Journal of Earth Science  2019, Vol. 30 Issue (2): 296-308   PDF    
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Geochemistry and Geochronology of the Gebunongba Iron Polymetallic Deposit in the Gangdese Belt, Tibet
Gao Shunbao 1, Zheng Youye 2,3, Jiang Junsheng 3, Tian Kan 3, Xu Jin 3     
1. Institute of Geological Survey, China University of Geosciences, Wuhan 430074, China;
2. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;
3. Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China
ABSTRACT: The Gebunongba iron polymetallic deposit is a typical skarn deposit located at the westernmost end of the discovered iron polymetallic deposits in the Gangdese metallogenic belt. Major and trace elements of the monzonite granite related to mineralization show that these rocks are peraluminous (ACNK=1.15-1.21) and are rich in Si (76.63 wt.%-76.93 wt.% SiO2), K (K2O/Na2O ratio of 1.80-2.30), LREE and LILEs (Rb, Ba, Th, U, Pb), but is depleted in high field strength elements (Nb, Ta, Ti and HREE). In addition, these rocks show obviously negative Eu anomalies (δEu=0.48-0.55). The LA-ICPMS U-Pb age of zircons in monzonite granite is 59.72±0.55 Ma (MSWD=0.79), whereas the 40Ar/39Ar age of muscovite in iron ores is 59.22±0.61 Ma (MSWD=16.20). This indicates that the deposit formed at the syn-collision stage of Lhasa-India terrane is later than the northward subduction of the Yajiang crust. The monzonite granite has been probably derived from the partial melting of ancient lower crustal materials, which is probably resulted from the underplating of mantle-derived magmas. It is favorable for the formation of iron polymetallic deposit. Iron polymetallic mineralization is prevalent in Gangdese metallogenic belt at syn-collision stage. Therefore, syn-collision stage is an important mineralization stage for iron polymetallic deposits. The results of this study proved that iron polymetallic mineralization still took place in the western segment of Gangdese metallogenic belt and provided basis for further prospecting the deposits of the same type.
KEY WORDS: western segment of the Gangdese belt    iron polymetallic deposit    geochronology    metallogenic environment    
0 INTRODUCTION

The Gangdese metallogenic belt is a very important area hosting copper-molybdenum, lead, zinc, iron and gold deposits. This belt is located in southern Lhasa terrane and structurally belongs to southern Gangdese volcanic arc. Since the beginning of the 21st century, numerous large and super-large deposits, such as Qulong, Jiama, Xiongcun, Zhunuo, Chongjiang, Tinggong, Cuibaizi, Demingding, Bangpu, Jiru, Pusangguo, Sinongduo and Narusongduo deposits, have been discovered in the Gangdese metallogenic belt due to the implementation of large-scale land and resources investigation project and the subsequent input of commercial prospection funds. These deposits are dominated by porphyry deposits, followed by skarn, volcanic hydrothermal and magmatic hydrothermal deposits. The metals are dominated by copper and molybdenum, followed by lead, zinc, iron and gold (Li et al., 2018; Meng et al., 2018; Ma et al., 2017; Tang et al., 2013, 2010, 2007; Li et al., 2010, 2003; Zheng et al., 2007, 2006a, b, 2004a, b, 2002; Li and Rui, 2004). These deposits mostly formed in Miocene, when the Brahmaputra oceanic basin subducted northward. Some deposits formed in Middle Jurassic and Eocene, when continental-continental collision between India and Lhasa terrane and subsequent post-collision extension occurred (Zhang et al., 2013, 2008; Tang et al., 2010; Wang et al., 2006; Zheng et al., 2006a, b; Lin et al., 2004; Rui et al., 2003).

In the Gangdese metallogenic belt, there are porphyry copper polymetallic deposits, skarn copper polymetallic deposits, volcanic hydrothermal lead-zinc-silver deposits, magmatic hydrothermal copper-zinc-lead deposits, and iron polymetallic deposits. The iron deposits are mainly distributed in central and northern part of the metallogenic belt, including Qiagong, Jiaduobule, Niangre, Jialapu, Leqingla and Lietinggang (Fig. 1). Some researchers have studied the metallogenic characteristics, genesis and formation age of these deposits (spots) and considered that they were skarn deposits formed in Late Cretaceous to Paleocene (50-68 Ma), with dominant metals of iron, associated with lead, zinc, copper, molybdenum (Yang et al., 2014; Fu et al., 2013; Gao et al., 2012; Li et al., 2011; Yu et al., 2011a, b). The formation of these deposits is later than the formation of skarn iron deposits in the Longgeer-Cuoqin area in the northern Gangdese metallogenic belt, which were formed at ~115 Ma (Fei et al., 2015).

