Geology, isotope geochemistry and ore genesis of the Shanshulin carbonate-hosted Pb-Zn deposit, southwest China

The Shanshulin Pb-Zn deposit occurs in Upper Carboniferous Huanglong Formation dolomitic limestone and dolostone, and is located in the western Yangtze Block, about 270 km west of Guiyang city in southwest China. Ore bodies occur along high angle thrust faults affiliated to the Weishui regional fault zone and within the northwestern part of the Guanyinshan anticline. Sulfide ores are composed of sphalerite, pyrite, and galena that are accompanied by calcite and subordinate dolomite. Twenty-two ore bodies have been found in the Shanshulin deposit area, with a combined 2.7 million tonnes of sulfide ores grading 0.54 to 8.94 wt.% Pb and 1.09 to 26.64 wt% Zn. Calcite samples have delta C-13(PDB) and delta O-18(SMOW) values ranging from -3.1 to +2.5 parts per thousand and + 18.8 to + 26.5 parts per thousand, respectively. These values are higher than mantle and sedimentary organic matter, but are similar to marine carbonate rocks in a delta C-13(PDB) vs. delta O-18(SMOW) diagram, suggesting that carbon in the hydrothermal fluid was most likely derived from the carbonate country rocks. The delta S-34(CDT) values of sphalerite and galena samples range from + 18.9 to + 203 parts per thousand and +15.6 to + 17.1 parts per thousand, respectively. These values suggest that evaporites are the most probable source of sulfur. The delta S-34(CDT) values of symbiotic sphalerite-galena mineral pairs indicate that deposition of sulfides took place under chemical equilibrium conditions. Calculated temperatures of S isotope thermodynamic equilibrium fractionation based on sphalerite-galena mineral pairs range from 135 to 292 degrees C, consistent with previous fluid inclusion studies. Temperatures above 100 degrees C preclude derivation of sulfur through bacterial sulfate reduction (BSR) and suggest that reduced sulfur in the hydrothermal fluid was most likely supplied through thermo-chemical sulfate reduction (TSR). Twelve sphalerite samples have delta Zn-66 values ranging from 0.00 to + 0.55 parts per thousand (mean + 0.25%.) relative to the JMC3-0749L zinc isotope standard. Stages Ito III sphalerite samples have delta Zn-66 values ranging from 0.00 to + 0.07 parts per thousand, +0.12 to + 0.23 parts per thousand, and +0.29 to +0.55%, respectively, showing the relatively heavier Zn isotopic compositions in later versus earlier sphalerite. The variations of Zn isotope values are likely due to kinetic Raleigh fractional crystallization. The (206)pb/(204)pb, (207)pb/(204)pb and (208)pb/(204)(Pb) ratios of the sulfide samples fall in the range of 18362 to 18.573, 15.505 to 15.769 and 38.302 to 39223, respectively. The Pb isotopic ratios of the studied deposit plot in the field that covers the upper crust, orogenic belt and mantle Pb evolution curves and overlaps with the age-corrected Proterozoic folded basement rocks, Devonian to Lower Permian sedimentary rocks and Middle Permian Emeishan flood basalts in a Pb-207/Pb-204 vs. Pb-206/Pb-204 diagram. This observation points to the derivation of Pb metal from mixed sources. Sphalerite samples have Sr-87/Sr-86(200 Ma) ratios ranging from 0.7107 to 0.7115 similar to the age-corrected Devonian to Lower Permian sedimentary rocks (0.7073 to 0.7111), higher than the age-corrected Middle Permian basalts (0.7039 to 0.7078), and lower than the age-corrected Proterozoic folded basement (0.7243 to 0.7288). Therefore, the Sr isotope data support a mixed source. Studies on the geology and isotope geochemistry suggest that the Shanshulin deposit is a carbonate-hosted, thrust fault-controlled, strata-bound, epigenetic, high grade deposit formed by fluids and metals of mixed origin. (C) 2014 Elsevier S.V. All rights reserved.