Mineralogy, structural control and age of the Incachule Sb epithermalveins, the Cerro Aguas Calientes collapse caldera, Central Puna
Natalia Salado Paz a, *, Iv�an Petrinovic b, Margarita Do Campo c, Jos�e Affonso Brod d,Fernando Nieto e, Valmir da Silva Souza f, Klauss Wemmer g, Patricio Payrola a,Roberto Ventura f
a Instituto de Bio y Geociencias del NOA (IBIGEO), Universidad Nacional de Salta-CONICET, Av. Bolivia 5150, Salta 4000, Argentinab Centro de Investigaciones en Ciencias de la Tierra (CICTERRA), Universidad Nacional de C�ordoba-CONICET, C�ordoba X5016GCA, Argentinac Instituto de Geocronología y Geología Isot�opica (INGEIS), Universidad Nacional de Buenos Aires- CONICET, Ciudad Universitaria C1428EHA CABA,Argentinad Facultade de Ciencias e Tecnologia (FC), Universidade Federal de Goi�as, Brazile Dpto de Mineralogía y Petrología e I.A.C.T, Universidad de Granada-CSIC, Avda. Fuentenueva s/n, 18002 Granada, Spainf Instituto de Geociencias, Universidade de Brasília, Campus Universitario Darcy Ribeiro, Asa Norte, CEP 70910-900, Brasília, DF, Brazilg University of Goettingen, Geoscience Centre, Goldschmidtstraße 3, 37077 Goettingen, Germany
a r t i c l e i n f o
Article history:Received 28 March 2017Received in revised form23 June 2017Accepted 9 July 2017Available online 26 August 2017
The Incachule Sb epithermal veins is located near to the N-E rim of the Cerro Aguas Calientes collapsecaldera (17.5e10.8 Ma), in the geologic province of Puna, Salta- Argentina. It is hosted in Miocene felsicvolcanic rocks with continental arc signature. The district includes twelve vein systems with minerali-zation of Sb occurring in hydrothermal breccias and stockwork. The veins are composed of quartz-sulfidewith pyrite, stibnite and arsenopyrite. All around the veins, wall rocks are variably altered to clayminerals and sulfates in an area of around 2.5 km wide by more than 7 km long. The hydrothermalalterations recognized are: silicic, phyllic and argillic.
The veins are characterized by high contents of Sb, As, and Tl and intermediate contents of Pb-Zn-Cu,and traces of Ag and Au. Homogenization and ice-melting temperatures of fluid inclusions vary from125 �C to 189 �C and �2.4 �C to �0.8 �C. The isotopic data indicated a range of d34S �3.04‰ to þ0.72‰consistent with a magmatic source for sulfur.
We present the firsts K-Ar ages for hydrothermal illite/smectite mixed layers (I/SR1, 60% illite layers)and illite that constrain the age of the ore deposit (8.5e6.7 ± 0.2 Ma).
The data shown here, let characterized the Incachule district as a shallow low sulfidation epithermalsystem hosted in a collapse caldera. Our data also indicate that mineralization is structurally controlledby a fault system related to the 10.3 Ma collapse of Aguas Calientes caldera. The interpreted local stressfield is consistent with the regional one.
In the Puna geological province profuse magmatic activityoccurred during the Miocene, giving place to different kinds ofvolcanic and subvolcanic forms comprising stratovolcanoes, cal-deras, and domes. Several metallogenetic major episodes, in somecases of economic interest, are linked with that magmatic activity(Coira, 1983; Caffe, 1999; Kay and Mpodozis, 2001; Chernicoff et al.,
. Salado Paz).
2002; Sillitoe, 2008). This applies particularly to the Central Puna(~24�S), which is one of the largest ore districts with silver, lead,copper and gold mineralizations in epithermal veins (Sureda et al.,1986; Pelayes, 1981; Sillitoe, 2008; Zappettini, 1999; Ramallo et al.,2011). Previous studies were focused on the mineralogy of the oredeposits, with the goal of defining whether or not they were of theepithermal type (Arga~naraz and Sureda, 1979; Morello, 1968).However, those studies did not explore the genetic linkage betweenthe mineralization and specific volcanic episodes and/or volcaniccenters.
In the case of the Incachule mine, located in the Central Puna
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(Fig. 1), previous works interpreted the mineralization as epi-thermal, and related it with the activity of the Quevar volcano (JICA,1993; Zappettini, 1999; Chernicoff et al., 2002). In this study, wepresent new data suggesting a temporal and spatial link betweenthe mineralization of the Incachule area and the cerro Aguas Cal-ientes collapse caldera and with the local and regional tectonicframework.
The relationship between epithermal deposits and collapsecalderas has been cited for various examples globally, where thoselandforms are important structural traps for localization and dis-tribution of ore deposits. In the case of the geological province ofthe Puna, hydrothermal activity hosted in collapse caldera werementioned for northern Puna (Coira, 1999), but the relationship ofmagmatism and structure of the caldera remains as a matter ofdebate. Regarding the Central Puna (~24�S), the first references thatlinks the origin of polymetallic veins with posthumous volcanicactivities and collapse caldera correspond to Petrinovic (1999) forthe Aguas Calientes and to Riller et al. (2001) for the Negra Muertacalderas. However, studies aimed to understand the relationshipbetween magmatic and/or collapse caldera event structures(doming, subsidence, resurgence) and the hydrothermal-mineralization episodes responsible for the Incachule ore districthave not yet been done.
The aim of this study is to characterize themineral association ofthe hydrothermal alteration halos and the mineralized zones, toconstrain the main features of the mineralizing fluids, based ongeochemical analyses of fresh and altered rocks and ore minerals,as well as on isotopic and fluid inclusions data.We discuss a geneticmodel for the Sb veins mineralization and its relation with thecaldera collapse and their structure.
Incachule is the first ore deposit in Central Puna for which thegenetic linkage with collapse caldera is investigated, thus the un-derstanding of the processes that originate this deposit can beuseful to understand the relationship of mineralized systems withcaldera events or structure in similar mineral ore deposits in theCentral Andes.
Fig. 1. A- Regional geological map of the study area. Modified from Blas
2. Regional geological setting
The Cerro Aguas Calientes collapse caldera hosts a Sb mineral-ization and is located in the geological province of Puna (Turner andM�endez, 1979), on the trace of the Calama-Olacapato-El ToroLineament, a major strikeeslip fault system oriented NWeSE andoblique to the NeS tectonics trends of the central Andes (Mon,1979; Salfity, 1985). The products of the collapse caldera cover anarea of 1700 Km2, with a late stage of resurgence in the calderacenter that uplifts 1000 m the outcrops of the intracaldera withrespect to outflow caldera ignimbrites. The caldera has 15 km indiameter with the major axis oriented N30E� in a left-lateraltranspressive setting (Petrinovic et al., 2010).
