ARTICLE

Lithologic controls on mineralization at the Lagunas Nortehigh-sulfidation epithermal gold deposit, northern Peru

Luis M. Cerpa & Thomas Bissig & Kurt Kyser &

Craig McEwan & Arturo Macassi & Hugo W. Rios

Received: 26 August 2011 /Accepted: 15 January 2013 /Published online: 5 February 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract The 13.1-Moz high-sulfidation epithermal golddeposit of Lagunas Norte, Alto Chicama District, northernPeru, is hosted in weakly metamorphosed quartzites of theUpper Jurassic to Lower Cretaceous Chimú Formation andin overlying Miocene volcanic rocks of dacitic to rhyoliticcomposition. The Dafne and Josefa diatremes crosscut thequartzites and are interpreted to be sources of the pyroclasticvolcanic rocks. Hydrothermal activity was centered on thediatremes and four hydrothermal stages have been defined,three of which introduced Au ± Ag mineralization. The firsthydrothermal stage is restricted to the quartzites of theChimú Formation and is characterized by silice parda, atan-colored aggregate of quartz-auriferous pyrite–rutile ±digenite infilling fractures and faults, partially replacingsilty beds and forming cement of small hydraulic brecciabodies. The δ34S values for pyrite (1.7–2.2‰) and digenite

(2.1‰) indicate a magmatic source for the sulfur. Thesecond hydrothermal stage resulted in the emplacement ofdiatremes and the related volcanic rocks. The Dafne diat-reme features a relatively impermeable core dominated bymilled slate from the Chicama Formation, whereas theJosefa diatreme only contains Chimú Formation quartziteclasts. The third hydrothermal stage introduced the bulk ofthe mineralization and affected the volcanic rocks, the diat-remes, and the Chimú Formation. In the volcanic rocks,classic high-sulfidation epithermal alteration zonationexhibiting vuggy quartz surrounded by a quartz–aluniteand a quartz–alunite–kaolinite zone is observed. Companydata suggest that gold is present in solid solution or microinclusions in pyrite. In the quartzite, the alteration is subtleand is manifested by the presence of pyrophyllite or kaolin-ite in the silty beds, the former resulting from relatively highsilica activities in the fluid. In the quartzite, gold minerali-zation is hosted in a fracture network filled with coarsealunite, auriferous pyrite, and enargite. Alteration and min-eralization in the breccias were controlled by permeability,which depends on the type and composition of the matrix,cement, and clast abundance. Coarse alunite from the mainmineralization stage in textural equilibrium with pyrite andenargite has δ34S values of 24.8–29.4‰ and d18OSO4 valuesof 6.8–13.9‰, consistent with H2S as the dominant sulfurspecies in the mostly magmatic fluid and constraining thefluid composition to low pH (0–2) and logfO2 of −28 to −30.Alunite–pyrite sulfur isotope thermometry records temper-atures of 190–260 °C; the highest temperaturescorresponding to samples from near the diatremes. Aluniteof the third hydrothermal stage has been dated by 40Ar/39Arat 17.0±0.22 Ma. The fourth hydrothermal stage introducedonly modest amounts of gold and is characterized by thepresence of massive alunite–pyrite in fractures, whereas

Editorial handling: F. Tornos

Electronic supplementary material The online version of this article(doi:10.1007/s00126-013-0455-6) contains supplementary material,which is available to authorized users.

L. M. Cerpa (*)Departamento de Ciencias Geológicas, Universidad Católica delNorte, Av. Angamos 0610, Antofagasta, Chilee-mail: lcerpa@gmail.com

T. BissigMineral Deposit Research Unit, University of British Columbia,Vancouver, Canada

K. KyserQueen’s University, Kingston, Ontario, Canada

C. McEwan :A. Macassi :H. W. RiosMinera Barrick Misquichilca, Av. Victor Andrés Belaunde,Lima, Peru

Miner Deposita (2013) 48:653–673DOI 10.1007/s00126-013-0455-6

barite, drusy quartz, and native sulfur were deposited in thevolcanic rocks. The d18OSO4 values of stage IV alunite varybetween 11.5 and 11.7‰ and indicate that the fluid wasmagmatic, an interpretation also supported by the isotopiccomposition of barite (δ34S=27.1 to 33.8‰ and d18OSO4 =8.1 to 12.7‰). The Δ34Spy–alu isotope thermometry recordstemperatures of 210 to 280 °C with the highest valuesconcentrated around the Josefa diatreme. The LagunasNorte deposit was oxidized to a depth of about 80 m belowthe current surface making exploitation by heap leach meth-ods viable.

Keywords Diatreme . Breccia . High sulfidation .

Epithermal . Central Andes .Miocene . Landscape evolution

Introduction

Lagunas Norte (7°56′ S, 78°15′ E) is one of the most recentdiscoveries of world class epithermal gold deposits in north-ern Peru and is, in contrast to other important epithermaldeposits of the region (e.g., Pierina: Rainbow 2009;Yanacocha: Longo et al. 2010), not only hosted in volcanicrocks but also in Upper Jurassic to Lower Cretaceousquartzites. The limited reactivity of the quartzites resultedin important challenges in mapping of the alteration andconsequently the relatively recent discovery of the deposit(Araneda et al. 2003), despite the fact that it is well exposedat surface.

In this article, we present a genetic model of the LagunasNorte deposit on the basis of the paragenetic evolution,mineralization, and alteration and its relationship with therocks that host the mineralization. We also present stableisotope data for pyrite, barite, and three types of alunite,commonly in textural equilibrium with auriferous pyrite,which constrain the origin of the mineralizing fluids andallow documentation of the evolution of the magmatic-hydrothermal system of Lagunas Norte.

Exploration history and reserves

Prior to the discovery of the Lagunas Norte Au–Ag deposit,significant coal mining had been carried out in the AltoChicama area since the end of the nineteenth century.Between 1880 and 1931, Compañia Minera Northernexploited the Callacuyan coal deposit, located 5 km to theNW of Lagunas Norte (Escudero 1979). Only small-scalecoal mining for local domestic use took place after 1931, aslarger scale operations were not profitable due to the highsulfur content of the coal (Manrique 1986).

In 1999, Centromin-Perú carried out preliminary studiesto evaluate the metallic mineral potential of the area, whichled to the identification of elevated gold values in stream

sediment samples (Dunin-Borkowski 2000). Minera BarrickMisquichilca acquired the property in 2001 and announcedthe discovery of the deposit in April 2002 with an initialresource of 3.5 Moz of Au with an average grade of 1.95 g/t(Araneda et al. 2003). Gold production commenced in late2005 and attained annual production of 1.2 Moz at a cashcost of $125/oz in 2008. The total production and currentreserves is ∼13.1 Moz Au (Barrick Gold Corp. AnnualReport 2011).

Regional geologic setting

The Mesozoic units of northern Peru (Fig. 1) consist ofsedimentary rocks deposited during the Andean cycle(Mégard 1987). Starting in the Tithonian, the westernPeruvian continental margin was dominated by the subsid-ing Chicama basin (Jaillard and Jacay 1989), where locallyas much as 2,500 m shale, intercalated with subordinate thinsandstone beds (Chicama Formation: Jaillard and Soler1996) was deposited in a dominantly deep marine sedimen-tary environment. In the Berriasian–Valanginian, a gradualtransition from deep marine to shallower siliciclastic sedi-mentation occurred, resulting in a succession of quartz sand-stones derived from the Guyana and Brasilia cratons(Moulin 1989). These sandstones were probably depositedin a fluvio-deltaic environment and in the study area arerepresented by the Chimú Formation (Benavides-Cáceres1956; Jaillard and Jacay 1989). The transition from a pelagicto shallow continental margin environment marks a changein paleogeography due to a drastic change in subductiongeometry at the northern Peruvian margin at that time(Jaillard et al. 2000).

