Biogeochemical anomaly response of circumboreal shrubs and juniper to the Juomasuo hydrothermal Au-Co deposit in northern Finland

Tree tissue chemistry has proven successful in guiding advanced exploration in the early stages of mineral exploration projects in Arctic and subarctic regions. In this paper, the biogeochemical response of three circumboreal shrubs, crowberry (Empetrum nigrum L.), Labrador tea (Ledum palustre) and bilberry (Vaccinium myrtillus L.), and one conifer, common juniper (Juniperus communis L.), to the underlying hydrothermal Juomasuo Au-Co deposit was assessed in southeast Finnish Lapland. A variety of contrasting spatial multi-elemental anomaly patterns were found for the elements Au and Co, along with Fe, Th, U and rare earth elements, such as Ce, La and Nd, in different plant tissue types over the subcropping lodes and also deep (blind) mineralizations down to a depth of 200 m in two seasonally varying campaigns: in late summer (August) 2013 and early summer (June) 2014. Besides these elements verified by lithogeochemistry, Ag, Bi, Mo, Se, Te, W and Ni exhibited anomalous spatial patterns over the mineralization. Based on the Mann-Whitney-Wilcoxon test, the Au concentrations in twigs/stems of crowberry, bilberry, Labrador tea and common juniper over the mineralization were found to be higher than the background, but the evergreen species gave the most consistent response to the mineralization. This also applied to U, W and Mo, as well as Nd and several other rare earth elements. Using unsupervised clustering with self-organizing maps and k-means, the location of the underlying mineralized zones could be determined with high overall accuracy (70-90%). This indicates that the biogeochemical anomaly patterns over the Juomasuo sulphidic lodes are strong, and the deposit would have been detected from the biogeochemical data even without prior knowledge gained from a diamond drilling campaign. The sampled vascular species are widely distributed over the pan-Arctic and circumboreal terrains, thus demonstrating their considerable significance to mineral exploration for hydrothermal Au ores at northern latitudes.