The time-space distribution of Eocene to Miocene magmatism in the central Peruvian polymetallic province and its metallogenetic implications

Journal of South American Earth Sciences 26 (2008) 16–35

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The time-space distribution of Eocene to Miocene magmatism in the central Peruvian polymetallic province and its metallogenetic implications Thomas Bissig a,b,*, Thomas D. Ullrich a, Richard M. Tosdal a, Richard Friedman c, Shane Ebert a a

Mineral Deposit Research Unit (MDRU), Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC, Canada V6T 1Z4 Depto. Ciencias Geológicas, Universidad Catolica del Norte, Av. Angamos 0610, Antofagasta, Chile c Pacific Centre for Isotopic and Geochemical Research (PCIGR), Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC, Canada V6T 1Z4 b

a r t i c l e

i n f o

Article history: Received 7 December 2005 Accepted 20 October 2007

Keywords: Central Peru Magmatism Metallogeny Neogene Flat Subduction Geochronology Carbonate hosted deposits

a b s t r a c t Eocene to late Miocene magmatism in the central Peruvian high-plain (approx. between Cerro de Pasco and Huancayo; Lats. 10.2–12°S) and east of the Cordillera Occidental is represented by scattered shallowlevel intrusions as well as subaerial domes and volcanic deposits. These igneous rocks are calc-alkalic and range from basalt to rhyolite in composition, and many of them are spatially, temporally and, by inference, genetically associated with varied styles of major polymetallic mineralization. Forty-four new 40 Ar–39Ar and three U/Pb zircon dates are presented, many for previously undated intrusions. Our new time constraints together with data from the literature now cover most of the Cenozoic igneous rocks of this Andean segment and provide foundation for geodynamic and metallogenetic research. The oldest Cenozoic bodies are of Eocene age and include dacitic domes to the west of Cerro de Pasco with ages ranging from 38.5 to 33.5 Ma. South of the Domo de Yauli structural dome, Eocene igneous rocks occur some 15 km east of the Cordillera Occidental and include a 39.34 ± 0.28 Ma granodioritic intrusion and a 40.14 ± 0.61 Ma rhyolite sill, whereas several diorite stocks were emplaced between 36 and 33 Ma. Eocene mineralization is restricted to the Quicay high-sulfidation epithermal deposit some 10 km to the west of Cerro de Pasco. Igneous activity in the earliest Oligocene was concentrated up to 70 km east of the Cordillera Occidental and is represented by a number of granodioritic intrusions in the Milpo–Atacocha area. Relatively voluminous early Oligocene dacitic to andesitic volcanism gave rise to the Astabamba Formation to the southeast of Domo de Yauli. Some stocks at Milpo and Atacocha generated important Zn–Pb (–Ag) skarn mineralization. After about 29.3 Ma, magmatism ceased throughout the study region. Late Oligocene igneous activity was restricted to andesitic and dacitic volcanic deposits and intrusions around Uchucchacua (approx. 25 Ma) and felsic rocks west of Tarma (21–20 Ma). A relationship between the Oligocene intrusions and polymetallic mineralization at Uchucchacua is possible, but evidence remains inconclusive. Widespread magmatism resumed in the middle Miocene and includes large igneous complexes in the Cordillera Occidental to the south of Domo de Yauli, and smaller scattered intrusive centers to the north thereof. Ore deposits of modest size are widely associated with middle Miocene intrusions along the Cordillera Occidental, north of Domo de Yauli. However, small volcanic centers were also active up to 50 km east of the continental divide and include dacitic dikes and domes, spatially associated with major base and precious metal mineralization at Cerro de Pasco and Colquijirca. Basaltic volcanism (14.54 ± 0.49 Ma) is locally observed in the back-arc domain south of Domo de Yauli approximately 30 km east of the Cordillera Occidental. After about 10 Ma intrusive activity decreased throughout Central Perú and ceased between 6 and 5 Ma. Late Miocene magmatism was locally related to important mineralization including San Cristobal (Domo de Yauli), Huarón and Yauricocha. Overall, there is no evidence for a systematic eastward migration of the magmatic arc through time. The arc broadened in the late Eocene to early Oligocene, and thereafter ceased over wide areas until the early Miocene, when magmatism resumed in a narrow arc. A renewed widening and subsequent cessation of the arc occurred in the late middle and late Miocene. The pattern of magmatism probably reflects two cycles of flattening of the subduction in the Oligocene and late Miocene. Contrasting crustal

* Corresponding author. Address: Mineral Deposit Research Unit (MDRU), Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC, Canada V6T 1Z4. E-mail address: [email protected] (T. Bissig). 0895-9811/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2008.03.004

T. Bissig et al. / Journal of South American Earth Sciences 26 (2008) 16–35

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architecture between areas south and north of Domo de Yauli probably account for the differences in the temporal and aerial distribution of magmatism in these areas. Ore deposits are most abundant between Domo de Yauli and Cerro de Pasco and were generally emplaced in the middle and late Miocene during the transition to flat subduction and prior to cessation of the arc. Eocene to early Oligocene mineralization also occurred, but was restricted to a broad east–west corridor from Uchucchacua to Milpo–Atacocha, indicating a major upper-plate metallogenetic control. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Geologists have long tried to recognize patterns in the temporal and spatial distribution of ore deposits and, thus, identify metallogenetic provinces where exploration efforts promise high rates of success. Identifying factors that lead to the observed distribution of mineral resources continues to be important in mineral exploration, and within this context, reliable geochronologic data are an important component for the understanding of metallogenetic relationships. In this contribution we present 44 new 40Ar/39Ar incrementalheating dates of intrusions and volcanic rocks, 32 of which were previously undated. Three new U/Pb zircon and 25 published dates complement the geochronologic database for the region in between Cerro de Pasco (10.2° Lat. S) and Huacravilca (12.5° Lat. S) from the Cordillera Occidental and up to 70 km to the east of it (Figs. 1–3). The study region hosts numerous large polymetallic deposits of different types which have been assigned to two roughly orogen parallel NW striking belts of approximately 700 km strike length and up to 70 km width overall (e.g., Petersen, 1965; Noble and McKee, 1999). The central portion of these belts between 10.2° and 12.5° Lat. S is the focus of this study (Fig. 1) and contains predominantly carbonate hosted Pb–Zn–Ag (±Cu, Au) deposits, but epithermal Au–Ag–Cu deposits are locally important. A direct spatial and, by inference, genetic relationship of the mineralization to shallow-level intrusions can be demonstrated or at least postulated for many of the region’s deposits. However, numerous other shallow-level intrusions lack evidence for metal enrichment, despite the similar carbonate and siliciclastic sedimentary host-rocks. We discuss the spatial and temporal distribution of magmatism and its metallogenetic significance in a more restricted area and thus in a less generalized way than the overview of Noble and McKee (1999). Published K–Ar geochronological studies (Soler and Bonhomme, 1988a,b; Noble and McKee, 1999) on the Cerro de PascoChurín transect (approximately 10.4–11° Lat. S) in the northern part of our study area have identified a previously unrecognized Oligocene metallogenetic episode comprising the Milpo and Ata-

Fig 3

Cerro de Pasco

Junín Iquitos

Co

Tarma

rd

La Oroya

ille

Trujillo

ra c Oc

Huancayo

ide

Lima

Domo de Yauli

al nt

Arequipa

Yauricocha

100 km

Fig. 2 N

Fig. 1. Location map of the area studied. The dashed line represents the continental divide of the Cordillera Occidental. The areas detailed in Figs. 2 and 3 are outlined, as well as the Domo de Yauli structural dome.

cocha Pb–Zn (–Ag) skarn systems (Fig. 3 and Table 1). Noble and McKee (1999) and Noble et al. (2004) compiled geochronological data spanning an area from the Castrovirreina district (13.5°S) to the Yanacocha area (6°S) and found that the overall ages of the polymetallic deposits range from late Eocene to late Miocene, but that the bulk of the mineralization occurred in the middle and late Miocene. However, the available geochronological data are largely conventional K–Ar dates and in many cases whole-rock samples were dated. Further, as emphasized by Noble and McKee (1999), few intrusions and ore deposits in central Perú have been dated. This is particularly the case in the areas between Domo de Yauli and the Huacravilca intrusion (Figs. 1 and 2), but also applies to numerous intrusions and domes between Cerro de Pasco and La Oroya (Figs. 1 and 3). Domo de Yauli is a structural dome that likely already influenced the geometry of Triassic and Jurassic sedimentary basins (Rosas et al., 2007) and is situated on an important ENE striking cross-strike structural discontinuity (Benavides, 1999; Love et al., 2004). The study area is therefore subdivided into ‘‘north” and ‘‘south of Domo de Yauli” herein. We focused on the areas from the continental divide to approximately 70 km east of it, but did not study and therefore do not discuss in detail the main Cenozoic volcanic arcs located to the west of the Cordillera Occidental. Many of our new data cover the area south of Domo de Yauli (Fig. 2) and correspond to intrusions where no previously published age constraints are available. New data were also obtained for a number of intrusions north of Domo de Yauli (Fig. 3), which are complemented by published data. In addition, we confirmed and refined the age constraints for the previously dated Milpo–Atacocha, Chungar and Uchucchacua districts as well as those for some apparently unmineralized domes. Age constraints are now available for the majority of upper Eocene to upper Miocene intrusions in the segment between about 10.2° and 12° Lat. S. 2. The polymetallic ore deposits of central Perú The types of ore deposits in the study region range from Pb–Zn skarn with an inferred depth of emplacement of 2–3 km (Milpo– Atacocha: Johnson, 1955; Gunnesch and Baumann, 1984; Soler, 1986) to shallow cordilleran base metal lode and high-sulfidation replacement deposits (e.g., Colquijirca and Cerro de Pasco: Baumgartner et al., 2003; Bendezú et al., 2003, 2004; Vidal and Ligarda, 2004; Baumgartner, 2007). Besides these carbonate rock-hosted deposits, epithermal mineralization hosted by volcanic rocks is known from the Carhuacayán and Quicay deposits (Fig. 3 and Table 1), whereas porphyry-related Cu mineralization is known at Morococha (Fig. 2 and Table 1). For most deposits it can be readily discerned which of the intrusive rocks are genetically or at least spatially and temporally related to the mineralization. It should be noted, however, that a syngenetic or diagenetic origin has been proposed for several deposits hosted by Triassic, Jurassic or Cretaceous sedimentary rocks. Thus, Rivera (2002) suggests such an origin for early stages of the mineralization at Cerro de Pasco, whereas Gunnesch and Baumann (1984) did not exclude a syngenetic origin for the ores at Atacocha and Milpo. These interpretations are largely based on the presence of stratiform Pb–Zn mineralization, discordant ore bodies at Milpo and Atacocha

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T. Bissig et al. / Journal of South American Earth Sciences 26 (2008) 16–35

Fig. 2. The area studied south of Domo de Yauli. The bedrock geology has been simplified from the 1:100,000 maps sheets published by the Instituto Geológico, Minero y Metalúrgico (INGEMMET, Mégard, 1968). Intrusive and volcanic rocks are differentiated according to the indicated new and published age constraints. UTM coordinates (Zone 18S, PSAD56) are given.

having been explained as remobilized syngenetic ore (Gunnesch and Baumann, 1984). South of Domo de Yauli, Cedillo and Tejada (1988) proposed that the stratiform lead–zinc deposit of Cercapuquio (Fig. 2) is syngenetic with the host Upper Jurassic strata. Our field observations, the regional relationships and more recent studies (e.g., Baumgartner, 2007), however, render unlikely a syngenetic origin for mineralization in these districts. Stratiform and stratabound ores in the San Cristóbal district (Beuchat, 2003) or at Azulcocha (Muñoz, 1994) are assumed to be distal features of intrusion related hydrothermal activity, although in the case of Cercapuquio it is not clear with which intrusion the mineralization was associated. At Milpo, field observations of alteration assemblages within and outside the associated intrusion leave little doubt that it is an intrusion-related skarn. For a comprehensive summary and collection of references we refer to Noble and McKee (1999) and Rosenbaum et al. (2005), but the major mineral deposits of the area discussed herein are summarized in Table 1 (see also Figs. 2 and 3). 3. Samples and analytical procedures Igneous rocks have been sampled in the entire study region. The sampled rocks represent the complete geographic range of the Neogene igneous province east of the main volcanic arcs within the Andean segment discussed herein. Regional geological maps published by INGEMETT at a scale of 1:100,000 were used as a field guide. Both apparently barren and mineralized intrusions were sampled and the freshest possible specimens from the igneous rocks were taken.

