LAUSR

Abstract Scheelite is the main ore mineral in skarn-type tungsten deposits, and a common accessory mineral in a variety of rock-types. The Baoshan deposit in South China is one of the most important polymetallic scheelite skarn deposits in China, hosting 40,000 t of WO3 with economic concentrations of Zn, Cu, and Ag. It is hosted by a calcic skarn that is zoned outwards mineralogically from garnet-clinopyroxene, through clinopyroxene-garnet, to wollastonite, and overprinted by retrograde minerals. Scheelite occurs in both the prograde and retrograde skarns, and is complexly zoned. On the basis of its textures, the scheelite was classified into three types. Scheelite I and II belong to the early and late prograde stages, respectively, and Scheelite III precipitated during the retrograde stage. The molybdenum (Mo) content of these scheelite types ranges from 54 ppm to 24 wt%, and the total rare earth element content ranges from 12 to 321 ppm. Rare earth element (REE) concentrations and chondrite-normalized REE profiles vary with the distribution of major elements. The profiles indicate variable degrees of REE enrichment, which correlates negatively with the Mo content. Molybdenum-rich scheelite displays a negative Eu anomaly, and Mo-poor scheelite a positive Eu anomaly. Crystal structure provided the first-order control on the minor and trace element composition of the scheelite. Incorporation of REE3 + into scheelite was controlled partly by a coupled substitution involving Mo. The lattice strain model was used to estimate scheelite-fluid partition coefficients for the REE from the contents of these elements in the scheelite and to predict the relative distributions of the REE in the ore-forming fluids. It is proposed that conditions were initially oxidizing, leading to strong incorporation of Mo in Scheelite I, that they became more reducing with the crystallization of Scheelite II containing lesser Mo, and that during retrograde skarn formation there was a return to oxidizing conditions due to an influx of meteoric waters, which altered Scheelite II giving rise to the formation of Scheelite III. The study shows that the composition of scheelite recorded the history of the Baoshan hydrothermal system, and that the behaviour of the REE could be used to quantitatively reconstruct the changing physicochemical conditions during ore formation.