Petrogenesis of Carbonatites in the Alnö Complex, Central Sweden

Sammanfattning: 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.