Cambrian Mount Read Volcanics Research Articles

Complex evolution of volcanic arcs: The lithofacies and palaeogeography of the Cambrian Stavely Arc, Delamerian Fold Belt, Western Victoria

The Mount Stavely Volcanic Complex is a Cambrian greenstone belt in western Victoria (Australia), which formed as a continental arc during subduction of the proto-Pacific plate under Gondwana. While the MSVC has been broadly described in literature, its lithofacies and palaeogeography remains unconstrained. The lithofacies that occur throughout the MSVC were characterised using field work, and diamond drill core from the Geological Survey of Victoria. The majority of the MSVC consists of fragmental facies that were deposited in a deep-marine environment. These deposits were subsequently intruded by melts, which formed the coherent facies; some became either shallow-intrusive or even extrusive. The volcanic processes that formed the Mount Stavely Volcanic Complex are similar to those currently occurring in the present-day Kermadec Arc. Furthermore the submarine nature of these volcanic facies is similar to the Cambrian Mount Read Volcanics in Tasmania.. However, in timing and cause of volcanic activity, not all of these ancient volcanic terranes are related. These findings help constrain the depositional environment in which the Mount Stavely Volcanic Complex was emplaced, which in turn provides important constraints for the tectonic reconstruction of Gondwana.

High-Precision U-Pb Zircon Chronostratigraphy of the Mount Read Volcanic Belt in Western Tasmania, Australia: Implications for VHMS Deposit Formation

Research Article| March 01, 2015 High-Precision U-Pb Zircon Chronostratigraphy of the Mount Read Volcanic Belt in Western Tasmania, Australia: Implications for VHMS Deposit Formation James K. Mortensen; James K. Mortensen † 1Pacific Centre for Isotopic and Geochemical Research, Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, Canada V6T 1Z4 †Corresponding author: e-mail, jmortensen@eos.ubc.ca Search for other works by this author on: GSW Google Scholar J. Bruce Gemmell; J. Bruce Gemmell 2CODES ARC Centre of Excellence in Ore Deposits, Private Bag 126, University of Tasmania, Tasmania 7001, Australia Search for other works by this author on: GSW Google Scholar Andrew W. McNeill; Andrew W. McNeill 2CODES ARC Centre of Excellence in Ore Deposits, Private Bag 126, University of Tasmania, Tasmania 7001, Australia Search for other works by this author on: GSW Google Scholar Richard M. Friedman Richard M. Friedman 1Pacific Centre for Isotopic and Geochemical Research, Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, Canada V6T 1Z4 Search for other works by this author on: GSW Google Scholar Economic Geology (2015) 110 (2): 445–468. https://doi.org/10.2113/econgeo.110.2.445 Article history received: 20 Aug 2012 accepted: 19 Jun 2014 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation James K. Mortensen, J. Bruce Gemmell, Andrew W. McNeill, Richard M. Friedman; High-Precision U-Pb Zircon Chronostratigraphy of the Mount Read Volcanic Belt in Western Tasmania, Australia: Implications for VHMS Deposit Formation. Economic Geology 2015;; 110 (2): 445–468. doi: https://doi.org/10.2113/econgeo.110.2.445 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract The Middle Cambrian Mount Read Volcanics of western Tasmania, Australia, host several world-class volcanic-hosted massive sulfide (VHMS) deposits, representing a wide range of deposit styles. Although the deposits and their host sequences are variably deformed and locally preserve spectacular examples of primary textures and structures, rapid lateral and vertical facies changes, and faults with uncertain sense and magnitude of displacement, have made it impossible to correlate stratigraphy across the belt. Previous dating studies in the area have yielded relatively imprecise crystallization ages. We have employed the chemical abrasion isotope dilution thermal ionization mass spectroscopy (ID-TIMS) U-Pb zircon method to obtain highly precise crystallization ages for a total of 18 samples of volcanic and intrusive rock units from throughout the Mount Read Volcanics and underlying mafic-ultramafic complexes. The new data permit detailed resolution of age relationships within the belt. The study establishes an age of 516.0 ± 0.9 Ma for the McIvor Hill gabbro, which is part of a mafic-ultramafic complex interpreted to underlie the Mount Read Volcanics. Magmatism in the central Mount Read Volcanics lasted at least 12.7 m.y., from 506.8 ± 1.0 Ma for a massive dacite unit in the lower part of the Central Volcanic Complex to 496.0 ± 0.9 Ma for a welded ignimbrite in the lower Tyndall Group. Together with previous age constraints, results of the study provide a precise chronostratigraphic framework for magmatism and VHMS deposit formation within the Mount Read Volcanics. The precise age data indicate that, north of the Henty fault, magmatism occurred in three discrete pulses, at least two of which were separated by periods of sedimentation. We demonstrate that VHMS deposits in the Mount Lyell, Roseberry-Hercules, and Que-Hellyer districts, comprising the majority of the known, significant VHMS deposits in the belt, formed within a narrow time interval at ~500 ± 1 Ma, at a relatively late stage in the evolution of the belt. Some of the larger intrusions in the belt (e.g., the Bonds Range porphyry; 500.4 ± 0.8 Ma) were emplaced contemporaneously with VHMS deposit formation; however, other bodies such as the Murchison granite (497.3 ± 0.9 Ma) are younger than the deposits and are unlikely to have been involved in their genesis as has previously been suggested. These conclusions will aid explorers in targeting VHMS mineralization in the Mount Read Volcanics. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Apparent welding textures in altered pumice-rich rocks

