Mineral systems anatomy linked to computational techniques for lithium mineral exploration targeting in Västernorrland region in Sweden

The Exploration Information Systems (EIS) ( https://eis-he.eu; Horizon Europe grant No. 101057357) project aims to create mineral prospectivity tools and a user-friendly GIS-wizard and to conduct real world testing of those tools on selected study sites containing known mineralisation. Geological Survey of Sweden (SGU) initiated several multidisciplinary mapping projects in Sweden on thematic mapping of energy, battery and innovation critical mineral and metals. The new data on geology, geophysics and geochemistry as well as new age dating from rocks are carried out within a framework of SGU’s mapping projects in conjunction with the EIS project.  A mineral system for lithium mineralisation has been defined which is presented partly in Sadeghi et al., (2024) and Lynch et al, (2024).

In Västernorrland, LCT pegmatites are dated to 1.8 Ga and may be linked to S-type granites formed around 1.84-1.85 Ga. Updated geological maps, incorporating geological fieldwork and geophysical data, show that lithium mineralisation often occurs in association with the contact between granites and preserved sedimentary rocks, and indicate these granites formed by partial melting of magmatic intrusions. Lithium bearing pegmatites typically align with preexisting planar structures in meta-supracrustal rocks, suggesting that earlier structures acted as pathways or traps for volatile-rich melts. Localised ductile deformation may have influenced pegmatite emplacement, as evidence by folded pegmatite forms. This structural information combined with fault kernel density maps, highlight the pathways critical for lithium mineralization. According to field observations, lithium pegmatite mineralisation in Västernorrland is linked to older mafic rocks (e.g., amphibolite, gabbro, andesite) that act as physical traps, and graphite schists that occur in the area interlayered with metasedimentary rocks that may act as chemical traps due to higher content of S and C. A detailed study on till geochemical dataset carried out by Sadeghi et al., (2024) concluded that the Principal Component-4 of a selected trace elements dataset (La-Mn-Li) can represent a proxy for such a chemical trap.

Using this mineral system approach and the EIS toolkit, a new prospectivity map has been generated using “fuzzy method”. Fuzzy operators were used to create fuzzy memberships for each dataset input into the model and then overlain using fuzzy modifiers. The model was validated using known LCT pegmatite occurrences and locations of exploration concessions for LCT pegmatites. The results are validated by the existing known mineralization, claim areas for prospecting and the distribution of know LCT-pegmatite dykes.  The model is well correlated to the validations area but there is space for improvement using more detailed data in the northwestern part of study area where a geological mapping project is ongoing.   

Sadeghi, M., Casey, P., Carranza, E.J.M., Lynch, E.P (2024) . Principal components analysis and K-means clustering of till geochemical data: Mapping and targeting of prospective areas for lithium exploration in Västernorrland Region, Sweden. Ore Geology Reviews 167, 106002.

Lynch, E.P., Andersson, J.B.H., Sadeghi, M., Bauer, T., & Bečelytė, I. (2024).  Stepwise magmatism and structural reactivation facilitates LCT pegmatite formation: Insights from central Sweden. 36th Nordic geological winter meeting, January 10–12 2024, Abstract volume. Geologiska Föreningen Specialpublikation 5, p. 131.