porfidosdecerroverdeysantarosa-160504205640

Economic Geology
Vol. 98, 2003, pp. 1683–1696
SCIENTIFIC COMMUNICATIONS
40
Ar-39Ar AGES OF HYPOGENE AND SUPERGENE MINERALIZATION IN
THE CERRO VERDE-SANTA ROSA PORPHYRY Cu-Mo CLUSTER, AREQUIPA, PERU
CHAN X. QUANG,† ALAN H. CLARK, JAMES K.W. LEE,
Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, ON K7L 3N6, Canada
AND JORGE
GUILLÉN B.
Sociedad Minera Cerro Verde S.A., Asiento Minero Cerro Verde-Uchumayo, Avenida Alfonso Ugarte 304, Casilla 299, Arequipa, Peru
Abstract
The contiguous Cerro Verde and Santa Rosa porphyry copper deposits are hosted by Paleogene granitoid
rocks and Precambrian gneiss, and spatially associated with 61 ± 1 Ma (U-Pb zircon: Mukasa, 1986) dacitic porphyry stocks. The age of hydrothermal activity is constrained by laser-induced incremental-heating 40Ar-39Ar
sericite (muscovite-2M1) dates of 61.8 ± 0.7 (2σ) and 62.0 ± 1.1 Ma for Cerro Verde, and 62.2 ± 2.9 Ma for
Santa Rosa, representing the terminal event in the Arequipa segment of the Coastal batholith.
The deposits crop out on the Santa Rosa erosional pediment, which itself is incised into the older La Caldera
surface. Two populations, of ages 36.1 to 38.8 Ma and 24.4 to 28.0 Ma, are identified by multiple analyses of a
sample from Cerro Verde comprising alunite partially replaced by natroalunite, demonstrating that supergene
activity had commenced by the latest Eocene, during the Incaic orogeny, thereafter continuing through the
Oligocene. In the Santa Rosa deposit, deep (ca. 300–350 m) leaching in the late Oligocene is recorded by ca. 26
Ma natroalunite that is inferred to have formed beneath the La Caldera surface. At the top of the Cerro Verde
pit (2738 m bench), veins of alunite (ca. 23 Ma) and natroalunite (ca. 21 Ma) in a hematitic leached zone are
truncated by the Santa Rosa surface, which is inferred to have developed after 21 Ma. Decreasing ages of alunitegroup minerals with increasing depth in the Cerro Verde pit (e.g., ca. 12 Ma at the 2648 m level, and 4.9–6.7 Ma
at the 2618 m level) are evidence for deepening of the supergene profile through the Miocene beneath this
pediment. Jarosite dates (0.7–1.3 Ma) record the persistence of minor supergene activity into the Pleistocene.
Introduction
The contiguous Cerro Verde and Santa Rosa deposits and
the neighboring Cerro Negro prospect constitute the northernmost demonstrably economic hydrothermal systems in the
central Andean upper Paleocene-middle Eocene porphyry
Cu-Mo belt, which parallels the South American plate boundary for at least 800 km in southern Peru and northern Chile
(Fig. 1a). Centered at latitude 16°33' S, longitude 71°34' W,
30 km southwest of the city of Arequipa, the Cerro Verde and
Santa Rosa open pits have been operated since 1994 by Sociedad Minera Cerro Verde, initially formed by Cyprus Amax
and now a subsidiary of Phelps Dodge Copper Corporation
(82.5%) and Compañía de Minas Buenaventura (9.2%). SXEW recovery attained 84,000 t of fine Cu at US $0.44/lb in
2001, and output was expected to rise to 86,900 t at US
$0.40/lb in 2002 (Ednie, 2002). Production, 70 percent from
Cerro Verde, is almost entirely from reserves of 331 Mt of supergene ore grading 0.52 percent copper, but the development of 464 Mt of largely hypogene material at 0.61 percent
copper is under consideration (Ednie, 2002).
The geology of the deposits is documented by Estrada
(1969, 1978), Kihien (1975), Le Bel (1985), Perea et al.
(1983), and Phelps Dodge (2000). Stewart (1968) provides a
detailed account of the “Caldera Complex,” the cluster of
granitoid intrusions that hosts much of the mineralization,
representing a segment of the Peruvian Coastal batholith
† Corresponding
author: e-mail, [email protected]
0361-0128/01/3398/1683-14 $6.00
(e.g., Pitcher et al., 1985). Age relationships in the district are,
in part, defined through U-Pb zircon (62–67 ± 1 Ma: Mukasa
and Tilton, 1985; Mukasa 1986) and Rb-Sr (68 ± 3 Ma: Le
Bel, 1985) dates for the precursor Yarabamba granodiorites
(Fig. 1b), and U-Pb zircon (61 ± 1 Ma: Mukasa, 1986) and KAr biotite (56-59 ± 2 Ma: Estrada, 1978) dates for the porphyry bodies most closely associated with hypogene activity.
However, no age data are recorded for the economically critical supergene oxide and sulfide mineralization.
In the present communication, we document new multistep, laser-induced 40Ar-39Ar dates for sericites directly associated with hypogene chalcopyrite-pyrite mineralization and
for supergene alunite-group minerals from both Cerro Verde
and Santa Rosa, the latter representing the first such data for
a porphyry deposit in southern Peru.
Geologic Framework
Host rocks and structural relationships
The Cerro Verde and Santa Rosa deposits crop out at elevations of 2,680 to 2,750 m a.s.l. on a subplanar pediment,
herein termed the Santa Rosa surface. This was eroded into
the older La Caldera surface (Jenks, 1948), which lies ca. 200
m higher (Fig. 2) and is now locally represented by isolated
summits, including Cerro Verde (2,904 m a.s.l.) and Cerro
Negro (2,910 m a.s.l.).
Whereas the Santa Rosa deposit (Fig. 1b) is hosted entirely
by Paleogene granitoid units, the Cerro Verde center straddles
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(b)
N 8172000
250
E 222000
0
LEGEND
500 m
Cerro Verde pit
A’
“Dacite Monzonite
Porphyry”
Tourmaline
Breccia
Yarabamba
Granodiorite
“Silica
Breccia”
Tiabaya
Granodiorite
Fault
A-A’
Pit Outline
in 2000
Location of
Dated Sample
Augite Diorite
•A
•
E 221000
N 8171000
SURF-111 ( sr, al, jr)
A - A’
••
•
••
SURF-109
SURF-114 ( na)
Yura Group
Charcani
Gneiss
al
na
jr
sr
Alunite
Natroalunite
Jarosite
Sericite
SURF-109 ( al,na, jr)
Santa Rosa pit
Arequipa
(a)
PERU
Chapi
Cuajone
SURF-110 ( sr, al )
200 km
CERRO VERDE
Quellaveco
Toquepala
BOLIVIA
SURF-119 ( sr)
SURF-113 ( na)
N 8170000
Arica
20°S
Cerro Colorado
CHILE
70°W
Spence
66°W
N 8169000
E 224000
SURF-112 ( na)
E 223000
Iquique
••
•
B
E 225000
B’
B - B’
FIG. 1. a. Locations of Cerro Verde-Santa Rosa and other Paleocene to middle Eocene porphyry Cu deposits of southern
Peru and northern Chile. B. Local geology of the Cerro Verde-Santa Rosa district, showing the locations of dated alunitegroup and sericite samples. Heavy lines show limits of panoramas in Figures 6 (A-A') and 8 (B-B'). Modified after Phelps
Dodge (2000).
El Misti
5822 m a.s.l. Cerro Verde
Santa Rosa deposit
2904 m a.s.l.
Cerro Verde deposit
Cerro Negro
2910 m a.s.l.
Looking NE (045°)
Cordillera Occidental
Dissected La Caldera Surface
Santa Rosa Surface
FIG. 2. Panoramic view, looking northeast (045°), of the Cerro Verde-Santa Rosa-Cerro Negro district, showing the Santa
Rosa pediment (in foreground), remnants of the La Caldera surface (accordant summits in middleground), and active and
dormant stratovolcanoes of the Cordillera occidental (skyline) in August, 2001. (NB. The term “La Caldera surface” reflects
the basinal local topography, with no volcanological implications). Approximate width of the foreground is 2 to 3 km.
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the contact between these and the Precambrian Charcani
gneiss. The latter comprises a series of amphibolite-facies
metasedimentary and metaigneous rocks, representing part
of the Mesoproterozoic (Wasteneys et al., 1995) Arequipa
Massif, which constitutes much of the Andean basement in
southern Peru and northernmost Chile. Each of the deposits
is associated with ca. 0.12-km2 steep-walled stock of hypabyssal quartz- and feldspar-phyric rock, traditionally
termed “dacite monzonite porphyry” or “quartz-bearing
monzonite porphyry” (Fig. 1b), but with quartz, alkali
feldspar, and plagioclase modal contents indicating a dacitic
composition. These are the youngest major intrusive units in
the district. However, a dike of postmineralization dacitic,
quartz-feldspar porphyry is exposed in the southern quadrant of the 2563 m level of the Cerro Verde pit, and a weakly
altered quartz porphyry exposed in the Santa Rosa pit may
represent the same late intrusive event. Tourmaline-cemented breccias are widespread only in the Cerro Verde
and Cerro Negro deposits, but small volumes of tourmalinefree “silica breccia” occur at all three centers (Fig. 1b).
Northwest-southeast– striking bodies of tourmaline-rich
granitic pegmatite and aplite widely cut all granitoid and
basement rocks, but their relationship to the tourmaline
breccias is uncertain.
The northwest-southeast elongation of the Santa Rosa
hydrothermal system (Fig. 1b), as well as the overall distribution of the Cerro Verde, Santa Rosa, and Cerro Negro deposits, represent a segment of a linear array of porphyry,
breccia, and vein copper deposits that extends at least to the
Chapi mine, 7 km to the southeast. This trend parallels a system of northwest-striking and steeply northeast dipping regional faults (Phelps Dodge, 2000), plausibly the northwestern equivalent of the major Incapuquio fault system in the
Cuajone-Quellaveco-Toquepala district, ca. 115 km to the
southeast (Fig. 1a). As at Toquepala (Zweng and Clark,
1995), shallow intrusion and mineralization at Cerro Verde
and Santa Rosa are considered to have been controlled by
the intersections of these northwest-southeast structures and
local northeast-southwest tensional faults. Widespread
northeast-southwest and east-west postmineral fractures and
tensional faults cut the northwest-southeast structures and
were associated with reverse reactivation (Fig. 1b; Phelps
Dodge, 2000).
