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Corresponding author: e-mail, dcn@kori.reno.nv.us

Abstract

A 40Ar/39Ar age of 16.94 ± 0.34 Ma on hypogene alunite from the La Virgen sedimentary and volcanic rockhosted Au deposit and new ages of 16.06 ± 0.11 to 15.58 ± 0.12 Ma on porphyry Cu-Au deposits at Minas Conga confirm the existence of early Miocene and earliest middle Miocene mineralization in the eastern part of the Miocene metallogenic belt of northern Peru. A new K-Ar age of 15.3 ± 0.3 Ma for the Magistral porphyry-skarn Cu-Mo deposit extends the belt of known early and middle Miocene deposits southeast of La Virgen. A 40Ar/39Ar age of 20.02 ± 0.15 Ma is reported for a late intrusive phase at the Michiquillay porphyry Cu deposit.

A 40Ar/39Ar plateau age of 15.61 ± 0.12 Ma on alunite from the San Pedro Sur zone at the La Zanja epithermal Au district is consistent with the location of the district within the middle Miocene Quiruvilca-Pierina subbelt of the Miocene metallogenic belt. 40Ar/39Ar mineralization ages of 20.79 ± 0.10 and 21.78 ± 0.11 Ma have been obtained on mineral deposits of the Malvas and Huinac districts in the Cordillera Negra to the south. Together with a published age of 18.7 ± 0.6 Ma on the Churropampa Au prospect in the eastern part of the Cordillera Negra and a new Re-Os age of 18.15 ± 0.06 Ma on the Pachagón porphyry Cu-Ag prospect northeast of Trujillo, an early Miocene mineral activity in the western part of the metallogenic belt is indicated. The new ages show that clearly magmatically related mineralization was formed at the same latitudes in both the eastern and western parts of the Miocene metallogenic belt during early Miocene and earliest middle Miocene times.

The deposits of the giant Yanacocha Au district, which were formed at about 11 Ma, are younger than deposits to the west and to the east. Deposits of the Hualgayoc district to the north, which have yielded mineralization ages from 12.4 ± 0.4 to 14.4 ± 0.2 Ma, may overlap in time with those of Yanacocha. These districts, which are located within large coeval volcanic fields, could reflect a younger episode of magmatic activity of late middle Miocene age.

Introduction

Noble and McKee (1999) pointed out that two mining districts in northern Peru, Michiquillay and El Toro (Fig. 1), were significantly older than districts of middle Miocene age to the west. These authors suggested the possible existence of a north-south–trending belt of mineral deposits of early Miocene age, which they termed the Michiquillay-El Toro metallogenic subbelt. They also recognized a younger, better defined middle Miocene Quiruvilca-Pierina subbelt to the west. The present communication presents new isotopic ages on altered and/or mineralized rocks from seven mining districts and prospects, Michiquillay, La Virgen, Magistral, La Zanja, and Pachagón, Huinac, and Malvas, that together with new ages on the Minas Conga district provide additional support for the existence of early to early middle Miocene mineralization in the eastern and western parts of the Miocene metallogenic belt of northern Peru. The younger Yanacocha and Hualgayoc districts are located in the central part of the metallogenic belt (Fig. 1). 40Ar/39Ar heating spectra and isochron plots are given in Figures 2 and 3. The data for K-Ar and Re-Os determinations are given in the text.

Fig. 1.

Map showing location of dated mineral deposits and other selected deposits in northern Perú. Districts are as follows: 1 = Cañariaco; 2 = La Granja; 3 = Tantahuatay-Sinchao; 4 = Hualgayoc; 5 = La Zanja; 6 = Sipán; 7 = Minas Conga; 8 = La Carpa; 9 = El Galeno; 10 = Yanacocha; 11 = Michiquillay; 12 = Aurora Patricia; 13 = Sayapuyo; 14 = Algamarca; 15 = Igor; 16 = El Toro; 17 = La Arena; 18 = La Virgen; 19 = Quiruvilca; 20 = Milluachaqui-Salpo; 21 = Mundo Nuevo-Tamboras; 22 = Angasmarca; 23 = Pasto Bueno; 24 = Magistral; 25 = Pashpap; 26 = Nueva California; 27 = El Extraño; 28 = Jacabamba; 29 = Huarangayoc; 30 Pierina; 31 = Antamina; 32 = Huinac; 33 = Ticapampa; 34 = Huanzalá; 35 = Malvas; 36 = Chururopampa. Locations of the several subbelts of the Miocene metallogenic belt are from Noble and McKee (1999). Note that these lines show the trends of the subbelts and in no way indicate their known or possible widths.

