U–Pb, 39Ar/40Ar geochronology of the metamorphosed volcanic rocks of the Bainaimiao Group in central Inner Mongolia and its implications for ore genesis and geodynamic setting

Abstract

The greenschist facies volcano-sedimentary sequence of the Bainaimiao Group in central Inner Mongolia hosts the Bainaimiao Cu–Mo–Au deposit and the Bainaimiao lode gold deposit. But the ore classification and genetic process of the Bainaimiao Cu–Mo–Au deposit is still disputed. In this study we have selected the interbedded metamorphosed volcanic rocks of the Bainaimiao Group for LA-ICP-MS zircon U–Pb analysis. 17 analyses form a tight cluster on the concordia and yield a weighted mean 206Pb/238U age of 465±1Ma (95% confidence level, MSWD=0.95, n=17), which represents the best estimate of the crystallization age of the volcanic rocks of the Bainaimiao Group. Biotite from the greenschist of the Bainaimiao Group yielded a well-defined plateau with an age of 429.2±4.1Ma. The inverse isochron age is 429.1±4.2Ma with an elevated (40Ar/36Ar)i ratio of 299.9±22.7, which represents the time of regional greenschist facies metamorphism. Previous studies showed that the Re–Os isochron age of the molybdenite from the Bainaimiao Cu–Mo–Au deposit is 445.0±3.4Ma and the mineralized granodiorite porphyry yielded a weighted mean 206Pb/238U age of 445±6Ma. These data indicate that the Bainaimiao volcanic rocks were formed at about 465Ma and the porphyry Cu–Mo–Au mineralization took place at about 445Ma.Volcanic rock emplacement is 20Ma older than the Cu–Mo–Au mineralization indicates it is a porphyry deposit rather than VMS deposit. The dynamic setting for the porphyry mineralization is the Paleo-Asian Oceanic plate subduction along the northern margin of the North China Craton in early Paleozoic. Subsequently, the collision between the Bainaimiao arc and the North China Craton led to regional metamorphism and deformation at about 430Ma and the old porphyry system may have been overprinted during this metamorphic process.