DISCUSSION. GEOLOGY, MINERALIZATION, ALTERATION, AND STRUCTURAL EVOLUTION OF THE EL TENIENTE PORPHYRY Cu-Mo DEPOSIT A DISCUSSION

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1 2007 Society of Economic Geologists, Inc. Economic Geology, v. 102, pp DISCUSSION GEOLOGY, MINERALIZATION, ALTERATION, AND STRUCTURAL EVOLUTION OF THE EL TENIENTE PORPHYRY Cu-Mo DEPOSIT A DISCUSSION M. ALEXANDRA SKEWES AND CHARLES R. STERN Department of Geological Sciences, University of Colorado, Boulder, Colorado Sir: In their paper concerning the geology, mineralization, alteration, and structural evolution of the El Teniente Cu-Mo deposit in Chile, Cannell et al. (2005) concluded that El Teniente, despite its anomalously large size, is a typical porphyry Cu-Mo deposit with regard to its alteration and sulfide assemblage zonation and the genetic association of mineralization with dacite intrusions. We do not agree that the dacite porphyries were the causative intrusions for copper mineralization. Instead, most of the mineralization was emplaced in conjunction with the formation of multiple hydrothermal breccia pipes and their associated veins that were derived from a large, deep magma chamber (Skewes et al., 2002, 2005; Stern and Skewes, 2005), and this deposit should be classified as a megabreccia rather than a porphyry deposit. At El Teniente most Cu mineralization (80%) is intimately associated with biotitized mafic intrusive rocks (Camus, 1975; Cuadra, 1986; A. Arévalo, R. Floody, and A. Olivares, unpub. report for CODELCO-Chile, 1998, 76 p.). Cu mineralization occurs disseminated in the biotitized mafic rocks, in different generations of highly visible anhydrite and quartz veins, in less visible but abundant biotite veins, and also in the matrices of the multiple hydrothermal breccia complexes that occur in the deposit. A significant proportion of high-grade (>1%; Fig. 1) hypogene Cu mineralization at El Teniente was emplaced both within and in veins around these multiple hydrothermal breccia complexes in direct genetic association with their formation. Alteration and sulfide assemblages in these multiple breccia complexes are similar to those in typical porphyry systems, but at El Teniente these assemblages and mineralization grades are not zoned around felsic intrusions as in a typical porphyry system. Also, the timing of Cu mineralization (Fig. 2) is not correlated with the age of felsic intrusions as suggested by Cannell et al. (2005). Felsic intrusions occur at El Teniente, but these intrusions are in general low grade or barren, too small to have been the source of the enormous amount of Cu in the deposit, and are spatially associated with high-grade Cu mineralization only when they intrude into or are cut by mineralized hydrothermal breccias. Cannell et al. (2005) base their interpretation on logging of 20 km of drill core and examination of approximately 200 thin sections. Our conclusions are based on a synthesis of geologic studies since 1986 of hundreds of kilometers of new exposure in both tunnels and drill holes in the hypogene zone. At El Teniente core logging, assays, and mapping are Corresponding author: , Skewes@colorado.edu done systematically and daily by mine geologists in order to maintain and expand mine operations. Cannell et al. do not take into account some of the most significant aspects of the work done in the hypogene zone during the last two decades by mine geologists at El Teniente. Their conclusions, which are identical to those presented 30 years ago by Camus (1975), reflect this. FIG. 1. Copper grades between levels Teniente 4 and 5 in the El Teniente mine (Skewes et al., 2002, 2005). Copper grades surrounding the Teniente Dacite Porphyry dike north of the Braden pipe are enhanced by supergene enrichment (diagonally lined area) that penetrated below level Teniente 5 in this area of the mine. Grades in the central rock-flour breccia of the Braden pipe are generally <0.5 percent but 0.75 to >1.5 percent in the tourmalinerich Marginal breccia rim of this pipe. Areas of high-grade (>1%) copper east and northeast of the pipe are totally within the hypogene zone and correspond to the location of multiple breccias. 1165

