Geochemistry of Hydrothermal Alteration in Volcanic Rocks

Hydrothermal alteration is a chemical replacement of the original minerals in a rock by new minerals where a hydrothermal fluid delivers the chemical reactants and removes the aqueous reaction products. An understanding of hydrothermal alteration is of value because it provides insights into the chemical attributes and origins of ore fluids and the physical conditions of ore formation (Reed M. 1997).


Introduction
Hydrothermal alteration is a chemical replacement of the original minerals in a rock by new minerals where a hydrothermal fluid delivers the chemical reactants and removes the aqueous reaction products. An understanding of hydrothermal alteration is of value because it provides insights into the chemical attributes and origins of ore fluids and the physical conditions of ore formation (Reed M. 1997).
Within a mineral deposit, the solution channelways are usually obvious because precipitated minerals and altered wallrocks remain as evidence. The direction in which the solutions flowed, especially in flat-lying deposits, is usually less obvious but in many cases can be enferred from mineral zoning or similar evidence (Skinner B. 1997).
The mobility of major, minor and rare-earth elements (REE) during alteration processes in different environments has been documented by numerous authors and has been used to discriminate the origin of kaolin deposits. (Sturchio et al., 1986;De Groot & Baker, 1992;Gouveia et al., 1993;van der Weijden & van der Weijden, 1995;Condie et al., 1995;Dill et al.,1997Dill et al., , 2000Galán et al., 1998Galán et al., , 2007Pandarinath et al., 2008, among others). Terakado & Fujitani (1998) studied the REE and other trace elements in silicastones, alunites and related rocks in order to examine the behaviour of trace elements in the acidic hydrothermal alteration of silicic volcanic rocks. They found that most of the elements such as Na, Fe, Ba and LREE were leached from the silicastones, while HREE, Th, Hf and Zr were retained in the rocks, even under strongly acidic hydrothermal processes. Alunite samples have LREEenriched and HREE-depleted features. According to Dill et al. (1997), the ratios TiO 2 + Fe 2 O 3 vs. Cr + Nb, Zr vs. TiO 2 and Ba + Sr vs. Ce + Y + La in kaolinites allow discrimination between hypogene and supergene kaolinization processes. The APS-bearing argillaceous zones that formed during supergene processes are significantly enriched in REE relative to hypogene equivalents (Dill, 2000). Pandarinath et al. (2008) studied the effects of hydrothermal alteration on major, rare-earth, and other trace-element concentrations in rhyolitic rocks of the Los Azufres geothermal field, Mexico. They concluded that the hydrothermal alteration resulted in a decrease in MnO, P 2 O 5 , Ta, Rb/Zr and Rb/Nb, and an increase in Zr, Nb and Nb/Y. The greater variances of Y, Ce, Pr, Nd, Sm, Lu and Pb in altered rocks are probably due to hydrothermal The aim of the study described in this chapter was to evaluate the relation between the chemical composition of major, minor and trace elements, the stable isotopes in kaolin (O and D) and the mineralogical alteration zonation to confirm the hydrothermal genesis of the deposits, working with volcanic rocks from Patagonia Argentina.
Knowing the genesis of such deposits is crucial in order to establish exploration criteria and evaluate reserves. These Patagonian deposits are derived from Mesozoic rhyolitic and andesitic rocks. There have been several studies aimed at discovering the origin of the primary deposits (Domínguez & Murray, 1995Cravero et al., 1991Cravero et al., , 2001Domínguez et al., 2008, Marfil et al., 2005. Most of the studies deal with the mineralogy, chemical composition, the deposit structure and oxygen-deuterium isotopic data.
The deposits examined in this study are located in different areas of Patagonia in the Provinces of Río Negro and Chubut and they are studied by Cravero et al. (2010), Maiza et al. (2009) and Marfil et al. (2005. Geologically, the area is characterized by the presence of a set of volcanic rocks and tuffs with minor clastic sediments that overlie a basement of Mesozoic age, essentially constituted by granites. At the base, the volcanic complex is composed of andesites, which is known as www.intechopen.com Vera formation. This complex is followed by a succession of sandstone tuffs, rich in fossil plants (Dicrodium Flora formation) and finally by a suite of ignimbritic tuffs and flows known as the Sierra Colorada formation, deposited at the top of the series. The age of this volcano-sedimetary complex ranges from Triassic to Middle Jurassic. Kaolinite deposits are enclosed in rhyolitic tuffs of the Sierra Colorada formation. An important silicification, developed at the top of the formation, seems to have protected the altered areas from erosive processes giving rise to smooth elevations in the landscape.