In Fig. 1 we summarize the geological background of the area.Thus, the stratigraphic basement of the studied area comprises theNeoproterozoic-Lower Cambrian Puncoviscana Formation (marinelow grade metasedimentary sequences, Turner, 1960) and a lowPaleozoic plutonic-volcano-sedimentary marine sequence(Omarini et al., 1984; Coira et al., 2001; Viramonte et al., 2007).Northward of the study area, Cretaceous-Paleocene continentalsedimentary sequences of the Salta Group crop out (Turner, 1960).The Cretaceous Pirgua Subgroup is the best exposed unit of theSalta Group in the area of the Piedra Caída-Caj�on creeks (Vilela,1969). To the north and west of the study area, younger continen-tal sedimentary sequences, the Pastos Grandes Group occurs(Turner, 1972). Miocene-Pliocene volcanic rocks of dacitic-andesiticcomposition, generally showing high potassium contents, crop outextensively in the study area. They comprise ignimbrites originatedfrom cerro Aguas Calientes collapse caldera, ignimbrites and lavasfrom central eruptions of Quevar and Tuzgle stratovolcanos andvolcanic rocks from minor eruptive centers (Petrinovic et al., 1999),such as the domes and subvolcanic bodies of Concordia, El Morro,Organullo, and Rupasca (Fig.1), with ages ranging between 12.1 and13.5 Ma obtained on biotite using the K-Ar method (JICA, 1993;Petrinovic et al., 1999). The latest records of volcanic activity inthe area correspond to the monogenetic basaltic centers of theNegro de Chorrillos and San Jer�onimo, which were dated (K/Ar
co et al. (1996), Petrinovic et al. (1999), and Petrinovic et al. (2010).
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whole rock) by Aquater (1980) at 0.2 ± 0.08 Ma and 0.78 ± 0.1 Ma,respectively, and the explosive rhyolitic centers in Tocomar withages between 1.15 and 0.5 Ma (Coira and Paris, 1981; Petrinovic andColombo Pi~nol, 2006).
According to Petrinovic et al. (2010) the evolution of the AguasCalientes collapse caldera comprises two caldera-forming episodesthat occurred at 17.15 Ma and 10.3 Ma. Both caldera collapses arequite similar in shape, location and characteristics. The 17.5 Maproduced a thick intra-caldera ignimbrite (Verde Ignimbrite)without an evident and associated extra-caldera facies (Petrinovicet al., 2010). At Cerro Verde hill evidence of a local contractions ofthe caldera moat (tectonic/volcanic resurgence) and postcalderavolcanism can be observed (Fig. 1).
The 10.3 Ma collapse event (Chorrillos and Tajamar Ignimbritesfrom Petrinovic et al., 2010) was accompanied by base andprecious-metal mineralization (Salado Paz, 2014), in a conspicuousvolcano-tectonic framework. The veins of Pb-Ag-Zn are hosted inthe NNE caldera border and Sb veins crops out near to the orientalcaldera border (Petrinovic et al., 2010; Salado Paz, 2014). Theseveins are posthumous to the last resurgence event (Petrinovic et al.,2010). Both mineralizations are close to each other (~4 Km), so issuggested that they form part of the same system, formed atdifferent depths (Coira and Paris, 1981).
3. Geology of the Sb-Au deposits
The mineralization of the Incachule district occurs in vetiformbodies which strikes between N 290� and N330�, are 600 m inlength and have thicknesses between 2.5 and 5 m. This deposit ishosted in the 17.3 Ma intracaldera facies (Verde Ignimbrite) andalso in the younger Chorrillos and Tajamar ignimbrites (10.3 Ma)(Fig. 2A).
The intra-caldera rocks show hydrothermal alteration associ-ated with the mineralization, evidenced by light colours and thepresence of veinlets on fracture planes. Themineralization is placedbetween the eastern collapse rim and the resurgent dome of thelast caldera cycle (Fig. 2B); there are no outcrops westward of theresurgence that attested the continuity of the mineralization to thewestern caldera rim. The geothermal system that produced themineralization is still active, it has migrated southward given placeto the present Incachule geothermal field, that depicts similarcharacteristics than the old one (Fig. 2C). This is particularly evidentfrom the observation of a deep negative gravimetric anomaly(12Mgal), close to the present geothermal field (G€otze et al., 1988).
Two types of mineralization have been distinguished: 1-Mineralized hydrothermal breccias (Fig. 3A and B). 2- Quartz withsparse mineralization, stockworks formed by veinlets of sulfursinside the silicificated wall rock (Fig. 3C and D). The hydrothermalbreccias are the mineralized structure most important, where thesulfurs minerals are disseminated in the matrix. The zone ofstockworks mineralization and veinlets are near to the mineralizedbodies breccias. Both mineralization styles are composed of purestibnite, arsenical pyrite, marcasite and pyrite in a gangue of quartzand chalcedony with breccia, massive, lattice bladed and colloformtextures (Fig. 3E, F and 3G).
The hydrothermal alteration zones around the veins are presentin an area a 7 by 2.5 km direction N-S elongated band (JICA, 1993;Zappettini, 1999; Salado Paz, 2014). Three halos of hydrothermalalteration could be distinguished: silicic, phyllic and argillic(Fig. 2A). Silicic alteration affects the dacites and the pyroclasticdykes of the Verde ignimbrite and also the pyroclastic dykes of theChorillos ignimbrite. The alteration reaches up to 15 m in thickness,from mineralized veins to wall rock, making the wall rock highlycompetent (Fig. 3A and B). Moreover, siliceous sinter up to 20 cm inthickness crops out close to the mineralized zone. The sinter shows
a laminated structure consisting of 2 cm-long sheets (Fig. 2C).Phyllic alteration affects the Verde Ignimbrite, surrounding the
silicic alteration halos as an elliptical zone (Fig. 3H). The alterationis intense to moderate and pervasive.
Argillic alteration is located in the northwest and south bordersof the phyllic alteration halo. In the field it can be identified by itswhitish and yellowish colours. This zone shows intense andpervasive alteration (Fig. 3H).