The Valanginian was dominated by marine transgressionsand regressions giving rise to the Santa–Carhuaz Formation(Benavides-Cáceres 1956) which consists of alternatingsandy and shaly beds. Carbonate and black shales of theChulec and Pariatambo Formations, respectively, overlie theSanta–Carhuaz Formation and indicate a progressivelydeepening depositional environment. In the Aptian, thewestern border of the Chicama basin was the site of intensevolcanic arc activity (Casma Group; Atherton et al. 1985;Soler 1991). Marine sedimentation and volcanism ended inthe Albian during dextral transpressive deformation in thearc (Soler and Bonhomme 1990). Volcanic activity wassucceeded by voluminous intrusive activity leading to theemplacement of the 100–55 Ma Coastal Batholith (Cobbinget al. 1981; Soler 1991).

In the late Cretaceous, the Mariana-type subduction wasreplaced by the present-day Andean-type subduction(Benavides-Cáceres 1999). This resulted in a tectonic inver-sion and intense compressive deformation which gave rise tothe Marañon Fold and Thrust Belt (Benavides-Cáceres 1999).

654 Miner Deposita (2013) 48:653–673

The folded Mesozoic rocks are unconformably overlain by theEocene to Miocene Calipuy Group volcanic and volcaniclasticrocks (Cossío and Jaén 1967; Wilson 1975; Rivera et al. 2005;Montgomery 2012). Volcanism ceased in the middle to lateMiocene along most of northern and central Peru. This cessa-tion has been attributed to the onset of flat subduction along the

Peruvian margin due to the subduction of the aseismic Nazcaridge and oceanic Inca plateau (Gutscher et al. 1999; Hampel2002). The emplacement of many ore deposits in Peru may bedirectly related to these changes in subduction geometry(Rosenbaum et al. 2005; Bissig et al. 2008; Bissig andTosdal 2009).

Lower Cretaceoussiliciclastic rocks

N

Late Cretaceous, siliciclasticand carbonaceous rocks

Coastal Batholith

Oligocene-Miocene

Miocene

LagunasNorte

Yanacocha

CAJAMARCA

TRUJILLO

La Virgen

8

7

7 9 7 8

8

7

7 9 7 8

Quiruvilca

PA C IF ICO C E A N

Cordillera Blanca Batholith

Lower Jurassic, siliciclasticrocks and shales

Upper Jurassic, shales

Permo-Triassic limestones

Paleozoic, metamorphic rocks

Quaternary

Callacuyan

P E R U

E c u a d o r C o lo m b ia

B ra s il

L im a

C u sco

80 70

80 70

0

10º

0

10

Tru jillo

Calipuy Gp.volcanic rocks

Chimú Fm.

Chicama Fm.

Fig. 1 Simplified geological map of northwestern Peru and locations of Lagunas Norte and other deposits of the Miocene metallogenetic belt ofPeru (modified from INGEMMET (1999) and Noble and McKee (1999))

Miner Deposita (2013) 48:653–673 655

Deposit geology

Mesozoic basement

The basement at Lagunas Norte is dominated by Mesozoicpelitic and siliciclastic rocks belonging to the Chicama andChimú Formations, respectively (Reyes 1980). TheseMesozoic rocks are thrusted and folded into NW strikingeast-verging folds (the Marañon Fold and Thrust Belt:Benavides-Cáceres 1999) and are weakly metamorphosed toslate and quartzite. LowerMiocene volcanic rocks assigned tothe Calipuy Group were deposited unconformably over thefolded Mesozoic strata. Gold mineralization is hosted by thesiliciclastic Chimú Formation and the overlying volcanic stra-ta. The deposit stratigraphy is described below in detail(Figs. 2 and 3).

The Jurassic Chicama Formation (Stappenbeck 1929;Cossío and Jaén 1967) crops out to the west and north of themining operations (Fig. 2). Its thickness is unknown in thestudy area but has been estimated to be up to 1,500 m thick40 km S of Lagunas Norte (Cossío and Jaén 1967; Jaillard andJacay 1989). It consists of a succession of dark carbonaceousshale and siltstone with occasional thin beds of fine-grainedsandstone (see electronic supplementary data). The unit hasbeen weakly metamorphosed to slate and features an intensecleavage subparallel to the bedding. The transition to theoverlying Chimú Formation (see below) is gradual and char-acterized by increasing abundance of quartzite intercalations.The Chicama Formation does not crop out at the deposit, butits presence below the mineralized zone is evident from clastsof slate in the Dafne breccia (see below) crosscutting theMesozoic strata.

The Upper Jurassic to Lower Cretaceous Chimú Formation(Benavides-Cáceres 1956) is the principal ore host (Figs. 2and 3). It consists of compositionally mature quartz sandstone(typically ∼95 % SiO2; see electronic supplementary data) butcontains occasional coal beds, which historically have beenexploited, as well as scarce siltstone and shale intercalations.The sandstone has undergone weak metamorphism whichresulted in some recrystallization and cementation of quartzgrains to form quartzite. The thickness of the ChimúFormation is estimated to about 450–600 m in the LagunasNorte area (Benavides-Cáceres 1956).

Volcanic rocks of the Calipuy Group

A sequence of volcanic and volcaniclastic rocks assigned tothe Calipuy Group (Cossío and Jaén 1967; Rivera et al. 2005;Montgomery 2012) overlies the Mesozoic strata in an angularunconformity. At Lagunas Norte, four subunits from oldest toyoungest, the Quesquenda, Dafne, Josefa, and Shulcahuangaunits, can be distinguished. The Josefa and Dafne units areclosely related to magmatic-hydrothermal breccia bodies

interpreted as diatremes from which the eruptive productsprobably originated.

The Quesquenda unit is named after an eruptive center4 km to the north of the deposit (Rivera et al. 2005) and cropsout in the easternmost portions of the deposit. There it consistsof more than 150 m pyroclastic and volcaniclastic rocks ofandesitic composition with interstratified lithic-rich tuffaceousdeposits containing carbonized wood (see electronic supple-mentary data). The Quesquenda unit is interpreted a productof pyroclastic eruptions interbedded with lahar deposits.

The Dafne unit consists of a series of breccias in thewestern part of the mineralized area. The breccia bodyoverall has a subvertical inverted cone shape and in planview a NW elongated ellipsoid shape measuring up to1 km along its long axis. The breccia body cuts theMesozoic basement (Chicama and Chimú Formations)and is thought to be a diatreme. Four principal lithofaciesassociations have been identified and their characteris-tics, spatial relationships, and alteration are describedbelow, following the classification scheme proposed byDavies et al. (2000, 2008) and Gifkins et al. (2005). Thefour breccia lithofacies are the diatreme margin, mainbody, crater, and apron lithofacies and are summarizedin Table 1 and illustrated in Figs. 4 and 5.

The diatreme margin lithofacies generally consists ofclast-supported monomict and polymict breccias in arock flour or juvenile volcanic matrix. Hydrothermalcement is ubiquitous (Table 1). The breccias show coarsestratification parallel to the diatreme margin. No clasts ofbreccia within breccia have been observed. Threedomains of monomictic breccias (Table 1) contain, re-spectively, quartzite, siltstone, or tan-colored hydrother-mal quartz clasts (locally termed silice parda, seebelow). The clasts are angular to subangular and thebreccias have jigsaw-fit to slightly rotated textures. Thepolymictic breccias contain subangular-rotated clasts ofquartzite and siltstone. They have a rock flour matrix andcement of hydrothermal quartz and, at depth, pyrite. Thediatreme margin lithofacies has a gradual transition to thewall rock with jigsaw-fit textures, angular clasts, andlocally monomictic breccia chimneys (Fig. 5b). Hydrothermalcement of quartz and pyrite dominates over matrix.

