A Downgoing Indian Lithosphere Control on Along-Strike Variability of Porphyry Mineralization in the Gangdese Belt of Southern Tibet

Porphyry-type mineralization above subduction zones generally shows some variability of mineralization styles and ages and geochemistry of mineralizing intrusions along and across strike of magmatic arcs (Kay et al., 1999; Hollings et al., 2005; Sillitoe and Perelló, 2005; Jones et al., 2015; Angerer et al., 2018). However, it is not clear whether such variability occurs in continental collision zones. Many porphyry copper deposits have been recognized in the continental collision zones of the Tethys orogen, including the Gangdese belt in southern Tibet, the Chagai belt in Pakistan, and the Arasbaran-Kerman belt in Iran (Hou and Zhang, 2015). Variability of collision-related porphyry copper deposits along or across these orogens has been poorly understood. In this study, we focus on porphyry mineralization in the Gangdese belt. It was recently noted that the Gangdese porphyries exhibit distinct across-strike metallogenic variability characterized by porphyry Cu mineralization in the southern Lhasa subterrane and porphyry Mo mineralization in the central Lhasa subterrane (Fig. 1B; Sun et al., 2017b). However, it is unclear whether there is along-strike variability of porphyry copper mineralization in the Gangdese belt, despite many studies providing geochronological and geochemical data for ore-related intrusions and associated mineralization (Hou et al., 2004, 2015a; Li et al., 2011; Leng et al., 2013; Lu et al., 2015; Wang et al., 2015; Yang et al., 2015, 2016a; Sun et al., 2017a, 2018).

Here, we present new zircon U-Pb and Hf-O and bulkrock Sr-Nd isotope data for the Miocene ore-related intrusions from five Gangdese porphyry copper deposits (Qulong, Lakange, Chongjiang, Jiru, Zhunuo) and Eocene felsic and gabbroic rocks from the western Gangdese belt. Combined with literature data, our new results enable recognition of the contrasting isotopic characteristics of porphyry copper mineralization along the strike of the Gangdese belt, which has important implications for exploration of collision-related porphyry copper deposits.

The Tibetan Plateau consists primarily of four terranes; from north to south, they are the Songpan-Ganzi, Qiangtang, Lhasa, and Himalaya terranes (Fig. 1A). The Lhasa terrane is bounded by the Bangong-Nujiang suture zone to the north and by the Indus-Yarlung Zangbo suture zone to the south. The Lhasa terrane is composed of northern, central, and southern subterranes, separated by the Shiquanhe-Nam Tso mélange zone and Luobadui-Milashan fault, respectively (Yin and Harrison, 2000; Zhu et al., 2011; Deng et al., 2014). The Gangdese belt in the southern Lhasa subterrane experienced northward-directed subduction of the Neo-Tethyan oceanic plate from the Middle Triassic to Paleocene, after which the Indo-Asian continental collision occurred at ~65 to 50 Ma (Ding et al., 2005; Leech et al., 2005; Replumaz et al., 2010; Wu, F.Y., et al., 2014; Hu, X.M., et al., 2015, 2016; Zhu et al., 2015). The Middle Triassic to Eocene Gangdese batholiths, Jurassic-Cretaceous volcanic rocks, and ~60 to 52 Ma Linzizong volcanic rocks were emplaced over a large area in the southern and central Lhasa subterranes (Fig. 1B; Mo et al., 2008; Lee et al., 2009; Zhu et al., 2015). Adakite-like Oligocene to Miocene (33–13 Ma) intrusions mainly occur in the southern Lhasa subterrane (e.g., Hou et al., 2004, 2015b; Yang et al., 2016a). Miocene (27–10 Ma) ultrapotassic volcanic rocks were emplaced in the central Lhasa subterrane and to a lesser extent in the southern Lhasa subterrane (Miller et al., 1999; Zhao et al., 2009; Liu et al., 2014; Sun et al., 2018). These Oligocene to Miocene igneous rocks formed in response to slab breakoff of the Indian continent or convective removal of lithospheric mantle (Chung et al., 1998; Miller et al., 1999; Williams et al., 2004; Hou et al., 2015a).

Many Miocene adakite-like intrusions are spatially and temporally associated with porphyry copper deposits in the 50-km-wide, 700-km-long E-trending Gangdese porphyry Cu belt (Fig. 1B). This belt, which is located in the south of the Lhasa subterrane, includes the giant Qulong deposit and many smaller deposits such as Zhunuo, Dabu, Chongjiang, Lakange, and Jiru (Fig. 1B). The geology, hydrothermal alteration, and mineralization features of these six deposits are summarized below (Table 1).

