Redox state of subducted sediments controls porphyry copper mineralization along the Tethyan belt

Domain: earth_science
Published: 2025-07-12
Editorial outcome: minor_revision

Abstract
The Tethyan orogenic belt hosts numerous world-class porphyry copper deposits, with most forming during the Cenozoic continental collision and fewer during earlier Mesozoic subduction. To understand this pattern, we integrate redox indicators from detrital zircon grains with constraints from sedimentary geology and granite geochemistry during these times. Our analysis reveals a major shift from reduced magmas forming during the Mesozoic to more oxidized intrusive systems in the Cenozoic. Here we show that subduction of organic-rich, reduced marine sediments in the Mesozoic suppressed the oxidation state of arc magmas, locking chalcophile elements in the lower crust and inhibiting the formation of porphyry Cu deposits. In contrast, the subduction of more oxidized continental sediments during Cenozoic collision elevated the mantle’s oxidation state, releasing stored copper to melts that form porphyry deposits. These findings highlight the critical role of redox state of subducted sediments and tectonic history in shaping the distribution of porphyry mineralization along the Tethyan belt.

Introduction
The Tethyan orogenic belt, extending for 12,000 km from the Pyrenees through the Alps and the Turkish–Iranian plateau, across Pakistan and the Himalayan–Tibetan Plateau, and into the Indochina Peninsula (Fig. 1a), is one of the world’s most important porphyry Cu belts1–4. This orogen formed through successive stages of continental breakup, ocean basin formation, and eventual collision between Gondwana-derived continents and Eurasia (Supplementary Fig. 1 and Supplementary Material). The Tethys realm can be divided chronologically into the Proto-, Paleo-, and Neo-Tethys oceans, formed during the Early Paleozoic, Late Paleozoic, and Mesozoic, respectively5. Of particular interest is the Neo-Tethys Ocean, which opened in the Early Permian and underwent protracted subduction during the Mesozoic, culminating in extensive Cenozoic continental collisions that led to the formation of porphyry Cu deposits in regions such as the Gangdese and Yulong belts (Tibet), the Kerman belt (Iran), and the Anatolides belt (Turkey)1,2,5–8 (Fig. 1a and Supplementary Material).

Despite a long Mesozoic subduction history, significantly fewer and smaller porphyry Cu deposits formed during subduction-related magmatism than during Cenozoic collisional or post-collisional magmatic stages in the Tethyan belt (Figs. 1b, 2c). The reason for this disparity remains elusive. A key control on porphyry copper formation is the oxidation state of the magmatic system, i.e., magmatic oxygen fugacity, commonly expressed as ΔFMQ (the log of oxygen fugacity relative to the fayalite–magnetite–quartz buffer). Oxidized arc magmas (ΔFMQ ≈ +1 to +2) favor the mobilization of chalcophile elements, enabling their transport to the upper crust9,10. Detrital zircon provides a valuable record of magmatic redox conditions over geologic timescales. Global geological events–particularly oceanic anoxic events and seawater incursion events–can influence the geochemical composition of subducted sediments11,12, altering mantle wedge oxygen fugacity and affecting porphyry mineralization potential.

Here, we apply a multidisciplinary approach to track the oxygen fugacity evolution of Tethyan magmas from the Early Jurassic to the Miocene. Using the novel zircon oxybarometer, we reconstruct ΔFMQ through time and integrate these data with sedimentary rocks, global sea-level and temperature changes, whole-rock geochemical data, and the timing of major geological events. Our findings illuminate the mechanisms responsible for redox variations in the mantle wedge and their implications for subduction- vs. collision-related porphyry Cu deposits.

Results
Redox variation from Mesozoic to Cenozoic
Detrital zircon oxybarometry reveals an overall increase in ΔFMQ from the Early Jurassic through the Miocene in the Tethyan belt (Fig. 2d). Although data density during the Jurassic to Early Cretaceous is relatively low and thus more variable, Mesozoic ΔFMQ values are predominantly below +1. By contrast, Cenozoic ΔFMQ values commonly exceed +1, consistent with a transition to more oxidized magmatic conditions (Fig. 2d). The V/Sc ratios are also widely used as a redox proxy in igneous rocks, with higher ratios indicating more oxidized magmas19. The oxidation states of mid-oceanic ridge basalts (MORBs) close to ΔFMQ ~ 0 have V/Sc ratios of 6.74 ± 1.11. We compiled V/Sc data from Tethyan magmatic rocks (Supplementary Data 3). The results shows the lower V/Sc ratios of Mesozoic rocks (median = 6.9, n = 631) are like MORBs, while the Cenozoic rocks have higher V/Sc ratios (median = 8.4, n = 786) (Supplementary Fig. 2). The V/Sc results are consistent with our zircon ΔFMQ results, both indicated the Cenozoic ƒO2 is higher than the Mesozoic. Under low ƒO2, sulfur tends to form sulfides. The chalcophile elements (e.g., Cu, Au) have high partition coefficients between sulfide and silicate melts10. As a result, the chalcophile elements were sequestered in the lower crust via sulfide accumulation under low ƒO2, resulting in rare mineralization in the upper crust9,10,20. This process parallels the anoxic conditions inferred for the Paleo-Tethys Ocean basin during the Permian when the basin was in a restricted environment near the equator21,22. The correlation between redox state and porphyry Cu deposit frequency (Fig. 2c, d) supports redox-driven control over the spatiotemporal distribution of Tethyan porphyry Cu mineralization.

Methods
Materials
The zircon age and trace elements are compiled from the global detrital zircon database68 and our newly acquired data in Tibet. The complete database includes 13981 detrital zircons (see details in Supplementary Data 2).

