davies galeno 05

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ARTICLE R. Charlie Davies Patrick J. Williams The El Galeno and Michiquillay porphyry Cu–Au–Mo deposits: geological descriptions and comparison of Miocene porphyry systems in the Cajamarca district, northern Peru Received: 29 September 2004 / Accepted: 29 August 2005 / Published online: 18 November 2005 Ó Springer-Verlag 2005 Abstract El Galeno and Michiquillay are early to middle Miocene Cu–Au–Mo porphyry-related deposits located in the auriferous Cajamarca district of northern Peru. The El Galeno deposit (486 Mt at 0.57% Cu, 0.14 g/t Au and 150 ppm Mo) is associated with multiple dioritic intrusions hosted within Lower Cretaceous quartzites and shales. Emplacement of the porphyry stocks (17.5– 16.5 Ma) in a hanging wall anticline was structurally controlled by oblique faults superimposed on early WNW-trending fold-thrust structures. Early K-feld- spar–biotite–magnetite (potassic) alteration was associ- ated with pyrite and chalcopyrite mineralisation. A quartz–magnetite assemblage that occurs at depth has completely replaced potassically altered rocks. Late- and post-mineralisation stocks are spatially and temporally related to weak quartz–muscovite (phyllic) alteration. High Au grades are associated with early intrusive phases located near the centre of the deposit. Highest Cu grades (0.9% Cu) are mostly associated with a super- gene enrichment blanket, whilst high Mo grades are restricted to contacts with the metasedimentary rocks. The Michiquillay Cu–Au–Mo deposit (631 Mt at 0.69% Cu, 0.15 g/t Au, 100–200 ppm Mo) is associated with a Miocene (20.0–19.8 Ma) dioritic complex that was em- placed within the hanging wall of a back thrust fault. The intrusive complex is hosted in quartzites and lime- stones. The NE-trending deposit is crosscut by NNW- trending prospect-scale faults that influenced both alteration and metal distribution. In the SW and NE of the deposit, potassic alteration zones contain moderate hypogene grades (0.14 g/t Au and 0.8% Cu) and are characterised by chalcopyrite and pyrite mineralisation. The core of the deposit is defined by a lower grade (0.08 g/t Au and 0.57% Cu) phyllic alteration that overprinted early potassic alteration. Michiquillay con- tains a supergene enrichment blanket of 45–80 m thickness with an average Cu grade of 1.15%, which is overlain by a deep leached cap (up to 150 m). Cu–Au– Mo (El Galeno-Michiquillay) and Au-rich (Minas Conga) deposits in the Cajamarca region are of similar age (early–middle Miocene) and intrusive rock type (dioritic) associations. Despite these geochronological and geochemical similarities, findings from this study suggest variation in metal grade between the hybrid-type and Au-rich deposits result from a combination of physio-chemical factors. These include variations in temperature and oxygen fugacity conditions during hypogene mineralisation resulting in varied sulphide assemblages, host rock type, precipitation of ubiquitous hydrothermal magnetite, and late hydrothermal fluid flow resulting in a well-developed phyllic alteration zone. Keywords Northern Peru Copper Gold Miocene Porphyry Introduction The Cajamarca mining region, located in northern Peru, has one of the largest gold inventories in South America with the high-sulphidation giant Yanacocha Au mine, plus over 30 smaller auriferous epithermal and porphyry deposits exposed at different erosion depths (Gustafson et al. 2004). Gold proven reserves for the Yanacocha district are 69.4 Moz (Newmont 2004). The Minas Conga (Au–Cu), Michiquillay (Cu– Au–Mo) and El Galeno (Cu–Au–Mo) prospects are located 10–15 km east of Yanacocha. These three early Miocene intrusion-related systems have a combined Editorial handling: A. Boyce R. C. Davies (&) Ivanhoe Mines Mongolia Inc, PO Box 353, 210646A Ulaanbaatar, Mongolia E-mail: [email protected] Tel.: +976-9976-5248 Fax: +976-1134-5634 P. J. Williams School of Earth Science, James Cook University, 4810 Townsville, QLD, Australia Mineralium Deposita (2005) 40: 598–616 DOI 10.1007/s00126-005-0026-6

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Page 1: Davies Galeno 05

ARTICLE

R. Charlie Davies Æ Patrick J. Williams

The El Galeno and Michiquillay porphyry Cu–Au–Mo deposits: geologicaldescriptions and comparison of Miocene porphyry systemsin the Cajamarca district, northern Peru

Received: 29 September 2004 / Accepted: 29 August 2005 / Published online: 18 November 2005� Springer-Verlag 2005

Abstract El Galeno and Michiquillay are early to middleMiocene Cu–Au–Mo porphyry-related deposits locatedin the auriferous Cajamarca district of northern Peru.The El Galeno deposit (486 Mt at 0.57% Cu, 0.14 g/tAu and 150 ppm Mo) is associated with multiple dioriticintrusions hosted within Lower Cretaceous quartzitesand shales. Emplacement of the porphyry stocks (17.5–16.5 Ma) in a hanging wall anticline was structurallycontrolled by oblique faults superimposed on earlyWNW-trending fold-thrust structures. Early K-feld-spar–biotite–magnetite (potassic) alteration was associ-ated with pyrite and chalcopyrite mineralisation. Aquartz–magnetite assemblage that occurs at depth hascompletely replaced potassically altered rocks. Late- andpost-mineralisation stocks are spatially and temporallyrelated to weak quartz–muscovite (phyllic) alteration.High Au grades are associated with early intrusivephases located near the centre of the deposit. Highest Cugrades (�0.9% Cu) are mostly associated with a super-gene enrichment blanket, whilst high Mo grades arerestricted to contacts with the metasedimentary rocks.The Michiquillay Cu–Au–Mo deposit (631 Mt at 0.69%Cu, 0.15 g/t Au, 100–200 ppm Mo) is associated with aMiocene (20.0–19.8 Ma) dioritic complex that was em-placed within the hanging wall of a back thrust fault.The intrusive complex is hosted in quartzites and lime-stones. The NE-trending deposit is crosscut by NNW-trending prospect-scale faults that influenced bothalteration and metal distribution. In the SW and NE of

the deposit, potassic alteration zones contain moderatehypogene grades (0.14 g/t Au and 0.8% Cu) and arecharacterised by chalcopyrite and pyrite mineralisation.The core of the deposit is defined by a lower grade(0.08 g/t Au and 0.57% Cu) phyllic alteration thatoverprinted early potassic alteration. Michiquillay con-tains a supergene enrichment blanket of 45–80 mthickness with an average Cu grade of 1.15%, which isoverlain by a deep leached cap (up to 150 m). Cu–Au–Mo (El Galeno-Michiquillay) and Au-rich (MinasConga) deposits in the Cajamarca region are of similarage (early–middle Miocene) and intrusive rock type(dioritic) associations. Despite these geochronologicaland geochemical similarities, findings from this studysuggest variation in metal grade between the hybrid-typeand Au-rich deposits result from a combination ofphysio-chemical factors. These include variations intemperature and oxygen fugacity conditions duringhypogene mineralisation resulting in varied sulphideassemblages, host rock type, precipitation of ubiquitoushydrothermal magnetite, and late hydrothermal fluidflow resulting in a well-developed phyllic alterationzone.