Hypogene alteration-mineralization relationships
Hydrothermal alteration extends over a northwest-elongated, 5- by 1.5-km area in the Cerro Verde-Santa Rosa district, potassic and phyllic zones lying within a propylitic envelope. Potassic alteration, characteristically with a blotchy
development, is most extensively preserved at depth, but persists to shallower levels at Cerro Verde. Two main subfacies
are represented: orthoclase with lesser biotite (ca. 30%) and
magnetite (<5%) in Yarabamba granodiorite and the dacitic
porphyries; and biotite-magnetite, best developed in Charcani gneiss and andesite. In addition, pit exposures and drill
intersections at Cerro Verde reveal magnetite-biotite-albite
alteration, which may be characteristic of the deeper, lowgrade (0.1–0.15% Cu) subfacies of the early alteration. Magnetite-cemented hydrothermal breccias at Santa Rosa probably developed contemporaneously. Perea et al. (1983)
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estimate an average sulfide content of 3 percent in the potassic zones, with a chalcopyrite/pyrite ratio of ca. 3.
Phyllic (i.e., quartz-sericite-pyrite) alteration surrounds the
potassic zones in the upper portions of the deposits and
represents the major host of economic mineralization, with
average sulfide contents of 5 to 7 percent and a chalcopyrite/
pyrite ratio of 0.3 to 0.7 (Perea et al., 1983). The characteristic pervasive assemblage is best developed in Yarabamba granodiorite and the dacitic porphyries, but is also represented
in Charcani gneiss. The age relationships between the phyllic
alteration and the tourmaline breccia bodies in the Cerro
Verde deposit are uncertain. Clasts in these breccias exhibit
intense quartz > sericite, pyrite-free alteration, appearing silicified, as at Toquepala (cf. Zweng and Clark, 1995). The Bonanza breccia, the main body of silica breccia at Santa Rosa
(Fig. 1b), comprises angular fragments with intense sericite >
quartz alteration and disseminated chalcopyrite in a matrix
dominated by massive chalcopyrite and minor pyrite, magnetite, and ferberite.
Supergene mineralization
The earliest large-scale mining in the district was initiated
in 1968 by Mineroperú, who exploited brochantite-dominated oxide ores at Cerro Verde, averaging 1 percent Cu, but
with restricted zones exceeding 2 percent. The supergene
activity attained depths of over 300 m (from ca. 2,750 to
2,438 m a.s.l.) within the main body of tourmaline breccia,
probably a reflection of its high permeability, but the supergene profile is significantly thinner at Santa Rosa. In most
areas of the deposits, the main zone of chalcocite mineralization exhibits a very irregular and discontinuous distribution. Moreover, the southwestern sector of the Cerro Verde
pit reveals the presence of at least two sulfide enrichment
blankets: an older horizon, ca. 15 m thick, discontinuously
preserved on and above the 2648 m bench; and a younger,
more localized, but thicker zone located within a large body
of tourmaline breccia and juxtaposed with hypogene mineralization between the 2573 and 2633 m benches. This lower
blanket averages 60 to 80 m in thickness but increases to 100
m, and locally 150 m, in the main tourmaline breccia body.
In general, the supergene sulfide zone thins to the north and
northeast. At Santa Rosa, the single preserved blanket ranges
from 20 to 45 m thick, with an underlying transitional section
in which chalcopyrite is partially replaced by chalcocite, covellite, and bornite.
The chalcocite blankets at both Cerro Verde and Santa
Rosa are overlain by the brochantite subzone of the oxide
zone, in which minor chrysocolla occurs as veins cutting a
brochantite stockwork. Chalcedony, antlerite, and malachite
are minor constituents. The highest oxide-Cu grades occur in
the matrix of hydrothermal breccias, and the thickest development of such ores is in the eastern half of the Cerro Verde
deposit. The brochantite ores are, in turn, overlain by the
copper pitch subzone, in which Cu-Fe-Mn oxides predominate. The distribution of the oxide ores is erratic and commonly spatially associated with chalcocite-kaolinite or
hematitic zones. The uppermost part of the profile is the
leached zone, which averages 70 m in thickness, but locally
attains depths of 250 m in the Cerro Verde deposit. Hematite,
goethite, and minor jarosite are most abundantly developed
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in breccia zones. The widespread hematite is evidence for the
previous existence of chalcocite (Anderson, 1982), and relict
zones of chalcocite-kaolinite occur throughout the supergene
profiles. Jarosite is locally abundant over the originally more
pyritic margins of the two deposits.
(a)
Age (Ma)
100
(b)
SURF 110 , Sericite
Ar/40Ar
60
39
40
20
0.04
0.02
Integrated Age: 61.8±3.3 Ma
0
0.0
120
SURF 110 , Sericite
Correlation Age: 61.8±0.7 Ma
0.06
PA = 62.3±2.0
80
(c)
100
0
1.0
39
Fraction Ar
0
0.002
36
(d)
SURF 111 , Sericite
0.004
Ar/40Ar
SURF 111 , Sericite
Correlation Age: 62.0±1.1 Ma
0.06
PA = 61.3±2.7
80
0.04
60
40
0.02
20
Integrated Age: 56.2±5.5 Ma
0
0.0
100
(e)
1 0 0
0
1.0
0
SURF 119 , Sericite
0.10
0 . 1 0
PA = 61.2±1.4
8 0
0.002
(f)
9 0
80
0.004
SURF 119 , Sericite
Correlation Age: 62.2±2.9 Ma
0.08
0 . 0 8
7 0
60
6 0
39Ar/40Ar
Age(M
a)
Sampling and Geochronology
Samples rich in sericite were selected from the Cerro
Verde and Santa Rosa pits to determine the age of the preponderant hypogene Cu mineralization. Fine-grained (<50
µm) sericite is distributed throughout a matrix of quartz (±
tourmaline) and iron oxides with residual disseminated pyrite.
Pure separates, yielding muscovite(-2M1) X-ray powder diffraction patterns, were prepared for 40Ar-39Ar incrementalheating analysis. The locations, hypogene and supergene contexts, and inferred ages of the sericites are summarized in
Table 1, and the Ar-Ar analytical data are summarized as age
spectra and inverse isochron plots in Figure 3.
The occurrence of supergene alunite-group minerals at
Cerro Verde and Santa Rosa has been described by Kihien
(1975), Cedillo et al. (1979), and Cedillo and Wolf (1982).
Representative samples of alunite-group minerals were collected from different levels of the supergene profiles that
were superimposed on the phyllic alteration zones of the
Cerro Verde and Santa Rosa deposits (Fig. 1b). The locations,
petrographic relationships, and inferred ages of the dated supergene minerals are recorded in Table 2 and in Figure 1.
One critical sample, SURF-110, from the 2648 m bench of
the Cerro Verde pit, is discussed separately. Full 40Ar-39Ar analytical data are recorded in Appendix 1. All dates are quoted
with an uncertainty of ±2σ. For the purposes of this paper, an
age plateau is defined as three or more separate outgassing
steps with ages that are concordant at 2σ errors and that account for at least 50 percent of the 39Ar released; ideally,
120
5 0
40
4 0
3 0
20
2 0
0
0 . 0 6
0.04
0 . 0 4
0.02
0 . 0 2
Integrated Age: 60.7±1.5 Ma
1 0
0.06
0
0 . 0
0.0
1 . 0
F r a c t i o n
3 9 A r
1.0
0 . 0 0
0
0 . 0 0 0
0
0 . 0 0
1
0 . 0 0
2
0.002
3 6 A r / 4 0 A r
0 . 0 0 3
0 . 0 0 4
0.004
FIG. 3. 40Ar-39Ar step-heating isochron plots and age spectra for hydrothermal sericites from Cerro Verde (a-d) and Santa Rosa (e, f). PA = plateau
age.
there should be no consistent increase or decrease in apparent age across the plateau.
Age of Hypogene Mineralization
The age of hypogene mineralization in the district is established by 40Ar-39Ar hydrothermal sericite dates (Fig. 3).
SURF-110 and SURF-111, both from Cerro Verde, yielded
identical correlation ages of 61.8 ± 0.7 Ma and 62.0 ± 1.1 Ma
(Fig. 3b, d; Table 1). These dates are preferred to the plateau
TABLE 1. Locations and Ages of Hydrothermal Sericite Samples from Cerro Verde-Santa Rosa
Sample
Elevation
(m a.s.l.) Location
Latitude (°S)
Longitude (°W) Hypogene setting
SURF-110
2,648
Cerro Verde pit,
upper benches on
southern wall
16.5326
71.5980
Q-s-p (quartzsericite-pyrite)
altered quartz
monzodiorite
Hematite-goethite
capping with slightly
weathered pyrite
and minor "sooty"
chalcocite
and covellite
SURF-111
2,738
Cerro Verde pit,
top of western
wall
16.5311
70.6074
Q-s-p altered
tourmaline
breccia
SURF-119
2,558
Santa Rosa pit,
southwestern
wall
16.5420
71.5841
Bonanza breccia
comprising intense
sericite-quartz–
altered fragments
with disseminated
chalcopyrite ±
pyrite cemented
by massive
chalcopyrite
associated with
sericite, molybdenite
± pyrite
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Mineralogy by
powder X-ray
diffraction
Habit
Age ± 2σ
(Ma)
Muscovite-(2M1)
Finegrained,
pervasive
61.8 ± 0.7
correlation
age
Hematite
leached cap
Muscovite-(2M1)
Finegrained,
pervasive
62.0 ± 1.1
correlation
age
Transitional zone
between secondary
sulfide enrichment
and hypogene
zones
Muscovite-(2M1)
Fine- to
mediumgrained,
pervasive
61.0 ± 1.2
plateau
age
Supergene setting
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TABLE 2. Locations and Ages of Supergene Alunite-Group Minerals
a. Cerro Verde
Sample
Elevation
(m a.s.l.) Location
Latitude (°S)
Hypogene
Longitude (°W) setting
Supergene
setting
SURF-111
2,738
Cerro Verde pit,
western margin
16.5311
70.6074
Q-s-p–altered
tourmaline
breccia
Hematite
leached cap
SURF-114
2,648
Cerro Verde pit,
eastern slopes
16.5285
71.5963
SURF-109
2,618
Cerro Verde pit,
southeastern wall
SURF-110
2,648
Cerro Verde pit,
upper benches
on southern wall
SURF-112
SURF-113
Age ± 2σ
(Ma)
Mineral
Habit
(a) Natroalunite
(b) Alunite
(c) Jarosite
4-cm-wide, white
to pale yellow
porcelaneous
vein comprising
aggregates of up to
20-µm-size alunite
(b) crystals cut by
finer-grained,
massive natroalunite
(a); both are cut by
mm-scale jarosite
(c) veins
(a) 20.7 ± 0.3
(a) 21.2 ± 0.3
(b) 23.3 ± 0.3
(c) 0.74 ± 0.03
plateau ages
Silicified and
Hematitesericite-altered goethite
intrusive
leached Cap
Natroalunite
1-cm-wide, pale
green-light brown,
porcelaneous vein
12.0 ± 0.5
12.6 ± 0.9
plateau ages
16.5308
71.5955
Q-s-p–altered
dacite
porphyry
Jarositic
leached cap
(a) Natroalunite
(b) Alunite
(c) Jarosite
Millimeter-wide,
pale yellow,
porcelaneous alunite
(b) veinlets cut by
cm-scale, tan-white
porcelaneous
natroalunite (a) veins;
both are cut by thin,
fine-grained jarosite
(c) veinlets
(a) 4.9 ± 0.3
(b) 6.7 ± 0.2
(c) 1.3 ± 0.2
plateau ages
16.5326
71.5980
Q-s-p–altered
quartz
monzodiorite
Hematitegoethite
capping with
slightly
weathered
pyrite and
minor 'sooty'
chalcocite
and covellite
(a) Alunite
(b) Natroalunite
Pink, porcelaneous,
fine-grained
(1–3-µm size),
zoned alunite
(a) veins cut by
cryptocrystalline,
natroalunite
(b) veinlets
(a) (36.1 ± 0.3)
to (38.8 ± 0.7)
minimum ages
(b) (24.4 ± 0.3)
to (28.0 ± 0.4)
maximum ages
2,588
Santa Rosa pit,
16.5426
southwestern wall 71.5862
Strong quartzsericite-clay–
altered
intrusive
Lower portion
of the
secondary
sulfide
enrichment
zone
Natroalunite
Tan, massive clots
in leached quartz
veinlets
26.8 ± 1.7
26.2 ± 0.8
plateau ages
2,573
Santa Rosa pit,
16.5410
southwestern wall 71.5854
Q-s-p–altered
quartz
monzodiorite
Transitional
Natroalunite
zone between
secondary
sulfide enrichment and
hypogene zones
Gray-tan, massive
patches in quartztourmaline-sulfide
vein with
chalcocite-covellite
enrichment
(26.9 ± 0.3)
(27.4 ± 0.3)
maximum ages
b. Santa Rosa
ages (Fig. 3a, c) because the isochron plots display linear
arrays that clearly define the age of the samples, whereas the
spectra exhibit atmospheric argon and slight recoil effects in
the lower temperature increments, resulting in greater
uncertainty in the plateaus. Sericite (SURF-119), from the
matrix of the Bonanza breccia at Santa Rosa, gave a correlation age of 62.2 ± 2.9 Ma (Fig. 3f), which corresponds closely
to those from Cerro Verde. Although composed of only four
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steps, the age spectrum defines a plateau (Fig. 3e), even
though much of the 39Ar was released in a single step and the
lowest- and highest-temperature steps are associated with
large errors due to the small amounts of gas released.
