Identification and characterisation of mining residues through field surveys at a decommissioned Cu-Sb(-Ag) mine site (Monte Avanza, Carnic Alps, NE Italy)

Decommissioned mining sites are frequently characterised by the occurrence of waste materials and tailings piles as a result of past mineral extraction and processing. Although these materials potentially still contain high amounts of minerals, they are still usually poorly characterised, resulting in a lack of information about their spatial distribution and content of several elements, which may include also potentially toxic elements and critical raw materials. Field investigations through portable devices integrating both geochemical and geophysical techniques represent a powerful tool for detecting mine residues occurrence and for their preliminary characterisation. The data obtained through this approach can indeed be useful to support the management of these materials, helping to assess the need for the implementation of monitoring or remediation plans or, conversely, the opportunity of their reprocessing for secondary recovery plans (Barago et al., 2021). This multidisciplinary approach was used within the area nearby the former fahlore Cu-Sb(-Ag) mine at Mt. Avanza (Carnic Alps, NE Italy), intermittently exploited until 1952. The main exploited mineral is the tetrahedrite (Cu6[Cu4(Fe,Zn)2]Sb4S13), which can host several other elements in its lattice structure. Among these elements, the ore at Mt. Avanza contains notable amounts of mercury (Hg) (Casali, 1996), which can easily volatilise to the atmosphere as gaseous elemental mercury (GEM) depending on its abundance in the substrate. A preliminary real-time survey of atmospheric GEM concentrations within the study area was thus performed using a portable analyser to detect the presence of GEM sources, which can represent a good proxy of the occurrence of mine residues thanks to the Hg enrichment of the ore. Moreover, samples of mine residues, soils, stream sediments, and water were collected for a first geochemical characterisation. Relatively high amounts of various elements (Cu, Sb, As, Pb, Zn, Hg) were observed, mostly in samples from waste rock piles (Barago et al., 2023). For a more detailed analysis of mine residues dispersed within the area, a second field investigation was conducted. For each previously identified waste rock pile, sub-surface samples were collected both on a regular grid and in correspondence to peaks in atmospheric GEM concentrations. Trace element concentrations were preliminarly determined in the field using a portable X-ray Fluorescence (XRF) analyser and the results were used to select samples for further laboratory analysis. At the same locations, Electrical Resistivity Tomography (ERT) and Frequency Domain Electro-Magnetic (FDEM) geophysical surveys were performed to estimate the thickness of mine residues and evaluate potential correlations with the detected metal concentrations. Combined results provide useful information for a preliminary and rapid estimation of metal contents and distribution in mine residues, allowing optimisation of sampling for a subsequent more detailed characterisations.