Keywords Northern Peru Æ Copper Æ Gold ÆMiocene Æ Porphyry

Introduction

The Cajamarca mining region, located in northernPeru, has one of the largest gold inventories in SouthAmerica with the high-sulphidation giant YanacochaAu mine, plus over 30 smaller auriferous epithermaland porphyry deposits exposed at different erosiondepths (Gustafson et al. 2004). Gold proven reservesfor the Yanacocha district are �69.4 Moz (Newmont2004). The Minas Conga (Au–Cu), Michiquillay (Cu–Au–Mo) and El Galeno (Cu–Au–Mo) prospects arelocated 10–15 km east of Yanacocha. These three earlyMiocene intrusion-related systems have a combined

Editorial handling: A. Boyce

R. C. Davies (&)Ivanhoe Mines Mongolia Inc, PO Box 353,210646A Ulaanbaatar, MongoliaE-mail: [email protected].: +976-9976-5248Fax: +976-1134-5634

P. J. WilliamsSchool of Earth Science, James Cook University,4810 Townsville, QLD, Australia

Mineralium Deposita (2005) 40: 598–616DOI 10.1007/s00126-005-0026-6

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mineral resource in excess of 1,600 Mt (Llosa and Veliz2000; P. McInnes, unpublished data; Cordova andHoyos 2000). Several barren intrusions of older orsimilar age and composition also crop out in the re-gion. To the north of the Cajamarca district are anumber of high-sulphidation Au deposits that includeSipan, La Zanja, Tantahuatay, and the porphyry-skarn-manto polymetallic Hualgayoc district that hoststhe Cerro Corona deposit (Fig. 1). The district is lo-cated in the Andes with elevations ranging between2,700 and 4,400 m above sea level.

This paper presents a detailed geological descriptionof the El Galeno (Cu–Mo–Au) porphyry prospect withemphasis on documentation of the relative timing ofdifferent intrusive phases, as well as characterisation ofthe alteration, mineralisation and vein infill paragenesis.Revised intrusive, structural, alteration and mineralisa-tion data from the Michiquillay porphyry prospect arealso presented. Finally, El Galeno and Michiquillay arecompared with the relatively well-documented Au-richMinas Conga porphyry complex.

Regional geological setting of the miocene deposits

The Cajamarca district is located in the Western Cor-dillera of northern Peruvian Andes. The region is char-acterised by deformed Cretaceous marine sedimentaryrocks that have undergone several phases of compressivedeformation since Palaeogene time and been intruded bymultiple magmatic phases (Fig. 2). Hydrothermal andmineralisation events were temporally related to twomajor magmatic episodes, i.e. early to middle Mioceneporphyry Cu–Au–Mo formation and late Miocene epi-thermal activity (R. C. Davies, unpublished data). Thegeological setting of northern Peru and the Cajamarcaregion is outlined in Benavides (1956), Reyes (1980),Wilson (1985a, 1985b), Cobbing et al. (1981), S. J.Turner (unpublished data) and Wilson (2000).

Cretaceous to Oligocene rocks

The oldest rocks that crop out in the Cajamarca regionare a thick package of lower- to upper-Cretaceousplatform sedimentary rocks. These sedimentary unitsinclude lower- Cretaceous sandstone and quartzite se-quences at the base of the package that fine upward toUpper-Cretaceous limestone, marl and shale sequences(Benavides 1956). Intense folding and thrusting of themarine sediments from 59 to 55 Ma resulted in emer-gence of the Cretaceous sedimentary units and devel-opment of open, upright folds and imbricate thrustsheets (Megard 1984, 1987).

The deformed sedimentary rocks are unconformablyoverlain by predominantly basaltic to andesitic volcanicsequences known as the Llama Formation (Noble et al.1990; Atherton et al. 1985). The Lower Llama VolcanicSequence forms an aerially-extensive volcanic successionthat range in age from 54.8 to 42.6 Ma (Atherton et al.1985; Noble et al. 1990; R. C. Davies, unpublished data).Intrusions in the Minas Conga, Cerro Perol and CruzConga regions indicate several stocks were emplacedbetween 57 and 43 Ma (Llosa et al. 1996; R. C. Davies,unpublished data).

Miocene rocks

Early to middle Miocene magmatic units (23.2–15.6 Ma)are composed of intermediate, calc-alkaline porphyriticintrusive and volcanic rocks that contain plagioclase,mafic±quartz phenocrysts. Geochronological studies ofmineralised porphyry systems indicate emplacement,main stage alteration and mineralisation at these centresoccurred during this magmatic interval (Llosa et al.1996; R. C. Davies, unpublished data; Gustafson et al.2004; Noble et al. 2004). Coeval barren porphyriticstocks of similar composition display weak propyliticalteration (R. C. Davies, unpublished data). The youn-gest intrusive age for this second magmatic interval is

Fig. 1 Map of Peru showing the major mineralised centres near thetownship of Cajamarca

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defined by an 40Ar/39Ar date of 16.5 Ma from a post-mineralisation intrusion at El Galeno (R. C. Davies,unpublished data), whilst the Upper Llama VolcanicSequence has been dated at 15.8 Ma also using40Ar/39Ar (S. J. Turner, unpublished data). An intervalfrom ca. 16 to 12 Ma is defined by an apparent decreasein magmatic and deformation activity in the Cajamarcaregion. However, the period between 14 and 10 Macorresponded with major magmatic and hydrothermalactivity in the polymetallic Hualgayoc district to thenorth (Macfarlane et al. 1994).

Deposition of andesitic lava flows of the RegaladoFormation (12.3–11.6 Ma; S. J. Turner, unpublisheddata) represented the initiation of middle to late Mio-cene magmatic activity. Economically, the most pro-ductive interval for the Cajamarca region occurredbetween 12 and 10 Ma with formation of the high-sul-phidation Yanacocha Au deposit. Main stage minerali-sation at Yanacocha took place between 11.5 and10.9 Ma (S. J. Turner, unpublished data). Magmatic andhydrothermal activity in the region ceased in the lateMiocene (�8 Ma) with the onset of a shallowing sub-duction zone (Gutscher et al. 1999).

Previous work at mineralised centres in the Caja-marca region includes several studies at the Michiquillaydeposit by Laughlin et al. (1968), Hollister and Sirvas(1974), and reports by the Metal Mining Agency, JapanInternational Cooperation Agency (MMA, unpublisheddata) and P. McInnes (unpublished data). Harvey et al.

(1999) and Longo (2000) provided detailed discussionson gold deposits and trends within the Yanacocha dis-trict, whilst Llosa and Veliz (2000) documented alter-ation and mineralisation styles at the Au–Cu MinasConga prospect. Unpublished reports (R. Hammond; J.B. Garcia; R. H. Sillitoe; R. C. Davies) and publishedwork by Cordova and Hoyos (2000) and Davies (2000)describe the geology of El Galeno. Turner (unpublisheddata, 1999) discussed the volcanic setting and styles ofmineralisation of the late Miocene Yanacocha district aswell as other high-sulphidation systems such as Sipan,La Zanja and Tantahuatay. More recently, Gustafsonet al. (2004) discussed porphyry-epithermal transitionalfeatures observed in a variety of deposits within theCajamarca region.

Regional structure

The Cajamarca area is located within the southern partof the Huancabamba Deflection zone where Andeanstructural fabric rotates from NNW to near E–W(Megard 1984). Structural features observed in the de-formed Cretaceous sedimentary rocks include largewavelength (>5 km) open folds that gently plungeWNW or ESE. In the northeastern part of the studyarea, a thrust fault known as the Puntre Fault is spatiallyassociated with numerous magmatic units of bothPalaeogene and Miocene age (Fig. 2). The thrust fault

Fig. 2 Simplified geological map of the Cajamarca region showingthe distribution of the major mineralised Miocene centres (modifiedfrom Reyes 1980). Age dates: (1) Llosa et al. 1996; (2) Gustafson

et al. 2004; (3) Turner, unpublished data; (4) R.C. Davies(unpublished data); (5) Noble et al. 2004

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displays a major deflection in trend from near E–W tonear N–S (see below). To the east and west of thedeflection, the Puntre fault has a NW trend consistentwith local regional structures. Faults and fractures in theCretaceous sedimentary rocks are generally subverticalwith dominant normal slip ranging from a few to tens ofmetres. In the study area, subvertical fault planes plot inthree general domains, which include N–S, E–W andESE-trending. N-striking faults are the most abundantorientation recognised from aerial-photo interpretation(R. C. Davies, unpublished data).

