Gold-copper fertile intrusions in the Hualgayoc mining district, Peru

M Viala1, K Hattori1, P Gomez2

1Department of Earth and Environmental Sciences, University of Ottawa, Ottawa, Ontario, Canada; 2Gold Fields La Cima, Lima, Peru

Introduction Geological setting

Summary and on-going work

Bulk rock composition

Cerium anomaly in zircon: indication of magmaoxidation state

238U/206Pb ages of intrusions from zircons

Zircon textures

Implications for exploration

Lithology and alteration

1 2

3 4

5 6

7

8 9

Aknowledgement

References

0

50

100

150

200

250

300

350

San Jo

se

Caball

erisa

Co Coro

na 1

San M

iguel

Choro B

lanco

Co Coro

na 4

Co Coro

na 6

Cienag

a

Co Coro

na 5

Jesu

s

Hualgay

oc

Las G

ordas

Coymolac

he

San N

icolas

Ce/

Ce*

Mor

e O

xidi

zed

Porphyry Au-Cu deposit

Porphyry-style mineralization High sulphidation Au-style mineralization

Apparently barren

Ce* = (NdN )2 / SmN

8 9 10 11 12 13 14 15 16

Cerro Jesus

Cerro San Jose

San Miguel andesite

Cerro Cienaga

San Miguel diorite

Age (Ma)Apparently barren

Porphyry Au-Cu deposit

Porphyry-style mineralization

High sulfidation-style Au mineralization

High sulfidation Au depositSkarn mineralization

1 sample; N=6

2 samples; N=35

1 sample; N=13

3 samples; N=51

1 sample; N=21

Cerro Corona 6 samples; SHRIMP

Coymolache 1 sample; N=21

Tantahuatay 2 samples; N=42

AntaKori porphyry 2 1 sample; N=26

Hualgayoc rhyodacite

1 sample; N=22

(From Longo et al. 2010)

Yanacocha district

Alunite age

Yanacocha volcanic rocks

AntaKori porphyry 1 1 sample; N=22

Calipuy andesite 1 sample; N=25

Calipuy rhyolite 1 sample; N=22

0,00007

Ì

Cajamarca

HualgayocMina Congas

La CarpaGaleno

Michiquillay

Cerro CoronaCerro CoronaTantahuatay

Sipan

Yanacocha

La Zanja

Peru

25 Km

E 0,00008E

9200,000 N

9300,000 NÌ

Cajamarca

LEGEND

PRECAMBRIAN METAMORPHIC ROCKS

PALEOZOIC INTRUSIVE ROCKSPALEOZOIC METASEDIMENTARY ROCKS

TRIASSIC JURASIC CARBONATES

CRETACEOUS CARBONATESJURASSIC VOLCANIC ROCKS

UPPER CRETACEOUS BATHOLITH

OLIGO MIOCENE VOLCANIC ROCKS

PALEOCENE SEDIMENTARY ROCKS

OLIGO MIOCENE INTRUSION

QUATERNARY

MIOCENE VOLCANIC ROCKS

Cerro CoronaCerro Corona

MAJOR FAULTS

CHICAMA-YANACOCHASTRUCTURAL CORRIDOR

PORPHYRY Au-CuDEPOSIT

HIGH SULFIDATION Au-AgDEPOSIT

Yanacocha

PeruPeru

25 Km25 Km

N

Tantahuatay

TOWN AND VILLAGE

Hualgayoc

20°

7°

40°30°

56°

35°

30°

24°

30°

10°

25°

40°

30°

45°

30°

8°

12°

30° 30°

18°

28°

N

9250000

9255000

000557

000067

000567

24°

4 Km

Alluvial (Quaternary)

Postmineral tuffPostmineral rhyodaciteAndesite domePyroclastic rocksQuartz-phyric dacite Quartz-diorite porphyry Porphyritic diorite

LimestoneSandstone

BeddingFaultVeins (Ag, Cu)Porphyry Au-Cu

Oxide AuMassive pyrite-enargite

Mio

cene

Cret

aceo

us

San Miguel

Tantahuatays

Co. Corona

LEGEND

Sill Coymolache

AntaKori

Co. Hualgayoc

Co. San Jose

Co. Jesus

Co. Cienaga

San Nicolas

Co. Quijote

0 5 10 15 200

20

40

60

80

100

Sr/Y

Y

“Adakite”-like rocks

Normal arc rocks

0 50 100 150 2000

20

40

60

80

100

Sr/Y

Ce/Ce* in zircon

Cerro Corona

Co. Caballerisa

Sill Coymolache

Co. Choro Blanco

San Nicolas

Co. Hualgayoc

San Miguel andesite

San Miguel diorite

Co. Quijote

LEGENDa) b)

0 1 20

10

20

30

La/Y

b

Yb

Garnet residue

Normal arc rocks

0.5 1.50 50 100 150 2000

20

40

60

80

Mg#

Ce/Ce* in zircon

c) d)

200um 100um

100um 50um

Pinkish euhedral zircons with pyrite after heavy liquid separation.

