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1. Variable Modes of Formation for Tonalite–Trondhjemite–Granodiorite–Diorite (TTG)-related Porphyry-type Cu±Au Deposits in the Neoarchean Southern Abitibi Subprovince (Canada): Evidence from Petrochronology and Oxybarometry

   Xuyang Meng, Jeremy P. (September 2022, Journal of petrology)

   Most known porphyry Cu±Au deposits are associated with moderately oxidized and sulfur-rich, calc-alkaline to mildly alkalic arc-related magmas in the Phanerozoic. In contrast, sodium-enriched tonalite–trondhjemite–granodiorite–diorite (TTG) magmas predominant in the Archean are hypothesized to be unoxidized and sulfur-poor, which together preclude porphyry Cu deposit formation. Here, we test this hypothesis by interrogating the causative magmas for the ∼2·7 Ga TTG-related Côté Gold, St-Jude, and Clifford porphyry-type Cu±Au deposit settings in the Neoarchean southern Abitibi subprovince. New and previously published geochronological results constrain the age of emplacement of the causative magmas at ∼2·74 Ga, ∼2·70 Ga, and∼2·69 Ga, respectively. The dioritic and trondhjemitic magmas associated with Côté Gold and St-Jude evolved along a plagioclase-dominated fractionation trend, in contrast to amphibole-dominated fractionation for tonalitic magma at Clifford. Analyses of zircon grains from the Côté Gold, St-Jude, and Clifford igneous rocks yielded εHf(t)±SD values of 4·5±0·3, 4·2±0·6, and 4·3±0·4, and δ18O±SD values of 5·40±0·11  , 3·91±0·13  , and 4·83±0·12  , respectively. These isotopic signatures indicate that, although these magmas are mantle-sourced with minimal crustal contamination, for the St- Jude and Clifford settings the magmas or their sources may have undergone variable alteration by heated seawater or meteoric fluids. Primary barometric minerals (i.e. zircon, amphibole, apatite, and magnetite–ilmenite) that survived variable alteration and metamorphism (up to greenschist facies) were used for estimating fO2 of the causative magmas. Estimation of magmatic fO2 values, reported relative to the fayalite–magnetite–quartz buffer as  FMQ, using zircon geochemistry indicates that the fO2 values of the St-Jude, Côté Gold, and Clifford magmas increase from  FMQ –0·3±0·6 to  FMQ +0·8±0·4 and to  FMQ +1·2±0·4, respectively. In contrast, amphibole chemistry yielded systematically higher fO2 values of  FMQ +1·6±0·3 and  FMQ +2·6±0·1 for Côté Gold and Clifford, respectively, which are consistent with previous studies that indicate that amphibole may overestimate the fO2 of intrusive rocks by up to 1 log unit. Micro X-ray absorption near edge structure (μ-XANES) spectrometric determination of sulfur (i.e. S6+/ S) in primary apatite yielded ≥ FMQ−0·3 and FMQ+1·4–1·8 for St-Jude and Clifford, respectively. The magnetite–ilmenite mineral pairs from the Clifford tonalite yielded  FMQ +3·3±1·3 at equilibrium temperatures of 634±21 ◦C, recording the redox state of the late stage of magma crystallization. Electron probe microanalyses revealed that apatite grains from Clifford are enriched in S (up to 0·1 wt%) relative to those of Côté Gold and St-Jude (below the detection limit), which is attributed to either relatively oxidized or sulfur-rich features of the Clifford tonalite. We interpret these results to indicate that the deposits at Côté Gold and Clifford formed from mildly (∼ FMQ +0·8±0·4) to moderately (∼ FMQ +1·5) oxidized magmas where voluminous early sulfide saturation was probably limited, whereas the St-Jude deposit represents a rare case whereby the ingress of externally derived hydrothermal fluids facilitated metal fertility in a relatively reduced magma chamber (∼ FMQ +0). Furthermore, we conclude that variable modes of formation for these deposits and, in addition, the apparent rarity of porphyry-type Cu–Au deposits in the Archean may be attributed to either local restriction of favorable metallogenic conditions, and/or preservation, or an exploration bias. 

