The Cenozoic History of Volcanism and Hydrothermal

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The Cenozoic History of Volcanism and Hydrothermal Alteration in the Central 40

39

Andean Flat-Slab Region: New Ar- Ar Constraints from the El Indio–Pascua Au (-Ag, Cu) Belt, 29°20′–30°30′ S a

a

a

Thomas Bissig , James K. W. Lee , Alan H. Clark & Kevin B. Heather

b

a

Department of Geological Sciences and Geological Engineering , Queens University, Kingston, Ontario, K7L 3N6, Canada b

Barrick Chile Ltda., Barrio Industrial Late 58, Alto Penuelas, Coquimbo, Chile Available online: 06 Jul 2010

To cite this article: Thomas Bissig, James K. W. Lee, Alan H. Clark & Kevin B. Heather (2001): The Cenozoic History of Volcanism and Hydrothermal Alteration in the Central Andean Flat-Slab Region: 40

39

New Ar- Ar Constraints from the El Indio–Pascua Au (-Ag, Cu) Belt, 29°20′–30°30′ S, International Geology Review, 43:4, 312-340 To link to this article: http://dx.doi.org/10.1080/00206810109465016

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The Cenozoic History of Volcanism and Hydrothermal Alteration in the Central Andean Flat-Slab Region: New 4 0 Ar- 3 9 Ar Constraints from the El Indio—Pascua Au (-Ag, Cu) Belt, 29°20'-30°30' S THOMAS BISSIG,1 JAMES K. W. LEE, ALAN H. CLARK, Department of Geological Sciences and Geological Engineering, Queens University, Kingston, Ontario, Canada, K7L 3N6 AND KEVIN B. HEATHER

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Barrick Chile Ltda., Barrio Industrial Late 58, Alto Peñuelas, Coquimbo, Chile

Abstract Ninety-three new 40 Ar- 39 Ar laser step-heating plateau dates for igneous rocks and alteration minerals from the El Indio-Pascua Au-Ag belt permit significant refinement of the Tertiary volcanic stratigraphy and the definition of a succession of alteration events in this major mineralized district. Eight distinct Tertiary stratigraphic units are now recognized (two newly defined in this research): (1) the 30-36 Ma Bocatoma Unit, comprising dioritic and granodioritic shallow-level intrusions; (2) the voluminous 23-26 Ma Tilito Formation, consisting predominantly of dacitic tuffs; (3) the 1 7 . 5 21 Ma Escabroso Group made up of extensive successions of andesitic flows and coeval hypabyssal intrusions; (4) the 14-17 Ma, andesitic, Cerro de las T6rtolas Formation and its intrusive lithodeme, the Infiernillo Unit; (5) the dacitic, 11.0-12.7 Ma, Vacas Heladas Formation; (6) the rhyodacitic 7.5-8 Ma Pascua Formation, defined in this study; (7) the 5.5-6.2 Ma rhyolitic Vallecito Formation; and (8) the recently defined rhyolitic 2 Ma Cerro de Vidrio Formation. Magmatic activity decreased markedly following the eruption of the Cerro de las T6rtolas Formation. Hydrothermal activity occurred at least from the late Eocene to the Late Miocene, but economic Au-Ag-Cu mineralization was confined to the 6-9.5 Ma interval, the only observed contemporaneous igneous unit being the restricted Pascua Formation. Epithermal Au-Ag-Cu deposits and major prospects emplaced in this period include, from north to south, Pascua-Lama, Veladero, Sancarrón, Río del Medio, El Indio, Tambo, and Vacas Heladas. The widespread, albeit barren, alteration associated with the Bocatoma, Escabroso, Infiernillo, and Vacas Heladas magmatism indicates that the availability of hydrothermal fluid was not the controlling factor for ore formation, emphasising instead the role of the metal content of the magmas associated with epithermal mineralization, and/ or the requirement for favorable physiographic conditions at the site of ore deposition.

Introduction THE CENTRAL ANDEAN flat-slab region, an amagmatic segment of the orogen separating the Central and Southern volcanic zones between Latitudes 28° and 33° S (Fig. 1), is of both geological and economic interest because it hosts several world-class Miocene and Pliocene precious- and base-metal mineral districts. These include the Maricunga AuAg belt at ~27°30' in the Cordillera Principal at the northern limit of the flat slab, the Farallón Negro Cu-Au district 300 km to the east, and the Los Pelambres-El Pachón and Río Blanco-Disputada por1

Corresponding author; email: [email protected]

0020-6814/01/517/312-29 $10.00

phyry Cu districts at the southern boundary. The subject of the present discussion, the El Indio-Pascua Au (-Ag, Cu) belt, is situated in the center of the flat-slab segment at the crest of the Cordillera Principal (Fig. 1). The Neogene tectonic, volcanological, and hence geodynamic evolution of the region has been discussed in a number of studies (Kay et al., 1988, 1 9 9 1 ; Kay and Abruzzi, 1996; G u t s c h e r et al., 2000), and metallogenetic aspects, linking petrochemical changes in the volcanic arc to the formation of the ore deposits in the region, are addressed by Kay et al. (1999) and Kay and Mpodozis (2001). It has been generally recognized that reliable stratigraphic and geochronological data for the volcanic

312


VOLCANISM AND HYDROTHERMAL ALTERATION

313

FIG. 1. Location map of the El Indio-Paseua belt. Depth contour lines on the Wadati-Benioff zone are taken from Cahill and Isaeks (1992), and outline the segment of flat subduction. Note that the El Indio—Pascua belt is located in the center of this segment, whereas the other major Mio-Pliocene mineral districts are situated near its northern and southern boundaries.

and intrusive rooks, as well as accurate age constraints on hydrothermal activity, are prerequisites for meaningful geological and, particularly, metallogenetic models. The aforementioned studies were extensively underpinned by K-Ar mineral and whole-rock dates presented by Maksaev et al. (1984), Jannas (1995), Martin et al. (1995, 1997), and Clavero et al. (1997). However, the conventional K-Ar method has some important acknowledged disadvantages in an area such as the El Indio-Pascua belt, which has experienced widespread and multi-stage hydrothermal activity. For instance, whole-rock K-Ar ages for even slightly altered volcanic rocks may be inaccurate, because the minerals are likely to have lost Ar, while alteration minerals may be overprinted by younger hydrothermal activity, yielding arbitrarily mixed ages. In the present context, we communicate the results of a new geochronological study, involving the 40Ar-39Ar laser step-heating technique, considered more robust than the K-Ar method, and undertaken alongside detailed field investigations of the volcanic stratigraphy (Heather and Diaz, 2000). More than 90 40Ar-39Ar plateau ages refine the volcanic and hypabyssal chronology in the El IndioPascua belt and establish a new history of hydro-

thermal alteration and ore deposition that significantly modifies earlier accounts. The new data were obtained from specimens from carefully documented outcrops or underground mine exposures on the Chilean and Argentinian flanks of the Cordillera Principal, and will be discussed together with the previously available K-Ar dates. The geochronological data presented herein further provide a foundation for clarification of the geomorphological development of the region (Bissig, Clark, Lee, and Hodgson, submitted), and have important implications for both metallogenetic and ore-genetic modeling. Following the discovery and development of the world-class El Indio epithermal Au (-Cu, Ag) vein deposit (~5 million ounces [Moz] Au) in the Cordillera Principal in 1976, exploration companies have investigated a large number of alteration systems between 29° and 30°30' S, north and south of the El Indio mine. Additional ore bodies, all of epithermal type, with ca. 1 Moz contained Au overall, were soon found within 10 km of El Indio, and have subsequently been mined. These include the acid-sulfate type Kimberly and Wendy breccia pipes of the Tambo district and the Río del Medio low-sulfidation vein (Fig. 2). Several other alteration systems in


314

BISSIG ET AL

FIG. 2. Simplified geology and the major faults of the El Indio-Pascua belt. Oligocene to Middle Miocene volcanic and intrusive units are undifferentiated (see Fig. 3), whereas units younger than late Middle Miocene are shown in more detail. Geological information is largely from Martin et al. (1995) for Chile; no comprehensive regional map at an appropriate scale is available for Argentina, for which information was taken from Ramos et al. (1989). our field observations, and LANDSAT TM interpretation. Latitude/Longitude and UTM coordinates (Zone 19) are indicated. Ages obtained for igneous rocks are indicated (see Table 1 for complete listing). Abbreviations: BdTF = Baños del Toro fault: PdlD = Portezuelo de los Despoblados.

