Mineralogy and Geochemistry (HFSE and REE) of the Present-Day Acid-Sulfate Types Alteration from the Active Hydrothermal System of Furnas Volcano, S ã o Miguel Island, the Azores Archipelago

: Acid-sulfate alteration is comprised by clays, sulfate, sinter, and native sulphur minerals crystallized as neoformation products from the dissolution of primary minerals during water–rock interaction. Smectite, kaolinite, halloysite-7 Å, and opal-A occur in assemblages with alunite. Smectite represents a mechanical mixture between two (propylitic and acid-sulfate) alteration types. High amounts of high-ﬁeld strength elements (HFSE) and rare earth elements (REE) were measured in acid-sulfate rocks. The Nb vs. Ta and Zr vs. Hf show a positive trend and widely scattered relationships, suggesting a large fractionation during acid-sulfate alteration. Higher Σ REE amounts (up to 934.5 ppm) were found in clay-sulfate assemblages and lower Σ REE amounts in sinter (opal-A ± sulfate, 169.05 ppm) than fresh rocks (up to 751.2 ppm). The acid-sulfate rocks reveal a distinctive gull-wing chondrite-normalized pattern with a negative Eu anomaly and light- and heavy-REE “wings” similar to the gull-wing pattern of fresh rocks. The Eu/Eu* shows a large fractionation of acid sulfate rocks from 0.16 to 0.78 with respect to fresh trachyte products (0.10–0.38). The variation of (La/Sm) N and (La/Yb) N ratios show a large fractionation of light-REE and heavy-REE. The Y vs. Dy and Y vs. Ho show a very good positive correlation coefﬁcient and a large Y fractionation in acid-sulfate rocks with respect to fresh trachyte rocks.


Introduction
Acid sulfate type alteration is associated with geothermal systems related to volcanoplutonic activity along convergent and divergent plate boundaries, where steam condensate forms and mixes with meteoric water encouraging intensive water-rock interaction and silicate hydrolysis, typically at high water-to-rock ratios [1][2][3][4][5][6]. Fluid discharged at the surface corresponds to near-neutral chloride-rich hot spring waters or to acid-sulfate boiling pools related to steam that separates from deeper chloride-rich boiling fluids in vapour dominated systems resulting in fumarolic activity containing CO 2 and H 2 S [6][7][8].
Low pressures favor high SO 2 /H 2 S and HCl/NaCl in the emitted magmatic vapors, increasing the acidity of near-surface condensate [9,10]. The acid formation is further favored where crater lakes occur at the surface, as the supply of capping groundwater to minimizes the possibility that the gas-steam mixture can escape directly to the atmosphere [11]. Thermodynamically, the acid-sulfate alteration stage is connected with the low-enthalpy (T < 200 • C) water resources, representing a much larger potential and a wider regional distribution than high-enthalpy resources [12].
Such environments are highly analogous and inseparable from the high-sulfidation environments responsible for some types of epithermal ore formation [5,13,14]. Steadily,

Geology
The Azores archipelago is located close to the Mid-Atlantic Ridge, where the three lithospheric plates (American, Eurasian, and African) are joined as a triple junction [31][32][33]. The nine islands of the Azores archipelago ( Figure 1a) are constituted by trachyte pyroclastic rocks deposited during several phases of Plinian and sub-Plinian eruption type [34]. The volcanic products range in composition from basanite to trachyte, where the products erupted within caldera complex are trachytic in composition consisting of trachyte and latite [34,35].
The São Miguel Island, one of the nine islands, is constituted by three major trachytic central volcanoes of Furnas, Fogo, and Sete Cidades (Figure 1b) linked by rift zones, where the volcanic activity along the rift is represented by basaltic effusive eruptions accompanied by a strombolian cone building [34]. Since 1970, detailed studies led to considerable advances regarding to a thorough knowledge of the explosive volcanism explosive [35] and of the tephra deposits stratigraphy from all volcanic eruptions in the São Miguel island [36].
Furnas, one of the most active and hazardous volcanoes, has exhibited effusive (dome forming) to highly explosive (caldera-forming) eruptions of felsic magmas [37][38][39] during its subaerial existence. Volcanism within the caldera complex has been exclusively trachyte in composition, with mafic products being limited to vents on the volcano's flanks. The geology of the Furnas stratovolcano ( Figure 1c) is constituted by an accumulation of trachyte and trachybasaltic lava flows with an episodic character, changing during 15,000 years from Plinian to sub-Plinian type [34].