2 1166 DISCUSSIONS FIG. 2. Summary of U-Pb ages (solid diamonds) for igneous rocks and Re- Os ages (solid circles) of molybdenite from Maksaev et al. (2004) and Cannell et al. (2005). Crosses are K-Ar ages for the Sewell Tonalite and La Huifa pluton from Cuadra (1986), and K-Ar and Ar-Ar dates on sericite from the Braden breccia pipe (Cuadra, 1986; Maksaev et al., 2004). No Re-Os mineralization ages obtained to date are older than 6.3 Ma, although the Sewell Tonalite, which is >6.46 Ma, cuts preexisting mineralized veins in Teniente Mafic Complex rocks. The 6.3, 5.89, and 5.6 Ma Re-Os ages of molybdenite are correlated with the emplacement of specific breccia complexes because the molybdenite was collected from within these specific breccias, including the Porphyry A igneous breccia. No Re-Os ages correspond to the 6.1 Ma age of intrusion of the Central and Northern Diorites or the 5.3 Ma age of intrusion of the Teniente Dacite Porphyry, as indicated by the? symbol. Most of the Re-Os ages fall in the range 5.0 to 4.7 Ma, which does correlate with the age of the barren latite ring dikes around the Braden pipe, but also overlaps the age determined for the emplacement of the Braden breccia pipe itself, which is much larger and more copper-rich unit than the ring dikes. The figure illustrates that four of the five Re-Os age intervals correlate directly with breccia emplacement, and neither the Northern or Central Diorites nor the Teniente Dacite Porphyry are associated with mineralization events. We detail below some of the main differences between the description and interpretation of the genesis of the El Teniente Cu-Mo deposit presented in Cannell et al. (2005), the data of Maksaev et al. (2004), and our own interpretation. In their paper, Cannell et al. (2005) use the outdated map of hypogene mineralization (see their fig. 3) presented by Camus (1975), which divides Cu grades only between more or less than 0.5 percent Cu. The hypogene ore distribution has important implications for the understanding of the genesis of the deposit. However, the map used by Cannell et al. (2005) was produced over three decades ago, when only the supergene zones were mined, and little was known of the deeper hypogene zones where most of the copper in the deposit is actually concentrated. In 1975, less than 96 km of core, mostly from within the supergene zone, was available, and tunnels within the hypogene zone were limited in extent. Mining operations began in the hypogene zone in During the last two decades many hundreds of new tunnels have been constructed and well over 500 km of core drilled in the hypogene zone. Ore-grade determinations for these new exposures, done in conjunction with daily mapping and core-logging, have been updated in a map of Cu grade (Fig. 1) synthesized by mine geologists and published by Skewes et al. (2002, 2005). Failure to consider this new information and what is currently known about the spatial distribution of Cu ore in this deposit is a key limitation of the paper by Cannell et al. (2005). The new exposures developed over the last 20 years of mine development show that hypogene copper is concentrated around multiple high-grade (>1%) centers within biotitized mafic intrusions (Fig. 1). These high-grade centers correspond to hydrothermal breccia complexes, with copper contained both in the matrices of these breccias and veins surrounding these complexes. Arredondo (1994), in her study of four mineralogically different breccia complexes, notes that the matrix of these breccias is generally comprised of 5 to 20 percent sulfides. These multiple copper-rich breccia complexes are clearly not zoned around the Teniente Dacite Porphyry, which in the past was considered the productive felsic porphyry intrusion in the Teniente deposit (Camus, 1975; Cuadra, 1986). The multiple copper-rich breccias occur along two main trends, northeast and northwest, located east and south of the Teniente Dacite Porphyry. The most prominent of the giant breccia complexes at El Teniente is the Braden pipe. Although Cannell et al. refer to the Braden pipe as largely barren, it contains over 1 million tonnes (Mt) of fine copper at grades of >0.7 percent Cu, and a total of >5 Mt of Cu at an average grade of 0.4 percent, with an additional >1 Mt of