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Corresponding author: e-mail, srive003@codelco.cl

*
Deceased 8 April 2004.
**
Present address: Codelco-Chile, Superintendencia de Geología, El Salvador, Chile.

Abstract

The porphyry copper systems of the El Salvador region, represented mainly by El Salvador, Potrerillos, Exploradora, Sierra Jardín, and Coya, are located in the southern part of the middle Eocene to early Oligocene porphyry copper belt of northern Chile. They have distinctively higher gold grades (between 0.1 and 0.5 g/t) and slightly lower molybdenum grades (<100 ppm) than the systems located in the northern parts of the belt. The El Salvador and Potrerillos deposits each contains resources of >600 million metric tons (Mt) at >0.6 percent Cu and 0.1 to 0.2 g/t Au. The Exploradora porphyry copper-gold prospect contains geologic resources of ~100 Mt at 0.3 percent Cu and 0.2 g/t Au, with an overlying leached capping enriched in gold, averaging ~0.5 g/t. The Sierra Jardín and Coya prospects show the lowest Cu contents (0.1–0.2 %), but the Au tenor is in the 0.1- to 0.5-g/t range.

The deposits are related to discrete magmatic pulses emplaced in different lithotectonic environments, including the borders of Paleocene volcanic structures (El Salvador, Sierra Jardín; 45–40 Ma) and dilational jogs and reverse and transfer faults (Exploradora, Potrerillos, and Coya; 37–31 Ma). Host rocks are mainly volcanic and marine sedimentary rocks of Jurassic to Cretaceous age. The most remarkable feature of the deposits is their relationship to multiphase, syntectonic tonalite to granodiorite intrusions. There is strong superimposition (telescoping) of intrusion and alteration-mineralization phases, including early, magnetite-rich, potassic alteration-mineralization, moderately to weakly developed sericitic alteration, and late-stage overprinted advanced argillic alteration. Postmineral phreatomagmatic activity is characterized by pebble dikes and/or diatreme breccias.

The appreciable number of gold-rich porphyries (as opposed to their gold-poor counterparts) discovered during the past ten years in the Chilean Andes (e.g., La Fortuna, Cerro Casale, Esperanza) implies that geologic conditions that favor their development are more widespread than previously considered. Several key characteristics of gold-rich porphyry copper deposits in the El Salvador region can be used for exploration purposes and open new ground to prospecting.

Introduction

The El Salvador region occupies an important segment of the middle Eocene to early Oligocene porphyry copper belt of northern Chile between latitudes 25°30′ and 27°00′ S (Fig. 1) and is centered on the well-described El Salvador porphyry copper deposit (Gustafson and Hunt, 1975; Gustafson and Quiroga, 1995; Gustafson et al., 2001). Exploration and evaluation of several porphyry copper prospects in the region (Fig. 2) revealed above-average gold grades (>0.1 g/t Au). These relatively high grades contrast with the gold-poor nature of the typical Chilean molybdenum-rich porphyry copper deposit (>0.02% Mo; Camus, 2003). Although with substantially lower gold contents than those of the nearby gold-rich porphyry systems of the Maricunga belt (Vila and Sillitoe, 1991; King, 1992; Muntean and Einaudi, 2000, 2001; Camus, 2003), the porphyry centers of the El Salvador region display many geologic characteristics similar to the gold-rich (molybdenum-poor) systems described from elsewhere in the world (Sillitoe, 1998, 2000).

Fig. 1.

Location of the porphyry copper and copper-gold deposits discussed in the text. Numbers in parentheses correspond to the alteration-mineralization age in Ma; compiled from Cornejo et al. (1997), Camus (2003), and Perelló et al. (2004).

Fig. 1.

Location of the porphyry copper and copper-gold deposits discussed in the text. Numbers in parentheses correspond to the alteration-mineralization age in Ma; compiled from Cornejo et al. (1997), Camus (2003), and Perelló et al. (2004).

Fig. 2.

Summary structural and intrusive framework of the El Salvador region, showing the location and age of the porphyry copper-gold deposits described. Note the different structural environment for the deposits west of the Sierra Castillo fault compared to those located along east side of fault. Modified from Cornejo et al. (1993) and Cornejo and Mpodozis (1996). María Delia is an immature low sulfide porphyry system prospect lacking gold.

Fig. 2.

Summary structural and intrusive framework of the El Salvador region, showing the location and age of the porphyry copper-gold deposits described. Note the different structural environment for the deposits west of the Sierra Castillo fault compared to those located along east side of fault. Modified from Cornejo et al. (1993) and Cornejo and Mpodozis (1996). María Delia is an immature low sulfide porphyry system prospect lacking gold.

In this paper, the term gold rich is employed in the context of the Andean porphyry copper province. For detailed resources and copper-, molybdenum-, and gold-grade data for several Andean porphyry copper deposits, the reader is referred to Camus (2003). The porphyry copper deposits of the El Salvador region contain elevated gold grades when compared to other more molybdenum-rich deposits from the same province but obviously lower than other porphyry copper centers in the Southwest Pacific region and elsewhere, including the nearby gold-only porphyries of the Maricunga belt (Sillitoe, 1998, 2000).

Future copper mining faces a high degree of competition and sustained decreases in grade, thereby challenging both technology and ore quality. In this context, ecological or metallurgically simple deposits, as well as those with additional credits that increase their ore value, will be favored. In this respect, porphyry deposits may provide the alternatives of clean oxidized ore, amenable to leaching, or sulfide ore containing the classic molybdenum credit and the more recently appreciated precious metal, mainly gold, subproduct (Gilmour, 1982; Long, 1995).