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2. Serpentinite-derived slab fluids control the oxidation state of the subarc mantle

   https://doi.org/10.1126/sciadv.abj2515  

   Zhang, Yuxiang; Gazel, Esteban; Gaetani, Glenn A.; Klein, Frieder (November 2021, Science Advances)

   Recent geochemical evidence confirms the oxidized nature of arc magmas, but the underlying processes that regulate the redox state of the subarc mantle remain yet to be determined. We established a link between deep subduction-related fluids derived from dehydration of serpentinite ± altered oceanic crust (AOC) using B isotopes and B/Nb as fluid proxies, and the oxidized nature of arc magmas as indicated by Cu enrichment during magma evolution and V/Yb. Our results suggest that arc magmas derived from source regions influenced by a greater serpentinite (±AOC) fluid component record higher oxygen fugacity. The incorporation of this component into the subarc mantle is controlled by the subduction system’s thermodynamic conditions and geometry. Our results suggest that the redox state of the subarc mantle is not homogeneous globally: Primitive arc magmas associated with flat, warm subduction are less oxidized overall than those generated in steep, cold subduction zones. 

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3. Oxidized Sulfur Species in Slab Fluids as a Source of Enriched Sulfur Isotope Signatures in Arcs

   https://doi.org/10.1029/2024GC011542  

   Beaudry, Patrick; Sverjensky, Dimitri A (July 2024, Geochemistry, Geophysics, Geosystems)

   Abstract Recycling of oxidized sulfur from subducting slabs to the mantle wedge provides simultaneous explanations for the elevated oxygen fugacity (fO2) in subduction zones, their high hydrothermal and magmatic sulfur outputs, and the enriched sulfur isotopic signatures (i.e., δ34S > 0‰) of these outputs. However, a quantitative understanding of the abundance and speciation of sulfur in slab fluids consistent with high pressure experiments is lacking. Here we analyze published experimental data for anhydrite solubility in H₂O‐NaCl solutions to calibrate a high‐pressure aqueous speciation model of sulfur within the framework of the deep earth water model. We characterize aqueous complexes, required to account for the high experimental anhydrite solubilities. We then use this framework to predict the speciation and solubility of sulfur in chemically complex fluids in equilibrium with model subducting mafic and ultramafic lithologies, from 2 to 3 GPa and 400 to 800°C at logfO₂from FMQ‐2 to FMQ+4. We show that sulfate complexes of calcium and sodium markedly enhance the stability of sulfate in moderately oxidized fluids in equilibrium with pyrite atfO₂conditions of FMQ+1 to +2, causing large sulfur isotope fractionations up to 10‰ in the fluid relative to the slab. Such fluids could impart oxidized, sulfur‐rich and high δ³⁴S signatures to the mantle wedge that are ultimately transferred to arc magmas, without the need to invoke³⁴S‐rich subducted lithologies. 

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4. 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

   https://doi.org/10.5382/econgeo.5084  

   Meng, Xuyang; Mao, Jingwen; Simon, Adam; Duan, Chao; Xie, Guiqing; Su, Huimin; Hou, Tong; Shi, Ke; Chen, Nian (August 2024, Economic Geology)

