Age and fluid source of the sub-volcanic Zhaiping Ag–Pb–Zn deposit in the eastern Cathaysia Block (Fujian Province, Southeastern China)

The Zhaiping Ag–Pb–Zn vein deposit in the Coastal Volcanic Belt of southeastern China, which resulted from subduction of the paleo-Pacific oceanic lithosphere beneath the South China Block, is hosted by Early Cretaceous sub-volcanic rocks (rhyolite porphyry) and younger granite porphyry and quartz porphyry. Three generations of veins have been recognized: (i) pre-ore quartz + pyrite, (ii) syn-ore silver + pyrite + pyrrhotite + base metal sulfides + quartz + sericite, and (iii) post-ore quartz + calcite + fluorite veins. Rubidium–Sr dating of syn-ore pyrite yielded an isochron age of 132.2 ± 2.1 Ma (2σ, MSWD = 2.0, n = 4), which is roughly coincidental with the age of the granite porphyry and quartz porphyry (zircon U–Pb age: ca. 133 Ma). The age data, together with Pb, S, H, and O isotopic compositions, indicate a close temporal and possibly genetic relationship between mineralization and porphyry magmatism at Zhaiping. Geochemical and isotope data demonstrate that the Zhaiping sub-volcanic rocks were derived from partial melting of Proterozoic crustal sources and underwent fractionation of feldspars, monazite, and/or allanite. The 87Sr/86Sr ratios of pre-ore (Aph1) and syn-ore (Aph2) stage apatite are in the ranges of 0.71350 to 0.71441 and 0.71260 to 0.71328, respectively, which are distinct from those of the sub-volcanic intrusive rocks (0.7119–0.7125). The Sr isotope signature, together with lower fluid temperatures (< 360 °C) relative to typical magmatic fluid, points at addition of an external fluid. It is suggested that the sub-volcanic intrusion acted as a heat source that drove the dehydration of, and subsequent fluid circulation within, the host rocks. Lower 87Sr/86Sr ratio but higher Sr content of Aph2 relative to Aph1 possibly indicates continuous fluid–rock interaction and progressive decomposition of feldspar in the host rocks, thus supplying large amounts of Sr and probably Pb to the ore fluid.