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Revista geológica de Chile

versión impresa ISSN 0716-0208

Rev. geol. Chile v.34 n.2 Santiago jul. 2007

http://dx.doi.org/10.4067/S0716-02082007000200007 

 

Revista Geológica de Chile, Vol. 34. No. 2. p. 291-303. July 2007.

Vein carbonates in the low sulfidation epithermal Au-Ag District of El Peñón, II Región, Chile: environment of formation and exploration implications

Carbonatos en vetas en el distrito epitermal de baja sulfuración de El Peñón, II Región, Chile: ambiente de formación e implicancias para la exploración.

 

Thomas Bissig
Departamento de Ciencias Geológicas, Universidad Católica del Norte, Av. Angarrios 0610, Antofagasta, Chile, tbissig@ucn.cl

Daisy Donoso
Minera Meridian Limitada, El Peñón, Antofagasta, Chile, daisy.donoso@meridiangold.com

Nelson C. Guerra
Departamento de Ciencias Geológicas, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile, nguerra@ucn.cl

Gregory M. Dipple
Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Rd, Vancouver, B.C., V6T 1Z4, Canada, gdipple@eos.ubc.ca


ABSTRACT

Carbonate minerals are common gangue minerals in the low sulfidation epithermal Au-Ag District of El Peñón, II Región, Chile. They can be subdivided into two principal groups on the basis of the paragenetic relationships. Paragenetically early carbonates occur together with quartz and sulfide minerals in banded veins and are closely associated with precious metal mineralization. These carbonates are compositionally complex, consisting of calcite, finely intermixed with ankerite, kutnohorite, rhodochrosite and dolomite, but also contain up to 0.8 wt% PbC03 and up to 4.1 wt% ZnC03. In contrast, the paragenetically late carbonates are relatively pure calcite, although dolomite has been recognized locally. Late calcites contain less than 3.5 wt% MnC03, less than 0.5 wt% ZnC03 and less than 2.7 wt% FeC03 and are also distinct from the early carbonates in that they exhibit moderate to intense red fluorescence under short-wave ultraviolet light. The red fluorescence color is likely related to moderate Mn contents in calcite, combined with trace amounts of Pb, but low Fe and Mg concentrations. Late carbonates are interpreted to have precipitated from C02-rich steam-heated fluids and are not directly associated with ore. 8180 compositions of both, early and late carbonates generally range between 21.1 and 14.1 %o relative to the Standard Mean Ocean Water (VSMOW). However, two samples of late calcite yielded 8180 values of 3 and 5%o, respectively. The calculated composition of water in equilibrium with calcite at the estimated temperature of deposition of 230°C yielded 8180 values between 6 and 13%o, but negative values of -3.1 and -5.1%o for two samples of late calcite. Although the low 8180 values for two of the late calcitesand oxygen isotopic data of vein quartz from earlier studies suggest precipitation from a meteoric fluid, the heavy isotopic signature of most carbonates indicates that the mineralizing fluid was overall heterogeneous. 813C values range between -8.9%o and +1.5%o relative to the Pee Dee Belemnite (VPDB) standard and are consistent with a meteoric or magmatic origin, in some samples probably with a minor contribution of organic carbon. Periodic involvement of a fluid with a strong magmatic component or, alternatively, meteoric water which has undergone extensive evaporation probably played an important role in the hydrothermal system. The complex stable isotope signature of El Peñón is interpreted as indicative for arid climatic conditions as a meteoric origin of the fluid is not as evident as in similar deposits emplaced in more pluvial regions.

Key words: Epithermal, Low sulfidation, Carbonates, Ultraviolet fluorescence, Stable isotopes, El Peñón, Chile.