Fig. 1.

Map showing location of dated mineral deposits and other selected deposits in northern Perú. Districts are as follows: 1 = Cañariaco; 2 = La Granja; 3 = Tantahuatay-Sinchao; 4 = Hualgayoc; 5 = La Zanja; 6 = Sipán; 7 = Minas Conga; 8 = La Carpa; 9 = El Galeno; 10 = Yanacocha; 11 = Michiquillay; 12 = Aurora Patricia; 13 = Sayapuyo; 14 = Algamarca; 15 = Igor; 16 = El Toro; 17 = La Arena; 18 = La Virgen; 19 = Quiruvilca; 20 = Milluachaqui-Salpo; 21 = Mundo Nuevo-Tamboras; 22 = Angasmarca; 23 = Pasto Bueno; 24 = Magistral; 25 = Pashpap; 26 = Nueva California; 27 = El Extraño; 28 = Jacabamba; 29 = Huarangayoc; 30 Pierina; 31 = Antamina; 32 = Huinac; 33 = Ticapampa; 34 = Huanzalá; 35 = Malvas; 36 = Chururopampa. Locations of the several subbelts of the Miocene metallogenic belt are from Noble and McKee (1999). Note that these lines show the trends of the subbelts and in no way indicate their known or possible widths.

Fig. 2.

Incremental-heating spectra and isochron plots for samples from Michiquillay (J-28/584FT), La Virgen (LAVIRAL) and La Zanja (SPS-340-AL and LZD-3-BIO). Sample number for the Michiquillay specimen indicates drill hole and depth in feet. Analytical uncertainties are given at the 1σ level.

Fig. 2.

Incremental-heating spectra and isochron plots for samples from Michiquillay (J-28/584FT), La Virgen (LAVIRAL) and La Zanja (SPS-340-AL and LZD-3-BIO). Sample number for the Michiquillay specimen indicates drill hole and depth in feet. Analytical uncertainties are given at the 1σ level.

Fig. 3.

Incremental-heating spectra for samples from the Malvas (MALVIS) and Huinac (HUINAC-5) districts, Cordillera Negra. Analytical uncertainties are given at the 2σ level.

Fig. 3.

Incremental-heating spectra for samples from the Malvas (MALVIS) and Huinac (HUINAC-5) districts, Cordillera Negra. Analytical uncertainties are given at the 2σ level.

Mineral Districts

Minas Conga

Minas Conga is a major mineral district, with recently explored porphyry Au-Cu deposits at Chailhuagon and Perol (Llosa et al., 1996; Gustafson et al., 2004). Potassium silicate alteration at Perol and Chailhuagón is dated at 15.80 ± 0.09 (biotite) and 15.58 ± 0.12 (orthoclase) Ma, respectively, by 40Ar/39Ar isochron methods (Gustafson et al., 2004). Coarsegrained hypogene alunite from advanced argillically altered volcanic rock in the Cocañes zone, directly north of Perol, has yielded an age of 16.06 ± 0.11 Ma.

