Hydrothermal Aluminum-Phosphate-Sulfates in Ash from the 2014 Hydrothermal Eruption at Ontake Volcano, Central Honshu, Japan

: Aluminum-phosphate-sulfates (APS) of the alunite supergroup occur in igneous rocks within zones of advanced argillic and silicic alteration in porphyry and epithermal ore environments. In this study we report on the presence of woodhouseite-rich APS in ash from the 27 September 2014 hydrothermal eruption of Ontake volcano. Scanning electron microscope coupled with energy dispersive X-ray spectrometer (SEM-EDS) and ﬁeld emission (FE)-SEM-EDS observations show two types of occurrence of woodhouseite: (a) as cores within chemically zoned alunite-APS crystals ( Zoned-alunite-woodhouseite-APS ), and (b) as a coherent single-phase mineral in micro-veinlets intergrown with similar micro-veinlets of silica minerals ( Micro-wormy-vein woodhouseite-APS ). a highly acidic hydrothermal system existing beneath the volcano summit, formed by condensation in magmatic steam and / or ground waters of sulfur-rich magmatic volatiles exsolved from the magma chamber beneath Mt. Ontake. Under these conditions, an advanced argillic alteration assemblage forms, which is composed of silica, pyrophyllite, alunite and kaolinite / dickite, plus APS, among other minerals. The discovery of woodhouseite in the volcanic ash of the Ontake 2014 hydrothermal eruption represents the ﬁrst reported presence of APS within an active volcano. Other volcanoes in Japan and elsewhere with similar phreatic eruptions ejecting altered ash fragments will likely contain APS minerals derived from magmatic-hydrothermal systems within the subvolcanic environment. The presence of APS minerals within the advanced argillic zone below the summit vent of Ontake volcano, together with the prior documentation of phyllic and potassically altered ash fragments, provides evidence for the existence within an active volcano in Japan of an alteration column comparable to that of porphyry copper systems globally.

Given the similarity in geologic setting, APS minerals should also occur in altered rocks associated with active volcanoes. Samples from some active volcanoes are indeed rich in hydrothermal alunite [26][27][28][29]. These products have been interpreted to be derived from sulfuric acid-rich subvolcanic hydrothermal systems. APS from within an active volcano will help understand subvolcanic hydrothermal processes, providing a mineralogical record of the pre-eruptive physicochemical conditions within the subvolcanic hydrothermal system. However, reported occurrences of APS in active volcanic systems have not been reported. This study aims to document the presence of APS mineral in volcanic products from active volcanoes by examining samples of alunite-bearing altered rock from the 2014 hydrothermal eruption of Ontake volcano in central Japan [29].
During the Holocene, phreatic (or hydrothermal) eruptions have occurred more frequently at Ontake volcano than magmatic eruptions. The frequency of the phreatic (or hydrothermal) eruptions has been estimated to be~0.6/Ky, double that of the magmatic eruptions [37,38]. Before the 2014 eruption, three phreatic (or hydrothermal) eruptions were witnessed in 1979, 1991, and 2007 [37,39,40], suggesting a higher frequency than 0.6/Ky. Geothermal manifestations have developed on the southwestern flank of Kengamine cone for at least the last 250 years [41]. Hydrothermally altered rocks were exposed in the same area prior to the 2014 eruption.

Volcanic Ash from the 2014 Hydrothermal Eruption
The eruption on 27 September 2014 took place on the southwestern flank of Kengamine peak. This eruption ejected approximately one million tonnes of volcanic ash; an estimated volume similar to that of the 1979 eruption [42]. The Volcanic Explosivity Index (VEI) of the 2014 eruption was two [31,43]. The volcanic ash draped the surface of the summit area and the eastern flank [43][44][45]. Geophysical studies reported precursory seismicity linked to the eruption [46][47][48].
The ash of the 2014 eruption is composed of abundant altered lithic fragments and minor unaltered volcanic rock fragments [49]. Based on the study of individual ash particles, Minami et al. (2016) [29] classified the alteration into five types: silica-pyrite, silica-pyrite ± alunite ± kaolin, silicapyrophyllite-pyrite, silica-muscovite ± chlorite, and silica-K-feldspar ± albite ± garnet ± biotite. These results indicate that the ash grains were derived from an active subvolcanic magmatic-hydrothermal system existing under the crater and consisting of silicic, advanced argillic, phyllic, and potassic alteration zones. Minami et al. (2016) [29] interpreted these zones as comparable to the alteration zones in a porphyry copper system [50]. Another study on the volcanic products reported a sulfur isotopic equilibrium temperature of ca. 286 °C, based on the assumption of equilibrium between sulfate (gypsum and anhydrite) and pyrite [51]. These results clearly show that the 2014 eruption was similar to hydrothermal eruption [52] driven by a convecting hot water or steam-dominated hydrothermal system. In this paper we use "hydrothermal eruption [52]" to best describe the 2014 Ontake eruption.

