Using barium isotopes to distinguish metamorphic and magmatic fluids for the gold deposits

The orogenic and intrusion-related gold deposits represent the two most significant types of gold reserves globally, collectively accounting for over half of the total and are formed associated with metamorphic and magmatic hydrous fluids, respectively. Theoretically, gold deposits should be a result of activities of the metamorphic and magmatic hydrous fluids that sourced, carried, and reserved gold. However, distinguishing differences between the metamorphic and magmatic fluids proves challenging mainly due to overlapping of trace elements, S, and Re-Os isotopes of sulfides and C, H, O, He, and Ar isotopes of the ore-forming fluids. Barium, as a fluid sensitive cation, is believed to faithfully record the sources and evolution of fluids. In this work, we presented the Ba isotopes of various ore-rocks from the Hadamen and Haoyaoerhudong gold deposits along the northern margin of the North China Craton. These two deposits have been well-defined the ore-forming fluids that originated from crystal-melt separation in alkaline granitic magma and dehydration of chlorite and mica from the black sales during metamorphism, respectively. The Ba isotopes exhibit significantly fractionation among the various ore-rocks from above two gold deposits. With increasing SiO₂ content, δ^138/134Ba values increased from −0.28 ‰ in the potassium silicate alteration zone, crossed −0.21 ‰ ∼ −0.19 ‰ in the potassium silicate alteration zone filled with sulfide-quartz veins, to −0.13 ‰ ∼ +0.01 ‰ in the sulfide-quartz veins. This suggests that the heavier Ba isotopes were preferentially incorporated into the evolving magmatic fluids primarily due to the crystallization of K-feldspar and barite. In contrast, δ^138/134Ba values decreased from the carbonaceous slate (+0.73 ‰ ∼ +0.95 %) to sulfide veins (−0.28 ‰ ∼ +0.07 ‰), then increased to sulfide-quartz veins (+0.01 ‰). This phenomenon results from the continuously enhanced dissolution of diagenetic barite accompanying metamorphism and the crystallization of barite during evolution of metamorphic fluids. The microstructural characteristics also support that crystallization and dissolution of barite control significant Ba isotope fractionation in two types of fluids with different features. Furthermore, the magmatic and metamorphic fluids exhibit relative positive and negative relationships between Ba content and δ^138/134Ba values, respectively. These geochemical features are also useful in defining the origin of the ore-forming fluids. Therefore, we propose that Ba isotope composition will be a new tool for deciphering the evolution of the Au-bearing ore-forming fluids and distinguishing the origin of the ore-forming fluids.