Skarn, Porphyry, Vein, and Replacement Mineralization in the Toqui District, Southern Chile
Published:January 01, 2010
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Steven D. Bussey, Alejandro Kakarieka, Lawrence D. Meinert, 2010. "Skarn, Porphyry, Vein, and Replacement Mineralization in the Toqui District, Southern Chile", The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries, Richard J. Goldfarb, Erin E. Marsh, Thomas Monecke
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The Toqui district is located in southern Chile, 1,350 km south of Santiago. The total geological resource for the district is 20 million tonnes (Mt) grading 8.2 percent Zn and 1.5 g/t Au, with zones of significantly higher Au grades. All orebodies in the district are being developed by the Toqui mine, an underground room and pillar operation that has an average annual production of 500,000 t per year.
The Toqui district contains a series of skarn and replacement orebodies within a 24 km2 area. Oldest rocks include Jurassic andesite and Cretaceous volcanic sandstone and tuff of the Toqui Formation, with a basal 5-to 30-m-thick limestone unit, rich in oyster fossils and forming the main ore host. Above these units is 800 m of black shale of the Katterfeld Formation, overlain by andesite of the Cretaceous lower Divisadero Group, which is then overlain unconformably by rhyolite ignimbrite of the upper Divisadero Group. Intrusive rocks include rhyolite, dacite, and andesite sills emplaced into all the Cretaceous rock units. Multiple periods of magmatic and hydrothermal activity have been documented from 120 to 105 Ma.
At district scale, Fe, As, Au, Bi, and Co are highest in the southeast, associated with garnet, pyroxene, and amphibole alteration, whereas Pb and Ag are highest in the northwest, associated with chlorite and sericite. Zinc grades are fairly uniform across the district, but sphalerite is zoned from high Fe in the southeast to low Fe in the northwest. Economically significant gold mineralization was superimposed on earlier base metal-rich skarn in the southeastern part of the district. Late hydrothermal fluids entered the skarn system along pre-existing northwest-trending structures. Gold occurs as electrum associated with native bismuth, cobaltite, and a variety of sulfosalts. Gold-rich ore generally contains abundant arsenopyrite, but arsenopyrite-rich ores are not necessarily gold rich. Gold and cobaltite deposition was accompanied by extensive retrograde amphibole formation, with clay minerals more abundant at the periphery of the gold zones. Deep drilling has encountered two areas of subeconomic pyrite-chalcopyrite-molybdenite stockworks. One is beneath the skarn orebodies in the southeastern part of the district and the other is beneath mineralization in the northwestern part. The emerging picture is one of a large porphyry-skarn district with multiple pulses of intrusion and alteration, resulting in multiple orebodies and mineralization styles.
The toqui Zn-Au district is located in Region XI, southern Chile, approximately 1,350 km south of Santiago (Fig. 1A). It is situated along the eastern flank of the southern Andes Mountains within the Toqui River valley and between 620 and 1,650 m elevation. Within the district, Breakwater Resources Ltd. owns and operates the Toqui mine through its wholly owned Chilean subsidiary, Sociedad Contractual Minera El Toqui. By road, the mine site is approximately 120 km north of the regional capital, Coyhaique, and an equal distance northeast of the port of Chacabuco (Fig. 1B).
The district covers a 24-km2 area and consists of 11 orebodies that have been or will be developed by the Toqui mine. Two of the orebodies, Antolín and Zúñiga, were discovered and developed early in the history of the district and are no longer accessible. They are described in Wellmer et al. (1983) as discordant veins of sphalerite and galena with minor amounts of pyrite and chalcopyrite in a gangue of quartz and calcite. We refer to them as the Antolín and Zúñiga Zn-Pb-Ag veins. They have not been a focus of recent exploration and are not discussed in detail in this report. Of the remaining orebodies, two are replacement deposits and seven are skarns. Skarn and replacement mineralization is hosted principally in a 5- to 30-m-thick limestone bed at the base of the Cretaceous Toqui Formation, although minor amounts of the mineralization are also hosted in other units. Historically important areas of mineralization include the Doña Rosa Zn-Au orebody; San Antonio, Mallín-Mónica, and Estatuas Zn orebodies; Concordia Zn-Pb-Ag orebody; and the previously mentioned Antolín and Zúñiga Zn-Pb-Ag orebodies. More recently, the Porvenir Zn orebody was discovered in 2006, and of particular importance, the Au-Co-Zn Aserradero and Mina Profunda orebodies were discovered in 2002 and 2007, respectively.
The Toqui mine has been in continuous production since 1983, except for during 1986 and a brief period in 1998. Between 1998 and 2008, exploration doubled the size of the known resource from 10 to 20 million tonnes (Mt). At the end of 2008, the total geological resource for the district was 20 Mt grading 8.2 percent Zn and 1.5 g/t Au. This reported resource includes the Aserradero orebody with 1.5 Mt grading 6.25 g/t Au, 7.35 percent Zn, and 0.2 percent Co, and Mina Profunda with 0.8 Mt grading 6.73 g/t Au, 2.98 percent Zn, and 0.2 percent Co (Table 1). The most recent discovery in the Toqui district, Mina Profunda, is important for two reasons. First, it is gold-rich and mining Mina Profunda has kept the mine profitable during times of low zinc prices. Second, the orebody is hosted in rocks located stratigraphically beneath the principal limestone-host in the district, which expands the prospective area for additional mineralization. The goal of this paper is to summarize the geology of the district based on exploration activities from 1998 to 2008 and focus on the gold-bearing orebodies of Aserradero and Mina Profunda. Isotopic age determinations for geologic units and hydrothermal minerals in the district are also presented.
History and Production
The Toqui district was discovered in the early 1970s when two small zinc-rich veins, Zuñiga and Antolín, were sampled from outcrops along the Toqui River. Metallgesellschaft AG signed a joint venture early in 1974 with the owner Ignacio Walker, a Chilean entrepreneur, to explore and develop the property. Metalgesellschaft dropped the option and returned the property to Walker, who in 1979 constructed a mine with a starting resource of 1.2 Mt @ 10 percent Zn. Initial production at Toqui started on November 25, 1983. By 1986, the San Antonio orebody had been discovered and was being mined in conjunction with the high-grade veins at Zuñiga and Antolín.
LAC Minerals Ltd. (LAC) acquired a majority interest in 1987 and become the sole owner in 1989. LAC subsequently drilled off the Mallín-Mónica area and significantly expanded the reserves of the project with a large drilling program. It was during this time that the Doña Rosa orebody was discovered with initial reserves estimated at 4.4 Mt containing 9.72 percent zinc and 1.8 g/t gold.
In 1994, Barrick Gold Corporation acquired LAC Minerals and three years later Breakwater Resources Ltd. purchased all outstanding common shares of Toqui from Compania Minera Barrick Ltda. and Sociedad Contractual Minera Barrick, wholly owned subsidiaries of Barrick Gold Corporation. Subsequent systematic exploration of the district by Breakwater Resources Ltd. resulted in the discovery of significant extensions of known orebodies (San Antonio East and Mallín South) and several new orebodies (Estatuas, Aserradero, Concordia, Porvenir, and Mina Profunda). Several new exploration strategies were used successfully during this time and are discussed where appropriate below. Between 1998 and 2008 more than 200,000 m were drilled at Toqui. In 2004, Breakwater Resources Ltd. increased mill capacity to 520,000 t per year and in 2006 installed a small intensive leaching plant to recover gold from the Aserradero and Mina Profunda skarn orebodies.
