Stratigraphic-volcanic setting of massive sulfide deposits in the Cambrian Mount Read Volcanics, Tasmania

Abstract

The Cambrian Mount Read Volcanics of western Tasmania host the volcanic-hosted massive sulfide (VHMS) deposits at Hellyer, Que River, Rosebery, Hercules, and Mount Lyell. The volcanics were erupted along, and partly onto, the western margin of a Precambrian basement terrane now represented by the Tyennan region. The volcanic belt is asymmetrical in section, with a main eastern zone of relatively massive lava-rich sequences, accompanied by abundant intrusions and a broader western zone of volcano-sedimentary sequences rich in volcaniclastic mass-flow sandstone and breccia. The rocks have been strongly deformed, regionally metamorphosed to lower greenschist facies, and hydrothermally altered in many areas.Stratigraphic and facies relationships within the highly complex volcanic belt are critically reviewed and reassessed. The main volcanic zone is considered to comprise two interfingering lava-rich associations: the quartz-feldspar-porphyritic Eastern sequence and the largely feldspar-porphyritic Central Volcanic Complex. This lava-rich zone interfingers with the western volcano-sedimentary sequences, herein referred to as: the Yolande River sequence, the Dundas Group, and the Mount Charter Group (new term).Recent volcanologic studies indicate that the lava-rich sequences, and indeed the bulk of the preserved Mount Read Volcanics, are submarine in character, despite the presence of abundant ignimbritelike pumiceous deposits.The early volcanism was predominantly rhyolitic-dacitic in composition but was followed by a period of active andesitic-basaltic volcanism during which the massive sulfide deposits at Hellyer and Que River, and some of those at Mount Lyell, were formed. This widespread andesitic-basaltic volcanism appears to coincide with a period of active extension and rifting along the belt, focused to some extent on the Henty fault system. The rifting episode is evidenced by abundant tholeiitic dikes along the Henty fault and throughout the northern Central Volcanic Complex and by tholeiitic pillow lavas within the Henty fault wedge.The andesitic-basaltic volcanism appears to have culminated in distinctive P 2 O 5 and light rare earth element (LREE)-enriched basalts at Hellyer, and similar rocks at Lynch Creek and Howards Plains near Queenstown may be correlates. The Hellyer basalts were buried by black shale, and the final volcanic phase was marked by a return to active felsic volcanism which produced widespread crystal- and pumice-rich mass-flow deposits of the Southwell Subgroup and Tyndall Group.