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Figure 1. Distribution of the major iron deposits in the Gangdese area, Tibet (modified from Zheng et al., 2014). The ore deposits with serial number in the map: (1) Chajiasi Fe Deposit; (2) Gebunongba Fe Deposit; (3) Longgeer Fe Deposit; (4) Bangbule Pb-Zn-Cu-Fe Deposit; (5) Nixiong Fe Deposit; (6) Chagele Pb-Zn-Cu-Mo Deposit; (7) Qiagong Fe Deposit; (8) Jiaduobule Fe Deposit; (9) Shinongduo Pb-Zn-Ag Deposit; (10) Narusongduo Pb-Zn-Ag Deposit; (11) Lietinggang Fe Deposit; (12) Leqingla Fe Deposit; (13) Jialapu Fe Deposit; (14) Zhunuo Cu-Mo Deposit; (15) Xiongcun Cu-Au Deposit; (16) Jiru Cu-Mo Deposit; (17) Pusangguo Cu-Pb-Zn Deposit; (18) Chongjiang Cu Deposit; (19) Tinggong Cu Deposit; (20) Niangre Fe Deposit; (21) Duodigou Fe Deposit; (22) Qulong Cu-Mo Deposit; (23) Jiama Cu-Mo-Pb-Zn Deposit; (24) Bangpu Cu-Mo Deposit; (25) Demingding Mo-Cu Deposit; (26) Cuibaizi Cu-Mo Deposit.

Since Pre-Paleogene strata (especially the Jurassic-Cretaceous limestone related to mineralization in the east) are absent in a large area, there are a few skarn iron polymetallic deposits in the west of the Xietongmen County. However, at the westernmost end of the southern Gangdese volcanic arc, Cretaceous limestone is deposited due to various sedimentary environments. Moreover, a large number of iron polymetallic deposits (spots) have been discovered in this area in recent years (Zhang et al., 2006), indicating a greater potential for prospecting iron polymetallic deposits than the eastern segment of Gangdese. However, there was little study on the metallogenic time of iron polymetallic deposits. Therefore, it is still unclear whether the iron polymetallic deposits discovered in the western and the eastern segments of Gangdese are formed at the same time and whether the iron polymetallic deposits with an age of about 50-60 Ma can be discovered in the western segment of Gangdese.

Therefore, in this study, we perform detailed geological field work and the systematics petrology and mineralogy study on the largest iron polymetallic deposit (Gebunongba Deposit) in the western Gangdese. We report the whole rock geochemistry and zircon U-Pb data of the ore-related intrusions in order to constrain the ages and petrogenesis of ore-related granite, and report the muscovite 40Ar/39Ar ages in order to address the ages of mineralization and metallogenic environment in the Gebunongba Deposit, and further provide some new evidences to discuss the mineralization regularity in the western Gangdese.

1 METALLOGENIC GEOLOGICAL BACKGROUND

The Gebunongba iron polymetallic deposit is located in the southern of Geji County in the Ali area, Tibet. It structurally belongs to the western part of the southern Gangdese volcanic-magmatic arc. The Jurassic-Paleogene volcanic-sedimentary strata are outcropped, which are the Zenong Group, Jiega Formation and Linzizong Group. Upper Jurassic to Lower Cretaceous strata are composed of purple and gray conglomerates, sandstones, siltstones, claystones and intermediate-acid pyroclastic rocks and formed in shore shallow sea-transitional environment with intense arc volcanic activities. The Lower Cretaceous Jiega Formation is composed of limestone, bioclastic limestone and fine clastic rocks interbeded with basalt and mudstone and formed in the shore shallow sea environment. The Linzizong Group is dominated by intermediate-acid volcanic rocks, interbeded with pyroclastic sedimentary rocks, which is a set of volcanic rocks formed in the continental environment in collision period. Intrusive rocks dominated by intermediate-acid rocks can be found as batholite in the south and stock in the north. Their main emplacement age is Cretaceous, followed by Paleocene-Eocene. Regional structure is mainly of NW-trending. Fault structures are extremely developed in the vicinity of Longboer-Gongbujiangda arc-back fault belt. The boundary faults (Gar-Lunggar-Cuoqin fault) are mainly south-dipping thrust faults, with long-term activities (Pan et al., 2006).

