Soil gas geochemistry for the study of far- and near-field gas migration mechanisms.

Abstract

Detailed soil-gas surveys have been conducted at two mine districts to better comprehend gas migration mechanisms from deposits buried at different depths. The Tolfa (Lazio, Central Italy) and Neves-Corvo (Baixo Alentejo, Portugal) mine districts have different characteristics: the former is superficial (30-100 m) while the latter is located at a depth of 400-500 m and covered by low-permeability metamorphic rocks. The soil-gas results from these two different investigated areas provide interesting results related to the study of far- and near-field gas migration mechanisms. In particular, the anomalous concentrations of sulphur compounds (COS, CS2 e SO2), CO2, and Rn confirmed the presence of the two ore deposits. Furthermore, radon was found to be sensitive to soil permeability variations and to be associated with migration pathways (faults and/or fractures). Overall the results indicate the potential for using the studied gas species as indicators of deep seated ore deposits (i.e. in the Neves-Corvo mine district), even at locations where there is thick overburden and no surface evidence.