Hydrothermal mineral geochemistry of the Qianchen gold deposit and its implications, Jiaodong Peninsula

The Qianchen gold deposit, situated in the southern segment of the Jiaojia-Xincheng ore-forming belt in the Jiaodong Peninsula, exhibits mineralization predominantly at depths between -920m and -1920m, rendering much difficulty for its exploration. Chlorite and sericite are hydrothermal minerals that are sensitive to the change of physicochemical conditions during fluid flow. They effectively record mineralization processes, and therefore it is of a great significance to identify mineralized centers. Based on systematic drill-core logging and alteration zonation studies, this work conducted mineralogical and electron microprobe composition analyses of chlorite and sericite from alteration zones at different elevations. Field observations reveal a distinct alteration zonation from the orebody outward, characterized successively by pyrite-sericite-quartz alteration zone, sericite-quartz alteration zone, and K-feldspar alteration zone. Petrographic observations show that chlorite and sericite in the ore occur mainly as anhedral fine scales with no relict precursor minerals, closely associated with pyrite, suggesting direct precipitation or complete replacement origin. In contrast, those in the wall rocks often preserve pseudomorphs after feldspar or biotite, indicative of formation predominantly via dissolution-crystallization processes. Geochemical analyses reveal that composition of chlorite shifts from Fe-rich brunsvigite in the orebody toward Mg-rich clinochlore in the wall rocks, with Fe/(Fe+Mg) ratios decreasing from 0.62 to 0.48. Ion substitution in sericite shifts from Tschermak substitution toward Fe3+VI=Al3+VI substitution, reflecting an increase in Fe3+ within the crystal lattice. These compositional variations jointly indicate a low-oxygen-fugacity environment during mineralization. The ore-forming fluid of the Qianchen deposit belongs to a medium-low temperature, low-salinity H2O-NaCl-CO2±CH4 system, in which gold was transported primarily as Au(HS)2- complexes. Thermodynamic modeling suggests that a decrease in oxygen fugacity destabilizes Au(HS)2- complexes, triggering gold precipitation. Combined with consumption of reduced sulfur due to pyrite precipitation, this study proposes that gold deposition at the Qianchen deposit was driven by both decreasing oxygen fugacity and sulfidation. The data presented here demonstrate that chlorite Al, Si, and Mg contents along with Fe/(Fe+Mg) ratios, as well as sericite Mg/Fe ratios and Tschermak-dominated substitution mechanisms, can serve as potential geochemical indicators to distinguish mineralized from barren rocks. These findings offer valuable insights for targeting deep-seated and peripheral concealed orebodies in the Qianchen gold deposit.