the region, such as Sancarrón and Vacas Heladas, have proven Au mineralization but are subeconomic at this stage. However, the giant Pascua-Lama (~18.6 Moz Au, 630 Moz Ag; see Mining Journal, London, April 2001) and Veladero (~15.6 Moz Au, 230 Moz Ag) projects some 50 km farther north are currently under development by Barrick Gold Corporation and Homestake Mining, respectively, and are expected eventually to replace El Indio as the focus of mining activity in the region. Genetic aspects of the El Indio and Tambo deposits are doc-

umented by Jannas et al. (1990, 1999), Jannas (1995), and Deyell et al. (2000, submitted), while the enormous Pascua deposit is currently under investigation (Chouinard, 2001). 40

Ar- 3 9 Ar Geochronology

Sample preparation For the igneous units, fresh magmatic biotite, hornblende, plagioclase, or sanidine were handpicked from crushed, 0.25-0.5 mm material. Hydro-



thermal alteration assemblages and hence mineralization were dated using sericite, illite, hydrothermal biotite, and, more widely, clearly hypogene alunite. The alteration minerals were separated from crushed rocks in size fractions typically less than 0.25 mm, but pure alunite was extracted directly from the uncrushed rock where possible in order to maintain the microscopic context. All mineral separates were examined by petrographic and X-ray powder diffraction methods, and were, apart from some quartz-bearing sericite and alunite samples, more than 9 5 % pure. In addition, most dated alunites were also analyzed by electron microprobe. Analytical

methods

All analyses were carried out in the Queen's University 40 Ar- 39 Ar Laboratory. For each mineral separate, ~10 mg of material were wrapped in Al foil and stacked vertically into 11.5 cm long x 2.0 cm diameter containers, which were then irradiated with fast neutrons for 7.5 hours in the McMaster University Nuclear Reactor in Hamilton, Canada. 40 Ar- 3 9 Ar flux monitors—MAC-83 biotite with a published age of 24.36 ± 0.17 Ma (2a) (Sandeman et al., 1999)—were inserted at ~ 0 . 5 cm intervals along the irradiation containers. Following irradiation, the samples and monitors were placed in small pits, ~2 mm in diameter, drilled in a Cu sample-holder. This was placed beneath the sapphire view-port of a small, bakeable, stainlesssteel c h a m b e r connected to an ultra-high vacuum purification system. Monitors were fused in a single step, using a focused LEXEL 3 5 0 0 argon-ion laser beam. The J-values for individual samples were subsequently determined by second-order polynomial interpolation. These values varied for the samples discussed herein from ~2.04 x 10-3 to 1.60 x 1 0 - 3 , with errors of typically less than 1.3% ( 2 a ) . For step-heating experiments on silicate minerals, the beam was defocused to heat the entire sample, but refocused to fuse the sample in the final step. Samples were heated for ~3 minutes for each step at increasing power settings (0.25 to 7.0 W). Alunite samples were, however, heated only to 3 to 4 W, thereby precluding fusion and h e n c e sulfur contamination of the analytical system. These power settings were generally adequate to release over 8 0 % of the Ar. The evolved gases were purified using a coldtrap with liquid N 2 and a SAES C50 getter for ~5 minutes. Argon isotopes were measured using a

315

MAP 216 mass spectrometer, with a Bäur Signer source and an electron multiplier. All data were corrected for blanks, atmospheric contamination, and neutron-induced interferences (Onstott and Peacock, 1987; Roddick, 1983). All errors are reported as ±2a, and dates were calculated using the decay constants recommended by Steiger and Jager (1977).

Geological Setting and Revised Volcanic Stratigraphy The Cenozoic hydrothermal systems of the El Indio-Pascua belt were emplaced within a NNESSW-striking tectonic depression bounded by highangle reverse and normal faults (Fig. 2). The western limit of the study area is represented by the approximately N-S-striking Banos del Toro fault, 5 - 1 0 km west of the El Indio and Tambo deposits, and the less well defined eastern boundary by the Cordilleras Colangüil and de la Brea faults, ~ 3 0 - 4 0 km east of the Chile-Argentina border (Fig. 2). The majority of the known alteration centers, whether mineralized or barren, are hosted by a succession of Oligocene to Upper Miocene subaerial, volcanic, and volcaniclastic rocks, which are widely preserved in the southern parts of the El Indio belt, where they overlie an Upper Paleozoic to Lower Jurassic basement. However, mineralization at Pascua-Lama and Veladero at the apparent northern limit of the belt is hosted by hydrothermal breccias intruding basement granitoid rocks. The first comprehensive geological study of the region was carried out over 35 years ago (Thiele, 1964), and the volcanic stratigraphy of the district is now relatively well known from studies by Maksaev et al. (1984) and Martin et al. (1995), carried out in the Chilean part of the study area. Recent 1:10,000 regional mapping by Barrick Gold Corporation geologists in the wider El Indio area (e.g., Heather and Diaz, 2000) provide, together with the new 4 0 Ar- 3 9 Ar data presented here, a basis for a significant refinement of the volcanic stratigraphy. In contrast to that of the Chilean side of the Cordillera, the geology of contiguous Argentina is s p a r s e l y d o c u m e n t e d (Groeber, 1951; Aparicio, 1984; Ramos et al., 1989; Limarino et al., 1999). For this area we present reconnaissance-scale age constraints for volcanic rocks that are tentatively integrated into the stratigraphic scheme established for the western part of the region.

316

BISSIG ET AL.

Upper Paleozoic

to Lower Jurassic

The Pastos Blancos

basement:

Group


The Pastos Blancos Group is subdivided into two volcanic sequences and at least two distinct intrusive units, and constitutes, together with minor Paleozoic gneisses, a composite basement for the Mesozoic and Tertiary strata in the El Indio belt (Martin et al., 1999). The individual subunits proposed by Martin et al. (ibid.) are described below. The Permian Guanaco-Sonso Sequence consists of red to brown, welded, rhyolitic to dacitic ash-flow tuffs, volcaniclastic s e d i m e n t s , and minor lava flows. The rocks are almost everywhere slightly altered. The Permian to Lower Triassic intrusive Chollay—El León Unit c o n s i s t s of m e d i u m - to coarse-grained granites, monzonites, diorites, and dacitic porphyries and is assigned to the Chollay Batholith, the northerly equivalent of the ElquiLimari Batholith. The granitoid rocks of the Choll a y - E l León Unit i n t r u d e t h e G u a n a c o - S o n s o Sequence, and abundant mafic dikes cut both the Guanaco-Sonso volcanic and Chollay intrusive rocks. The Los Tilos Sequence comprises a wide variety of rock types, including bimodal basaltic/rhyolitic lava flows, hypabyssal intrusive rocks, and rhyodacitic welded ash-flow tuffs, as well as partly calcareous and gypsiferous volcaniclastic sandstones and conglomerates. This unit rests on, and intrudes, the Paleozoic units and is assigned a Middle Triassic to Early Jurassic age. The Upper Triassic to Lower Jurassic Colorado Unit represents an intrusive complex consisting of reddish to orange, fine- to coarsegrained granitoids and quartz-feldspar porphyries. These felsic intrusive rocks are locally commingled with mafic dikes and hypabyssal bodies and clearly intrude the Chollay Unit. The Colorado intrusive rocks are probably directly related to the similarly b i m o d a l Los Tilos v o l c a n i c s e q u e n c e a n d are assumed to constitute much of the basement in the Pascua area. Middle Jurassic to Eocene units are very rare in the immediate El Indio belt itself, but have been recognized to the west of the Baños del Toro Fault. They are not further discussed in this paper. Oligocene

