Regional Metallogenic Structure Based on Aeromagnetic Data in Northern Chile*

Home , Domeyko Fault, La Negra Formation

APPLIED GEOPHYSICS, Vol.13, No.4 (December 2016), P. 721-735, 9 Figures. DOI:10.1007/s11770-016-0593-6

Regional metallogenic structure based on aeromagnetic data in northern Chile*

Zhu Xiao-San1 and Lu Min-Jie♦1

Abstract: Chile is a very important country that forms part of the Andean metallogenic belts. The Atacama and Domeyko fault systems in northern Chile control the tectonic– magmatic activities that migrate eastward and the types of mineral resources. In this paper, we processed and interpreted aeromagnetic data from northern Chile using reduction to pole, upward ﬁ eld continuation, the second derivative calculation in the vertical direction, inclination angle calculation, and analytical signal amplitude analysis. We revealed the locations and planar distribution characteristics of the regional deep faults along the NNE and NS directions. Furthermore, we observed that the major reasons for the formation of the tectonic–magmatic rocks belts were the nearly parallel deep faults distributed from west to east and multiple magmatic activities along these faults. We ascertained the locations of volcanic mechanisms and the relationships between them using these regional deep faults. We deduced the spatial distributions of the basic–intermediate, basic, and acidic igneous rocks, intrusive rocks, and sedimentary sequences. We showed the linear positive magnetic anomalies and magnetic anomaly gradient zones by slowly varying the background, negative magnetic anomaly field, which indicated the presence of strong magmatic activities in these regional deep faults; it also revealed the favorable areas of copper and polymetallic mineralization. This study provides some basic information for further research on the geology, structural characteristics, and mineral resource prospecting in northern Chile. Keywords: Andean metallogenic belt, aeromagnetic anomaly, Atacama fault system, Domeyko fault system, structural interpretation, volcanic mechanism

Introduction for 32.7% of the global share and the gold reserve accounted for 7.5% of the global reserve according to the preliminary statistics of the Chilean copper The Andean metallogenic belt is one of the most association in 2012, which ranked the fourth in the important metallogenic belts worldwide and Chile is world in 2013. Chile is also rich in reserves of iron an important country with respect to mining in the and other metals in Chile and the production of these Andes. Chile is abundant in mineral resources of copper, metals is high (Mei et al., 2009; Li et al., 2011a). The molybdenum, and gold; copper production accounted occurrence of these mineral resources is related to the

Manuscript received by the Editor April 14, 2016; revised manuscript received October 14, 2016. *This work was jointly supported by the National Science Foundation of China (No. 41404070) and China Geological Survey (No. DD20160102-02). 1. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China. ♦Corresponding author: Lu Min-Jie (Email: [email protected]) © 2016 The Editorial Department of APPLIED GEOPHYSICS. All rights reserved. 721 Regional metallogenic structure in northern Chile parallel-arc strike-slip faults and the basement structures ashes; (3) the volcanic mechanisms that generate in the NW direction that formed during the subduction of magnetic anomalies with sharp gradient changes and the Pacifi c plate in the Andes; in other words, the spatial disorderly, banded anomalies with alternating positive distributions of mineral resources are controlled by and negative values caused by volcano depressions or these faults systems (Ren et al., 1993; Camus and Dilles, volcano domes; (4) volcanic mechanisms that generate a 2001; Richards et al., 2001; Fu, 2013). beaded arrangement of strong magnetic anomalies belts, The Atacama Fault Zone (AFZ) (Armijo and Thiele, caused by fi ssures eruptions controlled by the basement 1990; Brown et al., 1993; Taylor et al., 1998; Lucassen faults; and (5) volcanic mechanisms that generate a et al., 2001; Li et al., 2009, 2011a; He et al., 2014) is single strong negative anomaly, caused by rocks with located in the west of northern Chile and the Domeyko reverse magnetization in smooth, negative magnetic Fault Zone (DFZ) (Elderry et al., 1996; Niemeyer and fi elds. These classifi cations of volcanic mechanisms also Urrutia, 2009) is situated in the east. The Atacama and provided effective methods for recognizing them using Domeyko fault systems have very outstanding features in aeromagnetic data. the Andean metallogenic belt and control the eastward- Although the domestic geological survey and the migrating tectonic–magmatic activities, mineralization, corresponding geological research in Chile were and the regional metallogenic zonings. Geologists conducted relatively well compared with those conducted have studied the Atacama and Domeyko faults systems in other countries in the Andes (Elderry et al., 1996; using satellite images (Chorowicz et al., 1996), surface Taylor et al., 1998; Niemeyer and Urrutia, 2009), the investigation together with both kinematic models and study of the two regional fault systems using geophysical dynamic data (Jensen et al., 1996; Susie et al., 1996), data has rarely been conducted. We studied the two and geochemical analysis of rocks (Scheuber, 1990). fault systems in detail using the newest aeromagnetic Chorowicz et al. (1996) analyzed new structural faults data integrated with the regional geology, structures, by improving the accuracy of the satellite images. Jensen and mineral resource information in northern Chile to et al. (1996) predicted the geometric position of the further reveal the spatial distribution characteristics of Atacama fault system and the distribution of fractures on these faults systems, identify and delineate the volcanic different scales according to the kinematic model of fault mechanisms, analyze the geology of the volcanic development. Susie et al. (1996) analyzed the dominant mechanisms and the features of magmatic activities, and dynamic data of the western Domeyko fault system and understand their restrictions on mineralization in the summarized these problems related to the data. Scheuber study area. (1990) discussed the horizontal and vertical movements of the Atacama fault system, the magmatic–hydrothermal activities, and mineralization on the basis of the analysis Geological setting of geochemical data along with surface investigations. Most volcanic mechanisms are residuals of ancient volcanic mechanisms that undergo drastic changes during Andean tectonism has an obvious feature of migrating geological processes, and the different types include eastward, which reveals the convergence process of volcano-intrusive rocks, sub-volcanic rocks, volcanic plates. It formed five parallel-arc tectonic–magmatic rocks in volcanic vents, and erupted volcaniclastic rocks. belts that belong to five different periods including In magnetic anomaly maps, most volcanic mechanisms the Jurassic–Early Cretaceous period, the Paleocene show magnetic anomalies with alternating positive and epoch, the late Eocene–Oligocene epoch, the early– negative values, circular shapes (or shapes similar to a middle Miocene epoch, and the middle Miocene–early circle) or chain-like shapes distributed along faults (Zhou Pliocene epochs (Figure 1). Furthermore, it formed fi ve and Wu, 1983). Zhou and Wu (1983) summarized five corresponding giant porphyry copper–gold belts. The types of volcanic mechanisms: (1) volcanic mechanisms tectonic deformations generated a series of thrust-slip that generate magnetic anomalies with almost equal fault belts in the NS direction and folds dipping to the areas and in single-axial shapes (or similar to single- north. axial shapes), caused by ultrabasic, basic, or intermediate The tectonic structure of Chile mainly comprises rocks in volcanic vents; (2) volcanic mechanisms five first-grade tectonic units that include the Nazca that generate multi-peak or multi-axial, radial, strong plate and three ridge subduction belts (including the magnetic anomalies, formed because of volcanic vents Ridge de Nazca, the Iquiqua, and the Juan Fernande with dominant intermediate acid–acid erupted volcanic ridges subduction zones), the Chile–Peru trench belt,

