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Introduction

Porphyry copper (Cu) deposits (PCDs) provide ∼75 % of the global Cu resources. Oceanic subduction and continental collision trigger the large metallogeny of PCDs at convergent plate boundaries. PCDs related to Pacific plate subduction typically occur in the Circum-Pacific belt, particularly in the Andes, Papua New Guinea, and the Philippines (Sillitoe, 2010

Sillitoe, R.H. (2010) Porphyry Copper Systems. Economic Geology 105, 3–41. https://doi.org/10.2113/gsecongeo.105.1.3

; Wilkinson, 2013

Wilkinson, J.J. (2013) Triggers for the formation of porphyry ore deposits in magmatic arcs. Nature Geoscience 6, 917–925. https://doi.org/10.1038/ngeo1940

). PCDs related to collision of the African, Arabian and Indian Plates with the Eurasian Plate occur in the Alpine-Himalayan belt (Richards, 2009

Richards, J.P. (2009) Postsubduction porphyry Cu-Au and epithermal Au deposits: Products of remelting of subduction-modified lithosphere. Geology 37, 247–250. https://doi.org/10.1130/G25451A.1

). PCDs in these two belts provide a window to probe plate subduction and associated effects on large Cu endowments. A variety of geochemical tools (e.g., Sr, Nd, Pb and Cu isotopes) have been employed to constrain the sources of metals in PCDs of the Alpine-Himalayan belt, but it remains debated whether the ore-forming metals are sourced from the partial melting of sulfide-rich juvenile lower crustal rocks (Li et al., 2011

Li, J.-X., Qin, K.-Z., Li, G.-M., Xiao, B., Chen, L., Zhao, J.-X. (2011) Post-collisional ore-bearing adakitic porphyries from Gangdese porphyry copper belt, southern Tibet: Melting of thickened juvenile arc lower crust. Lithos 126, 265–277. https://doi.org/10.1016/j.lithos.2011.07.018

; Hou et al., 2020

Hou, Z., Yang, Z., Wang, R., Zheng, Y. (2020) Further discussion on porphyry Cu-Mo-Au deposit formation in mainland China. Earth Science Frontiers 27, 20–44. https://doi.org/10.13745/j.esf.sf.2020.3.8

) or the fertile subcontinental lithospheric mantle (SCLM) (Xu et al., 2016

Xu, L., Bi, X., Hu, R., Qi, Y., Tang, Y., Wang, X., Zhu, J. (2016) Redox states and genesis of magmas associated with intra-continental porphyry Cu–Au mineralization within the Jinshajiang–Red River alkaline igneous belt, SW China. Ore Geology Reviews 73, 330–345. https://doi.org/10.1016/j.oregeorev.2015.05.007

; Zheng et al., 2019

Zheng, Y.-C., Liu, S.-A., Wu, C.-D., Griffin, W.L., Li, Z.-Q., Xu, B., Yang, Z.-M., Hou, Z.-Q., O’Reilly, S.Y. (2019) Cu isotopes reveal initial Cu enrichment in sources of giant porphyry deposits in a collisional setting. Geology 47, 135–138. https://doi.org/10.1130/G45362.1

).

Mercury (Hg) is a chalcophile metal and an important component in PCDs (Xu et al., 2024

Xu, B., Yin, R.-S., Chiaradia, M., Miao, Z., Griffin, W.L., Hou, Z.-Q., Yang, Z.-M., O’Reilly, S.Y. (2024) Mercury isotope evidence for the importance of recycled fluids in collisional ore systems. Science Advances 10, eadp7383. https://doi.org/10.1126/sciadv.adp7383

). The seven natural stable isotopes of Hg (¹⁹⁶Hg, ¹⁹⁸Hg, ¹⁹⁹Hg, ²⁰⁰Hg, ²⁰¹Hg, ²⁰²Hg, ²⁰⁴Hg) display mass-dependent fractionation (Hg-MDF, reported as δ²⁰²Hg) and mass-independent fractionation (Hg-MIF, reported as Δ¹⁹⁹Hg). Hg-MDF occurs during various geochemical processes (e.g., microbial Hg(II) reduction, Hg(II) methylation, Hg(0) volatilisation), whereas Hg-MIF occurs mainly during photochemical reactions (Blum et al., 2014