Fiamme and eutaxitic texture are common in ancient non-welded, pumice breccias. The fiamme are phyllosilicate-rich lenses that define a bedding-parallel foliation resembling eutaxitic texture in welded ignimbrites. Pumice breccias in the Cambrian Mount Read Volcanics (Australia) are composed of tube pumice clasts, bubble-wall shards, plagioclase crystal fragments and volcanic lithic clasts, and contain dark green fiamme and stylolites. These sericite or chlorite+sericite fiamme are aligned roughly parallel to regional bedding, and are enclosed in pale pink feldspar+quartz-altered domains of uncompacted, randomly oriented, tube pumice clasts and bubble-wall shards. Similarly, pumice breccias in the Miocene Green Tuff Belt in the Hokuroku Basin (Japan) comprise tube pumice clasts, fiamme, bubble-wall shards, crystal fragments and non-vesicular volcanic clasts. The dark green, wispy smectite or chlorite fiamme are aligned parallel to bedding, and are enclosed in domains of variably clay-, zeolite- and sericite-altered, uncompacted pumice clasts and shards. The fiamme in these examples vary in length (0.5 mm to 1 m), length to height ratio (3:1 to 40:1), shape (e.g. flame-like, feathery, branching, bow tie, wedge), and internal texture (porphyritic, massive, vesicular or stylolitic). The fiamme define an anastomosing bedding-parallel, pre-tectonic eutaxitic texture. Delicate feathery terminations and locally preserved vesicles indicate that the fiamme are altered compacted pumice clasts. The presence of bedding-parallel stylolites and solution seams, undeformed shards and pumice clasts adjacent to the fiamme, and the overprinting of fiamme by the regional cleavage suggest that the fiamme formed during diagenesis, prior to tectonic deformation, and that compaction did not involve welding. Fiamme and eutaxitic texture in these examples resulted from the alteration, dissolution and mechanical compaction of cold tube pumice clasts during diagenesis and lithification.

Pumiceous rhyolitic peperite in ancient submarine volcanic successions

Pumiceous peperite is associated with a rhyolitic sill that intruded wet, unconsolidated, submarine stratified pumice breccia in the Cambrian Mount Read Volcanics, Australia. Other examples of pumiceous peperite described in this paper occur at the pumiceous base of a rhyolitic lava and the margins of a rhyolitic cryptodome complex in the Miocene Green Tuff Belt, Japan. Intervals of pumiceous peperite are thin (<15 m), laterally discontinuous, massive, poorly sorted and the clast-to-matrix ratio varies significantly over short distances. They are composed of feldspar-phyric tube pumice clasts and domains of clastic sediment that include stratified pumice breccia, pumiceous siltstone and bioturbated mudstone. The sediment domains may be massive or may display relic depositional structures. Sediment adjacent to pumice clasts is silicified, possibly reflecting induration on contact with hot pumice. Pumiceous peperite has gradational contacts with in situ and clast-rotated pumiceous hyaloclastite and coherent pumiceous rhyolite. Gradational relationships between pumiceous peperite and coherent pumiceous rhyolite, pumice clasts with chilled margins and curviplanar surfaces, and abundant jigsaw-fit texture imply that quench fragmentation and autobrecciation were the dominant mechanisms of clast formation. The formation of pumiceous peperite is favoured by a combination of low confining pressure and delayed quenching allowing vesiculation. Pumiceous peperite may easily be misinterpreted or overlooked as it resembles other pumice-rich facies that are common in submarine volcanic successions. The correct identification of pumiceous peperite can provide evidence to constrain the timing of intrusive episodes in volcanic successions.