722 Zhu et al. the Andean-type active continental margin, the Andean the Devonian–Carboniferous sedimentary unit of the orogenic belt, and the South America shield zone. It can passive continental margin (belonging to the tectonic- be divided into ﬁ ve third-grade tectonic units from west lithofacies restoration unit and an important object in the to east according to the structures and landforms in the study of regional tectonic evolution), the Carboniferous middle and north of Chile, and these units include the deep magmatic-arc unit, the Permian volcanic-island- Arica forearc basin and forearc accretionary wedge, the arc unit, the major island arc and back-arc basin rock Chilean Cordillera coast belt, the central Chile basin unit that formed during the tectonic transition period zone, the major Cordillera orogenic belt, and the western in the late Triassic-Early Jurassic period (the dominant Cordillera area (Lucassen et al., 2001; Li et al., 2011a, volcano island belt and the back-arc basins belonging 2011b). Their corresponding tectonic-rock stratigraphic to two third-grade units of the trench-arc-basin system; units and rock stratigraphic units include the pre- the two units are closely associated with each other in Devonian tectonic rock stratigraphic unit (mainly for the space), and the tectonic unit extruded and contracted by units with tectonic deformations and metamorphisms the Andean-type active continental margin since the Late that occurred before the Carboniferous period; it is the Jurassic period. dominant unit of the forearc accretionary wedge units),

(a)80° W 70° W 60° W (b) 72° W 70° W 68° W

Argentina Early Paleozoic platformPrecambrian basement Iquique Oaxaquia N South America N Amazon Craton Chile 10° S AFZ Mainly early Paleozoic rocks 0 100 km 22° S DFZ Arequipa Calca Tocopilla Calama

Antofagasta DFZ 20° S Paleozoic terranes with 24° S Grenville basement Pacific Ocean Antofalla Figure 1b AFZ Western Pampeanas arc Chile Amazon Craton Lengend Pampia Taltal Argentina Magmatic arc Volcanic arc 26° S Chañaral DFZ

Cuyania Study area 30° S Chilenia Paleozoic terrane with unknown affinities Copiapo 28° S Mesozoic-Cenozoic Patagonia Vallenar accreted terranes Legend AFZ Fault 0 1000 km Coastline Country border La Serena City 30° S 80° W 70° W 60° W Fig.1 Simpliﬁ ed distribution map of the tectonic basements in the middle Andes (a) and cartoon showed the Atacama Fault Zone (AFZ) and the Domeyko Fault Zone (DFZ) in the north of Chile (b) (Armijo and Thiele, 1990; Elderry et al., 1996; Taylor et al., 1998; Lucassen et al., 2001; Li et al., 2011a) .

The Precambrian metamorphic rocks and granites strata formed in the Precambrian-Ordovician and the were mainly distributed in the coastal area of Chile, Silurian-Carboniferous periods were distributed along and the Precambrian-early Paleozoic metamorphic the coastal belt, which were strongly metamorphosed rocks found in the area were common in lamellar and distributed in sheet shape (Figure 2). The Cambrian- and mylonitic structures (Figure 2) and distributed in Permian (328–235Ma) igneous rocks were mainly belts. The Paleozoic ilmenite series granite belts were including granites, granodiorites, tonalitic diorites, and associated with the Paleozoic rhyolite belts in the diorites (Fang and Li, 2014). western Santiago-Concepcion (Li et al., 2011a). The The Permian-Triassic (270–205Ma) igneous rocks 723 Regional metallogenic structure in northern Chile were mainly including granites, granite porphyries, clastic sedimentary series and the volcanic-sedimentary and biotite granite diorites. The Triassic-Jurassic series. The strata in the northern Chile were dominated (212–180Ma) igneous rocks were dominantly including by the La Negra formation, which was the major ores monzodiorites, monzogranites, granodiorite granites, bearing strata of the iron-oxide-copper-gold (IOCG) diorites and babbros (Figure 2) (Li et al., 2011a, 2011b; deposits and iron oxide deposits (such as, the erupted- Li and Fang, 2011). type magnetite-hematite ores deposits) (Zhou et al., The Mesozoic-Tertiary strata were distributed widely 2010; Li et al., 2011a; He et al., 2014). in Chile and they were composed by the shallow marine The Jurassic-Cretaceous strata were spread along the

70° W 65° W 25° S 25° S N

South America

Study area

27.5° S 27.5° S

0 200 km 30° S 30° S 70° W 65° W Legend Intrusive rock Volcanic rock Granite Granodiorite Tonalite Alkali granite Intermediate rock Basic rock Ultrabasic rock Alkaline rock Ophiolite rock