RESUMEN

En el distrito epitermal de Au-Ag de baja sulfuración de El Peñón, II Región, Chile, los carbonatos son minerales de ganga comunes. Estos minerales pueden ser subdivididos en dos grupos principales sobre la base de sus relaciones paragenéticas. Carbonatos paragenéticamente tempranos se encuentran junto a cuarzo y sulfuras en vetas bandeadas y están estrechamente asociados con la mineralización de metales preciosos. Estos carbonatos son composicionalmente complejos, consistiendo en calcita entrecrecida con ankerita, kutnohorita, rhodocrosita y dolomita, pero también contienen hasta 0,8% en peso de PbC03 y hasta 4,1% en peso de ZnC03. En contraste, los carbonatos paragenéticamente tardíos son calcita relativamente pura, aunque localmente también se ha reconocido dolomita. Las calcitas tardías contienen menos de 3,5% en peso MnC03, menos de 0,5% en peso de ZnC03 y menos de 2,7% en peso de FeC03 y también son distintas a los carbonatos tempranos en que se caracterizan por una fluorescencia roja moderada a intensa bajo luz ultravioleta de onda corta. El color rojo de fluorescencia bajo luz ultravioleta probablemente está relacionado con contenidos en algún porcentaje de Mn, combinado con trazas de Pb y bajos contenidos de Fe. Los carbonatos tardíos se interpretan como precipitados de fluidos ricos en C02 condensados de vapor ('steam-heated') y no tienen relación directa con la mena. Las composiciones de 8180 de carbonatos, tanto tempranos como tardíos, varían entre 21,1 y 14,1 %0 relativo al estándar VSMOW 'Standard Mean Ocean Water'. Sin embargo, dos muestras de calcita tardía arrojan valores de 8180 de 3 y 5%o. Los valores calculados para la composición de agua en equilibrio con calcita en las temperaturas estimadas de precipitación de 230°C entregan valores de 8180 entre 6 y 13%o pero valores negativos de -3,1 y -5,1%o para dos muestras de calcita tardía. A pesar de que el valor bajo de 8180 de algunas calcitas tardías y datos isotópicos para cuarzo en vetas de estudios anteriores, sugieren precipitación desde un fluido meteórico, la signatura isotópica pesada de la mayoría de las muestras de carbonato indican que el fluido en general ha sido heterogéneo. Los valores de 813C varían entre -8,9%o y +1,5%o relativo al estándar VPDB 'Pee Dee Belemnite' son consistentes con un origen meteórico o magmático, en algunas muestras probablemente con una contribución menor de carbono orgánico. Un aporte periódico de fluido con un importante componente magmático o, alternativamente, agua meteórica que ha sufrido extensa evaporación probablemente jugó un rol importante en el sistema hidrotermal. Los isótopos estables en carbonatos en El Peñón se interpretan como evidencia de condiciones climáticas áridas, ya que el origen meteórico del fluido no está tan bien definido como en yacimientos epitermales similares en regiones más pluviales.

Palabras claves: Epitermal, Baja sulfuración, Carbonatos, Fluorescencia ultravioleta, Isótopos estables, El Peñón, Chile.


 

INTRODUCTION

Carbonates are common gangue minerals in low sulfidation epithermal deposits (e.g., Cooke and Simmons, 2000; Simmons etal., 2005). They may occur in the wall rock as part of the alteration assemblage, or as gangue in veins or breccias hosting the mineralization. Carbonates in veins may grow as tabular crystals, commonly replaced in part by quartz (lattice texture), banded quartz-carbonate veins or, alternatively as late open space infill (Simmons etal., 2000, 2005). Thus, carbonate occurrence and textures are varied and provide constraints on the depositional environment within epithermal districts. The lattice texture is widely interpreted as evidence for fluid boiling and is normally found close to the mineralized zone (Cooke and Simmons, 2000; Simmons etal., 2005); similarly the carbonate which occurs together with quartz and/or chalcedony in banded colloform-crustiform veins is spatially related to mineralization. In turn, the late, commonly coarse calcite filling open space has been demonstrated to have been precipitated from steam-heated meteoric waters, rich in C02 (Simmons etal., 2000). The lattertype of carbonate is therefore not directly related to precious metal deposition.