El Galeno deposit

El Galeno is a Cu–Au–Mo porphyry deposit with anestimated geological resource of 486 Mt at 0.57% Cu,0.14 g/t Au and 150 ppm Mo (Cordova and Hoyos2000). It is exposed at an altitude between 3,850 and4,100 m. Multiple intrusive stocks were emplaced be-tween ca. 17.50 and 16.53 Ma (R. C. Davies, unpub-lished data). The intrusions are hosted by folded LowerCretaceous sedimentary rocks and were emplaced in ahanging wall anticline (Fig. 2). Mineralisation is char-acterised by a hypogene zone overlain by a supergeneenrichment blanket up to 120 m thick. Several unmi-neralised gabbroic dykes crop out to the southwest andnorthwest of the mineralised centre, and display weak tomoderate chlorite alteration. The dykes intruded bothsubvertical N- to NW-trending fault planes and steeplydipping bedding planes. 40Ar/39Ar age dating of horn-blende phenocrysts from a gabbroic dyke yielded anapproximate age of 29.4±1.4 Ma (R. C. Davies,unpublished data), indicating the dykes are Oligocene inage and intruded prior to emplacement of the El Galenoporphyry complex.

Lithology

Field evidence indicates that the El Galeno porphyrycomplex is mainly hosted within quartzite and siltstonesedimentary units (Fig. 3). Observed contacts betweenintrusions and sedimentary host rocks are subvertical.Furthermore, drilling indicates the intrusive complexdoes not extend laterally beyond the outcrop limit(Sillitoe, unpublished data; R. C. Davies, unpublisheddata). The intrusive complex is elliptical in shape,�1,250 m in length by 600 m wide, and its long axis isoriented NW–SE. This orientation is roughly parallel tothe WNW trend of the hanging wall anticline (El GalenoAnticline) that hosts the mineralised centre. The com-plex comprises at least four intrusive phases (Fig. 4),three of which are identifiable in outcrop and the otherin drill core. Timing for emplacement of the fourintrusions has been established from crosscutting,overprinting, alteration and vein relationships observedin both drill core and outcrop.

P1 porphyry

The oldest intrusive phase is a crowded medium-grainedporphyritic diorite composed of euhedral to subhedralplagioclase, with minor euhedral biotite, hornblende androunded quartz phenocrysts (Fig. 5a). Plagioclase phe-nocrysts (An44-46; 35–45 vol%) range between 0.3 and5.0 mm in length and show multiple twinning plusoscillatory zoning. Magmatic biotite and hornblendephenocrysts (1 vol%) of 0.5–2.0 mm length are mostlyreplaced by secondary biotite. The P1 porphyry is thelargest body of the four recognised phases and laterintrusive units were emplaced toward the core of thisstock.

P2 porphyry

The second intrusive phase is also a porphyritic dioritebut texturally heterogeneous. It is dominantly charac-terised by a crowded porphyritic texture (Fig. 5b) withfine-grained plagioclase (0.5–1.0 mm; An42-60) and lessabundant rounded quartz grains, but varies to a med-ium-grained (1.5–3.0 mm) weakly porphyritic rock.Phenocrysts show similar characteristics to magmaticgrains in the P1 porphyry. Hydrothermal biotite fromthis intrusion yielded an age of 17.5±0.3 Ma using40Ar/39Ar (R. C. Davies, unpublished data).

P3 porphyry

P3 porphyry stocks are coarse-grained crowded por-phyritic quartz-diorite with euhedral plagioclase, quartz,biotite and hornblende phenocrysts (Fig. 5c). Plagio-clase phenocrysts (An42-51; 30–35 vol%) display oscilla-tory zoning and range in length from 1.0 up to 8.0 mm.Rounded quartz phenocrysts are 2.0–10.0 mm in lengthand may have minor embayments. Euhedral biotitebooks (6 vol%) range from 1.0 to 10.0 mm in length andhornblende (�3 vol%) phenocrysts are between 1.0 and5.0 mm. This porphyry contains rare xenoliths ofquartz–magnetite altered fragments. In drill core, theoccurrence of these intrusions was sporadic, varying inthickness from a few metres to tens of metres and wasmainly intersected toward the centre of the P1 porphyry(Fig. 4). In outcrop (Fig. 3), contacts of a P3 porphyrybody are near vertical from which it is inferred that thisintrusive phase occurs as a number of small sized verticaldykes.

P3 porphyritic breccia

Hydrothermal breccias (P3 porphyritic breccia) arelocated along the contacts of some of the P3 porphyrybodies (Fig. 4). Fragments observed within the hydro-thermal breccia include angular to subrounded quartzand quartz–magnetite veins that range in size from

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millimeters up to several centimetres. The matrix isdominantly composed of quartz, muscovite and pyrite.

Thin (0.3–2.0 m wide) dioritic dykes are characterisedby crowded porphyritic texture with abundant feldspar

(0.6–2.5 mm) and minor mafic phenocrysts set within alight grey feldspathic groundmass. The dykes were notobserved in outcrop and rarely intersected in drill core.The dykes are considered to have formed late in the

Fig. 3 Geological map of ElGaleno with the majorlithological units and prospect-scale structures. Also, shownare the outer limits of Cu gradezones and the locations of drillcore logged

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Fig. 5 Photographs of thedifferent intrusive phases withinthe Galeno complex. a P1porphyry composed dominantlyof plagioclase (Pl), hornblende(Hbl), biotite (Bt) and quartz(Qtz) phenocrysts. Plagioclaseshave been partially replaced bymuscovite and illite. b P2porphyry with a medium to fine-grained crowded porphyritictexture consisting of plagioclase,biotite phenocrysts and fine-grained hornblende. Crosscuttingquartz-K-feldspar veins andminor sulphides are related todark K-feldspar–biotitealteration that is overprinted byweak propylitic alteration. c P3porphyry with coarse-grainedplagioclase, biotite andhornblende and quartzphenocrysts. d Biotite alteredMBx porphyry with Pl and Btphenocrysts. MBx containsfragments of altered andmineralised P3 porphyry. Dark,fine-grained selvages composedof hydrothermal biotite rim somefragments

Fig. 4 Near E–W section through the El Galeno porphyry complexshowing the distribution of the major lithological units and outerlimits of alteration assemblages [based on drill core logging R.H.

Sillitoe (unpublished data) and R.C. Davies (unpublished data)].The lower limits of the leached cap, chalcocite–covellite zone andsulphide enrichment blanket are also shown

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evolution of the intrusive complex, probably followingemplacement of the third intrusive phase. However,their exact timing is unclear.

MBx porphyry

The youngest intrusive phase is a magmatic brecciathat is weakly porphyritic, with phenocrysts andxenoliths set in a feldspar-rich matrix (Fig. 5d). Euhe-dral plagioclase phenocrysts (An40–56; �10 vol%; 0.5–9.0 mm) show multiple twinning textures and generallycontain sieve-textured rims that suggest plagioclaseinstability (refer to insert of Fig. 11b). Biotite books(�2 vol%) are euhedral and vary from 0.3 to 3.0 mmin size. Rounded quartz phenocrysts (�1 vol%) are�3.0 mm in length and acicular hornblende grains(0.3–2.0 mm) are partially replaced by secondary bio-tite. This unit was intersected at depth in the northerncentral part of the P1 porphyry. Xenoliths are generallyrounded and range in size from a few to tens of cen-timetres. Altered and mineralised xenoliths of the pre-vious three intrusive phases are the dominant clasts.Many clasts have thin (�1 mm) fine-grained dark sel-vages composed of hydrothermal biotite. Magmaticbiotite books from this intrusion yielded a 40Ar/39Arplateau age of 16.53±0.18 Ma (R. C. Davies, unpub-lished data).