Blum, J.D., Sherman, L.S., Johnson, M.W. (2014) Mercury Isotopes in Earth and Environmental Sciences. Annual Review of Earth and Planetary Sciences 42, 249–269. https://doi.org/10.1146/annurev-earth-050212-124107

). The primitive mantle displays near-zero Δ¹⁹⁹Hg values (0.00 ± 0.10 ‰; 2 s.d.; Moynier et al., 2021

Moynier, F., Jackson, M.G., Zhang, K., Cai, H., Halldórsson, S.A., Pik, R., Day, J.M.D., Chen, J. (2021) The Mercury Isotopic Composition of Earth’s Mantle and the Use of Mass Independently Fractionated Hg to Test for Recycled Crust. Geophysical Research Letters 48, e2021GL094301. https://doi.org/10.1029/2021GL094301

). In contrast to magmatic Hg isotope signatures, Hg(II) photoreduction results in negative Δ¹⁹⁹Hg signals in terrestrial systems (−0.6 to 0 ‰; soil and vegetation) and positive Δ¹⁹⁹Hg values in marine systems (0 to +0.3 ‰; sediments and seawater) (Blum et al., 2014

Blum, J.D., Sherman, L.S., Johnson, M.W. (2014) Mercury Isotopes in Earth and Environmental Sciences. Annual Review of Earth and Planetary Sciences 42, 249–269. https://doi.org/10.1146/annurev-earth-050212-124107

). The distinctive Δ¹⁹⁹Hg signatures of terrestrial, marine and mantle reservoirs allow for tracing the source of metals in magmatic and hydrothermal systems (Yin et al., 2024

Yin, R., Wang, X., Sun, R., Gao, L., Deng, C., Tian, Z., Luo, A., Lehmann, B. (2024) Linking the mercury biogeochemical cycle to the deep mercury cycle: A mercury isotope perspective. Chemical Geology 654, 122063. https://doi.org/10.1016/j.chemgeo.2024.122063

). Positive Δ¹⁹⁹Hg values have been observed in arc basalts and hydrothermal systems (0 to +0.3 ‰; epithermal Au deposits and porphyry Mo deposits) in the Circum-Pacific belt, reflecting large scale recycling of metals from marine reservoirs via oceanic subduction (Deng et al., 2021

Deng, C., Sun, G., Rong, Y., Sun, R., Sun, D., Lehmann, B., Yin, R. (2021) Recycling of mercury from the atmosphere-ocean system into volcanic-arc–associated epithermal gold systems. Geology 49, 309–313. https://doi.org/10.1130/G48132.1

; Yin et al., 2022

Yin, R., Chen, D., Pan, X., Deng, C., Chen, L., Song, X., Yu, S., Zhu, C., Wei, X., Xu, Y., Feng, X., Blum, J.D., Lehmann, B. (2022) Mantle Hg isotopic heterogeneity and evidence of oceanic Hg recycling into the mantle. Nature Communications 13, 948. https://doi.org/10.1038/s41467-022-28577-1

; Gao et al., 2024a

Gao, L., Sun, D., Tian, Z., Luo, A., Lehmann, B., Yin, R. (2024a) Positive Δ¹⁹⁹Hg anomalies in Mesozoic porphyry Mo deposits of Northeastern China and their implications to the metallogeny of arc-related hydrothermal systems at convergent margins. Chemical Geology 645, 121880. https://doi.org/10.1016/j.chemgeo.2023.121880

).

A recent study observed highly variable Δ¹⁹⁹Hg in fertile porphyry-related magmas (−0.30 to +0.22 ‰) and coeval ultrapotassic mafic rocks (−0.54 to +0.25 ‰) in the Himalayan-Tibetan orogenic belt, highlighting that PCDs receive metals or volatiles from the SCLM modified by subducted materials (Xu et al., 2024

Xu, B., Yin, R.-S., Chiaradia, M., Miao, Z., Griffin, W.L., Hou, Z.-Q., Yang, Z.-M., O’Reilly, S.Y. (2024) Mercury isotope evidence for the importance of recycled fluids in collisional ore systems. Science Advances 10, eadp7383. https://doi.org/10.1126/sciadv.adp7383

). However, there are yet undefined Hg isotopic reservoirs related to PCDs, including lower crustal rocks and the ore-forming sulfides. In this work, we investigate the Hg isotopic composition of these reservoirs in the Himalayan-Tibetan orogenic belt to gain an understanding of the metal sources of PCDs.