Zircon U–Pb dating and trace element analyses
Zircon grains were separated from two rivers in Tibet and mounted in epoxy resin for age and trace element determinations. Zircon U–Pb dating and trace element analyses were conducted simultaneously by laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) in the Mineral and Fluid Inclusion Microanalysis Laboratory, Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China. The NWR193UC laser ablation system (Elemental Scientific Lasers, USA) was equipped with a Coherent Excistar 200 excimer laser and a two-volume ablation cell. The laser ablation system was coupled to an Agilent 7900 ICP–MS instrument (Agilent, USA). Zircons were mounted in epoxy resin discs, polished to expose grain interiors, cleaned ultrasonically in ultrapure water, and then cleaned again before analysis using analytical-grade methanol. Pre-ablation was conducted before each spot analysis using five laser shots (~0.3 μm in depth) to remove potential surface contamination. The analyses used a 30 μm laser beam diameter with a laser frequency of 8 Hz and fluence of 2 J/cm2. The Iolite software package was used for data reduction69. Zircons 91500 and GJ-1 were the primary and secondary reference materials, respectively. The exponential function was used to correct for down-hole fractionation70. NIST 610 and 91Zr were used to calibrate the trace element concentrations as an external reference material and internal standard, respectively. Zircon age and trace element data are listed in Supplementary Data 2.

Calculation of oxygen fugacity
Data points outside the Tethys domain were excluded. Zircon ages younger than 200 Ma are used for the following discussion to minimize the influence of the Paleo-Tethys Oceanic subduction because the closure of the Paleo-Tethys Ocean occurred in the Late Permian–Late Triassic5. Zircons with a Th/U ratio of <0.1 were used to exclude metamorphic zircon71. Moreover, zircons derived from S-type granites were screened out using zircon P and REE contents72. After screening, 3010 zircon grains were used for reconstructing the Tethyan oxygen fugacity variation (Supplementary Data 2). The novel magmatic oxybarometer using ratios of Ce, U, and Ti in zircon is independent of temperature and pressure, with a standard error of ±0.6 log unit ƒO2 73. The ΔFMQ values can be calculated by the equation:

ΔFMQ = 3.998 (±0.124) × log[Ce / (U × T)] + 2.284 (±0.101) (1)

where Ui denotes age-corrected initial U content73. The ΔFMQ values are plotted as binned averages (bin size = 5 Myr).

Monte Carlo isotopic simulation
The Sr-Nd concentrations and isotopic ratios of the sediment–depleted mantle mixture can be calculated using the following equations:

Cx = Cs × F + Cm × (1 − F) (2)

Rx = Rs × F × Cs / Cx + Rm × (1 − F) × Cm / Cx (3)

where x is Sr or Nd, Cx, Cs, and Cm are element concentrations of x after mixing, sediment, and depleted mantle, respectively. Rx, Rs, and Rm are isotopic ratios of x after mixing, sediment, and depleted mantle, respectively, and F is the proportion of sediment-derived melt.

To obtain as many as possible mixture scenarios, random two pelagic or terrigenous sediments were selected and mixed to create a new pelagic or terrigenous sediment end-member. Then the new sediment end-member was mixed with the depleted mantle in random proportions. 30,000 pelagic sediment–mantle mixtures and 30,000 terrigenous sediment–mantle mixtures were generated using this method. The simulation results are shown in Fig. 4c, d.

