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

Abstract

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 (30–100m) whereas the latter is at a depth of 400–500m 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 233Bq/L, CO2 9.5%, CH4 12.3ppm, COS 3.7ppm; and at Neves-Corvo: Rn 130Bq/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.