Ore-forming Fluid and Metallogenic Mechanism of Wolframite-Quartz Vein-type Tungsten Deposits in South China

摘要

South China is endowed with copious wolframite-quartz vein-type W deposits that provide a significant contribution to the world‘s tungsten production.Mineralization is spatially associated with highly evolved granites,which have been interpreted as products of ancient crustal anatexis.Ore veins are mainly hosted in low-grade metamorphosed quartz sandstone,slate and granitic rocks.The ore minerals mainly comprise wolframite,cassiterite,scheelite and pyrite,with minor molybdenite,arsenopyrite and chalcopyrite.Typical steeply dipping veins can be divided into five zones from top to the bottom,namely:(Ⅰ)thread,(Ⅱ)veinlet,(Ⅲ)moderate vein,(Ⅳ)thick vein,and(Ⅴ)thin out zones.In general,three types of fluid inclusions at room temperature are commonly recognized in wolframite and/or quartz from these veins:two-phase liquid-rich(type L),two-phase CO2-bearing(type CB),and CO2-rich(type C).Comparative microthermometry performed on fluid inclusions hosted in wolframite and associated quartz indicates that most wolframite was not coprecipitated with the coexisting quartz.Detailed petrographic observation and cathodoluminescence(CL)imaging on coexisting wolframite and quartz of the Yaogangxian deposit,show repeated precipitation of quartz,wolframite,and muscovite,suggesting a more complex fluid process forming these veins.Previous studies of H-O isotopes and fluid inclusions suggested that the main ore-forming fluids forming the wolframite-quartz vein-type deposits had a magmatic source,whereas an unresolved debate is centered on whether mantle material supplemented the ore-forming fluids.The variable CO2 contents in the ore-forming fluids also implies that CO2 might have had a positive effect on ore formation.Fluid inclusion studies indicate that wolframite was most likely deposited during cooling from an initial H2 O+Na Cl±CO2 magmatic fluid.In addition,fluid-phase separation and/or mixing with sedimentary fluid might also have played an important role in promoting wolframite deposition.We speculate that these processes determine the precipitation of W to varying degrees whereas the leading mechanistic cause remains an open question.Comprehensive studies on spatial variation of fluid inclusions show that both the steeply and gently dipping veins are consistent with the"five floors"model that may have broader applications to exploration of wolframite-quartz vein-type deposits.Recent quantitative analysis of wolframite-and quartz-hosted fluid inclusions by laser ablation inductively-coupled plasma mass spectrometry shows enhanced advantages in revealing fluid evolution,tracing the fluid source and dissecting the ore precipitation process.Further studies on wolframite-quartz vein-type W deposits to bring a deeper understanding on ore-forming fluids and the metallogenic mechanism involved.