Geological interpretation of Bouguer gravity and aeromagnetic data from the Gobi-desert covered area, Eastern Tianshan, China: Implications for porphyry Cu-Mo polymetallic deposits exploration

Ore Geology Reviews 80 (2017) 1042–1055

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Geological interpretation of Bouguer gravity and aeromagnetic data from the Gobi-desert covered area, Eastern Tianshan, China: Implications for porphyry Cu-Mo polymetallic deposits exploration Fan Xiao ⁎, Zhenghai Wang School of Earth Sciences and Geological Engineering, Sun Yat-sen University, Guangzhou 510275, China Guangdong Key Laboratory of Geological Process and Mineral Resources Exploration, Sun Yat-sen University, Guangzhou 510275, China

a r t i c l e

i n f o

Article history: Received 3 July 2016 Received in revised form 26 August 2016 Accepted 29 August 2016 Available online 30 August 2016 Keywords: Reduced-to-pole Upward continuation transformation Correlation analysis Tuwu-Yandong porphyry ore belt Gobi-desert cover

a b s t r a c t In this study, Bouguer gravity and aeromagnetic data have been used to better understand the geology and mineral resources near the late Carboniferous-late Permian porphyry Cu-Mo polymetallic mineralization in the Chinese Eastern Tianshan belt, which is extensively covered by Gobi-desert. The reduced-to-pole (RTP) transformation of regional-scale aeromagnetic data shows that the porphyry Cu-Mo deposit is within a cluster of magnetic anomaly highs that overprint on a northeast trending magnetic gradient belt generally along the crustal-scale Kanguertag-Huangshan fault. The 10 km upward continuation transformation of both Bouguer gravity and aeromagnetic data indicates that the known porphyry Cu-Mo polymetallic deposits are located on the flanks of prominent gravity and magnetic anomaly highs. These anomalies are spatially correlated with the late Carboniferous-late Permian igneous rocks and in the Tuwu-Yandong mineralization district are centered over the granodiorite rocks genetically related to porphyry copper systems. In order to minimize interpretational ambiguities, a useful approach that is correlation analysis (CA) based on correlation coefficient (CC) given by gravity and magnetic data was employed to separate positively and negatively correlated anomalies features. The CA procedure is applied to 10 km upward continuation transformation of both Bouguer gravity and RTP transformed aeromagnetic data for mapping correlative magnetization and density contrast anomalies from deep sources, which may be associated with the porphyry Cu-Mo polymetallic mineralization. Five prominent CC positive anomalies have been found in the southern margin of Dananhu-Tousuquan arc. Those anomalies zones could be interpreted to reflect a late Carboniferous-late Permian magmatic belt that is favorable for additional discoveries of late Carboniferous to late Permian porphyry copper systems in north region of Eastern Tianshan. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The Chinese Eastern Tianshan is located at easternmost part of Tianshan Mountain Range in the southern Altaids (Chen et al., 2008; Xiao, 2004). It has been considered to be one of the most significant polymetallic mineralization belts in China since it produces a lot of metal minerals including Cu, Au, Ni, Fe, Sn, Cr, Ag, Pb, and Zn (Chen et al., 2008; Chen et al., 2007; Zhang and Liu, 2006). The late period of early Carboniferous (Chen et al., 2005; Han et al., 2006b; Liu et al., 2003; Rui et al., 2002b; Wang et al., 2014b; Zhang et al., 2006) Tuwu and Yandong superlarge porphyry Cu-Mo polymetallic deposits, with resources of 4.7 million mt copper at an average grade of 0.67% Cu and significant amounts of molybdenum, gold and silver (Liu et al., 2003; Liu et al., 2005; Zhang et al., 2006), were discovered in this region. Hence it is ⁎ Corresponding author at: School of Earth Sciences and Geological Engineering, Sun Yat-sen University, Guangzhou 510275, China. E-mail address: [email protected] (F. Xiao).

http://dx.doi.org/10.1016/j.oregeorev.2016.08.034 0169-1368/© 2016 Elsevier B.V. All rights reserved.

promising to prospect for porphyry Cu-Mo polymetallic deposits of similar age and tectonic environment in this remote district of northwestern China. The Tuwu and Yandong ore deposits were discovered by No.1 Geological Party of Xinjiang Bureau of Geology and Mineral Exploration in 1997, prospected during the period 1998–2002, and have been commercially exploited since 2003 (Chen et al., 2007; Liu et al., 2005). Drilling has shown that the deposits contain the largest copper resource of the known porphyry deposits in Chinese Tianshan metallogenic domain (Chen et al., 2008; Chen et al., 2007; Liu et al., 2005). Due to the extensive coverage of Gobi-desert (i.e. Tertiary and Quarternary deposits) (Chen et al., 2008; Xiao et al., 2014; Zhao et al., 2012), mineral exploration in Eastern Tianshan therefore relies mostly on geophysical prospecting techniques such as gravity, magnetic and electronic (e.g. Chen et al., 2008; Deng, 2002; Li and Eaton, 2005; Wang et al., 2014a; Wang et al., 2001b; Zeng et al., 2013; Zhang et al., 2014). Regional-scale Bouguer gravity and aeromagnetic data may be particularly useful for the identification of the favorable region to explore for potential porphyry copper polymetallic deposits. This is

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because the Bouguer gravity and aeromagnetic data cover a large area and porphyry-style copper polymetallic deposits are commonly found in linear, orogen-parallel belts (Sillitoe, 2010), where magmatic and hydrothermal systems associated with such deposits are usually considered to be dominantly of density and magnetic affinity (Bookstrom, 1989; Fichler et al., 1999; Grant, 1985; Roy and Clowes, 2000; Shah et al., 2013; Woods and Webster, 1985), and these can be respectively identified using gravity and magnetic data. In the present study, it shows that regional-scale Bouguer gravity and aeromagnetic data are propitious to revealing the tectonic and geological structures that are required to locate possible porphyry affinity plutonic rocks, which are genetically associated with porphyry Cu-Mo polymetallic mineralization hydrothermal system during the processes of their differention. By using the upward continuation filtering and correlation analysis techniques, it permits mapping of the igneous rocks associated with the Tuwu and Yandong deposits and allows the recognition of potential areas with gravity and magnetic anomaly patterns similar to Tuwu and Yandong. Several of those potential areas are geologically mapped to be associated with the outcrops of intrusive rocks that are age equivalent or near to those hosting the Tuwu-Yandong ore deposits. These data suggest the presence of a regional and largely concealed late Carboniferous-late Permian magmatic arc in Eastern Tianshan that is highly prospective for porphyry Cu-Mo polymetallic deposits similar in age to the 330 Ma Tuwu and Yandong deposits. 2. Geological setting 2.1. Regional geology The Eastern Tianshan is in the southern part of the world famous orogenic belt that Central Asia orogenic belt (Xiao, 2004). It was formed

