**3. Methods**

Morphological and structural investigations at the microscale were carried out using field emission scanning electron microscopy (FESEM-JSM-6500F) at the National Institute of Geophysics and Volcanology (INGV, Rome, Italy). Secondary electron (SE) images, backscattered electron microscopy (BSEM) images, and X-ray element distribution maps were performed to detail wall incrustations. X-ray diffraction (XRD) patterns were obtained employing different instrumentations at the Department of Earth Science (University of Pisa, Pisa, Italy) using a Philips PW 1050/1710 with a conventional Bragg-Brentano (BB) parafocusing geometry equipped with a copper tube and a secondary graphite monochromator. The data were collected in the angular range 4◦ ≤ 2θ ≤ 60◦, with 0.02◦ 2θ scan step and counting time 2 s/step operating at 40 kV and 25 mA. Mineral chemical composition was analyzed using a JXA 8200 WD/ED electron microprobe (EMP) at the INGV (Rome, Italy). The analyses were performed using 15 kV acceleration voltage, 2.6 nA beam current, and probe diameter of 5 μm with a counting time of 10 s on the peak and 5 s on the background on both sides of the peak. Estimated precision is 0.05 wt %. The detection limit is 0.01 wt % for Cl, 0.03 wt % for Al2O3 and Na2O, 0.04 wt % for SiO2, MgO and CaO, 0.05 wt % for FeOtot, MnO, NiO, 0.06 wt % for TiO2 and Cr2O3, 0.09 wt % for SO3, 0.10 wt % for K2O.

The concentrations of Ba, Sc, V, Cr, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Ce, rare earth elements (REEs), Pb, Th, and U of carbonate and serpentine were determined with a sector-field, single-collector Element 2 XR ICP-MS at the University of Lausanne in laser ablation mode (LA-ICP-MS), i.e., interfaced to a NewWave UP-193 ArF excimer ablation system (ESI). Spot size varied between 75 and 100 μm with a frequency of 20 Hz and energy of 6.0 J/cm2, using as standard NIST SRM 612. Raw data were reduced off-line using the LAMTRACE software [16]. The analytical precision is better than 8% RSD.

Seasonal mine water sampling was performed at the following two sites: (i) mine water 1, in a deeper part of the mine; (ii) mine water 2, close to the carbonate crust along the main adit. For our model we used mine water 2, because it is associated with the carbonate crust. In the field, the temperature, pH, and electrical conductivity were measured with a portable multiparameter data logger calibrated in the laboratory; total alkalinity was determined by acidimetric titration. Accuracy is 0.5% for conductivity, 0.25% for temperature, 0.05 for pH, and 0.1 meq/L for alkalinity. After the physico-chemical measurements, water samples were collected in different modes depending on the specific chemical and isotopic analyses, performed in the laboratory. Water samples for anions (SO4 2− and Cl−) and cations (Na<sup>+</sup> and K+) analyses were collected without any pretreatment, while samples for Ca2<sup>+</sup>, Mg2<sup>+</sup>, trace elements, and silica were filtered in the field through 0.45 μm acetate-cellulose membrane filters and acidified in order to prevent precipitation. In addition, they were stored in bottles previously washed with diluted HNO3. All water samples were stored at 4 ◦C prior to processing. The anions and cations were analyzed, at the Institute of Geoscience and Earth Resources of the CNR in Pisa (IGG-CNR; Italy), using a Dionex DX100 ion chromatograph and a Perkin-Elmer 3110 atomic absorption spectrometer. The analytical precision is 3% for both species. Silica determination was performed via spectrophotometric method. Trace elements were analyzed by ICP-MS at the IGG-CNR. The analytical precision is better than 2%.

Aqueous species speciation and mineral saturation indices (SI) were calculated using the geochemical speciation code PHREEQC [17] and the Base de Donnee Thermoddem\_V1.10 database from the Bureau de Recherches Géologiques et Minières (BRGM Institute, Fontenay-aux-Roses, France; http://thermoddem.brgm.fr.) All the solutions were calculated based on Cl-charge balance.