The occurrence of hydrothermal events in the region is also proven by the presence of fluorite and Pb-Cu-Zn veins (Labudia & Hayase, 1975). Fluorite veins are hosted by rhyolites presenting an alteration mineralogy with sericite, carbonates, silica and kaolinite (Labudia & Hayase, 1975). According to Manera (1972) and Hayase & Manera (1973), homogenization temperatures of fluid inclusions in fluorite are between 150 and 240 ºC (without correction for pressure). The presence of kaolinite and the formation temperatures of these veins suggest a possible relationship between fluorite-base metal veins and the studied kaolinite deposits of the area. In this case, kaolinite occurrences could be used as a prospecting tool for other types of mineral deposits in the region.

Geological setting
The deposits examined in this study are hydrothermal origin, located in the provinces of Río Negro (Blanquita, Equivocada and Loma Blanca mines) and Chubut (Estrella Gaucha mine) (Patagonia, Argentina) ( Fig. 1). Fig. 1. Localization of the kaolin deposits in a sketch map of Patagonia (Cravero et al. 2010, modified).
In the provinces of Chubut and Santa Cruz there are many kaolin deposits, some of them very extensive, locates in the Chubut river valley and lote 8, 18, 19 and Cerro Rubio respectively. These deposits are sedimentary and residual origin, formed from the alteration of volcanic materials, especially Jurassic rhyolitic tuffs.
In the province of Chubut there is another group of kaolin deposit related with hydrothermal alteration. Estrella Gaucha mine is one of them. It is situated in western Chubut Province, 70 km from Alto Río Senguer and 20 km from Aldea Apeleg. It is located at SW from Cerro Bayo at 1620 meters above sea level. Ploszkiewicz & Ramos (1977), found andesites and dacites of Ñirehuao Formation and dacitic ignimbrites of Gato Formation. These volcanic rocks are Cretaceous edge. The outcropping rocks in the area studied are conglomeratic sandstones with moderate alteration and stratified. In their composition are common rhyolites, andesites and siliceous sandstones. These rocks are the base of the deposit. Iron oxides are abundant and they are chloritized. The lithic components are selective altered related with the textures and mineralogical composition. It is possible to observe particles replaced pseudomorphologicaly by chlorite, kaolinite, alunite and quartz. Some grains conserve the original textures while in others the reemplacement is massive forming monomineral particles. The mineralization is developed from the alteration of rhyolitic tuffs of the Lower Cretaceous Payaniyeu Formation in contact with tuff sandstone of Apeleg Formation. The relation between both Formations is not clear because the alteration processes developed in the transition zone (Ploszkiewicz & Ramos 1977). The mineralogy consists mainly of dickite with rare alunite and variable amounts of quartz distributed in four alteration zones, from a silicified upper zone, grading downwards to an alunite zone, then a kaolinized zone and, finally, a sericite-chlorite zone (Hayase et al. 1971, Maiza 1972, 1981, Maiza & Hayase 1975and Maiza et al. 2009). (Fig 2a) The Chubut River Valley deposits and those of the Santa Cruz province are developed on Jurassic volcanic rocks which are predominantly rhyolitic and form one of the world's most voluminous silicic provinces (Pankurst & Rapela, 1995). It is called Chon-Aike Province (Pankurst et al., 1998), which ranges in age from Early Jurassic to Early Cretaceous. This province comprises several formations that, depending on the geographical locality, receive different names (Marifiland Chon-Aike, among others). In Chubut, the deposits were formed on the Marifil Formation (Malvicini & Llambias, 1974) and are found along the Chubut River in an area of ~60 km 2 . In Santa Cruz, the primary deposits came from the alteration of rhyolitic ash-fall tuffs and ignimbrites of the Chon-Aike Formation which formerly belonged to the Bahía Laura Group) (Domínguez & Murray, 1997;Cravero et al., 2001). In both the Marifil and Chon-Aike rocks, the kaolinized areas have a wide horizontal extension, limited thickness (8-12 m at most) and a downward decreasing degree of alteration. The mineralogical composition is kaolinite with minor halloysite. illite and relict quartz and feldspars (Dominguez & Murray, 1997;Cravero et al., 2001;Dominguez et al., 2008). Table 1 synthesizes the geology and mineralogy of the deposits (Cravero et al. 2010).
