Abstract

The Mount Morgan ore deposit was originally a 50 million metric ton body of quartz-pyrite containing some 280 metric tons of gold and appreciable amounts of copper.The quartz-pyrite ore is hosted by a roof pendant of Middle Devonian felsic volcanics in the Mount Morgan Tonalitc. The ore is discordant to both bedding and early (possibly thrust-related) deformation, enclosed by arcuate inward-dipping listric faults with normal displacements and partly encircled by a half dome of outward-dipping bedding. This striking arrangement is interpreted as a collapse feature, probably related genetically to the hydrothermal system.Assemblages and textures within the altered and mineralized area are related to the evolution of the hydrothermal system. The alteration zone has early-formed contact metamorphic and metasomatic assemblages in both the volcanics and the marginal parts of the tonalitc. These are overprinted by intense quartz-sericite-chlorite alteration crosscut by hydrothermal breccias. The ore consisted of early-formed quartz and pyrite aggregates crosscut by veins of quartz and pyrite, subvertical shoots of gold and silver tellurides, and pyrrhotite-bearing hydrothermal breccias.The orebody is interpreted as a structurally controlled replacement deposit related to the contiguous plutonic complex, a view accepted up to 20 years ago but abandoned for interpretations as, first, a porphyry copper-related deposit and, more recently, a volcanogenic massive sulfide deposit.

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