After examination under a petrographic microscope, 44 samples from 42 intrusive bodies or volcanic domes were chosen for 40 Ar/39Ar geochronology. Among these, a statistically significant plateau age was obtained for 41 samples, and all except two of the rocks dated were of Cenozoic age (Tables 2 and 3). The 40Ar/39Ar database is complemented by three new U–Pb zircon analyses. See electronic supplementary material for complete analytical data. 3.1. Ar–Ar geochronology After crushing the rocks with a steel mortar, approximately 10 mg of biotite and hornblende with grain sizes between 0.25 and 0.5 mm were handpicked and subsequently washed in deionized water, rinsed and then air-dried at room temperature. Whole-rock samples were chosen in only one case (2PYB524: fine-grained basalt), and plagioclase and nepheline were dated from one sample each. A hand magnet was passed over the samples to remove magnetic minerals and metallic crusher fragments. The samples were wrapped in aluminum foil with similar-aged samples and with neutron flux monitors (Fish Canyon Tuff sanidine, 28.02 Ma (Renne et al., 1998). The samples were irradiated at the McMaster Nuclear Reactor in Hamilton, Ont., for 44 MWH, with a neutron flux of approximately 3  1016 neutrons/cm2. Analyses (n = 54) of 18 neutron flux monitor positions produced uncertainties of <0.5% in the J value. The samples were analyzed at the Noble Gas Laboratory, Pacific Centre for Isotopic and Geochemical Research, University of British Columbia, Vancouver, BC, Canada. The separates were step-heated at increasing laser powers in the defocused beam of a 10-W CO2

19

T. Bissig et al. / Journal of South American Earth Sciences 26 (2008) 16–35

Fig. 3. The area studied north of Domo de Yauli. The bedrock geology has been simplified form the 1:100,000 maps sheets published by the Instituto Geológico, Minero y Metalúrgico (INGEMMET, Cobbing, 1973). Intrusive and volcanic rocks are differentiated according to the indicated new and published age constraints. UTM coordinates (Zone 18S, PSAD56) are given.

Table 1 Ore deposits and major prospects in the region of interest Deposit or district

Type

Selected References

Uchucchacua Colquijirca Milpo–Atacocha Cerro de Pasco

Ag–Mn–Pb–Zn vein, carbonate-replacement and skarn Cordilleran base metal lode and carbonate-replacement high-sulfidation Au–Ag Zn–Pb (–Cu, –Ag, –Au) skarn hosted by Jurassic limestone Cordilleran base-metal lode and carbonate-replacement

Chungar Huarón Río Pallanga Carhuacayán Santander Yauliyacu–Casapalca, Rosaura Morococha Domo de Yauli (San Cristóbal) Rey Salomon Mario Azulcocha–Chuquipita– Jatunhuasi Yauricocha

Zn (–Cu, –Au, –Ag, –Pb, –Mo) skarn Zn–Pb–Cu–Ag veins Zn (–Pb, –Ag, –Au, –Cu) carbonate-replacement vein deposit. Epithermal Au–Ag–Zn–Pb veins and stockwork Polymetallic skarn without direct association with intrusion Pb, Zn (–Ag) polymetallic veins

Bussell et al. (1990), Petersen et al. (2004) Bendezú et al. (2003), Vidal and Ligarda (2004) Johnson (1955), Gunnesch and Baumann (1984) Baumgartner et al., 2003; Bendezú et al., 2004; Baumgartner, 2007 Soler and Bonhomme (1988a) Thouvenin (1983) Farrar and Noble (1976) Noble and McKee (1999) Zimmerninck (1983) N/A

Porphyry Cu and associated base-metal veins W–Cu–Zn–Pb veins, hosted largely by Paleozoic volcanic rocks and metapelites

Eyzaguirre et al. (1975), Beuchat (2003) Beuchat (2003)

Cu-skarn Epithermal (?) Zn–Cu–Mn (–Au) skarn and manto type mineralization

N/A N/A Muñoz (1994)

Pb–Zn skarn and carbonate replacement deposit. Carlin-type (?) Au–Ag veins (Purisima Concepción), distal polymetallic veins. Stratiform Pb–Zn mineralization

Alvarez and Noble (1988), Jurado et al. (2004).

Cercapuquio

Cedillo and Tejada (1988)

Deposits in italics represent abandoned but historically producing deposits or currently subeconomic prospects.

laser (New Wave Research MIR10) until fused. The gas evolved from each step was analyzed using a VG5400 mass spectrometer

equipped with an ion-counting electron multiplier. All measurements were corrected for total system blank, mass spectrometer

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T. Bissig et al. / Journal of South American Earth Sciences 26 (2008) 16–35

Table 2 New age constraints from the region between Domo de Yauli and Huacravilca Sample

Rock/mineral

Cretaceous 2PYB529 Alkali gabbro/neph

Plateau age

Plateau39Ar/steps

Correlation age

Integrated age

470.944/ 8640.569

115.32 ± 0.55

74.2%/10 of 15 steps

116.7 ± 1.8

114.54 ± 0.6

412.554/ 8689.785 456.672/ 8622.151 422.369/ 8650.514 429.611/ 8652.552

40.14 ± 0.61

40.3%/6 of 16 steps

38.3 ± 2.65

31.5 ± 1.62

39.34 ± 0.28

90.5%/7 of 11 steps

39.43 ± 0.85

39.25 ± 0.31

Location

Coord.-UTM

Chicchce

Eocene 2PCB600

Rhyolite sill/bi

2PYB518

Granodiorite/bi

Cerro Maraypaquina Huacravilca

2PYB536

Diorite/zircon

W of Chaucha

2PYB540

Diorite/hb

Chaucha

Oligocene 2PYB532

Dacite/bi

Yanacancha

2PYB531

Andesite flow/hb

W of Yanacancha

Miocene (Aquitanian and Burdigalian) 2PCB594 Gt-bearing rhyolite/ SW of Canchayllo bi 2PYB539a Tonalite/bi W of Vitis 2PYB539b

Monzodiorite/bi

W of Vitis

2PYB544

Granodiorite/bi

Chuquipita

Miocene (Langhian and Seravallian) 2PCB608 Gt-bearing dacite/bi Laguna Tunshu 2PCT-56

Diorite/bi

Laguna Tunshu

2PCT-57

Granodiorite/bi

Laguna Tunshu

2PCB607

Diorite/bi

Laguna Tunshu

2PCB602

Diorite/bi

Laguna Vicecocha

2PYB524

Basalt/WR

Rio de la Virgen

2PYB525

Dacite/bi

Rio de la Virgen

2PCB609

Granodiorite/bi

Cerro Portachuelo

2PCB611

Tonalite/bi

Mina Rey Salomon

Miocene (Tortonian and Messinian) 2PYB503 Qz-monzonite/bi Yauricocha 2PYB505

Qz-monzonite/bi

Yauricocha (Exito)

2PYB512

Dacite dome/bi

Huasicancha

2PYB514

Dacite dome/bi

Huasicancha

Assoc. min.

32.01 ± 0.20

424.055/ 8693.006 409.590/ 8648.150 411.395/ 8647.743 421.755/ 8665.461

422.798/ 8637.969 423.734/ 8635.257 470.768/ 8635.223 471.103/ 8634.445

Ar loss

U/Pb zirocn age: 36.07 ± 0.05 based on weighted mean of 206Pb/238U results (3 fractions) 33.09 ± 0.43 63.7%/6 of 11 steps 33.03 ± 1.22 31.87 ± 0.54

451.578/ 8654.256 454.903/ 8650.156

395.335/ 8684.496 395.818/ 8683.517 395.491/ 8684.246 394.775/ 8685.187 406.414/ 8686.610 464.155/ 8629.957 463.998/ 8630.068 398.103/ 8672.938 390.600/ 8679.614

Comment

95.5%/8 of 10 steps

32.13 ± 0.34

31.9 ± 0.22

100%/8 steps

31.55 ± 0.73

31.2 ± 0.5

18.42 ± 0.15

31.8%/3 of 12 steps

18.19 ± 0.76

18.22 ± 0.13

17.02 ± 0.11

57.6%/5 of 12 steps

17.00 ± 0.24

16.8 ± 0.12

16.66 ± 0.13

16.37 ± 0.34

16.54 ± 0.18

16.20 ± 0.20

98.9%/10 of 11 steps 73.7%/6 of 12 steps

16.75 ± 0.24

15.89 ± 0.18

14.31 ± 0.10

59.5%/4 of 10 steps

14.46 ± 0.96

14.55 ± 0.18

Excess

14.24 ± 0.09

42.6%/3 of 15 steps

13.99 ± 0.69

13.98 ± 0.36

Minor Ar loss?

13.95 ± 0.12

13.47 ± 0.28

14.03 ± 0.57

13.67 ± 0.11

93.1%/11 of 12 steps 82.7%/7 of 10 steps

13.93 ± 0.11

13.45 ± 0.10

13.67 ± 0.13

48.6%/3 of 7 steps

13.73 ± 1.64

13.45 ± 0.18

14.54 ± 0.49

35%/4 of 10 steps

15.5 ± 1.3

15.92 ± 0.42

31.2 ± 0.5

± Skarn

Plag, qz incl. Minor Ar loss

40

Ar?

Inherited Ar

40

± Skarn

CRD CRD

13.85 ± 0.15

69.7%/5 of 12 steps

14.26 ± 2.55

13.86 ± 0.14

12.41 ± 0.13

55.5%/6 of 12 steps

12.41 ± 0.59

12.25 ± 0.13

13.00 ± 0.13

70%/5 of 11 steps

13.07 ± 0.27

12.9 ± 0.11

10 steps 9 to 7Ma

6.21 ± 0.31

7.56 ± 0.09

7.47 ± 0.06

78.8%/6 of 10 steps

7.3 ± 0.35

7.52 ± 0.10

5.87 ± 0.08

57.4%/3 of 14 steps

5.59 ± 1.02

5.28 ± 0.08

Inclusions

5.40 ± 0.25

59.7%/4 of 14 steps

5.38 ± 1.52

5.51 ± 1.32

High atmos.

