Apatite and REE minerals petrochronology and Sr-Nd isotopic signatures: Age and hydrothermal evolution in the formation of the Mengya’a skarn deposit, eastern Nyainqêntanglha belt, Xizang

Abstract

Skarn Pb-Zn deposits are important sources of lead, zinc, and other rare dispersed metals. However, determining the origins and evolution of ore-forming fluids, as well as the timing of mineralization associated with Pb-Zn mineralization, remains challenging. Apatite and rare earth minerals, commonly found in a variety of ore deposits, offer valuable insights into the genesis of these deposits. This study presents detailed textures, high-precision in-situ U-Pb geochronology, element chemistry, and Sr-Nd isotopic analyses of multistage hydrothermal apatites from the Mengya’a Pb-Zn polymetallic deposit. Combined with U-Pb dates of coexisting rare earth minerals, these data allow us to precisely constrain the timing of mineralization and reconstruct the evolutionary history of the ore-forming fluids. Hydrothermal apatites are classified into four types based on mineral assemblages, CL imaging, and trace element compositions: Ap1 (early retrograde stage, with epidote), Ap2 (early retrograde stage, with fluorite), and Ap3-1 core and Ap3-2 rim (late retrograde stage, with sulfides and rare earth minerals). The lower intercept age of Ap1 (∼54.4 Ma), which aligns with the ages of coexisting monazite, xenotime, and parisite, suggesting that the deposit formed during the early Eocene main collision orogeny, challenging previous hypotheses of a Miocene or Paleocene origin. The variation in element chemistry and Sr-Nd isotopes provides evidence for the contributions of both magma and wall rock to ore formation. Ap1 exhibits high ⁸⁷Sr/⁸⁶Sr ratios (0.7329–0.7439) and ε_Nd(t) values (−17.2 to −10.9), consistent with derivation from the ancient Lhasa terrane or sedimentary strata, indicating a significant contribution of the wall rock. The influx of magmatic-hydrothermal fluids is reflected in the weakened negative Eu anomalies of Ap2 (Eu/Eu* = 0.22–0.42), the increased Sr concentrations in Ap3-1 (320–2139 ppm), and the left-leaning REE patterns of Ap3-2 (LREE/HREE = 0.20–0.08). The variation in ⁸⁷Sr/⁸⁶Sr ratios (0.7168–0.7274 for Ap2, 0.7278–0.7301 for Ap3-1, 0.7194–0.7217 for Ap3-2) and ε_Nd(t) values (−17.2 to −10.9 for Ap2, −18.0 to −12.7 for Ap3-1, −14.4 to −13.7 for Ap3-2) closely resemble the characteristics of skarn-related mineralization in regional granite, which is largely attributed to magma-derived fluids. This study highlights the variation in the fluid-to-rock ratio for skarn deposits, revealing a progression from wall-rock-dominated contributions in early stages to primarily magmatic fluid contributions in later stages, with meteoric water mixing occurring specifically during the mineralization stage. Overall, this study underscores the reliability of apatite as an indicator for geochronology, geochemistry, and isotopic analysis in complex mineralization processes of skarn Pb-Zn deposits.