The importance of gold in porphyry deposits was emphasized in the 1970s because of the increase in gold price and the intensive exploration carried out in island arcs (Kesler, 1973; Titley, 1978; Perelló and Cabello, 1989; Sillitoe, 1990, 1993). For many years, the high gold contents of some deposits in island arcs were considered a distinctive feature that contrasted with gold-poor porphyry deposits, hosted by continental crust (Kesler, 1973, Titley, 1978). Alternative explanations for the variability in gold content include differences in erosion level (Titley and Beane, 1981), difference in magmatic evolution of individual deposits (Hollister, 1975), depth of emplacement (Sillitoe, 1998, 2000), and magmatic-fluid derivation and chemistry (Sillitoe, 1998, 2000; Ulrich et al., 1999; Simon et al., 2000). In the case of the Andean Cordillera, Lowell (1989) and others pointed out that some Chilean porphyry copper deposits have somewhat higher gold contents (e.g., El Bolsón [also named Exploradora] and Potrerillos) and that gold-bearing and gold-deficient deposits coexist in the same belt. The widespread occurrence of gold-rich porphyry systems in Chile was described only fairly recently in the Maricunga belt (Sillitoe, 1991; Vila and Sillitoe, 1991; King, 1992; Muntean and Einaudi, 2000, 2001), at Carmen (Rivera and Peri, 1991; Palacios et al., 1993), La Fortuna (Perelló et al., 1996), and Esperanza (Perelló et al., 2004) in Chile and the Farallón Negro district in Argentina (Sillitoe, 1979; Sasso et al., 1995; Perelló et al., 1998; Rojas et al., 1998; Sasso and Clark, 1998; Ulrich and Heinrich, 2002). These studies prompt a revision of genetic concepts in order to explain why porphyry systems with significant differences in gold and molybdenum contents coexist along the same belt and even in the same cluster.

This paper provides geologic information for Eocene to Oligocene porphyry copper systems with elevated gold contents in the El Salvador region, northern Chile. The geologic data were gathered during fieldwork done by the authors on behalf of CODELCO-Chile during the late 1990s. The information presented is largely unpublished, except for that concerning El Salvador and Potrerillos, where the gold assays were routinely obtained by classic fire-assay procedures in the laboratories of the Potrerillos smelter. Chemical information from the other deposits was obtained in accordance with current standards. The new K-Ar age for the Sierra Jardín deposit was determined at the Servicio Nacional de Geología y Minería (SERNAGEOMIN), Santiago, Chile (Table 1).

Table 1.

Potassium-Argon Age Determination, Sierra Jardín Porphyry Gold-Copper Prospect

Sample location UTM coordinates (east-north)Dated mineralK (%)Ar rad (nl/g)Ar atm (%)Age (Ma)Error (2 σ)Observations
451,3907,135,150Quartz-sericite-gypsum concentrate3,1055,1372243.31.5Sericite from D vein halo
451.2977,135,158Quartz-sericite-gypsum concentrate2,2643,6744041.32.3Sericite from D vein halo
451,2907,135,220Biotite6,31410,4204842.01.5Biotitized andesite
451,2807,135,085Whole rock3,6505,9032541.11.4Sericitized rhyolite
Sample location UTM coordinates (east-north)Dated mineralK (%)Ar rad (nl/g)Ar atm (%)Age (Ma)Error (2 σ)Observations
451,3907,135,150Quartz-sericite-gypsum concentrate3,1055,1372243.31.5Sericite from D vein halo
451.2977,135,158Quartz-sericite-gypsum concentrate2,2643,6744041.32.3Sericite from D vein halo
451,2907,135,220Biotite6,31410,4204842.01.5Biotitized andesite
451,2807,135,085Whole rock3,6505,9032541.11.4Sericitized rhyolite

Emplacement Environment of Porphyry Systems in the El Salvador Region

The structural framework and distribution of Tertiary intrusions in the El Salvador segment of the middle Eocene to early Oligocene porphyry belt is summarized in Figure 2 (Cornejo et al., 1993, 1997; Mpodozis et al., 1993, 1994; Cornejo and Mpodozis, 1996). The Sierra Castillo fault represents the southern extension of the Domeyko fault system (Maksaev, 1990; Mpodozis et al., 1993) with which giant porphyry copper deposits (Chuquicamata, Escondida, Collahuasi) are closely related. In the El Salvador region, the Sierra Castillo fault, associated with a thermal event dated at 160 Ma (Mpodozis et al., 1993), separates two lithotectonic domains. The western domain includes the volcanic and lesser sedimentary rocks that filled a Jurassic to Cretaceous intra-arc basin, which was deformed during the Late Cretaceous to Paleocene and covered by early Tertiary volcanic rocks, including calderas like those described from the El Salvador porphyry copper district (Cornejo et al., 1997). The eastern domain consists mainly of Jurassic marine sedimentary rocks that cover a Paleozoic granitic basement. These rocks were deformed to produce north-south—oriented fold-thrust belts during both the Late Cretaceous to Paleocene (Exploradora fold and thrust belt; Fig. 2; Cornejo and Mpodozis, 1996) and the middle Eocene to early Oligocene Incaic tectonic event (Potrerillos fold and thrust belt; Fig. 2; Mpodozis et al., 1993). The remnants of a Triassic rift, the basement to the Maricunga belt, extend into the northern part of the region, between the Salar de Punta Negra and Carrizo canyon (Cornejo and Mpodozis, 1996), where the Exploradora and Sierra Jardín prospects are located. Therefore the gold-bearing porphyry deposits in this area coincide with the Triassic rift environment, characterized by the presence of carbon-rich sedimentary rocks, which may have contributed to their gold-bearing nature (Camus, 2003).

The porphyry systems in the El Salvador region are related to stocks emplaced during two discrete periods: 45 to 40 Ma (e.g., Sierra Jardín, El Salvador) and 37 to 31 Ma (e.g., Exploradora, María Delia, Potrerillos, Coya). An east-ward-younging trend of the intrusions characterizes the region. The older prospects are hosted by Late Cretaceous to early Tertiary volcanic successions west of the Sierra Castillo fault. Their emplacement was controlled by the 60-m.y.-old caldera at El Salvador (Cornejo et al., 1997) or by central-vent volcanic edifices with ages of ~55 to 48 Ma at Sierra Jardín (Cornejo and Mpodozis, 1996). Minor coeval volcanic rocks are associated with these porphyry systems. Coeval volcanic rocks are largely absent in the gold-poor middle Eocene to early Oligocene porphyry copper belt farther north (e.g., Collahuasi, Chuquicamata, Escondida; Camus, 2003). The younger porphyry systems, located east of the Sierra Castillo fault or within it, are structurally controlled by jogs (Exploradora), splays (María Delia), reverse faults (Potrerillos), or transfer faults (Coya). The younger prospects intruded Paleozoic intrusive complexes as well as Mesozoic sedimentary rocks (Tomlinson et al., 1993; Mpodozis et al., 1994). Therefore the larger deposits, El Salvador (11.3 million metric tons (Mt) of contained copper and approx 100 t of contained gold; Camus, 2003) and Potrerillos (5.8 Mt of contained copper and approx 100 t of contained gold; Camus, 2003), were formed at different times under different structural conditions. Their elevated (>0.1 g/t) gold contents are not related to any particular regional condition, such as the Triassic rift, but rather are a response to the individual evolutionary histories of the host intrusive complexes, including magma crystallization processes (e.g., Ulrich et al., 1999).

