Strike-slip fault system and fluorite mineralization in the Hongjianbing area, Mazong Mountain, Gansu China

Abstract: The Beishan Orogenic Belt, located along the southern margin of the Central Asian Orogenic Belt, is one of the key mineral resource regions in northwestern China. The Hongjianbing fluorite deposit, located in the northern part of this belt, is a well-known quartz-vein-type fluorite deposit that has attracted considerable attention from researchers. Through field geological mapping, UAV and remote sensing measurements, and borehole structural recording, two east–west-trending strike-slip fault systems were identified in the study area, separated by a distance of 5 km, with nearly vertical dips. These faults exhibit multi-stage activity, with early deformation characterized by dextral strike-slip motion. The intervening blocks experienced ductile deformation, with S-C fabric development in the shear zones.39Ar-40Ar dating of biotite from the mylonite in the ductile shear zone yielded a plateau age of approximately 330 Ma, marking the timing of ductile deformation. Later, these two faults evolved into brittle left-lateral strike-slip faults, forming a Riedel shear system (R, R', T shears), and displaying fault breccia and fault gouge. K–Ar dating of authigenic illite from the fault gouge yielded an age of approximately 220 Ma, indicating a transition from ductile to brittle deformation over time.The host rocks of the fluorite deposit are mainly intermediate to acidic volcanic rocks from the Carboniferous Baishan Formation. Zircon U–Pb dating of these rocks yielded ages of approximately 330 Ma, suggesting that the host rocks may have formed during contemporaneous magmatic activity. All fluorite orebodies are located within the main damage zone of the southern brittle fault system, which exhibits left-lateral, right-stepping characteristics. The development of this brittle fault system provided the necessary space and conduits for ore-forming fluid migration, facilitating fluorite mineralization.The cataclastic texture of the ores further suggests that mineralization occurred after significant faulting, reflecting high fluid mobility within the fault damage zones. This high fluid mobility is also reflected in the characteristics of the fluorite ore bodies. After migrating along the brittle fault zones, mineralizing fluids precipitated fluorite as fine veinlets, which is the current form of mineralization observed in the deposit. A three-dimensional geological model of the ore body was constructed using GOCAD software, revealing the close temporal and spatial relationship between fluorite mineralization and fault activity.Keywords: Structural control of mineralization; Fluorite deposit; Geochronology; Three-dimensional mineralization model