Regionally extensive semiconformable zones of silicified, Fe-Mg metasomatized and epidotized, dominantly mafic, volcaniclastic strata and lava flows are exposed 1 to 2 km stratigraphically beneath the Chisel Lake Zn-Cu massive sulfide deposit, Snow Lake district, Manitoba. The alteration zones occur within a Lower Proterozoic medium-grade regional metamorphic terrane of felsic and mafic volcanics and marine sediments, and lie between subvolcanic tonalite sills and the massive sulfide deposit.Semiconformable alteration occurs at two main stratigraphic positions. The lower zone of silicification in pillowed and massive mafic lavas is inferred to have occurred close to the sea floor and probably was not related directly to formation of the Chisel Lake massive sulfide deposit. The stratigraphically higher zone, which may be spatially and temporally associated with massive sulfide deposition, occurs in a approximately 300-m-thick heterolithic mafic volcanic breccia and wacke unit. This volcaniclastic-hosted alteration is zoned laterally from dominantly silicification and epidotization to mainly Fe-Mg metasomatized, garnet-chlorite + or - biotite + or - staurolite rocks nearer the Chisel Lake sulfide deposit. A discordant footwall Fe-Mg alteration zone directly beneath the sulfide deposit extends toward, and may meet, the semiconformable Fe-Mg metasomatized zone.Silicification contributed to partial to complete replacement of volcanic clasts, beds in the volcaniclastic unit, rocks adjacent to some felsic dikes and pillow interiors by quartz and sodic plagioclase. Mass balance calculations for silicified rocks and equivalent least altered parts of volcaniclastic beds, dikes, and pillows indicate that SiO 2 increased by up to 50 percent of its initial value, and Na 2 O by up to 30 percent. Up to 80 percent of the FeO, MgO, CaO, and Zn was removed during silicification. Elemental fluxes during Fe-Mg metasomatism are generally opposite those characterizing silicification and are of comparable magnitude. Epidotization resulted in depletion of Na, total Fe (but increased the Fe (super 2+) /Fe (super 3+) ratio), Mg, Mn, K, Zn, and Ba, and enrichment in Ca and Sr relative to least altered rocks. Almost constant interelement ratios of Ti, Zr, and Al in altered and less altered rocks indicate that these elements were essentially immobile during metasomatism and subsequent medium-grade regional metamorphisrm. Limited data suggest that the heavy REE were also immobile during silicification.The subconcordant silicification in the mafic volcaniclastic unit is interpreted to have formed at subsea-floor depths of 1 to 2 km where a felsic dike swarm and subvolcanic tonalite sills heated Si-rich evolved seawater above the temperature of the silica solubility maximum ( approximately 340 degrees -450 degrees C at pressures below 900 bars) causing silica deposition and metal leaching. A portion of the Fe, Mg, and possibly Zn that was leached from the subconcordant silicified zone may have been transported laterally away from this environment, thereby producing the semiconformable Fe-Mg metasomatized zones. Cross stratal structures similar to the hydrothermally altered synvolcanic faults that are known to cut the semiconformable alteration may represent fluid flow paths from a subconcordant metal reservoir in the volcaniclastic unit to the sea floor, where massive sulfides were deposited.Semiconformable pervasively silicified zones, particularly those associated with Fe-Mg metasomatized and/or epidotized rocks, are significantly larger exploration targets than areas of proximal alteration, and indicate large-scale hydrothermal mass transfer. Zonation of silicification to Fe-Mg metasomatism laterally within the alteration may provide a vector toward the discordant Fe-Mg-enriched alteration zones that commonly underlie volcanic-associated massive sulfide deposits.