Structural geology

El Galeno is situated at the intersection of a hangingwall anticline and a NE-striking tear fault (Fig. 2). Thehanging wall anticline, called El Galeno Anticline, has agentle plunge to the NW (25� fi 303�) and is locatedimmediately above the Puntre Thrust fault. The lowerCretaceous rocks in the hanging wall of the fault arejuxtaposed against middle-upper Cretaceous rocks in thefootwall, indicating over 2,000 m of reverse offset basedon unit thicknesses measured by Benavides (1956). Tothe east and northwest of the intrusive system, the re-verse fault strikes approximately WNW and dips at alow angle (�30�) to the SSW, suggesting that the faultforms a low-angle thrust ramp. However, to the north-east of the complex, the reverse fault deflects in aclockwise direction to near NNW. Within this NNWtrending zone of the fault, argillic-altered sedimentaryrocks have a subvertical dip and display polyphasedeformation. These features include early WNW shal-lowly plunging folds and subhorizontal calcite veins thatare both crosscut by later extensional structuresincluding subvertical calcite veins and small-scale nor-mal faults (R. C. Davies, unpublished data). Other fea-tures observed in near proximity to the thrust zoneinclude a bend in the hanging wall anticline axis towardsthe deflection zone and abundant fractures in quartziteson the northern limb of the anticline. These featuressuggest a change from a low-angle frontal ramp, to the

east and northwest of El Galeno, to a high-angle obliqueramp within the zone of deflection.

Southwest of the intrusive complex, exposures onSenal Guaguayo mountain contain subvertical normalfaults that range from NW- to near E-trending. Dis-placement along the fault planes varies from roughly 2to 5 m. Fault breccias and altered Oligocene gabbroicdykes (1–3 m wide) occur along some of these faultplanes. Normal faults measured at Cerro Listo have auniform orientation with a general NE trend and offsetsup to 10 m. These subvertical faults are characterised byminor brecciation along fault planes, although gabbroicdykes are noticeably absent at Cerro Listo. Aerial photointerpretation suggests a NE-trending tear fault bisectsthe valley between topographic peaks of Senal Gua-guayo and Cerro Listo (R. C. Davies, unpublisheddata). This tear fault is interpreted to crosscut the ElGaleno Anticline.

To the west of Laguna Rinconada (Fig. 3), a series ofsmall-scale normal faults was observed within the FarratFormation. These subvertical faults have an approxi-mate E–W strike and display 0.5–2.0 m displacement.Drill cores along strike of these faults are stronglyfractured and characterised by intense clay alteration todepths in excess of 300 m (R. C. Davies, unpublisheddata). It is inferred that the fault set west of LagunaRinconada extends into the intrusive centre. The natureand history of this fault with respect to plutonemplacement, controls on mineralisation plus possiblepost-mineralisation offset is uncertain.

Alteration, mineralisation and vein paragenesis

Four separate alteration events are identifiable at the ElGaleno prospect based on drill core logging and petro-graphic studies. The first of these is temporally andspatially related to the earliest two intrusive phases. P3porphyry stocks truncate mineralised P1 and P2 por-phyries indicating that hydrothermal events occurredprior to its emplacement. Altered P3 porphyry is in turncrosscut by P3 hydrothermal breccias, with MBx por-phyry containing mineralized fragments of all earlierintrusive phases. Alteration assemblages associated witheach of the intrusive phases display zoned distributions.

Mineralisation is divided into hypogene mineralisa-tion, which includes up to three separate stages, andsupergene mineralisation (Fig. 6). Hypogene minerali-sation occurs in all four generations of porphyry intru-sions as well as extending ten to a few hundreds ofmetres into the host sedimentary rocks.

Stage 1

Alteration The oldest and most widespread alterationidentified in drill core is observed within the P1 and P2porphyries. The dominant assemblage consists ofK-feldspar and biotite (potassic alteration). This is

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characterised by replacement of plagioclase pheno-crysts by K-feldspar (Fig. 7a), while primary horn-blende and biotite phenocrysts are replaced byhydrothermal biotite, and fine-grained secondary bio-tite formed within the matrix. Minor amounts of fine-grained magnetite occur along the rims of alteredmafic minerals and disseminated within the matrix.The earliest vein types include biotite-quartz fractureinfillings (Fig. 7b), followed by quartz veins 0.5 mm upto 10 cm width. High density of quartz stockwork (up

to 50 vol%) is related to this alteration stage andgenerally extends about 200 m below the current sur-face (R. C. Davies, unpublished data). Stockworkdensity generally decreases with depth and toward theperiphery of the intrusive system, although quartzstockwork locally extends up to 100 m into the hostrocks. Minor silicification and a weak propyliticalteration are also evident along the contact marginswith sedimentary host rocks and in localised zones ofthe P1 porphyry.

Fig. 6 Intrusion, alteration and vein infill paragenesis at El Galeno

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Pervasive magnetite-quartz alteration (Fig. 7c) occursat depth and toward the central zone of the P1 porphyry(Fig. 3). Quartz–magnetite-altered fragments occur asxenoliths in the P3 porphyry, although a weak magnetitealteration is evident along its contacts. These relation-ships indicate this quartz-magnetite alteration domi-nantly occurred at depth and prior to emplacement ofthe third intrusive phase.Hypogene sulphides Hypogene mineralisation identi-fied at El Galeno is both fracture infill and disseminatedin character. The earliest mineralisation was molybde-nite (Fig. 7d) deposited in re-opened early quartz veinswithin both the P1 porphyry and host sedimentaryrocks. These quartz veins range in thickness from a fewmillimetres to centimetres. The highest molybdeniteabundances occur along the contacts between the hostsedimentary rocks and P1 porphyry (Fig. 8a). Thisoldest mineralisation phase was followed by the depo-sition of magnetite and pyrite (Fig. 7d) with varyingproportions of arsenopyrite and pyrrhotite. Later chal-copyrite (Fig. 7d) and minor amounts of bornite cross-cut these earlier phases. These later mineralisationphases are evident in both the P1 and P2 intrusions.Hypogene pyrite, chalcopyrite and molybdenite alsoextend into the sedimentary host rocks. Based on log-ging and assay data (R. C. Davies, unpublished data),high-grade hypogene zones are associated with intense

hydrothermal biotite alteration zones and localisedcontacts between the host units and P1 porphyry(Fig. 8b–c).

Stage 2

Alteration The second alteration stage was temporallyand spatially related to emplacement of the P3 por-phyry P3 hydrothermal breccias and diorite dykes(Fig. 6). The earliest alteration identified with this stageinvolved intense K-feldspar replacement of thegroundmass and primary plagioclase grains, andhydrothermal biotite replacement of primary maficminerals. This potassic alteration has a smoky, lightgrey appearance that is a unique characteristic withinP3 porphyries (Fig. 5c). The diorite dykes have a sim-ilar alteration assemblage but a slightly darkerappearance. Development of new quartz veins andreopening of older veins was also related to this alter-ation phase. This alteration has a substantially lowervein density than the Stage 1 alteration.

