Sedimentary Fe deposits are both scientifically and economically important. As a major ore mineral of these deposits, siderite is generally assumed to have been formed via diagenetic transformation of other Fe-bearing minerals. The Devonian Daxigou sedimentary siderite deposit, Central China, contains ca. 500 Mt Fe with an average ore grade of ca. 30 wt% FeO^T but is poorly known in the literature. Different from most sedimentary Fe deposits that contain multiple generations of Fe-bearing minerals, the ore mineral in this deposit is solely siderite, and thus may provide valuable information about the processes of siderite mineralization. Stratiform orebodies of the Daxigou deposit are hosted in a turbidite sequence formed in the Devonian Zhashui-Shanyang intraplate rift basin. Orebodies are composed of interbedded ore and mudstone layers. The ore mineral is siderite and gangue minerals are quartz and clay minerals (mainly muscovite and illite). Siderite has shale-normalized REE+Y patterns with positive Eu anomalies (Eu/Eu*_PAAS = 1.19–1.59) and low Y/Ho ratios (Y/Ho = 27.5–32.6) indicative of involvement of seafloor hydrothermal fluids. Siderite separates have εNd_(t) values from − 9.9 to -8.9, suggesting that Fe was leached from underlying clastic rocks. Siderite has δ¹³C_PDB values from − 3.45 to -1.09‰ and δ⁵⁶Fe_IRMM014 values from − 0.72‰ to -0.27‰, with only limited fractionations relative to dissolved inorganic carbon in seawaters and to hydrothermally derived Fe²⁺. High resolution transmission electron microscopic images reveal that siderite grains were nucleated on the surface of clay minerals. Thus, we conclude that siderite of the Daxigou deposit was precipitated directly from ferruginous seawaters via heterogeneous nucleation on clay minerals at elevated temperatures, instead of formation through diagenetic transformation from other Fe-bearing minerals. The Daxigou deposit can be considered as a unique primary sedimentary siderite deposit. It was formed under an extensional regime of the South China Craton during the breakup of Gondwana. Our study provides new insights about the mineralization pathways of sedimentary Fe deposits in the geological past.

沉积铁矿床在科学和经济上都很重要。作为这些矿床的主要矿石矿物，菱铁矿通常被认为是通过其他含铁矿物的成岩转变形成的。位于华中地区的泥盆纪大溪沟沉积菱铁矿床含有约 500 Mt Fe，矿石平均品位约为 30 wt% FeO^T，但在文献中知之甚少。与大多数包含多代含铁矿物的沉积铁矿床不同，该矿床中的矿石矿物完全是菱铁矿，因此可能提供有关菱铁矿矿化过程的宝贵信息。Daxigou 矿床的层状矿体赋存于泥盆纪 Zhashui-Shanyang 板内裂谷盆地形成的浊积岩层序中。矿体由互层矿石和泥岩层组成。矿石矿物是菱铁矿，脉石矿物是石英和粘土矿物（主要是白云母和伊利石）。菱铁矿具有页岩归一化的 REE+Y 模式，具有正 Eu 异常 （Eu/Eu*_PAAS = 1.19–1.59） 和低 Y/Ho 比值 （Y/Ho = 27.5–32.6），表明涉及海底热液流体。菱铁矿分离体的 εNd_（t） 值从 -9.9 到 -8.9，表明 Fe 是从下面的碎屑岩中浸出的。菱铁矿的¹³C_PDB 值δ − 3.45 至 -1.09‰，δ⁵⁶Fe_IRMM014值从 − 0.72‰ 到 -0.27‰，相对于海水中溶解的无机碳和热液衍生的 Fe²⁺，馏分有限。高分辨率透射电子显微镜图像显示，菱铁矿颗粒在粘土矿物表面成核。 因此，我们得出结论，大溪沟矿床的菱铁矿是在高温下通过粘土矿物上的非均相成核直接从含铁海水中沉淀出来的，而不是通过其他含铁矿物的成岩转变形成的。Daxigou 矿床可以被认为是一个独特的原生沉积菱铁矿矿床。它是在冈瓦纳解体期间在南华克拉通的延伸制度下形成的。我们的研究为地质历史中沉积铁矿床的矿化途径提供了新的见解。