Base metal exploration of the Mount Read Volcanics, western Tasmania: Pt. I. Geology and exploration, Elliott Bay

Volcanogenic massive sulfide deposits in eastern Australia range from less than 0.1 to 110 million metric tons; the median is 2.7 million metric tons. Of the 42 known deposits, 19 are in the Cambrian Mount Read Volcanics of Tasmania: these make up 62 percent of the total tonnage. The major types of deposits in the Mount Read Volcanics are: (1) lead-zinc-rich polymetallic (Pb-Zn-Cu-Ag-Au) ore depositsuch as Rosebery, Hercules, Que River, and Hellyer, and (2) copper-rich massive and disseminated ore depositsuch as Prince Lyell and North Lyell. The polymetallic ores average 21 percent combined Pb + Zn plus 160 g/metric ton Ag and 3 g/metric ton Au and the mean size is about 10 million metric tons; therefore, they represent an extremely attractive exploration target. Mineral exploration in the remote, and previously little-explored, southern part of the Mount Read Volcanics at Elliott Bay has revealed a geologic setting and mineralization styles similar to those in the main part of the volcanic belt between Mount Darwin and Que River. The volcanic pile at Elliott Bay is composedominantly of rhyolitic pyroclastics and quartz porphyry intrusions overlain on the western side by a sequence of epiclastics and marine sediments. The volcanics are dominantly quartz porphyritic whereas feldspar porphyritic assemblages are dominant in the central volcanic sequence which hosts the polymetallic massive sulfide ores to the north. A combination of geologic mapping (with particular emphasis on alteration styles), gold distribution in stream sediments and pan concentrates, and base metal soil geochemistry has proved successful in locating several types of mineralization in various horizons of the volcanic pile. The two major mineralization types, stratiform lead-zinc-silver massive sulfide lenses and strata-bound low-grade gold-carbonate deposits, occur toward the top of the rhyolitic pyroclastic pile 200 to 1,000 m below the contact with the western epiclastic sequence. Formation of the massive sulfide lenses is considered to relate to hydrothermal circulation along fault structures that are interpreted as marginal rift faults which developed early in the formation of the volcanic pile. The strata-bound gold and associated vein-style and disseminated galena-sphalerite occur with high-level quartz porphyry bodies and granitoids emplaced during the final phases of volcanic activity.