Examensarbete vid Institutionen för geovetenskaper Stable Isotope Systematics of Skarn-hosted REE-silicate - ISSN 1650-6553 Nr 279 Magnetite Mineralisations in Central Bergslagen, Sweden Fredrik Sahlström Stable Isotope Systematics of The metasupracrustal-hosted, often polymetallic REE-Fe-deposits of Bastnäs- Skarn-hosted REE-silicate - type are found along the “REE-line” in the Palaeoproterozoic Bergslagen ore province, south central Sweden. They essentially comprise REE silicate- Magnetite Mineralisations in bearing magnetite skarn mineralisations with variable contents of other metals. Even though these deposits have been important for mining and research for Central Bergslagen, Sweden centuries, their origin still remains unclear. In this study, samples from 10 different deposits along the REE-line have been charactarised as to mineralogy, petrography and bulk geochemistry, in addition to their isotope systematics. Mineral separates of magnetite and, when present, co-existing quartz or carbonates have been analysed for their oxygen and (for carbonates) carbon isotope compositions, in order to put constraints on the sources for metals and fluids in these deposits. Magnetites have δ18O-values of -1.79 to 1.12 ‰, while quartzes lie between 7.19 and 8.28 ‰. Carbonates have δ18O- Fredrik Sahlström values between 5.77 and 7.15 ‰ and δ13C-values between -5.35 and -3.32 ‰. Thermometric calculations based on mineral pairs (magnetite-quartz, magnetite-calcite/dolomite), combined with available fluid inclusion data, indicate formation of primary magnetite assemblages between c. 650 to 400 °C. At these temperatures, magnetites from some of the deposits would have been in equilibrium with a magmatic fluid (δ18O = 6-8 ‰), while magnetites from other deposits would have been in equilibrium with fluids of lower δ18O (4-6 ‰). Oxygen and carbon isotope trends in carbonates can be explained by interaction between original host carbonates and a fluid of magmatic composition. The combined results indicate that the Bastnäs-type magnetite- REE mineralisations were deposited from an originally magmatic fluid at relatively high temperatures. At local scale, variable modification of the fluid isotopic composition can be explained by mixing with seawater-dominated fluids. Uppsala University, Department of Earth Sciences Master Thesis, 30 hp Solid Earth Geology ISSN 1650-6553 Nr 279 Printed by Geotryckeriet, Uppsala University, Uppsala, 2014. Examensarbete vid Institutionen för geovetenskaper ISSN 1650-6553 Nr 279 Stable Isotope Systematics of Skarn-hosted REE-silicate - Magnetite Mineralisations in Central Bergslagen, Sweden Fredrik Sahlström Supervisors: Erik Jonsson & Karin Högdahl Copyright © Fredrik Sahlström and the Department of Earth Sciences Uppsala University Published at Department of Earth Sciences, Geotryckeriet Uppsala University, Uppsala, 2014 Stable isotope systematics of skarn-hosted REE-silicate - magnetite mineralisations in central Bergslagen, Sweden Table of Contents Abstract ................................................................................................................................................... 3 Sammanfattning på svenska ................................................................................................................... 4 1. Introduction ......................................................................................................................................... 5 1.1. Rare earth elements ..................................................................................................................... 5 1.2. Purpose of study and hypothesis ................................................................................................. 6 2. Geological Background ........................................................................................................................ 7 2.1. Regional geology and mineralisations of the Bergslagen ore province ....................................... 7 2.2. Skarn deposits .............................................................................................................................. 9 2.3. Local geology of the “REE-line” .................................................................................................. 10 2.3.1. Genesis of Bastnäs-type deposits ........................................................................................ 11 3. Materials and methods ..................................................................................................................... 12 3.1. Sample preparation .................................................................................................................... 13 3.1.1. Preparation of polished sections and thin sections ............................................................ 13 3.1.2. Preparation of samples for stable isotope analysis ............................................................. 14 3.2. Analytical methods ..................................................................................................................... 15 3.2.1. Powder x-ray diffraction ...................................................................................................... 15 3.2.2. Bulk geochemical analysis ................................................................................................... 15 3.2.3. Optical microscopy .............................................................................................................. 16 3.2.4. SEM-EDS .............................................................................................................................. 16 3.2.5. Electron probe microanalyser (EPMA) ................................................................................ 17 3.2.6. Stable isotope analysis ........................................................................................................ 19 4. Results ............................................................................................................................................... 22 4.1. Bulk geochemistry of REE-mineralised assemblages ................................................................. 22 4.2. Mineralogy and mineral chemistry ............................................................................................ 24 4.2.1. Östra Gyttorpsgruvan .......................................................................................................... 24 4.2.2. Johannagruvan .................................................................................................................... 32 4.2.3. Högforsfältet........................................................................................................................ 36 4.2.4. Danielsgruvan ...................................................................................................................... 37 4.2.5. Myrbacksfältet ..................................................................................................................... 38 1 4.2.6. Södra Hackspiksgruvan ........................................................................................................ 39 4.2.7. Östanmossagruvan .............................................................................................................. 41 4.2.8. Bastnäsfältet ........................................................................................................................ 44 4.2.9. Malmkärragruvan ................................................................................................................ 46 4.2.10. Rödbergsgruvan ................................................................................................................ 48 4.3. Chemical dating of uraninites ..................................................................................................... 49 4.4. Results stable isotopes ............................................................................................................... 50 5. Discussion .......................................................................................................................................... 52 5.1. Mineralogy, mineral chemistry and bulk geochemistry ............................................................. 52 5.2. Uraninite geochronology ............................................................................................................ 53 5.3. Stable isotope systematics ......................................................................................................... 54 5.3.1. Thermometry....................................................................................................................... 54 5.3.2. Fluid modeling ..................................................................................................................... 56 5.3.3. Effects of metamorphism .................................................................................................... 61 5.4. Fluid chemistry and ore mineralogy ........................................................................................... 61 6. Conclusions ........................................................................................................................................ 63 7. Acknowledgements ........................................................................................................................... 64 8. References ........................................................................................................................................