Energy, Structures, Metals, and Fluids: A Mineral Systems Study of Cu ± Au Mineralization in the Kiruna Mining District

In light of the increased demand for raw materials to support the transition away from a fossil-fuel society, mineral exploration for future raw material sources is imperative, requiring a holistic mineral systems approach. This thesis focuses on the Pahtohavare area in the Kiruna mining district where three epigenetic Cu ± Au deposits of unknown age and structural setting occur within 0.5 km of a pre-orogenic stratiform Cu (Fe ± Zn) deposit, 2 km of the Rakkurijärvi iron oxide-copper-gold (IOCG) deposit, and 5 km of the giant Kiirunavaara iron oxide-apatite (IOA) deposit, which may have shared mineral system ingredients during ore formation. The aim of the thesis is to characterize the mineral system ingredients of the epigenetic Pahtohavare deposits utilizing an up-to-date regional tectonic framework and to assess parallels between other mineral systems in the district. The following ingredients were investigated in this study: 1) driving force, using U-Pb zircon geochronology of local igneous intrusions, 2) transport pathways and structural traps, utilizing structural analysis and field mapping, 3) metal sources, based on sulfide trace element and carbonate REE geochemistry, and 4) ligand and fluid sources, using S, Sr, and Nd-isotope analyses. U-Pb zircon geochronology of intrusions in the district where Cu-mineralization is spatially proximal yields ages unanimously from the early phase of the Svecokarelian orogeny (ca. 1923-1867 Ma including age uncertainties). These ages suggest magmatism provided a thermal energy drive for the mineral systems developing during this time such as the Kiirunavaara IOA and Rakkurijärvi IOCG. Structural analysis and petrographic investigation from the Pahtohavare area show that the host rocks for the Pahtohavare deposits are folded into a noncylindrical, SE-plunging anticline and that cleavage measurements mirror the fold geometry. Foliation trails, mylonitic fabrics, and porphyroclasts with pressure shadows characterize the S0/S1 fabric as tectonic and constrains the fold as F2, indicating the folding event occurred during the late phase of the Svecokarelian orogeny (ca. 1.81-1.78 Ga). The epigenetic Pahtohavare deposits sit in brittle-ductile second-order structures that cross cut earlier foliation and the F2 fold, indicating that the deposits formed ca. 80 m.y. after the IOA and IOCG deposits in a distinct tectonic regime. Petrographic investigation of microstructures identifies remobilization textures linking the pre-orogenic stratiform Cu (Fe ± Zn) and late-orogenic Pahtohavare Cu ± Au deposits. Comparison of Co/Ni ratios between structurally constrained early and late sulfide generations (0.1-10 vs. 0.1-100) indicate the late-orogenic fluids were capable of efficiently transporting and concentrating Co, implying hot and saline conditions. However, a weak Co/Ni ratio mixing trend between the two generations suggest the late-orogenic fluids primarily sourced metals from the pre-existing host rocks and sulfides, rather than an external reservoir. This is supported by flat REE (chondrite normalized) patterns from ore-related carbonate, reflecting a source from the mafic host rocks. Both the pre- and late-orogenic sulfide generations have overlapping δ34S values ranging between ca. -20‰ and ca. -5‰ to near-zero values. The sulfur source for both mineral systems likely originated from local graphite schist horizons (ca. -22‰ to -21‰) and the volcanic host rocks. 87Sr/86Sr values for ore-related carbonates from the epigenetic Pahtohavare deposits are radiogenic (ca. 0.7095-0.7154) and form a mixing trend with strongly radiogenic (ca. 0.7134-0.7740) non-ore-related syn-sedimentary carbonate in biotite-altered rocks. This suggests a metamorphic fluid derived from crustal and Rb-rich rocks was responsible for the formation of the ore-related carbonate. Therefore, the Pahtohavare epigenetic deposits are suggested to have formed from metamorphic fluids during prograde metamorphism modified by the pre-enriched and altered host rocks in the Pahtohavare area. The results of this thesis suggest that late-orogenic tectonism played a key role in reactivating mineral system ingredients, highlighting the potential of second-order structures near pre-existing mineral deposits as a strategic exploration target for Cu-mineralization in the region.