Orogenic Au deposits formed in metamorphic belts over most of the Earth history and the genetic models developed since the three last decades aim to provide a general model for these gold-only deposits. However, some deposits, known as orogenic gold deposits with atypical metal associations, show enrichment in base and critical metals, mainly Cu, Co, sometimes Ni, Sb, U and (REE) that are not explained by the current models (i.e. metamorphic devolatilisation, subcrustal fluids, magmatic-hydrothermal model). The Pohjanmaa Belt in western Finland hosts both typical and atypical gold mineralization and is a natural laboratory to decipher the genesis of orogenic Au-Cu-Co-Ni deposits and establish a genetic deposit model including both type of deposits. This study explores the origin and nature of the mineralizing fluids and precipitation mechanisms by combining multiple methods as follows:
(1) microthermometry and in-situ geochemistry on well preserved primary fluid inclusion assemblages;
(2) whole rock geochemistry for basic geochemical characterization and identification of metal enrichments in hydrothermal alteration zones;
(3) in-situ geochemistry on hydrothermal mineral separates (amphibole, biotite, chlorite, sericite) to highlight potential sources of fluid and metals;
(4) Halogen ratios (Cl, Br, I) for identification of potential fluid sources, including evaporites;
(5) In-situ S isotope analysis and geochemistry of sulfides to characterize the S source, redox-related precipitation mechanisms and link with the mineralizing fluids.