Abstract The Neoarchean Abitibi subprovince hosts more than half of all known Archean volcanogenic massive sulfide (VMS) deposits worldwide. Although most classify as bimodal mafic, reflecting a dominance of mafic volcanic rocks, they are commonly proximal to or hosted by felsic volcanic successions. These felsic volcanic rocks provide key insights into VMS formation, yet it remains unclear whether they are anomalously abundant in areas with VMS endowment compared with the broader volcanic stratigraphy. This study uses new assemblage and lithologic map compilations of the Abitibi subprovince that enable quantitative estimates of felsic volcanic rock proportions. Volcanogenic massive sulfide zones are defined as areas within a 15-km radius of the largest deposit. By this definition, ~70% of zones contain felsic proportions exceeding the subprovince-wide volcanic background of ~7%. Monte Carlo simulations (n = 1,000) confirm that this enrichment is highly unlikely under random placement. The results reinforce a robust empirical association between VMS deposits and localized felsic volcanic centers. These centers are interpreted as regions of unusually evolved crust within their larger assemblages, where crustal growth was concentrated and accelerated through rifting, creating deep crustal permeability and accommodating the extension required for thermally mature crust. Within or proximal to felsic centers, deep crustal structures coupled with elevated heat flow from increased volumes of volcanic and subvolcanic felsic and mafic magma produced an environment favorable for VMS formation. Collectively, this analysis provides new evidence for the role of felsic centers in VMS formation and offers an additional method for assessing VMS potential in the Abitibi and other greenstone belts.
Abstract Magnesium and nickel isotopes provide insights into weathering processes and lateritic Ni enrichment; however, their coupled behavior during garnierite formation remains poorly constrained. We present Mg-Ni isotope compositions for serpentinized bedrock, garnierite, and laterite from the Morowali Ni laterite deposit, Sulawesi, Indonesia, to elucidate weathering intensity, solid-solution interaction, and mechanisms of Ni-bearing phyllosilicate precipitation. Weathering from bedrock (δ26Mg = –0.33 to –0.11‰) through saprolite (–0.20 to 0.14‰) to laterite (0.47‰) indicates preferential leaching of light Mg and retention of heavy Mg. Conversely, δ60Ni decreases from bedrock (–0.30 to 0.14‰) through saprolite (–0.78 to 0.03‰) to laterite (–1.34‰), reflecting mobilization of heavy 60Ni and adsorption of light 58Ni onto Fe oxyhydroxides. Garnierite occurs as two mineralogically and isotopically distinct groups: serpentine-like (δ26Mg = –0.10 to 0.12‰; δ60Ni = 0.98–1.46‰) and talc-like types (δ26Mg = 0.54–1.12‰; δ60Ni = 1.08–1.67‰), comparable to Ni isotope ratios in seawater (δ60Ni ≈ 1.44‰). Covariation among Mg-Ni isotopes and Ni/Mg ratios along the weathering profile suggests that both isotopes record weathering intensity and dissolution-precipitation processes. Infiltration of 60Ni-enriched fluids into the saprolite promoted garnierite precipitation and produced 60Ni-enriched ores. Petrography and Fe contents indicate that serpentine-like garnierite formed first, followed by talc-like garnierite during downward groundwater percolation. Color zoning within talc-like garnierite, from emerald rims to white cores, reflects progressive Ni and 26Mg depletion from evolving fluids. Elevated δ26Mg and depleted δ60Ni values in thick laterite horizons mark advanced weathering and may serve as geochemical indicators of underlying Ni-rich zones in Sulawesi.
