Geological Setting And Genesis Of The Dongping Gold Deposit,Hebei Province, North China

The Dongping gold deposit, located in Chongli County (Hebei Province) about 200 km 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 ± 3.2 Ma), composed chiefly of highly alkaline syenite and quartz syenite. This study reveals the age of the Carboniferous in the deposit to be 351.7±2.8 Ma (MSWD=1.9). The Dongping deposit is locally hosted in Cretaceous (～143±1 Ma) alkali granites that intruded the older rocks, and the gold mineralization is closely associated genetically with this event. Magmatic evolution in this district is characterized by multi-stage intrusions, which formed most of the composite granitoids bodies and mineralization series. Hydrothermal zircons in the alkali granites have Th/U ratios mostly ranging between 0.01-0.7 indicating oscillatory zoning. A few grains with high Th/U ratios (1.31-2.07) may be from metamorphic domains. Hydrothermal zircons of the first stage at Devonian have been distinguished from those magmatic zircons of the second stage at Cretaceous based on internal textures, CL images and rare earth element patterns. Negative sHf(t) values of the zircon mainly range between-19.75 to -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 2.67 Ga, (mainly 2.2 to 2.3 Ga) suggest that recycled Neoarchean basement was also present in the source region. TDM2 indicate that the magma may have been derived from a very old crustal basement (-2.67 Ga) in the northern margin of North China Craton by partial melting. The Shuiquangou syenite complex was intruded at Neo-Archean Sanggan metamorphic rocks. The Devonian syenites and quartz syenites have TDM1 ages ranging from 1.96 to 2.08 Ga. Zircons from these rocks have eHf(t) values of- 11.9 to-18.9. Certain zircons from the gold-bearing granite of Paleozoic age have an initial 176Hf/177Hf ratios 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 vary from 1.33 to 1.59 Ga and 1.72 to 2.11 Ga, respectively, indicating that the Neoproterozoic to Mesoproterozoic rocks of this area were an important source for the younger magma which was important in forming the ore deposits. TheThe 40Ar/39Ar ages of ore-related of the K-feldspar samples indicates two episodes of gold genesis at 154.89 ± 0.70 Ma and 176.93 ± 4.66 Ma, respectively (Late to middle Jurassic). These ages of mineralization are younger than the intrusion at the Devonian-Carboniferous and close in time to the Mesozoic events. Gold mineralization is hosted mainly by K-feldspar-quartz (stockworks and veins) and disseminated sulfides. The deposit contains three ore types that are distinguished by their mineral associations: quartzose gold, telluride gold, and disseminated gold. Most gold is present in the telluride minerals calaverite (43% Au,38% Ag) and petzite (23% Au,46% Ag). The paragenesis of the ores exhibits three distinct hydrothermal stages, of which the second was the main ore-enrichment stage. The homogenization temperature of most fluid inclusions varies from 240 to 400℃ and even lower (<240 to 120℃). These inclusions in vein quartz are characterized by low salinity (average 6.04- 8.8 wt% NaCl equivalent) and high CO2. The laser Raman spectrum of the inclusions shows the fluid compositions are dominated in H2O and CO2. Hydrogen isotope compositions of fluid inclusions in quartz are (-100.3 to -66.5%o (δD)) and oxygen isotope compositions for the fluids are-0.3 to +6.9%o. These values indicate that the ore-forming fluid came from a deep magmatic hydrothermal system and meteoric water, with involvement of organic water. The sulfur isotopes are mainly negative 34S from -0.3 to- 13.6%o VCDT but a few pyrite grains have positive values ranging between 0.4 to 0.5%o VCDT, showing that the sulfur has a homogenous magmatic source in ore bearing quartz veins under relatively oxidizing conditions of ore-formation. The 87Sr/86Sr ratios of ore-forming fluids at Middle Devonian vary between 0.705-0.706. The 87Sr/86Sr plot versus 8DH2O (SMOW) shows that the fluid inclusions and host granitoids rocks had a mixed magmatic-meteoric water source. The lead isotope ratios plot mainly in the orogene zone between mantle to upper crust. The characteristics of the Dongping gold deposit (i.e., continental margin location, spatial association of gold ore, post-deformational timing of gold mineralization, low grades of base metals, and anomalous association with W, Sn, and Mo), the ore-forming conditions (PTX), low salinity, CO2-rich fluids, and the late-magmatic hydrothermal origin at Late Jurassic-Early Cretaceous process of the deposit, indicate that it belongs to the group of orogenic gold deposits (OGD) rather than an Intrusion related gold system (IRGS).