Fluid evolution of the Neoarchean Pampalo orogenic gold deposit (E Finland): Constraints from LA-ICPMS fluid inclusion microanalysis

The sources of fluids and precious metals in orogenic gold deposits remain controversial, and a key question is whether auriferous magmatic-hydrothermal fluids are a decisive ingredient for economic orogenic gold mineralization. Contributions of magmatic-hydrothermal fluids have been proposed based on indirect arguments such as isotope data or emplacement of granites coevally with gold deposits. By contrast, complete elemental concentration data for orogenic gold fluids, which could be used to compare them directly with those of magmatic-hydrothermal fluids, are not yet available. This study reports the major and trace element chemistry of fluid inclusions from the Neoarchean Pampalo orogenic gold deposit, located in the Hattu schist belt in Eastern Finland. Three successive generations of gold-mineralized and barren veins provide the paragenetic and chronological framework for establishing the fluid evolution history of the Pampalo deposit. Based on petrography, microthermometry and LA-ICPMS fluid inclusion microanalysis, five major fluid types were identified. The data shows that all fluid types, despite characteristic differences, carry the signature of orogenic gold fluids, i.e., low- to intermediate-salinity, presence of CO2, CH4 or N2, high S contents, enrichment in B, As, Sb and W, and low Pb and Zn concentrations. Gold concentrations vary significantly between the oldest (ca. 0.6 μg/g) fluid related to the zones with highest gold grades in the deposit and later fluid types (< 0.03 μg/g) unrelated to gold mineralization. This shows that gold mineralization was caused by episodic input of exceptionally gold-rich fluids and not by continuous and long-lasting hydrothermal activity. Comparing the fluid composition data from Pampalo with the well-established major and trace element characteristics of magmatic-hydrothermal fluids, the potential role of magmatic fluids in orogenic gold systems can be quantitatively evaluated. Mass balance calculations using metal concentrations as well as Cl/Br ratios demonstrate that the composition of the early Au-rich fluid at Pampalo is not compatible with any significant magmatic-hydrothermal fluid contribution, and that all fluids at Pampalo are metamorphogenic in origin. Molar Cl/Br ratios as low as 9-20 are found in some of the fluid types, which groups them among the most Br enriched crustal fluids reported so far. The exceptional Br enrichment is inconsistent with an evaporative origin but instead points to Br uptake from organic matter in metasedimentary rocks during metamorphism and fluid production. Taken together, the selective Au enrichment of only the earliest fluid type is well explained by metamorphic fluid production from Au-rich source rock lithologies, without any significant contributions from magmatic-hydrothermal fluids.