++++ ++++ $$$$$ ++++ ++++ Neogene period ++++Ȗ1 Ȗį1++++ ȖȠ11 į1 $$$$$ $$$$ Neutral volcanic volcanic Neutral rock classified Non rock volcanic ++++ ++++ $$$$$ volcanic Acid rock acidic Middle rock volcanic basic Middle rock volcanic volcanic Basic rock ++++ ++++ $$$$ Paleogene period ++++Ȗ( Ȗį(++++2 ȖȠ( į( Ȟ( ??? ++++ ?? ??? $$$$$ ??? $$$$ ?? ȣ14 ++++ ++++ $$$$$ Quaternary period Į14 Įȕ14 ȕ14??? ++++Ȗ. ++++Ȗį. ȖȠ. į. Ȟ. ı. ȟ-. Ƞĳ. Cretaceous period ++++ ++++ ??? ++++ \\\\\ ?? //// \\ ??? ++++ $$$$ \\\\\ ?? ??? ++++ $$$$$ Ȝ4 ȜĮ4 Į4 Įȕ4 ȕ4 ȣ4 ++++ ++++ $$$$ //// \\ ++++Ȗ- Ȗį-++++ ȖȠ- į- Ȟ- Jurassic period ++++ //// ++++ ++++ \\\\\Ȝ1 2 ++++ ++++ ////Ȝ1 Į1 Trassic period ++++Ȗ7 Ȗį7++++2 į7 ++++ ??? //// ?? ??? \\ \\\\ \ ?? ??? ++++ ////Ȝ11 ȜĮ1 Į1 Įȕ1 ȕ11 ++++Ȗ0] ı0] Ƞĳ0] Neogene period \\\\\ \\ ??? Mesozoic era ++++ $$$$ ?? ??? \\\\\ \\ ??? ++++ ++++ $$$$$ //// ?? ++++ $$$$ Ȝ1 ȜĮ1\\ Į1 Įȕ1 ???ȕ1 ȣ1 Permian period ++++Ȗ3 Ȗį3++++ ȖȠ3 \\\\\Ȝ1 ++++ ++++ ?? ++++ ++++ ??? ++++ ?? ??? ++++Ȗ& ++++ Įȕ( ȕ( Carboniferious period ++++ ++++Ȗį& ??? $$$$ ?? ++++ $$$$$ /// / ++++ $$$$ ?? Ȗ' ȖȠ' \\\\\2 Devonian period ++++ Paleogene period ///Ȝ( / ȜĮ(\\ Įȕ(

++++ ?? ??? ++++Ȗ3] ?? ??? Later Paleozoic era ++++ Į.( Įȕ( ???ȕ( ȣ( ++++ ??? /// / ++++ \\\\\ ??? Silurian period ++++Ȗ6 /// / ??? Ȝ. Į. ȕ. \\\\\ ??? ++++ ++++ Ordovician period ++++Ȗ2 į2 ȟ2 \\\\\ ?? /// / \\\\\Ȝ. Cretaceous period /// / Įȕ.?? ++++ ++++ ??? Cambrian period ++++Ȗę ??? /// / \\\\\ \\ ??? 1 ///Ȝ. / ȜĮ. Į. ???ȕ. ȣ. ++++ ++++ ++++ ++++ ++++Ȗ3] Ȗį3]++++1 į3] Paleozoic era ++++ /// / \\\\\ ///Ȝ- / \\\\\ Neo-proterozoic era ı3W \\\\\ /// / \\ \\\\\Ȝ- ȜĮ- ++++ !! Jurassic period ++2 2 2 ++++Ȗ3W2 țȖ3W Ȟ3W ı3W ȟ3W ?? ? Meso-proterozoic era !!! ++++ ++ ??? ??ȕ- ? ++++ !!! ??? ++++Ȗ3W1 țȖ3W++1 Ȟ3W1 Paleo-proterozoic era !! ++++ ++ /// / ?? \\ ++++ \\\\\Ȝ- ȜĮ- ??Įȕ- ȣ- Archean-Proterozoic eon Ȗ$U3W1 /// / \\ /// / \\\\\ \\ Trassic period ///Ȝ37 / ȜĮ37 \\ ?? ? ??? ??ȕ0] ? Normal fault Inferred normal fault Reversed fault ?? ? ??? \\ ??ȕ3] ? Later Mesozoic era ȜĮ&3Ú ??? Inferred fault fault (not distinguish) Inferred fault (not distinguish) \\ \\ ȜĮ26\\ ȣ3] Glacial drift or \\ Early Mesozoic era ȜĮ2\\ Įę Lake City aqueoglacial deposit ?? //// \\ \\ //// ??Įȕ3W1 ȣ3W 1 Paleo-proterozoic era \\\\\Ȝ3W1

Fig.2 Structure-Magmatic rocks map of the northern Chile.