Previous Geologic Studies
Wellmer et al. (1983) published one of the first geologic descriptions of the Toqui district. At that time, the district consisted of mineralized outcrops to the north of Cerro Estatuas, the Zuñiga and Antolin veins, several drill holes in the central part of the Concordia deposit, and the newly-discovered San Antonio orebody. The stratiform Zn-Pb-Ag-Cu massive sulfide mineralization at Concordia, the veins at Zuñiga and Antolin in the footwall, and the interbedded nature of volcanic units within the Toqui Formation led them to propose a volcanogenic exhalative origin for the mineralization. At San Antonio, they attributed the presence of skarn minerals to reaction with the underlying San Antonio rhyolite sill and acknowledged the difficulty in distinguishing between syngenetic and epigenetic styles of mineralization. Their detailed descriptions of the stratigraphy within the Toqui Formation, at Concordia and San Antonio, still provide important information because much of the old core has been lost.
Palacios et al. (1996) presented an update on the geology following the discovery of Mallín-Monica and Doña Rosa orebodies. A skarn classification was suggested, along with excellent paragenetic studies of sulfides and gangue minerals, fluid inclusion measurements, and several isotopic age determinations. Three stages of hydrothermal alteration and mineralization were related to progressive metasomatism of the limestone host. Palacios et al. (1996) presented the first two K-Ar ages on samples of the altered San Antonio rhyolite sill and concluded that the deposit formed between 108 and 100 Ma.
Townley and Palacios (1999) and Townley and Godwin (2001) synthesized metallogenic data for the Aysén region of southern Chile, carried out systematic isotopic dating, and studied the lead isotope characteristics of galena from the deposits in the region, including samples from the Toqui district. They presented 40Ar/39Ar ages of 105 ± 3 Ma on wholerock San Antonio rhyolite and 107 ± 18 Ma on actinolite from Doña Rosa, and concluded that the northern cluster of deposits in Aysén, including the Toqui district, are slightly younger than and formed from a less radiogenic source than deposits farther south.
The Toqui district is situated in the Patagonian Andes at 45° S and is hosted in Early Cretaceous sedimentary rocks at the eastern edge of the Patagonia batholith (Fig. 1). The batholith is 1,200-km-long, with emplacement ages that span 160 to 11 Ma (Bruce et al., 1991). In this part of Patagonia, the main phase of emplacement was between 120 and 90 Ma (Pankhurst et al., 1999). The Early Cretaceous sediments were deposited in a back-arc basin called the Aysén basin, also known as the Rio Mayo embayment (Bell and Suárez, 1997), which developed above a sequence of Jurassic volcanic rocks. The Jurassic volcanic rocks are part of the Chon Aike province, one of the largest silicic volcanic provinces in the world (Pankhurst and Rapela, 1995). Basement to the Jurassic volcanic rocks is the Eastern Andes Metamorphic complex that consists of early Paleozoic passive margin turbidites and platform carbonates, now metamorphosed to lower greenschist facies (Hervé et al., 1998).
The stratigraphy of the Aysén basin includes rocks of the Upper Jurassic Ibañez Group and Lago La Plata Formation, the Early Cretaceous Coyhaique Group, Late Cretaceous Divisidero Group, and the Late Cretaceous to early Tertiary Alto Coyhaique volcanic sequence. The Ibañez Group, described principally from locations in Chile, consists mainly of calc-alkaline subaerial felsic volcanic rocks (Niemeyer et al., 1984) that formed when subduction was initiated on the western side of South America as a result of the opening of the South Atlantic Ocean in the Late Jurassic. The Ibañez Group is correlated with the Lago La Plata Formation, described mainly from locations in Argentina, which consists of andesitic to rhyolitic pyroclastic rocks and flows, with minor black shale and limestone near the top of the unit (Ramos, 1976). Both units formed along the western limit of the Chon Aike volcanic province, but are related to subduction processes rather than plume-driven crustal melting (Rapela et al., 2005).
The Coyhaique Group is consists of marine and continental-derived sedimentary rocks, which overlie the volcanic rocks of the Ibañez Group, and which represents shallow- to deep- water marine facies deposited in the Aysén basin. It includes rocks of the Toqui, Katterfeld, and Apeleg Formations (Suárez et al., 1996). At the type location in the Toqui mining district, the Toqui Formation consists of green volcaniclastic siltstone and sandstone, with minor fossil shell fragments. Interlaminated thin sandstone layers and finer grained units often have well-developed graded bedding within thin laminations and are thought to be turbidites. Also included are pumice lapilli tuff beds that are typically less than 10 cm thick, tuffaceous sandstone, debris flow deposits, and oyster shell-rich limestone beds (Suarez et al., 1996). This latter fossiliferous zone is host to most of the mineralization in the Toqui district. Rocks of the Toqui Formation were deposited in a shallow-water marine environment with nearby active volcanoes (Hervé et al., 2000). Conformably overlying and interfingering with rocks of the Toqui Formation are those of the Katterfeld Formation. They correspond to a sequence of fossiliferous marine black shales, with minor sandstone and calcareous horizons (Ramos, 1981) that represents a deepening of the Aysén basin. Above rocks of the Katterfeld Formation are the youngest rocks of the Coyhaique Group, which define the Apeleg Formation (Ploszkiewicz and Ramos, 1977), a sequence of sandstone, mudstone, and minor conglomerate. Rocks of the Apeleg Formation were deposited in an open marine shelf environment in shallow water and represent the final filling of the Aysén basin (Bell and Suárez, 1997). The distribution and thickness of rocks of the Apeleg Formation suggest the shallow marine sandstone progressively filled the Aysén basin from east to west. A local unconformity separates sedimentary rocks of the Coyhaique Group from the mainly volcanic rocks of the Divisadero Group (Heim, 1940; Ramos, 1981; Bell and Suarez, 1997).
Tectonic inversion of the Aysén basin occurred at about 120 to 110 Ma as a result of a major lithospheric reorganization during the breakup of southern Gondwana. It resulted in the formation of the Lagos La Plata and Fontana fold-and-thrust belt in Argentina (Fig. 1B), only a few kilometers north of the Toqui district (Folguera and Iannizzotto, 2004). The first phases of tectonic inversion are marked by an unconformity within the Divisadero Group that separates andesitic flows and subaqueous volcaniclastic units from overlying subaerial felsic flows and ignimbrites (Folguera and Iannizzotto, 2004). Folguera and Ramos (2008) postulate that this phase of orogenic contraction and basin inversion is related to an episode of shallow subduction from 130 to 110 Ma and restricted to between 42° and 48° S beneath the South American margin.