The main body lithofacies association occupies the centralportions of the diatreme and is volumetrically the most im-portant. It is largely composed of polymictic, matrix-supported breccias (Fig 5c, d and Table 1) featuring quartzite,siltstone, slate, and juvenile volcanic clasts (fiamme-likewhispy clast shapes) as well as occasional silice parda hydro-thermal quartz fragments. The clasts are subangular to sub-rounded and the breccias have no apparent internalorganization or clast sorting. The matrix generally is rockflour derived from siltstone and shale and fine-grainedreworked volcanic material.

656 Miner Deposita (2013) 48:653–673

In the upper central part of the diatreme, polymicticunstratified and massive breccias are assigned to the craterlithofacies. Their distinguishing characteristic is the

presence of large (up to 1.7 m in diameter) rounded tosubrounded quartzite and andesite blocks featuring striaeon the clast surfaces (Fig. 5e and Table 1). Smaller clasts

803000 803500 804000 804500

9122

000

9122000

9121

500

9121500

9121

000

9120

500

9120

000

9121000

9120500

9120000

803000 803500 804000 804500

N

C Shulcahuanga

L i

s

A’

A

Chicama Fm.

Chimú Fm.

Josefa Unit

Dafne Unit

Quesquenda Unit

A A’

DAFNE

JOSEFA

ALEXA

Shulcahuanga Unit

Tectonic Breccias Cross Section

Main Faults

Pit Limits

> 1000 g/T * m Au

250 g/T * m Au

125 g/T * m Au

Grade/Thickness contour(Au g/t x meters)

Cenozoic

Mesozoic

Fig. 2 Geological map of the Lagunas Norte deposit based on Barrick’s regional and local mapping. The three principal ore zones are labelled

Miner Deposita (2013) 48:653–673 657

include quartzite and siltstone, and the matrix consists ofrock flour and juvenile volcanic material.

The apron lithofacies located in upper peripheral parts ofthe diatreme is characterized by an intercalation of polymic-tic and monomictic clast-supported breccias with rock flourand volcanic matrix (Fig. 5f and Table 1). This lithofaciesshows coarse bedding (also known as tephra stratification:Lorenz 2003). Quartzite and siltstone clasts are subroundedto rounded and locally tabular. Clast imbrication is common

and fiamme are widely observed in the most peripheralpolymictic parts of the breccias (Fig. 5g). These polymicticbreccias are recognized up to 1 km north of the diatreme.Clast size in this unit decreases with increasing distancefrom the diatreme.

The Josefa unit is subdivided into two subunits. The firstunit is a breccia body interpreted as a diatreme and thesecond consists of a volcanic and volcano-sedimentary suc-cession probably related to the same diatreme, althoughother source(s) are possible. The diatreme was emplaced inthe eastern part of the deposit in the Josefa area (Fig. 2),whereas the eruptive products have also been preserved inthe Alexa area, 1 km north of Josefa, and overlying theperipheral parts of the Dafne breccia, 0.5 km to the west(Fig. 2).

The Josefa diatreme is approximately 45 by 30 m in planview and was only recognized in the open pit after mininghad started. It has an inverted cone shape (Fig. 6a), but incontrast to the larger Dafne diatreme, no siltstone, shale, andcarbonaceous material is present. Quartz crystals up to5 mm in diameter occur in the largely juvenile volcanicmatrix.

The breccias at Josefa have been classified using thesame scheme as for the Dafne diatreme (Table 1). Thebreccia margin lithofacies consist of monolithic clast-supported breccias with angular quartzite clasts and quartzcrystals in a volcanic matrix (Fig. 6b). This breccia alsocontains quartz–alunite cement. The main body of the

Chicama Fm.

Chimú Fm.

Quesquenda Unit

Dafne Diatreme

Josefa Diatreme

QPF Unit

Shulcahuanga Unit

CA

LIP

UY

GR

OU

P

ME

SO

ZO

ICC

EN

OZ

OIC

Fig. 3 Generalized stratigraphic column showing the principal litho-logic units of Lagunas Norte deposit and cross-cutting relationships

Table 1 Summary of lithofacies and distribution in the Dafne and Josefa diatremes

Characteristics Characteristics Distribution InterpretationDafne Josefa

Marginlithofacies

Layered monomictic andpolymictic breccias,subrounded to subangularclasts, crude stratificationparallel to breccia margin;matrix and cement support;advanced argillic alteration

Layered polymictic breccias,subangular to subroundedclasts, crude stratificationparallel to breccias margin;cement support; advancedargillic alteration

At the border of bothdiatremes, in contact withthe bedrock; contactsteeply dipping towardsthe center of diatremes

Successive phreatic andphreatomagmatic explosionsgenerate crude stratification;later overprinted byhydrothermal activity

Mainbodylithofacies

Polymictic breccias, matrixsupported, not stratified, chaoticdistribution, subrounded torounded clasts; argillicalteration

Polymictic breccias, matrix andcement supported not stratified,chaotic distribution,subrounded to rounded clasts;advanced argillic alteration

In the central part of bothdiatremes

Mainly phreatomagmaticexplosions which reworkedmatrix and clasts

Craterlithofacies

Massive body, polymictic, andmatrix supported; rounded clast;chaotic to crude stratification atborder; contains large andesiteblocks with striae on surfaces;argillic alteration

Massive body, polymictic,matrix, and cement supported;rounded clast; crudestratification at border; containslarge quartzite blocks;advanced argillic alteration

In upper central partof both diatremes

Succession of violentphreatomagmatic explosions,capable of ejecting largebedrock blocks

Apronlithofacies

Gently dipping tephrastratification; stratified,polymictic, and clast, matrix,and cement supported, roundedto subrounded clast; advancedargillic alteration

Gently dipping tephrastratification; stratified,polymictic and clast supported,rounded to subrounded clast;advanced argillic alteration

Located in the northwestpart of the Dafnediatreme; also similarfacies in the southern partof the Josefa diatreme

Succession of phreatic andphreatomagmatic eventsresulting in bedded succession;each bed representing anexplosive event and airfalldeposition

658 Miner Deposita (2013) 48:653–673

breccia is polymictic and matrix supported. Clasts are sub-angular to subrounded and juvenile clasts and quartz crys-tals are present in a tuffaceous matrix (Fig. 6c). As in themarginal facies, quartz–alunite cement is present.

The crater lithofacies in the upper part of the Josefadiatreme is characterized by large quartzite blocks up to80 cm in diameter (Fig. 6d) in a tuffaceous matrix withabundant quartz crystals and juvenile volcanic clasts.The apron lithofacies is only partly preserved at thesouthern margin of the diatreme where it consists of aseries of crudely stratified beds (Fig. 6e). These depositsare overlain by pyroclastic flow deposits which areinferred to be related to the eruptive activity at Josefa,on the basis of lithologic similarities of the juvenilecomponents in the diatreme. Two principal units havebeen recognized: a quartz feldspar phyric unit (QFPunit) and an overlying dacitic unit; the latter character-ized by the absence of quartz phenocrysts. These vol-canic units crop out at Josefa and Alexa as well as atDafne where they overlie the apron lithofacies breccias(Figs. 2 and 3) and are generally affected by advancedargillic alteration.

The QFP unit is characterized by monomictic brecciascontaining quartzite clasts. The clast sizes increase towardsthe Josefa diatreme (locally termed paleosurface breccia,Fig. 7a). Overlying this breccia is a pyroclastic flow depositwith small (<2 cm) altered pumice fragments and quartzcrystals up to 5 mm (Fig. 7b). This pyroclastic deposit isoverlain by lithic lapilli tuff containing small quartz crystalsand rare accretionary lapilli. The upper part of this tuff unitshows planar stratification.

Overlying the QFP unit, the Dacitic units are a series ofpyroclastic and volcaniclastic deposits with pumice andlithic fragments but no quartz crystals (Fig. 7c). The lowerpart of this unit locally shows planar stratification (Fig. 7d)and small paleochannels filled with lithic clasts up to 2 cmin size.

