Oxidized sulfur-rich arc magmas formed porphyry Cu deposits by 1.88 Ga

Porphyry Cu systems account for a major share of global Cu production and are predominantly Phanerozoic, linked to hydrous, moderately oxidized, sulfur-rich arc magmas. Whether analogous metallogenic processes operated in the Precambrian has been uncertain due to poor preservation and pervasive alteration of ancient rocks. Precambrian tectono-magmatic conditions have been argued to be unfavorable for porphyry Cu formation, given reduced conditions in oceanic basalts and sediments that could release H2S to the mantle wedge, stabilizing mantle sulfides, depleting melts in chalcophile metals, and suppressing ore formation. The Haib porphyry Cu deposit (southern Namibia) is one of the largest, best-preserved Paleoproterozoic systems and offers an opportunity to directly test the redox state and sulfur content of ore-related magmas. The study aims to determine the timing, sources, oxidation state (fO2), and sulfur contents of the Haib calc-alkaline magmas, and to evaluate whether Phanerozoic-like porphyry-forming processes were active by ca. 1.88 Ga.

Prior work shows Phanerozoic porphyry Cu deposits are generated by hydrous, moderately oxidized (approximately FMQ +1 to +2) and sulfur-rich arc magmas derived from a mantle wedge metasomatized by slab-derived oxidized fluids. The scarcity of documented Precambrian porphyry Cu deposits has been attributed to poor preservation and/or reduced subduction environments dominated by ferrous iron and sulfide in sediments and oceanic crust, which could lead to H2S-rich fluxes to the sub-arc mantle, stabilizing mantle sulfides, depleting magmas in chalcophile elements, and hindering porphyry fertility. Models invoking deep-crustal garnet fractionation have been proposed to auto-oxidize magmas to porphyry-favorable redox states, but require specific differentiation paths often recorded in trace-element systematics. Geological evidence suggests that at least local modern-style plate tectonics operated by 1.9–1.8 Ga. Independent studies document thick Paleoproterozoic evaporites and elevated marine sulfate around ~2.0 Ga, implying potential recycling of sulfate-rich surface reservoirs into the mantle and the possibility of generating oxidized arc magmas earlier than commonly assumed.

• Geological context and sampling: Sixty-two volcanic and intrusive samples were collected from drill core and outcrop across the Haib system (Richtersveld Magmatic Arc). Nineteen of the least-altered samples were selected for analysis. Petrography and alteration were assessed to target robust phases and minimize post-magmatic overprints.
• Whole-rock geochemistry: Major and trace elements were measured by XRF and ICP-MS (lithium metaborate pre-fusion) at ALS. QA/QC included analysis of a secondary reference sample (LK-NIP-1) and duplicates for selected samples.
• U-Pb geochronology: Magmatic zircons from pre-, syn-, and post-ore units were dated by LA-MC-ICP-MS for initial constraints, and high-precision CA-ID-TIMS for key units (mineralized granodiorite porphyry, leucogranodiorite porphyry, aplite) to mitigate Pb-loss and refine emplacement ages. Hydrothermal rutile associated with chalcopyrite was dated in situ by LA-MC-ICP-MS.
• Zircon isotopes and trace elements: Zircon Hf isotopes (Lu-Hf) were analyzed by LA-MC-ICP-MS (split-stream where applicable) with multiple standards; oxygen isotopes (δ18O) were measured by SIMS (Cameca IMS 1280) with monitoring of 16O1H/16O to screen for alteration. Zircon trace elements (including Ce, Ti, U) were measured to apply a zircon-based magmatic oxybarometer.
• Mineral inclusion approach: Robust zircon and titanite crystals were imaged (BSE, CL) to identify pristine apatite inclusions. This strategy minimizes effects of metamorphism/alteration by analyzing inclusions shielded within refractory hosts.
• Electron microprobe (EPMA): Major/minor elements of apatite were measured (Cameca SX100). Analyses with high ZrO2 (>1%) indicating zircon contamination were excluded. Beam damage tests constrained reliability of S and Cl determinations.
• μ-XANES at S K-edge: Sulfur redox in apatite inclusions was determined using synchrotron μ-XANES (APS 13-ID-E; SLS PHOENIX X07MA/B). Spectra were collected on mapped inclusions, normalized, and peak-fit to quantify S6+/ΣS. Contributions from host zircon and epoxy were assessed and contaminated spectra discarded.
• Magmatic fO2 estimation: Three independent oxybarometers were applied: (1) apatite S6+/ΣS-based calibration (1000 °C, 300 MPa) with P–T corrections; (2) zircon trace-element empirical oxybarometer (Ce, Ti, initial U) with screening criteria; (3) qualitative constraint from titanite + magnetite + quartz assemblage.
• Sulfur contents of melt: Apatite S concentrations were converted to melt S contents using two independent approaches for apatite/melt partitioning (Dap/m): Method 1 combining an fO2-dependent calibration (for mafic melts at ~1000 °C, 300 MPa) with a temperature correction to apatite saturation temperature; Method 2 a thermodynamic/activity-based approach relating Dap/m to temperature and activities, assuming a linear negative T-dependence and parameterization at 930 °C. Apatite saturation temperatures were estimated for each sample.
• Additional constraints: Ti-in-zircon thermometry provided temperatures relevant to oxybarometry and partitioning calculations. Structural and petrological observations (REE patterns, Dy/Yb vs SiO2) constrained differentiation paths (amphibole vs garnet fractionation).

