Based on fluid inclusion and oxygen isotope data, this paper holds that high18O ore-forming fluids were formed by mixture of magmatic fluid and evolved seawater. High salinity [w(NaCleq)=5.7%~21.3%] and rich gas indicate the contribution of magmatic fluid to oreforming fluids. Background felsic volcanics at Gacun are extremely enriched in18O compared to isotopically normal felsic rocks (14.2‰~17.5‰vs 6‰~10‰).This has been interpreted in terms oy pervasive and widespread exchange with marine water under the condition of low temperature and a high water/rock ratio, which supports the mineralogical evidence of intense hydrothermal alteration. Wholerockδ18O values of stockwork samples from Gacun orebodies are relatively uniform (10.5‰~13.1‰), showing depletion of18O relative to the other Gacun volcanics less intensely affected by the ore-forming solutions. These relations signify that the ore-forming process postdated the early hydrothermal event that enriched the Gacun volcanics with18O. The presentδ18O values of 10.5‰~13.1‰for the stockwork rocks are therefore believed to have been reached through two successilve stages of hydrothermal alteration: early18O enrichment to values near 16.2‰in the regional,pervasive and low temperature event, followed by18 O loss to around 10.5‰~13.1‰induced by evolved marine water in stockwork vents.