN GEOSCIENCES Master’s thesis 2019 TIIVISTELMÄ OPINNÄYTETYÖSTÄ Oulun yliopisto Teknillinen tiedekunta Koulutusohjelma (kandidaatintyö, diplomityö) Pääaineopintojen ala (lisensiaatintyö) Geologian ja mineralogian koulutusohjelma Tekijä Työn ohjaaja yliopistolla Mäkisalo, Anssi Johannes Kärenlampi, K., tohtorikoulutettava, Hanski, E., professori emeritus Työn nimi Otanmäen intruusion anortosiittisten kivien geologinen karakterisointi ja merkitys magmaattisessa evoluutiossa sekä Fe-Ti-V-oksidimalmin synnyssä Opintosuunta Työn laji Aika Sivumäärä Geotieteet Pro gradu Maaliskuu 2019 128 s., 6 liitettä Tiivistelmä Kainuussa sijaitseva Otanmäen intruusio on differentioitunut mafinen intruusio, jossa esiintyy kerrosmyötäistä Fe-Ti- V-oksidimalmia. Malmi ilmenee <1–200 metriä leveinä linsseinä heterogeenisessä vyöhykkeessä, joka koostuu eri gabrotyypeistä sekä anortosiitista. Anortosiitti muodostaa muutamia metrejä halkaisijaltaan olevia runsaslukuisia, muodoltaan vaihtelevia sulkeumia. Sulkeumat ovat merkittävä komponentti intruusion malmia sisältävässä osassa, mutta niitä ei ole tutkittu tarkasti ja niiden merkitys intruusion geologisessa historiassa on osin epäselvä. Tämän pro gradu- tutkielman tarkoitus on karakterisoida anortosiittiset kivet geologisen kartoituksen sekä kokokivi- ja mineraalikemiallisten menetelmien avulla, verrata niitä intruusion muihin mafisiin kiviin sekä vastaaviin intruusioi- hin globaalisti ja tutkia niiden merkitystä magmaattisen evoluution ja Fe-Ti-V-oksidimalmin synnyn kannalta. Työtä varten valmistettiin 20 ohuthiettä, joista tehtiin mikroskooppisten tutkimusten lisäksi 64 elektronimikroanalyysiä plagioklaasin koostumuksen selvittämiseksi. Geokemiallinen aineisto sisältää 18 ICP-OES/ICP-MS analyysiä sekä 58 kannettavalla XRF-analysaattorilla tehtyä koostumusmittausta. Lisäksi hyödynnettiin aikaisempien tutkimusten aineistoja, joihin kuuluu 941 kallioperähavaintopistettä, 139 ohuthiettä, 197 kokokivigeokemiallista analyysia, 110 elektronimikroanalyysia sekä Rautaruukki Oy:n laatimia karttoja ja kairasydänraportteja. Otanmäen intruusiosta laadittiin vuosina 2017–18 suoritetun kartoitustyön perusteella 20 km2:n kattava, 1:50 000- mittakaavainen geologinen kartta sekä stratigrafiapylväs. Lisäksi laadittiin 1:200-mittakaavassa yksityiskohtaisia geologisia karttoja anortosiittisia sulkeumia sisältäviltä alueilta hyödyntäen drone-ilmakuvausta. Työssä esitetään uusi, yhtenäistetty nimistö Otanmäen intruusiolle. Intruusio jaetaan kolmeen tektoniseen lohkoon: Otanneva, Otan- mäki ja Vuorokas. Intruusiolohkot sisältävät kolme magmaattisen stratigrafian pääyksikköä: alavyöhyke, malmi- vyöhyke ja ylävyöhyke. Lisäksi intruusion reunoilla esiintyy kahdessa kohtaa hienorakeisia amfiboliittisia kiviä. Mineralogisten havaintojen sekä pää- ja hivenalkuainekoostumuksen perusteella anortosiittiset kivet ovat voimakkaasti differentioituneita plagioklaasiadkumulaatteja, jotka ovat syntyneet samasta magmasta muiden intruusion kivien kanssa. Malmi- ja ylävyöhykkeiden kiteytymisen aikana plagioklaasi on ainakin ajoittain kellunut magmassa. Tämä on mahdollistanut plagioklaasikiteiden kasautumisen magmasäiliön yläosaan. Osa tästä yläosan massiivisesta anortosiitista upposi takaisin magmasäiliön alempiin osiin vaihtelevasti kiinteytyneinä autoliitteina, jotka kerrostuivat osin jähmettyneeseen kumulaattikerrokseen. Oksidimalmin kiteytyminen ja siitä seurannut tiheyden lasku jäännössulassa ovat todennäköisesti vaikuttaneet autoliittien uppoamiseen. Fe-Ti-V-oksidimalmin syntyyn on vaikuttanut osittain kiteytyneiden kumulaattien valuminen kohti magmasäiliön vajonnutta keskustaa. Malmivyöhykkeessä tapahtuneet magmaattiset liikunnot ovat tapahtuneet ainakin osittain anortosiittisten autoliittien kerrostumisen jälkeen. Autoliittien osalta Otanmäen intruusiolla on merkittäviä yhteneväisyyksiä Skaergaardin ja Sept Ilesin intruusioihin. Muita tietoja Asiasanat: Otanmäki, intruusio, anortosiitit, Fe-Ti-V-malmit, magmaattinen evoluutio, paleproterotsooinen ABSTRACT FOR THESIS University of Oulu Faculty of Technology Degree Programme (Bachelor's Thesis, Master’s Thesis) Major Subject (Licentiate Thesis) Degree Programme in Geology and Mineralogy Author Thesis Supervisor Mäkisalo, Anssi Johannes Kärenlampi, K., Doctoral student, Hanski, E., Professor emeritus Title of Thesis Geological characterization of anorthositic rocks in the Otanmäki intrusion, central Finland: constraints on magma evolution and Fe-Ti-V oxide ore genesis Major Subject Type of Thesis Submission Date Number of Pages Geosciences Master's Thesis March 2019 128 p., 6 App. Abstract The 2.06 Ga Otanmäki intrusion in central Finland is a differentiated mafic intrusion hosting stratabound Fe-Ti-V oxide ore. The