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Geological Background

The eastern part of the Alpine-Himalayan orogenic belt comprises a series of blocks and micro-continents (Fig. 1a). Subduction of the Palaeo-Tethys Ocean in the Palaeozoic caused collision of these blocks and micro-continents (Metcalfe, 2021

Metcalfe, I. (2021) Multiple Tethyan ocean basins and orogenic belts in Asia. Gondwana Research 100, 87–130. https://doi.org/10.1016/j.gr.2021.01.012

). The Neo-Tethys Ocean was closed in the Late Cretaceous, followed by the collision of the Indian and Eurasian continents (Metcalfe, 2021

Metcalfe, I. (2021) Multiple Tethyan ocean basins and orogenic belts in Asia. Gondwana Research 100, 87–130. https://doi.org/10.1016/j.gr.2021.01.012

). The Songpan-Ganzi, Qiangtang, Lhasa and Himalaya Terranes, divided by the Jinsha, Bangong-Nujiang and Yarlung-Zangbo suture zones, mark the sequential closure of the Palaeo-, Meso- and Neo-Tethys oceans, respectively (Zheng et al., 2021

Zheng, Y.-C., Shen, Y., Wang, L., Griffin, W.L., Hou, Z.-Q. (2021) Collision‐related porphyry Cu deposits formed by input of ultrapotassic melts into the sulfide‐rich lower crust. Terra Nova 33, 582–589. https://doi.org/10.1111/ter.12550

). The Tibetan Plateau hosts the Yulong and the Gangdese porphyry Cu belts, which contain numerous world-class PCDs formed during the Indian-Eurasian collisional orogeny.

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The Yulong porphyry Cu belt (Fig. 1b) follows the northwestward trend of the Red River-Ailao Shan Fault, i.e. a sinistral fault system that emerged due to the Indian-Eurasian continental collision. It comprises Proterozoic to early Palaeozoic crystalline folded basement and middle to late Palaeozoic carbonate and clastic sedimentary rocks, and distributes >20 PCDs (e.g., Yulong) with mineralisation ages (∼40 Ma) synchronous with nearby Eocene alkaline magmatism (Yang and Cooke, 2019

Yang, Z., Cooke, D.R. (2019) Porphyry Copper Deposits in China. In: Chang, Z., Goldfarb, R.J. (Eds.) Mineral Deposits of China. Society of Economic Geologists, Littleton, 133–187. https://doi.org/10.5382/SP.22.05

). Ore-bearing porphyries at Yulong are mainly monzonite granite porphyries containing assemblages of pyrite, chalcopyrite and molybdenite (Fig. S-1). The Yulong PCD is associated with potassic, sericitic, and epidote alterations, manifested as the widespread distribution of quartz-biotite veins, secondary K-feldspar, and epidote-altered zones, accompanied by localised development of sulfide mineralisation (Fig. S-2).

The Gangdese porphyry Cu belt is located in the Lhasa terrane (Fig. 1c), which is a microcontinental fragment comprising Precambrian basement rocks, Palaeozoic to Mesozoic sedimentary formations, and Mesozoic to Cenozoic igneous rocks (Sun et al., 2024

Sun, J.-L., Bai, Z.-J., Zhong, H., Liu, X., Zhu, J.-J., Chen, L., Zhu, W.-G. (2024) Sulfide saturation in reduced magmas during generation of the Gangdese juvenile lower crust: Implications for porphyry Cu–Au mineralization in the Gangdese belt, Tibet. Mineralium Deposita 59, 1387–1405. https://doi.org/10.1007/s00126-024-01269-0

). This belt was formed during post-collision between the Indian and Eurasian continents and hosts abundant Miocene ultrapotassic magmatic rocks and numerous PCDs (e.g., Jiama, Qulong, and Zhibula) with mineralisation ages of 16 to 14 Ma (Hou et al., 2011

Hou, Z., Zhang, H., Pan, X., Yang, Z. (2011) Porphyry Cu (–Mo–Au) deposits related to melting of thickened mafic lower crust: Examples from the eastern Tethyan metallogenic domain. Ore Geology Reviews 39, 21–45. https://doi.org/10.1016/j.oregeorev.2010.09.002

). Mineralised intrusions in the Gangdese PCDs are mainly monzonite granite and quartz monzonite porphyries, containing assemblages of pyrite, chalcopyrite, molybdenite, magnetite and bornite (Fig. S-3). The deposits are characterised by pyrite-sericitisation alteration in Qulong, sericitic and skarn-related alteration in Jiama, and epidote-garnet alteration in Zhibula (Fig. S-4).