A quartz–muscovite/illite ± pyrite (phyllic) alter-ation assemblage overprints the potassic-silicate alter-ation and early quartz ±molybdenite veins. The quartz–muscovite/illite alteration is defined by muscovite

Fig. 7 Photographs of alteration and mineralisation features at ElGaleno. a Photomicrograph of a zoned plagioclase phenocrystcrosscut and partially replaced by early Stage 1 potassic alteration.b Stage 1 early Bt vein in the P1 porphyry crosscut by a Qtz vein(middle left of image). Thin magnetite (Mag) veins post-date boththe biotite and quartz veins. c Stage 1 quartz–magnetite alterationcrosscut by quartz and quartz–magnetite veins. Magnetite infill ofreopened veins and late pyrite (Pyr) veinlets are also evident. d

Photomicrograph of Stage 1 mineralisation with early molybdenite,followed by pyrite and finally chalcopyrite. e Photomicrographshowing Stage 2 K-feldspar (bottom left to top centre) has partiallyreplaced a plagioclase phenocryst. Fine-grained hydrothermalbiotite (top right) also precipitated during this alteration stage.Both these minerals are partially replaced by muscovite/illite. fStage 2 pyrite vein with a quartz–muscovite selvedge in a P3 dioriticdyke

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replacement of feldspar grains (Fig. 7e), quartzreplacement of the groundmass and quartz infill in newor reopened fractures. Weak quartz–muscovite alter-ation is evident within the first three intrusive phases anddominantly restricted to the upper 15 m of the intrusivecentre.

Hypogene sulphides Late quartz–muscovite–pyriteveins and thin pyrite veinlets overprint earlier stock-work. The quartz–pyrite veins with muscovite halos(Fig. 7f) are related to the weak phyllic alteration andveins mostly comprise pyrite infill. Minor chalcopyrite,bornite and molybdenite also precipitated along fracture

Fig. 8 Section A–A1 showingmetal grade distributions. HighAu and Cu grades are located inthe upper eastern section of theporphyry complex. Moderate Cugrades also occur on the westernflank of the complex. High Mogrades are mostly located on theedges of the complex. Thequartz–magnetite alteration andlate porphyries (P3 and MBx) arepoorly mineralised

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planes. These veins are dominantly found within, or inclose proximity, to the third intrusive phase. The P3hydrothermal breccias have a quartz muscovite–pyritematrix with xenoliths of quartz–magnetite (Fig. 9a).This mineralisation event is distinguishable from stage1 by a substantial decrease in Au, Cu and to a lesserextent Mo, but produced zones with anomalously highZn and Pb content. Anomalously high Ag, Pb, and Zngrades are also associated with the hydrothermalbreccia. It is inferred that main stage deposition of Au,Cu and Mo occurred prior to emplacement of thisphase.

Stage 3

Alteration The youngest alteration event identifiedwas temporally and spatially related to the emplace-ment of the final intrusive phase. Alteration is darkgrey in appearance (Fig. 9b) and dominantly charac-terised by intense fine-grained hydrothermal biotitereplacement of magmatic mafic phenocrysts andgroundmass. Weak K-feldspar replacement of plagio-clase and the groundmass was also associated with thisevent. Quartz muscovite–pyrite (phyllic) alteration isevident near contacts between xenoliths and the

magmatic breccia. This phyllic alteration is character-ised by a high abundance (up to 15 vol%) of dissemi-nated fine-grained pyrite and intense muscovitereplacement of feldspar (Fig. 9c).

Hypogene sulphides Several late veining events aretemporally related with this alteration phase and containsimilar mineralogy of the previous stages. The fragmentsdisplay barren and mineralised veins that terminate atthe fragment boundaries, which suggest that this min-eralisation phase represents a new hypogene mineraldeposition phase.

Sulphide minerals are commonly associated withzones of intense biotite and K-feldspar alteration. Earlymagnetite precipitation was followed by developmentof thin (�3 mm), wavy quartz veins. Quartz veins werelater reopened and molybdenite, followed by pyrite,then chalcopyrite and bornite were deposited (Fig. 9b–e). These assemblages are crosscut by hairline pyriteveinlets (Fig. 9c). This late stage alteration also in-volved partial to complete hematite replacement ofchalcopyrite and bornite (Fig. 9d–e). This was possiblyassociated with late fluorite–epidote–carbonate–quartzinfill.

Fig. 9 Photographs of alteration and mineralisation features at ElGaleno. a Xenolith fragments of P1, P2, P3 and quartz–magnetiteveins in the P3 hydrothermal breccia. The matrix of the brecciadominantly comprises quartz, muscovite and pyrite. b BiotisedMBx porphyry that contains thin quartz veins related to Stage 3alteration. These veins have been reopened and filled in withmolybdenite (Mo) and chalcopyrite (Ccp). Insert, plagioclasephenocryst with sieved-texture rim. c Phyllic altered MBx porphyrywith P1 porphyry fragment. Late pyrite veinlets crosscuts contact

between the P1 fragment and MBX porphyry. Fine-grainedplagioclase grains have been near completely replaced by musco-vite/illite. d Photomicrograph of stage 3 molybdenite that has beenpartially replaced by hematite (Hem). e Photomicrograph of stage 3molybdenite, chalcopyrite and bornite (Bn). The chalcopyrite–bornite grain is rimmed by late overprinting hematite. f Chalco-pyrite grain rimmed by late chalcocite (Cc) during secondaryenrichment

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Secondary enrichment and weathering

Weak clay (argillic) alteration mostly occurs within theupper 10 m of the intrusive complex or zones withabundant fractures. In these zones, relict feldspar andmafic minerals have been replaced by kaolinite.

Secondary mineralisation is restricted to a super-gene blanket that extends �120 m below the currentsurface (Fig. 4; Sillitoe, unpublished data). Sulphideminerals present within the supergene blanket includechalcocite and minor covellite. Chalcocite replacementis mostly observed along the rims of both hypogenechalcopyrite and bornite (Fig. 9f). Metal grades withinthe supergene blanket range, from 0.7 up to 1.2% Cu.These grades are considerably higher than those re-lated to hypogene mineralisation, which generallyrange between 0.2 and 0.3% Cu. El Galeno alsocontains a leached cap that extends �10 m below thesurface.

Michiquillay deposit

Michiquillay is a Cu–Au–Mo intrusive porphyry systemwith an indicated resource of 631 Mt at 0.69% Cu,0.15 g/t Au and between 100 and 200 ppm Mo with asupergene enrichment zone containing an estimated re-source of 46.2 Mt at 1.15% Cu (P. McInnes, unpub-lished data). The Michiquillay porphyry contains aleached cap (varying from a few metres up to 150 mthick) that overlies a secondary enrichment blanket(1.15% Cu) extending up to a further 80 m in thickness.Exposures of the mineralised complex occur at altitudesfrom 3,500 to 3,750 m (Fig. 10).

Extensive mapping and drilling in the mid 1970s bythe Metal Mining Agency (MMA) of Japan defined thedimensions of the intrusive complex as being �5 km inlength and 1.5 km wide (Fig. 10). The main intrusivebody is roughly parallel to the local NW structuraltrend. Laughlin et al. (1968) dated two intrusions in theMichiquillay region using the K–Ar technique. Theseinclude a quartz–biotite monzonite from the Michi-quillay prospect that yielded an age of 20.6±0.6 Ma(biotite) and a quartz–hornblende monzonite from theMichiquillay suite to the north that produced an age of46.4±1.8 Ma (hornblende). Llosa et al. (1996) ob-tained an age date of 18.8±1.6 Ma (magmatic biotite)from an intrusion at the Michiquillay prospect alsousing the K–Ar technique. 40Ar/39Ar data by R. C.Davies (unpublished data) indicate emplacement of asynmineralisation stock at the Michiquillay prospectoccurred 19.77±0.05 Ma. More recently, Noble et al.(2004) dated a biotite phenocryst from a biotite alteredporphyry dyke at 20.02±0.15 Ma. To the north of theprospect, a barren intrusion was emplaced20.6±0.14 Ma (R. C. Davies, unpublished data). Thisindicates synmineralisation stocks at the deposit areslightly younger than unmineralised intrusions to thenorth.