ore occurs as <1–200-m-wide lenses in a heterogeneous zone comprising different gabbro varieties and anorthosite. Anorthosite forms ubiquitous amoeboid-shaped inclusions a few meters in diameter, constituting a significant component in the ore-bearing part of the intrusion. The anorthositic rocks have not been studied earlier in detail and their role in the geological history of the intrusion is not well understood. The purpose of this Master’s thesis is to characterize the anorthositic rocks by means of mapping and geochemical and minerochemical methods, compare them to other mafic rocks in the Otanmäki intrusion and corresponding mafic intrusions globally and study their role in the magmatic evolution and Fe-Ti-V oxide ore genesis. A total of 20 thin sections were prepared for petrographic microscopy and 64 EPMA analyses were made to determine the plagioclase composition. The geochem- ical data include 18 ICP-OES/ICP-MS analyses and 58 portable XRF measurements. Pre-existing data, which include 941 bedrock observation points, 139 thin sections, 197 geochemical analyses and 110 electron microanalyses, were utilized as well as historical maps and drill core logs by the mining company Rautaruukki Oy. Based on the mapping campaign carried out in 2017–2018, a 1:50 000 geological map of the Otanmäki intrusion covering 20 km2 and a related stratigraphical column are presented. Detailed maps in 1:200 scale, which are based on aerial drone photography from anorthosite-bearing areas, were also compiled. A revised nomenclature for the Otanmäki intrusion is presented, in which the intrusion is divided into three tectonic blocks: Otanneva, Otanmäki, and Vuorokas. All these blocks comprise three major stratigraphical units: the Lower Zone, Ore Zone and Upper Zone. Furthermore, fine-grained marginal amphibolites are reported from two locations. Based on the mineralogical observations and major and trace element data, the anorthositic rocks represent strongly differentiated plagioclase adcumulates and are cognate to the Otanmäki intrusion. Crystallization of the Ore and Upper Zones has likely included one or several periods of plagioclase buoyancy, enabling accumulation of plagio- clase crystals in the roof part of the magma chamber. Parts of the massive roof anorthosite sank gravitationally into the lower parts of the magma chamber as variably solidified autoliths and were deposited on a semi-solidified sub- stratum. The sinking was likely affected by Fe-Ti-oxide crystallization and subsequent decrease in the density of the residual melt. The genesis of the Fe-Ti-V oxide ore is suggested to have been affected by gravitational movements of partially solidified cumulate slurry towards a subsided magma chamber center. At least parts of these movements postdate the deposition of the anorthositic autoliths. The occurrence of anorthositic autoliths bears many similarities with the Skergaard and Sept Iles mafic intrusions. Additional Information Keywords: Otanmäki, mafic intrusion, anorthosite, Fe-Ti-V mineralization, magmatic evolution, Paleoproterozoic CONTENTS 1 INTRODUCTION .............................................................................................................. 1 2 THEORETICAL CONSIDERATIONS ............................................................................. 4 2.1 Mafic magmatism ......................................................................................................... 4 2.1.1 Layered mafic intrusions .................................................................................... 5 2.1.2 Origin of layering in mafic intrusions ................................................................ 5 2.1.3 Mafic pegmatites ................................................................................................ 7 2.2 Magmatic Fe-Ti-V oxide deposits ............................................................................... 8 2.2.1 Deposit types ...................................................................................................... 8 2.2.2 Genesis ................................................................................................................ 9 2.2.3 Mineralogy ........................................................................................................ 11 2.3 Anorthosites ............................................................................................................... 12 2.3.1 Definition .......................................................................................................... 12 2.3.2 Anorthosite occurrences ..................................................................................
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