Origin and evolution of ore-forming fluids in a tungsten mineralization system, Middle Jiangnan orogenic belt, South China: Constraints from in-situ LA-ICP-MS analyses of scheelite

Abstract

The Middle Jiangnan orogenic belt is a very important tungsten (W) polymetallic belt in China. There are many types of W deposits in the belt, including veinlet-disseminated and stratabound deposits, which have different origins. The veinlet-disseminated Dahutang W deposit has undergone extensive alteration, such as biotitization (potassic alteration), and greisenization. Scheelite in veinlet-disseminated W mineralization displays a zoned texture indicating at least two generations. The early generation (D1) from the Dahutang W deposit exhibits magmatic hydrothermal characteristics with comparatively high Nb, Ta, and Mo concentrations, but a low Sr concentration (44 to 95 ppm). In contrast, the scheelite samples from the Xi’an W deposit has relatively low Nb, Ta, and Mo concentrations, but a high Sr concentration (582 to 861 ppm), mainly originating from a metamorphic fluid that mixed with meteoric water. The composition of the late generation of scheelite (D2) samples from the Dahutang deposit are intermediate between the composition of scheelite (D1) and scheelite of the Xi’an depsoit. The mineral chemistry of the scheelite samples indicate that the ore-forming fluids of the Dahutang deposit were dominated by magmatic hydrothermal fluids during the early stage, and that meteoric water was added during fluid evolution. The rare earth elements (REEs) in the scheelite samples, especially the variation of the δEu (EuN/((SmN × GdN)0.5)) values, record the change in oxygen fugacity. The ore-forming fluids in the early stage of the Dahutang deposit were reduced becoming oxidized during the precipitation of the D2 scheelite, whereas the ore-forming fluids of the Xi’an deposit were oxidized. An increased oxygen fugacity of the ore-forming fluids of the Dahutang deposit restricted the precipitation of wolframite and promoted the formation of scheelite. Extensive alteration resulted in the decomposition of plagioclase, thus releasing Ca and Sr for the crystallization of scheelite at the Dahutang tungsten deposit.