bayo district_chilean patagonia_late jurassic to cretaceous magmatism and protracted history of...

©2014 Society of Economic Geologists, Inc.Economic Geology, v. 109, pp. 487–502

The Cerro Bayo District, Chilean Patagonia: Late Jurassic to Cretaceous Magmatism and Protracted History of Epithermal Ag-Au Mineralization*

Jaime a. Poblete,1,† thomas bissig,1 James K. mortensen,2 Janet gabites,3 richard Friedman,3 and manuel rodriguez4

1Mineral Deposit Research Unit, Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC, Canada

2Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC, Canada3Pacific Centre for Isotope and Geochemical Research, Department of Earth and Ocean Sciences,

University of British Columbia, 6339 Stores Road, Vancouver, BC, Canada4Coeur South America, Av. Vitacura 5250, Of. 404 Vitacura, Santiago, Chile

AbstractThe Cerro Bayo low sulfidation epithermal district is located in the Aysén region, Chilean Patagonia, at

the western tip of the Deseado Massif epithermal silver-gold province. Mineralization is hosted in the mainly rhyolitic fragmental successions of the Jurassic Ibáñez Formation. New biotite 40Ar/39Ar and zircon U-Pb ages together with published ages constrain the eruption of this unit to between 158 and 144 Ma. Rhyolitic domes and dikes, including Cerro Bayo proper, aligned on the prominent N-S–striking Cerro Bayo fault and intrud-ing rocks of the Ibáñez Formation were dated by U-Pb on zircon at 146.5 ± 0.2 and 146.3 ± 0.2 Ma. Dacitic domes of 83.0 ± 0.2 and 82.6 ± 0.2 Ma (U-Pb on zircon) are present in the western part of the district at Laguna Verde. Silver and gold mineralization is hosted by steeply dipping, dominantly N to NW and subordi-nate W-striking quartz veins and, to a lesser extent, in breccias, which, on the basis of new adularia 40Ar/39Ar ages, were emplaced in three main episodes: (1) Mallines: ca. 144 to 142 Ma, (2) Bahía Jara and Brillantes: ca. 137 to 124 Ma, and (3) Laguna Verde: 114 to 111 Ma, overall spanning 33 m.y. Most Ag-Au has been produced from veins emplaced in the latter two episodes. Veins at Mallines and Bahía Jara are spatially related to the Cerro Bayo fault. The oldest episode of mineralization in the district is similar in age to the youngest epithermal deposits in the Deseado Massif, whereas the economically most important veins are age equivalent to skarn and polymetallic vein mineralization in the Patagonian Andes of southern Chile. The protracted history of epither-mal processes at Cerro Bayo records the evolution from continental-scale extension related to the Gondwana breakup to the establishment of Andean-type arc and back-arc environments. The largely extensional tectonics throughout this period resulted in similar epithermal mineralization styles emplaced episodically over a large time interval.

IntroductionPatagonia contains numerous vein-hosted epithermal Ag-Au deposits of generally Late Jurassic to Early Cretaceous age (e.g., Schalamuk et al., 1997; Fernández et al., 2008; Dietrich et al., 2012). Most deposits are hosted by Jurassic basaltic-andesitic to rhyolitic rocks of the Deseado and North Pata-gonian (a.k.a. Somun Cura) massifs and occur over the entire width of the South American continent from the Chilean bor-der to the Atlantic coast of Argentina (Fig. 1). Precious metal-rich deposits are also distributed along the Andes, parallel to the modern magmatic arc related to the subduction of the Nazca plate (Townley et al., 2000). Here, despite the similar host rocks, mineralization ages vary from Late Jurassic (e.g., El Faldeo: Palacios et al., 1997) to Late Cretaceous (e.g., El Toqui: Townley, 1997; Bussey et al., 2010) and mineralization styles are more varied and include skarn, epithermal precious metal, and base metal sulfide veins (Townley and Palacios, 1999; Townley et al., 2000). The Cerro Bayo district is situ-ated at 46°30'S and 71°50'W in Chile, near the international border with Argentina and 14 km west of the town of Chile

Chico. It is located at the western tip of the Deseado Mas-sif, but occurs within the Andean domain of South America (Fig. 1). It contains numerous mostly N to NW striking veins distributed over an area of 16 by 12 km at the southern shore of Lake General Carrera (Fig. 2). Published geochronologic data are scarce and the temporal relationship of mineraliza-tion to key igneous units remains obscure. Previously pub-lished K/Ar and 40Ar/39Ar ages (Townley, 1997; De la Cruz and Suárez, 2008) of alteration minerals associated with epi-thermal veins in the district span 34 m.y., which is an unusu-ally large age range when compared to other low sulfidation epithermal districts (e.g., Arancibia et al., 2006; Velador et al., 2010), although spatially overlapping episodes of mineraliza-tion spanning more than 10 m.y. are reported from Mexico (Camprubí et al., 2003). In this paper we present new data that clarify the temporal relationship of igneous activity and epithermal mineralization at Cerro Bayo and establish the age range of veins using 40Ar/39Ar geochronology on vein adularia, thereby eliminating the possibility of mixing between primary host rock and hydrothermal ages. Our data indicate that epi-thermal mineralization occurred in the district throughout much of the Lower Cretaceous and are, in broad terms, con-sistent with the previously established range of mineralization ages.

0361-0128/14/4197/487-16 487Submitted: March 22, 2012

Accepted: June 3, 2013

†Corresponding author: e-mail, [email protected]*A digital supplement to this paper is available at http://economicgeology.

org/ and at http://econgeol.geoscienceworld.org/.

488 POBlEtE Et Al.

Mining and Exploration HistoryThe Cerro Bayo district historically has produced about

0.52 Moz Au and 28 Moz Ag, and a measured plus indi-cated resource of 1.6 Mt at 3.2 g/t Au and 373 g/t Ag (using a 163 g/t Ageq cutoff grade) has been outlined (March 2012, www.mandalayresources.com).

Ag-Au mineralization at Cerro Bayo (previously also referred to as Fachinal) was discovered by Freeport Chilean Explora-tion Company (FCEC) in 1984. Early in 1990, Coeur d’Alene Mines Corporation acquired FCEC and extensive exploration concessions in the district and continued evaluation of the area with infill and step-out drilling and tunneling, which led to a production decision in mid-1994. Production started in late May 1995 from veins in the Laguna Verde area. Mining was concentrated on several breccia bodies, large veins, and stockworks. Three open pits were developed, but some veins were exploited using underground methods. Due to declin-ing metal prices and depletion of reserves, the mine opera-tions were suspended in November 2000. A drilling program in 2000, prior to mine suspension, outlined the high-grade Lucero vein system at Bahía Jara, near the Cerro Bayo dome, located 12 km east from Laguna Verde. Production from this vein system recommenced in April 2002. Exploration and development drilling continued in the district during 2003 and production continued until late 2008, when operations were suspended due to diminishing reserves and the need to

evaluate the viability of mining several new veins discovered within the Coihues Este area of Laguna Verde (Fig. 2). Explo-ration continued during 2009 and 2010, culminating with the delineation of new mineral resources and reserves in veins at Coihues Este. Mandalay Resources Corp. purchased 100% of Compañía Minera Cerro Bayo in August 2010, continued exploration in the district, and reinitiated production in 2011.