to Upper Pliocene

units

T h e m o d e r n A n d e a n s u b d u c t i o n c y c l e is assumed to have started at ~26 Ma, in the latest Oligocene, with the transition from oblique, and relatively slow, convergence to orthogonal subduction and rapid convergence (Pilger, 1984) when the Farallón Plate broke up into the Cocos and Nazca

plates. The lower Oligocene Bocatoma Intrusive Unit (Martin et al., 1995) pre-dates this plate tectonic rearrangement, while the inception of the new convergence pattern is represented in the region by the voluminous volcanism of the upper Oligocene — Lower Miocene Tilito Formation (Martin et al., 1995). The subsequent evolution of the volcanic arc was characterized by markedly decreasing magma volumes. Detailed descriptions of the Oligocene to Upper Pliocene volcanic units, largely following the stratigraphic subdivisions suggested by Martin et al. (1995), are given below (see also Figs. 2 and 3). The locations of the dated samples are recorded in Table 1. Middle-to-upper Oligocene: the Bocatoma intrusive unit. This entirely intrusive assemblage consists of stocks approximately 1.5 km in diameter, most commonly exposed in the area north of Río Potrerillos, in the wider Pascua-Lama area, where OligoMiocene volcanic units do not completely cover the basement. The intrusive rocks range from finegrained to coarsely porphyritic diorites and granodiorites and c h a r a c t e r i s t i c a l l y e x h i b i t 1-2 cm poikilitic hornblende and biotite phenocrysts. Martin et al. (1995) reported K-Ar biotite and wholerock dates ranging from 31 to 39.5 Ma for Bocatoma stocks. This age range is supported herein by three new 4 0 Ar- 3 9 Ar plateau ages (Table 1; Fig. 2). A date of 35.9 ± 1.2 Ma was obtained for hornblende from a diorite at Potrerillos, 7 km S of Pascua, and a granodiorite from the Lama prospect yielded a biotite age of 35.5 ± 1 . 2 Ma. A similar hornblende date of 30.0 ± 1 . 9 Ma was determined for relatively fresh diorite from the Lama prospect area, but the age spectrum shows evidence of some Ar loss. Upper Oligocene to Lower Miocene: The Tilito Formation. This unit consists mainly of variably welded dacitic, and less abundant andesitic and rhyolitic, ash-flow and lithic-crystal tuffs, as well as associated volcaniclastic sediments. Minor basalts have also been assigned to this unit (Martin et al., 1995). It is the most voluminous Cenozoic formation in the region, attaining thicknesses of more than 1200 m. A more distal sedimentary facies is represented by few hundred meters of conglomerates and sandstones intercalated with subordinate dacitic tuffs, in the Valle del Cura area on the Argentinian slope. Plagioclase and biotite phenocrysts are associated in the Tilito Formation volcanic rocks with variable proportions of quartz, augite, hornblende and, locally, sanidine, embedded in a commonly devitrified and/or argillized aphanitic matrix. The


VOLCANISM AND HYDRO THERMAL ALTERATION

317

FIG. 3. Oligocene to Upper Pliocene volcanic stratigraphy of the El Indio belt. Age ranges indicated are from 40ArAr data from this study. Note that the erupted volumes decreased after the eruption of the Escabroso Group volcanics and the thicknesses are not to scale. 39

felsic units of the Tilito Formation are quite similar in petrography to the Paleozoic Guanaco—Sonso rhyolitic tuffs, but the latter contain more abundant brownish quartz phenocrysts and generally lack translucent feldspars, fresh biotite, augite, and h o r n b l e n d e . Tilito Formation strata have been folded into open, N-S-striking anticlines and synclines a few hundred m in wavelength in the Valle del Cura and Cordillera Sancarrón areas in Argentina (Fig. 4), as well as the Río Apolinario area north of Sancarrón in Chile (Martin et al., 1995). K-Ar ages ranging from 27.2 ± 1.0 to 21.0 ± 1.5 Ma were reported by Martin et al. (1995, 1997, and references therein) who, however, preferred a narrower age range of 2 3 - 2 7 Ma for the eruption of the dacites. Three new 40 Ar- 39 Ar dates (Table 1, Fig. 2) obtained in this study support the interpretation that this volcanic episode terminated at ~23 Ma. Biotite from a non-welded dacitic tuff near Despoblados in the Cordillera Sancarrón, 15 km east of Veladero, yielded an age of 25.1 ± 0.3 Ma, whereas that from a similar tuff from Valle del Cura was dated at 23.9 ± 0.3 Ma. A welded tuff, exposed in a higher stratigraphic position between the Río Apolinario and

Sancarrón valleys in Chile, gave a slightly younger biotite age of 23.1 ± 0.4 Ma. Lower Miocene: The Escabroso Group. Maksaev et al. (1984) and Martin et al. (1995) defined a suite of intermediate volcanic rocks overlying the Tilito Formation as the Escabroso Formation, itself assigned to the now-obsolete Dona Ana Group. On the basis of detailed mapping in the wider El Indio mine area, however, Heather and Diaz (2000) subdivided this package, elevating the formation to group status. The volcanic and sedimentary rocks of the Escabroso Group are separated from the Tilito Formation by an important angular unconformity and a persistent regolith horizon, and can be subdivided locally into up to five lithostratigraphic formations (Heather and Diaz, 2000). On the basis of petrographic similarities to a 18.6 ± 0.9 Ma dike in a comparable geologic setting ~5 km to the southeast, a basaltic andesite dike is herein assigned to the Escabroso Group; it cuts a N-S-striking anticline developed in Tilito Formation tuffs in the northern Cordillera Sancarrón (Fig. 4). We take this as evidence for a regional deformation event between the deposition of the two volcanic series.

421.626/6740.170 429.467/6690.292

Lama Lama

Sancar.-Apolinario

Cord. Sancar. N V. Del Cura

99thb213a 00thb238a

99thbl46a 99thbl96a

3 km W, El Indio

98thb74a 99thb110b

Lama

Lama

00thb259a

Potrerillos

Libra Apolinario

India Solitaria India Solitaria

99thbl85a 00thb253a

99thbl63b

99thb112a

98thb89b

98thb89a

99thb201a

99thbl97a 99thb197a

P. Deidad P. Deidad

98thb57a 98thb57a

Q. Vacas Heladas Cord. Sancar. N Cord. Sancar. N Cord. Sancar. N

N of P. Deidad

98thb41a 98thb42c 98thb54b

99thb221a

SE Co. Torta SE Co. Torta

402.900/6728.575

Potrerillos

405.910/6752.550

400.675/6746.175 406.100/6757.650

405.552/6716.128 402.975/6735.000

406.242/6712.074 406.242/6712.074

420.822/6736.333

425.483/6734.170 425.483/6734.170

413.607/6707.811 413.607/6707.811 403.994/6708.579 413.531/6696.900

409.493/6710.573 412.755/6709.911

409.234/6710.486

400.266/6747.049 402.861/6756.152 405.601/6755.208

417.820/6746.560

99thb184a

00thb265a

403.372/6758.319

Coord. UTM-19

Nevada N, Pascua Despoblados N

Location

99thb139a

Sample

V. 39 Ar

18.6 ± 0.9 18.9 ± 1.3 19.0 ± 1.1

Andesite dike/bi Andesite dike/hbl Andesite lava/hbl

Diorite/plag

Andesite lava/bi Diorite/bi

Andesite poiph./bi Andesite porph./hbl

15.3 ± 0.3

16.0 ± 0.2 15.7 ± 0 . 4

15.2 ± 2.5 14.9 ± 0.7

93.9%/4 of 6 steps 91.6%/1 of 2 steps 11.63

92.4%/4 of 8 steps

92.7%/3 of 5 steps

Total gas age

77.4%/6 of 10 steps Total fusion age

80.9%/2 of 5 steps 91.0%/4 of 5 steps

80.9%/2 of 7 steps 87.4%/4 of 7 steps

23.4%/4 of 7 steps

2 aliquots, excess

38.9%/5 of 9 steps 61.6%/4 of 14 steps 77.5%/2 of 5 steps 92.8%/best of 3 steps

40

40

Ar

Ar

39

Ar

Second of 2 steps

Recoil loss of

2 aliquots, excess 40Ar Best steps from 2 aliquots

Most 39 Ar in 2 steps

2 aliquots, excess

Best steps from 2 aliquots

Disturbed spectrum System overload, last step lost

Comment

32.4%/4 of 9 steps

96.4%/7 of 19 steps

92.8%/3 of 7 steps 93. 2%/4 of 9 steps

76.9%/3 of 7 steps 34.2%/2 of 12 steps 81.0%/4 of 7 steps

27.7%/4 of 14 steps Second of 3 steps

Ar/steps

Plateau 39

20.17 20.69

6.91 5.07

Cerro de las Tórtolas Formation and Infiernillo Unit 38.68 Granodiorite/bi 15.4 ± 0.2 1.45 Andesite porph./bi 15.7 ± 0.3

