Ma, Wang, Liu, Yingchao, Hou, Zengqian, Yang, Zhusen, Huizenga, Jan Marten, Li, Huajian, Yue, Longlong, Li, Zhenqing, and Zhao, Sibo (2025) Evolution of ore fluids in the magmatic-hydrothermal Pb-Zn metallogenic system: A case study from Narusongduo deposit in the Himalayan-Tibetan orogen. Geological Society of America Bulletin. (In Press)

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Abstract

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.

Item ID:	84537
Item Type:	Article (Research - C1)
ISSN:	1943-2674
Copyright Information:	© 2025 Geological Society of America
Date Deposited:	29 Jan 2025 00:52
FoR Codes:	37 EARTH SCIENCES > 3705 Geology > 370508 Resource geoscience @ 100%
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