4. Material and methods
Fifty five samples were collected in representative outcrops ofhydrothermal altered rocks and mineralized breccias. We per-formed standard petrographical analyses from 38 rock samples and15 samples representative of the mineralization to determine li-thology and the primary and secondary (alteration) mineralogy.Eight representative samples were chosen for mineralogical studyby reflectance spectrometry analyses (SWIR) with the equipmentPIMA II SP (Portable Infrared Mineral Analizer), which employedwave lengths between 1200 and 2600 nm, in the SEGEMAR labo-ratory, Buenos Aires. This methodology is particularly sensitive forminerals that contain certain molecules and radicals, includingH2O, OH�, NH3, CO3
2�, SO42�, which produce diagnostic absorption
features at certain wavelengths in the SWIR spectra (Pontual et al.,1997). For the case of ammonium-containing minerals, the spectrahave depression in the region between 1900 and 2200 nm(Thompson et al., 1999).
The mineralogical composition of the<2 mm sub-fraction wastested in 8 samples by X-ray diffraction (XRD) using an X'Pert Prodiffractometer (CuKa radiation, 40 kV, 40 mA) (Departamento deFísico Química, UNC, Argentina), and a PANalytical X'Pert Prodiffractometer (CuKa radiation, 45 kV, 40 mA) equipped with anX'Celerator solid-state linear detector (Department of Mineralogyand Petrology, University of Granada). Clay sub-samples (<2 mm)were prepared in accordance with the guidelines of Moore andReynolds (1997). Afterwards, four representative samples (I 12B, I30, I8, I31A), were chosen for detailed study with a scanning elec-tron microscopy (SEM), employing polished thin sections usingback-scattered electron imaging and X-ray dispersive (EDS) anal-ysis carried out with a ZEISS DSM 950 equipment (Scientific In-strument Centre, University of Granada, CIC) and Thermo ElectronNORAMNSS-100 (LaSem Universidad Nacional de Salta). Moreover,five samples representative of the mineralized zones were studiedwith a JEOL superprobe JXA-8230electron probe micro analyser(EPMA) (Instituto de Geociencias, Universidade de Brasília).
Atomic concentration ratios were converted into formulae ac-cording to stoichiometry. Accordingly, the structural formulae ofdioctahedral micas and illite/smectitemixed-layers were calculatedon the basis of 22 negative charges (O10 (OH)2). In the cases ofsulfates-phosphates structural formulae were calculated on thebasis of 11 negative charges ((O4)2 (OH) 6).
Chemical analyses of major, minor and trace elements wereperformed for twenty two rock samples from hydrothermal alter-ation and mineralized zones with FRX and ICP-MS and acidicdigestion for the metals at ACMELABS laboratories (Canada). Inorder to study the mobility of elements during the alteration pro-cesses and to identify the elements that could be used as path-finders of Au anomalies; we compared our results with previousanalyses of unaltered Verde and Tajamar ignimbrite fromPetrinovicet al. (1999).
A total of 163 microthermometric analyses on fluid inclusions inquartz from the mineralized veins were performed on four selectedrocks using a Linkam fluid inclusion cooling-heating stage at theFluid Inclusion Laboratory of the Universidad Nacional del Sur,Bahía Blanca, Argentina.
Fig. 2. A- Geological map of the area of Incachule mine depicting the hydrothermal alteration zones. The silicic zone is very small and could not be represented in this scale. B- Crosssection W-E and relationship between epithermal breccias, dome resurgence and edge collapse caldera C- Cross section N-S from Incachule mine area, indicating the outcrops fromfossil and actual sinters.
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Sulfur isotopic compositions were determined on handpickedsulfides at the Geochronology Laboratory of the Institute of Geo-sciences, University of Brasilia, by LA-MC-ICP-MS (Laser Ablation-Multi Collector-Inductively Coupled Plasma Mass Spectrometry).
In order to determine the structural controls on hydrothermalalteration and mineralized zones we collected structural data infault veins of hydrothermal breccias in a total of 40 stations and instockwork veins. In the first case, two types of data were obtained:a-faults with kinematic indicators (fault plane, dip, slickensides andstep), and b-faults without kinematic indicators (fault plane anddip). For vein stockworks, we measured average thickness, strikeand dip. Stereonet (Allmendinger, 2002) and FaultKinwin freewerepackage were used for the interpretation of statistical data(Allmendinger, 2001).
K/Ar ages were determined on <2 mm size fractions at theGeoscience Centre of the University of G€ottingen by the followingprocedure: The argon isotopic composition was measured in aPyrex glass extraction and purification line coupled to an ARGUS VImulti-collector noble gas mass spectrometer operating in staticmode. The amount of radiogenic 40Ar was determined by isotopedilution method using a highly enriched 38Ar spike from
Schumacher (1975). The age calculations were based on the con-stants recommended by the IUGS quoted in Steiger and J€ager(1977). Potassium was determined in duplicate using a BWB XPflame photometer. The samples were dissolved in a mixture of HFand HNO3 according to the technique of Heinrich and Herrmann(1990). The analytical error for the K/Ar age calculations is givenon a 95% confidence level (2s).
5. Results
5.1. Petrography and mineralogy of hydrothermal alteration
5.1.1. SEM and optical microscopyFour volcanic rocks, representative of the silicic alteration (I30,
I31A), phyllic to argillic transition alteration (I12B) and argillicalteration zones (I8) were studied with SEM and opticalmicroscopy.
Sample I30 contains primary phenocrysts of potassium feldsparand biotite, strongly replaced by secondary phases and showingabundant dissolution voids. Potassium feldspar is replaced by illite/smectite mixed-layers (I/S) or, less frequently, by micron-scale
Fig. 3. A-Outcrops of hydrothermal breccias. B- Hydrothermal breccia with altered clasts and chalcedonic matrix. C- Outcrops of disseminate veins. D- Stibnite crystal fromdisseminate veins. E-Texture lattice bladed in quartz from vein. F- Stibnite crystal in colloform quartz texture. G- Pure crystal of stibnite in massive quartz texture. H- Hydrothermalalteration in host rocks.