Data availability
The data that support the findings of this study are available at Figshare (https://doi.org/10.6084/m9.figshare.28424204).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript \"Types of Subducted Material Controlling Tethyan Porphyry Copper Mineralization\" presents a compelling\nmultidisciplinary investigation of redox evolution in the Tethyan orogenic belt and its critical role in porphyry copper\nmineralization. By innovatively integrating detrital zircon oxybarmetry, geochemical analyses, stratigraphic constraints, and\nglobal geological events, the authors establish a robust model linking subducted sediment redox states to porphyry copper\nformation. This work successfully addresses the long-standing enigma of porphyry copper scarcity in the Tethyan belt. The\nmethodology is rigorous and the writing is clear. I recommend publication in Nature Communication after addressing the\nfollowing improvements.\n1. In this manuscript, the authors proposed a model about the redox of subducted sediments controlling the Tethyan\nporphyry Cu mineralization. Is the model a universal or regionally confined process? Is it applicable for other porphyry Cu\nbelts (e.g., Andes or Circum-Pacific arcs where porphyry deposits are primarily related to oceanic subduction)?\n2. The authors focus narrowly on porphyry Cu deposits but does not explore implications for other redox-sensitive mineral\nsystems. If possible, provide a brief discussion on how redox-controlled processes in the Tethyan belt might inform\nexploration for other deposit types globally.\n3. The authors use zircon oxybarometry to estimate the variation of mantle wedge oxygen station from the Early Jurassic to\nthe Miocene, and further link this variation to subduction input of oxidized or reduced sediments. However, zircon is mainly\ncrystallized from felsic rocks that are not always generated via differentiation of mantle-derived magmas, but also can form\nby partial melting of crustal materials. How to discriminate the detrital zircons in river sediments that were crystallized from\nmantle- or crustal-derived melts? Moreover, magmatic rocks formed in different tectonic settings (e.g., arc vs. back-arc) may\nalso have distinct oxygen fugacity.\nOther comments:\nLine 73: This manuscript states the scarcity of Mesozoic subduction-related porphyry Cu deposits. What about the giant\nCretaceous porphyry Cu deposits in western Tethys (e.g., Romania, Serbia)?\nLine 86–88: The discussion briefly mentions sulfur speciation (e.g., sulfides vs. sulfates) but does not elaborate on how this\naffects metal mobility. Please include a more detailed discussion of sulfur speciation under different redox conditions and\nhow this influences the transport of chalcophile elements (e.g., Cu, Au).\nLine 94-98: Please Elaborate on how sediments (e.g., organic-rich /oxidized) release reduced/oxidizing fluids during\nsubduction, affecting mantle wedge oxygen fugacity. If possible, provide redox reactions of subducted sediments in the\nmantle source.\nLine 217–236: What is the feasibility of zircon as an exploration tool? What is the difference between detrital zircons and\nmagmatic zircons as exploration implications, and their limitation?\nLine 261: The detrital zircon oxybarometry method is applied here to trace the oxidation state evolution, while the results are\nnot cross-validated with other redox proxies (e.g., whole-rock V/Sc ratios) or zircons from magmatic rocks. Compare ∆FMQ\nvalues with other redox indicators from Tethyan magmatic rocks (if available).\nFigure 1: Add a scale bar in Figure 1a to facilitate distance estimation, even though it already includes latitude and\nlongitude.\nFigure 2: Add text in Figure 2e to indicate the position of the Neo-Tethys Ocean and major continents.\nOverall, this work represents a significant advancement in understanding metallogenic controls in collisional settings. Timely\npublication would substantially benefit both academic research and mineral exploration communities. I look forward to\nseeing the revised version.", "reviewer_2": "This manuscript reads well. It is a good compilation of a large amount of data that confirms the paper published in Geology\nby Richards and Sengor, 2017. Richards and Sengor showed that very few porphyry copper deposits formed during the\nPaleo-Tethyan subduction due to organic-rich sediments subducted to the mantle, and that large deposits formed during\nlater Neo-Tethyan subduction. This manuscript provides the compilation of data related to the oxidation conditions of\nmagmas using the well-established method based on zircon trace-element composition, to confirm the shift in oxidation\nconditions of igneous rocks, which reflects the source mantle.\nThis manuscript compiles types of sediments, the location and size of porphyry copper deposits, and data of host igneous\nrocks (Supplementary text and data). In this sense, this manuscript provides a useful compilation of data from many porphyry\ncopper deposits along the Tethyan subduction zone. As mentioned, the data compilation confirms the conclusion of\nRichards and Sengor, that there was a change in the oxidation conditions of the mantle and in the resultant magmas. The\nmanuscript also confirms other earlier publications, that oxidized magmas are conducive for porphyry copper mineralization\n(e.g., Sillitoe, 2010; Shen et al., 2015; Loucks et al., 2024), and that subducted sediments influence the oxidation conditions\nof the mantle and magmas (e.g., Wang et al., 2007; Evans, 2012),\nThe authors stress formation of large porphyry copper deposits during continental collision; the Neo-Tethys plate and\noverlying sediments were subducted long after the closure of the Neo-Tethys, resulting in mantle metasomatization. The\ncontinental collision and closure of the sea does not mean the end of subduction of oceanic plate. This has been well\nillustrated by many tomographic images of subducted slabs below the Asian continent. Therefore, emphasizing the collision\nstage for large porphyry copper deposits is misleading and confuses the issue.