5.20

5.36 2.62

12.46 6.80

19.1 ± 1.2

Andesite lava/plag Andesite lava/plag

22.1 ± 2.5

5.77 6.30

12.46 10.61 48.27

Escabroso Group 21.9 ± 0.9 Andesite lava/plag Andesite lava/plag 21.7 ± 1.5 Granodiorite/bi 18.7 ± 0.2 Andesite porph./hbl 18.0 ± 0.7 Andesite porph./bi 17.6 ± 0.5

23.9 ± 0.3

10.73 28.79 46.96

Tilito Formation 23.1 ± 0 . 4 25.1+0.4

4.01

13.58

x10-3cm 3 , NTP

3.73 6.75

Bocatoma unit 35.9 ± 1.2

190.1 ± 3.2 261.0 ± 5 . 4

Basement

Age (Ma)

30.0 ± 1.9 35.5 ± 1.2

Dacite pyrocl./bi Dacite pyrocl./bi Dacite pyrocl./bi

Granodiorite/bi

Diorite/hbl Diorite/hbl

Granite/bi Rhyolite pyrocl./Kfsp

Rock/mineral

TABLE 1. Geochronological data for the ingeous rocks


318 BISSIG ET A

5.8 ± 0.2

Rhyolitie pyrocl./bi

Rhyolite lava/bi Rhyolite lava/gl

414.899/6696.800

425.220/6690.000

415.820/6749.370

415.820/6749.370

P. Vacas Heladas

V. Del Cura

Despoblados N

Despoblados N

99thbl43b

00thb267a

00thb250a

00thb250a

2.0 + 0.2

2.1 ± 0.5

Cerro de Vidrio

6.1 + 0.4

28.31

20.88

64.94

95.2% 3 of 4 steps

83.8%/4 of 8 steps

85.9%/6 of 8 steps

78.6%/3 of 8 steps

58.1%/6 of 8 steps

10.62

54.5%/5 of 11 steps

8.42

93.8%/3 of 4 steps

Total fusion age

85.7%/6 of 8 steps

Total gas age

51.02

7.12

6.78

16.30

13.89

100%/7 of 7 steps

95.5%/6 of 8 steps

5.39

14.77

65.5%/9 of 16 steps

Total gas age

65.6%/5 of 9 steps

22.64

6.43

13.49

Total fusion age 97.9%/8 of 10 steps

7.45

99.2%/8 of 10 steps

Total fusion age

91.7%/7 of 10 steps

8.98

54.87

3.70

34.75

Ar

40

Ref. Bissig et al., 2001

Ref. Bissig et al., 2001

All steps >80% atmos

Excess 40 Ar at low T.

Unreliable, recoil loss of 39 Ar

2 reproducible aliquots

Ar- 39 Ar data for igneous rocks dated in this study. The sample locations and the number of steps and fraction of 39Ar in the plateau are tabulated. The total amount of 39Ar released is given as a measure of the K content in the specimens (all samples were irradiated for 7.5 hours). Mineral abbreviations: bi = biotite; hbl = hornblende; Kfsp = potassium feldspar; plag = plagioclase; san - sanidine; gl = glass. Full documentation of the data is provided in Bissig (2001).

l 40

5.5 ± 0.5

Granite clast/bi

414.899/6696.800

P. Vacas Heladas

99thb143a

Rhyolite pyrocl./bi

5.7 ± 0.3 6.2 ± 1.2

Rhyolite pyrocl./bi

405.061/6708.724

W of El Indio

98thb71b

Rhyolite pyrocl./plag

405.061/6708.724

W of El Indio

98thb71b

5.5 ± 0.1

6.0 ± 0.3

Rhyolite pyrocl./san

408.647/6700.122

SE of Azufreras

98thb29b

Vallecito Formation Rhyolite pyrocl./bi

SE of Azufreras

98thb29b

7.8 ± 0.3 7.1 ± 2 . 3

Dacite dike/bi

Pascua Formation Dacite dike/bi

408.647/6700.122

Pascua

186.5-186.75 m 401.605/6757.059

Pascua, DDH-108

99thbl30b

Inca 47

Dacite pyrocl./bi

413.620/6750.075

Despoblados N

11.0 ± 0.2

12.4 ± 0.5

Dacite pyrocl./bi

430.161/6695.100

00thb252a

11.9 ± 1.5

V. Del Cura

12.7 ± 0.9

Dacite porph./bi

409.546/6694.855

99thbl65a 99thb220a

Dacite pyrocl./bi

404.731/6758.045

N of Lama

Vacas Heladas

99thbl35a

11.5 ± 0.3

11.7 ± 0.1 11.2 ± 0.1

Dacite pyrocl./bi

12.0 + 0.2 11.5 + 0.2

Dacite pyrocl./bi

397.298/6756.067

406.515/6700.452

Azufreras

Dacite porph./bi

405.744/6700.964

S of Pascua

Azufreras

98thbl5b

Dacite pyrocl./bi

406.093/6700.369

98thb64b

Azufreras

98thb11a

Vacas Heladas Formation Dacite pyrocl./bi

406.193/6701.883

98thbl8a

Azufreras

98thbl0b


VOLCANISM AND H


320

BISSIG ET AL.

