Ore-forming mechanism of quartz-vein-type W-Sn deposits of the Xitian district in SE China: Implications from the trace element analysis of wolframite and investigation of fluid inclusions

摘要

The Xitian W-Sn district is located in eastern Hunan Province of SE China and comprises the Heshuxia, Goudalan, and Longshang W-Sn deposits. Our study on the occurrence, mineral assemblage, and geochemistry of the orebodies shows that quartz-vein orebodies are dominant mode of mineralization and can be divided into three stages: stage I, quartz-molybdenite-wolframite (I); stage II, quartz-wol-framite (II)-polymetallic sulfides; and stage III, quartz-pyrite-fluorite. The elements Nb, Ta, Sc, Sn, Zn, Cr, V, Mo, Cu, and Zr are abundant in both wolframite (I) and wolframite (II). Wolframite (I) contains higher concentrations of Nb (1408-9245 ppm), Ta (241-2407 ppm), and Sc (123-521 ppm) than wolframite (II) (Nb 144-1080 ppm, Ta 3.76-315 ppm, and Sc 1.86-11.44 ppm). These results indicate that wolframite (I) was formed closer to the host rock and at a greater ore-forming depth than wolframite (II). The total REE concentrations of wolframite (I) and (II) range from 34.96 to 133.36 ppm and 21.30 to 43.73 ppm, respectively. Both wolframite (I) and wolframite (II) have low LREE concentrations (0.00-0.05 and 0.08-8.18 ppm, respectively) and high HREE concentrations (34.96-133.31 and 21.23-41.87 ppm, respectively). The enrichment of HREEs in wolframite may be due to the fact that the sizes of the HREEs3+ (0.94-1.02 A) in combination with Nb5+ or Ta5+ (0.72 A) are closer to that of W6+ (0.68 A) in combination with Ca2+ (1.08 A) or Mg2+ (0.80 A) than that of the LREEs3+ (1.03-1.13 A), making the coupled substitution easier. A study of fluid inclusions in the coexisting gangue minerals shows that the homogenization temperatures of the fluid inclusions decrease from stage I to stage III (stage I, 187-382 degrees C, average 275 degrees C; stage II, 122-278 degrees C, average 175 degrees C; stage III, 92-172 degrees C, average 139 degrees C), whereas the salinities of the fluid inclusions increase from stages I to II and decrease from stages II to III (stage I, 1.8-18.2 wt% NaCleqv, average 10.3 wt% NaCleqv; stage II, 7.3-24.4 wt% NaCleqv, average 14.9 wt% NaCleqv; stage III, 0.2-5.3 wt% NaCleqv, average 1.5 wt% NaCleqv). Raman spectroscopy reveals that the fluid inclusions mainly contain H2O; CO2 is minor and H2S, CH4, and N-2 are rare. We conclude that the formation of wolframite was driven by post-magmatic thermodynamic processes. The ore forming fluid flowed through a multi -fractured low-pressure zone in the fault system in the district, and a temperature-pressure decrease led to fluid immiscibility characterized by CO2 escaping in a low-pH and high-Eh environment in stage I, crust-mantle mixed fluid and meteoric water mixing in stage II, and natural cooling of the fluid system in stage III. (C) 2016 Elsevier B.V. All rights reserved.