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 (2 sigma, 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 Sr-87/Sr-86 ratios of pre-ore (Ap(h1)) and syn-ore (Ap(h2)) 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 degrees 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 Sr-87/Sr-86 ratio but higher Sr content of Ap(h2) relative to Ap(h1) 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.