## 4 Discussion  
In general, according to Schaef et al. [39], Rosenbauer et al. [40], and Alfredsson et al. [41], the injection of carbon dioxide into basaltic rocks has several advantages in comparison with sedimentary basins if the storage of CO\({}_{2}\) is through its mineral carbonation. Effective mineral carbonation of CO\({}_{2}\) is best achieved with ultrabasic or basic volcanic rocks due to the high contents of Mg\({}^{2+}\), Fe\({}^{2+}\), and Ca\({}^{2+}\) in ferromagnesian mineral phases, which can react with CO\({}_{2}\) and precipitate as carbonates (e.g., magnesite, siderite, or calcite). Since basalt carbonation could therefore become an important carbon storage solution [15], we selected 10 ultramafic and mafic rock areas in this study as a potential area for CO\({}_{2}\) storage. In a preliminary phase of site characterization for CO\({}_{2}\) geological storage in ultrabasic and basic volcanic rocks, we selected one geological formation with the potential to be considered as adequate for CO\({}_{2}\) storage. Comparing the 10 studied areas, we found that the Sines massif had more favorable characteristics (Table 3) than the other areas with respect to all the relevant parameters evaluated (Table 2). Its location, only a few kilometers from the main CO\({}_{2}\) sources in the region, which would avoid excessive transport costs, coupled with the large expected volume of cumulate gabbro with a high percentage of mafic minerals, provided the best prospect in the study area for the implementation of mineral carbonation projects. Despite the subvolcanic (intrusive) nature of the Sines massif, the study of mafic lithologies, in particular the cumulate gabbros with more than 60% of ferromagnesian minerals, presented the chemical and mineralogical features theoretically required for mineral carbonation. Frequently, the boundaries between the cumulate gabbros and diorites were unclear, and thus it became necessary to test not only the more mafic lithologies but also the dioritic rocks in order to assess the mineral carbonation potential of the rock massif. As such, samples of both rock types were tested in laboratory experiments to quantify their reactivity with a natural brine highly saturated with CO\({}_{2}\), under supercritical conditions, to understand the chemical reactions that are expected to occur in the area immediately surrounding an injection well. These laboratory experiments were designed to assess whether the CO\({}_{2}\)-water-rock interaction would dissolve the cations of cumulate gabbros and gabbro-diorites necessary to react the CO\({}_{2}\) and promote fast mineral carbonation. The Sines massif is ideally located close to the CO\({}_{2}\) sources in the region, which are hundreds of meters to a few kilometers away from the cumulate gabbros outcrops. A further advantage is that the subvolcanic massif extends from those outcrops to the offshore, where it encompasses a much larger area, most of which is covered by recent sediments. A scenario of CO\({}_{2}\) injection in the cumulate gabbros in the near offshore could be used to assess whether the fracture permeability is sufficient to allow for injection. Geophysical surveys and reinterpretation of previous data were necessary to clarify the offshore area, volume, and fracture pattern. The studies of fracture density did not show a consistent and pervasive fracture pattern, and it is expected that fracture density decreases with depth, which may make an in situ carbonation process unviable. Mineral carbonation can also be achieved ex situ by resorting to the mining of the cumulate gabbros in quarries and promoting its reaction with CO\({}_{2}\) in specifically built facilities. A large quarry already exists in the Sines massif. Enhanced weathering is another possibility. The biggest contributor to the natural cycle of removing carbon dioxide from the atmosphere is the chemical weathering of certain types of rocks.