Zinc isotopes trace the metal source and fluid flow pathways of the large Caixiashan Pb-Zn deposit, China

The ore-forming fluids responsible for the formation of distal carbonate replacement Pb-Zn deposits have the capacity to migrate over considerable distances. Pinpointing the source of metals and tracing fluid flow pathways for such distal mineralization can, therefore, be challenging. Zinc isotopes can serve as a tool to assess these ambiguities since different fluid reservoirs are characterized by distinct Zn isotopic compositions and metal precipitation processes cause systematic changes to Zn isotope ratios. In this contribution, we utilize newly acquired Zn isotopic data of the large Caixiashan distal carbonate replacement Pb-Zn deposit (131 million tons of ore at 3.95 wt% Pb + Zn) in the Eastern Tianshan to investigate the source of metals and fluid flow pathways. The S66Zn values of sphalerite from the four ore zones (i.e., ore zones I, II, III, and IV) range from -0.04 to 0.18 %o, -0.04 to 0.07 %o, -0.42 to -0.32 %o and -0.39 to 0.07 %o, respectively. The average S66Zn values of the sphalerite samples from the four ore zones increase systematically from south to north, specifically from the zones III to II and IV to I. The felsic intrusions and barren country rocks have S66Zn values ranging from -0.11 to -0.06 %o and -0.32 to -0.02 %o, respectively. The Zn isotopic ratios of sphalerite are positively correlated with Zn/Cd ratios, demonstrating that the S66Zn values of sphalerite shift towards heavier values as the hydrothermal fluid from which they precipitated evolved. This systematic increase in S66Zn values with fluid evolution resulted from continued precipitation of sphalerite, which is typically isotopically light. Based on Rayleigh distillation modeling, sphalerite with S66Zn values as light as -0.48 %o would have required that the initial ore-forming fluid had S66Zn values of about -0.2 %o. A fluid with this isotopic composition could not have exsolved from the Carboniferous felsic intrusions, and is more likely to have been derived from remobilization of earlier Zn-rich Precambrian basement. This contribution highlights that the formation of large Pb-Zn deposits, such as Caixiashan, may depend on the remobilization of previously precipitated Zn.