Advanced mineral exploration in the glaciated terrain using topsoil geochemistry and the compositional statistical data analysis

Mineral exploration in northern, vulnerable nature areas demands on the development of new, environmentally friendly sampling and analyses techniques. Those areas are typically covered by the transported cover of which the glacigenic sediments such as a till are the most dominant as a result of several glaciation stages. To avoid conventional basal till and bedrock sampling using heavy machines, the use of different surface geochemical sampling media and techniques have increased recently. Particularly, the development of selective and weak leach techniques for the topsoil (Ah and B horizons) geochemistry has been intensive, and the use of those techniques has led to the observation of new mineralization.
In this research, carried out under the project New Exploration Technologies (NEXT), funded by the European Union’s Horizon 2020 research and innovation programme, we used stratified random sampling strategy for creating a sampling grid and developed novel compositional statistical data analysis for the interpretation of geochemical data obtained by the multi-source surface geochemical techniques. The test area located in northern Finland where there is an active exploration campaign going on for Au-U-Co mineralizations in the Rajapalot area, Ylitornio by the Mawson Oy. Glacial morphology in the study area is dominated by the ribbed moraine ridges with peatlands in between. The thickness of till cover varied from some metres to 15 m. A sampling network for both the Ah and B horizon samples comprised 98 proper and 10 field duplicate sampling stations from the mineral soil dominated by the Podsol-type soil horizon. The chemical analyses methods used were Ultratrace 1:1:1 Aqua Regia leach and 0.1 M sodium pyrophosphate leach for the Ah horizon samples, and Ionic leach and Super Trace Aqua Regia leach methods for the B horizon samples. The laboratory analyses were supported by the portable X-Ray Fluorescence (pXRF) analyses done directly in the field. The statistical analysis methods of the results were based on conventional supervised and unsupervised classification techniques using as explanatory variables: a) log-ratio transformations of the geochemical compositions, b) enrichment factors between sampled media, and c) the summaries of the two preceding systems of variables provided by the parallel principal component analysis.
The preliminary results of the topsoil geochemistry show a significant response to many elements to known geochemical features and elevated contents in the base-of-till and underlying bedrock geochemical data. Anomaly patterns are also reflecting the lithological variations of the rock units in the bedrock. Based on the results, it is obvious that a) there is good correlation between the surface geochemistry and underlying bedrock, b) stratified randomization in the planning phase and statistical methods in data interpretation stage increases the quality of the data and the reliability of geochemical exploration, and c) topsoil sampling with selective analysis methods is effective and environmentally friendly geochemical exploration technique in the glaciated terrains.