Plantilla de artículo 2013
Andean Geology 46 (3): 445-470, September, 2019
Andean Geology
doi: 10.5027/andgeoV46n3-3041
Early Jurassic arc related magmatism associated with porphyry copper mineralization at Zafranal, Southern Peru unraveled by zircon U-Pb ages
*Alan Santos1, 4, Guo Weimin2, Fernando Rivera3, Colombo Tassinari4, Luis Cerpa5, Shoji Kojima1,

1 Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile.;

2 Nanjing Center, China Geological Survey, 534 Este Calle Zhongshan, China.

3 Universidad Nacional Mayor de San Marcos, Lima distrito 15081, Perú.

4 Instituto de Geociencias de la Universidad de Sao Paulo, Rua do Lago, 526, Sao Paulo Brasil..

5 Instituto Geológico Minero y Metalúrgico, Av. Canadá 1470-San Borja, Lima, Perú.

* Corresponding author:

Early Jurassic arc-related igneous rocks host porphyry copper prospects and gold-bearing quartz vein deposits in southern Peru. Ten new zircon U-Pb ages for wall rocks of gold-bearing quartz veins, Jurassic rocks and copper-mineralized porphyry bodies in Zafranal porphyry copper, together with published ages for Jurassic rocks, reveal a continuous magmatic evolution of the early Jurassic arc. The Jurassic rocks and gold-bearing quartz vein systems in the western flank of the Western Cordillera are hosted by Paleo- and Meso-proterozoic orthogneisses of the Arequipa Massif (1.75-1.44 Ga) that underwent Grenville-age metamorphism ~1 Ga. The early mafic magmatism is recorded between 199.6-193.2 Ma, and was followed by dominantly felsic magmatism from 184.1-174.9 Ma. Both magmatic events have formed the thinnest intrusive belt (<15 km wide) of the Coastal Batholith in southern Peru. The last magmatic event of the early Jurassic (181.0-174.9 Ma) is represented by several phases of porphyries associated with copper mineralization in the Zafranal porphyry copper deposit. The published ages indicate that the magmatic arc migrated along eastern limit of the Arequipa Massif during Middle Jurassic. In the late Jurassic (~146 Ma) the magmatic locus returned near early Jurassic intrusion. Overall, the plutonic intrusive rocks and porphyry bodies with copper mineralization represent the oldest magmatic events of the Coastal Batholith of Peru formed during the early Jurassic.

Keywords: Coastal Batholith, Zafranal porphyry, Early Jurassic, U-Pb zircon age, SHRIMP.



1. Introduction

Considerable volumes of magma in the Peruvian convergent margin magmatic-arc, that formed the Coastal Batholith, were generated during a protracted time span (Jenks and Harris, 1953; Cobbing and Pitcher, 1972; Cobbing et al., 1977). Mesozoic plutons were emplaced within metamorphic rocks of Arequipa Massif (Stewart et al., 1974; Cobbing et al., 1977; Casquet et al., 2010). In the Arequipa Segment the emplacement zone (<15 km wide) was controlled by large faults (Caldas, 1993; Demouy et al., 2012). These faults integrated in the Cincha-Lluta fault system, are thought to have generated crustal weakness zones that allowed repetitive injections of magma (Caldas, 1993) and associated copper-gold mineralization (Carlotto et al., 2009a). The mineralization includes gold-bearing quartz veins and porphyry type deposits, and thus a significant number of porphyry copper and associated Mesozoic magmatic-hydrothermal deposits prospects have been identified (Fig. 1). The porphyry copper prospects show a marked tendency to occur in typically orogen-parallel linear belts (Sillitoe and Perelló, 2005) or within magmatic arc (Seedorff et al., 2005). Each belt corresponds to a magmatic arc of broadly similar overall dimensions (Sillitoe, 2010); furthermore, each metallogenic belt migrates systematically farther east, related to the magmatic arc (Sillitoe, 2003). Determining the temporal and spatial evolution of the magmatic arcs of Coastal Batholith (Arequipa Segment; Cobbing et al., 1977), in southern Peru will contribute to our understanding of metallogenic epoch, as a first order approach for exploration targets. According to the geochronological data the Coastal Batholith was considered to have been emplaced in late Cretaceous (Cobbing and Pitcher, 1972; Stewart et al., 1974; Cobbing et al., 1977; Weibel et al., 1978; Cordani et al., 1985; Beckinsale et al., 1985; Mukasa, 1986; Caldas, 1993; Schildgen et al., 2009; Carlotto et al., 2009a), but afterwards divided into two groups one early Jurassic and other Upper Cretaceous (Demouy et al., 2012; Boeckhout, 2012). However, the emplacement of the first magmatic suites of the Coastal Batholith, occur during the early Jurassic and is located from Arequipa to Chuquibamba (Fig. 1). Early Jurassic rocks are registered in southern Arequipa by Mukasa (1986), who considered that the extent of this arc remains unsolved. Thus, more recent studies (Demouy et al., 2012; Boeckhout, 2012) have focused on the Jurassic arc extent close to Arequipa. Therefore, this study aims to determine the early Jurassic arc extent to the northwest of Arequipa and crystallization age of porphyry bodies related to copper mineralization in Zafranal porphyry copper deposit by means of zircon U-Pb ages with sensitive high-resolution ion microprobe (SHRIMP) and laser ablation inductively-coupled plasma spectrometry (LA-ICP-MS) determinations. Furthermore, this paper describes the regional geological setting of Jurassic intrusive rocks (Coastal Batholith-Arequipa Segment), with particular emphasis on the porphyritic bodies that host the copper mineralization in Zafranal, based on more than four years of fieldwork by the authors during a study of the Coastal Batholith program that included 1:25,000-scale regional mapping.



Fig. 1. A. Regional geologic map of the northern sector of the Arequipa Segment with the location of the main porphyry copper deposit (modified from Santos et al., 2016). The age data of blue letters are compiled from Stewart et al. (1974); Cordani et al. (1985); JICA (1986); Mukasa (1986); Demouy et al. (2012); Rivera (2012a); Huamán et al. (2014); Santos et al. (2016), and those of black letters are obtained in this work. B. Geological section. The location is in map.


