Contrasting Tectonomagmatic Conditions for Coexisting Iron Oxide-Apatite Deposits and Porphyry and Skarn Cu ± Au Deposits in the Middle-Lower Yangtze River Metallogenic Belt, China

Porphyry Cu ± Au ± Mo deposits (hereafter simply “porphyry Cu deposits”) are the greatest sources of Cu and Mo and supply significant Au, Ag, and metals such as Te (Sillitoe, 2003, 2010; Cooke et al., 2005, 2014; Richards and Mumin, 2013a, b; Kesler and Simon, 2015). Iron oxide-apatite (IOA) deposits are important sources of iron and phosphates and have the potential to supply significant rare earth elements (REEs) hosted in apatite (Barton, 2013; Simon et al., 2018; Troll et al., 2019; Reich et al., 2022). Porphyry Cu deposits typically form in compressional to transpressional settings of crustal-thickening convergent margins (Richards, 2003; Sillitoe, 2010; Meng et al., 2021a, b, 2022), whereas IOA deposits commonly form under extensional to transtensional back-arc settings (Sillitoe, 2003; Groves et al., 2005, 2010; Mao et al., 2011; Barton, 2013; Richards and Mumin, 2013a, b; Reich et al., 2022; Skirrow, 2022). While it is well documented that porphyry Cu and IOA deposits are not genetically related, both mineral deposit types are observed to have occurred in spatial and temporal proximity in the Mesozoic Coastal Cordillera of northern Chile and southern Peru (Sillitoe, 2003; Richards et al., 2017) and Middle-Lower Yangtze river belt in China (Mao et al., 2011; Zhou et al., 2013).

The parental magmas for the magmatic-hydrothermal ore fluids that form porphyry Cu and IOA deposits are generally thought to be derived from partial melting of metasomatized mantle lithosphere (Richards, 2003; Groves et al., 2010). The observation that IOA deposits contain much less Cu-Fe sulfide ore relative to porphyry Cu deposits has been interpreted to indicate that their source magmas were S poor (Richards and Mumin, 2013a, b). This hypothesis can explain the preferential occurrence of IOA versus porphyry Cu deposits in extensional back-arc settings, because magmas formed during asthenospheric upwelling may be S poor owing to the subdued contribution of oxidized sulfur from the subducting slab to the mantle (Richards et al., 2017). However, it remains unclear as to whether and how contrasting tectonomagmatic conditions controlled the formation of these two distinct mineral systems in an evolving arc setting.

To better understand the spatiotemporal relationship among porphyry Cu and IOA deposits, we systematically investigated intrusive rocks for the P-T-$fO2$ conditions and S-Cl concentrations of the source magmas related to representative well-characterized porphyry and skarn Cu ± Au and IOA deposits in the Middle-Lower Yangtze River metallogenic belt. In comparison to the Coastal Cordillera of northern Chile and Peru, the belt only comprises IOA deposits with minor, non-economic Cu-Fe sulfide mineralization (Table 1). These new belt-scale data sets are interpreted in a geologic background to reflect the fundamental tectonic and magmatic control on the two types of mineralization in the Middle-Lower Yangtze River metallogenic belt. A combination of differences in magmatic P-T-$fO2$ conditions and volatile compositions is proposed here to have favored the formation of porphyry and skarn Cu ± Au and IOA deposits under broadly compressional and extensional arc settings in the metallogenic belt, respectively. Evaporite assimilation is suggested to be a key external trigger for IOA deposit formation.

The arcuate-shaped Middle-Lower Yangtze River metallogenic belt hosts ~200 polymetallic deposits and has a lateral extent of approximately ~550 km along the Yangtze River at the northeastern margin of the Yangtze craton in eastern China (Fig. 1; Chang et al., 1991; Zhai et al., 1992; Pan and Dong, 1999; Mao et al., 2011). A total indicated and measured metal resource of 13.9 million tonnes (Mt) Cu, >600 t Au, and 2.49 billion tonnes (Bt) Fe has been defined (Zhao et al., 1999; Yin et al., 2016). It is bound to the north by the Dabie orogen and North China craton along the Xiangfan-Guangji and Tancheng-Lujiang faults and to the south by the Yangtze craton along the Yangxin-Changzhou fault (Chang et al., 1991; Zhai et al., 1992). The Precambrian crystalline basement in the Middle-Lower Yangtze River metallogenic belt mainly includes metamorphosed late Archean to Paleoproterozoic tonalite-trondhjemite-granodiorite rocks. This contrasts with the crystalline basement of Neoproterozoic metamorphosed flysh-like clastic sedimentary sequences in the south Yangtze craton (Fig. 1; Qiu et al., 2000; Dong et al., 2011).

The crystalline basement rocks in the Middle-Lower Yantze River metallogenic belt are overlain by Paleoproterozoic to Neoproterozoic (1850–990 Ma) volcanosedimentary and Paleozoic-Early Triassic clastic and dolomitic successions, including siltstone, shale, and limestone (Chang et al., 1991). Extensive fault networks developed during the Neoproterozoic Jiangnan orogeny due to subduction-related collision of the Cathaysia terrane and Yangtze craton (Yao et al., 2014; Goldfarb et al., 2021). Shallow marine carbonate and evaporite sequences deposited in the Triassic are unconformably overlain by late Triassic-Jurassic terrestrial coal, sandstone, and mudstone (Chang et al., 1991; Zhai et al., 1992).

Plate reconstruction shows that the paleo-Pacific oceanic plate has subducted beneath the Eurasian plate (Fig. 2a) since the early Jurassic (Li et al., 2019); this may be flat or low-angle subduction (Li and Li, 2007; Wu et al., 2019; Liu et al., 2021; Qiu et al., 2023). The velocity of the paleo-Pacific plate is suggested to have steadily increased from ~155 to 137 Ma, and then decreased abruptly after ~135 Ma (Fig. 2b). Although the geodynamic setting for forming the Middle-Lower Yangtze River metallogenic belt remains debated (Table 1), the change in the plate velocity is consistent with the evolving kinematic regime in the belt that changed from transpression prior to ~135 Ma to strike-slip extension from 135 to 127 Ma, and then to purely extension at ~126 to 123 Ma (Chang et al., 2012), consistent with the evolving tectonic framework of the East Asian continental margin (Li, J., et al., 2014; Zhou et al., 2015; Li et al., 2019). Chen et al. (2020) noted a slight northeastward migration of the high-K calc-alkaline mafic magmatism in the Middle-Lower Yangtze River metallogenic belt and interpreted this as the manifestation of local extension caused by slab rollback beginning around 140 Ma. However, the compiled geochronological data suggest the age variation of high-K calc-alkaline magmatism across the Middle-Lower Yangtze River metallogenic belt is exceptionally limited (Fig. 3). Lithospheric extension is supported by decreasing crustal thickness in the Middle-Lower Yangtze River metallogenic belt from 62 ± 6 km (1σ, n = 254) to 49 ± 8 km from ~150 to 135 Ma to <133 Ma (1σ, n = 40; Fig. 2c), which was estimated using published whole-rock La/Yb ratios following the method of Profeta et al. (2015). Extension likely commenced at ~140 Ma, but was not widespread until ~135 Ma in response to eastward retreating subduction of the paleo-Pacific oceanic plate, rollback of a steeper slab, upwelling of asthenospheric mantle, and/or lithospheric delamination at the East Asian continental margin (Zhu and Xu, 2019; Zhang et al., 2020; Lü et al., 2021; Mao et al., 2021).