2 DEPOSIT GEOLOGY

The outcropped geologic bodies in the mining area are complex. The volcanic rocks of Upper Jurassic-Lower Cretaceous Zenong Group, limestones of Lower Cretaceous Jiegar Formation and volcanic rocks of Paleocene Dianzhong Formation are outcropped in the southwestern and northeastern part (Fig. 2). Medium-coarse-grained porphyritic monzonite granite, medium-fine-grained porphyritic monzonite granite, medium-fine-grained porphyritic moyite, gabbro-diorite, dioritic porphyrite, quartz diorite, coloradoite, dioritic porphyrite veins, granite porphyry and beschtauite are outcropped in the central-southern part and northeastern part, among which, medium-fine-grained porphyritic monzonite granite, gabbro-diorite, dioritic porphyrite, and quartz diorite intrude the Jiega Formation limestone. There is a clear genesis association between the ores and monzonite granite. Ore bodies are dominantly hosted in the contact zones between the limestone and monzonite granite. As a result, different degrees of skarnization occurs in contact zone and high-grade iron polymetallic ore bodies are formed. Fold and fault structures are well developed in the mining area. Fold structures represent as NW-trending anticlines and fault structures consist of NW, nearly NS and NE-trending faults.

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Figure 2. Geological map of Gebunongba Deposit. Q. Quaternary; E1-2d. volcanic rocks of Dianzhong Formation; K1jg. limestone of Jiega Formation; J3k1z. volcanic rocks of Zenong Group; mb. marble; γπ. granite porphyry; πηοδ. porphyritic quartz monzodiorite; δο. quartz drorite; ξoλ. quartz-orthophyre; πhg. monzonitic granite; qλ. rhyolite; qλτα. coloradoite; ξoπ. quartz orthophyre veins; δμ. dioritic porphyrite veins; γπ. granite porphyry veins.

According to the results of prospection and field observations, there are many ore bodies in the mining area, the vast majority of which are iron ore bodies, followed by copper-lead-zinc ore bodies (Figs. 2, 3). Most ore bodies (mineralized bodies) are discontinuously outcropped and have zonations in the mining area. The ore bodies are different in scales, with length ranging from several meters to hundreds of meters and width of several meters to tens of meters, and the ore bodies (mineralized bodies) show layered, vein, lenticular and cystic shape, most of which have consistent attitude with surrounding rocks. Ores are dominated compact massive, disseminated, mottled or vein structures, minor brecciated, vein-like, thin film and crusty structures, and the ore textures are mainly presented as metasomatic relict, reticulated, stockwork, automorphic-hypautomorphic granular and allotriomorphic granular. Ores are mainly composed of magnetite and chalcopyrite, with minor sphalerite, galena, pyrite, malachite, hematite, limonite, garnet, pyroxene, actinolite, epidote and calcite, minor quartz, chlorite, muscovite and biotite as gangue minerals. With the monzonite granite as the center outward form complex alteration zoning (garnet to pyroxene-actinolite-epidote) and mineralization zoning (magnetite-chalcopyrite to sphalerite-galena).

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Figure 3. Typical characteristics of intrusive rocks and ores in Gebunongba Deposit. (a) Outcrop photos of monzonite granite; (b) microscope photographs of monzonite granite; (c) dense massive magnetite ore; (d) vein magnetite ore; (e) skarn iron-lead-zinc ore; (f) magnetite formed in actinolite skarn; (g)-(h) galena replaced sphalerite, sphalerite replaced hematite.
3 SAMPLE COLLECTION AND ANALYTICAL METHODS

In this paper, the major elements, rare earth, and trace elements in three medium-fine-grained monzonite granite samples related with mineralization were analyzed, the LA-ICP-MS U-Pb dating of the zircon in one medium-fine-grained monzonite granite sample (GB-11-1) and the Ar-Ar dating of one muscovite sample (GB-9-1) was performed. The fresh, less-tectonic thermal disturbed samples of monzonitic granite were collected from the intrusion, which is directly contact with magnetite ore bodies in the southeastern of the Gebunongba Deposit. The samples of monzonitic granite, offwhite or yellow-white in color, show a massive structure and medium-fine grained granitic texture, mainly consist of quartz, plagioclase, potassium feldspar, and minor biotite (Fig. 3). The subhedral to anhedral quartz grains, accounting for about 30% in volume, are colorless and transparent, with the 2-4 mm grain sizes. The subhedral columnar potassium feldspargrains, accounting for about 25% in volume, are shallow fleshy red in color, with 3-5 mm grain sizes and minor Carlsbad twin. The white or offwhite plagioclase grains, accounting for about 40% in volume, are subhedral columnar, tabular and short columnar, with 3-5 mm grain sizes. The mousy, scalybiotite, accounting for about 4% in volume, are pleochroism, with 0.1-1.0 mm grain sizes. The muscovite sample (GB-9-1) for Ar-Ar dating was collected from the muscovite-actinolite-epidote-bearing iron ores from the southeastern part of Gebunongba Deposit.

