The Mattabi deposit, a typical zoned, copper-zinc massive sulfide orebody, is situated conformably in a steeply dipping, moderately metamorphosed, and slightly deformed Archean felsic volcanic pile. The footwall beneath the massive zone includes rhyolite agglomerate and poorly sorted lapilli tuff. Dolomite is a minor constituent of all rock types and forms approximately 10 percent of the lapilli tuff. An alteration zone is overprinted on the footwall rocks and includes vein-type chalcopyrite-pyrite ore which cuts (1) local patches of chlorite within and immediately below the ore, (2) a chert-silicified rhyolite-andalusite "tufa" stratum conformable with the base of the ore, and (3) a diagnostic siderite zone which extends to at least 1,000 feet stratigraphically below the ore. Coarse chloritoid is an important accessory mineral in the siderite zone.The siderite zone is characterized by addition of FeO, Fe 2 O 3 , MnO, minor MgO, CO 2 , and S, and distinct depletion of CaO and Na 2 O. Copper and zinc are bimodally distributed in the alteration zone, with the statistically anomalous population occurring dominantly in the central portion of the siderite zone. Factor analysis of quantitative trace element and mineralogical data indicates a siderite-Mn-Fe versus quartz-dolomite factor which accounts for most of the variability of the data, and best characterizes the products of the alteration process.The zonal arrangement of chlorite and silicification is similar to that observed in the Japanese stockwork deposits. The Mattabi alteration zone differs significantly from the Noranda deposits in its abundance of siderite and paucity of chlorite as a wide-spread indicator of alteration. Siderite has probably formed due to replacement of dolomite by iron from a rising, aqueous, metal-rich solution.