Regional Geologic SettingThe Mesozoic Chon Aike large igneous province (LIP) of

Patagonia (Fig. 1) covers an area of about 1,000,000 km2 and is widely interpreted as the result of regional extension lead-ing to the opening of the South Atlantic Ocean (Pankhurst and Rapela, 1995; Pankhurst et al., 1998; König and Jokat, 2006). Three episodes of silicic magmatism have been defined (Pankhurst et al., 2000): an early episode between 188 and 178 Ma, interpreted as the result of widespread crustal ana-texis shortly after the Karoo-Ferrar mafic magmatism, fol-lowed by episodes between 172 and 162 Ma and between 157 and 152 Ma. The latter two present an increasing sub-duction component and focus of igneous activity shifted west-ward (Pankhurst et al., 2000; Dietrich et al., 2012). In the Patagonian Cordillera, these Jurassic felsic rocks correspond to the largely rhyolitic Ibáñez Formation and its southern equivalent, the Tobífera Formation (Figs. 1–4), which over-lie a Paleozoic metamorphic basement (Parada et al., 1997).

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Fig. 1. Regional map showing the Deseado and North Patagonian massifs and the distribution of the principal Jurassic to Cretaceous igneous units. The locations of Cerro Bayo and other mineral districts mentioned in the text are shown. Fm. = formation.

thE CERRO BAyO DIStRICt, ChIlEAn PAtAGOnIA: MAGMAtISM AnD EPIthERMAl Ag-Au MInERAlIzAtIOn 489

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Ages from ~158 to as young as ~140 Ma were obtained for the Ibáñez Formation (Palacios et al., 1997; De la Cruz and Suárez, 2008; Suárez et al., 2009; see also below). The Jurassic and Cretaceous rocks were locally faulted, tilted, and thrust during Cretaceous-Tertiary Andean deformation (Suárez and De la Cruz, 2000) along the Patagonian Cordillera.

Metallogeny of the Argentinean and Chilean Patagonia

The felsic rocks of the Chon-Aike large igneous prov-ince, particularly those from the Deseado Massif, contain

numerous vein-hosted epithermal Ag-Au deposits of low to intermediate sulfidation type (Schalamuk et al., 1997; Echa-varría et al., 2005; Fernández et al., 2008). The Lower Juras-sic(?) Pingüino deposit, containing vein-hosted indium-rich polymetallic mineralization (Jovic et al., 2011a, b), probably represents the oldest metallogenetic event in the region. The majority of the Ag-Au epithermal veins were emplaced in the Late Jurassic to Early Cretaceous and include those cur-rently mined at Cerro Vanguardia (Schalamuk et al., 1997), Manatial Espejo (154 Ma: Wallier, 2009), Mina Martha

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Fig. 3. Photographs of characteristic rocks and field relationships that show the Ibáñez Formation stratigraphy and domes. A) Photograph taken from Bahía Jara looking south. N-S–aligned rhyolitic subvolcanic domes intrude the Ibáñez Formation. B) Microphotograph of rhyolite of the Cerro Bayo dome. Devitrified fluidal texture showing rotated quartz (Qtz) phenocrysts. C) Microphotograph of dacite from the Laguna Verde dome (270353E/4841771N) where illite veinlets cut the flow banding. D) N355° gray quartz with finely disseminated pyrite crosscutting N-striking rhyolitic dikes south of the Cerro Bayo dome. E) Panoramic view showing the contact between unit 4 of the Ibáñez Formation and the Toqui Formation in the Cerro Torta area. Note the red color of the upper levels of unit 4, indicating the base of the mainly shallow marine Toqui Formation.


(Páez et al., 2011), and San José (141 Ma: Dietrich et al., 2012). Numerous other prospects with similar characteristics and ages are present in the region (Echavarría et al., 2005; Fernández et al., 2008). The veins in all districts strike domi-nantly WNW to NNW and are steeply dipping. NW-striking extensional faults are also thought to have controlled much of the silicic volcanism (Pankhurst et al., 2000; Guido and Campbell, 2011).

Silver-gold and polymetallic vein mineralization, as well as Pb-Zn– and Au-rich skarn mineralization, is present along the Austral Patagonian Cordillera in Chile south of 44°S and west of the Deseado Massif (Townley, 1997; Townley et al., 2000; Fernández et al., 2008). These prospects and mines are distributed in three geographically distinct clus-ters, including El Faldeo and surrounding areas (south of 47°05'S), Cerro Bayo and Mina Silva (46°05'–47°05'S), and El Toqui (44°30'–46°05'S). The mineralization of this region has been summarized in Townley (1997) and Townley et al. (2000; Fig. 1) and ages vary widely. Besides the Cerro Bayo district (see below), the oldest mineralization is documented from the 142 to 140 Ma El Faldeo Zn-Au epithermal and skarn district (Palacios et al., 1997; Townley et al., 2000), located 75 km south-southwest from Cerro Bayo. Slightly

younger mineralization has been reported from the ~130 Ma Halcones-Leones epithermal prospect some 30 km south from Cerro Bayo (Townley, 1997). However, the majority of mineralization ages reported for Chilean Patagonia (Town-ley, 1997; Townley et al., 2000) fall into the Aptian and Albian of the Early Cretaceous. The most significant of these depos-its include the El Toqui Zn-Au (ca. 120–100 Ma, Bussey et al., 2010) and Mina Silva Pb-Zn-Ag skarn deposits, located 150 km north and 30 km west of Cerro Bayo, respectively (Townley and Palacios, 1999).

MethodologyStandard isotope dilution-thermal ionization mass spec-

trometry (ID-TIMS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb geo-chronology has been applied to date felsic domes and ignim-brite deposits. Mineralization and additional ignimbrite deposits have been dated by 40Ar/39Ar on vein adularia and biotite, respectively. Analytical methods are described in a digital supplement (see online Appendices 1–5).

Volcanic Stratigraphy and Subvolcanic Intrusions in the Cerro Bayo District

Regional mapping was carried out by Suárez and De la Cruz (1997) and De la Cruz and Suárez (2008), who provided a basic volcanic stratigraphy, but also reported several geochro-nological constraints, which will be discussed below. Addi-tionally, unpublished Couer d’Alene maps, largely generated by D. Williams in 2006, are available and formed the basis of Figure 2. The district is dominated by the andesitic to rhyo-litic Upper Jurassic Ibáñez Formation, which is intruded by rhyolitic domes and dikes at Bahía Jara and Mallines, as well as dacitic domes near Laguna Verde (Figs. 2–4). The Ibáñez Formation is subhorizontal to shallowly (<20°) E dipping and hosts all epithermal veins in the district. These Upper Juras-sic and Lower Cretaceous rocks are described in more detail below.

A gradual transition from the deposition of felsic pyro-clastic rocks of the Ibáñez Formation to the Vanlanginian to Hauterivian shallow marine limestones and quartzites of the Toqui Formation has been documented in the Aysén basin (Suárez et al., 2009, 2010b). The Toqui Formation is only pre-served as a ~30-m-thick unit in the study area (Fig. 3E) and was deposited after the Hauterivian (De la Cruz and Suárez, 2008) following a marine transgression in a subsiding back-arc environment (the Aysén basin: Suárez et al., 2009, 2010a). The Toqui Formation is covered by subaerial felsic pyroclastic rocks of the Aptian Divisadero Group, which has been docu-mented 20 km south of Cerro Bayo (De la Cruz and Suárez, 2008). The resumption of subaerial felsic volcanism is taken as evidence for basin inversion at that time, which, however, did not result in significant deformation in the Cerro Bayo area (De la Cruz and Suárez, 2008). Eocene to Pliocene basaltic plugs and lavas intrude and overlie the Mesozoic sequence (Fig. 3A).

Stratigraphy of the Ibáñez Formation in the Cerro Bayo district

Based on detailed documentation of stratigraphic columns throughout the Cerro Bayo district (Poblete, 2011), the

UNIT 1:(Minimum thickness: 112 m)Andesitic to dacitic coherent

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Fig. 4. Generalized stratigraphic column for the Ibáñez Formation in the Cerro Bayo district, including dome intrusion events. See text for detailed descriptions.

