The acid-sulfate zone and the mineral alteration styles of the Roman Puteolis (Neapolitan area, Italy): clues on fluid fracturing progression at the Campi Flegrei volcano

Active fumarolic solfataric zones represent important structures of dormant volcanoes, but unlike emitted fluids, their mineralizations are omitted in the usual monitoring activity. This is the case of the Campi Flegrei caldera in Italy, among 10 the most hazardous and best-monitored explosive volcanoes in the world, where the landscape of Puteolis is characterized by an acid sulfate alteration that is active at least since Roman time. This paper provides temperature, mineralogical, textural, compositional and stable isotope data for those solfataric terrains sampled at the crater and Pisciarelli slope of the Solfatara volcano between 2013 and 2019. Temperatures vary between 40 ° and 95 °C. Minerals include alunite with grain sizes generally larger than 20 μm, alunogen, native sulfur, well-ordered kaolinite, and, common at Pisciarelli, pyrite, illite and NH415 sulfates. Sulfate terrains have higher contents of Ti, Ba, Au, As, Hg and Tl relative to their parent substrate. The Pisciarelli slope is anomalous in terms of the presence of NH4. δS values for sulfides and native S range between -3.00 and 0.49 ‰ and from -4.42 to 0.80 ‰, respectively. Sulfates show δ S and δ O values in the range of -3.35 to 3.80 ‰ and between 0.3 and 31.33 ‰, respectively. The style of mineralization and the stable isotope geochemistry do produce complex and not completely consistent classifications and genetic constraints. We merge our data with volcanological information, data from exploration 20 drillings and geophysical results. With the conceptual model, we suggest a series of shallow and deep aquifers interconnected like “communicating vessels” through a main fault system that downthrows Solfatara with respect to Pisciarelli. Fluid outflow from the different discrete aquifers hosted in sediments – and possibly bearing organic imprints – is the main dataset that allows determination of the steam-heated environment with a supergene setting superimposed. Supergene conditions and highsulfidation relicts, together with the narrow sulfate alteration zone buried under the youngest volcanic deposits, point to the 25 existence of an evolving paleo-conduit. The data will contribute to monitor and evaluate the volcanic hazards.

160 °C (e.g., Cardellini et al., 2017;Gresse et al., 2017), in agreement (or locally lower respect) with measurements reported by Sicardi (1959) in the range of 141-215 °C at Bocca Grande andof 99-110 °C at other sites, between 1921 and1951. Pisciarelli waters exhibited temperatures mostly around 95 °C with minimum values of 84 °C between 1978 and 1999 (Celico, 1992;Valentino and Stanzione, 2004). A geyser-like vent at Pisciarelli has a temperature of up to 116 °C (Chiodini et al., 5 2016). Only, the mud pool ("La Fangaia"; Fig. 1d) was hotter in the past with values up to 100 °C (Sicardi, 1959). This author describes a mud vent in the southeastern area of the crater that is no longer present.

Sampling, sample preparation and analytical techniques
Sampling was conducted within the Solfatara crater and in the Pisciarelli and Cinofilo areas (Fig. 1a,d,e) with additional sites compared to Piochi et al. (2015); the crater floor, except the pool, was intentionally avoided because the reworking in historical time (Photos-Jones et al., 2016) and thus a possible anthropogenic contamination. This study enlarges the dataset on the acid 25 sulfate alteration zone of the Phlegraean area, in order to understand the quiescent dynamics of the volcano. Similar observations and data are also available for Ischia island (Piochi et al., 2019) that belongs to the Phlegraean Volcanic District (Piochi et al., 2005).
Our new collection is, therefore, widening the observation period for the Puteolis sulfate area that now spans between January Furthermore, amorphous phases are largely present at various sites (Table S1, Table 1), particularly, in muds and in the samples from Bocca Grande and L1 vent (Fig. 1d,e). The widespread amorphous phases could correspond to material from the both original volcanic rock and alteration. General assumptions (Piochi et al., 2015;Montanaro et al., 2017) indicate amorphous silica, although it merits a more rigorous examination. 5 Finally, Fe-oxide, and fresh to variably altered feldspar and biotite are the most common primary volcanic mineral phases.
DRIFT-FTIR spectra collected on selected samples (Fig. S2) produce data consistent with XRDP results (Fig. S1) and furthermore allow useful details on structure and eventual minor phases or impurities (Supplement). Table S2 lists the relevant vibration modes of spectra and the proposed mineral assignments. The crystals formed by evaporation of water in the Pisciarelli pool ( Fig. S2a; Supplement) show a sharp band at 1422-1411 cm -1 that is in the region of the ν4(NH4 + ), as described in the 10 literature (e.g., Weis and Ewing, 1996;Parafiniuk and Kruszewky, 2010) and in agreement with XRDP mineral data (Table   S1; Fig S1). Native S from two different samples (PINT S tozzo 18/10/17 and PINT S 18/1/18 in Table 1; Fig. S2b) is evident in the DRIFT-FTIR spectra at < 2950 cm -1 , with the strongest bands at 843 and 468 cm -1 that coincide with those of sulfur in the both the USGS (see Sulfur GDS94; Clark et al., 2007) and RRUFF (http://rruff.info/) databases. The spectra differ in the OH stretching 15 region, likely indicating the occurrence of impurities, although water absorption by the KBr solution can be a further possibility.
As expected (Clark et al., 1990), alunite can be determined through its major band at 3483 cm -1 coupled with a smaller one at 3513 cm -1 in the OH-stretching region (Fig. S2c,d,e). Accordingly, the spectra show a small band at ca. 4605 cm -1 from the Al-OH combination mode that should be ascribed to alunite. Furthermore, it is possible recognizing the doublet mode at 1088 20 ÷ 1099 cm -1 and at 1028 ÷ 1025 cm -1 (Toumi and Tlili, 2008) and the mode at 3971÷3978 cm -1 (see USGS database; Clark et al., 2007) from alunite. However, in the OH-stretching region there are some other vibrations. Based on Madejovà and Komadel (2001), illite is likely producing the vibration at 3622 ÷ 3629 cm -1 .
The new pool at Solfatara characterizes for peculiar DRIFT-FTIR spectra in the OH-stretching region ( Fig. S2e; note the inset) due the presence of kaolinite, in addition to alunite, and minor (or occasional) S, feldspar, pyrite and amorphous phases.
Specifically, these are i) alunite (Clark et al., 1990) with a major band at 3483 cm -1 coupled with a smaller one at 3510 cm -1 , and ii) kaolinite (Madejova, 2003) with two minor bands at 3667 and 3651 cm -1 between two major vibrational modes at 3695 30 and 3620 cm -1 . Accordingly, it is possible recognizing the OH-deformation at ca. 915 and 938 cm -1 , the Si-O stretch at 1008 and 1026 cm -1 , and the Al-OH modes at ca. 4605 cm -1 and 4523 cm -1 from kaolinite.

Stable isotopes of sulfur and oxygen (SGI)
A new set of δ 34 S and δ 18 O values for sulfur-bearing minerals is listed in Table 2. δ 34 S values of sulfides and native S range between -3.00 and 0.49 ‰ and between -4.42 and 0.80 ‰, respectively. Sulfates are characterized by δ 34  studies earlier than 2000 (Cortecci et al., 1978;Valentino et al., 1999) also show positive δ 34 S values (Fig. 5a,b). In comparison to previous studies: 1) our new S-isotope data for Pisciarelli include few positive values (Fig. 5b,c).
2) the new O isotope values for sulfate are the highest obtained until now (Fig. 5d). To note, the muds generally have the least 5 heavy oxygen isotopes, except samples from 2013 -2014 for which O-isotope determinations are lacking. The diagram also indicates a lowering in δ 34 S for neoformed minerals reported later than 2000.
3) the sulfides at Pisciarelli show δ 34 S values mostly at 0 ‰ (Fig. 6). 4) the different sites display a homogeneous range in δ 34 S (Fig. 5a,b,c); however, the variability for the ASA and L1 sites reflects different sample heights along slope (Fig. 1d) and wall (Fig. 1e), respectively. 10 5) a likely appearance of a positive correlation between S isotope results for pyrite and for sulfate phases coexisting at Solfatara, with two from the 1994 data outside the trend (Fig. 6a). 6) new isotope data for sulfate reveal a difference compared to studies older than 1990 and the most recent one (Fig. 7).
In addition, the δ 18 O values for sulfates at Pisciarelli show a slight tendency to increase with topographic elevation at the sampling site. Specifically, the highest values generally occur at Solfatara that is at > 96 m asl while Pisciarelli lies at 66 -74 15 m asl.

Content of major and trace elements (WRG)
Table S3 reports the whole-rock geochemical compositions of selected samples. As expected, samples are highly hydrated and sulfur-rich, due the OH-group and/or S in the crystalline network (see ideal formula in Table 1), and/or the presence of native S in the analysed sample. LOI can be up to 80 wt% (sample L20 camino 18.9.17), although most commonly at 20-30 20 wt%, and the S content is up to more than 50 wt%. Carbon is generally low (< 1.25 wt%), and always < wt 22 %. Notably, the SiO2 content is highly variable. Depending on the sample mineralogy, it can be as high as 70-80 wt%. MnO is always < 0.1 wt.%.
With respect to the local volcanic rock substrate (Table S3; Fig. 8a), some lithophile elements (Si, Al, P, Sc, Ti, V, Zr, Ba, Yb, Th, Hf) are comparable or depleted (for examples, Be at < 1-5 ppm vs > 4 ppm, Ga < 23.8 ppm, Rb < 95.6 ppm, Y < 8.1 ppm). 25 Depletion concerns with Na, Mg, K, Ca, Rb, Eu, U content. Only Ba displays a significant level of enrichment reaching very high values up to thousands of ppm. Cs shows concentrations of up to tens of ppm. Siderophiles and chalcophiles ratios (Fig. 8b) mostly lie at 1 or slightly above, with depletions for Fe, Zn, Ga, and enrichments for S, Au, As, Tl, Hg (Table S3). Sb can be higher than the primary rock composition.

