breccia hosted porphyry cu-mo-au progressive magmatic hydro thermal system

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IntroductionIN DIVERSE ORE DEPOSITS of magmatic affiliation, includingthe Olympic Dam Cu-U-Au deposit (Roberts and Hudson,1983), the Ardlethan Sn deposit (Ren et al., 1988), and the

Kidston Au deposit in Australia (Baker and Andrew, 1991),breccia bodies serve as the main ore hosts. Breccias also con-tain significant parts of orebodies in many porphyry deposits,for example, at Cananea (Mexico; Bushnell, 1988), Los Bronces-Río Blanco (Chile; Warnaars et al., 1985; Vargas et al., 1999), andOyu Tolgoi (Mongolia; Perelló et al., 2001). Breccias provideeasy access for permeating fluids and may thereby control

Evolution of the Breccia-Hosted Porphyry Cu-Mo-Au Deposit at Agua Rica, Argentina: Progressive Unroofing of a Magmatic Hydrothermal System

MARIANNE R. LANDTWING,†

ETH Zürich, Isotope Geochemistry and Mineral Resources, Department of Earth Sciences, Swiss Federal Institute of Technology, ETH Zentrum NO, 8092 Zürich, Switzerland

ERIC D. DILLENBECK,*Northern Orion Explorations Limited, Suite 1400, 570 Granville Street, Vancouver, British Columbia, Canada V6C 3P1

MARTIN H. LEAKE,BHP Billiton Minerals Exploration, Suite 2300, 1111 West Georgia Street, Vancouver, British Columbia, Canada V6E 4M3

AND CHRISTOPH A. HEINRICH

ETH Zürich, Isotope Geochemistry and Mineral Resources, Department of Earth Sciences, Swiss Federal Institute of Technology, ETH Zentrum NO, 8092 Zürich, Switzerland and Faculty of Mathematics and Natural Sciences,

University of Zürich, Rämistrasse 17, 8006 Zürich, Switzerland

AbstractDetailed geologic mapping has been used to show that Agua Rica is a porphyry-style Cu-Mo-Au deposit that was

first overprinted by polystage brecciation associated with a high sulfidation epithermal event and then by a barrensurface-venting phreatomagmatic diatreme, prior to a final stage of supergene enrichment. It was emplaced in theMiocene (~8–5 Ma) as an outlier of the Farallón Negro Volcanic Complex in northwestern Argentina.

The Agua Rica deposit lies next to the contact between Precambrian or lower Paleozoic metasedimentary rocksand coarse-grained Ordovician granites. In a first pulse of Miocene magmatism, equigranular to porphyritic in-trusions were emplaced, with minor potassic alteration and weak Cu-Mo mineralization. Subsequent intrusion offeldspar porphyries was associated with intense porphyry-style stockwork veining, potassic and propylitic alter-ation, and disseminated Cu-Mo-Au mineralization (molybdenite, chalcopyrite ± bornite ± pyrite). The present al-teration and mineralization pattern is dominated by an almost pervasive overprint of high sulfidation epithermalassemblages (phyllic and advanced argillic alteration and Cu-Au-Ag-As-Pb-Zn mineralization) in breccia cementsand as void fillings. Covellite is the dominant copper mineral in the ore and seems to have partly or completelyreplaced chalcopyrite and bornite of the earlier porphyry events. The high sulfidation epithermal assemblages areclosely related to the emplacement of a largely clast-supported hydrothermal breccia. Three major bodies of thisbreccia have been mapped on the basis of clast lithology, clast shape and size, degree of alteration, and composi-tion of breccia matrix. Igneous breccia with a fine-grained porphyritic matrix is intimately associated and in-terfingers with the base of the hydrothermal breccia columns. A final phase of magmatic hydrothermal activityformed a matrix-supported and commonly bedded crater infill breccia. It formed by a surface-ventingphreatomagmatic eruption, as shown by a continuous downward transition from bedded breccias to clast-sup-ported breccias with sandy or pumiceous matrix to a solid igneous breccia with a fine-grained porphyritic matrixin the lower core of the conical crater infill breccia body. Graded, matrix-rich epiclastic sediments subsequentlyfilled the crater. Magmatic activity was terminated by a dike of unmineralized biotite porphyry, which intrudedthe crater infill breccia. Talus breccia was shed into the crater from the rim. Supergene leaching and enrichment,which replaced covellite, pyrite, chalcopyrite, and bornite by chalcocite and secondary covellite, formed an en-richment blanket that was dissected by the present-day, steeply incised topography.

The distinctive feature of the Agua Rica hydrothermal system is the occurrence of early, weakly mineralizedintrusions, later feldspar porphyries with stockwork-hosted chalcopyrite-bornite-molybdenite mineralization,hydrothermal breccias with an epithermal pyrite-covellite overprint, and barren surface-venting breccias—allexposed at one location within 1,000 m of vertical exposure. Reconstruction of the time sequence of these geo-logic elements indicates that Agua Rica is the result of a protracted history of magmatic hydrothermal activitywith superposition of several intrusion events that probably extended over several million years during pro-gressive regional uplift, erosion, and explosive unroofing.

Economic GeologyVol. 97, 2002, pp. 1273–1292

† Corresponding author: e-mail, [email protected]*Present address: Colorado School of Mines, Department of Geology and

Geological Engineering, Golden, Colorado 80401.

high-grade mineralization (Taylor and Pollard, 1993), butbrecciation may also overprint and thereby mechanically re-distribute or chemically reconstitute preexisting orebodies.

The character and origin of breccias in magmatic hy-drothermal environments vary widely. Igneous, magmatic hy-drothermal, hydromagmatic, and intrusive breccias as well astectonic breccias are distinguished according to the classifica-tion scheme of Sillitoe (1985), and several of these brecciatypes can occur in one deposit. To understand the formationof a complex breccia-hosted deposit, systematic documenta-tion of breccia characteristics is required for interpreting thetemporal succession of multiply overprinting brecciationevents. Agua Rica poses a particular challenge for the study ofbreccia development in high sulfidation epithermal and por-phyry-style ore deposits, because a considerable part of theorebody is hosted by breccias of highly variable character.The aim of this study is to document several superimposedevents of magmatism, brecciation, and hydrothermal miner-alization, by detailed mapping in an exceptionally well ex-posed geologic setting.

Agua Rica is one of two major copper-molybdenum-goldresources that are temporally and spatially associated with theFarallón Negro Volcanic Complex (Sasso, 1997; Perelló et al.,1998; Sasso and Clark, 1998), together with the Cu-Au por-phyry of Bajo de la Alumbrera (Guilbert, 1995; Wall, 1997;Profett et al., 1998: Ulrich and Heinrich, 2001; Ulrich et al.,2001). Copper oxide and rhodochrosite deposits in the AguaRica area had been mined intermittently on a small scale(Rojas et al., 1998), before Compañia Cities Service Ar-gentina S.A. undertook the first significant exploration from1970 to 1972 (Koukharsky and Mirré, 1976; Navarro, 1986).From 1994 to 1999, the Agua Rica deposit was explored indetail in a joint venture between Broken Hill ProprietaryLimited (BHP) and Northern Orion Explorations Limited.An extended drilling program delineated a sum of measured,indicated, and inferred resources of 750 Mt at an averagegrade of 0.62 percent Cu, 0.23 g/t Au, 0.037 percent Mo, andminor Ag, calculated at a cutoff grade of 0.4 percent Cu(Table 1; Perelló et al., 1998, Rojas et al., 1998).

Koukharsky and Mirré (1976) published the first detailedgeologic description of Agua Rica (previously called Mi Vida)and recognized that it shared characteristics of porphyry cop-per deposits and near-surface, volcanic-related deposits. Theydescribed one breccia body with wide variations in clast com-position, clast size, clast shape, alteration, and mineralization.Navarro (1986) distinguished three breccia types and inferredprocesses of fluidization and violent rupture through explo-sive forces of a rock mass subjected to a reduced lithostaticpressure. More recently, BHP geologists identified severaladditional breccia types in drill core and noted that differentbreccias are associated with characteristic types of hydrother-mal alteration and sulfide mineralization (B. Jones, F. Urzúa,R. Roco, and A Lasry, pers. commun., 1996; Perelló et al.,1998; Rojas et al., 1998).

This paper is primarily based on detailed surface mapping,supplemented by geologic observations in drill core. It docu-ments a complex history of several stages of subvolcanic in-trusion, mineralization, brecciation, and successive erosionat Agua Rica. An evolution is proposed that explains most ofthe observed features in the context of the general geologic

models for high sulfidation and porphyry-style ore depositsthat have been suggested by Sillitoe (1973), Hedenquist andLowenstern (1994), and White and Hedenquist (1995). Ofspecial interest at Agua Rica is the clear field evidence for astepwise superposition of deep, intermediate, and shallowprocesses at the current level of exposure, resulting from suc-cessive unroofing and erosion of an evolving magmatic hy-drothermal system.

Geographic and Geologic Overview

Geographic setting

Agua Rica is situated in northwestern Argentina (ca. lat27º26' S and long 66º16' W), in Catamarca Province. It is lo-cated 25 km north of the town of Andalgalá and about 15 kmsouthwest of Cerro Nevado del Candado (5,450 m asl), thehighest peak of the Sierra de Aconquija (Fig. 1). The Sierrade Aconquija, one of the basement-cored uplifts of the Sier-ras Pampeanas, is situated 200 km east of the main Andeancordillera, forming the easternmost mountain chain of theAndes. Elevations in the Agua Rica area range from 2,900 to3,500 m asl. The area is dissected by very steep, V-shaped val-ley topography and continues to be actively eroded. Uncon-solidated to partially consolidated layered scree locally attain-ing a thickness of several tens of meters and poorly tomoderately developed soils cover parts of the area.

