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Petrogenesis of Carbonatites in the Alnö Complex, Central Sweden Sherissa Roopnarain

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Petrogenesis of Carbonatites in the Alnö Complex, Central Sweden Sherissa Roopnarain Petrogenesis of Carbonatites in the Examensarbete i Hållbar Utveckling 176 Alnö Complex, Central Sweden Master thesis in Sustainable Development Petrogenesis of Carbonatites in the Alnö Complex, Central Sweden Sherissa Roopnarain Sherissa Roopnarain Uppsala University, Department of Earth Sciences Master Thesis E, in Sustainable Development, 30 credits Printed at Department of Earth Sciences, Master’s Thesis Geotryckeriet, Uppsala University, Uppsala, 2014. E, 30 credits Examensarbete i Hållbar Utveckling 176 Master thesis in Sustainable Development Petrogenesis of Carbonatites in the Alnö Complex, Central Sweden Sherissa Roopnarain Supervisor: Valentin R. Troll Evaluator: Abigail Barker Master thesis in Sustainable Development Uppsala University Department of Earth Sciences Petrogenesis of Carbonatites in the Alnö Complex, Central Sweden SHERISSA ROOPNARAIN Roopnarain, S., 2013: Petrogenesis of Carbonatites in the Alnö Complex, Central Sweden. Master Thesis in Sustainable Development at Uppsala University No. 176, 100 pp., 30 ECTS/ hp. Abstract: The Alnö Complex is a Late Precambrian alkaline and carbonatite intrusion (c. 30km2) into Early Proterozoic country rock that extends from the north east, to the north western shoulder of Alnö Island. Carbonatites are rare among volcanic provinces, with Oldoinyo Lengai of northern Tanzania being the only active carbonatite volcano in the world today. The high carbonate mineral volumes and rare earth element (REE) concentrations of carbonatites, in combination with the intrusive-extrusive nature of their suites contribute to the rarity of these rocks. Carbonatites, through their peculiar petrological and geochemical compositions, provide vital insights to the composition and condition of the Earth’s mantle. The genesis of the Alnö carbonatites and their relation to other lithological units at the complex is however, only partially understood. This stems from the epistemological division of carbonatites as having either a ‘magmatic’ or ‘reactive’ origin. This study focuses on sampled carbonatites from the Alnö Complex, employing an oxygen and carbon isotope approach on their native calcite, complemented with petrological and mineralogical methods in order to constrain petrogenesis. As a reference, oxygen and carbon isotope data of calcite from an earlier Alnö investigation as well as from an array of data from comparative alkaline complexes elsewhere are also discussed. The combined data and the derived findings support a scenario that is consistent with the ‘magmatic’ model wherein carbonatites have a primary mantle-derived origin, and prospectively stem from a parent magma akin to that of Oldoinyo Lengai, but have experienced a degree of silicate and sedimentary assimilation. The extraction of the Alnö carbonatites for their rare earth metals is a looming possibility due to the current volatility in the rare earth market. The risks and opportunities involved in this kind of natural resource extraction provide a context wherein sustainable development paradigms can be applied. The capacity of the Alnö environment to withstand the impact of development in the mining sector is discussed through a perspective of establishing a quarry, and quarry-related methods for rare earth extraction. Keywords: Petrogenesis, Carbonatite, Alnö Complex, Stable Isotope, Rare Earth Element (REE), Kimberlite, Sustainable Development, Resource Mining, Rural Landscape, Natural Heritage. Sherissa Roopnarain, Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala, Sweden. i Petrogenesis of Carbonatites in the Alnö Complex, Central Sweden SHERISSA ROOPNARAIN Roopnarain, S., 2013: Petrogenesis of Carbonatites in the Alnö Complex, Central Sweden. Master Thesis in Sustainable Development at Uppsala University, No. 176, 100 pp., 30 ECTS/ hp. Popular Summary: In the Gulf of Bothnia, there is an island known as Alnö. The island was formed in a volcanic eruption that occurred almost six hundred million years ago. The release of ash and lava from the eruption has carved an alkaline volcanic landscape, and when the magma solidified, igneous rocks were formed. Such rocks, borne out of complex volcanic processes are typically composed of essential elements and minerals. The Alnö igneous rocks are particularly captivating because they are rich in rare elements and minerals. This study analyses an array of these rocks that are known as carbonatites. Carbonatites are a most unusual breed of igneous rock, as intricate in texture as they are in composition. They are particularly enriched with carbonate minerals, the analysis of which is able to construct deeper insights on their origin, composition, structure and