492 POBlEtE Et Al.

Ibáñez Formation can be subdivided into four stratigraphic units which can be traced throughout the district (Figs. 2, 4). The basal unit 1 consists of andesitic to dacitic coherent lavas and volcaniclastic successions with an estimated minimum stratigraphic thickness of 112 m. It principally crops out in the northwestern part of the Cerro Bayo district, north and south of Laguna Verde and in the Brillantes area (Fig. 2). The veins at Brillantes are hosted by this unit. Overlying andesitic to dacitic rocks of unit 1 is a variably welded rhyolitic fragmental succession (unit 2). The rocks contain about 5 to 10% quartz and plagioclase and, in some areas, K-feldspar crystals in an argillized ash matrix. Quartz makes up about 30 to 50% of the crystal content. Feldspars are partly replaced by illite ± smectite and carbonate. Mafic minerals are generally altered to hematite and other Fe oxide phases. Argillized fiamme containing quartz phenocrysts and devitrification textures are common. An overall thickness of ~150 m has been estimated for unit 2 (Poblete, 2011), and it is an important ore host in the Laguna Verde area. Unit 3 is a volcanosedimentary-dom-inated unit containing laminated volcaniclastic crystal-rich sandstones to conglomerates likely representing detritus from unit 2. It is widely distributed in the district and separates the felsic pyroclastic units 2 and 4. An overall thickness of 20 to 60 m has been estimated (Poblete, 2011). Locally at the Lucero vein system, this stratigraphic horizon is an important ore host (Fig. 2). Unit 4 is petrographically similar to unit 2 and consists of variably welded rhyolitic to rhyodacitic pyro-clastic deposits. However, quartz is overall less abundant than in unit 2 rocks, making up only 15 to 50% of the crystal con-tent, and, in contrast to the underlying units, granitic lithic clasts have only been observed in this unit. A minimum thick-ness of ~300 m is estimated for unit 4 and epithermal veins in the Mallines and Bahía Jara area are hosted by this unit.

Jurassic and Cretaceous intrusive rocks

The main intrusive rocks present in the district include a series of N-S–aligned subvolcanic domes that extend from Mallines to the Bahía Jara area (Figs. 2, 3). A volumetrically less important series of roughly NE-SW aligned domes also crop out from the Laguna Verde to the Cañadón Verde area. The N-S–aligned domes have an aphyric to fluidal porphy-ritic texture with phenocryst content generally increasing from south to north. Phenocrysts make up to 1 to 10% of the rock and include quartz (60–85% of crystals, ≤2 mm) and K-feldspar (15–40%, ≤4 mm; partly replaced by clay and quartz). The groundmass is mainly represented by devitri-fied (bow-tie texture) bands with central rotated, strongly embayed quartz grains (Fig. 3B), as well as spherulites, which have been replaced by K-feldspar. South of the Cerro Bayo dome, N-trending rhyolite dikes that are texturally similar to the Cerro Bayo dome crop out. Flow banding striking from 140° to 220° and dipping from 55°W to 64°E (right-hand rule convention applied to all structural data) is locally observed; however, the dominant orientation is 180°/~70°, parallel to the principal structure along which the domes are aligned. Up to 5-cm-wide 175°-striking quartz veins with fine-grained dis-seminated euhedral pyrite cut the dikes (Fig. 3D) whereas barite veins cutting the main Cerro Bayo dome have also been observed, indicating that domes and dikes were emplaced prior to the hydrothermal activity at Cerro Bayo.

A series of domes crop out between the Laguna Verde and the Cañadón Verde areas (Fig. 2) along an only poorly defined NE trend. A porphyric fluidal texture with 20% phenocrysts including quartz (45% of the crystals), K-feldspar (45%, partly replaced by clays ± sericite), and biotite (10%) pheno-crysts is present in the Cañadón Verde dome. In the domes near Laguna Verde light and dark bands can be observed in thin section, where the dark color is due to the presence of opaque minerals (Fig. 3C). The rock contains only about 1% largely unaltered phenocrysts consisting of 90% K-feldspar, 9% quartz, <1% biotite, immersed in a fluidal fine-grained potassium-rich matrix.

Geochronology of Igneous RocksLimited geochronological data are available in the literature

for the volcanic succession in the Cerro Bayo area. Suárez and De la Cruz (1997) present seven K-Ar biotite ages for the upper part of the Ibáñez Formation, ranging between 150 ± 4 Ma and 144 ± 3 Ma (Table 1), and De la Cruz and Suárez (2008) reported two SHRIMP U-Pb ages of 149.1 ± 3.2 (Ma llines area) and 141.9 ± 3.2 Ma (~5 km east of the Cerro Bayo dome). These ages all correspond to the Oxfordian-Kim-meridgian interval (Upper Jurassic). Suárez and De la Cruz (1997) also report a K-Ar biotite age of 132 ± 3 Ma (Berria-sian-Lower Cretaceous) for tuffaceous sandstone that is inter-preted as the basal level of the Toqui Formation. Apart from the age constraints for the Ibáñez Formation, three K-Ar ages are available for the Cerro Bayo dome (De la Cruz and Suárez, 2008). One analysis on feldspar gave an age of 111 ± 4 Ma, and the other two on whole rock gave ages of 92 ± 2 and 97 ± 2 Ma (Table 1). These are considered minimum ages because the feldspars and rocks that were analyzed are partly sericitized.

We here present four new zircon U-Pb ID-TIMS dates for intrusive rocks as well as one LA-ICP-MS date for the Ibáñez Formation. In addition, we also present four new 40Ar/39Ar biotite ages for the Ibáñez Formation in the Cerro Torta area (Fig. 5, Table 1). Analytical results for ID-TIMS fall on the concordia curve. The zircons yield interpreted ages of 146.50 ± 0.21 Ma for the Cerro Bayo dome (Fig. 5) and 146.3 ± 0.2 Ma for the Cerro Lápiz dome (Fig. 5). These ages are both based on the weighted averages of 206Pb/238U dates for four zircon grains for each dome. The zircons from the Laguna Verde and the Cañadón Verde domes gave ages of 82.6 ± 0.2 and 83.0 ± 0.2 Ma, respectively (Fig. 5, Table 1), and provide the first evi-dence for Late Cretaceous igneous activity in the study area. Minor inheritance is evident in one zircon grain of the Laguna Verde sample. The interpreted ages are based on weighted averages of 206Pb/238U dates for two and three zircon grains for the Laguna Verde and Cañadón Verde domes, respectively. To constrain unit 4 of the Ibáñez Formation, a sample of lithic fragment-poor rhyolite tuff was taken at Pampa la Perra imme-diately north of the plane of an ENE-striking normal fault. This sample yielded an age of 146.4 ± 1.1 Ma (LA-ICP-MS U-Pb on zircon), within error of the ages of the N-aligned rhyolite domes of Cerro Bayo and Cerro Lápiz.