Overlying the pumice- and lithic clast-bearing daciticdeposits there is an ash tuff with only scarce lithicfragments. Within those strata between Dafne andJosefa, abundant fossilized leaves and tree trunks thatare still in vertical position have locally been found(Fig. 7e). The pinnate leaves (Fig. 7f) are of campto-drome shape, which indicates a humid and tropical flora(Alvarez-Ramis 1999) comparable to that present now atlower elevations in eastern Peru. The stratigraphicallyhighest rock type consists of pyroclastic deposits withscarce quartz crystals and clasts of pumice and quartz-ite. The alteration of the dacitic unit is dominated byadvanced argillic assemblages (see below).

Shulcahuanga unit

The Shulcahuanga unit consists of porphyritic andesite lavasand andesitic to dacitic domes that crop out to the west andsouth of the deposit around Cerro Shulcahuanga. Andesiticlavas assigned to this unit overlie the Dafne unit (Figs. 2 and3) and have been affected by only weak chlorite-smectite/illite alteration. No conclusive stratigraphic rela-tionships with the Josefa unit have been observed, but dueto the weaker alteration, the Shulcahuanga unit is interpretedto be younger.

Shulcahuanga dome

Chimú Fm. quarzite

Chimú Fm. quarzite

Shulcahuangaandesite flows

Milled and reworkeddiatreme facies

crater and main body lithofaciesargilic alteration

Milled and reworkeddiatreme facies

crater and main body lithofaciesargilic alteration

Stratified and silicified diatreme faciesapron lithofacies

Stratified and silicified diatreme faciesapron lithofacies

FaultFault

Fault

Fig. 4 Panoramic view of the Dafne diatreme. Photograph shows the pit exposure in 2007, looking from the northeast

Miner Deposita (2013) 48:653–673 659

Two lithologies are recognized. Firstly, andesite, locallyknown as Andesitas Azules due to the pale blue–green hueimposed by clay alteration, is characterized by a fine-grained aphanitic groundmass with hornblende phenocrystsand occurs as dykes that crosscut the southern margin of the

Dafne diatreme. The second lithology forms the Shulcahuangadome (Figs. 2 and 3) and adjacent lava flows to the east andexhibits a porphyritic texture with plagioclase, biotite, andhornblende phenocrysts. These rocks are characterized byprominent flow banding (Macassi 2005). An age range of

L

Q

B C

L

CJSP

Q

DL

CJ

Q

E

A

Tectonic breccias

Shulcahuanga Unit

Volcanic facies

Dia

trem

eLi

thof

acie

s

Chicama Fm.

Chimú Fm.

Margin

Main Body

Crater

Apron

A

F

A

GL

CJ

Q

0 1cm

Fig. 5 Lithofacies associationsof the Dafne Diatreme complex.a Cross section (see Fig. 1 forlocation of cross section),looking northwest, through theDafne diatreme, showinglithofacies distribution. bMonomictic clast-supportedbreccia showing pyrite cementwith angular clasts of quartzite(Q) and siltstone (L). cPolymictic breccia withcarbonaceous matrix-supportedjuvenile clast (CJ), quartzite (Q),silice parda (SP), and siltstone(L) from central diatreme body. dPolymictic clast-supportedbreccia with clasts of siltstone(L), juvenile clasts (CJ), andquartzite (Q). e Polymicticmatrix-supported breccia withlarge andesite boulders (A) up to1.70 m in diameter; correspondsto the crater lithofacies. f Close-up photograph of a coarselystratified polymictic breccia withandesite blocks (A); correspondsto the diatreme apron facies. gDetail of clast-supportedpolymictic breccia, showingsiltstone (L), quartzite (Q), andjuvenile clast (CJ) from the distalapron breccia lithofacies

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16.8 to 17.3 Ma for the Shulcahuanga unit has been establishedon the basis of 40Ar/39Ar data on biotite and hornblende(Montgomery 2012).

Analytical methods

The alteration paragenesis defined by previous workers(Guerra 2001; Araneda et al. 2003; Macassi 2005; Ríos2005) has been refined on the basis of field observationsand detailed petrography. Mineral assemblages have beenidentified by standard optical microscopy and, where appro-priate, by scanning electron microscopy (SEM) and X-raydiffraction at the Universidad Católica del Norte. Theseanalyses have been complemented by infrared spectroscopyusing a Portable Infrared Mineral Analyzer (PIMA) and

using the SPECMIN database (Thompson et al. 1999) andlocal databases for interpretation of infrared spectra.

Stable O, S, and H isotopic analyses were carried out atthe Queen’s University Facility for Isotope Research,Kingston, Ontario, Canada. The δ34S and d18OSO4 valuesfor alunite were determined using a method modified fromWasserman et al. (1992) and Arehart et al. (1992). Sulfurwas extracted online with continuous flow technology, us-ing a Finnigan MAT 252 isotope ratio mass spectrometer.Sulfate oxygen was extracted using the BrF5 technique ofClayton and Mayeda (1963). Hydrogen isotopic composi-tions were measured using a Thermal Finnigan TCEA cou-pled to a Thermo Finnigan Delta+ XP mass spectrometerand continuous flow technology (ConFlo III) as described inRainbow et al. (2005). All values are reported in units of permil (‰) relative to Vienna Standard Mean Ocean Water for

A4090

4080

4070

4060

4090

4080

4070

4060

ED

CB

BC

DE

Fig. 6 Field photographs of theJosefa breccia. a The Josefabreccia body in pit exposure(4,060 to 4,090 m.a.s.l. bench).Letters B to E refer to close-upphotographs. bVolcanic and rockflour matrix crackle breccias withquartz–alunite cement, herelacking juvenile fragments. cDetail of polymictic breccia withquartzite and juvenile pumicefragments. d Phreatomagmaticbreccia with a quartzite blockfrom the crater lithofacies. eClose-up of the apron lithofaciesshowing coarse bedding (markedby red dashed lines)

Miner Deposita (2013) 48:653–673 661

O and H and Cañon Diablo Troilite for S isotopic composi-tions. Accuracy was monitored using standards calibrated toNIST 8556 and 8557 for sulfur and oxygen and NIST 8538biotite for hydrogen. Analytical precision for both δ34Sa n d d18OSO4 v a l u e s i s 0 . 3‰ , f o r δD 3‰ .Paleotemperature for coexisting alunite–pyrite pairs iscalculated using the following fractionation factors: 103

ln apy�H2S ¼ 0:40� 106T�2 (Ohmoto and Rye 1979) and103 ln aalun SO4ð Þ�H2S ¼ 6:463� 106T�2 þ 0:56 (Ohmoto andLasaga 1982).

One sample has been dated by the 40Ar/39Ar method atthe Pacific Centre for Isotopic and Geochemical Research.Alunite was handpicked and analyzed as described in Bissiget al. (2008). The data are included as digital appendix(ESM).

Hydrothermal evolution and mineralization

The hydrothermal alteration at Lagunas Norte manifestsitself in very distinctive ways, depending on the hostrock compositions and textures. In the upper volcanic-hosted part of the deposit, the alteration developed a

zonation pattern typical for high-sulfidation systems(e.g., Simmons et al. 2005) with a nucleus of vuggyquartz, surrounded by quartz–alunite and dickite–kaolin-ite ± alunite zones which indicate acidic fluids thatbecame progressively neutralized during reaction withthe host rock. Contrasting the volcanic units, alterationaffecting the quartzite is subtle and difficult to detect(see electronic supplementary data) but kaolinite and, inmore silty strata, pyrophyllite have been detected byPIMA.

Four hydrothermal stages have been defined at LagunasNorte (Fig. 8) and are described below. Gold was introducedduring stages 1 and 3; the latter being the principal miner-alization stage. Minor additional gold was also introducedduring stage 4. Supergene oxidation to depths of up to 80 mbelow the current surface made the ore amenable to heapleaching methods.

