Geochronology of Hydrothermal Processes Leading to the Formation of the Au–U Mineralization at the Rompas Prospect, Peräpohja Belt, Northern Finland: Application of Paired U–Pb Dating of Uraninite and Re–Os Dating of Molybdenite to the Identification of Multiple Hydrothermal Events in a Metamorphic Terrane

Abstract

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.

Epigenetic gold occurrence in a Paleoproterozoic meta-evaporitic sequence in the Rompas-Rajapalot Au system, Peräpohja belt, northern Finland

“…Chemical and U-Pb ages of uraninite from Rompas suggest that gold deposition took place around 1.75 Ga (Molnár et al, 2016;2017a). In addition, Re-Os-dating of molybdenite associated with gold at Palokas has yielded an age of 1.78 ± 0.01 Ga (Molnár et al, 2017a) indicating that the mineralization may be synchronous with the emplacement of late-to post-orogenic granitoids in the vicinity of the mineralization. This relationship is also supported by the identical boron isotope composition of the tourmaline (from -4 to 0 ‰ δ 11 B) that is associated with gold at Rajapalot and occurs in a 1.78 Ga pegmatite granite (Ranta et al, 2017), as well as in the tourmaline-bearing alteration zones at Rompas (from -2 to +0.5 ‰ δ 11 B; F. Molnár, unpublished data).…”

“…As a result, the deposits have been termed "Rompas-type Au deposits" (Molnár et al, 2016). Furthermore, the intense multiple deformation and episodes of pre-, syn-and post-metamorphic hydrothermal alteration (Molnár et al, 2017a) make the identi fica tion of the protoliths in the northern part of the Peräpohja belt problematic.…”

“…Genetic classifi cation of the mineralization has been problematic due to the atypical features and lack of information on this newly discovered area (Eilu, 2015). Chemical and U-Pb ages of uraninite from Rompas suggest that gold deposition took place around 1.75 Ga (Molnár et al, 2016;2017a). In addition, Re-Os-dating of molybdenite associated with gold at Palokas has yielded an age of 1.78 ± 0.01 Ga (Molnár et al, 2017a) indicating that the mineralization may be synchronous with the emplacement of late-to post-orogenic granitoids in the vicinity of the mineralization.…”

Editorial for Special Issue “Geochemistry and Mineralogy of Hydrothermal Metallic Mineral Deposits”

“…Advances in analytical methods over the past few decades have lowered detection limits and allowed determination of ages and isotopic compositions that were previously not possible. Molnar et al [1] use isotopic analyses of uraninite and molybdenite to assist in unraveling hydrothermal events related to Rompas gold-uranium mineralization in northern Finland. The mineralization is hosted by Paleoproterozoic rocks with a complex magmatic, metamorphic, and structural history, and the detailed isotopic analyses provide insight into the relation between mineralization and orogenic events [1].…”

“…Molnar et al [1] use isotopic analyses of uraninite and molybdenite to assist in unraveling hydrothermal events related to Rompas gold-uranium mineralization in northern Finland. The mineralization is hosted by Paleoproterozoic rocks with a complex magmatic, metamorphic, and structural history, and the detailed isotopic analyses provide insight into the relation between mineralization and orogenic events [1]. This study also illustrates the importance of detailed field and mineralogical studies as the basis for isotopic analyses.The study of platinum-group elements (PGE) mineralization in the Great Dyke of Zimbabwe at the Unki mine, by Chalumba [2] shows how detailed mineralogical relations can separate distinct processes.…”