Andean Geology 37 (1): 144-176. January, 2010
Andean Geology formerly Revista Geológica de Chile
www.scielo.cl/andgeol.htm

 

Geochronological and thermochronological constraints on porphyry copper mineralization in the Domeyko alteration zone, northern Chile

Determinaciones geocronológicas y termocronológicas para la mineralización de cobre porfídico en la zona de alteración de Domeyko, norte de Chile.

 

Víctor Maksaev1, Tomás A. Almonacid2, Francisco Munizaga1, Víctor Valencia3, Michael McWilliams4, Fernando Barra3

1 Departamento de Geología, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile. vmaksaev@ing.uchile.cl; fmunizag@ing.uchile.cl
2 Minera Peñoles de Perú S.A., Av. Central No. 643 Of. 201, San Isidro, Lima, Perú. Tomas_Almonacid@penoles.com.mx
3 Department of Geosciences, University of Arizona, 1040E. 4th Street, Bldg. #77, Tucson, AZ 85721, USA. victorv@email.arizona edu; fbarra@email.arizona.edu
4 CSIRO Exploration and Mining, 26 Dick Perry Avenue, Kensington WA 6151, Australia; PO Box 1130, Bentley WA 6102, Australia. Mike.Mcwilliams@csiro.au

ABSTRACT. At Domeyko, 40 km south of Vallenar in northern Chile (28°57'S-70°53'W), the Dos Amigos and Tricolor porphyry copper centers are located within a north-south-elongated hydrothermal alteration zone 6x1.5 km of surface dimensions. The centers are related to tonalite to granodiorite porphyry stocks displaying potassic alteration, which are surrounded by Lower Cretaceous andesitic volcanic rocks with sericitic, kaolinite-illite and propylitic alteration zones. The western boundary of the alteration zone is marked by the post-mineralization Cachiyuyo Batholith of granodioritic to dioritic composition. U-Pb zircon ages for the Dos Amigos porphyry are of 106.Ü3.5 and 104.0±3.5 Ma; and 108.5±3.4 for the nearby Tricolor porphyry. The Cachiyuyo Batholith yielded U-Pb zircon ages of 99.6±1.8 and 99.1±1.9 Ma; and 40Ar/39Ar ages for biotite of 96.9±3.9 and 94.8±0.9 Ma. These dates indicate that batholith emplacement postdated the Dos Amigos and Tricolor porphyries, in agreement with geological relationships. Although copper mineralization is spatially and genetically related to the Lower Cretaceous (Albian) porphyry stocks, most of the dated hydrothermal micas from the Dos Amigos and Tricolor porphyries yielded 40Ar/39Ar ages between 97.1±2.5 and 96.0±1.4 Ma, which overlap within error with the cooling ages obtained for the neighboring batholith. 40Ar/39Ar dating of micas revealed significant disturbance of their K-Ar isotopic systematics that complicates accurate determination of the timing of hydrothermal activity at Domeyko. Nevertheless, the 40Ar/39Ar data establish a minimum Late Cretaceous age for this activity. A fission track age of 59.8±9.8 Ma of apatite from the Dos Amigos porphyry indicates cooling through the temperature range of the apatite partial annealing zone (~125-60°C) during the Paleocene; and an (U-Th)/He age of 44.7±3.7 Ma of apatite from the same porphyry sample shows cooling through the temperature range of the apatite He partial retention zone (~85-40°C) during the Eocene. These ages correspond to the exhumation of the porphyry, and the latter provides a maximum age for the supergene enrichment processes that formed the chalcocite blanket currently mined at Dos Amigos.

Keywords: Porphyry copper, Andes, Chile, Geochronology, Thermochronology Coastal Cordillera.

RESUMEN. En Domeyko, a 40 km al sur de la ciudad de Vallenar, en el norte de Chile (28°57'S-70°53'O), se ubican los pórfidos cupríferos Dos Amigos y Tricolor dentro de una zona de alteración hidrotermal elongada en sentido norte-sur de 6x1,5 km. Estos están relacionados con stocks porfídicos tonalíticos a granodioríticos con alteración potásica, rodeados por zonas de alteraciones sericítica, caolinita-illita y propilítica, las cuales afectaron principalmente rocas volcánicas del Cretácico Inferior. El límite oeste de la zona de alteración lo constituye el Batolito Cachiyuyo postmineral de composición granodiorítica a diorítica. Edades U-Pb en circón para el pórfido Dos Amigos son de 106,1±3,5 Ma y 104,0±3,5 Ma y de 108,5±3,4 Ma para el pórfido Tricolor. El Batolito Cachiyuyo adyacente dio edades U-Pb en circón de 99,6±1,9 y 99,1±1,9 Ma y edades 40Ar/39Ar en biotita de 96,9±3,9 y 94,8±0,9 Ma. De acuerdo a estas edades el emplazamiento del batolito fue posterior a los pórfidos Dos Amigos y Tricolor, consistente con las relaciones geológicas. Aunque la mineralización de cobre está relacionada espacialmente y genéticamente con los stocks porfídicos del Cretácico Inferior (Albiano), la mayoría de las micas datadas de los pórfidos Dos Amigos y Tricolor dieron edades 40Ar/39Ar entre 97,1±2,5 y 96,0±1,4 Ma, las cuales son coincidentes con las edades de enfriamiento obtenidas para el batolito. La datación 40Ar/39Ar por pasos de micas reveló una importante perturbación del sistema isotópico K-Ar de las mismas, lo que complica una determinación exacta de la edad de la actividad hidrotermal en Domeyko, pero los datos 40Ar/39Ar establecen con certeza una edad mínima Cretácico Tardío para ella. Una edad de trazas de fisión en apatita de 59,8±9,8 Ma para el pórfido Dos Amigos indica enfriamiento a través del rango de temperatura de la zona de acortamiento parcial de trazas en apatita (~125-60°C) durante el Paleoceno y una edad (U-Th)/He en apatita de 44,7±3,7 Ma obtenida para el mismo pórfido revela enfriamiento a través del rango de la zona de retención parcial de He en apatita (~85-40°C) durante el Eoceno. Estas edades corresponden a la exhumación del pórfido y la última provee una edad máxima para los procesos de enriquecimiento supérgeno que formaron el nivel enriquecido con calcosina actualmente en explotación en la mina Dos Amigos.

