1. Introduction

The distribution of metallic ore deposits along the southern central Andes has been typically characterized as various margin-parallel NS trending metallogenic belts of specific ages and broad geological setting (e.g., Sillitoe, 1981, 1991, 2003; Camus, 2003; Sillitoe and Perelló, 2005). The Jurassic-Cretaceous belt of the Coastal Cordillera in Chile is characterized mostly by Cu strata-bound, stratabound and vein-type Iron oxide-copper-gold (IOCG) and iron oxide-apatite (IOA) deposits, all of these associated with the Jurassic and Cretaceous arc magmatism and characteristically associated with extensional tectonics in an active continental margin (e.g., Vivallo y Henríquez, 1998; Sillitoe, 2003). Farther east, along the Domeyko Cordillera, in northern Chile, porphyry copper deposits of Eocene-Oligocene age are distributed in a longitudinal belt, spatially associated with the Eocene-Oligocene arc and the transpressive/compressive Domeyko Fault System (Boric et al., 1990), which points to a major structural control on the magmatism and mineralization for this time span (e.g., Mpodozis and Cornejo, 2012). A younger belt of world-class porphyry-Cu-Mo deposits of Late Miocene age is located east of the Eocene-Oligocene arc, along the high Andes of central Chile, including the Los Pelambres, Río Blanco-Los Bronces, Los Sulfatos and El Teniente porphyry copper deposits. These ore deposits are spatially associated to the Neogene magmatic arc, developed in a thickened crust.

Porphyry copper deposits are outstanding metallogenic features of active continental margins and several world-class metal deposits are present in this environment. These deposits are mostly present in active continental margins and cordilleran margins (Sillitoe, 1998; Kloppenburg et al., 2010; Mpodozis and Cornejo, 2012), but minor occurrences also have been recognized in extensional settings, such those described in Turkey and southeastern China (Sánchez et al., 2016; Piquer et al., 2017) and collisional settings (i.e., Zhang and Hou, 2018; Richards, 2009). Several hypothesis has been proposed to explain the genesis of large (giant) porphyry copper deposits, and for the case of the Chilean Andes, these can be limited for the Cenozoic examples, where specific compositional and tectonic features such as thickened crust, compressional/transpressive setting, high water content in magmas or high oxidation state for it, has been postulated (Oyarzún et al., 2001; Bissig et al., 2003; Reich et al., 2003, among others). The case is different for a still not well known belt of Cretaceous Cu-porphyries recognized at north Chile (e.g., Reyes, 1991; Maksaev et al., 2007, 2010; Sillitoe and Perelló, 2005; Richards et al., 2017) which are largely smaller and lower in grade relative to Cenozoic deposits, and are spatially and temporally associated to Cretaceous arc magmatism, which overall developed under extensional conditions. In the same way, this arc magmatism and the Cretaceous Cu-porphyries are also spatially associated with a well-known Cretaceous metallogenic belt characterized mostly by IOCG and IOA deposits (i.e., Sillitoe, 2003; Maksaev et al., 2007).

In recent years, exploration activities have turned to a renewed attention to this cretaceous porphyry copper belt of northern Chile, distributed mostly between 26º and 31º S. This belt is characterized by the presence of small (<5 km²) porphyry intrusions and subvolcanic rocks of dacite/granodiorite to diorite composition of Lower to Upper Cretaceous age. Most known examples in this kind of ore deposits correspond to the Inca de Oro and Dos Amigos districts (Matthews et al., 2006; Maksaev et al., 2007, 2010), but the largest producer from this belt is the Andacollo Cu-Au porphyry, located at ~30 ° 30’ S (Llaumett, 1975; Reyes, 1991; Richards et al., 2017).

In this work, we present new constrains on the age of some of these Early and Late Cretaceous Cu-Au porphyries located between 28º and 30º S, throughout seven new zircon U-Pb ages, which were obtained in the Geochronology Laboratory of University of Tasmania and the Isotope Geology Laboratory of SERNAGEOMIN, with the aim of refining the crystallization age of these ore-bearing dacite porphyries. Moreover, we present new geological constraints with the objective to refine our knowledge on the tectonic regime associated with the emplacement of these deposits, based on a preliminar work presented by Creixell et al. (2015) and a more advanced analysis of the Cretaceous regional geology and stratigraphy, through field observations of regional-scale structures and key stratigraphic relationship among Cretaceous geological units, including the Chañarcillo Group and the Punta del Cobre, Cerrillos and Viñita formations. Also, the compositional evolution of the Cretaceous magmatism has been investigated throughout a set of 71 samples of Cretaceous volcanic rocks and dacite porhyries, analyzed for major, trace and rare earth elements. Most of these observations have been focused to investigate the spatial and temporal relationship between the emplacement of the Cretaceous intrusions related to porphyry copper mineralization and discrete events of transpressive and compressive deformation between 28º and 30º S. The final aim of this paper is to establish genetic temporal and spatial relationship between these relatively small porphyry copper deposits and tectonic deformation events that occurred during the Cretaceous.

Analytical procedures for U-Pb geochronology and geochemistry are included, together with analytical data, as Appendixes (Table 1A for U-Pb geochronology and Table 2A for geochemical analyses).

2. Regional tectonic setting

The early evolution of the magmatic arc of the southern central Andes, from Triassic to Early Cretaceous time span, was associated with crustal extension along the continental margin (Mpodozis and Ramos, 1989; Grocott and Taylor, 2002; Haschke et al., 2002). The magmatic source during this stage was dominated by additions of primitive melts from the asthenospheric mantle to the crust (e.g., Lucassen et al., 2006). The paleogeographic configuration of the active continental margin consisted of a submarine to subaerial magmatic arc, characterized by effusion of large volume of intermediate to mafic lavas (e.g., La Negra Formation, García, 1967; Oliveros et al., 2007; Punta del Cobre Formation; Segerstrom and Ruiz, 1962) and emplacement of composite batholiths. An eastward parallel back-arc basin was dominated by marine sedimentation during the Jurassic to Early Cretaceous (Tarapacá basin), but with restricted episodes of subaerial deposition (Oliveros et al., 2012). This geodynamic setting remained in steady state during the Jurassic and Early Cretaceous, except for some discrete episodes of intra-arc transpression along major fault systems for the Late Jurassic (Scheuber et al., 1994; Creixell et al., 2011; Ring et al., 2012) and Early Cretaceous (Scheuber and González, 1999; Arévalo and Creixell, 2009). Since about 100-85 Ma, several compressive to transpressive events were registered along the southern Central Andes margin, marking a change from marine to subaerial deposition along the backarc, inception of a foreland basin instead of a back-arc, progressive eastward shift of the axis of the magmatic arc and dominance of more felsic compositions of igneous rocks (e.g., Richards et al., 2017). These Late Cretaceous deformation episodes have been proposed as the first pulses of Andean orogenesis in the central Andes by several authors (e.g., Coutand et al., 2001; Cobbold and Rosello, 2003; Arriagada et al., 2006; Bascuñan et al., 2015). These events led to recognition of regional-scale unconformity surfaces or displacements along major fault systems (e.g., Sánchez and Emparán, 2006). Moreover, discrete changes in trace element ratios (e.g., La/Yb) of arc magmas have been described associated with Cretaceous deformation events and also for Cenozoic ones, in the last case associated with crustal thickening (Kay and Mpodozis, 2001; Haschke et al., 2002; Mpodozis and Cornejo, 2012).