Environmental sub-zones: stationary phenomena and runoff processes
Throughout the years, the various Solfatara and Pisciarelli sampling sites display a nearly constant mineral alteration assemblage (Table 1). Commonly, the mineral neogenesis variably develops on sub-mm-to dm-to m-scales, in relation to the outgassing dynamics, runoff, weather conditions, outcropping substrate, and anthropogenic activity. 35 However, the various sites further display reproducible rock geochemistry and stable isotope compositions at the timescale of survey, and with respect to oldest data (e.g., Valentino et al., 1999) as well; they can be considered reference points for future investigations.
Based on the presented dataset, we propose the existence of major alteration sub-zones, in which some (minor/peculiar) mineral phases appear or disappear, in response to changing physical-chemical boundary conditions mainly associated to weather 40 mineralogy, rock chemistry, and isotopic compositions, and characterized by temperature variations in a narrow range. Such a constancy is revealed when comparing results reported by Sicardi (1959) (see Geological setting at 2.1 section) with the present results, corroborating the existence of "stationary" sub-zones that are presented in the following. The only exception is the mud pool in the crater. 5 The Pisciarelli and Solfatara pools (Fig. 1d,e) are the two major and distinct sub-zones. They display persistent differences in dynamics, temperature and mud (solute plus water) mineralogy. The main pool at Pisciarelli shows vigorous boiling (Fig. 1b,e), with temperatures ranging from 63.9 to 94.3 °C (Tables S1 and 2) and a relative dominance of water vapour. The mud is typically grey in colour, mostly ash to sand up to mm-sized grains, with generally rounded/smoothed shapes as a function of the boiling vigour and material supply from the nearby slopes. On the other hand, the main Solfatara pool is characterized 10 by degassing with a temperature at around 49 °C (Tables S1 and 2). The mud is beige and fine grained, always with a fetid odour. We recurrently detected an enrichment in pyrite, illite and feldspar at Pisciarelli and in native sulfur at Solfatara. The latter is enriched in As, Hg, Nb, W, Zr, Sb and depleted in Sr, Ba, Co compared to the original deposits (Fig. 8). Sulfides and sulfates display nearly similar δ 34 S values at Solfatara, whereas they are different at the Pisciarelli mud pool (Fig. 6a, inset).
The DRIFT-FTIR spectra of muds from Pisciarelli, in contrast to those from Solfatara, always gave the vibration at 1430 cm -15 1 related to the NH4 (see section 3.1). Notably, the La Fangaia mud pool (Fig. 1d) is likely cooling as Sicardi (1959) reported up 100 °C, several tens of degrees higher than at the present (Tables S1 and 2). A slight cooling is discernible when comparing our data (Table S1) with those in the literature (Martini et al., 1991;Celico et al., 1992;Valentino and Stanzione, 2003). In contrast, the Pisciarelli area should be hotter, although only by a few degrees Celsius (Tables S1 and 2) if compared with increasing temperature values at the geyser-vent (Chiodini et al., 2016). 20 The PINT-PEXT sub-zone (Fig. 1e) -an isolated morphological heightis composed of an alunitic-rich low-cohesive reddish terrain with a temperature around 95 °C, typically comprising kaolinite (Tables S1 and 2). The kaolinite is easily discriminated also by DRIFT-FTIR spectra (not shown) that, in agreement with XRDP traces, point for a well-ordered structure. This terrain bears variably sized (up to few dm) clasts and is subject to slumping and sinking.
A hole up to 2-3 m deep represents a distinct sub-zone that we emphasize because it opened 180 m North from the main pool 25 within the crater in May 2017, by surface collapse. A grey viscously boiling mud fills the hole (Fig. 1c,d), with a minimum temperature of 70 °C; a temperature of 91 °C was measured in April 2019. The mud is dominated by alunite with subordinate native sulfur, showing vague similarity to Solfatara mud (Table S1), although its grey colour and the occasional pyrite are reminiscent of the Pisciarelli mud. However, kaolinite is the main clay mineral in the new pool ( Fig. S1e; Fig. S2e, note the inset). Mud geochemistry reveals peculiarly high concentrations of Sr, Ga, Co, Th, V, REE, Sb (Fig. 9). 30 Finally, a rather broad sub-zone includes the other various sampling sites that are characterized by encrustations of alunite with a well-defined, although relatively large, range of δ 34 S values between ca. -4 and 1 ‰ (Tables S1,2; Fig. 5c). These occurrences reflect the nearby presence of vapour degassing. We suspect that those at the ASA, M. Olibano and SSt sampling sites along the slopes of the Solfatara crater (Fig. 1a,d) are ascribable to long-lived encrustations; further investigations are useful to ascertain this suspect. The minor fumarolic vents around the old baths (Sst site) seem unchanged in respect to the 35 description by Sicardi (1959), particularly showing the occurrence of native S and a comparable temperature at around 95 °C.
These show variable bulk-rock geochemical compositions.
Widespread alunite formation reflects the potassium and feldspar-rich rock substrate on which they develop (see Piochi et al., 2014 and references therein).
Vapour effluents around the various geysers/vents at Pisciarelli are the most important factors affecting the mineral neogenesis 40 at the alunite-dominated sub-zones. Pisciarelli is a decametres-deep incision on the NE Solfatara slope and the degassing vents are constrained in a gorge-like morphology. This setting favours the stagnation of the hydrothermal steam that impregnates the rock substratum and supplies elements to the formation of a variety of Na and NH4 sulphates. We detected high abundance as droplets in the humid air, as revealed by experimental desiccation tests of the water. Aerosol particles from inside and nearby the Solfatara crater that bear NH4 + (and Cl -, possibly in the form of NH4Cl) as major ion (Mather et al., 2004), as well as the NH4Cl inside the BG and BN orange-yellow encrustations (Fig. 3h,i), furthermore support the widespread presence of ammonium species. Notably, N2 is emitted at the main intra-crater fumarole of Bocca Grande (see Chiodini et al., 2010). 5 S1a,d), however, point to the presence of NH4 in both the mud and water pools at Pisciarelli, but not at Solfatara (except for BG and BN). Alum -(K) has also been found in relation to the wet conditions at Pisciarelli.
Vapour emissions outflow and the conditions of hydrothermal steam stagnation are dependent on atmospheric pressure and wind conditions. 10 So that, it is likely the meteoric weather the main cause for appearance to disappearance (and vice versa) of some phases. This is also particularly evident for the PINT, PEXT, L19, L20 (Fig. 1e) and the SMO, ASA, SSt (Fig. 1d) sub-zones that may typically present bipyramid and/or fine dendritic sulfur crystallites (Fig. 2a). Their crystallization seems to be favoured by relatively strong exhalations and porous terrain (PINT, PEXT, L19, L20; Fig. 1e) or conditions where gases remain briefly trapped (SMO, some places along ASA, Sst; Fig. 1d). Respective conditions also prevail in close proximity (< dm scales) to 15 the main fumaroles (L1 vent, BG, BN; Fig. 1d), where sulfur forms a cream-like patina reflecting the condensing gas flow along the host fractures and fissures.
However, native S disappears during run off and we have macroscopically tested at several places that re-crystallization needs one-to-two months, if not longer (i.e., sample L20 camino; Fig. 1e).
Furthermore, periods of intense rainfall determine the timing areal extent and depth of the mud pools, as well as the generation 20 of secondary mud vents and the erosion in Pisciarelli and its periodic water puddle. Sicardi (1959) already noted the occurrence of mud vents and black mud pools following rainy periods. Notably, pools at Pisciarelli are supported by anthropogenic embankment.
Meteoric and surface waters can dilute the aggressive endogenous fluids determining alteration degree conditions low enough for the generation of iIllite, or other clays (Pirajno, 2008) at Pisciarelli. Further studies need to be performed in order to better 25 characterize clays as they can bear information useful to further constrain the hydrothermal setting.
The distribution of sulfates appears irregular, and this should be an object of future investigations.

Classification of alteration and genetic environments: the contradictory data
The style of mineralization (Arribas, 1995;Sillitoe, 1993;Pirajno, 2008;Ercan et al.; and the stable isotope results (Rye 30 et al., 1992) allow classification of alteration and differentiation of genetic environments. Table 3 summarizes characteristic mineralogical, lithological and isotopic features of these environments, in comparison to observations made at the study sites.
Several contrasting interpretations can result from the data.
Alunite plus kaolinite form in steam-heated environments at 100 to 160 °C where fumarolic vapour condenses above the boiling zone of nearly neutral-pH, H2S-rich fluids representing a low sulfidation environment. 35 Nevertheless, alunite shows grain sizes in the range of 50 to 100 µm (Fig. 3a,e,f), unlike the finest (<20 µm) ones detected in high-temperature (>40-50 °C) steam-heated (Simón et al., 2005) or supergene (Arribas, 1995) environments. Those coarser sizes usually occur in low-temperature steam-heated (and hypogene, as well) environments (Hedenquist et al., 2000), or could directly crystallize from a SO2-rich magmatic vapour that rapidly ascent through fractures (namely high sulfidation setting; Rye et al., 1992;Stoffregen and Alpers, 1992). The occurrence of kaolinite and alunite at several Solfatara and Pisciarelli sub-40 zones (particularly, new pool and PINT, Table 1) fits with the high sulfidation environment; in fact, the two phases usually coexist in the advanced argillic alteration zones proxy to ascent plumes (e.g., Pirajno, 2008). The evidence of K-feldspar support the classification as high sulfidation -magmatic hydrothermal environment (Rye et al., 1992). Nevertheless, illites/montmorillonites are most common in intermediate argillic alteration zones (Pirajno, 2008) and their widespread occurrence in the various studied sites, as well as in the local subsurface (Valentino and Stanzione, 2003;, is among the results that contrasts this high sulfidation environment. Indeed, sampling temperatures are higher than 40 °C (Tables 2 and S1)  5 and SO2 is rare or absent (Aiuppa et al., 2013) at Campi Flegrei. Also, the lack (or rare detection) of lower temperatures (< 40 °C), disordered polymorphs (i.e., halloysite) point to limited supergene alteration.
Only, the alunite coexisting with kaolinite in the new hole pool exhibits the finest grain size. Accordingly, the XRDP and DRIFT-FTIR analyses of CF samples point to slightly ordered kaolinite forms that usually occur at temperatures < 150 °C in epithermal systems (Sillitoe, 1993), but could also represent a metastable form in hotter settings (Zotov et al., 1998). In this 10 later case, the new pool sub-zone with kaolinite and alunite can represent local, well-circumscribed advanced argillic alteration condition indicative of a proxy plume.
However, when considering litho-geochemical parameters, schematic diagrams further produce contrasting visions. For example, following Ercan et al. (2016), the clay-bearing muds can be ascribed to variable supergene to hypogene alteration field in the binary diagram of immobile Zr vs. TiO2 (Fig. 9a). They also have high (above 1,000 ppm) Ba+Sr and low (<200 15 ppm) Ce+Y+La concentrations (Fig. 9b).
The stable isotope geochemistry of minerals supports an interpretation of steam-heated to supergene environments (Fig. 7). Sisotope equilibrium occurs between sulfides and sulfates, with reliable re-calculated temperatures in high-sulfidation environments (Arribas, 1986). In contrast, this equilibrium cannot be accounted for at Campi Flegrei and any reliable temperatures result from the S-isotope fractionation between sulfates and H2S. In fact, δ 34 S values of alunite and pyrite roughly 20 overlap. Instead, sulfur-H2S and sulfides-H2S at the sampling temperature reflect equilibrium S-isotope fractionation: the theoretical δ 34 S value of dissolved H2S fluid is between -3.84 and -0.84 ‰ (Allard et al., 1991). This also implies that the sulfate altered rocks are not magmatic hydrothermal in origin, in agreement with the lack of typical mineral phases that show a high oxidation state of S (SO2-rich, high-sulfidation according to Henley and Ellis, 1983). Based on Rye et al. (1992), SO2 disproportionation results in the formation of 34 S-enriched H2SO4 and 34 S-depleted H2S. In addition, the intense Al-leaching 25 in a high sulfidation system is not typical for the Campi Flegrei setting ( Fig. 8; Table 3).
Actually, Campi Flegrei lacks the occurrence of enargite and luzonite, both diagnostic for high-sulfidation environments.
Significantly lower δ 34 S values (< 15 ‰) for alunite can derive from: (i) the light sulfur isotopic composition of H2S during 30 boiling (steam-heated or low-sulfidation setting), (ii) δ 34 S of sulfides (supergene) or (iii) the bulk sulfur isotope composition of magmatic steam (Rye et al., 1992). The first possibility could partly account for the isotopic composition of alunite-pyrite and alunite-H2O pairs (Fig. 10), although contrasting with conclusions based on texture, mineral assemblage and bulk-rock geochemistry ( Table 3). The presence of kaolinite in the subsurface, under an alunitic cover and the occurrence of argillic alteration at depth suggest a vertical zonation at the Solfatara crater and, following Rye et al. (1992), a steam heated setting. 35 Furthermore, δ 34 S values for the pyrite-H2S pairs further support a supergene setting (Fig. 10). Finally, we are not able to directly identify any microbial sulphur cycling, although FT-IR and rock geochemistry corroborate the absence or limited biota contribution. The analysed samples do not exhibit bands attributable to C=H ligands (Supplement) and the carbon content is <1.25 wt% (most common < 0.2 wt%; Table S3). Yet, some higher δ 34 S and δ 18 O values for sulfates could be indicative of microbial sulfur cycling, particularly considering the dry-wet alternating conditions. 40