2. Tectonomagmatic setting

The early Jurassic intrusive rocks of southern Peru extends for ̴130 km between latitudes 15°48’ and 16°36’ S and contains copper and gold-bearing mineral deposits, such as porphyry copper and gold-bearing quartz veins (Fig. 1). The oldest rocks within the study area are assigned to the metamorphic Arequipa Massif (Cobbing and Pitcher, 1972) of Proterozoic age (Cobbing et al., 1977; Dalmayrac et al., 1977; Shackleton et al., 1979). This massif represents the site of a complex history of sedimentation, magmatism and metamorphism in the Paleoproterozoic, and development of a sedimentary basin in the Mesoproterozoic (Casquet et al., 2010). The Arequipa Massif was accreted to the western margin of Gondwana, in the late Proterozoic related to the Sunsas orogeny (Wasteneys et al., 1995; Loewy et al., 2004; Chew et al., 2007; Ramos, 2008). Following, terrane accretion, between 850 to 650 Ma the Andean margin of Peru was quiescent (James, 1971; Chew et al., 2008; Acosta and Sempere, 2017). During in the Early Paleozoic (Ordovician and Silurian-Devonian) it was affected by extensive tectonic events and development of coast-parallel magmatic arcs (Shackleton et al., 1979; Mukasa and Henry, 1990; Bahlburg et al., 2006; Reitsma, 2012). Subsequently, Permian-Triassic rifting along the western margin of Gondwana marks the beginning of the Andean orogenesis (Davidson and Mpodozis 1991; Benavides, 1999).

Since ca. Mid-Mesozoic (200-90 Ma), the Andean evolution seems to be more directly controlled by plate convergence (Coira et al., 1982; Thorpe, 1984; Allmendinger et al., 1997; Ramos and Aleman, 2000). It was dominated by periods of intracontinental rifting (Petford and Atherton, 2003) and formation of a well-defined intra-arc and back-arc basins (Coira et al., 1982; Atherton et al., 1983; Davidson and Mpodozis, 1991). Mesozoic intra-arc and back-arc rifts were filled vertically up through axial faults by magmas derived from the upper mantle or lower crust (Couch et al., 1981; Atherton et al., 1983, 1985), as well as sedimentary detritus derived from the rift margins, volcanic edifices (Benavides, 1956) and locally interbedded calcareous sedimentary rocks (Benavides, 1962). Marine sedimentation continue more or less uninterruptedly from the Jurassic into the early Cretaceous (Benavides, 1956). The rifting along the western margin of Peru were controlled by regional-scale axial fault systems, which correspond to the NW-striking Cincha-Lluta Fault System (CLFS) (Carlotto et al., 2009a; Simmons et al., 2013), which was active during batholith construction (Demouy et al., 2012). This fault system is equivalent to the Icapuquio Fault System (IFS) farther south in Peru (Wilson and García, 1962; Vargas, 1970; Vicente et al., 1979; Jacay et al., 2002), which controls the spatial distribution of most of the porphyry Cu-Mo deposits, including the Zafranal porphyry district (Fig. 1). Furthermore, Zafranal porphyry copper is located very close to the intersection with the Iquipi-Clavelinas Fault System (ICFS) of EW orientation.

The late Cretaceous-Paleocene (90-52 Ma), Peruvian continental margin was dominated by compressional tectonics (Isacks, 1988, and references therein; Camus, 2003). This marks the beginning of crustal thickening (Vicente, 1989; Jaillard, 1992; Carlotto et al., 2009b), produced by structural shortening of the crust and uplift due to thermal thinning of the lithosphere by delamination (James, 1971; Isacks, 1988; Sobolev and Babeyko, 2005; Oncken et al., 2006; Wipf, 2006). The old fault systems (CLFS and IFS) changed from normal to reverse thrust regime, and as a result the magmatic arc installed within the Arequipa Massif begins to uplift and separates the Arequipa Basin from the ocean (Carlotto et al., 2009a), starting the accumulation of the entirely subaerial volcanic rocks linked to Toquepala Group (Clark et al., 1990a). Plutonic bodies of the Yarabamba Super-unit (Coastal Batholith) intruded the Toquepala Group (Stewart et al., 1974; Mukasa, 1986; Beckinsale et al., 1985; Boily et al., 1989; Clark et al., 1990a; Martínez and Cervantes, 2003), and obscured much of the earlier rift sequence and late Cretaceous fold-and-thrust belt. Porphyry copper deposits are associated with Paleocene and Early Eocene granite and granodiorite porphyry stocks (Simmons et al., 2013).

Eocene-Oligocene (52-30 Ma), the tectono-magmatic events were controlled by the evolution of a flat-slab subduction regime (Sandeman et al., 1995; Noury et al., 2017), which caused a farther northeast migration of the locus of magmatism (Andahuaylas-Yauri Batholith; Sandeman et al., 1995; Perelló et al., 2003). Furthermore, uplift in the Precordillera, the Arequipa Massif is overturned and thrusted over the Mesozoic rocks (Vicente, 1989; Caldas, 1993), followed by erosion-degradation of the older magmatic arc and Jurassic basin allowed the development of the syn-orogenic Querque foreland and Moquegua forearc basins (Vicente, 1989; Jaillard, 1992; Carlotto et al., 2009b).

Early Miocene (24-17 Ma), extensive explosive volcanism of the Huaylillas Formation mantled the degradational Altos de Camilaca surface recorded in the Moquegua Formation (Tosdal et al., 1981, 1984; Quang et al., 2003, 2005). A late Miocene to Holocene, phase of Andean uplift (Isack, 1988; Roperch et al., 2006), was accompanied by widespread ignimbrite eruptions of the Chuntacala Formation (14-8 Ma), and andesitic volcanism of the Pliocene-Holocene Barroso Group (<5 Ma) and exhumation of the giant porphyry copper deposits (Tosdal et al., 1981, 1984; Clark et al., 1990b; Quang et al., 2005), based on the size classification proposed by Clark (1993).

