Tungsten mineralization process and oxygen fugacity evolution in the Weijia W deposit, South China: Constraints from the microstructures, geochemistry, and oxygen isotopes of scheelite, garnet, and calcite

Scheelite is a major (W)-bearing mineral enriched in rare earth elements (REEs) and is widely used to investigate the mineralization process, fluid source and redox environment of W deposits. In the Weijia skarn-type W deposit in South China, scheelite was formed during the greisen, retrograde skarn, oxidation, and sulfide-quartz vein stages. Various characteristics of cathodoluminescence (CL) images of scheelite grains (e.g., light to dark color, core-rim texture and oscillatory zonation) have recorded their detailed crystallization process. Scheelite grains formed in the greisen and retrograde skarn stages have similar LREE patterns, and the gradual fractionation in HREEs may be a result of the precipitation of garnet (And(38-70)Gr(29-52)Pr(1-3)) in the prograde skarn stage. In addition, the similar REE patterns of scheelite from the greisen stage and the granite porphyry pluton suggest a genetic relationship. The oxygen isotopic compositions of scheelite and quartz are all plotted near the edge of the magmatic fluid field, confirming their derivation from the same magmatic hydrothermal fluid. Elemental mapping and spot analyses using an electron probe microanalyzer indicated that extensive isomorphic substitution of W by molybdenum (Mo) occurs in scheelite, and the darker color in the CL images corresponds to a higher Mo content and lower W content. The Mo content of scheelite gradually increased from the greisen stage (mean = 0.46%), through the retrograde stage (mean = 2.89%), to the oxidation stage (mean =19.8%), and then dropped sharply in the sulfide-quartz vein stage (mean = 0.20%), indicating a change in oxygen fugacity. Moreover, scheelite formed in the sulfide-quartz vein stage has an oscillating Mo content, thus implying a slight or periodical change in oxygen fugacity during the same mineralization stage.