GENESIS OF DONGPING GOLD-TELLURIDE DEPOSIT BASED ON GEOCHEMICAL CHARACTERISTICS OF FLUIDS, 40Ar/39Ar DATING, STABLE AND RADIOGENIC ISOTOPES (NORTH CHINA)

Abstract

The Dongping goldfield is located within the Shuiquangou alkaline complex of the western Yanshan Mountains of Hebei Province, on the northern margin of the North China Craton. It is one of the largest gold deposits in China, with a planned gold production of 2.57 tons annually over a lifespan of 12 years. The Dongping gold deposit is enriched in the elements Au, Te, Ag, Pb, Bi, Sb and As. Most of the gold is present in the telluride minerals calaverite (43% Au, 38% Ag) and petzite (23% Au, 46% Ag). Gold mineralization is hosted mainly by K-feldspar-quartz stockworks, veins and disseminated sulfides. The deposit contains three ore types that are distinguished by their mineral associations: vein quartz gold, telluride gold, and disseminated gold. The paragenesis of the ores exhibits three distinct hydrothermal stages, of which the second one was the main ore-enrichment stage. The ore-related 40Ar/39Ar ages determined on K-feldspar samples indicate two episodes of gold mineralization, at 154.89 ± 0.70 Ma and 176.93 ± 4.66 Ma. The gold mineralization ages thus postdate the Devonian-age granite intrusion but overlap possible JurassicCretaceous magmatic activity. The homogenization temperatures of the majority of inclusions range from 120 to 240°C and from 240 to 400°C. The inclusions in quartz veins are CO2-rich and characterized by low salinity (average 6.0–8.8 wt% NaCl eq.). The laser Raman spectrum of the inclusions shows that the fluid compositions are dominantly waterrich but also contain CO2. The hydrogen isotope compositions (δ2H) of the fluid inclusions range from ‒100.3 to ‒66.5 ‰, and the calculated oxygen isotope compositions (δ18O) for source fluids range from ‒0.3 to +6.9 ‰ "Standard Mean Ocean Water" (SMOW). These values indicate that the ore-forming fluid came from a deep magmatic hydrothermal system, with involvement of meteoric water and possibly water affected by organic matter. The sulphur isotope compositions (δ34S) of pyrite are mainly from ‒0.3 to ‒13.6 ‰ Vienna Cañon Diablo Troilite (VCDT), suggesting homogeneity of sulphur in the magmatic source with subsequent fractionation under relatively oxidizing conditions in ore-bearing quartz veins. The relationship of δ2HH2O to 87Sr/86Sr indicates that the fluid inclusions and host granitoid rocks were influenced by mixing of magmatic and meteoric waters.

Key words: Chongli county; Dongping deposit; 40Ar/39Ar dating; stable and radiogenic isotopes; ore-forming fluids; gold deposit; North China