2.1. Current knowledge on Cretaceous tectonics of the southern central Andes

During the Early Cretaceous, some authors have identified the occurrence of one or more tectonic events that disturbed the dominant extensional/transtensional tectonic setting along the arc and backarc. Along the western part of the study area, Arévalo and Creixell (2009), based on kinematic analysis of brittle and ductile faults, including S-C mylonites and porphyroclasts, and radiometric dating of mylonites, inferred that a sinistral transpressive regime of deformation along the Atacama Fault System (AFZ) took place as early as 121 Ma and extended until at least 117 Ma, controlling the emplacement of plutonic complexes and IOCG deposits and following a stage of along-arc extension, suggested by geometry of fault-bounded plutons. For the same AFZ system farther north (22º-26º S), Scheuber and González (1999) proposed that a sinistral transpression stage started roughly at 125 Ma, based on changes in the orientation of dike swarms, and after a stage of arc-normal extension. Dallmeyer et al. (1996), on the basis of ⁴⁰Ar/³⁹Ar dating on mylonites, inferred a similar age ~126 Ma for the change from vertical to left-lateral displacements along the AFZ. Arancibia (2004), based on kinematic analysis and ⁴⁰Ar/³⁹Ar dating of neoformed minerals, showed that the Silla del Gobernador Shear Zone, a regional-scale fault with ductile deformation located along the Coastal Cordillera at about 32º S, displayed west-directed reverse shearing between 109 and 97.8 Ma.

At regional scale, several sources of data indicate indirectly that the plutonic rocks of the Coastal Cordillera were exhumed during the Cretaceous, pointing to possible deformation events during this period. Gana and Zentilli (2000) inferred that Carboniferous granitoids exposed along the Coastal Cordillera at 33º S were exhumed at about 106-98 Ma, based on apatite fission track ages. Sedimentary record of Cretaceous stratified rock sequences of central Chile also suggests that exhumation and subsequent erosion of uplifted blocks took place during the Cretaceous. Recently, Boyce et al. (2020), based on the stratigraphic record of terrigenous strata of the Early Cretaceous Las Chilcas Formation in central Chile (33º S), proposed that this unit represents the sedimentary filling of a proximal foreland basin, associated with compressive pulses, basin inversion and orogenic processes that took place between 105 and 83 Ma. In addition, Merino et al. (2013) interpreted that stratigraphy and detrital zircon ages of Late Cretaceous continental sedimentary deposits exposed in the Frontal Cordillera of Chile between 28º30’ and 29º30’ S (ascribed to the Pucalume Formation) registered the deposition of synorogenic sedimentary rocks associated with a regional tectonic deformation event (“Peruvian phase”) with basement exhumation at about 90 Ma. In the same way, Bascuñan et al. (2015) based on stratigraphy and detrital zircon ages of Late Cretaceous sedimentary rocks in the Antofagasta region of northern Chile, suggested that at least two orogenic pulses occurred at 107 and ~80 Ma. South of 36º S, the Upper Cretaceous Neuquén Group in Argentina has been also interpreted as synorogenic deposits associated with compression and uplift of the Early Cretaceous arc located to the west, in the Chilean territory (Tunik et al., 2010; Mescua et al., 2013, and references therein). At regional scale, the Cretaceous deformation events indicated by the geological record of north-central Chile can be associated with two major tectonic events described for the arc and back-arc basins associated with the active continental margin: A first event, mostly known as Mochica Orogenic Phase, with an early stage around 110-105 Ma and a widespread second pulse between 100 and 95 Ma (e.g., Jaillard, 1994; Jaillard and Soler, 1996; Tunik et al., 2010; Menegazzo et al., 2016). The second event is the Peruvian Orogenic Phase, which started around 90 Ma, and probably extended during most Late Cretaceous time (Jaillard, 1992; Haschke et al., 2002; Menegazzo et al., 2016).

3. Main stratigraphic and structural features in the studied segment

The geology around the study area (Fig. 1), located south of Vallenar city, comprises mostly Early to Late Cretaceous volcanic, sedimentary and plutonic rock units. These volcanic and sedimentary rocks, between 28° and 30° S, are distributed at regional scale, as N-S oriented belts dipping east (Fig. 1), interrupted by N-S to NNE-SSW extensional and compressional faults and fold systems. Plutonic rocks are organized along complexes of dominant dioritic composition, with progressively younger ages eastward.

 

Fig. 1. Simplified geological map of coastal region of north Chile between 28° and 29°30’ S, with emphasis on the Cretaceous stratified and plutonic rocks units and major structures, modified from Arévalo and Welkner (2008); Welkner et al. (2006), Arévalo et al. (2009) and Creixell et al. (2012, 2013). The location of studied cretaceous porphyry copper deposits is marked by green circles (Early Cretaceous) and red hexagons (Late Cretaceous). Some of the main fault systems have abbreviated names, as follows: ISZ: Infiernillo Shear Zone; LCFTB: Los Colorados Fold and Thrust Belt: ASZ: Algarrobo Shear Zone; LCETF: Las Cañas-El Torito Fault. Numbered black squares indicate the location of key stratigraphic localities mentioned in the text: 1: Algarrobal creek; 2: La Silla; 3: La Verde Creek; 4: Tres Cruces.

3.1. Cretaceous stratigraphy

The Cretaceous stratigraphy of the area is synthetized in figure 2 and it is characterized by a lower unit corresponding to the Late Jurassic-Early Cretaceous rocks of the Punta del Cobre Formation (Segerstrom and Ruiz, 1962), mostly constituted by pyroxene-bearing andesites. In the stratigraphically lower and upper sections of the formation, these lavas are interbedded with sedimentary breccias, marine sandstones and limestones, indicating that volcanism took place under submarine conditions in several levels of the unit. An important change in facies is noted at the upper section of the Punta del Cobre Formation, where minor lavas are associated with red beds (sandstones, pyroclastic breccias, and conglomerates), with some well-recognized subaerial facies associated with explosive volcanism, dated at 118-112 Ma by Creixell et al. (2013). This unit, together with intermediate to mafic plutonic rocks, is ranging in age between 143 and 110 Ma (Creixell et al., 2012), form the core of the Coastal Cordillera at this latitude and all these units represents the Late Jurassic to Early Cretaceous magmatic arc. A relevant feature of this arc association is that numerous IOA and IOCG deposits are present along the pluton-lava interface or along faults of the AFS (Vivallo et al., 2008; Arévalo and Creixell, 2009; Arévalo et al., 2009; Arredondo et al., 2018). Limited age data of the mineralization and hydrothermal alteration of these deposits indicate Early Cretaceous ages that are coetaneous with arc magmatism (Vivallo et al., 2008; Creixell et al., 2012; Veloso et al., 2016). The origin of these IOA deposits was interpreted as magmatic hydrothermal (Ruiz, 1967; Ruiz et al., 1968; Bookstrom, 1977; Menard, 1995), whereas other authors favored an iron oxide ore magmas as source for these deposits (Nyström y Henríquez, 1994; Travisany et al., 1995). In addition, a recent magmatic-hydrothermal model has also been proposed for the IOA deposits (Knipping et al., 2015; Reich et al., 2016).

 

Fig. 2. Schematic and generalized stratigraphy of the study area (left column), with red ballons indicating the temporal location of main plutonic events. The tectonic setting and different styles and events of mineralization are located in the right columns.  Based on Arévalo et al. (2009); Creixell et al. (2012, 2013); Salazar et al. (2013) and references in the text. Cu-P: porphyry copper deposits; IOCG: iron oxide copper gold deposits; IOA: iron oxide apatite deposits. Geological Time Scale according to Gradstein et al. (2012).

