Hydrothermal origin of syndepositional chert bands and nodules in the Mesoproterozoic Wumishan Formation: Implications for the evolution of Mesoproterozoic cratonic basin, North China

Abundant chert bands and nodules are discovered throughout the Mesoproterozoic Wumishan Formation (∼1.5 Ga) in the North China Block. The middle part of the Wumishan Formation consists of cyclic deposition of calcareous shale, massive microbial dolostone (thrombolites and stromatolites), and laminated dolostone. Chert bands occur most abundantly in the laminated dolostone, although they are also found with lower abundance in other lithologies. Chert bands are normally parallel to the bedding plane, and show soft-deformation structures, suggesting early silicification at or near the water-sediment interface. The Wumishan chert is composed of two types of silica fabric: the microcrystalline and cryptocrystalline quartz that derives from the replacement of host dolomitic rock, and the mosaic/chalcedonic quartz, representing typical void-filling precipitation. We measured the germanium/silica ratios (Ge/Si) and the REE compositions of the chert bands/nodules collected from a 60 m interval in the middle part of the Wumishan Formation in the Mopanyu section. Ge/Si ratios of the buck samples, measured by ICP-MS, range from 13.6 to 35.6 μmol/mol. Additional LA-ICP-MS analyses allow the measurements of individual silica fabric, providing more detailed information about the geochemistry of chert nodules and bands. Ge/Si ratios of the replacive chert vary between ∼10 and ∼20 μmol/mol, while the mosaic/chalcedonic quartz has wider range of variation (1.1-93.2 μmol/mol). These values are>1 order of magnitude higher than the modern seawater value (0.72 μmol/mol), but is within the range of hydrothermal fluids (3.4-51.3 μmol/mol, average = ∼13 μmol/mol, n  =  101), suggesting hydrothermal fluid as an important Si source. This conclusion is consistent with the positive Eu anomaly in the REE pattern. A mass balance calculation indicates that hydrothermal fluid would account for 10-15%, 20-25%, 40-55%, and >75% of Si for the fluid temperature of 100 °C, 150 °C, 200 °C, and 250 °C, respectively. Although large igneous provinces are not discovered in the North China Block during the deposition of the Wumishan Formation, the studied Wumishan cherts have significant contribution from hydrothermal fluid, which might be related to the early rifting activities in the North China Block before the breakup of supercontinent Nuna.