Stage I: early hydrothermal activity

The first hydrothermal event at Lagunas Norte is character-ized by fine-grained yellowish to tan-colored aggregates ofquartz, pyrite, and minor rutile, which is referred to as silice

A B C

D E F

A B C

D E F

50cm

Fig. 7 Volcanic stratigraphy ofthe Josefa volcanic unit. aMonomictic breccia withhydrothermal cement (by minegeologists also referred to asPaleosurface Breccia)representing the basal portion. bDetail of pyroclastic flow withpumice fragments and tinyquartz crystals, affected byadvanced argillic alteration. cPumice and quartzite clast-bearing pyroclastic flow depositaffected by advanced argillicalteration. d Fine-grainedlaminated ash fall deposit of theupper volcanic member of theJosefa volcanic unit, affected bypervasive advanced argillicalteration. e Remnant of acarbonized tree (yellow arrow)in upright position in the ashfall deposit of the upper Josefavolcanic unit. f Detail offossilized leaf present in an ashfall deposit in the upper Josefavolcanic unit

662 Miner Deposita (2013) 48:653–673

parda by mine geologists, a term also used herein. Thisassemblage is restricted to the Chimú Formation where itwas generally emplaced along a network of preexistingfractures and is best developed in silty layers, but also formsthe cement of small fault controlled monomictic brecciabodies (Fig. 9a, b).

In the area between Josefa and Dafne and in the southernpart of the Josefa zone, silice parda is accompanied bychalcopyrite and digenite (Fig. 9c, d). Gold is not visibleby SEM or optical microscopy, but company internal min-eralogical studies show that gold is associated with pyrite,and we assume that gold is present in solid solution or asnanoparticles in the pyrite, as in other Andean high-sulfidation epithermal deposits (e.g., Pascua; Chouinard etal. 2005a). As indicated by the presence of silice pardaclasts in the Dafne diatreme, the first mineralization stagepreceded the diatreme emplacement. Absolute age con-straints for silice parda were not determined due to a lackof dateable minerals in this assemblage. However,Montgomery (2012) reports an age of paragenetically earlyalunite hosted in the Chimú Formation of 17.36±0.14 Ma,which may be considered a minimum age for this stage.

Stage II: phreatic and phreatomagmatic activity

The breccia lithofacies present in the Dafne and Josefadiatremes suggest that they formed by phreatic and phrea-tomagmatic activity which here is defined as the secondhydrothermal stage. This stage was important as groundpreparation for subsequent mineralization by fracturing theadjacent rock and as host of a portion of the ore.Mineralization in the diatremes is controlled by the perme-ability, which in turn is controlled by matrix type and

abundance, type, shape, and size of clasts. A minimum agefor the brecciation events is given by the oldest age ofalunite within the overlying volcanic sequence of 17.05±0.12 Ma (Montgomery 2012).

Stage III: main mineralization stage

Most of the gold was introduced during this stage and iscontained within the pyrite but not visible optically. Themain mineralization and alteration stage is difficult to detectin the quartzite. However, fracture infill of coarse alunite(Fig. 10a) associated with pyrite and enargite (Fig. 10b), atdepths below 80 m from the present surface, is observed. Inthe quartzite, disseminated kaolinite has been detected byPIMA. In the more silty beds of the Chimú Formation in thecore of Lagunas Norte, pyrophyllite is present, whereaskaolinite occurs in the periphery of the deposit. In bedswhere coal is present, a sulfide assemblage containing py-rite, stibnite, and arsenopyrite is observed locally.

In both diatremes alteration patterns are lithologically con-trolled. The margin of the Dafne breccia is intensely silicifiedwith minor alunite, whereas in the main body, dickite–kaolin-ite alteration affected juvenile fragments, and fracture con-trolled silicification is present locally. The crater facies showsa weak dickite–kaolinite alteration restricted to matrix andjuvenile fragments. In the apron lithofacies, the matrix com-position determines the alteration intensity and assemblages.Where the matrix is predominantly carbonaceous, the juvenilefragments are preferentially altered to alunite–dickite–kaolin-ite, whereas in beds with volcanic matrix, quartz–alunite is thedominant alteration assemblage. The Josefa breccias are per-vasively altered to quartz–alunite and juvenile fragments havecommonly been replaced by pyrite and alunite.

STAGE I STAGE II STAGE III STAGE IV STAGE V

Pyrite

Alunite

Enargite

Pyrophylite

Silice Parda

Digenite

Stibnite

Chalcopyrite

Rutile

Daf

ne a

nd J

osef

a D

iatr

emes

empl

acem

ent

Gold

Chimú Fm.

Sulfur

Barite

Jarosite

Arsenopyrite

Diaspore

Drusy Quartz

Scorodite

Hematite

Goethite

Coarse Massive Disseminated

Carbonaceous layers

Chimú Fm. /Volcanic unitsChimú Fm. Volcanic unitsChimúFm. /

Volcanic units

Fig. 8 Paragenetic sequence from Lagunas Norte deposit; thickness of lines shows the relative abundance of minerals

Miner Deposita (2013) 48:653–673 663

In the volcano-sedimentary levels at Dafne, Josefa, andAlexa as well as in the Josefa marginal facies and the Dafnebreccia, an alteration zoning pattern typical for high-sulfidation epithermal deposits is observed. The distributionof vuggy quartz zones is controlled by small E-orientedfaults and the permeability of volcanic or breccia facies.Within the volcanic package, vuggy quartz is best developedin pumice- and crystal-rich pyroclastic flow deposits wherepumice fragments and feldspar phenocrysts were leachedand the volcanic matrix was completely replaced by residualquartz (Fig. 10c, d). Surrounding the vuggy quartz zone, theassemblage quartz–alunite–pyrite altered the rocks. Alunitehas replaced feldspars and pumice clasts (Fig. 10e, f) and thegroundmass has been replaced by fine-grained quartz andpyrite. Pyrite has generally been oxidized but is preservedtogether with alunite in some silicified strata at Alexa(Fig. 11a), whereas at Josefa, it occurs together with rutile.The matrix of the breccia at the base of the volcanic pile hasbeen affected by pervasive quartz–alunite ± kaolinite–dick-ite alteration. Disseminated alunite from the volcanic Josefaunit representing this main mineralization stage gives an

40Ar/39Ar plateau age of 17.0±0.22 Ma (Fig. 12), which isconsistent with the age range of 16.7 to 17.1 Ma inferred forthe main hydrothermal activity (Montgomery 2012). Theandesitic volcanic rocks surrounding the deposit have beenaffected by weak to moderate argillic alteration where illitepartly replaces hornblende and quartz–chlorite veinlets havebeen observed (Fig. 11b).

Stage IV: late-stage alteration

Late-stage alteration is characterized by massive aluniteforming the cement of local fault breccias and filling thinfractures in Chimú quartzite. This alunite is white toyellowish in color (Fig. 11c), forming fine-grained andmassive aggregates. Pyrite in textural equilibrium withthis type of alunite is observed (Fig. 11d). Traces ofkaolinite and, at Alexa, diaspore are generally presentas well. The massive alunite has cut pyrophyllite-alteredsiltstone beds and overgrown coarse-grained alunite. Thelast manifestation of hydrothermal activity is barite, ru-tile, and drusy quartz as well as late native sulfur filling

DC

BA

Di

Py

PyQz

0 0,01mm0 0,1mm

Fig. 9 Photographs showingthe principal characteristics ofthe first stage of mineralization.a Monomictic breccias withtan-colored quartz cement(silice parda, see text fordetails). b Replacement of siltylayers of the Chimú Formationby silice parda. c Polishedsection photograph of first stageof mineralization showing thegranular texture of silice pardaand interspersed small pyritecrystals (Py). d Detail ofpolished section photograph ofpyrite crystals (Py) in digenite(Di) present in quartzite (Qz)

664 Miner Deposita (2013) 48:653–673

open spaces in the volcanic rocks. Barite-filled fracturesoccur in the quartzite at Alexa and Josefa. Only modestgold mineralization is associated with this late-stage al-teration. The youngest 40Ar/39Ar age on alunite reportedby Montgomery (2012) is 16.45±0.28 Ma and is hereininterpreted to constrain hydrothermal stage IV.