high-grade (>1%) Cu in the Marginal breccia rim to this pipe (R. Floody, unpub, report for CODELCO-Chile, 2000, 90 p.). It is >600 m in diameter at the depth of the deepest exploration drill holes, and its total contained copper is still unknown. Furthermore, an unknown amount of copper and molybdenum occurs around this breccia pipe in radial and concentric veins that formed in association with its emplacement. The pipe truncated the southern end of the Teniente Dacite Porphyry and postdates the intrusion of this felsic porphyry by over 500,000 years (Fig. 2; Maksaev et al., 2004). It is therefore clearly unrelated to this porphyry. Crosscutting lithological relationships and chronological data reveal that the multiple breccia complexes at EI Teniente developed over an extended >2 m.y. period (Fig. 2; Skewes et al., 2002, 2005). The current distribution of copper sulfides within the El Teniente deposit is the product of superposition of multiple mineralization and brecciation events and intrusion of felsic plutons on an already Cu-mineralized and biotitized mafic intrusive complex. The spatial distribution of mining activity at El Teniente directly reflects the spatial distribution of these multiple copper-rich breccia complexes. Specific extraction centers, from which Cannell et al. (2005) present vein orientation data collected by El Teniente geologists, are shown in their figure 14. These include the Mina Esmeralda (Morales, 1997), Sub-6 mine (Arredondo, 1994), Ten-4 south mine, and the Regimiento mine. Each of these mining centers is located around a breccia complex, as can be seen by comparing the location of these centers with the location of hydrothermal breccia complexes in figure 3 of Cannell et al. (2005) and figures 4 and 6 in Skewes et al. (2002, 2005). Some of these breccia complexes are intruded by younger, barren, or weakly mineralized felsic porphyry pipes. Each of these breccia complexes, as well as the central Braden pipe, has at least > Mt of hypogene Cu. In each of these mining areas, the vertical extent of highgrade copper mineralization corresponds to the vertical extent of the breccia complexes, and neither the roots of these breccias nor the depth to which high-grade copper extends has yet been determined /98/000/ $

3 DISCUSSIONS 1167 Biotitization is the most abundant and widespread alteration type at El Teniente, occupying from 20 to more than 50 percent of volume of the altered mafic rocks (Camus, 1975; A. Arévalo, R. Floody, and A. Olivares, unpub. report for CODELCO-Chile, 1998, 76 p.). Biotitization is significant because it is directly associated with copper mineralization, particularly chalcopyrite, which is the most important sulfide in the deposit. Eighty percent of copper is associated with biotitized mafic rocks at El Teniente (Howell and Molloy, 1960; Camus, 1975), and biotitization was the background alteration upon which all later alteration and mineralization events were superimposed (Skewes et al., 2002, 2005). The paper by Cannell et al. (2005) focuses on late alteration and mineralization events, veins and structures, and late barren felsic intrusions that are superimposed on the biotite-altered copper-rich mafic rocks that host most of the copper in the deposit. Detailed mapping by El Teniente geologists demonstrates that many Cu-rich veins that occur in the highly mineralized Teniente Mafic Complex (see fig.12b of Skewes et al., 2002) are absent in the late felsic intrusions (A. Brzovic and D. Benado, 2003, unpub. report for CODELCO-Chile, 119 p.). Biotitization at El Teniente is most intense around biotite breccias, but these breccias and their associated biotite veins are difficult to identify in the dark El Teniente Mafic Complex host rocks because of the lack of color contrast and the texturally destructive nature of the alteration. The intensity of biotite alteration led to the misinterpretation of the host rock at El Teniente as extrusive andesite for over three decades (Skewes et al., 2002, 2005). However, biotite breccias are readily identifiable in felsic intrusions. One such biotite-anhydrite breccia complex occurs within the Sewell Tonalite (see fig. 12A of Skewes et al., 2002; Seguel et al., 2006). Seguel et al. (2006) note that the