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Methods

A total of 43 sulfide-bearing bulk ore samples (i.e. mineralised porphyries) were collected from the Yulong, Qulong, Jiama, and Zhibula deposits. Sulfides (pyrite, n = 26; chalcopyrite, n = 16; bornite, n = 4) were separated from the bulk ore samples. Sulfides were pre-concentrated using flotation with xanthate as the collector, followed by hand picking under a binocular microscope to ensure high purity. Magmatic rocks (n = 19) representing the juvenile lower crust and ultrapotassic mafic rocks (n = 5) representing the SCLM from the Gangdese area were also collected. Detailed information for the samples can be found in Supplementary Tables S-1 and S-2. All the samples were prewashed with 18.2 MΩ cm water, air dried, ground to a particle size of 200 mesh, and homogenised in an agate mortar for petrochemical analysis.

The major elements of rock powders were analysed using X-ray fluorescence (XRF) spectrometry at ALS Guangzhou with a PANalytical PW5400 instrument, which yielded an analytical uncertainty of <5 % for sample duplicates. The loss on ignition (LOI) was measured according to the percentage weight loss before and after heating the samples at 1000 °C in a muffle furnace. LOI values of all samples are <3 wt. % (Table S-3), indicating limited alteration effects. The major element concentrations of the lower crustal rocks (n = 19) have been recently reported (Sun et al., 2024

Sun, J.-L., Bai, Z.-J., Zhong, H., Liu, X., Zhu, J.-J., Chen, L., Zhu, W.-G. (2024) Sulfide saturation in reduced magmas during generation of the Gangdese juvenile lower crust: Implications for porphyry Cu–Au mineralization in the Gangdese belt, Tibet. Mineralium Deposita 59, 1387–1405. https://doi.org/10.1007/s00126-024-01269-0

).

Total Hg (THg) concentrations and Hg isotopic compositions were determined at the Institute of Geochemistry, Chinese Academy of Sciences. Concentrations of THg were directly measured using a Lumex R915+ Hg analyser (detection limit 50 pg Hg), which yielded Hg recoveries of 90–110 % (n = 8) for GSS-4 standard reference material and uncertainties of <10 % (2 s.d.) for sample duplicates. Based on the measured THg concentrations, variable amounts of sample powders containing 10 ng Hg were weighed and prepared using a two stage combustion furnace for preconcentrating Hg into 5 mL of 40 % acid mixture (HNO₃/HCl = 2/1, v/v), following previous methods (Zerkle et al., 2020

Zerkle, A.L., Yin, R., Chen, C., Li, X., Izon, G.J., Grasby, S.E. (2020) Anomalous fractionation of mercury isotopes in the Late Archean atmosphere. Nature Communications 11, 1709. https://doi.org/10.1038/s41467-020-15495-3

; Gao et al., 2024b

Gao, L., Sun, D., Wang, X., Chen, D., Tian, Z., Luo, A., Yin, R. (2024b) A Catalyst Tube-Equipped Dual-Stage Tube Furnace System for Accurate Hg Isotopic Determination of Ore Samples Using Neptune Plus Multicollector Inductively Coupled Plasma Mass Spectrometry. Analytical Chemistry 96, 17560–17566. https://doi.org/10.1021/acs.analchem.4c03041

). The Hg blank of the acid mixture is below 0.05 ng/mL. Preparation of zero sample powders yielded a method blank of less than 50 pg Hg in the sample solution. Pretreatment of standard reference material (GSS-4, soil) yielded Hg recoveries of 90–110 % (n = 8). The Hg preconcentrated solutions were diluted to 1 ng/mL with 10–20 % of acid mixture (HNO₃/HCl = 2/1, v/v) before Hg isotopic analysis at the Institute of Geochemistry, Chinese Academy of Sciences, using a Thermo Scientific Neptune Plus multi-collector inductively coupled plasma mass spectrometry (Supplementary Text S-1; Yin et al., 2016

Yin, R., Krabbenhoft, D.P., Bergquist, B.A., Zheng, W., Lepak, R.F., Hurley, J.P. (2016) Effects of mercury and thallium concentrations on high precision determination of mercury isotopic composition by Neptune Plus multiple collector inductively coupled plasma mass spectrometry. Journal of Analytical Atomic Spectrometry 31, 2060–2068. https://doi.org/10.1039/C6JA00107F