Merging information and unravelling the setting
Merging all available information, it appears that observations concerning the both an apparent "stationarity" sub-zones' and a seemingly contradictory classification environment reflect the evolving conditions that have followed the last magma intrusion and eruption and that probably are overlapping through time.
The solfataric alteration zone has a strongly limited extent within the central sector of the Campi Flegrei caldera. It coincides 5 with the area of eruptive vents (e.g., Mt. Olibano, Accademia, Solfatara; Fig. 1a) and uplift of the most recent period of volcanism . The zone appears to be limited under the later Fossa Lupara and Astroni vents, while outgassing and thermal aquifers occur within the caldera. However, there is an indication for their discrete, more than their continuous distribution, both across the caldera and through depth (Guglielminetti, 1986).
The alteration zone locally presents high Ti, Ba, Au, As, Hg, Tl, S concentrations relative to the above parent basement lithology (Fig. 8).
The zone also appears anomalous in terms of ammonium content. Therefore, we here adopt ammonium as a possible tracer, but we have no information yet about the various contributing sources for the N-species and the cycling of nitrogen at the local 15 scale. The presence of NH4-bearing sulfates is particularly abundant at Pisciarelli. Those sulfates systematically form by drying water collected at the various pools of the area, in relation to the abundance of nitrogen species (0.2-1 g/l) in those waters (Martini et al., 1991;Celico, 1992;Valentino et al., 1999;Holloway and Dahlgren, 2002;Valentino and Stanzione, 2003;Aiuppa et al., 2006). Actually, the NH4 + content in the shallowest Phlegraean ground waters is generally low (<0.03 g/l; Martini et al., 1991;Celico, 1992;Valentino et al., 1999;Valentino and Stanzione, 2004;Aiuppa et al., 2006). Yet, Mather et 20 al. (2004) measured a significant abundance of ammonium chloride particles/aerosols at the Solfatara crater. Some realgar encrustations sampled at BG and BN also associate with ClNH4 (Table S1; Fig. 3h,i).
The concentration of some metals and metalloids requires sources different from the parent basement. Anthropogenic contributions are obviously possible (Alloway, 2012), e.g. when considering that NH4 is generally attributed to agricultural (i.e., fertilizer) and urban soils. However, Hg and As have been detected at the main fumaroles with similar concentrations 25 today and in the last century (Ferrara et al. 1994;Bagnato et al., 2014). These fumaroles continuously emit H2S and CO2 (Allard et al., 1991;Aiuppa et al., 2013;Chiodini et al., 2016) and are the preferred location of crystallization of native S and alunite. NH4 + emissions are also present at Solfatara (Chiodini et al., 2010), although ammonium is of limited importance inside the crater waters (<0.001 g/l; Aiuppa et al. 2006) and crystallization of alum instead of NH4 + -sulfates has been obtained in the water from Solfatara pool. In agreement with these authors, the plausible source is the conventional geothermal reservoir. 30 High concentrations (20-100 g/l) of ammonium are reported in the deeper (>500 m; Carlino et al., 2012) aquifers at the Mofete wells, located on the western side of the Campi Flegrei (Chiodini et al., 1988). Those deeper aquifers are located within a sequence of tuffs and marine sediments also drilled by the CF23 well (1000-1200 m depth; Piochi et al., 2014;, i.e. nearby the solfataric area. In summary, we propose an environmental setting that merges all collected information (Fig. 11). Fluid outflows from discrete 35 aquifers hosted in sedimentsand bearing organic imprintsfeed the Pisciarelli site giving its ammonium peculiarity. Our purpose does not exclude the possible biological contribution that has been ascertained in the studied sites (e.g., Ciniglia et al., 2005;Glamoclija et al., 2004). However, marine strata and a volcano-clastic sequence intercepted by deep drillings (San Vito1, Mofete and CF23 wells; Rosi and Sbrana, 1987;Piochi et al., 2014) are considered as the key sediments for the NH4 species.
An additional supply can originate from the swampy sediments encountered in shallowest boreholes (de Vita et al., 1999) 40 located in the central sector of the caldera (Fig. 11), nearest to the study area. Results from cluster analysis of resistivity, Pwave velocity and density parameters, from Pisciarelli across the Solfatara crater (Di Giuseppe et al., 2017) lithologically constrain the model section. These authors highlight a sudden upraise up to ca. 1500 m depth of buried rocks through a tectonic structure just beneath Pisciarelli. The dislocated rocks are the fossiliferous marine and volcano-clastic sequences drilled across the caldera. The deep aquifer is represented by a confined body with a high electrical conductibility (logρ = 0.97 Ωm). At the deepest position in the model, we correlate thermo-metamorphic rocks with the brines characterized by logρ at 2.7 Ωm, Vp = 3800 m/s and ∆σ -38.8kg/m3. These physical features are indicative for the occurrence of voids/fracturing and the migration of gases. Therefore, we infer a deep source of gases emitted at the surface, which likely also indicates the location of the heat 5 source.
Shallow and deeper aquifers are interconnected via a network of "communicating vessels" through a fault system, allowing deeper and shallower water to mix and being expelled at Pisciarelli. This justifies an apparent persistence of thermal springs around the Agnano Plain also in presence of the desiccating lake described by Ventriglia (1942). It also supports the depth of the water table, being at a higher topographic position in the Solfatara area in respect to the surroundings (Bruno et al., 2007). 10 In the model, we further speculate that the acid-sulfate alteration zone at the Campi Flegrei is actually evidence of a paleoconduit. This is based on field observations showing that alteration deposits locally underlie the most recent eruptive units (e.g., Astroni) that are unaltered. Therefore, the texture of the mineral assemblage, the enrichment in some metals and the lithogeochemical parameters are relict of a "high-sulfidation system". The evolutionary dynamics within the conduit and, in particular, the water overflows from the aquifers alternating with runoff processes, explains the contradictory mineral 15 environments with superimposed intermediate and advanced argillic alteration.
At present, a steam-heated (or low-sulfidation) environment (as derived by most isotope data on alunites; see previous section) is developing in relation to the presence of aquifers and their chemical compositions. This is in agreement with previous studies (e.g., Aiuppa et al., 2017;Piochi et al., 2015;Gresse et al., 2017). Following Hedenquist and Lowensten (1994), this is also in agreement with the shift in δ 18 O at constant δ 2 H values of the emitting fluids (Caliro et al., 2007). Based on the δ 18 O values of 20 alunite, the recalculated environmental temperature is ≤ 200 °C (Fig. 8, inset a).
Furthermore, the presence of NH4 + is considered strictly connected to the surface environment and likely to organic/biological processes (Jaffe, 2000), which is consistent with S-isotope values of pyrite-H2S pairs, the heavier δ 34 S values suggesting microbial sulfur cycling and the supergene environment that is locally developing.

Conclusions and Outlook 25
The acid sulfate alteration zone at Pisciarelli and Solfatara is located in the sector of the Campi Flegrei caldera that was the most volcanically active area in the last 5 ka. The alteration zone includes discrete sub-zones with very constant mineralogy, temperature and chemistry, considering the studied time interval. Outgassing dynamics, weather conditions, and runoff are the most important factors affecting the generation of new mineral phases at the sub-mm-to dm-to m-scales.
The new minerals include alunite, alunogen, native sulfur, pyrite, kaolinite and subordinately mascagnite. 30 The limited areal extent of the alteration zone underlying the most recent unaltered volcanic units, its mineralization texture and style, the δ 18 O and δ 34 S values of S-bearing minerals, and the enrichment in Ti, Ba, Au, As, Hg and Tl, are attributed to the evolution of a paleo-conduit. Our mineralogical and isotopical results overlap with those in Valentino et al. (1999), favouring a stability in the hydrothermal dynamics over the past 20 years. The zone is anomalous in terms of the presence of NH4 + . These features result from the mixing between waters that overflow through a fault system intercepting discrete aquifers 35 supplied by surface water and deep fluids. Most of the alunite forms above the water table at a temperature ≤ 200 °C. At present, the dominant steam-heated environment coexists with local supergenic conditions. Based on presently available data, several key aspects await further investigations.
In particular, a detailed survey of the distribution of aquifers in the subsurface will foster our understanding of caldera dynamics and contributes to the debate existing between a "hydrothermal" (Moretti et al., 2017) vs. a "magmatic" (Cardellini et al., 40 2017) unrest. Assessing the composition and spatial extent of aquifers -also including the contribution from rain fall -is crucial are condensing in the shallower aquifer system (Valentino and Stanzione, 2003;Aiuppa et al., 2007;Caliro et al., 2007;Vaselli et al., 2011;Piochi et al., 2014;Chiodini et al., 2016). However, the ability to buffer these magmatic fluids clearly depends on the water availability in relation to the volume of fluid, with implications on the sourced magma volume(s) evolving/degassing at depth. On the other hand, the circulation of fluids in the subsurface, sourced from both the downward 5 surface infiltration and the ascent of deep fluids, would contribute to the pressurization that is evident through shallow seismicity as previously suggested (Saccorotti et al., 2007;D'Auria et al., 2011;Di Luccio et al., 2015). Most important, knowing the water availability in the subsurface is crucial for evaluating the volcanic hazard in an area dominated by phreatomagmatic events, such as the Campi Flegrei caldera (Rosi and Sbrana, 1987;Di Vito et al., 1999).
What causes the presence of NH4 + is still rather elusive. Based on Moretti et al. (2017), it appears useful for evaluating the 10 temperature-dependence of N2 vs. ammonia production and the relative role of hydrothermal vs. magmatic systems. We add the possible role of organic materials and bacteria, and atmospheric sources to the terrestrial cycle. Furthermore, ammonia NH3 is toxic (Fromm and Gillete, 1968) and this requires ascertaining its concentration level in an inhabited environment.
Finally, the Pisciarelli site appears suitable for studies related to biota and the origin and evolution of life. Here, the waterdominance, nitrogen richness, ≤ 200 °C temperatures and supergenic conditions are all considered important ingredients for 15 the formation of organic substances and the ultimate source of organisms (Jaffe, 2000). Consequently, this site could become a natural laboratory for investigating the complex organic-inorganic interface/relations through multidisciplinary collaborations among mineralogists, geochemists, petrologists, volcanologists, and biologists.

