Tungsten mineralization during the evolution of a magmatic-hydrothermal system: Mineralogical evidence from the Xihuashan rare-metal granite in South China

Abstract

Abstract Tungsten deposits are usually associated with granitic intrusions that record a long and complex evolution of the magmatic-hydrothermal system. However, the genetic link between magmatic-hydrothermal evolution and tungsten mineralization remains unclear. The Xihuashan tungsten deposit in South China, an important vein-type wolframite deposit, is closely associated with greisen and multi-phase intrusive activity that produced biotite granite, two-mica granite, and muscovite granite. From the biotite granite to the two-mica granite to the muscovite granite, micas vary from siderophyllite to lithian siderophyllite, with decreasing K/Rb and Nb/Ta ratios and increasing Rb and Cs contents. The zoned micas in the muscovite granite and greisen display fluorine-depleted rims, reflecting subsolidus replacement by external aqueous fluids. The presence of siderite indicates a Fe-, Mn-, and CO2-rich fluid under reducing conditions. The micas in the greisen have higher-F contents and lower Fe3+/Fe2+ ratios than those in the muscovite granite, suggesting that the fluids contributing to greisen formation had a relatively high-fluorine content and were reduced. The increase of CO2 in the fluid enhanced its ability to unlock W from melts/rocks into fluids. The reducing environment also facilitated the tungsten mineralization. During greisenization, the pH value of the fluid increased, which destabilized the polymeric tungstates to form WO42–. The mixture of W-rich solution and Fe-, Mn-rich external fluid eventually precipitated as vein-type wolframite in favorable locations. An empirical equation (Li2O = 0.0748 × F2 + 0.0893 × F) was introduced for estimating the Li2O contents of hydrothermal micas using the F contents determined by EPMA.