Isotopic geochronology of the late Paleozoic Kanggur gold deposit of East Tianshan mountains, Xinjiang, NW China

Abstract

The Kanggur gold deposit lies in East Tianshan mountains, eastern section of Central Asia orogenic belt. The gold mineralization occurs on the northern margin of the Aqishan-Yamansu Paleozoic island arc in the Tarim Plate. It was hosted mainly in Middle-Lower Carboniferous calc-alkaline volcanic rocks, and controlled by the distributions of syn-tectonic intrusions and ductile shear zones. In order to determine ore-forming age of the Kanggur deposit, samples were collected from ores, wall rocks, altered rocks and intrusions. The dating methods include Rb-Sr isochron and Sm-Nd isochron, and secondly 40Ar/39Ar age spectrum, U-Pb and Pb-Pb methods. Based on the mineral assemblage and crosscutting relationship of ore veins, five mineralization stages are identified. This result is confirmed by isotope geochronologic data. The first stage featuring formation of pyrite-bearing phyllic rock, is mineralogically represented by pyrite, sericite and quartz with poor native gold. The Rb-Sr isochron age of this stage is 290±5 Ma. The second stage represents the main oreforming stage and is characterized by native gold-quartz-pyrite-magnetite-chlorite assemblage. Magnetite and pyrite of this stage are dated by Sm-Nd isochron at 290.4±7.2 Ma and fluid inclusion in quartz is dated by Rb-Sr isochron at 282.3±5 Ma. The third mineralization stage features native gold-quartz-pyrite vein. In the fourth stage, Au-bearing polymetallic sulfide-quartz veins formed. Fluid inclusions in quartz are dated by Rb-Sr isochron method at 258±21 Ma. The fifth stage is composed of sulfide-free quartz-carbonate veins with Rb-Sr age of 254±7 Ma. The first and second stages are related to ductile-brittle deformation of shear zones, and are named dynamo-metamorphic hydrothermal period. The third to fifth stages related to intrusive processes of tonalite and brittle fracturing of the shear zones, are called magmato-hydrothermal mineralization period. The Rb-Sr isochron age of 290±5 Ma of the altered andesite in the Kanggur mine area may reflect timing of regional ductile shear zone. The Rb-Sr isochron age of 282±16 Ma of the quartz-syenite porphyry and the zircon U-Pb age of 275±7 Ma of tonalite in the north of Kanggur gold mine area are consistent with the age of gold mineralization (290-254 Ma). This correspondence indicates that the tonalité and subvolcanic rocks may have been related to gold mineralization. The Rb-Sr, Sm-Nd and U-Pb ages and regional geology support the hypothesis that the Kanggur gold deposit was formed during collisional orogenesis process in Late Variscan.