The Qulong porphyry Cu-Mo deposit is situated ~50 km east of Lhasa (Fig. 1) and has a resource of 2,200 million tonnes (Mt) ore at average grades of 0.5% Cu and 0.04% Mo (Qin et al., 2014). It is associated with the Miocene Rongmucuola pluton, which varies from granodiorite to monzogranite in composition and has zircon U-Pb ages of 19.5 ± 0.4 to 16.4 ± 0.4 Ma (Wang et al., 2006; Yang, 2008). The pluton intruded the Early Jurassic Yeba Formation volcanic-sedimentary rocks and Jurassic granite porphyry (zircon U-Pb age of 182.3 ± 1.5 Ma; Yang et al., 2009). An intermineralization monzogranite porphyry with zircon U-Pb ages of 17.6 ± 0.7 to 16.2 ± 0.3 Ma intruded the Rongmucuola pluton (Hou et al., 2004; Zhao et al., 2016), and a postmineralization high-Mg diorite porphyry with zircon U-Pb ages of 15.7 ± 0.2 to 15.3 ± 0.3 Ma crosscut the Rongmucuola pluton and monzogranite porphyry (Yang et al., 2015; Zhao et al., 2016). Paleocene monzogranite (64.6 ± 2.5 Ma), quartz gabbro (64.0 ± 1.4 Ma), and Late Cretaceous mafic rocks (~92–85 Ma) crop out near Qulong (Wen, 2007; Ma et al., 2015). Porphyry-type Cu and minor skarn-type Cu-Pb-Zn mineralization occurred at Qulong. Porphyry-related hydrothermal alteration produced an inner potassic zone centered on the Miocene monzogranite porphyry and the adjacent Rongmucuola pluton and an outer propylitic zone in the Yeba Formation (Yang et al., 2009). Potassic alteration includes early-stage K-feldspar alteration (quartz + K-feldspar + local anhydrite) and late-stage biotite alteration. Veins associated with potassic alteration include K-feldspar ± quartz, quartz ± K-feldspar ± anhydrite ± chalcopyrite ± pyrite ± bornite, quartz-K-feldspar-molybdenite ± anhydrite ± chalcopyrite ± pyrite, and biotite ± chalcopyrite veins. Propylitic alteration affected the Yeba Formation rocks and is characterized by replacement of plagioclase by epidote. The associated veins are composed of epidote and quartz. Late phyllic alteration (quartz + muscovite + illite + chlorite + pyrite) was generally superimposed on potassic-altered rocks and occurred as pervasive alteration and vein halos. Veins associated with phyllic alteration include chalcopyrite-pyrite ± anhydrite, pyrite-quartz ± anhydrite veins. The main Cu-bearing mineral at Qulong is chalcopyrite with lesser bornite, which mainly occur as disseminations within phyllic-altered rocks, or as chalcopyrite ± biotite and chalcopyrite-pyrite ± anhydrite veins/veinlets (Yang and Cooke, 2019). Skarn mineralization formed within interbeds between tuff and marble of the Yeba Formation. Re-Os model ages of molybdenite from the Qulong porphyry and skarn mineralization occurred at 16.4 ± 0.5 and 16.9 ± 0.6 Ma, respectively (Meng et al., 2003; Li et al., 2005; Wang et al., 2006).

The Lakange porphyry Cu deposit is located ~20 km east of Lhasa (Fig. 1). Its Cu tonnage and grade are unknown at this preliminary stage of exploration. Early Jurassic Yeba Formation andesite, tuff, volcaniclastic rocks, and interbedded sandstones and slates crop out in the center and north of the district. Limestones and sandstones of the Late Jurassic Duodigou Formation crop out in the south of the district and are in fault contact with the Yeba Formation. Miocene granodiorite porphyry (zircon U-Pb age = 13.6 ± 0.4 Ma) was emplaced into the Yeba Formation (Leng et al., 2016). An Re-Os molybdenite model age constrains copper mineralization to 13.4 ± 0.2 Ma (Leng et al., 2015). Potassic alteration of the granodiorite porphyry produced secondary K-feldspar, biotite, and quartz. Phyllic alteration (muscovite + illite + quartz + pyrite) typically occurred along the contact zones between granodiorite porphyry and wall rocks of the Yeba Formation. Chalcopyrite is the dominant hypogene copper sulfide, occurring as disseminations within potassic- and phyllic-altered rocks, and in quartz-chalcopyrite ± pyrite ± molybdenite and chalcopyrite ± pyrite veins.