Tectonic setting and regional geology of the Farallón Negro Volcanic Complex

The tectonic setting of the Farallón Negro Volcanic Com-plex has been described by Sasso (1997) and Sasso and Clark(1998). The Agua Rica deposit is spatially associated with thecoeval Farallón Negro Volcanic Complex, which is located at27° S latitude, above the transition zone between two distincttectonomagmatic Andean segments. In the volcanic segmentbetween 15° S and 24° S latitude, the Benioff zone is inclinedabout 30° to the east, but between 27° S and 33° S latitude itdips only about 5° to 10° and Quaternary volcanism is lacking(Sillitoe, 1976; Allmendinger et al., 1983; Jordan et al., 1983;Jordan and Allmendinger, 1986; Sasso and Clark, 1998;Mahlburg Kay et al., 1999). The area is located at the inter-face between the northernmost Sierras Pampeanas and thePuna physiographic and tectonic provinces (Fig. 1). ThePuna, a high mountainous plateau, is the extension of the Al-tiplano of Peru and Bolivia into northwestern Argentina. TheSierras Pampeanas are characterized by irregular basementblocks separated by wide intramontane basins (Allmendingeret al., 1983; Allmendinger, 1986).

1274 LANDTWING ET AL.

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TABLE 1. Mineral Resources of the Agua Rica Deposit1

MineralCut-off grade resources Cu Mo Au

(% Cu) (Mt) (%) (%) (g/t)

1.0 60 1.31 0.040 0.350.7 167 0.99 0.036 0.32 0.4 750 0.62 0.037 0.23 0.2 1,714 0.43 0.032 0.17

1Data are based on the 150 drill hole block model of BHP made in Feb-ruary 1998

The early volcanic history of the Farallón Negro VolcanicComplex, ranging in age from 12.6 to 8.5 Ma and covering700 km2, was characterized by the eruption of basalts, basalticandesites, and dacites from several volcanic centers, possiblyfocused by the intersections of regional structures (Sasso,

1997). Major extrusive, intrusive, and hydrothermal activityextended from 8.5 to 5.5 Ma, including the construction ofthe interpreted Farallón Negro stratocone (Llambías, 1970).Rapid uplift of the basement blocks in the Miocene andPliocene (Jordan et al., 1983) resulted in the formation of

PROGRESSIVE UNROOFING OF A MAGMATIC-HYDROTHERMAL SYSTEM: AGUA RICA, ARGENTINA 1275

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FIG. 1. Simplified geologic map of the Farallón Negro-Capillitas-Agua Rica area in northwestern Argentina. Precambrianand Early Paleozoic basement is intruded by the Late Ordovician to Early Silurian Capillitas-Belén granite. Miocene redbeds of the Calchaquense (or Calchaquí or Morterito) Formation overlie this basement with marked unconformity, followedby Miocene high K calc-alkaline volcanic and intrusive rocks of the Farallón Negro Volcanic Complex (Llambías, 1970; Sassoand Clark, 1998). Modified from Martínez et al. (1995), Llambías (1970), and Sasso (1997).

pediment surfaces and, eventually, in the exposure and localsupergene alteration of the hydrothermal centers (McBride,1972; Perelló et al., 1998; Sasso and Clark, 1998). Today theFarallón Negro Volcanic Complex consists of a deeply erodedremnant of a stratovolcano and small outlying intrusive and/orvolcanic centers. Two Cu-Au-(Mo) porphyries, the Bajo de laAlumbrera mine (Guilbert, 1995; Wall, 1997; Ulrich et al.,1999, 2001; Ulrich and Heinrich, 2001) and the Agua Rica de-posit (Koukharsky and Mirré, 1976; Perelló et al., 1998, Rojaset al., 1998) have been studied in detail. Smaller epithermaldeposits such as the Capillitas polymetallic vein deposit, minedfor gemstone rhodochrosite (Angelelli, 1974; Indri, 1986;Breitenmoser, 1999; Hug, 1999), and the Farallón Negro sil-ver and gold mine (Malvinci and Llambías, 1963; Llambías,1970) are associated with the same magmatic complex.Whereas Bajo de la Alumbrera lies within the base of the in-ferred stratovolcano, Agua Rica is associated with outlying in-trusions in Paleozoic basement and has, at the present erosionlevel, no major extrusive units (Sasso and Clark, 1998).

Basement Rocks

Metasedimentary rocks of the Sierra Aconquija Complex

Metasedimentary rocks are the main country rocks to theMiocene porphyry intrusions and breccias of the Agua Ricadeposit. Black to dark gray, fine- to medium-grained quartzarenites, subarkoses, graywackes, and siltstones were region-ally metamorphosed to greenschist facies and are consideredto be Precambrian or Early Paleozoic (Koukharsky and Mirré,1976). In some places, the metasedimentary rocks are cut bymilky quartz veins up to 1 m in thickness of inferred meta-morphic origin. The metasedimentary rocks show complextextural and compositional variations at a 10- to 100-m scale,including transitions from fine-grained, foliated and crenu-lated, schistose metasedimentary rocks to massive quartziticmetasedimentary rocks.

The metasedimentary rocks at Agua Rica were folded be-fore the Miocene magmatic overprint. Bedding and composi-tional layering indicate a large open fold structure. The axialplane of this fold is approximately vertical and strikes south-southeast–north-northwest. The fold axis is slightly inclinedtoward south-southeast.

Capillitas-Belén granite

Paleozoic granitoids, defined throughout the AconquijaRange as the Capillitas-Belén granite suite (Caelles et al.,1971; Koukharsky and Mirré, 1976), intrude metasedimen-tary rocks as major plutons and associated pegmatites (Fig.1). They have been dated as Late Ordovician to Early Sil-urian (422.7 ± 6.1 and 438.4 ±6.3, a minimum K-Ar age ofmuscovite; Caelles et al., 1971); the geologic context pro-vided by Caelles (1979) and McBride (1972). The Pabellóngranite, a member of this suite (Koukharsky and Mirré,1976), crops out west of Agua Rica, overthrust uponmetasedimentary rocks (Fig. 2). Within the deposit area, Pa-leozoic granitoids are restricted to small outcrops that cutthrough the metasedimentary rocks. They also occur as raftsto sand-sized clasts in most breccias. Coarse muscovite flakesare a characteristic indication of a granite-derived matrixcomponent in many breccias.

The granites are mainly composed of plagioclase, K-feldspar, quartz, biotite, muscovite, and tourmaline. Texturesvary from equigranular to porphyritic, with K-feldspar phe-nocrysts up to 15 cm in size. Schlieren and gneissic metasedi-mentary xenoliths are common. These enclaves are pebble toboulder sized and consist of quartz, feldspar, muscovite,abundant biotite, and commonly andalusite and cordierite.Pegmatites with coarse-grained porphyritic textures and largeidiomorphic muscovites and K-feldspars are also present, lo-cally containing black tourmaline or green apatite crystals upto several centimeters in size.

Tertiary Intrusions Series of porphyritic stocks intruded the Agua Rica area in

at least three different pulses (early, syn, and postminerali-zation) during the Miocene period (Navarro, 1986; Sasso,1997; Perelló et al., 1998, Sasso and Clark, 1998). Stockworkquartz veins, potassic alteration, and Cu-Mo mineralizationwere closely related to this multistage igneous activity.

Melcho intrusions

The early Melcho intrusions, exposed in the southern partof the deposit (Fig. 2), have been described by Koukharskyand Mirré (1976) and Perelló et al. (1998) as syenodioritesand monzonites. Emplacement has been dated at 8.56 ± 0.48Ma (Ar-Ar age; Sasso, 1997; Sasso and Clark, 1998). Fourvariants have been recognized (B. Jones, written commun.,1996):

1. Amphibole-feldspar porphyry has a medium- to coarse-grained, equigranular to strongly porphyritic texture and iscomposed of plagioclase (≈25%), amphibole up to 5 mm insize (≈10%), and K-feldspar (≈10%) up to 2 cm in size. Inplaces, it has a poikilitic texture with K-feldspar enclosing pla-gioclase and amphibole crystals.

2. Fine-grained amphibole-quartz-plagioclase porphyry isinterpreted as a possible contact facies of the amphibole-feldspar porphyry. It has an equigranular to weakly por-phyritic texture with phenocrysts of plagioclase (≈20%),quartz (≈10%), and amphibole (≈5%).

3. Quartz-feldspar porphyry has a medium- to coarse-grained, moderate to strongly porphyritic texture. Plagioclase(≈35%), quartz (10–20%), and minor biotite and amphiboleare the phenocryst phases.

4. Fine-grained quartz-feldspar intrusion has a fine- tomedium-grained equigranular texture, with plagioclase (20–30%), quartz (10–20%), and minor biotite and amphibolephenocrysts.

The amphibole-feldspar porphyry (1) and the fine-grainedamphibole-quartz-plagioclase porphyry (2) are cut by boththe quartz-feldspar porphyry (3) and the fine-grained quartz-feldspar intrusion (4). The latter two variants are associatedwith porphyry-style stockwork veining, potassic and propyliticalteration, and disseminated Cu-Mo-Au mineralization andcould possibly be apophyses or time equivalents of the fol-lowing unit.