Methodology
For this study we used an ash sample documented in Minami et al., (2016) [29]. The sample was collected four days after the eruption, at a roadside point (35°54'29.00" N, 137°34'06.23" E) 8 km northeast from the vent (Figure 1). The sample consists mainly of fine (<250 μm) ash. A relatively coarse fraction (70-125 μm) obtained by sieving was prepared for a polished section using epoxy resin. The polished section was observed with a JEOL JSM-6610LV scanning electron microscope (SEM) coupled with an Oxford Instruments energy dispersive X-ray spectrometer (EDS) at Akita University and a JEOL JSM-7100F field emission (FE) SEM and Oxford Instruments EDS at Hokkaido

Volcanic Ash from the 2014 Hydrothermal Eruption
The eruption on 27 September 2014 took place on the southwestern flank of Kengamine peak. This eruption ejected approximately one million tonnes of volcanic ash; an estimated volume similar to that of the 1979 eruption [42]. The Volcanic Explosivity Index (VEI) of the 2014 eruption was two [31,43]. The volcanic ash draped the surface of the summit area and the eastern flank [43][44][45]. Geophysical studies reported precursory seismicity linked to the eruption [46][47][48].
The ash of the 2014 eruption is composed of abundant altered lithic fragments and minor unaltered volcanic rock fragments [49]. Based on the study of individual ash particles, Minami et al. (2016) [29] classified the alteration into five types: silica-pyrite, silica-pyrite ± alunite ± kaolin, silica-pyrophyllite-pyrite, silica-muscovite ± chlorite, and silica-K-feldspar ± albite ± garnet ± biotite. These results indicate that the ash grains were derived from an active subvolcanic magmatic-hydrothermal system existing under the crater and consisting of silicic, advanced argillic, phyllic, and potassic alteration zones. Minami et al. (2016) [29] interpreted these zones as comparable to the alteration zones in a porphyry copper system [50]. Another study on the volcanic products reported a sulfur isotopic equilibrium temperature of ca. 286 • C, based on the assumption of equilibrium between sulfate (gypsum and anhydrite) and pyrite [51]. These results clearly show that the 2014 eruption was similar to hydrothermal eruption [52] driven by a convecting hot water or steam-dominated hydrothermal system. In this paper we use "hydrothermal eruption [52]" to best describe the 2014 Ontake eruption.

Methodology
For this study we used an ash sample documented in Minami et al., (2016) [29]. The sample was collected four days after the eruption, at a roadside point (35 • 54 29.00" N, 137 • 34 06.23" E) 8 km northeast from the vent (Figure 1). The sample consists mainly of fine (<250 µm) ash. A relatively coarse fraction (70-125 µm) obtained by sieving was prepared for a polished section using epoxy resin. The polished section was observed with a JEOL JSM-6610LV scanning electron microscope (SEM) coupled with an Oxford Instruments energy dispersive X-ray spectrometer (EDS) at Akita University and a JEOL JSM-7100F field emission (FE) SEM and Oxford Instruments EDS at Hokkaido University. Grain morphological, textural, and petrographic observations were made using backscattered electron images (BEI). Qualitative and semi-quantitative chemical analyses were obtained using the EDS spectra. The analytical instrumental conditions were: 15 kV acceleration voltage, probe current of 2.2 nA (SEM-EDS) and 0.5 nA (FE-SEM-EDS), 10 mm working distance, and a 20 s live time.