The Spectrum of Ore Deposit Types, Volcanic Environments, Alteration Halos, and Related Exploration Vectors in Submarine Volcanic Successions: Some Examples from Australia

Variations in shape, metal content, alteration mineralogy, and volcanic host rocks of the ore deposits in the two major volcanic-hosted massive sulfide (VHMS) districts of eastern Australia, the Cambrian Mount Read Volcanics and the Cambro-Ordovician Mount Windsor subprovince, strongly reflect their volcanic environment, conditions of ore formation, and hydrothermal alteration processes. Lens and sheet-style polymetallic zinc-rich deposits such as Rosebery, Hellyer, Que River, and Thalanga are considered to have formed in moderate to relatively deep water environments (500-1,000+m). These deposits probably formed either on the sea floor (e.g., Hellyer, Que River) or by replacement of porous volcaniclastic units directly below the sea floor (e.g., Rosebery). The footwall alteration associated with these polymetallic VHMS deposits was controlled by host-rock permeability and porosity, which are in turn related to volcanic facies type, degree of fracturing, and synvolcanic structural architecture. Focusing of hydrothermal fluids along synvolcanic structures has resulted in well-zoned chlorite-sericite footwall alteration pipes within footwall lavas at Hellyer. On the other hand, diffuse fluid flow through very thick pumice breccia at Rosebery and Hercules has resulted in strata-bound, sericite-dominated footwall alteration zones parallel to the paleosea floor and the ore lenses. Massive and disseminated, pyritic Cu-Au deposits, such as those in the Mount Lyell field and at Highway-Reward, formed by subsea-floor replacement and are associated with only minor zinc-lead massive sulfide ore. These deposits formed from higher temperature fluids (>300 degrees C), in which copper transport is enhanced, and are commonly located in felsic volcanic centers dominated by shallow porphyritic intrusions (e.g., Highway-Reward). The Cu-Au ore lenses may be strata-bound (e.g., Mount Lyell) or crosscutting pipes (e.g., Highway-Reward) depending on the structure and permeability characteristics of the felsic volcanic host rocks. The presence of high-sulfidation alteration minerals (e.g., pyrophyllite, zunyite) in some of the Cu-Au deposits (e.g., Mount Lyell field) indicates that fluids were relatively acidic and suggests the possibility of magmatic fluid input into the hydrothermal system. Alteration zonation associated with the Cu-Au VHMS deposits is more symmetrical than that of the Zn-rich deposits, with sericite-rich alteration extending into the hanging wall, in keeping with the subsurface replacement origin of these deposits. Synvolcanic gold-rich deposits, with high gold/base metal ratios are less common than the Cu-Au and Zn-rich VHMS ore types. The gold-rich ores (e.g., Henty, South Hercules) are strata bound in nature, have low sulfide contents, and are associated with central zones of intense silicification, surrounded by envelopes of sericite-pyrite and carbonate alteration. Volcanological and geochemical studies at Henty indicate the gold-rich ore formed by the replacement of particular volcanic units deposited in a relatively shallow water environment dominated by volcaniclastic facies, lavas, and limestones. This spectrum of Cu-Au, Zn-rich, and Au-only deposits in the Mount Read Volcanics and the Mount Windsor subprovince is interpreted to represent a continuum from classic sea-floor VHMS ores toward those with features more akin to porphyry Cu-Au and epithermal Au-Ag deposits. This spectrum relates to the interplay between factors in the submarine volcanic environment and the character of the hydrothermal fluid as follows: (1) proportions of volcaniclastic, lava, and subvolcanic intrusive facies; (2) depth of seawater; (3) permeability and porosity of volcanic host rocks; (4) balance between magmatic components and seawater components in the ore fluid; and (5) temperature and acidity of the ore fluid. Mineralogical, lithogeochemical, and isotopic studies have revealed a range of alteration vectors useful in exploration for both the Zn-rich and Cu-Au VHMS deposits. Carbonate and white mica compositional variations are highlighted as important mineralogical vectors; thallium and antimony halos may be useful trace element vectors; and oxygen and sulfur provide important isotope vectors toward the center of the hydrothermal system.