tered plagioclases from massive andesitic flows occurring near the base of the Escabroso Group yield distinctly older ages. Although the spectra exhibit excess 40Ar, indicated by U-shaped configurations with apparent ages of ~40-60 Ma at the lowest and highest power settings, geologicallyreasonable ages of 22.4 ± 0.8, 21.7 ± 1.5, and 21.7 ± 2.2 Ma were obtained from two aliquots of each sample at intermediate laser powers. Five further, less disturbed ages from the Cordillera Sancarrón and areas near the border east and southeast of El Indio range from 19.1 ± 1.2 to 17.6 ± 0.5 Ma (see Table 1 and Fig. 2). Middle Miocene: The Cerro de las Tórtolas ForFIG. 4. View to the north from the northern part of Cordillera Sancarrón. An anticline developed in Tilito Formation mation and Infiernillo Intrusive Unit. The volcanic tuffs (T) is outlined. The fold is intersected by a basaltic ande- and hypabyssal rocks of this magmatic episode are site dike assigned to the Escabroso Group (D: coord. UTM-19: volumetrically subordinate to those of the Escabroso 420.640/6738.480). Escabroso Volcanic rocks (E) unconform- Group. They originate from several eruptive centers ably overlie the Tilito Formation Tuffs. The Cordillera Ortíga and compose the upper part of Cerro de las Tórtolas (5648 m a.s.l.) is visible in the background. (6380 m), the type locality (Maksaev et al., 1984). The Cerro de las Tórtolas Formation consists The Escabroso Group comprises andesitic to mainly of biotite, augite, hornblende, and plagiodacitic lava flows, autoclastic breccias, subordinate clase-phyric andesite flows, commonly with relavolcaniclastic sediments ranging from breccia to tively large phenocrysts in a fine-grained to sandstone, and minor dacitic ash-flow tuffs. Small aphanitic matrix. The intrusive bodies assigned to granodioritic to dioritic intrusive bodies are associ- the Infiernillo Unit include relatively coarse plagioated with this volcanic episode. Remnants of large clase-porphyritic and equigranular, and finevolcanic edifices representing plausible sources for grained porphyritic granodiorites and diorites, with the Escabroso Group rocks are recognized on Cerro abundant green hornblende and biotite and minor Doña Ana and the type locality Cerro Escabroso, augite. Biotite and hornblende are generally more west of the Baños del Toro Fault near El Indio, and abundant, and augite rarer, than in the older Escaon the lower and intermediate slopes of Cerro de las broso Group, but it is difficult to distinguish these Tórtolas (6380 m a.s.l.), as well as in the Cordillera units in outcrop or thin section. An angular unconSancarrón in Argentina (Fig. 2). The characteristic formity is recognized between these two predomiextrusive rocks are massive or flow-banded, augite- nantly andesitic units west of Cerro de las Tórtolas and plagioclase-phyric andesites, with hornblende (Martin et a l , 1995), but the K-Ar ages of 13.3 ± 1.1 and biotite as minor constituents. Distinctive ande- to 18.2 ± 0.6 Ma determined for apparent Cerro de site flows exhibiting glomeroporphyritic plagioclase las Tórtolas and Infiernillo rocks by Martin et al. occur locally. Granodioritic intrusive rocks exhibit (ibid.) overlap with those recorded for the Escabroso the same mineral assemblage as the extrusive rocks, Group volcanism. In this study, due to the general but contain more abundant biotite and interstitial lack of unambiguous field relationships between these two rarely juxtaposed stratigraphic units, we quartz and K-feldspar. The Early Miocene age range of 17 to 21 Ma assign rocks to the Infiernillo Unit and Cerro de las inferred by Martin et al. (1995, 1997) is slightly T6rtolas Formation if they yield ages between 14 modified herein to 17.5—21 Ma on the basis of new and 17 Ma, the range suggested by Martin et al. 40 Ar-39Ar geochronological data. Locally, in the area (1995, 1997). Our age constraints were obtained of Cerro Torta, north and east of the El Indio mine, from several small dioritic and granodioritic intruandesitic rocks previously included in the younger sive bodies from the Lama property and the Río Cerro de las Tórtolas Formation and Infiernillo Unit Apolinario valley, which all yield dates of between are now assigned to this group. Whereas Martin et 15.7 ± 0.4 and 14.9 ± 0.7 Ma. A dacite flow from al. (1995) reported K-Ar whole-rock ages of between Potrerillos, 7 km south of Pascua, was dated at 16.0 13.1 ± 1.0 and 16.9 ± 0.6 Ma from the area, unal- ± 0.2 Ma and a hypabyssal andesite, in gradational



contact with a coeval granodiorite, from India Solitaria was dated at 15.7 ± 0.3 Ma. Elsewhere partly chloritized biotite from a dacitic porphyry spatially related to 16.8 Ma potassic alteration at the Libra prospect (described later in this article) yielded a total gas age of 15.2 ± 2.5 Ma, but no reliable plateau was obtained (Table 2). On the basis of our new data, a very brief eruptive hiatus of ~0.5 m.y. is inferred to have occurred at the Escabroso Group-Cerro de las T6rtolas Formation stratigraphic boundary. Upper Miocene: The Vacas Heladas Formation. Relatively small volumes of dacitic (and subordinate andesitic) ignimbrites. as well as local domes and block-and-ash deposits, crop out at several locations including Azufreras (west of Tambo), the area south and northeast of Pascua, and the Fabiana prospect 10 km southeast of Pascua. Small outcrops of non-welded tuffs preserved in the Valle del Cura are also assigned to this unit, but probably represent a landslide deposit. The nomenclature of the rocks described in this section has been inconsistent. They were originally included in the Cerro de las Tórtolas Formation by Maksaev et al. (1984), but because of their distinctive trace-element chemical composition, were referred to as the "Cerro de las Tórtolas II Formation" by Kay et al. (1999). The term "Vacas Heladas Formation" was introduced by Martin et al. (1995) b e c a u s e tuffs of this unit crop out northwest of Arroyo Vacas Heladas, south of Tambo, but Martin et al. (1997) employed yet another name, Tambo Formation. Although none of these designations reflect a precise type locality, we maintain herein the usage "Vacas Heladas Formation," which is current among the majority of geologists working in the area. The formation largely consists of amphibole- and quartz-bearing, biotite and plagioclase-phyric, dacitic to andesitic welded ignimbrites and air-fall tuffs. Lithic fragments are rare. Augite has been observed as an accessory phase in one sample. Rocks of the Vacas Heladas Formation can be readily distinguished from those of the Cerro de las Tórtolas Formation and older units by the presence of quartz phenocrysts, a normally fresh appearance, a general absence of pyroxene, and the high proportion of biotite. An overall age range from 9.7 ± 0.5 to 12.8 ± 0.7 Ma was suggested by Martin et al. (1995), and is b r o a d l y s u p p o r t e d h e r e i n . The 4 0 A r - 3 9 A r data obtained in this study (see Table 1, Fig. 2) permit the recognition of several centers of eruption. Thus,

321

the tuff and block-and-ash deposits on the high plain of Azufreras, southwest of Tambo, range in age from 11.5 ± 0.3 to 12.0 ± 0.2 Ma, whereas the ignimbrite ~3 km south of Pascua and the texturally similar rocks near Fabiana, probably originating from the same source, were dated at 11.3 ± 0.1 and 11.0 ± 0.2 Ma, respectively. The unwelded tuff near the Porfiada exploration zone, ~2.5 km northeast of Pascua, yielded a biotite age of 12.7 ± 0.9 Ma, similar to the 12.4 ± 0.5 Ma age of a sample collected in the Valle del Cura. Mid-Upper Miocene: The Pascua formation. A dacitic dike cutting the main Brecha Central ore body at Pascua and intersected by diamond drilling is herein dated at 7.8 ± 0.3 Ma (biotite: Fig. 5). The dike therefore significantly postdated the Vacas Heladas Formation and is also older than the oldest Vallecito Formation tuff (see below). The dated sample is a relatively coarse, biotite-, quartz-, and plagioclase-phyric dacite, the large, subhedral, quartz phenocrysts exhibiting embayed margins (Fig. 5). The abundance of quartz (~10%) and the lack of hornblende pheonocrysts distinguishes this rock from the Vacas Heladas Formation ash flows. This dike is the only igneous unit sensibly coeval with the mineralization in the El Indio-Pascua district confirmed in the present study. However, Martin et al. (1995) reported a biotite K-Ar age of 7.6 + 0.7 Ma for a volcanic rock from Paso Chollay, 12 km northeast of Pascua. This dacitic tuff was originally assigned to the Vallecito Formation by Martin et al. (op. cit.), but its affiliation with that sequence was already questioned by those authors. We therefore suggest that these rocks, despite their restricted o c c u r r e n c e , be i n c l u d e d in a s e p a r a t e , newly defined, Pascua Formation, reflecting their apparent metallogenetic significance. Upper Miocene: The Vallecito Formation. The poorly to moderately welded rhyolitic tuffs of the Vallecito Formation represent the youngest previously recognized volcanic rocks in the El Indio belt. These ash-flow tuffs, locally covering red- and yellow-weathering volcaniclastic sandstones, conglomerates, and breccias assigned to the same unit (Martin et al., 1995), are biotite-, sanidine-, quartz-, and plagioclase-phyric. Lithic fragments compose up to ~5% of the rocks and are usually derived from altered units, but fresh fragments of quartz, sanidine, and plagioclase-porphyritic granite occurring locally in the tuffs at Paso Vacas Heladas are considered to represent an unexposed intrusive facies of the unit. Distinguishing features are the


322

BISSIG ET AL.