Mineral Identification
Semi-quantitative chemical analyses by EDS were carried out to identify APS minerals. APS minerals are defined by the stoichiometric formula (Na, K, Ag, H 3 O, NH 4 , Pb, Ca, Ba, Sr, REE) (Al, Fe, Cu, and Zn) 3 ((S, P, As) O 4 ) (OH) 6 [1][2][3][4][5]. Crystals with spectra consisting of O, S, P, Ca, and Al were identified as APS minerals ( Figure 2). In this paper, we use the general name of "APS" to express the designate s.s. between endmember compositions listed in Table 1. For example, the APS crystals consisting mainly of P, S, Ca are denoted by "woodhouseite-APS" (woodhouseite composition-rich APS). Alunite crystals with spectra consisting of O, S, Al, Na, K, and Ca were distinguished from APS ( Figure 2; Figure 3). In many cases, these alunite crystals consist of Na-Ca-K in various proportion. We simply express the alunite as the designate s.s. of Na, K, and Ca endmembers (Table 1). For example, Na-K rich alunite is expressed as "Na-K-alunite". The selected minerals in Table 1 are referred from [4,6,7].

Mineral Identification
Semi-quantitative chemical analyses by EDS were carried out to identify APS minerals. APS minerals are defined by the stoichiometric formula (Na, K, Ag, H3O, NH4, Pb, Ca, Ba, Sr, REE) (Al, Fe, Cu, and Zn)3 ((S, P, As) O4) (OH)6 [1-5]. Crystals with spectra consisting of O, S, P, Ca, and Al were identified as APS minerals ( Figure 2). In this paper, we use the general name of "APS" to express the designate s.s. between endmember compositions listed in Table 1. For example, the APS crystals consisting mainly of P, S, Ca are denoted by "woodhouseite-APS" (woodhouseite composition-rich APS). Alunite crystals with spectra consisting of O, S, Al, Na, K, and Ca were distinguished from APS ( Figure 2; Figure 3). In many cases, these alunite crystals consist of Na-Ca-K in various proportion. We simply express the alunite as the designate s.s. of Na, K, and Ca endmembers (Table 1). For example, Na-K rich alunite is expressed as "Na-K-alunite".

Petrography of Woodhouseite-APS-Bearing Volcanic Ash Grains
Woodhouseite-APS were observed in the volcanic ash grains altered to advanced argillic and silicic assemblages, as reported by Minami et al. (2016) [29]. The mineral assemblage, alteration type, and occurrence of the woodhouseite-APS in the ash grains studied is summarized in Table 2. Woodhouseite-APS crystals commonly occur as cores of euhedral alunite crystals. Two crystal textures were observed: Zoned-alunite-woodhouseite-APS and Micro-wormy-vein woodhouseite-APS. The former is more abundant in the examined altered ash grains (Table 2).