Michiquillay

Michiquillay is a well-known porphyry Cu deposit with an average Au content of about 0.25 g/t. Pertinent references are listed in Noble and McKee (1999). We have obtained a preferred 40Ar/39Ar isochron age of 20.02 ± 0.15 Ma on phenocrystic biotite (sample J-28/584FT) from a specimen of a weakly biotitized porphyry dike lacking quartz veins in the central part of the deposit (Fig. 2). The sharp contrast in alteration and quartz veining with the strongly mineralized wall rocks indicates that this is a late intramineral intrusion. Plateau and total gas ages of 20.10 ± 0.13 and 19.95 ± 0.13 Ma, respectively, are concordant with the isochron age, and the new determination can be confidently taken as dating the later stages of magmatic activity of the Michiquillay porphyry system. The new age is consistent with, and supercedes, the less precise K-Ar ages of 21.0 ± 0.6 and 18.8 ± 1.6 Ma reported by Laughlin et al. (1968) and Llosa et al. (1996), respectively.

El Toro

The El Toro Au deposit is exposed on a hill of the same name near the town of Huamachuco. Gold values are widely distributed both in sandstone and argillite of the Chimú Formation and in silicic volcanic rocks, with pockets of very high grade ore hosted by organic-rich argillite (Montoya et al., 1995). The presence of alunite, zunyite, and kaolinite argue for the involvement of mineralizing solutions of high-sulfidation character (Montoya et al., 1995). Noble and McKee (1999) have presented a K-Ar age on hypogene alunite of 18.1 ± 0.5 Ma.

La Virgen

La Virgen, at the southern end of the belt, includes sandstone-hosted gold deposits that are presently being mined, situated directly in contact with strongly acid-sulfate–altered lavas of silicic composition that in places host significant Au values, although the presence of ore-grade mineralization in the volcanic rocks has yet to be demonstrated. The district provides strong support for the interpretation that sandstone-hosted Au deposits are related to magmatically generated hydrothermal systems of high-sulfidation affiliation (Montoya et al. 1995). An analytically excellent isochron age of 16.94 ± 0.34 Ma (Fig. 3) obtained on hypogene alunite (LAVIR-AL) from acid-sulfate–altered lava in a zone containing anomalous Au values (UTM coordinates 822600E; 911720N) is considered to date the time of mineralization-related hydrothermal alteration. The isochron age is preferred to the slightly older plateau age of 17.25 ± 0.18 Ma because of the probable presence of excess argon, as suggested by the 40Ar/36Ar ratio of 307 ± 11 and the total gas age of 17.41 ± 0.18 Ma.

Magistral

Perelló et al. (2001) have discussed a K-Ar age of 15.3 ± 0.7 Ma (earliest middle Miocene) for hydrothermal biotite (specimen MAG01) from the potassium-silicate–altered San Ernesto porphyry of the Magistral porphyry-skarn Cu-Mo deposit (8°12′56″ lat S, 77°46′18″ long W; see Noble and McKee (1999) for older references bearing on this deposit). Analytical data are as follows: K = 7.054 percent, Ar* = 4.212 nl/g; Ar* = 45 percent, analytical uncertainty given at the 2σ level. This age shows that this deposit is considerably older than the deposits of the late Miocene metallogenic subbelt, within which it is located.

La Zanja

The La Zanja district is comprised of a number of foci of hydrothermal alteration and Au mineralization about 45 km northwest of the Yanacocha district (Tanabe and Turner, 2000; Rodríguez and Ccasa, 2002). The La Zanja district is situated within a volcanic field composed mainly of units of silicic ashflow tuff that is probably part of the northern extension of the early Miocene volcanic fields exposed in the Cordillera Negra and Santiago de Chuco area to the south. Alteration is mainly of a high-level acid-sulfate character, although rocks exposed at lower elevations represent the uppermost parts of one or more porphyry systems. The plateau age on hypogene alunite (sample SPS-340-AL) from the San Pedro Sur zone of the district of 15.61 ± 0.12 Ma incorporates 61.2 percent of the released 39Ar. A small silicic volcanic center northeast of Maki-Maki in the Yanacocha district, on which a 40Ar/39Ar isochron age of 15.62 ± 0.06 Ma was reported by Turner (1997), probably belongs to the same magmatic pulse that produced mineralization at La Zanja. Conversely, a total gas age of 11.91 ± 0.06 Ma on biotite from an unaltered volcanic dome (sample LZD-3-BIO) in the western part of the La Zanja district (UTM coordinates 733823E; 9243363N) shows that igneous activity of about the same age responsible for mineralization at Yanacocha is present as far west as La Zanja.

