Geology and mineralization of the Rosebery-Hercules area, Tasmania
The sequence hosting the Rosebery and Hercules orebodies is interpreted as a caldera complex. The felsic "footwall pyroclastics" represent caldera-forming ash-flow tuffs, in part deposited subaerially. Collapse of the caldera led to deposition of tuffaceous sediments over much of the area, and triggered formation of the massive sulphide deposits. A series of quartz-bearing mass flows interrupted the ensuing quiet sedimentary regime, but rapid and systematic variations in thickness of this sequence indicate resurgence of the caldera. The Mt. Black Volcanics, of massive dacitic, andesitic and minor basaltic lavas, with subordinate tuffs and limestone, were erupted during renewed volcanism.
Overlying the "central sequence" volcanics unconformably is the White Spur Formation at the base of the Dundas Group. This is compo§ed of epiclastic mass flows with an increasing sedimentary component, culminating in the argillaceous Chamberlain Shale, which is followed by Stitt Quartzite, dolomitic Westcott Argillite, and interfingering Natone Volcanics and Salisbury Conglomerate.
The Rosebery Fault is a major thrust, with a down-dip displacement of at least 1.5 km, which juxtaposes White Spur Formation with "footwall pyroclastics" west of Rosebery. The Dundas Group west of the fault is , disrupted by faults and tectonic melange zones.
Rosebery is a well-documented stratiform massive sulphide deposit. Footwall to ore is often strongly altered "quartz schist", but a chlorite-chalcopyrite-pyrite-magnetite stringer zone has recently been located beneath F lens. The ore lenses typically consist of basal massive pyrite-chalcopyrite directly overlain by banded pyrite-galena-sphalerite ore, and a separate, stratigraphically higher barite-sulphide lens. In F lens, barite occurs at the top of the pyrite - galena - sphalerite ore. Replacive tourmaline - pyrite - pyrrhotite - magnetite assemblages appear to be related to Devonian granite metasomatism.
The smaller Hercules orebody consists of a number of discrete but adjacent lenses oriented parallel to regional cleavage. The lenses show zonation from chalcopyrite-pyrite stringers at the base, to massive chalcopyrite - pyrite, then massive galena - sphalerite ore. Textures of the ore are often porphyroblastic, but primary features of "spotty ore" at the extremitites and below massive sulphide lenses, indicate a cavity - filling or replacement origin for some of the sulphides. The ore lenses are interpreted as a deformed massive sulphide orebody. Spheroidal carbonates are closely associated with ore and represent strong carbonatization of original tuff. Precious metal mineralization at South Hercules has unusual textures, distribution, mineralogy and relationships with carbonate alteration that suggest a replacement origin.
- Master's Thesis