The subduction of the paleo-Pacific oceanic plate and the subsequent rollback or retreating produced voluminous dioritic to granodioritic magmas in the Jurassic to Early Cretaceous that intruded sedimentary sequences in the Middle-Lower Yangtze River metallogenic belt along reactivated basement-penetrating faults (Pan and Dong, 1999; Mao et al., 2011; Yang and Cooke, 2019). The Jurassic-Cretaceous magmatism in the belt occurred in three episodes (Mao et al., 2011; Zhou et al., 2013; Chen et al., 2020): (1) ~152 to 135 Ma high-K calc-alkaline I-type granitoid associated with porphyry and skarn Cu ± Au deposits (Figs. 2b, 3), (2) ~133 to 125 Ma shoshonitic intrusive and volcanic sequences associated with IOA deposits (Figs. 2b, 3), and (3) slightly younger A-type intrusive rocks with granitic–syenitic compositions (<130 Ma) associated with subeconomic Au mineralization. The porphyry and skarn Cu ± Au and IOA deposits mainly formed at broadly transpressional (or in transition to transtension) and transtensional settings at ~140 and ~130 Ma, respectively (Fig. 2b).

These mineral deposits have been grouped into seven discrete ore districts from west to east, including Edong, Jiujiang-Ruichang (Jiurui), Anqing-Guichi, Tongling, Lujiang-Zongyang (Luzong), Nanjing-Wuhu (Ningwu), and Nanjing-Zhenjiang (Ningzhen; Fig. 1; Mao et al., 2011; Pirajno and Zhou, 2015). The porphyry and skarn Cu ± Au deposits are mainly clustered in Daye, Jiurui, Anqing-Guichi, and Tongling (i.e., at latitude of 29.5°E–31°E), whereas the IOA deposits are restricted to the northeastern part of the belt (Fig. 3), including the Ningwu (Nanjing-Wuhu) and Luzong (Lujiang-Zongyang) ore districts (Fig. 1). A few skarn Fe deposits have also been reported in the Edong and Luzong ore districts, which mainly formed at the same time as the IOA deposits (Fig. 3). The porphyry and skarn Cu ± Au deposits of the Middle-Lower Yangtze River metallogenic belt are genetically associated with granodioritic to quartz dioritic stocks that were emplaced into thick carbonate and clastic sedimentary sequences. The contrasting alteration features of the porphyry and skarn Cu ± Au deposits are mainly affected by host rocks (clastic versus carbonate; Table 1). Massive, disseminated, and veinlet mineralization are primarily hosted in skarns along the contacts between the intrusions and the carbonate sequences or are stratabound between the limestone-dolomite sequences (Tables 1, 2). Mineralization is also hosted in quartz dioritic to granodioritic stocks with potassic and phyllic alteration in Tongshankou, Baoshan, Chengmenshan, Wushan, Matou, and Dongguashan where the hosts are clastic rocks (Table 2).

The Middle-Lower Yangtze River metallogenic belt IOA deposits are spatially and temporally associated with synvolcanic gabbroic to dioritic intrusions emplaced in the Early Cretaceous volcanic basins or Triassic sedimentary sequences (Mao et al., 2011; Zhao et al., 2020). Mineralization styles include massive, breccia-hosted dissemination and veinlets that occur in the apical parts of the dioritic intrusions or at their contacts with the carbonate sequences, where skarns have formed (Tables 1, 2). High-temperature sodic alteration was followed by ore-stage actinolite ± apatite ± diopside ± chlorite ± epidote alteration and sulfide (pyrite + chalcopyrite) ± sulfate (gypsum + anhydrite) veins (Zhou et al., 2013; Duan et al., 2021). Representative IOA deposits include Washan, Gaocun, Heshangqiao, Baixiangshan, and Zhongjiu-Gushan in the Ningwu basin and the Nihe deposit in the Luzong basin. Magnetite from the massive and vein ores in the Washan and Gaocun deposits has been estimated to crystallize at temperatures of 550° to 800°C, consistent with mineralization temperature for IOA deposits worldwide (Zeng et al., 2022). The iron ores in these deposits are mainly interpreted to have formed from high-temperature magmatic-hydrothermal saline fluids rather than silicate or Fe oxide melts (Su et al., 2019; Zhao et al., 2020; Zeng et al., 2022).

The petrogenesis of the igneous rocks in the Middle-Lower Yangtze River metallogenic belt has been studied extensively using lithogeochemical and isotopic methods (cf. Chen et al., 2020; Yang et al., 2021). On primitive mantle-normalized trace element and chondrite-normalized rare earth element (REE) diagrams, both the high-K calc-alkaline and shoshonitic igneous rocks exhibit enrichments in large ion lithophile elements (LILE) and light rare earth elements (LREEs); depletion in Nb, Ta, Ti, P; and a listric-shaped pattern from middle REEs to heavy REEs (Mao et al., 2011; Chen et al., 2020). Such elemental patterns are typical of subduction-related magmas and reflect the fractionation of amphibole, apatite, and Fe-Ti oxides during magma evolution. Most of the high-K calc-alkaline rocks are adakite-like with high La/Yb and Sr/Y ratios and weak to positive Eu anomalies (0.8–1.2), whereas the shoshonitic rocks yielded features of normal arc rocks with lower La/Yb and Sr/Y ratios as well as negative Eu anomalies. Both groups of intrusive rocks yield indistinguishable initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7050 to 0.7100, suggesting crustal contamination in the source magmas (Mao et al., 2011). The high-K calc-alkaline rocks yielded wider ranges of zircon ε_Hf(t) values (–20 to +2) and whole-rock ε_Nd(t) values (–18 to –2) compared to the shoshonitic rocks, with zircon ε_Hf(t) and whole-rock ε_Nd(t) values clustering at values of –13 to +2 and –10 to –1, respectively (Chen et al., 2020). The most depleted zircon ε_Hf(t) values are from the high-K calc-alkaline granitoids in the Tongling area (Yan et al., 2015; Yang et al., 2021).

Various petrogenetic models have been proposed to explain the origin of the high-K calc-alkaline magmas (Table 1), including remelting of the thickened or delaminated lower crust (Wang et al., 2007; Hou, Z.Q., et al., 2011), interaction of crustal materials with partial melts from the shallow subducted paleo-Pacific slab or from the metasomatized mantle lithosphere at that time (Liu, S.A., et al., 2010; Mao et al., 2011; Yan et al., 2021), or partial melting of mantle lithosphere that was previously metasomatized by slab-derived melts or fluids in the Neoproterozoic (Li et al., 2008; Wang et al., 2016; Chen et al., 2020). In contrast, the shoshonitic rocks associated with IOA deposits were derived mainly from remelting of the lithospheric mantle that was metasomatized by slab-derived fluids (Yuan et al., 2011) either during Mesozoic paleo-Pacific subduction with contamination of crustal materials (Mao et al., 2011; Zhou et al., 2013) or during the Neoproterozoic Jiangnan orogeny (Chen et al., 2020).

Samples were collected from open pits, underground mines, and drill cores from representative porphyry and skarn Cu ± Au and IOA deposits in the Middle-Lower Yangtze River metallogenic belt. One hundred and twenty samples of diorite, quartz diorite, quartz monzonite, and granodiorite with equigranular or porphyritic textures were collected from the ore-forming plutonic stocks (Fig. 4). Thin sections were prepared for petrographic examination, and most of the collected samples are shown to have been variably altered. Detailed descriptions of these samples are provided in Table 3. Zircon grains from the variably altered samples were separated using conventional magnetic and density methods at the Geological Surveying and Mapping Institute of Hebei Province, China. A total of ~15,000 representative zircon grains were handpicked, mounted on epoxy resins, and polished to expose their internal structures.