Appendix
Appendix A 20 XRDP and DRIFT-FTIR patterns were acquired at the Osservatorio Vesuviano (Istituto Nazionale di Geofisica e Vulcanologia, Naples, Italy).
The XRDP instrument was a PANalytical X'Pert equipped with a high speed PIXcel detector . The configuration includes Ni-filter, CuKα radiation, pyrolytic graphite crystal monochromator, 40 kV and 40 mA current, 3-70°, 2θ range, 0.02° steps and 8 s/step. X'Pert HIGH Score Plus computer program and JCPDS PDF-2 database allowed the 25 interpretation of diffraction patterns.
DRIFT was mounted on a Nicolet 670 NexusTM both by ThermoFisher Scientific S.p.a.. The FTIR comprises a heated ceramic (Globar) source, a 670 Laser unit, a KBr beamsplitter, and an MCT detector, constantly purged from a high-pressure Nitrox dry air and CO2-trapping 280 generator by Domnick Hunter. The OMNIC Data Collector 5.2© allows data collection and interpretation in the investigated range of 5000 -400 cm -1 (resolution: <0.1 cm -1 ). Data collection was conducted on KBr 30 mixed with 5to -10 % of sample by grounding in an agate mortar, following the background acquisition for the KBr powder.
Additional acquisition on no diluted samples allowed checking for possible hygroscopic effects, obtaining similar results.
The appearance, morphology and chemical composition of minerals were determined on selected samples prepared as opaque mounts coated by cord and rod graphite, by JEOL and ZEISS electron microscope (EDS-BSEM) facilities. The JEOL-JSM 5310, equipped with a Link EDS and a Inca 4.08 software (CISAG Laboratory University of Napoli Federico II), has operating 35 conditions of 15 kV accelerating voltage, 50-100 mA filament current, variable spot size and 50 s net acquisition time. ZEISS instrument is a SIGMA field emission scanning electron microscopy (Osservatorio Vesuviano, department of Istituto Nazionale di Geofisica e Vulcanologia, Naples, Italy), equipped with XMAN micro-analysis system by Oxford, controlled by a SMARTSEM and AZTEC softwares. Operating conditions for SIGMA were 15 kV accelerating voltage, 50-100 mA filament current, 5-10 nm spot size and variable time acquisition time (several to tens of seconds). 40 by Inductively Coupled Plasma Emission Spectrometer (ICP-ES) using LiBO2/Li2B4O7 fusion, minor and trace elements were determined by inductively coupled plasma-mass spectrometry (ICP-MS) using a four acid (HNO3-HClO4-HF-HCl) digestion.
The uncertainty is generally <1% for major/minor oxides, <5-20% for trace elements. LECO was used for determining the C and S abundances. Loss on ignition (LOI) was calculated by weight loss after ignition at 1000°C. 5 Sulfur and oxygen isotope measurements were performed directly on pure mineral separates without and with further chemical preparation in the stable isotope laboratory at the Institut für Geologie und Paläontologie (University of Münster). Chemical preparation was different depending on sample type: i.e., sulfates+elemental S+sulfides, elemental S +sulfides, or muds. Oxidized S-bearing and multi-phase samples first required the extraction of sulfate by sample leaching in a 10% NaCl solution, filtration through a 0.45 micron cellulose acetate filter, and followed by the addition of 8.5% barium chloride solution to 10 precipitate dissolved sulfate as barium sulfate for isotope measurements. Elemental S and pyrite extraction was performed on sulfate-free powders. This extraction consisted of a wet chemical treatment (acidic chromous chloride solution at sub-boiling conditions) that liberates sulfur as hydrogen sulfide which will ultimately be precipitated as silver sulfide, ready for isotope measurements. Elemental sulphur was liberated from each sample via acetone leaching and subsequently converted to silver sulfide applying the acidic chromous chloride treatment (Canfield et al., 1986). Again, resulting hydrogen sulfide was 15 precipitated as silver sulfide (Ag2S). For S-isotope measurements, mineral separates as well as silver sulphide and barium sulfate precipitates were homogenously mixed with vanadium pentaoxide in tin capsules and combusted in a Carlo Erba elemental analyzer interfaced to a ThermoFinnigan Delta Plus mass spectrometer (EAIRMS: Elemental Analyzer-Isotope Ratio Mass Spectrometry). Results are reported in the standard delta notation (δ 34 S) as per mil difference to the Vienna Canyon Diablo Troilite (VCDT) standard. Reproducibility as determined through replicate measurements was generally better than ± 20 0.3 ‰. Sulfates were also measured for O-isotopes by using a ThermoFinnigan TC/EA interfaced with a ThermoFinnigan Delta Plus XL. Results are reported in the standard delta notation (δ 18 O) as per mil difference to the Vienna Standard Mean Ocean Water (VSMOW) standard. Reproducibility as determined through replicate measurements was generally better than ± 0.5 ‰.

Supplement 25
Three supplementary tables: Table S1 -List of samples, collection date, temperature and mineralogical associations as resulting by XRDP analyses and corroborated by FTIR and EDS-BSEM study. The sampling includes water spring sampled at Stufe di Nerone. In the temperature column: tc, thermo couple (see chapter 2.2 Sampling, sample preparation and analytical techniques), infr, infrared 30 gun; infrared derived values are in red. In the mineralogy column: ?, for minerals to be validated; minerals in red are approximate attribution based on XRDP patterns. The orange cells evidence water samples. Selected XRDP traces are in Fig.   S1. Further details in this supplement. Table S2 -Vibration modes and related tentative assignment of functional groups, and mineral attribution for selected samples 35 by DRIFT-FTIR investigations. Alu = alunite, Clay = essentially illite, Masc = NH4 -bearing sulfates, am = amorphous, Kao = kaolinite, KAl = alum -(K). ?, uncertain attribution. Note: assignments and attributions are based on mineralogy derived by XRDP study and corroborated by EDS-BSEM analyses. Further details in this supplement.
in Fig. 1) within the Pisciarelli and Solfatara areas and at different times. MDL indicates the detection limit for major, trace, C and S contents. Figure S1 -Selected XRDP traces of NH4-sulfates dominating the assemblage formed from drying the Pisciarelli water (a, b) 5 and of various multiphase muds from Solfatara (c,e) and Pisciarelli (d,f). Each panel reports the sample name in Table S1. The muds show the large hump between 18 ° and 30 ° 2θ degree attributed to the amorphous phase and the signals from the dominant alunite, feldspars (except MS new 9/17) and other minor phases (sulfur, pyrite and clays; Table S1). The smaller panels evidence the reflection intensity in the most significant range useful to discriminate illite in d) and kaolinites in e). Some XRDP trace has a corresponding infrared spectrum in Fig Table S2 for vibrational modes and relative assignments. stable isotope values and contributed to data elaboration; MP did data representations and stable isotope data modelling. MP prepared the manuscript. All authors contributed into the final manuscript.

Acknowledgements 25
Osservatorio Vesuviano (Istituto Nazionale di Geofisica e Vulcanologia) funded analyses of whole-rock geochemistry; we are therefore grateful to the Directors, namely Giuseppe De Natale and Francesca Bianco. The study also benefits of the INGV fund FIRS 08-6-5-056 granted to M. Piochi. We are also grateful to colleagues at the Osservatorio Vesuviano: Rosario Avino is kindly thanked for BG sample collections in 2018, Enrica Marotta and Pasquale Belviso who provided thermos-probe. Dr Angarano, Tennis Hotel and Stufe di Nerone allowed the free access at the sampling sites. We appreciated comments and 30 suggestions from Franco Pirajno and an anonymous reviewer that improved the data presentation and discussion. We would also like to thank the Editor Kei Ogata and editorial staff for managing this manuscript.

10
Table 2 -δ 34 S vs. δ 18 O values of sulfur-bearing minerals. Sample name as in Table 1 and S1; the muds are in italic. * in Fig.   1; 'from google earth. Table 3 -Summary of the mineralogical and isotopical features at the acid sulfate area following Rye et al. (1992);Hedenquist and Lowerstern (1994). *highest 34 S and 18 O during bacteriogenic reduction of sulfates with maximum fractionation in dry-15 wet alternating conditions. ^ δ 34 S reflects the H2S/SO2 and temperature of fluid. 1 always present, 2 may be associated. 3 from Valentino and Stanzione (2003;, Gresse et al. (2017). 'halloysite is indicated in Montanaro et al. (2017) and included here for completeness. "Montmorillonite needs of specific validation (Supplement) and is reported considering its detection in the local subsurface (Rosi and Sbrana, 1987;Valentino and Stanzione, 2003;. water. All images were taken using a binocular microscope. Refer to Fig. 1a,d,e for listed sites.  Table S1. Refer to Fig. 1e for site location. Results from cores collected in 1965, and 1971, 1984, 2013 and 2014 are compiled from the literature (i.e., Ref: Cortecci et al., 1978;Valentino et al., 1999;Valentino and Stanzione, 2003;Piochi et al., 2015). Sampling sites are shown in Fig. 1a,d,e. Note the values are plotted based on sample sites, from west to east.   Chiodini et al. (2008) and Piochi et al. (2015). δ 18 O values of local outgassing is -2 to 0‰. Fields and processes based on Rye et al. (1992) and Rye (2005). Panel a) highlights Solfatara and Pisciarelli samples; shaded areas define the δ 18 O values we recalculated based on Rye et al. (1992) at the indicated temperature.

Figures and Tables
Fields and processes based on Rye et al. (1992) and Rye (2005). Panel a) highlights Solfatara and Pisciarelli samples; shaded areas define the δ 18 O values we recalculated based on Rye et al. (1992) at the indicated temperature.    Rye (1979) and Rye et al. (1992). Fields for steam-heated (white) and supergene (gray) environments are from Rye et al. (1992): dashed envelop for Alunite-Pyrite (circle) or Alunite-H2S (rhombus) pairs, continuous envelop for Alunite-H2O. Figure 11-Sketch of the acid-sulfate alteration zone at the Campi Flegrei caldera (Fig. 1a). Subsurface is constrained by borehole (deep from Rosi and Sbrana, 1987;Piochi et al., 2014 andshallow from de Vita et al., 1999) and geophysic (Di Giuseppe et al., 2017) information. The presence of NH4-rich aquifers correlate with their occurrence in marine sequences (Rosi and Sbrana, 1987;Piochi et al., 2014) at the Mofete wells (Chiodini et al., 1988). In the legend P-wave velocity, resistivity and density (with respect to 2.4 g/cm 3 ) from Di Giuseppe et al. (2017); the geophysically explored area is in the dashed rectangle. BG = bocca grande (Fig. 1d); LF = La Fangaia mud pool (Fig. 1a,d).  Fig. 1a,d,e). The complete set of minerals is in Table S1. Refer to the supplement for details.   Table 3 -Summary of the mineralogical and isotopical features at the acid sulfate area following Rye et al. (1992); Hedenquist and Lowerstern (1994). *highest 34 S and 18 O during bacteriogenic reduction of sulfates with maximum fractionation in dry-wet alternating conditions. ^ δ 34 S reflects the H2S/SO2 and temperature of fluid. 1 always present, 2 may be associated. 3 from Valentino and Stanzione (2003;, Gresse et al. (2017). 'halloysite is indicated in Montanaro et al. (2017) and included here for completeness. "Montmorillonite needs of specific validation (Supplement) and is reported considering its detection in the local subsurface (Rosi and Sbrana, 1987;Valentino and Stanzione, 2003; Abstract. Active fumarolic solfataric zones represent important structures of dormant volcanoes, but unlike emitted fluids, their mineralizations are omitted in the usual monitoring activity. This is the case of the Campi Flegrei caldera in Italy, among 10 the most hazardous and best-monitored explosive volcanoes in the world, where the landscape of Puteolis is characterized by an acid sulfate alteration that is active at least since Roman time. This paper provides temperature, mineralogical, textural, compositional and stable isotope data for those solfataric terrains sampled at the crater and Pisciarelli slope of the Solfatara volcano between 2013 and 2019. Temperatures vary between 40 ° and 95 °C. Minerals include alunite with grain sizes generally larger than 20 µm, alunogen, native sulfur, well-ordered kaolinite, and, common at Pisciarelli, pyrite, illite and NH4-15

Phase
sulfates. Sulfate terrains have higher contents of Ti, Ba, Au, As, Hg and Tl relative to their parent substrate. The Pisciarelli slope is anomalous in terms of the presence of NH4. δ 34 S values for sulfides and native S range between -3.00 and 0.49 ‰ and from -4.42 to 0.80 ‰, respectively. Sulfates show δ 34 S and δ 18 O values in the range of -3.35 to 3.80 ‰ and between 0.3 and 31.33 ‰, respectively. The style of mineralization and the stable isotope geochemistry do produce complex and not completely consistent classifications and genetic constraints. We merge our data with volcanological information, data from exploration 20 drillings and geophysical results. With the conceptual model, we suggest a series of shallow and deep aquifers interconnected like "communicating vessels" through a main fault system that downthrows Solfatara with respect to Pisciarelli. Fluid outflow from the different discrete aquifers hosted in sedimentsand possibly bearing organic imprintsis the main dataset that allows determination of the steam-heated environment with a supergene setting superimposed. Supergene conditions and highsulfidation relicts, together with the narrow sulfate alteration zone buried under the youngest volcanic deposits, point to the 25 existence of an evolving paleo-conduit. The data will contribute to monitor and evaluate the volcanic hazards.

