Signatures of Cu (-Au) mineralisation reflected in inorganic and heavy mineral stream sediments at Vähäkurkkio, north-western Finland

In this orientation study, stream sediments were assessed as a sample medium to detect Cu (-Au) occurrences discovered in Geological Survey of Finland (GTK) investigations at Vähäkurkkio in the Lätäseno Schist Belt. The Vähäkurkkio target area is situated in an environmentally sensitive Arctic area, thus requiring an exploration method with minimal environmental impact, such as analysis of stream sediments. Cu (-Au) mineralisation was detected by GTK in drill holes at two different sites near the Lätäseno and Piippujoki rivers. The geochemistry results for the <177 μm fraction of inorganic stream sediment showed a 2.7 km long section of elevated Cu, S, Te and Bi concentrations immediately downstream from the mineralised site by the Lätäseno river. Stream sediments from the same section contained chalcopyrite and other Cu-bearing mineral grains, detected by automated scanning electron microscopy (SEM-EDS and FE-SEM-EDS), in the fine fraction (<63 μm) of heavy mineral samples. Inorganic stream sediment geochemistry was thus capable of revealing Cu (-Au) occurrences in the Lätäseno Schist Belt, but only when the topographical and geological characteristics were favourable. Heavy mineral analyses showed that the fine fraction (<63 μm) is beneficial for automated SEM-EDS studies of stream sediments, but sample processing procedures need to be tailored to suit the target of the investigation. At present, only native Cu and Cu-sulphides can be considered reliable indicator minerals for Cu (-Au) mineralisation in the study area.