**2. Geological Background and Topographic-Microclimatic Characteritics of the Mine**

Southern Tuscany is characterized by several outcrops of ultramafic and mafic rocks belonging to the Ligurian units. These comprise serpentinized harzburgites and dunites, gabbros and basalts, with their original sedimentary cover [1,9–11]. A large ophiolite body outcrops near the village of Montecastelli Pisano. Here, the Pavone river has eroded a canyon through the ultramafic units exposing outcrops of harzburgite and dunite which locally have been strongly carbonated [1]. The Montecastelli ultramafic body has been affected by the following two main stages of deformation: (i) early ductile shearing recorded by mylonitic gabbros and (ii) late brittle deformation with the development of cataclastic zones and the recrystallization of serpentinite-gabbro-basalt assemblage. A major cataclastic zone, which runs through the entire ultramafic body WNW–ESE, hosts a reworked Cu-Fe sulphide mineralization, mainly represented by bornite, chalcopyrite, and chalcocite in veins and nodules [12–14]. The deposit was intermittently explored and mined between 1832 and 1942 through the excavation of small open pits and underground works, providing a negligible, total production of 40 tons of Cu-Fe sulphides.

The Montecastelli copper mine (43◦16.4' N; 10◦56.7' E) consists of about 1500 m of adits and drifts distributed over three levels from 195 m to 245 m above sea level (a.s.l.) connected by a main shaft and smaller inclined shafts (Figure 1). The three main adits have been excavated in serpentinized harzburgite and dunite reaching a NW-trending mineralized cataclastic zone from which the surface (ca. 320 m a.s.l.) plunges towards NE. The upper part of the cataclastic zone (from 320 m down to 245 m) is poorly mineralized and there are no underground works directly connecting the uppermost exploitation drifts to the surface. The entrance of the uppermost level (Santori adit) is located at 245 m a.s.l and it is connected by an inclined shaft to the intermediate level (Isabella adit, 215 m a.s.l). Both adits are excavated through serpentinized harzburgite, with a progressive increase of serpentinized dunite and gabbro lenses towards the mineralized cataclastic zone.

A vertical internal shaft connects the Isabella adit to the lowermost level of the mine, the Ribasso adit (205 m a.s.l), which emerges 700 m further north in the Pavone valley. Most of the drifts excavated in the mineralized cataclastic zone have collapsed due to the soft and soapy character of the ore body (chlorite-, serpentine-rich); hence observation is possible only in few stopes. The Isabella adit offers

the best and safest exposures of the ultramafic host rocks, crosscutting about 200 m of serpentinized harzburgite and reaching the footwall of the mineralized cataclastic zone. The intermediate part of this adit, between the entrance and the main internal shaft, displays walls and ceiling covered by white to creamy-white crusts composed mainly of carbonates (Figure 2). Something similar also occurs in the first part of the Santori adit. In particular, the whitish crusts are noticeable looking inward into the Isabella and Santori adits (Figure 2A); whereas they are not visible looking outward from the adits (Figure 2B). The Ribasso adit is flooded and observations there have been precluded. Although dripping water has been locally observed and sampled, there is not a direct spatial association between carbonate crusts and fractures discharging dripping water. Most of the walls covered by carbonates are homogeneously wet and they do not display any laminar water flow; they resemble cold surfaces covered by condensed water.

**Figure 1.** Cross section of the Montecastelli copper mine, showing the geological features and the overall air circulation pattern during the two main upward (winter) and downward (summer) airflow stages.

**Figure 2.** Isabella adit of the Montecastelli mine. (**A**) View of the adit looking inward where the white coating of hydromagnesite + kerolite and aragonite is visible and (**B**) view from the adit looking outward where the withe coating is not visible.

The hill hosting the mine is covered by a pine and oak forest and variably well-drained soils rich in organic matter. The soil allows a significant infiltration of meteoric water inside the fractured serpentinites, forming small confined aquifers which, in turn, slowly feed small perennial drippings along the underground walls, as well as small pools on the floor. The average external air temperature at Montecastelli [15] displays a typical seasonal variation with a maximum of 25 ◦C during August and a minimum of 6 ◦C during January and February (Figure 3). The maximum humidity of the external air and soil is reached during the rainy season, from October to the end of April, while the period between mid-June and late August is characterized by severe soil aridity. Air temperature and humidity inside the mine are relatively constant throughout the year (15 to 17 ◦C and 80% to 100%), with major fluctuations near the entrance of Isabella and Santori adits.

Owing to the configuration of the mining works, the Montecastelli mine can be classified as a dynamic underground complex [1,9–11] where ventilation is triggered by the so-called "chimney effect". The difference in elevation between the uppermost adits (Santori and Isabella) and the lowermost adit (Ribasso) triggers the air circulation, which changes seasonally. During the summer, the relatively cold air inside the mine flows out from the lowermost adit, drawing in warmer and drier air from the outside through the uppermost levels (downward airflow, Figures 1 and 3). Contrarily, during the winter, the relatively warm air inside the mine flows out from the uppermost adits (upward airflow), drawing in colder and humid air from the outside through the lowermost level (Figures 1 and 3). The direct, on-site observation of stagnant circulation during April to May and during October (weak or zero airflow), complies with the intersection between the annual cycle of external air temperature and the almost constant air temperature in the cave (Figure 3).

**Figure 3.** Diagram showing the annual cycle of external air temperature (average mean, maximum, and minimum), the air temperature in the cave, and other meteorological data with respect to the microclimatic dynamic conditions observed in the underground system. Meteorological data for Montecastelli and Castelnuovo as well as other data and information from this study are from the Tuscan Hydrologic Survey [15].

In summary, the Montecastelli underground system behaves like many multiple entrance karst undergrounds placed at midlatitudes, but with a major difference of decoupling between air and water circulation. Airflow is strictly controlled by the topography of the adits/shafts network coupled with the annual cycle of external air temperature, while the lack of large vertical fractures in the overlying rock mass (typical of carbonate-hosted karst systems) precludes the direct arrival of CO2-enriched air during the spring and summer time. Contrarily, meteoric water interacting with soils above the mine can dissolve an aliquot of soil CO2, bringing it into the mine by slow infiltration through a network of tiny fractures. The lack of a massive influx of CO2-rich air during the warm season is pivotal to the understanding of mechanisms of serpentinite carbonation along the Isabella adit.
