Gas migration from two mine districts: The Tolfa (Lazio, Central Italy) and the Neves-Corvo (Baixo Alentejo, Portugal) case studies

- Soil-gas technique is applied to study gas migration over two mine districts.
- The association Rn-CO2 confirms the presence of high gas microseeps.
- Gas release on the surface is controlled mainly by zones of enhanced permeability (faults/fractures).

Detailed soil gas surveys were conducted at two mine districts to understand 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 relatively shallow (30-100 m) whereas the latter is at a depth of 400-500 m and covered by low-permeability metamorphic rocks. The studied gases included major (N2, O2, CO2) and trace (4He, 222Rn) gases, hydrocarbons (CH4, C2H6 and C3H8) and S compounds (H2S, COS, SO2). The measured concentrations (some examples of max values at Tolfa: Rn 233 Bq/L, CO2 9.5%, CH4 12.3 ppm, COS 3.7 ppm; and at Neves-Corvo: Rn 130 Bq/L, CO2 24.3%, CH4 0.1%) indicate that gases migrate preferentially through zones of brittle deformation by advective processes, as suggested by the relatively high rate of migration needed to obtain anomalies of short-lived 222Rn in the soil pores. Considering the different depths of the two ore deposits, obtained results can be considered as features of near-field (Tolfa) and far-field (Neves-Corvo) gas migration.