Early Miocene districts in the Cordillera Negra

Total gas 40Ar/39Ar ages of 20.79 ± 0.10 and 21.78 ± 0.11 Ma, respectively, have been obtained on hydrothermal sericite from the Malvas (sample MALVIS; UTM coordinates 0210000E, 8902000N) and Huinac (sample HUINAC-5; UTM coordinates 0205500E, 8927500N) mining districts in the western part of the Cordillera Negra (Bodenlos and Strackzek, 1957). Analytical uncertainties are at the 2σ level. The incremental heating curves are irregular, and neither plateaus nor isochrons can be defined (Fig. 3). The somewhat old apparent age of step two of the sample HUINAC-5 analysis and the slightly old age of step 1 of the heating spectrum of the sample from Malvas in combination with the slightly low apparent ages of the last steps of the analysis suggest recoil loss of 39Ar combined with partial capture of 39Ar by associated quartz. The total gas ages are considered to provide the best estimates of the time of formation of the sericite. These early Miocene ages are consistent with the K-Ar age of 18.7 ± 0.6 Ma reported for alunite from the Chururopampa epithermal Au prospect in the eastern part of the Cordillera Negra by Noble and McKee (1999) and with the report by Strusievicz et al. (2000) of early Miocene mineralization in the Cordillera Negra.

Early Miocene porphyry mineralization near Otuzco

Perelló et al. (2003) have described the Ag-rich Pachagón porphyry copper system located at 7°45′56″ lat S, 78°36′56″ long W, about 70 km northeast of Trujillo and 15 km northwest of Otuzco. The prospect is situated on the extreme western margin of the Miocene metallogenic belt. A K-Ar age of 18.8 ± 1.6 Ma (analytical uncertainty at the 2σ level) has been obtained on hydrothermal sericite (sample PACHKAR01); analytical data are K = 1.571 percent, Ar* = 1.157 nl/g, and Ar* = 27 percent. A much more accurate and precise Re-Os age of 18.15 ± 0.06 Ma has been obtained on molybdenite from sample PCH01/127.3. Analytical data are Re = 319.4(3) ppm and 187Os = 60.69(8) ppb.

Discussion and Conclusions

The 15.61 ± 0.12 Ma alunite age from La Zanja and the age of about 18 Ma previously reported for the Pachagón porphyry prospect provide additional support for the existence of the middle Miocene Quiruvilca-Pierina metallogenic subbelt in the western part of the Miocene metallogenic belt. The Sipán Au deposit about 17 to 18 km southeast of La Zanja (Candiotti and Guerrero, 1997), which is in the same volcanic field as La Zanja, is probably of similar middle Miocene age. Although the Malvas and Huinac districts as well as the Pachagón prospect are in the westernmost part of the Miocene volcanic field, the Chururopampa prospect dated at 18.7 ± 0.6 Ma is in the eastern part of the Cordillera Negra. The Re-Os age of 18.15 ± 0.06 Ma on Pachagón, moreover, extends the distribution of early Miocene mineral activity well to the northwest of the occurrences in the Cordillera Negra.

The present work adds two additional districts older than 15.5 Ma to the eastern part of the Miocene metallogenic belt and confirms earlier, less accurate K-Ar ages for mineralization at Michiquillay. The age of 15.3 ± 0.7 Ma reported for Magistral extends the distribution of mineralization of this age about 45 km to the southeast. The five dated deposits range from about 20.0 Ma for Michiquillay to 15.6 Ma for nearby Minas Conga and 15.3 Ma for Magistral to the south. Although the presence of early Neogene mineralization along the eastern edge of the metallogenic belt is supported, the ages differ sufficiently so that a single magmatic pulse cannot be invoked to produce all the deposits.