Stage V: supergene stage

Lagunas Norte has been affected by extensive supergeneoxidation (up to 80 m below the current surface) which hasproduced hematite, goethite, and locally jarosite and scor-odite. Iron oxides occur mainly as cement of tectonic andhydrothermal breccias as well as on fracture surfaces wherethey commonly present iridescent colors. Oxidation wascrucial for liberating the gold which made exploitation ofthe deposit economically viable using cyanide leachingmethods.

Stable isotopes

Stable isotopic compositions were obtained for each para-genetic stage. δ34S values (Fig. 13) were obtained forsulfides from all paragenetic stages, whereas δ34S, δD,and d18OSO4 values were obtained for alunite from stagesIII and IV (Fig. 14). The δ34S values and Δ34Salu–pyprecipitation temperatures were calculated for alunite andpyrite occurring in textural equilibrium at different loca-tions in the deposit (Table 2 and Fig. 15).

Stage I

Three δ34S values for sulfides were obtained. Pyrite hasvalues of 1.7 and 2.2‰, and coexisting digenite has a valueof 2.1‰. A maximum fluid temperature of 360 °C is given byΔ34S thermometry on the digenite–pyrite pair (Hubberten1980).

Al

Al

Al

En

Py

Qz

0 1mm0 1mm

0 0,25mm

DC

BA

FE

1cm

Fig. 10 Photographs showingcharacteristics of hydrothermalstage III. a Coarse alunitecrystals in fracture of Chimúquartzite. b Polished sectionmicrophotograph of enargite(En) and pyrite (Py) in quartz(Qz) gangue, indicative ofalunite–pyrite–enargite relatedfluids (Deyell et al. 2005). cVuggy quartz texture involcanic unit at Alexa. dDisseminated alunite (Al)replacing feldspar, Josefavolcanic unit. e Alunite (Al)replacing feldspar crystals andpumice fragments in a tuffdeposit from the Josefa volcanicunit. f Thin-sectionmicrophotograph of tabularalunite crystals (Al) replacingplagioclase phenocrysts

Miner Deposita (2013) 48:653–673 665

Stage III

Both coarse and disseminated alunite from stage III wasanalyzed. Coarse alunite is translucent to pale pink in color,with a tabular crystal habit. Eight samples were analyzedand show a range of δ34S values between 24.8 and 29.4‰.Six of these alunite samples are in textural equilibrium withpyrite δ34S values of 4 to −0.5‰ and locally with enargite(δ34S=−1.2‰). The temperatures calculated for alunite–pyrite pairs for this hydrothermal stage are between 190and 270 °C (Table 2 and Fig. 15); the highest temperatureshave been recorded near the diatremes at ∼200 m depth

below the current surface, while the lowest values comefrom samples near the surface. Fluid inclusion microther-mometry was not possible because of a lack of suitablematerial. The δD vs. d18OSO4 values of the alunite and thecalculated isotopic compositions of their apparent fluids areconsistent with magmatic vapors and a predominantly mag-matic origin for the fluid (Fig. 14).

Disseminated alunite from stage III is usually white, butlocally pale pink. Eight samples were analyzed and gave δ34Svalues between 21.7 and 28‰ and a range of d18OSO4 valuesfrom 6.8 to 13.9‰. These values are typical for aluniteprecipitated from a dominantly magmatic fluid (Fig. 14; Ryeet al. 1992; Rye 2005). However, one of the samples locatednear the eastern margin of the deposit has values of d18OSO4 =6.8‰ and δD=−40.2‰, which suggest influence of a mete-oric fluid component. This sample has a δ34S value of 27.2‰.

Stage IV

The massive alunite of this stage is white and has an earthytexture. Seven samples were analyzed and have δ34S valuesbetween 19.1 and 29.2‰ (Fig. 14) and d18OSO4 valuesbetween 11.5 and 11.6‰ (Fig. 14). Pyrite is commonlypresent in textural equilibrium with stage IV alunite andhas δ34S values between −1.4 and 1.4‰ (n = 5).Temperatures calculated from the Δ34Salu–py pair (Table 2)range between 210 and 280 °C, with higher temperaturesnear the diatremes and lower temperatures near the surface(Fig. 15). For this stage, barite has δ34S values from 27.1 to33.8‰ and d18OSO4 values from 8.1 to 12.7‰.

AlAl

Py

Qzt

0 1mm0 1mm

BA

DC

Fig. 11 Photographs showingcharacteristics of hydrothermalstages III and IV. a Monomicticbreccia with dark quartz–pyritecement. b Detail of weaklypropylitically altered andesiteof Shulcahuanga unit showingquartz veinlet with Fe oxidehalo. c Massive alunite (Al)filling open spaces inmonomictic breccia. d Thin-section microphotographshowing a monomictic brecciawith quartzite clasts (Qzt) infine-grained clastic matrix, withalunite (Al) cement

0

10

20

30

0 20 40 60 80 100

MSWD = 1.4, probability=0.2339Includes 91.2% of the Ar

Plateau age = 17.00+/- 0.22 Ma

Age

(Ma)

39Cummulative Ar percent

Sample Lc5014: Stage III Alunite

Fig. 12 40Ar/39Ar age spectrum for alunite sample of stage III fromLagunas Norte. Errors are given at the 2σ level

666 Miner Deposita (2013) 48:653–673

Supergene stage

Two samples of supergene goethite were analyzed; theyhave δD values of −187 and −183‰ and δ18O compositionsof −5.9 and −5.4‰. These values likely reflect the isotopiccomposition of local meteoric water in equilibrium withgoethite well after hydrothermal processes ended.

Discussion

Most high-sulfidation epithermal deposits are related tomagmatic-hydrothermal activity affecting volcanic or igne-ous rocks (e.g., Cooke and Simmons 2000), but LagunasNorte differs because part of the mineralization is hosted inunreactive quartzites. Four different hydrothermal stageshave been defined.

The first stage is restricted to the Mesozoic rocks and ischaracterized by silice parda (quartz ± pyrite ± rutile),which was precipitated in fractures and preexisting faultsand also replaced siltstone beds of the Chimú Formation.Silice parda locally cemented monomictic breccias, whichsuggests that the magmatic-hydrothermal fluids of this stagehad enough pressure to hydraulically fracture the quartzite.Locally and 40 m below the surface, pyrite, chalcopyrite,and digenite form the sulfide assemblage. The presence ofdigenite suggests a high sulfidation state for the fluid(Einaudi et al. 2003; Rye 2005). Sulfur isotopic composi-tions (δ34S 1.7 to 2.2‰ and 2.1‰ for pyrite and digenite,respectively) are consistent with a magmatic source of sul-fur. The fluid temperature of 360 °C estimated on the basisof sulfur isotope fractionation between sulfide (pyrite–digenite pair) species likely overestimates the true paleo-temperature which is unreasonably high for epithermal

0

50-5

5

1 0

I II

II

III

IIIIIIIII III

III

IIIIII

IIIIIIIII

IIIIII

III

III

IIIIV IV

IV

IVIVIV

IVIII

III

IIIIII

IIIIIIIIIIII

III

1 0 1 5 2 0 3 02 5 3 5

III IIIIII IIIIIIIII

IIIIII IIIIIIIII

IIIIII

IIIIII

III

IIIIII

III

III

IIIIV

IV

IV

IV IVIVIV

IVIV

IV

IV

IVIVIV

IVIV

IV

IV

IV IVIV

IV

IVIVIVIV

IVIV

PyriteI

I Digenite

III

III

III

III

Pyrite

Enargite

Alunite (coarse)