Biotite fresh enough for 40Ar/39Ar was identified only in unit 4 from the Ibáñez Formation rocks of the Cerro Torta area in the southeastern part of the Cerro Bayo district (Fig. 3E). Samples JP-168 and JP-197 (Figs. 3E, 5) yielded statistically reliable plateau spectra, indicating minimal disturbance of the


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agun

a Ve

rde

134

± 3

Suár

ez a

nd D

e la

Cru

z, 1

997

CC

-331

Ig

nim

brite

K

-Ar

Bio

tite

4834

974

2739

21

Lag

una

Verd

e 14

4 ±

4 Su

árez

and

De

la C

ruz,

199

7C

C-3

28

Igni

mbr

ite

K-A

r B

iotit

e 48

3520

5 27

2878

L

agun

a Ve

rde

144

± 3

Suár

ez a

nd D

e la

Cru

z, 1

997

CC

-330

Ig

nim

brite

K

-Ar

Bio

tite

4835

307

2722

09

Lag

una

Verd

e 14

5 ±

3 Su

árez

and

De

la C

ruz,

199

7C

C-1

90

Igni

mbr

ite

K-A

r B

iotit

e 48

3634

6 28

5820

M

allin

es

146

± 4

Suár

ez a

nd D

e la

Cru

z, 1

997

CC

-93-

D

Igni

mbr

ite

K-A

r B

iotit

e 48

1919

4 27

6298

~2

0 km

S o

f Lag

una

Verd

e 14

9 ±

4 Su

árez

and

De

la C

ruz,

199

7C

C-4

7-2

Igni

mbr

ite

K-A

r B

iotit

e 48

5945

0 28

1100

N

orth

sho

re o

f Lak

e

G

ener

al C

arre

ra

150

± 4

Suár

ez a

nd D

e la

Cru

z, 1

997

CC

-191

Ig

nim

brite

K

-Ar

Bio

tite

Nea

r C

C-1

90

M

allin

es

132

± 3

Suár

ez a

nd D

e la

Cru

z, 1

997

CH

I-88

C

erro

Bay

o do

me

K-A

r K

-fel

dpsa

r 48

4122

1 28

1345

B

ahía

Jar

a 11

1 ±

2 D

e la

Cru

z an

d Su

árez

, 200

8C

HI-

88

Cer

ro B

ayo

dom

e K

-Ar

Who

le r

ock

4841

221

2813

45

Bah

ía J

ara

99

± 2

De

la C

ruz

and

Suár

ez, 2

008

CH

I-88

C

erro

Bay

o do

me

K-A

r W

hole

roc

k 48

4122

1 28

1345

B

ahía

Jar

a 97

±

2 D

e la

Cru

z an

d Su

árez

, 200

8

JP-6

5A

Cer

ro B

ayo

dom

e U

-Pb

Zirc

on

4841

563

2814

08

Bah

ía J

ara

146.

5 ±

0.21

T

his

stud

yJP

-78A

C

erro

Láp

iz d

ome

U-P

b Zi

rcon

48

3765

9 28

1524

M

allin

es

146.

3 ±

0.20

T

his

stud

yJP

-141

A

Lag

una

Verd

e do

me

U-P

b Zi

rcon

48

4177

1 27

0353

L

agun

a Ve

rde

82.6

±

0.20

T

his

stud

yJP

-112

A

Cañ

adón

Ver

de d

ome

U-P

b Zi

rcon

48

4283

9 27

4647

L

agun

a Ve

rde

83.0

±

0.20

T

his

stud

yJP

-72

Igni

mbr

ite

U-P

b Zi

rcon

48

3927

7 27

5367

B

ahía

Jar

a 14

6.4

± 1.

10

Thi

s st

udy

JP-1

67

Igni

mbr

ite

40A

r / 3

9 Ar

Bio

tite

4836

270

2858

88

Mal

lines

15

4.0

± 1.

50

Thi

s st

udy

JP-1

68

Igni

mbr

ite

40A

r / 3

9 Ar

Bio

tite

4836

275

2857

32

Mal

lines

14

9.74

±

0.72

T

his

stud

yJP

-194

Ig

nim

brite

40

Ar

/ 39 A

r B

iotit

e 48

3714

5 28

4938

M

allin

es

153.

27

± 0.

98

Thi

s st

udy

JP-1

97

Igni

mbr

ite

40A

r / 3

9 Ar

Bio

tite

4837

063

2850

26

Mal

lines

14

4.90

±

0.46

T

his

stud

y

CH

I-50

01

Bre

ccia

ted

quar

tz-

K-A

r A

dula

ria

4837

632

2819

48

Mal

lines

15

6 ±

5 D

e la

Cru

z an

d Su

árez

, 200

8

adu

lari

a ve

inC

HI-

50-1

Q

uart

z-ad

ular

ia v

ein

40A

r / 3

9 Ar

Adu

lari

a 48

3763

2 28

1948

M

allin

es

144.

2 ±

1 D

e la

Cru

z an

d Su

árez

, 200

8C

HI-

33

Qua

rtz-

adul

aria

vei

n K

-Ar

Adu

lari

a 48

4167

1 27

8767

B

ahía

Jar

a 14

5 ±

5 D

e la

Cru

z an

d Su

árez

, 200

8C

HI-

18C

B

recc

iate

d qu

artz

vei

n 40

Ar

/ 39 A

r A

dula

ria

4841

957

2775

00

Bah

ía J

ara

137

± 1.

4 D

e la

Cru

z an

d Su

árez

, 200

8C

HI-

20

Qua

rtz-

adul

aria

vei

n 40

Ar

/ 39 A

r A

dula

ria

4841

780

2810

34

Bah

ía J

ara

128.

5 ±

2.6

De

la C

ruz

and

Suár

ez, 2

008

CH

I-20

Q

uart

z-ad

ular

ia v

ein

K-A

r A

dula

ria

4841

780

2810

34

Bah

ía J

ara

128

± 9

De

la C

ruz

and

Suár

ez, 2

008

N/D

A

ltere

d ig

nim

brite

K

-Ar

Seri

cite

48

4190

0 72

9900

L

agun

a Ve

rde

114

± 3

Tipp

et e

t al.,

199

1N

/D

Alte

red

igni

mbr

ite

K-A

r W

hole

roc

k N

/D

N/D

L

agun

a Ve

rde

113

± 2

Tow

nley

, 199

7

JP80

Q

uart

z-ad

ular

ia v

ein

40A

r / 3

9 Ar

Adu

lari

a 48

3808

8 28

1316

M

allin

es (4

) 14

4.4

± 1.

1 T

his

stud

yJP

79B

Q

uart

z-ad

ular

ia v

ein

40A

r / 3

9 Ar

Adu

lari

a 48

3802

8 28

1381

M

allin

es (2

) 14

4.4

± 1.

6 T

his

stud

yJP

233

Qua

rtz-

adul

aria

vei

n 40

Ar

/ 39 A

r A

dula

ria

4837

795

2817

82

Mal

lines

(3)

142.

3 ±

1.6

Thi

s st

udy

JP19

0 Q

uart

z-ad

ular

ia v

ein

40A

r / 3

9 Ar

Adu

lari

a 48

4534

7 27

8781

R

ober

ta v

ein

133

± 1.

5 T

his

stud

yJP

247

Qua

rtz-

adul

aria

vei

n 40

Ar

/ 39 A

r A

dula

ria

4840

434

2785

52

Gua

naco

I v

ein

124.

9 ±

1.1

Thi

s st

udy

JP25

9 Q

uart

z-ad

ular

ia v

ein

40A

r / 3

9 Ar

Adu

lari

a 48

4147

5 27

7529

G

uana

co I

II v

ein

111.

7 ±

1.2

Thi

s st

udy

JP26

5 Q

uart

z-ad

ular

ia v

ein

40A

r / 3

9 Ar

Adu

lari

a 48

4112

9 72

9627

Ta

itao

111.

9 ±

2 T

his

stud

y

Coo

rdin

ates

are

giv

en in

UT

M z

one

19, e

xcep

t for

JP2

65, w

hich

falls

in z

one

18; d

atum

is P

SAD

56N

/D =

no

data

494 POBlEtE Et Al.

Ar systematics of the samples. They gave ages of 149.74 ± 0.72 and 144.90 ± 0.46 Ma, respectively. Sample JP-197 from Cerro Torta W directly underlies the Toqui Formation and represents the top of the Ibáñez Formation. Samples JP-194 and JP-167 gave ages of 153.27 ± 0.98 and 154.0 ± 1.5 Ma, respectively. The argon loss in samples JP-194 and JP-167 (Fig. 5F, H) pro-vides evidence for a later hydrothermal overprint leading to 40Ar loss in the biotite. Sample JP-194 was collected within 500 m from the Cascada vein (Fig. 2), providing a reasonable explanation for the localized thermal resetting due to hydro-thermal effects, although no detailed information on alteration and mineralization characteristics is available for this vein. The age of the hydrothermal perturbation is not well constrained, but, on the basis of the apparent ages of the lower-tempera-ture steps, it was probably younger than ~130 Ma.