All the major elements and rare earth trace elements were analyzed in Wuhan Rock and Mineral Comprehensive Testing Center. The major elements were analyzed by X-ray fluorescence spectroscopy (XRF) and the rare earth and trace elements were analyzed by inductively coupled plasma emission spectrometer (ICP-AES).

Zircon sorting was completed in the Research Institute of Regional Geology and Mineral Resources, Langfang, Hebei Province. When zircon was made as test target, cathodoluminescence (CL) photography was firstly completed in National Ion Probe Center, Chinese Academy of Geological Sciences. The LA-ICPMS U-Pb isotopic analyses of zircon were performed using standard test procedures at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan). The laser ablation system is GeoLas 2005 and the ICP-MS is Agilent 7500a. During laser ablation process, helium was used as carrier gas and argon was used as compensated gas in order to adjust the sensitivity. The two types of gas were mixed through a T-connector before entering the ICP. A small amount of nitrogen is added to plasma center gas stream (Ar+He) to improve instrument sensitivity, reduce detection limit, and improve analytical precision (Hu et al., 2008). In addition, the laser ablation system was equipped with a signal smoothing device to obtain a smooth analytical signal even if the laser pulse frequency is as low as 1 Hz (Hu et al., 2012). The resolution analytical data of each time interval includes about 20-30 s blank signals and 50 s sample signals. The off-line processing of analytical data (including the selection of sample and blank signals, instrument sensitivity drift correction, elemental content and U-Th-Pb isotope ratios and age calculations) was performed using ICPMS DataCal software (Liu et al., 2008a). Detailed instrument operating conditions and data processing methods are shown in Liu et al.(2010, 2009, 2008a, b).

Muscovite sorting was completed using binocular. The purity of samples is greater than 98%. Samples were sent to Scientific Research Institute of Atomic Energy to exposure for 90 h. The 40Ar-39Ar dating of muscovite was performed by using laser melting stage heating method on the GV5400 mass spectrometer in the Key Laboratory of Isotopic Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. The test methods and detailed experimental procedure were described in the paper of Qiu (2006) and Jiang et al. (2006). Age calculation and mapping were completed using Ar-Ar data processing software, and ArARCALCohol (Version 2.2c) (Koppers, 2002).

4 RESULTS 4.1 Geochemistry of Metallogenic Rocks

The major elements, rare earth and trace elements of the three medium-fine-grained monzonite granite samples collected from Gebunongba are shown in Table 1.

Table 1 Major elements (wt.%) and trace elements (10-6) data of monzonitic granite in Gebunongba Deposit

Medium-fine-grained monzonite granite has SiO2 content of 76.63 wt.%-76.93 wt.%, Al2O3 content of 12.53 wt.%-12.69 wt.%, MgO content of 0.14 wt.%-0.15 wt.%, Na2O content of 2.42 wt.%-2.83 wt.% and K2O content of 5.03 wt.%-5.56 wt.%. It is enriched in K and belongs to cal-alkalic rock, with K2O/Na2O of 1.80-2.30, Ritman Index (σ) of 1.79-1.89 (Fig. 4a) and aluminum saturation index (ANCK) of 1.15-1.21. Corundum molecules (C) appear in all the calculated results of CIPW standard minerals and fall within the peraluminous region in ACNK-ANK plot (Fig. 4b), which is similar to the metallogenic rocks with an age of 53-65 Ma in the Gangdese metallogenic belt. The content of corundum molecules ranges between 1.76% and 2.31%.

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Figure 4. Classification diagrams of monzonitic granite in Gebunongba Deposit. (a) Base map refers to Peccerillo and Taylor (1976); (b) base map of ANK-ACNK diagram refers to Maniar and Piccoli (1989). The data of the metallogenic rocks of 55-65 Ma is the data of Qiagong, Jiaduobule and Jialabu deposits, which is quoted from the unpublished data of the authors and Fu et al. (2013). The data of the metallogenic rock mass of ±115 Ma are the data of Xiongni Deposit, which is quoted from the unpublished data of the authors and Yu et al.(2011a, b).