To constrain the crystallization conditions and original compositions of the ore-forming magmas, we restricted our analyses to the relatively fresh minerals in the least-altered samples (Fig. 5). Zircon grains and the host mineral inclusions from some altered samples were also analyzed. Amphibole, apatite, biotite, and zircon were analyzed for major and/or trace element abundances using electron probe microanalysis (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Primary amphibole and biotite grains are common in the thin sections for the least-altered high-K calc-alkaline intrusive rock samples but are only rarely observed in the IOA-related dioritic samples (Fig. 4). Magnetite is common as an interstitial phase or as inclusions in other minerals, particularly amphibole. Coexisting magnetite and ilmenite are rarely present as inclusions in zircon grains (e.g., DYJGZ-01, Fig. 5a-b) but seem to form by exsolution from original Fe-Ti oxides. We prioritized analyzing zircon-hosted apatite inclusions (Fig. 5c) that preserve primary chemical zoning (Fig. 5d), or compositionally homogeneous apatite grains hosted in mineral phases such as amphibole and biotite. Porous apatite grains or those intergrown with hydrothermal minerals (e.g., sample NWHSQ-04) are interpreted to have been altered (Fig. 5e) or crystallized from magmatic fluids exsolved from silicate melt (Fig. 5f). All studied zircon grains exhibit oscillatory or sector zonings, as revealed by cathodoluminescence (CL) imaging (Fig. 5a, c; App. Figs. A1 and A2).

Zircon grains were analyzed for U-Pb isotope and trace element abundances using LA-ICP-MS at the Collaborative Innovation Center for Exploration of Strategic Mineral Resources, China University of Geosciences (Wuhan). A 193-nm NWR HE excimer laser coupled with an Agilent 7900 ICP-MS was used to ablate and analyze the zircon grains. The parameters for the laser were a fluence of 3.5 J/cm², a repetition rate of 8 Hz, and a spot size of 32 μm. Thirty seconds of background were acquired followed by each analysis of 40 s. Primary and secondary reference materials for the zircon U-Pb isotope and trace element analyses were analyzed regularly during the analytical sessions.

Correction of laser-induced element fractionation, instrumental drift, and downhole fractionation was performed with ICPMSDataCal software (Liu, Y., et al., 2010). The U-Pb isotope ratios were normalized to the primary zircon reference material 91500. The secondary reference materials SA-01 or Qinghu-01 were analyzed to monitor the accuracy and reproducibility of the unknown isotopic analyses. The trace element data were normalized using an internal standard scheme to the synthetic glass NIST610 with an assumption of the stoichiometric concentration of Si in zircon as 15.32 wt %. The synthetic glass NIST612 and zircon 91500 were used to monitor the accuracy of the unknown trace element analyses. The isotopic and trace element data for the secondary reference materials are consistent with the reported standard values (App. Table A1). Concordia or intercept ages for the samples were calculated using Isoplot v. 4.15 (Ludwig, 2012), in which the uncertainties of the uranium decay constants are considered. The values of the mean square of weighted deviates (MSWD) for the studied samples are calculated to measure the ratio of the scatter of the data points to the predicted scatter due to the analytical uncertainty (Wendt and Carl, 1991). To minimize the effects of hydrothermal alteration and contamination of subsurface mineral/melt inclusions on T-$fO2$ results, the zircon trace element data were screened using criteria as follows: (1) La content < 1 ppm, (2) LREE index (Dy/Nd + Dy/Sm) > 10 (Bell et al., 2016); (3) Ti < 50 ppm (Lu, 2016).

Major and trace element abundances of amphibole, biotite, and apatite were acquired using a JOEL JXA-iHP200F field emission EPMA at the Institute of Mineral Resources, China Academy of Geological Sciences. The analytical parameters varied for the different analyzed minerals. Amphibole and biotite were analyzed for twelve elements (i.e., Si, Ti, Al, Cr, Fe, Mn, Mg, Can, Na, K, F, and Cl) at the same conditions: 15-kV accelerating voltage, 20-nA beam current, and 5-μm spot beam size.

Apatite grains were analyzed for fourteen elements (i.e., P, Ca, Al, Si, Mn, Fe, Sr, Y, Na, K, F, Cl, S, and Zr) using the following conditions: 15-kV accelerating voltage, 10-nA beam current, and beam sizes from 2 to 5 μm depending on the size of the analyzed grain. Meng et al. (2021a) used these analytical conditions for apatite and reported that (1) the measured S and Cl contents of apatite are more reliable than F content when using a 2-μm beam, and (2) damage of apatite using a beam size of 5 μm can be minimized, particularly for apatite grains with the c-axis perpendicular to the electron beam (Meng et al., 2021a). In this study, we only report the F content of apatite for analyses that used a beam size of 5 μm or for grains where the c-axis was perpendicular to the 2-μm beam.

X-ray lines, crystals, internal standards, and counting times for peak and background measurements are provided in the notes of Appendix Tables A2 through A4. We excluded all analyses of zircon-hosted apatite grains with ZrO₂ concentration of >1 wt %, as these are interpreted to reflect contamination of the inclusion by the host.

The crystallization temperatures of apatite, zircon, and amphibole were estimated using the methods in Piccoli and Candela (1994), Loucks et al. (2020), and Ridolfi (2021), respectively. For estimating apatite saturation temperature (AST), we used previously compiled lithogeochemical data for the studied intrusive rocks (Chen et al., 2020) and assumed the whole-rock SiO₂ and P₂O₅ abundances approximate those in the silicate melts at the time of apatite crystallization. A revised Ti-in-zircon thermometer by Loucks et al. (2020) was used to constrain zircon crystallization temperatures (T_Ti-Zr), considering it includes the effect of pressure on the calibration of Ferry and Watson (2007). The activities of SiO₂ and TiO₂ were assumed to be 1 and 0.6, respectively, based on the presence of primary quartz and titanite as well as zircon-hosted ilmenite in the samples (Tables 3, 4; Dilles et al., 2015; Schiller and Finger, 2019). The crystallization pressure of the amphibole grains was estimated using the amphibole barometer of Mutch et al. (2016).

Values of $fO2$ for the ore-forming magmas were estimated from the compositions of zircon and amphibole using the oxybarometers of Loucks et al. (2020) and Ridolfi (2021). We attempted to use the zircon oxybarometers calibrated by Smythe and Brenan (2016) and Loucks et al. (2020). The method of Smythe and Brenan (2016) requires quantitative determination of trace element abundances in zircon and the zircon-equilibrated melt composition in addition to water content and the activities of SiO₂ and TiO₂ (i.e., aSiO₂ and aTiO₂). This method is sensitive to water content, where a variation of 1 wt % H₂O in the melt can lead to a deviation of ~0.5 log units of $fO2$⁠. Considering that we currently lack a robust method for accurately estimating the absolute water content at the time of zircon crystallization, the empirical zircon oxybarometer of Loucks et al. (2020) that only involves the measured concentrations of Ti and Ce and the age-corrected initial U concentration of zircon was used. The zircon Ce-Ti-U_i oxybarometer was calibrated by Loucks et al. (2020) using $fO2$ values determined from coeval magnetite-ilmenite pairs and amphibole, melt Fe³⁺/Fe²⁺ ratios, and experimental run products at controlled $fO2$⁠. Loucks et al.’s (2020) method is applicable to igneous rocks with a broad compositional range (i.e., calc-alkaline, tholeiitic, adakitic, shoshonitic, metaluminous to mildly peraluminous, and mildly peralkaline) with $fO2$ values of ΔFMQ –4.9 to ΔFMQ +2.9. The estimated $fO2$ value yields a standard error of ~ ±0.6 log unit.

