Mineralium Deposita (2001) 36: 45±57 Ó Springer-Verlag 2001

ARTICLE

Brian K. Townley á Collin I. Godwin

Isotope characterization of lead in galena from ore deposits

of the AyseÂn Region, southern Chile

Received: 10 July 1999 / Accepted: 15 July 2000

Abstract Lead isotope analyses of galena from ®ve ore 18.56, 207Pb/204Pb ˆ 15.63, 208Pb/204Pb ˆ 38.52. Aver-
deposits and six prospects in the AyseÂn region of southern ages for group 3, the southernmost group with the El
Chile are reported. Most of the deposits are either low Faldeo, Lago Chacabuco and Lago Cochrane pros-
sul®dation epithermal gold±silver veins or skarn and pects, are 206Pb/204Pb ˆ 18.83, 207Pb/204Pb ˆ 15.65,
manto deposits; the majority are either suspected to be, or 208
Pb/204Pb ˆ 38.63. The Cretaceous deposits (groups 1
dated as, Late Jurassic to mid-Cretaceous. Galena lead and 2) contain orogene-type lead that becomes increas-
isotope data for most of the deposits from southern Chile ingly radiogenic southward. Lead from the Late Jurassic
cluster near the ``orogene'' within a ``plumbotectonic'' deposits (group 3) appears to re¯ect mixing of orogene
model framework. Average values (206Pb/204Pb ˆ 18.53, lead with highly radiogenic lead. The observed linear ar-
207
Pb/204Pb ˆ 15.63, and 208Pb/204Pb ˆ 38.50) are near ray of lead in group 3 probably re¯ects mixing of orogene
Jurassic to Cretaceous model ages on the ``orogene'' curve lead with highly radiogenic lead, which was likely ex-
of Zartman and Doe (1981) and the second-stage curve of tracted by selective leaching of mineralizing hydrothermal
Stacey and Kramers (1975) on a 206Pb/204Pb versus solutions from the metamorphic basement.
207
Pb/204Pb plot. These model ages are compatible with
absolute ages as currently known. The elongate trends in
the general cluster indicate mainly an orogenic model ®t, Introduction
suggesting variable mixing of lead from di€erent sources,
mainly model upper crust and lesser model mantle and This paper presents galena lead isotope ratios of 25
lower crust reservoirs. Galena lead associated with one samples from ®ve ore deposits and seven prospects in the
deposit (El Faldeo) is relatively radiogenic, and lies near a AyseÂn region of southern Chile. A few additional galena
Jurassic age on the ``upper crustal'' curve of Zartman and lead isotope analyses were obtained from other sources.
Doe (1981), which is compatible with the Ar/Ar age of the Three of the deposits are currently active, signi®cant
deposit. Galena lead isotope clusters de®ne three main mines. Most of the deposits sampled in this study are
groups of deposits. These three groups appear to be re- either low sul®dation epithermal gold±silver veins and
lated to three mineralizing events, dated by K±Ar and Ar/ breccias, or skarn and manto deposits. Unclassi®ed
Ar, in the Late Jurassic (group 3), and in the Early and sul®de disseminations in breccia and base metal veins
mid-Cretaceous (groups 1 and 2 respectively). Averages were also sampled. The deposits sampled are either
for group 1, the northern group including El Toqui and suspected to be, or dated as, Late Jurassic to mid-
Katerfeld, are 206Pb/204Pb ˆ 18.51, 207Pb/204Pb ˆ 15.62, Cretaceous.
208
Pb/204Pb ˆ 38.48. Averages for group 2, the southern The AyseÂn region, shown in Fig. 1, is located in
group with Fachinal and Mina Silva, are 206Pb/204Pb ˆ southern Chile. It is ¯anked to the east by Argentina and
to the west by the Paci®c coast. Important towns in the
region include Coihaique and Puerto AyseÂn. Deposits in
Editorial handling: B. Lehmann the study area occur between latitude 44.46 and 47.25°S,
and longitude 71.35 and 72.46°W.
B. K. Townley (&)
Departamento de GeologõÂ a, Universidad de Chile,
P.O. Box 13518, Santiago, Chile
Geological setting and tectonic evolution
C. I. Godwin
Department of Earth and Ocean Science, The University
of British Columbia, Vancouver, British Columbia, The AyseÂn region (Fig. 1) consists of a Carboniferous to
V6T 1Z4 Canada Upper Triassic metamorphic basement. The basement is
46
 47
b evolved into a back-arc basin during the Lower Creta-
Fig. 1 Regional geology and location of deposits sampled for lead ceous (Baker et al. 1981; De Wit and Stern 1981).
isotope study of AyseÂn region, southern Chile (modi®ed from Early Cretaceous±Late Cretaceous rocks formed in a
Townley 1997). Geological units are described in Table 1; deposit
geology is in Table 2; and galena lead isotope data are in Table 3 wedge-shaped back-arc basin that widened to the south.
Volcanism di€ered from north to south. In the north,
andesite of calc-alkaline anity is interbedded with ma-
overlain discordantly by Upper Cretaceous to Tertiary rine sediments, but in the south, basalt of ophiolite anity
basalt, andesite and tu€ associated with lesser marine is observed (De Wit and Stern 1981). In the AyseÂn region
and continental sedimentary rocks. Paleozoic to Creta- the back-arc basin was a series of narrow, small marine
ceous rocks are intruded by stocks related to the mainly basins that are now represented by Coihaique Group
Upper Jurassic to Lower Cretaceous Patagonian marine sedimentary rocks interstrati®ed with calc-alka-
batholith. Minor Quaternary and active volcanism line andesitic volcanic rocks. Closure of the back-arc ba-
occurs in the area. Table 1 de®nes the regional geolog- sin in the AyseÂn region was accompanied and followed by
ical framework for the study area. Figure 1 shows the uplift of the main volcanic arc in the Late Cretaceous.
regional geology and locations of deposits studied. Intrusive activity associated with andesitic volcanics of
The Carboniferous to Upper Triassic metamorphic the Coihaique Group and mainly dacitic volcanics of the
basement represents accretion of a sedimentary wedge Divisadero Formation migrated slightly to the east. Many
along the western margin of the South American plate. mid-Cretaceous (~100 Ma) stocks intrude rocks of the
This terrain has been deformed by a series of orogenies IbanÄez Formation and the Coihaique Group.
(Miller 1984). Arc volcanism and uplift continued until the Late
Upper Jurassic±Lower Cretaceous basaltic andesite Cretaceous, represented by dacitic volcanic, felsic vol-
to andesite volcanic arc rocks of the IbanÄez Formation caniclastic, pyroclastic and epiclastic rocks of the
are rooted in granitic intrusive rocks of the Patagonian Divisadero Formation. Incipient extension in the Late
batholith. The volcanic arc formed in response to Cretaceous, triggered by lower crustal melting, followed
underthrusting of the Nazca plate below South America felsic calc-alkaline volcanism along the volcanic arc
(Bartholemew and Tarney 1984). The volcanics are bi- when volcanism shifted eastward (Baker et al. 1981).
modal. Basaltic andesite is more abundant in the west Late Cretaceous±Tertiary back-arc volcanism is char-
within the volcanic arc. Rhyolite, more abundant in the acterized by mainly tholeiitic basalt in some places int-
east where they are interbedded with andesite, is also erbedded with clastic marine and continental deposits.
more abundant in upper portions of the volcanic This activity ± represented by the Patagonian Plateau
sequence. Baker et al. (1981) proposed that the basaltic basalts and the Guadal and Galera Formations ± took
andesite and andesite originated by partial melting of the place from Late Cretaceous to Late Pliocene.
tholeiitic subducting slab and the mantle wedge, and The Mid- to Late Miocene (14 to 10 Ma) was marked
that the rhyolite originated by lower crustal melting by collision of the Chile Ridge, which was initiated by
associated with ma®c magma underplating. This lower subduction of a long segment of the ridge between 55 and
crustal melting process and trench-rollback (Davidson 48°S latitude. Three other segments of the ridge collided in
and Mpodozis 1991) triggered back-arc extension that Late Miocene to Pleistocene at 6, 3 and 0.5 Ma (Forsythe