Seca Norte-Trampeadero feldspar porphyries

Two irregular bodies of feldspar porphyry form the mainhost of the porphyry-style ores (chalcopyrite, molybdenite,


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FIG. 2. Geologic map of the Agua Rica exploration area. The map is based on detailed surface mapping by M. Landtwing(central part of Fig. 3; 1998) and by BHP geologists (periphery of the deposit). The three main groups of rock types includethe basement, the intrusions and porphyries, and several, multiple-stage breccia bodies (UTM coordinates, WGS84, UTMzone 19).

and bornite) at Agua Rica: the Trampeadero feldspar por-phyry in the eastern part of the deposit and the Seca Nortefeldspar porphyry in the western part (Fig. 2). The contacts offeldspar porphyries with surrounding rocks are vertical tosteeply dipping. The Trampeadero feldspar porphyry has pre-served intrusive contacts with quartzitic metasedimentaryrocks to the east and south and with the Seca Norte feldsparporphyry to the west, but most other contact zones are occu-pied by later breccias. No unambiguous intrusive contactswith the Melcho porphyries exist, although the feldsparporphyries are interpreted to be significantly younger on thebasis of K-Ar dating (biotite-rich K-silicate alteration at SecaNorte dated at 5.10 ± 0.05 Ma; Perelló et al., 1998). Based ontextural relicts of quartz, feldspar, amphibole, and biotite asthe principal phenocrysts, the Seca Norte feldspar porphyryand the Trampeadero feldspar porphyry are inferred to havea dacitic to monzonitic composition (Perelló et al., 1998). Thetwo units differ mainly in the style and degree of quartz stock-work veining, alteration, and mineralization, rather than intheir poorly preserved original magmatic character. Crosscut-ting relations in outcrop and drill core observations show sig-nificant textural internal variations in the Seca Norte andTrampeadero feldspar porphyries, indicating multiple phasesof intrusion for both (Landtwing, 1998; Perelló et al., 1998).Two variants can be distinguished where subsequent feldspar-destructive alteration has not totally obliterated original tex-tures. One is a feldspar porphyry, which is the dominant vari-ant in the Seca Norte and the Trampeadero area and has amedium- to coarse-grained, moderately to strongly por-phyritic texture. The main phenocryst phases are zoned pla-gioclase (≈35%), biotite and amphibole (<5%), and minorsmall quartz phenocrysts (<5%). Scattered, large K-feldsparphenocrysts up to 3 cm in size are locally present. Thegroundmass is fine-grained quartz and feldspar. The other isa fine-grained, mafic feldspar porphyry which has a medium-to fine-grained, moderately porphyritic texture with plagio-clase (≈15%), amphibole (5–10%), biotite (5–10%), andquartz (<5%) phenocrysts. It may represent a fine-grained,more mafic phase of the feldspar porphyry.

Distal porphyries

Several small bodies of feldspar porphyry containing tracesof pyrite have been mapped around the margins of the min-eralized system and are referred to here as distal porphyries.Outcrops of this unit typically occur within a few hundredmeters of the main mineralized porphyries but may be as faras 1 km from known copper mineralization. Distal porphyrieshave not been mapped in contact with the main mineralizedporphyries, and their age relations are unknown. They occuron the western edge of the project area and on the northernand southeastern limits of the mineralized system.

Biotite porphyryA biotite porphyry dike exposed next to the Agua Rica camp

site has a width of several tens of meters and can be followedfor 200 m. Contacts with the surrounding breccia bodies arevertical to subvertical. In places the biotite porphyry has achilled margin against the crater infill breccia. Its texturevaries from medium- to coarse-grained porphyritic, comprisingmillimeter-sized plagioclase (≈30%), K-feldspar phenocrystsup to 4 cm in size (5–10%), hypidiomorphic biotite plates to7 mm (≈10%), and characteristic round quartz grains (<5%)in a pale gray, aphanitic groundmass. In places, the dike con-tains pebble-sized xenoliths of metasedimentary rocks andfeldspar porphyry. The biotite porphyry is weakly overprintedby clay minerals and very minor pyrite but is totally barren ofcopper. Field relations show that this is a very late intrusivephase postdating porphyry-style and high sulfidation epither-mal mineralization, despite an old Ar-Ar age of 12.67 ± 0.30Ma (Sasso, 1997) which is considered to be spurious.

Breccia TypesClassifications of breccia types commonly include both

partly genetic and purely descriptive nomenclature, as dis-cussed by Sillitoe (1985) and Baker et al. (1986). In thispaper, process-related names are used for easier overview ofbreccia units, but detailed descriptions of the characteristicfeatures of each unit (Table 2) should permit reinterpretationby future readers, if necessary. The units were derived fromdetailed clast and matrix mapping at a 1:4,000 scale of most ofthe area shown in Figure 2, using a consistent data recordingscheme illustrated in Figure 3 (Landtwing, 1998).

Hydrothermal brecciaHydrothermal breccia is the volumetrically most prominent

breccia type of the Agua Rica deposit and is exposed over avertical extent of several hundred meters. Common charac-teristics to all textural variations of hydothermal breccia arethe absence of any layering and a matrix consisting of variableamounts of silt- to sand-sized clastic material cemented by hy-drothermal minerals. Contacts to the wall rocks vary fromsubvertical to subhorizontal but generally converge down-ward. At lowermost known elevations, the contacts commonlybecome shallow toward a flat bottom. Subhorizontal or out-ward-dipping upper contacts against metasedimentary rocks(e.g., in the Trampeadero area) indicate that some of thebreccia columns did not extend to the land surface at the timeof emplacement.

The hydrothermal breccia is mainly clast supported andrarely clast to matrix supported, with 60 to 85 vol percent clasts,10 to 30 vol percent matrix, and 0 to 10 vol percent openspace (Fig. 4A, B). Generally the lithologies of fragmentsclosely reflect those of the closest wall rocks. Transitions from


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FIG. 3. Data-recording legend and a small excerpt of a 1:4,000 map covering the entire breccia complex, which formedthe basis for the separation and interpretation of breccia units shown in Figure 2. Systematically recorded breccia charac-teristics include primary clast lithology, size and shape of clasts, internal mineralization and alteration of transported brecciaclasts, the nature of the matrix, and several pre- and postbrecciation structures (Landtwing, 1998). The dark colors show thelimits of outcrops; the pale colors show the inferred occurrence of the various units and their contacts. The figure documentsthe specific mapping method established for the Agua Rica area. Adaptations of this mapping approach may be applicableto other breccia complexes with interacting hydrothermal, volcanic, and sedimentary breccias.


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TABLE 2. Breccia Types at Agua Rica

Hydrothermal Sand and pebble dikes breccia Igneous breccia Crater infill breccia Granite breccia Talus breccia (breccia dikes)

Geometry Various irregular Various irregular Circular body, 0.7-1 Either an isolated Fan-shaped body, Irregular dike-shaped of breccia bodies bodies in the central km diam in the 200- to 100-m-sized overlying the bodies mm–m in widthbody part of the deposit northern part of the block or in contact other lithologies

Agua Rica area to the subjacent basement

Vertical Several hundred Up to several Several hundred Unknown Up to 50 m Extension of several extension meters hundred meters meters hundred meters

supposedContact to Variable from Subvertical to Subvertical to Subvertical to Variable from low Vertical to subvertical, the wall subvertical to vertical vertical vertical angle up to vertical rarely inclinedrock subhorizontalClast/matrix 60-85 vol % clasts, 5-30 vol % clasts, 40-70 vol % clasts, 40-70 vol % clasts, 75-95 vol % clasts, 5-50 vol % clasts, ratio predominantly clast matrix supported matrix supported clast to matrix clast supported matrix supported

supported, rarely supportedclast to matrix supported

Clast com- Monomictic or Polymictic; clasts of Highly polymictic; Monomictic; pre- Monomictic; clasts Polymictic; clasts of position polymictic; clasts several different clasts of several dominance of of foliated or metasedimentary

reflect wall-rock lithologies (including different lithologies; granite clasts; crenulated meta- rocks, feldspar composition, pre- older igneous brec- predominance of minor clasts of sedimentary rocks porphyry, granite and dominance of clasts cia); predominance clasts of meta- pegmatite and and minor milky stockwork, and milky of metasedimentary of clasts of metasedimentary rocks quartz-feldspar- white vein quartz white vein quartzrocks, and feldspar sedimentary rocks, and feldspar muscovite-biotite and quartzitic meta-porphyry feldspar porphyry porphyry schist sedimentary rocks

Size of Heterogeneous; Homogeneous; Heterogeneous; Homogeneous; Heterogeneous; Homogeneous; clasts 1 predominance of small- to medium- changing clast size medium-sized to small-sized to large sand dikes: small-sized

small- to medium- sized clasts, rare in different zones: large clasts clasts, predominance clasts, rare medium-sized clasts, some large clasts (1) bedded zones of medium-sized sized clasts; pebble large clasts, boulders, with small- to clasts dikes: small- and and blocks up to medium-sized clasts, medium-sized clasts20 m in size (2) unstratified (megaclasts) at zones with small-Seca Norte sized to large clasts

Rounding Angular (meta- Angular to Subrounded to well- Subrounded to Angular to Rounded, rare of clasts sedimentary rocks) subrounded rounded (small- to well rounded subrounded subrounded or angular

to subrounded medium-sized clasts); (feldspar porphyry angular to sub-and granite) rounded (large clasts)

Matrix Silt and sand-sized Magmatic aphanitic, Silt- and sand-sized Comminuted and Silt, sand, and Silt- and sand-sized clastic grains, light gray ground- clastic grains, ground granitic limonite, brownish clastic grains, hydro-hydrothermal mass with pheno- clay minerals material; mm- to to greenish colored thermal mineralsminerals as open- crysts of feldspar, cm-sized grains of space infill amphibole, biotite, biotite, muscovite,

and rare quartz feldspar, quartz, and tourmaline; some clay minerals

Subunits Three subunits: Enormous variation Two interbedded Clasts with closely Texture locally Some vertically graded and char- (1) Trampeadero, in clast content; clast subunits: (1) fine- spaced concentric jigsaw-like dikes with coarse clasts acteristic (2) Agua Dulce, and of older igneous grained, well-sorted, fractures; texture in the center and features (3) Seca Norte breccia indicating normal-graded, or locally jigsaw-like laminated rock flour at

hydrothermal the formation in crossbedded zones, the margins; cross-breccia;complex several pulses (2) poorly sorted cutting relationships textural and zones with highly and occurrence of sand compositional variable clast sizes; and pebble dike variations; in dipping of bedding material as breccia-Seca Norte area, planes in (1) indicating clast indicating blocks with closely a basin structure, multiple formation; spaced concentric inclination of bedding occurring commonly in fractures (hypogene varies from 70° to 30°, contact zones between exfoliation) and flattening toward the two different megaclasts center of the basin lithologies

Interpreted Intense fracturing Magmatic intrusion Crater formed by In situ brecciation Consolidated Fluidization of rock formation due to the with xenoliths phreatomagmatic to caused by fluids or scree and talus fragments in upward-process mechanical energy phreatic explosions, fracturing due to the escaping fluid

released from subsequent sedi- mechanical energy channelwayshydrous magmas mentary infill released from

hydrous magmas

1 Definitions of the clast sizes used are the following: small sized: <3 cm; medium sized: 3-30 cm; large sized: 30 cm-1 m; megaclast: >1 m (Fig. 3)