724 Zhu et al. coastal belt and were mainly composed of igneous rocks Table 1 Magnetic susceptibility statistics of the rocks and ores and sedimentary sequences rocks, which were including bodies in the northern Chile (Wang et al., 2012; Lu et al., 2013; Li neutral, intermediate-acidic and acidic igneous rocks, et al., 2015). volcaniclastic rocks, terrestrial-neritic igneous rocks, Magnetic susceptibility pyroclastic rocks and marine limestones (Figure 2). The Serial Names of rock and Samples /4π×10-3 SI number ore body number Average strata were also closely related with both the iron-oxide- Value scope copper-gold (IOCG) deposits and the epithermal gold- value silver and polymetallic deposits (Li et al., 2011b). 1 magnetite 30 77.1–718 215.1 The Paleogene and Neogene strata in the eastern Chile Silicifi ed andesitic breccias with 2 18 28.6–117 63.2 were primarily composed of continental igneous rocks magnetite and volcanic clastic rocks. These strata were closely mineralization related with the porphyry copper deposits and epithermal Andesite with gold-silver and polymetallic deposits. The igneous rocks 3 magnetite 28 24.8–124 55.3 of these strata were predominantly including acidic mineralization granites, granodiorites, monzodiorites, monzogranites 4 Magnetic hematite 22 37.8–88 51.2 etc. (Figure 2). 5 Diorite 162 14.7–68.5 39.1 The Quaternary strata were distributed in the river 6 Andesite 34 9.47–25.5 14.81 valleys, lakes, deserts, and piedmont areas in Chile, Andesitic breccias which were mainly composed of the sediments of recent 7 containing iron and 15 0.93–27.2 12.42 copper volcanic evaporations. The Quaternary evaporites rocks Silicifi ed andesitic and sediments of salt lakes were the mineral sources of 8 breccia with 114 0.66–27 8.6 potassium-lithium-saltpeter ores deposits (Figure 2). hematitization 9 Hematite 65 0.141–21.7 6.95 Rock magnetic characteristics in the north of 10 Andesitic breccia 37 1.06–16.8 6.88 Chile 11 Tectonic breccia 33 0.13–8.27 3.7 Silicifi ed tectonic 12 51 0.002–9.71 3.15 The rocks and ores bodies in the northern Chile were breccias including diorites, andesites, altered andesite, silicified 13 Altered andesite 31 0.221–6.84 2.62 andesites, silicified breccias, specularite, magnetites, 14 Specularite ore body 60 0.15–2.30 0.78 magnetites containing copper, silicifi ed andesitic breccias 15 Skarn 50 0.20–1.6 0.70 with magnetite mineralization, magnetite mineralized 16 Pyrite ore body 60 0.59–0.90 0.67 andesites, magnetic hematites, pyrites, andesitic breccias 17 Aeolian sand 4 0.154–0.963 0.241 containing iron and copper, hematite, hematization 18 Silicifi ed vein 120 0.01–0.42 0.18 silicifi ed andesitic breccias, andesitic breccias, tectonic breccias, silicifi ed tectonic breccias, skarns, eolian sand, be regarded as the expanding features mark of the rock silicified veins etc. All those rocks mentioned above mass in the area (Lu et al., 2013). Andesites, andesitic contained certain amounts of magnetite particles and breccias containing iron and copper, silicifi ed andesitic pyrites. The differences of the magnetic susceptibilities breccias with pyrite mineralization had intermediate of these rocks in this area were obvious (Table 1). values of magnetic susceptibilities and the magnetic It was shown in Table 1 that the magnetites had the anomalies caused by them were regarded as the biggest values of magnetic susceptibilities and those of background values in this area. Due to magnetites and the magnetites containing copper had the second big magnetites containing copper being closely related with values (Hinz and Frese, 1990). The silicified andesitic each other in space, the magnetic anomalies caused by breccias with magnetite mineralization, Andesite with the andesitic breccias containing iron and copper and the magnetite mineralization and magnetic hematite had pyritization silicifi ed andesites were usually superposed big values of magnetic susceptibilities (Lu et al., 2013). with strong magnetic anomalies. The hematitization These rocks and ores bodies had strong magnetic silicified andesitic breccias, hematites and andesitic susceptibilities in this area, which were the dominant breccias had intermediate to small values of magnetic sources of strong magnetic anomalies. The magnetic susceptibilities and the magnetic anomalies caused by susceptibility of diorite was strong and diorites could them were diffi cult to be recognized and separated from cause low magnetic anomalies amplitudes, which could each other (Liu, 2007). According to the relationships 725 Regional metallogenic structure in northern Chile between magnetic susceptibilities and the types of data griding method, reduction to pole, upward field rocks and ores bodies, the big values of magnetic continuation, second derivatives calculation in vertical susceptibilities were corresponding to magnetites and direction, inclination angles calculation and analytical magnetites containing copper, and the small values were signal amplitude analysis during the processing of corresponding to hematites and breccias (Wang et al., aeromagnetic data (Liu et al., 2006; Ranganai et al., 2012; Li et al., 2015). 2015; Al Kadasi, 2015). The magnetic susceptibilities of rocks and ores The principal processing procedures were shown in bodies in the north of Chile were obvious different detail as follows. and they showed the obvious congruent relationships (1) Grid processing. We generated the gridded between the element zoning sequences and the magnetic aeromagnetic data using the Kriging method based on characteristics of ores, that is, the big values of magnetic the aeromagnetic data in the scale of 1:100,000 and the susceptibilities were corresponding to the rocks of size of unit was 100 m×100 m. The rest of processing magnetite (or hematite) types, the small values were procedures were based on the gridded magnetic ΔT corresponding to either the hematite (or specularite) anomaly. types or the broken alterated rock types (Wang et al., (2) Reduction to pole. The study area was located 2012). in the low latitude zone of the southern hemisphere and it was very important to perform reduction to pole procedure on the data of magnetic ΔT anomaly. Processing and interpretation of The basic parameters of magnetic field were chosen aeromagnetic data according to the international reference geomagnetic fi eld (Thébault et al., 2010). The declination (D) and the inclination (I) of the normal magnetic fi eld in the study Processing of aeromagnetic data area were −1.2º and −25.8º, respectively. (3) Upward field continuation. Field continuation The aeromagnetic data (Figure 3) used in the procedure of aeromagnetic data changed the data to the study were from the Servicio Nacional de Geología y situation, which had been collected at another height Minería (in Spanish) in Chile and the data acquisition rather than the exact height (Al Kadasi, 2015). The work was done in the scale of 1:100,000 in 2012. The magnetic anomalies caused by the magnetic bodies in line distance of data acquisition was 500 m and the shallow and the high frequency magnetic interferences fl ying height above the surface was 150 m. A series of near surface could be depressed through the upward processing procedures were adopted in order to suppress field continuation procedure. The shapes of magnetic noises, extract and strengthen useful information, anomalies would gradually become monotonous with the and enhance the geological intepretation ability of upward fi eld continuation procedure and the anomalies the mangetic anomalies. In this study, we used the with low frequency bandwidth caused by the large magnetic bodies could be enhanced and highlighted. 71.5° W70.5° W 69.5° W 68.5° W 26° S The upward field continuation procedure was good N Chanaral for understanding the magnetic characteristics of deep 26.5° S geological bodies, and it was also help for deducing the C19 C20 C21 Atacama region depths and spatial distributions of geological bodies. Caldera 27° S It was signifi cant on deducing the basement structures, C25 C26 C27 inferring the concealed rock bodies and the distributions 27.5° S of sedimentary layers. In this study, we conducted the Pacific ocean Pacific C31 C32 C33 Chile Argentina upward fi eld continuation with four different heights (1 28° S km, 2.5 km, 5 km, and 10 km) based on the magnetic ΔT 100 km 0 anomaly with reduction to pole. Legend (4) Derivative calculation. Derivative calculation Study area Coastline Country border based on the magnetic ΔT anomaly with reduction to pole could eliminate the background anomalies values Fig.3 Cartoon showed the distributions of the aeromagnetic of the normal fi elds, depress the effects of the regional maps in the study area. There were eight aeromagnetic fi eld, separate the superposed anomalies and distinguish maps in the scale of 1:100,000 and they were C19, C20, C21, C25, C26, C27, C31_32, and C33, respectively. the anomalies caused by adjacent magnetic bodies. The

