Geochronology and geochemical characteristics of the Dongping ore-bearing granite, North China: Sources and implications for its tectonic setting

Abstract

The Dongping gold deposit, located in Chongli County (Hebei Province) about 200km northwest of Beijing, is one of the largest gold-producing areas along the northern margin of the North China Craton. It is located in the of Shuiquangou alkaline igneous complex of Middle Devonian age (394.3±3.2Ma), composed chiefly of highly alkaline syentite and quartz syenites. This study reveals the age of the Carboniferous in the deposit at 351.7±2.8Ma (MSWD=1.9). The Dongping deposit is locally hosted in Cretaceous (~143±1Ma) alkali granites that intruded the older and the gold mineralization is closely associated genetically with this event. Hydrothermal zircons in the alkali granites have Th/U ratios mostly ranging between 0.01 and 0.7 indicating oscillatory zoning. A few grains with high Th/U ratios (1.31–2.07) may be from metamorphic domains. Negative εHf(t) values of the zircon mainly range between −19.75 and −16.93, suggesting that they originated principally by the melting of recycled continental crust. Less abundant zircons with εHf(t) ranging from −25.76 to −23.46, with Hf model ages (TDM2) of 2.54 to 2.67Ga, (mainly 2.2 to 2.3Ga) suggest that recycled Neoarchean basement was also present in the source region. The Devonian syenites and quartz syenites have TDM1 ages ranging from 1.96 to 2.08Ga. Zircons from these rocks have εHf(t) values of −11.9 to −18.9. Certain zircons from the gold-bearing granite of Paleozoic age have an initial 176Hf/177Hf ratio of 0.281816 to 0.282058 and 0.282147 to 0.282348, reflecting a homogenous distribution of hafnium isotopes typical of magmatic sources. The TDM1 and TDM2 of the latest intrusion varying 1.33 to 1.59Ga and 1.72 to 2.11Ga respectively, indicating that the Neoproterozoic to Mesoproterozoic rocks of this area are an important source for the younger magma which are important to forming ore deposits. The TDM2 indicate that the magma may be derived from a very old crustal basement (~2.67Ga) in the northern margin of North China Craton by partial melting.