Ionic geochemistry: enhanced soil analysis for mineral exploration via high-efficiency ligand-based mobile ion extraction with simultaneous detection of 57 elements in a single extraction

Ligand-based extraction of mobile ions is a precise and efficient technique for uncovering and tracing buried mineralization sources by extracting mobile ions physiosorbed on to soil particles. This study focuses on developing a streamlined method that employs strong ligands, EDTA, citric acid, potassium cyanide, ammonium hydroxide and ammonium chloride, in a single digestion process. Inductively coupled plasma mass spectrometry (ICP-MS) was chosen as the analytical method to achieve low detection limits while enhancing the efficiency of exploration in terms of time. Two hundred samples were collected from a semi-arid region in Eastern Anti-Atlas, Morocco. Samples were prepared by sieving to obtain a particle size of <180 µm without prior crushing or grinding. Ligand quantities and the extraction pH were optimized using a Box–Behnken experimental design, with the pH set to 8.5. The resulting leachate was analysed for 57 elements using ICP-MS. This method successfully detected elevated concentrations of gold (Au; up to 40 ppb) and its pathfinder elements, including silver (Ag, up to 1279.04 ppb), copper (Cu, up to 9633 ppb), cobalt (Co, up to 896.30 ppb), lead (Pb, up to 10 765.30 ppb), selenium (Se, up to 68 ppb), nickel (Ni, up to 368 ppb) and arsenic (As, up to 1277.95 ppb), revealing potential anomalies that were not identifiable using conventional techniques (aqua regia, four acid digestion, fire assay). Our method provided superior geochemical contrast with reduced background noise as evidenced by signal-to-noise (S/N) ratios of up to 3944 for Au, and high S/N values for pathfinder elements including Pb (40.8), Ag (29.2), As (21.1), Cu (16.7), Co (14.1), Se (13.6) and Ni (6.3). Principal component analysis validated the sampling reliability, with a significant Kaiser–Meyer–Olkin coefficient of 0.74 and strong correlations between Au and its pathfinders. Additionally, the mild digestion process was benchmarked against more aggressive methods, such as aqua regia and four-acid digestion, confirming its efficacy. This innovative method will enhance mineral exploration strategies by improving geochemical resolution, allowing for faster and more accurate identification of geochemical anomalies. By providing clearer contrast between background and mineralized zones, it reduces the need for extensive follow-up sampling and accelerates the selection of drilling targets.