N/A

Disturbed

See Fig. 2 for locations. Coordinates are given in UTM zone 18, PSAD 56. Full dataset is presented in the electronic supplementary material.

sensitivity, mass discrimination, radioactive decay during and subsequent to irradiation, as well as interfering Ar from atmospheric contamination and the irradiation of Ca, Cl and K (isotope production ratios: (40Ar/39Ar)K = 0.0302, (37Ar/39Ar)Ca = 1416.4306, (36Ar/39Ar)Ca = 0.3952, Ca/K = 1.83 (37ArCa/39ArK). The plateau and correlation ages were calculated using Isoplot ver.3.09 (Ludwig, 2003). Errors are quoted at the 2-sigma (95% confidence) level and are propagated from all sources except mass spectrometer sensitivity and age of the flux monitor. 3.2. U/Pb geochronology All mineral separations, geochemical separations and mass spectrometry were done at the Pacific Centre for Isotopic and Geochemical Research in the Department of Earth and Ocean Sciences,

University of British Columbia. Standard methodology for zircon grain selection, abrasion, dissolution, geochemical preparation and conventional U–Pb zircon isotope dilution thermal ionization mass spectrometry (ID-TIMS) was applied as described in Mortensen et al. (1995). All age errors are quoted at 2r (Ma) level and errors for isotopic ratios are quoted at 1r (%). 4. Geologic evolution of central Perú Geological features related to the Andean geodynamic cycle dominate in the central Peruvian Cordillera Occidental and adjacent areas studied herein (Benavides, 1999). The Andean cycle is characterized by a subduction setting fundamentally similar to that of the present day; that is, the oceanic Nazca Plate and its predecessor, the Farallón Plate, subducting beneath the South American

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T. Bissig et al. / Journal of South American Earth Sciences 26 (2008) 16–35 Table 3 New age constraints from the region between Domo de Yauli and Cerro de Pasco Sample

Rock/mineral

Location

Coord.-UTM

Cretaceous 3PSB633 Hb diorite/hb

Calhuacocha

330.545/ 8778.712

Eocene 2PPB562

Diorite stock/plag

Cerro Señal Raco*

3PPB710

Andesite/hb

Huangur (n. Quicay)

Early Oligocene (Rupelian) 3PYB715 Porph. diorite/hb

Ticlio/Señal*

2PMB582

Milpo stock*

K-alt. diorite/bi

G.-diorite porphyry/ bi Late Oligocene (Chattian) 3PUA558 Dacite porphyry/bi

Socorro (Milpo)*

2PUB560

E of Uchucchacua*

2PMB585

Porphyritic dacite/bi

Uchucchacua*

Early Miocene 2PTB612 Rhyolite/bi

Mina Santa Sabina

2PTB613

Granite/bi

Cerro Santa Ana

2PTB615

Dacite porphyry/bi

Soccochuccho

Middle Miocene 3PSB636 Diorite/bi

Marcapomacocha

3PSB624

Huacracancha

2PCE249

Andesite porphyry/ bi Dacite porph. dike/bi

2PUE246

Dacite porpyry/bi

Iskaycruz

2PUB550

Rhyolite dike/zircon

N of Uchucchacua

Granite/bi

Chungar*

3PSB631

W of Colquijirca*

Late Miocene 3PYB724 Monzonite/bi

Señal Carrizal

3PSB630

Granodiorite/bi

Calhuacocha*

3PSB617

Dacite dome/bi

Carhuacayán*

2PUB553

Rhyolite dike/bi

NE of Uchucchacua

2PUT26

Diorite/zircon

Anamaray

2PPB589

Dacite porphyry/bi

Alpamarca

Correlation age (Ma)

77.8 ± 1.3

95%/6 of 11 steps

81.2 ± 3.1

77.22 ± 0.88

348.992/ 8808.030 352.654/ 8819.271

34.6 ± 1.3

100%/7 of 7 steps

34.1 ± 3.0

35.21 ± 1.97

33.5 ± 1.5

95.8%/8 of 10 steps

32.1 ± 1.9

34.58 ± 3.13

369.607/ 8715.577 368.305/ 8827.975 367.851/ 8829.003

31.6 ± 1.3

80.5%/5 of 8 steps

33.1 ± 2.9

25.92 ± 1.6

29.64 ± 0.26

29.69 ± 0.22

31.02 ± 5.33

30.72 ± 0.72

Skarn

356.599/ 8709.914 330.262/ 8778.340 363.459/ 8760.442 318.360/ 8828.950 311.774/ 8831.511 333.743/ 8806.494

29.59 ± 0.20

Plateau

98.6%/14 of 16 steps

N/A

Skarn (?)

Integrated age (Ma)

Comment

25.28 ± 0.44

98.1/11 of 13 steps

25.55 ± 0.72

25.16 ± 0.25

24.49 ± 0.40

41.2%/3 of 11 steps

25.73 ± 0.89

24.25 ± 0.23

21.00 ± 0.21

47.7%/3 of 11 steps

22.37 ± 0.52

20.92 ± 0.16

21.04 ± 0.20

68.8%/5 of 12 steps

21.19 ± 0.32

20.59 ± 0.19

20.54 ± 0.25

61.8%/6 of 14 steps

20.02 ± 2.29

20.31 ± 0.2

14.96 ± 0.30

98.4%/7 of 10 steps

15.20 ± 0.39

14.7 ± 0.21

14.55 ± 0.26

99.9%/9 of 10 steps

14.37 ± 0.39

14.41 ± 0.19

14.13 ± 0.24

85.8%/8 of 12 steps

14.41 ± 0.4

13.89 ± 0.35

Skarn (?)

13.49 ± 0.30

91.8%/8 of 10 steps

13.53 ± 0.35

13.20 ± 0.20

Skarn (?)

U/Pb zircon age: 13.63 ± 0.11 Ma. Age based on 3 concordant fractions

Skarn (?)

12.88 ± 0.36

38.7%/5 of 10 steps

13.11 ± 0.64

13.64 ± 0.22

10.92 ± 0.4

90.5%/6 of 9 steps

11.16 ± 0.98

10.92 ± 0.23

9.83 ± 0.26

9.34 ± 0.22

408.160/ 7839.589 414.536/ 8743.007 403.577/ 8751.765 355.490/ 8741.209 348.407/ 8761.788 358.806/ 8808.684 308.308/ 8813.241 315.683/ 8831.349 332.366/ 8770.240

Plateau age (Ma)

39

Ar/steps

316.643/ 8825.516 325.320/ 8826.184

Assoc. Min

9.74 ± 0.24 Epith.? Skarn? Skarn (?) Manto (?)

99.16%/7of 8 steps

Min. loss

40

Ar-

± excess Ar

8.2 ± 0.18

99.9%/ 12 of 13 8.18 ± 0.23 8.11 ± 0.14 steps N/A Most steps near 7.3 ± 0.47 7.98 ± 0.11 Age 7 Ma 7Ma U/Pb zircon age: 6.8 ± 1.0. Age estimate based on one concordant fraction 5.75 ± 0.09

44.1%/ 4 of 10 steps

5.53 ± 0.47

6.07 ± 0.1

6-7 Ma

See Fig. 3 for locations. Coordinates are given in UTM zone 18, PSAD 56. Asterisks indicate rocks previously dated by other authors (Table 4). Full dataset is presented in the electronic supplementary material.

continent. The cycle initiated with the break-up of Pangea in the Triassic (Coira et al., 1982) and persists to the present. A Mariana-type subduction with an extensional back arc domain located east of an ensialic arc dominated much of the Mesozoic (Benavides, 1999). Mainly shallow marine sedimentary sequences, composed of limestone and lesser siliciclastic rocks, dominate the stratigraphy of the units deposited in the back arc basin during this time (Benavides, 1999; Rosas et al., 2007). An alkali-gabbro intruding Lower Cretaceous rocks of the Goyllarisquizga group and ChulecPariatambo Formation was dated in this study at 115.3 ± 0.6 Ma (nepheline plateau age: Table 2; Figs. 2 and 4A). This date provides a minimum age for these largely siliciclastic rocks, but also indicates the presence of an extensional back-arc regime up to at least the Aptian. Mid to late Creataceous alkaline magmatism is also documented for eastern Ecuador (Barragán et al., 2005) and Colom-

bia (Vásquez and Altenberger, 2005) as well as in northwestern Argentina (Lucassen et al., 2002). Initiation of spreading in the South Atlantic in the middle Cretaceous led to deformation defined as the Mochica tectonic phase (Mégard, 1984; Jaillard and Soler, 1996) in Central Perú. Based on evidence from the magmatic rock record, the character of arc magmatism changed from voluminous volcanism with the products partially deposited in a subaqueous setting in the fore-arc (Casma Group) up to the Early Cretaceous, to plutonism represented by the 100–60 Ma Coastal Batholith (Cobbing, 1973). However, numerous basaltic dikes, sills and laccoliths intrude the Casapalca Formation red beds in the area of Chungar and Calhuacocha (Fig. 3). These basalts are considered calcalkaline, as they are hornblende and plagioclase-phyric, and lack petrographic evidence for alkaline character. Hornblende from one of these rocks yielded a plateau age of 77.8 ± 1.3 Ma (Table 3

22

T. Bissig et al. / Journal of South American Earth Sciences 26 (2008) 16–35 40

A

39

Ar- Ar age spectra for Cretaceous intrusions

150

2PSB-529, Nepheline, Chicche

Age (Ma)

100

Plateau age = 115.32 ± 0.55 Ma 50

0

B

0

20

40 60 80 Cumulative 39Ar Percent

100

140

2PSB-633 Hornblende Calhuacocha

120

Age (Ma)

100 80 60

Plateau age = 77.8±1.3 Ma

graphic mapping supported by geochronology within the volcanic sequences to the west of the continental divide. Much of the detailed work on the Cenozoic volcanic arcs in Perú has been carried out in northern (e.g., Noble et al., 1990), south central (e.g., Noble et al., 1979a) or southern Perú (Sébrier and Soler, 1991; Sandeman et al., 1995) whereas only limited data are available for central Perú (Noble et al., 1979b, 1999a) In this segment of the Andes, the largest volumes of volcanic rocks were deposited west of the Cordillera Occidental and thus west of the area concerned herein, a notable exception being the early Oligocene Astabamba Formation composed of andesitic and dacitic volcanic rocks which crops out west of Hunacayo (Fig. 2, Mégard, 1968). Volcanic activity west of the Cordillera Occidental of central Perú is represented by the Calipuy Supergroup (Strusievicz et al., 2000) volcanic rocks, a widespread sequence that covers the post Incaic erosional surface (Noble et al., 1979b) in the Cordillera Negra west of the Cordillera Blanca and was assigned a late Eocene age (Noble et al., 1999a). The Calipuy Supergroup was subdivided into the lower Choruro Group (P28 Ma) and the late Oligocene–Miocene Huaraz Group (Strusievicz et al., 2000). On the basis of the available data it is still unclear how volumetrically important Oligocene and Miocene volcanism was west of the Cordillera Occidental of the Andean segment discussed herein. 5. Age of magmatism south of Domo de Yauli

40 20 0 0

20

40

60

80

100

Cumulative 39Ar Percent Fig. 4. 40Ar–39Ar step heating spectra for Mesozoic igneous rocks. Box heights are 2r. See electronic supplementary material for analytical data.

and Figs. 3 and 4B), which is within the age range of the Coastal Batholith. Cenozoic contractional deformation, crustal thickening and uplift in the Andes are related to the balance between upper plate motion and slab roll-back velocity, but not to net convergence (Oncken et al., 2006). The deformation has been defined as a series of orogenic phases (e.g., Mégard, 1984; Sébrier and Soler, 1991; Sandeman et al., 1995; Jaillard and Soler, 1996; Benavides, 1999) which, however, vary in timing both along and across the orogen (Oncken et al., 2006). Within the study area, the late Paleocene to Eocene Incaic orogenic phase formed a fold and thrust belt with the most intense deformation between the rigid block represented by the coastal batholith and the Marañón arch tectonic high constituting the western border of the South American continent at that time (Benavides, 1999). The current Cordillera Occidental represents the backbone of the Incaic orogeny, and most of the folds and thrust faults in the study area have their origin in this orogenic phase. Deformation, subsequent uplift and erosion were most intense in the Cordillera Occidental and decreased in intensity towards the east. The Oligocene and Miocene phases of uplift and contractile deformation in the Central Andes have been summarized by numerous authors (Tosdal et al., 1984; Sébrier and Soler, 1991; Sandeman et al., 1995; Benavides, 1999; Oncken et al., 2006). Within the study area, important mid-Miocene deformation has been constrained to between 14.5 and 5.2 Ma by Farrar and Noble (1976) for areas south of Cerro de Pasco. 4.1. The Paleogene and Neogene volcanic arcs of central Perú The timing of the Cenozoic arc volcanism of central Perú is still relatively poorly understood due to the lack of detailed strati-

The arc segment between Domo de Yauli (Lat., 11.68°S; Long., 76.05°W) and Huacravilca (Lat., 12.48°S; Long., 75.41°W; Fig. 2) is like the northern segment (see below), represented by isolated shallow intrusive centers and volcanic domes. However, compared to the areas north of Domo de Yauli, fewer polymetallic ore deposits are documented in this segment (Table 1). In contrast to the area north of Domo de Yauli, where a number of age constraints have been available prior to this study, only one K–Ar age, for the Yauricocha stock (7.5 Ma; K–Ar biotite: Giletti and Day, 1968; recalculated: Noble and McKee, 1999), has previously been recorded for this part of the study area. Our new data are summarized in Table 2, and age spectra are illustrated in figures as referred to below. 5.1. Late Eocene and early Oligocene intrusive activity The central portion of this area contains a number of upper Eocene intrusive centers. Magmatism of this age has not been recorded previously. The Huacravilca granodiorite intrusion yielded an undisturbed biotite plateau age of 39.34 ± 0.28 Ma (Figs. 2 and 5A). This intrusion features only minor calc–silicate alteration with minor magnetite skarn at its margin. Seventy kilometers farther northwest, at Cerro Maraypaquina (Fig. 2), a coarsely porphyritic rhyolite sill intruding strata of the Casapalca Formation yielded a somewhat disturbed argon release pattern, but a statistically significant biotite plateau age of 40.14 ± 0.61 Ma (Table 2 and Fig. 5B). A similar, slightly argillically altered rhyolite sill cropping out near the small and undated Mario epithermal prospect some 60 km southeast may represent the same intrusive phase, but there is no evidence for mineralization associated with these Eocene intrusions south of Domo de Yauli. All middle Eocene intrusive rocks are calc-alkaline and dacitic to rhyolitic (SiO2 = 68–72 wt.%) in composition. In the late Eocene to early Oligocene, the style and composition of igneous activity changed with the intrusion of diorite stocks near Yauricocha. The largest of these is a coarse-grained hornblende and plagioclase porphyritic diorite exposed at Chaucha (Fig. 2), which yielded a hornblende plateau age of 33.09 ± 0.43 Ma (Fig. 5C), whereas a smaller diorite intrusion approximately

23

T. Bissig et al. / Journal of South American Earth Sciences 26 (2008) 16–35 40

39

Ar- Ar age spectra for Eocene to early Oligocene igneous rocks,south of Domo de Yauli

Age (Ma)

40 20

PA = 40.14 ± 0.61 Ma

120 80 PA = 33.09 0.43 Ma 40

E 60

2PYB531, Hornblende, 60 Astabamba Fm.

