Syndeposition and epigenetic modification of the strata-bound Pb–Zn–Cu deposits associated with carbonate rocks in western Kunlun, Xinjiang, China

Abstract

Lead–zinc–copper (Pb–Zn–Cu) deposits in the western Kunlun Orogen of China are mostly distributed along the Keziletao–Kusilafu fault zones in the Palaeozoic clastic–carbonate rocks, which formed in a late Palaeozoic subsidence belt at the south-west margin of the Tarim platform. Geological and geochemical studies were conducted on these deposits including detailed field observation, petrographic study, and geochemical and isotopic analyses of sulfur, lead, strontium, rhenium–osmium, and argon isotopes, as well as fluid inclusion geochemistry. The characteristics of the deposits include: (1) Except for the Tiekelike Pb–Cu deposit associated with mid-Devonian rocks, all of the other deposits were developed in the lower layers of the early Carboniferous strata. These strata mostly comprise sedimentary dolomitic limestone, sandy limestone, black limestone, and calcareous shale. (2) The ore bodies are stratiform and host veins. The stratiform ores are coeval with host rocks (yielding ore-forming ages of 331–337Ma), while the vein ore bodies are mostly massive deposits formed at 206–235Ma. The isotopic compositions of the sulfur and lead reveal that the ore-forming elements originated from various sources. (3) The mineralization in cements of brecciated ore is a veinlet type, while in the breccias it is a disseminated type. The stratoid, disseminated, and framboidal structures of the ore reveal the synsedimentary ore-formation. However, the veins, mesh-veins, and massive structures indicate late stage hydrothermal mineralization. (4) The principal metallic minerals include sphalerite, galena, and pyrite, while the secondary oxide minerals include variable distributions of cerussite, anglesite and limonite. The gangue minerals include dolomite, calcite, quartz, sericite, and gypsum, with minor chert and barite. (5) The original host rocks likely experienced dolomitization, calcitization, and minor silicification. The ore deposits underwent hydrothermal alterations, sub-greenschist facies metamorphism, and brecciation. The ore deposits may have initially formed via syndeposition in a subsidence belt along the southwest margin of the late Paleozoic Tarim platform; these deposits were further modified during later hydrothermal alteration and orogenesis during the Triassic period.