). Hg-MDF is expressed as δ²⁰²Hg in ‰ in reference to the NIST-3133 Hg standard:

MIF is reported in Δ notation as the difference between the measured δ^xxxHg values and theoretically predicted values:

where xxx represents 199, 200, or 201, and β is 0.252 for ¹⁹⁹Hg, 0.5024 for ²⁰⁰Hg, and 0.752 for ²⁰¹Hg (Blum and Bergquist, 2007

Blum, J.D., Bergquist, B.A. (2007) Reporting of variations in the natural isotopic composition of mercury. Analytical and Bioanalytical Chemistry 388, 353–359. https://doi.org/10.1007/s00216-007-1236-9

). NIST-3177 secondary standard solutions were measured every 10 samples. The overall average and uncertainty of NIST-3177 (δ²⁰²Hg = −0.53 ± 0.06 ‰, Δ¹⁹⁹Hg = −0.02 ± 0.04 ‰; Δ²⁰⁰Hg = +0.01 ± 0.03 ‰; Δ²⁰¹Hg = −0.02 ± 0.06 ‰; 2 s.d., n = 10) and GSS-4 (δ²⁰²Hg = −1.69 ± 0.13 ‰; Δ¹⁹⁹Hg = −0.41 ± 0.06 ‰; Δ²⁰⁰Hg = −0.02 ± 0.06 ‰; Δ²⁰¹Hg = −0.37 ± 0.06 ‰; 2 s.d., n = 8) agree well with previous results (Table S-4; Blum and Berquist, 2007

Blum, J.D., Bergquist, B.A. (2007) Reporting of variations in the natural isotopic composition of mercury. Analytical and Bioanalytical Chemistry 388, 353–359. https://doi.org/10.1007/s00216-007-1236-9

; Deng et al., 2021

Deng, C., Sun, G., Rong, Y., Sun, R., Sun, D., Lehmann, B., Yin, R. (2021) Recycling of mercury from the atmosphere-ocean system into volcanic-arc–associated epithermal gold systems. Geology 49, 309–313. https://doi.org/10.1130/G48132.1

; Gao et al., 2024b

Gao, L., Sun, D., Wang, X., Chen, D., Tian, Z., Luo, A., Yin, R. (2024b) A Catalyst Tube-Equipped Dual-Stage Tube Furnace System for Accurate Hg Isotopic Determination of Ore Samples Using Neptune Plus Multicollector Inductively Coupled Plasma Mass Spectrometry. Analytical Chemistry 96, 17560–17566. https://doi.org/10.1021/acs.analchem.4c03041

). The larger values of standard deviation (2 s.d.) for either NIST-3177 or GSS-4 are used to reflect analytical uncertainties.

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Results

As shown in Tables S-1 and S-2 and Figure 2a, the studied sulfide samples show large variations in Hg concentration (0.77 to 2253 ng/g), δ²⁰²Hg (−2.59 to +0.85 ‰), and Δ¹⁹⁹Hg values (−0.29 to +0.21 ‰). Sulfides (pyrite, chalcopyrite and bornite) from the three Gangdese PCDs (Qulong, Jiama, and Zhibula) display δ²⁰²Hg values ranging from −2.14 to +0.69 ‰ and Δ¹⁹⁹Hg from −0.15 to +0.10 ‰. Sulfides (pyrite) from Yulong PCD display a similar range in δ²⁰²Hg (−2.59 to +0.85 ‰), but a larger range for Δ¹⁹⁹Hg values (−0.29 to +0.21 ‰). The juvenile lower crustal rocks from the Milin area display low Hg concentrations (0.12 to 2.67 ng/g), mainly positive δ²⁰²Hg (−0.33 to +1.59 ‰) and negative Δ¹⁹⁹Hg values (−0.20 to +0.01 ‰). Ultrapotassic mafic rocks display low Hg levels (0.29 to 6.03 ng/g), negative δ²⁰²Hg (−1.48 to −0.48 ‰) and mainly negative Δ¹⁹⁹Hg (−0.42 to +0.06 ‰). All samples show a positive correlation between Δ²⁰¹Hg and Δ¹⁹⁹Hg with the Δ¹⁹⁹Hg/Δ²⁰¹Hg ≈ 1 (R² = 0.68) (Fig. 2b), consistent with that observed in the atmosphere-land-ocean system (Blum et al., 2014

Blum, J.D., Sherman, L.S., Johnson, M.W. (2014) Mercury Isotopes in Earth and Environmental Sciences. Annual Review of Earth and Planetary Sciences 42, 249–269. https://doi.org/10.1146/annurev-earth-050212-124107

). The LOI values of the juvenile lower crustal rocks and ultrapotassic mafic rocks are less than 3 wt. %, indicating that these samples are fresh and have not undergone significant weathering or alteration.