The amphibole oxybarometer was calibrated using experimental data of amphibole in equilibrium with the calc-alkaline and alkaline silicate melts under various $fO2$ conditions (Ridolfi et al., 2010; Ridolfi and Renzulli, 2012; Ridolfi, 2021). We used the most recently updated amphibole oxybarometer that was calibrated by filtering out poor-quality experimental data (Ridolfi, 2021). The oxybarometer is P-T-independent and is suitable for Mg-rich calcic amphibole in calc-alkaline and alkaline melts across a wide P-T range (up to 2,200 MPa and 1,130°C). The spreadsheet Amp-TB2 in Ridolfi (2021) was used to assess the reliability and suitability of the amphibole EPMA analyses for the oxybarometer. We excluded analyses that were outside the compositional range for calibrating the amphibole oxybarometer. The standard error for the amphibole oxybarometer is ±0.3 log units.

We tried to use multiple mineral-based methods to estimate concentrations of H₂O, S, and Cl in the silicate melts from which the ore-forming magmatic-hydrothermal fluids exsolved. The H₂O content was approximated using the amphibole hygrometer of Ridolfi (2021). However, because the coefficient of determination for the linear relationship between experimental and calculated H₂O contents using the amphibole hygrometer is low (R² = 0.645; Ridolfi, 2021), we suggest that the H₂O results can only be regarded as semiquantitative. The S concentration in the melt was estimated using a model for the partition coefficient of S between apatite and melt (D_S^ap/m; Meng et al., 2021a) based on experimental studies that demonstrated the S concentration in apatite is controlled by the melt S content, $fO2$⁠, and temperature (Parat and Holtz, 2004; Konecke et al., 2019). The melt Cl/H₂O ratio was estimated using methods as follows: (1) an exchange partitioning model (Li and Hermann, 2017) established based on experimental results of felsic silicate melts by Webster et al. (2009), (2) a thermodynamic exchange partitioning model that considers the nonideality in apatite solid solution (Li and Costa, 2020), (3) an exchange coefficient model for partitioning of Cl-OH between amphibole and melt as a function of amphibole composition (Giesting and Filiberto, 2014), and (4) an empirical exchange coefficient model for Cl-F-OH partitioning between biotite and silicate melt as functions of the compositions of the biotite and the equilibrated melt (Zhang, C., et al., 2022). For method (4), we assumed the whole-rock compositions summarized by Chen et al. (2020) represent the melt composition.

The absolute timing of the igneous rocks has been well constrained mainly using zircon U-Pb dating by a number of authors (Fig. 3; App. Table A5). We supplement the cathodoluminescence (CL) images and U-Pb dates for the zircon samples in order to distinguish the antecrystic, autocrystic, and xenocrystic zircon grains. A total of 429 isotopic analyses were performed. Most analyses yielded Th/U ratios of >0.3 (App. Table A1), and the analyzed grains typically exhibit oscillatory and sector zonings consistent with a magmatic origin (App. Figs. A1 and A2). The U-Pb isotope data for the xenocrystic and antecrystic cores were distinguished based on their morphologies (Corfu et al., 2003). Zircon xenocrysts typically yielded older ²⁰⁶Pb/²³⁸U dates compared to the concordia or intercept U-Pb ages, whereas the antecrystic zircon cores yielded indistinguishable ²⁰⁶Pb/²³⁸U dates (App. Table A1) but contrasting chemical zoning in CL responses that contrast the rims (Fig. 5a).

Geochronological data are summarized in Table 4 and illustrated in Appendix Figures A3 and A4. Concordia, or intercept U-Pb ages, were calculated to define the crystallization ages of the dated zircon grains and the host intrusive rocks. Consistent with previous studies (Figs. 2b, 3; App. Tables A5–A6), the geochronological results suggest that the magmas related to porphyry and skarn Cu deposits and IOA deposits in the Middle-Lower Yangtze River metallogenic belt intruded the upper crust at 150 to 135 and ~130 Ma, respectively (Table 4). The porphyry- and skarn-related igneous rocks contain abundant zircon xenocrysts that yielded U-Pb dates of 3443 to 739 Ma (n = 32), which is consistent with the age spectrum of the zircon xenocrysts entrapped by lamprophyres in Middle-Lower Yangtze River metallogenic belt (Zhang, S., et al., 2021). Nineteen Jurassic xenocrystic zircon grains were identified in porphyry- and skarn-related intrusive rocks, whereas only one Triassic zircon grain was identified in the IOA-related samples (App. Table A1).

The estimated crystallization temperatures of various mineral phases and the estimated crystallization pressures of amphibole for representative samples are summarized in Table 5 and illustrated in Figures 6a, b, 7a, 8, 9a, and 10. The zircon and amphibole record comparable crystallization temperatures of 700° to 950°C (Figs. 6a, b, 8a) and 650° to 900°C (Figs. 6a, b, 8b), respectively. Assuming aTiO₂ = 0.6 and aSiO₂ = 1.0, the T_Ti-Zr values for dioritic rocks associated with IOA deposits are greater than T_Ti-Zr values for granodioritic to quartz dioritic rocks from porphyry and skarn deposits (890° ± 50° vs. 810° ± 50°C; 1σ; Table 5; Fig. 6a). No time-space variations in T_Ti-Zr values are observed for the igneous rocks associated with porphyry and skarn Cu ± Au deposits across the four porphyry and skarn ore districts (Figs. 6a, b, 7). In contrast, the crystallization temperatures of amphibole in the intrusive rocks associated with the porphyry and skarn and IOA deposits are comparable at 770° ± 40°C (1σ) and 750° ± 50°C (1σ), respectively (Table 5).

The amphibole grains are classified as Mg-hornblende except for one sample that contains Mg-hastingsite and Tschemakitic pargasite (App. Table A2). The crystallization pressures for amphibole from the porphyry- and skarn-related intrusive rocks range from 527 ± 60 MPa (1σ) to 118 ± 30 MPa (1σ), whereas amphibole in the IOA-related diorite samples crystallized at pressures of 189 ± 72 to 85 ± 16 MPa (1σ; Table 5), corresponding to crystallization paleo-depths of 5 to 20 and 3 to 7 km, respectively. Amphibole barometry indicates that amphibole in the porphyry- and skarn-related intrusive rocks crystallized along a steeper P-T trajectory compared to amphibole from the magmas associated with IOA mineralization (Fig. 10a).

The magmatic $fO2$ estimates for representative intrusive samples from porphyry and skarn Cu and IOA deposits are reported in Table 5 and illustrated in Figures 6c, d, 7b, 8, and 10b. Geochemical data for zircon from Wang et al. (2013), Wang et al. (2015), Wen et al. (2020), and Zhang, J., et al. (2021) were included in the reported $fO2$ values.

