Mid-Silurian back-arc spreading at the northeastern margin of Gondwana: The Dapingzhang dacite-hosted massive sulfide deposit, Lancangjiang zone, southwestern Yunnan, China

The Dapingzhang dacite-hosted volcanogenic massive sulfide deposit is located in a late Paleozoic collisional zone, the Sanjiang fold-and-thrust belt, characterized by a collage of Gondwana-derived terranes at the southern margin of the South China (Yangtze) block. This area has experienced repeated Paleozoic and Mesozoic collisional and extensional events, and is overprinted by Himalayan strike-slip and thrust movements. The Dapingzhang orebody is strongly sheared into discontinuous ore lenses and blocks, and is hosted by a several-hundred-meter-thick volcanic rock sequence of dacite, with minor metabasalt (spilite), rhyolite, and chert. Bulk-rock Nd isotope data give εNd (429 Ma) values of + 2 to + 5, and indicate a dominantly mantle source, or origin from young continental crust (TDM ~ 800–1000 Ma). The dacites have distinctly low abundances of Ti, Nb, Ta, Zr, Th, and REEs, which is typical of subduction-related volcanism. se-metal mineralization occurs as polymetallic massive sulfide mineralization, mainly pyrite, sphalerite, galena, chalcopyrite, tetrahedrite–tennantite, and locally barite, and as pyrite–chalcopyrite stockworks. Pervasive hydrothermal alteration is dominantly quartz–sericite. Maximum enrichment in gold, bismuth, selenium, and tellurium is at the interface between the stockwork and massive sulfide mineralization styles. Deeper parts of the stockwork zone are characterized by elevated molybdenum and rhenium, whereas the distal parts to the massive sulfide mineralization have high mercury, antimony, zinc, cadmium, arsenic, lead and silver. The metal association and mineral assemblage are typical of volcanic rock-hosted massive sulfides in back-arc volcanic settings. m–Pb dating on zircon by LA-ICP-MS defines an age of 429 ± 3 Ma (2σ) (n = 19) for the dacite sequence. Rhenium–Os isotope data on Mo-rich bulk ore samples define an isochron of 429 ± 10 Ma (2σ) (n = 9; MSWD 0.21; initial 187Os/188Os 3.1 ± 1.9). Common Os is very low, and the absolute abundances of 187Re and 187Os define a model age of 429 ± 4 Ma (n = 9; 95% confidence). The mid-Silurian dacite sequence and associated massive sulfide mineralization identify a hitherto unknown early rifting and back-arc seafloor spreading event at the northeastern Gondwana margin related to the early evolution of the Paleotethys Ocean.