Due to the different possible associations with ore, a characterization of carbonates present in low sulfidation epithermal prospects may help focussing exploration efforts. This is particularly valid for the El Peñón district of northern Chile, located near the eastern margin of the Central Depression, some 165 km southeast of Antofagasta. The El Peñón Au-Ag deposit (>4.5 Moz Au; >70 Moz Ag), owned and operated by Meridian Gold Corporation, is considered a typical low sulfidation epithermal Au-Ag deposit (Warren et al., 2004). The volcanic rocks and epithermal veins are not well exposed due to a cover of an evaporitic crust essentially consisting of nitrate cemented gravel typically found throughout northern Chile and commonly referred to as 'caliche' in the region (cf. international usage of the term which describes carbonate cemented soil). Geological mapping of the district is to a large part based on the distribution of clasts in the float (Zuluaga, 2004; Warren etal., 2004). Quartz-carbonate vein material is also commonly found in the float and knowing the origin of the carbonate in such clasts will help establishing exploration guidelines for non-outcropping veins. In this study, we present the results of vein-carbonate characterization in the El Peñón district. Along with observations under short-wave ultraviolet light, geochemical data, including carbon and oxygen stable isotopes from a limited number of samples are presented. The results yield new insights into the origin of the fluids from which the carbonate minerals precipitated. This study also demonstrates the possible use of ultraviolet fluorescence as an inexpensive tool for future exploration in the El Peñón district or areas with similar geological characteristics.

GEOLOGICAL SETTING OF EL PENON

The El Peñón deposit is located within a north-south trending belt of epithermal deposits of Paleocene age (Sillitoe 1991; Fig. 1), within the Central Depression of northern Chile. The deposit is hosted by late Upper Paleocene to Lower Eocene subaerial rhyolitic flows and domes, overlying Upper Cretaceous and Paleocene andesite and dacite flows (Fig. 1; Zuluaga, 2004; Warren et al., 2004). The Cretaceous and Paleocene belt of volcanic rocks are confined to the Central Depression, here only approximately 20 km wide (Arancibia et al., 2006a). Recent geochronology (Arancibia et al., 2006a) yields 40Ar/39Ar ages of 54.5 ±0.6 Ma for the rhyolitic dome complex hosting the veins; an age range from 53.1±0.5 to 51.0±0.6 Ma for vein-adularia; and broadly coincident K-Ar ages for late dacitic to rhyodacitic domes. 40Ar/39Ar geochronology on supergene alunite and Mn-oxides indicates that oxidation of the El Peñón deposit occurred between -27 and -14 Ma, but continued with less intensity until ca. 9 Ma (Arancibia etal., 2006b). A change from arid to hyper arid climate in the Late Miocene is held responsible for the cessation of supergene processes (Arancibia et al., 2006b; Rech etal., 2006).

ALTERATION AND MINERALI ZATION

The following description of the El Peñón epithermal system is summarized from Warren et al. (2004). The Au-Ag mineralization is hosted by at least four distinct, steeply dipping N to NE striking veins (Fig. 1) emplaced in pre-existing normal faults. The gold-silver mineralization is restricted to banded quartziadularia veins, hydrothermal breccias and minorquartzstockwork. Veins are normally 0.5 to 4 m, but locally up to 20 m wide. Veinlets up to 10 cm in thickness with similar gangue mineralogy occur commonly within 10 m of the principal veins. Primary sulfide ore minerals include pyrite, chalcopyrite, galena, sphalerite, and electrum which typically account for less than 3% of the vein material. The primary sulfide minerals were oxidized locally as deep as 400 m below the actual surface and principal ore minerals in the oxidized zones are electrum, gold, silver and acanthite. The veins exhibit textures typical for low sulfidation epithermal deposits including banded, colloform-crustiform, massive, brecciated and lattice textures.