Abundance and Distribution of Gold in the El Salvador and Potrerillos Porphyry Deposits

The Potrerillos porphyry copper deposit was mined by Andes Copper Co., a subsidiary of Anaconda Copper Corp., between 1926 and 1959, the year that the El Salvador deposit was put into production (Gustafson and Hunt, 1975). Although they have been considered as standard porphyry copper systems, exploration, and production data as well as evaluation of the hypogene ore show high gold contents relative to those of traditional porphyry copper-molybdenum deposits (e.g., Cox, 1986). The unpublished data of CODELCO show that about 300 Mt, averaging 1.0 percent Cu and 0.1 g/t Au, has been produced from El Salvador. Production data for Potrerillos are less well documented but it is estimated that ~200 Mt of 1.0 percent Cu and 0.2 g/t Au were mined to 1959 from the hypogene ore.

El Salvador

The geology of El Salvador has been described in detail by several authors (Gustafson and Hunt, 1975; Gustafson and Quiroga, 1995; Cornejo et al., 1997; Gustafson et al., 2001; Watanabe and Hedenquist, 2001). Copper mineralization at El Salvador is related to the emplacement of a northeast-oriented cluster of subvertical, cylindrical porphyry stocks hosted by Cretaceous volcanic rocks of andesitic composition, which in turn are covered by early Tertiary volcanic and volcaniclastics rocks.

Most of the copper produced at El Salvador was located in the Quebrada Turquesa area, where the ore was formed during cooling of granodiorite porphyry stocks dated at 42 to 41 Ma (Cornejo et al., 1997; Gustafson et al., 2001). Hydrothermal alteration centered on these stocks is characterized by a central zone of potassic alteration with chalcopyrite-bornite mineralization, accompanied by anhydrite and magnetite, that grades laterally and vertically into zones with higher sulfide contents associated with quartz-sericite alteration.The system was capped by a late-stage, pyrite-rich, advanced argillic alteration zone, the roots of which are now exposed in the upper parts of Cerro Indio Muerto (Watanabe and Hedenquist, 2001), representing the basal remnants of a lithocap (e.g., Sillitoe 1993, 1995).

Gustafson and Hunt (1975, p. 886) first pointed out that the distribution of gold and silver correlated with that of hypogene copper and that the higher gold values (>0.15 g/t) are restricted “exclusively to the central chalcopyrite-bornite zone.” The supergene immobility of gold and molybdenum was also stressed by these authors, because both display similar contents above and below the top of sulfides. Later Colley et al. (1989) cited gold values in the deposit ranging from 0.14 to 1.07 g/t.

Roeschmann (1979) studied the gold distribution in the central part of the Quebrada Turquesa area, concluding that there was a close spatial relationship between gold and the hypogene mineralization. At upper levels (2,710 m; Fig. 3), zones with gold grades of >0.17 g/t are coincident with the productive K porphyry which, in turn, hosts most of the chalcopyrite-bornite mineralization along with intense quartz veining of early and transitional types (A and B veins, respectively; Gustafson and Hunt, 1975; Gustafson and Quiroga, 1995). A zone containing >0.08 g/t Au surrounds the highest grade gold zone, suggesting that intrusion of the late L porphyry partially modified the distribution of hypogene mineralization. This relationship between porphyry intrusions and gold content is also observed at lower levels (2,400 m; Fig. 4), where gold-rich zones are almost perfectly coincident with the chalcopyrite-bornite center. The close relationship between the gold mineralization and the contact of the productive K and X porphyries and the subvertical central position of the late L porphyry is shown in Figure 5. The same gold distribution is also recognized in other mineralized centers within the El Salvador cluster, such as Quebrada M-Colina del Cobre (Roeschmann, 1979; Rojas, 1994) and Cerro Pelado (Gustafson et al., 2001), as well as in several gold and copper-gold porphyry deposits elsewhere, as at Lobo in the Maricunga belt (Vila and Sillitoe, 1991) and Carmen in the Inca de Oro district (Rivera and Peri, 1991). This pattern is interpreted as a result of the simultaneous deposition of copper and gold with bornite and magnetite (Simon et al., 2000) in the carapaces of porphyry stocks during successive intrusions of broadly coaxial intermineral bodies. The pattern is considered of prime importance in the exploration of clustered gold-bearing porphyry copper systems.

Fig. 3.

El Salvador 2,710-m level, showing main rock types and contoured gold values (modified from Roeschmann, 1979). The gold is reasonably coincident with the hypogene copper sulfide zonation and, consequently, shows a positive correlation with copper grades. There is a clear relationship between gold mineralization and the K porphyry.

Fig. 3.

El Salvador 2,710-m level, showing main rock types and contoured gold values (modified from Roeschmann, 1979). The gold is reasonably coincident with the hypogene copper sulfide zonation and, consequently, shows a positive correlation with copper grades. There is a clear relationship between gold mineralization and the K porphyry.

Fig. 4.

El Salvador 2,400-m level, showing rock types, sulfide zonation, and contoured gold values (modified from Roeschmann, 1979). Note that the general shape of the gold zone is similar to that of the 2,710-m level, implying that there is no difference in gold distribution between the supergene enrichment and protore zones. As in Figure 3, gold is coincident with copper and the K porphyry.

Fig. 4.

El Salvador 2,400-m level, showing rock types, sulfide zonation, and contoured gold values (modified from Roeschmann, 1979). Note that the general shape of the gold zone is similar to that of the 2,710-m level, implying that there is no difference in gold distribution between the supergene enrichment and protore zones. As in Figure 3, gold is coincident with copper and the K porphyry.

Fig. 5.

El Salvador, section 19,950 N, summarizing rock types and contoured gold values (modified from Roeschmann, 1979). There is no vertical variation of gold with changes in the supergene leaching-enrichment profile. Note the clearer correlation of gold with the K and X porphyries and the central position of the intermineral L porphyry.

Fig. 5.

El Salvador, section 19,950 N, summarizing rock types and contoured gold values (modified from Roeschmann, 1979). There is no vertical variation of gold with changes in the supergene leaching-enrichment profile. Note the clearer correlation of gold with the K and X porphyries and the central position of the intermineral L porphyry.