Early Cretaceous rock sequences exposed to the east (inland) of the magmatic arc are considered by several authors as representing a back-arc environment (e.g., Mpodozis and Ramos, 1989). The Chañarcillo Group (Upper Hauterivian-Aptian, Segerstrom and Parker, 1959) is constituted by sedimentary rocks grouped in the Nantoco, Totoralillo and Pabellón formations, all these with lithologies dominated by marine limestones, with minor amounts of shales, calcareous sandstones and volcanic rocks. This unit is contemporaneous with the upper portion of the Punta del Cobre Formation, exposed ca. 20 km west. The Bandurrias Formation comprises brownish-red sandstones and breccias with local intercalations of andesites and Aptian-Albian fossiliferous limestones. Outcrops of the Pabellón Formation along the Pelícano and Algarrobal creeks show that upper levels of this unit are characterized by progressive variation from calcareous strata to terrigenous deposits (sandstones and conglomerates) with dominant volcanic detritus. The Early Cretaceous sequences mentioned above are covered by rocks of the Cerrillos Formation. The contact between these units is variably unconformable or conformable along the studied segment (Arévalo et al., 2009; Creixell et al., 2013) and is described and analyzed in more detail below (chapter 3.2.). In general terms, the Cerrillos Formation, consists of an association of subaerial terrigenous deposits and a dominant volume of volcanic rocks (lavas and tuffs) with about 1,000 meters of thickness. The age of the unit, considering several localities between Copiapó and Vallenar, was previously assigned between the Albian and the Maastrichtian (Arévalo et al., 2009; Maksaev et al., 2009), but for the studied segment, has been restricted between the Albian and the Coniacian by Creixell et al. (2013), on the basis of the recognition of a regional scale unconformity at the top of the sequence and several zircon U-Pb ages. The base of the unit has been dated by several zircon U-Pb ages 112 and 110 Ma (Maksaev et al., 2009; Creixell et al., 2013). The Cerrillos Formation has been separated in a lower dominantly siliciclastic member (Checo de Cobre member), and an upper volcanic member. However, the lower sedimentary member interfingers laterally, northward and southward, with volcanic facies. At Pelicano creek (La Silla locality at figure 1), the Checo de Cobre member is composed mostly of well-stratified volcanic rocks, medium to coarse grained sandstones with intercalation of green limestones, tuffs and volcanic breccias, conglomerates, and minor mudstones, (Salazar et al., 2015). At Los Choros creek (Tres Cruces Locality at Figure 1), the Cerrillos Formation, consists of conglomerates and tuffs (Fig. 3A), and is exposed conformable over brown-red sandstones and andesites of the Bandurrias Formation. In this locality the upper volcanic member of the Cerrillos Formation is composed mostly of andesite lavas with intercalations of rhyolite tuffs and volcanic breccias. Andesites are pyroxene bearing, but locally contain also oxidized olivine phenocrysts.

The Cerrillos Formation varies locally to the east to a sedimentary sequence of about 1,085 m thickness consisting of sedimentary breccias, lithic sandstones and conglomerates and minor intercalations of calcareous limolites of Late Cretaceous age (Turonian-Coniacian) ascribed by Creixell et al. (2013) to the Pucalume Formation. Both the Cerrillos and the Pucalume formations are covered unconformably by an eastward gently-dipping plateau of andesites and dacitic and rhyolitic tuffs with U-Pb ages between 84 and 78 Ma assigned to the Viñita Formation (Creixell et al., 2013; Salazar et al., 2013).

 

Fig. 3. Field photographs of the study area. A. Basal sequence of the Cerrillos Formation near Los Choros creek, consisting in gently east-dipping conglomerates, sandstones and tuffaceous sandstones, with well-developed stratification, view to the SE; B. Profile view of the Las Cañas fault, with limestones of the Totoralillo Formation (Kch) on the hangingwall, yuxtaposed over younger andesites of the upper member of the Cerrillos Formation (Kc2) in the footwall, view to the north. Geologist for scale at the left border of the photograph; C. Hydrothermal breccia located along the trace of the El Tofo fault, dated at 111 Ma by Creixell et al. (2012), consisting lithic clasts strongly replaced by albite and quartz (ab), in a matrix of actinolite and quartz (amp); D. Supergene alteration on the Early Cretaceous Cachiyuyo porphyry copper, with main replacement by silica, with late pods of hematite (hm) and kaolinite (ka) and chrysocolla veins (Cu); E. Unconformable stratigraphic contact between the strongly deformed Pabellón Formation (Kch) and the east-dipping Lower Member of the Cerrillos Formation (Kc1). Stratification is marked along black lines. The unconformity is marked along the white line in the figure; Algarrobo creek, view to the south. Approximate width of the photo is 70 m.

3.2. Unconformities

Two main unconformities are recognized along the study area. The older one is registered at the base of the Cerrillos Formation, and changes its character from north to south and west to east. In the western half of the study area, at Tres Cruces locality (Fig. 1 and 3A) this unconformity is almost absent between the Bandurrias Formation and the overlying Cerrillos Formation. The same situation is observed along the La Verde Creek locality (Fig. 1), between Pabellón and Cerrillos formations. In the eastern part of the study area, the basal contact of the Cerrillos Formation with the underlying Pabellón Formation is variable along strike both geometrically and in the facies involved. Along the Algarrobo creek (about 15 km east of the Tricolor deposit, Fig. 1), this contact is largely unconformable, where folded limestones of the Pabellón Formation are covered by sandstones and conglomerates disposed in onlap geometry over the limestones (Fig. 3E). Deformation on the limestones slightly decreases to the south, where at La Silla locality (Fig. 1) we can observe a less angularity but an erosive contact at the base of the Cerrillos Formation over the Pabellón Formation.

The second regional scale unconformity is observed at the contact between Cerrillos (or locally Pucalume) and the overlying Viñita Formation. The angular unconformity between these units is variable, but locally is over 40º, especially near Las Campanas observatory, where a gently inclined eastward dipping plateau of andesite lavas and acidic tuffs is disposed over largely deformed beds of the Cerrillos Formation.

In order to better constrain the age of the older unconformity and the source of detrital material involved in the upper levels of the Pabellón Formation, we obtained three U-Pb ages near the top of that sequence in two localities. One sample (MJC-81, Appendix Table 1A, Fig. 4A) of coarse-grained sandstones at La Verde Creek locality yielded a mean average zircon U-Pb age of 118.6±1.0 Ma, considering a Gaussian distribution of individual ages between 113 and 124 Ma (n=30). The nearly unimodal distribution of ages indicates that the zircon and therefore detrital sources for this sandstones are derived from volcanic materials exposed along the same section, below or directly west of the sampled levels, belonging mostly to the upper levels of the Punta del Cobre Formation. Another sample (PLC-52, Appendix Table 1A), obtained near the top of the Pabellón Formation at La Silla locality, yields a weighted mean average zircon U-Pb age of 126.1±0.1 Ma (Fig. 4B) from a single Gaussian age distribution (from a unimodal age distribution), largely older than a sample of pyroclastic breccia from the overlying sequence (Cerrillos Formation), that yields a mean average of 114.7 ±0. 3 Ma (sample PLC-51, Appendix Table 1A, Fig. 4C), and a zircon U-Pb crystallization age of 111.5 ± 1.2 Ma from a tuff at the same stratigraphic level (Creixell et al., 2013). The unimodal character of the ages for the Pabellón Formation reflects that source of detrital material comes from local volcanic sources, probably the explosive eruptions characteristics from this period of time, well preserved in the top levels of the Punta del Cobre and Bandurrias formations, and locally registered in the top levels of the Pabellón Formation, as interbedded dacite and rhyolite tuffs (Creixell et al., 2013). Taking into account these data with previous paleontological and U-Pb data from Creixell et al. (2013) from the Pabellón and Cerrillos formations, we can restrict the age for the unconformity between both units between 116 and 110 Ma.

 

Fig. 4. Tera-Wasserburg diagrams for zircon U-Pb ages of this study; A-B. U-Pb ages on sedimentary rocks of the upper Pabellón Formation from the Chañarcillo Group (samples MJC-81 and PLC-52, respectively); C. Sample from a  volcanic breccia strata of the basal part of the Cerrillos Formation (sample PLC-51); D. La Unión porphyry (sample UN-9675); E. Johana porphyry (sample PY-01); F. Las Campanas porphyry (sample SVD-08); G. Mina Elisa porphyry (sample EL-9674). Crosses and ellipses for data-point errors are at 1σ. Weighted mean age (intercept) error is at 2σ .