Epithermal Veins, Mineralization, and Vein Adularia 40Ar/39Ar Geochronology

Mineralization in the Cerro Bayo district occurs in a large number of low to intermediate sulfidation epithermal veins associated with a quartz ± adularia ± calcite ± illite-smectite gangue assemblage within veins and as metric vein halos. Only limited fluid inclusion data are available from the literature (Townley, 1997; Pizarro, 2000), but the range in homogeniza-tion temperatures (123°–327°C) and salinities (0.79–4.29 NaCl equiv) is consistent with low sulfidation epithermal veins else-where (Simmons et al., 2005). The mineralization can be subdi-vided into four areas, which can be distinguished on the basis of structural setting, sulfide assemblages (Towney, 1997; Pizarro, 2000), clay alteration mineralogy (Poblete, 2011), and age.

82.6 ± 0.2 Ma

207Pb/235U

data-point error ell ipses are 2sigma

0.01315

0.01295

0.080 0.084 0.088

82

83

0.01275

84

0.01335

0.092

20

6P

b/2

38U

207Pb/235U

145.5

146.5


0.02306

0.02282

0.02290

0.02298

0.150 0.152 0.156 0.158 0.1600.154

207Pb/235U


0.0231

0.0230

0.0227

0.0228

0.0229

0.148 0.152 0.156 0.160 0.164

145

146

83.0 ± 0.2 Ma

207Pb/235U


0.01306

0.01302

0.01290

0.01294

0.01298

0.081 0.083 0.085 0.087 0.089

82.4

82.8

0.01286

83.2

20

6P

b/2

38U

D. Cañadón Verde Dome (JP-112A)

B. Cerro Lapiz Dome (JP-78A) 146.3 ± 0.2 Ma

A. Cerro Bayo Dome (JP-65A) 146.50 ± 0.21 Ma

C. Laguna Verde Dome (JP-141A)

0.15.10.150.154

153

151

149

147

145

143

141

139

206 P

b /

238 U

Age

(Ma)

Mean = 146.4 ± 1.1 [0.76%] 95% conf.Wtd by data-pt error only, 0 of 17 rej

MSWD = 5.6, probability 0.000(error bars are 2σ)

E. JP-72 - Pampa la Perra

Age

(Ma)

Cumulative 39Ar Fraction

Plateau steps are grey, rejected steps are black

Plateau age = 154.0 ± 1.5 Ma

(2σ, including J-error of 0.1%)

MSWD = 1.9, probability = 0.11

Includes 84.5% of the 39Ar

F. JP-167 - Cerro Torta E Unit 4160

120

140

0.0 0.2 0.4 0.6 0.8 1.0



Plateau age = 149.74± 0.72 Ma




240

80

160

0.0 0.2 0.4 0.6 0.8 1.00

G. JP-168 - Cerro Torta E Unit 4

Age

(Ma)







160

80

120

0.0 0.2 0.4 0.6 0.8 1.040

H. JP-194 - Cerro Torta W Unit 4







0.0 0.2 0.4 0.6 0.8 1.0

I. JP-197 - Cerro Torta W Unit 4

160

80

240

0

Fig. 5. Concordia diagrams for igneous intrusive and extrusive rocks. A) JP-65A (Cerro Bayo dome), B) JP-78A (Cerro Lápiz dome), C) JP-141A (Laguna Verde dome), and D) JP-112A (Cañadón Verde dome), with calculated ages. Gray ellipses reflect the 2σ errors of the individual grains. E) Plot of 206Pb/238U zircon ages for individual LA-ICP-MS analysis from sample JP-72 (unit 4) taken from Pampa la Perra (error bars are at ±2σ; rej = rejected). F, G, H, I) 40Ar/39Ar age spectra of biotite samples from Cerro Torta E JP-167 and JP-168, and from samples from Cerro Torta W JP-194 and JP-197.


These four areas are summarized below. Note that only very limited drill core intercepts of vein material or underground exposures were available for study and most of the mineraliza-tion information is compiled from literature sources (Townley, 1997; Pizarro, 2000) and Coeur D’Alene company data.

Mallines

Mallines is the area located in the southeastern part of the district (Figs. 2, 6) and is currently exposed at the highest topo graphic elevation (ca. 900 masl) of all areas discussed herein. With the exception of the Cascada vein (Fig. 3E), no resources large enough to warrant mining operations have been delineated in this area to date. Vein orientations vary considerably and four vein systems, each with differ-ent dominant orientations, were mapped (Fig. 6). The oldest veins, which are subvertical and strike 150° to 160°, corre-spond to system 1; system 2 veins crosscut system 1 and have a predominant 110° strike with subvertical to 60° dips with meter-scale sinistral dilatational jogs. System 3 veins cross-cut systems 1 and 2 (Fig. 7A) and have strike orientations of 165° to 180° with vein dips from vertical to 60° and dextral or sinistral offset up to 60 cm. System 4 (Fig. 7B), which is cut by system 3, while no conclusive temporal relationship to systems 1 and 2 has been observed, has subvertical E-W– to NE-striking vein orientations with unknown kinematics and offset.

Veins from system 1, with widths from 0.5 to 1 m, pres-ent mainly saccharoidal, drusy, and lesser gray quartz with minor lattice texture of quartz replacing tabular calcite. The hydrothermal alteration adjacent to the veins corresponds to silicification with illite, minor kaolinite and, locally, chlorite

(Poblete, 2011). Drusy quartz, regular boxwork (with hema-tite filling), and fine-grained sulfides are observed. System 2 veins have widths of up to ~1 m. Chalcedonic, saccharoidal, minor drusy and gray quartz, and locally crustiform textures including quartz and adularia are present in these veins. The alteration minerals are mainly illite, minor kaolinite, and silici-fication (Poblete, 2011). System 3 veins are between 0.2 and 10 m wide, with Veta Madre being the widest. They contain saccharoidal, chalcedonic, drusy, bladed, crustiform (quartz + adularia), and gray quartz. Veins have been affected by minor brecciation. Sulfides include highly corroded euhe-dral and subhedral disseminated pyrite and lesser arseno-pyrite, overgrown by proustite-tetrahedrite (Townley, 1997). Fine-grained disseminated pyrite is associated with gray and bladed quartz. The hydrothermal alteration adjacent to system 3 veins consists mostly of silicification accompanied by illite, smectite, and minor kaolinite (Poblete, 2011). All veins at Mallines have been oxidized at surface and sulfides are replaced by variable amounts of hematite, goethite, and jarosite, commonly as boxwork. Based on the evidence of oxi-dation, some of the kaolinite is interpreted as supergene with a lesser component of kaolinite formed in the steam-heated environment because some kaolinite is associated with chal-cedonic quartz and K-rich mica (illite).

From Mallines, one K-Ar age of 156 ± 5 Ma and one 40Ar/39Ar age of 142.1 ± 1 Ma, both on adularia, have been reported previously (De la Cruz and Suárez, 2008) for this area. In this study, three additional 40Ar/39Ar ages for vein adu-laria associated with crustiform texture were obtained (Fig. 8, Table 1, App. 1). Sample JP79B corresponds to a 113°-strik-ing vein (system 2) crosscutting 150°- to 160°-striking veins

555374

62

80

55

65

6555

55

80

80

6080

60

75

Veins + structures

80 Quartz veins / dipping

Normal faulting

Strike Slip faultingSilicification

Boxwork

Outcrop map around

Madre vein

Alteration in wallrock

Limonite

Illite + kaolinite (S3)

Illite - kaolinite - smectite (S2)

Illite ± kaolinite (S1)

Inferred fault

4838200

4838100

4838000

4837900

4837800

4837700

4837600

281900

282000

281800

281900

281800

Madre vein

System 4

N

0 50 m

Fig. 6. Detailed map of crosscutting veins and associated alteration assemblages in the Mallines area. Vein halos are color coded according to alteration assemblage in immediate wall rock. Note that all vein halos contain strong amount of silicifica-tion along with clay alteration assemblage; also note that alteration assemblages have been determined by X-ray diffraction and short-wave infrared reflectancy techniques in samples immediately adjacent to individual veins (Poblete, 2011).