Palabras clave: Pórfido cuprífero, Andes, Chile, Geocronología, Termocronología, Cordillera de la Costa.

1.       Introduction

The Domeyko alteration zone is located in the southern part of the Atacama Desert, 40 km south of Vallenar (28°57'S-70°53'W; Fig. 1). It includes the Dos Amigos porphyry copper deposit, currently being exploited, and the Tricolor porphyry copper occurrence (Fig. 2). Regionally, these deposits are part of a Mid-Late Cretaceous belt of porphyry copper deposits that extends along the eastern flank of the Coastal Cordillera of northern Chile, between latitudes 26° and 31°S (Llaumett, 1975; Camus, 2002, 2003; Sillitoe and Perelló, 2005; Maksaev et al., 2006a, 2007) (Fig. 3).


The Domeyko alteration zone was first ex-plored for its porphyry copper potential from 1962 to 1964, during a 'Program for Development' funded by the United Nations (Kents. 19621), which detected copper mineralization occurrences at Tricolor and Dos Amigos (Fig. 2); six drill holes (totaling 683 m) were completed at Tricolor intercepting low-grade hypogene copper intervals between 0.13 to 0.32 percent. It was followed by exploration during the years 1968 to 1971 by the governmental institution 'Corporación Nacional de Fomento' (CORFO) in the Dos Amigos area, including 43 diamond drill holes (totaling 5,361 m) and 4 percussion drill holes (236 m). This program identified a supergene copper enrichment zone for which a resource of 3.5 million metric tons averaging 1.18 percent copper was estimated (Palafox, 19752). Further exploration at Dos Amigos by Shell Chile between 1982 and 1983 expanded the supergene resource to 5 million metric tons at about 1 percent copper and 0.25 grams per metric ton gold and discovered additional hypogene mineralization of 36 million metric tons at 0.36 percent copper and 0.26 grams per ton gold. Since 1996, the enrichment blanket at Dos Amigos has been the objective of open pit mining by Compañía Explotadora de Minas (CEMIN), with annual average extraction of 1 million ton of ore averaging 1 percent copper, which is processed by heap leaching and solvent extraction-electrowinning (SX/EW) methods. There are no previous publications on the geology of the Domeyko alteration zone, except for two whole rock K-Ar determinations of 106±10 and 97±20 Ma for altered rocks reported by Munizaga et al., (1985). Consequently, the present contribution constitutes the f irst geological description of the area. It provides new U-Pb, 40Ar/39Ar, fission track and (U-Th)/He data that confirm its position within a mid-Cretaceous metallogenic episode of porphyry copper mineralization (e.g., Sillitoe and Perelló, 2005). The present work also provides a time-temperature model for the low-temperature cooling of the Dos Amigos porphyry, which in turn helps to constrain the timing of the supergene processes and associated chalcocite enrichment.