A stratigraphic section for rocks in the Toqui district is shown in Figure 2. Known mineralization in the district extends over a 6 × 4 km area, elongate in a northwest-southeast orientation (Fig. 3). Units that host economic mineralization include those of the uppermost part of the Ibañez Group and the lower part of the Toqui Formation. These units are well exposed along the Toqui River in the northern part of the district, where they are relatively flat lying. However, in the southern part of the district, units have moderate dip to the southeast and the mineralized layer has been encountered in drill holes at more than 1,000 m below surface (see cross section A-A', Fig. 4).
The upper part of the Ibañez Group is dominated by bedded andesitic pumice-lapilli tuff and pumice-lithic lapilli tuff. Deeper drill holes into the Ibañez Group show a volcanic rock-dominated sequence of andesite to dacite tuff, breccia, and flows. Within the district, no rhyolite ignimbrite units have been recognized in the upper 1,000 m of the Ibañez Group. A zircon extracted from dacite tuff in a drill hole into the Ibañez Group was dated by U-Pb at 137.8 ± 0.4 Ma (Bussey et al., 2010), which is consistent with previous ages for the unit (Pankhurst et al., 2003). The transition from volcanic rocks of the Ibañez Group to sedimentary rocks of the Toqui Formation is marked by calcareous sandstone layers interbedded with thin layers of andesitic tuff over a 10-m-thick interval.
Rocks of the Early Cretaceous Toqui Formation conformably overlie those of the Ibañez Group. They are dominated by volcanic material deposited in a sedimentary environment. Stratigraphic variation within the Toqui Formation is shown schematically in Figure 2. The base of the formation is calcareous sandstone that is locally absent and, in places, as thick as 15 m and is an important ore host. The calcareous sandstone grades upward into fossiliferous limestone, with the upper part of the limestone rich in large Gryphaea (oyster) fossil shells identified as Exogyra couloni (Tapia, 1984). The fossiliferous limestone, which is the principal ore host in the district, varies from 5 to 30 m in thickness and is referred to as the main manto (Fig. 5A). A sharp but conformable contact separates the limestone from the overlying, informally termed banded tuff (Fig. 5C-E). This is a sequence of coarse to fine ash tuff (≤2 mm-size fragments) comprising thin size-graded layers with a combined thickness that varies from a few tens of centimeters to more than 4 m. Soft sediment deformation features are common. Above the banded tuff unit, the Toqui Formation is characterized by another fossiliferous limestone layer, less than 2 m thick, followed by a sequence of massive to bedded volcanic sandstone that varies from 20 to 30 m in thickness and is overlain by massive to weakly bedded ash tuff that varies from 30 to 120 m in thickness across the district. Rare layers of andesitic pumice lapilli tuff, from 10 cm to 1 m in thickness, can be found throughout the Toqui Formation, including in the main manto. They are comprised of flattened pumice lapilli with fiamme-like texture (Fig. 5B) and are thought to be airfall deposits that settled underwater, but the pumice lapilli were flattened by later sedimentary compaction, rather than welding.
Rocks of the Katterfeld Formation were conformably deposited on top of the weakly bedded ash tuff that makes up the top of the Toqui Formation. The Katterfeld Formation varies from less than 5 to 500 m in thickness across the district and is dominated by black shale, with minor thin beds of fine-grained volcanic sandstone and rare thin limestone layers (Fig. 5F). Thicknesses are estimated by subtracting intervals of rhyolite sill in the section, which locally inflate the apparent thickness of Katterfeld Formation by more than 100 percent. The thinnest sections are located at Cerros Estatuas and Concordia (Fig. 3), and the thickest sections are in the southeastern part of the district. Beds of well-sorted andesite pumice lapilli tuff are present in the uppermost part of the Katterfeld Formation in the southeastern part of the district. The interbedded nature of the pumice beds and black shale indicates increased volcanic activity towards the end of black shale deposition.
Overlying rocks of the Katterfeld Formation, volcanic units of the Divisadero Group are subdivided into a lower unit of andesitic flows, laharic breccias (Fig. 5G), and associated sedimentary rocks, and an upper unit dominated by felsic ignimbrite. In the western part of the district, near Estatuas, the preserved Lower Divisadero section consists of 300 m of andesitic flows and breccias, with very little sedimentary material, and is in sharp contact with a thin underlying section of Katterfeld Formation. An unaltered sample of massive andesite from a drill hole in Cerro Estatuas provided a wholerock 40Ar/39Ar age of 122.6 ± 0.8 Ma (Bussey et al., 2010). In the southeastern part of the district, the Lower Divisadero section is at least 800 m thick. The contact between the Upper and Lower Divisadero units appears conformable in some places, but in other places it is marked by a notable angular unconformity (Kakarieka and Bussey, 2007) indicating an episode of deformation and erosion occurred prior to deposition of rocks of the Upper Divisadero Group. This unconformity is correlated with a similar unconformity in the Lagos La Plata and Fontana area just north of the Toqui district, where it has been interpreted as marking the beginning of a period of orogenic shortening (Folguera and Iannizzotto, 2004). Two kilometers south of the Toqui district, the Upper Divisadero unit consists of a uniform sequence of weakly welded rhyolite ignimbrite with moderately developed compaction foliation defined by fiamme. Based on the elevation of the highest exposures, the rhyolite ignimbrite is estimated to be at least 500-m-thick. Age determinations on samples of the Upper Divisadero unit elsewhere in the region indicate a range of 120 to 106 Ma (Ramos, 1981; Folguera and Iannizzotto, 2004).
The Gemelos andesite is the oldest intrusive unit known in the district. It forms hornblende andesite dikes that cut related peperite breccia bodies, the largest of which is 450 m in diameter, that are emplaced into black shale of the Katterfeld Formation. The dikes are as much as 30 m wide and consist of 1 to 2 mm of plagioclase and 10 to 30 mm of hornblende phenocrysts in a fine-grained groundmass. Analyses of hornblende phenocrysts from two dikes yielded 40Ar/39Ar ages of 124 ± 0.7 Ma and 125.1 ± 1.2 Ma (Bussey et al., 2010).
The most prominent intrusive rock in the Toqui district is the 600-m-thick San Antonio rhyolite sill exposed in a cliff on the northwest side of Cerro San Antonio, in the southeastern part of the district (Figs. 3, 4). Numerous sills of San Antonio rhyolite were emplaced into rocks of the Toqui and Katterfeld Formations within the district. The sills range in thickness from 600 m to less than 2 m, with thicker sills in the southeastern part of the district and higher in the section. The sill-forming rhyolite contains 15 percent phenocrysts of euhedral quartz, K-feldspar and trace biotite in a fine-grained groundmass that makes up the remaining 85 percent of the rock (Fig. 5H). Zircons extracted from the San Antonio rhyolite returned a U-Pb age of 120.0 ± 0.1 Ma (Bussey et al., 2010). Every exposure of rhyolite in the district shows effects of alteration; biotite is altered to sericite and K-feldspar is partially to completely altered to sericite plus calcite. The groundmass is now a microgranular intergrowth of K-feldspar, albite, quartz, sericite, and calcite. Locally, the rhyolite contains microgranular enclaves, as much as 20 cm in diameter, consisting of acicular hornblende, plagioclase, and K-feldspar. The microgranular enclaves can be rounded, ellipsoidal, discoid, elongated, or lenticular in shape. Contacts with the enclosing rhyolite can be sharp to diffuse and the enclaves tend to occur in clusters. The microgranular enclaves have not been studied in detail, but are thought to reflect mixing with more mafic magma.