Table 1 Table of geological formations for the AyseÂn Region in southern Chile (Fig. 1)

Unit Age Description

Strati®ed rocks
TraigueÂn Formation Neogene Welded tu€, tu€aceous breccia interstrati®ed with basalt,
and andesite and marine sedimentary rocks
Galera Formation Lower to mid-Miocene Continental sandstone and conglomerate interstrati®ed
with basalt of the Patagonian Plateau
Guadal Formation Upper Oligocene to Marine sandstone and fossiliferous shale interstrati®ed
Lower Miocene with Patagonian Plateau basalt
Patagonian Plateau basalt Upper Cretaceous Tholeiitic and minor calc-alkalic basalt and andesitic basalt;
to Recent lesser rhyolitic ¯ows and plugs and associated subvolcanic intrusives
Divisadero Formation Lower to Upper Welded tu€, tu€aceous breccia and subvolcanic intrusions of
Cretaceous mainly dacitic but also andesitic composition
Coihaique Group Lower Cretaceous Marine sediment, fossiliferous limestone, shale and
sandstone, and minor andesitic volcanics
IbanÄez Formation Upper Jurassic to Basaltic andesite to andesite and lesser dacitic to
Lower Cretaceous rhyolitic volcanics and epiclastic deposits
Metamorphic basement Carboniferous to Schist, phyllite, quartzite, slate, marble and metavolcanic rocks
Upper Triassic
Intrusive rocks
Patagonian batholith Upper Jurassic to Paleocene, Granite, granodiorite, diorite and gabbro
and Miocene
48