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Discussion

The existence of terrestrial Hg in the lithospheric mantle and juvenile lower crust. Ultrapotassic mafic rocks and juvenile lower crustal rocks display low LOI values (<3 wt. %; Table S-3). A lack of correlation between the LOI and Hg concentrations, and δ²⁰²Hg or Δ¹⁹⁹Hg values of these rocks (Fig. S-5) suggests that the variations of Hg abundance and isotopic composition do not correlate with alteration. Since magmatic processes trigger minor Hg-MDF but not Hg-MIF (Moynier et al., 2021

Moynier, F., Jackson, M.G., Zhang, K., Cai, H., Halldórsson, S.A., Pik, R., Day, J.M.D., Chen, J. (2021) The Mercury Isotopic Composition of Earth’s Mantle and the Use of Mass Independently Fractionated Hg to Test for Recycled Crust. Geophysical Research Letters 48, e2021GL094301. https://doi.org/10.1029/2021GL094301

), we use Δ¹⁹⁹Hg instead of δ²⁰²Hg to constrain the sources of Hg in the rock samples.

The primitive mantle displays low Hg concentrations (0.4–0.6 ng/g) and near-zero Δ¹⁹⁹Hg (0.00 ± 0.10 ‰; 2 s.d.) (Moynier et al., 2021

Moynier, F., Jackson, M.G., Zhang, K., Cai, H., Halldórsson, S.A., Pik, R., Day, J.M.D., Chen, J. (2021) The Mercury Isotopic Composition of Earth’s Mantle and the Use of Mass Independently Fractionated Hg to Test for Recycled Crust. Geophysical Research Letters 48, e2021GL094301. https://doi.org/10.1029/2021GL094301

). The elevated Hg concentrations (1.79 ± 4.28 ng/g; 2 s.d., n = 5) and negative Δ¹⁹⁹Hg (−0.12 ± 0.33 ‰; 2 s.d., n = 5) values in ultrapotassic mafic rocks suggest the existence of recycled terrestrially derived Hg in their magma source, since Hg in terrestrial reservoirs displays negative Δ¹⁹⁹Hg values (Blum et al., 2014

Blum, J.D., Sherman, L.S., Johnson, M.W. (2014) Mercury Isotopes in Earth and Environmental Sciences. Annual Review of Earth and Planetary Sciences 42, 249–269. https://doi.org/10.1146/annurev-earth-050212-124107

). Measurements of Sr-Nd-Pb isotopes confirmed that these ultrapotassic mafic rocks were derived from the SCLM fertilised by terrestrial components via Indian Plate continental subduction (Wang et al., 2020

Wang, P., Zhao, G., Han, Y., Liu, Q., Zhou, N., Yao, J., Li, J., Li, Y. (2020) Post-collisional potassic rocks in Western Kunlun, NW Tibet Plateau: Insights into lateral variations in the crust-mantle structure beneath the India-Asia collision zone. Lithos 370–371, 105645. https://doi.org/10.1016/j.lithos.2020.105645

). Furthermore, Sr-Nd-Pb isotope data suggest that crustal material was incorporated early into the juvenile lower crust during the melting-assimilation-storage-homogenisation process (Miller et al., 1999

Miller, C., Schuster, R., Klötzli, U., Frank, W., Purtscheller, F. (1999) Post-Collisional Potassic and Ultrapotassic Magmatism in SW Tibet: Geochemical and Sr–Nd–Pb–O Isotopic Constraints for Mantle Source Characteristics and Petrogenesis. Journal of Petrology 40, 1399–1424. https://doi.org/10.1093/petroj/40.9.1399

), which could explain the negative Δ¹⁹⁹Hg values in juvenile lower crustal rocks.

