Oxidized, Water-rich Magmatism in the Hualgayoc Au-Cu Mining Camp, Northern Peruvian Cordillera

1Department of Earth Sciences, University of Ottawa, Ontario, Canada; 2Gold Fields La Cima, Lima, Peru; 3Compania de Minas Buenaventura S.A.A., Lima, Peru; 4Regulus Resources Inc., Lima, Peru

Introduction1 Geological setting2

U-Pb zircon dates4

Lithology and alteration 3

Bulk rock composition5

Zircon composition6 7

8 Magma oxygen fugacity estimated using Ce4+/Ce3+ in zircon

Ce and Eu anomaly in zircon

AknowledgementsReferences

9 Implication for exploration 10 Summary and on-going work

M Viala1, K Hattori1, P Gomez2, J Trujillo3, K Heather4

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 Corona

MAJOR FAULTS

CHICAMA-YANACOCHASTRUCTURAL CORRIDOR

PORPHYRY Au-CuDEPOSIT

HIGH SULFIDATION Au-AgDEPOSIT

Yanacocha

PeruPeru

25 km25

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

9 10 11 12 13 14 15 16

Hualgayoc

Calipuy rhyolite

AntaKori porphyry 2

Calipuy andesite

Cienaga north

Sill Coymolache

AntaKori porphyry 1

Cerro Corona

San Miguel

Cienaga south

San Jose

Jesus

Tantahuatayigneous rocks

Ma

Sill Coymolache

Pl

Bt

Hbl

1cm

Cerro Hualgayoc rhyodacite

BtQz

Pl1cm

Cerro Corona – Phase 3 (strong potassic alteration)

Hbl

Bt

Pl

QzKfs-Qz veinlets

1cm

San Jose (weak potassic alter-ation)

Pl

Bt

Kfs

1cm

San Miguel Diorite (weak chlorite alteration)

Pl

Chl after Hbl

1cm

N

9250000

9255000

000557

000067

000567

LEGEND13.5-15 Ma

11-13.5 Ma

9-10 Ma

San Jose (strong white mica alteration)

Pl (WM) Py

1cm

Hbl (WM)

1cm

QzFe-O-Oh

WM

Zorro (intense acidic alteration)

1cm

AlnFe-O-Oh

Prl

Cerro Cienaga (intense alunite + pyrophyllite alteration)

Tantahuatay intrusive rock (in-tense white mica alteration)

QzPy

1cm

WM

Tantahuatay intrusive rock (strong white mica+pyrophyllite alteration)

QzPy

1cmWM + Prl

1cm

Py+Qz

Ccp+PyMag

HemChl

2cm

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

Pyrite+enargite vein – Tantahuatay

Choro Blanco Caballerisa

CC phase 1

Mineralized intrusions

CC phase 4CC phase 6

San Miguel

Coymolache San Nicolas

Apparently barren intrusions

-18

-17

-16

-15

-14

660 680 700 720 740 760

Log

fO2

T (C°)

Max. s

olubili

ty of

Au in

magm

a

Median zircon values

FMQ

FMQ +

1

FMQ +

2

-19

-18

-17

-16

-15

-14

-13

600 650 700 750 800

Log

fO2

T (C°)

Max.

solub

ility of

Au in

mag

ma

All zircons

FMQ

FMQ +

1 FM

Q +2

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu.001

.01

.1

1

10

100

1000

10000Rock/Chondrites

a)

Median zircon value of each intrusions

Zircon field

Inherited core from Cerro Hualgayoc

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8600

700

800

900

T°(C

)

Th/U

c)

HualgayocCoymolache

San Miguel San JoseCienagaCerro Corona

CaballerisaChoro Blanco

Cerro Jesus

Las GordasSan Nicolas

0 5 10 15 200

20

40

60

80

100

Sr/Y

Y

“Adakite”-like rocks

Normal arc rocks

d)

1

10

100

Rock/chondrite

La PrCe Nd

PmSm

EuGd

TbDy

HoEr

TmYb

Lu

a)

.001 .01 .1 1 10 100 1000.01

.1

1

10

100

1000

10000

(Sm

/La)

N

La

Hydrothermal zircon field

b)

50 60 70 800

10

20

30

40

50

60

70

Mg-

#

SiO2

b)

HualgayocCoymolache

San Miguel

Cerro Corona

Caballerisa

Choro BlancoSan NicolasCerro Quijote

1.0 1.2 1.4 1.6 1.81

2

3

4

5

(La/

Sm) N

(Dy/Yb)N

c)