Age (Ma)

40 20

Cumulative 39Ar Percent

0

100 39

Cumulative Ar Percent

F

2PCB532, Biotite, Astabamba Fm.

.0058

PA = 31.94 ± 0.31 Ma

40

0

100

Pb/ 238 U

0

100

2PYB540, Hornblende, Chaucha

160

0 Cumulative 39Ar Percent

PA = 31.20 ± 0.50 Ma Age (Ma)

C

2PCB600, Biotite, 60 Co Maraypaquina

206

32 0

D

B

46 2PYB518, Biotite, 44 Huacravilca 42 40 38 36 PA = 39.34 ± 0.28 Ma 34

Age (Ma)

Age (Ma)

A

2PYB536, zircon W of Chaucha

36 D

.0056

20 0

100

0 39

Cumulative Ar Percent

100

0

.0054 .034

39

B E 36.07 ± 0.05 Ma

35

0

37

.036 235 Pb/ U

.038

207

Cumulative Ar Percent

Fig. 5. (A–E) 40Ar–39Ar step heating spectra for Eocene and Oligocene igneous rocks from South of Domo de Yauli. Box heights are 2r. PA: Plateau age. (F) Concordia age diagram for Sample 2PYB536. Age is based on weighted mean of fractions B, D and E. Error given at 2r.

8 km farther west was dated by U/Pb on zircon and yielded a concordant age of 36.07 ± 0.05 Ma (Fig. 5F). In the earliest Oligocene, comparatively large volumes of andesitic to dacitic lava flows, ignimbrites and tuffs of the Astabamba Formation (Fig. 2) erupted approximately 20 km east of the late Eocene intrusive centers mentioned above. Two samples from this volcanic unit were dated: hornblende from an andesite flow yielded an age of 31.2 ± 0.5 Ma, whereas a dacite dome yielded a biotite plateau age of 32.01 ± 0.2 Ma (Fig. 5D and E). Both samples yielded undisturbed age spectra. Evidence for significant polymetallic mineralization associated with either the Astabamba Formation or the diorite stocks is lacking. 5.2. Miocene magmatism The late Oligocene and earliest Miocene, i.e., 31–18.5 Ma, are characterized by an apparent volcanic lull. Magmatic activity resumed with the intrusion of rhyolite sills, domes and cryptodomes a few kilometers south of the town of Canchayllo (Fig. 2

A

U.Cret.

and 6). Phenocrystic biotite from these rhyolites yielded a quasiplateau age of 18.42 ± 0.15 Ma (Fig. 7A). All heating steps yielded ages between 19 and 17 Ma, and the three-step plateau age, although representing only 32% of the 39Ar released, lies within the error range of the isotope correlation (18.19 ± 0.76 Ma) and the integrated ages (18.22 ± 0.13 Ma) and is considered meaningful. The rhyolites at Canchayllo contain red-brownish almandinerich garnet which is interpreted as evidence for assimilation of argillaceous upper crustal material into the melt (Fig. 6). Igneous activity thereafter shifted west, as demonstrated by the large tonalite to granodiorite intrusions near Vitis, 12 km northwest of Yauricocha. Two biotite plateau ages (Fig. 7B and C) were obtained from this intrusive complex: 17.02 ± 0.11 Ma from a tonalite and 16.66 ± 0.13 Ma from a monzodiorite sample. The large intrusions generated a contact metamorphic halo of considerable size, consisting mostly of grey marbles, but no significant polymetallic mineralization has been identified. Intrusive centers younger than 17 Ma are mostly distributed in the western part of the area. Age constraints have been obtained from the Chuquipita intrusion (biotite: 16.20 ± 0.20 Ma; Fig. 7D)

B

garnets

Rhyolitic sill 18.42 ± 0.15 Ma

Fig. 6. Magmatic garnet bearing rhyolite sill near Canchayllo. (A) The rhyolite sill intruding upper cretaceous marly limestones (Celendín Formation). Cliff height approx. 100 m. The rhyolite was dated at 18.42 ± 0.15 Ma. (B) Close up of the garnet bearing rhyolite at Canchallyo showing the garnet crystals which attain a diameter of ca. 5 mm.

24

T. Bissig et al. / Journal of South American Earth Sciences 26 (2008) 16–35

18 16

PA = 18.42 ± 0.15 Ma

14

Cumulative 39Ar Percent

PA = 17.02 ± 0.11 Ma

16

100

E

18 2PYB544, Biotite, Chuquipita 16 14

PA = 16.20 ± 0.15 Ma

12 10

12 PA = 16.66 ± 0.13 Ma

8

0 Cumulative 39Ar Percent

0

100

20

F

2PCB608, Biotite, 18 Cerro Tunshu 16 14 12

PA = 14.31 ± 0.10 Ma

Cumulative 39Ar Percent

100

60 2PCT56, Biotite, Cerro Tunshu 40 PA= 14.24 ± 0.10 Ma

20 0

8 39

100

0

Cumulative Ar Percent

Cumulative Ar Percent

G 28 24

H

2PCT57, Biotite, Cerro Tunshu

Age (Ma)

20 16 12 8

12 PA = 13.67 ± 0.11 Ma

8

0

39

PA = 13.00 ± 0.13 Ma

Cumulative Ar Percent

8

Age (Ma)

12 PA = 12.41 ± 0.13 Ma

4

Cumulative 39Ar Percent

L

16 2PCB602, Biotite, Laguna Vicecocha 14 12

0

39

PA = 13.67 ± 0.13 Ma

10

39

100

Cumulative Ar Percent

Cumulative Ar Percent

Age (Ma)

M

Cumulative 39Ar Percent

100

30 2PYB524, Whole rock, Río de la Virgen PA = 14.54 ± 0.49 Ma

20 10

APA = 16.64 ± 0.26 Ma

0

0

100

0 0

100

8

0

12

4 0

K

16

8

100

16 2PCB609, Biotite, Cerro Portachuelo

100

2PCB611, Biotite 20 Mina Rey Salomon

0

0

Cumulative 39Ar Percent

24

I

16 2PCB607, Biotite, Cerro Tunshu

4

PA = 13.95 ± 0.12 Ma

4

0

100

39

Age (Ma)

0

Age (Ma)

16

10

8

Age (Ma)

2PYB539b, biotite, W of Vitis

4

8 0

Age (Ma)

Age (Ma)

24 20

24 20

Age (Ma)

0

J

C

2PYB539a, Biotite, 28 W of Vitis

12

12

D

32

Age (Ma)

B

22 2PCB594, Biotite, Canchayllo 20

Age (Ma)

Age (Ma)

A

Ar- 39Ar age spectra for early and middle Miocene igneous rocks, south of Domo de Yauli

Age (Ma)

40

0

Cumulative 39Ar Percent

100

2PYB525, Biotite, 16 Río de la Virgen 12 PA = 13.85 ± 0.15 Ma

8 4 0 0

Cumulative 39Ar Percent

100

Fig. 7. 40Ar–39Ar step heating spectra for early and middle Miocene igneous rocks from South of Domo de Yauli. Box heights are 2r. PA, plateau age; APA, apparent plateau age.

where the Chuquipita, Jatunhuasi and Azulcocha mines formerly exploited minor stratiform replacement and skarn Mn–Zn (–As, –Au, –Cu) (Muñoz, 1994). Along the strike of the Andean range, about 20–30 km northwest of Chuquipita, near Cerro Tunshu, six intrusive rocks from a large intrusive complex yielded ages between 14.31 ± 0.10 Ma and 12.41 ± 0.13 Ma (Fig. 7E–J). The rocks range from dioritic to granodioritic compositions. One of these samples taken from a granodiorite porphyry dike contains dark

red almandine-rich magmatic garnet, indicating a peraluminous composition. Calc–silicate alteration, albeit without evidence for sulfide mineralization, is widespread. However, at Mina Rey Salomon (Fig. 2), significant but subeconomic copper skarn lies along the contact of the large tonalite intrusion which forms part of the Cerro Tunshu intrusive complex. This tonalite yielded a biotite plateau age of 13.0 ± 0.13 Ma (Sample 2PCB611; Fig. 7I). Granodiorite from the same complex was dated at 12.41 ± 0.13 Ma and

25

T. Bissig et al. / Journal of South American Earth Sciences 26 (2008) 16–35

represents the youngest intrusive rock of the wider Cerro Tunshu area (Sample 2PCB609; Fig. 7J). Some 10 km east of Cerro Tunshu at Laguna Vicecocha (Fig. 2) several smaller but compositionally similar intrusions crop out. One sample from this area yielded an age of 13.67 ± 0.13 Ma, i.e., within the range of the Cerro Tunshu intrusive complex (Fig. 7K). Basaltic flows occur adjacent to dacite flow-domes near the Río de la Virgen farmstead in the southeastern corner of the study area (Figs. 2 and 8). This whole rock sample is characterized by two apparent plateaus in the age spectrum; the first includes the lower heating steps and yielded an age of 16.64 ± 0.26 Ma, whereas the remaining steps at higher temperature define a 14.54 ± 0.49 Ma plateau age (Fig. 7L). This complex age spectrum is likely due to excess argon. The plateau age defined by the last four heating steps is probably more reliable, but should be considered a maximum. To complement the whole-rock analysis, a plagioclase separate was attempted to date, but yielded only a poor plateau age of 17.9 ± 4.6 Ma. One dacite dome spatially associated with the basalt

was dated at 13.85 ± 0.15 Ma (biotite plateau, Figs. 7M and 8) and may be considered of similar age as the basalt. Magmatism resumed in the late Miocene with the emplacement of the Yauricocha and coeval Exito quartz monzonite to granodiorite intrusions in the Cordillera Occidental. An undisturbed 7.47 ± 0.06 Ma biotite plateau age (Fig. 9A) was obtained from the Exito intrusion, whereas a sample from the compositionally equivalent nearby Yauricocha intrusion yielded an integrated age of 7.56 ± 0.09 Ma, but no interpretable plateau age (Fig. 9B). Both ages agree with the published K–Ar biotite age of 7.5 Ma (Giletti and Day, 1968; recalculated by Noble and McKee, 1999). The youngest currently known magmatic event in the region is represented by the dacite domes of the Herú Formation (Mégard, 1968), which extruded at the eastern limit of the study area near the town of Huasicancha (Fig. 2). Two samples from different domes were analyzed and yielded somewhat disturbed age spectra, probably due to plagioclase inclusions in the magmatic biotite. Nevertheless, statistically significant biotite plateau ages of 5.87 ± 0.08 Ma and 5.4 ± 0.25 Ma were obtained (Fig. 9C and D). Magmatism ceased after the extrusion of these domes. 6. Timing of magmatism north of Domo de Yauli

Dacite dome field 13.65 +/- 0.15 Ma

Owing to the relatively large number of polymetallic deposits and prospects in the area north of Domo de Yauli, many of the intrusive and volcanic rocks have been dated in previous studies (summarized by Noble and McKee, 1999; Table 4). The evolution of the magmatic arc is therefore well established.