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intergrowths of I/S and kaolinite. It could also depict replacementby quartz in the borders and overgrowth of secondary potassiumfeldspar. Biotite is mostly replaced by I/S and secondary potassiumfeldspar (Fig. 4B and C), I/S and kaolinite frequently occur asirregular laths, up to 10 mm long, with open spaces between them,and radial morphology (Fig. 4A and B). K is the prevailing cation in
the interlayer sites of I/S. Pyrite and rutile occurs as anhedral in-clusions in altered phenocrysts. Minor tiny euhedral or anhedral Fesulfates-phosphates (destinezite?), depicting some substitution ofP and/or S for As, also occur as secondary phases (Fig. 4B and C).Sample I31A depict a pervasive replacement by secondary quartz,chalcedony and adularia. The primary mineralogy is no longer
Fig. 4. BSE images (A-C and F-I), optical microscopy (DeE) and SE (J, K) BSE images (AeI) and SEM (J,K) and A-Illite with radial morphology and oxides of Ti inclusion (I30 sample).B- Biotite replaced by I/S with potassic feldspars inclusion (I30 sample). C- Biotite replaced by I/S and potassic feldspars with SO4AsFe inclusions (I30 sample). D- Chalcedony andquartz in pervasive silicic alteration zone (sample I31A). E� Rhombic crystals of adulary from silic alteration (sample I31A). F- biotite replaced by Kln þ I/S with TiO2 inclusion(sample I12). G- Potassic feldespars replaced by Kln þ I/S, with FeO2 inclusion, illite and sulfates (sample I12). H- SO4FePbAs filling void in a rock matrix composed of quartz, potassicfeldspar and biotite (sample I12). I- Potassic feldspar fromthe matrix altered to I/S plus kaolinite, patches of secondary FeAsO and sulfates-phosphates are also observed (sampleI12). J- Handpicked crystals of jarosite from argillic alteration zone(I8 sample). K- Crystals of alunite from argillic alteration sample (I8 sample). Mineral abreviations: I/S: illite-smectite mixed-layers; Kln: kaolinite. Ox: oxides; Ill: illite; Bt: biotite; Kfs: K-feldespars; Jrs: jarosite; Aln: alunite; Chc: chalcedony; Adl: adulary; Qz: quartz.
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recognizable, but the volcanic texture is preserved. Quartz andchalcedony are abundant, it occurs in grains up to 500 mm in size(Fig. 4D). Adularia conform rhombic crystals up to 25 mm in size andalso constitute veins and veinlets (Fig. 4E).
Sample I12B is composed by phenocrysts of biotite and potas-sium feldspar and accessory zircon in a groundmass replaced byphyllosilicates. Potassium feldspar crystals show abundant frac-tures and are pervasively altered to kaolinite or to micron-scaleintergrowths of kaolinite and I/S, but also to illite (Fig. 4F, G and
H) and less frequently to quartz. Biotite also depicts abundantfractures and open spaces along cleavage planes, and is profuselyaltered to micron-scale intergrowths of kaolinite and I/S (Fig. 4F).The rock presents veinlets and fractures filled with complex as-semblages of sulfates, phosphates, arseniates and zoned oxidesindicating successive and sometimes cyclic changes in fluidcomposition (Fig. 4G, H and I). The zones that appear whitest in BSEimages are those showing the highest lead contents in EDS. Rutileand iron oxides, and less commonly Fe-As oxide and scarce As-Ce
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phosphate were also identified. These phases also occur as thincoatings around primary crystals, rimming voids, forming irregularpatches, as well as disseminated or depicting dendritic textures inthe groundmass, in close association with phyllosilicates (Fig. 4Hand I).
Sample I8 is entirely composed by secondary minerals such asopale, jarosite and alunite. The jarosite and alunite occur in fine-grained euhedral and subhedral crystals associated with kaolinite(Fig. 4 J and K).
5.1.2. Clay minerals and associated secondary mineralsThe clay minerals assemblages occurring in the different alter-
ation zones were studied by SWIR, XRD and SEM, results are shownin Table 1.
The clay minerals identified in the argillic alteration halo werekaolinite þ I/S associated with alunite, jarosite, and opale. Ac-cording to XRD data in most of the samples kaolinite and I/S are insimilar proportions in the clay fraction XRD, although lessfrequently it only contains, kaolinite (samples I 24, I38, I34). TheSWIR study identified reflection in 1461 nm indicating alunitecontaining ammonium ions in some samples (I7, I8) (Godeas, 2010).
The phyllic alteration halo is characterized by a clay assemblageof illite þ (I/S) þ smectite, associated with pyrite and quartz (I17,I19A, I26, I27, I20, I12). The reflections of I/S in the region between16 and 17�2q in XRD patterns indicate R1 order and illite contents of60e75%, depending of the sample (Moore and Reynolds, 1997).
Some samples from silicic alteration halo present kaolinite (I30,I31A) associated with quartz and adularia. The SWIR study alsoidentified reflections in 1912, 2013 and 2112 nm indicating thepresence of buddingtonite (I33, I29) and reflections in 1554, 1410,2180, 2228 nm indicating illite (I9) with ammonium ions (peaks in1912 and 2013 nm), (Pontual et al., 1997).
Table 1Mineralogy of altered rocks based on XRD, SWIR, and SEM-EDX studies.
5.1.3. Mineral compositionThe I/S from silicic halo (I30) depicts higher substitution along
illitic and Tschermack compositional vectors than in the samplecorresponding to the phyllic alteration halo (I12B). In both cases K isthe prevailing cation in the interlayer site, with minor Na and Ca(Table 2). Many analyses show high Al and low K contents, probablydue to contamination resulting from fine intergrowths of I/S andkaolinite below the resolution of the EDS. On the other side, sampleI12B contains secondary illite in addition to I/S, the two phasesdepict a clear contrast in compositional diagrams, as illite showsless illitic and less Tschermack substitution than I/S (Fig. 5).
The EDS analyses for sulfates from phyllic alteration halo (I12B)indicate intermediate members of the alunite supergroup depictingchemically variable zones that frequently are too thin to beanalyzed individually with the EDS. The analytical results agree,within reasonably limits, with the general formula for this super-group: DG3(TO4)2(OH,H2O)6 (Jambor, 1999), In this case D is occu-pied by K andminor Na, and divalent Pb, G is typically A13þ or Fe3þ,and T is S6þ, P5þ or As5þ. A substantial part of the analysis corre-spond to the plumbojarosite-jarosite family, the hinsdalite-corkiteseries or the beudantite group (Table 2), but others depict com-plex chemical composition corresponding to intimate mixtures orsolid solutions amongmore than two compositional end-members.
5.2. Geochemistry of the wall rock
All the rocks in the mineralized area are altered to a variabledegree, so in order to analyze the mobility of elements during thehydrothermal alteration we included in the plots representativeanalyses of unaltered rocks from two caldera cycles taken fromPetrinovic et al. (1999) (Table 3).
The unaltered rocks plot within the andesite and rhyodacite/dacites fields in the diagram of Winchester and Floyd (1977)
Table 2Structural formulae for illite, I/S, kaolinite, intergrows of illite plus kaolinite, and sulfates according to EDX data.
Fig. 5. Compositional diagrams for micaceous phases. Red circles: sample I30, yellow crosses: sample I12B. Mineral abreviations: I/S: illite-smectite mixed-layers; Kln: kaolinite.Composition expressed in atoms per formula unit (a.p.f.u). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of thisarticle.)