and Nelson 1985; Cande and Leslie 1986; Forsythe et al. ``plumbotectonic'' models of Zartman and Doe (1981)
1986). Wide fault zones separate these segments. and Doe and Zartman (1979). The position of the cluster
Recent and active volcanism (the Hudson, Maca and and the elongation trend of data stretch from the oro-
Mentolat volcanoes in the central-western part of Fig. 1) gene curve towards the upper crust curve, suggesting a
is superimposed on the Jurassic±Cretaceous arc and mixed source for lead ± as expected within an orogene.
back-arc. Volcanism is related to activity along the The cluster is near Jurassic to Cretaceous model ages on
coastal-trending LiquinÄe±Ofqui fault zone. the orogene curve, and the second-stage curve of Stacey
and Kramers (1975). These model ages are compatible
with absolute ages as currently known (Table 2). Aver-
Deposit geology age lead isotope values for the AyseÂn orogene in the
Jurassic to Cretaceous approximate (average of groups 1
Deposit locations are indicated in Fig. 1. Geology of each sampled
area is summarized in Table 2 and galena lead isotope data are and 2, Table 3): 206Pb/204Pb ˆ 18.53, 207Pb/204Pb ˆ
contained in Table 3. Active mine areas sampled, from north to 15.63, and 208Pb/204Pb ˆ 38.50.
south, are the following: Galena lead associated with part of the El Faldeo
1. Toqui (Tables 2 and 3, group 1, nos. 6 to 13) is a stratiform Au- deposit is relatively radiogenic (Table 3, group 3E:
rich massive sul®de replacement skarn manto hosted by co-
206
Pb/204Pb ˆ 19.11, 207Pb/204Pb ˆ 15.70, and 208Pb/
204
quinoid limestone and lesser veins hosted by dacitic volcanic Pb ˆ 38.73). This lead models as Jurassic on the up-
rocks. Past production and reserves ~10 million tonnes aver- per crustal curve in Fig. 2, which is compatible with the
aging 8% Zn, 0.6% Cu, 1.5% Pb, 1.5 ppm Au and 50 ppm Ag
(Palacios et al. 1994). age of the deposit (Table 2).
2. Mina Silva and Manto Rosillo (Tables 2 and 3, group 2, nos. 27
and 28) is a skarn±manto zinc-rich, but gold-poor, massive
sul®de marble and black schist replacement. Past production Lead isotope Groups 1 to 3
and reserves are uncertain, but the area has been worked since
the early 1940s.
3. Fachinal (Tables 2 and 3, group 2, nos. 17 and 18) is a low Two clusters and one linear array respectively, called
sul®dation epithermal base metal and gold±silver-rich vein and group 1, group 2 and group 3 (Tables 2 and 3), are
breccia deposit. Reserves are estimated at 18.7 million tonnes, apparent on the detailed plots of galena lead isotope
averaging 1.11 ppm Au and 32 ppm Ag (Tippett et al. 1991). data in Figs. 3 to 5. Group 1 (deposit nos. 1 to 13 and
Mineralization in the AyseÂn region consists of epithermal vein 16), the northern group, averages: 206Pb/204Pb ˆ 18.51,
207
and breccia deposits (all of the low sul®dation type), mesothermal Pb/204Pb ˆ 15.62, 208Pb/204Pb ˆ 38.48. Group 2
base metal vein deposits and base metal skarns. Porphyry associ-
ated mineralization of unknown potential has been described in
(deposits 14 to 15 and 17 to 29), the southern group,
Cerro Las Torres, Lago Azul and El Faldeo (Townley 1997). averages: 206Pb/204Pb ˆ 18.56, 207Pb/204Pb ˆ 15.63,
208
Pb/204Pb ˆ 38.52. Group 1 is slightly less radiogenic
than group 2. The division between the occurrence of
Galena lead isotope analyses group 1 and 2 deposits occurs near latitude 46°S. Av-
erages for group 3 (deposits 30 to 42), the southernmost
Galena lead isotope analyses (Table 3) from the AyseÂn region of group, are: 206Pb/204Pb ˆ 18.83, 207Pb/204Pb ˆ 15.65,
southern Chile were done mainly at the Geochronology Laboratory,
Department of Earth and Ocean Sciences, The University of British
208
Pb/204Pb ˆ 38.63. Group 3 data exhibit a linear trend
Columbia (for analytical details see Godwin et al. 1988). A small, with much variation that has been subdivided into
clean cleavage cube of galena (<0.1 g), handpicked under a binoc- groups 3A±E. Group 3A plots generally within the ®eld
ular microscope from each sample, was washed and dissolved in of groups 1 and 2, but groups 3B±E are relatively radi-
dilute hydrochloric acid. After evaporation to lead chloride, ~10± ogenic with values that plot progressively closer to the
25 ng of the lead was loaded on a single rhenium ®lament with
phosphoric acid and silica gel. Isotopic compositions were deter- upper crust growth curve. Group 3E plots beyond the
mined using a modi®ed Vacuum Generators Ltd. Isomass 54R solid- right-hand side of Figs. 3 to 5, and is near the Jurassic
source mass spectrometer. Measured ratios were normalized for point on the upper crustal curve in Fig. 2.
instrument mass fractionation based on replicate analyses of the
NBS-981 common lead standard. Reported ratios in Table 3 are
generally averages of at least two runs on a single, or sometimes
multiple, dissolution. The total standard variations observed in du- Characteristics of group 1
plicate analyses at 2r are generally less than 0.1% for 206Pb/204Pb,
207
Pb/204Pb and 208Pb/204Pb, and 0.05% for 207Pb/206Pb and Group 1 data (deposit nos. 1 to 13 and 16) exhibit a
208
Pb/206Pb. Lead isotopic data and relevant errors are presented in
Figs. 3 and 4.
trend (Fig. 3) that is steeper than a time-related growth
model curve. The trend of data extending from the
orogene growth curve towards the upper crust curve
Characteristics of galena lead isotopes from (Figs. 3 and 4) suggests a mixed source for lead.
the AyseÂn region Most of the deposits in group 1 consist of skarn,
mesothermal and epithermal veins, and gold-rich
Orogene lead polymetallic epithermal-type mineralization. Katerfeld
(Tables 2 and 3, deposit nos. 1 and 2) is a near-surface
Galena lead isotopes of AyseÂn region ores (Fig. 2) gold-rich polymetallic epithermal system. Mina Santa
cluster slightly above the ``orogene'' curve within the Teresa (deposit nos. 4 and 5) is a deep, fault-hosted,
 49

Table 2 Geology of deposits and description of samples analyzed for galena lead isotopes, AyseÂn Region, southern Chile. Locations of
sites are in Fig. 1; analyses are in Table 3