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FIG. 4. Macroscopic features of the main breccia types distinguished in the Agua Rica deposit. A. Hydrothermal breccia(Seca Norte). The occurrence of concentric clasts isolated within angular clasts is a feature of this breccia; the matrix of thebreccia at this outcrop is made of both rock flour and infill by hydrothermal minerals. B. Hydrothermal breccia (Tram-peadero). The breccia consists of clasts of quartzitic metasedimentary rock in a matrix of hydrothermal minerals (quartz, alu-nite, and pyrophyllite). C. Igneous breccia. The breccia is polymictic with angular clasts of metasedimentary rocks in astrongly porphyritic matrix of fine- to medium-grained phenocrysts of feldspar; amphibole occurs in an aphanitic, pale graygroundmass. D. Small Y-shaped sand dike cutting Trampeadero feldspar porphyry with small porphyry-stage stockwerkquartz veins. E. Granite breccia. Granite and pegmatite clasts (arrows) in a granitic matrix consist of millimeter- to centime-ter-sized grains of granite rock flour (biotite, muscovite, feldspar, quartz, and tourmaline) and clay minerals. F. Crater infillbreccia. Accretionary lapilli indicate a subaerial formation for this unit. G. Fine-grained crater infill breccia. The breccia sub-unit shows stratification with normal graded layers (parallel to the arrow); clasts in this sample are small and well rounded.H. Talus breccia. The two drill core samples show angular clasts, foliated and crenulated metasedimentary rock, and milkywhite vein quartz; the matrix is fine grained, consisting of silt, sand, and limonite. I. Mi Vida conglomerate. It is polymicticand poorly sorted, with clasts of schistose and quartzitic metasedimentary rock, feldspar porphyry, and granite.

mainly monomictic to polymictic occur where several rocktypes adjoin a pipe, but there can be significant clast mixingaway from contacts. Clasts of quartzitic metasedimentaryrock and feldspar porphyry are dominant. Schistose metased-imentary rock, milky white vein quartz, granite, other brecciatypes (igneous breccia, sand dikes, older hydrothermal brec-cia), vuggy silica, and crystalline alunite vein material (seebelow) also occur as clasts. Most of the clasts are small tomedium sized (Fig. 3), but boulder-sized fragments andblocks also occur. Most clasts are angular (especially thosefrom metasedimentary rocks) to subrounded (especially thoseof feldspar porphyry and granite).

Based on field mapping of outcrop distribution and slightdifferences in matrix compositions, the hydrothermal brecciahas been divided into three units (Fig. 2 and Table 2) whichcommonly show gradational contacts to adjacent hydrother-mal breccia units. The Trampeadero hydrothermal brecciahas a matrix consisting predominantly of hydrothermal min-erals filling former open space. The hydrothermal infill variesdepending on the dominant alteration assemblage and in-cludes pyrophyllite, clay minerals (dickite and kaolinite), alu-nite, jarosite, quartz, sericite, native sulfur, pyrite, covellite,sphalerite, and/or galena. Textures in this unit vary from jig-saw breccia with angular clasts and only minor displacementof material against wall rock to strongly rotated and trans-ported breccia clasts without preferred orientation. Centime-ter- to meter-sized dikes of Trampeadero hydrothermal brec-cia cut the metasedimentary rock or interfinger with igneousbreccia. Clasts are small to medium sized. The Agua Dulcehydrothermal breccia has a matrix of mainly silt- to sand-sizedclastic material (rock flour) with only minor infill by hy-drothermal minerals. The unit is nearly monomictic with an-gular, medium-sized clasts predominantly of quartziticmetasedimentary rock. Additionally, rare clasts of feldsparporphyry, igneous breccia, and granite occur. With a matrixcontent of less than 25 vol percent, the breccia is clast sup-ported. The Seca Norte hydrothermal breccia has a matrix ofboth rock flour and infill by hydrothermal minerals in variableproportion. The clasts vary from millimeter sized to mega-clasts of more than 10 m in diameter and are predominantlyangular. Notable features are the occurrence of clasts of crys-talline alunite veins and 0.2- to 1-m-sized blocks with closelyspaced concentric fractures, known as onion-skin or hypo-gene exfoliation (Farmin, 1937).

Generally all three units of hydrothermal breccia—bothmatrix and clasts—are strongly overprinted by feldspar-de-structive (phyllic, advanced argillic) alteration and pyrite, cov-ellite, enargite, sphalerite, and galena mineralization. Someclasts preserve evidence of earlier stages of alteration (potas-sic, phyllic, and advanced argillic), veining (quartz stockworkveins, crystalline alunite veins), and mineralization (chalcopy-rite-bornite, pyrite-covellite) that predate brecciation. Quartzstockwork veins cutting both clasts and matrix have neverbeen seen in the breccia.

Igneous breccia

The central part of the deposit consists of irregular bodiesof igneous breccia with a strongly porphyritic igneous matrix.Clasts of older igneous breccia indicate that the igneous brec-cia unit mapped in Figure 2 was formed during at least two

pulses, each probably related to one of the major stages in theevolution of Agua Rica, but textures of all igneous brecciasare too similar to allow mapping of internal contacts.

All igneous breccias contain 5 to 30 vol percent clasts andare matrix supported, with a poorly sorted clast content (Fig.4C). The proportion of clasts varies in outcrop over short dis-tances. The breccias contain abundant clasts of quartziticand schistose metasedimentary rocks and feldspar porphyry,with or without stockwork veins. Clasts of milky vein quartz,granite, sand dikes (see below), vuggy silica, chalcedony, andolder igneous breccia are rarer. The angular to subroundedclasts are generally small to medium sized (Fig. 3). The ig-neous matrix consists of fine- to medium-grained small phe-nocrysts of (<2 mm) feldspar (25–35%), amphibole (~5%),biotite (~5%), and rare quartz in an aphanitic pale graygroundmass. Stockwork-style quartz veins cutting the matrixare absent.

In the Trampeadero area, extending south and east fromthe Agua Rica camp (Fig. 2), igneous breccia is exposed in thelower and central parts of the hydrothermal breccia columns,showing gradual and diffuse contacts with these probably co-eval hydrothermal breccias. Here, the igneous breccia com-monly shows pervasive advanced argillic alteration, with localzones of intense silicification and vuggy silica development.Clay minerals, sericite, and alunite replace feldspar phe-nocrysts. Sulfides locally replace amphiboles. Both clasts andmatrix of this breccia are variably mineralized with pyrite,covellite, enargite, and supergene chalcocite.

Outcrops of petrographically similar igneous breccia, con-taining advanced argillic altered and mineralized clasts ofolder igneous breccia, occur west of Agua Rica camp in Que-brada Minas. This igneous breccia forms a core in the base ofthe crater infill breccia (below), to which it shows gradualtransitions and probably an intimate temporal and geneticrelationship.

Crater infill breccia

The crater infill breccia occupies a circular area 0.7 by 1 kmin diameter in the northern part of the Agua Rica area (Figs.2 and 3). The crater infill breccia is exposed in outcrop anddrill core over a depth interval of at least 700 m. The contactswith the wall rocks (mainly basement) are steep, commonlysharp, and generally dip toward the center of the circularbody of the crater infill breccia. The topographically higherand more central parts of this breccia are stratified with nor-mal graded (Fig. 4G) and crossbedded layers, defining a basinstructure. Inclination of bedding varies from about 70° to 30°,dipping and flattening toward the center of the breccia body.In the central to southern part of the breccia body, west ofAgua Rica camp, the crater infill breccia grades downward toan increasingly clast-supported breccia and finally into ig-neous breccia, containing clasts of earlier igneous brecciawith sulfides and advanced argillic or phyllic alteration.

Except for these basal parts, most of the crater infill brec-cia is matrix supported with 40 to 70 vol percent clasts. Thebreccia components are everywhere polymictic. Clasts ofquartzitic and schistose metasedimentary rock and feldsparporphyry are dominant, but milky white vein quartz, granite,pegmatite, igneous breccia, sand dikes, and vuggy silica clastsalso occur. Additionally, two clasts of a pale-colored, slightly


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quartz porphyritic volcanic rock similar to the basal igneousbreccia matrix were found. Some clasts of quartzitic metased-imentary rock, feldspar porphyry, and igneous breccia haveundergone various stages of potassic, phyllic, and/or advancedargillic alteration. Pyrite, covellite, chalcocite, and molybden-ite are disseminated in clasts or replaced entire clasts beforetheir incorporation in the crater infill breccia. Some clasts ofquartzitic metasedimentary rock and feldspar porphyry con-tain earlier quartz stockwork veins.

The crater infill breccia has two interbedded subunits (Table2): (1) fine-grained, well-sorted, normal-graded or crossbed-ded layers with small- to medium-sized clasts, and (2) poorlyto moderately sorted, unstratified zones with some boulder-sized clasts (Fig. 3). Accretionary lapilli (Fig. 4F) and channelstructures locally occur in the fine-grained layers. Small- andmedium-sized clasts are subrounded to well rounded andoccur in both subunits. Larger angular to subrounded clasts,up to 1 m in size, with possible impact depressions, occur lo-cally in poorly sorted, unstratified zones. The matrix of bothsubunits is composed of clay minerals and subangular to well-rounded clastic sand and silt grains of the same compositionsas the clasts. In some outcrops, the matrix contains well-rounded millimeter-sized grains of pyrite. Locally the entirebreccia is silicified, but alunite and covellite are absent fromthe matrix. Minor veinlets of fine-grained, dusty chalcocite lo-cally cut the clasts and the matrix.

