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Journal of South American Earth Sciences 43 (2013) 86e100

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Journal of South American Earth Sciences

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Precious metal-bearing epithermal deposits in western Patagonia (NE LagoFontana region), Argentina

Mabel Elena Lanfranchini a,b,*, Ricardo Oscar Etcheverry a,c,1, Raúl Ernesto de Barrio a,1,Clemente Recio Hernández d

a Instituto de Recursos Minerales, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, Calle 64, esquina 120, 1900 La Plata, ArgentinabComisión de Investigaciones Científicas de la Provincia de Buenos Aires, ArgentinacConsejo Nacional de Investigaciones Científicas y Técnicas, Argentinad Facultad de Ciencias, Universidad de Salamanca, Spain

a r t i c l e i n f o

Article history:Received 2 August 2012Accepted 28 January 2013

Keywords:Precious metalEpithermal systemWestern PatagoniaArgentina

* Corresponding author. Instituto de Recursos MinNaturales y Museo, Universidad Nacional de La Plata, CPlata, Argentina. Tel./fax: þ54 221 4225648.

E-mail addresses: [email protected], mlan(M.E. Lanfranchini).

1 Tel./fax: þ54 221 4225648.

0895-9811/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.jsames.2013.01.005

a b s t r a c t

Precious metal-bearing quartz veins occur at the northeastern sector of the Lago Fontana region insouthwestern Argentina, within the context of the Andean continental magmatic arc environment.The deposits and their associated alteration zones are spatially related to a Cretaceous calc-alkalinemagmatism represented by silicic dikes and hypabyssal intrusions, and hosted by a Late Jurassic toCretaceous volcano-sedimentary sequence. The veins and related veinlets crop out discontinuously, ingeneral terms in a NW-SE belt. The primary vein mineral assemblage is composed mostly of pyr-ite � galena � chalcopyrite > hematite � arsenopyrite in silica gangue minerals. Chemical analyses ofgrab samples from selected quartz veins show as much as 5.7 ppm Au and 224 ppm Ag, as well aselevated Pb, Cu, and Zn. Hydrothermal fluids caused an innermost silicification and adularia-sericitealteration assemblage, and an external propylitic halo.

Sulfur isotope values measured for sulfides (d34SH2S from �1.90 to þ1.56&), and oxygen and hydrogenisotopes measured on quartz crystals and extracted primary fluid inclusion waters (d18OH2O ¼ �2.85toþ5.40&; dDH2O ¼�106.0 to�103.4&) indicate thatmineralizationprobably formed frommagmaticfluids,whichweremixedwithmeteoricwaters.Also,fluid inclusiondata fromquartz veins pointout that thesefluidshad low salinity (1.7e4.2 wt% NaCl equiv.), and temperatures of homogenization between 180 and 325 �C.

Mineralogical, petrographic and geochemical features for mineralized surface exposures indicatea typical adularia-sericite, low sulfidation epithermal system in the Lago Fontana area that representsa promising target for further exploration programs.

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1. Introduction

Epithermal deposits commonly develop in association with calc-alkaline magmatism in volcanic arcs at convergent plate margins(White and Hedenquist, 1990, 1995; Simmons et al., 2005). Manyimportant ore deposits were concentrated around the Pacific Rimduring the Cretaceous-Tertiary metallogenic epoch, among severalregions, in the Cordillera de los Andes of South America (Sillitoe,1997; Hedenquist et al., 2000). Some of these deposits, such asHuemules (Secretaría de Minería, 1984; Viera and Hughes, 1999), La

erales, Facultad de Cienciasalle 64, esquina 120, 1900 La

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Ferrocarrilera (Rolando, 2001), Mina Angela (Márquez, 1999b;Dejonghe et al., 2002), El Toqui (Wellmer and Reeve,1990), and CerroBayo (Poblete et al., 2011) have been well studied in theArgentinianeChilean part of the Cordillera Patagónica, but otherprospective areas in the region have not been studied in detail.Among these are the epithermal precious metal-bearing quartzveins, and their related alteration zones, in the northeastern sector ofthe Lago Fontana area, southwestern Chubut Province (Fig. 1).

The southern side of Lago Fontana area has been recognized forits conspicuous hydrothermal alteration zones and base and pre-cious metals occurrences during the last decades of the XX century(Domínguez, 1981; Fondo Rotatorio de las Naciones Unidas para laexploración de los Recursos Naturales, 1982; Márquez, 1995;Marchionni et al., 1997; Japan International Cooperation Agency andMetal Mining Agency of Japan, 2000). Since then, there have beenseveral exploration projects and research studies in the region.

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Fig. 1. Regional geology of the northeastern sector of the Lago Fontana area and surrounding zones (Modified from Ramos, 1981; Ploszkiewicz, 1987; Lanfranchini, 2002). Mainstudy areas are marked with rectangles, and named as follows: A. Area of Fig. 5 that includes the southern portion of the Cordillera de Sakmata. B. Area of Fig. 7 that comprises thesouthern sector of the Sierra de Payaniyeu, and Cerro Pepita. C. Area of Fig. 9, toward the eastern sector, includes the Cerros Teta, Colorado, and Pedrero.