2.       Metallogenic setting

The porphyry copper belt that extends along the eastern f lank of the Coastal Cordillera of northern Chile was initially recognized as the 'Pacific Belt: of Chilean porphyry copper deposits by Llaumett (1975). Subsequent geochronological work has shown that the porphyry copper systems located between latitudes 21° and 23°S form a sub-belt with ages from 142 to 132Ma(Munizaga et al., 1985; Reyes, 1991; Boric et al., 1990; Perelló et al., 2003; Sillitoe and Perelló, 2005; Maksaev et al., 2006a); this belt seems to re-appear south of latitude 33°S (Fig. 3), but there are insufficient geochronological data to prove it. The porphyry copper deposits and prospects located in the Coastal Cordillera between latitudes 26° and 31°S form another sub-belt with U-Pb ages from 108 to 88 Ma (Maksaev et al., 2006b) (Fig. 3). The largest historie and current copper producer of the last sub-belt is the Andacollo porphyry copper-gold deposit (Llaumett et al., 1975; Reyes, 1991). Ithas been operated since 1996 by the Compañía Minera Carmen de Andacollo (ownership: 90% Teck and 10% 'Empresa Nacional de Minería'), with an average annual production of 21,000 tons cathode copper. Open pit mining at Andacollo to date has exclusively concentrated on leachable resources amounting to 34.6 million metric tons of 0.73 percent copper in the supergene chalcocite enrichment blanket, but its hypogene zone with resources of 311 million metric tons averaging 0.46 percent copper and 0.15 grams per ton gold is currently being prepared for production. In addition to the production at Dos Amigos deposit, a number of the other porphyry copper prospects from the sub-belt have been explored to varying degree, but have not attained production status (e.g., Los Toros, Los Loros; Fig. 3). In general, the Cretaceous porphyry deposits of the Coastal Cordillera are much smaller (resources <~300 million metric tons) and possess lower hypogene grades (<0.4% Cu) than those of the Cenozoic porphyry copper belts located farther east and at higher elevations in the Chilean Andes (e.g., Sillitoe and Perelló, 2005). The Cretaceous deposits are related to small stocks of quartz diorite to granodiorite porphyry emplaced into arc-related plutonic and volcanic rocks. They tend to be dominated by potassic alteration (biotite, K-feldspar) with a variably developed intermedíate argillic overprint (illite, and/or smectite, chlorite, sericite). In addition, sericitic alteration is present at Andacollo, Antucoya-Buey Muerto, and in the Domeyko alteration zone (Reyes, 1991; Perelló et al., 2003; Maksaev et al., 2006a).


During the Jurassic to Early Cretaceous a sub-duction-related magmatic are developed along the area of the Coastal Cordillera in westernmost Chile. The are was flanked eastward (inland) by a sedimentary-marine back-arc basin represented by carbonate strata of the Chañarcillo Group (Segerstrom and Parker, 1959; Moscoso et al., 1982; Arévalo et al., 2005; Arévalo, 2005a, b). The porphyry copper sub-belt with U-Pb ages from 108 to 88 Ma extends along the eastern edge of this magmatic are. The copper deposits were formed during and after the mid-Cretaceous (Albian) tectonic inversión of the back-arc basin, as shownby the end of marine sedimentation of the Chañarcillo Group and the onset of coarse-grained alluvial sedimentation and subaerial volcanism of the Cerrillos Formation during the late Aptian (e.g., Marschik and Fonboté, 2001; Marschik and Sollner, 2006; Charrier et al., 2007; Maksaev et al., 2009). This abrupt change in the sedimentary environment represents a signif icant modification of the tectonic regime on the continental margin from tensional to compressive (Maksaev et al., 2009), in turn related to a major reorganization of the Andean orogen involving the closure of the back-arc basins all along the western margin of South America (Dalziel, 1986; Bourgois et al., 1987; Mpodozis and Ramos, 1990).

This porphyry copper sub-belt runs parallel to, but some 10-15 km to the east of the 'Chilean Iron Belt' (Ruiz et al., 1965) made of a number of iron oxide-apatite and iron oxide-copper-gold deposits distributed along the soufhernmost segment (26o-30°S) of the Atacama Fault Zone (Fig. 4) (Ruiz et al., 1965;Espinoza, 1990; Nyström and Henríquez, 1994; Sillitoe, 2003; Gelciche et al., 2005; Maksaev et al., 2007). The Atacama Fault Zone is a major sinistral, strike-slip fault system that extends along the Coastal Cordillera for more than 1,000 kmbetween latitudes 20° and 30° (Arabasz, 1971; Scheuberand González, 1999) (Fig. 4). It developed in the Jurassic with sinistral shear persisting into the Early Cretaceous in an overall transtensional tectonic setting along the magmatic are, in cióse association with regional pluton emplacement, as well as crustal thinning and subsidence(Taylor et al., 1998;Brown et al., 1993; Dallmeyer et al., 1996; Scheuber and Andriessen, 1990; Scheuber et al., 1995; Scheuberand González, 1999; Grocott and Taylor, 2002). K-Ar and 40Ar/39Ar ages for actinolite from magnetite-apatite deposits of 129 to 96 Ma have been published (e.g., Munizaga et al., 1985; Oyarzún et al., 2003; Díaz et al., 2003) and suggest chronological overlapping with the 108 to 88 Ma range of the porphyry copper sub-belt. However, older and more accurate U-Pb ages for magnetite and apatite from 131 to 127 Ma have also been reported for deposits of the Chilean Iron Belt (Gelcich et al., 2005). Therefore, the deposits of the Chilean Iron Belt represent a metallogenic episode that preceded in time the formation of the porphyry copper deposits. Whole-rock K-Ar ages of of 117±3 and 121±3 Ma for altered andesites and dykes at Manto Verde were initially taken to indicate the age of the primary mineralization of iron oxide-copper-gold deposits (Vila et al., 1996), but more precise U-Pb ages of 128.9±0.6 and 126.4±0.5 Ma for a quartz monzonite to granodioritic dyke withpotassic alteration, led Gelcich et al., (2003) to conclude that mineralization at Manto Verde is most probably even older. Likewise, dates older than those of the porphyry copper sub-belt have been reported for the Candelaria iron oxide-copper-gold deposit, including Re/Os molybdenite dates of 114.2±0.6 and 115.2±0.6 Ma interpreted as mineralization ages by Mathur et al., (2002). These are coincident with the 115.1±0.2 Ma 40Ar/39Ar plateau age forbiotite asso-ciated with chalcopyrite-pyrite reported by Marschik and Fontboté (2001) and the 40Ar/39Ar plateau ages of 114.2±0.8 and 114.1± 0.7 Maforbiotite of Ullrich & Clark (1999). However, a younger 40Ar/39Ar plateau age of 111.7±0.8 Ma for amphibole (Ullrich and Clark, 1999) and similar 40Ar/39Ar ages of 111.0±1.7 and 110.7± 1.6 Maforbiotite (Arévalo et al., 2006) probably represent a later event of alteration at Candelaria, which overlaps within error with the oldest ages of the porphyry sub-belt. Thus, it is possible that mineralization at Candelaria represents a transition between iron oxide-copper-gold and porphyry copper metallogenic events during the mid-Cretaceous basin inversión along the Coastal Cordillera of northern Chile between latitudes 26° and 31°S.