Potrerillos

Potrerillos is one of the more interesting porphyry systems in Chile because the porphyry stock intruded calcareous sedimentary sequences during synmineralization reverse faulting. Geologic publications include the original work by March (1935) and several studies on district geology (Olson, 1989; Tomlinson, 1994; Marsh et al., 1997). Information on gold distribution at Potrerillos is by Reyes (1981) and Parra (1983) as well as work by one of the authors (TV) during reevaluation of the deposit.

The Potrerillos porphyry copper system is centered on a monzodiorite to granodiorite porphyry stock (Cobre porphyry) that intruded a succession of marine calcareous rocks of Jurassic age (March, 1935; Reyes, 1981). Two smaller, weakly mineralized porphyry stocks (Norte and Gonzales porphyries) as well as strata-bound, volcanic- and sedimentary rock-hosted gold mineralization at El Hueso (Marsh et al., 1997), Agua de la Falda-Jerónimo (Lazcano and Fuentes, 1997; Marsh et al., 1997; Thompson et al., 2004), and Coya are located nearby in the district (Fig. 2; see also Fig. 10 below). It is generally accepted that the Cobre porphyry was emplaced syntectonically along a north-northeast—oriented, flat-lying reverse fault dated at about 36 Ma (Reyes, 1981; Tomlinson, 1994; Marsh et al., 1997). A halo of garnet-actinolite—bearing hornfels was developed in the rocks around the stock, with local development of sulfide mineralization (Reyes, 1981; Parra, 1983). Early hypogene copper mineralization was related to potassic alteration with disseminated and vein-hosted chalcopyrite and bornite but subordinate pyrite and magnetite. Late enargite-pyrite mineralization was related to a quartz-sericite alteration assemblage interpreted to represent the roots of an advanced argillic-altered lithocap that crops out at the nearby El Hueso gold deposit (Marsh et al., 1997). Late, postmineral pebble dikes and minor faults follow a north-northwest orientation (Reyes, 1981; Parra, 1983).

Fig. 10.

Sketch geology of the Coya porphyry gold-copper cluster. The close spatial relationship with the Potrerillos porphyry system is emphasized. The northwest-trending fault with down-to-the-northeast vertical offset, protected the upper portions of the Coya system from erosion and exposed the gold mineralized coquine level located deep in the stratigraphic column, as shown in schematic section A-A?. There are several small magnetite-quartz stockwork centers containing gold-copper mineralization in the Coya area.

Fig. 10.

Sketch geology of the Coya porphyry gold-copper cluster. The close spatial relationship with the Potrerillos porphyry system is emphasized. The northwest-trending fault with down-to-the-northeast vertical offset, protected the upper portions of the Coya system from erosion and exposed the gold mineralized coquine level located deep in the stratigraphic column, as shown in schematic section A-A?. There are several small magnetite-quartz stockwork centers containing gold-copper mineralization in the Coya area.

Although the presence of gold is not mentioned in the old reports, sample maps for the underground workings show that the hypogene mineralization in the Cobre porphyry typically contains between 0.1 and 0.3 g/t Au, and locally up to 5 g/t, and that the gold mineralization extends into the calcareous wall rocks. Colley et al. (1989) quoted a resource figure of 278 Mt, averaging 1.1 to 1.2 percent Cu and 0.3 to 1.25 g/t Au. A recent review showed that an average grade of >0.2 g/t Au zones is almost coincident with >0.9 percent Cu zones, which in turn coincide with a fine-grained facies of the Cobre porphyry (Fig. 6a-b), characterized by intense potassic alteration (Parra, 1983). This fine-grained facies occurs as slivers within the Cobre porphyry. Therefore, gold mineralization at Potrerillos appears to be related to early stages of copper mineralization characterized by bornite-chalcopyrite-magnetite and only minor pyrite during early potassic alteration stages. There is no clear relationship between the late, high-sulfidation stage and the gold mineralization, which is reflected by the absence of a linear correlation between gold and arsenic values. The remaining geologic resource at Potrerillos is estimated as several hundred million metric tons, averaging >0.5 percent Cu and >0.1 g/t Au. Several tens of million metric tons with similar copper and gold grades are estimated at the nearby Norte and Gonzales porphyry prospects (see Fig. 10 below).

Fig. 6.

a. Rock types, sulfide zoning, and contoured gold values at the 2,983-m level, Potrerillos. Modified from Reyes (1981) and Parra (1983). Gold zones coincide with the highest copper grades in the chalcopyrite-bornite zone, which occurs with potassic alteration in fine-grained porphyry. b. Rock types, sulfide zoning, and contoured copper values at the 2,983-m level, Potrerillos. Note coincidence in the distribution of copper and gold, as in (a). The copper is related to the same finegrained porphyry.

Fig. 6.

a. Rock types, sulfide zoning, and contoured gold values at the 2,983-m level, Potrerillos. Modified from Reyes (1981) and Parra (1983). Gold zones coincide with the highest copper grades in the chalcopyrite-bornite zone, which occurs with potassic alteration in fine-grained porphyry. b. Rock types, sulfide zoning, and contoured copper values at the 2,983-m level, Potrerillos. Note coincidence in the distribution of copper and gold, as in (a). The copper is related to the same finegrained porphyry.

Geologic Descriptions of Other Gold-Rich Porphyry Systems in the El Salvador Region

Exploradora

The Exploradora (or Bolsón) prospect is located about 65 km north of El Salvador, in an area characterized by old small-scale copper, gold, and silver mine workings (Figs. 12). Exploration work carried by CODELCO and, more recently, Minera Los Andes, discovered a low-grade porphyry copper-gold center (Cáceres, 1997).

The Exploradora stock, consisting of equigranular monzodiorite to diorite, is emplaced in a dilational jog along the Sierra Castillo dextral fault system (Fig. 2) within a succession of Jurassic marine sedimentary rocks unconformably overlain by Eocene volcanic rocks. The intrusion, dated at 35 to 36 Ma (K-Ar whole rock; Cornejo and Mpodozis, 1996), generated endo- and exoskarn. In the northern part of the intrusion, a north-northeast—trending, dikelike, syntectonic, fine-grained diorite porphyry surrounded by a carapace of orthomagmatic breccia hosts copper-gold mineralization. Intermineral quartz diorite porphyry dikes, as well as a northwest-striking swarm of postmineral granodiorite porphyry dikes, cut the intrusion, which has been dated at 32.2 to 33.4 Ma (Cornejo and Mpodozis, 1996). A diatreme complex, roughly 1 km in diameter, consisting of phreatomagmatic breccia, igneous breccia of latitic composition, and pyroclastic material of possible tuff-ring origin, along with northwest-trending pebble dikes, crops out in the central part of the prospect. The diatreme complex is believed to be the product of a late-stage phreatomagmatic event (Figs. 78).