M.E. Lanfranchini et al. / Journal of South American Earth Sciences 43 (2013) 86e100 87

In the northeastern Lago Fontana area, reconnaissancemapping,satellite image processing, field checking, and geochemical sam-pling resulted in the discovery of new quartz veins containingprecious metals at levels worth of further exploration(Lanfranchini, 2002). This paper presents the results of reconnais-sance studies at various scales of these precious metal vein occur-rences and the geology of the region where they crop out, togetherwith a preliminary metallogenic model for the region.

2. Previous work

Small placer gold deposits were mined in the Lago Fontanaarea in the early 20th century. The La Ferrocarrilera

polymetallic vein deposit (Fig. 1) was also mined during the1950s, but there is no geological information from thatactivity.

Regional geologic studies were conducted in the Lago Fontanaand Lago La Plata areas as well as in the Aldea Apeleg region byPloszkiewicz and Ramos (1977), Haller and Lapido (1980), Ramos(1981, 1983), and Ploszkiewicz (1987), who defined the mainstratigraphical and morphostructural units. Later, Folguera et al.(2000), and Folguera and Iannizzotto (2004) proposed a tectonicevolution model for the Lagos Fontana and La Plata area. Theseauthors defined extensional structure systems during the Mid-LateJurassic to the Upper Cretaceous which allowed the generation ofthe quartz vein epithermal deposits.

Fig. 2. Stratigraphic section of the study area. Not to scale.

M.E. Lanfranchini et al. / Journal of South American Earth Sciences 43 (2013) 86e10088

Detailed metallogenic investigations in the southern part of theLago Fontana area were carried out by Domínguez (1981), andRolando et al. (2002), who studied the polymetallic mineralizationof the La Ferrocarrilera mine region. Hayase et al. (1971) and Maiza(1981) carried out the first studies of the alteration zones at theCordillera de Sakmata and Cordillera del Gato. Lanfranchini andEtcheverry (1999), and Lanfranchini (2002) characterized somegeological and metallogenic aspects of the precious metal lowsulfidation epithermal veins in the northeastern part of the LagoFontana area. Lanfranchini et al. (2007) studied the geology andgeochemistry of the El Abuelo calcic Fe-skarn, located in thesouthern part of the area, a deposit that was mined between 1940and 1960. Giacosa et al. (1988), Márquez (1999a) and Dejongheet al. (2002) summarized the metallogenic features of pollymetal-lic mineralizations of the Lago Fontana area and throughout theChubut province. Furthermore, these last authors compared themineralization with the gold- and silver-bearing, base metalsulfide-rich quartz veins discovered in the Macizo del Deseado,southern Argentina (Genini,1990; Zubia et al., 1999), ametallogenicsub-province that contains important precious-metal resources.

3. Regional geological setting

An extensional structural regime characterized the AndesPatagónicos from the Mid-Late Jurassic to the Lower Cretaceous. Inthe Lago Fontana region, an extensive basic to andesitic volcanismwas developed along a magmatic arc during the Middle to UpperJurassic. It is represented by the lava flows of the Lago La PlataFormation (Ramos, 1976, 1981). At the end of this period, crustalstretching of the basement, controlled by regional NW strikingsubvertical normal faults, formed several basins associatedwith theopening of theWeddell Sea (Mpodozis and Ramos, 2008), that wereinundated by marine transgressions represented by the Cotidianoand Tres Lagunas Formations. Toward the eastern sector the TresLagunas Formation reaches up to the Valanginian (Ramos, 1981).Later, during the Berriasian-Hauterivian, the fossiliferous blackshales of the Katterfeld Formation were deposited. This event in-dicates a general subsidence of the western sector. Subsequently, inthe Barremian, a deltaic complex was installed. It is represented bythe Apeleg Formation which is a widely distributed, 1500 m-thick,deltaic sandstone (Ramos, 1981).

During the interval Late Lower-Early Upper Cretaceous rhyoliticto basaltic lava flows and related ash flow tuffs erupted, con-comitant with the intrusion of small stocks and dykes. Thesemagmatic rocks conform the Ñirehuao Formation (Skarmeta andCharrier, 1976) and the El Gato Formation (Ploszkiewicz andRamos, 1977). The subsequent Andean Orogeny that began in theUpper Cretaceous, and continued through the Tertiary resulted inemplacement and attendant uplift of the Patagonian Batholith(Giacosa and Márquez, 1999; Pankhurst et al., 2003; Hervé et al.,2007), constituted by granitic to mafic rocks (La Magdalena Fm,Muzzio Fm and others). Despite this, Weaver et al. (1990) proposeda Mid-Jurassic to Upper Cretaceous ages for the tonalitic to graniticintrusions dating the Patagonian Batolith in Chile, at 48� latitude.This event was followed by WNW compression related to sub-duction of the Nazca Plate beneath the South American Plate(Sillitoe, 1974). Compression led to inversion of older faults andcrustal shortening accommodated by folding and development ofnew faults (Folguera et al., 2000; Folguera and Ianizzotto, 2004;Ianizzotto et al., 2004).

