| 1. |
- Andersson, Joel
(författare)
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Paleoproterozoic deformation in the Kiruna‑Gällivare area in northern Norrbotten, Sweden : Setting, character, age, and control of iron oxide-apatite deposits
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis covers the structural evolution of the Kiruna‑Gällivare area in the northern Norrbotten ore province, Sweden. The study area hosts several economically significant iron oxide-apatite (IOA) deposits and includes the type locality for this ore type. Despite the abundant work on the genesis of IOA-systems, their structural setting and control is poorly constrained. This highlights the need for multi-scale structural studies that can help to unravel structural controls on the genesis and overprinting deformation histories. Four IOA-hosting key study areas were under investigation covering multi-scale structural controls from regional to deposit scale. Extensive geological mapping focused on structures, stratigraphy, and hydrothermal alteration, combined with multi-scale structural analysis and U-Pb geochronology was conducted. Results are synthesized in a time-constrained tectonothermal model for IOA deposits and host rocks of the Kiruna‑Gällivare area.The results indicate that the IOA deposits in Norrbotten formed in an overall extensional regime coeval with basin development in a backarc setting. The onset of basin development is indicated by a U-Pb age in zircon from a volcanic intercalation in a stratigraphically basal alluvial conglomerate of the ore-bearing sequence. A titanite age indicates that an ore-proximal cataclastic fault has syn-volcanic origin and formed coeval with basin development and ore formation. A similar origin is postulated for ore-proximal biotite-bearing structures at the Malmberget IOA deposit. In comparison with Kiruna, the Malmberget area experienced higher metamorphic conditions and records a more complex deformation history. Following backarc extension, subsequent crustal shortening resulted in basin inversion and re-activation of structures. Crustal scale, reverse shear zones developed in favourable lithologies and inferred pre-existing structures during D1. The timing of D1 crustal shortening coincides with peak metamorphism (M1) and is bracketed by crosscutting relationships. In contrast, the timing of an overprinting D2 crustal shortening is directly constrained by U-Pb geochronology in titanite indicating an age of approx. 1.8 Ga during an event tentatively interpreted to have lasted up to 20 m.y. This time span is coeval with the exhumation of the Kiruna mining district as recorded by an U‑Pb reset age in apatite in association to an IOA deposit. The D2 deformation is characterized by reactivation of older structures and responsible for juxtaposition of blocks from different crustal levels and tectonic exhumation into upper crustal domains. Transposition of fabrics and ore bodies into re-activated listric faults during basin inversion explains sub-parallel relationships between the ore-proximal structures, bedding, and stratiform/stratabound orebodies.Sodic-calcic + Fe ± Cl alteration is widespread and generally sits in early structural positions and interpreted as pre‑ to syn‑D1. However, U-Pb titanite results indicate that sodic-calcic alteration was developed also during the younger D2 event and shows that the alteration style is temporally and spatially widely distributed. Commonly, the alteration styles associated with D2 deformation are potassic in character and associated to Fe- and Cu-sulphide minerals. These potassic alteration assemblages sit in structurally late positions, often brittle in character. Sulphides were remobilized into D2-structures and the entrapment style is mainly controlled by rock competency.A least two additional overprinting deformation phases are identified (D3 and D4). Clockwise rotation of the overall crustal shortening direction resulted in a gentle refolding of the inverted basin and influences the shape of some IOA deposits in the Kiruna mining district. Dominant joint structures at the Malmberget IOA deposit are indicated as relatively early features and their development is controlled by pre-existing foliation and crosscut by hydrothermally altered structures, that may be coeval with hydraulic fracturing in the Kiruna mining district that crosscut all other fabrics.
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| 2. |
- Hällström, Lina
(författare)
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Source, mobility and fate of critical Be, Bi, F and W from historical sulfidic-oxidic skarn tailings : Re-mining as remediation method?
