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The Jiama deposit, a significant porphyry‐skarn‐type copper polymetallic deposit located within the Gangdese metallogenic belt in Tibet, China, exemplifies a typical porphyry metallogenic system. However, the mineral chemistry of its accessory minerals remains under‐examined, posing challenges for resource assessment and ore prospecting. Utilizing electron microprobe analysis and LA‐ICP‐MS analysis, this study investigated the geochemical characteristics of apatite in ore‐bearing granite and monzogranite porphyries, as well as granodiorite, quartz diorite, and dark diorite porphyries in the deposit. It also delved into the diagenetic and metallogenic information from these geochemical signatures. Key findings include: (1) The SiO2 content, rare earth element (REE) contents, and REE partition coefficients of apatite indicate that the dark diorite porphyry possibly does not share a cogenetic magma source with the other four types of porphyries; (2) the volatile F and Cl contents in apatite, along with their ratio, indicate the Jiama deposit, formed in a collisional setting, demonstrates lower Cl/F ratios in apatite than the same type of deposits formed in a subduction environment; (3) compared to non‐ore‐bearing rock bodies in other deposits formed in a collisional setting, apatite in the Jiama deposit exhibits lower Ce and Ga contents. This might indicate that rock bodies in the Jiama deposit have higher oxygen fugacity. Nevertheless, the marginal variation in oxygen fugacity between ore‐bearing and non‐ore‐bearing rock bodies within the deposit suggests oxygen fugacity may not serve as the decisive factor in the ore‐hosting potential of rock bodies in the Jiama deposit. [ABSTRACT FROM AUTHOR]

• CH 4 was generated by the thermal decomposition of the carbonaceous wall-rocks. • CH 4 added, which contributed to the system from oxidized to reduced conditions. • Ore elements were deposited via fluid boiling process. High f O 2 conditions characterize the majority of global porphyry copper deposits and contain highly oxidized minerals, such as magnetite and anhydrite. In contrast, the No. 1 porphyry Cu–Au deposit in the Xiongcun district (Tibet, China) has abundant pyrrhotite, reduced fluids (CH 4 ≫ CO 2), and a relative lack of highly oxidized minerals, which are indicative of low f O 2 conditions. Scanning electron microscopy, fluid inclusion, C–H–O–He–Ar isotopes, and whole-rock organic carbon contents and isotope analysis were used to constrain the evolution of ore-forming fluid, the origin of CH 4 and metal deposition mechanisms for the No. 1 deposit. The He–Ar isotopic compositions (3He/4He = 0.11–0.96 Ra, 40Ar/36Ar = 418.7–2920.2) suggest that the ore-forming fluids predominantly derived from crust source with minor mantle input. The H–O isotopic analysis results (δ18O H2O = −1.8 to +5.2‰, δD = −106 to −89.9‰) indicate that the ore-forming fluids were derived from a magmatic source that mixed with some meteoric waters. The element compositions of zircons and fluid/melt inclusions from the mineralized Middle Jurassic quartz diorite porphyry reveal that the primary magma was characterized by high log f O 2 (>NNO) conditions. The quartz diorite porphyry intruded into the carbon-bearing wall rocks produced abundant CH 4 by thermal decomposition of organic matter (δ13C CH4 = −26.3 to −28.5‰), which changed the redox state of the porphyry copper system from oxidized to reduced condition. Ore elements were deposited via fluid boiling as a consequence of the rapid decrease in temperature and pressure. [ABSTRACT FROM AUTHOR]

• Initial ore-forming fluids were low-salinity, CO 2 -rich, single-phase fluids. • The fluids evolved from magmatic in origin to meteoric water-dominated. • Fluid immiscibility was the prime factor resulting in chalcopyrite precipitation. • Fluid-cooling was probably responsible for molybdenite precipitation. The No. 2 porphyry Cu–Au deposit in the Xiongcun district forms part of the Gangdese porphyry copper belt of Tibet. This study clarifies the fluid evolution and ore precipitation mechanisms of the magmatic-hydrothermal system that formed the deposit by combining the results of fluid inclusion petrography, microthermometry, scanning electron microscope-cathodoluminescence imaging, and stable isotope analyses of multiple quartz-dominated veins. Four distinct sets of quartz-dominated veins and eight generations of quartz (Q1–Q8) within the veins were identified, namely from early to late, early barren quartz veins (V1) comprised of equigranular CL-bright Q1 quartz, quartz–pyrite–chalcopyrite ± magnetite veins (V2) containing early equigranular CL-bright Q1 quartz and later CL-gray or CL-dark Q2–3 (Q2 + Q3) quartz grains intergrown with Cu–Fe sulfides, quartz–molybdenite ± pyrite ± chalcopyrite veins (V3) comprised of quartz grains (Q4–Q6) characterized by subhedral comb-like textures with well-developed oscillatory growth zoning, and late quartz veins (V4) containing CL-gray Q7 quartz with euhedral growth zones and later CL-dark Q8 quartz. Fluid inclusions in these veins can be classified into six types: B40, B40H, B80, B15H, B15H+, and B15. The B40 and B40H inclusions; B80 inclusions; and B15, B15H, and B15H+ inclusions contain vapor bubbles occupying 30–60 vol%, >70 vol%, and 5–25 vol%, respectively; in addition, B15H, B15H+, and B40H inclusions contain transparent daughter minerals (halite, and in some cases, sylvite). Raman spectra showed that CO 2 is only present in B40 and B80 inclusions. In the Q1 quartz in V1 and V2 veins, the dominant type of inclusions is B40, which have homogenization temperatures (Th) of 278 °C–389 °C (peaking at 310 °C–330 °C) and salinities of 2.0–12.8 wt% NaCl equiv. In Q2–3 quartz in V2 veins, all inclusion types were observed. These were trapped at temperatures of approximately 325 °C–340 °C and had salinities of 1.8–69.7 wt% NaCl equiv. In V3 veins, only B15H and B15 inclusions were observed, which had Th values of 171 °C–320 °C (peaking at 240 °C–260 °C) and 173 °C–317 °C (peaking at 210 °C–230 °C) and salinities of 28.7–33.9 wt% NaCl equiv and 5.6–23.1 wt% NaCl equiv, respectively. In V4 veins, only B15 inclusions were observed. Primary B15 inclusions in V4 veins yielded Th values of 166 °C–229 °C (peaking at 170 °C–200 °C) and salinities of 5.1–17.1 wt% NaCl equiv. The 3He/4He and 40Ar/36Ar ratios of the fluid inclusions exhibited the ranges of 0.08–0.84 Ra and 451.3–1567.8, respectively, and the δ18O fluid and δD fluid values varied from −3.2‰ to 5.8‰ and −92.8‰ to −80.3‰, respectively. By integrating all results from the fluid inclusion, cathodoluminescence, and isotopic analyses, we conclude that the initial ore-forming fluids of the No. 2 deposit were low-salinity, CO 2 -rich single-phase fluids of magmatic origin. Subsequently, fluid immiscibility developed in the initial ore-forming fluids, generating hypersaline liquid and low-salinity vapor phases and leading to the separation of a CO 2 phase plus chalcopyrite precipitation from the fluids. As meteoric water was injected into the hydrothermal system, the ore-forming fluids gradually evolved to become meteoric water-dominated, low temperature, low-salinity, and CO 2 -poor; in addition, fluid-cooling due to the meteoric water input resulted in molybdenite precipitation. [ABSTRACT FROM AUTHOR]