Genesis of High-Grade Gold Mineralization at the Guocheng Deposit, Jiaodong Peninsula: Constraints from Magnetite Geochemistry

The processes responsible for high-grade disseminated gold mineralization remain poorly constrained, hindering effective exploration. This study integrates petrography, BPMA, and LA-ICP-MS analysis of magnetite from marble- and granite-hosted ores with contrasting gold grades, to constrain wall-rock-induced changes in the thermodynamic environment. BPMA results show distinct mineral assemblages: granite-hosted ores are characterized by quartz (52.31%)-K-feldspar (19.65%)-sericite (9.56%)-pyrite (8.36%), whereas marble-hosted ores feature pyrrhotite (33.90%)-chlorite (27.50%)-pyrite (15.22%)-magnetite (1.94%). The closed intergrowths of magnetite with gold and sulfides, along with the magnetite Ga-V (Grant-Vaughan) discrimination diagram, indicate a hydrothermal origin for magnetite formed during the mineralization stage. Geochemical data show that marble-hosted magnetite has lower V and chalcophile element (Co, Ni, Sn, Zn) concentrations than granite-hosted magnetite. Considering the partitioning behavior of these elements in magnetite, these differences indicate magnetite crystallization under increasing oxygen fugacity (fO2) and decreasing sulfur fugacity (fS2). Thermodynamic modeling results demonstrate that these changes in fO2 and fS2 destabilized gold-sulfur complexes in the ore-forming fluid, significantly enhancing gold precipitation efficiency and ultimately leading to the formation of high-grade ores in marble.