Introduction
Active solfataric landscapes are among the most peculiar and fascinating environments on the Earth that may be considered as planetary analogues (e.g., White and Hedenquist, 1990;Rye et al., 1992;Lowe et al. 1993;Zillig et al., 1996;Ciniglia et al., 30 2005;Rye, 2005;Glamoclija et al., 2004;Sgavetti et al., 2008). Their peculiarity arises from the stringent interaction between inorganic (mineral assemblages and geochemistry) and organic (biota) substances under extreme ambient conditions (pH, temperature, salinity, oxygen deficiency, etc.) associated with endogenous degassing (i.e., H2O, CO2, CH4, H2S, SO2, HCl, HF, etc.) and hot water fluid circulation (hydrothermal/geothermal systems) on dormant volcanoes. They allow investigating a variety of processes in the field of geology (i.e., magma and volcano dynamics), biology (i.e., physiological adaptation to 35 environmental stresses and the origin of the life), medicine, astrology and archaeology (i.e., thermal bath and antibacterial applications), with possible future medical and biotechnological applications.
The solfataric area has been exploited for centuries for its alum occurrences (Photos-Jones et al., 2016 and references therein).
Intense mining during Roman and Medieval times modified their original context (Photos-Jones et al., 2016): the Pisciarelli gorge-valley was a quarry, while caving activity exposed the eastern (the Monte Olibano inner wall) and northern flanks of 30 the Solfatara volcano, and rework deposits in the crater floor.
Old pictures and descriptions (Sicardi, 1959) suggest that the most evident manifestations along the SE and NE rim remain roughly the same: 1) the main Bocca Grande fumarole (Fig. 1d) with various exhalative branches northward along the morphological heights; 2) the minor fumarolic vents around the old thermal baths (Sst site; Fig. 1d) and 3) the mud pools (Fig.   1b,c,d,e). Also, the thermal spring in Pisciarelli (Fig. 1a,b,e), known as the "Bulla", i.e., the bubbling one, was known at least 35 since Medieval time (Photos-Jones et al., 2016). Moreover, the same descriptions indicate the presence of a lake in the Agnano Plain (Fig. 1a). According to Ventriglia (1942), the lake extended up to the slope base of the Solfatara volcano and had a maximum depth of 15 m; drillings recovered related sediments . Ventriglia (1942) also indicated high temperatures in the lake preventing fish from living. Today, the area shows several mud pools and thermal springs, while some ("de Pisis" and "Sprudel" springs in the Terme of Agnano; Fig. 1a) disappeared. Yet, high temperatures can be still detected. 40 160 °C (e.g., Cardellini et al., 2017;Gresse et al., 2017), in agreement (or locally lower respect) with measurements reported by Sicardi (1959) in the range of 141-215 °C at Bocca Grande andof 99-110 °C at other sites, between 1921 and1951. Pisciarelli waters exhibited temperatures mostly around 95 °C with minimum values of 84 °C between 197884 °C between and 199984 °C between (Celico, 1992Valentino and Stanzione, 2004). A geyser-like vent at Pisciarelli has a temperature of up to 116 °C (Chiodini et al., 5 2016). Only, the mud pool ("La Fangaia"; Fig. 1d) was hotter in the past with values up to 100 °C (Sicardi, 1959). This author describes a mud vent in the southeastern area of the crater that is no longer present.

Sampling, sample preparation and analytical techniques
Sampling was conducted within the Solfatara crater and in the Pisciarelli and Cinofilo areas (Fig. 1a,d,e) with additional sites compared to Piochi et al. (2015); the crater floor, except the pool, was intentionally avoided because the reworking in historical time (Photos-Jones et al., 2016) and thus a possible anthropogenic contamination. This study enlarges the dataset on the acid 25 sulfate alteration zone of the Phlegraean area, in order to understand the quiescent dynamics of the volcano. Similar observations and data are also available for Ischia island (Piochi et al., 2019) that belongs to the Phlegraean Volcanic District (Piochi et al., 2005).
Our new collection is, therefore, widening the observation period for the Puteolis sulfate area that now spans between January 2013 and April 2019 (Table S1). Selection of sampling sites (herein after referred using the acronyms in Fig. 1d,e) is based 30 on variable macroscopic features including outgassing "magnitude", tectonics and fracturing evidences, mineral occurrences and exhalative vents locations, as visible in the field and described in the literature (Allard et al., 1991;Ferrara et al. 1994;Valentino and Stanzione, 2003;Aiuppa et al., 2013;Bagnato et al., 2014;Chiodini et al., 2016). A thermo-couple digital probe 51/52 II by Fluke with precision of ± 0.3 °C was used to measure temperature in the field, contextually to sample collection.
Samples were air-dried for several days to one week. Subsequently, these were studied under the Optical Microscope (OM) in 35 order to assess their general mineral assemblages. Where possible, the various S-bearing phases (or enriched portions) were handpicked for subsequent isotopic analyses. Figures 2 and 3 show the appearance of most representative samples. in the Supplement (S1) together with representative patterns (Fig. S1, S2).

Mineral data by OM, XRDP, BSEM, DRIFT-FT-IR results
New and previously published (Piochi et al., 2015) mineralogical data for the Solfatara-Pisciarelli area (Table S1, Table 1) provide information on a yearly to monthly basis since 2013 along with measurements of temperature. The mineral assemblage dataset derives from XRDP analyses (Supplement) corroborated by textural and chemical information obtained at the EDS-5 BSEM. DRIFT-FT-IR spectra determined on representative samples, display characteristic bands of minerals they include (see below and Supplement), and help in material characterization.
Through time, K, Al sulphates (alunite) and native S (Fig. 3a,b,c,d,e,f) are the main and widely distributed secondary mineral phases associated to surface degassing. Alunogen and pyrite (Fig. 3a,b) are second in abundance. All these mineral phases can form single phase concretion or coexist in up to mm-sized grains. Alunogen oftenif not generallyassociates with alunite 10 and occurs in two distinct morphologies (Fig. 3a,d,e). Most commonly, it consists of fibrous tangled masses of white crystals.
Where coexisting, alunogen fibres grow from the edges of alunite crystals (e.g., sample L100 zucc in Table 1; Fig. 3d). This appearance seems usual along the fault scarp, north of the pool (L1 site, Fig. 1e). Secondly, alunogen has thin, platy crystal habits (Fig. 3b,d,e). Many of these crystal groups show rounded to corroded edges suggesting alteration after crystallization ( Fig. 3e). Dendritic and/or sometimes bi-pyramidal crystallites (Figs 2a,3a,3c) are ubiquitous habits for native sulfur (typically 15 sampled at L1, SMO, some places along ASA, Sst in Fig. 1d and Table 1) that mostly cluster within the alunitic surface and the rock voids (Fig. 2c). Along the fracture, S may form a yellow ductile patina (L1 vent, BG, BN in Fig. 1d,e and Table 1).
Pyrite (Fig. 2d) occurs as smaller (≤mm-sized) rhombododecahedric grains (Fig. 3a) or as fine-grained mineral uniformly 20 coating other components (i.e., feldspars, lava, etc.; Figs 2d,3i). It has a particular high abundance within the Pisciarelli muds (Figs 1e,2d), i.e., both within the main pool where it could reach cm-sizes and in the geyser (G site in Fig. 1e) emission. The blackish colour of the muds should also derive from the pyrite abundance. Pyrite also occurs around the degassing areas. Barite can be further detected at Pisciarelli.
Clays have a low relative abundance in the studied samples (Supplement). They are mostly kaolinite and illite (Table S1, Table  25 1), as derived by the XRDP traces (see Fig. S1c,d,e) and supported by EDS-BSEM and DRIFT-FT-IR study (see below; Fig.   S2 and Supplement). In particular, the infrared technique is suitable to detect the kaolinite and the related bands in the OHregion, in agreement with Madejová et al. (2002). Illite usually occurs in the muds at Pisciarelli (from geyser and around other emissive vents) and occasionally at Solfatara (Table 1, S1). Kaolinite characterizes the newly formed pool within the Solfatara crater and occurs locally at Pisciarelli (Fig. 1c,d,e and Table 1). Figure 4 illustrates the platy particles of kaolinite with typical 30 widths of < 10 μm that assembly in packages and associated with alunite crystals.
Other efflorescent phases (Fig. 3c,i) occur randomly. Rarely, Al, Fe sulphates (halotrichite) have been identified nearby the Pisciarelli geyser as crust-like aggregates. Na and NH4 sulphates induce the pale orange painting on efflorescences and encrustations, and generally of soils. Sulphates, bearing Na, Ca, Mg are least common, and represent a typical occurrence in the new Solfatara mud. Alum has been detected at Pisciarelli. 35 Air-dried evaporation of water sampled at the Pisciarelli pool resulted in the precipitation of mascagnite, tschermigite, and letovicite (Fig. 2e,f, S1a and Table S1, Table 1). Figure 3i shows the euhedral tschermingite that coexists with native S in the sample L30 eff-blocchetto (Table S1, Table 1). Instead, evaporation of Solfatara mud pool water produced alum, as documented already in the Medieval and Roman times (Photo-Jones et al., 2016). Water from the Stufe di Nerone (west side not shown in figure) crystallized halite . 40 Realgar (detected at the EDS-BSEM and not listed in Table S1) and ammonium chloride (Fig. 3g,h) appears as peculiar precipitates at the Bocca Grande and Bocca Nuova sites (Fig. 1d).
Furthermore, amorphous phases are largely present at various sites (Table S1, Table 1), particularly, in muds and in the samples from Bocca Grande and L1 vent (Fig. 1d,e). The widespread amorphous phases could correspond to material from the both original volcanic rock and alteration. General assumptions (Piochi et al., 2015;Montanaro et al., 2017) indicate amorphous silica, although it merits a more rigorous examination. 5 Finally, Fe-oxide, and fresh to variably altered feldspar and biotite are the most common primary volcanic mineral phases.
DRIFT-FTIR spectra collected on selected samples (Fig. S2) produce data consistent with XRDP results (Fig. S1) and furthermore allow useful details on structure and eventual minor phases or impurities (Supplement). Table S2 lists the relevant vibration modes of spectra and the proposed mineral assignments. The crystals formed by evaporation of water in the Pisciarelli pool ( Fig. S2a; Supplement) show a sharp band at 1422-1411 cm -1 that is in the region of the ν4(NH4 + ), as described in the 10 literature (e.g., Weis and Ewing, 1996;Parafiniuk and Kruszewky, 2010) and in agreement with XRDP mineral data (Table   S1; Fig S1). Native S from two different samples (PINT S tozzo 18/10/17 and PINT S 18/1/18 in Table 1; Fig. S2b) is evident in the DRIFT-FTIR spectra at < 2950 cm -1 , with the strongest bands at 843 and 468 cm -1 that coincide with those of sulfur in the both the USGS (see Sulfur GDS94; Clark et al., 2007) and RRUFF (http://rruff.info/) databases. The spectra differ in the OH stretching 15 region, likely indicating the occurrence of impurities, although water absorption by the KBr solution can be a further possibility.
As expected (Clark et al., 1990), alunite can be determined through its major band at 3483 cm -1 coupled with a smaller one at 3513 cm -1 in the OH-stretching region (Fig. S2c,d,e). Accordingly, the spectra show a small band at ca. 4605 cm -1 from the Al-OH combination mode that should be ascribed to alunite. Furthermore, it is possible recognizing the doublet mode at 1088 20 ÷ 1099 cm -1 and at 1028 ÷ 1025 cm -1 (Toumi and Tlili, 2008)  Notably, the DRIF-FTIR spectra of muds from Pisciarelli (Fig. S2d) show a vibration in the region of 1430 cm -1 . Because the muds were separated from water, as before, the band can be related to the NH4 (Weis and Ewing, 1996;Parafiniuk and 25 Kruszewski, 2010) in tschermigite, mascagnite and letovicite (Table S1, Fig. S2, Fig. S1, Supplement).
The new pool at Solfatara characterizes for peculiar DRIFT-FTIR spectra in the OH-stretching region ( Fig. S2e; note the inset) due the presence of kaolinite, in addition to alunite, and minor (or occasional) S, feldspar, pyrite and amorphous phases.
Specifically, these are i) alunite (Clark et al., 1990) with a major band at 3483 cm -1 coupled with a smaller one at 3510 cm -1 , and ii) kaolinite (Madejova, 2003) with two minor bands at 3667 and 3651 cm -1 between two major vibrational modes at 3695 30 and 3620 cm -1 . Accordingly, it is possible recognizing the OH-deformation at ca. 915 and 938 cm -1 , the Si-O stretch at 1008 and 1026 cm -1 , and the Al-OH modes at ca. 4605 cm -1 and 4523 cm -1 from kaolinite.

