Genesis of the Xuebaoding W-Sn-Be Crystal Deposits in Southwest China: Evidence from Fluid Inclusions, Stable Isotopes and Ore Elements

The Xuebaoding crystal deposit, located in northern Longmenshan, Sichuan Province, China, is well known for producing coarse-grained crystals of scheelite, beryl, cassiterite, fluorite and other minerals. The orebody occurs between the Pankou and Pukouling granites, and a typical ore vein is divided into three parts: muscovite and beryl within granite (Part I); beryl, cassiterite and muscovite in the host transition from granite to marble (Part II); and the main mineralization part, an assemblage of beryl, cassiterite, scheelite, fluorite, apatite and needle-like tourmaline within marble (Part III). No evidence of crosscutting or overlapping of these ore veins by others suggests that the orebody was formed by single fluid activity. The contents of Be, W, Sn, Li, Cs, Rb, B, and F in the Pankou and Pukouling granites are similar to those of the granites that host Nanling W–Sn deposits. The calculated isotopic compositions of beryl, scheelite and cassiterite (δD, −69.3‰ to −107.2‰ and δ18OH2O, 8.2‰ to 15.0‰) indicate that the ore-forming fluids were mainly composed of magmatic water with minor meteoric water and CO2 derived from decarbonation of marble. Primary fluid inclusions are CO2− CH4+ H2O ± CO2 (vapor), with or without clathrates and halites. We estimate the fluid trapping condition at T = 220 to 360°C and P > 0.9 kbar. Fluid inclusions are rich in H2O, F- and Cl-. Evidence for fluid-phase immiscibility during mineralization includes variable L/V ratios in the inclusions and inclusions containing different phase proportions. Fluid immiscibility may have been induced by the pressure released by extension joints, thereby facilitating the mineralization found in Part III. Based on the geochemical data, geological occurrence, and fluid inclusion studies, we hypothesize that the coarse-grained crystals were formed by: (i) the high content of ore elements and volatile elements such as F in ore-forming fluids; (ii) occurrence of fluid immiscibility and Ca-bearing minerals after wall rock transition from granite to marble making the ore elements deposit completely; (iii) pure host marble as host rock without impure elements such as Fe; and (iv) sufficient space in ore veins to allow growth.