These dates overlap within error with the 61 ± 1 Ma UPb zircon date of Mukasa (1986) for “dacitic-monzonitic
porphyries” at Cerro Verde and a 62 ± 2 Ma Rb-Sr wholerock isochron age reported by Beckinsale et al. (1985) for
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13 samples of intrusive rocks in the vicinity of Cerro Verde. In
contrast, the conventional K-Ar dates of Estrada (1969, 1978),
56 to 59 Ma, are markedly younger and may reflect the loss of
radiogenic 40Ar*.
Hypogene mineralization in the Cerro Verde-Santa Rosa
district was, at ca. 62 Ma, the final event in the evolution of
the Arequipa segment of the Coastal batholith (e.g., Pitcher
et al., 1985).
(a)
Age (Ma)
40
(b)
SURF 111b, Alunite
30
SURF 111a, Natroalunite
30
PA = 23.3±0.3
PA = 20.7±0.3
20
20
10
10
Integrated Age: 24.1±0.4 Ma
0
0.0
0
0.0
1.0
Fraction 39Ar
(c) SURF 111a, Natroalunite (2)
5
Integrated Age: 19.5±0.4 Ma
1.0
(d)
SURF 111c, Jarosite
5
4 Integrated Age: 0.72±0.03 Ma
20
Age and Geomorphologic Setting of
Supergene Mineralization
Whereas the first recorded conventional K-Ar dates for
supergene alunite group minerals (Gustafson and Hunt,
1975) were interpreted as problematic, subsequent K-Ar
(Alpers and Brimhall, 1988; Sillitoe and McKee, 1996) and,
particularly, 40Ar-39Ar incremental-heating (Vasconcelos et al.,
1994; Bouzari and Clark, 2000, 2002; Mote et al., 2001) studies have convincingly demonstrated the efficacy of this approach in the elucidation of the history of weathering profiles.
The Ar systematics of alunite are documented by Love et al.
(1998) and Vasconcelos (1999).
The age spectra determined herein for alunite-group minerals reveal complex relationships between the Cerro Verde
and Santa Rosa supergene profiles, and the data are therefore
discussed separately below. The dates provide, in turn, age
constraints for the local landforms that controlled subjacent
supergene processes.
4
Age(M
a)
PA = 21.2±0.3
10
30
(e)
3
PA = 0.74±0.03
2
1
Integrated Age: 20.1±0.3 Ma
0
0.0
3
2
1
1.0
SURF 114, Natroalunite
0
0.0
0
0 .
0
(f)
40
1 . 0
F r a c t i o n
3 9 A r
1.0
SURF 114, Natroalunite (2)
4 0
30
3 0
PA = 12.6±0.9
Age(M
a)
20
10
PA = 12.0±0.5
20
2 0
10
1 0
Integrated Age: 12.5±1.1 Ma
0
0.0
(g)
40
Integrated Age: 11.9±0.6 Ma
0
0.0
0
1
.0
SURF 109b, Alunite
0 .
15
30
0
1 . 0
F r a c t i o n
(h)
10
PA = 6.7±0.2
0
0.0
5
Integrated Age: 5.4±2.4 Ma
0
0.0
1.0
Integrated Age: 4.7±0.4 Ma
1.0
SURF 109c, Jarosite
(i)
10
1.0
PA = 4.9±0.3
20
10
3 9 A r
SURF 109a, Natroalunite
1 0
9
Integrated Age: 1.6±0.4 Ma
8
8
Cerro Verde
Plateau ages for seven samples broadly decrease with
depth (Fig. 4; Table 2a). White to pale yellow alunite (SURF111b) from the 2738 m level gave an age of 23.3 ± 0.3 Ma,
derived from a four-step plateau comprising 74.6 percent of
the 39Ar released (Fig. 4a). A supergene origin for this sample is supported by a δ34S value of 7.5 per mil, which is similar to the 8.3 per mil determined for hypogene pyrite from
SURF-110, a value slightly higher than those reported by Le
Bel (1985) for hypogene pyrite (5.1–6.9‰) from the deposit.
Duplicate analyses of a natroalunite vein (SURF-111a) that
cuts, and contains fragments of, the SURF-111b alunite
veins (Fig. 5) yielded ages of 20.7 ± 0.3 and 21.2 ± 0.3 Ma
from five-step plateaus (Fig. 4b, c) comprising 83.0 and 86.7
percent of the 39Ar released, respectively. This implies a ca.
2- to 3-m.y. history of leaching and alunite-group mineral
precipitation at this horizon of the supergene profile. Moreover, a four-step plateau comprising 72.4 percent of the 39Ar
released (Fig. 4d) gave an age of 0.74 ± 0.03 Ma for finegrained jarosite (SURF-111c), coating fracture surfaces and
forming millimeter-scale veinlets that cut both the alunite
(SURF-111b) and natroalunite (SURF-111a) veins (Fig. 5),
providing evidence for a persistence of supergene activity
into the Pleistocene.
Ninety meters deeper, on the 2648 m mine bench, natroalunite (SURF-114) gave duplicate ages of 12.6 ± 0.9 and
12.0 ± 0.5 Ma from three-step plateaus comprising 87.2 and
82.8 percent of the 39Ar released, respectively (Fig. 4e, f).
Pale yellow alunite (SURF-109b) from close to the bottom of
the Cerro Verde pit, at 2618 m, yielded a plateau age of 6.7 ±
0.2 Ma, from five steps representing 94.7 percent of the 39Ar
released (Fig. 4g). These veins are cut by cm-wide veins of
0361-0128/98/000/000-00 $6.00
Age(M
a)
7
6
6
5
4
4
PA = 1.3±0.2
3
2
2
1
0
0
0 .
0
1 .
0.0
F r a c t i o n
0
1.0
3 9 A r
FIG. 4. 40Ar-39Ar step-heating age spectra for supergene alunite group
minerals from Cerro Verde. PA = plateau age.
Alunite
L
Natroalunite
L
L
T
L
L
Jarosite
L
M
0
1
2
3
4
5 cm
FIG. 5. Slabbed surface of SURF-111 (2738 m level, Cerro Verde pit),
showing crosscutting relationships among supergene alunite, natroalunite,
and jarosite veins. Fragments of paler alunite and wall rock are included in
the natroalunite vein. L = leached wall rock, M = hypogene molybdenitequartz vein, T = hypogene tourmaline veinlet.
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white to tan natroalunite (SURF-109a), which yielded a
plateau age of 4.9 ± 0.3 Ma from three steps representing
96.7 percent of the 39Ar released (Fig. 4h). The natroalunite
veins are, in turn, cut by thin jarosite (SURF-109c) veinlets,
dated at 1.3 ± 0.2 Ma from two contiguous steps that represent 63.6 percent of the 39Ar released (Fig. 4i).
These data indicate that the major leached, oxidized, and
supergene sulfide zones exposed at Cerro Verde developed
over an interval of at least 18 m.y., from the latest Oligocene
to the Late Miocene. The data do not discriminate between
quasicontinuous or episodic downward encroachment of supergene processes, but we favor the latter model in view of
the clearly episodic landform history of the region, in which
abrupt uplift and erosional events were separated by more
quiescent intervals (Tosdal et al., 1984). At two sites, natroalunite precipitation followed that of alunite.
40
Ar-39Ar dates for alunite (23 Ma) and natroalunite (21 Ma)
veins associated with pulverulent, supergene hematite at the
top of the Cerro Verde pit, record the time of oxidation and
leaching of a preexisting chalcocite blanket (Fig. 6). This
older, enriched blanket and its associated higher-altitude
leached zone were eroded by the Santa Rosa pediment that,
therefore, is inferred to have formed after 21 Ma. The ca. 23
Ma alunite (SURF-111b) and 21 Ma natroalunite (SURF111a) veins are inferred to have formed beneath the older La
Caldera surface, represented by remanent accordant summits
overlooking the Santa Rosa pediment. Natroalunite and alunite dated at 12.6 to 4.9 Ma were subsequently generated in
the course of continued or, more probably, renewed leaching
(a)
A
2738 m
2723 m
2708 m
2693 m
2678 m
2663 m
Santa Rosa
Two natroalunites from the Santa Rosa deposit gave ca. 26
Ma dates (Figs. 7 and 8; Table 2b). Duplicate analyses of
SURF-112 from the chalcocite zone (2588 m level) yielded
concordant plateau ages of 26.2 ± 0.8 and 26.8 ± 1.7 Ma, representing 95.7 and 100 percent, respectively, of the total 39Ar
released (Fig. 7a, b). In contrast, the two age spectra for
SURF-113, from the slightly deeper transitional zone (2573
m level), display progressively decreasing apparent ages toward higher-temperature steps. Such complex spectra may either record 39Ar recoil or the occurrence of two or more alunite-group minerals with different Ar-retention temperatures
(Vasconcelos et al., 1994; Bouzari and Clark, 2002). In the
former case, the highest-temperature steps would yield maximum crystallization ages (25.5 ± 0.6 and 26.4 ± 0.8 Ma; Fig.