Zircon: Zircon grains from the porphyry- and skarn-related intrusive rocks yielded average magmatic $fO2$ values of ΔFMQ + 0.5 ± 0.6 (1σ) to + 2.2 ± 0.3 (1σ) at temperatures of 930° to 740°C (Figs. 6c, d, 7b, 8a, b). There are no time-space variations in ΔFMQ values for the igneous rocks associated with porphyry and skarn Cu ± Au deposits across the four ore districts (Figs. 6c, d, 7b). With the exception of one sample, the average magmatic $fO2$ values for samples from the Edong, Jiurui, and Anqing-Guichi ore districts increase from ~ΔFMQ +0.7 at 930°C to ~ΔFMQ +2.5 at ~800°C, and then decline to ~ΔFMQ +1 at ~740°C (Fig. 8a; Table 5). A strong negative correlation between ΔFMQ values and T_Ti-Zr is observed for samples from the Tongling district, culminating in an ΔFMQ value of +2.2 ± 0.3 at 775° ± 37°C (1σ; Fig. 8a). The negative correlation of estimated $fO2$ with T_Ti-Zr was also detected for single samples from the other ore districts studied. Five antecrystic zircon cores from Jiguanzui in the Daye district yielded a ΔFMQ value of +0.7 ± 0.2 (1σ) at 933° ± 21°C (1σ), whereas 10 analyses of autocrystic zircon grains from the same sample yielded higher average ΔFMQ value of +1.9 ± 0.3 (1σ) at lower T_Ti-Zr of 788° ± 28°C (1σ; Table 5). Similarly, three analyses of the antecrystic zircon cores from Tongshan in the Anqing-Guichi district yielded a ΔFMQ value of +0.7 ± 0.3 (1σ) at 896° ± 10°C (1σ) compared to autocrystic zircon grains from the same sample that yielded a ΔFMQ value of +1.6 ± 0.4 (1σ) at 817° ± 29°C (1σ; Table 5).

The estimated magmatic $fO2$ values of the IOA-related magmas are highly variable (Table 5) and are on average lower than the porphyry- and skarn-related magmas (Figs. 6b, 7b). The IOA-related samples have been divided into three groups (Figs. 6b, 7b, 8b): (1) a sample from Nihe in the Luzong ore district yielded ΔFMQ –1.4 ± 0.2 (1σ); (2) four samples from the northern Ningwu ore district yielded increasing $fO2$ values (from ΔFMQ –0.8 ± 0.4 to ΔFMQ +0.6 ± 0.3) with decreasing temperatures; (3) two samples from the southern Ningwu ore district yielded greater FMQ values of 1.2 ± 0.7 (1σ) and 2.1 ± 0.4 (1σ; Table 5).

Thirty-two Archean-Proterozoic zircon xenocrysts from the porphyry- and skarn-related intrusive rocks yielded an average magmatic $fO2$ value of ~ΔFMQ +0.1 ± 1.6 (1σ, n = 32; Fig. 8c; App. Table A1). Most of the zircon grains yielded U_i/Nb ratios of ≥40 (U_i represents the age-corrected initial U concentration in zircon calculated following the method in Loucks et al., 2020; Fig. 8c). In comparison, fourteen xenocrystic zircon grains of Jurassic ages yielded a higher average $fO2$ value of ΔFMQ +1.4 ± 0.5 (1σ, n = 14; Fig. 8c; App. Table A1). The Triassic xenocrystic zircon grain identified in one sample from the Heshangqiao IOA deposit yielded an $fO2$ value of ΔFMQ +0.4 ± 0.4 (2SE).

All the analyzed zircon grains from the studied samples yielded negative Eu anomalies (Figs. 6e, f, 7c, 9). The porphyry- and skarn-related intrusive rocks typically yielded higher zircon Eu/Eu* ratios of 0.62 ± 0.08 (ranging from 0.43 ± 0.04 to 0.77 ± 0.08; 1σ) compared to those for IOA deposits (0.43 ± 0.09 on average; ranging from 0.33 ± 0.04 to 0.53 ± 0.09; 1σ). No time-space variations in zircon Eu/Eu* values are observed for the igneous rocks associated with porphyry and skarn Cu ± Au deposits across the four ore districts (Figs. 6e, f, 7c). A positive correlation between the Eu/Eu* and ΔFMQ values has not been observed (Fig. 9b), suggesting the magmatic $fO2$ may not be the main factor affecting the zircon Eu/Eu* anomalies reported here.

Amphibole: The average magmatic $fO2$ value estimated from amphibole compositions for the porphyry- and skarn-related intrusive rock samples is ΔFMQ +1.6 ± 0.4 (1σ), which is lower than that for IOA deposits (ΔFMQ +2.4 ± 0.3, 1σ; Table 5). The magmatic $fO2$ values for the samples from porphyry- and skarn-related deposits decrease from ΔFMQ ~2.3 ± 0.4 at 838° ± 42°C to a scattered range of ΔFMQ +1.0 to 1.5 at lower temperatures (Fig. 10b). In contrast, the magmatic $fO2$ of the IOA-related intrusive rocks increases from ΔFMQ +2.1 ± 0.3 (1σ) to ΔFMQ +2.7 ± 0.2 (1σ) with decreasing temperature (Fig. 10b).

H₂O content approximation: The H₂O concentrations of the silicate melts were approximated using the amphibole hygrometer (Ridolfi, 2021). These range from 5.6 ± 1.5 to 8.8 ± 0.5 wt % (avg. = 6.3 ± 1.0 wt %, 1σ) for porphyry- and skarn-related rocks, and 4.9 ± 0.3 to 5.7 ± 0.3 wt. % (avg. = 5.2 ± 0.5 wt %, 1σ; Fig. 10c) for IOA-related rocks. Given that the standard deviation is 2.4 wt % for H₂O estimated using the amphibole hygrometer (Ridolfi, 2021), our results are consistent with the average H₂O contents in global mafic-intermediate arc magmas (4.0 ± 1.3 wt %; Rasmussen et al., 2022). The positive correlation of the crystallization pressure with melt H₂O contents (Fig. 10c) is consistent with increasing magma storage depth with melt H₂O contents (Rasmussen et al., 2022).

S-Cl contents: For porphyry- and skarn-related intrusions, the S concentrations in apatite range from 0.08 ± 0.08 to 0.28 ± 0.03 wt % (avg. = 0.13 ± 0.06 wt %; 1σ), and the Cl concentrations are from 0.09 ± 0.02 to 0.89 ± 0.29 wt % (avg. = 0.35 ± 0.28 wt %; 1σ), respectively (Table 6). The molar fractions of F, Cl, and OH and ratios of X_F/X_OH, X_Cl/X_OH, and X_F/X_Cl were used with the thermodynamic models from Stock et al. (2018) to distinguish apatite grains crystallized prior to volatile exsolution (i.e., predegassed) and after volatile exsolution (i.e., postdegassed; see App. Fig. A5). The results indicate that most apatite grains from the porphyry- and skarn-related intrusive rocks crystallized prior to volatile exsolution.