The Dabu (also known as Nanmu) porphyry Cu-Mo deposit is located ~40 km west of Lhasa (Fig. 1) and has a resource of 250 Mt at average grades of 0.2% Cu and 0.02% Mo. It has an Eocene monzogranite (zircon U-Pb age = 46 ± 1 Ma), Miocene intermineralization monzogranite porphyry (zircon U-Pb age = 15 ± 1 Ma), and Miocene postmineralization lamprophyre dike (Wu, S., et al., 2014, 2016). An Eocene quartz gabbro and granodiorite (zircon U-Pb age = 46.4 ± 1.0 Ma) has also been reported to the north of the district (Wen, 2007). Re-Os molybdenite ages indicate that Dabu mineralization occurred at 14.7 ± 0.2 Ma (Hou et al., 2003). Hydrothermal alteration produced early potassic alteration within the monzogranite porphyry and late phyllic alteration within monzogranite porphyry and the adjacent monzogranite. Potassic alteration caused pervasive alteration of plagioclase to K-feldspar and hornblende and magmatic biotite to hydrothermal biotite. Veins associated with potassic alteration include quartz-biotite ± K-feldspar ± chalcopyrite ± pyrite and quartz-K-feldspar ± chalcopyrite veins. Phyllic alteration caused replacement of silicate minerals by muscovite, illite, quartz, and pyrite and was associated with chalcopyrite-pyrite ± bornite ± molybdenite ± quartz and pyrite-quartz veins. The main Cu-bearing mineral is chalcopyrite with minor bornite—these sulfides typically occur as disseminations and veins in potassic- and phyllic-altered rocks.

The Chongjiang porphyry Cu-Mo deposit is located ~100 km west of Lhasa (Fig. 1). It has a resource of 45 Mt at average grades of 0.4% Cu and 0.02% Mo (Yang et al., 2016a). A Miocene monzogranite pluton intruded volcanic-sedimentary rocks, lavas, and clastic rocks of the Late Cretaceous Shexing Formation and was intruded by intermineralization monzogranite porphyry with zircon U-Pb ages of 15.6 ± 0.5 to 14.0 ± 0.2 Ma (Hou et al., 2004; Mo et al., 2006; Xu et al., 2010; Hu et al., 2017). Older (~50 Ma) granitoids crop out near Chongjiang (Wen, 2007; Huang et al., 2015). A molybdenite Re-Os model age indicates that mineralization occurred at 14.0 ± 0.2 Ma (Hou et al., 2003). Hydrothermal alteration produced an inner potassic zone centered on the monzogranite porphyry and the adjacent monzogranite pluton. An outer propylitic zone affected the Shexing Formation, whereas phyllic alteration generally overprinted the potassic zone. Potassic alteration produced hydrothermal K-feldspar, biotite, quartz, and local anhydrite and was related to quartz ± anhydrite ± chalcopyrite ± pyrite, quartz-K-feldspar ± chalcopyrite ± pyrite ± molybdenite and quartz-biotite ± chalcopyrite ± pyrite veins. Phyllic alteration is characterized by the presence of muscovite, illite, chlorite, and quartz and was associated with chalcopyrite ± pyrite ± quartz and pyrite-quartz veins. Chalcopyrite is the main Cu-bearing mineral, which typically occurs as disseminations and veins in potassic- and phyllic-altered rocks.

The Jiru porphyry Cu deposit is located ~20 km west of Nanmulin (Fig. 1). It has a resource of 42 Mt at an average grade of 0.4% Cu (Zheng et al., 2014). An early Eocene granitoid with zircon U-Pb age of 48.6 ± 0.6 Ma (Zheng et al., 2014) was intruded by a late Eocene granite (zircon U-Pb age of 36.3 ± 0.9 Ma; Wang et al., 2014) and Miocene porphyry with zircon U-Pb ages of 16.2 ± 0.4 to 15.5 ± 0.3 Ma (Wang et al., 2014; Zheng et al., 2014; Yang et al., 2016b). Five molybdenite samples obtained from quartz-molybdenite veins that crosscut the Miocene monzogranite porphyry yielded an Re-Os weighted mean age of 15.3 ± 0.1 Ma; by contrast, another three molybdenite samples collected from quartz-molybdenite veins that crosscut the Eocene monzogranite yielded an Re-Os weighted mean age of 48.3 ± 0.4 Ma (Zheng et al., 2014). The Eocene and Miocene Re-Os ages are similar to the zircon U-Pb ages of the crosscutting intrusions, indicating that two episodes of copper mineralization may have occurred at Jiru. The hydrothermal alteration affected the Miocene monzogranite porphyry and was characterized by early potassic alteration with mineral assemblages of hydrothermal K-feldspar, biotite, and quartz and late phyllic alteration with mineral assemblages of muscovite, illite, and quartz. Chalcopyrite is the main Cu-bearing mineral, occurring as disseminations within potassic- and phyllic-altered rocks and in veins composed of quartz, chalcopyrite, and pyrite. In combination with previous literature (Zheng et al., 2014; Yang et al., 2016b), our observations of the hydrothermal alteration and mineralization assemblages do not support the occurrence of two porphyry copper systems at Jiru; we infer the deposit to be Miocene in age.