Stable isotopes of sulfur and oxygen (SGI)
A new set of δ 34 S and δ 18 O values for sulfur-bearing minerals is listed in Table 2. δ 34 S values of sulfides and native S range between -3.00 and 0.49 ‰ and between -4.42 and 0.80 ‰, respectively. Sulfates are characterized by δ 34 S and δ 18 O values ranging from -3.35 to 3.80 and from 0.3 to 31.33‰, respectively. Temporal variations in δ 34 S for different sulfur-bearing phases at the different locations reveal a dominantly negative signature, regardless of their mineralogy, with native S showing 40 the most negative values (Fig. 5). studies earlier than 2000 (Cortecci et al., 1978;Valentino et al., 1999) also show positive δ 34 S values (Fig. 5a,b). In comparison to previous studies: 1) our new S-isotope data for Pisciarelli include few positive values (Fig. 5b,c).
2) the new O isotope values for sulfate are the highest obtained until now (Fig. 5d). To note, the muds generally have the least 5 heavy oxygen isotopes, except samples from 2013 -2014 for which O-isotope determinations are lacking. The diagram also indicates a lowering in δ 34 S for neoformed minerals reported later than 2000.
3) the sulfides at Pisciarelli show δ 34 S values mostly at 0 ‰ (Fig. 6). 4) the different sites display a homogeneous range in δ 34 S (Fig. 5a,b,c); however, the variability for the ASA and L1 sites reflects different sample heights along slope (Fig. 1d) and wall (Fig. 1e), respectively. 10 5) a likely appearance of a positive correlation between S isotope results for pyrite and for sulfate phases coexisting at Solfatara, with two from the 1994 data outside the trend (Fig. 6a). 6) new isotope data for sulfate reveal a difference compared to studies older than 1990 and the most recent one (Fig. 7).
In addition, the δ 18 O values for sulfates at Pisciarelli show a slight tendency to increase with topographic elevation at the sampling site. Specifically, the highest values generally occur at Solfatara that is at > 96 m asl while Pisciarelli lies at 66 -74 15 m asl.

Content of major and trace elements (WRG)
Table S3 reports the whole-rock geochemical compositions of selected samples. As expected, samples are highly hydrated and sulfur-rich, due the OH-group and/or S in the crystalline network (see ideal formula in Table 1), and/or the presence of native S in the analysed sample. LOI can be up to 80 wt% (sample L20 camino 18.9.17), although most commonly at 20-30 20 wt%, and the S content is up to more than 50 wt%. Carbon is generally low (< 1.25 wt%), and always < wt 22 %. Notably, the SiO2 content is highly variable. Depending on the sample mineralogy, it can be as high as 70-80 wt%. MnO is always < 0.1 wt.%.
With respect to the local volcanic rock substrate (Table S3; Fig. 8a), some lithophile elements (Si, Al, P, Sc, Ti, V, Zr, Ba, Yb, Th, Hf) are comparable or depleted (for examples, Be at < 1-5 ppm vs > 4 ppm, Ga < 23.8 ppm, Rb < 95.6 ppm, Y < 8.1 ppm). 25 Depletion concerns with Na, Mg, K, Ca, Rb, Eu, U content. Only Ba displays a significant level of enrichment reaching very high values up to thousands of ppm. Cs shows concentrations of up to tens of ppm. Siderophiles and chalcophiles ratios (Fig.   8b) mostly lie at 1 or slightly above, with depletions for Fe, Zn, Ga, and enrichments for S, Au, As, Tl, Hg (Table S3). Sb can be higher than the primary rock composition.

Environmental sub-zones: stationary phenomena and runoff processes
Throughout the years, the various Solfatara and Pisciarelli sampling sites display a nearly constant mineral alteration assemblage (Table 1). Commonly, the mineral neogenesis variably develops on sub-mm-to dm-to m-scales, in relation to the outgassing dynamics, runoff, weather conditions, outcropping substrate, and anthropogenic activity. 35 However, the various sites further display reproducible rock geochemistry and stable isotope compositions at the timescale of survey, and with respect to oldest data (e.g., Valentino et al., 1999) as well; they can be considered reference points for future investigations.
Based on the presented dataset, we propose the existence of major alteration sub-zones, in which some (minor/peculiar) mineral phases appear or disappear, in response to changing physical-chemical boundary conditions mainly associated to weather 40 mineralogy, rock chemistry, and isotopic compositions, and characterized by temperature variations in a narrow range. Such a constancy is revealed when comparing results reported by Sicardi (1959) (see Geological setting at 2.1 section) with the present results, corroborating the existence of "stationary" sub-zones that are presented in the following. The only exception is the mud pool in the crater. 5 The Pisciarelli and Solfatara pools (Fig. 1d,e) are the two major and distinct sub-zones. They display persistent differences in dynamics, temperature and mud (solute plus water) mineralogy. The main pool at Pisciarelli shows vigorous boiling (Fig.   1b,e), with temperatures ranging from 63.9 to 94.3 °C (Tables S1 and 2) and a relative dominance of water vapour. The mud is typically grey in colour, mostly ash to sand up to mm-sized grains, with generally rounded/smoothed shapes as a function of the boiling vigour and material supply from the nearby slopes. On the other hand, the main Solfatara pool is characterized 10 by degassing with a temperature at around 49 °C (Tables S1 and 2). The mud is beige and fine grained, always with a fetid odour. We recurrently detected an enrichment in pyrite, illite and feldspar at Pisciarelli and in native sulfur at Solfatara. The latter is enriched in As, Hg, Nb, W, Zr, Sb and depleted in Sr, Ba, Co compared to the original deposits (Fig. 8). Sulfides and sulfates display nearly similar δ 34 S values at Solfatara, whereas they are different at the Pisciarelli mud pool (Fig. 6a, inset).
The DRIFT-FTIR spectra of muds from Pisciarelli, in contrast to those from Solfatara, always gave the vibration at 1430 cm -15 1 related to the NH4 (see section 3.1). Notably, the La Fangaia mud pool (Fig. 1d) is likely cooling as Sicardi (1959) reported up 100 °C, several tens of degrees higher than at the present (Tables S1 and 2). A slight cooling is discernible when comparing our data (Table S1) with those in the literature (Martini et al., 1991;Celico et al., 1992;Valentino and Stanzione, 2003). In contrast, the Pisciarelli area should be hotter, although only by a few degrees Celsius (Tables S1 and 2) if compared with increasing temperature values at the geyser-vent (Chiodini et al., 2016). 20 The PINT-PEXT sub-zone (Fig. 1e) -an isolated morphological heightis composed of an alunitic-rich low-cohesive reddish terrain with a temperature around 95 °C, typically comprising kaolinite (Tables S1 and 2). The kaolinite is easily discriminated also by DRIFT-FTIR spectra (not shown) that, in agreement with XRDP traces, point for a well-ordered structure. This terrain bears variably sized (up to few dm) clasts and is subject to slumping and sinking.
A hole up to 2-3 m deep represents a distinct sub-zone that we emphasize because it opened 180 m North from the main pool 25 within the crater in May 2017, by surface collapse. A grey viscously boiling mud fills the hole (Fig. 1c,d), with a minimum temperature of 70 °C; a temperature of 91 °C was measured in April 2019. The mud is dominated by alunite with subordinate native sulfur, showing vague similarity to Solfatara mud (Table S1), although its grey colour and the occasional pyrite are reminiscent of the Pisciarelli mud. However, kaolinite is the main clay mineral in the new pool ( Fig. S1e; Fig. S2e, note the inset). Mud geochemistry reveals peculiarly high concentrations of Sr, Ga, Co, Th, V, REE, Sb (Fig. 9). 30 Finally, a rather broad sub-zone includes the other various sampling sites that are characterized by encrustations of alunite with a well-defined, although relatively large, range of δ 34 S values between ca. -4 and 1 ‰ (Tables S1,2; Fig. 5c). These occurrences reflect the nearby presence of vapour degassing. We suspect that those at the ASA, M. Olibano and SSt sampling sites along the slopes of the Solfatara crater (Fig. 1a,d) are ascribable to long-lived encrustations; further investigations are useful to ascertain this suspect. The minor fumarolic vents around the old baths (Sst site) seem unchanged in respect to the 35 description by Sicardi (1959), particularly showing the occurrence of native S and a comparable temperature at around 95 °C.
These show variable bulk-rock geochemical compositions.
Widespread alunite formation reflects the potassium and feldspar-rich rock substrate on which they develop (see Piochi et al., 2014 and references therein).
Vapour effluents around the various geysers/vents at Pisciarelli are the most important factors affecting the mineral neogenesis 40 at the alunite-dominated sub-zones. Pisciarelli is a decametres-deep incision on the NE Solfatara slope and the degassing vents are constrained in a gorge-like morphology. This setting favours the stagnation of the hydrothermal steam that impregnates the rock substratum and supplies elements to the formation of a variety of Na and NH4 sulphates. We detected high abundance as droplets in the humid air, as revealed by experimental desiccation tests of the water. Aerosol particles from inside and nearby the Solfatara crater that bear NH4 + (and Cl -, possibly in the form of NH4Cl) as major ion (Mather et al., 2004), as well as the NH4Cl inside the BG and BN orange-yellow encrustations (Fig. 3h,i), furthermore support the widespread presence of ammonium species. Notably, N2 is emitted at the main intra-crater fumarole of Bocca Grande (see Chiodini et al., 2010). 5 DRIFT-FTIR and XRDP patterns (Fig. S2d, S1a,d), however, point to the presence of NH4 in both the mud and water pools at Pisciarelli, but not at Solfatara (except for BG and BN). Alum -(K) has also been found in relation to the wet conditions at Pisciarelli.
Vapour emissions outflow and the conditions of hydrothermal steam stagnation are dependent on atmospheric pressure and wind conditions. 10 So that, it is likely the meteoric weather the main cause for appearance to disappearance (and vice versa) of some phases. This is also particularly evident for the PINT, PEXT, L19, L20 (Fig. 1e) and the SMO, ASA, SSt (Fig. 1d) sub-zones that may typically present bipyramid and/or fine dendritic sulfur crystallites (Fig. 2a). Their crystallization seems to be favoured by relatively strong exhalations and porous terrain (PINT, PEXT, L19, L20; Fig. 1e) or conditions where gases remain briefly trapped (SMO, some places along ASA, Sst; Fig. 1d). Respective conditions also prevail in close proximity (< dm scales) to 15 the main fumaroles (L1 vent, BG, BN; Fig. 1d), where sulfur forms a cream-like patina reflecting the condensing gas flow along the host fractures and fissures.
However, native S disappears during run off and we have macroscopically tested at several places that re-crystallization needs one-to-two months, if not longer (i.e., sample L20 camino; Fig. 1e).
Furthermore, periods of intense rainfall determine the timing areal extent and depth of the mud pools, as well as the generation 20 of secondary mud vents and the erosion in Pisciarelli and its periodic water puddle. Sicardi (1959) already noted the occurrence of mud vents and black mud pools following rainy periods. Notably, pools at Pisciarelli are supported by anthropogenic embankment.
Meteoric and surface waters can dilute the aggressive endogenous fluids determining alteration degree conditions low enough for the generation of iIllite, or other clays (Pirajno, 2008) at Pisciarelli. Further studies need to be performed in order to better 25 characterize clays as they can bear information useful to further constrain the hydrothermal setting.
The distribution of sulfates appears irregular, and this should be an object of future investigations.

