Geochemical characteristics and Sm-Nd geochronology of scheelite in the Baojinshan ore district, central Hunan

The Baojinshan gold-tungsten deposit, located in the eastern segement of the Baimashan-Longshan-Ziyunshan EW-trending gold belt, is a typical gold deposit hosted in the Precambrian metasedimentary rocks, central Hunan Province. Based on the field investigations and indoor observations, different analytical methods including XRD, EMPA, ICP-MS and ID-MS have been adopted in this paper to study the geochemical compositions of the scheelite samples collected from the Baojinshan ore district in order to reveal their elemental and isotopic signatures. Metallogenic time and ore genesis of gold deposits hosted in metasedimentary rocks in central Hunan are further discussed. It is revealed that, the studied scheelite samples are relatively homogenous in chemical compositions: their WO, concentrations are basically consistent with the theoretical value of scheelite (CaWO4), and their CaO concentrations are slightly less than its theoretical value, which can be ascribed to the substitution of Sr, REE, Na and other trace elements into Ca site in scheelite lattice. Expect for Sr, REE and As, the concentrations of other trace elements are less than the corresponding Clarke values in the continental crust, especially those elements including Mo, Bi, Sn, Be, Nb and Ta, are less than 1 x 10(-6), obviously different from those scheelite occurring in the tungsten deposits related to Late Mesozoic granite in South China. The REE concentrations of scheelite in this study vary in the range of 484.5 x 10(-6) similar to 972.9 x 10(-6), with an average of 721.3 x 10(-6), significantly higher than those of scheelite samples from other Au-W deposits or W deposits in central and western Hunan. All scheelite samples in the Baojinshan ore district are characterized by the convex MREE-rich pattern. The Sm-147/Nd-144 values of scheelite samples in this study vary from 0.2079 to 0.4070, and their Nd-144/Nd-144 values change from 0.512126 to 0.512398. On the (SM)-S-147/Nd-144 - Nd-143/Nd-144 diagram, the scheelite samples from the Baojinshan deposit display an excellent linear relationship, which corresponds to an isochron age of 207.8 +/- 1.5Ma (MSWD = 1.03), thus, the gold-tungsten mineralization at the Baojinshan ore district took place during the Late Triassic. The Sr-87/Sr-86 values of the studied scheelite samples vary in the range of 0.73164 similar to 0.73974, roughly consistent with those of the Zhazixi deposit, higher than those of the Liaojiaping and Longshan deposits, but are obviously less than those of the Woxi and Xi'an deposits, revealing there exists some distinct differences for gold deposits or tungsten deposits formed at different times in central Hunan, on their ore-forming materials and mineralization mechanism. It is revealed that Sr-Nd isotope compositions of the scheelite in this study, obviously different from those of the Banxi Group and the main part of the Ziyunshan pluton, and partly overlap with those of the complement part in the Ziyunshan pluton, which reveals that gold-tungsten mineralization is related to the late magmatic activity. Combined with the regional date data, it is concluded that the Late Triassic Au-W mineralization is widespread throughout central Hunan, and is intimately associated with the Indosinian granitic activities, the gold deposits hosted in metasedimentary rocks in this region, genetically belong to the reduced intrusion-related gold system (IRGS) rather than orogenic deposits.

Automated summary

The Baojinshan gold-tungsten deposit, located in central Hunan Province, is a typical gold deposit hosted in Precambrian metasedimentary rocks. The geochemical compositions of scheelite samples from the deposit were studied using various analytical methods, revealing their elemental and isotopic signatures. The gold-tungsten mineralization at Baojinshan occurred during the Late Triassic, associated with late magmatic activity.