496 POBlEtE Et Al.

(system 1). The age spectrum shows evidence for minor excess 40Ar but contains a reliable plateau and preferred age of 144.4 ± 1.6 Ma (Fig. 8). Sample JP233 was taken from a paragenetically late N-striking vein (system 3). A reliable plateau age of 142.3 ± 1.6 Ma is taken as the preferred age for this sample (Fig. 8). Sample JP80 was taken in the Veta Segunda area at Mallines and corresponds to a vein with a 015° strike orientation (system 4; Fig. 7C). The age spectrum for this sample exhibits eight individual fractions with ages between ~140 and ~145 Ma containing more than 80% 39Ar. The >140 Ma steps are bordered by younger-aged steps at low and high temperature. The spectrum overall has a slight

bell shape, but includes a small plateau containing three steps with equivalent ages that jointly define an age of 144.6 ± 1.5 Ma, which is the preferred age for this sample (Fig. 8).

Taking all ages together, it is evident that hydrothermal activity at Mallines was concentrated at about 144 Ma with the paragenetically younger veins possibly postdating the older veins by ~1 m.y., although the 40Ar/39Ar data do not per-mit distinguishing the two events on a strictly statistical basis. The adularia ages also indicate that the veins were emplaced slightly after the 146 Ma rhyolite domes at Cerro Bayo and Cerro Lápiz. The older K-Ar age reported by De la Cruz and Suárez (2008) is considered geologically meaningless because

N

Drusy quartz texture

Crustiform texture

Drusy quartz cross-cutting crustiform texture

E

DC

Adularia in crustiform texture

BAN

Fig. 7. Vein textures and crosscutting relationships of veins from throughout the district. A. Late N-S–striking system 3 veins crosscutting a system 2 vein at Mallines to the west of Cerro Bayo fault. B. Quartz replacing calcite showing lattice texture, taken as evidence for fluid boiling at Segunda vein, Mallines area. C. Crusti-form texture showing gray quartz, white quartz, and adularia from sample JP-80, taken from Mallines. This adularia was dated by 40Ar/39Ar. D. Sample JP-247 taken from Guanaco I vein before and after potassium staining, where the presence of adularia is indicated by the yellow stain; 40Ar/39Ar geochronology results for this sample are presented in Figure 9. E. Roberta vein (Brillantes) where crust-iform texture is observed, suggesting fluid boiling during mineral deposition (Dong et al., 1995). The first stage of crustiform quartz deposition is followed by a second stage of drusy quartz and post-mineralization dextral fault displacement (in blue dashed line).


it is older than the age of the host rocks. This sample may have been affected by excess 40Ar.

Bahía Jara

Several veins have been exploited at Bahía Jara, and some of the highest-grade veins of the district (e.g., Lucero vein) are located immediately west of the Cerro Bayo dome and the N-striking Cerro Bayo fault (Fig. 2). Here the veins have strike orientations of 315° to 330°, steeply dipping to the northeast, some displaying slickensides with sinistral strike-slip movement. Farther west, the Guanaco I-IV veins have similar strike orientations (135°–150°) but dip steeply to the southwest.

The veins near Cerro Bayo contain chalcedonic, drusy, and minor gray quartz. Locally, quartz intergrown with barite and quartz replacing tabular calcite (lattice texture) can be observed. Veins in the Guanaco area present gray, saccharoi-dal and crustiform quartz textures. In ore shoots, saccharoi-dal quartz and locally hydrothermal breccia, where the clasts consist of chalcedonic quartz and the cement to gray quartz, pyrite, and proustite, are the dominant vein infill. The sulfide assemblage of the Guanaco I vein (Townley, 1997) consists

of disseminated pyrite, overgrown by and intergrown with sphalerite, the latter containing chalcopyrite inclusions. Dis-seminated anhedral bornite occurs in fractures; bornite is partly replaced by covellite along cleavage planes and may be of supergene origin. Arsenopyrite is rare, only present as subhedral disseminated grains, while native silver and gold are present as small (<20 μm) anhedral and subhedral grains. Throughout the Bahía Jara area, late coarse-bladed barite is common either as late vein infill or as infill of NE- to E-strik-ing fractures.

Two samples have been dated from Bahía Jara. Sample JP247 (Fig. 7D) corresponds to the Guanaco I vein. A pla-teau age of 124.9 ± 1.1 Ma with a mean standard weighted deviation (MSWD) of 2.1, based on six steps and 53.4% 39Ar, was defined. Sample JP259 was taken from the Guanaco III vein. Ages of the individual heating increments are stepwise increasing at low and middle temperatures but attain a three-step plateau of 111.7 ± 1.2 Ma and a slightly older age of 124.9 ± 3.1 Ma for the last heating step. This age spectrum is less reliable than all others and may reflect thermal resetting of a 125 Ma age (i.e., similar to Guanaco I) during a hydrothermal pulse at ~111 Ma (Fig. 8).

For the Guanaco and Cerro Bayo area, additional samples were dated by De la Cruz and Suárez (2008). For the Gua-naco area, they obtained an adularia K-Ar age of 145 ± 5 Ma and a 40Ar/39Ar age of 137.0 ± 1.4 Ma, whereas 1 km north of Cerro Bayo dome a K-Ar age of 128 ± 9 Ma and a 40Ar/39Ar age of 128.4 ± 2.6 Ma were reported for the same adularia sample.

Brillantes

The Brillantes area is located in the northern part of the dis-trict (Fig. 2) and contains, from east to west, the Constanza, the Roberta, the Francisca, and the Brillantes veins hosted by units 1 and 2 of the Ibáñez Formation. To date, no precious metal production has come from these veins. The Roberta vein (Fig. 7E) strikes 190° and dips 75°, with an average width of 20 cm at surface. Observed mineralization at surface (500 masl) consists mainly of fine-grained disseminated sub-hedral pyrite crystals associated with chalcedonic quartz and adularia (Fig. 7E). Galena, malachite, and azurite are present in the Constanza vein surface outcrops. The Roberta vein has been dated in this study (sample JP190). Vein adularia yielded a plateau age of 133.0 ± 1.5 Ma that is considered reliable (Fig. 8).

laguna Verde area

The Laguna Verde area occupies the northwestern portion of the district and contains a large number of veins (Fig. 2; Table 2). Veins here have a dominant northwesterly strike, although some mineralized structures have a northerly or a west-northwesterly strike. Hydrothermal brecciation was comparatively important and a significant portion of the min-eralization is breccia hosted. Vein textures are dominated by white saccharoidal quartz and gray banded quartz as well as calcite, adularia and, locally, fluorite (Townley, 1997; Pizarro, 2000). Bladed textures are only locally observed in the Cai-quenes vein system. Gold and silver are typically present as electrum or native silver or gold. Sulfide content generally increases with vein depth and includes pyrite, sphalerite,

Age

(Ma)

P A = 144.4 ± 1.6 MaIncl. 75.5% of the 39Ar

MSWD = 2.4

JP-79B-Mallines area System 2

300

200

0

100

Age

(Ma)


MSWD = 0.67

JP-233-Mallines areaSystem 3

80

0

240

160


MSWD = 1.3

JP-190-Roberta VeinBrillantes area

80

0

240

160

Age

(Ma)


MSWD = 2.1

JP-247-Guanaco I Vein Bahía Jara area

80

0

160



MSWD = 0.38

JP-259-Guanaco III VeinBahía Jara area

80

0.2 .4 .6 .8

160

Age

(M

a)


P A = 111.9 ± 2 MaIncl. 71.2% of the 39Ar

MSWD = 9.26

JP-265-Taitao system Laguna Verde area

120

0

40

.2 .4 .6 .8

200


MSWD = 2.2

JP-80-Mallines areaSystem 4

80

0

160

Fig. 8. Diagrams showing the step heating pattern of 40Ar/39Ar analyses on vein adularia. Ages and mean standard weighted deviations (MSWD) are indicated. All errors are given at the 2σ level. PA = plateau age.