The S and Cl concentrations in igneous apatite from the IOA-related silicate melts range from 0.01 ± 0.01 to 0.11 ± 0.02 wt % (1σ) and 0.39 ± 0.26 to 2.94 ± 1.71 wt % (1σ), respectively (Table 6). The data reveal two populations of apatite in these samples, consistent with predegassed and postdegassed apatite crystallization (Fig. 11). The average S and Cl concentrations in the predegassed apatite grains in the Ningwu IOA ore district are 0.10 ± 0.01 wt % (ranging from 0.09 ± 0.06 to 0.11 ± 0.02 wt %; 1σ) and 2.22 ± 0.77 wt % (ranging from 1.26 ± 0.03 to 2.94 ± 1.71 wt %; 1σ), respectively (Table 6). Apatite that crystallized after degassing yielded lower average S and Cl concentrations of 0.02 ± 0.01 wt % (1σ) and 1.03 ± 0.55 wt %, respectively (1σ; Table 6). In contrast, hydrothermal apatite grains in the albitized diorite sample NWHSQ-04 contain homogenous S and Cl contents of 0.16 ± 0.06 wt % (1σ) and 0.36 ± 0.07 wt % (1σ), respectively. These S and Cl concentrations are consistent with data from Zeng et al. (2016) for apatite crystallized from early-stage magmatic-hydrothermal fluids in the Ningwu ore district (Fig. 11).

The estimated average concentrations of S in the porphyry- and skarn-related and IOA-related silicate melts in equilibrium with predegassed apatite are comparable at 0.13 ± 0.06 wt % (1σ) and 0.16 ± 0.09 wt % (1σ), respectively (Fig. 12; Table 6). The average melt Cl/H₂O ratios estimated using apatite compositions for the intrusive rocks associated with the porphyry and skarn Cu and IOA deposits are 0.05 ± 0.03 (1σ) and 0.14 ± 0.02 (1σ) using the model of Li and Hermann (2017) and 0.04 ± 0.03 (1σ) and 0.11 ± 0.03 (1σ) using the model of Li and Costa (2020). Using the exchange partition coefficients of OH-Cl between amphibole and melt (Giesting and Filiberto, 2014), the melt Cl/H₂O ratios for porphyry and skarn Cu deposits are estimated to be lower than those for IOA deposits (0.02 ± 0.01 vs. 0.12 ± 0.04; 1σ; Fig. 12; Table 6). Similarly, the melt Cl/H₂O ratios estimated using biotite and bulk-rock compositions (Zhang, C., et al., 2022) are 0.03 ± 0.01 and 0.34 ± 0.03 for the porphyry- and skarn-related and IOA-related rocks, respectively (Table 6). These results are internally consistent, suggesting that the Cl/H₂O ratios of the IOA-related melts were a factor of 3 to 11 higher than those of the porphyry- and skarn-related melts.

The temperature-independent, mineral-based oxybarometers indicate that $fO2$⁠, reported as ΔFMQ, varied in the source magmas that evolved ore-forming fluids for porphyry Cu deposits and IOA deposits in the Middle-Lower Yangtze River metallogenic belt (Fig. 13). The $fO2$ data reported here for the causative magmas for the porphyry and skarn Cu ± Au deposits indicate that they were oxidized throughout their evolution (Fig. 13), consistent with published studies of arc magmas in general and those that formed porphyry Cu systems (Richards, 2015; Cottrell et al., 2021; Meng et al., 2021a; 2022). The porphyry- and skarn-related high-K calc-alkaline magmas in the Middle-Lower Yangtze River metallogenic belt are thought to have been derived from (1) thickened or delaminated lower crust (Wang et al., 2007; Hou, Z.Q., et al., 2011) or (2) partial melting of sub-arc mantle that was metasomatized in the Neoproterozoic (Li et al., 2008; Wang et al., 2016; Chen et al., 2020) and/or the Mesozoic (Mao et al., 2011). The estimated magmatic $fO2$ values reported here discount the possibility of the former model considering that the analyzed Archean-Neoproterozoic zircon xenocrysts that are probably entrained from the Precambrian crystalline basement are constrained to be relatively reduced (~ΔFMQ +0; Fig. 5a), so that pure remobilization of the Precambrian crystalline basement (i.e., dominated by late Archean to Paleoproterozoic tonalite-trondhjemite-granodiorite; Dong et al., 2011) and their mafic-ultramafic counterparts during thickening or delamination of the lower crust should have produced relatively reduced and S-poor magmas. A significant modification and oxidation of the lithospheric mantle is therefore required. Compared to the scattered ΔFMQ values (~0 on average; Fig. 8c) estimated using the compositions of the Neoproterozoic arc-like xenocrystic zircon grains (U_i/Nb ratios of ≥40; Grimes et al., 2015), the Jurassic arc-like zircon xenocrysts yielded relatively high magmatic $fO2$ of ΔFMQ +1.4 ± 0.5 (1σ, n = 14). We therefore suggest that the fluids or melts released during the paleo-Pacific flat-slab subduction may have contributed to metasomatism and oxidation of the mantle from which the oxidized porphyry- and skarn-related magmas formed in the Middle-Lower Yangtze River metallogenic belt.

The relatively high $fO2$ values of ΔFMQ +0.5 to ΔFMQ +2.5 for the porphyry- and skarn-related magmas predict the coexistence of sulfate and sulfide in the silicate melt by following the experimentally determined sulfide-sulfate transition in $fO2$ space for basaltic-dacitic melt at 1,000°C and 300 MPa (Jugo et al., 2010; Botcharnikov et al., 2011; Kleinsasser et al., 2022). This is supported by the presence of magmatic sulfide and sulfate minerals in igneous rocks from the Tongling (Du and Audétat, 2020) and Daye districts (Table 3), respectively. The magmatic $fO2$ increased with magma cooling, which may be attributed to the fractionation of Fe²⁺-bearing minerals (Ulmer et al., 2018; Tang et al., 2018; Zhang, J.B., et al., 2022). As the magma evolved, the estimated $fO2$ increased to ΔFMQ +2.5 at 770°C and then decreased to ~ΔFMQ +1 at ~700°C (Fig. 13).

Exsolution of magmatic-hydrothermal fluids from hydrous melts can occur during their emplacement in the upper crust (e.g., ~300 MPa; Edmonds and Woods, 2018). The decrease in $fO2$ as the porphyry- and skarn-related magmas cooled from 800° to 700°C could be explained by mass transfer of sulfur from the melt to the exsolved magmatic-hydrothermal fluid, where sulfur in the melt at a lower temperature is predicted to be both sulfate (SO₄)^2– and sulfide (H₂S) and sulfur in the fluid is sulfite (SO₂) at ~ΔFMQ +2.5 (Jugo et al., 2010; Audétat and Simon, 2012; Nash et al., 2019). The iron reduction and H₂S oxidation is described by the reaction:

The $fO2$ values for the intrusive rocks in the northern Ningwu ore district increased by nearly two orders of magnitude from ΔFMQ –1 at 970°C to ΔFMQ +0.5 at 800°C, and then increased to ΔFMQ +2.5 at <800°C (Fig. 13). The variable $fO2$ values for the two groups of samples may be attributed to the difference in the amount of evaporite assimilated, whereas the increasing trend in $fO2$ can be explained by fractionation of Fe²⁺-bearing minerals and two reactions that occur during degassing of the melt that yielded high Cl/H₂O ratios. This increases the Fe³⁺/Fe²⁺ ratio of the melt resulting in oxidation via the following reactions:

The latter involves the mass transfer of Fe²⁺ from the melt to the exsolved fluid phase as FeCl₂ (Simon et al., 2004; Bell and Simon, 2011).