In the southern part of the district, a sill of San Antonio rhyolite, is encountered at the base of the main manto in all the orebodies known as of 1998. When Breakwater Resources started exploration in 1998, geologists noted that shallow drilling southeast of the San Antonio orebody showed that the rhyolite at that location was emplaced not at the base of the main manto, but at the base of the banded tuff unit. They drilled a hole to test for the presence of the main manto limestone beneath the rhyolite sill and discovered the San Antonio southeastern orebody. Subsequent exploration continued the practice of drilling through the San Antonio rhyolite sill to test the base of the Toqui Formation for mineralization. Drilling at the northern end of Aserradero encountered gold mineralization in calcareous sandstone below the rhyolite sill and led to the discovery of Mina Profunda.
The Altazor andesite porphyry sill was emplaced into the upper part of the Katterfeld Formation, stratigraphically above the thick San Antonio rhyolite sill described at Cerro San Antonio. The andesite sill is 45 m thick, can be traced for more than 1 km along strike, and is comprised of abundant plagioclase and hornblende phenocrysts, with lesser pyroxene and K-feldspar, in a small amount of aphanitic groundmass. A U-Pb age determination on extracted zircons returned an age of 119.7 ± 0.7 Ma (Bussey et al., 2010). Weak but pervasive alteration to chlorite, with trace epidote and disseminated pyrite, can be found in all exposures.
The Porvenir dacite forms a sill emplaced into rocks of the Katterfeld and Toqui Formations, between the Aserradero, Porvenir, and Estatuas orebodies (Fig. 4). The sill extends to the southwest and beyond the Porvenir orebody for an unknown distance. The dacite is porphyritic and crystal-rich, with phenocrysts making up more than 60 percent of the rock (Fig. 5I). Phenocrysts of plagioclase, hornblende, and biotite are most abundant, with less than 2 percent rounded quartz phenocrysts present. A U-Pb age of 113.2 ± 0.5 Ma was obtained on zircons extracted from the Porvenir dacite (Bussey et al., 2010). Alteration is pervasive; plagioclase is extensively replaced by very fine grained sericite, actinolite, and quartz, and hornblende and biotite are replaced by actinolite. The groundmass is a fine-grained intergrowth of quartz, Kfeldspar, and actinolite. Quartz-actinolite veinlets are common. The Porvenir dacite contains mafic microgranular enclaves (Fig. 5I), like those described in the San Antonio rhyolite.
Fine-grained, late andesite dikes have been documented in underground mapping and drill holes across the Toqui district, but they are essentially unrecognized at the surface. They crosscut sills of San Antonio rhyolite and Porvenir dacite, as well as skarn mineralization, and may be the youngest igneous unit in the district. Underground mapping shows they are often clustered in swarms with east-west to northwest-southeast orientations and subvertical dips. The widest mapped dike is 10 m thick, but most are 5 m or less.
North-south–trending faults are the oldest structures in the Toqui district, and are offset by nearly all other fault orientations. Between the San Antonio and Mallin-Monica orebodies, a north-trending, dike-like body of rhyolite porphyry connects sills of San Antonio rhyolite that were emplaced at different stratigraphic levels. The Zuñiga and Antolin veins also strike north-south.
The most significant structure recognized in the district is the Melchor fault, a northwest-trending, high-angle structure that runs along the western and southern edge of the district (Figs. 3, 4). The fault has an apparent vertical throw of at least 1 km, but there have been no detailed kinematic studies of this fault to determine the sense of motion. Northwest-trending faults are the most dominant structural features and are associated with ore mineralization, acting as feeders for the hydrothermal fluids. Palacios et al. (1996) describe a mineralized breccia that occupies a northwest-trending fault on the western side of the Doña Rosa orebody, which they interpreted as the main conduit for the ore-forming fluids. Kakarieka and Bussey (2007) documented mineralogical and metal zonation away from specific northwest-trending faults at the Porvenir and Aserradero orebodies. Orebodies in the Toqui district tend to be elongate in a northwest-southeast direction parallel to faults thought to be conduits for mineralizing fluids. This observation was used to help design a series of drill programs from 2001 to 2006 to evaluate isolated mineralized drill intercepts south of the Doña Rosa orebody and led to the discovery of the Aserradero and Porvenir orebodies. In the Estatuas orebody, early north-south faults are followed by development of northwest- to west-northwest– trending faults, and finally northeast-trending faults that include bedding plane thrusts. Kinematic studies (Nelson, 2006) on the faults in Estatuas orebody indicate early north-south normal faults may have controlled depositional thickening of the main manto limestone, but that later motion on the north-south faults was sinistral. Northwest-trending faults show mostly sinistral oblique slip movement and northeast faults show mostly dip-slip to sinistral oblique-slip movement. Although not consistently evident, bedding plane thrust faulting appears to be the youngest structural activity, as a thrust fault can be seen cutting a northeast-trending fault in the Estatuas orebody. Several drill holes in the Altazor area encountered two intervals of the main ore-hosting manto limestone and these are explained by local, low-angle reverse faulting within the Toqui Formation.
Geologic Setting of New Discoveries
Stratigraphic relationships at the most recent discoveries in the district are illustrated in Figure 6. Units dip gently to the southeast and postmineral faulting has disrupted individual orebodies. The San Antonio rhyolite intruded along the base of the Toqui Formation throughout the district, typically at the base of the main manto limestone, although it has locally been found immediately above the main manto and as sills within the upper part of the Ibañez Group. The Porvenir dacite was emplaced as a sill within rocks of the Toqui Formation above the main manto limestone, followed a fault upsection in the vicinity of the Porvenir orebody, and then is present to the northeast as a sill within rocks of the Katterfeld Formation. Other important stratigraphic relationships within the Toqui Formation are not readily apparent at this scale, but can be seen in cross sections with vertical exaggeration (Fig. 7).
The Mina Profunda orebody is hosted in calcareous sandstone at the boundary between the lower Toqui Formation and the top of the Ibañez Group (Fig. 7, section C-C'). Thin lenses and layers of andesite tuff, often thinly laminated, are interbedded with calcareous sandstone which grades downward into bedded lithic lapilli tuffs and breccias of andesitic composition. The top of the Mina Profunda orebody occurs along the base of a thin pumice lapilli tuff layer within the calcareous sandstone.
The Porvenir and Aserradero orebodies are hosted in the middle to upper part of the main manto limestone (Fig. 7, section D-D') with the base of the banded tuff forming the upper limit of ore. The calcareous sandstone below the main manto limestone is relatively thick in the Aserradero area and thins rapidly to the southwest beneath the Porvenir orebody. The three pumice lapilli tuff beds present at the base of the main manto at Porvenir and Aserradero are similar to three tuff beds located directly above Mina Profunda (Fig. 7), but no detailed stratigraphic studies have been done to prove a correlation.