The ΣREE ranges between 106×10-6 and 125×10-6 and the LREE/HREE ratio ranges between 5.24 and 7.55, indicative of LREE enrichment with right-dipping pattern (Fig. 5a). δEu=0.48-0.55, indicating an obviously negative Eu anomaly. (La/Sm)N is 3.43-4.35, indicating that light rare earth elements are highly fractionated. (Gd/Yb)N is 0.74-1.06, indicating weak fractionation in heavy rare earth elements.

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Figure 5. REE distribution curve and trace element spider diagrams of the monzonitic granite in Gebunongba Deposit. The chondrite and primitive mantle normalized values are quoted from Sun and McDonough (1989). The data of the metallogenic rocks of 55-65 Ma are the data of Qiagong, Jiaduobule and Jialabu deposits, which are quoted from the unpublished data of the authors and Fu et al. (2013). The data of the metallogenic rocks of ±115 Ma are the data of Xiongni Deposit, which are quoted from the unpublished data of the authors and Yu et al.(2011a, b).

The rocks are enriched in large ion lithophile elements (LILE), such as trace elements Rb, Ba, Pb, Th and K, and depleted in high field strength elements (HFSE), such as Nb, Ta and Ti (Fig. 5b).

4.2 LA-ICPMS U-Pb Age of the Zircon in Metallogenic Rocks

The cathodoluminescence photograph of zircon (Fig. 6) shows that the zircon crystal in the monzonite granite associated with mineralization represents automorphic short and long columnar morphology, with a length of 85-210 μm and a length-width ratio of about 2 : 1-3 : 1. The growth rings in zircon crystals are well developed and can be clearly seen, indicating the zircons are typical zircons with magmatic genesis (Wu and Zheng, 2004; Crofu et al., 2003).

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Figure 6. CL images of zircons (a) and U-Pb concordancy diagram (b) of monzonitic granite in Gebunongba Deposit.

U-Pb dating results (Table 2) show that in the zircon of monzonite granite, the content of Pb ranges between 11.4×10-6 and 35.2×10-6, the content of Th ranges between 221×10-6 and 836×10-6, and the content of U ranges between 318×10-6 and 587×10-6. Th/U ratio ranges between 0.55 and 1.43, averaging on 0.79. All the zircon samples are included in the calculation of weighted average age except the three zircon samples with low concordant test results. The 206Pb/238U age that is corrected by isotopic ratios ranges from 58.20 to 67.77 Ma, mainly between 58 and 60 Ma and rarely more than 60 Ma. The weighted average age is 59.72±0.55 Ma (MSWD=0.79) (Fig. 6), indicating the crystallization age of monzonite granite is Paleocene.

Table 2 U-Pb LA-ICP-MS analyses data of the zircon from monzonitic granite in Gebunongba Deposit
4.3 Muscovite Ar-Ar Age

The 40Ar/39Ar isotope analyses results of the muscovite in Gebunongba are shown in Table 3 and Fig. 7. The obtained Ar-Ar age spectrum was flat and the plateau age is 59.22±0.61 Ma (MSWD=16.20). The isochron age is 59.32±0.72 Ma (MSWD=14.97), which is consistent with plateau age, indicating the 40Ar/39Ar age of the muscovite samples in Gebunongba is reliable. Plateau age represents the formation time of the muscovite in Gebunongba Deposit.

Table 3 40Ar/39Ar dating analytical results of the muscovite from Gebunongba Deposit
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Figure 7. 40Ar/39Ar plateau age (a) and isochron age (b) of muscovite from Gebunongba Deposit.
5 DISCUSSIONS 5.1 Ages and Petrogenesis of Ore-Related Granite

The LA-ICPMS U-Pb age of the zircon in the monzonite granite associated with the iron-polymetallic mineralization in Gebunongba Deposit is 59.72±0.55 Ma (MSWD=0.79) indicating that the monzonite granite was formed at Paleocene.

The monzonite granite in the Gebunongba Deposit is obviously rich in silicon, potassium, peraluminous, LREE and LILE, but depleted in HFSE. It shows a distinct negative Eu anomaly. These geochemical characteristics are very similar to those of other skarn iron polymetallic deposits formed during 53-65 Ma in the Gangdese metallogenic belt (Figs. 4, 5) and consistent with those of the collision granite defined by Pearce et al. (1984).

