Multistage mineralisation in the Raja orogenic Au-Co prospect, Peräpohja belt, Finland: constraints from S isotopes and trace elements in pyrite, pyrrhotite and chalcopyrite
2025
Tepsell, J. | Lahaye, Y. | Molnár, F. | Rämö, O. T. | Cook, N. | Geologian tutkimuskeskus | Geological Survey of Finland
This study applies a multi-proxy geochemical approach to the Raja Au-Co prospect in northern Finland integrating trace element, S isotope, and textural data on sulphides and assessing these data against previously reported Ni, Cu and Fe isotope data. The combined dataset offers a more nuanced understanding of fluid evolution and mineralisation processes at Raja, classified as orogenic gold with atypical metal association. Our results support a multistage process, beginning with Co-only mineralisation driven by high-temperature, Co-rich metamorphic fluids, possibly from the evaporite-bearing Petäjäskoski formation, followed by a later hydrothermal overprint enriching Au. Early Co-rich fluids deposited euhedral pyrite (type A) with δ34S median of + 2.7‰, high Co/Ni ratio, and enrichment in Co, As, Se, Te, Bi, and Au. Subsequent metamorphism resulted in partial desulphidation of pyrite A, liberating Au, Co, and other trace elements and forming massive pyrrhotite, chalcopyrite, cobaltite, and Au-Bi-Te phases. Late-stage, Au-enriched fluids overprinted parts of the earlier Co-only mineralisation and deposited pyrite (type C) with δ34S median of + 5.6‰. Pyrite B, found in barren rocks, is unrelated to the mineralisation. Isotope and compositional variations in pyrite C suggest fluid mixing or pulsed activity involving magmatic-hydrothermal fluids and sedimentary-derived S. Our findings show that orogenic gold deposits with atypical metal association may form when Au-rich fluids overprint pre-existing base metal systems. This study illustrates how multivariate isotope geochemical methods can refine genetic models and unravel fluid evolution in epigenetic hydrothermal Au mineralisation in metamorphic belts, with implications for similar deposits in northern Finland and beyond.
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