2.1. Tectonics and stratigraphy

Arequipa Segment of the Coastal Batholith comprises three main tectonic domains bounded by high-angle NW-striking faults (Fig. 1). The eastern domain, east of the Cincha-Lluta Fault System (CLFS), contains basement blocks, composed of migmatite and gneiss (Wasteneys et al., 1995). These basement blocks, were intruded by diorites, sienogranites-monzogranites of Ordovician age (466.1±2.6 Ma; Santos et al., 2016). Both the basement and intrusive rocks are overlain by volcanic breccias and andesitic flows of Chocolate Formation, and limestones of Socosani Formation (Romero and Ticona, 2003). Towards the Mesozoic Basin (Fig. 1B), the sedimentary outcrops are approximately >5 km thick (Romero and Ticona, 2003), divided into Chocolate, Socosani, Puente, Cachios, Labra, Gramadal, Hualhuani, Murco, Arcurquina, and Ashua Formations, which are strongly deformed showing sub-isoclinal folds, vertical and overturned strata. These strata are unconformably overlain by >2 km-thick of conglomerates, sandstones, and some volcanic breccias of the Huanca Formation (Querque Basin). The main Cincha-Lluta fault is characterized by high-angle NE-emergent, NW-striking, reverse fault and with a sinistral component.

The central domain (Fig. 1B), is located farther west of the Western Cordillera, and here, the basement rocks (Arequipa Massif) are intruded by plutonic complexes of the Coastal Batholith. These rocks together have an elongate geometry with orientation N145°, parallel to the main faults and orogen of NW-orientation. Furthermore, in areas where intrusive rocks were cut by sinistral faults, many mylonites and shear zones occur in some place. There are roof pendants of basement rocks (e.g., Cerro Gandolfo; Fig. 2), as well as volcano-sedimentary sequences of the Chocolate Formation (e.g., Zafranal porphyry). Plutonic rocks, ranging in composition from diorites and granites through quartz diorites to tonalites-granodiorites, were emplaced throughout the Arequipa Massif of relatively variable large to short pulses, each estimated to last roughly 4 to 12 Ma (Pitcher, 1985; Santos et al., 2016).

In the western domain, the Mesozoic rocks are covered by sedimentary rocks of the Moquegua Formation (Fig. 1B). This unit comprises mainly conglomerates, sandstones, siltstones with evaporite intercalations and tuffs with ages K-Ar of 25.3 to 22.8 Ma (Tosdal et al., 1981; Quang, 2005). A number of high-angle reverse faults of the Western Andean Escarpment cut both Mesozoic sedimentary bedrock and lower Moquegua red beds. Near the town of Aplao, a reverse fault dipping approximately 70° NE places Chocolate Formation over lower Moquegua, but deformation decreases up section, with upper Moquegua group strata deformed into a monocline and erosionally stripped (Schildgen et al., 2009). In the early Miocene (23-17 Ma), ignimbrite flows of the Huaylillas Formation were deposited on the surface of erosion of the Moquegua Formation (Tosdal et al., 1981; Quang et al., 2005).

3. Regional geology of ore deposits

The studied area is a thin strip (<15 km wide) of rocks belonging to the Coastal Batholith. This area contains gold-bearing quartz veins hosted in metamorphic and volcano-sedimentary rocks, and porphyry copper deposits (Fig. 1).

3.1. Intrusion-related gold-bearing quartz vein systems

Vein-type deposits are by far the most abundant and common mineralization style in the Coastal Cordillera (Sillitoe, 2003). The Rinconada de Chapi and Copacabana mining districts are located in the Quebrada Ongoro, 18 km southeast of the town of Chuquibamba (Figs. 1A and 2), and include gold-bearing quartz vein mineralization (e.g., Palpa-Ocoña, southern Peru; Sillitoe and Thompson, 1998). These quartz veins occur as NW-SE strycking swarms and with NEE-SWW orientations related to the main fault systems (CLFS and ICFS). In Copacabana, the gold-bearing quartz veins have the orthogneiss of the Arequipa Massif as wall rocks. These orthogneiss show a granoblastic texture, whit quartz, K-feldspar and plagioclase with biotite in thin bands and lenticular structures. The mineralization occurs as tabular quartz veins (dips >60°S), with a marked structural control in a N70°E trend. The major veins are 0.3 to 0.5 m thick. They consist mainly of quartz containing pyrite, chalcopyrite and native gold, and as supergene mineral contain hematite, goethite and copper-oxides (mainly chrysocolla). The higher grades range from 10 to 12 g/t Au. Hydrothermal alteration of the host rocks is common in many areas around the quartz veins, and consists of kaolinization and subordinate chloritization. At Rinconada de Chapi the veins are hosted in a volcano-sedimentary sequence and dikes complex intensely deformed, as indicated by vertical and overturned strata, anastomosing and thrust-bounded tectonic lenses of the Chocolate Formation. The ore deposits occur as steep-dipping tabular quartz veins (dips >80°N), with marked structural control and discontinuous longitudinal development in a NW-SE direction (N140°) reaching 1 km along strike. The major quartz veins are <0.8 m thick. They consist mainly of quartz, pyrite, chalcopyrite and native gold, and as secondary mineral contains calcite, hematite, chrysocolla and malachite. The higher grades range from 10 to 15 g/t Au. Also there are some thin veins (<10 cm), with quartz and galena.



Fig. 2. Arequipa Massif and Chocolate Formation rocks-hosted gold-bearing quartz veins and controlling faults in the Quebrada Ongoro (Modified from INGEMMET, 2001). Radiometric age obtained in this work. Map location is in figure 1.