Blanquita and Equivocada mines are situated 30 km SE of Los Menucos in the province of Río Negro (Fig. 1). The mineralized zone is distributed along a belt 5 to 8 km wide and 20 km long with an approximate area of 110 km 2 . Kaolinite deposits are enclosed in rhyolitic tuffs of the Sierra Colorada Formation, Triassic to Middle Jurassic in age. Blanquita mine is characterized by the presence of kaolinite and alunite with scarce dickite and pyrophyllite, whereas in the Equivocada mine, kaolinite is associated with dickite and traces of alunite (without pyrophyllite) (Marfil et al., 2005).
The rhyolitic tuffs, which host most of the kaolinized bodies, were deposited on top of the porphyritic rhyolites. The texture of the tuff is variable, from fine-grained up to agglomerate levels, and shows a microcrystalline matrix with abundant quartz, sanidine, plagioclase, hornblende and biotite. Aleration is moderate with sericitizacion and kaolinization of feldspars and chloritization of amphiboles and biotites (Marfil et al. 2005) (Fig. 2b Blanquita Mine and Fig. 2c Equivocada Mine). Loma Blanca is situated 70 km NW of Los Menucos (Río Negro Province, Argentina) (Fig. 1). The parent rocks are andesites and their tuffs from the Vera Formation (Lower Triassic) (Los Menucos Group), which lie unconformably on the La Esperanza granite and Colo Niyeu metamorphites. 150 m to the northeast of the deposit there is an outcrop of granite intruded by aplitic dykes. Its geological importance arises from the mineralogy, structure and setting of the strongly mineralized area and its relationship with the different lithologic units (Fig  2d). Hayase & Maiza (1972a) studied the mineralogy of the Loma Blanca deposit using X-ray diffraction (XRD), thermogravimetric (TG) and differential thermal analysis (DTA), and microscopy. They concluded that the deposit was formed by the activity of an acid  Table 1. Host-rock composition, alteration mineralogy, age and origin of the Patagonia kaolin deposits (Cravero et al. 2010, modified).
hydrothermal solution or superheated solfatara. They proposed a concentric zonation model. From the parent rock outward, different alteration patterns were recognized: a zone with sericite, chlorite and montmorillonite; a zone with kaolinite and dickite; a zone with dickite, pyrophyllite and alunite-natroalunite; and a zone with quartz, disseminated sulphides and diaspore (Fig. 3). ). The structure is not clearly exposed due to the scarce outcrops and smooth geomorphical topography covered by recent sediments. Geological evidence from the area indicates the development of tectonic fracturing and differential block movements affecting the heterogeneous volcanosedimentary pile that channelled the hydrothermal solutions responsible for the mineralization. The mineral assemblage, with high-temperature minerals (diaspore, pyrophyllite and dickite), the abovementioned sulphides, the areal structure and favourable lithology related to mineralization processes identify this zone as suitable for metallic mineral prospecting. The floor of the Loma Blanca mine consists of andesite that shows progressive alteration and development of a propylitic zone. Tuffaceous levels are transitionally interlayered; they are affected by tectonics and have developed breccias zones that, owing to their greater permeability, provide pathways for mineralizing solutions. Towards the top of the mineralized zone, the texture is obliterated due to the intense silicification that has erased the lithological characteristics of the affected rocks (Maiza et al., 2009). The oldest formation in the area is Colo Niyeu, which is a low-grade metasedimentary basement. According to Labudia & Bjerg (1994), it could only be assigned a pre-Permian age. This unit was intruded by different plutonites of the so-called Complejo Plutónico La Esperanza. Close to the deposit, outcrops attributed to the Donosa granite, due to their mineralogical and textural characteristics, were recognized; the age of the intrusion, according to Pankurst et al. (1992), will be Late Permian. Over these formations lie the lithological components of the Los Menucos Group, of Triassic age; they are intruded by andesitic dykes of the Taquetren Formation. The field relations of these units have been obscured by tectonism, covered by recent sediments and masked by the mineralizing process ).