The volcanic products range from basanite through alkali olivine basalt, potassic trachybasalt, basalt trachyandesite (shoshonite), trachyandesite (latite), to trachyte [38]. The pyroclastic rocks are trachyte in composition, consisting of latite and trachyte rocks within the caldera complex of Furnas volcano, where vents erupting basic lava are restricted to the flanks of the volcano.
The Upper Furnas Group (UFG) is build-up of at least ten intra-caldera, sub-Plinian eruptions of trachyte pumice, named Furnas A-J, where the latter, younger in age [36], is also known as Furnas AD 1630 [37,39]. Three of these eruptions correspond to lava domes in their final stages (Furnas E, I, and J) [37]. The dominant lithologies of the UFG consist of inter-bedded pumice lapilli and ash beds, inferred to be the result of a complex transition between magmatic and phreatomagmatic activity during eruption, whereas Furnas H is consisted exclusively by magmatic activity [37,38].
The main WNW-ESE fracture systems identified cross the volcanic structure, including a conjugate faults system with N-S and NE-SW trends well-represented to the south coast [38].
Post-volcanic activity is characterized by hot springs, boiling pools, gassy cold springs as well as fumarolic fields referring to the areas of several hydrothermal systems related to the Quaternary volcanoes of Furnas, Fogo, and Sete Cidades [40]. The post-volcanic activity in the Furnas volcanic complex was first described by Zbyszewski [41], where hot springs related probably to caldera-bounding fault [35] can be observed around The stratigraphy of Furnas volcanic complex was divided in three groups [38], such as: Lower Furnas group (Amoras Formation and Povoação ignimbrite Formation, −30,000 BP), Middle Furnas group (Mouco Formation, Ponta Graça Ignimbrite Formation and Cancelinha Formation, 12,000 BP) and Upper Furnas group (1630 AD).
The Upper Furnas Group (UFG) is build-up of at least ten intra-caldera, sub-Plinian eruptions of trachyte pumice, named Furnas A-J, where the latter, younger in age [36], is also known as Furnas AD 1630 [37,39]. Three of these eruptions correspond to lava domes in their final stages (Furnas E, I, and J) [37]. The dominant lithologies of the UFG consist of inter-bedded pumice lapilli and ash beds, inferred to be the result of a complex transition between magmatic and phreatomagmatic activity during eruption, whereas Furnas H is consisted exclusively by magmatic activity [37,38].
The main WNW-ESE fracture systems identified cross the volcanic structure, including a conjugate faults system with N-S and NE-SW trends well-represented to the south coast [38].
Post-volcanic activity is characterized by hot springs, boiling pools, gassy cold springs as well as fumarolic fields referring to the areas of several hydrothermal systems related to the Quaternary volcanoes of Furnas, Fogo, and Sete Cidades [40]. The post-volcanic activity in the Furnas volcanic complex was first described by Zbyszewski [41], where hot springs related probably to caldera-bounding fault [35] can be observed around to the Lagoa of Furnas, the village of Furnas, and along the upper Ribeira Quente brook at Ribeira dos Tambores close to the western bank of the brook. Hydrothermal features are naturally discharging water, e.g., including water-rich like boiling pools, thermal springs, as well as steam vents in total dissolved solids. The physic and chemical characteristics of thermal waters from fumaroles, boiling pools or hot springs are exposed in several works [29,30,42]. The gaseous composition of fumarolic grounds consist mainly of CO 2 with minor amounts of H 2 S and N 2 [30], where chemical flux of carbon dioxide estimated is about 9358 tons/yr [30].

Sampling and Field Observation
Altered volcanic rocks were collected from outcrops around to low-temperature fumaroles and steaming grounds from Caldeiras-Furnas village) and Lagoa das Furnas, both located in the intracraterial area of Furnas's volcano. Two fresh trachytic rocks were collected for mineralogical and chemical analyses; one of this is a glassy pumice and is defined hereafter as "trachyte pumice".
Caldeiras-Furnas village area, ca +229 m altitude: The largest and representative area (several hundred square meters) of fumarole activity in the São Miguel islands occurs in the Furnas village located within the craterial area of Furnas volcano. Boiling fumaroles with steam explosions and steam heated pools (Figure 2a) of a mixed hydrothermal and surface waters are discharged at the surface. Interaction between fumarolic acid plume and the ground surface ( Figure 2b) and outcropping rocks (e.g., trachyte pumice and trachyte) at a pH about 2-3 is well observed by the widespread acid-sulfate alteration in the proximal areas. Colour changes from yellowish, reddish or bloody to white of the acid-sulfate rocks was observed as an impressive peculiarity ( Figure 2c). Also, finest veins exhibiting green to blue colour within white sulfate or argillic rocks were observed. Sinter interbeddings of 2-3 cm thickness precipitated directly from solutions occurs near to stream lines of the Ribeira Quente brook.