Rocks of the Arequipa Massif and Chocolate Formation were intruded by diorites to quartz diorite of the Punta Coles Super-unit, cropping out largely in the Quebrada Ongoro, but also to the northeast part of gold-bearing quartz vein district. Based on the fact that the diorite-quartz diorite intrusive truncates the gold-bearing quartz veins hosted in rocks of the Arequipa Massif, it is inferred that the formation of these mineral deposits occurred before the emplacement of the dioritic-quartz dioritic intrusive (Cerro Gandolfo, Fig. 2). The bulk of the Punta Coles super-unit is made up of light- to medium-dark gray to greenish, hypidiomorphic granular diorite in the central part (following IUGS classification, Streckeisen, 1973), towards the boundaries increases the quartz content and reduces its grain size (Quebrada Ongoro). In general, these rocks are composed of a dominance of plagioclase over chloritized amphibole and traces of biotite. Minor quartz and sericitized K-feldspar occur interstitially in the equigranular phase.

3.2. Zafranal porphyry copper

The Zafranal porphyry copper deposit is located approximately 90 km northwest of the city of Arequipa in southern Peru, it was first described by Tejada (2010). Measured and indicated resources amount >472.1 Mt, 0.36% Cu and @ 0.08 g/t Au (Fernandez-Baca, 2011). The Zafranal local geology was described by geologists of AQM Peru S.A.C. (De Ruijter et al., 2013) and summarized by Rivera et al. (2010), Rivera (2012a, b). Metamorphic rocks of the Arequipa Massif are the oldest rocks at Zafranal, cropping out largely along the shear zone of the NW-striking Zafranal reverse fault. The next younger units are volcano-sedimentary sequences of the Chocolate Formation comprised of sandstone and siltstones, which in turn are overlain by tuffs, breccias and andesitic lava flows (Rivera et al., 2010; Fig. 3), which are present as roof pendants in igneous intrusions. Several roof pendants and large blocks of volcano-sedimentary sequences are present in the pre-mineralization diorites-granodiorites, which form the northern margin of the Zafranal porphyry. Alteration and mineralization at Zafranal is centered on porphyritic dykes and stock complexes, ranging in composition from diorite to quartz diorite, which show potassic alteration dominated by biotite (Tejada, 2010). The older porphyry suite (Zafranal diorite) host copper mineralization, displaying intense sericitic alteration overprinted on potassic alteration (Rivera et al., 2010), including typical A-type chalcopyrite-pyrite and B-type chalcopyrite-molybdenite quartz veinlets, and D-type quartz-pyrite-chalcopyrite veinlets with sericitic halos (cf. Gustafson and Hunt, 1975). An outer propylitic halo with epidote-chlorite, approximately 400 m wide, is present to the north (Rivera, 2012a). A 40Ar/39Ar plateau age of 82.41±0.43 Ma was obtained for hydrothermal biotite of the Zafranal diorite (Rivera, 2012a), located close to the east of the Zafranal porphyry system (Fig. 3). A microdiorite porphyry cut the Zafranal diorite, and is characterized by a much weaker potassic alteration towards depth, sparse A-, and B-type veinlets, containing 0.35-0.40 percent Cu in the hypogene mineralization (Rivera et al., 2010). A 40Ar/39Ar plateau ages of 83.370±0.54 and 81.16±0.43 Ma were obtained for hydrothermal biotites (Rivera, 2012a), in a microdiorite located in the central part of the Zafranal porphyry system (Fig. 3). Post-mineral intrusions diorite and diorite-monzodiorite dykes, displaying only weakly propylitic alteration with pyrite and chalcopyrite dissemination (Rivera, 2012a). However, U-Pb data presented herein demonstrates that the porphyries are temporally related to an early Jurassic magmatism.

Hydrothermal alteration and mineralization. Zafranal porphyry partly conform to the classic Lowell and Guilbert (1970) hydrothermal alteration zonation model, in which a potassic altered core grade laterally to an annular sericitic zone surrounded by a fringe of propylitic alteration (Rivera et al., 2010). The potassic alteration is defined by the presence of hydrothermal biotite, quartz and K-feldspar together with anhydrite, chalcopyrite±pyrite, and molybdenite, which make up the hypogene mineralization (Rivera, 2012a). Hornblende and magmatic biotite in the igneous rocks are almost totally replaced by fine-grained, brown biotite, and plagioclase is partially replaced by K-feldspar. The sericitic alteration clearly overprinted and destroys the earlier formed potassic assemblage and comprises quartz, sericite, and pyrite. Chalcopyrite is partially replaced by supergene chalcocite and covelite (Tejada, 2010). Intermediate argillic alteration is characterized by the presence of illite, chlorite and sericite together with kaolinite and smectite. Locally, internal parts of the southern limit of the deposit contain quartz, alunite, kaolinite and illite, minerals denoting the existence of advanced argillic alteration.

Supergene alteration and mineralization. The upper oxidized zone averages ~60 m thick, which is located within the leached capping (30-200 m thick), associated with the zone of sericitic alteration (Rivera et al., 2010). In the oxidized zone, chrysocolla, malachite, chalcanthite and neotocite are the ore minerals. The supergene enriched blanket average ~75 m thick, but locally attains a maximum of 150 m (Rivera et al., 2010), where the chalcocite and covellite occur as replacement coating on chalcopyrite, and minor pyrite grains. This supergene profile contains copper grades from 0.8 to 1% (Tejada, 2010).



Fig. 3. Geologic map of Zafranal porphyry copper (De Ruijter et al., 2013). The age data in blue letters are compiled from Rivera (2012a) and those in black letters are obtained in this work. Map location is in figure 1.


3.3. Late intrusions

At Cerro de Puerto (Huano village, Fig. 2), a NW elongated pluton composed of hypidiomorphic, granular, amphibole-rich tonalite (Torrecillas Super-unit, Fig. 1), extends from Chuquibamba up to northern Aplao. This pluton is cropping out close to Campanero porphyry copper prospect. The Campanero porphyry prospect (Rivera et al., 2008) is located approximately 14 km west of the Zafranal porphyry (Fig. 1).

