An Early Cretaceous magmatic-hydrothermal origin of the Shanhou stratabound Pb-Zn deposit in Fujian Province of southeast China: Constraints from hydrothermal titanite U-Pb dating and C-O-S-Pb isotopes

Abstract

Stratabound base metal sulfide deposits within Neoproterozoic metavolcanic rocks and skarns in the Fujian Province are important Zn-Pb producers in China. To date, the inability to determine absolute ages and proper sources of Pb-Zn mineralization has led to conflicting ore genetic models. These include the metamorphosed volcanogenic massive sulfide (VMS) model, which predicts that the Pb-Zn mineralization was synchronous with submarine volcanic activity, and the Early Cretaceous magmatic-hydrothermal skarn model. The Shanhou deposit (2.56 Mt ore @ 6.8 % Pb + Zn) in the central Fujian Province, the southeastern coastal area of China, consists of twelve stratabound massive Pb-Zn sulfide orebodies hosted in hydrothermally altered metavolcanic rocks of the Neoproterozoic Dongyan Formation. No intrusive body that could have contributed to the formation of the calc-silicate mineral-dominated wall rocks and Pb-Zn ores has been found spatially associated with the Shanhou deposit, and the origin of the mineralizing fluids, sulfur, and metals has not yet been identified previously. Here we present in-situ U-Pb ages of hydrothermal titanite and C-O-S-Pb isotopes of gangue and ore minerals to constrain the timing and origin of the Pb-Zn mineralization. Three hydrothermal stages can be identified: pyroxene (prograde stage I), epidote + chlorite + titanite (retrograde stage II), and quartz + calcite + polymetallic sulfides (quartz-calcite-sulfide stage III). In-situ LA-ICP-MS U-Pb isotopic analyses of hydrothermal titanites coexisting with sulfides in two massive ore samples yield indistinguishable ages of 145 ± 13 Ma and 144 ± 9 Ma, overlapping the 152 to 145 Ma emplacement age of granitic rocks in the central Fujian area. The results show that the Shanhou Pb-Zn deposit is significantly younger than the Neoproterozoic host (meta)volcanic rocks, arguing against a Neoproterozoic VMS genetic model as suggested by some previous researchers. The δ34S (V-CDT) values of pyrrhotite, pyrite, marcasite, chalcopyrite, sphalerite, and galena in the massive sulfide ores range from −2.9 to 6.9 ‰, indicating that the sulfur at Shanhou is mainly of magmatic origin. Sphalerite separates have 206Pb/204Pb ratios of 17.782 to 17.782, 207Pb/204Pb ratios of 15.585 to 15.593, and 208Pb/204Pb ratios of 38.376 to 38.407, which are similar to those of the host rocks, indicating that the ore-forming materials were derived from the host rocks. Calcite in sulfide ores has δ13CV-PDB values of −7.8 to −4.3 ‰ and δ18OV-SMOW values of 6.1 to 9.5 ‰, suggesting a magmatic origin for the ore-forming fluids. It is concluded that the Shanhou stratabound Pb-Zn deposit is a metasomatic skarn-like deposit associated with a possible hidden Early Cretaceous felsic intrusion at depth, rather than a metamorphosed VMS deposit.