Basalt

6.1. Late Eocene and early Oligocene intrusive activity

Basalt

14.54 +/- 0.49 Ma

Fig. 8. Middle Miocene volcanism near Río de la Virgen. Basalt, dated at 14.54 ± 0.49 Ma (whole rock, maximum age); the basalt extruded along a N–S striking fissure and is surrounded by dacitic domes dated at 13.85 ± 0.15 Ma.

A number of late Eocene to early Oligocene domes of dacitic and andesitic composition underlie prominent topographic features rising above the plains to the west of Cerro de Pasco. The oldest magmatic activity was dated indirectly at Quicay where hypogene alunite yielded a K–Ar age of 37.5 Ma (A. Alvarez and D.C. Noble, in Noble and McKee, 1999). A mid-Eocene K–Ar whole-rock age of 38.5 ± 1 Ma was also reported by Soler and Bonhomme (1988b) for the nearby Huangoc quartz monzonite stock; this rock appears

40

Ar- 39Ar age spectra for late Miocene igneous rocks, south of Domo deYauli

B 10

12 2PYB505, Biotite, 10 Yauricocha, Exito

6 PA = 7.47 ± 0.06 Ma

4

0 Cumulative

C

39

No Plateau IA = 7.56 ± 0.09 Ma

0

100

D

10

2PYB514, Biotite, Herú Fm.

Age (Ma)

8

6 4 Pa = 5.87

2

0.08 Ma

Cumulative 39 Ar Percent

6 4 2

0 0

100 Cumulative 39 Ar Percent

Ar Percent

2PYB512, Biotite Herú Fm.

8 Age (Ma)

4

0 0

40

6

2

2

Fig. 9.

2PYB503, Biotite Yauricocha

8

8

Age (Ma)

Age (Ma)

A

100

0

PA = 5.40 0

0.25 Ma

Cumulative 39 Ar Percent

100

Ar–39Ar step heating spectra for late Miocene igneous rocks from South of Domo de Yauli. Box heights are 2r. PA, plateau age; IA, integrated age.

26

T. Bissig et al. / Journal of South American Earth Sciences 26 (2008) 16–35

Table 4 Compiled age constraints for intrusions and ore deposits in the study area Location

Rock/mineral

Age

Method

Reference

Our Ar–Ar age/comment (see Tab. 2, 3)

Uchucchacua Uchucchacua N Cerro de Pasco Cerro de Pasco Yanamate Colquijirca Colquijirca

Pre-mineral dacite dike/sanidine Rhyolite dike/bi

24.5 9.3 ± 0.6

K–Ar K–Ar

Noble and McKee, 1999 Romani, 1982

25.28 ± 0.44/considered equivalent Not dated in this study

14.5 12.4–10.9 15.2 ± 0.4 12.43 ± 0.06 11.6 to 10.5

K–Ar Ar–39Ar K–Ar 40 Ar–39Ar 40 Ar–39Ar

Silberman and Noble, 1977 Bendezú et al., 2004 Soler and Bonhomme, 1988a Bendezú et al., 2003 Bendezú et al., 2003

Not Not Not Not Not

Socorro Milpo

Dacite dome/avg of bi, san, plag Alunite (n = 3) Granodiorite porh./WR Dacite dome/biotite Au–Ag and Base metal mineralization/alunite (n = 7) Granodiorite porph./plagioclase

29.8 ± 1.4

K–Ar

Soler and Bonhomme, 1988a

Sunkullo Mariac Atacocha Raco Huangoc Quicay

Granodiorite porph./biotite Granodiorite porph./biotite Granodiorite porph./WR Diorite stock/WR Quartz Monzonite/WR Hypogene alunite

30.9 ± 0.4 31.1 ± 0.4 29.3 ± 0.5 35.2 ± 0.1 38.5 ± 1.0 37.5

K–Ar K–Ar K–Ar K–Ar K–Ar K–Ar

Chungar Calhuacocha Río Pallanga Bosque de Piedra Carhuacayán Morococha Morococha Chumpe/San Cristóbal Chumpe/San Cristóbal Yauricocha

Granite/biotite Granodiorite/biotite Diorite/biotite Dacite ignimbrite/biotite

13.3 ± 0.3 10.0 ± 0.3 14.6 ± 0.5 5.2 ± 0.2

K–Ar K–Ar K–Ar K–Ar

Soler and Bonhomme, 1988a Soler and Bonhomme, 1988a Soler and Bonhomme, 1988a Soler and Bonhomme, 1988b Soler and Bonhomme, 1988b Alvarez, A. and Noble, D.C., unpub.; Noble and McKee, 1999 Soler and Bonhomme, 1988a Soler and Bonhomme, 1988a Farrar and Noble, 1976 Farrar and Noble, 1976

Our total gas age 30.72 ± 0.72 within error, but disturbed Ar-release pattern Not dated in this study Not dated in this study Not dated in this study 34.6 ± 1.3/ages within error Not dated in this study Not dated in this study

Magmatic steam alunite Quartz monzonite/zircon Quartz monzonite Granodiorite intrusion/zircon

7.8 ± 0.2 8.8–9.1 8.2–8.3 6.6 +1/–3.6

K–Ar U–Pb K–Ar U–Pb

Noble and McKee, 1999 Beuchat, 2003 Eyzaguirre et al., 1975 Beuchat, 2003

Dacite dome Not dated in Not dated in Not dated in

Granodiorite intrusion

5.4 ± 0.3

K–Ar

Unpublished, Noble and McKee, 1999

Not dated in this study

Quartz monzonite/biotite

7.5

K–Ar, recalc

Giletti and Day, 1968; Noble and McKee, 1999

7.47 ± 0.06/coincides well

40

unrelated to mineralization. During this study, we obtained two late Eocene ages for unmineralized domes. A 40Ar/39Ar plagioclase plateau age of 34.6 ± 1.3 Ma (Fig. 10A) was determined for a diorite stock of Cerro Señal Raco (Fig. 3). This stock is unaltered, and lacks hornblende and biotite phenocrysts. Soler and Bonhomme (1988b) reported a similar K–Ar whole-rock age of 35.2 ± 0.1 Ma. At Huangur, 5 km east of Quicay, an andesite flow yielded a hornblende plateau age of 33.5 ± 1.5 Ma (Figs. 3 and 10B), which agrees within the 2r confidence interval with the age for the Cerro Señal Raco diorite. Magmatic activity in the early Oligocene appears to have migrated to the east. A number of intrusions have been dated as early Oligocene in the wider Milpo–Atacocha district. Near the Milpo and Atacocha mines which exploit Pb–Zn skarn mineralization, numerous small intrusive bodies of plagioclase–biotite as well as quartz plagioclase and biotite porphyritic granodiorite stocks intrude into steeply dipping Jurassic limestone. However, most of these intrusive bodies, including the quartz–phyric Socorro stock approximately 500 m north of the Milpo skarn deposit (Fig. 11), lack alteration haloes in the limestone. Soler and Bonhomme (1988a) report a 29.8 ± 1.4 Ma K–Ar plagioclase age for the Socorro stock. In this study, biotite yielded a disturbed argon release pattern with no age plateau (Fig. 10C). However, the integrated age of 30.72 ± 0.72 Ma lies within the 2r confidence level of the published K–Ar age. The skarn at Milpo was emplaced at the intrusive contact to a granodiorite stock (Fig. 11) which differs from Socorro in that it lacks phenocrystic quartz. A potassium–feldspar altered sample containing magmatic biotite yielded an age of 29.59 ± 0.20 Ma (Fig. 10D). Crystallization and potassic alteration of this rock are considered to be closely related in time. Soler and Bonhomme (1988a,b) report a number of additional age constraints for intrusions in the area. The Sunkullo and Mariac intrusions to the southeast of Milpo have ages of about 31 Ma and thus may be considered contemporaneous with the Socorro stock.

dated dated dated dated dated

in in in in in

this this this this this

study study study study study

12.88 ± 0.36/difference explained by excess40Ar 9.74 ± 0.24/ages within error of each other date Not dated in this study Not dated in this study yields similar age of 8.2 ± 0.18 Ma this study this study this study

The Atacocha–San Gerardo stock, in contrast, yielded a slightly younger K–Ar whole-rock age of 29.3 ± 0.5 Ma. Although the age constraints are not unambiguous, the data suggest that the intrusions related to skarn mineralization may have post-dated the barren granodiorite stocks of the district. Only one sample from outside the Uchucchacua to Milpo transect yielded an Oligocene age. This hornblende and plagioclase porphyritic diorite hosted by Casapalca Formation red beds at Ticlio west of Domo de Yauli yielded a hornblende plateau age of 31.6 ± 1.3 Ma (Figs. 2 and 10E). Late Oligocene magmatic activity was restricted to the wider Uchucchacua area. A potassium–feldspar altered dacite porphyry dike from the cross-cut 473-NE at level 450 yielded a magmatic biotite plateau date of 25.28 ± 0.44 Ma (Fig. 10F), which is similar to an 40Ar–39Ar date of 24.5 Ma for a large relict phenocryst of sanidine (unpublished age reported by Noble and McKee, 1999). This dike has some garnet skarn at its margin, but is cut by the Ag-mineralized Sandra vein. Dacitic and andesitic flows and volcaniclastic rocks were deposited to the east of Uchucchacua and crop out near the road to Cerro de Pasco. One sample from an autobrecciated dacitic flow was dated at 24.49 ± 0.40 Ma (Figs. 3, 10G and 12) and thus coincides with the intrusion at Uchucchacua. The volcanic rocks east of Uchucchacua were assigned to the Calipuy Group on the regional maps (Cobbing, 1973), and correspond to the base of the Huaraz Group of Strusievicz et al. (2000). After an apparent lull in magmatism until 21 Ma, igneous activity resumed locally with the emplacement of few small intrusive and volcanic centers 15–30 km southeast of the town of Junín, approximately 50–70 km southeast of Uchucchauca, the region where magmatic activity was concentrated previously (Fig. 3). Biotite plateau ages of these rocks were obtained from a small rhyolite dome at Santa Sabina near the road from La Oroya to Tarma (21.00 ± 0.21 Ma, Fig. 10H), from granite some 6 km to the north-

27

T. Bissig et al. / Journal of South American Earth Sciences 26 (2008) 16–35 40

Ar- 39Ar age spectra for Eocene to early Miocene igneous rocks,north of Domo de Yauli

Age (Ma)

50

B

2PPB-562, Plagioclase, Señal Raco

40 30 20

PA= 34.6 ± 1.3 Ma

C

3PPB-710, Hornblende, Huangur PA = 33.5 ± 1.5 Ma

60

Age (Ma)

60

Age (Ma)

A

40 20

40 2PMB585, Biotite, Milpo (Socorro) 30 20

No Plateau, IA = 30.72 ± 0.72 Ma

10

10 0 100

50

30 20

10

10

0

0

PA = 31.6 ± 1.3 Ma

50 3PUA-588, Biotite, Uchucchacua 40 PA = 25.28 ± 0.44 Ma 30 20

0

39

Cumulative Ar Percent

H Age (Ma)

Age (Ma)

36 2PUB560, Biotite, 32 E of Uchucchacua 28 24 20 PA = 24.49 ± 0.40 Ma 16 12 8 4 0 100 39 Cumulative Ar Percent

100

0

100

0

100

Cumulative 39Ar Percent

10

Cumulative Ar Percent

G

F

3PYB715, Hornblende, Ticlio

40

PA = 29.59 ± 0.2 Ma

39

0

100

Age (Ma)

E

30

0

39

Cumulative Ar Percent

40 2PMB582, Biotite, Milpo stock

20

0

26

I

2PTB612, Biotite, 24 Santa Sabina 22 20 18

PA = 21.0 ± 0.21 Ma

16

Cumulative 39Ar Percent

100

2PCB613, Biotite, 30 Cerro Santa Ana Age (Ma)

Cumulative 39Ar Percent

Age (Ma)

Age (Ma)

D

0

0 0

20 PA = 21.04 ± 0.20 Ma 10

14 0

12 100

0

39

Cumulative Ar Percent

Age (Ma)

J

0

Cumulative 39Ar Percent

100

40 PTB615, Biotite, Soccochuccho 30 20 10

PA = 20.54 ± 0.25 Ma

0 0 Cumulative 39Ar Percent Fig. 10.