3.       Local Geology

The Domeyko alteration zone is located in a región characterized by rolling hills and intermontane depressions at an average elevation of 800 m. It is exposed along a ridge that rises to 1243 m in elevation at the Domeyko hill, with dimensions of 6 km in the NS direction and 1 to 1.5 km in the EW direction (Fig. 2). The ridge stands above relies of extensive west-sloping terraces of Miocene gravels formed by coalescent alluvial fans, developed at elevations between 950 and 780 m (Atacama Gravels ; Mortimer, 1973; Moscoso et al., 1982).

An unaltered, granodioritic to dioritic batholith (herein referred to as the Cachiyuyo Batholith) constitutes the abrupt western boundary of the Domeyko alteration zone. The batholith intruded the altered volcanic rocks, but its contact also coincides locally with a north-trending regional fault (Fig. 2). The majority of the altered rocks are part of a NNW-striking and E-dipping sueeession of Neocomian age assigned to the Bandurrias Group (Moscoso etal., 1982) and composedof andesitic lavas and volcanic breccias, with subordinate dacite. Minor intrusive bodies and dikes of fine-grained, green-colored andesite are emplaced in the volcanic sueeession.

Two porphyry stocks intrude the volcanic rocks at Tricolor and Dos Amigos (Figs. 3 and 5). The Dos Amigos porphyry is tonalitic to granodioritic in composition with plagioclase and quartz pheno-crysts, up to 4 mm in diameter, in a microcrystalline groundmass composed of an aggregate of plagioclase and quartz; some plagioclase phenocryst margins display vermicular intergrowths with K-feldspar. The porphyry contains fine-grained hydrothermal biotite profusely disseminated and microcrystalline biotite aggregates replacing amphibole. The Tricolor porphyry is of similar composition with plagioclase and minor quartz phenocrysts in a microcrystalline groundmass composed of an aggregate of similar components. The porphyry contains abundant fine-grained opaque minerals and hydrothermal biotite profusely disseminated; the latter typically altered to chlorite.


A composite hydrothermal breccia body (Marisol breccia) is exposed over a surface area of 500x600 m immediately north of the Dos Amigos mine (Fig. 5); its central part is polymictic and matrix-support-ed, with sericitically-altered angular fragments of volcanic rocks and porphyry in a matrix (30-40%) of tourmaline and rock flour; abundant pyrite, but only minor chalcopyrite are visible in the breccia matrix in exploration adits. The marginal part of the Marisol breccia is formed by subrounded clasts of volcanic rocks in a matrix of silicified rock flour with sericite; underground this matrix includes pyrite. In an exploration adit 150 m beneath the surface, the breccia shows a higher proportion of strongly seriticized porphyry fragments in a matrix of tourmaline, pyrite, and quartz.

A longitudinal fault zone traverses the whole altered area (Fig. 5), accompanied by a number of subsidiary northwest-trending subvertical faults; locally biotite-bearing porphyry is exposed at Tricolor with N-S/vertical foliation. The overall geometric fault pattern in the Domeyko alteration zone is compatible with a longitudinal sinistral shear (Almonacid, 2007).

3.1.       Alteration types

The Domeyko alteration zone includes potassic. sericitic, kaolinite-illite, and propylitic alteration assemblages (Fig. 6). Potassic alteration is present at both Dos Amigos and Tricolor porphyries and in the immediately surrounding volcanic rocks east of Tricolor. The potassic zone at Dos Amigos is 750x750 m in surface area, but that at Tricolor is only 250x250 m. The characteristic alteration mineral assemblage includes biotite and magnetite, with subordinate K-feldspar. The primary texture of the porphyries is largely preserved, but volcanic rocks adjacent to the porphyry at Tricolor are strongly biotitized, black in color and show texture obliteration.


