Epigenetic gold occurrence in a Paleoproterozoic meta-evaporitic sequence in the Rompas-Rajapalot Au system, Peräpohja belt, northern Finland

Abstract

The Rompas-Rajapalot gold prospect is located in the northern part of the Paleoproterozoic Peräpohja belt. It covers an area of at least 10 x 10 km and comprises various styles of gold mineralization ranging from localized high-grade Au pockets in uraniniteand pyrobitumen-bearing calcsilicate-carbonate-quartz veins in mafic metavolcanic rocks (Rompas area) to disseminated gold grains in Fe-Mg-rich metasediments and quartz-tourmaline-sulfide-native gold veins (Palokas area). This study deals with the petrography and mineral chemistry of the gold mineralization at Palokas, which occurs in the eastern part of the Rompas-Rajapalot prospect. Major and trace element data and fluid inclusion characteristics of tourmaline are used to evaluate the origin and the pressure-temperature-fluid composition parameters of hydrothermal fluids. Whole-rock geochemical analyses are utilized to evaluate the nature of the protolith of the host rocks. Gold occurs in a native form in at least two different textural settings: 1) single, relatively coarse grains disseminated among the rock-forming silicates in cordierite-orthoamphibole rocks and 2) smaller grains occurring in fractures of tourmaline in quartz-sulfide-tourmaline breccias and in fractures of chloritized cordierite-orthoamphibole rocks adjacent to the tourmaline-rich breccias. Fracture-related gold is associated with Bi-Se-S-bearing tellurides, native Bi, molybdenite, chalcopyrite, and pyrrhotite. Coarser-grained disseminated gold were not found to be clearly associated with sulfides nor any fractures. Statistical correlations show that the Au concentration correlates strongly with Te, Cu, Co, Se, Bi, Mo, and Ag (ρ = 0.730-0.619) whereas Au correlates moderately with As, Fe, W (ρ = 0.523-0.511) and to a lesser extent with U, Pb, and Ni (ρ = 0.492-0.407). Gold has the strongest negative correlations with Sr and Ca. The chondrite-normalized REE patterns of tourmaline from the Au-mineralized rocks (both vein type and host-rock tourmaline) and the late-to post-orogenic granite partly overlap and show similar LREE-enriched trends, with the enrichment being lower in tourmaline from the granite. Fluid inclusion studies from tourmaline in gold-bearing

Application of lithogeochemical and pyrite trace element data for the determination of vectors to ore in the Raja Au-Co prospect, northern Finland

“…2; Ranta et al, 2018). This is in good agreement with the extensive pre-orogenic albitization event, which is widespread in the PB and typically initiated during sedimentation and diagenetic processes in evaporitic environments (Eilu et al, 2003;Ranta et al, 2018). This pre-orogenic Na-metasomatism mechanically prepared the rocks for the Co-rich mineralization within the Co-Only zones.…”

“…Considering the intratectonic basinal setting at Rajapalot, the source rocks for the Co-transporting high-salinity fluids, could have been the evaporitic Petäjäskoski Formation (see Fig. 2; Ranta et al, 2018). This is in good agreement with the extensive pre-orogenic albitization event, which is widespread in the PB and typically initiated during sedimentation and diagenetic processes in evaporitic environments (Eilu et al, 2003;Ranta et al, 2018).…”

“…1c). This age was also confirmed by Ranta et al (2017Ranta et al ( , 2018, who conducted a trace element, boron isotope and fluid inclusion study on tourmaline from quartz-tourmaline-sulfide-native gold veins at Rajapalot (Palokas prospect). By using tourmaline, the authors could evaluate the physico-chemical characteristics of the mineralizing fluid, which provide a genetic link to the 1.78 Ga granitoid magmatism and the fracture-related gold mineralization (Ranta et.…”

“…The area of the Rajapalot Au-Co project hosts an epigenetic hydrothermal Au-Co-system that has properties comparable to an orogenic gold deposit with atypical metal association. Considering that the supracrustal sequences of the PB formed in an old intracratonic basin, multiple hydrothermal events could have invoked metal precipitation during pre-, syn-and post-orogenic processes (Lahtinen et al, 2012;Eilu, 2015;Molnár et al, 2016Molnár et al, , 2017Ranta et al, 2018). There is even the possibility that prior to regional metamorphism the episodic intrusion of felsic and mafic magmas locally altered the supracrustal sequences https://doi.org/10.5194/se-2021-119 Preprint.…”

“…However, the supracrustal metamorphic model is not in favor of genetically linking the post-tectonic intrusions with fracture-related gold. In association with the tectonic evolution of the PB, three emplacement periods of granitic intrusions are distinguished: (i) pre-orogenic Kierovaara granite (1.99 Ga; Ranta et al, 2018),…”