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(Fig. 6A). The altered rocks plot within rhyodacite/dacite andrhyolite fields showing similar or higher SiO2 contents than theunaltered rocks (Fig. 6A).
Normalized rare earth elements (REE) patterns of unalteredrocks are characterized by an enrichment of LREE and relatively flatHREE patterns. Such features are common in calc-alkaline rocks ofthe Andes. Altered rocks depict similar REE patterns, although, theyshow slight depletion in LREE and stronger depletion in HREE(Fig. 6B) relatively to fresh volcanic rock.
Unaltered rocks show negative correlations between CaO, Fe2O3,Na2O, MgO and SiO2 in Harker plots, and positive correlations be-tween K2O and SiO2, all typical of magmatic differentiation trends.The altered rocks show more scattering in Harker plots, suggestingelement mobility, although broad negative correlations betweenFe2O3, Al2O3, MgO and SiO2 and positive correlations between K2Oand SiO2 could be observed. Na2O shows a slight negative corre-lation with SiO2, whereas CaO and SiO2 show no correlation (Fig. 6C).
Combining the AI index defined by Ishikawa et al. (1976) withthe carbonate-chlorite-pyrite index from Large et al. (2001) in aboxplot allows the recognition of the main minerals that wererespectively destroyed and formed as a result of the hydrothermalprocess. These indices were used in some massive sulfur ore de-posits and were also applied in epithermal ore deposits (Gemmelland Large, 1992; Gemmell, 2007).
The alteration index AI ¼ 100*(MgO þ K2O)/(MgO þ K2O þ Na2O þ CaO) defined by Ishikawa et al. (1976),quantifies K or Mg enrichment or depletion relative to Ca and Na.The AI increase as a result of formation of K feldspars and/or Kphyllosilicates (muscovite or illite), and decrease due to calcareousalteration. In the other hand, the carbonate-chlorite-pyrite indexCCPI ¼ 100*(MgO þ FeO)/(MgO þ FeO þ Na2O þ K2O) from Largeet al. (2001), measures total alkali depletion, relative to Mg andFe enrichment associated with formation of secondary chlorite,pyrite, dolomite or siderite.
The unaltered volcanic rocks (first and second collapse calderaevent) have AI values between 40 and 60 and CCPI values between40 and 60 reflecting high primary concentrations of alkalis, Fe andMg. On the other hand, the altered rocks have AI > 90 and CCPI <40,the trend showed by these rocks is consistent with the alteration ofmafic minerals (mainly biotite) and plagioclase and the formationof secondary K-micas (illite-muscovite, I/S) and K-feldspars(adularia) (Fig. 7A).
In order to compare base and transition metal concentrationsbetween unaltered and altered rocks, metal abundances wereplotted against K2O contents, as the geochemistry of the alteredrocks suggest that the hydrothermal fluids associated with themineralization have high concentration of K2O. The concentrationsof base metals (Pb, Zn) are noticeably lower in the altered rocksfrom the Incachule ore deposits than in the fresh volcanic rocks
Table 3Geochemical analysis of major, minor and trace elements of altered and unaltered rocks. Concentration of trace elements expressed in ppm.
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(Fig. 7B). The Cu concentration is high in two samples from unal-tered rocks. On the contrary, altered rocks depict higher abun-dances of Tl, As, Sb and Cs than the unaltered rocks. It is remarkablethat several altered rocks display gains in Ag and Au.
5.3. Ore mineralization
The opaque minerals identified under the chalcographic mi-croscope were pyrite, arsenian pyrite, stibnite and marcasite. Theseminerals occur in veinlets as stockworks and as massive accumu-lations in the cement of hydrothermal breccias. Microscopically finegrained sulfide minerals occur as discrete aggregates in the matrixof the silicified breccia clast. Averages of mineral compositions aregiven in Table 4.
Stibnite is the most abundant ore mineral, it forms euhedralcrystals up to 3.5 cm in size and subhedral to anhedral crystals ofvariable sizes (between 100 and 300 mm) but always show largergrain sizes than the other sulfides (Fig. 8 A, B). Pyrite occurs in tinyeuhedral crystals up to 20 mm (Fig. 8 C, D), as small inclusions inbiotite (Fig. 8 E), as anhedral mosaic, and forming veinlets in thebreccia cement and silicified breccia clast. Arsenical pyrite occursscarcely as anhedral disseminated crystals in the breccia cementand forming veinlets (Fig. 8 F).
The veins are characterized by high contents of Sb, As and Tl, andintermediate contents of Ag, Pb-Zn-Cu and traces of Au. Whereas,hydrothermal breccias in some cases depict high Ag/Au ratios(Table 5). Some samples from stockworks (I30v) and hydrothermalbreccia (I12v) have higher Cu-Pb-Zn concentration compare withother samples.
5.3.1. Fluid inclusionsMicrothermometric measurements were done in quartz veinlets
associated with the mineralization. The studied fluid inclusionsconsist of primary biphasic inclusions (liquid þ vapor), with pre-dominant liquid phase, showing ovoid, irregular and regularshapes, less than 15 mm in size. Homogenization temperaturesranging from 125 to 189 �C, with an average of 157.64 �C, andfreezing temperatures from �0.8 to �2.4 were determined. Fluidsalinities between 1.5 and 4.5wt% NaCl equivalent where estimatedfor the inclusions showing the lower freezing temperatures, fromlast ice melting temperatures using the equations of Bodnar (1993),whereas a mode salinity of 1.5 wt% NaCl equivalent was obtained(Table 6).
5.3.2. Sulfur isotopesSulfur isotopic compositions were measured in hypogene stib-
nite, pyrite, and arsenopyrite, free of mineral inclusions, from theveins and hydrothermal breccias. The range of d34S obtained, from�7.10e2.88‰ (Table 7), is compatible with a magmatic source forsulfur (Ohmoto and Rye, 1979). The d34S values of the ore-formingfluid, �3.04 to þ0.72‰, was calculated from the d34S values of thesulfide minerals for which the appropriate temperature of forma-tion were reasonably estimated by fluid inclusion data employingthe empirical equation from Ohmoto and Rye (1979).
5.4. Structural analysis
Themajor mineralized structures are the hydrothermal-brecciasemplaced along the strike slip faults, having a northwest trending(N330�) with left-lateral component and 50� to 80� dip. However,some faults have E-W trending with high dip angle (80-90�: I42,I47). Just a few are located on normal faults with a strike slipcomponent (P63, I49, I46) and NW-SE, E-Wor NE-SW trend (Fig. 9).