\nThe manuscript has stratigraphic columns of sediments, to provide support for the authors’ proposal, but the sections are\nessentially similar to Figure 3 of the paper by Richards and Sengor. During the Mesozoic, sediments were reduced, such\nthat their subduction led to the mantle being reduced, resulting in the formation of reduced magmas.", "reviewer_3": "Overall, this paper is well-written and presents a compelling narrative. The authors have compiled a comprehensive dataset\nencompassing porphyry deposits, sedimentary rocks, global sea-level and temperature fluctuations, whole-rock\ngeochemistry, and major geological events. Their analysis reveals a strong correlation between redox variations and key\ngeological events in the Tethyan domain. Furthermore, they highlight the critical role of subducted sediment composition in\nmodulating redox conditions and controlling porphyry Cu deposit formation. Given the novelty and significance of these\nfindings, I believe this manuscript is well-suited for Nature Communications. I recommend minor revisions and suggest a few\npoints that could further strengthen the manuscript during the revision process:\n1. Lines 173–174: The authors state that as mantle-derived ultrapotassic magmas ascend from ~56 km to ~15 km depth,\ntheir ΔFMQ increases from +0.8 to +3.0, yet their ability to significantly oxidize magma in the lower crust remains limited.\nHowever, in Lines 50–51, they also mention that oxidized arc magmas (ΔFMQ ≈ +1 to +2) promote the mobilization of\nchalcophile elements, facilitating their transport to the upper crust. Considering the uncertainties in oxygen fugacity\ncalculations, FMQ +0.8 is nearly equivalent to FMQ +1. Additionally, oxygen fugacity estimations from detrital zircons\ninherently carry errors. The authors could cite more recent references, as the oxygen fugacity of porphyry-related magmas is\ngenerally considered to be higher in the deep source with high pressure.\n2. The authors should briefly describe the zircon screening method and oxygen fugacity calculation in the main text. This\nwould provide better context for the Ce-U-Ti data and the europium anomaly-to-ytterbium ratio introduced later in the paper.\n3. Figure 1b is highly informative, but the legends require clearer explanations to enhance readability and facilitate\ninterpretation.\n4. Lines 182–183: The authors dismiss the autoxidation models of garnet and other minerals, arguing that these processes\ndo not apply to mantle source oxidation. However, in Figure 5b, they propose that sulfides were oxidized to sulfates in the\nlower crust resulted from injection of oxidized ultrapotassic magmas. The authors need to give relative more detailed\nintroduction on how oxidizing agents are introduced during continental subduction, leading to high-fO2 mantle-derived\nmagmas that subsequently oxidize the juvenile lower crust."}, "author_response": "Response to Reviewer #1\nThe manuscript \"Types of Subducted Material Controlling Tethyan Porphyry Copper\nMineralization\" presents a compelling multidisciplinary investigation of redox\nevolution in the Tethyan orogenic belt and its critical role in porphyry copper\nmineralization. By innovatively integrating detrital zircon oxybarometry, geochemical\nanalyses, stratigraphic constraints, and global geological events, the authors establish a\nrobust model linking subducted sediment redox states to porphyry copper formation.\nThis work successfully addresses the long-standing enigma of porphyry copper scarcity\nin the Tethyan belt. The methodology is rigorous and the writing is clear. I recommend\npublication in Nature Communication after addressing the following improvements.\n1. In this manuscript, the authors proposed a model about the redox of subducted\nsediments controlling the Tethyan porphyry Cu mineralization. Is the model a universal\nor regionally confined process? Is it applicable for other porphyry Cu belts (e.g., Andes\nor Circum-Pacific arcs where porphyry deposits are primarily related to oceanic\nsubduction)?\nResponse: We agree that clarifying the applicability of our model is critical. The Neo-\nTethyan Ocean is an east-west trend that has been near the equator for a long time and\nhas undergone oceanic opening, oceanic subduction, and closing processes. The Andes\nor Circum-Pacific porphyry copper belt is related to the subduction of the (Paleo-)\nPacific Ocean, which has a south-north trend and is always in an open environment and\nless prone to anoxia. In addition, the oxygen fugacity of the north-south trending Andes\narc may be latitude-dependent (Hu et al., 2024, NC). Therefore, the redox state is a\nuniversal control for porphyry deposits, while the drivers of redox variations differ\nregionally. Please see the new Lines 100–105.\n2. The authors focus narrowly on porphyry Cu deposits but does not explore\nimplications for other redox-sensitive mineral systems. If possible, provide a brief\ndiscussion on how redox-controlled processes in the Tethyan belt might inform\nexploration for other deposit types globally.\nResponse: Thanks for the suggestion. We added a brief discussion on how redox-\ncontrolled processes could guide exploration for W-Sn deposits (New Lines 251–256).\nFor example, high oxygen fugacity environments favor porphyry Cu mineralization,\nwhereas leads to massive precipitation of cassiterite and scheelite and hinders an\nefficient transport of Sn-W elements from the magma source to the crust (Wu et al.,\n2021, OGR). This expands the model’s relevance beyond porphyry Cu systems.\n3. The authors use zircon oxybarometry to estimate the variation of mantle wedge\noxygen station from the Early Jurassic to the Miocene, and further link this variation to\nsubduction input of oxidized or reduced sediments. However, zircon is mainly\ncrystallized from felsic rocks that are not always generated via differentiation of\nmantle-derived magmas, but also can form by partial melting of crustal materials. How\nto discriminate the detrital zircons in river sediments that were crystallized from\nmantle- or crustal-derived melts? Moreover, magmatic rocks formed in different\ntectonic settings (e.g., arc vs. back-arc) may also have distinct oxygen fugacity.\nResponse: We thank the reviewer for raising this critical methodological concern. We\nclarified in the Methods Section that detrital zircons were filtered using zircon P and\nREE contents to exclude crustal sources (Zhu et al., 2020, EPSL). It is challenging to\nidentify the tectonic setting with detrital zircon. This manuscript focuses on the\nstatistical variation of oxygen fugacity with time.\nLine 73: This manuscript states the scarcity of Mesozoic subduction-related porphyry\nCu deposits. What about the giant Cretaceous porphyry Cu deposits in western Tethys\n(e.g., Romania, Serbia)?