3.3. Faults

A western domain of structures is part of the Atacama Fault System (AFS), where preserved stratigraphic relationship and some drag folds associated to extensional displacements along older plutons, suggest that these structures were previously active as normal faults, during the Early Cretaceous, around 130 Ma (Creixell et al., 2012). The younger well-documented tectonic activity on these structures is lower Cretaceous in age (121-117 Ma; Arévalo and Creixell, 2009), that is younger than the age for sinistral displacements in the AFZ (ca. 126 Ma) further north (Dallmeyer et al., 1996; Scheuber and González, 1999). Some of these faults contain ductile shear zones at pluton margins, indicating deformation coeval with pluton emplacement, whereas other faults exhibit brittle deformation and commonly occurrences of hydrothermal alteration zones. Along these structures, left-lateral transpression has been recognized, adjacent to Lower Cretaceous plutons and IOCG deposits. Main structures of this domain are grouped into the El Tofo Fault System (ETFS) where the main structures correspond to El Tofo and Las Leñas faults and La Higuera Shear Zone. To the east of the Early Cretaceous magmatic arc, normal faults spatially associated with Early Cretaceous sedimentary and volcanic sequences were documented by Arévalo et al. (2009) and Creixell et al. (2013). These faults control and affect the distribution of Lower Cretaceous back-arc sedimentary units (Bandurrias Formation and Chañarcillo Group).

Faults recognized along the eastern part of the study area exhibit mostly reverse sense of displacements (Fig. 1). The most prominent structure corresponds to the east-dipping Las Cañas-El Torito Fault (LCETF). The trace of the fault extends by about 50 km trending roughly N-S, between 28º50’ and 29º30’ S. South of 29º S, the LCETF juxtaposes calcareous marine beds of the Chañarcillo Group (Early Cretaceous) on its hanging wall over volcanic rocks of the Upper Member of the Cerrillos Formation (Creixell et al., 2013), through a master fault associated with minor synthetic reverse faults over the Chañarcillo Group. Directly east of La Verde porphyry, the geometry of the master fault is exposed as a reverse fault dipping 30° east, parallel to the dip of the strata of the Totoralillo Formation over Cerrillos Formation, configuring a ramp-like geometry for the fault (Fig. 3B). The exposed footwall of the LCETF consists of the subhorizontal volcanic rocks of the upper member of the Cerrillos Formation truncated eastward by the fault. In the hanging wall block, the Cerrillos Formation is disposed conformably and unconformably over the Chañarcillo Group as an east-dipping succession, passively-folded over the fault ramp. Locally, folds with near vertical limbs are recognized around 29º15’ S on the fault hanging wall. These contact relationship allows inferring a post-89 Ma activity for the LCETF. On the other hand, in the same hanging wall, the Cerrillos Formation is unconformably covered by the subhorizontal Viñita Formation (dated at 84 Ma, Creixell et al., 2013), thus bracketing the activity of the LCETF between 89 and 84 Ma.

South of this locality, the LCETF exhibits progressively less stratigraphic displacement and near 29º30’ S, deformation along the fault system is almost negligible, with no regional folding and minor stratigraphic displacement. The trace of the fault system is not recognized farther south.

4. Cretaceous dacitic and dioritic porphyry intrusives

4.1. Early Cretaceous porphyritic intrusions

Several small intrusive bodies (<5 km²) have been recognized, hosted by volcanic rocks of the upper part of the Punta del Cobre Formation (Fig. 1). Most of these intrusives contain variable amounts and types of Cu mineralization and are spatially associated with hydrothermally altered rocks with sericitic to intermediate argillic mineral associations in several generations of events. All these intrusive bodies typically have >25% phenocrysts, fine-grained groundmass and composition ranging from diorite to granodiorite/dacite. Some of these have been recognized in porphyry copper exploration prospects at Punta Colorada, Cachiyuyo, El Pleito, Pajonales, but also in already exploited deposits, such as Mina Unión (Frontera) and Dos Amigos-Tricolor porphyry copper deposits.

At Punta Colorada prospect (Fig. 1), a dacite porphyry is intruded into andesites of the Punta del Cobre Formation. The porphyry has withish grey color, largely visible along old exploration trenches and is intruded by andesite dikes. This intrusive is also spatially related to dacite dikes intruded into the andesite of the Punta del Cobre Formation. The dacite displays a porphyritic texture and mafic inclusions with acicular amphibole (quenching texture). Phenocrysts correspond to albitized plagioclase, corroded quartz, biotite partially replaced by epidote, smectite and titanite and chloritized amphibole. The fine-grained groundmass is composed by quartz, biotite and plagioclase. Advanced argillic alteration zones are widespread in the host rock close to the dacite porphyry, with kaolinite, diaspore, silica and pyrophyllite. Chrysocolla is the main ore mineral recognized in the field, disposed widespread along rock fractures in the dacite.

At Cachiyuyo (Fig. 1), a porphyritic dacite is intruded into tuffs of the Cerrillos Formation with a remarkable reddish outcrop color. This dacite is composed by quartz, sericite altered plagioclase and chloritized amphibole phenocrysts, but the intrusive is also affected by pervasive silicification and by supergene alteration characterized by widespread limonites, kaolinite, alunite, hematite-goethite and chrysocolla veins (Fig 3D).

At Mina La Unión (Fig. 1, Frontera prospect), there is an old open pit and several small sites of past mining activity. In this prospect, a Cu-Au porphyry deposit is characterized by several porphyritic diorite intrusions hosted by andesite of the Punta del Cobre Formation, with biotite (potassic alteration) and late hematite that contains Cu (oxides and sulfides) and Au (Maksaev and Llaumett, 2015).

An important number of dacitic dikes and stocks have been recognized along the El Pleito-Melón and Los Cristales mining districts (south of the Pajonales prospect, Fig. 1), partially overprinting the Early Cretaceous IOA and IOCG deposits. In the vicinities of El Pleito mine, high temperature mineral associations associated to iron oxide deposits are partially overprinted by widespread silica+albite alteration zones spatially associated with porphyritic dacite dikes and stocks, that intrude into andesite sequences and Early Cretaceous diorite plutons. In the same area, localized advanced argillic alteration zones (andalusite+pyrophyllite+kaolinite+dickite), common for IOCG deposits are spatially associated with dacite dikes and stocks (some pyrite-bearing). Specularite-K-feldspar veins and actinolite-albite hydrothermal breccias (Fig. 3C), all these dated around 111-110 Ma (Creixell et al., 2012) are apparently emplaced along Early Cretaceous faults. The dacite dikes are similar in composition to those described at Punta Colorada (Fig. 1). Morelli (2008) reported the occurrence of dacite porphyries, dated at 116 Ma, associated to relict sericitic alteration zones, surrounded by a large argillic alteration zone at Pajonales (Fig. 1), but without evidences of porphyry-type mineralization. At the northern end of the Los Cristales district, the Totora prospect shows evidences of porphyry-type Cu-Au mineralization, associated with a quartz-diorite porphyritic intrusive with surrounding potassic alteration, but also a porphyrytic dacite intrusion. (Maksaev and Llaumett, 2015).

A slightly younger group of porphyry copper type deposits associated with porphyritic intrusives of granodiorite to tonalite composition has been described in the Dos Amigos district, close to the town of Domeyko (Fig. 1). These consist of two porphyry copper deposit centers (Tricolor and Dos Amigos), located within a large hydrothermal alteration zone of 6x1.5 km (Maksaev et al., 2010). The porphyries intrude into volcanic rocks of the Punta del Cobre Formation. The granodioritic and tonalitic porphyries (At Tricolor and Dos Amigos) are characterized by presence of quartz and plagioclase phenocrysts and a fine-grained groundmass that display potassic alteration characterized by the presence of fine-grained biotite aggregates. By the other hand, the volcanic country rock, mostly andesites, display variable hydrothermal alteration, including sericitic, kaolinite-illite and propylitic mineral assemblages (Maksaev et al., 2010). A large hydrothermal breccia body is present close to Dos Amigos porphyry copper center, and is characterized by polymictic clasts of sericitized volcanis rocks and porphyries in a matrix of rock flour, pyrite, quartz and tourmaline, but also containing minor amounts of chalcopyrite (Maksaev et al., 2010). The Dos Amigos deposit, specifically its chalcocite secondary enrichment blanket was mined until 2015 (Maksaev and Llaumett, 2015).