FIG. 5. A. Photomicrograph of the Paseua Formation dacite. Large phenocrysts of clear, euhedral plagioclase (plag) and embayed quartz (qtz), as well as smaller hiotite flakes (hi), are embedded in a fine-grained, partly argillized matrix (sample Inca 47, DDH-108, 186.5 m; see Table 1). Plane-polarized transmitted light. B. Argon release pattern of the biotite of the Paseua Formation dacite dike, which cuts the mineralization at Paseua. The pattern is not disturbed and the age is assumed to be reliable.

large, bipyramidal quartz euhedra, the lack of hornblende, and the presence of phenocrystic sanidine. K-Ar ages for welded and non-welded tuffs in the El Indio-Tambo area range from 5.4 ± 0.4 to 6.6 ± 0.4 Ma (Martin et al., 1995). Our 40 Ar- 39 Ar data (Table 1, Fig. 2) suggest a more restricted age range of 5.5 + 0.1 to 6.0 ± 0.3 Ma. An additional biotite age of 6.16 ± 1.18 Ma was determined for the tuffs at Paso Vacas Heladas, the large error being the result of a high content of atmospheric Ar. Biotite from a granitic clast in the tuffs at Paso Vacas Heladas was dated at 5.5 ± 0.5 Ma, which implies that it represents an unexposed intrusive lithodeme of the formation. A large (~40 km 2 ) ignimbrite sheet in the upper Valle del Cura on the Argentinian side of the frontier, originally termed the "Vacas Heladas ignimbrite" by Ramos et al. (1989), is herein assigned to the Vallecito Formation. It originated from a small c a l d e r a east of Cerro Vacas H e l a d a s [sic] and yielded a biotite K-Ar age of 6.0 ± 0.4 Ma (ibid.), which has been confirmed herein by a 4 0 Ar- 3 9 Ar biotite age of 5.8 ± 0.2 Ma. Upper Pliocene: Cerro de Vidrio Formation. An upper Pliocene age has been obtained for a rhyolite dome located 15 km southeast of Paseua (Bissig, Clark, and Lee, submitted), but similarly young volcanic rocks may also occur in small volumes elsewhere in the El I n d i o - P a s c u a district. The rock assigned to the Cerro de Vidrio Formation consists of generally undevitrified glass of variable porosity containing 3 km 2 ) alteration system ~5 km west of Paso



325

FIG. 7. Escabroso Group-related alteration. A. Feldspar-porphyritie dacite at the Sanoo prospect, strongly altered to quartz-topaz and locally foliated, probably by shear deformation, as indicated by the stippled line (foliation strikes 325/ 80). Microcrystalline topaz occurs in white bands parallel to the foliation. Note scale on hammer handle is in cm. B. Backscattered electron image of topaz (top) and alunite (al) growing in cavity of strongly altered dacitic tuffs. Paso Deidad (Fig. 6); sample 98thb57c (Table 2). Alunite appears to cement the topaz grains.

Sancarrón. This zone exhibits many of the aforementioned high-temperature minerals (Fig. 7) and is spatially related to a dacitic to andesitic shallowlevel intrusion. The dated alunite is finely crystalline and occurs with small amounts of topaz as disseminations in strongly silicified tuffs of the Tilito Formation. In the area of Paso Deidad, ~7 km east of El Indio, several small hypabyssal andesitic intrusive bodies (hornblende: 18.0 ± 0.7 Ma) are spatially associated with strongly silicified and quartzalunite-altered Tilito Formation dacitic lithic tuffs. Medium-grained, yellowish, hypogene alunite occurring with topaz in vugs (Fig. 7) and as veins was dated at 17.2 ± 0.2 Ma, and is considered to represent the youngest alteration related to Escabroso Group magmatism. Middle Miocene alteration related to the Cerro de las Tórtolas Formation and Infiernillo Intrusive Unit As with the hypabyssal rocks of the Escabroso and Infiernillo Intrusive Units, distinction between the two corresponding episodes of alteration can be difficult and is based largely on geochronology. However, the alteration assemblages reveal a temporal change toward lower temperatures and a shallower depth of formation. The age of the major, but barren, episode of alteration associated with the Infiernillo Unit is constrained by new 40Ar-39Ar dates from throughout the district, supported by KAr ages (Martin et al., 1995; Clavero et al., 1997) previously obtained in the El Indio-Tambo area.

Widespread, weak to moderate propylitic and argillic alteration in the El Indio-Pascua belt is also attributed to Infiernillo intrusions (Martin et al., 1995; Jannas et al., 1999). The Libra alteration system, 2-3 km2 in area, is exposed ~7 km north of El Indio on the eastern flank of the Río del Medio valley. The higher levels of the zone, at ~4350 m a.s.l. exhibit strong silicification, whereas weak potassic alteration (hydrothermal biotite) and tourmaline (Gallardo, 1996) occur at 4000 m a.s.l., within 200 m of the valley bottom. The presence of topaz and andalusite, occurring at intermediate levels ~100 m higher than the hydrothermal biotite, points to assemblages comparable to those in the older Sanco system. The potassic alteration is associated with a hornblende, biotite, and plagioclase-porphyritic hypabyssal andesitic intrusion. Age constraints for hydrothermal activity (Table 2; Fig. 6) are provided by hydrothermal biotite and sericite dated at 16.8 ± 0.4 Ma and 16.6 ± 0.5 Ma, respectively. Slightly younger alteration ages were obtained from a site in the upper Río Apolinario valley, ~8 km NNW of Sancarrón (alunite, 14.9 + 0.5 Ma, spatially related to a 14.9 + 0.7 Ma andesitic porphyry), and from Veladero Sur, 10 km south of Veladero proper in Argentina (alunite, 15.7 ± 0.8 Ma: a minimum age due to contamination with late jarosite). The alteration assemblages in these areas do not exhibit diagnostically high-temperature minerals but are dominated by pervasive silicification and more local advanced argillic alteration, with coarse, hypogene


326

BISSIG

ET AL

FIG. 8. Topographic map of the Pascua-Lama-Veladero district showing the location of prospects. Alunite ages obtained in this study are indicated (UTM coordinates are given, contour interval 100 m).

alunite. Lama Central, a small alteration system ~5 km southeast of the main ore body of the Pascua project, is characterized by intensely silicified granites and fine-grained lithic tuffs and volcaniclastic sediments of the Paleozoic to Lower Jurassic basement. The silicified rocks host a stockwork of medium-grained yellow alunite veins and bodies of crackle breccia with an alunite matrix. Later generations of breccias exhibit a rock-flour matrix and rounded fragments. Coarse, pink alunite veins locally cut the second generation of breccia and exhibit native sulfur in their centers. The yellow and pink alunites yield similar ages of 13.3 ± 0.3 and 13.6 + 0.8 Ma, respectively. The native sulfur growing in the pink alunite veins represents the oldest steam-heated and, hence, shallow-seated alteration known to be preserved in the district. Late Miocene: Alteration related to the Vacas Heladas Formation Apparently barren alteration between 10.0 and 12.8 Ma in age, and therefore contemporaneous with the volcanism of the Vacas Heladas Formation, is commonly found in the vicinity of the major deposits of the district. However, local high, but incoherent,

Au and Ag values of this age are known only from the Filo Federico exploration target at the northern rim of the Veladero deposit (Fig. 8). Alunites from this area have been dated at 10.7 ± 0.2 Ma from a ~2 m wide vein of coarse-grained alunite (Fig. 9), and 10.9 ± 0 . 1 Ma from a zone of pervasive feldspar replacement in felsic lithic-crystal tuff. However, the context of these alunites with respect to the major Veladero mineralized zones is not clear. Additional dates for apparently barren, Vacas Heladas Formation-related alteration in the wider Pascua-Lama-Veladero area have been obtained from two other locations, as listed in Table 2. Alunite replacing feldspars in an andesitic to dacitic tuff from a small alteration zone at Veladero Sur, 8 km south of Veladero proper (Fig. 6), yielded an age of 12.8 ± 0.3 Ma, and medium-grained yellow alunite filling the pore spaces of a coarse volcaniclastic sandstone at Fabiana, 3 km east of Veladero (Fig. 8), was dated at 10.4 ± 0 . 1 Ma. Porcelaneous alunite veins, inferred to be of steam-heated origin, are also widespread at Fabiana and two were dated at 10.3 ± 0.2 and 10.0 ± 0.4 Ma (Fig. 9). Barren alteration of this episode has also been recognized in the El Indio and Tambo districts, typ-



327

FIG. 9. Vacas Heladas Formation-related alteration. A. Coarsely crystalline, massive alunite from Filo Federieo, between the Pascua-Lama and Veladero properties (see Fig. 8). Sample site is 00thb245a (see Table 2). B. Porcelaneous alunite vein (v) at the Fabiana prospect (Fig. 8); br = a zone where similar alunite forms a hydrothermal breccia matrix. Photo taken in the northern part of the prospect, looking northeaast. Sample site is 00thb280a.