Zircon with apatite inclusions in transmitted light.

Rounded inherited core in zircon from Cerro Hualgayoc.

Zircon with oscillatory zoning.

Cerro Coymolache

Pl

Bt

Hbl

1cm

Pl

Bt

Strong Kfs halo

Wavy Qz-Py vein

1cm

K-staining showing pervasive Kfs alter-ation (yellow color) – Cerro Corona.

Ccp+PyMag

HemChl

2cm

High-grade ore with Ccp, Py, Mag and Hem – Cerro Corona.

Pl

Hbl

Chl veinlet

400um

Chl forming veinlets and replacing Hbl and Bt – San Miguel Diorite.

Qz

White mica

400um

Intense white mica alteration under crossed polars – AntaKori

Fine grained Aln

Qz-Prl matrix

Fe-O-OH

1cm

Advanced argilic alteration forming Aln and Prl – Cerro Cienaga.

Py

Anh

Limestone

1cm

High Temp alteration forming anhydrite veins – AntaKori.

Cerro Hualgayoc rhyodacite

BtQz

Pl1cm

San Miguel andesite

Pl

Hbl

Cpx

1cm

Blue sapphire xenocrysts in theSan Miguel volcanic rocks

Py

400um

Cerro Corona – Phase 3 (strong potassic alteration)

Hbl

Bt

Pl

QzKfs-Qz veinlets

1cm

Cerro Corona – Phase 1 (weak potassic alteration)

1cm Hbl

Pl

Bt

Qz

San Miguel Diorite1cmPl

Chl veinletChl after Hbl

Cerro Quijote

1cm

Pl

Chl

Hbl

N

9250000

9255000

000557

000067

000567

LEGEND14-15 Ma

11-13 Ma

8-9 Ma

Sample location

100um

Zircon with sector and oscillatory zoning. This grain is unsuitable for trace elements analysis.

LA-ICP-MS analysis spot

The Hualgayoc mining district consists of weakly deformed Cretaceous sedimentary rocks (mainly limestone, with minor sandstone and shale). These formations were intruded by several Miocene dioritic bodies including Cerro Corona, and overlaid by andesitic to rhyolitic flows, domes and tuffs. The AntaKori and Tantahuatay deposits are partially hosted in the Calipuy volcanic formation in the western part of the district, south-west of the San Miguel diorite. The Cerro Corona porphyry intruded Cretaceous limestones, west of Cerro Jesus and Cerro San Jose intrusions, which host historic mines of silver-rich intermediate sulfidation veins.

Fig. 2: Simplified geology of the Hualgayoc mining district, modified from Gustafson et al. (2004), after S. Canchaya,J. Paredes and R. Tosdal (1996)

The Hualgayoc mining district is located in the Andean Cordillera of northern Peru, 30km north of the Yanacocha high-sulphidation Au district. The district hosts numerous Au-Cu deposits, including the Cerro Corona Au-Cu porphyry, the Tantahuatay high sulfidation Au, and the AntaKori skarn/high sulphidation Au-Cu deposits. In this study we characterize the igneous rocks in the Hualgayoc mining district and identify the features associated with Au-Cu fertile magmas.

Fig. 1: Regional geological map of the Cajamarca province, from Cerro Corona technical report

Zircon is a common accessory mineral in most intrusions. The grains are euhedral with a pinkish color. Most grains range from 50um to 300um and commonly contain apatite and feldspar inclusions.All zircons show typical magmatic oscillatory zoning, and common sector zoning.Inherited cores are present but rare, and are only found so far in zircons from Cerro Corona and Cerro Hualgayoc.

The magmatic activity in the Huagayoc mining district was previously thought to range from Paleocene to Miocene in age. New U-Pb zircon ages indicate that magmatic activity ranged from 14.8Ma to 9.7Ma, similar to the ages of igneous and hydrothermal activity of the Yanacocha high-sulfidation Au district. Most intrusions formed in a 1 m.y. period between 14-15Ma. Some are associated with mineralization (Cerro Corona) while others appear to be barren (Coymolache). Late magmatism at 9-10Ma consists of barren rhyodacite-rhyolite domes.

Unlike the other Rare Earth Elements, which only exist at the 3+ state, cerium (Ce) can also exist in the 4+ state. In zircon, Ce4+ readily subtitutes for Zr4+ while Ce3+ is strongly excluded. Therefore, the Ce anomaly in zircon can be used as a tracer of magmatic redox state (Ballard et al, 2002).We observe that all intrusions associated with mineralization have a median Ce/Ce* value between 100-170. In contrast, most apparently barren intrusions have a lower median Ce/Ce* value between 50-100. The data suggest that mineralized intrusions are characterized by intrinsically oxidized parental magma, which may be an factor for the Au-Cu mineralization.