The hydrothermal-breccia veins form mineralized lenticularbodies with quartz and laminated chalcedony, commonly separated
Fig. 6. Major and trace elements data for altered (triangle) and unaltered whole rocks of the Cerro Aguas Calientes collapse caldera, first collapse event (square) and second collapsecaldera (rhombus). A- Total silica versus Zr/TiO2 diagram (Winchester and Floyd, 1977). B- Spider diagram for rare earth elements. C-Harker variation diagrams showing thevariations of major and trace elements of altered and unaltered rocks of Cerro Aguas Calientes collapse caldera.
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Fig. 7. A- AI vs CCPI binary diagram, discriminating altered rocks from Incachule mine from the least altered ignimbrite host rocks. B-Cu, Sb, Pb, Cs, Zn, As, Ag, Au, Tl contents versusK2O variation diagrams.
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Fig. 8. Chalcographic photomicrographs of the main ore minerals. A- Euhedral crystal of stibnite (10X). B- Anhedral crystal of stibnite (20X). C- Small crystal of pyrite as anhedralmosaic (20X). D- Abundant subhedral crystals of pyrite in quartz matrix (20X). F- Tiny inclusions of pyrite in biotite (20X). G- Arsenical pyrite in veins and crystal (20X). Mineralabbreviations: Py: pyrite; Stb: stibnite; Apy: arsenical pyrite.
Table 5Trace metal contents in veins (breccias) and disseminated mineralization.
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by slivers of foliated altered wall rock. These breccias are composedof non-rotated variable sized (0.5e3.7 cm) angular clasts of alteredwall rock and chalcedony rich fragments (in some cases foliated)with jigsaw textures, enclosed in a hydrothermal matrix of
chalcedony or quartz.Kinematic indicators such as slickensides, steps, and quartz
slickenfibres are commonly observed along altered wall rockslivers, with a main plunge of 120e145� and 15-45� dip (Fig. 9).
Table 7Sulfur isotopes data data for sulfide minerals. The d34S values of the ore-formingfluid were calculated using the empirical equation worked out by Ohmoto andRye (1979).
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The hydrothermal breccias are emplaced along the same faultplanes as the previous pyroclastic dykes from the 10.3 Ma collapsecycle. The Chorillos ignimbrite pyroclastic dykes have N330� trend,up to 2 m width and a maximum length of 200 m. Some of thedykes have sigmoid shapes. However, in some cases the hydro-thermal breccias cut pseudo-parallel or obliquely the pyroclasticdyke, showing different strikes (Fig. 10A and B and C).
The minor mineralized structures are stockworks zones orveinlets with sulfides (I39, I47), that are located close to the hy-drothermal breccias (I12, I7, I8). All veins show a similar trend(N�330.) and 70-85� dip, similar to those of the mineralized faults(Fig. 10 D). The stockworks include pyrite-quartz and stibnite veinswith competent altered host rocks, up to 10 cm wide and up to80 cm long.
Other structures not related with the mineralization wereobserved in the area, like reverse (I51, I71) and normal (I48, I49)faults.
5.5. K-Ar ages
K-Ar analyses were performed for two clay fractions from rocksof the phyllic alteration zone (Table 8 A). Sample I20A, whichcontains disseminated mineralization, and whose <2 mm fraction iscomposed of I/S R1 (y 60% illite layers) and kaolinite yield a date of8.5 ± 0.2 Ma. Sample I12B, which contains vein breccia minerali-zation and whose <2 mm fraction is composed of I/S R1 (y 60% illitelayers) and illite yielded an age of 6.7 ± 0.2 Ma.
Therefore, K-Ar ages constrain the hydrothermal mineralizationprocess at the Incachule mine to the Miocene (Tortonian-Messinian).
6. Discussion
6.1. Mineralogy characteristics and physical-chemical variation
The typical models of epithermal deposits focus on the changesof minerals assemblages, physical and geochemical parameters,their variations with depth can be used to determinate the level oferosion in each case (Buchanan, 1981; Hayba et al., 1985; Healdet al., 1987). However, the type and distribution of secondarymineral assemblages produced by hydrothermal alterationsdepend of many variables, including fluid composition, tempera-ture, composition and permeability of the primary rocks, positionof the water table, deep boiling zone and structural controls of fluidflow. This complex interaction makes it hard to interpret the gen-esis. Such is the case of hot spring and near surface systems wherecooling, boiling and condensations processes are present.
Three main mineral assemblages were identified in the Inca-chule ore deposits. Quartz, adularia and illite, associated with
Fig. 9. Detailed map with structural data. Stereograms for each data point, plane faults and striaes.
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stibnite, pyrite and arsenical pyrite constitute one group. The sec-ond group is represented by chalcedony, I/S and smectite, and thelast group comprise kaolinite, alunite, jarosite, marcasite and opal.The first association suggests that boiling had occurred (Dong andMorrison, 1995; Hedenquist, 1990) thereby increasing pH and Kþ/Hþ with the precipitation of adularia and K-micas associated withsulfides. The association is characteristic of low temperature andcompatible with the values determined by fluid inclusion in thiswork (~158 �C). Then, the cooling of the fluid let attain silicasaturation and precipitation of chalcedony, I/S (R1) and smectitesuggesting a temperature range between 80� and 120 �C(Hedenquist, 1990; Pollastro, 1993; Inoue, 1995). The last mineralassemblages are formed by oxidation of H2S to H2SO4 giving placeto argillic alteration zone characterized by the occurrence ofkaolinite, opal, alunite and other complex-sulfates. These sulfates(hinsdalite, coorkite, jarosite, alunite, plumbojarosite, beudanite)are common in the supergene zone of pyrite bearing deposits.However, these minerals can also form by hypogenic processes inhydrothermal altered zone, as occurs in present hot springs andgeothermal zones (Raymahashay, 1969; Dutrizac and Jambor,2000). In the Incachule area the association of the acid suite min-erals with silica sinters as well as the occurrence of hydrothermalbreccias point to an origin by steam-heated acid-sulfate waters attemperatures <120 �C, near the water table, in the shallowestepithermal environment. Neither Fe-oxides, cellular textures norsecondary sulfide that could indicate supergenic processes wereobserved. Moroever, pyrite crystals are not altered nor stibniteshave insignificant alteration. The occurrence of jarosite-plumbojarosite in several samples (I12B, I8, I7 and I38, I50) in-dicates oxidation of H2S when the vapor phase generated by boilingof the deep waters interacts with the atmosphere just above thewater table (Simmons et al., 2005). Moreover, textures observed inBSE images indicate that alteration mainly proceed by dissolution-
crystallization processes. Some cyclic textures of I/S þ Kln and I/Sobserved in sample I12B, suggest compositional fluctuations in thefluid, with variations in Kþ/Hþ, Ca2þ/Hþ and Si/Al ratios. Kaoliniteassociated with Pb-sulfates (I12B) was also observed, indicatingacid conditions, as this mineral assemblage is stable at low pH. Theacid alteration produced by this mechanism is superficial, rarelyextending even 50 m below the surface, and are generally notpreserved in ore deposits (Hedenquist et al., 2000). However, in theIncachule mine part of the superficial system is preserved. Probablybecause the arid climate prevailing since the late Miocene in thecentral Puna (Quade et al., 2014) reduced weathering erosion andsupergenesis in the Incachule ore deposits.