A similar geological-metallotectic scheme is observed in theCordillera Patagónica Chilena where geological investigations wereconducted by several researchers during the last decade. Thesestudies allowed establishing with precision the stratigraphy andthe geochronology of the different Mesozoic volcanic events and

their hydrothermal mineralizations associated (Parada et al., 2001;Hervé et al., 2007; Suárez et al., 2008). Therefore, the ChileanJurassic-Cretaceous volcano-sedimentary stratigraphy can be wellcorrelated with that of the Argentinian side. The Divisadero Groupof Chile, which correlates with the Ñirehuao and El Gato Forma-tions in Argentina, was dated by Pankhurst et al. (2003) obtainingages between 118 and 116 Ma for acid ignimbrites, while Suárezet al. (2008) got 118.5 � 0.8 and 116.7 � 0.7 Ma for similar rocksusing UePb SHRIMP. The Ñirehuao Formation was analyzed using40Are39Ar method by Parada et al. (2001) who obtained an age of96.3 � 1 Ma. This Cretaceous magmatism is closely associated withseveral hydrothermal ore deposits in Chile, such as Cerro BayoMinein Laguna Verde area, where adularia from epithermal veins wasdated at 114-111 Ma by the 40Are39Ar method (Poblete et al., 2011).In the argentinian side, Márquez (1999a) points out that there isa temporary relationship between the majority of the mineral oc-currences in the Lago Fontana region and the acid to mesosilicicCretaceous volcanism developed during the uppermost LowerCretaceous and the beginning of the Upper Cretaceous.

3.1. Local geology

The Lago La Plata Formation is the oldest unit (Fig. 2) in thestudied areas. It is composed of basalt and basaltic andesites whichforms small and isolated outcrops with a poor topographicexpression where it is difficult to recognize lava flow structures. Inthin sections, these rocks contain pale green pyroxene and pla-gioclase (An50e52) phenocrysts, enclosed in a pilotaxitic ground-mass. This sequence is overlain by the sedimentary rocks of the TresLagunas Formation. This unit is composed of thin greenish sand-stone layers interbedded with dark gray marls and minor lime-stones. Usually, it is transitionally followed by the sandstones of theApeleg Formation (Fig. 3A) which conforms thick outcrops con-stituted mainly by light brown, fine to medium size quartz sand-stone with a clast-supported texture, and minor conglomerate andshales with tabular layout.

Fig. 3. Field photographs: A. Sandstone of the Apeleg Formation, eastern slope of the Sierra de Payaniyeu. B. Rhyolitic dike of the El Gato Formation, Sierra de Payaniyeu. C. Gabbro-dioritic rocks of the Muzzio Formation, Cerro del Finadito area. D. Quartz veins hosted by sandstone of the Apeleg Formation, Cerro del Finadito. E. Quartz veins from the easternslope of the Sierra de Payaniyeu. F. Main quartz vein from the Cerro del Finadito. G. Quartz stockwork from the Cerro Teta, Cordillera de Sakmata. H. Photograph of polished sectionshowing galena grains (gn) replacing pyrite crystal (py), from Lote 15 quartz veins.


Ñirehuao volcanic rocks form rounded hills composed ofstrongly altered greenish to brown porphyritic andesite containingpyroxene and plagioclase phenocrysts in a pilotaxitic groundmass.The El Gato Formation crops out extensively constituting mainlynumerous rhyolitic and dacitic dikes and small subvolcanic stocks.These igneous rocks form the Cerros Teta, Pedrero, and Colorado(Fig. 1), which are several kilometers in diameter. The predominantNW-striking rhyolitic dikes of the El Gato Formation appear as

elongated crests 0.5e8.0 m-thick and as much as 1000 m-long(Fig. 3B and D). The rhyolitic rocks are light brown in color, and havea porphyritic texture, composed of bipyramidal quartz phenocrysts,up to 5 mm in length, as well as K-feldspar, plagioclase, euhedralbiotite, and Fe-oxides, with accessory apatite and titanite in a fel-sitic groundmass.

Geochemical data of igneous rocks from the Ñirehuao and El GatoFormations indicate a generally subalkalinemagmatism,mostlywith


a calc-alkaline signature (Lanfranchini, 2002). Andesites of ÑirehuaoFormation are meta-aluminous, while the El Gato volcanics are per-aluminous. According to the Rb vs Nb þ Y tectonic discriminationdiagram they correspond to a magmatic arc environment(Lanfranchini et al., 2006).

The Patagonian Batholith is poorly exposed in this region, onlya few gabbroic to dioritic rocks of the Muzzio Formation (UpperCretaceous, Ramos, 1976) which crop out as relatively small in-trusions mainly within the sandstones of the Apeleg Formation(Fig. 3C). These intrusions formed low grade hornfels aureoles inthe Apeleg sandstone at the Cerro del Finadito area (Figs. 1 and 5).

Finally, PlioceneePleistocene basalts and recent glaciofluvialefluvial deposits overlie the older rocks. It conform extensively ter-raced levels essentially toward the eastern region.

4. Satellite image analysis

Several scenes of ascending and descending orbits of a geore-ferenced RADARSAT SAR image, with fine resolution, 37e40� angleof incidence, a pixel spacing of 6.25 m in both azimuth and cover-age, and an image resolution of 10� 10mwere used (Fig. 4A and B).These scenes, as well as the aerial photographs of the area, wererectified using a previously georeferenced LANDSAT-TM Image.RADAR image enhancement was performed by conventionalmethods and structural features extraction was done by visualinterpretation (Marchionni et al., 1999). In the same area, LANDSATTM satellite image analysis showed color alteration zones and alsoseveral linear morphological features with positive topographicexpression. The complete satellite image processing allowed theidentification of the main lineaments of the study area, which afterbeen field checked, resulted in several mineralized structures andsurrounding hydrothermal alteration zones. Linear structures havebeen effective exploration guides in the study area. These linea-ments have a dominant NW-trending orientation and a subordi-nate NE-striking orientation. This structural scheme is consistentwith that proposed by Ploszkiewicz and Ramos (1977), who rec-ognized the lineaments defined block-faulting within the crystal-line basement. The continuation of the lineaments into youngersedimentary rocks (Upper MioceneeLower Pliocene) indicatesa reactivation of the normal faults during recent times.