Lithology

The Michiquillay intrusive centre is hosted withinquartzite units to the north and limestone units to thesouth (Fig. 10). At least two major intrusive phases wererecognised during drill core logging based on texturalevidence. The most common lithology is a medium-grained crowded porphyritic diorite (termed D1) withplagioclase, biotite and hornblende phenocrysts set in afeldspathic groundmass (Fig. 11a). Quartz phenocrystsare rounded, have a low abundance (between 0.5 and 4.0vol%) and occur in localised zones. Euhedral plagioclasephenocrysts (An42-52) range from 0.3 to 5.0 mm and arethe most abundant minerals (�35–45 vol%). Book-shaped euhedral biotite phenocrysts (�3 vol%) varyfrom 0.5 to 8.0 mm and generally contain minor feldsparinclusions. Hornblende phenocrysts (�1–3 vol%) areacicular and mostly 0.3–1.0 mm in length. Slight varia-tions in grain-size and mineral abundance occurthroughout the intrusive body, although no clear trun-cations or crosscutting relationships were observedmaking it difficult to distinguish discrete intrusive pha-ses. Hollister and Sirvas (1974) referred to this unit as aquartz–biotite monzonite. However, petrographic evi-dence indicates primary feldspar phenocrysts are pla-gioclase and primary hornblende grains have beenreplaced by secondary biotite. Therefore, it is suggestedthat this intrusive unit be classified as a biotite–horn-blende diorite.

The second intrusive phase is weakly porphyritic andcharacterised by low vein abundances and intense biotitealteration (Fig. 11b–c). This medium- to fine-grainedintrusive phase dominantly contains euhedral plagio-clase phenocrysts (�15 vol%), plus biotite alteredhornblende phenocrysts and euhedral biotite books(Fig. 11b). This second intrusive phase forms dykes thatrange in thickness from 1 to 6 m (Fig. 12a). The dykescrosscut and truncate the potassic-altered main intrusionand contain xenoliths of the altered biotite–hornblendediorite (Fig. 11b). The dykes generally lack well-devel-oped quartz stockwork veins. No quenched marginswere observed along the rims of xenoliths. The dykeswere only observed in the northeastern and southwest-ern zones of the complex. This phase is inferred to be aseries of syn- to late-mineralisation vertical dioritedykes.

Structural geology

The Michiquillay intrusive complex is located in thehanging wall of a NW-trending back thrust (Fig. 10)that was interpreted by Hollister and Sirvas (1974) tobe crosscut by the NE-trending Encanada Fault. Thelatter fault was not recognised during aerial photoanalysis or fieldwork by the author and its existence issuspect. The thrust dips at �60� towards the NE and isreferred to as the Michiquillay Fault (Hollister andSirvas 1974; MMA, unpublished data). It is charac-

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terised by a matrix-supported breccia that containsboth quartzite and limestone fragments, though nointrusive fragments were observed. The MichiquillayFault was not intersected in drill core. However, in thesouthern zone of the deposit, the reoccurrence ofstockwork and change from potassic to phyllic alter-ation toward the bottom of some drill holes is inter-preted to indicate the Michiquillay Fault is nearby(R. C. Davies, unpublished data). In outcrop, obliqueprospect-scale faults crosscut the porphyry deposit andwere previously mapped by MMA (unpublished data).These dominantly occur toward the centre of thedeposit (Fig. 10) and are recognisable in outcrop by an

increase in fracture density toward the central part ofthe fault zone. The faults are subvertical and domi-nantly strike either to the NNW or NNE.

The NNW-striking prospect-scale faults separatepotassic alteration zones in the NE and SW of thecomplex, and define the outer limit of a strong phyllicalteration zone. An increase in stockwork density wasnoted along these fault planes. Some NNW fault planescontain localised zones of vuggy quartz that arespatially-associated with strong gossan-like oxidationfeatures. The NNW-striking faults are the dominantfault set at the deposit and are inferred to have stronglyinfluenced both alteration and mineralisation. Steeply

Fig. 10 Simplified geological map of theMichiquillay prospect [modified fromprevious mapping by MMA(unpublished data)]

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plunging slicken lines on some fault planes indicate latesubvertical displacement.

Alteration and mineralisation

Both intrusive phases contain a well-developed K-feld-spar–biotite–quartz–magnetite alteration assemblage.The potassic alteration is strongest in the northeasternand southwestern parts of the prospect (Fig. 12b), butextends only a few metres in the host rock. Surfaceexpressions of the potassic alteration zones have a NEtrend (Fig. 10). This alteration is characterised by thereplacement of plagioclase by K-feldspar and the pre-cipitation of fine-grained hydrothermal biotite that isstrongly developed in the southwestern zone. Thenortheastern zone contains only moderate hydrothermalbiotite but stronger K-feldspar–quartz alteration. De-spite these slight variations, both the southwestern andnortheastern zones display very similar alteration styles.Veins associated with the potassic alteration includeearly 1–2 mm wavy magnetite and biotite veins that arecrosscut by quartz veins. Hypogene sulphides associatedwith the potassic alteration include chalcopyrite,molybdenite (Fig. 11d), pyrite and minor bornite. Theseminerals occur as infill in veins and fractures, as well asdisseminated in the groundmass. Microscopic studies byMMA (unpublished data) reported that pyrrhotite and

sphalerite also precipitated during this stage. Based onlogging results and corresponding assay data, averagehypogene grades for these zones are �0.14 g/t Au and0.8% Cu (R. C. Davies, unpublished data).

Intense quartz–muscovite–pyrite (phyllic) and latekaolinite (argillic) alteration zones occupy the core ofthe complex (Fig. 12b). Quartz–pyrite veins withmuscovite/illite selvedges are observed throughout theintrusive complex and crosscut all veins (Fig. 11e). In-tense phyllic alteration has destructively overprintedolder textures (Fig. 11f). Intensity of the quartz–muscovite–pyrite alteration however weakens towardthe edges of the core where it has partially replacedpotassic-altered rocks. As mentioned above, NNW-striking faults are inferred to have controlled the dis-tribution of these alteration assemblages. Sulphidesprecipitated during this alteration include largeamounts of pyrite (Fig. 11f), plus minor molybdeniteand chalcopyrite (P. McInnes, unpublished data).MMA (unpublished data) also identified luzonite andtetrahedrite–tennantite within the central upper parts ofthe phyllic alteration zone. Au and Cu grades in thiszone (0.08 g/t Au and 0.57% Cu; R. C. Davies,unpublished data) are lower than in the potassicalteration zones.

Michiquillay contains a supergene enrichment zonecharacterised by covellite and chalcocite replacement ofhypogene chalcopyrite (Fig. 12c). Grades associated

Fig. 11 Photographs of intrusive units, alteration and mineralisa-tion features at Michiquillay. a The D1 porphyry consists ofplagioclase, biotite and hornblende phenocrysts with a feldspathicgroundmass. The groundmass has been partially to stronglyoverprinted by hydrothermal biotite (dark grey bottom left). Minorchalcopyrite (middle top) and pyrite (top right) is also present. bMedium-grained crowded D1 porphyry (left) truncated by a fine-grained dyke (right). Both intrusions contain minor quartzstockwork. c A dyke with plagioclase and biotite phenocrysts set

in a groundmass that has been intensely replaced by hydrothermalbiotite. Late pyrite and hematite veins crosscut the dyke. dChalcopyrite and molybdenite mineralisation in a quartz vein. eLate pyrite veins with quartz–sericite halos crosscut the potassicaltered D1 porphyry. f Quartz–sericite alteration has destructivelyreplaced most primary igneous textures, although some grainboundaries are evident. Late thick pyrite veins are associated withthe quartz–sericite alteration

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with this enrichment zone range from 2.2 to 0.5% Cu, atan average of �1.09% Cu (P. McInnes, unpublisheddata). In parts, late clay (argillic) alteration partially tomoderately overprints the quartz–muscovite alteration.Clay alteration is spatially associated with fault zones

and extends to depths of 100 m below the surface. Thecomposition of the clay was determined as kaolinitefrom X-ray diffraction analysis (R. C. Davies, unpub-lished data). Late development of malachite is evident inboth outcrop and on drill core.