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by amalgamation of various accreted terranes that involve continental blocks, arc complexes and accretionary wedges between the Siberian and Tarim-North China cratons (Allen et al., 1993; Carroll et al., 1995; Chen et al., 1999; Windley et al., 1990; Xiao, 2004). Previous studies indicate that the Eastern Tianshan belt has experienced a complex geodynamic evolution process including Paleozoic accretion and collision, Mesozoic thermal subsidence, and Cenozoic thrusting and uplift (Allen et al., 1993; Chen et al., 2011; Chen et al., 2013; Gu et al., 2006; Guo et al., 2010; Han et al., 2011; He et al., 2014; Laurent-Charvet et al., 2002; Shu and Wang, 2003; Windley et al., 1990; Xiao, 2004; Xiao et al., 2008; Zhang et al., 2004b; Zhou et al., 2010b). The outcrops of strata in this district are mainly composed of Devonian and Carboniferous, with minor Ordovician, Silurian and Jurassic (Fig. 1C, Table 1). In terms of tectonic, the Eastern Tianshan orogenic belt could be generally divided into four major structural units: the Dananhu-Tousuquan arc belt, the Kanguertag-Huangshan forearc/intra-arc basin belt, the Aqishan-Yamansu forearc/arc belt and the Central Tianshan arc belt, which are separated by the E–W trending, regional-scale KanguertagHuangshan, Yamansu-Kushui and Aqikuduke-Shaquanzi deep faults, respectively (Fig.1B) (Han et al., 2013; Xiao, 2004). The Dananhu-Tousuquan arc belt is situated north of the Kanguertag-Huangshan fault. It is mainly composed of Ordovician to Silurian volcanic rocks, Devonian to Carboniferous volcanic-intrusive rocks, Jurassic sedimentary rocks and Cenozoic covers (Fig. 1C, Table 1). The early-middle Ordovician Formation of Qiaganbulake and the late Silurian-early Devonian Group of Hongliuhe mainly consist of calc-alkaline volcanic rocks, which are exposed along the southern edge of Turpan-Hami basin and interpreted to be of positive active margin origin (Qin et al., 2002a; Xiao, 2004). The Devonian Formations of Dananhu and Kanguertag mainly consist of basic lavas, pyroclastic rocks, clastic sediments, and calc-alkali felsic volcanic lavas and tuffs.

Fig. 1. Location and regional geology maps of the Eastern Tianshan belt. A. The geographical location of the study area; B. Sketch map showing geological units of the study area (modified after Xiao, 2004; Zhang et al., 2015); C. Simplified geological map of the study area (modified from BGEDXP, 2009).

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Table 1 The major lithostratigraphic units of the tectonic belts in the Eastern Tianshan district (BGEDXP, 2009). Tectonic units

Epoch

Group/formation Rocks

Dananhu-Tousuquan arc belt

Quaternary-Tertiary Middle Jurassic Early Jurassic Early Jurassic Late Carboniferous Late Carboniferous Late Carboniferous Late Carboniferous

Xishanyao Badaowan Sangonghe Dikan'er Qishan Wutongwozi Qi'eshan

Early Carboniferous Late Devonian

Kalatake Kanguertag

Early Devonian Late Silurian–early Devonian Early–middle Ordovician Quaternary-Tertiary Early Permian

Dananhu Hongliugou

Sand, soil and gravel Sandstone, siltstone, and conglomerate interceded with shale and coal seals Conglomerate, sandstone, and mudstone interlayered with limestone and coal beds Sandstone, conglomerate, and conglomerate interlayered with coal seals Tuff, sandstone, and phyllite with minor limestone, chert and basalt Tuff, andesite, basalt, sandstone and limestone Basalt, andesite, tuff, and phyllite interceded with siliceous rocks, limestone and marble Basalt, andesite, spilite, keratophyre, and andesitic brecciated lavas interlayered with lithic sandstone, and conglomerate Sandstone and tuff interlayered with limestone Volcanic breccia, altered tholeiite, andesite, dacite, and rhyolite interlayered with pyroclastic rocks and marble Andesite, basalt, dacite, and tuff with local lithic sandstone and limestone Tholeiite, andesite, tuff interlayered with sandstone

Qiaganbulake

Basalt, andesite, dacite interlayered with tuff and siliceous rocks

Haerjiawu

Late Carboniferous Late Carboniferous Early Carboniferous

Dikan'er Wutongwozi Yamansu

Early Carboniferous Quaternary-Tertiary Middle Permian Late Carboniferous Early Carboniferous

Gandun Aqikebulake Tugutubulake Yamansu

Early Carboniferous Early Carboniferous

Gandun Aqishan

Paleoproterozoic Mesoproterozoic Mesoproterozoic Neoarchean

Kushitai Kawabulake Xinxinxia Donggualing

Kanguertag-Huangshan forearc/arc basin belt

Aqishan-Yamansu forearc/arc belt

Central Tianshan arc belt

Sand, soil and gravel Tuff, andesitic porphyrite and volcanic breccia with local felsite porphyry, pyroclastic rock and basalt Tuff, sandstone, and phyllite with minor limestone, chert and basalt Basalt, andesite, tuff, and phyllite interceded with siliceous rocks, limestone and marble Tuff, volcanic breccia, felsite, sandstone, and conglomerate with local andesite, basalt and limestone Tuff, phyllite and sandstone interlayered with siliceous rocks, limestone and basalt Sand, soil and gravel Conglomerate and sandstone interlayered with limestone, basalt and tuff Tuff, andesite, basalt and volcanic breccia with local rhyolite and limestone Tuff, volcanic breccia, felsite, sandstone, and conglomerate with local andesite, basalt and limestone Tuff, phyllite and sandstone interlayered with siliceous rocks, limestone and basalt Basalt, andesite, rhyolite, volcanic breccia, tuff and quartz porphyry with local andesite and limestone Quartz schist, marble, and quartzite with hornblendic schist and mica schist Marble and dolomite, with metamorphic clastic rock Schist, metamorphic sandstone, quartzite, hornblendic schist and mica schist, with marble Granulite and gneiss

The Carboniferous Kalatake Formation, the Qi'eshan Group, and the Dikan'er Formation mainly consist of lavas, pyroclastic rocks, graywacke, and carbonates. The Devonian-Carboniferous tholeiitic basalts and calc-alkaline andesites were considered to have the characteristics of island arc volcanic rocks (Xiao, 2004; Yang et al., 1996; Zhou et al., 2001). Almost all of the discovered porphyry Cu-Mo polymetallic deposits including Tuwu and Yandong superlarge porphyry Cu-Mo polymetallic mines are located in this arc belt (Fig. 1). A number of isotopic dates on the arc associated intrusions such as granodiorite and plagioclase grano-porphyry in this tectonic domain indicate that the Dananhu-Tousuquan arc has a middle Devonian-early Carboniferous age (Song et al., 2002; Wang et al., 2006), which is generally in agreement with mineralization age of porphyry Cu-Mo polymetallic deposits that include the Tuwu and Yandong mines (Liu et al., 2003). The Kanguertag-Huangshan forearc/intra-arc basin belt, lying between the Kanguertag-Huangshan and Yamansu-Kushui faults, consists predominantly of Carboniferous marine lavas and pyroclastic rocks that were thrust southward over the Aqishan-Yamansu forearc/arc (Fig.1C, Table 1) (Ma et al., 1997; Xiao, 2004). These rocks could be subdivided into two principal tectonic assemblages (Ma et al., 1997; Xiao, 2004; Yang et al., 1996). The first one are volcano-sedimentary rocks that make up several lower-middle Carboniferous formations in the southern part, and the other one is mélanges and broken formations that mainly include several Devonian-Carboniferous volcano-sedimentary rock formations in the northern part. In some literature, geologists favor to term this tectonic domain as Kanguertag-Huangshan ductile shear belt because of the Devonian-Carboniferous strata have been strongly deformed and partly mylonitized, which is characterized by a series of mylonites and mylonitized rocks, tectonic lenses, and breccias formed along early extensional faults due to north-southwards