726 Zhu et al. procedure could decrease the background magnetic with reduction to pole aimed to deduce the boundaries interferences superposed in local magnetic fields and and center of magnetic bodies. The positions of peak those magnetic interferences from surrounding rocks signal amplitude values could be regarded as the with weak magnetic susceptibilities (Wang et al., 2009, boundaries of magnetic bodies in the situation that 2010, 2014b; Al Kadasi, 2015; Eppelbaum, 2015). The the recording plane was close to the magnetic bodies; zero-line of the second derivative in vertical direction otherwise, if the recording plane was far from the based on the magnetic ΔT anomaly with reduction magnetic bodies, these positions would be considered to pole could be used to trace the covered magnetic the center of magnetic anomaly bodies (Huang and anomalies bodies and deduce the scopes and locations of Guan, 1998; Wang, 2012; Wang et al., 2014a). In this them (Zhang et al., 2014). study we conducted the processing procedure based on (5) Inclination angle calculation. We could strengthen the aeromagnetic ΔT anomaly with reduction to pole fracture structure information, weaken the non-structural and the upward fi eld continuations of the aeromagnetic information. and identify the positions of fractures or ΔT anomaly with different heights (2.5km, 5.0km and the boundaries of magnetic bodies by calculating the 10km), which had been reduced to pole. inclination angle of the magnetic ΔT anomaly with reduction to pole (Miller and Singh, 1994). Combining Structural interpretation of aeromagnetic data the inclination angle calculation with the upward The comprehensive structural interpretation was field continuation procedure during the magnetic data based on the magnetic anomaly characteristics of the processing could overcome the infl uences of the shallow aeromagnetic data with reduction to pole integrated with and hidden sources of deep interference and obtain even, the regional geological information in the northern Chile. accurate fracture information (Wang et al., 2014a; Wei Through the analysis of the aeromagnetic anomalies et al., 2016). In this paper, we calculated the inclination properties (Figure 4), the strong magnetic anomalies angles based on the magnetic ΔT anomaly with reduction were mainly distributed in the center of study area and to pole and the upward fi eld continuation with different but present in the whole area along the NS direction. heights (2.5 km, 5 km, and 10 km) of the magnetic ΔT The magnetic anomalies were spread along the NNE or anomaly with reduction to pole. NS directions and distributed in the shapes of irregular (6) Analytic signal amplitude calculation. Analytic strips that were massive or bead-like. The low magnetic signal amplitude analysis of the magnetic ΔT anomaly

(a) (b) 71.4° W 71° W 70.6° W 70.2° W 69.8° W 69.4° W 71.4° W 71° W 70.6° W 70.2° W 69.8° W 69.4° W 26.4° S 26.4° S

26.6° S 26.6° S

26.8° S 26.8° S

27° S 27° S

27.2° S nT 27.2° S nT 1873 5451 1500 4698 27.4° S 1127 27.4° S 3944 754 3190 2436 27.6° S 382 27.6° S 9 1683 -364 929 27.8° S -737 27.8° S 175 -1109 -579 -1482 -1332 28° S 28° S

Fig.4 Aeromagnetic ΔT anomaly map (a) and aeromagnetic ΔT anomaly map with reduction to pole (b) in the study area. anomalies were observed to expand along the coastal Figure 5 showed the second derivative in the vertical belt in the west of study area and in the eastern basin, direction (Figure 5a) and the analytic signal amplitude and were predominantly in massive strips. analysis (Figure 5b) based on the aeromagnetic ΔT 727 Regional metallogenic structure in northern Chile anomaly with reduction to pole. The positive and according to the analytic signal amplitude analysis negative values of the second derivative in the vertical based on the aeromagnetic ΔT anomaly with reduction direction all showed rough variations, the anomaly to pole. These positive anomalies were big and changed gradients varied sharply and the absolute values of them sharply. The peak values were sharp and the gradients of were big and varied quickly (Figure 5). The number anomalies varied quickly. These positive anomalies were of local anomalies was large and most of them were shaped either as strips or were lead-like. predominantly increased positive values accompanied Figure 6 shows the second derivative calculation in the by decreased negative anomalies. The anomaly values vertical direction based on the upward ﬁ eld continuation on the two sides of the map were small and most of the with different heights (1 km, 2.5 km, 5 km, and 10 km) anomalies were either massive or shaped like strips. of aeromagnetic ΔT anomaly, which had been reduced It was shown in Figure 5b that the dominant positive to pole. Figures 7 and 8 show the inclination angle anomaly belts was distributed along the NNE direction calculations (Figure 7) and analytic signal amplitude

(a) (b) 71.4° W 71° W 70.6° W 70.2° W 69.8° W 69.4° W 71.4° W 71° W 70.6° W 70.2° W 69.8° W 69.4° W 26.4° S 26.4° S F1 F2 F4 F5 F6 F1 F2 F4 F5 F6 C1 C2 C1 C2 26.6° S 26.6° S C3 C3 C7 C7 C4 C6 C4 C6 26.8° S C5 26.8° S C5 C8 C9 C8 C9 F3 F7 F3 F7 27° S C10 27° S C10 C11 C11 2VD AS 27.2° S C13 27.2° S C14 C12 0.086 C12 C13 12.5 0.071 11.1 C15 0.057 C16 27.4° S C14 27.4° S C14 9.7 0.042 C15 8.3 0.028 27.6° S 27.6° S 6.9 0.013 5.6 C16 -0.001 C17 4.2 27.8° S -0.016 27.8° S 2.8 -0.030 1.4 -0.045 28° S C17 28° S C18 0.0

Fig.5 The second derivative in vertical direction map based on the aeromagnetic ΔT anomaly reduction to pole (a), and the analytic signal amplitude analysis and structural interpretation map based on the aeromagnetic ΔT anomaly reduction to pole (b) in the study area. In these maps, the black lines showed the locations of interpreted faults and some faults were expressed in F1, F2, …, F6, and F7. The red dashed lines showed the locations of volcanic mechanisms and they were expressed in C1, C2, …, C17, and C18.

