Application of the weak leach method in the Sokli carbonatite massif

Abstract

In the central area of the Sokli carbonatite massif, a weak leach study was planned and conducted for the purpose of testing the functionality of the techniques in ore exploration in the area. The focus of the weak leach method testing project was to map out phoscorite units and late carbonatite dykes known to be present in the centre of the complex. The target area was chosen to be around the central fracture zone which according to old drill core data contains REE rich late carbonatite dykes. The central fracture zone is covered by the Sokliaapa bog and thus the weak leach lines were planned on the outskirts. Subsequently, outside the fracture zone are several known and indicated phoscorite rocks that the sampling lines go over. Ionic leach and ammonium acetate methods were used alongside conventional geochemistry, aqua regia. All analyses were provided by ALS laboratory.

Overall, 8 weak leach sampling lines were completed going over the specific areas of interest, mainly magnetic anomalies. Many old roads, pits and drillholes in the area further decreased the sampling area and shortened the lines. The sampling was therefore focused only on short lines on both sides of the Sokliaapa bog. Due to the lines not going through the whole complex, three background samples were collected separately from outside the carbonatite area. Duplicate samples were collected for quality assurance purposes. The results of the project were assessed by calculating statistical values for the main results and RSD% for the duplicate pairs. Response ratios were calculated to observe the anomalous metal concentrations for each sample point. Further, the results were projected on the magnetic maps to compare the phoscorite metals and REEs to the magnetic anomalies.

The results of the weak leach methods showed that the methods work well in the Sokli environment, backed by the aqua regia method proving the difference of the soil composition and the mobile metal signal measured by the weak leach methods. Especially the REE results showed clear anomalous signals that are situated linearly on the areas having lower magnetism. In the magnetic maps, phoscorite metals also showed good distribution on the positive magnetic anomalies.

The results provided means for further interpretations where magnetic maps, drill core data and a single weak leach line completed by GTK in 2011, were utilized to draw a map of REE structures and phoscorites of the central area of the Sokli carbonatite complex.