Methods and instrumentation
The petrographic studies were carried out with an Olympus B2-UMA trinocular petrographic microscope with a built-in Sony 151A video camera, a high-resolution monitor and Image-Pro Plus image processing software were used. The mineralogical composition of bulk samples was determined by XRD, using a Rigaku D-Max III, with Cu-Kα radiation and a graphite monochromator operated at 35 kV and 15 mA. The XRD patterns were recorded from 2º to 60º 2θ. Chemical analyses of bulk samples for major, minor and trace elements were carried out by inductively-coupled plasma mass-spectrometry (ICP

Mineralogical composition 4.1 Estrella Gaucha mine
The mineralogy of the kaolinized zone in this deposit is very simple. The mineral more abundant is dickite, with minor amount of alunite and quartz and scarce pyrophillite and diaspore. The presence of dickite was mentioned in other kaolin deposits: Tres Picos Mines (Province of Neuquen) (Hayase & Maiza 1970, Losada et al. 1975, Loma Blanca Mine (Province of Río Negro) (Hayase & Maiza 1972a) and Adelita Mine (Province of Río Negro) (Hayase & Maiza 1972b) buy in any case is the more abundant.
In Estrella Gaucha mine dickite is associated with fine size quartz. To the top of the mineralization the more abundant mineral is alunite, with relictic inclusions of dickte, in a 20-30 meters thickness zone. The deposit culminates with a silicified zone with quartz with saccharoidal texture. Massive dickite, constitutes tabular crystals of 200 microns (Fig. 4a). It has parallel extinction (to 4º) and very low birefringence (0.006 to 0.008).

Blanquita mine
The kaolinization processes have completely obliterated the original textures of the rock, leaving only quartz (Fig. 4b). The mineralogy includes dickite, kaolinite, pyrophyllite, variable amounts of quartz and scarce alunite (Marfil et al. 2005).

Equivocada mine
The alteration processes is similar to that mentioned in Blanquita mine. It has almost erased the original texture and mineralogy of the tuffs. Only quartz and some biotite remnants are still visible (Fig. 4c). The lithic particles have been pseudomorphically replaced by kaolinite and dickite. The alteration mineralogy consist of a core of alunite grading outwards to an association of kaolinite-illite, illite-zeolite and finally fresh rock (Hayase & Maiza 1970, Marfil et al. 2005. The whole mineralization was later discordantly overprinted by natroalunite (Maiza & Mas 1981).

Loma Blanca mine
In the propylitic alteration zone the original texture is preserved. Plagioclase (andesine) crystals are altered, mainly to illite and calcite. Chlorite pseudomorphically replaces the mafic minerals, and goethite is formed from the remaining Fe (Fig. 4d). Saccharoidal quartz is found as patches in the groundmass.
In sericitic alteration zone the groundmass is completely argillized with secondary quartz. Iron oxides stain the whole rock. Sericite is the main alteration product. Quartz forms saccharoidal textures where three or more anhedral crystals are grouped. No original minerals are preserved.
In the intermediate argillic alteration the andesitic tuff is completely kaolinized. There is a stockwork of pure dickite veins, where the area between the veins is composed of a quartzdickite association with variable amounts of illite, and scarce diaspore (Fig. 4e). There are veins of dickite associated with bohemite crystals. Diaspore was identified in silicified zone and is closely related to dickite.
Advanced argillic alteration zone coincides with the greatest mineralization. The texture resembles that of tuff with lithic clasts replaced by natroalunite. There is no relict quartz. The groundmass is an association of quartz-kaolin and metallic minerals. Hydroxides of Al can be recognized in certain areas. Natroalunite is abundant in this area and is associated with dickite and pyrophyllite (Fig. 4f).