Genesis of PCDs in the Himalayan-Tibetan belt. Due to the chalcophile behaviour of Hg and the lack of Hg-MIF during metallogenic processes (Deng et al., 2021

Deng, C., Sun, G., Rong, Y., Sun, R., Sun, D., Lehmann, B., Yin, R. (2021) Recycling of mercury from the atmosphere-ocean system into volcanic-arc–associated epithermal gold systems. Geology 49, 309–313. https://doi.org/10.1130/G48132.1

), Δ¹⁹⁹Hg can be used to trace the source of Hg and other metals in PCDs. The Δ¹⁹⁹Hg signals of sulfides in Yulong and Gangdese PCDs show negative to positive Δ¹⁹⁹Hg values of −0.29 to +0.21 ‰. These values are different from the overall positive Δ¹⁹⁹Hg values for epithermal Au deposits (−0.03 to +0.27 ‰) and porphyry Mo deposits (−0.05 to +0.24 ‰) in the Circum-Pacific belt (Fig. 2), which receive Hg from subducted marine sediments (Deng et al., 2021

Deng, C., Sun, G., Rong, Y., Sun, R., Sun, D., Lehmann, B., Yin, R. (2021) Recycling of mercury from the atmosphere-ocean system into volcanic-arc–associated epithermal gold systems. Geology 49, 309–313. https://doi.org/10.1130/G48132.1

; Gao et al., 2024a

Gao, L., Sun, D., Tian, Z., Luo, A., Lehmann, B., Yin, R. (2024a) Positive Δ¹⁹⁹Hg anomalies in Mesozoic porphyry Mo deposits of Northeastern China and their implications to the metallogeny of arc-related hydrothermal systems at convergent margins. Chemical Geology 645, 121880. https://doi.org/10.1016/j.chemgeo.2023.121880

).

Potential metal sources for PCDs of the Himalayan-Tibetan orogenic belt include the juvenile lower crust and SCLM (Wang et al., 2018

Wang, R., Weinberg, R.F., Collins, W.J., Richards, J.P., Zhu, D.-c. (2018) Origin of postcollisional magmas and formation of porphyry Cu deposits in southern Tibet. Earth-Science Reviews 181, 122–143. https://doi.org/10.1016/j.earscirev.2018.02.019

; Xu et al., 2023

Xu, L.-L., Zhu, J.-J., Huang, M.-L., Pan, L.-C., Hu, R., Bi, X.-W. (2023) Genesis of hydrous-oxidized parental magmas for porphyry Cu (Mo, Au) deposits in a postcollisional setting: examples from the Sanjiang region, SW China. Mineralium Deposita 58, 161–196. https://doi.org/10.1007/s00126-022-01143-x

). In Figure 2, 17 sulfides from the Gangdese PCDs and 9 sulfides from the Yulong PCD display Δ¹⁹⁹Hg values within the range of juvenile lower crustal rocks, suggesting that they received large fractions of Hg from the juvenile lower crust. However, the non-zero Δ¹⁹⁹Hg values in other sulfides in these PCDs, either below −0.1 ‰ or above +0.1 ‰, suggest that the juvenile lower crust was not the sole source of Hg. Photoreduction of Hg(II) results in negative and positive Δ¹⁹⁹Hg values in terrestrial and marine systems, respectively (Blum et al., 2014

Blum, J.D., Sherman, L.S., Johnson, M.W. (2014) Mercury Isotopes in Earth and Environmental Sciences. Annual Review of Earth and Planetary Sciences 42, 249–269. https://doi.org/10.1146/annurev-earth-050212-124107

). Three sulfides (chalcopyrite, pyrite) display positive Δ¹⁹⁹Hg values above +0.1 ‰, and 8 sulfides (chalcopyrite, pyrite) display negative Δ¹⁹⁹Hg values below −0.1 ‰ (Fig. 2), suggesting the existence of recycled Hg from both marine and terrestrial systems. Since the Himalayan-Tibetan orogenic belt was influenced by the Neo-Tethys oceanic subduction followed by the Indian-Eurasian continental collision, the coexistence of positive and negative Δ¹⁹⁹Hg values in the Gangdese and Yulong PCDs may be a consequence of dual stage fertilisation of the SCLM.

