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The origin of sediment-hosted copper-cobalt deposits (SCDs) within metamorphic terranes remains contentious, particularly in regard to the timing of mineralization relative to basin evolution. Here, we link the timing of Cu-Co mineralization in the Zhongtiao Mountains district, central China, to basin closure during development of the Trans-North China Orogen. Metamorphic apatite from meta-evaporite has a U-Pb age (1844 ± 25 Ma) within error of a Re-Os age for molybdenite from Cu- and Co-bearing veins (1819 ± 10 Ma), implying that mineralization and metamorphism were coincident. In situ trace element and Fe-S isotope analyses of deformed sedimentary pyrite (Py I) and younger euhedral metamorphic pyrite (Py II) in pyritic graphite schist indicate that Co was mobilized from Py I via fluid-mediated dissolution and reprecipitation. In the graphite schist, representing a sulfidic shale metamorphosed at upper greenschist facies conditions, relic Py I has high δ34S values (22.9 ± 0.4‰, n = 10) and δ56Fe values (0.90 ± 0.16‰, n = 8). These values are consistent with bacterial sulfate reduction and pyrite formation in a Paleoproterozoic sulfidic sea (i.e., after the first great oxygenation event). In addition, Py I has unusually high Co contents (0.8–3.0 wt.%), suggesting that transition metals, including Co and Ni, were originally concentrated in sedimentary pyrite in an evaporative setting. The evaporitic setting is supported by the presence of sylvite in sulfidic schists and adjacent scapolitic calc-silicates. Younger Co-poor Py II has slightly lower δ34S values (21.4 ± 0.2‰, n = 19) and higher δ56Fe values (1.61 ± 0.15‰, n = 9), supporting the hypothesis that Co was released from sedimentary sulfide during dissolution and reprecipitation. The great variability of δ34S (14.8 to 22.9‰) and δ56Fe (0.13 to 2.30‰) in hydrothermal pyrite (Py III) associated with Cu-Co mineralization suggests that an external Cu-rich oxidizing fluid was involved in the ore-forming process. Copper and Co in this type of deposit are typically both considered to be derived from fertile basement rocks. This study, however, highlights another potential source for the Co, with its enrichment in a sulfidic sea along an evaporative margin and its upgrading through mobilization during orogenesis that leads to formation of a relatively high-temperature group of SCDs. Tectonometamorphic processes causing such Co enrichments took place during the Paleoproterozoic and Neoproterozoic oxygenation events, implying that the most prospective targets for Co resources are concentrated in ca. 2.2–2.0 Ga and 0.9–0.7 Ga deformed passive margin marine sequences.

Isotopic dating studies have revealed that many carbonatite-related REE ore systems have protracted and complex temporal evolution histories. The mechanisms that cause these complex dating results remain poorly understood. Here, we present an in-situ Sm-Nd and Th-Pb isotopic investigation of monazite and bastnasite from a geologically-complicated deposit, the world-class Bayan Obo REE deposit of China, with the aim of defining the general mechanisms accounting for the protracted age spread observed in this and many other REE deposits. Monazite and bastnasite are two principle REE ore minerals in the Bayan Obo deposit. Most of the studied monazite and bastnasite grains have experienced fluid-aided modification. The modified grains have generally homogeneous Sm and Nd contents and Sm-Nd isotopic compositions, implying low mobility of Sm and Nd during the modification process. Thus, the Sm-Nd chronometer can be used to constrain the primary crystallization timing of the REE minerals at 1293 ± 48 Ma. However, altered REE minerals have apparent 208Pb-232Th ages (1248–261 Ma) that are variably younger than the primary crystallization age. In most samples, Th contents have no correlations with 208Pb-232Th ages, while Pb contents tend to decrease along with the younging trend of 208Pb-232Th ages. NanoSIMS mapping shows that 208Pb and 208Pb/232Th are heterogeneous, and irregularly distributed in the altered grains. Such results are indicative of variable Pb loss during the fluid-aided modification process. Thus, many Th-Pb ages represent partial isotopic resetting and, hence, are geologically meaningless. Completely reset ages cluster at 450–400 Ma or 280–260 Ma, representing two discrete episodes of post-ore hydrothermal activity. Small amounts of monazite and bastnasite that were newly crystallized during these two overprinting episodes have initial Nd isotopic signatures approaching the time-evolved isotopic compositions of primary ores, which suggests insignificant REE input to the primary ores during these events. This study provides critical evidence to resolve complex dating results and disparate ore genesis models for the giant Bayan Obo deposit. Moreover, our results indicate the resistance of Sm-Nd isotopes, but the susceptibility of Th-(U)-Pb isotopes, to fluid-aided modification. Therefore, younger Th-U-Pb dates in geologically-complicated deposits should be treated with caution as they may represent partial resetting results and/or may not represent events of increased metal tenor.

