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

• Main Authors: Ranta, J.-P. (Jukka-Pekka), Molnár, F. (Ferenc), Hanski, E. (Eero), Cook, N. (Nick)
• Format: Article in Journal/Newspaper
• Language: English
• Published: The Geological Society of Finland 2018
• Subjects: Finland; Paleoproterozoic; Palokas; Peräpohja belt; Rompas-Rajapalot; fluid inclusions; gold; Northern Finland
• Online Access: http://urn.fi/urn:nbn:fi-fe2019062421614

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 × 10 km and comprises various styles of gold mineralization ranging from localized high-grade Au pockets in uraninite- and 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 ...