Granite breccia

Outcrops of granite breccia are concentrated in the centerof the Agua Rica prospect, within an area of about 100 by 200m, remote from any solid granite. The vertical extent of thebreccia is unknown, and it is unclear if this breccia body formsan isolated block or is in contact with the subjacent basement.The granite breccia is clast to matrix supported and contains40 to 70 vol percent clasts. The breccia is monomictic andconsists of more than 90 vol percent granitic material. Most ofthe clasts consist of coarse-grained Pabellón granite. Someclasts are pegmatitic (Fig. 4E), with centimeter to decimeter-sized crystals of muscovite and black tourmaline. Rarely,clasts of quartz-feldspar-muscovite-biotite schists occur andare interpreted as schlieren and gneissic metasedimentaryxenoliths in the granites. The clasts range from medium tolarge in size (Fig. 3) and are subrounded to well rounded. Insome cases, the granite breccia shows jigsaw breccia textures;a few clasts show onion-skin exfoliation with concentric frac-tures. The matrix consists of comminuted granitic materialcomposed of millimeter- to centimeter-sized grains of biotite,muscovite, feldspar, quartz, and tourmaline, with some clayminerals. The granite breccia is argillized in all outcrops. Inplaces, the matrix of the breccia contains millimeter-sized,subrounded grains of pyrite, but the clasts are unmineralized.

Talus breccia

The talus breccia occurs north of the Agua Rica camp (Figs.2 and 3), as a fan-shaped deposit with a thickness of up to 50m that overlies the crater infill breccia, the igneous breccia,the granite breccia, the hydrothermal breccia, and metasedi-mentary rocks. Contacts with the surrounding granite brec-cia, igneous breccia, crater infill breccia, and sand dikes rangefrom low angle to vertical.

The talus breccia has 75 to 95 vol percent clasts and is in-variably clast supported (Fig. 4H). It is essentially monomic-tic, with clasts of foliated and crenulated metasedimentaryrock and minor milky white vein quartz clasts. Rare clasts ofquartzitic metasedimentary rock occur next to the contactwith the quartzitic wall rocks. The talus breccia is poorlysorted; clasts range in size between sand and boulder, butmost are medium sized. All clasts are angular to subrounded,and characteristically slab to block shaped but locally jigsaw-like. The matrix is fine grained, brownish to greenish, con-sisting of silt, sand, and limonite variably cementing thebreccia.

Clasts and matrix of the talus breccia generally are un-mineralized with the exception of rare pyrite grains. Most ofthe outcrops are unaltered, but in some outcrops dark-col-ored patches overprint the matrix. These patches wereshown by X-ray diffractometry to consist mainly of quartzand some biotite.

Sand and pebble dikes

Irregular dike-shaped sand and pebble dikes occur over theentire Agua Rica area. Their strike directions lack any pre-ferred orientation. The sand dikes are typically several mil-limeters to several centimeters in width but locally attain sev-eral meters. Sand and pebble dikes commonly occur incontact zones between two different breccia types or betweena breccia unit and its wall rock. Contacts with the surround-ing rock are sharp (Fig. 4D), commonly vertical to subverti-cal, but locally moderately dipping. Intrusions of sand andpebble dikes commonly exhibit alteration halos in the sur-rounding rocks.

The sand and pebble dikes have a matrix content of 50 to95 vol percent and are matrix supported. They are het-erolithic, with clasts of metasedimentary rock, feldspar por-phyry with or without quartz stockwork veins, granite, vuggysilica, and vein quartz. The clast size is dependent on thewidth of the dikes, commonly being smaller than one-tenth ofthe dike width. In sand dikes, clasts are sand to fine pebblesized; in pebble dikes clasts range in size up to cobbles. Theclasts are predominantly rounded but in places are sub-rounded or angular. The matrix consists of silt- and sand-sizedclastic grains (rock flour) and clay minerals. Some of the dikesare graded, with coarse clasts in the center of the dike andlaminated rock flour at the margins.

Sand and pebble dikes must have been formed throughoutmost of the hydrothermal history at Agua Rica. One dike iscut by quartz stockwork veins of the type normally observedin the feldspar porphyries (drill hole AR 74, Quebrada Minas;B. Jones, pers. commun., 1998). Clasts of sand and pebbledikes occur in the igneous breccia, the hydrothermal breccia,and the crater infill breccia. Sand and pebble dikes also cutmetasedimentary rocks, feldspar porphyries, igneous breccia,hydrothermal breccia, crater infill breccia, granite breccia,and even talus breccia.

Alteration and mineralization of sand and pebble dikesvaries widely. Some exhibit phyllic or advanced argillic alter-ation and may contain abundant covellite and pyrite, andsome are unaltered and not mineralized. Due to their perme-able matrix, they locally contain considerable chalcocite.


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Fault breccias and structures

In the Agua Rica area, the net displacement and the senseof movement of faults is commonly difficult to determine, butbrittle, syn-, and postalteration and mineralization faultingcan be observed as fault breccias, pseudotachylite, zones ofgoethite, clay, and native sulfur, and centimeter-wide oreveins containing pyrite, galena, covellite. and chalcocite. Sev-eral structural trends have been identified at Agua Rica (Fig.2). The entire complex was emplaced along the northwest-southeast-striking Quebrada Minas lineament, which is sub-parallel to the inferred fold axis in the metasedimentaryrocks. East-west-striking, south-dipping normal faults wereactive both syn- and postmineralization. The Quebrada Secafault offsets leached material, and epithermal, arsenic-richfluids moved along the same structure. The precise locationof the Quebrada Seca fault is not clear east of the Minasstructure but is obvious to the west. The Agua Rica campstructure has the same orientation as the Quebrada Seca faultand may have controlled emplacement of the biotite por-phyry. West-southwest-dipping, low-angle reverse faults havestructurally thickened the leached capping on the west side ofthe deposit, and the thrusted granite-metasedimentary rockcontact has the same geometry. Late, north-south-strikingstructures have resulted in pockets of deep leaching in theTrampeadero area.

Small zones of fault gouge and fault breccia that are sur-rounded by unfractured wall rocks and breccias are wide-spread in the Agua Rica area. Contacts with the unfracturedwall rock are subvertical to moderately dipping. The percent-age of clasts, matrix, and open space in the fault brecciavaries, from matrix supported with 20 vol percent clasts toclast-supported with 70 vol percent clasts. Clasts of these ir-regular-shaped zones of strongly fractured rock usually reflectthe wall-rock composition. Fragments are predominantly an-gular and small to medium sized, with rare large-sized clasts.The matrix consists of silt- and sand-sized clastic grains (rockflour and fault gouge) and clay minerals. Altered and miner-alized clasts indicate that all stages of alteration, veining, andmineralization (including supergene leaching and enrich-ment, see below) occurred prior to fault brecciation.

Strata Postdating Tertiary Igneous Activity

Mi Vida conglomerate

The Mi Vida conglomerate, first described by Koukharskyand Mirré (1976), was deposited on a paleorelief similar to,but perhaps less steep than, the present-day V-shaped valleytopography. It rests on all previously described rock units withhorizontal to slightly inclined contacts. The Mi Vida con-glomerate reaches a maximum thickness of 50 m and occursas irregular bodies in terraces both above and at the present-day stream course, mainly along Quebrada Minas and Que-brada Yeguas. The conglomerate is clast to matrix supportedand contains 50 to 75 vol percent clasts. It is polymictic andpoorly sorted, with clasts of all previously described rock unitsexcept the Melcho intrusions, plus fragments of biotite schist(consisting of quartz, feldspar, muscovite, and biotite), ash-fall tuff, and older Mi Vida conglomerate, indicating local re-deposition. The size of the clasts ranges from sand to blocksto tens of meters. The clasts are angular to well rounded. The

conglomerate shows a porosity of up to 20 vol percent. Thematrix consists of silt- to coarse sand-sized fragments andrusty red, limonitic cement. The main components of the ce-ment are goethite, hematite, and illite, as identified by X-raydiffractometry. The matrix of the Mi Vida conglomerate con-tains no sulfide, but some of the clasts are altered and miner-alized, showing all alteration and mineralization stages.

Ash-fall tuff

A sandy ash-fall tuff was deposited on a paleorelief that wasessentially identical to the present-day topography. It occursas clasts in the Mi Vida conglomerate and as an interbeddedlayer in the overburden. This windblown, pyroclastic, coarsecrystal-lithic ash layer is gray to brownish and whitish in colorand consists mainly of plagioclase, quartz, biotite, and mag-netite. It commonly shows prominent bedding or crossbed-ding and is only weakly consolidated. In an outcrop next tothe Agua Rica camp, the ash-fall tuff is offset 50 cm by arecent fault. A sample of this outcrop has been dated at 0.52± 0.02 Ma (K-Ar age; Perello et al., 1998), and Sasso (1997)obtained a similar age for an ash layer in the region of Bajo ElDurazno, 35 km west-northwest of Agua Rica (0.51 ± 0.06Ma, an Ar-Ar biotite age; Table 3).

Alteration and MineralizationThe polymetallic Agua Rica system was affected by three

alteration and mineralization stages: potassic, advancedargillic, and supergene (Fig. 5). The economic Cu-Mo-(Au)mineralized zone of Agua Rica is roughly elongate east-west,2 km long, and 1 km wide. Three areas of mineralization ofapproximately equal size form the main resource. From westto east these are Seca Norte, Quebrada Minas (with FiloAmarillo), and Trampeadero (Fig. 2). South of the main min-eralized body, chalcopyrite ± bornite mineralization and lo-cally weak supergene enrichment are present in the Melchoarea. The following short description is based on observationsobtained during mapping (Landtwing, 1998) and drill corelogging by BHP geologists. In addition to hand lens observa-tions, short-wave infrared spectrometry was used by BHP ge-ologists to determine the alteration mineralogy. Further in-formation on the alteration mineralogy has been published byTakagi and Brimhall (1998) and Takagi (1999).