Quartzite and siltstone of the Labra Formation are the oldest rocks at Campanero prospect, cropping out largely in the Rio Majes, but also in the central part of the prospect. Pre-mineralization granodiorite-tonalite are widely distributed to the northwest of the prospect. Copper mineralization is spatially and temporally related to a hornblende-rich quartz diorite with porphyritic texture. It presents high content of plagioclase, biotitized amphibole, and quartz. In general, the outcrops present strong sericitic alteration with hematite dissemination in matrix and veinlets. A 40Ar/39Ar plateau age of 141.4±0.8 Ma was obtained in hydrothermal biotite (Rivera, written communication, 2016).

4. Sampling at Zafranal Porphyry copper deposit

Sampling was conducted mainly in two transects (Fig. 1) and 10 representative samples of metamorphic and plutonic rocks as well as intrusives associated with porphyry copper mineralization in Zafranal deposit were selected for the U-Pb dating. Sample locations are shown in figure 1. Coordinates, unit, description, abbreviated mineralogy and the analytical work carried out on each sample are presented in table 1. Unit designations is following the nomenclature proposal of Cobbing et al. (1977) and Santos et al. (2016) (see Fig. 1).

5. Analytical method

New U-Pb ages were obtained using a sensitive high-resolution ion microprobe (SHRIMP) at the Institute of Geosciences of the University of Sao Paulo-Brazil and laser ablation inductively coupled plasma spectrometry (LA-ICP-MS) at the Laboratory of China University of Geosciences (Wuhan).

Zircons grains were separated by standard procedures; including crushing, Wilfley table and a magnetic separation (Frantz), and gravimetric separation by dense liquids to obtain a zircon concentrate. Finally zircon grains were hand-picked under a binocular microscope. A representative set of zircons grains (including the different morphologies of each zircon population) were arranged in rows in a mounting tablet, and fixed with epoxy resin and were polished to standardize the external surfaces. Cathode-luminescence (CL) SEM images, were used to decipher the internal structures of the sectioned grains and to target specific areas within the zircons. The U-Pb isotope analysis were made, using a sensitive high-resolution ion microprobe (SHRIMP) in the same technique described by Sato et al. (2014) and laser ablation inductively coupled plasma spectrometry (LA-ICP-MS), the analytical method is reported in Liu (2011). Concordia ages and diagrams were generated using the Isoplot/Ex V.2.49 software package by Ludwig (2001). U-Pb data obtained are in Appendix (Table A1-A2) and results are described below as shown in figures 4-7.

6. Zircon SHRIMP and LA_ICP-MS samples and results

6.1 Wall rock and gold-bearing quartz vein mineralization

Wall rock. A granoblastic orthogneiss (sample 76, table 1) at Copacabana, taken from the wall rock of the quartz veins. The zircons from this sample are equant elongate, to sub-round in shape and less than 200 µm length. The CL images show large relict cores sub-round in shape, in some cases present igneous zoning, surrounded by mantles of variable thickness with homogeneous zoning rims, discordant to the earlier oscillatory zoning (Fig. 4A). Twenty spots were analyzed on 20 zircon grains, included cores and rims. Six of eleven cores plot near concordia and yield a mean U-Pb age of 1747±21 Ma (MSWD=6.8), which is interpreted as the crystallization age of the igneous protolith. The very high Th/U values of cores (1.69-1.05), typical of igneous zircons (Hoskin and Schaltegger, 2003), reinforce this interpretation. The remaining nine zircons were analyzed in the rims yielded U-Pb ages between 1594-1076 Ma, the rims are high-U (mostly over 311-1240 ppm) and with low Th/U ratios (mostly <0.9) indicative of an igneous origin with partially recrystallized rims. This decrease correlates with a decrease in the 207Pb/206Pb age of the zircon (Hoskin and Schaltegger, 2003).

The other sample of granoblastic orthogneiss (sample 77, table 1) from approximately 2 km southeast of the Chuquibamba (Fig. 1); The zircons from this sample are sub-round in shape and less than 170 µm length. The CL images reveal small few relict cores, some cores preserve igneous zoning, and other cores show low-luminescence, in both cases the cores are girded by areas broad and homogeneous (Fig. 4B). Twenty spots were analyzed on 20 zircon grains. Three cores yielded U-Pb ages between 1444-1383 Ma, containing zircon Th/U ratios from <0.7. The rims yielded U-Pb ages 1236-827 Ma, if the more imprecise rims analyses are excluded, the rest plot yielded a zircon U-Pb age of 1005.6±4.7 Ma (MSWD=5.5), containing low Th/U ratios from 0.80 to 0.23. This calculated age probably corresponds to a Grenvillian metamorphism age that started at ca. 1 Ga (Casquet et al., 2010), whereas the igneous protolith have crystallized in the Mesoproterozoic (1444 Ma).

Post-mineral rocks in gold-bearing quartz vein mineralization area. An equigranular quartz diorite (sample 75, table 1) from approximately 2.5 km southeast of the main quartz veins (Quebrada Ongoro, Fig. 2). Separated zircons are mostly 150 µm in length, equant to slightly elongated, and sub-round in form. CL images display an oscillatory zoning (Fig. 4C), yielded a zircon U-Pb age of 198.3±1.3 Ma based on fourteen of twenty zircon crystals, with Th/U from 1.4 to 3. Six crystals excluded from the age calculation; one zircon is very old, the textural features and isotopic measurements indicate this crystal is an inherited xenocryst dated at 1583±47 Ma, with a zircon Th/U ratio (0.94), probably incorporated from the Arequipa Massif (Fig. 4C). The other sample of diorite (sample 82, table 1) from village of Angostura (Quebrada Tacya, Fig. 1). The zircons from this sample are sub-round to elongated grains (less than 120 µm in length). The CL images show a slight oscillatory zoning towards the edges; many grains generally have clear cores. Based on ten spots of twenty grains analyzed, yielded a zircon U-Pb age of 195.2±2.0 Ma, with zircon Th/U ratios from 1 to 2.1. Ten spots analyses were excluded from the calculated age, as yielded discordant dates (Fig. 4D).