San J

ose

Caba

lleris

a

Co C

oron

a 1

San M

iguel

Chor

o Blan

co

Co C

oron

a 4

Co C

oron

a 6

Cien

aga

Co C

oron

a 5

Jesu

s

Hualg

ayoc

Las G

orda

s

Coym

olach

e

San N

icolas

0

200

400

600

800

Ce4+

/Ce3

+

Porphyry Au-Cu mineralization

High-sulfidation style mineralization

Apparently barren

c)1000

0.1 0.3 0.5 0.7 0.90

500

Ce4+

/Ce3

+

Eu/Eu*

a)

6000 8000 10000 12000 140000

500

1000

Hf

Magma evolution

Ce4+

/Ce3

+

b)

50 60 70 800

1

2

3

4

5

6

7

Th

SiO2

e)

The Hualgayoc mining district in the Peruvian Cordillera is located 30km north of the Yanaco-cha high-sulphidation Au deposit. The district hosts numerous Au-Cu deposits.This study characterizes the igneous rocks in the district to evaluate the features associated with Au-Cu fertile magmas.

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

Cretaceous sedimentary rocks were intruded by dioritic rocks, including the Cerro Corona porphyry, and overlaid by andesitic to rhyolitic flows, domes and tuffs. Mineral deposits include the Cerro Corona porphyry Cu-Au mine, Tantahuatay high-sulfidation Au mine, and the AntKori Cu skarn deposit

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

The dominant phase of intrusive rocks in the district is hornblende±biotite porphyritic diorite with magnetite micro-phenocrysts, suggesting relatively oxidized parental magmas. Volcanic rocks include rhyodacite domes north of Cerro Corona, and the andesitic to rhyolitic Calipuy formation which partially hosts the Tantahuatay and AntaKori deposits.Weak to medium chlorite±epidote alteration affects the San Miguel and Cerro Quijote intrusions. Intense white mica alteration occurs in the San Jose and Cerro Jesus intrusion and rocks within the Tantahuatay and AntaKori deposits. Acidic alteration forming pyrophylite±alunite is present in Cerro Cienaga intrusion and within the Tantahuatay and AntaKori deposits. Potassic alteration forms K-feldspar + biotite + magnetite occurs at Cerro Corona, and locally in the San Jose intrusion.

REEs are mostly +3, but Ce can be +4 under oxidized conditions and Eu +2 under re-duced conditions. Zircon readily incorpo-rates Ce4+ into the Zr4+ site. Therefore, the Ce and Eu anomaly in zircon may be used to evaluate magmatic redox state. All zircons have consistently low anomalies of Eu (Eu/Eu*=0.5-0.7) (Fig. a) and variable Ce4+/Ce3+ (10-900). Ce4+/Ce3+ appears to in-crease with magma evolution (fig.b).

All mineralized intrusions including the Cerro Corona porphyry have high median Ce4+/Ce3+ values (360-625) while all appar-ently barren intrusion have low median Ce4+/Ce3+ values (200-290), except the San Miguel intrusion (~ 500) (Fig. c).

•Botcharnikov, R. E., Linnen, R. L., Wilke, M., Holtz, F., Jugo, P. J., & Berndt, J. (2011). High gold concentrations in sulphide-bearing magma under oxidizing conditions. Nature Geoscience, 4(2), 112.•Gustafson, L. B., Vidal, C. E., Pinto, R., & Noble, D. C. (2004). Porphyry-epithermal transition, Cajamarca region, northern Peru. Society of Economic Geologists, 11, 279-299.•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.•Smythe, D. J., & Brenan, J. M. (2016). Magmatic oxygen fugacity estimated using zircon-melt partitioning of cerium. Earth and Planetary Science Letters, 453, 260-266.•Zajacz, Z., Candela, P. A., Piccoli, P. M., Wälle, M., & Sanchez-Valle, C. (2012). Gold and copper in volatile saturated mafic to intermediate magmas: Solubilities, partitioning, and implications for ore deposit formation. Geochimica et Cosmochimica Acta, 91, 140-159.

We thank Gold Fields Cerro Corona mine staff for their logistic support during our field work; Buenaventura and Regulus Resources Inc. for their help with sampling; Samuel Morfin, Glenn Poirier and Alain Mauviel for their analytical assistance; and Jeffrey Hedenquist for his advice on the project. The project was supported by Natural Science and Engineering Council of Canada Discovery Grant to K.H.

