Emplacement, alteration history, ore genesis, and timing of mineralization, of iron oxide apatite ores and their host rocks in the Norrbotten region of northern Sweden

• Main Author: Westhues, Anne
• Format: Thesis
• Language: English
• Published: Memorial University of Newfoundland 2017
• Subjects: Kiruna; Northern Sweden; Norrbotten
• Online Access: https://research.library.mun.ca/12702/; https://research.library.mun.ca/12702/1/thesis.pdf

The origin of the iron oxide apatite (IOA) deposits around Kiruna in the Norrbotten region of northern Sweden, and similar deposits worldwide, has been debated for decades. Contrasting theories include: 1) iron oxide extracted from an immiscible silicate liquid-iron oxide melt; or 2) iron oxide transported and subsequently emplaced by hydrothermal fluids. This study is the first to combine detailed geochemical studies and in situ U-Pb dating on accessory minerals with tracer isotope geochemistry on the whole rock and mineral scale to explore the different hypotheses. In situ U-Pb geochronology of zircon and titanite confirms a previously documented event around 1880-1900 Ma in the Norrbotten region, but dates the altered metavolcanic rocks in the footwall and hanging wall of the ore and a syenite more accurately at 1884 to 1880 Ma. A granite pluton has an age of 1874 Ma, in close overlap with zircon from two ore bodies (1877 – 1874 Ma). Zircon from the host rocks and the granite exhibit typical igneous growth zoning, whereas all zircon from the iron ore and some from intrusions show hydrothermal influences in texture and chemistry, including weight percent of water. Younger U-Pb dates from monazite and titanite suggest later events influencing the Kiruna IOA deposits between 1772 and 1628 Ma. The isotopic systems used in this study, ranging from whole rock Sm-Nd and Lu-Hf to in situ oxygen and Lu-Hf in zircon, all show significant differences between samples from host rocks and ore. These differences can be best explained when invoking hydrothermal fluids at the magmatic-to-hydrothermal transition (T > 600 ºC) possibly exsolved from the intrusions, to mobilise and concentrate iron oxides as the massive IOA deposits. The main ore forming event is thought to have occurred around 1874 Ma, and Hf and Nd isotopic signatures suggest a depleted mantle source for these fluids, potentially related to the Kiruna greenstone group, which could also be an important iron source. Later hydrothermal events influenced the ...