Sericitic alteration surrounds both potassic alteration centers; it is characterized by bleaching of the rocks, almost complete destruction of original rock texture and an assemblage of quartz, f ine-grained white mica, pyrite, and minor andalusite. The sericitic zones grade outward to rocks with preserved original texture, but with feldspars and mafic minerals altered to kao-linite, illite and quartz, with minor montmorillonite; this zone extends irregularly north-southfor~5 km. An external propylitic zone (chlorite, epidote and calcite) is mostly restricted to the eastern part of the Domeyko alteration zone (Fig. 6).

A supergene alteration overprint is apparent in most of the Domeyko alteration zone with common presence of halloysite and kaolinite, and fracture filling with supergene alunite and gypsum.

3.2.       Mineralization

The Domeyko alteration zone exposes a leached cap characterized by the presence of profuse limonite staining and impregnation comprising goethite and hematite, which give an overall reddish color to the rocks of the area. This leached cap has an average thickness of 100 m over the Dos Amigos deposit and Marisol hydrothermal breccia body. A limited zone with oxidized copper minerals is preserved at the bottom of the leached cap at Dos Amigos; its thickness is irregular, ranging from a few meters to ~30 m within fault zones. The main copper-bearing oxidized minerals are chry socolla, atacamite and minor brochantite, which are accompaniedby goethite and minor amarantite. At Tricolor a number of small shafts and adits along NNW and NW-trending fracture zones, from 0.3 to 1.2 mwide, contain chrysocolla and minor chalcocite.

A supergene chalcocite-enriched blanket is developed at Dos Amigos between 740 and 800 m elevation, with an average thickness of ~30 m and up to 60 m in faults and fractured zones. Within this blanket black, sooty chalcocite has replaced the margins of pyrite and chalcopyrite grains. Minor covellite and digenite exist from the middle part to the bottom of the blanket, also mostly as fine coatings to pyrite, chalcopyrite and bornite, with covellite becoming increasingly abundant in the lowermost part of the enrichment zone. Although the supergene sulfides are largely restricted to rimming of the hypogene sulfides, the copper grade of the enrichment zone get to 1.25 percent, for an overall enrichment factor of up to 3 times the hypogene copper grade. However, the supergene enrichment within the Marisol hydrothermal breccia body averages copper grade of less than 0.5 percent, due to the lower hypogene grade of this unit (0.2%; Almonacid, 2007).

Hypogene copper-bearing minerals are mostly chalcopyrite and lesser bornite, within a stockwork of quartz veins hosted by the Dos Amigos porphyry displaying biotite-dominated potassic alteration ex-posed on the pit floor of the mine (740 m level); its vertical extent is currently unknown and copper grade typically averages less than 0.36 percent, according to CEMIN data. Irregular and discontinuous biotitic veins, 0.02 to 2 mm thick, are the earliest veins in the porphyry, and contain abundant magnetite, but lack sulfides. These veins are cut by wavy, irregular and discontinuous, quartz-bearing veins with biotite, 0.2 to 6 mm thick, which contain pyrite, chalcopyrite, bornite, and magnetite. Both vein sets are, in turn, cut by straight and continuous quartz-bearing veins; mostly composed of anhedral and euhedral quartz with either central or parallel bands of pyrite, chalcopyrite, and magnetite, together with minor bornite, biotite and sericite. Late veins are composed of pyrite, quartz and minor muscovite with sericitic alteration envelopes; only rare pyrite-chalcopyrite intergrowths exist in these late veins.

The potassic-altered tonalitic to granodioritic porphyry that crops out at Tricolor also displays a stockwork of sulfide-bearing quartz veins beneath the leach capping, which are apparent in the dumps of an exploration adit at 800 m elevation.

Gold mineralization at Dos Amigos and Tricolor is poorly constrained. However, limited surface sampling reveals anomalous values mostly less than 0.3 grams per ton (between 0.11 and 0.88 g/t Au) for porphyries displaying potassic alteration; similarly, assays for 22 samples have returned average molybdenum values of only 15 parts per million (Almonacid, 2007).

4.       Geochronology

4.1.     Analytical procedures

4.1.1.   U-Pb dating

Zircon grains from the Dos Amigos tonalite porphyry, the granodiorite porphyry of the Tricolor area, and the unaltered granodiorite and diorite of the Cachiyuyo Batholith were dated by L A-ICP-MS U-Pb. The new U-Pb zircon geochronological results are summarized in Table 1, and are plotted with error bars (±2σ) in figures 7 and 8. The U-Pb analytical data are included in appendix A.


The analytical work was performed at the University of Arizona using the laser ablation ICP-MS technique following procedures described by Gehrels et al., (2008) and Maksaev et al., (2009), who provided a detailed discussion of the sample-preparation techniques, analytical methods, and data analysis. The reported ages are based on 206Pb/238Pb ratios because they are better constrained foryoung rocks than the 276Pb/235Pb and 206Pb/207Pb ratios, which present significantly higher errors; all reported final ages and weighted mean ages have uncertainties at the two-sigma level.