Petrography of Woodhouseite-APS-Bearing Volcanic Ash Grains
Woodhouseite-APS were observed in the volcanic ash grains altered to advanced argillic and silicic assemblages, as reported by Minami et al. (2016) [29]. The mineral assemblage, alteration type, and occurrence of the woodhouseite-APS in the ash grains studied is summarized in Table 2. Woodhouseite-APS crystals commonly occur as cores of euhedral alunite crystals. Two crystal textures were observed: Zoned-alunite-woodhouseite-APS and Micro-wormy-vein woodhouseite-APS. The former is more abundant in the examined altered ash grains ( Table 2).  Zoned-alunite-woodhouseite-APS: This type of occurrence is characterized by compositionally zoned alunite crystals. On BEI, the crystal consists of a bright core and a dark rim. As shown in Figure 4 (the EDS elemental intensity profile corresponds to the scan line in the BEI image), the X-ray counts per second (cps) for Ca and P Kα1 peaks are high in the core and low in the rim, whereas those for S, Na, and K show an inverse relationship. This profile indicates that the woodhouseite-APS component (Ca-P and low S) is more abundant in the core than in the rim, which has an alunite composition. Most alunite rims were found to have chemical compositions close to Na-K-alunite.  Zoned-alunite-woodhouseite-APS appears in a variety of occurrences within ash grains classified as advanced argillically-or residual/vuggy silica-altered, according to Minami et al. (2016) [29]. In one type of occurrence, individual Zoned-alunite-woodhouseite-APS crystals typically range between 10 and 50 micrometers in size (Figure 5a,b, Table 2). Within these Zoned-alunite-woodhouseite-APS crystals, an internally homogenous core of woodhouseite-APS, which often shows a texture suggesting partial dissolution, is surrounded by concentric polygons of woodhouseite-APS and, further out, by euhedral alunite (Figure 5b). A fine mixture of silica mineral(s) and pyrophyllite fill the interstitial spaces.
Zoned-alunite-woodhouseite-APS is also observed in irregular aggregates of coarse alunite crystals (Figure 5c,d). In this case, woodhouseite-APS occur as what appear to be partially dissolved, often fibrous, clusters within the surrounding alunite, which is typically concentrically zoned. Within the alunite surrounding the woodhouseite-APS, the inner rims have a chemical composition closer to K-alunite and the outer rims are closer to Na-Ca-alunite.
Another type of occurrence of Zoned-alunite-woodhouseite-APS is shown in Figure 5e   Zoned-alunite-woodhouseite-APS appears in a variety of occurrences within ash grains classified as advanced argillically-or residual/vuggy silica-altered, according to Minami et al. (2016) [29]. In one type of occurrence, individual Zoned-alunite-woodhouseite-APS crystals typically range between 10 and 50 micrometers in size (Figure 5a,b, Table 2). Within these Zoned-alunite-woodhouseite-APS crystals, an internally homogenous core of woodhouseite-APS, which often shows a texture suggesting partial dissolution, is surrounded by concentric polygons of woodhouseite-APS and, further out, by euhedral alunite (Figure 5b). A fine mixture of silica mineral(s) and pyrophyllite fill the interstitial spaces.
Zoned-alunite-woodhouseite-APS is also observed in irregular aggregates of coarse alunite crystals (Figure 5c,d). In this case, woodhouseite-APS occur as what appear to be partially dissolved, often fibrous, clusters within the surrounding alunite, which is typically concentrically zoned. Within the alunite surrounding the woodhouseite-APS, the inner rims have a chemical composition closer to K-alunite and the outer rims are closer to Na-Ca-alunite.
Another type of occurrence of Zoned-alunite-woodhouseite-APS is shown in Figure 5e,f. The ash grain comprises a massive silicified part and irregular or vein-like open spaces or vugs, which are partially filled with an aggregate of Zoned-alunite-woodhouseite-APS. Similar to those described previously, the alunite crystals contain woodhouseite cores. The crystals are smaller (submicron to ca. 10 µm) than the other two types (Figure 5f).  Table 2). (b) A finegrained silica-pyrophyllite mixture interstitially fills among the zoned alunite crystals with a woodhouseite core. (c) An aggregate of coarse zoned alunite crystals. (d) Zoned alunite containing a fibrous-woodhouseite core (ONTK-VA-004 in Table 2). The interstitial silica and Si-Al clay minerals are not accompanied with the zoned alunite. (e) Zoned alunite filling the vugs in a massive silicified rock fragment. Irregular or vein-shaped vugs are incompletely filled with tiny zoned alunite crystals (ONTK-VA-001 in Table 2). (f) An aggregate of tiny crystals of zoned alunite in the vugs.

Micro-wormy-vein woodhouseite-APS:
In this type of occurrence, the APS mineral forms a coherent single-phase in microveinlets reminiscent of the wormy texture used to describe intergrowths between quartz and advanced argillic alteration minerals, such as pyrophyllite, dickite or alunite, in porphyry copper systems of the Cajamarca region of northern Peru [53]. At Ontake a similar texture is observed in mineral mixtures of silica and woodhousite-APS or kaolinite/dickite (Figure 6a,b). The  Table 2). (b) A fine-grained silica-pyrophyllite mixture interstitially fills among the zoned alunite crystals with a woodhouseite core. (c) An aggregate of coarse zoned alunite crystals. (d) Zoned alunite containing a fibrous-woodhouseite core (ONTK-VA-004 in Table 2). The interstitial silica and Si-Al clay minerals are not accompanied with the zoned alunite. (e) Zoned alunite filling the vugs in a massive silicified rock fragment. Irregular or vein-shaped vugs are incompletely filled with tiny zoned alunite crystals (ONTK-VA-001 in Table 2). (f) An aggregate of tiny crystals of zoned alunite in the vugs.
Micro-wormy-vein woodhouseite-APS: In this type of occurrence, the APS mineral forms a coherent single-phase in microveinlets reminiscent of the wormy texture used to describe intergrowths between quartz and advanced argillic alteration minerals, such as pyrophyllite, dickite or alunite, in porphyry copper systems of the Cajamarca region of northern Peru [53]. At Ontake a similar texture is observed in mineral mixtures of silica and woodhousite-APS or kaolinite/dickite (Figure 6a,b). The typical size of the wormy-veins is less than 10 µm in width (Figure 6b). The micro wormy-vein woodhousite-APS texture occurs as an intricate network of silica and woodhouseite-APS with silica occasionally crosscutting the woodhousite-APS.
Minerals 2019, 9, x FOR PEER REVIEW 9 of 15 typical size of the wormy-veins is less than 10 μm in width (Figure 6b). The micro wormy-vein woodhousite-APS texture occurs as an intricate network of silica and woodhouseite-APS with silica occasionally crosscutting the woodhousite-APS.  Table 2) (b) Micro-wormy vein APS cross-cut by siliceous microwormy veins in the matrix of fine-grained silica-kaolin mixtures.

