Mount Read Research Articles

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.

Base metal exploration of the Mount Read Volcanics, western Tasmania; Pt. III, Application of lead isotopes at Elliott Bay

Lead isotope studies were carried out in the southern part of the Mount Read Volcanics on rock chip, drill core, and bed-rock geochemical samples in order to improve the genetic model of mineralization and to assign exploration priorities. Samples analyzed from 20 separate mineralized occurrences fall into three major isotopic groups which correspond to three different styles and episodes of mineralization.Samples of the Cambrian stratiform massive sulfide Pb-Zn-Ag mineralization at the stratigraphic top of the rhyolite volcanic pile constitute the least radiogenic group ( 206 Pb/ 204 Pb = 18.06 to 18.14), and samples from the Devonian vein-style Pb-Zn-As mineralization emplaced along major faults in the western epiclastic and sedimentary rocks form the most radiogenic group ( 206 Pb/ 204 Pb = 18.44 to 18.53). A third group consists of samples with isotopic ratios mostly intermediate between the other two; the majority of the 206 Pb/ 204 Pb ratios range from 18.18 to 18.30. The samples come from disseminated and vein-type Pb-Zn mineralization associated with bodies of quartz porphyry which intrude the pyroclastic sequence and are considered to be later than the episode of massive sulfide formation. The samples also contain strata-bound gold mineralization which is typically associated with carbonate alteration in the host volcanics. A plot of lead contents versus isotopic ratios of these samples gives an inverse correlation and indicates that the spread in ratios results from radioactive decay since mineralization. The samples as a whole indicate one source of lead emplaced in the Cambrian rather than a mixing of two sources, one Cambrian and the other Devonian.Lead isotope data for all samples of mineralization and volcanics studies can be interpreted in a multistage model to imply an ultimate source for the lead from the Precambrian basement. The ratios of massive sulfide mineralization at Elliott Bay are distinctly less radiogenic than for massive sulfide mineralization farther north in the Mount Read Volcanics at Rosebery and Que River and this implies the possibility of different ages for the separate mineralizing systems or a variation in the U-Th-Pb characteristics of source regions along the volcanic belt.

Tasmanian Cambrian biostratigraphy—a preliminary report

Abstract In Tasmania shelly fossils are known from Middle and Upper Cambrian sediments of the Dundas Trough, Fossey Mountain Trough, Dial Range Trough, Beaconsfield Trough, Smithton Basin, Adamsfield Trough and from within sediments associated with the Mount Read Volcanics of Western Tasmania. In the Dundas Trough fossils range in age from early Middle Cambrian (Ptychagnostus gibbus Zone) to the middle Late Cambrian (pre‐Payntonian A or B). Late Middle Cambrian fossils occur in sediments associated with the Mount Read Volcanics in two places in Western Tasmania. Late Middle Cambrian fossils only are known from the Smithton Basin and the Beaconsfield Trough. Late Middle to early Late Cambrian faunas are known from the Dial Range Trough; the Adamsfield Trough contains middle Middle to middle Late Cambrian fossils. Tasmanian Cambrian faunas show affinities with those of Queensland, China, the northwest Siberian Platform and northern Victoria Land, Antarctica.

The Mount Read Volcanics and Cambrian‐Ordovician relationships at Queenstown, Tasmania

Abstract The Cambrian Mount Read Volcanics at Queenstown can be subdivided into three major units: (i) a central belt of quartz‐keratophyric lavas, consisting mainly of quartz and feldspar porphyries with lenses of massive potassic lava along the axial zone; (ii) a flanking belt of acid pyroclastic rocks which interfinger with sedimentary sequences of the Dundas Trough to the west and with the central lavas, and which are here termed the Queenstown Pyroclastics; and (iii) an unconformable Late Cambrian sequence of volcanic and sedimentary rocks here defined as the Tyndall Group. Important minor units are quartz‐feldspar porphyry and hornblende‐pyroxene porphyry intrusives and some newly‐discovered granitic bodies. The Tyndall Group includes the Jukes Formation of earlier workers and a lower formation, which is missing in some areas, of acid tuff, breccia, and shale defined as the Comstock Tuff. A fossiliferous limestone occurs at the unconformable base of this group at Comstock, and gives a minimum age fo...

Large-scale Horizontal Displacement within Australo-Antarctica in the Ordovician

Crawford and Campbell have postulated1 (page 11) that the southern continental margin of Australia between longitude 131° ? and 143° ? “is the relic of a major shear which in Early Ordovician time extended still farther across what is now north-eastern Tasmania and that dextral movement of at least 300 km occurred”. We believe that this hypothesis is without foundation and that the curvature of the southern portion of the Adelaide Orogen and the curvilinear pattern of the Mount Read Volcanics in Tasmania, which were cited to support their arguments, can be explained without invoking1 (page 12) “fault-drag on an immense scale” on opposite sides of the postulated shear (see line AB in Fig. 1).

The Structure of the bedded rocks west of Rosebery, Tasmania

Abstract The Rosebery “Series”, due west of Rosebery, Tasmania, dips west and is the right way up. Along strike to the north and south there are marked changes in rock type, and possibly infolding and interbedding of sediments similar to the Crimson Creek Argillite. The Rosebery “Series” is younger than part of the Mount Read Volcanics, which may imply that some of the Volcanics are equivalent to the Oonah Quartzite and Slate.

On the Felsites and associated rocks of Mount Read and vicinity

On the Felsites and associated rocks of Mount Read and vicinity