highest grades of copper anywhere in the Sewell Tonalite occur within this specific biotite-anhydrite breccia complex. This complex has been dated at 6.3 Ma, based on Re-Os ages of molybdenite (Fig. 2; Maksaev et al., 2004), and was found to be older than any of the felsic porphyries in the deposit by over 200,000 years. Cannell et al. (2005) present nine new Re-Os ages of molybdenite in samples that are not fully described or located. These ages include one of 5.89 Ma and eight that fall in the range 5.0 to 4.7 Ma. Nine other Re-Os ages, determined by Maksaev et al. (2004), span a somewhat larger range, from 6.3 to 4.42 Ma, and fall within four clusters at 6.31 Ma (one date), 5.60 Ma (one date), 5.0 to 4.78 Ma (four dates), and at 4.42 Ma (three dates). Comparing all 18 Re-Os mineralization ages with U-Pb ages for zircons from five felsic intrusions in El Teniente presented by Maksaev et al. (2004), Cannell et al. (2005) conclude that the timing of Mo mineralization, and by extrapolation Cu mineralization, is correlated with the timing of felsic intrusions. We argue that there is a better association between the timing of mineralization and emplacement of specific breccia complexes than the timing of felsic intrusions. The five felsic intrusions dated by U-Pb ages of zircons by Maksaev et al. (2004) include the Sewell Tonalite, Northern and Central Quartz Diorites, Porphyry A, Teniente Dacite Porphyry dike, and latite (or dacite) ring dikes concentric to the Braden pipe. Only the age of Porphyry A, which is in fact an igneous breccia and not a felsic porphyry (Fig. 3; Stern et al., 2006), and the latite dikes, which are contemporaneous with the much larger Braden breccia pipe, correspond to Re- Os molybdenite ages (Fig. 2). No Re-Os molybdenite ages correspond to the age of intrusion of the Sewell Tonalite, the Central and Northern Diorites, or the Teniente Dacite Porphyry dike, which historically has been considered the causative pluton for the deposit. Also, no U-Pb ages of felsic intrusions correspond to the 6.31, 5.89, 5.6, or 4.42 Ma Re-Os ages determined by Maksaev et al. (2004) and Cannell et al. (2005) (Fig. 2). Only the 5.0 to 4.78 Ma Re-Os ages correspond within ±200,000 to the age of any felsic porphyry the very small volume and essentially barren latite ring dikes around the Braden pipe. Maksaev et al. (2004) dated the Sewell Tonalite, the largest felsic intrusion in the deposit, in a strongly faulted, altered, and mineralized sample (TT-I0l) along the contact between this intrusion and the older Teniente Mafic Complex rocks. FIG. 3. Photograph of the Porphyry A microdiorite igneous breccia with clasts of Sewell Tonalite and Teniente Mafic Complex rocks in an igneous-textured matrix which contains primary igneous anhydrite (Stern et al., 2006). Porphyry A sample TT150 dated by Maksaev et al., (2004) contains a greater abundance of clasts of the Sewell Tonalite, and we interpret the bimodal U-Pb ages they obtained to reflect two different groups of zircons; the older one (6.46 Ma) from the Sewell Tonalite clasts and the younger one (5.67 Ma) from the igneous matrix of the Porphyry A breccia /98/000/ $

4 1168 DISCUSSIONS Zircons from this sample define bimodal ages of 6.15 and 5.59 Ma. These ages are both much younger than previous K-Ar ages for the Sewell Tonalite (7.4 and 7.1 Ma; Fig. 2; Cuadra, 1986). They are also much younger than a 40 Ar/ 39 Ar step heating age for the La Huifa pluton (plateau age of 6.97 Ma and total gas age of 7.05 Ma, both in the range of the K-Ar age of 7.0 Ma obtained for this pluton by Cuadra, 1986; Fig. 2), a pluton petrologically similar to the Sewell Tonalite which outcrops 2 km north of the deposit (Reich, 2001). Moreover, these ages are also both younger than an Re-Os age of 6.3 Ma for molybdenite in a biotite breccia that cuts the Sewell Tonalite (see fig. 12A in Skewes et al., 2002; Seguel et al., 2006). Therefore, these ages are clearly younger than the true age of the Sewell Tonalite, which must be >6.3 Ma, and thus older than the oldest of the Re-Os ages obtained in the deposit. Maksaev et al. (2004) also dated zircons from the Porphyry A igneous breccia containing abundant clasts of Sewell Tonalite, obtained in 2 m of core from 384 