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The Longzhoushan and Ertan basalts in the Pan-Xi area were generated by relatively low degrees of partial melting (<7%), during which Ir, Ru, and Rh are compatible while Pt and Pd are incompatible. Most of these samples have high PGE concentrations and much higher Pd/lr and Cu/Pd ratios than those of primitive mantle. A small amount of sulfides (∼0.007%) remained in the mantle source during partial melting, whereas part of the magmas reached sulfide saturation and segregated PGE-rich sulfides (∼0.020%–0.033%) during ascent period. Osmium, Ir, and Pt could form platinum-group metallic alloys hosted in chromites or sulfides during fractional crystallization process, leading to the decoupling of Pt and Pd contents and considerable variations of Pr contents. Sulfides segregated from the basaltic magmas may provide important material sources for the formation of the Cu-Ni-PGE deposits (mineralization) in the mafic-ultramafic intrusions near the magma conduits. [ABSTRACT FROM AUTHOR]

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]