Classification of alteration and genetic environments: the contradictory data
The style of mineralization (Arribas, 1995;Sillitoe, 1993;Pirajno, 2008;Ercan et al.; and the stable isotope results (Rye 30 et al., 1992) allow classification of alteration and differentiation of genetic environments. Table 3 summarizes characteristic mineralogical, lithological and isotopic features of these environments, in comparison to observations made at the study sites.
Several contrasting interpretations can result from the data.
Alunite plus kaolinite form in steam-heated environments at 100 to 160 °C where fumarolic vapour condenses above the boiling zone of nearly neutral-pH, H2S-rich fluids representing a low sulfidation environment. 35 Nevertheless, alunite shows grain sizes in the range of 50 to 100 µm (Fig. 3a,e,f), unlike the finest (<20 µm) ones detected in high-temperature (>40-50 °C) steam-heated (Simón et al., 2005) or supergene (Arribas, 1995) environments. Those coarser sizes usually occur in low-temperature steam-heated (and hypogene, as well) environments (Hedenquist et al., 2000), or could directly crystallize from a SO2-rich magmatic vapour that rapidly ascent through fractures (namely high sulfidation setting; Rye et al., 1992;Stoffregen and Alpers, 1992). The occurrence of kaolinite and alunite at several Solfatara and Pisciarelli sub-40 zones (particularly, new pool and PINT, Table 1) fits with the high sulfidation environment; in fact, the two phases usually coexist in the advanced argillic alteration zones proxy to ascent plumes (e.g., Pirajno, 2008). The evidence of K-feldspar support the classification as high sulfidation -magmatic hydrothermal environment (Rye et al., 1992). Nevertheless, illites/montmorillonites are most common in intermediate argillic alteration zones (Pirajno, 2008) and their widespread occurrence in the various studied sites, as well as in the local subsurface (Valentino and Stanzione, 2003;, is among the results that contrasts this high sulfidation environment. Indeed, sampling temperatures are higher than 40 °C (Tables 2 and S1)  5 and SO2 is rare or absent (Aiuppa et al., 2013) at Campi Flegrei. Also, the lack (or rare detection) of lower temperatures (< 40 °C), disordered polymorphs (i.e., halloysite) point to limited supergene alteration.
Only, the alunite coexisting with kaolinite in the new hole pool exhibits the finest grain size. Accordingly, the XRDP and DRIFT-FTIR analyses of CF samples point to slightly ordered kaolinite forms that usually occur at temperatures < 150 °C in epithermal systems (Sillitoe, 1993), but could also represent a metastable form in hotter settings (Zotov et al., 1998). In this 10 later case, the new pool sub-zone with kaolinite and alunite can represent local, well-circumscribed advanced argillic alteration condition indicative of a proxy plume.
However, when considering litho-geochemical parameters, schematic diagrams further produce contrasting visions. For example, following Ercan et al. (2016), the clay-bearing muds can be ascribed to variable supergene to hypogene alteration field in the binary diagram of immobile Zr vs. TiO2 (Fig. 9a). They also have high (above 1,000 ppm) Ba+Sr and low (<200 15 ppm) Ce+Y+La concentrations (Fig. 9b).
The stable isotope geochemistry of minerals supports an interpretation of steam-heated to supergene environments (Fig. 7). Sisotope equilibrium occurs between sulfides and sulfates, with reliable re-calculated temperatures in high-sulfidation environments (Arribas, 1986). In contrast, this equilibrium cannot be accounted for at Campi Flegrei and any reliable temperatures result from the S-isotope fractionation between sulfates and H2S. In fact, δ 34 S values of alunite and pyrite roughly 20 overlap. Instead, sulfur-H2S and sulfides-H2S at the sampling temperature reflect equilibrium S-isotope fractionation: the theoretical δ 34 S value of dissolved H2S fluid is between -3.84 and -0.84 ‰ (Allard et al., 1991). This also implies that the sulfate altered rocks are not magmatic hydrothermal in origin, in agreement with the lack of typical mineral phases that show a high oxidation state of S (SO2-rich, high-sulfidation according to Henley and Ellis, 1983). Based on Rye et al. (1992), SO2 disproportionation results in the formation of 34 S-enriched H2SO4 and 34 S-depleted H2S. In addition, the intense Al-leaching 25 in a high sulfidation system is not typical for the Campi Flegrei setting (Fig. 8; Table 3).
Actually, Campi Flegrei lacks the occurrence of enargite and luzonite, both diagnostic for high-sulfidation environments.
Significantly lower δ 34 S values (< 15 ‰) for alunite can derive from: (i) the light sulfur isotopic composition of H2S during 30 boiling (steam-heated or low-sulfidation setting), (ii) δ 34 S of sulfides (supergene) or (iii) the bulk sulfur isotope composition of magmatic steam (Rye et al., 1992). The first possibility could partly account for the isotopic composition of alunite-pyrite and alunite-H2O pairs (Fig. 10), although contrasting with conclusions based on texture, mineral assemblage and bulk-rock geochemistry ( Table 3). The presence of kaolinite in the subsurface, under an alunitic cover and the occurrence of argillic alteration at depth suggest a vertical zonation at the Solfatara crater and, following Rye et al. (1992), a steam heated setting. 35 Furthermore, δ 34 S values for the pyrite-H2S pairs further support a supergene setting (Fig. 10). Finally, we are not able to directly identify any microbial sulphur cycling, although FT-IR and rock geochemistry corroborate the absence or limited biota contribution. The analysed samples do not exhibit bands attributable to C=H ligands (Supplement) and the carbon content is <1.25 wt% (most common < 0.2 wt%; Table S3). Yet, some higher δ 34 S and δ 18 O values for sulfates could be indicative of microbial sulfur cycling, particularly considering the dry-wet alternating conditions. 40 Merging all available information, it appears that observations concerning the both an apparent "stationarity" sub-zones' and a seemingly contradictory classification environment reflect the evolving conditions that have followed the last magma intrusion and eruption and that probably are overlapping through time.
The solfataric alteration zone has a strongly limited extent within the central sector of the Campi Flegrei caldera. It coincides 5 with the area of eruptive vents (e.g., Mt. Olibano, Accademia, Solfatara; Fig. 1a) and uplift of the most recent period of volcanism . The zone appears to be limited under the later Fossa Lupara and Astroni vents, while outgassing and thermal aquifers occur within the caldera. However, there is an indication for their discrete, more than their continuous distribution, both across the caldera and through depth (Guglielminetti, 1986).
The alteration zone locally presents high Ti, Ba, Au, As, Hg, Tl, S concentrations relative to the above parent basement lithology (Fig. 8).
The zone also appears anomalous in terms of ammonium content. Therefore, we here adopt ammonium as a possible tracer, but we have no information yet about the various contributing sources for the N-species and the cycling of nitrogen at the local 15 scale. The presence of NH4-bearing sulfates is particularly abundant at Pisciarelli. Those sulfates systematically form by drying water collected at the various pools of the area, in relation to the abundance of nitrogen species (0.2-1 g/l) in those waters (Martini et al., 1991;Celico, 1992;Valentino et al., 1999;Holloway and Dahlgren, 2002;Valentino and Stanzione, 2003;Aiuppa et al., 2006). Actually, the NH4 + content in the shallowest Phlegraean ground waters is generally low (<0.03 g/l; Martini et al., 1991;Celico, 1992;Valentino et al., 1999;Valentino and Stanzione, 2004;Aiuppa et al., 2006). Yet, Mather et 20 al. (2004) measured a significant abundance of ammonium chloride particles/aerosols at the Solfatara crater. Some realgar encrustations sampled at BG and BN also associate with ClNH4 (Table S1; Fig. 3h,i).
The concentration of some metals and metalloids requires sources different from the parent basement. Anthropogenic contributions are obviously possible (Alloway, 2012), e.g. when considering that NH4 is generally attributed to agricultural (i.e., fertilizer) and urban soils. However, Hg and As have been detected at the main fumaroles with similar concentrations 25 today and in the last century (Ferrara et al. 1994;Bagnato et al., 2014). These fumaroles continuously emit H2S and CO2 (Allard et al., 1991;Aiuppa et al., 2013;Chiodini et al., 2016) and are the preferred location of crystallization of native S and alunite. NH4 + emissions are also present at Solfatara (Chiodini et al., 2010), although ammonium is of limited importance inside the crater waters (<0.001 g/l; Aiuppa et al. 2006) and crystallization of alum instead of NH4 + -sulfates has been obtained in the water from Solfatara pool. In agreement with these authors, the plausible source is the conventional geothermal reservoir. 30 High concentrations (20-100 g/l) of ammonium are reported in the deeper (>500 m; Carlino et al., 2012) aquifers at the Mofete wells, located on the western side of the Campi Flegrei (Chiodini et al., 1988). Those deeper aquifers are located within a sequence of tuffs and marine sediments also drilled by the CF23 well (1000-1200 m depth; Piochi et al., 2014;, i.e. nearby the solfataric area. In summary, we propose an environmental setting that merges all collected information (Fig. 11). Fluid outflows from discrete 35 aquifers hosted in sedimentsand bearing organic imprintsfeed the Pisciarelli site giving its ammonium peculiarity. Our purpose does not exclude the possible biological contribution that has been ascertained in the studied sites (e.g., Ciniglia et al., 2005;Glamoclija et al., 2004). However, marine strata and a volcano-clastic sequence intercepted by deep drillings (San Vito1, Mofete and CF23 wells; Rosi and Sbrana, 1987;Piochi et al., 2014) are considered as the key sediments for the NH4 species.
An additional supply can originate from the swampy sediments encountered in shallowest boreholes (de Vita et al., 1999) 40 located in the central sector of the caldera (Fig. 11), nearest to the study area. Results from cluster analysis of resistivity, Pwave velocity and density parameters, from Pisciarelli across the Solfatara crater (Di Giuseppe et al., 2017) lithologically constrain the model section. These authors highlight a sudden upraise up to ca. 1500 m depth of buried rocks through a tectonic structure just beneath Pisciarelli. The dislocated rocks are the fossiliferous marine and volcano-clastic sequences drilled across the caldera. The deep aquifer is represented by a confined body with a high electrical conductibility (logρ = 0.97 Ωm). At the deepest position in the model, we correlate thermo-metamorphic rocks with the brines characterized by logρ at 2.7 Ωm, Vp = 3800 m/s and ∆σ -38.8kg/m3. These physical features are indicative for the occurrence of voids/fracturing and the migration of gases. Therefore, we infer a deep source of gases emitted at the surface, which likely also indicates the location of the heat 5 source.
Shallow and deeper aquifers are interconnected via a network of "communicating vessels" through a fault system, allowing deeper and shallower water to mix and being expelled at Pisciarelli. This justifies an apparent persistence of thermal springs around the Agnano Plain also in presence of the desiccating lake described by Ventriglia (1942). It also supports the depth of the water table, being at a higher topographic position in the Solfatara area in respect to the surroundings (Bruno et al., 2007). 10 In the model, we further speculate that the acid-sulfate alteration zone at the Campi Flegrei is actually evidence of a paleoconduit. This is based on field observations showing that alteration deposits locally underlie the most recent eruptive units (e.g., Astroni) that are unaltered. Therefore, the texture of the mineral assemblage, the enrichment in some metals and the lithogeochemical parameters are relict of a "high-sulfidation system". The evolutionary dynamics within the conduit and, in particular, the water overflows from the aquifers alternating with runoff processes, explains the contradictory mineral 15 environments with superimposed intermediate and advanced argillic alteration.
At present, a steam-heated (or low-sulfidation) environment (as derived by most isotope data on alunites; see previous section) is developing in relation to the presence of aquifers and their chemical compositions. This is in agreement with previous studies (e.g., Aiuppa et al., 2017;Piochi et al., 2015;Gresse et al., 2017). Following Hedenquist and Lowensten (1994), this is also in agreement with the shift in δ 18 O at constant δ 2 H values of the emitting fluids (Caliro et al., 2007). Based on the δ 18 O values of 20 alunite, the recalculated environmental temperature is ≤ 200 °C (Fig. 8, inset a).
Furthermore, the presence of NH4 + is considered strictly connected to the surface environment and likely to organic/biological processes (Jaffe, 2000), which is consistent with S-isotope values of pyrite-H2S pairs, the heavier δ 34 S values suggesting microbial sulfur cycling and the supergene environment that is locally developing.