No. Mine or Deposit typea Mineralization Sample Host rock Ageb

prospect

Group 1
01, 02 Katerfeld Epithermal Hydrothermal breccia; Vein breccia Highly fractured Early Cretaceous:
Au±Ag-base quartz veinlets (01); quartz andesitic porphyry 97.9 ‹ 1.5 Ma
metals and stockwork vein (02) intruded into micro- (Ar/Ar, whole rock)
dioritic porphyry
03 Lago Aro Base metals in Quartz veins and Quartz vein Dacite and dacitic Early Cretaceous
quartz veins vein breccias (03) tu€
04, 05 Mina Santa Epithermal Quartz veins, and Quartz vein Fault hosted veins in Early Cretaceous(?)
Teresa quartz±Au±Ag- hydrothermal and (04); hydro- granodiorite and
base metal veins tectonic breccia thermal andesitic porphyry
veins breccia (05)
06, 07, Toqui: Base metal- Quartz veins and Quartz vein Dacites and dacitic Early Cretaceous
08 Zuniga quartz veins vein breccias (06, 07 tu€s
and 08)
09 Toqui: DonÄa Zn (Au) skarn Selective limestone Massive Limestone and Early Cretaceous:
Rosa replacement by sul®de (09) andesitic tu€ 108 ‹ 4,
massive sul®de intruded by felsic 100 ‹ 2 Ma (K±Ar
lenses and hydro- sills whole rock; Palacios
thermal breccias et al. 1994);
105 ‹ 3 (Ar/Ar
WR); 107 ‹ 18 Ma
(Ar/Ar Act.)
10, 11 Toqui: San Zn (Au) skarn Selective limestone Skarn miner- Limestones and Early Cretaceous:
Antonio replacement by alization in andesitic tu€s 108 ‹ 4, 100 ‹ 2
massive sul®de limestone intruded by (K±Ar WR, Palacios
lenses and hydro- (10 and 11) felsic sill et al. 1994),
thermal breccias 105 ‹ 3 (Ar/Ar
WR), 107 ‹ 18
(Ar/Ar Act.)
12, 13 Toqui: Base metal-quartz Quartz vein and Quartz vein Dacite and dacitic Early Cretaceous
Antolin vein vein breccia (12 and 13) tu€
16 Cordillera Base metal quartz Quartz veins and Quartz vein Epiclastic rocks and Early Cretaceous(?)
La veins hydrothermal (16) dacitic tu€s
Campana: breccia intruded by
Cerro granite
Negro
Group 2
14, 15 Rio Epithermal (?) Quartz veins, Calcite vein Basaltic to rhyolitic Early Cretaceous(?)
Amarillo Au±Ag-base hydrothermal (14 and 15) volcanics, dacitic
metal veins breccias, pebble tu€s and calcareous
and breccia dikes, rich epiclastic rocks
and calcite veins
17 Fachinal: Epithermal Quartz veins Quartz vein Dacitic and rhyolitic Early Cretaceous:
Horquetas Au±Ag-base (17) tu€, silici®ed 114 ‹ 3 Ma (K/Ar
metal veins conglomerates and WR, Tippett et al.
epiclastic rocks 1991), 113 ‹ 2 (Ar/
Ar WR)
18 Fachinal: Epithermal Quartz veins, breccia Quartz vein Dacitic and rhyolitic Early Cretaceous:
Caiquenes Au±Ag-base veins, hydrothermal (18) tu€, silici®ed 114 ‹ 3 Ma (K/Ar
metal veins veins and diatreme conglomerates and WR, Tippett et al.
breccia epiclastic rocks 1991), 113 ‹ 2 (Ar/
Ar WR)
19 Halcones- Epithermal (?) Quartz-calcite hydro- Hydrothermal Rhyolitic and dactic Late Jurassic±Early
Leones: Au±Ag-base thermal breccia and breccia (19) tu€s, epiclastic Cretaceous:
Est. La metal veins vein breccias rocks and silici®ed 130 ‹ 2 Ma
Pintura conglomerates (Ar/Ar WR)
50

Table 2 (Contd.)

No. Mine or Deposit typea Mineralization Sample Host rock Ageb

prospect

20 Halcones- Base metal Quartz vein, banded Quartz vein Andesite, dacitic tu€, Late Jurassic±Early
Leones: quartz veins quartz veins and (20) epiclastic rocks Cretaceous
Est. La hydrothermal and silici®ed
Calera breccia conglomerate near
a granodioritic
intrusion
21 Halcones- Base metal Quartz vein and Quartz vein Dacitic tu€, andesitic Late Jurassic±Early
Leones: quartz veins hydrothermal (21) breccia and Cretaceous
Est. La breccia conglomerate near a
Leona granitic intrusive
and a dacitic dome
22 La Poza Base metal, manto Stratabound Veinlet (22) Tu€, hematitic chert Early Cretaceous(?)
disseminated and and altered andesite
veinlet
mineralization
23 Paula. Unknown Unknown Unknown Volcanics (?) Early Cretaceous(?)
(23)
24, 25, Qda. Chica Epithermal Disseminated Rhyolitic tu€ Dacitic and rhyolitic Early Cretaceous(?)
26 Au±Ag-base mineralization in (24), hydro- tu€, and epiclastic
metal veins quartz veins, hydro- thermal rocks
thermal and breccia (25)
tectonic breccias and quartz
vein (26)
27, 28 Mina Silva Zn manto and Hydrothermal Hydrothermal Black and green Mid-Cretaceous
skarn breccias, quartz breccia schists, and marbles granite: 72 Ma
veins and massive (27 and 28) near a contact with (Hasegawa et al.
sul®de lenses granitic and monzo- 1971); 69 ‹ 4 Ma
granitic intrusions (Ar/Ar whole rock).
Monzogranite:
100 ‹ 3 Ma
(Toloza 1987)
29 Pto. Sanchez Unknown Unknown Unknown Shale and sandstone Early Cretaceous(?)
Group 3
Group 3A
30 Lago Chaca- Au±Ag and Quartz veins, Massive Black schist Host rock is Paleozoic.
buco massive sul®de hydrothermal sul®de Age of the veins
rich quartz veins breccias and vein (30) has not been
massive sul®de determined
veins
31 Lago Base metal rich Quartz veins Quartz Black schist Host rock is Paleozoic.
Cochrane quartz veins vein (31) Age of the veins has
not been determined
Group 3B
32, 33, El Faldeo Au±Ag and base Disseminated mineral- Hydrothermal Andesite, dacite, epic- Late Jurassic:
34 metal rich veins ization in hydro- breccia (32, lastic rocks and con- 161 ‹ 5 Ma
thermal breccias, 33 and 34) glomerate overlain (Ar/Ar whole rock)
and quartz-barite by black schist and
and quartz veins intruded by a
dacitic sill
Group 3C
35, 36, El Faldeo See group 3B See group 3B Veins (35, 36 See group 3B See group 3B
37 description description and 37) description description
Group 3D
38, 39 El Faldeo See group 3B See group 3B Hydrothermal See group 3B See group 3B
description description breccia (38 description description
and 39)
Group 3E
40, 41, El Faldeo See group 3B See group 3B Veins (40, See group 3B See group 3B
42 description description 41 and 42) description description
a
All epithermal mineralization is of the low sul®dation type
b
Dates without reference are unpublished data in Townley (1997)
 51

Table 3 Galena lead isotope analyses from deposits sampled in analyses, de®ned by groups, are plotted in Figs. 2 to 5. Only ita-
AyseÂn Region, southern Chile. Deposits are located in Fig. 1. licized analyses are used in the plots
Deposits and related samples are described in Table 2. Deposit