Although precious metals are not present in noticeableamounts, the ammonium ion contained in someminerals phases ofthe mineralized zone (buddingtonite, brammalite and alunite) is anindicator sensitive to the presence of Ag and Au (Ridgway et al.,1990), so it is considered an excellent pathfinder in mining andgeothermal exploration for this area.
6.2. Geochemical changes, and alteration mineralogy and type ofmineralization
The compositions of the altered and fresh caldera rocks aresimilar, with a wider range of SiO2 in altered rocks, indicatingthat most of the altered rocks were enriched in Si during fluid-rock interaction. Evidence for enrichment of Si can be seen inthe intense silicic alteration and quartz stockworks in thealtered rocks. All the rocks exhibit similar REE patterns char-acterized by a gently dipping pattern from LREE to HREE.However altered rocks are depleted in HREE relatively to thefresh ones, suggesting its mobility during the interaction withhydrothermal fluids. Although REE are commonly consideredimmobile during water-rock interactions, there are numerous
Fig. 10. Relationship between pyroclastic dykes and breccia veins. A- Breccia cutting pyroclastic dyke. B- Breccia cutting pyroclastic dyke with horizontal strike. C- Stereographicdiagram from strike plane of faults (without striae data) in breccia zone and veinlets stockwork data in schematic cross section.
Table 8Analytical results of K/Ar analyses of the fine fractions (<2 mm) of hydrothermallyaltered rocks.
Sample K2O (Wt%) 40 Ar(nl/g) 40 Ar (%) Age (Ma)
I12 7.42 1.603 25.85 6.69 ± 0.26
I20 5.04 1.388 28.12 8.51 ± 0.20
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studies which show that REE can be mobilized during hydro-thermal alteration (Oreskes and Einaudi, 1990; Taylor and Fryer,1983; Cathelineau, 1987; Lottermoser, 1992). In the case of theIncachule ore deposits, the mobility of REE appears to increasein the late stages of the hydrothermal process (phyllic andargillic zones), probably as a consequence of the increase influid/rock ratios.
During the hydrothermal alteration, the host rocks lost NaO2,CaO, Fe2O3 and MgO indicating mobility and leaching of these el-ements by hydrothermal fluids, consistent with the widespreadalteration of primary biotite and K-feldspar observed at the opticalmicroscope and SEM scale. On the other hand, K2O shows higher
concentration in altered rocks suggesting significant secondaryaddition during water-rock interactions, in coincidence with thewide occurrence of secondary K-rich phyllosilicates and adularia inaltered rocks. Al2O3 shows lower contents in altered rocks andnegative correlation with SiO2.
Altered rocks have higher Tl, As, Sb, Cs, Au and Ag contentsthan unaltered ones; moreover, these elements show a positivecorrelation with K2O, indicating that hydrothermal fluids wereenriched in such elements. The enrichment in Sb, As and Tl is awell-known pathfinder in epithermal Au-Ag deposits (Taylor,2007), and typical of shallow environment epithermal deposits(Henley et al., 1984; Hayba et al., 1985; Heald et al., 1987;Hedenquist, 1986; Berger and Silberman, 1985; Hedenquistet al., 2000). The altered rocks depict lower Pb and Zn contentsthan the fresh volcanic rocks, indicating leaching of these ele-ments by acid fluids (Brookins, 1988). Sample I12A represents anexception, as it shows gains of Pb. However, it corresponds to theouter zone of the phyllic alteration halo close to the contact withthe Verde Ignimbrite. The higher Pb content of this sample alongwith their clay mineral assemblage, consisting of illite þ I/S andlacking kaolinite suggests alteration by fluids with intermediate
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to high alkaline/Hþ activity ratios, conditions under whichkaolinite is not stable (Inoue, 1995). The mobility of Pb is alsoindicated by the occurrence of sulfates, arseniates, and phos-phates of Pb-Fe at SEM scale in samples corresponding to thephyllic-argillic transition (I12B) and silicic alteration zones (I30).These secondary minerals are formed through oxidation of thebase metal sulfides by acid hydrothermal fluids (Silberman andBerger, 1985).
The breccias and stockworks zones show Ag/Au ratios andmetalassociation typical of low sulfidation ore deposits (Hedenquistet al., 2000; Sillitoe and Hedenquist, 2003; Simmons et al., 2005).Higher Pb, Zn, Cu and Ag contents in vein samples I12v and I30v,could be indicating a vertical zonation in the epithermal system assuggested by Coira and Paris (1981). According to the micro-thermometric study of fluids inclusions the epithermal mineral-izing fluids at Incachule were dilute (<2 wt% NaCl equivalent) andhad low temperatures, with a 157.5 �C average. This data isconcordant with JICA (1993) who determinate an average tem-perature of 166 �C in quartz. As regards of the origin of sulfur, thed34S of pyrite arsenopyrite and stibnite indicate a predominance ofmagmatic sulfur (d34S values near 0‰, Ohmoto and Rye, 1979)(Table 7).
6.3. Relationship with collapse caldera and age of themineralization
Mineralization is frequently hosted in permeable zones of thecalderas, commonly at the collapse margins or in fault/fractureplanes into the caldera structure. These are zones of weakness thatcould be related with pre-caldera stages in a radial pattern or at thepostcaldera stages. Commonly they follow a radial to concentricpattern (Lipman, 1997), but if regional tectonics drives the collapseof the caldera, as suggested by Petrinovic et al. (2010), minerali-zation will be tectonically hosted. Aguas Calientes collapse calderais emplaced on the Calama-Olacapato-Toro strike-fault zone, whichacts as trigger of caldera-forming eruptions (Petrinovic et al., 2010).
Fig. 11. Evolution scheme of collapse caldera
In this work we observed that all mineralization of Sb have astructurally control with the NW-SE dilatational structure, as thepyroclastic dykes of the second event eruption of the collapsecaldera (Figs. 9 and 10). The relationships between hydrothermalbreccias that cut pyroclastic dykes pseudo-parallel or obliquely,indicate that theywas formed later. Also, there are faults with strikeNW-SE outside collapse caldera without mineralization, suggestingthat the Incachule mineralization is structural and geneticallyassociated with the collapse caldera.
