Genesis of Early Cambrian cherts of the Yurtus Formation in northwestern Tarim Basin

This study employed petrological, elemental and silicon isotopic geochemical studies of the cherts from the lowest Cambrian Yurtus Formation in the northwestern margin of the Tarim Basin. The genesis and sedimentary model of the cherts were discussed. Petrological observations indicate that the cherts in the middle Yurtus Formation are mostly composed of cryptocrystalline and microcrystalline quartz without clastic quartz, which excludes the influence of terrigenous input on the chert deposition. The cherts in the lower Yurtus Formation are relatively Fe-rich and have a significantly positive Eu anomaly, which suggests that the cherts were likely originated from submarine hydrothermal fluids. In contrast, the middle Yurtus Formation cherts show high Y/Ho ratio, moderate negative Ce anomaly, LREE depletion, and positive Y anomaly, indicating that the cherts were mainly sourced from seawater. This is also consistent with their δ³⁰Si values that bias to hot water sources. In addition, the redox-sensitive element proxies indicate that the sedimentary environment of the cherts of the Yurtus Formation was probably oxidized for the surface seawater, while is anoxic or even euxinic for the bottom seawater. This study proposes that a sedimentary model for the cherts of the Yurtus Formation as follows: the cherts in the lower section were directly sourced from large amounts of silica-rich hydrothermal fluids under the influence of upwelling. Subsequently, the hydrothermal activities weakened and the redox-stratified ocean facilitated the coupled cycling of dissolved silicon and iron in seawater, and the silicon was co-deposited in the form of Fe³⁺-Si colloidal. During the dominance of the euxinic zone, the iron and silicon cycle was decoupled, leading to the deposition of black shale and the formation of interbedded cherts and black shales in the middle section.