4.1.2.   40Ar/39Ar dating

7 mica samples (biotite, muscovite, sericite) from the altered Dos Amigos and Tricolor por-phyries and 2 biotite samples from the Cachiyuyo Batholith were dated by the step-heating 40Ar/39Ar method. The 40Ar/39Ar ages are summarized in Table 2 and the analytical data are included in appendix B. The analytical work was performed at the Stanford University using the step-heating technique following procedures of Marsh et al., (1997), who provided a detailed discussion of the sample-preparation techniques, analytical methods, and data analysis. Plateaus were defined using the criteria of Dalrymple and Lamphere (1971) and Fleck et al., (1977), specifying the presence of at least three contiguous gas fractions that together represent more than 50 percent of the total 39Ar released from the sample and with apparent ages within error of each other. All 40Ar/39Ar plateau ages and weighted mean 40Ar/39Ar ages are reported with errors at the two-sigma level; besides, in order to avoid under-estimate analytical uncertainties, the errors have further been enhanced multiplying by (MSWD)1/2for those weighted mean ages with a mean square of weighted deviates higher than 2.


4.1.3.   Fission-track and (U-Th)/He dating

Apatite from one sample of the Dos Amigos porphyry (KP-14) was dated by fission-track chro-nology at the laboratory of Apatite and Zircon Inc. (Viola, Idaho, USA) using laser ablation ICP-MS to estimate the uranium concentrations of the apatite grains for which spontaneous fission tracks were counted (e.g., Hasebe et al., 2004; Donelick et al., 2005); a summary of analytical data are included in appendix C. The analysis included the measurement of the maximum fission-track etch-pit diameters oriented within 5o of the c axis of the apatite crystal (Dpar) in order to consider fission-track annealing variability among different apatite species in thermal history modeling (Carlson et al., 1999). Irradiation of the apatite sample with 252Cf was used to increase the amount of etched confined track for length measurement. The AFTSolve multi-kinetic inverse modelling program of apatite fission track data (Ketcham et al., 2000) was used to derive time-temperature history for the Dos Amigos porphyry from the apatite fission-track data. This program implements various laboratory calibrations of the behavior of fission tracks in apatite in response to heating and cooling histories, and calculates the range of thermal histories that are potentially consistent with the measured age and the measured frequeney distribution of confined track lengths.

Full details concerning these calibrations and the various uses of AFTsolve are given in Carlson et al., (1999), Donelick et al., (1999), Ketcham et al., (1999,2000). 20,000 random time-temperature paths are created by a Monte Cario scheme, and for each path the resulting f ission-track age and track length distribution are calculated, and the goodness-of-f it between calculated and measured data is evaluated by a Kolmogorov-Smirnov test. The program maps out the time-temperature regions that envelop all thermal histories with 'good' and 'acceptable' fit, corresponding to goodness-of-fit values from 0.5 to 1 and from 0.05 to 0.5, respectively.

Apatite from the same sample (KP-14) from the Dos Amigos porphyry was also dated by the (U-Th)/He method at Stanford University by argon laser heating for He extraction and at UC Santa Cruz by sector ICP-MS for U-Th determinations; an analytical uncertainty of 7 percent is estimated for the apatite analyses; the analytical data are included in appendix C. Replicate analyses yielded concordant ages and the final (U-Th)/He ages reported include an alpha-ejection correction that accounts for diffusion-domain-dependent loss of the daughter nuclide (after Farley et al., 1996 and Farley, 2002).

4.2.      Results

4.2.1.   U-Pb dating

Two samples from the potassic-altered, mineralized porphyry, collected at the bottom of the open pit in the Dos Amigos mine (KP-13, KP-14; Fig. 5), yielded U-Pb ages of 106.1±3.5 and 104.0±3.5 Ma, respectively. In addition, a sample from the Tricolor porphyry (KP-08) also potassic-altered yielded a U-Pb age of 108.5±3.4 Ma (Figs. 5 and 7). These ages are indistinguishable from each other, as they overlap within analytical error; they correspond to the Albian according to the International Stratigraphic Chart, 2008.


A sample of granodiorite from the Cachiyuyo Batholith immediately west of Tricolor (KP-25) yielded a U-Pb age of 99.6±1.8 Ma (Fig. 5) and a diorite sample from the same batholith, but collected 19 km to the southwest (KP-20) yielded an indistinguishable U-Pb age of 99.1±9 Ma (Fig. 8).


4.2.2.  40Ar/39Ar dating

Most of the age spectra obtained are irregular implying disturbance of the K-Ar isotopic system of the dated micas (Figs. 9 and 10). Only four age spectra define plateaus with at least 50 percent of the released argon and with apparent ages within error of each other (Table 2a). The samples from the Domeyko alteration zone yielded 40Ar/39Ar plateau ages for sericite and muscovite from 96.3±3.7 to 85.8±1.2Ma, and a 40Ar/39Ar plateau age of 94.8±0.9 Ma was obtained for biotite from the Cachiyuyo batholith (Table 2a; Fig. 9). Yet, the 40Ar/39Ar plateau ages of 96.3±3.7 and 96.1±1.0 Ma for muscovite and sericite from the altered porphyry stocks (KP-10, KP-16; Table 2a) are much younger than their respective U-Pb ages of 108.5±3.4 and 106.1±3.5 Ma, and the 40Ar/39Ar plateau age of 94.8±0.9 Ma obtained for biotite from the batholith (KP-20) is also younger than its U-Pb age of 99.1±1.9 Ma. Thus, these 40Ar/39Ar plateaus represent minimum cooling ages; in fact, some of the spectra (KP-10, KP-16) have relatively large errors of the apparent ages of individual steps and their respective inverse isochrons show initial 40Ar/36Ar ratios lower than the 295.5 valueof atmospheric argon, which is consistent with argon loss (Fig. 9). The youngest 40Ar/39Ar plateau age of 85.8±1.2 Ma for sericite from the Marisol tourmaline breccia probably ref lect argon loss as well.


