Petrogenesis of Paleoproterozoic A1-type felsic to intermediate igneous rocks and co-genetic REE-HFSE mineralization in central Finland, Fennoscandian Shield: Evidence
from whole-rock geochemistry, Sm-Nd and U-Pb isotope data and thermodynamic modeling

Thesis event information

Date and time of the thesis defence

Place of the thesis defence

Linnanmaa, lecture room L10. Remote connection: https://oulu.zoom.us/j/62656363154

Topic of the dissertation

Petrogenesis of Paleoproterozoic A1-type felsic to intermediate igneous rocks and co-genetic REE-HFSE mineralization in central Finland, Fennoscandian Shield: Evidence
from whole-rock geochemistry, Sm-Nd and U-Pb isotope data and thermodynamic modeling

Doctoral candidate

Master of philosophy Kimmo Kärenlampi

Faculty and unit

University of Oulu Graduate School, Faculty of Technology, Oulu Mining School

Subject of study

Geosciences

Opponent

Professor Tapani Rämö, University of Helsinki

Custos

Professor Kari Strand, University of Oulu

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Geochronology and origin of granites and associated REE-HFSE-mineralization in the Otanmäki area, central Finland

The subject of the dissertation research project were the granitic rocks (“the Otanmäki suite”) and the related mineralization of rare earth metals in the Otanmäki area, central Finland. The research was carried out in collaboration with the University of Oulu, the Geological Survey of Finland and Otanmäki Mine Oy and was financially supported by the University of Oulu's ADMA-DP doctoral program, K.H. Renlund Foundation and Tauno Tönning Foundation.

The granites assigned to the Otanmäki suite were discovered as early as the 1950s, but their geological significance and origin have remained unclear. The aim of the study was to characterize the rock types of the Otanmäki suite and their spatial distribution and chemical characteristics and to make conclusions about their geotectonic environment and origin.

In addition, the mineralization of rare earth metals and mineral potential associated with these rock types were investigated. Field and drill core observations, petrography, mineral and whole-rock chemistry, isotope analyses, and thermodynamic numerical modeling with Magma Chamber Simulator modeling software were used as research methods.

The results of the study show that the Otanmäki suite extend over much larger area than was previously thought and that it contains a spectrum of granitic and intermediate plutonic rocks. The geochemical characteristics of these rocks suggest that they were formed in a continental rift. Based on geochemical and thermodynamic modeling, the rocks in the Otanmäki suite were formed when magmas derived from the Earth’s mantle intruded the crust and slowly crystallized at a depth of about 7-15 km.

The new uranium-lead age determination results for zircons specify the age of the Otanmäki suite rock types, which was unclear in previous studies, and show that they are 2.04-2.06 billion years old. At that time, the ancient continent of Karelia went through a significant crustal extension, which eventually led to its complete break-up. Rocks with a similar composition and age are not known elsewhere in Finland or in the entire Fennoscandian Shield area.

The scarcity of the Otanmäki suite rocks is explained by the interpretation that after their emplacement in the crust, they were thrust into their current position as rare thrust units from the distant, now disappeared western edge of the Karelian continent, which was tectonically removed about 1.9 billion years ago.

The Otanmäki suite granites are also economically interesting, as they host two mineralized zones
(Katajakangas and Kontioaho), which contain high concentrations of rare earth elements (REE), such as lanthanum (La), cerium (Ce), neodymium (Nd) and yttrium (Y), as well as high-field strength elements (HFSEs) such as zirconium (Zr), niobium (Nb), thorium (Th) and uranium (U). Some of these metals are so-called high-tech metals, which are widely used in many high-tech applications, such as wind turbines, electric and hybrid cars, and smartphones.

The new research results show that in Katajakangas, the REE-HFSE mineralization is localized in mineralized dikes intruded into the host granite and at Kontioaho in a sheet-like intrusive body. They contain Ce-rich allanite, zircon, titanite and various Nb-rich oxide minerals. Allanite-(Ce) is the most significant rare earth carrier mineral in both mineralized zones. Based on trace element compositions, zircon U-Pb ages, and numerical modeling, the study suggests that REE-HFSE mineralization is closely related to the granite magmatism of the Otanmäki suite and that the mineralized dikes and sheet-like intrusion represent highly differentiated end products of granitic magmas. The new information can be used as a source material for mineral exploration.
Last updated: 20.8.2021