(a) (b) 71.4° W 71° W 70.6° W 70.2° W 69.8° W 69.4° W 71.4° W 71° W 70.6° W 70.2° W 69.8° W 69.4° W 26.4° S 26.4° S

26.6° S F19 26.6° S

26.8° S 26.8° S

27° S 27° S

F3 1000_2VD 5000_2VD 27.2° S 0.00177 27.2° S 0.000038 0.00149 0.000031 27.4° S 0.00122 27.4° S 0.000024 0.00095 0.000017 0.00067 0.000011 27.6° S 0.00040 27.6° S 0.000004 -0.00013 -0.000003 27.8° S -0.00015 27.8° S -0.000010 -0.00042 -0.000016 F18 F24 F17 -0.00069 -0.000023 28° S 28° S F17

728 Zhu et al.

26.6° S 26.6° S

26.8° S 26.8° S F9

27° S 27° S

2600_2VD F3 10000_2VD 27.2° S 0.000205 27.2° S 0.0000073 0.000170 0.0000060 27.4° S 0.000135 27.4° S 0.0000047 0.000101 0.0000034 0.000066 0.0000021 27.6° S 0.000031 27.6° S 0.0000008 -0.000004 -0.0000005 27.8° S -0.000039 27.8° S -0.0000018 -0.000073 F16 -0.0000031 -0.000108 F18 -0.0000044 28° S 28° S F17

Fig.6 The second derivative in the vertical direction maps based on the upward field continuation of the aeromagnetic ΔT anomaly with different heights in the study area, which had been reduced to pole. (a) The height of upward fi eld continuation of 1 km; (b) the height of upward fi eld continuation of 2.5 km; (c) the height of upward fi eld continuation of 5 km; (d) the height of upward fi eld continuation of 10 km. In these maps, the black lines show the locations of interpreted faults and some faults were expressed in F1, F2, …, F6, and F7. The red dashed lines show the locations of volcanic mechanisms and they were expressed in C1, C2, …, C17, and C18. analysis results (Figure 8) based on the aeromagnetic mafic igneous rocks from the Atacama and Domeyko ΔT anomaly with reduction to pole and the upward fi eld faults systems, which cut through the mantle and continuations of the aeromagnetic ΔT anomaly with were parallel to the island-arc and orogenic belts. The different heights (2.5 km, 5.0 km, and 10 km) that had anomalies revealed the deep extension of these faults been reduced to pole, respectively. Figures 6, 7, and 8 over long distances along the NNE direction, which were show that the sources of the positive anomalies were obviously shown in Figures 7b, 7c, and 7d. deep magnetic bodies or magnetic bodies with strong The Atacama fault belt represents the Mesozoic magnetic susceptibilities, which indicated that anomaly regional fault system. The system is a ductile-brittle belts were generated by the deep-extending geological sinistral strike-slip fault system composed by main faults bodies. The anomalies might be caused by the intruded along the nearly NS direction and sub-faults along the

(a) (b) 71.4° W 71° W 70.6° W 70.2° W 69.8° W 69.4° W 71.4° W 71° W 70.6° W 70.2° W 69.8° W 69.4° W 26.4° S 26.4° S F1 F2 F4 F5 F6

C1 C2 C1 26.6° S 26.6° S C2 C3 C3 C7 C7 C4 C6 C4 C6 26.8° S C5 26.8° S C5 C8 C9 C8 C9 F3 F7 27° S C10 27° S C10 C11 C11 V_edge 27.2° S 27.2° S 5000_VE C13 C14 34096 C13 C14 C12 C12 594 27664 469 21232 27.4° S C16 27.4° S C16 343 C14 C14 C15 14800 C15 218 8367 93 27.6° S 1935 27.6° S -32 C17 -4497 C17 -157 27.8° S -10929 27.8° S -282 -17362 -408 -23794 -533 28° S C18 28° S

729 Regional metallogenic structure in northern Chile

C1 C2 C1 C2 26.6° S 26.6° S C3 C3 C7 C7 C4 C6 C4 C6 26.8° S C5 26.8° S C5 C8 C9 C8 C9

27° S C10 27° S C10 C11 C11 2500_VE 10000_VE 27.2° S C14 27.2° S C14 C12 C13 1737 C12 C13 134.7 1390 96.3 C16 C16 27.4° S C14 1043 27.4° S C14 58.2 C15 697 C15 20.1 350 -18.0 27.6° S 3 27.6° S -56.1 C17 -344 C17 -94.2 27.8° S -691 27.8° S -132.3 -1038 -170.3 C18 -1385 -208.4 28° S 28° S C18

Fig.7 Inclination angle calculation and structural interpretation maps based on the upward field continuations of the aeromagnetic ΔT anomaly with different heights in the study area, which had been reduced to pole. (a) Without field continuation; (b) the height of upward fi eld continuation of 2.5 km; (c) the height of upward fi eld continuation of 5 km; (d) the height of upward fi eld continuation of 10 km. In these maps, the black lines show the locations of interpreted faults and some faults were expressed in F1, F2, …, F6, and F7. The red dashed lines show the locations of volcanic mechanisms and they were expressed in C1, C2, …, C17, and C18.