7c, d) and, in the latter, would reveal the presence of separate
minerals with ages of ca. 28 to 29 and 26 Ma.
The natroalunites are interpreted as recording deep (ca.
300–350 m) penetration of supergene solutions beneath the
La Caldera surface, remnants of which are preserved on ca.
2,900 m a.s.l. summits. A minimum age for this surface is constrained by the ca. 26 Ma natroalunites associated with chalcocite mineralization in the Santa Rosa pit on the 2588 m
2753 m
2723 m 2708 m
2693 m
2678 m
2663 m
2648 m
2633 m
2618 m
m
2603 m 2603
2588
m m
2588
26032603
m m
2588 m
2588
2588 m
2738 m
2648 m
2633 m 2618 m
2603 m
SURF-109, 2618 m
A’
2738 m
2708 m
2693 m 2678 m
2663 m 2648 m
2633 m
2618 m
2603 m
SURF-111, 2723 m
SURF-114, 2648 m
2573 m
2573 m
La Caldera Surface
Santa Rosa Surface
(b)
beneath the Santa Rosa pediment. The 1.3 Ma (SURF-109c)
and 0.74 Ma (SURF-111c) jarosite veins reflect episodes of
minor supergene activity under hyperarid conditions beneath
the fossilized landscape, and were probably not associated
with significant Cu mobility.
SURF-111a, na 20.6±0.3 Ma, 21.2±0.3 Ma
SURF-111b, al 23.3±0.3Ma
SURF-111c, ja 0.74±0.05 Ma
SURF-110, al 36.1-38.8 Ma,
na 24.4-28.0 Ma
SURF-109a, na 4.9±0.3 Ma
SURF-109b, al 6.7±0.2 Ma, 6.5±0.2 Ma
SURF-109c, ja 1.32±0.15 Ma
SURF-114, na 12.0±0.5 Ma, 12.6±0.9 Ma
Hypogene Zone
Transition Zone
Chalcocite Zone
Hematite
Leached Cap
Hematite-Goethite
Leached Cap
Jarositic
Leached Cap
SURF-111a Sample number
Oxide Zone
2738 m
Bench level (m a.s.l.)
Unconsolidated 20.6±0.3 Ma Plateau age
(37.3±3.3 Ma) Minimum age
Material
al Alunite
na Natroalunite
ja Jarosite
FIG. 6. a. Panorama of Cerro Verde pit, looking southwest (245°), in August, 2001, showing the bench elevations (m a.s.l.).
b. Sketch showing the distribution of supergene facies and locations of 40Ar-39Ar dates discussed in the text. The leached cap
is dominated by hematite with local jarosite. Local faulting coupled with the presence of highly permeable breccia bodies
may have caused the complexities in the spatial distribution of supergene facies (e.g., chalcocite enrichment occurs on the
2618 m bench adjacent to hypogene mineralization). Dashed lines show remnants of the La Caldera surface and the subplanar Santa Rosa pediment.
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(a) SURF 112, Natroalunite
(b) SURF 112, Natroalunite (2)
40
60
Integrated Age: 26.8±1.7 Ma
30
PA = 26.2±0.8
40
20
10
20
Integrated Age: 27.2±0.9 Ma
0
0.0
Fraction Ar
(c)
SURF 113, Natroalunite
PA = 26.8±1.7
0
0.0
1.0
39
40
1.0
(d) SURF 113, Natroalunite (2)
30
30
20
20
25.5±0.6 Ma
10
26.4±0.8 Ma
10
0
0.0
Integrated Age: 26.9±0.3 Ma
Integrated Age: 27.4±0.3 Ma
0
1.0
0.0
1.0
FIG. 7. 40Ar-39Ar step-heating age spectra for supergene alunite group
minerals from Santa Rosa. PA = plateau age.
(a) B
B’
2678 m
2663 m
2648 m
2633 m
2618 m
2603 m
2588
2573 m
(b)
La Caldera surface
Santa Rosa surface
SURF-112, na 26.8±1.7 Ma, 26.2±0.8 Ma
Landform correlations
A simple landform chronology for the Cerro Verde-Santa
Rosa district is implied by the supergene 40Ar-39Ar data presented above. The La Caldera surface, represented by remnant accordant summits, is inferred to have reached its final
configuration before ca. 26 Ma. The Santa Rosa pediment,
comprising broad, subplanar valley floors on which the Cerro
Verde and Santa Rosa deposits crop out, is no older than 21
Ma. This landform extends west-southwest along the Quebrada Cerro Verde drainage, to merge with the vast Pampa de
La Joya surface to the southwest, the dominant landform in
this area of the Pacific piedmont.
Evidence for Eocene supergene activity
Six analyses of alunite from sample SURF-110, from the
2648 m level of the Cerro Verde pit, yielded age spectra with
consistent configurations that record unexpectedly old dates
(Table 2a; Appendix 1). The spectra (Fig. 9) are defined by a
staircase pattern in the lower-temperature steps and a flatter
profile in the mid- to high-temperature steps, exhibiting progressively rising ages with increasing temperature. Two apparent populations of ages, 36.1 ± 0.3 to 38.8 ± 0.7 Ma and
24.4 ± 0.3 to 28.0 ± 0.4 Ma, are recognized in, respectively,
the highest-temperature steps and in the lowest-temperature
steps that record less than 10 percent atmospheric argon
(Table 2a). The spectra would be in permissive agreement
with the occurrence of a subordinate younger species, with an
age of ≤ 28 Ma, and a dominant species at least 38 to 39 Ma
in age (cf. Vasconcelos et al., 1994; Bouzari and Clark, 2002).
Whereas cathodoluminescence imaging did not distinguish
two phases because of pervasive quenching by Fe, backscattered electron images reveal a network of dark (i.e., low aggregate at. wt) veinlets and patches that cut an extremely fine
grained (1–3 µm) intergrowth of zoned crystals with pale
SURF-113, na ( 25.5 ± 0.6 Ma), (26.4 ± 0.8 Ma)
(a)
Oxide Zone
Hematite-Goethite
Leached Cap
Hematite
na
Leached Cap
6 0
5 0
FIG. 8. a. Panorama of Santa Rosa pit, looking southwest (205°), in August 2001. b. Sketch showing the distribution of supergene facies and 40Ar39
Ar dates. The leached cap is dominated by hematite, with minor goethite.
Relatively thin zones of oxide and secondary chalcocite enrichment occur
below the leached cap and pass downward to a zone of transitional sulfide
mineralization.
4 0
3 0
2 0
1 0
0
0 . 0
1 . 0
F r a c t i o n
(c)
60
50
40
30
37.3±3.3 (al)
20
27.5±2.0 (na)
10
Integrated Age: 34.2±0.5 Ma
0
0.0
(d)
6 0
5 0
4 0
1 0
0
0 . 0
(e)
4 0
3 0
2 0
1 0
0
1 . 0
3 9 A r
1 . 0
F r a c t i o n
5 0
F r a c t i o n
3 0
2 0
60
50
40
30
37.2±0.4 (al)
20
28.0±0.4 (na)
10
0 Integrated Age: 33.8±0.3 Ma
0.0
1.0
0 . 0
3 9 A r
60
50
40
30
20
36.1±0.3 (al)
10 26.3±0.4 (na)
0 Integrated Age: 33.8±0.3 Ma
0.0
1.0
1.0
6 0
Age( M
a)
mine bench, inferred to represent the lower limit of the
supergene chalcocite development beneath the La Caldera
surface in the late Oligocene. The lack of fault offset of either
the Santa Rosa or La Caldera surface in the area of the mines
precludes significant relative vertical displacement of the two
porphyry centers in the Oligo-Miocene. The occurrence of
these upper Oligocene natroalunites more than 150 m below
the ca. 23 Ma alunite and ca. 21 Ma natroalunites of SURF111 at Cerro Verde is therefore interpreted as reflecting the
importance of local controls on permeability, such as the distribution of breccia bodies and fracture systems, in focusing
meteoric fluid flow.
0361-0128/98/000/000-00 $6.00
60
60
50
50
40
40
30
30
37.7±0.4 (al)
38.8±0.7 (al)
20
20
27.4±1.6
(na)
24.4±0.3 (na)
10
10
Integrated Age: 36.2±0.3 Ma
Integrated Age: 34.3±0.2 Ma
0
0
0.0
1.0
39
0.0
Fraction Ar 1 .0
Age( M
a)
Chalcocite Zone
SURF-112
Sample number
Unconsolidated 2588 m
Bench level (m a.s.l.)
Material
26.8±1.7 Ma Plateau age
Natroalunite
(27.4±0.3 Ma) Maximum age
Age(M
a)
Transition Zone
Age (Ma)
Bonanza Breccia
SURF 110, Alunite (al) & N atroalunite (na)
(b)
Age(M
a)
Age (Ma)
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SCIENTIFIC COMMUNICATIONS
3 9 A r
(f)
60
50
40
30
37.0±0.4 (al)
20
25.8±0.7 (na)
10
Integrated Age: 34.3±0.2 Ma
0
0.0
1 .0
6 0
5 0
4 0
3 0
2 0
1 0
0
0 . 0
1 . 0
F r a c t i o
n
3 9 A r
FIG. 9. 40Ar-39Ar step-heating age spectra displaying two populations of
ages for porcelaneous alunite veins in sample SURF-110, from the 2648 m
level, Cerro Verde.
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cores and darker rims, recording remanent compositional
zoning (Fig. 10). Qualitative microprobe analyses (EDS) confirm the presence of phosphorus and strontium in the centers
of the zoned alunite crystals. In a similar setting, Stoffregen
and Alpers (1987) assigned a supergene origin to cryptocrystalline aluminum-phosphate-sulfate (APS) minerals in late
fractures from the leached cap of the La Escondida porphyry
deposit on the basis of their grain size and geologic setting.
Comparative backscattered electron imaging and microprobe
analysis of ca. 6.5 Ma alunite (SURF-109b) also revealed
zoned crystals and the presence of phosphorus and calcium.
We tentatively accept this as evidence that these elements can
occur in supergene alunite, whereas Watanabe and Hedenquist (2001) considered the occurrence of svanbergite and
woodhouseite cores in supergene alunite from El Salvador to
represent relict hypogene alunite. Elemental X-ray mapping
of the Cerro Verde assemblages is precluded by their extremely fine grain size, but the EDS spectra and the modest
intensity contrasts in the backscattered electron images (Fig.