Alunite (disseminated) IV

IV

IV

IV

Pyrite

Alunite (massive)

Barite

Sulfur (native)

STAGE I STAGE III STAGE IV

Fre

quen

cy

34S (‰)

Fig. 13 Histogram of δ34S values of sulfides and sulfates in the Lagunas Norte deposit. Alunite samples are colored according to paragenetic stages

KA

OLI

NIT

ELI

NE

MW

L

-15 -5 5 15 25-100

-80

-60

-40

-20

0

VV

FMW

D

18OSO4

Coarse AluniteDisseminated AluniteMassive Alunite

Alunite fluids(200°to 280°C)

Fluids

STAGE III

STAGE IV

(‰)

(‰)

Fig. 14 δDandd18OSO4 diagramshowing data for stage III and IValunite. The fluid data werecalculated according to Stoffregenet al. (1994) at 200–280 °C (basedon temperature range obtainedfromΔ34Salu–py). The lines andfields are:MWL = meteoric waterline (Craig 1961), kaolinite line(Savin and Epstein 1970), FMW =felsic magmatic water (Taylor1988), VV = volcanic vapor, i.e.,range of fumarole water(Giggenbach 1992)

Miner Deposita (2013) 48:653–673 667

systems and inconsistent with the sulfide and alterationparagenesis and the near-surface geomorphologic settingof the deposit where mineralization directly underlies the25–26 Ma subplanar Pampa la Julia erosional surface(Montgomery 2012). This indicates that the calculated fluidtemperature probably has no geological meaning and thatthe two sulfide species are not in isotopic equilibrium.

The second hydrothermal stage reflects the emplacementof diatreme breccias. Diatremes have widely been docu-mented in epithermal deposits (e.g., Sillitoe 1985; Kelian,Davies et al. 2008; Pascua; Chouinard et al. 2005b). AtLagunas Norte, the emplacement of the diatremes was in-strumental for ground preparation for the mineralizationintroduced during stage III. The Dafne diatreme intersectsshale of the Chicama Formation as well as quartzite of theChimú Formation. The involvement of shale resulted insome milled breccia lithofacies being relatively imperme-able to later fluid circulation which is reflected by thelimited alteration of the central parts of the diatreme.Similar relatively impermeable breccias have been docu-mented from Kelian, Indonesia (Davies et al. 2008). TheJosefa Diatreme, in contrast, only cuts quartzite whichresulted in a more permeable breccia body that wascemented throughout by quartz–alunite. The smaller diame-ter of the clasts in the center of both diatremes together withthe rounding of clasts and crude stratification near the marginof the breccias suggest multiple explosive brecciation events(Lorenz and Kurszlaukis 2007;Walters 2006), but the absenceof breccia-in-breccia clasts indicates that the breccias have notbeen consolidated or cemented hydrothermally between theindividual explosions and that brecciation probably occurredover a short time interval. The violent emplacement of thediatremes not only generated permeability along their borders,but probably also improved the secondary permeability in thesurrounding host rocks by means of fracturing.

The main stage hydrothermal alteration and mineraliza-tion (stage III) affected the volcanic rocks as well as theunderlying Mesozoic basement. The alteration mineralogyand sulfide assemblages are generally as expected for high-sulfidation epithermal deposits (e.g., Simmons et al. 2005).The distribution of gold and the alteration zonation aremainly controlled by the permeability of the host rock.However, a number of deviations from the norm exist andcan be related to host rock characteristics. For example,locally in silty beds where organic carbon is present, theassemblage pyrite–arsenopyrite and stibnite is present. Thissulfide assemblage would be expected in a low-sulfidationenvironment (e.g., Cooke and Simmons 2000; Einaudi et al.2003), but at Lagunas Norte, it can readily be explained bythe locally strong reducing conditions.

The d18OSO4 values for disseminated alunite from thevolcanic levels reflect a large component of magmatic fluidwhich is common for Andean high-sulfidation systems (Rye2005; Deyell et al. 2005; Rainbow et al. 2005). However, nearthe periphery of the deposit, the hydrothermal fluid had ameteoric component. The coarse alunite samples from depthsof more than 80 m below the current surface have δ34S of 24.8to 29.4‰, values consistent with a H2S-dominated fluid,which again is typical for Andean high-sulfidation systems(Baumgartner et al. 2009; Rainbow 2009; Rye 1993, 2005).The observed sulfide and alteration assemblage indicates

Table 2 Summary of isotopicdata for coexisting alunite–pyritepairs. T is calculated using thefollowing fractionation factors: 103 lnapy�H2S ¼ 0:40� 106T�2

(Ohmoto and Rye 1979); 103 lnaalunðSO4Þ�H2S ¼ 6:463� 106

T�2 þ 0:56 (Ohmoto andLasaga 1982)

Sample name Type Alunite δ34S Pyrite δ34S Δ34Salu–py T (°C)

LD6-006 Coarse 28.93 0.76 28.17 195

LD6-196 Coarse 25.7 −0.52 26.22 213

LD6-104 Coarse 24.82 2.61 22.21 256

LD6-070 Coarse 27.39 0.33 27.06 205

LD6-176 Coarse 25.65 −0.59 26.24 213

LD7-178 Coarse 23.39 −4.43 27.82 198

LD6-111 Disseminated 25.25 −0.73 25.98 215

LC5-014 Disseminated 26.82 2.01 24.81 227

LD7-002 Disseminated 22.7 −4.34 27.04 205

LD6-177 Massive 25.48 −1.39 26.87 207

LD6-101 Massive 23.88 1.43 22.45 253

LD6-112 Massive 24.64 −1.77 26.41 211

LD6-068 Massive 19.13 −1.37 20.5 278

LD7-168 Massive 27.19 2.02 25.17 223

�Fig. 15 a Distribution map of principal alteration zones and fluidtemperatures obtained using the Δ34Salu–py thermometer at LagunasNorte. Red numbers correspond to stage III and blue numbers to stageIV. Note that the highest temperatures were obtained near the Dafneand Josefa diatremes. b Schematic cross section (see A for section line)showing the lithology and calculated fluid temperatures from samplesprojected onto the section plane. Red letters correspond to temperaturesfrom stage III and blue letters to stage IV

668 Miner Deposita (2013) 48:653–673

803000 804000

91

22

00

09

12

10

00

91

22

00

09

12

10

00

803000 804000

WeaklyAltered (argillic-propylitic)

Fm. Chimú (dk+kao+po)

Vuggy quartz

Quartz+Alunite

Alunite+Dickite+Kaolinite

Fault

Pit Limit

A

Cross SectionA A’

A’

A

253ºTemp. Al. (IV)

A A’

4200

4100

4000

3900

? Chicama Fm.

Chimú Fm.

Josefa Unit

Dafne Unit

Quesquenda Unit

Main Faults

Shulcahuanga Unit

Silice parda

Santa-Carhuaz Fm.

B

Open Pit limit

Oxide/Sulfides limit

Sample LocationProjected Sample

Location

SampleLocation(Projected)

256

DAFNE

JOSEFA

ALEXA

Miner Deposita (2013) 48:653–673 669

relatively acidic and moderately oxidizing conditions at 230 °C(pH=0–2 and logfO2=−28 to −30: Fig. 16).

The last hydrothermal event (stage IV) is characterizedby massive alunite–pyrite in fractures in the quartzites,whereas in the volcanic rocks, late barite and quartz aswell as late native sulfur precipitated in open spaces.Isotopic data indicate that hydrothermal stages III andIV had overall similar characteristics and a magmaticorigin for the fluids.