Deposit Sample no.a Deposit name 206

Pb/204Pb 207
Pb/204Pb 208
Pb/204Pb 207
Pbá100/206Pb 208
Pbá10/206Pb

Group 1 (averages of latitude and longitude are 45.08°S and 71.84°W)

01 BT080394-07 Katerfeld 18.517 15.625 38.445 84.384 20.763
02 BT080394-04 Katerfeld 18.560 15.674 38.589 84.450 20.791
AVG-K (n = 2) Katerfeld 18.539 15.650 38.517 84.417 20.777
03 BTLA-GOAR Lago Aro 18.497 15.613 38.410 84.410 20.766
04 BT221194-06 Mina St. Teresa 18.489 15.604 38.392 84.400 20.770
05 BT221194-05 Mina St. Teresa 18.481 15.600 38.394 84.405 20.780
AVG-R (n = 2) Mina St. Teresa 18.485 15.602 38.393 84.403 20.775
06 TOQUI-2b Toqui (Zuniga) 18.502 15.599 38.340 84.310 20.722
07 TOQUI-1b Toqui (Zuniga) 18.513 15.627 38.497 84.411 20.795
08 T-003 Toqui (Zuniga) 18.504 15.622 38.481 84.427 20.796
AVG-T (n = 3) Toqui (Zuniga) 18.506 15.616 38.481 84.383 20.771
09 T-004 Toqui (D. Rosa) 18.497 15.610 38.420 84.395 20.771
10 TOQUI-3c Toqui (S. Antonio) 18.534 15.642 38.539 84.396 20.794
11 T-005 Toqui (S. Antonio) 18.502 15.618 38.442 84.415 20.778
AVG-V (n = 2) Toqui (S. Antonio) 18.518 15.630 38.490 84.404 20.786
12 T-002 Toqui (Antolin) 18.522 15.639 38.511 84.435 20.792
13 T-001 Toqui (Antolin) 18.516 15.635 38.489 84.440 20.787
AVG-W (n = 2) Toqui (Antolin) 18.519 15.637 38.500 84.437 20.789
16 BT022494-02 Cord. La Campana 18.519 15.621 38.474 84.348 20.775
1 AVG-1 (n = 8) Group 1 (n = 8) 18.510 15.622 38.478 84.404 20.775
Group 2 (averages of latitude and longitude are 46.52°S and 72.03°W)
14 B119436 Rio Amarillo 18.571 15.629 38.523 84.159 20.743
15 B119436 Rio Amarillo 18.579 15.638 38.552 84.171 20.751
AVG-A (n = 2) Rio Amarillo 18.575 15.634 38.538 84.165 20.747
17 BT160494-29 Fachinal (Horquetas) 18.548 15.635 38.516 84.293 20.765
18 BT160494-28 Fachinal (Caiquenes) 18.542 15.632 38.509 84.305 20.768
19 B117785 La Pintura 18.564 15.642 38.542 84.259 20.762
20 BT270294-01 Est. La Calera 18.553 15.636 38.508 84.274 20.756
21 B117813 Est. La Leona 18.560 15.643 38.549 84.282 20.770
22 POZAd La Poza 18.555 15.624 38.508 84.204 20.753
23 PAULAb Paula 18.570 15.630 38.520 84.168 20.743
24 B119461 Qda. Chica 18.542 15.625 38.474 84.266 20.750
25 B119465 Qda. Chica 18.549 15.633 38.499 84.279 20.756
26 B119465 Qda. Chica 18.551 15.635 38.509 84.281 20.759
AVG-Q (n = 3) Qda. Chica 18.547 15.631 38.494 84.275 20.755
27 BT151094-20 Mina Silva 18.549 15.629 38.500 84.261 20.756
28 BT151094-17 Mina Silva 18.549 15.630 38.501 84.264 20.756
AVG-S (n = 2) Mina Silva 18.549 15.630 38.501 84.263 20.756
b
29 Sanchez Pto. Sanchez 18.550 15.620 38.480 84.205 20.744
2 AVG-2 (n = 11) Group 2 18.556 15.632 38.515 84.245 20.756
Group 3e (averages of latitude and longitude are 47.20°S and 72.28°W)
Group 3A
30 BT271194-01 Lago Chacabu 18.522 15.647 38.601 84.477 20.840
31 TOQUI-4b Lago Cochran 18.510 15.640 38.600 84.495 20.854
3A AVG-B,N and 3A (n = 2) Group 3A: Faldeo 18.516 15.644 38.601 84.486 20.847
Group 3B
32 62040f El Faldeo 18.530 15.634 38.555 84.371 20.807
33 62555f El Faldeo 18.517 15.627 38.537 84.393 20.812
34 FALDEO1 El Faldeo 18.508 15.627 38.526 84.436 20.816
3B AVG-D and 3B (n = 3) Group 3B: Faldeo 18.518 15.629 38.539 84.400 20.811
Group 3C
35 62656f El Faldeo 18.577 15.643 38.606 84.206 20.782
36 62640f El Faldeo 18.567 15.642 38.623 84.246 20.802
37 62856f El Faldeo 18.573 15.647 38.609 84.246 20.788
3C AVG-E and 3C (n = 3) Group 3C: Faldeo 18.572 15.644 38.613 84.233 20.790
Group 3D
38 62577f El Faldeo 18.627 15.634 38.562 83.932 20.702
39 62056f El Faldeo 18.612 15.640 38.742 84.032 20.816
3D AVG-X and 3D (n = 2) Group 3D: Faldeo 18.620 15.637 38.652 83.982 20.759
52

Table 3 (Contd.)

