At the Echo Bay mine, vein type U-Ni-Ag-Cu deposits occur along NE-SW-trending fracture zones in ca. 1,800 m.y. old andesitic tuffs. The temperatures of mineralization suggested by the occurrence of native bismuth and mckinstryite, the fluid inclusion filling temperatures, the sulphur isotopic compositions of coexisting sphalerite and galena, and the oxygen isotopic compositions of coexisting quartz and hematite were: around 120 degrees C for the first four stages, about 200 degrees C for stages 5 and 6, and below about 95 degrees C for the last stage. Data on the mineralogy, temperature, and the sulphur and carbon isotopic compositions of the hydrothermal minerals were used to evaluate the chemistry of hydrothermal fluids. The delta O 18H2O and delta S 34 (sub sigma S) values and the initial high oxidation state of the hydrothermal fluids possibly reflect an origin as surface water. A model which involves the circulation and boiling of sea water caused by the intrusion of diabase at about 1,450 m.y. ago is presented to explain the concentration of salts, enrichment of O 18 , reduction of oxidation state, decreasing of pH of the fluids, and leaching of heavy metals from volcanic rocks.