498 POBlEtE Et Al.

chalcopyrite, and galena as well as proustite/pyrargyrite (Townley, 1997; Pizarro, 2000).

From Laguna Verde, a sample (JP265) from the Taitao vein system was dated. A plateau age of 111.9 ± 2 Ma is, despite the large MSWD of 9.26, taken as the preferred age, but, due to the evidence for excess argon, must be considered a maximum age (Fig. 8). The true age may be slightly lower. Tippet et al. (1991) reported a K-Ar age of 114 ± 3 Ma for intensely illite altered wall rock adjacent to a vein of the Taitao system, Laguna Verde (Tippet et al., 1991, in De la Cruz and Suárez, 2008).

DiscussionLow to intermediate sulfidation epithermal mineralization

at Cerro Bayo occurred periodically over more than 30 m.y., an age range that is based on the results presented herein but supported by data from earlier literature (Townley, 1997; De la Cruz and Suárez, 2008). The mineralization style through-out this period remained remarkably consistent, although subtle changes are evident. Early mineralization at Mallines, in the southeastern part of the district, occurred immediately after or concurrent with the final stages of the Ibáñez Forma-tion felsic volcanism, and a large part of the mineralization is hosted in veins located within 2 km of the major N-oriented Cerro Bayo fault, controlling felsic dome emplacement at ~146 Ma (Fig. 9). Mineralization following within 2 m.y. from felsic volcanism or dome emplacement is common in other low to intermediate sulfidation epithermal districts of Patago-nia or elsewhere (e.g., El Peñón: Arancibia et al., 2006; Man-antial Espejo: Wallier, 2009). Vein orientations at Mallines vary from W to N striking and are more variable than elsewhere in the district. Earlier veins have more westerly orientations whereas later veins have a dominant northerly strike (Fig. 6). Although structural mapping was limited, the small offset in vein-hosting structures might reflect a rearrangement of the stress field from an NE-SW extension to an E-W extension, which is in permissive agreement with the initiation of subsid-ence and E-W extension in the back-arc environment of the Aysén basin at that time (Suárez et al., 2009).

After an early episode of vein emplacement at Mallines, the focus of hydrothermal activity shifted north. Economically important mineralization occurred periodically between 137 and about 125 Ma and was concentrated in the Bahía Jara and Brillantes area to the north of Mallínes. At this time, a shal-low ocean may have covered the area, but marine sediment accumulation was apparently limited and no evidence for up to 200 m of limestone deposition as reported from farther north (Bussey et al., 2010) is present in the Cerro Bayo dis-trict. After an apparent lull between 125 and 112 Ma, epither-mal mineralization resumed in the western half of the Cerro Bayo district around Laguna Verde, west of Bahía Jara and Bri llantes (Fig. 9).

Veins have dominantly NW to N strike orientations and, in the Bahía Jara area, exhibit evidence for sinistral fault move-ment. These observations are consistent with the structural arrangement for other districts in the Deseado Massif, such as Huevos Verdes, where the NW-striking veins are interpreted as tension fractures and sinistral transtensional fault infill related to NNE-oriented regional sinistral faults (Dietrich et al., 2012).

In the Deseado Massif, epithermal Ag-Au mineralization postdates but closely follows large-scale magmatism of the Chon Aike LIP (Guido and Campbell, 2011). A general west-ward migration of the focus of magmatism is evident and is mimicked by the mineralization ages (Dietrich et al., 2012). The earliest mineralization documented in the Deseado Mas-sif consists of probably Early Jurassic vein-hosted polymetallic mineralization rich in indium (Jovic et al., 2011a), which, on the basis of geologic and alteration features, has a considerable magmatic fluid input and may be related to the earlier felsic volcanism resulting from widespread melting of the lower crust (Pankhurst et al., 2000). This is followed by Late Juras-sic mineralization, largely restricted to the Argentinian part of the Deseado Massif (Schalamuk et al., 1997; Wallier, 2009). Upper Jurassic silicic volcanic deposits exhibit an increas-ing subduction component, although the magmatism at this time was still widely distributed across the Deseado Massif

table 2. Summary of Ore and Alteration Mineralogy for Selected Veins from the Deep Eroded Laguna Verde Area

Vein system Elevation (masl) Mineralization Alteration Reference

Temer Sur vein 305 Py N/D Pizarro (2000)Temer Sur vein 280 Py N/D Pizarro (2000)Taitao I vein 305 Py-AgSS-El-Arg-Cpy-Aspy-Trh N/D Pizarro (2000)Taitao II vein 415 Py-Cpy-Sph-Ag-Au N/D Townley (1997)Cristal vein 480 Au-Ag-El-Hm-Lm N/D Townley (1997)Cóndor I vein 440 Py(Hm)-Ag-Au N/D Townley (1997)Cóndor I vein 354 Py N/D Pizarro (2000)Cóndor I vein 305 Py-Sph-Cpy-Trh N/D Townley (1997)Temer I vein 410 Ag(Lm)-Au(Lm) N/D Townley (1997)Temer I vein 370 Py(Hm)-Sph-Cpy-Ag-Au-<<Trh N/D Townley (1997)Temer vein 270 Py(Hm-Lm)-<<Ag-<<Au N/D Townley (1997)Breccia zone 425 Py(Lm-Hm)-Sph-Au-Ag-El N/D Townley (1997)Carla and Marisol sector 900 Py(Lm-Hm)-Au-Ag-El N/D Townley (1997)Caiquenes sector 530 Hm-Lm-Au-Ag-El N/D Townley (1997)Coihues sector diatreme (?) 420 Py(Hm-Lm)-Au-Ag N/D Townley (1997)Fabiola vein 297 Py-AgSS-Cpy N/D This studyDelia vein 180 N/D Silicification + illite This studyDelia vein 80 N/D Silicification + illite This study

Abbreviations: Ag = native silver, AgSS = silver sulfosalt, Arg = argentite, Aspy = arsenopyrite, Au = native gold, Cpy = chalcopyrite, El = electrum, Hm = hematite, Lm = limonite, N/D = no data, Py = pyrite, Sph = sphalerite, Trh = tetrahedrite


(Pankhurst et al., 2000). Early Cretaceous ages for epithermal veins are documented from the San José district in Argentina (Dietrich et al., 2012), located about 150 km east of Cerro Bayo, as well as from Mallines and El Faldeo in the Chilean Cordillera. Magmatism in the Cerro Bayo district apparently outlasted silicic magmatism elsewhere in the Deseado Mas-sif; somewhat younger mineralization ages between 137 and

124 Ma are restricted to Cerro Bayo and a number of pros-pects within 75 km to the south of it in the Chilean part of the Cordillera. Magmatism at this time is reported from the north-ern Aysén basin some 100 km north of Cerro Bayo, where tuff cones erupted in shallow marine basins (Suárez et al., 2010b). Epithermal and skarn mineralization emplaced in the middle Cretaceous (Aptian-Albian) is distributed along the Cordillera

4842000

4840000

4838000

4836000

4834000

4844000

4846000

4848000

282000

284000

286000

288000

280000

278000

276000

274000

272000

270000

268000

266000

264000

262000

4832000

Lake General Carrera

Laguna Verde

Mallines

Bahía Jara

LagunaVerde

4 km

Brillantes

Ibáñez Formation (UJz)

Toqui Formation (LK)

Meseta Chile Chico Lower Basalts (Eo) Subvolcanic Rhyolitic Domes (UJz)

Subvolcanic Dacitic Domes (UK)

Diorite and Gabbros (Eo)

Quaternary

Veins (UJz-LK) Fault

80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155

IBÁÑEZ FM.

