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摘要
In situ trace element characteristics of scheelite and garnet provide the evolution information of ore-forming fluids. The Longjiaoshan deposit is a large skarn tungsten deposit discovered in southeastern Edong orefield of the Middle-Lower Yangtze River Metallogenic Belt (MLYB). The fluid evolution history of the deposit has not been systematically studied, which restricts the prospecting work and theoretical research of the MLYB. Based on detailed field work and previous studies, this study carried out in situ major and trace elements analyses of scheelite and garnet formed at different stages. The composition of red-brown garnet (Grt-1), yellow-green garnet (Grt-2)pyroxene skarn and veined wollastonite-garnet (Grt-3) from the Longjiaoshan deposit is Adr30.6-84.1Gro13.9-50.7Pyr1.5-30.3, Adr38.3-100Gro0.0-39.4Pyr0.0-22.4 and Adr75.3-100.0Gro0.0-13.9Pyr0.0-12.7, respectively. These three types of garnet are rich in LILEs and LREE, depleted in HFSEs and show positive Eu anomaly. The content of U element in garnet from the Longjiaoshan deposit gradually decreases, which is consistent with the composition change of andradite, indicating that the oxygen fugacity of ore-forming fluid gradually increases in the skarn stage. From retrograde to quartz-sulfide stage the Mo content of scheelite increases and then decreases, as well as the oxygen fugacity increases and then decreases, indicating the complex pulse oxygen fugacity change characteristics in the mineralization process. The scheelite formed in the retrograde alteration stage and quartz sulfide stage shows decreasing in Eu negative anomaly but increasing in Eu positive anomaly, indicating that the pH value of ore-forming fluids gradually increases. Therefore, pH value is not the main factor affecting scheelite precipitation. In the Skarn stage, there is a positive correlation between the total REE and Y of Grt-1, but there is no positive correlation between the total REE and Y of Grt-2 and Grt-3, and Grt-3 is rich in Fe, indicating that the degree of water-rock reaction increases gradually as the crystallization of garnet changes from fluid equilibrium state to non-equilibrium condition. Moreover, the irregular growth zones of Sch-1 in the retrograde stage (main mineralization stage) is more developed than Sch-2 in the quartz sulfide stage, indicating that the water-rock reaction degree reaches the peak in the main mineralization stage, that is, the water-rock reaction is an important factor controlling the ore mineral precipitation. Fluid mixing and water-rock reaction are the main mechanisms controlling tungsten precipitation in the Longjiaoshan deposit. The Longjiaoshan area is a prospective terrane for the formation of large and high grade W ore deposits. ? 2022 Editorial Department of Mineral Deposits.
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