\nResponse: The oxidation state of the mantle wedge and related arc magmas in\nsubduction zones is heterogeneous due to the input of fluids and melts derived from the\nsubducted slab (Kelley and Cottrell, 2009, Science; Brounce et al., 2015, Geology;\nLi et al., 2020, Geology). The magmatic zircon trace elements indicate the oxygen fugacity\nfrom the Cretaceous porphyry deposits in Serbia are mainly ∆FMQ ≈ 0 to +1, with\nminor ∆FMQ >1 (Klimentyeva et al., 2022, Geosciences). Overall, the magmatic\noxidation state in the Mesozoic is lower than Cenozoic.\nLine 86–88: The discussion briefly mentions sulfur speciation (e.g., sulfides vs. sulfates)\nbut does not elaborate on how this affects metal mobility. Please include a more detailed\ndiscussion of sulfur speciation under different redox conditions and\nhow this influences the transport of chalcophile elements (e.g., Cu, Au).\nResponse: The oxygen fugacity influences sulfur speciation. Under low oxygen\nfugacity, the sulfur exits in the form of sulfide (S2-) and promotes precipitation of Cu2+\nin the magma source, as a result, hindering the transport of chalcophile elements to the\nshallow crust. In contrast, high oxygen fugacity causes sulfides (S2-) to oxidize to\nsulfates (SO 2-), facilitating chalcophile elements' transport to the shallow crust (Sun et\nal., 2015, OGR). We have added detail in the main text (New Lines 91–93).\nLine 94-98: Please Elaborate on how sediments (e.g., organic-rich /oxidized) release\nreduced/oxidizing fluids during subduction, affecting mantle wedge oxygen fugacity.\nIf possible, provide redox reactions of subducted sediments in the mantle source.\nResponse: We thank the reviewer for this critical question. Below, we elaborate on\nhow organic-rich (reduced) and oxidized sediments release contrasting fluids during\nsubduction, modulating mantle wedge oxygen fugacity with redox reactions: See\nrevised texts in new Lines 120–123.\n(i) The breakdown of black shales with organic carbon (kerogen):\nKerogen→CH +H O+CO , and CH acts as a reducing agent, consuming Fe3+ and O\n4 2 2 4 2\nvia: CH +Fe O +O →FeO+CO +H (Thakurta et al., 2008; Wang et al., 2007).\n4 2 3 2 2 2\n(ii) The subducted carbonate interacts with mantle peridotite/pyroxenite:\nOrthopyroxene + Carbonate → Clinopyroxene + Olivine + CO (e.g., at 1.5–3.0 GPa,\n2\n1150–1300 ℃), which would typically consume reduced Fe2+ and increase mantle’s\noxygen fugacity (Dalton and Wood, 1993; Foley et al., 2009; Rohrbach and Schmidt,\n2011).\nLine 217–236: What is the feasibility of zircon as an exploration tool? What is the\ndifference between detrital zircons and magmatic zircons as exploration implications,\nand their limitation?\nResponse: We appreciate the reviewer’s inquiry regarding the utility of zircon in\nmineral exploration. Zircon is a robust and versatile exploration tool due to its chemical\nand physical resilience; preserving critical factors for porphyry Cu formation (e.g.,\noxidation and hydration state); and cost-effectiveness.\nDetrital zircons come from multiple source regions, so their geochemical characteristics\nreflect the variation over long timescales in regional geological background. While\nmagmatic zircons are directly from the specific magmatic unit, which can more directly\nreflect the nature of specific parent magma, they require exposure to intrusive rocks,\nlimiting application in covered terrains. Please see the new Lines 237–239.\nLine 261: The detrital zircon oxybarometry method is applied here to trace the oxidation\nstate evolution, while the results are not cross-validated with other redox proxies (e.g.,\nwhole-rock V/Sc ratios) or zircons from magmatic rocks. Compare ∆FMQ values with\nother redox indicators from Tethyan magmatic rocks (if available).\nResponse: We appreciate the reviewer’s critical suggestion to cross-validate our zircon\nΔFMQ results with other redox proxies. The V/Sc ratios are widely used as a redox\nproxy in igneous rocks, with higher ratios indicating more oxidized magmas (Li and\nLee, 2004, EPSL). The oxidation states of mid-oceanic ridge basalts (MORBs) close to\n∆FMQ ~ 0 have V/Sc ratios of 6.74 ± 1.11. We compiled V/Sc data from Tethyan\nmagmatic rocks (Supplementary Fig. 3). The result shows the lower V/Sc ratios of\nJurassic and Cretaceous rocks are similar to MORBs, while the Cenozoic rocks have\nhigher V/Sc ratios. The V/Sc results are consistent with our zircon ΔFMQ results.\nPlease see new Lines 83–90 and New Supplementary Fig. 2.\nSupplementary Fig. 2 whole-rock V/Sc ratios in Neo-Tethyan igneous rocks. Data\nsource: GEOROC database and include samples with SiO between 58–70 wt%.\nalready includes latitude and longitude.\nResponse: Thanks for the suggestion. A scale bar has been added in Fig 1a.\nFigure 2: Add text in Figure 2e to indicate the position of the Neo-Tethys Ocean and\nmajor continents.\nResponse: Thanks for the suggestion. We have labeled the Neo-Tethys Ocean and\nmajor continents in Fig 2e.\nOverall, this work represents a significant advancement in understanding metallogenic\ncontrols in collisional settings. Timely publication would substantially benefit both\nacademic research and mineral exploration communities. I look forward to seeing the\nrevised version.\nResponse to Reviewer #2\nThis manuscript reads well. It is a good compilation of a large amount of data that\nconfirms the paper published in Geology by Richards and Sengor, 2017. Richards and\nSengor showed that very few porphyry copper deposits formed during the Paleo-\nTethyan subduction due to organic-rich sediments subducted to the mantle, and that\nlarge deposits formed during later Neo-Tethyan subduction. This manuscript provides\nthe compilation of data related to the oxidation conditions of magmas using the well-\nestablished method based on zircon trace-element composition, to confirm the shift in\noxidation conditions of igneous rocks, which reflects the source mantle.\nResponse: We appreciate the reviewer’s comparison of our work to Richards & Şengör\n(2017), and we acknowledge their foundational contributions to understanding Paleo-\nTethyan anoxia and its impact on magma fertility. However, our work is distinct from\nRichards and Şengör (2017) in several ways:\n(i) Scope of Study: Richards and Şengör (2017) focused on the sparsity of porphyry\ncopper deposits associated with Paleo-Tethyan (Permian to Jurassic) ocean subduction,\nwhereas our work focuses on the Neo-Tethyan (Cretaceous-Cenozoic) ocean\nsubduction. The Paleo-Tethys and Neo-Tethys oceans are completely different entities.