Lagoa das Furnas (Furnas lake) area, ca +323 m altitude: Fumarole activity and steaming grounds are well spread around the Furnas's lakes (Figure 2d). The area is well exploited within magnify scenery, where local people take advantage of fumarole activity for cooking (the so-called "cozido") of the traditional gastronomy. Steaming grounds are characterized by mud-rich amorphous phase and hallloysite-7Å. Several locations were recognized where a mixture of steaming grounds and sulfate-clay rocks do occur.

Analytical Methods and Samples Preparations
X-ray diffraction. Bulk samples were crushed in an agate mortar and pestle to pass through a 30-mesh sieve. The <2 µm clay fractions were separated at an initial clay (+sulfate)/water concentration of 10 g/L by the sedimentation method according to Stokes law, where the clay fractions were concentrated by centrifugation. No chemical treatments were used. The mineralogy of bulk samples and clay fractions was determined by X-ray diffraction (XRD) using a Rigaku Geigerflex D/max (Rigaku Corporation, Tokyo, Japan). C series X-ray diffractometer machine equipped with a CuKα radiation, an automatic divergence slit and a 0.5-receiving slit. The X-ray diffraction of random samples was analyzed in the range 2-70 • 2θ, with a step size of 0.05 • 2θ and 5 s counting time. Oriented clay aggregates corresponding to smectite clay fractions were X-ray run in air-dried and ethylene glycol conditions. Rich opal samples were scanned at 0.6 • 2θ from 10 to 70 • 2θ with a step of 0.01 • [18] with a Philips X'Pert diffractometer machine. The intensity of full-width half maximum (FWHM) of the distinctive broad peak at~4-Å attributed to opal was measured using Profile Fitting program (version 2016, Philips X'Pert ©, Philips, Amsterdam, the Netherlands). The nomenclature of the non-crystalline silica phase used is according with literature data [43][44][45].
Scanning electron microscopy: Morphological study of minerals was performed using a Hitachi S-4100 scanning electron microscope (SEM) (Hitachi, Tokyo, Japan), working at 25 kV. Freshly cleaved rock chips were mounted on a carbon holder and sputter-coated with a thin carbon film.
Electron microprobe analysis: Quantitative mineral chemical compositions were obtained on doubly-polished thick sections (200 µm) of acid-sulfate samples using a Jeol Hyperprobe JXA-8500F electron microprobe (EPMA) (JEOL Ltd., Tokyo, Japan) operated at 15 kV accelerating voltage and 10 nA beam current in the case of silicates and sulfates. The intensity data were corrected with a ZAF program and detection limits (3τ) above mean background were 0.03 wt.% for most oxides with counting times of 80 s. Chemical analysis: Samples selected from the acid-sulfate alteration for chemical analysis correspond to different mineral assemblages identified previously by XRD (Table 1). Major, trace and rare earth elements chemistry were measured on whole altered and freshly rocks by inductive-coupled plasma-mass spectrometry (ICP-MS), using the Li-tetraborate procedure at ACTLab (Vancouver, BC, Canada). The REE patterns were plotted against the chondrite normalisation reference [46] in order to visualise the fractionation processes of either light-(LREE) relative to middle-(MREE) or heavy-rare earth elements (HREE) and Eu-anomaly.
Sinter interbeddings of 3 to 5 cm thick containing alunite + opal-A or opal-A + alunite assemblages were observed in acid-sulfate altered pumice rocks. The main component of sinters is opal-A identified by XRD, where a very broad reflection centred at about~4 Å with a raw intensity of about 700 counts s −1 was observed ( Figure 3c). The FWHM value corresponding to~4 Å reflection is considered to indicate the degree of order-disorder of opal-A structure [18]. The FWHM measured is about 0.9 • 2θ under the operating conditions for the samples collected.
Lagoa das Furnas: The XRD pattern of the <2 µm clay fractions extracted from the mud pools shows smectite, kaolinite, and alunite. The XRD pattern of smectite is characterized by the d 001 peak at 15 Å in air-dried conditions, which expanded after EG solvated at 15.9 Å (Figure 3d). A small amount of feldspar occurs in the <2 µm fractions.
Supergene alteration: Pale green and blue colour fine veins 1-3 mm thick were observed in bearing rich-alunitic rocks. These fine veins correspond to melanterite (FeSO 4 .7H 2 O) precipitation, where these soluble sulfates (i.e., melanterite, etc) represent unstable phase which stored only Fe 2+ . The colour of these finest veins disappeared some hours later from the samples collected in plastic bags.
