Prospectivity evaluation of the Myrmalm Target, a potential VMS mineralisation in Southern Trøndelag, Norway

The Myrmalm Target is located in the southern Trøndelag, a region consisting of a sequence of nappes, stratigraphically overturned and emplaced on the autochthonous crystalline basement. Trøndelag is a historically important mining region in Norway. In the past, it produced Cu, Zn and Au from Volcanogenic Massive Sulphide (VMS) mineralisation, located within the tectonostratigraphic units of Caledonides.  Although the last operational mine was closed at the end of the 20th century, the potential of this metallogenic province has not yet been exploited. In modern days this metallogenic province is considered under-explored. Increased demand for metals and an introduction of new European strategies including the transition to the green economy brought attention back to Trøndelag. The Geological Survey of Norway performed a range of exploration projects including mapping, sampling campaigns and geophysical surveys to promote the area and increase the understanding of its units. Several junior exploration companies, such as Kuniko Limited established exploration projects to identify drilling targets, potentially hosting economic mineralisation. One of them is the Myrmalm target, appointed for drilling in 2023, which resulted in obtaining a wealth of geochemical data. This study aims to investigate the prospectivity of the Myrmalm Target to host an economic VMS-type mineralisation. The initial logging identified the main lithology groups such as metasediments, metavolcanics and mineralisation horizons, all cut by intrusions of Trondhjemite. A clustering methodology based on major elements Na, Mg, Al, P, S, K, Ca, Mn, Fe, and Ti utilising centred log ratio – principal component analysis – k-means clustering was performed to classify the geochemical samples. Each of the clusters was analysed based on microscopy observations and geochemical data. Cluster one was identified to portray Trondhjemite, defined to be post-volcanic, and thus cannot be considered as an indicator of a prospective VMS system. The second cluster, representing metasediments was defined as a turbiditic sequence of metamorphosed greywacke beds intercalated by phyllites and lesser ribbon cherts. Cluster 3, identified as metavolcanics of basaltic composition is divided into two groups. The older one consists of calc-alkaline metabasalts associated with oceanic arc volcanism while the younger one is of tholeiitic composition representing enriched basalts. Both volcanics and metasediments were metamorphosed under greenschist facies conditions and cooccur in a metamorphosed volcanic-sedimentary sequence. This stratigraphy implies an evolution from an oceanic arc environment into the back-arc basin. Mineralisation is portrayed by cluster 4 and consists of two major massive sulphide horizons. The shallower horizon is spatially associated with the OA metavolcanics and is interpreted to be an iron sulphide formation of organic origin, while the deeper horizon associated with E-MORBs is potentially an expression of VMS processes. Both are however barren of base metals and locally, cannot be considered prospective for hosting an economic VMS mineralisation.