XRD
When kaolinite and dickite are associated, it is very difficult to differentiate them by XRD. The two triplets located between 34º and 40º2, were studied in detail. Dickite appears in very small proportions compared with kaolinite in the samples Loma Blanca, Blanquita and Equivocada mines while in Estrella Gaucha mine dickite is the predominant mineral. In the last one, dickite with variable amounts of quartz (between 0 to 25 %) was determined. In some samples scarce alunite was identified.
In "Loma Blanca" in the advanced argillic alteration zone, besides alunite-natroalunite, diaspore at the top and pyrophyllite in the highest area of the profile were identified. These minerals are found in isolated masses within the area of greatest hydrothermal activity as relicts of a first stage of higher pressure and temperature. In the propylitic alteration zone, chlorite and scarce illite were identified. Feldspar from the andesitic rock is still present. The predominant alteration mineral is illite together with abundant quartz. In the intermediate argillic alteration zone, kaolinite and quartz reflections were identified. Natroalunite composition was determined by XRD using basal reflections (003, 006 and 009). Maiza & Mas (1980), synthesized the minerals of the series from sericite alteration in a H 2 SO 4 solution with varying amounts of KCl and NaCl. They determined the c o values in each of the products obtained and checked them by chemical analysis. With these values they fitted a straight line by the least-squares method. The calculated value for natroalunite from Loma Blanca is 16.83 Å, which corresponds to 87% Na. The formula calculated on this basis is (Na 0.13 K 0.87 )Al 3 (SO 4 ) 2 (OH) 6 . ).

Chemical composition
The kaolinte content in Blanquita, calculated from de mineralogical composition and major element analysis ranges between 27 and 73 % and between 40 and 97% in Equivocada. Samples with a greater silica content correspond to the silicified zone of the deposits. The alunite content, estimated from S concentration, varied from 0.3 to 2.1 % in both deposits, the larger contents being related to the more intense kaolinization. On the other hand, an increase in the alumina content can be related to an increase in the degree of alteration of the fresh rock, reaching values close to theoretical ones for pure kaolinite. In these samples, the alkali content is also small ( Table 2). The concentration of MnO and MgO in the samples analysed are < 0.01 wt % (Marfil et al. 2005). The trace element contents of kaolinite helps to understand the origin. Kaolinites formed from hydrothermal alteration of acid-type igneous rock show enrichments in S, Ba and Sr, whereas Cr, Nb, Ti and the lanthanides tend to concentrate in kaolinites formed from meteoric processes (Dill et al. 1997). The S contents was determined to ranges from 0.03 to 0.36 wt %, this may reflect the presence of small amounts of alunite in the samples analysed.The Zr and Ti contents show a positive linear correlation with greater values of both elements found in samples from Equivocada mine. The (Ti + Fe) contens are less than 1 wt% and the (Cr + Nb) contents range from 0 and 174 ppm with no significant difference between samples from the two deposits. The (Sr + Ba) concentrations vary between 1000 and 10000 ppm, with the larger values probably related to the presence of trace amount of barite. The Ce + Y + La concentraton is also variable (from 3 to 323 ppm). (Fig 5a, b, c and d) (Table 3). (Marfil et al. 2005). The chondrite-normalized REE diagrams are shown in figure 6a and 6b corresponding to Blanquita and Equivocada respectively.
In Loma Blanca mine Fe 2 O 3 , CaO, Na 2 O and K 2 O contents decrease from propylitic to advanced argillic alteration zones, whereas Al 2 O 3 and LOI increase in the kaolinite natroalunite zone (   The chondrite-normalized REE diagram according to Boynton, 1984(in Rollinson, 1992, for the samples of the propylitic and sericitic alteration zones are shown in Fig. 6c and 6d for the samples of intermediate and advanced argillic alteration zones. In the latter, especially in pure kaolin samples, there is marked LREE impoverishment with respect to HREE, especially when compared with the less altered samples. There is no evidence of a positive Ce anomaly, which is typical of the deposits of residual and/or meteoric origin (Cravero et al., 2001). LREE are more abundant than HREE in the intermediate argillic alteration and advanced argillic alteration zones .