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- There is a potential risk that geochemical cycles of several critical metals will be affected in the pristine environment, when mining of these metals increases to meet the demand in green technology. The identification of critical metals is based on the economic importance and vulnerability to supply restrictions. In the past, naturally low concentrations in the environment, and instrumental analysis with higher detection limits, has limited research regarding several of these critical metals. However, to understand their geochemical behavior and potential environmental impact are of high importance to ensure a responsible development of mine waste- and water management. Skarn ores can contain high amounts of Fe-sulfides, carbonates and fluorite, together with enriched concentrations of critical metals such as Be, Bi and W. Nevertheless, little attention has been paid to mine drainage from skarn tailings and their environmental impact, compared to tailings from sulfidic deposits. At Yxsjö mine site, Sweden, skarn tailings enriched in the major elements C, F, S (1.0, 1.9 and 1.2 wt.%.) and Be, Bi, and W (average 280, 500 and 960 ppm, respectively) were deposited in Smaltjärnen repository (1918-1963). The tailings were stored in ambient conditions until 1993 when the tailings were covered by sewage sludge. In-between 1969-1989, tailings were discharged into Morkulltjärnen repository, which was covered with sewage sludge and partly water saturated directly after closure. This thesis focuses on the Smaltjärnen tailings. The element distribution in the tailings were identified by combining 1) total concentrations of nine targeted minerals from rock drilled cores, 2) total concentrations of 99 samples from four intact tailings cores, and 3) environmental mineralogy (EM) conducted on one of the cores. The environmental mineralogy included paste-pH, sequential extractions, optical microscopy, scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), Raman vibrational spectroscopy and X-ray diffraction (XRD). Environmental mineralogy was also used to reveal geochemical processes affecting the mobility of elements in the tailings. Monthly water samples (May-October, 2018) were taken in three groundwater wells in the tailings, and at five surface water locations downstream the tailings. At three surface water locations, the diatom taxonomy response to the water quality was used to evaluate the impact on ecosystems. The quality of the mine drainage was compared to surface water downstream Morkulltjärnens repository and to a reference point. The overall results were used to evaluate the need for remediation, and particularly, the possibility to use re-mining as remediation method. The Smaltjärnen tailings contained 88 wt.%. of Ca-rich silicates accompanied by minerals such as calcite [CaCO3], fluorite [CaF2], monoclinic and hexagonal pyrrhotite [Fe1-xS)], danalite [Be3(Fe4.4Mn0.95Zn0.4)(SiO4)3.2S1.4], scheelite [CaWO4] and bismuthinite [Bi3S2] (average 5.7, 3.6, 2.4, 0.3, 0.1 wt.%. and 0.1 wt.%., respectively). Both pyrrhotite and danalite had oxidized in the upper parts of the tailings down to 2.5m depth, and calcite had partly neutralized the acid produced resulting in a pH decrease from 8 to 4 in the upper parts of the tailings. Weathering of danalite was intensified by the more acidic conditions (pH<6), in which Be hydrolyzes. The lowered pH enabled dissolution of fluorite, resulting in severely high concentrations of F in the groundwater (average 73 mg/L) and surface water (average 1.6 mg/L). In the uppermost tailings, secondary gypsum [CaSO4], Al-complexes and hydrous ferric oxides (HFO) had formed. The geochemical behavior of Be was complex in the tailings and in surface water downstream the tailings. According to the sequential extraction, Be released from danalite in the upper most tailings were present in water soluble phases, as exchangeable phases and had co-precipitated with Al- and Fe-oxyhydroxides. A strong correlation between Be, Ca and S in water soluble phases and in the surface water downstream the tailings indicated that Be partly substituted for Ca in secondary gypsum. In two groundwater wells, secondary precipitates of a white sludge containing Be, Al, F and Zn were found, indicating that Be was partly removed from the groundwater by Al-complexes. In the third groundwater well, the globally highest dissolved concentrations of Be were measured (average 4.5 mg/L), and in the surface water the concentrations (average 41 µg/L) were well above thresholds values for aquatic organisms (1 µg/L). In these pH-conditions (average 5.7-6.5) and oxygenated waters, Be is expected to precipitate as Be(OH)2 if complexing ligands are absent. A strong correlation between dissolved Be and F was found in the surface water, indicating that Be-fluorocomplexes had formed. Bismuth and W have previously been considered as relatively immobile elements. However, the results showed that both Bi and W had partly been mobilized from their primary minerals (bismuthinite and scheelite) in the tailings. Weathered bismuthinite and scheelite grains with rims of goethite and water soluble phases of Bi and W were found in the deeper tailings with pH>7. The release of WO42- was hypothetically attributed to anion exchange with CO32- on surfaces of scheelite. Because, at the same depth where W was mobilized, solid C was accumulated and secondary orthogonal calcite was frequently detected with Raman spectroscopy. Bismuth was scavenged in the tailings by exchangeable phases and co-precipitation with HFO in the upper-most tailings. In the groundwater, Bi was just above the detection limit in all groundwater wells, while W was found in elevated concentrations. In the surface water, Bi and W were transported in the particulate phase together with Fe, and settled in the sediments a few 100 meters from the tailings outlet. Surface water downstream Morkulltjärnen had a near-neutral pH (average 