alteration. Such a task is encompassed by the term ‘petrogenesis’. Constraining petrogenesis involves deciphering between a ‘magmatic’ (i.e. of the mantle) or ‘reactive’ (i.e. non-mantle) origin. The carbonate mineral, calcite is encoded with signals (isotopes) that are able to indicate magmatic or reactive sources and influences. Hence, oxygen and carbon isotopes from calcite are analysed along with the textural and mineralogical properties of the carbonatite specimens. A distinction between the two types of origin remains controversial for carbonatites, derived in part from the overlap of mantle and reactive signals that sometimes occurs, and that invariably represents the lithological connection between carbonatites and other alkaline rocks that are accumulated in a particular site (e.g. the Alnö Complex). Comparative analysis, with earlier investigations on Alnö alkaline rocks as well as carbonatites from international alkaline volcanic provinces, similar to Alnö, buffers the retrieved data from elusive petrogenetic distinctions, and hence forms an integral part of the study. The research findings support a scenario that is consistent with a ‘magmatic’ model wherein carbonatites have a mantle-derived origin, and prospectively stem from a magma similar to that of Oldoinyo Lengai in Tanzania, which is currently the only known active carbonatite volcano in the world. The findings discuss that the Alnö carbonatite-forming magma had likely incorporated silicates as well as sedimentary material and gas on its journey through the mantle and even up to the point of intrusion at the crust. The extraction of the Alnö carbonatites for their rare earth metals is a looming possibility due to the current volatility in the rare earth market. The risks and opportunities involved in this kind of natural resource extraction provide a context wherein sustainable development paradigms can be applied. The capacity of the Alnö environment to withstand the impact of development in the mining sector is discussed through a perspective of establishing a quarry, and quarry-related methods for rare earth extraction. Keywords: Petrogenesis, Carbonatite, Alnö Complex, Stable Isotope, Rare Earth Element (REE), Kimberlite, Sustainable Development, Resource Mining, Rural Landscape, Natural Heritage. Sherissa Roopnarain, Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala, Sweden. ii Plagiarism Declaration 1. I know that plagiarism is wrong. Plagiarism is to use another’s work and to pretend that it is one’s own. 2. I have used the Author-Date convention for citation and referencing. Each significant contribution to, and quotation in this dissertation from the work(s) of other writers has been thoroughly acknowledged through citation and reference. 3. This dissertation is my own work. 4. I have not allowed, and will not allow, anyone to copy this work with the intention of passing it off as his or her own. 5. I have completed the electronic formatting and uploading of this dissertation by myself. I understand that the electronic publishing of this work, releases it as research belonging to Uppsala University and that it is therefore wrong for another to upload it for me. iii ‘Earth, water, air and fire. The question of whether they are endowed with an intellect, enabling them to comprehend, cannot be solved without deep research. Each of the four occupies a certain position of its own, assigned to it by nature. They are forced to move and to change their respective positions, so that fire and air are driven into the water, and again these three elements enter the depths of the earth. The elements will come together, to react upon each other and combine. When they return to their respective places, parts of the earth will sail together with the water. And parts will sail with the air, and with the fire. For the elements have the property of navigating back to their origin in a straight line, but they have no properties which would cause them to remain where they are, or to move otherwise than in a straight line. These rectilinear motions of the four elements, on their return voyage, are of two kinds: either centrifugal — the motion of the air and the fire, or centripetal — the motion of the earth and the water. And when the elements have reached their origins, they remain at rest.’ Moshe ben Maimon (1135-1204) iv Table of Contents 1. Introduction ............................................................................................................................ 1 1.1 Research Objectives ......................................................................................................... 3 2. Geological Background .......................................................................................................... 4 2.1 Alnö Island Volcanism ....................................................................................................
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