Potassic and propylitic alteration, quartz-chalcopyrite ±bornite mineralization, and stockwork veins

Potassic alteration at Agua Rica is preserved in limited areasonly but was probably more extensive, as indicated by char-acteristic stockwork veining that can be mapped even in areasof pervasive advanced argillic overprint. Potassic alteration con-sisting of quartz, hydrothermal biotite, K-feldspar, and mag-netite (Perelló et al., 1998) is best developed in the Melchointrusions and in a small outcrop of breccia in the Melcho areawith clasts showing pervasive quartz-biotite alteration. Twosamples of secondary biotite in this area have been dated at6.29 ± 0.06 Ma (brecciated unit) and 7.03 ± 0.10 Ma (Ar-Arages; Sasso, 1997). Subeconomic quartz-pyrite ± chalcopyrite± molybdenite veins with selvages of biotite, and rarely, K-feldspar occur in this zone. In the potassically altered areas ofMelcho, amphibole is altered to biotite and pyrite. Elsewhereat Melcho, propylitic alteration is indicated by alteration of


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amphiboles to chlorite and pyrite and the occurrence of clotsof epidote. Propylitic alteration commonly occurs distally tothe Agua Rica complex but is difficult to recognize due to thegreenschist facies metamorphism of the surrounding meta-sedimentary rocks.

Within the area of economically interesting mineralization(Seca Norte and Trampeadero areas), remnants of potassic al-teration and chalcopyrite ± bornite mineralization are presentas patches in the Seca Norte and Trampeadero feldspar por-phyries (Perelló et al., 1998) and as clasts in igneous breccia,hydrothermal breccia, crater infill breccia, and fault breccia.Potassic alteration consists of pale brown biotite, usually al-tered to chlorite or sericite. Minor K-feldspar may also bepreserved. A sample of biotite-rich, potassic alteration fromSeca Norte has been dated at 5.10 ± 0.05 Ma (Perelló et al.,1998). Rare magnetite-chalcopyrite veins in which magnetitehas been partially altered to hematite are present in SecaNorte. Early, A-type (Gustafson and Hunt, 1975), irregular,discontinuous quartz ± pyrite ± chalcopyrite ± bornite stock-work veins are intensely developed in the Seca Norte feldsparporphyry but are weakly developed or absent in the Tram-peadero feldspar porphyry. Crosscutting B-type (Gustafsonand Hunt, 1975), straight, continuous quartz-molybdenite-pyrite veins with cockscomb-textured quartz are found

throughout the Seca Norte and Trampeadero porphyries andin the metasedimentary rocks adjacent to the porphyries.Molybdenite and pyrite occur as centerlines and at the mar-gins of these B-type veins. Molybdenite is also found as thinand discontinuous fracture fillings.

Molybdenite was mainly deposited during this initial min-eralization event and is only seen as veinlets within clasts inthe hydrothermal breccias. Disseminated pyrite is common inthe porphyries, but chalcopyrite and bornite are rarely recog-nizable in hand samples. An area of significant chalcopyriteand bornite occurs on the west side of the Seca Norte feldsparporphyry, where this initial phase of ore contains up to 0.8percent Cu and 0.5 g/t Au. Elsewhere, chalcopyrite ± borniteore contains about 0.3 percent Cu and 0.25 g/t Au.

Phyllic alteration

Phyllic alteration consisting mainly of quartz, sericite (fine-grained illite or white mica, identified by short-wave infraredspectroscopy), and pyrite overprints the potassic and propy-litic alteration. A sample of sericite from the Seca Norte-Trampeadero area has been dated by K-Ar methods byPerelló et al. (1998) at 5.38 ± 0.05 Ma (Table 3). In the SecaNorte feldspar porphyry, where A- and B-type veins are abun-dant, fine-grained quartz and sericite with disseminated


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TABLE 3. Radiogenic Ages of the Agua Rica Deposit1

Location Description Age (Ma) Method Reference Interpretation of age

Melcho Melcho intrusions 8.56 ± 0.48 Ar-Ar, hornblende Sasso (1997) Emplacement, Melcho intrusions

Melcho Melcho intrusions, secondary biotite 7.03 ± 0.10, Ar-Ar, biotite Sasso (1997) Potassic alteration, Melcho 6.29 ± 0.06 intrusions

Melcho Melcho intrusions, sericitized 6.10 ± 0.04 Ar-Ar, whole rock, Sasso (1997) Phyllic alteration, Melcho intrusions (?)

Seca Norte- Biotite-rich K-silicate alteration 5.10 ± 0.05 K-Ar Perelló et al. (1998) Potassic alteration, Seca Trampeadero Norte-Trampeadero

Seca Norte- Hypogene alunite 5.35 ± 0.04 Ar-Ar, alunite Sasso (1997) Advanced argillic alteration, Trampeadero Seca Norte-Trampeadero

Trampeadero Hypogene alunite from the alunite 4.88 ± 0.08, K-Ar, alunite Perelló et al. (1998) Advanced argillic alteration, zone around a vuggy silica ledge 4.96 ± 0.08 Seca Norte-Trampeadero

Seca Norte- Alunite 6.18 ± 0.06 K-Ar, alunite Perelló et al. (1998) Advanced argillic alteration, Trampeadero Seca Norte-Trampeadero (?)

Seca Norte- Sericite 5.38 ± 0.05 K-Ar, sericite Perelló et al. (1998) Phyllic alteration, Seca Trampeadero Norte-Trampeadero

Agua Rica camp Biotite porphyry 12.67 ± 0.30 Ar-Ar, biotite Sasso (1997) Spuriously old age, field relations indicate that the biotite porphyry was formed in a very late phase of magmatic hydrothermal activity

Seca Norte Alunite from a massive cm-wide 3.94 ± 0.05 K-Ar, alunite Perelló et al. (1998) Supergene alteration (?)banded vein

Agua Rica camp Ash-fall tuff (biotite rich) 0.52 ± 0.02 K-Ar Perelló et al. (1998) Formation of ash-fall tuff

1Summarized from Perelló et al. (1998) and Sasso (1997)

pyrite completely destroys the primary rock texture. Outsidethe Seca Norte porphyry, phyllic alteration is less intense andprimary rock textures are usually preserved. In Trampeaderofeldspar porphyry and metasedimentary rocks, straight, con-tinuous pyrite veins with quartz-sericite selvages are well

developed. Locally, the quartz-sericite-pyrite veins coalesceinto massive stockworks that destroy the rock texture. Quartz-sericite-pyrite veins cut A- and B-type quartz stockwork veinsbut do not cut hydrothermal breccia. Phyllic alteration alsodominates in the Melcho area, where a whole-rock sample of


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768500 769000 769500

a) Rock Types

b) Ore Distribution

2500

3000

3500

768500 769000 769500 770000

770000

c) Ore grades EEW

EEW

EEW

2500

3000

3500

768500 769000 769500

770000

2500

3000

3500

Paleozoic basementMetasedimenary rocks

Wall rock lithologiesTertiary intrusions

Seca Norte feldspar porphyry

Trampeadero feldspar porphyry

Breccia typesHydrothermal breccia; matrix of sand andsilty material and hydrothermal minerals

Igneous breccia

Hydrothermal breccia; matrix of hydro-thermal minerals, minor sand and silty material

LeachedSupergene-stage mineralization

Partial leachedEnriched (chalcocite, covellite, digenite)

Primary / epithermal covelliteEpithermal-stage mineralization

Pyrite (minor chalcopyrite, covellite)

Porphyry-stage mineralization

Coarse covellite

Primary chalcopyrite (± bornite)

* pyrite is the dominant sulfide present in all zones except leached zones

Alteration

StructuresFault

Limit of advanced argillic alteration (upper part / center) and transition to phyllic alteration (lower part /outside)

E - W Section 6969400N

advanced argillic alteration

phyllicalteration

> 1% Cu

> 0.4% Cu

Mineralogy

Mineralization

> 0.5g/t Au

> 0.2g/t Au

0 500

m

FIG. 5. East-west section of the Agua Rica deposit along 6969400N, showing the relationships between the three miner-alizing units (Seca Norte feldspar porphyry, Trampeadero feldspar porphyry, and hydrothermal breccia), the ore distribution(primary porphyry and high sulfidation epithermal stage, and overlying supergene enrichment blanket), and the approximatetransition from advanced argillic to phyllic alteration. Potassic alteration in the Seca Norte and Trampeadero areas is onlypreserved as remnants and was not included in this compilation but is closely associated with intense quartz stockwork vein-ing which is well preserved and was systematically mapped. Relicts of potassic alteration with pale brown secondary biotiteare best preserved in the deeper parts of the Seca Norte and Trampeadero feldspar porphyries (see also Perelló et al., 1998).

the sericitized Melcho intrusion has been dated at 6.10 ± 0.04Ma (Ar-Ar age; Sasso, 1997). Phyllic alteration also affectslarge parts of the northern section of the prospect, includingthe crater infill breccia.

Advanced argillic alteration and covellite overprint

Advanced argillic alteration with zones of vuggy silica andmassive silicification is centered on the hydrothermal brecciapipes (Fig. 5). Hypogene alunite samples have been dated bySasso (1997) at 5.35 ± 0.04 Ma (Ar-Ar age) and by Perelló etal. (1998) at 4.88 ± 0.08 Ma and 4.96 ± 0.08 Ma (K-Ar ages;Table 3). An additional sample of alunite was dated by Perellóet al. (1998) at 6.18 ± 0.06 Ma (K-Ar age). At Agua Rica, theadvanced argillic alteration zone is defined by the presence ofpyrophyllite and alunite. Other alteration minerals present in-clude quartz, kaolinite, dickite, diaspore, zunyite, topaz,rhodochrosite, and commonly some sericite. Hypogene alu-nite is abundant in the advanced argillic zone but also occurswell out into the phyllic zone. Minerals of the advancedargillic alteration assemblage partly or entirely form the ma-trix of hydrothermal breccia, occur in veins without sulfides,and variably replace the groundmass and phenocrysts in theporphyries. Pervasive replacement of feldspar by slightlypink-colored, crystalline to amorphous alunite is especiallywell developed in the igneous breccia. Alunite can be sparry,granular, or microcrystalline. Crystalline alunite veins cut thequartzitic metasedimentary unit as well as the feldspar por-phyries and occur as clasts in the Seca Norte hydrothermalbreccia. The millimeter- to centimeter-wide alunite crystalsforming these veins are coarse grained, sparry, and pale pink,yellow, or beige colored.

