*4.2. Wall Coating*

The mineral paragenesis forming the wall coating, identified by XRD and EMP analyses, is represented by hydromagnesite, with minor aragonite and a Mg-rich clay silicate (Figure S1, Table S1 and S2 in Supplementary Materials, SM). Macroscopically, hydromagnesite occurs mostly as coatings, crusts, and spherules lining the rock surfaces and tiny fractures propagating for a few millimeters into the serpentinite walls (Figure 6). Coatings, crusts, and spherules can have a large areal extension but with a limited thickness, up to few millimeters.

**Figure 6.** Examples of carbonate coatings: (**a**) Hydromagnesite ± kerolite coating (first precipitation), (**b**) hydromagnesite rosettes (second precipitation), (**c**) early precipitation of hydromagnesite along the rock fractures, and (**d**) preferential precipitation of hydromagnesite on the surfaces facing outwards of the mine adit.

At the microscale, two episodes of hydromagnesite precipitation are recognized (Figures 7 and 8). The first precipitation is characterized by hydromagnesite layers alternated with Mg-rich clays layers (with thicknesses ranging from 100 μm to <5 μm; Figure 8). In particular, the Mg-clays layers, generally deposited in the late stage, and showed an irregular and wavy texture with the presence of shrinkage cracks (<5 μm in width). The second episode of precipitation formed an aggregate of hydromagnesite rosettes, each made by fibrous-radiating acicular crystals (with size ranging from ~1 to >8 μm). This generation forms an external coating, distinguished from the early precipitation by a strong increase in porosity (Figure 8). Minor amounts of anhedral aragonite (with size ranging from ~500 μm to a few micrometers) have been observed in association with both hydromagnesite μm (Figure 8). We found partially dissolved serpentine fines, with a size variable from 5 to 200 microns, forming a layer between the serpentinite and the crusts (Figure 8b–f). This powdered serpentine was most likely produced during the mine excavation.

The chemical composition of Mg-clays (Table S2) can be attributed to either stevensite or a hydrated and highly disordered variety of talc-like phase, named "kerolite" Mg3Si4O10(OH)2·*n*H2O [18–20]. Kerolite-like minerals are usually associated with carbonates derived from alteration of ultramafic rocks at high pH [20], whereas stevensite is more common in low pH environments. In addition, the XRD data points to the presence of "kerolite" rather than stevensite (Figure S1). "Kerolite" has previously been found mixed with poorly crystalline serpentine, both in natural settings and in experimental reaction products [15]. The term "deweylite" has been used to describe intimate mixtures of a fine-grained, highly disordered, kerolite-like mineral and a disordered serpentine mineral, typically chrysotile, in varying proportions. This is in agreement with our variable chemical analyses, indicating an excess of Mg and a deficiency of Si, probably because of the presence of serpentine and (or) disordered kerolite (Table S2).

**Figure 7.** Microphotographs of various carbonate occurrences showing: (**a**) Banded hydromagnesite and kerolite; (**b**) aragonite spherules growth onto carbonate coating + bastite porphyroclasts; (**c**) carbonate coating composed of hydromagnesite, kerolite, and late aragonite; and (**d**) overgrowths of banded hydromagnesite.

**Figure 8.** SEM images and X-ray element distribution maps of wall incrustations: (**a**) Secondary electron (SE) image showing intergrowth of lenticular crystals of hydromagnesite on the outer layer of the crusts; (**b**) backscattered electron microscopy (BSEM) image showing the serpentine fines embedded into hydromagnesite ± kerolite. Kerolite layers are distinguishable from hydromagnesite layers because they appear light grey in color in the BSEM images; (**c**–**f**) BSEM images (**c** and **e**) and X-ray element distribution maps for Si (**d** and **f**) showing the following: (i) serpentinite substratum, (ii) serpentinite fines at the external surface of the rock, (iii) hydromagnesite and kerolite layered crust, and (iv) late hydromagnesite rosettes. The yellow color in Figure 8d,f highlights the presence of kerolite layers and serpentinite fines on the wall surface.

#### *4.3. Trace Element Composition of Mineral Phases*

Trace element composition of serpentine and hydromagnesite analyzed by LA-ICP-MS is reported in Tables 1 and 2. Serpentine minerals have high Cr and Ni content, up to ~2600 ppm and ~1400 ppm, respectively, relatively high content of V and Cu, and detectable content of Pb, U, Th, Y, Rb, Sr, Zr, and Nb. REE serpentine patterns span over a relatively small range of contents; from ~0.1 to more ~1 times those of the chondritic values, with LREE depleted patterns and a positive Eu anomaly (Figure 9).