Discussion
Phreatic (or hydrothermal) and phreatomagmatic (or magmatic-hydrothermal) eruptions [52] frequently bring to the surface altered lithic fragments from sub-volcanic hydrothermal systems [28,[54][55][56]. The woodhouseite-APS-bearing ash erupted from the September 2014 hydrothermal eruption of Ontake volcano is derived from pre-existing altered rocks under the Kengamine summit crater (Figure 7). They formed in the sub-volcanic environment within an active magmatic-hydrothermal system. The ash grains containing APS minerals consist mainly of hydrothermal minerals including silica, pyrophyllite, kaolinite/dickite, and alunite. The stability temperature conditions of the mineral assemblages in that style of hydrothermal environment ranges between ~150 and 350 °C under highly acidic conditions [57,58]. This temperature range is consistent with the temperature of 286 °C determined by sulfur isotopic fractionation between sulfate and sulfide minerals in volcanic products also from the Ontake 2014 eruption [51]. These genetic conditions indicate a magmatic volatile-rich hydrothermal environment, which is directly comparable with that observed in the early alteration stage of high-sulfidation epithermal ore Au-Cu-Ag-As deposits [59] and the advanced argillic alteration lithocaps above porphyry copper deposits [23,50,60]. They confirm the genetic association proposed among porphyry coppers, some epithermal deposits, and hydrothermal systems within the core of active volcanoes in magmatic arcs [61].
The occurrences described here of both Zoned-alunite-woodhouseite-APS and Micro-wormy-vein woodhouseite-APS are similar to hydrothermal APS and alunite from epithermal-porphyry ore systems [17,18,22]. For example, both at the Rodalquilar gold-alunite epithermal deposit in Spain [16] and the worldclass Far Southeast (FSE) porphyry Cu-Au-Ag deposit in the Philippines [23], euhedralbladed hydrothermal alunite contains identical cores of APS minerals (Figure 8). Similarly, hydrothermal APS has also been observed as a monomineralic vein [15] comparable to the Microwormy vein woodhouseite-APS at Ontake volcano. Although those two types found by this study show different textures, both types formed under similar genetic conditions within subvolcanic advanced argillic and silicic alteration zones [29].  Table 2) (b) Micro-wormy vein APS cross-cut by siliceous micro-wormy veins in the matrix of fine-grained silica-kaolin mixtures.