Also, several places were identified in the Furnas village (e.g., Calderia dos Vimes, along the Ribeira Quente brook) and Caldeiras da Lagoa das Furnas where the acid-sulfate rocks supported changes of colours from white to yellowish and red colour related to Fe 2+ oxidation and precipitation of FeO(OH). Jarosite precipitated in the supergene conditions at a low pH (<3), with high Eh and sulfate available [48].
Supergene alteration: Pale green and blue colour fine veins 1-3 mm thick were observed in bearing rich-alunitic rocks. These fine veins correspond to melanterite (FeSO4.7H2O) precipitation, where these soluble sulfates (i.e., melanterite, etc) represent unstable phase which stored only Fe 2+ . The colour of these finest veins disappeared some hours later from the samples collected in plastic bags.
Also, several places were identified in the Furnas village (e.g., Calderia dos Vimes, along the Ribeira Quente brook) and Caldeiras da Lagoa das Furnas where the acid-sulfate rocks supported changes of colours from white to yellowish and red colour related to Fe 2+ oxidation and precipitation of FeO(OH). Jarosite precipitated in the supergene conditions at a low pH (<3), with high Eh and sulfate available [48].

Scanning Electron Microscopy and Electron Microprobe Analysis
Scanning electron microscopy observations show well pseudoheganal plates of kaolinite sometimes with undefined plate shapes (Figure 4a), where plates varying in size (a × b) from 0.25 × 1.25 to 4 × 6.5 µ m 2 (estimation carried out on 30 crystals). Kaolinite with a book-shape plates morphology wasn't observed in the samples studied. Long tubes of halloysite-7 Å with lengths about 25 µ m were also identified in the alunite and opal-A assemblage (Figure 4b). Alunite crystals display either pseudo-cubic or rhomboidal habit

Scanning Electron Microscopy and Electron Microprobe Analysis
Scanning electron microscopy observations show well pseudoheganal plates of kaolinite sometimes with undefined plate shapes (Figure 4a), where plates varying in size (a × b) from 0.25 × 1.25 to 4 × 6.5 µm 2 (estimation carried out on 30 crystals). Kaolinite with a book-shape plates morphology wasn't observed in the samples studied. Long tubes of halloysite-7 Å with lengths about 25 µm were also identified in the alunite and opal-A assemblage (Figure 4b). Alunite crystals display either pseudo-cubic or rhomboidal habit (Figure 4c,d). Coalesced aggregates of opal-A exhibit a lepisphere shape (Figure 4e). Finally, pumices exhibit evident devitrification processes originating from the embryonic opal lepishere (Figure 4f), where successive coalescences of small lepispheres show the overgrowths in a gel-like texture. Kaolinite, alunite, and feldspar were analyzed by EPMA ( Table 2). Kaolinite (normalization at 14 oxygen atoms) contains only Si and Al in tetrahedral and octahedral sheets. Alunite (normalization at 14 oxygen atoms) contains both K + and Na + fixed cations in different proportions beside Al 3+ and SO 2 . Undissolved feldspar (normalization at 8 oxygen atoms) found in acid-sulfate rocks shows a mean normative composition of An 4 Ab 58 . 2 Or 37 . 8 (Table 2) which lies at the orthoclase-sanidine boundary in the ternary diagram of the An-Ab-Or system (not shown).

HFSE and REE Geochemistry
Trace and rare earth elements were measured on altered and freshly trachyte (Table 3) from the Furnas craterial volcanic area. Concentrations of lithophile (Cr, Rb, Cs), siderophile (Co, Ni, W, U), and chalcophile (Cu, Pb, Zn, As, Ga, Sn) elements show values close to those values measured in fresh rocks.
High-field-strain elements measured in altered acid-sulfate sulfate rocks show high concentrations with respect to fresh trachytic rocks, being reliable geochemical indicators own to their immobility in most geological settings. Nevertheless, there is clear evidences which suggested their mobility in fluid-driven systems due to the influence of halogens, e.g., fluorine and chlorine [49,50]. High amounts of Nb (78.77-623.8 ppm), Zr (424.6-3192.5 ppm), Hf (9.8-70 ppm), and Y (9.4-53.9 ppm) were identified in acid-sulfate rocks, where mud samples contain higher concentrations than clay-sulfate or sinter rocks. The Nb vs. Ta and Zr vs. Hf diagrams (Figure 5a   The REE chondrite normalized [46] patterns corresponding to sulfate-kaolin minerals ± opal-A (alunite-clay rocks), opal-A ± sulfate (sinter) and smectite + kaolinite ± alunite (mud) assemblages were compared with those of fresh trachyte and pumice lapilli rocks (Table 4).
