Ore-forming processes of the Huangniping metasediment-hosted gold deposit, northern Longmenshan orogenic belt: constraints from textures, trace elements and sulfur isotopes of pyrite

The Huangniping Au deposit represents a typical example of metasediment-hosted gold mineralization in the northern Longmenshan orogenic belt, yet the processes controlling gold enrichment and redistribution remain poorly constrained. In particular, the role of multistage pyrite growth in recording fluid evolution, metal remobilization, and gold precipitation has not been systematically evaluated. Here we integrate petrographic observations, back-scattered electron (BSE) imaging, in-situ LA–ICP–MS trace element analysis, and LA–MC–ICP–MS sulfur isotope data to investigate seven generations of pyrite formed during sedimentary-diagenetic (Py0a–Py0b), metamorphic hydrothermal (Py1–Py3), and hydrothermal vein (Py4–Py5) stages. Gold is predominantly hosted as lattice-bound solid solution within pyrite, but exhibits systematic decoupling from As across successive pyrite generations, indicating that Au incorporation was not solely controlled by As substitution. Elemental zoning and relic cores in Py1 indicate inheritance and mobilization of Au and trace elements from early diagenetic pyrite during metamorphism. Py2 and Py3 display rhythmic zoning and enhanced Au, As, Cu, and Sb contents, suggesting progressive fluid enrichment. Py4 contains the highest Au contents (average 167 ppm, up to 393 ppm), with textures and zoning patterns indicative of episodic injections of Au-rich fluids and metal remobilization. In contrast, Py5 is characterized by low Au and base metal depletion. Sulfur isotope compositions of metamorphic hydrothermal stage (δ 34 S ranging from 15.0 to 27.5‰) and hydrothermal vein stage (19.3–22.8‰) overlap significantly, indicating a common ore-forming fluid source likely derived from metamorphism of the Cambrian Niutitang Formation sediments. This study demonstrates that multistage pyrite growth effectively archives the physicochemical evolution of ore-forming fluids in metasediment-hosted gold systems and highlights metasomatic overprinting, progressive metal remobilization, and cyclic fluid ingress as key processes governing gold enrichment in the northern Longmenshan orogenic belt.