compressing. Along the Kanguertag-Huangshan fault, there is an ultramafic-mafic complex belt, which dominantly consists of peridotite, lherzolite, gabbro, olivine gabbro, honblende gabbroic norite, pyroxenite diorite, and diorite, and extends hundreds of kilometers long (Xiao, 2004; Zhou et al., 2001). So far, the exact tectonic environment that the Kanguertag-Huangshan belt should belong to is still in dispute. Some geologists are prone to suggest that the Kanguertag-Huangshan tectonic belt represents a suture zone separating the Siberia Craton to the north from the Tarim-North China Craton to the south due to it contains ophiolitic fragments composed predominately of serpentinite, pillowed basalt, meta-gabbro, meta-basalt, meta-diabase, metaplagiogranite, quartz keratophyre, and chert (e.g. Huang et al., 2013; Ji et al., 1994; Rui et al., 2002a; Yang et al., 1996; Zhang et al., 2004b; Zhou et al., 2001). Others, however, are inclined to interpret that it has an inter-arc basin origin because of its distribution between the Dananhu-Tousuquan and Aqishan-Yamansu arcs, which could be in good agreement with gold-copper polymetallic mineralization and ultramafic-mafic complex (e.g. Ma et al., 1997; Shu et al., 2002; Xiao, 2004). The Aqishan-Yamansu forearc/arc belt, located between the Yamansu-Kushui and Aqikuduke-Shaquanzi faults, is mainly made up of bimodal volcanic rocks of the lower Carboniferous Yamansu Formation, and calc-alkaline volcanic rocks of the lower Carboniferous Aqishan Formation, the middle Carboniferous Shaquanzi Formation of clastic rocks and andesitic tuff, and the upper Carboniferous Tugutublak Formation of volcaniclastics, andesites, basalts and volcanic breccia intercalated carbonates (Fig. 1C, Table 1). Permian volcanic-sedimentary rocks of Aqikebulake Formation characterized by marine and terrestrial clastic rocks intercalated with bimodal volcanic rocks and carbonates are sparsely exposed in the eastern part of Aqishan-Yamansu forearc/

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arc. Although Devonian radiolarian chert, which indicates subduction probably had started at least in Devonian time, has been reported to be found in the Aqishan-Yamansu tectonic belt (Li et al., 2002b), almost all the isotopic ages of the volcanic rocks from this belt suggest that subduction may have lasted to the late Carboniferous (Ji et al., 1994; Li et al., 1998; Yang et al., 1998; Zhang et al., 2000). The Central Tianshan arc belt, located south of the Aqikuduke fault, is mainly composed of calc-alkaline volcanic and plutonic rocks, volcaniclastics and Precambrian metamorphic-crystalline basement rocks in greenschist-amphibolite facies (Fig. 1C, Table 1). The Precambrian basement of this arc belt consists of Neoarchean and MesoNeoproterozonic metamorphic rocks with calc-alkalic geochemistry, which are dominantly composed of gneiss, quartz schist, migmatite, and marbles and imbricated with deformed volcanics, clastics, limestones, and ultramafic rocks, suggesting that the Central Tianshan tectonic belt was remnant of Andean-type magmatic arc (Hu et al., 2000; Xiao, 2004; Zhou et al., 2001). The high-resolution isotopic dating from both volcanic and plutonic rocks indicates that the Central Tianshan arc magmatism possibly began in the late Ordovician-Silurian to Devonian-early Carboniferous (Chen et al., 1994; Hu et al., 2000; Li et al., 2001; Xia et al., 2004). Much of the magmatism in the East Tianshan belt occurred during the process of Paleozoic accretion and collision between the Siberian and Tarim-North China cratons, which formed in a convergent continental margin environment during the early period of the middle Carboniferous to early Permian (Ma et al., 1997; Xiao, 2004). Intrusive and sub-vocalic rocks in the Eastern Tianshan belt were therefore characterized by a widespread of Carboniferous to Permian felsic intrusions of diorite, granodiorite and adamellite, with lesser Devonian and Triassic granitoids (Fig. 1C) (Chen et al., 2005; Wang et al., 2014b; Wu et al., 2006a; Zhou et al., 2010b). Previous studies have shown that, from early to late, the magmatic activities could be divided into four epochs: the late Devonian (386–369 Ma) (Wang et al., 2009; Zhou et al., 2008), the early Carboniferous (349–330 Ma) (Gu et al., 2006; Wang and Xu, 2006; Zhou et al., 2008), the late Carboniferous-late Permian (320– 252 Ma) (Han et al., 2006a; Wang and Xu, 2006) and the early-middle Triassic (246–230 Ma) (Gu et al., 2006; Liu, 2000; Sylvester, 1998; Zhou et al., 2008). The first three epochs of the magmatism have the evolution character of being gradually longer in duration, greater in strength, and peaking in the third epoch of the late Carboniferous-late Permian, when the intensity of magmatism in the fourth epoch of the early-middle Triassic became significantly weaker (Wang et al., 2014b). Without considering the first magmatism epoch of the late Devonian, there are three mineralization events that are generally corresponding to the other three magmatism epochs have been identified: the subduction-island arc stage with porphyry-type and volcano-sedimentary copper deposits, the collisional-accretionary stage with orogenic-type gold deposits, and the post-collisional-extension stage with mafic-ultramafic copper-nickel and epithermal gold deposits (Mao et al., 2002; Wang et al., 2014c; Zhang et al., 2008; Zhou et al., 2010a). 2.2. Known porphyry Cu-Mo deposits in the eastern Tianshan district In the past few decades, it has been reported that there were about twelve porphyry copper polymetallic ore deposits or/and occurrences (Chen et al., 2008; Liu et al., 2005), from west to east, containing the Western Yandong, Yandong, Tuwu, Western Linlong, Linlong, Chihu, Eastern Gobi, Eastern Tudun, Southwestern Sanchakou, Sanchakou, Eastern Sanchakou and Baishan, have been discovered in the Eastern Tianshan area (Fig.1 C). The porphyry deposits mainly occurred in the middle of highly mineralized Devonian and Lower Carboniferous accreted volcanic island arcs of Dananhu-Tousuquan (Fig. 1C). They generally have similar geological features and form a porphyry copper polymetallic mineralization belt that spatially locates in the south margin of late Paleozoic Dananhu-Tousuquan arc. The wall rocks, ore-bearing rocks, wall-rock alterations, ore minerals and ages of some of the

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known porphyry deposits are given in Table 2. Among them, TuwuYandong superlarge deposit is the most representative and well studied in detail, the major features of which are summarized as follows. The Tuwu and Yandong porphyry Cu-Mo deposits are located in the middle of the Dananhu-Tousuquan arc belt and are about 2–5 km away from the northern margin of the Kanguertag-Huangshan fault (Fig. 2). The outcrops of strata in the Tuwu-Yandong mining district consist predominantly of lower Devonian, Carboniferous, middle Jurassic and Quandary deposits (Fig. 2). The lower Devonian Dananhu Formation is mainly composed of basalt and volcaniclastics. The lower Carboniferous Qi'eshan Group is predominately made up by intermediate to mafic volcanic rocks, which strike approximately EW-trending, dip to the south at 25–65°, and have well-developed schistosity and meta-andesitization (Chen et al., 2005). It can be further divided into three lithologic sections including volcaniclastics and tuff in the lower, basalt and andesite with intercalated dacite and basaltic andesite in the middle, as well as sandstone and polymictic conglomerate intercalated with tuff and andesite in the upper parts (Hou et al., 2005; Wang et al., 2001a). The upper Carboniferous Tuwu Formation and Gandun Formation mainly consist of feldspathic lithic sandstone, biolithite and sedimentary tuff. The middle Jurassic of Xishanyao Formation chiefly consists of sandstone, siltite, mudstone, and conglomerate, intercalated with coal layers, which overlay and form an angular unconformity with the strata of the Qi'eshan Group. The Tuwu-Yandong mineral belt contains numerous inter-acid intrusions, which were emplaced into the early Carboniferous volcanic rocks and mostly are irregular in shape where exposed (Fig. 2). These intrusions primarily involve the early Carboniferous hornblende gabbro, quartz gabbro, gabbro diorite, quartz diorite, granodiorite, and adamellite, as well as the late Carboniferous moyite and plagioclase granite (Fig. 2). The Tuwu and Yandong deposits are mainly hosted in the dioritic and plagiogranitic porphyries that intruded the Qi'eshan Group (Chen et al., 2005). The isotopic geochemical tracing suggest that the magmas of the dioritic and plagiogranitic porphyries were likely evolved from partial melting of the subducted, mafic-ultramafic oceanic slab, which was hybridized subsequently by peridotite in the mantle wedge (Zhang et al., 2004a, 2006; Wang et al., 2014b, 2014c; Shen et al., 2014; Gao et al., 2015). The mineralized intrusions of the Tuwu and Yandong deposits contain abundant quartz-sulfide and biotite-chlorite-sulfide veins (Gao et al., 2015). Both dioritic and plagiogranitic porphyries have undergone extensive hydrothermal alteration of silicification, chloritization, epidotization, sericitization and carbonatization (Han et al., 2006b; Zhang et al., 2004a; Wang et al., 2001a). Three alteration zones associated with the porphyry mineralization that are phyllic zone, chlorite-biotite zone and propylitic zone have been recognized surrounding the orebodies from inside to outside (Han et al., 2006b). The ore minerals in the Tuwu and Yandong deposits mainly include chalcopyrite and pyrite, with minor bornite, pyrite, molybdenite, magnetite, sphalerite and hessite, covellite, and the gangue minerals are dominated by quartz and plagioclase, with minor sericite, chlorite, biotite amphibole and epidote (Wang et al., 2014b; Zhang et al., 2008). The ores are characterized by medium-fine grained, subhedra-euhedral textures, disseminated and fine-veinlet structures (Wang et al., 2014b). In terms of geophysics, the ore-bearing rocks of Tuwu and Yandong porphyry copper deposits that both dioritic and plagiogranitic porphyries would produce relatively low gravity anomaly and moderate- to high- magnetic anomaly, because the mineralized intermediate-acid intrusions are mainly composed of felsic minerals with low densities and magnetite with high susceptibilities (Wang et al., 2001b; Wu et al., 2008; Zhang et al., 2010b). At the same time, the Tuwu and Yandong porphyry deposits are hosted in the volcanic rocks of basaltic affinity with relatively high densities and magnetic susceptibilities, which would produce high gravity and magnetic anomaly (Wang et al., 2001b; Wu et al., 2008; Zhang et al., 2010b). Therefore, both the analysis of regional geophysical data and the geophysical surveys including