Deposit Sample no.a Deposit name 206

Pb/204Pb 207
Pb/204Pb 208
Pb/204Pb 207
Pbá100/206Pb 208
Pbá10/206Pb

Group 3E
40 22676f El Faldeo 19.236 15.703 38.826 81.633 20.184
41 22076f El Faldeo 19.088 15.688 38.691 82.188 20.270
7 62076f El Faldeo 19.006 15.694 38.668 82.574 20.345
3E AVG-Y and 3E (n = 3) Group 3E: Faldeo 19.110 15.695 38.728 82.132 20.266
a c
Analyses by A. Pickering, Geochronology Laboratory, Depart- Puig (1988)
d
ment of Earth and Ocean Sciences, The University of British Co- Zentilli et al. (1988)
e
lumbia Analyses in group 3 with similar ratios are grouped into 3A to 3D
b f
Puig (1990) Palacios et al. (1996)

polymetallic epithermal system. The largest producing The group 1 deposits are hosted by a broad strati-
mine within the study area, the Toqui gold±zinc-rich graphic range of andesites, andesite porphyries and
skarn±manto and vein (deposit nos. 6 to 13), belongs to dacites of the IbanÄez Formation, by granodioritic in-
this group. trusions of the Patagonian batholith, by limestones and

Fig. 2 General 206Pb/204Pb versus 207Pb/204Pb and 206Pb/204Pb versus

208
Pb/204Pb plots of galena lead isotopes from deposits in southern Fig. 3 Detailed 206Pb/204Pb versus 207Pb/204Pb and 206Pb/204Pb versus
Chile. Plumbotectonic framework trends are plotted for reference 208
Pb/204Pb plots of galena lead isotopes from deposits in AyseÂn
(upper crust, orogene, lower crust and mantle: Stacey and Kramers region, southern Chile (Table 3). Group 1 (circles), from the northern
1975; Zartman and Doe 1981). Deposit data from Table 3 plot within part of the study area, plots along a distinct line that distinguishes it
a narrow ®eld along the orogene. Exception: El Faldeo (black from the more equant group 2 cluster (squares) that occur in the
triangle). Symbols along each growth trend represent the base of the southern part of the studied area (Fig. 5). Group 3 (triangles), from
following time intervals: n Present day, r Tertiary, k Cretaceous, j the southernmost part of the studied area (Fig. 5), crosses the top of
Jurassic, t Triassic, p Permian, b Carboniferous, d Devonian, s the ®gure and extends beyond it to the right (Fig. 2). Crosses locate
Silurian, o Ordovician, and c Cambrian the arithmetic mean for each data group
 53

their usual close spatial and temporal relationship to the

Patagonian batholith.

Characteristics of group 2

Group 2 (deposits nos. 14 and 15, and 17 to 29) data

have a similar trend in Fig. 3 to group 1. The trend and
position to the right of group 1 is probably caused by
mixing, similar to group 1, but with a larger upper
crustal component, and possibly, a slightly younger age.
Group 2 is represented mainly by gold-rich polyme-
tallic epithermal vein type mineralization. The Mina Silva
deposit is a zinc (with lead, copper and silver) skarn±
manto deposit. It does not contain economic amounts of
gold. This suggests that deposits of group 2 occurred ei-
ther as high-level gold-bearing systems, or as gold-poor
base metal systems at depth. Producing mines of this
group are the Mina Silva zinc skarn±manto (Tables 2 and
3, deposit nos. 27 and 28) and the Fachinal epithermal
gold±silver vein-breccia (deposit nos. 17 and 18).
Fig. 4 Detailed 207Pb/206Pbá100 versus 208Pb/206Pbá10 plot of galena Group 2 deposits are (1) hosted by rhyolitic and da-
lead isotopes from deposits in AyseÂn region (Table 3). Group 1 citic tu€s of the Divisadero Formation, (2) at contacts
(circles), from the northern part of the study area, plots in a tight between andesites or andesitic basalt of the IbanÄez
cluster that distinguishes it from the more elongate cluster of group 2 Formation with rhyolitic tu€s of the Divisadero For-
(squares) from the southern part of the studied area (Fig. 5). Group 3
(triangles), from the southernmost part of the studied area (Fig. 5), mation, (3) hosted by dacitic and rhyolitic tu€s of the
crosses the bottom of the ®gure and extends beyond it (Fig. 2). IbanÄez Formation, (4) at contacts between dacitic tu€s
Crosses locate the arithmetic mean for each data group of the Divisadero Formation and granites of the Pata-
gonian batholith, and (5) hosted by marbles and schists
of the metamorphic basement spatially related to gran-
by andesites of the Coihaique Group (Tables 1 and 2).
ites and felsic dikes of the Patagonian batholith (Ta-
This sequence represents a Late Jurassic±Cretaceous
bles 1 and 2). Thus, deposits of this group occur within
volcanic arc to back-arc pair in the northern part of the
rhyolitic and dacitic volcaniclastic and epiclastic rocks
study area. Another feature of the deposits of group 1 is
of the volcanic arc, and are spatially close to and within
rocks of the metamorphic basement. The deposits of
group 2 are at, or south of, Lake General Carrera
(Fig. 1; 46°S latitude).

Characteristics of group 3

Group 3 deposits (nos. 30 to 42) are characterized by a

long linear trend with the relatively low slopes for
206
Pb/204Pb versus 207Pb/204Pb (Fig. 3) and the
207
Pb/206Pbá100 versus 208Pb/206Pbá10 (Fig. 4). The slopes
of linear trends in the plots of Figs. 3 and 4 are charac-
teristic of linear arrays associated with lead in galena from
many carbonate-hosted lead±zinc deposits (Heyl et al.
1974; Godwin et al. 1982; Crocetti et al. 1988).
Group 3 deposits appear to be associated with the
earliest stages of volcanic arc development where dacitic
volcanic and subvolcanic complexes intrude and are in
close proximity to the metamorphic basement. They also
can be hosted by rocks of the metamorphic basement.