Chondrite-normalized REE patterns of selected samples ( Figure 6) display a gull-wing shapes [52] where water-rock interaction process plays a significant role in this shape-type formation [53]. The Eu negative anomaly (the gull body) and light (La/Sm) and heavy (Gd-Lu) "wings" suggest an association with acid-sulfate systems observed elsewhere (i.e., Taupo, Vulcano, etc.) [54,55].
The ΣREE (  (Table 5) is very large ranging from 0.16 to 0.78 in acid-sulfate rocks (including sinter and muds). The Eu depletion in whole acid-sulfate rocks compared with unaltered rocks, imply a dissolution of sanidine mineral and glass during acid sulfate alteration of trachyte (Eu/Eu* = 0.38) and pumice lapilli (Eu/Eu* = 0.10) rocks. Higher Ce/Ce* ratio values (Table 5) from 0.94 to 1.45 were found in acid-sulfate rocks than in fresh volcanic rocks (0.84 to 0.99). In the case of a possible positive Gd anomaly, the interpolation occurs between Sm and Tb, where the Eu shows a negative anomaly. However, the Gd/Gd* (Gd/Gd* = Gd*/(0.33Sm N + 0.67Tb N ) calculated ratio is <1 (Table 5), which means no positive anomaly [56].  The (La/Sm) N and (La/Yb) N ratios (Table 5) of acid sulfate rocks compared show two distinct fields with a large fractionation also observed in the (La/Sm) N vs. (La/Yb) N diagram (Figure 7).
A strongly depletion of MREE (55.04 to 9.76) and HREE (15.25 to 2.54) show the acidsulfate rocks comparatively to trachyte (53.96 to 45.89) and trachyte pumice rocks (15.29 to 12.47), respectively. As we expected, the (Tb/Yb) N ratio shows a large fractionation from 0.96 to 1.84 in acid-sulfate rocks relatively to fresh rocks (1.67 to 1.57), as well as no (Tb/Lu) N fractionation was observed (Table 5).  Yttrium(III) content, often considered an analog of HREE due to the fact of it having a radius close to Dy(III) or Ho(III), ranges from 9.4 to 53.9 ppm in acid sulfate rocks (Table 5). Y and HREE become fractionated in aqueous systems, resulting in non-chondritic Y/Dy and Y/Ho ratios [57]. The Y/Dy ratio shows a large fractionation from 3.45 to 5.79 in acid-sulfate rocks with respect to fresh trachyte pumice and trachyte rocks (6.72 and 8.11). Also, Y/Ho ratio ranges from 23.77 to 28.48 (in fresh rocks from 28.82 to 32.17). The data plotted in the Y vs. Dy and Y vs. Ho diagrams (Figure 8a,b) show a very good positive correlation coefficient, where Y and Dy(Ho) are large fractionated in acid-sulfate rocks with respect to fresh rocks. The Y/Dy and Y/Ho ratios (Figure 8c) show a lower ratio and a weak fractionation with a low correlation coefficient with respect to fresh rocks.

Discussion
Mineralogy. The thermal water rising to the surface in hot springs is entirely meteoric water heated by vapours emanating from the magma chamber. The acidity of the fumarole and thermal springs is a consequence of H 2 S and S oxidation with production of H 2 SO 4 at low temperature (below 200 • C) in a convective meteoric system. The acid-sulfate meteoric fluid circulation in a volcanic or post-volcanic environment interacted with the minerals from trachyte and trachyte pumice rocks, generating a suite of neoformation minerals that represents a high potential for REE concentrations in geothermal waters. Sulphuric acid production during sulphur oxidation dissolved silicates from host rocks producing clays, muds, sulfates and sinter.
Acid-sulfate alteration is comprised by clays, sulfate, sinter, and native sulphur minerals crystallized as neoformation products from the dissolution of primary minerals during water-rock interaction at temperatures around 100-150 • C [30]. Kaolinite and halloysite-7Å crystallization is a function of the degree of solution supersaturation within a pH~3. The kaolinite shape morphology observed in acid-sulfate alteration from the Furnas area is different than the morphology observed during kaolinite formation from mica or feldspar in granitic rocks [58,59]. Halloysite-7Å was found associated always within the alunite + opal-A assemblage, whereas well-ordered kaolinite occurs either as single mineral phase in the <2 µm fraction or in assemblage with alunite, quartz or smectite. Quartz resulted as a by-product during kaolinite precipitation. Also, in weathered pumice, rocks occur halloysite-10Å [60].
