Geology, fluid inclusions, and H–O–S isotopes of the Yaozhuang Pb–Zn deposit in the Northern Qinling Belt, Central China

The genetic relationship between distal Pb–Zn–Ag vein-type deposits and porphyry molybdenum systems in orogenic belts is ambiguous, especially for ore-forming fluid sources, metal transport mechanisms, and magmatic–hydrothermal process. This study combines field observations, fluid inclusions, and H–O–S isotopic analyses from the Yaozhuang Pb–Zn deposit in the Northern Qinling Belt, Central China, in which a concealed porphyry molybdenum-related granitic intrusion exists, to clarify the genetic link between Pb–Zn mineralization and the deeper porphyry system. The Pb–Zn orebodies are hosted in the Mesoproterozoic metamorphic marine volcanic rocks and clastic rocks. Alteration and mineralization of the Yaozhuang Pb–Zn deposit could be categorized: Quartz–pyrite–arsenopyrite stage (Stage I), quartz–calcite–polymetallic sulfide stage (Stage II), and calcite–quartz stage (Stage III). Stage I is defined by quartz–pyrite–arsenopyrite and quartz–pyrite veins. Pyrite typically exhibits subhedral to euhedral and was commonly altered or filled by later galena and sphalerite along its fractures. Stage II is featured by notable lead and zinc mineralization, with extensive sulfides such as pyrite, galena, sphalerite, chalcopyrite, and pyrrhotite, coexisting with quartz–calcite ± biotite ± sericite and generally occurring as veins. Locally, chalcopyrite and pyrrhotite of this stage can occur as droplets in sphalerite, indicating simultaneous formation with Pb–Zn mineralization. Stage III is characterized by calcite–quartz veins that commonly crosscut previous formed sulfide-bearing veins (Stage I and II), indicating they formed later than the main mineralization stage (Stage II). From Stage I to III, four types of fluid inclusions are observed: CH₄-rich (LC-type), aqueous (W-type), CO₂-rich (C-type), and multi-phase solid-bearing (S-type) fluid inclusions. The microthermometric result from different types of fluid inclusions at Yaozhuang indicates a decline on temperature and salinity from Stage I (peaks at ca. 400–445 °C, 7.0–9.0 and 10.5–12.0 wt% NaCl eqv.), through Stage II (peaks at ca. 265–295 °C and 5.0–5.5 wt% NaCl eqv.) to Stage III (peaks at ca. 145–175 °C and 0.5–1.5 wt% NaCl eqv.). The H–O isotopic compositions of Stage II quartz (δD_fluid = –96 ‰ to –80 ‰ and δ¹⁸O_fluid = 4.9–5.4 ‰) indicate that fluids of this stage mostly originate from a magmatic–hydrothermal origin, but it was also injected by meteoric water, leading to fluid mixing. This inference is consistent with fluid evolution by temperature and salinity decreasing from Stage I to II. In-situ sulfur isotopes of sulfides from Stage I and II are comparable (–3.6 ‰ to 6.4 ‰), also suggesting a predominant magmatic–hydrothermal origin. In combination of recent discovery of the concealed porphyry molybdenum-related granitic intrusion under the Yaozhuang area by comprehensive investigation including drill core verification and geophysical survey, we propose that the Yaozhuang Pb–Zn deposit is a hydrothermal vein-type deposit, probably associated with this porphyry molybdenum-related granitic intrusion.