According to zircon U-Pb dates, the age of Early Cretaceous porphyry intrusions range from 120 to 104 Ma. The oldest ones correspond to Totora diorite porphyries (120.8 and 119.5 Ma, V. Maksaev unpublished U-Pb data, in Maksaev and Llaumett, 2015) and the dacite porphyry at Pajonales (116.6±4.0 Ma, Morelli, 2008). For Punta Colorada dacite porphyry, Creixell et al. (2012) obtained a zircon U-Pb age of 109.7±0.9 Ma, and for the dacite porphyry at Cachiyuyo prospect, Creixell et al. (2013) obtained a zircon U-Pb age of 111.0 ± 1.9 Ma. The younger (Albian) ages, between 108.5 and 104.0 Ma where obtained by Maksaev et al. (2010) for Dos Amigos and Tricolor porphyries, respectively. In this study, a new zircon U-Pb age of 112.0 ± 2.1 Ma was obtained for the diorite porphyry at Mina La Unión or Frontera Prospect (Fig. 4D, sample UN-9675 in Table 1A Appendix), that is concordant with those obtained for porphyritic intrusives at Punta Colorada and Cachiyuyo prospects.

4.2. Late Cretaceous porphyritic intrusions

The porphyritic intrusives of this group are stocks of reduced exposure (<10 km²), with associated porphyry-Cu-like deposits, mostly with past mining or present exploration activities. All of these intrusives are emplaced into andesite and dacite-andesite lavas of the Upper Member of the Cerrillos Formation. Three of these porphyry copper bearing prospects are located between 2 and 4 km to the west of the trace of the Las Cañas-El Torito Fault System (Fig. 1). The only exception corresponds to Mina Elisa, located 15 km to the west of the fault and emplaced into the Late Cretaceous Domeyko Plutonic Complex (Creixell et al., 2013).

The Late Cretaceous porphyries corresponds to dacites, with variable amount of phenocrysts of embayed quartz, plagioclase and amphibole replaced by chlorite, in a fine-grained groundmass of the same minerals, often replaced by secondary quartz and sericite. Outcrops are associated with small (<3 km²) zones with hydrothermally altered volcanic country rocks. These zones consist of sericitic alteration with hematite and late carbonate veins and variably developed supergene argillic alteration overimposed or, in the case of Las Campanas, surrounding the rocks affected by sericitic alteration.

The age of the dacitic intrusives is constrained by one zircon U-Pb age of 88.4±1.2 Ma reported by Creixell et al. (2013) for a dacite porphyry in the La Verde porphyry copper prospect (Fig. 1). Approximately 30 km north, in the Johana (Cortadera) porphyry copper prospect, a similar zircon U-Pb age of 87.4±1.2 Ma was obtained (Fig. 4E, sample PY-01 in Table 1 Appendix), whereas at Las Campanas (Fig. 1), a dacitic porphyry displays a zircon U-Pb age of 90.13+0.91 Ma (Fig. 4F, sample SVD-08). For a dacite porphyry intrusion at Mina Elisa (Fig. 1), we obtained a slightly older zircon U-Pb age of 92.4+1.1 Ma (Fig. 4G, sample EL-9674). The age of these intrusives is close to the age of their volcanic country rocks, in this case the upper volcanic member of the Cerrillos Formation (92-89 Ma, Creixell et al., 2013), compatible with their subvolcanic nature.

5. Geochemical trends in the Cretaceous volcanism

With the aim to study the compositional evolution through time of the Cretaceous magmatism, from 140 to 80 Ma (Punta del Cobre to Viñita formations), as a tool to trace possible changes at the source region of the magmas, we analyzed a set of 71 whole-rock samples (see Table 2A of Appendix for detailed results) for major, trace and rare earth elements (REE). Selected trace element ratios, sensitive to pressure conditions (e.g., by variations in tectonic conditions or crustal thickness) were also used. The sample set include a large registry of Cretaceous volcanism including Early Cretaceous lavas from the Punta del Cobre and Bandurrias formations, and lavas and tuffs from the Cerrillos and Viñita formations, but also selected samples from porphyrytic intrusions. Details on the results and Analytical procedures are presented in the Appendix.

The general composition of Cretaceous volcanic rocks shows a large range in terms of SiO₂ contents, covering the entire range between basalt and rhyolite. For most major elements, a good correlation with SiO₂ content is observed for every sample group. It is worth to note that the Mesozoic volcanic sequences of the Central Andes are affected by low-grade regional metamorphism from low zeolite to greenschist facies (e.g., Levi et al., 1989), so that a significant number of samples of volcanic rocks and porphyries (~35%) evidences effects of low temperature alteration, reflected by LOI>2%. In spite of this, petrographic inspection of the samples, together with relatively homogeneous behavior of strongly mobile elements, such as Ce and Rb, suggest that the effects of alteration are mostly incipient and do not affect the elements distribution for petrological analysis. Moreover, samples of porphyries affected by alteration (LOI between 3.3 and 3.5%) are indistinguishable in terms of trace element contents with respect to less altered samples of the same group. In order to minimize the possible effects of alteration or metamorphism, we focused on the composition of immobile elements, such as Th, Zr and REE. In the classification scheme based on immobile elements (Winchester and Floyd, 1977), basaltic andesites and andesites are dominant in Early Cretaceous lavas, and andesites and dacites in Late Cretaceous rocks. The youngest Cretaceous units (Viñita Formation, Fig. 5) show widespread compositions, from basalt to rhyolite. Together with progressive increase in SiO₂, the Zr/TiO₂ ratio also shows higher values towards younger units. Most samples from porphyrytic intrusions fall into the field of rhyolite/dacite.

 

Fig. 5. Rock classification diagram (SiO2 wt% versus Zr/TiO2, Winchester and Floyd, 1977) for Cretaceous volcanic rocks, including samples from the Punta del Cobre, Bandurrias, Cerrillos and Viñita formations, and lower and upper Cretaceous Cu-bearing porphyry intrusions. Sub-AB: Sub-alkaline basalt; Alk-Bas: Alkaline basalt.

As also noted by previous studies (e.g., Morata and Aguirre, 2003; Richards et al., 2017), trace elements distribution (here normalized to Primitive Mantle of Sun and McDonough, 1989) are the typical for magmas originated in subduction-related margins, with enrichment in LILE (Large Ion Lithophile Elements) relative to HFSE (High Field Strength Elements), Nb-Ta, Ti and P negative anomalies and positive Pb (Fig. 6A). Early Cretaceous lavas of Punta del Cobre Formation show the most primitive compositions in the studied section, but do not represent primary mantle melts, as suggested by low MgO contents in the andesites (most samples <4 wt%). However, the data of Morata and Aguirre (2003) for the upper section of the Punta del Cobre Formation (considered as Arqueros Formation by these authors) display relatively higher Th and U contents with respect to our data. These authors and Richards et al. (2017) also note the presence of Early Cretaceous samples with primary alkaline composition, not clearly distinguished in our analyses. Our data also reflect differences with composition of Early Cretaceous volcanic rocks described in detail by Marschik and Fontboté (2001) for the Punta del Cobre district at 27°30’ S, where they report compositions between basalts and dacites for this unit and a slight enrichment in Zr/TiO₂ ratios with respect to our data. REE patterns tends to be similar for all volcanic units as normalized relative to Chondrite (Nakamura, 1974), with the higher REE contents in the upper member of the Cerrillos Formation. Most REE patterns of volcanic rocks are nearly flat and Eu negative anomalies are slightly marked in all units, including dacite porphyries. La/Sm increases toward younger units, whereas La_N/Yb_N values show a slight increase in Late Cretaceous dacite pophyries and Viñita Formation (Fig. 7A). The Sr contents are largely scattered in all sample groups, varying largely around 100 and 700 ppm, whereas Sr/Y ratios are mostly below 30 in all groups, as typical for arc magmas (Fig. 7B and C). The HFSE (High Field Strength Elements), especially Zr, Y and Nb, as well as Nb/Ta, Th/Ta and Ce/Pb ratios, are slightly increased for samples of the upper member of the Cerrillos Formation and some dacite porphyries (Fig. 7D).