RHYOLITIC DOMES

QTZ-ADULARIA VEINS

DACITIC DOMES

40Ar / 39Ar in biotite

40Ar / 39Ar in adularia

U - Pb in zircon

Ma

B

133.0 ± 1.5

144.4 ± 1.1 144.4 ± 1.1

111.9 ± 2

111.7 ± 1.2 124.9 ± 1.1

142.3 ± 1.6

144.4 ± 1.6 144.2 ± 1.0 (1)

128 ± 9 (1)145 ± 5 (1)

137 ± 1 (1)114 ± 3 (3)113 ± 2 (2)

128.4 ± 2.6 (1)

146.50 ± 0.21

146.3 ± 0.2

82.6 ± 0.2

83.0 ± 0.2

144.90 ± 0.46

153.27 ± 0.98

149.74 ± 0.72

154.0 ± 1.5

146.4 ± 1.1

N

A

Fig. 9. A. Simplified Cerro Bayo district map showing all adularia 40Ar/39Ar and K-Ar geochronology results for this study and literature sources (see text and Table 1 for details). Bold: this study. Italic: (1) De la Cruz and Suárez, 2008; (2) Townley, 1997; (3) Tippet et al., 1991. Pink and yellow rectangles correspond to zircon U-Pb ages for rhyolitic and dacitic domes, respectively, and green rectangles represent zircon U-Pb and biotite 40Ar/39Ar. UTM projection, Provisional South American Datum (PSAD), 1969, zone 19. Modified from Coeur South America. B. Summary of geochronological ages obtained in this study. Eo = Eocene, LK = Lower Cretaceous, UJz = Upper Jurassic, UK = Upper Cretaceous.

500 POBlEtE Et Al.

between about 44°30' and 47°30' (Townley, 1997; Townley et al., 2000; Fig. 10) whereas mineralization of this age is appar-ently absent farther east. Hauterivian- to Aptian-aged mag-matism is regionally represented by the Divisadero Group,

which consists of silicic subaerial ash flow deposits covering the sediments of the Lower Cretaceous Coihaique Group (De la Cruz and Suárez, 2008). This unit does not crop out in the Cerro Bayo district.

44°00’

15’

45’

45°00’

45’

15’

30’

46°00’

15’

30’

45’

47°00’

15’

30’

45’

71°00’45’ 15’30’72°00’15’30’45’73°00’15’30’

Non-consolidated deposits (Recent)

Permanent ice cover (Recent)

Legend

Main city or town

Symbols

10 0 10 20 30 40 50 km

Scale

44° - 48°S and 71° - 73°45’W

Skarn

Epithermal

Base metal polymetallic veins

Cu porphyry

Liquiñe-Ofqui Fault System

Fault

Structures and deposits of the Aysén region between

Deposit type

Metallogenetic epoch

1. From ca. 145 Ma to 141 Ma (UJ)

2. From ca. 137 Ma to 124 Ma (LK)

3. From ca . 116 Ma to 106 Ma (LK)

Chile Chico

Cochrane

Coihaique

Pto. Aysén

Pto. Cisnes

AR

GE

NT

INA

Pac

ific

Oce

an

Lake General Carrera

Ice

Fiel

ds

( )

El FaldeoUJ

Lago AzulUJ / LK

Lago ChacabucoUJ ?

Lago CochraneUJ ?

Halcones-LeonesLK

Rio AmarilloLK ?

Qda. ChicaLK ?

Cerro BayoUJ / LKMina Silva

and Manto Rosillo LK

Cord. La CampanaLK ?

El ToquiLK

KaterfeldLK

Lago AroLK ?

Sta. TeresaLK ?

Fig. 10. Metallogenetic map of the Aysén region, Chilean Patagonia. Three defined metallogenic epochs are indicated. The red line defines the deposits of the first, Late Jurassic metallogenetic episode, widespread in the Deseado Massif; the orange circles define the second metallogenetic episode (Lower Cretaceous); and the green lines define the third, also Lower Cretaceous metallogenetic episode. Modified from Townley and Palacios (1999). LK = Lower Cretaceous; UJ = Upper Jurassic.


Veins from all areas of the Cerro Bayo district present adu-laria, lattice-textured quartz, probably replacing calcite, and banded veins, which are taken as evidence for fluid boiling (e.g., Dong et al., 1995; Simmons et al., 2005). Subtle varia-tions in the style of mineralization and vein geochemistry through time are evident (Townley, 1997; Pizarro, 2000; Poblete, 2011). At Mallines the presence of arsenopyrite indi-cates a low sulfidation state of the ore mineral assemblage, whereas in the other areas sphalerite, galena, silver sulfosalts, and fahlore, together with variable amounts of chalcopyrite, indicate a low to intermediate sulfidation state. Vein textural and mineralogical evidence for boiling (Fig. 7B-E), together with limited fluid inclusion data from the literature (Town-ley, 1997; Pizarro, 2000), is consistent with shallow levels of emplacement of less than 1,000 m to as little as ~100 m, assuming hydrostatic conditions. However, stratigraphic and alteration observations suggest that Mallines is less eroded compared to Brillantes and Laguna Verde. Mallines is hosted in the stratigraphically highest unit 4 of the Ibáñez Formation and alteration in the wall rock adjacent to the veins contains dominantly smectite and kaolinite with lesser poorly crystal-lized illite. In contrast, the Laguna Verde and Brillantes veins are located in units 1 and 2 of the Ibáñez Formation and the clay mineralogy adjacent to the veins is dominated by well-crystallized illite (Poblete, 2011). Thus, there is no correlation between mineralization age and level of exposure. The old-est veins at Mallines are the least eroded, whereas the young-est veins at Laguna Verde are the most deeply eroded ones. This indicates that differential uplift along the NE-striking postmineral faults and subsequent erosion occurred after the mineralization at Laguna Verde, i.e., in the Late Cretaceous or later.

ConclusionsSilver-rich low to intermediate sulfidation epithermal min-

eralization at Cerro Bayo was emplaced episodically over a 33-m.y. period between ~144 and 111 Ma, corresponding to the earliest Cretaceous to middle Cretaceous. The oldest veins in the district coincide with late stages of epithermal vein emplacement in the western Deseado Massif, whereas subsequent mineralizing events are restricted to an arc-paral-lel belt at the active western South American continental mar-gin. Early vein mineralization was concurrent with the waning stages of silicic volcanism whereas subsequent mineralization is apparently unrelated to age-equivalent volcanic rocks in the Cerro Bayo district.

AcknowledgmentsThis article is largely based on the senior author’s M.Sc.

thesis, completed in 2011. JP wants to thank Coeur d’Alene Mines for funding this project and logistical support through-out. Don Birak, Alfredo Cruzat, and Claudio Romo from Coeur d’Alene Mines are acknowledged specifically for their help in making this project happen. JP would also like to thank the Society of Economic Geologists for a Student Research Grant from the Alberto Terrones L. Fund, which contrib-uted greatly to the results presented in this manuscript. The authors also thank the reviewers Steve Bussey, Leandro Echa-varría, and associate editor Raymond Jannas, who with their

valuable comments improved this manuscript. This is MDRU publication no. 325.

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