Formation of Pb-Zn deposits in the Sichuan-Yunnan-Guizhou triangle linked to the Youjiang foreland basin: Evidence from Rb-Sr age and in situ sulfur isotope analysis of the Maoping Pb-Zn deposit in northeastern Yunnan Province, southeast China

摘要

The Maoping Pb-Zn deposit is located in the northeastern Yunnan province of the Sichuan-Yunnan-Guizhou (SYG) MVT lead-zinc district at the southwestern margin of the Yangtze Craton, with total Zn + Pb metal reserves of > 3 Mt (average grade of Zn + Pb > 20%). The orebodies, hosted in the dolostone of the Devonian Zaige Formation, the Carboniferous Baizuo and the Weining Formations, are mainly vein and massive ores composed of pyrite, sphalerite, galena, quartz, and calcite. To constrain the metallogenic age and the source of the reduced sulfur of the Maoping lead-zinc deposit, direct Rb-Sr chronometry of sphalerite and in situ sulfur isotope analysis of sulfides of all paragenetic stages was studied. A sphalerite Rb-Sr age of 202.5 +/- 8.5 Ma for sulfide mineralization obtained from a quantitative geochronological two-component paleomixing model coincides with a well-constrained episode of the evolution of the Youjiang foreland basin from the Middle Triassic to Early Jurassic. In situ sulfur isotope analysis shows that sulfur isotope values are distributed between 14.7 and 23.2 parts per thousand, with a maximum difference of 8.5 parts per thousand. However, the delta S-34 values of pyrite, sphalerite, and galena are closely clustered and vary from 20.9 to 23.2 parts per thousand, 19.3 to 21.0 parts per thousand, and 14.7 to 17.2 parts per thousand, respectively. The relation of the delta S-34(pyrite) > delta S-34(sphalerlte) > delta S-34(galena) in all samples indicates that sulfur isotope equilibrium fractionation between coexisting pyrite, sphalerite, and galena may have been reached at temperatures of approximately 120-250 degrees C. From this, the total delta S-34 of the ore-forming fluids was calculated to be 19.7 to 22.0 parts per thousand, which is interpreted as the result of thermochemical reduction of aqueous sulfate in the Youjiang Basin basinal brine. Based on these results and previously reported metallogenic ages, the features of the ore-controlling structures, ore-body features, and the characteristics of the ore-forming fluids of other typical lead-zinc deposits in the SYG triangle, the Zn-Pb deposits located within the SYG triangle are genetically related to migration of basinal brines caused by the contraction and then closure of the Youjiang foreland basin in the Middle Triassic to Early Jurassic.