Silicate melts that produced porphyry and skarn Cu ± Au deposits in the Middle-Lower Yangtze River metallogenic belt are distinct from those that produced the IOA deposits. The porphyry- and skarn-related melts were hydrous (~5–8 wt % H₂O) and S-rich (~0.13 ± 0.06 wt %) with moderate Cl/H₂O ratios (~0.04 ± 0.03), consistent with porphyry Cu systems and arc magmas globally (Candela and Piccoli, 1995; Meinert et al., 2005; Audétat and Simon, 2012; Richards, 2015; Meng et al., 2021a; Fig. 12). The IOA-related melts were less hydrous (~4–5 wt % H₂O), with significantly higher Cl/H₂O ratios (~0.11 ± 0.03) and highly variable S contents (<0.16 ± 0.09 wt %; Fig. 12).

The hydrous ore-forming melts for the porphyry and skarn Cu ± Au deposits lack whole-rock Eu anomalies and have elevated whole-rock Sr/Y ratios (see summary in Chen et al., 2020) and higher zircon Eu/Eu* ratios (Figs. 6–8) that are consistent with fractionation of amphibole and suppression of plagioclase (Richards and Kerrich, 2007). In comparison, the IOA-related dioritic rocks typically yielded negative whole-rock Eu anomalies, lower whole-rock Sr/Y ratios (see summary in Chen et al., 2020), and lower zircon Eu/Eu* ratios (Figs. 6–8), suggesting early fractionation of plagioclase that may be favored in the relatively dry magmas (Richards, 2011). These systematic changes in geochemical features are consistent with lithospheric extension at ~135 Ma (Figs. 2, 14; Li et al., 2019).

Exhaustion of the volatile ingredients in the previously metasomatized mantle source without a continuous flux of oxidized slab-derived fluids is not capable of maintaining the S contents in the derivative silicate melts because reduced sulfur will be separated early from the magmas as sulfide liquid (Wallace and Edmonds, 2011; Richards et al., 2017; Meng et al., 2022). Slab-derived fluids typically yielded high Cl/H₂O ratios, and the decline in the slab-derived flux to the mantle (e.g., during slab rollback or remobilization of previously metasomatized mantle for the Middle-Lower Yangtze River metallogenic belt) will decrease the Cl/H₂O ratios in the derivative melts (Kent et al., 2002; Candela and Piccoli, 2005). However, the predegassed S concentrations in ore-forming melts for the porphyry and skarn Cu ± Au deposits are indistinguishable from those in the magmas associated with IOA deposits, whereas the Cl/H₂O ratios of the predegassed IOA-related melts estimated using apatite compositions are a factor of ~3 higher than those estimated for porphyry- and skarn-related melts and those for most Phanerozoic arc magmas (Fig. 12). We therefore suggest that an additional source with a higher S concentration and Cl/H₂O ratio is required for forming IOA-related melts (Fig. 14).

One possible source to explain the S- and Cl-rich nature of the predegassed silicate melts associated with IOA deposits is the assimilation of evaporite sequences. This is supported by (1) the presence of hypersaline brine inclusions hosted in pyroxene-garnet skarns in the Ningwu basin (Li, W., et al., 2015), in which the brine inclusions are SO₄-rich (3–39 wt %) and have Cl/Br, Na/K, and Na/B ratios consistent with the assimilation of sedimentary halite (Li, W., et al., 2015), and (2) the heavy sulfur isotope values in hydrothermal anhydrite δ³⁴S_Anh = +15.2 to +16.9‰) and pyrite (δ³⁴S_Py = +4.6 to +12.1‰; Li, W., et al., 2015; Duan et al., 2021). The hypersaline brine inclusions were trapped at temperatures of ~740° to 860°C (Ma et al., 2006; Li, W., et al., 2015), whereas the hydrothermal anhydrite and pyrite precipitated from the ore fluid at 450° to 540°C (Duan et al., 2021). The results indicate that assimilation of the evaporite sequences probably occurred before saturation of apatite at 871° ± 31°C during IOA magma crystallization (App. Fig. A6). The significantly low sulfur isotope values for the high-K calc-alkaline intrusive rocks and sulfide minerals (δ³⁴S = –2 to +5‰) from most of the studied porphyry and skarn Cu ± Au deposits suggest much stronger assimilation of evaporite sequences (as suggested by the high pyrite δ³⁴S = +4.6 to + 12.1‰) during formation of the IOA deposits rather than porphyry and skarn Cu ± Au deposits (Pan and Dong, 1999; Li, W., et al., 2015; Zhou et al., 2015; Fan et al., 2019; Duan et al., 2021).

Porphyry Cu and IOA systems preferentially formed under broadly compressional and extensional environments, respectively, in response to the secular tectonic evolution in subduction-related settings (Sillitoe, 2003; Mao et al., 2011; Richards et al., 2017). The tectonic setting of the Middle-Lower Yangtze River metallogenic belt at >135 Ma has been debated, with subduction and intraplate models being proposed (Table 1). The interpreted broadly compressional setting at >135 Ma, which was a common feature in eastern China and adjacent countries (e.g., Korea, Japan, and northern Vietnam; Mao et al., 2021), is incompatible with the intraplate model in which the magmas associated with porphyry and skarn Cu ± Au deposits are thought to be derived mainly from partial melting of the previously metasomatized mantle (e.g., in the Neoproterozoic) in an extensional environment (Table 1). Neoproterozoic magmatism has barely been identified in the northeastern Yangtze craton, along which the Middle-Lower Yangtze River metallogenic belt formed (Zhou et al., 2002). The scattered and low ΔFMQ values of ~0 on average (Fig. 8c) estimated from the Neoproterozoic zircon xenocrysts of arc affinity are inconsistent with the relatively oxidized conditions of the mantle source of the Middle-Lower Yangtze River metallogenic belt in the Neoproterozoic. In contrast, the magmatic $fO2$ data presented in this study suggest that the mantle source may have been metasomatized and oxidized since ~170 Ma, coincident with the operation of the previously proposed paleo-Pacific flat-slab subduction (Li et al., 2019; Wu et al., 2019; Liu et al., 2021; Qiu et al., 2023). The temporal coincidence suggests that the paleo-Pacific flat-slab subduction should have contributed to the oxidation of the mantle lithosphere. The long distance (~1,000 km) of the paleo-Pacific flat-slab subduction is comparable to the Farallon flat slab during the Laramide orogeny in North America (Liu, L., et al., 2010; Axen et al., 2018; Yan et al., 2020). Fluids and/or melts released from the paleo-Pacific oceanic plate may have infiltrated, weakened, and triggered the partial melting of the overlying mantle lithosphere to produce adakitic magmas that ascend along suture zones (Fig. 14).

A simple model is put forward here to accommodate the tectonomagmatic evolution at the time of porphyry and skarn Cu ± Au and IOA deposit formation in the Middle-Lower Yangtze River metallogenic belt (Figs. 14, 15). Under a transpressional (or in transition to transtensional) setting (Table 1), primitive basaltic magmas were produced by partial melting of the relatively oxidized sub-arc mantle previously metasomatized by slab-derived fluids released from paleo-Pacific flat slab (Fig. 14a). Hot, hydrous basaltic magmas interacted with the upper plate lithosphere (e.g., Archean to Neoproterozoic crystalline basement) and underwent melting, assimilation, storage, and homogenization (Hildreth and Moorbath, 1988). These processes are commonly associated with the fractionation of mafic minerals and will progressively make the relatively oxidized primitive arc magmas geochemically evolved, more oxidized, and enriched in volatile elements such as H₂O and S, as well as variable Cl concentrations (Richards et al., 2012; Loucks, 2021). A moderately compressional setting inhibits rapid magma ascent (Watanabe et al., 1999) and favors the accumulation of andesitic-dacitic magmas in mid- to upper-crustal reservoirs, followed by exsolution of magmatic-hydrothermal fluids upon further cooling and interaction with the preexisting igneous or carbonate rocks to form porphyry and skarn Cu deposits (Richards, 2003; Sillitoe, 2010; Cooke et al., 2014).

