Volcanic-hosted sulfide deposits at Mt. Lyell are confined to a complex silicic volcanic unit (mine sequence) which is up to 800 m thick and includes laterally impersistent lavas, ash-flow and ash-fall deposits, as well as abundant, open-framework, volcanic, mass-flow breccias. Pervasive hydrothermal alteration involving Na depletion is associated with the sulfide mineralization and persists with minor disseminated pyrite mineralization over a strike length of 8 km within the mine sequence. Stratigraphic relations together with lithofacies and thickness variations in volcanic units suggest that the sulfide mineralization and hydrothermal alteration occurred at a major eruptive center and that ore deposition occurred over an extended time interval during the accumulation of the mine sequence.Two orientation groups of Middle Devonian folds are present in the area. First generation (D 1 ) folds are closed to tight and upright north-northwest-trending structures which largely control the distribution and orientation of rock units in the volcanic sequence but have not been associated with significant microstructural modification of the ores or host rocks. Open west- to west-northwest-trending upright second generation (D 2 ) folds are well developed in sedimentary sequences overlying the volcanic sequence but are poorly developed in the volcanic sequence itself. D 2 strain has been accommodated in the volcanic sequence largely by the development of a mesoscopically penetrative west- to west-northwest-trending upright foliation (S 2 ) and associated downdip lineation (L 2 ). Shortenings of up to 60 percent perpendicular to S 2 and elongations of up to 150 percent parallel to L 2 have modified primary sulfide microstructures and are responsible for many of the sulfide deposits in the area being elongate in the L 2 direction.

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