Fig. 4. A. RADARSAT SAR image, descending orbit. B. Main

5. Hydrothermal alteration

Narrow hydrothermal alteration zones surround the mineralizedveins. Silicification, the most common alteration type, is welldeveloped in the sandstones of the Apeleg Formation. Silicification isevident both as pervasive replacement (Fig. 6a and b) composed ofminute quartz and chalcedony grains, and as numerous very thinveinlets that are accompanied by minor sericite patches. Withinthese veinlets, minor amounts of euhedral rhombic adularia crystals(Fig. 6h), as much as 350 mm in length, are intergrown with quartz.Sericite frequently forms small patches in the wall rock. Scarce,50 mm to 1 mm in diameter, xenomorphic grains of disseminatedpyrite are associatedwith the patchy sericite alteration. Veinlet wallsare locally coated with radiating sprays of iron oxide tinted sericite.

Argillically altered rocks are not easily recognized, because theyare frequently buried by thin detrital cover due to weathering ofsoft argillically altered rocks. Argillic alteration is weakly developedadjacent to many of the veins and extends a few meters into thewall rocks. It is mainly characterized by kaolinite replacement offeldspars and pervasive illite-smectite.

Strongpropylitic alteration,definedbydarkgreyish-greenish rock,several meters in width, forms distal to the veins, mainly in theandesitic flows of the Ñirehuao Formation. It is characterized by cal-cite and epidote replacement of plagioclase, and chlorite (ripidiolite-brunsvigite) both pervasive (Fig. 6g) as replacement of pyroxene.

In the Cordillera del Gato, at thewest of the studyarea, there is anextensive alteration zone. It forms an kaoliniteealuniteesiliceousblanket that is mainly replacing acid volcanics of El Gato Forma-tion. Despite this area is not the aim of this study we consider,preliminarily, that this deposit would correspond to a steam heatedsystem related to the Cretaceousmagmatic activity, according to theobserved mineralogical and textural features. This interpretation isalso supported by some stable isotope ðd18OH2OÞ data, calculated indickite from Susana Mine (Lanfranchini, 2002) that yielded valuesbetween �2.81& and �2.71& (at 230 �C, Hayase and Maiza, 1973).

6. Vein deposits

Epithermal quartz veins occur in several places of the Payaniyeuregion within the Late Jurassic-Cretaceous volcano-sedimentary

structural lineament map of the NE Lago Fontana area.

Fig. 5. Geologic map of the Cordillera de Sakmata, and northern part of the Sierra de Payaniyeu. Modified from Ramos, 1981; Lanfranchini, 2002.


sequence. They are spatially related to rhyolitic and rhyodaciticdikes of the El Gato Formation. Although the quartz veins constitutediscontinuous outcrops, they form a sinuous outcrop pattern thatgenerally trends NW-SE in an approximately 30-km-long by 15-km-wide belt. The veins are near vertical, ranging from centime-ters up to several meters thick, and tens to hundreds of meters long

(Fig. 3E and F). The main veins locally contain anomalous concen-trations of precious and base metals (Table 1).

The principal quartz veins crop out at the Lote 15 sector in theSierra de Payaniyeu (Figs.1B and 7) and at theMina de Plomo sectorin the Cordillera de Sakmata (Figs. 1A and 5). They are near vertical,and have N 30e40�W and minor N 50�E (Mina de Plomo) strikes.

Fig. 6. Field photographs showing silicified structures at Mina de Plomo (a) and Cerro Teta (b). c. Quartz banded texture in Lote 15 vein. d. Hand specimen photograph of brecciatexture from the Cerro Pepita, coin diameter ¼ 25 mm. eef. Photographs of thin sections showing vein quartz textures in Lote 15. e. Mosaic texture. f. Drusy texture toward thecrystal rim and recrystallized amorphous silica (chalcedony). g. Chlorite aggregates at Lote 15 veins. h. Rhombic adularia crystals in quartz veinlets from Lote 15. Abbreviations:adul ¼ adularia, cal ¼ calcite, qz ¼ quartz.


The veins crop out discontinuously for almost 900 m, are as muchas 5 m wide, and are commonly hosted by rhyodacitic dikes of theEl Gato Formation, which intruded the volcano-sedimentary suc-cession. Wall rocks immediately adjacent to the vein system,mostly rhyodacite, andesite and sedimentary rocks, have beenaltered to a quartz-sericite assemblage, and are locally brecciatedand cut by microcrystalline quartz veins (Fig. 8). This alteration

extends for at least 15 m from the veins, where it grades outward toa propylitic assemblage.

