In situ geochemistry and Sr–O isotopic composition of wolframite and scheelite from the Yaogangxian quartz vein-type W(–Sn) deposit, South China

Abstract

Wolframite and scheelite are primary W–bearing minerals and form in multiple metallogenic stages in quartz-vein type W(–Sn) deposits, recording copious metallogenic information. This study investigates the geochemistry, geochronology and Sr–O isotope of these minerals in the Yaogangxian W(–Sn) deposit, South China, to constrain the metallogenic age and decipher the mineralization process. Three wolframite and scheelite phases were identified in the greisen and quartz-vein type ores with different cathodoluminescence (CL) response, rare-earth element (REE) characteristics, and Sr–O isotopic compositions. The Cassiterite U–Pb dating in the greisen phase yielded a 156.0 ± 0.7 Ma isochron age and a 147.5 ± 0.6 Ma isochron age in the oxide phase, both of which are within error or slightly younger than Jurassic granites. From the early to late stage, the ΣREE contents of scheelite and wolframite decreased sharply, and the REE patterns of the scheelite changed from the LREE-enriched with negative Eu anomalies to HREE-enriched with positive Eu anomalies. The δ18OH2O values of the fluid (8.5 ‰–8.8 ‰) in the greisen phase calculated using the scheelite–H2O fractionation equation are identical to the magmatic–hydrothermal fluid, whereas the δ18OH2O values of the fluid (1.4 ‰–1.6 ‰) in the sulfide phase indicate the fluid mixing with meteoric water. The (87Sr/86Sr)0 ratios of wolframite and scheelite in the quartz-vein type ores range from the 0.74972 to 0.78397, higher than the Jurassic granites ((87Sr/86Sr)0 = 0.71593–0.73188) but lower than the sandstone in the host rock ((87Sr/86Sr)0 = 0.785227–0.813912), indicating an intense fluid-rock interaction. Therefore, although the W(–Sn) mineralization is primarily related to Jurassic granites, fluid mixing and fluid–rock interaction in the quartz-vein type ores might have been a trigger mechanism for W(–Sn) precipitation.