The formation mechanism of the Xilekuduke porphyry Mo‐Cu deposit, NW China, revealed by the fluid inclusions and H‐O‐S isotopes

The Xilekuduke porphyry Mo‐Cu deposit is located in the Altay‐East Junggar region of the Central Asian Orogenic Belt, northwest China. The orebodies occurring as vein type are host within the monzogranite and granite porphyry. Ore minerals include mainly molybdenite, pyrite, and chalcopyrite, whilst the major alteration include potassic, sericite, carbonate, and silicic. Mineralization can be divided into three stages: quartz‐K‐feldspar–polymetallic stage (Stage I), quartz‐polymetallic stage (Stage II), and quartz–calcite–pyrite (minor) stage (Stage III). Three types of fluid inclusion are present in the Mo‐Cu sulfide–calcite–quartz veins: CO2‐bearing (C‐type), aqueous (W‐type), and daughter mineral‐bearing (S‐type). Petrographic and microthermometric analyses of the fluid inclusions yielded homogenization temperatures for Stage I, II, and III to be 402–499°C, 214–391°C, and 136–254°C, respectively, with corresponding salinities of 39.2–59.6, 3.7–44.9 and 4.1–14.4 wt% NaCl equivalent. The δ18OH₂O and δD values of fluid inclusions in quartz are determined to be 5.3–6.0 ‰ and −76 to −60 ‰ (Stage I), 1.7–3.2 ‰ and −96 to −90 ‰ (Stage II), and −2.6 to −2.4 ‰ and −106 ‰ (Stage III), respectively. These results indicate that the primary ore‐forming fluids (stages I and II) were derived from granitic magma and were mixed with meteoric water in stage III. For the sulfide and sulfate (anhydrite), their δ34S values are of 0.4–5.8 ‰, 13.9–14.4 ‰, respectively, also that suggest a magmatic source. Fluid immiscibility, meteoric water interaction, and ore fluid‐wallrock interactions may have been critical for molybdenum precipitation.