The veins are composed of milky to translucent quartz withlesser sparry carbonate. The most frequent microscopic primarytextures are massive, comb and zoned crystals (Fig. 6e and f); evi-dence of recrystallization includes mosaic and flamboyant textures.Banded and breccia textures (Fig. 6c and d) are developed

Table 1Variations in geochemical analyses of main quartz veins from Lote 15, Mina de Plomo, Sierra de Payaniyeu, Cerro del Finadito and Cerro Teta, Western Patagonia Argentina.

Quartz veins and veinlets Au Ag Cu Pb Zn Fe As Sb Hg

(ppb) (ppm) (ppm) (ppm) (ppm) (pct) (ppm) (ppm) (ppb)

Lote 15 (N: 24) Average 340.1 4.9 775.1 2994.8 461.6 4.6 92.5 5.7 1.3Max. value 5728.0 26.5 4549.0 16,200 1950.0 16.3 380.0 42.6 4.0Min. value 6.0 0.1 11.0 65.0 21.0 0.5 4.6 1.7 0.0

Mina de Plomo (N: 10) Average 691.8 120.7 803.6 22,751 351.4 6.2 184.4 13.6 51.0Max. value 1560.0 223.8 1180.0 46,633 648.0 11.9 429.0 46.0 110.0Min. value 5.0 0.3 12.0 128.0 34.0 0.6 2.0 2.0 10.0

Sierra de Payaniyeu (N: 6) Average 46.8 5.7 5.0 20.8 81.3 1.7 33.7 2.5 0.0Max. value 107.0 17.0 7.0 54.0 126.0 2.5 82.0 2.5 0.0Min. value 2.5 0.1 3.0 2.3 19.0 1.3 5.0 2.5 0.0

Cerro del Finadito (N: 10) Average 5.0 0.5 23.8 98.4 94.4 1.3 23.8 2.3 21.0Max. value 5.0 0.9 59.0 199.0 189.0 3.1 79.0 3.0 35.0Min. value 5.0 0.3 2.0 5.0 3.0 0.1 2.0 2.0 10.0

Cerro Teta (N: 13) Average 124.0 9.0 39.6 30.5 30.1 1.1 38.3 4.3 0.0Max. value 524.0 76.5 162.0 84.0 69.0 1.9 112.0 6.0 0.1Min. value 2.5 0.1 7.0 7.0 10.0 0.4 2.5 2.5 0.0

Fig. 7. Geologic map of the southern sector of the Sierra de Payaniyeu and the Cerro Pepita. Modified from Ploszkiewicz, 1987; Lanfranchini, 2002.


Fig. 9. Geologic map of the Cerros Teta, Colorado and Pedrero Modified fromPloszkiewicz, 1987; Lanfranchini, 2002.


surrounding rhyodacite and andesite wall rock fragments, eacha few centimeters in diameter, that are located near both the cen-ters and the margins of the quartz veins. In addition, several thinquartz veinlet systems cross-cut the mineralized zones.

The hypogene mineral assemblage is composed of pyrite� galena � chalcopyrite > hematite � arsenopyrite � chlorite� fluorite � rutile, with variable amounts of gold and silver. Goldappears as native grains as long as 10 mm or as micro-inclusions inthe sulfide minerals. Pyrite forms several generations of crystals,which show euhedral square cross sections, up to 5 mm in diam-eter. It occurs disseminated in quartz veins or in the adjacent wallrocks and is commonly oxidized to Fe-oxide boxworks. Galenacrystals are as much as 3 cm in diameter and frequently containexsolved chalcopyrite. Some galena partially replaces pyrite crys-tals (Fig. 3H) and is intergrown with quartz grains. Chalcopyrite isscarce and erratically distributed as irregular masses of anhedralcrystals as much as 2 mm in diameter. In polished sections, he-matite is observed as inclusions in pyrite or in the gangue mineralsas lamellas up to 500-mm-long or forming aggregates of radiatingcrystals. Some arsenopyrite appears as 400-mm-long euhedralrombic sections. Chlorite, needle-like rutile, and fluorite are presentin minor amounts filling vugs and small fractures.

Several quartz veinlets, almost parallel to the main veins, thatare 0.3e0.8 cm-wide, also contain sulfide minerals. Sometimes,they form stockwork zones (Fig. 3G), 5 to 15 m-wide, that cut theCretaceous volcanic rocks. The edges of these veins are sharp andare marked by euhedral pyrite and dark brown limonite. In addi-tion, numerous, very thin quartz-epidote-chlorite-sericite stringersare frequently present in the zones.

A thin supergene zone is developed along the main mineralizedstructures. These are marked by a very colorful mineral assemblagecomprising secondary sulfides (covellite, chalcocite), and oxideminerals (anglesite, cerusite, and Fe-oxides) in veinlets, and par-ticularly replacing hypogene chalcopyrite and galena grains.

In the Cerro Teta area (Fig. 9), several near vertical quartz veins,each as much as 5 m-wide, strike approximately EeW. At CerroColorado, the silicified zones are also near vertical, and reach up to4m-wide but have a variable strike between NeS and NWeSE.Despite the absence of visible sulfides in the outcrops of milkyquartz veins, geochemical analyses reveal up to 524 ppb Au and76 ppm Ag.

The general sulfide mineral paragenetic sequence has 3 mainstages: (1) an early formation of euhedral pyrite that precipitatedbefore the generation of scarce irregular chalcopyrite grains, (2)afterward, main galena deposition occurred, sometimes includingchalcopyrite, and (3) a second generation of cubic pyrite crystals. Inaddition, fine-grained pyrite appears frequently disseminated to-ward the vein margins and in adjacent wall rocks but there are noclear evidences about its paragenetic relationship.

