The Urucum area of Brazil hosts a series of Cryogenian ironstones intercalated by oxide-dominated manganese layers. The Urucum iron and manganese formations (IF-MnF) are among the largest sedimentary iron and associated manganese deposits of the Neoproterozoic, however, the depositional model and the source of metals for the IF-MnF in this area are highly controversial. In this study, we performed systematic Fe isotope analysis on fresh and geochemically characterized drill core samples of the Urucum iron and manganese formation deposited in the center of the ancient Urucum graben system. The samples have a large variation in Fe isotope composition, with a δ⁵⁶Fe range of −2.04‰ to +0.75‰, and exhibit a general trend of decreasing δ⁵⁶Fe values with increasing manganese contents. The low δ⁵⁶Fe values of the IF and MnF samples reflect Rayleigh fractionation processes of contineous partial oxidation of aqueous Fe(II) prior to deposition at the sampling site. Using a mixing model and previously published Nd isotope data on the same samples, we estimated that benthic (i.e., porewaters released from submarine sediments in the Urucum basin) Fe fluxes provided 7–50% of total Fe in the Urucum IF-MnF, and the rest of Fe source was from low-temperature hydrothermal vents. Based on combined Fe and Nd isotope data of the Urucum IF-MnF, we propose that low-temperature hydrothermal fluids and benthic fluxes of pore waters were mixed and transported by an upwelling current. The fluid subsequently experienced partial oxidation during the transportation process and became enriched in light Fe isotopes. In the Urucum graben basin, the iron- and manganese-rich oxides deposition occurred progressively under increasingly oxidizing conditions, and such process could have operated repeatedly to produce the alternation of iron and manganese formations. The chemical sediments of the Urucum IF-MnF deposits thus reflect the existence of a sharp redox gradient in the marine environment during the late Cryogenian period.

巴西的 Urucum 地区拥有一系列 Cryogenian 铁岩，中间夹有氧化物为主的锰层。Urucum 铁锰地层 (IF-MnF) 是新元古代最大的沉积铁和伴生锰矿床之一，然而，该地区 IF-MnF 的沉积模式和金属来源存在很大争议。在这项研究中，我们对沉积在古 Urucum 地堑系统中心的 Urucum 铁和锰地层的新鲜和地球化学特征钻芯样品进行了系统的 Fe 同位素分析。样品Fe同位素组成变化较大，δ ⁵⁶ Fe范围为-2.04‰～+0.75‰，总体呈δ ⁵⁶下降趋势Fe 值随着锰含量的增加而增加。低δ ⁵⁶IF 和 MnF 样品的 Fe 值反映了在采样点沉积之前连续部分氧化 Fe(II) 水溶液的瑞利分馏过程。使用混合模型和先前发布的相同样品的 Nd 同位素数据，我们估计底栖（即乌鲁库姆盆地海底沉积物释放的孔隙水）铁通量提供了乌鲁库姆 IF-MnF 中总 Fe 的 7-50%，并且其余铁源来自低温热液喷口。基于 Urucum IF-MnF 的 Fe 和 Nd 同位素组合数据，我们提出低温热液流体和孔隙水的底栖通量通过上升流混合和输送。该流体随后在运输过程中经历了部分氧化，并富含轻铁同位素。在乌鲁库姆地堑盆地，富含铁和锰的氧化物沉积在日益氧化的条件下逐渐发生，这种过程可以反复运行以产生铁和锰的交替。因此，Urucum IF-MnF 沉积物的化学沉积物反映了低温纪晚期海洋环境中存在急剧的氧化还原梯度。