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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 ill, 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/86 Sr 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 2445 µg/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/86 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. [ABSTRACT FROM AUTHOR]

BACKGROUND: The Nanyangtian scheelite deposit is an important skarn scheelite deposit in Yunnan Province, which is located in Laojunshan W-Sn deposit area, Southeast Yunnan. Due to its complex geological background and multi-stage metallogenic characteristics, its mineralization age and genesis remain controversial. OBJECTIVES: To explore the metallogenic age, genesis and material origins of two types of scheelite deposits in order to explore formation patterns. METHODS: Molybdenum Re-Os isotope dating was used to constrain the age, whereas in situ trace element and in situ Sr isotopes of scheelite were used to determine the composition of trace elements and Sr isotopes in scheelite. RESULTS: The Re-Os isochron age of molybdenite associated with scheelite in the feldspar-quartz mineral vein of Nanyantian was 151.0±1.3Ma, younger than the age of skarn mineralization, indicating a later mineralization event. The skarn-type scheelite was enriched in light rare earth elements with negative Eu anomaly (δEu=0.46). The average content of ∑REE, Mo and Sr in skarn scheelite were 65.60, 240.16 and 883.43μg/g, respectively. Feldspar-quartz vein-type scheelite showed a flat rare earth pattern with positive Eu anomaly (δEu=2.8) and average content of ∑REE, Mo and Sr were 194.40, 16.01 and 129.26μg/g, respectively. respectively. The skarn scheelite had a relatively low and uniform 87Sr/86Sr value of 0.71319 to 0.71491, indicating that the ore-forming fluids were mainly magmatic-hydrothermal in origin, whereas feldspar-quartz vein type scheelite had a wide 87Sr/86Sr range of 0.71537 to 0.72803, with an average of 0.72079, characteristic of metamorphic fluids. CONCLUSIONS: The differences in trace and rare earth element contents between two types of mineralization indicate that they have different fluid sources. The negative Eu anomaly of the skarn-type scheelite indicates a high oxygen fugacity environment, whereas the feldspar-quartz vein-type scheelite is formed in a reductive environment in terms of positive Eu anomaly. Sr isotopes of two different types of scheelite display a feature of binary mixing, indicating that feldspar-quartz vein-type scheelite has a superimposed transformation effect on skarn-type scheelite, and the strong metasomatism of ore-forming fluids and surrounding rocks is the key to the formation of scheelite. [ABSTRACT FROM AUTHOR]