Discussion
Phreatic (or hydrothermal) and phreatomagmatic (or magmatic-hydrothermal) eruptions [52] frequently bring to the surface altered lithic fragments from sub-volcanic hydrothermal systems [28,[54][55][56]. The woodhouseite-APS-bearing ash erupted from the September 2014 hydrothermal eruption of Ontake volcano is derived from pre-existing altered rocks under the Kengamine summit crater (Figure 7). They formed in the sub-volcanic environment within an active magmatic-hydrothermal system. The ash grains containing APS minerals consist mainly of hydrothermal minerals including silica, pyrophyllite, kaolinite/dickite, and alunite. The stability temperature conditions of the mineral assemblages in that style of hydrothermal environment ranges between~150 and 350 • C under highly acidic conditions [57,58]. This temperature range is consistent with the temperature of 286 • C determined by sulfur isotopic fractionation between sulfate and sulfide minerals in volcanic products also from the Ontake 2014 eruption [51]. These genetic conditions indicate a magmatic volatile-rich hydrothermal environment, which is directly comparable with that observed in the early alteration stage of high-sulfidation epithermal ore Au-Cu-Ag-As deposits [59] and the advanced argillic alteration lithocaps above porphyry copper deposits [23,50,60]. They confirm the genetic association proposed among porphyry coppers, some epithermal deposits, and hydrothermal systems within the core of active volcanoes in magmatic arcs [61].
The occurrences described here of both Zoned-alunite-woodhouseite-APS and Micro-wormy-vein woodhouseite-APS are similar to hydrothermal APS and alunite from epithermal-porphyry ore systems [17,18,22]. For example, both at the Rodalquilar gold-alunite epithermal deposit in Spain [16] and the worldclass Far Southeast (FSE) porphyry Cu-Au-Ag deposit in the Philippines [23], euhedral-bladed hydrothermal alunite contains identical cores of APS minerals (Figure 8). Similarly, hydrothermal APS has also been observed as a monomineralic vein [15] comparable to the Micro-wormy vein woodhouseite-APS at Ontake volcano. Although those two types found by this study show different textures, both types formed under similar genetic conditions within subvolcanic advanced argillic and silicic alteration zones [29].    [16]. (b) Backscattered electron image of alunite and APS minerals from the advanced argillic zone immediately above the FSE porphyry Cu-Au-Ag deposit, Philippines [23]. These photographs are modified and referred from [16] and [23], respectively.

Conclusions
The petrographical and mineralogical study of ash grains from the 2014 Ontake volcano hydrothermal eruption resulted in identification of APS minerals such as woodhouseite. Two types of woodhouseite were observed: Zoned-alunite-woodhouseite-APS and Micro-wormy-vein woodhouseite-APS. The genetic environment of APS minerals is proposed to be highly acidic hydrothermal fluids existing beneath the volcanic summit, formed by condensation with magmatic and/or ground waters of magmatic volatiles exsolved from the magma chamber underneath Ontake volcano. Under these conditions, an advanced argillic alteration assemblage formed consisting of silica, pyrophyllite, alunite, and kaolinite/dickite, plus APS, among other minerals. The 2014 hydrothermal eruption served to bring to the surface samples of this advanced argillic zone as well as deeper, higher  [16]. (b) Backscattered electron image of alunite and APS minerals from the advanced argillic zone immediately above the FSE porphyry Cu-Au-Ag deposit, Philippines [23]. These photographs are modified and referred from [16] and [23], respectively.

Conclusions
The petrographical and mineralogical study of ash grains from the 2014 Ontake volcano hydrothermal eruption resulted in identification of APS minerals such as woodhouseite. Two types of woodhouseite were observed: Zoned-alunite-woodhouseite-APS and Micro-wormy-vein woodhouseite-APS. The genetic environment of APS minerals is proposed to be highly acidic hydrothermal fluids existing beneath the volcanic summit, formed by condensation with magmatic and/or ground waters of magmatic volatiles exsolved from the magma chamber underneath Ontake volcano. Under these conditions, an advanced argillic alteration assemblage formed consisting of silica, pyrophyllite, alunite, and kaolinite/dickite, plus APS, among other minerals. The 2014 hydrothermal eruption served to bring to the surface samples of this advanced argillic zone as well as deeper, higher temperature alteration zones [29]. The presence of APS minerals within the advanced argillic zone below the summit vent of Ontake volcano, together with the description of phyllic and potassically altered ash fragments, provides first time evidence for the existence in Japan of an alteration column identical to that of porphyry copper systems globally.
The discovery of woodhouseite in the volcanic ash of the Ontake 2014 hydrothermal eruption represents the first reported presence of APS within an active volcano. As shown in previous studies [26][27][28][29] other volcanoes with phreatic (or hydrothermal) eruptions similar to that of Ontake in September 2014 eject altered volcanic products rich in hydrothermal alunite and associated alteration minerals. We believe that further detailed studies will prove that the presence of APS at Ontake is not an exception, but likely commonplace among such active volcanoes.