The K2O/Na2O ratio of the monzonite granite in the Gebunongba Deposit is significantly higher than 1 (1.80-2.30) and the aluminum saturation index (ANCK) is 1.15-1.21, indicating that crust-derived materials are partial melting products (Chappell and White, 1992). The Th/U ratio of trace elements is 12.15-14.82, significantly higher than that of the lower crust (6.00, Rudnick and Gao, 2003). The Nb/Ta ratio is 7.13-8.83, closed to that of the lower crust (8.3, Rudnick and Gao, 2003). The Nb/Ta ratio is 9.62-17.45, close to that of lower crust (13, Rudnick and Gao, 2003). The above characteristics show that the monzonite granite in the Gebunongba Deposit may be derived from the partial melting of the lower crust. Based on the fact that there are many intermediate-basic intrusive rocks formed at the same time in Gebunongba Deposit including gabbro-diorite, dioritic porphyrite, and quartz diorite, we consider that the ore-related rocks in Gebunongba Deposit have been probably derived from the partial melting of ancient lower crustal materials, which is probably resulted from the underplating of mantle-derived magmas.

In Rb-(Y+Nb), Th/Yb-Ta/Yb, and Th/Ta-Yb diagrams, the monzonite granites in the Gebunongba Deposit fall in the same syn-collisional area with the ore-related rocks of other skarn iron polymetallic deposits formed during 65-53 Ma in the Gangdese metallogenic belt. These rocks are formed in the active continental margin, different from the ore-relate intrusions formed at Early Cretaceous (±115 Ma) in skarn iron deposits (Fig. 8). The structural location and regional tectonic evolution show that the Bangong Lake-Nujiang River oceanic basin in the north of the Lhasa terrane was closed before 59 Ma in a post-collision environment. The Brahmaputra oceanic basin in the southern side was closed at about 65 Ma. However, the syn-collision of India-Eurasia occurred between 65 and 40/45 Ma, lasting for about 20 Ma (Mo and Pan, 2006). Therefore, the formation of the Gebunongba Deposit may be closely related with the magmatic activities occurred at the syn-collision stage of Lhasa-India terrane after the closure of Brahmaputra oceanic basin in the southern side.

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Figure 8. Rb-(Y+Nb) diagram (base map refers to Pearce, 1996), Th/Yb-Ta/Yb diagram (base map refers to Pearce and Peate, 1995) and Th/Ta-Yb diagram (base map is from Gorton and Schandl, 2000) of the monzonitic granite in Gebunongba Deposit. The data of the metallogenic rocks of 55-65 Ma is the data of Qiagong, Jiaduobule and Jialabu deposits, which is quoted from the unpublished data of the author and Fu et al., 2013. The data of the metallogenic rocks of ±115 Ma is the data of Xiongni Deposit, which is quoted from the unpublished data of the author and Yu et al., 2011a.
5.2 Ages of Mineralization

The 40Ar/39Ar plateau age of the muscovite in iron ores is 59.22±0.61 Ma (MSWD=16.20). In the Gangdese copper polymetallic metallogenic belt, in addition to the Gebunongba Deposit developed in the western part, a large number of skarn iron polymetallic deposits in central-eastern part, include Qiagong, Jiaduobule, Niangre, Jialapu, Leqingla, and Lietinggang. Iron polymetallic deposits in the Xietongmen-Lhasa area of the Gangdese metallogenic belt were mainly formed during Late Cretaceous-Paleocene, peaking at 50-68 Ma (Yang et al., 2014; Fu et al., 2013; Li et al., 2011; Yu et al., 2011a, b). The rock and mineralization ages of the Gebunongba Deposit are about 59 Ma, consistent with the ages of other iron polymetallic deposits in the east of the Gangdese metallogenic belt.