 

Fig. 6. Whole-rock trace elements distribution (spider diagram) normalized on the Primitive Mantle composition of Sun and McDonough (1989). In A. The Lower and Upper Cretaceous porphyries (red open circles) are compared with composition of the Punta del Cobre and Cerrillos formations volcanic rocks; B. Cretaceous porphyries (red open circles) compared to Upper Cretaceous Viñita Formation.

 

Fig. 7. Geochemical bivariant diagrams for whole-rock composition of Cretaceous volcanic rocks and porphyry intrusives. A. LaN/YbN (normalized to C1-Chondrite of Nakamura, 1974), versus Age (My=million years); B. Sr/Y ratios versus Age; C. Sr/Y ratios versus Y contents in ppm. Compositional fields after Defant and Drummond (1990); D. Nb/Ta ratios versus Age; E. Th/Ta versus Age; F. Ce/Pb ratios versus Age. Symbols legend as in figure 5.

6. Discussion

6.1. Major tectonic events during the Early to Late Cretaceous

Field evidences obtained in this study allowed to detect two major transpressive/compressive tectonic events that disrupt relative dominant extensional conditions prevalent in the overriding plate during at least the Early Cretaceous (i.e., Aberg et al., 1984; Mpodozis and Ramos, 1989; Lucassen et al., 2006, among others). The nature of the evidences of these events are dependent on their paleogeographic domain, since in the Lower Cretaceous magmatic arc domain, current Coastal Range, these correspond to ductile deformation zones related to the emplacement of Early Cretaceous plutonic complexes and lava sequences (i.e., Arévalo and Welkner, 2008; Arévalo et al., 2009), while farther east, in the Cretaceous back-arc basin domain, these events are recorded as unconformities, crosscutting relationships with faults and sedimentary provenance indicators as demonstrated in this study. Our interpretation is in part contradictory with that of Richards et al. (2017), who proposed that the Cretaceous arc/backarc developed in three separated stages: >125 Ma extension, 125-110 Ma back-arc extension and rifting and 110 Ma and younger contraction.

The first identified tectonic deformation event detected along the studied segment is recorded in the magmatic arc domain and corresponds to left-lateral transpressive deformation registered along major segments of the Atacama Fault System (AFS). These segments are represented by Las Leñas, El Tofo, La Higuera and Algarrobo faults (Arévalo and Creixell, 2009). While El Tofo and Algarrobo faults display pure left-lateral displacement observed in cataclasites and mylonites, the La Higuera fault exhibits ductile vertical displacements along S-C shear zones and evidenced in sigmoidal porphyroclasts in mylonites from the margin of the El Trapiche Plutonic Complex. According to Arévalo and Creixell (2009) and Creixell et al. (2012), these faults, together with subsidiary faults that host hydrothermal IOCG deposits configure a whole left-lateral transpressional field that started around 121 Ma. All these ductile and brittle faults that are recognized along the Early Cretaceous arc domain have direct spatial relationship with magmatic rocks and alteration zones dated between 121 and 117 Ma, and show an overprint by alteration zones dated around 110 Ma (Creixell et al., 2012). In the back-arc domain, the Early Cretaceous deformation is recorded as an angular and erosional unconformity between the Pabellón and Cerrillos formations that, according to stratigraphic and geochronological data, represent a tectonic deformation event occurred between 116 and 110 Ma (Creixell et al., 2013). This unconformity represents a major change of marine to continental conditions of sedimentation, reflected by the shift from marine carbonate precipitation to siliciclastic deposition, and, consequently, the onset of subaerial drainage systems. These data suggest that the Early Cretaceous deformation front migrated from west to east after 117 Ma, following a progressive migration of the volcanic front in the same direction (Punta del Cobre to Cerrillos formations). The character of the deformation probably changed in time from sinistral transpressive deformation along the arc to east-west compressive one in the eastern back-arc domain. After the transpression ceased in the arc at 117 Ma and moved to the back-arc, both areas were ubiquitously invaded by magmas and hydrothermal fluids between 116 and 110 Ma, as is registered by the dacite and diorite intrusives related to porphyry copper-like mineralization, and dike swarms that crosscut older alteration zones related to IOCG and IOA deposits (Travisany et al., 1995; Creixell et al., 2012). With these data we can establish that the first episode of Cu-bearing dacite porphyry intrusions took place during this tectonic deformation event, and not during an extensional/transtensional event, as proposed by Richards et al. (2017) for north Chile. The whole Early Cretaceous tectonic event registered here between 121 and 110 Ma can be correlated with the first stage of development of the Mochica Deformation Event, well documented in the Peruvian Andes (Jaillard, 1994; Jaillard and Soler, 1996).

The second relevant tectonic deformation event has been recognized only along the eastern part of the segment considered in this study, where folding and faulting of the Chañarcillo Group in the north, and Cerrillos Formation to the south, is recognized along the Las Cañas-El Torito Fault (LCETF in Fig. 1). As described in previous sections, deformation corresponds to the overthrusting of the Chañarcillo Group limestones above the Cerrillos Formation volcanic rocks. Also, a large unconformity surface, recognized along the contact between the rocks of the Cerrillos and overlying Viñita formations, is observed in the study area. A large portion of the stratigraphy of the Cretaceous rock sequence is also repeated along this fault, but this displacement diminished towards south, disappearing almost completely around 30° S. The geological data (Creixell et al., 2013; Salazar et al., 2013) indicates that the deformation took place between 89 and 84 Ma mostly along the LCETF, accompanied by migration of the magmatic foci to the east. As we note that three Cu-bearing dacite porphyries occur close to the fault in time and space, we can hypothesize that this Late Cretaceous tectonic deformation event excerted control on the localization of these Cu-bearing porphyries. At a broad scale, this Late Cretaceous compressive event coincides with the early pulse of the Peruvian compressive phase highlighted by several authors (e.g., Jaillard, 1992; Haschke et al., 2002). In the eastern margin of the Cretaceous intraarc basin, provenance studies with detrital zircon U-Pb ages on the Pucalume Formation sedimentary rocks, evidence denudation of an uplifted eastern Permian (Cisuralian) basement block between 91 and 80 Ma (Merino et al., 2013). Such basement block is very well constrained through regional mapping and corresponds mostly to Early Permian plutonic complexes, bounded by the San Félix and Pinte reverse faults in the eastern part of the studied area (Salazar et al., 2013). The latter shows that the Late Cretaceous compressive deformation event at this latitude was spread into the foreland and that it involved basement blocks. The regional importance of the Late Cretaceous “Peruvian” compressive event in the margin of the southern central Andes has been highlighted by several authors (e.g., Jaillard, 1992; Mescua et al., 2013; Bascuñan et al., 2015), as the start of the continuous compressive conditions of the margin until the present times.

6.2. Tectonic significance of compositional trends in magmatism

A remarkable feature of Cretaceous magmatic rocks in the study area is the tendency to more evolved composition through time, from dominantly basaltic andesites in Lower Cretaceous rocks to andesite and dacites in Upper Cretaceous.

Several studies has pointed out that porphyry copper deposits are derived from magmatic suites, largely oxidized and enriched in sulphur but most importantly in water (e.g., Richards, 2011, and references therein). A relevant feature detected in this study is that the suite of Early Cretaceous dacite intrusions has indications of having been derived from differentiated magmas (compared to volcanic rocks) that were also richer in water (>4 wt% H₂O in the magma, Ridolfi et al., 2010) as indicated by the persistent presence of amphibole as phenocryst in these rocks (Fig. 3C). Recent work by Richards et al. (2017) confirms that the studied Cretaceous Cu-porphyries are comparatively richer in S than Early Cretaceous IOCG deposits. On the other hand, some studies have noted that a direct relationship between porphyry copper genesis and stages of crustal shortening can be envisaged, especially for the Cenozoic Andean environment where a correlation between large deformation events and geochemical changes in magma composition have been detected (Kay and Mpodozis, 2001; Bissig et al., 2003; Rabbia et al., 2003; Mpodozis and Cornejo, 2012).