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Table 2 Porphyry copper-molybdenum polymetallic deposits in the Eastern Tianshan district. Deposits

Ore-bearing rocks

Wall rock alterations

Ore minerals

Dating samples

Method

Age/Ma

References

Western Qi'eshan group: andesite, Yandong basalt and tuff

Plagioclase granite porphyry

Silicification, epidotization, sericitization, kaolinization

Plagioclase granite porphyry

Re-Os

326 ± 4.5

Zhang et al., 2010a

Yandong

Plagioclase granite porphyry, dioritic porphyry

Silicification, chloritization, epidotization, sericitization, carbonatization

Chalcopyrite, bornite, molybdenite, pyrite, magnetite, hematite, covellite, chalcocite Chalcopyrite, pyrite, bornite, molybdenite, chalcocite, digenite, sphalerite, magnetite, rickardite

Plagioclase granite porphyry Plagioclase granite porphyry Plagioclase granite porphyry Molybdenite

SHRIMP U-Pb SHRIMP U-Pb SHRIMP U-Pb Re-Os

356 ± 8

322.7 ±

Albite granite porphyry Sericite

U-Pb

2.3 356 ± 8

Rui et al., 2002b Liu et al., 2003 Chen et al., 2005 Rui et al., 2002b

39Ar/40Ar

341.2 ±

Tuwu

Chihu

Wall rocks

Qi'eshan group: andesite and basalt intercalated with tuff

Qi'eshan group: basalt, basaltic andesite and andesite

Kushui Formation (Kalatake formation?): mafic volcanic-pyroclastic rocks and basalt andesites

Sanchakou Wutongwozi formation: clastic rocks, basalt, andesitic tuff, spilite, and keratophyre

Eastern Gobi

Baishan

Plagioclase granite porphyry, dioritic porphyry

Plagioclase granite porphyry, dioritic porphyry

Silicification, chloritization, epidotization, sericitization, carbonatization

Chalcopyrite, pyrite, bornite, molybdenite, chalcocite, digenite, sphalerite, magnetite, rickardite

Silicification, chloritization, sericitization, kaolinization

Molybdenite, chalcopyrite, lindgrenite

Granodiorite-porphyry, Silicification, granodiorite, quartz sericitization, dioritic porphyry chloritization, epidotization, kaolinization

Chalcopyrite, pyrite, molybdenite, chalcocite

Gandun formation: Porphyritic granite, metasandstone, meta-sandy granite porphyry mudstone, meta-argillaceous sandstone, meta-mudstone, meta andesites, tuff and hornfels

Potassic alteration, silicification, pyritization, tourmalinization, carbonatization, fluoritization, sericitization, biotitization

Molybdenite, pyrite, chalcopyrite, galena, magnetite, scheelite, wolframite

Gandun formation: Tuff, graywacke, siltstone, interlayered with carbonaceous shale and basalt.

Phyllic and potassic alteration, biotitization, chloritization, carbonatization

Molybdenite, pyrite, marcasite, chalcopyrite, galena, sphalerite, pyrrhotite, magnetite, ilmenite

Granite porphyry

334 ± 2 333 ± 4

4.9 361 ± 8

Plagioclase granite porphyry Plagioclase granite porphyry Plagioclase granite porphyry Quartz

SHRIMP U-Pb SHRIMP 334 ± 3 U-Pb LA-ICP-MS 301 ± 13 U-Pb 39Ar/40Ar 347.3 ±

Plagioclase granite porphyry

39Ar/40Ar

2.1 310.95 ±

Qin, 2000 Qin, 2000 Rui et al., 2002b Chen et al., 2005 Li et al., 2002b Qin et al., 2002b Qin, 2000

Plagioclase granite porphyry

4.57 LA-ICP-MS 292.1 ± Li et al., U-Pb 33.5/283.5 2002b

Plagioclase granite porphyry

SHRIMP U-Pb

Granite porphyry

SHRIMP U-Pb Rb-Sr

278 ± 4

U-Pb

245

Molybdenite

Re-Os

235.4 ±

Molybdenite

Re-Os

2.5 231.9 ±

Granite porphyry

6.5 LA-ICP-MS 233.2 ± U-Pb 4.1

Plagiogranite granite porphyry Pyrite

Granite porphyry

± 3.5 322 ± 10

276

Molybdenite

LA-ICP-MS 227.6 ± U-Pb 1.3 Re-Os 223.2 ±

Molybdenite

Re-Os

2.7 227.7 ±

Re-Os

4.3 229 ± 2

Molybdenite/Pyrite Re-Os

224.8 ±

Molybdenite

4.5/225 ±

Wu et al., 2006a, 2006b Li et al., 2004 Lang et al., 1992 Sun et al., 2009 Wu et al., 2013a Wu et al., 2013b Wu et al., 2013a Huang et al., 2011 Tu et al., 2014 Zhang et al., 2009 Li et al., 2006a Zhang et al., 2005

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gravity and magnetic from the Tuwu-Yandong porphyry Cu-Mo polymetallic mineralization district have shown that this porphyry mineralization belt is located in gradient gravity and magnetic anomaly zone (i.e. on the flanks of high gravity and magnetic anomaly background) overprinted with local gravity anomaly low to moderate and magnetic anomaly moderate to high (Long et al., 2001; Wang et al., 2001b; Zhuang et al., 2003; Wu et al., 2008; Xiao et al., 2009; Zhang et al., 2010b; Xiao, 2013). The other porphyry deposits of the Eastern Tianshan porphyry copper mineralization belt should generally have similar geophysical anomaly patterns with Tuwu and Yandong deposits due to their similar geological features (Table 2). As a result, the characterized gravity and magnetic anomaly patterns of Tuwu and Yandong

deposits have been widely used as a geophysical prospecting model for porphyry-type copper deposits in the Eastern Tianshan belt and proved to be useful in mineral exploration and potential mapping for porphyry copper polymetallic deposits similar to the Tuwu and Yandong mines in this region (Zhu et al., 2003; Xiao et al., 2009; Xiao, 2013; Zhuang, 2005; Zhang et al., 2010b). 3. Bouguer gravity and aeromagnetic data In the past several decades, regional-scale investigations using airborne geophysics such as gravity and magnetic surveys have been widely conducted throughout China including the Eastern Tianshan district

