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Orogenic gold deposits contribute the largest proportion of the world's gold reserves, and the source of their ore-forming components has been recognized as the metamorphic devolatilization of metapelites or metabasites across the greenschist-to amphibolite-facies transition. However, hypozonal orogenic gold deposits represent an enigma in this context. Some of these apparently formed in higher-grade metamorphic rocks when temperatures were beyond the wet solidus of quartz-feldspar-bearing rocks; it is therefore puzzling how these fluids were generated in the source and migrated through the crust without causing partial melting. Here, we show that devolatilization of hydrated komatiites, a volumetrically significant lithological unit in Precambrian greenstone belts, is a viable model that can plausibly lead to gold mineralization at amphibolite-facies conditions. Our thermodynamic simulations indicate that subsolidus metamorphic devolatilization of komatiites at ~700 °C (upper amphibolite facies) can unlock significant amounts of gold via dehydration of talc and chlorite. This genetic model is supported by the geochemical characteristics of, and estimated pressure-temperature (P-T) formation conditions of, hypozonal gold deposits and the intimate spatiotemporal association between hypozonal deposits and komatiites in greenstone belts. This work expands the P-T range of the metamorphic devolatilization model and enhances its robustness in explaining gold mineralization in metamorphic terranes. [ABSTRACT FROM AUTHOR]

The presence of sulfate-rich fluids in natural magmatic hydrothermal systems and some carbonatite-related rare earth element (REE) deposits is paradoxical, because sulfate salts are known for their retrograde solubility, implying that they should be insoluble in hightemperature geofluids. Here, we show that the presence of quartz can significantly change the dissolution behavior of Na2SO4, leading to the formation of extremely sulfate-rich fluids (at least 42.8 wt% Na2SO4) at temperatures >~330 °C. The elevated Na2SO4 solubility results from prograde dissolution of immiscible sulfate melt, the water-saturated solidus of which decreases from ≥~450 °C in the binary Na2SO4-H2O system to ~270 °C in the presence of silica. This implies that sulfate-rich fluids should be common in quartz-saturated crustal environments. Furthermore, we found that the sulfate-rich fluid is a highly effective medium for Nd mobilization. Thermodynamic modeling predicts that sulfate ions are more effective in complexing REE(III) than chloride ions. This reinforces the idea that REEs can be transported as sulfate complexes in sulfate-rich fluids, providing an alternative to the current REE transport paradigm, wherein chloride complexing accounts for REE solubility in ore fluids. [ABSTRACT FROM AUTHOR]