The results of the chemical analysis of major elements in Estrella Gaucha mine, show that SiO 2 ranges from 44.50 to 69.06 % and Al 2 O 3 38.34 to 21.70 %. The SO 3 content (between 0.08 and 0.93 %) was adjudicated to alunite. The alumina amount allowed determining kaolin content between 75 and 100 %. The MnO is less than 0.01 % (Table 2) (Maiza et al. 2009).
The results of trace element contents are shown in Table 3 and plotted in Fig. 5. The content of TiO 2 +Fe 2 O 3 are lower than 1%wt, Cr+Nb range between 1 and 214 ppm, Sr+Ba between 1.000 and 10.000 while Ce+Y+La is low (15 -160 ppm). These results are closed with that determined in Blanquita, Equivocada and Loma Blanca mines. The content of As, Rb, Ag, In, Sn, Cs and Bi are insignificant.
In Fig. 6e the chondrite-normalized REE are shown for the samples from Estrella Gaucha. It is possible to observe a LREE impoverishment with respect to HREE, similar to that observed in Blanquita, Equivocada and Loma Blanca mines. Negative Eu anomaly was identified and Ce anomaly was no observed.

O and H isotopes
The O and H stable isotope composition of kaolinites samples from the four mines studied are shown in Fig. 7. According to Savin & Lee (1988), the isotopic composition of kaolinite may reflect the geological conditions during its formation, provide the mineral did not suffer isotopic changes after its deposition. Thus, the O isotope composition in kaolinites of sedimentary origin usually varies from +19 to 23 ‰ and kaolinites from residual deposits has a δ 18 O between +15 and +19‰ (Murray & Janssen, 1984). These values are compatible with a meteoric origin at temperatures between 20 and 25 ºC. It is important to note than the δ 18 O values of the analyzed samples from the four mines, are much lower than those assumed for kaolinite deposit formed under superficial conditions, thus suggesting a different origin for kaolinization fluids (Marfil et al. 2005).
δ 18 O and δD values of kaolinites from Blanquita and Equivocada ranges from +4.8 to +10.3 ‰ and from -88 to -116‰ respectively (Table 4). While the O isotope composition is similar in both deposits, the δD values of Blanquite mine are slightly more negative. (Martil et al 2005). Both δ 18 O and δD values are clearly different from those reported by Cravero et al (1991) in kaolinites from deposits of residual origin in the Santa Cruz and Chubut provinces (δ 18 O from +16.5 to +18.8‰ and δD from -57.5 to -86.5‰). In a later paper, Cravero et al. (2001)  In Loma Blanca mine δ 18 O values in kaolinites range from 10.8 ‰ to 13.2 ‰ and δD from -83‰ to -85‰ (Table 4). Although these values are within the range of variation for hydrothermal kaolins (Murray & Janssen, 1984), they are larger than those determined for Blanquita and Equivocada mines.  (Table 4). They are similar to that obtained in Blanquita and Equivocada mines. The absence of primary minerals with fluid inclusions, not allowed determining the formation temperature of the deposit. However the presence of diaspore and pyrophyllite in the alteration minerals allows narrow the conditions between 250 and 350 ºC, because they are the stability limits of those minerals. Dill et al.(1997) used the relationships between SO 3 vs. P 2 O 3 , Zr vs. TiO 2 , Ti + Fe vs. Cr + Nb, and Ba + Sr vs. Ce + Y + La to discriminate between kaolin deposits of different origins in Peru. They found that S, Ba and Sr are enriched during hydrothermal alteration, whereas Cr, Nb, Ti and lanthanide elements are concentrated mainly during weathering.