All intrusions except Cerro Quijote show an “adakitic”-like signature with high Sr/Y ratios (40-90) and low Y (5-16ppm) (Fig. a). This can be explained by high water contents in parental magmas (>4wt% H2O) that suppress plagioclase crystallization (Sisson and Grove, 1993). This is consistent with the presence of biotite and hornblende phenocrysts in most intrusions. Samples from Cerro Corona, which host the Au-Cu deposit, are among the highest Sr/Y and Ce/Ce* in zircon, along with Cerro Choro Blanco and Cerro Caballerisa (Fig. b). Oxidation condition does not seem to correlate with magma evolution, suggesting that the magmas were intrinsically oxidized (Fig. c).Low La/Yb ratio (<20) of most intrusion indicate that these rocks are not adakites, partial melt of the subducting slab with a garnet residue (Fig. d).

The age range of intrusions in the district was previously uncertain, with some suggestion of at least Eocene to Miocene ages. This study showed that the dated samples range from ~15 to 9Ma; the Cerro Corona porphyry that hosts the Au-Cu deposit is amongst the oldest intrusions. Also, zircon grains from mineralized intrusions have higher Ce/Ce* ratio than most zircon grains from barren intrusions. This suggests that the cerium anomaly in zircon can be used to identify more oxidized intrusions that may potentially be Au-Cu fertile.

The dominant phase of intrusive rocks in the Hualgayoc mining district consist of hornblende±biotite-bearing porphyritic diorite with magnetite micro-phenocrysts, indicating relatively oxidized parental magma. This includes Cerro Corona, the Coymolache sill, the San Miguel diorite, and the San Nicolas, Cerro Jesus and Cerro San Jose intrusions.Volcanic rocks include the Hualgayoc rhyodacite north of Cerro Corona, the San Miguel andesite which contains clinopyroxene with rare xenocrysts of blue sapphire, and the andesitic to rhyolitic Calipuy formation which partially hosts the Tantahuatay and AntaKori deposits.

Alteration is prevalent in all intrusions exceptthe Coyomolache sill, the San Nicolas intrusion and the Hualgayoc rhyodacite. Weak to medium chlorite±epidote alteration affects the San Miguel diorite and Cerro Quijote intrusions. Strong white mica alteration occurs at San Jose, Cerro Jesus, Tantahuatay and AntaKori. Acidic alteration of pyrophylite±alunite is present in Cerro Cienaga, Cerro Tantahuatay and AntaKori. Potassic alteration of K-feldspar+biotite+magnetite occurs at Cerro Corona, and locally in the San Jose intrusion.

Abbreviations: Bt: biotite – Hbl: hornblende – Pl: plagioclase – Qz: quartz Chl: chlorite – Kfs: potassic feldspar – Cpx: clinopyroxene – Anh: anhydrite Aln: alunite – Prl: pyrophyllite – Py: pyrite – Ccp: chalcopyrite – Mag: magnetite Hem: hematite

The Hualgayoc mining district has been affected by Miocene magmatism, from 14.8Ma to 9.7Ma. Most intrusions formed early, in a ~1 m.y. period, and have a variable bulk-rock composition and degree of magma oxidation state. This suggests they could have been formed from different batches of parental magma, which could explain contemporanuous barren and mineralized intrusions. The younger intrusions and volcanic rocks from Tantahuatay and AntaKori plus the Hualgayoc rhyodacite coincide with ages of volcanism and hydrothermal activity associated with the nearby Yanacocha high-sulfidation Au district. This suggests that the similar aged igneous rocks of both districts may originate from the same regional magmatic event. On-going work includes: more zircon dating to better understand the chronological relationship between the intrusions; hornblende geothermobarometry to define the depth of emplacement and temperature of crystallization; Sr and Nd isotopes to characterize the magmatic signatures and contamination from country rocks.

We thank Gold Fields Cerro Corona staff for accomodation at mine site and assistance with sampling; Buenaventura and Regulus Resources Inc. staff for assistance with sampling; and Jeffrey Hedenquist, Samuel Morfin, Glenn Poirier and Alain Mauviel for their assistance.

•Ballard, J. R., Palin, M. J., & Campbell, I. H. (2002). Relative oxidation states of magmas inferred from Ce (IV)/Ce (III) in zircon: application to porphyry copper deposits of northern Chile. Contributions to Mineralogy and Petrology, 144(3), 347-364.

•Longo, A. A., Dilles, J. H., Grunder, A. L., & Duncan, R. (2010). Evolution of calc-alkaline volcanism and associated hydrothermal gold deposits at Yanacocha, Peru. Economic Geology, 105(7), 1191-1241.

•Sisson, T. W., & Grove, T. L. (1993). Experimental investigations of the role of H2O in calc-alkaline differentiation and subduction zone magmatism. Contributions to mineralogy and petrology, 113(2), 143-166.