Smectite occurs in steamy grounds near the Furnas lake in assemblage with kaolinite (halloysite-7Å) + alunite, representing a mechanical mixture between two alteration types. This means that smectite crystallized under different conditions related to an older crystallisation event imposed by a propylitic stage than by the present-day hot fluid circulations which generated acid-sulfate clay. Smectite crystallized from glassy dissolution of pumice rocks rather than feldspar, where the reaction of smectite formation depends on glass chemical composition, the fluid pH and fluid/rock ratio [61][62][63].
In a sulfate-rich system, alunite forms when H + activity decreases and both K + /Na + and SO 4 − activities increase. Alkali-ion concentrations might rise to high values to stabilise alunite formation. Native sulphur resulted from subsequent oxidation of H 2 S at the surface of the alteration zone. Native sulphur is one of the last minerals which precipitate in a sulfate-rich system related to a post-volcanic activity, being a common mineral found also in other active hydrothermal systems [24,64,65]. Also, the sulfuric acid is produced by hot springs due to near surface oxidation of effluent S-bearing gases close to~95 • C in an environment with pH at 4.5 [66]. Interbeds of sinter were deposited at the surface as the result of silica excess polymerised forming a colloidal suspension that subsequently produced either silica amorphous or opal. Alternately, silica may form complexes with sulfate, where the excess of high-silica supersaturation may precipitate in the acid environment, as sulfate is removed from solution forming alunite. During supergene alteration, the oxidation reaction rates of Fe 2+ into Fe 3+ increased either due to microbial activity or oxygen from air or water conditions (Fe 2+ + 1/4O 2 + H + = Fe 3+ + 1/2H 2 O). The hydrolysis and precipitation of FeO(OH) will produce most of the acid in this process. If pH is less than about 3.5, Fe(OH)3 is not stable and Fe 3+ remains in solution [Fe 3+ + 3 H 2 O = Fe(OH) 3 (s) + 3H + ]. Jarosite is generally formed at oxidizing low-pH conditions (pH < 3) being the first phase precipitating subsequently to alunite formation during hydrothermal conditions. Jarosite formation is limited by the availability of K or Na deriving from feldspar alteration in a more acidic micro-environment [67], being disseminated in the oxidation zone outer of fumarole and hot springs areas. Melanterite is one of the first simple sulfates precipitated in the high SO 4 2− concentration in extreme low pH, especially in acid mine drainage environments where it promotes the speciation of Me n+ with SO 4 2− [68,69]. The dehydration process of melanterite is accompanied by its dissolution, generating acid production upon dissolution and hydrolysis [70].
High field strength elements. The HFSE can be transported by a variety of fluids under magmatic, metamorphic and hydrothermal environments. Consequently, they are also used as tracers to distinguish the protoliths of altered rocks based on its "conservative" nature [71]. The Nb, Ta and Zr have significantly higher concentrations in hydrothermal altered rocks as compared to fresh rocks. The geochemical behaviour of Nb and Ta is intimately linked to that of the more abundant titanium in oxide minerals. Thus, the dominant mineral hosts of Ti, Nb and Ta in crustal rocks are Fe-Ti oxides such as: rutile, ilmenite and Ti-magnetite. Both Ti-magnetite and ilmenite occur in the trachyte rocks (sub-Plinian eruption) of the UFG [72], where the Nb and Ta budget was stored in these minerals during magmatic crystallization. However, both Nb and Ta are insoluble at low temperatures, remaining to be incorporated probably into hydrothermal rutile formed after oxidation of Fe-Ti-oxides.
The Nb vs. Ta and Zr vs. Hf diagrams (Figure 5a,b) display a positive correlation trend and a widely scattered positive relationships suggesting a large fractionation during acid-sulfate alteration. Otherwise, the Nb/Ta and Zr/Hf ratios (Figure 5c) remained rather constant with lower values than to fresh rocks, confirming a fractionation in an acid low-temperature environment fraction.
No secondary Zr-or Hf-minerals were reported after dissolution of primary and accessory silicates in acid-sulfate rocks altered. However, the breakdown of sodic pyroxene and amphibole released Zr to the hydrothermal fluids, resulting in a gain of Zr in the hydrothermally altered rocks [73]. The presence of SO 4 complexes in hydrothermal systems could facilitate the transport of Zr and Hf under acidic and oxidizing conditions [69,70], where dominating Zr species are hydroxides such as Zr(OH) n [74].
Rare earth elements: The REE geochemistry of acid-sulfate alteration provide information concerning the water-rock reaction, where the relative abundances of REE in solid phases depend upon the temperature, fluid chemistry, and the nature of neoformation phases [75,76].
Glassy matrix from trachyte pumice rocks and feldspar (sanidine), pyroxene and amphibole from trachyte rocks are the main sources of REE during cold water/rocks interaction. Higher temperature and lower pH caused dissolution of glassy and minerals in the host rocks, increasing the REE concentration as pH decreased.
