Albian–Cenomanian A-type granite-related Ag–Pb–Zn veins in the central Yidun Terrane, SW China: constraints from the Xiasai deposit

Abstract

The Xiasai–Lianlong metallogenic belt in the central Yidun Terrane (China) hosts several Sn–Ag and (Sn–) Ag–Pb–Zn deposits, which are spatially associated with Cretaceous granites. The Xiasai Ag–Pb–Zn deposit (about 0.27\xa0Mt Pb+Zn, 1028\xa0t Ag, and 20,000\xa0t Sn) is the largest deposit in the belt and is related to a monzogranite intrusion. The deposit consists of sulfides in hydrothermal veins, resulting from three successive mineralization stages (I–III). A Rb–Sr isochron of sphalerite samples yielded an age of 99\u2009±\u20093\xa0Ma, and zircon U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) dating of the Xiasai monzogranite gave ages between 102\u2009±\u20091 and 101\u2009±\u20091\xa0Ma. The Xiasai monzogranite has a weakly peraluminous A-type granite composition, with high SiO 2 , alkali, and FeO t contents and low CaO, MgO, and TiO 2 contents. It is marked by enrichments in Rb, Th, and U, with depletions in Ba, Nb, Sr, P, and Ti relative to the primitive mantle. The REE patterns show a negative slope due to LREE enrichment with negative Eu anomalies (Eu/Eu*\u2009=\u20090.1–0.3). The Xiasai monzogranite, with variable ε Hf ( t ) values (−\u20092.7 to 0.6) and Meso- to Neoproterozoic two-stages Hf model ages ( T DM2 \u2009=\u2009925–1095\xa0Ma), was probably derived from mixing of melts derived from partial melting of the Paleoproterozoic crustal basement and the Early Cretaceous mantle in an extensional tectonic setting. Hydrothermal fluids are characterized by high temperatures (423–481\xa0°C) and salinities (14.9–19.0\xa0wt% NaCl) for Sn mineralization (stage I), moderate temperatures (285–386\xa0°C) and salinities (3.5–8.0\xa0wt% NaCl) for the Cu–Zn mineralization (substage II-2), and low temperatures (158–242\xa0°C) and salinities (3.4–5.7\xa0wt% NaCl) for the Ag–Pb mineralization (substage II-3). The H–O isotopes indicate that the hydrothermal fluid likely has a magmatic component that mixed over time with infiltrating meteoric water and organic matter during fluid–rock interactions. The δ 34 S values of sphalerite range from −\u20099.7 to −\u20093.1‰ and those from galena from −\u200910.5 to −\u20094.9‰. The estimated δ 34 S value of the hydrothermal fluid is −\u20098.5‰, using the sulfur isotopic fractionation of sphalerite–galena pairs that are interpreted to be in equilibrium. Sulfides from substages II-2 and II-3 have relatively homogeneous Pb isotopic compositions, which are similar with those of K-feldspars from the Xiasai monzogranite. Geochronological and isotope data support a magmatic–hydrothermal origin for the Xiasai Ag–Pb–Zn deposit.