\n(ii) Findings: Richards and Şengör (2017) emphasized that most porphyry copper\ndeposits in the Tethyan belt formed during subduction of the Neo-Tethyan ocean basin\nin the late Mesozoic–Cenozoic due to high magmatic oxidation states. Our findings are\nopposite; we identified that most porphyry deposits in the Tethyan belt are associated\nwith Cenozoic continental collision, while fewer are formed during subduction of the\nNeo-Tethyan ocean. The selected Neo-Tethyan suites in Richards and Şengör (2017)\nare from Cenozoic continental collision-related magmas, which cannot reflect the\noxidation state during Neo-Tethyan oceanic subduction. We recalculated the V/Sc\nratios for the pre-collisional Neo-Tethyan Mesozoic suites and found low V/Sc ratios\n(median=6.9, n=786), contrasting with the high V/Sc ratios from Richards and Şengör\n(2017), validating our independent redox constraints by zircon oxybarometry (Fig. 2d).\nThis manuscript compiles types of sediments, the location and size of porphyry copper\ndeposits, and data of host igneous rocks (Supplementary text and data). In this sense,\nthis manuscript provides a useful compilation of data from many porphyry copper\ndeposits along the Tethyan subduction zone. As mentioned, the data compilation\nconfirms the conclusion of Richards and Sengor, that there was a change in the\noxidation conditions of the mantle and in the resultant magmas. The manuscript also\nconfirms other earlier publications, that oxidized magmas are conducive for porphyry\ncopper mineralization (e.g., Sillitoe, 2010; Shen et al., 2015; Loucks et al., 2024), and\nthat subducted sediments influence the oxidation conditions of the mantle and magmas\n(e.g., Wang et al., 2007; Evans, 2012),\nResponse: We appreciate the reviewer’s acknowledgment of our comprehensive data\ncompilation. While our work confirms key principles linking magmatic oxidation to\nporphyry Cu fertility, it advances the field in three novel ways, addressing unresolved\nquestions and introducing new methodologies:\nPrior studies (e.g., Richards and Şengör, 2017; Sillitoe, 2010) established broad\ncorrelations between oxidation state and mineralization. However, our detrital zircon\nΔFMQ (Fig. 2d) provides the first quantitative record of redox evolution across the\nTethyan belt. We found a transition from a reduced environment during Mesozoic\noceanic subduction (ΔFMQ < +1) to an oxidized environment during Cenozoic\ncontinental collision (ΔFMQ > +1), which has not been previously constrained. By\ncorrelating zircon ΔFMQ with stratigraphic records of organic-rich vs. carbonate\nsediments (Fig. 3), we demonstrate that sediment composition directly modulates\nmantle redox.\nThe earlier works focused on single qualitative parameters (e.g., whole-rock V/Sc ratio),\nour study integrates diverse quantitative and qualitative proxies to trace the variation of\noxygen fugacity.\nA key unresolved issue in Richards and Şengör (2017) was how Neo-Tethyan Mesozoic\nbarren magmas relate to Cenozoic mineralization. Our two-stage model (Fig. 5)\naddresses this: Reduced magmas sequestered Cu in lower-crustal sulfides during the\nMesozoic and increase of oxygen fugacity during the Cenozoic remobilized these\nsulfides, enabling large-scale mineralization.\nThe authors stress formation of large porphyry copper deposits during continental\ncollision; the Neo-Tethys plate and overlying sediments were subducted long after the\nclosure of the Neo-Tethys, resulting in mantle metasomatization. The continental\ncollision and closure of the sea does not mean the end of subduction of oceanic plate.\nThis has been well illustrated by many tomographic images of subducted slabs below\nthe Asian continent. Therefore, emphasizing the collision stage for large porphyry\ncopper deposits is misleading and confuses the issue.\nResponse: We agree that the Neo-Tethys oceanic plate and overlying sediments would\nsubduct long after the closure of the Neo-Tethys Ocean. The continental collision-\nrelated porphyry deposits in our manuscript mainly refer to post-collisional porphyry\ndeposits. For example, the Gangdese porphyry Cu deposits mainly formed in the\nMiocene, which is about 40~45 Myr later than the initial collision between Indian and\nAsian continents (~55–60 Ma). The breakoff of the oceanic plate happened in ~45 Ma\n(Zhu et al., 2022). Thus, it is unlikely the post-collision porphyry mineralization is\nrelated to the mantle metasomatism by the Neo-Tethys oceanic slab. Instead, the mantle\nis more likely metasomatized by the subducted continental plate. We intended to\nemphasize that the composition of subducted sediments changed fundamentally from\noceanic subduction to the post-collisional stage, which directly drove mantle oxidation\nand porphyry Cu formation.\nThe manuscript has stratigraphic columns of sediments, to provide support for the\nauthors’ proposal, but the sections are essentially similar to Figure 3 of the paper by\nRichards and Sengor. During the Mesozoic, sediments were reduced, such that their\nsubduction led to the mantle being reduced, resulting in the formation of reduced\nmagmas.\nResponse: We appreciate this comment. However, our stratigraphic columns differ\nfrom Figure 3 of Richards and Şengör (2007), which is evident in the ages of these\ncolumns. For example, Figure 3 of Richards and Şengör (2007) covers Carboniferous\nto Triassic stratigraphic columns from the Paleo-Tethyan Ocean basin. In contrast, our\nFigure 3 provides representative stratigraphic columns from Jurassic to Neogene for the\nNeo-Tethys. Additionally, our stratigraphic columns are located on the passive\ncontinental margins to the south of the Neo-Tethys Ocean (Fig. S1C). Therefore, the\nnorthward subduction of these sediments is most likely to influence the mantle source\nbeneath the Tethyan Cu belt. Consequently, our stratigraphic columns support the\nsediment-driven oxidation variation.