Influence of magmatic-hydrothermal fluid evolution on rhenium enrichment in porphyry molybdenum deposits: Examples from the Suyunhe and Baishan deposits, Xinjiang

Porphyry molybdenum deposits are among the most important industrial sources of rhenium (Re). Rhenium mainly occurs in molybdenite by isomorphous substitution and is highly sensitive to the physicochemical conditions of ore-forming fluids. In this study, we investigate the Suyunhe Mo and Baishan Re-Mo deposits in Xinjiang, NW China. Petrography, ore microscopy and LA-ICP-MS trace-element analyses of molybdenite and quartz are integrated to compare Re enrichment in different mineralization stages and to constrain the controlling factors. Molybdenite from both deposits can be subdivided into type-Ⅰ plate- or lath-shaped grains and type-Ⅱ fine-grained aggregates. Type-Ⅱ molybdenite systematically contains higher Re contents than type-Ⅰ molybdenite, and no obvious core-rim zoning of Re is observed. The presence of kink-band deformation in molybdenite indicates superimposed mineralization. In the Suyunhe deposit, Re contents in molybdenite show large inter-stage variations, with markedly higher Re contents in the magmatic-hydrothermal transition stage (T2), where locally elevated S/Mo ratios suggest local Re enrichment and partial early crystallization. In contrast, molybdenite from the Baishan deposit displays relatively small variations in Re contents among stages, and the fHCl/fHF ratio of the ore-forming fluids remains relatively constant, favoring continuous transport of Re in the fluid. Combined constraints from Ti-in-quartz thermometry, fluid inclusions and trace-element data indicate that decreasing temperature is the primary factor controlling the partitioning of Re into molybdenite, whereas pH and halogen composition further modulate the speciation of Re in the ore-forming fluids and, consequently, the Re distribution between fluids and molybdenite. These factors together govern the differential enrichment of Re in the Suyunhe and Baishan porphyry deposits.