A compositional trace element study between ore-gangue mineral pairs in iron oxide apatite ores of the Kiirunavaara and Per Geijer ore bodies
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Insinööritieteiden korkeakoulu |
Master's thesis
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Date
2021-06-14
Department
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European Mining Course (EMC)
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European Mining, Minerals and Environmental Programme (EMMEP)
Language
en
Pages
132
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Abstract
IOA (iron oxide apatite) or Kiruna-type deposits are an important source of iron and potentially other elements (e.g., REE, P, Co). However, their genesis still remains an unanswered question with theories ranging from a magmatic to a hydrothermal origin. Trace element distribution in magnetite allows to differentiate between magmatic and hydrothermal origin of Kiruna-type deposits. Since apatite plays an important role in IOA formation and its chemistry is used as a monitor of halogen behaviour in magmatic-hydrothermal systems, apatite mineralogy can set new constraints on the formation of IOA deposits. Therefore, trace elements in apatite and magnetite are investigated in this work in order to gain a better understanding of IOA formation. A total of 34 samples are collected from five cores from the Swedish Kiirunavara deposit and from four cores from the neighbouring Per Geijer (Henry) IOA deposit. The samples are investigated regarding their mineralogy and their major, minor and trace element composition. Conventional microscopy is used for mineralogical characterisation, electron microprobe analysis (EMPA) for spot analysis of apatite and magnetite grains and inductively coupled plasma mass spectrometry (ICP-MS) for bulk chemical composition of pulp samples. Apatites are additionally studied with a SEM. Trace elements in magnetite have combined magmatic and hydrothermal signatures with relatively high contents of Co (0.177 - 0.2440 wt\% CoO) and V (0.067 - 0.175 wt\% V2O3) pointing towards a magmatic origin. Low contents, mostly below detection limit, of Ti, Ni and Cr indicate a hydrothermal origin. Trace element discrimination diagrams reveal that high-T rather than low-T hydrothermal fluids account for the hydrothermal signature. Apatites from Kiirunavaara and Per Geijer are all fluorapatites suggesting a magmatic origin. Several apatites exceed F end member composition (>3.77 wt\% F). Fluorine tends to overfill the apatite halogen site, when enough CO32-is present within the system. In addition, fluorine plays an important role by generating liquid immiscibility between silicic melt and Fe-P rich melt. This leads to the assumption that the IOA deposits from Kiirunavaara and Per Geijer are generated due to 1) injection of a magma into the crust,2) assimilation of carbonates and evaporates that change the initial composition of the magma, 3) magma immiscibility and separation of a P-Fe volatile-rich melt from the fractionating silicate magma due to oxidizing conditions caused by the high F content and high water activity within the magma, 4) concentration of volatiles within the residual melts causes temperature and pressure to drop, which shifts the conditions towards a hydrothermal signature. The presence of a former evaporite bed is supported by the regional occurrence of scapolite. Monazite inclusions within the apatites that are believed to crystallize at high temperature around 800°C are another hint for a magmatic origin of the Kiirunavara and the Per Geijer ores.Description
Supervisor
Leveinen, JussiThesis advisor
Lohmeier, StephanieKeywords
iron ore, trace elements, ICP-MS, Per Geijer, Kiirunavaara, ore genesis