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Fig. 2. Generalized geological map of Tuwu-Yandong mining district (modified after Chen et al., 2005).

by the former Ministry of Geology and Mineral Resources (MGMR, now Ministry of Land and Resource-MLR) (Wang et al., 1997; Yang et al., 1994; Zhao et al., 1989). The airborne gravity measurements and magnetic survey were flown in the Eastern Tianshan region along NStrending flight lines, generally perpendicular to the tectonic belts. The data was gridded with a cell size of 2 km × 2 km and processed using Chinese industry-standard techniques, particularly Bouguer gravity data were derived from Bouguer correction that has taken into account height, mass and terrain effects in the measurement of natural gravity (Zhao et al., 1989). Bouguer gravity and aeromagnetic data set of the Eastern Tianshan used in this study covers approximately 52,300 km2 over seven 1: 250, 000-scale quadrangle map sheets. 4. Densities and magnetic properties of the major rock units As is well known, knowledge of densities and magnetic properties of rock units within a study area, respectively, play an important role in the interpretation of gravity and magnetic data (Guan, 2005; William et al., 2013; Zeng, 2005). Therefore, to aid interpretation of the Bouguer gravity and aeromagnetic maps, nonetheless without sampling and measurement in this study, the density data and magnetic properties data including both susceptibilities and remnant magnetizations of the major rock units in the Eastern Tianshan region were collected from the previous studies (Shao, 2012; Zhuang, 2005), and the results are shown in Table 3. Although the properties of the data were not comprehensive and they probably do not represent true values of all the geophysical significant units, some useful information on the values of these properties could be obtained for the bedrock units. The unconsolidated sediments of Gobi-desert (i.e. Tertiary and Quarternary deposits) have the lowest densities, the weakest magnetic susceptibility and the lowest remanent magnetization. Thus, the Gobidesert covered layer is usually considered to produce low gravity and magnetic anomalies. The normal sedimentary rocks, e.g. sandstone,

limestone and conglomerate, usually have relatively low- to moderatedensities, weak magnetic susceptibility and low remanent magnetization. As a result, they generally would produce low- to moderate- gravity and low amplitude magnetic anomalies. The volcanic and intrusive rocks vary greatly in both density and magnetism, and their densities, susceptibilities, and remnant magnetizations, are mainly dependent on constituent proportions of the minerals present. In general, the more mafic and fewer felsic minerals the volcanic and intrusive rocks contain, the higher densities, stronger susceptibility and higher remanent magnetization the rocks will have, and vice versa (Table 3). Consequently, felsic volcanic and intrusive rocks such as tuff, porphyries, volcanic breccia, granite and granodiorite usually could make low gravity and magnetic anomalies, whereas mafic-ultramafic volcanic and intrusive rocks that involve andesite, diorite, gabbro and ultrabasic rock commonly could cause moderate- to high- amplitude gravity and magnetic anomalies. The metamorphic rocks that involve gneiss, schist, metamorphic sandstone, marble and hornstone generally have extremely weak magnetic susceptibility and very low remanent magnetization, whereas they have relatively moderate to high densities (Table 3), which depend primarily on the original mineral composition of the rocks and are strongly influenced by the degree and type of metamorphism that the rocks have undergone (Zhuang, 2005). Accordingly, the metamorphic rocks usually could produce low magnetic anomalies and moderate- to high- gravity anomalies. 5. Correlation analysis method The geological interpretation of gravity and magnetic anomalies is usually hindered by the effects of anomaly superposition and source ambiguity that is inherent to potential analysis (von Frese et al., 1997a, 1997b). A significant attempt to minimizing interpretational ambiguities is to consider analyses of anomaly correlations by using the correlation analysis (CA) method, which is a common

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Table 3 The density and magnetic properties of rocks in the Eastern Tianshan district. Lithos

Rock types

Density (103 kg/m3)

Magnetism Samples

Unconsolidated sediments Sedimentary rocks

Vocalic rocks

Intrusive rocks

Metamorphic rocks

Quaternary and Tertiary depositsb Tuffaceous sandstonea Sandstonea Siliceousa Limestonea Limestoneb Conglomeratea Conglomerateb Porphyrya Tuffa Tuffb Volcanic brecciaa Andesitea Andesiteb Basaltb Granita Graniteb Granodioritea Dioriteb Gabbroa Gabbrob Diabaseb Ultrabasic rocka Gneissa Gneissb Schista Metamorphic sandstonea Marblea Hornstonea Hornstoneb

Samples

Range

Average

N.A. 557 1029 181 328 18 13 53 283 904 567 154 524 74 52 2616 774 353 169 220 92 49 166 226 138 758 47 321 120 23

N.A. 2.55–2.69 2.40–2.68 N.A. 2.66–2.72 2.62–2.72 2.66–2.73 2.62–2.91 2.61–2.67 2.65–2.73 2.45–2.93. 2.69–2.73 2.73–2.82 2.63–2.86 2.58–2.90 2.61–2.63 2.50–2.71 2.66–2.83 2.64–3.15 2.83–2.95 2.81–3.07 2.80–3.04 2.90–2.98 2.64–2.71 2.57–2.99 2.68–2.71 2.64–2.68 2.74–2.79 2.72–2.90 2.70–2.95

1.77 2.61 2.66 2.67 2.70 2.69 2.71 2.67 2.66 2.69 2.66 2.71 2.76 2.73 2.77 2.62 2.61 2.68 2.79 2.86 2.93 2.86 2.93 2.65 2.69 2.68 2.68 2.77 2.84 2.73

N.A. 186 189 40 108 N.A. 94 N.A. N.A. 923 N.A. 20 N.A. N.A. N.A. 1160 71 104 91 20 65 27 262 N.A. N.A. 395 67 22 N.A. N.A.

Magnetic Susceptibility (104piSI)

Remnant Polarization (10−3 A/m)

Range

Average

Range

Average

N.A. 0–3900 N.A. 0–231 N.A. N.A. N.A. N.A. N.A. 0–21,800 N.A. 0–2600 N.A. N.A. N.A. 0–8400 10–10,000 0–1191 180–52,800 1940–15,000 160–120,000 340–705,000 0–20,500 N.A. N.A. 0–550 N.A. N.A. N.A. N.A.

0 550 0 0 0 N.A. 0 N.A. N.A. 950 N.A. 950 N.A. N.A. N.A. 340 1079 50 2872 5010 11,650 58,510 3850 N.A. N.A. 76 0 0 N.A. N.A.

N.A. 0–1500 N.A. 0–125 N.A. N.A. N.A. N.A. N.A. 0–44,100 N.A. 0–1700 N.A. N.A. N.A. 0–5200 N.A. 0–60 N.A. 280–3640 N.A. N.A. 0–15,000 N.A. N.A. 0–582 N.A. N.A. N.A. N.A.

0 170 0 0 0 N.A. 0 N.A. N.A. 550 N.A. 240 N.A. N.A. N.A. 110 N.A. 0 N.A. 1500 N.A. N.A. 4600 N.A. N.A. 60 0 0 N.A. N.A.