The Neo-Tethys oceanic subduction may have fertilised the SCLM by marine-derived Hg released from subducted marine sediments (Fig. 3a), as supported by positive Δ¹⁹⁹Hg values of ultrapotassic mafic rocks (Xu et al., 2024

Xu, B., Yin, R.-S., Chiaradia, M., Miao, Z., Griffin, W.L., Hou, Z.-Q., Yang, Z.-M., O’Reilly, S.Y. (2024) Mercury isotope evidence for the importance of recycled fluids in collisional ore systems. Science Advances 10, eadp7383. https://doi.org/10.1126/sciadv.adp7383

). The positive Δ¹⁹⁹Hg values above +0.1 ‰ in the studied Gangdese and Yulong PCDs confirm the contribution of recycled marine Hg released from the fertilised SCLM. Crustal assimilation, which produces S-type granites, may impart negative Δ¹⁹⁹Hg values to Gangdese and Yulong PCDs. However, the lack of coeval S-type melts in the eastern Gangdese area suggests limited crustal assimilation (Li et al., 2022

Li, W.-T., Ding, H.-X., Zhang, Z.-M., Larson, K.P. (2022) Petrogenesis of meta-sedimentary rocks in the deep crust of the eastern Gangdese arc. Lithos 430–431, 106884. https://doi.org/10.1016/j.lithos.2022.106884

). The negative Δ¹⁹⁹Hg values of Gangdese and Yulong PCDs are more reasonably explained by the Indian-Eurasian continental collision, which fertilised the SCLM by introducing abundant terrestrially derived Hg (Fig. 3b), as supported by the negative Δ¹⁹⁹Hg values of ultrapotassic mafic rocks (Xu et al., 2024

Xu, B., Yin, R.-S., Chiaradia, M., Miao, Z., Griffin, W.L., Hou, Z.-Q., Yang, Z.-M., O’Reilly, S.Y. (2024) Mercury isotope evidence for the importance of recycled fluids in collisional ore systems. Science Advances 10, eadp7383. https://doi.org/10.1126/sciadv.adp7383

). Following the dual stage fertilisation processes, the subducted Indian continental lithosphere underwent tearing, triggering upwelling of the asthenosphere, leading to melting of the metasomatised SCLM and juvenile lower crust, favouring the formation of large volumes of metal- and volatile-rich magma and an associated large number of PCDs (Hou et al., 2015

Hou, Z., Yang, Z., Lu, Y., Kemp, A., Zheng, Y., Li, Q., Tang, J., Yang, Z., Duan, L. (2015) A genetic linkage between subduction- and collision-related porphyry Cu deposits in continental collision zones. Geology 43, 247–250. https://doi.org/10.1130/G36362.1

; Zheng et al., 2019

Zheng, Y.-C., Liu, S.-A., Wu, C.-D., Griffin, W.L., Li, Z.-Q., Xu, B., Yang, Z.-M., Hou, Z.-Q., O’Reilly, S.Y. (2019) Cu isotopes reveal initial Cu enrichment in sources of giant porphyry deposits in a collisional setting. Geology 47, 135–138. https://doi.org/10.1130/G45362.1

; Xu et al., 2024

Xu, B., Yin, R.-S., Chiaradia, M., Miao, Z., Griffin, W.L., Hou, Z.-Q., Yang, Z.-M., O’Reilly, S.Y. (2024) Mercury isotope evidence for the importance of recycled fluids in collisional ore systems. Science Advances 10, eadp7383. https://doi.org/10.1126/sciadv.adp7383

).

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Conclusions

The formation of massive PCDs in the Tibetan Plateau is to be expected. Based on Hg isotopes, this study demonstrates that the Neo-Tethys oceanic subduction and Indian-Eurasia continental collision have extensively fertilised the SCLM beneath the Tibetan Plateau. These two processes transported large amounts of metals and volatiles from recycled terrestrial and marine sediments to the SCLM, providing a material base for the metallogeny of PCDs. This study highlights the powerful use of Hg isotopes in tracing recycled metals and volatiles in the mantle, and highlights the crucial role of multistage plate subduction and mantle fertilisation in the generation of PCDs in the Himalayan-Tibetan orogenic belt.

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Acknowledgements

This work is supported by the Hundred Talent Plan through the Chinese Academy of Sciences. We thank Rui Wang for providing some of the samples, Massimo Chiaradia for his assistance in data interpretation, and Manuel Keith and an anonymous reviewer for providing valuable comments.

Editor: Raul O.C. Fonseca

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References

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Supplementary Information

The Supplementary Information includes:

• Text S-1
• Tables S-1 to S-4
• Figures S-1 to S-5
• Supplementary Information References

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