The remaining five biotite samples show signi-ficant disparity in their apparent ages of individual degassing steps, even for selected portions of the respective age spectra (MSWD>2; Fig. 10). There-fore, their analytical uncertainty has been enhan-ced yielding weighted mean 40Ar/39Ar ages from 105.4±4.9 to 96.9±1.4 Ma (Table 2b). Despite of disturbance and imprecisión these ages for biotite overlap within error with the U-Pb dates that were obtained for the Domeyko alteration zone and the Cachiyuyo Batholith (Fig. 11).




 4.2.3.   Fission-track and(U-Th)/He thermochronology

Sample KP-14 from the Dos Amigos porphyry yielded a LA-ICP-MS apatite fission-track age of 59.8±9.8 Ma (±2σ). The apatite track length distribution is unimodal, relatively narrow and negatively skewed (Skewness=-1.66) with a mean track length of 13.66±0.17 µm and a standard deviation of 1.98 µm (Fig. 12). Its Dpar is 1.58 µm.


Duplicate (U-Th)/He ages of 44.7±3.7 and 44.0J=4.2 Ma were obtained on the same apatite sample from the Dos Amigos porphyry (KP-14), attesting to analytical reproducibility.

4.3.     Discussion

The U-Pb zircon ages are interpreted as crystallization ages for the intrusions considering that zircon has the highest known closure temperature for Pb diffusion, which exceeds 900°C for zircons of typical sizes (Cherniak and Watson, 2000 and references therein). Thus two thermal events have occurred related to the emplacement of intrusive bodies; the Dos Amigos and Tricolor porphyry stocks of the Domeyko alteration zone crystallized during the Albian (between 108.5±3.4 and 104.0±3.5 Ma), and the neighboring, unaltered Cachiyuyo Batholith crystallized later, during the Cenomanian between 99.6±1.8 and 99.1±1.9 Ma, thereby confirming geological relationships.

The 40Ar/39Ar data for hydrothermal micas indi-cate a minimum Late Cretaceous age for hydrothermal activity in the Domeyko alteration zone, but the disparity and/or imprecisión of the 40Ar/39Ar ages precludes a more accurate age determination. These micas may have been formed during cooling of the porphyry stocks and/or later, during the cooling stage of the neighboring Cachiyuyo Batholith (Fig. 11). Nevertheless, despite disturbance, most 40Ar/39Ar ages for alteration micas coincide within error limits of the U-Pb ages, especially those obtained for the batholith (Fig. 11), indicating that these 40Ar/39Ar ages record cooling of the Cachiyuyo Batholith. Thus, the thermal event related to the emplacement of the Cachiyuyo Batholith is inferred to have partially or completely reset the isotopic clock of the hydrothermal alteration micas in the adjacent Domeyko Alteration zone.

The crystallization ages obtained for the Dos Amigos porphyry of the Domeyko alteration zone are comparable with the whole rock K-Ar age of 104±3 Ma reportedby Reyes (1991) for sericitized porphyry of the Andacollo copper-gold porphyry deposit and with a U-Pb zircon age of 104. 0±3.3 Ma for the altered Culebrón porphyry stock located in the center of the Andacollo deposit (our unpublished data). These ages confirm that both deposits are part of the same regional mid-Cretaceous porphyry copper mineralization episode.

The apatite fission-track age of 59.8±9.8 Ma (±2σ) for the Dos Amigos porphyry is significantly younger than the U-Pb and 40Ar/39Ar ages of 104.0±3.5 Ma and 96.0±1.4 Ma obtained for this mineralized intrusión, respectively. In addition, the track length distribution (Fig. 12) is comparable to the typical track length distribution of 'undisturbed basement' (Gleadow et al., 1986; Green et al., 1989), which normally results from a progressive monotonic cooling through the temperature range of the apatite partial annealing zone (~125-60°C; Laslett et al., 1987; Reiners et al., 2005). It is apparent that cooling through the ~125-60°C temperature range occurred considerably later than the igneous and hydrothermal thermal events detected in the Domeyko alteration zone, which is consistent with the apatite fission-track age record of exhumation-cooling. Assuming a present-day temperature of 15°C, a model time-temperature path was generated from the fission-track data of the FT-14 apatite sample using the AFTSolve multi-kinetic inverse modeling software (Ketcham et al., 2000). According to this model the apatite sample started to accumulate tracks at 62.6Ü0.2 Ma and progressively cooled with time through the temperature range of the apatite partial annealing zone (APAZ: -125° to 60°C) during the Paleocene (Fig. 13). Therefore it is inferred that the Dos Amigos porphyry cooled through the -125-60°C temperature range during the Paleocene in response to exhumation.


