The Mineral Characteristics And Metallogenic Mechanism Of Scheelite In The Zhaishang Gold Deposit In The Western Qinling Orogenic Blet

Located in the western part of the Min-Li metallogenetic belt, western Qinling Mountains, the Zhaishang gold deposit is a very extraordinary Carlin-type ore system characterized by the enrichment of scheelite. In term of space distribution, the tungsten mineralization either coincides with or separates from gold orebodies. Scheelite is the only significant ore mineral, although the tungsten ore contains a host of major minerals, including quartz, calcite, siderite and ankerite, together with rhodochrosite, muscovite, barite, gypsum, kaolinite, apatite, rutile, hematite and pyrite as subordinate or accessory minerals.Generally tungsten is prone to accumulate in magmatic fluid and precipitate in mesothermal to hypothermal condition. However, fluid inclusions in scheelites homogenize at 180～220℃ and reveal that formation of the deposit involved fluids containing H2O with or without carbonic species and low concentrations of several salts. Furthermore, trace element geochemistry of scheelite shows high concentrations of Sr and Y compared to Mo, with a depletion of high field strength elements such as Nb, Ta, Ti and Bi, which eliminate an immediate influence form igneous intrusions. The scheelites, in general, have high contents of REE with bell-shaped chondrite-normalized REE patterns accompanied by slightly even no Eu anomalies that are identical to the anomalies of diorite porphyrite. Microthermometric data and trace elements composition imply a role of long-evolved magmatic water in distal intrusion-related settings but nevertheless consistent with a magmatic and crustal source for metal. For example, element ratios, such as Rb/Sr, Nb/Ta and Zr/Hf, are much lower than typical mantle values. Additionally, a zoned pattern also observed in a single scheelite grain, which records distinctive differences in chemistry, probably represents distinct pulses of hydrothermal fluids that varied significantly in their compositions.Accordingly and in view of experimental research findings on the solubility of tungsten minerals and speciation in hydrothermal solutions, a conclusion can be safely drawn that the tungsten would probably be carried by tungstate species in hydrothermal systems and the crystallization of scheelites is attribute to the increase of pH caused by the reaction between ore-forming fluids and calcareous wall rocks.