The first author had previously speculated that the spacetime relationships of early Miocene mineralization activity might reflect a westward relocation of the Michiquillay-El To ro subbelt sometime in the latest early or early middle Miocene—perhaps during or following the Quechua I compressive tectonic pulse (Benavides-Cáceres, 1999). The data now available indicate that at various times during the early and middle Miocene, mineral deposits clearly related to magmatism formed at about the same time at markedly different distances from the trench. For example, the Pachagón prospect lies some 70 km to the west-southwest of the Michiquillay-El Toro subbelt, as shown by Noble and McKee (1999). In addition, activity at Magistral, dated at 15.3 ± 0.7 Ma, took place concurrently, within the limits of analytical uncertainty, with mineralization at Pashpap (14.7 ± 0.2 Ma) and Pierina (14.5 ± 0.4 Ma) some 40 km to the west. It is unclear whether there are two discrete loci or whether dating of additional deposits will show that magmatic activity and related early to early middle Miocene mineralization took place over the entire width of the Miocene metallogenic belt during this period. The location of the Algamarca district, somewhat to the east of the trend defined by other deposits of the Quiruvilca-Pierina subbelt, is consistent with the latter interpretation. In contrast, new age determinations (D.C. Noble and C.E. Vidal, unpub. data) support the narrow locus of late Miocene mineralization described by Noble and McKee (1999).

Mineralization at the Hualgayoc district north of Yanacocha took place between about 14.4 ± 0.2 and 12.4 ± 0.4 Ma (Macfarlane et al., 1994; James, 1995; Noble and McKee, 1999). The district lies between the Quiruvilca-Pierina subbelt to the west and the northward projection of the Michiquillay-El To ro subbelt to the east. No significant mineral deposits are present east of Hualgayoc, and perhaps the district is located near where there was a gradual or abrupt change in the distance from the trench of early Miocene magmatism and mineralization.

The giant Yanacocha Au district (Longo, 2000; Gustafson et al., 2004), on which isotopic ages on alunite of about 11 Ma have been reported (Turner, 1997; Noble and McKee, 1999), was formed later than the deposits to the east and west. The district is situated within a large coeval volcanic field. Yanacocha and the Hualgayoc district to the north perhaps reflect another shift, this time to the east, or a broadening, of the locus of magma generation.

As shown by major districts such as Ucchuchacua (Noble and McKee, 1999) and Magistral, the fact that a deposit falls along a well-defined locus of magmatic activity, such as the late Miocene subbelt, does not mean that the age of such a prospect or deposit can be considered as known. The age of a large number of prospects and deposits remains unknown. As only one example, undated deposits and prospects within and near the Michiquillay-El Toro subbelt include La Arena (Díaz and Quirita, 1998) and La Florida between La Virgen and El To ro, and El Galeno, Aurora Patricia, and La Carpa in the vicinity of Michiquillay.

Changes in the location and nature of magmatic and tectonic activity and mineralization can be very rapid. The causes of these changes are unclear, although they presumably were controlled by the location of the subducting plate and the angle of subduction, the form and degree of continuity of the subducting plate, and/or other aspects of plate tectonics. The elucidation of these controls will require better knowledge of the space-time distribution of igneous activity and mineralization.

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Acknowledgments

With the exception of the sample from Malvas, which was collected by the late Jorge Injoque E., samples for 40Ar/39Ar dating were collected and prepared for dating by the first author. Samples from the Cordillera Negra were analyzed at the Geochronology Laboratory of the New Mexico Bureau of Mines and Mineral Resources, Socorro, New Mexico. The remainder of the 40Ar/39Ar analyses were done at the Nevada Isotope Geochronology Laboratory, University of Nevada, Las Vegas, Las Vegas, Nevada. The K-Ar age determinations were done at the Servicio Nacional de Geología y Minería (SERNAGEOMIN) Santiago, Chile. The Re-Os age was obtained by Holly Stein, AIRIE Program, Department of Earth Resources, Colorado State University, Fort Collins, Colorado. We thank the personnel involved.