The northern and southern groups of orebodies have distinct styles of alteration. The northern orebodies (Concordia and Mallines) are characterized by massive sulfide replacement of the main manto limestone and chlorite and sericite alteration, with lesser adularia and rhodochrosite, in and surrounding the orebodies (Fig. 8). Sericite and adularia are more abundant in places where the main manto limestone had higher siliciclastic content, such as in the thin tuffaceous layers. Altered rock is separated from fresh limestone by local zones of intense silicification. All the northern deposits hosted in the main manto limestone are characterized by a lack of skarn formation.
The southern group of orebodies is characterized by well-developed calc-silicate alteration minerals (Palacios et al., 1996). The San Antonio, Mallin-Monica, and Doña Rosa orebodies have unaltered limestone mapped along their northern borders (Fig. 8) and the transition to calc-silicate skarn is abrupt, occurring over a few tens of centimeters. Skarn near this front is pyroxene-dominant and contains minor amounts of bustamite and ilvaite.
Biotite hornfels developed in an ash-rich volcanic siltstone horizon that directly overlies the upper manto limestone. The thickness of biotite hornfels decreases from south to north, being almost 6 m thick over the Porvenir orebody, 3 to 4 m thick over Aserradero, and less than 1 m thick over Doña Rosa and San Antonio. Biotite hornfels is often cut by pyroxene-amphibole veins, with white pyroxene halos (Fig. 9A).
Most of the orebodies in the southern part of the district contain at least minor amounts of garnet. Garnet is typically zoned with a colored core and birefringent, nearly colorless rims (Fig. 10A). In orebodies with significant garnet, the color of the garnet changes systematically across the orebody. For example, at Porvenir (Fig. 11, D-D') there is red-brown garnet at the base of the main manto (Fig. 9B), which grades upward into brown garnet (Fig. 9C), and then into green garnet. The amount of pyroxene gradually increases from the red to brown to green garnet zones until pyroxene is dominant over garnet (Fig. 9D). Away from the ore zones, epidote is the dominant calc-silicate alteration mineral.
All the southern orebodies contain significant amounts of early-formed pyroxene. As in most skarn systems (Meinert et al., 1997, 2005), the pyroxene (Fig. 9C-G) contains fluid inclusions with multiple daughter minerals (Fig. 10B), indicative of forming from high-temperature, high-salinity hydro -thermal fluids. A systematic study of skarn mineral chemistry has not been done at Toqui, but petrographic examinations have been made on samples from every significant orebody in the district (Meinert, 2004, 2007, 2008). Subtle compositional variation of skarn minerals across the district cannot be ruled out, but it is clear that nearly all pyroxene in the district is Fe rich. Representative compositions of pyroxene, garnet, and amphibole are shown in Table 2.
Early garnet-pyroxene skarn alteration of the main manto extends from the Estatuas orebody in the west to the Altazor area in the east (Fig. 8), covering a 15-km2 area. The alteration remains open to the northwest, southeast, and southwest. Cutting through the middle of the early skarn is a narrow, north-northeast –trending zone of retrograde amphibole alteration that is >2-km-long and as much as 300-m-wide. It extends from the central part of the Doña Rosa orebody in the north, through Mina Profunda, and to the southern tip of Aserradero (Fig. 8). The southeastern edge of the Porvenir orebody is also altered to amphibole.
At Mina Profunda (Fig. 11, C-C'), gold mineralization is associated with a central area of retrograde amphibole. Amphibole forms fine- to coarse-grained masses and occurs with minor lath-shaped epidote and traces of chlorite, calcite, and quartz (Fig. 10C). Locally, amphibole and pyroxene exhibit delicate banding after compositional layering within the calcareous sandstone (Fig. 9G). In places, euhedral amphibole is completely surrounded by massive arsenopyrite ± cobaltite (Fig. 10E). Locally well-developed bladed calcite in vugs and veins, often partially replaced by quartz, is associated with amphibole, arsenopyrite, and cobaltite (Fig. 9I). Surrounding the central area of amphibole replacement and associated gold mineralization is a halo of similar-looking rock overprinted by texturally late, low-temperature alteration characterized by chlorite-clay replacement of some earlier skarn minerals (Fig. 10D), and vein calcite. Sericite, with minor chlorite and epidote, are the dominant alteration minerals within the calcareous sandstone unit outside the orebody.
Aserradero (Fig. 11 D-D') shows the same skarn mineral zonation as described at Mina Profunda, with distinctive dark green retrograde amphibole replacement of pyroxene associated with economic gold mineralization. Strong amphibole alteration formed along the base of individual pumice tuff layers indicating the layers were barriers to fluid flow. The thickest and most intense zone of alteration and mineralization is directly beneath the banded tuff that overlies the main manto. The banded tuff appears to have formed a seal on the hydrothermal system and focused fluid flow within the main manto.
In the Concordia area, a deep drill hole intercepted (1) several hundred meters of chlorite-epidote–altered volcanic rocks, with quartz-epidote-pyrite-chalcopyrite veinlets, (2) 400 m of quartz-pyrite altered rock cut by quartz-pyrite-chalcopyrite-molybdenite-tourmaline veinlets with quartz-sericitepyrite halos, and (3) 300 m of pervasively chloritized volcanic rock, with quartz-pyrite-anhydrite veinlets, magnetite-pyrite veinlets, and quartz-chalcopyrite-molybdenite-epidote veinlets. Although Cu grades are subeconomic, this porphyrystyle stockwork mineralization is evidence of a significant intrusion-related hydrothermal system. Widely spaced deep drill holes across the district have identified two centers of stockwork-style mineralization: one beneath the Concordia area as described above and an area of similar alteration and veining beneath the Altazor area (Fig. 8). Intrusions responsible for the porphyry-style mineralization have not been recognized but would be expected at depth below the areas of stockwork veins.
Geochronology of alteration and mineralization
Alteration and ore minerals, as well as zircons from igneous rocks, were collected from several sites across the district for isotopic dating; the results are summarized in Figure 12. Resulting 40Ar/39Ar and zircon U-Pb (TIMS) dates were determined at the Pacific Center for Isotopic and Geochemical Research in Vancouver, Canada, and Re-Os dates at the AIRIE laboratory, Colorado State University. Samples included sericite from altered pumice lapilli tuff layers from the Concordia and San Antonio orebodies, actinolite from Aserradero, biotite from hornfels above the Porvenir orebody, and molybdenite from the deep quartz-molybdenite-pyrite-chalcopyrite veinlets beneath Concordia and Altazor. The Re-Os ages of molybdenite from the deep veinlets beneath Concordia and Altazor range from 120.1 ± 0.4 Ma to 118.0 ± 0.4 Ma and overlap with an 40Ar/39Ar age of sericite of 117.8 ± 0.7 Ma from Concordia (Bussey et al., 2010). Samples from the southern orebodies returned ages that cluster at ca. 110 Ma, with actinolite from Aserradero at 111.3 ± 0.7 Ma, biotite hornfels from Porvenir at 109.0 ± 0.3 Ma, and sericite from San Antonio at 108.8 ± 0.7 Ma (Bussey et al., 2010).