Conclusions and Outlook 25
The acid sulfate alteration zone at Pisciarelli and Solfatara is located in the sector of the Campi Flegrei caldera that was the most volcanically active area in the last 5 ka. The alteration zone includes discrete sub-zones with very constant mineralogy, temperature and chemistry, considering the studied time interval. Outgassing dynamics, weather conditions, and runoff are the most important factors affecting the generation of new mineral phases at the sub-mm-to dm-to m-scales.
The new minerals include alunite, alunogen, native sulfur, pyrite, kaolinite and subordinately mascagnite. 30 The limited areal extent of the alteration zone underlying the most recent unaltered volcanic units, its mineralization texture and style, the δ 18 O and δ 34 S values of S-bearing minerals, and the enrichment in Ti, Ba, Au, As, Hg and Tl, are attributed to the evolution of a paleo-conduit. Our mineralogical and isotopical results overlap with those in Valentino et al. (1999), favouring a stability in the hydrothermal dynamics over the past 20 years. The zone is anomalous in terms of the presence of NH4 + . These features result from the mixing between waters that overflow through a fault system intercepting discrete aquifers 35 supplied by surface water and deep fluids. Most of the alunite forms above the water table at a temperature ≤ 200 °C. At present, the dominant steam-heated environment coexists with local supergenic conditions. Based on presently available data, several key aspects await further investigations.
In particular, a detailed survey of the distribution of aquifers in the subsurface will foster our understanding of caldera dynamics and contributes to the debate existing between a "hydrothermal" (Moretti et al., 2017) vs. a "magmatic" (Cardellini et al., 40 2017) unrest. Assessing the composition and spatial extent of aquifers -also including the contribution from rain fall -is crucial are condensing in the shallower aquifer system (Valentino and Stanzione, 2003;Aiuppa et al., 2007;Caliro et al., 2007;Vaselli et al., 2011;Piochi et al., 2014;Chiodini et al., 2016). However, the ability to buffer these magmatic fluids clearly depends on the water availability in relation to the volume of fluid, with implications on the sourced magma volume(s) evolving/degassing at depth. On the other hand, the circulation of fluids in the subsurface, sourced from both the downward 5 surface infiltration and the ascent of deep fluids, would contribute to the pressurization that is evident through shallow seismicity as previously suggested (Saccorotti et al., 2007;D'Auria et al., 2011;Di Luccio et al., 2015). Most important, knowing the water availability in the subsurface is crucial for evaluating the volcanic hazard in an area dominated by phreatomagmatic events, such as the Campi Flegrei caldera (Rosi and Sbrana, 1987;Di Vito et al., 1999).
What causes the presence of NH4 + is still rather elusive. Based on Moretti et al. (2017), it appears useful for evaluating the 10 temperature-dependence of N2 vs. ammonia production and the relative role of hydrothermal vs. magmatic systems. We add the possible role of organic materials and bacteria, and atmospheric sources to the terrestrial cycle. Furthermore, ammonia NH3 is toxic (Fromm and Gillete, 1968) and this requires ascertaining its concentration level in an inhabited environment.
Finally, the Pisciarelli site appears suitable for studies related to biota and the origin and evolution of life. Here, the waterdominance, nitrogen richness, ≤ 200 °C temperatures and supergenic conditions are all considered important ingredients for 15 the formation of organic substances and the ultimate source of organisms (Jaffe, 2000). Consequently, this site could become a natural laboratory for investigating the complex organic-inorganic interface/relations through multidisciplinary collaborations among mineralogists, geochemists, petrologists, volcanologists, and biologists.

Appendix
Appendix A 20 XRDP and DRIFT-FTIR patterns were acquired at the Osservatorio Vesuviano (Istituto Nazionale di Geofisica e Vulcanologia, Naples, Italy).
The XRDP instrument was a PANalytical X'Pert equipped with a high speed PIXcel detector . The configuration includes Ni-filter, CuKα radiation, pyrolytic graphite crystal monochromator, 40 kV and 40 mA current, 3-70°, 2θ range, 0.02° steps and 8 s/step. X'Pert HIGH Score Plus computer program and JCPDS PDF-2 database allowed the 25 interpretation of diffraction patterns.
DRIFT was mounted on a Nicolet 670 NexusTM both by ThermoFisher Scientific S.p.a.. The FTIR comprises a heated ceramic (Globar) source, a 670 Laser unit, a KBr beamsplitter, and an MCT detector, constantly purged from a high-pressure Nitrox dry air and CO2-trapping 280 generator by Domnick Hunter. The OMNIC Data Collector 5.2© allows data collection and interpretation in the investigated range of 5000 -400 cm -1 (resolution: <0.1 cm -1 ). Data collection was conducted on KBr 30 mixed with 5to -10 % of sample by grounding in an agate mortar, following the background acquisition for the KBr powder.
Additional acquisition on no diluted samples allowed checking for possible hygroscopic effects, obtaining similar results.
The appearance, morphology and chemical composition of minerals were determined on selected samples prepared as opaque mounts coated by cord and rod graphite, by JEOL and ZEISS electron microscope (EDS-BSEM) facilities. The JEOL-JSM 5310, equipped with a Link EDS and a Inca 4.08 software (CISAG Laboratory University of Napoli Federico II), has operating 35 conditions of 15 kV accelerating voltage, 50-100 mA filament current, variable spot size and 50 s net acquisition time. ZEISS instrument is a SIGMA field emission scanning electron microscopy (Osservatorio Vesuviano, department of Istituto Nazionale di Geofisica e Vulcanologia, Naples, Italy), equipped with XMAN micro-analysis system by Oxford, controlled by a SMARTSEM and AZTEC softwares. Operating conditions for SIGMA were 15 kV accelerating voltage, 50-100 mA filament current, 5-10 nm spot size and variable time acquisition time (several to tens of seconds). 40 by Inductively Coupled Plasma Emission Spectrometer (ICP-ES) using LiBO2/Li2B4O7 fusion, minor and trace elements were determined by inductively coupled plasma-mass spectrometry (ICP-MS) using a four acid (HNO3-HClO4-HF-HCl) digestion.
The uncertainty is generally <1% for major/minor oxides, <5-20% for trace elements. LECO was used for determining the C and S abundances. Loss on ignition (LOI) was calculated by weight loss after ignition at 1000°C. 5 Sulfur and oxygen isotope measurements were performed directly on pure mineral separates without and with further chemical preparation in the stable isotope laboratory at the Institut für Geologie und Paläontologie (University of Münster). Chemical preparation was different depending on sample type: i.e., sulfates+elemental S+sulfides, elemental S +sulfides, or muds. Oxidized S-bearing and multi-phase samples first required the extraction of sulfate by sample leaching in a 10% NaCl solution, filtration through a 0.45 micron cellulose acetate filter, and followed by the addition of 8.5% barium chloride solution to 10 precipitate dissolved sulfate as barium sulfate for isotope measurements. Elemental S and pyrite extraction was performed on sulfate-free powders. This extraction consisted of a wet chemical treatment (acidic chromous chloride solution at sub-boiling conditions) that liberates sulfur as hydrogen sulfide which will ultimately be precipitated as silver sulfide, ready for isotope measurements. Elemental sulphur was liberated from each sample via acetone leaching and subsequently converted to silver sulfide applying the acidic chromous chloride treatment (Canfield et al., 1986). Again, resulting hydrogen sulfide was 15 precipitated as silver sulfide (Ag2S). For S-isotope measurements, mineral separates as well as silver sulphide and barium sulfate precipitates were homogenously mixed with vanadium pentaoxide in tin capsules and combusted in a Carlo Erba elemental analyzer interfaced to a ThermoFinnigan Delta Plus mass spectrometer (EAIRMS: Elemental Analyzer-Isotope Ratio Mass Spectrometry). Results are reported in the standard delta notation (δ 34 S) as per mil difference to the Vienna Canyon Diablo Troilite (VCDT) standard. Reproducibility as determined through replicate measurements was generally better than ± 20 0.3 ‰. Sulfates were also measured for O-isotopes by using a ThermoFinnigan TC/EA interfaced with a ThermoFinnigan Delta Plus XL. Results are reported in the standard delta notation (δ 18 O) as per mil difference to the Vienna Standard Mean Ocean Water (VSMOW) standard. Reproducibility as determined through replicate measurements was generally better than ± 0.5 ‰.

Supplement 25
Three supplementary tables: Table S1 -List of samples, collection date, temperature and mineralogical associations as resulting by XRDP analyses and corroborated by FTIR and EDS-BSEM study. The sampling includes water spring sampled at Stufe di Nerone. In the temperature column: tc, thermo couple (see chapter 2.2 Sampling, sample preparation and analytical techniques), infr, infrared 30 gun; infrared derived values are in red. In the mineralogy column: ?, for minerals to be validated; minerals in red are approximate attribution based on XRDP patterns. The orange cells evidence water samples. Selected XRDP traces are in Fig.   S1. Further details in this supplement. Table S2 -Vibration modes and related tentative assignment of functional groups, and mineral attribution for selected samples 35 by DRIFT-FTIR investigations. Alu = alunite, Clay = essentially illite, Masc = NH4 -bearing sulfates, am = amorphous, Kao = kaolinite, KAl = alum -(K). ?, uncertain attribution. Note: assignments and attributions are based on mineralogy derived by XRDP study and corroborated by EDS-BSEM analyses. Further details in this supplement.
in Fig. 1) within the Pisciarelli and Solfatara areas and at different times. MDL indicates the detection limit for major, trace, C and S contents. Figure S1 -Selected XRDP traces of NH4-sulfates dominating the assemblage formed from drying the Pisciarelli water (a, b) 5 and of various multiphase muds from Solfatara (c,e) and Pisciarelli (d,f). Each panel reports the sample name in Table S1. The muds show the large hump between 18 ° and 30 ° 2θ degree attributed to the amorphous phase and the signals from the dominant alunite, feldspars (except MS new 9/17) and other minor phases (sulfur, pyrite and clays; Table S1). The smaller panels evidence the reflection intensity in the most significant range useful to discriminate illite in d) and kaolinites in e). Some XRDP trace has a corresponding infrared spectrum in Fig. S2: the sample with mascagnite is the same of Fig. S2a, the Solfatara 10 muds in c) produced the FT-IR spectra in Fig. S2b, the Pisciarelli mud MP 6_16 is in Fig. S2d, the samples in e) are the same of Fig. S2e. Abbreviations (c, f): S = Sulfur; Al = Alunite; Kfd =Alkali feldspar. Note: montmorillonite needs of validation by specific studies, as the very small signals (Fig. S2f).  Table S2 for vibrational modes and relative assignments.

Team list
Monica Piochi, Angela Mormone, Harald Strauss, Giuseppina Balassone 9 Author contribution MP and AM conducted sampling campaigns and prepared samples for analyses. GB participated in some of the sampling 20 campaigns. AM conducted the XRPD analyses and interpreted the patterns. MP acquired, elaborated and interpreted the DRIFT-FT-IR spectra and, in collaboration with AM and GB, performed the EDS-BSEM investigations. HS determined the stable isotope values and contributed to data elaboration; MP did data representations and stable isotope data modelling. MP prepared the manuscript. All authors contributed into the final manuscript.

Acknowledgements 25
Osservatorio Vesuviano (Istituto Nazionale di Geofisica e Vulcanologia) funded analyses of whole-rock geochemistry; we are therefore grateful to the Directors, namely Giuseppe De Natale and Francesca Bianco. The study also benefits of the INGV fund FIRS 08-6-5-056 granted to M. Piochi. We are also grateful to colleagues at the Osservatorio Vesuviano: Rosario Avino is kindly thanked for BG sample collections in 2018, Enrica Marotta and Pasquale Belviso who provided thermos-probe. Dr Angarano, Tennis Hotel and Stufe di Nerone allowed the free access at the sampling sites. We appreciated comments and 30 suggestions from Franco Pirajno and an anonymous reviewer that improved the data presentation and discussion. We would also like to thank the Editor Kei Ogata and editorial staff for managing this manuscript.