Rollback of the paleo-Pacific flat slab from ~135 Ma may have led to an extensional to transtensional setting of the upper plate lithosphere in the Middle-Lower Yangtze River metallogenic belt during IOA formation (Figs. 14, 15; Table 1; Chang et al., 1991, 2012; Mao et al., 2006, 2011; Li et al., 2019) and caused a decrease in the mass transfer of aqueous fluids from the subducting slab to the mantle wedge that reduced the Cl and S in the source magmas. Upwelling metasomatized asthenosphere mantle may have interacted with the base of the crust, but the flux of hot, less-hydrous primitive basaltic magmas cannot be maintained at the base of the crust for extensive interaction, making the derivative magmas relatively mafic and less hydrous, so that they could ascend rapidly to the upper crust or erupt at the surface (Loucks, 2021), as evidenced by the voluminous volcanic deposits along the Middle-Lower Yangtze River metallogenic belt (Figs. 1, 14b, 15b). The Cl enrichment in the intermediate to mafic source magmas for IOA deposits during evaporite assimilation resulted in the mass transfer of significant quantities of Fe from the silicate melt to the exsolved ore fluid (Simon et al., 2004; Reich et al., 2022). Normal faulting networks developed in extensional settings during slab rollback can serve as conduits for the highly focused ascent of FeCl₂-rich magmatic-hydrothermal fluids (Fig. 15b; Reich et al., 2022). Magnetite will precipitate during the rapid ascent of an evolved magmatic-hydrothermal fluid via a reaction such as the following:

because the solubility of FeCl₂ is strongly pressure dependent (Chou and Eugster, 1977; Boctor et al., 1980; Simon et al., 2004; Zajacz et al., 2008; Reich et al., 2022). The P-T paths estimated using amphibole compositions (Fig. 10a) are consistent with the steeper geotherms of extensional settings as opposed to those for arc settings (Hopkins et al., 2008). Owing to a steep geothermal gradient in the extensional setting (Richards and Mumin, 2013b), as well as to thermal convection during asthenospheric upwelling, the high-heat–producing plutons may develop high-temperature alteration zones extensively in their apical parts for the IOA deposit formation (Figs. 14b, 15b; Table 1).

The data reported here indicate that the silicate melts for porphyry and skarn Cu ± Au deposits and IOA deposits have similar predegassed S contents. However, early volatile exsolution in the IOA-related magmas, as monitored by apatite composition (Fig. 11), would emit S, H₂O, and volatile metals (e.g., Cu) to the surface (Edmonds et al., 2022) at the time of magnetite precipitation at 500° to 800°C (Reich et al., 2022; Zeng et al., 2022). The concomitant emission of S, H₂O, and Cu with Cl would limit the residual melts for large-scale Fe ± Cu sulfide mineralization at ~400°C owing to SO₂ disproportionation (Rye, 1993). We here suggest that tectonic-driven processes are the best possible explanation for the different modes for IOA versus porphyry and skarn Cu ± Au deposits in the Middle-Lower Yangtze River metallogenic belt.

The petrogenetic studies presented here on the ore-forming source magmas for the representative porphyry and skarn Cu ± Au and IOA deposits in the Middle-Lower Yangtze River metallogenic belt suggest that their magmatic $fO2$ values vary systematically with crustal assimilation, crystal fractionation, and magmatic degassing. Because the estimated predegassed S concentrations are indistinguishable, the contrasting S concentrations in the primitive magmas were not the fundamental cause for the contrasting metal endowments in the porphyry and skarn Cu ± Au and IOA deposits of the Middle-Lower Yangtze River metallogenic belt. Instead, the magma composition and evolution paths (e.g., assimilation, decompression, cooling, and degassing) controlled by kinematic settings and geothermal gradients in the upper plate lithosphere exerted a first-order control on forming porphyry and skarn Cu ± Au and IOA deposit types in the Middle-Lower Yangtze River metallogenic belt.

The coexistence of an IOA deposit and a porphyry Cu system is rare on a global scale, such as in the Middle-Lower Yangtze River metallogenic belt, the Coastal Cordillera of northern Chile and Peru, and northern Sweden. Although slab rollback or retreating, asthenospheric upwelling, and extensional settings commonly follow compressional settings related to normal subduction, the IOA deposits are only rarely identified where evaporite sequences are identified or inferred based on geochemical evidence. This observation predicts that evaporite assimilation is probably a key ingredient for IOA deposit formation under a broadly extensional setting.

The data underlying this article are available in the article and its online supplementary materials. The supplementary materials include six appendix figures (App. Figs. A1–A6) and six appendix tables (App. Tables A1–A6).

The authors declare no competing interests.

The research was supported by the National Natural Science Foundation of China (Grant # 41820104010, J.M.); the U.S. National Science Foundation EAR (Grants 2214119 and 2233425, A.C.S.); a start-up research grant from China University of Geosciences (Beijing) to X. Meng (#3-8-2023-008); and a grant from the Natural Science Foundation of Anhui Province (#2208085QD111) to one of the co-authors (Shi, K).

We appreciate the field assistance from our colleagues at many institutions, including the First Brigade of the Hubei Geological Bureau (D.Q. Liu, L.Wang, and Y.D. Ruan), the Northwestern Brigade of the Jiangxi Geological Bureau (X.Z. Chen, D.B. Kong, D.S. Feng), the Geological Survey of Anhui Province (Z.T. Li) and 324 Brigade of Bureau of Geology and Mineral Exploration of Anhui Province (Y.S. Zhu), China University of Geosciences (Beijing; Y.L. Jin, Y.X. Zhang), the Institute of Mineral Resources of CAGS (Q.Q. Zhu), and the University of Science and Technology of China (Y. Wang), as well as those from the mining companies including Tongling Nonferrous Metals Group. Co., Ltd. (S. Wu) and the Nanshan Mine Company of the Maanshan Iron and Steel Group Incorporation (C.L. Wang and B.Y. Yang). We thank Z.Y. Chen, X.D. Chen, and C.H. Liu at the Mineral Institute of Mineral Resources (CAGS) for assistance with electron probe microanalyses. The graduate students, including J.H. Wang, Y. Zheng, H.X. Zhang, and J.W. Xiao, are appreciated for their assistance with LA-ICP-MS zircon isotope and trace element analyses at the China University of Geosciences (Wuhan). We sincerely thank D. Cooke, H. Chen, and an anonymous reviewer for detailed reviews that significantly improved the manuscript.

Xuyang Meng is a full professor at the China University of Geosciences (Beijing) in China. He started his Ph.D. program at the University of Alberta, and received the Ph.D. degree from Laurentian University in 2021, followed by a one-year postdoctoral study at the University of Michigan (Ann Arbor). His research focuses on the metallogeny of magmatic-hydrothermal deposits using a variety of geologic and geochemical tools including field mapping, oxybarometry, thermobarometry, EMPA, LA-ICP-MS, SIMS, and μ-XANES. He has conducted research on Archean to Miocene magmatic-hydrothermal Cu ± Au ± Fe deposits in China, Canada, Namibia, Serbia, and South America.