Fig. 7.

Surface geology of the Exploradora prospect. Copper-gold mineralization is related to the breccia-porphyry complex in the south-central part of the area. Ignimbrite and alluvium in the central part mostly cover the late mineral diatreme breccia. The extensions of this body at depth are interpreted from drill hole data. Note the extension of the advanced argillic alteration and the position of the late veins and dikes along a northwest trend, in contrast to the north-northeast alignment of the porphyry complex.

Fig. 7.

Surface geology of the Exploradora prospect. Copper-gold mineralization is related to the breccia-porphyry complex in the south-central part of the area. Ignimbrite and alluvium in the central part mostly cover the late mineral diatreme breccia. The extensions of this body at depth are interpreted from drill hole data. Note the extension of the advanced argillic alteration and the position of the late veins and dikes along a northwest trend, in contrast to the north-northeast alignment of the porphyry complex.

Fig. 8.

Cross section showing rock types, hydrothermal alteration, and main mineralized units, Exploradora prospect. The tabular shape of the mineralized breccia-porphyry complex is apparent. Hypogene copper-gold mineralization is related to potassic-altered rocks containing early quartz veinlets carrying copper sulfides and magnetite. The supergene enrichment blanket is located near the base of the lithocap. The leached zone contains gold values as high as 0.5 g/t. Section line shown in Figure 7.

Fig. 8.

Cross section showing rock types, hydrothermal alteration, and main mineralized units, Exploradora prospect. The tabular shape of the mineralized breccia-porphyry complex is apparent. Hypogene copper-gold mineralization is related to potassic-altered rocks containing early quartz veinlets carrying copper sulfides and magnetite. The supergene enrichment blanket is located near the base of the lithocap. The leached zone contains gold values as high as 0.5 g/t. Section line shown in Figure 7.

Drilling reveals that advanced argillic alteration constitutes a 300-m-thick lithocap (Figs. 78), below which potassic alteration characterized by secondary biotite, mostly retrograded to chlorite, and intense quartz-K-feldspar veining is developed. Remnants of the chlorite-rich propylitic halo are present to the east, while to the north and south there is interfingering of early potassic alteration with prograde skarn.

Hypogene copper mineralization is characterized mainly by fine-grained disseminations of chalcopyrite and rare bornite, with an average grade of 0.3 percent Cu and 0.2 g/t Au. Most of this mineralization is related to an early set of well-developed quartz-K-feldspar veinlets occurring in both the diorite porphyry and related orthomagmatic breccia. Magnetite commonly occurs in these veinlets and is overprinted by finegrained specularite related to development of the advanced argillic lithocap. The latter is characterized by widespread pyrite and intense replacement by pyrophyllite, quartz, and alunite, an association that grades downward into sericite.

An immature, sooty chalcocite-bearing supergene enrichment blanket located at the base of the advanced argillic lithocap overlies hypogene sulfides (Fig. 8). This blanket contains copper values in the 0.4 to 0.6 percent Cu range, with gold contents similar to those present in the protore. Nevertheless, the 200-m-thick overlying leached capping is enriched in gold, reaching 0.5 g/t, which is interpreted as a result of residual enrichment due to volume loss during the leaching.

Northwest-trending epithermal veins, parallel to the granodiorite porphyry dikes (Fig. 7), represent late mineralization events at Exploradora. To the south, these fracture-filling veins are of a high-sulfidation type, with abundant quartz and alunite carrying elevated gold (1–2 g/t) and silver (up to 30 g/t) values. However, to the north and east, the veins are of a low-sulfidation type, with banded quartz textures dominated by lead-zinc-(silver) mineralization.

The Exploradora porphyry system resulted from superposition of several intrusive events, during rapid exhumation in an active tectonic environment. Different erosion levels are suggested for the rocks and alteration in the main stock (~35 Ma; Cornejo and Mpodozis, 1996) and in the Exploradora porphyry complex (32–34 Ma; Cornejo and Mpodozis, 1996). The well-preserved advanced argillic lithocap and the apparently near-surface preservation of the diatreme, as indicated by the inferred tuff ring, suggest superposition of shallow processes on deeper, orthomagmatic parts of the porphyry system. This telescoping is common in porphyry systems elsewhere in the middle Eocene to early Oligocene porphyry copper belt of northern Chile (e.g., Perelló, 1999), as well as in gold-rich porphyry systems elsewhere (Sillitoe, 2000).

Sierra Jardín

The Sierra Jardín porphyry gold-copper prospect is located 40 km north of El Salvador (Figs. 12). The area was mined on a small scale for gold in the past, and since the 1960s has been explored for gold, silver, and copper. Exploration carried out in the early 1990s by CODELCO (J. Osorio, unpub. data, 1995), along with regional studies (Cornejo and Mpodozis, 1996), showed the prospect to be a somewhat unusual porphyry system in which gold contents predominate over those of copper, similar to the gold-rich porphyries in the Maricunga belt (Vila and Sillitoe, 1991; Vila et al., 1991; Muntean and Einaudi, 2000, 2001).

Three superimposed Tertiary volcanic units occur at Sierra Jardín. The oldest is represented by andesite-dacite flows and intrusions dated at 61 to 55 Ma (Cornejo and Mpodozis, 1996), which are covered by andesite and basaltic andesite related to radial dikes dated at 55 to 48 Ma (Cornejo and Mpodozis, 1996). The youngest unit is a volcanoplutonic complex of andesitic to dacitic composition dated at 37 Ma (Cornejo and Mpodozis, 1996). Several 35 Ma volcanic edifices, showing remnants of surficial solfataric alteration, are also well preserved in the area.

At Sierra Jardín (Fig. 9), gold-copper mineralization is associated with a fine-grained diorite porphyry complex, emplaced into folded Cretaceous volcanic and sedimentary rocks, and the subhorizontal aphanitic andesite of the 55 to 48 Ma unit. The porphyry complex comprises small stocks and northwest-trending dikes of diorite to quartz diorite, intermineral diorite dikes, and late mineral andesite and pebble dikes. Radiometric dating (Table 1) suggests an age of 42 to 41 Ma for the main mineralized stock and 38 Ma for the intermineral phase (Cornejo and Mpodozis, 1996).

Fig. 9.

