Geochemistry of garnet and scheelite as indicators for skarn-type Mo-W mineralization: A case study from the Shibaogou deposit, Qinling Orogen, China

To understand the mechanism of skarn-type Mo-W mineralization, it is crucial to study fluid evolution. In-situ elemental analysis was conducted on garnet and scheelite from the Shibaogou deposit, a representative large-scale skarn Mo-W deposit in the Luanchuan orefield, Qinling orogen, aiming to constrain the origin and evolution of ore-forming fluid and to analyze the skarn mineralization process. Multiple generations of garnet and scheelite were observed along with the multistage of skarnization. I. Anhydrous skarn stage, three generations of garnet (Grt-c, Grt-m, Grt-r) with molybdenite (Mo-I) occurring around Grt-r and scheelite occurring in coarse-grained (primary Sch-I, altered rim Sch-Ia). II. Hydrous skarn stage, minor scheelite (Sch-II) coexisted with actinolite. III. Oxide stage, scheelite (Sch-III) occurred in feldspar-fluorite veins, with minor molybdenite (Mo-II). IV. Quartz sulfide stage, early quartz-molybdenite (Mo-III) veins and later quartz Fe-Zn-Pb-sulfide veins occurred with a few scheelites (Sch-IV). V. Carbonite stage. The Y/Ho ratios of garnet and scheelite indicate that the ore-forming fluid was originally magmatic-hydrothermal. The variations in U and Sn contents of garnet indicate a gradual increase in oxidation state during the anhydrous skarn stage. The high andradite proportion in Grt-m resulted from an increased pH of the fluid, related to the presence of Ca(HCO3)2. This increased pH and Ca2+ activity of fluid led to the main tungsten mineralization (Sch-I). The subordinate Mo mineralization formed at the end of stage I, as the expansion of the molybdenite logfO2-pH stability field, controlled by the decrease in temperature. The redox state evolutions indicated by the consistent Mo contents and (EuN vs. EuN*) of scheelite, with the highest oxidation state occurring in the hydrous skarn stage. The oxidation state reached the lowest level in the sulfide stage with sufficient reduced sulfur, leading to main Mo mineralization. The REE patterns, Nb, and ∑REE contents of scheelite suggest a change in fluid composition to F enrichment after anhydrous skarn, which enhanced the fluid's ability to dissolve W. In the oxide stage, scheelite formed a second significant mineralization with fluorite: H3WO4F2– + 2Ca2+ = CaWO4 + CaF2 + 3H+. The carbonate wall rocks played a controlling role in the evolution of ore-forming fluid and mineralization mechanism through the fluid-rock interactions. © 2024