The Metallogenic Mechanism Of Distal Contact Pb–Zn–Ag Veins In Shizhuyuan Ore District, Hunan Province, China

Shizhuyuan mining district consists of the proximal W–Sn–Mo–Bi skarns and greisens and distal Pb–Zn–Ag veins. The distal Pb–Zn–Ag veins were controlled by the NE-trending faults, and consist of sulfides and sulfosalts that formed during three separate stages:(1) early pyrite and arsenopyrite,(2) a middle stage of sphalerite and chalcopyrite, and(3) late galena, Ag-, Sn-, and Bi-bearing sulfosalt, and pyrrhotite. Fluid inclusion and electron microprobe analyses indicate that the early stage of mineralization occurred at a temperature of ~400°C and involved boiling under hydrostatic pressure(~200 bar), with the temperature of the system dropping during the late stage to ~200°C. The sulfur fugacity during the early stage of mineralization was between 10–8 and 10–6 bars, whereas the sulfur fugacity at the late stage of mineralization was between 10–16 and 10–13 bars. The δ13C and δ18O values of calcites from the proximal skarns are between-6.5‰ and-3.6‰ and between 4.8‰ and 8.4‰, respectively, and they from the distal Pb–Zn–Ag range from-3.5‰ to-1.5‰ and from 8.1‰ to 15.8‰, respectively, which reflect skarn formed from the infiltration metasomatism and distal carbonate veins formed from water/rock reactions. The δDH2O and δ18OH2O values of quartzs from proximal greisens are-77‰ and 5.1‰, respectively, δDH2O values of garnets from proximal skarns are-97‰, while δ18OH2O values of them range between 9.2‰ and 9.4‰, and the δDH2O and δ18OH2O values of quartzs from distal quartz veins are-83‰ and 2.8‰. We propose that the fluids formed proximal skarn–greisen and distal quartz veins are characterized by magmatic-water. The range of ?34S of 3.8–8.9‰ for sulfides within the Pb–Zn–Ag veins reflects sulfur derived from mainly a magmatic source, containing minor amounts of sedimentary rock-derived sulfur. The 206Pb/204 Pb values of galena from proximal–distal mineralization areas were between 18.565 and 18.622, 207Pb/204 Pb between 15.694 and 15.738, and 208Pb/204 Pb between 38.819~38.986, which reflect that ore-forming elements were derived from the upper crust. Combined with previous studies, it can be concluded that the large range of molten crust induced by upwelling mantle was form containing W–Sn polymetallic magma, then emplaced into the shallow crust and caused the large-scale metallogenese in the Nanling region. The Shizhuyuan proximal skarn–greisen W–Sn–Mo–Bi ores to distal Pb–Zn–Ag veins were experienced multistage mineralization processes: early skarns and mineralization formed from infiltration metasomatism, followed by retrograde reaction, late stage distal Pb–Zn–Ag veins filling of fractures and alteration of surrounding rocks by metal-rich fluids. During final crystallization of the Qianlishan pluton, a liquid exsolved from the melt and ascended along fracture zones. As this fluid ascended, it cooled to <400°C and boiled under hydrostatic conditions. As the temperature cooled to 200°C and sulfur fugacity decreased, different sulfide and sulfosalt minerals precipitated from the hydrothermal fluids.