Association of gold with uraninite and pyrobitumen in the metavolcanic rock hosted hydrothermal Au-U mineralisation at Rompas, Peräpohja Schist Belt, northern Finland

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

“…The identical range of boron isotope data for tourmaline from the 1.78 Ga granites (from −4‰ to 0‰, δ 11 B) and tourmaline from the metasomatized rocks at Palokas [25] also suggests that there is not only spatial, but also genetic relationships between the post-orogenic granitoids (1.76-1.79 Ga, samples A2160 to A20163 on Figure 2) [11] and accumulation of gold in the area. This is also indicated by the lead isotope model age of galena and altaite associated with the late fracture-filling native gold at Rompas [6]. The range of boron isotope values (e.g., δ 11 B data between −2‰ and +0.5‰; Molnár, unpublished data) for the occasionally occurring tourmaline at Rompas also appear to point towards to the common origin of gold mineralization in the various parts of the Rompas-Palokas area.…”

“…Distribution of uranium in the calcsilicate veins is more regular, and most of the uranium-bearing zones do not contain elevated concentrations of gold. According to the results of our previous studies [6], the most common chemical ages for uraninite suggest a 1.95-1.90 Ga minimum age for the deposition of the primary hydrothermal uranium mineralization. The original mineral assemblage of the primary uranium-bearing vein consists of uraninite, dolomite, quartz, albite, with minor amounts of pyrrhotite, pyrite, pentlandite, chalcopyrite, galena, molybdenite, and titanite.…”

“…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). In this paper, 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 Au-U deposit at Rompas.…”

“…Ductile deformation and re-crystallisation of the uraninite-bearing quartz-dolomite veins into calcsilicate mineral assemblages was followed by brittle fracturing and a hydrocarbon-bearing fluid flow event with radiolytic polymerization of hydrocarbons around grains of uraninite. Native gold, together with highly radiogenic galena (up to 3423 and 1992 206 Pb/ 204 Pb, respectively), as well as hunchunite, nickeline with rare cobaltite, pyrite, pyrrhotite, chalcopyrite, molybdenite, and Bi-telluride minerals, precipitated during a subsequent hydrothermal event in the fractures of uraninite and in the cracks and on the surfaces of uraninite-pyrobitumen nodules (Pb isotope ratios are from [6]). Re-mobilization and re-deposition of uranium by these late hydrothermal events produced secondary uraninite grains with chemical ages between 1.85 and 1.65 Ga, whereas calculated model ages for radiogenic lead-bearing minerals associated with native gold indicate 1.70-1.75 Ga for deposition of gold.…”

“…All of the in situ analytical techniques have the advantage of direct dating from a polished thin section of ore/rock specimen, thereby taking the textural-paragenetic properties of uraninite into consideration in the interpretation of the U-Pb age data. In this study, LA-ICP-MS was used, and we compare the results of U-Pb dating by LA-ICP-MS with U-Th-Pb chemical ages calculated from electron microprobe analyses of uraninite grains [6] in the same set of thin sections. …”