Zircon spot U-Pb analyses from regional equigranular tonalite (sample 89, table 1), collected from village of Andamayo (Río Colca). Eight of twenty analyzed zircons yield an age of 182.3±1.7 Ma (Fig. 4E).

6.2 Zafranal porphyry copper

The denomination of the different magmatic phases (precursor, pre-, inter-, and post-mineralization) were made during the geological mapping and drill-core logging stages, performed by geologists of AQM Peru S.A.C. and summarized by Tejada (2010), Fernandez-Baca (2011), Rivera et al. (2008, 2010), Rivera (2012a, b), and De Ruijter et al. (2013). This observation has allowed to obtain relative ages, and here were included the approximate duration of different porphyry bodies related to the copper mineralization in Zafranal, determined by using exclusively U-Pb zircon dating.

Precursor pluton. A hypidiomorphic granular granodiorite (95A, Zafranal Super-unit) at the Zafranal porphyry district, taken from approximately 4 km east of the center Zafranal porphyry and 2.5 km northwest of the Santo Domingo mining (gold-bearing quartz veins). In general it consists of slightly NW elongated pluton (Fig. 1), which extends to Cerro Torconta and reach the south of Arequipa. Based on field relations, the granodiorite intruded to gneiss and early diorites. The zircons of granodiorite are elongate (less than 210 µm in length) euhedral in shape. CL images show that most crystals with a prominent oscillatory zoning in the entire zircon population, but some few grains show core and rims with low-luminescence (Fig. 5A). The granodiorite yielded a U-Pb age of 183.0±1.1 Ma (Fig. 5A), based in eleven zircons of twenty points analyzed. Three spot analyses yielded younger dates (178 Ma), presumably due to Pb loss. The all crystal zircons analyzed display high-U (810-1900 ppm) and low Th/U values (0.5-0.8).

Early-mineral rocks. A quartz diorite porphyry (Q-DIO) at the Zafranal porphyry, taken from approximately 0.5 km east of the center of the porphyry copper (Fig. 3) present lower degree of hydrothermal alteration. The zircon crystals from this sample generally are clear and colorless and mostly elongate (less than 400 µm in length) sub-rounded to euhedral in shape, this combination of zircon shape suggests relatively rapid to moderate crystallization with a high-level emplacement. The CL images reveal an ocillatory zoning and some irregular zoning, but one zircon xenocrystal has internal structure different from the remaining ninteen zircons. The xenocrystal has homogeneous CL, with outermost invariably being a bright CL rim. Therefore, largely given the common nature of the zircon population. The quartz diorite porphyry, yielded a zircon U-Pb age 179.8±1.2 Ma (Fig. 5B), based on twelve of nineteen. Six zircons are younger due to Pb loss. The xenocrystal indentified in CL is much older, isotopic measurements indicate that it is an inherited xenocrystal dated at 1032±11 Ma, with low-U (350 ppm) a low-Th/U ratio (0.51), similar in age to those of the Arequipa Massif (Fig. 5B).



Fig. 4. U-Pb ages for samples from rocks associated with gold-bearing quartz veins at Copacabana mining distric. Ages are calculated based on the Concordia diagrams and weighted mean 206Pb/238U ages histograms. Furthermore, ages distribution for metamorphic zircons (A and B) and Concordia diagram are shown, and for each sample one or more representative analyzed zircons. Histogram diagrams are also shown the distribution for Pb loss (sky-blue bars), and inheritance (gray color) in some of the analyzed zircons (C and E). A. 76: orthogneiss and B. 77: orthogneiss, both samples from Arequipa Massif. C. 75: quartz diorites and D. 82: diorite, both samples from Coastal Batholith (Punta Coles Super-unit). E. 89: tonalite from Coastal Batholith (Zafranal Super-unit).



Fig. 5. U-Pb ages for rock samples from the Zafranal porphyry. Ages are calculated based on the Concordia diagrams and weighted mean 206Pb/238U ages histograms. Furthermore, in histogram diagrams are also shown the distribution for Pb loss (sky-blue bars), and inheritance (gray bars) in some of the analyzed zircons (A, B, and C). A. 95A: granodiorite; B. Q-DIO: quartz diorite porphyry; C. ZAF-DIO: quartz monzodiorite and D. M-DIO: microdiorite. All samples from Coastal Batholith (Zafranal Super-unit).


Inter-mineral porphyry 1. The Zafranal quartz-monzodiorite porphyry (ZAF-DIO) from approximately 1 km east of the center of the main orebody (Fig. 3) pervasively hydrothermally altered and mineralized. The zircons from this sample are elongate to sub-round in shape and less than 300 µm in length. CL images mainly show strong oscillatory zoning and irregular zoning. Other zircons display small relict cores, which are elongate euhedral prisms with oscillatory zoning surrounded by rim with resorption features. Rims show different orientation oscillatory zoning and are large enough to be analyzed (5C). Twenty-one points were analyzed including rims and cores. Three cores yielded U-Pb ages between 180-184 Ma, which are interpreted as the inherited zircons from the precursor pluton granodiorite (sample 95A). The analyzed rims yielded U-Pb age of 176 Ma, very close to age established by the remaining sixteen zircons (175.9 Ma). One of the crystal give an age slightly younger (169 Ma). We interpret the statistically more robust and age of 175.9±1.0 Ma to more accurately reflect the crystallization age for quartz-monzodiorite porphyry.