As suggested by some compositional characteristics such as LILE enrichment over HFSE, Nb-Ta depletion, Pb enrichment and Sr/Y<30, the magmas that originated the Cretaceous volcanic rocks show typical geochemical features for magmas generated in a subduction-related active margin. In general terms, REE patterns are flat, with (La_N/Yb_N) ratios <10. These values suggest an origin from melting of a depleted mantle-derived magma, with minor involvement of crustal component in their genesis (e.g., Pearce, 1982). However, variable but intermediate #mg values between 25 and 50 indicate that magmas were not primary mantle melts, but fractionated or differentiated after melting from a possible mantelic source (e.g., Klein and Langmuir, 1987). This is consistent with previous interpretations for the Cretaceous magmatism based on trace elements and Sr-Nd isotopic ratios that suggest that an origin from mantle melts modified in low degree by crustal melts and more shallow processes such as magma differentiation (e.g., Vergara et al., 1995; Morata and Aguirre, 2003). Richards et al. (2017) note that the Early Cretaceous (125-110 Ma) igneous rocks from north Chile show a primitive isotopic composition (higher εNd values towards N-MORB), that is consistent with our major and trace elements data that suggest that the Punta del Cobre lavas are the least differentiated in composition. The lack of a strongly marked Eu anomalies (Eu/Eu*<0.2; Fig. 6) suggest moderate to high O₂ fugacity of the magma or plagioclase fractionation (e.g., Burnham et al., 2015).

When compared with the older volcanics of the Punta del Cobre Formation and from the lower member of the Cerrillos Formation, the volcanic rocks of the upper member of this last unit display some minor geochemical changes, by example an increase in Th/Ta ratios (Fig. 7E), that can be interpreted as an increase in crustal contamination (e.g., Condie, 2003) around the 89-92 Ma period, coincident with the onset of the Peruvian deformation stage. Similarly, Ce/Pb, Th/La and Nb/Ta ratios also show a slight increase around 90 Ma. The increase in these ratios and particularly the increase in Th, Ce and possibly Nb point out towards a modification of the primary mantle source by an external source like crustal fluids or slab-derived fluids (see by example Oliveros et al., 2007). Isotopic data from Richards et al. (2017) show a slight scatter of Nd-Sr isotope ratios for Late Cretaceous igneous rocks that can be interpreted as a small degree of interaction between MORB-like and crustal sources.

In summary, our geochemical data point to subtle changes in trace elements ratios and magma sources as mentioned above around 90 Ma, but also only slight changes in, for example, La_N/Yb_N (Fig. 7A) or Sm/Yb ratios, so we can discard a major crustal thickening processes as responsible for these changes in geochemical composition around 90 Ma. The Yb values normalized to C1 chondrite (Sun and McDonough, 1989) over 10, exclude the presence of stable garnet in the source of magmas (e.g., Gromet and Silver, 1987), and by consequence suggest that relevant crustal thickening (>40 km) during the Cretaceous and especially during the transpressive or compressional stages described above, was not present. In the same way, Sm/Yb for the more basic rocks are always low (<3), and according to Kay and Mpodozis (2001), such low values are indicative of a low pressure mantle source with clinopyroxene fractionation in the lower crust.

As a reference, we use the geochemical parameters of La/Yb or Sr/Y for estimation of crustal thickness during magma genesis (Chapman et al., 2015; Profeta et al., 2015) as these trace element ratios are largely controlled by crustal thickness in arc settings (see also Chiaradia, 2015). After filtering data to the restriction given by Profeta et al. (2015), we can note a two consecutive steplike increases in calculated crustal thicknesses from La/Yb ratios, from less than 30 km in Punta del Cobre Formation to around 40 km between 120 and 110 Ma, and from around 40 to more than 50 km between 80 and 90 Ma (Fig. 8), coincident with both, the Early Cretaceous transpressive event (120-110 Ma) and the Peruvian deformation (80-90 Ma),described in previous sections . Calculated thickness from Sr/Y give more disperse values, but still showing a tendency to increase around 90 Ma. Finally we can hypothesize that the Late Cretaceous compressive event, at least at the studied segment and domains, had a traceable imprint on the magma composition, but minor when compared with composition of Neogene magmatic rocks associated to major orogenic phases in the Eocene or Miocene (e.g., Kay and Mpodozis, 2001; Haschke et al., 2002; Bissig et al., 2003).

 

Fig. 8. Calculated thickness (crustal thickness in kilometers) versus Age of the rocks. Calculations using La/Yb ratios and data filtering, following Profeta et al. (2015).

6.3. Possible implications for mineral exploration of Cu resources.

The geochronological and geochemical data presented in this contribution points to a direct relationship between the occurrence of porphyry copper deposits, porphyritic dacite intrusions and Cretaceous deformed belts, since most dacite porphyry and porphyry copper occurrences are restricted to a tectonic block limited by Cretaceous fault systems, the El Tofo Fault System by the west and the LCETF by the east. The older mineralization/alteration event is characterized by the occurrence of Cu-bearing dacite to diorite porphyritic intrusions with ages variable between 116 and 109 Ma, with a main cluster around 110-109 Ma and 106-104 Ma (Dos Amigos and Tricolor Cu-porphyries). These ages are also consistent with a prolongation of this belt to the south, considering the Andacollo mining district, dated at around 104 Ma (Maksaev et al., 2010; Richards et al., 2017). Most of these occurrences represent the relatively deep portion of Cu-porphyry systems, including the intrusive porphyries and locally potassic alteration zones. The westernmost of these intrusive porphyries (as dykes or stocks), are overimposed on the IOA and IOCG belts, generating complex patterns of alteration zones, with younger (110 Ma) alteration assemblages occurring close to older IOA- and IOCG-related alteration zones.

In the case of Late Cretaceous porphyry intrusions, its distribution is clearly limited to the east by the LCETF. These porphyries also show a close temporal and spatial relationship with the volcanic country rocks of the Cerrillos Formation and the location of the Chañarcillo Group marine deposits. This suggests that these mineralized intrusions are intimately linked with the host volcanic rocks, probably as hypabyssal intrusions. Therefore, we can propose that both the magmatic and structural systems were active during the Late Cretaceous. Furthermore, we can suggest the Late Cretaceous volcanic belt of the Upper Cerrillos Formation (dated between 92 and 88 Ma) as a first order control on the distribution on the Late Cretaceous porphyritic intrusions. At regional scale, the period of emplacement of the Late Cretaceous, Cu-bearing, dacite porphyry intrusions was coeval with the Peruvian orogenic phase, that in the study area, registered compressional deformation, with a subtle, but detectable change in the magma composition with respect to Early Cretaceous magmatism.

7. Concluding Remarks

Two main episodes of emplacement of dacite to diorite porphyries with associated porphyry copper type deposits have been identified in north Chile between 28º00’ and 29º30’ S. These episodes took place during the Early Cretaceous (116 to 104 Ma) and the Late Cretaceous (92 to 87 Ma). These intrusive porphyries are distributed within a tectonic block limited to the west by the eastern segments of the Atacama Fault System (El Tofo Fault System) and to east by the Las Cañas-El Torito Fault. The emplacement of the Early Cretaceous porphyries was contemporaneous with sinistral transpressive deformation that started at 121 Ma in the arc domain and migrated eastward to the back-arc domain until 111 Ma, when a regional-scale unconformity was produced between the Pabellón and Cerrillos formations. The Late Cretaceous intrusive porphyries were emplaced coetaneous with a compressive deformative phase that, in the study area, was shown as reverse displacements along the LCETF, but at regional-scale, is correlated with the Peruvian Orogenic Phase (Jaillard, 1992).