The advanced argillic assemblage is intimately associatedwith pyrite, covellite, enargite, sphalerite, galena, and minormolybdenite. These sulfides occur in veins in the porphyriesand metasedimentary rocks and as open-space fillings in thehydrothermal and igneous breccia. Pyrite is the most abun-dant sulfide throughout the deposit, averaging 3 to 7 vol per-cent in the ore zones. Covellite is the dominant copper sul-fide. Spectacular specimens of coarse-grained covellite occuras euhedral hexagonal plates up to 1 cm in veins and as ce-ments or open-space fillings in the hydrothermal and igneousbreccias. Copper grades in areas with abundant, coarse-grained covellite exceed 2 percent Cu, whereas average dis-seminated copper grades throughout the Quebrada Minasbreccia pipe are about 0.5 percent Cu.

Supergene overprint

Supergene copper enrichment formed a well-developedblanket at Seca Norte and Trampeadero (Fig. 5). If significantcopper enrichment was ever developed over the hydrother-mal breccia pipe in Quebrada Minas, it has been removedduring later erosion of this deeply incised drainage. Theleached capping mimics topography except for the struc-turally thickened area on the west side of Seca Norte, in anumber of anomalously deeply leached areas in Tram-peadero, and in local surface outcrops in Seca Norte andTrampeadero. The leached capping at Agua Rica is domi-nated by jarosite with lesser amounts of goethite and minorhematite. The lower boundary of supergene enrichment issubhorizontal and flatter than present valley topography. No

evidence for multiple episodes of enrichment has been ob-served. Enriched copper grades are typically twice and rarelythree times as high as underlying primary grades. Chalcociteis the dominant supergene enrichment mineral at Tram-peadero, but fine-grained covellite predominates at SecaNorte. Defining the limit of supergene enrichment is difficultin areas that have undergone advanced argillic overprint dueto the presence of both fine-grained supergene and hypogenecovellite.

Two types of supergene veins occur. Amorphous aluniteand the chalcocite veins cut all lithologies except the Mi Vidaconglomerate and the ash-fall tuff. Chalcocite veins are gen-erally 1 mm wide, discontinuous, and wispy. Amorphous alu-nite veins are soft and very fine grained, varying in color fromwhite to yellow and beige. A sample of alunite from a massive,centimeter-wide, banded vein within the chalcocite blanketwas dated by Perelló et al. (1998) at 3.94 ± 0.05 Ma (K-Ar age;Table 3).

Discussion and Interpretation: Progressively Shallowing Magmatic Hydrothermal Activity

Based on geologic mapping, petrography, and publishedgeochronological data, the Miocene magmatic hydrothermalhistory of the Agua Rica system can be reconstructed in con-siderable detail. It comprises four main stages spanning arange of magmatic hydrothermal environments from initialplutonic magma emplacement in the Melcho intrusion stage,through the feldspar porphyry and likely main copper intro-duction stage, to a third stage of hydrothermal brecciationand epithermal overprinting, and closing with a postminerali-zation crater stage where hydrous magmas and diatremebreccias vented directly to the surface (Figs. 6 and 7). So farno fluid pressure estimates are available to infer absolute pa-leodepths for the first three stages (Fig. 7), but a successiveunroofing of the magmatic system, from a deep subvolcanicenvironment to surface-venting activity near the present ero-sion level, is required to explain the combined observations.

Melcho intrusions, weak mineralization, and potassic alteration

In a first stage of Miocene magmatism, the Melcho intru-sions were emplaced in the Agua Rica area. These earlyintrusions are associated with weak potassic and propylitic al-teration and with subeconomic chalcopyrite-bornite minerali-zation associated with widely spaced quartz stockwork veins.The Melcho intrusions show no unambiguous direct contactwith the subsequent main mineralizing intrusions, either inoutcrop or in drill core, but are interpreted as separate pre-cursor intrusions based on their older Ar-Ar ages (Sasso,1997). The depth of the Melcho intrusions relative to thepaleosurface is unknown, but their holocrystalline texture andthe spaced veining with individual alteration halos may indi-cate emplacement near the lower end of the typical produc-tive porphyry copper level (Sillitoe, 1973).

Feldspar porphyries, stockwork mineralization, and potassic alteration

During a second stage of magmatism, the Seca Norte andTrampeadero feldspar porphyries were emplaced. Potassic andpropylitic alteration with intense quartz stockwork veining,


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typical of most economic porphyry copper deposits, is closelylinked with the emplacement of these feldspar porphyries;the associated chalcopyrite-bornite-molybdenite mineraliza-tion probably represents the main phase of economic oremetal (Cu, Mo, Au) introduction into the deposit. The earli-est sand and pebble dikes were also formed at this stage byfluidization of rock fragments in upward-escaping fluid chan-nelways (e.g., Wolfe, 1980; Laznicka, 1988), but similarevents also occurred throughout all subsequent brecciationstages. It is possible that a significant volcanic edifice hasoverlain the porphyries at this stage, similar to the productiveporphyry stage emplaced into the base of a major volcanic se-quence at Bajo de la Alumbrera (lithostatic fluid pressures upto ~1.1 kbars; Ulrich et al., 2001).

Hydrothermal breccia, covellite mineralization, and advanced argillic alteration

In the third stage of igneous activity, hydrothermal brecciaoverprinted the feldspar porphyries as a result of hydrother-mal fluid release from magmas that clearly postdated the al-ready stockwork-veined, potassically altered, and chalocopy-rite-bornite-molybdenite mineralized porphyries. This eventgenerated a continuous suite of interfingering igneous andhydrothermal breccias. We infer that the igneous brecciaserved as the essential source of heat, fluid, and mechanicalenergy for the formation of the coeval hydrothermal breccias.Similar igneous breccias were described by Sillitoe (1985),commonly occurring as irregular patches near the roofs of


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MELCHOINTRUSIONS

FELD-SPAR

PORPHY-RIES

HYDRO-THERMAL

BREC-CIA

CRATER INFILL BRECCIASUPERGENE ENRICHMENT

ALTERATION

VEINS

Potassic

Amorphous alunite veinsChalcocite veins

Pyrite veins with quartz-sericite selvages Pyrophyllite-alunite veinsCrystalline alunite veins

Quartz-pyrite±chalcopyrite±molybdenite

Advanced argillic

Pyrite-covellite-enargite-galena-sphalerite veins

Phyllic

Porphyry styleminerali-zation

MINERALIZATION

UNITS

APPROXIMATE TIME [Ma]

9 8 6 5 47 3 2 1

Ash-fall tuff


Magnetite-chalcopyrite veinsType A quartz-pyrite±chalcopyrite±bornite veins


Type B quartz-molybdenite-pyrite veins

Biotite porphyryTalus breccia

Hydrothermal brecciasGranite breccia ??

Fault breccia ? ?

Porphyry stage

Overburden (scree, gravel, soil)

Mi Vida conglomerate

Igneous breccia

Leaching and supergene enrichment stage

Sand and pebble dikesCrater infill breccia

High sulfidation epithermal stage

Melcho intrusions8.56

7.03 6.29

0.52

5.35 6.18

3.94

4.88 and 4.96 6.10 5.38

5.10

FIG. 6. Evolution of the Agua Rica deposit. The time table is based on geologic mapping of wall rock to breccia relation-ships (Landtwing, 1998) and drill core logging of alteration and mineralization assemblages by BHP geologists. Ar-Ar dates(open squares and and circles) by Sasso (1997) and K-Ar dates (filled squares and and circles) by Perelló et al. (1998) arelisted in Table 3. Rectangular symbols are used for measurement of the Melcho intrusions; circles for measurements in theSeca Norte-Trampeadero area.

subvolcanic stocks in volcanic-hosted porphyry copper de-posits, grading downward into solid intrusive rocks and un-fractured wall rocks. Fluids from an apophysis of a magmachamber are exsolved by saturation of the melt as a result ofdecompression. The mechanical energy released from the

hydrous magmas causes fragmentation, which leads to furtherdecompression once the lithostatic load is breached by fluidchannelways to the surface. Rapid propagation of fracturesinto higher levels leads to increased energy release, mixing andmilling of fragments, production of rock flour, and varying


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crater infillbreccia

talusbreccia

igneousbreccia

inferred depthbelow paleosurface

2 km?

1 km?

3 km?

4 km?

hydrothermalbreccia

igneousbreccia

3 km?Seca Norte feldspar

porphyry

Trampeadero feldspar

porphyry ?

?

? ?


Melchointrusions

Metasedimentaryrocks


biotiteporphyry

2 km?

3400

3200

3000

2800

2600

2400

2200

3600

Granite

5000

m

Paleozoic basement

Metasedimentary rocks

Stockwork veins

Wall rock lithologiesTertiary intrusions

Biotite porphyry

Melcho intrusions



Breccia types

Breccia with matrix of hydrothermal minerals and minor sand and silty material

Breccia with matrix of sand and siltymaterial and hydrothermal minerals

Granite breccia

Igneous breccia

Crater infill breccia

Talus breccia

Granite

Present-day surface

SENW

SENW

SENWpresent-day

elevation (m asl.)