The Zhunuo porphyry Cu deposit is located at Angren (Fig. 1). It has a resource of 403 Mt at average grades of 0.57% Cu and 0.02% Mo. The Miocene intrusive complex was emplaced into Eocene quartz porphyry and rhyolite, which have zircon U-Pb ages of 51.6 ± 1.0 and 49.1 ± 0.6 Ma, respectively (Sun et al., 2020). The Miocene intrusive complex is composed of premineralization monzogranite (zircon U-Pb age = 14.7 ± 0.3 Ma), intermineralization monzogranite porphyry (zircon U-Pb age = 14.5 ± 0.2 Ma), late-mineralization diorite porphyry (zircon U-Pb age = 14.2 ± 0.2 Ma), and postmineralization lamprophyre and granite porphyry (zircon U-Pb ages of 12.2 ± 0.1 and 12.0 ± 0.2 Ma, respectively; Sun et al., 2018, 2020). The Re-Os ages of molybdenite from Zhunuo ores range from 14.78 ± 0.11 to 13.51 ± 0.07 Ma (Zheng et al., 2007; Sun et al., 2020). Early potassic alteration affected the monzogranite porphyry and adjacent monzogranite, producing hydrothermal K-feldspar, biotite, and quartz and local anhydrite and magnetite. Veins related to potassic alteration include biotite ± quartz ± K-feldspar ± pyrite ± chalcopyrite ± bornite, quartz-K-feldspar ± pyrite, quartz-K-feldspar ± chalcopyrite ± pyrite, quartz-K-feldspar ± anhydrite ± biotite ± chalcopyrite ± molybdenite ± pyrite, and quartz-K-feldspar-anhydrite-molybdenite-chalcopyrite veins. Distal propylitic alteration formed in the rhyolite and is characterized by replacement of plagioclase by epidote. Late phyllic alteration produced pyrite, muscovite, illite, chlorite, quartz, and tourmaline in the monzogranite porphyry and adjacent monzogranite and quartz porphyry and generally overprinted potassic alteration. Veins associated with phyllic alteration include tourmaline ± quartz ± chalcopyrite ± pyrite, quartz-molybdenite ± chalcopyrite ± pyrite, pyrite ± quartz ± chalcopyrite, and quartz-muscovite-illite-pyrite ± chalcopyrite veins (Sun et al., 2020). The main Cu-bearing mineral is chalcopyrite with lesser bornite, which occurs as disseminations within potassic- and phyllic-altered rocks and associated quartz veins.

Samples of Miocene intrusions, including the Chongjiang premineralization monzogranite, Jiru intermineralization monzogranite porphyry, and Zhunuo intermineralization monzogranite porphyry, were collected for zircon U-Pb dating and Hf-O isotope analyses. Additional samples of Miocene intrusions from Qulong (premineralization monzogranite and intermineralization monzogranite porphyry), Lakange (intermineralization granodiorite porphyry), and Chongjiang (premineralization monzogranite and intermineralization monzogranite porphyry) were collected for bulk-rock Sr-Nd isotope analyses. Eocene igneous rocks from Zhunuo and Jiru were collected for zircon Hf isotope analyses. Eocene igneous rocks from the western Gangdese belt (Dejilin dacite, Xiani granite and gabbro, Zhongba monzogranite) were collected for zircon U-Pb dating, zircon Hf isotope, and bulk-rock Sr-Nd isotope analyses. The locations and characteristics of the newly studied samples are listed in Table 2, which includes 35 Sr-Nd samples, 10 zircon Hf samples, five zircon U-Pb geochronology, and three zircon O samples.

U-Pb dating of zircons from the Jiru monzogranite porphyry (JR11-1) was performed using a sensitive high-resolution ion microprobe (SHRIMP) from the SHRIMP II Consortium at Curtin University in Perth, Western Australia. The analytical spot size is approximately 20 to 30 μm in diameter. The ratios and absolute abundances of U, Th, and Pb isotopes were determined relative to the BR266 zircon standard (²⁰⁶Pb/²³⁸U age = 559.0 ± 0.3 Ma; Stern, 2001). The Plešovice standard with a ²⁰⁶Pb/²³⁸U age of 337.13 ± 0.37 Ma (Sláma et al., 2008) was used to monitor performance. During dating, several measurements of the Plešovice zircon standard gave weighted mean ²⁰⁶Pb/²³⁸U age of 335 ± 2 Ma (MSWD = 3, n = 15). Details about the analytical procedures are provided in Compston et al. (1984) and Lu et al. (2012).

