Two Episodes of Postpeak Metamorphic High- to Moderate-Temperature Orogenic Gold Mineralization at Danba, Southwest China: Timing and Fluid Source Constraints from Thermodynamic Modeling and U-Pb, Re-Os, and S Isotopes

Abstract Orogenic gold deposits are unique in that they formed at a wide variety of crustal depths from deeply sourced fluids. They are enigmatic in terms of their timing relative to crustal peak metamorphism and the coexistence of both high-T (temperature) and medium-T mineralization. The hypozonal Danba gold deposit in southwestern China provides an important opportunity to investigate both the depth and timing of formation of gold mineralization. In situ U-Pb dating on monazite from distal garnet-biotite-schist wall rocks combined with thermodynamic modeling reveal an age of 201.1 ± 1.4 Ma for the peak metamorphism at amphibolite facies, with up to ~5.0 wt % H2O loss. Also identified are two subsequent regional-scale fluid modification events at ~194 and ~187 Ma, compatible with the timing and pressure-temperature (P-T) conditions of two gold mineralization episodes in the deposit. High-precision Re-Os dating on molybdenite from high-T ore assemblages hosted in shear fractures at depths of ~16 to 20 km yields ages between 194.8 ± 0.3 and 193.3 ± 0.3 Ma, whereas molybdenite from later medium-T ore assemblages hosted in extensional joints provides ages of 190.9 ± 0.3 to 186.0 ± 0.3 Ma. The Danba gold deposit clearly formed in two mineralization episodes during rapid crustal uplift along a retrograde metamorphic trajectory, and thus the crustal host sequences that were previously devolatilized are not the preferred ore fluid source. Supporting evidence also includes systematic in situ sulfur isotope ratios of molybdenite and associated sulfides, which indicate a concentration of δ34S values of 3 to 5‰ for the two episodes of mineralization. These contrast with δ34S values of products of the regional upper and lower crustal metamorphic and granitic end members but are similar to those of sulfides from a contemporaneous potassic mafic complex. Therefore, the superimposition of two discrete gold mineralization events at Danba, due to episodic release of deep auriferous fluids, is more likely to have a similar source to the potassic basic magma, mainly derived from metasomatized mantle lithosphere. This research suggests that Danba and the giant Jiaodong gold province, among others, share common characteristics of postmetamorphic timing, primitive high oxidation state, and positive sulfur isotope ratios, which are potentially indicative of metasomatized mantle-related orogenic gold deposits. It also implies the potential for new exploration spaces in Phanerozoic orogenic gold deposits hosted in high-grade metamorphic terranes.