10) suggest that the P and Sr in SURF-110 are present in
solid solution in the alunite structure rather than as discrete
aluminum-phosphate-sulfate minerals. A supergene origin
for alunite from this specimen is supported by bulk δ34S values of 6.4 and 7.7 per mil from duplicate analyses, which are
similar to the 8.3 per mil determined for hypogene pyrite in
the same sample and overlap with the 5.1 to 6.9 per mil values reported by Le Bel (1985) for hypogene pyrite at Cerro
Verde. Although the analytical data are probably insufficient
to rule out the presence of relict hypogene alunite (see Vasconselos et al., 1994), there is no record of late Eocene-early
Oligocene hydrothermal activity in this Andean transect
(Clark et al., 1990), and we tentatively conclude that the alunite group minerals in SURF-110 are wholly supergene.
The 36.1 to 38.8 Ma dates for alunite from SURF-110
therefore imply that supergene activity was underway by the
latest Eocene, probably in response to uplift and erosion during Incaic tectonism (Sandeman et al., 1995). This early
episode of deep leaching and sulfate precipitation, controlled
by highly permeable structures, occurred long before the
late-Oligocene development of the La Caldera surface and
the concomitant initial development of the main supergene
profile at Cerro Verde and Santa Rosa. Similar late-Eocene to
early-Oligocene ages for supergene activity are documented
(b)
(a)
2µm
Pits on
surface
with edge
effects
Natroalunite
Zoned alunite
with APS-rich
core
FIG. 10. a. Backscattered electron image of pink porcelaneous alunite vein
(SURF-110). b. Sketch of the same field, showing zoned crystals of alunite
with pale centers, reflecting concentrations of phosphorus and strontium,
and irregular veinlets of a “darker” material, inferred to be natroalunite.
0361-0128/98/000/000-00 $6.00
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in northern Chile. Thus, at the ca. 51.8 Ma Cerro Colorado
deposit, Bouzari and Clark (2000, 2002) recorded a 35.3 ± 0.7
Ma 40Ar-39Ar age for alunite from a hematitic leached cap,
and Sillitoe and McKee (1996) determined a K-Ar date of
34.3 ± 1.1 Ma for alunite from jarositic leached cap. Farther
south, in the ca. 57.0 Ma Spence deposit, supergene
processes were active as early as 44.4 ± 0.5 Ma (Rowland and
Clark, 2001). Comparably early initial supergene activity
(43.9 ± 2.6 Ma) has been inferred for the El Salvador deposit,
near the southern limit of the Atacama Desert, but the significance of the older age data for supergene minerals in this
upper Eocene center (Gustafson and Hunt, 1975; Gustafson
et al., 2001; Mote et al., 2001) has been questioned by
Bouzari and Clark (2002).
The 24.4 to 28.0 Ma dates in SURF-110 for the late natroalunite veinlets correspond to the ca. 26 Ma natroalunite
dates from Santa Rosa, providing further evidence of leaching
and sulfate precipitation during the late Oligocene.
Conclusions
Porphyry intrusion and hypogene mineralization in the
Cerro Verde-Santa Rosa district occurred at ca. 62 Ma, marking the local termination of magmatic activity. Subsequently,
supergene activity controlled by local subplanar landforms, as
well as by highly permeable breccia zones, generated profiles
in the Cerro Verde and Santa Rosa deposits with variable
thicknesses and complex age relationships (Fig. 11). Dates of
36.1 to 38.8 Ma for alunite from Cerro Verde are evidence
that deeply penetrating supergene processes were in progress
locally by the late Eocene, to be subsequently overprinted by
24.4 to 28.0 Ma activity that generated minor natroalunite
veinlets. Equivalent ages of 25.5 to 26.8 Ma are also documented for natroalunite from the bottom of the Santa Rosa
supergene profile, evidence for ongoing leaching and penetration of supergene solutions at both Cerro Verde and Santa
Rosa. The late Oligocene leaching inferred to have occurred
beneath the La Caldera surface provides a minimum age of
ca. 26 Ma for the final configuration of this landform, which
is now preserved as accordant summits including Cerro Verde
and Cerro Negro (Fig. 11). The Santa Rosa pediment, comprising broad, subplanar valley floors, subsequently degraded
the La Caldera surface, truncating a previously hematitized
chalcocite horizon with 21 Ma natroalunite veins now exposed at the top of the Cerro Verde pit. These relationships
provide a maximum age of ca. 21 Ma for this erosional surface, beneath which the Cerro Verde profile continued to
deepen through the Miocene (Fig. 11). Minor supergene activity persisted into the Pleistocene.
Acknowledgments
This study, a component of the senior author’s M.Sc. thesis,
was funded by Rio Tinto Mining and Exploration Ltd., Lima,
Peru, who also provided a graduate bursary to C.X.Q., and by
grants to A.H.C. and J.K.W.L. from the Natural Sciences and
Engineering Research Council of Canada (NSERC). The dated
samples were collected by C.X.Q. in 2001 during a mapping
project sponsored by the Society of Economic Geologists: its
leaders, William X. Chávez, Jr., and Erich Petersen, provided stimulating discussions in the field. Doug Archibald,
Kerry Klassen, Dave Kempson, and Alan Grant assisted with
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SCIENTIFIC COMMUNICATIONS
SE
Santa Rosa deposit
NW
Cerro Verde deposit
Cerro Verde
La Caldera surface (> 26 Ma)
36.1 - 38.8 Ma
24.4 - 28.0 Ma
(SURF 110)
?
10.9 - 12.6 Ma
20.6 - 23.3 Ma
(SURF 111a, b)
4.9 - 6.7 Ma
(SURF 114)
?
(SURF 109a, b)
?
Ma)
Santa Rosa surface (< 21
?
?
100 m
26.9 - 27.4 Ma
?
(SURF 113)
26.2 - 26.8 Ma
(SURF 112)
0
?
Hypogene Zone
Transition Zone
Chalcocite Zone
Hematite
Leached Cap
Hematite-Goethite
Leached Cap
Jarositic
Leached Cap
100 m
Oxide Zone
?
Inferred Fault
Preserved
Landforms
FIG. 11. Idealized cross section, looking southwest, of the Cerro Verde-Santa Rosa supergene profile, summarizing the
relationships between supergene mineralization and post-hypogene landform development.
Ar-Ar, sulfur isotope, microprobe, and X-ray analyses, respectively. Dave Andrews, Bob Harrington, and Tim Moody at
Rio Tinto are thanked for their unstinting support of this research. Paulo Vasconcelos provided an insightful and constructive review of the original manuscript.
Permission to publish this study, a contribution to the
Queen’s University Central Andean Metallogenetic Project
(QCAMP), has been given by Sociedad Minera Cerro Verde
S.A. We thank Randy Davenport and Jim Jones for their cooperation.
October 22 2002; June 4, 2003
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Appendix 1
40Ar-39Ar
Laser1
Power
(watts)
40Ar/39Ar
Isotope ratios
38Ar/39Ar
37Ar/39Ar
Analytical Data
36Ar/39Ar
Ca/K
Cl/K
40Ar
(%)
39Ar
(%)
40Ar*/39Ar
K
Age ± 2σ
a. Hydrothermal sericites from Cerro Verde and Santa Rosa
SURF-110, sericite, J = 0.002426 ± 0.000072; volume 39ArK = 20.07 × 10–10cm3, integrated age = 61.80 ± 3.27 Ma (2σ)
0.75
1831.466 ± 0.329 1.326 ± 0.367 0.168 ± 1.688 6.472 ± 0.330
1.052
0.024
101.67
0.83 -32.215 ± 64.791 -146.81 ± 307.61
0.90>
1250.463 0.041
0.912 0.090
0.057 0.941
4.283 0.044
0.357
0.023
98.42
0.79
20.931 24.246
89.36 100.99
1.10>
512.848 0.033
0.370 0.082
0.021 1.134
1.714 0.036
0.116
0.007
95.98
1.61
21.230 8.666
90.61 36.07
1.30>
331.933 0.018
0.240 0.084
0.019 0.928
1.086 0.025
0.105
0.004
93.83
1.77
21.065 6.155
89.92 25.63
<1.60>
142.716 0.010
0.106 0.103
0.007 0.604
0.444 0.022
0.028
0.001
88.98
3.39
15.998 2.590
68.69 10.91
<2.00>
67.648 0.007
0.057 0.072
0.004 0.714
0.185 0.024
0.022
0.002
78.27
6.90
14.840 1.307
63.81 5.52
<3.00>
29.978 0.005
0.026 0.068
0.002 0.353
0.054 0.022
0.012
0.000
51.87
20.06
14.513 0.356
62.43 1.51
<4.00>
20.291 0.004
0.020 0.063
0.002 0.531
0.021 0.044
0.010
0.000
29.52
19.70
14.379 0.287
61.86 1.22
<5.00>
18.508 0.004
0.019 0.089
0.001 0.799
0.015 0.053
0.006
0.000
21.80
17.07
14.564 0.241
62.64 1.02
<7.00>
18.893 0.004
0.019 0.069
0.002 0.240
0.017 0.053
0.009
0.000
25.27
27.88
14.190 0.272
61.06 1.15
SURF-111, sericite, J = 0.002426 ± 0.000064; volume 39ArK = 26.70 × 10–10cm3, integrated age = 56.22 ± 5.54 Ma (2σ)
0.50>
669.039 ± 0.050
0.525 ± 0.179 0.021 ± 1.787 2.304 ± 0.062
0.119
0.019
99.09
3.22
6.161 ± 25.857
0.75>
502.677 0.034
0.394 0.141
0.038 1.245
1.719 0.043
0.227
0.014
98.47
3.92
7.770 14.006
1.00>
345.413 0.037
0.279 0.187
0.027 0.828
1.197 0.046
0.158
0.010
99.71
5.21
0.994 9.872
<1.25>
387.986 0.022
0.294 0.087
0.014 0.614
1.298 0.031
0.069
0.009
96.24
3.11
14.797 8.631
<1.50>
330.103 0.018
0.252 0.084
0.012 1.394
1.099 0.024
0.055
0.007
95.64
2.43
14.609 5.416
<1.75>
210.627 0.012
0.171 0.100
0.008 1.450
0.693 0.024
0.034
0.006
94.64
3.99
11.414 4.371
<2.00>
162.036 0.010
0.127 0.057
0.008 0.800
0.519 0.018
0.035
0.004
92.00
4.08
13.091 2.343
<2.25>
134.685 0.014
0.109 0.075
0.006 0.477
0.422 0.030
0.024
0.003
90.12
3.48
13.477 3.348
<2.50>
92.503 0.010
0.077 0.081
0.004 1.254
0.274 0.030
0.006
0.003
85.10
4.28
13.938 2.368
<3.00>
57.729 0.007
0.050 0.080
0.005 0.271
0.151 0.019
0.026
0.002
74.79
8.62
14.657 0.843
<4.00>
33.782 0.005
0.033 0.062
0.003 0.390
0.067 0.030
0.013
0.002
56.38
14.18
14.819 0.594
<5.00>
33.111 0.006
0.033 0.066
0.002 0.638
0.066 0.033
0.008
0.001
56.48
13.10
14.498 0.640
<6.00>
31.609 0.005
0.031 0.061
0.002 0.613
0.061 0.022
0.008
0.001
54.79
13.48
14.376 0.400
<8.00>
25.511 0.005
0.028 0.077
0.002 0.471
0.040 0.024
0.011
0.001
44.14
16.90
14.319 0.293
26.77 ± 111.50
33.69 60.17
4.34 43.10
63.63 36.47
62.83 22.89
49.28 18.61
56.40 9.94
58.04 14.19
59.99 10.02
63.04 3.56
63.72 2.51
62.36 2.71
61.85 1.69
61.61 1.24
SURF-119, sericite, J = 0.002195 ± 0.000014; volume 39ArK = 0.72 × 10–10cm3, integrated age = 60.71 ± 1.51 Ma (2σ)
<1.00>
16.534 ± 0.047
0.104 ± 0.473 0.069 ± 0.326 0.067 ± 0.293
0.109
0.005
9.21
2.69
<3.00>
16.330 0.008
0.029 0.122
0.008 0.239
0.006 0.208
0.022
0.002
5.13
46.55
<5.00>
15.925 0.007
0.032 0.159
0.007 0.275
0.006 0.227
0.013
0.003
2.68
42.26
7.00>
15.783 0.018
0.060 0.251
0.029 0.269
0.026 0.287
0.047
0.006
12.46
8.51
66.85 ± 27.54
60.91 1.47
61.05 1.68
56.00 9.20
0361-0128/98/000/000-00 $6.00
1693
17.197 ± 7.216
15.644 0.383
15.681 0.439
14.364 2.397
1694
SCIENTIFIC COMMUNICATIONS
Appendix (Cont.)
