Tracing P-REE mineralization in till from the southern Oslo Rift, Norway

The exponential growth of the high-tech industry, and the current environmental challenges affecting the agricultural sector result in a global demand for critical raw materials, such as rare earth elements (REE) and P. REEs are prioritized given their use for green-energy technologies, whereas P is key for the fertilizer industry.

Southern Norway is host to several magmatic REE occurrences where Fen, Europe’s largest REE deposit, is the most relevant of all. East of the Fen deposit, the southern Oslo Rift area contains several magmatic P-REE occurrences (e.g., Kodal deposit). The P-REE mineralization is mainly contained in small (cm to <1 m) titanomagnetite–apatite–ilmenite-rich pockets irregularly distributed within narrow areas (<1 km²) hosted by monzonite. A recent multidisciplinary project lead by the Geological Survey of Norway has re-defined the ore genesis model of the area and assessed its regional mineral potential.

The restricted size and erratic occurrence of the P-REE mineralization along with the glaciated nature of the terrane pose significant challenges for mineral exploration in the area. If not properly assessed, mineralized areas can be overlooked. In this contribution, we test the performance of field and laboratory measurements of till geochemical and physical properties in the Siljan occurrence as means to detect P-REE mineralization in the broader Southern Oslo Rift area.

The Siljan occurrence is one of the most prospective areas for P-REE mineralization indicated by the regional prospectivity study. Here, we carried out a targeted till sampling transect (1 km long; 50 m x 50 m grid, n = 110) and drone magnetic survey (8 km²; 5 m x 5m resolution). Till material was analyzed in the field using a portable XRF and magnetic susceptibility meter. The samples were sieved to a size of <2 mm, analyzed for major and trace elements using ICP-MS, and subjected to magnetic susceptibility measurements at the Geological Survey of Norway laboratory. First, several geochemical vectors were selected separately for the field and laboratory datasets using raw data and multivariate statistics. Then, geochemical and magnetic susceptibility anomalies from both datasets were decoupled from background using fractal analysis. Our results show that geochemical vectors involving Ca, Fe and P, and magnetic susceptibility measurements from both datasets can efficiently detect the Siljan anomaly and spatially correlate with a magnetic high indicated by the drone survey. Notably, REE-driven anomalies are less well-defined in the field dataset, nonetheless, these remain useful to detect the mineralized zone. The survey results also indicate that till transport is minimal in Siljan, which makes this media a very robust vector for mineral exploration surveys.

Overall, this study shows that characterization of soil geochemistry and -magnetic susceptibility combined with an appropriate exploration grid design, can efficiently trace the fingerprint of magmatic P-REE mineralization in southern Norway, and likely, in similar glaciated terranes elsewhere. Moreover, our results indicate that field measurements can provide equally reliable results when compared to the more costly and time-consuming laboratory analyses.