Sketch geology of the Sierra Jardín porphyry gold cluster. Note the controls of the mineralized centers by both circular structural features derived from preexistent volcanic edifices and northwest-trending fractures. There are at least six small dioritic to quartz dioritic porphyry centers exhibiting intense potassic alteration and gold-copper mineralization in magnetite-quartz-anhydrite stockworks. Prograde actinolite-biotite-magnetite halos are also shown.

Fig. 9.

Sketch geology of the Sierra Jardín porphyry gold cluster. Note the controls of the mineralized centers by both circular structural features derived from preexistent volcanic edifices and northwest-trending fractures. There are at least six small dioritic to quartz dioritic porphyry centers exhibiting intense potassic alteration and gold-copper mineralization in magnetite-quartz-anhydrite stockworks. Prograde actinolite-biotite-magnetite halos are also shown.

Most of the area shows strong chloritic alteration of andesite. Biotite-anhydrite and early quartz-feldspar veining affected the porphyry center, which is surrounded by an acti-nolite-biotite-magnetite halo. Remnants of an advanced argillic lithocap are well preserved in nearby dacitic pyroclastic rocks, which display widespread vuggy quartz and alunite carrying anomalous gold (0.1–0.5 g/t) and arsenic (up to 500 ppm) values.

The potassic-altered porphyry contains mineralization averaging 0.2 percent Cu and up to 0.5 g/t Au, with grades directly related to the abundance of quartz-magnetite veinlets. Deep drill holes (up to 500 m) reveal intense replacement of the intrusive and volcanic rocks by biotite and magnetite, along with late-magmatic quartz-anhydrite-magnetite veinlets containing scarce chalcopyrite and bornite. The extent of these stockwork zones is not clear because of the limited number of drill holes, but surface exposures reveal a clear spatial relationship between veining and high gold grades and the fine-grained, potassic-altered dioritic porphyry. The intermineral porphyries cut the hypogene mineralization.

No supergene enrichment is present at Sierra Jardín due to the very low sulfide content of the hypogene mineralization and limited preservation of the pyritic lithocap. Nevertheless, there is an upper zone of supergene oxide copper mineralization with similar copper and gold grades to the underlying protore.

Coya

The Coya prospect is located 3 km southeast of Potrerillos (Figs. 1–2, 10). The geologic setting comprises a strongly folded and faulted Jurassic marine sequence overlying Paleozoic intrusive and volcanic rocks, all covered unconformably by felsic to intermediate-composition volcanic rocks of Paleocene age. The district is part of the Potrerillos fold and thrust belt, characterized by reverse faults produced during 42 to 37 Ma compression (Tomlinson et al., 1993). The main structural feature is a northwest-striking transfer fault, down-thrown into the northeastern block, in which the higher parts of the Coya system are preserved (Fig. 10).

Quartz diorite porphyry stocks, containing relatively high gold grades, occur in three centers, controlled by reverse faults. An extensive skarn halo, which includes jasperoid, is developed in Jurassic sedimentary rocks around the porphyry stocks. An extensive advanced argillic lithocap is developed in pyroclastic rocks overlying the porphyry centers. Late phreatomagmatic activity at Coya is represented by pebble dikes and a diatreme. A similar intrusion and mineralization age to that of the nearby Potrerillos and El Hueso deposits (37–38 Ma; Marsh et al., 1997) is proposed, with a minimum age of 31 to 32 Ma provided by postmineral intrusions immediately west of the main mineralized area (e.g., late intrusions in Fig. 10).

Gold mineralization accompanies biotite-quartz-magnetite-chalcopyrite stockworks in strongly biotitized, fine-grained diorite. Geochemical results from rock and talus-fines samples show copper (0.2–0.3%) and gold (0.2–0.5 g/t) anomalies in potassic-altered porphyry stocks with abundant veins. Drill holes suggest that the original distribution of the hypogene mineralization was modified by the inter- to postmineral intrusions as well as probably being cut by low-angle reverse faults. Particularly high gold contents (up to 18 g/t) accompanied by arsenic and antimony (up to 1,000 ppm), but without copper, are found in a relatively thin coquina unit within the Jurassic sedimentary sequence, which is partially replaced by sulfosalts. The coquina unit is considered equivalent to one described at the nearby Agua de la Falda-Jerónimo deposit (Lazcano and Fuentes, 1997). Some jasperoid and skarnified sedimentary rocks also contain anomalously high (up to 10 g/t) gold contents.

At the northwestern corner of the prospect, a monzodiorite to granodiorite porphyry stock (Gonzales Sur porphyry; Fig. 10) is affected by potassic and quartz-sericite alteration similar to that at Potrerillos. Drilling in areas with intense quartz-magnetite-copper sulfide stockworks returned 0.5 percent Cu and 0.6 g/t Au. This particular mineralization is considered to be transitional between the gold-rich porphyry copper system described at Potrerillos and the gold-only (Maricunga-style) porphyry deposits, like Coya, suggesting the presence of spatial variations or zoning in metal contents at the district or cluster scale.

Geologic Significance of the Gold-Rich Porphyry Systems in the El Salvador Region

The geologic features of the gold-rich porphyry deposits and prospects in the El Salvador region may be summarized as follows:

  1. The porphyry copper-gold systems formed in different geologic and structural settings. Those with ages of 40 to 43 Ma (El Salvador, Sierra Jardín) were emplaced west of the Sierra Castillo fault system within a structural block dominated by Cretaceous and Tertiary volcanic and sedimentary rocks. A strong structural control on emplacement is evident, which is inherited from preexisting Paleocene volcanic edifices (Cornejo et al., 1993; Mpodozis et al., 1994). In contrast, the systems dated at 38 to 31 Ma were emplaced east of the Sierra Castillo fault into Paleozoic intrusive complexes and/or Jurassic marine sedimentary rocks and display a clear structural control by reverse faults.

  2. At a regional scale, a general decrease in the age of porphyry systems from northwest to southeast is apparent. However, at a more detailed scale, the stocks show no clear temporal polarity, except at El Salvador where a younging trend is apparent from north to south (Gustafson et al., 2001). This age trend suggests differences in the timing and style of the structural openings, which permitted magma emplacement as discrete pulses. The same complex intrusive history is also reflected at the deposit scale, where early inter-, and late mineral phases are broadly cospatial, giving rise to variations in both the geometry of the intrusions and in the structural control imposed upon emplacement.