Inter-mineral porphyry 2. The microdiorite (M-DIO) forms a second porphyry copper-related, from approximately 0.5 km from east part of the main porphyry body (Fig. 3). Microdiorite cut the Zafranal quartz-monzodiorite porphyry, and is characterized by showing a pervasive hydrothermal alteration, mineralization and dynamic metamorphism features. Zircons are elongate to sub-rounded (less than 150 µm in length), subhedral in shape. The CL images reveal a varied internal structure and features (Fig. 5D). Some grains display small cores surrounded with mantle weak oscillatory zoning. Also commonly show crystals with prominent oscillatory zoning. Eighteen spots were analyzed on fourteen zircon grains, included rim and core. The cores do not show substantial variation in isotopic measurements in relation to the rims. The older population of zircons have a late Paleproterozoic 206Pb/238U age of 1801-1710 Ma (four crystal zircons), whereas other zircons with more homogeneous rim area has a Mesoproterozoic 206Pb/238U age of 1575-1064 Ma (six crystal zircons). The other younger population of zircons with a marked oscillatory zoning. Three crystal zircons have an Ordovician (478-481 Ma) U-Pb age. Four analyzed in three zircons have Devonian-Carboniferous U-Pb age of 396-319 Ma. Furthermore, one zircon yield an age of 179.7 ± 3 Ma (Fig. 5D), and therefore is interpreted as maximum crystallization age of the microdiorite.

6.3 Late Jurassic Zone in Cuyania

Late intrusion. At Huano village, a granular tonalite (86, Torrecillas Super-unit). The zircons from this sample are slightly elongate to sub-round (less than 120 µm in length), subhedral in shape. CL image show, some crystal highlights very high concentrations of U in the crystal center, more moderate values in an intermediate shell and high concentrations in the rim. Twenty spots were analyzed including center and rim of zircons. Nine crystal center yielded U-Pb ages between 140-144 Ma, with very high U concentrations of 2481-1000 ppm and also high Th/U values of center (0.8-1.57). Eleven rims yielded U-Pb ages in 149-146 Ma, with U concentrations of 875-513 ppm and Th/U values between 0.61-0.94. If the less concordant analyses are excluded, the rest plot yielded a zircon U-Pb age of 145.9±1.4 Ma (Fig. 6).

7. Discussion

Four views emerge from the data obtained: 1. Arequipa Massif, 2. Early Jurassic arc extent of the Coastal Batholith, 3. Intrusive bodies associated with ore mineralization in the quartz veins and Zafranal porphyry deposit, and 4. late Jurassic magmatic pulse.

7.1. Arequipa Massif

Arequipa Massif includes rocks displaying protolith ages of 1.9 Ga (Dalmayrac et al., 1977; Cobbing et al., 1977) and other rocks showing Mesoproterozoic protolith and metamorphism ages of 1.2-1.0 Ga (Wasteneys et al., 1995; Loewy et al., 2004). Along the Cincha-Lluta Fault System, the metamorphic rocks are thrusted over the Mesozoic series of the Western Peruvian Mesozoic Basin (Caldas, 1993; Carlotto et al., 2008, Fig. 1). The Arequipa Massif in this area contains Paleoproterozoic zircons (samples 76 and 77) and core zircons with rims plot on discordia resulting from Pb-loss during Grenvillian metamorphism (Fig. 4A, B), similar to migmatitic gneisses of the Camaná-Mollendo domain, described by Casquet et al. (2010). This confirms the Arequipa Massif extent to the western flank of the Western Cordillera.




Fig. 6. U-Pb ages for rock samples from the Zafranal porphyry. Ages are calculated based on the Concordia diagrams and weighted mean 206Pb/238U ages histograms. Furthermore, in concordia diagram is also shown representative analyzed zircons, and in histogram diagram is shown the distribution for Pb loss (sky-blue bars) in two of the analyzed zircon. (86) Tonalite from Coastal Batholith (Torrecillas Super-unit).


7.2. Early Jurassic arc extent of the Coastal Batholith

The U-Pb ages reported herein of mafic and felsic intrusive rocks from Chuquibamba to Arequipa in southern Peru (Fig. 1, Coastal Batholith), shows that first magmatic activities occurred during 200-174 Ma.

For the magmatic belt in the study area (Fig. 1) Cordani et al. (1985) indicated an early Jurassic K-Ar ages (189±11 Ma), which is considered as the minimum age of the tectonomagmatic process. Later, Mukasa (1986) published two early Jurassic U-Pb ages (188 and 184 Ma) from northwest of Arequipa. He indicated that these rocks represent the plutonic substructure of a Jurassic continental arc of unknown extent. In the south of Arequipa, Demouy et al. (2012) got new U-Pb ages for gabbros-diorites, which restricted early Jurassic age (200-175 Ma) to a mafic magmatism (Fig. 1). Our results show that the first magmatic activity in this study area occurred during 198.3-195.2 Ma (Sinemurian; Fig. 4C, D), and is associated with diorite-quartz diorite rocks of Punta Coles Super-unit (Table 1). A second magmatic phase encloses tonalite-granodiorite rocks (Table 1) emplaced at 183.0-175.9 Ma (Pliensbachian-Toarcian; Figs. 4E and 5) of Zafranal Super-unit. The second magmatic phase include small microdiorites, quartz diorites and quartz monzodiorite porphyritic stocks and dikes (Table 1) with an EW-trend at 179.8-175.9 Ma (Fig. 5B, C). The two magmatic suites make up a thin magmatic belt (15x130 km) that extends from southern Chuquibamba to the southern Arequipa (Fig. 1).

7.3. Intrusive associated with gold-bearing quartz veins and Zafranal porphyry deposit

7.3.1. Gold-bearing quartz vein mineralization
The gold-bearing quartz vein systems in Copacabana and Rinconada de Chapi mining districts have metamorphic and volcano-sedimentary host rocks, respectively. Early diorites (198.3-195.2 Ma; Fig. 2) of Punta Coles Super-unit intruded the metamorphic host rocks of the gold-bearing quartz veins and restrict their extent (Cerro Gandolfo, Fig. 2). Therefore it is inferred that the gold-bearing quartz veins were formed before ca. 198.3 Ma.