Discussion
The trace element content and P vs. S, Zr vs. Ti, (Cr + Nb) vs. (Ti + Fe) and (Ce + Y + La) vs. (Ba + Sr) ratios of kaolin samples from the four mines studied do not differ significantly. This suggests that all the deposits might hav e f o r m e d b y t h e s a m e g e n e t i c p r o c e s s . However, although the Ti + Fe and Sr + Ba contents indicate a hypogenic origin (Dill at al. 1997), the presence of large Cr + Nb and Ce + Y + La concentrations in some samples could indicate the presence of kaolinite of supergenic origin as well. Titanium may be released from a primary mineral in the parent rock (e.g. biotite) during either hypogenic or supergenic kaolinization. However, as supergenic alteration seems to be more efficient, the Ti content in kaolinite has been used to discriminate between the two processes (Dill et al 1997). Because it behaves a geochemically immobile element in superficial conditions, Zr is also a good indicator of the degree of meteorization of the parent rock. Consequently, kaolin samples with high Ti and Zr contents point to a superficial environment of formation (Marfil et al. 2005). Cravero et al. (2010) compared kaolin deposit from hypogene and supergene origin from Patagonia Argentina. In the hypogene deposits, S, Sr, Pb, V, P 2 O 5 , and LREE are more abundant, whereas Fe 2 O 3 , Y, Rb, U, Zr and HREE predominate in the supergene (weathered) deposits. It is important to consider that the samples are altered and the degree of alteration is not the same in all of them. The main alteration products in all the samples are clay minerals of the kaolin group. The approach adopted is to consider that the greater the LOI, the greater the resulting alteration degree. The concentrations of S, Pb and Sr, present only in the hypogene deposits, vary with the degree of alteration, indicating that these elements are mainly concentrated in the clay minerals. Zr does not show a clear behaviour; in the hypogene deposits it appears to be mobile during alteration, whereas under weathering conditions no clear relationship can be established. The V and U contents increase in both types of deposits with the degree of alteration; hence they can be considered to be immobile in both environments. P 2 O 5 and S are only present in hypogene deposits and their contents increase with the alteration degree. Y shows no relation with the degree of alteration.
The greater content of HREE in the supergene deposits is more easily explained by considering that these elements are fractionated during alteration in the hydrothermal deposits and remain unchanged during weathering, thereby giving greater values in the supergene deposits, where no fractionation has occurred. Regarding the mobility of elements during hydrothermal alteration, other authors have found different behaviours. In rhyolitic rocks from Yellowstone drill cores, Ti, Al, Fe, Sc, Co, Y, Zr, REE, Hf, Ta and Th remained relatively immobile (Sturchio et al., 1986).
During the hydrothermal alteration of rhyolitic rocks of the Los Azufres geothermal field, Mexico (Pandarinath et al., 2008), REE concentrations were not significantly different between fresh and altered rocks. The immobile role of REE during hydrothermal processes in rhyolitic rocks has been reported by De Groot & Baker (1992). Nevertheless, in the advanced argillic alteration zone of Rangan area (Central Iran), composed mainly of alunitejarosite and pyrophylite, LREE are relatively immobile in the rocks of this zone and depleted in LREE and HREE (Parsapoor et al., 2009). These authors considered that this behaviour may be due to the entry of these elements into the lattice of alunite-jarosite. LREE can in fact substitute for K in the large-radius cations (A) of the alunite-jarosite formula. Alunite is a common component of the hypogene deposits studied here.
The samples from Patagonia studied by Cravero et al. 2010 have been taken as representative of the whole altered area, so the element contents do not come from a specific part of the weathering profile; therefore it can be considered that the conservative behaviour of the REE elements arises from the fact that they have fractionation could have occurred within the profile. Another characteristic of the REE that supports a supergene origin is the presence of a Ce anomaly, produced when Ce 3+ is oxidized to Ce 4+ . When these data are plotted in the graphical style of Dill et al. (1997Dill et al. ( , 2000, some of the relationships are not as clear as in their work, except for SO 3 vs. P 2 O 3 . Their supergene deposits are characterized by much greater contents of Ce + Y + La than those formed under hypogene conditions, whereas in Patagonia, these values are dispersed. In Peru, supergene kaolins are characterized by large values of both TiO 2 and Zr, whereas in Patagonia only Zr shows the same behaviour. Cr + Nb also show the opposite trend, while in Peru the greatest contents are related to a supergene origin; in Patagonia they are associated with hydrothermal deposits. In both areas, the same behaviour is found for S, P 2 O 5 , Ba and Sr, with the greater values being found in the deposits formed in the hypogene environment. The greater amounts of S and V in the hypogene deposits are explained by the presence of minerals such as alunite (sulphate) and mottramite (vanadate).
