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Abstract: 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 (429Ma) values of +2 to +5, and indicate a dominantly mantle source, or origin from young continental crust (TDM ~800–1000Ma). The dacites have distinctly low abundances of Ti, Nb, Ta, Zr, Th, and REEs, which is typical of subduction-related volcanism. The base-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. Uranium–Pb dating on zircon by LA-ICP-MS defines an age of 429±3Ma (2σ) (n=19) for the dacite sequence. Rhenium–Os isotope data on Mo-rich bulk ore samples define an isochron of 429±10Ma (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±4Ma (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. [Copyright &y& Elsevier]

Abstract: The Tianqiao Pb–Zn deposit in the western Yangtze Block, southwest China, is part of the Sichuan–Yunnan–Guizhou (SYG) Pb–Zn metallogenic province. Ore bodies are hosted in Devonian and Carboniferous carbonate rocks, structurally controlled by a thrust fault and anticline, and carried about 0.38million tons Pb and Zn metals grading >15% Pb+Zn. Both massive and disseminated Pb–Zn ores occur either as veinlets or disseminations in dolomitic rocks. They are composed of ore minerals, pyrite, sphalerite and galena, and gangue minerals, calcite and dolomite. δ34S values of sulfide minerals range from +8.4 to +14.4‰ and display a decreasing trend from pyrite, sphalerite to galena (δ34Spyrite >δ34Ssphalerite >δ34Sgalena). We interpret that reduced sulfur derived from sedimentary sulfate (gypsum and barite) of the host Devonian to Carboniferous carbonate rocks by thermal–chemical sulfate reduction (TSR). δ13CPDB and δ18OSMOW values of hydrothermal calcite range from –5.3 to –3.4‰ and +14.9 to +19.6‰, respectively, and fall in the field between mantle and marine carbonate rocks. They display a negative correlation, suggesting that CO2 in the hydrothermal fluid was a mixture origin of mantle, marine carbonate rocks and sedimentary organic matter. Sulfide minerals have homogeneous and low radiogenic Pb isotope compositions (206Pb/204Pb=18.378 to 18.601, 207Pb/204Pb=15.519 to 15.811 and 208Pb/204Pb=38.666 to 39.571) that are plotted in the upper crust Pb evolution curve and overlap with that of Devonian to Carboniferous carbonate rocks and Proterozoic basement rocks in the SYG province. Pb isotope compositions suggest derivation of Pb metal from mixed sources. Sulfide minerals have 87Sr/86Sr ratios ranging from 0.7125 to 0.7167, higher than Sinian to Permian sedimentary rocks and Permian Emeishan flood basalts, but lower than basement rocks. Again, Sr isotope compositions are supportive of a mixture origin of Sr. They have an Rb–Sr isotopic age of 191.9±6.9Ma, possibly reflecting the timing of Pb–Zn mineralization. C–O–S–Pb–Sr isotope compositions of the Tianqiao Pb–Zn deposit indicate a mixed origin of ore-forming fluids, which have Pb–Sr isotope homogenized before the mineralization. The Permian flood basalts acted as an impermeable layer for the Pb–Zn mineralization hosted in the Devonian–Carboniferous carbonate rocks. [Copyright &y& Elsevier]