Metal-rich todorokite as a pathfinder mineral in base and precious metal deposits: A mineralogical and microchemical approach to ore exploration

Abstract

Todorokite is a unique tunnel-structured hydrous manganese oxide common in terrestrial manganese deposits. It is renowned for its excellent adsorption capacity, associated with a high specific surface area and microporous structure. Due to its role in metal accumulation in the oxidation zone of some ore bodies, todorokite may act as a vectoring tool for concealed ore deposits. This research uses a multi-method analytical approach to investigate the mineral chemistry of todorokite-rich samples from six hydrothermal deposits in different geological settings in Greece: Mavro Xylo-Karposluk (Drama), Aspro gialoudi-Vani (Milos Island), Piavitsa (Chalkidiki), Trikorfo (Thasos Island), Thapsana (Paros Island), and Koskina (Evia Island). Portable X-ray fluorescence (pXRF) was used as the primary analytical method of various Mn-ore samples, complemented by XRD, SEM and electron microprobe analysis (EMPA) for mineral characterization and finally, micro-XRF and inductively coupled plasma – mass spectrometry (ICP-MS) to determine the concentration and distribution of economically important metals(loids) such as Zn, Pb, Ag, and Sb. In this research, we demonstrate that todorokite from manganese oxide deposits adjacent to intrusion related (Piavitsa, Mavro Xylo-Karposluk) and hybrid epithermal-VMS deposits (Milos Island) shows significant enrichment in Zn, Pb, Ag, As and Sb, in contrast to samples from less mineralized settings (Trikorfo, Thapsana, Koskina) that are base and precious metal-poor. EMPA cross-sections across mineral boundaries and micro-XRF mapping confirm metal zonation and phase-specific enrichment, particularly in Ag- and Zn-bearing todorokite. Additionally, geochemical correlations between pXRF and bulk ICP-MS data of Mn-oxide ore samples highlight the value of portable XRF as a rapid tool for detecting metal anomalies in the field. This research shows that combining portable XRF with focused mineralogical analyses can significantly improve geochemical exploration of hypogene hydrothermal and concealed deposits of base and precious metals.