Evolution of the magmatic–hydrothermal system and formation of the giant Zhuxi W–Cu deposit in South China

摘要

The Zhuxi deposit is a recently discovered W–Cu deposit located in the Jiangnan porphyry–skarn W belt in South China. The deposit has a resource of 3.44 million tonnes of WO3, making it the largest on Earth,however its origin and the evolution of its magmatic–hydrothermal system remain unclear, largely because alteration–mineralization types in this giant deposit have been less well-studied, apart from a study of the calcic skarn orebodies. The different types of mineralization can be classified into magnesian skarn, calcic skarn, and scheelite–quartz–muscovite(SQM) vein types. Field investigations and mineralogical analyses show that the magnesian skarn hosted by dolomitic limestone is characterized by garnet of the grossular–pyralspite(pyrope, almandine, and spessartine) series, diopside, serpentine,and Mg-rich chlorite. The calcic skarn hosted by limestone is characterized by garnet of the grossular–andradite series, hedenbergite, wollastonite, epidote, and Fe-rich chlorite. The SQM veins host highgrade W–Cu mineralization and have overprinted the magnesian and calcic skarn orebodies. Scheelite is intergrown with hydrous silicates in the retrograde skarn, or occurs with quartz, chalcopyrite, sulfide minerals, fluorite, and muscovite in the SQM veins.Fluid inclusion investigations of the gangue and ore minerals revealed the evolution of the ore-forming fluids, which involved:(1) melt and coexisting high–moderate-salinity, high-temperature, high-pressure(>450 ℃and >1.68 kbar), methane-bearing aqueous fluids that were trapped in prograde skarn minerals;(2) moderate–low-salinity, moderate-temperature, moderate-pressure(~210–300 ℃and ~0.64 kbar),methane-rich aqueous fluids that formed the retrograde skarn-type W orebodies;(3) low-salinity,moderate–low-temperature, moderate-pressure(~150–240 ℃and ~0.56 kbar), methane-rich aqueous fluids that formed the quartz–sulfide Cu(–W) orebodies in skarn;(4) moderate–low-salinity,moderate-temperature, low-pressure(~150–250 ℃and ~0.34 kbar) alkanes-dominated aqueous fluids in the SQM vein stage, which led to the formation of high-grade W–Cu orebodies. The S–Pb isotopic compositions of the sulfides suggest that the ore-forming materials were mainly derived from magma generated by crustal anatexis, with minor addition of a mantle component. The H–O isotopic compositions of quartz and scheelite indicate that the ore-forming fluids originated mainly from magmatic water with later addition of meteoric water. The C–O isotopic compositions of calcite indicate that the ore-forming fluid was originally derived from granitic magma, and then mixed with reduced fluid exsolved from local carbonate strata. Depressurization and resultant fluid boiling were key to precipitation of W in the retrograde skarn stage. Mixing of residual fluid with meteoric water led to a decrease in fluid salinity and Cu(–W) mineralization in the quartz–sulfide stage in skarn. The high-grade W–Cu mineralization in the SQM veins formed by multiple mechanisms, including fracturing, and fluid immiscibility, boiling, and mixing.