Laser1
Power
(watts)
40Ar/39Ar
Isotope ratios
38Ar/39Ar
37Ar/39Ar
36Ar/39Ar
Ca/K
Cl/K
40Ar
(%)
39Ar
(%)
40Ar*/39Ar
K
Age ± 2σ
b. Supergene alunite-group minerals from Cerro Verde
SURF-111a, natroalunite, J = 0.002413 ± 0.000010; volume 39ArK = 64.05 × 10–10cm3, integrated age = 19.50 ± 0.45 Ma (2σ)
0.30
181.047 ± 0.023
0.154 ± 0.133 0.012 ± 1.417 0.685 ± 0.035
0.004
0.002
107.10
0.27
-13.397 ± 5.779
0.75
18.961 0.004
0.059 0.023
0.001 0.991
0.055 0.030
0.003
0.008
82.61
15.57
3.287 0.493
<1.00>
9.052 0.004
0.063 0.017
0.001 0.648
0.015 0.037
0.002
0.010
48.67
23.16
4.641 0.170
<1.25>
7.621 0.004
0.060 0.013
0.001 0.375
0.010 0.057
0.003
0.010
36.84
17.62
4.806 0.170
<1.50>
6.734 0.004
0.065 0.013
0.001 0.570
0.007 0.071
0.002
0.011
28.74
13.19
4.787 0.149
<2.00>
8.759 0.005
0.062 0.023
0.003 0.396
0.014 0.083
0.005
0.010
43.20
4.03
4.948 0.351
<4.00>
6.052 0.004
0.068 0.011
0.001 0.415
0.004 0.091
0.002
0.012
20.12
24.96
4.827 0.120
6.00
14.791 0.007
0.069 0.065
0.005 0.726
0.034 0.123
0.008
0.011
59.19
1.19
5.999 1.252
-59.28 ± 26.00
14.25 2.13
20.09 0.73
20.80 0.73
20.72 0.64
21.41 1.51
20.89 0.52
25.93 5.37
SURF-111a, natroalunite, J = 0.002413 ± 0.000010; volume 39ArK = 90.39 × 10–10cm3, integrated age = 20.08 ± 0.34 Ma (2σ)
0.50
188.629 ± 0.041
0.186 ± 0.187 0.020 ± 1.635 0.671 ± 0.072
0.000
0.007
99.42
0.06
1.155 ± 12.748
0.75
194.660 0.016
0.168 0.081
0.008 1.487
0.715 0.029
0.000
0.003
104.45
0.17
-8.994 5.459
1.00
66.329 0.006
0.064 0.059
0.001 2.458
0.226 0.017
0.001
0.002
97.67
2.35
1.530 1.084
1.20
40.133 0.005
0.050 0.039
0.001 1.321
0.135 0.020
0.002
0.002
96.47
2.66
1.397 0.799
1.50
38.400 0.005
0.054 0.040
0.002 1.160
0.125 0.018
0.005
0.004
93.19
3.42
2.605 0.671
<1.75>
13.665 0.003
0.055 0.026
0.001 1.022
0.031 0.023
0.002
0.008
65.60
11.57
4.694 0.220
<2.00>
8.141 0.003
0.059 0.018
0.000 1.226
0.011 0.036
0.001
0.010
39.84
27.43
4.892 0.124
<2.20>
7.242 0.003
0.061 0.020
0.000 0.851
0.008 0.044
0.002
0.010
31.20
26.13
4.975 0.106
<2.40>
8.640 0.003
0.064 0.018
0.001 0.521
0.013 0.038
0.003
0.011
43.04
12.09
4.910 0.148
<3.00>
8.879 0.003
0.063 0.021
0.001 0.370
0.014 0.033
0.005
0.011
44.13
9.52
4.947 0.139
4.20
9.785 0.003
0.064 0.020
0.002 0.391
0.016 0.039
0.006
0.011
46.16
4.59
5.242 0.190
5.02 ± 55.34
-39.59 24.29
6.65 4.70
6.07 3.47
11.31 2.90
20.32 0.95
21.17 0.53
21.53 0.46
21.25 0.64
21.41 0.60
22.67 0.82
SURF-111b, alunite, J = 0.002417 ± 0.000012; volume 39ArK = 138.16 × 10–10cm3, integrated age = 24.06 ± 0.39 Ma (2σ)
0.50
39.941 ± 0.162
0.253 ± 0.558 0.166 ± 0.726 0.227 ± 0.497
0.006
0.013
79.05
0.00 34.947 ± 181.043 146.29 ± 727.95
0.75
152.370 0.099
0.233 0.355
0.098 0.792
0.594 0.150
0.011
0.003
101.58
0.01
-4.712 40.037
-20.66 176.59
1.00
273.501 0.045
0.259 0.130
0.043 0.768
0.948 0.057
0.021
0.008
96.74
0.04
11.247 12.397
48.39 52.63
2.00
345.202 0.045
0.261 0.082
0.008 1.402
1.152 0.048
0.041
0.007
95.91
0.75
14.272 5.231
61.18 22.05
2.50
33.942 0.006
0.056 0.041
0.001 1.571
0.096 0.017
0.005
0.005
81.51
8.17
6.286 0.480
27.21 2.06
2.75
11.786 0.003
0.040 0.028
0.001 0.327
0.022 0.020
0.003
0.005
52.69
16.41
5.581 0.132
24.17 0.57
<3.00>
8.670 0.003
0.040 0.028
0.001 0.343
0.011 0.037
0.004
0.005
38.11
30.36
5.368 0.127
23.26 0.55
<3.20>
9.091 0.003
0.042 0.025
0.001 0.477
0.013 0.024
0.005
0.006
40.94
28.63
5.372 0.095
23.27 0.41
<3.40>
10.258 0.003
0.043 0.020
0.001 0.652
0.017 0.031
0.005
0.006
47.47
11.51
5.392 0.160
23.36 0.69
<3.60>
12.890 0.004
0.047 0.032
0.002 0.256
0.026 0.029
0.008
0.006
57.59
4.12
5.473 0.224
23.71 0.96
SURF-111c, jarosite, J = 0.002188 ± 0.000014; volume 39ArK = 51.09 × 10–10cm3, integrated age = 0.72 ± 0.03 Ma (2σ)
0.50
2.004 ± 0.003
0.015 ± 0.027 0.001 ± 0.188 0.006 ± 0.018
0.002
-0.000
91.54
15.19
<1.00>
0.567 0.007
0.014 0.030
0.001 0.105
0.001 0.030
0.002
-0.000
63.67
29.62
<1.50>
0.361 0.004
0.013 0.019
0.000 0.255
0.001 0.059
0.000
-0.000
36.57
30.76
<2.00>
0.428 0.004
0.013 0.027
0.000 0.359
0.001 0.073
0.000
-0.000
48.66
12.06
2.50
0.437 0.006
0.013 0.036
0.000 0.280
0.001 0.118
0.001
-0.000
36.30
9.36
3.00
0.605 0.021
0.016 0.060
0.002 0.185
0.002 0.123
0.006
0.000
66.45
3.02
0.140 ± 0.035
0.175 0.013
0.198 0.009
0.186 0.019
0.241 0.029
0.160 0.077
0.55 ± 0.14
0.69 0.05
0.78 0.04
0.73 0.08
0.95 0.11
0.63 0.31
SURF-114, natroalunite, J = 0.002147 ± 0.000018; volume 39ArK = 28.28 × 10–10cm3, integrated age = 11.86 ± 0.56 Ma (2σ)
0.50> 499.571 ± 0.024
0.440 ± 0.061 0.054 ± 0.178 1.805 ± 0.028
0.132
0.016
103.61
0.26
-19.556 ± 8.597
1.00
99.825 0.007
0.187 0.044
0.011 0.178
0.340 0.016
0.037
0.025
98.29
5.30
1.689 1.466
<1.50>
15.820 0.004
0.109 0.022
0.003 0.128
0.044 0.015
0.012
0.020
81.07
30.85
2.979 0.199
<2.00>
13.759 0.004
0.107 0.022
0.004 0.066
0.037 0.016
0.014
0.020
77.48
27.87
3.083 0.179
<2.50>
16.035 0.004
0.126 0.016
0.005 0.067
0.044 0.016
0.018
0.024
79.13
24.05
3.334 0.204
3.00
22.508 0.005
0.154 0.018
0.007 0.076
0.065 0.017
0.024
0.029
83.37
8.34
3.736 0.323
5.00
45.269 0.005
0.169 0.032
0.010 0.097
0.143 0.016
0.032
0.029
90.67
3.33
4.228 0.658
-77.38 ± 34.76
6.53 5.66
11.50 0.77
11.90 0.69
12.87 0.78
14.41 1.24
16.30 2.53
SURF-114, natroalunite, J = 0.002425 ± 0.000028; volume 39ArK = 17.76 × 10–10cm3, integrated age = 12.51 ± 1.09 Ma (2σ)
0.50
377.362 ± 0.092
0.411 ± 0.384 0.110 ± 1.233 1.373 ± 0.138
0.456
0.044
95.90
0.06 21.987 ± 59.974
1.00
1230.844 0.049
0.903 0.102
0.011 6.713
4.352 0.052
0.024
0.020
101.52
0.60
-19.568 24.905
<1.50>
30.918 0.005
0.126 0.026
0.002 0.346
0.098 0.016
0.014
0.021
91.23
23.65
2.701 0.447
<1.75>
16.613 0.004
0.120 0.022
0.003 0.231
0.048 0.027
0.014
0.022
81.97
29.40
2.984 0.382
<2.00>
11.127 0.004
0.108 0.017
0.003 0.151
0.029 0.021
0.018
0.020
73.38
34.12
2.949 0.184
2.25
22.698 0.006
0.142 0.025
0.005 0.273
0.067 0.028
0.027
0.026
83.71
12.17
3.696 0.555
93.71 ± 249.10
-87.71 114.39
11.78 1.94
13.01 1.66
12.86 0.80
16.10 2.40
SURF-109a, natroalunite, J = 0.002403 ± 0.000014; volume 39ArK = 50.96 × 10–10cm3, integrated age = 4.68 ± 0.38 Ma (2σ)
0.50
307.469 ± 0.022
0.304 ± 0.111 0.032 ± 0.458 1.103 ± 0.031
0.011
0.018
101.38
0.18
-4.780 ± 7.746
1.00
170.257 0.015
0.169 0.060
0.012 0.427
0.602 0.022
0.054
0.009
101.41
1.67
-2.482 3.057
<1.50>
8.252 0.003
0.076 0.017
0.003 0.074
0.024 0.017
0.017
0.013
85.02
42.52
1.213 0.122
<1.70>
4.240 0.003
0.081 0.014
0.002 0.099
0.011 0.021
0.011
0.015
74.61
36.47
1.052 0.072
-20.84 ± 33.97
-10.79 13.33
5.25 0.53
4.56 0.31
0361-0128/98/000/000-00 $6.00
1694
1695
SCIENTIFIC COMMUNICATIONS
Appendix (Cont.)