For the most part, the ages presented here compare favorably with ages published by Palacios et al. (1996) and Townley and Godwin (2001). Palacios et al. (1996) presented three whole rock K-Ar age determinations on igneous rocks from the Toqui district. Two of those were samples of sericite-altered San Antonio rhyolite (from San Antonio and Doña Rosa orebodies) and the resulting ages were interpreted by Palacios et al. (1996) to reflect the age of mineralization. Their San Antonio orebody sample returned an age of 108 ± 4 Ma, which is essentially identical to the 108.8 ± 0.7 Ma age on sericite presented here. Their Doña Rosa orebody sample returned an age of 100 ± 2 Ma, which is significantly younger than all of the ages on hydrothermal minerals presented here and is thought to have suffered some argon loss or partial thermal resetting. The ages published by Townley and Godwin (2001) are only slightly younger than ages presented here for alteration at the southern orebodies. They include a whole-rock 40Ar/39Ar age of 105 ± 3 Ma on a sample of San Antonio rhyolite and a 40Ar/39Ar age of 107 ± 18 Ma on actinolite from the Doña Rosa orebody.
Sulfide and metal zonation
Sphalerite is the most abundant base metal sulfide mineral in the Toqui district. Other abundant sulfide and oxide minerals include pyrrhotite, pyrite, magnetite, and arsenopyrite, with minor chalcopyrite and galena, and lesser amounts of cobaltite, electrum, native bismuth, and a variety of sulfosalts. The orebodies in the district can be characterized by their dominant sulfide mineralogy (Fig. 13) and, thus, metal associations.
Concordia, in the northern part of the district, contains relatively high grades of Ag (avg about 70 g/t) and Pb (4%), in addition to 10 percent Zn, with local areas of high Cu. The highest Cu grade drill hole intercept is 3.9 percent Cu over 5 m, but the orebody average is 0.3 percent. Mineralization is mostly disseminated and massive sulfide replacement of limestone. Sphalerite has a very low Fe content and pyrite is the dominant sulfide gangue mineral. Mallines, located just southeast of Concordia, has similar mineralogy to Concordia, but the Zn grade is lower, averaging 4 percent.
In the southern part of the district, the garnet-rich Altazor area is mineralized with a distinctive magnetite-chalcopyrite assemblage, along with minor amounts of sphalerite and galena. Estatuas is a sphalerite-dominated orebody (avg 8% Zn), with very low grades of Ag (avg 7 g/t) and Pb (less than 0.1%). The sphalerite is Fe-rich and there is a relatively low percentage of pyrite and pyrrhotite, estimated to be less than 3 percent. The San Antonio and Mallín-Monica orebodies are characterized by Fe-rich sphalerite, pyrrhotite greater than pyrite, and locally abundant galena. Doña Rosa is similar to San Antonio and Mallín-Monica in that Fe-rich sphalerite and pyrrhotite was dominant over pyrite, but it also contained locally abundant arsenopyrite and galena. Gold grades were greater than 2 g/t in the western part of the orebody affected by retrograde amphibole alteration. In their paragenetic study of the Doña Rosa orebody, Palacios et al. (1996) recognized early pyrite, pyrrhotite, and scheelite, followed by sphalerite, galena, and chalcopyrite, and finally a late event with arsenopyrite, gold, hessite, and maldonite. Porvenir, the southernmost of the orebodies, contains Fe-rich sphalerite, abundant arsenopyrite (up to 4.75% As over 6 m in some drill holes; orebody average is 0.53%), and relatively low amounts of galena (orebody averages less than 0.01% Pb) and Ag-bearing minerals (avg 2.5 g/t Ag).
Aserradero and Mina Profunda are unique in the Toqui district in that the gold is the most important economic metal. They are best classified as gold skarns with by-product Zn. Aserradero is hosted in the main manto limestone and contains about 6 percent Zn, but Mina Profunda, hosted in the underlying calcareous sandstone and separated from the overlying main manto by a 30-m-thick intervening rhyolite sill, contains less than 3 percent Zn. Cobaltite is common in these two orebodies and both orebodies average about 0.2% Co (a Co reserve has not been calculated). Gold and cobaltite are always associated with arsenopyrite, but arsenopyrite is not consistently associated with gold or cobaltite. For example, the Porvenir, Aserradero, and Mina Profunda orebodies are characterized by high volumes of arsenopyrite (0.53, 0.93, and 1.6%, respectively), but Porvenir has essentially no gold or cobaltite, except along a small part of the southeastern edge of the orebody, whereas Aserradero and Mina Profunda have relatively high-grade gold (6.25 and 6.73 g/t, respectively), with local zones exceeding 20 g/t.
Petrographic study indicates deposition of arsenopyrite and cobaltite overlapped with and continued after deposition of amphibole. Native gold, in grains as much as 30 μm in diameter, is most commonly found within arsenopyrite or pyrrhotite along fractures or crystal facies. It is associated with native bismuth and an unidentified mineral thought to be a Ag-rich sulfosalt (Fig. 10F). Cobaltite is difficult to discern from arsenopyrite, both in hand specimen and thin section, unless it has its characteristic cubic habit, but the two phases appear to be coeval.
Metal zonation within the main manto has been compiled for the central part of the southern Toqui district (Fig. 14). For each drill hole, a weighted average metal value was calculated for the full thickness of the main manto limestone, although in many holes the entire thickness of manto is not mineralized. Many older drill holes did not have a full suite of elements analyzed. Metal zonation at the Mina Profunda orebody is not shown because it is hosted in the underlying calcareous sandstone unit. A gridded surface was created from the point data, using a search radius of 100 m, which tends to exaggerate higher grades along the margins of drilled areas and in isolated drill holes, but overall provides an accurate representation of metal distribution, particularly in areas with good drill hole coverage.
Zinc grades are fairly uniform across most of the known orebodies and, therefore, not shown in Figure 14. Similar Zn grades are present in pyroxene-rich orebodies, such as San Antonio and Mallin-Monica to the north, and the garnet-rich Porvenir orebody in the south. In contrast, Pb shows a strong north-south zonation, being enriched in the northern part of the area (Fig. 14A), particularly the western part of Doña Rosa orebody where most of the orebody contains greater than 4,000 ppm Pb. The Porvenir orebody in the south has less than 500 ppm Pb. Silver (not shown) has a very similar distribution to that of Pb. Copper (Fig. 14B) has a much different distribution than the other base metals, with values greater than 0.8 percent Cu located along the western margin of the Doña Rosa orebody and the northwest end of the Aserradero orebody, where it occurs as chalcopyrite and tends to be associated with epidote- or chlorite-bearing alteration. Arsenic shows values greater than 1 percent in the central parts of the Aserradero and Porvenir orebodies (Fig. 14C) and along the western edge of the Doña Rosa orebody. Gold shows a strong correlation with the distribution of amphibole-rich alteration, particularly at Aserradero and Porvenir. Only a small percentage of drill holes at Doña Rosa have As or Au data, so the distribution of these elements at Doña Rosa is not accurately represented in Figure 14C, D. The western half of the orebody shows the highest Au values, which is consistent with production history that indicates the western part of Doña Rosa averaged over 2 g/t Au. Note that there is a good correlation between As and Au at Aserradero, but a very poor correlation at Porvenir. Almost no Co or Bi data are available for the San Antonio, Mallin-Monica, or Doña Rosa orebodies, but at Aserradero and Porvenir these elements have a very similar distribution to that of Au.