The superscript letters of a and b indicate the data referenced from Zhang et al., 2005 and Shao, 2012, respectively. N. A. means no data.

but useful approach for comprehensively analyzing multiple geopotential fields such as gravity and magnetic for possible correlations (Cohen, 1981; De Ritis et al., 2010; von Frese et al., 1997a, 1997b; Zhang et al., 2014). The correlation coefficient (CC) between Bouguer gravity and aeromagnetic within a local domain Ω is given by: σ 2Ω ðG; M Þ ∑Ω ðg−g Þðm−mÞ ffi ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi CCðG; MÞ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 σ Ω ðGÞσ Ω ðM Þ ∑Ω ðg−g Þ2 ∑Ω ðm−mÞ2 where σΩ2(G), σΩ2(M) and σΩ2(G, M) are the variance of Bouguer gravity, variance of aeromagnetic, and the covariance between Bouguer gravity and aeromagnetic in a local domain Ω, respectively. Values of CC(G, M) vary between + 1.0 and − 1.0, with + 1.0 meaning perfect positive or direct correlation and − 1.0 indicating perfect negative or inverse correlation between gravity and magnetic. CC(G, M) values near or equal zero are interpretated conventionally to mean that the variations in gravity and magnetic do not match each other. 6. Geological interpretation of the Bouguer gravity and aeromagnetic data from eastern Tianshan 6.1. Bouguer gravity data The gravity data provide an useful geophysical signature that has been widely used in regional characterization of the earth for identifying potentially favorable regions for mineral exploration including porphyry-type deposits (e.g. Bookstrom, 1989; De Oliveira et al., 2008; Shahabpour, 1999; Tweto and Case, 1972; Yang et al., 2006; Zhu et al., 2013; Zhuang et al., 2003). The gravity anomaly, numerically defined

as the arithmetic difference between the observed vertical acceleration of gravity and the predicted acceleration at the observation site in geophysical exploration, can be identified where the density of the material varies laterally, and is associated with changes in the nature and structure of the subsurface. As a result, the magnitudes of gravity anomaly could be measured by subtracting the theoretical gravity (i.e. normal field) from the observed data, which has been adjusted by both temporal and spatial corrections (Mallick et al., 2012; William et al., 2013). The gravity anomaly may be either positive or negative depending on the presence of mass excesses or deficiencies, which in turn are considered to be controlled by the volume and contrasting densities of anomalous targets. Numerous types of gravity anomalies with different geological significances, e.g. free-air, Bouguer and isostatic residual anomalies have been produced by employing a variety of models assumed in the calculation of the theoretical gravity field (William et al., 2013; Zeng, 2005). The Bouguer gravity anomaly, which is determined by including height, terrain, and the mass of material between the elevation datum and the site of the observation in the theoretical model, is one of the most important and widely used gravity anomalies in geophysical exploration because of it has the advantage to be not much dependent on the local scales of topography so as to bring out geological effects clearly. In general, although Bouguer anomalies do not correlate with local topography unless these features are associated with structural or stratigraphic variations in density below the elevation datum level, they do correlate inversely with regional topography due to isostatic compensation, which is supposed to be related to the convexity and concavity of lower crust. Therefore, in geological analysis of the Bouguer gravity anomaly, it should be understood as superimposed effect of both the shallower and more local anomaly, as well as the deeper and more regional anomaly (Mallick et al., 2012; William et al., 2013; Zeng, 2005). The shallow and local anomaly is mainly the response of features within a depth of a few tens of meters to a few kilometers. The deep and

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regional anomaly is mostly originated from deep-seated structures and anomalous targets in density at lower crustal or even Moho depth. The values of the regional Bouguer gravity data from Eastern Tianshan district are negative ranging from roughly − 250 mGal to − 120 mGal and they show several significant high and low gravity anomalies (Fig. 3). A broad gravity anomaly high mainly lies in both the north and southwest, whereas the lowest gravity anomaly lies to the east. In general, the gravity highs correspond to the topographic elevation lows (i.e. alluvial basins), very prominent on the north part of the Eastern Tianshan region, whereas the gravity lows are mainly associated with topographic elevation highs (i.e. mountain terrains) on the eastern-southeastern part of this district (Deng, 2002; Zeng et al., 2013). As a result, it seems to indicate that the crust beneath of the Eastern Tianshan has been isostatically compensated due to the fact that the Bouguer gravity anomalies decreasing with increasing mean topographic elevation. The northern Bouguer gravity high may be caused by crustal thinning, whereas the east-southeastern Bouguer gravity low perhaps is produced by crustal thickening. The Dananhu-Tousuqian arc belt is generally dominated by a huge Bouguer gravity anomaly high, which in general has a negative correlation to the topographic elevation in this region. The axis of the gravity high is nearly convex towards southeast in western segment and southwest in eastern segment. The southward convexity indicating the piling up of rocks, both at low and deep levels, may be due to the southward acting compressive forces. The fact that the broad gravity anomaly high was produced over Gobi-desert, as well as a variety of sedimentary and volcanic-plutonic rocks with low to moderate densities such as sandstone, conglomerate, tuff and andesite, suggests that this strong anomaly high may reflect a huge concealed plutonic mass with high density, which is most likely related to Carboniferous-Permian Kanguertag-Sanchakou volcanic structures (Li et al., 2006b; Feng et al., 2009; Xu et al., 2011) that could be interpreted to be possibly genetically associated with porphyry Cu-Mo mineralization hydrothermal systems due to the known Tuwu-Yandong porphyry belt located at the margin of this anomaly zone. The steep gravity anomaly gradients on the south may define the edge between Dananhu-Tousuquan arc and Kanguertag-Huangshan forearc/arc basin coinciding KanguertagHuangshan fault. The region containing the Tuwu and Yandong porphyry mineralization belt is characterized by N–E trending, high-gradient gravity anomaly, perhaps indicating either Dacaotan-Dananhu fault to the north or Kanguertag-Huangshan ductile shear zone to the south. The Kanguertag-Huangshan forearc/arc basin belt is predominately characterized by moderate-low Bouguer gravity anomaly, which is gradually descended in E–W direction (Fig. 3). The western part shows moderate Bouguer gravity anomaly. This may be produced by the Carboniferous volcanic rocks with moderate densities. In the middle part, the gravity anomaly becomes lower due to emplacement of granitoids with relative low densities, which is equivalent to decreasing the

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superimposed gravity effect. The eastern part displays strong gravity anomaly low. This is probably caused by both the Gobi-desert coverage within the alluvial basin, as well as a wide range of granitoids both outcropping and inferred. The Yamansu-Kushui fault, which is the tectonic boundary between Kanguertag-Huangshan forearc/arc basin and Aqishan-Yamansu forearc/arc, is not characterized by steep gradient gravity anomaly. This anomaly pattern seems to signify that the fault is not very deep. The Aqishan-Yamansu forearc/arc belt displays a wide range of Bouguer gravity anomalies, which appear to be prominent highs on the west and gradually descend to significant lows on the east. In the western part, the prominent gravity anomaly high showing northward convexity mostly like corresponding to the presence of mafic-ultramafic volcanic rocks, which are most likely to be related to Carboniferous Aqishan volcanic structures and a number of genetically associated volcanic-sedimentary iron deposits that have been mapped and investigated in previous studies (Ma et al., 1997; Feng et al., 2009; Xu et al., 2011). In the middle part, the moderate Bouguer gravity anomaly indicates the normal Carboniferous volcanic rocks with moderate densities. In the eastern part, the gravity anomaly decreases to very low. This is due to the Gobi-desert coverage and emplacement of granitoids with low densities. The Aqikekuduke-Shaquanzi fault, which is the tectonic boundary between Aqishan-Yamansu forearc/arc and Central Tianshan arc, similarly with Yamansu-Kushui fault, does not show steep gradient gravity anomaly, which is likely to indicate that the fault is not very deep. The Central Tianshan arc belt shows a relatively complex Bouguer gravity anomaly pattern. The eastern part of this arc belt shows Bouguer gravity anomaly high, perhaps is caused by the mafic-ultramafic volcanic rocks, which are also correlated with the porphyry Cu-Mo polymetallic mineralization system. The presence of a goose-shaped, closured Bouguer gravity anomaly high in the central part of the Central Tianshan arc indicates possible interference of a later magma event, which is associated with the mafic-ultramafic intrusive complex there (Fig. 1C). The Bouguer gravity anomaly low in the eastern part of this arc belt is due to the crystalline basement of metamorphic rocks and the emplacement of granitoids with low densities. In order to isolate the Bouguer gravity anomaly, an upward continuation filter of the Bouguer gravity data was performed to calculate the gravity field at an elevation higher than that at which it was originally measured. This transformation technique could reduce near-surface effects, while enhancing the gravity anomalies from deeper mass sources (Blakely, 1995; William et al., 2013; Zeng, 2005). Therefore, this transformation highlights the longer spatial wavelength of regional anomalies at the expense of the shorter wavelength of local anomalies. The Bouguer gravity data from Eastern Tianshan were upward continued from the nominal survey height to 10 km (Fig. 4) and significant features could be observed. There exist two prominent gravity anomaly highs. The first one is situated on the middle and western part of

Fig. 3. Map showing Bouguer gravity data of the Eastern Tianshan district.