Fig. 12 Section A–A1 lookingNW, showing the majorlithological units, alteration andlogged drill cores and Cu grademap from assay data

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Discussion

Interpretation of the El Galeno and michiquillaydeposits

Previous interpretations of El Galeno geology include acomplex series of stratigraphically controlled sills andlaccoliths that intruded along bedding planes or zones ofweakness (R. Hammond, unpublished data). Sillitoe(unpublished data) suggested that several small, vertical,annular dykes intruded an early porphyry body thatcontains the majority of hypogene mineralisation. Basedon observations from this study, a revised version of theSillitoe (unpublished data) model is presented for ElGaleno. Emplacement of the principal and earliest por-phyry occurred at the intersection of a regional structurecrosscut by an oblique secondary structure. The P1porphyry is elliptical in plan view with a northwesttrend, similar to the localised structural fabric. Thesefeatures indicate that emplacement of the principal ElGaleno porphyry (P1) was dominantly structurallycontrolled.

Early Mo precipitation occurred along the contactsof the P1 porphyry and host rocks. Candela and Piccoli(1995) showed that at low initial water content and a lowCl/water ratio, Mo is preferentially partitioned from themelt into exsolved fluid compared to Cu. This suggestsearly Mo mineralisation is temporally associated withemplacement of the oldest and driest porphyry. Mainstage Cu and Au deposition occurred during, or shortlyafter emplacement of the P2 porphyry, towards the coreof the complex where temperatures would have beenelevated. A pervasive quartz–magnetite alteration atdepth and towards the core of the P1 porphyry possiblydeveloped contemporaneously with emplacement of theP2 porphyry. Both the P1 and P2 porphyries have well-developed potassic alteration hosting modest hypogenemineralisation. The younger intrusions (P3 and MBxporphyries) have potassic and phyllic alteration phases,are poorly mineralised and have a lower abundance ofquartz stockwork than the earlier P1 and P2 porphyries.These intrusions are inferred to be late- to post-miner-alisation. A substantial proportion of the higher Cugrades are associated with a supergene enrichment zonethat extends up to 150 m beneath a thin leached cap.

Emplacement of the Michiquillay porphyry duringearly to middle Miocene times (20.0–19.8 Ma) wasstructurally controlled by the Michiquillay Fault. Theexistence or significance of the Encanada Fault proposedby Hollister and Sirvas (1974) is unclear. Early potassicalteration developed along a NE trend and was associ-ated with chalcopyrite, pyrite, magnetite, bornite andminor molybdenite mineralisation. Significant hypogeneCu and Au grades are preserved in the northeastern andsouthwestern potassic alteration zones. Late hydrother-mal fluids related to phyllic and argillic alteration weretransported along the NNW fault planes and over-printed earlier potassic alteration. An intense phyllic

alteration zone located toward the centre of the depositdefines a pyritic-rich, low-grade zone. It is inferred thatthese late fluids remobilised some Cu and Au associatedwith early potassic alteration. The deposit contains asupergene enrichment zone located beneath a leachedcap of varying thickness.

Comparison of Miocene porphyry deposits

The Cajamarca district hosts a number of mineralisedporphyry and high-sulphidation systems that displayvarying mineralisation and alteration styles. Turner(1999) suggested one of the principal differences betweenthe various late Miocene high-sulphidation systems inthe Cajamarca region relates to level of erosion. Heargued that Yanacocha represents a high level epither-mal system characterised by phreatic breccias and has apoorly understood structural control, whereas Sipan andLa Zanja are relatively deeply eroded systems wheremagmatic plus hydrothermal breccias predominate andstructural controls are more clearly recognised. Agedates from three early to middle Miocene mineralisedporphyry deposits, El Galeno (Cu–Au–Mo), Michi-quillay (Cu–Au–Mo) and Chailhuagon-Perol (Au–Cu)at Minas Conga, indicate the intrusive complexes wereemplaced over a period of �6 my but have significantlydifferent metal grades (Table 1). The following para-graphs compare and contrast these porphyry deposits inthe Cajamarca region.

El Galeno is dominantly hosted in quartzites, Mich-iquillay in both quartzites and limestones, whereas theMinas Conga deposits (Chailhuagon and Perol) aremostly hosted in limestones and marls (Table 1). Hostrocks with low permeability, such as marbleised lime-stone, minimise lateral flow of hydrothermal fluids,thereby focusing metalliferous fluids to the host intru-sion and possibly increasing metal concentrations(Sillitoe 2000). El Galeno, Michiquillay and Chailhuagondeposits consist of early to middle Miocene hornblende–biotite diorites that are geochemically very similar (R. C.Davies, unpublished data). Chailhuagon is the oldestand possibly longest lived complex with the main por-phyritic stock emplaced �23.2 Ma (Llosa et al. 1996)and a potassic altered porphyry dated at 15.6 Ma(Gustafson et al. 2004). Drilling at both Chailhuagonand Perol has defined post-mineralisation stocks atdepths that significantly reduce the Au–Cu grade of bothdeposits (Llosa and Veliz 2000). Michiquillay containstwo recognisable synmineralisation intrusions that wereemplaced between 20.0 and 19.8 Ma (R. C. Davies,unpublished data; Noble et al. 2004). El Galeno is theyoungest system (17.5–16.5 Ma) and has the mostcomplex intrusive history, with at least four intrusivephases present. The earliest two intrusions at El Galenoare synmineralisation, whilst the final two represent late-to post-mineralisation porphyries. Post-mineralisationlow-grade or barren stocks are, however, common fea-tures of both Au-rich and Cu–Au–Mo porphyry

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deposits (e.g. Maricunga belt, Vila and Sillitoe 1991;Bajo de la Alumbrera, Ulrich and Heinrich 2001; Sillitoe2000).

Multiple alteration stages at El Galeno were tempo-rally related to the emplacement of different intrusions.The earliest two porphyries are characterised byK-feldspar-biotite alteration and intense quartz–mag-netite alteration at depth. Quartz–muscovite–pyritealteration overprints the potassic alteration and isassociated with emplacement of later intrusions. AtMichiquillay, the distribution of the potassic and phyllicalteration assemblages was controlled by prospect-scalefaults. Intense potassic alteration consisting of K-feld-spar–biotite and widespread magnetite defines the Cha-ilhuagon main porphyritic stock. Both Galeno andMichiquillay lack widespread hydrothermal magnetitethroughout the deposit. Au-rich porphyry deposits, suchas Grasberg (Meinert et al. 1997; Pollard and Taylor2002), Far South East (Hedenquist et al. 1998) and Bajode la Alumbrera (Ulrich and Heinrich 2001), arecommonly characterised by abundant hydrothermalmagnetite associated with an early potassic alteration

(Clark and Arancibia 1995; Sillitoe 2000). All threemineralized porphyry centres in the Cajamarca districtcontain weak quartz, propylitic and late carbonatealteration assemblages.