m depth in drill hole DDH-1337 (sample TT-150). This igneous breccia intrudes along the contact between the Sewell Tonalite and the biotitized rocks of the Teniente Mafic Complex that host the bulk of the mineralization in the deposit, and contains clasts of both these rocks (Fig. 3). The zircons from the sample collected by Maksaev et al. (2004) define a bimodal population of ages of 6.46 and 5.67 Ma. We interpret the older age to reflect zircons derived from the Sewell Tonalite clasts in this breccia, and suggest this is at best a minimum age for this pluton. Mapping and petrological studies at El Teniente show that at least one widespread event of biotitization and mineralization predates emplacement of the Sewell Tonalite and that this >6.46 Ma felsic intrusion cuts biotitized Teniente Mafic Complex rocks and cuts mineralized biotite veins. This implies that the 18 Re-Os ages for molybdenite obtained to date in the deposit only records the last 1.9 m.y. of mineralization, from 6.3 to 4.4 Ma, and the temporal extent of mineralization was in fact greater. This may reflect the fact that the early widespread mineralization associated with biotitization of the Teniente Mafic Complex generally does not result in the deposition of considerable molybdenite. We consider the younger 5.67 Ma age, determined by Maksaev et al. (2004) for Porphyry A, to be that of zircons crystallized in the igneous matrix of this breccia, which therefore date its emplacement. Maksaev et al. (2004) also obtained an Re-Os age of 5.6 Ma for molybdenite (sample tt-mo-l) from the Porphyry A igneous breccia complex, and Cannell et al. obtained an Re-Os age of 5.89 Ma from the larger hydrothermal anhydrite breccia that Porphyry A intrudes. They attribute this mineralization age to the formation of the Porphyry A breccia, which they refer to as Gray porphyry, and for which they determine a basaltic composition (51 wt % SiO 2 but with 11 wt % volatile content LOI). However, this composition, our petrologic work (Stern et al., 2006), and mapping by Teniente geologists indicate that Porphyry A is not a felsic porphyry, but part of a ~5.7 Ma mineralized breccia complex. Maksaev et al. (2004) obtained a U-Pb age of 6.11 Ma for zircons from the Northern Quartz Diorite (sample TT-I02) and a very similar overall average age of 6.08 Ma for the Central Quartz Diorite (sample TT-90). However, they suggest that the zircons from this latter sample yield a skewed distribution, which can be separated into two populations, one with an age of 6.28 Ma and one with an age of 5.50 Ma. We consider this interpretation of the data to be incorrect, and argue that they define a simple unimodal peak at 6.08 Ma (Fig. 4). The essentially similar 6.1 Ma ages of these two quartz diorites does not correspond to any Re-Os ages for molybdenite in the El Teniente deposit (Fig. 2), despite the fact that these two felsic intrusions occur in the center of Cu-rich hydrothermal breccia complexes. Maksaev et al. (2004) also obtained an age of 5.28 Ma for zircons from the Teniente Dacite Porphyry dike, which has frequently been cited as the causative pluton for mineralization in the deposit. However, not a single Re-Os age for molybdenites within the range ±300,000 years of 5.28 Ma was found among the 18 samples dated (Fig. 2), many of which were taken from directly along the border of this pluton (fig. 6 in Maksaev et al., 2004). As noted by Cannell et al. (2005), most of these Re-Os ages cluster around 5.0 to 4.8 Ma, >300,000 younger than the age of the Teniente Dacite Porphyry. This is consistent with the geologic observations that the Teniente Dacite Porphyry cuts (see fig. 12 in Skewes et al., 2005) and is cut by mineralized breccias and veins. Cannell et al. (2005, p. 1000) explain this by stating that there may be an as yet undated dacite phase that FIG. 4. Distribution of U-Pb ages of the 20 zircons from the Central Quartz Diorite Porphyry sample TT90 reproduced from Maksaev et al. (2004). Maksaev et al. (2004) considered this to be a bimodal distribution of two different ages; one that averages 6.28 Ma (the light gray shaded points) and one that averages 5.50 Ma (the dark gray shaded points). These two ages are close to two Re-Os ages (6.3 and 5.6 Ma) they obtain from molybdenite in the deposit, although the molybdenite samples with these ages are from within two hydrothermal breccia complexes that are not located anywhere near the Central Quartz Diorite porphyry intrusion. We see no evidence for a bimodal distribution of ages and consider the age of this porphyry to be 6.08 Ma, based on the average of the unimodal distribution of the 20 individual zircon ages, which all overlap within the analytical uncertainty. There are no Re-Os ages for any molybdenite in the deposit that are within ±200,000 years of this 6.08 Ma age of this felsic porphyry /98/000/ $

