<p>Although recent research has proposed models for enrichment of gold in some mineral deposits, current hypotheses fail to explain exceptionally high-grade mineralization in reduced intrusion-related gold deposits. This study proposes the mechanisms that enriched gold to unusually high levels in the Ludousou deposit. The ores formed in two stages, a low-grade Stage 1 (&lt; 3&#xa0;g/t Au) and a high-grade Stage 2 (avg. 30&#xa0;g/t Au). In Stage 1, pyrite (Py1) and arsenopyrite (Apy1) host sub micro-meter scale Au with median concentrations of 0.01 and 0.1&#xa0;ppm, respectively, with negative median δ³⁴S values of -4.8 and -5.3 ‰, respectively. Fluid inclusion in quartz (Qz1) indicate phase separation and ores formed at ~ 240&#xa0;°C and 0.5 kbar, from a fluid with a median salinity of 22.9 wt.% NaCl equiv. The median δ¹⁸O_Qz1 is 19.9 ‰. In Stage 2, Py2 and Apy2 contain median concentrations of 1.4 and 1,492&#xa0;ppm Au, and median δ³⁴S values of 4.2 and 4.9 ‰, respectively. Microthermometric data of Qz2 show ores formed at 0.3 kbar, ~ 200&#xa0;°C from a fluid with a median salinity of 7.6 wt.% NaCl equiv. The median δ¹⁸O_Qz2 is 13.7 ‰. Phase separation in Stage 1 promotes low-grade gold formation. Decreases in salinity, temperature, δ¹⁸O, and an increase in δ³⁴S from Stage 1 to Stage 2 suggest high-grade mineralization was produced by mixing ore fluid with seawater. This triggered a decrease in both salinity and temperature, as well as an increase in <i>f</i>O₂, consequently decreasing gold solubility. In addition, seawater introduced cationic flocculants, which promoted the aggregation of gold and thereby enhanced the efficiency of gold precipitation. This study demonstrates that mixing ore fluids with seawater was critical to achieving exceptionally high-grade enrichment.</p>