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

Abstract

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 according 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 δEu varying from negative (Sch Ⅰb) to positive (Sch Ⅰa). Sch Ⅱ grains show complex inner textures, 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 δEu (Sch Ⅱa and Sch Ⅱb) to MREE-depleted with positive δEu (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 δEu, while Sch IIIb is MREE depleted with positive δEu and negative δCe. 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 δEu, 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.