Metal grades for Zn, As, and Au at the Mina Profunda, Porvenir, and Aserradero orebodies, superimposed on alteration data, are contoured on schematic cross sections in Figure 15. Zinc grades for the Mina Profunda orebody (C-C', Fig. 15A) are quite low, with only a 1-m-thick layer at the top of the orebody being greater than 5 percent Zn. At Aserradero (D-D', Fig 15A), strong control on mineralization by the overlying banded tuff and the thin internal pumice lapilli tuff layers can be seen. At Porvenir, the central part of the orebody is a zone grading greater than 30 percent Zn and it is clear that the red-brown garnet zone at the base of the orebody is not mineralized. Arsenic is widely distributed surrounding the Mina Profunda orebody (C-C', Fig. 15B), with values greater than 0.1 percent extending over 5 m above and below the economic limit of the orebody. Arsenic concentrations of as much as 10 percent extend nearly 50 m laterally beyond the edge of the orebody on the right (northeast) side of the section.
The two distinct skarn styles displayed at Porvenir and Aserradero (e.g., Fig. 11, section D-D') are mirrored by distinct mineralization characteristics. At Porvenir, a zone of greater than 1 percent As is situated at the top of the economic orebody with local areas greater than 10 percent As lying immediately above the orebody and hosted in the banded tuff and upper manto limestone (D-D', Fig. 15B). Arsenic concentrations in the Aserradero orebody are very consistent and these enrichments continue into the calcareous sandstone below the Main Manto. The distribution of gold is quite different between the Porvenir and Aserradero orebodies (Fig. 15C). Along cross section D-D', there is no anomalous gold in the Porvenir orebody, whereas gold concentrations at Aserradero range from 2 to >20 g/t, and are associated with strong amphibole-rich alteration within the main manto limestone.
Two clusters of ages are present in the geochronology data: an older group at about 119 Ma (120.1 ± 0.4 to 117.8 ± 0.7 Ma) and a younger cluster at 110 Ma (113.2 ± 0.5 to 105 ± 3 Ma). The older group includes igneous ages for the San Antonio rhyolite and Altazor andesite, as well as hydrothermal ages for the molybdenite in the porphyry-style stockworks and sericite in the Concordia orebody. The younger cluster consists of seven ages that average about 110 Ma, including an igneous age on the Porvenir dacite and hydrothermal ages on actinolite, sericite, and biotite from skarn orebodies in the southern part of the district.
The coincidence of the Re-Os ages on molybdenite in the two stockwork vein systems with the magmatic ages of the San Antonio rhyolite and Altazor andesite intrusions indicates porphyry-style mineralization in the district was associated with a pulse of magmatic activity at about 120 Ma. The 40Ar/39Ar age of 117.8 ± 0.7 Ma on sericite from the Concordia orebody suggests the replacement-style mineralization in the north part of the district may have formed at the same time as the porphyry-style mineralization. However, additional geochronology on the northern orebodies needs to be done to have confidence in this interpretation. Skarn mineralization in the southern part of the district may be correlated to the younger cluster of ages that averages 110 Ma and may be related to another pulse of magmatic activity represented by the Porvenir dacite at 113.2 ± 0.5 Ma. Although all the UPb, Re-Os, and 40Ar/39Ar determinations on igneous minerals that give ages in the range of 130 to 120 Ma are believed to be robust and reliable, the range of 40Ar/39Ar and K-Ar ages, both from this and previous studies, which give younger ages in the range of 111 to 100 Ma, are questionable and may reflect resetting, Ar loss, or analytical problems. If there are separate igneous-hydrothermal events from 111 to 100 Ma, then more data will be required to unambiguously establish their existence.
Development of the unconformity between rocks of the lower and upper Divisadero Group is bracketed by an age of 122.6 ± 0.8 Ma from the Lower Divisadero Group at Cerro Estatuas and ages of 120 to 106 Ma on rhyolite from the Upper Divisadero Group. This marks the beginning of contractional deformation and basin inversion in the Lagos La Plata and Fontana fold-and-thrust belt (Folguera and Iannizzotto, 2004). Although there is little evidence for major thrusting in the Toqui district, bedding plane thrusts in the Estatuas orebody, repeated intervals of main manto limestone in the Altazor area, and the presence of the Divisadero Group unconformity at the southern end of the district indicate that the Toqui district was affected by the Lagos La Plata and Fontana deformation event. The timing of magmatic and hydrothermal activity in the district overlaps with the age of compressional deformation in the Lagos La Plata and Fontana fold-and-thrust belt and the interpreted period of shallow subduction in this part of the southern Andes from 130 to 110 Ma (Folguera and Iannizzotto, 2004; Folguera and Ramos, 2008).
Similar to most large skarn systems (e.g., Meinert et al., 2005), the Toqui district contains well-developed mineral zonation that is a function of the temperature and chemistry of the hydrothermal fluids and the nature of the country rock. Skarn mineralogy changes from garnet-rich in the south (Porvenir and Altazor), to pyroxene-rich (San Antonio, Monica, Doña Rosa, Estatuas) farther north, with retrograde skarn between the orebodies. Although chemical conditions that suppress calc-silicate formation have been documented at high temperature (e.g., Johnson and Norton, 1991), it is thought that the chlorite-sericite-adularia-rhodochrosite alteration surrounding the northernmost orebodies in the district (Concordia and Mallines) was formed by fluids that had cooled below the stability fields for calc-silicate minerals. These orebodies are, therefore, considered to be low-temperature carbonate replacement deposits.
In the southern part of the district, the garnet/pyroxene ratio may be used to determine the proximal vs. distal setting of alteration with respect to the fluid conduits. In most skarn deposits, the garnet/pyroxene ratio reflects both the temperature of the skarn-forming fluid and the oxidation state of the system. Higher temperature and/or more oxidized parts of a system tend to be more garnet rich than lower temperature and more reduced parts of the system (Meinert et al., 2005). Thus, proximal parts of skarn systems have higher garnet/pyroxene ratios than distal parts. Therefore, the large area of high garnet/pyroxene ratios surrounding Altazor, an area that also contains significant magnetite, is considered the most proximal skarn and center to the district. The relatively garnet-rich Porvenir and Estatuas orebodies are also considered proximal, whereas Doña Rosa, Mallin-Monica, and San Antonio have distal pyroxene-dominated skarn assemblages. At the scale of individual orebodies, proximal to distal relationships help identify feeder structures through which hydrothermal fluids accessed the main manto limestone. Cutting through the middle of this zonation is a north-northwest–trending zone of Au-Co mineralization that is associated with retrograde amphibole replacement of early pyroxene (Fig. 8). In terms of district-scale zonation, there is a systematic progression to distal skarn and sulfide assemblages going from south to north. The simplest explanation involves a single mineralizing event whose fluid source was beneath the Altazor area. Fluids moved up through volcanic rocks of the Ibañez Group, along mostly northeast-trending faults, then laterally into rocks of the Toqui Formation, presumably cooling as they moved away from their source.