Geochronology of the Mount Read Volcanics, Tasmania, Australia

The late Middle Cambrian Mount Read Volcanics, Tasmania, Australia, comprise submarine rhyolite to basalt lavas, intrusions, and volcaniclastic rocks which host significant polymetallic massive sulfide deposits. Isotopic U-Pb and 40 Ar/ 39 Ar dating of magmatic zircons and hornblendes, respectively, from different parts of the Mount Read Volcanics yields a concordant age of 502.6 + or - 3.5 Ma (1Sigma ), which constrains the time of volcanogenic massive sulfide mineralization. The Comstock Tuff from the Tyndall Group in the upper part of the volcanic sequence and the Mount Black Dacite from the Central Volcanic Complex have weighted mean 206 Pb/ 238 U ages of 494.4 + or - 3.8 Ma and 494.9 + or - 4.3 Ma, respectively, and may be slightly younger than the other rocks. Fauna in the Mount Read Volcanics is diagnostic of a late Middle Cambrian (middle to late Menevian stage) age, and the isotopic dates therefore imply that the Late Cambrian is younger than 500 Ma. Inherited zircons in the Mount Read Volcanics have ages similar to those found in granites and sediments in the Tasman fold belt, southeastern Australia, which suggests similar source material for the two areas.

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

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.

Facies architecture of mineralized submarine volcanic sequences; Cambrian Mount Read Volcanics, western Tasmania

The Middle to Late Cambrian Mount Read Volcanics, western Tasmania, comprise compositionally and texturally diverse lavas and volcaniclastic rocks, most of which were emplaced in submarine environments below wave base. The facies architecture reflects the contrasting character and geometry of primary volcanic and volcaniclastic facies which are strongly controlled by eruption style and eraplacement processes. In the northern Mount Read Volcanics the principal volcanic facies are (1) silicic, intermediate, and mafic lavas; (2) juvenile volcaniclastic deposits, generated in association with the extrusion of lava flows, or else produced by explosive eruptions; and (3) synvolcanic intrusions involving silicic and mafic magmas emplaced into unconsolidated host sequences. Distribution and lithofacies characteristics suggest that deposits from both intrabasinal submarine and extrabasinal subaerial and/or shallow-marine volcanic centers are present. The volcanic facies are interbedded with a sedimentary facies association comprising black mudstone and/or graded bedded sandstone of mixed volcanic and Precambrian basement provenance. The predicted facies geometry of the principal volcanic facies has guided our approach to correlation in the Mount Read Volcanics. Mass-fiow-emplaced pumiceous volcaniclastic facies provide the best framework for correlation, because these are produced in large volumes, erupted infrequently, eraplaced rapidly, and are widely distributed. Distinctive units of this type that occur near Hellyer (in the Southwell Subgroup) and 40 km to the south at White Spur-Howards Road (White Spur Formation) could each be part of the same regionally extensive volcaniclastic facies association. A similar volcaniclastic facies association hosts the Hercules and Rosebery massive sulfide deposits and clearly demonstrates that such associations are prospective for this style of mineralization.

Geochemistry of the Cambrian volcanic-hosted massive sulfide-rich Mount Read Volcanics, Tasmania, and some tectonic implications

The Cambrian Mount Read Volcanics belt in western Tasmania hosts five major gold-rich massive sulfide deposits and remains the focus of intense exploration activity and geologic research. A geochemical study of least altered lavas and shallow intrusions, with emphasis on basaltic to andesitic compositions, was carried out to determine the primary magmatic affinities and tectonic setting of eruption of the Mount Read Volcanics, to aid in internal correlations within the belt, and to provide starting-material estimates for mineralization-related alteration studies.Three calc-alkaline suites (one extending to shoshonitic compositions) and two tholeitiic suites have been distinguished within the Mount Read belt. Suite I is voluminous and includes the Eastern sequence, Central Volcanic Complex, Tyndall Group, the intrusive quartz-feld-spar porphyries and granitoids, and the andesitic lavas of the Que-Hellyer footwall sequence. Suite I andesites are transitional, medium to high K calc-alkaline rocks ((La/Yb) N = 5-12, avg 8.1) and are the least light REE-enriched calc-alkaline lavas in the Mount Read Volcanics.Suite II comprises intrusive and extrusive, often hornblende-porphyritic andesites and dacites mainly from the upper part of the southern Central Volcanic Complex. They are more P 2 O 5 and light REE enriched ((La/Yb) N = 10-26, avg 16.7) than suite I and have high K calc-alkaline affinities.Suite III includes basaltic and andesitic lavas from the Que-Hellyer hanging-wall sequence in the northern part of the belt, the Lynch Creek basalts near Queenstown, and intrusive basalts in the Howards Plains area northwest of Queenstown. Suite III rocks show a striking compositional range, from low TiO 2 (0.4-0.5%), low P 2 O 5 (<0.1%) basalts with (La/Yb) N values from 8 to 12 and almost flat heavy REE patterns, to low TiO 2 (0.4-0.8%) but strongly to exceptionally P 2 O 5 and light REE-enriched basalts with up to 350 times chondritic La and (La/Yb) N values up to 34. The latter are regarded as shoshonites.Tholeiitic pillow basalts of the Henty fault wedge, and the closely related dikes of the Henty dike swarm, constitute suite IV: these have TiO 2 contents from 1.0 to 1.6 percent, very low Nb contents (<3 ppm), and only slightly light REE-enriched REE patterns ((La/Yb) N = 1.4-3.4). They are considered to be most similar to suprasubduction zone basalts erupted during the early phase of arc splitting and back-arc basin development and indicate a period of tension and rifting of the Mount Read belt in post-Central Volcanic Complex but pre-Tyndall Group time.Suite V is constituted by the Miners Ridge basalts and includes low TiO 2 (<0.7%) basalts with strong light REE depletion ((La/Yb) N <0.5) and higher TiO 2 (to 2.6%) lavas with only slight light REE depletion ((La/Yb) N = 0.9). Suite V basalts are tentatively correlated with rift tholeiite sequences associated with the Crimson Creek Formation, and therefore, are regarded as part of the pre-Mount Read Volcanics basement.The three calc-alkaline suites occur in a general stratigraphic order (with suites I and II being more or less coeval near Queenstown); they indicate a magmatic evolution from transitional medium to high K rocks through high K rocks, to strongly enriched shoshonitic rocks. Closest modern analogues are postcollisional volcanics in regions of arc-continent collision (e.g., East Papua and the Roman province).The occurrence of boninite-derived chromites (probably derived from the allochthonous mafic-ultramafic complexes outcropping just west of the Mount Read belt) in two sedimentary units within the Mount Read Volcanics sequence, below suite III basalts, supports a postcollisional origin for at least the major part of the Mount Read Volcanics. Other similar postcollisional sequences, particularly if formed in a submarine setting, must be considered highly prospective for VHMS deposits.