Wall-rock alteration is dominated by quartz-adularia-illiteidisseminated pyrite in the rhyolitic rocks in the immediate vicinity of the veins, grading into more smectite-rich and low quartz-adularia assemblages at distances of more than -10 m from the vein (Warren et al., 2004). Dacitic rocks present chlorite and calcite, as well as illite/ smectite in the alteration assemblage.

OCCURRENCE OF CARBONATES IN THE VEINS OF EL PENON

Carbonate minerals have generally been tested in the field using dilute HCI and calcite has been found to be present in most samples. X-ray diffraction identified principally calcite, in some samples mixed with lesser amounts of ankerite, dolomite, rhodochrosite and kutnohorite. Carbonates most commonly occur as gangue in the main mineralized veins as well as fracture infill within a few meters from the principal veins. Fine-grained calcite mixed with lesser dolomite, kutnohorite and ankerite are commonly found in banded to massive veins, overgrown or partially replaced by quartz ('early carbonate' hereafter). Lattice texture is mostly preserved as pseudomorphs of quartz after calcite, but the latter is locally still preserved. Calcite, locally together with minor amounts of rhodochrosite and/or dolomite, also occurs as late open space infill, overgrowing drusy quartz or as late stage veinlets cutting earlier banded quartz or quartz-carbonate veins ('late calcite' hereafter). Late calcite is also present as massive calcitei quartz veins up to 50 cm wide, featuring individual calcite crystals of up to 6 cm in diameter (see also Warren etal., 2004). The carbonates found in the float or in exploration trenches on the surface can on textural grounds commonly be assigned to the latter type, but examples of probable early calcite have also been observed at the Tres Tontos prospect (Fig. 1). Vein carbonates within or near known mineralized veins are mostly hosted by dacitic rather than rhyolitic host rocks, but carbonate bearing veins also occur in rhyolitic wall rock. Within the currently mined mineralized structures, carbonates are commonly found at depths greater than 250 m. Above this level supergene oxidation of pyrite probably has generated sufficient acid to dissolve a large part of the vein carbonates.

CHEMISTRY OF THE CARBONATES AT EL PENON

Eight carbonate samples from diamond drill holes, complemented by one calcite sample from the float on the surface were analyzed in the laboratories of the Universidad Católica del Norte, Antofagasta, Chile for carbonate compositions (Table 1, Fig .2). Additionally, one limestone sample from the El Profeta Formation (Chong, 1973) collected some 15 km east of El Peñón was analyzed for comparison. The chemical analyses of the carbonates are aimed to identifying differences in compositions of carbonates from different evolutionary stages and are compared to the fluorescence colors of the carbonate minerals observed under ultraviolet light (Fig. 3). The fluorescence under ultraviolet light has widely been used for almost a century in the prospecting for scheelite {e.g., Greenwood, 1943), but is only occasionally applied to carbonate minerals in the context of mineral exploration (e.g., Escalante et al., 2006). Under short-wave ultraviolet light (UV; 254 nm wavelength) and in absence of other light sources, calcite crystals can present fluorescent colors varying from pale blue over red and yellow to greenish, but may also lack fluorescence (e.g., Chang et al., 1996). The fluorescence colors are attributed to minor trace element contents such as Mn, Pb, Zn and REE or defects in the crystal structure whereas in calcites with elevated contents of Mg or Fe the fluorescence under UV light is suppressed (Gies, 1975).

In addition, a total of 17 samples, including all of those for which the carbonate compositions were determined, were analyzed for stable oxygen and carbon isotopes. The data provide constraints for the nature of the fluids from which the carbonate precipitated.




ANALYTICAL PROCEDURES

CARBONATE COMPOSITIONS

Amounts of between 0.017 to 0.057 g of material have been drilled out from the samples using a Dremel micro-drill tool. Care has been taken to select carbonate material as pure as possible, but in some cases up to 50 wt% quartz and other insoluble mineral grains were incorporated. To the extracted samples, 25 ml of deionised water and 2 ml of concentrated hydrochloric acid were added.