Two hydrothermal quartz vein systems related to a CaeFe-magnetite skarn (Lanfranchini et al., 2007), named El Abuelo, and El

Fig. 8. Schematic cross section through the Lo

Solcito are located at Cerro Pepita, in the southern part of the studyarea. Ore grades vary between 40 and 63 wt% Fe. Anomalous metalcontents of>1% Cu and as much as 81 ppm Ag have been measured.

7. Sample selection and analytical methods

Representative quartz veins and altered wall rocks were sam-pled for whole rock geochemical analysis in order to detect thepresence of metal anomalies. Minor and trace element contentswere analyzed on 63 selected samples by inductively coupledplasma-mass spectrometry (ICP-MS), and by instrumental neutronactivation analysis (INAA) at Actlabs Ltd. (Canada).

Electron microprobe analyses were carried out on sulfide min-erals with the aim of establishing anomalous contents of preciousmetals, using a Cameca Camebax SX-100 at the Departamento deGeología, Universidad de Oviedo, Spain. Operating conditions were15e20 kV accelerating voltage, beam current of 15e20 mA, andbeam width of 1e2 mm. Natural mineral standards certificated byMAC (Micro Analysis Consultants Ltd., United Kingdom) wereused for calculating element concentrations.

Heating and freezing analyses of fluid inclusions were obtainedfrom approximately 50- to 100-mm-thick doubly polished thinsections using a LINKAM THM SG 600 heating-freezing stage in theFluid Inclusion Laboratory at the Centro de Desenvolvimento de

te 15 mineralized structure (not to scale).

Fig. 10. Photomicrographs of FI in quartz vein, Lote 15, Sierra de Payaniyeu. A. Quartzcrystal showing secondary and primary fluid inclusions along healed fracture planesand growth planes, respectively. B. and C. Close-up of previous photograph showing L(liquid) rich primary FI.


Tecnologia Nuclear, Belo Horizonte, Brazil and at the Instituto deRecursos Minerales, Universidad Nacional de La Plata, Argentina.Calibration with standards of known melting points indicates anaccuracy of �2 �C for liquidevapor homogenization on heating and�0.5 �C for freezing point depressions. Low temperature phasechanges were measured first in order to minimize the possibility ofinclusion decrepitation. The samples were initially cooledto �120 �C, and then slowly warmed with the purpose of meas-uring first melting (Te) and final melting (Tm) temperatures. Furtherheating allowed the determination of liquidevapor homogeniza-tion temperatures (Th).

Representative samples of silicate and sulfide minerals wereselected for stable isotope ratio determinations at the ServicioGeneral de Isótopos Estables, Facultad de Ciencias, Universidad deSalamanca, Spain. Extracted gasses were analyzed in two VG Iso-tech SIRA-II mass spectrometers, equipped with a cryogenic system(“cold finger”). The measured isotopic ratios are reported in theconventional delta notation (d&) relative to SMOW (d18O, dD) orCDT (d34S). Mineral oxygen was analyzed on CO2 obtained byconventional fluorination (Clayton and Mayeda, 1963), employinga loading technique as described by Friedman and Gleason (1973)and ClF3 as reagent (Borthwick and Harmon, 1982). Long-termprecision, as determined by repeated analysis of NBS-28 quartz,averages �0.2&. Both, O and H isotope ratios were measured forfluids extracted from primary fluid inclusions hosted in quartzcrystals by the method of decrepitation. D/H ratios were measuredon H2 gas obtained by reduction of water over hot depleted U(Godfrey, 1962). d18O of fluid inclusion water was determined onCO2 obtained by the guanidine hydrochloride method (Dugan et al.,1985). Average reproducibility of water extraction plus isotopicratio determination is thought to be within �0.8& for d18O and�1& for dD. d34S of sulfides (galena and pyrite) was measured onSO2 extracted off-line following Robinson and Kusakabe (1975),employing a second, dedicated mass spectrometer. Long-termprecision, as indicated by repeated analysis of NBS-123 sphalerite,is better than �0.27&. The d18OH2O values were calculated fromisotopic measurements on quartz crystals, using the fractionationequations of Zheng (1993) at 280 �C, as based on the fluid inclusions(FI) data. dDH2O and some d

18OH2O data were measured directlyfrom the extracted fluid phase of primary fluid inclusions in quartzcrystals, as there were few coarse hydroxyl-bearing minerals suit-able for dDH2O determination. The fluid was obtained by decrep-itation of primary fluid inclusions, hosted in quartz crystals withonly one generation of FI. The fluid extractionwas done in a tubularoven, under controlled temperature and with a small decrepitationtemperature range. There is always concern over the reliability ofdD analyses of fluids obtained by decrepitation methods, due topossible contamination of the primary fluid by fluids from sec-ondary inclusions that were unrelated to the vein-forming event.However, our results seem reliable because they show consistencybetween analyses and because the d18OH2O results directly meas-ured on FI and those calculated for the fluid in equilibrium withquartz crystals were identical.

The d34SH2S was recalculated using the fractionation equationsof Ohmoto and Rye (1979) on pyrite, and Li and Liu (2006) ongalena, both at 280 �C.

