Trace elements and textures of scheelite in porphyry-skarn Cu-Au systems: The example of Dongguashan deposit, eastern China

Porphyry-skarn deposits are the major sources of Cu and Au in eastern China, and W is a common byproduct of these mines. Scheelite is the W-bearing mineral in these deposits, and is widespread in both porphyry and skarn ore, however, systematic variations in the textures and geochemistry of the scheelite are not well constrained. Here, multiple generations of scheelite in Dongguashan deposit provide an opportunity for systematic study of the evolution of scheelite textures and chemistry, and their implications for ore-forming processes in a porphyry-skarn Cu-Au system. Scheelite is widespread in the porphyry and skarn zones at Dongguashan, and five types of scheelite are identified. Type1 scheelite occurs in quartz-sulfide veins from the porphyry potassic alteration stage; it displays oscillatory zoning and patchy cathodoluminescent (CL) zones. Type2 and 3 scheelite show relatively homogeneous CL image and occur interstitial to garnet and pyroxene. Type4 scheelite from the skarn ore has patchy CL zones; type5 scheelite shows patchy (core-rim) CL texture and occurs in quartz-sulfide veins that cut massive skarn or skarn sulfide ore. We conclude that the oscillatory banding of type1 scheelite was controlled by crystal kinetics, whereas the patchy, chaotic and sector CL textures in other types were mainly produced by extrinsic fluids of variable composition. The trace elements Mo, Na, Nb, V and REE strongly affect the CL intensity of scheelite at Dongguashan, and trace elements Na, Nb, V and n-expressionry sumexpressiontion REE decrease in the order type1 and 5 -> type4 -> type2 and 3. All scheelite types show Y/Ho ratios (12.5-30) similar to igneous rocks (20-27.5), indicating the tungsten mineralization is genetically related to the associated quartz monzodiorite. High Mo, Nb, V and the negative Eu anomaly in type1 scheelite indicate that the fluid related to the porphyry ore was oxidized and F-rich. The homogeneous compositions of type2, 3 and 4 scheelite, together with their weakly negative Eu anomalies show that they formed in a stable high T oxidized environment. The hydrothermal vein features with enrichment of REE (1 wt%) in the late type5 scheelite indicate it formed by injection of a late magmatic-hydrothermal pulse, and the variations in Eu anomalies in type5 suggest the fO(2) gradually decreased in late fluids. Our study highlights that scheelite CL textures and geochemistry are effective for tracing multiple ore-forming processes in porphyry-skarn Cu-Au systems.