The apatite (U-Th)/He age of 44.7±3.7 Ma provides further support to the above interpretation considering the even lower temperature range of the apatite He partial retention zone (~85-40°C; Wolf et al., 1998; Shuster et al., 2006). The apatite cooled through the ~85-40°C temperature range during the Eocene, which is coherent with the modeled cooling path from the apatite fission-track data for the Dos Amigos porphyry (Fig. 13). Thus the combined fission-track and (U-Th)/He thermochronological data indicate that the Dos Amigos porphyry was exhumed during the Paleocene-Eocene period. The exhumation during this time probably was an effect of denudation, which in turn could be consequence of surface uplift and erosión, resulting from major tectonic compressive events in northern Chile, such as the 'K-T' tectonic event near the Cretaceous-Tertiary boundary in the región (Cornejo et al., 2003; Charrier et al., 2007) and the important Eocene Incaic compressive tectonism that affected northern Chile and Pera (Charrier and Vicente. 1972; Maksaev, 1978, 1979).

The exhumation cooling of the Dos Amigos porphyry stock through the apatite He partial retention zone (~85-40°C; Wolf et al., 1998) at 44.7±3.7 Ma additionally signifies that a máximum of some 2 km of rock cover may have been removed during the last 44 Myr, accepting a geothermal gradient of 30°C/km. Although the actual paleogeothermal gradient is uncertain, this implies a very low mean exhumation rate since the mid-Eocene (<0.05 mm/yr). Furthermore, the apatite (U-Th)/He age of 44.7±3.7 Ma also provides a máximum age for the formation of the supergene enrichment blanket at Dos Amigos, because the porphyry had to be exhumed to expose sulfides to the effects of oxidative weathering and chalcocite precipitation within the zone of cool groundwater at the time.

The chalcocite blanket at Dos Amigos, located between 740 and 800 m elevation probably developed beneath a low hill within the Miocene Atacama pediplain at roughly the same time as the enrichment at Andacollo (e.g., Sillitoe, 2005), considering that terrace relies of the Miocene Atacama Gravels partly surround the Domeyko alteration zone and slope gently westwards from 950 m to 780 m above sea level.

5.       Conclusions

The tonalitic to granodioritic porphyry stocks of Dos Amigos and Tricolor in the Domeyko alteration zone crystallized during the Albian (U-Pb ages from 108.5±3.4 to 104.0±3.5 Ma). Hydrothermal alteration of the types: potassic, sericitic, kaolinite-illite and propylitic are zoned around these stocks, and stockwork copper mineralization is fundamentally restricted to these porphyries. Therefore the data confirm that these porphyry copper centers are part of the regional, mid-Cretaceous porphyry copper mineralization episode recognized along the eastern part of the Coastal Cordillera of northern Chile, and with identical U-Pb ages as the Culebrón porphyry of the Andacollo copper-gold deposit.

The Cachiyuyo Batholith that marks the western border of the Domeyko alteration zone crystallized later during the Cenomanian (U-Pb ages 99.1±1.9 and 99.6±1.8 Ma). Mostof 40Ar/39Arages obtained for hydrothermal biotite and sericite from the Tricolor and Dos Amigos porphyry centers overlap with the U-Pb ages obtained for the batholith. They establish a minimum Late Cretaceous age for hydrothermal activity, even though it is inferred that they reflect the effect of the thermal overprint imposed by post-mineralization batholith emplacement.

The apatite fission-track and (U-Th)/He thermochronological data are compatible with exhumation-cooling of the Dos Amigos porphyry during the Paleocene-Eocene, probably related to denudation resulting from uplift imposed by the K-T and Incaic compressive tectonism. Furthermore, the apatite (U-Th)/He age of 44.7±3.7 Ma provides a máximum age for the supergene enrichment processes that formed the chalcocite blanket of this porphyry system, but also implies a very low mean exhumation rate of the porphyry since the late Eocene.

Acknowledgments

Conicyt, Chile, through Fondecyt Grant 1040492 to V Maksaev and F. Munizaga, provided financial support for this study. The investigation of the Domeyko zone was part of the M.Sc. Thesis of A. Almonacid. The CEMTN mining company granted access to the Dos Amigos mine; we are particularly thankful to D. Ibaceta, Mine Manager; Mr. L. Castro, Mine Administrator, and to Mr. A. Álvarez, Mine Supervisor. Reviews by J. Perelló, RJ. Pankhurst, and R.H. Sillitoe contributed to improve this paper; a previous versión also benefited from evaluations by K. Hickey and an anonymous reviewer

 

Notes

1            Kents, P. 1962. Domeyko hydrothermal development, Departaent Chañaral, Province of Atacama (unpublished report), United Nations Special Fund: 6 p.

 

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Manuscript received: March 12, 2009; revised/accepted: October 16, 2009.

©  2011  Servicio Nacional de Geología y Minería