Ore-forming processes in the Xingjiashan W-Mo skarn deposit (Jiaodong, China): Insights from multi-generation scheelite and garnet geochemistry

The Xingjiashan W-Mo skarn deposit in the Jiaodong Peninsula, eastern China, is hosted by the Paleoproterozoic metamorphic rocks and Mesozoic granitic stock. Four generations of scheelite (Sch-1, Sch-2, Sch-3, Sch-4) were recognized based on their occurrences, mineral paragenesis, fluorescence and texture. The Mo contents (MoO3) of scheelite show a decrease from 30.9 to 4.1% (Sch-1) to 2.24-0.11% (Sch-2), then to 0.44-0% (Sch-3), finally with an increase to 1.11-0.85% (Sch-4). Garnet grains, disseminated in ores, show a decrease of andradite contents (44.1-25.5%) from the core to rim. These variations of composition in scheelite and garnet indicate that the ore-forming fluids evolved from relatively oxidizing to reducing condition, then back to oxidizing condition at the end of W mineralization. At least two-stage Mo precipitation events happened within the deposit, and the changing redox state played a key role in Mo mineralization. Partial Mo firstly incorporated into scheelite to form high-Mo scheelite at the prograde stage, and then was released from scheelite crystal, precipitated as molybdenite at the retrograde stage. The relatively constant Y/Ho ratio among different generations of scheelite suggests that the ore-forming fluids responsible for Sch-1 to Sch-4 were derived from a single hydrothermal system, and probably lack of fluid mixing event during the ore-forming process. Varieties of REE patterns in scheelite can reflect the crystallization of REE-enriched minerals (garnet, epidote and scheelite) during the oreforming process. A geochemical and textural model in terms of the different generation scheelite is established to reveal the ore-forming process at Xingjiashan.