In situ Trace Element and Sr Isotope Composition of Scheelite in the Xiuwacu Molybdenum-Tungsten Deposit, Northwest Yunnan: Constraints on Mineralization

BACKGROUND: The Xiuwacu Mo-W deposit is a typical hydrothermal quartz vein deposit in the Yidun island arc Cu-Mo metallogenic belt, northwest Yunnan. Numerous studies have been conducted on rock and ore chronology and petrogenesis, dynamics, but its ore-forming fluid evolution has been rarely studied.
OBJECTIVES: To reveal the origin and evolution of ore-forming fluids of the Xiuwacu Mo-W deposit.
METHODS: Scanning electron microscope (SEM) cathodoluminescence method, and in situ trace element (LA-ICP-MS) and in situ Sr (fs-LA-MC-ICP-MS) isotope analyses of scheelite.
RESULTS: The occurrences of scheelite and the cathodoluminescence images indicated three generations of scheelite, Ⅰ, Ⅱ, and Ⅲ, with the middle stage scheelite being the most developed. The rare earth element pattern of the scheelite in the early stage was similar to that of the porphyritic granite. It showed a right-inclined pattern with light rare earth enrichment and a moderate negative Eu anomaly (δEu=0.42). The average Mo content was 3.0%, and the average ⁸⁷Sr/⁸⁶Sr was 0.7098, close to the specular feldspar granite (0.7075-0.7098). Compared with scheelite in the early stage, the content of light rare earth elements in the scheelite from the middle stage was low, and Eu also had a medium negative anomaly (δEu=0.37). Mo content was reduced to an average of 2445μg/g, and ⁸⁷Sr/⁸⁶Sr increased to 0.7113. The rare earth distribution pattern of scheelite in the later stage showed an arched pattern with relative enrichment of middle rare earth and no Eu anomaly (δEu=0.93). Mo content of scheelite decreased to 56μg/g, and average ⁸⁷Sr/⁸⁶Sr was 0.7083.
CONCLUSIONS: From early to late, the gradual decrease of light rare-earth elements, especially La and Ce, in scheelite indicates the crystallization of bastnaesite. The increase in δEu and the sharp decrease in Mo content indicate the transformation of ore-forming fluids from oxidation to reduction. The change of Sr isotope composition indicates the change in the source of ore-forming materials. The magmatic fluid contributed a lot in the early stage, and the large-scale interaction between the magma-hydrothermal fluids and surrounding rocks is responsible for the formation of scheelite in the middle stage. The strata provide amounts of Ca for scheelite formation, indicating that strong water-rock interaction played an important role in the formation of the deposit.