Magmatic evolution and sources of metals and fluids in the large granite-related Xiasai vein-type Ag-Pb-Zn(-Sn) deposit, northern Yidun terrane, SW China

The Xiasai Ag-Pb-Zn-(Sn) deposit in the northern Yidun terrane (eastern Tethys), hosted in Late Triassic metasandstones, is genetically associated with Late Cretaceous granites. We present chemical and Sr-Nd-Pb-Li-B isotope data of Late Cretaceous granites and their mafic microgranular enclaves (MME), late aplites, and Late Triassic metasedimentary rocks. The narrow range of Sr-87/Sr-86(99) (0.70627-0.70953) and epsilon Nd-99 (-6.1 to -5.2) values of most Late Cretaceous granites reflects a mixed source with contributions from the mantle and Paleoproterozoic felsic crust. These granites have high SiO2 contents (>69 wt%) and strong depletions in Sr, Ba, and Eu. Major and trace element contents correlate with the fractionation index 1/TiO2, indicating that they experienced extensive fractionation. Local late aplite dikes were derived from similar source rocks, but seem to be younger. The Pb isotopic compositions reflect that the metal Pb was derived from the granite with variable contributions from the Late Triassic sedimentary rocks. Weakly and moderately altered metasandstones have similar delta Li-7 and delta B-11 values as most granites (delta Li-7: 0.00 to 2.68 parts per thousand; delta B-11 values: -14.93 to -12.36 parts per thousand), indicating magmatic fluid sources. The magmatic fluids resulted in the initial enrichment of Sn, Cu, Pb, and Zn. Strong alteration of feldspar and biotite resulted in the metasandstones in negligible change of Li concentrations, significant loss of Sr, Na2O, CaO, MgO, and Fe2O3, but significant addition of B. This alteration lowered delta Li-7 from 1.01 to -3.65 parts per thousand and lowered delta B-11 from -9.43 to -19.18 parts per thousand. The low delta Li-7 values and extremely low delta B-11 values reflect external fluids from the Late Triassic sedimentary wall rocks, which interacted with granites and mobilized metals (e.g., Sn, Ag, Pb, and Zn) from the granite and/or the sedimentary wall rocks. The location of the Xiasai deposit is controlled by regional NNW-trending faults induced by Late Cretaceous extension.