U-Pb dating of zircons from the Chongjiang monzogranite (CJ001) was performed using a SHRIMP II at the Beijing SHRIMP Center, Chinese Academy of Geological Sciences. U-Th-Pb ratios were determined relative to the Temora standard zircon (417 Ma; Black et al., 2003), and the absolute abundances were calibrated to the standard zircon M257. During the analytical period, several measurements of the Temora zircon standard gave weighted mean ²⁰⁶Pb/²³⁸U ages of 416.6 ± 1.5 Ma (mean square of weighted deviates [MSWD] = 2.6, n = 54). Measured compositions were corrected for common Pb using the ²⁰⁷Pb method.

U-Pb dating of zircons from the Xiani granite (XL-21) and gabbro (XL-07/14), and Zhongba monzogranite (ZB-28) was performed on the laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. This instrument couples a quadrupole ICP-MS (Agilient 7500a) and a UP-193 solid-state laser (193-nm; New Wave Research) with an automatic positioning system. For this study, laser spot size was set to ~32 µm, laser energy density was set to 8.5 J cm^–2, and repetition rate was set to 10 Hz. Zircon 91500 (Wiedenbeck et al., 1995) was used as external standard for U-Pb dating and was analyzed twice every five analyses. They yielded a ²⁰⁶Pb/²³⁸U age of ~1064 Ma, which is similar to the recommended age of 1065 Ma (Wiedenbeck et al., 1995). Isotopic ratios and element concentrations of zircons were calculated using ICPMSDataCal (Liu et al., 2008). Common Pb correction and ages of the samples were calculated using LA-ICP-MS common lead correction, according to the method of Andersen (2002). Details of the analytical procedures are provided in Liu et al. (2010).

In situ zircon oxygen isotopes of mineralizing intrusions from Chongjiang (CJ001), Jiru (JR11-1), and Zhunuo (ZN005-189) were analyzed using the SHRIMP-IIe/MC at the Beijing SHRIMP Center. The analytical spot size is approximately 23 μm in diameter. Values of oxygen isotope ratios were standardized to Vienna-standard mean ocean water (V-SMOW, ¹⁸O/¹⁶O = 0.0020052) and reported in standard per mil notation. The instrumental mass fractionation factor was corrected using the Penglai and Qinghu zircon standards with δ¹⁸O values of 5.31 ± 0.10‰ (2SD) and 5.4 ± 0.2‰ (2SD), respectively (Li et al., 2010, 2013). Detailed instrument conditions, analytical procedures and data reduction procedures are described in Ickert et al. (2008).

In situ zircon Hf isotope compositions of the studied Miocene and Eocene igneous rocks were determined using a Neptune multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) instrument equipped with a Geolas-193 laser ablation system at the National Research Center of Geoanalysis and the State Key Laboratory of Geological Process and Mineral Resources. Details about the analytical procedures and data acquisition are provided in Hu et al. (2012), Zhou et al. (2018), and Sun et al. (2018). Model ages and ɛ_Hf values of the samples were calculated with an assumption that the ¹⁷⁶Lu/¹⁷⁷Hf ratio of average crust is 0.015, and the ¹⁷⁶Hf/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf ratios of present chondrite and depleted mantle are 0.282772 and 0.0332, 0.28325 and 0.0384, respectively (Vervoort et al., 1996; Blichert-Toft et al., 1997; Griffin et al., 2002). The decay constant of ¹⁷⁶Lu used for this study is 1.865 × 10^–11 (Scherer et al., 2001).

The Sr and Nd isotope compositions of the Miocene and Eocene igneous rocks were determined by thermal ionization mass spectrometry (TIMS) in the State Key Laboratory Geological Process and Mineral Resources. The detailed analytical procedures are provided in Liu et al. (2004) and Sun et al. (2018). The measured Sr and Nd isotope ratios were normalized against ⁸⁶Sr/⁸⁸Sr = 0.1194 and ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219.

Zircon U-Pb geochronological data for five intrusions are listed in Appendix Table A1 and shown in Figure 2. All the zircon samples have numerous narrow oscillatory zones and Th/U ratios of 0.4 to 2.0, indicating their magmatic origin (Hoskin and Schaltegger, 2003). The Chongjiang monzogranite sample (CJ001) and Jiru monzogranite porphyry sample (JR11) yield weighted mean ²⁰⁷Pb-corrected ²³⁸U/²⁰⁶Pb dates of 14.4 ± 0.4 Ma (MSWD = 1.2; n = 13) and 14.9 ± 0.2 Ma (MSWD = 0.77; n = 12), respectively, which are interpreted as their magmatic crystallization ages (Fig. 2A, B).