The collapse caldera rim, despite being a permeable zone, doesnot present mineralization. However, there are NW-SE hydrother-mal veins emplaced into the caldera moat, close to the orientalcaldera rim, due to the intense breaking of the rocks in the fault andthe rheological contrast between precaldera (granitic rocks) andintracaldera rocks (ignimbrites). Others minor structures, as thestockworks zone, show a similar trend (N�330) to the hydrothermalbreccias evidencing the same stress field.
The last-collapse cycle of the Aguas Calientes caldera (10.3 Ma,>100 km3) probably erupt all its magma to the surface quickly, aspredicted in theoretical models (Gualda et al., 2013), attesting thatthe longevity of the magma chamber could not be up to hundredsof thousands years (Wilson and Charlier, 2009). Notwithstanding,the hydrothermal activity related with caldera systems may beginmillions of years after the volcanism (Branney and Acocella, 2015).Taking into account these considerations, we have delineated anevolution model for the Inacachule ore deposits.
The hydrothermal system started after the eruption of theTajamar ignimbrite, when a deformation episode produced thecontractional/resurgent stage of the cerro Aguas Calientes thatcould have driven the hydrothermal activity. Then, the fluids werechanneled in weakness zones produced by fault planes and pyro-clastic dykes (Fig. 11). We estimate that a further increase in localand/or regional deformation caused the reactivation of NW struc-tures, allowing the sudden release of hydraulic pressure and pro-ducing the hydrothermal breccias (Salado Paz et al., 2013; SaladoPaz, 2014), which are hosted in the same trend as the pyroclastic
, tectonic events and mineralized veins.
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dykes of the caldera. This means that the regional and local stressfield that promoted the collapse and conduit arranges alsocontrolled the mineralization. So, the transpressive setting inter-preted for the caldera stage probably persisted during the 8.3e6Maperiodwith the same style and intensity and probably continued upto the Quaternary in this area (Petrinovic et al., 2006).
In this way, the two K-Ar ages determined in hydrothermal claysare the first radiometric ages obtained for epithermal systems ofthe Central Puna. The older age was obtained for sample I20 (8.51Ma), that corresponds to the stockworks mineralization, whereas asample representative of the vein zone yielded a younger age(sample I12B, 6.69 Ma). We interpret this temporal variation as twodifferent mineralization episodes. In the early stage, the increase oftectonic stress produced boiling generating breccias with jigsawtexture and not rotated clast, dispersing mineral (stockworks) nearto the principal veins with secondary minerals precipitated infractures of the host rock (I20). Then, probably new hydraulicfracturing in breccias happened, evidenced by textures consistingof clasts of chalcedony immersed in chalcedony hydrothermalmatrix in the breccias (Salado Paz et al., 2013), (I12b).
The ages obtained for secondary clay minerals from the Inca-chule alteration zones are similar to the ages of Cerro Quevar(Wilson et al., 1998), thus supporting previous studies that relatedthe Incachule ore deposits with the Quevarmagmatism (Zappettini,1999). However, the connection between the argillic alterationzone and silicic sinter demonstrates that Incachule is hosted andrelated with post-magmatic activity from the Aguas Calientescollapse caldera. As well the mineralization is hosted in structuresof caldera (pyroclastic dykes) near to the collapse border. The lackof continuity of the mineralization and hydrothermal alteration inthe western sector of resurgence is an evidence of the unrelatedcharacter of the mineralization with the magmatism of the Quevarvolcano. Ever though, the genetic link between ore deposits andcaldera formation is controversial, because the geochronologicaldata indicate that the mineralization could be several million yearsyounger than the caldera formation (Guillou-Frottier et al., 2000).Some authors accepts discrepancy between the mineralization andthe magmatism in the order of 0.5e2 Ma (McKee et al., 1992; Eatonand Setterfield, 1993). In this case, the age of 8.5 Ma is 1.8 Mayounger than the age reported for the Tajamar ignimbrite, so itcould correspond to a magma reservoir at depth. Such relationshipmust be further investigated with the determination of moreradiometric ages.
7. Conclusion
The Incachule ore deposits are an example of low sulfidationepithermal system, formed as shallow and surface expressions of ahydrothermal system associated with acidic steam-heated fluidsgiving place to alteration zones, siliceous sinters, stockwors andbrecciated root zones. The ore deposits present different types ofalteration:1) Silicic composed by quartz-chalcedony-adularia, 2)Phyllic composed by illite, I/S, smectite, pyrite and quartz 3) Argilliccomposed by kaolinite, jarosite, opalo, alunite, complex-sulfates.The sulfur ore comprising stibnite, pyrite, arsenian pyrite andmarcasite was deposited in association with the hydrothermalalteration.
The epithermal textures such as breccias, stockwork veins, col-loform and lattice bladed textures, the low temperature (average157.5� C) and low salinity (average 1.5 wt% NaCl equiv.) determinedin fluid inclusions, and sulfur isotopic composition are typical oflow sulfidation epithermal deposits.
The presence of Sb veins characterizes shallow levels with ver-tical zonation to base metals, and traces of Ag and Au. Secondaryminerals such as sulfates, arseniates and phosphates of Pb indicate
leaching and lateral dispersion of this element that overprintedearlier alteration patterns.
The K-Ar ages obtained indicate that alterations are youngerthan the last collapse caldera cycle. Notwithstanding, the geneticmodel outlined in this paper suggests a genetic relationship be-tween the hydrothermal activity and the structure of the collapsecaldera. The hydrothermal fluids related to Au mineralizationascended along structures resulting from the same stress fieldwhich governed caldera collapse.
Hot spring systems are surface expressions very susceptible toerosion, which makes the case of the Incachule mine an excep-tionally preserved example.
Acknowledgments
This workwas funded by grants CONICET PIP 0781, ANPCYT PICT381, CAPES-MINCyT 009/12, ANPCYT PICT 0407, CONICET PIP 0489.The authors thank SEGEMAR, Dra M. Godeas for performing SWIRspectometry analyses and the Geochronology laboratory fromUniversidade de Brasilia (Brasil). The help of I. Guerra with the SEM(Centro de Instrumentaci�on Científica, University of Granada,Espa~na), Carlos GomezeSilvia Blanco (LaSem Universidad Nacionalde Salta) and Luis Mancini (Universidade de Brasilia) was essentialfor the present work. The authors would like to acknowledge theuse of the diffractometers from Departamento de Físico Química,UNC, Argentina, y Departamento de Mineralogía y Petrología de laUniversidad de Granada. We thank the two anonymous reviewers,whose suggestions helped improve and clarify this manuscript.
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