6.6) and of all elements analyzed, only dissolved W (average 1.1 µg/L) were high compared to threshold values (0.8 µg/L) and the reference sample. Dissolved Be, Ca, F and S from Smaltjärnen, and dissolved W from Morkulltjärnen were found in elevated concentrations more than 2 km from the mine site. Along this distance, metal tolerant diatom species (Achnanthidium minutissimum group II and Brachysira neoexilis, respectively) were dominant (>50%), indicating a negative impact on ecosystems. The mine drainage from Smaltjärnen had a larger negative impact on the diatom taxonomy with higher abundance of metal tolerant species, lower richness and evenness, more than 1% of deformed valves and the taxonomy was affected by the lower pH, compared to diatoms downstream Morkulltjärnen repository. In conclusion, pyrrhotite oxidation was the direct or indirect cause of Be, Bi, F and W mobilization in the Smaltjärnen tailings, resulting in low quality mine drainage. The oxidation rate decrease with time, but weathering of the Smaltjärnen tailings is expected to be ongoing for hundreds of years since only a small part had weathered during the 50-100 years of storage. The low water quality and negative impact on diatoms, stress the need for remediation. Low concentrations of Be, Bi, Ca, F, Fe and S, accompanied by a near-neutral pH (average pH 6.6) downstream Morkulltjärnen, suggested that cover and water saturation could inhibit sulfide and danalite oxidation, and indirectly prevent fluorite weathering. However, high concentrations of dissolved W downstream Morkulltjärnen displayed that cover and water saturation can increase the mobility of W in the Smaltjärnen tailings, which needs to be taken into consideration. This thesis shows the importance of understanding the complex mineral and element matrix in skarn tailings before choosing remediation technique. Re-mining could be a beneficial remediation method since most W were found in intact scheelite grains. However, more research regarding the mineral processing and metallurgy is needed to ensure a sustainable extraction technique that separates sulfides, carbonates, danalite and fluorite, and deposits them in a proper way.
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| 3. |
- Martinsson, Olof
(författare)
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Tectonic setting and metallogeny of the Kiruna greenstones
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Paleoproterozoic Kiruna Greenstones belong to a large c. 2.1 Ga tholeiitic province in the north-eastern part of the Baltic Shield. The mainly basaltic volcanism was related to an event of continental rifting, which ended up with continental rupture. The occurrence of komattites, picrites and thick piles of tholeiites in the northern parts of Sweden, Norway and Finland is in contrast to the sedimentary dominated areas in central-eastern Finland. This suggests the location of a mantle plume to the northern area, which generated the large volumes of mantle melts. The stratigraphical record of the well preserved Kiruna Greenstone Group demonstrates a change from initial clastic sedimentation, evaporate deposition and WPB-type volcanism to later extensive volcanism of flood basalt character. Subsequent crustal thinning generated MORB-type magmas by decompressional mantle melting. The later development of a subaqueous basin was accompanied by a change to explosive volcanism, and large amounts of volcaniclastic material was formed by Surtseyan eruptions. During ocean opening along a line from Ladoga to Lofoten a NNE-directed failed rift-arm was formed. This is expressed by rapid basin subsidence and voluminous eruption of MORB-type pillowlava, which created an anomalous environment of local extent within the greenstone domain. Basin shoaling and subsequent uplift and erosion of the rifted margin marks an end of the rift event. Two different types of economic sulfide deposits occur in the Kiruna Greenstones, syngenetic Cu-(Zn) ores of exhalative origin (Viscaria-type), and epigenetic Cu-Au ores (Pahtohavare-type). Both types are formed from highly saline hydrothermal fluids, but they are clearly different in metal association, ore related alteration and ore character. Conspicuous for the Viscaria-type is the occurrence of stacked blanket-shaped mineralizations of magnetite and sulfides, and the layered structure of high-grade Cu-ore, which is explained by repeated exhalative activity and deposition of the ores in brinepools. The most productive ore was formed in association with the main stage of basin subsidence and MORB-type volcanism in the failed rift. Faults parallel with the rift axis acted as channels for the ore fluids, and controlled the shape and location of brine-pool in which the ore was precipitated. Ores of the Pahtohavare-type have formed in zones of active ductile to brittle shearing. In detail the location of ores are mainly lithologically controlled by black schists, which has acted as chemical traps. The ores are hosted by albite felsites, and surrounded by characteristic zones of scapolite-biotite alteration. Calcopyrite and pyrite are the main ore minerals and they occur mainly as veinlets, veins and matrix to brecciated albite felsite. Ferro-dolomite is a characteristic ore related mineral formed in several generations from early dissemination in albite felsite, gangue to ore minerals and late barren veins. The Viscaria and Pahtohavare deposits are different in many respects to typical massive sulfide deposits and Au-ores in most other greenstone terrains. This is mainly due to their formation from highly saline solution, which is a common feature of both exhalative and epigenetic sulfide deposits formed in continental rift environments. Thus, the existence of evaporitic sediments at the base of the Kiruna Greenstones may be of major metallogenetic importance for this region, serving as a source for saline hydrothermal fluids.
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