SENW2. Feldspar porphyries, quartz stockwork

veining, mineralization and potassic, alteration

3. Hydrothermal breccia, covellite mineralization and advanced argillic alteration

4. Post-mineralization crater infill brecciaand supergene enrichment

1. Melcho intrusions,weak mineralization and potassic alteration

FIG. 7. Schematic cross sections from N 6970500, E 768000 to N 6969000, and E 769500, summarizing the four mainstages in the evolution of the Agua Rica deposit. 1. The Melcho intrusions were emplaced, associated with weak potassic al-teration and minor Cu-Mo mineralization. 2. Intrusion of the Seca Norte and Trampeadero feldspar porphyries, associatedwith porphyry-style quartz stockwork veins, potassic alteration, and disseminated Cu-Mo-Au mineralization with chalcopy-rite ± bornite. 3. Formation of hydrothermal breccias grading down into igneous breccias, followed by pervasive hydrother-mal overprint to high sulfidation epithermal (pyrite-covellite) and advanced argillic alteration assemblages. 4. Phreatomag-matic eruption forming crater infill breccia and related second generation of igneous breccia, followed by talus breccia shedfrom the crater rim and intrusion of barren biotite porphyry into the base of the diatreme. Indicated depths below the pale-osurface in (1) to (3) are indications only, based on geologic mapping, petrography, and general geologic models (Sillitoe,1973; Hedenquist and Lowenstern, 1994); they illustrate that the successive stages in the evolution of Agua Rica may be ex-plained by progressive unroofing and surface erosion during protracted magmatic hydrothermal activity.

degrees of upward transport of the material as slurry. Clastswith closely spaced, concentric fractures (hypogene exfolia-tion) indicate instantaneous drop in confining pressure(Farmin, 1937), as hot clasts move upward, or a drop of am-bient fluid pressure in the breccia column. The highest de-gree of mixing and block rotation may have occurred in thelower core portions of the breccia pipes at Agua Rica. Here,initial permeability generated by hydraulic fracturing is high-est and fluid flow is most focused, leading to a large produc-tion of rock flour and eventually leaving only minor openspace. In their outer and upper parts, the breccia chimneysare affected by less milling and rounding of fragments, lead-ing to more open breccias cemented predominantly by chem-ical precipitation of hydrothermal minerals. Sheet fracturingparallel to the walls and jigsaw breccias develop at the tops ofthe breccia columns and in lateral transition zones to nor-mally fractured country rock (jigsaw breccia; Bryner, 1961;Norton and Cathles, 1973). There is no evidence that the hy-drothermal breccias of Agua Rica ejected any magma or solidmaterial to the land surface, but the style of emplacementsimilar to the subsequent surface venting crater stage is mosteasily explained a result of shallower magma emplacement,following some unroofing between the porphyry and the hy-drothermal breccia stages. Shallow fluid exsolution from mag-mas otherwise similar to those associated with the precedingporphyry stage would lead to greater energy release andcould explain the different magmatic hydrothermal products(Burnham and Ohmoto, 1980).

Advanced argillic alteration is closely related to the hy-drothermal breccia and dominantly fracture and void con-trolled, e.g., as cavity fillings in the hydrothermal breccia. Theassociated covellite-pyrite ± native sulfur assemblage, formedat least partly by sulfidation of chalcopyrite and bornite, pre-cipitated earlier in veins associated with the feldspar por-phyries, and included the addition of As (in enargite) andprobably Pb and Zn. Mass-balance data are not available toassess whether significant amounts of Cu, Mo, and/or Au wereadded or removed at this stage, but highly variable Cu/Au ra-tios at Agua Rica (in contrast to Typical Cu-Au porphyriessuch as those at Bajo de la Alumbrera; Ulrich and Heinrich,2001) indicate significant redistribution of the economic oremetals and probably local introduction of higher grade Auzones during the high sulfidation epithermal overprint.

Postmineralization crater infill breccia and supergene enrichment

In a fourth and final phase of magmatic hydrothermal ac-tivity, the surface-venting crater infill breccia transgressed,partly incorporating altered fragments of igneus breccia andall earlier alteration stages. The crater is interpreted to be theresult of shallow-level phreatomagmatic eruption, involvinglarge amounts of preexisting rock and a clear input (butpoorly defined proportion) of juvenile magma. In its upperpart, the crater infill breccia consists of water-transported andpartly landslide-derived mass flow sediments, with channelstructures and graded bedding dipping toward the center ofthe vent. Accretionary lapilli (Fig. 4G), rare clasts of juvenileorigin, a possibly significant proportion of ash, and a gradualdownward transition from bedded though clast-supportedbreccias into an igneous breccia preserved in the lower core

of the conical breccia body demonstrate a phreatomagmaticorigin.

A barren biotite porphyry dike intruding the base of thecrater infill breccia and the associated igneous breccia con-cluded the magmatic activity at Agua Rica. Fans of talus brec-cia overlying all magmatic and hydrothermally altered rocksresulted from surface degradation by rock falls shed from thecrater escarpment, after termination of all hydrothermal ac-tivity with the exception of some late pebble dikes.

Supergene leaching and secondary enrichment of covelliteore probably occurred while the current drainage pattern wasbeing established, but at a stage when the topography wasconsiderably less steep than the present incision of V-shapedgullies (Fig. 5b). Structural thickening of leached capping onthe west side of the Seca orebody indicates that reverse fault-ing partly postdated hydrothermal activity and possibly someof the supergene enrichment. The Mi Vida conglomerateformed essentially in the present topographic environment,by cementation of alluvial sand, scree, and river gravel due tonatural acid water drainage.

Summary and ConclusionsDetailed prospect-scale mapping and sample-scale petro-

graphy focusing on the wide variety of breccia types at AguaRica have identified at least four stages in the magmatic hy-drothermal evolution of this geometrically complex Cu-Mo-Au deposit. Formation of the Melcho intrusions with associ-ated subeconomic potassic and propylitic alteration and weakpyrite ± chalcopyrite ± molybdenite mineralization precededthe intrusion of the Seca Norte and Trampeadero feldsparporphyries. Intense quartz stockwork veining, potassic alter-ation, and chalcopyrite-molybdenite ± bornite mineralizationwere associated with these feldspar porphyries and probablyrepresent the main metal-introducing stage in the deposit.This initial porphyry Cu-Mo-Au system was then intenselyoverprinted by a major event of hydrothermal brecciation dri-ven by fluid exsolution from subjacent igneous breccias. Ad-vanced argillic and phyllic alteration and almost completesulfidation of Cu-Fe sulfides to the present hypogene covel-lite-pyrite ore was associated with hydrothermal brecciation.A surface-venting phreatomagmatic crater eruption termi-nated the protracted magmatic hydrothermal activity, prior tosupergene alteration and development of the steeply incisedpresent topography.

K-Ar dating of alteration minerals (Sasso, 1997; Perelló etal., 1998) suggests that the preore intrusions were emplacedbetween 8.6 and 6.3 Ma, the porphyry-stage chalcopyrite-molybdenite ± bornite mineralization occurred around 5 Ma,and the high sulfidation epithermal overprint with advancedargillic alteration was completed by about 4.9 Ma. No datesare available for the crater stage, but the indistinguishablepetrographic features of the igneous breccia matrices(groundmass and phenocrysts) in the hydrothermal brecciaand crater stages indicate that the two events probably fol-lowed each other in short succession.

Comparison of the geologic history of Agua Rica with thatof other deposits exhibiting superimposition of epithermal-style associations over higher temperature assemblages typi-cal for porphyry copper environments (e.g., Perelló et al.,2001; Müller et al., 2002) and with general models for


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porphyry and epithermal environments (Sillitoe, 1973; White,1991; Dilles and Einaudi, 1992; Hedenquist and Lowenstern,1994; White and Hedenquist, 1995; Hedenquist et al., 1998)indicates that the four evolution stages of Agua Rica tookplace in an environment of successively decreasing paleo-depth. The depth labels in the synoptic cartoons of Figure 7are indicative estimations only, but removal of several kilo-meters of overburden between the porphyry stages (em-placement of Melcho intrusions and the feldspar porphyries)and the formation of the surface-venting crater infill brecciais established by field observation. Surface erosion duringprotracted magmatic hydrothermal activity may have oc-curred continuously at an average rate of millimeters peryear, but more dramatic events of eruptive removal of over-burden associated with some or all of the magmatic hy-drothermal stages seem more likely in this geologic setting.The telescoping at Agua Rica of early intrusions, mineralizedfeldspar porphyries, hydrothermal breccias, and surface-vent-ing explosion breccias, now all exposed at the same level oferosion, was probably driven by the progressive degradationof the paleosurface imposed by differential uplift of the Sier-ras Pampeanas, in an environment of regional uplift of base-ment blocks along northeast-striking thrust faults (All-mendinger, 1986; Jordan and Allmendinger, 1986; Sasso,1997; Sasso and Clark, 1998; Bissig et al., 2001). Future high-precision dating of magmatic (U-Pb on zircon) and hy-drothermal events (Ar-Ar and Re-Os of alteration and sulfideminerals) in conjunction with fluid inclusion studies may beused to calibrate the temporal evolution of magmatism andhydrothermal fluid pressures further. This study shows thatdetailed geologic mapping of clast-matrix relations and stan-dard petrography alone can identify several distinct stages inthe initially bewildering complexity of a breccia-hosted oresystem. At Agua Rica, field observations allowed us to seethrough several overprinting events back to the probablemain stage of economic porphyry copper mineralization.Such geologic reconstruction can be essential for explorationand economic mine development in a brecciated deposit suchas Agua Rica. Even where overprinting fluid activity may nothave introduced or removed large quantities of the main oremetals, significant mechanical redistribution and variable de-grees of mineralogical reconstitution of the ore components islikely to have important consequences for orebody delin-eation, mining, metallurgy, and environmental management.

AcknowledgmentsM. Landtwing’s M.Sc. thesis at ETH Zürich formed the

basis of this paper. The project could not have been realizedwithout the generous financial, logistic, and geologic supportof the Agua Rica project of BHP Copper. We would like tothank Noel White for his initial help and John Mortimer andPablo Marcet for their subsequent organization of this re-search project and for their cooperation and permission topublish the results. M. Landtwing would personally like tothank the present and former BHP geologists Raúl Roco,Tom Paterson, Andrés Lasry, Ben Jones, Felipe Urzúa,Gabriel López Vázques, Lars Beggerow, Sandra Troutman,and Noel White for discussions (geologic and nongeologic)and for their instructive advice. There are many others of theAgua Rica camp staff who deserve thanks for their help and

assistance. Without their background work, this project wouldnot have been possible. Many people at the Swiss Federal In-stitute of Technology in Zürich have helped during this pro-ject. Special thanks are due to Thomas Ulrich who con-tributed in the field and laboratory and reviewed earlyversions of this paper. Detailed comments by Eric Seedorffand constructive journal reviews by Alan Clark, José Perelló,Marco Einaudi, and Mark Hannington helped to clarify andimprove the manuscript. This project was partly supported byETH project grants (0-20041-95 and 0-20663-99) and by theSwiss National Science Foundation (grants 21-45548.95 and20-52265.97).

May 3, 2000; May 23, 2002

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