100

40

Ar–39Ar step heating spectra for Eocene to lowermost Miocene igneous rocks from North of Domo de Yauli. Box heights are 2r. PA, plateau age; IA, integrated age.

east (21.04 ± 0.20 Ma, Fig. 10I) and from the slightly younger Soccochuccho dacite porphyry located 15 km southeast of Junín (20.54 ± 0.25 Ma, Fig. 10J). Minor copper oxide mineralization at Soccochuccho has been mined, but the other bodies are apparently barren. 6.2. Miocene magmatic activity After the brief late Oligocene–early Miocene magmatic episode, igneous activity ceased until about 16 Ma, when it resumed along the entire length of the segment north of Domo de Yauli. Magmatism was concentrated in the Cordillera Occidental and extended up to 50 km west of the limit of the late Oligocene–early Miocene igneous activity. However, magmatism in the middle Miocene was not restricted to the Cordillera Occidental as it also has been recognized in the major Colquijirca and Cerro de Pasco mineral districts, about 40–50 km east of the continental divide. Magmatic activity in the Cerro de Pasco area began with the 15.2 ± 0.4 Ma

granodiorite intrusion at Yanamate (K–Ar, whole-rock, Soler and Bonhomme, 1988a) whereas at Cerro de Pasco, Baumgartner (2007) reports an age range of 15.4–15.16 Ma (U/Pb single zircons) for the igneous rocks of a dacitic diatreme dome complex spatially related to the Cerro de Pasco deposit. Earlier K–Ar studies (Silberman and Noble, 1977) yielded a somewhat younger age of 14.5 Ma for the same rocks. Slightly later a 14.13 ± 0.24 Ma dacite porphyry dike (40Ar/39Ar biotite; Fig. 13A and Table 3) intruded west of Colquijirca. Dacite from the Marcapunta diatreme–dome complex at Colquijirca yielded magmatic biotite ages of between 12.7 and 12.4 Ma (Bendezú et al., 2003), which predated two pulses of mineralization dated at 11.6–11.3 and 10.9–10.5 Ma (Bendezú et al., 2003, 2004). Alunite related to lead–zinc ores at Cerro de Pasco were deposited at a similar time as those at Colquijirca, i.e., between 12.4 and 10.9 Ma (Bendezú et al. 2004). However, Baumgartner (2007) also reports five alunite ages between 14.54 and 14.41 Ma for the northern part of the diatreme dome complex, but outside the main mineralized zones.

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T. Bissig et al. / Journal of South American Earth Sciences 26 (2008) 16–35

A

Unaltered jurassic limestone

Socorro granodiorite stock: 30.72 ± 0.72 Ma (Integrated age)

B Milpo granodiorite stock: 29.59 ± 0.20 Ma

Alteration

Fig. 11. Intrusions of the Milpo area. (A) The slightly propilitically altered Socorro stock intruding steeply dipping and unaltered Jurassic limestones. The intrusive contact is shown by the dashed line. Geologist for scale. (B) The small open pit operation of the Milpo deposit which roughly outlines the surface outcrop of the Milpo stock (dashed line) and surrounding skarn, hornfels and marble halo (stippled line).

A

Middle Miocene magmatism in the Uchucchacua area is manifested by a rhyolite dike, which locally generated some Pb–Zn– Ag skarn mineralization to the northeast of Uchucchacua (Fig. 12). A concordant U/Pb zircon age of 13.63 ± 0.11 Ma was obtained for this rhyolite dike (Fig. 13K). A dacite porphyry located adjacent to the northern, distal end of the Iskaycruz skarn and replacement deposit, which is located west of the main axis of the Cordillera Occidental, yielded a similar age of 13.49 ± 0.30 Ma (Fig. 13B; biotite plateau). This porphyry, which forms part of a dome, does not appear to be related directly to the polymetallic deposit. Smaller middle Miocene mineralized centers are located south of Cerro de Pasco. These include the abandoned Río Pallanga carbonate replacement deposit (Fig. 3; K–Ar biotite age of 14.6 ± 0.5 Ma reported by Farrar and Noble, 1976, for a spatially associated dacitic dome) and the Chungar skarn deposit (12.88 ± 0.36 Ma, biotite plateau age of a granite, Fig. 13C). Some apparently barren mid-Miocene intrusions are also present. The large diorite intrusion at Marcapomacocha and a small diorite stock at Huacracancha west of Carhuacayán (Fig. 3) yielded biotite plateau ages of 14.96 ± 0.30 Ma and 14.55 ± 0.26 Ma, respectively (Fig. 13D and E). A granodioritic stock at Ticlio was dated at 14.11 ± 0.04 Ma (U–Pb, zircon; Beuchat, 2003), whereas 20 km west of Ticlio (Fig. 2), at Cerro Señal Carrizal a monzonite intrusion apparently unrelated to alteration or mineralization yielded an age of 10.92 ± 0.4 Ma (Fig. 13F). At Calhuacocha, ca. 6 km north of Chungar (Fig. 3), a granodiorite apparently unrelated to significant mineralization was dated at 9.74 ± 0.24 Ma (Fig. 13G). Magmatism younger than 10 Ma generated a number of mineralized centers, most importantly the Toromocho Cu-porphyry at Ticlio dated at 9.11 ± 0.1 Ma (U–Pb zircon, Beuchat, 2003). The historically mined epithermal Pb–Zn–Ag (–Cu) deposit of Carhuacayán 15 km south of Huayllay was dated at age of 7.8 ± 0.3 Ma (K–Ar on alunite, Noble and McKee, 1999), whereas

B

Oligocene andesite, dacite

Rhyolite dike 9.32 ± 0.57 Ma Rhyolite dike, 13.63 ± 0.11 Ma

Rhyolite dyke: 7.3 ± 0.47 Ma (correlation age) Skarn

Andesite

Andesite

Fig. 12. Geological relationships near Uchucchacua. (A) Rhyolite dyke intruding Casapalca Formation red beds northeast of Uchucchacua. The peak in the background represents volcanic rocks which were dated at 24.49 ± 0.40 in outcrops to the southeast of the visible ones. The Dike yielded a biotite isotope correlation age of 7.3 ± 0.47. (B) Middle Miocene rhyolite dyke intruding Cretaceous limestones and Eocene to Oligocene (?), slightly propilitized andesites. The rhyolite dike is similar to the one shown in 7A but was dated at 13.63 ± 0.23 Ma (U/Pb on zircon). The dike on the skyline is one of a series of dikes of which one has been dated by K–Ar at 9.32 ± 0.57 (Romani, 1982). The 6.8 Ma Anamaray skarn lies behind the ridgeline.

29

T. Bissig et al. / Journal of South American Earth Sciences 26 (2008) 16–35

40

B

2PCE249, Biotite, W of Colquijirca

30

PA = 14.13 ± 0.24 Ma

20

16 12 8

10

0

39

100

12 8

F

16 12 8

20

8

PA = 14.75 ± 0.21 Ma

0 0

39

Cumulative Ar Percent

H

Cumulative 39Ar Percent

Age (Ma)

8 PA = 9.74 ± 0.24 Ma 4

22

I

PA = 8.20 ± 0.18 Ma

8

2PUB553, Biotite 18 Uchucchacua No Plateau, IA= 7.98 ± 0.11 Ma 14 10 6

4

0

0 Cumulative Ar Percent

K .0028

14 10

Pb/ 238U

2PPB589, Biotite, Alpamarca

12

PA = 5.746 ± 0.09 Ma

8

Cumulative 39Ar Percent

A

.0024

4

.0032

2PUT26, Zircon, 18 Anamaray ~6.8 ± 1.0 Ma A16 B (Fraction D) 14 C

.0020 10

14 B

2

D 13.63 ± 0.23 Ma

C

0 0

100 Cumulative 39Ar Percent

100 Cumulative 39Ar Percent

L

2PUB550, Zircon, N of Uchucchacua 16

0

100

206

6

2 0

100

Pb/ 238 U

39

206

0

100 39

Cumulative Ar Percent

16 3PSB617, Biotite, Carhuacayan 12

12

0

100

Age (Ma)

3PSB630, Biotite, Calhuacocha

16

PA = 10.92 ± 0.40 Ma

4

0 100

3PYB-724, Biotite, Carrizal

12

4 0

100

Cumulative 39Ar Percent

16

20

PA = 14.96 ± 0.30 Ma

PA = 12.88±0.36 Ma 0

Age (Ma)

16

100

Cumulative 39Ar Percent

28 3PSB624, Biotite, 24 Huacracancha Age (Ma)

Age (Ma)

E

24 3PSB636, Biotite, Marcapomacocha 20

0

Age (Ma)

8 0

0

4

Age (Ma)

12 4

Cumulative Ar Percent

J

16

0

0

G

24 3PSB-631, Biotite Chungar 20

PA = 13.49 ± 0.30 Ma

4

D

28

C

24 2PUE-246, Biotite, 20 Iskaycruz Age (Ma)

Age (Ma)

A

39

Ar- A rage spectra for Miocene igneous rocks,north of Domo de Yauli

Age (Ma)

40

.0020 .011

.015

.019

207

Pb/235U

6

D

.0008 .005

8

.00245

Detail of older 17 results 16 A B

15 .00225 0.014

C 0.016

.015 207

Pb/ 235 U

Fig. 13. (A–J) 40Ar–39Ar step heating spectra for Eocene to lowermost Miocene igneous rocks from North of Domo de Yauli. Box heights are 2r. PA, plateau age; IA, integrated age. (I) Concordia age diagram for a rhyolite dike NE of Uchucchacua, sample 2PUB550. Age based on concordant and equivalent fractions C and D. Fractions A and B show evidence for inherited Pb. (J) Concordia age diagram for diorite at Anamaray, N of Uchcucchacua, sample 2PUT26. The age is estimated to 6.8 Ma based on the concordant age of 6.76 ± 0.02 Ma of fraction D. The inset shows the fractions A, B and C in detail. These fractions are interpreted to contain Tertiary and/or Cretaceous inherited Pb.

the associated dacite dome yielded a biotite plateau age of 8.2 ± 0.18 Ma in this study (Fig. 13H). At Uchucchacua, rhyolite dikes of different ages are present. Besides the middle Miocene dike mentioned above, a petrographically similar dike that intruded into Casapalca Formation red beds northeast of Uchucchacua yielded a slightly disturbed (Fig. 13I) biotite age spectrum with an isotope correlation age of 7.30 ± 0.47 Ma and an integrated age of 7.98 ± 0.11 Ma. A further rhyolite dike in the area has a reported K–Ar age of 9.32 ± 0.57 Ma (Romani, 1982). The age of the dioritic Anamaray intrusion, which generated a minor skarn north of Uchucchacua, is defined by one concordant zircon fraction dated at 6.8 ± 1 Ma by the U/Pb method (Table 3 and Fig. 13L). Despite the large number of igneous rocks dated in the Uchucchacua area it still remains unclear which of these magmatic events is responsible for the bulk of the mineralization in the main Ag–Mn–Pb–Zn vein systems.