\nResponse to Reviewer #3\nOverall, this paper is well-written and presents a compelling narrative. The authors have\ncompiled a comprehensive dataset encompassing porphyry deposits, sedimentary rocks,\nglobal sea-level and temperature fluctuations, whole-rock geochemistry, and major\ngeological events. Their analysis reveals a strong correlation between redox variations\nand key geological events in the Tethyan domain. Furthermore, they highlight the\ncritical role of subducted sediment composition in modulating redox conditions and\ncontrolling porphyry Cu deposit formation. Given the novelty and significance of these\nfindings, I believe this manuscript is well-suited for Nature Communications. I\nrecommend minor revisions and suggest a few points that could further strengthen the\nmanuscript during the revision process:\n1. Lines 173–174: The authors state that as mantle-derived ultrapotassic magmas\nascend from ~56 km to ~15 km depth, their ΔFMQ increases from +0.8 to +3.0, yet\ntheir ability to significantly oxidize magma in the lower crust remains limited. However,\nin Lines 50–51, they also mention that oxidized arc magmas (ΔFMQ ≈ +1 to +2) promote the mobilization of chalcophile elements, facilitating their transport to the upper crust. Considering the uncertainties in oxygen fugacity calculations, FMQ +0.8\nis nearly equivalent to FMQ +1. Additionally, oxygen fugacity estimations from detrital\nzircons inherently carry errors. The authors could cite more recent references, as the\noxygen fugacity of porphyry-related magmas is generally considered to be higher in the\ndeep source with high pressure.\nResponse: The stability field of sulfide shifts significantly towards more oxidized\nconditions with increasing pressure (Matjuschkin et al., 2016). At the depth of the lower\ncrust (1~2 GPa), oxidizing sulfides to sulfates requires higher oxygen fugacity\n(ΔFMQ>3). Therefore, the ability of ultrapotassic magma to oxidize magma in the\nlower crust (ΔFMQ ≈ +0.8) is limited. See revised texts in new Lines 185–189.\n2. The authors should briefly describe the zircon screening method and oxygen fugacity\ncalculation in the main text. This would provide better context for the Ce-U-Ti data and\nthe europium anomaly-to-ytterbium ratio introduced later in the paper.\nResponse: Thanks for the suggestion. A description of zircon screening and\noxybarometry calculations has been provided in Method.\n3. Figure 1b is highly informative, but the legends require clearer explanations to\nenhance readability and facilitate interpretation.\nResponse: Thanks for the suggestion. Legends have been clearly explained in the\ncaption.\n4. Lines 182–183: The authors dismiss the autoxidation models of garnet and other\nminerals, arguing that these processes do not apply to mantle source oxidation.\nHowever, in Figure 5b, they propose that sulfides were oxidized to sulfates in the lower\ncrust resulted from injection of oxidized ultrapotassic magmas. The authors need to\ngive relative more detailed introduction on how oxidizing agents are introduced during\ncontinental subduction, leading to high-fO mantle-derived magmas that subsequently\n2\noxidize the juvenile lower crust.\nResponse: Thanks for your comment. We didn’t refer to the ultrapotassic magmas in\nFigure 5b, instead, we think the evaporite or carbonate serve as agents to increase\noxygen fugacity. More detail has been added in the main text (New Lines 179–183,\n228–233)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I have checked the detailed point-by-point responses and the revised manuscript; I found the authors have adequately dealt with most of the reviewer’s concerns and critiques. Thus, I recommend the paper be published in Nature Communications.", "reviewer_2": "The revised manuscript is clearer in presenting the authors idea. The authors report a change in the oxidation conditions of magmas from the Paleozoic-Mesozoic to Tertiary periods, based zircon trace elements with porphyry copper deposits formed in the Tertiary. The authors propose that the change in the oxidation conditions of magmas was a critical factor in the formation of porphyry copper deposits. The authors argue that earlier magmas formed metal sulphide cumulates in the lower crust, whereas subsequently oxidized Tertiary magmas were able to release these metals to form porphyry copper deposits.The concentration of metals as cumulates in earlier intrusions and later remelting of the metal-containing cumulate is an interesting idea in the process of forming porphyry copper deposits; this idea was first proposed by Richards, 2009, and Hou et al., 2015, both in Geology. However it is not clear, what is the evidence for the formation of metal sulphide cumulates? and how can this be established, formation of sulphide cumulates during the Paleozoic and Mesozoic, up until the Tertiary period.\nThe authors imply that this is a globally applicable model, but large porphyry copper deposits occur in North and South America, Indonesia, and Philippines. There is no record of temporal change in oxidation conditions of magmas in these areas that contain giant porphyry copper deposits.\nDuring revision, the authors added new statements to the manuscript. Some are not correct.\nFor example, in their rebuttal note, the authors suggest that the Andes and the Circum-Pacific porphyry copper belts are related to the subduction of a (paleo-) Pacific plate. This is not correct.\nThe authors wrote that “magma oxygen fugacity of the north-south trending Andes arc may be latitutde-dependent” in the rebuttal note and L.102- of the main text). In the South American Andes, porphyry copper deposits and oxidized igneous rocks have formed from close to the equator to 35 S. Therefore, there is no evidence to support this sentence.\nIn the revised manuscript, the authors suggest that an injection of oxidized ultrapotassic rocks in the lower crust contributed to porphyry copper mineralization (L. 182). Ultrapotassic rocks are not common around porphyry copper deposits. There is no evidence for this statement.", "reviewer_3": "The authors have addressed all my comments thoroughly and provided well-reasoned responses. I believe the manuscript is now well-suited for publication in Nature Communications.\nThank you for the opportunity to review this insightful work."}, "author_response": "Response letter\nWe thank the reviewers for their thoughtful comments and constructive feedback.\nBelow, we respond point-by-point to Reviewer #2’s additional remarks following the\nrevision. We are also grateful for the supportive evaluations from Reviewers #1 and #3.\nReviewer #1 (Remarks to the Author):\nI have checked the detailed point-by-point responses and the revised manuscript; I\nfound the authors have adequately dealt with most of the reviewer’s concerns and\ncritiques. Thus, I recommend the paper be published in Nature Communications.