  3. The mineralized intrusive complexes are generally subvertical and cylindrical or dikelike in form, typically granodioritic through monzodioritic to dioritic in composition, with porphyritic to microporphyritic textures. The stocks are cut by inter- and/or postmineral dikes. Late phreatomagmatic events include pebble dikes and diatreme complexes. In all cases, a complex history of superimposed intrusions is apparent, and a progressively shallower environment of emplacement is interpreted from the rock types and alteration-mineralization features.

  4. The copper and gold mineralization is associated with early potassic alteration containing biotite, K-feldspar, and anhydrite along with stockworks of EB (early biotite) and Atype quartz veinlets (Gustafson and Quiroga, 1995), with predominance of chalcopyrite-bornite mineralization. The shape and dimensions of the stockworked area are a function of the size and form of the productive stock and invading intermineral intrusions, as observed in other gold or copper-gold porphyry centers (Rivera and Peri, 1991; Vila and Sillitoe, 1991; Vila et al., 1991). Magnetite is always present in the early phases of alteration-mineralization and it should be considered as a distinctive characteristic of these systems (e.g., Sillitoe, 1979; Perelló and Cabello, 1989). Prograde metamorphism generated biotite-actinolite-magnetite hornfels in andesitic country rocks and skarn in sedimentary rocks. Telescoping is generally present, as shown by the superimposition of advanced argillic over potassic alteration. Except for El Salvador and Potrerillos, where sericitic assemblages are well developed, most gold-rich porphyry copper systems, such as Sierra Jardín, Exploradora, and Coya, have minor sericitic zones (cf. Sillitoe, 2000). This may be taken to suggest the existence of a gap between the prograde potassic and late advanced argillic alteration.

  5. At the district scale, especially in the Potrerillos-Coya area, there is a remarkable spatial coexistence of high copper/gold ratio centers and Maricunga-type gold-rich (copper-poor) porphyries. This situation, together with the occurrence of the molybdenum-rich Cerro Pelado system at El Salvador (Gustafson et al., 2001), suggests that the relative gold enrichment of a particular productive porphyry intrusion may be explained by local, rather than by regional or even continental parameters (Sillitoe, 1979; Perelló and Cabello, 1989; Ulrich et al., 1999; Simon et al., 2000).

Exploration Significance of the Gold-Rich Porphyry Systems in the El Salvador Region

This study stresses that in any porphyry copper belt there may be exploration opportunities for gold-rich varieties (Sillitoe, 1998, 2000). For instance, low-grade porphyry copper systems may have been inadequately tested for gold or advanced argillic lithocaps may be underexplored for underlying porphyry copper-gold mineralization. In contrast, some sulfide-poor systems may have passed undetected due to the lack of a prominent surface expression or color anomaly. In northern Chile, several recently described occurrences of porphyry copper-gold and gold-only types, shown in Figure 1, such as Anillo (Marquardt et al., 1994; Rojas, 1994) in the Paleocene belt, and Esperanza south of Chuquicamata (Perelló et al., 2004) and La Fortuna (Perelló et al., 1996) in the middle Eocene to early Oligocene belt, have widened the scope of exploration.

Even though the total gold contents of El Salvador and Potrerillos are low (2–3 Moz) compared to many porphyry copper-gold deposits (Sillitoe, 1998, 2000), there may be considerable gold potential in undiscovered deposits when it is realized that the existence of gold-rich porphyry copper deposits in Chile is not widely recognized and the key characteristics of the model are not clearly understood and applied to exploration.

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Acknowledgments

Many geologists have made contributions to the present understanding of the deposits described and recognition is given here to their work. Permission to publish was given by. Francisco Camus (CODELCO) and Nicolás Saric (formerly Minera Los Andes). Andrew Hodgkin, Heinz Gröpper, and Daphne Petersen corrected the English of an early version of the manuscript and made useful comments and suggestions. Drawings were produced by Alejandro Toranzo and Rodolfo Heuser. Detailed reviews by Spencer Titley and Francisco Camus are greatly appreciated. The extremely patient and dedicated editing and final reviews by Richard Sillitoe and José Perelló are also acknowledged.

Figures & Tables

Fig. 1.

Location of the porphyry copper and copper-gold deposits discussed in the text. Numbers in parentheses correspond to the alteration-mineralization age in Ma; compiled from Cornejo et al. (1997), Camus (2003), and Perelló et al. (2004).

Fig. 1.

Location of the porphyry copper and copper-gold deposits discussed in the text. Numbers in parentheses correspond to the alteration-mineralization age in Ma; compiled from Cornejo et al. (1997), Camus (2003), and Perelló et al. (2004).

Fig. 2.

Summary structural and intrusive framework of the El Salvador region, showing the location and age of the porphyry copper-gold deposits described. Note the different structural environment for the deposits west of the Sierra Castillo fault compared to those located along east side of fault. Modified from Cornejo et al. (1993) and Cornejo and Mpodozis (1996). María Delia is an immature low sulfide porphyry system prospect lacking gold.

Fig. 2.

Summary structural and intrusive framework of the El Salvador region, showing the location and age of the porphyry copper-gold deposits described. Note the different structural environment for the deposits west of the Sierra Castillo fault compared to those located along east side of fault. Modified from Cornejo et al. (1993) and Cornejo and Mpodozis (1996). María Delia is an immature low sulfide porphyry system prospect lacking gold.

Fig. 3.

El Salvador 2,710-m level, showing main rock types and contoured gold values (modified from Roeschmann, 1979). The gold is reasonably coincident with the hypogene copper sulfide zonation and, consequently, shows a positive correlation with copper grades. There is a clear relationship between gold mineralization and the K porphyry.

Fig. 3.

El Salvador 2,710-m level, showing main rock types and contoured gold values (modified from Roeschmann, 1979). The gold is reasonably coincident with the hypogene copper sulfide zonation and, consequently, shows a positive correlation with copper grades. There is a clear relationship between gold mineralization and the K porphyry.

Fig. 4.

El Salvador 2,400-m level, showing rock types, sulfide zonation, and contoured gold values (modified from Roeschmann, 1979). Note that the general shape of the gold zone is similar to that of the 2,710-m level, implying that there is no difference in gold distribution between the supergene enrichment and protore zones. As in Figure 3, gold is coincident with copper and the K porphyry.

Fig. 4.

El Salvador 2,400-m level, showing rock types, sulfide zonation, and contoured gold values (modified from Roeschmann, 1979). Note that the general shape of the gold zone is similar to that of the 2,710-m level, implying that there is no difference in gold distribution between the supergene enrichment and protore zones. As in Figure 3, gold is coincident with copper and the K porphyry.