The youngest intrusive events include the dacite porphyry dome complex of Alpamarca (5.75 ± 0.09 Ma biotite plateau, Fig. 13J) and the Chumpe intrusion at Domo de Yauli (6.6 + 1/ 3.6 U–Pb zircon age: Beuchat, 2003; 5.4 ± 0.3 Ma commercial K–Ar date reported by Noble and McKee, 1999). The Alpamarca dome is located 20 km due west of Colquijirca and minor Pb–Zn mineralization has historically been mined. The Chumpe intrusion is thought to be related to the mineralization in the San Cristóbal district. Magmatism apparently ceased after the eruption of the Huayllay ignimbrite, for which a K–Ar date of 5.2 ± 0.2 Ma was reported by Farrar and Noble (1976). 7. Discussion Cenozoic magmatic activity was not uniformly distributed in time and space, and important differences in temporal (Fig. 14)

30

A

T. Bissig et al. / Journal of South American Earth Sciences 26 (2008) 16–35

7.1. Late Eocene and Oligocene magmatic activity

7

Age distribution of intrusions south of Domo de Yauli 5

3

1 Y

5

B

7

9

11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41

7 CP IC

5 SC 3 U

Age distribution of intrusions north of Domo de Yauli, including Domo de Yauli

Mi At

Mo

Q

Co

1

U

5

7

9

11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41

Intrusions contiguous with currently producing mines (as indicated)

The oldest documented Cenozoic intrusions occur south of Domo de Yauli where isolated felsic magmas intruded at ca. 40 Ma in a roughly orogen-parallel belt approx 15 km east of the current main Cordillera Occidental, which forms the continental divide (Fig. 15). Shallow intrusive and extrusive magmatism in the northern segment initiated slightly later and only occurs in planar areas west of Cerro de Pasco. It is represented by the Quicay dome and the stocks of Cerro Señal Raco and Huangoc, as well as the andesite flows at Huangur. The Eocene magmatism of the Cerro de Pasco area is spatially more restricted compared to the Eocene magmatism south of Domo de Yauli. Moreover, the presence of volcanic domes indicates that the areas west of Cerro de Pasco experienced only limited erosion and geomorphologic modification since the Eocene. Voluminous late Eocene volcanism is represented by the Chururo Group of the Calipuy Supergroup in the Cordillera Negra (Noble et al., 1999a; Strusievicz et al., 2000). Thus, Eocene magmatism evidently occurred on both sides of the present Cordillera Occidental, although it is still poorly documented west of the Cordillera Occidental of the Andean segment discussed herein. Middle Eocene to early Oligocene magmatism in southern Peru is represented by the Andahuaylas–Yauri Batholith (Perelló et al., 2003).

Intrusions contiguous with small, historically producing deposits Intrusions with no apparent relation to significant mineralization Fig. 14. Histograms of age distribution intrusive rocks from this study and published information (see Tables 1 and 4 for references). The height of the column reflects the number of intrusions or volcanic units dated and does not reflect erupted volumes. (A) The intrusions south of Domo de Yauli (B) the intrusions north of Domo de Yauli Names of the deposits include: Y, Yauricocha; SC, San Cristóbal; U, Uchucchacua; Mo, Morococha; Co, Colquijirca; CP, Cerro de Pasco; Mi, Milpo; At, Atacocha; Q, Quicay.

and spatial (Fig. 15) distribution of volcanic and intrusive rocks are evident between the segments separated by Domo de Yauli. Temporal variations in the distribution of arc volcanism and deformation can be attributed to instability in the equilibrium between absolute motion of the upper plate, i.e., westward drift of the south American continent, and the roll-back velocity of the Nazca plate (e.g., Sébrier and Soler, 1991; Oncken et al., 2006). In contrast, the intensity of magmatic activity appears to be independent of the crustal stress regime (Sébrier and Soler, 1991; Trumbull et al., 2006). The width of the arc is controlled by subduction angle combined with other geologic factors that influence the pathways of ascending magmas, such as the stress regime in the upper plate, crustal architecture and lithospheric delamination (Trumbull et al., 2006). The effect of subduction angle on arc magmatism has been extensively demonstrated for the Pampean flat-slab segment northern Chile (e.g., Kay et al., 1999; Kay and Mpodozis, 2002; Bissig et al., 2003). Areas across the strike of the orogen where magmatism was concentrated over extended periods of time, on the other hand, may be related to cross-strike discontinuities as documented for Antamina, some 120 km north–northwest of Cerro de Pasco, where igneous bodies are more abundant along the discontinuity (Love et al., 2004). A broad west–east aligned belt where intrusions and volcanic rocks of different ages are clustered is now documented between Uchucchacua and Milpo–Atacocha, and a first order crustal control, similar to Antamina is postulated for this transect. Although the evolution of Paleogene and Neogene volcanic arcs to the west of the study area is still insufficiently documented, we present ideas and possible interpretations for the evolution of the arc-magmatism based on the temporal and spatial distribution of the igneous activity from the Cordillera Occidental to the east.

7.2. Oligocene magmatic activity In the early Oligocene intrusion of significant volumes of granodioritic rocks was concentrated in the wider Milpo–Atacocha area, ca. 50–60 km east of the continental divide, and by the intrusion of diorite at Ticlio. Diorite intrusions at Chaucha and the voluminous volcanic Astabamba Formation, in the southern segment of our study area, also were emplaced in the late Eocene and early Oligocene. Compared to the early and middle Eocene, magmatism south of Domo de Yauli appears to have shifted to more mafic compositions in the early Oligocene. Current data suggest a general middle Oligocene magmatic lull from approximately 29.3–25.5 Ma in central Perú. However, magmatism may have been restricted from the early Oligocene (31 Ma) to the early Miocene (18.5 Ma) in the southern portion of the study area. An early to middle Oligocene magmatic quiescence (ca. 31– 26 Ma) all along the Peruvian margin has been proposed by Sébrier and Soler (1991). Magmatism in this period was restricted but has been reported from southern Peru (Sandeman et al., 1995; Clark et al., 1990), although it appears to have been most important in the southern Peruvian Cordillera Oriental and Altiplano (Sébrier and Soler, 1991; Sandeman et al., 1995) as well as the Precordillera of northern Chile (Trumbull et al., 2006). These relationships suggest a regional geodynamic control on the reduction of magma output. Indeed, paleomagnetic data indicate slow rates of convergence between the Nazca and South American plates in the middle Oligocene (Pardo-Casas and Molnar, 1987), which may explain the observed reduction of magma production in central Perú. Magmatism in the study area resumed to the east of Uchucchacua in the late Oligocene, around 25 Ma. It appears that this renewed magmatic activity was, again, restricted to the Uchucchacua–Cerro de Pasco–Milpo transect, but this may be an artifact of the scarcity of geochronological constraints of volcanic rocks along the western slope of the Cordillera Occidental between Domo de Yauli and Uchucchacua. Indeed, based on sparse data, Sébrier and Soler (1991) suggest an increase of arc magmatism at 26 Ma over the entire length of, but restricted to the Cordillera Occidental. This contrasts with the data compilation by Rosenbaum et al. (2005) which indicates only scarce volcanic activity in south-

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T. Bissig et al. / Journal of South American Earth Sciences 26 (2008) 16–35

Late Eocene

Late Oligocene

Early Oligocene

25 Ma

CP 31 - 29.5 Ma

38.5 - 33.5 Ma J

21 Ma

T LO

Domo de Yauli

32 Ma

40 Ma H 33.5 - 31 Ma

39.5 Ma

Early to Mid. Miocene

Late Miocene

14.5-11 Ma

Continental divide/ Cordillera Occidental

8.5-5 Ma

Mineralization - operating mine Mineralization- historic mine

18.5 Ma

11-5 Ma

N

50 km

5 2. -1 17 a M

7.5 Ma Bimodal, 14 Ma

5 Ma

Fig. 15. Schematic representation of arc volcanism in central Peru through time. The grey shaded areas represent the extent, but not the volume of volcanism active at different times. Abbreviations: CP, Cerro de Pasco; J, Junín; T, Tarma; LO, La Oroya; H, Huancayo.

ern and central Peru in the late Oligocene. In southern Peru and northern Chile, the arc evolution was distinct. Clark et al. (1990) document an abrupt broadening of the arc at 28.5 ± 1 Ma in southernmost Peru. Recent comprehensive data compilation (Trumbull et al., 2006) indicates increased magmatic activity around 25 Ma in a broadened arc in southern Perú and northern Chile, whereas further south in the actual Pampean flat-slab segment vigorous arc volcanism initiated after a volcanic lull at 26 Ma, but was concentrated in a narrow arc (e.g., Bissig et al., 2001; Kay and Mpodozis, 2002). Thus, the magmatic evolution of central Perú is probably fundamentally distinct from southern Perú, in that late Oligocene arc volcanism was aerially restricted or, alternatively, concentrated in a still insufficiently documented narrow arc west of the Cordillera Occidental. The increase in magma output after 26 Ma in northern Chile has been attributed to a change in direction of subduction of the Nazca plate from a northeasterly to a more easterly motion and an increase of the rate of convergence (Pilger, 1981; Pardo-Casas and Molnar, 1987). In the northern Chilean Andes, this reconfiguration resulted in a more orthogonal convergence vector (Pilger, 1981; Kay and Mpodozis, 2002). The same change of the direction in relative plate motion resulted in an increase of the obliquity of the subduction beneath the central Peruvian Andes, a convergence geometry, which could explain the generally reduced volcanism and the complete lack thereof south of Domo de Yauli in the late Oligocene. However, reduced magma output, combined with arc widening is also commonly attributed to low subduction angles (e.g., Trumbull et al., 2006). Based on plate reoconstruction models (Yañez et al., 2001) it can be speculated that the Juan Fernandez ridge was subducted below central Peru in the late Oligo-

cene, causing flat subduction and associated widening and volume reduction in the arc. Evidence for a local eastward broadening of the magmatic activity in the latest Oligocene is given by small volumes of ca. 21 Ma felsic rocks which occur approximately 70 km SE from Uchucchacua where 24.5 Ma volcanic rocks have been dated in this study. The differences along strike between the areas north and south of Domo de Yauli in distribution of pre-Miocene magmatism is most likely related to differences in crustal architecture and deformation influencing ascent paths of magmas (Oncken et al., 2006), rather than fundamental differences in subduction parameters. 7.3. Early Miocene magmatic activity Magmatism south of Domo de Yauli resumed with the intrusion of the 18.5 Ma rhyolite sill near Canchayllo, about 35 km east of the continental divide. This rock has a peraluminous composition, and thus differs from most other Neogene rocks of the area. It represents the first magmatic product after a period of quiescence and may reflect partial melting, or extensive assimilation, of crustal material at the initial stages of renewed arc-magmatism. Thereafter, the magmatic arc appears to have contracted to the west, towards the continental divide, at about 17 Ma. Magmatic activity flared up around 15 Ma over the entire strike-length of the Andes within the study area. This general evolution, from a magmatic lull over large areas and over an extended period of time, to relatively wide-spread magmatism that, moreover, moved towards the trench in the earliest middle Miocene may indicate a transition from a flat subduction regime to a more normal subduction setting.

