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Application of Airborne Electromagnetics and Magnetics for Mineral Exploration in the Baishiquan–Hongliujing Area, Northwest China

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Application of Airborne Electromagnetics and Magnetics for Mineral Exploration in the Baishiquan–Hongliujing Area, Northwest China remote sensing Article Application of Airborne Electromagnetics and Magnetics for Mineral Exploration in the Baishiquan–Hongliujing Area, Northwest China Shengjun Liang 1, Siyuan Sun 1,2,* and Hongfei Lu 3 1 China Aero Geophysical Survey & Remote Sensing Center for Natural Resources, Beijing 100083, China; [email protected] 2 College of Geo-Exploration Sciences and Technology, Jilin University, Changchun 130026, China 3 Geological Team Six of Xinjiang Geological and Mineral Bureau, Hami 839000, China; [email protected] * Correspondence: [email protected]; Tel.: +86-010-6206-0135 Abstract: Airborne electromagnetics is an effective and efficient exploration tool in shallow mineral exploration for its high efficiency and low cost. In 2016, airborne electromagnetic and airborne magnetic surveys have been carried out at the border of Xinjiang Uygur Autonomous Region and Gansu Province, the Northwest China. With an integrated system, the airborne electromagnetics and airborne magnetic data were collected simultaneously by AreoTEM-IV system from Aeroquest International Limited in Vancouver, BC, Canada, and the CS3 Cesium Vapor magnetometer from Scintrex in Concord, ON, Canada. About 3149 line-km of both data with 250 m line space were acquired. After data processing, the comprehensive analysis and interpretation of resistivity and magnetic anomalies has been carried out to infer lithological structure and outline the potential ore Citation: Liang, S.; Sun, S.; Lu, H. deposits. Verified by the ground surveys, seven outlined anomalies are consistent with the known Application of Airborne ore sites, and one new gold deposit and several mineralization clues were found. The prospective Electromagnetics and Magnetics for reserves of gold are expected to exceed 10 tons. Besides, some prospecting target areas were outlined Mineral Exploration in the as the possible locations of copper–nickel deposits. The successful case shows the airborne magnetic Baishiquan–Hongliujing Area, data accords with geological structures, and the airborne electromagnetic method is effective in Northwest China. Remote Sens. 2021, finding metal mineral resources, which can help to quickly identify potential ore targets with no 13, 903. https://doi.org/10.3390/ surface outcrop. rs13050903 Keywords: airborne electromagnetics; airborne magnetics; mineral exploration; geophysical survey; Academic Editor: Bernhard Siemon gold ore; deposit Received: 22 January 2021 Accepted: 23 February 2021 Published: 27 February 2021 1. Introduction Publisher’s Note: MDPI stays neutral Geophysical exploration has played a vital role in mineral exploration, engineering with regard to jurisdictional claims in geological survey, environment investigation, tectonic geology, and archaeological research. published maps and institutional affil- In China, with the vast lands and the rapid development of economy, ground-based iations. geophysical exploration is getting more and more difficult to meet the requirements of energy and mineral explorations for its low efficiency and limited access caused by terrible terrain and traffic [1,2]. Airborne geophysical exploration has become an important tool to quickly investigate large-scale subsurface with lower time and economic costs in remote, Copyright: © 2021 by the authors. uninhabitable areas [3]. Licensee MDPI, Basel, Switzerland. Airborne electromagnetics (AEM) is one of the most widely used airborne geophysical This article is an open access article explorations, especially in groundwater and geotechnical studies [4]. With instruments on distributed under the terms and an aircraft, it transmits electromagnetic waves and receives the electromagnetic signals from conditions of the Creative Commons the earth by coils to detect the subsurface target body. There are two types of AEM methods: Attribution (CC BY) license (https:// frequency-domain method (AFEM) and time-domain method (ATEM). The AFEM method creativecommons.org/licenses/by/ provides high resolution at shallower depths, while the ATEM method is more suitable 4.0/). Remote Sens. 2021, 13, 903. https://doi.org/10.3390/rs13050903 https://www.mdpi.com/journal/remotesensing Remote Sens. 2021, 13, 903 2 of 15 for large coverage areas and deeper exploration depths [5]. Major AEM systems in the world basically come from a few companies [2,4]. These companies developed many AEM systems and successfully applied in in mineral exploration [6,7], groundwater study [8,9], engineering geology [10], and environment investigations [11] of shallow subsurface. The most commonly used AFEM systems include IMPULE from Geotech, and