5 DISCUSSIONS 1169 temporarily correlates with these 5.0 Ma ages. On this basis, they conclude that mineralization at El Teniente was sourced directly from felsic intrusions. However, this conclusion is not supported by the available data and, in fact, contradicts the available information. Maksaev et al. (2004) obtained an age of 4.82 Ma for zircons from latite ring dikes concentric to the Braden breccia pipe. This age correlates with a number of the Re-Os ages of molybdenite that fall in the range 4.7 to 4.8 Ma (Fig. 2). However, these very small volume, essentially barren, dikes are contemporaneous with the much larger Braden breccia pipe (Maksaev et al., 2004). It is clear from the structural information summarized by Cannell et al. (2005) that a significant proportion of mineralized veins in the deposit are distributed both radially and concentrically to the Braden breccia pipe, and genetically related to it, not to the minor concentric latite dikes, which are highly altered but Cu poor. The three oldest Re-Os ages determined by Maksaev et al. (2004) and Cannell et al. (2005), 6.31, 5.89, and 5.60 Ma, are for molybdenite sampled directly from the matrices of mineralized breccia complexes (Fig. 2). The 6.31 Ma age is for a sample from a biotite breccia cutting the Sewell Tonalite east of the Braden pipe (fig. 12A in Skewes et al., 2002; Seguel et al., 2006), and both the 5.89 and the 5.60 Ma ages are for molybdenite from a hydrothermal anhydrite breccia complex associated with the Porphyry A igneous breccia (Stern et al., 2006). The greatest concentration of Re-Os ages, which include 12 determinations between 5.0 to 4.7 Ma, are from a variety of samples distributed around the Braden breccia pipe that formed in this same age range. Therefore, overall, there is a clear spatial and temporal relation between three of the Re-Os mineralization age intervals and the age of formation of specific breccia complexes (Fig. 2). Cannell et al. (2005) state that most of the copper in El Teniente was emplaced contemporaneously with intrusion of the Teniente Dacite Porphyry dike and other dacite pipes between 5.9 and 4.9 Ma. However, in detail this is not the case. The dacite pipes (Northern and Central Diorites) were emplaced at 6.1 Ma. The 5.67 Ma Porphyry A is not a felsic porphyry, but an igneous breccia. And, although Re-Os isotope dating of Mo mineralization has been overly concentrated in and around the Teniente Dacite Porphyry intrusion, not a single mineralization age obtained is within ±300,000 years of the age of this intrusion. The 4.82 Ma barren latite ring dikes correspond in age to the majority of the Re-Os molybdenite ages obtained by both Cannell et al. (2005) and Maksaev et al. (2004), but this is also the age of formation of the Braden breccia pipe, a much larger and more copper-rich lithological unit in the deposit. Thus, there is no correlation between the age of mineralization and the intrusion of felsic porphyries at El Teniente. Both the Teniente Dacite Porphyry and the Northern and Central Diorites (the dacite pipes of Cannell et al., 2005) cut mineralized mafic rocks (see fig.12a in Skewes et al., 2005) and truncate sulfide-rich veins (see fig. 12B in Skewes et al., 2002). Thus, a significant amount of mineralization predates the intrusion of the Northern and Central Diorites at 6.1 Ma and the Teniente Dacite Porphyry at 5.3 Ma. Maksaev et al. (2004) have determined one 6.3 Ma Re-Os age from a biotite breccia complex, which was not considered by Cannell et al. (2005), but very few Re-Os dates have been attempted in the mafic rocks that host 80 percent of the