Abstract
The Peräpohja belt comprises a greenschist to amphibolite facies; multiply-folded supracrustal sequence of quartzites; mafic volcanics; carbonate rocks; black shales; mica schists and greywackes deposited from ca. 2.44 Ga to 1.92 Ga; during protracted rifting of the Archaean basement. Metamorphism and multiple folding of the basin fill occurred during the Svecofennian orogeny (1.92–1.80 Ga). The Rompas Au–U mineralization is hosted within deformed and metamorphosed calcsilicate veins in mafic volcanics. Textural evidence suggests that deposition and periods of uraninite re-mobilization were followed by localized hydrocarbon-bearing fluid flow which produced pyrobitumen crusts around grains of uraninite. Gold precipitated during the latest hydrothermal event at around 1.75 Ga. In situ U–Pb dating of uraninite by laser ablation inductively coupled mass spectroscopy (LA-ICP-MS), and Re–Os dating of molybdenite, indicate that primary hydrothermal uranium mineralization forms two age clusters; about 2.03–2.01 and 1.95–1.94 Ga. Resetting of the U–Pb system and precipitation of new generations of uraninite are associated with major deformation and metamorphic stages of the Svecofennian orogeny at 1.91–1.89 Ga, 1.85 Ga, and 1.80 Ga. Gold deposition was synchronous with the emplacement of the 1.75–1.78 Ga late/post-orogenic granitoids. The gold-producing hydrothermal event is also recorded by Re–Os dating of molybdenite from the gold-bearing Mg-metasomatized metasedimentary and metavolcanic units at the Palokas prospect; a few kilometres from Rompas. Results of this study confirm that some domains in the structure of uraninite may preserve the original crystallization age, despite an overprinting amphibolite facies metamorphic and other hydrothermal events. The study supports the utility of in situ U–Pb dating of uraninite and the ability of Re–Os dating to assist in sorting out different hydrothermal events in areas with complex tectonic; magmatic and metamorphic histories.
Highlights
Uraninite is theoretically an ideal mineral for U–Pb radiometric dating, as it incorporates very little or no structural lead at the time of crystallization and the lead found in ancient uraninite consists of overwhelmingly radiogenic 207 Pb, 206 Pb, and 208 Pb isotopes, the decay product of radioactive isotopes of uranium (235 U and 238 U) and thorium (232 Th), respectively.radiogenic lead is incompatible in the structure of uraninite and mobilized during post-crystallization fluid/mineral interactions [1,2]
This peculiarity of uraninite complicates or may even exclude direct determination of the crystallization age, but, on the other hand, opens the opportunity to evaluate the timing of processes leading to modification of the primary mineralization
Our previous studies on the Rompas Au–U deposit [6] in the Paleoproterozoic Peräpohja belt, northern Finland (Figure 1) showed that accumulation of gold was superimposed on a pre-existing uraninite-bearing hydrothermal vein system during the late/post-orogenic stages of the Svecofennian orogeny (1.92–1.80 Ga)
Summary
Uraninite is theoretically an ideal mineral for U–Pb radiometric dating, as it incorporates very little or no structural lead at the time of crystallization and the lead found in ancient uraninite consists of overwhelmingly radiogenic 207 Pb, 206 Pb, and 208 Pb isotopes, the decay product of radioactive isotopes of uranium (235 U and 238 U) and thorium (232 Th), respectively.radiogenic (and common) lead is incompatible in the structure of uraninite and mobilized during post-crystallization fluid/mineral interactions [1,2]. Alteration-recrystallization may locally reorganize the uranium budget, and uraninite may be subject to isotopic disturbance, resetting the radiometric clock in some domains of the uraninite crystals This peculiarity of uraninite complicates or may even exclude direct determination of the crystallization age, but, on the other hand, opens the opportunity to evaluate the timing of processes leading to modification of the primary mineralization. This latter is especially interesting in Precambrian terranes where uraninite-bearing ores frequently show mineralogical–textural evidences for overprinting by metamorphism, deformation and/or fluid exchange [3,4,5]. We investigate how major stages in the Svecofennian tectonic evolution in the Peräpohja belt are reflected in the U–Pb ages of uraninite from the polygenetic
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