Eighteen zircon analyses from the Xiani granite sample (XL-21) yielded a weighted mean ²⁰⁷Pb-corrected ²³⁸U/²⁰⁶Pb date of 50.6 ± 1.0 Ma (MSWD = 0.91), which is taken as the igneous crystallization age of the granite (Fig. 2C). The Xiani gabbro sample (XL-07/14) gave a weighted mean ²⁰⁷Pb-corrected ²³⁸U/²⁰⁶Pb date of 49.6 ± 0.6 Ma (MSWD = 1.4; n = 18), interpreted as the magmatic crystallization age of the gabbro (Fig. 2D).

Fifteen zircons from the Zhongba monzogranite sample (ZB-28) yielded a weighted mean ²⁰⁷Pb-corrected ²³⁸U/²⁰⁶Pb date of 47.2 ± 1.0 Ma (MSWD = 0.75; n=15), which is considered the igneous crystallization age of the monzogranite (Fig. 2E).

Results for in situ zircon Hf-O isotope analyses for three Miocene mineralizing intrusions are listed in Appendix Table A2. The Chongjiang monzogranite (CJ001) has zircon ɛ_Hf(t) values of 1.7 to 5.7 and δ¹⁸O values of 5.8 to 6.7‰. The Jiru monzogranite porphyry (JR11) has zircon ɛ_Hf(t) values of −4.5 to 4.5 and δ¹⁸O values of 6.3 to 7.0‰. The Zhunuo monzogranite porphyry (ZN005-189) has zircon ɛ_Hf (t) values of −2.8 to 0 and δ¹⁸O values of 5.5 to 8.4‰.

Results for in situ zircon Hf isotope analyses for the Eocene igneous rocks are listed in Appendix Table A3. The Zhunuo rhyolite (ZN11-7) and quartz porphyry (ZN1511-15) have ɛ_Hf(t) values of −0.2 to 3.2 and −0.2 to 4.1, and second-stage Hf depleted mantle model ages (T2DM) of 1140 to 923 and 1142 to 866 Ma, respectively. The Jiru monzogranite (JR11-8) has ɛ_Hf(t) values of 2.6 to 7.1 and T2DM of 958 to 669 Ma. The Dejilin dacite (DJL17-2) has ɛ_Hf(t) values of 1.0 to 4.4 and T2DM of 1060 to 846 Ma. The Xiani granite (XL-21) and gabbro (XL-07/14) have ɛ_Hf(t) values of −0.7 to 3.9 and −1.2 to 1.5, and T2DM of 1171 to 875 and 1202 to 1030 Ma, respectively. The Zhongba granite (ZB-28) has ɛ_Hf(t) values of 1.2 to 3.8 and T_2DM of 1045 to 884 Ma.

Results for 35 bulk-rock Sr-Nd isotope analyses for the Eocene and Miocene igneous rocks are listed in Appendix Table A4. All the Miocene samples have evolved Sr-Nd isotope compositions, with initial ⁸⁷Sr/⁸⁶Sr and ɛ_Nd(t) values ranging from 0.70489 to 0.70565 and −4.2 to 0.0 for the Qulong monzogranite and granodiorite, 0.70517 to 0.70532 and −0.2 to 0.2 for the Lakange granodiorite, and 0.70489 to 0.70565 and −4.2 to −2.8 for the Chongjiang monzogranite.

The Eocene samples have evolved isotopic compositions. The initial ⁸⁷Sr/⁸⁶Sr ratios and ɛ_Nd(t) values are 0.70326 to 0.70675 and −3.8 to −1.7 for the 10 rhyolite and quartz porphyry samples from Zhunuo, 0.70558 to 0.70562 and −2.7 to −2.2 for the three dacite samples at Dejilin, and 0.71121 to 0.71146 and −8.4 to −8.2 for the two monzogranite samples at Zhongba. At Xiani, the three granite samples (initial ⁸⁷Sr/⁸⁶Sr = 0.70584–0.70614; ɛ_Nd(t) = −3.9 to −3.6) yielded more evolved isotopic compositions than the four gabbro samples (initial ⁸⁷Sr/⁸⁶Sr = 0.70558–0.70562; ɛ_Nd(t) = −2.7 to −2.2).

The Gangdese batholiths in the southern Lhasa subterrane were emplaced from the Middle Triassic to the Miocene (Ji et al., 2009; Zhu et al., 2011, 2015; Hou et al., 2015b, and references therein). Our syntheses of all published and new Nd-Hf isotopes indicate that there are distinct spatial isotopic variations along the Gangdese belt from precollisional to collisional magmatism (Fig. 3).