NW and NE directions. These faults developed because In the east side of maps (Figures 4, 5, 6, 7, and 8), of the extension and compression of the local structures there were some low positive anomalies in massive during the subduction of the Aulk plate. The Atacama shapes. The variations among these anomalies were fault belt is formed based on the magmatites intruding low, their peak values were small, the widths of these from deeper layers along the subduction belts in the peak values were broad, and the anomaly gradients late Triassic–early Cretaceous periods, and it controls were smooth. The local anomalies were not obvious and the Mesozoic tectonic–magmatic system (Brown et al., were infrequent; they were in the EW and NE directions 1993). The Domeyko fault belt is composed of a series and were either massive or in strips. These positive of faults in the NS direction and the major faults are anomalies with low and smooth peak values might have deep. The rotation directions of the fault system changed some relationships with the altered and metamorphic from right-hand to left-hand in the Eocene–Oligocene monzodiorites and granodiorites, which had weak to epochs, and then the sub-faults in different directions intermediate magnetic susceptibilities. They might led to the formation of special fault–fracture belts. The also be those volcanic clastic sedimentary series with intersection parts of these faults in different directions low magnetic properties superposed by the regional has been active since the Eocene epoch. These spaces hydrothermal alterations. are usually intruded by the Paleogene–Neogene acidic The negative anomalies (Figures 4, 5, 6, 7, and 8) magnetites and these intrusion magmas superposed on were distributed in strip shape and along the NW and NE hydrothermal fluid materials. The fault belt is closely directions. These negative anomalies in the center were related with the huge porphyry copper deposits belt large and showed rapid variations. The absolute values of in Chile. There were also some regional deep faults the anomaly gradients were also large. The local negative along the EW direction (Figure 8), and these faults anomalies were easily to be recognized and their number might be formed during the eastward subduction of the was large. The negative anomalies on the two sides were Paciﬁ c Nazca plate, which caused the formations of the distributed broadly in massive shapes. Their anomaly longitudinal extension fault zones. These fault zones values were small and showed gradual changes, and belonged to the transform faults, which were caused by their peak values varied smoothly. That the negative the horizontal components in the EW direction of the anomalies along the nearly EW direction cut through regional stresses and companied with the intrusion of the positive anomaly belts (Figure 8), which might maﬁ c magmatites. be caused by the sedimentary and tectonic sequences 730 Zhu et al. with weak magnetic susceptibilities distributed along caused by rocks, which were predominantly sandstones, the regional deep fault belt in the EW direction. The red mudstones, volcanic-sedimentary rocks, and negative anomalies in the NE direction were primarily sedimentary sequences composed of red conglomerates

(a) (b) 71.4° W 71° W 70.6° W 70.2° W 69.8° W 69.4° W 71.4° W 71° W 70.6° W 70.2° W 69.8° W 69.4° W 26.4° S 26.4° S F1 F2 F4 F5 F6 C1 C1 C2 26.6° S C2 26.6° S C3 C3 C7 C7 C4 C6 C4 C6 26.8° S C5 26.8° S C5 C9 C8 C9 C8 F3 F7 27° S C10 27° S C10 C11 C11 H_edge 5000_HE C13 C13 27.2° S C14 27.2° S C14 C12 19079 C12 321.8 14974 261.7 27.4° S C16 10870 27.4° S C16 201.5 C14 C14 C15 6766 C15 141.4 27.6° S 2661 27.6° S 81.2 -1443 21.1 C17 -5547 C17 -39.1 27.8° S -9652 27.8° S -99.2 -13756 -159.4 -17860 -219.5 28° S C18 28° S C18

(c) (d) 71.4° W 71° W 70.6° W 70.2° W 69.8° W 69.4° W 71.4° W 71° W 70.6° W 70.2° W 69.8° W 69.4° W 26.4° S 26.4° S F1 F2 F4 F5 F6 C1 C1 C2 26.6° S C2 26.6° S C3 C3 C7 C7 C4 C6 C4 C6 26.8° S C5 26.8° S C5 C8 C9 C8 C9 F3 F7 F3 F7 27° S C10 27° S C10 C11 C11 2500_HE 10000_HE 27.2° S C14 27.2° S C14 106.2 C12 C13 708 C12 C13 565 86.1 27.4° S C16 27.4° S C16 65.9 C14 422 C14 C15 279 C15 45.7 25.6 27.6° S 136 27.6° S -7 5.4 C17 -150 C17 -14.8 27.8° S -293 27.8° S -34.9 -436 -55.1 -75.3 28° S C18 -579 28° S C18

Fig.8 Analytic signal amplitude analysis and structural interpretation maps based on the upward field continuations of the aeromagnetic ΔT anomaly with different heights in the study area, which had been reduced to pole. (a) without fi eld continuation; (b) the height of upward field continuation of 2.5 km; (c) the height of upward field continuation of 5 km; (d) the height of upward fi eld continuation of 10 km. In these maps, the black lines show the locations of interpreted faults and some faults were expressed in F1, F2, …, F6, and F7. The red dashed lines show the locations of volcanic mechanisms and they were expressed in C1, C2, …, C17, and C18. and clastic rocks. These sedimentary rocks with weak relative short times. The late tectonic movements magnetic susceptibilities caused the negative magnetic and weathering erosions had less effect on them; anomalies in this region. therefore, most of the volcanic mechanisms in the Most of the volcanic mechanisms in the Andes were study area basically retained their original shapes formed after the subduction of the Pacifi c plate toward and characteristics, which were easily identified in the South American plate, and they were formed in aeromagnetic anomaly maps. The volcanic mechanisms

731 Regional metallogenic structure in northern Chile in the study area usually appeared as the rings of the intrusive rocks, and these anomalies in the directions second derivative in vertical direction, which were of NNE, NE and nearly EW were predominantly combined by strong positive magnetic anomalies caused by the deep or shallow magnetic features of alternating with negative ones (Figure 5a). The peak the strike-slip Acatama and Domeyko fault systems analytic signal amplitudes in the center were large along the nearly NS direction, where there were many and most of them were distributed in the rings with regional faults and tectonic structures along the NS and multiple centers or in shapes close to equal axials (Figure NNE directions (Table 2). It revealed the existence of 5b). It was shown in Fig. 7 that most of the volcanic regional deep faults in this region and the deep positions mechanisms were bead-like and distributed along the and shape characteristics of the strata composed by regional deep faults in the NS direction; some of them the abovementioned rocks. The smooth negative were distributed at the intersection areas of the deep magnetic anomalies with low absolute values were faults in the NS and EW directions, especially in the related with the sedimentary rocks, the metamorphic east side of the study area (Figure 8). Figure 9 shows the sedimentary rocks and the tectonic rocks, which had distribution map of near surface structures, which were weak magnetic susceptibilities. The negative anomalies summarized from the interpreted faults and volcanic in the NW direction revealed the distribution directions mechanisms mentioned above. Eighteen small volcanic and possible scopes of the deep basement faults and mechanisms (C1, C2, …, C17, and C18) were identiﬁ ed tectonic structures. The negative magnetic anomalies and most of them were distributed in the east of the study area. We had also interpreted 13 regional deep Table 2 Attributes of regional deep faults deduced from the faults (F1, F2, …, F12, and F13), and the strikes and aeromagnetic data. some aeromagnetic anomalies properties of these faults Series Fault Characteristics of are shown in Table 2. Trend of fault number name aeromagnetic ﬁ eld Gradient variation 71° W 70.6° W 70.2° W 69.8° W 69.4° W 1F1SN belt, discontinuity and

