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

Abstract

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 north- western part of the Guanyinshan anticline. Sulfide ores are composed of sphalerite, pyrite, and galena that are ac- companied 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 δ 13 C PDB and δ 18 O SMOW values ranging from −3.1 to +2.5‰ and +18.8 to +26.5‰, respectively. These values are higher than mantle and sedimentary organic matter, but are similar to ma- rine carbonate rocks in a δ 13 C PDB vs. δ 18 O SMOW diagram, suggesting that carbon in the hydrothermal fluid was most likely derived from the carbonate country rocks. The δ 34 S CDT values of sphalerite and galena samples range from +18.9 to +20.3‰ and +15.6 to +17.1‰, respectively.These values suggest that evaporites are themost probable source of sulfur. The δ 34 S CDT values of symbiotic sphalerite–galena mineral pairs indicate that deposition of sulfides tookplaceunderchemicalequilibriumconditions.CalculatedtemperaturesofSisotopethermodynamicequilibrium fractionation based on sphalerite–galena mineral pairs range from 135 to 292 °C, consistent with previous fluid in- clusion studies. Temperatures above 100 °C preclude derivation of sulfur through bacterial sulfate reduction (BSR) andsuggestthatreducedsulfurinthehydrothermalfluidwasmostlikelysuppliedthroughthermo-chemicalsulfate reduction (TSR). Twelve sphalerite samples have δ 66 Zn values ranging from 0.00 to +0.55‰ (mean +0.25‰) rel- ativetotheJMC3-0749Lzincisotopestandard.StagesItoIIIsphaleritesampleshaveδ 66 Znvaluesrangingfrom0.00 to +0.07‰, +0.12 to +0.23‰, 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 18.362 to 18.573, 15.505 to 15.769 and 38.302 to 39.223, respectively. The Pb isotopic ratios of the studied depositplotinthe fieldthatcovers theuppercrust,orogenic beltandmantlePb evolution curvesandoverlapswith the age-corrected Proterozoic folded basement rocks, Devonian to Lower Permian sedimentary rocks and Middle Permian Emeishan flood basalts in a 207 Pb/ 204 Pb vs. 206 Pb/ 204 Pb diagram. This observation points to the derivation ofPbmetalfrommixedsources.Sphaleritesampleshave 87 Sr/ 86 Sr 200 Ma ratiosrangingfrom0.7107to0.7115similar to the age-corrected Devonian to Lower Permian sedimentary rocks (0.7073 to 0.7111), higher than the age- correctedMiddlePermianbasalts(0.7039to0.7078),andlowerthantheage-correctedProterozoicfoldedbasement (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.