The solutions obtained were heated to approximately 80°C on a heating plate and subsequently diluted to 50 ml. It is assumed that during this procedure only carbonates were dissolved in some samples leaving a significant insoluble silicate fraction. The solutions obtained were analyzed for Ca, Mg, Fe, Mn, Zn, Pb, Sr and Ba by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). The analytical data were normalized to 100% for better comparison and are presented as carbonate wt% (Table 1). This data processing was necessary because the insoluble fraction was not analyzed, but is part of the weight considered for percent calculations before normalization. The analytical results show, however, that the majority ofthe samples contained less than 18wt% insoluble material (Table 1), mostly quartz, but from two finegrained banded calcite-quartz veins up to 50 wt% of the material was insoluble.

STABLE ISOTOPES

Fourteen samples from diamond drill cores, complemented by one sample of late calcite from the surface were analyzed for stable oxygen and carbon isotopes (Table 2). Additionally, two limestone samples taken from the Jurassic El Profeta Formation, were analyzed for comparison. The analyses were carried out at the Pacific Centre for Isotope Geochemical Research (PCIGR) at the University of British Columbia, Vancouver, Canada. Carbonate samples were analysed using a Gas Bench connected to a Finnigan Delta Plus XL mass spectrometer. Between 150 and 300ug of crushed sample material was placed in the bottom of a clean exetainer, and sealed with a piercable septum. Afterflushing the exetainerwithheliumfor5 minutes, 7 drops of 99% phosphoric acid were introduced and then left to equilibrate at 72°C for an hour. Subsequently the C02gas produced was sampled and analyzed. The sample run consists of 5 aliquots of reference C02 gas of known composition, 10 aliquots of sample gas, and one final aliquot of reference gas. In-house rock standards were distributed throughout the samples in the gas bench after every eight samples.


Measured 180/160 ratios were corrected for fractionation between phosphoric acid and calcite following the procedure of Das Sharma etal. (2002), and were adjusted for machine fractionation using a factor calculated from repeated analyses of internal UBC standards BN 13, BN 83-2, H6M, which are calibrated against two international standards, NBS 18 and NBS 19. The final results 813C (VPDB) and 8180 (VSMOW) were corrected to VPBD and VSMOW based on an average of multiple analyses of NBS 18 and 19. The standard deviation on the average analyses of NBS 18 and 19 is <0.1 per mil at the 2 sigma level for both 813C (VPDB) and 8180 (VSMOW).

ANALYTICAL RESULTS

COMPOSITIONAL VARIATIONS

The samples analyzed for carbonate compositions can be separated into two groups based on their calcite content (values given aswt%and ppm). Samples with high calcite contents (87-97.5%), but low FeC03(<2.8%), MgC03(<2%, except sample QV004-91.87 with 7.46%), PbC03 (<130ppm), ZnCO3(<0.49%),aswell as MnC03 contents between 0.78-3.5% are characteristicfor late stage carbonates (Fig. 2). Sample QV004-91.87 represents coarse white calcite overgrowing fine-grained yellowish calcite, the latter coexisting with sphalerite. Thus, the high Mg content of this sample may be explained by contamination with early, yellowish magnesian-calcite. An exception to the general rule is sample QM-53-10.2 which corresponds to paragenetically late dolomite. This sample contains 33.53% MgC03, 8.02% FeC03, but similar to the other late carbonates, low MnC03, PbC03and ZnC03 contents. In contrast, early carbonates, representing the second group, exhibit calcite contents of only 43 to 54.3%, combined with elevated FeC03(4.6-12.7%), MnC03 (25-41%), MgC03 (4.7-10.8%), PbC03 (491-8218 ppm) and ZnC03 (1.8-4.11 %) concentrations (Fig. 2, Table 1). The content of SrC03 and BaC03for both groups does not show any correlation with calcite content. The Jurassic limestone exhibits concentrations similar overall to those of the late calcite, but has with only 0.2% a distinctly lower Mn carbonate content, but at 1.28% a relatively high Zn carbonate concentration. Although the limestone analyzed is an apparently unaltered oolithic limestone, the unusually high Zn content may be a distal feature of polymetallic vein mineralization which has been observed in the wider area from where the limestone has been sampled (S. Matthews, personal communication, 2006).