**Table 1.** Trace element composition (ppm) of serpentine determined by LA-ICP-MS.

serp = serpentine.

**Table 2.** Trace element composition (ppm) of hydromagnesite and aragonite determined by LA-ICP-MS.


d.l. = detection limit; - = not determined; Hmg = Hydromagnesite; Ar = aragonite.

**Figure 9.** Trace element composition normalized to chondrite of hydromagnesite (red lines), serpentine (green lines), and aragonite (yellow lines).

Hydromagnesite shows up to 45 ppm of Cr, significant amounts of Ni, Zn, Sr, V, Ba, and Pb, and detectable REE, U, Th, Y. Hydromagnesite displays a depleted and scattered REE pattern with slightly enriched LREE (LaN/YbN(av) = 1.86).

Hydromagnesite trace elements indicate that the fluid responsible for its deposition is a fluid that interacted with serpentinite host rocks. The low-temperature weathering of serpentinites produces spring waters containing a significant content of Cr, as well as Ni, Co, and Mn [1,9,21].

#### *4.4. Chemical Composition of Mine Waters*

Waters emerging from the mine walls and ceiling have a mean temperature of ~17.5 ◦C and a pH of ~8.4 throughout the year and can be classified as bicarbonate-magnesium type waters, because of their high Mg2<sup>+</sup> and HCO3 − concentrations, ranging from ~90 to 132 mg/L and ~7.7 to 10.0 mg/L, respectively (Table 3 and Figure 10) and show low Ca/Mg molar ratio (<0.04, Figure 10a). In the Mg-SiO2-HCO3 triangular diagram (Figure 10b), a mixing line between an aqueous phase in equilibrium with carbonates and Mg-rich silicates is reported. Mine waters fall to the left side of the line, indicating that such waters are in equilibrium with hydrous Mg-carbonates. Mine waters show low concentrations of trace elements, locally below detection limits, as reported also in other emerging waters in Tuscany [11]. Significant amounts of Cr (~24 ppb), Sr (~20 ppb), Ba (~85 ppb), Fe (~25 ppb) and Zn (~7 ppb) have been detected (Table 3). Other trace elements (Ni, Mn, B, V, Al, Th, and U) have very low concentration between <0.01 and up to 5 ppb. The pCO2 and saturation indices (SI) of the relevant phases (hydromagnesite and aragonite) were calculated using the chemistry of mine waters using the PHREEQC code. Mine waters 2, collected near the carbonate crusts, show log pCO2 varying from –3.06 to –3.04 and they are undersaturated with respect to hydromagnesite (SIhmg ~–4.81) and saturated with respect to aragonite (SIarag –0.03).




**Figure 10.** Chemistry of mine waters. (**a**) Mg vs. Ca plot computed from concentrations in meq/L and (**b**) Mg-SiO2-HCO3 triangular plot, computed from concentrations in mg/L. Red triangular points represent from another mine excavated in serpentinites from Southern Tuscany (Querceto, [11]) and stairs represent the expected compositions of aqueous phases controlled by CO2-driven dissolution of Mg-bearing solid phases (i.e., sepiolite, talc, serpentine, and hydrated carbonates).

#### **5. Discussion**

Underground environments, such as mine adits and natural caves, have peculiar characteristics, such as an almost constant temperature throughout the year, high humidity, and seasonal variation of the direction of ventilation [22]. Cyclical variations of these parameters can enhance CO2 degassing, as well as water evaporation and condensation that can trigger carbonate precipitation [22–24]. Even though the formation of underground carbonate concretions in natural caves has been well understood, the formation of various types of concretions in mining underground works, especially with ultramafic substratum, is still lacking a thorough investigation. The study of this latter type of concretion in an ultramafic underground environment, such as the Montecastelli Cu mine, can give the opportunity to explore the peculiar conditions at which CO2-rich water spontaneously react with serpentinite rocks.

#### *5.1. PHREEQC Models*

Petrographic and XRD data shows that the wall coating paragenesis in the Montecastelli mine is composed of hydromagnesite, kerolite, and aragonite precipitated. In order to understand whether the coating precipitated directly from dripping waters we performed geochemical modeling with PHREEQC using as starting fluid waters emerging from the fractures in the mine wall close to the carbonate crusts (mine water 2). Our model indicates that the CO2-rich dripping water (log pCO2 = −3.05) moves towards equilibrium with the pCO2 of the mine atmosphere (log pCO2 = −3.40 corresponding to ~418 ppmv) measured along the mine adits (Table 4). After the pCO2 of the dripping water equilibrates with that of the mine, it becomes saturated with aragonite (SIarag = 0.25) and undersaturated in hydromagnesite (SIhmg = −3.09).