Lewis B. Gustafson provided information on the Minas Conga and Michiquillay districts and reviewed an early version of the manuscript. Alan Clark provided a constructive review of the manuscript. We thank Compañía. de Minas Buenaventura S.A.A. for permission to publish the La Zanja and Cordillera Negra ages.

Figures & Tables

Fig. 1.

Map showing location of dated mineral deposits and other selected deposits in northern Perú. Districts are as follows: 1 = Cañariaco; 2 = La Granja; 3 = Tantahuatay-Sinchao; 4 = Hualgayoc; 5 = La Zanja; 6 = Sipán; 7 = Minas Conga; 8 = La Carpa; 9 = El Galeno; 10 = Yanacocha; 11 = Michiquillay; 12 = Aurora Patricia; 13 = Sayapuyo; 14 = Algamarca; 15 = Igor; 16 = El Toro; 17 = La Arena; 18 = La Virgen; 19 = Quiruvilca; 20 = Milluachaqui-Salpo; 21 = Mundo Nuevo-Tamboras; 22 = Angasmarca; 23 = Pasto Bueno; 24 = Magistral; 25 = Pashpap; 26 = Nueva California; 27 = El Extraño; 28 = Jacabamba; 29 = Huarangayoc; 30 Pierina; 31 = Antamina; 32 = Huinac; 33 = Ticapampa; 34 = Huanzalá; 35 = Malvas; 36 = Chururopampa. Locations of the several subbelts of the Miocene metallogenic belt are from Noble and McKee (1999). Note that these lines show the trends of the subbelts and in no way indicate their known or possible widths.

Fig. 1.

Map showing location of dated mineral deposits and other selected deposits in northern Perú. Districts are as follows: 1 = Cañariaco; 2 = La Granja; 3 = Tantahuatay-Sinchao; 4 = Hualgayoc; 5 = La Zanja; 6 = Sipán; 7 = Minas Conga; 8 = La Carpa; 9 = El Galeno; 10 = Yanacocha; 11 = Michiquillay; 12 = Aurora Patricia; 13 = Sayapuyo; 14 = Algamarca; 15 = Igor; 16 = El Toro; 17 = La Arena; 18 = La Virgen; 19 = Quiruvilca; 20 = Milluachaqui-Salpo; 21 = Mundo Nuevo-Tamboras; 22 = Angasmarca; 23 = Pasto Bueno; 24 = Magistral; 25 = Pashpap; 26 = Nueva California; 27 = El Extraño; 28 = Jacabamba; 29 = Huarangayoc; 30 Pierina; 31 = Antamina; 32 = Huinac; 33 = Ticapampa; 34 = Huanzalá; 35 = Malvas; 36 = Chururopampa. Locations of the several subbelts of the Miocene metallogenic belt are from Noble and McKee (1999). Note that these lines show the trends of the subbelts and in no way indicate their known or possible widths.

Fig. 2.

Incremental-heating spectra and isochron plots for samples from Michiquillay (J-28/584FT), La Virgen (LAVIRAL) and La Zanja (SPS-340-AL and LZD-3-BIO). Sample number for the Michiquillay specimen indicates drill hole and depth in feet. Analytical uncertainties are given at the 1σ level.

Fig. 2.

Incremental-heating spectra and isochron plots for samples from Michiquillay (J-28/584FT), La Virgen (LAVIRAL) and La Zanja (SPS-340-AL and LZD-3-BIO). Sample number for the Michiquillay specimen indicates drill hole and depth in feet. Analytical uncertainties are given at the 1σ level.

Fig. 3.

Incremental-heating spectra for samples from the Malvas (MALVIS) and Huinac (HUINAC-5) districts, Cordillera Negra. Analytical uncertainties are given at the 2σ level.

Fig. 3.

Incremental-heating spectra for samples from the Malvas (MALVIS) and Huinac (HUINAC-5) districts, Cordillera Negra. Analytical uncertainties are given at the 2σ level.

Contents

GeoRef

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