The magmatic oxygen fugacity of the intrusions calculated following the method of Smythe and Brenan (2016) show moderately oxidized values, FMQ +0.5 to +2, independent of the association with mineralization. Mineralized Cerro Corona porphyry intrusions appear to be less oxidized with median value of FMQ +0.8 to +1.3.Experimental data shows maximum solubility of Au in andesitic melt at around FMQ +1.5 and decreases at higher and lower fO2 (Botcharnikov et al. 2010). In contrast, the solubility of Cu in melt increases with increasing oxidation conditions (Zajacz et al. 2012). The median fO2 value of magmas from the Hualgayoc mining district, FMQ +1.28, correspond to the condition for relatively high Au solubility and appears to be consistent with the abundant Au mineralization in the district including the high Au/Cu ratio, ~1.7 x10-4, of the Cerro Corona deposit.

Zircon grains from mineralized intrusions have higher Ce4+/Ce3+ than most zircon grains from barren intrusions. This suggests that the Ce anomaly in zircon can be used to identify intrusions that may be potentially Au-Cu fertile within a district. Our results also suggest that Au-fertile districts are characterized by moderate magma oxidation conditions (FMQ +1 to +2). This may be useful to identify potentially fertile districts.

Au-Cu mineralization in the district is associated with hydrous, moder-ately oxidized magmas that originate from amphibole-bearing source rocks, with little to no crustal assimilation. Contemporaneous em-placement of mineralized and barren intrusions in the district suggest that oxidized magma do not necessarily produced Cu and Au mineral-ization. The mineralization requires other factors including focused in-jections of magmas and hydrothermal activity to concentrate the metals to economic values.On-going work includes more U-Pb zircon dating, and trace element analysis of zircon and bulk rocks to evaluate any differences for magmas associated with high-sulfidation Au deposits, skarn and por-phyry Au-Cu deposits.

The igneous activity in the district was previously thought to range from Paleocene to Miocene in age. New U-Pb zircon ages obtained in this study indicate that igneous activity ranged from 14.8Ma to 9.7Ma, similar to that at the Yanacocha high-sulfidation Au deposit. Most intrusions formed between 14 and 15Ma. Some are associated with mineralization (Cerro Corona) whereas others appear to be barren (Coymolache). Magmatic activity from 13.5 to 11 Ma is focused in the Tantahuatay and AntaKori areas, and consists of porphyritic intrusions and the Calipuy volcanic formation.Late magmatism at 9-10Ma consists of barren rhyodacite-rhyolite domes near Cerro Corona.

All the intrusions have similar Mg-# (0.30-0.55) and SiO2 (59-65 wt%) except the early phase of the Cerro Corona intrusive complex with Mg-# (0.65), and the Cerro Hualgayoc rhyolite which shows high SiO2 content (70 wt%) (fig.b).

All intrusions show listric-shaped REE pattern (Fig. a), and low [Dy]n/[Yb]n ratio (1.4-1.1) (Fig. c), reflecting preferential retention of middle REEs by amphibole in the source. Intrusions show a weak Eu anomaly (0.8-1.1) reflecting essentially no plagioclase fractionation.

All intrusions except Cerro Quijote show an “adakitic”-like geochemical signature with high Sr/Y ratios (40-90) and low Y (5-16ppm) (Fig. d). High Sr/Y can be explained by high water contents in parental magmas that suppress plagioclase crystallization (Sisson and Grove, 1993). This is consistent with the presence of phenocrysts of biotite and hornblende in most intrusions.

Low Th content in samples (3-7 ppm) indicates essentially no assimilation of siliciclastic rocks during magma ascent through the thick continental crust (Fig. e).

Sharp oscillatory zoning and low light REEs concentration in zircons confirm magmatic origin (fig. b).

Zircon grains from all intrusions show a similar REE pattern with relatively flat middle to heavy REEs, weak negative Eu anomaly and high Ce anomaly. In con-trast, inherited cores show con-cave-shaped heavy REEs profile and a strong negative Eu anomaly, suggesting the derivation from the basement rocks (fig. a).

The Ti-in-zircon thermometer yielded crystallization temperatures between 620-720 C° except the San Nicolas in-trusion which appears to have crystal-ized at higher temperature (720-800 C°) (fig.b).