Onset and duration of Zn—Pb mineralization in the Talate Pb—Zn (—Fe) skarn deposit, NW China: Constraints from spessartine U—Pb dating

Abstract

Distal Pb—Zn skarn commonly displays ambiguous relationships with the nearby intrusions, leading to poorly constrained and often controversial skarn metallogenic models. In this study, we discuss the potential usage of spessartine, a common U-bearing gangue mineral in many Pb—Zn skarn deposits, in U—Pb dating. Spessartine from the Talate skarn Pb—Zn deposit (Xinjiang, NW China) can be divided into two types (GI and GII) based on mineral assemblage. GI spessartine (Sp56.5 Al28.3 Gr12.4 to Sp66.0 Al22.3 Gr8.5) is coarse-grained with core-rim texture occasionally well-preserved, disseminated magnetite and magmatic TI titanite grains are hosted within the spessartine GI core, indicating that the spessartine GI is slightly postdate the magmatic titanite TI and disseminated magnetite. GII massive spessartine (Sp64.2 Al24.8 Gr2.7 to Sp69.6 Al20.1 Gr1.2) occurs with hydrothermal titanite TII and shows a close relationship with coarse-grained hydrothermal magnetite, and is cut by quartz or fine sulfide (e.g., pyrite, pyrrhotite and chalcopyrite) veins.The flat time-resolved signals obtained from the depth profile analyses and inclusion-free trace element mapping of U in spessartine indicate that the U is structurally bounded in the lattice. Both GI and GII spessartine show clear positively correlation between U, Al and Mn, but no apparent correlation is present between U and Fe, which indicates that Al and Mn are more important in the U incorporation in spessartine. Integrating with the positive correlations with U and LREEs in spessartine, which suggest U is incorporated in the lattice by a substitution of [U4+]VIII + 2[Al3+]IV − [Ca2+]VIII + 2[Si4+]IV. In-situ U—Pb dating of the two spessartine types yielded a weighted average 206Pb/238U age of 231.7 ± 7.2 Ma (GI; MSWD = 0.56; n = 35) and 211.5 ± 5.8 Ma (GII; MSWD = 1.1; n = 47). Considering the errors caused by common Pb and matrix mismatch, the corrected GI and GII spessartine ages are consistent with the TI (228.0 ± 4.6 Ma; MSWD = 2; n = 28) and TII (209.8 ± 3.6 Ma; MSWD = 1.03; n = 39) titanite ages, suggesting that spessartine U—Pb dating is robust and reliable. The least 8.1 My gap between the formation of the two titanite and spessartite types reflects two mineralization events, suggesting an influx of magmatic fluids during the retrograde alteration/mineralization. Our study represents a new way of directly constraining the timing of skarn alteration/mineralization.