The genesis of unilateral mineralization in the main ore body of Jiaodong-type gold deposits: As evidenced by potassic alteration

The formation and spatial distribution of gold deposits on the Jiaodong Peninsula are strictly fault-controlled, with >85% of orebodies concentrated in the footwall of principal fault zones; however, the mechanism producing this pronounced one-sided zoning remains unclear. In this study we selected potassic-alteration zones on both the hanging- and footwall sides of the Jiaojia master fault and, by integrating field mapping, petrographic observation, whole-rock geochemistry and zircon U-Pb geochronology, systematically characterized their alteration styles and implications for mineralization. Zircon U-Pb dating indicates that the protolith of the footwall potassically altered rock is a biotite granite (~163Ma), whereas that of the hanging-wall potassically altered rock is a plagioclase-hornblende rock (~2715Ma). Petrographic and mass-balance analyses indicate that in the footwall potassic alteration is dominated by hydrothermal replacement of primary plagioclase by K-feldspar (with development of striated/lamellar textures in K-feldspar); whole-rock changes are characterized by relative gains in Na2O, Fe2O3 and K2O and losses in MgO and SiO2. By contrast, hanging-wall rocks experienced amphibole → chlorite alteration (chloritization) and show net depletion of MnO, K2O, CaO and Fe2O3, consistent with elemental loss to a migrating fluid. We attribute these contrastive alteration patterns mainly to differences in structure, fluid pathways and rock physico-chemical properties. The footwall Linglong granite hosts an extensive network of secondary faults, joints and fracture-vein systems and displays higher chemical reactivity, favoring pervasive potassic alteration that increases porosity/permeability and establishes favourable physico-chemical conditions for subsequent ore-forming fluid migration and focusing. The hanging-wall plagioclase-hornblende rock, characterized by low porosity, poor permeability, and limited chemical reactivity, underwent only weak potassic alteration, and the associated loss of ore-related elements further inhibited gold mineralization. This study reveals that variations in mineral replacement reactions and Fe enrichment between the hanging- and footwall rocks during potassic alteration account for the asymmetric, footwall-dominated distribution of the Jiaojia gold orebodies. Consequently, rock tensile strength and reactivity (reaction-activation) indices constitute effective exploration parameters for targeting Jiaodong-type gold mineralization.