Structural framework and hydrothermal event history in relation to uranium mineralization along the southeastern margin of the Fennoscandian Shield

The southeastern Fennoscandian Shield has been relative stable since the last major crust-forming event at 1.8-1.9 Ga. Episodic anorogenic magmatism, transgression, burial and exhumation has occurred from Mesoproterozoic to Mesozoic time, but metamorphic grade in cover sequences is greenschist facies or lower and tectonic disruption is only significant in localized deformation zones. Because granitoids and migmatized metasediments of the Svecofennian basement in southern Finland are associated with regional radiometric anomalies, the area has attracted attention as potentially analogous to the Mesoproterozoic uranium systems found across basin-basement interfaces in Canada and Australia. If we consider events and opportunities for uranium dissolution, transport, dispersion and enrichment in terms of sedimentary and structurally enhanced permeability, fluid composition and thermal regime, then a number of alternative mineralization scenarios emerge.
Localized magmatic-hydrothermal and structurally focused epidote-K-feldspar-chlorite-hematite alteration, with leaching by oxidizing fluids have been recognized in proximity to 1.6 Ga rapakivi granite plutons, with the most prominent structural trends defining a linked network of N and NNE-ENE structures, in part representing reactivation of Svecofennian structures. While there is no evidence for uranium mobilization or enrichment at this time, subsequent reactivation of this structural template has been significant in localizing later low-temperature hydrothermal processes. Evidence for the previous existence of Paleozoic sediments in southern Finland and for fluid flow across the basement — cover interface comes from fossils in fracture fill, dating of galena in calcite-fluorite-sulfide veins and dating of uraninite. Paleomagnetic studies also show a Paleozoic remanence component, while fission track and U-Th-He data, though contentious, support Paleozoic reheating, usually attributed to burial by a transient foreland basin formed in response to Caledonian orogeny.
Further constraints on the timing of these events and the extent of structural reactivation can be obtained from analysis of the sedimentary record and deformation of Paleozoic rocks in adjacent Estonia. The predominantly tidal to estuarine Cambrian and Ordovician lithofacies, and the presence of paleokarst formed prior to the Devonian, imply that southern Finland could not have been deeply buried during early Paleozoic time. Devonian burial is however tenable, and the extent of faulting and disruption of sediments in Estonia implies that structurally enhanced permeability could have been effective in the basement in southern Finland, allowing fluid infiltration and metal transport. The importance of topographically or tectonically driven fluid migration, or redox contrast in the cover sequence need to be assessed and isotope data, which are currently consistent with Ordovician and Devonian mineralization, need to be further refined.