Both the Middle Triassic-Jurassic (~237–152 Ma) and the Cretaceous (~137–65 Ma) arc magmatic rocks have depleted Nd-Hf isotope compositions, consistent with their derivation mainly from the juvenile Neo-Tethyan mantle wedge (Fig. 3A-D). Exceptions are at Daggyai Co and from Lhasa to Gongbogymda (~91°–93°E; Fig. 1B), where there are large variations of ɛ_Nd(t) and ɛ_Hf(t), suggesting probable crustal contamination by local Archean-Paleoproterozoic Lhasa basement (Fig. 3A-D; Hou et al., 2015b).

The Paleocene-Eocene (~65–40 Ma) and Miocene (~21–13 Ma) intrusions were emplaced during the India-Asia continental collision over a large area from Gaer to Nyingchi (Fig. 1B). It was previously shown that the Paleocene-Eocene rocks in the western Gangdese belt (west of ~89°E) generally have higher initial ⁸⁷Sr/⁸⁶Sr ratios and lower ɛ_Nd(t) values than those in the eastern Gangdese belt (east of ~89°E; Wang et al., 2015). Our new isotopic data such as the ca. 50 Ma Xiani gabbros and granites at ~86°E are consistent with previous interpretations (Fig. 3E, F). More importantly, both the mafic and felsic Paleocene-Eocene igneous rocks show similar spatial isotopic variations with their ɛ_Nd(t) and ɛ_Hf(t) values increasing from ~84° to ~92°E and then decreasing toward ~95°E (Fig. 3E, F). These spatial trends are also exhibited by the Miocene (~21–13 Ma) mineralizing and barren granitoids (Fig. 3G, H), but not the precollisional Triassic-Cretaceous intrusions (Fig. 3A-D). Given that the precollisional arc rocks at ~88° to 95°E have depleted Nd-Hf isotopes (Fig. 3A-D), the more unradiogenic Nd-Hf isotopes of the Paleocene to Miocene rocks most likely reflect the involvement of the subducted Indian continental materials with low ɛ_Nd(t) and ɛ_Hf(t) values (Chu et al., 2011). This interpretation is supported by geophysical results showing a N-dipping subducted Indian continental lithosphere and transport of Indian crust north of the Yarlung Zangbo suture (Replumaz et al., 2010; Gao et al., 2016). In addition, the contributions of subducted Indian components to collisional magmas along the Gangdese belt probably decreased from ~84° to ~92°E and then increased from ~92° to ~95°E, in keeping with the spatial isotopic variations of the Paleocene to Miocene igneous rocks (Fig. 3E-H).

The origin of Miocene ore-related intrusions in the Gangdese belt remains highly controversial, but it is generally accepted that either subduction-modified juvenile Tibetan lower crust or metasomatized mantle-derived mafic magmas played major roles in their formation (Hou et al., 2004, 2015a; Lu et al., 2015; Yang et al., 2015, 2016a; Sun et al., 2018; Wang et al., 2018). The Paleocene-Eocene intrusions crop out within or nearby all known Miocene porphyry copper deposits (Fig. 4A) and have more juvenile Nd and Hf isotope compositions than the Miocene mineralizing granitoids at individual deposits when all data are recalculated to 15 Ma (Fig. 4B, C). It suggests the Miocene rocks involved a less radiogenic component, which is most likely the subducted Indian continental materials with unradiogenic Nd and Hf isotopes (Guo et al., 2013; Yang et al., 2016a; Sun et al., 2018). The Miocene mineralizing intrusions have large variations in zircon oxygen isotope ratios, which range from mantle-like (5.3 ± 0.6‰, 2σ; Valley et al., 1998) to supracrustal δ¹⁸O values (up to 8‰; Fig. 5), consistent with involvement of mantle and supracrustal components in their genesis (Kemp et al., 2006).

We propose a three-component mixing model to explain the large variations of Sr-Nd isotopes of the Miocene mineralizing intrusions ([⁸⁷Sr/⁸⁶Sr]_{(t = 15 Ma)} = 0.7048–0.7091; ɛ_{Nd(t = 15 Ma)} = −6.9 to 2.4; Figs. 6, 7). The three components include Indian plate-released fluids, subduction-modified Tibetan lower crust, and mafic magmas derived from mantle metasomatized by Indian continental materials (Fig. 7). Given the subduction-modified Tibetan lower crust is heterogeneous, as shown by the spatial isotopic variations of Paleocene-Eocene igneous rocks (Fig. 3E, F), the local Paleocene-Eocene gabbros were chosen to represent Tibetan lower crustal end member at different localities (Table 3; Fig. 7). The modeling results show that the proportion of Indian plate-released fluids increased from Qulong, Jiama, and Lakange (0–10%) in the east to other deposits (Dabu, Chongjiang, Tinggong, and Zhunuo; 10–25%) in the west. Similarly, the proportion of metasomatized mantle-derived magmas also increased from Qulong, Jiama, Lakange, and Dabu (mostly <5%) in the east to Chongjiang, Tinggong, and Zhunuo (mostly 10–25%) in the west (Fig. 7). This westward-increasing contribution of Indian continental components during Miocene mineralization could reflect increasingly shallower angle of subducted Indian lithosphere from east (Qulong) to west (Zhunuo) as imaged by geophysical data (Zhao et al., 2010; Chen et al., 2015; Guo et al., 2018). Along-strike variation of subduction angle during the Miocene could have led to slab tearing (Chen et al., 2015; Guo et al., 2018; Li and Song, 2018; Liu et al., 2020), leading to formation of five distinct porphyry copper districts in the Gangdese belt (Fig. 8).