8. Geochemical analyses

Geochemical analyses on surface grab samples from the prin-cipal quartz veins reveal anomalous values in precious and basemetals. The highest precious metal contents were found at the Lote15 andMina de Plomo occurrences, where Au values are as much as5.7 and 1.6 ppm, and Ag is up to 26.5 and 223.8 ppm, respectively(Table 1). The highest Au and Ag values correlate moderately with

FeePb > Cu > Zn at Lote 15 and Mina de Plomo, which are con-sistent with the relative abundance of recognized sulfide minerals.Other pathfinder elements are enriched, including As, which rea-ches up to 429 ppm, Sb (46 ppm) and Hg (110 ppb), and positivelycorrelate with the Au and Ag values.

Electron microprobe analyses of pyrite, galena, and chalcopyritecrystals reveal anomalous Au and Ag contents specially in galena(as much as 0.15 and 0.31 atomic percent, respectively) that areprobably due to submicroscopic inclusions.

9. Fluid inclusions

9.1. Petrography

Different textural varieties of euhedral to subhedral quartzcrystals were studied for FI. Eighteen quartz samples from the Lote15 and Mina de Plomo occurrences in the Sierra de Payaniyeu wereexamined to estimate the temperature and composition of hydro-thermal fluids. Quartz from the main veins and silicified wall rockscontained zones of primary FI, and planes of secondary and pseu-dosecondary FI. Primary FI data from euhedral, 1- to 3-cm-longquartz crystals, which are intergrownwith sulfide grains and smallrhombic adularia crystals with 90 mm average length, were ana-lyzed. These quartz crystals contain growth zones (Fig. 10) mainlydefined by an abundant two-phase FI association, and a few smallmonophase (

Fig. 11. Histograms of thermometric data for phase changes observed in primary FI in quartz: Homogenization temperature of L-V inclusions from Lote 15 (A, 11 samples), and Minade Plomo (B, 7 samples); calculated salinity of aqueous inclusions from Lote 15 (C), and Mina de Plomo (D).


in vapor phase volume. Although these FI have different pro-portions of vapor phase, they homogenize at the same temperatureto the liquid phase, suggesting trapping of boiling fluid.

Table 3Stable isotope data from Lote 15, Cerro Pepita (El Abuelo skarn and El Solcito vein),and Mina de Plomo sectors.

Sample Location Mineral d18Omin(&)

d18OH2O(&)

dDmin(&)

dDH2O(&)

9.2. Microthermometric measurements

Microthermometric measurements were restricted to quartz-hosted inclusions, because these inclusions are large and a pri-mary origin could be assigned to them, and phase changes are easyto observe. During freezing measurements no phase changes thatwould indicate the presence of carbon-bearing volatile phases inthe vapor were observed in any of the vein systems studied. Thefluid inclusions had first melting temperatures (Te) between �22and �24 �C, similar to the eutectic for the system NaCleH20, withminor Kþ that would have been responsible for lowering theeutectic point of the system below the theoretical �20.8 �C(Shepherd et al., 1985). The final melting temperatures (Tm) for icerange between�2.5 �C and�1 �C, indicating low salinities that varybetween 1.7 and 4.8 wt % NaCl equiv., with a mode in the range of3.5e4.5 wt % NaCl equiv. (Fig. 11 and Table 2).

During heating experiments, the two-phase FI homogenizedinto the liquid phase at temperatures between 180� and 300 �C,

Table 2Summary of primary fluid inclusion microthermometric data in quartz grains fromrepresentative veins at Lote 15, and Mina de Plomo.

Location Type of mineralization Th (L þ V) Te Tm SalinityLote 15 Quartz vein 180e325


considered to have similar latitude, altitude, and continental con-ditions than those prevailing during the Cretaceous in the area ofstudy (Fig. 12). In general it is assumed that the current meteoricwater line is analogous to possible meteoric water in the geologicpast, as the isotopic value of the oceans is buffered by interaction ofseawater with oceanic crustal material in ridge areas(Muehlenbachs, 1986).

Sulfide sulfur isotopes (d34Smin) from the Lote 15, Cerro Pepita (ElAbuelo skarn and El Solcito vein) and Mina de Plomo quartz veinsand veinlets, yielded values from �1.90& to þ1.56& (Table 3).

10.1. Stable isotope thermometry

Oxygen isotope systematics were used to get an independentestimate of the conditions of vein formation. The measured d18Ovalues of quartz and fluid phase extracted from quartz crystalpairs give vein temperatures of about 265 �C, using experimentalcoefficients from Clayton et al. (1972). This estimate agrees verywell with the range of 220�e280 �C estimated from fluidinclusion homogenization temperatures for the quartzdeposition.

Sulfur isotope thermometry was attempted without much suc-cess on pyrite-galena pairs. Galena and intergrown pyrite pairs givereasonable temperatures, but even these do not unequivocally meettextural criteria for equilibrium deposition. Similarly, in rare pyrite-galena contact associations, the galena involved is a younger gen-eration overgrown on older pyrite. Of all the associations examined,only two (samples P8 and 1187) seem to have coexisting sulfides thatare products of the same paragenetic stage and, even in these, thepyrite appears somewhat older. These samples give d34Spy e d34Sgavalues corresponding to equilibrium temperatures in the range of345e485 �C (from the equations of Ohmoto and Rye, 1979; Li andLiu, 2006), clearly higher than the fluid inclusion data. We inferthat these two carefully selected specimens represent non-equilibrium phases, probably as a result of non-contemporaneousdeposition.

