Geochemistry of scheelite from Jiangligou skarn W-(Cu-Mo) deposit in the West Qinling orogenic belt, Northwest China: Implication on the multistage ore-forming processes

The Jiangligou skarn W-(Cu-Mo) deposit is a typical scheelite-dominated skarn deposit located in the West Qinling orogenic belt, northwestern China, hosting around 42, 000 tons of WO3 at an average grade of 1% with byproducts of Cu and Mo. Four generations of scheelite (Sch I, Sch II, SchIII, and SchIV) were recognized ac-cording to the mineral assemblages, inner textural, and geochemistry features. REE patterns of Sch I have a close relationship with the garnet and diopside-hedenbergite and show significant LREE-enrichment and HREE-depletion, with & delta;Eu varying from negative (Sch Ib) to positive (Sch Ia). Sch II grains show complex inner tex-tures, zoning, syn-crystallization fractures, and filling, thus can be divided into four sub-generations with characteristic flat REE patterns. The early generation of scheelite gradually shifts from MREE-enriched with negative & delta;Eu (Sch IIa and Sch IIb) to MREE-depleted with positive & delta;Eu (Sch IIc) or to lower REE with the same REE patterns (Sch IId). Sch III grains are commonly found in the sulfides overprinted ores, however, they have distinct REE patterns: Sch IIIa has relatively enriched MREE with negative & delta;Eu, while Sch IIIb is MREE depleted with positive & delta;Eu and negative & delta;Ce. The Sch IV has the largest (La/Lu)N with HREE down to the detection limit. Texture evidence implies that this may be due to later hydrothermal alteration which significantly removed the HREE. The Ca vacancy and Nb-REE coupled substitution dominantly account for the REE pattern of Sch I, III, IV and Sch II respectively. Revised batch crystallization modelling calculation on Sch II and Sch III proved that the variation of & delta;Eu, MREE and Ce are controlled by the relatively low Eu2+/Eu3+ and Ce3+/Ce4+ ratios under relatively reduced conditions, and high Eu2+/Eu3+ and low Ce3+/Ce4+ ratios under relative oxidized conditions during scheelite precipitation. The extra HREE depletion of Sch II from the measured data is interpreted as the low concentration of HREE in the ore-forming fluid caused by the precipitation of previous skarn minerals. This is also the main control in the depletion of HREE in Sch I. Besides, the oxide condition change is also reflected by the Mo content change of the scheelite. Varied REE patterns of the four scheelite generations imply that at least four large volumes of flux of ore-forming fluids account for the whole scheelite mineralization.

Automated summary

The Jiangligou skarn W-(Cu-Mo) deposit is a scheelite-dominated skarn deposit in China, containing 42,000 tons of WO3 with byproducts of Cu and Mo. Four generations of scheelite (Sch I, Sch II, Sch III, and Sch IV) were identified based on mineral assemblages, textures, and geochemistry. Each generation has different REE patterns and characteristics. The variation in δEu, MREE, and Ce can be explained by the different oxidation-reduction conditions during scheelite precipitation. The depletion of HREE in Sch II and Sch I is attributed to the low concentration of HREE in the ore-forming fluid. The varied REE patterns suggest multiple episodes of ore-forming fluids during scheelite mineralization.