Hydrothermal fluids in orogenic and intrusion-related (mesothermal) gold deposits are dominated by saline-aqueous-carbonic fluids, commonly represented by the ternary system H2O-NaCl-CO2. The phase equilibria, thermodynamic properties (PVTx, density, compositions of fluid phases) and quartz dissolution-precipitation behavior in the H2O-NaCl-CO2 system, which are still poorly constrained, are of great importance to understanding the process of hydrothermal gold mineralization. Here, we conducted thermodynamic modeling to constrain the fluid properties under single- and two-phase conditions in the H2O-NaCl-CO2 system at temperatures of 300 to 500 °C and pressures of 0.001 to 3.5 kbar. Our results illustrate thermodynamic controls rooted in the equilibria of the H2O-NaCl-CO2 system. Increasing CO2 and/or NaCl contents shift the solvus to higher pressures and temperatures, expanding the pressure–temperature region of L + V immiscibility. Calculated isopleths of CO2 content in coexisting, immiscible vapor and liquid describe the maximum amount of CO2 that may be present in fluid inclusions through the studied P-T-x ranges. Quartz solubility in the H2O-NaCl-CO2 fluids shows strong dependence on temperature, pressure, and CO2 content, with several potential triggers for vein mineral deposition. Specifically, solubility of quartz generally decreases with decreasing temperature, pressure, and increasing CO2 content both in the single- and two-phase fluids, but exhibits retrograde behavior in the L + V field or at the phase-transition boundary. In detail, the dependence of quartz solubility on pressure is weak at low temperatures (300 °C), and becomes progressively stronger at high temperatures (400 °C and 500 °C), vice versa for temperature dependency at different pressures. The present study reviews two mechanisms of fluid immiscibility and constrain the conditions at which distinct fluid inclusion types, bedding-parallel shear veins and fault-related extension veins are formed in mesothermal gold deposits. Thermodynamic modeling of the solvus of the H2O-NaCl-CO2 system is consistent with the hypothesis that decompression is an efficient mechanism driving fluid immiscibility at specific ranges of temperature and composition, producing coexisting liquid- and vapor-phase fluids, both with low density and relatively low content of CO2. For fluids with high CO2 contents (>10 mol. %), cooling at relatively high pressure may also lead to fluid immiscibility, producing a liquid with low CO2 content in equilibrium with a vapor of medium to high CO2 content. Very CO2-rich, or “pure CO2” fluid inclusions (>90 mol. % CO2, type I) cannot be produced by immiscibility under these conditions, but may rather be the result of decrepitation of primary H2O-NaCl-CO2 fluid inclusions. Within the P-T-x ranges studied here, cooling-dominated phase separation accounts for the formation of fluid inclusions with moderate to high CO2 contents (50–80 mol. % CO2, type IIa). Fluid inclusions with low to moderate CO2 contents (5–30 mol. % CO2, type IIb) can represent the original ore-forming fluids, or can be produced by decompression-dominated phase separation. And H2O-NaCl fluid inclusions (type III) generally represent the latest-stage ore-forming fluids, or can be produced by decompression-induced immiscibility at high temperature. In orogenic (mesothermal) gold deposits, decompression-induced quartz precipitation during pressure fluctuation is dominant in bedding-parallel shear veins. Fault-related extension veins are associated with initial decompression-induced quartz precipitation and subsequent cooling-dominated deposition.