ULTRAVIOLET (UV) FLUORESCENCE OBSERVATIONS

The carbonates of El Peñón have been characterized under short-wave UV light in dark rooms and can be assigned to two principal groups, based on their fluorescence under UV light. Late stage carbonate is most commonly calcite which typically presents moderate to intense red fluorescence, the intensity of which varies with crystal zoning (Fig. 3). However, locally paragenetically late dolomite which does not exhibit fluorescence has been observed (sample QM053-10.20). Contrastingly, early carbonate is characterized by a complete lack or barely detectable fluorescence in yellowish-greenish or reddish colors (Fig. 3). The use of UV light combined with XRD on samples from El Peñón also led to the identification of scheelite, present as late stage mineral in the Quebrada Colorado vein.

STABLE ISOTOPE COMPOSITIONS

The limestones of the El Profeta Formation present isotopic values typical for marine carbonates (8180= +22.9 and +26%o, 813C=-0.8 and +0.5%o) with no indication for hydrothermal isotopic signatures, whereas the values of 8180 of the majority of the early as well as late calcites from El Peñón are ¡sotopically somewhat lighter (between +14.1 and +21.1%o) (Fig. 4). However, the sample from the surface (DDH-002) and one sample of late calcite from the Quebrada Colorada vein (PX384-405) have a distinctly lighter oxygen isotopic signature of +2.96%o and +4.97%o, respectively. The corresponding isotopic composition of water in equilibrium with calcite was calculated for 230°C using the fractionation factors between calcite or dolomite and water given in Friedman and O'Neil (1977).

Because no microthermometric data are available forfluid inclusions in carbonate minerals, the temperature of 230°C was inferred from fluid inclusion data from vein quartz (Warren et at., 2004) and represents the lower end of the temperature range forthe Au-Ag mineralization at El Peñón (Warren et at., 2004). Equilibrium water 8180 compositions fall between +6%o and +13.1%o, the only exception being the samples DDH-002 and PX384-405 which have values of -5.1%o and -3.1%o respectively (Fig. 4, Table 2).

Most of the carbonates analyzed fall in the range of 813C values between -2.1%o and +1.53%o, albeit three samples of early carbonates exhibit distinctly more negative values between -8.9%o and -3.15%o. Relatively low 813C values are also characteristic for those late calcite which have low 8180 values (813C = -3.1%ofor DDH-002, and -4.3%o for PX384-405). Within the dataset there is no apparent overall correlation of oxygen or carbon ¡sotopic compositions with distance from mineralized veins or depth below surface.


DISCUSSION

GEOCHEMISTRY AND FLUORESCENCE COLOR

The fluorescence color of calcite is a function of defects or trace elements in the crystalline structure (Gies, 1975). Red colors are generally believed to be indicative for Mn contents between 1-15 mole percent, combined with trace contents of Pb (Gies, 1975; Chang etal., 1996). The paragenetically late stage calcites exhibiting moderate and strong red fluorescence colors from El Peñón contain between 0.77 and 3.54 wt% MnC03 but low MgC03 and FeC03as well as low Pb and Zn contents. In contrast, the non fluorescing early stage carbonate samples have much higher Mn, Mg, Fe, Pb and Zn carbonate contents combined with less than 60% calcite. The lack of fluorescence can probably be attributed to the high Fe and Mg contents in the calcite structure or could be an indication that calcite is a relatively minor constituent of the early carbonate material. Thus, in the El Peñón district intense red fluorescence is most likely related to MnC03 contents between 0.7 and 3.6% in otherwise relatively pure calcite samples. Complementing other exploration techniques, the UVfluorescence color of carbonate minerals in the El Peñón district and other areas in the Paleocene epithermal belt of northern Chile is a potential low cost exploration tool which can routinely be applied forthe classification of vein carbonate samples and identification of carbonate material most likely spatially associated with mineralization. Fluorescence colors of carbonate minerals have been proved to be useful tools for research and exploration in the context of carbonate rock hosted deposits in Central Perú (Escalante et al., 2006).