The apparent fluid temperatures calculated for stagesIII and IV from the sulfur isotopic composition ofalunite–pyrite pairs range from 190 to 280 °C and arehighest near the diatremes (Fig. 15) and lowest near thepresent surface some distance away from the diatremes.Based on the calculated fluid temperatures, hydrother-mal activity was probably focused around the Dafnediatreme for stage III, whereas stage IV was centeredat Josefa. This is also consistent with the observationthat volcanic deposits likely originating from the Josefadiatreme post-date those sourced from Dafne. Coarsealunite–pyrite pairs (Δ34Salu–py) consistently have higherfluid temperatures than disseminated alunite–pyritepairs. This is in agreement with the interpretation thatcoarse-bladed alunite habits may indicate fluid boiling,whereas fine-grained disseminated alunite was precipi-tated from cooler, non-boiling fluids, although no sup-porting fluid inclusion evidence is available.

Boiling could have been enhanced by water-table loweringdue to erosion near the hydrothermal system. The depositdirectly underlies the 25–26-Ma subplanar Pampa la Juliaerosional surface (Montgomery 2012), but the deposit liesimmediately southeast of the Rio Chicama valley pedimentwhich likely incised concurrently with mineralization(Montgomery 2012) and erosion may have enhanced mineral-izing processes at the steep back scarp of the valley, much likesuggested for the El Indio belt in Chile (Bissig et al. 2002).

The calculated fluid temperatures for stage III are in appar-ent disagreement with the presence of pyrophyllite in siltybeds of the Chimú Formation, since pyrophyllite normallyforms above 300 °C (Hemley et al. 1980). However, in a fluidwith high silica activity, which is a reasonable assumption forLagunas Norte given the abundance of quartzite, pyrophyllitemay be metastable to significantly lower temperatures(Hemley et al. 1980; Mojares et al. 2001), such as those of280 °C from Δ34Salu–py thermometry.

The isotopic compositions of goethite, particularly thestrongly negative δD, are indicative of high elevations of3,000–4,000 m (Poage and Chamberlain 2001). Similarisotopic signatures for goethite have also been documentedby Montgomery (2012) who suggested that the goethiteformed in response to rapid late Miocene uplift. The paleo-botanic evidence, on the other hand, suggests significantlylower elevations or warmer climate at the time of volcanismand hypogene mineralization than at present, indicatingsurface uplift of 2,000–3,000 m after 17 Ma (cf., Garzioneet al. 2008; Montgomery 2012).

The hydrothermal activity at Lagunas Norte pre-dates theintermediate sulfidation polymetallic vein system ofQuiruvilca, 10 km to the west, for which an age of 15.2 to15.7 has been determined by 40Ar/39Ar on muscovite/illitefrom the selvage of a base metal quartz–carbonate vein(Montgomery 2012). However, all other important high-sulfidation epithermal deposits in northern Peru, includingPierina (14 Ma; Rainbow 2009) and Yanacocha (13–8 Ma;Longo et al. 2010) were emplaced significantly later thanLagunas Norte and also post-date Quiruvilca. Further south,in Central Peru, the majority of magmatic-hydrothermaldeposits have been emplaced during the middle to lateMiocene (e.g., Bissig et al. 2008; Baumgartner et al. 2009;Kouzmanov et al. 2008). The flattening of the subductionangle between about 14 and 10 Ma that generally coincidedwith the onset of Nazca ridge subduction (Hampel 2002) isthought to have played a role in generating favorable con-ditions for mineralization in central and northern Peru(Rosenbaum et al. 2005; Bissig et al. 2008; Bissig andTosdal 2009). The slightly older age of Lagunas Nortecannot confidently be assigned to initiation of ridge sub-duction, but the leading edge of the Nazca ridge may havecollided with the trench at that time (cf., Rosenbaum et al.2005).

HSO4-

SO42-

H2S0 HS-

-24

-26

-28

-30

-32

-34

-36

-38

-40

-42

-44

-46

0 2 4 6 8 10 12

Py

Po

Hem

Mag

-2

Tn

En

T=230 oC

K=0.01 m

Log

O2

pH

Al KaoS=0.01 m

Fig. 16 Log fO2–pH diagram at 230 °C and saturated vapor pressure,showing the stability fields of alunite (Al, gray box), kaolinite (Kao),enargite (En), tennantite (Tn), hematite (Hem), magnetite (Mag), pyrrho-tite (Po), pyrite (Py), and sulfur species from Lagunas Norte deposit. Theprobable fluid composition for stage III is indicated by the gray box

670 Miner Deposita (2013) 48:653–673

Conclusions

The Lagunas Norte deposit is a high-sulfidation epithermalsystem that is hosted in both normally unreactive rocks (quartz-ite of the Chimú Formation) and dacitic to rhyolitic volcanicrocks (Miocene). It is at 17Ma likely the oldest high-sulfidationdeposits in the middle Miocene metallogenic belt of Peru. Themagmatic and hydrothermal evolution was controlled by atleast two diatremes (Dafne and Josefa), which cut the basementcomposed of quartzite of the Chimú Formation and in the caseof Dafne also slate of the Chicama Formation.

At Lagunas Norte, four hydrothermal stages are recog-nized, and most of the gold and silver were introducedduring stages I and III. Stage I is restricted to quartzite,where the gold is associated to pyrite–digenite ± chalcopy-rite in a quartz ± rutile (silice parda) gangue assemblagemainly in fractures and faults, as well as replacing somesiltstone levels. Isotopic data are consistent with a magmaticorigin of the sulfur. The second stage is the emplacement ofthe Dafne and Josefa diatremes, in addition to the volcanicrocks and their products. This phreatic and phreatomagmaticactivity was instrumental for enhancing fracture-controlledpermeability of the otherwise impermeable quartzitic hostrock.

Stage III contains the bulk of the alteration and mineral-ization. In rocks of the Chimú Formation, coarse alunite–enargite–pyrite precipitated in fractures, but alteration isrestricted to traces of kaolinite and pyrophyllite in somebeds within the quartzite. Locally, where coal is present,stibnite and arsenopyrite are observed. In contrast, the brec-cias and Miocene volcano-sedimentary units overlying theCretaceous rocks have been affected by alteration assemb-lages typically described for high-sulfidation epithermalsystems. The alteration and mineralization is largely con-trolled by permeability. The central portion of the Dafnediatreme is relatively impermeable due to the matrix beinglargely composed of milled slate of the Chicama Formation,whereas quartzite clasts are dominant at Josefa whichresulted in better permeability for the fluids.

Isotopic compositions of alunite–pyrite–enargite and al-unite–pyrite from stages III and IV, respectively, indicatethat these alteration minerals, and by inference the gold,precipitated from fluids that were acidic, H2S dominant,and largely magmatic in origin. Fluid temperatures basedon Δ34Salu–py thermometry range between 190 and 280 °C,with the highest values near the diatremes which are inter-preted to be the focus of hydrothermal activity.

Acknowledgments This research is part of the PhD research of LuisCerpa. Funds for this study were provided by Minera BarrickMisquichilca S.A., with additional support of the Hugh E. McKinstryStudents Research Fund from the Society of Economic Geologists.This research would not have been possible without the experience andknowledge of Lagunas Norte staff, particularly Nick Teasdale, Jose

Nizama, and several geologists from Servicios Técnicos of LagunasNorte—Barrick. Sulfur and oxygen analysis was carried out in collab-oration with Kerry Klassen and QFIR Lab in Queen’s University,which is supported by NSERC Discovery, CFI, and OIT grants toKurt Kyser. Teresa Velarde’s continuous assistance in spectrometricdata is very much appreciated. Víctor Carlotto, Luis Miguel Muñoz,GR-13 team, and my co-workers of Regional Geology from GeologicalSurvey of Perú (INGEMMET) have greatly helped with this project,and their continuing enthusiasm and support are appreciated.Discussions with Allan Montgomery, Amelia Rainbow, HuayongChen, and Fernando Tornos have greatly helped with this project andreviewers Regina Baumgartner, Noel White, and David Cooke as wellas Editor Bernd Lehmann are thanked for their constructive reviews.

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