The Mt. Lyell copper deposits occur within 10 km 3 of hydrothermally altered, calc-alkaline felsic pyroclastics and lavas that form the overturned, steeply west-dipping limb of a north-south-trending anticline within the Mt. Read Volcanics.Exhalative pyritic massive sulfide deposits (e.g., The Blow) and siliceous barite-bornite-chalcopyrite mineralization (e.g., North Lyell) occur at or near the top of the volcanic sequence. The bulk of the mineralization occurs as disseminated pyrite-chalcopyrite lenses within the volcanics (e.g., Prince Lyell). The disseminated deposits are lenticular, broadly concordant with the stratigraphy and subparallel to the cleavage developed during the Tabberabberan orogeny (Middle Devonian). They are associated with quartz-sericite and quartz-sericite-chlorite rocks, the latter commonly containing minor siderite-hematite-magnetite, barite, sphalerite, apatite, and monazite and more rarely inclusions of bornite-chalcopyrite and chalco-pyrite-pyrrhotite in the pyrite.The host volcanics may have developed in a subaerial environment, but the exhalative sulfide deposits require water depths approaching 1 km. North-south- and east-west-trending faults, which were active during the Tabberabberan orogeny and probably controlled sedimentation patterns in the Early Ordovician and the Late Cambrian, may have controlled a shift to a partial or total(?) submarine environment at the time of mineralization and acted as conduits for solution movement. The disseminated deposits are thought to have formed from solutions percolating laterally along permeable horizons within the volcanic sequence underlying the exhalative massive sulfide deposits. The deposits at the top of the sequence were probably partly eroded during the Late Cambrian and/or Early Ordovician, and the entire sequence was metamorphosed to lower greenschist grade during the Devonian.Determination of the physicochemical conditions of formation of the disseminated pyrite-chalcopyrite mineralization has been attempted using the composition of chlorite coexisting with quartz, pyrite, and chalcopyrite, by means of a six-thermodynamic-component solid solution model for chlorite, and assuming ideal mixing for cations on energetically equivalent sites. The model is calibrated on the composition of the chlorite from the OH vein, Creede, Colorado, and estimates of the oxidation state and temperature during vein formation. Log Sigma SO 4 /Sigma H 2 S versus pH and log Sigma SO 4 /Sigma H 2 S versus T diagrams have been constructed to illustrate the variation in chlorite stoichiometry as a function of temperature and solution conditions.The temperatures of 260 degrees to 290 degrees C calculated from chlorite compositions at Prince Lyell, assuming quartz-iron oxide-chlorite equilibrium and the f (sub O 2 ) conditions above the hematite-magnetite buffer, probably reflect metamorphic reequilibration. Assuming quartz-chlorite-pyrite equilibrium and that each chlorite is a closed system for four of the six components, it is possible to estimate ore deposition temperatures of 270 degrees to 310 degrees C, initial log f (sub S 2 ) values of about -10 to -11.5, and initial Sigma S values between 0.0005 and 0.003--the latter estimate involving additional assumptions about the concentration of the major components in the ore fluid and the pH which was taken as 1 unit below neutrality. At 300 degrees C and a Sigma S value of about 0.001, a pH of between 4 and 4.25 and a log Sigma SO 4 /Sigma H 2 S value between 0 and -2 are required to maintain about 10 ppm Fe and 1 ppm Cu in a 1-molal chloride solution in equilibrium with pyrite and chalcopyrite.Bornite-bearing ores in the North Lyell area may have formed from acid, oxidized ground waters that derived their copper from leaching of preexisting chalcopyrite.The variation in delta 34 S py values from Prince Lyell (10 to 5.2ppm) to Cape Horn (+6.4 to -0.4ppm) to Crown Lyell-North Lyell (+0.8 to -5.3ppm) is consistent with a progressive increase in the oxidation conditions at which these deposits formed. The delta 34 S (sub py-ccp) values vary from +1.8 to -1.5 per mil and can be reproduced by mass transfer modeling of possible reaction paths in a feldspar-ore solution system at constant temperature and varying initial oxidation conditions. The calculations suggest an initial value of delta 34 S (sub Sigma S) of 7 per mil for the ore fluid(s).