Rytkynkylä volcanite sequence, a preliminary lithological assessment of a potentially mineralized locality on the Raahe-Ladoga shear complex

ntroduction
The Raahe-Ladoga shear complex is one of the most important ore potential regions in Finland with multiple
historical volcanogenic massive sulphide (VMS) deposits eg. Vihanti, Pyhäsalmi and Rauhala located on it in
Northern-Ostrobothnia. The aforementioned deposits have produced in total ca. 90 Mt of ore, of which the most
common commodities have been Zn, Cu, Pb, S and Au. In addition to the large deposits, multiple smaller satellite
VMS and orogenic gold deposits locate around the Vihanti-Pyhäsalmi region, primarily hosted in volcanite sequences
(Geological survey of Finland, 2024).
Aside from Svecofennian plutonic rocks, the lithology in the Vihanti-Pyhäsalmi region is dominated by
volcano-sedimentary rocks of three primary suites, the Vihanti-, Pyhäsalmi- and Ylivieska suites. Vihanti- and
Pyhäsalmi suites are older, ca. 1.93–1.92 Ga, characterized by felsic to intermediate, commonly rhyolitic plagioclase
porphyritic volcanites, while the Ylivieska suite, ca. 1.89–1.88 Ga, is characterized by a variety of felsic to mafic
metavolcanites where the mafic endmembers are commonly andesitic uralite plagioclase porphyrites (Mäki et al.,
2015; Kousa, 2018). The Rytkynkylä volcanite sequence locates in the middle of the Vihanti-Pyhäsalmi region, but
it is not clear whether it belongs to Pyhäsalmi or Ylivieska suite. Thus, the goal of this study is to clarify the suite to
which the Rytkynkylä volcanic sequence belongs to and explore its potential for mineralization.
Results
Field observations show that the Rytkynkylä mafic/intermediate volcanite sequence is controlled by the dextral
Rauhanperä shear zone on the northeastern side of the volcanite (Mäki et al., 2015). The southwestern side of the
volcanite has an intrusive contact with a Svecofennian granitoid. It has been observed that the Rytkynkylä volcanite
sequence contains small mineralization localities of arsenopyrite, pyrite, pyrrhotite, chalcopyrite and ilmenite. In
addition to the volcanite sequence mineralizations, the southwestern granitoid has minor disseminated pyrite
mineralization and the northeastern shear zone has minor galena mineralization.
Petrographic analysis shows that the volcanite sequence is composed of mafic to intermediate uralite
plagioclase porphyrite that is andesitic by composition. The volcanite sequence has undergone localized, sometimes
extreme, hydrothermal metamorphosis that has, in some samples, eg. altered all plagioclase into epidote. The southern
granitoid bordering the volcanite has a tonalitic/granodioritic composition. Like the volcanite, the northern granite
has undergone extreme alteration near and on the Rauhanperä shear zone with one locality having unakite, and
mylonitization of the granite and the volcanite by the lithology contact.
U–Pb age determination on the volcanite, using a sample deemed as unaltered as possible on the field, was
attempted. However, electron microscopy (SEM) studies showed that no zircon grains were found from the sample
and the age determination could not be accomplished.
Conclusions
Based on dominant uralite plagioclase porphyrite lithologies of the Rytkynkylä volcanite sequence and the local shear
zone dynamics, the preliminary assessment of the Rytkynkylä volcanite sequence thus far favours it being a part of
the younger Ylivieska suite ca. 1.89–1.88 Ga. This would imply that it is not a continuation of the older Pyhäsalmi
suite VMS rich island arc volcanites. Nevertheless, some potential mineralizations have been found in the Rytkynkylä
volcanite and will be analysed further in the future