5.3 Origin of the Skarn Deposits in Gangdese Belt

There are two types of intrusions in the Gangdese belt, corresponding to different types of mineralization. The small-scaled quartz porphyry and (monzonitic) granite porphyry are related to the skarn lead-zinc deposits formed during Late Cretaceous- Paleocene in the Gangdese metallogenic belt (such as Sinongduo, Chagele, Longgen, Mengyae deposits), while the ore-related rocks of the skarn iron polymetallic deposits (such as Gebunongba, Qiagong, Jiaduobule, Lietinggang and Jialapu) are mainly granodiorite, (biotite) monzonite granite and coarse-grained granite. The two types of rocks are formed in similar tectonic environment and are both related to the syn-collision magmatism after the closure of the Brahmaputra oceanic basin. However, there are some differences on the magmatic source areas of the two types of ore-related intrusive rocks in the Gangdese metallogenic belt. The ore-related intrusive rocks in the skarn Pb-Zn deposits tend to be more acid, and were mainly derived from the partial melting of ancient crust materials (Zheng et al., 2015; Duan et al., 2014; Fu et al., 2013; Wang et al., 2012). While these ore-related intrusive rocks in the skarn Fe-Cu deposits are typical of moderate-acidic that were dominantly related to the partial melting of lower crust induced by the invasion of mantle source materials, with a small amount of mantle source materials mixing into them (Yang et al., 2014; Li et al., 2011; Yu et al., 2011b). Fu et al. (2013) even suggested that the intrusive rocks associated with the Fe-Cu mineralization in the Gangdese metallogenic belt shows more contribution from mantle-derived materials, while the Pb-Zn-Ag mineralization was mainly controlled by the granite related to crust source material. These characteristics are basically consistent with the metallogenic specificity of the ore-forming rocks in the skarn type deposits (Zhao et al., 2017, 2012; Zhao, 2015).

In addition, the metallogenic rocks of the skarn iron polymetallic deposits in the Gangdese metallogenic belt have the characteristics of the intermediate-acidic rocks formed in syncollision environment. Moreover, in addition to iron minerals, there are many associated copper, lead, zinc and molybdenum minerals, all of which are ecnomic byproducts. The skarn iron polymetallic deposits in the Gangdese metallogenic belt are significantly different from the skarn iron deposits formed in island arc environment in Lunggar-Cuoqin in northern Gangdese metallogenic belt.

5.4 Implication for Regional Mineralization

Large quantities of mineral resources, such as copper, molybdenum, lead, zinc, gold, iron and silver, are discovered in the Gangdese metallogenic belt. Porphyry and skarn deposits are dominant, followed by volcanic hydrothermal, magmatic hydrothermal and epithermal deposits. In recent years, researchers have performed extensive study on the petrogenetic and metallogenic epoch and genesis of discovered deposits (Fu et al., 2015, 2013; Duan et al., 2014; Ji et al., 2014; Wang et al., 2014; Yang et al., 2014; Zhang et al., 2013; Li et al., 2011; Yu et al., 2011; Tang et al., 2010; Zheng et al., 2007; Li and Rui; 2004; Lin et al., 2004; Rui et al., 2003) and found that these deposits formed during Middle Jurassic-Miocene, especially in early Middle Jurassic (175-173 Ma), end Late Cretaceous-Eocene (70-50 Ma) and Miocene (20-15 Ma), which correspond to the early northward subduction stage of Brahmaputra oceanic basin, the end of the northward subduction stage of Brahmaputra oceanic basin and syn-collision stage of Lhasa-Himalaya terrane, and post-collision stage of Lhasa-India terrane, respectively.

Large amounts of metals are concentrated during these stages. During early Middle Jurassic time, large to super-large-scaled porphyry-epithermal copper-gold deposits (represented by the Xiongcun Deposit) were formed when the brahmaputra oceanic basin subducted northwards. During Miocene, large to super-large-scaled porphyry-skarn copper-molybdenum polymetallic deposits (represented by the Qulong and Jiama depoists) were formed at the post-collision stage at the Lhasa-India terrane. Previous prospecting work and research results suggest that the deposits formed at the end of the northward subduction stage have smaller scale than the two types of deposits mentioned above. However, in recent years, more and more deposits that are formed during syn-collision period are discovered and identified, making it a very important lead, zinc, iron and copper metallogenic event in the Gangdese metallogenic belt (Table 4, Fig. 9).

Table 4 Diagentic and metallogenic epoch of the major Late Cretaceous-Eocene deposits in Gangdese metallogenic belt
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Figure 9. Formation time of the major deposits of Late Cretaceous-Eocene in Gangdese metallogenic belt (the data is quoted from Table 4).