26.4° S dislocation belt N F1 F2 F4 F5 F6 F8 F9 Gradient variation C1 C2 26.6° S 2 F2 SN-NE belt, discontinuity and C3

F11 C7 F10 dislocation belt C4 C6 26.8° S C5 Gradient variation C8 C9 F3 F7 3 F3 SN-NE belt, discontinuity and 27° S C10 F12 dislocation belt C11 Bead-like anomaly 27.2° S C13 C12 C14 4 F4 SN-NE belt, discontinuity and dislocation belt 27.4° S C16 C15 Bead-like anomaly

27.6° S F13 Legend 5 F5 SN-NE belt, discontinuity and C17 Big deep fault dislocation belt Intermediate deep 27.8° S and small fault Bead-like anomaly Volcanic mechanism 6 F6 NE-SN belt, discontinuity and 28° S C18 0 50 km F1,F2, ..., F13 Faults name dislocation belt. C1,C2, ...,C18 Volcanic mechanisms name Bead-like anomaly Fig.9 Cartoon shows the interpreted faults and volcanic 7 F7 NE-SN belt, discontinuity and mechanisms based on the aeromagnetic data in the north of dislocation belt Chile. The faults were expressed in F1, F2, …, F6, and F13. The Bead-like anomaly volcanic mechanisms were shown in C1, C2, …, C16, and C18. 8 F8 SN-NE belt, discontinuity and dislocation belt 9 F9 SN Bead-like anomaly belt Because of multiple tectonic activities, the igneous 10 F10 SN Bead-like anomaly belt discontinuity and rocks accounted for approximately 70% of all kinds 11 F11 EW of rocks in Chile. The magnetic activities in different dislocation belt discontinuity and periods from west to east formed obvious migrating 12 F12 EW-NW dislocation belt eastward magnetic activities and lithologic zonings in discontinuity and 13 F13 EW Chile. The positive magnetic anomalies were related to dislocation belt the basic-intermediate basic or acidic igneous rocks and 732 Zhu et al. in the NE direction indicated the distribution scopes slip Atacama and Domeyko fault systems in the nearly of the sedimentary sequences with weak magnetic NS direction. The negative magnetic anomalies were susceptibilities. related to the sedimentary rocks, the metamorphic There were some positive magnetic anomalies in sedimentary rocks, and the tectonic rocks, which had this region that were massive of shaped like strips; weak magnetic susceptibilities. The negative anomalies however, most of them were stable decreasing negative in the NE direction revealed the distribution scopes magnetic anomalies caused by the intrusive rocks of the sedimentary sequences with weak magnetic with weak magnetic susceptibilities (the west side of susceptibilities. Figures 4 and 5). The Jurassic and Lower Cretaceous (2) We deduced the regional deep faults along the magmatic arcs were developed in the center of NNE and NS directions, and we revealed the planar Cordillera coastal belt and the two boundaries of the expanding properties of the basic to intermediate basic Chilean central basin, and there were several types of or acidic igneous rocks and intrusive rocks layers, which magmatic rocks. The mafic igneous rocks had large were distributed along these faults. The near parallel values of magnetic susceptibilities and their remanences faults structures from west to east presented the obvious were still strong, which caused the jumps and rough banding properties of tectono-magmatite. variations of magnetic anomalies and formed strong (3) We ascertained the locations of volcanic local isolated or strip anomalies in this area. The neutral mechanisms and studied the relationships between these igneous rocks represented by andesites had intermediate volcanic mechanisms and the regional deep faults. magnetic susceptibilities and could cause magnetic ﬁ eld (4) We showed the linear positive magnetic anomalies ﬂ uctuations of a certain intensity, which could generate and magnetic anomaly gradient zones based on slowly jump-like magnetic anomalies in the study area (Figure varying background negative magnetic anomaly field, 4). In the major Cordillera orogenic belt there were which indicated there were massive mafic and felsic the Paleogene–Neogene acidic granites, granodiorites, magmatic activities in these regional deep faults. It also monzodiorites, and monzogranites, which constituted revealed the cross areas of main faults and secondary the western Cordillera metallogenic belt (or the volcanic faults, and the locations of secondary faults were areas arc-porphyry belt in the Andes). Most of these acidic favorable for copper and polymetallic mineralization. igneous rocks had weak magnetic susceptibilities and This study provided some basic information on the formed regional decreasing negative magnetic anomalies geology, structural characteristics, and prospect for (the east side of Figures 4b and 5b). delineating ores in the north of Chile.

Conclusions Acknowledgments

Most of mineral resources in the Andean metallogenic The authors thank the reviewers for their constructive belt were related with the parallel-arc strike-slip faults comments and suggestions! The authors thank all the and the northwestern basement structures. The Atacama people who joined in the acquisition of the aeromagnetic and Domeyko fault systems in the northern Chile data in the north of Chile! controlled the tectonic–magmatic activities, which seen to migrate eastward and the types of mineral resources. In this paper, we achieved several conclusions References on the basis of the processing and interpretation of the aeromagnetic data integrated with the regional geological, structural, and mineral information of the Al Kadasi, A. N., 2015, Interpretation of aeromagnetic data study area in the northern Chile. in terms of surface and subsurface geologic structures, (1) The positive magnetic anomalies were related southwestern Yemen: Arab J. Geosci., 8, 1163–1179. to the strata composed of the basic to intermediate Armijo, R., and Thiele, R., 1990, Active faulting in northern basic or acidic igneous rocks and intrusive rocks in Chile: ramp stacking and lateral decoupling along a the study area. The positive anomalies in the NNE, subduction plate boundary?: Earth and Planetary Science NS, and nearly ES directions were predominantly the Letters, 98, 40−61. shallow and deep magnetic responses of the strike- Brown, M., Diáz, F., and Grocott, J., 1993, Displacement

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