Fig. 5 Latitude shift in galena lead isotope data (206Pb/204Pb) from Latitudinal variations in galena lead isotopes
AyseÂn region, southern Chile (Table 3). The horizontal trend of
group 3 (triangles) extends to markedly radiogenic values beyond the
right side of the ®gure (Fig. 2). Crosses locate the arithmetic mean for Variation of galena lead isotopes with latitude are
each data group plotted in Fig. 5. Di€erences among groups 1 to 3 are
54

apparent. Group 1 deposits, the least radiogenic, occur Coihaique Group. The origin of andesites of the IbanÄez
north of Lake General Carrera near latitude 45°S Formation are related to the evolution of the Upper
(Fig. 1). Group 2 deposits, which are slightly more Jurassic to Lower Cretaceous volcanic arc, rooted at
radiogenic than group 1, generally occur south of lati- depth by the Patagonian batholith. On the basis of
tude 46°S (Fig. 1). Thus, there is a change in between the geochemical interpretations (Baker et al. 1981), these
50 km north±south distance from Lake General Carrera ma®c rocks are believed to have originated as a partial
to Cordillera La Campana. Lead isotopes become more melt of the tholeiitic subducted slab that was modi®ed
radiogenic southward in both groups. Group 3 has through open system fractionation and possible crustal
isotopic ratios that vary from the low values of groups 1 contamination. Partial melting of the mantle above the
and 2 (Figs. 3 to 5) to highly radiogenic values that plot subduction zone is considered less important (Baker
near the upper crustal curve (Fig. 2). All deposits in this et al. 1981).
group occur in the southernmost part of the study area Mineralization associated with the Cretaceous events
at an average latitude of 47.2°S. Analyses from the El has a mixed lead source. It is dominantly orogene in
Faldeo deposit, in the southern part of the study area, character re¯ecting varying proportions of the plumbo-
has highly variable lead isotope ratios (groups 3A±E) tectonic end members: mantle, lower crust and upper
that do not all plot on the latitude shift line (Fig. 5). crust. This mixed source is re¯ected by the location and
A comparison of latitudinal variations to the regional trend of data in Figs. 2 and 3. Puig (1988, 1990) also
geological setting (Table 2, Fig. 1) reveals more di€er- indicates this mixed source characteristic of lead in ores
ences. Group 1 deposits, in the northern part of the of the Chilean Andes, with data clustering and trends
study area, are hosted by the Late Jurassic±Early Cre- not unlike the results presented here. In addition,
taceous volcanic arc and have no known relationship to evidence of similar widespread crustal involvement in
the metamorphic basement. Speci®cally, they are hosted magmatism has been noted by Davidson et al. (1990)
mainly by ma®c volcanic rocks and ma®c intrusions of and Macfarlane and Petersen (1990). Hendquist and
the volcanic arc and back-arc marine sedimentary rocks Gulson (1992) and Mukasa et al. (1990) also argue for
of Early Cretaceous age. Group 2 deposits, in the intrusive, basement and host rock sources of lead iso-
southern part of the study area, are hosted by more felsic topes in hydrothermal systems.
volcanic rocks and are within or spatially close to the The linear trend in group 1 data in Fig. 3 might re-
metamorphic basement, which is widespread south of ¯ect magmatic incorporation of the metamorphic base-
Lake General Carrera. Group 3 deposits have galena ment and of rhyolites and felsic rocks formed by crustal
lead isotope values that overlap groups 1 and 2, but also melting. These sources could have contributed relatively
are highly radiogenic. They occur in, or are surrounded radiogenic lead, but ma®c magma underplating might
by, metamorphic basement. have contributed more primitive lead. The tectonic set-
ting indicates a broad range of potential sources for the
volcanic rocks. In general, Figs. 2 to 5 indicate mixing of
Discussion lead from, at least, the upper crust±mantle and the lower
crust±upper crust.
The three linear clusters of galena lead isotope data,
groups 1 to 3, re¯ect di€erences in the tectono-magmatic
evolution and geology of the deposits sampled in the Group 2
AyseÂn region. Similar clustering of lead isotopes to de-
®ne fundamental geologic provinces has been observed Group 2 deposits are associated most closely with epi-
in the central Andean Cordillera by Macfarlane et al. clastic rocks, and rhyolitic and dacitic volcaniclastic
(1990) and in the Chilean Andes by Puig (1988, 1990). rocks of the IbanÄez Formation and the Divisadero
Groups 1 and 2 re¯ect Cretaceous events, and group 3 Formation, or with intrusions of the Patagonian
re¯ects a Jurassic mineralizing event, based on limited batholith. They are restricted to the southern portions of
absolute radiometric dating and stratigraphic arguments the region, within the Late Jurassic±Cretaceous volcanic
(Table 2). Details about groups 1 to 3 are discussed arc. Group 2 deposits also are associated spatially with
below. the metamorphic basement, which is only known south
of latitude 46.15°S (Fig. 1).
The origin of rhyolites within the IbanÄez Formation
Group 1 and Divisadero Formation is related to the evolution of
the Upper Jurassic±Cretaceous volcanic arc. These rocks
Group 1 is restricted to the northern portions of the are believed to be the product of crustal fusion triggered
AyseÂn region, within the Upper Jurassic±Cretaceous by ma®c magma underplating (Baker et al. 1981). This
volcanic arc. Their characteristics indicate a close an- tectonic setting would imply melting of the lower crust,
ity with ma®c to intermediate volcanic and intrusive and widespread felsic volcanism and hydrothermal
rocks (andesites and andesitic porphyries, dacites, activity as a surface expression. The lead±lead mixing
granodiorites) of the IbanÄez Formation and the Pata- trends of Figs. 2 to 4, thus might appropriately re¯ect
gonian batholith, and limestones and andesites of the magmatic mixing of upper crust, lower crust and mantle
 55

