Scheelite geochemistry implications for ore-forming fluid evolution of Zhuxiling and Xiaoyao tungsten deposits, southern Anhui Province

In recent years, a batch of porphyry-skarn tungsten deposits have been successively discovered in the northern Jiangxi and southern Anhui area. Those deposits consist of a new world-class tungsten metallogenic belt, which is called the Jiangnan Tungsten Belt. There are a large number of W-Mo deposits related to weakly differentiated oxidative granites in the southern Anhui area in the northeastern part of the belt. Researches on such deposits are relatively limited and lack of studies on their mineral chemistry, and as a result, the nature and evolution of their ore-forming fluids remain unclear. In this paper, two representative tungsten polymetallic deposits in the southern Anhui area, the Zhuxiling and Xiaoyao deposits, are selected to carry out in situ compositional analyses on the scheelite to reveal the nature of mineralizing fluids in the skarn system, the change of physochemical conditions and the influence of water-rock reaction of these two depsoits. We also discussed the differences of the tungsten mineralization chatacteristics between these two deposits and other ones found in the Jiangnan Tungsten Belt. According to our study, the in situ compositional analysis of the scheelites could reflect not only the changes of oxygen fugacity, pH values, temperatures and compositions of the ore-forming fluids, but also the water-rock reaction process during the skarn formation. Furthermore, the scheelites of Xiaoyao and Zhuxiling deposits can be divided into three stages, among which the early-stage scheelite (Sch-1) is formed in the prograde skarn stage and coexists with garnet and diopside, while the Sch-2 happened in the retrograde metamorphic stage of the skarn, and the Sch-3 was formed the late quartz-sulfide stage. The Mo content of Sch-1 is the highest and the REE shows a right-dipping curve with a negative Eu anomaly, which is similar to the REE pattern of the parent rock, indicating that the early fluid was magmatic exsolved Cl-rich fluid. Compared to Sch-1, Sch-2 shows a weaker REE differentiation and positive Eu anomaly, whose REE and Mo contents are lower. As crystallization of Sch-1 took away abundant LREE, the ratio of LREE/HREE in Sch-2 decreases. The total amounts of REE, Nb, Ta, Y of the late stage Sch-3 are significantly higher than those of Sch-1 and Sch-2, whose REE pattern displays a flat curve with weak differentiation between the LREE and HREE and a weak positive Eu anomaly. These features imply that late-stage fluid of Sch-3 is F-rich. This study proposed that the Eu anomaly of the scheelites might not reflect the oxygen fugacity, but more likely reflects the influence of water-rock reaction. The Mo content of the scheelite is closely related to the oxygen fugacity and is more effective than the Eu anomaly as an oxidation indicator. Compared with the Xiaoyao deposit, the Zhuxiling garnet has a lower content of andradite and a higher content of spessartite. Furthermore, the Mo content of the scheelite in the Zhuxiling deposit (630 x 10(-6) similar to 18813 x 10(-6)) is significantly lower than that in the Xiaoyao deposit (8700 x 10(-6) similar to 74478 x 10(-6)), from which we inferred that the Zhuxiling deposit is a relatively reduced tungsten skarn system compared with the Xiaoyao deposit. Comparsion of Zhuxiling and Xiaoyao reveals that redox states of tungsten deposits are different within the southern Anhui segment, although they are considered to be generally oxidized in constrast to those reduced ones in the northern Jiangxi segment. The oxygen fugacity and the degree of differentiation of magmatic rocks are important factors controlling the tonnage of the deposit.