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Dananhu-Tousuquan arc belt. The presence of this prominent Bouguer gravity anomaly high further indicates that a huge amount of mafic-ultramafic affiliated plutonic rocks has been emplaced deep in DananhuTousuqian arc. The other one is mainly located on the western part of both Aqishan-Yamansu forearc/arc and Central Tianshan arc belts. This anomaly high is most likely related with volcanic activities above mentioned, which have a broad range and deep sources. The KanguertagHuangshan fault characterized earlier by steep gravity gradient is still present in the upward continuation gravity anomaly map. This anomaly pattern indicates that the fault may have extended to a very deep level, which is reasonably considered to be the tectonic boundary between the Siberian craton to the north and Tarim-North China craton to the south. Nevertheless, the Aqikekuduke-Shaquanzi fault does not show as a gravity gradient anomaly belt in the upward continuation gravity anomaly map. This anomaly pattern is most likely to suggest that the Aqikekuduke-Shaquanzi fault was not extended to a deep level and it does not seem to indicate that this fault is the tectonic boundary separating the Siberian craton to the north and Tarim-North China craton to the south (e.g. Ma et al., 1997; Li et al., 2002a; Xiao et al., 2004). 6.2. Aeromagnetic data Similar to gravity data, magnetic data also have long been in existence and successfully used as an aid for mapping and understanding subsurface for dealing with mineral prospecting problems that involve porphyry-type deposits exploration (e.g. Anderson et al., 2013; Anderson et al., 2014; De Oliveira et al., 2008; Roy and Clowes, 2000; Woods and Webster, 1985; Zhuang et al., 2003). In terms of geophysics, it is well known that magnetic anomalies could be caused by two distinct types of magnetism: induced and remanent (Guan, 2005; William et al., 2013). The induced component mostly depends on the magnetic susceptibility of the rocks that measures the ease with which the rock becomes magnetized (Guan, 2005; William et al., 2013). The remanent component mainly reflects the past magnetic history of the rocks (Guan, 2005; William et al., 2013). In general, the induced component is considered to dominate and the remanent component could be assumed to be absent and demagnetization effects are negligible in rocks, particularly plutonic and sedimentary rocks, but the reverse is usually true in many volcanic rocks (Table 3) (Guan, 2005; William et al., 2013). Therefore, rocks with higher magnetic susceptibilities commonly produce magnetic anomalies with larger variation in the observed magnetic field than do rocks with lower magnetic susceptibilities (Table 3) (Anderson et al., 2013; Guan, 2005; William et al., 2013). In order to better align magnetic anomalies with causative geological targets, the regional aeromagnetic data were firstly processed utilizing

the reduced-to-pole (RTP) transformation method (Baranov and Naudy, 1964; Guan, 2005; William et al., 2013), which is a common technique that recalculates total magnetic intensity data as if the inducing magnetic field had a 90° inclination, as is the case at the north magnetic pole. This operation could remove the dependence of magnetic data on the magnetic inclination and thus minimizes anomaly asymmetry due to magnetic inclination and locates anomalies above their causative bodies. The RTP transformation for the aeromagnetic data from Eastern Tianshan was applied using an inclination of 61.2° and declination of 1.1° that referred to the World Magnetic Model (WMM) (Chulliat et al., 2014). Because of the relatively steep inclination of the magnetic field in Eastern Tianshan, there is only minimal difference between the observed magnetic anomaly and the RTP transformation. The RTP transformation of the regional aeromagnetic data from Eastern Tianshan district shows a number of contrasting magnetic anomalies (Fig. 3). The dominant aeromagnetic is EW-trending, parallel to the major crustal-scale faults and belts of exposed igneous rocks. The patterns in the aeromagnetic anomaly data are generally spatially correlated with known lithotectonic terranes. The Dananhu-Tousuqian arc belt is generally dominated by aeromagnetic anomaly highs. The high magnetic anomalies have been referred to as the northern magnetic high zone in the Eastern Tianshan region and interpreted to reflect arc-related rocks and their basement (Deng, 2002). There are two linear magnetic lows, the west of which is N–W trending and the east of which is N–E trending, located in the magnetic highs belt and they generally correlate with the west and east segments of the deep Dacaotan-Dananhu fault, respectively (Fig. 3). The southern linear aeromagnetic low correlates with steeply dipping Carboniferous sedimentary rocks along Kanguertag-Huangshan fault. The Tuwu and Yandong porphyry deposits are contained within high- to moderate-amplitude, scattered aeromagnetic anomalies between Dacaotan-Dananhu and Kanguertag-Huangshan faults, where the early Carboniferous igneous rocks are exposed (Fig. 1), suggesting that Carboniferous igneous rocks have both a low and high magnetic signature depending on their contends of mafic-ultramafic minerals. The Kanguertag-Huangshan forearc/arc basin belt shows a complex aeromagnetic anomaly pattern. The ductile shear belt has a relatively low magnetic background with several moderate- to high-amplitude, linear trend scattered magnetic highs. The long wavelength, broad low-amplitude aeromagnetic signals mainly correlate with both sedimentary rocks and some Carboniferous to Permian igneous rocks (Fig. 1). The broad magnetic low has been referred to as the northern magnetic trough in the Eastern Tianshan region and interpreted to reflect Carboniferous sedimentary rocks. In sharp contrast, clusters of magnetic anomaly highs align approximately E–W trending along the western part of the Kanguertag-Huangshan fault. The highs are ovoid in shape

Fig. 4. Map showing Bouguer gravity data upward continued to 10 km. Also shown radiometric age dates (BGEDXP, 2009), which are represented by the colored squares.

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Fig. 5. Map showing RTP transformed aeromagnetic data of the Eastern Tianshan district.

with axial dimensions of about 10 × 5 km and long axes trending N–W, and thus are transverse to the Kanguertag-Huangshan fault. The highs are spaced between 30 and 40 km apart. Between the highs there are low- to moderate-amplitude, short-wavelength anomalies that are commonly associated with mapped Carboniferous igneous rocks. The Aqishan-Yamansu forearc/arc belt is predominately characterized by aeromagnetic anomaly highs. The high-amplitude, short- to moderate-wavelength anomalies extend for more than 400 km along an E–W trending zone. The broad high aeromagnetic highs have been referred as caused by Carboniferous rocks, and many marine volcanic sedimentary hosted iron deposits are located in this aeromagnetic high anomaly belt generally along the Aqikekuduke-Shaquanzi fault (Feng et al., 2009; Xu et al., 2011). The Central Tianshan arc belt displays a complicated aeromagnetic anomaly pattern. There exist three prominent high-amplitude, short- to moderate-wavelength high magnetic anomalies in the middle part of the Central Tianshan block. These aeromagnetic highs are inferred to be related with mafic-ultramafic volcanic rocks, although they probably have different origin. The western magnetic high is likely caused by Carboniferous volcanics of the Aqishan Formation that predominately accumulated in the Aqishan volcanic basin (Feng et al., 2009; Xu et al., 2011). The middle magnetic high is probably produced by the Devonian mafic-ultramafic intrusions, which were emplaced in crystalline basement rocks of the Archean gneiss with relatively low magnetic anomaly (Fig. 1). The eastern magnetic is most likely related to the Carboniferous volcanic rocks of Yamansu Formation and Permian mafic-ultramafic intrusions that predominately accumulated in the Yamansu and Baishanquan volcanic basins (Feng et al., 2009; Xu et al., 2011).

