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.

Carbonatites are rare igneous rocks, which contain at least 50 %, carbonate minerals. They are often found along with alkaline silicate rocks, meaning that they contain relatively large amounts of Na2O and K2O, and important tools to understand mantle processes. Importantly, carbonatites are known to be rich in REE (Rare Earth Elements), compared to other magmatic rocks. The Alnö complex is located in the northern part of the island of Alnö, northeast of Sundsvall and is one of the biggest ring-shaped intrusions of the alkaline and carbonatite sort. The origin of carbonatites is not fully known yet but they may have resulted from a mantle plume, and absolute dating methods indicate that the age of the Alnö carbonatites are around 600 Ma. A large amount of carbonatites have been found in the Alnö complex and the purpose of this thesis is to assess whether the Alnö complex and its carbonatites can become a potential source of REEs and if it can be profitable to mine these. The research has been conducted by first analyzing samples from the Alnö complex in thin sections through a light polarising microscope. The thin sections were photographed with a focus on apatite crystals, since apatite are hosts of REE. The apatites in the thin sections are crystals with high relief, sub-rounded and white-grey in color in a calcitic matrix. The images of the apatite were then edited with photoshop, to graphically isolate the apatite. The processed images were then analyzed in a program called “ImageJ” to calculate the total area of apatites in the thin sections and the area percentage of apatites. The area percentage helps to give an estimation of how much REE that can occur in the carbonatites of the Alnö Complex. A recent 3D- analysis of the Alnö complex using petrophysical and geophysical methods indicated the volume of the complex, which when coupled with our apatite data, allows us to estimate the total REE volume. The estimation of occurrences of apatites was calculated to around 13 % of the carbonatites in the Alnö complex. The cost to mine the REEs was much higher than the market price of the REEs. So the Alnö carbonatites are not profitable to mine for REEs today. / Karbonatit är en ovanlig bergart som innehåller minst 50 procent karbonater, därav namnet. De hittas ofta i samband med alkaliska silikat-bergarter, vilket innebär att de innehåller till stor del natriumoxid och kaliumoxid samt kisel, och är viktiga för att kunna förstå processer i manteln. Karbonatiter är kända för att vara innehållsrika på sällsynta jordartsmetaller, även kända som REE, jämfört med andra magmatiska bergarter. Alnökomplexet ligger i den norra delen av Alnön, nordost om Sundsvall och är ett av världens största alkaliska och karbonatit-ringkomplex, med en radie på 2,5 km. Dess ursprung i jordens inre är okänt men det tros vara ett resultat av en mantelplym, smältor från manteln som stiger mot ytan, och åldersdatering via absoluta dateringsmetoder tyder på att karbonatiterna är nästan 600 Ma. Stora mängder karbonatiter har hittats i Alnökomplexet och syftet med detta arbete är att bedöma om Alnökomplexet potentiellt kan bli gynnsam som källa för prospektering av sällsynta jordartsmetaller, ur ekonomisk synpunkt. Detta utfördes genom att analysera prover från Alnökomplexet samt studera data från Magnus Anderssons arbete om Alnökomplexet. Proverna analyserades med hjälp av en mikroprob som fotograferade apatiten, vita kristaller i ett mörkgrå matrix, då apatit indikerar på hög sannolikhet för REE-förekomst. Sedan redigerades dessa bilder med Photoshop och Paint, där andra kristaller redigerades bort så att det enda som var kvar var de vit-gråa utåtstickande kristallerna mot ett kalcitrikt matrix. Med ett annat program som heter ImageJ beräknades arean av dessa kristaller samt procenten av arean som apatiterna utgör i tunnslipen. Detta gav en viss uppskattning på hur stor mängd REE som kan förekomma i Alnökarbonatiter. Resultatet jämfördes med data från Magnus Andersson som har gjort en 3D- undersökning av karbonatiter under Alnö-komplexet. Apatiten utgjorde en area på 13 % och mängden REE var inte tillräckligt stor mängd relativ marknadspriserna samt utvinningskostnader för att räknas som vinstgivande.