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Regional Metallogenic Structure Based on Aeromagnetic Data in Northern Chile*

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Regional Metallogenic Structure Based on Aeromagnetic Data in Northern Chile* 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
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