The REE chondrite normalized patterns of acid-sulfate samples inherited the trend of trachyte and pumice rocks, where a chondrite normalization is revealed by Eu depletion. Also, a chondrite-normalized REE pattern similar to the "gull-wing" trend shows the host rocks, where the less Eu-rich feldspar during cold water-rocks interaction is well highlighted in all neoformation products related to acid-sulfate envelope. The REE pattern of acid sulfate rocks is dominated by the REE trend of whole fresh rocks.
Higher ΣREE amounts (up to 934.5 ppm) were found in clay-sulfate assemblages and lower ΣREE amounts in sinter (opal ± sulfate, 169.05 ppm) than to fresh rocks (up to 751.2 ppm). The total concentration of dissolved REE in waters (fumaroles and boiling pools) from the São Miguel island ranges between 8.0 and 3169 nmol L −1 [42], reflecting a near congruent dissolution of trachyte rocks.
The clay-sulfate samples show an enrichment in LREE, with a high (LREE/HREE) N ratio (up to 18.35), suggesting a very large fractionation across to REE series. However, the opal ± sulfate (sinter) assemblage has a lower (LREE/HREE) N at about 4.22. Opposing to hydrated silica, lower amounts of REE are incorporated in quartz [77]. The fluid pH had controlled the take-up of REE into sinter (390.08 ppm) or mud rocks (934.5 ppm). Furthermore, Woitischek [42] identified a high LREE depletion vs. HREE in boiling pools waters (Furnas area) characterized by lower pH and higher SO 4 contents, being explained by the alunite precipitation.
The Eu/Eu* shows a large fractionation of acid sulfate rocks from 0.16 to 0.78 with respect to fresh trachyte and trachyte pumice rocks (0.10-0.38), indicating a negative Eu anomaly. Also, a negative Eu anomaly (0.13-0.60) was found in waters [42] that leached the trachytic rocks (Eu/Eu* = 0.15 and 0.56) from the Furnas area. The presence of the Eu anomaly can be used as a water-rock interaction proxy reflecting the dominance of trachyte leaching in the present case [78].
The pronounced negative Eu anomaly trend is interpreted as a partial degree of feldspar alteration leading to lower Eu content. In fact, Eu is fractionated during fluidrock interaction under suitable conditions of oxygen fugacity, temperature and pH as a consequence of Eu 3+ to Eu 2+ changes allowing Eu 2+ to form hydroxide complexes more stably than Eu 3+ [79,80].
The LREE enrichments and positive Eu anomaly [81] imply the dissolved REE complexes, REE mobility and fractionation induced during the secondary mineral crystallization. Also, a positive Eu anomaly in low pH sulfate-rich hydrothermal fluids from Yellowstone was reported [82], suggesting a preferential breakdown of Eu-rich sanidine.
Variation of (La/Sm) N and (La/Yb) N ratio show a larger fractionation during the acid-sulfate alteration compared to fresh trachite and trachyte pumice rocks, where two distinct trends are observed. One from fresh rocks to sinter and mud and another towards kaolin alunite ± opal-A. The (La/Sm) N ratio ranges from 3.46 to 20.31 (fresh rocks from 5.40 to 5.99) and (La/Yb) N ratio from 16.77 to 73.28 (fresh pumice and trachyte rocks 21.44 to 19.03). In addition, a larger fractionation of (La/Ce) N ratio of 0.85-1.91 of acid-sulfate rocks than fresh rocks (1.64-1.32) was also observed.
Yttrium amount measured reflects the REE abundances and mobility in acid-sulfate rocks. In fact, Y and Ho or Dy fractionation (with similarly charged and sized) is observed during the precipitation of alunite and clays where a positive correlation was highlighted ( Figure 8). This means that during acid-sulfate alteration process the non-chondritic Y and Ho or Dy remained tightly coupled towards sinter formation where lower amounts were found. Furthermore, non-chondritic Y/Dy and Y/Ho ratios of acid sulfate rocks remained constant and lower values than the fresh rocks, providing good evidence for Y vs. Ho(Dy) fractionation at low temperature.
HFSE show a large fractionation during acid-sulfate alteration stage and the REE concentrations increased in acid-sulfate rocks in the convective hydrothermal system of Furnas. The REE are released from primary minerals (i.e., feldspars) and taken up by the alunite and clays. The REE are sorbed by sulfate complexes, but the relative proportions of Ln 3+ and LnSO 4 are related to pH and the type of crystal-chemistry of the sulfate precipitated [83]. REE adsorption onto kaolinite surfaces shows a clear pH dependence, where dominant electrostatic interaction and specific site binding due to the negatively charged kaolinite surface occur at low pH from 3 to 4 (needed for kaolin formation), which enhanced the REE adsorption [84,85].
