Conference Agenda

Vegetation cover poses a significant challenge in geological surveys. Utilizing plant chemistry to detect underlying geological features offers a non-invasive exploration tool. Here, we employ a novel approach in biogeochemical data analysis to distinguish between mineral deposits and bedrock void of mineralization. Different plant species and tissues exhibit distinct element compositions in a multivariate hyperspace, making direct comparisons in terms of absolute concentrations challenging. Our approach aims to overcome this traditional constraint by analyzing biogeochemical datasets irrespective of the specific species and tissue collected, as we will account for the physiological variations through data preprocessing. We use principles of compositional data analysis to transform biogeochemical datasets, enabling comparability across different sample materials and other plant datasets.

The corrected biogeochemical dataset for the sample type was analyzed using Linear Discriminant Analysis to identify element associations indicative of differences between samples collected over mineralized bedrock and those from rocks void of mineralization (barren). Results show differences in group means between samples sourced from pillow basalts and mafic rocks compared to their mineralized counterparts, distinguishing between mineralized and mineralization-free samples. For example, when comparing element ratios of Cu, Ca, and Rb to P and Sr, differentiation is observed across six distinct plant materials between barren pillow lavas and the corresponding mineralized bedrock.

The Pohjanmaa Belt in western Finland hosts orogenic Au deposits with enrichment in base metals, such as the Jouhineva Cu-Co-Au-Ag deposit (0.448 Mt at 0.811% Cu, 0.182% Co, 0.88 ppm Au, and 7.864 ppm Ag measured reserves). It is located 3 km NW of the Au-only Huhta occurrence along the same NNW-trending fault. We test if the different metal association in both deposits is related to temporally and structurally different mineralization events. Four deformation events are distinguished: D₁ formed a now EW- and NW-trending S₁, which is transposed during subsequent deformation; D₂ folded S₁ into a NW-SE to NNW-SSE trending S₂ crenulation cleavage; D₃ refolded S₂ into close to isoclinal folds and generated a NNW-SSE trending, axial planar S₃; D₄ formed a centimeter-spaced, EW-trending S₄ foliation. Huhta hosts syn-D₃ Lö-Apy-Au-Qtz veins with multiple stages of sulfide, arsenide, and sulfarsenide mineralization. Jouhineva hosts syn-D₃ Apy-Au-Ccp-Qtz and syn-D₄ Ccp-Au-Qtz veins. Titanite U-Pb and structural data indicate synchronous formation of Lö-Apy-Au-Qtz veins in Huhta and Apy-Au-Ccp-Qtz veins in Jouhineva at 1850-1820 Ma during D₃. Thus, the different mineralogy and metal endowments formed during the same tectonic event during the Svecofennian Orogeny. Only Jouhineva hosts additional Cu-Au mineralization that formed later during the retrograde evolution. The different metal association in both occurrences may be explained by different metal and fluid sources and compartmentalized fluid migration, distinct mineralization events during the 30 m.y. evolution that cannot be resolved by titanite geochronology, or different precipitation mechanisms although host rocks and PT conditions are similar.

The Gorno mining district is an example of Mississippi Valley-type (MVT) deposits in the Italian Orobic Alps. Spanning an area of ~100 km², it consists of stratabound Zn-Pb-Ag (± fluorite ± barite) mineralization hosted in a lower Carnian stratigraphic succession. Despite Vedra Metals S.r.l.´s acquisition of the exploration license for the deposits has reignited an economic interest, an updated metallogenic model has yet to be developed.

In the carbonate host rocks at Gorno, a complex series of dolomitization, silicification, brecciation, dissolution, and cementation occurred. Microthermometry of primary fluid inclusions in sphalerite and fluorite, alongside sphalerite trace-element geothermometry, indicates formation temperatures ranging from 80 to 140 °C (mean value: ~100 °C). Moreover, fluid inclusion microthermometry and micro-Raman spectroscopy document the involvement of high-salinity brines (up to ~25 eq.wt% NaCl) and gaseous hydrocarbons (e.g. CH₄) in ore deposition. Isotopic signatures from ore-related carbonates for carbon (0.5 to 2.5 ‰ PDB), oxygen (-6.6 to -12.1 ‰ PDB), and strontium (0.70840-0.70943) indicate that the ore fluid was likely seawater modified through interaction with the underlying Permian clastic sediments and/or with the metamorphic basement.

The presence of sulfide bodies in association with organic-rich shales implies a notable role of organic carbon in ore deposition. Organic matter and associated hydrocarbons likely served as reactive barriers, leading to the reduction of the ore fluid and initiating the precipitation of sulfide minerals.

Efficient transfer of S and chalcophile metals through the Earth’s crust in arc systems is paramount for the formation of large magmatic-hydrothermal ore deposits and can strongly affect the Earth’s climate. The formation of sulfide-volatile compound drops has been recognized as a potential key mechanism for such transfer but their fate during dynamic arc magmatism remains cryptic. We report evidence of compound drops preserved in the active Christiana-Santorini-Kolumbo volcanic field. The observed compound drops are micrometric sulfide blebs associated with vesicles trapped within silicate phenocrysts. The compound drops accumulate and coalesce at mafic-felsic melt interfaces where larger sulfide ovoids form. These ovoids are subsequently oxidized to magnetite during sulfide-volatile interaction. Comparison of metal concentrations between the sulfide phases and magnetite allows for determination of element mobility during oxidation. The formation and evolution of compound drops is an efficient mechanism for transferring S and chalcophile metals into shallow magmatic-hydrothermal arc systems.

There is still no consensus in the literature concerning the most critical processes for rare earth elements (REE) enrichment in carbonatite rocks. One-third of alkaline‑carbonatite complexes are associated with ultrabasic and basic rocks. Some basic rocks contain similarly high concentrations of LREE as carbonatites where perovskite [CaTiO3] is responsible for this enrichment. Carbonatites form later than ultrabasic and basic rocks, and carbonate melt penetrates into silicate rocks and reacts with them. The reaction was named “antiskarn reaction”. Until now there is no detailed study on whether perovskite from pyroxenites can become a source for REE enrichment of carbonatite melt during antiskarn reactions.

We studied three ultrabasic-alkaline carbonatite complexes from the Kola Alkaline Province (Russia) to focus on the comparison of REE contents from pyroxenites and carbonatites and the fate of perovskite. Ultrabasic/basic rocks and carbonatites show the same broad range of REE contents. In most analyzed pyroxenites, perovskite is the main REE carrier mineral. For carbonatites/phoscorites, apatite and calcite are the minerals that control rocks' REE enrichment. Several samples show considerable variations of REE concentrations related to carbonatite infiltration, sometimes even at a very low local scale (mm²). If perovskite was present it was replaced near the carbonatite melt by titanite that has much lower REE contents. Calcites and apatites from the infiltrated carbonatite are highly enriched in LREE but show large local element variations. Thus, we propose that LREE liberated by the perovskite replacement can be dissolved in the carbonatite melt and transferred to its crystallizing minerals (calcite, apatite).