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Browsing by Author "Dimakopoulos, Georgios"

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  • Dimakopoulos, Georgios (2022)
    The metamorphosed Kutemajärvi gold deposit is located near the town of Orivesi, at the eastern flank of the Tampere Schist Belt, which constitutes part of the Svecofennian domain of southern Finland, and it is hosted in the volcanic rocks of the Koskuenjärvi formation. Previous isotopic studies have mainly focused on the igneous and metamorphic rocks of the Tampere Schist Belt and only a few of them have presented ages for the area of Kutemajärvi. This study aims to shed light on the timing of mineralization by employing the single-grain U-Pb dating method of monazite and zircon, in order to evaluate the relationship between the ore and its host rocks. Based on the results from the SEM mineral identification, monazite grains are divided into metamorphic and hydrothermal grains. In the case of zircon grains, a third magmatic type has been identified. Results from U-Pb dating of single monazite and zircon grains are well constrained and document four distinct stages of geodynamic evolution in the region. Ages older than 1.91 Ga represent detrital material transported during the stage of rifting that led to the opening of the Tampere basin. Subsequent subduction resulted in active volcanism which is expressed with the extrusion of the Koskuenjärvi formation at 1904 Ma. At the late stages of subduction or at the outset of the collision stage, the subvolcanic Pukala porphyry intruded into the volcanic sequence of the Tampere Schist Belt at 1890 Ma, which provides the maximum mineralization age. Release of hydrothermal fluids, due to the crystallization of the Pukala intrusion caused pervasive hydrothermal alteration of the Kutemajärvi host rocks and deposition of epithermal gold and other elements. However, the participation of hydrothermal fluids, released by high-temperature metamorphism of the lower crust, cannot be ruled out. Ages between 1890‒1878 Ma record the syn-collision stage, during which the deposit, the Pukala intrusion and its adjoining rocks were deformed and metamorphosed at greenschist to lower-amphibolite facies. The majority of ages fall within the 1880‒1878 Ma time-interval, characterizing the metamorphic peak that marks the culmination of the Svecofennian orogeny and provides a minimum age of the mineralization. This major orogenic event is partly overlapped by the collision of the Central Svecofennian Arc Complex with the Southern Svecofennian Arc Complex that transpired at 1880‒1860 Ma, as indicated by ample age data. Monazite and zircon also yield lower ages (<1860 Ma), which record retrograde metamorphic and subordinate cooling events, and resonate recurring tectonothermal activity, associated with the syn- and post-collisional magmatism of Southern Svecofennia and the emplacement of rapakivi intrusions in southern Finland. Single-grain U-Pb dating of monazite and zircon from polished thin sections, in tandem with collation of the obtained ages with earlier published data, establishes a spatial and temporal framework with respect to the tectonometamorphic evolution of the Kutemajärvi gold deposit and the Tampere Schist Belt. Precise temporal constraints substantiate the intricate geological history of the area and can be used to discriminate magmatic, metamorphic and hydrothermal events, with a view to breaking ground on the exploration of other epithermal deposits in the metamorphic terranes of southern Finland.