• Timing: LA-MC-ICP-MS zircon U-Pb upper intercept ages: andesite porphyry 1912 ± 10 Ma and 1892 ± 9 Ma; rhyolitic tuff 1892 ± 6 Ma; equigranular granodiorite 1887 ± 7 Ma; quartz-monzonite enclave 1886 ± 5 Ma; diorite 1875 ± 10 Ma. CA-ID-TIMS weighted mean 207Pb/206Pb ages (Th-corrected): mineralized granodiorite porphyry 1885.47 ± 0.93 Ma; leucocratic granodiorite porphyry 1886.0 ± 1.6 Ma; aplite 1881.02 ± 0.71 Ma; carbonate-altered aplite dike 1881.8 ± 1.2 Ma. Mineralization bracketed at 1883.6 ± 1.8 Ma; hydrothermal rutile LA-ICP-MS age 1891 ± 34 Ma.
• Magma sources: Zircon εHf(initial at 1880 Ma) from −2.6 ± 0.6 to +1.0 ± 0.9 (mean −2.0 ± 1.6; n=99) and δ18O 5.56 ± 0.64‰ to 7.10 ± 0.39‰ (mean 6.14 ± 1.19‰; n=102) indicate mantle-dominated magmas with minor incorporation of recycled upper crustal material; more elevated δ18O in volcanic/porphyritic rocks implies greater low-temperature altered crustal input. Limited Hf variation suggests minimal assimilation during ascent.
• Differentiation: Listric REE patterns, moderate negative Eu anomalies, and decreasing Dy/Yb with increasing SiO2 implicate low-pressure fractionation dominated by amphibole ± plagioclase, with little to no garnet influence.
• Oxidation state (fO2): Apatite μ-XANES S6+/ΣS yields AFMQ +0.85 ± 0.08 to +1.07 ± 0.11 (mean +0.89 ± 0.04). After P–T correction, plutonic fO2 ~ AFMQ +1.24 ± 0.01; volcanic fO2 ~ AFMQ +1.37 ± 0.02. Zircon trace-element oxybarometer indicates AFMQ +1.3 ± 0.3 to +2.1 ± 0.6 (average +1.9 ± 0.6) for syn-ore plutonics. Titanite + magnetite + quartz assemblage requires > AFMQ +0.7 ± 0.1 at 768 ± 23 °C (Ti-in-zircon).
• Sulfur budget: Apatite inclusions in zircon/titanite contain 0.11 ± 0.04 to 0.19 ± 0.08 wt.% S, comparable to apatite in Phanerozoic arc and porphyry-related rocks. Estimated melt S contents: Method 1 = 0.07 ± 0.03 to 0.17 ± 0.08 wt.% (mean 0.11 ± 0.06 wt.%); Method 2 = 0.02 ± 0.001 to 0.08 ± 0.04 wt.% (mean 0.04 ± 0.03 wt.%), broadly overlapping S in Phanerozoic arc melts with limited degassing (SiO2 ≥ 52 wt.%).
• Emplacement conditions and system character: Plutonic host and mineralization emplaced at ~200–300 MPa, consistent with deeper levels of porphyry systems. Hydrothermal anhydrite with chalcopyrite in potassic alteration indicates an oxidized ore fluid exsolved from oxidized magma.
• Metallogenic implication: The Haib ore-forming magmas at 1.886–1.881 Ga were moderately oxidized and S-rich, akin to Phanerozoic porphyry-fertile arc magmas, and derived from mantle modified by oxidized slab-derived fluids.

