Geochronology, isotopic chemistry, and gold mineralization of the black slate-hosted Haoyaoerhudong gold deposit, northern North China Craton

Abstract

The Haoyaoerhudong gold deposit is the largest gold deposit in the north margin of the North China Craton gold province, and contains over 7 Moz of gold at an average grade of 0.62 g/t. The deposit is hosted in the carbonaceous and pyritic slate, phyllite, and schist and is controlled by a tight syncline and shear zones. The high-grade orebodies contains abundant pyrite veins and pyrite-quartz veins. Three stages of pyrite have been identified, including the diagenetic disseminated pyrite, pyrite veins caused by peak matamorphism, and pyrite-quartz veins forming during post-peak metamorphism. Native gold has been observed in pyrite veins and pyrite-quartz veins.The 40Ar/39Ar plateau age of a biotite separate from a tails of boudinaged pyrite-quartz veins is 260.1 ± 2.9 Ma. Combined with previously published 40Ar/39Ar mica age data and low closure temperature of mica, these results suggest that pyrite veins and pyrite-quartz veins were formed during the peak, to post-peak metamorphism during 285–260 Ma, respectively. The lower limit of the formation age of sedimentary disseminated pyrites has constrained to 1670–1560 Ma by the intruded mafic–ultramafic dikes.Disseminated pyrite separates have δ34S values ranging from −39.40‰ to + 17.85‰, 206Pb/204Pb of 19.144–21.892, 207Pb/204Pb of 15.681–15.864, and 208Pb/204Pb of 37.502–38.925, suggesting they were formed from seawater sulfate and has experienced strong sulfur isotopic fractionation. In contrast, hydrothermal pyrites from pyrite veins and pyrite-quartz veins have δ34S values ranging from +6.8‰ to +16.47‰, 206Pb/204Pb of 18.566–18.922, 207Pb/204Pb of 15.645–15.684, and 208Pb/204Pb of 38.924–38.983, which may reflect dissolution-reprecipitation of disseminated sulfides from the pre-existing organic-rich sediments. The mineral paragenetic, geometric, and cross-cutting relationships of pyrite veins and pyrite-quartz veins at Haoyaoerhudong suggest that gold was most likely introduced into pyrite, accompanying sedimentation of the organic-rich shales, and then became enriched during diagenesis. Subsequently, the hydrothermal fluids following metamorphism and shear zone activity make dissolution of the gold in the diagenetic pyrite and precipitated in the intersection of shear zone and tight syncline.