Abstract Porphyry deposits represent globally significant sources of Cu and Mo, yet the magmatic volatile controls on Cu versus Mo metallogenic divergence remain poorly quantified in collisional settings. Here, we compare porphyry Cu systems from the Gangdese belt, southern Tibet, with porphyry Mo systems from the southern margin of the North China craton—two collisional belts with contrasting magma sources and metal endowments. By integrating apatite geochemistry with thermodynamic forward modeling, we reconstructed volatile evolution during ore-forming magma differentiation. Copper-forming magmas were more hydrous (initial H2O contents of ~5–6 wt %) than Mo-forming magmas (~3.8–4.7 wt %), whereas both were moderately oxidized. Modeled fluid-saturation thresholds are ~11 wt % H2O for Cu systems (~0.5 GPa) and ~9.2 wt % H2O for Mo systems (~0.4 GPa). Copper-related magmas were initially enriched in Cl (0.25–0.35 wt %) and F (0.40–0.55 wt %) but underwent significant F depletion (bulk DFc/m≈1.8) driven by early, extensive amphibole fractionation. In contrast, Mo-related magmas had lower initial F (0.05–0.25 wt %) and comparable Cl (0.20–0.35 wt %) but became progressively enriched in both halogens DFc/m≈0.7; DClc/m≈0.14 during prolonged crystallization with limited amphibole crystallization. This contrasting H2O-F-Cl evolution links source-imposed volatile budgets to Cu-dominated outcomes versus Mo-dominated outcomes: Cl-enriched residual melts favor Cu-transporting fluids, whereas late-stage F enrichment likely lowers melt viscosity, prolongs differentiation, and promotes Mo enrichment in evolved residual melts.
Abstract The Brucejack epithermal Au-Ag deposit in northwestern British Columbia, Canada, is host to some of the world’s highest-grade bonanza gold ores. The trace element composition of pyrite in the deposit records a series of abrupt changes in the chemistry of the hydrothermal ore fluids that preceded and ultimately led to ultrahigh-grade gold mineralization. Pyrite cores in preelectrum stage I veins are Co-Ni rich and record the passage of early porphyry-type fluids, which evolved to epithermal-type fluids during telescoping of the system, as evidenced by oscillatory, colloform Au-As growth zones that mantle the Co-Ni cores. The vertical extent of this telescoping, as determined from the deepest pyrite samples that display composite epithermal-porphyry compositions and associated zoning, is ~1,000 to >1,300 m. This hydrothermal activity predated the emplacement of bonanza-grade epithermal stage III to stage V carbonate-quartz-electrum veins that crosscut stage I to II veins throughout the deposit. The time between the emplacement of prebonanza and bonanza vein stages was brief, as demonstrated by the respective 182.7 ± 1.0 and 182.6 ± 1.1 Ma U-Pb zircon ages of syn- to postmineral trachybasaltic dikes and the youngest mineralized host rock, an andesitic crystal tuff unit. Secondary ion mass spectrometry (SIMS) sulfur isotope exchange thermometric analyses show that the Co-Ni pyrite formed in a porphyry environment, whereas bonanza-stage veins formed under epithermal conditions. Our study reveals how pyrite chemistry integrated with geochronology can be used to map the evolution of hydrothermal activity in porphyry-epithermal environments in four dimensions (x-y-z-t), even if early alteration has been overprinted, or obliterated, by the products of telescoping.
Abstract Scheelite (CaWO4) is considered a useful mineral for classifying ore deposit types because its trace element concentrations, especially rare earth elements (REEs), As, Eu, Mo, and Sr, can inform on paragenesis. In this study, this concept is tested by integrating trace elements, in situ Sr and Nd isotopes, and δ18O data from two quartz-scheelite vein swarms cutting Paleozoic plutons in New Zealand. In both cases, the scheelite has been inferred by previous researchers to be of magmatic-hydrothermal origin. However, the scheelite in both deposits exhibits highly variable inter- and intrasample trace element concentrations, which we interpret to relate to different substitution mechanisms and oxidation states of the mineralizing fluids. Moreover, age-corrected scheelite in situ 87Sr/86Sr and 143Nd/144Nd values diverge from those of the host granitoids despite some veins occurring entirely within the plutons. Quartz-scheelite oxygen isotope thermometry indicates equilibration at relatively low temperatures (~170°–280℃), and the calculated δ18OH2O values suggest mixing between W-bearing fluids and meteoric waters. The scheelite trace element chemistries, therefore, reflect a complex interplay between local element reservoirs and oxidation states. Since these processes appear to have operated at the deposit scale, scheelite-based trace element paragenesis discrimination models should be applied with caution. For example, a metamorphic W-bearing fluid may acquire a magmatic-like trace element signature through interaction with granitoids shortly before scheelite crystallization; conversely, a magmatic-hydrothermal W-bearing fluid may develop a nonmagmatic signature by reacting with the metamorphic country rocks.