Acid-sulfate alteration model: The alteration of primary minerals and direct precipitation from oversaturated solutions in active geothermal fields depends on temperature, composition and pH of solutions, fluid/rock ratio, where several clay formation episodes may be recorded in the same rock.
The acid-sulfate alteration model ( Figure 9) proposed for the Furnas area is based on the classical model [86] where the distribution of isotherms above the heat source is influenced by the convective system recharged by meteoric or/and sea waters. Isotopic (δ 18 O H2O = -3.3 ‰ and δD H2O = −16.9‰) composition of the discharged waters (spring and boiling pools) from the Furnas correspond to meteoric water or mixture of seawater and meteoric water, whereas the sulfate source comes from the shallow H 2 S oxidation [42].
The acid-sulfate alteration identified, where steam heated overprint on several steam vents, hot springs or mud pools, is the main alteration type in the Furnas craterial area. The acid stage (interaction fluid/trachyte rocks) is characterized by a kaolinite + alunite + halloysite-7Å + opal-A + native sulphur assemblage. The total leaching of cations from primary minerals (i.e., sanidine) of trachite rocks near the vents, including Al, is due to the pH stabilization in the very acid domain by H 2 S oxidation. Under acidic hydrothermal fluids (pH < 4 at 20 • C) circulation in a convective meteoric system, the Al becomes more soluble than silicon. Low or high supersaturated solution of Si and Al will favor the crystallization of kaolinite or halloysite-7 Å [87].
Since a major part of the Al dissolved in solutions is consumed by alunite precipitation (due to the H 2 S oxidation) and the growth of kaolinite is limited [88], alunite is the most stable solid phase with respect to solubility controls on Al [89], where the Al concentration in solution is controlled by the growth kinetics of alunite. The Si concentration is independently controlled by the solubility of amorphous silica in particular glassy from pumice rocks. Opal-A lepispheres form in silica-saturated hot-spring fluids, where silica polymerisation promotes growth and precipitation of colloids aggregate to form a friable, porous, weakly cemented deposit occurs in young sinter [90,91].
The presence of smectite in assemblages with alunite + kaolinite from mud pools and steaming grounds (mud clay) near the Furnas lake suggests different crystallization conditions (intermediate-type system with intermediate ratio Ca 2+ , Na + , Mg 2+ /H + ) than the acid sulfate alteration stage previously described. Smectite, besides opal-A, zeolites, etc., is one of the neoformation minerals crystalized during glass alteration [59,60,63] at a low temperature related to a propylitic alteration supposed to occur below 100 m depth underground. Smectite occurs in a mechanical mixture with kaolinite and alunite crystallized during the acid-sulfate stage, probably being discharged along the fracture zones by hot convective waters.
HFSE and REE provide important insights into the active hydrothermal-meteoric system of Furnas volcano, where their mobilization within the acid-sulfate alteration was controlled by temperature, pH, and solution chemistry. The results suggest that acid-sulfate alteration plays an important role in the REE controlling and fractionation, being a transient REE storage within this alteration stage.

Conclusions
High-field strain elements and REE provide important insights into the active hydrothermal meteoric system of the Furnas volcano, where the confirmed mobilization within the acid-sulfate alteration was controlled by temperature, pH, and solution chemistry. This study shows that HFSE are significantly fractionated in acidic, low-temperature environments and oxidizing conditions, where higher amounts were stored during acidsulfate stage formation after the breakdown of pre-existing HFSE-bearing accessory minerals and probably formation of new minerals (i.e., hydrothermal rutile, etc).
Also, the acid-sulfate alteration plays an important role in the REE controlling and fractionation, providing transient REE storage in clays and sulfate minerals. Non-chondritic Y vs. Ho(Dy) displays a clear fractionation generated during fluid migration where different neoformation phases were discriminated within the acid-sulfate stage.
There are no differences of the REE trends between fresh rocks, fluid, and acid-sulfate rocks where subparallel trends have resulted owing to the low alteration intensity and low fluid/rock ratios. A significant negative Eu anomaly (the gull body) observed in all samples (including sinter) confirms less alteration of Eu-rich feldspar. The REE are differentially mobilized during acid-sulfate alteration where large negative Eu-anomaly and LREE enriched were observed in clay-sulfate assemblages. Kaolin + alunite and smectite + kaolin + alunite (mud) assemblages retained higher amounts of LREE than opal-A + alunite (sinter), whereas the HREE are strongly depleted in sinter.