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The arguments for such a geodynamic change are most convincing for the area south of Domo de Yauli, where large middle Miocene intrusions form part of the Cordillera Occidental, but where 14 Ma basaltic volcanism locally occurs in the back arc domain. Such basaltic volcanism would have been facilitated by extension induced by slab steepening and increased roll-back velocity. Basaltic rocks of Eocene, Miocene, and Pliocene age have been reported from the Huancavelica area south of the region discussed herein (Noble et al., 1999b), and provide further evidence for middle Miocene extension in the Huancayo–Huancavelica area. The size of middle Miocene intrusions along the Cordillera Occidental decreases markedly north of Domo de Yauli and basaltic middle Miocene volcanism has not been observed anywhere else in the study area. North of Domo de Yauli, instead, magmatic activity is recorded for the two largest mineralized districts of the study area, Cerro de Pasco and Colquijirca, in the late-middle Miocene, about 40–50 km east of the continental divide. 7.4. Late Miocene magmatic activity After a short episode of comparatively intense middle Miocene magmatism, magma production ceased in the areas south of Domo de Yauli, where no igneous rocks of ages between 12.4 and 7.5 Ma have been recognized. In contrast, from Domo de Yauli to the north magmatism appears to have been more continuous, but at reduced intensity, and several small intrusions and volcanic domes yielded ages between 12 and 9 Ma. Intrusions of this interval are located close to the continental divide and include Cerro Carrizal at Rosaura, the Toromocho porphyry at Morococha, both located west of Domo de Yauli, as well as a granodiorite at Calhuacocha some 60 km farther north near the older Chungar granite intrusion. However, mineralization ages of 10.5–10.9 Ma were obtained for Colquijirca and Cerro de Pasco (Bendezú et al., 2003, 2004), representing magmatic-hydrothermal activity that locally occurred significantly east of the Cordillera Occidental. In the southern portion of the study area, magmatism resumed with the intrusion of the Yauricocha and Exito stocks at 7.5 Ma. At approximately the same time, dacite domes, dated at 8.2 Ma, were emplaced at Carhuacayán and felsic dikes generated skarn in the Uchucchacua district. Indirect evidence for magmatism at this time comes from alteration minerals at Huarón (Thouvenin, 1983 in Noble and McKee, 1999). Both Huarón and Carhuacayán are located approximately 20 km east of the continental divide, indicating that magmatism locally shifted to the east at this time. The youngest recorded magmatic events south of Domo de Yauli are represented by a number of small dacite domes, dated at 5.9– 5.4 Ma, almost 50 km E of the continental divide. At the same time, magmatism north of Domo de Yauli is represented by dacitic to dioritic domes at Alpamarca, as well as the dacitic ignimbrites of Bosque de Piedra. Magmatism occurred throughout the region in a position some tens of km east of the continental divide before it ceased at about 5 Ma. The magmatic evolution of the late Miocene may be interpreted as a function of the flat subduction regime of central and northern Perú currently thought to be responsible for the general lack of volcanism. Plate reconstruction models (Hampel, 2002; Rosenbaum et al., 2005) predict the onset of the subduction of the Nazca ridge to between 11.2 and 14 Ma at the latitude of Cerro de Pasco. Flat subduction is commonly attributed to increased buoyancy of the slab due to the subduction of aseismic ridges such as the Nazca ridge (Gutscher et al., 2000; Yañez et al., 2001; Van Hunen et al., 2002) Other workers suggest that high differential velocity between the upper plate and oceanic plate roll-back velocities are responsible for low subduction angles which may be enhanced during subduction of aseismic ridges (Kay and Mpodozis, 2002; Oncken et al., 2006).

The general decrease in magmatic activity after 11–10 Ma and in the latest Miocene, as well as the eastward shift of magmatism may be interpreted as evidence for slab flattening due to the subduction of the Nazca Ridge. The subducting ridge did apparently not affect areas farther north to the same degree, since magmatism was important in the Cordillera Blanca through the late Miocene (Petford and Atherton, 1992). Differences in distribution of Miocene magmatism between areas south and north of Domo de Yauli are best explained by interplay of upper-plate structural control and the southward more flat subduction geometry. 7.5. Metallogenetic implications of observed magmatism Epithermal mineralization at Quicay supports the only currently producing mine related to Eocene magmatic activity. This mineralization is thought to coincide in time with the eruption of the Chururo group volcanic rocks of the Calipuy Supergroup farther west and represents an isolated metallogenetic event. Presumably these magmatic centers are an extension of the much more continuous metallogenic province in the Cordillera Andahuaylas south of Cusco, which is host to numerous porphyry copper and Fe–Cu skarn deposits and prospects, including Tintaya and Las Bambas (Perelló et al., 2003). Lower Oligocene skarn at Milpo and Atacocha is associated with the last pulses of the late Eocene to early Oligocene magmatic episode observed within the study area before the onset of a magmatic lull in the middle Oligocene. This cessation of magmatism Reconstructed plate convergence rates and vectors (Pilger, 1981; Pardo-Casas and Molnar, 1987) suggest that the timing of mineralization and reduction of magma production rates coincided with decreasing subduction rates. Such a scenario potentially allows heating the slab to reach temperatures, which permit production of highly oxidized, sulfur and metal rich supercritical fluids in the slab (Mungall, 2002). A high-oxidation state at the site of magma generation in the mantle wedge de-stabilizes sulfides and consequently sulfur and ore metals can be incorporated in the melt (Mungall, 2002). We propose that the Milpo and Atacocha–San Gerardo skarn represent a brief metallogenetic event immediately before the cessation of a magmatic episode. Mantle derived magmas were rich in sulfur and oxidized. Lead isotopes from Atacocha indicate that ore metals likely derived from the magma as well as the Jurassic host rocks (Gunnesch et al., 1990) and thus are consistent with an important contribution of ore metals from the arc magma. Late Oligocene mineralization may have been emplaced at Uchucchacua, but evidence is inconclusive and mineralization of similar age has nowhere else been documented within the area discussed. However, if late Oligocene mineralization indeed is present it likely would have been emplaced at a time of limited magmatic activity, possibly associated with a flat subduction configuration. It is important to note that all mineralization from Eocene to late Oligocene age was emplaced in a broad E–W trending belt between Uchucchacua and Milpo–Atacocha. This, points to an important upper-plate metallogenetic control and subduction geometry cannot by itself account for the observed distribution of mineralization. The most prolific regional metallogenetic episode was initiated in the middle Miocene, as has been pointed out previously (Noble and McKee, 1999; Rosenbaum et al., 2005). The onset of this episode also coincided with a generally increased and more widely distributed magma production. In the southern segment, where the geologic relationships suggest a slab steepening and westward migration of magmatic activity, restricted mineralization in the abandoned mines of the Azulcocha district and at Rey Salomón formed at this time. Relatively small middle Miocene deposits,

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including Río Pallanga and Chungar, formed along the Cordillera Occidental farther north. Some intrusive complexes south of Domo de Yauli are quite large, being up to 10 by 5 km in outcrop but the tops of these intrusions have been eroded. The absence or modest size of mineralization (e.g., Rey Salomón) may be attributed to the deep level of exposure. However, mineral deposits, including Río Pallanga, Santander and Chungar farther north along continental divide are of modest size as well, despite the presumably lesser degree of exhumation which has so far failed to expose large intrusive bodies in those areas. The apparently largest middle Miocene ore deposit in the Cordillera Occidental is Iskaycruz, situated close to the transect where all pre-Miocene mineralization was concentrated. Contrasting the deposits along the continental divide, the largest middle Miocene deposits formed in the Cerro de Pasco and Colquijirca districts at a distance of 40–50 km east of the Cordillera Occidental. These two major mineral districts are located significantly east of the axis of the middle Miocene arc farther south and also lie in the broad east-west trending metallogenetic belt where the Iskaycruz, Uchucchaucua, Quicay and Milpo–Atacocha districts are located. Cerro de Pasco and Colquijirca formed in a setting that is unrepresented in the middle Miocene magmatic arc farther south. The clustering of ore deposits of different ages in the Uchucchacua–Cerro de Pasco transect argues for metallogeny controlled by upper plate parameters such as crustal-scale structures across the strike of the structures of the Incaic fold and thrust belt. After about 14–11 Ma, renewed flattening of the subducted slab caused a broadening as well as a decrease of intensity of the magmatic arc, which ended in the cessation of arc magmatism at approximately 5 Ma. Mineralization at Yauricocha, Carhuacayán, Huarón and most importantly at Morococha and San Cristóbal and other deposits near Domo de Yauli can be related to this renewed flat subduction regime. Similarly, the giant Cu– Zn (–Ag, –Mo) skarn deposit of Antamina, some 120 km NW of Cerro de Pasco and in a position approximately 20 km east of the Cordillera Blanca Batholith was emplaced at 10.7–10.1 Ma (Love et al., 2004, and unpublished data). World-class mineralization in a flat subduction environment is widespread in the Pampean flat-slab segment (Kay and Mpodozis, 2001). For the El Indio-Pascua Belt in the centre of this segment Bissig et al. (2003) proposed a metallogenetic model in which highly oxidized supercritical fluids were introduced from the subhorizontal slab to the lower crustal site of melt generation. This configuration had the potential to generate sulfur and metal rich magmas which generated the widespread epithermal mineralization in the early late Miocene in the El Indio-Pascua Belt. The plate configuration and magmatic arc in Central Peru is fundamentally similar. However, the Peruvian flat slab segment has a wider variety of host rocks and a more complex basement architecture compared to the Pampean flat-slab (e.g., Macfarlane, 1999), which leads to the observed wider variety of mineralization styles and episodes in Central Perú.

8. Conclusions Scattered igneous activity occurred in the Central Peruvian Cordillera Occidental and the adjacent high-plains between the late Eocene (40.2 Ma) and the late Miocene (5.2 Ma), but is characterized by regional and temporal variations in intensity. Widespread but minor late Eocene and early Oligocene magmatism was followed by a magmatic lull that lasted from 29.3 to 25.3 Ma north of Domo de Yauli, but persisted until the early Miocene farther south. The reduced magmatic output in the Oligocene correlates with a period of slow convergence between the Nazca

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plate and South American continent and, after about 25 Ma, more oblique subduction and a possible flat slab configuration. Magmatism increased in intensity in the middle Miocene. South of Domo de Yauli, felsic to intermediate rocks intruded near the axis of the Cordillera Occidental and local basaltic volcanism took place in the back-arc domain, a pattern that is interpreted as reflecting the steepening of the subducted slab following a possible late Oligocene episode of flat subduction. North of Domo de Yauli, middle Miocene magmatism is represented by small felsic to intermediate bodies near the continental divide, but also shallow felsic intrusions and domes in the Cerro de Pasco and Colquijirca area, some 40–50 km east of the Cordillera Occidental. The different patterns probably reflect an upper-plate structural control on magma ascent. Magma output rates decreased in the late Miocene and magmatism locally expanded to the east, a pattern that may be interpreted as a renewed onset of flat subduction beneath central Perú. Plate reconstruction models predict the onset of Nazca Ridge subduction in the study area between 14 and 11 Ma, a timing that agrees well with the onset of flat subduction thereafter. The magmatic arc overall expanded and contracted several times from the Eocene to Miocene but did not migrate systematically to the east. Several episodes of intrusion-related ore formation are recognized in the study area. The most important area is a broad, easterly striking belt between Uchucchacua and Milpo–Atacocha where late Eocene (Quicay), early Oligocene (Milpo–Atacocha), late Oligocene (Uchucchacua?), middle Miocene (Iskaycruz (?), Cerro de Pasco, Colquijirca) and late Miocene (Uchucchacua (?), Alpamarca) mineralization events are concentrated. Economic mineralization of Eocene and Oligocene age has not been confirmed anywhere else in the study area, but the abandoned and yet undated Cercapuquio deposit and the Mario prospect could be of this age. Middle Miocene mineralization, which is with the exception of Colquijirca and Cerro de Pasco, of generally modest size, developed along the entire length of the Cordillera Occidental in the study area, whereas, late Miocene mineralization is concentrated at Domo de Yauli, but includes that at the operating mines of Yauricocha to the south and Huarón to the north. We conclude that the Uchucchacua to Milpo transect represents a first-order metallotect where mineralization occurred in several episodes since the late Eocene. However middle and late Miocene deposits of generally more modest size formed throughout central Perú. Exploration should therefore be concentrated around intrusions with ages between 14 and 7 Ma. Intrusions of other ages appear to have had the potential for generating mineralization only in the Uchucchacua–Milpo transect. Acknowledgements The results presented herein are part of the outcomes of a three year MDRU project entitled ‘‘Sources and Exhausts in Polymetallic Carbonate Rock-hosted Ore Deposits”. Financial and logistical support was provided by Anglo American Exploration, Cia. de Minas Buenaventura, Cia. Minera Antamina, BHP Billiton, Falconbridge, Phelps Dodge and Teck Cominco as well as by the Natural Science and Engineering Council of Canada (NSERC). The senior author would also like to thank the Swiss National Science Foundation for a stipend during 2002. Cia. Minera Corona and Cia. Minera Volcan are thanked for granting access to their properties and for logistical support. Field assistance by Plácido Pariguana is gratefully acknowledged. This manuscript greatly benefited from comments by JSAMES reviewers Alan H. Clark and Victor A. Ramos. This article represents MDRU Publication No. P-219.

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