Aragonite supersaturation (SI > 0.45) is reached through evaporation (more than 20%, Table 5), allowing its precipitation. Dripping water becomes supersaturated in hydromagnesite (SI = 0.46) with higher incremental evaporation (more than 55%), letting hydromagnesite precipitation after aragonite. Therefore, the observed mineral paragenesis of the wall coating, where hydromagnesite is the first phase to precipitate, cannot be generated directly from dripping waters.


**Table 4.** CO2 measured along the mine adits.

St.dev. = Standard deviation.

**Table 5.** Saturation indeces of hydromagnesite and aragonite, pH and Mg/Ca molar ratio of dripping and condensed waters computed in PHREEQC.


Previously, it has been shown that the formation of Mg-carbonate wall coating in underground mines, excavated in ultramafic lithologies, can also occur through direct precipitation from dripping waters [2]. However, the dripping waters in this case [2] interacted mainly with brucite, and therefore became enriched in Mg2<sup>+</sup> but not in Ca2<sup>+</sup> (0.36 to 1.02 mg/L) or Si4<sup>+</sup> (0.0 to 9.3 mg/L) which prevented the precipitation of aragonite and kerolite. At Montecastelli, instead, dripping waters interacted prevalently with serpentinized harzburgite (Figure 4 and Table 3), and in a minor amount with gabbro lenses which outcrop in the area, therefore, they became relatively enriched not only in Mg2<sup>+</sup> but also in Ca2<sup>+</sup> (5.4 to 8.7 mg/L) and Si (13.6 to 32.4 mg/L).

An alternative model has been elaborated considering the water film on the adits' walls that could have been derived from evaporation of dripping and pooled waters, as well as by ingress of humid air from outside. The chemical composition of this "condensed" water is assumed corresponding to a distilled water (Mg2<sup>+</sup> <sup>≤</sup> 0.01, Si4<sup>+</sup> <sup>≤</sup> 0.01, and Ca2<sup>+</sup> <sup>≤</sup> 0.01 in mg/L) in equilibrium with pCO2 of the air in the mine. We hypothesized that the condensed water became enriched in ions after the dissolution of serpentine fines and the equilibration with pCO2 of the air. The mineral paragenesis, dissolution reactions, and equilibrium constants (log k) at 25 ◦C and 1.013 bar are listed in Table S3. Our model shows that the condensed water on the mine walls after interacting with the serpentine fines would have a content of Mg2<sup>+</sup> ~22 mg/L, SiO2 ~36.2 mg/L, and HCO3 − ~105 mg/L, with a pH of 8.38. This fluid is undersaturated in hydromagnesite (SIhmg ~–9.02). Hydromagnesite supersaturation can be slowly reached by evaporation (up to 85%, SIhmg ~0.81), whereas aragonite cannot precipitate due to the absence of Ca2+. Therefore, this process can explain the observed wall coating paragenesis, where hydromagnesite is the first mineral to precipitate.

#### *5.2. Interaction between Condensed Water and Mine Wall Serpentinite*

Even though the dissolution of serpentine could provide the cations required for the precipitation of the wall coating paragenesis, serpentine is known to have low solubility at low-temperature conditions [25–28]. This is confirmed by our microscopic investigations showing that there is no evidence for the dissolution of serpentine in the substratum of the carbonate crusts (Figure 8). However, we have observed a discontinuous layer of serpentine fines at the interface between mine walls and carbonate crusts in all the studied samples. The presence of a layer of serpentine fines on the mine wall is most likely a consequence of the shafts and adits excavation. Underground excavation in the early half of the XIX century was done using dry jack hammers which generated a high amount of powder from the serpentine rocks which coated the adit walls and was progressively glued by condensing waters.

It is known that the dissolution rate, and reactivity, of serpentine with CO2 is strongly enhanced by the increase of the reactive surface area [27,28]. Therefore, serpentine fines represent an efficient reagent from which CO2-rich condensed water could have progressively stripped Mg and Si required for the formation of the wall coating paragenesis. As observed by FEG-SEM, both the serpentinite fine layer and the new hydromagnesite crusts are highly porous (Figures 7 and 8). They could play as a capillary-drying system where the condensed water infiltrates through the porosity, mostly uptaking Mg and Si along the surfaces of the serpentine particles and crystallizing new minerals (hydromagnesite or kerolite) on the outer surface of the fine layer and the previously crystallized crusts.