As shown in Table 4, the Mengya, Narusongduo, Sinongduo, Dexin, Chagele, Longgen, Qiagong, Jiaduobule, Jialapu, Lietinggang and Gebunongba deposits formed at this stage were distributed throughout the central-northern Gangdese metallogenic belt, with a length of over 1 000 km (W-E direction) and a width of about 80 km (N-S direction). Skarn lead-zinc deposits and volcanic hydrothermal-magmatic hydrothermal lead-zinc deposits were generally distributed in northern-central Gangdese metallogenic belt, and their structural units spanned the central-northern Gangdese volcanic arc and southern Longgeer-Gongbujiangda arc back fault uplift zone. While skarn iron polymetallic deposits were generally distributed in central-southern Gangdese metallogenic belt, and their structural units spanned the central-northern volcanic arc of southern Gangdese metallogenic belt. The metallogenic time of these deposits ranged between 48.3 and 68.2 Ma, especially between 55 and 63 Ma. The metallogenic time of skarn lead-zinc deposits ranged between 60.44 and 68.2 Ma (Sinongduo, Chagele, Longgen and Mengya deposits), the metallogenic time of skarn iron polymetallic deposits ranged between 50.9 and 63.4 Ma (Gebunongba, Qiagong, Jiaduobule, Lietinggang and Jialapu deposits) and the metallogenic time of volcanic hydrothermal-magmatic hydrothermal lead-zinc deposits ranged from 56.57 to 57.81 Ma (Narusongduo and Dexin deposits). In general, skarn lead-zinc deposits formed early, followed by skarn iron polymetallic deposits, and volcanic hydrothermal-magmatic hydrothermal lead-zinc deposits in the last. Previous study on the genesis and metallogenic environment of metallogenic rocks revealed that these deposits were formed in the syn-collision environment of Lhasa-Indian terrane at the end of the northward subduction period of Brahmaputra oceanic basin. The mineralization might be related with the partial melting of ancient crustal materials in Lhasa terrane. The mantle-derived magma underplating was generated by the post-arc extension and collision of continental margin (Yang et al., 2015; Duan et al., 2014; Wang et al., 2012). At the same time, highly variable Hf isotopic compositions of zircon revealed that the metallogenic rocks resulted from the mixture of mantle- and crust-dericed materials (Fu et al., 2013; Yu et al., 2011a). Based on the same metallogenic characteristics, geochemical characteristics of ore-related intrusive rocks, ore-forming ages, and metallogenic dynamics with other iron-polymetallic deposits (such as Qiagong, Jiaduobule, Letinggang, Jialapu) in the Gangdese belt, we proposed that the ore-related intrusive rocks of Gebunongba Deposit possibly have the similar deep magmatic process with those of iron-polymetallic deposits in this metallogenic belt.

The large and medium-scaled lead-zinc-iron copper deposits formed during syn-collision period are widespread in the Gangdese metallogenic belt, indicating syn-collision period is a very important polymetallic metallogenic stage. The Gebunongba Deposit is located at the westernmost end of the discovered iron polymetallic deposits in the Gangdese metallogenic belt. Its metallogenic characteristics, metallogenic geological background and metallogenic time are very similar to other deposits in the belt, indicating iron polymetallic mineralization is similar in the whole Gangdese metallogenic belt. More importantly, compared with the iron deposits previously discovered in western and northern Gangdese metallogenic belt (Nixiong and Longgeer deposits), the Gebunongba Deposit is associated with lead-zinc-copper mineralization, which is more suitable for development and utilization due to higher economic value.

6 CONCLUSIONS

(1) The monzonite granites at Gebunongba is peraluminous and is rich in Si, K, LREE and LILEs, but is depleted in HFSE and HREE, with obviously Eu negative anomalies (δEu=0.48-0.55). It is probably derived from the partial melting of ancient lower crustal materials triggered by underplating of mantle-derived magmas.

(2) The age of monzonite granites is 59.72±0.55 Ma and the 40Ar/39Ar plateau age of the muscovite in iron ores is 59.22±0.61 Ma, indicating that both the emplacement of intrusive and timing of mineralization are Paleocene.

(3) The formation of the Gebunongba Deposit is related to the magmatic activities occurred during the syn-collision period of Lhasa-Indian terrane after the closure of Brahmaputra oceanic basin, sharing similar feature of other deposits in Gangdese metallogenic belt.

(4) Gebunongba Deposit has similar metallogenic time and environment with the other iron-polymetallic deposits in eastern Gangdese metallogenic belt. The discovery of the Gebunongba Deposit conforms that the iron polymetallic mineralization of Gangdese metallogenic belt extends westward greatly, which provides a basis for continuing prospecting the same type deposits in the western of Gangdese metallogenic belt.

ACKNOWLEDGMENTS

This work was jointly funded by the National Key Research and Development Program (No. 2016YFC0600300), the Fundamental Scientific Research Fund for Central Universities, Changjiang Scholars Program and Innovation Team Development Plan (No. IRT1083). We thank anonymous reviewers for their constructive comments. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0984-0.


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