\nWe sincerely thank the reviewer for the positive evaluation and support for our revised\nmanuscript.\nReviewer #2 (Remarks to the Author):\nThe revised manuscript is clearer in presenting the authors idea. The authors report a\nchange in the oxidation conditions of magmas from the Paleozoic-Mesozoic to Tertiary\nperiods, based zircon trace elements with porphyry copper deposits formed in the\nTertiary. The authors propose that the change in the oxidation conditions of magmas\nwas a critical factor in the formation of porphyry copper deposits. The authors argue\nthat earlier magmas formed metal sulphide cumulates in the lower crust, whereas\nsubsequently oxidized Tertiary magmas were able to release these metals to form\nporphyry copper deposits. The concentration of metals as cumulates in earlier intrusions\nand later remelting of the metal-containing cumulate is an interesting idea in the process\nof forming porphyry copper deposits; this idea was first proposed by Richards, 2009,\nand Hou et al., 2015, both in Geology. However it is not clear, what is the evidence for\nthe formation of metal sulphide cumulates? and how can this be established, formation\nof sulphide cumulates during the Paleozoic and Mesozoic, up until the Tertiary period.\nThank you for this valuable comment. The evidence for the formation of metal sulphide\ncumulates has been comprehensively reviewed in Yang and Cao (2024, Earth-Science\nReviews). For example, petrological investigations of Mesozoic deep arc roots in the\nGangdese magmatic belt (southern Tibet) have systematically documented widespread\noccurrences of magmatic sulfides (pyrrhotite–pentlandite–chalcopyrite) hosted within\nlower-crustal hornblendite cumulates. The precipitation of these metal sulfides in the\nlower crust is interpreted to result from the low oxidation state of arc magmas during\nthe Mesozoic–a main finding of our study.\nThe authors imply that this is a globally applicable model, but large porphyry copper\ndeposits occur in North and South America, Indonesia, and Philippines. There is no\nrecord of temporal change in oxidation conditions of magmas in these areas that contain\ngiant porphyry copper deposits.\nWe appreciate this important point. Our model primarily addresses the temporal\nscarcity of porphyry Cu deposits during subduction-related magmatism and their\nabundance in post-subduction settings worldwide. In regions such as North and South\nAmerica, Indonesia, and Philippines, subduction-related arc magmas are\ncharacteristically oxidized and therefore conducive to the formation of giant porphyry\ncopper deposits. This observation is fully consistent with our model: high magmatic\noxidation states–regardless of tectonic setting–are key to porphyry Cu fertility.\nDuring revision, the authors added new statements to the manuscript. Some are not\ncorrect.\nFor example, in their rebuttal note, the authors suggest that the Andes and the Circum-\nPacific porphyry copper belts are related to the subduction of a (paleo-) Pacific plate.\nThis is not correct.\nThank you for pointing this out. We agree that not all Circum-Pacific porphyry copper\ndeposits are associated with subduction of the (paleo-) Pacific plate. The Andes, for\ninstance, are linked to the subduction of the Nazca plate, while porphyry systems in\nIndonesia and Philippines reflect more complex interactions involving subduction of\nmultiple plates. (see Cooke and Hollings, 2005, Economic Geology; Sillitoe, 2010,\nEconomic Geology). As this statement originally appeared in the response to Reviewer\n#1, no change to the main text is necessary.\nThe authors wrote that “magma oxygen fugacity of the north-south trending Andes arc\nmay be latitutde-dependent” in the rebuttal note and L.102- of the main text). In the\nSouth American Andes, porphyry copper deposits and oxidized igneous rocks have\nformed from close to the equator to 35 S. Therefore, there is no evidence to support this\nsentence.\nWe sincerely appreciate this clarification. The concept of latitude-dependent oxygen\nfugacity in global arc magmas was recently proposed by Hu et al. (2024, Nature\nCommunications), who found that “the calculated average ΔFMQ values of different\narc segments by different methods all display a tendency to increase with latitude,\nranging from ~+1.0 around the equator to ~+2.0 at high-latitude (>30 °N/S) regions”.\nWhile the Andes host porphyry Cu deposits over a wide latitudinal range, this\nobservation does not necessarily contradict Hu et al. (2024)’s global-scale mantle\noxidation gradient. Importantly, recent studies have emphasized that moderately\noxidized magmas (ΔFMQ = +1.3 ± 0.7) are optimal for porphyry Cu deposit formation\n(Wang et al., 2025, Science Bulletin). We have rephrased the sentence to “This is\nconsistent with the high ∆FMQ (> +1) in the Andes belt, which may display a tendency\nto increase in ∆FMQ from +1.0 around the equator to +2.0 at high latitude” (Line 103-\n104 in revised main text).\nIn the revised manuscript, the authors suggest that an injection of oxidized ultrapotassic\nrocks in the lower crust contributed to porphyry copper mineralization (L. 182).\nUltrapotassic rocks are not common around porphyry copper deposits. There is no\nevidence for this statement.\nWe agree that ultrapotassic magmas are not commonly associated with porphyry\nsystems in typical subduction zones. However, in post-collisional settings in the\nTethyan belt, such as southern Tibet, ultrapotassic rocks are frequently observed in\nspatial and temporal association with porphyry Cu mineralization. This has been\ndocumented in multiple studies (Yang et al., 2015, Journal of Petrology; Sun et al., 2018,\nJournal of Petrology; Xu et al., 2017, Geology; Zheng et al., 2021, Terra Nova). We\nhave added a sentence to clarify that “Although ultrapotassic magmas are not\ncommonly associated with porphyry systems in oceanic subduction zones (e.g. Andes\nbelt), they are frequently observed in spatial and temporal association with porphyry\nCu mineralisation in collisional settings” (Line 183-186 in revised main text).\nReviewer #3 (Remarks to the Author):\nThe authors have addressed all my comments thoroughly and provided well-reasoned\nresponses. I believe the manuscript is now well-suited for publication in Nature\nCommunications. Thank you for the opportunity to review this insightful work.\nWe greatly appreciate the reviewer’s feedback and endorsement of our revised\nmanuscript."}]

Decision letter