## 2 Geological Setting
### Geological Setting of Mining Area  
The Mahuagou gold deposit stands out as one of the most prominent gold deposits in the core area of the Huangling Anticline in Yichang, Hubei. This deposit is a medium- to low-temperature quartz vein-alteration/structural type gold deposit. It is governed by  
Figure 2: Geological map of the Huangling Anticline and sample point locations: (**a**) geological map of the Huangling Anticline and some gold deposit locations in the Huangling Anticline (the map was modified according to [PERSON] et al., 2022 [35], [PERSON] et al., 2023 [37], Institute of Geology, Chinese Academy of Geological Sciences, and Seventh Geological Brigade, Hubei Geological Bureau); (**b**) geological map of Mahuagou gold deposit and sample point locations.  
northwest-trending brittle-ductile shear zones and stands out for small scale, high grade, and ease of beneficiation [39]. Since exploration commenced in the 1990s, two major north-northwest-trending gold-bearing structural zones have been identified and labeled as FAul and FAul (as illustrated in Figure 2b) [40]. At a drilling elevation of around 800 m, the No. I and No. II ore-bodies within the FAul zone are notable for their stability, high-grade content, significant thickness, and rapid thinning out [41]. As a result, the deeper gold mineral resources within this mining region show considerable exploration potential; this makes uncovering concealed ore bodies at greater depths a pivotal challenge for future exploration breakthroughs in the Mahuagou mining district.  
The geological characteristics of the Mahuagou mining area are primarily defined by the contact zone between the biotite plagioclase gneiss in the Gucunping Formation (Pt2g) and the biotite monzonitic granite in Zongxifang Unit (\(\gamma\)Pt3\(Zx\)). The Gucunping Formation (Pt2g) is divided into two segments: Gucunping Formation Segment 1 (Pt2g1) and Gucunping Formation Segment 2 (Pt2g2). Both segments have similar lithology, characterized by alternating layers of biotite plagioclase gneiss and plagioclase amphibolite. Within the biotite monzonitic granite (\(\gamma\)Pt3\(Zx\)), there are numerous lens-shaped metamorphic enclaves and bits of early plagioclase granite enclaves. Plagioclase granite enclaves often exhibit mylonitization, and the biotite monzonitic granite locally displays foliation, indicating that the rock body was emplaced under the influence of strong shearing stresses. Within the mining area, various types of veins with diverse morphologies and orientations are exposed, including granite pegmatite veins, diorite porphyrite veins (\(\delta\mu\)), diabase veins (\(\beta\mu\)), and other mafic veins. These veins have been interpreted as the filler of early faults and fracture structures, with prevalent orientations in the nearly east-west and northwest-west orientations. The Mahuagou brittle-ductile shear zone is a significant mineralized structural zone formed during the early Neoproterozoic, situated at the contact zone between biotite monzonitic granite (\(\gamma\)Pt3\(Zx\)) and biotite plagioclase gneiss (Pt2g). During the intrusion of the granite (\(\gamma\)Pt3\(Zx\)), lateral compression and north-northwest orientation right lateral shearing persisted, giving rise to early compressional mineralized structures, with later tectonic activities displaying evident extensional and torsional features. Gold mineralization primarily occurs within the felsic cataclastic rock in the mineralized structural zone, with the occurrence of quartz micro veins and sulfide micro vein infillings.