Genesis and exploration potential of gold deposits in Archean high-amphibolite facies metamorphic rocks of the Jiaodong Peninsula: Geological and geochemical constraints from the Majiayao gold deposit

• Medium temperatures, medium–low salinity, and CO 2 -rich H 2 O-CO 2 -NaCl ± CH 4 orefluids. • Sulfidation and fluid phase separation are dominant mineralization mechanisms. • Gold mineralization at 3.3–6.0 km depth suggests deep exploration is promising. The Jiaodong Peninsula, China, hosts the giant Jiaodong gold province, containing over 5,800 tonnes of proven gold resources. While 87% of these deposits reside in Mesozoic granites, the remaining 11% occur within Archean metamorphic terranes. Notably, the origin, enrichment processes, and genetic model of the Majiayao gold deposit, the largest hosted in Archean metamorphic rocks, remain poorly constrained. This study comprehensively analyzes the deposit geology, fluid inclusions, isotopes, and trace elements of the Majiayao deposit, aiming at clarifying the genesis of the deposit and perfecting the overall genetic model of Jiaodong gold deposits. The Majiayao mineralization exhibited a four-stage sequence: (1) milky quartz-sericite, (2) quartz-pyrite, (3) quartz-siderite-polymetallic sulfide, and (4) quartz-carbonate. Detailed petrographic analyses identified three fluid inclusion types: H 2 O-CO 2 -NaCl ± CH 4 (Type I), CO 2 (Type II), and H 2 O-NaCl (Type III). Homogenization temperatures of fluid inclusions in the four stages (from early to late) ranged from 198-298 °C, 136–289 °C, 112–264 °C, and 139–179 °C, corresponding to salinities of 3.33–11.05 wt%, 2.77–12.51 wt%, 2.41–12.51 wt%, and 0.18–4.03 wt% NaCl equivalent. These data collectively suggest that the ore-forming fluids belong to a medium-temperature, medium–low salinity H 2 O-CO 2 -NaCl ± CH 4 system, enriched in CO 2 with minor CH 4 . This system evolved from medium to medium–low temperatures, shifting from CO 2 -rich to H 2 O-rich compositions, and from fluid immiscibility to fluid mixing. LA-ICP-MS analysis of pyrite revealed Au concentrations ranging from 0.008 to 12.909 ppm (median = 0.024 ppm) in Py2 and 0.025 to 2.330 ppm (median = 0.129 ppm) in py3. These results indicated extremely low invisible gold content associated with brittle deformation. The key mechanisms for gold mineralization were inferred to be sulfidation, characterized by the saturation and precipitation of abundant low-As pyrite and other sulfide minerals, alongside fluid phase separation. Lead isotope compositions ( 206 Pb/ 204 Pb = 16.451–16.674, 207 Pb/ 204 Pb = 15.211–15.353, 208 Pb/ 204 Pb = 36.979–37.803) indicated that ore-forming materials likely originated from the lower crust. Compared to typical Jiaodong-type gold deposits, the Majiayao deposit exhibited similar fluid inclusion characteristics and H-O-S isotopic signatures, but with slightly lower mineralization temperatures, a narrower range of δ 18 O H2O values, and distinct lead isotope compositions. Compared to the depth of 5–9 km in Zhaoyuan-Laizhou gold belt, the Majiayao deposit displays a comparatively shallow estimated mineralization depth of 3.3–6.0 km, suggesting that the deep part of this region still has significant exploration potential.