The multi-proxy evidence (apatite μ-XANES, zircon oxybarometry, titanite–magnetite–quartz) demonstrates that Paleoproterozoic Haib magmas were moderately oxidized (about FMQ +1 to +2) and sulfur-rich prior to volatile degassing, conditions favorable for retaining chalcophile elements and for efficient transfer of Cu into exsolved S–Cl-bearing magmatic-hydrothermal fluids in the upper crust. Trace-element systematics (Dy/Yb vs SiO2, REE patterns) indicate differentiation dominated by amphibole at low pressure, excluding significant garnet fractionation and therefore discounting auto-oxidation via deep crustal garnet fractionation as the main driver of elevated fO2. Zircon isotopes show limited crustal assimilation during ascent, implying that the redox signature reflects source characteristics rather than contamination. The inferred oxidized mantle source is consistent with subduction-zone recycling of sulfate-rich fluids released from oxidized sediments and/or seawater in the subducted slab, plausible given evidence for thick Paleoproterozoic evaporites and elevated marine sulfate by ~2.0 Ga. These findings resolve the question of whether Precambrian tectono-magmatic systems could generate porphyry Cu deposits: similar processes to those in the Phanerozoic were operating by ~1.88 Ga, with oxidized, S-rich arc magmas capable of delaying deep sulfide saturation, preserving Cu in ascending magmas, and producing porphyry-style mineralization at upper crustal depths.

This study provides direct mineral-scale constraints showing that by 1.88 Ga, calc-alkaline arc magmas were moderately oxidized (FMQ +1 to +2) and sulfur-rich, and that such magmas formed the Haib porphyry Cu deposit in a mature island-arc setting. High-precision geochronology brackets mineralization at 1883.6 ± 1.8 Ma. Apatite μ-XANES, zircon oxybarometry, and phase equilibria converge on elevated magmatic fO2, while apatite-based estimates indicate S-rich melts comparable to Phanerozoic porphyry-fertile systems. Isotopic data indicate mantle-dominated sources with minor recycled crustal contributions, and differentiation trends exclude garnet-driven auto-oxidation. The results support a Paleoproterozoic onset of efficient sulfur recycling into the mantle via subduction of sulfate-rich sediments/fluids, enabling porphyry metallogeny earlier than widely assumed. Future work should expand similar multi-proxy redox and sulfur budgets to other Precambrian arcs and deposits, refine temperature- and fO2-dependent apatite–melt sulfur partitioning models, and integrate volatile budgets to assess the prevalence and preservation potential of early porphyry systems.

• Afflicted Precambrian terrains can be variably altered/metamorphosed; although inclusion-based approaches in zircon/titanite minimize overprints, they may not capture all magmatic variability.
• The apatite μ-XANES oxybarometer is calibrated at specific P–T; P–T corrections introduce uncertainty, and the assumption that silicate glass trends apply to apatite incorporation may not be exact.
• Limited number and small sizes of apatite inclusions can introduce analytical challenges; spectra with zircon/epoxy contributions were discarded, potentially reducing sample representation.
• The titanite + magnetite + quartz oxybarometric constraint is less robust due to difficulty confirming equilibrium assemblages.
• Sulfur partitioning (Dap/m) lacks a universally accepted model capturing simultaneous fO2, T, and composition effects; two different estimation methods agree within an order of magnitude but still impart significant uncertainty to melt S contents.
• Hydrothermal degassing can modify volcanic products; plutonic samples are interpreted as best pre-degassing estimates, but some variability may persist.