7.3.2. Zafranal porphyry deposit
The first radiometric ages for the Zafranal porphyry copper deposit were published by Rivera (2012a), who reported 40Ar/39Ar plateau ages for hydrothermal biotite of 82.41±0.43 Ma from Zafranal diorite and 83.37±0.54 and 81.16±0.43 Ma from microdiorite. According to this author, the Zafranal diorite and microdiorite are associated with copper mineralization, and he concluded that the alteration and mineralization developed in the late Cretaceous. However, the 40Ar/39Ar ages for Zafranal hydrothermal biotites only record cooling under 320±30°C; biotite closure temperature estimated using Dodson (1973) and parameters presented by Harrison et al. (1985) and McDougall and Harrison (1999). Whether this thermal event was related or not to the hydrothermal copper mineralization of Zafranal is considered uncertain here, as no other, isotopic data are available for constraint ore formation. While, the use of Zircon U-Pb dating from intrusions associated with porphyry copper mineralization makes possible to us confidently assign crystallization ages to the various intrusive units, as zircon has the highest known closure temperature for Pb diffusion, which exceeds 900°C for zircons of typical sizes (Cherniak and Watson, 2000, and references therein). At Zafranal, three phases of porphyry intrusions were dated by U-Pb, with the oldest being the granodiorite precursor batholith at 183.0±1.1 Ma. The second phase is a quartz diorite intrusion at 179.8±1.2 Ma, and the third phase is a quartz monzodiorite at 175.9±1.0 Ma (Fig. 7). As the multiphase porphyry complex at Zafranal is hosting a copper mineralized stockwork, we postulate that copper mineralization also was introduced during the early Jurassic, while the late Cretaceous 40Ar/39Ar biotite ages of Rivera (2012a) represent a thermal overprint, which could be either a late hydrothermal activity or just heating by Cretaceous magmatic processes of the Arequipa Segment of Coastal Batholith of Peru (i.e., Demouy et al., 2012).

7.3.3. Late Jurassicn
Cordani et al. (1985) reported a K-Ar age of late Jurassic intrusion east of the CLFS (157±14 Ma). Later Demouy et al. (2012) published a late Jurassic age (U-Pb, 154.7±1.0 Ma) for a monzonite sill from the south of Arequipa, which is associated with sedimentary formations without any presence of magmatic activity between 160.5 to 90 Ma. However, our data set combined with the recently published U-Pb ages (Santos et al., 2016), show that two late Jurassic magmatic events developed west of the CLFS, and cover a wide area west of Chuquibamba (Fig. 1). First magmatic activity occurred during 158-157 Ma (Oxfordian), containing biotite-rich granodiorite and granite-monzogranite rocks of Tembladera and Chillihuay units, respectively. A second magmatic phase encloses amphibole-rich tonalite rocks emplaced at 145.9 Ma (Tithonian) of Torrecillas Super-unit. Both magmatic events probably ended with injection of copper mineralizing fluid (e.g., Tinajas prospect 152 Ma and Campanero prospect 141 Ma, Fig. 1).

Overall, our geochronology data coincide with the first magmatic pulse from 200-175 Ma previously identified in the southern sector of the Arequipa Segment of the Coastal Batholith of Peru by Demouy et al. (2012). Later the early Jurassic arc was overprint by felsic Cretaceous intrusive bodies (90-60 Ma, Demouy et al., 2012), hence the biotites of some early Jurassic rocks register the thermal overprint of these two main magmatic pulses (e.g., Zafranal porphyry copper).




Fig. 7. Summary of magmatic ages determined at Zafranal porphyry copper based on weighted mean averages of 207Pb-corrected 206Pb/238U spot ages using SHRIMP and LA-ICP-MS. Errors shown at 2σ levels.


8. Conclusions

Outcrops of metamorphic rocks in Western Cordillera (Western Peruvian Mesozoic Basin) correspond to Arequipa Massif with Paleoproterozoic igneous protolith and Mesoproterozoic Grenville-age metamorphism (1.75 Ga and 1 Ga).

The Coastal Batholith exposed in the study area, along the fault-bounded thin magmatic belt (<15 km wide) was emplaced into Arequipa Massif during the early Jurassic. Two main magmatic events are identified: (1) Early mafic magmatic event from ca. 200-194 Ma that correspond to the Punta Coles Super-unit (200-188 Ma). It consists mainly of diorites and quartz diorites, and (2) A second event of dominantly felsic magmatism from ca. 184-175 Ma that corresponds to the Zafranal Super-unit (188-176 Ma). The porphyritic stocks with copper mineralization at Zafranal crystallized from 181 to 175 Ma and provide a maximum age for the hydrothermal processes.

Late Jurassic magmatic events comprised of granodiorites and granitic-monzogranitic bodies emplaced from 158 to 157 Ma, which correspond to Tembladera and Chillihuay units. A late magmatic phase encloses tonalite plutonics emplaced at ca. 145.9 Ma, corresponding to Torrecillas Super-unit (150-145 Ma).

Mineralizing processes in this area occurred in the early Jurassic. Gold-bearing quartz vein mineralization formed in early stages of the Coastal batholith (>200 Ma), while copper minerals in the Zafranal porphyritic host rocks may have been introduced from 181.1 to 174.7 Ma. A late Cretaceous thermal event registered by biotite with 40Ar/39Ar age from 83 to 81 Ma (previously considered as the mineralizing event) constitutes just a thermal, overprint, which could be either a late hydrothermal alteration or a local heating from an unidentified magmatic source. Overall, much of copper mineralization in this area is thought to be linked to the last stages of each magmatic suite (e.g., Zafranal, Tinajas and Campanero).

We wish to express our gratitude to INGEMMET and AQM Peru Copper S.A.C. for the support in the logistics of obtaining the Coastal Batholith samples and information generated during their exploration stages in the Zafranal cluster. John Cervantes and Paola LLamoca of Regional Geology from Geological Survey of Peru (INGEMMET) have greatly helped with this project. This work is part of the master thesis of the first author, with the support of the high-resolution geochronology laboratory of the Institute of Geosciences of the University of Sao Paulo-Brazil and Laboratory of China University of Geosciences (Wuhan).


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