mineralization, except along the margins of the felsic intrusions. We suggest that the available Re-Os ages date only the later stages of mineralization at El Teniente. We agree with the conclusion of Cannell et al. (2005) that the formation of the deposit was associated with the intrusion of a large, deep magma chamber that is interpreted to be the source of the dacites, the Braden pipe, and ultimately, the copper and molybdenum mineralization. The amount of copper in the deposit (> Mt) implies a source of magma >600 km 3 (Skewes and Stern, 1995; Cloos, 2001; Richards, 2003; Stern and Skewes, 2005). We disagree about the process by which copper and molybdenum were transferred from the magma in the large magma chamber below the deposit into the host rocks where it is now mined. Cannell et al. (2005) suggest that there was an intimate spatial and temporal association between all stages of mineralization and latest Miocene to early Pliocene felsic intrusions at Teniente and that mineralization was associated with the intrusion of the felsic porphyries into the mafic host rocks of the deposit. However, the geologic and geochronologic data (Fig. 2) indicate clearly that in detail all stages of mineralization were not intimately associated with felsic intrusions, and that mineralization was not emplaced by the intrusion of the small volume of late, barren felsic porphyries that occur in the deposit. Furthermore, we consider the volume of late porphyries in the El Teniente deposit to be far too small to have been the source of the metals in this deposit (Stern and Skewes, 2005), and these porphyries are in general barren except where cut by younger hydrothermal breccias. Instead, there is an intimate spatial and temporal association of different stages of mineralization at El Teniente with the emplacement of multiple hydrothermal breccia complexes. We interpret these breccia pipes to have been generated by the exsolution of hot, saline, metal-rich magmatic fluids from the roof of a deep magma chamber (Skewes et al., 2002, 2005; Stern and Skewes, 2005). The roots of these breccias and the remnants of the large productive magma chamber below the deposit occur at a depth still below the deepest exploration drill holes in the deposit. For the reasons detailed above, we consider El Teniente to be a megabreccia deposit, not a typical porphyry deposit (Skewes et al., 2002, 2005). Other giant Miocene and Pliocene Cu deposits in central Chile, such as Pelambres (Atkinson et al., 1996) and Los Bronces-Río Blanco (Warnaars et al., 1985; Serrano et al., 1996; Vargas et al., 1999; Skewes et al., 2003; Frikken et al., 2005) also contain most of their mineralization in multiple giant breccia pipes (Skewes and Stern, 1995). Our classification of El Teniente as a megabreccia deposit reflects not only the presence of multiple metal-rich breccias in the deposit, but, more fundamentally, the important genetic role of breccia emplacement in making this such a large deposit. El Teniente is not merely larger, but also different, and formed differently, from typical porphyry Cu-Mo deposits. Hydrothermal breccias occur in many typical porphyry deposits, but their dominant role as a mechanism of emplacement of mineralization in El Teniente and the other giant Miocene and Pliocene Cu deposits in central Chile is /98/000/ $

6 1170 DISCUSSIONS one of the most significant genetic processes in these giant deposits. This is because the generation of hydrothermal breccias by the exsolution of saline, metal-rich magmatic fluids from the roof of a large, deep (>6 km) magma chamber is considered to be a more efficient mechanism for transferring large quantities of metals from magmas than exsolution of magmatic fluids from small volume, shallow (<3 km) felsic porphyry intrusions, as pressure increases the solubility of metal and sulfur in saline fluids (Luhr, 1990; Newton and Manning, 2005). Finally, igneous processes involving recharge by mafic magmas into the base of a large, deep chamber may result in higher concentrations of saline fluids and metals in the roof of this chamber than can normally be dissolved in felsic magmas (Streck and Dilles, 1998; Hattori and Keith, 2001). 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