In addition to RTP transformation, the upward continuation filter was also applied to the aeromagnetic data for calculating the magnetic field at an elevation higher than that at which it was originally measured so as to highlight the longer spatial wavelength anomalies. The aeromagnetic data from Eastern Tianshan were also upward continued from the nominal survey height to 10 km (Fig. 4). Several significant features could be found on the upward continuation aeromagnetic anomaly map. The linear trend of upward continuation magnetic anomaly in the DananhuTousuquan arc belt are correlated with mafic-ultramafic igneous rocks of Carboniferous age, which has been also characterized by Bouguer gravity anomaly and is associated with known porphyry mineralization belt that could have provided ore-bearing hydrothermal systems during the process of their differentiation. Intrusions at Tuwu and Yandong deposits have similar early Carboniferous isotopic ages. North of the KanguertagHuangshan fault and south of the Yamansu-Kushui fault, predominately magnetic lows overprinted on moderate magnetic anomaly background in both the Kanguertag-Huangshan forearc/intra-arc and AqishanYamansu forearc/arc belts, correlate with a cluster of intrusive rocks with ages similar to those of the Tuwu and Yandong deposits. The presence of prominent magnetic highs that are produced by the mafic-ultramafic igneous rocks predominately accumulate in the Aqishan, Yamansu and Baishanquan volcanic basins along Aqishan-Yamansu forearc/arc belt, indicates that the mafic-ultramafic volcanic strata have large thickness and may origin from relatively deep level magma resources. 7. Discussion and conclusions Regional Bouguer gravity and aeromagnetic data of the Eastern Tianshan have been interpreted to supplement geological knowledge

Fig. 6. Map showing aeromagnetic data upward continued to 10 km. Also shown radiometric age dates (BGEDXP, 2009), which are represented by the colored squares.

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Fig. 7. The correlation coefficient map between the 10 km upward continued Bouguer gravity and aeromagnetic data at a corresponding scale of 30 km. Also shown are radiometric age dates (BGEDXP, 2009), which are represented by the colored squares.

for understanding porphyry Cu-Mo polymetallic mineralization similar to Tuwu and Yandong deposits, especially as large areas within the Eastern Tianshan are without outcrop due to the coverage of Gobi-desert. The regional Bouguer gravity shows that the Tuwu-Yandong porphyry Cu-Mo polymetallic mineralization belt locates at a NEE-trending, steep gradient gravity anomaly zone along the Kanguertag-Huangshan fault. The aeromagnetic data indicate that the porphyry belt is characterized by a cluster of magnetic anomaly highs, which are superposed in the NEE-trending gradient magnetic anomaly belt. Both the upward continuation Bouguer gravity and aeromagnetic anomalies suggest that the Tuwu-Yandong porphyry belt is situated at the edge of a prominent gravity and magnetic high, which is most likely to be produced by a relatively deep density contrast and magnetic source that is genetically related to porphyry mineralization associated hydrothermal systems. In order to further explore the interrelationship between Bouguer gravity and aeromagnetic data, which probably could minimize interpretational ambiguities and provide more significant geophysical information for identifying potential porphyry mineralization in the Eastern Tianshan district, the CA method (Figs. 5 and 6) (Cohen, 1981; De Ritis et al., 2010; von Frese et al., 1997a, 1997b; Zhang et al., 2014) was employed. The correlation coefficients between the 10 km upward continued Bouguer gravity and aeromagnetic data at a corresponding scale of 30 km were calculated and they are shown in Fig. 7. The positive correlation anomalies (CC = 1) predominantly are present in the north part of the Eastern Tianshan belt, generally along the KanguertagHuangshan fault. These positive correlation anomalies mainly reflect volcanic structures and intrusive plutonic rocks. The negative correlation anomalies (CC = −1) mainly are present in the northeastern and northernmost part of the Eastern Tianshan belt and they mostly mark sediment-filled structural basins related to the coverage of Gobi-desert. There exist five prominent positive correlation anomaly zones in the southern margin of Dananhu-Tousuquan arc, which have been circled in the Fig. 7. Almost all the known porphyry Cu-Mo polymetallic deposits, particularly those in the Tuwu-Yandong mineralization belt, are spatially associated with these circled anomaly zones, where the deposits are found to be located at the inside or near the edges. Furthermore, the radiometric ages indicate that, close to the prominent positive correlation anomaly zones, late Carboniferous to late Permian intrusive rocks could be found. This anomaly pattern probably suggests that there likely exists a deep, higher density and magnetization magma source along the prominent positive correlation anomaly belt, which is likely to be related to porphyry Cu-Mo polymetallic mineralization associated hydrothermal systems. To further validate the geological significance of the prominent positive correlation anomaly zones, the porphyry Cu-Mo mineralization associated geochemical anomaly in the same region from our previous study (Xiao et al., 2014) has been considered. The geochemical anomaly map has shown that, in the south margin of Dananhu-Tousuquan arc (i.e. along the Kanguertag-Huangshan fault), there also exist several strong geochemical anomaly zones, which are

spatially associated with the prominent positive correlation anomaly zones and known porphyry Cu-Mo polymetallic deposits. As a result, the circled positive correlation anomaly zones in the southern margin of Dananhu-Tousuquan arc could be interpreted to reflect a late Carboniferous to late Permian, subduction-related oxidized magmatic belt that is favorable for additional discoveries of late Carboniferous to late Permian porphyry copper systems in north region of Eastern Tianshan. In this study, the patterns of Bouguer gravity and magnetic anomalies and the geological features with occurrences of porphyry mineralization in the Eastern Tianshan that are extensively covered by Gobidesert, have been considered in order to chart a strategy for exploration for potential porphyry-style deposits similar to the Tuwu and Yandong superlarge porphyry Cu-Mo polymetallic mines. Although it seems to be not so easy to directly delineate possible buried mineralized porphyries using regional gravity and magnetic data, judicious interpretation of the geophysical anomalies based on geology and tectonics, may suggest favorable zones for porphyry Cu-Mo polymetallic deposits exploration. Analysis of regional Bouguer gravity and aeromagnetic data indicates that the Tuwu and Yandong deposits are located in a steep gradient gravity and magnetic anomaly zone, within a cluster of high- to moderate- magnetic and moderate- to low- gravity anomalies, which are concentrated along the crustal scale Kanguertag-Huangshan fault. Upward continuation analysis of these anomalies suggests that they are associated with relatively deep gravity and magnetic sources and the porphyry hydrothermal systems are located on the flanks of the anomalies. The upward continuation anomalies are thus interpreted to reflect a mainly buried, late Carboniferous to late Permian magmatic arc that is generally parallel to the southern margin of Dananhu-Tousuquan arc belt. The correlation analysis method has been applied to the 10 km upward continuation signatures of both gravity and magnetic data for comprehensively analyzing the two geopotential fields, and it has been proved to be a useful approach for minimizing interpretational ambiguities of Bouguer gravity and aeromagnetic data. The delineated prominent positive correlation anomaly zones may represent late Carboniferous to late Permian, subduction-related oxidized magmatism prior to landward arc migration where the intrusions become more reduced in character. These anomaly zones are favorable for additional discoveries of late Carboniferous to late Permian porphyry copper systems in north region of Eastern Tianshan.

Acknowledgments Thanks are due to Prof Franco Pirajno and two anonymous reviewers for their constructive comments and suggestions. This research was financially supported by the National Natural Science Foundation of China (No. 41502310) and the Guandong Natural Science Foundation (No. 2015A030310246).

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