ically in close vicinity to the later-mineralized systems. Thus, coarse alunite has been dated from the cement of Brecha Silvestre near the economic Wendy Breccia of the Tambo mine (10.4 ± 0.3 Ma), while on the upper slopes of Cerro la Campana near the El Indio mine, pervasive powdery alunite, inferred to be of steam-heated origin, was dated at 12.1 ± 0.4 Ma. The alteration assemblages of this episode lack higher-temperature minerals such as tourmaline, topaz, and andalusite, but the abundant steamheated alunite and native sulfur occurring immediately beneath the high plains common in the region reveal lower temperatures of formation and a generally shallower setting than for the previous phases of hydrothermal activity. Hydrothermal Activity: Economic and Subeconomic Mineralization Detailed 40Ar-39Ar age constraints for the major ore deposits of the district are now available from the Pascua and Lama prospects as well as for the El Indio district, including the Río del Medio low-sulfidation vein, and for the Tambo district. Data from Veladero and the apparently smaller Sancarrón and Vacas Heladas prospects complete the database for the Au-Ag mineralization. All economically significant precious-metal enrichment in this region may now be assigned to a single episode between ~6 and 9.5 Ma. The geochronological results and the geological context of the individual samples are described below for the various mines and prospects

from north to south; the analytical data and sample locations are listed in Table 3. Pascua and Lama The Pascua-Lama district embraces several ore bodies, exhibiting different styles of high-sulfidation epithermal alteration and mineralization. In the Argentinian (i.e., Lama) half of the deposit, it is difficult to relate dateable alteration minerals, such as alunite, to specific ore zones, because Au is hosted by strongly silicified rocks. However, alunite is clearly contemporaneous with the mineralization in the Brecha Central, the main Pascua orebody. Two samples of alunite from veins cutting Choilay basement granites were collected from the Lama prospect at ~4500 m a.s.l., 1.5 km east of the ChileArgentina border (Fig. 9). Microcrystalline, yellowish alunite from a N-S vein was dated at 9.4 ± 0.2 Ma (Fig. 10), and a similar but less precise age of 9.54 ± 0.85 Ma was obtained from a NW-SE-striking, coarse, pinkish vein from the southern flank of the Río Turbio valley running through the Lama prospect area (Fig. 8). Farther west, in the "Caracoles Norte" sector (Fig. 8), ~1 km northeast of Brecha Central, a 9.0 + 0.2 Ma date was obtained for medium-grained alunite pervasively replacing the feldspars of a coarse-grained Chollay granite. A series of six alunite plateau ages were obtained for magmatic-hydrothermal alunite from Brecha Central and its immediate envelope. Although the alteration, brecciation, and mineralization occurred in several distinct stages (A. Chouinard, pers. commun., 2000), plateau dates for alunites belonging to

89.7%/6 of 8 steps 76.8%/5 of 7 steps 96.6%/5 of 6 steps 98.6%/6 of 8 steps

93.3%/4 of 6 steps 73.5%/4 of 9 steps

34.56 28.75 24.78 13.14 17.01 35.49 15.24

6.01 8.83 36.27 16.17 14.52 7.30 23.58 9.04 21.77

8.4 ± 0.2 8.7 ± 0.2 8.3 ± 0.3 8.6 ± 0.4 8.5 ± 0.3 8.1 ± 0.2 6.7 ± 0.8

9.4 ± 0.6 7.6 + 0.4 6.2 ± 0.3 7.7 ± 0.3 6.6 ± 0.3 3.5 ± 0.4 7.0 ± 0.3 7.8 ± 0.4 7.5 ± 0.6

Al (HY) Al (HY) Al (HY) Al (HY) Al (HY) Al (HY)

Al (HY)

Al (HY) Al (HY)

Ser Ser Al (HY) Ser Ser Ser/ill Ser Ser

400.535/6756.503 400.353/6756.503 400.490/6756.503 400.295/6756.565 400.295/6756.565 400.320/6756.360

409.410/6747.200

407.999/6727.652 407.721/6728.427

406.875/6712.765 406.875/6712.765

Pascua, lev. 4680 m

Pascua, lev. 4680 m

Pascua, lev. 4680 m

Pascua, lev. 4360 m

Pascua, lev. 4360 m

Pascua, lev. 4360 m

Veladero

Cerro Tío Pepe

Cerro Don Lucho

Río del Medio

Río del Medio

El Indio, 4130 m level (Campana B)

El Indio, 3845 m level (Mula Muerta)

El Indio, 3865 m level (Paihuano)

El Indio, 3925 m level (Jalene)

El Indio, 3885 m level (Viento)

El Indio, DDH-JA8 (Jalene)

Montura

Inca25

Inca26a

Inca33

Inca41

Int:a42

Inca45

00thb282a

99thbl73a

99thb174a

98thb4a

99thb4d

PP12cbb

Inca14

Inca15

Inca21

Inca23

Jay1

99thb71c

405.061/6708.724

Al (HY)

El Indio-Viento, Río del Medio

7.9 ± 0.2

7.7 ± 0.2

Sancarron

8.0 ± 0.2

Veladero

Pascua Al (HY)

400.740/6756.232

Pascua surface

PS-26c

27.28

23.85

59.48

17.17

9.0 ± 0.2

Al (HY)

402.328/6756.562

Lama

99thb209a

26.61

9.4 ± 0.2

Al (HY)

403.025/6755.870

87.7%/3 of 5 steps

67.0%/best step of 5

84.3%/3 of 6 steps

97.4%/5 of 6 steps

95.0%/9 of 12 steps

85.7%/3 of 8 steps

Total gas age

86.6%/5 of 7 steps

98.4%/4 of 6 steps

78.8%/2 of 5 steps

65.9%/5 of 8 steps

85.9%/4 of 6 steps

91.2%/6 of 10 steps

97.0%/4 of 7 steps

97.7%/4 of 6 steps

78.5%/3 of 10 steps

Lama

6.32

9.5 ± 0.9

99thb214a

Lama Al (HY)

Plateau Ar%/ steps

39

V. Ar, 10 - 3 cm 3 , NTP

403.526/6755.104

Age (Ma)

Lama

Mineral

99thb217a

UTM-19

Location

Sample

Coord.,

39

TABLE 3. Geochronolgical Data for Alteration Minerals from Mineralized Alteration Systems1


Ar > 59%

40

Fine-gr., gas released at low T

No plateau

Mixed age, Tab. 2

Atmos 40 Ar > 8 0 %

Atmos

Comment

328 BISSIG ET


329


different episodes of alteration span a relatively narrow range between 8.7 ± 0.2 and 8.1 ± 0.2 Ma. The alunite most clearly related to Au-Ag-Cu mineralization was from an enargite-alunite vein cutting the breccia and yielded an age of 8.6 ± 0.4 Ma (Fig. 11). The 7.8 Ma dacitic dike of the Pascua Formation described above, the only igneous rock similar in age to the mineralization in the district, cuts the Brecha Central. Veladero Only one sample from the Veladero prospect was dated in this study. Collected at an elevation of 4200 m in the southeast part of the exploration area in the mineralized Cerro Colorado zone (Fig. 8), the dated coarse-grained alunite occupies a vein cutting pervasively quartz-alunite altered quartz-phyric crystal tuffs, probably belonging to the Guanaco-Sonso s e q u e n c e of the b a s e m e n t . T h e age s p e c t r u m obtained is complex (2-14A). Two heating steps released 7 8 . 8 % of the 39Ar at intermediate laser powers and yielded ages of 7.8 ± 0.3 and 8.1 ± 0.3 Ma. However, the highest-temperature step, comprising 1 1 % of the 39Ar, yielded an age of 10.7 ± 0.9 Ma. Electron microprobe analysis of this sample revealed a strong zoning of the alunite: Na-rich cores, containing more Na than K per formula unit, are overgrown by alunite with distinctly higher K contents (Fig. 12). The age spectrum, therefore, may represent a mixture and two episodes of alunite growth. It remains unclear, however, whether either of the two generations was associated with the main Au-Ag mineralization at Cerro Colorado. Sancarrón Small-scale mineralization is found on either side of the Chile-Argentina border in the Sancarrón district, situated halfway between El Indio and Pascua (Figs. 2 and 6; Heresmann and Davicino, 1990; Williams, 1998). Two samples have been dated from the Chilean part of the prospect ("Sancarrón Chileno"), ~2 km north of Paso Sancarrón (Fig. 13). The main exploration targets in that area are two 4500 m h i l l s , Cerro Tío P e p e and Cerro Don L u c h o . Coarsely crystalline alunite cementing a breccia of strongly silicified felsic tuff clasts, cropping out on the west flank of Cerro Tio Pepe at an elevation of 4290 m a. s. 1., was dated at 7.6 ± 0.2 Ma (Fig. 13). Scorodite veins, up to 3 0 - 4 0 cm wide and repres e n t i n g oxidized e n a r g i t e m i n e r a l i z a t i o n , a r e observed in the vicinity of the sample site. On the summit of Cerro Don Lucho, slightly finer grained


