## 2 Study Area and Data  
### Geology Setting  
The eastern Hebei area is situated in the northeastern part of the Jizhong-Jidong micro-block, which lies in the central segment of the northern margin of the North China Craton (Figure 1a) [38]. After undergoing a series of geological evolution stages, the area developed several large-scale Banded Iron Formation (BIF)-type iron ore deposits,including the Sijiaying, Macheng, Shuichang, Shiregou, Xingshan, Zuolanzhangzi, and Malanzhuang deposits [39]. This area has an estimated iron ore reserve of approximately 6.3 billion tons, and it still holds significant potential for future exploration [40]. The Yanshan iron deposit, located in the southern part of Hebei Province, China, and covering an area of approximately 4 km\({}^{2}\), is situated to the north of the N18 exploration line, which separates it from the super-large Sijiaying BIF-type iron ore deposit (Figure 1b). The Yanshan iron deposit, an Algoroma-type Banded Iron Formation (BIF) deposit, experienced mineralization during the late Neoarchean. The initial source of the ore minerals was the mixing of high-temperature submarine hydrothermal fluids and seawater. Subsequently, it underwent primary sedimentation, metamorphism, multiple phases of tectonic activity, and hydrothermal alteration, ultimately resulting in the accumulation of iron-rich ores [41]. The basement stratum of the Yanshan iron deposit consists of the Neoarchean Lu-anxian Group, while the overlying strata of this deposit include the Mesoproterozoicoic to Neoproterozoic Changcheng, Jixian, and Qingbaikou systems, followed by the Cambrian, Ordovician, Carboniferous, Permian, and Quaternary systems. The Luanxian Group serves as the primary ore-hosting formation, reaching low-amphibolite facies metamorphism. The main lithologies are biotite granulite, magnetite (or hematite) quartzite, and plagioclase amphibolite [42]. After undergoing two phases of folding, the basement structure of the area is now composed of the nearly N-S trending Yangshan anticlinorium and Sima synclinorium, extending from east to west. Among them, the Sima synclinorium is larger in scale and is further divided into five subordinate folds: the Macheng synchronium, Xinxian anticlinorium, Sijiaying synclinorium, Damazhuang anticlinorium, and Gaoguanying-Lixiazhuang synclinorium. These folds control the morphology, spatial distribution, scale, and occurrence of the major sedimentary-metamorphic iron ore deposits  
Figure 1: (**a**) Tectonic subdivision of the North China C ration and location of the study area in the southern margin of the C ration; (**b**) geological map of the Sijiaying BIF iron deposit; (**c**) geological A–B cross-section of the Yanshan BIF. in the surrounding area. The ore body of the Yanshan iron deposit is thickened near the core of the fold [43]. The fault structures in the mining area are primarily a set of northeast to north-trending faults that run nearly parallel to the ore body, exhibiting a wave-like surface pattern. This compressive-shear reverse fault dips southeastward along its strike, crossing the ore body from south to north, leading to displacement along the fault and causing shifts in the position and morphology of the ore body. Magmatic activity in the area is minimal, with only small-scale pegmatite veins and segmental metamorphosed gabbro-diabase veins observed [44]. The main rock type of the Yanshan iron deposit is magnetite quartzite, while the primary ore type in the surface oxidized zone is hematite quartzite [45]. Based on geological characteristics, the high-grade iron ore in the mining area can be roughly divided into two categories [46; 47]. The first type of high-grade iron ore is formed by primary sedimentation, with limited exposure, mainly found in the lower part of thick, low-grade ore bodies. The second type of high-grade iron ore is formed through hydrothermal alteration of low-grade ores.