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

摘要

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 (8D, -69.3 per tousand to -107.2 per tousand and ?~(18)O_(H2O), 8.2 per tousand to 15.0 per tousand) indicate that the ore-forming fluids were mainly composed of magmatic water with minor meteoric water and CO_2 derived from decarbonation of marble. Primary fluid inclusions are CO_2 - CH4 + H_2O ± CO_2 (vapor), with or without clathrates and halites. We estimate the fluid trapping condition at T = 220 to 360°C and P > 0.9 kbar.