components, consistent with a markedly orogenic plored±especially to the south of the town of Cochrane
source. Thus, groups 1 and 2 data are comparable both (Fig. 1).
in trend and clustering to galena lead isotopic ratios of
other Andean ores, indicated as mainly orogenic (Puig
1988, 1990). Latitudinal variations
Mineralizing events of groups 1 and 2 are not nec-
essarily separated in time, but temporal relationship of Latitudinal variations of lead isotope ratios probably
andesites with respect to rhyolites suggest the later are re¯ect di€erent degree and style of involvement of the
slightly younger than, and become dominant within, the metamorphic basement. North±south variations in
Divisadero Formation. groups 1 and 2 could be related to variable degrees of
magmatic assimilation of metamorphic basement.
Group 1 is associated with volcanic arc and back arc
Group 3 rocks that are well represented north of Lake General
Carrera. The metamorphic basement may be thin, lo-
The southern part of the study area is marked by com- cally absent or absent underneath this area. Groups 2
mencement in mid- to Late Jurassic of a volcanic arc and 3 are associated with the metamorphic basement
superimposed on the Paleozoic metamorphic basement. that is dominant south of Lake General Carrera.
This is represented by volcanic and epiclastic rocks of Group 2 trends probably re¯ect magmatic assimilation
the IbanÄez Formation. The oldest dated volcanic rocks of the metamorphic basement. Similar latitude varia-
of this formation are Middle Jurassic and host the El tions in Andean Cenozoic volcanic centers have been
Faldeo prospect of group 3 (Table 2; 161 ‹ 5 Ma). related to di€erences in basement isotopic domains by
These volcanic rocks discordantly overly the metamor- WoÈrner et al. (1992) and McMillan et al. (1993).
phic basement. Thus, this early stage of activity of the Group 3 has one relatively non-radiogenic end-member
volcanic arc is associated with polymetallic, mainly zinc that is similar to values of groups 1 and 2. However, it is
and lesser gold mineralization. However, the lead iso- marked by another relatively radiogenic end-member
topes in the El Faldeo deposit vary widely and form a that is probably related to mixing with markedly radi-
linear array of low slope on 206Pb/204Pb versus ogenic lead that has been selectively and hydrothermally
207
Pb/204Pb plot (Fig. 3) and the 207Pb/206Pbá100 versus leached from, most likely, the adjacent metamorphic
208
Pb/206Pbá10 plot (Fig. 4). Long linear arrays of low basement.
slope can be interpreted as mixing in which the lead end-
members are (1) relatively non-radiogenic orogene lead,
and (2) markedly radiogenic lead obtained by selective Conclusion
leaching of the peripherally associated metamorphic
basement. This interpretation is consistent with ®eld Plots in Fig. 2 of galena lead isotopes for most of the
observations and ore deposit modeling for the El Faldeo deposits from southern Chile within plumbotectonic
district. Two di€erent types of mineralization are inter- models (Doe and Zartman 1979; Zartman and Doe
preted, porphyry related Zn±Pb±Cu-skarn and meso- 1981) cluster near the orogene. Average values
thermal vein, overprinted by Au±Ag-rich polymetallic (206Pb/204Pb ˆ 18.53, 207Pb/204Pb ˆ 15.63, and 208Pb/
204
epithermal mineralization (Townley 1997). Samples Pb ˆ 38.50) are near Jurassic to Cretaceous model
from the ®rst type of mineralization have lead isotopic ages on the ``orogene'' curve of Zartman and Doe (1981)
ratios within the range of groups 1 and 2, whereas and the second-stage curve of Stacey and Kramers
samples from epithermal mineralization have more (1975) in Fig. 2. Elongation trends in the general cluster
radiogenic and variable lead isotopic ratios. This second indicate variable mixing of lead from mantle, upper
type of mineralization is compatible with a concept of crust and lower crust reservoirs ± as expected within an
hydrothermal leaching of lead from rocks of the meta- orogene. This same conclusion has been indicated for
morphic basement, which underlie the hosting Late other ores of the Chilean Andes, but di€ers markedly
Jurassic volcanic and sedimentary rocks. In addition, from Argentinian ores (Puig 1988, 1990). Galena lead
such radiogenic lead enrichment has been observed in associated with part of the El Faldeo deposit is relatively
many studies (e.g. Russell and Farquhar 1960; Heyl radiogenic (206Pb/204Pb ˆ 19.11, 207Pb/204Pb ˆ 15.70,
et al. 1974; Crocetti et al. 1988), and this process prob- and 208Pb/204Pb ˆ 38.73).
ably occurs because radiogenic lead isotopes, produced Detailed plots reveal that three main groups of de-
by decay of large interstitial U and Th atoms, are more posits may be discerned based on galena lead isotope
easily available to ore ¯uids (especially because the studies: groups 1 to 3. These three groups appear to be
lattices surrounding radioactive sites become damaged related to three mineralizing events, one in the Late
by radiation; see Sinclair 1968) than the tightly held Jurassic (group 3), and two in the Early Cretaceous
stable 204Pb atoms (Godwin et al. 1982, 1988). The (groups 1 and 2). The Late Jurassic event (group 3) is
group 3 deposits have distinctive lead isotopes, and related to the initial development of volcanic arc evo-
indicate potential for mineralization within the meta- lution. The Cretaceous mineralizing events (groups 1
morphic basement, which is widespread and unex- and 2) are isotopically distinct because of di€erent
56

degrees and types of mixing with crustal rocks, as in- Canada), and C. Palacios, A. Lahsen and M.A. Parada (Departa-
dicated by trends in lead±lead data and latitudinal shift. mento de GeologõÂ a, Universidad de Chile, Chile) for support and
encouragement throughout the project. A. Pickering, J. Gabites,
Speci®cally, group 1 deposits are (1) generally less J. Mortenson and C. Godwin were involved with the analysis of the
radiogenic than group 2, (2) restricted to the northern samples submitted to the Geochronology Laboratory, Department
area of the region, (3) hosted within rocks of a well- of Earth and Ocean Science, The University of British Columbia,
developed volcanic arc-back arc system, and (4) exhibit Canada. C. Palacios provided the analyses from the Faldeo area;
these analyses were done by the University of Alberta, Alberta,
a highly mixed source for lead involving, orogene, Canada. Field, analytical and personal support was obtained from
upper and lower crust and mantle lead. In contrast, Proyecto Fondef (MI-15), Facultad de Ciencias FõÂ sicas y Mate-
group 2 deposits are (1) generally more radiogenic than maÂticas, Universidad de Chile, Santiago, Chile. E€orts by review-
group 1, (2) restricted to the southern area of the re- ers of this paper are gratefully acknowledged.
gion, (3) hosted within felsic volcanic rocks of the arc
system or within metamorphic basement, and (4) al-
ways close spatially to the metamorphic basement. The References
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