Such an interpretive model can be further discussed considering the boundary layer effect in the water film forming on the external surface of the crust. Here, the hydromagnesite precipitation induces a Mg-depleted boundary layer. The higher Si/Mg ratio could temporarily stabilize the precipitation of a kerolite band. The interplay between dissolution/crystallization and diffusion kinetics could generate the complex hydromagnesite-kerolite banding. Condensed water could also propagate into small fractures from the rock surface dissolving chrysotile. In this case, the chemistry of the resulting fluids would be comparable with that resulting from the interaction with the serpentine fines. However, considering the aerial extension of these veins, the overall effect contribution to the carbonation process is expected to be limited as compared with the fines layer. Therefore, we consider the spontaneous carbonation in the Montecastelli mine mainly driven by the presence of the serpentinite fines.

#### *5.3. Seasonally Driven Deposition of the Carbonate Crusts*

Geochemical modeling showed that precipitation of wall carbonates is driven by evaporation of modified condensed water on the adit walls. The most peculiar feature of the crusts is their distribution along both sides of the adit walls. Adit walls have an irregular surface with peaks and throughs of variable sizes resulting from their excavation. These irregularities produce a continuous alternation of rock surfaces, which are facing inward and outward from the adit entrance. The carbonate-kerolite crusts are present only on the outward looking faces (Figure 6d).

Considering that the mine environment has a stable temperature and humidity, such an asymmetric distribution needs to be explained by a selective depositional process. Seasonal changes of air circulation inside the mine can explain this observation because they can trigger the preferential evaporation of condensed water on the outward looking surfaces. As described before, during summer the relatively colder and heavier air inside the mine flows out from the lowermost level, drawing the hot and dry air from outside into the upper adits. Contrarily during winter, the relatively warm and light air inside the mine flows out from the upper levels, while drawing in colder and humid air from the outside through the lowermost adit. In the latter case, incoming air becomes progressively more humid flowing through the flooded lowermost adit. Therefore, the optimal condition for selective evaporation of water condensed on the outward looking surfaces of the upper adits is attained during summer because the air flow is dry (Figure 1). Instead, winter air circulation is dominated by humid air, which cannot promote evaporation of condensed water (Figure 1).

Typical carbonate-hosted caves at similar midlatitudes behave differently. Seasonal cyclicity in temperature and air density, coupled with enhanced CO2 production in soils during the warm season, is commonly responsible for the relevant summer CO2 buildup in cave air and the inhibition of calcite precipitation by elevated CO2 levels in the cave waters. Maximum speleothem growth occurs during the cold season when the airflow inversion introduces CO2-poor air in the cave and the CO2 level in cave water is strongly reduced [22–24] The reverse behavior at the Montecastelli mine is easily understood considering the different precipitation process (evaporation-condensation-evaporation) and the peculiar topographic and geologic characters of this artificial underground system that prevent extreme CO2 buildup during the warm season.

The interaction between condensed water and serpentine minerals on the wall surface can be summarized as by Equation (1), that is compatible with the precipitation of alternating bands of hydromagnesite and kerolite:

$$\begin{aligned} \text{(6\text{Mg}\_3\text{Si}\_2\text{O}\_5(\text{OH})\_2\text{ (Serpentite)} + 4\text{HCO}^-\text{}\_3 + 5\text{H}\_2\text{O} \rightarrow 3\text{Mg}\_3\text{Si}\_4\text{O}\_{10}(\text{OH})\_2\text{-H}\_2\text{O (Korolite)} + \text{Mg}(\text{CO}\_3)\_4(\text{OH})\_2\text{-H}\_2\text{O} \ (\text{MnO})\_2\\ \text{Mg}(\text{CO}\_3)\_4(\text{OH})\_2\text{-4H}\_2\text{O (hydroagnosticite)} + 4\text{Mg}^{2+} + 2(\text{OH})\_2 \end{aligned} \tag{1}$$

The presence of small amounts of aragonite can be related both to the partial dissolution of Ca-rich minerals, present in very minor amount in the serpentinites. Kerolite precipitation is thought to be promoted by low temperature, high pH, and high silica and magnesium activity [20]. Saturation of amorphous silica at high pH (>8.2) has also been proposed as a key condition for the precipitation of Mg-silicates [18–20]. Condensed waters in the Montecastelli mine has high pH (~8.9), high Mg content (>70 mg/L), and high dissolved silica activity, which are required for the precipitation of kerolite, and thus further supporting the genetic process.