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FIG. 10. Alunite from Lama. A. A ~2 cm wide vein (striking 175°, steeply dipping) of fine-grained alunite (immediately above scale) cutting fine-grained granite. Sample site is 99thb214a (Table 3), collected at 4500 m below the Penelope zone. B. The age spectrum for 99thb214a. The plateau includes four steps and the spectrum is not disturbed.

FIG. 11. Brecha Central, Pascua. A. Outcrop of Brecha Central (BC) at ~4950 m a.s.l.. above the main mineralized zone. The breccia intersects granite (G) and is associated with a stockwork zone (not clearly visible in this view) with veins of alunite and gypsum at the contact. The breccia matrix contains abundant coarse alunite (sample site of PS-26c). B. The age spectrum for sample Inca33, a syn-mineralization alunite from an enargite-alunite vein cutting Brecha Central at Pascua (4860 m level). Minor Ar loss is apparent in the first heating increments, but a reliable plateau of five steps is attained.

alunite from the cement of a similar hydrothermal breccia yielded an age of 7.9 ± 0.2 Ma. The El Indio—Viento-Campana and Río del Medio veins The El Indio-Viento-Campana Au (-Ag, Cu) deposit (Fig. 14) comprises a series of major veins, attaining widths of up to 20 m and largely NNESSW striking, hosted by Tilito Formation tuffs (Amiga Tuff in Jannas et al., 1999). The alteration is predominantly argillic and phyllic (Jannas et al.,

1999) advanced argillic facies occurring only at the shallowest levels. The mineralized structures have traditionally been assigned to enargite ± pyrite-rich, "copper-stage", and younger quartz-rich "goldstage" veins on the basis of cross-cutting relationships (Jannas et al., 1990, 1999). However, more recent work by mine geologists and our new age data suggest that this is oversimplified. Some veins, e.g. Quarzo Uno, adjacent to the Cu-rich Viento vein, and Jalene, 500 m east of Viento (Fig. 14), exhibit mineral assemblages typical of low-sulfidation



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FIG. 12. Alunite from Veladero. A. The complex age spectrum of sample O0thb282a from Cerro Colorado, Veladero, showing the two steps that yielded a combined age of 8.0 ± 0.2 Ma and the last step with a distinctly older age. This may be an effect of the two generations of alunite in this sample (see B). B. Backscattered electron image of sample 00thb281a. The Na-rich cores appear slightly darker, and contain to 0.53 Na per formula unit, whereas the rims are. Na poor (0.02 Na pfu).

FIG. 13. A. Topographic map of the Sancanón prospect. The locations of the. dated alunites are indicated. Dashed lines outline the streams. UTM coordinates are given; contour interval is SO m. B. Age spectrum for 99thl)173a. For location see Figure 13A. The plateau includes five steps.

deposits, and including sphalerite, chalcopyrite, tennantite, and, locally, rhodochrosite. For many of the veins, direct age constraints must be based on sericite from wall-rock alteration zones immediately adjacent to the mineralized structures. The Jalene vein was thereby dated at 7.8 ± 0.4 Ma, Viento at 7.0 ± 0.2 Ma, the copper-rich Mula Muerta vein at 7.6 + 0.3, and the quartz-and gold-rich Paihuano vein (between the Viento vein and the main El Indio

veins) at 6.6 ± 0.3 Ma (Fig. 15). A slightly younger K-Ar sericite age of 6.4 ± 0.2 Ma is reported by Jannas et al. (1999) for the Indio Sur 3500 Vein, representing by far the richest part of the deposit. The E-W-oriented Campana B vein, the only major mineralized structure in the hanging wall of the Inca Norte fault (see Jannas et al., 1999), exhibits bands of medium-grained alunite intercalated with enargite. The alunite yielded an age of 6.2 ±


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FIG. 14. Topographic map of the wider El Indio-Tambo district. Locations of dated alteration minerals are indicated. UTM coordinates are given, contour interval is 50 m.

0.3 Ma (Fig. 15), the youngest for an alteration mineral clearly related to economic mineralization in the district. The exploited Río del Medio low-sulfidation vein, hosted by Escabroso Group andesites, crops out at ~3950 m a. s. 1., 3 km north of the El Indio mine (Fig. 14). Sericite from an altered wall-rock fragment within the vein yielded a plateau date of 7.6 ± 0.4 Ma (Fig. 15). An additional, but disturbed, age spectrum was obtained from sericite separated from the andesitic wall rock, ~1 m away from the vein. This sample yields an apparent total gas age of 9.4 ± 0.6 Ma, but the spectrum lacks a plateau. It is inferred that this sericite may record an older epi-

sode of alteration partly reset during the emplacement of the Río del Medio vein. The Tambo deposit A variety of small ore bodies, predominantly hosted by phreatic breccias, have been mined in the area of Cerro Elefante, ~6 km SSE of El Indio (Figs. 14 and 16). The mineralized bodies all exhibit acidsulfate characteristics and are generally poor in sulfides (Jannas et al., 1999). Dates for hypogene alunite cementing the fragments or altering the immediate wallrock of the mineralized breccias record the age of the mineralization. The emplacement of Au in the Kimberly and Wendy breccias and



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FIG. 15. Selected age spectra for sericite and alunite from El Indio and Rio del Medio. The samples were collected from within, or in the immediate vicinity of. the mineralized veins. From upper left to lower right: Río del Medio low sulfidation vein; Mula Muerta enargite vein; Paihuano quartz-gold vein; and Campana B enargite ± alunite vein. The minor disturbance in the high-T steps of Inca 15 is probably due to outgassing of fluid inclusions in contaminating quartz.

the Indigena vein took place between 8.9 ± 0.4 and 8.0 ± 0.4 Ma (Fig. 17; further data are reported in Deyell et al., submitted). Veta Veronica (Fig. 16), a thin, but Au-rich, quartzose vein hosted by Tilito Formation dacitic tuffs, ~3 km northeast of the Kimberly breccia, has been dated using alunite replacing feldspar from the immediate envelope of the vein. The 8.5 ± 0.2 Ma date is similar to the other ages obtained for Tambo district mineralization. However, alunites from the veins and breccia bodies at Canto Sur, ~1.5 km northwest of Kimberly (Fig. 14), have been dated at 7.1 ± 0.2 Ma and 7.3 ± 0.1 (Deyell et al., 2001), ages significantly younger than those for the other breccia bodies in the area, although coeval with ore deposition at El Indio. On Azufreras, ~3 km west of the Wendy and Kimberly ore bodies (Fig. 14), a porcelaneous alunite vein, associated with native sulfur occurring in a fault zone, represents the steam-heated facies of alteration in the wider Tambo area. This yielded an age of 7.7 ± 0.2 Ma, thus predating the Canto Sur miner-

alization but probably postdating the other mineralized centers. Vacas Heladas Situated ~9 km south of Tambo (Fig. 14), this relatively small alteration system apparently lacks economically significant m i n e r a l i z a t i o n , although erratic high gold grades are reported (Arcos, 1997). Pervasive acid-sulfate alteration has been dated from the northeastern part of the prospect, the site of local Au anomalies (