Significant differences between the three porphyrycentres relate to the hypogene sulphide assemblage andthe presence or absence of a well-developed phyllicalteration. At El Galeno, elevated hypogene Cu and Aumineralisation were temporally and spatially associatedwith potassic alteration and high-density quartz stock-work veining in early intrusions. Pyrite, molybdeniteand to a slightly lesser extent chalcopyrite largely formthe hypogene sulphide assemblage. At Michiquillay,high hypogene grades occupy zones of intense potassicalteration and are associated with chalcopyrite, pyrite,molybdenite and minor bornite. Late stage fluidstransported along faults resulted in development ofa low-grade, pyrite-rich phyllic core. Gammons andWilliams-Jones (1997) suggest late-stage fluids associ-ated with phyllic overprinting have the potential toremobilise significant quantities of Au and Cu. Such ascenario is proposed for Michiquillay with late-stage

Table 1 Summary of the El Galeno, Michiquillay and Minas Conga porphyry centres

E1 Galenoa Michiquillaya,b Chailhuagon(Minas Conga)c,d

Perol(Minas Conga)c,d

Metal association Cu–Au–Mo Cu–Au–Mo Au-Cu Au-CuResource 486Mt @ 0.57%

Cu+ 0.14g/t Au631 Mt @ 0.69%Cu+ 0.15g/t Au

190Mt @ 0.77g/tAu+ 0.28%Cu

428 Mt @ 0.78g/tAu + 0.31% Cu

Age (Ma) 17.5–16.5 20.6–19.8 23.2–15.6 23.2–15.8Intrusion type Hornblende–biotite

dioriteHornblende–biotitediorite

Hornblende–biotitediorite

Hornblende–biotitediorite

No. intrusions recognised 4 2 2 3Dimensions of principalintrusive

1,250·600 m(NW oriented)

5,000·1,500 m(NW oriented)

2,000·500 m(N–S oriented)

1,600·650 m(NW oriented)

Host rock type Qtzte, Sst and Sh Lst and Qtzte Lst and Marl Lst, Marl and MGDElevation (m.a.s.l) 3,850–4,000 3,500–3,750 3,840–3,900 3,870–3,960Structural control ofporphyry emplacement

Located at theintersection ofseveral structures

Emplaced within thehanging wallof reverse fault

Located at the intersectionof N- and NW-trendingfaults

NW-trending perolthrust fault

Brecciation Minor hydrothermalbreccia,post-mineralisationmagmatic breccia

Minor magmatic breccia,generally absent

Absent Absent

Principal AlterationPhases

K, QM, QS, CbEF K, M, QS, E, Cl K(+M), QS, Skn, Q, CbE K(+M), QS, Skn, AA

Quartz stockwork Moderate–strong Moderate–strong Strong Strong–moderateControlson mineralisation

Dominantly intrusionrelated

NNW faults andNE alteration trend

Intrusion andvein related

Intrusion andvein related

Main Hypogene sulphides Pyr, Ccp, Mo Ccp, Pyr, Mo Ccp, Bn, Pyr Ccp, Bn, PyrSupergene blanket Present (0–12 m) Present (0–40 m) Absent Poorly developed

(<50 m)Supergene sulphides Cc, Cv Cc, Cv Cc, Cv Cc, CvLeached cap Thin (<10 m) Moderate–thick

(up to 80 m)Absent Absent

Rock types: Qtzte quartzite; Sst sandstone; Sh shale; Lst limestone; MGD microgranodioriteAlteration phases: K potassic; Q quartz/silicic; M magnetite; S sericite; Skn skarn; AA advanced argillic; Cb carbonate; E epidote;F fluorite; Cl claySulphides: Pyr pyrite; Ccp chalcopyrite; Mo molybdenite; Bn bornite; Cc chalcocite; Cv CovelliteaThis studybMMA (unpublished data), McInnes (unpublished data)cLlosa and Veliz (2000)dGustafson et al. (2004)

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fluids remobilising potassic alteration related Cu–Aumineralisation away from the centre of the deposit. Incontrast to Michiquillay and El Galeno, hypogenemineralisation at Chailhuagon was deficient in molyb-denum. High hypogene Au and Cu grades are charac-terised by abundant bornite, magnetite and chalcopyrite.A strong association between elevated gold and bornitehas been documented at other Au-rich porphyrydeposits such as Bingham (Ballantyne et al. 1997) andGrasberg (Rubin and Kyle 1997). However, wherebornite is absent or chalcopyrite is more abundant thanbornite, gold distribution is generally closely associatedwith chalcopyrite (Ballantyne et al. 1997, Ulrich andHeinrich 2001; Kesler et al. 2002). Experimental studiesof Au distribution for high temperature (400–700�C)porphyry copper deposits indicate that bornite is thedominant host of Au at temperatures of 700�C orgreater (Simon et al. 2000). Additionally, chalcopyritepreferentially precipitates over bornite and magnetite atlower temperatures and oxygen fugacity. These authorsalso showed that when experimentally equilibrated withAu, chalcopyrite was found to contain Au which wasless by one order of magnitude than bornite.

The highest Cu grades at El Galeno are spatially-restricted to a secondary enrichment blanket that mayhave been partially removed during glaciation. Michi-quillay also contains supergene enrichment zone of�45 m thickness that is located beneath a moderatelythick (up to 80 m) leached cap. In contrast, supergeneenrichment and chalcocite–covellite replacement is veryrare at the relatively pyrite-poor Minas Conga deposits.

Conclusion

The Cajamarca region hosts a number of significantMiocene porphyry-related deposits, including El Galeno(Cu–Au–Mo), Michiquillay (Cu–Au–Mo) and MinasConga (Au–Cu). Despite significantly varying in Au andCu concentrations, synmineralisation intrusions at thesedeposits have similar geochemistry. Based on observa-tions presented in this paper, a key difference betweenAu and Cu concentrations at the deposits is proposed tobe the physio-chemical conditions associated with earlystage hypogene mineralisation. Deposits at the Au-richMinas Conga centre have a well-developed potassicalteration zone comprised of ubiquitous hydrothermalmagnetite and a high temperature-oxygen fugacityhypogene sulphide assemblage dominated by borniteand chalcopyrite. These features conform to a general-ised model for most Au-rich porphyry-related depositsfound along the Andes and worldwide (Sillitoe 2000).Michiquillay and El Galeno are Cu–Au–Mo depositsthat fit to generalised model for Andean porphyry Cudeposits proposed by Sillitoe (1989). These hybrid-typedeposits display early potassic alteration overprinted byphyllic alteration, and in contrast to Minas Conga,lower temperature hypogene sulphide assemblagescharacterised by chalcopyrite and pyrite. Other factors

that influenced metal grades between the deposits mayinclude host rock type, presence of widespread hydro-thermal magnetite, and the occurrence of late-stage flu-ids resulting in remobilisation of early potassicalteration-related metals.

Acknowledgements This paper has been largely taken from achapter of the first author’s doctorate. The study was originallysponsored by North Ltd. and later by Rio Tinto (Peru). Bothcompanies provide field support and funded the analytical research.Bob Beeson, Jose Machare, Jesus Cordova and Tim Moody sig-nificantly contributed to the set up and design of the project.Additional analytical funds were provided through a James CookUniversity–Industry collaborative grant. Newmont and Buen-aventura are thanked for allowing access and samples to be col-lected from Yanacocha and Minas Conga, respectively. Specialthanks are extended to Cesar Vidal for coordinating various visits.Reviews by Tim Baker, Dick Tosdal, David Cooke, Jeremy Rich-ards, Lluis Fontbote, Massimo Chiaradia and Eric Marcoux sig-nificantly improved the manuscript.

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