Fig. 5.

El Salvador, section 19,950 N, summarizing rock types and contoured gold values (modified from Roeschmann, 1979). There is no vertical variation of gold with changes in the supergene leaching-enrichment profile. Note the clearer correlation of gold with the K and X porphyries and the central position of the intermineral L porphyry.

Fig. 5.

El Salvador, section 19,950 N, summarizing rock types and contoured gold values (modified from Roeschmann, 1979). There is no vertical variation of gold with changes in the supergene leaching-enrichment profile. Note the clearer correlation of gold with the K and X porphyries and the central position of the intermineral L porphyry.

Fig. 10.

Sketch geology of the Coya porphyry gold-copper cluster. The close spatial relationship with the Potrerillos porphyry system is emphasized. The northwest-trending fault with down-to-the-northeast vertical offset, protected the upper portions of the Coya system from erosion and exposed the gold mineralized coquine level located deep in the stratigraphic column, as shown in schematic section A-A?. There are several small magnetite-quartz stockwork centers containing gold-copper mineralization in the Coya area.

Fig. 10.

Sketch geology of the Coya porphyry gold-copper cluster. The close spatial relationship with the Potrerillos porphyry system is emphasized. The northwest-trending fault with down-to-the-northeast vertical offset, protected the upper portions of the Coya system from erosion and exposed the gold mineralized coquine level located deep in the stratigraphic column, as shown in schematic section A-A?. There are several small magnetite-quartz stockwork centers containing gold-copper mineralization in the Coya area.

Fig. 6.

a. Rock types, sulfide zoning, and contoured gold values at the 2,983-m level, Potrerillos. Modified from Reyes (1981) and Parra (1983). Gold zones coincide with the highest copper grades in the chalcopyrite-bornite zone, which occurs with potassic alteration in fine-grained porphyry. b. Rock types, sulfide zoning, and contoured copper values at the 2,983-m level, Potrerillos. Note coincidence in the distribution of copper and gold, as in (a). The copper is related to the same finegrained porphyry.

Fig. 6.

a. Rock types, sulfide zoning, and contoured gold values at the 2,983-m level, Potrerillos. Modified from Reyes (1981) and Parra (1983). Gold zones coincide with the highest copper grades in the chalcopyrite-bornite zone, which occurs with potassic alteration in fine-grained porphyry. b. Rock types, sulfide zoning, and contoured copper values at the 2,983-m level, Potrerillos. Note coincidence in the distribution of copper and gold, as in (a). The copper is related to the same finegrained porphyry.

Fig. 7.

Surface geology of the Exploradora prospect. Copper-gold mineralization is related to the breccia-porphyry complex in the south-central part of the area. Ignimbrite and alluvium in the central part mostly cover the late mineral diatreme breccia. The extensions of this body at depth are interpreted from drill hole data. Note the extension of the advanced argillic alteration and the position of the late veins and dikes along a northwest trend, in contrast to the north-northeast alignment of the porphyry complex.

Fig. 7.

Surface geology of the Exploradora prospect. Copper-gold mineralization is related to the breccia-porphyry complex in the south-central part of the area. Ignimbrite and alluvium in the central part mostly cover the late mineral diatreme breccia. The extensions of this body at depth are interpreted from drill hole data. Note the extension of the advanced argillic alteration and the position of the late veins and dikes along a northwest trend, in contrast to the north-northeast alignment of the porphyry complex.

Fig. 8.

Cross section showing rock types, hydrothermal alteration, and main mineralized units, Exploradora prospect. The tabular shape of the mineralized breccia-porphyry complex is apparent. Hypogene copper-gold mineralization is related to potassic-altered rocks containing early quartz veinlets carrying copper sulfides and magnetite. The supergene enrichment blanket is located near the base of the lithocap. The leached zone contains gold values as high as 0.5 g/t. Section line shown in Figure 7.

Fig. 8.

Cross section showing rock types, hydrothermal alteration, and main mineralized units, Exploradora prospect. The tabular shape of the mineralized breccia-porphyry complex is apparent. Hypogene copper-gold mineralization is related to potassic-altered rocks containing early quartz veinlets carrying copper sulfides and magnetite. The supergene enrichment blanket is located near the base of the lithocap. The leached zone contains gold values as high as 0.5 g/t. Section line shown in Figure 7.

Fig. 9.

Sketch geology of the Sierra Jardín porphyry gold cluster. Note the controls of the mineralized centers by both circular structural features derived from preexistent volcanic edifices and northwest-trending fractures. There are at least six small dioritic to quartz dioritic porphyry centers exhibiting intense potassic alteration and gold-copper mineralization in magnetite-quartz-anhydrite stockworks. Prograde actinolite-biotite-magnetite halos are also shown.

Fig. 9.

Sketch geology of the Sierra Jardín porphyry gold cluster. Note the controls of the mineralized centers by both circular structural features derived from preexistent volcanic edifices and northwest-trending fractures. There are at least six small dioritic to quartz dioritic porphyry centers exhibiting intense potassic alteration and gold-copper mineralization in magnetite-quartz-anhydrite stockworks. Prograde actinolite-biotite-magnetite halos are also shown.

Table 1.

Potassium-Argon Age Determination, Sierra Jardín Porphyry Gold-Copper Prospect

Sample location UTM coordinates (east-north)Dated mineralK (%)Ar rad (nl/g)Ar atm (%)Age (Ma)Error (2 σ)Observations
451,3907,135,150Quartz-sericite-gypsum concentrate3,1055,1372243.31.5Sericite from D vein halo
451.2977,135,158Quartz-sericite-gypsum concentrate2,2643,6744041.32.3Sericite from D vein halo
451,2907,135,220Biotite6,31410,4204842.01.5Biotitized andesite
451,2807,135,085Whole rock3,6505,9032541.11.4Sericitized rhyolite
Sample location UTM coordinates (east-north)Dated mineralK (%)Ar rad (nl/g)Ar atm (%)Age (Ma)Error (2 σ)Observations
451,3907,135,150Quartz-sericite-gypsum concentrate3,1055,1372243.31.5Sericite from D vein halo
451.2977,135,158Quartz-sericite-gypsum concentrate2,2643,6744041.32.3Sericite from D vein halo
451,2907,135,220Biotite6,31410,4204842.01.5Biotitized andesite
451,2807,135,085Whole rock3,6505,9032541.11.4Sericitized rhyolite

Contents

GeoRef

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