The integrated geological observations, together with geochemical data on the Cretaceous volcanism from the Early to the Late Cretaceous, indicates that both Early and Late Cretaceous transpressive to compressional deformation episodes were not significant in terms of changes in the genesis of the magmatism, but trace element ratios such as Th/Ta, Nb/Ta and La/Yb suggest that the Late Cretaceous event was related to an increase in crustal thickness with respect to the Early Cretaceous, but minor compared to major Cenozoic orogenic phases, at least along the arc to back-arc domain.

Our findings suggest that regional mapping of the Cretaceous structural systems can be used as a first-order guide to the exploration of porphyry copper deposits associated to the dacite/diorite porphyries.

Acknowledgments
Most of the field observations were made during development of geological cartography programs of SERNAGEOMIN, funded by FNDR Project Nº BIP30068454-0 and PNG Project (Plan Nacional de Geología). The authors wish to thank J. Bustamante for geochemical analyses and R. Tello and H. Neira for their assistance in the field. We thank valuable comments on this paper by V. Maksaev and F. Henríquez. T. Bissig also made suggestions on a previous version of the manuscript. This work is part of the Postgraduate Thesis of the second author (J. Fuentes), and he wishes to thanks to the Postgraduate Program in Economic Geology from Universidad Católica del Norte.

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Appendix

Table 1A. ANALYTICAL DATA FOR ZIRCON LA-ICPMS U-Pb AGE DETERMINATIONS OBTAINED IN THE UNIVERSITY OF TASMANIA, WITH EXCEPTION OF SAMPLES PLC-52 AND PLC-51, ANALYZED AT GEOCHRONOLOGY LABORATORY OF SERNAGEOMIN.

Analytical Procedures Table 1A

LA-ICPMS zircon geochronology at the University of Tasmania

The LA-ICPMS method is now widely used for measuring U, Th and Pb isotopic data.

Approximately 100 g of rock was crushed in a Cr-steel ring mill to a grain size <400 micron. Non magnetic heavy minerals were then separated using a gold pan and a Fe-B-Nd hand magnet for the magnetic fraction. The zircons were hand picked from the heavy mineral concentrate under the microscope in cross-polarised transmitted light. The selected crystals were placed on double sided sticky tape and epoxy glue was then poured into a 2.5 cm diameter mould on top of the zircons. The mount was dried for 12 hours and polished using clean sandpaper and a clean polishing lap. The samples were then washed in distilled water in an ultrasonic bath.

The analyses in this study were performed on an Agilent 7500cs quadrupole ICPMS with a 193 nm Coherent Ar-F gas laser and the Resonetics S155 ablation cell at the University of Tasmania in Hobart. The downhole fractionation, instrument drift and mass bias correction factors for Pb/U ratios on zircons were calculated using 2 analyses on the primary (91500 standard of Wiendenbeck et al. (1995)) and checked on 1 analysis on each of the secondary standard zircons (Temora standard of Black et al. (2003) and JG1 of Jackson et al. (2004)) analysed at the beginning of the session and every 15 unknown zircons (roughly every 1/2 hour) using the same spot size and conditions as used on the samples. Additional secondary standards (The Mud Tank Zircon of Black and Gulson (1978), Penglai zircons of Li et al. (2010), and the Plesovice zircon of Slama et al. (2008)) were also analysed. The correction factor for the ²⁰⁷Pb/²⁰⁶Pb ratio was calculated using large spots of NIST610 analysed every 30 unknowns and corrected using the values recommended by Baker et al. (2004).

Each analysis on the zircons began with a 30 second blank gas measurement followed by a further 30 seconds of analysis time when the laser was switched on. Zircons were sampled on 32 micron spots using the laser at 5 Hz and a density of approximately 2 J/cm². A flow of He carrier gas at a rate of 0,35 litres/minute carried particles ablated by the laser out of the chamber to be mixed with Ar gas and carried to the plasma torch. Isotopes measured were ⁴⁹Ti, ⁵⁶Fe, ⁹⁰Zr, ¹⁷⁸Hf, ²⁰²Hg, ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th and ²³⁸U with each element being measured every 0.16 s with longer counting time on the Pb isotopes compared to the other elements. The data reduction used was based on the method outlined in detail in Meffre et al. (2008) and Sack et al. (2011) similar to that outlined in Black et al. (2004) and Paton et al. (2010). Uncertainties were calculated using methods similar to that outlined Paton et al. (2010).

Element abundances on zircons were calculated using the method outlined by Kosler (2001) using Zr as the internal standard element, assuming stoichiometric proportions and using the NIST610 to standard correct for mass bias and drift.

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LA-ICPMS zircon geochronology at Sernageomin

Samples were previously crushed under 500 µm and heavy minerals were concentrated using a Gemini water table. The resulting sample was examined under a UV lamp in a microscope and zircon crystals were selected by hand picking. A number between 30 and 120 zircon crystals were mounted in a 2.5 cm diameter epoxy briquette, together with a Temora 2 (Black et al., 2004) crystal. After mounting, cathodoluminiscence and backscattered electron images were obtained under electronic microscope (Zeiss MA-10), to obtain an detailed image of zoning patterns, inherited cores, inclusions and fractures in zircon crystals.

Samples for U-Pb dating were analyzed by laser ablation in a Photon-Machines equipment, Analyte 193.G2 eximer model. Several briquette samples are mounted together with a briquette with GJ-1 primary standar (Jackson et al., 2004) and Mud Tank (Black and Gulson, 1978) and Plesovice (Sláma et al., 2008) as secondary standars. The ablation diameter in the zircon crystals was 30 µm, with a pulse frequency of 9 Hz and energy density about 2 mJ/cm, producing a crater of 15-20 µm depth in the zircon crystals.

After ablation, isotopes and elements concentration (U, Pb and Th) were measured in a Thermo Fischer Element XR double focus mass spectrometer, with an electron multiplier. Data reduction done using Iolite software (Petrus and Kamber, 2012). Isotope fractionation produced during ablation and along the spectrometer, instrumental drift and calibration of isotope ratios, were controlled through analyses on the primary standar GJ-1. Second order variations related to sample position on the ablation chamber on the Photon-Machines equipment, were corrected by measurements on the Temora 2 estándar mounted in every simple briquette. Secondary standards (Plesovice and Mud Tank) are considered as unknown samples and therefore as quality control on the analyses. For Phanerozoic samples, 206Pb/238U ages are corrected for common-Pb using the 207-method of Williams (1998). Final results, age calculations and graphics have been extracted using the Isoplot tool for Microsoft Excel (Ludwig, 2012). The decay constants are those defined by Steiger and Jäger (1977).

Table 2A. MAJOR AND TRACE ELEMENTS CONTENTS IN ROCK SAMPLES OF PUNTA DEL COBRE, BANDURRIAS, CERRILLOS AND VIÑITA FORMATIONS AND DACITE PORPHYRIES.

Analytical Procedures Table 2A

Rock sample for geochemical analyses were crushed and pulverized under 60 µm. The rock sample was mixed with a 1:3 flux mixture of  lithium metaborate and tetraborate. The final proportion of sample and flux mixture is 1:10 and then melt in a glass pearl, alternatively the sample was mixed with binder material (vegetal wax and boric acid) and pressed in a briquette, for major or trace elements (Cu, Pb, Ni, Co, Zn, Cr, Sc, V, Zr, Rb, Sr, Ba) , respectively. The concentration of major elements represented as oxides in weight percent (wt%) has been measured by X-ray Fluorescence in the chemical Laboratory of SERNAGEOMIN using a Panalytical AXIOS equipment. The concentration of a set of trace elements indicated above was measured in the same equipment, and expressed in parts per million (ppm). Another part of the crushed sample was dissolved by fussion using a 4:1 mixture of Na₂CO₃:L₂B₄O₇ and acid attack with HNO₃ and used to measure the concentration in ppm of rare Earth Elements (REE: La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Nb, Ho, Er, Tm, Yb, Lu) and some trace elements (U, Th, Y, Cs, Hf, Ta) by ICP-MS (Inductive Coupled Plasma Mass Spectromety) using an Agilent 7500 equipment.