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Style and setting of volcanic-hosted massive sulphide mineralisation in the Early Permian Berserker beds, Mount Chalmers, Queensland


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Hunns, Steven Robert 2001 , 'Style and setting of volcanic-hosted massive sulphide mineralisation in the Early Permian Berserker beds, Mount Chalmers, Queensland', PhD thesis, University of Tasmania.

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Mount Chalmers is a Kuroko-style volcanic-hosted massive sulphide (VHMS) deposit that is hosted
within the Early Permian Berserker beds of central, eastern Queensland. The Berserker beds occur within
tectonic units that comprise the New England Orogen. During the Early Permian horst-graben style
extension occurred throughout the New England Orogen and across the back arc region. These fault
basins accumulated marine sediments, commonly diamictites and volcanics. It is in one of these
extensional basins that the sediments. volcaniclastics and volcanics that form the Berserker beds were
probably deposited.
The Berserker beds are a complex unit of graded, mass flow emplaced, pumiceous breccias, volcanolithic
sandstone units, graded polymict, feldspar phyric pumiceous breccias, rhyolite intrusives, coherent
to auto-brecciated rhyolite flows and their autoclastic products, andesitic intrusives, lavas and their
autoclastic products. In the vicinity of the Mount Chalmers, mine the Berserker beds are represented by a
dynamic and constantly evolving stratigraphic succession of proximal and distal volcanics interbedded
with distally derived turbidites. A significant feature of this evolving and variable stratigraphy is the
cyclic nature of the volcanism that is represented by the change in volcanism from rhyolite dominant to
andesitic dominant and back to rhyolite again. The differing volcanic facies indicate that both subaerial
and submarine volcanism were occurring within the general vicinity of each other.
The Berserker beds contain a diverse invertebrate fauna and trace fossil assemblage. The invertebrate
fauna contains bryozoans, echinoderms, brachiopods and molluscs. The fossil assemblage and its mode of
preservation are typical of a shallow shelf (near wave base) sand and silt environment. The faunal
assemblage indicates a water depth in the range of 50 - 300 m. The trace fossils are mainly temporary
fodinichnia (feeding traces) structures and comprise mainly Teichichnus and Planolites, with scattered
Rhizocorallium and Zoophycus type burrows. The trace fossils have a restricted faunal diversity, and may
be assigned to the Cruziana ichnofacies. This ichnofacies is normally understood as characterising the
region between daily wave base and storm wave base, in low to moderate energy regimes.
The mineralisation at Mount Chalmers occurs in two main ore lenses, the Main Lode and the West Lode,
and one minor lens, the South Lode. The massive sulphide mineralisation occurs on the flank of a rhyolite
dome. The massive sulphide mineralisation consists of massive, layered and fragmental sulphides.
Underlying the massive sulphide mineralisation is an extensive well developed network stringer veins.
Footwall alteration at Mount Chalmers is dominated by silica along with extensive zones of chlorite
alteration. Dolomite, sericite and kaolinite alterations are spatially restricted to high angle normal faults. Textural, petrographic and microthermometric investigations of primary fluid inclusions yielded
homogenisation temperatures of 160-268°C and salinities of 5-8 NaC1 equiv. wt %. Semi-quantitative
SEM/WDS microprobe analyses of fluid inclusion decrepitates indicate that the Mt. Chalmers ore fluids
were enriched in potassium and calcium but depleted in magnesium relative to seawater. PIXE
microanalysis of fluid inclusions in quartz also indicates a significant base metal concentration in these
fluids. Cation composition and higher salinities relative to seawater suggests that a magmatic input of ore
metals during seawater leaching of the footwall volcanic pile was a distinct possibility.
The 634S values for pyrite from Mount Chalmers range from -17.6 to -1.6 960, with a median value of-5.7
'Yoo Chalcopyrite has a narrower range of 8 34S values compared to that for pyrite Both sphalerite and
galena have restricted ranges in their 634S values compared to pyrite and chalcopyrite. Barite has a broad
range in 834S values and has a skewed distribution. In combination with the fluid inclusion evidence, the
favoured model to explain the 8 34S values in sulphides and barite was hydrothermal fluid that was
probably dominated by evolved seawater, but one that had a minor, but significant input of sulphur from a
magmatic source. The departures from the "normal" range of 8 34S values to values as low as -17.1 960 can
be explained by the hydrothermal fluid interacting with biogenic sulphur within microniches within the
sediments as the fluids passed through the volcano-sedimentary pile.
Mount Chalmers has a very narrow range of 6 180 values (+9.1 to +9.8 960). Modelled w/r ratios for both
open and closed systems indicate that the Mount Chalmers hydrothermal fluid was dominated by
seawater, with the possibility of the minor input of magmatic fluid. Calculated high water fluxes over a
short time period (5,000 years) indicates that high water/rock ratios (>1) prevailed throughout most of the
life the Mount Chalmers hydrothermal system.
Palaeontological evidence shows that the Berserker beds were deposited in a shallow-submarine
environment (<300 m). Contact relationships between the volcanics and sedimentary facies reveals that
peperitic facies are widespread. Silicic and mafic magmas have intruded at shallow levels into volcano-sedimentary
pile. The Mount Chalmers VHMS mineralisation is spatially and temporally associated with
the intrusion of a rhyolitic lava dome(s) into the volcano-sedimentary pile. Studies to date on modern and
ancient VMS deposits have indicated that a minimum water depth between 1,000 to 1,500 m is required
to prevent boiling of hydrothermal fluids and therefore concentrate metal deposition on the seafloor.
However, these minimum water depths are in strong disagreement with the minimum water depth as
suggested by the palaeontological and fluid inclusion evidence for the Mt. Chalmers VHMS deposit,
indicating that is possible for a hydrothermal system to form an exhalative VHMS in a shallow-marine
environment. Fluid inclusion and isotopic evidence suggests that there was a minor but significant input
of metals, vapour and S from a magmatic source.

Item Type: Thesis - PhD
Authors/Creators:Hunns, Steven Robert
Keywords: Geology, Stratigraphic, Volcanism
Copyright Holders: The Author
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Copyright 2001 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Charts in pocket at front of vol. Thesis (Ph.D.)--University of Tasmania, 2001. Includes bibliographical references

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