Laser1
Power
(watts)
<1.90>
2.50>
40Ar/39Ar
Isotope ratios
38Ar/39Ar
37Ar/39Ar
36Ar/39Ar
Ca/K
Cl/K
6.212 0.003
13.140 0.007
0.107 0.015
0.106 0.043
0.004 0.116
0.016 0.164
0.018 0.023
0.041 0.061
0.022
0.070
0.020
0.019
40Ar
(%)
81.29
84.70
39Ar
(%)
17.72
1.44
40Ar*/39Ar
K
Age ± 2σ
1.139 0.123
2.010 0.758
4.93 0.53
8.69 3.27
SURF-109b, alunite, J = 0.002397 ± 0.000018; volume 39ArK = 50.61 × 10–10cm3, integrated age = 5.35 ± 2.44 Ma (2σ)
0.75> 420.870 ± 0.060
0.349 ± 0.128 0.071 ± 0.663 1.497 ± 0.072
0.198
0.013
102.40
3.12 -10.205 ± 18.067
<1.00>
7.100 0.004
0.039 0.016
0.024 0.028
0.019 0.034
0.067
0.005
76.05
17.61
1.677 0.195
<1.25>
2.349 0.003
0.032 0.010
0.016 0.016
0.003 0.064
0.045
0.004
34.01
32.21
1.524 0.056
<1.50>
2.036 0.003
0.030 0.013
0.017 0.021
0.002 0.149
0.048
0.004
23.90
20.82
1.518 0.085
<2.00>
1.904 0.004
0.029 0.021
0.019 0.036
0.002 0.295
0.053
0.003
20.91
9.99
1.462 0.160
4.00
5.005 0.006
0.033 0.086
0.026 0.103
0.006 0.331
0.071
0.004
25.73
2.23
3.624 0.622
<7.00>
2.083 0.004
0.031 0.017
0.016 0.030
0.002 0.203
0.045
0.004
20.18
14.03
1.627 0.110
SURF-109c, jarosite, J = 0.002210 ± 0.000012; volume 39ArK = 36.28 × 10–10cm3, integrated age = 1.56 ± 0.42 Ma (2σ)
0.50>
27.797 ± 0.004
0.057 ± 0.031 0.000 ± 0.515 0.094 ± 0.015
0.001
0.006
98.14
25.14
<1.00>
3.435 0.003
0.060 0.014
0.000 0.226
0.011 0.016
0.001
0.010
89.93
42.18
<1.50>
2.934 0.003
0.071 0.012
0.000 0.231
0.009 0.017
0.001
0.013
86.67
21.43
2.50
5.078 0.004
0.077 0.026
0.001 0.227
0.016 0.021
0.003
0.014
89.68
6.05
4.00
4.070 0.005
0.078 0.025
0.001 0.281
0.013 0.029
0.004
0.014
86.94
5.20
0.489 ± 0.403
0.317 0.051
0.362 0.046
0.494 0.103
0.501 0.109
-44.68 ± 80.09
7.24 0.84
6.58 0.24
6.55 0.37
6.31 0.69
15.60 2.67
7.02 0.47
1.95 ± 1.61
1.26 0.20
1.44 0.18
1.97 0.41
2.00 0.44
c. Supergene alunite-group minerals from Santa Rosa
SURF-112, natroalunite, J = 0.002420 ± 0.000014; volume 39ArK = 6.58 × 10–10cm3, integrated age = 26.82 ± 1.69 Ma (2σ)
0.50>
14.300 ± 0.016
0.683 ± 0.025 0.555 ± 0.025 0.036 ± 0.278
1.614
0.159
61.20
4.93
5.459 ± 3.135
<1.00>
10.353 0.005
1.243 0.006
0.581 0.008
0.016 0.073
1.635
0.282
42.74
42.91
5.911 0.363
<1.25>
10.797 0.009
1.115 0.012
0.591 0.013
0.018 0.270
1.689
0.257
36.59
10.56
6.745 1.510
<2.50>
12.217 0.010
1.078 0.013
0.568 0.016
0.023 0.274
1.632
0.249
46.05
8.81
6.509 1.905
<5.00>
11.704 0.005
1.090 0.007
0.600 0.009
0.019 0.093
1.698
0.248
43.89
27.59
6.549 0.541
7.00>
10.828 0.015
1.118 0.019
0.581 0.022
0.024 0.437
1.693
0.264
46.52
5.20
5.611 3.302
23.68 ± 13.51
25.62 1.56
29.21 6.49
28.19 8.19
28.37 2.33
24.33 14.22
SURF-112, natroalunite, J = 0.002420 ± 0.000014; volume 39ArK = 6.29 × 10–10cm3, integrated age = 27.25 ± 0.92 Ma (2σ)
0.50
27.377 ± 0.114
0.362 ± 0.624 0.223 ± 0.451 0.193 ± 0.536
1.724
0.108
73.98
0.11 11.232 ± 60.543
1.00
18.736 0.022
0.576 0.060
0.232 0.087
0.066 0.223
1.361
0.131
64.79
1.46
6.782 4.678
<1.50>
7.917 0.007
1.002 0.011
0.243 0.019
0.010 0.173
1.426
0.227
28.10
12.59
5.676 0.537
<2.00>
9.651 0.004
1.325 0.007
0.236 0.011
0.014 0.040
1.390
0.300
38.37
54.12
5.955 0.169
<2.25>
10.726 0.004
1.030 0.008
0.226 0.011
0.017 0.090
1.331
0.233
40.58
28.98
6.387 0.446
3.00
16.739 0.016
0.278 0.079
0.098 0.096
0.022 0.388
0.554
0.061
16.71
2.73
14.254 2.640
48.38 ± 257.33
29.37 20.09
24.61 2.31
25.81 0.73
27.67 1.92
61.18 11.14
SURF-113, natroalunite, J = 0.002423 ± 0.000020; volume 39ArK = 34.07 × 10–10cm3, integrated age = 26.88 ± 0.32 Ma (2σ)
0.50
7.998 ± 0.011
0.259 ± 0.052 0.067 ± 0.072 0.016 ± 0.402
0.362
0.055
29.95
0.90
5.565 ± 1.964
1.00
7.572 0.005
0.271 0.012
0.053 0.023
0.005 0.164
0.300
0.059
13.64
6.98
6.541 0.245
1.25
7.229 0.004
0.309 0.013
0.048 0.022
0.003 0.162
0.273
0.067
9.50
12.57
6.548 0.152
1.50
7.052 0.003
0.313 0.011
0.048 0.020
0.003 0.116
0.273
0.068
10.10
19.58
6.345 0.106
1.75
6.905 0.003
0.301 0.009
0.048 0.017
0.003 0.102
0.276
0.066
10.38
31.35
6.195 0.090
2.00
6.657 0.003
0.287 0.011
0.051 0.017
0.003 0.074
0.294
0.062
11.90
27.34
5.869 0.072
2.50
6.644 0.009
0.221 0.046
0.074 0.050
0.009 0.474
0.408
0.046
9.04
1.27
6.013 1.233
24.16 ± 8.47
28.37 1.06
28.40 0.66
27.53 0.46
26.88 0.39
25.47 0.31
26.09 5.31
SURF-113, natroalunite, J = 0.002423 ± 0.000020; volume 39ArK = 31.12 × 10–10cm3, integrated age = 27.37 ± 0.32 Ma (2σ)
0.50
8.327 ± 0.010
0.281 ± 0.031 0.071 ± 0.059 0.017 ± 0.247
0.377
0.061
37.42
1.11
5.150 ± 1.260
1.00
7.758 0.004
0.282 0.014
0.057 0.032
0.006 0.154
0.331
0.061
17.52
5.43
6.384 0.299
1.25
7.486 0.003
0.306 0.008
0.048 0.015
0.004 0.118
0.281
0.067
11.32
14.68
6.640 0.127
1.50
7.280 0.003
0.326 0.011
0.049 0.018
0.003 0.133
0.288
0.071
10.69
20.92
6.504 0.132
1.75
7.127 0.003
0.326 0.008
0.051 0.017
0.003 0.058
0.300
0.071
12.23
29.62
6.259 0.063
3.00
6.988 0.003
0.313 0.009
0.053 0.015
0.004 0.088
0.312
0.068
13.04
28.23
6.080 0.095
22.37 ± 5.44
27.69 1.29
28.79 0.55
28.21 0.57
27.15 0.27
26.38 0.41
d. Alunite sample SURF-110 from Cerro Verde
SURF-110, alunite, J = 0.002409 ± 0.000012; volume 39ArK = 115.45 × 10–10cm3, integrated age = 36.17 ± 0.25 Ma (2σ)
0.75
35.254 ± 0.038
0.083 ± 0.440 0.056 ± 0.448 0.136 ± 0.158
0.270
-0.004
93.56
0.04
2.583 ± 7.628
1.50
82.325 0.031
0.111 0.178
0.043 0.323
0.300 0.076
0.142
0.005
103.30
0.07
-3.054 6.936
2.50
9.625 0.004
0.030 0.039
0.014 0.049
0.012 0.053
0.091
0.003
34.00
2.79
6.352 0.188
3.00
8.484 0.004
0.023 0.036
0.011 0.052
0.002 0.138
0.069
0.002
5.98
6.66
7.989 0.088
3.50
8.802 0.003
0.022 0.041
0.012 0.044
0.001 0.326
0.079
0.002
3.09
6.47
8.558 0.125
4.25
7.832 0.003
0.023 0.034
0.011 0.038
0.001 0.352
0.072
0.002
2.56
10.40
7.651 0.095
5.00
8.383 0.003
0.021 0.037
0.009 0.062
0.001 0.293
0.058
0.002
1.43
21.26
8.285 0.051
6.00
8.730 0.003
0.019 0.034
0.007 0.050
0.000 0.869
0.045
0.001
1.29
24.23
8.641 0.125
7.00
8.818 0.003
0.019 0.043
0.005 0.072
0.001 0.557
0.036
0.001
1.17
13.46
8.741 0.090
8.00
9.033 0.003
0.018 0.061
0.006 0.070
0.000 0.759
0.040
0.001
0.53
14.61
9.013 0.083
0361-0128/98/000/000-00 $6.00
1695
11.19 ± 32.95
-13.32 30.37
27.40 0.81
34.39 0.37
36.82 0.53
32.95 0.41
35.65 0.22
37.17 0.53
37.60 0.38
38.75 0.35
1696
SCIENTIFIC COMMUNICATIONS
Appendix (Cont.)
