U-Pb geochronology, tourmaline geochemistry, and stable (B, S) isotope constraints from the Hirvilavanmaa Au-only and the polymetallic Naakenavaara orogenic gold deposits, Central Lapland belt, northern Finland

Abstract

In this study of the Hirvilavanmaa Au-only and the polymetallic (Au-Cu-Co-Ni) Naakenavaara orogenic gold deposits from the Central Lapland belt (CLB) we present precise U-Pb ages obtained by single collector LA-ICP-MS from hydrothermal monazite and xenotime associated with different stages of hydrothermal activity in the deposits. We also use crystal chemical data from tourmaline, and in situ sulfur isotopes from sulfides, and boron isotope data from tourmaline obtained by LA-MC-ICP-MS to study sources of ore-forming components and conditions during ore deposition. Ages of texturally and paragenetically constrained hydrothermal xenotime indicate that Co deposition (1816 ± 10 Ma) and subsequent Cu deposition (1804 ± 3.3 Ma) at Naakenavaara took place during deformation events at ca. 1.82–1.80 Ga, and that ore-related hydrothermal activity had also begun at Hirvilavanmaa (1822 ± 9 Ma) during the same events, which can be related to late-orogenic Svecofennian tectonism and deformation in the CLB. Hydrothermal activity that led to Au deposition in both deposits took place during the waning stages of the Svecofennian orogeny, with hydrothermal monazite giving ages of 1785 ± 10 Ma and 1752 ± 10 Ma for Hirvilavanmaa and Naakenavaara, respectively. Tourmaline crystal chemistry indicates low fluid/rock ratios during tourmaline deposition in both deposits, with relatively oxidizing conditions during ore formation at Hirvilavanmaa and during the early Co-rich mineralizing stage at Naakenavaara but more reducing conditions for the subsequent Cu-rich mineralizing stage at Naakenavaara. Sulfur and boron-isotope compositions of sulfide minerals and tourmaline, respectively, point towards a mafic metavolcanic rock source for the ore-forming components at Hirvilavanmaa, and a metasedimentary source at Naakenavaara. There has only been limited previous work on the precise dating of mineralizing stages and on the sources of ore-forming components in individual orogenic deposits in the CLB. Results from our study provide important constraints on the timing of hydrothermal mineralization and different sources of ore-forming components in both Au-only and base metal-rich orogenic deposits in the CLB, one of the most important metallogenic belts in the Fennoscandian Shield with significant potential for new discoveries.