Most of the known sulfide mineralization in the Mt. Read Volcanics occurs within a central belt of rhyolitic-dacitic-andesitic volcanics, 9 to 12 km wide and roughly 90 km long, in which felsophyric lavas and ash flows are common but sediments are rare. The volcanics resemble rhyolitic caldera complexes in part, and large sections appear to be subaerial. Sediments and tuffs which probably represent marine embayments are also present, and the exhalative massive sulfides tend to coincide with local marine units. Preliminary radiometric dating indicates a Middle Cambrian age for the belt at Queenstown.The central belt is flanked on the western side by folded volcano-sedimentary marine sequences, partly of Middle Cambrian age, in which volcanic-lithic turbidites and quartz-phyric tuffs are common. Contacts with the central belt range from abrupt and faulted to conformable and possibly interfingering, and interpretation of the relationship is difficult. Some evidence suggests that the mineralized belt may represent a volcano-tectonic rift structure superimposed on a larger marine basin.Overlying the central belt in the Queenstown-Mt. Darwin area is an unmineralized sequence of volcaniclastic conglomerates and quartz-phyric volcanics of late Middle Cambrian to Late Cambrian age. Deposition of this sequence occurred largely in local graben structures and was accompanied by considerable erosion of the earlier volcanics. Some mineralization in the Darwin-Mt. Lyell area was probably removed at this stage.