The along-strike isotopic variation along the Gangdese belt is consistent with new geophysical evidence suggesting a segmented subducting Indian slab (Figs. 3E, F, 8; Li and Song, 2018; Liu et al., 2020). The segment between ~86° and ~92°E hosts all known Gangdese Miocene porphyry copper deposits, which are found above a steeper Indian slab segment that was less metasomatized by Indian continental materials than other segments (Figs. 3E, F, 8). By contrast, the segments to the west of ~86°E and east of ~92° to ~95°E were strongly metasomatized by Indian continental components above flatter slabs—these segments do not host any Miocene porphyry deposits. The lack of porphyry deposits above flatter Indian slab is at odds with previous observations that young giant porphyry deposits coincide with domains of flat slab subduction in circum-Pacific arcs (Cooke et al., 2005). We speculate that this discrepancy is due to the continental nature of the subducting Indian slab, which likely contains less chlorine, sulfur, and water than subducting oceanic slab.

The Nd-Hf isotopes are effective in imaging large-scale lithospheric architecture through time (Hou et al., 2015b). Previous studies have proposed that isotopic values of ore-related intrusions are correlated with metal endowment of porphyry copper deposits in collisional zones (Hou et al., 2013; Asadi, 2018; Deng et al., 2018). However, our larger Nd-Hf isotope data show that there are no obvious positive correlations between copper metal tonnage and bulk-rock ɛ_Nd(t) and zircon ɛ_Hf(t) values on a global scale (Fig. 9). We advise caution when using Nd-Hf isotopes for predicting metal endowment of porphyry deposits, because other factors such as tectonic setting, crustal thickening, magma differentiation, fluid exsolution, and ore-forming processes all play roles in determining Cu endowment and grades (Tosdal and Richards, 2001; Chiaradia, 2014; Zhang and Audetat, 2017).

Gangdese Paleocene-Eocene igneous rocks and Miocene granitoids have systematic along-arc isotopic variations, characterized by bulk-rock ɛ_Nd(t) and zircon ɛ_Hf(t) values increasing from ~84° to ~92°E and decreasing from ~92° to ~95°E. The amounts of subducted Indian continental materials involved in mantle metasomatism decreased from ~84° to ~92°E and then increased from ~92° to ~95°E during the Paleocene-Eocene due to variations in the subduction angle of the segmented Indian slab. The Miocene porphyry Cu deposits are located above a steeper Indian slab segment, and their genesis involved Indian plate-released fluids, metasomatized mantle-derived mafic magmas, and Tibetan lower crust. Our results highlight that caution must be taken when using Nd-Hf isotopes for predicting metal endowment of porphyry deposits.

Constructive comments by David Cooke, Zhi-ming Yang, and an anonymous reviewer are gratefully acknowledged. Funding for this project was jointly granted by the National Key Research and Development Project of China (2016YFC0600305, 2017YFC0601506), National Natural Science Foundation of China (41772077, 41973040), Changjiang Scholars and Innovative Research Team in University (IRT14R54), the 111 Project of the Ministry of Science and Technology (BP0719021), and Fundamental Research Funds for the Central Universities. This is contribution 1407 from the ARC Centre of Excellence for Core to Crust Fluid Systems (www.CCFS.mq.edu.au). YL publishes with permission of the executive director of the Geological Survey of Western Australia.

Xiang Sun is a professor at the China University of Geosciences, Beijing. He received a B.Sc. degree in 2002 and a Ph.D. degree in 2008 from the Liaoning Technical University. Xiang conducted postdoctoral research at the China University of Geosciences, Beijing (July 2008 to July 2010), and cooperative research at the University of Western Australia (September 2015 to September 2016) and the Australian National University (January to March 2020). His research interests focus on magmatic-hydrothermal mineralization, particularly porphyry copper mineralization.