The calc-silicate mineralogy at Toqui is Mn poor compared to typical Zn skarns (Meinert et al., 2005), although minor rhodochrosite is present in the Concordia orebodies and traces of bustamite are present in the skarn front at San Antonio. The position of mineralization at the outer edge of skarn at Toqui is typical for Zn skarns. The gold-rich orebodies at Aserradero and Mina Profunda, as well as the gold-rich parts of Doña Rosa and Porvenir, are dominated by paragentically late actinolitic amphibole. The fact that amphibole, arsenopyrite, and cobaltite are intimately associated with bladed calcite is strong evidence that they were deposited by boiling, low-salinity fluids. The gold-rich zones have anomalous concentrations of Bi, Te, As, and Co, which is common for gold skarns (Meinert et al., 2005). The distinct alteration and metal signatures of these orebodies, compared to the remainder of the district, suggests a separate source for the gold-bearing fluids.
Gold skarns tend to be associated with reduced diorite-granodiorite plutons and dike-sill complexes, whereas Zn skarns are related to oxidized felsic intrusions (Meinert, 2000; Meinert et al., 2005). The presence of a separate mafic-intermediate composition magma body beneath the district, indicated by locally abundant hornblende-rich microgranular enclaves in the Porvenir dacite sill, raises the possibility that late-stage gold-rich hydrothermal fluids from this magma could be responsible for the gold-rich orebodies at Toqui. Mafic magmas emplaced at the base of a fractionating felsic magma chamber can provide heat, volatiles, and metals to the overlying chamber allowing generation of a metal-rich hydrothermal fluid (Keith et al., 1997, 1998).
The preferred model for the evolution of mineralization in the southern Toqui district is along the lines of the genetic model for porphyry copper systems summarized by Sillitoe (2010). Early generation of a single-phase liquid from magma took place under lithostatic conditions, with subsequent phase separation to a hyper-saline liquid and vapor. High temperature hyper-saline fluid entered the limestone of the lower Toqui Formation and formed early garnet-pyroxene skarn. Cooling and uplift of the system resulted in a transition to hydrostatic conditions and generation of a single-phase aqueous fluid that formed retrograde epidote-chlorite-actinolite skarn and Zn-rich base metal sulfide mineralization (Meinert et al., 2003). Late generation of magmatic fluid under increasing depth and pressure conditions formed a gold-rich fluid (Hedenquist et al., 1998; Heinrich et al., 2004) that ascended along a north-northwest–trending fault system. Upon entering the lower Toqui Formation, it reacted with early skarn to form retrograde amphibole. Boiling of the hydrothermal fluids may have aided deposition of gold and cobaltite as suggested by the association of amphibole, arsenopyrite, and cobaltite with bladed calcite in the Mina Profunda orebody. On-going compressive deformation associated with the Lagos La Plata and Fontana fold-and-thrust belt was probably important to maintain elevated pressure required for generation of a late gold-rich fluid.
The Toqui district consists of seven skarn orebodies (San Antonio, Mallin-Monica, Doña Rosa, Estatuas, Mina Profunda, Aserradero, and Porvenir), two replacement deposits (Concordia and Mallines), and two Zn-Pb-Ag vein deposits (Zuñiga and Antolin). The total geological resource for the district is 20 Mt grading 8.2 percent Zn and 1.5 g/t Au, one-half of which has been discovered in the past ten years, and mineralization remains open to the northwest, east, and southeast of the known orebodies. Mineralization is hosted predominately in a 5- to 30-m-thick limestone at the base of the Cretaceous Toqui Formation. Skarn and alteration mineral zonation across the 24 square kilometer district reveals progressively distal assemblages from southeast to northwest, suggesting the source of hydrothermal fluids was under the southeastern part of the district. Recently discovered porphyry-style alteration and veining beneath the district demonstrate that it is a large integrated mineral system, which is zoned from a porphyry center to distal, skarns, mantos, and veins.
The Toqui district is large and complex and the relationships among the various igneous events and styles of mineralization are not yet fully understood. Geochronological evidence demonstrates multiple episodes of magmatic-hydrothermal activity in the district peaking at 120.1 ± 0.4 to 117.8 ± 0.7 Ma and possibly 113.2 ± 0.5 to 105 ± 3 Ma, which are synchronous with regional compressive deformation represented by the Lagos La Plata and Fontana fold-and-thrust belt. Northwest-trending faults focused hydrothermal fluids from depth into the limestone host. A paragenetically late stage of gold mineralization in the southern part of the district formed the Aserradero and Mina Profunda Au skarn orebodies and was associated with retrograde amphibole alteration of early garnet-pyroxene skarn. As exploration continues to expand the size of the district, new information will undoubtedly lead to a better understanding of the origin of this important ore district in southern Chile.
The authors thank Breakwater Resources Ltd. for permission to publish this paper, and are grateful for discussions with former and current mine geologists whose careful mapping, logging and documentation of all aspects of the geology at Toqui were instrumental in the exploration success. In particular, Eduardo Esparza, Roberto Castañón, Franco Lay-Son, José L. Jara, Gonzalo Henríquez, Rodrigo Molina, Mauricio Arce, Aldo Crino, and Marcelo Alfaro are thanked for their contributions. Critical and constructive reviews by Zhaoshan Chang and Peter Megaw resulted in a substantially improved manuscript.
Figures & Tables
The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries
VOLCANIC-ASSOCIATED and sedimentary-exhalative massive sulfide deposits on land account for more than one-half of the world's total past production and current reserves of zinc and lead, 7 percent of the copper, 18 percent of the silver, and a significant amount of gold and other by-product metals (Singer, 1995). A new source of these metals is now being considered for exploitation from deep-sea massive sulfide deposits. Because the oceans cover more than 70 percent of the Earth's surface, many expect the ocean floor to host a proportionately large number of these deposits. However, there have been few attempts to estimate the global mineral potential. Significant accumulations of metals from hydrothermal vents have been documented at some locations (e.g., 91.7 Mt of 2.06% Zn, 0.46% Cu, 58.5 g/t Co, 40.95 g/t Ag, and 0.51 g/t Au in the Atlantis II Deep of the Red Sea: Mustafa et al., 1984; Nawab, 1984; Guney et al., 1988). Even more metal is contained in deep-sea manganese nodules. Current estimates in the U.S. Geological Survey (USGS) mineral commodities summaries indicate a global resource of copper in deep-sea nodules of about 700 Mt. In the Pacific "high-grade" area, an estimated 34,000 Mt of nodules contain 7,500 Mt of Mn, 340 Mt of Ni, 265 Mt of Cu, and 78 Mt of Co (Morgan, 2000; Rona, 2003). A number of countries, including China, Japan, Korea, Russia, France, and Germany, are actively exploring this area.