11. Discussion

Precious metal-bearing quartz vein formation in the Lago Fon-tana area occurred during the Cretaceous magmatic activity. Thismagmatism is represented by basic to acid subvolcanic bodies andlava flows into the Late Jurassic to Cretaceous volcano-sedimentarysequence.

These vein deposits extend mainly along a N30� to 60�W di-rection, suggesting a structural control for the circulation of

Fig. 12. Plot of dDH2O vs d18OH2O with boxes showing the variation for mineralizing

fluids for the Lote 15 sector of the Sierra de Payaniyeu (after Sheppard, 1986).

hydrothermal fluids along older faults that were reactivated laterin the Lower Cretaceous. These vein systems are interpreted tohave formed at relatively high levels. The depth at which theyformed can be inferred from fluid inclusion data, considering thatthey were trapped during boiling processes and therefore do notrequire a pressure correction (Haas, 1971; Bodnar et al., 1985).Indirect evidence for boiling is the existence of primary fluid in-clusions with different proportions of vapor phase homogenizedat the same temperatures in some groups (Fig. 13). Pressures ofabout 60e70 bars can be calculated for these solutions, from dataobtained for the NaCleH2O system, assuming

Fig. 14. Schematic cross sections illustrating the metallogenic evolution at Lago Fontana area during the Mid-Upper Jurassic to Recent (not to scale). A. Mid-Upper Jurassic: Thedevelopment of an extensive basaltic plateau (Lago La Plata Formation) formed a calc-alkaline volcanic arc of NeS trend. B. Lower-Upper Cretaceous: After formation of the volcano-sedimentary sequence, emplacement of Cretaceous rhyolitic bodies and related quartz veins occurred, together with a skarn formation in thin interbeded limestone. Arrows denotefluid flow patterns (mixing of magmatic and meteoric waters?) and crossed circle the heat source. C. After a long period of denudation the vein deposits are exposed.


This increase in pH could also have favored base metal sulfide pre-cipitation. The resulting H2S depletion in the hydrothermal fluidswould destabilize Au and Ag bisulfide complexes, resulting in theprecipitation of precious metals together with sulfides (Reed andSpycher, 1985).

An intrusion-related model of this epithermal deposits is sche-matically shown in Fig. 14, through cross sections that illustrate themetallogenic evolution at the Lago Fontana area during the Mid-Upper Jurassic to Upper Cretaceous, beginning with the formationof a volcanic arc, followed by deposition of a sedimentary sequenceand by the emplacement of rhyolitic bodies, corresponding to a latearc in the area, and related quartz veins.

12. Conclusions

Widespread calc-alkaline magmatism, part of a NeS-trendingvolcanic arc, occurred during the Jurassic - Cretaceous in south-western South America. In the NE Lago Fontana area, this igneousactivity formed several intrusive bodies that are spatially andgenetically associated with precious metal-bearing low sulfidationepithermal systems. This magmatism represents a useful guide forexplorationists because it is linked to a considerable area of min-eralization described in this paper.

Although current knowledge of these epithermal deposits in thestudy area is incomplete, in particular due to lack of drilling to

define deeper levels, our surface geological studies allow somegeneralizations. These include: (1) the presence of an evidentregional geochemical signal, with anomalous contents of Au, Ag, As,and Hg, (2) a significant distribution of quartz veins which conformdifferent epithermal systems located across a 450 km2 area, and (3)mineralogical and textural features that suggest relatively shallowpaleodepths of the exposed veins.

The presence of a favorable scenario like the NE Lago Fontanaregion, with clear and defined metallotects represented mainly bythe existence of several mineral occurrences and related hydro-thermal alteration zones allows to encourage further explorationprograms with promising expectation.

Acknowledgments

This work was financially supported by the Consejo Nacional deInvestigaciones Científicas y Tecnológicas of Argentina (CONICET-PIP2727). The microprobe analyses have been carried out with theassistance of Dr. A. Martín-Izard in the Departamento de Geología,Universidad de Oviedo, Spain and the FI measurements with thesupervision of Dr. F.J. Ríos in the Centro de Desenvolvimento deTecnologia Nuclear, Belo Horizonte, Brazil. We are very thankful toDr. R. Goldfarb, and Dr. H.Ostera for their kind and constructive help.We also thank the reviewers (Dr. A. Folguera and anonymous) fortheir useful comments.


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Precious metal-bearing epithermal deposits in western Patagonia (NE Lago Fontana region), Argentina1. Introduction2. Previous work3. Regional geological setting3.1. Local geology

4. Satellite image analysis5. Hydrothermal alteration6. Vein deposits7. Sample selection and analytical methods8. Geochemical analyses9. Fluid inclusions9.1. Petrography9.2. Microthermometric measurements

10. Stable isotope10.1. Stable isotope thermometry

11. Discussion12. ConclusionsAcknowledgmentsReferences

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Journal of South American Earth Sciences - UNLPnaturalis.fcnym.unlp.edu.ar/repositorio/_documentos/... · 2013. 11. 20. · Small placer gold deposits were mined in the Lago Fontana