Paragenetically early carbonates contain large amounts of Fe, Mn, Mg, Pb and Zn, whereas late carbonates consist largely of calcite with minor Mn, but generally low Fe, Mg, Pb and Zn contents. The carbonate compositions of the main stage of hydro-thermal activity reflect the generally high concentrations of metals typically abundant in epithermal fluids. These early carbonates probably precipitated from fluids with higher temperatures as the more pure late carbonates as the incorporation of ions other than Ca is a function of temperature (Chang et al., 1996). In the waning stages of the hydro-thermal activity, the concentration of these elements decreased dramatically, which is reflected in the relatively pure late stage calcites presentthroughout the district. It also is probable that the fluid temperatures were lower during the late stages of hydrothermal activity but direct evidence is lacking. Abundant massive white calcite cross-cutting the mineralization has been studied at the Golden Cross epithermal system in New Zealand, where this type of calcite has been inferred to have precipitated from steam-heated fluids in the waning stages of hydrothermal activity (Simmons et al., 2000). The composition, commonly coarse grained nature and paragenetic relationships of the late calcite at El Peñón are similar to the calcites precipitated from steam-heated waters at Golden Cross (Simmons et al., 2000).

STABLE ISOTOPE DATA

The stable isotopes provide constraints on the nature of fluids from which the carbonates precipitated. The ore fluids of low sulfidation epithermal deposits are typically dominated by meteoric water characterized byvariably negative 8180values (e.g., Cooke and Simmons, 2000; Simmons etal., 2000; Simmons et al., 2005). However, at El Peñón, the majority of the 8180 values for water in equilibrium with calcite calculated at temperatures of 230-260°C (Warren et al., 2004) lie between +6 and + 13%o and are thus significantly heavier than expected. Only two samples of late calcite have oxygen isotope compositions consistent with meteoric water. The isotopic data of carbonates obtained in this study contrast with the oxygen isotope compositions of quartz from three samples of the Quebrada Colorada vein reported by Warren (2005). The data of Warren (2005) suggest that the quartz precipitated from meteoric waters with 8180 between -1.3%o and -4.9%o (calculated for 230°C), i.e., values similar to those for the two late calcite samples from this study. However, all other carbonate samples analyzed in this study have significantly heavier isotope compositions in which early and late carbonates cannot be distinguished. The origin of the heavy carbonate isotope compositions is somewhat difficult to understand and we favor a model in which extensive surface-near evaporation of meteoric water generated 180 enriched waters with a high content of dissolved carbonate. This enriched meteoric fluid was incorporated periodically in the hydrothermal cycle and may have led to increased calcite precipitation. An alternative explanation for heavy oxygen signatures would be an oxygen source from magmaticC02 in the carbonates. C02would have been contributed periodically to the otherwise meteoric water dominated hydrothermal system and isotopic equilibration between H20 and C02 was not attained prior to carbonate precipitation. In contrast, partial or complete isotopic exchange between meteoric fluids and the host volcanic rocks, while a possible cause for heavy oxygen isotopic signature, cannot explain the discrepancy between quartz and carbonate isotopic composition and we therefore discard this explanation. We also discard low temperature isotopic re-equilibration of carbonate minerals. Although this may lead to overestimated 8180 values when calculated at 230°C, it does not explain the factthat not all samples were affected bythis process. A juvenile magmatic component is also suggested by strontium isotopic data (unpublished data, S. Matthews, 2006) and consistent with documented magmatic activity coeval with the mineralization in the district (Arancibia efa/.,2006a). The discrepancy between quartz and carbonate isotopic compositions may be due to fluctuations in the overall fluid budget. Carbonate precipitation may have been enhanced after periods of increased surface evaporation or, alternatively due to magmatic fluid flashing. Although low sulfidation epithermal systems are most commonly dominated by meteoricfluids (Simmons etal., 2005), significant magmatic contributions have also been reported [e.g., Faure et al., 2002). An important difference between El Peñón and other low sulfidation deposits such as Golden Cross (New Zealand) or Hishikari (Japan) is the climate at the time of mineralization. The climate in northern Chile is characterized by its aridity which likely has dominated as far back as the Late Jurassicoreven Triassic (Hartley ef a/., 2005; Clarke, 2005). Ground water of meteoric origin was subject to strong evaporation and probably replaced much slower than at Golden Cross or Hishikari where the climate is more pluvial. Thus, periods of strong surface evaporation or pulses of magmatic degassing may have a considerable effect on the overall oxygen isotopic composition.

