University of Oulu

Ranta, J.-P., Molnár, F., Hanski, E., & Cook, N. (2018). Epigenetic gold occurrence in a Paleoproterozoic meta-evaporitic sequence in the Rompas-Rajapalot Au system, Peräpohja belt, northern Finland. Bulletin of the Geological Society of Finland, 90(1), 69–108.

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

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Author: Ranta, Jukka-Pekka1; Molnár, Ferenc2; Hanski, Eero1;
Organizations: 1Oulu Mining School, University of Oulu, P.O.BOX 3000, FI-90014, Finland
2Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland
3Mawson Resources Ltd., 1305–1090 West Georgia Street Vancouver, BC V6E 3V7, Vancouver, Canada
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 4.4 MB)
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Language: English
Published: The Geological Society of Finland, 2018
Publish Date: 2019-06-24


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 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 quartz-tourmaline-sulfide veins indicate that the veins were formed from H₂O-Na₂O-CO₂-CH₄-(H₂S) fluids in a boiling system under pressure conditions ranging from lithostatic to hydrostatic, with the depth being ~5 km and the temperature ~300°C. The properties of the ore-forming fluids support the genetic link between the late- to post-orogenic granitoid magmatism at ~1.78 Ga and the formation of the fracture-hosted gold mineralization, suggested based on earlier studies (including Re-Os-molybdenite age and boron isotope data from tourmaline). Based on the whole-rock geochemistry, it is highly plausible that the cordierite-orthoamphibole rocks and interlayered calcsilicatealbite rocks are part of a basin-wide lacustrine, at least partly evaporitic, sequence. The protolith of the cordierite-orthoamphibole rock was most probably a lake-margin sedimentary pile with abundant Mg-rich clays.

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Series: Bulletin of the Geological Society of Finland
ISSN: 0367-5211
ISSN-E: 1799-4632
ISSN-L: 0367-5211
Volume: 90
Issue: 1-2
Pages: 69 - 108
DOI: 10.17741/bgsf/90.1.004
Type of Publication: A1 Journal article – refereed
Field of Science: 1171 Geosciences
Funding: This work was supported by the K.H. Renlund Foundation, Mawson Resources Limited, Advanced Material Doctoral Program (University of Oulu), and project No. 281670 “Mineral Systems and Mineral Prospectivity in Lapland—MinSysPro” co-funded by the Academy of Finland and Geological Survey of Finland.
Academy of Finland Grant Number: 281670
Detailed Information: 281670 (Academy of Finland Funding decision)
Copyright information: © 2018 The Authors. All published content of BGSF is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.