Evolution of ore fluids in the magmatic-hydrothermal Pb-Zn metallogenic system: A case study from Narusongduo deposit in the Himalayan-Tibetan orogen

Most skarn-type Pb-Zn deposits have at least two alteration stages: an early prograde and a later retrograde. However, whether these stages were formed by a single pulse or multiple pulses of magmatic fluids remains unclear. The Narusongduo deposit, a large magmatic-hydrothermal Pb-Zn deposit in the Gangdese polymetallic belt of the Himalayan-Tibetan orogen, provides an ideal vehicle to answer this issue. Skarn- and cryptoexplosive breccia−type are two major types of mineralization in this deposit. Three hydrothermal stages are distinguished for the skarn-type ore: pre-ore stage with garnet, diopside, rhodonite, and wollastonite; syn-ore stage with actinolite, epidote, chlorite, quartz, calcite plus sulfides; and post-ore stage with quartz, calcite, and pyrite. Contrastingly, two stages—namely, the syn-ore stage with epidote-chlorite-sericite-quartz-calcite-sphalerite-galena, and the post-ore stage with quartz-calcite-tetrahedrite-tennantite-anglesite—are found for the cryptoexplosive breccia−type ore. Primary fluid inclusions trapped in garnet from skarn-type ores have relatively higher homogenization temperatures of 430−440 °C, higher salinities of 42−43 wt% NaCl equivalent, and higher concentrations of Na (8.26 wt%), K (1.64 wt%), Pb (18,118 ppm), and Zn (11,196 ppm) than those in sphalerite and calcite (300−310 °C, 4−5 wt% NaCl equivalent, 1.27 wt%, 0.03 wt%, 649 ppm, and 664 ppm). Primary fluid inclusions trapped in sphalerite and quartz from cryptoexplosive breccia−type ores share similar temperature, salinity, and element compositions (300−310 °C, 4−5 wt% NaCl equivalent, 1.13 wt% Na, 0.04 wt% K, 705 ppm Pb, and 981 ppm Zn) with that in the same stage of the skarn-type ores. These microthermal and trace element data from single fluid inclusions point to the syn-ore fluid for the skarn-type ores being different from the pre-ore fluid for the same ore types but similar to the syn-ore stage of the cryptoexplosive breccia−type ores. This suggests magmatic fluid origins for the fluids from all stages in the deposit. Geochemical reaction path modeling of the pre-ore fluid from skarn-type ores predicted large amounts of skarn and alteration mineral precipitation with no Pb and Zn sulfides. Together with varying fluid inclusion compositions, we suggest that the pre-ore versus syn-ore stages are different pulses of hydrothermal fluids exsolved from a magma chamber. The magmatic-hydrothermal process should be as follows. The early supercritical fluid exsolved from the magma reservoir upwelling to ∼1.0 km under lithostatic conditions, separated into a hypersaline fluid and a low-salinity vapor, and produced the earliest skarn alteration. Subsequently, the second pulses of the supercritical fluid intersected the saturated vapor-pressure curve and boiled at the same depth of ∼1.0 km under hydrostatic conditions, resulting in the residual liquid with moderate to low temperature and low salinity and the formation of the skarn-type and cryptoexplosive breccia−type mineralization in different lithologies and structures. This work disclosed a two-stage pulse of fluids in the magmatic-hydrothermal Pb-Zn metallogenic system, indicating that maybe this kind of fluid involution is responsible for the formation of this kind of deposit.