The relatively restricted range of 813C values between -2.1%o and +1.53%o for most samples indicates that carbon originating from organic matterdid not playa significant role in the ore fluid. The compositional range is consistent with a magmatic origin orfrom interaction of thefluid with arc magmatic rocks. However, three samples of early carbonates exhibit distinctly more negative values between -8.9%o and -3.15%o. These negative values may indicate an origin from a relatively reduced magma or a component of organic carbon (seeFaure, 1986). The slightly negative 813C values of the two calcite samples exhibiting also low 8180 values are consistent with values expected for meteoric water, possibly characterized by minor organic carbon (see Faure, 1986).

CONCLUD NG REMARKS

The carbonate minerals present in the veins at El Peñón can, on the basis of their fluorescence color under UV light, be distinguished into two groups. Paragenetically early carbonate minerals have no or only weak fluorescence whereas paragenetically late carbonates exhibit moderate to intense red fluorescence colors. The fluorescence is controlled by the carbonate composition and is strongest in relatively pure calcites with Mn carbonate contents of in the range of up to a few percent combined with traces of Pb. Thus, the UV fluorescence may serve as an exploration tool for classifying vein carbonate material found throughout the district in the float. Fluorescing calcites have a less direct relationship with epithermal mineralization and are probably of steam-heated origin, whereas non-fluorescing carbonate minerals are more directly related to mineralization.

The oxygen isotope values of both early and late carbonate minerals at El Peñón are in general significantly heavier than expected for low sulfida-tion epithermal systems and distinct from those obtained for vein quartz in earlier studies. The unusually heavy oxygen isotope signatures of carbonates may be explained by fluctuations in the fluid budget. Unusually heavy 8180 values can be explained by periods of extensive surface evaporation and bicarbonate enrichment of meteoric water or, alternatively, by magmatic C02 episodically contributed to the hydrothermal system. Unlike in other epithermal districts around the world where the climate is more pluvial, the arid climate of northern Chile favored the generation of isotopically heavy fluids by strong evaporation and/or restricted dilution of magmatic C02in the ore fluid.

ACKNOWLEDGEMENTS

C. Cubelli (Meridian Gold Ltda.) and S. Kasaneva are thanked for the support and permission to access the El Peñón property and drill-cores. J. Gabites (PCIGR, Vancouver) is acknowledged for isotopic analyses; while thanks are extended to I. Alfaro who performed the chemical analyses. L. Jofré helped with drafting of the figures. Comments provided by the reviewers R. Tosdal, V. Maksaev, F. Henriquezand one anonymous reviewer helped to improve and clarify the manuscript. This study has partially been financed by the Dirección General delnvestigacionesyPostgrado(DGIP), Universidad Católica del Norte, Antofagasta. Field support was provided by Meridian Gold Ltda. and stable isotope analyses were financed by the Departamento de Ciencias Geológicas of the Universidad Católica del Norte.

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Received: December7, 2006; accepted: May 22,2007