The Arctic volcanogenic massive sulfide prospect, located in the Ambler mineral district of northwestern Alaska, includes three types of hydrothermally altered rocks overlying, underlying, and interlayered with semimassive sulfide mineralization. Hydrothermal alteration of wall rocks and deposition of sulfide and gangue minerals were contemporaneous with Late Devonian or Early Mississippian basalt-rhyolite volcanism. Whole-rock analyses of altered rocks surrounding the prospect indicate that strong chemical gradients exist in major and minor oxides and volatiles. These were not homogenized or overprinted by Late Jurassic (?) to Middle Cretaceous greenschist to glaucophane-schist facies metamorphism. Very low alkali contents and moderate to very high contents of Ba, F, and Mg are the major chemical characteristics of the pervasively altered rocks. The strong Mg metasomatism, high Ba contents, and limited extent of altered rocks suggest a rapid influx of relatively cold seawater into a hot hydrothermal vent system. Alteration developed asymmetrically around a linear fissure, suggesting fracture control of ore fluids rather than a point source. Micro-probe analyses of phyllosilicates from the Arctic area indicate two discrete mineral populations. Magnesian chlorite, barian white mica, and barian fiuorphlogopite in hydrothermally altered rocks have compositions distinct from similar minerals (chlorite, phengite, biotite) developed during high-pressure metamorphism in metapelitic and metavolcanic lithologies. These differences in mineral chemistry are the result of differences in protolith composition caused by hydrothermal alteration-metasomatism. Without mineral composition data, the contacts between some alteration assemblages and relatively unaltered metavolcanic and metasedimentary rocks with similar mineralogy cannot be determined.