Laser1
Power
(watts)
40Ar/39Ar
Isotope ratios
38Ar/39Ar
37Ar/39Ar
36Ar/39Ar
Ca/K
Cl/K
40Ar
(%)
39Ar
(%)
40Ar*/39Ar
K
Age ± 2σ
SURF-110, alunite, J = 0.002173 ± 0.000014; volume 39ArK = 120.24 × 10–10cm3, integrated age = 34.28 ± 0.23 Ma (2σ)
1.00
11.735 ± 0.006
0.036 ± 0.069 0.022 ± 0.042 0.027 ± 0.028
0.090
0.003
64.70
0.73
4.142 ± 0.226
1.50
6.892 0.003
0.027 0.022
0.009 0.028
0.002 0.047
0.039
0.003
8.99
4.40
6.273 0.040
2.00
7.459 0.003
0.023 0.030
0.010 0.024
0.001 0.097
0.041
0.002
3.40
11.02
7.211 0.038
2.50
8.887 0.003
0.019 0.032
0.007 0.029
0.000 0.189
0.030
0.001
1.25
17.41
8.789 0.039
3.00
9.153 0.003
0.019 0.041
0.008 0.033
0.000 0.296
0.032
0.001
1.18
14.52
9.059 0.051
3.50
9.274 0.003
0.019 0.040
0.008 0.027
0.000 0.292
0.034
0.001
0.95
16.54
9.200 0.045
4.00
9.575 0.003
0.018 0.040
0.011 0.033
0.001 0.145
0.046
0.001
1.49
10.93
9.449 0.042
5.00
9.424 0.003
0.019 0.040
0.014 0.020
0.000 0.180
0.059
0.001
1.08
13.00
9.338 0.040
6.00
9.801 0.003
0.018 0.042
0.011 0.026
0.000 0.234
0.048
0.001
1.06
11.45
9.714 0.047
16.17 ± 0.88
24.42 0.15
28.05 0.15
34.13 0.15
35.17 0.20
35.71 0.17
36.67 0.16
36.24 0.15
37.69 0.18
SURF-110, alunite, J = 0.002409 ± 0.000012; volume 39ArK = 45.41 × 10–10cm3, integrated age = 34.20 ± 0.45 Ma (2σ)
0.40
20.642 ± 0.094
0.107 ± 0.773 0.102 ± 0.775 0.102 ± 0.776
0.167
0.003
112.52
0.07
-2.224 ± 29.984
0.75
37.848 0.073
0.118 0.595
0.106 0.655
0.170 0.371
0.155
0.004
112.09
0.09
-4.780 22.409
1.00
63.703 0.057
0.122 0.420
0.101 0.459
0.248 0.182
0.212
0.006
103.93
0.13
-2.776 14.869
2.00
7.572 0.005
0.023 0.041
0.020 0.044
0.005 0.171
0.055
0.002
15.43
8.57
6.366 0.234
3.00
8.449 0.004
0.020 0.028
0.021 0.032
0.001 0.550
0.057
0.001
2.86
45.50
8.222 0.161
3.25
8.109 0.005
0.022 0.036
0.024 0.031
0.002 0.319
0.065
0.002
5.04
14.63
7.695 0.180
3.60
8.034 0.005
0.022 0.033
0.020 0.042
0.001 0.601
0.056
0.002
2.21
9.43
7.837 0.212
4.50
8.414 0.005
0.020 0.035
0.017 0.044
0.001 0.725
0.048
0.001
1.73
17.20
8.270 0.171
6.00
8.736 0.005
0.020 0.084
0.017 0.096
0.001 0.986
0.046
0.001
-0.02
4.39
8.681 0.390
-9.69 ± 131.01
-20.90 98.53
-12.10 65.06
27.46 1.00
35.38 0.69
33.13 0.77
33.74 0.90
35.59 0.73
37.34 1.66
SURF-110, alunite, J = 0.002152 ± 0.000016; volume 39ArK = 61.63 × 10–10cm3, integrated age = 33.81 ± 0.26 Ma (2σ)
0.50
25.732 ± 0.017
0.070 ± 0.070 0.063 ± 0.053 0.091 ± 0.031
0.220
0.008
94.05
0.27
1.00
8.331 0.004
0.024 0.013
0.023 0.015
0.006 0.023
0.085
0.002
17.97
9.81
1.50
8.977 0.003
0.020 0.013
0.022 0.013
0.001 0.065
0.081
0.001
3.49
20.39
2.75
9.065 0.003
0.020 0.013
0.020 0.014
0.001 0.255
0.076
0.001
1.93
25.66
2.00
9.318 0.003
0.021 0.015
0.024 0.015
0.001 0.092
0.091
0.002
2.36
11.23
2.50
9.449 0.003
0.021 0.015
0.028 0.013
0.001 0.141
0.104
0.002
1.93
7.93
3.50
9.340 0.003
0.020 0.016
0.023 0.014
0.001 0.160
0.087
0.001
1.48
9.56
5.00
9.528 0.003
0.021 0.015
0.028 0.013
0.001 0.122
0.106
0.001
1.53
15.16
1.533 ± 0.768
6.835 0.049
8.675 0.039
8.902 0.065
9.111 0.042
9.279 0.048
9.214 0.048
9.396 0.040
5.94 ± 2.97
26.34 0.19
33.37 0.15
34.24 0.25
35.03 0.16
35.67 0.18
35.42 0.18
36.11 0.15
SURF-110, alunite, J = 0.002147 ± 0.000018; volume 39ArK = 86.75 × 10–10cm3, integrated age = 33.83 ± 0.29 Ma (2σ)
0.50
36.487 ± 0.040
0.095 ± 0.426 0.083 ± 0.270 0.142 ± 0.128
0.259
0.008
88.76
0.06
0.75
36.385 0.021
0.085 0.286
0.070 0.201
0.139 0.076
0.204
0.005
91.41
0.06
1.25
15.489 0.005
0.040 0.071
0.032 0.046
0.039 0.033
0.110
0.004
67.95
0.70
2.00
7.836 0.003
0.026 0.037
0.021 0.019
0.002 0.073
0.078
0.003
7.06
13.67
2.25
8.108 0.003
0.023 0.028
0.012 0.025
0.001 0.109
0.044
0.002
2.27
11.40
2.50
8.879 0.003
0.020 0.036
0.013 0.021
0.001 0.160
0.048
0.001
1.21
10.01
2.75
9.165 0.003
0.019 0.044
0.010 0.029
0.000 0.212
0.036
0.001
0.68
9.81
3.00
9.347 0.003
0.020 0.035
0.016 0.022
0.000 0.213
0.057
0.001
0.84
12.62
3.25
9.351 0.003
0.019 0.036
0.014 0.022
0.000 0.331
0.051
0.001
0.90
11.08
3.50
9.587 0.004
0.020 0.033
0.018 0.022
0.001 0.190
0.068
0.001
1.21
11.62
4.00
9.391 0.003
0.022 0.034
0.026 0.016
0.001 0.275
0.097
0.002
0.91
9.57
5.00
9.777 0.003
0.020 0.038
0.020 0.018
0.001 0.267
0.075
0.001
0.90
9.40
4.411 ± 5.747
3.288 3.297
4.965 0.383
7.285 0.051
7.929 0.038
8.780 0.041
9.113 0.043
9.282 0.044
9.279 0.056
9.485 0.047
9.318 0.055
9.703 0.054
17.01 ± 22.05
12.69 12.68
19.13 1.47
28.00 0.19
30.45 0.14
33.69 0.16
34.96 0.16
35.60 0.17
35.59 0.21
36.37 0.18
35.73 0.21
37.20 0.21
SURF-110, alunite, J = 0.002173 ± 0.000014; volume 39ArK = 46.44 × 10–10cm3, integrated age = 34.27 ± 0.23 Ma (2σ)
0.50
11.612 ± 0.021
0.066 ± 0.348 0.059 ± 0.283 0.055 ± 0.161
0.115
0.007
49.22
0.09
1.00
8.714 0.005
0.031 0.087
0.020 0.064
0.013 0.061
0.069
0.003
36.02
1.38
1.50
7.089 0.004
0.027 0.047
0.015 0.030
0.002 0.131
0.052
0.003
6.46
5.62
2.00
7.766 0.003
0.023 0.034
0.011 0.026
0.001 0.085
0.041
0.002
3.03
11.51
2.25
8.716 0.003
0.021 0.041
0.009 0.031
0.001 0.160
0.034
0.001
1.24
10.55
2.50
8.834 0.003
0.020 0.040
0.009 0.031
0.000 0.173
0.032
0.001
0.90
14.67
2.75
9.528 0.003
0.017 0.040
0.008 0.031
0.000 0.352
0.030
0.001
0.59
19.08
3.00
9.758 0.003
0.018 0.041
0.011 0.029
0.000 0.234
0.039
0.001
0.75
11.17
3.50
9.146 0.003
0.019 0.053
0.010 0.029
0.001 0.261
0.037
0.001
1.26
11.44
5.00
9.607 0.003
0.018 0.046
0.013 0.024
0.000 0.188
0.049
0.001
0.80
14.48
5.774 ± 2.942
5.542 0.242
6.618 0.091
7.533 0.038
8.616 0.044
8.763 0.040
9.486 0.049
9.698 0.049
9.040 0.056
9.543 0.043
22.49 ± 11.39
21.60 0.94
25.76 0.35
29.29 0.15
33.46 0.17
34.03 0.15
36.81 0.19
37.63 0.19
35.10 0.21
37.03 0.17
1 "<"
indicates step used in plateau age calculations, ">" indicates step used in inverse correlation calculations
Neutron flux monitor: 24.36 ± 0.17 Ma MAC-83 biotite (Sandeman et al., 1999)
Isotope production ratios: (40Ar/39Ar)K = 0.0302, (37Ar/39Ar)Ca = 1416.4306, (36Ar/39Ar)Ca = 0.3952, Ca/K = 1.83(37ArCa/39ArK)
0361-0128/98/000/000-00 $6.00
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