   Porphyry Cu ± Mo ± Au and iron oxide-apatite (IOA) deposits rarely occur in spatial and temporal proximity in Phanerozoic arc-related settings, and the formation of these mineral deposit types in an evolving arc setting remains poorly understood. Specifically, the roles of magma composition and the tectonic regime remain the subject of some debate. Here, we systematically estimated the P-T-fO2 conditions and H2O-S-Cl contents for dioritic to granodioritic source magmas for porphyry and skarn Cu ± Au (150–135 Ma) and IOA deposits (~130 Ma) that formed in transpressional and transtensional settings in the Middle-Lower Yangtze River metallogenic belt, China. Our estimates show that, compared to IOA deposits, the porphyry- and skarn-related magmas were relatively felsic, cooler, and more hydrous. These geochemical features are consistent with the tectonic transition from subduction to slab rollback of the paleo-Pacific plate in the East Asia continental margin at <135 Ma and concomitant crustal extension and steepening of the regional geothermal gradient. Apatite data reveal that the silicate melts associated with the porphyry and skarn Cu ± Au and IOA deposits had comparable predegassed S concentrations (~0.13 ± 0.06 wt % vs. ~0.16 ± 0.09 wt % on average), but that IOA-related melts contained higher predegassed Cl/H2O ratios (~0.11 ± 0.03 vs. ~0.04 ± 0.03 for porphyry- and skarn-related magmas) that decreased by one order of magnitude after magmatic degassing. Magmatic fO2 estimated using zircon and amphibole, reported in log units relative to the fayalite-magnetite-quartz (FMQ) redox buffer, gradually increased during cooling of the porphyry- and skarn-related magmas (ΔFMQ +0.7 to +2.5) at 950° to 800°C and decreased to ΔFMQ +1 at 700°C owing to fractionation of Fe2+-rich minerals and subsequent S degassing, respectively. In contrast, the magmatic fO2 values for the IOA-related source magmas varied significantly from ΔFMQ –1.5 to ΔFMQ +2.5 but generally show an increasing trend with cooling from 970° to 700°C that probably resulted from variable degrees of evaporite assimilation, fractionation of Fe2+-rich minerals, and Cl degassing. These results are consistent with the hypothesis that Cl enrichment of the IOA-related source magmas played a determinant role in their formation. We propose that the porphyry and skarn Cu ± Au deposits in the Middle-Lower Yangtze River metallogenic belt formed in a transpressional setting in response to paleo-Pacific flat-slab subduction that favored storage and evolution of S-rich hydrous ore-forming magmas at variable crustal levels. A subsequent extensional setting formed due to slab rollback, leading to rapid degassing of Cl-rich IOA-related magmas. For the latter scenario, assimilation of evaporite by mafic to intermediate magmas would lead to an enrichment of Cl in the predegassed magmas and subsequent exsolution of hypersaline magmatic-hydrothermal fluid enriched in Fe as FeCl2. This Fe-rich ore fluid efficiently transported Fe to the apical parts of the magma bodies and overlying extensional normal faults where IOA mineralization was localized. The concomitant loss of S, H2O, and Cu with Cl by volcanic outgassing may have inhibited sulfide mineralization at lower temperatures. 

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5. Unusual sulfide-rich magmatic apatite crystals from >2.7 Ga Abitibi Greenstone Belt, Canada

   Meng, X; Mole, DR; Simon, AC; Mao, J; Kontak, DJ; Jugo, PJ; Kleinsasser, J (April 2025, American Mineralogist)

   Sodic volcano-plutonic terranes in the Archean can be well preserved, but why oxidized S-rich sodic magmas and porphyry-type Cu-Au deposits are so rare remains poorly understood. Here we addressed this issue by measuring the S concentration and S6+/ΣS ratio of primary apatite grains in >2.7 Ga felsic volcanic rocks from the well-characterized Neoarchean Abitibi Greenstone Belt of the Superior Province, Canada. Whereas apatite grains in most samples contain low-S concentrations (<0.01 wt%, n = 24), a few apatite samples are S-rich (0.14 ± 0.03 wt%, 1σ) and have low-S6+/ΣS ratios (0.56 ± 0.17; 1σ, n = 4). Samples with S-poor apatite have variable whole-rock La/Yb ratios (generally <30) and zircon 10 000*(Eu/Eu*)/Yb ratios of 11 ± 8 (1σ), which may be products of plume-driven or over-thickened crustal melting. In contrast, the samples with S-rich apatite have elevated La/Yb ratios of 49 ± 15 (1σ), zircon 10 000*(Eu/EuN*)/Yb ratios of 26 ± 7 (1σ), and zircon δ18O values of 5.8 ± 0.1 ‰ (1σ), consistent with a deep, hydrous and homogeneous mantle-like source for the melts dominated by amphibole ± garnet fractionation that is reminiscent of subduction-like process. These are the first reported results documenting the predominant accommodation of relatively reduced S in S-rich apatite grains crystallized from terrestrial silicate melts, possibly reflecting slight oxidation associated with the hydration of Neoarchean mantle and crystal fractionation over the magma evolution. The more common S-poor apatite data suggest that suppressed oxidation of the parental sodic magmas led to weak S emission from Earth’s interior to its evolving surface, explaining the rarity of porphyry-type Cu deposits in >2.7 Ga Archean sodic volcano-plutonic terranes. 

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