The Antamok and Acupan gold vein systems of the Baguio district have produced approximately 3 X 10 5 kg of gold and represent one of the major gold-producing complexes of the circum-Pacific belt. The deposits are of late Tertiary age and may have formed during the Pleistocene. At the present time there is considerable geothermal activity in the area and investigations of these thermal waters form an integral part of this study.The intrusive, volcanic, and sedimentary rocks that host the veins are typical of island-arc assemblages. The veins average about 1 m in thickness and consist mostly of quartz with subordinate pyrite. Significant tonnages of ore are also found in breccias adjacent to a fragmental volcanic plug in the Acupan mine. The Baguio erosion surface, now deeply dissected, appears to have been the extant surface at the time of mineralization and the veins tend to die out upwards as they approach it. Alteration assemblages are dominated by chlorite.Fluid inclusion studies on quartz indicate deposition throughout the vein systems in the range 250 degrees to 300 degrees C, by low salinity fluids (0-6 equiv. wt % NaCl). Gas analyses of fluid inclusions demonstrate that CO 2 , CO, H 2 , NO, H 2 S, SO 2 , and CH 4 were present in the fluids but that the combined total of these gases was always less than 1 mole percent relative to H 2 O.Chemical and isotopic analyses of the modern geothermal waters and isotopic analyses of vein and host-rock material suggest considerable similarities between the ore fluids and present thermal waters. The delta D values of fluid inclusion waters range from --72 to --82 versus --58 to --75 for the modern waters. The delta 18 O values of vein quartz (3.2-6.1) are light, given the temperature of quartz deposition. Complex partial mixing patterns of the modern geothermal waters are suggested by the isotopic data, and similar conditions probably prevailed during the earlier gold mineralization.The source of the gold in the vein systems could be either the country rocks, older low-grade gold-copper mineralization within them, or contributions from magmas at depth. The gold/silver ratios in the ores, however, are much higher ( approximately 1:1) than they are in the host rocks ( approximately 1:130).