The δ18O values in syntectonic gold-bearing quartz veins hosted by schist, mylonite, gneiss, and migmatite from the Altay accretionary prism, Xinjiang province, northwest China, range from 3.4‰–17.8‰ along a 20 km traverse. Plots of δ18O values in veins hosted by schist across several thrust sheets display a sawtooth pattern in which isotopic values decrease with increasing metamorphic grade across each thrust sheet and show abrupt increases of 5.3‰ to 6.8‰ across faults in the hanging wall. Models in which the host schist or clays buffer the isotopic composition of the veins provide a poor fit to the data. On the other hand, a fluid-buffered model with δ18Ofluid = 0 to +2‰ (standard mean ocean water, SMOW) provides a good fit to the data over a 5–10 km depth interval (150–300 °C). Posttectonic normal reactivation of the thrusts with offsets averaging 5 km explains the abrupt increase in δ18O across the faults in the hanging wall. A model in which seawater is tectonically ingested into the accretionary prism to a depth of 5 to 10 km, followed by hydraulic fracturing and advection of hot fluid back to the sea floor along high-permeability pathways, is presented.