Geochemistry and Origin of Scheelites from the Xiaoyao Tungsten Skarn Deposit in the Jiangnan Tungsten Belt, SE China

Abstract

The type, association, variations, and valence states of several metal elements of scheelite can trace the source and evolution of the ore-forming fluids. There are four types of scheelite from the Xiaoyao deposit: (1) scheelite intergrown with garnet in the proximal zone (Sch1a) and with pyroxene in the distal zone (Sch1b), (2) scheelite replaced Sch1a (Sch2a) and crystallized as rims around Sch1b (Sch2b), (3) quartz vein scheelite with oscillatory zoning (Sch3), and 4) scheelite (Sch4) within micro-fractures of Sch3. Substitutions involving Mo and Cd are of particular relevance, and both elements are redox-sensitive and oxidized Sch1a, Sch2b, Sch3 are Mo and Cd enriched, relatively reduced Sch1b, Sch2a, Sch4 are depleted Mo and Cd. Sch1a, Sch2a, Sch3, and Sch4 are characterized by a typical right-inclined rare earth element (REE) pattern, inherited from ore-related granodiorite and modified by the precipitation of skarn minerals. Sch1b and Sch2b are characterized by low light rare earth element/heavy rare earth element (LREE/HREE) ratios, influenced by a shift in fO 2 during fluid-rock alteration. Sch1b, Sch2b and Sch3 have higher Sr contents than those of Sch1a and Sch2a, reveal that host-rock alteration and fluid-rock interaction have elevated Sr contents. The Y/Ho ratios of scheelite gradually increase from skarn to quartz vein stages, due to fluid fractionation caused by fluid-rock interaction. Thus, the variation in REE and trace elements in scheelite in time and space reflects a complex magmatic-hydrothermal process involving various fluid-rock interactions and fluid mixing.Minerals 2020, 10, 271 2 of 22 (CL) imaging has been proven to be an effective technique to reveal internal textures, zoning and trace element distributions in scheelite [3]. However, previous studies did not evaluate dissolution and re-precipitation processes in scheelite from tungsten skarn deposits due to the changing hydrothermal fluids [13,14]. The detailed textural characterization and systematic investigations of scheelite from each stage prior to in situ chemical analysis are critical to constraint the evolution of skarn systems and W ore precipitation.The Jiangnan tungsten belt in the southeastern margin of the Yangtze Craton is a new giant tungsten province with WO 3 reserves over 5.0 million tons [19][20][21][22][23], including the Dahutang (reserves of 1.10 Mt and grade of 0.18%) and the Zhuxi (reserves of 3.44 Mt and grade of 0.54%) deposits, as well as other large-scale tungsten deposits, such as Yangchuling, Xiaoyao, Dongyuan, Zhuxiling deposits [24][25][26]. The present study focuses on the Xiaoyao tungsten skarn deposit, characterized by complex scheelite textural features. The Xiaoyao deposit has reserves of 75,000 tons of WO 3 with an average grade of 0.2% [27]. We present both textural and compositional features for scheelite from the Xiaoyao deposit to evaluate the importance of dissolution and re-precipitation processes in the formation of skarn-related scheelite.