DIGHEM and RESOLVE from CGG. Major ATEM systems include TEMPEST from CGG, SkyTEM series from SkyTEM Surveys, and AeroTEM and VTEM series from Geotech. These systems were designed and developed with different features for focusing on different concerns. The processing and interpretation of AEM data is more complex than ground EM data because of the large data volume, variation of flight height, and attitude changes of aircraft. As the 1D inversion and some processing technologies have matured, some commercial software were developed and applied in AEM surveys, such as Geosoft Oasis Montaj [12], Aarhus Workbench [13], and Maxwell from EMIT [14] and so on. With the development of computer and technology, 3D and Quasi-3D inversion are gradually applied to AEM explorations [15–17], and the machine learning has also shown a good effect on data processing [18–20]. Since the airborne electromagnetic technology was imported into China, it has brought great promotion for China’s geophysical exploration [2]. AEM’s early applications mainly focused on mineral exploration in Heilongjiang Province, Hubei Province, and Xinjiang Uygur Autonomous Region [21,22]. Now it gradually applied to environment, groundwa- ter and engineering investigations [23–25]. In recent decades, China constantly impel the research and development of AEM system. Some systems are under testing, and they are continuously improved before they are mature enough to be used in surveys. In this paper, a successful case is presented to illustrate the effectiveness of AEM on mineral exploration in China. In 2016, China Aero Geophysical Survey & Remote Sensing Center for Natural Resources (AGRS) led a comprehensive geophysical survey at Baishiquan–Hongliujing area which locates at the border of Xinjiang Uygur Autonomous Region and Gansu Province, the Northwest China. With an integrated system, about 3149 line-km of both AEM and aeromagnetic data with 250 m line space were recorded simultaneously. The AEM data were acquired by the AeroTEM-IV system, which is a time-domain system imported from Canada. The aeromagnetic data were acquired by an optically pumped cesium vapor magnetometer (Scintrex CS3). The AeroTEM-IV system is used to reveal electrical conductivities within the upper several hundred meters of the subsurface, and the airborne magnetic system helps to distinguish magnetic rocks. Simple techniques were used in AEM data processing, such as filtering, correction, stacking, and one-dimensional inversions. Magnetic data are processed through reduction to pole, derivative calculation and the continuation. On these bases, a comprehensive interpretation of resistivity and magnetic anomalies is carried out to outline the possible ore deposits. A total of 39 new anomalies in AEM data were outlined, and then ground investigations were implemented to verify these anomalies, including trenching and sample collection. Through the ground investigations, seven outlined anomalies are consistent with the known ore sites, and one new gold ore was determined. The prospective reserves of gold are expected to exceed 10 tons. Several mineralization clues were also found, and some prospecting target areas are outlined as the possible locations of copper–nickel deposits. The successful exploration shows airborne electromagnetic method is effective in finding metal mineral resources, and can provide support for solving the regional basic geological problems. 2. Survey Area The Baishiquan–Hongliujing area is located in Hami City, Xinjiang Uygur Autonomous Region, which is circled by red box in Figure1. The survey area covers an area of about 700 km2, and is underlain by Proterozoic metamorphic rocks [26]. Baishiquan is a magmatic Cu-Ni deposit in the southern Central Asian Orogenic Belt (CAOB), located in the northern margin of the Central Tianshan Terrane and to the south of the Shaquanzi Fault [27]. The Remote Sens. 2021, 13, x FOR PEER REVIEW 3 of 16 2. Survey Area The Baishiquan–Hongliujing area is located in Hami City, Xinjiang Uygur Autono- mous Region, which is circled by red box in Figure 1. The survey area covers an area of about 700 km2, and is underlain by Proterozoic metamorphic rocks [26]. Baishiquan is a magmatic Cu-Ni deposit in the southern Central Asian Orogenic Belt (CAOB), located in the northern margin of the Central Tianshan Terrane and to the south of the Shaquanzi Fault [27]. The CAOB is the largest Late Paleozoic juvenile orogenic belt in the world, and Late Paleozoic magmatism in Baishiquan resulted from paleo-Asian ocean closure, colli- sion of multiple continents and post-collisional extension [28]. As a result, intrusive rocks are developed well in this area, including gneissic granite, diorite, minor granite, and mafic–ultramafic rocks [29]. The main structure in the ore district is the Shaquanzi deep fracture and its subsidiary structures (Figure 2) [26]. The Baishiquan mafic–ultramafic intrusions
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