More information about the formation conditions of the studied deposits can be obtained from their mineralogy. Kaolin deposits of hydrothermal origin are constituted by kaolinite with dickite, alunite and pyrophyllite (Murray, 1984). However, in kaolin deposits of sedimentary origin only kaolinite is present. In the mines studied the presence of dickite, pyrophyllite with alunite and diaspore in Loma Blanca, indicate a hydrothermal origin. In order to estimate the isotopic composition of fluids responsible for the kaolinization process a temperature of 350 ºC was assumed. Higher temperatures would not be reasonable as kaolinite coexists with pyropyillite and lower values would not be compatible with the presence of pyrophillite. Numerous authors have synthesized pyrophyllite at temperatures above 260ºC (Grim, 1969;Roy & Osborn, 1954;Hemley, 1959;Reed & Hemley, 1967;Tzuzuki & Mizutani, 1971).
The coexistence of pyrophyllite and kaolinite allows us to estimate a maximum temperature of 350ºC. Diaspore can be recognized frequently in kaolin deposits of hydrothermal origin. According to Roy & Osborn (1954), the bohemite-diaspore transformation temperature is between 270 and 300ºC a t 175-1500 atm. water pressure.  A schematic summary of the paragenetic sequence from Loma Blanca mine is given in Table 5. The main Al minerals of higher temperature occur predominantly in the central area below the silicified zone, which is responsible for protecting the deposit from subsequent erosion processes. Dickite and nacrite commonly occur in deposits of hydrothermal origin (Murray, 1984). Numerous authors have synthesized pyrophyllite at temperatures above 260ºC (Grim, 1969;Roy & Osborn, 1954;Hemley, 1959;Reed & Hemley, 1967;and Tzuzuki & Mizutani, 1971). The coexistence of pyrophyllite and kaolinite allows us to estimate a maximum temperature of 350ºC. Diaspore can be recognized frequently in kaolin deposits of hydrothermal origin. According to Roy & Osborn (1954), the boehmite-diaspore transformation temperature is between 270 and 300ºCa at 175-1500 atm. water pressure.
Thus, using the O isotope fractionation equation of Shepard & Gilg (1996) indicate that O and H isotope composition of the fluids involved in the kaolinization process are of magmatic origin or of superficial origin but isotopically equilibrated with magmatic rocks at magmatic temperatures. (Marfil et al. 2005). Table 5. Summary of the paragenetic sequence.

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The deposits studied consist of a set of irregular bodies and veins developed on tuffs of rhyolitic and andesitic composition.  The alteration minerals present include kaolinite, dickite, pyrophyllite, alunite, diaspore, illite and chlorite.  The trace elements contents of the kaolin samples in all the deposits studied are very similar. Most of the comparative diagrams (P vs. S, Zr vs. Ti, (Cr + Nb) vs. (Ti + Fe) and (Ce + Y + La) vs. (Ba + Sr) suggest that kaolinite formed from hydrothermal alteration of volcanic rocks. However the high (Nb + Cr) and (Ce + Y + La) contents do not exclude the presence of kaolinite of residual origin.  The mineral assemblage (dickite-natroalunitepyrophyllite-diaspore), the alteration zonation pattern, the geochemistry of trace elements, the relation between LREE and HREE and the small δ 18 O values suggest that the deposits studied were formed by hydrothermal processes.  Mineralization developed in two stages: the first stage, of higher temperature, led to the formation of dickite, pyrophyllite and diaspore. Then the temperature decreased due to contamination with meteoric water, and alunite and kaolinite developed.

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The mineral assemblage allows us to estimate a formation temperature between 270 and 350ºC.  The hypogene deposits are characterized by greater contents of Sr, Pb, V, S and P 2 O 5 , all of them, apart from V, increasing as alteration proceeds. REE, probably forming part of the alunite structure, are fractionated during alteration.  The combination of chemical composition, mineralogical association and O and D isotopes data suggest that the deposits studied were formed because the circulation of hydrothermal fluids, discounting superficial processes at low temperatures.