Geochemistry of the host rocks of the volcanogenic massive sulfide deposit at Que River, Tasmania

Que River is a moderate-sized, high-grade polymetallic volcanogenic massive sulfide deposit located in the Cambrian Mount Read Volcanics of western Tasmania. Several subvertical lenses of massive sulfide occur in a sequence dominated by variably altered intermediate to silicic volcaniclastic rocks. The sequence was deformed and regionally metamorphosed to prehnite-pumpellyite facies during the Devonian. Approximately 240 host-rock samples have been analyzed in an attempt to determine their primary geochemical affinities and to evaluate the geochemical effects of alteration.On binary variation diagrams, elements such as Zr, Nb, Y, Ti, and Al define linear fields that characteristically project toward the origin. The linear behavior of these elements on such plots is in marked contrast to that of almost all other elements, which show very complex distribution patterns. The linear patterns are interpreted to reflect immobility of the components mentioned, with movement up and down the linear arrays being controlled by concentration or dilution due to subtraction or addition of other components. Thus, primary ratios between immobile components have been preserved and can therefore be used to infer the primary geochemical affinities of the altered rocks. The intersection point of the linear trends defined by the Que River rocks with a low-pressure fractional crystallization trend defined by Cenozoic orogenic lavas probably provides the best estimate of the initial composition of the rocks prior to alteration.In a plot of Zr/TiO 2 vs. Nb/Y the Que River rocks define a restricted area dominantly in the fields defined by modern andesite and dacite-rhyodacite. Basalts and rhyolites are apparently absent. In terms of their primary geochemical affinities, the host rocks of the mineralization are probably best described as a high K, calc-alkaline suite, similar in many ways to modern continental margin andesitic rocks. However, the Que River rocks are by comparison abnormally depleted in Ti and enriched in transition metals such as Ni, Cr, and V.The geochemistry of alteration is complex and not readily amenable to quantitative evaluation. Elements such as Fe, Mg, and Mn have been added to the system whereas Ca and Na have been removed. Both Si and K have been partly remobilized during alteration. Among the major and minor elements, Mn shows the greatest relative enrichment over its inferred primary abundance. However, the Mn anomalies are sporadic and do not define a neat halo. Although most of the observed geochemical alteration appears to have accompanied mineralization, the andesites have been modified from their inferred primary compositions, probably during the regional metamorphism. Hydrothermal alteration is not symmetrical about the ore lenses. Although the presently observed displacement between the massive sulfide lenses and the alteration halo is probably due largely to the Devonian deformation, it is possible that there is also a primary component.The apparent immobility of Al implies that the abundant phyllosilicates, whose development is clearly related to mineralization, do not represent simple chemical precipitates or sediments. Rather, their development would appear to be controlled by the local availability of immobile Al.

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.

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...