Geology, lithogeochemistry and paleotectonic setting of the host sequence to the Kangasjärvi Zn-Cu deposit, central Finland: Implications for volcanogenic massive sulphide exploration in the Vihanti-Pyhäsalmi district

Abstract

The Kangasjärvi Zn-Cu deposit is a highly deformed and metamorphosed Paleoproterozoic volcanogenic massive sulphide (VMS) deposit located in the Vihanti-Pyhäsalmi base metal mining district of central Finland. The host sequence to the deposit, referred to as the Inner Volcanic Sequence (IVS), is comprised of a bimodal suite of metavolcanic rocks and a regionally extensive tonalite-trondhjemite gneiss (sub-volcanic intrusions?). A separate and perhaps younger sequence of mafi c volcanic rocks, with irregular intervals of undifferentiated intermediate to felsic schists and metalimestones, referred to as the Outer Volcanic Sequence (OVS), are separated from the IVS sequence by intervals of metagreywacke and UP bearing graphitic schists. A stratigraphic scheme for rocks within the IVS is proposed based on outcrop observations, locally preserved volcanic textures, aspects of seafl oor-related hydrothermal alteration and lithogeochemistry. In this scheme, rare andesites form the lowermost volcanic stratigraphy and are overlain by typical island-arc basalts that were erupted in a subaqueous setting. Tonalite-trondhjemite subvolcanic intrusions were locally emplaced within andesites and coeval rhyolites were extruded on the basaltic substrate. The extrusion of rhyolites, including high-silica rhyolites, was coeval with regional-scale, pre-metamorphic seafl oor hydrothermal alteration and local sulphide mineralization. Extensively altered rhyolites envelope massive sulphides and are underlain by altered basalts. The latter rocks are now characterized by a variety of low-variance metamorphic mineral assemblages (e.g. orthoamphibole-cordierite rocks) and defi ne a domain of intense pre-metamorphic chlorite ± sericite alteration in the stratigraphic footwall of the deposit. The altered nature of these rocks is attributed to reaction with seawater-related hydrothermal fl uids within a zone of upfl ow at or near the seafl oor. The fundamental controls on convective hydrothermal circulation and subsequent alteration and massive sulphide mineralization at Kangasjärvi, and possibly elsewhere in the district, share many characteristics with other well-described, ancient VMS deposits (e.g. massive sulphide deposits in the Flin Flon Belt, Manitoba, Canada). These characteristics include: 1) an association with bimodal volcanism developed in extensional settings; 2) a close spatial association with regionally extensive felsic subvolcanic intrusions; and 3) petrogenesis of ore-associated volcanic rocks (e.g. high-silica rhyolites, felsic subvolcanic intrusions) indicative of substantial heat transfer from the mantle to the upper crust and the development of anomalous thermal corridors. These features translate into geochemically distinctive rock types that, when combined with aspects of stratigraphy and pre-metamorphic alteration, may be used to develop regional exploration strategies in the Vihanti-Pyhäsalmi district.

Geology, alteration and lithogeochemistry of the Paleoproterozoic Korpela VMS occurrence in Eastern Finland

“…Similar conclusions were drawn by Roberts et al ( , 2004 from the Ruostesuo Zn-Cu and Kangasjärvi Zn-Cu deposits, located at the southern extension of the Vihanti-Pyhäsalmi belt. The altered rocks at the vicinity of the ore deposits in the Pyhäsalmi region (Kangasjärvi, Ruostesuo and Mullikkoräme) are mainly surrounded by sericite-cordierite and garnet-cordierite-anthophyllite assemblages without any evidence of aluminous alteration (Roberts et al , 2004Mäki et al 2015). The chemostratigraphy at the Pyhäsalmi deposit indicates the immediate protoliths to massive sulphide mineralisation as rhyolites A and B of transitional magmatic affinity (Mäki et al 2015).…”

“…At Pyhäsalmi, the volcanic rocks are of transitional magmatic affinity, and the basalts and basaltic andesites are tholeiitic and have Island Arc Basalts (IAB) signatures (Rasilainen et al 2003;Mäki et al 2015). Similar conclusions were drawn by Roberts et al ( , 2004 from the Ruostesuo Zn-Cu and Kangasjärvi Zn-Cu deposits, located at the southern extension of the Vihanti-Pyhäsalmi belt. The altered rocks at the vicinity of the ore deposits in the Pyhäsalmi region (Kangasjärvi, Ruostesuo and Mullikkoräme) are mainly surrounded by sericite-cordierite and garnet-cordierite-anthophyllite assemblages without any evidence of aluminous alteration (Roberts et al , 2004Mäki et al 2015).…”

“…All the Paleoproterozoic VMS deposits in Finland and elsewhere have been affected by deformation and regional metamorphism, resulting in recrystallisation of the original alteration minerals to variable metamorphic assemblages (e.g. Roberts et al 2004;Latvalahti 1979;Mäki et al 2015;Barrett et al 2005;Caté 2016;Kampmann et al 2017;Hollis et al 2018). This often leads to complex classification of metamorphic mineral assemblages and difficulties in discriminating between lithological units.…”

“…Author's opinion is that these methods have not been utilised widely enough in brownfield or greenfield VMS exploration in Finland. Lithogeochemistry, especially the use of chemostratigraphy and mobile-element geochemistry, has mainly been applied in the Vihanti-Pyhäsalmi belt, in Central Finland (Mäki 1986;Roberts et al 2004;Imaña et al 2013;Mäki et al 2015). The study of mobile element ratios and concentrations related to the proximal zone of the Pyhäsalmi deposit (Mäki 1986) served as a successful exploration tool resulting in the discovery of Mullikkoräme VMS deposit in 1987, within the Vihanti-Pyhäsalmi belt (Mäki 1986;Mäki et al 2015).…”

Synthesis of the Geological Evolution and Metallogeny of Finland

“…2.2) is the most productive among the Svecofennian mineral deposits (Mäki and Puustjärvi, 2003;Roberts et al, 2004;Mäki et al, 2015). 2.2) is the most productive among the Svecofennian mineral deposits (Mäki and Puustjärvi, 2003;Roberts et al, 2004;Mäki et al, 2015).…”